Renaming ‘Chemosensory’ Proteins (CSPs): Lipid/Nucleotide-Binding Proteins — Molecular Nomenclature, Structure, Expression, Function, Evolutionary Networks, Clinical Diseases and Associated Molecular Medicine

1. Definition

The term “CSPs” stands for “Chemosensory Proteins”. They typically refer to small, water-soluble binding proteins, also known as odor-binding proteins (obps), that are strongly thought to mediate the recognition of odor molecules, odorants, and ligands to olfactory receptors (ORs), at the periphery of sensory dendrites in the insect sensillum (Vogt and Riddiford, 1981; Picimbon and Leal, 1999; Angeli et al., 1999; Picimbon, 2003). According to Lartigue et al. (2002), CSPs are composed of six α-helical chains with an appropriate molecular weight of 10-12 kDa (or 110-120 amino acid residues), four cysteines that form two tiny loops, two nearby disulfide bridges, and a globular “prism-like” functional structure (Figure 1). Four CSP structures have so far been identified in locusts (Schistocerca gregaria; Tomaselli et al., 2006) and moths (Mamestra brassicae, Bombyx mori, and Spodoptera litura; Lartigue et al., 2002; Jansen et al., 2006, 2007; Jia et al., 2021). Drosophila ananassae, D. erecta, D. grimshawi, D. melanogaster, D. mojavensis, D. persimilis, D. pseudoobscura, D. sechellia, D. simulans, D. virilis, D. yakuba, D. willistoni (Droso), Aedes aegypti (AAEL), Anophaeles gambiae (AgCG), Culex pipiens (CPIJ), Bombyx mori (Bmor), Apis mellifera (Amel), Nasonia vitripennis (NV), Pediculus humanis corporis (Phum), and Tribolium castaneum (AAJJ) genomic sequences are reported in Xuan et al. (2015), Liu et al. (2017, 2019, 2020), and Picimbon (2020). Sequences for Acyrthosiphon pisum (ACYPI) are presented in Table S1 (Genome Assembly and Protein Identity) and contrasted with those for Pediculus (Phum) and Bombyx (Bmor). While Pediculus CSPs show higher identity to molecular orthologs in Diptera, Hymenoptera, and Coleoptera, Acyrthosiphum CSPs show more identity to Hemipteran orthologs. Bombyx CSPs, in contrast, show the greatest degree of resemblance to their molecular orthologs in the kingdoms of bacteria and lepidopterans. (Table S1). They are highly similar to Allergen Tha p 1 and PAN-1-like protein molecules (BmorCSP5, BmorCSP14, BmorCSP17, BmorCSP18, BmorCSP11, BmorCSP10, and BmorCSP16; following the arrangement of the genes in the genome, Xuan et al., 2015, Table S1), which is an interesting finding. Allergen Tha p 1 is connected to the molecular sequence of three truncated pseudogenes (BmorCSP5, BmorCSP16, and BmorCSP18). The protein gene family’s ancestral function, which has nothing to do with olfaction, may have been immune system activation.

It’s also noteworthy that the molecule contains different amounts of amino acid residues between Cys29-Cys37 and Cys56-Cys59 (6-8, 18-19, respectively; Figure 1). For Culex CPIJ002628, and Nasonia NV16080, an amino acid insertion (two residues) between Cys29-Cys37 has been found (Liu et al., 2020; Picimbon, 2003, 2005, 2019, 2020). Locusta migratoria OSDs have a similar amino acid insertion (Picimbon et al., 2000a; Picimbon, 2005). For Nasonia NV16076 and NV16077, Apis GB17875, and Acyrthosiphon ACYPI000345, an amino acid insertion (one residue) between Cys56-Cys59 has been found. The intercysteine gaps in Drosophila, Anopheles, Aedes, Bombyx, Tribolium, and Pediculus are strictly preserved. Nasonia in particular exhibits amino acid insertion (or deletion) in many various regions of the CSP protein (Figure 1). As with B. mori ribosome peptide changes, these motif insertion mutations in jewel wasp and Culex primarily affect the N-terminal region and the loop between α2 and α3 (Figure 1; Xuan et al., 2014, 2016; Picimbon, 2019; Yue et al., 2023). All CSPs have two disulfide bridges and four cysteines, which keep the molecule stable (Figure 1). However, mutations in the ribosome and genome are probably what give the CSP protein molecule family its versatility for a variety of biological functions.

2. Protein structure and RNA sequence adaptability

Because of the CSP structure’s extreme flexibility, multifunctional features are heavily supported. RNA editing and/or post-translational changes, which were found in the silkworm moth B. mori are characteristics of CSPs (Xuan et al., 2014, 2015, 2016, 2019; Picimbon, 2017, 2019, 2023). The presence of recoding at the level of protein synthesis in the CSP family is clearly supported by the inclusion of a Glycine residue next to a Cysteine residue at certain positions, amino acid inversion, and motif insertion in protein sequence (Xuan et al., 2014, 2015, 2016, 2019; Picimbon, 2017, 2019, 2023). They can also ‘breathe’ or modify their conformation in certain ways in response to ligand binding, which may be another important characteristic of the ancestor multifunctional soluble binding protein (Campanacci et al., 2003; Mosbah et al., 2003).

3. Expression profiling in development, organisms, and tissues

CSP molecules are present in insects at all steps of their life cycles, from eggs and larvae through nymphs and adults (Picimbon, 2003, 2020; Picimbon et al., 2000a,b, 2001; Wanner et al., 2005). They are mostly expressed in the antennae, mouth, pedipalps, and legs of locusts, and they have been connected to phase shift (phenotypic plasticity) in those insects (Angeli et al., 1999; Picimbon et al., 2000a; Guo et al., 2011; Martín-Blázquez et al., 2018).

CSP molecules are not an insect’s apnage. They are also expressed in a wide range of organisms, including many species of arthropods, crustaceans, shrimp, crab, lobster, and copepods (Zhu et al., 2019; Picimbon, 2023). They don’t, however, only exist in arthropod species. They are clearly seen to exist in both eukaryotes and prokaryotes by the fact that they are very widely expressed at the level of the bacteria superkingdom (Liu & Picimbon, 2017; Picimbon, 2019; Liu et al., 2019; Picimbon, 2023; also see WP_149730592 in multi-species). Prokaryote CSPs are twins or identical twins to insect CSPs (Liu et al., 2019; Picimbon, 2023; WP_149730592), which is significant when discussing the function of these molecules. They have been reported on bacterial species, including Lysobacter and Escherichia coli (E. coli), coccobacillus Acinetobacter baumannii, Macrococcus/Staphylococcus caseolyticus, the filamentous actinomycete Kitasatospora griseola, the Actinobacteria genus in the families Entereobacteriaceae, Nocardioidaceae, Pseudonocardiaceae (Solhabitans fulvus), and Streptomycetaceae (Picimbon, 2023). According to RNA and genomic reports, CSP molecules are also found in firmicutes, aeromonadales, alteromonadales, eubacteriales (Clostridium perfringens), and hyphomicrobiales (MDK0835621, MDK0841570; Picimbon, 2023). Based on this, a chemosensing-related function is strongly disputed. These microbes are recognized as typical digestive tract bacteria, primary prokaryotic secondary metabolites, multi-drug resistant opportunistic pathogens, highly positive cytochrome c oxidase reactions, and multi-species symbionts in insects, and plants, but rather not for their olfactory acuity. Acinetobacteria are significant soil and aquatic microbes that aid in the mineralization of molecules like aromatic compounds (benzene). The smallest free-living prokaryotic cell (0.013 μm³) with very low GC (33%) is found in marine Actinobacteria (Candidatus actinomarinidae; Ghai et al. 2013). Given that their geographic distribution is similar to that of picocyanobacteria, there appears to be a strong relationship between the Candidatus and picocyanobacteria microbial groups. Based on the existing literature, it appears more likely that these two types of microorganisms exchange or share molecular modules and toxin-antitoxin systems rather than pheromones (Zhao et al., 2019; Doré et al., 2023). Clostridium perfringens (formerly known as C. welchii, or Bacillus welchii) is known for α-toxin, the toxin involved in gas gangrene (clostridial myonecrosis). It’s possible that CSPs participate in quorum sensing, a process where actinobacterial microbes produce and react to signaling molecules (autoinducers) to detect the presence of picocyanobacteria in their environment. But in this case, quorum-sensing controlled activities like bioluminescence, virulence factor secretion (toxins, hemolysins, proteases), biofilm formation, sporulation, conjugation, secondary metabolism, plasmid transfer, and/or pigment production would only work if a number of bacteria carried the others in synchrony. This might not include molecules like volatile organic compounds (VOCs), volatile carboxylic acids (VACs), pyrazines, chemosensory signals, aggregation odors, cohesion or sex pheromones, which have only seldom been linked to insect-bacteria associations (Taga and Bassler, 2003; Silva-Junior et al., 2018). Even while it would be intriguing to investigate the variety and binding property of CSP proteins in axenic insects (which lack bacterial populations in the gut system), it is still improbable that fecal bacterial CSP molecules are involved in the transmission of pheromone signals. The specific binding of VOCs, VACs, or locust cohesion pheromones has not been demonstrated for any CSP proteins (Dillon et al., 2000; Wada-Katsumata et al., 2015). Numerous bacteria produce CSPs that are strictly identical to those found in silkworms, however, the silkworm CSPs are all widely distributed throughout the entire body of the insect, not just found in its sense organs. They experience a striking up-regulation in response to pesticide drug exposure (Xuan et al., 2015; Picimbon, 2023).

Similarly to fungus, some Candidatus bacterial species express rhizobial Nod factors such as the canonical nod gene nodABC in symbiosis with their host plant (Persson et al., 2015). These bacteria also fix nitrogen-producing Nod for the plant. Thus, a multi-purpose activator and transporter molecule like CSP, which could bind to DNA and lipids and transport lipids and carbohydrates to bacteria, could be very helpful in the two organismal systems, activating rhizobial genes like nod through bacterial factors as multiple mechanisms of symbiotic interaction for adaptation and co-evolution (Siebers et al., 2016). Similar to this, Actinobacteria engage in numerous endosymbiotic interactions with insects in addition to interacting with plants and/or other microorganisms (Gopikrishnan et al., 2023). The insect gut bacterial communities may perform a variety of tasks, including cellulose degradation, nitrogen fixation, detoxification of protective plant compounds, pesticide degradation, inhibition of pathogens, and transport of nutrients, as shown by the gut bacteriome of Ips (Coleoptera, Curculionidae, Scolytinae; Chakraborty et al., 2020; Siddiqi et al., 2022). As a result, it is highly likely that bacteria, such as actinobacteria, share and exchange CSP molecule in the rhizobium of plants and the guts of insects for signaling, cell-cell communication, gene expression regulation, and/or the degradation of various toxins, all of which are necessary for the successful adaptation to specific ecological niches. Although this has not yet been experimentally demonstrated, the theory of horizontal transfer of DNA/RNA from bacteria to hosts strongly suggests that CSPs exist in plants (Han and Luan, 2015; Liu et al., 2016a; Zhu et al., 2016a; Lacroix and Citovsky, 2019). However, the widespread demonstration of CSPs in bacterial strains and insect guts across a wide range of species and genera suggests multi-duplication at a very early stage of evolution (Liu et al., 2020; Picimbon, 2023). To refer to an entire family with functions which are solely focused on chemical detection as ‘CSP’ is very incongruous.

‘CSPs’ are found in phyllosoma, in larval and adult stages, and in the adult stage, in many different non-sensory organs of many different crustacean species, including Antarctic copepod, crab, crayfish, lobster, salmon louse, prawn, shrimp, and water flea (Picimbon, 2023). This situation is pretty similar to how insect CSPs are being described right now. Insect venom can also contain CSPs, in addition to the sensillum (Angeli et al., 1999; Perkin et al., 2015). The sex pheromone gland in moths is where the majority of CSP molecules are expressed (Xuan et al., 2014, 2015; Picimbon, 2017). However, in addition to the female moth pheromone gland, CSP-expressing tissues and secretions also include the antennal branches, mouth, mandibles, salivae, proboscis, cephalic capsula, eyes, head, thorax, abdomen, epidermis, fat body, gut, wings and legs, i.e., a variety of reproductive and non-reproductive, sensory and non-sensory fluids, but in particular the majority of metabolic tissues (Celorio-Mancera et al., 2012; González-Caballero et al., 2013; Liu et al., 2014; Zhu et al., 2016b; Liu et al., 2020). EST-base data analysis in Pediculus and Acyrthosiphon (Figure 2) reveals distribution of CSP-RNAs throughout the entire body, in the antennae but also in the head, thorax, and abdomen, similar to Coleoptera, Diptera, and Hymenoptera (Liu et al., 2017, 2020; Picimbon, 2020). Six transcripts from first instar P. humanus corporis larvae and engorged adults were used to identify the CSPs in the body louse (Pedra et al., 2003; Kirkness et al., 2010; Liu et al., 2019). The International Aphid Genomics Consortium (2010) reported that the CSPs from the aphid A. pisum are also found at mixed stages, in the head and the antennae of third instar nymphs, and in winged and wingless parthenogenetic females inoculated with bacteria or treated with ampicillin for removal of pathogenic bacteria (Hunter et al., 2004; Stern et al., 2005; Richards et al., 2007; Shigenobu et al., 2010). Finding out how CSPs are expressed in different tissues can provide information on how they function. Pioneering Northern blot experiments, Western blotting, PCR, and advanced real-time PCR analysis reveal that moth CSP molecules are widely distributed throughout the body of the insect, including the head (epidermis, brain, and eyes), thorax, abdomen (gut and fat body), and wings in addition to the antennae, pheromone gland, and legs (Picimbon et al., 2000a,b, 2001; Xuan et al., 2014, 2015). By definition, the insect EST database, which contains information about the relationships of molecules and tissues of origin, contains more than 30,000 mRNA sequences from n tissue libraries (Pittendrigh et al., 2015; Ollivier et al., 2019). Therefore, we ”dissected” the honey bee, Apis mellifera, the silkworm moth, Bombyx mori, the jewel wasp, Nasonia vitripennis, the red flour beetle, Tribolium castaneum, the drosophilid dipterans Drosophila ananassae, D. erecta, D. grimshawi, D. melanogaster, D. mojavensis, D. persimilis, D. pseudoobscura, D. sechellia, D. simulans, D. virilis, D. willistoni, D. yakuba, and the mosquitoes, Aedes aegypti, Anophaeles gambiae, and Culex pipiens Flybase and specific databases such as VectorBase or KAIKO/SilkDB (http://flybase.org/blast, http://www.vectorbase.org, http://sgp.dna.affrc.go.jp, http://silkworm.genomics.org.cn; Xuan et al., 2015; Liu et al., 2017, 2020; Picimbon, 2020). The same methodology was utilized to analyze the aphid and Pediculus CSPs from Flybase and VectorBase (Figure 2). The EST-cDNA library from 21 insect species was analyzed, and the results revealed that the CSPs are widely expressed and are present in both chemosensory and non-chemosensory organs throughout the whole insect body (antennae, head, thorax, and abdomen, Figure 2). Following exposure to avermectin pesticide molecules, nearly all CSPs are up regulated in the majority of insect body tissues, especially in the gut, and fat body, which is crucial for repelling an olfactory function (Xuan et al., 2015).

4. The evolution and diversity of CSP genes

A function in chemical communication is also at odds with the diversity, evolution, quantity, and number of CSP genes. The 4-8 CSPs found in pediculus lice, honeybees, emerald cockroach (jewel) wasps, drosophila flies, and anopheles mosquitoes show that insects generally have very few CSP-coding genes (Picimbon, 2003, 2020, 2023; Wanner et al., 2004; Forêt et al., 2007; Liu et al., 2019, 2020). Does this imply that these insects have poor sense of smell or that they don’t communicate chemically with one another? Instead, the small number of CSPs disproves their potential for chemical communication (4 in Drosophila). For chemical communication, pheromones, and mate recognition, flies are known to use a complex mixture of long chain epicuticular hydrocarbons (Savarit et al., 1999; Wicker-Thomas et al., 2015; Yew and Chung, 2015; Blomquist et al., 2018). Bees, lice, and wasps only maintain 6-8 CSP genes, making it impossible for them to distinguish a variety of scents or complex chemical cuticular if CSP molecules only have one distinct function in chemosensing (Liu et al., 2019, 2020). According to Liu et al. (2020), CSPs are distinctly arranged in pairs of duplicates on individual chromosomes in bees. When examining the aphid CSP genes (ACYPI000094-ACYPI009116, ACYPI000093-ACYPI002311, and ACYPI000096-ACYPI003368), we find a similar distribution of duplication on particular scaffolds (Figure 3). CSPs may therefore cooperate to cause cellular function. These CSPs in aphids resemble DNA sequences that have been copied invertedly (Figure 3). They have a junction between them and are directed in opposing directions. They are therefore anticipated to increase translocation rates (Spealman et al., 2020). The most prevalent type of chromosome rearrangement, inverted duplications, also known as foldback inversions, is frequently seen in the emergence of new phenotypes, including particular developmental features in the brain. Chromosomal rearrangement and the degree of synteny play a role in pesticide resistance in aphids, which has long been known (Blackman et al., 1978; Mandrioli et al., 2019). As seen in aphids and ants, chromosome rearrangements are undoubtedly the most dramatic type of mutation, frequently resulting in rapid evolution, adaptation, and speciation (Mathers et al., 2021; Vela et al., 2022). Although the aphid CSP genes lack any specific retroposon elements, the ACYPI000095 intron resembles the Solanum lycopersicum cultivar I-3 on chromosome 8 (CP023764). The I-3 gene is responsible for tomato’s resistance to fusarium and increased susceptibility to bacterial spots, which may indicate that CSPs were transferred from aphids to plants, and boosted host resistance.

The number of CSPs and retroposon elements in the genomes of butterflies, moths, and beetles is significantly higher (about 19-20 containing SINE Bm1, Bm2, BMC1, BmRTE, L1Bm, MLE, Taguchi, Kendo, and/or Woot retroposons; Ozaki et al., 2008; Xuan et al., 2015; Liu et al., 2017). With Feilai and Wujin-Aa4/Wuneng retroposons, Culex and Aedes mosquitoes have even more CSP genes, indicating a high rate of gene duplication (Picimbon, 2020). The number of CSP genes varies between 27 and 83 in different Culex mosquito species (Mei et al., 2018; Picimbon, 2020). Aedes aegypti mosquitoes still carry about 69 CSP genes (Picimbon, 2020). This will eventually lead to discussions about pheromones, chemical communication, and long chain epicuticular hydrocarbon fatty acid diversity in these mosquito species (Wang et al., 2019). However, it is important to remember that the majority of these genes are strictly identical copies of the same gene due to tandem duplication, transposition to new chromosomes, or whole-genome duplication (polyploidy; Picimbon, 2020). As a result, this probably reinforces the same phenotype or function at the molecular level rather than introducing a new function like the binding of a new fatty acid. Several copies of the same gene that encode for the same molecule and are located on the same chromosome surely have an impact on how the cell functions.

The CSP protein may only be able to achieve “chemosensing” from such a limited set of genes through splicing, RNA editing, and protein recoding (Picimbon, 2019). Through retrotransposition, translocation, duplication, post-translational modifications and/or RNA + peptide editing mutation, CSPs can produce over hundreds of protein variants, as shown in Dscam and cochlear sensory genes of the bird auditory system (Neves et al., 2004; Xuan et al., 2014, 2015, 2016, 2019; Picimbon, 2017, 2019, 2023; Hou et al., 2022; Liu et al., 2023; Yue et al., 2023). Therefore, assuming that these mechanisms are limited to the antennae or sensory organs, which remains to be proven (see Xuan et al., 2014), it would be interesting to investigate the change of function in relation to chemosensing after gene manipulation, DNA splicing, RNA editing, and/or protein recoding and molecular changes through ribosome peptide mutations.

The CSP genes in aphids are composed of two exons separated by a single, variable-length intron, just like in most other insect species (Figure 3; Xuan et al., 2015, Liu et al., 2016a, 2017, 2019, 2020; Picimbon, 2019, 2020). A few nucleotides following the start codon that codes for the amino acid methionine, one more intron is inserted in ACYPI005842 and genes like AAJJ1196A, BmorCSP19, GB19453 and PHUM594410. This intron (phase 0 intron) is inserted after the third base and does not disrupt the codon. This demonstrates the tight regulation of the splicing of the signal peptide region and supports the notion put forth by Blobel (2000) that the functional significance of a signal peptide molecule is directly related to its length. The first six amino acids could be the needle tip that breaks through the cellular or subcellular membrane. The other amino acid residues might be crucial for the localization and/or transit of particular molecular partners from the Signal Recognition Particle complex within the cell (Matlin, 2002; Doudna and Batey, 2004). It’s interesting to note that ACYPI genes like ACYPI000094, ACYPI000345 and ACYPI003368 that have large signal peptides (large exon1 of 292-439 nucleotides) are connected to large intron (3103-8697 nucleotides; Figure 3). The length of exon 1 and its flancking intron in aphids vary together. This is a fundamental pattern regarding exon and intron motif differences in eukaryotic genomes (Zhu et al., 2009). This is a fundamental pattern regarding the arrangement of exons and introns in insect CSP genes (Xuan et al., 2015; Liu et al., 2016a, 2017, 2019, 2020; Picimbon, 2020; Figure 3). However, in CSP genes, the length of the flancking intron is not always correlated with the length of exon2. While the exon2 of the gene AAJJ0269C is enormous at 532 nucleotides, its intron length is 5063 bps. In contrast, the exon2 of the genes CPIJ002608, which has 312 nucleotides, and AgCG50174, which has 317 nucleotides, has intron lengths of only 77 and 97 bps, respectively, in their introns (Liu et al., 2017; Picimbon, 2020). It seems that in flies, an increase in average exon2 length is correlated with a decrease in intron length. The exon2 of the Drosophila genes DmelOSD, GD12430, GM24353, GE22174, GG15834, GA19747, GL15312, GK24936, GI14872, GJ19536, and GH12519 measures 214-299 bps, and the intron measures 52-191. The Drosophila ananassae GF20186 gene that is associated with these fly genes is composed of a single exon only. A conserved exon2 (143 bps) and an intron length of 52-436 bps are found in a number of genes, including CG30172, GD24980, GM18223, GE11495, GG19963, GA10970, GL10431, GK19383, GI20684, GJ20437, GH20166, and GF12039. This indicates that Drosophila CSP genes develop through exon1/intron deletion and exon2 expansion (Picimbon, 2020). Whether structural and functional properties of the molecule DanaGF20186 (exon2) remain, and whether these properties can be expanded through RNA editing and/or peptide mutation, would be an intriguing avenue to investigate.

Three exons and two introns are found to be common among genes encoding moth pheromone-binding proteins (Vogt et al., 2002; Picimbon, 2003; Abraham et al., 2005), in the Acyrthosiphon gene ACYPI005842, AAEL012383 (Aedes), AgCG50175 (Anopheles), GB19453 (Apis), NV16079 (Nasonia), PHUM594410 (Pediculus), AAJJ1796 (Tribolium), and three Bombyx genes, BmorCSP4, BmorCSP10, and BmorCSP19 (Figure 3; Xuan et al., 2015; Liu et al., 2017, 2019, 2020; Picimbon, 2020). Different positions are used to introduce the extra intron. The additional intron follows Ala82 in PBPs (Abraham et al., 2005). It is inserted into the nucleotide region that codes for the signal peptide in ACYPI003368, AAJJ1796A, BmorCSP19, CPIJ017094, GB19453, and PHUM594410. It is inserted at the codon that codes for Arg87 in BmorCSP4 while it is inserted at the codon that codes for Arg8, Arg76, and Ser115 in NV16079, AgCG50175, and BmorCSP10, respectively. This implies that the two exons and three introns genes in the “CSP” gene family, despite their structural resemblance, did not originate from the duplication of a common ancestor but rather from independent intron insertions that happened in different insect species during their evolutionary history. The Aedes gene AAEL001957 and the Culex gene CPIJ002607 both have notable arrangements and dizzying sizes. At the codons for the signal peptide residues, Arg9, Lys46, Ala165, Ser190, and Pro298 in that order, introns build up (Picimbon, 2020).

The fact that the introns in ACYPI000094, ACYPI009116, ACYPI000097, ACYPI000345, ACYPI000093, ACYPI002311, ACYPI000096, ACYPI003368, ACYPI000095, AAJJ0012A-I, AAJJ0283A, AAJJ0283B, ASP3c, GB10389, GB13325, GB19242, GB17875, PHUM594420, PHUM594430, PHUM594540, PHUM594550, PHUM594660, OS-D, CG30172, GD24980, GD12430, GM18283, GM24353, GE11495, GE22174, GG19963, GG15834, GF12039, GA15697, GA19747, GL10431, GL15312, GK19383, GK24936, GI20684, GI14872, GJ20437, GJ19536, GH20166, GH12519, AgCG50174, AgCG50200, AgCG50208, AAEL001963, AAEL001985, CPIJ002608, CPIJ002605, NV16075, NV16076, NV16077, NV16080, NV16079, NV16108, NV16109, BmorCSP1, BmorCSP2, BmorCSP3, BmorCSP6, BmorCSP7, BmorCSP8, BmorCSP9, BmorCSP11, BmorCSP12, BmorCSP13, BmorCSP15, BmorCSP17, BmorCSP20, and all other single-intron CSP genes vary in length but are always found in the same place in aphids, beetles, bees, body louses, flies, mosquitoes, moths, and wasps suggests that all these genes share a very ancient common ancestor (Xuan et al., 2015; Liu et al., 2017, 2019, 2020; Picimbon, 2019, 2020, 2023; Figure 3). The Ordovician, when terrestrial plants first appeared, is thought to be when the class of insects first appeared on Earth 480 million years ago. Overall, the current findings would be compatible with a shared heritage for the CSPs of all insect species, including those belonging to the orders Coleoptera, Diptera, Homoptera, Hymenoptera, Lepidoptera, and Neoptera. There is a very long history of the CSP molecule. After that, every grouping started to exhibit unique patterns, maybe as a result of the appearance of unique phenotypic and/or functional traits (Liu et al., 2020).

Intron loss and gain, retrotransposition, RNA editing + retrotransposition, and duplication were all factors in the evolution of CSP genes (Picimbon, 2003, 2019, 2020, 2023; Forêt et al., 2007; Kulmuni et al., 2013; Xuan et al., 2015, 2019; Liu et al., 2017). In moths, a single unifying hypothesis of RNA editing and retrotransposition-driven evolution of CSPs has recently been put out (Xuan et al., 2019). This hypothesis states that new CSP protein motifs are initially produced via DNA- and RNA-dependent RNA polymerization before edited CSP-RNA variations are retrotransposed in the genome. This evolutionary route might be able to explain the new abilities, skills, and tasks that the CSP molecule developed.

7. Terminology: function or structure?

This molecule family’s first member was given the name p10 in honor of the size and molecular weight (in kDa) of a soluble protein found in the regenerated legs of insects. The same protein, called Pam, was found in the fully developed antennae and legs of the adult American cockroach P. americana in both sexes (Nomura et al., 1992; Picimbon and Leal, 1999). Similar clones identified in the fruit fly D. melanogaster and the migratory locust (Locusta migratoria) during a search for olfactory genes were dubbed Olfactory-Sensory type D protein (OS-D or Pheromone Binding Protein A10; McKenna et al., 1994; Pikielny et al., 1994; Picimbon et al., 2000a). Without any concrete proof of specificity towards olfactory sensilla, that was merely an initial attempt to rename this molecule protein gene family in favor of olfactory or chemosensory function. Based on the remarkably high expression of this protein family in sensory structures like the legs and antennae, this nomenclature already addressed chemosensory functions. Likewise, to differentiate themselves from another family of soluble proteins already referred to as odor/pheromone-binding proteins (OBPs/PBPs, 16-17 kDa, six cysteines), later clones of p10, OS-D, or A10 found in the antennae of the tobacco hornworm (the sphingid moth Manduca sexta) were renamed and changed to “Sensory Appendage Proteins” (SAPs; Robertson et al., 1999; Ingham et al., 2020). The history of the different names used to characterize this molecule protein family as early as 1999 is compiled in Table 2. Even though p10 was found in leg regeneration tissue rather than antennal sensory structures, it already underwent three nomenclatures and three changes in 1999–2000, all of which had the same clear obvious meaning (chemosensory function), neglecting the first member of this protein gene family to be reported in the literature. Consequently, the protein gene family was renamed to exclude developmental functions and solely reflect putative chemosensory functions. At the time, it was believed that there was no connection at all between p10 relatives and mucins, cytoskeleton complexes, genetic elements, and intracellular processes (Figure 6).

After one (polyclonal) antibody against p10 labeled some chemosensory structures in the adult antennae of the desert locust S. gregaria, Angeli et al. changed the name of the p10/OS-D protein family to “ChemoSensory Protein” (“CSP”; Angeli et al., 1999). This marked the fourth change to the p10 nomenclature, which still heavily downplays the developmental role in favor of a chemosensory function. Clones that resemble “CSP” in microbe bacterial species are referred to as “B-CSP” (Liu et al., 2019, 2020; Picimbon, 2023). On the other hand, the fact that bacterial species contain “CSPs” begs the question of what function these might serve in chemical detection. What role, if any, CSP molecules play in chemosensing or olfaction is still unknown. This protein molecule family has been shown to function independently of the chemosensory system since 2003 (Picimbon, 2003; Sabatier et al., 2003). The molecules known as “Pherokines” are those that are highly present in the hemolymph of flies as a result of microbial, bacterial, or viral infection (Sabatier et al., 2003; Einhorn & Imler, 2019; Table 2), introducing the idea of p10 or CSP molecule in the immune response. Eventually, it was proposed to rename these molecules as “Cuticular Sensory Proteins” (“CSPs”) to highlight their strong expression in immune barriers, exoskeleton (to which muscles are attached), epidermis, epicuticles, insect major outer surfaces, antennae, legs, and sensory organs (Picimbon and Regnault-Roger, 2008; Einhorn & Imler, 2019). Thus, as information about this molecule protein gene family has grown, new names for it have also appeared, reporting functions far beyond the antennal sensory branch; however, none of them have addressed the possible involvement of “CSP” molecule in connection to actin-related protein complex, RNA/DNA, larger precursor molecules, mucins, nuclear complex proteins, transcription initiation, nucleoside kinase, and genetic regulation as we have here (Figure 4, Figure 5 and Figure 6 & Figures S1–S3).

What distinguishes OS-D, A10, SAP, and CSP from one another? Many synonyms (see Table 2) entirely overlook the most remarkable characteristic of this molecule protein family, which is expressed throughout the entire body of the insect. The entire molecule family, “OS-D”, “SAP”, or “CSP”, needs to be renamed in order to account for all family members across the different arthropod and insect species as well as across the different species in the bacterial prokaryotic superkingdom (Liu et al., 2020; Picimbon, 2023). The first “physiological” or “pharmacological” effect that is observed for a neuropeptide is usually used to name it, and this naming convention frequently turns out to be fairly inaccurate. It is probably best to rename all naturally occurring peptides, carrier proteins, “CSPs”, and similar molecules in a more methodical and objective manner. The location and wide range of activities listed for this molecule protein family conflict with the different names used to identify SAP or CSP members. It is necessary to come up with a new term or nomenclature without assuming any known functions already in place or limiting it to chemosensory or olfactory structures.

The definition of “SAP” and “CSP” is no longer valid, as there is growing evidence that CSPs express in the gut and metabolic organs and do not play a unique and crucial role in chemosensing. Instead, it now refers to membership in a subset of soluble protein molecules that share a high degree of structural similarity and a particular four-cysteine pattern (see Figure 1), but are all found at high levels in a large variety of organs and tissues (see Figure 2; Xuan et al., 2015; Liu et al., 2020). When referring to the entire molecule protein family, it becomes somewhat inappropriate to use the terms “SAP” and “CSP”, which exactly translate to “Sensory Appendage Proteins” and “Chemosensory Proteins” expressed in sensillum, respectively. This term should not be used to refer to all the genes and proteins expressed in a variety of sensory and non-sensory (metabolic) cell tissues and hemolymph, as well as those that have evolutionary ancestry with organisms ranging from bacterial microbes to honeybees (Picimbon, 2023). Perhaps the information needed to correctly name this molecule protein family is now available, heartfelt thanks to the analysis of the genome and Expressed Sequence Tag (EST) databases of bacteria, insects, sea crustaceans, and arthropods in the context of molecular data demonstrating that CSP proteins are not exclusively tuned to olfactory and taste chemosensory organs (Figure 2, Figure 3, Figure 4, Figure 5 and Figure 6 & Figures S1–S3).

This circumstance is comparable to that of “lipocalins”, a term for a superfamily of widely distributed, heterogeneous proteins that transports small hydrophobic molecules such as lipids and steroids (from the Greek lipos, which means fat, and Greek kalyx, which means cup). Unlike lipocalins, the term “CSP” family describes a group of homogenous, evolutionary well-conserved molecules with a characteristic sequence of four cysteines, tissue profiling (ubiquitously expressed), and rather diverse binding properties (not only to long fatty acids and straight lipid chains, C18:2, but also to cyclic compounds like cinnamonaldehyde; Liu et al., 2016b). In contrast, molecules belonging to the lipocalin family are diverse, heterogeneous, and poorly conserved throughout evolution (Diez-Hermano et al., 2021). Therefore, even though the gut and the fat body—which are believed to be the insect body’s main organs for storing lipids and fatty acids as energy—produce the majority of the CSP molecules, it is difficult to distinguish between groups and sub-groups within the CSP family. Through the process of lipolysis, these lipids (C18:2) are made available to other organs for use as fuel in their development, regeneration, or growth, as well as in their response to an infectious agent or insecticide (Xuan et al., 2015; Liu et al., 2014; 2016b, 2020; Picimbon, 2019; Einhorn and Imler, 2019). According to Xuan et al. (2015), the great majority of CSPs are taking part in the moth response to avermectin insecticide. According to Loftus et al. (2005), Verjovski-Almeida et al. (2005), Noriega et al. (2006), Nene et al. (2007), Plasmodium gallinaceum, Brugua malayi, and Dengue virus infections in Aedes, and bacterial infections in Culex, CSPs are also highly prevalent. Rather than being related to chemosensing, the abundance of CSP genes in moths and mosquitoes is likely related to toxin, insecticide, viral, or microbial resistance (Picimbon, 2020). Like in all other mosquito species, Ae. Aegypti, the dengue fever mosquito, has another use for CSPs. These molecules are found in both adult Aedes and larvae, but they are primarily concentrated in the adult’s fat bodies, corpora allata (sources of JH), and salivary glands (Picimbon, 2020). This suggests that, at least in the yellow fever mosquito Ae. aegypti, “CSPs” are involved in the binding, transport, and biosynthesis of juvenile hormone (JH). The function of CSP molecule in relation to juvenoids is strongly suggested by the widespread tissue and developmental expression of CSPs as well as the fact that CSP is present in other arthropod species such as copepods, shrimps, and water fleas in addition to insects. According to Kim et al. (2017), this function would be comparable to that of the OBPs in the hemolymphatic transport of JH molecule. JH is known to control the processes of development in insects and crustaceans (Laufer et al., 1987). Therefore, JH-related proteins, or JHRPs would be a better name for these proteins than SAPs or CSPs. There are no sensory appendages on corpora allata. Nearly every facet of physiology in both systems is regulated by JH function in both larvae and adults. Caste differentiation, pheromone production, development, male genital morphogenesis, female ovarian egg production, social and mating behaviors, and immune function are all regulated by JH (Jindra et al., 2013). Specifically, it has been shown that JH controls neuronal plasticity linked to brain structure, neurogenesis, and behavioral maturation; this is rather similar to the pattern of CSP expression in crickets, moths and honeybees (Cayre et al., 1994; Liu et al., 2020; Anton and Rossler, 2021). According to Bian et al. (2019), even the prothoracic glands (ecdysone) express CSP, which is incompatible with a function in olfaction. The fact that prokaryotic cells and bacteria share the CSPs and JH relationship is a crucial point in this analysis. Phurelipids are secondary metabolites that bacteria produce. Phurelipids and JH have a similar molecular structure. Not only did JH and other phurelipids hinder insects’ immune responses, but they also prevented pupae from developing into adults (Ahmed et al., 2022). From the binding of a wide range of exogenous foreign toxic chemicals to the intracellular transport of endogenous fuel C18:2, linoleic acid, lipids, hormones, and fatty acid precursors, all of these observations strongly imply that “CSP” molecules serve a variety of functions in the immune defense system of microbes, bacteria, crustaceans, insects and arthropods (Liu et al. 2016a,b, 2017, 2020; see Figure 6). This is accurate, unless it can be shown that CSPs are part of the microbe bacterial olfactory hoedonics. Even then, it’s not clear how “CSPs” connect to the transcription factors, RNA-BPs, Mucins, nuclear pores, DNA-regulatory proteins, SAmkC, WAS/WASL-like, CWP, actin-linkers, and other transcriptional factors that have been discussed thus far (Figure 4, Figure 5 and Figure 6 & Figures S1–S3). It is even less clear how CSPs could have evolved and been subject to natural selection in viruses prior to cell-based life (see Figure 4B and Figure 5B; Moelling and Broeker, 2019).

It is critical to look into the relationship between CSP, Mucin, NPCP, and other cell regulator molecules rather than just concentrating on chemosensing (see Figure 6). The intriguing and significant attachment of CSP proteins to larger molecules, like mucins, has implications for both evolutionary and functional issues. If the exon theory of genes—also referred to as the introns-early theory— is correct, it will depend on how CSPs can connect to mucin and/or what role CSP plays as mucin’s N-tail. According to this theory, exons may correspond to the boundaries of autonomous modules or small structural domains that assembled to form protein molecules during their evolutionary process. A CSP module, a block from the amino acid pattern around Cys53–Cys56, is present in both the large mRNA molecules that code for the protein TSSC1 and the pol-like protein (reverse transcriptase, RNAse H, and exo/endo phosphatase; ACYPI000612 and ACYPI005051, respectively). Furthermore, transcription factors, mucin variants, GP Ib-like, NPCPs, ASRPs, and an extensive list of nucleotide-binding proteins are just a few of the many large molecules that we assess in our comparative analysis of CSP-RNA binding proteins (Table 1 and Figure 4, Figure 5 and Figure 6 & Figures S1–S3). The N-terminal module in this instance consists of the entire CSP, which runs counter to the protein’s intended function of odor sensing (see Figure 5 & Figure S3).

Long lipid fatty acid chains and CSPs are activated for female sex pheromone synthesis in a variety of insect species, though moths in particular (Ohnishi et al., 2009; Xuan et al., 2014, 2016, 2019; Picimbon, 2017, 2019). Gene expression patterns and intracellular signaling are just two of the many cellular processes in bacteria that are greatly impacted by exogenous long lipid fatty acid chains. DiRusso and Black (2004) state that these transcription patterns primarily affect the proteins needed for fatty acid biosynthesis and breakdown. This puts the “CSP” molecules inside the cell rather than in the extracellular environment of the chemosensory neurons, which is consistent with the current findings: a very ancient gene with a highly conserved structure, that is expressed throughout an insect’s or crustacean’s entire body, spreads throughout many developmental stages, and is found in bacteria and possibly viruses (Picimbon, 2023; Figure 1, Figure 2, Figure 3, Figure 4, Figure 5 and Figure 6). In addition, it binds to a broad range of substrates, lipids, fatty acids, transcription regulator families, DNA and intracellular elements (Liu et al., 2016b; Figure 4, Figure 5 and Figure 6). Therefore, it is imperative that this protein family be given a new name. [on the basis of their existence in viruses, bacteria and arthropods, their widespread expression profiling, their regulation by xenobiotics, drugs, toxins, viruses, and microbial agents, in many different sensory and non-sensory organs of the insect body, and their ability to bind numerous non-semiochemicals, non-sensory endogenous ligands like long fatty acid lipidic chains and genetic components like DNA or RNA]. The new name should ideally not refer to any particular function because the evidence for a given function may be weak and it may change quickly, even in related groups of proteins. CSP/SAP is an example of a peptide molecule that, in many cases, would be better categorized according to its structure rather than just its function, particularly in cases where the physiological function is still up for debate.

It’s probably best if the peptide is just described in the new nomenclature (Figure 1). We could suggest using the general term “lipoclistins” (from Greek Λίπος lipos=fat and Greek κλειστό kleistó=closed in) for CSP and OBP molecules (“lipoclistins are proteins that enclose lipoid ligands, just like lipocalins do”; citing Rudolf Alexander Steinbrecht, 2003, 2020; Max-Planck Institute for Biological Intelligence, Seewiesen, Germany). The term “lip-anoiktins” (perhaps “lip-aniktins”, as oi in modern Greek is pronounced i; derived from Greek words for “fat” and “open”, respectively; open=ανoικτος anoiktos) may be a better way to describe the main structural component of the CSP —an open-air structure— according to this system of naming based on words and names’ Greek ancestry, following the nomenclature of COVID-19. World Health organization has designated key strains of SARS-CoV-2, the virus that causes COVID-19, with labels that are clear, easy to say, and memorable. From now on, all virus variations will be called by their Greek alphabetic names, like “Alpha”, “Beta”, “Gamma”, “Epsilon”, or “Omicron”. For CSP protein, the Latin word “apertus” might be a simpler, more tasteful term (citing Rudolf Alexander Steinbrecht).

A generic name such as “arthrolipin” would be inappropriate for CSP molecule since it is not specific to the arthropod family; it can also be found in bacteria and prokaryotic cells (Liu et al., 2019, 2020; Picimbon, 2023; WP_149730592). Not to be forgotten, in honor of the early literature and the old name, Alejandro P. Rooney (USDA-ARS, Lubbock, Texas, USA) proposed “4CSP” (Four Cys Soluble Proteins) or CSP/4Cys without drawing any broad conclusions regarding their physiological task. At the next international conference on very broad nomenclature, this would be a name to discuss. A good starting point for renaming all molecules or molecule families whose names don’t accurately reflect their functions would be the renaming of CSPs.

CSPs currently have a completely disorganized nomenclature. Even worse now that we have so much more knowledge than we did twenty years ago when we only considered expression in the antennae. “Chemosensory” does not refer to molecules that are expressed widely throughout the insect and linked to intracellular systems (Figure 6).

8. Evolutionary Networks and Associated Diseases

Originally, the idea of binding protein matrices for chemo-sensors was developed through research on the potential of “CSPs” as semio-chemical carriers (Lu et al., 2014; Zhu et al., 2019). But if we accept to take into account the entire current status of data gathered on this molecule family, CSP matrices could greatly aid in the development of other types of sensors, such as lipid, RNA mutation, and DNA sensors, as well as sensors of insecticide and bio-environmental pollutants (Liu et al., 2017, 2019, 2020; Kramer et al., 2022). In order to capture, bind, and transport hydrophobic odor molecules across the sensillum in sensillar lymph and control insect behavior, chemosensory proteins (CSPs) are not significant molecular components of the insect olfactory system (Liu et al., 2020). Since CSPs have long been willingly restricted to insects and other arthropods, no biomedical application was anticipated. This was unfortunate since these binding proteins had a high affinity for non-semiochemicals like linoleic acid, as reported by Liu et al. (2016b). This information may pave the way for new applications of CSP research, particularly in biomedicine and disorders linked to lipids. Another example is the study of the moth “pheromone”-binding protein (PBP), which has been shown to bind to specific vitamin compounds with a higher affinity (Guo et al., 2021). This finding is crucial in order to challenge “Reverse Chemical Ecology” (the idea that PBP may be used to discover pheromones; Zhu et al., 2017; Choo et al., 2018; Zaremska et al., 2022) and suggest using PBP in both health and sickness instead (Delrue and Speeckaert, 2023). Vitamin K transport is necessary for strong bones (Kohlmeier et al., 1996). Furthermore, because CSP molecules and other “olfactory binding proteins” can transfer C18-lipids—that is, oleic acid, palmitic acid, and linoleic acid—to degradative enzymes, they can potentially treat COVID-19 (Picimbon, 2023b). Restricting CSPs’ (and OBPs’) involvement in chemosensing would limit their use to anosmia and agueusia (Picimbon, 2002, 2003; 2023b; Ozaki, 2019). Restricting their involvement in insects and other arthropods, as well as too willingly ignoring their existence in the microbe bacterial kingdom, may result in excluding and ignoring their strong potential impact in infectious diseases and human medicine. Besides, if we can verify that CSPs are a component of glycoproteins in the viral outer surface membrane envelope, the potential in infectious diseases and human medicine will become even more significant.

Several studies show that CSP molecules are found in Eukaryotes, arthropods, insects, winged insects like moths and whiteflies, wingless insects like head and body lice, and Prokaryotes, the most primitive cellular systems lacking lymph and chemosensory dendrites. Many different bacterial strains express CSPs, including very small marine Actinobacteria and Proteobacteria such as Acinetobacter, Aeromonas, Escherichia, Klebsellia, Lysobacter, Macroccocus, Salinarimonas, and Vibrio (Picimbon, 2023a). These strains are known to cause serious infectious diseases in both animals and humans (Bendary et al., 2022). The majority of the bacterial species that harbor CSPs, including Moraxella, Staphylococcus, Streptomyces, Enterobacter and Xanthomonadales, are also known as critical symbionts of the insect gut, which is crucial for insect control (Rupawate et al., 2023). Among many other physiological systems, the gut microbiota is necessary for the growth, development, detoxification, feeding, immunological response, and environmental adaptation of insect pest hosts (Zhao et al., 2022; Suenami et al., 2023). Combining different bacterial species with CSP molecules could be a big step toward microbial control of insect pests in agriculture and potential human vectors (Hunter, 2019; Vega Rúa and Okech, 2019; Rooney et al., 2019).

Many types of bacteria, such as E. coli, Staphylococcus aureus, and A. baumannii, can cause harm to both humans and invertebrates. Some bug species are potential sources of severe human infection when the pathogen is not environmental in origin. This is the case, for instance, with the multi-resistant Pseudomonadales A. baumannii, which is known to be a symbiotic species of the head louse and human body louse (Pediculus) as well as a bacterial pathogen linked to a variety of diseases (pneumonia, meningitis, urinary tract and bloodstream infections; Liu et al., 2019; Morris et al., 2019; Hammoud et al., 2021; Larkin et al., 2023). Risky bacterial strains including Bartonella quintana, Borrelia recurrentis, and Rickettsia prowazekii cause fever, trench fever, relapsing fever, and epidemic typhus, which are all primarily spread by Pediculus lice (Badiaga and Brouqui, 2012; Cheslock and Embers, 2019). Therefore, investigating the potential genes that were transferred from the bacterium to the louse may be crucial to stopping many infectious diseases that affect humans, such as typhus. Research on CSPs—prospective molecules that can be transferred from human to louse—is necessary to understand infectious diseases spread by louses, investigate bacterial immunity in the body, and investigate epidemic diseases carried by vectors.

According to cutting-edge molecular research on insects, toxicity tolerance to chemical and biological toxins is directly correlated with binding protein molecules like CSPs and OBPs (Sabatier et al., 2003; Liu et al., 2014, 2016, 2020; Xuan et al., 2015; Einhorn and Imler, 2019; Guo et al., 2021). The widespread use of insecticide products like plant-derived pyrethrin neurotoxins (Permethrin) for control has led to a variety of lice developing resistance to many families of insecticide molecules, much like mosquitoes (Le Goff and Giraudo, 2019). This has caused the emergence or more re-emergence of lice and louse-borne infections in many countries around the world (Thakur and Chauhan, 2022). The small evolutionary network of CSP-encoding genes, which are present in the insect immune response primary pathways of Drosophila, Anopheles, Apis, Nasonia, and Pediculus, does not imply that these molecules are converted into trillions of odorants, but rather to a small repertoire of lipids and/or genetic elements (Liu et al., 2019, 2020; Picimbon, 2020). Although the evolutionary network of CSPs is much richer in plant-eating insects and mosquitoes such as Aedes and Culex, most CSP genes are duplicated copies of the same gene that are strictly identical in most cases (Picimbon, 2020). When the number of a gene varies between species due to a phenomenon in which portions of the genome are repeated, it is not chemosensory system growth or development; rather, it is copy number variation or gene dosage (Hastings et al., 2009). It would be interesting to look for the specific gene dosage within the evolutionary network of CSPs in bacteria, as gene dosage would represent the number of copies of the gene in a bacterial cell and gene expression would primarily depend on higher and lower dosage. CSP cannot recognize a broad range of ligands in bacteria if the dosage is the same as that of Drosophila (4) and Pediculus (6). The question of whether CSP can activate or regulate specific genetic elements is raised by the gene dosage. It doesn’t require a wide range of CSP molecules to play a control regulatory role in the expression of the genome if it can accomplish both (Figure 3). It would be especially important to investigate CSP molecules in bacterial species such as A. baumannii, M./S. caseolyticus, K. griseola, and E. coli, as they may offer an essential gene regulatory mechanism for toxin bactericide resistance in certain dangerous strains that are the main cause of serious infectious diseases. It is likely that early in their evolutionary histories, both microbial and insect host species experienced comparable resistance mechanisms, and it is possible that bacteria transmitted CSPs to insects through horizontal transfer (Liu et al., 2016a, 2017, 2020; Liu and Picimbon, 2017). This would explain the similarity between the CSP molecules found in silkworm moths and bacterial symbionts (Xuan et al., 2015; Picimbon, 2023a). CSPs may be involved in the transfer of resistance from one bacterial cell to another, as demonstrated by E. coli strains that can spread their resistance to the same antibiotic to nearby bacterial cells (Poirel et al., 2018). This would primarily involve how CSPs interact with promoter sequences and/or other regulatory components of gene expression involved in a cell’s or group of cells’ response to a hazardous environment, based on the similarity revealed here between CSPs, transcription factors, RNA-BPs, DNA elements, actin complexes, NPCPs, allergens, and mucins (Figure 4, Figure 5 and Figure 6 & Figures S1–S3).

If CSPs existed before the world of cells and before viruses? If CSP genes are present in bacterial cells and all of their progeny, including birds and reptiles, even though it is unknown how insects evolved from bacteria? That is probably the cause of the great diversity of CSPs that we found in crab species that are known to store lipids and to switch between sexual and asexual reproduction based on the activation of specific genes (Ye et al., 2023; Picimbon, 2023a). Furthermore, it is recognized that they trigger the activation of “CSP” genes in reaction to specific environmental stressors, including elevated temperatures, elevated salinities, and chemical toxicity (Picimbon, 2023a). The common pattern of evolution in CSP molecules that we report here in a comparative analysis of “chemosensory proteins”, mucins, TIFs, and “DNA-binding proteins” is quite intriguing, despite the fact that the protein structure, in particular the N-terminus, is largely retained from insects to bacteria (Picimbon, 2023a). Although further research is required, our analysis reveals a common function for TIFs, “CSPs”, and DNA/RNA binding proteins that must be none other than regulating crucial elements of gene expression.

It is worth noting that the CSP molecule (Mp10, no ligand) induces plant immunological responses when injected into the phloem, in a manner akin to that of microbial species associated with honeydew (Bos et al., 2010; Elzinga et al., 2014; Rodriguez et al., 2014; Wari et al., 2019; Pirtilla et al., 2023). Although the release of phytoalexins, jasmonic acid, salicylic acid, and/or volatile organic compounds (VOCs) from leaves is facilitated by CSP proteins, other salivary effector proteins, and/or honeydew microbes (Cui et al., 2019), it would be interesting to find out if specific CSP structures, the N-terminal tail, and/or the inoculation of specific bacteria, such as A. baumannii, M./S. caseolyticus, K. griseola, and E. coli, activate defense genes similar to those reported for plant-virus-vector interactions in human tissue, as well as target transcription factors and/or some protein degradation pathways (Ray and Casteel, 2022). The ability of CSPs to transport linoleic and benzoic acids may mediate the effects of CSP injection in a plant or human body, according to the results in B. tabaci whiteflies (Liu et al., 2016b). However, our comparison of CSPs to DNA/RNA-binding proteins (this study) may also suggest that CSPs activate specific genes via their N-terminal tail (Figure 4, Figure 5 and Figure 6 & Figures S1–S3). Benzoic acid and lipidic acid are the sources of plant hormones belonging to the jasmonate and salycilate classes, respectively. According to Bajguz and Piotrowska-Niczyporuk (2023), “CSP” or “Mp10” injection may help stimulate specific hormone biosynthesis pathways. During metabolism, dihomo-γ-linolenic acid (DHA) is produced from linoleic acid (LA), an essential fatty acid for humans, crustaceans, and many insect species. This molecule (DHA) is an essential part of phospholipids found in neuronal membranes and serves as a substrate for the synthesis of prostaglandins, which are believed to be essential for preserving nerve blood flow. LA performs numerous functions both directly and indirectly through its metabolites. Energy production can occur in both sensory and non-sensory tissues. It functions as one of the structural components of the cell membrane. It is thought to be an essential precursor to arachidonic acid, which is vital for various physiological processes, including neurodevelopment. LA maintains the epidermis of the skin resistant to water loss and is also necessary for the health of the human heart. When saturated fat is substituted with LA, randomized clinical trials have shown that it lowers both total and LDL cholesterol (Farvis et al., 2014). There is some proof that LA raises blood pressure and insulin sensitivity (Moloney et al., 2004; den Hartig, 2019). Consequently, “CSP” molecules like Mp10 may be viewed as lipid transporters and human health mediators rather than odor scavengers in insects. Although most organisms, including fish, crabs, lobsters, prawns, shrimp, marine crustaceans, molluscs, and terrestrial isopods, are thought to be incapable of producing LA on their own, it is readily produced by bacteria, protozoa, and plants (Malcicka et al., 2018). For these “animals”, odor chemical communication is therefore unlikely to be significantly influenced by LA. This lipid molecule family and dietary linoleic acid serve as metabolic precursors for the synthesis of a wide range of different chemicals, and animals’ ability to survive and grow depends largely on their nutrition (Liu et al., 2020). CSPs play a critical role in this process. However, given the striking similarity between CSPs and the N-tail of TIFs, Mucins, and numerous other proteins (Figures S2–S3), it is also possible that “CSP” directly activates genes associated with hormones, enzymes, jasmonates, and salycilates. Further research is required to determine the effects of injecting CSP or Mucin-N-termini through or via the skin of humans, shrimp, or plants on diseases related to lipid metabolism, hormone production, or genetic issues in plants, animals, and humans. Though gene activators have specific activities that are highly dependent on cell or tissue function, developmental stage, or pathological situation, fatty acid lipid transporters are present in almost all intracellular organelles (Ma, 2011; Ko et al., 2017; Cramer, 2019). It’s possible that CSP therapy enhances the dispersion of lipid droplets following ingestion, akin to a chylomicron, or during tissue growth or hormone synthesis by promoting the transport of lipids in the blood, lymph, sap, gut, glandular secretion, and body fluids. It is also possible that the use of CSP molecules controls genes related to biosynthesis, which leads to the synthesis of hormones and/or oxidation of lipids. This dual activity—lipid transport and contact with RNA/DNA—may be essential for the functional regulation of prokaryotes and eukaryotic cells (see Figure 6). The fields of clinical care, human health, and plant protection, as well as agriculture, could all benefit greatly from more research on CSP linked to DNA. Many clues point to the presence of a well-established active lipid metabolism in bacteria, plant, or human cells and tissues (Teng et al., 2017; Lamichhane et al., 2021; Jian et al., 2022; Macabuhay et al., 2022). The hypothesis behind this bacterial lipid hijacking, which is similar to that of a viral particle, has been supported by several studies (van der Meer-Janssen et al., 2010; Toledo and Benach, 2015; Allen and Martinez, 2020; O’Neal et al., 2020). It implies that the pathogen can spread to different tissues and become even more severely infected by breaking down the lipid molecules of the host while enriching its own stocks. Should CSP molecules possess the capacity to simultaneously interact with lipids and impact the expression of metabolic genes, they could play a pivotal role in impeding the process of lipid hijacking and vector-borne illnesses. Lipid molecules are primarily involved in controlling cellular function in bacteria, as they are in many other organisms, such as humans and insects. Furthermore, it is recognized that viruses regulate lipid synthesis, signaling, and metabolism in order to modify their host cells into the ideal conditions for virus replication (Heaton and Randall, 2011). The functions of membrane cells, such as cell division and protection, and selective permeability, which allows all water-soluble nutrients to be exchanged with the environment, depend on fatty acids (Barák and Muchová, 2013; Diomandé et al., 2015; de Carvalho and Caramujo, 2018). As such, the synthesis of fatty acid lipids in bacteria is either a strictly regulated “take” or “make” process. The use of a specific pool of lipids with particular physicochemical characteristics may be required in the event of a major alteration in the environment or in the lifestyle of the microbe. This can be accomplished through the modification of pre-existing fatty acid lipids, the synthesis of new ones, or the efficient extraction of them from their environment. After infection, fatty acid- and “niche-specific” changes in lipid fluxes were observed. The infectious agent is primarily dependent on the host’s lipid uptake, with the exception of blood, where the pathogen A. baumannii synthesizes its own fatty acids (Adams et al., 2021). Given that CSPs disrupt both lipid transport and the activation of metabolic genes involved in biosynthetic lipid synthesis, it is plausible that they have a substantial effect on the equilibrium between these processes. Modifying CSP molecules may interfere with the lipid pathways in the bacterial cell and impact both cellular and membrane activities, thereby blocking, decreasing, or eliminating the defense mechanisms of a particularly dangerous microbial strain.

9. Last reflections and conclusions

The existence of “CSPs” in bacteria and their interactions with fatty acids like LA disprove the overwhelming theory that these proteins have “chemosensory” capabilities. The low gene count, wide tissue distribution, expression at different developmental stages, sensitivity to chemical stress primarily in the gut and fat body, need for adaptation to any change in the cell environment, and excretion with saliva and hemolymph are some other arguments in favor of renaming the molecule family that should be taken into consideration in addition to this. Contrary to the “chemosensory” theory proposed for this molecule, CSP builds the N-terminal tail of some viral envelope glycoproteins (see Figure S3). Functional genomics, genetic engineering, clinical genetic cancer, cell regulation, eukaryotic, prokaryotic, and viral transcriptional mechanisms, medical genetics, medical microbiology, cell regulation, infection, and management of hereditary disorders may all benefit from further research on the role and binding of “CSPs” to lipids and genetic components, such as promoter regions that control gene expression in various systems.

Through the direct regulation of defense gene expression, novel methods to knockdown ‘CSP’ genes in pathogenic bacteria or insect pest species might potentially be developed. This would change the target species’ lipid transport (fuel), lower virulence in drug- or insecticide-resistant strains, and decrease resistance to antibiotics, bactericides, and germicides. While CSP-LA may aid in understanding the connections between lipid metabolism and insulin resistance, CSP-RNA or -DNA may introduce some crucial ideas on the drug resistance of cancer cells. In numerous ways, controlling the activity of bacterial “CSPs”—which are more appropriately thought of as lipid scavengers and gene activators than as chemosensory molecules—looks to be a novel and extremely promising method for observing how lipids, “made or taken”, host-virus interaction, RNAs, genes, genome, and chemical medication interact, as well as for lessening the pathogenicity of dangerous bacteria like A. baumannii, E. coli, Klebsiella pneumonia, Mycobacterium tuberculosis, Pseudomonas, Staphylococcus, and Streptococcus, among many others. Changing the physiology of the symbiotic bacterial cell to cut off the parasite’s main source of lipid fuel is another possible modern parasite management strategy for Pediculus or Aedes. Our finding that linoleic acid interacts with Bemisia ‘CSPs’ (Liu et al., 2016b, 2020) and their resemblance to Allergens, Transcription Initiation Factors, Cell Walls Proteins, DNA/RNA-binding proteins, Nuclear Pore Complex Proteins (NPCPs), Actin Skeleton Regulatory Proteins (ASRPs), and Mucins, as reported here (Figure 4, Figure 5 and Figure 6 & Figures S1–S3), suggests two approaches to treat infectious or vector-borne diseases: either focusing on a specific lipid component of bacterial cells, or eliminating the infectious agent that controls the metabolic genes of numerous insect pests and parasites.

The final conclusion is that too many variables preclude “CSPs” from having a part in chemosensing. In 2023, a chemosensory role may be supported only by the name of the particular molecule protein gene family and all of its synonymous terms. Even after thirty years, clones of this protein have been found in a wide range of organisms, including bacteria, arthropods, crustaceans, and insects, but its actual function is still unclear and the topic of much discussion. Starting from scratch, studies of the CSP molecule in these organisms (and viruses) may be able to solve the puzzle and provide new approaches for controlling transcription and gene expression. These approaches may involve utilizing the novel hypothesis that these proteins are intracellular, linked to lipid transport, long-chain fatty uptake, hormone biosynthesis pathways, actin-linked complexes, nuclar pore protein complex, cell wall, phosphorylation of G-protein, channels, enzymes, and receptors (see Figure 6).

As demonstrated here, CSPs can exist as small modules made up of six α-helices or as a small module linked to a larger molecule, such as a mucin (which plays a variety of roles in protecting mucosal surfaces), a transcriptional initiation factor (TIF), which increases transcription of a gene or set of genes, or a positive regulator of the genome.

It would be irresponsible to overlook the potential that these new analyses bring to the fields of cell physiology, development, and clinical medicine. CSP molecules attach to linoleic acid to provide energy to all of the bacteria and cells in an insect’s body; gel-forming mucins are required for the lubrication and removal of debris, pathogens, and allergens. Based on these two unavoidable facts, we must relaunch p10’s plot.