Advances in the Evolution and Function of the RSPO Gene Family

1. Introduction

The RSPO gene family, encompassing members such as RSPO1, RSPO2, RSPO3, and RSPO4, consists of secreted glycoproteins rich in cysteine that serve as essential activators of the Wnt signaling pathway. This pathway functions as a critical mechanism for various developmental processes, including embryogenesis and organ formation, alongside cellular functions like stem cell maintenance, proliferation, and differentiation.[5] Multiple studies indicate that Rspo interacts with extracellular elements of the Wnt signaling pathway (Figure 1).

Initial research identified the RSPO family, proteins important for developmental signaling, in model organisms like mice and zebrafish, yielding key insights into their biological roles.[1] However, the evolutionary history of these genes remains unclear, largely due to complex patterns of gene duplication, loss, and adaptation across lineages, as well as gaps in fossil records and genomic databases. Scientists actively investigate their origins, diversification, and functional roles across species using comparative genomics, phylogenetic reconstructions, and functional assays to uncover mechanisms behind conservation and variation in different taxa.

The research significance of studying the RSPO family is profoundly important, as these genes encode key regulators of the Wnt signaling pathway, which orchestrates essential cellular processes in development and homeostasis. By meticulously elucidating the evolutionary trajectories and functional divergence of RSPO genes across diverse taxa, researchers can gain invaluable insights into the intricate molecular mechanisms that underpin developmental biology, including cell fate determination, tissue morphogenesis, and regenerative capabilities. Moreover, understanding the adaptive evolution of these genes and their conserved or divergent functions in various model organisms provides critical clues for advancing disease research. This is especially pivotal in fields like oncology and congenital disorder studies, where aberrant Wnt signaling is frequently implicated in tumor progression, metastasis, and structural birth defects, thereby opening avenues for identifying novel biomarkers and therapeutic strategies.[3]

Recent advances (2023-2025) in RSPO family research have highlighted subtype-specific roles in disease pathogenesis. RSPO1 has been identified as a key regulator in liver fibrosis, where it drives hepatic stellate cell activation and collagen deposition through Wnt/β-catenin pathway activation [7,18]. Additionally, its role in promoting intestinal epithelial proliferation has been validated in murine models, emphasizing potential therapeutic applications for mucosal repair [8]. For RSPO2, emerging evidence indicates its oncogenic function in colorectal cancer (CRC) by potentiating Wnt signaling through ZNRF3 inhibition, with copy number amplifications and gene fusions frequently detected in CRC patients [7,15,31]. Recombinant RSPO proteins, such as human RSPO1 expressed in CHO cells, have shown promise in preclinical models for modulating β-catenin nuclear translocation, offering novel strategies for treating Wnt-related disorders [8].

2. Classification and Evolutionary Analysis of RSPO Gene Family

Comprehensive Gene Characterization & Family Clustering Analysis

The R-spondin (RSPO) gene family has become a key regulatory element in Wnt signaling pathway studies. Consisting of four members (Figure 2) – RSPO1, RSPO2, RSPO3, and RSPO4 – this family amplifies Wnt signaling by interacting with its receptors, fulfilling critical roles in diverse biological functions. RSPO proteins significantly regulate numerous physiological processes, such as embryonic development, stem cell maintenance, tissue regeneration, and tumor formation. Acting as upstream regulators, they primarily activate the Wnt/β-catenin pathway through binding to LRP5/6 receptors. Specifically, RSPO proteins engage LGR4/5/6 receptors to block the ubiquitin-mediated proteasomal degradation of Wnt receptor complexes. This action stabilizes β-catenin, promoting its accumulation and facilitating cell proliferation and tissue growth, thus underpinning RSPO’s vital function in early embryonic and organ development. Furthermore, by modulating Wnt signaling, RSPO enhances stem cell proliferation and self-renewal, which is crucial for tissue repair and regeneration [9,14].

The robust maximum likelihood phylogenetic tree clearly illustrated in Figure 2B reveals the evolutionary relationships among the identified genes . Significantly, our findings indicated that RSPO family members were entirely absent in all examined species from key phyla including Arthropoda, Porifera, Placozoa, Cnidaria, and the specific organism Helobdella robusta, highlighting potential evolutionary gaps. To rigorously assess the homology of RSPO isoforms, we systematically mapped the highest sequence identity derived from Blastp comparisons between each RSPO family member and the human RSPO variants (hRSPO1 to hRSPO4), positioning these identity values around the periphery of the phylogenetic tree for intuitive visualization in Figure 2C. Additionally, to accurately estimate and vividly depict sequence divergence within the RSPO gene families, we applied the R package ‘dgfr’. This involved generating a comprehensive distance matrix through pairwise comparisons of all gene sequences within each family, followed by dimensionality reduction to simplify the complex data. Subsequently, we calculated both the mean and median similarity metrics alongside precise protein sequence counts for each emerging cluster, ultimately determining that the optimal number of clusters for RSPO is four, as effectively demonstrated in Figure 2A.

Phylogenetic Tree Construction of the Four Major Subtypes (RSPO1-4): Mapping the Genetic Evolution.

Building upon our comprehensive gene identification efforts, which involved high-throughput sequencing and comparative genomic analyses, we meticulously constructed detailed phylogenetic trees for the four major subtypes of RSPO genes, namely RSPO1, RSPO2, RSPO3, and RSPO4, employing robust maximum likelihood algorithms and multiple sequence alignments to ensure accuracy. (Figure 1A, B) This intricate work allowed us to visualize and trace the evolutionary relationships, divergence patterns, and ancestral connections within these subtypes across diverse species, thereby providing invaluable insights into their genetic evolution, such as key adaptive mutations and conserved functional domains that elucidate their roles in developmental pathways.[35]

Investigating the Evolutionary Roots and Divergence: Revealing the Origins of RSPO Genes [2]

Delving deeper into the evolutionary history of RSPO genes, which are critical components in Wnt signaling pathways regulating development and homeostasis, we focused on the conservation of the RSPO4 subtype across diverse animal lineages. Our findings, derived from extensive phylogenetic comparisons, indicate that RSPO4 is present not only in Gnathostomata, such as vertebrates, but also in Protostomia, including arthropods and mollusks, suggesting that it potentially originated from the common ancestor of bilaterian animals prior to their divergence. This discovery sheds light on the ancient origins and subsequent divergence of RSPO genes, highlighting their fundamental role in the evolutionary process by preserving essential regulatory functions that have persisted through metazoan history.[14]

Comprehensive Analysis of Copy Number Distribution Patterns for Each Subtype Across Diverse Species

The four RSPO subtypes, which play critical roles in regulating Wnt signaling pathways, exhibit markedly distinct evolutionary patterns across animal lineages. Specifically, RSPO1, RSPO2, and RSPO3 isoforms have undergone evolutionary loss in protostomes and remain exclusively confined to the deuterostome lineage, suggesting a specialized functional adaptation within this group. In contrast, RSPO4 demonstrates a highly conserved presence across both protostome and deuterostome groups, indicating its fundamental role in core biological processes. This divergence underscores the varied adaptive pathways that have shaped evolutionary history, reflecting differential selective pressures and functional innovations among these molecular regulators. This involved systematically examining how these patterns vary among different organisms, such as mammals, birds, and insects, thereby providing a clearer picture of the genetic diversity, including variations in gene expression and functional adaptations, and the evolutionary pressures, like selective pressures and gene family expansions, that have shaped the RSPO gene family over time.[55]

Our research on gene identification and family clustering has yielded key advances, including the expansion of the catalog of known RSPO homologous genes through the discovery of novel variants, detailed characterization of their structural domains such as furin-like repeats and cysteine-rich regions, and functional annotations. Simultaneously, we elucidated the evolutionary trajectory of these genes by tracing lineage divergences, adaptive changes, and selective pressures across diverse organisms, from vertebrates to invertebrates, revealing conserved mechanisms. By employing comprehensive phylogenetic analyses using advanced methods like maximum likelihood and Bayesian inference, we investigated evolutionary origins at ancestral nodes, reconstructed ancestral sequences, and evaluated distribution patterns across extensive genomic datasets, including whole-genome assemblies and transcriptomes. This integrated approach has significantly deepened the understanding of these critical genetic elements, highlighting their conserved functions in Wnt signaling pathways and their potential importance for developmental biology, particularly in organogenesis and tissue regeneration contexts.

3. Structural Characteristics and Functional Analysis of RSPO Gene

The principle of structural conservation is fundamental for investigating the FU Repeat Motif within the RSPO3 subtype of ray-finned fishes, as it underpins the analysis of evolutionary stability and functional adaptations. This motif is precisely defined by the presence of three conserved cysteine residues, which are crucial for preserving the protein’s structural integrity through the formation of disulfide bonds that stabilize the overall fold. By systematically assessing amino acid conservation patterns across diverse species and examining tertiary structures using advanced modeling techniques, researchers can gain profound insights into evolutionary ties and functional parallels or distinctions among different subtypes, thereby revealing key mechanisms of protein evolution and specialization.[10,24]

Members of the RSPO family share 40%-60% structural homology across their domains, with each mammalian protein (including human and mouse) encoded by five exons. Structurally, they possess an N-terminal signal sequence, two adjacent fibrinogen-like cysteine-rich domains (FU-CR1 and FU-CR2), a conserved thrombospondin type 1 repeat (TSR-1), and a variable C-terminal domain rich in positively charged basic amino acids (BR). Figure 3 schematically depicts these four R-spondin protein domains, their functions, and interacting receptors.The FU-CR1, FU-CR2, and TSR-1 domains show significant sequence similarity across all four mammalian RSPO members, implying nearly identical functional roles for these regions. Crucially, the two N-terminal FU-CR domains (spanning ~90-134 amino acids in RSPO2) are exclusively responsible for Wnt signal activation; their absence inactivates downstream Wnt signaling [11,26,37].R-spondin’s Wnt-amplifying activity emerges more clearly through its interactions with membrane receptors, including LGR4-6, LRP5/6, ZNRF3/RNF43, Frizzled, and heparan sulfate proteoglycan (HSPG) [27]. Foundational studies revealed that ZNRF3/RNF43 directly binds the R-spondin FU-CR1 domain via two conserved residues (arginine and glutamine) [34]. The adjacent FU-CR2 domain contains two highly conserved phenylalanines essential for high-affinity binding to LGR4-6 [38,39,40,41].

Notably, FU-CR1 mutations (R66A or Q71A) disrupt RSPO1 interaction with ZNRF3/RNF43, while FU-CR2 mutations (F106A or F110A) abolish contact with LGR4. This demonstrates that RSPO binding to both receptors constitutes a key regulatory mechanism for Wnt pathway activation. These findings confirm two critical points: (i) R-spondin engages distinct receptors through different FU-CR domains, and (ii) these domains function independently without compromising interactions with other RSPO receptors [42,45].

Furthermore, research indicates that substituting tyrosine and arginine at residues 78 and 113 in the FU-CR domain may impair RSPO2 secretion through the cell surface and extracellular matrix, highlighting this domain’s role in standard RSPO2 secretion [46]. Deciphering structure-function relationships is vital to this work, as it establishes a foundational framework for unraveling molecular-level biological mechanisms. This process involves associating the protein’s 3D architecture, elucidated via techniques like X-ray crystallography or computational modeling, with its functions such as enzymatic activity or ligand binding. Consequently, investigators can anticipate how structural modifications, including point mutations or disruptions, influence protein function and organismal physiology, potentially yielding insights into disease conditions or adaptive traits. For example, in ray-finned fishes, this methodology uncovers how structural differences support survival in varied aquatic environments. Such knowledge is essential not only for comprehending ray-finned fish biology but also carries significant implications for evolutionary biology, where it assists in mapping protein evolution across lineages, and bioengineering, where it enables the intentional creation of synthetic proteins for industrial purposes [10,24].

4. Functional and Biological Significance of RSPO Gene

Role and Significance in the Wnt Signaling Pathway

The Wnt signaling pathway plays a crucial role in biological processes such as cell fate determination, development, and proliferation. Secreted glycoproteins from the conserved Wnt gene family activate downstream pathways through receptors, including Frizzled and LRP5/6. These pathways include the canonical β-catenin-dependent route for gene transcription and non-canonical pathways like Wnt/Ca²⁺ and Wnt/PCP. As a result, Wnt signaling impacts gene expression, cytoskeletal organization, cell polarity, migration, and overall cellular behavior. Tightly regulated Wnt activity is essential for proper development and tissue homeostasis, whereas dysregulation is associated with diseases including cancers, degenerative disorders, and developmental defects. This underscores the significant scientific and clinical importance of studying key molecules within this conserved pathway [5,6].

When Wnt ligands are not present (Figure 4A), cytoplasmic β-catenin levels are maintained low due to degradation by a destruction complex. This complex, situated at the apical cell surface, is composed of the scaffold proteins Axin, Adenomatous Polyposis Coli (APC), Casein Kinase 1α (CK1α), and Glycogen Synthase Kinase 3 (GSK3) [46]. Within this complex, β-catenin is phosphorylated by GSK3β, which targets it for subsequent degradation through the ubiquitin-proteasome pathway. GSK3β, a complex kinase participating in numerous cellular functions, can phosphorylate hundreds of potential substrates, including β-catenin [47]. It is a key regulator of β-catenin stability [49,50]. The degradation complex ensures the continuous elimination of β-catenin, thereby preventing its movement into the nucleus and ultimately suppressing the expression of Wnt target genes.

Regulatory Mechanisms and Interactions with Other Factors

The regulatory mechanisms governing the Wnt signaling pathway are complex and multifaceted, involving multiple levels of precise control to maintain cellular homeostasis. Firstly, the secretion and release of Wnt proteins are tightly regulated at the cellular level, with various secretory proteins, such as chaperone proteins like Wntless (Wls), and specific extracellular matrix components participating actively in this process, ensuring Wnt ligands are properly processed and presented. Secondly, upon ligand binding, Wnt signal transduction critically requires the assembly and stabilization of large protein complexes at the plasma membrane; these complexes include the serpentine Frizzled receptors, LRP5/6 co-receptors, and intracellular Dishevelled (Dvl) scaffolding proteins, which collectively act as a signalosome for signal propagation. Furthermore, the Wnt signaling pathway is subject to numerous negative and positive feedback mechanisms operating at transcriptional, translational, and post-translational levels to ensure precise spatiotemporal control of signal transmission and prevent aberrant activation; key negative regulators include proteins like Axin and APC, while positive feedback can involve target gene products. Extensive crosstalk exists between the Wnt pathway and other fundamental signaling cascades such as Hedgehog, Notch, and TGF-β pathways, creating intricate signaling networks that collectively regulate essential cellular processes like proliferation, differentiation, migration, and apoptosis, thereby orchestrating complex tissue developmental processes and maintaining adult tissue integrity through synergistic or antagonistic effects.[6,16,17]

Functions in Development and Disease

The Wnt signaling pathway, mediated by key molecules like Wnt ligands and β-catenin, plays a pivotal role during embryonic development, particularly in essential processes such as cell differentiation, tissue polarity establishment, and organogenesis of complex structures. For instance, during early embryonic development, the Wnt pathway is critically involved in regulating dorsoventral axis formation and neural tube development, where it orchestrates cell fate specification and morphogenetic movements. In adult tissues, the Wnt signaling pathway continues to function in maintaining stem cell self-renewal and tissue homeostasis, such as in the intestinal crypts where it promotes epithelial regeneration and balances proliferation with differentiation. However, aberrant activation or inhibition of Wnt signaling can lead to various diseases, including cancer[3,15,31], osteoporosis[62], cardiovascular diseases[53], and neurodegenerative disorders[50,78].

Expression Patterns and Functional Studies in Established Model Organisms (e.g., Mice, Zebrafish)

Significant progress has been made in studying the Wnt signaling pathway using model organisms, which provide versatile platforms for dissecting complex biological processes. In mouse models, researchers have employed sophisticated techniques such as gene knockout and transgenic technologies to reveal the critical roles of Wnt signaling in embryonic development and adult tissue homeostasis, including its involvement in organ formation, stem cell regulation, and wound healing. Zebrafish, as an important model organism, leverages transparent embryos and facile genetic manipulation to enable real-time, high-resolution observation of dynamic changes in the Wnt signaling pathway during early development, offering valuable insights into cell migration, patterning, and morphogenesis. Additionally, other model systems like Drosophila have complemented these studies by highlighting evolutionarily conserved aspects of Wnt signaling. Research utilizing these diverse model organisms has not only elucidated fundamental mechanisms, such as ligand-receptor interactions and intracellular cascades, but also identified numerous novel regulatory factors, crosstalk with other pathways like Notch and Hedgehog, and implications for disease mechanisms, thereby advancing potential therapeutic strategies..[7,30]

Association with Human Diseases (e.g., Cancer, Developmental Disorders)

Dysregulation of the Wnt signaling pathway is closely associated with various human diseases. In cancer research, aberrant activation of Wnt signaling is recognized as one of the key factors in the initiation and progression of many tumors[15,31]. For example, in common cancers such as colorectal cancer[15], breast cancer[13], and liver cancer[20], dysregulation of the Wnt pathway is often associated with abnormal accumulation of β-catenin and altered gene expression. Additionally, Wnt signaling abnormalities are linked to various developmental disorders, including congenital heart disease[53], skeletal dysplasia[74], and neurodevelopmental disorders[80]. Therefore, in-depth investigation of the mechanisms underlying Wnt signaling in disease pathogenesis will facilitate the development of novel diagnostic and therapeutic strategies, offering new hope for patients.[3]

5. Future Research Directions and Prospects

Functional validation of the RSPO family in non-model organisms represents a critical research frontier, as it enables scientists to gain deeper insights into the biological roles of these proteins beyond traditional laboratory species, where their functions may be less representative of natural diversity. This frontier is crucial for addressing gaps in evolutionary biology and molecular mechanisms. By conducting targeted studies in diverse organisms such as amphioxus, lampreys, and agricultural species, researchers can systematically uncover conserved functional mechanisms while identifying lineage-specific adaptations, thus elucidating how these proteins have evolved under different environmental pressures. For example, examining RSPO in basal chordates like amphioxus can reveal ancestral roles, whereas studies in agricultural species might highlight adaptations relevant to growth and development. This approach not only validates the evolutionary significance of RSPO proteins across phylogenies but also provides robust comparative perspectives essential for understanding their broader biological relevance, including implications for disease modeling and therapeutic innovations.[4]

Furthermore, advancing our understanding requires in-depth of molecular mechanisms governing RSPO function, including protein-protein interaction networks and post-translational modifications. Investigating how RSPO proteins interact with Frizzled receptors, LRP co-receptors, and antagonists like DKK proteins will clarify their regulatory specificity within the Wnt pathway. Concurrently, studying post-translational modifications such as glycosylation, phosphorylation, and ubiquitination is crucial, as these modifications dynamically modulate protein stability, subcellular localization, and signaling activity, ultimately dictating functional outcomes in different cellular contexts.[16,17]

Evolution-driven functional adaptation studies stand to reveal how selective pressures have shaped RSPO family diversification across metazoan lineages. Comparative genomic analyses combined with functional assays can identify amino acid residues under positive selection that correlate with novel functional traits, such as ligand-binding specificity or tissue-specific expression patterns. These evolutionary insights not only illuminate the adaptive significance of RSPO proteins in ecological niches but also provide a framework for predicting functional divergence and conserved motifs critical for therapeutic targeting.[35,41,54]

6. Conclusions

Phylogenetic analysis of the RSPO gene family has unveiled its complex evolutionary history, tracing intricate diversification patterns across metazoan species and pinpointing key ancestral nodes that herald the emergence of distinct subtypes. This comprehensive evolutionary framework not only clarifies orthologous relationships between vertebrate and invertebrate RSPO genes but also highlights conserved motifs and lineage-specific expansions driving functional specialization. By integrating comparative genomic data with molecular clock analyses, researchers have reconstructed the RSPO family’s evolutionary timeline, identifying critical periods of adaptive radiation and functional divergence coinciding with major leaps in animal body plan complexity.

Delving deeper, structure-function studies provide novel insights into how RSPO proteins execute their biological roles in development and disease pathogenesis. High-resolution crystallography and molecular dynamics simulations illuminate the three-dimensional architecture of RSPO-ligand complexes, revealing conserved binding interfaces with Frizzled receptors and LRP co-receptors that dictate Wnt pathway activation specificity. These structural revelations, combined with site-directed mutagenesis and functional assays, identify critical amino acid residues governing protein-protein interactions and post-translational modifications—events dynamically regulating RSPO activity within tissue-specific contexts. Such mechanistic clarity profoundly enhances our grasp of RSPO-mediated signaling in embryonic development and adult tissue homeostasis while revealing promising therapeutic targets for diseases rooted in pathway dysregulation, including cancer and congenital malformations.

Continuous refinement of these methodologies paves the way for rationally designing precision medicines targeting RSPO-mediated Wnt signaling. As we probe the molecular intricacies of RSPO function, the potential for personalized therapeutic interventions grows increasingly tangible. By leveraging evolutionary conservation of RSPO motifs and understanding their context-dependent modulation, scientists can devise strategies to selectively inhibit or enhance Wnt signaling in specific tissues, addressing a broad spectrum of pathologies. Moreover, integrating bioinformatics tools and machine learning algorithms will accelerate the discovery of novel RSPO modulators, enabling high-throughput screening of therapeutic candidates and hastening the translation of basic research into clinical practice. Ultimately, concerted efforts to unravel RSPO biology and Wnt signaling complexities will herald a new era of precision medicine, transforming the diagnostic and therapeutic landscape for patients battling Wnt-related disorders.