The central nervous system (CNS) is an essential hub for neuronal communication. As a major component of the CNS, glial cells are vital in the maintenance and regulation of neuronal network dynamics. Research on microglia, the resident innate immune cells of the CNS, has advanced considerably in recent years, and our understanding of their diverse functions continues to grow. Microglia play critical roles in the formation and regulation of neuronal synapses, myelination, responses to injury, neurogenesis, inflammation and many other physiological processes. In parallel with advances in microglial biology, cutting-edge techniques for the characterization of microglial properties have emerged with increasing depth and precision. Labeling tools and reporter models are important for the study of microglial morphology, ultrastructure and dynamics, but also for microglial isolation, which is required to glean key phenotypic information through single-cell transcriptomics and other emerging approaches. Strategies for selective microglial depletion and modulation can provide novel insights into microglia-targeted treatment strategies in models of neuropsychiatric and neurodegenerative conditions, cancer and autoimmunity. Finally, fate mapping has emerged as an important tool to answer fundamental questions about microglial biology, including their origin, migration and proliferation throughout the lifetime of an organism. This review aims to provide a comprehensive discussion of these emerging techniques, with applications to the study of microglia in development, homeostasis and CNS pathologies.

Sepsis-associated encephalopathy (SAE) is a common brain dysfunction following sepsis that often results in severe cognitive and neurological sequelae and increases the mortality rate in patients with sepsis. Microglia are resident macrophages in the brain that play essential roles in the pathological and physiological processes of SAE. Depending on the nature of the stimulus, microglia can adopt two polarization states (M1/M2), which correspond to altered microglial morphology, gene expression, and function. Therefore, we systematically described the pathogenesis, morphology, function, and phenotype of microglial activation in SAE. We demonstrated that microglia are closely related to occurrence and development of SAE and concomitant cognitive impairement. Finally, we outlined some potential therapeutic approaches that can prime microglia and neuroinflammation towards the beneficial restorative microglial phenotype in SAE.

The pharmacological effects of the fruiting body of Ophiocordyceps sinensis (Os) exerted antioxidant, anti-virus, and immunomodulatory, whereas the anti-inflammatory effects and active components of Os mycelial submerged culture (OMSC) are not known. This study aimed to investigate the active compounds in the fermented liquid (FLOS), hot water (WEOS), and 50-95% (EEOS-50, EEOS-95) ethanol extracts of OMSC and their anti-inflammatory effects and potential mechanisms in LPS-stimulated microglial BV2 cells. The results demonstrated that all of the OMSC extracts could inhibit NO production in BV2 cells. EEOS-95 exhibited the strongest inhibitory effects (41 % inhibitory ability at 500 µg/mL), and its ergosterol, GABA, total phenolic, and total flavonoid contents were significantly higher than those of the other extracts (18.60, 18.60, 2.28, and 2.14 mg/g, p < 0.05, respectively). EEOS-95 also has a strong inhibitory ability against IL-6, IL-1β and TNF-α with IC50 of 617, 277 and 507 µg/mL, respectively, which is higher than that of 1 mM melatonin. The anti-inflammatory mechanism of EEOS-95 seems to be associated with up-regulation of PPAR-γ/Nrf-2/HO-1 antioxidant-related expression and down-regulation of NF-κB/COX-2/iNOS pro-inflammatory expression signaling. Therefore, we hypothesized that EEOS-95 can prevent neurodegenerative diseases, but the effect on the application in vivo should be further investigated.

Pituitary adenylate cyclase-activating polypeptide (PACAP) and vasoactive intestinal peptide (VIP) are two structurally-related immunosuppressive peptides. However, the underlying mechanisms through which these peptides regulate microglial activity are not fully understood. Using lipopolysaccharide (LPS) to induce an inflammatory challenge, we tested whether PACAP or VIP differentially affected microglial activation, morphology and cell migration. We found that both peptides attenuated LPS-induced expression of the microglial activation markers Iba1 and iNOS (###p<0.001), as well as the pro-inflammatory mediators IL-1β, IL-6, Itgam and CD68 (###p<0.001). In contrast, treatment with PACAP or VIP exerted distinct effects on microglial morphology and migration. PACAP reversed LPS-induced soma enlargement and increased the percentage of small-sized, rounded cells (54.09% vs 12.05% in LPS-treated cells), whereas VIP promoted a phenotypic shift towards cell subpopulations with mid-sized, spindle-shaped soma (48.41% vs 31.36% in LPS-treated). Additionally, PACAP was more efficient than VIP in restoring LPS-induced impairment of cell migration and the expression of urokinase plasminogen activator (uPA) in BV2 cells compared with VIP. These results suggest that whilst both PACAP and VIP exert similar immunosuppressive effects in activated BV2 microglia, each peptide triggers distinctive shifts towards phenotypes of differing morphologies and with differing migration capacities.

Adeno-associated viral (AAV) vectors can be used for gene delivery. AAV.CAP-Mac was recently developed; it can cross the blood-brain barrier and transduce cells throughout the CNS. However, some brain regions were not transduced in adult rhesus monkeys. Additionally, AAV vectors can only package 5 kb of DNA. Also, AAV vectors may be genotoxic and cytotoxic, especially at high doses. Finally, AAV vectors are very expensive to produce at sufficiently high titers for treatment. Off-the-shelf cell-based delivery systems wherein the cells can infiltrate the target tissue and be hyper-motile would be ideal. Also, mRNA-based gene editing would be a transient treatment, and thus be much safer.

Microglial cells, the resident macrophages of the CNS, exist in a process-bearing, ramified/surveying phenotype under resting conditions. Upon activation by cell damaging factors they get transformed to an amoeboid phenotype releasing various cell products including pro-inflammatory cytokines, chemokines, proteases, reactive oxygen/nitrogen species and the excytotoxic ATP and glutamate. In addition, they engulf pathogenic bacteria or cell debris and phagocytose them. However, already resting/surveying microglia has a number of important physiological functions in the CNS; they e.g. shield small disruptions of the blood-brain barrier by their processes, dynamically interact with synaptic structures and clear surplus synapses during development. In neurodegenerative illnesses they aggravate the original disease by a microglia-based compulsory neuroinflammatory reaction. Therefore, the blockade of this reaction improves the outcome of Alzheimer’s disease, Parkinson’s disease, multiple sclerosis, amyotrophic lateral sclerosis, etc. The function of microglia is regulated by a whole array of purinergic receptors classified as P2Y12, P2Y6, P2Y4, P2X4, P2X7, A2A, A3, and being targets for endogenous ATP, ADP, or adenosine. ATP is sequentially degraded by the ecto-nucleotidases and 5’-nucleotidase enzymes to the weak adenosine agonist inosine as an end-product. The appropriate selective agonists/antagonists for purinergic receptors as well as the respective enzyme inhibitors may profoundly interfere with microglial functions and reconstitute the homeostasis of the CNS disturbed by neuroinflammation.