Spatiotemporal regulation of the GPCR activity of BAI3 by C1qL4 and Stabilin-2 controls myoblast fusion

Transcriptomic and epigenomic insights into pectoral muscle fiber formation at the late embryonic development in pure chicken lines

Long-term intensive genetic selection has led to significant differences between broiler and layer chickens, which are evident during the embryonic period. Despite this, there is a paucity of research on the genetic regulation of the initial formation of muscle fiber morphology in chick embryos. Embryonic d 17 (E17) is the key time point for myoblast fusion completion and muscle fiber morphology formation in chickens. This study aimed to explore the genetic regulatory mechanisms underlying the early muscle fiber morphology establishment in broiler chickens of Cornish (CC) and White Plymouth Rock (RR) and layer chickens of White Leghorn (WW) at E17 using the transcriptomic and chromatin accessibility sequencing of pectoral major muscles. The results showed that broiler chickens exhibited significant higher embryo weight and pectoral major muscle weight at E17 compared to layer chickens (P = 0.000). A total of 1,278, 1,248, and 892 differentially expressed genes (DEGs) of RNA-seq data were identified between CC vs. WW, RR vs. WW, and CC vs. RR, separately. All DEGs were combined for cluster analysis and they were divided into 6 clusters, including cluster 1 with higher expression in broilers and cluster 6 with higher expression in layers. DEGs in cluster 1 were enriched in terms related to macrophage activation (P = 0.002) and defense response to bacteria (P = 0.002), while DEGs in cluster 6 showed enrichment in protein-DNA complex (P = 0.003) and monooxygenase activity (P = 0.000). ATAC-seq data analysis identified a total of 38,603 peaks, with 13,051 peaks for CC, 18,780 peaks for RR, and 6,772 peaks for WW. Integrative analysis of transcriptomic and chromatin accessibility data revealed GOLM1, ISLR2, and TOPAZ1 were commonly upregulated genes in CC and RR. Furthermore, screening of all upregulated DEGs in cluster 1 from CC and RR identified GOLM1, ISLR2, and HNMT genes associated with neuroimmune functions and MYOM3 linked to muscle morphology development, showing significantly elevated expression in broiler chickens compared to layer chickens. These findings suggest active neural system connectivity during the initial formation of muscle fiber morphology in embryonic period, highlighting the early interaction between muscle fiber formation morphology and the nervous system. This study provides novel insights into late chick embryo development and lays a deeper foundation for further research.

CTRP13-Mediated Effects on Endothelial Cell Function and Their Potential Role in Obesity

BACKGROUND

Obesity, a major component of cardiometabolic syndrome, contributes to the imbalance between pro- and anti-atherosclerotic factors via dysregulation of adipocytokine secretion. Among these adipocytokines, the C1q/TNF-related proteins (CTRPs) play a role in the modulation of atherosclerosis development and progression. Here, we investigated the vascular effects of CTRP13.

RESULTS

CTRP13 is not only expressed in adipose tissue but also in vessels/endothelial cells (ECs) of mice, rats, and humans. Obese individuals (mice, rats, and humans) showed higher vascular CTRP13 expression. Human Umbilical Vein Endothelial Cells (HUVECs), cultured in the presence of serum from obese mice, mimicked this obesity-associated effect on CTRP13 protein expression. Similarly, high glucose conditions and TNF-alpha, but not insulin, resulted in a strong increase in CTRP13 in these cells. Recombinant CTRP13 induced a reduction in EC proliferation via AMPK. In addition, CTRP13 reduced cell cycle progression and increased p53 phosphorylation and p21 protein expression, but reduced Rb phosphorylation, with the effects largely depending on alpha-2 AMPK as suggested by adenoviral overexpression of dominant-negative (DN) or wild-type (WT) alpha 1/alpha 2 AMPK.

CONCLUSION

The present study demonstrates that CTRP13 expression is induced in ECs under diabetic conditions and that CTRP13 possesses significant vaso-modulatory properties which may have an impact on vascular disease progression in patients.

Kainate receptors regulate synaptic integrity and plasticity by forming a complex with synaptic organizers in the cerebellum

Kainate (KA)-type glutamate receptors (KARs) are implicated in various neuropsychiatric and neurological disorders through their ionotropic and metabotropic actions. However, compared to AMPA- and NMDA-type receptor functions, many aspects of KAR biology remain incompletely understood. Our study demonstrates an important role of KARs in organizing climbing fiber (CF)-Purkinje cell (PC) synapses and synaptic plasticity in the cerebellum, independently of their ion channel or metabotropic functions. The amino-terminal domain (ATD) of the GluK4 KAR subunit binds to C1ql1, provided by CFs, and associates with Bai3, an adhesion-type G protein-coupled receptor expressed in PC dendrites. Mice lacking GluK4 exhibit no KAR-mediated responses, reduced C1ql1 and Bai3 levels, and fewer CF-PC synapses, along with impaired long-term depression and oculomotor learning. Remarkably, introduction of the ATD of GluK4 significantly improves all these phenotypes. These findings demonstrate that KARs act as synaptic scaffolds, orchestrating synapses by forming a KAR-C1ql1-Bai3 complex in the cerebellum.

The dark sides of the GPCR tree - research progress on understudied GPCRs

A large portion of the human GPCRome is still in the dark and understudied, consisting even of entire subfamilies of GPCRs such as odorant receptors, class A and C orphans, adhesion GPCRs, Frizzleds and taste receptors. However, it is undeniable that these GPCRs bring an untapped therapeutic potential that should be explored further. Open questions on these GPCRs span diverse topics such as deorphanisation, the development of tool compounds and tools for studying these GPCRs, as well as understanding basic signalling mechanisms. This review gives an overview of the current state of knowledge for each of the diverse subfamilies of understudied receptors regarding their physiological relevance, molecular mechanisms, endogenous ligands and pharmacological tools. Furthermore, it identifies some of the largest knowledge gaps that should be addressed in the foreseeable future and lists some general strategies that might be helpful in this process.

Brain-Specific Angiogenesis Inhibitor 3 Is Expressed in the Cochlea and Is Necessary for Hearing Function in Mice

Mammalian auditory hair cells transduce sound-evoked traveling waves in the cochlea into nerve stimuli, which are essential for hearing function. Pillar cells located between the inner and outer hair cells are involved in the formation of the tunnel of Corti, which incorporates outer-hair-cell-driven fluid oscillation and basilar membrane movement, leading to the fine-tuned frequency-specific perception of sounds by the inner hair cells. However, the detailed molecular mechanism underlying the development and maintenance of pillar cells remains to be elucidated. In this study, we examined the expression and function of brain-specific angiogenesis inhibitor 3 (Bai3), an adhesion G-protein-coupled receptor, in the cochlea. We found that Bai3 was expressed in hair cells in neonatal mice and pillar cells in adult mice, and, interestingly, Bai3 knockout mice revealed the abnormal formation of pillar cells, with the elevation of the hearing threshold in a frequency-dependent manner. Furthermore, old Bai3 knockout mice showed the degeneration of hair cells and spiral ganglion neurons in the basal turn. The results suggest that Bai3 plays a crucial role in the development and/or maintenance of pillar cells, which, in turn, are necessary for normal hearing function. Our results may contribute to understanding the mechanisms of hearing loss in human patients.

CTRP13 ablation improves systemic glucose and lipid metabolism

OBJECTIVE

Tissue crosstalk mediated by secreted hormones underlies the integrative control of metabolism. We previously showed that CTRP13/C1QL3, a secreted protein of the C1q family, can improve glucose metabolism and insulin action in vitro and reduce food intake and body weight in mice when centrally delivered. A role for CTRP13 in regulating insulin secretion in isolated islets has also been demonstrated. It remains unclear, however, whether the effects of CTRP13 on cultured cells and in mice reflect the physiological function of the protein. Here, we use a loss-of-function mouse model to address whether CTRP13 is required for metabolic homeostasis.

METHODS

WT and Ctrp13 knockout (KO) mice fed a standard chow or a high-fat diet were subjected to comprehensive metabolic phenotyping. Transcriptomic analyses were carried out on visceral and subcutaneous fat, liver, and skeletal muscle to identify pathways altered by CTRP13 deficiency. RNA-seq data was further integrated with the Metabolic Syndrome in Man (METSIM) cohort data. Adjusted regression analysis was used to demonstrate that genetic variation of CTRP13 expression accounts for a significant proportion of variance between differentially expressed genes (DEGs) in adipose tissue and metabolic traits in humans.

RESULTS

Contrary to expectation, chow-fed Ctrp13-KO male mice had elevated physical activity, lower body weight, and improved lipid handling. On a high-fat diet (HFD), Ctrp13-KO mice of either sex were consistently more active and leaner. Loss of CTRP13 reduced hepatic glucose output and improved glucose tolerance, insulin sensitivity, and triglyceride clearance, though with notable sex differences. Consistent with the lean phenotype, transcriptomic analyses revealed a lower inflammatory profile in visceral fat and liver. Reduced hepatic steatosis was correlated with the suppression of lipid synthesis and enhanced lipid catabolism gene expression. Visceral fat had the largest number of DEGs and mediation analyses on the human orthologs of the DEGs suggested the potential causal contribution of CTRP13 to human metabolic syndrome.

CONCLUSIONS

Our results suggest that CTRP13 is a negative metabolic regulator, and its deficiency improves systemic metabolic profiles. Our data also suggest the reduction in circulating human CTRP13 levels seen in obesity and diabetes may reflect a compensatory physiologic response to counteract insulin resistance.

Postnatal skeletal muscle myogenesis governed by signal transduction networks: MAPKs and PI3K-Akt control multiple steps

Skeletal muscle myogenesis represents one of the most intensively and extensively examined systems of cell differentiation, tissue formation, and regeneration. Muscle regeneration provides an in vivo model system of postnatal myogenesis. It comprises multiple steps including muscle stem cell (or satellite cell) quiescence, activation, migration, myogenic determination, myoblast proliferation, myocyte differentiation, myofiber maturation, and hypertrophy. A variety of extracellular signaling and subsequent intracellular signal transduction pathways or networks govern the individual steps of postnatal myogenesis. Among them, MAPK pathways (the ERK, JNK, p38 MAPK, and ERK5 pathways) and PI3K-Akt signaling regulate multiple steps of myogenesis. Ca2+, cytokine, and Wnt signaling also participate in several myogenesis steps. These signaling pathways often control cell cycle regulatory proteins or the muscle-specific MyoD family and the MEF2 family of transcription factors. This article comprehensively reviews molecular mechanisms of the individual steps of postnatal skeletal muscle myogenesis by focusing on signal transduction pathways or networks. Nevertheless, no or only a partial signaling molecules or pathways have been identified in some responses during myogenesis. The elucidation of these unidentified signaling molecules and pathways leads to an extensive understanding of the molecular mechanisms of myogenesis.

Loss of CTRP10 results in female obesity with preserved metabolic health

Obesity is a major risk factor for type 2 diabetes, dyslipidemia, cardiovascular disease, and hypertension. Intriguingly, there is a subset of metabolically healthy obese (MHO) individuals who are seemingly able to maintain a healthy metabolic profile free of metabolic syndrome. The molecular underpinnings of MHO, however, are not well understood. Here, we report that CTRP10/C1QL2-deficient mice represent a unique female model of MHO. CTRP10 modulates weight gain in a striking and sexually dimorphic manner. Female, but not male, mice lacking CTRP10 develop obesity with age on a low-fat diet while maintaining an otherwise healthy metabolic profile. When fed an obesogenic diet, female Ctrp10 knockout (KO) mice show rapid weight gain. Despite pronounced obesity, Ctrp10 KO female mice do not develop steatosis, dyslipidemia, glucose intolerance, insulin resistance, oxidative stress, or low-grade inflammation. Obesity is largely uncoupled from metabolic dysregulation in female KO mice. Multi-tissue transcriptomic analyses highlighted gene expression changes and pathways associated with insulin-sensitive obesity. Transcriptional correlation of the differentially expressed gene (DEG) orthologous in humans also show sex differences in gene connectivity within and across metabolic tissues, underscoring the conserved sex-dependent function of CTRP10. Collectively, our findings suggest that CTRP10 negatively regulates body weight in females, and that loss of CTRP10 results in benign obesity with largely preserved insulin sensitivity and metabolic health. This female MHO mouse model is valuable for understanding sex-biased mechanisms that uncouple obesity from metabolic dysfunction.

C1ql1-Bai3 signaling is necessary for climbing fiber synapse formation in mature Purkinje cells in coordination with neuronal activity

Changes in neural activity induced by learning and novel environments have been reported to lead to the formation of new synapses in the adult brain. However, the underlying molecular mechanism is not well understood. Here, we show that Purkinje cells (PCs), which have established adult-type monosynaptic innervation by climbing fibers (CFs) after elimination of weak CFs during development, can be reinnervated by multiple CFs by increased expression of the synaptic organizer C1ql1 in CFs or Bai3, a receptor for C1ql1, in PCs. In the adult cerebellum, CFs are known to have transverse branches that run in a mediolateral direction without forming synapses with PCs. Electrophysiological, Ca²⁺-imaging and immunohistochemical studies showed that overexpression of C1ql1 or Bai3 caused these CF transverse branches to elongate and synapse on the distal dendrites of mature PCs. Mature PCs were also reinnervated by multiple CFs when the glutamate receptor GluD2, which is essential for the maintenance of synapses between granule cells and PCs, was deleted. Interestingly, the effect of GluD2 knockout was not observed in Bai3 knockout PCs. In addition, C1ql1 levels were significantly upregulated in CFs of GluD2 knockout mice, suggesting that endogenous, not overexpressed, C1ql1-Bai3 signaling could regulate the reinnervation of mature PCs by CFs. Furthermore, the effects of C1ql1 and Bai3 overexpression required neuronal activity in the PC and CF, respectively. C1ql1 immunoreactivity at CF-PC synapses was reduced when the neuronal activity of CFs was suppressed. These results suggest that C1ql1-Bai3 signaling may mediate CF synaptogenesis in mature PCs, potentially in concert with neuronal activity.

Phospholipid metabolism in the liver - Implications for phosphatidylserine in non-alcoholic fatty liver disease

Mammalian cells contain more than a thousand different glycerophospholipid species that are essential membrane components and signalling molecules, with phosphatidylserine (PS) giving membranes their negative surface charge. Depending on the tissue, PS is important in apoptosis, blood clotting, cancer pathogenesis, as well as muscle and brain function, processes that are dependent on the asymmetrical distribution of PS on the plasma membrane and/or the capacity of PS to act as anchorage for various signalling proteins. Recent studies have implicated hepatic PS in the progression of non-alcoholic fatty liver disease (NAFLD), either as beneficial in the context of suppressing hepatic steatosis and fibrosis, or on the other hand as a potential contributor to the progression of liver cancer. This review provides an extensive overview of hepatic phospholipid metabolism, including its biosynthetic pathways, intracellular trafficking and roles in health and disease, further taking a deeper dive into PS metabolism, including associate and causative evidence of the role of PS in advanced liver disease.

Noncanonical Regulation of cAMP-Dependent Insulin Secretion and Its Implications in Type 2 Diabetes

Impaired glucose tolerance (IGT) and β-cell dysfunction in insulin resistance associated with obesity lead to type 2 diabetes (T2D). Glucose-stimulated insulin secretion (GSIS) from β-cells occurs via a canonical pathway that involves glucose metabolism, ATP generation, inactivation of K ATP channels, plasma membrane depolarization, and increases in cytosolic concentrations of [Ca 2+ ] c . However, optimal insulin secretion requires amplification of GSIS by increases in cyclic adenosine monophosphate (cAMP) signaling. The cAMP effectors protein kinase A (PKA) and exchange factor activated by cyclic-AMP (Epac) regulate membrane depolarization, gene expression, and trafficking and fusion of insulin granules to the plasma membrane for amplifying GSIS. The widely recognized lipid signaling generated within β-cells by the β-isoform of Ca 2+ -independent phospholipase A 2 enzyme (iPLA 2 β) participates in cAMP-stimulated insulin secretion (cSIS). Recent work has identified the role of a G-protein coupled receptor (GPCR) activated signaling by the complement 1q like-3 (C1ql3) secreted protein in inhibiting cSIS. In the IGT state, cSIS is attenuated, and the β-cell function is reduced. Interestingly, while β-cell-specific deletion of iPLA 2 β reduces cAMP-mediated amplification of GSIS, the loss of iPLA 2 β in macrophages (MØ) confers protection against the development of glucose intolerance associated with diet-induced obesity (DIO). In this article, we discuss canonical (glucose and cAMP) and novel noncanonical (iPLA 2 β and C1ql3) pathways and how they may affect β-cell (dys)function in the context of impaired glucose intolerance associated with obesity and T2D. In conclusion, we provide a perspective that in IGT states, targeting noncanonical pathways along with canonical pathways could be a more comprehensive approach for restoring β-cell function in T2D. © 2023 American Physiological Society. Compr Physiol 13:5023-5049, 2023.

Loss of Brain Angiogenesis Inhibitor-3 (BAI3) G-Protein Coupled Receptor in Mice Regulates Adaptive Thermogenesis by Enhancing Energy Expenditure

Effective energy expenditure is critical for maintaining body weight (BW). However, underlying mechanisms contributing to increased BW remain unknown. We characterized the role of brain angiogenesis inhibitor-3 (BAI3/ADGRB3), an adhesion G-protein coupled receptor (aGPCR), in regulating BW. A CRISPR/Cas9 gene editing approach was utilized to generate a whole-body deletion of the BAI3 gene (BAI3-/-). In both BAI3-/- male and female mice, a significant reduction in BW was observed compared to BAI3+/+ control mice. Quantitative magnetic imaging analysis showed that lean and fat masses were reduced in male and female mice with BAI3 deficiency. Total activity, food intake, energy expenditure (EE), and respiratory exchange ratio (RER) were assessed in mice housed at room temperature using a Comprehensive Lab Animal Monitoring System (CLAMS). While no differences were observed in the activity between the two genotypes in male or female mice, energy expenditure was increased in both sexes with BAI3 deficiency. However, at thermoneutrality (30 °C), no differences in energy expenditure were observed between the two genotypes for either sex, suggesting a role for BAI3 in adaptive thermogenesis. Notably, in male BAI3-/- mice, food intake was reduced, and RER was increased, but these attributes remained unchanged in the female mice upon BAI3 loss. Gene expression analysis showed increased mRNA abundance of thermogenic genes Ucp1, Pgc1α, Prdm16, and Elov3 in brown adipose tissue (BAT). These outcomes suggest that adaptive thermogenesis due to enhanced BAT activity contributes to increased energy expenditure and reduced BW with BAI3 deficiency. Additionally, sex-dependent differences were observed in food intake and RER. These studies identify BAI3 as a novel regulator of BW that can be potentially targeted to improve whole-body energy expenditure.

Pleiotropy of C1QL proteins across physiological systems and their emerging role in synapse homeostasis

The C1q/TNF superfamily of proteins engages in a pleiotropy of physiological functions associated with various diseases. C1QL proteins demonstrate important protective and regulatory roles in the endocrine, immune, cardiovascular, and nervous systems in both human and rodent studies. Studies in the central nervous system (CNS), adipose, and muscle tissue reveal several C1QL protein and receptor pathways altering multiple cellular responses, including cell fusion, morphology, and adhesion. This review examines C1QL proteins across these systems, summarizing functional and disease associations and highlighting cellular responses based on in vitro and in vivo data, receptor interaction partners, and C1QL-associated protein signaling pathways. We highlight the functions of C1QL proteins in organizing CNS synapses, regulating synapse homeostasis, maintaining excitatory synapses, and mediating signaling and trans-synaptic connections. Yet, while these associations are known, present studies provide insufficient insight into the underlying molecular mechanism of their pleiotropy, including specific protein interactions and functional pathways. Thus, we suggest several areas for more in-depth and interdisciplinary hypothesis testing.

Biasing the conformation of ELMO2 reveals that myoblast fusion can be exploited to improve muscle regeneration

Myoblast fusion is fundamental for the development of multinucleated myofibers. Evolutionarily conserved proteins required for myoblast fusion include RAC1 and its activator DOCK1. In the current study we analyzed the contribution of the DOCK1-interacting ELMO scaffold proteins to myoblast fusion. When Elmo1-/- mice underwent muscle-specific Elmo2 genetic ablation, they exhibited severe myoblast fusion defects. A mutation in the Elmo2 gene that reduced signaling resulted in a decrease in myoblast fusion. Conversely, a mutation in Elmo2 coding for a protein with an open conformation increased myoblast fusion during development and in muscle regeneration. Finally, we showed that the dystrophic features of the Dysferlin-null mice, a model of limb-girdle muscular dystrophy type 2B, were reversed when expressing ELMO2 in an open conformation. These data provide direct evidence that the myoblast fusion process could be exploited for regenerative purposes and improve the outcome of muscle diseases.

Skeletal Muscle Regeneration in Cardiotoxin-Induced Muscle Injury Models

Skeletal muscle injuries occur frequently in daily life and exercise. Understanding the mechanisms of regeneration is critical for accelerating the repair and regeneration of muscle. Therefore, this article reviews knowledge on the mechanisms of skeletal muscle regeneration after cardiotoxin-induced injury. The process of regeneration is similar in different mouse strains and is inhibited by aging, obesity, and diabetes. Exercise, microcurrent electrical neuromuscular stimulation, and mechanical loading improve regeneration. The mechanisms of regeneration are complex and strain-dependent, and changes in functional proteins involved in the processes of necrotic fiber debris clearance, M1 to M2 macrophage conversion, SC activation, myoblast proliferation, differentiation and fusion, and fibrosis and calcification influence the final outcome of the regenerative activity.

Adhesion G protein-coupled receptors: structure, signaling, physiology, and pathophysiology

Adhesion G protein-coupled receptors (AGPCRs) are a family of 33 receptors in humans exhibiting a conserved general structure but diverse expression patterns and physiological functions. The large NH2 termini characteristic of AGPCRs confer unique properties to each receptor and possess a variety of distinct domains that can bind to a diverse array of extracellular proteins and components of the extracellular matrix. The traditional view of AGPCRs, as implied by their name, is that their core function is the mediation of adhesion. In recent years, though, many surprising advances have been made regarding AGPCR signaling mechanisms, activation by mechanosensory forces, and stimulation by small-molecule ligands such as steroid hormones and bioactive lipids. Thus, a new view of AGPCRs has begun to emerge in which these receptors are seen as massive signaling platforms that are crucial for the integration of adhesive, mechanosensory, and chemical stimuli. This review article describes the recent advances that have led to this new understanding of AGPCR function and also discusses new insights into the physiological actions of these receptors as well as their roles in human disease.

Phosphatidylserine orchestrates Myomerger membrane insertions to drive myoblast fusion

Muscle cell fusion is a multistep process where the final step of the reaction drives progression beyond early hemifusion events to complete fusion. This step requires activity of the muscle-specific fusogen Myomerger, a single-pass transmembrane protein containing 84 amino acids with an ectodomain that includes two α-helices. Previous studies have demonstrated that Myomerger acts by destabilizing membranes through generation of elastic stresses in the outer leaflet of the plasma membrane. An obvious question is how such destabilizing activity might be regulated to avoid membrane and cellular damage, and how the two juxtaposed helices cooperate in fusion. Using cellular fusion assays and in vitro liposome assays, we report that the two helices possess unique characteristics, both of which are needed for full activity of the protein. We demonstrate that externalized phosphatidylserine (PS), a lipid previously implicated in myoblast fusion, has a determinant role in the regulation of Myomerger activity. The membrane-proximal, amphipathic Helix-1 is normally disordered and its α-helical structure is induced by PS, making membrane interactions more efficacious. The distal, more hydrophobic Helix-2 is intrinsically ordered, possesses an ability to insert into membranes, and augments the membrane-stressing effects of Helix-1. These data reveal that Myomerger fusogenic activity is an exquisitely orchestrated event involving its two ectodomain helices, which are controlled by membrane lipid composition, providing an explanation as to how its membrane-stressing activity is spatially and temporally regulated during the final step of myoblast fusion.

Involvement of phosphatidylserine receptors in the skeletal muscle regeneration: therapeutic implications

Sarcopenia is a progressive loss of muscle mass and strength with a risk of adverse outcomes such as disability, poor quality of life, and death. Increasing evidence indicates that diminished ability of the muscle to activate satellite cell-dependent regeneration is one of the factors that might contribute to its development. Skeletal muscle regeneration following myogenic cell death results from the proliferation and differentiation of myogenic stem cells, called satellite cells, located beneath the basal lamina of the muscle fibres. Satellite cell differentiation is not a satellite cell-autonomous process but depends on signals provided by the surrounding cells. Infiltrating macrophages play a key role in the process partly by clearing the necrotic cell debris, partly by producing cytokines and growth factors that guide myogenesis. At the beginning of the muscle regeneration process, macrophages are pro-inflammatory, and the cytokines produced by them trigger the proliferation and differentiation of satellite cells. Following the uptake of dead cells, however, a transcriptionally regulated phenotypic change (macrophage polarization) is induced in them resulting in their transformation into healing macrophages that guide resolution of inflammation, completion of myoblast differentiation, myoblast fusion and growth, and return to homeostasis. Impaired efferocytosis results in delayed cell death clearance, delayed macrophage polarization, prolonged inflammation, and impaired muscle regeneration. Thus, proper efferocytosis by macrophages is a determining factor during muscle repair. Here we review that both efferocytosis and myogenesis are dependent on the cell surface phosphatidylserine (PS), and surprisingly, these two processes share a number of common PS receptors and signalling pathways. Based on these findings, we propose that stimulating the function of PS receptors for facilitating muscle repair following injury could be a successful approach, as it would enhance efferocytosis and myogenesis simultaneously. Because increasing evidence indicates a pathophysiological role of impaired efferocytosis in the development of chronic inflammatory conditions, as well as in impaired muscle regeneration both contributing to the development of sarcopenia, improving efferocytosis should be considered also in its management. Again applying or combining those treatments that target PS receptors would be expected to be the most effective, because they would also promote myogenesis. A potential PS receptor-triggering candidate molecule is milk fat globule-EGF-factor 8 (MFG-E8), which not only stimulates PS-dependent efferocytosis and myoblast fusion but also promotes extracellular signal-regulated kinase (ERK) and Akt activation-mediated cell proliferation and cell cycle progression in myoblasts.

Regulation of the myoblast fusion reaction for muscle development, regeneration, and adaptations

Fusion of plasma membranes is essential for skeletal muscle development, regeneration, exercise-induced adaptations, and results in a cell that contains hundreds to thousands of nuclei within a shared cytoplasm. The differentiation process in myocytes culminates in their fusion to form a new myofiber or fusion to an existing myofiber thereby contributing more synthetic material to the syncytium. The choice for two cells to fuse and become one could be a dangerous event if the two cells are not committed to an allied function. Thus, fusion events are highly regulated with positive and negative factors to fine-tune the process, and requires muscle-specific fusogens (Myomaker and Myomerger) as well as general cellular machinery to achieve the union of membranes. While a unified vertebrate myoblast fusion pathway is not yet established, recent discoveries should make this pursuit attainable. Not only does myocyte fusion impact the normal biology of skeletal muscle, but new evidence indicates dysregulation of the process impacts pathologies of skeletal muscle. Here, I will highlight the molecular players and biochemical mechanisms that drive fusion events in muscle, and discuss how this key myogenic process impacts skeletal muscle diseases.

CTRP11 contributes modestly to systemic metabolism and energy balance

C1q/TNF‐related proteins (CTRP1‐15) constitute a conserved group of secreted proteins of the C1q family with diverse functions. In vitro studies have shown that CTRP11/C1QL4 can inhibit adipogenesis, antagonize myoblast fusion, and promote testosterone synthesis and secretion. Whether CTRP11 is required for these processes in vivo remains unknown. Here, we show that knockout (KO) mice lacking CTRP11 have normal skeletal muscle mass and function, and testosterone level, suggesting that CTRP11 is dispensable for skeletal muscle development and testosterone production. We focused our analysis on whether this nutrient‐responsive secreted protein plays a role in controlling sugar and fat metabolism. At baseline when mice are fed a standard chow, CTRP11 deficiency affects metabolic parameters in a sexually dimorphic manner. Only Ctrp11‐KO female mice have significantly higher fasting serum ketones and reduced physical activity. In the refeeding phase following food withdrawal, Ctrp11‐KO female mice have reduced food intake and increased metabolic rate and energy expenditure, highlighting CTRP11’s role in fasting–refeeding response. When challenged with a high‐fat diet to induce obesity and metabolic dysfunction, CTRP11 deficiency modestly exacerbates obesity‐induced glucose intolerance, with more pronounced effects seen in Ctrp11‐KO male mice. Switching to a low‐fat diet after obesity induction results in greater fat loss in wild type relative to KO male mice, suggesting impaired response to obesity reversal and reduced metabolic flexibility in the absence of CTRP11. Collectively, our data provide genetic evidence for novel sex‐dependent metabolic regulation by CTRP11, but note the overall modest contribution of CTRP11 to systemic energy homeostasis.

Biallelic ELMO3 mutations and loss of function for DOCK-mediated RAC1 activation result in intellectual disability

The engulfment and cell motility 3 (ELMO3) protein belongs to the ELMO-family of proteins. ELMO proteins form a tight complex with the DOCK1-5 guanine nucleotide exchange factors that regulate RAC1 spatiotemporal activation and signalling. DOCK proteins and RAC1 are known to have fundamental roles in central nervous system development. Here, we searched for homozygous or compound heterozygous mutations in the ELMO3 gene in 390 whole exomes sequenced in trio in individuals with neurodevelopmental disorders compatible with a genetic origin. We found a compound heterozygous mutation in ELMO3 (c.1153A>T, p.Ser385Cys and c.1009 G > A, p.Val337Ile) in a 5 year old male child with autism spectrum disorder (ASD) and developmental delay. These mutations did not interfere with the formation of an ELMO3/DOCK1 complex, but markedly impaired the ability of the complex to promote RAC1-GTP-loading. Consequently, cells expressing DOCK1 and either of the ELMO3 mutants displayed impaired migration and invasion. Collectively, our results suggest that biallelic loss-of-function mutations in ELMO3 may cause a developmental delay and provide new insight into the role of ELMO3 in neurodevelopmental as well as the pathological consequences of ELMO3 mutations.

RTN4/NoGo-receptor binding to BAI adhesion-GPCRs regulates neuronal development

RTN4-binding proteins were widely studied as "NoGo" receptors, but their physiological interactors and roles remain elusive. Similarly, BAI adhesion-GPCRs were associated with numerous activities, but their ligands and functions remain unclear. Using unbiased approaches, we observed an unexpected convergence: RTN4 receptors are high-affinity ligands for BAI adhesion-GPCRs. A single thrombospondin type 1-repeat (TSR) domain of BAIs binds to the leucine-rich repeat domain of all three RTN4-receptor isoforms with nanomolar affinity. In the 1.65 Å crystal structure of the BAI1/RTN4-receptor complex, C-mannosylation of tryptophan and O-fucosylation of threonine in the BAI TSR-domains creates a RTN4-receptor/BAI interface shaped by unusual glycoconjugates that enables high-affinity interactions. In human neurons, RTN4 receptors regulate dendritic arborization, axonal elongation, and synapse formation by differential binding to glial versus neuronal BAIs, thereby controlling neural network activity. Thus, BAI binding to RTN4/NoGo receptors represents a receptor-ligand axis that, enabled by rare post-translational modifications, controls development of synaptic circuits.

Rho GTPases in Skeletal Muscle Development and Homeostasis

Rho guanosine triphosphate hydrolases (GTPases) are molecular switches that cycle between an inactive guanosine diphosphate (GDP)-bound and an active guanosine triphosphate (GTP)-bound state during signal transduction. As such, they regulate a wide range of both cellular and physiological processes. In this review, we will summarize recent work on the role of Rho GTPase-regulated pathways in skeletal muscle development, regeneration, tissue mass homeostatic balance, and metabolism. In addition, we will present current evidence that links the dysregulation of these GTPases with diseases caused by skeletal muscle dysfunction. Overall, this information underscores the critical role of a number of members of the Rho GTPase subfamily in muscle development and the overall metabolic balance of mammalian species.

CTRP4 ablation impairs associative learning and memory

C1q/TNF-related protein (CTRP) family comprises fifteen highly conserved secretory proteins with diverse central and peripheral functions. In zebrafish, mouse, and human, CTRP4 is most highly expressed in the brain. We previously showed that CTRP4 is a metabolically responsive regulator of food intake and energy balance, and mice lacking CTRP4 exhibit sexually dimorphic changes in ingestive behaviors and systemic metabolism. Recent single-cell RNA sequencing also revealed Ctrp4/C1qtnf4 expression in diverse neuronal cell types across distinct anatomical brain regions, hinting at additional roles in the central nervous system not previously characterized. To uncover additional central functions of CTRP4, we subjected Ctrp4 knockout (KO) mice to a battery of behavioral tests. Relative to wild-type (WT) littermates, loss of CTRP4 does not alter exploratory, anxiety-, or depressive-like behaviors, motor function and balance, sensorimotor gating, novel object recognition, and spatial memory. While pain-sensing mechanisms in response to thermal stress and mild shock are intact, both male and female Ctrp4 KO mice have increased sensitivity to pain induced by higher-level shock, suggesting altered nociceptive function. Importantly, CTRP4 deficiency impairs hippocampal-dependent associative learning and memory as assessed by trace fear conditioning paradigm. This deficit is sex-dependent, affects only female mice, and is associated with altered expression of learning and memory genes (Arc, c-fos, and Pde4d) in the hippocampus and cortex. Altogether, our behavioral and gene expression analyses have uncovered novel aspects of the CTRP4 function and provided a physiological context to further investigate its mechanism of action in the central and peripheral nervous system.

Systemic Glycosaminoglycan Clearance by HARE/Stabilin-2 Activates Intracellular Signaling

Scavenger receptors perform essential functions, critical to maintaining mammalian physiologic homeostasis by continuously clearing vast numbers of biomolecules from blood, interstitial fluid and lymph. Stabilin-2 (Stab2) and the Hyaluronic Acid Receptor for Endocytosis (HARE), a proteolytic isoform of Stab2, are important scavenger receptors responsible for the specific binding and internalization (leading to degradation) of 22 discrete molecules, macromolecular complexes and cell types. One-third of these ligands are glycosaminoglycans (GAGs). Full-length Stab2, but not HARE, mediates efficient phagocytosis of apoptotic cells and bacteria via binding to target surface ligands. HARE, the C-terminal half of Stab2, mediates endocytosis of all the known soluble ligands. HA was the first ligand identified, in 1981, prior to receptor purification or cloning. Seven other GAG ligands were subsequently identified: heparin, dermatan sulfate, chondroitin and chondroitin sulfates A, C, D and E. Synthetic dextran sulfate is also a GAG mimic and ligand. HARE signaling during HA endocytosis was first discovered in 2008, and we now know that activation of HARE/Stab2 signaling is stimulated by receptor-mediated endocytosis or phagocytosis of many, but not all, of its ligands. This review focuses on the HARE-mediated GAG activation of intracellular signaling, particularly the Extracellular Signal-Regulated Kinase 1/2 pathway.

Mechanisms of adhesion G protein-coupled receptor activation

Adhesion G protein-coupled receptors (AGPCRs) are a thirty-three-member subfamily of Class B GPCRs that control a wide array of physiological processes and are implicated in disease. AGPCRs uniquely contain large, self-proteolyzing extracellular regions that range from hundreds to thousands of residues in length. AGPCR autoproteolysis occurs within the extracellular GPCR autoproteolysis-inducing (GAIN) domain that is proximal to the N terminus of the G protein-coupling seven-transmembrane-spanning bundle. GAIN domain-mediated self-cleavage is constitutive and produces two-fragment holoreceptors that remain bound at the cell surface. It has been of recent interest to understand how AGPCRs are activated in relation to their two-fragment topologies. Dissociation of the AGPCR fragments stimulates G protein signaling through the action of the tethered-peptide agonist stalk that is occluded within the GAIN domain in the holoreceptor form. AGPCRs can also signal independently of fragment dissociation, and a few receptors possess GAIN domains incapable of self-proteolysis. This has resulted in complex theories as to how these receptors are activated in vivo, complicating pharmacological advances. Currently, there is no existing structure of an activated AGPCR to support any of the theories. Further confounding AGPCR research is that many of the receptors remain orphans and lack identified activating ligands. In this review, we provide a detailed layout of the current theorized modes of AGPCR activation with discussion of potential parallels to mechanisms used by other GPCR classes. We provide a classification means for the ligands that have been identified and discuss how these ligands may activate AGPCRs in physiological contexts.

Structure of the DOCK2-ELMO1 complex provides insights into regulation of the auto-inhibited state

DOCK (dedicator of cytokinesis) proteins are multidomain guanine nucleotide exchange factors (GEFs) for RHO GTPases that regulate intracellular actin dynamics. DOCK proteins share catalytic (DOCKDHR2) and membrane-associated (DOCKDHR1) domains. The structurally-related DOCK1 and DOCK2 GEFs are specific for RAC, and require ELMO (engulfment and cell motility) proteins for function. The N-terminal RAS-binding domain (RBD) of ELMO (ELMORBD) interacts with RHOG to modulate DOCK1/2 activity. Here, we determine the cryo-EM structures of DOCK2-ELMO1 alone, and as a ternary complex with RAC1, together with the crystal structure of a RHOG-ELMO2RBD complex. The binary DOCK2-ELMO1 complex adopts a closed, auto-inhibited conformation. Relief of auto-inhibition to an active, open state, due to a conformational change of the ELMO1 subunit, exposes binding sites for RAC1 on DOCK2DHR2, and RHOG and BAI GPCRs on ELMO1. Our structure explains how up-stream effectors, including DOCK2 and ELMO1 phosphorylation, destabilise the auto-inhibited state to promote an active GEF.

Role of the Hyaluronan Receptor, Stabilin-2/HARE, in Health and Disease

Stabilin-2/HARE is the primary clearance receptor for circulating hyaluronan (HA), a polysaccharide found in the extracellular matrix (ECM) of metazoans. HA has many biological functions including joint lubrication, ocular turgor pressure, skin elasticity and hydration, cell motility, and intercellular signaling, among many others. The regulatory system for HA content in the tissues, lymphatics, and circulatory systems is due, in part, to Stabilin-2/HARE. The activity of this receptor was discovered about 40 years ago (early 1980s), cloned in the mid-1990s, and has been characterized since then. Here, we discuss the overall domain organization of this receptor and how it correlates to ligand binding, cellular signaling, and its role in known physiological disorders such as cancer.

Adhesion GPCRs as a paradigm for understanding polycystin-1 G protein regulation

Polycystin-1, whose mutation is the most frequent cause of autosomal dominant polycystic kidney disease, is an extremely large and multi-faceted membrane protein whose primary or proximal cyst-preventing function remains undetermined. Accumulating evidence supports the idea that modulation of cellular signaling by heterotrimeric G proteins is a critical function of polycystin-1. The presence of a cis-autocatalyzed, G protein-coupled receptor (GPCR) proteolytic cleavage site, or GPS, in its extracellular N-terminal domain immediately preceding the first transmembrane domain is one of the notable conserved features of the polycystin-1-like protein family, and also of the family of cell adhesion GPCRs. Adhesion GPCRs are one of five families within the GPCR superfamily and are distinguished by a large N-terminal extracellular region consisting of multiple adhesion modules with a GPS-containing GAIN domain and bimodal functions in cell adhesion and signal transduction. Recent advances from studies of adhesion GPCRs provide a new paradigm for unraveling the mechanisms by which polycystin-1-associated G protein signaling contributes to the pathogenesis of polycystic kidney disease. This review highlights the structural and functional features shared by polycystin-1 and the adhesion GPCRs and discusses the implications of such similarities for our further understanding of the functions of this complicated protein.

The Emerging Role of Adhesion GPCRs in Cancer

Aberrant expression, function, and mutation of G protein-coupled receptors (GPCRs) and their signaling partners, G proteins, have been well documented in many forms of cancer. These cell surface receptors and their endogenous ligands are implicated in all aspects of cancer including proliferation, angiogenesis, invasion, and metastasis. Adhesion GPCRs (aGPCRs) form the second largest family of GPCRs, most of which are orphan receptors with unknown physiological functions. This is mainly due to our limited insight into their structure, natural ligands, signaling pathways, and tissue expression profiles. Nevertheless, recent studies show that aGPCRs play important roles in cell adhesion to the extracellular matrix and cell-cell communication, processes that are dysregulated in cancer. Emerging evidence suggests that aGPCRs are implicated in migration, proliferation, and survival of tumor cells. We here review the role of aGPCRs in the five most common types of cancer (lung, breast, colorectal, prostate, and gastric) and emphasize the importance of further translational studies in this field.

Arrestin-independent constitutive endocytosis of GPR125/ADGRA3

The orphan receptor GPR125 (ADGRA3) belongs to subgroup III of the adhesion G protein−coupled receptor (aGPCR) family. aGPCRs, also known as class B2 GPCRs, share basic structural and functional properties with other GPCRs. Many of them couple to G proteins and activate G protein−dependent and −independent signaling pathways, but little is known about aGPCR internalization and β‐arrestin recruitment. GPR125 was originally described as a spermatogonial stem cell marker and studied for its role in Wnt signaling and cell polarity. Here, using cell‐based assays and confocal microscopy, we show that GPR125 is expressed on the cell surface and undergoes constitutive endocytosis in a β‐arrestin−independent, but clathrin‐dependent manner, as indicated by colocalization with transferrin receptor 1, an early endosome marker. These data support that the constitutive internalization of GPR125 contributes to its biological functions by controlling receptor surface expression and accessibility for ligands. Our study sheds light on a new property of aGPCRs, namely internalization; a property described to be important for signal propagation, signal termination, and desensitization of class A (rhodopsin‐like) and B1 (VIP/secretin) GPCRs.

Adhesion G protein-coupled receptors: opportunities for drug discovery

Adhesion G protein-coupled receptors (aGPCRs) - one of the five main families in the GPCR superfamily - have several atypical characteristics, including large, multi-domain N termini and a highly conserved region that can be autoproteolytically cleaved. Although GPCRs overall have well-established pharmacological tractability, currently no therapies that target any of the 33 members of the aGPCR family are either approved or in clinical trials. However, human genetics and preclinical research have strengthened the links between aGPCRs and disease in recent years. This, together with a greater understanding of their functional complexity, has led to growing interest in aGPCRs as drug targets. A framework for prioritizing aGPCR targets and supporting approaches to develop aGPCR modulators could therefore be valuable in harnessing the untapped therapeutic potential of this family. With this in mind, here we discuss the unique opportunities and challenges for drug discovery in modulating aGPCR functions, including target identification, target validation, assay development and safety considerations, using ADGRG1 as an illustrative example.

Stabilin Receptors: Role as Phosphatidylserine Receptors

Phosphatidylserine is a membrane phospholipid that is localized to the inner leaflet of the plasma membrane. Phosphatidylserine externalization to the outer leaflet of the plasma membrane is an important signal for various physiological processes, including apoptosis, platelet activation, cell fusion, lymphocyte activation, and regenerative axonal fusion. Stabilin-1 and stabilin-2 are membrane receptors that recognize phosphatidylserine on the cell surface. Here, we discuss the functions of Stabilin-1 and stabilin-2 as phosphatidylserine receptors in apoptotic cell clearance (efferocytosis) and cell fusion, and their ligand-recognition and signaling pathways.

Voices from the dead: The complex vocabulary and intricate grammar of dead cells

Of the roughly one million cells per second dying throughout the body, the vast majority dies by apoptosis, the predominant form of regulated cell death in higher organisms. Long regarded as mere waste, apoptotic cells are now recognized as playing a prominent and active role in homeostatic maintenance, especially resolution of inflammation, and in the sculpting of tissues during development. The activities associated with apoptotic cells are continually expanding, with more recent studies demonstrating their ability to modulate such vital functions as proliferation, survival, differentiation, metabolism, migration, and angiogenesis. In each case, the role of apoptotic cells is active, exerting their effects via new activities acquired during the apoptotic program. Moreover, the capacity to recognize and respond to apoptotic cells is not limited to professional phagocytes. Most, if not all, cells receive and integrate an array of signals from cells dying in their vicinity. These signals comprise a form of biochemical communication. As reviewed in this chapter, this communication is remarkably sophisticated; each of its three critical steps-encoding, transmission, and decoding of the apoptotic cell's "message"-is endowed with exquisite robustness. Together, the abundance and intricacy of the variables at each step comprise the vocabulary and grammar of the language by which dead cells achieve their post-mortem voice. The combinatorial complexity of the resulting communication network permits dying cells, through the signals they emit and the responses those signals elicit, to partake of an expanded role in homeostasis, acting as both sentinels of environmental change and agents of adaptation.

How cells fuse

Brukman et al. review cell–cell fusion mechanisms, focusing on the identity of the fusogens that mediate these processes and the regulation of their activities.

Cell–cell fusion remains the least understood type of membrane fusion process. However, the last few years have brought about major advances in understanding fusion between gametes, myoblasts, macrophages, trophoblasts, epithelial, cancer, and other cells in normal development and in diseases. While different cell fusion processes appear to proceed via similar membrane rearrangements, proteins that have been identified as necessary and sufficient for cell fusion (fusogens) use diverse mechanisms. Some fusions are controlled by a single fusogen; other fusions depend on several proteins that either work together throughout the fusion pathway or drive distinct stages. Furthermore, some fusions require fusogens to be present on both fusing membranes, and in other fusions, fusogens have to be on only one of the membranes. Remarkably, some of the proteins that fuse cells also sculpt single cells, repair neurons, promote scission of endocytic vesicles, and seal phagosomes. In this review, we discuss the properties and diversity of the known proteins mediating cell–cell fusion and highlight their different working mechanisms in various contexts.

Adhesion G-protein coupled receptors: Implications for metabolic function

Adhesion G-protein coupled receptors (aGPCRs) are emerging as important actors in energy homeostasis. Recent biochemical and functional studies using transgenic mice indicate that aGPCRs play important roles in endocrine and metabolic functions including β-cell differentiation, insulin secretion, adipogenesis and whole body fuel homeostasis. Most aGPCRs are orphans, for which endogenous ligands have not yet been identified, and many of the endogenous ligands of the already de-orphanised aGPCRs are components of the extracellular matrix (ECM). In this review we focus on aGPCR expression in metabolically active tissues, their activation by ECM proteins, and current knowledge of their potential roles in islet development, insulin secretion, adipogenesis and muscle function.

Expression patterns of C1ql4 and its cell-adhesion GPCR Bai3 in the murine testis and functional roles in steroidogenesis

C1q-like 4 (C1QL4), a novel member of the C1q- and TNF-related protein family, was found to be highly expressed in rodent and human testis. However, the localization, developmental, and hormonally regulated expression and biologic function of C1ql4 in the testis have not been investigated. Here, we demonstrated that C1ql4 mRNA and protein levels in murine testes gradually increased from the postnatal period to the adult stage and were up-regulated by LH in vivo. In situ hybridization demonstrated that the distribution and expression levels of C1ql4 mRNA varied at different developmental stages, although C1ql4 mRNA was detected in the seminiferous tubule and interstitial Leydig cells. Recombinant C1QL4 did not affect cell proliferation but did increase testosterone production in TM3 Leydig cells, as well as in cultured seminiferous tubules. C1QL4-induced testosterone secretion in Leydig cells was accompanied by increased expression of steroidogenic acute regulatory (StAR) protein and steroidogenic enzymes. During this process, the c-Raf/extracellular signal-regulated protein kinase kinases 1 and 2/ERK1/2/mitogen- and stress-activated protein kinase-1 and cAMP/PKA/cAMP-responsive element binding protein signaling cascades were activated by C1QL4. The cell-adhesion GPCR brain-specific angiogenesis inhibitor 3 (BAI3), a putative receptor of C1QL4, was detected in the seminiferous tubule and interstitial Leydig cells during testicular development. Knockdown of Bai3 expression in Leydig cells led to a reduction in Star expression, accompanied by increases in phosphorylation of ERK1/2 and intercellular cAMP levels. However, C1QL4-induced StAR expression was not completely suppressed in the Bai3-deficient Leydig cells, and phosphorylation of ERK1/2 and intercellular cAMP levels were not significantly changed before and after C1QL4 stimulation. Our results suggested that although BAI3 played a role in C1QL4-induced steroidogenesis, there was an unidentified receptor that mediated C1QL4-activated testosterone secretion in Leydig cells through phosphorylation of ERK1/2 and up-regulation of intracellular cAMP levels. Taken together, our results showed, for the first time to our knowledge, that C1QL4 served as a novel acute regulator of testosterone secretion, and BAI3 functioned as a new receptor that is involved in steroidogenesis in Leydig cells. BAI3-independent ERK1/2 activation and cAMP activation mediated C1QL4-induced testosterone secretion. This study expanded the reproductive roles and mechanisms of C1QL4 and BAI3 signaling pathways.-Tan, A., Ke, S., Chen, Y., Chen, L., Lu, X., Ding, F., Yang, L., Tang, Y., Yu, Y. Expression patterns of C1ql4 and its cell-adhesion GPCR Bai3 in the murine testis and functional roles in steroidogenesis.

Understanding the Role of the BAI Subfamily of Adhesion G Protein-Coupled Receptors (GPCRs) in Pathological and Physiological Conditions

Brain-specific angiogenesis inhibitors (BAIs) 1, 2, and 3 are members of the adhesion G protein-coupled receptors, subfamily B, which share a conserved seven-transmembrane structure and an N-terminal extracellular domain. In cell- and animal-based studies, these receptors have been shown to play diverse roles under physiological and pathological conditions. BAI1 is an engulfment receptor and performs major functions in apoptotic-cell clearance and interacts (as a pattern recognition receptor) with pathogen components. BAI1 and -3 also participate in myoblast fusion. Furthermore, BAI1–3 have been linked to tumor progression and neurological diseases. In this review, we summarize the current understanding of the functions of BAI1–3 in pathological and physiological conditions and discuss future directions in terms of the importance of BAIs as pharmacological targets in diseases.