The adhesion-GPCR BAI3, a gene linked to psychiatric disorders, regulates dendrite morphogenesis in neurons

Emerging Roles of BAI Adhesion-GPCRs in Synapse Development and Plasticity

Synapses mediate communication between neurons and enable the brain to change in response to experience, which is essential for learning and memory. The sites of most excitatory synapses in the brain, dendritic spines, undergo rapid remodeling that is important for neural circuit formation and synaptic plasticity. Abnormalities in synapse and spine formation and plasticity are associated with a broad range of brain disorders, including intellectual disabilities, autism spectrum disorders (ASD), and schizophrenia. Thus, elucidating the mechanisms that regulate these neuronal processes is critical for understanding brain function and disease. The brain-specific angiogenesis inhibitor (BAI) subfamily of adhesion G-protein-coupled receptors (adhesion-GPCRs) has recently emerged as central regulators of synapse development and plasticity. In this review, we will summarize the current knowledge regarding the roles of BAIs at synapses, highlighting their regulation, downstream signaling, and physiological functions, while noting the roles of other adhesion-GPCRs at synapses. We will also discuss the relevance of BAIs in various neurological and psychiatric disorders and consider their potential importance as pharmacological targets in the treatment of these diseases.

Versatile Signaling Activity of Adhesion GPCRs

The adhesion G protein-coupled receptors (aGPCRs) are a family of 33 receptors in humans that are widely expressed in various tissues and involved in many diverse biological processes. These receptors possess extremely large N-termini (NT) containing a variety of adhesion domains. A distinguishing feature of these receptors is the presence within the NT of a highly conserved GPCR autoproteolysis-inducing (GAIN) domain, which mediates autoproteolysis of the receptors into N-terminal and C-terminal fragments that stay non-covalently associated. The downstream signaling pathways and G protein-coupling preferences of many aGPCRs have recently been elucidated, and putative endogenous ligands for some aGPCRs have also been discovered and characterized in recent years. A pivotal observation for aGPCRs has been that deletion or removal of the NT up the point of GAIN cleavage results in constitutive receptor activation. For at least some aGPCRs, this activation is dependent on the unmasking of specific agonistic peptide sequences within the N-terminal stalk region (i.e., the region between the site of GAIN domain cleavage and the first transmembrane domain). However, the specific peptide sequences involved and the overall importance of the stalk region for activation can vary greatly from receptor to receptor. An emerging theme of work in this area is that aGPCRs are capable of versatile signaling activity that may be fine-tuned to suit the specific physiological roles played by the various members of this family.

Adhesion GPCRs as Novel Actors in Neural and Glial Cell Functions: From Synaptogenesis to Myelination

Adhesion G-protein-coupled receptors (aGPCRs) are emerging as key regulators of nervous system development and health. aGPCRs can regulate many aspects of neural development, including cell signaling, cell-cell and cell-matrix interactions, and, potentially, mechanosensation. Here, we specifically focus on the roles of several aGPCRs in synapse biology, dendritogenesis, and myelinating glial cell development. The lessons learned from these examples may be extrapolated to other contexts in the nervous system and beyond.

Adhesion GPCRs as Modulators of Immune Cell Function

Immune cells express several adhesion G protein-coupled receptors (aGPCRs), including the ADGRE subfamily members EMR1 (F4/80, ADGRE1), EMR2 (ADGRE2), EMR3 (ADGRE3), EMR4 (FIRE, ADGRE4), and CD97 (ADGRE5), the ADGRB subfamily member BAI1 (ADGRB1), and the ADGRG subfamily members GPR56 (ADGRG1), GPR97 (Pb99, ADGRG3), and GPR114 (ADGRG5). Expression of these molecules in hematopoietic stem and progenitor cells, monocytes/macrophages (Mφs), dendritic cells, granulocytes, and lymphocytes depends on lineage diversification and maturation, making them suitable markers for individual leukocyte subsets (e.g., F4/80 on mouse Mφs). Recent studies revealed intriguing activities of aGPCRs in tolerance induction (EMR1), granulopoiesis (CD97), engulfment of apoptotic cells and bacteria (BAI1), hematopoietic stem cell formation (GPR56), and control of cytotoxicity (GPR56). Here, we review these findings and discuss their biological and translational implications.

The role of G protein-coupled receptors in the early evolution of neurotransmission and the nervous system

The origin and evolution of the nervous system is one of the most intriguing and enigmatic events in biology. The recent sequencing of complete genomes from early metazoan organisms provides a new platform to study the origins of neuronal gene families. This review explores the early metazoan expansion of the largest integral transmembrane protein family, the G protein-coupled receptors (GPCRs), which serve as molecular targets for a large subset of neurotransmitters and neuropeptides in higher animals. GPCR repertories from four pre-bilaterian metazoan genomes were compared. This includes the cnidarian Nematostella vectensis and the ctenophore Mnemiopsis leidyi, which have primitive nervous systems (nerve nets), the demosponge Amphimedon queenslandica and the placozoan Trichoplax adhaerens, which lack nerve and muscle cells. Comparative genomics demonstrate that the rhodopsin and glutamate receptor families, known to be involved in neurotransmission in higher animals are also widely found in pre-bilaterian metazoans and possess substantial expansions of rhodopsin-family-like GPCRs. Furthermore, the emerging knowledge on the functions of adhesion GPCRs in the vertebrate nervous system provides a platform to examine possible analogous roles of their closest homologues in pre-bilaterians. Intriguingly, the presence of molecular components required for GPCR-mediated neurotransmission in pre-bilaterians reveals that they exist in both primitive nervous systems and nerve-cell-free environments, providing essential comparative models to better understand the origins of the nervous system and neurotransmission.

International Union of Basic and Clinical Pharmacology. XCIV. Adhesion G protein-coupled receptors

The Adhesion family forms a large branch of the pharmacologically important superfamily of G protein-coupled receptors (GPCRs). As Adhesion GPCRs increasingly receive attention from a wide spectrum of biomedical fields, the Adhesion GPCR Consortium, together with the International Union of Basic and Clinical Pharmacology Committee on Receptor Nomenclature and Drug Classification, proposes a unified nomenclature for Adhesion GPCRs. The new names have ADGR as common dominator followed by a letter and a number to denote each subfamily and subtype, respectively. The new names, with old and alternative names within parentheses, are: ADGRA1 (GPR123), ADGRA2 (GPR124), ADGRA3 (GPR125), ADGRB1 (BAI1), ADGRB2 (BAI2), ADGRB3 (BAI3), ADGRC1 (CELSR1), ADGRC2 (CELSR2), ADGRC3 (CELSR3), ADGRD1 (GPR133), ADGRD2 (GPR144), ADGRE1 (EMR1, F4/80), ADGRE2 (EMR2), ADGRE3 (EMR3), ADGRE4 (EMR4), ADGRE5 (CD97), ADGRF1 (GPR110), ADGRF2 (GPR111), ADGRF3 (GPR113), ADGRF4 (GPR115), ADGRF5 (GPR116, Ig-Hepta), ADGRG1 (GPR56), ADGRG2 (GPR64, HE6), ADGRG3 (GPR97), ADGRG4 (GPR112), ADGRG5 (GPR114), ADGRG6 (GPR126), ADGRG7 (GPR128), ADGRL1 (latrophilin-1, CIRL-1, CL1), ADGRL2 (latrophilin-2, CIRL-2, CL2), ADGRL3 (latrophilin-3, CIRL-3, CL3), ADGRL4 (ELTD1, ETL), and ADGRV1 (VLGR1, GPR98). This review covers all major biologic aspects of Adhesion GPCRs, including evolutionary origins, interaction partners, signaling, expression, physiologic functions, and therapeutic potential.

Regulation of dendrite morphogenesis by extrinsic cues

Dendrites play a central role in the integration and flow of information in the nervous system. The morphogenesis and maturation of dendrites is hence an essential step in the establishment of neuronal connectivity. Recent studies have uncovered crucial functions for extrinsic cues in the development of dendrites. We review the contribution of secreted polypeptide growth factors, contact-mediated proteins, and neuronal activity in distinct phases of dendrite development. We also highlight how extrinsic cues influence local and global intracellular mechanisms of dendrite morphogenesis. Finally, we discuss how these studies have advanced our understanding of neuronal connectivity and have shed light on the pathogenesis of neurodevelopmental disorders.

New functions and signaling mechanisms for the class of adhesion G protein-coupled receptors

The class of adhesion G protein-coupled receptors (aGPCRs), with 33 human homologs, is the second largest family of GPCRs. In addition to a seven-transmembrane α-helix-a structural feature of all GPCRs-the class of aGPCRs is characterized by the presence of a large N-terminal extracellular region. In addition, all aGPCRs but one (GPR123) contain a GPCR autoproteolysis-inducing (GAIN) domain that mediates autoproteolytic cleavage at the GPCR autoproteolysis site motif to generate N- and a C-terminal fragments (NTF and CTF, respectively) during protein maturation. Subsequently, the NTF and CTF are associated noncovalently as a heterodimer at the plasma membrane. While the biological function of the GAIN domain-mediated autocleavage is not fully understood, mounting evidence suggests that the NTF and CTF possess distinct biological activities in addition to their function as a receptor unit. We discuss recent advances in understanding the biological functions, signaling mechanisms, and disease associations of the aGPCRs.

Development of anxiety-like behavior via hippocampal IGF-2 signaling in the offspring of parental morphine exposure: effect of enriched environment

Opioid addiction is a major social, economic, and medical problem worldwide. Long-term adverse consequences of chronic opiate exposure not only involve the individuals themselves but also their offspring. Adolescent maternal morphine exposure results in behavior and morphologic changes in the brain of their adult offspring. However, few studies investigate the effect of adult opiate exposure on their offspring. Furthermore, the underlying molecular signals regulating the intergenerational effects of morphine exposure are still elusive. We report here that morphine exposure of adult male and female rats resulted in anxiety-like behavior and dendritic retraction in the dentate gyrus (DG) region of the hippocampus in their adult offspring. The behavior and morphologic changes were concomitant with the downregulation of insulin-like growth factor (IGF)-2 signaling in the granular zone of DG. Overexpression of hippocampal IGF-2 by bilateral intra-DG injection of lentivirus encoding the IGF-2 gene prevented anxiety-like behaviors in the offspring. Furthermore, exposure to an enriched environment during adolescence corrected the reduction of hippocampal IGF-2 expression, normalized anxiety-like behavior and reversed dendritic retraction in the adult offspring. Thus, parental morphine exposure can lead to the downregulation of hippocampal IGF-2, which contributed to the anxiety and hippocampal dendritic retraction in their offspring. An adolescent-enriched environment experience prevented the behavior and morphologic changes in their offspring through hippocampal IGF-2 signaling. IGF-2 and an enriched environment may be a potential intervention to prevention of anxiety and brain atrophy in the offspring of parental opioid exposure.

CD44 regulates dendrite morphogenesis through Src tyrosine kinase-dependent positioning of the Golgi

The acquisition of proper dendrite morphology is a crucial aspect of neuronal development towards the formation of a functional network. The role of the extracellular matrix and its cellular receptors in this process has remained enigmatic. We report that the CD44 adhesion molecule, the main hyaluronan receptor, is localized in dendrites and plays a crucial inhibitory role in dendritic tree arborization in vitro and in vivo. This novel function is exerted by the activation of Src tyrosine kinase, leading to the alteration of Golgi morphology. The mechanism operates during normal brain development, but its inhibition might have a protective influence on dendritic trees under toxic conditions, during which the silencing of CD44 expression prevents dendritic shortening induced by glutamate exposure. Overall, our results indicate a novel role for CD44 as an essential regulator of dendritic arbor complexity in both health and disease.

The BAI subfamily of adhesion GPCRs: synaptic regulation and beyond

The brain-specific angiogenesis inhibitors 1-3 (BAI1-3) comprise a subfamily of adhesion G-protein-coupled receptors (GPCRs). These receptors are highly expressed in the brain and were first studied for their ability to inhibit angiogenesis and tumor formation. Subsequently, BAI1 was found to play roles in apoptotic cell phagocytosis and myoblast fusion. Until recently, however, little was known about the physiological importance of the BAI subfamily in the context of normal brain function. Recent work has provided evidence for key roles of BAI1-3 in the regulation of synaptogenesis and dendritic spine formation. In this review, we summarize the current understanding of the BAI subfamily with regard to downstream signaling pathways, physiological actions, and potential importance as novel drug targets in the treatment of psychiatric and neurological diseases.

Adhesion G-protein-coupled receptors: elusive hybrids come of age

Adhesion G-protein-coupled receptors (GPCRs) are the most recently identified and least understood subfamily of GPCRs. Adhesion GPCRs are characterized by unusually long ectodomains with adhesion-related repeats that facilitate cell- cell and cell-cell matrix contact, as well as a proteolytic cleavage site-containing domain that is a structural hallmark of the family. Their unusual chimeric structure of adhesion-related ectodomain with a seven-pass transmembrane domain and cytoplasmic signaling makes these proteins highly versatile in mediating cellular signaling in response to extracellular adhesion or cell motility events. The ligand binding and cytoplasmic signaling modes for members of this family are beginning to be elucidated, and recent studies have demonstrated critical roles for Adhesion GPCRs in planar polarity and other important cell-cell and cell-matrix interactions during development and morphogenesis, as well as heritable diseases and cancer.

Dock protein family in brain development and neurological disease

The family of dedicator of cytokinesis (Dock), a protein family that belongs to the atypical Rho guanine nucleotide exchange factors (GEFs) for Rac and/or Cdc42 GTPases, plays pivotal roles in various processes of brain development. To date, 11 members of Docks have been identified in the mammalian system. Emerging evidence has suggested that members of the Dock family are associated with several neurodegenerative and neuropsychiatric diseases, including Alzheimer disease and autism spectrum disorders. This review summarizes recent advances on the understanding of the roles of the Dock protein family in normal and diseased processes in the nervous system. Furthermore, interacting proteins and the molecular regulation of Docks are discussed.

Brain-specific angiogenesis inhibitor-1 signaling, regulation, and enrichment in the postsynaptic density

Brain-specific angiogenesis inhibitor-1 (BAI1) is an adhesion G protein-coupled receptor that has been studied primarily for its anti-angiogenic and anti-tumorigenic properties. We found that overexpression of BAI1 results in activation of the Rho pathway via a Gα(12/13)-dependent mechanism, with truncation of the BAI1 N terminus resulting in a dramatic enhancement in receptor signaling. This constitutive activity of the truncated BAI1 mutant also resulted in enhanced downstream phosphorylation of ERK as well as increased receptor association with β-arrestin2 and increased ubiquitination of the receptor. To gain insights into the regulation of BAI1 signaling, we screened the C terminus of BAI1 against a proteomic array of PDZ domains to identify novel interacting partners. These screens revealed that the BAI1 C terminus interacts with a variety of PDZ domains from synaptic proteins, including MAGI-3. Removal of the BAI1 PDZ-binding motif resulted in attenuation of receptor signaling to Rho but had no effect on ERK activation. Conversely, co-expression with MAGI-3 was found to potentiate signaling to ERK by constitutively active BAI1 in a manner that was dependent on the PDZ-binding motif of the receptor. Biochemical fractionation studies revealed that BAI1 is highly enriched in post-synaptic density fractions, a finding consistent with our observations that BAI1 can interact with PDZ proteins known to be concentrated in the post-synaptic density. These findings demonstrate that BAI1 is a synaptic receptor that can activate both the Rho and ERK pathways, with the N-terminal and C-terminal regions of the receptor playing key roles in the regulation of BAI1 signaling activity.