Latrophilins: A Neuro-Centric View of an Evolutionary Conserved Adhesion G Protein-Coupled Receptor Subfamily

Cartography of teneurin and latrophilin expression reveals spatiotemporal axis heterogeneity in the mouse hippocampus during development

Synaptic adhesion molecules (SAMs) are evolutionarily conserved proteins that play an important role in the form and function of neuronal synapses. Teneurins (Tenms) and latrophilins (Lphns) are well-known cell adhesion molecules that form a transsynaptic complex. Recent studies suggest that Tenm3 and Lphn2 (gene symbol Adgrl2) are involved in hippocampal circuit assembly via their topographical expression. However, it is not known whether other teneurins and latrophilins display similar topographically restricted expression patterns during embryonic and postnatal development. Here, we reveal the cartography of all teneurin (Tenm1-4) and latrophilin (Lphn1-3 [Adgrl1-3]) paralog expression in the mouse hippocampus across prenatal and postnatal development as monitored by large-scale single-molecule RNA in situ hybridization mapping. Our results identify a striking heterogeneity in teneurin and latrophilin expression along the spatiotemporal axis of the hippocampus. Tenm2 and Tenm4 expression levels peak at the neonatal stage when compared to Tenm1 and Tenm3, while Tenm1 expression is restricted to the postnatal pyramidal cell layer. Tenm4 expression in the dentate gyrus (DG) exhibits an opposing topographical expression pattern in the embryonic and neonatal hippocampus. Our findings were validated by analyses of multiple RNA-seq datasets at bulk, single-cell, and spatial levels. Thus, our study presents a comprehensive spatiotemporal map of Tenm and Lphn expression in the hippocampus, showcasing their diverse expression patterns across developmental stages in distinct spatial axes.

Dysfunction of the adhesion G protein-coupled receptor latrophilin 1 (ADGRL1/LPHN1) increases the risk of obesity

Obesity is one of the diseases with severe health consequences and rapidly increasing worldwide prevalence. Understanding the complex network of food intake and energy balance regulation is an essential prerequisite for pharmacological intervention with obesity. G protein-coupled receptors (GPCRs) are among the main modulators of metabolism and energy balance. They, for instance, regulate appetite and satiety in certain hypothalamic neurons, as well as glucose and lipid metabolism and hormone secretion from adipocytes. Mutations in some GPCRs, such as the melanocortin receptor type 4 (MC4R), have been associated with early-onset obesity. Here, we identified the adhesion GPCR latrophilin 1 (ADGRL1/LPHN1) as a member of the regulating network governing food intake and the maintenance of energy balance. Deficiency of the highly conserved receptor in mice results in increased food consumption and severe obesity, accompanied by dysregulation of glucose homeostasis. Consistently, we identified a partially inactivating mutation in human ADGRL1/LPHN1 in a patient suffering from obesity. Therefore, we propose that LPHN1 dysfunction is a risk factor for obesity development.

Structural analysis and conformational dynamics of a holo-adhesion GPCR reveal interplay between extracellular and transmembrane domains

Adhesion G Protein-Coupled Receptors (aGPCRs) are key cell-adhesion molecules involved in numerous physiological functions. aGPCRs have large multi-domain extracellular regions (ECR) containing a conserved GAIN domain that precedes their seven-pass transmembrane domain (7TM). Ligand binding and mechanical force applied on the ECR regulate receptor function. However, how the ECR communicates with the 7TM remains elusive, because the relative orientation and dynamics of the ECR and 7TM within a holoreceptor is unclear. Here, we describe the cryo-EM reconstruction of an aGPCR, Latrophilin3/ADGRL3, and reveal that the GAIN domain adopts a parallel orientation to the membrane and has constrained movement. Single-molecule FRET experiments unveil three slow-exchanging FRET states of the ECR relative to the 7TM within the holoreceptor. GAIN-targeted antibodies, and cancer-associated mutations at the GAIN-7TM interface, alter FRET states, cryo-EM conformations, and receptor signaling. Altogether, this data demonstrates conformational and functional coupling between the ECR and 7TM, suggesting an ECR-mediated mechanism of aGPCR activation.

Piconewton Forces Mediate GAIN Domain Dissociation of the Latrophilin-3 Adhesion GPCR

Latrophilins are adhesion G-protein coupled receptors (aGPCRs) that control excitatory synapse formation. Most aGPCRs, including latrophilins, are autoproteolytically cleaved at their GPCR-autoproteolysis inducing (GAIN) domain, but the two resulting fragments remain noncovalently associated on the cell surface. Force-mediated dissociation of the fragments is thought to activate G-protein signaling, but how this mechanosensitivity arises is poorly understood. Here, we use magnetic tweezer assays to show that physiologically relevant forces in the 1-10 pN range lead to dissociation of the latrophilin-3 GAIN domain on the seconds-to-minutes time scale, compared to days in the absence of force. In addition, we find that the GAIN domain undergoes large changes in length in response to increasing mechanical load. These data are consistent with a model in which a force-sensitive equilibrium between compact and extended GAIN domain states precedes dissociation, suggesting a mechanism by which latrophilins and other aGPCRs may mediate mechanically induced signal transduction.

Biased signalling is structurally encoded as an autoproteolysis event in adhesion G protein-coupled receptor Latrophilin-3/ADGRL3

Adhesion G protein-coupled receptors (aGPCRs) possess a unique topology, including the presence of a GPCR proteolysis site (GPS), which, upon autoproteolysis, generates two functionally distinct fragments that remain non-covalently associated at the plasma membrane. A proposed activation mechanism for aGPCRs involves the exposure of a tethered agonist, which depends on cleavage at the GPS. However, this hypothesis has been challenged by the observation that non-cleavable aGPCRs exhibit constitutive activity, thus making the function of GPS cleavage widely enigmatic. In this study, we sought to elucidate the function of GPS-mediated cleavage through the study of G protein coupling with Latrophilin-3/ADGRL3, a prototypical aGPCR involved in synapse formation and function. Using BRET-based G protein biosensors, we reveal that an autoproteolysis-deficient mutant of ADGRL3 retains constitutive activity. Surprisingly, we uncover that cleavage deficiency leads to a signalling bias directed at potentiating the activity of select G proteins such as Gi2 and G12/13. These data unveil the underpinnings of biased signalling for aGPCRs defined by GPS autoproteolysis.

Spatially resolved multiomics of human cardiac niches

The function of a cell is defined by its intrinsic characteristics and its niche: the tissue microenvironment in which it dwells. Here we combine single-cell and spatial transcriptomics data to discover cellular niches within eight regions of the human heart. We map cells to microanatomical locations and integrate knowledge-based and unsupervised structural annotations. We also profile the cells of the human cardiac conduction system1. The results revealed their distinctive repertoire of ion channels, G-protein-coupled receptors (GPCRs) and regulatory networks, and implicated FOXP2 in the pacemaker phenotype. We show that the sinoatrial node is compartmentalized, with a core of pacemaker cells, fibroblasts and glial cells supporting glutamatergic signalling. Using a custom CellPhoneDB.org module, we identify trans-synaptic pacemaker cell interactions with glia. We introduce a druggable target prediction tool, drug2cell, which leverages single-cell profiles and drug-target interactions to provide mechanistic insights into the chronotropic effects of drugs, including GLP-1 analogues. In the epicardium, we show enrichment of both IgG+ and IgA+ plasma cells forming immune niches that may contribute to infection defence. Overall, we provide new clarity to cardiac electro-anatomy and immunology, and our suite of computational approaches can be applied to other tissues and organs.

A compound PCP scheme underlies sequential rosettes-based cell intercalation

The formation of sequential rosettes is a type of collective cell behavior recently discovered in the Caenorhabditis elegans embryo that mediates directional cell migration through sequential formation and resolution of multicellular rosettes involving the migrating cell and its neighboring cells along the way. Here, we show that a planar cell polarity (PCP)-based polarity scheme regulates sequential rosettes, which is distinct from the known mode of PCP regulation in multicellular rosettes during the process of convergent extension. Specifically, non-muscle myosin (NMY) localization and edge contraction are perpendicular to that of Van Gogh as opposed to colocalizing with Van Gogh. Further analyses suggest a two-component polarity scheme: one being the canonical PCP pathway with MIG-1/Frizzled and VANG-1/Van Gogh localized to the vertical edges, the other being MIG-1/Frizzled and NMY-2 localized to the midline/contracting edges. The NMY-2 localization and contraction of the midline edges also required LAT-1/Latrophilin, an adhesion G protein-coupled receptor that has not been shown to regulate multicellular rosettes. Our results establish a distinct mode of PCP-mediated cell intercalation and shed light on the versatile nature of the PCP pathway.

Context-dependent requirement of G protein coupling for Latrophilin-2 in target selection of hippocampal axons

The formation of neural circuits requires extensive interactions of cell-surface proteins to guide axons to their correct target neurons. Trans-cellular interactions of the adhesion G protein-coupled receptor latrophilin-2 (Lphn2) with its partner teneurin-3 instruct the precise assembly of hippocampal networks by reciprocal repulsion. Lphn2 acts as a repulsive receptor in distal CA1 neurons to direct their axons to the proximal subiculum, and as a repulsive ligand in the proximal subiculum to direct proximal CA1 axons to the distal subiculum. It remains unclear if Lphn2-mediated intracellular signaling is required for its role in either context. Here, we show that Lphn2 couples to Gα12/13 in heterologous cells; this coupling is increased by constitutive exposure of the tethered agonist. Specific mutations of Lphn2's tethered agonist region disrupt its G protein coupling and autoproteolytic cleavage, whereas mutating the autoproteolytic cleavage site alone prevents cleavage but preserves a functional tethered agonist. Using an in vivo misexpression assay, we demonstrate that wild-type Lphn2 misdirects proximal CA1 axons to the proximal subiculum and that Lphn2 tethered agonist activity is required for its role as a repulsive receptor in axons. By contrast, neither tethered agonist activity nor autoproteolysis were necessary for Lphn2's role as a repulsive ligand in the subiculum target neurons. Thus, tethered agonist activity is required for Lphn2-mediated neural circuit assembly in a context-dependent manner.

Isoform- and ligand-specific modulation of the adhesion GPCR ADGRL3/Latrophilin3 by a synthetic binder

Adhesion G protein-coupled receptors (aGPCRs) are cell-surface proteins with large extracellular regions that bind to multiple ligands to regulate key biological functions including neurodevelopment and organogenesis. Modulating a single function of a specific aGPCR isoform while affecting no other function and no other receptor is not trivial. Here, we engineered an antibody, termed LK30, that binds to the extracellular region of the aGPCR ADGRL3, and specifically acts as an agonist for ADGRL3 but not for its isoform, ADGRL1. The LK30/ADGRL3 complex structure revealed that the LK30 binding site on ADGRL3 overlaps with the binding site for an ADGRL3 ligand - teneurin. In cellular-adhesion assays, LK30 specifically broke the trans-cellular interaction of ADGRL3 with teneurin, but not with another ADGRL3 ligand - FLRT3. Our work provides proof of concept for the modulation of isoform- and ligand-specific aGPCR functions using unique tools, and thus establishes a foundation for the development of fine-tuned aGPCR-targeted therapeutics.

GPR56 C-terminal fragment mediates signal received by N-terminal fragment of another adhesion GPCR Latrophilin1 in neurons

Adhesion GPCRs (aGPCRs) are a subfamily of GPCRs that are involved in cell adhesion, cell proliferation, and cell migration in various tissues. G protein-coupled receptor proteolytic site (GPS) of aGPCR is required to cleave the extracellular domain autocatalytically, generating two fragments; a N-terminal fragment (NTF) and a C-terminal fragment (CTF) containing seven transmembrane structure. NTF can interact with CTF non-covalently after cleavage, however the physiological significance of the cleavage of aGPCR at GPS, and also the interaction between NTF and CTF have not been fully clarified yet. In this study, we first investigated the expression profiles of two aGPCRs, GPR56/ADGRG1, and LPHN1/ADGRL1 in mouse brain, and found that the NTF and CTF of GPR56 independently expressed in different brain region at different developmental stages. Immunoprecipitation of GPR56CTF co-immunoprecipitated LPHN1NTF from mouse brain and HEK293T cells expressing both fragments. Stimulation with LPHN1 ligand, α-Latrotoxin N4C (αLTXN4C), to cells expressing LPHN1NTF and GPR56CTF increased intracellular Ca²⁺ concentration ([Ca²⁺ ]i). We also demonstrated that GPR56KO mouse neurons attenuated their Ca²⁺ response to αLTXN4C. These results suggest the possibility of functional and chimeric complex containing LPHN1NTF and GPR56CTF in neuronal signal transduction.

Piconewton forces mediate GAIN domain dissociation of the latrophilin-3 adhesion GPCR

Latrophilins are adhesion G-protein coupled receptors (aGPCRs) that control excitatory synapse formation. aGPCRs, including latrophilins, are autoproteolytically cleaved at their GPCR-Autoproteolysis Inducing (GAIN) domain, but the two resulting fragments remain associated on the cell surface. It is thought that force-mediated dissociation of the fragments exposes a peptide that activates G-protein signaling of aGPCRs, but whether GAIN domain dissociation can occur on biologically relevant timescales and at physiological forces is unknown. Here, we show using magnetic tweezers that physiological forces dramatically accelerate the dissociation of the latrophilin-3 GAIN domain. Forces in the 1-10 pN range were sufficient to dissociate the GAIN domain on a seconds-to-minutes timescale, and the GAIN domain fragments reversibly reassociated after dissociation. Thus, mechanical force may be a key driver of latrophilin signaling during synapse formation, suggesting a physiological mechanism by which aGPCRs may mediate mechanically-induced signal transduction.

Disentangling autoproteolytic cleavage from tethered agonist-dependent activation of the adhesion receptor ADGRL3

Adhesion G protein-coupled receptor latrophilin 3 (ADGRL3), a cell adhesion molecule highly expressed in the central nervous system, acts in synapse formation through trans interactions with its ligands. It is largely unknown if these interactions serve a purely adhesive function or can modulate G protein signaling. To assess how different structural elements of ADGRL3 (e.g., the adhesive domains, autoproteolytic cleavage site, or tethered agonist (TA)) impact receptor function, we require constructs that disrupt specific receptor features without impacting others. While we showed previously that mutating conserved Phe and Met residues in the TA of ADGRL3-C-terminal fragment (CTF), a CTF truncated to the G protein-coupled receptor proteolysis site, abolishes receptor-mediated G protein activation, we now find that autoproteolytic cleavage is disrupted in the full-length version of this construct. To identify a construct that disrupts TA-dependent activity without impacting proteolysis, we explored other mutations in the TA. We found that mutating the sixth and seventh residues of the TA, Leu and Met, to Ala impaired activity in a serum response element activity assay for both full-length and CTF constructs. We confirmed this activity loss results from impaired G protein coupling using an assay that acutely exposes the TA through controlled proteolysis. The ADGRL3 mutant expresses normally at the cell surface, and immunoblotting shows that it undergoes normal autoproteolysis. Thus, we found a construct that disrupts tethered agonism while retaining autoproteolytic cleavage, providing a tool to disentangle these functions in vivo. Our approach and specific findings are likely to be broadly applicable to other adhesion receptors.

A guide to adhesion GPCR research

Adhesion G protein-coupled receptors (aGPCRs) are a class of structurally and functionally highly intriguing cell surface receptors with essential functions in health and disease. Thus, they display a vastly unexploited pharmacological potential. Our current understanding of the physiological functions and signaling mechanisms of aGPCRs form the basis for elucidating further molecular aspects. Combining these with novel tools and methodologies from different fields tailored for studying these unusual receptors yields a powerful potential for pushing aGPCR research from singular approaches toward building up an in-depth knowledge that will facilitate its translation to applied science. In this review, we summarize the state-of-the-art knowledge on aGPCRs in respect to structure-function relations, physiology, and clinical aspects, as well as the latest advances in the field. We highlight the upcoming most pressing topics in aGPCR research and identify strategies to tackle them. Furthermore, we discuss approaches how to promote, stimulate, and translate research on aGPCRs 'from bench to bedside' in the future.

ADGRL3 genomic variation implicated in neurogenesis and ADHD links functional effects to the incretin polypeptide GIP

Attention deficit/hyperactivity disorder (ADHD) is the most common childhood neurodevelopmental disorder. Single nucleotide polymorphisms (SNPs) in the Adhesion G Protein-Coupled Receptor L3 (ADGRL3) gene are associated with increased susceptibility to developing ADHD worldwide. However, the effect of ADGRL3 non-synonymous SNPs (nsSNPs) on the ADGRL3 protein function is vastly unknown. Using several bioinformatics tools to evaluate the impact of mutations, we found that nsSNPs rs35106420, rs61747658, and rs734644, previously reported to be associated and in linkage with ADHD in disparate populations from the world over, are predicted as pathogenic variants. Docking analysis of rs35106420, harbored in the ADGLR3-hormone receptor domain (HRM, a common extracellular domain of the secretin-like GPCRs family), showed that HRM interacts with the Glucose-dependent insulinotropic polypeptide (GIP), part of the incretin hormones family. GIP has been linked to the pathogenesis of diabetes mellitus, and our analyses suggest a potential link to ADHD. Overall, the comprehensive application of bioinformatics tools showed that functional mutations in the ADGLR3 gene disrupt the standard and wild ADGRL3 structure, most likely affecting its metabolic regulation. Further in vitro experiments are granted to evaluate these in silico predictions of the ADGRL3-GIP interaction and dissect the complexity underlying the development of ADHD.

ADGRL1 haploinsufficiency causes a variable spectrum of neurodevelopmental disorders in humans and alters synaptic activity and behavior in a mouse model

ADGRL1 (latrophilin 1), a well-characterized adhesion G protein-coupled receptor, has been implicated in synaptic development, maturation, and activity. However, the role of ADGRL1 in human disease has been elusive. Here, we describe ten individuals with variable neurodevelopmental features including developmental delay, intellectual disability, attention deficit hyperactivity and autism spectrum disorders, and epilepsy, all heterozygous for variants in ADGRL1. In vitro, human ADGRL1 variants expressed in neuroblastoma cells showed faulty ligand-induced regulation of intracellular Ca²⁺ influx, consistent with haploinsufficiency. In vivo, Adgrl1 was knocked out in mice and studied on two genetic backgrounds. On a non-permissive background, mice carrying a heterozygous Adgrl1 null allele exhibited neurological and developmental abnormalities, while homozygous mice were non-viable. On a permissive background, knockout animals were also born at sub-Mendelian ratios, but many Adgrl1 null mice survived gestation and reached adulthood. Adgrl1^-/- mice demonstrated stereotypic behaviors, sexual dysfunction, bimodal extremes of locomotion, augmented startle reflex, and attenuated pre-pulse inhibition, which responded to risperidone. Ex vivo synaptic preparations displayed increased spontaneous exocytosis of dopamine, acetylcholine, and glutamate, but Adgrl1^-/- neurons formed synapses in vitro poorly. Overall, our findings demonstrate that ADGRL1 haploinsufficiency leads to consistent developmental, neurological, and behavioral abnormalities in mice and humans.

Thwarting of Lphn3 Functions in Cell Motility and Signaling by Cancer-Related GAIN Domain Somatic Mutations

Cancer progression relies on cellular transition states accompanied by changes in the functionality of adhesion molecules. The gene for adhesion G protein-coupled receptor latrophilin-3 (aGPCR Lphn3 or ADGRL3) is targeted by tumor-specific somatic mutations predominantly affecting the conserved GAIN domain where most aGPCRs are cleaved. However, it is unclear how these GAIN domain-altering mutations impact Lphn3 function. Here, we studied Lphn3 cancer-related mutations as a proxy for revealing unknown GAIN domain functions. We found that while intra-GAIN cleavage efficiency was unaltered, most mutations produced a ligand-specific impairment of Lphn3 intercellular adhesion profile paralleled by an increase in cell-matrix actin-dependent contact structures for cells expressing the select S810L mutation. Aberrant remodeling of the intermediate filament vimentin, which was found to coincide with Lphn3-induced modification of nuclear morphology, had less impact on the nuclei of S810L expressing cells. Notoriously, receptor signaling through G13 protein was deficient for all variants bearing non-homologous amino acid substitutions, including the S810L variant. Analysis of cell migration paradigms revealed a non-cell-autonomous impairment in collective cell migration indistinctly of Lphn3 or its cancer-related variants expression, while cell-autonomous motility was potentiated in the presence of Lphn3, but this effect was abolished in S810L GAIN mutant-expressing cells. These data identify the GAIN domain as an important regulator of Lphn3-dependent cell motility, thus furthering our understanding of cellular and molecular events linking Lphn3 genetic somatic mutations to cancer-relevant pathogenesis mechanisms.

Latrophilin-2 is a novel receptor of LRG1 that rescues vascular and neurological abnormalities and restores diabetic erectile function

Diabetes mellitus (DM) is a chronic metabolic disorder characterized by inappropriate hyperglycemia, which causes endothelial dysfunction and peripheral neuropathy, ultimately leading to multiple complications. One prevalent complication is diabetic erectile dysfunction (ED), which is more severe and more resistant to treatment than nondiabetic ED. The serum glycoprotein leucine-rich ɑ-2-glycoprotein 1 (LRG1) is a modulator of TGF-β-mediated angiogenesis and has been proposed as a biomarker for a variety of diseases, including DM. Here, we found that the adhesion GPCR latrophilin-2 (LPHN2) is a TGF-β-independent receptor of LRG1. By interacting with LPHN2, LRG1 promotes both angiogenic and neurotrophic processes in mouse tissue explants under hyperglycemic conditions. Preclinical studies in a diabetic ED mouse model showed that LRG1 administration into the penile tissue, which exhibits significantly increased LPHN2 expression, fully restores erectile function by rescuing vascular and neurological abnormalities. Further investigations revealed that PI3K, AKT, and NF-κB p65 constitute the key intracellular signaling pathway of the LRG1/LPHN2 axis, providing important mechanistic insights into LRG1-mediated angiogenesis and nerve regeneration in DM. Our findings suggest that LRG1 can be a potential new therapeutic option for treating aberrant peripheral blood vessels and neuropathy associated with diabetic complications, such as diabetic ED.

Ligands and Beyond: Mechanosensitive Adhesion GPCRs

Cells respond to diverse types of mechanical stimuli using a wide range of plasma membrane-associated mechanosensitive receptors to convert extracellular mechanical cues into intracellular signaling. G protein-coupled receptors (GPCRs) represent the largest cell surface protein superfamily that function as versatile sensors for a broad spectrum of bio/chemical messages. In recent years, accumulating evidence has shown that GPCRs can also engage in mechano-transduction. According to the GRAFS classification system of GPCRs, adhesion GPCRs (aGPCRs) constitute the second largest GPCR subfamily with a unique modular protein architecture and post-translational modification that are well adapted for mechanosensory functions. Here, we present a critical review of current evidence on mechanosensitive aGPCRs.

Modeling dopamine dysfunction in autism spectrum disorder: From invertebrates to vertebrates

Autism Spectrum Disorder (ASD) is a highly heterogeneous neurodevelopmental disorder characterized by deficits in social communication and by patterns of restricted interests and/or repetitive behaviors. The Simons Foundation Autism Research Initiative's Human Gene and CNV Modules now list over 1000 genes implicated in ASD and over 2000 copy number variant loci reported in individuals with ASD. Given this ever-growing list of genetic changes associated with ASD, it has become evident that there is likely not a single genetic cause of this disorder nor a single neurobiological basis of this disorder. Instead, it is likely that many different neurobiological perturbations (which may represent subtypes of ASD) can result in the set of behavioral symptoms that we called ASD. One such of possible subtype of ASD may be associated with dopamine dysfunction. Precise regulation of synaptic dopamine (DA) is required for reward processing and behavioral learning, behaviors which are disrupted in ASD. Here we review evidence for DA dysfunction in ASD and in animal models of ASD. Further, we propose that these studies provide a scaffold for scientists and clinicians to consider subcategorizing the ASD diagnosis based on the genetic changes, neurobiological difference, and behavioral features identified in individuals with ASD.

Machine Learning Prediction of ADHD Severity: Association and Linkage to ADGRL3, DRD4, and SNAP25

OBJECTIVE

To investigate whether single nucleotide polymorphisms (SNPs) in the ADGRL3, DRD4, and SNAP25 genes are associated with and predict ADHD severity in families from a Caribbean community.

METHOD

ADHD severity was derived using latent class cluster analysis of DSM-IV symptomatology. Family-based association tests were conducted to detect associations between SNPs and ADHD severity latent phenotypes. Machine learning algorithms were used to build predictive models of ADHD severity based on demographic and genetic data.

RESULTS

Individuals with ADHD exhibited two seemingly independent latent class severity configurations. SNPs harbored in DRD4, SNAP25, and ADGRL3 showed evidence of linkage and association to symptoms severity and a potential pleiotropic effect on distinct domains of ADHD severity. Predictive models discriminate severe from non-severe ADHD in specific symptom domains.

CONCLUSION

This study supports the role of DRD4, SNAP25, and ADGRL3 genes in outlining ADHD severity, and a new prediction framework with potential clinical use.

NanoBRET in C. elegans illuminates functional receptor interactions in real time

Background

Protein-protein interactions form the basis of every organism and thus, investigating their dynamics, intracellular protein localization, trafficking and interactions of distinct proteins such as receptors and their ligand-binding are of general interest. Bioluminescence resonance energy transfer (BRET) is a powerful tool to investigate these aspects in vitro. Since in vitro approaches mostly neglect the more complex in vivo situation, we established BRET as an in vivo tool for studying protein interactions in the nematode C. elegans.

Results

We generated worms expressing NanoBRET sensors and elucidated the interaction of two ligand-G protein-coupled receptor (GPCR) pairs, the neuropeptide receptor NPR-11 and the Adhesion GPCR LAT-1. Furthermore, we adapted the enhanced bystander BRET technology to measure subcellular protein localization. Using this approach, we traced ligand-induced internalization of NPR-11 in vivo.

Conclusions

Our results indicate that in vivo NanoBRET is a tool to investigate specific protein interactions and localization in a physiological setting in real time in the living organism C. elegans.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12860-022-00405-w.

GPCRs Are Optimal Regulators of Complex Biological Systems and Orchestrate the Interface between Health and Disease

GPCRs arguably represent the most effective current therapeutic targets for a plethora of diseases. GPCRs also possess a pivotal role in the regulation of the physiological balance between healthy and pathological conditions; thus, their importance in systems biology cannot be underestimated. The molecular diversity of GPCR signaling systems is likely to be closely associated with disease-associated changes in organismal tissue complexity and compartmentalization, thus enabling a nuanced GPCR-based capacity to interdict multiple disease pathomechanisms at a systemic level. GPCRs have been long considered as controllers of communication between tissues and cells. This communication involves the ligand-mediated control of cell surface receptors that then direct their stimuli to impact cell physiology. Given the tremendous success of GPCRs as therapeutic targets, considerable focus has been placed on the ability of these therapeutics to modulate diseases by acting at cell surface receptors. In the past decade, however, attention has focused upon how stable multiprotein GPCR superstructures, termed receptorsomes, both at the cell surface membrane and in the intracellular domain dictate and condition long-term GPCR activities associated with the regulation of protein expression patterns, cellular stress responses and DNA integrity management. The ability of these receptorsomes (often in the absence of typical cell surface ligands) to control complex cellular activities implicates them as key controllers of the functional balance between health and disease. A greater understanding of this function of GPCRs is likely to significantly augment our ability to further employ these proteins in a multitude of diseases.

Adhesion GPCR Latrophilin 3 regulates synaptic function of cone photoreceptors in a trans-synaptic manner

Cone photoreceptors mediate daylight vision in vertebrates. Changes in neurotransmitter release at cone synapses encode visual information and is subject to precise control by negative feedback from enigmatic horizontal cells. However, the mechanisms that orchestrate this modulation are poorly understood due to a virtually unknown landscape of molecular players. Here, we report a molecular player operating selectively at cone synapses that modulates effects of horizontal cells on synaptic release. Using an unbiased proteomic screen, we identified an adhesion GPCR Latrophilin3 (LPHN3) in horizontal cell dendrites that engages in transsynaptic control of cones. We detected and characterized a prominent splice isoform of LPHN3 that excludes a element with inhibitory influence on transsynaptic interactions. A gain-of-function mouse model specifically routing LPHN3 splicing to this isoform but not knockout of LPHN3 diminished Ca_V1.4 calcium channel activity profoundly disrupted synaptic release by cones and resulted in synaptic transmission deficits. These findings offer molecular insight into horizontal cell modulation on cone synaptic function and more broadly demonstrate the importance of alternative splicing in adhesion GPCRs for their physiological function.

An assessment of executive function in two different rat models of attention-deficit hyperactivity disorder: Spontaneously hypertensive versus Lphn3 knockout rats

Attention-deficit/hyperactivity disorder (ADHD) a common neurodevelopmental disorder of childhood and often comorbid with other externalizing disorders (EDs). There is evidence that externalizing behaviors share a common genetic etiology. Recently, a genome-wide, multigenerational sample linked variants in the Lphn3 gene to ADHD and other externalizing behaviors. Likewise, limited research in animal models has provided converging evidence that Lphn3 plays a role in EDs. This study examined the impact of Lphn3 deletion (i.e., Lphn3-/- ) in rats on measures of behavioral control associated with externalizing behavior. Impulsivity was assessed for 30 days via a differential reinforcement of low rates (DRL) task and working memory evaluated for 25 days using a delayed spatial alternation (DSA) task. Data from both tasks were averaged into 5-day testing blocks. We analyzed overall performance, as well as response patterns in just the first and last blocks to assess acquisition and steady-state performance, respectively. "Positive control" measures on the same tasks were measured in an accepted animal model of ADHD-the spontaneously hypertensive rat (SHR). Compared with wildtype controls, Lphn3-/- rats exhibited deficits on both the DRL and DSA tasks, indicative of deficits in impulsive action and working memory, respectively. These deficits were less severe than those in the SHRs, who were profoundly impaired on both tasks compared with their control strain, Wistar-Kyoto rats. The results provide evidence supporting a role for Lphn3 in modulating inhibitory control and working memory, and suggest additional research evaluating the role of Lphn3 in the manifestation of EDs more broadly is warranted.

Developmental Perspectives on Arterial Fate Specification

Blood vessel acquisition of arterial or venous fate is an adaptive phenomenon in response to increasing blood circulation during vascular morphogenesis. The past two decades of effort in this field led to development of a widely accepted paradigm of molecular regulators centering on VEGF and Notch signaling. More recent findings focused on shear stress-induced cell cycle arrest as a prerequisite for arterial specification substantially modify this traditional understanding. This review aims to summarize key molecular mechanisms that work in concert to drive the acquisition of arterial fate in two distinct developmental settings of vascular morphogenesis: de novo vasculogenesis of the dorsal aorta and postnatal retinal angiogenesis. We will also discuss the questions and conceptual controversies that potentially point to novel directions of investigation and possible clinical relevance.

Latrophilin-3 disruption: Effects on brain and behavior

Latrophilin-3 (LPHN3), a G-protein-coupled receptor belonging to the adhesion subfamily, is a regulator of synaptic function and maintenance in brain regions that mediate locomotor activity, attention, and memory for location and path. Variants of LPHN3 are associated with increased risk for attention deficit hyperactivity disorder (ADHD) in some patients. Here we review the role of LPHN3 in the central nervous system (CNS). We describe synaptic localization of LPHN3, its trans-synaptic binding partners, links to neurodevelopmental disorders, animal models of Lphn3 disruption in different species, and evidence that LPHN3 is involved in cognition as well as activity and attention. The evidence shows that LPHN3 plays a more significant role in neuroplasticity than previously appreciated.

Cell Adhesion Molecules Involved in Neurodevelopmental Pathways Implicated in 3p-Deletion Syndrome and Autism Spectrum Disorder

Autism spectrum disorder (ASD) is characterized by impaired social interaction, language delay and repetitive or restrictive behaviors. With increasing prevalence, ASD is currently estimated to affect 0.5–2.0% of the global population. However, its etiology remains unclear due to high genetic and phenotypic heterogeneity. Copy number variations (CNVs) are implicated in several forms of syndromic ASD and have been demonstrated to contribute toward ASD development by altering gene dosage and expression. Increasing evidence points toward the p-arm of chromosome 3 (chromosome 3p) as an ASD risk locus. Deletions occurring at chromosome 3p result in 3p-deletion syndrome (Del3p), a rare genetic disorder characterized by developmental delay, intellectual disability, facial dysmorphisms and often, ASD or ASD-associated behaviors. Therefore, we hypothesize that overlapping molecular mechanisms underlie the pathogenesis of Del3p and ASD. To investigate which genes encoded in chromosome 3p could contribute toward Del3p and ASD, we performed a comprehensive literature review and collated reports investigating the phenotypes of individuals with chromosome 3p CNVs. We observe that high frequencies of CNVs occur in the 3p26.3 region, the terminal cytoband of chromosome 3p. This suggests that CNVs disrupting genes encoded within the 3p26.3 region are likely to contribute toward the neurodevelopmental phenotypes observed in individuals affected by Del3p. The 3p26.3 region contains three consecutive genes encoding closely related neuronal immunoglobulin cell adhesion molecules (IgCAMs): Close Homolog of L1 (CHL1), Contactin-6 (CNTN6), and Contactin-4 (CNTN4). CNVs disrupting these neuronal IgCAMs may contribute toward ASD phenotypes as they have been associated with key roles in neurodevelopment. CHL1, CNTN6, and CNTN4 have been observed to promote neurogenesis and neuronal survival, and regulate neuritogenesis and synaptic function. Furthermore, there is evidence that these neuronal IgCAMs possess overlapping interactomes and participate in common signaling pathways regulating axon guidance. Notably, mouse models deficient for these neuronal IgCAMs do not display strong deficits in axonal migration or behavioral phenotypes, which is in contrast to the pronounced defects in neuritogenesis and axon guidance observed in vitro. This suggests that when CHL1, CNTN6, or CNTN4 function is disrupted by CNVs, other neuronal IgCAMs may suppress behavioral phenotypes by compensating for the loss of function.

Proper synaptic adhesion signaling in the control of neural circuit architecture and brain function

Trans-synaptic cell-adhesion molecules are critical for governing various stages of synapse development and specifying neural circuit properties via the formation of multifarious signaling pathways. Recent studies have pinpointed the putative roles of trans-synaptic cell-adhesion molecules in mediating various cognitive functions. Here, we review the literature on the roles of a diverse group of central synaptic organizers, including neurexins (Nrxns), leukocyte common antigen-related receptor protein tyrosine phosphatases (LAR-RPTPs), and their associated binding proteins, in regulating properties of specific type of synapses and neural circuits. In addition, we highlight the findings that aberrant synaptic adhesion signaling leads to alterations in the structures, transmission, and plasticity of specific synapses across diverse brain areas. These results seem to suggest that proper trans-synaptic signaling pathways by Nrxns, LAR-RPTPs, and their interacting network is likely to constitute central molecular complexes that form the basis for cognitive functions, and that these complexes are heterogeneously and complexly disrupted in many neuropsychiatric and neurodevelopmental disorders.

Recent advances in inter-cellular interactions during neural circuit assembly

Neural circuit assembly is regulated by inter-cellular interactions involving secreted and surface-tethered guidance molecules. Here, we review recent progress in our understanding of their mechanisms-of-action and biological effects. We discuss mechanisms through which the secreted cue Netrin-1 regulates neuron migration and highlight novel roles for axon-derived secreted guidance cues. We cover recent structural work at atomic resolution that provides new insights into the activation mechanisms of axon guidance receptors and into protein complexes containing cell adhesion molecules, such as Teneurin (Ten), Latrophilin (Lphn) and FLRT. Ten-Ten homophilic, Ten-Lphn heterophilic, and Ten-Lphn-FLRT tripartite complexes seem to elicit distinct context-dependent cellular responses. Seemingly opposite responses can also be triggered by the Eph/ephrin signaling system. Here, recent work provides a simple mechanism for the decision of forming a new synapse versus rejection of the pre-synaptic partner. These studies identify novel regulatory mechanisms and biological functions that will apply generally in developing neural systems.

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.