alpha-Latrotoxin receptor CIRL/latrophilin 1 (CL1) defines an unusual family of ubiquitous G-protein-linked receptors. G-protein coupling not required for triggering exocytosis

Signaling by latrophilin adhesion-GPCRs in synapse assembly

Latrophilins are evolutionarily conserved adhesion-GPCRs with diverse roles, including a prominent function in synapse organization. In mammals, the primary transcripts of three latrophilin genes (ADGRL1-3) are extensively alternatively spliced, producing hundreds of isoforms with diverse cytoplasmic sequences. Extracellularly, latrophilins feature N-terminal lectin- and olfactomedin-like domains that bind to Teneurin and FLRT adhesion molecules, respectively, and are followed by an autoproteolytic GAIN domain typical for adhesion-GPCRs. Since Teneurins and FLRTs in turn interact with other ligands, latrophilins form a large trans-cellular protein interaction network. Intracellularly, latrophilins bind to G proteins, arrestins, and postsynaptic scaffold proteins. Latrophilins stimulate all Gα proteins tested, with the Gα isoform preference regulated by alternative splicing. In brain, latrophilins act as essential postsynaptic organizers that functionally require extracellular binding to teneurins and FLRTs, intracellular activation of GαS, and recruitment of postsynaptic scaffolds. Thus, latrophilins are signaling platforms that connect trans-cellular interactions to cellular responses in a manner regulated by alternative splicing.

Latrophilin-3 as a downstream effector of the androgen receptor induces urothelial tumorigenesis

Emerging evidence indicates that androgen receptor (AR) signaling plays a critical role in the pathogenesis of male-dominant urothelial cancer. Meanwhile, latrophilins (LPHNs), a group of the G-protein-coupled receptor to which a spider venom latrotoxin is known to bind, remain largely uncharacterized in neoplastic diseases. The present study aimed to determine the functional role of LPHN3 (encoded by the ADGRL3 gene), in association with AR signaling, in urothelial tumorigenesis. In human normal urothelial SVHUC cells, AR overexpression and androgen treatment considerably increased the expression levels of ADGRL3/LPHN3, while chromatin immunoprecipitation assay revealed the binding of AR to the promoter region of ADGRL3. In SVHUC or SVHUC-AR cells with exposure to a chemical carcinogen 3-methylcholanthrene, LPHN3 activation via ligand (e.g., α-latrotoxin, FLRT3) treatment during the process of the neoplastic/malignant transformation or LPHN3 knockdown via shRNA virus infection induced or reduced, respectively, the oncogenic activity. In N-butyl-N-(4-hydroxybutyl)nitrosamine-treated female mice, α-latrotoxin or FLRT3 injection accelerated the development of bladder tumors. Immunohistochemistry in surgical specimens further showed the significantly elevated expression of LPHN3 in non-muscle-invasive bladder tumors, compared with adjacent normal urothelial tissues, which was associated with a marginally (p = 0.051) higher risk of disease recurrence after transurethral resection. In addition, positivity of LPHN3 and AR in these tumors was strongly correlated. These findings indicate that LPHN3 functions as a downstream effector of AR and promotes urothelial tumorigenesis.

Latrophilin-3 as a downstream effector of the androgen receptor induces bladder cancer progression

Emerging evidence indicates that androgen receptor (AR) signaling plays a critical role in the pathogenesis of male-dominant urothelial cancer and its outgrowth. Meanwhile, latrophilins (LPHNs), a group of the G-protein-coupled receptors to which a spider venom latrotoxin (LTX) is known to bind, remain largely uncharacterized in neoplastic diseases. The present study aimed to determine the functional role of LPHN3 (encoded by the ADGRL3 gene), in association with AR signaling, in the progression of bladder cancer. In AR-positive bladder cancer lines, dihydrotestosterone considerably increased the expression levels of ADGRL3 and LPHN3, while chromatin immunoprecipitation assay revealed the binding of AR to the promoter region of ADGRL3. Treatment with LPHN3 ligands (e.g. α-LTX, FLRT3) resulted in the induction of ADGRL3 expression, as well as cell viability, in bladder cancer lines. By contrast, LPHN3 knockdown via shRNA virus infection significantly reduced the viability and migration of these cells. Immunohistochemistry in transurethral resection specimens further showed a strong correlation between LPHN3 and AR expression. Moreover, LPHN3 positivity in muscle-invasive bladder tumors, as an independent prognosticator, was associated with a significantly higher risk of disease progression and disease-specific mortality following radical cystectomy. These findings suggest that LPHN3 functions as a downstream effector of AR and promotes the growth of bladder cancer.

Latrophilins as Downstream Effectors of Androgen Receptors including a Splice Variant, AR-V7, Induce Prostate Cancer Progression

Latrophilins (LPHNs), a group of the G-protein-coupled receptor to which a spider venom latrotoxin (LTX) is known to bind, remain largely uncharacterized in neoplastic diseases. In the present study, we aimed to determine the role of LPHNs in the progression of prostate cancer. We assessed the actions of LPHNs, including LPHN1, LPHN2, and LPHN3, in human prostate cancer lines via their ligand (e.g., α-LTX, FLRT3) treatment or shRNA infection, as well as in surgical specimens. In androgen receptor (AR)-positive LNCaP/C4-2/22Rv1 cells, dihydrotestosterone considerably increased the expression levels of LPHNs, while chromatin immunoprecipitation assay revealed the binding of endogenous ARs, including AR-V7, to the promoter region of each LPHN. Treatment with α-LTX or FLRT3 resulted in induction in the cell viability and migration of both AR-positive and AR-negative lines. α-LTX and FLRT3 also enhanced the expression of Bcl-2 and phosphorylated forms of JAK2 and STAT3. Meanwhile, the knockdown of each LPHN showed opposite effects on all of those mediated by ligand treatment. Immunohistochemistry in radical prostatectomy specimens further showed the significantly elevated expression of each LPHN in prostate cancer, compared with adjacent normal-appearing prostate, which was associated with a significantly higher risk of postoperative biochemical recurrence in both univariate and multivariable settings. These findings indicate that LPHNs function as downstream effectors of ARs and promote the growth of androgen-sensitive, castration-resistant, or even AR-negative prostate cancer.

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.

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.

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.

Latrophilin-3 heterozygous versus homozygous mutations in Sprague Dawley rats: Effects on egocentric and allocentric memory and locomotor activity

Latrophilin-3 (LPHN3) is a brain specific G-protein coupled receptor associated with increased risk of attention deficit hyperactivity disorder (ADHD) and cognitive deficits. CRISPR/Cas9 was used to generate a constitutive knockout (KO) rat of Lphn3 by deleting exon 3, based on human data that LPHN3 variants are associated with some cases of ADHD. Lphn3 KO rats are hyperactive with an attenuated response to ADHD medication and have cognitive deficits. Here, we tested KO, heterozygous (HET), and wildtype (WT) rats to determine if there was a gene-dosage effect. We tested the rats in home-cage activity starting at postnatal day (P)35 and P50, followed by tests of egocentric learning (Cincinnati water maze [CWM]), spatial learning (Morris water maze [MWM]), working memory (radial water maze [RWM]), incidental learning (novel object recognition [NOR]), acoustic startle response (ASR) habituation, tactile startle response (TSR) habituation, prepulse modification of acoustic startle, shuttle-box passive avoidance, conditioned freezing, and a mirror image version of the CWM. KO and HET rats were hyperactive. KO and HET rats had egocentric (CWM) and spatial deficits (MWM), increased startle response, and KO rats showed less conditioned freezing on contextual and cued memory; there were no effects on working memory (RWM) or passive avoidance. The selective gene-dosage effect in Lphn3 HET rats indicates that Lphn3 exhibits dominate expression on functions where it is most abundantly expressed (striatum, hippocampus) but not on behaviors mediated by regions of low expression. The data add further evidence to the impact of this synaptic protein on brain function and behavior.

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.

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.

Parahippocampal latrophilin-2 (ADGRL2) expression controls topographical presubiculum to entorhinal cortex circuit connectivity

Brain circuits are comprised of distinct interconnected neurons that are assembled by synaptic recognition molecules presented by defined pre- and post-synaptic neurons. This cell-cell recognition process is mediated by varying cellular adhesion molecules, including the latrophilin family of adhesion G-protein-coupled receptors. Focusing on parahippocampal circuitry, we find that latrophilin-2 (Lphn2; gene symbol ADGRL2) is specifically enriched in interconnected subregions of the medial entorhinal cortex (MEC), presubiculum (PrS), and parasubiculum (PaS). Retrograde viral tracing from the Lphn2-enriched region of the MEC reveals unique topographical patterning of inputs arising from the PrS and PaS that mirrors Lphn2 expression. Using a Lphn2 conditional knockout mouse model, we find that deletion of MEC Lphn2 expression selectively impairs retrograde viral labeling of inputs arising from the ipsilateral PrS. Combined with analysis of Lphn2 expression within the MEC, this study reveals Lphn2 to be selectively expressed by defined cell types and essential for MEC-PrS circuit connectivity.

The relevance of adhesion G protein-coupled receptors in metabolic functions

G protein-coupled receptors (GPCRs) modulate a variety of physiological functions and have been proven to be outstanding drug targets. However, approximately one-third of all non-olfactory GPCRs are still orphans in respect to their signal transduction and physiological functions. Receptors of the class of Adhesion GPCRs (aGPCRs) are among these orphan receptors. They are characterized by unique features in their structure and tissue-specific expression, which yields them interesting candidates for deorphanization and testing as potential therapeutic targets. Capable of G-protein coupling and non-G protein-mediated function, aGPCRs may extend our repertoire of influencing physiological function. Besides their described significance in the immune and central nervous systems, growing evidence indicates a high importance of these receptors in metabolic tissue. RNAseq analyses revealed high expression of several aGPCRs in pancreatic islets, adipose tissue, liver, and intestine but also in neurons governing food intake. In this review, we focus on aGPCRs and their function in regulating metabolic pathways. Based on current knowledge, this receptor class represents high potential for future pharmacological approaches addressing obesity and other metabolic diseases.

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.

Latrophilin GPCR signaling mediates synapse formation

Neural circuit assembly in the brain requires precise establishment of synaptic connections, but the mechanisms of synapse assembly remain incompletely understood. Latrophilins are postsynaptic adhesion-GPCRs that engage in trans-synaptic complexes with presynaptic teneurins and FLRTs. In mouse CA1-region neurons, Latrophilin-2 and Latrophilin-3 are essential for formation of entorhinal-cortex-derived and Schaffer-collateral-derived synapses, respectively. However, it is unknown whether latrophilins function as GPCRs in synapse formation. Here, we show that Latrophilin-2 and Latrophilin-3 exhibit constitutive GPCR activity that increases cAMP levels, which was blocked by a mutation interfering with G-protein and arrestin interactions of GPCRs. The same mutation impaired the ability of Latrophilin-2 and Latrophilin-3 to rescue the synapse-loss phenotype in Latrophilin-2 and Latrophilin-3 knockout neurons in vivo. Our results suggest that Latrophilin-2 and Latrophilin-3 require GPCR signaling in synapse formation, indicating that latrophilins promote synapse formation in the hippocampus by activating a classical GPCR-signaling pathway.

Disorders of synaptic vesicle fusion machinery

The revolution in genetic technology has ushered in a new age for our understanding of the underlying causes of neurodevelopmental, neuromuscular and neurodegenerative disorders, revealing that the presynaptic machinery governing synaptic vesicle fusion is compromised in many of these neurological disorders. This builds upon decades of research showing that disturbance to neurotransmitter release via toxins can cause acute neurological dysfunction. In this review, we focus on disorders of synaptic vesicle fusion caused either by toxic insult to the presynapse or alterations to genes encoding the key proteins that control and regulate fusion: the SNARE proteins (synaptobrevin, syntaxin-1 and SNAP-25), Munc18, Munc13, synaptotagmin, complexin, CSPα, α-synuclein, PRRT2 and tomosyn. We discuss the roles of these proteins and the cellular and molecular mechanisms underpinning neurological deficits in these disorders.

FLRTing Neurons in Cortical Migration During Cerebral Cortex Development

During development, two coordinated events shape the morphology of the mammalian cerebral cortex, leading to the cortex's columnar and layered structure: the proliferation of neuronal progenitors and cortical migration. Pyramidal neurons originating from germinal zones migrate along radial glial fibers to their final position in the cortical plate by both radial migration and tangential dispersion. These processes rely on the delicate balance of intercellular adhesive and repulsive signaling that takes place between neurons interacting with different substrates and guidance cues. Here, we focus on the function of the cell adhesion molecules fibronectin leucine-rich repeat transmembrane proteins (FLRTs) in regulating both the radial migration of neurons, as well as their tangential spread, and the impact these processes have on cortex morphogenesis. In combining structural and functional analysis, recent studies have begun to reveal how FLRT-mediated responses are precisely tuned - from forming different protein complexes to modulate either cell adhesion or repulsion in neurons. These approaches provide a deeper understanding of the context-dependent interactions of FLRTs with multiple receptors involved in axon guidance and synapse formation that contribute to finely regulated neuronal migration.

Alternative splicing controls teneurin-latrophilin interaction and synapse specificity by a shape-shifting mechanism

The trans-synaptic interaction of the cell-adhesion molecules teneurins (TENs) with latrophilins (LPHNs/ADGRLs) promotes excitatory synapse formation when LPHNs simultaneously interact with FLRTs. Insertion of a short alternatively-spliced region within TENs abolishes the TEN-LPHN interaction and switches TEN function to specify inhibitory synapses. How alternative-splicing regulates TEN-LPHN interaction remains unclear. Here, we report the 2.9 Å resolution cryo-EM structure of the TEN2-LPHN3 complex, and describe the trimeric TEN2-LPHN3-FLRT3 complex. The structure reveals that the N-terminal lectin domain of LPHN3 binds to the TEN2 barrel at a site far away from the alternatively spliced region. Alternative-splicing regulates the TEN2-LPHN3 interaction by hindering access to the LPHN-binding surface rather than altering it. Strikingly, mutagenesis of the LPHN-binding surface of TEN2 abolishes the LPHN3 interaction and impairs excitatory but not inhibitory synapse formation. These results suggest that a multi-level coincident binding mechanism mediated by a cryptic adhesion complex between TENs and LPHNs regulates synapse specificity.

The trans-synaptic interaction of the cell-adhesion molecules teneurins (TENs) with latrophilins (LPHNs) promotes excitatory synapse formation. Here authors report the high resolution cryo-EM structure of the TEN2-LPHN3 complex, describe the trimeric TEN2-LPHN3-FLRT3 complex and show how alternative-splicing regulates the TEN2-LPHN3 interaction.

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.

Latrophilin mediates insecticides susceptibility and fecundity through two carboxylesterases, esterase4 and esterase6, in Tribolium castaneum

Latrophilin (LPH) is known as an adhesion G-protein-coupled receptor which involved in multiple physiological processes in organisms. Previous studies showed that lph not only involved the susceptibility to anticholinesterase insecticides but also affected fecundity in Tribolium castaneum. However, its regulatory mechanisms in these biological processes are still not clear. Here, we identified two potential downstream carboxylesterase (cce) genes of Tclph, esterase4 and esterase6, and further characterized their interactions with Tclph. After treatment of T. castaneum larvae with carbofuran or dichlorvos insecticides, the transcript levels of Tcest4 and Tcest6 were significantly induced from 12 to 72 h. RNAi against Tcest4 or Tcest6 led to the higher mortality compared with the controls after the insecticides treatment, suggesting that these two genes play a vital role in detoxification of insecticides in T. castaneum. Furthermore, with insecticides exposure to Tclph knockdown beetles, the expression of Tcest4 was upregulated but Tcest6 was downregulated, indicating that beetles existed a compensatory response against the insecticides. Additionally, RNAi of Tcest6 resulted in 43% reductions in female egg laying and completely inhibited egg hatching, which showed the similar phenotype as that of Tclph knockdown. These results indicated that Tclph affected fecundity by positively regulating Tcest6 expression. Our findings will provide a new insight into the molecular mechanisms of Tclph involved in physiological functions in T. castaneum.

Revisiting the classification of adhesion GPCRs

G protein–coupled receptors (GPCRs) are encoded by over 800 genes in the human genome. Motivated by different scientific rationales, the two classification systems that are mainly in use, the ABC and GRAFS systems, organize GPCRs according to their pharmacological features and phylogenetic relations, respectively. Within those systems, adhesion GPCRs (aGPCRs) constitute a group of over 30 mammalian homologs, most of which are still orphans with undefined activating signals and signal transduction properties. Previous efforts have further subdivided mammalian aGPCRs into nine subfamilies to indicate phylogenetic relationships. However, this subclassification scheme has shortcomings and inconsistencies that require attention. Here, we have reassessed the phylogenetic relationships of aGPCRs from vertebrate and invertebrate species. Our findings confirm that secretin receptor–like GPCRs most probably emerged from ancestral aGPCRs. We show that reassignment of several aGPCRs to families essentially requires input from functional data. Our analyses establish the need for introducing novel aGPCR subfamilies due to aGPCR sequences from invertebrate species that are not readily assignable to any existing subfamily. We conclude that the current classification systems ought to be updated to consider an unambiguous taxonomy of a hierarchically organized classification and pharmacological properties, and to accommodate phylogenetic affiliations between aGPCR genes within mammals and across the animal kingdom.

Genetic basis of functional variability in adhesion G protein-coupled receptors

The enormous sizes of adhesion G protein-coupled receptors (aGPCRs) go along with complex genomic exon-intron architectures giving rise to multiple mRNA variants. There is a need for a comprehensive catalog of aGPCR variants for proper evaluation of the complex functions of aGPCRs found in structural, in vitro and animal model studies. We used an established bioinformatics pipeline to extract, quantify and visualize mRNA variants of aGPCRs from deeply sequenced transcriptomes. Data analysis showed that aGPCRs have multiple transcription start sites even within introns and that tissue-specific splicing is frequent. On average, 19 significantly expressed transcript variants are derived from a given aGPCR gene. The domain architecture of the N terminus encoded by transcript variants often differs and N termini without or with an incomplete seven-helix transmembrane anchor as well as separate seven-helix transmembrane domains are frequently derived from aGPCR genes. Experimental analyses of selected aGPCR transcript variants revealed marked functional differences. Our analysis has an impact on a rational design of aGPCR constructs for structural analyses and gene-deficient mouse lines and provides new support for independent functions of both, the large N terminus and the transmembrane domain of aGPCRs.

Alternative splicing event modifying ADGRL1/latrophilin-1 cytoplasmic tail promotes both opposing and dual cAMP signaling pathways

The adhesion G protein-coupled receptor ADGRL1/latrophilin-1 (LPHN1) stabilizes synapse formation through heterophilic interactions. A growing consensus is pointing to the role of LPHN1 in modulating intracellular levels of cAMP, although conflicting data exist. Variants of LPHN1 resulting from alternative splicing differ at multiple sites, two of which, designated as SSA and SSB, modify extracellular and intracellular receptor regions, respectively. While SSA splicing modulates receptor-ligand affinity, the function of SSB splicing remains elusive. Here, we explored the role of SSB in an attempt to unify current findings on LPHN1 signaling pathways by testing SSB-containing and SSB-deficient receptor variants in signaling paradigms involving interaction with their ligands neurexin and FLRT. cAMP competitive binding assays revealed that cells expressing either receptor variant exhibited a ligand-dependent decrease in the forskolin-induced cAMP accumulation. Surprisingly, the expression of SSB-containing LPHN1 promoted both constitutive and ligand-dependent cAMP production, whereas SSB-deficient LPHN1 did not. Pertussis toxin treatment unveiled a constitutive coupling to G_αi/o for SSB-containing LPHN1 while abrogating the ligand-mediated activation of G_αs . Importantly, neither receptor variant increased the intracellular concentration of Ca²⁺ nor MAP kinase activation in the presence of ligands. These results suggest that SSB splicing selectively affects the duality of LPHN1 signaling toward opposing cAMP pathways.

Latrophilin's Social Protein Network

Latrophilins (LPHNs) are adhesion GPCRs that are originally discovered as spider's toxin receptors, but are now known to be involved in brain development and linked to several neuronal and non-neuronal disorders. Latrophilins act in conjunction with other cell adhesion molecules and may play a leading role in its network organization. Here, we focus on the main protein partners of latrophilins, namely teneurins, FLRTs and contactins and summarize their respective temporal and spatial expression patterns, links to neurodevelopmental disorders as well as their structural characteristics. We discuss how more recent insights into the separate cell biological functions of these proteins shed light on the central role of latrophilins in this network. We postulate that latrophilins control the refinement of synaptic properties of specific subtypes of neurons, requiring discrete combinations of proteins.

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

The adhesion G protein-coupled receptors latrophilins have been in the limelight for more than 20 years since their discovery as calcium-independent receptors for α-latrotoxin, a spider venom toxin with potent activity directed at neurotransmitter release from a variety of synapse types. Latrophilins are highly expressed in the nervous system. Although a substantial amount of studies has been conducted to describe the role of latrophilins in the toxin-mediated action, the recent identification of endogenous ligands for these receptors helped confirm their function as mediators of adhesion events. Here we hypothesize a role for latrophilins in inter-neuronal contacts and the formation of neuronal networks and we review the most recent information on their role in neurons. We explore molecular, cellular and behavioral aspects related to latrophilin adhesion function in mice, zebrafish, Drosophila melanogaster and Caenorhabditis elegans, in physiological and pathophysiological conditions, including autism spectrum, bipolar, attention deficit and hyperactivity and substance use disorders.

Knockout of latrophilin-3 in Sprague-Dawley rats causes hyperactivity, hyper-reactivity, under-response to amphetamine, and disrupted dopamine markers

Attention deficit hyperactivity disorder is a pervasive developmental disorder characterized by inattention, impulsivity, and hyperactivity and is 75-90% heritable. Latrophilin-3 (LPHN3; or ADGRL(3)) is associated with a subtype of ADHD, but how it translates to symptoms is unknown. LPHN3 is a synaptic adhesion G protein coupled receptor that binds to fibronectin leucine rich transmembrane protein 3 and teneurin-3 (FLRT3 and TEN-3). We created a null mutation of Lphn3 (KO) in Sprague-Dawley rats using CRISPR/Cas9 to delete exon-3. The KO rats had no effects on reproduction or survival but reduced growth. KO females showed catch-up weight gain whereas KO males did not. We tested WT and KO littermates for home-cage activity, anxiety-like behavior, acoustic startle response, and activity after amphetamine challenge. Expression of Lphn3-related genes, monoamines, and receptors were determined. Lphn3 KO rats showed persistent hyperactivity, increased acoustic startle, reduced activity in response to amphetamine relative to baseline, and female-specific reduced anxiety-like behavior. Expression of Lphn1, Lphn2, and Flrt3 by qPCR and their protein products by western-blot analysis showed no compensatory upregulation. Striatal tyrosine hydroxylase, aromatic L-amino acid decarboxylase (AADC), and the dopamine transporter were increased and dopamine D1 receptor (DRD1) and dopamine- and cAMP-regulated neuronal phosphoprotein (DARPP-32) decreased with no changes in DRD2, DRD4, vesicular monoamine transporter-2, N-methyl-d-aspartate (NMDA)-NR1, -NR2A, or -NR2B. LPHN3 is expressed in many brain regions but its function is largely unknown. Data from human, mouse, zebrafish, Drosophila and our new Lphn3 KO rat data collectively show that its disruption is significantly correlated with hyperactivity and associated striatal changes in dopamine markers.

Actin cytoskeleton remodeling defines a distinct cellular function for adhesion G protein-coupled receptors ADGRL/latrophilins 1, 2 and 3

Latrophilins represent a subgroup of the adhesion G protein-coupled receptor family, which bind to actin-associated scaffolding proteins. They are expressed in various tissues, suggesting that they might participate in biological processes that are ubiquitous. Here we focus on actin cytoskeleton dynamics to explore the role of latrophilins in mammalian cells. Individual overexpression of each latrophilin isoform comparably increased cell volume while modifying the net profile of F-actin-dependent cell extensions, as evaluated by confocal microscopy analysis. Latrophilin deletion mutants evidenced that direct coupling to the intracellular machinery was a requirement for modulating cell extensions. The association between latrophilins and the actin cytoskeleton was detected by co-immunoprecipitation assays and corroborated with immunocytochemistry analysis. Consistent with the destabilization of F-actin structures, latrophilin isoforms constitutively induced a prominent increase in the activity of actin-depolymerizing factor, cofilin. Intercellular adhesion events stabilized by heterophilic Teneurin-4 trans-interactions disrupted latrophilin colocalization with F-actin and led to an isoform-specific rescue of cell extensions. Thus, we find that the actin cytoskeleton machinery constitutes an important component of constitutive as well as ligand-induced signaling for latrophilins.

This article has an associated First Person interview with the first author of the paper.

Summary: Synapses involve the adhesion function of latrophilins within existing neuronal extensions. We show that latrophilins engage the actin cytoskeleton, both constitutively and upon ligand stimulation, to dictate cell extension patterns.

Teneurins and latrophilins: two giants meet at the synapse

Teneurins and latrophilins are both conserved families of cell adhesion proteins that mediate cellular communication and play critical roles in embryonic and neural development. However, their mechanisms of action remain poorly understood. In the past several years, three-dimensional structures of teneurins and latrophilins have been reported at atomic resolutions and revealed distinct protein folds and unique structural features. In this review, we discuss these structures which, together with structure-guided biochemical and functional analyses, provide hints for the mechanisms of trans-cellular communication at the synapse and other cell-cell contact sites.

Latrophilins and Teneurins in Invertebrates: No Love for Each Other?

Transsynaptic connections enabling cell–cell adhesion and cellular communication are a vital part of synapse formation, maintenance and function. A recently discovered interaction between the Adhesion GPCRs Latrophilins and the type II single transmembrane proteins Teneurins at mammalian synapses is vital for synapse formation and dendrite branching. While the understanding of the effects and the molecular interplay of this Latrophilin-Teneurin partnership is not entirely understood, its significance is highlighted by behavioral and neurological phenotypes in various animal models. As both groups of molecules, Latrophilins and Teneurins, are generally highly conserved, have overlapping expression and often similar functions across phyla, it can be speculated that this interaction, which has been proven essential in mammalian systems, also occurs in invertebrates to control shaping of synapses. Knowledge of the generality of this interaction is especially of interest due to its possible involvement in neuropathologies. Further, several invertebrates serve as model organisms for addressing various neurobiological research questions. So far, an interaction of Latrophilins and Teneurins has not been observed in invertebrates, but our knowledge on both groups of molecules is by far not complete. In this review, we give an overview on existing experimental evidence arguing for as well as against a potential Latrophilin-Teneurin interaction beyond mammals. By combining these insights with evolutionary aspects on each of the interaction partners we provide and discuss a comprehensive picture on the functions of both molecules in invertebrates and the likeliness of an evolutionary conservation of their interaction.

Proteolytically released Lasso/teneurin-2 induces axonal attraction by interacting with latrophilin-1 on axonal growth cones

A presynaptic adhesion G-protein-coupled receptor, latrophilin-1, and a postsynaptic transmembrane protein, Lasso/teneurin-2, are implicated in trans-synaptic interaction that contributes to synapse formation. Surprisingly, during neuronal development, a substantial proportion of Lasso is released into the intercellular space by regulated proteolysis, potentially precluding its function in synaptogenesis. We found that released Lasso binds to cell-surface latrophilin-1 on axonal growth cones. Using microfluidic devices to create stable gradients of soluble Lasso, we show that it induces axonal attraction, without increasing neurite outgrowth. Using latrophilin-1 knockout in mice, we demonstrate that latrophilin-1 is required for this effect. After binding latrophilin-1, Lasso causes downstream signaling, which leads to an increase in cytosolic calcium and enhanced exocytosis, processes that are known to mediate growth cone steering. These findings reveal a novel mechanism of axonal pathfinding, whereby latrophilin-1 and Lasso mediate both short-range interaction that supports synaptogenesis, and long-range signaling that induces axonal attraction.

The brain is a complex mesh of interconnected neurons, with each cell making tens, hundreds, or even thousands of connections. These links can stretch over long distances, and establishing them correctly during development is essential. Developing neurons send out long and thin structures, called axons, to reach distant cells. To guide these growing axons, neurons release molecules that work as traffic signals: some attract axons whilst others repel them, helping the burgeoning structures to twist and turn along their travel paths.

When an axon reaches its target cell, the two cells join to each other by forming a structure called a synapse. To make the connection, surface proteins on the axon latch onto matching proteins on the target cell, zipping up the synapse. There are many different types of synapses in the brain, but we only know a few of the surface molecules involved in their creation – not enough to explain synaptic variety.

Two of these surface proteins are latrophilin-1, which is produced by the growing axon, and Lasso, which sits on the membrane of the target cell. The two proteins interact strongly, anchoring the axon to the target cell and allowing the synapse to form. However, a previous recent discovery by Vysokov et al. has revealed that an enzyme can also cut Lasso from the membrane of the target cell. The ‘free’ protein can still interact with latrophilin-1, but as it is shed by the target cell, it can no longer serve as an anchor for the synapse. Could it be that free Lasso acts as a traffic signal instead?

Here, Vysokov et al. tried to answer this by growing neurons from a part of the brain called the hippocampus in a special labyrinth dish. When free Lasso was gradually introduced in the culture through microscopic channels, it interacted with latrophilin-1 on the surface of the axons. This triggered internal changes that led the axons to add more membrane where they had sensed Lasso, making them grow towards the source of the signal.

The results demonstrate that a target cell can both carry and release Lasso, using this duplicitous protein to help attract growing axons as well as anchor them. The work by Vysokov et al. contributes to our knowledge of how neurons normally connect, which could shed light on how this process can go wrong. This may be relevant to understand conditions such as schizophrenia and ADHD, where patients’ brains often show incorrect wiring.

Towards an Understanding of Synapse Formation

Synapses are intercellular junctions specialized for fast, point-to-point information transfer from a presynaptic neuron to a postsynaptic cell. At a synapse, a presynaptic terminal secretes neurotransmitters via a canonical release machinery, while a postsynaptic specialization senses neurotransmitters via diverse receptors. Synaptic junctions are likely organized by trans-synaptic cell-adhesion molecules (CAMs) that bidirectionally orchestrate synapse formation, restructuring, and elimination. Many candidate synaptic CAMs were described, but which CAMs are central actors and which are bystanders remains unclear. Moreover, multiple genes encoding synaptic CAMs were linked to neuropsychiatric disorders, but the mechanisms involved are unresolved. Here, I propose that engagement of multifarious synaptic CAMs produces parallel trans-synaptic signals that mediate the establishment, organization, and plasticity of synapses, thereby controlling information processing by neural circuits. Among others, this hypothesis implies that synapse formation can be understood in terms of inter- and intracellular signaling, and that neuropsychiatric disorders involve an impairment in such signaling.

suREJ proteins: new signalling molecules in sea urchin spermatozoa

In Strongylocentrotus purpuratus, the fucose sulphate polymer (FSP) of egg jelly induces the sperm acrosome reaction (AR; Vacquier & Moy, 1997). Protease treatment of sperm renders the cells insensitive to FSP, indicating that sperm membrane receptors mediate the signal transduction events underlying the AR. Monoclonal antibodies to a 210 kDa membrane glycoprotein induce Ca2+ influx into sperm and trigger the AR (Trimmer et al., 1986; Moy et al., 1996). Purified 210 kDa protein binds species-specifically to egg jelly and blocks AR induction by antibody (Podell & Vacquier, 1985; Moy et al., 1996). FSP binds to the 210 kDa protein attached to Sepharose (Vacquier & Moy, 1997). Monoclonal antibodies localise the 210 kDa protein on the plasma membrane over the acrosome and also on the sperm flagellum. The 210 kDa protein has the attributes of a sperm receptor for egg jelly and is henceforth named suREJ1 (Moy et al., 1996). We describe here the three REJ proteins found thus far in S. purpuratus sperm.

Adhesion GPCRs in Kidney Development and Disease

Chronic kidney disease (CKD) represents the fastest growing pathology worldwide with a prevalence of >10% in many countries. In addition, kidney cancer represents 5% of all new diagnosed cancers. As currently no effective therapies exist to restore kidney function after CKD- as well as cancer-induced renal damage, it is important to elucidate new regulators of kidney development and disease as new therapeutic targets. G protein-coupled receptors (GPCRs) represent the most successful class of pharmaceutical targets. In recent years adhesion GPCRs (aGPCRs), the second largest GPCR family, gained significant attention as they are present on almost all mammalian cells, are associated to a plethora of diseases and regulate important cellular processes. aGPCRs regulate for example cell polarity, mitotic spindle orientation, cell migration, and cell aggregation; all processes that play important roles in kidney development and/or disease. Moreover, polycystin-1, a major regulator of kidney development and disease, contains a GAIN domain, which is otherwise only found in aGPCRs. In this review, we assess the potential of aGPCRs as therapeutic targets for kidney disease. For this purpose we have summarized the available literature and analyzed data from the databases The Human Protein Atlas, EURExpress, Nephroseq, FireBrowse, cBioPortal for Cancer Genomics and the National Cancer Institute Genomic Data Commons data portal (NCIGDC). Our data indicate that most aGPCRs are expressed in different spatio-temporal patterns during kidney development and that altered aGPCR expression is associated with a variety of kidney diseases including CKD, diabetic nephropathy, lupus nephritis as well as renal cell carcinoma. We conclude that aGPCRs present a promising new class of therapeutic targets and/or might be useful as diagnostic markers in kidney disease.

Synaptic Neurexin Complexes: A Molecular Code for the Logic of Neural Circuits

Synapses are specialized junctions between neurons in brain that transmit and compute information, thereby connecting neurons into millions of overlapping and interdigitated neural circuits. Here, we posit that the establishment, properties, and dynamics of synapses are governed by a molecular logic that is controlled by diverse trans-synaptic signaling molecules. Neurexins, expressed in thousands of alternatively spliced isoforms, are central components of this dynamic code. Presynaptic neurexins regulate synapse properties via differential binding to multifarious postsynaptic ligands, such as neuroligins, cerebellin/GluD complexes, and latrophilins, thereby shaping the input/output relations of their resident neural circuits. Mutations in genes encoding neurexins and their ligands are associated with diverse neuropsychiatric disorders, especially schizophrenia, autism, and Tourette syndrome. Thus, neurexins nucleate an overall trans-synaptic signaling network that controls synapse properties, which thereby determines the precise responses of synapses to spike patterns in a neuron and circuit and which is vulnerable to impairments in neuropsychiatric disorders.

Postsynaptic adhesion GPCR latrophilin-2 mediates target recognition in entorhinal-hippocampal synapse assembly

Synapse assembly likely requires postsynaptic target recognition by incoming presynaptic afferents. Using newly generated conditional knock-in and knockout mice, we show in this study that latrophilin-2 (Lphn2), a cell-adhesion G protein-coupled receptor and presumptive α-latrotoxin receptor, controls the numbers of a specific subset of synapses in CA1-region hippocampal neurons, suggesting that Lphn2 acts as a synaptic target-recognition molecule. In cultured hippocampal neurons, Lphn2 maintained synapse numbers via a postsynaptic instead of a presynaptic mechanism, which was surprising given its presumptive role as an α-latrotoxin receptor. In CA1-region neurons in vivo, Lphn2 was specifically targeted to dendritic spines in the stratum lacunosum-moleculare, which form synapses with presynaptic entorhinal cortex afferents. In this study, postsynaptic deletion of Lphn2 selectively decreased spine numbers and impaired synaptic inputs from entorhinal but not Schaffer-collateral afferents. Behaviorally, loss of Lphn2 from the CA1 region increased spatial memory retention but decreased learning of sequential spatial memory tasks. Thus, Lphn2 appears to control synapse numbers in the entorhinal cortex/CA1 region circuit by acting as a domain-specific postsynaptic target-recognition molecule.

Adhesion G Protein-Coupled Receptors as Drug Targets

The adhesion G protein-coupled receptors (aGPCRs) are an evolutionarily ancient family of receptors that play key roles in many different physiological processes. These receptors are notable for their exceptionally long ectodomains, which span several hundred to several thousand amino acids and contain various adhesion-related domains, as well as a GPCR autoproteolysis-inducing (GAIN) domain. The GAIN domain is conserved throughout almost the entire family and undergoes autoproteolysis to cleave the receptors into two noncovalently-associated protomers. Recent studies have revealed that the signaling activity of aGPCRs is largely determined by changes in the interactions among these protomers. We review recent advances in understanding aGPCR activation mechanisms and discuss the physiological roles and pharmacological properties of aGPCRs, with an eye toward the potential utility of these receptors as drug targets.

Brain imaging genetics in ADHD and beyond - Mapping pathways from gene to disorder at different levels of complexity

Attention-deficit/hyperactivity disorder (ADHD) is a common and often persistent neurodevelopmental disorder. Beyond gene-finding, neurobiological parameters, such as brain structure, connectivity, and function, have been used to link genetic variation to ADHD symptomatology. We performed a systematic review of brain imaging genetics studies involving 62 ADHD candidate genes in childhood and adult ADHD cohorts. Fifty-one eligible research articles described studies of 13 ADHD candidate genes. Almost exclusively, single genetic variants were studied, mostly focussing on dopamine-related genes. While promising results have been reported, imaging genetics studies are thus far hampered by methodological differences in study design and analysis methodology, as well as limited sample sizes. Beyond reviewing imaging genetics studies, we also discuss the need for complementary approaches at multiple levels of biological complexity and emphasize the importance of combining and integrating findings across levels for a better understanding of biological pathways from gene to disease. These may include multi-modal imaging genetics studies, bioinformatic analyses, and functional analyses of cell and animal models.

Behavioral and transcriptomic profiling of mice null for Lphn3, a gene implicated in ADHD and addiction

Background

The Latrophilin 3 (LPHN3) gene (recently renamed Adhesion G protein‐coupled receptor L3 (ADGRL3)) has been linked to susceptibility to attention deficit/hyperactivity disorder (ADHD) and vulnerability to addiction. However, its role and function are not well understood as there are no known functional variants.

Methods

To characterize the function of this little known gene, we phenotyped Lphn3 null mice. We assessed motivation for food reward and working memory via instrumental responding tasks, motor coordination via rotarod, and depressive‐like behavior via forced swim. We also measured neurite outgrowth of primary hippocampal and cortical neuron cultures. Standard blood chemistries and blood counts were performed. Finally, we also evaluated the transcriptome in several brain regions.

Results

Behaviorally, loss of Lphn3 increases both reward motivation and activity levels. Lphn3 null mice display significantly greater instrumental responding for food than wild‐type mice, particularly under high response ratios, and swim incessantly during a forced swim assay. However, loss of Lphn3 does not interfere with working memory or motor coordination. Primary hippocampal and cortical neuron cultures demonstrate that null neurons display comparatively enhanced neurite outgrowth after 2 and 3 days in vitro. Standard blood chemistry panels reveal that nulls have low serum calcium levels. Finally, analysis of the transcriptome from prefrontal cortical, striatal, and hippocampal tissue at different developmental time points shows that loss of Lphn3 results in genotype‐dependent differential gene expression (DGE), particularly for cell adhesion molecules and calcium signaling proteins. Much of the DGE is attenuated with age, and is consistent with the idea that ADHD is associated with delayed cortical maturation.

Conclusions

Transcriptome changes likely affect neuron structure and function, leading to behavioral anomalies consistent with both ADHD and addiction phenotypes. The data should further motivate analyses of Lphn3 function in the developmental timing of altered gene expression and calcium signaling, and their effects on neuronal structure/function during development.

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 GPCR-Related Protein Networks

Adhesion G protein-coupled receptors (aGPCRs/ADGRs) are unique receptors that combine cell adhesion and signaling functions. Protein networks related to ADGRs exert diverse functions, e.g., in tissue polarity, cell migration, nerve cell function, or immune response, and are regulated via different mechanisms. The large extracellular domain of ADGRs is capable of mediating cell-cell or cell-matrix protein interactions. Their intracellular surface and domains are coupled to downstream signaling pathways and often bind to scaffold proteins, organizing membrane-associated protein complexes. The cohesive interplay between ADGR-related network components is essential to prevent severe disease-causing damage in numerous cell types. Consequently, in recent years, attention has focused on the decipherment of the precise molecular composition of ADGR protein complexes and interactomes in various cellular modules. In this chapter, we discuss the affiliation of ADGR networks to cellular modules and how they can be regulated, pinpointing common features in the networks related to the diverse ADGRs. Detailed decipherment of the composition of protein networks should provide novel targets for the development of novel therapies with the aim to cure human diseases related to ADGRs.

Superfamily of genes encoding G protein-coupled receptors in the diamondback moth Plutella xylostella (Lepidoptera: Plutellidae)

G protein-coupled receptors (GPCRs) are the largest and most versatile superfamily of cell membrane proteins, which mediate various physiological processes including reproduction, development and behaviour. The diamondback moth, Plutella xylostella (Lepidoptera: Plutellidae), is one of the most notorious insect pests, preferentially feeding on cruciferous plants. P. xylostella is not only one of the world's most widespread lepidopteran insects, but has also developed resistance to nearly all classes of insecticides. Although the mechanisms of insecticide resistance have been studied extensively in many insect species, few investigations have been carried out on GPCRs in P. xylostella. In the present study, we identified 95 putative GPCRs in the P. xylostella genome. The identified GPCRs were compared with their homologues in Bombyx mori and Drosophila melanogaster. Our results suggest that GPCRs in different insect species may have evolved by a birth-and-death process. One of the differences among compared insects is the duplication of short neuropeptide F receptor and adipokinetic hormone receptors in P. xylostella and B. mori. Another divergence is the decrease in quantity and diversity of the stress-tolerance gene, Mth, in P. xylostella. The evolution by the birth-and-death process is probably involved in adaptation to the feeding behaviour, reproduction and stress responses of P. xylostella. Some of the genes identified in the present study could be potential targets for the development of novel pesticides.

Structural Basis of Latrophilin-FLRT-UNC5 Interaction in Cell Adhesion

Fibronectin leucine-rich repeat transmembrane proteins (FLRTs) are cell-adhesion molecules with emerging functions in cortical development and synapse formation. Their extracellular regions interact with latrophilins (LPHNs) to mediate synapse development, and with Uncoordinated-5 (UNC5)/netrin receptors to control the migration of neurons in the developing cortex. Here, we present the crystal structures of FLRT3 in isolation and in complex with LPHN3. The LPHN3/FLRT3 structure reveals that LPHN3 binds to FLRT3 at a site distinct from UNC5. Structure-based mutations specifically disrupt LPHN3/FLRT3 binding, but do not disturb their interactions with other proteins or their cell-membrane localization. Thus, they can be used as molecular tools to dissect the functions of FLRTs and LPHNs in vivo. Our results suggest that UNC5 and LPHN3 can simultaneously bind to FLRT3, forming a trimeric complex, and that FLRT3 may form transsynaptic complexes with both LPHN3 and UNC5. These findings provide molecular insights for understanding the role of cell-adhesion proteins in synapse function.

Association of LPHN3 rs6551665 A/G polymorphism with attention deficit and hyperactivity disorder in Korean children

Attention deficit hyperactivity disorder (ADHD) is a common and highly heritable disorder of school-age children. Its heritability was estimated at 80-90% but the genetic component underpinning this disorder remains to be disclosed. Recently, a highly consistent association between latrophilin3 (LPHN3) gene and ADHD was reported. In the present study, we examined the association between the LPHN3 rs6551665 A/G polymorphism and ADHD in Korea. The samples used in the study consisted of 150 ADHD children and 322 controls. The ADHD children were diagnosed according to DSM-IV. ADHD symptoms were evaluated with Dupaul Parent ADHD Rating Scales. LPHN3 rs6551665 SNP was determined by PCR-RFLP. Hardy-Weinberg equilibrium, genotype and allele frequency differences between the case and the control, and odds ratio were examined using the chi-square and exact tests. The LPHN3 gene locus was found to have no deviation from the Hardy-Weinberg expectation. We observed a significant association between the ADHD children and control group in genotype frequency (p=0.01) and allele frequency (p=0.02). The ADHD children appeared to have a surplus of GG genotype (OR 2.959, 95% CI 1.416-6.184, p=0.003) and G allele (OR 1.44, 95% CI 1.062-1.945, p=0.02). The association was more distinctive when analysis was confined to male samples (p=0.005), the OR of male controls and cases was 4.029 (95% CI 1.597-10.164, p=0.002) and the OR having G allele vs. A allele was 1.46 (95% CI 1.002-2.127, p=0.048). Thus our results imply that the LPHN3 rs6551665 GG genotype and G allele may provide a significant effect on the ADHD, although larger sample sizes and functional studies are necessary to further elucidate these findings.

Structural basis of latrophilin-FLRT interaction

Latrophilins, receptors for spider venom α-latrotoxin, are adhesion type G-protein-coupled receptors with emerging functions in synapse development. The N-terminal region binds the endogenous cell adhesion molecule FLRT, a major regulator of cortical and synapse development. We present crystallographic data for the mouse Latrophilin3 lectin and olfactomedin-like (Olf) domains, thereby revealing the Olf β-propeller fold and conserved calcium-binding site. We locate the FLRT-Latrophilin binding surfaces by a combination of sequence conservation analysis, point mutagenesis, and surface plasmon resonance experiments. In stripe assays, we show that wild-type Latrophilin3 and its high-affinity interactor FLRT2, but not the binding-impaired mutants we generated, promote HeLa cell adhesion. In contrast, cortical neurons expressing endogenous FLRTs are repelled by wild-type Latrophilin3 and not by the binding-impaired mutant. Taken together, we present molecular level insights into Latrophilin structure, its FLRT-binding mechanism, and a role for Latrophilin and FLRT that goes beyond a simply adhesive interaction.

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The LPHN olfactomedin-like domain forms a five-bladed β propeller

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A conserved calcium-binding site is located at the center of the protein

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Latrophilin-FLRT binding depends on a conserved binding site

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Mutations in the binding site inhibit Latrophilin-FLRT signaling

Dystroglycan binding to α-neurexin competes with neurexophilin-1 and neuroligin in the brain

α-Neurexins (α-Nrxn) are mostly presynaptic cell surface molecules essential for neurotransmission that are linked to neuro-developmental disorders as autism or schizophrenia. Several interaction partners of α-Nrxn are identified that depend on alternative splicing, including neuroligins (Nlgn) and dystroglycan (αDAG). The trans-synaptic complex with Nlgn1 was extensively characterized and shown to partially mediate α-Nrxn function. However, the interactions of α-Nrxn with αDAG, neurexophilins (Nxph1) and Nlgn2, ligands that occur specifically at inhibitory synapses, are incompletely understood. Using site-directed mutagenesis, we demonstrate the exact binding epitopes of αDAG and Nxph1 on Nrxn1α and show that their binding is mutually exclusive. Identification of an unusual cysteine bridge pattern and complex type glycans in Nxph1 ensure binding to the second laminin/neurexin/sex hormone binding (LNS2) domain of Nrxn1α, but this association does not interfere with Nlgn binding at LNS6. αDAG, in contrast, interacts with both LNS2 and LNS6 domains without inserts in splice sites SS#2 or SS#4 mostly via LARGE (like-acetylglucosaminyltransferase)-dependent glycans attached to the mucin region. Unexpectedly, binding of αDAG at LNS2 prevents interaction of Nlgn at LNS6 with or without splice insert in SS#4, presumably by sterically hindering each other in the u-form conformation of α-Nrxn. Thus, expression of αDAG and Nxph1 together with alternative splicing in Nrxn1α may prevent or facilitate formation of distinct trans-synaptic Nrxn·Nlgn complexes, revealing an unanticipated way to contribute to the identity of synaptic subpopulations.

Control of neural circuit formation by leucine-rich repeat proteins

The function of neural circuits depends on the precise connectivity between populations of neurons. Increasing evidence indicates that disruptions in excitatory or inhibitory synapse formation or function lead to excitation/inhibition (E/I) imbalances and contribute to neurodevelopmental and psychiatric disorders. Leucine-rich repeat (LRR)-containing surface proteins have emerged as key organizers of excitatory and inhibitory synapses. Distinct LRR proteins are expressed in different cell types and interact with key pre- and postsynaptic proteins. These protein interaction networks allow LRR proteins to coordinate pre- and postsynaptic elements during synapse formation and differentiation, pathway-specific synapse development, and synaptic plasticity. LRR proteins, therefore, play a critical role in organizing synaptic connections into functional neural circuits, and their dysfunction may contribute to neuropsychiatric disorders.

Novel functional complexity of polycystin-1 by GPS cleavage in vivo: role in polycystic kidney disease

Polycystin-1 (Pc1) cleavage at the G protein-coupled receptor (GPCR) proteolytic site (GPS) is required for normal kidney morphology in humans and mice. We found a complex pattern of endogenous Pc1 forms by GPS cleavage. GPS cleavage generates not only the heterodimeric cleaved full-length Pc1 (Pc1(cFL)) in which the N-terminal fragment (NTF) remains noncovalently associated with the C-terminal fragment (CTF) but also a novel (Pc1) form (Pc1(deN)) in which NTF becomes detached from CTF. Uncleaved Pc1 (Pc1(U)) resides primarily in the endoplasmic reticulum (ER), whereas both Pc1(cFL) and Pc1(deN) traffic through the secretory pathway in vivo. GPS cleavage is not a prerequisite, however, for Pc1 trafficking in vivo. Importantly, Pc1(deN) is predominantly found at the plasma membrane of renal epithelial cells. By functional genetic complementation with five Pkd1 mouse models, we discovered that CTF plays a crucial role in Pc1(deN) trafficking. Our studies support GPS cleavage as a critical regulatory mechanism of Pc1 biogenesis and trafficking for proper kidney development and homeostasis.

Latrophilins function as heterophilic cell-adhesion molecules by binding to teneurins: regulation by alternative splicing

Latrophilin-1, -2, and -3 are adhesion-type G protein-coupled receptors that are auxiliary α-latrotoxin receptors, suggesting that they may have a synaptic function. Using pulldowns, we here identify teneurins, type II transmembrane proteins that are also candidate synaptic cell-adhesion molecules, as interactors for the lectin-like domain of latrophilins. We show that teneurin binds to latrophilins with nanomolar affinity and that this binding mediates cell adhesion, consistent with a role of teneurin binding to latrophilins in trans-synaptic interactions. All latrophilins are subject to alternative splicing at an N-terminal site; in latrophilin-1, this alternative splicing modulates teneurin binding but has no effect on binding of latrophilin-1 to another ligand, FLRT3. Addition to cultured neurons of soluble teneurin-binding fragments of latrophilin-1 decreased synapse density, suggesting that latrophilin binding to teneurin may directly or indirectly influence synapse formation and/or maintenance. These observations are potentially intriguing in view of the proposed role for Drosophila teneurins in determining synapse specificity. However, teneurins in Drosophila were suggested to act as homophilic cell-adhesion molecules, whereas our findings suggest a heterophilic interaction mechanism. Thus, we tested whether mammalian teneurins also are homophilic cell-adhesion molecules, in addition to binding to latrophilins as heterophilic cell-adhesion molecules. Strikingly, we find that although teneurins bind to each other in solution, homophilic teneurin-teneurin binding is unable to support stable cell adhesion, different from heterophilic teneurin-latrophilin binding. Thus, mammalian teneurins act as heterophilic cell-adhesion molecules that may be involved in trans-neuronal interaction processes such as synapse formation or maintenance.