The adhesion GPCRs CELSR1-3 and LPHN3 engage G proteins via distinct activation mechanisms

Structural basis for regulation of CELSR1 by a compact module in its extracellular region

The Cadherin EGF Laminin G seven-pass G-type receptor subfamily (CELSR/ADGRC) is one of the most conserved among adhesion G protein-coupled receptors and is essential for animal development. The extracellular regions (ECRs) of CELSRs are large with 23 adhesion domains. However, molecular insight into CELSR function is sparsely available. Here, we report the 3.8 Å cryo-EM reconstruction of the mouse CELSR1 ECR and reveal that 14 domains form a compact module mediated by conserved interactions majorly between the CADH9 and C-terminal GAIN domains. In the presence of Ca2+, the CELSR1 ECR forms a dimer species mediated by the cadherin repeats putatively in an antiparallel fashion. Cell-based assays reveal the N-terminal CADH1-8 repeat is required for cell-cell adhesion and the C-terminal CADH9-GAIN compact module can regulate cellular adhesion. Our work provides molecular insight into how one of the largest GPCRs uses defined structural modules to regulate receptor function.

The Adhesion GPCR ADGRL2 engages Gα13 to Enable Epidermal Differentiation

Homeostasis relies on signaling networks controlled by cell membrane receptors. Although G-protein-coupled receptors (GPCRs) are the largest family of transmembrane receptors, their specific roles in the epidermis are not fully understood. Dual CRISPR-Flow and single cell Perturb-seq knockout screens of all epidermal GPCRs were thus performed, uncovering an essential requirement for adhesion GPCR ADGRL2 (latrophilin 2) in epidermal differentiation. Among potential downstream guanine nucleotide-binding G proteins, ADGRL2 selectively activated Gα13. Perturb-seq of epidermal G proteins and follow-up tissue knockouts verified that Gα13 is also required for epidermal differentiation. A cryo-electron microscopy (cryo-EM) structure in lipid nanodiscs showed that ADGRL2 engages with Gα13 at multiple interfaces, including via a novel interaction between ADGRL2 intracellular loop 3 (ICL3) and a Gα13-specific QQQ glutamine triplet sequence in its GTPase domain. In situ gene mutation of this interface sequence impaired epidermal differentiation, highlighting an essential new role for an ADGRL2-Gα13 axis in epidermal differentiation.

The N terminus-only function of adhesion GPCRs: emerging concepts

Adhesion G-protein-coupled receptors (aGPCRs) play key roles in health and disease. They are unique in that they not only activate G-protein pathways but also have distinct functions that rely solely on their N termini, making them complex drug targets. To date there have been only descriptive observations about these enigmatic N terminus-only functions. Emerging evidence from several aGPCRs now indicates that these are a defining characteristic of these receptors that allows them to operate bidirectionally across environments. Recent advances in characterizing aGPCR splice variants and receptor structure have revealed the G protein-independent mechanisms that underlie their N terminus-only functions. This review consolidates current findings, explores how the N termini integrate functions, and identifies common principles across aGPCRs. We consider the therapeutic implications and discuss how specifically targeting N terminus functions provides a novel perspective on the pharmacological potential of aGPCRs.

Extrinsic induction of apoptosis and tumor suppression via the p53-Reprimo-Hippo-YAP/TAZ-p73 pathway

Tumor progression is suppressed by inherent cellular mechanisms such as apoptosis. The p53 tumor suppressor gene is the most commonly mutated gene in human cancer and plays a pivotal role in tumor suppression. RPRM is a target gene of p53 known to be involved in tumor suppression, but its molecular function has remained elusive. Here, we report that Reprimo (the protein product of RPRM) is secreted and extrinsically induces apoptosis in recipient cells. We identified FAT1, FAT4, CELSR1, CELSR2, and CELSR3, members of the protocadherin family, as receptors for Reprimo. Subsequent analyses revealed that Reprimo acts upstream of the Hippo-YAP/TAZ-p73 axis and induces apoptosis by transactivating various proapoptotic genes. In vivo analyses further support the tumor-suppressive effects of secreted Reprimo. These findings identify the p53-Reprimo-Hippo-YAP/TAZ-p73 axis as an extrinsic apoptosis pathway that plays a crucial role in tumor suppression. Our finding of the innate tumor eliminator Reprimo and the downstream pathway offers a promising avenue for the pharmacological treatment of cancer.

Synaptic Gα12/13 signaling establishes hippocampal PV inhibitory circuits

Combinatorial networks of cell adhesion molecules and cell surface receptors drive fundamental aspects of neural circuit establishment and function. However, the intracellular signals orchestrated by these cell surface complexes remain less understood. Here, we report that the Gα12/13 pathway lies downstream of several GPCRs with critical synaptic functions. Impairment of the Gα12/13 pathway in postnatal hippocampal neurons diminishes inhibitory inputs without altering neuronal morphology or excitatory transmission. Gα12/13 signaling in hippocampal CA1 neurons in vivo selectively regulates PV interneuron synaptic connectivity, supporting an inhibitory synapse subtype-specific function of this pathway. Our studies establish Gα12/13 as a signaling node that shapes inhibitory hippocampal circuitry.

Conformational coupling between extracellular and transmembrane domains modulates holo-adhesion GPCR function

Adhesion G Protein-Coupled Receptors (aGPCRs) are key cell-adhesion molecules involved in numerous physiological functions. aGPCRs have large multi-domain extracellular regions (ECRs) 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 transmembrane region and has constrained movement. Single-molecule FRET experiments unveil three slow-exchanging FRET states of the ECR relative to the transmembrane region 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 for aGPCR activation.

Celsr3 drives development and connectivity of the acoustic startle hindbrain circuit

In the developing brain, groups of neurons organize into functional circuits that direct diverse behaviors. One such behavior is the evolutionarily conserved acoustic startle response, which in zebrafish is mediated by a well-defined hindbrain circuit. While numerous molecular pathways that guide neurons to their synaptic partners have been identified, it is unclear if and to what extent distinct neuron populations in the startle circuit utilize shared molecular pathways to ensure coordinated development. Here, we show that the planar cell polarity (PCP)-associated atypical cadherins Celsr3 and Celsr2, as well as the Celsr binding partner Frizzled 3a/Fzd3a, are critical for axon guidance of two neuron types that form synapses with each other: the command-like neuron Mauthner cells that drive the acoustic startle escape response, and spiral fiber neurons which provide excitatory input to Mauthner cells. We find that Mauthner axon growth towards synaptic targets is vital for Mauthner survival. We also demonstrate that symmetric spiral fiber input to Mauthner cells is critical for escape direction, which is necessary to respond to directional threats. Moreover, we identify distinct roles for Celsr3 and Celsr2, as Celsr3 is required for startle circuit development while Celsr2 is dispensable, though Celsr2 can partially compensate for loss of Celsr3 in Mauthner cells. This contrasts with facial branchiomotor neuron migration in the hindbrain, which requires Celsr2 while we find that Celsr3 is dispensable. Combined, our data uncover critical and distinct roles for individual PCP components during assembly of the acoustic startle hindbrain circuit.

Heterogeneity of tethered agonist signaling in adhesion G protein-coupled receptors

Adhesion G protein-coupled receptor (aGPCR) signaling influences development and homeostasis in a wide range of tissues. In the current model for aGPCR signaling, ligand binding liberates a conserved sequence that acts as an intramolecular, tethered agonist (TA), yet this model has not been evaluated systematically for all aGPCRs. Here, we assessed the TA-dependent activities of all 33 aGPCRs in a suite of transcriptional reporter, G protein activation, and β-arrestin recruitment assays using a new fusion protein platform. Strikingly, only ∼50% of aGPCRs exhibited robust TA-dependent activation, and unlike other GPCR families, aGPCRs showed a notable preference for G12/13 signaling. AlphaFold2 predictions assessing TA engagement in the predicted intramolecular binding pocket aligned with the TA dependence of the cellular responses. This dataset provides a comprehensive resource to inform the investigation of all human aGPCRs and for targeting aGPCRs therapeutically.

The far extracellular CUB domain of the adhesion GPCR ADGRG6/GPR126 is a key regulator of receptor signaling

Adhesion G Protein-coupled receptors (aGPCRs) transduce extracellular adhesion signals into cytoplasmic signaling pathways. ADGRG6/GPR126 is an aGPCR critical for axon myelination, heart development and ear development; and is associated with developmental diseases and cancers. ADGRG6 has a large, alternatively-spliced, five-domain extracellular region (ECR) that samples different conformations and regulates receptor signaling. However, the molecular details of how the ECR regulates signaling are unclear. Herein, we studied the conformational dynamics of the conserved CUB domain which is located at the distal N-terminus of the ECR and is deleted in an alternatively-spliced isoform ( Δ CUB). We showed that the Δ CUB isoform has decreased signaling. Molecular dynamics simulations suggest that the CUB domain is involved in interdomain contacts to maintain a compact ECR conformation. A cancer-associated CUB domain mutant, C94Y, drastically perturbs the ECR conformation and results in elevated signaling, whereas another CUB mutant, Y96A, located near a conserved Ca 2+ -binding site, decreases signaling. Our results suggest an ECR-mediated mechanism for ADGRG6 regulation in which the CUB domain instructs conformational changes within the ECR to regulate receptor signaling.

The adhesion GPCR and PCP component flamingo (FMI-1) alters body size and regulates the composition of the extracellular matrix

The extracellular matrix (ECM) is a network of macromolecules that presents a vital scaffold for cells and enables multiple ways of cellular communication. Thus, it is essential for many physiological processes such as development, tissue morphogenesis, homeostasis, the shape and partially the size of the body and its organs. To ensure these, the composition of the ECM is tissue-specific and highly dynamic. ECM homeostasis is therefore tightly controlled by several mechanisms. Here, we show that FMI-1, the homolog of the Adhesion GPCR Flamingo/CELSR/ADGRC in the nematode Caenorhabditis elegans, modulates the composition of the ECM by controlling the production both of ECM molecules such as collagens and also of ECM modifying enzymes. Thereby, FMI-1 affects the morphology and functionality of the nematode´s cuticle, which is mainly composed of ECM, and also modulates the body size. Mechanistic analyses highlight the fact that FMI-1 exerts its function from neurons non-cell autonomously (trans) solely via its extracellular N terminus. Our data support a model, by which the activity of the receptor, which has a well-described role in the planar cell polarity (PCP) pathway, involves the PCP molecule VANG-1, but seems to be independent of the DBL-1/BMP pathway.

Celsr3 drives development and connectivity of the acoustic startle hindbrain circuit

In the developing brain, groups of neurons organize into functional circuits that direct diverse behaviors. One such behavior is the evolutionarily conserved acoustic startle response, which in zebrafish is mediated by a well-defined hindbrain circuit. While numerous molecular pathways that guide neurons to their synaptic partners have been identified, it is unclear if and to what extent distinct neuron populations in the startle circuit utilize shared molecular pathways to ensure coordinated development. Here, we show that the planar cell polarity (PCP)-associated atypical cadherins Celsr3 and Celsr2, as well as the Celsr binding partner Frizzled 3a/Fzd3a, are critical for axon guidance of two neuron types that form synapses with each other: the command-like neuron Mauthner cells that drive the acoustic startle escape response, and spiral fiber neurons which provide excitatory input to Mauthner cells. We find that Mauthner axon growth towards synaptic targets is vital for Mauthner survival. We also demonstrate that symmetric spiral fiber input to Mauthner cells is critical for escape direction, which is necessary to respond to directional threats. Moreover, we identify distinct roles for Celsr3 and Celsr2, as Celsr3 is required for startle circuit development while Celsr2 is dispensable, though Celsr2 can partially compensate for loss of Celsr3 in Mauthner cells. This contrasts with facial branchiomotor neuron migration in the hindbrain, which requires Celsr2 while we find that Celsr3 is dispensable. Combined, our data uncover critical and distinct roles for individual PCP components during assembly of the acoustic startle hindbrain circuit.

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.

Alternative splicing of latrophilin-3 controls synapse formation

The assembly and specification of synapses in the brain is incompletely understood1-3. Latrophilin-3 (encoded by Adgrl3, also known as Lphn3)-a postsynaptic adhesion G-protein-coupled receptor-mediates synapse formation in the hippocampus4 but the mechanisms involved remain unclear. Here we show in mice that LPHN3 organizes synapses through a convergent dual-pathway mechanism: activation of Gαs signalling and recruitment of phase-separated postsynaptic protein scaffolds. We found that cell-type-specific alternative splicing of Lphn3 controls the LPHN3 G-protein-coupling mode, resulting in LPHN3 variants that predominantly signal through Gαs or Gα12/13. CRISPR-mediated manipulation of Lphn3 alternative splicing that shifts LPHN3 from a Gαs- to a Gα12/13-coupled mode impaired synaptic connectivity as severely as the overall deletion of Lphn3, suggesting that Gαs signalling by LPHN3 splice variants mediates synapse formation. Notably, Gαs-coupled, but not Gα12/13-coupled, splice variants of LPHN3 also recruit phase-transitioned postsynaptic protein scaffold condensates, such that these condensates are clustered by binding of presynaptic teneurin and FLRT ligands to LPHN3. Moreover, neuronal activity promotes alternative splicing of the synaptogenic Gαs-coupled variant of LPHN3. Together, these data suggest that activity-dependent alternative splicing of a key synaptic adhesion molecule controls synapse formation by parallel activation of two convergent pathways: Gαs signalling and clustered phase separation of postsynaptic protein scaffolds.

Structure of the extracellular region of the adhesion GPCR CELSR1 reveals a compact module which regulates G protein-coupling

Cadherin EGF Laminin G seven-pass G-type receptors (CELSRs or ADGRCs) are conserved adhesion G protein-coupled receptors which are essential for animal development. CELSRs have extracellular regions (ECRs) containing 23 adhesion domains which couple adhesion to intracellular signaling. However, molecular-level insight into CELSR function is sparsely available. We report the 4.3 Å cryo-EM reconstruction of the mCELSR1 ECR with 13 domains resolved in the structure. These domains form a compact module mediated by interdomain interactions with contact between the N- and C-terminal domains. We show the mCELSR1 ECR forms an extended species in the presence of Ca 2+ , which we propose represents the antiparallel cadherin repeat dimer. Using assays for adhesion and G protein-coupling, we assign the N-terminal CADH1-8 module as necessary for cell adhesion and we show the C-terminal CAHD9-GAIN module regulates signaling. Our work provides important molecular context to the literature on CELSR function and opens the door towards further mechanistic studies.

Intramolecular activity regulation of adhesion GPCRs in light of recent structural and evolutionary information

The class B2 of GPCRs known as adhesion G protein-coupled receptors (aGPCRs) has come under increasing academic and nonacademic research focus over the past decade due to their physiological importance as mechano-sensors in cell-cell and cell-matrix contexts. A major advance in understanding signal transduction of aGPCRs was achieved by the identification of the so-called Stachel sequence, which acts as an intramolecular agonist at the interface between the N terminus (Nt) and the seven-transmembrane helix domain (7TMD). Distinct extracellular signals received by the Nt are integrated at the Stachel into structural changes of the 7TMD towards an active state conformation. Until recently, little information was available on how the activation process of aGPCRs is realized at the molecular level. In the past three years several structures of the 7TMD plus the Stachel in complex with G proteins have been determined, which provide new insights into the architecture and molecular function of this receptor class. Herein, we review this structural information to extract common and distinct aGPCR features with particular focus on the Stachel binding site within the 7TMD. Our analysis extends the current view of aGPCR activation and exposes similarities and differences not only between diverse aGPCR members, but also compared to other GPCR classes.