Several members of the adhesion subfamily of G protein-coupled receptors (aGPCRs) are capable of self-activation by an internal agonist sequence (aka the Stachel ) that’s exposed upon removal or conformational changes of the N-terminal fragment of the receptor. Synthetic peptides derived from the Stachel sequence can be used as exogenous agonists. In the inactive form, the Stachel is sequestered as the β13-strand within the GPCR Autoproteolysis-INducing (GAIN) domain, but it engages the seven transmembrane region as a helix when it is either an intramolecular sequence or a synthetic peptide. Little is known about the molecular details underlying this transition, but we hypothesize that a disordered conformation is central to this intermediate state in receptor activation. Despite the primarily helical Stachel AlphaFold3 and Pepfold4 models predicted with high confidence for the entire aGPCR subfamily, disorder predictions and biophysical experiments reveal a predominantly disordered conformation in solution. Investigating the ADGRG6/GPR126 Stachel peptide, circular dichroism (CD) and nuclear magnetic resonance (NMR) experiments reveal a predominantly random coil conformation in aqueous buffer, polar detergent micelles, and zwitterionic lipids. Titration of trifluoroethanol uncovered a two-state equilibrium between an unfolded and helix-containing conformation with NMR localizing a single-turn helix to residues L846-L849. Taken together, these data indicate the ADGRG6/GPR126 Stachel peptide is primarily disordered, but small populations may adopt a helix-containing conformer that seems to support a conformational-selection activation mechanism.

Adhesion G protein-coupled receptors (aGPCRs) constitute a structurally and functionally distinct group within the superfamily of GPCRs. In 2015, the International Union of Pharmacology invited the Adhesion GPCR Consortium to publish a comprehensive review about aGPCRs and establish a unified nomenclature. Since then, substantial progress has been made in delineating the biological roles, molecular architecture, biochemical properties, expression profiles, ligand repertoire, and activation and signaling strategies of aGPCRs. Commensurate with these advances, their relevance to human pathophysiology has become increasingly apparent. In a coordinated effort, the Adhesion GPCR Consortium has reviewed recent progress in this field and provides a comprehensive assessment of the current understanding of aGPCR biology, including a focus on human and mammalian aGPCRs, their evolutionary origins, methodological approaches, and model systems for their investigation, as well as emerging approaches for their therapeutic targeting. SIGNIFICANCE STATEMENT: Adhesion G protein-coupled receptors are versatile cell-surface proteins that integrate structural, biochemical, and physiological functions, with major roles in health and disease. This review summarizes current knowledge of their molecular features, functions in diverse model systems, and emerging opportunities for therapeutic targeting, providing a comprehensive resource that connects basic biology with translational applications across multiple scientific disciplines.

Several members of the adhesion subfamily of G protein-coupled receptors (aGPCRs) are capable of self-activation by an internal agonist sequence (i.e., the Stachel) that is exposed upon removal or conformational changes of the N-terminal fragment of the receptor. Synthetic peptides derived from the Stachel sequence can be used as exogenous agonists. In the inactive form of the full-length receptor, the Stachel is sequestered as the β13 strand within the G protein-coupled receptor (GPCR) autoproteolysis-inducing (GAIN) domain, but it engages the seven transmembrane region as a helix when it is either an intramolecular sequence or a synthetic peptide. Little is known about the molecular details underlying this transition, but we hypothesize that a disordered conformation is central to this intermediate state in receptor activation. Despite the primarily helical Stachel AlphaFold3 and PEP-FOLD4 models predicted with high confidence for the entire aGPCR subfamily, computational predictions and biophysical experiments reveal a predominantly disordered conformation in solution. Investigating the ADGRG6 (also known as GPR126) Stachel peptide, circular dichroism (CD) and nuclear magnetic resonance (NMR) experiments reveal a predominantly random-coil conformation in aqueous buffer, polar detergent micelles, and zwitterionic lipids. Titration of trifluoroethanol uncovered a two-state equilibrium between an unfolded and helix-containing conformation with NMR localizing a single-turn helix to residues L846-L849. Taken together, these data indicate the ADGRG6 Stachel peptide is primarily disordered with a subset adopting partial helical structures, likely requiring the steric hindrance of the receptor binding pocket to fully induce helix formation in an induced-fit mechanism.

Osteoporosis represents an increasing health and socioeconomic burden on aging societies. Current therapeutic options often come with potentially severe side effects or lack long-term efficacy, highlighting the urgent need for more effective treatments. Identifying novel drug targets requires a thorough understanding of their physiological roles. Genome-wide association studies in humans have linked gene variants of the adhesion G protein-coupled receptor 133 (GPR133/ADGRD1) to variations in bone mineral density and body height. In this study, we explore the impact of GPR133/ADGRD1 on osteoblast differentiation and function. Constitutive and osteoblast-specific knockouts of Gpr133/Adgrd1 in mice lead to reduced cortical bone mass and trabecularization in the femurs and vertebrae — features characteristic of osteoporosis. This osteopenic phenotype in receptor-deficient mice is caused by impaired osteoblast function, which, in turn, promotes increased osteoclast activity. At the molecular level, GPR133/ADGRD1 regulates osteoblast function and differentiation through a combined activation mechanism involving interaction with its endogenous ligand, protein tyrosine kinase 7 (PTK7), and mechanical forces. This is demonstrated in vitro through stretch assays and in vivo via a mechanical loading experiment. Further in vitro analysis shows that GPR133/ADGRD1-mediated osteoblast differentiation is driven by cAMP-dependent activation of the β-catenin signaling pathway. Activation of GPR133/ADGRD1 with the receptor-specific ligand AP-970/43482503 (AP503) enhances osteoblast function and differentiation, both in vitro and in vivo, significantly alleviating osteoporosis in a mouse ovariectomy model. These findings position GPR133/ADGRD1 as a promising therapeutic target for osteoporosis and other diseases characterized by reduced bone mass.

Mitogen-activated protein kinase kinase kinase kinase 1 (MAP4K1) is a serine/threonine kinase that acts as an immune checkpoint downstream of T-cell receptor stimulation. MAP4K1 activity is enhanced by prostaglandin E2 (PGE2) and transforming growth factor beta (TGFβ), immune modulators commonly present in the tumor microenvironment. Therefore, its pharmacological inhibition is an attractive immuno-oncology concept for inducing therapeutic T-cell responses in cancer patients. Here, we describe the systematic optimization of azaindole-based lead compound 1, resulting in the discovery of potent and selective MAP4K1 inhibitor 38 (BAY-405) that displays nanomolar potency in biochemical and cellular assays as well as in vivo exposure after oral dosing. BAY-405 enhances T-cell immunity and overcomes the suppressive effect of PGE2 and TGFβ. Treatment of tumor-bearing mice shows T-cell-dependent antitumor efficacy. MAP4K1 inhibition in conjunction with PD-L1 blockade results in a superior antitumor impact, illustrating the complementarity of the single agent treatments.

Regulatory T cells (Tregs) are known to facilitate tumor progression by suppressing CD8+ T cells within the tumor microenvironment (TME), thereby also hampering the effectiveness of immune checkpoint inhibitors (ICIs). While systemic depletion of Tregs can enhance antitumor immunity, it also triggers undesirable autoimmune responses. Therefore, there is a need for therapeutic agents that selectively target Tregs within the TME without affecting systemic Tregs. In this study, as shown also by others, the chemokine (C–C motif) receptor 8 (CCR8) was found to be predominantly expressed on Tregs within the TME of both humans and mice, representing a unique target for selective depletion of tumor-residing Tregs. Based on this, we developed BAY 3375968, a novel anti-human CCR8 antibody, along with respective surrogate anti-mouse CCR8 antibodies, and demonstrated their in vitro mode-of-action through induction of potent antibody-dependent cellular cytotoxicity (ADCC) and phagocytosis (ADCP) activities. In vivo, anti-mouse CCR8 antibodies effectively depleted Tregs within the TME primarily via ADCP, leading to increased CD8+ T cell infiltration and subsequent tumor growth inhibition across various cancer models. This monotherapeutic efficacy was significantly enhanced in combination with ICIs. Collectively, these findings suggest that CCR8 targeting represents a promising strategy for Treg depletion in cancer therapies. BAY 3375968 is currently under investigation in a Phase I clinical trial (NCT05537740).

Alternative splicing and multiple transcription start and termination sites can produce a diverse repertoire of mRNA transcript variants from a given gene. While the full picture of the human transcriptome is still incomplete, publicly available RNA datasets have enabled the assembly of transcripts. Using publicly available deep sequencing data from 927 human samples across 48 tissues, we quantified known and new transcript variants, provide an interactive, browser-based application Splice-O-Mat and demonstrate its relevance using adhesion G protein-coupled receptors (aGPCRs) as an example. On average, 24 different transcript variants were detected for each of the 33 human aGPCR genes, and several dominant transcript variants were not yet annotated. Variable transcription starts and complex exon-intron structures encode a flexible protein domain architecture of the N- and C termini and the seven-transmembrane helix domain (7TMD). Notably, we discovered the first GPCR (ADGRG7/GPR128) with eight transmembrane helices. Both the N- and C terminus of this aGPCR were intracellularly oriented, anchoring the N terminus in the plasma membrane. Moreover, the assessment of tissue-specific transcript variants, also for other gene classes, in our application may change the evaluation of disease-causing mutations, as their position in different transcript variants may explain tissue-specific phenotypes.

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

The adhesion G protein-coupled receptor (aGPCR) GPR126/ADGRG6 plays an important role in several physiological functions, such as myelination or peripheral nerve repair. This renders the receptor an attractive pharmacological target. GPR126 is a mechano-sensor that translates the binding of extracellular matrix (ECM) molecules to its N terminus into a metabotropic intracellular signal. To date, the structural requirements and the character of the forces needed for this ECM-mediated receptor activation are largely unknown. In this study, we provide this information by combining classic second-messenger detection with single-cell atomic force microscopy. We established a monoclonal antibody targeting the N terminus to stimulate GPR126 and compared it to the activation through its known ECM ligands, collagen IV and laminin 211. As each ligand uses a distinct mode of action, the N terminus can be regarded as an allosteric module that can fine-tune receptor activation in a context-specific manner.

Regulatory T cells (Tregs) play an indispensable role in mediating peripheral tolerance to self-antigens. They can also promote tumor growth by suppressing the function of effector CD8+ and CD4+ T cells in the tumor microenvironment (TME). Furthermore, Tregs are identified as one of the key resistance mechanisms hampering the efficacy of immune checkpoint inhibitors (ICIs) across many tumor types. Therefore, there is a high need for safe and effective agents that would specifically deplete tumor-infiltrating Tregs while sparing both peripheral Tregs and effector T cells. Chemokine receptor 8 (CCR8) is predominantly expressed on activated tumor-infiltrating Tregs and marks the most suppressive and proliferative Treg population. CCR8+ Tregs are associated with high tumor grade and poor overall survival across many tumor types such as lung, breast, or head-neck cancer. Thus, unlike other approaches directed against Tregs, targeting CCR8 offers the opportunity to specifically deplete intra-tumoral Tregs without impacting peripheral Tregs or other immune cells. BAY 3375968 is a non-internalizing fully human glycoengineered (afucosylated) monoclonal IgG1 antibody, which in vitro selectively depleted human CCR8+ Tregs via antibody dependent cellular cytotoxicity (ADCC) and antibody dependent cellular phagocytosis (ADCP). In vivo efficacy studies using mouse surrogate antibodies showed strong monotherapeutic efficacy across a variety of murine tumor models with clear correlation of intratumoral CCR8+ Treg depletion and CD8+ T cell increase. The monotherapeutic efficacy of CCR8 depleting antibodies was further enhanced by combinations with ICIs. BAY 3375968 also showed a good safety profile in cynomolgus monkeys. In conclusion, CCR8 is a novel Treg depleting immunotherapy target, and due to its highly tumor-restricted expression profile, BAY 3375968 may provide superior clinical safety and efficacy profile comparing to other less specific Treg targeting approaches. Based on the promising pre-clinical data, preparations for a phase I clinical trial investigating the safety, tolerability, pharmacokinetics, pharmacodynamics, and preliminary anti-tumor activity of BAY 3375968 are underway. Citation Format: Helge Roider, Su-Yi Tseng, Sabine Hoff, Sandra Berndt, Katharina Filarski, Uwe Gritzan, Beatrix Stelte-Ludwig, Wiebke M. Nadler, Joanna Grudzinska-Goebel, Philipp Ellinger, Mark Trautwein, Matyas Gorjanacz. BAY 3375968: An afucosylated anti-CCR8 antibody depleting activated intratumoral regulatory T cells as a cancer immunotherapy [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 2866.

Antibody-drug conjugates (ADCs) are used to target cancer cells by means of antibodies directed to tumor-associated antigens, causing the incorporation of a cytotoxic payload into target cells. Here, we characterized the mode of action of ADC costing of a TWEAKR-specific monoclonal antibody conjugated to a small molecule kinesin spindle protein (KSP) inhibitor (KSPi). These TWEAKR-KSPi-ADCs showed strong efficacy in a TWEAKR expressing CT26 colon cancer model in mice. TWEAKR-KSPi-ADCs controlled the growth of CT26 colon cancers in immunodeficient as well as in immunocompetent mice. However, when treated with suboptimal doses, TWEAKR-KSPi-ADCs were still active in immunocompetent but not in immunodeficient mice, indicating that TWEAKR-KSPi-ADCs act - in addition to the cytotoxic mode of action - through an immunological mechanism. Indeed, in vitro experiments performed with a cell-permeable small molecule KSPi closely related to the active payload released from the TWEAKR-KSPi-ADCs revealed that KSPi was capable of stimulating several hallmarks of immunogenic cell death (ICD) on three different human cancer cell lines: cellular release of adenosine triphosphate (ATP) and high mobility group B1 protein (HMGB1), exposure of calreticulin on the cell surface as well as a transcriptional type-I interferon response. Further, in vivo experiments confirmed that treatment with TWEAKR-KSPi-ADCs activated immune responses via enhancing the infiltration of CD4+ and CD8+ T lymphocytes in tumors and the local production of interferon-γ, interleukin-2, and tumor necrosis factor-α. In conclusion, the antineoplastic effects of TWEAKR-KSPi-ADCs can partly be attributed to its ICD-stimulatory properties.

Inhibition of intracellular nicotinamide phosphoribosyltransferase (NAMPT) represents a new mode of action for cancer-targeting antibody-drug conjugates (ADCs) with activity also in slowly proliferating cells. To extend the repertoire of available effector chemistries, we have developed a novel structural class of NAMPT inhibitors as ADC payloads. A structure-activity relationship-driven approach supported by protein structural information was pursued to identify a suitable attachment point for the linker to connect the NAMPT inhibitor with the antibody. Optimization of scaffolds and linker structures led to highly potent effector chemistries which were conjugated to antibodies targeting C4.4a (LYPD3), HER2 (c-erbB2), or B7H3 (CD276) and tested on antigen-positive and -negative cancer cell lines. Pharmacokinetic studies, including metabolite profiling, were performed to optimize the stability and selectivity of the ADCs and to evaluate potential bystander effects. Optimized NAMPTi-ADCs demonstrated potent in vivo antitumor efficacy in target antigen-expressing xenograft mouse models. This led to the development of highly potent NAMPT inhibitor ADCs with a very good selectivity profile compared with the corresponding isotype control ADCs. Moreover, we demonstrate─to our knowledge for the first time─the generation of NAMPTi payload metabolites from the NAMPTi-ADCs in vitro and in vivo. In conclusion, NAMPTi-ADCs represent an attractive new payload class designed for use in ADCs for the treatment of solid and hematological cancers.

Background Targeted thorium-227 conjugates (TTCs) are an emerging class of targeted alpha therapies (TATs). Their unique mode of action (MoA) is the induction of difficult-to-repair clustered DNA double-strand breaks. However, thus far, their effects on the immune system are largely unknown. Here, we investigated the immunostimulatory effects of the mesothelin-targeted thorium-227 conjugate (MSLN-TTC) in vitro and in vivo in monotherapy and in combination with an inhibitor of the immune checkpoint programmed death receptor ligand 1 (PD-L1) in immunocompetent mice. Methods The murine cell line MC38 was transfected with the human gene encoding for MSLN (hMSLN) to enable binding of the non-cross-reactive MSLN-TTC. The immunostimulatory effects of MSLN-TTC were studied in vitro on human cancer cell lines and MC38-hMSLN cells. The efficacy and MoA of MSLN-TTC were studied in vivo as monotherapy or in combination with anti-PD-L1 in MC38-hMSLN tumor-bearing immunocompetent C57BL/6 mice. Experiments were supported by RNA sequencing, flow cytometry, immunohistochemistry, mesoscale, and TaqMan PCR analyses to study the underlying immunostimulatory effects. In vivo depletion of CD8+ T cells and studies with Rag2/Il2Rg double knockout C57BL/6 mice were conducted to investigate the importance of immune cells to the efficacy of MSLN-TTC. Results MSLN-TTC treatment induced upregulation of DNA sensing pathway transcripts (IL-6, CCL20, CXCL10, and stimulator of interferon genes (STING)-related genes) in vitro as determined by RNASeq analysis. The results, including phospho-STING activation, were confirmed on the protein level. Danger-associated molecular pattern molecules were upregulated in parallel, leading to dendritic cell (DC) activation in vitro. MSLN-TTC showed strong antitumor activity (T:C 0.38, p<0.05) as a single agent in human MSLN-expressing MC38 tumor-bearing immunocompetent mice. Combining MSLN-TTC with anti-PD-L1 further enhanced the efficacy (T:C 0.08, p<0.001) as evidenced by the increased number of tumor-free surviving animals. MSLN-TTC monotherapy caused migration of CD103+ cDC1 DCs and infiltration of CD8+ T cells into tumors, which was enhanced on combination with anti-PD-L1. Intriguingly, CD8+ T-cell depletion decreased antitumor efficacy. Conclusions These in vitro and in vivo data on MSLN-TTC demonstrate that the MoA of TTCs involves activation of the immune system. The findings are of relevance for other targeted radiotherapies and may guide clinical combination strategies.

G protein coupled receptors signal through G proteins or arrestins. A long-standing mystery in the field is why vertebrates have two non-visual arrestins, arrestin-2 and arrestin-3. These isoforms are ~75% identical and 85% similar; each binds numerous receptors, and appear to have many redundant functions, as demonstrated by studies of knockout mice. We previously showed that arrestin-3 can be activated by inositol-hexakisphosphate (IP6). IP6 interacts with the receptor-binding surface of arrestin-3, induces arrestin-3 oligomerization, and this oligomer stabilizes the active conformation of arrestin-3. Here, we compared the impact of IP6 on oligomerization and conformational equilibrium of the highly homologous arrestin-2 and arrestin-3 and found that these two isoforms are regulated differently. In the presence of IP6, arrestin-2 forms "infinite" chains, where each promoter remains in the basal conformation. In contrast, full length and truncated arrestin-3 form trimers and higher-order oligomers in the presence of IP6; we showed previously that trimeric state induces arrestin-3 activation (Chen et al., 2017). Thus, in response to IP6, the two non-visual arrestins oligomerize in different ways in distinct conformations. We identified an insertion of eight residues that is conserved across arrestin-2 homologs, but absent in arrestin-3 that likely accounts for the differences in the IP6 effect. Because IP6 is ubiquitously present in cells, this suggests physiological consequences, including differences in arrestin-2/3 trafficking and JNK3 activation. The functional differences between two non-visual arrestins are in part determined by distinct modes of their oligomerization. The mode of oligomerization might regulate the function of other signaling proteins.

Immune checkpoint blockade using antibodies targeting the cell surface expressed proteins CTLA-4, PD-1 and PD-L1 has revolutionized cancer care and its clinical impact in several indications has prompted a search for complementary immunostimulatory approaches that can further increase the efficacy of these drugs. Mitogen-activated protein kinase kinase kinase kinase 1 (MAP4K1; HPK1), a serine/threonine kinase expressed exclusively in hematopoietic cell lineages, mediates a negative feedback signal downstream of T-cell receptor stimulation. Its activity is enhanced by PGE2 and TGFβ, factors commonly present in the tumor microenvironment. Mice deficient for MAP4K1 or expressing a kinase-dead variant of MAP4K1 exhibit enhanced T-cell function, including increased anti-tumor immunity. We developed the small molecule inhibitor BAY-405 that displays potent nanomolar MAP4K1 inhibition in biochemical and cellular assays, good kinase selectivity, and in vivo exposure after oral dosing. Pharmacological inhibition of MAP4K1 enhances T-cell immunity and overcomes the suppressive impact of PGE2, TGFβ and CD4+ T-regulatory cells. Single agent treatment of tumor-bearing mice results in suppression of tumor outgrowth in several syngeneic models. This is accompanied by an increase in the anti-tumor T-cell response, dependent on an intact T-cell compartment, while not involving direct anti-tumor cytotoxicity. Inhibition of MAP4K1 in conjunction with PD-L1 blockade results in further suppression of tumor outgrowth. Moreover, we found that MAP4K1 is expressed in both PD-L1-high and PD-L1-low human cancers. In summary, our data show that selective inhibition of MAP4K1 by means of small molecule drugs may be used to expand the patient population responding to immune checkpoint inhibition. Citation Format: Gabriele Leder, Rafael Carretero, Jeffrey Mowat, Sandra Berndt, Roland Neuhaus, Nuria Aiguabella Font, Ulf Boemer, Oliver von Ahsen, Uwe Eberspaecher, Judith Guenther, Mareike Grees, Corinna Link, Barbara Nicke, Daniel Baumann, Martina Schaefer, Mine Oezcan-Wahlbrink, Nicolas D. Werbeck, Ingo Hartung, Bertolt Kreft, Rienk Offringa. Enhancement of anti-tumor T-cell immunity by means of an oral small molecule targeting the intracellular immune checkpoint MAP4K1 [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1722.

Src family kinases (SFKs) are key regulators of cell proliferation, differentiation, and survival. The expression of these non-receptor tyrosine kinases is strongly correlated with cancer development and tumor progression. Thus, this family of proteins serves as an attractive drug target. The activation of SFKs can occur via multiple signaling pathways, yet many of them are poorly understood. Here, we summarize the current knowledge on G protein-coupled receptor (GPCR)-mediated regulation of SFKs, which is of considerable interest because GPCRs are among the most widely used pharmaceutical targets. This type of activation can occur through a direct interaction between the two proteins or be allosterically regulated by arrestins and G proteins. We postulate that a rearrangement of binding motifs within the active conformation of arrestin-3 mediates Src regulation by comparison of available crystal structures. Therefore, we hypothesize a potentially different activation mechanism compared to arrestin-2. Furthermore, we discuss the probable direct regulation of SFK by GPCRs and investigate the intracellular domains of exemplary GPCRs with conserved polyproline binding motifs that might serve as scaffolding domains to allow such a direct interaction. Large intracellular domains in GPCRs are often understudied and, in general, not much is known of their contribution to different signaling pathways. The suggested direct interaction between a GPCR and a SFK could allow for a potential immediate allosteric regulation of SFKs by GPCRs and thereby unravel a novel mechanism of SFK signaling. This overview will help to identify new GPCR–SFK interactions, which could serve to explain biological functions or be used to modulate downstream effectors.

4 5 Jakob Mitgau*1, Julius Franke*1, Camilla Schinner2, Gabriele Stephan3, Sandra Berndt1, 6 Dimitris G. Placantonakis3,4, Hermann Kalwa5, Volker Spindler2, Caroline Wilde1 and Ines 7 Liebscher#1 8 9 1Rudolf Schönheimer Institute for Biochemistry, Molecular Biochemistry, University of 10 Leipzig, Leipzig, Germany 11 2Department of Biomedicine, University of Basel, Basel, Switzerland 12 3Department of Neurosurgery, NYU Grossman School of Medicine, New York, NY 10016, 13 USA 14 4Kimmel Center for Stem Cell Biology, Laura and Isaac Perlmutter Cancer Center, NYU 15 Grossman School of Medicine, New York, NY 10016, USA 16 5Rudolf-Boehm-Institute for Pharmacology and Toxicology, University of Leipzig, Leipzig, 17 Germany 18 19 *contributed equally 20 #Corresponding author: Ines Liebscher 21 E-mail: ines.liebscher@medizin.uni-leipzig.de. 22 23 Running title: Antibodies mimicking mechano-activation of GPR126/ADGRG6 24 25

Arrestins regulate a wide range of signaling events, most notably when bound to active G protein-coupled receptors (GPCRs). Among the known effectors recruited by GPCR-bound arrestins are Src family kinases, which regulate cellular growth and proliferation. Here, we focus on arrestin-3 interactions with Fgr kinase, a member of the Src family. Previous reports demonstrated that Fgr exhibits high constitutive activity, but can be further activated by both arrestin-dependent and arrestin-independent pathways. We report that arrestin-3 modulates Fgr activity with a hallmark bell-shaped concentration-dependence, consistent with a role as a signaling scaffold. We further demonstrate using NMR spectroscopy that a polyproline motif within arrestin-3 interacts directly with the SH3 domain of Fgr. To provide a framework for this interaction, we determined the crystal structure of the Fgr SH3 domain at 1.9 Å resolution and developed a model for the GPCR-arrestin-3-Fgr complex that is supported by mutagenesis. This model suggests that Fgr interacts with arrestin-3 at multiple sites and is consistent with the locations of disease-associated Fgr mutations. Collectively, these studies provide a structural framework for arrestin-dependent activation of Fgr.

Arrestins are multifunctional proteins that were originally identified as suppressors of G protein‐coupled receptors signaling. Later arrestins were found to serve as adapter molecules that bind numerous signaling proteins, such as protein kinases, ubiquitin ligases, deubiquitinases, etc. Here, we showed the direct interaction with arrestin‐3 and parkin, which is a ubiquitin E3 ligase that covalently attaches ubiquitin to specific substrates, including mitochondrial proteins. Parkin is essential for mitophagy – the removal of damaged mitochondria. Many mutations in parkin are linked to familial or sporadic Parkinson’s disease. We identified several binding hot spots on both arrestin‐3 and Parkin using high throughput peptide arrays. Some pathogenic parkin mutations map to these arrestin‐binding hot spots. One of the pathogenic mutations, R275W, significantly increased parkin binding to arrestin‐3 in cells, as compared to wild type parkin. Our data indicate that arrestin‐3 may participate in parkin‐mediated physiological processes that likely contribute to Parkinson’s disease.Support or Funding InformationThis work was supported by grant R35 GM122491 from the National Institutes of Health to VVG.