Beyond the Ligand: Extracellular and Transcellular G Protein-Coupled Receptor Complexes in Physiology and Pharmacology

ELFN1 is a new extracellular matrix (ECM)-associated protein

AIMS

The ELFN1, discovered in 2007, is a single-pass transmembrane protein. Studies conducted thus far to elucidate the function of the Elfn1 have been limited only to animal studies. These studies have reported that ELFN1 is a universal binding partner of metabotropic glutamate receptors (mGluRs) in the central nervous system and its functional deficiency has been associated with the pathogenesis of neurological and neuropsychiatric diseases. In 2021, we described the first disease-associated human ELFN1 pathogenic gene mutation. Severe joint laxity, which was the most striking finding of this new disease and was clearly seen in the patients since early infancy, showed that the ELFN1 may have a possible function in the connective tissue besides the nervous system. Here, we present the first experimental evidence of the extracellular matrix (ECM)-related function of the ELFN1.

MATERIALS AND METHODS

Primary skin fibroblasts were isolated from the skin biopsies of ELFN1 mutated patients and healthy foreskin donors. For the clinical trial in a dish, in vitro ECM and DEM (decellularized ECM) models were created from skin fibroblasts. All the in vitro models were comparatively characterized and analyzed.

KEY FINDINGS

The mutation in the ELFN1 signal peptide region of patients resulted in a severe lack of ELFN1 expression and dramatically altered the characteristic morphology and behavior (growth, proliferation, and motility) of fibroblasts.

SIGNIFICANCE

We propose that ELFN1 is involved in the cell-ECM attachment, and its deficiency is critical enough to cause a loss of cell motility and soft ECM stiffness.

Decrease in GPSM2 mediated by the natural product luteolin contributes to colon adenocarcinoma treatment and increases the sensitivity to fluorouracil

Luteolin, a monomeric substance, is a natural product of the Brucea javanica (BJ) plant. Brucea javanica oil emulsion injection (BJOEI) is a proprietary Chinese medicine purified from BJ that is widely used clinically as an anti-tumor treatment. Although a growing body of research suggests that luteolin and BJOEI have anti-tumor effects, the molecular mechanism of action has not been fully elucidated. In this study, through molecular docking technology, we found that luteolin can interact directly with GPSM2 and regulate the FoxO signaling pathway through GPSM2. In addition, the inhibitory effect of luteolin on colon adenocarcinoma (COAD) cells was found to be offset by knockdown of GPSM2. In contrast, the anti-proliferative effects of luteolin could be notably reversed by overexpression of GPSM2. The results reveal that GPSM2 is crucial in luteolin-mediated anti-proliferative effects. The mediation of anti-proliferative effects by GPSM2 has also been indirectly demonstrated in RKO and SW480 xenograft mice models. In addition, we verified that BJOEI inhibits the progression of COAD by mediating GPSM2 and regulating the FoxO signaling pathway. We also found that BJOEI achieved a better anti-tumor effect when combined with fluorouracil injection. Collectively, our data show that the anti-tumor effects of BJOEI and luteolin on COAD are GPSM2-dependent and downregulating the expression of GPSM2 to regulate the FoxO signaling pathway may be an effective way to treat COAD.

Heterodimers Revolutionize the Field of Metabotropic Glutamate Receptors

Identified 40 years ago, the metabotropic glutamate (mGlu) receptors play key roles in modulating many synapses in the brain, and are still considered as important drug targets to treat various brain diseases. Eight genes encoding mGlu subunits have been identified. They code for complex receptors composed of a large extracellular domain where glutamate binds, connected to a G protein activating membrane domain. They are covalently linked dimers, a quaternary structure needed for their activation by glutamate. For many years they have only been considered as homodimers, then limiting the number of mGlu receptors to 8 subtypes composed of twice the same subunit. Twelve years ago, mGlu subunits were shown to also form heterodimers with specific subunits combinations, increasing the family up to 19 different potential dimeric receptors. Since then, a number of studies brought evidence for the existence of such heterodimers in the brain, through various approaches. Structural and molecular dynamic studies helped understand their peculiar activation process. The present review summarizes the approaches used to study their activation process and their pharmacological properties and to demonstrate their existence in vivo. We will highlight how the existence of mGlu heterodimers revolutionizes the mGlu receptor field, opening new possibilities for therapeutic intervention for brain diseases. As illustrated by the number of possible mGlu heterodimers, this study will highlight the need for further research to fully understand their role in physiological and pathological conditions, and to develop more specific therapeutic tools.

Emerging modes of regulation of neuromodulatory G protein-coupled receptors

In the nervous system, G protein-coupled receptors (GPCRs) control neuronal excitability, synaptic transmission, synaptic plasticity, and, ultimately, behavior through spatiotemporally precise initiation of a variety of signaling pathways. However, despite their critical importance, there is incomplete understanding of how these receptors are regulated to tune their signaling to specific neurophysiological contexts. A deeper mechanistic picture of neuromodulatory GPCR function is needed to fully decipher their biological roles and effectively harness them for the treatment of neurological and psychiatric disorders. In this review, we highlight recent progress in identifying novel modes of regulation of neuromodulatory GPCRs, including G protein- and receptor-targeting mechanisms, receptor-receptor crosstalk, and unique features that emerge in the context of chemical synapses. These emerging principles of neuromodulatory GPCR tuning raise critical questions to be tackled at the molecular, cellular, synaptic, and neural circuit levels in the future.

Presynaptic nanoscale components of retrograde synaptic signaling

While our understanding of the nanoscale architecture of anterograde synaptic transmission is rapidly expanding, the qualitative and quantitative molecular principles underlying distinct mechanisms of retrograde synaptic communication remain elusive. We show that a particular form of tonic cannabinoid signaling is essential for setting target cell-dependent synaptic variability. It does not require the activity of the two major endocannabinoid-producing enzymes. Instead, by developing a workflow for physiological, anatomical, and molecular measurements at the same unitary synapse, we demonstrate that the nanoscale stoichiometric ratio of type 1 cannabinoid receptors (CB1Rs) to the release machinery is sufficient to predict synapse-specific release probability. Accordingly, selective decrease of extrasynaptic CB1Rs does not affect synaptic transmission, whereas in vivo exposure to the phytocannabinoid Δ9-tetrahydrocannabinol disrupts the intrasynaptic nanoscale stoichiometry and reduces synaptic variability. These findings imply that synapses leverage the nanoscale stoichiometry of presynaptic receptor coupling to the release machinery to establish synaptic strength in a target cell-dependent manner.

Possible contributions of fibrogenesis to recurrent miscarriages - A transcriptome analysis

Recurrent miscarriages (RM) generally refer to two or more consecutive pregnancy losses. The risk of miscarriages grows with its frequency of occurrences, so as the future obstetric complications or longer-term health problems for patients. Most previous researches sought to discover the etiology of RM by making comparisons between patients with RM and fertile women. Our study collected decidua tissues from patients with RM and single miscarriage (SM) for transcriptome sequencing analysis and aimed at identifying vital factors contributing to additional miscarriages after previous miscarriage. Between the RM and SM group, a total of 122 differentially expressed genes (DEGs) were detected and pathways associated with cell adhesion and ECM remodeling were particularly enriched in the RM group, which indicated abnormally activated fibrogenesis process. Particularly, the enhancement of ITGB6, EGFLAM and COL3A1 in the RM group were validated by RT-qPCR. Our study discovered that fibrogenesis, which might be caused by intrauterine manipulation, could lead to recurrent miscarriages after a previous miscarriage. Therefore, we encourage higher attention to thorough prevention and prompt remedies towards fibrotic disorders related diseases.

G protein-coupled receptors (GPCRs): advances in structures, mechanisms, and drug discovery

G protein-coupled receptors (GPCRs), the largest family of human membrane proteins and an important class of drug targets, play a role in maintaining numerous physiological processes. Agonist or antagonist, orthosteric effects or allosteric effects, and biased signaling or balanced signaling, characterize the complexity of GPCR dynamic features. In this study, we first review the structural advancements, activation mechanisms, and functional diversity of GPCRs. We then focus on GPCR drug discovery by revealing the detailed drug-target interactions and the underlying mechanisms of orthosteric drugs approved by the US Food and Drug Administration in the past five years. Particularly, an up-to-date analysis is performed on available GPCR structures complexed with synthetic small-molecule allosteric modulators to elucidate key receptor-ligand interactions and allosteric mechanisms. Finally, we highlight how the widespread GPCR-druggable allosteric sites can guide structure- or mechanism-based drug design and propose prospects of designing bitopic ligands for the future therapeutic potential of targeting this receptor family.

Domain coupling in activation of a family C GPCR

The G protein-coupled metabotropic glutamate receptors form homodimers and heterodimers with highly diverse responses to glutamate and varying physiological function. The molecular basis for this diversity remains poorly delineated. We employ molecular dynamics, single-molecule spectroscopy, and hydrogen-deuterium exchange to dissect the pathway of activation triggered by glutamate. We find that activation entails multiple loosely coupled steps and identify a novel pre-active intermediate whose transition to the active state forms dimer interactions that set signaling efficacy. Such subunit interactions generate functional diversity that differs across homodimers and heterodimers. The agonist-bound receptor is remarkably dynamic, with low occupancy of G protein-coupling conformations, providing considerable headroom for modulation of the landscape by allosteric ligands. Sites of sequence diversity within the dimerization interface and diverse coupling between activation rearrangements may contribute to precise decoding of glutamate signals and transients over broad spatial and temporal scales.

Targeting mGluR group III for the treatment of neurodegenerative diseases

Glutamate, an excitatory neurotransmitter, is essential for neuronal function, and it acts on ionotropic or metabotropic glutamate receptors (mGluRs). A disturbance in glutamatergic signaling is a hallmark of many neurodegenerative diseases. Developing disease-modifying treatments for neurodegenerative diseases targeting glutamate receptors is a promising avenue. The understudied group III mGluR 4, 6-8 are commonly found in the presynaptic membrane, and their activation inhibits glutamate release. Thus, targeted mGluRs therapies could aid in treating neurodegenerative diseases. This review describes group III mGluRs and their pharmacological ligands in the context of amyotrophic lateral sclerosis, Parkinson's, Alzheimer's, and Huntington's diseases. Attempts to evaluate the efficacy of these drugs in clinical trials are also discussed. Despite a growing list of group III mGluR-specific pharmacological ligands, research on the use of these drugs in neurodegenerative diseases is limited, except for Parkinson's disease. Future efforts should focus on delineating the contribution of group III mGluR to neurodegeneration and developing novel ligands with superior efficacy and a favorable side effect profile for the treatment of neurodegenerative diseases.

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.

Structure of the photoreceptor synaptic assembly of the extracellular matrix protein pikachurin with the orphan receptor GPR179

Precise synapse formation is essential for normal functioning of the nervous system. Retinal photoreceptors establish selective contacts with bipolar cells, aligning the neurotransmitter release apparatus with postsynaptic signaling cascades. This involves transsynaptic assembly between the dystroglycan-dystrophin complex on the photoreceptor and the orphan receptor GPR179 on the bipolar cell, which is mediated by the extracellular matrix protein pikachurin (also known as EGFLAM). This complex plays a critical role in the synaptic organization of photoreceptors and signal transmission, and mutations affecting its components cause blinding disorders in humans. Here, we investigated the structural organization and molecular mechanisms by which pikachurin orchestrates transsynaptic assembly and solved structures of the human pikachurin domains by x-ray crystallography and of the GPR179-pikachurin complex by single-particle, cryo-electron microscopy. The structures reveal molecular recognition principles of pikachurin by the Cache domains of GPR179 and show how the interaction is involved in the transsynaptic alignment of the signaling machinery. Together, these data provide a structural basis for understanding the synaptic organization of photoreceptors and ocular pathology.

Mechanosensitive membrane proteins: Usual and unusual suspects in mediating mechanotransduction

This Viewpoint, which accompanies a Special Issue focusing on membrane mechanosensors, discusses unifying and unique features of both established and emerging mechanosensitive (MS) membrane proteins, their distribution across protein families and phyla, and current and future challenges in the study of these important proteins and their partners. MS membrane proteins are essential for tissue development, cellular motion, osmotic homeostasis, and sensing external and self-generated mechanical cues like those responsible for touch and proprioception. Though researchers' attention and this Viewpoint focus on a few famous ion channels that are considered the usual suspects as MS mechanosensors, we also discuss some of the more unusual suspects, such as G-protein coupled receptors. As the field continues to grow, so too will the list of proteins suspected to function as mechanosensors and the diversity of known MS membrane proteins.

Probing the orphan receptors: Tools and directions

The endogenous ligands activating a large fraction of the G Protein Coupled Receptor (GPCR) family members have yet to be identified. These receptors are commonly labeled as orphans (oGPCRs), and because of the absence of available pharmacological tools they are currently understudied. Nonetheless, genome wide association studies, together with research using animal models identified many physiological functions regulated by oGPCRs. Similarly, mutations in some oGPCRs have been associated with rare genetic disorders or with an increased risk of developing pathologies. The once underestimated pharmacological potential of targeting oGPCRs is increasingly being exploited by the development of novel tools to understand their biology and by drug discovery endeavors aimed at identifying new modulators of their activity. Here, we summarize recent advancements in the field of oGPCRs and future directions.

mGluR5 is transiently confined in perisynaptic nanodomains to shape synaptic function

The unique perisynaptic distribution of postsynaptic metabotropic glutamate receptors (mGluRs) at excitatory synapses is predicted to directly shape synaptic function, but mechanistic insight into how this distribution is regulated and impacts synaptic signaling is lacking. We used live-cell and super-resolution imaging approaches, and developed molecular tools to resolve and acutely manipulate the dynamic nanoscale distribution of mGluR5. Here we show that mGluR5 is dynamically organized in perisynaptic nanodomains that localize close to, but not in the synapse. The C-terminal domain of mGluR5 critically controlled perisynaptic confinement and prevented synaptic entry. We developed an inducible interaction system to overcome synaptic exclusion of mGluR5 and investigate the impact on synaptic function. We found that mGluR5 recruitment to the synapse acutely increased synaptic calcium responses. Altogether, we propose that transient confinement of mGluR5 in perisynaptic nanodomains allows flexible modulation of synaptic function.

The GPCR properties of polycystin-1- A new paradigm

Polycystin-1 (PC1) is an 11-transmembrane (TM) domain-containing protein encoded by the PKD1 gene, the most frequently mutated gene leading to autosomal dominant polycystic kidney disease (ADPKD). This large (> 462 kDal) protein has a complex posttranslational maturation process, with over five proteolytic cleavages having been described, and is found at multiple cellular locations. The initial description of the binding and activation of heterotrimeric Gαi/o by the juxtamembrane region of the PC1 cytosolic C-terminal tail (C-tail) more than 20 years ago opened the door to investigations, and controversies, into PC1's potential function as a novel G protein-coupled receptor (GPCR). Subsequent biochemical and cellular-based assays supported an ability of the PC1 C-tail to bind numerous members of the Gα protein family and to either inhibit or activate G protein-dependent pathways involved in the regulation of ion channel activity, transcription factor activation, and apoptosis. More recent work has demonstrated an essential role for PC1-mediated G protein regulation in preventing kidney cyst development; however, the mechanisms by which PC1 regulates G protein activity continue to be discovered. Similarities between PC1 and the adhesion class of 7-TM GPCRs, most notably a conserved GPCR proteolysis site (GPS) before the first TM domain, which undergoes autocatalyzed proteolytic cleavage, suggest potential mechanisms for PC1-mediated regulation of G protein signaling. This article reviews the evidence supporting GPCR-like functions of PC1 and their relevance to cystic disease, discusses the involvement of GPS cleavage and potential ligands in regulating PC1 GPCR function, and explores potential connections between PC1 GPCR-like activity and regulation of the channel properties of the polycystin receptor-channel complex.

Functions of G protein-coupled receptor 56 in health and disease

Human G protein-coupled receptor 56 (GPR56) is encoded by gene ADGRG1 from chromosome 16q21 and is homologously encoded in mice, at chromosome 8. Both 687 and 693 splice forms are present in humans and mice. GPR56 has a 381 amino acid-long N-terminal extracellular segment and a GPCR proteolysis site upstream from the first transmembrane domain. GPR56 is mainly expressed in the heart, brain, thyroid, platelets, and peripheral blood mononuclear cells. Accumulating evidence indicates that GPR56 promotes the formation of myelin sheaths and the development of oligodendrocytes in the cerebral cortex of the central nervous system. Moreover, GPR56 contributes to the development and differentiation of hematopoietic stem cells, induces adipogenesis, and regulates the function of immune cells. The lack of GPR56 leads to nervous system dysfunction, platelet disorders, and infertility. Abnormal expression of GPR56 is related to the malignant transformation and tumor metastasis of several cancers including melanoma, neuroglioma, and gastrointestinal cancer. Metabolic disorders and cardiovascular diseases are also associated with dysregulation of GPR56 expression, and GPR56 is involved in the pharmacological resistance to some antidepressant and cancer drug treatments. In this review, the molecular structure, expression profile, and signal transduction of GPR56 are introduced, and physiological and pathological functions of GRP56 are comprehensively summarized. Attributing to its significant biological functions and its long N-terminal extracellular region that interacts with multiple ligands, GPR56 is becoming an attractive therapeutic target in treating neurological and hematopoietic diseases.

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.

Genotype by environment interactions for chronic wasting disease in farmed US white-tailed deer

Despite implementation of enhanced management practices, chronic wasting disease in US white-tailed deer (Odocoileus virginianus) continues to expand geographically. Herein, we perform the largest genome-wide association analysis to date for chronic wasting disease (n = 412 chronic wasting disease-positive; n = 758 chronic wasting disease-nondetect) using a custom Affymetrix Axiom single-nucleotide polymorphism array (n = 121,010 single-nucleotide polymorphisms), and confirm that differential susceptibility to chronic wasting disease is a highly heritable (h2= 0.611 ± 0.056) polygenic trait in farmed US white-tailed deer, but with greater trait complexity than previously appreciated. We also confirm PRNP codon 96 (G96S) as having the largest-effects on risk (P ≤ 3.19E-08; phenotypic variance explained ≥ 0.025) across 3 US regions (Northeast, Midwest, South). However, 20 chronic wasting disease-positive white-tailed deer possessing codon 96SS genotypes were also observed, including one that was lymph node and obex positive. Beyond PRNP, we also detected 23 significant single-nucleotide polymorphisms (P-value ≤ 5E-05) implicating ≥24 positional candidate genes; many of which have been directly implicated in Parkinson’s, Alzheimer’s and prion diseases. Genotype-by-environment interaction genome-wide association analysis revealed a single-nucleotide polymorphism in the lysosomal enzyme gene ARSB as having the most significant regional heterogeneity of effects on chronic wasting disease (P ≤ 3.20E-06); with increasing copy number of the minor allele increasing susceptibility to chronic wasting disease in the Northeast and Midwest; but with opposite effects in the South. In addition to ARSB, 38 significant genotype-by-environment single-nucleotide polymorphisms (P-value ≤ 5E-05) were also detected, thereby implicating ≥ 36 positional candidate genes; the majority of which have also been associated with aspects of Parkinson’s, Alzheimer’s, and prion diseases.

Subsynaptic mobility of presynaptic mGluR types is differentially regulated by intra- and extracellular interactions

Presynaptic metabotropic glutamate receptors (mGluRs) are essential for the control of synaptic transmission. However, how the subsynaptic dynamics of these receptors is controlled and contributes to synaptic signaling remain poorly understood quantitatively. Particularly, since the affinity of individual mGluR subtypes for glutamate differs considerably, the activation of mGluR subtypes critically depends on their precise subsynaptic distribution. Here, using superresolution microscopy and single-molecule tracking, we unravel novel molecular mechanisms that control the nanoscale distribution and mobility of presynaptic mGluRs in hippocampal neurons. We demonstrate that the high-affinity group II receptor mGluR2 localizes diffusely along the axon, and is highly mobile, while the low-affinity group III receptor mGluR7 is stably anchored at the active zone. We demonstrate that intracellular interactions modulate surface diffusion of mGluR2, while immobilization of mGluR7 at the active zone relies on its extracellular domain. Receptor activation or increases in synaptic activity do not alter the surface mobility of presynaptic mGluRs. Finally, computational modeling of presynaptic mGluR activity revealed that this particular nanoscale arrangement directly impacts their ability to modulate neurotransmitter release. Altogether, this study demonstrates that distinct mechanisms control surface mobility of presynaptic mGluRs to contribute differentially to glutamatergic synaptic transmission.

Research Status of the Orphan G Protein Coupled Receptor 158 and Future Perspectives

G-protein-coupled receptors (GPCRs) remain one of the most successful targets for therapeutic drugs approved by the US Food and Drug Administration (FDA). Many novel orphan GPCRs have been identified by human genome sequencing and considered as putative targets for refractory diseases. Of note, a series of studies have been carried out involving GPCR 158 (or GPR158) since its identification in 2005, predominantly focusing on the characterization of its roles in the progression of cancer and mental illness. However, advances towards an in-depth understanding of the biological mechanism(s) involved for clinical application of GPR158 are lacking. In this paper, we clarify the origin of the GPR158 evolution in different species and summarize the relationship between GPR158 and different diseases towards potential drug target identification, through an analysis of the sequences and substructures of GPR158. Further, we discuss how recent studies set about unraveling the fundamental features and principles, followed by future perspectives and thoughts, which may lead to prospective therapies involving GPR158.

Simple but efficacious enrichment of integral membrane proteins and their interactions for in-depth membrane proteomics

Membrane proteins play essential roles in various cellular processes, such as nutrient transport, bioenergetic processes, cell adhesion, and signal transduction. Proteomics is one of the key approaches to exploring membrane proteins comprehensively. Bottom–up proteomics using LC–MS/MS has been widely used in membrane proteomics. However, the low abundance and hydrophobic features of membrane proteins, especially integral membrane proteins, make it difficult to handle the proteins and are the bottleneck for identification by LC–MS/MS. Herein, to improve the identification and quantification of membrane proteins, we have stepwisely evaluated methods of membrane enrichment for the sample preparation. The enrichment methods of membranes consisted of precipitation by ultracentrifugation and treatment by urea or alkaline solutions. The best enrichment method in the study, washing with urea after isolation of the membranes, resulted in the identification of almost twice as many membrane proteins compared with samples without the enrichment. Notably, the method significantly enhances the identified numbers of multispanning transmembrane proteins, such as solute carrier transporters, ABC transporters, and G-protein–coupled receptors, by almost sixfold. Using this method, we revealed the profiles of amino acid transport systems with the validation by functional assays and found more protein–protein interactions, including membrane protein complexes and clusters. Our protocol uses standard procedures in biochemistry, but the method was efficient for the in-depth analysis of membrane proteome in a wide range of samples.

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Fractionation of membranes improves the identification of membrane proteins.

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Membranes washed with urea or alkaline increase identified transmembrane proteins.

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Urea wash increases the detection of multispanning transmembrane proteins.

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Proteomics of urea-washed membranes keeps more protein–protein interactions.

Membrane trafficking and positioning of mGluRs at presynaptic and postsynaptic sites of excitatory synapses

The plethora of functions of glutamate in the brain are mediated by the complementary actions of ionotropic and metabotropic glutamate receptors (mGluRs). The ionotropic glutamate receptors carry most of the fast excitatory transmission, while mGluRs modulate transmission on longer timescales by triggering multiple intracellular signaling pathways. As such, mGluRs mediate critical aspects of synaptic transmission and plasticity. Interestingly, at synapses, mGluRs operate at both sides of the cleft, and thus bidirectionally exert the effects of glutamate. At postsynaptic sites, group I mGluRs act to modulate excitability and plasticity. At presynaptic sites, group II and III mGluRs act as auto-receptors, modulating release properties in an activity-dependent manner. Thus, synaptic mGluRs are essential signal integrators that functionally couple presynaptic and postsynaptic mechanisms of transmission and plasticity. Understanding how these receptors reach the membrane and are positioned relative to the presynaptic glutamate release site are therefore important aspects of synapse biology. In this review, we will discuss the currently known mechanisms underlying the trafficking and positioning of mGluRs at and around synapses, and how these mechanisms contribute to synaptic functioning. We will highlight outstanding questions and present an outlook on how recent technological developments will move this exciting research field forward.

Mechanisms Underlying Target Selectivity for Cell Types and Subcellular Domains in Developing Neocortical Circuits

The cerebral cortex contains numerous neuronal cell types, distinguished by their molecular identity as well as their electrophysiological and morphological properties. Cortical function is reliant on stereotyped patterns of synaptic connectivity and synaptic function among these neuron types, but how these patterns are established during development remains poorly understood. Selective targeting not only of different cell types but also of distinct postsynaptic neuronal domains occurs in many brain circuits and is directed by multiple mechanisms. These mechanisms include the regulation of axonal and dendritic guidance and fine-scale morphogenesis of pre- and postsynaptic processes, lineage relationships, activity dependent mechanisms and intercellular molecular determinants such as transmembrane and secreted molecules, many of which have also been implicated in neurodevelopmental disorders. However, many studies of synaptic targeting have focused on circuits in which neuronal processes target different lamina, such that cell-type-biased connectivity may be confounded with mechanisms of laminar specificity. In the cerebral cortex, each cortical layer contains cell bodies and processes from intermingled neuronal cell types, an arrangement that presents a challenge for the development of target-selective synapse formation. Here, we address progress and future directions in the study of cell-type-biased synaptic targeting in the cerebral cortex. We highlight challenges to identifying developmental mechanisms generating stereotyped patterns of intracortical connectivity, recent developments in uncovering the determinants of synaptic target selection during cortical synapse formation, and current gaps in the understanding of cortical synapse specificity.

The complement cascade in the regulation of neuroinflammation, nociceptive sensitization, and pain

The complement cascade is a key component of the innate immune system that is rapidly recruited through a cascade of enzymatic reactions to enable the recognition and clearance of pathogens and promote tissue repair. Despite its well-understood role in immunology, recent studies have highlighted new and unexpected roles of the complement cascade in neuroimmune interaction and in the regulation of neuronal processes during development, aging, and in disease states. Complement signaling is particularly important in directing neuronal responses to tissue injury, neurotrauma, and nerve lesions. Under physiological conditions, complement-dependent changes in neuronal excitability, synaptic strength, and neurite remodeling promote nerve regeneration, tissue repair, and healing. However, in a variety of pathologies, dysregulation of the complement cascade leads to chronic inflammation, persistent pain, and neural dysfunction. This review describes recent advances in our understanding of the multifaceted cross-communication that takes place between the complement system and neurons. In particular, we focus on the molecular and cellular mechanisms through which complement signaling regulates neuronal excitability and synaptic plasticity in the nociceptive pathways involved in pain processing in both health and disease. Finally, we discuss the future of this rapidly growing field and what we believe to be the significant knowledge gaps that need to be addressed.

Biased and allosteric modulation of bone cell-expressing G protein-coupled receptors as a novel approach to osteoporosis therapy

On the cellular level, osteoporosis (OP) is a result of imbalanced bone remodeling, in which osteoclastic bone resorption outcompetes osteoblastic bone formation. Currently available OP medications include both antiresorptive and bone-forming drugs. However, their long-term use in OP patients, mainly in postmenopausal women, is accompanied by severe side effects. Notably, the fundamental coupling between bone resorption and formation processes underlies the existence of an undesirable secondary outcome that bone anabolic or anti-resorptive drugs also reduce bone formation. This drawback requires the development of anti-OP drugs capable of selectively stimulating osteoblastogenesis and concomitantly reducing osteoclastogenesis. We propose that the application of small synthetic biased and allosteric modulators of bone cell receptors, which belong to the G-protein coupled receptors (GPCR) family, could be the key to resolving the undesired anti-OP drug selectivity. This approach is based on the capacity of these GPCR modulators, unlike the natural ligands, to trigger signaling pathways that promote beneficial effects on bone remodeling while blocking potentially deleterious effects. Under the settings of OP, an optimal anti-OP drug should provide fine-tuned regulation of downstream effects, for example, intermittent cyclic AMP (cAMP) elevation, preservation of Ca²⁺ balance, stimulation of osteoprotegerin (OPG) and estrogen production, suppression of sclerostin secretion, and/or preserved/enhanced canonical β-catenin/Wnt signaling pathway. As such, selective modulation of GPCRs involved in bone remodeling presents a promising approach in OP treatment. This review focuses on the evidence for the validity of our hypothesis.

Directed Evolution: Methodologies and Applications

Directed evolution aims to expedite the natural evolution process of biological molecules and systems in a test tube through iterative rounds of gene diversifications and library screening/selection. It has become one of the most powerful and widespread tools for engineering improved or novel functions in proteins, metabolic pathways, and even whole genomes. This review describes the commonly used gene diversification strategies, screening/selection methods, and recently developed continuous evolution strategies for directed evolution. Moreover, we highlight some representative applications of directed evolution in engineering nucleic acids, proteins, pathways, genetic circuits, viruses, and whole cells. Finally, we discuss the challenges and future perspectives in directed evolution.

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.

In vitro profiling of orphan G protein coupled receptor (GPCR) constitutive activity

BACKGROUND AND PURPOSE

Members of the GPCR family are targeted by a significant fraction of the available FDA-approved drugs. However, the physiological role and pharmacological properties of many GPCRs remain unknown, representing untapped potential in drug design. Of particular interest are ~100 less-studied GPCRs known as orphans because their endogenous ligands are unknown. Intriguingly, disease-causing mutations identified in patients, together with animal studies, have demonstrated that many orphan receptors play crucial physiological roles and, thus, represent attractive drug targets.

EXPERIMENTAL APPROACH

The majority of deorphanized GPCRs demonstrate coupling to G_i/o . However, a limited number of techniques allow the detection of intrinsically small constitutive activity associated with G_i/o protein activation, which represents a significant barrier in our ability to study orphan GPCR signalling. Using luciferase reporter assays, we effectively detected constitutive G_s , G_q and G_12/13 protein signalling by unliganded receptors and introducing various G protein chimeras, we provide a novel, highly sensitive tool capable of identifying G_i/o coupling in unliganded orphan GPCRs.

KEY RESULTS

Using this approach, we measured the constitutive activity of the entire class C GPCR family that includes eight orphan receptors and a subset of 20 prototypical class A GPCR members, including 11 orphans. Excitingly, this approach illuminated the G protein coupling profile of eight orphan GPCRs (GPR22, GPR137b, GPR88, GPR156, GPR158, GPR179, GPRC5D and GPRC6A) previously linked to pathophysiological processes.

CONCLUSION AND IMPLICATIONS

We provide a new platform that could be utilized in ongoing studies in orphan receptor signalling and de-orphanization efforts.

Predictable cholesterol binding sites in GPCRs lack consensus motifs

A rich diversity of transmembrane G protein-coupled receptors (GPCRs) are used by eukaryotes to sense physical and chemical signals. In humans alone, 800 GPCRs comprise the largest and most therapeutically targeted receptor class. Recent advances in GPCR structural biology have produced hundreds of GPCR structures solved by X-ray diffraction and increasingly, cryo-electron microscopy (cryo-EM). Many of these structures are stabilized by site-specific cholesterol binding, but it is unclear whether these interactions are a product of recurring cholesterol-binding motifs and if observed patterns of cholesterol binding differ by experimental technique. Here, we comprehensively analyze the location and composition of cholesterol binding sites in the current set of 473 human GPCR structural chains. Our findings establish that cholesterol binds similarly in cryo-EM and X-ray structures and show that 92% of cholesterol molecules on GPCR surfaces reside in predictable locations that lack discernable cholesterol-binding motifs.

A laminin-based local regulatory network in the testis that supports spermatogenesis

In adult rat testes, the basement membrane is structurally constituted by laminin and collagen chains that lay adjacent to the blood-testis barrier (BTB). It plays a crucial scaffolding role to support spermatogenesis. On the other hand, laminin-333 comprised of laminin-α3/ß3/γ3 at the apical ES (ectoplasmic specialization, a testis-specific cell-cell adherens junction at the Sertoli cell-step 8-19 spermatid interface) expressed by spermatids serves as a unique cell adhesion protein that forms an adhesion complex with α6ß1-integrin expressed by Sertoli cells to support spermiogenesis. Emerging evidence has shown that biologically active fragments are derived from basement membrane and apical ES laminin chains through proteolytic cleavage mediated by matrix metalloproteinase 9 (MMP9) and MMP2, respectively. Two of these laminin bioactive fragments: one from the basement membrane laminin-α2 chain called LG3/4/5-peptide, and one from the apical ES laminin-γ3 chain known as F5-peptide, are potent regulators that modify cell adhesion function at the Sertoli-spermatid interface (i.e., apical ES) but also at the Sertoli cell-cell interface designated basal ES at the blood-testis barrier (BTB) with contrasting effects. These findings not only highlight the physiological significance of these bioactive peptides that create a local regulatory network to support spermatogenesis, they also open a unique area of research. For instance, it is likely that several other bioactive peptides remain to be identified. These bioactive peptides including their downstream signaling proteins and cascades should be studied collectively in future investigations to elucidate the underlying mechanism(s) by which they coordinate with each other to maintain spermatogenesis. This is the goal of this review.

A local regulatory network in the testis mediated by laminin and collagen fragments that supports spermatogenesis

It is almost five decades since the discovery of the hypothalamic-pituitary-testicular axis. This refers to the hormonal axis that connects the hypothalamus, pituitary gland and testes, which in turn, regulates the production of spermatozoa through spermatogenesis in the seminiferous tubules, and testosterone through steroidogenesis by Leydig cells in the interstitium, of the testes. Emerging evidence has demonstrated the presence of a regulatory network across the seminiferous epithelium utilizing bioactive molecules produced locally at specific domains of the epithelium. Studies have shown that biologically active fragments are produced from structural laminin and collagen chains in the basement membrane. Additionally, bioactive peptides are also produced locally in non-basement membrane laminin chains at the Sertoli-spermatid interface known as apical ectoplasmic specialization (apical ES, a testis-specific actin-based anchoring junction type). These bioactive peptides are derived from structural laminins and/or collagens at the corresponding sites through proteolytic cleavage by matrix metalloproteinases (MMPs). They in turn serve as autocrine and/or paracrine factors to modulate and coordinate cellular events across the epithelium by linking the apical and basal compartments, the apical and basal ES, the blood-testis barrier (BTB), and the basement membrane of the tunica propria. The cellular events supported by these bioactive peptides/fragments include the release of spermatozoa at spermiation, remodeling of the immunological barrier to facilitate the transport of preleptotene spermatocytes across the BTB, and the transport of haploid spermatids across the epithelium to support spermiogenesis. In this review, we critically evaluate these findings. Our goal is to identify research areas that deserve attentions in future years. The proposed research also provides the much needed understanding on the biology of spermatogenesis supported by a local network of regulatory biomolecules.

Trans-Synaptic Regulation of Metabotropic Glutamate Receptors by Elfn Proteins in Health and Disease

Trans-regulation of G protein-coupled receptors (GPCRs) by leucine-rich repeat (LRR) transmembrane proteins has emerged as a novel type of synaptic molecular interaction in the last decade. Several studies on LRR–GPCR interactions have revealed their critical role in synapse formation and in establishing synaptic properties. Among them, LRR–GPCR interactions between extracellular LRR fibronectin domain-containing family proteins (Elfn1 and Elfn2) and metabotropic glutamate receptors (mGluRs) are particularly interesting as they can affect a broad range of synapses through the modulation of signaling by glutamate, the principal excitatory transmitter in the mammalian central nervous system (CNS). Elfn–mGluR interactions have been investigated in hippocampal, cortical, and retinal synapses. Postsynaptic Elfn1 in the hippocampus and cerebral cortex mediates the tonic regulation of excitatory input onto somatostatin-positive interneurons (INs) through recruitment of presynaptic mGluR7. In the retina, presynaptic Elfn1 binds to mGluR6 and is necessary for synapse formation between rod photoreceptor cells and rod-bipolar cells. The repertoire of binding partners for Elfn1 and Elfn2 includes all group III mGluRs (mGluR4, mGluR6, mGluR7, and mGluR8), and both Elfn1 and Elfn2 can alter mGluR-mediated signaling through trans-interaction. Importantly, both preclinical and clinical studies have provided support for the involvement of the Elfn1–mGluR7 interaction in attention-deficit hyperactivity disorder (ADHD), post-traumatic stress disorder (PTSD), and epilepsy. In fact, Elfn1–mGluR7-associated disorders may reflect the altered function of somatostatin-positive interneuron inhibitory neural circuits, the mesolimbic and nigrostriatal dopaminergic pathway, and habenular circuits, highlighting the need for further investigation into this interaction.

Psychedelics in Psychiatry: Neuroplastic, Immunomodulatory, and Neurotransmitter Mechanisms

Mounting evidence suggests safety and efficacy of psychedelic compounds as potential novel therapeutics in psychiatry. Ketamine has been approved by the Food and Drug Administration in a new class of antidepressants, and 3,4-methylenedioxymethamphetamine (MDMA) is undergoing phase III clinical trials for post-traumatic stress disorder. Psilocybin and lysergic acid diethylamide (LSD) are being investigated in several phase II and phase I clinical trials. Hence, the concept of psychedelics as therapeutics may be incorporated into modern society. Here, we discuss the main known neurobiological therapeutic mechanisms of psychedelics, which are thought to be mediated by the effects of these compounds on the serotonergic (via 5-HT_2A and 5-HT_1A receptors) and glutamatergic [via N-methyl-d-aspartate (NMDA) and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors] systems. We focus on 1) neuroplasticity mediated by the modulation of mammalian target of rapamycin-, brain-derived neurotrophic factor-, and early growth response-related pathways; 2) immunomodulation via effects on the hypothalamic-pituitary-adrenal axis, nuclear factor ĸB, and cytokines such as tumor necrosis factor-α and interleukin 1, 6, and 10 production and release; and 3) modulation of serotonergic, dopaminergic, glutamatergic, GABAergic, and norepinephrinergic receptors, transporters, and turnover systems. We discuss arising concerns and ways to assess potential neurobiological changes, dependence, and immunosuppression. Although larger cohorts are required to corroborate preliminary findings, the results obtained so far are promising and represent a critical opportunity for improvement of pharmacotherapies in psychiatry, an area that has seen limited therapeutic advancement in the last 20 years. Studies are underway that are trying to decouple the psychedelic effects from the therapeutic effects of these compounds. SIGNIFICANCE STATEMENT: Psychedelic compounds are emerging as potential novel therapeutics in psychiatry. However, understanding of molecular mechanisms mediating improvement remains limited. This paper reviews the available evidence concerning the effects of psychedelic compounds on pathways that modulate neuroplasticity, immunity, and neurotransmitter systems. This work aims to be a reference for psychiatrists who may soon be faced with the possibility of prescribing psychedelic compounds as medications, helping them assess which compound(s) and regimen could be most useful for decreasing specific psychiatric symptoms.

Aβ oligomers induce pathophysiological mGluR5 signaling in Alzheimer's disease model mice in a sex-selective manner

The prevalence, presentation, and progression of Alzheimer's disease (AD) differ between men and women, although β-amyloid (Aβ) deposition is a pathological hallmark of AD in both sexes. Aβ-induced activation of the neuronal glutamate receptor mGluR5 is linked to AD progression. However, we found that mGluR5 exhibits distinct sex-dependent profiles. Specifically, mGluR5 isolated from male mouse cortical and hippocampal tissues bound with high affinity to Aβ oligomers, whereas mGluR5 from female mice exhibited no such affinity. This sex-selective Aβ-mGluR5 interaction did not appear to depend on estrogen, but rather Aβ interaction with cellular prion protein (PrP^C), which was detected only in male mouse brain homogenates. The ternary complex between mGluR5, Aβ oligomers, and PrP^C was essential to elicit mGluR5-dependent pathological suppression of autophagy in primary neuronal cultures. Pharmacological inhibition of mGluR5 reactivated autophagy, mitigated Aβ pathology, and reversed cognitive decline in male APPswe/PS1ΔE9 mice, but not in their female counterparts. Aβ oligomers also bound with high affinity to human mGluR5 isolated from postmortem donor male cortical brain tissue, but not that from female samples, suggesting that this mechanism may be relevant to patients. Our findings indicate that mGluR5 does not contribute to Aβ pathology in females, highlighting the complexity of mGluR5 pharmacology and Aβ signaling that supports the need for sex-specific stratification in clinical trials assessing AD therapeutics.

Interplay between cell-adhesion molecules governs synaptic wiring of cone photoreceptors

Establishment of functional synaptic connections in a selective manner is essential for nervous system operation. In mammalian retinas, rod and cone photoreceptors form selective synaptic connections with different classes of bipolar cells (BCs) to propagate light signals. While there has been progress in elucidating rod wiring, molecular mechanisms used by cones to establish functional synapses with BCs have remained unknown. Using an unbiased proteomic strategy in cone-dominant species, we identified the cell-adhesion molecule ELFN2 to be pivotal for the functional wiring of cones with the ON type of BC. It is selectively expressed in cones and transsynaptically recruits the key neurotransmitter receptor mGluR6 in ON-BCs to enable synaptic transmission. Remarkably, ELFN2 in cone terminals functions in synergy with a related adhesion molecule, ELFN1, and their concerted interplay during development specifies selective wiring and transmission of cone signals. These findings identify a synaptic connectivity mechanism of cones and illustrate how interplay between adhesion molecules and postsynaptic transmitter receptors orchestrates functional synaptic specification in a neural circuit.

Localization of GPSM2 in the Nucleus of Invasive Breast Cancer Cells Indicates a Poor Prognosis

Purpose: GPSM2 (G protein signaling modulator 2) was reported to be involved in the cell division of breast cancer cells. Additionally, cytoplasmic dynein may mediate the transport process of GPSM2. DYNC1I1 (Cytoplasmic dynein 1 intermediate chain 1) is the most common cargo-binding subunit of dynein. However, the relationship between GPSM2 and DYNC1I1 and its clinical value is unclear.

Methods: Immunohistochemical staining was performed for assessment of GPSM2 and DYNC1I1 expression. Immunoprecipitation analysis was used to assess the interaction between GPSM2 and DYNC1I1.

Results: GPSM2 was correlated with clinical characteristics of breast cancer patients and is an unfavorable independent prognostic factor. In addition, nuclear expression of GPSM2 is an unfavorable independent prognostic factor (HR = 2.658, 95% CI = 1.490–4.741, p = 0.001). GPSM2 and DYNC1I1 are known to form a complex in breast cancer cells. Patients who were positive for expression of both DYNC1I1 and GPSM2 presented with shorter recurrence-free survival than other patients. Importantly, patients with GPSM2 nuclear expression showed higher DYNC1I1 expression.

Conclusion: GPSM2 was an independent prognostic factor in breast cancer and nuclear expression of GPSM2 was significantly associated with poor prognosis, which was related to the positive expression of DYNC1I1. Examination of both GPSM2 and DYNC1I1 is necessary to establish a prognosis in breast cancer patients.

Aβ oligomers induce sex-selective differences in mGluR5 pharmacology and pathophysiological signaling in Alzheimer mice.. [DOI: 10.1101/803262]

Sex is a key modifier of the prevalence and progression of Alzheimer’s disease (AD). β- Amyloid (Aβ) deposition is a pathological hallmark of AD and aberrant activation of metabotropic glutamate receptor 5 (mGluR5) by Aβ has been linked to AD progression. We find that mGluR5 exhibits distinct sex-dependent pharmacological profiles. Specifically, endogenous mGluR5 from male mouse cortex and hippocampus binds with high-affinity to Aβ oligomers whereas, female mGluR5 exhibits no affinity to Aβ oligomers. The binding affinity of mGluR5 to Aβ oligomer is dependent on its interaction with cellular prion protein (PrPC) as mGluR5 co-immunoprecipitates with PrPCfrom male, but not female, mouse brain. Aβ oligomers also bind with high-affinity to human mGluR5 in male, but not female, cortex. The mGluR5/Aβ oligomer/PrPCternary complex is essential to elicit mGluR5-dependent pathological signaling and as a consequence mGluR5-regulated GSK3β/ZBTB16 autophagic signaling is dysregulated in male, but not female, primary neuronal cultures. These sex-specific differences in mGluR5 signaling translate into in vivo differences in mGluR5-dependent pathological signaling between male and female AD mice. We show that the chronic inhibition of mGluR5 using a mGluR5-selective negative allosteric modulator reactivates GSK3β/ZBTB16-regulated autophagy, mitigates Aβ pathology and reverses cognitive decline in male, but not female, APPswe/PS1ΔE9 mice. Thus, it is evident that, unlike male brain, mGluR5 does not contribute to Aβ pathology in female AD mice. This study highlights the complexity of mGluR5 pharmacology and Aβ oligomer-activated pathological signaling and emphasizes the need for clinical trials redesign and analysis of sex-tailored treatment for AD.