Intermolecular Interaction between Anchoring Subunits Specify Subcellular Targeting and Function of RGS Proteins in Retina ON-Bipolar Neurons

The m6A reader YTHDC2 maintains visual function and retinal photoreceptor survival through modulating translation of PPEF2 and PDE6B

Inherited retinal dystrophies (IRDs) are major causes of visual impairment and irreversible blindness worldwide, while the precise molecular and genetic mechanisms are still elusive. N6-methyladenosine (m6A) modification is the most prevalent internal modification in eukaryotic mRNA. YTH domain containing 2 (YTHDC2), an m6A reader protein, has recently been identified as a key player in germline development and human cancer. However, its contribution to retinal function remains unknown. Here, we explore the role of YTHDC2 in the visual function of retinal rod photoreceptors by generating rod-specific Ythdc2 knockout mice. Results show that Ythdc2 deficiency in rods causes diminished scotopic ERG responses and progressive retinal degeneration. Multi-omics analysis further identifies Ppef2 and Pde6b as the potential targets of YTHDC2 in the retina. Specifically, via its YTH domain, YTHDC2 recognizes and binds m6A-modified Ppef2 mRNA at the coding sequence and Pde6b mRNA at the 5'-UTR, resulting in enhanced translation efficiency without affecting mRNA levels. Compromised translation efficiency of Ppef2 and Pde6b after YTHDC2 depletion ultimately leads to decreased protein levels in the retina, impaired retinal function, and progressive rod death. Collectively, our finding highlights the importance of YTHDC2 in visual function and photoreceptor survival, which provides an unreported elucidation of IRD pathogenesis via epitranscriptomics.

Homodimerization of a proximal region within the C-terminus of the orphan G-protein coupled receptor GPR179

G-protein coupled receptors exhibit numerous biological functions. The orphan G-protein coupled receptor GPR179 is a central component of a 1 Megadalton large signalling complex in the ON-pathway of the mammalian retina that assembles multiple proteins, including the metabotropic glutamate receptor mGluR6. Dimer formation is a hallmark of G-protein coupled receptors and some use intracellular C-termini for dimerization. Here we tested the dimerization properties of the intracellular C-terminal domains of mGluR6 and GPR179. While the C-termini of GPR179 and mGluR6 did not interact, we detected a robust homodimerization of a proximal region in the GPR179 C-terminus. Mapping studies defined a linear stretch of 64 amino acids as dimerization region. Bioinformatic analysis indicated that this dimerization region might adopt an α-helical structure that is predicted to dimerize by forming a coiled-coil. Based on these data, we speculate that homodimerization of GPR179 might contribute to the formation of large signalling complexes in the mammalian retina.

Development and maintenance of vision's first synapse

Synapses in the outer retina are the first information relay points in vision. Here, photoreceptors form synapses onto two types of interneurons, bipolar cells and horizontal cells. Because outer retina synapses are particularly large and highly ordered, they have been a useful system for the discovery of mechanisms underlying synapse specificity and maintenance. Understanding these processes is critical to efforts aimed at restoring visual function through repairing or replacing neurons and promoting their connectivity. We review outer retina neuron synapse architecture, neural migration modes, and the cellular and molecular pathways that play key roles in the development and maintenance of these connections. We further discuss how these mechanisms may impact connectivity in the retina.

AMIGO2 Scales Dendrite Arbors in the Retina

The size of dendrite arbors shapes their function and differs vastly between neuron types. The signals that control dendritic arbor size remain obscure. Here, we find that in the retina, starburst amacrine cells (SACs) and rod bipolar cells (RBCs) express the homophilic cell-surface protein AMIGO2. In Amigo2 knockout (KO) mice, SAC and RBC dendrites expand while arbors of other retinal neurons remain stable. SAC dendrites are divided into a central input region and a peripheral output region that provides asymmetric inhibition to direction-selective ganglion cells (DSGCs). Input and output compartments scale precisely with increased arbor size in Amigo2 KO mice, and SAC dendrites maintain asymmetric connectivity with DSGCs. Increased coverage of SAC dendrites is accompanied by increased direction selectivity of DSGCs without changes to other ganglion cells. Our results identify AMIGO2 as a cell-type-specific dendritic scaling factor and link dendrite size and coverage to visual feature detection.

Soto et al. find that two retinal interneurons express the cell-surface protein AMIGO2. Deletion of Amigo2 causes dendrites of these neurons, but not others, to expand, preserving branching patterns and connectivity. Increased interneuron dendrite coverage is accompanied by enhanced response selectivity of retinal output neurons.

LRRTM4 is a member of the transsynaptic complex between rod photoreceptors and bipolar cells

Leucine rich repeat transmembrane (LRRTM) proteins are synaptic adhesion molecules with roles in synapse formation and signaling. LRRTM4 transcripts were previously shown to be enriched in rod bipolar cells (BCs), secondary neurons of the retina that form synapses with rod photoreceptors. Using two different antibodies, LRRTM4 was found to reside primarily at rod BC dendritic tips, where it colocalized with the transduction channel protein, TRPM1. LRRTM4 was not detected at dendritic tips of ON-cone BCs. Following somatic knockout of LRRTM4 in BCs by subretinal injection and electroporation of CRISPR/Cas9, LRRTM4 was abolished or reduced in the dendritic tips of transfected cells. Knockout cells had a normal complement of TRPM1 at their dendritic tips, while GPR179 accumulation was partially reduced. In experiments with heterologously expressed protein, the extracellular domain of LRRTM4 was found to engage in heparan-sulfate dependent binding with pikachurin. These results implicate LRRTM4 in the GPR179-pikachurin-dystroglycan transsynaptic complex at rod synapses.

Integrative analysis of in vivo recording with single-cell RNA-seq data reveals molecular properties of light-sensitive neurons in mouse V1

Vision formation is classically based on projections from retinal ganglion cells (RGC) to the lateral geniculate nucleus (LGN) and the primary visual cortex (V1). Neurons in the mouse V1 are tuned to light stimuli. Although the cellular information of the retina and the LGN has been widely studied, the transcriptome profiles of single light-stimulated neuron in V1 remain unknown. In our study, in vivo calcium imaging and whole-cell electrophysiological patch-clamp recording were utilized to identify 53 individual cells from layer 2/3 of V1 as light-sensitive (LS) or non-light-sensitive (NS) by single-cell light-evoked calcium evaluation and action potential spiking. The contents of each cell after functional tests were aspirated in vivo through a patch-clamp pipette for mRNA sequencing. Moreover, the three-dimensional (3-D) morphological characterizations of the neurons were reconstructed in a live mouse after the whole-cell recordings. Our sequencing results indicated that V1 neurons with a high expression of genes related to transmission regulation, such as Rtn4r and Rgs7, and genes involved in membrane transport, such as Na⁺/K⁺ ATPase and NMDA-type glutamatergic receptors, preferentially responded to light stimulation. Furthermore, an antagonist that blocks Rtn4r signals could inactivate the neuronal responses to light stimulation in live mice. In conclusion, our findings of the vivo-seq analysis indicate the key role of the strength of synaptic transmission possesses neurons in V1 of light sensory.

Molecular mechanisms underlying selective synapse formation of vertebrate retinal photoreceptor cells

In vertebrate central nervous systems (CNSs), highly diverse neurons are selectively connected via synapses, which are essential for building an intricate neural network. The vertebrate retina is part of the CNS and is comprised of a distinct laminar organization, which serves as a good model system to study developmental synapse formation mechanisms. In the retina outer plexiform layer, rods and cones, two types of photoreceptor cells, elaborate selective synaptic contacts with ON- and/or OFF-bipolar cell terminals as well as with horizontal cell terminals. In the mouse retina, three photoreceptor subtypes and at least 15 bipolar subtypes exist. Previous and recent studies have significantly progressed our understanding of how selective synapse formation, between specific subtypes of photoreceptor and bipolar cells, is designed at the molecular level. In the ON pathway, photoreceptor-derived secreted and transmembrane proteins directly interact in trans with the GRM6 (mGluR6) complex, which is localized to ON-bipolar cell dendritic terminals, leading to selective synapse formation. Here, we review our current understanding of the key factors and mechanisms underlying selective synapse formation of photoreceptor cells with bipolar and horizontal cells in the retina. In addition, we describe how defects/mutations of the molecules involved in photoreceptor synapse formation are associated with human retinal diseases and visual disorders.

LRIT3 is Required for Nyctalopin Expression and Normal ON and OFF Pathway Signaling in the Retina

The first retinal synapse, photoreceptor→bipolar cell (BC), is both anatomically and functionally complex. Within the same synaptic region, a change in presynaptic glutamate release is sensed by both ON BCs (DBCs) via the metabotropic glutamate receptor 6 (mGluR6), and OFF BCs (HBCs) via ionotropic glutamate receptors to establish parallel signaling pathways that preferentially encode light increments (ON) or decrements (OFF), respectively. The synaptic structural organization of ON and OFF-type BCs at the photoreceptor terminal differs. DBCs make an invaginating synapse that contains a diverse but incompletely understood complex of interacting proteins (signalplex). HBCs make primarily flat contacts that contain an apparent different set of proteins that is equally uncharacterized. LRIT3 is a synaptic protein known to be essential for ON pathway visual function. In both male and female mice, we demonstrate that LRIT3 interacts with and is required for expression of nyctalopin, and thus TRPM1 at all DBC dendritic tips, but DBC signalplex components are not required for LRIT3 expression. Using whole-cell and multielectrode array (MEA) electrophysiology and glutamate imaging, we demonstrate that the loss of LRIT3 impacts both ON and OFF signaling pathway function. Without LRIT3, excitatory input to type 1 BCs is reduced, as are the visually evoked responses of many OFF retinal ganglion cells (RGCs). We conclude that the absence of LRIT3 expression disrupts excitatory input to OFF BCs and, thus disrupts the normal function of OFF RGCs.

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

G protein-coupled receptors (GPCRs) remain one of the most successful targets of U.S. Food and Drug Administration-approved drugs. GPCR research has predominantly focused on the characterization of the intracellular interactome's contribution to GPCR function and pharmacology. However, emerging evidence uncovers a new dimension in the biology of GPCRs involving their extracellular and transcellular interactions that critically impact GPCR function and pharmacology. The seminal examples include a variety of adhesion GPCRs, such as ADGRLs/latrophilins, ADGRBs/brain angiogenesis inhibitors, ADGRG1/GPR56, ADGRG6/GPR126, ADGRE5/CD97, and ADGRC3/CELSR3. However, recent advances have indicated that class C GPCRs that contain large extracellular domains, including group III metabotropic glutamate receptors (mGluR4, mGluR6, mGluR7, mGluR8), γ-aminobutyric acid receptors, and orphans GPR158 and GPR179, can also participate in this form of transcellular regulation. In this review, we will focus on a variety of identified extracellular and transcellular GPCR-interacting partners, including teneurins, neurexins, integrins, fibronectin leucine-rich transmembranes, contactin-6, neuroligin, laminins, collagens, major prion protein, amyloid precursor protein, complement C1q-likes, stabilin-2, pikachurin, dystroglycan, complement decay-accelerating factor CD55, cluster of differentiation CD36 and CD90, extracellular leucine-rich repeat and fibronectin type III domain containing 1, and leucine-rich repeat, immunoglobulin-like domain and transmembrane domains. We provide an account on the diversity of extracellular and transcellular GPCR complexes and their contribution to key cellular and physiologic processes, including cell migration, axon guidance, cellular and synaptic adhesion, and synaptogenesis. Furthermore, we discuss models and mechanisms by which extracellular GPCR assemblies may regulate communication at cellular junctions. SIGNIFICANCE STATEMENT: G protein-coupled receptors (GPCRs) continue to be the prominent focus of pharmacological intervention for a variety of human pathologies. Although the majority of GPCR research has focused on the intracellular interactome, recent advancements have identified an extracellular dimension of GPCR modulation that alters accepted pharmacological principles of GPCRs. Herein, we describe known endogenous allosteric modulators acting on GPCRs both in cis and in trans.

Homeostatic cAMP regulation by the RGS7 complex controls depression-related behaviors

Affective disorders arise from abnormal responses of the brain to prolonged exposure to challenging environmental stimuli. Recent work identified the orphan receptor GPR158 as a molecular link between chronic stress and depression. Here we reveal a non-canonical mechanism by which GPR158 exerts its effects on stress-induced depression by the complex formation with Regulator of G protein Signaling 7 (RGS7). Chronic stress promotes membrane recruitment of RGS7 via GPR158 in the medial prefrontal cortex (mPFC). The resultant complex suppresses homeostatic regulation of cAMP by inhibitory GPCRs in the region. Accordingly, RGS7 loss in mice induces an antidepressant-like phenotype and resiliency to stress, whereas its restoration within the mPFC is sufficient to rescue this phenotype in a GPR158-dependent way. These findings mechanistically link the unusual orphan receptor-RGS complex to a major stress mediator, the cAMP system and suggest new avenues for pharmacological interventions in affective disorders.

Inhibitory Signaling to Ion Channels in Hippocampal Neurons Is Differentially Regulated by Alternative Macromolecular Complexes of RGS7

The neuromodulatory effects of GABA on pyramidal neurons are mediated by GABA_B receptors (GABA_BRs) that signal via a conserved G-protein-coupled pathway. Two prominent effectors regulated by GABA_BRs include G-protein inwardly rectifying K⁺ (GIRK) and P/Q/N type voltage-gated Ca²⁺ (Ca_V2) ion channels that control excitability and synaptic output of these neurons, respectively. Regulator of G-protein signaling 7 (RGS7) has been shown to control GABA_B effects, yet the specificity of its impacts on effector channels and underlying molecular mechanisms is poorly understood. In this study, we show that hippocampal RGS7 forms two distinct complexes with alternative subunit configuration bound to either membrane protein R7BP (RGS7 binding protein) or orphan receptor GPR158. Quantitative biochemical experiments show that both complexes account for targeting nearly the entire pool of RGS7 to the plasma membrane. We analyzed the effect of genetic elimination in mice of both sexes and overexpression of various components of RGS7 complex by patch-clamp electrophysiology in cultured neurons and brain slices. We report that RGS7 prominently regulates GABA_BR signaling to Ca_V2, in addition to its known involvement in modulating GIRK. Strikingly, only complexes containing R7BP, but not GPR158, accelerated the kinetics of both GIRK and Ca_V2 modulation by GABA_BRs. In contrast, GPR158 overexpression exerted the opposite effect and inhibited RGS7-assisted temporal modulation of GIRK and Ca_V2 by GABA. Collectively, our data reveal mechanisms by which distinctly composed macromolecular complexes modulate the activity of key ion channels that mediate the inhibitory effects of GABA on hippocampal CA1 pyramidal neurons.SIGNIFICANCE STATEMENT This study identifies the contributions of distinct macromolecular complexes containing a major G-protein regulator to controlling key ion channel function in hippocampal neurons with implications for understanding molecular mechanisms underlying synaptic plasticity, learning, and memory.

The Transduction Cascade in Retinal ON-Bipolar Cells: Signal Processing and Disease

Our robust visual experience is based on the reliable transfer of information from our photoreceptor cells, the rods and cones, to higher brain centers. At the very first synapse of the visual system, information is split into two separate pathways, ON and OFF, which encode increments and decrements in light intensity, respectively. The importance of this segregation is borne out in the fact that receptive fields in higher visual centers maintain a separation between ON and OFF regions. In the past decade, the molecular mechanisms underlying the generation of ON signals have been identified, which are unique in their use of a G-protein signaling cascade. In this review, we consider advances in our understanding of G-protein signaling in ON-bipolar cell (BC) dendrites and how insights about signaling have emerged from visual deficits, mostly night blindness. Studies of G-protein signaling in ON-BCs reveal an intricate mechanism that permits the regulation of visual sensitivity over a wide dynamic range.