Dynamic regulation of ryanodine receptor type 1 (RyR1) channel activity by Homer 1

TRPC1: The housekeeper of the hippocampus

The non-selective cation channel TRPC1 is highly expressed in the brain. Recent research shows that neuronal TRPC1 forms heteromeric complexes with TRPC4 and TRPC5, with a small portion existing as homotetramers, primarily in the ER. Given that most studies have focused on the role of heteromeric TRPC1/4/5 complexes, it is crucial to investigate the specific role of homomeric TRPC1 in maintaining brain homeostasis. This review highlights recent findings on TRPC1 in the brain, with a focus on the hippocampus, and compiles the latest data on modulators and their binding sites within the TRPC1/4/5 subfamily to stimulate new research on more selective TRPC1 ligands.

The mGluR5-mediated Arc activation protects against experimental traumatic brain injury in rats

INTRODUCTION

Traumatic brain injury (TBI) is a complex pathophysiological process, and increasing attention has been paid to the important role of post-synaptic density (PSD) proteins, such as glutamate receptors. Our previous study showed that a PSD protein Arc/Arg3.1 (Arc) regulates endoplasmic reticulum (ER) stress and neuronal necroptosis in traumatic injury in vitro.

AIM

In this study, we investigated the expression, regulation and biological function of Arc in both in vivo and in vitro experimental TBI models.

RESULTS

Traumatic neuronal injury (TNI) induced a temporal upregulation of Arc in cortical neurons, while TBI resulted in sustained increase in Arc expression up to 24 h in rats. The increased expression of Arc was mediated by the activity of metabotropic glutamate receptor 5 (mGluR5), but not dependent on the intracellular calcium (Ca2+) release. By using inhibitors and antagonists, we found that TNI regulates Arc expression via Gq protein and protein turnover. In addition, overexpression of Arc protects against TBI-induced neuronal injury and motor dysfunction both in vivo and in vitro, whereas the long-term cognitive function was not altered. To determine the role of Arc in mGluR5-induced protection, lentivirus-mediated short hairpin RNA (shRNA) transfection was performed to knockdown Arc expression. The mGluR5 agonist (RS)-2-chloro-5-hydroxyphenylglycine (CHPG)-induced protection against TBI was partially prevented by Arc knockdown. Furthermore, the CHPG-induced attenuation of Ca2+ influx after TNI was dependent on Arc activation and followed regulation of AMPAR subunits. The results of Co-IP and Ca2+ imaging showed that the Arc-Homer1 interaction contributes to the CHPG-induced regulation of intracellular Ca2+ release.

CONCLUSION

In summary, the present data indicate that the mGluR5-mediated Arc activation is a protective mechanism that attenuates neurotoxicity following TBI through the regulation of intracellular Ca2+ hemostasis. The AMPAR-associated Ca2+ influx and ER Ca2+ release induced by Homer1-IP3R pathway might be involved in this protection.

Remodeling of the ryanodine receptor isoform 1 channel regulates the sweet and umami taste perception of Rattus norvegicus

Sweet and umami tastes are elicited by sweet and umami receptors on the tongue and palate epithelium, respectively. However, the molecular machinery allowing the taste reaction remains incompletely understood. Through a phosphoproteomic approach, we identified the key proteins that trigger taste mechanisms based on phosphorylation cascades. Ryanodine receptor isoform 1 (RYR1) was further verified by sensory and behavioral assays. We propose a model of RYR1-mediated sweet/umami signaling in which the RYR1 channel, which mediates Ca2+ release from the endoplasmic reticulum, is closed by dephosphorylation in bud tissue after sweet/umami treatment. The alteration in Ca2+ content in the cytosol induces transient membrane depolarization and generates a cell current for taste signal transduction. We demonstrate that RYR1 is a new channel involved in the regulation of sweet/umami signal transduction and propose a "metabolic clock" notion based on sweet/umami sensing. Our study provides a valuable foundation for a system-level understanding of the taste perception mechanism.

Excitation-contraction coupling in mammalian skeletal muscle: Blending old and last-decade research

The excitation–contraction coupling (ECC) in skeletal muscle refers to the Ca²⁺-mediated link between the membrane excitation and the mechanical contraction. The initiation and propagation of an action potential through the membranous system of the sarcolemma and the tubular network lead to the activation of the Ca²⁺-release units (CRU): tightly coupled dihydropyridine and ryanodine (RyR) receptors. The RyR gating allows a rapid, massive, and highly regulated release of Ca²⁺ from the sarcoplasmic reticulum (SR). The release from triadic places generates a sarcomeric gradient of Ca²⁺ concentrations ([Ca²⁺]) depending on the distance of a subcellular region from the CRU. Upon release, the diffusing Ca²⁺ has multiple fates: binds to troponin C thus activating the contractile machinery, binds to classical sarcoplasmic Ca²⁺ buffers such as parvalbumin, adenosine triphosphate and, experimentally, fluorescent dyes, enters the mitochondria and the SR, or is recycled through the Na⁺/Ca²⁺ exchanger and store-operated Ca²⁺ entry (SOCE) mechanisms. To commemorate the 7^th decade after being coined, we comprehensively and critically reviewed “old”, historical landmarks and well-established concepts, and blended them with recent advances to have a complete, quantitative-focused landscape of the ECC. We discuss the: 1) elucidation of the CRU structures at near-atomic resolution and its implications for functional coupling; 2) reliable quantification of peak sarcoplasmic [Ca²⁺] using fast, low affinity Ca²⁺ dyes and the relative contributions of the Ca²⁺-binding mechanisms to the whole concert of Ca²⁺ fluxes inside the fibre; 3) articulation of this novel quantitative information with the unveiled structural details of the molecular machinery involved in mitochondrial Ca²⁺ handing to understand how and how much Ca²⁺ enters the mitochondria; 4) presence of the SOCE machinery and its different modes of activation, which awaits understanding of its magnitude and relevance in situ; 5) pharmacology of the ECC, and 6) emerging topics such as the use and potential applications of super-resolution and induced pluripotent stem cells (iPSC) in ECC. Blending the old with the new works better!

Preliminary Observations on Skeletal Muscle Adaptation and Plasticity in Homer 2-/- Mice

Homer represents a diversified family of scaffold and transduction proteins made up of several isoforms. Here, we present preliminary observations on skeletal muscle adaptation and plasticity in a transgenic model of Homer 2^-/- mouse using a multifaceted approach entailing morphometry, quantitative RT-PCR (Reverse Transcription PCR), confocal immunofluorescence, and electrophysiology. Morphometry shows that Soleus muscle (SOL), at variance with Extensor digitorum longus muscle (EDL) and Flexor digitorum brevis muscle (FDB), displays sizable reduction of fibre cross-sectional area compared to the WT counterparts. In SOL of Homer 2^-/- mice, quantitative RT-PCR indicated the upregulation of Atrogin-1 and Muscle ring finger protein 1 (MuRF1) genes, and confocal immunofluorescence showed the decrease of neuromuscular junction (NMJ) Homer content. Electrophysiological measurements of isolated FDB fibres from Homer 2^-/- mice detected the exclusive presence of the adult ε-nAChR isoform excluding denervation. As for NMJ morphology, data were not conclusive, and further work is needed to ascertain whether the null Homer 2 phenotype induces any endplate remodelling. Within the context of adaptation and plasticity, the present data show that Homer 2 is a co-regulator of the normotrophic status in a muscle specific fashion.

Interaction of the Homer1 EVH1 domain and skeletal muscle ryanodine receptor

The skeletal muscle ryanodine receptor (RyR1) proteins are intracellular calcium (Ca²⁺) release channels on the membrane of the sarcoplasmic reticulum (SR) and required for skeletal muscle excitation-contraction coupling. Homer (Vesl) is a family of scaffolding proteins that modulate target proteins including RyRs (ryanodine receptors), mGluRs (group 1 metabotropic glutamate receptors) and IP₃Rs (inositol-1,4,5-trisphosphate receptors) through a conserved EVH1 (Ena/VASP homology 1) domain. Here, we examined the interaction between Homer1 EVH1 domain and RyR1 by co-immunoprecipitation, continuous sucrose density-gradient centrifugation, and bio-layer interferometry binding assay at different Ca²⁺ concentrations. Our results show that there exists a high-affinity binding between the Homer1 EVH1 domain and RyR1, especially at 1 mM of Ca²⁺. Based on our data and the known structures of Homer1 EVH1 domain and RyR1, we found two consensus proline-rich sequences in the structure of RyR1, PPHHF and FLPPP, and proposed two corresponding binding models to show mechanisms of recognition different from those used by other proline-rich motifs. The side proline residues of two proline-rich motifs from RyR1 are away from the hydrophobic surface of Homer1 EVH1, rather than buried in this hydrophobic surface. Our results provide evidence that Homer1 regulates RyR1 by direct interaction.

Transcriptomic profiling of mTOR and ryanodine receptor signaling molecules in developing zebrafish in the absence and presence of PCB 95

The mechanistic target of rapamycin (mTOR) and ryanodine receptor (RyR) signaling pathways regulate fundamental processes of neurodevelopment, and genetic mutations within these pathways have been linked to neurodevelopmental disorders. While previous studies have established that these signaling molecules are expressed in developing zebrafish, a detailed characterization of the ontogenetic profile of these signaling molecules is lacking. Thus, we evaluated the spatiotemporal expression of key transcripts in mTOR and RyR signaling pathways in wildtype zebrafish at 24, 72 and 120 hours post fertilization (hpf). We further determined whether transcriptional profiles of a subset of genes in both pathways were altered by exposure to PCB 95 (2,2′,3,5′,6-pentachlorobiphenyl), a pervasive environmental contaminant known to cause developmental neurotoxicity in mammalian systems via RyR-dependent mechanisms. Quantitative PCR revealed that transcription generally increased across development. Genes in the signaling pathway upstream of the mTORC1 complex, and the RyR-paralogs, ryr2a and ryr3, were robustly upregulated, and in situ hybridization of ryr3 coincided with a transcriptional shift from muscle to neuronal tissue after 24 hpf. Static waterborne exposure to PCB 95 beginning at 6 hpf significantly altered transcription of genes in both pathways. These changes were concentration- and time-dependent, and included downregulation of rptor, a member of the mTORC1 complex, at both 72 and 120 hpf, and increased transcript levels of the RyR paralog ryr2b and downstream target of RyR signaling, Wingless-type 2ba (wnt2ba) at 72 hpf. The detailed transcriptomic profiling of key genes within these two signaling pathways provides a baseline for identifying other environmental factors that modify normal spatiotemporal expression patterns of mTOR and RyR signaling pathways in the developing zebrafish, as illustrated here for PCB 95.

Ginsenoside Rg1 prevents starvation-induced muscle protein degradation via regulation of AKT/mTOR/FoxO signaling in C2C12 myotubes

Skeletal muscle atrophy is often caused by catabolic conditions including fasting, disuse, aging and chronic diseases, such as chronic obstructive pulmonary disease. Atrophy occurs when the protein degradation rate exceeds the rate of protein synthesis. Therefore, maintaining a balance between the synthesis and degradation of protein in muscle cells is a major way to prevent skeletal muscle atrophy. Ginsenoside Rg1 (Rg1) is a primary active ingredient in Panax ginseng, which is considered to be one of the most valuable herbs in traditional Chinese medicine. In the current study, Rg1 was observed to inhibit the expression of MuRF-1 and atrogin-1 in C2C12 muscle cells in a starvation model. Rg1 also activated the phosphorylation of mammalian target of rapamycin (mTOR), protein kinase B (AKT), and forkhead transcription factor O, subtypes 1 and 3a. This phosphorylation was inhibited by LY294002, a phosphatidylinositol 3-kinase inhibitor. These data suggest that Rg1 may participate in the regulation of the balance between protein synthesis and degradation, and that the function of Rg1 is associated with the AKT/mTOR/FoxO signaling pathway.

Smooth Muscle Ion Channels and Regulation of Vascular Tone in Resistance Arteries and Arterioles

Vascular tone of resistance arteries and arterioles determines peripheral vascular resistance, contributing to the regulation of blood pressure and blood flow to, and within the body's tissues and organs. Ion channels in the plasma membrane and endoplasmic reticulum of vascular smooth muscle cells (SMCs) in these blood vessels importantly contribute to the regulation of intracellular Ca2+ concentration, the primary determinant of SMC contractile activity and vascular tone. Ion channels provide the main source of activator Ca2+ that determines vascular tone, and strongly contribute to setting and regulating membrane potential, which, in turn, regulates the open-state-probability of voltage gated Ca2+ channels (VGCCs), the primary source of Ca2+ in resistance artery and arteriolar SMCs. Ion channel function is also modulated by vasoconstrictors and vasodilators, contributing to all aspects of the regulation of vascular tone. This review will focus on the physiology of VGCCs, voltage-gated K+ (KV) channels, large-conductance Ca2+-activated K+ (BKCa) channels, strong-inward-rectifier K+ (KIR) channels, ATP-sensitive K+ (KATP) channels, ryanodine receptors (RyRs), inositol 1,4,5-trisphosphate receptors (IP3Rs), and a variety of transient receptor potential (TRP) channels that contribute to pressure-induced myogenic tone in resistance arteries and arterioles, the modulation of the function of these ion channels by vasoconstrictors and vasodilators, their role in the functional regulation of tissue blood flow and their dysfunction in diseases such as hypertension, obesity, and diabetes. © 2017 American Physiological Society. Compr Physiol 7:485-581, 2017.

Regulation of postsynaptic plasticity genes' expression and topography by sustained dopamine perturbation and modulation by acute memantine: relevance to schizophrenia

A relevant role for dopamine-glutamate interaction has been reported in the pathophysiology and treatment of psychoses. Dopamine and glutamate may interact at multiple levels, including the glutamatergic postsynaptic density (PSD), an electron-dense thickening that has gained recent attention as a switchboard of dopamine-glutamate interactions and for its role in synaptic plasticity. Recently, glutamate-based strategies, such as memantine add-on to antipsychotics, have been proposed for refractory symptoms of schizophrenia, e.g. cognitive impairment. Both antipsychotics and memantine regulate PSD transcripts but sparse information is available on memantine's effects under dopamine perturbation. We tested gene expression changes of the Homer1 and PSD-95 PSD proteins in models of sustained dopamine perturbation, i.e. subchronic treatment by: a) GBR-12909, a dopamine receptor indirect agonist; b) haloperidol, a D2R antagonist; c) SCH-23390, a dopamine D1 receptor (D1R) antagonist; and d) SCH-23390+haloperidol. On the last day of treatment, rats were acutely treated with vehicle or memantine. The Homer1a immediate-early gene was significantly induced by haloperidol and by haloperidol+SCH-23390. The gene was not induced by SCH-23390 per se or by GBR-12909. Expression of the constitutive genes Homer1b/c and PSD-95 was less affected by these dopaminergic paradigms. Acute memantine administration significantly increased Homer1a expression by the dopaminergic compounds used herein. Both haloperidol and haloperidol+SCH-23390 shifted Homer1a/Homer1b/c ratio of expression toward Homer1a. This pattern was sharpened by acute memantine. Dopaminergic compounds and acute memantine also differentially affected topographic distribution of gene expression and coordinated expression of Homer1a among cortical-subcortical regions. These results indicate that dopaminergic perturbations may affect glutamatergic signaling in different directions. Memantine may help partially revert dopamine-mediated glutamatergic dysfunctions.

Gene delivery of Homer1c rescues spatial learning in a rodent model of cognitive aging

Homer1c has been shown to play a role in learning and memory. Overexpression of Homer1c in the hippocampus can improve memory in normal rats and can also rescue spatial learning deficits in Homer1 knockout mice. In a previous study, we found that Homer1c mRNA is upregulated after a spatial learning paradigm in aged rats that successfully learn the task, when compared to aged rats that are learning-impaired (AI). This study was designed to validate the role of Homer1c in successful cognitive aging. In this article, we report that gene delivery of Homer1c into the hippocampus of aged learning-impaired rats significantly improves individual performance on an object location memory task. The learning ability of these rats on the Morris Water Maze was also superior to that of AI control rats. In summary, using 2 independent spatial memory tasks, we demonstrate that Homer1c is sufficient to improve the spatial learning deficits in a rodent model of cognitive aging. These results point to Homer1c as a potential therapeutic target for improving age-related cognitive impairment.

Homer1 gene products orchestrate Ca(2+)-permeable AMPA receptor distribution and LTP expression

We studied the role of Homer1 gene products on the presence of synaptic Ca²⁺-permeable AMPA receptors (AMPARs) and long-term potentiation (LTP) generation in hippocampal CA1 pyramidal neurons, using mice either lacking all Homer1 isoforms (Homer1 KO) or overexpressing the immediate early gene (IEG) product Homer1a (H1aTG). We found that Homer1 KO caused a significant redistribution of the AMPAR subunit GluA2 from the dendritic compartment to the soma. Furthermore, deletion of Homer1 enhanced the AMPAR-mediated component of glutamatergic currents at Schaffer collateral synapses as demonstrated by increased AMPA/NMDA current ratios. Meanwhile, LTP generation appeared to be unaffected. Conversely, sustained overexpression of Homer1a strongly reduced AMPA/NMDA current ratios and polyamine sensitivity of synaptic AMPAR, indicating that the proportion of synaptic GluA2-containing AMPAR increased relative to WT. LTP maintenance was abolished in H1aTG. Notably, overexpression of Homer1a in Homer1 KO or GluA2 KO mice did not affect LTP expression, suggesting activity-dependent interaction between Homer1a and long Homer1 isoforms with GluA2-containing AMPAR. Thus, Homer1a is essential for the activity-dependent regulation of excitatory synaptic transmission.

Coordinated regulation of murine cardiomyocyte contractility by nanomolar (-)-epigallocatechin-3-gallate, the major green tea catechin

Green tea polyphenolic catechins exhibit biological activity in a wide variety of cell types. Although reports in the lay and scientific literature suggest therapeutic potential for improving cardiovascular health, the underlying molecular mechanisms of action remain unclear. Previous studies have implicated a wide range of molecular targets in cardiac muscle for the major green tea catechin, (-)-epigallocatechin-3-gallate (EGCG), but effects were observed only at micromolar concentrations of unclear clinical relevance. Here, we report that nanomolar concentrations of EGCG significantly enhance contractility of intact murine myocytes by increasing electrically evoked Ca(2+) transients, sarcoplasmic reticulum (SR) Ca(2+) content, and ryanodine receptor type 2 (RyR2) channel open probability. Voltage-clamp experiments demonstrate that 10 nM EGCG significantly inhibits the Na(+)-Ca(2+) exchanger. Of importance, other Na(+) and Ca(2+) handling proteins such as Ca(2+)-ATPase, Na(+)-H(+) exchanger, and Na(+)-K(+)-ATPase were not affected by EGCG ≤ 1 μM. Thus, nanomolar EGCG increases contractility in intact myocytes by coordinately modulating SR Ca(2+) loading, RyR2-mediated Ca(2+) release, and Na(+)-Ca(2+) exchange. Inhibition of Na(+)-K(+)-ATPase activity probably contributes to the positive inotropic effects observed at EGCG concentrations >1 μM. These newly recognized actions of nanomolar and micromolar EGCG should be considered when the therapeutic and toxicological potential of green tea supplementation is evaluated and may provide a novel therapeutic strategy for improving contractile function in heart failure.

Ryanodine receptors

Excitation-contraction coupling involves the faithful conversion of electrical stimuli to mechanical shortening in striated muscle cells, enabled by the ubiquitous second messenger, calcium. Crucial to this process are ryanodine receptors (RyRs), the sentinels of massive intracellular calcium stores contained within the sarcoplasmic reticulum. In response to sarcolemmal depolarization, RyRs release calcium into the cytosol, facilitating mobilization of the myofilaments and enabling cell contraction. In order for the cells to relax, calcium must be rapidly resequestered or extruded from the cytosol. The sustainability of this cycle is crucially dependent upon precise regulation of RyRs by numerous cytosolic metabolites and by proteins within the lumen of the sarcoplasmic reticulum and those directly associated with the receptors in a macromolecular complex. In addition to providing the majority of the calcium necessary for contraction of cardiac and skeletal muscle, RyRs act as molecular switchboards that integrate a multitude of cytosolic signals such as dynamic and steady calcium fluctuations, β-adrenergic stimulation (phosphorylation), nitrosylation and metabolic states, and transduce these signals to the channel pore to release appropriate amounts of calcium. Indeed, dysregulation of calcium release via RyRs is associated with life-threatening diseases in both skeletal and cardiac muscle. In this paper, we briefly review some of the most outstanding structural and functional attributes of RyRs and their mechanism of regulation. Further, we address pathogenic RyR dysfunction implicated in cardiovascular disease and skeletal myopathies.

Green tea catechins are potent sensitizers of ryanodine receptor type 1 (RyR1)

Catechins, polyphenols extracted from green tea leaves, have a broad range of biological activities although the specific molecular mechanisms responsible are not known. At the high experimental concentrations typically used polyphenols bind to membrane phospholipid and also are easily auto-oxidized to generate superoxide anion and semiquinones, and can adduct to protein thiols. We report that the type 1 ryanodine receptor (RyR1) is a molecular target that responds to nanomolar (-)-epigallocatechin-3-gallate (EGCG) and (-)-epicatechin-3-gallate (ECG). Single channel analyses demonstrate EGCG (5-10nM) increases channel open probability (Po) twofold, by lengthening open dwell time. The degree of channel activation is concentration-dependent and is rapidly and fully reversible. Four related catechins, EGCG, ECG, EGC ((-)-epigallocatechin) and EC ((-)-epicatechin) showed a rank order of activity toward RyR1 (EGCG>ECG>>EGC>>>EC). EGCG and ECG enhance the sensitivity of RyR1 to activation by < or =100microM cytoplasmic Ca(2+) without altering inhibitory potency by >100microM Ca(2+). EGCG as high as 10microM in the extracellular medium potentiated Ca(2+) transient amplitudes evoked by electrical stimuli applied to intact myotubes and adult FDB fibers, without eliciting spontaneous Ca(2+) release or slowing Ca(2+) transient recovery. The results identify RyR1 as a sensitive target for the major tea catechins EGCG and ECG, and this interaction is likely to contribute to their observed biological activities.

Minding the calcium store: Ryanodine receptor activation as a convergent mechanism of PCB toxicity

Chronic low-level polychlorinated biphenyl (PCB) exposures remain a significant public health concern since results from epidemiological studies indicate that PCB burden is associated with immune system dysfunction, cardiovascular disease, and impairment of the developing nervous system. Of these various adverse health effects, developmental neurotoxicity has emerged as a particularly vulnerable endpoint in PCB toxicity. Arguably the most pervasive biological effects of PCBs could be mediated by their ability to alter the spatial and temporal fidelity of Ca2+ signals through one or more receptor-mediated processes. This review will focus on our current knowledge of the structure and function of ryanodine receptors (RyRs) in muscle and nerve cells and how PCBs and related non-coplanar structures alter these functions. The molecular and cellular mechanisms by which non-coplanar PCBs and related structures alter local and global Ca2+ signaling properties and the possible short and long-term consequences of these perturbations on neurodevelopment and neurodegeneration are reviewed.

Regulation of store-operated channels by scaffold proteins in a431 cells

Store-operated channels are major calcium influx pathways in nonexitable cells. Homer scaffold proteins are well known for their role in regulating calcium signaling. Here we report on a detailed single-channel level characterization of native store-operated channels regulated by Homer scaffold proteins in A431 carcinoma cells. By applying the single-channel patch-clamp technique, we found that different types of store-operated calcium channels have different sensitivities to Homer proteins.

Role of ryanodine receptor subtypes in initiation and formation of calcium sparks in arterial smooth muscle: comparison with striated muscle

Calcium sparks represent local, rapid, and transient calcium release events from a cluster of ryanodine receptors (RyRs) in the sarcoplasmic reticulum. In arterial smooth muscle cells (SMCs), calcium sparks activate calcium-dependent potassium channels causing decrease in the global intracellular [Ca2+] and oppose vasoconstriction. This is in contrast to cardiac and skeletal muscle, where spatial and temporal summation of calcium sparks leads to global increases in intracellular [Ca2+] and myocyte contraction. We summarize the present data on local RyR calcium signaling in arterial SMCs in comparison to striated muscle and muscle-specific differences in coupling between L-type calcium channels and RyRs. Accordingly, arterial SMC Ca(v)1.2 L-type channels regulate intracellular calcium stores content, which in turn modulates calcium efflux though RyRs. Downregulation of RyR2 up to a certain degree is compensated by increased SR calcium content to normalize calcium sparks. This indirect coupling between Ca(v)1.2 and RyR in arterial SMCs is opposite to striated muscle, where triggering of calcium sparks is controlled by rapid and direct cross-talk between Ca(v)1.1/Ca(v)1.2 L-type channels and RyRs. We discuss the role of RyR isoforms in initiation and formation of calcium sparks in SMCs and their possible molecular binding partners and regulators, which differ compared to striated muscle.

Homer and the ryanodine receptor

Homer proteins have recently been identified as novel high-affinity ligands that modulate ryanodine receptor (RyR) Ca(2+) release channels in heart and skeletal muscle, through an EVH1 domain which binds to proline-rich regions in target proteins. Many Homer proteins can also self-associate through a coiled-coil domain that allows their multimerisation. In other tissues, especially neurons, Homer anchors proteins embedded in the surface membrane to the Ca(2+) release channel in the endoplasmic reticulum and can anchor membrane or cytosolic proteins to the cytoskeleton. Although this anchoring aspect of Homer function has not been extensively investigated in muscle, there are consensus sequences for Homer binding in the RyR and on many of the proteins that it interacts with in the massive RyR ion channel complex. In this review we explore the potential of Homer to contribute to a variety of cell processes in muscle and neurons that also involve RyR channels.

The Na+/Ca2+ exchange inhibitor 2-(2-(4-(4-nitrobenzyloxy)phenyl)ethyl)isothiourea methanesulfonate (KB-R7943) also blocks ryanodine receptors type 1 (RyR1) and type 2 (RyR2) channels

Na(+)/Ca(2+) exchanger (NCX) is a plasma membrane transporter that moves Ca(2+) in or out of the cell, depending on membrane potential and transmembrane ion gradients. NCX is the main pathway for Ca(2+) extrusion from excitable cells. NCX inhibitors can ameliorate cardiac ischemia-reperfusion injury and promote high-frequency fatigue of skeletal muscle, purportedly by inhibiting the Ca(2+) inward mode of NCX. Here we tested two known NCX inhibitors, 2-(2-(4-(4-nitrobenzyloxy)phenyl)ethyl)-isothiourea methanesulfonate (KB-R7943) and the structurally related 2-[[4-[(4-Nitrophenyl)methoxy]phenyl]methyl]-4-thiazoli dinecarboxylic acid ethyl ester (SN-6), for their influence on electrically or caffeine-evoked Ca(2+) transients in adult dissociated flexor digitorum brevis (FDB) skeletal muscle fibers and human embryonic kidney (HEK) 293 cells that have stable expression of type 1 ryanodine receptor (RyR1). KB-R7943 (< or = 10 microM) reversibly attenuates electrically evoked Ca(2+) transients in FDB and caffeine-induced Ca(2+) release in HEK 293, whereas the structurally related NCX inhibitor SN-6 does not, suggesting that KB-R7943 directly inhibits RyR1. In support of this interpretation, KB-R7943 inhibits high-affinity binding of [(3)H]ryanodine to RyR1 (IC(50) = 5.1 +/- 0.9 microM) and the cardiac isoform RyR2 (IC(50) = 13.4 +/- 1.8 microM). KB-R7943 interfered with the gating of reconstituted RyR1 and RyR2 channels, reducing open probability (P(o)), shortening mean open time, and prolonging mean closed time. KB-R7943 was more effective at blocking RyR1 with cytoplasmic conditions favoring high P(o) compared with those favoring low P(o). SN-6 has negligible activity toward altering [(3)H]ryanodine binding of RyR1 and RyR2. Our results identify that KB-R7943 is a reversible, activity-dependent blocker of the two most broadly expressed RyR channel isoforms and contributes to its pharmacological and therapeutic activities.

In vitro modulation of the cardiac ryanodine receptor activity by Homer1

The Homer protein family allows clustering and/or functional modulation of many proteins from different calcium signalling complexes including those formed by the ryanodine receptor (RyR) Ca(2+) release channel in skeletal muscle and the heart. Homer1b/c and the cardiac RyR (RyR2) are strongly expressed in the heart and neurons where their interaction with each other may modulate Ca(2+) signalling. However, functional interactions between Homer1b and RyR2 have been poorly defined. Our preliminary data and similar consensus binding sites for Homer in RyR2 and skeletal RyR (RyR1) proteins, led to the hypothesis that Homer may similarly regulate both RyR isoforms. Single-channel and [(3)H]ryanodine binding data showed that RyR2 and RyR1 activity increased to a maximum with ~50-100 nM Homer1b and fell with Homer1b > 200 nM. Homer1b (50 nM) activated RyR2 and RyR1 at all cytosolic [Ca(2+)]; estimated EC(50) value of RyR2 diminished from ~2.8 microM Ca(2+) (control) to ~1.9 microM Ca(2+) in the presence of 50 nM Homer1b. Short Homer1 (lacking the coiled-coil multimerisation domain) and Homer1b similarly modulated RyR2, indicating an action through ligand binding, not mutimerisation. These actions of Homer were generally similar in RyR2 and RyR1. The strong functional interactions suggest that Homer1 is likely to be an endogenous modulator of RyR channels in the heart and neurons as well as in skeletal muscle.

Targeting Homer genes using adeno-associated viral vector: lessons learned from behavioural and neurochemical studies

Over a decade of in-vitro data support a critical role for members of the Homer family of postsynaptic scaffolding proteins in regulating the functional architecture of glutamate synapses. Earlier studies of Homer knockout mice indicated a necessary role for Homer gene products in normal mesocorticolimbic glutamate transmission and behaviours associated therewith. The advent of adeno-associated viral vectors carrying cDNA for, or short hairpin RNA against, specific Homer isoforms enabled the site-directed targeting of Homers to neurons in the brain. This approach has allowed our groups to address developmental issues associated with conventional knockout mice, to confirm active roles for distinct Homer isoforms in regulating glutamate transmission in vivo, as well as in mediating a variety of behavioural processes. This review summarizes the existing data derived from our studies using adeno-associated viral vector-mediated neuronal targeting of Homer in rodents, implicating this family of proteins in drug and alcohol addiction, learning/memory and emotional processing.