Scaffolding protein Homer 1c mediates hypertrophic responses downstream of Gq in cardiomyocytes

Shank3 related muscular hypotonia is accompanied by increased intracellular calcium concentrations and ion channel dysregulation in striated muscle tissue

Phelan-McDermid syndrome (PMS) is a syndromic form of Autism Spectrum Disorders (ASD) classified as a rare genetic neurodevelopmental disorder featuring global developmental delay, absent or delayed speech, ASD-like behaviour and neonatal skeletal muscle hypotonia. PMS is caused by a heterozygous deletion of the distal end of chromosome 22q13.3 or SHANK3 mutations. We analyzed striated muscles of newborn Shank3Δ11(-/-) animals and found a significant enlargement of the sarcoplasmic reticulum as previously seen in adult Shank3Δ11(-/-) mice, indicative of a Shank3-dependent and not compensatory mechanism for this structural alteration. We analyzed transcriptional differences by RNA-sequencing of muscle tissue of neonatal Shank3Δ11(-/-) mice and compared those to Shank3(+/+) controls. We found significant differences in gene expression of ion channels crucial for muscle contraction and for molecules involved in calcium ion regulation. In addition, calcium storage- [i.e., Calsequestrin (CSQ)], calcium secretion- and calcium-related signaling-proteins were found to be affected. By immunostainings and Western blot analyses we could confirm these findings both in Shank3Δ11(-/-) mice and PMS patient muscle tissue. Moreover, alterations could be induced in vitro by the selective downregulation of Shank3 in C2C12 myotubes. Our results emphasize that SHANK3 levels directly or indirectly regulate calcium homeostasis in a cell autonomous manner that might contribute to muscular hypotonia especially seen in the newborn.

The emerging roles of Shank3 in cardiac function and dysfunction

Shank3 is a member of the Shank family proteins (Shank1-3), which are abundantly present in the postsynaptic density (PSD) of neuronal excitatory synapses. As a core scaffold in the PSD, Shank3 plays a critical role in organizing the macromolecular complex, ensuring proper synaptic development and function. Clinically, various mutations of the SHANK3 gene are causally associated with brain disorders such as autism spectrum disorders and schizophrenia. However, recent in vitro and in vivo functional studies and expression profiling in various tissues and cell types suggest that Shank3 also plays a role in cardiac function and dysfunction. For example, Shank3 interacts with phospholipase Cβ1b (PLCβ1b) in cardiomyocytes, regulating its localization to the sarcolemma and its role in mediating Gq-induced signaling. In addition, changes in cardiac morphology and function associated with myocardial infarction and aging have been investigated in a few Shank3 mutant mouse models. This review highlights these results and potential underlying mechanisms, and predicts additional molecular functions of Shank3 based on its protein interactors in the PSD, which are also highly expressed and function in the heart. Finally, we provide perspectives and possible directions for future studies to better understand the roles of Shank3 in the heart.

Molecular regulation of PLCβ signaling

Phospholipase C (PLC) enzymes convert the membrane phospholipid phosphatidylinositol-4,5-bisphosphate (PIP2) into inositol-1,4,5-triphosphate (IP3) and diacylglycerol (DAG). IP3 and DAG regulate numerous downstream pathways, eliciting diverse and profound cellular changes and physiological responses. In the six PLC subfamilies in higher eukaryotes, PLCβ is intensively studied due to its prominent role in regulating crucial cellular events underlying many processes including cardiovascular and neuronal signaling, and associated pathological conditions. In addition to GαqGTP, Gβγ generated upon G protein heterotrimer dissociation also regulates PLCβ activity. Here, we not only review how Gβγ directly activates PLCβ, and also extensively modulates Gαq-mediated PLCβ activity, but also provide a structure-function overview of PLC family members. Given that Gαq and PLCβ are oncogenes, and Gβγ shows unique cell-tissue-organ specific expression profiles, Gγ subtype-dependent signaling efficacies, and distinct subcellular activities, this review proposes that Gβγ is a major regulator of Gαq-dependent and independent PLCβ signaling.

Kainic acid-induced status epilepticus decreases mGlu5 receptor and phase-specifically downregulates Homer1b/c expression

Globally, over 50 million people are affected by epilepsy, which is characterized by the occurrence of spontaneous recurrent seizures. Almost one-third of the patients show resistance to current anti-epileptic drugs, making the exploration of new molecular targets necessary. An interesting target may be Homer1, due to its diverse roles in epileptogenesis and synaptic plasticity. Indeed, Homer1 regulates group I metabotropic glutamate (mGlu) receptors (i.e. mGlu₁ and mGlu₅) scaffolding and signaling in neurons. In the present work, using the systemic kainic acid (KA)-induced status epilepticus (SE) model in adult rats, we investigated the mRNA and protein expression patterns of the mGlu₅ receptor, Homer1a and Homer1b/c at 10, 80 and 120 days post-SE (i.e. T10, T80 and T120). Epileptogenesis was validated by electrophysiological recordings of seizures via electroencephalography (EEG) monitoring and through upregulation of glial fibrillary acidic protein. At the protein level, the mGlu₅ receptor was downregulated in the late latent phase (T10) and the early- and late exponential growth phase (T80 and T120, respectively), which was best observed in the hippocampal CA1 region. At mRNA level, significant downregulation of the mGlu₅ receptor was only detected in the late exponential growth phase. Homer1a expression did not change at any investigated time point. Interestingly, Homer1b/c was only downregulated in the late latent phase, a period where spontaneous seizures are extremely rare. Thus, this phase-specific downregulation may be indicative of an endogenous neuroprotective mechanism. In conclusion, these results suggest that Homer1b/c may be an interesting molecular target to prevent epileptogenesis and/or control seizures.

Phospholipase Cβ3 Membrane Adsorption and Activation Are Regulated by Its C-Terminal Domains and Phosphatidylinositol 4,5-Bisphosphate

Phospholipase Cβ (PLCβ) enzymes hydrolyze phosphatidylinositol 4,5-bisphosphate to produce second messengers that regulate intracellular Ca²⁺, cell proliferation, and survival. Their activity is dependent upon interfacial activation that occurs upon localization to cell membranes. However, the molecular basis for how these enzymes productively interact with the membrane is poorly understood. Herein, atomic force microscopy demonstrates that the ∼300-residue C-terminal domain promotes adsorption to monolayers and is required for spatial organization of the protein on the monolayer surface. PLCβ variants lacking this C-terminal domain display differences in their distribution on the surface. In addition, a previously identified autoinhibitory helix that binds to the PLCβ catalytic core negatively impacts membrane binding, providing an additional level of regulation for membrane adsorption. Lastly, defects in phosphatidylinositol 4,5-bisphosphate hydrolysis also alter monolayer adsorption, reflecting a role for the active site in this process. Together, these findings support a model in which multiple elements of PLCβ modulate adsorption, distribution, and catalysis at the cell membrane.

Chikusetsu saponin IVa confers cardioprotection via SIRT1/ERK1/2 and Homer1a pathway

Hyperglycemia-induced reactive oxygen species (ROS) generation and Ca²⁺ overload contribute to the development of diabetic cardiomyopathy. In this study, we aimed to study the protective effects of Chikusetsu saponin IVa (CHS) from Aralia taibaiensis against hyperglycemia-induced myocardial injuries. Treatment of H9c2 cells with high glucose (HG) for 24 h resulted in a loss of cell viability and increase of ROS, LDH and Ca²⁺ levels, and also induced cell apoptosis, and those changes were all markedly reversed by the administration of CHS. In further studies, CHS dose-dependently increased the expression of Homer1a, ERK1/2 and SIRT1 in both H9c2 cells and rat primary cardiomyocytes. However, transfection of Homer1a-specific siRNA abolished the ability of CHS in controlling the ROS and Ca²⁺ homeostasis. Moreover, specific SIRT1 inhibitors or siRNA significantly suppressed the enhanced phosphorylation of ERK1/2 and expression of Homer1a induced by CHS as well as its cytoprotective effect. CHS induced Homer1a expression was also suppressed by siERK1/2. Additionally, results in diabetic mice also showed that CHS protected myocardium from I/R-introduced apoptosis by activating the SIRT1/ERK1/2/Homer1a pathway. These results demonstrated that CHS protected against hyperglycemia-induced myocardial injury through SIRT1/ERK1/2 and Homer1a pathway in vivo and in vitro.

Genetic variants in PLCB4/PLCB1 as susceptibility loci for coronary artery aneurysm formation in Kawasaki disease in Han Chinese in Taiwan

Kawasaki disease (KD) is an acute, inflammatory, and self-limited vasculitis affecting infants and young children. Coronary artery aneurysm (CAA) formation is the major complication of KD and the leading cause of acquired cardiovascular disease among children. To identify susceptible loci that might predispose patients with KD to CAA formation, a genome-wide association screen was performed in a Taiwanese KD cohort. Patients with both KD and CAA had longer fever duration and delayed intravenous immunoglobulin treatment time. After adjusting for these factors, 100 susceptibility loci were identified. Four genes were identified from a single cluster of 35 using the Ingenuity Pathway Analysis (IPA) Knowledge Base. Silencing KCNQ5, PLCB1, PLCB4, and PLCL1 inhibited the effect of lipopolysaccharide-induced endothelial cell inflammation with varying degrees of proinflammatory cytokine expression. PLCB1 showed the most significant inhibition. Endothelial cell inflammation was also inhibited by using a phospholipase C (PLC) inhibitor. The single nucleotide polymorphism rs6140791 was identified between PLCB4 and PLCB1. Plasma PLC levels were higher in patients with KD and CC+CG rs6140791genotypes, and these genotypes were more prevalent in patients with KD who also had CAA. Our results suggest that polymorphism of the PLCB4/B1 genes might be involved in the CAA pathogenesis of KD.

Molecular mechanisms of phospholipase C β3 autoinhibition

Phospholipase C β (PLCβ) enzymes are dramatically activated by heterotrimeric G proteins. Central to this response is the robust autoinhibition of PLCβ by the X-Y linker region within its catalytic core and by the Hα2' helix in the C-terminal extension of the enzyme. The molecular mechanism of each and their mutual dependence are poorly understood. Herein, it is shown that distinct regions within the X-Y linker have specific roles in regulating activity. Most important,an acidic stretch within the linker stabilizes a lid that occludes the active site, consistent with crystal structures of variants lacking this region. Inhibition by the Hα2' helix is independent of the X-Y linker and likely regulates activity by limiting membrane interaction of the catalytic core. Full activation of PLCβ thus requires multiple independent molecular events induced by membrane association of the catalytic core and by the binding of regulatory proteins.

The atypical 'b' splice variant of phospholipase Cβ1 promotes cardiac contractile dysfunction

The activity of the early signaling enzyme, phospholipase Cβ1b (PLCβ1b), is selectively elevated in diseased myocardium and activity increases with disease progression. We aimed to establish the contribution of heightened PLCβ1b activity to cardiac pathology. PLCβ1b, the alternative splice variant, PLCβ1a, and a blank virus were expressed in mouse hearts using adeno-associated viral vectors (rAAV6-FLAG-PLCβ1b, rAAV6-FLAG-PLCβ1a, or rAAV6-blank) delivered intravenously (IV). Following viral delivery, FLAG-PLCβ1b was expressed in all of the chambers of the mouse heart and was localized to the sarcolemma. Heightened PLCβ1b expression caused a rapid loss of contractility, 4-6 weeks, that was fully reversed, within 5 days, by inhibition of protein kinase Cα (PKCα). PLCβ1a did not localize to the sarcolemma and did not affect contractile function. Expression of PLCβ1b, but not PLCβ1a, caused downstream dephosphorylation of phospholamban and depletion of the Ca(2+) stores of the sarcoplasmic reticulum. We conclude that heightened PLCβ1b activity observed in diseased myocardium contributes to pathology by PKCα-mediated contractile dysfunction. PLCβ1b is a cardiac-specific signaling system, and thus provides a potential therapeutic target for the development of well-tolerated inotropic agents for use in failing myocardium.

Characterization of the PLCB1 promoter and regulation by early growth response transcription factor EGR-1

The Gαq/-Gα11-PLCβ1 pathway is important for intracellular signalling and associated with pathological conditions, such as cardiac hypertrophy. The GNAQ and GNA11 promoters (encoding for Gαq and Gα11) have already been characterized and are both regulated by the transcription factor early growth response 1 (Egr-1). In contrast, the PLCB1 promoter (encoding for the direct downstream effector PLCβ1) has neither been cloned nor characterized. Therefore, the purpose of this study was to 1) characterize the PLCB1 promoter, and 2) assess its potential regulation by Egr-1. By means of 5'- Rapid Amplification of 5'-cDNA ends analysis in human heart tissue we found an initiation of transcription from multiple starting points, the main transcription starting point being located at nt-235 relative to the translation start point. The PLCB1 promoter was cloned and deletion constructs were generated. Luciferase assays were performed in three different cell lines and regulatory regions were identified between nt-595/nt-313 (Hek293: P=0.013; HASMC: P=0.019; H9c2: P=0.005). In electrophoretic mobility shift assays one specific Egr-1 binding site was identified at nt-451/-419 and PLCB1 promoter activity was increased more than 5-fold (Hek293: P=0.0008) and 1,6- fold (H9c2: P=0.0499) following overexpression of Egr-1. Thus, the PLCB1 promoter was characterized for the first time and a specific interaction with the transcription factor Egr-1 was shown. Our data provide a potential molecular mechanism relating to pathophysiological conditions such as cardiac hypertrophy where activation by Egr-1 of Gαq/Gα11-PLCβ1 plays an important role.

The phosphatidylinositol(4,5)bisphosphate-binding sequence of transient receptor potential channel canonical 4α is critical for its contribution to cardiomyocyte hypertrophy

Cardiomyocyte hypertrophy requires a source of Ca(2+) distinct from the Ca(2+) that regulates contraction. The canonical transient receptor potential channel (TrpC) family, a family of cation channels regulated by activation of phospholipase C (PLC), has been implicated in this response. Cardiomyocyte hypertrophy downstream of Gq-coupled receptors is mediated specifically by PLCβ1b that is scaffolded onto a SH3 and ankyrin repeat protein 3 (Shank3) complex at the sarcolemma. TrpC4 exists as two splice variants (TrpC4α and TrpC4β) that differ only in an 84-residue sequence that binds to phosphatidylinositol(4,5)bisphosphate (PIP2), the substrate of PLCβ1b. In neonatal rat cardiomyocytes, TrpC4α, but not TrpC4β, coimmunoprecipitated with both PLCβ1b and Shank3. Heightened PLCβ1b expression caused TrpC4α, but not TrpC4β, translocation to the sarcolemma, where it colocalized with PLCβ1b. When overexpressed in cardiomyocytes, TrpC4α, but not TrpC4β, increased cell area (893 ± 18 to 1497 ± 29 mm(2), P < 0.01) and marker gene expression (atrial natriuretic peptide increased by 409 ± 32%, and modulatory calcineurin inhibitory protein 1 by 315 ± 28%, P < 0.01). Dominant-negative TrpC4 reduced hypertrophy initiated by PLCβ1b, or PLCβ1b-coupled receptor activation, by 72 ± 8% and 39 ± 5 %, respectively. We conclude that TrpC4α is selectively involved in mechanisms downstream of PLCβ1b culminating in cardiomyocyte hypertrophy, and that the hypertrophic response is dependent on the TrpC4α splice variant-specific sequence that binds to PIP2.

Gαq signalling: the new and the old

In the last few years the interactome of Gαq has expanded considerably, contributing to improve our understanding of the cellular and physiological events controlled by this G alpha subunit. The availability of high-resolution crystal structures has led the identification of an effector-binding region within the surface of Gαq that is able to recognise a variety of effector proteins. Consequently, it has been possible to ascribe different Gαq functions to specific cellular players and to identify important processes that are triggered independently of the canonical activation of phospholipase Cβ (PLCβ), the first identified Gαq effector. Novel effectors include p63RhoGEF, that provides a link between G protein-coupled receptors and RhoA activation, phosphatidylinositol 3-kinase (PI3K), implicated in the regulation of the Akt pathway, or the cold-activated TRPM8 channel, which is directly inhibited upon Gαq binding. Recently, the activation of ERK5 MAPK by Gq-coupled receptors has also been described as a novel PLCβ-independent signalling axis that relies upon the interaction between this G protein and two novel effectors (PKCζ and MEK5). Additionally, the association of Gαq with different regulatory proteins can modulate its effector coupling ability and, therefore, its signalling potential. Regulators include accessory proteins that facilitate effector activation or, alternatively, inhibitory proteins that downregulate effector binding or promote signal termination. Moreover, Gαq is known to interact with several components of the cytoskeleton as well as with important organisers of membrane microdomains, which suggests that efficient signalling complexes might be confined to specific subcellular environments. Overall, the complex interaction network of Gαq underlies an ever-expanding functional diversity that puts forward this G alpha subunit as a major player in the control of physiological functions and in the development of different pathological situations.

Structural insights into phospholipase C-β function

Phospholipase C (PLC) enzymes convert phosphatidylinositol-4,5-bisphosphate into the second messengers diacylglycerol and inositol-1,4,5-triphosphate. The production of these molecules promotes the release of intracellular calcium and activation of protein kinase C, which results in profound cellular changes. The PLCβ subfamily is of particular interest given its prominent role in cardiovascular and neuronal signaling and its regulation by G protein-coupled receptors, as PLCβ is the canonical downstream target of the heterotrimeric G protein Gαq. However, this is not the only mechanism regulating PLCβ activity. Extensive structural and biochemical evidence has revealed regulatory roles for autoinhibitory elements within PLCβ, Gβγ, small molecular weight G proteins, and the lipid membrane itself. Such complex regulation highlights the central role that this enzyme plays in cell signaling. A better understanding of the molecular mechanisms underlying the control of its activity will greatly facilitate the search for selective small molecule modulators of PLCβ.

Negative feedback regulation of Homer 1a on norepinephrine-dependent cardiac hypertrophy

Homers are scaffolding proteins that modulate diverse cell functions being able to assemble signalling complexes. In this study, the presence, sub-cellular distribution and function of Homer 1 was investigated. Homer 1a and Homer 1b/c are constitutively expressed in cardiac muscle of both mouse and rat and in HL-1 cells, a cardiac cell line. As judged by confocal immunofluorescence microscopy, Homer 1a displays sarcomeric and peri-nuclear localization. In cardiomyocytes and cultured HL-1 cells, the hypertrophic agonist norepinephrine (NE) induces α1-adrenergic specific Homer 1a over-expression, with a two-to-three-fold increase within 1h, and no up-regulation of Homer 1b/c, as judged by Western blot and qPCR. In HL-1 cells, plasmid-driven over-expression of Homer 1a partially antagonizes activation of ERK phosphorylation and ANF up-regulation, two well-established, early markers of hypertrophy. At the morphometric level, NE-induced increase of cell size is likewise and partially counteracted by exogenous Homer 1a. Under the same experimental conditions, Homer 1b/c does not have any effect on ANF up-regulation nor on cell hypertrophy. Thus, Homer 1a up-regulation is associated to early stages of cardiac hypertrophy and appears to play a negative feedback regulation on molecular transducers of hypertrophy.