Protein phosphatase 2A anchoring disruptor gene therapy for familial dilated cardiomyopathy.

Familial dilated cardiomyopathy is a prevalent cause of heart failure that results from the mutation of genes encoding proteins of diverse function. Despite modern therapy, dilated cardiomyopathy typically has a poor outcome and is the leading cause of cardiac transplantation. The phosphatase PP2A at cardiomyocyte perinuclear mAKAPß signalosomes promotes pathological eccentric cardiac remodeling, as is characteristic of dilated cardiomyopathy. Displacement of PP2A from mAKAPß, inhibiting PP2A function in that intracellular compartment, can be achieved by expression of a mAKAPß-derived PP2A binding domain-derived peptide. To test whether PP2A anchoring disruption would be effective at preventing dilated cardiomyopathy-associated cardiac dysfunction, the adeno-associated virus gene therapy vector AAV9sc.PBD was devised to express the disrupting peptide in cardiomyocytes in vivo. Proof-of-concept is now provided that AAV9sc.PBD improves the cardiac structure and function of a cardiomyopathy mouse model involving transgenic expression of a mutant α-tropomyosin E54K Tpm1 allele, while AAV9sc.PBD has no effect on normal non-transgenic mice. At the cellular level, AAV9sc.PBD restores cardiomyocyte morphology and gene expression in the mutant Tpm1 mouse. As the mechanism of AAV9sc.PBD action suggests potential efficacy in dilated cardiomyopathy regardless of the underlying etiology, these data support the further testing of AAV9sc.PBD as a broad-based treatment for dilated cardiomyopathy.

Ca2+/Calmodulin-Dependent Protein Kinase II Enhances Retinal Ganglion Cell Survival But Suppresses Axon Regeneration after Optic Nerve Injury.

Neuroprotection after injury or in neurodegenerative disease remains a major goal for basic and translational neuroscience. Retinal ganglion cells (RGCs), the projection neurons of the eye, degenerate in optic neuropathies after axon injury, and there are no clinical therapies to prevent their loss or restore their connectivity to targets in the brain. Here we demonstrate a profound neuroprotective effect of the exogenous expression of various Ca2+/calmodulin-dependent protein kinase II (CaMKII) isoforms in mice. A dramatic increase in RGC survival following the optic nerve trauma was elicited by the expression of constitutively active variants of multiple CaMKII isoforms in RGCs using adeno-associated viral (AAV) vectors across a 100-fold range of AAV dosing in vivo. Despite this neuroprotection, however, short-distance RGC axon sprouting was suppressed by CaMKII, and long-distance axon regeneration elicited by several pro-axon growth treatments was likewise inhibited even as CaMKII further enhanced RGC survival. Notably, in a dose-escalation study, AAV-expressed CaMKII was more potent for axon growth suppression than the promotion of survival. That diffuse overexpression of constitutively active CaMKII strongly promotes RGC survival after axon injury may be clinically valuable for neuroprotection per se. However, the associated strong suppression of the optic nerve axon regeneration demonstrates the need for understanding the intracellular domain- and target-specific CaMKII activities to the development of CaMKII signaling pathway-directed strategies for the treatment of optic neuropathies.

Targeting mAKAPß expression as a therapeutic approach for ischemic cardiomyopathy.

Ischemic cardiomyopathy is a leading cause of death and an unmet clinical need. Adeno-associated virus (AAV) gene-based therapies hold great promise for treating and preventing heart failure. Previously we showed that muscle A-kinase Anchoring Protein ß (mAKAPß, AKAP6ß), a scaffold protein that organizes perinuclear signalosomes in the cardiomyocyte, is a critical regulator of pathological cardiac hypertrophy. Here, we show that inhibition of mAKAPß expression in stressed adult cardiomyocytes in vitro was cardioprotective, while conditional cardiomyocyte-specific mAKAP gene deletion in mice prevented pathological cardiac remodeling due to myocardial infarction. We developed a new self-complementary serotype 9 AAV gene therapy vector expressing a short hairpin RNA for mAKAPß under the control of a cardiomyocyte-specific promoter (AAV9sc.shmAKAP). This vector efficiently downregulated mAKAPß expression in the mouse heart in vivo. Expression of the shRNA also inhibited mAKAPß expression in human induced cardiomyocytes in vitro. Following myocardial infarction, systemic administration of AAV9sc.shmAKAP prevented the development of pathological cardiac remodeling and heart failure, providing long-term restoration of left ventricular ejection fraction. Our findings provide proof-of-concept for mAKAPß as a therapeutic target for ischemic cardiomyopathy and support the development of a translational pipeline for AAV9sc.shmAKAP for the treatment of heart failure.

A perinuclear calcium compartment regulates cardiac myocyte hypertrophy.

The pleiotropic Ca2+/calmodulin-dependent phosphatase calcineurin is a key regulator of pathological cardiac myocyte hypertrophy. The selective activation of hypertrophic calcineurin signaling under stress conditions has been attributed to compartmentation of Ca2+ signaling in cardiac myocytes. Here, perinuclear signalosomes organized by the scaffold protein muscle A-Kinase Anchoring Protein ß (mAKAPß/AKAP6ß) are shown to orchestrate local Ca2+ transients, inducing calcineurin-dependent NFATc nuclear localization and myocyte hypertrophy in response to ß-adrenergic receptor activation. Fluorescent biosensors for Ca2+ and calcineurin and protein kinase A (PKA) activity, both diffusely expressed and localized by nesprin-1α to the nuclear envelope, are used to define an autonomous mAKAPß signaling compartment in adult and neonatal rat ventricular myocytes. Notably, ß-adrenergic-stimulated perinuclear Ca2+ and PKA and CaN activity transients depended upon mAKAPß expression, while Ca2+ elevation and PKA and CaN activity in the cytosol were mAKAPß independent. Buffering perinuclear cAMP and Ca2+ prevented calcineurin-dependent NFATc nuclear translocation and myocyte hypertrophy, without affecting cardiac myocyte contractility. Additional findings suggest that the perinuclear Ca2+ transients were mediated by signalosome-associated ryanodine receptors regulated by local PKA phosphorylation. These results demonstrate the existence of a functionally independent Ca2+ signaling compartment in the cardiac myocyte regulating hypertrophy and provide a premise for targeting mAKAPß signalosomes to prevent selectively cardiac hypertrophy in disease.

Distribution of cardiomyocyte-selective adeno-associated virus serotype 9 vectors in swine following intracoronary and intravenous infusion.

Limited reports exist regarding adeno-associated virus (AAV) biodistribution in swine. This study assessed biodistribution following antegrade intracoronary and intravenous delivery of two self-complementary serotype 9 AAV (AAV9sc) biologics designed to target signaling in the cardiomyocyte considered important for the development of heart failure. Under the control of a cardiomyocyte-specific promoter, AAV9sc.shmAKAP and AAV9sc.RBD express a small hairpin RNA for the perinuclear scaffold protein muscle A-kinase anchoring protein ß (mAKAPß) and an anchoring disruptor peptide for p90 ribosomal S6 kinase type 3 (RSK3), respectively. Quantitative PCR was used to assess viral genome (vg) delivery and transcript expression in Ossabaw and Yorkshire swine tissues. Myocardial viral delivery was 2-5 × 105 vg/µg genomic DNA (gDNA) for both infusion techniques at a dose ∼1013 vg/kg body wt, demonstrating delivery of ∼1-3 viral particles per cardiac diploid genome. Myocardial RNA levels for each expressed transgene were generally proportional to dose and genomic delivery, and comparable with levels for moderately expressed endogenous genes. Despite significant AAV9sc delivery to other tissues, including the liver, neither biologic induced toxic effects as assessed using functional, structural, and circulating cardiac and systemic markers. These results indicate successful targeted delivery of cardiomyocyte-selective viral vectors in swine without negative side effects, an important step in establishing efficacy in a preclinical experimental setting.

MEF2 transcription factors differentially contribute to retinal ganglion cell loss after optic nerve injury.

Loss of retinal ganglion cells (RGCs) in optic neuropathies results in permanent partial or complete blindness. Myocyte enhancer factor 2 (MEF2) transcription factors have been shown to play a pivotal role in neuronal systems, and in particular MEF2A knockout was shown to enhance RGC survival after optic nerve crush injury. Here we expanded these prior data to study bi-allelic, tri-allelic and heterozygous allele deletion. We observed that deletion of all MEF2A, MEF2C, and MEF2D alleles had no effect on RGC survival during development. Our extended experiments suggest that the majority of the neuroprotective effect was conferred by complete deletion of MEF2A but that MEF2D knockout, although not sufficient to increase RGC survival on its own, increased the positive effect of MEF2A knockout. Conversely, MEF2A over-expression in wildtype mice worsened RGC survival after optic nerve crush. Interestingly, MEF2 transcription factors are regulated by post-translational modification, including by calcineurin-catalyzed dephosphorylation of MEF2A Ser-408 known to increase MEF2A-dependent transactivation in neurons. However, neither phospho-mimetic nor phospho-ablative mutation of MEF2A Ser-408 affected the ability of MEF2A to promote RGC death in vivo after optic nerve injury. Together these findings demonstrate that MEF2 gene expression opposes RGC survival following axon injury in a complex hierarchy, and further support the hypothesis that loss of or interference with MEF2A expression might be beneficial for RGC neuroprotection in diseases such as glaucoma and other optic neuropathies.

Signalosome-Regulated Serum Response Factor Phosphorylation Determining Myocyte Growth in Width Versus Length as a Therapeutic Target for Heart Failure.

BACKGROUND: Concentric and eccentric cardiac hypertrophy are associated with pressure and volume overload, respectively, in cardiovascular disease both conferring an increased risk of heart failure. These contrasting forms of hypertrophy are characterized by asymmetrical growth of the cardiac myocyte in mainly width or length, respectively. The molecular mechanisms determining myocyte preferential growth in width versus length remain poorly understood. Identification of the mechanisms governing asymmetrical myocyte growth could provide new therapeutic targets for the prevention or treatment of heart failure. METHODS: Primary adult rat ventricular myocytes, adeno-associated virus (AAV)-mediated gene delivery in mice, and human tissue samples were used to define a regulatory pathway controlling pathological myocyte hypertrophy. Chromatin immunoprecipitation assays with sequencing and precision nuclear run-on sequencing were used to define a transcriptional mechanism. RESULTS: We report that asymmetrical cardiac myocyte hypertrophy is modulated by SRF (serum response factor) phosphorylation, constituting an epigenomic switch balancing the growth in width versus length of adult ventricular myocytes in vitro and in vivo. SRF Ser103 phosphorylation is bidirectionally regulated by RSK3 (p90 ribosomal S6 kinase type 3) and PP2A (protein phosphatase 2A) at signalosomes organized by the scaffold protein mAKAPß (muscle A-kinase anchoring protein ß), such that increased SRF phosphorylation activates AP-1 (activator protein-1)-dependent enhancers that direct myocyte growth in width. AAV are used to express in vivo mAKAPß-derived RSK3 and PP2A anchoring disruptor peptides that block the association of the enzymes with the mAKAPß scaffold. Inhibition of RSK3 signaling prevents concentric cardiac remodeling induced by pressure overload, while inhibition of PP2A signaling prevents eccentric cardiac remodeling induced by myocardial infarction, in each case improving cardiac function. SRF Ser103 phosphorylation is significantly decreased in dilated human hearts, supporting the notion that modulation of the mAKAPß-SRF signalosome could be a new therapeutic approach for human heart failure. CONCLUSIONS: We have identified a new molecular switch, namely mAKAPß signalosome-regulated SRF phosphorylation, that controls a transcriptional program responsible for modulating changes in cardiac myocyte morphology that occur secondary to pathological stressors. Complementary AAV-based gene therapies constitute rationally-designed strategies for a new translational modality for heart failure.

Calcineurin Aß-Specific Anchoring Confers Isoform-Specific Compartmentation and Function in Pathological Cardiac Myocyte Hypertrophy.

BACKGROUND: The Ca2+/calmodulin-dependent phosphatase calcineurin is a key regulator of cardiac myocyte hypertrophy in disease. An unexplained paradox is how the ß isoform of the calcineurin catalytic A-subunit (CaNAß) is required for induction of pathological myocyte hypertrophy, despite calcineurin Aα expression in the same cells. It is unclear how the pleiotropic second messenger Ca2+ drives excitation-contraction coupling while not stimulating hypertrophy by calcineurin in the normal heart. Elucidation of the mechanisms conferring this selectivity in calcineurin signaling should reveal new strategies for targeting the phosphatase in disease. METHODS: Primary adult rat ventricular myocytes were studied for morphology and intracellular signaling. New Förster resonance energy transfer reporters were used to assay Ca2+ and calcineurin activity in living cells. Conditional gene deletion and adeno-associated virus-mediated gene delivery in the mouse were used to study calcineurin signaling after transverse aortic constriction in vivo. RESULTS: CIP4 (Cdc42-interacting protein 4)/TRIP10 (thyroid hormone receptor interactor 10) was identified as a new polyproline domain-dependent scaffold for CaNAß2 by yeast 2-hybrid screen. Cardiac myocyte-specific CIP4 gene deletion in mice attenuated pressure overload-induced pathological cardiac remodeling and heart failure. Blockade of CaNAß polyproline-dependent anchoring using a competing peptide inhibited concentric hypertrophy in cultured myocytes; disruption of anchoring in vivo using an adeno-associated virus gene therapy vector inhibited cardiac hypertrophy and improved systolic function after pressure overload. Live cell Förster resonance energy transfer biosensor imaging of cultured myocytes revealed that Ca2+ levels and calcineurin activity associated with the CIP4 compartment were increased by neurohormonal stimulation, but minimally by pacing. Conversely, Ca2+ levels and calcineurin activity detected by nonlocalized Förster resonance energy transfer sensors were induced by pacing and minimally by neurohormonal stimulation, providing functional evidence for differential intracellular compartmentation of Ca2+ and calcineurin signal transduction. CONCLUSIONS: These results support a structural model for Ca2+ and CaNAß compartmentation in cells based on an isoform-specific mechanism for calcineurin protein-protein interaction and localization. This mechanism provides an explanation for the specific role of CaNAß in hypertrophy and its selective activation under conditions of pathologic stress. Disruption of CaNAß polyproline-dependent anchoring constitutes a rational strategy for therapeutic targeting of CaNAß-specific signaling responsible for pathological cardiac remodeling in cardiovascular disease deserving of further preclinical investigation.

Regulation of Neuronal Survival and Axon Growth by a Perinuclear cAMP Compartment.

cAMP signaling is known to be critical in neuronal survival and axon growth. Increasingly the subcellular compartmentation of cAMP signaling has been appreciated, but outside of dendritic synaptic regulation, few cAMP compartments have been defined in terms of molecular composition or function in neurons. Specificity in cAMP signaling is conferred in large part by A-kinase anchoring proteins (AKAPs) that localize protein kinase A and other signaling enzymes to discrete intracellular compartments. We now reveal that cAMP signaling within a perinuclear neuronal compartment organized by the large multivalent scaffold protein mAKAPα promotes neuronal survival and axon growth. mAKAPα signalosome function is explored using new molecular tools designed to specifically alter local cAMP levels as studied by live-cell FRET imaging. In addition, enhancement of mAKAPα-associated cAMP signaling by isoform-specific displacement of bound phosphodiesterase is demonstrated to increase retinal ganglion cell survival in vivo in mice of both sexes following optic nerve crush injury. These findings define a novel neuronal compartment that confers cAMP regulation of neuroprotection and axon growth and that may be therapeutically targeted in disease.SIGNIFICANCE STATEMENT cAMP is a second messenger responsible for the regulation of diverse cellular processes including neuronal neurite extension and survival following injury. Signal transduction by cAMP is highly compartmentalized in large part because of the formation of discrete, localized multimolecular signaling complexes by A-kinase anchoring proteins. Although the concept of cAMP compartmentation is well established, the function and identity of these compartments remain poorly understood in neurons. In this study, we provide evidence for a neuronal perinuclear cAMP compartment organized by the scaffold protein mAKAPα that is necessary and sufficient for the induction of neurite outgrowth in vitro and for the survival of retinal ganglion cells in vivo following optic nerve injury.

Muscle A-kinase-anchoring protein-ß-bound calcineurin toggles active and repressive transcriptional complexes of myocyte enhancer factor 2D.

Myocyte enhancer factor 2 (MEF2) transcription factors are key regulators of the development and adult phenotype of diverse tissues, including skeletal and cardiac muscles. Controlled by multiple post-translational modifications, MEF2D is an effector for the Ca2+/calmodulin-dependent protein phosphatase calcineurin (CaN, PP2B, and PPP3). CaN-catalyzed dephosphorylation promotes the desumoylation and acetylation of MEF2D, increasing its transcriptional activity. Both MEF2D and CaN bind the scaffold protein muscle A-kinase-anchoring protein ß (mAKAPß), which is localized to the nuclear envelope, such that C2C12 skeletal myoblast differentiation and neonatal rat ventricular myocyte hypertrophy are inhibited by mAKAPß signalosome targeting. Using immunoprecipitation and DNA-binding assays, we now show that the formation of mAKAPß signalosomes is required for MEF2D dephosphorylation, desumoylation, and acetylation in C2C12 cells. Reduced MEF2D phosphorylation was coupled to a switch from type IIa histone deacetylase to p300 histone acetylase binding that correlated with increased MEF2D-dependent gene expression and ventricular myocyte hypertrophy. Together, these results highlight the importance of mAKAPß signalosomes for regulating MEF2D activity in striated muscle, affirming mAKAPß as a nodal regulator in the myocyte intracellular signaling network.

Bidirectional regulation of HDAC5 by mAKAPß signalosomes in cardiac myocytes.

Class IIa histone deacetylases (HDACs) are transcriptional repressors whose nuclear export in the cardiac myocyte is associated with the induction of pathological gene expression and cardiac remodeling. Class IIa HDACs are regulated by multiple, functionally opposing post-translational modifications, including phosphorylation by protein kinase D (PKD) that promotes nuclear export and phosphorylation by protein kinase A (PKA) that promotes nuclear import. We have previously shown that the scaffold protein muscle A-kinase anchoring protein ß (mAKAPß) orchestrates signaling in the cardiac myocyte required for pathological cardiac remodeling, including serving as a scaffold for both PKD and PKA. We now show that mAKAPß is a scaffold for HDAC5 in cardiac myocytes, forming signalosomes containing HDAC5, PKD, and PKA. Inhibition of mAKAPß expression attenuated the phosphorylation of HDAC5 by PKD and PKA in response to α- and ß-adrenergic receptor stimulation, respectively. Importantly, disruption of mAKAPß-HDAC5 anchoring prevented the induction of HDAC5 nuclear export by α-adrenergic receptor signaling and PKD phosphorylation. In addition, disruption of mAKAPß-PKA anchoring prevented the inhibition by ß-adrenergic receptor stimulation of α-adrenergic-induced HDAC5 nuclear export. Together, these data establish that mAKAPß signalosomes serve to bidirectionally regulate the nuclear-cytoplasmic localization of class IIa HDACs. Thus, the mAKAPß scaffold serves as a node in the myocyte regulatory network controlling both the repression and activation of pathological gene expression in health and disease, respectively.

Digitalis toxicity: ECG vignette.

"Digitalis toxicity, often candidly indexed as poisoning, has plagued the medical profession for over 200 years. The situation qualifies as a professional disgrace on the basis of three items: the situation persists, physicians are often slow to recognize it and, over the decades, writers have been harsh in their denunciation of fellow physicians when toxicity has occurred…." These are the opening remarks of an essay published in 1983 on the 2nd centenary of William Withering's 'magic potion from foxglove's extract for dropsy.' Even today, after many decades, these words appear relevant! We present and discuss an interesting ECG of digitalis toxicity.

RSK3 is required for concentric myocyte hypertrophy in an activated Raf1 model for Noonan syndrome.

Noonan syndrome (NS) is a congenital disorder resulting from mutations of the Ras-Raf signaling pathway. Hypertrophic cardiomyopathy associated with RAF1 "RASopathy" mutations is a major risk factor for heart failure and death in NS and has been attributed to activation of MEK1/2-ERK1/2 mitogen-activated protein kinases. We recently discovered that type 3 p90 ribosomal S6 kinase (RSK3) is an ERK effector that is required, like ERK1/2, for concentric myocyte hypertrophy in response to pathological stress such as pressure overload. In order to test whether RSK3 also contributes to NS-associated hypertrophic cardiomyopathy, RSK3 knock-out mice were crossed with mice bearing the Raf1(L613V) human NS mutation. We confirmed that Raf1(L613V) knock-in confers a NS-like phenotype, including cardiac hypertrophy. Active RSK3 was increased in Raf1(L613V) mice. Constitutive RSK3 gene deletion prevented the Raf1(L613V)-dependent concentric growth in width of the cardiac myocyte and attenuated cardiac hypertrophy in female mice. These results are consistent with RSK3 being an important mediator of ERK1/2-dependent growth in RASopathy. In conjunction with previously published data showing that RSK3 is important for pathological remodeling of the heart, these data suggest that targeting of this downstream MAP-kinase pathway effector should be considered in the treatment of RASopathy-associated hypertrophic cardiomyopathy.

The scaffold protein muscle A-kinase anchoring protein ß orchestrates cardiac myocyte hypertrophic signaling required for the development of heart failure.

BACKGROUND: Cardiac myocyte hypertrophy is regulated by an extensive intracellular signal transduction network. In vitro evidence suggests that the scaffold protein muscle A-kinase anchoring protein ß (mAKAPß) serves as a nodal organizer of hypertrophic signaling. However, the relevance of mAKAPß signalosomes to pathological remodeling and heart failure in vivo remains unknown. METHODS AND RESULTS: Using conditional, cardiac myocyte-specific gene deletion, we now demonstrate that mAKAPß expression in mice is important for the cardiac hypertrophy induced by pressure overload and catecholamine toxicity. mAKAPß targeting prevented the development of heart failure associated with long-term transverse aortic constriction, conferring a survival benefit. In contrast to 29% of control mice (n=24), only 6% of mAKAPß knockout mice (n=31) died in the 16 weeks of pressure overload (P=0.02). Accordingly, mAKAPß knockout inhibited myocardial apoptosis and the development of interstitial fibrosis, left atrial hypertrophy, and pulmonary edema. This improvement in cardiac status correlated with the attenuated activation of signaling pathways coordinated by the mAKAPß scaffold, including the decreased phosphorylation of protein kinase D1 and histone deacetylase 4 that we reveal to participate in a new mAKAP signaling module. Furthermore, mAKAPß knockout inhibited pathological gene expression directed by myocyte-enhancer factor-2 and nuclear factor of activated T-cell transcription factors that associate with the scaffold. CONCLUSIONS: mAKAPß orchestrates signaling that regulates pathological cardiac remodeling in mice. Targeting of the underlying physical architecture of signaling networks, including mAKAPß signalosome formation, may constitute an effective therapeutic strategy for the prevention and treatment of pathological remodeling and heart failure.

p90 ribosomal S6 kinase 3 contributes to cardiac insufficiency in α-tropomyosin Glu180Gly transgenic mice.

Myocardial interstitial fibrosis is an important contributor to the development of heart failure. Type 3 p90 ribosomal S6 kinase (RSK3) was recently shown to be required for concentric myocyte hypertrophy under in vivo pathological conditions. However, the role of RSK family members in myocardial fibrosis remains uninvestigated. Transgenic expression of α-tropomyosin containing a Glu180Gly mutation (TM180) in mice of a mixed C57BL/6:FVB/N background induces a cardiomyopathy characterized by a small left ventricle, interstitial fibrosis, and diminished systolic and diastolic function. Using this mouse model, we now show that RSK3 is required for the induction of interstitial fibrosis in vivo. TM180 transgenic mice were crossed to RSK3 constitutive knockout (RSK3(-/-)) mice. Although RSK3 knockout did not affect myocyte growth, the decreased cardiac function and mild pulmonary edema associated with the TM180 transgene were attenuated by RSK3 knockout. The improved cardiac function was consistent with reduced interstitial fibrosis in the TM180;RSK3(-/-) mice as shown by histology and gene expression analysis, including the decreased expression of collagens. The specific inhibition of RSK3 should be considered as a potential novel therapeutic strategy for improving cardiac function and the prevention of sudden cardiac death in diseases in which interstitial fibrosis contributes to the development of heart failure.

CIP4 is required for the hypertrophic growth of neonatal cardiac myocytes.

BACKGROUND: CIP4 is a scaffold protein that regulates membrane deformation and tubulation, organization of the actin cytoskeleton, endocytosis of growth factor receptors, and vesicle trafficking. Although expressed in the heart, CIP4 has not been studied with regards to its potential function in cardiac myocytes. RESULTS: We now show using RNA interference that CIP4 expression in neonatal rat ventricular myocytes is required for the induction of non-mitotic, hypertrophic growth by the α-adrenergic agonist phenylephrine, the IL-6 cytokine leukemia inhibitor factor, and fetal bovine serum, as assayed using morphometry, immunocytochemistry for the hypertrophic marker atrial natriuretic factor and [3H]leucine incorporation for de novo protein synthesis. This requirement was consistent with the induction of CIP4 expression by hypertrophic stimulation. The inhibition of myocyte hypertrophy by CIP4 small interfering oligonucleotides (siRNA) was rescued by expression of a recombinant CIP4 protein, but not by a mutant lacking the N-terminal FCH domain responsible for CIP4 intracellular localization. CONCLUSIONS: These results imply that CIP4 plays a significant role in the intracellular hypertrophic signal transduction network that controls the growth of cardiac myocytes in heart disease.

Anchored p90 ribosomal S6 kinase 3 is required for cardiac myocyte hypertrophy.

RATIONALE: Cardiac myocyte hypertrophy is the main compensatory response to chronic stress on the heart. p90 ribosomal S6 kinase (RSK) family members are effectors for extracellular signal-regulated kinases that induce myocyte growth. Although increased RSK activity has been observed in stressed myocytes, the functions of individual RSK family members have remained poorly defined, despite being potential therapeutic targets for cardiac disease. OBJECTIVE: To demonstrate that type 3 RSK (RSK3) is required for cardiac myocyte hypertrophy. METHODS AND RESULTS: RSK3 contains a unique N-terminal domain that is not conserved in other RSK family members. We show that this domain mediates the regulated binding of RSK3 to the muscle A-kinase anchoring protein scaffold, defining a novel kinase anchoring event. Disruption of both RSK3 expression using RNA interference and RSK3 anchoring using a competing muscle A-kinase anchoring protein peptide inhibited the hypertrophy of cultured myocytes. In vivo, RSK3 gene deletion in the mouse attenuated the concentric myocyte hypertrophy induced by pressure overload and catecholamine infusion. CONCLUSIONS: Taken together, these data demonstrate that anchored RSK3 transduces signals that modulate pathologic myocyte growth. Targeting of signaling complexes that contain select kinase isoforms should provide an approach for the specific inhibition of cardiac myocyte hypertrophy and for the development of novel strategies for the prevention and treatment of heart failure.