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

Abstract

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 asymmetric 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 asymmetric 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 are used to define a regulatory pathway controlling pathological myocyte hypertrophy. Chromatin Immunoprecipitation Assays with Sequencing (ChIP-seq) and Precision Nuclear Run-On Sequencing (PRO-seq) are used to define a transcriptional mechanism. Results: Here we report that asymmetric cardiac myocyte hypertrophy is modulated by serum response factor (SRF) phosphorylation, constituting an epigenomic switch balancing the growth in width versus length of adult ventricular myocytes in vitro and in vivo . SRF Ser 103 phosphorylation is bidirectionally regulated by p90 ribosomal S6 kinase type 3 (RSK3) and protein phosphatase 2A (PP2A) at signalosomes organized by the scaffold protein muscle A-kinase anchoring protein β (mAKAPβ), such that increased SRF phosphorylation activates Activator Protein 1 (AP1)-dependent enhancers that direct myocyte growth in width. AAV are used to express in vivo mAKAPβ-derived RSK3 and PP2A anchoring disruptor peptides that block the association of the enzymes with the mAKAPβ scaffold. Inhibition of RSK3 signaling prevents concentric cardiac remodeling due to pressure overload, while inhibition of PP2A signaling prevents eccentric cardiac remodeling induced by myocardial infarction, in each case improving cardiac function. SRF Ser 103 phosphorylation is significantly decreased in dilated human hearts, supporting the notion that modulation of the mAKAPβ-SRF signalosome could be a new therapeutic approach for human heart failure. Conclusions: We have identified a new molecular switch, namely mAKAPβ 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.

ERK1/2: An Integrator of Signals That Alters Cardiac Homeostasis and Growth

“…ERK activity is also controlled via several phosphatases to regulate the duration of the transduced signal. While the dephosphorylation of ERK may be achieved by Ser/Thr [ 12 ] or Tyr phosphatases [ 13 ], there also exist specialized dual specificity phosphatases (DUSPs) that function to dephosphorylate both residues concurrently. These DUSPs comprise a phosphatase domain with a conserved catalytic site with Asp, Cys, and Arg residues within a flexible enzymatic pocket to accommodate phosphorylated targets [ 14 ].…”

“…Genetic ablation of GATA4 precludes hypertrophy, and this phenotype was not found to be rescued in activated MEK1 transgenic mice, suggesting GATA4 as a critical mediator of the MEK1–ERK pathway in the heart [ 108 ]. Recently, ERK effector RSK3 was identified to phosphorylate another transcription regulator, serum-responsive factor (SRF), where downstream signaling modulates cardiomyocyte growth in width, which is essential to concentric hypertrophy [ 12 ]. Recent reports also indicate crosstalk between ERK and transcription factor STAT3 signaling to mediate cardioprotection in myocytes treated with doxorubicin, again highlighting the importance of the ERK pathway for the integration of stress responses in the heart [ 109 ].…”

A GSK3-SRF Axis Mediates Angiotensin II Induced Endothelin Transcription in Vascular Endothelial Cells

“…A recent study by Li et al have shown that dynamic regulation of SRF serine 103 phosphorylation by RSK3 and PP2A significantly impacts phenylephrine-induced gene transcription in cardiomyocytes. Specifically, the authors demonstrate, using ChIP-seq and PRO-seq, that the phosphomimetic SRF S103D appears to promote the association of SRF with enhancers, facilitate the recruitment of the basal transcription machinery, and preferentially augment the expression of early response and hypertrophic genes ( Li J. et al, 2020 ). Similar strategies could exploited to determine how the SRF S234D mutant influence the endothelial transcriptome.…”

FGF21-FGFR4 signaling in cardiac myocytes promotes concentric cardiac hypertrophy in mouse models of diabetes

“…We next considered that FGF21 might drive concentric remodeling in the presence of other stress stimuli. The α-adrenergic agonist phenylephrine induces a selective growth in width of cultured ARVM, while the β-adrenergic agonist isoproterenol induces growth in both width and length, resulting in a more symmetric ARVM hypertrophy (Li et al, 2020a). Co-treatment with FGF21 did not significantly further increase the prominent growth in width induced by phenylephrine or isoproterenol (Figure 2E-H).…”

“…Concentric cardiac hypertrophy is characterized by an increase in relative wall thickness (left ventricular wall thickness to internal diameter ratio) that is primarily a reflection of increased cardiac myocyte width, while, in contrast, eccentric cardiac hypertrophy is characterized by decreased relative wall thickness and relative myocyte lengthening. Concentric hypertrophy can be induced by activation of an extracellular signal-regulated kinase 1/2 (ERK1/2) -p90 ribosomal S6 kinase type 3 (RSK3) -serum response factor (SRF)-dependent gene expression regulatory pathway (Kehat et al, 2011;Li et al, 2020a;Passariello et al, 2016). It has been observed that in cardiac myocytes FGF21 activates ERK1/2 and, like ERK1/2 gene deletion, unstressed FGF21 knock-out mice exhibit eccentric cardiac hypertrophy (Planavila et al, 2013).…”