Research Article

E-C coupling structural protein junctophilin-2 encodes a stress-adaptive transcription regulator

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Science  21 Dec 2018:
Vol. 362, Issue 6421, eaan3303
DOI: 10.1126/science.aan3303

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Protecting the heart

Excitation-contraction (E-C) coupling is fundamental to heart contraction. Junctophilin-2 is a structural protein required for formation of the E-C coupling machinery. During heart disease, stress-activated calpain cleaves junctophilin-2, disrupting the E-C coupling machinery and calcium ion signaling, which compromises cell contraction. Guo et al. found that under stress conditions, calpain-mediated cleavage converted full-length junctophilin-2 from a structural protein into a transcriptional regulator that shuttled to the nucleus (see the Perspective by Padmanabhan and Haldar). Furthermore, failing cardiomyocytes in stressed myocardium transduced mechanical information (E-C uncoupling) into transcriptional reprogramming.

Science, this issue p. eaan3303; see also p. 1359

Structured Abstract

INTRODUCTION

Cardiac excitation-contraction (E-C) coupling refers to a cascade of Ca2+-mediated events whereby membrane depolarization leads to cell contraction. At the subcellular level, E-C coupling occurs within a microdomain of the cardiomyocyte, termed the cardiac dyad. In various forms of heart disease, such as pathological hypertrophy and heart failure, the E-C coupling process is abnormal, in part because of ultrastructural remodeling. Abnormal Ca2+ homeostasis (as a result of failed E-C coupling) triggers maladaptive remodeling at the transcriptional level, contributing to pathological myocardial remodeling, hypertrophy, and heart failure. However, it remains unclear whether cardiomyocytes possess a self-protective or homeostatic mechanism that mitigates adverse myocardial remodeling.

RATIONALE

Junctophilin-2 (JP2) is a structural protein that organizes the E-C coupling ultrastructural machinery in cardiomyocytes. We previously showed that calpain-mediated proteolytic cleavage of JP2 is key to its down-regulation in the diseased heart after cardiac stress. This cleavage contributes to loss of ultrastructural integrity at cardiac dyads, E-C uncoupling, and dysfunction of Ca2+ handling that results in heart failure. Computational analyses predicted that JP2 contains a nuclear localization signal (NLS), as well as an alanine-rich region (ARR) with characteristics of a helix-turn-helix structure, a DNA binding motif. We tested the hypothesis that JP2 encodes a stress-adaptive transcriptional regulator, which transduces mechanical information (E-C uncoupling) into transcriptional reprogramming in the myocardium in the setting of cardiac stress.

RESULTS

Biochemical, mutagenesis, and confocal imaging analyses revealed that stress-induced proteolysis of JP2 liberated an N-terminal fragment (JP2NT) that was imported into the nucleus through its NLS. Further biochemical and microarray assays showed that in the nucleus, JP2NT associated with chromatin and regulated transcription of a wide spectrum of genes via an evolutionarily conserved ARR located in the α-helix region of JP2. Chromatin immunoprecipitation sequencing (ChIP-seq) of JP2NT-overexpressing hearts revealed that it bound preferentially to the transcription start sites (TSSs) of genes, and gel shift studies defined the DNA binding motifs of JP2NT as the TATA box and a MEF2-response element (MRE). Elevation of JP2NT levels by JP2NT overexpression altered the in vivo genomic binding profile of TATA-box binding protein (TBP) and MEF2C. In addition, JP2NT suppressed the transcriptional activity of MEF2C by competing for MRE. Overexpression of JP2NT in mice led to reprogramming of the transcriptome in the setting of stress and attenuated hypertrophic remodeling and the progression of heart failure. Loss of JP2NT function by deletion of the JP2 NLS in mice accelerated the development of hypertrophy and heart failure after cardiac stress.

CONCLUSION

Our data reveal that calpain-mediated cleavage of JP2 transforms this E-C coupling structural protein into a transcriptional regulator that is shuttled into the nucleus and binds to promoters of target genes, inducing cardioprotective transcriptional reprogramming. These data reveal that cardiomyocytes possess a self-protective mechanism that counters pathological transcriptional remodeling after cardiac stress. Our findings also identify an intrinsic direct connection between ultrastructural remodeling and transcriptional reprogramming in the stressed heart.

Schematic depiction of the mechanism by which JP2NT converts a mechanical stress signal to transcriptional reprogramming in the stressed heart.

Left: E-C coupling under physiological conditions. Right: E-C coupling under pathologic conditions. Cardiac stress results in Ca2+ overload (1), promoting calpain activation (2). The resulting cleavage of JP2 liberates JP2NT from the T-tubule/sarcoplasmic reticulum junction, disrupting the ultrastructure of the E-C coupling machinery (3). JP2NT is shuttled into the nucleus via a conserved NLS (4). JP2NT binds to TATA-box elements via the ARR and associates with a MEF2 response element (MRE) to repress the transcription of genes that control deleterious cardiac remodeling (5).

Abstract

Junctophilin-2 (JP2) is a structural protein required for normal excitation-contraction (E-C) coupling. After cardiac stress, JP2 is cleaved by the calcium ion–dependent protease calpain, which disrupts the E-C coupling ultrastructural machinery and drives heart failure progression. We found that stress-induced proteolysis of JP2 liberates an N-terminal fragment (JP2NT) that translocates to the nucleus, binds to genomic DNA, and controls expression of a spectrum of genes in cardiomyocytes. Transgenic overexpression of JP2NT in mice modifies the transcriptional profile, resulting in attenuated pathological remodeling in response to cardiac stress. Conversely, loss of nuclear JP2NT function accelerates stress-induced development of hypertrophy and heart failure in mutant mice. These data reveal a self-protective mechanism in failing cardiomyocytes that transduce mechanical information (E-C uncoupling) into salutary transcriptional reprogramming in the stressed heart.

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