Modulation of cardiac contractility by the phospholamban/SERCA2a regulatome

Abstract

Heart disease remains the leading cause of death and disability in the Western world. Current therapies aim at treating the symptoms rather than the subcellular mechanisms, underlying the etiology and pathological remodeling in heart failure. A universal characteristic, contributing to the decreased contractile performance in human and experimental failing hearts, is impaired calcium sequestration into the sarcoplasmic reticulum (SR). SR calcium uptake is mediated by a Ca(2+)-ATPase (SERCA2), whose activity is reversibly regulated by phospholamban (PLN). Dephosphorylated PLN is an inhibitor of SERCA and phosphorylation of PLN relieves this inhibition. However, the initial simple view of a PLN/SERCA regulatory complex has been modified by our recent identification of SUMO, S100 and the histidine-rich Ca-binding protein as regulators of SERCA activity. In addition, PLN activity is regulated by 2 phosphoproteins, the inhibitor-1 of protein phosphatase 1 and the small heat shock protein 20, which affect the overall SERCA-mediated Ca-transport. This review will highlight the regulatory mechanisms of cardiac contractility by the multimeric SERCA/PLN-ensemble and the potential for new therapeutic avenues targeting this complex by using small molecules and gene transfer methods.

Figure 1. The SERCA2a/PLN regulatory complex in cardiac calcium cycling and survival

Injury to the ventricle such as myocardial infarction, ischemia, infection, valvular disease, familial, idiopathic activates the renin angiotensin and sympathetic nervous system along with cytokines. These in turn can cause direct damage to the individual cardiac myocytes resulting in contractile dysfunction and abnormal calcium cycling and eventual apoptosis and death. Targeting the SERCA2a/PLN proteome may have beneficial effects in abrogating the damage done to the individual cardiomyocytes.

Figure 2. Regulation of SR Ca-transport by a multimeric protein complex

SERCA2a activity is regulated by its reversible inhibitor PLN, SUMO and the histidine rich Ca-binding protein (HRC). Phosphorylation of PLN is mediated by cAMP-dependent or Ca-CAM-dependent PKs and dephosphorylation occurs by protein phosphatase 1 (PP1). The activity of PP1 is regulated by inhibitor-1 (I-1).

Figure 3. Modeling of PLN

There are three domains: cytosolic domain Ia, containing the phosphorylated Ser16 and Thr17 sites, cytosolic domain Ib and transmembrane domain II, containing AA that are important in pentamer stabilization and functional regulation of SERCA2a. The PLN monomer interacts with SERCA2a and inhibits its activity, while PLN phosphorylation leads to pentameric assembly and relief of the PLN inhibitory effects.

Figure 4. HAX-1 regulates PLN/SERCA activity

The anti-apoptotic protein HAX-1 interacts with PLN and enhances its inhibitory effects on SERCA2a and contractility. PKA phosphorylation of PLN or ablation of HAX-1 abolish the inhibition of HAX-1 on PLN/SERCA and Ca-cycling. Accordingly, overexpression of HAX-1 increases PLN inhibition and depresses contractility.

Figure 5. Beta adrenergic agonist stimulation and protein phosphatase 1 in regulation of the PLN/SERCA activity

PKA activation results in increased phosphorylation of PLN, Inhibitor-1 and Hsp20 amplifying the stimulatory effects b-AR stimulation on SR Ca-transport and contractility.

Figure 6. Strategies of improving calcium handling and SR Ca-content in heart failure

These strategies have focused pharmacologically on inhibiting the Na/K ATPase which results in increased intracellular Ca²⁺ and more recently on stabilizing the Ryanodine Receptor in resulting in decreased SR Ca²⁺ leak. Gene editing techniques have focused on enhancing SERCA2a’s activity by either increasing the level of SERCA2a or altering the expression of its partners.