Peptidyl-Prolyl Isomerase 1 Regulates Ca2+ Handling by Modulating Sarco(Endo)Plasmic Reticulum Calcium ATPase and Na2+/Ca2+ Exchanger 1 Protein Levels and Function

Phosphorylation of cardiac sodium channel at Ser571 anticipates manifestations of the aging myopathy

Diastolic dysfunction and delayed ventricular repolarization are typically observed in the elderly, but whether these defects are intimately associated with the progressive manifestation of the aging myopathy remains to be determined. In this regard, aging in experimental animals is coupled with increased late Na+ current (INa,L) in cardiomyocytes, raising the possibility that INa,L conditions the modality of electrical recovery and myocardial relaxation of the aged heart. For this purpose, aging male and female wild-type (WT) C57Bl/6 mice were studied together with genetically engineered mice with phosphomimetic (gain of function, GoF) or ablated (loss of function, LoF) mutations of the sodium channel Nav1.5 at Ser571 associated with, respectively, increased and stabilized INa,L. At ∼18 mo of age, WT mice developed prolonged duration of the QT interval of the electrocardiogram and impaired diastolic left ventricular (LV) filling, defects that were reversed by INa,L inhibition. Prolonged repolarization and impaired LV filling occurred prematurely in adult (∼5 mo) GoF mutant mice, whereas these alterations were largely attenuated in aging LoF mutant animals. Ca2+ transient decay and kinetics of myocyte shortening/relengthening were delayed in aged (∼24 mo) WT myocytes, with respect to adult cells. In contrast, delayed Ca2+ transients and contractile dynamics occurred at adult stage in GoF myocytes and further deteriorated in old age. Conversely, myocyte mechanics were minimally affected in aging LoF cells. Collectively, these results document that Nav1.5 phosphorylation at Ser571 and the late Na+ current modulate the modality of myocyte relaxation, constituting the mechanism linking delayed ventricular repolarization and diastolic dysfunction.NEW & NOTEWORTHY We have investigated the impact of the late Na current (INa,L) on cardiac and myocyte function with aging by using genetically engineered animals with enhanced or stabilized INa,L, due to phosphomimetic or phosphoablated mutations of Nav1.5. Our findings support the notion that phosphorylation of Nav1.5 at Ser571 prolongs myocardial repolarization and impairs diastolic function, contributing to the manifestations of the aging myopathy.

Scn1b expression in the adult mouse heart modulates Na+ influx in myocytes and reveals a mechanistic link between Na+ entry and diastolic function

Voltage-gated sodium channels (VGSCs) are macromolecular assemblies composed of a number of proteins regulating channel conductance and properties. VGSCs generate Na+ current (INa) in myocytes and play fundamental roles in excitability and impulse conduction in the heart. Moreover, VGSCs condition mechanical properties of the myocardium, a process that appears to involve the late component of INa. Variants in the gene SCN1B, encoding the VGSC β1- and β1B-subunits, result in inherited neurological disorders and cardiac arrhythmias. But the precise contributions of β1/β1B-subunits and VGSC integrity to the overall function of the adult heart remain to be clarified. For this purpose, adult mice with cardiac-restricted, inducible deletion of Scn1b (conditional knockout, cKO) were studied. Myocytes from cKO mice had increased densities of fast (+20%)- and slow (+140%)-inactivating components of INa, with respect to control cells. By echocardiography and invasive hemodynamics, systolic function was preserved in cKO mice, but diastolic properties and ventricular compliance were compromised, with respect to control animals. Importantly, inhibition of late INa with GS967 normalized left ventricular filling pattern and isovolumic relaxation time in cKO mice. At the cellular level, cKO myocytes presented delayed kinetics of Ca2+ transients and cell mechanics, defects that were corrected by inhibition of INa. Collectively, these results document that VGSC β1/β1B-subunits modulate electrical and mechanical function of the heart by regulating, at least in part, Na+ influx in cardiomyocytes.NEW & NOTEWORTHY We have investigated the consequences of deletion of Scn1b, the gene encoding voltage-gated sodium channel β1-subunits, on myocyte and cardiac function. Our findings support the notion that Scn1b expression controls properties of Na+ influx and Ca2+ cycling in cardiomyocytes affecting the modality of cell contraction and relaxation. These effects at the cellular level condition electrical recovery and diastolic function in vivo, substantiating the multifunctional role of β1-subunits in the physiology of the heart.

Retene, pyrene and phenanthrene cause distinct molecular-level changes in the cardiac tissue of rainbow trout (Oncorhynchus mykiss) larvae, part 2 - Proteomics and metabolomics

Polycyclic aromatic hydrocarbons (PAHs) are global contaminants of concern. Despite several decades of research, their mechanisms of toxicity are not very well understood. Early life stages of fish are particularly sensitive with the developing cardiac tissue being a main target of PAHs toxicity. The mechanisms of cardiotoxicity of the three widespread model polycyclic aromatic hydrocarbons (PAHs) retene, pyrene and phenanthrene were explored in rainbow trout (Oncorhynchus mykiss) early life stages. Newly hatched larvae were exposed to sublethal doses of each individual PAH causing no detectable morphometric alterations. Changes in the cardiac proteome and metabolome were assessed after 7 or 14 days of exposure to each PAH. Phase I and II enzymes regulated by the aryl hydrocarbon receptor were significantly induced by all PAHs, with retene being the most potent compound. Retene significantly altered the level of several proteins involved in key cardiac functions such as muscle contraction, cellular tight junctions or calcium homeostasis. Those findings were quite consistent with previous reports regarding the effects of retene on the cardiac transcriptome. Significant changes in proteins linked to iron and heme metabolism were observed following exposure to pyrene. While phenanthrene also altered the levels of several proteins in the cardiac tissue, no clear mechanisms or pathways could be highlighted. Due to high variability between samples, very few significant changes were detected in the cardiac metabolome overall. Slight but significant changes were still observed for pyrene and phenanthrene, suggesting possible effects on several energetic or signaling pathways. This study shows that early exposure to different PAHs can alter the expression of key proteins involved in the cardiac function, which could potentially affect negatively the fitness of the larvae and later of the juvenile fish.

Degradation of GRK2 and AKT is an early and detrimental event in myocardial ischemia/reperfusion

BACKGROUND

Identification of signaling pathways altered at early stages after cardiac ischemia/reperfusion (I/R) is crucial to develop timely therapies aimed at reducing I/R injury. The expression of G protein-coupled receptor kinase 2 (GRK2), a key signaling hub, is up-regulated in the long-term in patients and in experimental models of heart failure. However, whether GRK2 levels change at early time points following myocardial I/R and its functional impact during this period remain to be established.

METHODS

We have investigated the temporal changes of GRK2 expression and their potential relationships with the cardioprotective AKT pathway in isolated rat hearts and porcine preclinical models of I/R.

FINDINGS

Contrary to the maladaptive up-regulation of GRK2 reported at later times after myocardial infarction, successive GRK2 phosphorylation at specific sites during ischemia and early reperfusion elicits GRK2 degradation by the proteasome and calpains, respectively, thus keeping GRK2 levels low during early I/R in rat hearts. Concurrently, I/R promotes decay of the prolyl-isomerase Pin1, a positive regulator of AKT stability, and a marked loss of total AKT protein, resulting in an overall decreased activity of this pro-survival pathway. A similar pattern of concomitant down-modulation of GRK2/AKT/Pin1 protein levels in early I/R was observed in pig hearts. Calpain and proteasome inhibition prevents GRK2/Pin1/AKT degradation, restores bulk AKT pathway activity and attenuates myocardial I/R injury in isolated rat hearts.

INTERPRETATION

Preventing transient degradation of GRK2 and AKT during early I/R might improve the potential of endogenous cardioprotection mechanisms and of conditioning strategies.

Pin1 Promotes Regulated Necrosis Induced by Glutamate in Rat Retinal Neurons via CAST/Calpain2 Pathway

The purpose of the current study was to investigate whether peptidyl-prolyl cis/trans isomerase NIMA-interacting 1 (Pin1) can interact with calpastatin (CAST) and regulate CAST/calpain2, under excessive glutamate conditions, and subsequently regulate necrosis in rat retinal neurons. Glutamate triggered CAST/calpain2-mediated necrosis regulation in primary cultured retinal neurons, as demonstrated by propidium iodide-staining and lactate dehydrogenase assay. Co-IP results and a computer simulation suggested that Pin1 could bind to CAST. Western blot, real-time quantitative polymerase chain reaction, immunofluorescence, and phosphorylation analysis results demonstrated that CAST was regulated by Pin1, as proven by the application of juglone (i.e., a Pin1 specific inhibitor). The retinal ganglion cell 5 cell line, combined with siRNA approach and flow cytometry, was then used to verify the regulatory pathway of Pin1 in CAST/calpain2-modulated neuronal necrosis that was induced by glutamate. Finally, in vivo studies further confirmed the role of Pin1 in CAST/calpain2-modulated necrosis following glutamate excitation, in the rat retinal ganglion cell and inner nuclear layers. In addition, a flash electroretinogram study provided evidence for the recovery of impaired visual function, which was induced by glutamate, with juglone treatment. Our work aims to investigate the involvement of the Pin1-CAST/calpain2 pathway in glutamate-mediated excitotoxicity.