The ACTN2 gene encodes α-actinin 2, located in the Z-disc of the sarcomeres in striated muscle. In this study, we sought to investigate the effects of an ACTN2 missense variant of unknown significance (p.A868T) on cardiac muscle structure and function. Left ventricular free wall samples were obtained at the time of cardiac transplantation from a heart failure patient with the ACTN2 A868T heterozygous variant. This variant is in the EF 3–4 domain known to interact with titin and α-actinin. At the ultrastructural level, ACTN2 A868T cardiac samples presented small structural changes in cardiomyocytes when compared to healthy donor samples. However, contractile mechanics of permeabilized ACTN2 A868T variant cardiac tissue displayed higher myofilament Ca²⁺ sensitivity of isometric force, reduced sinusoidal stiffness, and faster rates of tension redevelopment at all Ca²⁺ levels. Small-angle X-ray diffraction indicated increased separation between thick and thin filaments, possibly contributing to changes in muscle kinetics. Molecular dynamics simulations indicated that while the mutation does not significantly impact the structure of α-actinin on its own, it likely alters the conformation associated with titin binding. Our results can be explained by two Z-disc mediated communication pathways: one pathway that involves α-actinin’s interaction with actin, affecting thin filament regulation, and the other pathway that involves α-actinin’s interaction with titin, affecting thick filament activation. This work establishes the role of α-actinin 2 in modulating cross-bridge kinetics and force development in the human myocardium as well as how it can be involved in the development of cardiac disease.

Arrhythmogenic Cardiomyopathy (ACM) is a familial heart disease, characterized by contractile dysfunction, ventricular arrhythmias (VAs), and the risk of sudden cardiac death. Currently, implantable cardioverter defibrillators and antiarrhythmics are the mainstays in ACM therapeutics. Angiotensin receptor blockers (ARBs) have been highlighted in the treatment of heart diseases, including ACM. Yet, recent research has additionally implicated ARBs in the genesis of VAs and myocardial lipolysis via the peroxisome proliferator-activated receptor gamma (PPARγ) pathway. The latter is of particular interest, as fibrofatty infiltration is a pathological hallmark in ACM. Here, we tested two ARBs, Valsartan and Telmisartan, and the PPAR agonist, Rosiglitazone, in an animal model of ACM, homozygous Desmoglein-2 mutant mice (Dsg2^mut/mut). Cardiac function, premature ventricular contractions (PVCs), fibrofatty scars, PPARα/γ protein levels, and PPAR-mediated mRNA transcripts were assessed. Of note, not a single mouse treated with Rosiglitazone made it to the study endpoint (i.e., 100% mortality: n = 5/5). Telmisartan-treated Dsg2^mut/mut mice displayed the preservation of contractile function (percent ejection fraction [%EF]; 74.8 ± 6.8%EF) compared to Vehicle- (42.5 ± 5.6%EF) and Valsartan-treated (63.1 ± 4.4%EF) mice. However, Telmisartan-treated Dsg2^mut/mut mice showed increased cardiac wall motion abnormalities, augmented %PVCs, electrocardiographic repolarization/depolarization abnormalities, larger fibrotic lesions, and increased expression of PPARy-regulated gene transcripts compared to their Dsg2^mut/mut counterparts. Alternatively, Valsartan-treated Dsg2^mut/mut mice harbored fewer myocardial scars, reduced %PVC, and increased Wnt-mediated transcripts. Considering our findings, caution should be taken by physicians when prescribing medications that may increase PPARy signaling in patients with ACM.

Regulation of myocardial mass is key for maintaining cardiovascular health. This review highlights the complex and regulatory relationship between mechanosignaling and myocardial mass, influenced by many internal and external factors including hemodynamic and microgravity, respectively. The heart is a dynamic organ constantly adapting to changes in workload (preload and afterload) and mechanical stress exerted on the myocardium, influencing both physiological adaptations and pathological remodeling. Mechanosignaling pathways, such as the mitogen-activated protein kinases (MAPKs) and the phosphoinositide 3-kinases and serine/threonine kinase (PI3K/Akt) pathways, mediate downstream effects on gene expression and play key roles in transducing mechanical cues into biochemical signals, thereby modulating cellular processes, including control of myocardial mass. Dysregulation of these processes can lead to pathological cardiac remodeling, such as hypertrophic cardiomyopathy. Furthermore, recent studies have highlighted the importance of protein quality control mechanisms, such as the ubiquitin-proteasome system, in settings of extreme physiological conditions that alter the heart workload such as pregnancy and microgravity. Overall, this review provides a thorough insight into how mechanical signals are converted into chemical signals to regulate myocardial mass in both healthy and diseased conditions.

The protein sarcospan (SSPN) is an integral member of the dystrophin-glycoprotein complex (DGC) and has been shown to be important in the heart during the development and the response to acute stress. In this study, we investigated the role of SSPN in the cardiac response to acute ischemia-reperfusion (IR) injury in SSPN-deficient (SSPN^−/−) mice. First, the hemodynamic response of SSPN^−/− mice was tested and was similar to SSPN^+/+ (wild-type) mice after isoproterenol injection. Using the in situ Langendorff perfusion method, SSPN^−/− hearts were subjected to IR injury and found to have increased infarct size and arrhythmia susceptibility compared to SSPN^+/+. Ca²⁺ handling was assessed in single cardiomyocytes and diastolic Ca²⁺ levels were increased after acute β-AR stimulation in SSPN^+/+ but not SSPN^−/−. It was also found that SSPN^−/− cardiomyocytes had reduced Ca²⁺ SR content compared to SSPN^+/+ but similar SR Ca²⁺ release. Next, we used qRT-PCR to examine gene expression of Ca²⁺ handling proteins after acute IR injury. SSPN^−/− hearts showed a significant decrease in L-type Ca²⁺ channels and a significant increase in Ca²⁺ release channel (RyR2) expression. Interestingly, under oxidizing conditions reminiscent of IR, SSPN^−/− cardiomyocytes, had increased H₂O₂-induced reactive oxygen species production compared to SSPN^+/+. Examination of oxidative stress proteins indicated that NADPH oxidase 4 and oxidized CAMKII were increased in SSPN^−/− hearts after acute IR injury. These results suggest that increased arrhythmia susceptibility in SSPN^−/− hearts post-IR injury may arise from alterations in Ca²⁺ handling and a reduced capacity to regulate oxidative stress pathways.

Young and aged sarcospan (SSPN)-deficient mice were examined to assess the role of SSPN in obesity and cardiometabolic disease. Both sexes displayed a “leaner” phenotype in response to high-fat diet (HFD). Notably, several sex differences were identified in aged SSPN-deficient mice: 1) females developed glucose intolerance (control and HFD) and 2) males exhibited increased left ventricular mass (control) and diastolic dysfunction (HFD). Therefore, we conclude that SSPN exerts a sex-dependent influence on obesity-associated diseases.