Laasmaa, M. | Search bioRxiv

bioRxiv2026

SIX1-dependent myofiber typology and metabolism controls muscle hypertrophy

The different types of muscle fibres respond in a specific way to hypertrophy or atrophy. The mechanisms underlying these heterogeneous adaptations remain poorly understood. Using single-nucleus RNA sequencing, we propose that fast glycolytic fibres show genetic limitations to hypertrophy induced by mechanical overload. We show that a prior fibre transition, achieved by reducing SIX1 protein expression (hypomorphism), enhances and accelerates overload-induced hypertrophy, bypassing the genetic limitations of fast glycolytic fibres. In contrast and unexpectedly, Six1 knockout in myofibers abolished overload-induced hypertrophy and instead caused atrophy of IIb/IIx fibers, despite the induction of a strong slow oxidative phenotype. In particular, Six1 deletion leads to metabolic defects caused by inhibition of glycolysis, AMPK and mitochondrial biogenesis. Our findings highlight the critical role of SIX1/AMPK/glycolysis-dependent aerobic metabolism in muscle growth and suggest that fibre type transitions, coupled with preserved metabolic function, may optimise hypertrophic responses.

physiology↗

bioRxiv2022

Prolonged β-Adrenergic Stimulation Disperses Ryanodine Receptor Clusters in Cardiomyocytes: Implications for Heart Failure

Ryanodine Receptors (RyRs) exhibit dynamic arrangements in cardiomyocytes, and we previously showed that "dispersion" of RyR clusters disrupts Ca2+ homeostasis during heart failure (HF) (Kolstad et al., eLife, 2018). Here, we investigated whether prolonged {beta}-adrenergic stimulation, a hallmark of HF, promotes RyR cluster dispersion, and examined the underlying mechanisms. We observed that treatment of healthy rat cardiomyocytes with isoproterenol for 1 hour triggered progressive fragmentation of RyR clusters. Pharmacological inhibition of CaMKII reversed these effects, while cluster dispersion was reproduced by specific activation of CaMKII, and in mice with constitutively active Ser2814-RyR. A similar role of protein kinase A (PKA) in promoting RyR cluster fragmentation was established by employing PKA activation or inhibition. Progressive cluster dispersion was linked to declining Ca2+ spark fidelity and magnitude, and slowed release kinetics from Ca2+ propagation between more numerous RyR clusters. In healthy cells, this served to dampen the stimulatory actions of {beta}-adrenergic stimulation over the longer term, and protect against pro-arrhythmic Ca2+ waves. However, during HF, RyR dispersion was linked to impaired Ca2+ release. Thus, RyR localization and function are intimately linked via channel phosphorylation by both CaMKII and PKA which, while finely tuned in healthy cardiomyocytes, underlies impaired cardiac function during pathology. Significance statementThe heartbeat is triggered by the release of Ca2+ from Ryanodine Receptors (RyRs) within cardiomyocytes. Recent data indicate RyR arrangement is highly malleable. However, mechanisms controlling RyR reorganisation and the subsequent impact on Ca2+ homeostasis remain unclear. Here, we show that prolonged {beta}-adrenergic stimulation causes RyR clusters to disperse, drastically altering the frequency and kinetics of Ca2+ release events called "Ca2+ sparks" in a process that is dependent on CaMKII and PKA. In healthy cells, these compensatory effects protect against arrhythmogenic Ca2+ over-activity. However, during heart failure, RyR hyper-phosphorylation and dispersion impairs Ca2+ release and cardiac performance. Thus, RyR localization and function are intimately linked via channel phosphorylation which, while finely tuned in health, underlies impaired cardiac function during pathology.

cell biology↗