Mitochondrial quality control is essential in highly structured cells such as neurons and muscles. In skeletal muscle the mitochondrial fission proteins are reduced in different physiopathological conditions including ageing sarcopenia, cancer cachexia and chemotherapy-induced muscle wasting. However, whether mitochondrial fission is essential for muscle homeostasis is still unclear. Here we show that muscle-specific loss of the pro-fission dynamin related protein (DRP) 1 induces muscle wasting and weakness. Constitutive Drp1 ablation in muscles reduces growth and causes animal death while inducible deletion results in atrophy and degeneration. Drp1 deficient mitochondria are morphologically bigger and functionally abnormal. The dysfunctional mitochondria signals to the nucleus to induce the ubiquitin-proteasome system and an Unfolded Protein Response while the change of mitochondrial volume results in an increase of mitochondrial Ca 2+ uptake and myofiber death. Our findings reveal that morphology of mitochondrial network is critical for several biological processes that control nuclear programs and Ca 2+ handling.
Calsequestrin (CS), the major Ca2+ -binding protein in the sarcoplasmic reticulum (SR), is thought to play a dual role in excitation-contraction coupling: buffering free Ca 2+ increasing SR capacity, and modulating the activity of the Ca 2+ release channels (RyRs). In this study, we generated and characterized the first murine model lacking the skeletal CS isoform (CS1). CS1-null mice are viable and fertile, even though skeletal muscles appear slightly atrophic compared to the control mice. No compensatory increase of the cardiac isoform CS2 is detectable in any type of skeletal muscle. CS1-null muscle fibres are characterized by structural and functional changes, which are much more evident in fast-twitch muscles (EDL) in which most fibres express only CS1, than in slow-twitch muscles (soleus), where CS2 is expressed in about 50% of the fibres. In isolated EDL muscle, force development is preserved, but characterized by prolonged time-to-peak and half-relaxation time, probably related to impaired calcium release from and re-uptake by the SR. Ca 2+ -imaging studies show that the amount of Ca 2+ released from the SR and the amplitude of the Ca 2+ transient are significantly reduced. The lack of CS1 also causes significant ultrastructural changes, which include: (i) striking proliferation of SR junctional domains; (ii) increased density of Ca 2+ -release channels (confirmed also by 3 H-ryanodine binding); (iii) decreased SR terminal cisternae volume; (iv) higher density of mitochondria. Taken together these results demonstrate that CS1 is essential for the normal development of the SR and its calcium release units and for the storage and release of appropriate amounts of SR Ca 2+ .
A better understanding of the signaling pathways that control muscle growth is required to identify appropriate countermeasures to prevent or reverse the loss of muscle mass and force induced by aging, disuse, or neuromuscular diseases. However, two major issues in this field have not yet been fully addressed. The first concerns the pathways involved in leading to physiological changes in muscle size. Muscle hypertrophy based on perturbations of specific signaling pathways is either characterized by impaired force generation, e.g., myostatin knockout, or incompletely studied from the physiological point of view, e.g., IGF-1 overexpression. A second issue is whether satellite cell proliferation and incorporation into growing muscle fibers is required for a functional hypertrophy. To address these issues, we used an inducible transgenic model of muscle hypertrophy by short-term Akt activation in adult skeletal muscle. In this model, Akt activation for 3 wk was followed by marked hypertrophy ( approximately 50% of muscle mass) and by increased force generation, as determined in vivo by ankle plantar flexor stimulation, ex vivo in intact isolated diaphragm strips, and in single-skinned muscle fibers. No changes in fiber-type distribution and resistance to fatigue were detectable. Bromodeoxyuridine incorporation experiments showed that Akt-dependent muscle hypertrophy was accompanied by proliferation of interstitial cells but not by satellite cell activation and new myonuclei incorporation, pointing to an increase in myonuclear domain size. We can conclude that during a fast hypertrophic growth myonuclear domain can increase without compromising muscle performance.
Calsequestrin-1 (CASQ1) is a moderate-affinity, high-capacity Ca(2+)-binding protein in the sarcoplasmic reticulum (SR) terminal cisternae of skeletal muscle. CASQ1 functions as both a Ca(2+)-binding protein and a luminal regulator of ryanodine receptor (RYR1)-mediated Ca(2+) release. Mice lacking skeletal CASQ1 are viable but exhibit reduced levels of releasable Ca(2+) and altered contractile properties. Here we report that CASQ1-null mice exhibit increased spontaneous mortality and susceptibility to heat- and anesthetic-induced sudden death. Exposure of CASQ1-null mice to either 2% halothane or heat stress triggers lethal episodes characterized by whole-body contractures, elevated core temperature, and severe rhabdomyolysis, which are prevented by prior dantrolene administration. The characteristics of these events are remarkably similar to analogous episodes observed in humans with malignant hyperthermia (MH) and animal models of MH and environmental heat stroke (EHS). In vitro studies indicate that CASQ1-null muscle exhibits increased contractile sensitivity to temperature and caffeine, temperature-dependent increases in resting Ca(2+), and an increase in the magnitude of depolarization-induced Ca(2+) release. These results demonstrate that CASQ1 deficiency alters proper control of RYR1 function and suggest CASQ1 as a potential candidate gene for linkage analysis in families with MH/EHS where mutations in the RYR1 gene are excluded.
The cytosolic free Ca 2+ transients elicited by muscle fiber excitation are well characterized, but little is known about the free [Ca 2+ ] dynamics within the sarcoplasmic reticulum (SR) ] did not differ between WT, CSQ-KO, and CSQ-DKO fibers. During sustained contractions, changes were rather small in WT, reflecting powerful buffering of CSQ, whereas in CSQ-KO fibers, significant drops in SR [Ca 2+ ] occurred. Their amplitude increased with stimulation frequency between 1 and 60 Hz. At 60 Hz, the SR became virtually depleted of Ca 2+ , both in CSQ-KO and CSQ-DKO fibers. In CSQ-KO fibers, cytosolic free calcium detected with Fura-2 declined during repetitive stimulation, indicating that SR calcium content was insufficient for sustained contractile activity. SR Ca 2+ reuptake during and after stimulation trains appeared to be governed by three temporally distinct processes with rate constants of 50, 1-5, and 0.3 s −1 (at 26°C), reflecting activity of the SR Ca 2+ pump and interplay of luminal and cytosolic Ca 2+ buffers and pointing to store-operated calcium entry (SOCE). SOCE might play an essential role during muscle contractures responsible for the malignant hyperthermia-like syndrome in mice lacking CSQ..excitation-contraction coupling | parvalbumin C alcium ions play an important signaling role in virtually every cell type. Ca 2+ is stored in a subcellular reticular organelle, and its release into the cytosol is triggered by action potentials and/or by second messengers, such as inositol triphosphate. In skeletal and cardiac muscle cells, Ca 2+ is stored inside the sarcoplasmic reticulum (SR), which is in close contact with invaginations of the cell surface membrane, the T-tubular system. Action potentials propagated through the T-tubular system are sensed by the voltage sensors (dihydropyridine receptors), which are located opposite to the ryanodine receptors (RyRs), the Ca 2+ release channels present in the terminal cisternae of the SR, and this causes calcium release (recently reviewed in 1, 2).The transient increase in cytosolic Ca 2+ concentration, which triggers the contractile response, has been intensively studied. Little is known about the dynamic changes of the free Ca 2+ concentration inside the SR, however. Calsequestrin (CSQ) is an acidic high-capacity Ca 2+ -binding protein located within the SR terminal cisternae. CSQ type 1 (CSQ1) is the major SR Ca 2+ buffer in fast muscle fibers, whereas CSQ2 predominates in cardiomyocytes and slow muscle fibers (3, 4). CSQ serves to keep the free [Ca 2+ ] low inside the SR while providing a pool of rapidly available bound calcium to maintain the free [Ca 2+ ] level (5, 6). This minimizes Ca 2+ leakage through the RyRs and reduces the dissipation of energy by the SR Ca 2+ pump, a function essential to maintain a low metabolic rate in quiescent excitable cells. In addition, CSQ has been shown to modulate RyRmediated Ca 2+ release from the SR (6). To understand the pivotal role of CSQ in SR function, it is critical to determine free SR Ca 2+ conce...
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.