The Effect of SERCA Activation on Functional Characteristics and Signaling of Rat Soleus Muscle upon 7 Days of Unloading

Abstract

Skeletal muscle abnormalities and atrophy during unloading are accompanied by the accumulation of excess calcium in the sarcoplasm. We hypothesized that calcium accumulation may occur, among other mechanisms, due to the inhibition of sarco/endoplasmic reticulum Ca2+-ATPase (SERCA) activity. Consequently, the use of the SERCA activator will reduce the level of calcium in the sarcoplasm and prevent the negative consequences of muscle unloading. Wistar rats were randomly assigned into one of three groups (eight rats per group): control rats with placebo (C), 7 days of unloading/hindlimb suspension with placebo (7HS), and 7 days of unloading treated with SERCA activator CDN1163 (7HSC). After seven days of unloading the soleus muscle, the 7HS group displayed increased fatigue in the ex vivo test, a significant increase in the level of calcium-dependent CaMK II phosphorylation and the level of tropomyosin oxidation, as well as a decrease in the content of mitochondrial DNA and protein, slow-type myosin mRNA, and the percentage of slow-type muscle fibers. All of these changes were prevented in the 7HSC group. Moreover, treatment with CDN1163 blocked a decrease in the phosphorylation of p70S6k, an increase in eEF2 phosphorylation, and an increase in MuRF-1 mRNA expression. Nevertheless, there were no differences in the degree of fast and slow muscle fiber atrophy between the 7HS and 7HSC groups. Conclusion: SERCA activation during 7 days of unloading prevented an increase in soleus fatigue, the decrease of slow-type myosin, mitochondrial markers, and markers of calcium homeostasis but had no effect on muscle atrophy.

Disuse-Induced Muscle Fatigue: Facts and Assumptions

“…In 2023, our laboratory showed that the administration of the L-type calcium channel blocker nifedipine, as well as the administration of the pharmacological activator SERCA, during 7-day rat hind limb suspension, prevented an increase in soleus muscle fatigue [81,90]. Also, as mentioned above, in this experiment, in the nifedipine-treated group, a decrease in a number of markers of mitochondrial density and mitochondrial biogenesis was prevented.…”

“…A number of experimental data indicate that excessive intracellular calcium accumulation may contribute to a decrease in mitochondrial density under muscle unloading. The administration of both the L-type calcium channel blocker nifedipine and the SERCA activator prevented a decrease in the content of mitochondrial DNA and the mitochondrial density marker protein TOM20 [81,90]. They both also prevented the decrease in the expression of mRNA of a number of mitochondrial biogenesis regulators, which was quite unexpected, since it is widely known that the accumulation, but not a decrease, in calcium levels leads to activation of mitochondrial biogenesis [119].…”

“…MAP-kinase p38, in turn, can be activated by CaMK II and ROS accumulation [80]. Administration of the SERCA activator during 7-day hind limb suspension prevented a decrease in the content of NFATc1 in myonuclei, and also led to a decrease in the level of phosphorylation of p38 MAP kinase, potentially due to CaMK II inactivation and ROS decrease [81]. So, under unloading conditions, the effect of SERCA activator administration on the downregulation of CaMK II/p38 seems to have a stronger activating effect on NFATc1 than its potential inhibition of calcineurin.…”

“…Five-week rat hind limb suspension leads to a decrease in the volumetric density of subsarcolemmal mitochondria and an increase in the volumetric density of intermyofibrillar mitochondria, with overall preservation of the total volumetric density of mitochondria [116]. A total of 24 h of rat hind limb suspension causes a decrease in the mRNA of the regulators of mitochondrial biogenesis and fusion [54]; by the 7th day, a decrease in the level of mitochondrial DNA and markers of mitochondrial density is also observed [21,81]. The same situation persists on the 14th day [117].…”

Spaceflight increases sarcoplasmic reticulum Ca2+leak and this cannot be counteracted with BuOE treatment

“…Regardless, targeting Ca 2+ leak from the SR has shown to be beneficial to muscle health in aged mice (52), hypoxia-induced atrophy in rodents (53), and murine dystrophic muscle (54) indicating that this may be a viable target for spaceflight induced changes in SR permeability and leak. Importantly, a recent study by Sharlo et al (55) treated rats with the SERCA activator, CDN1163, during hindlimb suspension (NASA’s simulated model of microgravity) and, while they did not measure SERCA function directly, they found improvements in several muscle parameters including soleus fatigue resistance, mitochondrial markers, and markers of Ca 2+ homeostasis. This finding, along with others targeting SERCA activation in situations of muscle impairment (11,12) demonstrate the importance of directly targeting SERCA in space.…”