No abstract

The sarco/endoplasmic reticulum Ca 2+ ‐ATPase (SERCA) is imperative for normal cardiac function regulating both muscle relaxation and contractility. SERCA2a is the predominant isoform in cardiac muscles and is inhibited by phospholamban (PLN). Under conditions of oxidative stress, SERCA2a may also be impaired by tyrosine nitration. Tafazzin (Taz) is a mitochondrial‐specific transacylase that regulates mature cardiolipin (CL) formation, and its absence leads to mitochondrial dysfunction and excessive production of reactive oxygen/nitrogen species (ROS/RNS). In the present study, we examined SERCA function, SERCA2a tyrosine nitration, and PLN expression/phosphorylation in left ventricles (LV) obtained from young (3‐5 months) and old (10‐12 months) wild‐type (WT) and Taz knockdown (Taz KD ) male mice. These mice are a mouse model for Barth syndrome, which is characterized by mitochondrial dysfunction, excessive ROS/RNS production, and dilated cardiomyopathy (DCM). Here, we show that maximal SERCA activity was impaired in both young and old Taz KD LV, a result that correlated with elevated SERCA2a tyrosine nitration. In addition PLN protein was decreased, and its phosphorylation was increased in Taz KD LV compared with control, which suggests that PLN may not contribute to the impairments in SERCA function. These changes in expression and phosphorylation of PLN may be an adaptive response aimed to improve SERCA function in Taz KD mice. Nonetheless, we demonstrate for the first time that SERCA function is impaired in LVs obtained from young and old Taz KD mice likely due to elevated ROS/RNS production. Future studies should determine whether improving SERCA function can improve cardiac contractility and pathology in Taz KD mice.

Neuronatin (NNAT) is a transmembrane protein in the endoplasmic reticulum involved in metabolic regulation. It shares sequence homology with sarcolipin (SLN), which negatively regulates the sarco(endo)plasmic reticulum Ca2+‐ATPase (SERCA) that maintains energy homeostasis in muscles. Here, we examined whether NNAT could uncouple the Ca2+ transport activity of SERCA from ATP hydrolysis, similarly to SLN. NNAT significantly reduced Ca2+ uptake without altering SERCA activity, ultimately lowering the apparent coupling ratio of SERCA. This effect of NNAT was reversed by the adenylyl cyclase activator forskolin. Furthermore, soleus muscles from high fat diet (HFD)‐fed mice showed a significant downregulation in NNAT content compared with chow‐fed mice, whereas an upregulation in NNAT content was observed in fast‐twitch muscles from HFD‐ versus chow‐ fed mice. Therefore, NNAT is a SERCA uncoupler in cells and may function in adaptive thermogenesis.

It is well established that microgravity exposure causes significant muscle weakness and atrophy via muscle unloading. On Earth, muscle unloading leads to a disproportionate loss in muscle force and size with the loss in muscle force occurring at a faster rate. Although the exact mechanisms are unknown, a role for Ca2+ dysregulation has been suggested. The sarco(endo)plasmic reticulum Ca2+ ATPase (SERCA) pump actively brings cytosolic Ca2+ into the SR, eliciting muscle relaxation and maintaining low intracellular Ca2+ ([Ca2+]i). SERCA dysfunction contributes to elevations in [Ca2+]i, leading to cellular damage, and may contribute to the muscle weakness and atrophy observed with spaceflight. Here, we investigated SERCA function, SERCA regulatory protein content, and reactive oxygen/nitrogen species (RONS) protein adduction in murine skeletal muscle after 35–37 days of spaceflight. In male and female soleus muscles, spaceflight led to drastic impairments in Ca2+ uptake despite significant increases in SERCA1a protein content. We attribute this impairment to an increase in RONS production and elevated total protein tyrosine (T) nitration and cysteine (S) nitrosylation. Contrarily, in the tibialis anterior (TA), we observed an enhancement in Ca2+ uptake, which we attribute to a shift towards a faster muscle fiber type (i.e., increased myosin heavy chain IIb and SERCA1a) without elevated total protein T-nitration and S-nitrosylation. Thus, spaceflight affects SERCA function differently between the soleus and TA.

The neuronatin (NNAT, 3.9 kb) gene is paternally inherited with the maternal allele silenced via DNA methylation. 1,2 Genomic imprinting such as this results in monoallelic expression-a characteristic of several other genes critical for growth, development, and metabolism. 3 Through alternative splicing, NNAT mRNA produces two isoforms, NNATα (81 amino acids) and NNATβ (54 amino acids), 4,5 though the extent with which these isoforms are functionally redundant or divergent remain to be uncovered. NNAT/Nnat mRNA is enriched in the developing brain, and while downregulated