Rapid decline in MyHC I(β) mRNA expression in rat soleus during hindlimb unloading is associated with AMPK dephosphorylation

One of the key events that occurs during skeletal muscle inactivation is a change in myosin phenotype, i.e. increased expression of fast isoforms and decreased expression of the slow isoform of myosin heavy chain (MyHC). It is known that calcineurin/nuclear factor of activated T-cells and AMP-activated protein kinase (AMPK) can regulate the expression of genes encoding MyHC slow isoform. Earlier, we found a significant decrease in phosphorylated AMPK in rat soleus after 24h of hindlimb unloading (HU). We hypothesized that a decrease in AMPK phosphorylation and subsequent histone deacetylase (HDAC) nuclear translocation can be one of the triggering events leading to a reduced expression of slow MyHC. To test this hypothesis, Wistar rats were treated with AMPK activator (AICAR) for 6days before HU as well as during 24h of HU. We discovered that AICAR treatment prevented a decrease in pre-mRNA and mRNA expression of MyHC I as well as MyHC IIa mRNA expression. Twenty-four hours of hindlimb suspension resulted in HDAC4 accumulation in the nuclei of rat soleus but AICAR pretreatment prevented this accumulation. The results of the study indicate that AMPK dephosphorylation after 24h of HU had a significant impact on the MyHC I and MyHC IIa mRNA expression in rat soleus. AMPK dephosphorylation also contributed to HDAC4 translocation to the nuclei of soleus muscle fibres, suggesting an important role of HDAC4 as an epigenetic regulator in the process of myosin phenotype transformation.