The Time Course of Muscle Nuclear Content of Transcription Factors Regulating the MyHC I(β) Expression in the Rat Soleus Muscle under Gravitational Unloading

Gravitational unloading leads to a slow-to-fast shift in the myosin phenotype. The reason for these changes is a decrease in the expression of the genes encoding heavy chains of myosin (MyHC) of the slow type, and an increase in the expression of the genes encoding MyHC of the fast types. Signal cascades such as calcineurin/NFAT and HDAC4/MEF-2D are known to regulate MyHC I gene expression. There is almost no data in the literature on the content of transcription factors NFATc1 and MEF-2D involved in the regulation of the gene expression of MyHC I, as well as histone deacetylase HDAC4 and MAP kinase ERK2 in the rat soleus muscle under the gravitational unloading. The aim of this study was to explore the role of various mechanisms controlling the expression of MyHC type I at each stage of gravitational unloading. The levels of proteins MEF-2D and ERK2 in the nuclear fraction were decreased significantly after 3 and 7 days of unloading, and the level of NFATc1 was decreased on the first and seventh days. However, the content of NFATc1, HDAC4, MEF-2D and ERK2 in the nuclear fraction after 14 days of gravitational unloading corresponded to the control level. These data indicate that the content of transcriptional regulators in the muscle nuclei at a late stage of gravitational unloading is not a limiting factor for maintaining their transcriptional activity.