The role of MAP-kinase p38 in the m. soleus slow myosin mRNA transcription regulation during short-term functional unloading.

The unloading of postural muscles leads to the changes in myosins heavy chains isoforms (MyHCs) mRNAs transcription pattern, that cause severe alterations of muscle functioning. Several transcription factors such as NFATc1 and TEAD1 upregulate slow MyHC mRNA transcription, and p38 MAP kinase can phosphorylate NFAT and TEAD1, causing their inactivation. However, the role p38 MAP kinase plays in MyHCs mRNAs transcription regulation in postural soleus muscle during unloading remains unclear. We aimed to investigate whether pharmacological inhibition of p38 MAPK during rat soleus unloading would prevent the unloading-induced slow-type MyHC mRNA transcription decrease by affecting calcineurin/NFATc1 or TEAD1 signaling. Male Wistar rats were randomly assigned to three groups: cage control (C), 3-day hindlimb suspended group (3HS) and 3-day hindlimb suspended group with the daily oral supplementation of 10 mg/kg p38 MAPK inhibitor VX-745 (3HS + VX-745). 3 days of hindlimb suspension caused the significant decreases of slow MyHC and slow-tonic myh7b mRNAs transcription as well as the decrease of NFATc1-dependent MCIP1.4 mRNA transcription in rat soleus muscles compared to the cage control. P38 MAP-kinase inhibition during hindlimb suspension completely prevented slow MyHC mRNA content decrease and partially prevented slow-tonic myh7b and MCIP1.4 mRNAs transcription decreases compared to the 3HS group. We also observed NFATc1 and TEAD1 myonuclear contents increases in the 3HS + VX-745 group compared to both 3HS and C groups (p < 0.05). Therefore, we found that p38 inhibition counteracts the unloading-induced slow MyHC mRNA transcription downregulation and leads to the activation of calcineurin/NFAT signaling cascade in unloaded rat soleus muscles.

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