Exertional heat stroke causes long-term skeletal muscle epigenetic reprogramming, altered gene expression, and impaired satellite cell function in mice

Kevin O Murray, Jason O Brant, Ray A Spradlin, Trace Thome, Orlando Laitano, Terence E Ryan, Alberto Riva, Michael P Kladde, Thomas L Clanton

The effect of exertional heat stroke (EHS) exposure on skeletal muscles is incompletely understood. Muscle weakness is an early symptom of EHS but is not considered a major target of multiorgan injury. Previously, in a preclinical mouse model of EHS, we observed vulnerability of limb muscles to a second EHS exposure, suggesting hidden processes contributing to declines in muscle resilience. Here, we evaluated possible molecular origins of EHS-induced declines in muscle resilience. Female C57BL/6 mice [total n =56; 28/condition, i.e. EHS & exercise control (EXC)] underwent forced wheel running at 37.5°C/40% relative humidity until symptom limitation (unconsciousness). EXC mice exercised identically at room temperature (22-23°C). After one month of recovery, the following were assessed: (1) specific force and caffeine-induced contracture in soleus (SOL) and extensor digitorum longus (EDL) muscles; (2) transcriptome and DNA methylome responses in gastrocnemius (GAST); and (3) primary satellite cell function (proliferation and differentiation). There were no differences in specific force in either SOL or EDL from EXC. Only EHS solei exhibited lower caffeine sensitivity. EHS GAST exhibited higher RNA expression of genes encoding structural proteins of slow fibers, heat shock proteins, and myogenesis. A total of ~2500 differentially methylated regions of DNA that could potentially affect many cell functions were identified. Primary satellite cells exhibited suppressed proliferation rates but normal differentiation responses. Results demonstrate long-term changes in skeletal muscles one month after EHS that could contribute to declines in muscle resilience. Skeletal muscle may join other, more recognized tissues, considered vulnerable to long-term effects of EHS.