Skeletal Muscle Cell Adaptations under Contractile, Pharmacological and Inflammatory Stress

Abstract

Skeletal muscle health is challenged by multiple factors that commonly coexist in ageing and chronic disease, including physical inactivity, inflammation, altered metabolic conditions and long-term medication use. Because these factors are difficult to disentangle in humans, this thesis used controlled C2C12 muscle-cell models to investigate their direct effects on skeletal muscle cells and to examine whether selected interventions could partially preserve the muscle-cell phenotype. The studies showed that skeletal muscle cells are sensitive to a wide range of external and intrinsic perturbations. Streptomycin, a commonly used cell-culture antibiotic, impaired myogenic differentiation, protein synthesis and mitochondrial organisation, demonstrating that routine culture conditions can themselves influence the experimental phenotype. Simvastatin directly impaired muscle-cell growth, differentiation and metabolic function, whereas metformin partially attenuated several structural and stress-related effects without restoring mitochondrial respiration or anabolic signalling. Inflammatory stress induced by LPS likewise impaired myogenic and metabolic responses. In contrast, intermittent electrical pulse stimulation induced favourable contraction-like adaptations and preserved selected muscle-cell features during inflammation, although the protective response remained incomplete. Finally, combined inhibition of the TGF-β type I receptors Acvr1b and Tgfbr1 unexpectedly reduced, rather than increased, myotube diameter, highlighting that modulation of classical growth-regulatory pathways does not necessarily translate directly into the expected cellular phenotype. Together, these findings show that skeletal muscle-cell responses to pharmacological, inflammatory, contractile and growth-regulatory stimuli are complex and cannot be explained by single signalling pathways alone. They also demonstrate that potentially protective interventions such as metformin and contraction-like stimulation may preserve selected aspects of muscle-cell phenotype without fully reversing the underlying disturbance. By combining structural, molecular, signalling and metabolic readouts, this thesis provides a more integrated understanding of how skeletal muscle cells respond to adverse conditions and how selected interventions may partially support their maintenance under stress.