Skeletal muscle carnitine-acylcarnitine translocase deletion reveals vulnerability of oxidative muscle to fatty acid oxidation deficiency

Fatty acid oxidation (FAO) is a critical bioenergetic source for skeletal muscle with FAO impairments being linked to metabolic and contractile dysfunction. FAO is regulated by the carnitine shuttle in which FAO eligible fatty acids, in the form of acylcarnitines, are transported into the mitochondrial matrix by carnitine‑acylcarnitine translocase (CACT), yet the role of CACT in muscle in vivo has remained unexplored. To determine the requirement of CACT in muscle FAO and its influence on muscle mitochondrial bioenergetics, lipid profile, and muscle contractility, a novel conditional skeletal muscle-specific CACT knockout mouse ( Cact Sk-/- ) was generated. The requirement of CACT for long-chain FAO was confirmed by the complete abrogation of FAO flux in Cact Sk-/- muscle mitochondria. CACT was also required for the oxidative flux of medium-chain octanoyl-carnitine and acetyl-carnitine. CACT loss disrupted the lipid profile of skeletal muscle with long-chain acylcarnitine accumulation and shifted saturation profile of phospholipids away from saturated and highly unsaturated and towards di- and tri-saturated phospholipids. Elevated mitochondrial content was demonstrated by increased phospholipid content and mitochondrial staining in Cact Sk-/- muscles, occurring to a greater extent in oxidative muscles. Loss of CACT reduced muscle specific force production by ~70% in oxidative soleus muscle despite increased fiber size, and compensatory mitochondrial accumulation that preserved muscle metabolic capacity. These findings demonstrate the crucial role of CACT in muscle FAO and that oxidative muscles, in particular, undergo extensive lipid compositional, metabolic, and structural remodeling that coincides with impaired contractile function.