LPLAT7 Regulates Fiber Type Phospholipid Acyl Chain Remodeling in Murine Skeletal Muscle

Skeletal muscle consists of both fast- and slow-twitch fibers. Phospholipids (PLs) are essential structural components of cellular membranes, and the diversity of their fatty acid compositions affects membrane fluidity and permeability. Although some studies have shown that PLs' acyl chain species differ among muscle fiber types, the mechanisms underlying these differences are unclear. To investigate this obscured point, we analyzed phosphatidylcholine (PC) and phosphatidylethanolamine (PE) molecules in the murine extensor digitorum longus (EDL; fast-twitch) and soleus (slow-twitch) muscles. We revealed that the acyl chain profiles of PLs varied greatly depending on the muscle fiber types and that the fast-twitch muscle was mainly composed of palmitate-containing PC (16:0-PC) and PE species, while the slow-twitch muscle was composed of not only 16:0 but also stearate-containing PC (18:0-PC) and PE species. The primary fatty acids at the sn-1 position of PLs were saturated fatty acids such as palmitic acid (C16:0) and stearic acid (C18:0). However, the molecular mechanism underlying the replacement of fatty acids at the sn-1 position in PLs was still not understood. Therefore, we analyzed genetically modified murine skeletal muscles to identify the principal enzyme for regulating the acyl chain profiles of PLs. This investigation identified lysophospholipid acyltransferases (LPLAT) 7, an acyltransferase involved in the remodeling pathway, as a candidate. Comparison of LPLAT7 gene expression in EDL and soleus by qRT-PCR showed higher expression levels in soleus, indicating variation of LPLAT7 expression level between EDL and soleus might provide the difference of 18:0-PC and PE levels. To determine whether LPLAT7 incorporated C18:0 into PC and PE in fast-twitch and slow-twitch muscles, we performed stable-isotope tracer experiments using d35-C18:0 in conditional LPLAT7 knockout (LPLAT7 cKO) mice. In addition, we explored that LPLAT7 incorporated C18:0 into the sn-1 position of lyso-PLs by in vitro enzyme activity measurement. Stable-isotope tracer and in vitro enzyme activity measurement showed that LPLAT7 mainly incorporated C18:0 into the sn-1 position of lyso-PC and lyso-PE and these acyl chain remodeling reactions were specifically reduced in the muscle from LPLAT7 cKO mice. Based on these investigations, we revealed that acyl chain profiles at sn-1 position of PLs varied greatly depending on the muscle fiber type and LPLAT7 regulated the muscle fiber type-specific acyl chain profiles. Grant-in-Aid for Scientific Research (KAKENHI) [grant numbers 21K21224]