Investigation of a Role for AGPAT4 in Mouse Skeletal Muscle

Abstract

Acylglycerophosphate acyltransferases (AGPATs) are a group of homologous proteins that function in the de novo synthesis and remodeling of glycerophospholipids, catalyzing the acyl‐CoA‐dependent acylation of lysophospholipids. We have recently determined that AGPAT4 is mitochondrial, and is a canonical AGPAT, preferentially utilizing lysophosphatidic acid as an acyl acceptor. However, we found that it functions in brain to regulate levels of phosphatidylinositol, phosphatidylcholine (PC), and phosphatdylethanolamine (PE), rather than phosphatidic acid per se. Whether this effect is tissue specific has not yet been investigated. The presence of multiple AGPATs suggests that different homologues may feed phosphatidic acid into unique phospholipid species synthesis pathways, although this remains poorly characterized. Phosphatidic acid (PA) metabolism is of particular interest in skeletal muscle, where levels of this phospholipid are involved in mediating muscle force contractility, muscle size, and initiation of mTOR signaling, among other effects. We have therefore investigated a role for AGPAT4 in skeletal muscle biology. We performed immunohistochemistry on AGPAT4 in cross sections from C57Bl/6J mouse soleus and extensor digitorum longus (EDL) muscle, and found that it is predominantly visualized in type IIa muscle fibres, as compared to type I, IIb, or IIx muscle fibres. We next investigated the impact of Agpat4 gene ablation on muscle physiology and function. Force contraction was significantly reduced in soleus muscle but not EDL from Agpat4−/− mice, in agreement with the higher proportion of type IIa fibres in the former muscle type. Interestingly, however, our studies indicate no difference between Agpat4−/− mice and wildtype littermates in measures of time to exhaustion during treadmill running tests. We found a significantly higher content of PA and PE in the soleus muscles of Agpat4−/− mice, with no difference in TAG content. As well, there were no apparent differences in mitochondrial content or function, suggesting that mitochondrial function is not impaired by Agpat4 deficiency in skeletal muscle. This work is the first to characterize the role of a bona fide AGPAT in skeletal muscle function.Support or Funding InformationThis work was supported by grants to RED from the Canada Foundation for Innovation – Leader's Opportunity Fund and Ontario Research Fund (Project#30259), and a Discovery Grant (#418213‐2012) from the Natural Sciences and Engineering Research Council (NSERC) of Canada. RMB, JJAH, DB are supported by an NSERC PGS‐D, VAF was supported by a CIHR Doctoral Scholarship, ASM was supported by an NSERC CGS‐D award, EBM was supported by an PGS‐M scholarship.