Characterization of Cardiac Mitochondrial Alterations in Mice Deficient in Phospholipase A/Acyltransferase-1 (Plaat1)

Abstract

Phospholipase A and acyltransferase 1 (PLAAT1) is a phospholipid remodeling enzyme that belongs to a paralogous family with orthologues documented in more than 500 species of vertebrates. In humans, Plaat1 is most highly expressed in the heart, brain, skeletal muscle, and testes. PLAAT1 can catalyze O-acyltransferase, N-acyltransferase and phospholipase A1/2 reactions in vitro, however its enzymatic and physiological functions in vivo are poorly understood. Our group discovered that mice deficient in Plaat1 (Plaat1-/-) have a significant ~1/3 reduction in cardiac cardiolipin content, suggesting PLAAT1 is a critical regulator of this lipid in the heart. Cardiolipin is an important phospholipid for mitochondrial morphology and is required for appropriate mitochondrial function, so the loss of cardiolipin in the Plaat1-/- heart has significant implications for mitochondrial content, morphology, and function. Research outside of our group has additionally shown that whole body PLAAT1 loss has significant implications for lipid metabolism, whereby PLAAT1 appears to be necessary for the development of non-alcoholic fatty liver and body weight gain associated with a high fat diet in mice. The purpose of this thesis project was to characterize Plaat1-/- cardiac mitochondria with a specific focus on examining parameters typically affected by the loss of cardiolipin content. We found that despite the reductions to cardiac cardiolipin content, there was no evidence of differences in mitochondrial content, electron transport chain supercomplex stability, inflammation, fibrosis, or expression of genes required for cardiolipin biosynthesis. However, Plaat1-/- cardiac mitochondria possess a mild increase in fused phenotype compared to their wildtype counterparts without changes to the content of various mediators of mitochondrial morphology. We additionally performed a preliminary investigation of outcome variables related to lipid metabolism in the heart, given previous research that has shown PLAAT1 to be important for lipid storage and metabolism. This investigation revealed an ~35% reduction in cardiac triacylglycerol concentration in male Plaat1-/- hearts, suggesting that PLAAT1 is also critical for normal cardiac neutral lipid metabolism in male mice, although the mechanism underlying this change was not investigated in the current thesis. Our finding of a significant perturbation in another important cardiac lipid with the loss of PLAAT1 indicates a novel role for this enzyme in the heart that merits further investigation for potential implications in cardiac health and disease.