Flawed Phospholipid Formation or Faulty Fatty Acid Oxidation: Determining the Cause of Mitochondrial Dysfunction in Hearts Lacking ACSL1

Cardiovascular disease is the number one cause of death worldwide. In the heart, mitochondria provide up to 95% of energy, with most of this energy coming from metabolism of fatty acids (FA). FA must be converted to acyl-CoAs by acyl-CoA synthetases (ACS) before entry into pathways of β- oxidation or glycerolipid synthesis. ACSL1 contributes more than 90% of total cardiac ACSL activity, and mice with an inducible knockout of ACSL1 (Acsl1T-/-) have impaired cardiac FA oxidation. The effects of loss of ACSL1 on mitochondrial respiratory function, phospholipid formation, or autophagic flux have not yet been studied. Acsl1T-/- hearts contained 3-fold more mitochondria with abnormal structure and displayed lower respiratory function. Because ACSL1 exhibited a strong substrate preference for linoleate (18:2), we investigated the composition of mitochondrial phospholipids. Acsl1T-/- hearts contained 83% less tetralinoleoyl-cardiolipin (CL), the major form present in control hearts. Modulating ACSL1 expression in cell lines confirmed that ACSL1 is necessary for linoleate incorporation into CL. To determine whether increasing content of linoleate in CL would improve mitochondrial respiratory function, control and Acsl1T-/- mice were fed a high linoleate diet, which normalized amount of tetralinoleoyl-CL, but did not improve respiratory function. The metabolic switch from FA use to high glucose use activates mechanistic target of rapamycin complex 1 (mTORC1), which initiates growth by increasing protein and RNA synthesis and FA metabolism while decreasing autophagy. Short-term mTORC1 inhibition normalized mitochondrial structure, number, and maximal respiration rate in Acsl1T-/- hearts but not ADP-stimulated oxygen consumption, which was likely caused by lower ATP synthase activity present in both vehicle- and rapamycin-treated Acsl1T-/- hearts. The autophagic rate was 88% lower in Acsl1T-/- hearts. mTORC1 inhibition increased autophagy to a rate that was 3.1-fold higher than in controls, allowing clearance of damaged mitochondria. ACSL1 deficiency in heart activated mTORC1, thereby inhibiting autophagy and increasing the number of damaged mitochondria with impaired respiratory capacity. ACSL1 is required for the normal composition of phospholipid species and maintenance of FA oxidation to prevent low autophagic rate. Loss of ACSL1 causes impaired mitochondrial respiratory function, which can be partially improved by clearing damaged mitochondria but not by normalizing CL.Doctor of Philosoph