ATGL-mediated lipolysis is essential for myocellular mitochondrial function, mitochondria-lipid droplet interaction and mitochondrial network connectivity

Defects in Adipose tissue TriGlyceride Lipase (ATGL)-mediated myocellular lipid droplet (LD) lipolysis results in mitochondrial dysfunction of unknown origin, which can be rescued by PPAR agonists. Here we examine if ATGL-mediated lipolysis is required to maintain mitochondrial network connectivity and function. Moreover, we explored if the functional implications of ATGL deficiency for mitochondrial network dynamics and function can be alleviated by promoting PPARα and/or PPARδ transcriptional activity. To this end, we cultured human primary myotubes from patients with neutral lipid storage disease with myopathy (NLSDM), a rare metabolic disorder caused by a mutation in the gene encoding for ATGL. These myotubes possess dysfunctional ATGL and compromised LD lipolysis. In addition, mitochondria-LD contact, mitochondrial network connectivity, and mitochondrial membrane potential were affected. Using a humanized ATGL inhibitor in myotubes (cultured form healthy donors) revealed similar results. Upon stimulating PPARδ transcriptional activity, mitochondrial respiration improved by more than 50 % in human primary myotubes from healthy lean individuals. This increase in respiration was dampened in myotubes with dysfunctional ATGL. Stimulation of PPARδ transcriptional activity had no effect on mitochondria-LD contacts, mitochondrial network connectivity, and mitochondrial membrane potential. Our results demonstrate that dysfunctional ATGL results in compromised mitochondrial-LD contacts and mitochondrial network connectivity, and that functional ATGL is required to improve mitochondrial respiratory capacity upon stimulation of PPARδ transcriptional activity.