The regulation of carnitine palmitoyltransferase 1 () mRNA splicing by nutrient availability in fat tissue.

After a meal, excess nutrients are stored within adipose tissue as triglycerides in lipid droplets. Previous genome-wide RNAi screens in cells have identified mRNA splicing factors as being important for lipid droplet formation. Our lab has previously shown that a class of mRNA splicing factors called serine/arginine-rich (SR) proteins, which help to identify intron/exon borders, are important for triglyceride storage in fat tissue, partially by regulating the splicing of the gene for carnitine palmitoyltransferase 1 (CPT1), an enzyme important for mitochondrial β-oxidation of fatty acids. The gene in generates two major isoforms, with transcripts that include exon 6A producing more active enzymes than ones made from transcripts containing exon 6B; however, whether nutrient availability regulates splicing in fly fat tissue is not known. During ad libitum feeding, control flies produce more transcripts containing exon 6B while fasting for 24 h results in a shift in splicing to generate more transcripts containing exon 6A. The SR protein 9G8 is necessary for regulating nutrient responsive splicing as decreasing 9G8 levels in fly fat tissue blocks the accumulation of transcripts including exon 6A during starvation. Protein kinase A (PKA), a mediator of starvation-induced lipid breakdown, also regulates splicing during starvation as transcripts including exon 6A did not accumulate when PKA was inhibited during starvation. Together, these results indicate that splicing in adipose tissue responds to changes in nutrient availability contributing to the overall control of lipid homeostasis.