The regulation of carnitine palmitoyltransferase 1 (CPT1) mRNA splicing by nutrient availability in Drosophila fat tissue

After a meal, excess nutrients are stored within adipose tissue as triglycerides in lipid droplets. Previous genomewide RNAi screens in Drosophila 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 Drosophila fat tissue, partially by regulating the splicing of the gene for carnitine palmitoyltransferase 1 (CPT1), an enzyme important for mitochondrial beta-oxidation of fatty acids. The CPT1 gene in Drosophila 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 CPT1 splicing in fly fat tissue is not known. During ad libitum feeding, control flies produce more CPT1 transcripts containing exon 6B while fasting for 24 h results in a shift in CPT1 splicing to generate more transcripts containing exon 6A. The SR protein 9G8 is necessary for regulating nutrient responsive CPT1 splicing as decreasing 9G8 levels in fly fat tissue blocks the accumulation of CPT1 transcripts including exon 6A during starvation. Protein kinase A (PKA), a mediator of starvation-induced lipid breakdown, also regulates CPT1 splicing during starvation as transcripts including exon 6A did not accumulate when PKA was inhibited during starvation. Together, these results indicate that CPT1 splicing in adipose tissue responds to changes in nutrient availability contributing to the overall control of lipid homeostasis.