Effects of Starvation on the Levels of Triglycerides, Diacylglycerol, and Activity of Lipase in Male and Female Drosophila Melanogaster

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 genome-wide 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 β-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.

The fruit fly acetyltransferase chameau promotes starvation resilience at the expense of longevity

Proteins involved in cellular metabolism and molecular regulation can extend lifespan of various organisms in the laboratory. However, any improvement in aging would only provide an evolutionary benefit if the organisms were able to survive under non-ideal conditions. We have previously shown that Drosophila melanogaster carrying a loss-of-function allele of the acetyltransferase chameau (chm) has an increased healthy lifespan when fed ad libitum. Here, we show that loss of chm and reduction in its activity results in a substantial reduction in weight and a decrease in starvation resistance. This phenotype is caused by failure to properly regulate the genes and proteins required for energy storage and expenditure. The previously observed increase in survival time thus comes with the inability to prepare for and cope with nutrient stress. As the ability to survive in environments with restricted food availability is likely a stronger evolutionary driver than the ability to live a long life, chm is still present in the organism's genome despite its apparent negative effect on lifespan.

Comparisons of lifespan and stress resistance between sexes in Drosophila melanogaster

Animals exhibit different extents of sexual dimorphism in a variety of phenotypes. Sex differences in longevity, one of the most complex life history traits, have also been reported. Although lifespan regulation has been studied extensively in the fruit fly, Drosophila melanogaster, the sex differences in lifespan have not been consistent in previous studies. To explore this issue, we revisited this question by examining the lifespan and stress resistance of both sexes among 15 inbred strains. We first found positive correlations between males and females from the same strain in terms of lifespan and resistance to starvation and desiccation stress. Although the lifespan difference between male and female flies varied greatly depending on the strain, males across all strains collectively had a longer lifespan. In contrast, females showed better resistance to starvation and desiccation stress. We also observed greater variation in lifespan and resistance to starvation and desiccation stress in females. Unexpectedly, there was no notable correlation observed between lifespan and the three types of stress resistance in either males or females. Overall, our study provides new data regarding sexual dimorphism in fly lifespan and stress resistance; this information may promote the investigation of mechanisms underlying longevity in future research.

Alterations in brain glycogen levels influence life-history traits and reduce the lifespan in female Drosophila melanogaster

Sexual dimorphism in lifespan, wherein females outlive males, is evident across all animal taxa. The longevity difference between sexes is controlled by multiple physiological processes with complex relationships to one another. In recent years, glycogen, the storage form of glucose, has been shown to cause rapid aging upon forced synthesis in healthy neurons. Glycogen in the form of corpora amylacea in the aging brain is also widely reported. While these studies did suggest a novel role for glycogen in aging, most of them have focused on pooled samples, and have not looked at sex-specific effects, if any. Given the widespread occurrence of sex-biased expression of genes and the underlying physiology, it is important to look at the sex-specific effects of metabolic processes. In the present study, using transgenic fly lines for the human glycogen synthase, we investigated the sex-specific effects of glycogen on stress resistance, fitness, and survival. We demonstrate that Drosophila melanogaster females with altered levels of glycogen in the brain display a shortened lifespan, increased resistance to starvation, and higher oxidative stress than male flies. The present study thus provides a novel insight into the sex-specific effect of glycogen in survival and aging and how differences in metabolic processes could contribute to sex-specific traits.