I’m Not Dead Yet (Indy) is a fly homologue of the mammalian SLC13A5 (mSLC13A5) plasma membrane citrate transporter, a key metabolic regulator and energy sensor involved in health, longevity, and disease. Reduction of Indy gene activity in flies, and its homologs in worms, modulates metabolism and extends longevity. The metabolic changes are similar to what is obtained with caloric restriction (dietary restriction). Similar effects on metabolism have been observed in mice and rats. As a citrate transporter, INDY regulates cytoplasmic citrate levels. Indy flies heterozygous for a P-element insertion have increased spontaneous physical activity, increased fecundity, reduced insulin signaling, increased mitochondrial biogenesis, preserved intestinal stem cell homeostasis, lower lipid levels, and increased stress resistance. Mammalian Indy knockout (mIndy-KO) mice have higher sensitivity to insulin signaling, lower blood pressure and heart rate, preserved memory and are protected from the negative effects of a high-fat diet and some of the negative effects of aging. Reducing mIndy expression in human hepatocarcinoma cells has recently been shown to inhibit cell proliferation. Reduced Indy expression in the fly intestine affects intestinal stem cell proliferation, and has recently been shown to also inhibit germ cell proliferation in males with delayed sperm maturation and decreased spermatocyte numbers. These results highlight a new connection between energy metabolism and cell proliferation. The overrall picture in a variety of species points to a conserved role of INDY for metabolism and health. This is illustrated by an association of high mIndy gene expression with non-alcoholic fatty liver disease in obese humans. mIndy (mSLC13A5) coding region mutations (e.g., loss-of-function) are also associated with adverse effects in humans, such as autosomal recessive early infantile epileptic encephalopathy and Kohlschütter−Tönz syndrome. The recent findings illustrate the importance of mIndy gene for human health and disease. Furthermore, recent work on small-molecule regulators of INDY highlights the promise of INDY-based treatments for ameliorating disease and promoting healthy aging.

Caloric restriction (CR) delays the onset of age-related changes and extends lifespan in most species, but how late in life organisms benefit from switching to a low-calorie (L) diet is unexplored. We transferred wild type male flies from a high- (H) to a L-calorie diet (HL) or vice versa (LH) at different times. Late-life HL shift immediately and profoundly reduces fly mortality rate to briefly lower rate than in flies on a constant L diet, and increases lifespan. Conversely, a LH shift increases mortality and hazard rate, which is temporarily higher than in flies aged on a H diet, and leads to shorter lifespan. Transcriptomic changes within 48 hours following diet shift uncover physiological adaptations to available nutrients. Unexpectedly, more abundant transcriptomic changes accompanied LH shift, including ribosome biogenesis, and promotion of growth, which likely contributes to higher mortality rate. Considering that the beneficial effects of CR on physiology and lifespan are conserved across many organisms, our findings suggest that CR interventions in older humans may counteract the detrimental effects of H diets even when initiated later in life.

Calorie restriction has many beneficial effects on healthspan and lifespan in a variety of species. However, how late in life application of caloric restriction can extend fly life is not clear. Here we show that late-life calorie restriction increases lifespan in female Drosophila melanogaster aged on a high calorie diet. This shift results in rapid decrease in mortality rate and extends fly lifespan. In contrast, shifting female flies from a low to a high calorie diet leads to a rapid increase in mortality and shorter lifespan. These changes are mediated by immediate metabolic and physiological adaptations. One of such adaptation is rapid adjustment in egg production, with flies directing excess energy towards egg production when shifted to a high diet, or away from reproduction in females shifted to low caloric diet. However, lifelong female fecundity reveals no associated fitness cost due to CR when flies are shifted to a high calorie diet. In view of high conservation of the beneficial effects of CR on physiology and lifespan in a wide variety of organisms, including humans, our findings could provide valuable insight into CR applications that could provide health benefits later in life

The Indy (I’m not dead yet) gene encodes a plasma membrane citrate transporter in Drosophila. INDY reduction affects metabolism and extends longevity of flies and worms. In flies, INDY is predominantly expressed in the midgut, fat body and oenocytes, tissues with a key role in metabolism. We hypothesize that INDY reduction in the midgut regulates citrate levels leading to metabolic changes that preserve intestinal stem cell (ISC) homeostasis and slows aging by modifying Insulin/Insulin-like signaling (IIS), which is a key nutrient sensing pathway. Our second goal was to examine the role of JAK/STAT signaling pathway, which activates epithelial renewal in the gut, in response to aging-related stressors. We hypothesize that Indy reduction has effects on the microbiome, preventing bacterial overgrowth and altering community diversity, leading to extended longevity in a JAK/STAT-mediated fashion. Our data suggest that effects of Indy reduction is mediated by reduced IIS and JAK/STAT pathways

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