Adaptive Physiological Response to Perceived Scarcity as a Mechanism of Sensory Modulation of Life Span

Abstract

Chemosensation is a potent modulator of organismal physiology and longevity. In Drosophila, loss of recognition of diverse tastants has significant and bidirectional life-span effects. Recently published results revealed that when flies were unable to taste water, they increased its internal generation, which may have subsequently altered life span. To determine whether similar adaptive responses occur in other contexts, we explored the impact of sensory deficiency of other metabolically important molecules. Trehalose is a major circulating carbohydrate in the fly that is recognized by the gustatory receptor Gr5a. Gr5a mutant flies are short lived, and we found that they specifically increased whole-body and circulating levels of trehalose, but not other carbohydrates, likely through upregulation of de novo synthesis. dILP2 transcript levels were increased in Gr5a mutants, a possible response intended to reduce hypertrehalosemia, and likely a contributing factor to their reduced life span. Together, these data suggest that compensatory physiological responses to perceived environmental scarcity, which are designed to alleviate the ostensive shortage, may be a common outcome of sensory manipulation. We suggest that future investigations into the mechanisms underlying sensory modulation of aging may benefit by focusing on direct or indirect consequences of physiological changes that are designed to correct perceived disparity with the environment.

Introduction, Results, and Discussion

The ability of chemosensory systems to alter longevity is both robust and conserved across invertebrate species (1–7). The rapid and acute nature of responses to sensory signals—required for surviving in a complex environment in which resources need to be recognized and physical danger avoided—likely provide the basis for such potent effects. The Drosophila gustatory system has emerged as a useful model by which to study this regulation. Taste perception is most often associated with allowing an organism to assess its nutritive environment and enact proper behavioral responses. It has only been recently appreciated that information emanating from these inputs inform internal physiologic mechanisms as a way to respond to the preponderance—and likely ingestion—or absence—and likely lack of ingestion—of a given stimulus. Thus, the gustatory system represents a powerful means of control of organismal health, and understanding how an organism responds to perceived nutrient abundance or scarcity is critical for exploiting this reality in a diet-independent manner.

A previous screen of gustatory gene mutants allowed for the characterization of the life-span effects of perceived dietary scarcity of a diverse range of tastants (3). A more detailed analysis of long-lived, water-insensitive pickpocket28 (ppk28) mutants provided an intriguing insight—that the inability to taste water promoted internal water storage (4). We wondered whether this exemplified a general response to lack of nutrient perception. As such, we investigated carbohydrate sensing as a separate modality. Flies recognize sugars through gustatory receptor neurons expressing Gr5a, along with one or more of a subset of other sweet taste receptors (8). Gr5a has been found to be required for the recognition of a small set of sugars (9), but it is specifically tuned toward the disaccharide, trehalose (10). Our original screen of gustatory receptors suggested that loss of Gr5a reduced life span (3). To establish the veracity of this result, we repeated life-span analysis using these same flies (ΔEP(X)-5; referred as Gr5a ^Δ for brevity) with a P element imprecise excision removing the initiation codon of translation (11) (Supplementary Figure S1). Indeed, we found Gr5a mutants were short lived and that genetic rescue of Gr5a expression restored flies to a normal life span (Figure 1A). Though short lived, we found young Gr5a mutants to have broadly similar activity (Figure 1B) and feeding (Figure 1C) patterns. We thus reasoned that Gr5a-mediated gustatory input normally contributes positively to fly longevity.

Figure 1.

Loss of Gr5a function decreases life span in Drosophila melanogaster independently of feeding behavior and activity. (A) Survival curves for Gr5a mutant (w,Gr5a ^Δ), Gal4/UAS rescue (w,Gr5a ^Δ; w,Gr5a-Gal4/+; UAS-Gr5a/+), and background control (w and w,Gr5a-Gal4/+) female flies. Pairwise comparison between rescue and background controls are statistically significant by log-rank test (p < 1×10^{− 10} in each case). (B) Average total activity counts for Gr5a mutant (w,Gr5a ^Δ) and control (w) female flies (n = 32 flies/genotype). **p < .01, *p < .05 by two-sided Student’s t test. Error bars indicate ±SEM. (C) Blue dye consumed in a 6-hour period by female Gr5a mutants (w,Gr5a ^Δ) and controls (w) on a 10% (n = 11 groups of 5 flies for each genotype; 12-16 days of age). Error bars indicate ±SEM.

To assess whether loss of Gr5a function led to altered carbohydrate homeostasis, we measured whole-organism quantities of an extended panel of sugars. Strikingly, we found that Gr5a mutants showed significantly high levels of trehalose specifically—the ligand of the Gr5a receptor (Figure 2A)—and that this phenotype persisted throughout life (Figure 2B). We also found no significant changes in total lipid stores or protein (Supplementary Figure S2). Trehalose is de novo synthesized in the fat body through the action of two enzymes, trehalose phosphate synthase 1 (tps1) and trehalose phosphate phosphatase (tpp), and released through a membrane-bound transporter, trehalose transporter 1 (Tret1-1) (12) (Supplementary Figure S3). Conversely, trehalose is catabolized by the enzyme trehalase (12). We determined that increased trehalose levels were more likely due to increased trehalose synthesis as there was no significant difference in the ability of whole-fly homogenates from control and Gr5a mutants to break down trehalose (Figure 2C), whereas mRNA transcript levels of tps1, tpp, and Tret1-1 were significantly increased in Gr5a mutants up to 100-fold (Figure 2D).

Figure 2.

Gr5a mutants maintain a hypertrehalosemic state. (A) Whole-organism carbohydrate levels in Gr5a null (w,Gr5a ^Δ) and control (w) female flies. **p < .01 by two-sided Student’s t test. Error bars indicate ±SEM. Gly = glycogen; Glu = glucose; Fru = fructose; Tre = trehalose. (B) Whole-organism trehalose in Gr5a null (w,Gr5a ^Δ) and control (w) female flies. **p < .01; *p < .05 by two-sided Student’s t test. Error bars indicate ±SEM. (C) Amount of trehalose converted to glucose in 1 hour by homogenate of Gr5a null (w,Gr5a ^Δ) and control (w) female flies. Error bars indicate ±SEM. (D) Relative mRNA transcript levels of trehalose synthesis genes in Gr5a null (w,Gr5a ^Δ) and control (w) female flies. ***p < .001 by two-sided Student’s t test. Error bars indicate ±SEM. (E) Hemolymph trehalose concentration of 12-16 day old Gr5a null (w,Gr5a ^Δ) and control (w) female flies. *p < .05 by two-sided Student’s t test. Error bars indicate ±SEM. (F) Survival curves for control (w) female flies on standard (SY10) or high trehalose (TY10) diet.

Trehalose has multiple functions in insects, including as a cellular protectant against stress and as a chitin precursor (12). Of particular interest, though, was its role as an energy reserve and major circulating carbohydrate analogous to mammalian blood glucose (13). Though hemolymph trehalose levels are not as tightly regulated—concentration can vary by up to an order of magnitude depending on energetic needs (13)—it was plausible that chronically high levels of circulating sugar represented a potentially adverse physiological state. To determine if Gr5a mutants showed evidence of increased circulating trehalose concentration (hypertrehalosemia), we assayed hemolymph trehalose levels of adult flies and found that Gr5a mutants were, indeed, hypertrehalosemic (Figure 2E). To determine whether this increased hemolymph trehalose concentration was sufficient to reduce life span, we induced hypertrehalosemia in control flies by feeding a high trehalose diet (Supplementary Figure S4). We found that the life-span trajectory of control flies on a high trehalose diet essentially mirrored that of flies measured concurrently on a standard diet (5.83% mean life-span decrease), despite increasing hemolymph trehalose substantially more than loss of Gr5a (Figure 2F), suggesting that the increased hemolymph trehalose concentration maintained by Gr5a mutants was not itself sufficient to reduce overall life span.

Though trehalosemic control in flies is not completely understood, there is evidence suggesting that hemolymph trehalose levels can be reduced by insulin-like peptide (ILP)–signaling pathway activity, particularly through the action of dILP2 (14). To discern whether loss of Gr5a function might induce indirect activation of dILP2—potentially as a mechanism to clear hemolymph trehalose levels to an acceptable, though increased level—we measured dILP2 mRNA transcript levels. We found dILP2 mRNA transcript levels to be significantly increased in Gr5a mutants (Supplementary Figure S5). Insulin/ILP-signaling pathway activity has been negatively correlated with life span in many species, including Drosophila (15). Thus, although the molecular mechanism underlying the shortened life span of Gr5a mutants requires further study, it is separable from their hypertrehalosemic state and may be a result of a homeostatic response to perceived sugar scarcity that involves insulin production.

This study, along with previous work (4), suggests that the connection between sensory perception and production of a given metabolite is, indeed, at least partially interconnected. It is, perhaps, not entirely surprising that such a close association exists as the components of food sources recognized by sensory systems serve as the substrates for organismal metabolism. Indeed, the positive effect on life span of Gr5a-mediated signals may potentially lie in their ability to control hemolymph carbohydrate concentration, accurately coupling the fly’s circulating trehalose levels to the perceived amount available for ingestion in its dietary environment. In this capacity, carbohydrate sensing represents an intriguing target for regulating blood sugar, in particular as it avoids any dietary modification. As complete of an understanding as possible of how external cues modulate organismal physiology is critical not only for the understanding of aging regulation but also how these mechanisms might be harnessed to improve both quality and length of life. Thus, in addition to carbohydrates, further investigation into the cellular and molecular basis of how other nutrients, including lipid and protein, are sensed, and whether neural inputs sensitive to these molecules also inform their internal metabolism, is ripe for study.

Supplementary Material

Supplementary material can be found at: http://biomedgerontology.oxfordjournals.org/

Funding

This work was funded by grants from the U.S. National Institutes of Health (5-T-32-GM007315, T-32-AG000114 to M.J.W.; F-31-AG033981, T-32-AG018316 to T.P.C.; R-01-AG030593, R-01-AG043972, R-01-AG023166 to S.D.P), the Glenn Foundation, the American Federation for Aging Research, and the Ellison Medical Foundation (S.D.P.). Additionally, this work utilized the Drosophila Aging Core of the Nathan Shock Center of Excellence in the Biology of Aging funded by the National Institute of Aging (P30-AG-013283).