Browsing by Subject "Starvation"

Nearly all animals live in environments with fluctuations in nutrient availability. The ability to sense and respond to these changes is essential for survival. Nutrition impacts physiology immediately, but can also have long-lasting effects across generations. The nematode Caenorhabditis elegans is particularly well-adapted to thrive in conditions of variable food availability. Here we find that starvation responses in C. elegans are largely independent of the larval stage at which worms experience starvation. Starvation in worms results in shrinkage, delayed growth upon recovery, and ultimately death. In order to adapt to starvation, metabolism is dramatically altered. At a gross level, this can be seen in a reduction of mitochondrial genomes and a more fragmented network of mitochondria.

Insulin-like signaling is a key cell signaling pathway controlling nutrient responses. We interrogate the role of insulin-like signaling in regulation of the acute starvation response. We show that daf-16/FoxO restructures carbohydrate metabolism by driving carbon flux through the glyoxylate shunt and gluconeogenesis and into synthesis of trehalose, a disaccharide of glucose. Trehalose is a well-known stress protectant, capable of preserving membrane organization and protein structure during abiotic stress. Metabolomic, genetic, and pharmacological analyses confirm increased trehalose synthesis and further show that trehalose not only supports survival as a stress protectant, but also serves as a glycolytic input. Further, we provide evidence that metabolic cycling between trehalose and glucose is necessary for this dual function of trehalose. This work demonstrates that daf-16/FoxO promotes starvation resistance by shifting carbon metabolism to drive trehalose synthesis, which in turn supports survival by providing an energy source and acting as a stress protectant.

In addition to acute changes in response to the nutrient environment, effects can persist intergenerationally. Maternal effects of environmental conditions produce intergenerational phenotypic plasticity. Adaptive value of these effects depends on appropriate anticipation of environmental conditions in the next generation, and mismatch between conditions may contribute to disease. However, regulation of intergenerational plasticity is poorly understood. Dietary restriction (DR) delays aging but maternal effects have not been investigated. We demonstrate maternal effects of DR in the roundworm C. elegans. Worms cultured in DR produce fewer but larger progeny. Nutrient availability is assessed in late larvae and young adults, rather than affecting a set point in young larvae, and maternal age independently affects progeny size. Reduced signaling through the insulin-like receptor daf-2/InsR in the maternal soma causes constitutively large progeny, and its effector daf-16/FoxO is required for this effect. nhr-49/Hnf4, pha-4/FoxA, and skn-1/Nrf also regulate progeny-size plasticity. Genetic analysis suggests that insulin-like signaling controls progeny size in part through regulation of nhr-49/Hnf4, and that pha-4/FoxA and skn-1/Nrf function in parallel to insulin-like signaling and nhr-49/Hnf4. Furthermore, progeny of DR worms are buffered from adverse consequences of early-larval starvation, growing faster and producing more off- spring than progeny of worms fed ad libitum. These results suggest a fitness advantage when mothers and their progeny experience nutrient stress, compared to an environmental mismatch where only progeny are stressed. This work reveals maternal provisioning as an organismal response to DR, demonstrates potentially adaptive intergenerational phenotypic plasticity, and identifies conserved pathways mediating these effects.

The roundworm C. elegans reversibly arrests larval development during starvation, but extended early-life starvation reduces reproductive success. Maternal dietary restriction (DR) buffers progeny from starvation as young larvae, preserving reproductive success. However, the developmental basis of reduced fertility following early-life starvation is unknown, and it is unclear how maternal diet modifies developmental physiology in progeny. We show here that extended starvation in first-stage (L1) larvae followed by unrestricted feeding results in a variety of developmental abnormalities in the reproductive system, including proliferative germ-cell tumors and uterine masses that express neuronal and epidermal cell-fate markers. We found that maternal DR and reduced maternal insulin/IGF signaling (IIS) increase oocyte provisioning of vitellogenin lipoprotein, reducing penetrance of starvation-induced abnormalities in progeny, including tumors. Furthermore, we show that maternal DR and reduced maternal IIS reduce IIS in progeny. daf-16/FoxO and skn-1/Nrf, transcriptional effectors of IIS, are required in progeny for maternal DR and increased vitellogenin provisioning to suppress starvation-induced abnormalities. daf-16/FoxO activity in somatic tissues is sufficient to suppress starvation-induced abnormalities, suggesting cell-nonautonomous regulation of reproductive system development. This work reveals that early-life starvation compromises reproductive development and that vitellogenin-mediated intergenerational insulin/IGF-to-insulin/IGF signaling mediates adaptation to nutrient availability. Using our SIA model, we go on to show that early-life starvation persistently activates PQM-1/SALL2 with pervasive effects on adult gene expression, including prominent effects on membrane biology. Early-life starvation increases fatty-acid synthetase fasn-1/FASN expression in pqm-1/SALL2-dependent fashion, and both genes promote SIA. Lipidomic analysis implicates phosphatidylcholine, and unsaturated phosphatidylcholine supplementation suppresses SIA. The fatty-acid desaturases fat-1 and fat-4 inhibit and promote SIA, respectively, revealing a role of arachidonic acid-containing phosphatidylcholine, the Lands cycle, and eicosanoid signaling. Indeed, fat-4 increases eicosanoid levels in adults subjected to early-life starvation, and N-acetylcysteine treatment suppresses SIA. This work shows that early-life starvation and IIS converge on PQM-1/SALL2 to affect adult lipid metabolism and eicosanoid signaling, affecting stem-cell proliferation and tumor formation.

Fluctuations in nutrient availability occur for nearly all species, and adaptation to endure starvation conditions is essential. Genetic pathways involved in regulating starvation resistance are implicated in aging and complex diseases such as cancer, diabetes, and obesity in humans. Consequences of experiencing starvation persist later in life and subsequent generations, suggesting epigenetic regulation. However, much is still unknown about how starvation resistance is regulated and the contributions of different types of regulation. The roundworm Caenorhabditis elegans reversibly arrests development in the absence of food and can endure starvation for several weeks. Here, we investigate how transcriptional, epigenetic, and genetic regulation impact starvation resistance during developmental arrest in C. elegans. Gene expression dynamics change quickly during the first few hours of starvation, and broadly conserved transcription factors are required for starvation survival. However, temporal- and tissue-specific requirements of transcription for supporting starvation survival and recovery are largely unknown. In chapter 2, we used mRNA-seq combined with temporal degradation of RNA Polymerase II in the soma and germline to better understand gene regulation throughout arrest. We find that transcription is required in the soma for survival early in starvation but is dispensable thereafter, and known transcriptional regulators primarily act early in arrest. In contrast, the germline is transcriptionally quiescent throughout starvation, but germline transcripts are relatively stable compared to somatic transcripts. This reveals alternative gene-regulatory strategies in the soma and germline during starvation-induced developmental arrest, with the soma relying on a robust early transcriptional response while the germline relies on mRNA stability to maintain integrity. Phenotypic plasticity is facilitated by epigenetic regulation, and remnants of such regulation may persist after plasticity-inducing cues are gone, even affecting germ cells to impact subsequent generations. However, the relationship between plasticity and transgenerational epigenetic memory is not understood. Dauer diapause provides an opportunity to determine how a plastic response to the early-life environment affects traits later in life and in subsequent generations. In chapter 3, we find that, after extended diapause, postdauer worms initially exhibit reduced reproductive success and greater interindividual variation. In contrast, F3 progeny of postdauers display increased starvation resistance and lifespan, revealing potentially adaptive transgenerational effects. Transgenerational effects are dependent on the duration of diapause, indicating an effect of extended starvation. In agreement, RNA-seq demonstrates a transgenerational effect on nutrient-responsive genes. This work reveals complex effects of nutrient stress over different time scales in an animal that evolved to thrive in feast and famine. Many conserved genes and pathways regulate starvation resistance, but most genetic analysis in C. elegans has been restricted to a single genetic background, potentially restricting identification of additional genes. Hundreds of genetically distinct wild strains of C. elegans have been whole-genome sequenced and can be used for GWAS. In chapters 4 and 5, we implemented two high-throughput sequencing approaches, RAD-seq and MIP-seq, to determine relative starvation resistance of over 100 wild strains over time. We used GWAS to identify QTL associated with starvation resistance, near-isogenic lines to validate QTL, and CRISPR gene editing to modify specific genes within QTL. We focused on genes in the insulin receptor-like domain (irld) family, as this family has been virtually uncharacterized, but the genes share homology with the sole known insulin-like receptor in C. elegans, DAF-2, which is a major regulator of starvation resistance. We found that specific variants in two members of the irld family confers increased starvation resistance in multiple genetic backgrounds, and this is dependent on the transcription factor downstream of insulin signaling, DAF-16/FOXO. Thus, this work shows that natural genetic variation in novel modifiers of insulin-signaling regulates starvation resistance.

Starvation during early human development produces epigenetic effects that could be adaptive if famine persists. We modeled the response to early starvation exposure in C. elegans using `L1 arrest,' a reversible developmental arrest in the first larval stage brought on by hatching in the absence of food. We found lifelong developmental effects following recovery from extended L1 arrest. Remarkably, some epigenetic effects persist for multiple generations. After extended starvation, development is delayed, producing smaller adults, fertility is reduced, and stress resistance increases. Starvation causes a striking amount of phenotypic variation among isogenic individuals, and those that develop slowest are least fertile and most stress resistant. However, increased stress resistance appears to be characteristic of recovering animals following any amount of L1 arrest but decreasing as the animal grows, possibly by size-dependent dilution. We assessed starved animals for signs of autophagy-related feeding defects and found low rates of pumping and feeding as well as grinding defects. A retrospective pumping assay revealed that after extended starvation, animals with lower pumping rates at the L1 stage tended to grow slower. Our work shows that environmental conditions and life history have transgenerational life history effects on several organismal traits, and that these traits appear to be rooted in nutrient-deprivation secondary to autophagy-related feeding defects. However, the manner by which these effects are transmitted transgenerationally remains an open and interesting question.