When nutrient availability becomes limited, animals must actively adjust their metabolism to allocate limited resources and maintain tissue homeostasis. However, it is poorly understood how tissues maintained by adult stem cells respond to chronic changes in metabolism. To begin to address this question, we fed flies a diet lacking protein (protein starvation) and assayed both germline and intestinal stem cells. Our results revealed a decrease in stem cell proliferation and a reduction in stem cell number; however, a small pool of active stem cells remained. Upon refeeding, stem cell number increased dramatically, indicating that the remaining stem cells are competent to respond quickly to changes in nutritional status. Stem cell maintenance is critically dependent upon intrinsic and extrinsic factors that act to regulate stem cell behavior. Activation of the insulin/IGF signaling pathway in stem cells and adjacent support cells in the germline was sufficient to suppress stem cell loss during starvation. Therefore, our data indicate that stem cells can directly sense changes in the systemic environment to coordinate their behavior with the nutritional status of the animal, providing a paradigm for maintaining tissue homeostasis under metabolic stress.
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