Control of reproductive aging by germline stem cells

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Project Summary/Abstract Adult stem cell exhaustion is a hallmark of aging. However, mechanisms of stem cell exhaustion during aging are largely unknown, and there are no therapies that can delay stem cell aging in humans. C. elegans is a premier model organism for studying aging; adult animals are short lived and only contain one stem cell pool, the germline stem cells that generate eggs and sperm. Extensive developmental studies have provided a rich description of the molecular and cellular events that control these stem cells in young animals. My goal is to understand stem cell exhaustion, and my strategy is to exploit the experimental power of C. elegans and the detailed knowledge of stem cell development to elucidate stem cell aging. The somatic distal tip cell (DTC) serves as the stem cell niche by expressing the Notch pathway ligands LAG-2 and APX-1, which bind and activate the GLP-1/Notch receptor in the stem cells. Notch signaling pathways are conserved during evolution and have been repeatedly implicated in regulating stem cells in mammals, suggesting the niche/stem cell system in worms is likely to be broadly relevant. Elucidating the regulatory logic of this system will advance the fields of reproductive aging and stem cell exhaustion. Preliminary results from our lab demonstrate that the number and activity of germline stem cells decline rapidly and progressively with age. Based on these results, I propose two innovative hypotheses. (1) An age-related decline in Notch signaling from the DTC niche causes stem cell exhaustion. (2) Neuronal TGF-β signaling mediates the activity of the DTC niche and contributes to the age- related decline of adult stem cells in the germline. To test these hypotheses, I propose two specific aims. Aim 1: Elucidate mechanisms of Notch pathway regulation during adult stem cell aging in the germline. I will monitor LAG-2 ligand expression in the DTC niche during aging and analyze LAG-2 ligand and Notch receptor function. The results will rigorously test my hypothesis by establishing how LAG-2 ligand expression is regulated during aging and whether LAG-2 ligand and/or GLP-1/Notch receptor are sufficient to sustain stem cell activity during aging. Aim 2: Determine how sensory neurons regulate the DTC niche to mediate germline stem cell aging. I will analyze the DAF-3 binding site in the lag-2 promoter, and the DAF-3 and DAF-5 transcription factors that are the effectors of TGF-β signaling. I will examine multiple levels of organization including protein expression, stem cell dynamics, and progeny production. The results will establish how neuronal signals control the niche and stem cells during aging. The mechanisms of stem cell exhaustion remain mysterious, and these experiments will advance the field by determining the contributions of neurons, the niche, and the stem cells themselves. The results will establish a foundation of knowledge that may stimulate innovative approaches to preserve stem cell function and promote healthy aging in humans.