Project Summary The hematopoietic stem cell (HSC) microenvironment in the bone marrow, termed the niche, provides a specialized microenvironment to control the proliferation, self-renewal, differentiation and migration of HSCs. HSC aging is accompanied by an expansion of myeloid-biased HSCs with declined self-renewal functions. Aging of HSCs is associated with various age-related blood diseases, such as clonal hematopoiesis of indeterminate potential (CHIP). Whether the aged microenvironment drives the aging of HSCs and age-related hematologic diseases, however, remains unclear. I recently demonstrated that the nociceptive nervous system is an essential HSC niche component that regulates granulocyte colony-stimulating factor (G-CSF)-induced HSC mobilization via the secretion of neuropeptide calcitonin gene-related peptide (CGRP) in the bone marrow. Our preliminary data reveal a significant reduction of CGRP levels in the aged bone marrow microenvironment, and administration of CGRP attenuates aging-associated phenotypes of HSCs in the old mice, resulting in a reduced number of HSCs and a restored myeloid versus lymphoid balance. In addition, we find that aged bone marrow microenvironment promotes the clonal expansion of genetically mutated HSCs (Asxl1tm/+ HSCs) over wildtype HSCs. Based on these preliminary data, we propose a 5-year experimental plan to characterize the functions of nociceptive neurons in the aged bonne marrow microenvironment and to dissect the contributions of aged bone marrow microenvironment to clonal hematopoiesis. In Specific Aim 1, we will evaluate the impact of aging on nociceptive neurons in the bone marrow. We will determine whether aging induces the loss of nociceptive fibers or reduces the CGRP levels without affecting the nociceptive innervation in the bone marrow. We will explore the mechanisms by which nociceptor-derived CGRP signals in the bone marrow drive the aging of HSCs. We will investigate whether modulation of CGRP signaling via ingestion of food containing capsaicin – a natural component of chili peppers that could trigger the activation of nociceptive neurons – could rejuvenate the functional and molecular aging signatures of HSCs. In Specific Aim 2, we will determine how aged bone marrow microenvironment interacts with HSCs to promote the Asxl1tm/+ mutant clonal hematopoiesis and explore whether targeting the aged bone marrow microenvironment could prevent Asxl1tm/+ HSC expansion. These proposed studies, focusing on the interactions between bone marrow niche and HSCs, will allow us to identify new extrinsic factors regulating HSC aging and potentially provide novel approaches to rejuvenate HSCs and prevent age- related hematologic diseases.