During aging, hematopoietic stem cells (HSCs) are able to function normally under homeostatic conditions but are unable to mount effective regenerative responses under conditions of acute damage. Thus, in response to bleeding, infection or chemotherapy, elderly people often fail to replenish their blood effectively,and have an increased risk of hematopoietic disorders such as anemia and neutropenia that can often be fatal. To understand why this hematopoietic dysfunction occurs during aging, it is important to identify the molecular mechanisms that allow young HSCs to respond to acute damage and determine how these mechanisms fail in the aging hematopoietic system. To understand the molecular signals that normally allow the effective regeneration of the blood, we have studied animals treated with the chemotherapeutic drug Cyclophosphamide (Cy) and the growth factor G-CSF which causes an acute loss of proliferating progenitors in the bone marrow followed by expansion of HSCs to regenerate the progenitor pool. Using this as a damage model, we have found that that the Wnt signaling is sharply upregulated in a large fraction of HSCs responding to Cy/G-CSF suggesting that Wnt signaling may be a critical mediator of HSC regeneration after damage. In order to test this hypothesis, we now propose to define the significance of Wnt activation in regenerating HSCs and determine if this activity changes as HSCs age. Identifying the mechanisms that underlie the regenerative response to Cyclophosphamide/ G-CSF treatment will enhance our ability to harvest and expand stem cells for transplantation therapy. Furthermore, understanding the basis of impaired regeneration in the aging hematopoietic system may allow us to design novel means to improve the health and quality of life of aging patients.