Collateral development compensates for atherosclerotic obstructive arterial disease, but natural mechanisms rarely restore maximal flow capacity. Clinical trials, employing single angiogenic cytokines, have failed to improve collateral flow. However, angiogenesis is complex, requiring multiple genes to be coordinately expressed in an appropriate time-dependent manner. Bone marrow-derived cells (BMCs) express numerous angiogenesis-related cytokines. This led us to test a cell-based approach to achieve optimal angiogenesis. We found 1) freshly aspirated autologous BMCs express multiple angiogenesis factors and increase flow and function in a porcine model of myocardial ischemia, 2) cultured BM-derived stromal cells (MSCs) express multiple angiogenesis factors, and when given to an ischemic mouse hindlimb increase collateral blood flow. However, confounding factors may compromise the capacity of MSCs to enhance collaterals. BMCs from CAD pts show marked variability in secreting angiogenesis-related cytokines. Moreover, aging impairs collateral development, and MSCs derived from old mice ("old" MSCs) have reduced capacity to secrete angiogenic cytokines and to increase collateral flow. HIF-1 transactivates multiple genes involved in the cell's hypoxia responses, including angiogenesis. We show that old MSCs, when compared to "young" MSCs, exhibit reduced HIF1a levels and secrete less hypoxia-induced angiogenic cytokines. Thus, inducing MSCs to overexpress HIF-1alpha seems a particularly appropriate angiogenic strategy, given that most candidates for angiogenic Rx are older pts. Our preliminary data supplies supportive evidence: 1) in young mice collateral flow in the ischemic hindlimb is > following injection of MSCs overexpressing HIF-1a vs. non-transduced MSCs; 2) HIF-1alpha transduction of MSCs derived from CAD patients converts these low VEGF-expressing MSCs to very high VEGF-expressing cells. Our major hypothesis is: The reduced capacity of old mice to develop collaterals can be increased by injecting MSCs, but can be optimized-approaching that achieved by young mice-by genetically engineering the MSCs so they overexpress a constitutively active form of HIF-1alpha. We will determine whether: Aim 1) The gene expression profiles of old MSCs and of ischemic tissue of old mice are different from those of young mice, and whether the expression profile of each is altered by HIF1alpha-transduced MSCs toward the young expression profile; Aim 2) The recovery of blood flow and limb function of aged mice, after injection of MSCs that have been genetically altered so they overexpress a constitutively active form of HIF-1alpha, improve so they are similar to that of young mice; Aim 3) Aging markedly alters collateral phenotype and HIF1alpha transduced MSCs alter phenotype toward the young mouse phenotype (using the high resolution of micro- CT). If this novel strategy is validated, the results will be of immense value in developing individualized therapies for improving collateral flow clinically, a goal that has to date proven elusive.