The development of atheroma is regulated in part by the localized expression of growth factors that promote the recruitment of hematopoietic cells as well as vascular smooth muscle cells into the neointima. We previously demonstrated that the neurotrophins and their receptors, the trk family of receptor tyrosine kinases and the p75 neurotrophin receptor (p75NTR), are highly expressed in atherosclerotic lesions. Three specific roles for the neurotrophins in regulating vessel development and in modulating the vascular response to injury have been identified: (1) neurotrophin-mediates survival of Trk B-expressing cardiac endothelial cells (2) neurotrophin-induced recruitment of Trk A and Trk B-expressing vascular smooth muscle cells to the developing neointima following injury; (3) neurotrophin-induced activation of p75NTR-expressing smooth muscle cells in the neointima induces apoptotic cell death. The paradox of neurotrophin actions in the vasculature, mediating both pro-survival and pro-death outcomes, has recently been clarified by our identification that the pro-forms of the neurotrophins selectively bind to the proapoptotic p75NTR, whereas the mature ligand selectively activates the chemotactic and survival promoting Trk receptors. Our preliminary studies indicate that both the pro- and mature forms of the neurotrophins NGF and BDNF are expressed in human atherosclerotic lesions and atheroma from murine models, and that selective MMPs and plasmin can cleave pro-forms to mature forms. The long term goals of this project are to understand how the differential expression of pro- and mature forms of neurotrophins regulate the dynamics of lesion formation and vascular remodeling. Studies in Specific Aim 1 will define the spatial and temporal expression of the pro- and mature forms of the neurotrophins in human lesions and in murine models of atheromata formation and correlate their expression with the co-ordinate expression of p75NTR and Trk receptors, as well as MMPs and components of the plasminogen protease system. Studies in Specific Aim 2 will identify the biological actions of the pro-neurotrophins on vascular smooth muscle cells, monocytes/macrophages and endothelial cells using in vitro analysis of chemotaxis, survival and apoptosis and compare them to the actions of the mature neurotrophins. Finally, in Specific Aim 3, we will genetically dissect the actions of the pro-neurotrophins from mature neurotrophins in lesion formation in vivo by replacing the native BDNF coding exon with a cleavage resistant mutant to generate only pro-BDNF. The effects of pro-BDNF overexpression in murine models of vascular injury will be assessed. The results of these studies may identify unique targets to regulate the microenvironment of the developing atheroma.
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