Cardiac Nerve Growth Factor Overexpression Induces Bone Marrow–derived Progenitor Cells Mobilization and Homing to the Infarcted Heart

One of the major therapeutic goals of modern cardiology is to design strategies aimed at minimizing myocardial necrosis and optimizing cardiac repair following myocardial infarction. However, a sound understanding of the biology is necessary before a specific intervention is pursued on a therapeutic basis. This review summarizes our current understanding of the cellular and molecular mechanisms regulating the inflammatory response following myocardial ischemia and reperfusion. Myocardial necrosis induces complement activation and free radical generation, triggering a cytokine cascade initiated by Tumor Necrosis Factor (TNF)-alpha release. If reperfusion of the infarcted area is initiated, it is attended by an intense inflammatory reaction. Interleukin (IL)-8 synthesis and C5a activation have a crucial role in recruiting neutrophils in the ischemic and reperfused myocardium. Neutrophil infiltration is regulated through a complex sequence of molecular steps involving the selectins and the integrins, which mediate leukocyte rolling and adhesion to the endothelium. Marginated neutrophils exert potent cytotoxic effects through the release of proteolytic enzymes and the adhesion with Intercellular Adhesion Molecule (ICAM)-1 expressing cardiomyocytes. Despite this potential injury, substantial evidence suggests that reperfusion enhances cardiac repair improving patient survival; this effect may be in part related to the inflammatory response. Monocyte Chemoattractant Protein (MCP)-1 is also markedly upregulated in the infarcted myocardium inducing recruitment of mononuclear cells in the injured areas. Monocyte-derived macrophages and mast cells may produce cytokines and growth factors necessary for fibroblast proliferation and neovascularization, leading to effective repair and scar formation. At this stage expression of inhibitory cytokines such as IL-10 may have a role in suppressing the acute inflammatory response and in regulating extracellular matrix metabolism. Fibroblasts in the healing scar undergo phenotypic changes expressing smooth muscle cell markers. Our previous review in this journal focused almost exclusively on reduction of the inflammatory injury. The current update is prompted by the potential therapeutic opportunity that the open vessel offers. By promoting more effective tissue repair, it may be possible to reduce the deleterious remodeling, that is the leading cause of heart failure and death. Elucidating the complex interactions and regulatory mechanisms responsible for cardiac repair may allow us to design effective inflammation-related interventions for the treatment of myocardial infarction.

Stem cells within the bone marrow (BM) exist in a quiescent state or are instructed to differentiate and mobilize to circulation following specific signals. Matrix metalloproteinase-9 (MMP-9), induced in BM cells, releases soluble Kit-ligand (sKitL), permitting the transfer of endothelial and hematopoietic stem cells (HSCs) from the quiescent to proliferative niche. BM ablation induces SDF-1, which upregulates MMP-9 expression, and causes shedding of sKitL and recruitment of c-Kit+ stem/progenitors. In MMP-9-/- mice, release of sKitL and HSC motility are impaired, resulting in failure of hematopoietic recovery and increased mortality, while exogenous sKitL restores hematopoiesis and survival after BM ablation. Release of sKitL by MMP-9 enables BM repopulating cells to translocate to a permissive vascular niche favoring differentiation and reconstitution of the stem/progenitor cell pool.

Endothelial progenitor cells (EPCs) have been isolated from circulating mononuclear cells in human peripheral blood and shown to be incorporated into foci of neovascularization, consistent with postnatal vasculogenesis. We determined whether endogenous stimuli (tissue ischemia) and exogenous cytokine therapy (granulocyte macrophage-colony stimulating factor, GM-CSF) mobilize EPCs and thereby contribute to neovascularization of ischemic tissues. The development of regional ischemia in both mice and rabbits increased the frequency of circulating EPCs. In mice, the effect of ischemia-induced EPC mobilization was demonstrated by enhanced ocular neovascularization after cornea micropocket surgery in mice with hindlimb ischemia compared with that in non-ischemic control mice. In rabbits with hindlimb ischemia, circulating EPCs were further augmented after pretreatment with GM-CSF, with a corresponding improvement in hindlimb neovascularization. There was direct evidence that EPCs that contributed to enhanced corneal neovascularization were specifically mobilized from the bone marrow in response to ischemia and GM-CSF in mice transplanted with bone marrow from transgenic donors expressing beta-galactosidase transcriptionally regulated by the endothelial cell-specific Tie-2 promoter. These findings indicate that circulating EPCs are mobilized endogenously in response to tissue ischemia or exogenously by cytokine therapy and thereby augment neovascularization of ischemic tissues.