Year of Publication

2012

Degree Name

Doctor of Philosophy (PhD)

Document Type

Doctoral Dissertation

College

Medicine

Department

Microbiology, Immunology, and Molecular Genetics

First Advisor

Dr. Donald A. Cohen

Abstract

Acute myocardial infarction (AMI) triggers the mobilization of stem/progenitor cells from bone marrow (BMSPCs) into peripheral blood (PB). The underlying mechanisms orchestrating this mobilization and subsequent homing of BMSPCs to the myocardium are poorly understood. While the role of traditional chemokines in the mobilization and homing of hematopoietic stem cell (HSCs) to BM niches is undisputed, their role in directing BMSPCs to the highly proteolytic environment of the ischemic myocardium is debatable and other redundant mechanism may exist. Based on our observation that bioactive lipids, such as sphingosine-1 phosphate (S1P) and ceramide-1 phosphate (C1P), play an important role in regulating trafficking of HSCs; we explored if they also direct trafficking of BMSPCs in the setting of myocardial ischemia. While BMSPCs expressed S1P receptors regardless of the source, the expression of S1P receptor 1 (S1PR1) and receptor 3 (S1PR3), which are responsible for migration and chemotaxis, was elevated in BMSPCs in naïve BM cells and was reduced following mobilization. This expression correlated to differential response of BMSPCs to S1P in chemotaxis assays. By employing flow cytometry analyses, we observed an increase in circulating PB CD34+, CD133+ and CXCR4+ lineage negative (Lin-)/CD45- cells that are enriched in non-HSCs (P < 0.05 vs. controls). This corroborated our mass spectrometry studies showing a temporal increase in S1P and C1P plasma levels. At the same time, plasma obtained in the early phases following AMI strongly chemoattracted human BM-derived CD34+/Lin- and CXCR4+/Lin- cells in Transwell chemotaxis assays in an S1P dependent fashion. We examined other mechanisms that may contribute to the homing of BMSPCs to the infarcted myocardium due to the reduction of S1PRs upon mobilization. We observed that hypoxia induced higher expression of cathelicidins in cardiac tissues. Indeed, PB cells isolated from patients with AMI migrated more efficiently to low, yet physiological, gradient of SDF-1 in Transwell migration assays compared to SDF-1 alone. Together, these observations suggest that while elevated S1P plasma levels early in the course of AMI may trigger mobilization of non-HSCs into PB, cathelicidins appear to play an important role in their homing to ischemic and damaged myocardium.

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