Methods: SDF-1 expression was analyzed utilizing SDF-1-EGFP reporter mice. SM-SDF-1 KO mice were generated using Cre/LoxP technology (SM22a-Cre; SDF-1fl/fl). Morphology was analysed with immunohistochemistry and immunofluorescence. Cardiac function was assessed utilizing echocardiography and millar tip catheterization. Whole transcriptome analysis, qRT-PCR and western blotting were performed. Further, apoptotic index and cell proliferation were quantified by TUNEL assay and PH3 immunostaining, respectively.
Results: SDF-1-EGFP lineage tracking and immunofluorescence analysis revealed high expression of SDF-1 particularly in smooth muscle cells and less frequently in perivascular and endothelial cells. Conditional SM-SDF-1 KO mice showed a high pre- and perinatal mortality (50%). Immunohistochemistry in surviving adult SM-SDF-1 KO mice revealed a severe cardiac hypertrophy phenotype, associated with increased cardiac fibrosis and apoptotic cell death. SM-SDF-1 KO mice revealed very thin and dilated arteries. Echocardiography measurements confimed concentric hypertrophy, and decreased stroke volume reflecting restrictive hypertrophic cardiomyopathy. Immunohistochemistry confirmed pronounced hypertrophy of cardiomyocytes. Additionally, we found evidence for enhanced proliferation markers in cardiomyocytes of SM-SDF-1 KO mice. Transcriptome analyses from KO hearts vs. non-ablated littermates identified over 150 significantly up- and downregulated genes. Western blot analysis for HIF-1a, AKT and ERK cell-signalling pathways were significantly elevated, whereas Rho Kinase signalling was specifically downregulated in SM-SDF-1 KO mice. As a possible reason for the hypertrophic phenotype, SDF-1 mutants exhibited aortic stenosis due to aortic valve thickening associated with upregulation of the extracellular proteoglycan versican anddownregulation of the SDF-1 co-receptor CXCR7. We further noticed increased plasma levels of SDF-1 in aortic stenosis patients suggesting a cardioprotective role.
Conclusion: Our data suggest that smooth muscle cell specific expression of SDF-1 plays a prominent role in cardiovascular development leadingto cardiac hypertrophy in adult animals. Our data further suggest that SDF-1 is involved in maintaining the homeostasis of the aortic valve, possibly by regulating versican.