Role of Cardiac Myocyte CXCR4 Expression in Development and Left Ventricular Remodeling After Acute Myocardial Infarction

Rationale: Stromal cell-derived factor (SDF)-1/CXCR4 axis has an instrumental role during cardiac development and has been shown to be a potential therapeutic target for optimizing ventricular remodeling after acute myocardial infarction (AMI) and in ischemic cardiomyopathy. Although a therapeutic target, the specific role of cardiac myocyte CXCR4 (CM-CXCR4) expression following cardiogenesis and survival of cardiac myocyte and left ventricular remodeling after AMI is unknown. Objective: We hypothesized that cardiac myocyte derived CXCR4 is critical for cardiac development, but it may have no role in adulthood secondary to the short transient expression of SDF-1 and the delayed expression of CM-CXCR4 following AMI. To address this issue, we developed congenital and conditional CM-CXCR4(-/-) mouse models. Methods and Results: Two strains of CM-CXCR4(flox/flox) mice were generated by crossing CXCR4(flox/flox) mice with MCM-Cre(+/-) mouse and MLC2v-Cre(+/-) mouse on the C57BL/6J background, yielding CXCR4(flox/flox) MCM-Cre(+/-) and CXCR4(flox/flox)MLC2v-Cre(+/-) mice. Studies demonstrated recombination in both models congenitally in the MLC2v-Cre(+/-) mice and following tamoxifen administration in the MCM-Cre(+/-) mice. Surprisingly the CXCR4(flox/flox)MLC2v-Cre(+/-) are viable, had normal cardiac function, and had no evidence of ventricular septal defect. CXCR4(flox/flox)MCM(+/-) treated with tamoxifen 2 weeks before AMI demonstrated 90% decrease in cardiac CXCR4 expression 48 hours after AMI. Twenty-one days post AMI, echocardiography revealed no statistically significant difference in the wall thickness, left ventricular dimensions or ejection fraction (40.9 +/- 7.5 versus 34.4 +/- 2.6%) in CXCR4(flox/flox) mice versus CM-CXCR4(-/-) mice regardless of strategy of Cre expression. No differences in vascular density (2369 +/- 131 versus 2471 +/- 126 vessels/mm(2); CXCR4(flox/flox) versus CM-CXCR4(-/-) mouse), infarct size, collagen content, or noninfarct zone cardiac myocyte size were observed 21 days after AMI. Conclusions: We conclude that cardiac myocyte-derived CXCR4 is not essential for cardiac development and, potentially because of the mismatch in timings of peaks of SDF-1 and CXCR4, has no major role in ventricular remodeling after AMI. (Circ Res. 2010;107:667-676.)