Deletion of the Semaphorin, Sema4D, but Not Inhibition of Sema4D Shedding by ADAM17, Impairs Platelet Function and Reduces Infarct Size After Myocardial Ischemia.

Abstract 771

Introduction. Platelets are activated early during the reperfusion of ischemic myocardium, potentially exacerbating the extent of ischemia/reperfusion (I/R) injury. We have recently shown (Zhu, et al., PNAS 2007) that platelets express the semaphorin family member, sema4D, as do T-cells. Sema4D is a cell surface protein whose receptors are expressed by B-cells, monocytes and endothelial cells as well as platelets. Loss of sema4D expression in mice causes a defect in signaling downstream of the platelet collagen receptor, glycoprotein (GP) VI, inhibiting platelet function in vitro and in vivo, and reducing the extent of platelet hyperresponsiveness and atherothrombosis in the setting of dyslipidemia (Zhu, et al. ATVB 2009). Because of the role played by platelets, leukocytes and endothelial cells in reperfusion injury, here we asked whether loss of sema4D expression can also protect the heart, reducing the extent of damage following temporary ischemia. Methods. The left anterior descending coronary artery of anaesthetized mice was ligated for 45 min. After reperfusion for 48 hours, the mice were re-anesthetized and perfused with 2,3,5-triphenyltetrazolium chloride to measure the area of infarction. Fluorescent microspheres were used to delineate the area at risk. Comparisons were made between sema4D(−/−) and wild type mice produced by breeding sema4D(+/−) heterozygotes. Results. Although there was no difference between the sema4D(−/−) and WT mice with respect to either heart size or area at risk, we observed a substantial (57%) decrease in infarct size in the sema4D(−/−) mice expressed as a fraction of the area at risk (N=7-9, p<0.005). Since sema4D is shed from the surface of activated platelets and T-cells by the metalloprotease, ADAM17, producing a large bioactive fragment, we next asked whether the protection against ischemia/reperfusion injury conferred by the sema4D knockout is due to the loss of cell-associated or soluble sema4D. Chimeric mice were produced in which hematopoiesis was reconstituted in irradiated sema4D(+/+) mice using fetal liver cells from mouse embryos that lack functional ADAM17. This produces mice in which sema4D is expressed as usual in the hematopoietic lineages, but unable to be shed. Chimerism, inhibition of sema4D shedding and recovery of normal cell counts were confirmed after transplantation. The ischemia/reperfusion studies were repeated comparing chimeras reconstituted with ADAM17-deficient and ADAM17-replete fetal liver cells. In contrast to the sema4D knockout, the extent of infarction was the same whether or not ADAM17 was functional and sema4D was shed. Conclusions. Although the role of sema4D and its receptors have been studied most extensively in the context of T-cell interactions with B-cells, our previous studies have made a case for the involvement of sema4D in platelet:platelet and platelet:endothelial cell interactions. We now show for the first time that 1) loss of sema4D expression in mice confers protection against ischemia/reperfusion injury in the myocardium, and 2) preventing the formation of soluble, bioactive sema4D is insufficient to recapitulate this effect. Since sema4D and its receptors are expressed on more than just platelets, it cannot be concluded that the observed protection in the knockout is solely due to the absence of platelet sema4D. However, experience with other knockouts that reduce platelet function suggests that the defects that we have observed in sema4D(−/−) platelet function are likely to contribute. Regardless of whether expression on platelets is entirely or only partly responsible for the observed phenotype, sema4D is an interesting target for therapeutic intervention.