Survival of Activated Myofibroblasts in Canine Myxomatous Mitral Valve Disease and the Role of Apoptosis

Myxomatous mitral valve disease (MMVD) is the single most important acquired cardiovascular disease of the dog. Much is known about the cellular changes and the contribution of activated myofibroblasts (valve interstitial cells (aVICs) to the valve extra-cellular matrix remodelling characteristic of the disease. However, little is known on how aVIC survival might contribute to disease pathogenesis. This study examined the temporal (disease severity-dependent) and spatial distribution of aVICs in MMVD valves, the expression of a range of apoptosis-related genes in cultured VICs from both normal (quiescent VIC (qVIC) and diseased (aVIC) valves, and the differential effects of doxorubicin treatment, as a trigger of apoptosis, on expression of the same genes. Activated myofibroblasts were identified in normal valves at the valve base only (the area closest to the annulus), and then became more numerous and apparent along the valve length as the disease progressed, with evidence of cell survival at the valve base. There were no significant differences in basal gene expression comparing qVICs and aVICs for CASP3, FAS, BID, BAX, BCL2, CASP8, DDIAS, XIAP and BIRC5. After doxorubicin treatment (2mM) for 8hrs there was significant difference (P<0.05) in the expression of BID, BCL2, DDIAS, and CASP8, but when assessed for interactions using a mixed model ANOVA only CASP8 was significantly different because of treatment (P<0.05). These data suggest aVIC survival in MMVD valves may be a consequence of heightened resistance of aVICs to apoptosis, but would require confirmation examining expression of the relevant proteins progression.