Myxomatous mitral valve disease is a common heart disease of dogs that is similar to myxomatous mitral valve disease in humans. The first hypothesis of this dissertation is that interstitial cell phenotype transformation described in human myxomatous valves also occurs in dogs with myxomatous mitral valves and correlates with disease severity. Normal and early-, intermediate-, and late-stage myxomatous canine mitral valves were examined by immunohistochemistry for cytoskeletal (vimentin, desmin, smooth muscle α-actin, smooth muscle myosin, and non-muscle myosin), collagenolytic (MMP-1, MMP-13), cell surface (CD-31, CD-45, CD-68) and proliferation (Ki-67) proteins. Normal canine mitral valve interstitial cells were positive for vimentin, but negative for α-actin, desmin, and non-muscle myosin (i.e. fibroblast phenotype). Interstitial cells from myxomatous valves showed increased positive staining for α-actin and desmin, but were negative for smooth muscle myosin (i.e. myofibroblast phenotype). Positive cells first appeared as clusters in the subendocardial region of the lamina atrialis and extended into deeper layers with increasing severity. Interstitial cells from myxomatous valves showed positive staining for non-muscle myosin (i.e. activated mesenchymal cell phenotype). Positive staining cells increased with disease severity and were dispersed throughout the valve layers. Expression of MMP-1 and MMP-13 correlated with disease severity. Interstitial cellularity increased in degenerative valves however Ki-67 staining was mildly increased. In conclusion, two patterns of interstitial cell phenotype transformation were identified in dogs with myxomatous mitral valve disease and both correlated with disease severity.

Myxomatous mitral valve disease in dogs shares similar pathological lesions to serotonin induced heart valve disease in humans. The second hypothesis of this dissertation is that serotonin and transforming growth factor β1 (TGFβ1) signaling play a role in canine myxomatous mitral valve disease. Septal mitral valve leaflets from dogs affected by late-stage myxomatous disease and normal dogs were collected. Immunohistochemistry and Western blotting were performed to determine the expression of target proteins involved in serotonin and TGFβ1 signaling in canine normal and myxomatous mitral valves. Immuohistochemistry and Western blotting demonstrated the up-regulation of 5HT_2B receptor, phosphorylated ERK1/2, and TGFβ1 receptor I and II, latent TGFβ1 and tryptophan hydroxylase 1 (TPH1) in myxomatous valves whereas the expression of serotonin transporter (SERT) was down-regulated in myxomatous valves. In conclusion, serotonin and TGFβ1 signaling are associated with canine myxomatous mitral valve disease.