Interstitial cells from dogs with naturally occurring myxomatous mitral valve disease undergo phenotype transformation.

BACKGROUND AND AIM OF THE STUDY

Myxomatous mitral valve disease is a common naturally occurring heart disease of dogs that is pathologically similar to myxomatous mitral valve disease in humans. It was hypothesized that interstitial cell phenotype transformation recently described in human myxomatous valves might also occur in dogs with myxomatous mitral valves, and correlate with disease severity.

METHODS

Normal and early-, intermediate- and late-stage myxomatous canine mitral valves were examined histologically and immunohistochemically for cytoskeletal (vimentin, desmin, smooth muscle alpha-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.

RESULTS

Normal canine mitral valve interstitial cells were positive for vimentin, but negative for alpha-actin, desmin and non-muscle myosin (i.e., fibroblast phenotype). Interstitial cells from myxomatous valves showed progressive positive staining for alpha-actin and desmin, but were negative for smooth muscle myosin (i.e., myofibroblast phenotype). Positive-staining cells first appeared as cellular 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. The expression of MMP-1 and MMP-13 increased in myxomatous mitral valves and correlated with disease severity. Interstitial cellularity increased dramatically in degenerative mitral valves, though Ki-67 staining was only mildly increased.

CONCLUSION

Two patterns of interstitial cell phenotype transformation were identified in dogs with myxomatous mitral valve disease, and both correlated with disease severity. Myofibroblast transformation characterized by positive staining for alpha-actin and desmin occurred in cellular clusters primarily in the lamina atrialis. Mesenchymal cell activation characterized by positive staining to non-muscle myosin occurred throughout the valve. The dog may be a natural model for studying the cell biology of progressive myxomatous valve disease.