Age-related changes in collagen synthesis and turnover in porcine heart valves

Although the six-month-old pig is commonly used as a model to study human heart valve biology and various age-specific valve diseases, the correlation of porcine valve biology and development with that of humans has not been thoroughly assessed. Given the important role of the matrix in valve function, the aim of this study was to characterize porcine valve matrix structure and collagen turnover during development and aging.

Porcine aortic valves (AV) and mitral valves (MV) were examined throughout fetal development and postnatally at six weeks, six months and six years, using Movat pentachrome staining and immunohistochemistry for collagen III, markers of collagen synthesis (molecular chaperone HSP47, hydroxylating enzyme prolyl-4-hydroxylase (P4H), cross-linking enzyme lysyl oxidase (LOX)) and collagen degradation (matrix metalloproteinase (MMP)-13), and a marker of an 'activated' cellular phenotype, smooth muscle alpha-actin (SMalphaA). An analysis of variance was used to compare the staining intensities.

Cell density measurements showed layer differentiation in the first trimester (p < 0.003), and this decreased ten-fold from the second trimester to six years of age (p < 0.025). Matrix turnover was identified by the co-localization of P4H, HSP47 and MMP13, and correlated to an 'activated' cellular phenotype. SMalphaA expression was noted on the inflow surface of both valves. P4H and LOX were maximally expressed around mature collagen (p < 0.001). P4H increased during fetal development (p < 0.01) and in the six-year-old AV fibrosa (p < 0.05). Collagen-related markers were consistently higher in the AV than MV (HSP47 in fetal; P4H, Col III, and LOX in six-year-old).

The substantial matrix changes shown in this porcine study provide further insight into the role of matrix turnover during development and aging and should be considered when using the porcine animal model to study age-specific human diseases.