Phenotypic plasticity of adult myocardium: molecular mechanisms.

Cardiac phenotypic plasticity (so-called cardiac remodelling, CR) is characterized by changes in myocardial structure that happen in response to either mechanical overload or a loss of substance such as that occurring after myocardial infarction. Mechanosensation is a widespread biological process and is inextricably mixed with other transduction systems from hormones and vasopeptides, which ultimately produce post-translational modifications of transcription factors. The expression of the four main transcription factors during cardiogenesis is also enhanced as a link to foetal reprogramming. CR results from re-expression of the foetal programme, which is mostly adaptive, but also from several other phenotypic modifications that are not usually adaptive, such as fibrosis. (i) The initial determinant is mechanical, and re-expression of the foetal programme includes a global increase in genetic expression with cardiac hypertrophy, re-expression of genes that are normally not expressed in the adult ventricles, repression of genes not expressed during the foetal life, and activation of pre-exisiting stem cells. Microarray technology has revealed a coordinated change in expression of genes pertaining to signal transduction, metabolic function, structure and motility, and cell organism defence. The physiological consequence is a better adapted muscle. (ii) During clinical conditions, the effects of mechanics are modified by several interfering determinants that modify CR, including senescence, obesity, diabetes, ischemia and the neurohormonal reaction. Each of these factors can alter myocardial gene expression and modify molecular remodelling of mechanical origin. Finally, as compared to evolutionary phenotypic plasticity described in plants and insects in response to variations in environmental conditions, in CR, the environmental factor is internal, plasticity is primarily adaptive, and it involves coordinated changes in over 1400 genes. Study of reaction norms showed that the genotypes from different animal species are similarly plastic, but there are transgenic models in which adaptation to mechanics is not caused by hypertrophy but by qualitative changes in gene expression.