The process of fibrosis, described as accumulation of myofibroblasts and excessive deposition of extracellular matrix components, is a key development in the progression of multiple different types of cardiac disease. Nevertheless, little is known about the molecular mechanisms which cause the onset of fibrosis in cardiac disease. Fibrosis is a significant component of arrhythmogenic cardiomyopathy (AC), a genetic disorder characterized by replacement of healthy cardiomyocytes (CMs) with fibrous tissue, leading to arrhythmia and in certain cases, sudden death. AC is often characterized as a “disease of the desmosome”, as mutations for all obligate desmosome proteins have been found in cases of AC, including the desmosome armadillo proteins Plakophilin-2 (PKP2) and Plakoglobin (PG). PKP2 and PG are multi-functional proteins involved in both mechanical stabilization of the cardiac area composita, as well as mediation of desmosome-related signaling pathways. We have determined that loss of PKP2 or PG in neonatal CMs causes an aberrant increase in gene expression of pro-fibrotic stimuli such as transforming growth factor beta 1 (TGF-beta1) and Interleukin-6 (IL-6). In addition, p38 MAPK, a known mediator of inflammatory fibrosis, is activated upon loss of PKP2/PG. We hypothesize that mutation or loss of PKP2 or PG cause the recruitment and activation of cardiac fibroblasts via pro-fibrotic TGF-beta and p38MAPK signaling, resulting in pathological fibrosis characteristic of AC. Indeed, conditioned media from PKP2-silenced CMs causes an increase in fibronectin gene expression by freshly isolated cardiac fibroblasts. Our future experiments will investigate whether inhibition of TGF-beta or p38MAPK signaling can alleviate fibrotic gene production. By highlighting a novel link between desmosome armadillo proteins and pro-fibrotic signaling in cardiac tissue, this study provides mechanistic insights into the pathogenesis of AC, as well as advances our knowledge of potential therapeutic targets for combating fibrosis in multiple different types of heart disease or injury.
Intracellular signaling pathways involved in post-mitotic dopaminergic PC12 cell death induced by 6-hydroxydopamine
