| Abstract: | Whereas aggregation of intracellular proteins was linked to the initiation of cardiac myopathy, the sequence of participating events, including myocyte apoptosis, autophagy, necrosis and fibrosis as the underlying mechanisms leading to heart failure, was not clear. Green fluorescent protein (GFP) and its derivatives induced cardiac dysfunction in mice when expressed in high quantity; however, the mechanism underlying the aggregation of fluorescent protein leading to heart failure remains unexplored.We created a transgenic mouse with switchable expression of the GFP monomer or the expression of DsRed, a red fluorescent protein (RFP) tetramer that tends to aggregate into a large protein complex. GFP mice were free of cardiac symptoms; in contrast, RFP mice with homozygous DsRed alleles developed myocyte necrosis, carditis, ventricular hypertrophy and fibrosis, left atrium thrombosis, dilated heart failure and death at the age of approximately five months. The hemizygote mice displayed similar symptoms at a later age. The expression of the microtubule-associated protein 1 light chain 3 cleaved isoform II (LC3 II) and transglutaminase 2, and the expression of many myopathy- and fibrosis-related genes were significantly induced in the hearts of two-month-old RFP mice. Together with the findings of increased autophagosomes, lysosomes and dysfunctional mitochondria, these results suggest a marked induction of myocyte autophagy and fibrosis as the main underlying mechanism of heart failure in RFP mice. Interestingly, apoptosis was not elevated in RFP hearts. One of the most up-regulated genes in the early stage RFP heart was the tissue inhibitor of matrix metalloproteinases type 1 (TIMP-1), corroborating the role of TIMP-1 in cardiac remodeling and anti-apoptotic activity. The heart-origin of the morbidity in RFP mice was confirmed by expressing DsRed tetramers specifically in cardiac tissues, and the same phenotypes as in RFP mice were observed. In summary, in cardiac myocytes under the stress of protein aggregation, strong induction of TIMP-1 and down-regulation of MMP activity may play a significant role in enhancing the synthesis of extracellular matrix, resulting in fibrosis and heart failure. |
