Combined cardiac overexpression of protein phosphatases PP5 and PP2C in young and old mice

We have previously reported that cardiac overexpression of protein phosphatases 2C or 5 (PP2C, PP5) alone leads to modest cardiac hypertrophy after six months of age in transgenic mice. Combined overexpression of both, PP2C and PP5, leads to hypertrophy, fibrosis and an impaired systolic function in six months old (young) mice. Here, we hypothesized that dual overexpression of PP5 and PP2C has an age dependent influence on heart failure. Cardiac myocytes-specific overexpression was driven by the cardiac α-myosin heavy chain promoter. By crossbreeding of mono-transgenic PP5 and PP2C mice, we obtained four genotypes namely wild type mice (WT), PP2C-TG, PP5-TG and DT (=PP2C X PP5). Now we followed the animals for twelve months (old) and compared the cardiac function of young and old mice. The relative heart weight in six months old mice was increased by 23 % in DT compared to WT (11.1 ± 0.68 vs. 8.23 ± 0.35, p < 0.05, n = 11-12) and by 30 % in twelve months old mice (11.8 ± 0.82 vs. 8.12 ± 0.38, p < 0.05, n = 6-8). This was accompanied by an increase in the expression of the mRNA for the atrial natriuretic peptide in DT (normalized expression: young: 1.00 ± 0.33 WT vs. 5.14 ± 0.75 DT; old: 1.00 ± 0.18 WT vs. 4.41 ± 1.70 DT; p < 0.05, n = 5-10) and by histologically measurable fibrosis as well as an increase of alpha-1 type I collagen (Col1a1). Additionally, systolic cardiac function was impaired as demonstrated by a decreased ejection fraction in young and old mice (young: 41.7 % ± 4.98 % DT vs. 65.2 % ± 3.42 % WT; old: 37.4 % ± 6.19 % DT vs. 64.8 % ± 4.3 % WT, p < 0.05, n = 5-10). The increase in ejection fraction after intraperitoneal injection of the β-adrenoceptor agonist isoprenaline (1 mM / 100 µl) was less in DT compared to WT in young and old mice (young: 64.5 % ± 4.24 % DT vs. 90.4 % ± 4.54 % WT; old: 67.7 % ± 7.57 % DT vs. 94.1 % ± 2.75 % WT, p < 0.05, n = 5-10). In Langendorff preparations from young DT, force of contraction after ischemia and reperfusion was decreased by 28 % compared to WT (86.7 % ± 6.36 % vs. 111 % ± 9.74 %, p < 0.05, n = 5-9). These effects were not detected in old DT (90.6 % ± 13.1 % DT vs. 98.3 % ± 9.93 % WT, n = 6-9). In summary, overexpression of both, PP2C and PP5, influences heart health and lead to hypertrophy with an impaired systolic cardiac function and fibrosis. We conclude that there is a deleterious interaction between PP2C and PP5 that is already evident in young animals, but there is no evidence for a further age-dependent deterioration. We speculate that a similar mechanism might be operative in some patients with perhaps genetically caused heart failure, and drugs that target protein phosphatases like PP5 and PP2C might be useful for some specific heart failure patients.