Cardiomyocyte TRPC6 overexpression as one of the myocardial hypertrophy mechanisms in chronic kidney dysfunction

BACKGROUND. Klotho is a transmembrane and circulating protein primarily synthesized by the kidney. Klotho deficiency characterizes chronic kidney disease (CKD), as myocardial hypertrophy (GM). The cardioprotective effect of the Klotho protein is due to the negative regulation of a variety of stress signals, leading to the activation of the hypertrophic intracellular signaling pathway calcineurin (CaN) / NFAT in the myocardium. The effect of Klotho may presumably be mediated by the modulation of Ca2 + channels and / or Foxo factors essential for CaN signaling.
THE AIM: to study the activity of CaN/ NFAT signaling pathway in the myocardium and to determine the molecular mechanisms of its regulation in conditions of Klotho level decrease in spontaneous hypertensive rats (SHR) with experimental CKD.
MATERIAL AND METHODS. The experimental model of CKD was 3/4 or 5/6 nephrectomy (Nx) in SHR. Sham-operated (SO) SHR, and Wistar Kyoto rats (WKY) were used as controls. In all animals were measured systolic blood pressure, myocardial mass index – MMI, creatinine clearance, cardiomyocyte (CM) diameter, Klotho levels in serum (ELISA) and kidney (IHC), myocardial expression of calcineurin (IHC, PCR), transcription factor NFAT (IHC), TRPC6 (IHC), FOXO3A (PCR) and phosphor-Foxo1/3/4 (IHC). The tissue expressions of calcineurin, TRPC6, and Klotho were calculated as the IHC specific product area to the field of view ratio. NFAT expression was evaluated as the positively stained nuclei to the number of nuclei ratio in the field of view. Measurements were performed in 10 fields of view for each histology slide. RESULTS. The model has corresponded to the initial stages of CKD. The increase in MMI (p = 0.005) and CM diameter (p = 0.002) were observed compared in Nx rats to SO. Renal Klotho expression (p < 0.001), and serum Klotho level (p = 0.019) were lower in the Nx. In multiple linear regression analyzes, the values of MMI and CM thickness were independently associated with the level of renal Klotho protein (β = -0.38 ± 0.16, p = 0.026, β = -0.64 ± 0.14, p <0.001, respectively). Nx and systemic hypertension were accompanied by an increase in the expression of the calcineurin gene (p = 0.005) and cytoplasmic calcineurin in CM (p = 0.004), the number of NFAT-positive nuclei (p = 0.007), and an increase in the expression of the FOXO3A gene (p <0.001) in the absence of accumulation of phosphorylated Foxo1/3/4 in CM cytoplasm. SHR rats were characterized by positive IHC staining for TRPC6 compared to WKY (p = 0.004). The expression of calcineurin and TRPC6 varied co-directionally (r = 0.69, p <0.001), and both of these indicators were associated with the Klotho levels (calcineurin vs Klotho in the kidney, r = -0.73, p <0.001; TRPC6 vs Klotho in serum, r = -0.43, p = 0.025).
CONCLUSION. The development of Klotho deficiency on early-stage CKD is associated with the expression of transient Ca2+ channels TRPC6 and activation of the calcineurin / NFAT
hypertrophic signaling pathway in cardiomyocytes.

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