Inhibitor of MyoD family isoform a (I-mfa) regulates contractile function of glomerular mesangial cells and glomerular filtration rate

Purpose: Glomerular filtration is a critical process for maintaining homeostasis of body fluid volume. This process in kidney is regulated by multiple factors inside glomeruli, including the surface area of the glomerular filtration membrane available for filtration. Glomerular mesangial cells (MCs) sit among the networks of glomerular capillaries and regulate glomerular filtration rate (GFR) by changing the surface area of filtration membrane through their contractile function. Inhibitor of MyoD family isoform a (I-mfa) was initially found as a transcription modulator. We recently found that I-mfa was present in MCs. However, the function of this protein in MCs is not known. The aim of this study was to examine if I-mfa regulated contractile response of MCs and GFR. Methods: Experiments were carried out in cultured human MCs and mice. In cultured cells, we examined the contractile function of MCs with and without overexpressing or knocking down I-mfa. I-mfa was overexpressed by transfecting MCs with I-mfa expression plasmids and the no functional I-mfb expression plasmids were used as the control. I-mfa was knocked down using siRNA approach and scramble siRNA was used as the control. MC contraction was evaluated by reduction of planar surface area of the cells in response to Ang II (1 µM) at different time points (10 - 60 min) after treatment. In animal study, we assessed glomerular filtration function in I-mfa+/+ (control) and I-mfa-/- mice (male at age of 12 weeks) using transdermal GFR measurement. Using this approach, we measured GFR by transcutaneous measurement of the elimination kinetics of the fluorescent renal marker FITC-sinistrin under conscious and freely moving conditions. Results: In cultured human MCs, Ang II (1 µM, 60 min) induced ~45% decrease in the planar surface area. This contractile response was significantly enhanced in MCs with I-mfa knocked down, but not in MCs treated with scramble siRNA. Furthermore, the Ang II-stimulated contraction was significantly blunted in MCs with overexpression of I-mfa, but not with overexpression of I-mfb. Consistent with the in vitro experiments, deletion of I-mfa (I-mfa-/-) significantly decreased GFR compared to I-mfa+/+ mice (control) (1351.4 ±51.9 vs. 890 ±166.9, P < 0.05, n= 7 vs. 4, I-mfa+/+ vs. I-mfa-/-). Conclusion: This study suggests that I-mfa inhibits the contractile response of MCs and promotes glomerular filtration function.