KEY POINTS: I-mfa is a multifunctional cytosolic protein and its function in kidney is unknown. The major finding in the present study was that I-mfa promoted glomerular filtration rate in both male and female mice. I-mfa suppressed contractile function of both human and mouse glomerular mesangial cells by decreasing TRPC1 channel protein abundance. BACKGROUND: Inhibitor of MyoD family A (I-mfa) is a cytosolic protein. Its function in the kidney is unknown. The aim of this study was to examine the regulatory role of I-mfa on GFR. METHODS: GFR was measured by transdermal measurement of fluorescein isothiocyanate–sinitrin clearance in conscious wild-type (WT) and I-mfa knockout (KO) mice. Cell contractility was assessed in a single human or mouse mesangial cell. Single-cell RNA sequence, Western blot, and Ca(2+) imaging were used to evaluate the effects of I-mfa on transient receptor potential canonical (TRPCs) at messenger, protein, and functional levels in mesangial cells. RESULTS: In KO mice, GFR was significantly lower than that in WT mice. In WT mice, knocking down I-mfa selectively in mesangial cells using targeted nanoparticle/small interfering RNA delivery system significantly decreased GFR. In human mesangial cells, overexpression of I-mfa significantly blunted the angiotensin II (Ang II)-stimulated contraction, and knockdown of I-mfa significantly enhanced the contractile response. Consistently, the Ang II–induced contraction was significantly augmented in primary mesangial cells isolated from KO mice. The exaggerated response was restored by reintroducing I-mfa. Furthermore, single-cell RNA sequence showed an increase in trpc1 messenger, and Western blot showed an increase in TRPC1 protein abundance in I-mfa KO mouse mesangial cells. TRPC1 protein abundance was decreased in human embryonic kidney cells overexpressing I-mfa. Ca(2+) imaging experiments showed that downregulation of I-mfa significantly enhanced Ang II–stimulated Ca(2+) entry in human mesangial cells. Finally, TRPC1 inhibitor Pico145 significantly blunted Ang II–induced mesangial cell contraction. CONCLUSIONS: I-mfa positively regulated GFR by decreasing mesangial cell contractile function through inhibition of TRPC1-mediated Ca(2+) signaling.