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The glomerular basement membrane (GBM) is a key component of the filtering unit in the kidney. Mutations involving any of the collagen IV genes (COL4A3, COL4A4, and COL4A5) affect GBM assembly and cause Alport syndrome, a progressive hereditary kidney disease with no definitive therapy. Previously, we have demonstrated that the bone morphogenetic protein (BMP) antagonist uterine sensitization-associated gene-1 (USAG-1) negatively regulates the renoprotective action of BMP-7 in a mouse model of tubular injury during acute renal failure. Here, we investigated the role of USAG-1 in renal function in Col4a3-/- mice, which model Alport syndrome. Ablation of Usag1 in Col4a3-/- mice led to substantial attenuation of disease progression, normalization of GBM ultrastructure, preservation of renal function, and extension of life span. Immunohistochemical analysis revealed that USAG-1 and BMP-7 colocalized in the macula densa in the distal tubules, lying in direct contact with glomerular mesangial cells. Furthermore, in cultured mesangial cells, BMP-7 attenuated and USAG-1 enhanced the expression of MMP-12, a protease that may contribute to GBM degradation. These data suggest that the pathogenetic role of USAG-1 in Col4a3-/- mice might involve crosstalk between kidney tubules and the glomerulus and that inhibition of USAG-1 may be a promising therapeutic approach for the treatment of Alport syndrome.

Dialysis dependency is one of the leading causes of morbidity and mortality in the world, and once end-stage renal disease develops, it cannot be reversed by currently available therapy. Although administration of large doses of bone morphogenetic protein-7 (BMP-7) has been shown to repair established renal injury and improve renal function, the pathophysiological role of endogenous BMP-7 and regulatory mechanism of its activities remain elusive. Here we show that the product of uterine sensitization-associated gene-1 (USAG1), a novel BMP antagonist abundantly expressed in the kidney, is the central negative regulator of BMP function in the kidney and that mice lacking USAG-1 (USAG1 mice) are resistant to renal injury. USAG1 mice exhibited prolonged survival and preserved renal function in acute and chronic renal injury models. Renal BMP signaling, assessed by phosphorylation of Smad proteins, was significantly enhanced in USAG1 mice with renal injury, indicating that the preservation of renal function is attributable to enhancement of endogenous BMP signaling. Furthermore, the administration of neutralizing antibody against BMP-7 abolished renoprotection in USAG1 mice, indicating that USAG-1 plays a critical role in the modulation of renoprotective action of BMP and that inhibition of USAG-1 is a promising means of development of novel treatment for renal diseases.