Tubular FSTL1 Aggravates Calcium Oxalate Kidney Stone Injury by Enhancing YAP1/CCN2 Signaling

Follistatin-like 1 (FSTL1) has been implicated in various biological and pathological processes, including lung development and fibrogenesis. However, its role in the progress of calcium oxalate (CaOx) stone nephropathy remains to be elucidated. Here, we found the increase of tubular FSTL1 in a mouse model of nephrolithiasis, and deficiency of fstl1 alleviated the kidney stone injury. RNA sequencing identified the significant differential gene expression associated with osteoclast differentiation and YAP1/CCN2 signaling pathway. We stimulated primary renal tubular cells with calcium oxalate monohydrate (COM) and collected conditioned media (CM) to treat the fibroblasts. Data showed that crystal stimulation promoted the expression of FSTL1 and activation of the YAP/CCN2 signaling pathway. CM from crystal-stimulated tubular cells induced osteogenic-like differentiation of fibroblasts. Knockdown and overexpression experiments revealed that CCN2 secretion mediates the pro-osteogenic effects of crystal-stimulated tubular cells. Crystal stimulation increased the nuclear translocation and transcription activity of YAP1, which could be mitigated by FSTL1 downregulation. Persistent YAP1 activation abolished the effect of FSTL1knockdow on the osteogenic differentiation of fibroblasts. Mechanically, immunoprecipitation assays showed that crystal stimulation promoted the interaction of FSTL1 and YAP1, thereby suppressing YAP1 phosphorylation and increasing its nuclear translocation and transcript activation for CCN2. Consistently, fstl1 deficiency abrogated the increase in osteogenic differentiation and activation of the YAP1/CCN2 axis in the mouse nephrolithiasis model. In summary, our study reveals that the activation of YAP1/CCN2 signaling contributes to the crystal induced osteogenesis-like differentiation of fibroblasts. We also describe a secretion-independent mechanism underlying the effects of FSTL1 on the progression of CaOx stone nephropathy.