The origin of renal fibroblasts/myofibroblasts and the signals that trigger fibrosis

Renal fibrosis is a common characteristic of chronic kidney disease (CKD). Aberrant and excessive depositions of extracellular matrix (ECM) proteins in both glomeruli and interstitial regions are typical hallmarks of renal fibrosis and amplify the severity of kidney injury. To date, an approved therapy specifically targeted to renal fibrosis is needed to mitigate or even retard renal fibrosis. Recent findings have identified a unique population of myofibroblasts as a primary source of ECM in scar tissue formation. However, the origin of myofibroblasts in renal fibrosis remains the subject of controversial debates. The advancement in lineage tracing and immunofluorescent microscopy technologies have suggested that myofibroblasts may arise from a number of sources such as activated renal fibroblasts, pericytes, epithelial-to-mesenchymal transition (EMT), endothelial-to-mesenchymal transition (EndoMT), bone marrow derived cells and fibrocytes. Recent studies also indicate that multiple ligands of TGF-β/Smads are the direct mediators for renal fibrosis. Consistently, inhibition of the TGF-β/Smads signaling pathway using various strategies significantly reduce renal fibrotic lesions and ameliorate kidney injury, suggesting that targeting the TGF-β/Smads signaling pathway could be a new strategy for effective therapies. In this review, we will briefly discuss the diverse origins of myofibroblasts and molecular pathways triggering renal fibrosis. Prospective therapeutic approaches based on those molecular mechanisms will hopefully offer exciting insights in the development of new therapeutic interventions for patients in the near future.