[Role of thrombospondin-1 in the development of kidney diseases]

Thrombospondin-1 (TSP-1) is a 450-kDa matricellar glycoprotein. By its various domains, it can interact with various partners and exhibit anti-angiogenic, pro-apoptotic and immunomodulatory activities. TSP-1 is also a major endogenous activator of the pro-fibrotic growth factor TGF-β. In healthy adult renal parenchyma, TSP-1 expression is very scarce and limited to Bowman's capsule and interstitium. During nephropathies, many cell types can express or secrete TSP-1 (mesangial, endothelial, smooth muscle, tubular cells, podocytes and fibroblasts) depending on the nature of injury and the evolutive stage of the disease. Inhibition of the different domains of TSP-1 using specific antibodies or peptides, blockade of TSP-1 expression by antisense oligonucleotides and use of knock-out mice, allowed to identify the role of TSP-1 in various models of experimental nephropathy. All these studies demonstrated a deleterious effect of TSP-1 on renal repair by inducing TGF-β and fibrosis, decreasing VEGF and capillary density, and enhancing inflammatory cells recruitment. Thus, TSP-1 represents a potential therapeutic target for the management of chronic kidney diseases.

The role of cell plasticity in progression and reversal of renal fibrosis

The need for novel insights into the mechanisms of progression of renal disease has become urgent during the last several years because of the increasing incidence of chronic renal disease worldwide. Independent of the underlying disease, the subsequent progression of renal fibrosis is characterized mainly by both an exaggerated synthesis and abnormal accumulation of extracellular matrix proteins produced by mesenchymal cells within the kidney. These cells are mainly myofibroblasts deriving from a variety of renal cells such as vascular smooth muscle, mesangial, resident stem, tubular epithelial, vascular endothelial cells or pericytes. The appearance of myofibroblasts is a reversible process, as suggested by studies in experimental models showing regression of renal fibrosis during therapy with antagonists and/or blockers of the renin-angiotensin system. An additional factor that can also affect the mechanisms of progression/regression of fibrosis is the plasticity of podocytes controlling glomerular filtration.

Tissue transglutaminase contributes to interstitial renal fibrosis by favoring accumulation of fibrillar collagen through TGF-beta activation and cell infiltration

Renal fibrosis is defined by the exaggerated accumulation of extracellular matrix proteins. Tissue transglutaminase (TG2) modifies the stability of extracellular matrix proteins and renders the extracellular matrix resistant to degradation. In addition, TG2 also activates transforming growth factor-beta (TGF-beta). We investigated the involvement of TG2 in the development of renal fibrosis using mice with a knockout of the TG2 gene (KO). These mice were studied at baseline and 12 days after unilateral ureteral obstruction, which induced a significant increase in interstitial TG2 expression in wild-type mice (P < 0.001). Interstitial fibrosis was evident in both groups, but total and fibrillar collagen was considerably lower in KO mice as compared with wild-type (P < 0.001). Similarly, mRNA and protein expression of collagen I were significantly lower in KO animals (P < 0.05). A statistically significant reduction in renal inflammation and fewer myofibroblasts were observed in KO mice (P < 0.01). Free active TGF-beta was decreased in KO mice (P < 0.05), although total (active + latent) TFG-beta concentration did not differ between groups. These results show that mice deficient in TG2 are protected against the development of fibrotic lesions in obstructive nephropathy. This protection results from reduced macrophage and myofibroblast infiltration, as well as from a decreased rate of collagen I synthesis because of decreased TGF-beta activation. Our results suggest that inhibition of TG2 may provide a new and important therapeutic target against the progression of renal fibrosis.