Our aim was to gain insight into mechanisms

Our aim was to gain insight into mechanisms that underlie the contribution of thrombospondin-1 (TSP1) to progression of CKD. TSP1 is the most-studied member of the thrombospondin family of matricellular proteins. Previous studies have demonstrated that pharmacological suppression or genetic depletion of TSP1 attenuates disease progression in animal models of CKD., , , , TSP1 is a 450-kDa trimeric ECM protein, which does not fulfill primarily structural roles in the matrix, but instead functions as an extracellular modulator of cell function., Most prominently, TSP1 is known to inhibit angiogenesis, inhibit inflammation, activate MMP-dependent ECM turnover, and facilitate zj to j migration and activation, all of which are considered important contributors to progression of CKD., To delineate through which of its known biological activities TSP1 impacts progression of CKD, we compared progression of kidney disease of knockout (KO) mice (deficient in type IV collagen α3 chain) with that of ; double-knockout (DKO) mutant mice. Here, we demonstrate that decrease of excretory renal function is delayed if TSP1 is absent. Furthermore, tissue analysis of plasma creatinine level–matched kidneys of KO and of DKO revealed that in KO mice disease progression is predominantly associated with fibrosis, whereas inflammation is the predominant interstitial pathology in DKO mice. We provide evidence that this altered disease progression is due to impaired activation of latent transforming growth factor-β1 (TGF-β1) in the absence of TSP1. Our findings provide evidence that both fibroproliferative injury and inflammation can independently cause expansion of the interstitium, leading to decline of excretory renal function. Materials and Methods
Results
Discussion We conducted the present study to gain mechanistic insight into the role of TSP1 in the progression of CKD. For this purpose, we compared kidney disease progression in Col4a3 KO mice with that of Col4a3;Tsp1 DKO mice. Decline of excretory renal function was delayed by the absence of TSP1, confirming previous reports.9, 10, 11, 12, 13 Unexpectedly, comparison of creatinine-matched kidneys of Col4a3 KO and Col4a3;Tsp1 DKO mice revealed that disease progression was not simply delayed, but that kidney disease followed different paths. Despite similar interstitial volumes at matched creatinine levels, absence of TSP1 in Col4a3;Tsp1 DKO mice was associated with lesser collagen deposition, lesser fibroblast accumulation, and lesser tubular atrophy, compared with Col4a3 KO mice, but increased Th1-mediated inflammation. The extent of glomerulosclerosis and the density of microvessels were not altered by the absence of TSP1. Our findings suggest that the altered course of disease progression in Col4a3;Tsp1 DKO mice is caused by an inability of kidney cells to activate latent TGF-β1 in the absence of TSP1. Our findings are consistent with the literature on the role of TSP1 in TGF-β1 activation. TGF-β1 is known to induce fibroblast proliferation and collagen deposition, and to contribute to formation of tubular atrophy, all of which are reduced in the absence of TGF-β1–activating TSP1.1, 37 TGF-β1–deficient mice exhibit severe inflammatory lesions, and TGF-β1 in general is known to be anti-inflammatory.38, 39 In the present study, inability to activate latent TGF-β1 was associated with an increased inflammatory response in chronic progressive kidney disease. With regard to progression of CKD, it is an unexpected finding that decline of kidney function is caused by different pathways, depending on the presence or absence of TSP1. Our findings suggest that fibroproliferative and inflammatory responses are distinct entities, both of which (and possibly independent of each another) can determine progression of CKD. Our present findings confirm earlier findings indicating that relative interstitial volume is the most accurate correlate of chronic progression of kidney disease, but challenge the concept of one common uniform pathway leading to ESRF.