Background
Connective Tissue Growth Factor (CTGF/CCN2) is an important mediator of kidney fibrosis. Previous observations indicated that attenuation of CCN2 expression sufficed to alleviate early kidney damage. However, little is known about the role of CCN2 in fibrosis of severely damaged and more chronically injured kidneys. Therefore, we examined the effects of CCN2 haploinsufficiency on the progression of renal scarring in long-term STZ-induced diabetic nephropathy, in a more advanced stage of obstructive nephropathy following unilateral ureteric obstruction (UUO), and in severe aristolochic acid (AA)-induced tubulotoxic nephritis.
Methods
Wild-type (WT, CCN2+/+) and hemizygous CCN2+/? C57Bl/6 mice were studied. In the diabetes experiment, streptozotocin-injected and control mice were followed for 6 months, with regular blood pressure, glycaemia and albuminuria recordings. In the UUO experiment, the left ureter was obstructed for 14 days with the contralateral kidney serving as control. For the AA experiment, mice were followed for 25 days after 5 intraperitoneal injections with AA and compared to control mice injected with buffer alone. Organs were harvested for histology, mRNA and protein measurements. Collagen content was determined by HPLC and expressed as hydroxyproline/proline ratio.
Results
CCN2 expression was significantly increased in the damaged as compared to control kidneys. In all three models, CCN2 levels in the damaged kidneys of CCN2+/? mice averaged about 50 % of those in damaged WT kidneys. After 6 months of diabetes, albuminuria was increased 2.5-fold in WT mice, compared to 1.5-fold in CCN2+/? mice, mesangial matrix was expanded 5-fold in WT and 4.4-fold in CCN2+/? mice and the glomerular basement membrane was thickened 1.3-fold in WT and 1.5-fold in CCN2+/? mice (all differences between WT and CCN2+/? mice are NS). Tubular damage and interstitial fibrosis scores were also not different between Wt and CCN2+/? mice in the diabetes (1.8 vs. 1.7), UUO (2.8 vs. 2.6), and AA (1.4 vs. 1.2) models, as was the case for macrophage influx and collagen content in these three models.
Conclusion
Unlike in mild and relatively early STZ-induced diabetic nephropathy, scarring of severely and chronically damaged kidneys is not attenuated by a 50 % reduction of CCN2 to (near) normal levels. This suggests that CCN2 is either redundant in severe and chronic kidney disease, or that it is a limiting factor only at subnormal concentrations requiring further reduction by available or emerging therapies to prevent fibrosis of the severely injured kidney.
