Loss of basolateral invaginations in proximal tubules of cadmium-intoxicated rats is independent of microtubules and clathrin

Abstract

Experimental cadmium (Cd) nephrotoxicity after treating rats with CdCl 2 (2 mg Cd/kg b.m./day) for 2 weeks (subchronic nephrotoxicity) or with Cd-metallothionein (CdMT, a single dose of 0.4 mg Cd/kg b.m.) for a few hours (acute nephrotoxicity) is characterized by significant damage to cortical proximal tubules (PT) that results in reabsorptive and secretory defects. Most of the damage, studied so far, has been reported at the PT cell apical domain. This includes the loss of apical transporters and brush-border microvilli, and is considered to be the main cause of the kidney malfunction seen in this condition. However, in some studies the loss of basolateral (BL) invaginations and the activity of Na/K-ATPase in PT cells was also observed, but this “basolateral” aspect of intracellular Cd toxicity has been poorly investigated. In this report we induced subchronic and acute Cd nephrotoxicity in rats, and we studied the expression and intracellular distribution of microtubules and clathrin, and the abundance of Na/K-ATPase associated with BL invaginations in renal cortical tubules. Methods used were immunofluorescence microscopy, transmission and immunogold microscopy and immunoblotting of tissue homogenates and isolated total cell membranes. In both experimental models, in the cortical PT we demonstrated: (a) significantly damaged morphology of the cells, (b) fragmentation and depolymerization of microtubules, (c) loss of clathrin in the subapical domain and its relocation into vesicles scattered throughout the cytoplasm and (d) loss of BL invaginations and the associated Na/K-ATPase immunostaining. A similar loss of microtubules and redistribution of clathrin in the cortical PT was observed in rats treated with microtubule depolymerizing agent colchicine, but without any detectable loss of BL invaginations. We conclude that the loss of BL invaginations and the associated Na/K-ATPase in the cortical PT of Cd-intoxicated rats may contribute to the loss of PT function that characterizes Cd nephrotoxicity. This loss is accompanied by, but it is not dependent on perturbation of microtubule organization and loss of membrane-associated clathrin.