T cell-specific STAT3 deficiency abrogates lupus nephritis.

Signal transducer and activator of transcription (STAT) 3 is a regulator of T-cell responses to external stimuli, such as pro-inflammatory cytokines and chemokines. We have previously shown that STAT3 is activated (phosphorylated) at high levels in systemic lupus erythematosus (SLE) T cells and mediates chemokine-induced migration and T:B cell interactions. Stattic, a small molecular STAT3 inhibitor, can partially ameliorate lupus nephritis in mice. To understand the role of STAT3 better in T-cell pathophysiology in lupus nephritis and its potential as a treatment target, we silenced its expression in T cells using a cd4-driven CRE-Flox model. We found that lupus-prone mice that do not express STAT3 in T cells did not develop lymphadenopathy, splenomegaly, or glomerulonephritis. Moreover, the production of anti-dsDNA antibodies was decreased in these mice compared to controls. To dissect the mechanism, we also used a nephrotoxic serum model of nephritis. In this model, T cell-specific silencing of STAT3 resulted in amelioration of nephrotoxic serum-induced kidney damage. Taken together, our results suggest that in mouse models of autoimmune nephritis, T cell-specific silencing of STAT3 can hamper their ability to help B cells to produce autoantibodies and induce cell tissue infiltration. We propose that STAT3 inhibition in T cells represents a novel approach in the treatment of SLE and lupus nephritis in particular.

Physiological role of CLC-K1 chloride channel in the kidney.

Although CLC chloride channels share a common structure, the sites of expression in the body and their intracellular localization are different among CLCs. CLC-K1 and -K2 are kidney-specific CLC chloride channels. We have clarified their localization in the plasma membranes of tubular cells by immunohistochemistry and proposed their roles in transepithelial chloride transport. Since there exit no good inhibitors for these channels, a gene knockout approach was the only way to establish their roles in kidney. While we were generating CLC-K knockout mice, Simon et al. reported that the mutations of CLC-K2 in human resulted in Bartter's syndrome. This had been anticipated since CLC-K2 is known to be present in the basolateral plasma membranes of the distal nephron where sodium-dependent chloride transporters are present in the apical membranes. Thus, CLC-K2 constitutes an important route for chloride reabsorption as an exit for chloride ions in the basolateral membrane. Another important finding in Simon's report was that no CLC-K (a human homologue of rat CLC-K1) mutation was found in patients with Bartter's syndrome. This suggested that CLC-K1 has a different role in kidney. We believed this to be true, based on the finding that the intrarenal localization of CLC-K1 and CLC-K2 are completely different. In the CLC-K1 knockout mice, we could clearly verify that (i) the high chloride permeability in the tAL was mediated by CLC-K1 and (ii) this chloride transport is necessary for urinary concentration. Further studies are necessary to elucidate the detailed mechanisms of the urinary-concentrating defect in Clcnk1 -/- mice. Exact clearance studies and measurements of osmolality and solute contents in the inner medulla will provide the answer to this question.

Responses of blood pressure and catecholamine metabolism to high salt loading in endothelin-1 knockout mice.

The molecular mechanism responsible for salt sensitivity is poorly understood. Mice heterozygous for the null mutation of the endothelin-1 (ET-1) gene, Edn1, may be a potential tool for studying this mechanism, because they have elevated blood pressure and disturbances in central sympathetic nerve regulation. In the present study, we used this mouse model to examine the degree to which ET-1 contributes to the responses of blood pressure and catecholamine metabolism to high salt loading. Male Edn1+/- heterozygous mice and Edn1+/+ wild-type littermates were given either a high salt (8%) or a normal salt (0.7%) diet for 4 wk. During the normal diet, renal ET-1 levels in Edn1+/- mice were approximately 50% lower than ET-1 levels in wild-type mice, whereas the high salt diet decreased renal ET-1 levels by about 50% in both Edn1+/- and wild-type mice. The high salt diet significantly increased urinary sodium excretion and fractional excretion of sodium (FENa) but did not affect circulating plasma volume, serum electrolytes, creatinine clearance, or systemic blood pressure. In addition, urinary norepinephrine and normetanephrine excretion were significantly increased, indicating that salt loading can increase sympathetic nerve activity in normal mice. These responses to salt loading did not differ between Edn1+/- mice and their wild-type littermates. We conclude that physiological changes in ET-1 production do not affect the responses of blood pressure and catecholamine metabolism to salt loading, although the renal ET-1 content is decreased by salt loading.