Dysregulation of ENaC in Animal Models of Nephrotic Syndrome and Liver Cirrhosis

Abstract

Nephrotic syndrome and liver cirrhosis are common clinical manifestations, and are associated with avid sodium retention leading to the development of edema and ascites. However, the mechanism for the sodium retention is still incompletely understood and the molecular basis remains undefined. We examined the changes of sodium (co)transporters and epithelial sodium channels (ENaCs) in the kidneys of experimental nephrotic syndrome and liver cirrhosis. The results demonstrated that puromycin- or HgCl2-induced nephrotic syndrome was associated with 1) sodium retention, decreased urinary sodium excretion, development of ascites, and increased plasma aldosterone level; 2) increased apical targeting of ENaC subunits in connecting tubule and collecting duct segments; and 3) decreased protein abundance of type 2 11β-hydroxysteroid dehydrogenase (11βHSD2). Experimental liver cirrhosis was induced in rats by CCl4 treatment or common bile duct ligation. An increased apical targeting of alpha-, beta-, and gamma-ENaC subunits in connecting tubule, and cortical and medullary collecting duct segments in sodium retaining phase of liver cirhosis but not in escape phase of sodium retention. Immunolabeling intensity of 11βHSD2 in the connecting tubule and cortical collecting duct was significantly reduced in sodium retaining phase of liver cirrhosis, and this was confirmed by immunoblotting. These observations therefore strongly support the view that the renal sodium retention associated with nephrotic syndrome and liver cirrhosis is caused by increased sodium reabsorption in the aldosterone sensitive distal nephron including the connecting tubule and collecting duct, and increased apical targeting of ENaC subunits plays a role in the development of sodium retention in nephrotic syndrome and liver cirrhosis.

Keywords: Aldosterone sensitive distal nephron; Edema; Epithelial sodium channel; Sodium retention.

Fig. 1

Semi-quantitative immunoblots of kidney protein prepared from cortex/outer stripe of outer medulla (OSOM) and inner stripe of outer medulla (ISOM) from control and HgCl₂-nephropathic rats (membranous glomerulonephritis, MGN). The protein abundance of αENaC was increased in the cortex/OSOM and ISOM. The protein abundances of βENaC and γENaC were decreased in the cortex/OSOM, while increased in the ISOM. ^*p<0.05 vs. control. (Kim et al., Am J Physiol Renal Physiol 2006).

Fig. 2

Semiquantitative immunoblotting and immunoperoxidase microscopy of 11βHSD2 from control and HgCl₂-nephropathic rats. A) Immunoblot was reacted with anti-type 2 isoform of 11β-hydroxysteroid dehydrogenase (11βHSD2) antibodies and revealed a single 44 kDa bands. B) densitometric analyses revealed that 11βHSD2 expression in the cortex/OSOM was significantly reduced in HgCl₂-nephropathic rats compared with controls. ^*p< 0.05 vs. control (Kim et al., Am J Physiol Renal Physiol 2006).

Fig. 3

Urinary sodium excretion (A) and Urinary Na/K ratio (B) from control (Control) and CCl₄-treated cirrhotic rats (Cirrhosis) in protocol 1. A) Urinary sodium excretion was similar in control and CCl₄-induced cirrhosis rats. Four (△, Group A) of the CCl4-induced cirrhosis rats showed markedly decreased urinary sodium excretion, while the other six rats (□, Group B) remained similar compared with controls. B) The urinary Na/K ratio was decreased in cirrhotic rats, indicating increased aldosterone effectiveness in the distal nephron. ^*p<0.05 vs. control (Kim et al., J Am Soc Nephrol 2005).

Fig. 4

Semi-quantitative immunoblots of kidney protein prepared from cortex/OSOM of control and CCl₄-treated cirrhosis rats subdivided into group A or group B liver cirrhosis in protocol 1. In group A, protein abundance of αENaC was unchanged, while βENaC abundance was decreased compared with controls. The γENaC underwent a complex change associated with the increased abundance of 70 kDa band with a concomitant decrease in the main 85 kDa band. In contrast, there were no significant changes of ENaC subunit expression in group 2. ^*p<0.05 vs. control, ^#p<0.05 vs. group A (Kim et al., J Am Soc Nephrol 2005).

Fig. 5

Immunoperoxidase microscopy of βENaC in the connecting tubule (CNT) and cortical collecting duct (CCD) from control and 6 wk CBDL-induced liver cirrhosis rats. Immunoperoxidase labeling of βENaC is mainly associated with the entire cytoplasm of principal cells of the CNT (A) and CCD (C) in control rats. In contrast, βENaC labeling was markedly redistributed to the apical plasma membrane domains in CNT (B) and CCD (D) in 6wk cirrhotic rats. Arrows indicate apical labeling. Magnification : ×630. (Kim et al., Kidney Int 2006)

Fig. 6

Semiquantitative immunoblotting of kidney protein prepared from cortex/OSOM from control and 8 wk CBDL-induced liver cirrhosis rats. The protein abundance of all ENaC subunits (αENaC, βENaC and γ ENaC) was markedly decreased in cortex/OSOM in 8 wk liver cirrhosis rats compared with controls. ^*p<0.05 vs. control (Kim et al., Kidney Int 2006).