Effect of renal tubule-specific knockdown of the Na⁺/H⁺exchanger NHE3 in Akita diabetic mice

Abstract

Na+/H+exchanger isoform 3 (NHE3) contributes to Na+/bicarbonate reabsorption and ammonium secretion in early proximal tubules. To determine its role in the diabetic kidney, type 1 diabetic Akita mice with tubular NHE3 knockdown [Pax8-Cre; NHE3-knockout (KO) mice] were generated. NHE3-KO mice had higher urine pH, more bicarbonaturia, and compensating increases in renal mRNA expression for genes associated with generation of ammonium, bicarbonate, and glucose (phosphoenolpyruvate carboxykinase) in proximal tubules and H+and ammonia secretion and glycolysis in distal tubules. This left blood pH and bicarbonate unaffected in nondiabetic and diabetic NHE3-KO versus wild-type mice but was associated with renal upregulation of proinflammatory markers. Higher renal phosphoenolpyruvate carboxykinase expression in NHE3-KO mice was associated with lower Na+-glucose cotransporter (SGLT)2 and higher SGLT1 expression, indicating a downward tubular shift in Na+and glucose reabsorption. NHE3-KO was associated with lesser kidney weight and glomerular filtration rate (GFR) independent of diabetes and prevented diabetes-associated albuminuria. NHE3-KO, however, did not attenuate hyperglycemia or prevent diabetes from increasing kidney weight and GFR. Higher renal gluconeogenesis may explain similar hyperglycemia despite lower SGLT2 expression and higher glucosuria in diabetic NHE3-KO versus wild-type mice; stronger SGLT1 engagement could have affected kidney weight and GFR responses. Chronic kidney disease in humans is associated with reduced urinary excretion of metabolites of branched-chain amino acids and the tricarboxylic acid cycle, a pattern mimicked in diabetic wild-type mice. This pattern was reversed in nondiabetic NHE3-KO mice, possibly reflecting branched-chain amino acids use for ammoniagenesis and tricarboxylic acid cycle upregulation to support formation of ammonia, bicarbonate, and glucose in proximal tubule. NHE3-KO, however, did not prevent the diabetes-induced urinary downregulation in these metabolites.

Osmotic diuresis by SGLT2 inhibition stimulates vasopressin‐induced water reabsorption to maintain body fluid volume

“…Upregulation of SGLT2 protein expression has previously been observed in response to SGLT2 inhibition (Masuda et al, ; Vallon et al, ). In contrast, upregulation of renal gluconeogenesis in mice lacking tubular NHE3 is associated with marked suppression of SGLT2 expression (Onishi et al, ), probably due to a negative feedback mechanism to prevent excessive intracellular glucose concentrations. In this regard, blocking SGLT2‐mediated glucose uptake would lower intracellular glucose levels and inhibit this negative feedback on SGLT2 expression.…”

Interplay of adenosine monophosphate‐activated protein kinase/sirtuin‐1 activation and sodium influx inhibition mediates the renal benefits of sodium‐glucose co‐transporter‐2 inhibitors in type 2 diabetes: A novel conceptual framework

“…Most of the proximal tubular reabsorption of sodium in diabetes is mediated by increased expression and activity of NHE‐3 and, interestingly, SGLT2 and NHE‐3 are co‐localized and functionally intertwined in the early proximal tubule (Figure ) . Experimental knockout of NHE‐3 depresses the expression of SGLT2, and inhibition of SGLT2 interferes with the activity of NHE‐3 . A similar interplay between NHE‐3 and SGLT also occurs in the small intestinal epithelium.…”

“…Accordingly, overactivity of NHE or the epithelial sodium channel (ENaC) (both mediating sodium influx), or suppression of Na + ‐K + ATPase (which mediates sodium efflux), can cause increases in intracellular sodium, leading to cellular demise . Diabetes increases the activity of both NHE and ENaC and decreases the activity of Na + ‐K + ATPase; the resulting increase in intracellular sodium promotes mitochondrial stress and may contribute to the development of nephropathy …”

Autophagy stimulation and intracellular sodium reduction as mediators of the cardioprotective effect of sodium–glucose cotransporter 2 inhibitors

“…Studies supporting an effect of SGLT2 inhibitors on NHE‐1 have relied entirely on measurements of the effects of SGLT2 inhibitors on intracellular sodium rather than on direct studies of an interaction with NHE‐1. Nevertheless, it is possible that SGLT2 inhibitors may exert a cardioprotective effect by inhibiting the deleterious actions of increased NHE‐1 in a manner that is analogous to their effect to inhibit the increased activity of NHE‐3 in the kidney . However, NHE inhibition may represent an indirect action that is related to other effects of SGLT2 inhibitors (see below).…”