PARACELLULAR EPITHELIAL TRANSPORT MAXIMIZES ENERGY EFFICIENCY IN THE KIDNEY

Abstract

Claudins are tight junction transmembrane proteins that act as paracellular ion channels. The proximal renal tubule reabsorbs 70% of glomerulus-filtered Na+. Of this Na+, up to 1/3 is reabsorbed passively via the paracellular pathway. Claudin-2 has been shown to mediate paracellular sodium reabsorption in the proximal tubule of the kidney. Claudin-2 null mice are reported to have markedly decreased proximal tubule Na+, Cl- and fluid reabsorption, but they have no apparent disturbance in overall Na+ balance. We investigated whether we can uncover the salt-wasting phenotype of claudin-2 knockout mice by challenging them with Na+ depletion. Even during profound dietary sodium depletion, claudin-2 null mice demonstrated the ability to conserve sodium to the same extent as wild-type mice. We investigated how the Na+ wasted from the proximal tubule was compensated in other segments of the renal tubule. We performed immunoblot on transcellular Na+ transporters Na-H antiporter 3, Na-K-2Cl cotransporter, NaCl cotransporter and epithelial Na+ channel. We found no upregulation of the protein abundance of any of these transcellular transporters. To test whether there was functional upregulation of transcellular Na+ transport distally, diuretic challenge tests were performed. The natriuresis 4 hours after intraperitoneal furosemide (but not after hydrochlorothiazide or benzamil) was 40% higher in claudin-2 null mice than in WT mice, indicating that the site of compensation was the thick ascending limb. We concluded that claudin-2 null mice are able to conserve sodium to the same extent as wild-type mice, even under extreme conditions, due to upregulation of transcellular Na-K-2Cl transport activity in the thick ascending limb of Henle. We therefore hypothesized that the shifting of sodium transport to transcellular pathways would lead to increased whole kidney oxygen consumption. Indeed we found that the kidneys of claudin-2 null mice have markedly increased oxygen consumption despite normal sodium reabsorption, and consequently have medullary hypoxia. Furthermore, when subjected to bilateral renal ischemia-reperfusion injury, kidneys of claudin-2 null mice exhibited more severe tubular injury. In summary, our findings suggest a more important reason for the existence of the paracellular transport pathway, namely that it evolved to enhance the efficiency of energy and oxygen utilization by the renal tubule epithelium. Our research demonstrates that claudin-2 is a key paracellular pathway for passive Na+ absorption in the proximal tubule that reduces the overall energy expenditure by the kidneys. This dissertation sheds light on a fundamental metabolic process and thereby provides insight on the process that may have broader significance for epithelial tissues.