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How to match dietary salt intake and renal salt excretion

by Silke Härteis (13.07.2009)

The kidney is the key organ for the maintenance of salt- and fluid homeostasis in the body. Table salt (NaCl) dissociates in solution into sodium (Na+) and chloride (Cl-) ions, and the amount of Na+ and its accompanying anion in the body critically determines the extracellular volume and hence arterial blood pressure. To maintain body Na+ content constant, urinary Na+ excretion has to match the daily Na+ intake via the food. This is accomplished by a highly regulated resorption of Na+ along the nephron which determines the amount of Na+ that is excreted by the kidney in the final urine. The fine tuning of Na+ resorption is accomplished in the so called aldosterone-sensitive distal nephron (includes the late distal convoluted tubule, the connecting tubule, and the collecting duct) and involves complex regulatory mechanisms. Na+ flux through the epithelial sodium channel (ENaC) represents the rate-limiting step of Na+ absorption.

Figure 1: Cellular transport model of Na+ absorption in the aldosterone-sensitive nephron in the kidney[Bildunterschrift / Subline]: Figure 1: Cellular transport model of Na+ absorption in the aldosterone-sensitive nephron in the kidney

Thus, the appropriate regulation of ENaC is critically important for the maintenance of body Na+ balance and hence for the long term control of arterial blood pressure. To study the function and regulation of ENaC is a long standing research focus of the laboratory of Professor Korbmacher. The ion channel ENaC is inhibited by the diuretic drug amiloride which is used clinically to promote renal salt and water excretion. The most important hormone to regulate ENaC is the mineralocorticoid hormone aldosterone. An abnormal production of aldosterone (hyperaldosteronism, Conn’s syndrome) leads to an excessive stimulation of ENaC which causes an inadequate renal Na+ retention and arterial hypertension. The analysis of two genetic diseases, the Liddle’s syndrome and the pseudohypo­aldosteronism type 1, provided direct evidence that the molecular dysfunction of ENaC affects the regulation of arterial blood pressure. Liddle’s syndrome, caused by gain-of-function mutations of ENaC, leads to an increased Na+ resorption and thereby to a rare form of severe hereditary arterial hypertension. In contrast, loss-of-function mutations of ENaC in patients with pseudo­hypo­­aldosteronism type 1 result in renal Na+ loss and hypotension. These genetic diseases are rare but provide proof of principle that the appropriate function of ENaC is essential for the long-term control of blood pressure. A better understanding of molecular mechanisms involved in ENaC regulation will hopefully provide novel insights into the physiology and pathophysiology of arterial hypertension and diseases associated with renal sodium retention. This ultimately may lead to new diagnostic and therapeutic concepts.

The aim of my PhD project is to investigate molecular mechanisms involved in the regulation of ENaC by using a combined biochemical and electrophysiological approach.

ENaC is a member of the ENaC/degenerin family of ion channels. Typically, ENaC consists of three well characterized subunits (αβγ). Interestingly in humans – but not in mouse or rat – an additional δ-subunit exists. So far little is known about the physiological role and the functional properties of this additional subunit. Therefore, one aspect of my PhD project is the functional characterization of the δ-subunit of human ENaC. In this project I investigate the functional properties of αβγ- and δβγ-ENaC. As a model system I use the Xenopus laevis oocyte expression system which is a well-established functional expression system to study ion channels.

African clawed frog (Xenopus laevis) (left), Xenopus laevis oocytes (right) African clawed frog (Xenopus laevis) (left), Xenopus laevis oocytes (right) [Bildunterschrift / Subline]: Figure 2: African clawed frog (Xenopus laevis) (left), Xenopus laevis oocytes (right)

The reason for that is that the oocytes contain a potent protein translation machinery. After injection of in vitro transcribed cRNA into the cytoplasm of the oocyte, this machinery can be ‘hijacked’ to produce the protein of interest. One or two days after cRNA injection the function of the protein can be measured by using the two-electrode voltage clamp technique. I observed that the amiloride-sensitive whole-cell current (corresponds to the ENaC mediated Na+ inward current and is a size for ENaC activity) was significantly larger in oocytes expressing δβγ-ENaC compared to oocytes expressing αβγ-ENaC. In my study I elucidate the underlying mechanisms by which the δ-subunit enhances ENaC activity.

The second topic of my PhD project is ENaC regulation by proteases. There are a number of molecular mechanisms that are involved in ENaC regulation. Recent evidence indicates that proteolytic processing of ENaC is an important mechanism that contributes to ENaC activation in a complex manner. Concerning this second project I have contributed to the discovery that plasmin in nephrotic urine can activate ENaC which may contribute to renal Na+ retention observed in patients suffering from nephrotic syndrome, a kidney disease characterized by proteinuria and edema. This project was accomplished in cooperation with the group of Prof. O. Skøtt from Denmark and the resulting manuscript has been published in the Journal of the American Society of Nephrology. I am a coauthor on this paper to which I have contributed the electrophysiological measurements using the Xenopus laevis oocyte expression system.

  • 2001-2006
  • Studies in Molecular Medicine, Friedrich-Alexander-University of Erlangen-Nuremberg, Germany
  • 07/2006
  • Diploma in Molecular Medicine, Friedrich-Alexander-University Erlangen-Nürnberg, Germany
  • since 09/2006
  • PhD student at the Department of Cellular and Molecular Physiology (Prof. Dr. C. Korbmacher), Friedrich-Alexander-University of Erlangen-Nuremberg, Germany
  • since 11/2006
  • Research scholarship of the Elite Network of Bavaria
  • since 03/2007
  • Member of the BioMedTec International Graduate School of Science (BIGSS) Lead Structures of Cell Function of the Elite Network of Bavaria
  • since 02/2008
  • Member of the graduate student program in the context of the SFB 423: Collaborative Research Center "Kidney Injury: Pathogenesis and Regenerative Mechanisms"

  • Svenningsen P, Bistrup C, Friis UG, Bertog M, Haerteis S et al (2009) Plasmin in nephrotic urine activates the epithelial Na+ channel (ENaC). Journal of the American Society of Nephrology, 20(2):299-310.