Research

The Kurtz group

Development, structure and function of renin secreting cells of the kidney

The protease renin is the regulatory key enzyme of the renin-angiotensin-aldosterone system (RAAS). Renin is mainly produced and secreted by the juxtaglomerular cells of the afferent arterioles of the kidneys. RAAS activity regulates extracellular salt and blood pressure homeostasis. Uncontrolled stimulation of renin synthesis and renin secretion is a clinically relevant reason for hypertension, edemas, vascular and kidney damage. The renin secreting cells of the kidney display several unusual characteristics, which are not yet understood. Thus, it is not clear how renin cells develop and differentiate from the metanephric mesenchyme. Renin cells, moreover, show a very high degree of plasticity leading to reversible 20-fold changes of the number of renin cells depending on the challenge of the RAAS. Renin cells first synthesize enzymatically inactive prorenin, which is sorted into huge electron dense lysosome related organelles, in which prorenin gets proteolytically activated to renin by a yet undefined protease. Renin is then released from these vesicles by a mode of secretion that is not yet completely understood. Finally, the release of renin is inversely related to the extracellular concentration of calcium, what stands in clear contrast to the classical stimulatory effect of calcium in endo- and exocrine cells.

Our running projects deal with these open questions.


Running projects

Mechanisms of renin cell hyperplasia

Genetic defects of the RAAS lead to a strong compensatory hyperplasia of renin cells, which then form cellular multilayers around afferent arterioles. The number of these ectopic, hyperplastic renin cells is strongly dependent on the rate of oral salt intake, in the way that high salt intake suppresses the cells, whilst low salt intake stimulates the formation of these cells. To investigate the mechanisms underlying the salt dependent formation of hyperplastic renin cells we use as a model mice lacking aldosterone-synthase. Using those mice we investigate the roles of cell proliferation and of apoptosis of renin cells and we aim to characterize the mediators that regulate these processes.

Renin (green) und smooth muscle actin (red) in kidney section G, glomerulus; aa, afferent arteriole; ea, efferent arteriole; RC, renin cell; RCC, perivascular renin cell cluster

Reference: Kurt B, Karger C, Wagner C, Kurtz A. Control of renin secretion from kidneys with renin cell hyperplasia. Am J Physiol Renal Physiol. 2014 Feb 1;306(3):F327-32.


Role of hypoxia inducible transcription factor 2 (HIF-2) for renin cell differentiation

Based on our previous observation that deletion of the von Hippel Lindau protein in juxtaglomerular renin secreting cells suppresses renin expression and induced expression of the hormone erythropoietin, we now try to elucidate the cellular mechanisms triggering this striking phenotype change and thus do essentially interfere with renin cell differentiation. We meanwhile know that stabilization of the transcription factor HIF-2 is centrally involved in the phenotype shift. We now investigate which signal pathways are affected by HIF-2 stabilization and may therefore be involved in fetal renin cell differentiation.

In situ-hybridizations (ISH) for renin-mRNA and erythropoietin-mRNA in kidneys of a wildtyp mouse and of a mouse with deletion of von Hippel Lindau protein in juxtaglomerular cells Stars mark glomeruli, arrow heads highlight positive ISH signals.

Reference: Kurt B, Paliege A, Willam C, Schwarzensteiner I, Schucht K, Neymeyer H,Sequeira-Lopez ML, Bachmann S, Gomez RA, Eckardt KU, Kurtz A. Deletion of vonHippel-Lindau protein converts renin-producing cells into erythropoietin-producing cells. J Am Soc Nephrol. 2013 Feb;24(3):433-44.


Cell biology of renin secretion

Renin is stored in big electron-dense lysosome related organelles and is relased from them in a regulated fashion. The release of renin is oppositely regulated by the cAMP-signaling pathway (stimulatory) and a calcium related pathway (inhibitory). Using electron-microscopical analysis we have already described structural changes of the renin storage vesicle during controlled modulation of secretion. It is yet unclear along which pathways renin passes the cell membrane and which proteins of the classical exocytosis machinery could be involved. These questions will be investigated by detailed electron microscopy and protein biochemistry of isolated renin storage vesicles.

a) elektron-microscopical (ELMI) analysis of renin storage vesicles
b,c) 3D-ELMI-reconstruction of single storage vesicles
d) 3D-ELMI-reconstruction of a renin cell. Non-interconnected vesicles are shown with individual colors.

Reference: Steppan D, Zügner A, Rachel R, Kurtz A. Structural analysis suggests that renin is released by compound exocytosis. Kidney Int. 2013 Feb;83(2):233-41.


Mechanisms of calcium triggered inhibition of renin secretion

Increases of the cytosolic concentration of calcium typically trigger, enhance and maintain secretory events. The release of renin, however is inhibited by calcium mobilizing hormones such as angiotensin II or endothelin in a strict dependency on extracellular calcium. Lowering of the extracellular concentration of calcium into the micromolar range strongly potentiates renin secretion. This particular behavior of renin secretion has coined the term “calcium paradox” of renin secretion. The mechanisms underlying this calcium paradox are still less understood. Another striking but unexplained observation in this context is that the potentiating effect of low extracellular calcium on renin secretion, but not the inhibitory action of calcium mobilizing hormones are strongly dependent on intact gap junction proteins (connexin 40). It is the aim of this project to unravel the mechanisms underlying the phenomenon of the calcium paradox of renin secretion.

Renin secretion from isolated perfused kidneys of a wildtype and a connexin 40 deficient mouse. After a control phase the catecholamine isoproterenol was added to the perfusate for the whole length of the experiment, what led to a stable stimulation of renin secretion. Addition of angiotensin II to the perfusate completely abolished the stimulatory effect of isoproterenol. Lowering of the extracellular concentration of calcium from 2 mMol/L(normal) to 1 μMol/L led to a strong overshooting release of the inhibitory effect of angiotensin II on renin secretion in wildtype kidneys, but only to a moderate disinhibtion of renin secretion in connexin 40 deficient kidneys.

References:
Wagner C, de Wit C, Kurtz L, Grünberger C, Kurtz A, Schweda F. Connexin40 is essential for the pressure control of renin synthesis and secretion. Circ Res.2007
Schweda F, Friis U, Wagner C, Skott O, Kurtz A. Renin release. Physiology (Bethesda). 2007 Oct;22:310-9. Review


Methods

Molecular biology In vitro In vivo

The Wagner group

Recruitment of renin producing cells in the kidney

The protease renin is predominantly produced in the juxtaglomerular epitheloid cells in the media layer of afferent arterioles nearby the vascular pole of the glomerulus. However, this classic localisation is only a characteristic of the adult kidney. During renal development renin is found in the wall of larger preglomerular vessels till the renal artery. With ongoing maturation of the kidneys renin synthesis stops in the large vessels and finally becomes more and more restricted to the juxtaglomerular position.
In the adult kidney renin cells change their quantity subject to the degree of stimulation of the renin system, whereas cells switch on renin synthesis in retrograde sections of the afferent arterioles, but also in cells of larger arteries.

Renin synthesis (green fluorescence) in the wall of the afferent arteriole (red fluorescence shows alpha-smooth muscle cell actin as vessel marker) under control conditions (A) and in recruited cells after stimulation of the RAAS system(B).

The mechanisms switching the renin expression during nephrogenesis on or off or leading to the retrograde recruitment in the adult kidney are unknown.
The aim of our group is to investigate the physiological role of important local and systemic regulators of the renin system for recruitment of renin producing cells in the fetal and the adult kidney. Particularly, the meaning of classic regulators such as the cAMP pathways, cyclooxygenase 2 or ANG II should be characterized. Especially the role of gap junctions should be considered, which are numerously found between renin producing cells themselves but also between endothelial cells and renin producing cells.

Main projects

Characterisation of the spatiotemporal development of renin expression in kidneys of wildtype mice.
From the data received we aimed to characterize renin expression in the normal developing mouse kidney to establish a reference system for investigations in mice with defined genetic defects in regulatory genes of renin gene expression.

3D-reconstruction of renal vessel tree (red) and renin expression (green) in the developing mouse kidney at the embryonic stage E18.

Importance of the cAMP pathway for the recruitment of renin producing cells during renal development
using mouse models with the tissue-specific knockout of the Gs alpha protein in the renin-producing cells and with double knockout for ß1 and ß2 adrenergic receptors. First findings show, that the cAMP pathway is an essential factor for development of the renin system.

The importance of cell-cell communication for the recruitment of renin producing cells during renal development and in the adult kidney
using knockout mice deficient in the gap junction protein connexin 40 (Cx40). Our data show, that intercellular coupling via gap junctions essentially influences the recruitment of renin producing cells in the adult kidney.

Importance of nitric oxide (NO) for the recruitment of renin producing cells during renal development and in the adult kidney.
For this question renin expression is investigated in knockout mice deficient in endothelial NO synthase (eNOS).

Methods

In vivo analysis: In vitro techniques Molecularbiology/Biochemistry Optical analysis