 |
 |
|
|
|
|
The conversion of arachidonic acid to prostaglandin (PG) H2 by cyclooxygenase (COX) is a key enzymatic and rate-limiting step in the regulation of prostaglandin synthesis. Two isoforms of COX have been identified: a constitutive (COX-1) and an inducible (COX-2) form. COX-1 is the predominant isoform in the mammalian kidney and has been localized to glomeruli and collecting ducts. COX-2 has been localized to arteries, collecting ducts, interstitial cells, cortical thick ascending limb of Henle (cTALH) cells, and macula densa cells (Fig. 1).
Prostaglandins have a major impact on kidney function. They are involved in the control of renal blood flow, glomerular filtration, salt excretion, and in the secretion and expression of renin. Due to a parallel regulation of COX-2 and renin expression in the renal cortex, specifically COX-2-derived prostanoids have been implicated in the regulation of renin release (Fig 2). Therefore, the goal of our research is to characterize the relevance of COX-2 -derived prostanoids for renin synthesis and renin release.
Acute renal failure (ARF) secondary to sepsis is a highly prevalent diagnosis in the intensive care unit setting and continues to be associated with a high rate of morbidity and mortality. The combination of acute renal failure and sepsis is associated with a 70 percent mortality, as compared with a 45 percent mortality among patients with acute renal failure alone. Thus, the combination of sepsis and acute renal failure constitutes a particularly serious medical problem. A possible explanation of why ARF is so common in severe sepsis and septic shock and why mortality has remained high might relate to our minimal understanding of septic ARF and its pathogenesis. Therefore, the goal of our research is to characterize septic ARF, its pathogenesis and pharmacologic treatments.
The protease renin is the key enzyme in Renin-Angiotensin-System (RAS). Renin is mainly produced by the epithelial-like
juxtaglomerular (JG) cells located in the afferent arteriole of the kidney. An inadequate activation of the renin synthesis
is a clinically relevant factor in the pathogenesis of arterial hypertension, edema, vascular and renal damage.
The transcription of renin gene is the first decisive regulated step in the overall renin production. The renin gene
transcription determines not only the release of renin stored in vesicles and secreted through a regulated exocytosis, but
also the amount of constitutively-secreted prorenin. The inactive precursor prorenin is gaining increasing recognition since
the recent discovery of a (pro)renin membrane receptor which binds specifically renin and prorenin. Thereby prorenin enzymatic
activity is boosted and intracellular signaling is elicited. Current research spans from molecular and cellular techniques to
in vivo kidney (patho)physiology including generation of transgenic mice.
Role of the cAMP target sequences in the regulation of human renin gene
In vitro studies have shown that cAMP signaling is important for the control of renin
transcription. However, the functional relevance of renin gene cAMP target sequences in vivo is still a matter of controversy.
We have generated a transgenic mouse line which expresses lacZ reporter driven by a 12.2 kb human renin promoter with mutated CRE
(cAMP Response Element) and CNRE (cAMP and Overlapping Negative Response Element). The regulation of the human renin transgene
will be compared with the endogenous renin gene expression at physiological (salt diets, adrenoreceptor stimulation) and
pathophysiological (renal artery stenosis, obesity) conditions. This approach will essentially contribute to understanding
the function of the cAMP target sequences and the role of cAMP signaling in the control of human renin gene.
The expression of the lacZ reporter is specifically targeted to the renin-producing JG cells of the kidney. G- glomerulus, α-actin is stained as an artery marker.
Nuclear receptor PPARgamma
PPARgamma is a transcription factor which plays key role in Metabolic syndrome (obesity accompanied by hyperlipidemia, insulin
resistance and hypertension). We have discovered an atypical, but highly active PPARgamma binding sequence (termed Pal3) in
the human renin promoter. The identification of renin as a PPARgamma-regulated gene is particularly interesting with regard
to the pathogenesis of hypertension of Metabolic syndrome. To study the role of PPARgamma in the regulation of human renin
gene in vivo we are generating "renin-humanized" mice carrying the native or the mutated Pal3 sequence.
PPARgamma- 3D structure
Role of nuclear receptors COUP-TF I and II in the molecular regulation of renin transcription
COUP-TF I and II are developmentally active transcription factors. They are expressed during the ontogenesis and adulthood in
the mouse kidney including the JG area.
Expression of COUP-TFII in the renin-producing JG-cells of adult mouse kidney.
Therefore the COUP-TF proteins could be relevant for the cellular control of renin gene not only at (patho)physiological conditions, but also for recruitment of renin-producing cells during kidney ontogenesis or during chronic stimulation of renin production in the adult 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.
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).
