Martine Imbert-Teboul

Epithelial sodium channel is a key mediator of growth hormone-induced sodium retention in acromegaly.

Acromegalic patients present with volume expansion and arterial hypertension, but the renal sites and molecular mechanisms of direct antinatriuretic action of GH remain unclear. Here, we show that acromegalic GC rats, which are chronically exposed to very high levels of GH, exhibited a decrease of furosemide-induced natriuresis and an increase of amiloride-stimulated natriuresis compared with controls. Enhanced Na(+),K(+)-ATPase activity and altered proteolytic maturation of epithelial sodium channel (ENaC) subunits in the cortical collecting ducts (CCDs) of GC rats provided additional evidence for an increased sodium reabsorption in the late distal nephron under chronic GH excess. In vitro experiments on KC3AC1 cells, a murine CCD cell model, revealed the expression of functional GH receptors and IGF-I receptors coupled to activation of Janus kinase 2/signal transducer and activator of transcription 5, ERK, and AKT signaling pathways. That GH directly controls sodium reabsorption in CCD cells is supported by: 1) stimulation of transepithelial sodium transport inhibited by GH receptor antagonist pegvisomant; 2) induction of alpha-ENaC mRNA expression; and 3) identification of signal transducer and activator of transcription 5 binding to a response element located in the alpha-ENaC promoter, indicative of the transcriptional regulation of alpha-ENaC by GH. Our findings provide the first evidence that GH, in concert with IGF-I, stimulates ENaC-mediated sodium transport in the late distal nephron, accounting for the pathogenesis of sodium retention in acromegaly.

Atlas of gene expression in the mouse kidney: new features of glomerular parietal cells

To gain molecular insight into kidney function, we performed a high-resolution quantitative analysis of gene expression in glomeruli and nine different nephron segments dissected from mouse kidney using Serial Analysis of Gene Expression (SAGE). We also developed dedicated bioinformatics tools and databases to annotate mRNA tags as transcripts. Over 800,000 mRNA SAGE tags were sequenced corresponding to >20,000 different mRNA tags present at least twice in at least one library. Hierarchical clustering analysis of tags demonstrated similarities between the three anatomical subsegments of the proximal tubule, between the cortical and medullary segments of the thick ascending limb of Henles loop, and between the three segments constituting the aldosterone-sensitive distal nephron segments, whereas the glomerulus and distal convoluted tubule clusterized independently. We also identified highly specific mRNA markers of each subgroup of nephron segments and of most nephron segments. Tag annotation also identified numbers of putative antisense mRNAs. This database constitutes a reference resource in which the quantitative expression of a given gene can be compared with that of other genes in the same nephron segment, or between different segments of the nephron. To illustrate possible applications of this database, we performed a deeper analysis of the glomerulus transcriptome that unexpectedly revealed expression of several ion and water carriers; within the glomerulus, they were found to be preferentially expressed in the parietal sheet. It also revealed the major role of the zinc finger transcription factor Wt1 in the specificity of gene expression in the glomerulus. Finally, functional annotation of glomerulus-specific transcripts suggested a high proliferation activity of glomerular cells. Immunolabeling for PCNA confirmed a high percentage of proliferating cells in the glomerulus parietal sheet.

Proteinase-activated Receptor 2 Stimulates Na,K-ATPase and Sodium Reabsorption in Native Kidney Epithelium

Proteinase-activated receptors 2 (PAR2) are expressed in kidney, but their function is mostly unknown. Since PAR2 control ion transport in several epithelia, we searched for an effect on sodium transport in the cortical thick ascending limb of Henles loop, a nephron segment that avidly reabsorbs NaCl, and for its signaling. Activation of PAR2, by either trypsin or a specific agonist peptide, increased the maximal activity of Na,K-ATPase, its apparent affinity for sodium, the sodium permeability of the paracellular pathway, and the lumen-positive transepithelial voltage, featuring increased NaCl reabsorption. PAR2 activation induced calcium signaling and phosphorylation of ERK1,2. PAR2-induced stimulation of Na,K-ATPase Vmax was fully prevented by inhibition of phospholipase C, of changes in intracellular concentration of calcium, of classical protein kinases C, and of ERK1,2 phosphorylation. PAR2-induced increase in paracellular sodium permeability was mediated by the same signaling cascade. In contrast, increase in the apparent affinity of Na,K-ATPase for sodium, although dependent on phospholipase C, was independent of calcium signaling, was insensitive to inhibitors of classical protein kinases C and of ERK1,2 phosphorylation, but was fully prevented by the nonspecific protein kinase inhibitor staurosporine, as was the increase in transepithelial voltage. In conclusion, PAR2 increases sodium reabsorption in rat thick ascending limb of Henles loop along both the transcellular and the paracellular pathway. PAR2 effects are mediated in part by a phospholipase C/protein kinase C/ERK1,2 cascade, which increases Na,K-ATPase maximal activity and the paracellular sodium permeability, and by a different phospholipase C-dependent, staurosporine-sensitive cascade that controls the sodium affinity of Na,K-ATPase.

François Morel: four decades of interdisciplinary search for the logic of life

Martine Imbert-Teboul1 and Alain Doucet1 1Laboratoire de Physiologie et Génomique Rénales, Paris, France Correspondence: Martine Imbert-Teboul, 15 rue de lEcole de Medecine, 75270 Paris, France. E-mail: martine.imbert-teboul@ bhdc.jussieu.fr One year after the death of François Morel (8 May 2007), emotions have receded, allowing for an objective analysis of his scientific contribution. Not only has physiology lost one of its great players and upholders, but with him also vanished his vision of scientific research exclusively based on thought and work far from the spotlights and media, a reflection of the modest and discreet man he was. François Morel was born on 22 February 1923, in Geneva. After obtaining his medical degree at the University of Geneva, he began to pursue a career as a research scientist. In 1948, he joined the laboratory of Professor Courrier, who had just synthesized diiodo-thyronine, the first radiolabeled molecule with biological activity, at the Collège de France in Paris. Two floors below Courrier’s laboratory was that of Frédéric Joliot, who discovered artificial radioactivity, and two floors further below was the first French cyclotron that produced radioactive elements. Naturally, Morel became interested in using radioisotopes in biology. During his first study on thyroid hormone, he identified the limits of contemporary methods using 131I, in that they lacked reproducibility and did not permit quantification of the rate of hormone secretion or its plasma concentration. This led him to develop the concept and method of isotopic equilibrium: when rats were given drinking water containing 131I of known specific activity as the sole source of iodine, endogenous iodine was progressively replaced by 131I until equilibrium was reached. At that point in time, which could be determined by following the radioactivity measured externally over the thyroid, radioactivity yielded the total amount of iodine in the sample. Combining measurements of the concentration of free and bound iodine in the plasma with the curve of iodine turnover allowed one to calculate the daily rate of hormone secretion. The ingenuity of this method reflected Morel’s capacity to develop original methods to reach his goals. Aside from being a brilliant thinker, Morel was also a creative handyman who designed and built the efficient and precise apparatuses that were required by his projects. Thus, to study the renal handling of sodium, using 24Na as tracer, he conceived an impulsion integrator to continuously measure urinary excretion of sodium. This was a remarkable breakthrough, as flame spectro photometry did not yet exist. In 1953, a Department of Biology was created at the Commissariat à l’Énergie Atomique, and Morel joined its Laboratory of Physical and Chemical Physiology, in Saclay, which he headed until 1975. At the time, he was 31 and had never published an article in an international journal, but that was an epoch when true values other than H factors were used to detect talent. In his new laboratory, he demonstrated the tubular origin of potassium excreted in the urine.1 He also studied the endocrine control of ion and water transport across epithelia and was the first to describe, in 1958, the effect of aldosterone on sodium transport.2 In the early 1960s, along with the giants of nephrology, R. Berliner, J. Orloff, K. Ulrich, G. Giebisch, and C. Gottschalk, Morel recognized the limits of clearance technique and realized that further advances to localize and explore renal transport processes required micromethods. He rapidly implemented in his laboratory the in vivo microinjection technique that he had developed with Carl Gottschalk at Chapel Hill. He also greatly improved the potency of in vivo micropuncture by developing techniques and devices for chemical analysis of samples. First, he designed the first flame micro-spectrophotometer to determine sodium and potassium concentrations in nanoliter-size biological fluid samples.3 Second, he imagined using the Castaing’s electron microprobe for chemical analysis of these tiny samples.4 This breakthrough made it possible, for the first time, to study the tubular handling of magnesium, for which there was no valuable radioactive tracer. During his Saclay period, Morel also worked as a professor of physiology at the Paris Faculty of Sciences, where enthusiastic students benefited François Morel: four decades of interdisciplinary search for the logic of life