Maintaining the water homeostasis of the body is crucial for its proper functioning. Accordingly, water and electrolytes are balanced by a carefully orchestrated interplay between volume-and osmoregulation that is regulated by multiple hormones. A key hormone in process is the antidiuretic hormone arginine-vasopressin (AVP), which is released from the pituitary in states of hypernatreamia or hypovolaemia (1, 2).The main target organ of AVP in the process of osmoregulation is the kidney, which is composed of approximately one million nephrons, the functional units in kidney. In these nephrons, ultrafiltration results in the formation of a daily volume of approximately 180 l of pro-urine. Of this large volume, approximately 99% of the water is reabsorbed by the tubular epithelial cells of the nephron, whereas only 1.5-2 l of water is excreted via the urine. Approximately 90% of the tubular water reabsorption occurs in the proximal tubules and descending loop of Henle, where water is reabsorbed iso-osmotically via the water channel aquaporin-1. The remaining 9% can be reabsorbed in the principal cells of the collecting duct, the final part of the nephron, and is regulated by AVP. Binding of AVP to its type 2 receptor on the basolateral (interstitial) side of the principal cells induces a cAMP cascade that increases protein levels of the water channel aquaporin-2 (AQP2) and triggers the protein kinase A-induced translocation of AQP2 storage vesicles to the apical (luminal) plasma membrane (Fig. 1A) (1). Insertion of AQP2 in this membrane facilitates its water permeability and allows water transport from the tubular lumen into the principal cells, and its subsequent flow into the interstitium via the water channels aquaporin-3 and 4 in the basolateral plasma membrane. Together, this allows the body to retain water and compensate for states of hypernatraemia or hypovolaemia. When the water balance is restored, AVP release will lead to decreased plasma AVP levels and thereby to the retrieval of AQP2 from the apical membrane, rendering the collecting duct impermeable to water (1).Disturbances in the above process will result in diabetes insipidus (DI), a disease characterised by polyuria and compensatory polydipsia. The underlying causes of DI are diverse and can be a central defect, in which no functional AVP is released from the pituitary, or may be caused by defects in the kidney (nephrogenic DI, NDI). Four Journal of NeuroendocrinologyCorrespondence to: Joris H. Robben, 286 Deptartment of Physiology, Nijmegen Centre for Molecular Life Sciences, Radboud University Nijmegen Medical Centre, Nijmegen, The Netherlands (e-mail: j.robben@fysiol.umcn.nl).According to the body's need, water is reabsorbed from the pro-urine that is formed by ultrafiltration in the kidney. This process is regulated by the antidiuretic hormone arginine-vasopressin (AVP), which binds to its type 2 receptor (V2R) in the kidney. Mutations in the gene encoding the V2R often lead to the X-linked inheritable form of nephrogenic diabetes insipidus (ND...

Stem cell factor (SCF) has a potent synergistic effect during megakaryopoiesis when administered in combination with the major megakaryocytic cytokine, thrombopoietin (TPO). In this study we analyzed the underlying mechanisms with regard to STAT5 activity. TPO stimulation of MO7e cells resulted in STAT5 transactivation, which could be enhanced 1.6-fold by costimulation with SCF, whereas SCF alone did not induce STAT5 transcriptional activity. This costimulatory effect of SCF was reflected in an increase in TPO-induced STAT5 DNA binding and increased and prolonged STAT5 tyrosine phosphorylation in both MO7e cells and primary human megakaryocyte progenitors. In contrast, serine phosphorylation of STAT5 was constitutive and associated with an inhibitory effect on STAT5 transactivation. Signal transduction pathways that might synergize in TPO-mediated STAT5 transactivation were analyzed using specific pharmacological inhibitors and indicated an essential role for Janus-activated kinase 2 (JAK2) and a partial role for Src-family kinases. Costimulation with SCF was found to increase and prolong tyrosine phosphorylation of JAK2 and the TPO receptor c-mpl. In addition, the Src kinase inhibitor SU6656 partially downregulated the additional effect of SCF costimulation on STAT5 tyrosine phosphorylation. SCF-induced enhancement of JAK2 phosphorylation was not affected by inhibition of Src kinase, suggesting that both JAK2 and Src kinase mediate STAT5 tyrosine phosphorylation. Synergistic activation of JAK2 and Src kinase may thus contribute to the enhanced STAT5 signaling in the presence of TPO and SCF. Stem Cells 2005;23:240-251

Essential fatty acid (EFA) deficiency in mice induces fat malabsorption. We previously reported indications that the underlying mechanism is located at the level of the intestinal mucosa. We have investigated the effects of EFA deficiency on small intestinal morphology and function. Mice were fed an EFA-deficient or control diet for 8 wk. A 72-h fat balance, the EFA status, and small intestinal histology were determined. Carbohydrate absorptive and digestive capacities were assessed by stable isotope methodology after administration of [U-(13)C]glucose and [1-(13)C]lactose. The mRNA expression and enzyme activity of lactase, and concentrations of the EFA linoleic acid (LA) were measured in small intestinal mucosa. Mice fed the EFA-deficient diet were markedly EFA-deficient with a profound fat malabsorption. EFA deficiency did not affect the histology or proliferative capacity of the small intestine. Blood [13C6]glucose appearance and disappearance were similar in both groups, indicating unaffected monosaccharide absorption. In contrast, blood appearance of [13C]glucose, originating from [1-(13)C]lactose, was delayed in EFA-deficient mice. EFA deficiency profoundly reduced lactase activity (-58%, P<0.01) and mRNA expression (-55%, P<0.01) in mid-small intestine. Both lactase activity and its mRNA expression strongly correlated with mucosal LA concentrations (r=0.77 and 0.79, respectively, P<0.01). EFA deficiency in mice inhibits the capacity to digest lactose but does not affect small intestinal histology. These data underscore the observation that EFA deficiency functionally impairs the small intestine, which in part may be mediated by low LA levels in the enterocytes.

Essential fatty acid (EFA) deficiency in mice has been associated with increased bile production, which is mainly determined by the enterohepatic circulation (EHC) of bile salts. To establish the mechanism underlying the increased bile production, we characterized in detail the EHC of bile salts in EFA-deficient mice using stable isotope technique, without interrupting the normal EHC. Farnesoid X receptor (FXR) has been proposed as an important regulator of bile salt synthesis and homeostasis. In Fxr(-/-) mice we additionally investigated to what extent alterations in bile production during EFA deficiency were FXR dependent. Furthermore, we tested in differentiating Caco-2 cells the effects of EFA deficiency on expression of FXR-target genes relevant for feedback regulation of bile salt synthesis. EFA deficiency-enhanced bile flow and biliary bile salt secretion were associated with elevated bile salt pool size and synthesis rate (+146 and +42%, respectively, P < 0.05), despite increased ileal bile salt reabsorption (+228%, P < 0.05). Cyp7a1 mRNA expression was unaffected in EFA-deficient mice. However, ileal mRNA expression of Fgf15 (inhibitor of bile salt synthesis) was significantly reduced, in agreement with absent inhibition of the hepatic bile salt synthesis. Bile flow and biliary secretion were enhanced to the same extent in EFA-deficient wild-type and Fxr(-/-) mice, indicating contribution of other factors besides FXR in regulation of EHC during EFA deficiency. In vitro experiments show reduced induction of mRNA expression of relevant genes upon chenodeoxycholic acid and a selective FXR agonist GW4064 stimulation in EFA-deficient Caco-2 cells. In conclusion, our data indicate that EFA deficiency is associated with interrupted negative feedback of bile salt synthesis, possibly because of reduced ileal Fgf15 expression.

Cholestatic liver disease (CLD) in children negatively affects nutritional status, growth and development, which all lead to an increased risk of morbidity and mortality. This is illustrated by the fact that the clinical outcome of children with CLD awaiting a liver transplantation is in part predicted by their nutritional status, which is integrated in the pediatric end-stage liver disease model. Preservation of the nutritional status becomes more relevant as the number of patients waiting for liver transplantation increases and the waiting time for a donor organ becomes prolonged. Nutritional strategies are available to optimize feeding of children with CLD. Patients with CLD, however, form a heterogeneous group and the clinical manifestations of their disease vary. This makes a tailor-made approach for these children crucial. Not all aspects of nutrient metabolism and absorption in children with CLD are well understood and studied. Experiments with stable isotope-labeled triglycerides and fatty acids have provided essential information about fat absorption under physiological and cholestatic conditions in animal models and humans. We expect that in the future, tests using other isotope-labeled macronutrients, i.e. carbohydrates and proteins, can be used to further assess nutritional status of children with CLD, thereby creating tailor-made nutritional therapies.