Pituitary growth hormone (GH)-secreting cells regulate growth and metabolism in animals and humans. To secrete highly ordered GH pulses (up to 1,000-fold rise in hormone levels in vivo), the pituitary GH cell population needs to mount coordinated responses to GH secretagogues, yet GH cells display an apparently heterogeneous scattered distribution in 2D histological studies. To address this paradox, we analyzed in 3D both positioning and signaling of GH cells using reconstructive, two-photon excitation microscopy to image the entire pituitary in GH-EGFP transgenic mice. Our results unveiled a homologous continuum of GH cells connected by adherens junctions that wired the whole gland and exhibited the three primary features of biological networks: robustness of architecture across lifespan, modularity correlated with pituitary GH contents and body growth, and connectivity with spatially stereotyped motifs of cell synchronization coordinating cell activity. These findings change our view of GH cells, from a collection of dispersed cells to a geometrically connected homotypic network of cells whose local morphology and connectivity can vary, to alter the timing of cellular responses to promote more coordinated pulsatile secretion. This large-scale 3D view of cell functioning provides a powerful approach to identify and understand other networks of endocrine cells that are thought to be scattered in situ. Many dispersed endocrine systems exhibit pulsatile outputs. We suggest that cell positioning and associated cell-cell connection mechanisms will be critical parameters that determine how well such systems can deliver a coordinated secretory pulse of hormone to their target tissues.biological rhythms ͉ endocrinology ͉ systems biology ͉ connectivity ͉ calcium

The early phase of the stimulatory action of aldosterone on sodium reabsorption in tight epithelia involves hormone-regulated genes that remain to be identified. Using a subtractive hybridization technique on isolated renal cortical collecting ducts from rats injected with a physiological dose of aldosterone, we have identified an early response cDNA highly homologous to human and murine NDRG2 (N-Myc downstream regulated gene 2), which consists of four isoforms and belongs to a new family of differentiation-related genes. NDRG2 mRNA was expressed in classical aldosterone target epithelia, and in the kidney, it was specifically located in the collecting duct, the site of aldosterone-regulated sodium absorption. NDRG2 mRNA was increased within 45 min by aldosterone in the kidney and distal colon, whereas it was unaffected in the heart. In the RCCD2 collecting duct cell line, NDRG2 mRNA was enhanced as early as 15 min after aldosterone addition by transcription-dependent effects. NDRG2 was induced by aldosterone concentrations as low as 10 ؊9 M, and a maximal effect was observed at 10 ؊8 M. In contrast, the glucocorticoid dexamethasone was ineffective in NDRG2 expression, whereas the glucocorticoid-regulated gene sgk was induced. Taken together, these results indicate that NDRG2 regulation by aldosterone is an early mineralocorticoid-specific effect. Interestingly, NDRG2 is homologous to Drosophila MESK2, a component of the Ras pathway, suggesting that activation of the Ras cascade may play a significant role in mineralocorticoid signaling.

Gastric acid secretion is mediated by the H/K-ATPase of parietal cells. Activation of acid secretion involves insertion of H/K-ATPase into the parietal cell plasmalemma, while its cessation is associated with reinternalization of the H/K-ATPase into an intracellular storage compartment. The cytoplasmic tail of the H/K-ATPase beta subunit includes a four residue sequence homologous to tyrosine-based endocytosis signals. We generated transgenic mice expressing H/K-ATPase beta subunit in which this motif's tyrosine residue is mutated to alanine. Gastric glands from animals expressing mutant beta subunit constitutively secrete acid and continuously express H/K-ATPase at their cell surfaces. Thus, the beta subunit's tyrosine-based signal is required for the internalization of H/K-ATPase and for the termination of acid secretion. As a consequence of chronic hyperacidity, the mice develop gastric ulcers and a hypertrophic gastropathy resembling Menetrier's disease.

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