Revealing the large-scale network organization of growth hormone-secreting cells

Abstract

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

Cellular in vivo imaging reveals coordinated regulation of pituitary microcirculation and GH cell network function

“…S1 and Movie S1). Spatial resolution, light excitation, and emission intensities were sufficient to visualize the cellular organization of individual somatotrophs (10) and to distinguish different colored fluorescent reporters for GH (eGFP, secretory vesicles) and Prolactin (DsRed, cytoplasmic) (Fig. 1D).…”

“…Using pituitary slice preparations, we have recently demonstrated the existence of a fully functional GH network in situ (10). However, the mechanisms involved in the shaping of hormone pulses by the GH network in vivo, in a dynamic environment, remain unknown.…”

Un réseau qui donne du relief à l’hypophyse

“…Ces cellules GH-GFP, apparemment réparties au hasard dans le plan, apparaissent en fait liées les unes aux autres dans la profondeur de la glande. Elles forment ainsi un vaste « squelette endocrine » dont l'existence demeurait insoupçonnée jusqu'alors [5] (Figure 1). …”

“…Pour tenter de répondre à cette question, nous avons mesuré la concentration en calcium cytosolique dans des cellules individuelles au sein de tranches d'hypophyse. En fonction de l'organisation locale entre les cellules GH-GFP, deux types de réponses au GHRH ont pu être distinguées [5]. D'une part, dans les zones latérales où les cellules GH sont fortement condensées, des vagues calciques lentes apparaissent en même temps dans la majorité (sinon toutes) des cellules GH.…”

“…Cependant, si l'hypothalamus impose un certain tempo aux sécrétions hypophysaires, un phénomène d'intégration dans l'hypophyse joue vraisemblablement un grand rôle dans l'amplitude des pics hormonaux qui ne peut être expliquée par la simple somme des activités unitaires. Dans une étude récente [5], nous avons montré que les cellules somatotropes (sécrétant la GH) de l'hypophyse peuvent communiquer via un réseau anatomique câblant toute la glande.…”