Volume 22, Issue 1, March 2006

Table of Contents 22-1

Pituitary GH-secretory Cells

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Bonnefont and colleagues answered a long-standing question: if growth hormone (GH)-secreting cells are heterogeneously distributed and scattered throughout the anterior pituitary, as shown by histology, how do they physiologically mount GH pulsatile release that is frequently a thousand-fold in magnitude, especially since their GH pulses are much smaller when studied in vitro?

Using GH-GFP transgenic mice and custom-made computer software, these investigators were able to identify and localize the 3-D position of the labeled somatotrophs within the pituitary gland. Examination of fixed pituitaries from adult male mice revealed a connected 3-D, multi-cellular system comprised of numerous intercrossing strands of single GH cells with larger cell clusters at the intersections. This GH multi-cellular assembly withstood dispersion by a high-pressure in vivo perfusion procedure, and was shown to be linked by focal adherens junctions containing ß-catenin.

The system was shown to be both functional and plastic. Comparing the volume-to-surface ratios of the GH cell clusters within the lateral and median pituitary zones, the ratios were similar in prepubertal animals. However, GH cell clusters increased in the lateral zones from puberty to adulthood, and then returned to prepubertal geometries in the oldest mice. Cell clustering was prevented by prepubertal castration of male mice, without a significant change in GH cell density in the lateral zones; organizational geometry was the important factor for the pubertal increase in growth. Multi-cellular calcium recordings of GH-EGFP cells in acute pituitary slices were measured as a marker of cell-cell connectivity in hormone release. No large-scale cell connectivity was observed during spontaneous electrical activity. This increased in the lateral pituitary zones following GH-releasing hormone (GHRH) stimulation, leading to temporally precise, synchronized, recurrent calcium spikes that correlated with the frequency of small GH pulses reported in other studies; enzymatic dispersion of the GH cells prevented GHRH-stimulated calcium spike synchronization. GHRH also increased calcium spiking in the median pituitary zone by changing the cell connectivity into small islets of more highly functionally connected GH cells at some points in the system interspersed with functionally less connected GH cells.

The authors concluded that, “GH cells function as a geometry-driven network of cells, connected to each other by adherens junctions.” It logically follows that disruption of network architecture constitutes a novel mechanism for impaired GH release in pathological conditions, an issue the authors are pursuing in follow-up experiments.

Bonnefont X, Lacampagne A, Sanchez-Hormigo A, et al. Revealing the large-scale network organization of growth hormone-secreting cells. Proc Natl Acad Sci. 2005;102:16880 - 16885.

Editor’s Comment: A 3-D approach to functional analysis of the GH cell network provided novel and interesting insights into its physiology that were heretofore unobtainable. Because it is noninvasive and provides sensitive, real-time data of cellular and molecular events within their biological context,¹ in vivo bioluminescent imaging has recently emerged as a powerful new approach to elucidate physiologic and pathophysiologic mechanisms. It can be used grossly, such as monitoring rejection of transplanted tissues^2,3 or growth of cancer metastases.⁴ It can also be used to study protein-protein interactions,⁵ transcription,⁶ and gene silencing.⁷ Bioluminescent or fluorescent imaging holds great promise as a means of drug testing, both for therapeutic efficacy⁸ and potential effects on normal tissues,⁹ as well as in vivo evaluation of gene therapy strategies.¹⁰

Adda Grimberg, MD

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