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

Related pituitary cell lineages develop into interdigitated 3D cell networks

The pituitary gland has long been considered to be a random patchwork of hormone-producing cells. By using pituitary-scale tridimensional imaging for two of the least abundant cell lineages, the corticotropes and gonadotropes, we have now uncovered highly organized and interdigitated cell networks that reflect homotypic and heterotypic interactions between cells. Although newly differentiated corticotrope cells appear on the ventral surface of the gland, they rapidly form homotypic strands of cells that extend from the lateral tips of the anterior pituitary along its ventral surface and into the medial gland. As the corticotrope network is established away from the microvasculature, cell morphology changes from rounded, to polygonal, and finally to cells with long cytoplasmic processes or cytonemes that connect corticotropes to the perivascular space. Gonadotropes differentiate later and are positioned in close proximity to corticotropes and capillaries. Blockade of corticotrope terminal differentiation produced by knockout of the gene encoding the transcription factor Tpit results in smaller gonadotropes within an expanded cell network, particularly in the lateral gland. Thus, pituitary-scale tridimensional imaging reveals highly structured cell networks of unique topology for each pituitary lineage. The sequential development of interdigitated cell networks during organogenesis indicate that extensive cell:cell interactions lead to a highly ordered cell positioning rather than random patchwork.

Functional importance of blood flow dynamics and partial oxygen pressure in the anterior pituitary

The pulsatile release of hormone is obligatory for the control of a range of important body homeostatic functions. To generate these pulses, endocrine organs have developed finely regulated mechanisms to modulate blood flow both to meet the metabolic demand associated with intense endocrine cell activity and to ensure the temporally precise uptake of secreted hormone into the bloodstream. With a particular focus on the pituitary gland as a model system, we review here the importance of the interplay between blood flow regulation and oxygen tensions in the functioning of endocrine systems, and the known regulatory signals involved in the modification of flow patterns under both normal physiological and pathological conditions.

Novel ontogenetic patterns of sexual differentiation in arcuate nucleus GHRH neurons revealed in GHRH-enhanced green fluorescent protein transgenic mice

GH secretion and growth rates are developmentally regulated and sexually dimorphic, but the neuroregulatory mechanisms between birth and puberty are unclear. Using the GHRH-enhanced green fluorescent protein (eGFP) transgenic mouse, in which eGFP provides a strong surrogate signal for identifying GHRH neurons, we showed that numbers in the male arcuate nucleus were double those seen in females at x postnatal day (P)1 and P10, during which time numbers increased 2- to 3-fold. Thereafter (P20, P30, P60, P365) there was a significant trend for numbers to decrease in males and increase in females, such that sex differences were, surprisingly, absent in young and late adulthood. Conversely, we identified the emergence of male-dominant sex differences in the number of processes extended per GHRH perikarya across puberty. Intriguingly, prepubertal gonadectomy (P28), unlike adult gonadectomy, caused a dramatic 40% loss of GHRH cells in both sexes in adulthood and a significant (30%) increase in processes emanating from cell bodies only in females. These findings establish a novel ontogenetic profile for GHRH neurons and suggest previously undiscovered roles for peripubertal gonadal factors in establishing population size in both sexes. They also provide the first demonstration of emergent sex-specific GHRH architecture, which may signal the onset of sex-dependent regulation of activity reported for adult GHRH-eGFP neurons, and its differential regulation by gonadal factors in males and females. This information adds to our knowledge of processes that underpin the emergence of sex-specific GH secretory dynamics and hence biological activity of this pleiotropic hormone.

N-cadherin loss in POMC-expressing cells leads to pituitary disorganization

Pituitary tumors are the third most common intracranial tumor in humans and can cause altered hormone secretions leading to hypercortisolism, acromegaly, and infertility. Reduced expression of the cell adhesion molecule N-cadherin has been linked with the formation of pituitary tumors, but its role in normal pituitary gland physiology or tumor initiation is unknown. In the murine pituitary, N-cadherin expression is detected in virtually all cells of the posterior, intermediate, and anterior lobes. N-cadherin may function to initiate important cues such as controlling proliferation, directing cell placement, and promoting formation of cell networks that coordinately release hormones into the bloodstream. To address this, we generated mice lacking N-cadherin in proopiomelanocortin-expressing melanotrope and corticotrope cells of the intermediate and anterior lobes of the pituitary. We observed that intermediate lobe cells can aberrantly displace SOX2-containing progenitor cells in the N-cadherin conditional knockout mice at postnatal d 1. By postnatal d 30, although a reduction in α- and β-catenin membrane staining occurs, there is little effect on intermediate lobe architecture with N-cadherin loss. Also, despite these changes in adherens junction molecules, no alterations in cell proliferation occur. In contrast, loss of N-cadherin in the corticotropes leads to aberrant cell clustering and a reduction in Pomc mRNA. Taken together, our data reveal important roles of N-cadherin in pituitary cell placement and that loss of N-cadherin alone does not lead to pituitary tumor formation.

Investigating and modelling pituitary endocrine network function

Endocrine cells in the mammalian pituitary are arranged into three-dimensional homotypic networks that wire the gland and act to optimise hormone output by allowing the transmission of information between cell ensembles in a temporally precise manner. Despite this, the structure-function relationships that allow cells belonging to these networks to display coordinated activity remain relatively uncharacterised. This review discusses the recent technological advances that have allowed endocrine cell network structure and function to be probed and the mathematical models that can be used to analyse and present the resulting data. In particular, we focus on the mechanisms that allow endocrine cells to dynamically function as a population to drive hormone release as well as the experimental and theoretical methods that are used to track and model information flow through the network.

Pituitary growth hormone network responses are sexually dimorphic and regulated by gonadal steroids in adulthood

There are well-recognized sex differences in many pituitary endocrine axes, usually thought to be generated by gonadal steroid imprinting of the neuroendocrine hypothalamus. However, the recognition that growth hormone (GH) cells are arranged in functionally organized networks raises the possibility that the responses of the network are different in males and females. We studied this by directly monitoring the calcium responses to an identical GH-releasing hormone (GHRH) stimulus in populations of individual GH cells in slices taken from male and female murine GH-eGFP pituitary glands. We found that the GH cell network responses are sexually dimorphic, with a higher proportion of responding cells in males than in females, correlated with greater GH release from male slices. Repetitive waves of calcium spiking activity were triggered by GHRH in some males, but were never observed in females. This was not due to a permanent difference in the network architecture between male and female mice; rather, the sex difference in the proportions of GH cells responding to GHRH were switched by postpubertal gonadectomy and reversed with hormone replacements, suggesting that the network responses are dynamically regulated in adulthood by gonadal steroids. Thus, the pituitary gland contributes to the sexually dimorphic patterns of GH secretion that play an important role in differences in growth and metabolism between the sexes.

The 7th International Congress of Neuroendocrinology (ICN2010) 11-15 July 2010, Rouen, France

Neuroendocrinology has moved far beyond its initial focus on the regulation of pituitary hormone secretion. It now embraces not only the actions on the brain of a diverse range of 'new' hormones, such as leptin and ghrelin, but also the expanding roles of peptides as hormone-like messengers within the brain, controlling many fundamental behaviors and physiological processes. A recent International Congress of Neuroendocrinology highlighted the translational importance of some of these new insights.