Nathalie C. Guérineau

Folliculostellate cell network: A route for long- distance communication in the anterior pituitary

All higher life forms critically depend on hormones being rhythmically released by the anterior pituitary. The proper functioning of this master gland is dynamically controlled by a complex set of regulatory mechanisms that ultimately determine the fine tuning of the excitable endocrine cells, all of them heterogeneously distributed throughout the gland. Here, we provide evidence for an intrapituitary communication system by which information is transferred via the network of nonendocrine folliculostellate (FS) cells. Local electrical stimulation of FS cells in acute pituitary slices triggered cytosolic calcium waves, which propagated to other FS cells by signaling through gap junctions. Calcium wave initiation was because of the membrane excitability of FS cells, hitherto classified as silent cells. FS cell coupling could relay information between opposite regions of the gland. Because FS cells respond to central and peripheral stimuli and dialogue with endocrine cells, the form of large-scale intrapituitary communication described here may provide an efficient mechanism that orchestrates anterior pituitary functioning in response to physiological needs.

Selenoprotein T is a PACAP-regulated gene involved in intracellular Ca2+ mobilization and neuroendocrine secretion

Selenoproteins contain the essential trace element selenium, the deficiency of which is associated with cancer or accelerated aging. Although selenoproteins are thought to be instrumental for the effects of selenium, the biological function of many of these proteins remains unknown. Here, we studied the role of selenoprotein T (SelT), a selenocysteine (Sec) ‐containing protein with no known function, which we have identified as a novel target gene of the neuropeptide pituitary adenylate cyclase‐activating polypeptide (PACAP) during PC12 cell differentiation. SelT was found to be ubiquitously expressed throughout embryonic development and in adulthood in rat. Immunocytochemical analysis revealed that SelT is mainly localized to the endoplasmic reticulum through a hydrophobic domain. PACAP and cAMP induced a rapid and long‐lasting increase in SelT gene expression in PC12 cells, in a Ca2+‐dependent manner. These results suggested a possible role of SelT in PACAP signaling during PC12 cell differentiation. Indeed, overexpression of SelT in PC12 cells provoked an increase in the concentration of intracellular Ca2+ ([Ca2+]i) that was dependent on the Sec residue. Conversely, SelT gene knockdown inhibited the PACAP‐induced increase in [Ca2+]i and reduced hormone secretion. These findings demonstrate the implication of a selenoprotein in the regulation of Ca2+ homeostasis and neuroendocrine secretion in response to a cAMP‐stimulating trophic factor.— Grumolato, L., Ghzili, H., Montero‐Hadjadje, M., Gasman, S., Lesage, J., Tanguy, Y., Galas, L., Ait‐Ali, D., Leprince, J., Guerineau, N. C., Elkahloun, A. G., Fournier, A., Vieau, D., Vaudry, H., Anouar, Y. Selenoprotein T is a PACAP‐regulated gene involved in intracellular Ca2+ mobilization and neuroendocrine secretion. FASEB J. 22, 1756–1768 (2008)

Exchange protein activated by cAMP (Epac) mediates cAMP activation of p38 MAPK and modulation of Ca2+-dependent K+ channels in cerebellar neurons.

The exchange factor directly activated by cAMP (Epac) is a newly discovered direct target for cAMP and a guanine-nucleotide exchange factor for the small GTPase Rap. Little is known about the neuronal functions of Epac. Here we show that activation of Epac by specific cAMP analogs or by the pituitary adenylate cyclase-activating polypeptide induces a potent activation of the Ca2+-sensitive big K+ channel, slight membrane hyperpolarization, and increased after-hyperpolarization in cultured cerebellar granule cells. These effects involve activation of Rap and p38 MAPK, which mobilizes intracellular Ca2+ stores. These findings reveal a cAMP Epac-dependent and protein kinase A-independent signaling cascade that controls neuronal excitability.

Ghrelin Stimulation of Growth Hormone-Releasing Hormone Neurons Is Direct in the Arcuate Nucleus

Background Ghrelin targets the arcuate nucleus, from where growth hormone releasing hormone (GHRH) neurones trigger GH secretion. This hypothalamic nucleus also contains neuropeptide Y (NPY) neurons which play a master role in the effect of ghrelin on feeding. Interestingly, connections between NPY and GHRH neurons have been reported, leading to the hypothesis that the GH axis and the feeding circuits might be co-regulated by ghrelin. Principal Findings Here, we show that ghrelin stimulates the firing rate of identified GHRH neurons, in transgenic GHRH-GFP mice. This stimulation is prevented by growth hormone secretagogue receptor-1 antagonism as well as by U-73122, a phospholipase C inhibitor and by calcium channels blockers. The effect of ghrelin does not require synaptic transmission, as it is not antagonized by γ-aminobutyric acid, glutamate and NPY receptor antagonists. In addition, this hypothalamic effect of ghrelin is independent of somatostatin, the inhibitor of the GH axis, since it is also found in somatostatin knockout mice. Indeed, ghrelin does not modify synaptic currents of GHRH neurons. However, ghrelin exerts a strong and direct depolarizing effect on GHRH neurons, which supports their increased firing rate. Conclusion Thus, GHRH neurons are a specific target for ghrelin within the brain, and not activated secondary to altered activity in feeding circuits. These results support the view that ghrelin related therapeutic approaches could be directed separately towards GH deficiency or feeding disorders.

Emerging role of G protein-coupled receptors in microvascular myogenic tone

Blood flow autoregulation results from the ability of resistance arteries to reduce or increase their diameters in response to changes in intravascular pressure. The mechanism by which arteries maintain a constant blood flow to organs over a range of pressures relies on this myogenic response, which defines the intrinsic property of the smooth muscle to contract in response to stretch. The resistance to flow created by myogenic tone (MT) prevents tissue damage and allows the maintenance of a constant perfusion, despite fluctuations in arterial pressure. Interventions targeting MT may provide a more rational therapeutic approach in vascular disorders, such as hypertension, vasospasm, chronic heart failure, or diabetes. Despite its early description by Bayliss in 1902, the cellular and molecular mechanisms underlying MT remain poorly understood. We now appreciate that MT requires a complex mechanotransduction converting a physical stimulus (pressure) into a biological response (change in vessel diameter). Although smooth muscle cell depolarization and a rise in intracellular calcium concentration are recognized as cornerstones of the myogenic response, the role of wall strain-induced formation of vasoactive mediators is less well established. The vascular system expresses a large variety of Class 1 G protein-coupled receptors (GPCR) activated by an eclectic range of chemical entities, including peptides, lipids, nucleotides, and amines. These messengers can function in blood vessels as vasoconstrictors. This review focuses on locally generated GPCR agonists and their proposed contributions to MT. Their interplay with pivotal G(q-11) and G(12-13) protein signalling is also discussed.

Functional Remodeling of Gap Junction-Mediated Electrical Communication between Adrenal Chromaffin Cells in Stressed Rats

An increase in circulating catecholamine levels represents one of the mechanisms whereby organisms cope with stress. In the periphery, catecholamines mainly originate from the sympathoadrenal system. As we reported, in addition to the central control through cholinergic innervation, a local gap junction-delineated route between adrenal chromaffin cells contributes to catecholamine exocytosis. Here, we investigated whether this intercellular communication is modified when the hormonal demand is increased as observed during cold stress. Our results show that in cold exposed rats, gap-junctional communication undergoes a functional plasticity, as evidenced by an increased number of dye-coupled cells. Of a physiological interest is that this upregulation of gap-junctional coupling results in the appearance of a robust electrical coupling between chromaffin cells that allows the transmission of action potentials between coupled cells. This enhancement of gap-junctional communication parallels an increase in expression levels of connexin36 (Cx36) and connexin43 (Cx43) proteins. Both transcriptional and posttranslational mechanisms are involved because Cx36 transcripts are increased in stressed rats and the expression of the scaffolding protein zonula occludens-1, known to interact with both Cx36 and Cx43, is also upregulated. Consistent with an upregulated coupling extent in stressed rats, the cytosolic Ca2+ concentration rises triggered in a single cell by an iontophoretic application of nicotine occur simultaneously in several neighboring cells. These results describe for the first time a functional plasticity of junctional coupling between adult chromaffin cells that should be crucial for adaptation to stress or sensitization to subsequent stressors.

Gap junction signalling is a stress-regulated component of adrenal neuroendocrine stimulus-secretion coupling in vivo

Elucidating the mechanisms whereby neuroendocrine tissues coordinate their input and output signals to ensure appropriate hormone secretion is currently a topical issue. In particular, whether a direct communication mediated by gap junctions between neurosecretory cells contributes to hormone release in vivo still remains unknown. Here we address this issue using a microsurgical approach allowing combined monitoring of adrenal catecholamine secretion and splanchnic nerve stimulation in anaesthetised mice. Pharmacological blockade of adrenal gap junctions by the uncoupling agent carbenoxolone reduces nerve stimulation-evoked catecholamine release in control mice and to a larger extent in stressed mice. In parallel, the gap junction-coupled cell network is extended in stressed mice. Altogether, this argues for a significant contribution of adrenomedullary gap junctions to catecholamine secretion in vivo. As such, gap junctional signalling appears to be a substantial component for neuroendocrine function in the adrenal medulla, as it may represent an additional lever regulating hormone release.

Revisiting the Stimulus-Secretion Coupling in the Adrenal Medulla: Role of Gap Junction-Mediated Intercellular Communication

The current view of stimulation-secretion coupling in adrenal neuroendocrine chromaffin cells holds that catecholamines are released upon transsynaptic sympathetic stimulation mediated by acetylcholine released from the splanchnic nerve terminals. However, this traditional vertical scheme would merit to be revisited in the light of recent data. Although electrical discharges invading the splanchnic nerve endings are the major physiological stimulus to trigger catecholamine release in vivo, growing evidence indicates that intercellular chromaffin cell communication mediated by gap junctions represents an additional route by which biological signals (electrical activity, changes in intracellular Ca2+ concentration,…) propagate between adjacent cells and trigger subsequent catecholamine exocytosis. Accordingly, it has been proposed that gap junctional communication efficiently helps synapses to lead chromaffin cell function and, in particular, hormone secretion. The experimental clues supporting this hypothesis are presented and discussed with regards to both interaction with the excitatory cholinergic synaptic transmission and physiopathology of the adrenal medulla.

Regulator of G protein signaling-4 controls fatty acid and glucose homeostasis.

Circulating free fatty acids are a reflection of the balance between lipogenesis and lipolysis that takes place mainly in adipose tissue. We found that mice deficient for regulator of G protein signaling (RGS)-4 have increased circulating catecholamines, and increased free fatty acids. Consequently, RGS4-/- mice have increased concentration of circulating free fatty acids; abnormally accumulate fatty acids in liver, resulting in liver steatosis; and show a higher degree of glucose intolerance and decreased insulin secretion in pancreas. We show in this study that RGS4 controls adipose tissue lipolysis through regulation of the secretion of catecholamines by adrenal glands. RGS4 controls the balance between adipose tissue lipolysis and lipogenesis, secondary to its role in the regulation of catecholamine secretion by adrenal glands. RGS4 therefore could be a good target for the treatment of metabolic diseases.

Gap junction-mediated intercellular communication in the adrenal medulla: an additional ingredient of stimulus-secretion coupling regulation.

The traditional understanding of stimulus-secretion coupling in adrenal neuroendocrine chromaffin cells states that catecholamines are released upon trans-synaptic sympathetic stimulation mediated by acetylcholine released from the splanchnic nerve terminals. Although this statement remains largely true, it deserves to be tempered. In addition to its neurogenic control, catecholamine secretion also depends on a local gap junction-mediated communication between chromaffin cells. We review here the insights gained since the first description of gap junctions in the adrenal medullary tissue. Adrenal stimulus-secretion coupling now appears far more intricate than was previously envisioned and its deciphering represents a challenge for neurobiologists engaged in the study of the regulation of neuroendocrine secretion. This article is part of a Special Issue entitled: The Communicating junctions, composition, structure and characteristics.