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Title:The role of notch signaling in the development of the hypothalamic-pituitary axis and arcuate nucleus
Author(s):Aujla, Paven
Director of Research:Raetzman, Lori T.
Doctoral Committee Chair(s):Raetzman, Lori T.
Doctoral Committee Member(s):Mahoney, Megan M.; Stubbs, Lisa J.; Sweedler, Jonathan V.
Department / Program:School of Molecular & Cell Bio
Degree Granting Institution:University of Illinois at Urbana-Champaign
Notch signaling
Abstract:Development of the hypothalamic-pituitary axis requires precise neuronal signaling to establish a network that regulates neuroendocrine homeostasis. The anterior lobe (AL) and intermediate lobe (IL) of the pituitary are formed from the primordial structure Rathke’s pouch, while the posterior lobe (PL) of the pituitary arises from an evagination of the developing hypothalamus. The PL contains terminal axons of arginine vasopressin (AVP) neurons from the hypothalamic paraventricular nucleus (PVN) and supraoptic nucleus (SON). As the pituitary develops, signals from the hypothalamus are necessary for pituitary induction and expansion. Little is known about the control of cues that regulate early embryonic signaling between the two structures to direct their cell specification. However, we do know that ligands and receptors of the Notch signaling pathway, known to control cell specification in other contexts, are found both in the hypothalamus and Rathke’s pouch. Additionally, the Notch signaling effector gene Hes1 is present in the developing hypothalamus and pituitary and is required for proper formation of the pituitary. Therefore, we hypothesized that Hes1 is necessary for the generation, placement and projection of AVP neurons. To test this hypothesis, we analyzed Hes1 null mice and found that by embryonic day 16.5 (e16.5) and at e18.5, AVP cell bodies are formed in the SON and PVN, but are abnormally placed, suggesting that Hes1 may be necessary for the migration of AVP neurons. Additionally, at e18.5 Hes1 null mice exhibit abnormal AVP axonal projection from the SON and PVN to the PL. Using mass spectrometry to characterize peptide content, we found that Hes1 null pituitaries have aberrant somatostatin (SS) peptide, which correlates with abnormal SS cells in the pituitary and misplaced SS axon tracts at e18.5. Our results indicate that Hes1 facilitates the migration of hypothalamic neurons, as well as neuropeptide content within the pituitary. The effects observed with global Hes1 loss could be due to the action of Hes1 in the hypothalamus, Rathke’s pouch or both. To determine the contribution of hypothalamic Notch signaling to pituitary organogenesis, we used mice with loss and gain of Notch function specifically within the developing hypothalamus in cells that express Nkx2.1. We demonstrate that loss of Notch signaling by conditional deletion of an important Notch co-factor, Rbpjκ, in the hypothalamus (Rbpjκ cKO) does not affect expression of Hes1 within the posterior hypothalamus, a region known to influence pituitary development, or expression of the Notch effector gene Hes5. In contrast, mice with expression of activated Notch1 intracellular domain (NICD) within the hypothalamus (NICD Tg) display ectopic Hes5 expression and increased Hes1 expression in the posterior hypothalamus, which is sufficient to cause loss of IL and PL structures, and disrupt postnatal pituitary expansion. Additionally, we show that persistent hypothalamic Notch activation results in reduced expression of hypothalamic transcription factors and morphogens crucial to pituitary development, such as OTX2 and Fgf10, coincident with reduced survival of pituitary progenitors. Taken together, our results indicate that Rbpjκ- dependent Notch signaling within the developing hypothalamus is not necessary for pituitary development, but persistent Notch signaling in hypothalamic progenitors affects pituitary induction and expansion. We also observed that Rbpjκ cKO and NICD Tg mice demonstrate significant changes in the development of the hypothalamic arcuate nucleus (Arc), found within the anterior hypothalamus. The Arc contains proopoiomelanocortin (POMC), neuropeptide Y (NPY) and growth hormone releasing hormone (GHRH) neurons, which regulate feeding, energy balance and body size. Developmental disruption of this homeostatic mediator underlies diseases ranging from growth failure to obesity. Despite considerable investigation regarding function of Arc neurons, mechanisms governing their development remain unclear. We found that loss of Rbpjκ results in absence of Hes1 but not Hes5 within the primordial Arc at e13.5. At e18.5, Rbpjκ cKO mice have few progenitors and increased numbers of differentiated Pomc, NPY and Ghrh neurons. In contrast, NICD Tg mice have increased hypothalamic progenitors and absence of differentiated Arc neurons. Subsequently, both Rbpjκ cKO and NICD Tg mice have changes in growth and body size during postnatal development. Our results demonstrate that Notch/Rbpjκ signaling regulates the generation and differentiation of Arc neurons, which contribute to homeostatic regulation of body size.
Issue Date:2014-05-30
Rights Information:Copyright 2014 Paven Aujla
Date Available in IDEALS:2014-05-30
Date Deposited:2014-05

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