Hes1 is required for pituitary growth and melanotrope specification

Rathke's pouch contains progenitor cells that differentiate into the endocrine cells of the pituitary gland. It gives rise to gonadotrope, thyrotrope, somatotrope, corticotrope and lactotrope cells in the anterior lobe and the intermediate lobe melanotropes. Pituitary precursor cells express many members of the Notch signaling pathway including the downstream effector gene Hes1. We hypothesized that Hes1 regulates the timing of precursor differentiation and cell fate determination. To test this idea, we expressed Hes1 in differentiating pituitary cells and found that it can inhibit gonadotrope and thyrotrope differentiation. Pituitaries of Hes1 deficient mice have anterior lobe hypoplasia. All cells in the anterior lobe are specified and differentiate, but an early period of increased cell death and reduced proliferation causes reduced growth, evident as early as e14.5. In addition, cells within the intermediate lobe differentiate into somatotropes instead of melanotropes. Thus, the Hes1 repressor is essential for melanotrope specification. These results demonstrate that Notch signaling plays multiple roles in pituitary development, influencing precursor number, organ size, cell differentiation and ultimately cell fate.     

Numb deletion in POMC-expressing cells impairs pituitary intermediate lobe cell adhesion, progenitor cell localization, and neuro-intermediate lobe boundary formation

The pituitary gland contains six distinct hormone-secreting cell types that are essential for basic physiological processes including fertility and responding to stress. Formation of hormone-secreting cells during development relies on Notch signaling to prevent progenitors from prematurely differentiating. The nature of the signal curtailing Notch signaling in the pituitary is unknown, but a good candidate is the endocytic adaptor protein NUMB. NUMB targets Notch for proteolytic degradation, but it also has a broad range of actions, including stabilizing adherens junctions through interactions with cadherins and influencing cell proliferation by stabilizing expression of the tumor suppressor protein p53. Here, we show that NUMB and its closely related homolog, NUMBLIKE, are expressed in undifferentiated cells during development and later in gonadotropes in the anterior lobe and melanotropes of the intermediate lobe. All four isoforms of NUMB, are detectable in the pituitary, with the shorter forms becoming more prominent after adolescence. Conditionally deleting Numb and Numblike in the intermediate lobe melanotropes with Pomc Cre mice dramatically alters the morphology of cells in the intermediate lobe, coincident with impaired localization of adherens junctions proteins including E-CADHERIN, N-CADHERIN, β-CATENIN, and α-CATENIN. Strikingly, the border between posterior and intermediate lobes is also disrupted. These mice also have disorganized progenitor cells, marked by SOX2, but proliferation is unaffected. Unexpectedly, Notch activity appears normal in conditional knockout mice. Thus, Numb is critical for maintaining cell-cell interactions in the pituitary intermediate lobe that are essential for proper cell placement.     

Premature differentiation and aberrant movement of pituitary cells lacking both Hes1 and Prop1

In the pituitary, the transition from proliferating progenitor cell into differentiated hormone producing cell is carefully regulated in a time-dependent and spatially-restricted manner. We report that two targets of Notch signaling, Hes1 and Prop1, are needed to maintain progenitors within Rathke's pouch and for the restriction of differentiated cells to the ventral pituitary. We observed ACTH and αGSU producing cells that had prematurely differentiated within Rathke's pouch along with correlated ectopic expression of Mash1 only when both Prop1 and Hes1 were lost. We also discovered that downregulation of N-cadherin expression in cells as they transition from Rathke's pouch to the anterior lobe appears to be essential for their movement. In the Prop1 mutant, cells are trapped in Rathke's pouch and N-cadherin expression remains high. Also, Slug, a marker of epithelial-to-mesenchymal transition, is absent in the dorsal anterior lobe. When Hes1 is lost in the Prop1 mutant, N-cadherin is downregulated and cells are able to exit Rathke's pouch but have lost their migrational cues and form ectopic foci surrounding Rathke's pouch. Our data reveal important overlapping functions of Hes1 and Prop1 in cell differentiation and movement that are critical for pituitary organogenesis.     

Hes1 and Hes3 regulate maintenance of the isthmic organizer and development of the mid/hindbrain

The isthmic organizer, which is located at the midbrain-hindbrain boundary, plays an essential role in development of the midbrain and anterior hindbrain. It has been shown that homeobox genes regulate establishment of the isthmic organizer, but the mechanism by which the organizer is maintained is not well understood. Here, we found that, in mice doubly mutant for the basic helix-loop-helix genes Hes1 and Hes3, the midbrain and anterior hindbrain structures are missing without any significant cell death. In these mutants, the isthmic organizer cells prematurely differentiate into neurons and terminate expression of secreting molecules such as Fgf8 and Wnt1 and the paired box genes Pax2/5, all of which are essential for the isthmic organizer function. These results indicate that Hes1 and Hes3 prevent premature differentiation and maintain the organizer activity of the isthmic cells, thereby regulating the development of the midbrain and anterior hindbrain.     

Immunohistochemical localization and comparison of carboxypeptidases D, E, and Z, alpha-MSH, ACTH, and MIB-1 between human anterior and corticotroph cell "basophil invasion" of the posterior pituitary.

Basophil invasion, i.e., invasion of basophilic corticotrophs from the residual intermediate lobe into the posterior lobe of the human pituitary gland, is believed to be a physiological phenomenon. This study evaluated the distribution of CPE, CPD, CPZ, alpha-MSH, ACTH, and Ki-67 immunoreactivity between human anterior pituitary and basophil invasion of the neurohypophysis. Mild to moderate immunoreactivities for CPE and CPZ were distributed relatively uniformly in the majority of the anterior pituitary cells and basophil invasion. In contrast, only corticotrophs exhibited intense CPD immunoreactivity. Basophil invasion showed similar immunoreactivities for alpha-MSH, ACTH, CPE, and CPZ as corticotrophs in the anterior pituitary, except for CPD, which was detected much less frequently. In the posterior lobe, CPE, CPD, and CPZ were present within the Herring bodies. Although no MIB-1 immunoreactivity was identified in anterior pituitary cells, limited MIB-1 labeling was detected in basophil invasion in five of ten cases. Highly selective expression of CPD in corticotrophs suggests that CPD plays a particularly important role in prohormone (POMC) processing in corticotrophs, with minimal or no significant roles in non-corticotrophs. Evidence that corticotrophs in basophil invasion are undergoing proliferation and are also phenotypically different from their counterpart in the anterior pituitary has further raised the possibility of some neoplastic potential.     

Hes1 and Hes5 regulate vascular remodeling and arterial specification of endothelial cells in brain vascular development

The vascular system is the first organ to form in the developing mammalian embryo. The Notch signaling pathway is an evolutionarily conserved signaling mechanism essential for proper embryonic development in almost all vertebrate organs. The analysis of targeted mouse mutants has demonstrated essential roles of the Notch signaling pathway in embryonic vascular development. However, Notch signaling-deficient mice have so far not been examined in detail in the head region. The bHLH genes Hes1 and Hes5 are essential effectors for Notch signaling, which regulate the maintenance of progenitor cells and the timing of their differentiation in various tissues and organs. Here, we report that endothelial-specific Hes1 and Hes5 mutant embryos exhibited defective vascular remodeling in the brain. In addition, arterial identity of endothelial cells was partially lost in the brain of these mutant mice. These data suggest that Hes1 and Hes5 regulate vascular remodeling and arterial fate specification of endothelial cells in the development of the brain. Hes1 and Hes5 represent critical transducers of Notch signals in brain vascular development.     

Involvement of pro-apoptotic and anti-apoptotic factors in the early development of the human pituitary gland

The spatial and temporal pattern of appearance of pro-apoptotic caspase-3 and p53 proteins, and anti-apoptotic bcl-2 protein was investigated in the developing pituitary gland of 6 human embryos 5-8-weeks old, using morphological and immunohistochemical techniques. Their dynamic appearance was analyzed in the Rathke's pouch (future adenohypophysis), mesenchyme, and in the developing neurohypophysis. In the 5th and 6th week, caspase-3 positive cells appeared in the Rathke's pouch (5%) and stalk (11%), in the mesenchyme, but not in the neurohypophysis. In the 6th and 7th week, apoptotic cells were more numerous in the caudal part of the Rathke's pouch due to its separation from the oral epithelium. Pro-apoptotic p53 protein was detected in all parts of the pituitary gland throughout the investigated period. Nuclear condensations characterized cells positive to caspase-3 and p53 proteins. Apoptotic cells displayed condensations of nuclear chromatin on an ultrastructural level as well. While caspase-3 dependent pathway of cell death participated in morphogenesis of the adenohypophysis and associated connective tissue, p53-mediated apoptosis most likely participates in morphogenesis of all parts of the gland, including neurohypophysis. The anti-apoptotic bcl-2 protein was also detected in all parts of the developing gland. With advancing development, the positivity to bcl-2 protein increased in the cells of the adenohypophysis, while it decreased in the neurohypophysis. Bcl-2 protein probably prevented cell death in all parts of the gland and enhanced cell differentiation. The described pattern of appearance of the investigated pro-apoptotic and anti-apoptotic factors might be important for normal morphogenesis and function of the pituitary gland.     

In situ hybridization analysis of the temporospatial expression of the midkine/pleiotrophin family in rat embryonic pituitary gland

Pituitary gland development is controlled by numerous signaling molecules, which are produced in the oral ectoderm and diencephalon. A newly described family of heparin-binding growth factors, namely midkine (MK)/pleiotrophin (PTN), is involved in regulating the growth and differentiation of many tissues and organs. Using in situ hybridization with digoxigenin-labeled cRNA probes, we detected cells expressing MK and PTN in the developing rat pituitary gland. At embryonic day 12.5 (E12.5), MK expression was localized in Rathke’s pouch (derived from the oral ectoderm) and in the neurohypophyseal bud (derived from the diencephalon). From E12.5 to E19.5, MK mRNA was expressed in the developing neurohypophysis, and expression gradually decreased in the developing adenohypophysis. To characterize MK-expressing cells, we performed double-staining of MK mRNA and anterior pituitary hormones. At E19.5, no MK-expressing cells were stained with any hormone. In contrast, PTN was expressed only in the neurohypophysis primordium during all embryonic stages. In situ hybridization clearly showed that MK was expressed in primitive (immature/undifferentiated) adenohypophyseal cells and neurohypophyseal cells, whereas PTN was expressed only in neurohypophyseal cells. Thus, MK and PTN might play roles as signaling molecules during pituitary development.     

Hes genes regulate size, shape and histogenesis of the nervous system by control of the timing of neural stem cell differentiation

Radial glial cells derive from neuroepithelial cells, and both cell types are identified as neural stem cells. Neural stem cells are known to change their competency over time during development: they initially undergo self-renewal only and then give rise to neurons first and glial cells later. Maintenance of neural stem cells until late stages is thus believed to be essential for generation of cells in correct numbers and diverse types, but little is known about how the timing of cell differentiation is regulated and how its deregulation influences brain organogenesis. Here, we report that inactivation of Hes1 and Hes5, known Notch effectors, and additional inactivation of Hes3 extensively accelerate cell differentiation and cause a wide range of defects in brain formation. In Hes-deficient embryos, initially formed neuroepithelial cells are not properly maintained, and radial glial cells are prematurely differentiated into neurons and depleted without generation of late-born cells. Furthermore, loss of radial glia disrupts the inner and outer barriers of the neural tube, disorganizing the histogenesis. In addition, the forebrain lacks the optic vesicles and the ganglionic eminences. Thus, Hes genes are essential for generation of brain structures of appropriate size, shape and cell arrangement by controlling the timing of cell differentiation. Our data also indicate that embryonic neural stem cells change their characters over time in the following order: Hes-independent neuroepithelial cells, transitory Hes-dependent neuroepithelial cells and Hes-dependent radial glial cells.     

Tissue specific antigen of the caudal lobe of the chicken adenohypophysis]

The tissue-specific water-soluble antigen of the chick pituitary gland was revealed by immunochemical analysis. The content of this antigen was found to be predominant in the caudal lobe of the adenohypophysis. The antigen was found from the 13th day of embryogenesis by immunoelectrophoresis and immunofluorescence. Two kinds of the antigen (with high and low relative electrophoretic mobility) were discovered in the chick adenohypophysis. A conclusion was drawn that the adenohypophysis cells differed by the differentiation level.     

Hes1 and Hes5 as notch effectors in mammalian neuronal differentiation

While the transmembrane protein Notch plays an important role in various aspects of development, and diseases including tumors and neurological disorders, the intracellular pathway of mammalian Notch remains very elusive. To understand the intracellular pathway of mammalian Notch, the role of the bHLH genes Hes1 and Hes5 (mammalian hairy and Enhancer-of-split homologues) was examined by retrovirally misexpressing the constitutively active form of Notch (caNotch) in neural precursor cells prepared from wild-type, Hes1-null, Hes5-null and Hes1-Hes5 double-null mouse embryos. We found that caNotch, which induced the endogenous Hes1 and Hes5 expression, inhibited neuronal differentiation in the wild-type, Hes1-null and Hes5-null background, but not in the Hes1-Hes5 double-null background. These results demonstrate that Hes1 and Hes5 are essential Notch effectors in regulation of mammalian neuronal differentiation.     

Hes1 regulates formations of the hypophyseal pars tuberalis and the hypothalamus

The hypophyseal pars tuberalis surrounds the median eminence and infundibular stalk of the hypothalamus as thin layers of cells. The pars tuberalis expresses MT1 melatonin receptor and participates in mediating the photoperiodic secretion of pituitary hormones. Both the rostral tip of Rathke’s pouch (pars tuberalis primordium) and the pars tuberalis expressed αGSU mRNA, and were immunoreactive for LH, chromogranin A, and TSHβ in mice. Hes genes control progenitor cell differentiation in many embryonic tissues and play a crucial role for neurulation in the central nervous system. We investigated the Hes1 function in outgrowth and differentiation of the pars tuberalis by using the markers for the pars tuberalis. In homozygous Hes1 null mutant embryos, the rostral tip was formed in the basal-ventral part of Rathke’s pouch at embryonic day (E)11.5 as well as in wild-type embryos. In contrast to the wild-type, the rostral tip of null mutants could not extend rostrally with age; it remained in the low extremity of Rathke’s pouch during E12.5–E13.5 and disappeared at E14.5, resulting in lack of the pars tuberalis. Development of the ventral diencephalon was impaired in the null mutants at early stages. Rathke’s pouch, therefore, could not link with the nervous tissue and failed to receive inductive signals from the diencephalon. In a very few mutant mice in which the ventral diencephalon was partially sustained, some pars tuberalis cells were distributed around the hypoplastic infundibulum. Thus, Hes1 is required for development of the pars tuberalis and its growth is dependent on the ventral diencephalon.     

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.