Inhibition of Sox2-dependent activation of Shh in the ventral diencephalon by Tbx3 is required for formation of the neurohypophysis

Control of tuberal hypothalamic development and its implications in metabolic disorders

The tuberal hypothalamus regulates a range of crucial physiological processes, including energy homeostasis and metabolism. In this Review, we explore the intricate molecular mechanisms and signalling pathways that control the development of the tuberal hypothalamus, focusing on aspects that shape metabolic outcomes. Major developmental events are discussed in the context of their effect on the establishment of both functional hypothalamic neuronal circuits and brain-body interfaces that are pivotal to the control of metabolism. Emerging evidence indicates that aberrations in molecular pathways during tuberal hypothalamic development contribute to metabolic dysregulation. Understanding the molecular underpinnings of tuberal hypothalamic development provides a comprehensive view of neurodevelopmental processes and offers a promising avenue for future targeted interventions to prevent and treat metabolic disorders.

Dissecting SOX2 expression and function reveals an association with multiple signaling pathways during embryonic development and in cancer progression

SRY (Sex Determining Region) box 2 (SOX2) is an essential transcription factor that plays crucial roles in activating genes involved in pre- and post-embryonic development, adult tissue homeostasis, and lineage specifications. SOX2 maintains the self-renewal property of stem cells and is involved in the generation of induced pluripotency stem cells. SOX2 protein contains a particular high-mobility group domain that enables SOX2 to achieve the capacity to participate in a broad variety of functions. The information about the involvement of SOX2 with gene regulatory elements, signaling networks, and microRNA is gradually emerging, and the higher expression of SOX2 is functionally relevant to various cancer types. SOX2 facilitates the oncogenic phenotype via cellular proliferation and enhancement of invasive tumor properties. Evidence are accumulating in favor of three dimensional (higher order) folding of chromatin and epigenetic control of the SOX2 gene by chromatin modifications, which implies that the expression level of SOX2 can be modulated by epigenetic regulatory mechanisms, specifically, via DNA methylation and histone H3 modification. In view of this, and to focus further insights into the roles SOX2 plays in physiological functions, involvement of SOX2 during development, precisely, the advances of our knowledge in pre- and post-embryonic development, and interactions of SOX2 in this scenario with various signaling pathways in tumor development and cancer progression, its potential as a therapeutic target against many cancers are summarized and discussed in this article.

Proteostatic reactivation of the developmental transcription factor TBX3 drives BRAF/MAPK-mediated tumorigenesis

MAPK pathway-driven tumorigenesis, often induced by BRAFV600E, relies on epithelial dedifferentiation. However, how lineage differentiation events are reprogrammed remains unexplored. Here, we demonstrate that proteostatic reactivation of developmental factor, TBX3, accounts for BRAF/MAPK-mediated dedifferentiation and tumorigenesis. During embryonic development, BRAF/MAPK upregulates USP15 to stabilize TBX3, which orchestrates organogenesis by restraining differentiation. The USP15-TBX3 axis is reactivated during tumorigenesis, and Usp15 knockout prohibits BRAFV600E-driven tumor development in a Tbx3-dependent manner. Deleting Tbx3 or Usp15 leads to tumor redifferentiation, which parallels their overdifferentiation tendency during development, exemplified by disrupted thyroid folliculogenesis and elevated differentiation factors such as Tpo, Nis, Tg. The clinical relevance is highlighted in that both USP15 and TBX3 highly correlates with BRAFV600E signature and poor tumor prognosis. Thus, USP15 stabilized TBX3 represents a critical proteostatic mechanism downstream of BRAF/MAPK-directed developmental homeostasis and pathological transformation, supporting that tumorigenesis largely relies on epithelial dedifferentiation achieved via embryonic regulatory program reinitiation.

The Fuzzy planar cell polarity protein (FUZ), necessary for primary cilium formation, is essential for pituitary development

The primary cilium is an essential organelle that is important for normal cell signalling during development and homeostasis but its role in pituitary development has not been reported. The primary cilium facilitates signal transduction for multiple pathways, the best-characterised being the SHH pathway, which is known to be necessary for correct pituitary gland development. FUZ is a planar cell polarity (PCP) effector that is essential for normal ciliogenesis, where the primary cilia of Fuz-/- mutants are shorter or non-functional. FUZ is part of a group of proteins required for recruiting retrograde intraflagellar transport proteins to the base of the organelle. Previous work has reported ciliopathy phenotypes in Fuz-/- homozygous null mouse mutants, including neural tube defects, craniofacial abnormalities, and polydactyly, alongside PCP defects including kinked/curly tails and heart defects. Interestingly, the pituitary gland was reported to be missing in Fuz-/- mutants at 14.5 dpc but the mechanisms underlying this phenotype were not investigated. Here, we have analysed the pituitary development of Fuz-/- mutants. Histological analyses reveal that Rathke's pouch (RP) is initially induced normally but is not specified and fails to express LHX3, resulting in hypoplasia and apoptosis. Characterisation of SHH signalling reveals reduced pathway activation in Fuz-/- mutant relative to control embryos, leading to deficient specification of anterior pituitary fate. Analyses of the key developmental signals FGF8 and BMP4, which are influenced by SHH, reveal abnormal patterning in the ventral diencephalon, contributing further to abnormal RP development. Taken together, our analyses suggest that primary cilia are required for normal pituitary specification through SHH signalling.

DNA Conserved in Diverse Animals Since the Precambrian Controls Genes for Embryonic Development

DNA that controls gene expression (e.g. enhancers, promoters) has seemed almost never to be conserved between distantly related animals, like vertebrates and arthropods. This is mysterious, because development of such animals is partly organized by homologous genes with similar complex expression patterns, termed "deep homology." Here, we report 25 regulatory DNA segments conserved across bilaterian animals, of which 7 are also conserved in cnidaria (coral and sea anemone). They control developmental genes (e.g. Nr2f, Ptch, Rfx1/3, Sall, Smad6, Sp5, Tbx2/3), including six homeobox genes: Gsx, Hmx, Meis, Msx, Six1/2, and Zfhx3/4. The segments contain perfectly or near-perfectly conserved CCAAT boxes, E-boxes, and other sequences recognized by regulatory proteins. More such DNA conservation will surely be found soon, as more genomes are published and sequence comparison is optimized. This reveals a control system for animal development conserved since the Precambrian.

Tbx2 knockdown alleviated sevoflurane-induced cognitive disorder and neuron damages in aged rats via suppressing oxidative stress and ferroptosis

Anesthesia with sevoflurane contributes to perioperative neurocognitive disorder (PND), which is characterized by the deficiency in study and memory. T-Box transcription factor 2 (Tbx2), which is involved in the development of hippocampus neurons, was upregulated in the hippocampus of rats exposed to sevoflurane. Our study aimed to explore the role of Tbx2 in sevoflurane-induced cognitive disorder and hippocampus neuron damages. The expression of Tbx2 in hippocampus was upregulated after sevoflurane exposure, which was accompanied by the accumulation of reactive oxygen species and lipid peroxidation, as well as the loss of neurons in hippocampus. In vitro, silencing Tbx2 suppressed oxidative stress and ferroptosis induced by sevoflurane, whereas exogenous overexpression of Tbx2 exacerbated these processes. Importantly, Tbx2 knockdown improved sevoflurane-induced cognitive disorder in aged rats, as evidenced by the increases in behavioral indexes. Mechanistically, the expression of brain-derived neurotrophic factor (BDNF), as well as the downstream nuclear factor erythroid 2-related factor 2/heme oxygenase 1 (Nrf2/HO-1) signaling, was repressed by Tbx2. Mimicking the activation of BDNF with 7,8-dihydroxyflavone rescued the effects of Tbx2 overexpression on oxidative stress and ferroptosis in vitro, indicating that the BDNF/Nrf2/HO-1 signaling may mediate the role of Tbx2 in sevoflurane-induced cognitive disorder and neuron damages. In summary, Tbx2 may contribute to neuronal damages via enhancing the oxidative stress and ferroptosis caused by sevoflurane. BDNF/Nrf2/HO-1 signaling mediates the role of Tbx2 in sevoflurane-induced cognitive disorder. Knockdown of Tbx2 improves sevoflurane-induced cognitive impairment. Our finding provides a novel insight for PND treatment.

PSmad3+/Olig2- expression defines a subpopulation of gfap-GFP+/Sox9+ neural progenitors and radial glia-like cells in mouse dentate gyrus through embryonic and postnatal development

In mouse dentate gyrus, radial glia-like cells (RGLs) persist throughout life and play a critical role in the generation of granule neurons. A large body of evidence has shown that the combinatorial expression of transcription factors (TFs) defines cell types in the developing central nervous system (CNS). As yet, the identification of specific TFs that exclusively define RGLs in the developing mouse dentate gyrus (DG) remains elusive. Here we show that phospho-Smad3 (PSmad3) is expressed in a subpopulation of neural progenitors in the DG. During embryonic stage (E14-15), PSmad3 was predominantly expressed in gfap-GFP-positive (GFP+)/Sox2+ progenitors located at the lower dentate notch (LDN). As the development proceeds (E16-17), the vast majority of PSmad3+ cells were GFP+/Sox2+/Prox1low+/Ki67+ proliferative progenitors that eventually differentiated into granule neurons. During postnatal stage (P1-P6) PSmad3 expression was observed in GFP+ progenitors and astrocytes. Subsequently, at P14-P60, PSmad3 expression was found both in GFP+ RGLs in the subgranular zone (SGZ) and astrocytes in the molecular layer (ML) and hilus. Notably, PSmad3+ SGZ cells did not express proliferation markers such as PCNA and phospho-vimentin, suggesting that they are predominantly quiescent from P14 onwards. Significantly PSmad3+/GFP+ astrocytes, but not SGZ cells, co-expressed Olig2 and S100β. Together, PSmad3+/Olig2- expression serves as an exclusive marker for a specific subpopulation of GFP+ neural progenitors and RGLs in the mouse DG during both embryonic and postnatal period.

Neural Progenitor Cells and the Hypothalamus

Neural progenitor cells (NPCs) are multipotent neural stem cells (NSCs) capable of self-renewing and differentiating into neurons, astrocytes and oligodendrocytes. In the postnatal/adult brain, NPCs are primarily located in the subventricular zone (SVZ) of the lateral ventricles (LVs) and subgranular zone (SGZ) of the hippocampal dentate gyrus (DG). There is evidence that NPCs are also present in the postnatal/adult hypothalamus, a highly conserved brain region involved in the regulation of core homeostatic processes, such as feeding, metabolism, reproduction, neuroendocrine integration and autonomic output. In the rodent postnatal/adult hypothalamus, NPCs mainly comprise different subtypes of tanycytes lining the wall of the 3^rd ventricle. In the postnatal/adult human hypothalamus, the neurogenic niche is constituted by tanycytes at the floor of the 3^rd ventricle, ependymal cells and ribbon cells (showing a gap-and-ribbon organization similar to that in the SVZ), as well as suprachiasmatic cells. We speculate that in the postnatal/adult human hypothalamus, neurogenesis occurs in a highly complex, exquisitely sophisticated neurogenic niche consisting of at least four subniches; this structure has a key role in the regulation of extrahypothalamic neurogenesis, and hypothalamic and extrahypothalamic neural circuits, partly through the release of neurotransmitters, neuropeptides, extracellular vesicles (EVs) and non-coding RNAs (ncRNAs).

The hidden hedgehog of the pituitary: hedgehog signaling in development, adulthood and disease of the hypothalamic-pituitary axis

Hedgehog signaling plays pivotal roles in embryonic development, adult homeostasis and tumorigenesis. However, its engagement in the pituitary gland has been long underestimated although Hedgehog signaling and pituitary embryogenic development are closely linked. Thus, deregulation of this signaling pathway during pituitary development results in malformation of the gland. Research of the last years further implicates a regulatory role of Hedgehog signaling in the function of the adult pituitary, because its activity is also interlinked with homeostasis, hormone production, and most likely also formation of neoplasms of the gland. The fact that this pathway can be efficiently targeted by validated therapeutic strategies makes it a promising candidate for treating pituitary diseases. We here summarize the current knowledge about the importance of Hedgehog signaling during pituitary development and review recent data that highlight the impact of Hedgehog signaling in the healthy and the diseased adult pituitary gland.

Heterozygous variants in SIX3 and POU1F1 cause pituitary hormone deficiency in mouse and man

Congenital hypopituitarism is a genetically heterogeneous condition that is part of a spectrum disorder that can include holoprosencephaly. Heterozygous mutations in SIX3 cause variable holoprosencephaly in humans and mice. We identified two children with neonatal hypopituitarism and thin pituitary stalk who were doubly heterozygous for rare, likely deleterious variants in the transcription factors SIX3 and POU1F1. We used genetically engineered mice to understand the disease pathophysiology. Pou1f1 loss-of-function heterozygotes are unaffected; Six3 heterozygotes have pituitary gland dysmorphology and incompletely ossified palate; and the Six3+/-; Pou1f1+/dw double heterozygote mice have a pronounced phenotype, including pituitary growth through the palate. The interaction of Pou1f1 and Six3 in mice supports the possibility of digenic pituitary disease in children. Disruption of Six3 expression in the oral ectoderm completely ablated anterior pituitary development, and deletion of Six3 in the neural ectoderm blocked the development of the pituitary stalk and both anterior and posterior pituitary lobes. Six3 is required in both oral and neural ectodermal tissues for the activation of signaling pathways and transcription factors necessary for pituitary cell fate. These studies clarify the mechanism of SIX3 action in pituitary development and provide support for a digenic basis for hypopituitarism.

Populational heterogeneity and partial migratory origin of the ventromedial hypothalamic nucleus: genoarchitectonic analysis in the mouse

The ventromedial hypothalamic nucleus (VMH) is one of the most distinctive hypothalamic tuberal structures, subject of numerous classic and modern functional studies. Commonly, the adult VMH has been divided in several portions, attending to differences in cell aggregation, cell type, connectivity, and function. Consensus VMH partitions in the literature comprise the dorsomedial (VMHdm), and ventrolateral (VMHvl) subnuclei, which are separated by an intermediate or central (VMHc) population (topographic names based on the columnar axis). However, some recent transcriptome analyses have identified a higher number of different cell types in the VMH, suggesting additional subdivisions, as well as the possibility of separate origins. We offer a topologic and genoarchitectonic developmental study of the mouse VMH complex using the prosomeric axis as a reference. We analyzed genes labeling specific VMH subpopulations, with particular focus upon the Nkx2.2 transcription factor, a marker of the alar-basal boundary territory of the prosencephalon, from where some cells seem to migrate dorsoventrally into VMH. We also identified separate neuroepithelial origins of a Nr2f1-positive subpopulation, and a new Six3-positive component, as well as subtle differences in origin of Nr5a1 positive versus Nkx2.2-positive cell populations entering dorsoventrally the VMH. Several of these migrating cell types are born in the dorsal tuberal domain and translocate ventralwards to reach the intermediate tuberal domain, where the adult VMH mass is located in the adult. This work provides a more detailed area map on the intrinsic organization of the postmigratory VMH complex, helpful for deeper functional studies of this basal hypothalamic entity.

Literature review, report, and analysis of genotype and clinical phenotype of a rare case of ulnar-mammary syndrome

Objective

The clinical characteristics of Ulnar-mammary syndrome (UMS) caused by mutations in TBX3 (T-Box transcription factor 3) were studied and the correlation between genotype and clinical phenotype were analyzed to improve awareness and early diagnosis of the disease.

Methods

The clinical data of a boy aged 13 years and 5 months with left forearm deformity and growth retardation as the main features were analyzed. Genomic exon detection was performed, and the results were verified by Sanger sequencing. Simultaneously, we performed literature review to analyze the correlation between clinical phenotypes and genotypes.

Results

The clinical manifestations in the child were short stature, ulnar hypoplasia of the forearm, hypohidrosis, retracted nipple, micropenis, and cryptorchidism. Laboratory examination revealed hyperthyroidism, growth hormone deficiency, and hypogonadotropic hypogonadism. Imaging results displayed delayed bone age, small pituitary gland, and persistence of Rathke's cleft cyst. The results of the exome sequencing revealed the deletion of AGA at positions 1121-1,124 of TBX3, which resulted in a frameshift mutation (c.1121-1124del AGAG; pGlu374fs). According to the American College of Medical Genetics (ACMG) assessment, the mutation is a pathogenic variant. A definitive diagnosis of UMS was made on the basis of the clinical phenotype of the patient. The Chinese and English literature were reviewed to analyze the correlation between TBX3 genotype and clinical phenotype.

Conclusion

UMS is a rare hereditary disease caused by mutations in TBX3. There is significant clinical heterogeneity associated with the variants of this gene. To our knowledge, this mutation site in TBX3 has been reported for the first time, thereby expanding the mutation spectrum of this gene.

Targeting myeloid derived suppressor cells reverts immune suppression and sensitizes BRAF-mutant papillary thyroid cancer to MAPK inhibitors

MAPK signaling inhibitor (MAPKi) therapies show limited efficacy for advanced thyroid cancers despite constitutive activation of the signaling correlates with disease recurrence and persistence. Understanding how BRAF pathway stimulates tumorigenesis could lead to new therapeutic targets. Here, through genetic and pathological approaches, we demonstrate that BRAF^V600E promotes thyroid cancer development by increasing myeloid-derived suppressor cells (MDSCs) penetrance. This BRAF^V600E-induced immune suppression involves re-activation of the developmental factor TBX3, which in turn up-regulates CXCR2 ligands in a TLR2-NFκB dependent manner, leading to MDSCs recruitment into the tumor microenvironment. CXCR2 inhibition or MDSCs repression improves MAPKi therapy effect. Clinically, high TBX3 expression correlates with BRAF^V600E mutation and increased CXCR2 ligands, along with abundant MDSCs infiltration. Thus, our study uncovers a BRAF^V600E-TBX3-CXCLs-MDSCs axis that guides patient stratification and could be targeted to improve the efficacy of MAPKi therapy in advanced thyroid cancer patients.

BRAF-V600E mutation is common in patients with papillary thyroid carcinoma (PTC) and has been associated with an aggressive phenotype. Here the authors show that the mutation supports cancer progression by reactivating the developmental factor TBX3 and promoting the recruitment of myeloid derived suppressive cells.

A Novel Osteochondrodysplasia With Empty Sella Associates With a TBX2 Variant

Skeletal dysplasias comprise a heterogenous group of developmental disorders of skeletal and cartilaginous tissues. Several different forms have been described and the full spectrum of their clinical manifestations and underlying genetic causes are still incompletely understood. We report a three-generation Finnish family with an unusual, autosomal dominant form of osteochondrodysplasia and an empty sella. Affected individuals (age range 24-44 years) exhibit unusual codfish-shaped vertebrae, severe early-onset and debilitating osteoarthritis and an empty sella without endocrine abnormalities. Clinical characteristics also include mild dysmorphic features, reduced sitting height ratio, and obesity. Whole-exome sequencing excluded known skeletal dysplasias and identified a novel heterozygous missense mutation c.899C>T (p.Thr300Met) in TBX2, confirmed by Sanger sequencing. TBX2 is important for development of the skeleton and the brain and three prior reports have described variations in TBX2 in patients portraying a complex phenotype with vertebral anomalies, craniofacial dysmorphism and endocrine dysfunctions. Our mutation lies near a previously reported disease-causing variant and is predicted pathogenic with deleterious effects on protein function. Our findings expand the current spectrum of skeletal dysplasias, support the association of TBX2 mutations with skeletal dysplasia and suggest a role for TBX2 in development of the spinal and craniofacial structures and the pituitary gland.

Activating mutations in BRAF disrupt the hypothalamo-pituitary axis leading to hypopituitarism in mice and humans

Germline mutations in BRAF and other components of the MAPK pathway are associated with the congenital syndromes collectively known as RASopathies. Here, we report the association of Septo-Optic Dysplasia (SOD) including hypopituitarism and Cardio-Facio-Cutaneous (CFC) syndrome in patients harbouring mutations in BRAF. Phosphoproteomic analyses demonstrate that these genetic variants are gain-of-function mutations leading to activation of the MAPK pathway. Activation of the MAPK pathway by conditional expression of the BrafV600E/+ allele, or the knock-in BrafQ241R/+ allele (corresponding to the most frequent human CFC-causing mutation, BRAF p.Q257R), leads to abnormal cell lineage determination and terminal differentiation of hormone-producing cells, causing hypopituitarism. Expression of the BrafV600E/+ allele in embryonic pituitary progenitors leads to an increased expression of cell cycle inhibitors, cell growth arrest and apoptosis, but not tumour formation. Our findings show a critical role of BRAF in hypothalamo-pituitary-axis development both in mouse and human and implicate mutations found in RASopathies as a cause of endocrine deficiencies in humans.

Dynamic Expression of Imprinted Genes in the Developing and Postnatal Pituitary Gland

In mammals, imprinted genes regulate many critical endocrine processes such as growth, the onset of puberty and maternal reproductive behaviour. Human imprinting disorders (IDs) are caused by genetic and epigenetic mechanisms that alter the expression dosage of imprinted genes. Due to improvements in diagnosis, increasing numbers of patients with IDs are now identified and monitored across their lifetimes. Seminal work has revealed that IDs have a strong endocrine component, yet the contribution of imprinted gene products in the development and function of the hypothalamo-pituitary axis are not well defined. Postnatal endocrine processes are dependent upon the production of hormones from the pituitary gland. While the actions of a few imprinted genes in pituitary development and function have been described, to date there has been no attempt to link the expression of these genes as a class to the formation and function of this essential organ. This is important because IDs show considerable overlap, and imprinted genes are known to define a transcriptional network related to organ growth. This knowledge deficit is partly due to technical difficulties in obtaining useful transcriptomic data from the pituitary gland, namely, its small size during development and cellular complexity in maturity. Here we utilise high-sensitivity RNA sequencing at the embryonic stages, and single-cell RNA sequencing data to describe the imprinted transcriptome of the pituitary gland. In concert, we provide a comprehensive literature review of the current knowledge of the role of imprinted genes in pituitary hormonal pathways and how these relate to IDs. We present new data that implicate imprinted gene networks in the development of the gland and in the stem cell compartment. Furthermore, we suggest novel roles for individual imprinted genes in the aetiology of IDs. Finally, we describe the dynamic regulation of imprinted genes in the pituitary gland of the pregnant mother, with implications for the regulation of maternal metabolic adaptations to pregnancy.

Genesis of the Master Circadian Pacemaker in Mice

The suprachiasmatic nucleus (SCN) of the hypothalamus is the central circadian clock of mammals. It is responsible for communicating temporal information to peripheral oscillators via humoral and endocrine signaling, ultimately controlling overt rhythms such as sleep-wake cycles, body temperature, and locomotor activity. Given the heterogeneity and complexity of the SCN, its genesis is tightly regulated by countless intrinsic and extrinsic factors. Here, we provide a brief overview of the development of the SCN, with special emphasis on the murine system.

Decoding neuronal composition and ontogeny of individual hypothalamic nuclei

The hypothalamus plays crucial roles in regulating endocrine, autonomic, and behavioral functions via its diverse nuclei and neuronal subtypes. The developmental mechanisms underlying ontogenetic establishment of different hypothalamic nuclei and generation of neuronal diversity remain largely unknown. Here, we show that combinatorial T-box 3 (TBX3), orthopedia homeobox (OTP), and distal-less homeobox (DLX) expression delineates all arcuate nucleus (Arc) neurons and defines four distinct subpopulations, whereas combinatorial NKX2.1/SF1 and OTP/DLX expression identifies ventromedial hypothalamus (VMH) and tuberal nucleus (TuN) neuronal subpopulations, respectively. Developmental analysis indicates that all four Arc subpopulations are mosaically and simultaneously generated from embryonic Arc progenitors, whereas glutamatergic VMH neurons and GABAergic TuN neurons are sequentially generated from common embryonic VMH progenitors. Moreover, clonal lineage-tracing analysis reveals that diverse lineages from multipotent radial glia progenitors orchestrate Arc and VMH-TuN establishment. Together, our study reveals cellular mechanisms underlying generation and organization of diverse neuronal subtypes and ontogenetic establishment of individual nuclei in the mammalian hypothalamus.

Gonadotroph tumours with a low SF-1 labelling index are more likely to recur and are associated with enrichment of the PI3K-AKT pathway

AIMS

The gonadotroph tumour (GT) is the most frequently resected pituitary neuroendocrine tumour. Although many symptomatic GT are successfully resected, some recur. We sought to identify histological biomarkers that may predict recurrence and explore biological mechanisms that explain this difference in behaviour.

METHODS

SF-1 immunohistochemistry of 51 GT, a subset belonging to a longitudinal prospective cohort study (n = 25), was reviewed. Four groups were defined: Group 1-recently diagnosed GT (n = 20), Group 2-non-recurrent GT with long-term follow up (n = 11), Group 3-initial resections of GT that recur (n = 7) and Group 4-recurrent GT (n = 13). The percentage of SF-1 immunolabelling in the lowest staining fields (SF-1 labelling index (SLI)) was assessed and RNA sequencing was performed on 5 GT with SLI <80% and 5 GT with SLI >80%.

RESULTS

Diffuse, strong SF-1 immunolabelling was the most frequent pattern in Groups 1/2, whereas patchy SF-1 staining predominated in Groups 3/4. There was a lower median SLI in Groups 3/4 than 1/2. Overall, GT with SLI <80% recurred earlier than GT with SLI >80%. Differential expression analysis identified 89 statistically significant differentially expressed genes (FDR <0.05) including over-expression of pituitary stem cell genes (SOX2, GFRA3) and various oncogenes (e.g. BCL2, ERRB4) in patchy SF-1 GT. Gene set enrichment analysis identified significant enrichment of genes involved in the PI3K-AKT pathway.

CONCLUSIONS

We speculate that patchy SF-1 labelling in GT reflects intratumoural heterogeneity and are less differentiated tumours than diffusely staining GT. SF-1 immunolabelling patterns may have prognostic significance in GT, but confirmatory studies are needed for further validation.

Developmental Genes and Malformations in the Hypothalamus

The hypothalamus is a heterogeneous rostral forebrain region that regulates physiological processes essential for survival, energy metabolism, and reproduction, mainly mediated by the pituitary gland. In the updated prosomeric model, the hypothalamus represents the rostralmost forebrain, composed of two segmental regions (terminal and peduncular hypothalamus), which extend respectively into the non-evaginated preoptic telencephalon and the evaginated pallio-subpallial telencephalon. Complex genetic cascades of transcription factors and signaling molecules rule their development. Alterations of some of these molecular mechanisms acting during forebrain development are associated with more or less severe hypothalamic and pituitary dysfunctions, which may be associated with brain malformations such as holoprosencephaly or septo-optic dysplasia. Studies on transgenic mice with mutated genes encoding critical transcription factors implicated in hypothalamic-pituitary development are contributing to understanding the high clinical complexity of these pathologies. In this review article, we will analyze first the complex molecular genoarchitecture of the hypothalamus resulting from the activity of previous morphogenetic signaling centers and secondly some malformations related to alterations in genes implicated in the development of the hypothalamus.

Neurog2 Acts as a Classical Proneural Gene in the Ventromedial Hypothalamus and Is Required for the Early Phase of Neurogenesis

The tuberal hypothalamus is comprised of the dorsomedial, ventromedial, and arcuate nuclei, as well as parts of the lateral hypothalamic area, and it governs a wide range of physiologies. During neurogenesis, tuberal hypothalamic neurons are thought to be born in a dorsal-to-ventral and outside-in pattern, although the accuracy of this description has been questioned over the years. Moreover, the intrinsic factors that control the timing of neurogenesis in this region are poorly characterized. Proneural genes, including Achate-scute-like 1 (Ascl1) and Neurogenin 3 (Neurog3) are widely expressed in hypothalamic progenitors and contribute to lineage commitment and subtype-specific neuronal identifies, but the potential role of Neurogenin 2 (Neurog2) remains unexplored. Birthdating in male and female mice showed that tuberal hypothalamic neurogenesis begins as early as E9.5 in the lateral hypothalamic and arcuate and rapidly expands to dorsomedial and ventromedial neurons by E10.5, peaking throughout the region by E11.5. We confirmed an outside-in trend, except for neurons born at E9.5, and uncovered a rostrocaudal progression but did not confirm a dorsal-ventral patterning to tuberal hypothalamic neuronal birth. In the absence of Neurog2, neurogenesis stalls, with a significant reduction in early-born BrdU⁺ cells but no change at later time points. Further, the loss of Ascl1 yielded a similar delay in neuronal birth, suggesting that Ascl1 cannot rescue the loss of Neurog2 and that these proneural genes act independently in the tuberal hypothalamus. Together, our findings show that Neurog2 functions as a classical proneural gene to regulate the temporal progression of tuberal hypothalamic neurogenesis.SIGNIFICANCE STATEMENT Here, we investigated the general timing and pattern of neurogenesis within the tuberal hypothalamus. Our results confirmed an outside-in trend of neurogenesis and uncovered a rostrocaudal progression. We also showed that Neurog2 acts as a classical proneural gene and is responsible for regulating the birth of early-born neurons within the ventromedial hypothalamus, acting independently of Ascl1 In addition, we revealed a role for Neurog2 in cell fate specification and differentiation of ventromedial -specific neurons. Last, Neurog2 does not have cross-inhibitory effects on Neurog1, Neurog3, and Ascl1 These findings are the first to reveal a role for Neurog2 in hypothalamic development.

SHH signaling mediated by a prechordal and brain enhancer controls forebrain organization

Sonic hedgehog (SHH) signaling plays a pivotal role in 2 different phases during brain development. Early SHH signaling derived from the prechordal plate (PrCP) triggers secondary Shh induction in the forebrain, which overlies the PrCP, and the induced SHH signaling, in turn, directs late neuronal differentiation of the forebrain. Consequently, Shh regulation in the PrCP is crucial for initiation of forebrain development. However, no enhancer that regulates prechordal Shh expression has yet been found. Here, we identified a prechordal enhancer, named SBE7, in the vicinity of a cluster of known forebrain enhancers for Shh This enhancer also directs Shh expression in the ventral midline of the forebrain, which receives the prechordal SHH signal. Thus, the identified enhancer acts not only for the initiation of Shh regulation in the PrCP but also for subsequent Shh induction in the forebrain. Indeed, removal of the enhancer from the mouse genome markedly down-regulated the expression of Shh in the rostral domains of the axial mesoderm and in the ventral midline of the forebrain and hypothalamus in the mouse embryo, and caused a craniofacial abnormality similar to human holoprosencephaly (HPE). These findings demonstrate that SHH signaling mediated by the newly identified enhancer is essential for development and growth of the ventral midline of the forebrain and hypothalamus. Understanding of the Shh regulation governed by this prechordal and brain enhancer provides an insight into the mechanism underlying craniofacial morphogenesis and the etiology of HPE.

Stem Cells, Self-Renewal, and Lineage Commitment in the Endocrine System

The endocrine system coordinates a wide array of body functions mainly through secretion of hormones and their actions on target tissues. Over the last decades, a collective effort between developmental biologists, geneticists, and stem cell biologists has generated a wealth of knowledge related to the contribution of stem/progenitor cells to both organogenesis and self-renewal of endocrine organs. This review provides an up-to-date and comprehensive overview of the role of tissue stem cells in the development and self-renewal of endocrine organs. Pathways governing crucial steps in both development and stemness maintenance, and that are known to be frequently altered in a wide array of endocrine disorders, including cancer, are also described. Crucially, this plethora of information is being channeled into the development of potential new cell-based treatment modalities for endocrine-related illnesses, some of which have made it through clinical trials.

Sox2: A Regulatory Factor in Tumorigenesis and Metastasis

The transcription factor Sox2 plays an important role in various phases of embryonic development, including cell fate and differentiation. These key regulatory functions are facilitated by binding to specific DNA sequences in combination with partner proteins to exert their effects. Recently, overexpression and gene amplification of Sox2 has been associated with tumor aggression and metastasis in various cancer types, including breast, prostate, lung, ovarian and colon cancer. All the different roles for Sox2 involve complicated regulatory networks consisting of protein-protein and protein-nucleic acid interactions. Their involvement in the EMT modulation is possibly enabled by Wnt/ β-catenin and other signaling pathways. There are number of in vivo models which show Sox2 association with increased cancer aggressiveness, resistance to chemo-radiation therapy and decreased survival rate suggesting Sox2 as a therapeutic target. This review will focus on the different roles for Sox2 in metastasis and tumorigenesis. We will also review the mechanism of action underlying the cooperative Sox2- DNA/partner factors binding where Sox2 can be potentially explored for a therapeutic opportunity to treat cancers.

Anterior CNS expansion driven by brain transcription factors

During CNS development, there is prominent expansion of the anterior region, the brain. In Drosophila, anterior CNS expansion emerges from three rostral features: (1) increased progenitor cell generation, (2) extended progenitor cell proliferation, (3) more proliferative daughters. We find that tailless (mouse Nr2E1/Tlx), otp/Rx/hbn (Otp/Arx/Rax) and Doc1/2/3 (Tbx2/3/6) are important for brain progenitor generation. These genes, and earmuff (FezF1/2), are also important for subsequent progenitor and/or daughter cell proliferation in the brain. Brain TF co-misexpression can drive brain-profile proliferation in the nerve cord, and can reprogram developing wing discs into brain neural progenitors. Brain TF expression is promoted by the PRC2 complex, acting to keep the brain free of anti-proliferative and repressive action of Hox homeotic genes. Hence, anterior expansion of the Drosophila CNS is mediated by brain TF driven ‘super-generation’ of progenitors, as well as ‘hyper-proliferation’ of progenitor and daughter cells, promoted by PRC2-mediated repression of Hox activity.

Development of the basal hypothalamus through anisotropic growth

The adult hypothalamus is subdivided into distinct domains: pre-optic, anterior, tuberal and mammillary. Each domain harbours an array of neurones that act together to regulate homeostasis. The embryonic origins and the development of hypothalamic neurones, however, remain enigmatic. Here, we summarise recent studies in model organisms that challenge current views of hypothalamic development, which traditionally have attempted to map adult domains to correspondingly located embryonic domains. Instead, new studies indicate that hypothalamic neurones arise from progenitor cells that undergo anisotropic growth, expanding to a greater extent than other progenitors, and grow in different dimensions. We describe in particular how a multipotent Shh/ Fgf10-expressing progenitor population gives rise to progenitors throughout the basal hypothalamus that grow anisotropically and sequentially: first, a subset displaced rostrally give rise to anterior-ventral/tuberal neuronal progenitors; then a subset displaced caudally give rise to mammillary neuronal progenitors; and, finally, a subset(s) displaced ventrally give rise to tuberal infundibular glial progenitors. As this occurs, stable populations of Shh+ive and Fgf10+ive progenitors form. We describe current understanding of the mechanisms that induce Shh+ive /Fgf10+ive progenitors and begin to direct their differentiation to anterior-ventral/tuberal neuronal progenitors, mammillary neuronal progenitors and tuberal infundibular progenitors. Taken together, these studies suggest a new model for hypothalamic development that we term the "anisotropic growth model". We discuss the implications of the model for understanding the origins of adult hypothalamic neurones.

Functional identity of hypothalamic melanocortin neurons depends on Tbx3

Heterogeneous populations of hypothalamic neurons orchestrate energy balance via the release of specific signatures of neuropeptides. However, how specific intracellular machinery controls peptidergic identities and function of individual hypothalamic neurons remains largely unknown. The transcription factor T-box 3 (Tbx3) is expressed in hypothalamic neurons sensing and governing energy status, whereas human TBX3 haploinsufficiency has been linked with obesity. Here, we demonstrate that loss of Tbx3 function in hypothalamic neurons causes weight gain and other metabolic disturbances by disrupting both the peptidergic identity and plasticity of Pomc/Cart and Agrp/Npy neurons. These alterations are observed after loss of Tbx3 in both immature hypothalamic neurons and terminally differentiated mouse neurons. We further establish the importance of Tbx3 for body weight regulation in Drosophila melanogaster and show that TBX3 is implicated in the differentiation of human embryonic stem cells into hypothalamic Pomc neurons. Our data indicate that Tbx3 directs the terminal specification of neurons as functional components of the melanocortin system and is required for maintaining their peptidergic identity. In summary, we report the discovery of a key mechanistic process underlying the functional heterogeneity of hypothalamic neurons governing body weight and systemic metabolism.

Hypogonadotropic hypogonadism and pituitary hypoplasia as recurrent features in Ulnar-Mammary syndrome

Ulnar-mammary syndrome (UMS) is characterized by ulnar defects, and nipple or apocrine gland hypoplasia, caused by TBX3 haploinsufficiency. Signs of hypogonadism were repeatedly reported, but the mechanisms remain elusive. We aim to assess the origin of hypogonadism in two families with UMS. UMS was suspected in two unrelated probands referred to an academic center with delayed puberty because of the evident ulnar ray and breast defects in their parents. Clinical, biochemical and genetic investigations proved the existence of congenital normosmic IHH (nIHH) associated with pituitary hypoplasia in the two probands who were heterozygous for novel TBX3 pathogenic variants. The mutations co-segregated with delayed puberty, midline defects (nose, teeth and tongue anomalies) and other variable features of UMS in the two families (absent axillary hairs and nipple hypoplasia, asymmetrical features including unilateral ulnar or renal abnormalities). The combined analysis of these findings and of the previous UMS reports showed delayed puberty and other signs of hypogonadism in 79 and 37% of UMS males, respectively. Proband 1 was followed up to adulthood with persistence of nIHH. In conclusion, UMS should be suspected in patients with delayed puberty and midline defects, including pituitary hypoplasia, in the presence of mild cues for TBX3 mutation, even in the absence of limb malformations. In addition, TBX3 should be included among candidate genes for congenital nIHH.

Distinct temporal requirements for Sonic hedgehog signaling in development of the tuberal hypothalamus

Sonic hedgehog (Shh) plays well characterized roles in brain and spinal cord development, but its functions in the hypothalamus have been more difficult to elucidate owing to the complex neuroanatomy of this brain area. Here, we use fate mapping and conditional deletion models in mice to define requirements for dynamic Shh activity at distinct developmental stages in the tuberal hypothalamus, a brain region with important homeostatic functions. At early time points, Shh signaling regulates dorsoventral patterning, neurogenesis and the size of the ventral midline. Fate-mapping experiments demonstrate that Shh-expressing and -responsive progenitors contribute to distinct neuronal subtypes, accounting for some of the cellular heterogeneity in tuberal hypothalamic nuclei. Conditional deletion of the hedgehog transducer smoothened (Smo), after dorsoventral patterning has been established, reveals that Shh signaling is necessary to maintain proliferation and progenitor identity during peak periods of hypothalamic neurogenesis. We also find that mosaic disruption of Smo causes a non-cell autonomous gain in Shh signaling activity in neighboring wild-type cells, suggesting a mechanism for the pathogenesis of hypothalamic hamartomas, benign tumors that form during hypothalamic development.

The Paraventricular Nucleus of the Hypothalamus: Development, Function, and Human Diseases

The paraventricular nucleus of the hypothalamus (PVH), located in the ventral diencephalon adjacent to the third ventricle, is a highly conserved brain region present in species from zebrafish to humans. The PVH is composed of three main types of neurons, magnocellular, parvocellular, and long-projecting neurons, which play imperative roles in the regulation of energy balance and various endocrinological activities. In this review, we focus mainly on recent findings about the early development of the hypothalamus and the PVH, the functions of the PVH in the modulation of energy homeostasis and in the hypothalamus-pituitary system, and human diseases associated with the PVH, such as obesity, short stature, hypertension, and diabetes insipidus. Thus, the investigations of the PVH will benefit not only understanding of the development of the central nervous system but also the etiology of and therapy for human diseases.

The origins of the circumventricular organs

The circumventricular organs (CVOs) are specialised neuroepithelial structures found in the midline of the brain, grouped around the third and fourth ventricles. They mediate the communication between the brain and the periphery by performing sensory and secretory roles, facilitated by increased vascularisation and the absence of a blood-brain barrier. Surprisingly little is known about the origins of the CVOs (both developmental and evolutionary), but their functional and organisational similarities raise the question of the extent of their relationship. Here, I review our current knowledge of the embryonic development of the seven major CVOs (area postrema, median eminence, neurohypophysis, organum vasculosum of the lamina terminalis, pineal organ, subcommissural organ, subfornical organ) in embryos of different vertebrate species. Although there are conspicuous similarities between subsets of CVOs, no unifying feature characteristic of their development has been identified. Cross-species comparisons suggest that CVOs also display a high degree of evolutionary flexibility. Thus, the term 'CVO' is merely a functional definition, and features shared by multiple CVOs may be the result of homoplasy rather than ontogenetic or phylogenetic relationships.

Development of the hypothalamus: conservation, modification and innovation

The hypothalamus, which regulates fundamental aspects of physiological homeostasis and behavior, is a brain region that exhibits highly conserved anatomy across vertebrate species. Its development involves conserved basic mechanisms of induction and patterning, combined with a more plastic process of neuronal fate specification, to produce brain circuits that mediate physiology and behavior according to the needs of each species. Here, we review the factors involved in the induction, patterning and neuronal differentiation of the hypothalamus, highlighting recent evidence that illustrates how changes in Wnt/β-catenin signaling during development may lead to species-specific form and function of this important brain structure.

Hypothalamic sonic hedgehog is required for cell specification and proliferation of LHX3/LHX4 pituitary embryonic precursors

Sonic hedgehog (SHH) is an essential morphogenetic signal that dictates cell fate decisions in several developing organs in mammals. In vitro data suggest that SHH is required to specify LHX3⁺/LHX4⁺ Rathke's pouch (RP) progenitor identity. However, in vivo studies have failed to reveal such a function, supporting instead a crucial role for SHH in promoting proliferation of these RP progenitors and for differentiation of pituitary cell types. Here, we have used a genetic approach to demonstrate that activation of the SHH pathway is necessary to induce LHX3⁺/LHX4⁺ RP identity in mouse embryos. First, we show that conditional deletion of Shh in the anterior hypothalamus results in a fully penetrant phenotype characterised by a complete arrest of RP development, with lack of Lhx3/Lhx4 expression in RP epithelium at 9.0 days post coitum (dpc) and total loss of pituitary tissue by 12.5 dpc. Conversely, overactivation of the SHH pathway by conditional deletion of Ptch1 in RP progenitors leads to severe hyperplasia and enlargement of the Sox2⁺ stem cell compartment by the end of gestation.

MAPK pathway control of stem cell proliferation and differentiation in the embryonic pituitary provides insights into the pathogenesis of papillary craniopharyngioma

Despite the importance of the RAS-RAF-MAPK pathway in normal physiology and disease of numerous organs, its role during pituitary development and tumourigenesis remains largely unknown. Here, we show that the over-activation of the MAPK pathway, through conditional expression of the gain-of-function alleles BrafV600E and KrasG12D in the developing mouse pituitary, results in severe hyperplasia and abnormal morphogenesis of the gland by the end of gestation. Cell-lineage commitment and terminal differentiation are disrupted, leading to a significant reduction in numbers of most of the hormone-producing cells before birth, with the exception of corticotrophs. Of note, Sox2+ stem cells and clonogenic potential are drastically increased in the mutant pituitaries. Finally, we reveal that papillary craniopharyngioma (PCP), a benign human pituitary tumour harbouring BRAF p.V600E also contains Sox2+ cells with sustained proliferative capacity and disrupted pituitary differentiation. Together, our data demonstrate a crucial function of the MAPK pathway in controlling the balance between proliferation and differentiation of Sox2+ cells and suggest that persistent proliferative capacity of Sox2+ cells may underlie the pathogenesis of PCP.

Pivotal role of median eminence tanycytes for hypothalamic function and neurogenesis

Along with the sub-ventricular zone of the forebrain lateral ventricles and the sub-granular zone of the dentate gyrus in the hippocampus, the hypothalamus has recently emerged as a third gliogenic and neurogenic niche in the central nervous system. The hypothalamus is the main regulator of body homeostasis because it centralizes peripheral information to regulate crucial physiological functions through the pituitary gland and the autonomic nervous system. Its ability to sense signals originating outside the brain relies on its exposure to blood-born molecules through the median eminence, which is localized outside the blood brain barrier. Within the hypothalamus, a population of specialized radial glial cells, the tanycytes, control exposure to blood-born signals by acting both as sensors and regulators of the hypothalamic input and output. In addition, lineage-tracing experiments have recently revealed that tanycytes represent a population of hypothalamic stem cells, defining them as a pivotal cell type within the hypothalamus. Hypothalamic neurogenesis has moreover been shown to have an important role in feeding control and energy metabolism, which challenges previous knowledge and offers new therapeutic options.

Regulation of pituitary stem cells by epithelial to mesenchymal transition events and signaling pathways

The anterior pituitary gland is comprised of specialized cell-types that produce and secrete polypeptide hormones in response to hypothalamic input and feedback from target organs. These specialized cells arise from stem cells that express SOX2 and the pituitary transcription factor PROP1, which is necessary to establish the stem cell pool and promote an epithelial to mesenchymal-like transition, releasing progenitors from the niche. The adult anterior pituitary responds to physiological challenge by mobilizing the SOX2-expressing progenitor pool and producing additional hormone-producing cells. Knowledge of the role of signaling pathways and extracellular matrix components in these processes may lead to improvements in the efficiency of differentiation of embryonic stem cells or induced pluripotent stem cells into hormone producing cells in vitro. Advances in our basic understanding of pituitary stem cell regulation and differentiation may lead to improved diagnosis and treatment for patients with hypopituitarism.

Molecular mechanisms regulating impaired neurogenesis of fragile X syndrome human embryonic stem cells

Fragile X syndrome (FXS) is the most common form of inherited cognitive impairment. It is caused by developmental inactivation of the FMR1 gene and the absence of its encoded protein FMRP, which plays pivotal roles in brain development and function. In FXS embryos with full FMR1 mutation, FMRP is expressed during early embryogenesis and is gradually downregulated at the third trimester of pregnancy. FX-human embryonic stem cells (FX-hESCs), derived from FX human blastocysts, demonstrate the same pattern of developmentally regulated FMR1 inactivation when subjected to in vitro neural differentiation (IVND). In this study, we used this in vitro human platform to explore the molecular mechanisms downstream to FMRP in the context of early human embryonic neurogenesis. Our results show a novel role for the SOX superfamily of transcription factors, specifically for SOX2 and SOX9, which could explain the reduced and delayed neurogenesis observed in FX cells. In addition, we assess in this study the "GSK3β theory of FXS" for the first time in a human-based model. We found no evidence for a pathological increase in GSK3β protein levels upon cellular loss of FMRP, in contrast to what was found in the brain of Fmr1 knockout mice. Our study adds novel data on potential downstream targets of FMRP and highlights the importance of the FX-hESC IVND system.

Persistent expression of activated notch in the developing hypothalamus affects survival of pituitary progenitors and alters pituitary structure

BACKGROUND

As the pituitary gland develops, signals from the hypothalamus are necessary for pituitary induction and expansion. Little is known about the control of cues that regulate early signaling between the two structures. Ligands and receptors of the Notch signaling pathway are found in both the hypothalamus and Rathke's pouch. The downstream Notch effector gene Hes1 is required for proper pituitary formation; however, these effects 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 within the developing hypothalamus.

RESULTS

We demonstrate that loss of Notch signaling by conditional deletion of Rbpj in the hypothalamus does not affect expression of Hes1 within the posterior hypothalamus or expression of Hes5. In contrast, expression of activated Notch within the hypothalamus results in ectopic Hes5 expression and increased Hes1 expression, which is sufficient to disrupt pituitary development and postnatal expansion.

CONCLUSIONS

Taken together, our results indicate that Rbpj-dependent Notch signaling within the developing hypothalamus is not necessary for pituitary development, but persistent Notch signaling and ectopic Hes5 expression in hypothalamic progenitors affects pituitary induction and expansion.

SOX2 is sequentially required for progenitor proliferation and lineage specification in the developing pituitary

Sox2 mutations are associated with pituitary hormone deficiencies and the protein is required for pituitary progenitor proliferation, but its function has not been well characterized in this context. SOX2 is known to activate expression of Six6, encoding a homeodomain transcription factor, in the ventral diencephalon. Here, we find that the same relationship likely exists in the pituitary. Moreover, because Six6 deletion is associated with a similar phenotype as described here for loss of Sox2, Six6 appears to be an essential downstream target of SOX2 in the gland. We also uncover a second role for SOX2. Whereas cell differentiation is reduced in Sox2 mutants, some endocrine cells are generated, such as POMC-positive cells in the intermediate lobe. However, loss of SOX2 here results in complete downregulation of the melanotroph pioneer factor PAX7, and subsequently a switch of identity from melanotrophs to ectopic corticotrophs. Rescuing proliferation by ablating the cell cycle negative regulator p27 (also known as Cdkn1b) in Sox2 mutants does not restore melanotroph emergence. Therefore, SOX2 has two independent roles during pituitary morphogenesis; firstly, promotion of progenitor proliferation, and subsequently, acquisition of melanotroph identity.

Summary: SOX2 has two independent roles during pituitary morphogenesis: promoting progenitor proliferation via SIX6 and determining melanotroph identity via PAX7.

Essential function of the transcription factor Rax in the early patterning of the mammalian hypothalamus

The hypothalamus is a region of the anterior forebrain that controls basic aspects of vertebrate physiology, but the genes involved in its development are still poorly understood. Here, we investigate the function of the homeobox gene Rax/Rx in early hypothalamic development using a conditional targeted inactivation strategy in the mouse. We found that lack of Rax expression prior to embryonic day 8.5 (E8.5) caused a general underdevelopment of the hypothalamic neuroepithelium, while inactivation at later timepoints had little effect. The early absence of Rax impaired neurogenesis and prevented the expression of molecular markers of the dorsomedial hypothalamus, including neuropeptides Proopiomelanocortin and Somatostatin. Interestingly, the expression domains of genes expressed in the ventromedial hypothalamus and infundibulum invaded dorsal hypothalamic territory, showing that Rax is needed for the proper dorsoventral patterning of the developing medial hypothalamus. The phenotypes caused by the early loss of Rax are similar to those of eliminating the expression of the morphogen Sonic hedgehog (Shh) specifically from the hypothalamus. Consistent with this similarity in phenotypes, we observed that Shh and Rax are coexpressed in the rostral forebrain at late head fold stages and that loss of Rax caused a downregulation of Shh expression in the dorsomedial portion of the hypothalamus.

Development of the Neuroendocrine Hypothalamus

The neuroendocrine hypothalamus is composed of the tuberal and anterodorsal hypothalamus, together with the median eminence/neurohypophysis. It centrally governs wide-ranging physiological processes, including homeostasis of energy balance, circadian rhythms and stress responses, as well as growth and reproductive behaviours. Homeostasis is maintained by integrating sensory inputs and effecting responses via autonomic, endocrine and behavioural outputs, over diverse time-scales and throughout the lifecourse of an individual. Here, we summarize studies that begin to reveal how different territories and cell types within the neuroendocrine hypothalamus are assembled in an integrated manner to enable function, thus supporting the organism's ability to survive and thrive. We discuss how signaling pathways and transcription factors dictate the appearance and regionalization of the hypothalamic primordium, the maintenance of progenitor cells, and their specification and differentiation into neurons. We comment on recent studies that harness such programmes for the directed differentiation of human ES/iPS cells. We summarize how developmental plasticity is maintained even into adulthood and how integration between the hypothalamus and peripheral body is established in the median eminence and neurohypophysis. Analysis of model organisms, including mouse, chick and zebrafish, provides a picture of how complex, yet elegantly coordinated, developmental programmes build glial and neuronal cells around the third ventricle of the brain. Such conserved processes enable the hypothalamus to mediate its function as a central integrating and response-control mediator for the homeostatic processes that are critical to life. Early indications suggest that deregulation of these events may underlie multifaceted pathological conditions and dysfunctional physiology in humans, such as obesity.

Transcription factor 7-like 1 is involved in hypothalamo-pituitary axis development in mice and humans

Aberrant embryonic development of the hypothalamus and/or pituitary gland in humans results in congenital hypopituitarism (CH). Transcription factor 7-like 1 (TCF7L1), an important regulator of the WNT/β-catenin signaling pathway, is expressed in the developing forebrain and pituitary gland, but its role during hypothalamo-pituitary (HP) axis formation or involvement in human CH remains elusive. Using a conditional genetic approach in the mouse, we first demonstrate that TCF7L1 is required in the prospective hypothalamus to maintain normal expression of the hypothalamic signals involved in the induction and subsequent expansion of Rathke's pouch progenitors. Next, we reveal that the function of TCF7L1 during HP axis development depends exclusively on the repressing activity of TCF7L1 and does not require its interaction with β-catenin. Finally, we report the identification of two independent missense variants in human TCF7L1, p.R92P and p.R400Q, in a cohort of patients with forebrain and/or pituitary defects. We demonstrate that these variants exhibit reduced repressing activity in vitro and in vivo relative to wild-type TCF7L1. Together, our data provide support for a conserved molecular function of TCF7L1 as a transcriptional repressor during HP axis development in mammals and identify variants in this transcription factor that are likely to contribute to the etiology of CH.

ProNodal acts via FGFR3 to govern duration of Shh expression in the prechordal mesoderm

The secreted glycoprotein sonic hedgehog (Shh) is expressed in the prechordal mesoderm, where it plays a crucial role in induction and patterning of the ventral forebrain. Currently little is known about how Shh is regulated in prechordal tissue. Here we show that in the embryonic chick, Shh is expressed transiently in prechordal mesoderm, and is governed by unprocessed Nodal. Exposure of prechordal mesoderm microcultures to Nodal-conditioned medium, the Nodal inhibitor CerS, or to an ALK4/5/7 inhibitor reveals that Nodal is required to maintain both Shh and Gsc expression, but whereas Gsc is largely maintained through canonical signalling, Nodal signals through a non-canonical route to maintain Shh. Further, Shh expression can be maintained by a recombinant Nodal cleavage mutant, proNodal, but not by purified mature Nodal. A number of lines of evidence suggest that proNodal acts via FGFR3. ProNodal and FGFR3 co-immunoprecipitate and proNodal increases FGFR3 tyrosine phosphorylation. In microcultures, soluble FGFR3 abolishes Shh without affecting Gsc expression. Further, prechordal mesoderm cells in which Fgfr3 expression is reduced by Fgfr3 siRNA fail to bind to proNodal. Finally, targeted electroporation of Fgfr3 siRNA to prechordal mesoderm in vivo results in premature Shh downregulation without affecting Gsc. We report an inverse correlation between proNodal-FGFR3 signalling and pSmad1/5/8, and show that proNodal-FGFR3 signalling antagonises BMP-mediated pSmad1/5/8 signalling, which is poised to downregulate Shh. Our studies suggest that proNodal/FGFR3 signalling governs Shh duration by repressing canonical BMP signalling, and that local BMPs rapidly silence Shh once endogenous Nodal-FGFR3 signalling is downregulated.

Highlighted article: In the chick prechordal mesoderm, the Nodal precursor proNodal acts via a non-canonical route to inhibit BMP signalling and thus maintain Shh expression

Hes1 and Hes5 are required for differentiation of pituicytes and formation of the neurohypophysis in pituitary development

The pituitary gland is a critical endocrine organ regulating diverse physiological functions, including homeostasis, metabolism, reproduction, and growth. It is composed of two distinct entities: the adenohypophysis, including the anterior and intermediate lobes, and the neurohypophysis known as the posterior lobe. The neurohypophysis is composed of pituicytes (glial cells) and axons projected from hypothalamic neurons. The adenohypophysis derives from Rathke's pouch, whereas the neurohypophysis derives from the infundibulum, an evagination of the ventral diencephalon. Molecular mechanisms of adenohypophysis development are much better understood, but little is known about mechanisms that regulate neurohypophysis development. Hes genes, known as Notch effectors, play a crucial role in specifying cellular fates during the development of various tissues and organs. Here, we report that the ventral diencephalon fails to evaginate resulting in complete loss of the posterior pituitary lobe in Hes1(-/-); Hes5(+/-) mutant embryos. In these mutant mice, progenitor cells are differentiated into neurons at the expense of pituicytes in the ventral diencephalon. In the developing neurohypophysis, the proliferative zone is located at the base of the infundibulum. Thus, Hes1 and Hes5 modulate not only maintenance of progenitor cells but also pituicyte versus neuron fate specification during neurohypophysis development.

Lhx5 controls mamillary differentiation in the developing hypothalamus of the mouse

Acquisition of specific neuronal identity by individual brain nuclei is a key step in brain development. However, how the mechanisms that confer neuronal identity are integrated with upstream regional specification networks is still mysterious. Expression of Sonic hedgehog (Shh), is required for hypothalamic specification and is later downregulated by Tbx3 to allow for the differentiation of the tubero-mamillary region. In this region, the mamillary body (MBO), is a large neuronal aggregate essential for memory formation. To clarify how MBO identity is acquired after regional specification, we investigated Lhx5, a transcription factor with restricted MBO expression. We first generated a hypomorph allele of Lhx5—in homozygotes, the MBO disappears after initial specification. Intriguingly, in these mutants, Tbx3 was downregulated and the Shh expression domain abnormally extended. Microarray analysis and chromatin immunoprecipitation indicated that Lhx5 appears to be involved in Shh downregulation through Tbx3 and activates several MBO-specific regulator and effector genes. Finally, by tracing the caudal hypothalamic cell lineage we show that, in the Lhx5 mutant, at least some MBO cells are present but lack characteristic marker expression. Our work shows how the Lhx5 locus contributes to integrate regional specification pathways with downstream acquisition of neuronal identity in the MBO.

Genetic regulation of murine pituitary development

Significant progress has been made recently in unravelling the embryonic events leading to pituitary morphogenesis, both in vivo and in vitro. This includes dissection of the molecular mechanisms controlling patterning of the ventral diencephalon that regulate formation of the pituitary anlagen or Rathke's pouch. There is also a better characterisation of processes that underlie maintenance of pituitary progenitors, specification of endocrine lineages and the three-dimensional organisation of newly differentiated endocrine cells. Furthermore, a population of adult pituitary stem cells (SCs), originating from embryonic progenitors, have been described and shown to have not only regenerative potential, but also the capacity to induce tumour formation. Finally, the successful recapitulation in vitro of embryonic events leading to generation of endocrine cells from embryonic SCs, and their subsequent transplantation, represents exciting advances towards the use of regenerative medicine to treat endocrine deficits. In this review, an up-to-date description of pituitary morphogenesis will be provided and discussed with particular reference to pituitary SC studies.

Differential requirements for Gli2 and Gli3 in the regional specification of the mouse hypothalamus

Secreted protein Sonic hedgehog (Shh) ventralizes the neural tube by modulating the crucial balance between activating and repressing functions (GliA, GliR) of transcription factors Gli2 and Gli3. This balance—the Shh-Gli code—is species- and context-dependent and has been elucidated for the mouse spinal cord. The hypothalamus, a forebrain region regulating vital functions like homeostasis and hormone secretion, shows dynamic and intricate Shh expression as well as complex regional differentiation. Here we asked if particular combinations of Gli2 and Gli3 and of GliA and GliR functions contribute to the variety of hypothalamic regions, i.e., we wanted to approach the question of a possible hypothalamic version of the Shh-Gli code. Based on mouse mutant analysis, we show that: (1) hypothalamic regional heterogeneity is based in part on differentially stringent requirements for Gli2 or Gli3; (2) another source of diversity are differential requirements for Shh of neural vs. non-neural origin; (3) the medial progenitor domain known to depend on Gli2 for its development generates several essential hypothalamic nuclei plus the pituitary and median eminence; (4) the suppression of Gli3R by neural and non-neural Shh is essential for hypothalamic specification. Finally, we have mapped our results on a recent model which considers the hypothalamus as a transverse region with alar and basal portions. Our data confirm the model and are explained by it.

Sonic hedgehog signaling in the development of the mouse hypothalamus

The expression pattern of Sonic Hedgehog (Shh) in the developing hypothalamus changes over time. Shh is initially expressed in the prechordal mesoderm and later in the hypothalamic neuroepithelium—first medially, and then in two off-medial domains. This dynamic expression suggests that Shh might regulate several aspects of hypothalamic development. To gain insight into them, lineage tracing, (conditional) gene inactivation in mouse, in ovo loss- and gain-of-function approaches in chick and analysis of Shh expression regulation have been employed. We will focus on mouse studies and refer to chick and fish when appropriate to clarify. These studies show that Shh-expressing neuroepithelial cells serve as a signaling center for neighboring precursors, and give rise to most of the basal hypothalamus (tuberal and mammillary regions). Shh signaling is initially essential for hypothalamic induction. Later, Shh signaling from the neuroepithelium controls specification of the lateral hypothalamic area and growth-patterning coordination in the basal hypothalamus. To further elucidate the role of Shh in hypothalamic development, it will be essential to understand how Shh regulates the downstream Gli transcription factors.

The T-box gene family: emerging roles in development, stem cells and cancer

The T-box family of transcription factors exhibits widespread involvement throughout development in all metazoans. T-box proteins are characterized by a DNA-binding motif known as the T-domain that binds DNA in a sequence-specific manner. In humans, mutations in many of the genes within the T-box family result in developmental syndromes, and there is increasing evidence to support a role for these factors in certain cancers. In addition, although early studies focused on the role of T-box factors in early embryogenesis, recent studies in mice have uncovered additional roles in unsuspected places, for example in adult stem cell populations. Here, I provide an overview of the key features of T-box transcription factors and highlight their roles and mechanisms of action during various stages of development and in stem/progenitor cell populations.