Hes genes and neurogenin regulate non-neural versus neural fate specification in the dorsal telencephalic midline

Oscillatory control of factors determining multipotency and fate in mouse neural progenitors

The basic helix-loop-helix transcription factors Ascl1/Mash1, Hes1, and Olig2 regulate fate choice of neurons, astrocytes, and oligodendrocytes, respectively. These same factors are coexpressed by neural progenitor cells. Here, we found by time-lapse imaging that these factors are expressed in an oscillatory manner by mouse neural progenitor cells. In each differentiation lineage, one of the factors becomes dominant. We used optogenetics to control expression of Ascl1 and found that, although sustained Ascl1 expression promotes neuronal fate determination, oscillatory Ascl1 expression maintains proliferating neural progenitor cells. Thus, the multipotent state correlates with oscillatory expression of several fate-determination factors, whereas the differentiated state correlates with sustained expression of a single factor.

Proneural genes in neocortical development

Neurons, astrocytes and oligodendrocytes arise from CNS progenitor cells at defined times and locations during development, with transcription factors serving as key determinants of these different neural cell fates. An emerging theme is that the transcription factors that specify CNS cell fates function in a context-dependent manner, regulated by post-translational modifications and epigenetic alterations that partition the genome (and hence target genes) into active or silent domains. Here we profile the critical roles of the proneural genes, which encode basic-helix-loop-helix (bHLH) transcription factors, in specifying neural cell identities in the developing neocortex. In particular, we focus on the proneural genes Neurogenin 1 (Neurog1), Neurog2 and Achaete scute-like 1 (Ascl1), which are each expressed in a distinct fashion in the progenitor cell pools that give rise to all of the neuronal and glial cell types of the mature neocortex. Notably, while the basic functions of these proneural genes have been elucidated, it is becoming increasingly evident that tight regulatory controls dictate when, where and how they function. Current efforts to better understand how proneural gene function is regulated will not only improve our understanding of neocortical development, but are also critical to the future development of regenerative therapies for the treatment of neuronal degeneration or disease.

Hedgehog signaling regulates prosensory cell properties during the basal-to-apical wave of hair cell differentiation in the mammalian cochlea

Mechanosensory hair cells and supporting cells develop from common precursors located in the prosensory domain of the developing cochlear epithelium. Prosensory cell differentiation into hair cells or supporting cells proceeds from the basal to the apical region of the cochleae, but the mechanism and significance of this basal-to-apical wave of differentiation remain to be elucidated. Here, we investigated the role of Hedgehog (Hh) signaling in cochlear development by examining the effects of up- and downregulation of Hh signaling in vivo. The Hh effector smoothened (Smo) was genetically activated or inactivated specifically in the developing cochlear epithelium after prosensory domain formation. Cochleae expressing a constitutively active allele of Smo showed only one row of inner hair cells with no outer hair cells (OHCs); abnormal undifferentiated prosensory-like cells were present in the lateral compartment instead of OHCs and their adjacent supporting cells. This suggests that Hh signaling inhibits prosensory cell differentiation into hair cells or supporting cells and maintains their properties as prosensory cells. Conversely, in cochlea with the Smo conditional knockout (Smo CKO), hair cell differentiation was preferentially accelerated in the apical region. Smo CKO mice survived after birth, and exhibited hair cell disarrangement in the apical region, a decrease in hair cell number, and hearing impairment. These results indicate that Hh signaling delays hair cell and supporting cell differentiation in the apical region, which forms the basal-to-apical wave of development, and is required for the proper differentiation, arrangement and survival of hair cells and for hearing ability.

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.

Mechanisms that determine the internal environment of the developing brain: a transcriptomic, functional and ultrastructural approach

We provide comprehensive identification of embryonic (E15) and adult rat lateral ventricular choroid plexus transcriptome, with focus on junction-associated proteins, ionic influx transporters and channels. Additionally, these data are related to new structural and previously published permeability studies. Results reveal that most genes associated with intercellular junctions are expressed at similar levels at both ages. In total, 32 molecules known to be associated with brain barrier interfaces were identified. Nine claudins showed unaltered expression, while two claudins (6 and 8) were expressed at higher levels in the embryo. Expression levels for most cytoplasmic/regulatory adaptors (10 of 12) were similar at the two ages. A few junctional genes displayed lower expression in embryos, including 5 claudins, occludin and one junctional adhesion molecule. Three gap junction genes were enriched in the embryo. The functional effectiveness of these junctions was assessed using blood-delivered water-soluble tracers at both the light and electron microscopic level: embryo and adult junctions halted movement of both 286Da and 3kDa molecules into the cerebrospinal fluid (CSF). The molecular identities of many ion channel and transporter genes previously reported as important for CSF formation and secretion in the adult were demonstrated in the embryonic choroid plexus (and validated with immunohistochemistry of protein products), but with some major age-related differences in expression. In addition, a large number of previously unidentified ion channel and transporter genes were identified for the first time in plexus epithelium. These results, in addition to data obtained from electron microscopical and physiological permeability experiments in immature brains, indicate that exchange between blood and CSF is mainly transcellular, as well-formed tight junctions restrict movement of small water-soluble molecules from early in development. These data strongly indicate the brain develops within a well-protected internal environment and the exchange between the blood, brain and CSF is transcellular and not through incomplete barriers.

Hippocampal development - old and new findings

The hippocampus, derived from medial regions of the telencephalon, constitutes a remarkable brain structure. It is part of the limbic system, and it plays important roles in information encoding, related to short-term and long-term memory, and spatial navigation. It has also attracted the attention of many clinicians and neuroscientists for its involvement in a wide spectrum of pathological conditions, including epilepsy, intellectual disability, Alzheimer disease and others. Here we address the topic of hippocampal development. As well as original landmark findings, modern techniques such as large-scale in situ hybridizations, in utero electroporation and the study of mouse mutants with hippocampal phenotypes, add further detail to our knowledge of the finely regulated processes which form this intricate structure. Molecular signatures are being revealed related to field, intra-field and laminar cell identity, as well as, cell compartments expressing surface proteins instrumental for connectivity. We summarize here old and new findings, and highlight elegant tools used to fine-study hippocampal development.

BMP signaling in telencephalic neural cell specification and maturation

Bone morphogenetic proteins (BMPs) make up a family of morphogens that are critical for patterning, development, and function of the central and peripheral nervous system. Their effects on neural cells are pleiotropic and highly dynamic depending on the stage of development and the local niche. Neural cells display a broad expression profile of BMP ligands, receptors, and transducer molecules. Moreover, interactions of BMP signaling with other incoming morphogens and signaling pathways are crucial for most of these processes. The key role of BMP signaling suggests that it includes many regulatory mechanisms that restrict BMP activity both temporally and spatially. BMPs affect neural cell fate specification in a dynamic fashion. Initially they inhibit proliferation of neural precursors and promote the first steps in neuronal differentiation. Later on, BMP signaling effects switch from neuronal induction to promotion of astroglial identity and inhibition of neuronal or oligodendroglial lineage commitment. Furthermore, in postmitotic cells, BMPs regulate cell survival and death, to modulate neuronal subtype specification, promote dendritic and axonal growth and induce synapse formation and stabilization. In this review, we examine the canonical and non-canonical mechanisms of BMP signal transduction. Moreover, we focus on the specific role of BMPs in the nervous system including their ability to regulate neural stem cell proliferation, self-renewal, lineage specification, and neuronal function.

Canonical Notch2 signaling determines biliary cell fates of embryonic hepatoblasts and adult hepatocytes independent of Hes1

Notch signaling through the Notch2 receptor is essential for normal biliary tubulogenesis during liver development. However, the signaling events downstream of Notch2 critical for this process are less well defined. Furthermore, whether Notch signaling also underlies adult hepatic cell fate decisions is largely unknown. By implementing different genetic mouse models, we provide a comprehensive analysis that defines the role of Notch in cell fate control in the developing and adult liver. We show that cell-specific activation of Notch2 signaling by a Notch2IC (N2IC) transgene leads to rapid biliary specification of embryonic hepatoblasts, but also-when expressed in up to 6-month-old adult livers-rapidly reprograms adult hepatocytes to biliary cells with formation of tubular-cystic structures. When directed specifically to the adult biliary and facultative liver progenitor cell compartment, Notch2 is capable of inducing a ductular reaction. Furthermore, we characterized the significance of key effectors of canonical Notch signaling during normal development and in N2IC-expressing models. We demonstrate that tubule formation of intrahepatic bile ducts during embryonic development as well as N2IC-induced specification and morphogenesis of embryonic hepatoblasts and biliary conversion of adult hepatocytes all critically rely on canonical Notch signaling via recombination signal binding protein (RBP)-Jκ but do not require Hes1.

CONCLUSION

Notch2 appears to be the main determinant not only of biliary commitment of embryonic hepatoblasts during development but also of biliary reprogramming of adult hepatocytes. Notch2-dictated cell fates and morphogenesis in both embryonic hepatoblasts and adult hepatocytes rely on canonical Notch signaling but do not require Hes1. Adult liver cells possess a remarkable plasticity to assume new cell fates when embryonic signaling pathways are active. (HEPATOLOGY 2013).

Dynamic interactions between intermediate neurogenic progenitors and radial glia in embryonic mouse neocortex: potential role in Dll1-Notch signaling

The mammalian neocortical progenitor cell niche is composed of a diverse repertoire of neuroepithelial cells, radial glia (RG), and intermediate neurogenic progenitors (INPs). Previously, live-cell imaging experiments have proved crucial in identifying these distinct progenitor populations, especially INPs, which amplify neural output by undergoing additional rounds of proliferation before differentiating into new neurons. INPs also provide feedback to the RG pool by serving as a source of Delta-like 1 (Dll1), a key ligand for activating Notch signaling in neighboring cells, a well-known mechanism for maintaining RG identity. While much is known about Dll1-Notch signaling at the molecular level, little is known about how this cell-cell contact dependent feedback is transmitted at the cellular level. To investigate how RG and INPs might interact to convey Notch signals, we used high-resolution live-cell multiphoton microscopy (MPM) to directly observe cellular interactions and dynamics, in conjunction with Notch-pathway specific reporters in the neocortical neural stem cell niche in organotypic brain slices from embryonic mice. We found that INPs and RG interact via dynamic and transient elongate processes, some apparently long-range (extending from the subventricular zone to the ventricular zone), and some short-range (filopodia-like). Gene expression profiling of RG and INPs revealed further progenitor cell diversification, including different subpopulations of Hes1+ and/or Hes5+ RG, and Dll1+ and/or Dll3+ INPs. Thus, the embryonic progenitor niche includes a network of dynamic cell-cell interactions, using different combinations of Notch signaling molecules to maintain and likely diversify progenitor pools.

Neocortical arealization: evolution, mechanisms, and open questions

The mammalian neocortex is a structure with no equals in the vertebrates and is the seat of the highest cerebral functions, such as thoughts and consciousness. It is radially organized into six layers and tangentially subdivided into functional areas deputed to the elaboration of sensory information, association between different stimuli, and selection and triggering of voluntary movements. The process subdividing the neocortical field into several functional areas is called "arealization". Each area has its own cytoarchitecture, connectivity, and peculiar functions. In the last century, several neuroscientists have investigated areal structure and the mechanisms that have led during evolution to the rising of the neocortex and its organization. The extreme conservation in the positioning and wiring of neocortical areas among different mammalian families suggests a conserved genetic program orchestrating neocortical patterning. However, the impressive plasticity of the neocortex, which is able to rewire and reorganize areal structures and connectivity after impairments of sensory pathways, argues for a more complex scenario. Indeed, even if genetics and molecular biology helped in identifying several genes involved in the arealization process, the logic underlying the neocortical bauplan is still beyond our comprehension. In this review, we will introduce the present knowledge and hypotheses on the ontogenesis and evolution of neocortical areas. Then, we will focus our attention on some open issues, which are still unresolved, and discuss some recent studies that might open new directions to be explored in the next few years.

Foxg1 coordinates the switch from nonradially to radially migrating glutamatergic subtypes in the neocortex through spatiotemporal repression

The specification of neuronal subtypes in the cerebral cortex proceeds in a temporal manner; however, the regulation of the transitions between the sequentially generated subtypes is poorly understood. Here, we report that the forkhead box transcription factor Foxg1 coordinates the production of neocortical projection neurons through the global repression of a default gene program. The delayed activation of Foxg1 was necessary and sufficient to induce deep-layer neurogenesis, followed by a sequential wave of upper-layer neurogenesis. A genome-wide analysis revealed that Foxg1 binds to mammalian-specific noncoding sequences to repress over 12 transcription factors expressed in early progenitors, including Ebf2/3, Dmrt3, Dmrta1, and Eya2. These findings reveal an unexpected prolonged competence of progenitors to initiate corticogenesis at a progressed stage during development and identify Foxg1 as a critical initiator of neocorticogenesis through spatiotemporal repression, a system that balances the production of nonradially and radially migrating glutamatergic subtypes during mammalian cortical expansion.

Role for Lhx2 in corticogenesis through regulation of progenitor differentiation

The neocortex represents the brain region that has undergone a major increase in its relative size during the course of mammalian evolution. The larger cortex results from a corresponding increase in progenitor cell number. The progenitors giving rise to neocortex are located in the ventricular zone of the dorsal telencephalon and highly express Lhx2, a LIM-homeodomain transcription factor. The neocortex fails to form in the Lhx2 constitutive knockout, indicating a role for Lhx2 in corticogenesis, but mid-embryonic lethality of the Lhx2 knockout requires the use of conditional strategies for further studies. Therefore, to explore Lhx2 function in neocortical progenitors, we generated mice with Lhx2 conditionally deleted from cortical progenitors at the onset of neurogenesis. We find that Lhx2 is critical for maintaining the proliferative state of neocortical progenitors during corticogenesis. In the conditional knockouts, the neocortex is formed but is significantly smaller than wild type. We find that deletion of Lhx2 leads to significantly decreased numbers of cortical progenitors and premature neuronal differentiation. A likely mechanism is indicated by our findings that Lhx2 is required for the expression of Hes1 in cortical progenitors, a key effector in the Notch signaling pathway that maintains the proliferative progenitor state. We conclude that Lhx2 regulates the balance between proliferation and differentiation in cortical progenitors and through this mechanism Lhx2 controls cortical size.

The transcription factor Otx2 regulates choroid plexus development and function

The choroid plexuses (ChPs) are the main regulators of cerebrospinal fluid (CSF) composition and thereby also control the composition of a principal source of signaling molecules that is in direct contact with neural stem cells in the developing brain. The regulators of ChP development mediating the acquisition of a fate that differs from the neighboring neuroepithelial cells are poorly understood. Here, we demonstrate in mice a crucial role for the transcription factor Otx2 in the development and maintenance of ChP cells. Deletion of Otx2 by the Otx2-CreERT2 driver line at E9 resulted in a lack of all ChPs, whereas deletion by the Gdf7-Cre driver line affected predominately the hindbrain ChP, which was reduced in size, primarily owing to an increase in apoptosis upon Otx2 deletion. Strikingly, Otx2 was still required for the maintenance of hindbrain ChP cells at later stages when Otx2 deletion was induced at E15, demonstrating a central role of Otx2 in ChP development and maintenance. Moreover, the predominant defects in the hindbrain ChP mediated by Gdf7-Cre deletion of Otx2 revealed its key role in regulating early CSF composition, which was altered in protein content, including the levels of Wnt4 and the Wnt modulator Tgm2. Accordingly, proliferation and Wnt signaling levels were increased in the distant cerebral cortex, suggesting a role of the hindbrain ChP in regulating CSF composition, including key signaling molecules. Thus, Otx2 acts as a master regulator of ChP development, thereby influencing one of the principal sources of signaling in the developing brain, the CSF.

Intrahepatic cholangiocarcinoma can arise from Notch-mediated conversion of hepatocytes

Intrahepatic cholangiocarcinoma (ICC) is the second most common primary malignancy in the liver. ICC has been classified as a malignant tumor arising from cholangiocytes; however, the co-occurrence of ICC and viral hepatitis suggests that ICC originates in hepatocytes. In order to determine the cellular origin of ICC, we used a mouse model of ICC in which hepatocytes and cholangiocytes were labeled with heritable, cell type–specific reporters. Our studies reveal that ICC is generated by biliary lineage cells derived from hepatocytes, rather than cholangiocytes. Additionally, we found that Notch activation is critical for hepatocyte conversion into biliary lineage cells during the onset of ICC and its subsequent malignancy and progression. These findings will help to elucidate the pathogenic mechanism of ICC and to develop therapeutic strategies for this refractory disease.

Lhx2 regulates the development of the forebrain hem system

Early brain development is regulated by the coordinated actions of multiple signaling centers at key boundaries between compartments. Three telencephalic midline structures are in a position to play such roles in forebrain patterning: The cortical hem, the septum, and the thalamic eminence at the diencephalic–telencephalic boundary. These structures express unique complements of signaling molecules, and they also produce distinct populations of Cajal–Retzius cells, which are thought to act as “mobile patterning units,” migrating tangentially to cover the telencephalic surface. We show that these 3 structures require the transcription factor Lhx2 to delimit their extent. In the absence of Lhx2 function, all 3 structures are greatly expanded, and the Cajal–Retzius cell population is dramatically increased. We propose that the hem, septum, and thalamic eminence together form a “forebrain hem system” that defines and regulates the formation of the telencephalic midline. Disruptions in the forebrain hem system may be implicated in severe brain malformations such as holoprosencephaly. Lhx2 functions as a central regulator of this system's development. Since all components of the forebrain hem system have been identified across several vertebrate species, the mechanisms that regulate them may have played a fundamental role in driving key aspects of forebrain evolution.

Dual origins of the mammalian accessory olfactory bulb revealed by an evolutionarily conserved migratory stream

The accessory olfactory bulb (AOB) is a critical olfactory structure that has been implicated in mediating social behavior. It receives input from the vomeronasal organ and projects to targets in the amygdaloid complex. Its anterior and posterior components (aAOB and pAOB) display molecular, connectional and functional segregation in processing reproductive and defensive and aggressive behaviors, respectively. We observed a dichotomy in the development of the projection neurons of the aAOB and pAOB in mice. We found that they had distinct sites of origin and that different regulatory molecules were required for their specification and migration. aAOB neurons arose locally in the rostral telencephalon, similar to main olfactory bulb neurons. In contrast, pAOB neurons arose caudally, from the neuroepithelium of the diencephalic-telencephalic boundary, from which they migrated rostrally to reach their destination. This unusual origin and migration is conserved in Xenopus, providing an insight into the origin of a key component of this system in evolution.

Hes repressors are essential regulators of hematopoietic stem cell development downstream of Notch signaling

Previous studies have identified Notch as a key regulator of hematopoietic stem cell (HSC) development, but the underlying downstream mechanisms remain unknown. The Notch target Hes1 is widely expressed in the aortic endothelium and hematopoietic clusters, though Hes1-deficient mice show no overt hematopoietic abnormalities. We now demonstrate that Hes is required for the development of HSC in the mouse embryo, a function previously undetected as the result of functional compensation by de novo expression of Hes5 in the aorta/gonad/mesonephros (AGM) region of Hes1 mutants. Analysis of embryos deficient for Hes1 and Hes5 reveals an intact arterial program with overproduction of nonfunctional hematopoietic precursors and total absence of HSC activity. These alterations were associated with increased expression of the hematopoietic regulators Runx1, c-myb, and the previously identified Notch target Gata2. By analyzing the Gata2 locus, we have identified functional RBPJ-binding sites, which mutation results in loss of Gata2 reporter expression in transgenic embryos, and functional Hes-binding sites, which mutation leads to specific Gata2 up-regulation in the hematopoietic precursors. Together, our findings show that Notch activation in the AGM triggers Gata2 and Hes1 transcription, and next HES-1 protein represses Gata2, creating an incoherent feed-forward loop required to restrict Gata2 expression in the emerging HSCs.

Essential roles of the histone methyltransferase ESET in the epigenetic control of neural progenitor cells during development

In the developing brain, neural progenitor cells switch differentiation competency by changing gene expression profiles that are governed partly by epigenetic control, such as histone modification, although the precise mechanism is unknown. Here we found that ESET (Setdb1), a histone H3 Lys9 (H3K9) methyltransferase, is highly expressed at early stages of mouse brain development but downregulated over time, and that ablation of ESET leads to decreased H3K9 trimethylation and the misregulation of genes, resulting in severe brain defects and early lethality. In the mutant brain, endogenous retrotransposons were derepressed and non-neural gene expression was activated. Furthermore, early neurogenesis was severely impaired, whereas astrocyte formation was enhanced. We conclude that there is an epigenetic role of ESET in the temporal and tissue-specific gene expression that results in proper control of brain development.

MicroRNA9 regulates neural stem cell differentiation by controlling Hes1 expression dynamics in the developing brain

Earlier studies show that Hes1 expression is oscillatory in neural stem cells but sustained and high in the roof plate and the floor plate, and that such different dynamics of Hes1 expression (oscillatory versus sustained) regulate different proliferation and differentiation characteristics of these cells (active in neural stem cells but rather dormant in roof/floor plate cells). The mechanism of how different dynamics of Hes1 expression is controlled remains to be determined. Here, we found that the seed sequence of microRNA-9 (miR-9) is complementary to the 3'-UTR sequence of Hes1 mRNA. MiR-9 is highly expressed in the ventricular zone of the developing brain, which contains neural stem cells, but it is not expressed in the roof plate or the floor plate. Over-expression of miR-9 negatively regulates the Hes1 protein expression by interacting with the 3'-UTR of Hes1 mRNA, thereby inducing cell cycle exit and neuronal differentiation. Conversely, knockdown of miR-9 inhibits neuronal differentiation. Furthermore, knockdown of miR-9 inhibits the oscillatory expression of Hes1 mRNA in neural stem cells. These results indicate that miR-9 regulates the proliferation and differentiation of neural stem cells by controlling the dynamics of Hes1 expression in the developing brain.

The roof plate boundary is a bi-directional organiser of dorsal neural tube and choroid plexus development

The roof plate is a signalling centre positioned at the dorsal midline of the central nervous system and generates dorsalising morphogenic signals along the length of the neuraxis. Within cranial ventricles, the roof plate gives rise to choroid plexus, which regulates the internal environment of the developing and adult brain and spinal cord via the secretion of cerebrospinal fluid. Using the fourth ventricle as our model, we show that the organiser properties of the roof plate are determined by its boundaries with the adjacent neuroepithelium. Through a combination of in ovo transplantation, co-culture and electroporation techniques in chick embryos between embryonic days 3 and 6, we demonstrate that organiser properties are maintained by interactions between the non-neural roof plate and the neural rhombic lip. At the molecular level, this interaction is mediated by Delta-Notch signalling and upregulation of the chick homologue of Hes1: chairy2. Gain- and loss-of-function approaches reveal that cdelta1 is both necessary and sufficient for organiser function. Our results also demonstrate that while chairy2 is specifically required for the maintenance of the organiser, its ectopic expression is not sufficient to recapitulate organiser properties. Expression of atonal1 in the rhombic lip adjacent at the roof plate boundary is acutely dependent on both boundary cell interactions and Delta-Notch signalling. Correspondingly, the roof plate boundary organiser also signals to the roof plate itself to specify the expression of early choroid plexus markers. Thus, the roof plate boundary organiser signals bi-directionally to acutely coordinate the development of adjacent neural and non-neural tissues.

Genetic visualization of notch signaling in mammalian neurogenesis

Notch signaling plays crucial roles in fate determination and the differentiation of neural stem cells in embryonic and adult brains. It is now clear that the notch pathway is under more complex and dynamic regulation than previously thought. To understand the functional details of notch signaling more precisely, it is important to reveal when, where, and how notch signaling is dynamically communicated between cells, for which the visualization of notch signaling is essential. In this review, we introduce recent technical advances in the visualization of notch signaling during neural development and in the adult brain, and we discuss the physiological significance of dynamic regulation of notch signaling.

The doublesex homolog Dmrt5 is required for the development of the caudomedial cerebral cortex in mammals

Regional patterning of the cerebral cortex is initiated by morphogens secreted by patterning centers that establish graded expression of transcription factors within cortical progenitors. Here, we show that Dmrt5 is expressed in cortical progenitors in a high-caudomedial to low-rostrolateral gradient. In its absence, the cortex is strongly reduced and exhibits severe abnormalities, including agenesis of the hippocampus and choroid plexus and defects in commissural and thalamocortical tracts. Loss of Dmrt5 results in decreased Wnt and Bmp in one of the major telencephalic patterning centers, the dorsomedial telencephalon, and in a reduction of Cajal-Retzius cells. Expression of the dorsal midline signaling center-dependent transcription factors is downregulated, including Emx2, which promotes caudomedial fates, while the rostral determinant Pax6, which is inhibited by midline signals, is upregulated. Consistently, Dmrt5(-/-) brains exhibit patterning defects with a dramatic reduction of the caudomedial cortex. Dmrt5 is increased upon the activation of Wnt signaling and downregulated in Gli3(xt/xt) mutants. We conclude that Dmrt5 is a novel Wnt-dependent transcription factor required for early cortical development and that it may regulate initial cortical patterning by promoting dorsal midline signaling center formation and thereby helping to establish the graded expression of the other transcription regulators of cortical identity.

POU-III transcription factors (Brn1, Brn2, and Oct6) influence neurogenesis, molecular identity, and migratory destination of upper-layer cells of the cerebral cortex

The upper layers (II-IV) are the most prominent distinguishing feature of mammalian neocortex compared with avian or reptilian dorsal cortex, and are vastly expanded in primates. Although the time-dependent embryonic generation of upper-layer cells is genetically instructed within their parental progenitors, mechanisms governing cell-intrinsic fate transitions remain obscure. POU-homeodomain transcription factors Pou3f3 and Pou3f2 (Brn1 and Brn2) are known to label postmitotic upper-layer cells, and are redundantly required for their production. We find that the onset of Pou3f3/2 expression actually occurs in ventricular zone (VZ) progenitors, and that Pou3f3/2 subsequently label neural progeny switching from deep-layer Ctip2(+) identity to Satb2(+) upper-layer fate as they migrate to proper superficial positions. By using an Engrailed dominant-negative repressor, we show that sustained neurogenesis after the deep- to upper-layer transition requires the proneual action of Pou3fs in VZ progenitors. Conversely, single-gene overexpression of any Pou3f in early neural progenitors is sufficient to specify the precocious birth of Satb2(+) daughter neurons that extend axons to the contralateral hemisphere, as well as exhibit robust pia-directed migration that is characteristic of upper-layer cells. Finally, we demonstrate that Pou3fs influence multiple stages of neurogenesis by suppressing Notch effector Hes5, and promoting the expression of proneural transcription factors Tbr2 and Tbr1.

NOTCH1 is required for regeneration of Clara cells during repair of airway injury

The airways of the mammalian lung are lined with highly specialized epithelial cell types that are the targets of airborne toxicants and injury. Notch signaling plays an important role in the ontogeny of airway epithelial cells, but its contributions to recruitment, expansion or differentiation of resident progenitor/stem cells, and repair and re-establishment of the normal composition of airway epithelium following injury have not been addressed. In this study, the role of a specific Notch receptor, Notch1, was investigated by targeted inactivation in the embryonic lung epithelium using the epithelial-specific Gata5-Cre driver line. Notch1-deficient mice are viable without discernible defects in pulmonary epithelial cell-fate determination and differentiation. However, in an experimental model of airway injury, activity of Notch1 is found to be required for normal repair of the airway epithelium. Absence of Notch1 reduced the ability of a population of cells distinguished by expression of PGP9.5, otherwise a marker of pulmonary neuroendocrine cells, which appears to serve as a reservoir for regeneration of Clara cells. Hairy/enhancer of split-5 (Hes5) and paired-box-containing gene 6 (Pax6) were found to be downstream targets of Notch1. Both Hes5 and Pax6 expressions were significantly increased in association with Clara cell regeneration in wild-type lungs. Ablation of Notch1 reduced Hes5 and Pax6 and inhibited airway epithelial repair. Thus, although dispensable in developmental ontogeny of airway epithelial cells, normal activity of Notch1 is required for repair of the airway epithelium. The signaling pathway by which Notch1 regulates the repair process includes stimulation of Hes5 and Pax6 gene expression.

Comparative and evolutionary analysis of the HES/HEY gene family reveal exon/intron loss and teleost specific duplication events

Background

HES/HEY genes encode a family of basic helix-loop-helix (bHLH) transcription factors with both bHLH and Orange domain. HES/HEY proteins are direct targets of the Notch signaling pathway and play an essential role in developmental decisions, such as the developments of nervous system, somitogenesis, blood vessel and heart. Despite their important functions, the origin and evolution of this HES/HEY gene family has yet to be elucidated.

Methods and Findings

In this study, we identified genes of the HES/HEY family in representative species and performed evolutionary analysis to elucidate their origin and evolutionary process. Our results showed that the HES/HEY genes only existed in metazoans and may originate from the common ancestor of metazoans. We identified HES/HEY genes in more than 10 species representing the main lineages. Combining the bHLH and Orange domain sequences, we constructed the phylogenetic trees by different methods (Bayesian, ML, NJ and ME) and classified the HES/HEY gene family into four groups. Our results indicated that this gene family had undergone three expansions, which were along with the origins of Eumetazoa, vertebrate, and teleost. Gene structure analysis revealed that the HES/HEY genes were involved in exon and/or intron loss in different species lineages. Genes of this family were duplicated in bony fishes and doubled than other vertebrates. Furthermore, we studied the teleost-specific duplications in zebrafish and investigated the expression pattern of duplicated genes in different tissues by RT-PCR. Finally, we proposed a model to show the evolution of this gene family with processes of expansion, exon/intron loss, and motif loss.

Conclusions

Our study revealed the evolution of HES/HEY gene family, the expression and function divergence of duplicated genes, which also provide clues for the research of Notch function in development. This study shows a model of gene family analysis with gene structure evolution and duplication.

Notch-Rbpj signaling is required for the development of noradrenergic neurons in the mouse locus coeruleus

The locus coeruleus (LC) is the main source of noradrenaline in the brain and is implicated in a broad spectrum of physiological and behavioral processes. However, genetic pathways controlling the development of noradrenergic neurons in the mammalian brain are largely unknown. We report here that Rbpj, a key nuclear effector in the Notch signaling pathway, plays an essential role in LC neuron development in the mouse. Conditional inactivation of Rbpj in the dorsal rhombomere (r) 1, where LC neurons are born, resulted in a dramatic increase in the number of Phox2a- and Phox2b-expressing early-differentiating LC neurons, and dopamine-β-hydroxylase- and tyrosine-hydroxylase-expressing late-differentiating LC neurons. In contrast, other neuronal populations derived from the dorsal r1 were either reduced or unchanged. In addition, a drastic upregulation of Ascl1, an essential factor for noradrenergic neurogenesis, was observed in dorsal r1 of conditional knockout mice. Through genomic sequence analysis and EMSA and ChIP assays, a conserved Rbpj-binding motif was identified within the Ascl1 promoter. A luciferase reporter assay revealed that Rbpj per se could induce Ascl1 transactivation but this effect was counteracted by its downstream-targeted gene Hes1. Moreover, our in vitro gene transfection and in ovo electroporation assays showed that Rbpj upregulated Ascl1 expression when Hes1 expression was knocked down, although it also exerted a repressive effect on Ascl1 expression in the presence of Hes1. Thus, our results provide the first evidence that Rbpj functions as a key modulator of LC neuron development by regulating Ascl1 expression directly, and indirectly through its target gene Hes1.

Mind bomb 1 is required for pancreatic β-cell formation

During early pancreatic development, Notch signaling represses differentiation of endocrine cells and promotes proliferation of Nkx6-1(+)Ptf1a(+) multipotent progenitor cells (MPCs). Later, antagonistic interactions between Nkx6 transcription factors and Ptf1a function to segregate MPCs into distal Nkx6-1(-)Ptf1a(+) acinar progenitors and proximal Nkx6-1(+)Ptf1a(-) duct and β-cell progenitors. Distal cells are initially multipotent, but evolve into unipotent, acinar cell progenitors. Conversely, proximal cells are bipotent and give rise to duct cells and late-born endocrine cells, including the insulin producing β-cells. However, signals that regulate proximodistal (P-D) patterning and thus formation of β-cell progenitors are unknown. Here we show that Mind bomb 1 (Mib1) is required for correct P-D patterning of the developing pancreas and β-cell formation. We found that endoderm-specific inactivation of Mib1 caused a loss of Nkx6-1(+)Ptf1a(-) and Hnf1β(+) cells and a corresponding loss of Neurog3(+) endocrine progenitors and β-cells. An accompanying increase in Nkx6-1(-)Ptf1a(+) and amylase(+) cells, occupying the proximal domain, suggests that proximal cells adopt a distal fate in the absence of Mib1 activity. Impeding Notch-mediated transcriptional activation by conditional expression of dominant negative Mastermind-like 1 (Maml1) resulted in a similarly distorted P-D patterning and suppressed β-cell formation, as did conditional inactivation of the Notch target gene Hes1. Our results reveal iterative use of Notch in pancreatic development to ensure correct P-D patterning and adequate β-cell formation.

Early B-cell factors 2 and 3 (EBF2/3) regulate early migration of Cajal-Retzius cells from the cortical hem

Cajal–Retzius (CR) cells play a crucial role in the formation of the cerebral cortex, yet the molecules that control their development are largely unknown. Here, we show that Ebf transcription factors are expressed in forebrain signalling centres—the septum, cortical hem and the pallial–subpallial boundary—known to generate CR cells. We identified Ebf2, through fate mapping studies, as a novel marker for cortical hem- and septum-derived CR cells. Loss of Ebf2 in vivo causes a transient decrease in CR cell numbers on the cortical surface due to a migratory defect in the cortical hem, and is accompanied by upregulation of Ebf3 in this and other forebrain territories that produce CR cells, without affecting proper cortical lamination. Accordingly, using in vitro preparations, we demonstrated that both Ebf2 and Ebf3, singly or together, control the migration of CR cells arising in the cortical hem. These findings provide evidence that Ebfs directly regulate CR cell development.

The role of Hes genes in intestinal development, homeostasis and tumor formation

Notch signaling regulates intestinal development, homeostasis and tumorigenesis, but its precise downstream mechanism remains largely unknown. Here we found that inactivation of the Notch effectors Hes1, Hes3 and Hes5, but not Hes1 alone, led to reduced cell proliferation, increased secretory cell formation and altered intestinal structures in adult mice. However, in Apc mutation-induced intestinal tumors, inactivation of Hes1 alone was sufficient for reducing tumor cell proliferation and inducing differentiation of tumor cells into all types of intestinal epithelial cells, but without affecting the homeostasis of normal crypts owing to genetic redundancy. These results indicated that Hes genes cooperatively regulate intestinal development and homeostasis and raised the possibility that Hes1 is a promising target to induce the differentiation of tumor cells.

NEPRO: a novel Notch effector for maintenance of neural progenitor cells in the neocortex

The Notch pathway is essential for maintaining neural progenitor cells (NPCs) in the developing brain. Activation of the pathway is sufficient to maintain NPCs, whereas loss-of-function mutations in the critical components of the pathway cause precocious neuronal differentiation and NPC depletion. Hairy and Enhancer of split (Hes)-type transcription factors have long been thought to be the only Notch effectors for the maintenance of NPCs. Recently, a novel nuclear protein, Nepro, has been identified as another critical effector of Notch. The Notch pathway is bifurcated into Nepro and Hes-type proteins in the early development of the neocortex. The combination of Nepro and Hes-type proteins is necessary and sufficient for maintaining NPCs downstream of Notch.

Neurogenin2 regulates the initial axon guidance of cortical pyramidal neurons projecting medially to the corpus callosum

Background

The formation of the mammalian central nervous system requires the establishment of complex neural circuits between a diverse array of neuronal subtypes. Here we report that the proneural transcription factor Neurogenin2 (Ngn2) is crucial for the proper specification of cortical axon projections.

Results

The genetic loss of Ngn2 in mice results in fewer callosal axons projecting towards the midline as well as abnormal midline crossing. shRNA-mediated knockdown of Ngn2 revealed its cell-autonomous requirement for the proper projection of axons from layer 2/3 pyramidal neurons to the midline in vivo. We found that the acute loss of Ngn2 in vivo induces the axon of superficial layer 2/3 neurons to project laterally towards aberrant cortical and subcortical targets.

Conclusions

These and previous results demonstrate that Ngn2 is required for the coordinated specification of cardinal features defining the phenotype of cortical pyramidal neurons, including their migration properties, dendritic morphology and axonal projection.

Mouse in utero electroporation: controlled spatiotemporal gene transfection

In order to understand the function of genes expressed in specific region of the developing brain, including signaling molecules and axon guidance molecules, local gene transfer or knock- out is required. Gene targeting knock-in or knock-out into local regions is possible to perform with combination with a specific CRE line, which is laborious, costly, and time consuming. Therefore, a simple transfection method, an in utero electroporation technique, which can be performed with short time, will be handy to test the possible function of candidate genes prior to the generation of transgenic animals. In addition to this, in utero electroporation targets areas of the brain where no specific CRE line exists, and will limit embryonic lethality. Here, we present a method of in utero electroporation combining two different types of electrodes for simple and convenient gene transfer into target areas of the developing brain. First, a unique holding method of embryos using an optic fiber optic light cable will make small embryos (from E9.5) visible for targeted DNA solution injection into ventricles and needle type electrodes insertion to the targeted brain area. The patterning of the brain such as cortical area occur at early embryonic stage, therefore, these early electroporation from E9.5 make a big contribution to understand entire area patterning event. Second, the precise shape of a capillary prevents uterine damage by making holes by insertion of the capillary. Furthermore, the precise shape of the needle electrodes are created with tungsten and platinum wire and sharpened using sand paper and insulated with nail polish, a method which is described in great detail in this protocol. This unique technique allows transfection of plasmid DNA into restricted areas of the brain and will enable small embryos to be electroporated. This will help to, open a new window for many scientists who are working on cell differentiation, cell migration, axon guidance in very early embryonic stage. Moreover, this technique will allow scientists to transfect plasmid DNA into deep parts of the developing brain such as thalamus and hypothalamus, where not many region-specific CRE lines exist for gain of function (GOF) or loss of function (LOF) analyses.

Pallio-pallial tangential migrations and growth signaling: new scenario for cortical evolution?

Observations accruing in recent years imply that the areal patterning and size dimensioning of the mammalian neocortex are influenced by diverse sets of tangentially migrating glutamatergic neurons that invade the cortical plate and, in so doing, modify the properties of the neopallial proliferative compartments. This developmental scenario sheds new light upon the old issue of how the mammalian neocortex evolved its more complex structure from nonmammalian antecedent forms. In reviewing these novelties, I first point out the topological position of the neopallial island as a central component of the pallium in all gnathostomes, surrounded by a ring of prospective allocortical pallial regions and a more distant set of peripheral neighboring forebrain areas. Early patterning arises from the periphery via passive planar signaling. This process probably establishes the pallium field and its basic island plus allocortical ring organization, as well as a rough prepatterning of some regional subareas. Afterwards, patterning and modulated growth are also actively influenced by the convergence of separate streams of tangentially migrating subpial cells (partly peripheral and partly allocortical in origin) which collectively form the Cajal-Retzius neuronal population in layer I. Effects of these cells include the inside-out stratification of the cortical plate and they may also contribute to the evolutionary emergence of the 6-layered neocortical structure. The most recent addition to our knowledge of pallio-pallial migrations is the existence of a subsequent deep tangential migration of ventropallial cells into the neopallial primordium, whose signaling influence upon local progenitors magnifies the cortex population by 20%. These glutamatergic cells dispersedly invade the entire cortex but largely die postnatally. The crucial implications of these data for comparative thinking on mammalian neocortex evolution and interpretation of potential homologs in sauropsids are explored. Finally, a new conjecture regarding a possible role of the hitherto disregarded lateral pallium is advanced.

Notch-Hes1 pathway is required for the development of IL-17-producing γδ T cells

Unlike conventional T cells, which are exported from the thymus as naive cells and acquire effector functions upon antigen encounter in the periphery, a subset of γδ T cells differentiates into effectors that produce IL-17 within the fetal thymus. We demonstrate here that intrathymic development of the naturally occurring IL-17-producing γδ T cells is independent of STAT3 and partly dependent on RORγt. Comparative gene-expression analysis identified Hes1, one of the basic helix-loop-helix proteins involved in Notch signaling, as a factor specifically expressed in IL-17-producing γδ T cells. Hes1 is critically involved in the development of IL-17-producing γδ T cells, as evidenced by their severe decrease in the thymi of Hes1-deficient fetal mice. Delta-like 4 (Dll4)-expressing stromal cells support the development of IL-17-producing γδ T cells in vitro. In addition, conditional Hes1 ablation in peripheral γδ T cells decreases their IL-17 production but not their IFN-γ production. These results reveal a unique differentiation pathway of IL-17-producing γδ T cells.

Genetic evidence that intestinal Notch functions vary regionally and operate through a common mechanism of Math1 repression

Notch signaling is active in many sites, and its diverse activities must require tissue-specific intermediaries, which are largely unknown. In the intestinal epithelium, Notch promotes crypt cell proliferation and inhibits goblet cell differentiation. Pharmacologic studies suggest that the latter effect occurs through the transcription factor Math1/Atoh1, which specifies all intestinal secretory cells. We tested this hypothesis using mouse mutants. Genetic loss of the Notch effector RBP-Jκ alone increases all intestinal secretory lineages, with variation between proximal and distal gut segments. This secretory cell excess observed with RBP-Jκ loss was blocked in the absence of Math1 in RBP-Jκ(Fl/Fl);Math1(Fl/Fl);Villin-Cre((ER-T2)) mice. Loss of both factors also restored progenitor replication, proving that Math1 is epistatic to Notch signaling in both secretory cell differentiation and crypt cell proliferation. Investigating mechanisms downstream of Math1, we found that expression of the known Notch effector protein Hes1 was predictably lost in RBP-Jκ(-/-) mice but surprisingly recovered in RBP-Jκ;Math1 compound conditional mutants. Furthermore, the cell cycle inhibitors p27(Kip1) and p57(Kip2) were selectively overexpressed in duodenal and ileal crypts, respectively, in RBP-Jκ-deficient mice. Regional activation of these products was completely abrogated in the absence of Math1. Thus, all intestinal Notch effects channel through the tissue-restricted factor Math1, which promotes secretory differentiation and cell cycle exit by regionally distinct mechanisms. Our data further suggest that, besides transmitting Notch signals, the transcription factor Hes1 acts downstream of Math1 to regulate expression of cell cycle inhibitors and intestinal crypt cell replication.

Building a human cortex: the evolutionary differentiation of Cajal-Retzius cells and the cortical hem

Cajal-Retzius (CR) cells are the most significant source of reelin, an extracellular matrix glycoprotein essential for cortical development. Strategically located in the marginal zone, CR cells control radial migration and laminar positioning of pyramidal neurons of the cortical plate. They degenerate and undergo cell death when cortical migration is completed. In human cortex development, reelin-expressing CR cells are already present in the early preplate, and continue to increase in number after the appearance of the cortical plate. In the course of the first half of gestation, the reelin signal in the marginal zone undergoes a huge amplification in parallel with the growth of the cortical plate and the expansion of the cortical surface. A significant source of CR cells is the cortical hem, a putative signalling centre at the interface of the prospective hippocampus and the choroid plexus. Hem-derived CR cells co-express reelin and p73, a transcription factor of the p53-family. They form the predominant CR cell population of the human neocortex. Characteristically, CR cells express the anti-apoptotic isoform DeltaNp73 which may be responsible for the protracted lifespan of human CR cells and the morphological differentiation of their axonal plexus. This dense fibre plexus, absent in lower mammals, amplifies the reelin-signal and establishes a physical boundary between the cortical plate and the marginal zone. In this review, we analyze the multiple sources of reelin/p73 positive CR cells at the interface of various telencephalic centres and the choroid plexus of the lateral ventricles. Additional populations of CR cells may derive from the thalamic eminence in the ventral thalamus and from the strionuclear neuroepithelium, or 'amygdalar hem'. Comparative studies in a variety of species indicate that the cortical hem is the main origin of CR cells destined for the neocortex, and is most highly developed in the human brain. The close association between cortical hem and choroid plexus suggests a concerted role in the evolutionary increase of CR cells, amplification of the reelin signal in the marginal zone, and cortical expansion.

Cooperative functions of Hes/Hey genes in auditory hair cell and supporting cell development

Notch-mediated lateral inhibition has been reported to regulate auditory hair cell and supporting cell development from common precursors. While the Notch effector genes Hes1, Hes5 and Hey1 are expressed in the developing cochlea, inactivation of either of them causes only mild abnormality, suggesting their functional redundancy. To explore the roles of Hes/Hey genes in cochlear development, we examined compound heterozygous or homozygous mutant mice that lacked Hes1, Hes5 and Hey1 alleles. We found that a reduction in Hes/Hey gene dosage led to graded increase of hair cell formation. However, if at least one allele of Hes1, Hes5 or Hey1 was intact, excessive hair cells were accompanied by overproduction of supporting cells, suggesting that the hair cell increase does not occur at the expense of supporting cells, and that each Hes/Hey gene functions to induce supporting cells. By contrast, when all alleles of Hes1, Hes5 and Hey1 were inactivated, the number of hair cells increased more drastically, whereas that of supporting cells was unchanged compared with control, suggesting that supporting cell formation was balanced by their overproduction and fate conversion into hair cells. The increase of the cell numbers seemed to occur after the prosensory domain formation in the mutants because the proliferation state and the size of the prosensory domain were not affected. Thus, Hes1, Hes5 and Hey1 cooperatively inhibit hair cell formation, and one allele of Hes1, Hes5 or Hey1 is sufficient for supporting cell production probably by lateral inhibition in the sensory epithelium. Strikingly, Hes/Hey mutations lead to disorganized cell alignment and polarity and to hearing loss despite hair cell overproduction. These results suggest that Hes/Hey gene dosage is essential not only for generation of appropriate numbers of hair cells and supporting cells by controlling cell proliferation and lateral inhibition but also for the hearing ability by regulating the cell alignment and polarity.

Differential regulation of proliferation and differentiation in neural precursor cells by the Jak pathway

Neuronal precursor cells (NPCs) are temporally regulated and have the ability to proliferate and differentiate into mature neurons, oligodendrocytes, and astrocytes in the presence of growth factors (GFs). In the present study, the role of the Jak pathway in brain development was investigated in NPCs derived from neurosphere cultures using Jak2 and Jak3 small interfering RNAs and specific inhibitors. Jak2 inhibition profoundly decreased NPC proliferation, preventing further differentiation into neurons and glial cells. However, Jak3 inhibition induced neuronal differentiation accompanied by neurite growth. This phenomenon was due to the Jak3 inhibition-mediated induction of neurogenin (Ngn)2 and NeuroD in NPCs. Jak3 inhibition induced NPCs to differentiate into scattered neurons and increased the expression of Tuj1, microtubule associated protein 2 (MAP2), Olig2, and neuroglial protein (NG)2, but decreased glial fibrillary acidic protein (GFAP) expression, with predominant neurogenesis/polydendrogenesis compared with astrogliogenesis. Therefore, Jak2 may be important for NPC proliferation and maintenance, whereas knocking-down of Jak3 signaling is essential for NPC differentiation into neurons and oligodendrocytes but does not lead to astrocyte differentiation. These results suggest that NPC proliferation and differentiation are differentially regulated by the Jak pathway.

Her9 represses neurogenic fate downstream of Tbx1 and retinoic acid signaling in the inner ear

Proper spatial control of neurogenesis in the inner ear ensures the precise innervation of mechanotransducing cells and the propagation of auditory and equilibrium stimuli to the brain. Members of the Hairy and enhancer of split (Hes) gene family regulate neurogenesis by inhibiting neuronal differentiation and maintaining neural stem cell pools in non-neurogenic zones. Remarkably, their role in the spatial control of neurogenesis in the ear is unknown. In this study, we identify her9, a zebrafish ortholog of Hes1, as a key gene in regulating otic neurogenesis through the definition of the posterolateral non-neurogenic field. First, her9 emerges as a novel otic patterning gene that represses proneural function and regulates the extent of the neurogenic domain. Second, we place Her9 downstream of Tbx1, linking these two families of transcription factors for the first time in the inner ear and suggesting that the reported role of Tbx1 in repressing neurogenesis is in part mediated by the bHLH transcriptional repressor Her9. Third, we have identified retinoic acid (RA) signaling as the upstream patterning signal of otic posterolateral genes such as tbx1 and her9. Finally, we show that at the level of the cranial otic field, opposing RA and Hedgehog signaling position the boundary between the neurogenic and non-neurogenic compartments. These findings permit modeling of the complex genetic cascade that underlies neural patterning of the otic vesicle.

HES1 (hairy and enhancer of split 1) is a determinant of bone mass

HES1 (hairy and enhancer of split) is a transcription factor that regulates osteoblastogenesis in vitro. The skeletal effects of HES1 misexpression were studied. Transgenic mice where a 3.6-kilobase fragment of the collagen type 1 α1 promoter directs HES1 overexpression were created. Transgenics were osteopenic due to decreased osteoblast function in female and increased bone resorption in male mice. HES1 impaired osteoblastogenesis in vitro, and transgenic osteoblasts enhanced the resorptive activity of co-cultured osteoclast precursors. Mice homozygous for a Hes1 loxP-targeted allele were bred to transgenics, where the paired-related homeobox gene enhancer or the osteocalcin promoter direct Cre recombinase expression to inactivate Hes1 in the limb bud or in osteoblasts. To avoid genetic compensation, Hes1 was inactivated in the context of the global deletion of Hes3 and Hes5. Hes3 and Hes5 null mice had no skeletal phenotype. Hes1 inactivation in the limb bud increased femoral length and trabecular number. Hes1 inactivation in osteoblasts increased trabecular bone volume, number, and connectivity due to increased mineral apposition rate and suppressed bone resorption. Hes1 inactivation in vitro increased alkaline phosphatase expression and suppressed the resorptive activity of co-cultured osteoclast precursors. In conclusion, by inhibiting osteoblast function and inducing bone resorption, HES1 is an intracellular determinant of bone mass and structure.

Notch as a molecular switch in neural stem cells

Notch signaling pathway, originally discovered in Drosophila, impinges on a wide array of cellular processes including cell proliferation, cell differentiation, and cell apoptosis. Recent accumulating evidences implicated the important roles of Notch signal pathway in different aspects of stem cell biology of neural stem cell (NSC). In vivo and in vitro studies illustrated that Notch signal pathway could promote gliogenesis, inhibit premature neurogenesis, and be involved in self-renew of NSC. This short review summarizes the roles of the Notch signaling pathway on gliogenesis, neurogenesis, and self-renew of NSC and their underlying molecular mechanisms.

Development of the lateral ventricular choroid plexus in a marsupial, Monodelphis domestica

BACKGROUND

Choroid plexus epithelial cells are the site of blood/cerebrospinal fluid (CSF) barrier and regulate molecular transfer between the two compartments. Their mitotic activity in the adult is low. During development, the pattern of growth and timing of acquisition of functional properties of plexus epithelium are not known.

METHODS

Numbers and size of choroid plexus epithelial cells and their nuclei were counted and measured in the lateral ventricular plexus from the first day of its appearance until adulthood. Newborn Monodelphis pups were injected with 5-bromo-2-deoxyuridine (BrdU) at postnatal day 3 (P3), P4 and P5. Additional animals were injected at P63, P64 and P65. BrdU-immunopositive nuclei were counted and their position mapped in the plexus structure at different ages after injections. Double-labelling immunocytochemistry with antibodies to plasma protein identified post-mitotic cells involved in protein transfer.

RESULTS

Numbers of choroid plexus epithelial cells increased 10-fold between the time of birth and adulthood. In newborn pups each consecutive injection of BrdU labelled 20-40 of epithelial cells counted. After 3 injections, numbers of BrdU positive cells remained constant for at least 2 months. BrdU injections at an older age (P63, P64, P65) resulted in a smaller number of labelled plexus cells. Numbers of plexus cells immunopositive for both BrdU and plasma protein increased with age indicating that protein transferring properties are acquired post mitotically. Labelled nuclei were only detected on the dorsal arm of the plexus as it grows from the neuroependyma, moving along the structure in a 'conveyor belt' like fashion.

CONCLUSIONS

The present study established that lateral ventricular choroid plexus epithelial cells are born on the dorsal side of the structure only. Cells born in the first few days after choroid plexus differentiation from the neuroependyma remain present even two months later. Protein-transferring properties are acquired post-mitotically and relatively early in plexus development.

LIM-homeobox gene Lhx5 is required for normal development of Cajal-Retzius cells

Cajal-Retzius (C-R) cells play important roles in the lamination of the mammalian cortex via reelin secretion. The genetic mechanisms underlying the development of these neurons have just begun to be unraveled. Here, we show that two closely related LIM-homeobox genes Lhx1 and Lhx5 are expressed in reelin+ cells in various regions in the mouse telencephalon at or adjacent to sites where the C-R cells are generated, including the cortical hem, the mantle region of the septal/retrobulbar area, and the ventral pallium. Whereas Lhx5 is expressed in all of these reelin-expressing domains, Lhx1 is preferentially expressed in the septal area and in a continuous domain spanning from lateral olfactory region to caudomedial territories. Genetic ablation of Lhx5 results in decreased reelin+ and p73+ cells in the neocortical anlage, in the cortical hem, and in the septal, olfactory, and caudomedial telencephalic regions. The overall reduction in number of C-R cells in Lhx5 mutants is accompanied by formation of ectopic reelin+ cell clusters at the caudal telencephalon. Based on differential expression of molecular markers and by fluorescent cell tracing in cultured embryos, we located the origin of reelin+ ectopic cell clusters at the caudomedial telencephalic region. We also confirmed the existence of a normal migration stream of reelin+ cells from the caudomedial area to telencephalic olfactory territories in wild-type embryos. These results reveal a complex role for Lhx5 in regulating the development and normal distribution of C-R cells in the developing forebrain.

A novel role for Dbx1-derived Cajal-Retzius cells in early regionalization of the cerebral cortical neuroepithelium

Patterning of the cerebral cortex during embryogenesis depends not only on passive diffusion of morphogens but also on signal delivery by Cajal-Retzius neurons that migrate over long distances.

Patterning of the cortical neuroepithelium occurs at early stages of embryonic development in response to secreted molecules from signaling centers. These signals have been shown to establish the graded expression of transcription factors in progenitors within the ventricular zone and to control the size and positioning of cortical areas. Cajal-Retzius (CR) cells are among the earliest generated cortical neurons and migrate from the borders of the developing pallium to cover the cortical primordium by E11.5. We show that molecularly distinct CR subtypes distribute in specific combinations in pallial territories at the time of cortical regionalization. By means of genetic ablation experiments in mice, we report that loss of septum Dbx1-derived CR cells in the rostromedial pallium between E10.5 and E11.5 results in the redistribution of CR subtypes. This leads to changes in the expression of transcription factors within the neuroepithelium and in the proliferation properties of medial and dorsal cortical progenitors. Early regionalization defects correlate with shifts in the positioning of cortical areas at postnatal stages in the absence of alterations of gene expression at signaling centers. We show that septum-derived CR neurons express a highly specific repertoire of signaling factors. Our results strongly suggest that these cells, migrating over long distances and positioned in the postmitotic compartment, signal to ventricular zone progenitors and, thus, function as modulators of early cortical patterning.

Patterning of the cerebral cortex occurs early during embryonic development in response to secreted molecules or morphogens produced at signaling centers. These morphogens establish the graded expression of transcription factors (TFs) in progenitor cells and control the size and positioning of cortical areas in the postnatal animal. CR cells are among the earliest born cortical neurons and play a crucial role in cortical lamination. They are generated at signaling centers and migrate over long distances to cover its entire surface. We show that three different CR subtypes distribute in specific proportions in cortical territories. Genetic ablation of one subpopulation leads to a highly dynamic redistribution of the two others. This results in defects in expression of transcription factors and in progenitor cell proliferation, which correlate with the resulting changes in the size and positioning of cortical areas. Given our additional evidence that CR subtypes express specific repertoires of signaling factors, the ablation phenotypes point to a novel early role for CR cells as mediators of cortical patterning and suggest that CR cells are able to signal to progenitor cells. Our data thus add to the conventional model that morphogens act by passive diffusion and point to a strategy of morphogen delivery over long distance by migrating cells.

Bone morphogenetic protein signaling in the developing telencephalon controls formation of the hippocampal dentate gyrus and modifies fear-related behavior

The cortical hem is an embryonic signaling center that generates bone morphogenetic proteins (BMPs) and acts as an organizer for the hippocampus. The role of BMP signaling in hippocampal neurogenesis, however, has not been established. We therefore generated mice that were deficient in Bmpr1b constitutively, and deficient in Bmpr1a conditionally in the dorsal telencephalon. In double mutant male and female mice, the dentate gyrus (DG) was dramatically smaller than in control mice, reflecting decreased production of granule neurons at the peak period of DG neurogenesis. Additionally, the pool of cells that generates new DG neurons throughout life was reduced, commensurate with the smaller size of the DG. Effects of diminished BMP signaling on the cortical hem were at least partly responsible for these defects in DG development. Reduction of the DG and its major extrinsic output to CA3 raised the possibility that the DG was functionally compromised. We therefore looked for behavioral deficits in double mutants and found that the mice were less responsive to fear- or anxiety-provoking stimuli, whether the association of the stimulus with fear or anxiety was learned or innate. Given that no anatomical defects appeared in the double mutant telencephalon outside the DG, our observations support a growing literature that implicates the hippocampus in circuitry mediating fear and anxiety. Our results additionally indicate a requirement for BMP signaling in generating the dorsalmost neuronal lineage of the telencephalon, DG granule neurons, and in the development of the stem cell niche that makes neurons in the adult hippocampus.