Notch1 is required for neuronal and glial differentiation in the cerebellum

Quiescent and active hippocampal neural stem cells with distinct morphologies respond selectively to physiological and pathological stimuli and aging

New neurons are generated in the adult hippocampus throughout life by neural stem/progenitor cells (NSCs), and neurogenesis is a plastic process responsive to external stimuli. We show that canonical Notch signaling through RBP-J is required for hippocampal neurogenesis. Notch signaling distinguishes morphologically distinct Sox2(+) NSCs, and within these pools subpopulations can shuttle between mitotically active or quiescent. Radial and horizontal NSCs respond selectively to neurogenic stimuli. Physical exercise activates the quiescent radial population whereas epileptic seizures induce expansion of the horizontal NSC pool. Surprisingly, reduced neurogenesis correlates with a loss of active horizontal NSCs in aged mice rather than a total loss of stem cells, and the transition to a quiescent state is reversible to rejuvenate neurogenesis in the brain. The discovery of multiple NSC populations with Notch dependence but selective responses to stimuli and reversible quiescence has important implications for the mechanisms of adaptive learning and also for regenerative therapy.

Notch signaling: key role in intrauterine infection/inflammation, embryonic development, and white matter damage?

The mechanisms or pathophysiologies that lead to cerebral white matter damage during development are complex and not fully understood. It is postulated that exposure of the preterm brain to inflammatory cytokines during intrauterine infection/inflammation contributes to brain white matter damage, and this damage may affect the function and differentiation of progenitor oligodendrocyte cells under physiological conditions. The Notch pathway, an important signaling pathway controlling various cells' differentiation, functions in the timing of oligodendrocyte differentiation, and Notch signaling may contribute to white matter damage and may mediate neurogenesis in a pathophysiological phase. Recent studies have led to recognition of the role of the Notch pathway in neurogenesis in cerebral ischemic damage and in myelination and axonal damage of neurodegenerative diseases. Moreover, Notch plays a critical role in steering an immune response toward inflammation by regulating expression of various cytokines and proinflammatory cytokines resulting in the activation of Notch signaling. Thus, the Notch signaling pathway likely plays a key role in intrauterine infection/inflammation, brain development, and white matter damage, and future research directed toward understanding its role will be important. Insofar as Notch signaling could have an important effect on neurogenesis, mobilization of progenitor cells is one strategy for compensating for the neuronal losses seen in white matter damage after intrauterine infection/inflammation.

Huwe1 ubiquitin ligase is essential to synchronize neuronal and glial differentiation in the developing cerebellum

We have generated a knockout mouse strain in which the gene coding for the ubiquitin ligase Huwe1 has been inactivated in cerebellar granule neuron precursors (CGNPs) and radial glia. These mice have a high rate of postnatal lethality and profound cerebellar abnormalities. The external granule layer of the cerebellum, which contains CGNPs, is expanded and displays aberrant proliferation and impaired differentiation of the progenitor cell population. The uncontrolled proliferation of the CGNPs is associated with accumulation of the N-Myc oncoprotein, a substrate of Huwe1, and con-sequent activation of the signaling events downstream to N-Myc. Furthermore, loss of Huwe1 in Bergmann glia leads to extensive disorganization of this cell population with layering aberrations, severe granule neuron migration defects, and persistence of ectopic clusters of granule neurons in the external granule layer. Our findings uncover an unexpected role for Huwe1 in regulating Berg-mann glia differentiation and indicate that this ubiquitin ligase orchestrates the programming of the neural progenitors that give rise to neurons and glia in the cerebellum.

DeltaA mRNA and protein distribution in the zebrafish nervous system

Physical interaction between the transmembrane proteins Delta and Notch allows only a subset of neural precursors to become neurons, as well as regulating other aspects of neural development. To examine the localization of Delta protein during neural development, we generated an antibody specific to zebrafish Delta A (Dla). Here, we describe for the first time the subcellular localization of Dla protein in distinct puncta at cell cortex and/or membrane, supporting the function of Dla in direct cell-cell communication. In situ RNA hybridization and immunohistochemistry revealed dynamic, coordinated expression patterns of dla mRNA and Dla protein in the developing and adult zebrafish nervous system. Dla expression is mostly excluded from differentiated neurons and is maintained in putative precursor cells at least until larval stages. In the adult brain, dla mRNA and Dla protein are expressed in proliferative zones normally associated with stem cells.

Transient activation of Notch signaling in the injured adult brain

Brain injury induces various kinds of cellular responses that lead to tissue regeneration and repair. Recent studies have demonstrated that resident progenitors proliferate and then differentiate into mature neuronal cells. We show here that proliferating cells in the cryo-injured cerebral cortex transiently expressed Notch1 immunoreactivity in their cytoplasm. Since activated Notch signaling regulates cellular fate in the developing nervous system, similar regulation may exist in the injured adult brain. To monitor the Notch signaling pathway, we examined whether components of the signaling pathway were co-expressed in Notch1-positive cells. Presenilin-1, a membrane-spanning protease that is required for the release of the Notch intracellular domain, was detected in the Notch1-positive cells and Hes1, a target of the Notch intracellular domain, also co-localized with Notch1 three days after cryo-injury. These results suggest that transient activity of the Notch signaling pathway is involved in the regulation of proliferation and differentiation of progenitors in the injured brain.

Neural progenitors of the postnatal and adult mouse forebrain retain the ability to self-replicate, form neurospheres, and undergo multipotent differentiation in vivo

Somatic stem cells are reservoirs to replace lost cells or damaged tissue. Cells with neural stem cell (NSC) characteristics can be isolated from the postnatal mammalian brain into adulthood and expanded as neurospheres. We addressed the ability of these in vitro expanded putative NSCs to retain progenitor characteristics in vivo, in analogy to hematopoietic stem cells. When transplanted in utero, both postnatal and adult neural progenitors colonize host brains and contribute neurons and glia. In stark contrast to what has been reported when transplanted in postnatal hosts, epidermal growth factor-expanded cells also remain self-replicating and multipotent in vivo over many months and can be serially transplanted into multiple hosts. Surprisingly, embryonically transplanted NSCs remain in the neurogenic regions in adult hosts, where they express progenitor cell markers and continue to proliferate even after 6 months without tumor formation. These data indicate that spherogenic cells of the postnatal and adult mammalian brain retain their potential in vitro and in vivo throughout the life of the organism and beyond transplantation, which has important implications for cell replacement strategies.

Multiple lineage-specific roles of Smad4 during neural crest development

During vertebrate development, neural crest cells are exposed to multiple extracellular cues that drive their differentiation into neural and non-neural cell lineages. Insights into the signals potentially involved in neural crest cell fate decisions in vivo have been gained by cell culture experiments that have allowed the identification of instructive growth factors promoting either proliferation of multipotent neural crest cells or acquisition of specific fates. For instance, members of the TGFbeta factor family induce neurogenesis and smooth muscle cell formation at the expense of other fates in culture. In vivo, conditional ablation of various TGFbeta signaling components resulted in malformations of non-neural derivatives of the neural crest, but it is unclear whether these phenotypes involved aberrant fate decisions. Moreover, it remains to be shown whether neuronal determination indeed requires TGFbeta factor activity in vivo. To address these issues, we conditionally deleted Smad4 in the neural crest, thus inactivating all canonical TGFbeta factor signaling. Surprisingly, neural crest cell fates were not affected in these mutants, with the exception of sensory neurogenesis in trigeminal ganglia. Rather, Smad4 regulates survival of smooth muscle and proliferation of autonomic and ENS neuronal progenitor cells. Thus, Smad signaling plays multiple, lineage-specific roles in vivo, many of which are elicited only after neural crest cell fate decision.

Neuron-derived FGF9 is essential for scaffold formation of Bergmann radial fibers and migration of granule neurons in the cerebellum

Although fibroblast growth factor 9 (FGF9) is widely expressed in the central nervous system (CNS), the function of FGF9 in neural development remains undefined. To address this question, we deleted the Fgf9 gene specifically in the neural tube and demonstrated that FGF9 plays a key role in the postnatal migration of cerebellar granule neurons. Fgf9-null mice showed severe ataxia associated with disrupted Bergmann fiber scaffold formation, impaired granule neuron migration, and upset Purkinje cell maturation. Ex vivo cultured wildtype or Fgf9-null glia displayed a stellate morphology. Coculture with wildtype neurons, but not Fgf9-deficient neurons, or treating with FGF1 or FGF9 induced the cells to adopt a radial glial morphology. In situ hybridization showed that Fgf9 was expressed in neurons and immunostaining revealed that FGF9 was broadly distributed in both neurons and Bergmann glial radial fibers. Genetic analyses revealed that the FGF9 activities in cerebellar development are primarily transduced by FGF receptors 1 and 2. Furthermore, inhibition of the MAP kinase pathway, but not the PI3K/AKT pathway, abrogated the FGF activity to induce glial morphological changes, suggesting that the activity is mediated by the MAP kinase pathway. This work demonstrates that granule neurons secrete FGF9 to control formation of the Bergmann fiber scaffold, which in turn, guides their own inward migration and maturation of Purkinje cells.

Isolation and manipulation of mammalian neural stem cells in vitro

Neural stem cells are potentially a source of cells not only for replacement therapy but also as drug vectors, bringing bioactive molecules into the brain. Stem cell-like cells can be isolated readily from the human brain, thus, it is important to find culture systems that enable expansion in a multipotent state to generate cells that are of potential use for therapy. Currently, two systems have been described for the maintenance and expansion of multipotent progenitors, an adhesive substrate bound and the neurosphere culture. Both systems have pros and cons, but the neurosphere may be able to simulate the three-dimensional environment of the niche in which the cells reside in vivo. Thus, the neurosphere, when used and cultured appropriately, can expand and provide important information about the mechanisms that potentially control neural stem cells in vivo.

The mouse F3/contactin glycoprotein: structural features, functional properties and developmental significance of its regulated expression

F3/Contactin is an immunoglobulin superfamily component expressed in the nervous tissue of several species. Here we focus on the structural and functional properties of its mouse relative, on the mechanisms driving its regulated expression and on its developmental role. F3/Contactin is differentially expressed in distinct populations of central and peripheral neurons and in some non-neuronal cells. Accordingly, the regulatory region of the underlying gene includes promoter elements undergoing differential activation, associated with an intricate splicing profile, indicating that transcriptional and posttranscriptional mechanisms contribute to its expression. Transgenic models allowed to follow F3/Contactin promoter activation in vivo and to modify F3/Contactin gene expression under a heterologous promoter, which resulted in morphological and functional phenotypes. Besides axonal growth and pathfinding, these concerned earlier events, including precursor proliferation and commitment. This wide role in neural ontogenesis is consistent with the recognized interaction of F3/Contactin with developmental control genes belonging to the Notch pathway.

Progenitors from the postnatal forebrain subventricular zone differentiate into cerebellar-like interneurons and cerebellar-specific astrocytes upon transplantation

Forebrain subventricular zone (SVZ) progenitor cells give rise to glia and olfactory bulb interneurons during early postnatal life in rats. We investigated the potential of SVZ cells to alter their fate by transplanting them into a heterotypic neurogenic and gliogenic environment-the cerebellum. Transplanted cells were examined 1 to 7 weeks and 6 months post transplantation. Forebrain progenitors populated the cerebellum and differentiated into oligodendrocytes, cerebellar-specific Bergmann glia and velate astrocytes, and neurons. The transplanted cells that differentiated into neurons maintained an interneuronal fate: they were GABA-positive, expressed interneuronal markers, such as calretinin, and exhibited membrane properties that are characteristic of interneurons. However, the transplanted interneurons lost the expression of the olfactory bulb transcription factors Tbr2 and Dlx1, and acquired a cerebellar-like morphology. Forebrain SVZ progenitors thus have the potential to adapt to a new environment and integrate into diverse regions, and may be a useful tool in transplantation strategies.

Notch1 signaling stimulates proliferation of immature cardiomyocytes

The identification of the molecular mechanisms controlling cardiomyocyte proliferation during the embryonic, fetal, and early neonatal life appears of paramount interest in regard to exploiting this information to promote cardiac regeneration. Here, we show that the proliferative potential of neonatal rat cardiomyocytes is powerfully stimulated by the sustained activation of the Notch pathway. We found that Notch1 is expressed in proliferating ventricular immature cardiac myocytes (ICMs) both in vitro and in vivo, and that the number of Notch1-positive cells in the heart declines with age. Notch1 expression in ICMs paralleled the expression of its Jagged1 ligand on non-myocyte supporting cells. The inhibition of Notch signaling in ICMs blocked their proliferation and induced apoptosis; in contrast, its activation by Jagged1 or by the constitutive expression of its activated form using an adeno-associated virus markedly stimulated proliferative signaling and promoted ICM expansion. Maintenance or reactivation of Notch signaling in cardiac myocytes might represent an interesting target for innovative regenerative therapy.

Cerebellar granule cells: insights into proliferation, differentiation, and role in medulloblastoma pathogenesis

Cerebellar granule cells originate from precursors located in the dorsal region of rhombomere one within the hindbrain of developing embryos. They undergo proliferation for an extensive period well into postnatal stages of development to form the major cell type of the cerebellum, the most populous structure within the mammalian brain. Granule cell development is highly dependent upon the cerebellar environment and contact with neighbouring cells. In recent years, the molecular basis of these interactions has started to be unravelled. Granule cell precursors and the molecular mechanisms involved in controlling their proliferation have been shown to be involved in the pathogenesis of medulloblastoma, the most common malignant pediatric brain tumour. Here, we review the control of granule cell generation with emphasis on the molecular regulators of cell proliferation and differentiation during normal and malignant development.

Recent advances in cardiovascular regenerative medicine: the induced pluripotent stem cell era

Induced pluripotent stem (iPS) cells have recently been established by transfecting mouse and human fibroblasts with the transcription factors Oct3/4, Sox2, Klf4 and c-Myc, known to be expressed at high levels in embryonic stem (ES) cells. These cells have great potential in regenerative medicine as they have the capacity to differentiate into all three germ layer-derived cells and are syngeneic. The differentiation of ES cells into cardiomyocytes mimics the early processes involved in heart development. Recent studies describe the contribution of various growth factors and corresponding inhibitors to heart development during embryogenesis. Bone morphogenetic proteins, Wnt protein and Notch signals play critical roles in heart development in a context- and time-dependent manner. Consistent with ES cells, the exposure of iPS cells to such growth factors is hypothesized to augment differentiation into cardiomyocytes. The combination of iPS cells and appropriate developmental signal information has the potential for providing the foundations for future regenerative medicine.

The origins of medulloblastoma subtypes

Childhood tumors containing cells that are morphologically and functionally similar to normal progenitor cells provide fertile ground for investigating the links between development and cancer. In this respect, integrated studies of normal cerebellar development and the medulloblastoma, a malignant embryonal tumor of the cerebellum, have proven especially fruitful. Emerging evidence indicates that the different precursor cell populations that form the cerebellum and the cell signaling pathways that regulate its development likely represent distinct compartments from which the various subtypes of medulloblastoma arise. Definitive characterization of each medulloblastoma subtype will undoubtedly improve treatment of this disease and provide important insights to the origins of cancer.

NOTCH3 signaling pathway plays crucial roles in the proliferation of ErbB2-negative human breast cancer cells

ErbB2-negative breast tumors represent a significant therapeutic hurdle because of a lack of effective molecular targets. Although NOTCH proteins are known to be involved in mammary tumorigenesis, the functional significance of these proteins in ErbB2-negative breast tumors is not clear. In the present study, we examined the expression of activated NOTCH receptors in human breast cancer cell lines, including ErbB2-negative and ErbB2-positive cell lines. Activated NOTCH1 and NOTCH3 proteins generated by gamma-secretase were detected in most of the cell lines tested, and both proteins activated CSL-mediated transcription. Down-regulation of NOTCH1 by RNA interference had little or no suppressive effect on the proliferation of either ErbB2-positive or ErbB2-negative cell lines. In contrast, down-regulation of NOTCH3 significantly suppressed proliferation and promoted apoptosis of the ErbB2-negative tumor cell lines. Down-regulation of NOTCH3 did not have a significant effect on the ErbB2-positive tumor cell lines. Down-regulation of CSL also suppressed the proliferation of ErbB2-negative breast tumor cell lines, indicating that the NOTCH-CSL signaling axis is involved in cell proliferation. Finally, NOTCH3 gene amplification was detected in a breast tumor cell line and one breast cancer tissue specimen even though the frequency of NOTCH3 gene amplification was low (<1%). Taken together, these findings indicate that NOTCH3-mediated signaling rather than NOTCH1-mediated signaling plays an important role in the proliferation of ErbB2-negative breast tumor cells and that targeted suppression of this signaling pathway may be a promising strategy for the treatment of ErbB2-negative breast cancers.

An obligatory role of mind bomb-1 in notch signaling of mammalian development

BACKGROUND

The Notch signaling pathway is an evolutionarily conserved intercellular signaling module essential for cell fate specification that requires endocytosis of Notch ligands. Structurally distinct E3 ubiquitin ligases, Neuralized (Neur) and Mind bomb (Mib), cooperatively regulate the endocytosis of Notch ligands in Drosophila. However, the respective roles of the mammalian E3 ubiquitin ligases, Neur1, Neur2, Mib1, and Mib2, in mammalian development are poorly understood.

METHODOLOGY/PRINCIPAL FINDINGS

Through extensive use of mammalian genetics, here we show that Neur1 and Neur2 double mutants and Mib2(-/-) mice were viable and grossly normal. In contrast, conditional inactivation of Mib1 in various tissues revealed the representative Notch phenotypes: defects of arterial specification as deltalike4 mutants, abnormal cerebellum and skin development as jagged1 conditional mutants, and syndactylism as jagged2 mutants.

CONCLUSIONS/SIGNIFICANCE

Our data provide the first evidence that Mib1 is essential for Jagged as well as Deltalike ligand-mediated Notch signaling in mammalian development, while Neur1, Neur2, and Mib2 are dispensable.

Notch 2 and Notch 1/3 segregate to neuronal and glial lineages of the developing olfactory epithelium

The murine olfactory epithelium (OE) generates olfactory receptor neurons (ORNs) throughout development and into adulthood, but only a few of the factors regulating olfactory neuro- and glio-genesis have been delineated. Notch receptors maintain CNS neuronal progenitors and drive glial differentiation, and the Notch effectors Hes 1 and 5 are expressed in the OE, but the Notch receptors that stimulate Hes gene activation in defined lineages during OE development have not been determined. Here, we first use RT-PCR to reveal which Notch receptors and ligands are expressed in the developing and adult OE. This is followed by immunofluorescent detection, combined with lineage-specific markers to define the stage-specific developmental expression of different Notch family members. We show that throughout development, Notch 1 and 3 are expressed in cells retained within the lamina propria, where Notch 3 is expressed in olfactory ensheathing cells (OECs). In contrast, Notch 2 is expressed in apical embryonic and early postnatal OE neuronal progenitors. In postnatal and adult OE, Notch 1 is expressed predominantly in Bowman's glands, and Notch 2 in sustentacular cells. Notch 2 and Notch 1/3 may, therefore, have different roles in the commitment and differentiation of neuronal and glial lineages of the OE during development, and the maintenance of non-neuronal phenotypes postnatally.

The monolayer formation of Bergmann glial cells is regulated by Notch/RBP-J signaling

The Bergmann glia is a unipolar astrocyte in the cerebellar cortex, displaying a tight association with Purkinje cells. The cell bodies of Bergmann glia are located in a row around Purkinje cell somata; they extend radially arranged Bergmann fibers which enwrap the synapses on the Purkinje cell dendrites. It is well known that Bergmann glial somata migrate from the ventricular zone through the mantle zone, forming an epithelium-like lining in the Purkinje cell layer during development. However, the mechanism of the monolayer formation of Bergmann glia is poorly understood. Several reports have suggested that Notch signaling plays instructive roles in promoting the identities of several types of glial cells, including Bergmann glia. Moreover, Notch receptors are expressed in Bergmann glia during development. Here, we have deleted the Notch1, Notch2 and RBP-J genes in the Bergmann glia by GFAP-driven, Cre-mediated recombination, to study the role of Notch-RBP-J-signaling in the monolayer formation of Bergmann glia. Notch1/2- and RBP-J-conditional mutant mice showed disorganization of Bergmann fibers, irregularities of the Bergmann glial lining and aberrant localization of Bergmann glia in the molecular layer. Thus, Notch-RBP-J signaling plays crucial roles in the monolayer formation and morphogenesis of Bergmann glia.

Identification of self-replicating multipotent progenitors in the embryonic nervous system by high Notch activity and Hes5 expression

Discrimination of neural stem cells from other progenitors in the developing mammalian brain has been hampered by the lack of specific markers. Identifying the progenitor pools and signalling pathways that guide mammalian neurogenesis are central to understanding the complex mechanisms that govern development of the nervous system. Notch signalling plays a pivotal role in the development of the mammalian nervous system by maintaining multipotent neural stem cells and regulating their fate. In order to identify putative neural stem cells in situ, we generated transgenic mice that express Green Fluorescent Protein (GFP) and report Notch signalling activity in the developing CNS. Here we show the subdivision of progenitors within the neural tube of these mice. We purify progenitors from the neural tube and show that cells with the highest levels of Notch-reporter activity have self-renewal capability and multipotency, whereas those lacking Hes5 expression do not form neurospheres in vitro. Using marker protein co-expression and cell sorting, we show that both neuroepithelial cells as well as some radial glia at all axial levels of the embryonic neural tube display active Notch signalling. However, Tbr2-positive basal progenitors of the developing telencephalon and differentiating Islet1/2- and Lim1-positive motor neurons outside the ventricular zone do not express Hes5-GFP. Quantitative analysis showed that Hes5 expression correlates better with neural stem cell potential than expression of the related gene Hes1. Thus, Notch activity through Hes5 identifies multipotent progenitors with stem cell properties and subdivides the different progenitors into defined pools.

Notch and MAML signaling drives Scl-dependent interneuron diversity in the spinal cord

The ventral spinal cord generates multiple inhibitory and excitatory interneuron subtypes from four cardinal progenitor domains (p0, p1, p2, p3). Here we show that cell-cell interactions mediated by the Notch receptor play a critical evolutionarily conserved role in the generation of excitatory v2aIN and inhibitory v2bIN interneurons. Lineage-tracing experiments show that the v2aIN and v2bIN develop from genetically identical p2 progenitors. The p2 daughter cell fate is controlled by Delta4 activation of Notch receptors together with MAML factors. Cells receiving Notch signals activate a transcription factor code that specifies the v2bIN fate, whereas cells deprived of Notch signaling express another code for v2aIN formation. Thus, our study provides insight into the cell-extrinsic signaling that controls combinatorial transcription factor profiles involved in regulating the process of interneuron subtype diversification.

Notch signaling in development and cancer

Notch is an evolutionarily conserved local cell signaling mechanism that participates in a variety of cellular processes: cell fate specification, differentiation, proliferation, apoptosis, adhesion, epithelial-mesenchymal transition, migration, and angiogenesis. These processes can be subverted in Notch-mediated pathological situations. In the first part of this review, we will discuss the role of Notch in vertebrate central nervous system development, somitogenesis, cardiovascular and endocrine development, with attention to the mechanisms by which Notch regulates cell fate specification and patterning in these tissues. In the second part, we will review the molecular aspects of Notch-mediated neoplasias, where Notch can act as an oncogene or as a tumor suppressor. From all these studies, it becomes evident that the outcome of Notch signaling is strictly context-dependent and differences in the strength, timing, cell type, and context of the signal may affect the final outcome. It is essential to understand how Notch integrates inputs from other signaling pathways and how specificity is achieved, because this knowledge may be relevant for future therapeutic applications.

Antagonism between Notch and bone morphogenetic protein receptor signaling regulates neurogenesis in the cerebellar rhombic lip

Background

During the embryonic development of the cerebellum, neurons are produced from progenitor cells located along a ventricular zone within dorsal rhombomere 1 that extends caudally to the roof plate of the fourth ventricle. The apposition of the caudal neuroepithelium and roof plate results in a unique inductive region termed the cerebellar rhombic lip, which gives rise to granule cell precursors and other glutamatergic neuronal lineages. Recently, we and others have shown that, at early embryonic stages prior to the emergence of granule cell precursors (E12), waves of neurogenesis in the cerebellar rhombic lip produce specific hindbrain nuclei followed by deep cerebellar neurons. How the induction of rhombic lip-derived neurons from cerebellar progenitors is regulated during this phase of cerebellar development to produce these temporally discrete neuronal populations while maintaining a progenitor pool for subsequent neurogenesis is not known.

Results

Employing both gain- and loss-of-function methods, we find that Notch1 signaling in the cerebellar primordium regulates the responsiveness of progenitor cells to bone morphogenetic proteins (BMPs) secreted from the roof plate that stimulate the production of rhombic lip-derived neurons. In the absence of Notch1, cerebellar progenitors are depleted during the early production of hindbrain neurons, resulting in a severe decrease in the deep cerebellar nuclei that are normally born subsequently. Mechanistically, we demonstrate that Notch1 activity prevents the induction of Math1 by antagonizing the BMP receptor-signaling pathway at the level of Msx2 expression.

Conclusion

Our results provide a mechanism by which a balance between neural induction and maintenance of neural progenitors is achieved in the rhombic lip throughout embryonic development.

Neural differentiation of embryonic stem cells is induced by signalling from non-neural niche cells

BACKGROUND/AIMS

Embryonic stem cell (ESC) transplantation offers new therapeutic strategies for neurodegenerative diseases and injury. However, the mechanisms underlying integration and differentiation of engrafted ESCs are poorly understood. This study elucidates the influence of exogenous signals on ESC differentiation using in vitro modelling of non-stem/stem cell interactions.

METHODS

Murine ESCs were co-cultured with endothelial cells and astrocytes or conditioned medium obtained from endothelial or astrocyte cultures. After 7 days of co-culture isolated RNA was analysed using RT-PCR for the expression of pluripotency marker oct-4, neural progenitor marker nestin, and neurofilament (NFL), an early marker of neuronal lineage commitment. The presence of the glial cell surface marker A2B5 was determined in ESCs by flow cytometry.

RESULTS

Neuronal differentiation was inhibited in ESCs when grown in close vicinity to cerebral endothelial or glial cells. Under these conditions, ESC differentiation was predominantly directed towards a glial fate. However, treatment of ESCs with endothelial cell- or astrocyte-conditioned medium promoted neuronal as well as glial differentiation.

CONCLUSION

Our results indicate that ESC fate is determined by endothelial and glial cells that comprise the environmental niche of these stem cells in vivo. The direction of differentiation processes appears to be dependent on humoral factors secreted by adjacent cell lines.

Influence of thyroid hormones on maturation of rat cerebellar astrocytes

Thyroid hormone influences brain maturation through interaction with nuclear receptors and regulation of gene expression. Their role on astrocyte maturation remains unclear. We have analyzed the role of thyroid hormone in rat cerebellar astrocyte maturation by comparing the sequential patterns of intermediate filament expression in normal and hypothyroid animals. During normal development astroglial cells sequentially express nestin, vimentin, and glial fibrillary acidic protein. Differentiated astrocytes appeared in the superior medullary vellum by postnatal day 2 and reached the white mater and internal granular layer by postnatal day 4. Intermediate filament marker expression was transiently lost from postnatal days 6 to 8 in anterior lobes, without an increased apoptosis. Vimentin expression was replaced by glial fibrillary acidic protein between postnatal days 10 and 32. The differentiated astrocytes were evenly distributed throughout the cerebellar slices, including the internal granular layer. Differences between normal and hypothyroid rats were observed starting from postnatal day 4, with lack of differentiated astrocytes in the internal granular layer. The transient decrease of astrocyte markers immunoreactivity in the anterior lobe did not take place in hypothyroid rats. The vimentin-glial fibrillary acidic protein transition was delayed and most differentiated astrocytes remained confined to the white matter. The results indicate that thyroid hormone deficiency induces a delay and a partial arrest of astrocyte differentiation. Astrocytes express thyroid hormone receptor alpha and beta subtypes suggesting that astrocytes are direct target cells of thyroid hormones.

Neural crest-derived cells with stem cell features can be traced back to multiple lineages in the adult skin

Given their accessibility, multipotent skin-derived cells might be useful for future cell replacement therapies. We describe the isolation of multipotent stem cell-like cells from the adult trunk skin of mice and humans that express the neural crest stem cell markers p75 and Sox10 and display extensive self-renewal capacity in sphere cultures. To determine the origin of these cells, we genetically mapped the fate of neural crest cells in face and trunk skin of mouse. In whisker follicles of the face, many mesenchymal structures are neural crest derived and appear to contain cells with sphere-forming potential. In the trunk skin, however, sphere-forming neural crest-derived cells are restricted to the glial and melanocyte lineages. Thus, self-renewing cells in the adult skin can be obtained from several neural crest derivatives, and these are of distinct nature in face and trunk skin. These findings are relevant for the design of therapeutic strategies because the potential of stem and progenitor cells in vivo likely depends on their nature and origin.

Bone marrow stromal cells induce BMP2/4 production in oxygen-glucose-deprived astrocytes, which promotes an astrocytic phenotype in adult subventricular progenitor cells

Bone morphogenetic proteins (BMPs) affect cell proliferation and differentiation. Astrocytes in ischemic brain are highly responsive to bone marrow stromal cell (BMSC) treatment. We investigated the effects of BMSCs on astrocytes cultured under oxygen- and glucose-deprived conditions, which in part simulate in vivo stroke conditions, to test the hypothesis that BMSCs alter astrocytic expression of BMPs which may contribute to neurological functional recovery of stroke. Quantitative real-time RT-PCR showed that the expression of BMP2/4 mRNAs decreased within ischemic astrocytes, In contrast, BMP2/4 mRNA was significantly increased after cocultured with BMSCs. Western blotting also confirmed this increase at the protein level in the medium of ischemic astrocytes after coculture with BMSCs. As a source of neural stem and progenitor cells, cultured subventricular zone (SVZ) neurospheres exposed to medium obtained from ischemic astrocytes cocultured with BMSCs were significantly enriched in cells expressing the astrocytic marker glial fibrillary acidic protein (GFAP), but not at the expense of beta-III-tubulin-positive SVZ neuroblasts. The expression of BMP2/4 subsequently increased the phosphorylation of downstream effector Smad1 and the expression of notch signal pathway-induced protein Hes1 in cultured SVZ neurospheres. BMP antagonist Noggin blocked the elevation of phosphorylated Smad1 and the expression of Hes1 as well as reducing the percentage of astrocytic SVZ progenitor cells. Our results indicate that BMSCs increase BMP2/4 expression in ischemic astrocytes. These changes enhance subventricular progenitor cell gliogenesis by activating relevant signaling pathways. BMSC-stimulated signaling of endogenous astrocytes may alter the ischemic environment, promoting remodeling of brain and hence, improve functional recovery after stroke.

A novel hes5/hes6 circuitry of negative regulation controls Notch activity during neurogenesis

HES transcriptional repressors are important components of the Notch pathway that regulates neurogenesis from Drosophila to vertebrates. These proteins are normally induced by Notch activity and inhibit neural commitment by antagonizing the activity of proneural genes. We describe here four chick hes genes that are expressed during neurogenesis: three hes5-like genes (hes5-1, hes5-2 and hes5-3) and one hes6-like (hes6-2). We show that hes6-2 represses transcription of the hes5 genes, thus functioning as a negative regulator of Notch signaling. Conversely, hes6-2 may be repressed by hes5 activity. In cells committing to differentiation, we find that hes6-2 is up-regulated by proneural genes and contributes to the proneural program of neuronal commitment by preventing Notch activity in these cells. In neural progenitors, Notch signaling produces an initial burst of hes5 activity, which represses hes6-2. However, as hes5 transcription declines due to negative auto-regulation, hes6-2 may become active and inhibit the remaining hes5 activity to end Notch signaling. These cells can then enter a new cycle of fate decisions and will be kept as progenitors if a new pulse of Notch activity occurs. Maintenance of progenitors during vertebrate neurogenesis therefore requires that these cells go through successive cycles of Notch activity. We propose that the hes5/hes6 circuitry of negative cross-regulations is a conserved feature of the Notch pathway that underlies these cycles in neural progenitors.

Notch signaling in stem cell systems

The Notch signaling pathway is among the most commonly used communication channels in animal cells. Recent studies have demonstrated that this pathway is indispensable for cells in various stages of maturation, including terminal differentiation. One main focus in mammalian studies is the role of Notch in embryonic and postembryonic stem cell systems. In this review, the roles of Notch signaling in various mammalian stem and early progenitor cells are summarized.

Notch synergizes with axin to regulate the activity of armadillo in Drosophila

Cell fate decisions require the integration of various signalling inputs at the level of transcription and signal transduction. Wnt and Notch signalling are two important signalling systems that operate in concert in a variety of systems in vertebrates and invertebrates. There is evidence that the Notch receptor can modulate Wnt signalling and that its target is the activity and levels of Armadillo/beta-catenin. Here, we characterize this function of Notch in relation to Axin, a key element in the regulation of Wnt signalling that acts as a scaffold for the Shaggy/GSK3beta-dependent phosphorylation of Armadillo/beta-catenin. While Notch can regulate ectopic Wingless signalling caused by loss of function of Shaggy, it can only partially regulate the ectopic Wnt signalling induced by the loss of Axin function. The same interactions are observed in tissue culture cells where we observe a synergy in between Axin and Notch in the regulation of Armadillo/beta-catenin. Our results provide evidence for a function of Axin in the regulation of Armadillo that is different from its role as a scaffold for GSK3beta.

Notch signaling plays a key role in cardiac cell differentiation

Results from lineage tracing studies indicate that precursor cells in the ventricles give rise to both cardiac muscle and conduction cells. Cardiac conduction cells are specialized cells responsible for orchestrating the rhythmic contractions of the heart. Here, we show that Notch signaling plays an important role in the differentiation of cardiac muscle and conduction cell lineages in the ventricles. Notch1 expression coincides with a conduction marker, HNK-1, at early stages. Misexpression of constitutively active Notch1 (NIC) in early heart tubes in chick exhibited multiple effects on cardiac cell differentiation. Cells expressing NIC had a significant decrease in expression of cardiac muscle markers, but an increase in expression of conduction cell markers, HNK-1, and SNAP-25. However, the expression of the conduction marker connexin 40 was inhibited. Loss-of-function study, using a dominant-negative form of Suppressor-of-Hairless, further supports that Notch1 signaling is important for the differentiation of these cardiac cell types. Functional studies show that the expression of constitutively active Notch1 resulted in abnormalities in ventricular conduction pathway patterns.

Persistent expression of Notch2 delays gonadotrope differentiation

Normal pituitary gland development requires coordination between maintenance of progenitor cell pools and selection of progenitors for differentiation. The spatial and temporal expression of Notch2 during pituitary development suggested that it could control progenitor cell differentiation in the pituitary. Consistent with this idea, Notch2 is not expressed in Prop1 mutants, and anterior pituitary progenitors in Prop1 mutants appear to be unable to transition from proliferation to differentiation properly, resulting in anterior lobe failed cell specification and evolving hypoplasia. To test the function of Notch2 directly, we used the alphaGSU subunit promoter to express activated NOTCH2 persistently in pre-gonadotropes and pre-thyrotropes of transgenic mice. At birth, there is a small reduction in the population of fully differentiated thyrotropes and almost no fully differentiated gonadotropes. The temporal and spatial expression of Hey1 suggests that it could be a mediator of this effect. Gonadotropes complete their differentiation program eventually, although expression of LH and FSH is mutually exclusive with NOTCH2 transgene expression. This demonstrates that activated Notch2 is sufficient to delay gonadotrope differentiation, and it supports the hypothesis that Notch2 regulates progenitor cell differentiation in the pituitary gland.

Stem cells as a source of regenerative cardiomyocytes

The realization of regenerative cardiac medicine depends on the availability of cardiomyocytes in sufficient numbers for transplantation of cardiac tissue and the accompanying blood vessels. Embryonic stem (ES) cells, bone marrow (BM) stem cells, and tissue-derived stem cells are all potential cell sources. Although ES cells are highly proliferative and suitable for mass production, an efficient protocol is yet to be established to ensure selective cardiomyocyte induction using these cells. Recent advances in developmental biology have clarified the involvement of critical factors in cardiomyocyte differentiation, including bone morphogenic protein and Wnt signaling proteins, and such factors have the potential to improve the efficiency of stem cell induction. Initial studies of the intracoronary administration of BM mononuclear cells after myocardial infarction has yielded promising results; however, intensive investigation of the underlying molecular mechanisms at play as well as double-blinded clinical trials will be necessary to establish the extent of both migration of the BM stem cells into the damaged cardiac tissue and their differentiation into cardiomyocytes. Several types of cardiac tissue stem cells have also been reported, but an accurate and extensive comparison of these cells with regard to their characteristics and multipotency remains to be done. An integrative study involving developmental biology, stem cell biology, and tissue engineering is required to achieve the full potential of cardiac regeneration.

Jagged1 ablation results in cerebellar granule cell migration defects and depletion of Bergmann glia

Jagged1 is a ligand for members of the Notch family of receptors. Mutations in the human JAG1 gene are the major cause of Alagille syndrome, an autosomal dominant disorder affecting the liver, heart, eye, skeleton, kidneys, and craniofacial structures. Although expressed throughout mammalian embryonic development and in the adult, the function of Jagged1 in the central nervous system is not clear. Jagged1 is broadly expressed in the cerebellum suggesting an important role in Notch signaling. In order to address the function of Jagged1 in the mouse central nervous system, we have inactivated the Jag1 gene in the cerebellar primordium at mid-embryogenesis. Loss of Jagged1 results in aberrant granule cell migration and ectopic differentiation in the external germinal layer and molecular layer of the early postnatal cerebellum. We show that Bergmann glia in the cerebellum lose contact to the pial surface and have stunted processes. In vitro analysis revealed a depletion of Bergmann glia in the Jagged1 mutant mice. Our findings suggest that Jagged1 plays a role in cell fate specification and survival in the cerebellum.

Loss of notch activity in the developing central nervous system leads to increased cell death

Many cells in the mammalian brain undergo apoptosis as a normal and critical part of development but the signals that regulate the survival and death of neural progenitor cells and the neurons they produce are not well understood. The Notch signaling pathway is involved in multiple decision points during development and has been proposed to regulate the survival and apoptosis of neural progenitor cells in the developing brain; however, previous experiments have not resolved whether Notch activity is pro- or anti-apoptotic. To elucidate the function of Notch signaling in the survival and death of cells in the nervous system, we have produced single and compound Notch conditional mutants in which Notch1 and Notch3 are removed at different times during brain development and in different populations of cells. We show here that a large number of neural progenitor cells, as well as differentiating neurons, undergo apoptosis in the absence of Notch1 and Notch3, suggesting that Notch activity promotes the survival of both progenitors and newly differentiating cells in the developing nervous system. Finally, we show that postmitotic neurons do not require Notch activity indefinitely to regulate their survival since elevated levels of cell death are observed only during embryogenesis in the Notch mutants and are not detected in neonates.

Cross-talk between F3/contactin and Notch at axoglial interface: a role in oligodendrocyte development

Increasing evidence has shown that the Notch signalling pathway regulates oligodendrogliogenesis. Upon binding to classical Delta/Serrate/Lag-2 ligands, Notch signalling promotes generation of oligodendrocyte precursor cells while inhibiting their further differentiation into myelinating oligodendrocytes. In our recent studies, we have found that two neural cell adhesion molecules, F3/contactin and NB-3 interact with Notch receptors and promote oligodendrocyte development. Remarkably, all these F3 and NB-3/Notch cascade-related events required Deltex1 as the intermediate element. Experiments using several animal models further imply the function of F3/Notch signalling in vivo, which designates Notch signalling as a ligand-dependent, multipotential cascade involved in oligodendrocyte development.

Molecular evolution of protein O-fucosyltransferase genes and splice variants

O-Fucose has been described on both epidermal growth factor-like (EGF-like) repeats and Thrombospondin type 1 repeats (TSRs). The enzyme adding fucose to EGF-like repeats, protein O-fucosyltransferase 1 (Pofut1), is a soluble protein located in the lumen of endoplasmic reticulum (ER). A second protein O-fucosyltransferase, Pofut2, quite divergent from its homolog Pofut1, has recently been shown to O-fucosylate TSRs but not EGF-like repeats. To date, Pofut1 genes have only been characterized in human, mouse, and fly, and Pofut2 in mouse, fly, and partially in the nematode Caenorhabditis elegans. Here, we report cDNA sequences and genomic structures of bovine Pofut1 and Pofut2 genes and describe for the first time five alternative spliced transcripts for each gene. Only one transcript for both Pofut1 and Pofut2 encodes an active bovine O-fucosyltransferase. Variant transcript distribution was examined in 13 bovine tissues. Transcripts encoding active forms are ubiquitous, whereas other forms possess a more restricted tissue-expression profile. Sequence comparison and phylogenetic analyses revealed that both Pofut genes are present as a single copy in animal genomes, and their exon-intron organizations are conserved among vertebrates. The last common ancestor of all analyzed bilaterian species would be predicted to possess polyexonic Pofut genes in their genome.

Differential expression of cell fate determinants in neurons and glial cells of adult mouse spinal cord after compression injury

Cellular responses after spinal cord injury include activation of astrocytes, degeneration of neurons and oligodendrocytes, and reactions of the ependymal layer and meningeal cells. Because it has been suggested that tissue repair partially recapitulates morphogenesis, we have investigated the expression of several developmentally prominent molecules after spinal cord injury of adult mice where neurogenesis does not occur after injury. Cell fate determinants Numb, Notch-1, Shh and BMPs are abundantly expressed during development but mostly decline in the adult. In the present study, we investigated whether these genes are triggered by spinal cord injury as a sign of attempted recapitulation of development. Expression of Numb, Notch, Shh, BMP2/4 and Msx1/2 was analysed in the adult mouse spinal cord after compression injury by in situ hybridization up to 1 month after injury. The mRNA expression levels of Notch-1, Numb, Shh, BMP4 and Msx2 increased in the grey matter and/or white matter and in the ependyma rostral and caudal to the lesion site after injury. However, BMP2 and Msx1 were not up-regulated. Combining immunohistochemistry of cell type-specific markers with in situ hybridization we found that all the up-regulated genes were expressed in neurons. Moreover, Numb, BMP4 and Msx2 were also expressed by GFAP-positive astrocytes, while Shh was expressed by MBP-positive oligodendrocytes. In conclusion, the cell fate determinants Notch-1, Numb, Shh, BMP4 and Msx2 are expressed in neurons and/or glial cells after injury in a time-dependent manner, suggesting that these genes reflect to some extent an endogenous self-repair potential by recapitulating some features of development.

Paradigms of Notch Signaling in Mammals

Notch proteins regulate a broad spectrum of cell fate decisions and differentiation processes during fetal and postnatal life. These proteins are involved in organogenesis during embryonic development as well as in the maintenance of homeostasis of self-renewing systems. The paradigms of Notch function, such as stem and progenitor cell maintenance, lineage specification mediated by binary cell fate decisions, and induction of terminal differentiation, were initially established in invertebrates and subsequently confirmed in mammals. Moreover, aberrant Notch signaling is linked to tumorigenesis. In this review, we discuss the origin of postulated Notch functions, give examples from different mammalian organ systems, and try to relate them to the hematopoietic system.

Jagged1 signals in the postnatal subventricular zone are required for neural stem cell self-renewal

Neural stem cells (NSCs) in the postnatal mammalian brain self-renew and are a source of neurons and glia. To date, little is known about the molecular and cellular mechanisms regulating the maintenance and differentiation of these multipotent progenitors. We show that Jagged1 is required by mitotic cells in the subventricular zone (SVZ) and stimulates self-renewal of multipotent epidermal growth factor-dependent NSCs. Jagged1-expressing cells line the adult SVZ and are juxtaposed to Notch1-expressing cells, some of which are putative NSCs. In vitro, endogenous Jagged1 acts through Notch1 to promote NSC maintenance and multipotency. In vivo, reducing Jagged1/Notch1 signaling decreases the number of proliferating cells in the SVZ. In addition, soluble Jagged1 promotes self-renewal and neurogenic potential of multipotent neural progenitors in vitro. Our findings suggest a central role for Jagged1 in the NSC niche in the SVZ for maintaining a population of NSCs in the postnatal brain.

EphB2 induces proliferation and promotes a neuronal fate in adult subventricular neural precursor cells

The subventricular germinal zone (SVZ) retains an active population of stem cells and neural precursor cells throughout adulthood. EphrinB signaling mediates angiogenesis and vasculogenesis in the developing and adult brain. Recent studies indicate that molecules involved in angiogenesis often influence neurogenesis as well. However, little work has been done considering a role for EphB2/EphrinB in adult neural precursor cells. We therefore examined whether the EphB2 receptor tyrosine kinase could directly effect proliferation of SVZ neural precursors and/or direct the cell fate of SVZ cells in vitro. Here, we found that clustered EphB2 increased bromodeoxyuridine (BrdU) incorporation and proliferation of SVZ neurosphere cultures. Immunostaining and RT-PCR analysis for beta-tubulin III (Tuj1) and GFAP indicated 4-day treatment with EphB2 promoted a neuronal phenotype, suggesting that the EphB2 receptor might also direct SVZ cell fate. EphB2 transiently down-regulated SVZ cell mRNA of Notch1 and Zic1, genes that regulate neurogenesis and neuronal differentiation. Notch1 has been implicated in apoptosis of neural precursors, however, a cell viability assay revealed no statistical difference between EphB2-treated and control cultures. When SVZ neurospheres were cultured upon Matrigel, EphB2 attenuated radial migration of SVZ cells in vitro. These results demonstrate that EphB2/EphrinB signaling directly induces SVZ proliferation, decreases migration, and promotes a neuronal fate of SVZ neural precursors independent of cell survival.

Notch signaling in the mammalian central nervous system: insights from mouse mutants

The Notch pathway, although originally identified in fruit flies, is now among the most heavily studied in mammalian biology. In mice, loss-of-function and gain-of-function work has demonstrated that Notch signaling is essential both during development and in the adult in a multitude of tissues. Prominent among these is the CNS, where Notch has been implicated in processes ranging from neural stem cell regulation to learning and memory. Here we review the role of Notch in the mammalian CNS by focusing specifically on mutations generated in mice. These mutations have provided critical insight into Notch function in the CNS and have led to the identification of promising new directions that are likely to generate important discoveries in the future.