Murine numb regulates granule cell maturation in the cerebellum

NUMB alternative splicing and isoform-specific functions in development and disease

The NUMB gene encodes a conserved adaptor protein with roles in asymmetric cell division and cell fate determination. First described as an inhibitor of Notch signaling, multifunctional NUMB proteins regulate multiple cellular pathways through protein complexes with ubiquitin ligases, polarity proteins and the endocytic machinery. The vertebrate NUMB protein isoforms were identified over 2 decades ago, yet the majority of functional studies exploring NUMB function in endocytosis, cell migration and adhesion, development and disease have largely neglected the potential for distinct isoform activity in design and interpretation. In this review we consolidate the literature that has directly addressed individual NUMB isoform functions, as well as interpret other functional studies through the lens of the specific isoforms that were utilized. We also summarize the emerging literature on the mechanisms that regulate alternative splicing of NUMB, and how this is subverted in disease. Finally, the importance of relative NUMB isoform expression as a determinant of activity and considerations for future studies of NUMB isoforms as unique proteins with distinct functions are discussed.

Numb positively regulates Hedgehog signaling at the ciliary pocket

Hedgehog (Hh) signaling relies on the primary cilium, a cell surface organelle that serves as a signaling hub for the cell. Using proximity labeling and quantitative proteomics, we identify Numb as a ciliary protein that positively regulates Hh signaling. Numb localizes to the ciliary pocket and acts as an endocytic adaptor to incorporate Ptch1 into clathrin-coated vesicles, thereby promoting Ptch1 exit from the cilium, a key step in Hh signaling activation. Numb loss impedes Sonic hedgehog (Shh)-induced Ptch1 exit from the cilium, resulting in reduced Hh signaling. Numb loss in spinal neural progenitors reduces Shh-induced differentiation into cell fates reliant on high Hh activity. Genetic ablation of Numb in the developing cerebellum impairs the proliferation of granule cell precursors, a Hh-dependent process, resulting in reduced cerebellar size. This study highlights Numb as a regulator of ciliary Ptch1 levels during Hh signal activation and demonstrates the key role of ciliary pocket-mediated endocytosis in cell signaling.

Brain-specific functions of the endocytic machinery

Endocytosis is an essential cellular process required for multiple physiological functions, including communication with the extracellular environment, nutrient uptake, and signaling by the cell surface receptors. In a broad sense, endocytosis is accomplished through either constitutive or ligand-induced invagination of the plasma membrane, which results in the formation of the plasma membrane-retrieved endocytic vesicles, which can either be sent for degradation to the lysosomes or recycled back to the PM. This additional function of endocytosis in membrane retrieval has been adopted by excitable cells, such as neurons, for membrane equilibrium maintenance at synapses. The last two decades were especially productive with respect to the identification of brain-specific functions of the endocytic machinery, which additionally include but not limited to regulation of neuronal differentiation and migration, maintenance of neuron morphology and synaptic plasticity, and prevention of neurotoxic aggregates spreading. In this review, we highlight the current knowledge of brain-specific functions of endocytic machinery with a specific focus on three brain cell types, neuronal progenitor cells, neurons, and glial cells.

Inhibitory effects of cigarette smoke extracts on neural differentiation of mouse embryonic stem cells

Maternal smoking during the perinatal period is linked to adverse neonatal outcomes such as low birth weight and birth defects. Numerous studies have shown that cigarette smoke or nicotine exposure has a widespread effect on fetal nerve development. However, there exists a lack of understanding of what specific changes occur at the cellular level on persistent exposure to cigarette smoke during the differentiation of embryonic stem cells (ESCs) into neural cells. We previously investigated the effects of cigarette smoke extract (CSE) and its major component, nicotine, on the neural differentiation of mouse embryonic stem cells (mESCs). Differentiation of mESCs into neural progenitor cells (NPCs) or neural crest cells (NCCs) was induced with chemically defined media, and the cells were continuously exposed to CSE or nicotine during neural differentiation and development. Disturbed balance of the pluripotency state was observed in the NPCs, with consequent inhibition of neurite outgrowth and glial fibrillary acidic protein (Gfap) expression. These inhibitions correlated with the altered expression of proteins involved in the Notch-1 signaling pathways. The migration ability of NCCs was significantly decreased by CSE or nicotine exposure, which was associated with reduced protein expression of migration-related proteins. Taken together, we concluded that CSE and nicotine inhibit differentiation of mESCs into NPCs or NCCs, and may disrupt functional development of neural cells. These results imply that cigarette smoking during the perinatal period potentially inhibits neural differentiation and development of ESCs cells, leading to neonatal abnormal brain development and behavioral abnormalities.

Post-Developmental Roles of Notch Signaling in the Nervous System

Since its discovery in Drosophila, the Notch signaling pathway has been studied in numerous developmental contexts in diverse multicellular organisms. The role of Notch signaling in nervous system development has been extensively investigated by numerous scientists, partially because many of the core Notch signaling components were initially identified through their dramatic ‘neurogenic’ phenotype of developing fruit fly embryos. Components of the Notch signaling pathway continue to be expressed in mature neurons and glia cells, which is suggestive of a role in the post-developmental nervous system. The Notch pathway has been, so far, implicated in learning and memory, social behavior, addiction, and other complex behaviors using genetic model organisms including Drosophila and mice. Additionally, Notch signaling has been shown to play a modulatory role in several neurodegenerative disease model animals and in mediating neural toxicity of several environmental factors. In this paper, we summarize the knowledge pertaining to the post-developmental roles of Notch signaling in the nervous system with a focus on discoveries made using the fruit fly as a model system as well as relevant studies in C elegans, mouse, rat, and cellular models. Since components of this pathway have been implicated in the pathogenesis of numerous psychiatric and neurodegenerative disorders in human, understanding the role of Notch signaling in the mature brain using model organisms will likely provide novel insights into the mechanisms underlying these diseases.

Boc Acts via Numb as a Shh-Dependent Endocytic Platform for Ptch1 Internalization and Shh-Mediated Axon Guidance

During development, Shh attracts commissural axons toward the floor plate through a non-canonical, transcription-independent signaling pathway that requires the receptor Boc. Here, we find that Shh induces Boc internalization into early endosomes and that endocytosis is required for Shh-mediated growth-cone turning. Numb, an endocytic adaptor, binds to Boc and is required for Boc internalization, Shh-mediated growth-cone turning in vitro, and commissural axon guidance in vivo. Similar to Boc, Ptch1 is also internalized by Shh in a Numb-dependent manner; however, the binding of Shh to Ptch1 alone is not sufficient to induce Ptch1 internalization nor growth-cone turning. Therefore, the binding of Shh to Boc is required for Ptch1 internalization and growth-cone turning. Our data support a model where Boc endocytosis via Numb is required for Ptch1 internalization and Shh signaling in axon guidance. Thus, Boc acts as a Shh-dependent endocytic platform gating Ptch1 internalization and Shh signaling.

E3 Ubiquitin Ligases Neurobiological Mechanisms: Development to Degeneration

Cells regularly synthesize new proteins to replace old or damaged proteins. Deposition of various aberrant proteins in specific brain regions leads to neurodegeneration and aging. The cellular protein quality control system develop various defense mechanisms against the accumulation of misfolded and aggregated proteins. The mechanisms underlying the selective recognition of specific crucial protein or misfolded proteins are majorly governed by quality control E3 ubiquitin ligases mediated through ubiquitin-proteasome system. Few known E3 ubiquitin ligases have shown prominent neurodevelopmental functions, but their interactions with different developmental proteins play critical roles in neurodevelopmental disorders. Several questions are yet to be understood properly. How E3 ubiquitin ligases determine the specificity and regulate degradation of a particular substrate involved in neuronal proliferation and differentiation is certainly the one, which needs detailed investigations. Another important question is how neurodevelopmental E3 ubiquitin ligases specifically differentiate between their versatile range of substrates and timing of their functional modulations during different phases of development. The premise of this article is to understand how few E3 ubiquitin ligases sense major molecular events, which are crucial for human brain development from its early embryonic stages to throughout adolescence period. A better understanding of these few E3 ubiquitin ligases and their interactions with other potential proteins will provide invaluable insight into disease mechanisms to approach toward therapeutic interventions.

Numb deficiency in cerebellar Purkinje cells impairs synaptic expression of metabotropic glutamate receptor and motor coordination

Protein Numb, first identified as a cell-fate determinant in Drosophila, has been shown to promote the development of neurites in mammals and to be cotransported with endocytic receptors in clathrin-coated vesicles in vitro. Nevertheless, its function in mature neurons has not yet been elucidated. Here we show that cerebellar Purkinje cells (PCs) express high levels of Numb during adulthood and that conditional deletion of Numb in PCs is sufficient to impair motor coordination despite maintenance of a normal cerebellar cyto-architecture. Numb proved to be critical for internalization and recycling of metabotropic glutamate 1 receptor (mGlu1) in PCs. A significant decrease of mGlu1 and an inhibition of long-term depression at the parallel fiber-PC synapse were observed in conditional Numb knockout mice. Indeed, the trafficking of mGlu1 induced by agonists was inhibited significantly in these mutants, but the expression of ionotropic glutamate receptor subunits and of mGlu1-associated proteins was not affected by the loss of Numb. Moreover, transient and persistent forms of mGlu1 plasticity were robustly induced in mutant PCs, suggesting that they do not require mGlu1 trafficking. Together, our data demonstrate that Numb is a regulator for constitutive expression and dynamic transport of mGlu1.

Selective multifaceted E3 ubiquitin ligases barricade extreme defense: Potential therapeutic targets for neurodegeneration and ageing

Efficient and regular performance of Ubiquitin Proteasome System and Autophagy continuously eliminate deleterious accumulation of nonnative protiens. In cellular quality control system, E3 ubiquitin ligases are significant employees for defense mechanism against abnormal toxic proteins. Few findings indicate that lack of functions of E3 ubiquitin ligases can be a causative factor of neurodevelopmental disorders, neurodegeneration, cancer and ageing. However, the detailed molecular pathomechanism implying E3 ubiquitin ligases in cellular functions in multifactorial disease conditions are not well understood. This article systematically represents the unique characteristics, molecular nature, and recent developments in the knowledge of neurobiological functions of few crucial E3 ubiquitin ligases. Here, we review recent literature on the roles of E6-AP, HRD1 and ITCH E3 ubiquitin ligases in the neuro-pathobiological mechanisms, with precise focus on the processes of neurodegeneration, and thereby propose new lines of potential targets for therapeutic interventions.

Adapting for endocytosis: roles for endocytic sorting adaptors in directing neural development

Proper cortical development depends on the orchestrated actions of a multitude of guidance receptors and adhesion molecules and their downstream signaling. The levels of these receptors on the surface and their precise locations can greatly affect guidance outcomes. Trafficking of receptors to a particular surface locale and removal by endocytosis thus feed crucially into the final guidance outcomes. In addition, endocytosis of receptors can affect downstream signaling (both quantitatively and qualitatively) and regulated endocytosis of guidance receptors is thus an important component of ensuring proper neural development. We will discuss the cell biology of regulated endocytosis and the impact on neural development. We focus our discussion on endocytic accessory proteins (EAPs) (such as numb and disabled) and how they regulate endocytosis and subsequent post-endocytic trafficking of their cognate receptors (such as Notch, TrkB, β-APP, VLDLR, and ApoER2).

Numb is not a critical regulator of Notch-mediated cell fate decisions in the developing chick inner ear

The Notch signaling pathway controls differentiation of hair cells and supporting cells in the vertebrate inner ear. Here, we have investigated whether Numb, a known regulator of Notch activity in Drosophila, is involved in this process in the embryonic chick. The chicken homolog of Numb is expressed throughout the otocyst at early stages of development and is concentrated at the basal pole of the cells. It is asymmetrically allocated at some cell divisions, as in Drosophila, suggesting that it could act as a determinant inherited by one of the two daughter cells and favoring adoption of a hair-cell fate. To test the implication of Numb in hair cell fate decisions and the regulation of Notch signaling, we used different methods to overexpress Numb at different stages of inner ear development. We found that sustained or late Numb overexpression does not promote hair cell differentiation, and Numb does not prevent the reception of Notch signaling. Surprisingly, none of the Numb-overexpressing cells differentiated into hair cells, suggesting that high levels of Numb protein could interfere with intracellular processes essential for hair cell survival. However, when Numb was overexpressed early and more transiently during ear development, no effect on hair cell formation was seen. These results suggest that in the inner ear at least, Numb does not significantly repress Notch activity and that its asymmetric distribution in dividing precursor cells does not govern the choice between hair cell and supporting cell fates.

The clathrin adaptor Numb regulates intestinal cholesterol absorption through dynamic interaction with NPC1L1

Hypercholesterolemia, typically due to excessive cholesterol uptake, is a major risk factor for cardiovascular disease, which is responsible for ∼50% of all deaths in developed societies. Although it has been shown that intestinal cholesterol absorption is mediated by vesicular endocytosis of the Niemann-Pick C1-like 1 (NPC1L1) protein, the mechanism of sterol-stimulated NPC1L1 internalization is still mysterious. Here, we identified an endocytic peptide signal, YVNXXF (where X stands for any amino acid), in the cytoplasmic C-terminal tail of NPC1L1. Cholesterol binding on the N-terminal domain of NPC1L1 released the YVNXXF-containing region of NPC1L1 from association with the plasma membrane and enabled Numb binding. We also found that Numb, a clathrin adaptor, specifically recognized this motif and recruited clathrin for internalization. Disrupting the NPC1L1-Numb interaction decreased cholesterol uptake. Ablation of Numb in mouse intestine significantly reduced dietary cholesterol absorption and plasma cholesterol level. Together, these data show that Numb is a pivotal protein for intestinal cholesterol absorption and may provide a therapeutic target for hypercholesterolemia.

Control of stem cells and cancer stem cells by Hedgehog signaling: pharmacologic clues from pathway dissection

Hedgehog is a key morphogen regulating embryonic development and tissue repair. Remarkably, when misregulated, it leads to tumorigenesis. Hedgehog signaling is triggered by binding of ligands with transmembrane receptor Ptch and is subsequently mediated by transcriptional effectors belonging to the Gli family, whose functions is tuned by a number of molecular interactions and post-synthetic modifications. The complex of these regulatory circuitries provides a tight control of developmental processes, mainly involving the modulation of genes determining the fate of stem cells. Similarly, Hedgehog regulates cancer stem cells fostering tumorigenesis. To this regard, these processes represent promising targets for novel therapeutic strategies aiming at the control of stemness reactivation and maintenance in cancer.

Harnessing the power of the endosome to regulate neural development

Endocytosis and endosomal trafficking play a multitude of roles in cellular function beyond regulating entry of essential nutrients. In this review, we discuss the cell biological principles of endosomal trafficking, the neuronal adaptations to endosomal organization, and the role of endosomal trafficking in neural development. In particular, we consider how cell fate decisions, polarity, migration, and axon outgrowth and guidance are influenced by five endosomal tricks: dynamic modulation of receptor levels by endocytosis and recycling, cargo-specific responses via cargo-specific endocytic regulators, cell-type-specific endocytic regulation, ligand-specific endocytic regulation, and endosomal regulation of ligand processing and trafficking.

Golgi during development

The Golgi is essential for processing proteins and sorting them, as well as plasma membrane components, to their final destinations. Not surprisingly, this organelle, a major compartment of the secretory pathway, is an important venue for regulating many aspects of development in both invertebrates and vertebrates. Through its role as a site for protein cleavage and glycosylation as well as through changes in its spatial organization and secretory trafficking, the Golgi exerts highly specific effects on cellular differentiation and morphogenesis by spatially and temporally constraining developmental pathways.

Numb links extracellular cues to intracellular polarity machinery to promote chemotaxis

Cell polarization is essential throughout development for proliferation, migration, and differentiation. However, it is not known how extracellular cues correctly orient cell polarity at distinct stages of development. Here, we show that the endocytic adaptor protein Numb, previously characterized for its role in cell proliferation, subsequently plays an important role in cell migration. In neural precursors stimulated with the chemotactic factor BDNF, Numb binds to activated TrkB, the BDNF receptor, and functions both as an endocytic regulator for TrkB and as a scaffold for aPKC (aPKC). Thus, Numb promotes BDNF-dependent aPKC activation. Interestingly, Numb is also a substrate of aPKC. When phosphorylated, Numb exhibits increased efficacy in binding TrkB and in promoting a chemotactic response to BDNF. Therefore, Numb functions in a feed-forward loop to promote chemotaxis of neural precursors, linking BDNF, an extracellular cue, to aPKC, a critical component of the intrinsic polarity machinery.

Tmem176B and Tmem176A are associated with the immature state of dendritic cells

DCs play a central role in the development of innate and adaptive immunity but also in the induction and maintenance of immune tolerance. Identification of factors that govern DC activation, their maturation state, and their capacity to induce proinflammatory or tolerogeneic responses therefore represents a crucial aim of research. We previously identified a new molecule, Tmem176B (which we named TORID initially), as highly expressed in a model of allograft tolerance in the rat. We showed that its overexpression in rat DCs blocked their maturation, suggesting a role for this molecule in the maturation process. To characterize the function of Tmem176B further, we used a split-ubiquitin yeast, two-hybrid system to identify interacting partners and found that Tmem176B associated with itself but also with Tmem176A, a membrane protein similar to Tmem176B. Interestingly, these two molecules showed similar mRNA expression patterns among various murine tissues and immune cells and were both down-regulated following DC maturation. In addition, we showed that in using RNAi, these molecules are both involved in the maintenance of the immature state of the DCs. Taken together, these data suggest that Tmem176B and Tmem176A associate to form multimers and restrain DC maturation. Therefore, these two molecules may represent valid targets to regulate DC function.

The multiple functions of Numb

Numb is an evolutionary conserved protein that plays critical roles in cell fate determination. Mammalian Numb displays a higher degree of structural complexity compared to the Drosophila homolog based on the number of encoding genes (Numb and Numb-like) and of alternative spliced isoforms. Accordingly, Numb proteins display a complex pattern of functions such as the control of asymmetric cell division and cell fate choice, endocytosis, cell adhesion, cell migration, ubiquitination of specific substrates and a number of signaling pathways (i.e. Notch, Hedgehog, p53). Recent findings indicate that, besides controlling such physiologic developmental processes, subversion of the above Numb-dependent events plays a critical role in disease (e.g. cancer). We will review here the multiple functions of mNumb and their underlying molecular mechanisms in development and disease.

Trafficking and cell migration

The migration of single cells and epithelial sheets is of great importance for gastrulation and organ formation in developing embryos and, if misregulated, can have dire consequences e.g. during cancer metastasis. A keystone of cell migration is the regulation of adhesive contacts, which are dynamically assembled and disassembled via endocytosis. Here, we discuss some of the basic concepts about the function of endocytic trafficking during cell migration: transport of integrins from the cell rear to the leading edge in fibroblasts; confinement of signalling to the front of single cells by endocytic transport of growth factors; regulation of movement coherence in multicellular sheets by cadherin turnover; and shaping of extracellular chemokine gradients. Taken together, endocytosis enables migrating cells and tissues to dynamically modulate their adhesion and signalling, allowing them to efficiently migrate through their extracellular environment.

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.

SHH pathway and cerebellar development

The morphogenetic factor Sonic hedgehog (SHH) has been discovered as one of the masterplayers in cerebellar patterning and was subjected to intensive investigation during the last decade. During early postnatal development, this continuously secreted cholesterol-modified protein drives the expansion of the largest neuronal population of the brain, the granular cells. Moreover, it acts on Bergmann glia differentiation and would potentially affect Purkinje cells homeostasis at adult age. The cerebellar cortex constituted an ideal developmental model to dissect out the upstream mechanisms and downstream targets of this complex pathway. Its deep understanding discloses some of the mechanistic disorders underlying pediatric tumorigenesis, congenital ataxia, and mental retardation. Therapeutical use of its regulators has been consolidated on murine transgenic models and is now considered as a realistic human clinical application. Here, we will review the most recent advances made in the comprehensive understanding of SHH involvement in cerebellar development and pathology.

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.

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.

Hedgehog signaling pathway in neural development and disease

A number of developmental cues underlie proper brain morphogenesis and plasticity. Hedgehog (Hh) signaling pathway plays a critical role in determining proper embryonic patterning and cell fate determination in the central nervous system. Embryonic and adult neural progenitor cells are mostly responsive to Hh signaling, thereby sustaining developmental and tissue repair processes. Hh signaling pathway is finely tuned in order to maintain physiological cell functions during the development of the nervous system and afterwards. Its deregulation is responsible for a number of diseases (e.g. cancer, neurodegenerative disorders). Recently identified Hh pathway regulatory signals involved in the maintenance of nervous tissue shape and their relevance in human pathology are discussed here.

Numb controls integrin endocytosis for directional cell migration with aPKC and PAR-3

Migrating cells extend protrusions to establish new adhesion sites at their leading edges. One of the driving forces for cell migration is the directional trafficking of cell-adhesion molecules such as integrins. Here, we show that the endocytic adaptor protein Numb is an important component of the machinery for directional integrin trafficking in migrating cells. Numb binds to integrin-betas and localizes to clathrin-coated structures (CCSs) at the substratum-facing surface of the leading edge. Numb inhibition by RNAi impairs both integrin endocytosis and cell migration toward integrin substrates. Numb is regulated by phosphorylation since the protein is released from CCSs and no longer binds integrins when phosphorylated by atypical protein kinase C (aPKC). Because Numb interacts with the aPKC binding partner PAR-3, we propose a model in which polarized Numb phosphorylation contributes to cell migration by directing integrin endocytosis to the leading edge.

Numb is a suppressor of Hedgehog signalling and targets Gli1 for Itch-dependent ubiquitination

The developmental protein Numb is a major determinant of binary cell fates. It is also required for the differentiation of cerebellar granule cell progenitors (GCPs) at a stage of development responsive to the morphogenic glycoprotein Hedehog. Hedgehog signalling is crucial for the physiological maintenance and self-renewal of neural stem cells and its deregulation is responsible for their progression towards tumorigenesis. The mechanisms that inhibit this pathway during the differentiation stage are poorly understood. Here, we identify Numb as a Hedgehog-pathway inhibitor that is downregulated in early GCPs and GCP-derived cancer cells. We demonstrate that the Hedgehog transcription factor Gli1 is targeted by Numb for Itch-dependent ubiquitination, which suppresses Hedgehog signals, thus arresting growth and promoting cell differentiation. This novel Numb-dependent regulatory loop may limit the extent and duration of Hedgehog signalling during neural-progenitor differentiation, and its subversion may be a relevant event in brain tumorigenesis.

Role of Clast1 in development of cerebellar granule cells

We have identified the murine Clast1/LR8 gene by subtraction of cDNA derived from CD40 ligand-activated and naive B cells. The Clast1 gene is ubiquitously expressed in various organs of adult mice. However, its physiological function was largely unknown. To study a role of Clast1, we established Clast1-deficient (Clast1-KO) mice. Here, we reveal that approximately 65% of Clast1-KO mice showed severe ataxia. The Clast1-KO cerebellum with ataxia is small in size and revealed a severely aberrant lobulation, loss of the internal granule cell layer, and the disorganized Purkinje cells. Clast1 mRNA is expressed in the cerebellar granule cells of normal adult mice. Developmentally, Clast1 mRNA is also detected in the external germinal layer of the embryonic cerebellum, indicating its expression in granule cell precursors. Histopathological analysis of the developing Clast1-KO cerebellum demonstrated the reduced number of cells in the external germinal layer. Thus, Clast1 is required for development of cerebellar granule cells.

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.

Methylmercury induces activation of Notch signaling

Methylmercury (MeHg) toxicity in humans manifests deficits in neurological function. Cases of prenatal exposure to mercury have established that the developing nervous system is most highly susceptible to perturbation by MeHg. At a cellular level, MeHg-induced defects result from altered neuronal proliferation, migration and pathfinding. However, the molecular targets of MeHg that give rise to these outcomes are not fully understood. In an overall effort to identify the fundamental molecular targets of MeHg in neural development, we have explored the effects of MeHg on cell surface receptor function using the simplified Drosophila model. In this study, we investigated the potential role of MeHg to alter activity of the Notch receptor pathway, a highly conserved cell-cell signaling mechanism that controls cell fate decisions, proliferation, migration and neurite outgrowth in neural development. Notch receptor activation requires proteolysis by a cell surface ADAM metalloprotease. ADAM proteases are required for normal neural development and are activated by organomercurials, thus presenting a possible mechanism for MeHg neurotoxicity. Here, we demonstrate a concentration- and time-dependent increase in Notch receptor activity with MeHg exposure in three distinct Drosophila cell lines. Ten micromolar MeHg results in a 4-5.5-fold increase in Notch signaling as measured by the upregulation of two enhancer of split (E(spl)) target genes. MeHg-induced Notch activity also correlates with receptor proteolysis. Targeted knockdown of Notch protein expression demonstrates that MeHg induced E(spl) activation specifically requires the Notch receptor. Furthermore, MeHg-induced Notch activity is partially attenuated by the metalloprotease inhibitor, GM6001, consistent with a model in which MeHg promotes activation of ADAM metalloproteases. Finally, we demonstrate that inorganic HgCl(2) is significantly less active in inducing Notch activity, suggesting a mechanism specific to organic species of mercury. Overall, these data identify Notch as a potential target for MeHg toxicity in the developing nervous system.

Impaired cerebellar functions in mutant mice lacking DNER

DNER is a transmembrane protein carrying extracellular EGF repeats and is strongly expressed in Purkinje cells (PCs) in the cerebellum. Current study indicated that DNER functions as a new Notch ligand and mediates the functional communication via cell-cell interaction. By producing and analyzing knockout mice lacking DNER, we demonstrate its essential roles in functional and morphological maturation of the cerebellum. The knockout mice exhibited motor discoordination in the fixed bar and rota-rod tests. The cerebellum from the knockout mice showed significant retardation in morphogenesis and persistent abnormality in fissure organization. Histochemical and electrophysiological analyses detected that PCs retained multiple innervations from climbing fibers (CFs) in the mutant cerebellum. Synaptic transmission from parallel fibers (PFs) or CFs to PCs was apparently normal, while glutamate clearance at the PF-PC synapses was significantly impaired in the mutant mice. Moreover, the protein level of GLAST, the glutamate transporter predominantly expressed in Bergmann glia (BG), was reduced in the mutant cerebellum. Our results indicate that DNER takes part in stimulation of BG maturation via intercellular communication and is essential for precise cerebellar development.

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.

Intravenous and oral practolol in the acute stages of myocardial infarction

The influence of routine administration of the beta adrenoceptor blocking drug practolol on the outcome of acute myocardial infarction has been studied in 94 patients. The study was restricted to patients under the age of 70 experiencing their first myocardial infarction and in whom there was no contraindication to beta blockade. In the treated group an initial dose intravenous practolol 15 mg was followed by five oral doses of practolol 200 mg at 12 h intervals. A significant reduction in heart rate and systolic blood pressure was apparent in the treated group within 2 h. No difference was detectable in the course of the acute stage of the illness between treated and control patients, apart from a significant reduction in the incidence of atrial fibrillation among those receiving practolol. Patients with inferior infarctions showed a tendency to develop potentially harmful bradycardia and hypotension on receiving practolol which lead to withdrawal of the drug in many cases. At regular review over 7 mth no detectable difference emerged between the treated and control groups in the incidence of cardiac failure, death or reinfarction.