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H J Bellen

Publications and source records attributed to H J Bellen.

At least 37 records · Page 2Linked to original sources
PubMed1999

Math1: an essential gene for the generation of inner ear hair cells.

The mammalian inner ear contains the cochlea and vestibular organs, which are responsible for hearing and balance, respectively. The epithelia of these sensory organs contain hair cells that function as mechanoreceptors to transduce sound and head motion. The molecular mechanisms underlying hair cell development and differentiation are poorly understood. Math1, a mouse homolog of the Drosophila proneural gene atonal, is expressed in inner ear sensory epithelia. Embryonic Math1-null mice failed to generate cochlear and vestibular hair cells. This gene is thus required for the genesis of hair cells.

Animals↗
PubMed1999

Discs Lost, a novel multi-PDZ domain protein, establishes and maintains epithelial polarity.

Polarization of epithelial cells depends on a hierarchical process whereby specific membrane-associated proteins become targeted to specialized membrane domains. Here, we describe a novel Drosophila protein, Discs Lost (DLT), that plays a crucial role in the polarization of embryonic epithelia during cellular blastoderm formation. At subsequent stages of development, DLT interacts with the apical determinant Crumbs (CRB) and the laterally localized protein Neurexin IV (NRX IV). Mutations in dlt or double-stranded RNA interference lead to aberrant localization of CRB and NRX IV and cause a concomitant loss of epithelial cell polarity. Hence, DLT is required to establish and maintain cell polarity and participates in different molecular complexes that define apical and lateral membrane domains.

Amino Acid Sequence↗
PubMed1999

Syntaxin 1A interacts with multiple exocytic proteins to regulate neurotransmitter release in vivo.

Biochemical studies suggest that syntaxin 1A participates in multiple protein-protein interactions in the synaptic terminal, but the in vivo significance of these interactions is poorly understood. We used a targeted mutagenesis approach to eliminate specific syntaxin binding interactions and demonstrate that Drosophila syntaxin 1A plays multiple regulatory roles in neurotransmission in vivo. Syntaxin mutations that eliminate ROP/Munc-18 binding display increased neurotransmitter release, suggesting that ROP inhibits neurosecretion through its interaction with syntaxin. Syntaxin mutations that block Ca2+ channel binding also cause an increase in neurotransmitter release, suggesting that syntaxin normally functions in inhibiting Ca2+ channel opening. Additionally, we identify and characterize a syntaxin Ca2+ effector domain, which may spatially organize the Ca2+ channel, cysteine string protein, and synaptotagmin for effective excitation-secretion coupling in the presynaptic terminal.

Animals↗
PubMed1998

pavarotti encodes a kinesin-like protein required to organize the central spindle and contractile ring for cytokinesis.

Mutations in the Drosophila gene pavarotti result in the formation of abnormally large cells in the embryonic nervous system. In mitotic cycle 16, cells of pav mutant embryos undergo normal anaphase but then develop an abnormal telophase spindle and fail to undertake cytokinesis. We show that the septin Peanut, actin, and the actin-associated protein Anillin, do not become correctly localized in pav mutants. pav encodes a kinesin-like protein, PAV-KLP, related to the mammalian MKLP-1. In cellularized embryos, the protein is localized to centrosomes early in mitosis, and to the midbody region of the spindle in late anaphase and telophase. We show that Polo kinase associates with PAV-KLP with which it shows an overlapping pattern of subcellular localization during the mitotic cycle and this distribution is disrupted in pav mutants. We suggest that PAV-KLP is required both to establish the structure of the telophase spindle to provide a framework for the assembly of the contractile ring, and to mobilize mitotic regulator proteins.

Actins↗
PubMed1998

ROP, the Drosophila Sec1 homolog, interacts with syntaxin and regulates neurotransmitter release in a dosage-dependent manner.

The Sec1 family of proteins is thought to function in both non-neuronal and neuronal secretion, although the precise role of this protein family has not been defined. Here, we study the function of ROP, the Drosophila Sec1 homolog, in neurotransmitter release. Electrophysiological analyses of transgenic lines overexpressing ROP and syntaxin, a presynaptic membrane protein, indicate that ROP interacts with syntaxin in vivo. Characterization of four point mutations in ROP shows that they fall into two phenotypic classes. Two mutations cause a dramatic reduction in both evoked and spontaneous neurotransmitter release. In contrast, the other two mutations reveal an increase in evoked neurotransmission. Our data further show that neurotransmission is highly sensitive to the levels of ROP function. Studies on heterozygote animals indicate that half the amount of wild-type ROP results in a dramatic decrease in evoked and spontaneous exocytosis. Taken together, these results suggest that ROP interacts with syntaxin in vivo and is a rate-limiting regulator of exocytosis that performs both positive and inhibitory functions in neurotransmission.

Animals↗
PubMed1998

Neurexin IV, caspr and paranodin--novel members of the neurexin family: encounters of axons and glia.

Axonal insulation is of key importance for the proper propagation of action potentials. In Drosophila and other invertebrates, it has recently been demonstrated that septate junctions play an essential role in axonal insulation or blood-brain-barrier formation. Neurexin IV, a molecular component of Drosophila septate junctions, has been shown to be essential for axonal insulation in the PNS in embryos and larvae. Interestingly, a vertebrate homolog of Neurexin IV, caspr--also named paranodin--has been shown to localize to septate-like junctional structures. These vertebrate junctions are localized to the paranodal region of the nodes of Ranvier, between axons and Schwann cells. Caspr/paranodin might play an important role in barrier formation, and link neuronal membrane components with the axonal cytoskeletal network.

Action Potentials↗
PubMed1998

Synaptic vesicle size and number are regulated by a clathrin adaptor protein required for endocytosis.

Clathrin-mediated endocytosis is thought to involve the activity of the clathrin adaptor protein AP180. However, the role of this protein in endocytosis in vivo remains unknown. Here, we show that a mutation that eliminates an AP180 homolog (LAP) in Drosophila severely impairs the efficiency of synaptic vesicle endocytosis and alters the normal localization of clathrin in nerve terminals. Most importantly, the size of both synaptic vesicles and quanta is significantly increased in lap mutants. These results provide novel insights into the molecular mechanism of endocytosis and reveal a role for AP180 in regulating vesicle size through a clathrin-dependent reassembly process.

Adaptor Proteins, Vesicular Transport↗
PubMed1998

skittles, a Drosophila phosphatidylinositol 4-phosphate 5-kinase, is required for cell viability, germline development and bristle morphology, but not for neurotransmitter release.

The phosphatidylinositol pathway is implicated in the regulation of numerous cellular functions and responses to extracellular signals. An important branching point in the pathway is the phosphorylation of phosphatidylinositol 4-phosphate by the phosphatidylinositol 4-phosphate 5-kinase (PIP5K) to generate the second messenger phosphatidylinositol 4,5-bis-phosphate (PIP2). PIP5K and PIP2 have been implicated in signal transduction, cytoskeletal regulation, DNA synthesis, and vesicular trafficking. We have cloned and generated mutations in a Drosophila PIP5K type I (skittles). Our analysis indicates that skittles is required for cell viability, germline development, and the proper structural development of sensory bristles. Surprisingly, we found no evidence for PIP5KI involvement in neural secretion.

Animals↗
PubMed1998

The Drosophila sanpodo gene controls sibling cell fate and encodes a tropomodulin homolog, an actin/tropomyosin-associated protein.

Notch signaling is required in many invertebrate and vertebrate cells to promote proper cell fate determination. Mutations in sanpodo cause many different neuronal peripheral nervous system precursor cells to generate two identical daughter neurons, instead of a neuron and sibling cell. This phenotype is similar to that observed when Notch function is lost late in embryonic development and opposite to the numb loss-of-function phenotype. Genetic interaction studies show that sanpodo is epistatic to numb. sanpodo encodes a homolog of tropomodulin, an actin/tropomyosin-associated protein. Loss of sanpodo leads to an aberrant F-actin distribution and causes differentiation defects of actin-containing sensory structures. Our data suggest that an actin-based process is involved in Notch signaling.

Actins↗
PubMed1997

Math1 is essential for genesis of cerebellar granule neurons.

The cerebellum is essential for fine motor control of movement and posture, and its dysfunction disrupts balance and impairs control of speech, limb and eye movements. The developing cerebellum consists mainly of three types of neuronal cells: granule cells in the external germinal layer, Purkinje cells, and neurons of the deep nuclei. The molecular mechanisms that underlie the specific determination and the differentiation of each of these neuronal subtypes are unknown. Math1, the mouse homologue of the Drosophila gene atonal, encodes a basic helix-loop-helix transcription factor that is specifically expressed in the precursors of the external germinal layer and their derivatives. Here we report that mice lacking Math1 fail to form granule cells and are born with a cerebellum that is devoid of an external germinal layer. To our knowledge, Math1 is the first gene to be shown to be required in vivo for the genesis of granule cells, and hence the predominant neuronal population in the cerebellum.

Animals↗
PubMed1997

Genetic dissection of synaptic transmission in Drosophila.

Recent studies of mutations of Drosophila proteins implicated in synaptic transmission have yielded new insights into the roles of these proteins and the pathways in which they function. Analysis of mutant embryos lacking syntaxin or synaptobrevin suggests that these proteins perform distinct functions after vesicle docking with the presynaptic membrane. In addition, characterization of Drosophila endocytotic mutants provides in vivo evidence for the presence of different endocytotic pathways at a single synapse.

Animals↗
PubMed1997

P-element insertion alleles of essential genes on the third chromosome of Drosophila melanogaster: mutations affecting embryonic PNS development.

To identify novel genes and to isolate tagged mutations in known genes that are required for the development of the peripheral nervous system (PNS), we have screened a novel collection of 2460 strains carrying lethal or semilethal P element insertions on the third chromosome. Monoclonal antibody 22C10 was used as a marker to visualize the embryonic PNS. We identified 109 mutant strains that exhibited reproducible phenotypes in the PNS. Cytological and genetic analyses of these strains indicated that 87 mutations affect previously identified genes: tramtrack (n = 18 alleles), string (n = 15), cyclin A (n = 13), single-minded (n = 13), Delta (n = 9), neuralized (n = 4), pointed (n = 4), extra macrochaetae (n = 4), prospero (n = 3), tartan (n = 2), and pebble (n = 2). In addition, 13 mutations affect genes that we identified recently in a chemical mutagenesis screen designed to isolate similar mutants: hearty (n = 3), dorsotonals (n = 2), pavarotti (n = 2), sanpodo (n = 2), dalmatian (n = 1), missensed (n = 1), senseless (n = 1), and sticky ch1 (n = 1). The remaining nine mutations define seven novel complementation groups. The data presented here demonstrate that this collection of P elements will be useful for the identification and cloning of novel genes on the third chromosome, since >70% of mutations identified in the screen are caused by the insertion of a P element. A comparison between this screen and a chemical mutagenesis screen undertaken earlier highlights the complementarity of the two types of genetic screens.

Alleles↗
PubMed1996

A Drosophila neurexin is required for septate junction and blood-nerve barrier formation and function.

Septate and tight junctions are thought to seal neighboring cells together and to function as barriers between epithelial cells. We have characterized a novel member of the neurexin family, Neurexin IV (NRX), which is localized to septate junctions (SJs) of epithelial and glial cells. NRX is a transmembrane protein with a cytoplasmic domain homologous to glycophorin C, a protein required for anchoring protein 4.1 in the red blood cell. Absence of NRX results in mislocalization of Coracle, a Drosophila protein 4.1 homolog, at SJs and causes dorsal closure defects similar to those observed in coracle mutants. nrx mutant embryos are paralyzed, and electrophysiological studies indicate that the lack of NRX in glial-glial SJs causes a breakdown of the blood-brain barrier. Electron microscopy demonstrates that nrx mutants lack the ladder-like intercellular septa characteristic of pleated SJs (pSJs). These studies identify NRX as the first transmembrane protein of SJ and demonstrate a requirement for NRX in the formation of septate-junction septa and intercellular barriers.

Amino Acid Sequence↗
PubMed1996

Chromatid segregation at anaphase requires the barren product, a novel chromosome-associated protein that interacts with Topoisomerase II.

We have isolated a Drosophila gene, barren (barr), required for sister-chromatid segregation in mitosis. barr encodes a novel protein that is present in proliferating cells and has homologs in yeast and human. Mitotic defects in barr embryos become apparent during cycle 16, resulting in a loss of PNS and CNS neurons. Centromeres move apart at the metaphase-anaphase transition and Cyclin B is degraded, but sister chromatids remain connected, resulting in chromatin bridging. This phenotype is similar to that described in TOP2 mutants in yeast. Barren protein localizes to chromatin throughout mitosis. Colocalization and biochemical experiments indicate that Barren associates with Topoisomerase II throughout mitosis and alters the activity of Topoisomerase II. We propose that this association is required for proper chromosomal segregation by facilitating the decatenation of chromatids at anaphase.

Anaphase↗