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M E Halpern

Publications and source records attributed to M E Halpern.

30 records · Page 2Linked to original sources
PubMed1993

Induction of muscle pioneers and floor plate is distinguished by the zebrafish no tail mutation.

Dorsal mesoderm is thought to provide important signals for axis formation and neural differentiation in vertebrate embryos. We have examined induction and patterning in a zebrafish mutant, no tail, that lacks a derivative of dorsal mesoderm, the notochord. Despite the absence of a differentiated notochord, development of the central nervous system including floor plate appears normal, likely owing to the presence of notochord precursor cells. In contrast, somites are misshapen, and muscle pioneer cells are absent. Wild-type cells transplanted into mutant hosts can autonomously differentiate into notochord and thereby rescue somitic defects, suggesting that interactions between notochord and paraxial mesoderm are necessary for proper somite patterning. Thus, cells derived from dorsal mesoderm may have multiple signaling functions during zebrafish embryogenesis.

Animals↗
PubMed1993

Cellular mechanisms governing synaptic development in Drosophila melanogaster.

The neuromuscular connections of Drosophila are ideally suited for studying synaptic function and development. Hypotheses about cell recognition can be tested in a simple array of pre- and postsynaptic elements. Drosophila muscle fibers are multiply innervated by individually identifiable motoneurons. The neurons express several synaptic cotransmitters, including glutamate, proctolin, and octopamine, and are specialized by their synaptic morphology, neurotransmitters, and connectivity. During larval development the initial motoneuron endings grow extensively over the surface of the muscle fibers, and differentiate synaptic boutons of characteristic morphology. While considerable growth occurs postembryonically, the initial wiring of motoneurons to muscle fibers is accomplished during mid-to-late embryogenesis (stages 15-17). Efferent growth cones sample multiple muscle fibers with rapidly moving filopodia. Upon reaching their target muscle fibers, the growth cones rapidly differentiate into synaptic contacts whose morphology prefigures that of the larval junction. Mismatch experiments show that growth cones recognize specific muscle fibers, and can do so when the surrounding musculature is radically altered. However, when denied their normal targets, motoneurons can establish functional synapses on alternate muscle fibers. Blocking synaptic activity with either injected toxins or ion channel mutants does not derange synaptogenesis, but may influence the number of motor ending processes. The molecular mechanisms governing cellular recognition during synaptogenesis remain to be identified. However, several cell surface glycoproteins known to mediate cellular adhesion events in vitro are expressed by the developing synapses. Furthermore, enhancer detector lines have identified genes with expression restricted to small subsets of muscle fibers and/or motoneurons during the period of synaptogenesis. These observations suggest that in Drosophila a mechanism of target chemoaffinity may be involved in the genesis of stereotypic synaptic wiring.

Animals↗
PubMed1991

Growth cone behavior underlying the development of stereotypic synaptic connections in Drosophila embryos.

Each muscle fiber in the segmented body wall of Drosophila larvae is innervated by anatomically stereotyped neuromuscular junctions. These synapses arise through the selective choices of motoneuronal growth cones at their peripheral targets. Using digital optical microscopy of staged intracellular dye fills, we have singly identified embryonic motoneurons and have examined individual growth cones when they contact and differentiate at the target cells. There is a precise connectivity between motoneuron and muscle fiber, which is the direct consequence of growth cone behavior. We have also found that Drosophila muscle fibers possess molecularly heterogeneous cell surfaces that may be involved in growth cone recognition of appropriate targets. Fasciclin III, a homophilic adhesion molecule, is coexpressed by several of the efferent growth cones and in a site-specific fashion by the target muscle fiber's membrane. The fasciclin III expression is transient, corresponding to the period in embryogenesis when the first neuromuscular contacts are made. Upon encountering the target cell surface, the growth cones can sprout stereotypically arrayed filopodial processes, orient along the anterior-posterior axis, and turn in predictable directions. Subsequently, terminal branches are established in a nonrandom order. These phenomena were found to occur in two motoneurons that innervate adjacent muscle fiber targets, and may be general features of neuromuscular synaptogenesis in Drosophila.

Aging↗
PubMed1989

Axonal guidance and the development of muscle fiber-specific innervation in Drosophila embryos.

The outgrowth of peripheral nerves and the development of muscle fiber-specific neuromuscular junctions were examined in Drosophila embryos using immunocytochemistry and computer-enhanced digital optical microscopy. We find that the pioneering of the peripheral nerves and the formation of the neuromuscular junctions occur through a precisely orchestrated sequence of stereotyped axonal trajectories, mediated by the selective growth cone choices of pioneer motoneurons. We have also examined the establishment of the embryonic muscle fibers and, using intracellular dye fills, have identified cells that are putative muscle pioneers. The muscle fibers of the bodywall have completed their morphogenesis prior to the initiation of synaptic contacts, and owing to the timing of neurite outgrowth from the CNS, synaptogenesis is synchronous at muscle fibers throughout the bodywall. At each muscle fiber the innervating axons make their initial contacts on a characteristic surface domain of the target cell's membrane. Through stereotyped growth cone-mediated trajectories the motoneurons actively establish the basic anatomical features of the mature neuromuscular junction, including the stereotyped, muscle fiber-specific branch anatomy. These events occur without significant process pruning or apparent synapse elimination. Our results suggest that the basic elements of the mature neuromuscular innervation, including the details of the ending trajectory on the target cell's surface, are formed by the precise navigation and presumed recognition by the motoneuron growth cones of muscle membrane surface features.

Animals↗
PubMed1989

Stereotypic morphology of glutamatergic synapses on identified muscle cells of Drosophila larvae.

The distribution and morphology of glutamatergic synapses on Drosophila bodywall muscle fibers were examined at the single-synapse level using immunocytochemistry and electrophysiology. We find that glutamate-immunoreactive motor endings innervate the entire larval bodywall musculature, with each muscle fiber receiving at least one glutamatergic ending. The innervation is initiated at stereotyped locations on each muscle fiber from where moderately branched varicose nerve processes project over the internally facing muscle surface. Individual muscle fibers have distinct stereotypic patterns of nerve endings that occupy characteristic regions on the cell surface. The muscle-specific branching pattern of motor endings is reiterated by segmentally homologous fibers. Two morphological types of innervating nerve processes can be distinguished by their bouton size distributions: (1) Type I processes, which have localized branching and a broad size distribution of relatively large varicosities ranging up to 8 microns (mean diameter, 3.1 +/- 1.6 microns; +/- SD, n = 521), and (2) thinner Type II processes, which have a narrower distribution of small varicosities with a mean diameter of only 1.4 +/- 0.6 microns (+/- SD, n = 214). Immunoelectron microscopy with peroxidase-labeled second antibody demonstrates that the varicosities are surrounded by a subsynaptic reticulum, that they contain immunoreactive vesicles of about 30-50 nm, and thus probably represent synaptic release sites. By iontophoretic application of glutamate we mapped the responsive sites on the muscle surface and found an excellent correspondence between transmitter sensitivity and the patterns of endings as described by immunocytochemistry. In contrast to our finding of numerous glutamate iontophoresis-sensitive sites, we did not detect any aspartate-responsive muscles. These data provide strong new evidence for glutamate being an endogenous transmitter at the Drosophila larval neuromuscular junction.

Animals↗
PubMed1988

Identification of the neuropeptide transmitter proctolin in Drosophila larvae: characterization of muscle fiber-specific neuromuscular endings.

The cellular localization of the peptide neurotransmitter proctolin was determined for larvae of the fruitfly Drosophila melanogaster. Proctolin was recovered from the CNS, hindgut, and segmental bodywall using reverse-phase HPLC, and characterized by bioassay, immunoassay, and enzymatic analysis. A small, stereotyped population of proctolin-immunoreactive neurons was found in the larval CNS. Several of the identified neurons may be efferents. In the periphery, proctolin-immunoreactive neuromuscular endings were identified on both visceral and skeletal muscle fibers. On the hindgut, the neuropeptide is associated with endings on intrinsic circular muscle fibers. We propose that the hindgut muscle fibers are innervated by central neurons homologous to previously described proctolinergic efferents of grasshoppers. The segmental bodywall innervation consists of a pattern of segment-specific junctions on several singly identifiable muscle fibers. While it is generally accepted that Drosophila muscle fibers are innervated by glutamatergic motoneurons, our data indicate that a specialized subset of muscle fibers are also innervated by peptidergic efferents.

Abdomen↗
PubMed1984

Effects of deletions on expression of the herpes simplex virus thymidine kinase gene from the intact viral genome: the amino terminus of the enzyme is dispensable for catalytic activity.

We have transferred two deletions affecting the 5' end of the herpes simplex virus thymidine kinase (TK) gene into the intact viral genome. One, extending from -12 to +189, had no effect on TK mRNA synthesis and only a small effect on TK activity, although the first 27 codons of the TK polypeptide were deleted. The other, extending from -85 to +85, severely impaired TK mRNA synthesis. We conclude that the amino terminus of the TK polypeptide is dispensable for catalytic activity, and that expression of TK in viral infections requires some of the same promoter elements used in uninfected cells.

Animals↗
PubMed1983

Positive control of the herpes simplex virus thymidine kinase gene requires upstream DNA sequences.

We examined the ability of deleted versions of the herpes simplex virus type 1 thymidine kinase gene, present in biochemically transformed mouse cells, to be induced in trans to a higher level of expression by superinfecting herpes simplex virus immediate early gene products. The results demonstrate that sequences mapping between -200 and -80 and between -70 and -12 are required for induction. As these regions are largely coincident with the previously identified thymidine kinase gene promoter, the results suggest that herpes simplex virus immediate early gene products or their metabolic product activate thymidine kinase expression by acting at the promoter region to increase the rate of transcription.

Base Sequence↗
PubMed1979

Origins of phage T4 DNA replication as revealed by hybridization to cloned genes.

[3H]Thymidine-labeled progeny DNA was isolated after infection of Escherichia coli with two different bacteriophage T4 mutants. These strands were isolated shortly after the initiation of DNA replication and hybridized to 15 different (EcoRI) T4 restriction fragments cloned in plasmid vectors. Uniformly labeled T4 [32P]DNA extracted from phage particles was cohybridized as a normalizing reference. The results obtained lead to the conclusions that, among the loci tested, initiation occurs predominantly in the area of genes 50-5 and less prominently in the area of genes 25-29. However, our data do not support the idea of initiation in the area of genes 40-43. In contrast, this area displays the least replication among the genes tested.

Chromosome Mapping↗
PubMed1976

Intratesticular site of aromatization in the human.

To determine the intratesticular site of aromatization, homogenates of separated seminiferous tubules and intact tissue from human testes were incubated with [3H] androstenedione or [3H] testosterone. [3H] Estrogens were isolated and identified, and the amounts synthesized were expressed as pmol/mg protein incubated. In testicular tissue from the three adult subjects investigated, the total estrogen per mg protein was 1.5-2.7 times greater in intact tissue than in isolated seminiferous tubules. This suggests that the major site of aromatization in human testes is the interstitial tissue. Estradiol was by far the major estrogenic product. No differences were observed in the amount of estrogens synthesized whether the substrate was androstenedione or testosterone.

Adult↗