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M Poo

Publications and source records attributed to M Poo.

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

Growth-cone attraction to netrin-1 is converted to repulsion by laminin-1.

Growing axons are guided by both diffusible and substrate-bound factors. Growth cones of retinal neurons exhibit chemoattractive turning towards the diffusible factor netrin-1 in vitro and are guided into the optic nerve head (ONH) by localized netrin-1. Here we report that, in Xenopus, laminin-1 from the extracellular matrix (ECM), converts netrin-mediated attraction into repulsion. A soluble peptide fragment of laminin-1 (YIGSR) mimics this laminin-induced conversion. Low levels of cyclic AMP in growth cones also lead to the conversion of netrin-induced attraction into repulsion, and we show that the amount of cAMP decreases in the presence of laminin-1 or YIGSR, suggesting a possible mechanism for laminin's effect. At the netrin-1-rich ONH, where axons turn sharply to leave the eye, laminin-1 is confined to the retinal surface. Repulsion from the region in which laminin and netrin are coexpressed may help to drive axons into the region where only netrin is present, providing a mechanism for their escape from the retinal surface. Consistent with this idea, YIGSR peptides applied to the developing retina cause axons to be misdirected at the ONH. These findings indicate that ECM molecules not only promote axon outgrowth, but also modify the behaviour of growth cones in response to diffusible guidance cues.

Animals↗
PubMed1999

Phospholipase C-gamma and phosphoinositide 3-kinase mediate cytoplasmic signaling in nerve growth cone guidance.

Expression of rat TrkA in Xenopus spinal neurons confers responsiveness of these neurons to nerve growth factor (NGF) in assays of neuronal survival and growth cone chemotropism. Mutational analysis indicates that coactivation of phospholipase C-gamma (PLC-gamma) and phosphoinositide 3-kinase (PI3-kinase) by specific cytoplasmic domains of TrkA is essential for triggering chemoattraction of the growth cone in an NGF gradient. Uniform exposure of TrkA-expressing neurons to NGF resulted in a cross-desensitization of turning responses induced by a gradient of netrin-1, brain-derived neurotrophic factor (BDNF), or myelin-associated glycoprotein (MAG) but not by a gradient of collapsin-1/semaphorin III/D or neurotrophin-3 (NT-3). These results, together with the effects of pharmacological inhibitors, support the notion that there are common cytosolic signaling pathways for two separate groups of guidance cues, one of which requires coactivation of PLC-gamma and PI3-kinase pathways.

Animals↗
PubMed1998

Conversion of neuronal growth cone responses from repulsion to attraction by cyclic nucleotides.

Nerve growth is regulated by attractive and repulsive factors in the nervous system. Microscopic gradients of Collapsin-1/Semaphorin III/D (Sema III) and myelin-associated glycoprotein trigger repulsive turning responses by growth cones of cultured Xenopus spinal neurons; the repulsion can be converted to attraction by pharmacological activation of the guanosine 3',5'-monophosphate (cGMP) and adenosine 3',5'-monophosphate signaling pathways, respectively. Sema III also causes the collapse of cultured rat sensory growth cones, which can be inhibited by activation of the cGMP pathway. Thus cyclic nucleotides can regulate growth cone behaviors and may be targets for designing treatments to alleviate the inhibition of nerve regeneration by repulsive factors.

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PubMed1998

A critical window for cooperation and competition among developing retinotectal synapses.

In the developing frog visual system, topographic refinement of the retinotectal projection depends on electrical activity. In vivo whole-cell recording from developing Xenopus tectal neurons shows that convergent retinotectal synapses undergo activity-dependent cooperation and competition following correlated pre- and postsynaptic spiking within a narrow time window. Synaptic inputs activated repetitively within 20 ms before spiking of the tectal neuron become potentiated, whereas subthreshold inputs activated within 20 ms after spiking become depressed. Thus both the initial synaptic strength and the temporal order of activation are critical for heterosynaptic interactions among convergent synaptic inputs during activity-dependent refinement of developing neural networks.

Animals↗
PubMed1998

Localized synaptic actions of neurotrophin-4.

Neurotrophins secreted by the postsynaptic target cell may participate in activity-dependent synaptic modification during development and in the mature brain. A fundamental question of how neurotrophins can sculpt synaptic connections is whether neurotrophin-induced synaptic changes are spatially restricted to the site of neurotrophin secretion or whether they can spread to neighboring synapses. Using a model system of nerve-muscle coculture in which neurotrophin-4 (NT-4) is overexpressed in a subpopulation of postsynaptic myocytes, we demonstrated that presynaptic potentiation is restricted to synapses on myocytes overexpressing NT-4 without affecting nearby synapses formed by the same neuron on control myocytes. Likewise, postsynaptic modulation of acetylcholine channels by secreted NT-4 is spatially restricted to <60 micron from the site of NT-4 secretion. Therefore, activity-dependent secretion of neurotrophins can result in highly localized modification of neuronal connections.

Animals↗
PubMed1998

Acetylcholinesterase enhances neurite growth and synapse development through alternative contributions of its hydrolytic capacity, core protein, and variable C termini.

Accumulated indirect evidence suggests nerve growth-promoting activities for acetylcholinesterase (AChE). To determine unequivocally whether such activities exist, whether they are related to the capacities of this enzyme to hydrolyze acetylcholine and enhance synapse development, and whether they are associated with alternative splicing variants of AChEmRNA, we used four recombinant human AChEDNA vectors. When Xenopus laevis embryos were injected with a vector expressing the synapse-characteristic human AChE-E6, which contains the exon 6-encoded C terminus, cultured spinal neurons expressing this enzyme grew threefold faster than co-cultured control neurons. Similar enhancement occurred in neurons expressing an insertion-inactivated human AChE-E6-IN protein, containing the same C terminus, and displaying indistinguishable immunochemical and electrophoretic migration properties from AChE-E6, but incapable of hydrolyzing acetylcholine. In contrast, the nonsynaptic secretory human AChE-I4, which contains the pseudointron 4-derived C terminus, did not affect neurite growth. Moreover, no growth promotion occurred in neurons expressing the catalytically active C-terminally truncated human AChE-E4, demonstrating a dominant role for the E6-derived C terminus in neurite extension. Also, AChE-E6 was the only active enzyme variant to be associated with Xenopus membranes. However, postsynaptic length measurements demonstrated that both AChE-E6 and AChE-E4 enhanced the development of neuromuscular junctions in vivo, unlike the catalytically inert AChE-E6-IN and the nonsynaptic AChE-I4. These findings demonstrate an evolutionarily conserved synaptogenic activity for AChE that depends on its hydrolytic capacity but not on its membrane association. Moreover, this synaptogenic effect differs from the growth-promoting activity of AChE, which is unrelated to its hydrolytic capacity yet depends on its exon 6-mediated membrane association.

Acetylcholinesterase↗
PubMed1997

Quantal neurotransmitter secretion rate exhibits fractal behavior.

The rate of exocytic events from both neurons and non-neuronal cells exhibits fluctuations consistent with fractal (self-similar) behavior in time, as evidenced by a number of statistical measures. We explicitly demonstrate this for neurotransmitter secretion at Xenopus neuromuscular junctions and for rat hippocampal synapses in culture; the exocytosis of exogenously supplied neurotransmitter from cultured Xenopus myocytes and from rat fibroblasts behaves similarly. The magnitude of the fluctuations of the rate of exocytic events about the mean decreases slowly as the rate is computed over longer and longer time periods, the periodogram decreases in power-law manner with frequency, and the Allan factor (relative variance of the number of exocytic events) increases as a power-law function of the counting time. These features are hallmarks of self-similar behavior. Their description requires models that exhibit long-range correlation (memory) in event occurrences. We have developed a physiologically plausible model that accords with all of the statistical measures that we have examined. The appearance of fractal behavior at synapses, as well as in systems comprising collections of synapses, indicates that such behavior is ubiquitous in neural signaling.

Acetylcholine↗
PubMed1997

Expression of a putative vesicular acetylcholine transporter facilitates quantal transmitter packaging.

A putative vesicular acetylcholine transporter (VAChT) was overexpressed in developing Xenopus spinal neurons by injection of rat VAChT cDNA or synthetic mRNA into Xenopus embryos. This resulted in a marked increase in the amplitude and frequency of miniature excitatory postsynaptic currents at neuromuscular synapses, reflecting an over 10-fold increase in the vesicular packaging of acetylcholine (ACh). The effect appeared in developing neurons even before synaptogenesis and was blocked by L-vesamicol, a specific blocker of ACh uptake into synaptic vesicles. Mutational studies showed that two highly conserved aspartate residues within putative transmembrane domains 4 and 10 are essential for the transport activity. These results provide direct evidence for the physiological function of a putative VAChT and demonstrate that quantal size can be regulated by changes in vesicular transporter activity.

Acetylcholine↗
PubMed1994

Long-range signaling in growing neurons after local elevation of cyclic AMP-dependent activity.

Cyclic AMP-dependent activity at the growth cone or the soma of cultured Xenopus spinal neurons was elevated by local extracellular perfusion of the neuron with culture medium containing 8-bromoadenosine 3',5'-cyclic monophosphate (8-br-cAMP) or forskolin. During local perfusion of one of the growth cones of multipolar neurons with these drugs, the perfused growth cone showed further extension, while the distant, unperfused growth cones were inhibited in their growth. Local perfusion of the growth cone with culture medium or local perfusion with 8-br-cAMP at a cell-free region 100 microns away from the growth cone did not produce any effect on the extension of the growth cone. Reduced extension of all growth cones was observed when the perfusion with 8-br-cAMP was restricted to the soma. The distant inhibitory effect does not depend on the growth of the perfused growth cone since local coperfusion of the growth cone with 8-br-cAMP and colchicine inhibited growth on both perfused and unperfused growth cones, while local perfusion with colchicine alone inhibited only the perfused growth cone. The distant inhibitory effect was abolished when the perfusion of 8-br-cAMP was carried out together with kinase inhibitor H-8, suggesting the involvement of cAMP-dependent protein kinase and/or its downstream factors in the long-range inhibitory signaling. Uniform exposure of the entire neuron to bath-applied 8-br-cAMP, however, led to enhanced growth activity at all growth cones. Thus, local elevation of cAMP-dependent activity produces long-range and opposite effects on distant parts of the neuron, and a cytosolic gradient of second messengers may produce effects distinctly different from those following uniform global elevation of the messenger, leading to differential growth regulation at different regions of the same neuron.

8-Bromo Cyclic Adenosine Monophosphate↗
PubMed1992

Antibodies to synaptophysin interfere with transmitter secretion at neuromuscular synapses.

The involvement of synaptophysin, a synaptic vesicle-specific protein, in transmitter release at neuromuscular synapses was studied by intracellular application of synaptophysin antibodies into presynaptic neurons. Polyclonal antibodies or their Fab fragments were loaded into spinal neurons by injection into one of the early blastomeres of Xenopus embryos 1 day prior to culturing or, alternatively, directly through a whole-cell recording pipette at the soma of cultured neurons. At synapses made by antibody-loaded neurons in culture, the spontaneous synaptic currents showed marked reduction in frequency without significant change in their mean amplitude. The impulse-evoked synaptic currents showed reduced amplitude and increased failure rate. These results suggest that interference with synaptophysin function by antibody binding inhibits transmitter secretion.

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PubMed1985

Response of nerve growth cone to focal electric currents.

Monopolar electric current pulses were focally applied through a micropipette to the growth cone of Xenopus embryonic neurons in culture. Application of the current directly in front of the growth cone modulated the rate of growth cone extension: Negative (sink) currents increased the growth rate, while positive (source) currents reduced the growth rate. When the currents were applied in a direction perpendicular to the direction of the neurite growth, both negative and positive currents produced inhibitory effects. Application of a negative focal current at a 45 degree angle with respect to the direction of neurite growth resulted in an oriented growth of the neurite toward the current sink. However, after the growth cone had been attracted to the vicinity of a current sink, further extension of the neurite was inhibited. These current effects occur rapidly after the onset of the current application, and are at least partially reversible within 1 hr after the termination of the current. The magnitude of current density required to induce a growth cone response was found to be in the order of a few pA per micron2. Such current density is close to that which may be generated at the muscle cell surface by the acetylcholine molecules released from the growth cone during the early phase of nerve-muscle contact.

Animals↗
PubMed1979

Electrophoresis and diffusion in the plane of the cell membrane.

Electrophoretic and diffusional movements of concanavalin A (Con A) receptors and acetylcholine (ACh) receptors in the plane of the plasma membrane of mononucleate, spherical Xenopus myoblasts were studied by microfluorimetry and iontophoresis. We found that (a) a uniform electric field of 10 V/cm applied along the cell surface produces a partial accumulation of both types of receptors toward the cathodal pole of the cell within 30 min: (b) post-field relaxation of the culture results in the complete recovery of the uniform distribution of the Con A receptors within 10 min; and (c) in contrast to the Con A receptor in general, accumulation of ACh receptors by the electric field results in the formation of stable, localized receptor aggregates. Theoretical analyses were carried out for the distribution of charged membrane receptors at equilibrium between electrophoresis and diffusion, and for the rate of back diffusion after the removal of the field. These analyses indicated that, at 22 degrees C, the average electrophoretic mobility of the electrophoretically mobile population of the Con A receptors is about 1.9 X 10(-3) micron/s per V/cm, while their average diffusion coefficient is 5.1 X 10(-9) cm2/s.

Animals↗
PubMed1977

Electrophoresis of concanavalin A receptors along embryonic muscle cell membrane.

Fluorescent concanavalin A (con A)-labelling showed that an electric field of 4 V cm-1 grossly redistributed con A receptors alone the plasma membranes of living muscle cells within 4 h. This field produced a voltage drop of 12 mV across these 30 micronm-wide cells. The movement of receptors was independent of cell metabolism and seemed to be electrophoretic in nature.

Animals↗