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Biomedical subjects

M M Poo

Publications and source records attributed to M M Poo.

At least 19 recordsLinked to original sources
PubMed1999

Retracted: A ligand-gated association between cytoplasmic domains of UNC5 and DCC family receptors converts netrin-induced growth cone attraction to repulsion.

Netrins are bifunctional: they attract some axons and repel others. Netrin receptors of the Deleted in Colorectal Cancer (DCC) family are implicated in attraction and those of the UNC5 family in repulsion, but genetic evidence also suggests involvement of the DCC protein UNC-40 in some cases of repulsion. To test whether these proteins form a receptor complex for repulsion, we studied the attractive responses of Xenopus spinal axons to netrin-1, which are mediated by DCC. We show that attraction is converted to repulsion by expression of UNC5 proteins in these cells, that this repulsion requires DCC function, that the UNC5 cytoplasmic domain is sufficient to effect the conversion, and that repulsion can be initiated by netrin-1 binding to either UNC5 or DCC. The isolated cytoplasmic domains of DCC and UNC5 proteins interact directly, but this interaction is repressed in the context of the full-length proteins. We provide evidence that netrin-1 triggers the formation of a receptor complex of DCC and UNC5 proteins and simultaneously derepresses the interaction between their cytoplasmic domains, thereby converting DCC-mediated attraction to UNC5/DCC-mediated repulsion.

Amino Acid Sequence↗
PubMed1999

Gating of BDNF-induced synaptic potentiation by cAMP.

Neurotrophins have been implicated in activity-dependent synaptic plasticity, but the underlying intracellular mechanisms remain largely unknown. Synaptic potentiation induced by brain-derived neurotrophic factor (BDNF), but not neurotrophin 3, was prevented by blockers of adenosine 3',5'-monophosphate (cAMP) signaling. Activators of cAMP signaling alone were ineffective in modifying synaptic efficacy but greatly enhanced the potentiation effect of BDNF. Blocking cAMP signaling abolished the facilitation of BDNF-induced potentiation by presynaptic activity. Thus synaptic actions of BDNF are gated by cAMP. Activity and other coincident signals that modulate cAMP concentrations may specify the action of secreted neurotrophins on developing nerve terminals.

Animals↗
PubMed1999

Signal transduction underlying growth cone guidance by diffusible factors.

Many diffusible axon guidance cues and their receptors have been identified recently. These cues are often found to be bifunctional, acting as attractants or repellents under different circumstances. Studies of cytoplasmic signaling mechanisms have led to the notion that the response of a growth cone to a particular guidance cue depends on the internal state of the neuron, which, in turn, is under the influence of other coincident signals received by the neuron. Furthermore, many diffusible guidance cues appear to share common cytoplasmic signaling pathways.

Animals↗
PubMed1999

Presynaptic depolarization facilitates neurotrophin-induced synaptic potentiation.

Neurotrophins have been proposed to participate in activity-dependent modifications of neuronal connectivity and synaptic efficacy. Preferential strengthening of active inputs requires restriction of putative neurotrophin-mediated synaptic potentiation to active synapses. Here we report that potentiation of synaptic efficacy by brain-derived neurotrophic factor (BDNF) is greatly facilitated by presynaptic depolarization at developing neuromuscular synapses. Brief depolarization in the presence of low-level BDNF results in a marked potentiation of both evoked and spontaneous synaptic transmission, whereas exposure to either BDNF or depolarization alone is without effect. This potentiation depends on the relative timing of depolarization and reflects an enhancement of transmitter secretion from the presynaptic neuron. Thus synapses made by active inputs may be selectively strengthened by secreted neurotrophins as part of activity-dependent refinement of developing connections or of mature synapses.

Action Potentials↗
PubMed1998

Synaptic modifications in cultured hippocampal neurons: dependence on spike timing, synaptic strength, and postsynaptic cell type.

In cultures of dissociated rat hippocampal neurons, persistent potentiation and depression of glutamatergic synapses were induced by correlated spiking of presynaptic and postsynaptic neurons. The relative timing between the presynaptic and postsynaptic spiking determined the direction and the extent of synaptic changes. Repetitive postsynaptic spiking within a time window of 20 msec after presynaptic activation resulted in long-term potentiation (LTP), whereas postsynaptic spiking within a window of 20 msec before the repetitive presynaptic activation led to long-term depression (LTD). Significant LTP occurred only at synapses with relatively low initial strength, whereas the extent of LTD did not show obvious dependence on the initial synaptic strength. Both LTP and LTD depended on the activation of NMDA receptors and were absent in cases in which the postsynaptic neurons were GABAergic in nature. Blockade of L-type calcium channels with nimodipine abolished the induction of LTD and reduced the extent of LTP. These results underscore the importance of precise spike timing, synaptic strength, and postsynaptic cell type in the activity-induced modification of central synapses and suggest that Hebb's rule may need to incorporate a quantitative consideration of spike timing that reflects the narrow and asymmetric window for the induction of synaptic modification.

6-Cyano-7-nitroquinoxaline-2,3-dione↗
PubMed1998

Overexpression of synaptotagmin modulates short-term synaptic plasticity at developing neuromuscular junctions.

The level of synaptotagmin I or II in developing spinal neurons was increased by injection of synaptotagmin messenger RNA into early blastomeres of Xenopus embryos. The effect of overexpression of synaptotagmin on synaptic function was assayed in Xenopus nerve-muscle cultures within two days after injection. At neuromuscular synapses made by synaptotagmin-overexpressing neurons, the frequency of miniature postsynaptic currents was markedly reduced, while their mean amplitude was unchanged, as compared to those of control neurons in the same culture. The amplitude of evoked postsynaptic currents elicited by low-frequency test stimuli was not affected by overexpression. However, synapses made by synaptotagmin-overexpressing neurons exhibited significantly higher paired-pulse facilitation and reduced tetanus-induced depression of the synaptic response, and there was also an increased number of synaptic vesicles at regions 100-300 nm from the plasmalemma at such synapses. These results show that synaptotagmins can exert an inhibitory action on the spontaneous exocytosis of synaptic vesicles. The effects on short-term plasticity suggest that synaptotagmin may facilitate vesicular supply for the evoked release during higher frequency transmission.

Animals↗
PubMed1998

Retrograde signaling in the development and modification of synapses.

Retrograde signaling from the postsynaptic cell to the presynaptic neuron is essential for the development, maintenance, and activity-dependent modification of synaptic connections. This review covers various forms of retrograde interactions at developing and mature synapses. First, we discuss evidence for early retrograde inductive events during synaptogenesis and how maturation of presynaptic structure and function is affected by signals from the postsynaptic cell. Second, we review the evidence that retrograde interactions are involved in activity-dependent synapse competition and elimination in developing nervous systems and in long-term potentiation and depression at mature synapses. Third, we review evidence for various forms of retrograde signaling via membrane-permeant factors, secreted factors, and membrane-bound factors. Finally, we discuss the evidence and physiological implications of the long-range propagation of retrograde signals to the cell body and other parts of the presynaptic neuron.

Animals↗
PubMed1997

Propagation of activity-dependent synaptic depression in simple neural networks.

Triple whole-cell recordings from simple networks of cultured hippocampal neurons show that Induction of long-term depression at glutamatergic synapses is accompanied by a back propagation of depression to Input synapses on the dendrite of the presynaptic neuron. The depression also propagates laterally to divergent outputs of the presynaptic neuron and to convergent inputs on the postsynaptic neuron. There is no forward propagation of depression to the output of the postsynaptic neuron and no presynaptic propagation accompanying long-term depression at GABAergic synapses. Activity-induced synaptic modification is therefore not restricted to the activated synapse, but selectively propagates throughout the neural network.

Animals↗
PubMed1997

cAMP-induced switching in turning direction of nerve growth cones.

Development of the nervous system depends on the correct pathfinding and target recognition by the growing tip of an axon, the growth cone. Diffusible or substrate-bound molecules present in the environment may serve as either attractants or repellents to influence the direction of growth-cone extension. Here we report that differences in cyclic-AMP-dependent activity in a neuron may result in opposite turning of the growth cone in response to the same guidance cue. A gradient of brain-derived neurotrophic factor normally triggers an attractive turning response of the growth cone of Xenopus spinal neurons in culture, but the same gradient induces repulsive turning of these growth cones in the presence of a competitive analogue of cAMP or of a specific inhibitor of protein kinase A. This cAMP-dependent switch of the turning response was also found for turning induced by acetylcholine, but not for the turning induced by neurotrophin-3 (NT-3). Thus, in the presence of other factors that modulate neuronal cAMP-dependent activity, the same guidance cue may trigger opposite turning behaviours of the growth cone during its pathfinding in the nervous system.

Animals↗
PubMed1997

Turning of retinal growth cones in a netrin-1 gradient mediated by the netrin receptor DCC.

Netrin-1 promotes outgrowth of axons in vitro through the receptor Deleted in Colorectal Cancer (DCC) and elicits turning of axons within embryonic explants when presented as a point source. It is not known whether netrin-1 alone can elicit turning nor whether DCC mediates the turning response. We show that Xenopus retinal ganglion cell growth cones orient rapidly toward a pipette ejecting netrin-1, an effect blocked by antibodies to DCC. In vitro, netrin-1 induces a complex growth cone morphology reminiscent of that at the optic nerve head, a site of netrin-1 expression in vivo. These results demonstrate that netrin-1 can function alone to induce turning, implicate DCC in this response, and support the idea that netrin-1 contributes to steering axons out of the retina.

Amino Acid Sequence↗
PubMed1997

cAMP-dependent growth cone guidance by netrin-1.

Netrin-1 is known to function as a chemoattractant for several classes of developing axons and as a chemorepellent for other classes of axons, apparently dependent on the receptor type expressed by responsive cells. In culture, growth cones of embryonic Xenopus spinal neurons exhibited chemoattractive turning toward the source of netrin-1 but showed chemorepulsive responses in the presence of a competitive analog of cAMP or an inhibitor of protein kinase A. Both attractive and repulsive responses were abolished by depleting extracellular calcium and by adding a blocking antibody against the netrin-1 receptor Deleted in Colorectal Cancer. Thus, nerve growth cones may respond to the same guidance cue with opposite turning behavior, dependent on other coincident signals that set the level of cytosolic cAMP.

Animals↗
PubMed1997

Potentiation of developing synapses by postsynaptic release of neurotrophin-4.

The hypothesis that synaptic functions can be regulated by neurotrophins secreted from the postsynaptic cell was examined in Xenopus nerve-muscle cultures. Neuromuscular synapses formed on myocytes overexpressing neurotrophin-4 (M+ synapses) exhibited a higher level of spontaneous synaptic activity and enhanced evoked synaptic transmission as compared to those formed on normal control myocytes (M- synapses). The NT-4 effects involve a potentiation of presynaptic transmitter secretion as well as a lengthening of the mean burst duration of postsynaptic low conductance acetylcholine channels. Repetitive stimulation of either the presynaptic neuron or the postsynaptic myocyte led to a potentiation of synaptic transmission at M+ synapses. All potentiation effects of NT-4 overexpression were abolished by the extracellular presence of TrkB-IgG but not by the presence of TrkA-IgG, indicating that postsynaptic secretion of NT-4 was responsible for the synaptic modification.

Animals↗
PubMed1996

Spread of synaptic depression mediated by presynaptic cytoplasmic signaling.

Postsynaptic activity may modulate presynaptic functions by transsynaptic retrograde signals. At developing neuromuscular synapses in Xenopus nerve-muscle cultures, a brief increase in the cytosolic calcium ion (Ca2+) concentration in postsynaptic myocytes induced persistent depression of presynaptic transmitter secretion. This depression spread to distant synapses formed by the same neuron. Clearance of extracellular fluid did not prevent the spread of depression, and depression could not be induced by increasing the Ca2+ concentration in a nearby myocyte not in contact with the presynaptic neuron. Thus, the spread of depression is mediated by signaling in the presynaptic cytoplasm, rather than by a retrograde factor in the extracellular space.

Animals↗
PubMed1996

Synaptic modulation by neurotrophic factors: differential and synergistic effects of brain-derived neurotrophic factor and ciliary neurotrophic factor.

Extracellular application of brain-derived neurotrophic factor (BDNF) and ciliary neurotrophic factor (CNTF) to developing neuromuscular junctions in Xenopus nerve-muscle cultures resulted in an increase in the frequency of spontaneous synaptic currents (SSCs) and in the amplitude of nerve-evoked synaptic currents. Analyses of the amplitude and time course of the SSCs suggest that these effects are attributable to elevation of presynaptic transmitter release. The actions of these two factors on the transmitter secretion process, however, are distinctly different. Fura-2 Ca2+ imaging showed that an increase in presynaptic cytosolic Ca2+ ([Ca2+]i) accompanied the synaptic potentiation by BDNF, whereas no change in [Ca2+]i was observed during synaptic potentiation by CNTF. Removing external Ca2+ also abolished the potentiating effect of BDNF but did not influence the CNTF effect. Moreover, the two factors exerted different effects on the short-term synaptic plasticity. Paired-pulse facilitation normally found at these synapses was reduced by BDNF but unaffected by CNTF; CNTF, but not BDNF, reduced the extent of synaptic depression during high-frequency tetanic stimulation. Finally, the potentiation effect of BDNF and CNTF on spontaneous transmitter release was additive when both factors were applied together to the synapse at saturating concentrations (100 ng/ml) and was highly synergistic when low doses (1 and 10 ng/ml) of both factors were used. These results suggest that because of their differential effects on the secretory machinery, BDNF and CNTF may act cooperatively in modulating the development and functioning of synapses.

Animals↗
PubMed1996

Effects of cytochalasin treatment on short-term synaptic plasticity at developing neuromuscular junctions in frogs.

1. The role of actin microfilaments in synaptic transmission was tested by monitoring spontaneous and evoked transmitter release from developing neuromuscular synapses in Xenopus nerve-muscle cultures, using whole-cell recording of synaptic currents in the absence and presence of microfilament-disrupting agents cytochalasins B and D. 2. Treatment with cytochalasins resulted in disruption of microfilament networks in the growth cone and the presynaptic nerve terminal of spinal neurons in Xenopus nerve-muscle cultures, as revealed by rhodamine-phalloidin staining. 3. The same cytochalasin treatment did not significantly affect the spontaneous or evoked synaptic currents during low-frequency stimulation at 0.05 Hz in these Xenopus cultures. Synaptic depression induced by high-frequency (5 Hz) stimulation, however, was reduced by this treatment. Paired-pulse facilitation for short interpulse intervals was also increased by the treatment. 4. These results indicate that disruption of microfilaments alters short-term changes in transmitter release induced by repetitive activity, without affecting normal synaptic transmission at low frequency. 5. Our results support the notion that actin microfilaments impose a barrier for mobilization of synaptic vesicles from the reserve pool, but do not affect the exocytosis of immediately available synaptic vesicles at the active zone.

Acetylcholine↗
PubMed1996

Essential role of filopodia in chemotropic turning of nerve growth cone induced by a glutamate gradient.

Pathfinding of growing neurites depends on turning of the growth cone in response to extracellular cues. Motile filopodia of the growth cone are known to be critical for mediating contact-dependent guidance of the growth cone. However, whether filopodia also play an essential role in growth cone turning response induced by a diffusible chemotropic substance is unclear. Growth cones of cultured Xenopus spinal neurons exhibited chemotropic turning responses in a gradient of glutamate within a limited range of concentrations. This turning response depends on the activation of the NMDA subtype of glutamate receptors and requires the presence of extracellular Ca2+. Time-lapse differential interference contrast microscopy with quantitative analysis of filopodia dynamics showed a close correlation between an increased number of filopodia on the side of the growth cone facing the glutamate source and the turning. Such filopodia asymmetry was observed within minutes after the onset of the glutamate gradient, before any detectable turning of the growth cone. In Ca(2+)-free medium, no filopodia asymmetry was induced by the glutamate gradient, and no growth cone turning was observed. Furthermore, elimination of filopodia with a low concentration of cytochalasin B completely abolished the turning response without substantially affecting neurite extension. Thus, filopodia may be required for chemotropic guidance of the growth cone, and an asymmetry in filopodia distribution may be an early cellular event responsible for determining the direction the growth cone advances.

Actins↗
PubMed1996

Expression of synapsin I correlates with maturation of the neuromuscular synapse.

Synapsins are a family of neuron-specific phosphoproteins that are localized within the presynaptic terminals in adult brain. Previous work has demonstrated that introduction of exogenous synapsins I(a + b) or IIa into Xenopus spinal neurons promoted maturation of the neuromuscular synapse in a nerve-muscle co-culture system. We have now studied the expression of endogenous Xenopus synapsin I during synaptic maturation in vivo and in culture, using a polyclonal antibody raised against Xenopus synapsin I. Immunoprecipitation experiments indicated that synapsin I was not detectable during the early phase of synaptogenesis in vivo, and exhibited a marked increase during the period of synaptic maturation. In contrast, the expression of synaptophysin, another synaptic vesicle protein, was detected at the start of nervous system formation, and remained at a high level thereafter. Similar expression profiles for the two proteins were also observed in immunocytochemical studies of Xenopus spinal neurons in culture: intense staining of synaptophysin was found on the first day, while synapsin I was not detected until after three days in culture. The expression of synapsin I correlated very well with the appearance of a bell-shaped amplitude distribution of spontaneous synaptic currents, a physiological parameter which reflects functional maturation of the neuromuscular synapse. In one-day-old cultures grown in the absence of laminin, an extracellular matrix protein known to be present at the neuromuscular junction, the amplitude distribution of virtually all synapses was skewed towards smaller values. In contrast, when laminin was used as a culture substrate, many synapses exhibited a bell-shaped amplitude distribution. Laminin treatment also induced synapsin I expression in one-day-old cultures. These results suggest that the expression of endogenous synapsin I may regulate maturation at neuromuscular synapses.

Animals↗