Neuron death in the developing avian isthmo-optic nucleus, and its relation to the establishment of functional circuitry.

The present review covers all the published data on neuron death in the developing avian isthmo-optic nucleus (ION), which provides a particularly convenient situation for studying the causes and consequences of neuron death in the development of the vertebrate central nervous system. The main conclusions are as follows: The naturally occurring neuron death in the ION is related both temporally and causally to the ION's formation of afferent and efferent connections. The ION neurons need to obtain both anterograde and retrograde survival signals in order to survive during a critical period in embryogenesis. They may compete, at least for the retrograde signals, but the nature of the competition is still unclear. The retrograde signals are modified by action potentials. Neurons dying from a lack of anterograde survival signals can be distinguished morphologically from ones dying from a lack of retrograde signals. The neuron death refines circuitry by selectively eliminating neurons with "aberrant" axons projecting to the "wrong" (i.e., ipsilateral) retina or to the "wrong" (topographically inappropriate) part of the contralateral retina.     

Neuron death in the developing avian isthmo–optic nucleus,and its relation to the establishment of functional circuitry

The present review covers all the published data on neuron death in the developing avian isthmo–optic nucleus (ION), which provides a particularly convenient situation for studying the causes and consequences of neuron death in the development of the vertebrate central nervous system. The main conclusions are as follows: The naturally occurring neuron death in the ION is related both temporally and causally to the ION's formation of afferent and efferent connections. The ION neurons need to obtain both anterograde and retrograde survival signals in order to survive during a critical period in embryogenesis. They may compete, at least for the retrograde signals, but the nature of the competition is still unclear. The retrograde signals are modified by action potentials. Neurons dying from a lack of anterograde survival signals can be distinguished morphologically from ones dying from a lack of retrograde signals. The neuron death refines circuitry by selectively eliminating neurons with “aberrant” axons projecting to the “wrong” (i.e., ipsilateral) retina or to the “wrong” (topographically inappropriate) part of the contralateral retina. © 1992 John Wiley & Sons, Inc.     

Snapin recruits dynein to BDNF-TrkB signaling endosomes for retrograde axonal transport and is essential for dendrite growth of cortical neurons

Neurotrophin signaling is crucial for neuron growth. While the "signaling endosomes" hypothesis is one of the accepted models, the molecular machinery that drives retrograde axonal transport of TrkB signaling endosomes is largely unknown. In particular, mechanisms recruiting dynein to TrkB signaling endosomes have not been elucidated. Here, using snapin deficient mice and gene rescue experiments combined with compartmentalized cultures of live cortical neurons, we reveal that Snapin, as a dynein adaptor, mediates retrograde axonal transport of TrkB signaling endosomes. Such a role is essential for dendritic growth of cortical neurons. Deleting snapin or disrupting Snapin-dynein interaction abolishes TrkB retrograde transport, impairs BDNF-induced retrograde signaling from axonal terminals to the nucleus, and decreases dendritic growth. Such defects were rescued by reintroducing the snapin gene. Our study indicates that Snapin-dynein coupling is one of the primary mechanisms driving BDNF-TrkB retrograde transport, thus providing mechanistic insights into the regulation of neuronal growth and survival.     

Two viral kinases are required for sustained long distance axon transport of a neuroinvasive herpesvirus

Axonal transport is essential for the successful establishment of neuroinvasive herpesvirus infections in peripheral ganglia (retrograde transport) and the subsequent spread to exposed body surfaces following reactivation from latency (anterograde transport). We examined two components of pseudorabies virus (US3 and UL13), both of which are protein kinases, as potential regulators of axon transport. Following replication of mutant viruses lacking kinase activity, newly assembled capsids displayed an increase in retrograde motion that prevented efficient delivery of capsids to the distal axon. The aberrant increase in retrograde motion was accompanied by loss of a viral membrane marker from the transported capsids, indicating that the viral kinases allow for efficient anterograde transport by stabilizing membrane–capsid interactions during the long transit from the neuron cell body to the distal axon.     

Chloroplast-to-nucleus communication: Current knowledge,experimental strategies and relationship to drought stress signaling

In order for plant cells to function efficiently under different environmental conditions, chloroplastic processes have to be tightly regulated by the nucleus. It is widely believed that there is inter-organelle communication from the chloroplast to the nucleus, called retrograde signaling. Although some pathways of communication have been identified, the actual signals that move between the two cellular compartments are largely unknown. This review provides an overview of retrograde signaling including its importance to the cell, candidate signals, recent advances and current experimental systems. In addition, we highlight the potential of using drought stress as a model for studying retrograde signaling.Key words: retrograde, chloroplast, signals, drought, stress, high light, abiotic, excess light, photosynthesis     

Rab5 and Rab7 control endocytic sorting along the axonal retrograde transport pathway

Vesicular pathways coupling the neuromuscular junction with the motor neuron soma are essential for neuronal function and survival. To characterize the organelles responsible for this long-distance crosstalk, we developed a purification strategy based on a fragment of tetanus neurotoxin (TeNT H(C)) conjugated to paramagnetic beads. This approach enabled us to identify, among other factors, the small GTPase Rab7 as a functional marker of a specific pool of axonal retrograde carriers, which transport neurotrophins and their receptors. Furthermore, Rab5 is essential for an early step in TeNT H(C) sorting but is absent from axonally transported vesicles. Our data demonstrate that TeNT H(C) uses a retrograde transport pathway shared with p75(NTR), TrkB, and BDNF, which is strictly dependent on the activities of both Rab5 and Rab7. Therefore, Rab7 plays an essential role in axonal retrograde transport by controlling a vesicular compartment implicated in neurotrophin traffic.     

GABA-like immunoreactivity in a common inhibitory neuron of the antennal motor system of crickets

Summary In crickets, a deutocerebral motoneuron sends axon collaterals to 6 of the 7 antennal muscles. Previous results indicated that this neuron exerts inhibition on these muscles and thus may be a common inhibitory motoneuron. In our present study, we show by doublelabelling, i.e. retrograde cobalt-filling and GABA-immunocytochemistry, that this neuron is GABA-immunoreactive, thus demonstrating that one common inhibitory motoneuron is part of the antennal motor system of crickets.     

Learning-related synaptic plasticity: LTP and LTD.

The past several years have seen studies of synaptic plasticity in both invertebrate and vertebrate nervous systems come of age and lead to important new findings. In particular, current evidence points to a possible presynaptic site for long-term potentiation and the involvement of a retrograde messenger from the postsynaptic neuron. Recent advances in both cerebellar and cortical forms of long-term depression are also discussed.     

Motor neuron degeneration after sciatic nerve avulsion in adult rat evolves with oxidative stress and is apoptosis.

The mechanisms for motor neuron degeneration and regeneration in adult spinal cord following axotomy and target deprivation are not fully understood. We used a unilateral sciatic nerve avulsion model in adult rats to test the hypothesis that retrograde degeneration of motor neurons resembles apoptosis. By 21 days postlesion, the number of large motor neurons in lumbar spinal cord was reduced by approximately 30%. The death of motor neurons was confirmed using the terminal transferase-mediated deoxyuridine triphosphate-biotin nick-end labeling method for detecting fragmentation of nuclear DNA. Motor neuron degeneration was characterized by aberrant accumulation of perikaryal phosphorylated neurofilaments. Structurally, motor neuron death was apoptosis. Apoptotic motor neurons undergo chromatolysis followed by progressive cytoplasmic and nuclear condensation with chromatin compaction into uniformly large round clumps. Prior to apoptosis, functionally active mitochondria accumulate within chromatolytic motor neurons, as determined by cytochrome c oxidase activity. These dying motor neurons sustain oxidative damage to proteins and nucleic acids within the first 7 days after injury during the progression of apoptosis, as identified by immunodetection of nitrotyrosine and hydroxyl-modified deoxyguanosine and guanosine. We conclude that the retrograde death of motor neurons in the adult spinal cord after sciatic nerve avulsion is apoptosis. Accumulation of active mitochondria within the perikaryon and oxidative damage to nucleic acids and proteins may contribute to the mechanisms for apoptosis of motor neurons in the adult spinal cord.     

Disruption of dynein/dynactin inhibits axonal transport in motor neurons causing late-onset progressive degeneration

To test the hypothesis that inhibition of axonal transport is sufficient to cause motor neuron degeneration such as that observed in amyotrophic lateral sclerosis (ALS), we engineered a targeted disruption of the dynein-dynactin complex in postnatal motor neurons of transgenic mice. Dynamitin overexpression was found to disassemble dynactin, a required activator of cytoplasmic dynein, resulting in an inhibition of retrograde axonal transport. Mice overexpressing dynamitin demonstrate a late-onset progressive motor neuron degenerative disease characterized by decreased strength and endurance, motor neuron degeneration and loss, and denervation of muscle. Previous transgenic mouse models of ALS have shown abnormalities in microtubule-based axonal transport. In this report, we describe a mouse model that confirms the critical role of disrupted axonal transport in the pathogenesis of motor neuron degenerative disease.     

Developmentally regulated tissue-associated cues influence axon sprouting and outgrowth and may contribute to target specificity.

The heart circuitry of the medicinal leech (Hirudo medicinalis) is a highly stereotyped circuit in the adult, but selection of the heart tube (HT) as a definitive target by heart excitor (HE) motor neurons during embryogenesis involves redirection of axonal arbors. In the present study we have confirmed the specificity of mature innervation using a retrograde marker and have used a combination of tissue/organ coculture and in situ manipulations to test the ability of HT and body wall to support axon outgrowth compared to CNS associated tissue. We also examined the temporal limits of target influence and the specificity of its action. Embryonic and young juvenile HT and body wall, but not adult HT, support or stimulate marked axon outgrowth from CNS ganglia, including those that would not innervate these tissues in vivo. Outgrowth support/stimulation by young tissue is largely contact based with little or no overt selectivity. Thus, outgrowth-supporting cues are developmentally regulated in the periphery, decreasing in efficacy with age while adult CNS-derived tissues consistently provide effective substrates supporting extensive axon outgrowth and regrowth. The HE motor neuron was very discriminating in that it showed little axon extension onto the HT compared to that of other neurons generally. These studies support a role for bidirectional communication in target selection. We suggest a working hypothesis that the HE motor neuron may initially select HT in response to a hierarchy of outgrowth supporting cues that have very broad influence and subsequently responds to selective signals for slowing or stopping growth and terminating on the functionally appropriate target.     

Mycalolide B dissociates dynactin and abolishes retrograde axonal transport of dense-core vesicles

Axonal transport is critical for maintaining synaptic transmission. Of interest, anterograde and retrograde axonal transport appear to be interdependent, as perturbing one directional motor often impairs movement in the opposite direction. Here live imaging of Drosophila and hippocampal neuron dense-core vesicles (DCVs) containing a neuropeptide or brain-derived neurotrophic factor shows that the F-actin depolymerizing macrolide toxin mycalolide B (MB) rapidly and selectively abolishes retrograde, but not anterograde, transport in the axon and the nerve terminal. Latrunculin A does not mimic MB, demonstrating that F-actin depolymerization is not responsible for unidirectional transport inhibition. Given that dynactin initiates retrograde transport and that amino acid sequences implicated in macrolide toxin binding are found in the dynactin component actin-related protein 1, we examined dynactin integrity. Remarkably, cell extract and purified protein experiments show that MB induces disassembly of the dynactin complex. Thus imaging selective retrograde transport inhibition led to the discovery of a small-molecule dynactin disruptor. The rapid unidirectional inhibition by MB suggests that dynactin is absolutely required for retrograde DCV transport but does not directly facilitate ongoing anterograde DCV transport in the axon or nerve terminal. More generally, MB''s effects bolster the conclusion that anterograde and retrograde axonal transport are not necessarily interdependent.     

Microtubule nucleation and organization in dendrites

Dendrite branching is an essential process for building complex nervous systems. It determines the number, distribution and integration of inputs into a neuron, and is regulated to create the diverse dendrite arbor branching patterns characteristic of different neuron types. The microtubule cytoskeleton is critical to provide structure and exert force during dendrite branching. It also supports the functional requirements of dendrites, reflected by differential microtubule architectural organization between neuron types, illustrated here for sensory neurons. Both anterograde and retrograde microtubule polymerization occur within growing dendrites, and recent studies indicate that branching is enhanced by anterograde microtubule polymerization events in nascent branches. The polarities of microtubule polymerization events are regulated by the position and orientation of microtubule nucleation events in the dendrite arbor. Golgi outposts are a primary microtubule nucleation center in dendrites and share common nucleation machinery with the centrosome. In addition, pre-existing dendrite microtubules may act as nucleation sites. We discuss how balancing the activities of distinct nucleation machineries within the growing dendrite can alter microtubule polymerization polarity and dendrite branching, and how regulating this balance can generate neuron type-specific morphologies.     

Nitric oxide acts as a retrograde messenger during long-term potentiation in cultured hippocampal neurons

We examined long-term potentiation (LTP) at synapses between hippocampal neurons in dissociated cell culture following presynaptic, postsynaptic, or extracellular application of a nitric oxide (NO) scavenger, an inhibitor of NO synthase, and a membrane-impermeant NO donor that releases NO only upon photolysis with UV light. Our results indicate that NO is produced in the postsynaptic neuron, travels through the extracellular space, and acts directly in the presynaptic neuron to produce long-term potentiation, supporting the hypothesis that NO acts a retrograde messenger during LTP.     

Velocity measurements of particulate neuroplasmic flow in organized mammalian CNS tissue cultures.

The movements of spherical particles in the range of 0.3 to 0.8 mum diameter within neurities of cultured embryonic mouse spinal cord fragments were observed and recorded by means of Nomarski optics and time-lapse photocinematography at high power. Particulate movements were measured by projecting the motion pictures onto a calibrated screen and recording the distances moved with time of linearly moving particles and making note of the direction (toward or away from the neuron soma) of movement. In all, 128 particles were measured in six cultures. These measurements were taken away from the neuron soma near the periphery of the neurites. Eight-three particles were noted to be moving toward the neuron at a mean velocity of 1.03 +/- 0.38 (SD) mum/sec while 45 anterograde moving particles were noted to move at 1.07 +/- 0.62 (SD) mum/sec. Statistical analysis of these veolcities revealed no significant difference between them. Particles which were elongated and probably represented mitochondria moved more sluggishly and could not be measured accurately by the techniques employed. It appeared the spherical particles moving in a retrograde direction originated at the neurite tip apparently by pinocytosis. There was a suggestion that anterograde flow and retrograde flow may have been affected unequally by factors which develop in the observation chamber over a period of 2 hr or more. The most likely factor responsible was probably hypoxia.     

Velocity measurements of particulate neuroplasmic flow in organized mammalian CNS tissue cultures

The movements of spherical particles in the range of 0.3 to 0.8 μm diameter within neurities of cultured embryonic mouse spinal cord fragments were observed and recorded by means of Nomarski optics and time-lapse photocinematography at high power. Particulate movements were measured by projecting the motion pictures onto a calibrated screen and recording the distances moved with time of linearly moving particles and making note of the direction (toward or away from the neuron soma) of movement. In all, 128 particles were measured in six cultures. These measurements were taken away from the neuron soma near the periphery of the neurites. Eighty-three particles were noted to be moving toward the neuron at a mean velocity of 1.03 ± 0.38 (SD) μm/sec while 45 anterograde moving particles were noted to move at 1.07 ± 0.62 (SD) μm/sec. Statistical analysis of these velocities revealed no significant difference between them. Particles which were elongated and probably represented mitochondria moved more sluggishly and could not be measured accurately by the techniques employed. It appeared the spherical particles moving in a retrograde direction originated at the neurite tip apparently by pinocytosis. There was a suggestion that anterograde flow and retrograde flow may have been affected unequally by factors which develop in the observation chamber over a period of 2 hr or more. The most likely factor responsible was probably hypoxia.