The survival of transected axonal segments of cultured Aplysia neurons is prolonged by contact with intact nerve cells

Transcription factor Pebbled/RREB1 regulates injury-induced axon degeneration

Genetic studies of Wallerian degeneration have led to the identification of signaling molecules (e.g., dSarm/Sarm1, Axundead, and Highwire) that function locally in axons to drive degeneration. Here we identify a role for the Drosophila C2H2 zinc finger transcription factor Pebbled [Peb, Ras-responsive element binding protein 1 (RREB1) in mammals] in axon death. Loss of Peb in Drosophila glutamatergic sensory neurons results in either complete preservation of severed axons, or an axon death phenotype where axons fragment into large, continuous segments, rather than completely disintegrate. Peb is expressed in developing and mature sensory neurons, suggesting it is required to establish or maintain their competence to undergo axon death. peb mutant phenotypes can be rescued by human RREB1, and they exhibit dominant genetic interactions with dsarm mutants, linking peb/RREB1 to the axon death signaling cascade. Surprisingly, Peb is only able to fully block axon death signaling in glutamatergic, but not cholinergic sensory neurons, arguing for genetic diversity in axon death signaling programs in different neuronal subtypes. Our findings identify a transcription factor that regulates axon death signaling, and peb mutant phenotypes of partial fragmentation reveal a genetically accessible step in axon death signaling.

On-chip electroporation, membrane repair dynamics and transient in-cell recordings by arrays of gold mushroom-shaped microelectrodes

This study demonstrates the use of on-chip gold mushroom-shaped microelectrodes (gMμEs) to generate localized electropores in the plasma membrane of adhering cultured neurons and to electrophysiologically monitor the ensuing membrane repair dynamics. Delivery of an alternating voltage pulse (0.5-1 V, 100 Hz, 300 ms) through an extracellularly positioned micrometer-sized gMμE electroporates the patch of plasma membrane facing the microelectrode. The repair dynamics of the electropores were analyzed by continuous monitoring of the neuron transmembrane potential, input resistance (R(in)) and action potential (AP) amplitude with an intracellular microelectrode and a number of neighbouring extracellular gMμEs. Electroporation by a gMμE is associated with local elevation of the free intracellular calcium concentration ([Ca(2+)](i)) around the gMμE. The membrane repair kinetics proceeds as an exponential process interrupted by abrupt recovery steps. These abrupt events are consistent with the "membrane patch model" of membrane repair in which patches of intracellular membrane fuse with the plasma membrane at the site of injury. Membrane electroporation by a single gMμE generates a neuron-gMμE configuration that permits recordings of attenuated intracellular action potentials. We conclude that the use of on-chip cultured neurons via a gMμE configuration provides a unique neuroelectronic interface that enables the selection of individual cells for electroporation, generates a confined electroporated membrane patch, monitors membrane repair dynamics and records attenuated intracellular action potentials.

Wallerian degeneration, wld(s), and nmnat

Traditionally, researchers have believed that axons are highly dependent on their cell bodies for long-term survival. However, recent studies point to the existence of axon-autonomous mechanism(s) that regulate rapid axon degeneration after axotomy. Here, we review the cellular and molecular events that underlie this process, termed Wallerian degeneration. We describe the biphasic nature of axon degeneration after axotomy and our current understanding of how Wld(S)--an extraordinary protein formed by fusing a Ube4b sequence to Nmnat1--acts to protect severed axons. Interestingly, the neuroprotective effects of Wld(S) span all species tested, which suggests that there is an ancient, Wld(S)-sensitive axon destruction program. Recent studies with Wld(S) also reveal that Wallerian degeneration is genetically related to several dying back axonopathies, thus arguing that Wallerian degeneration can serve as a useful model to understand, and potentially treat, axon degeneration in diverse traumatic or disease contexts.

Long-term, multisite, parallel, in-cell recording and stimulation by an array of extracellular microelectrodes

Here we report on the development of a novel neuroelectronic interface consisting of an array of noninvasive gold-mushroom-shaped microelectrodes (gMmicroEs) that practically provide intracellular recordings and stimulation of many individual neurons, while the electrodes maintain an extracellular position. The development of this interface allows simultaneous, multisite, long-term recordings of action potentials and subthreshold potentials with matching quality and signal-to-noise ratio of conventional intracellular sharp glass microelectrodes or patch electrodes. We refer to the novel approach as "in-cell recording and stimulation by extracellular electrodes" to differentiate it from the classical intracellular recording and stimulation methods. This novel technique is expected to revolutionize the analysis of neuronal networks in relations to learning, information storage and can be used to develop novel drugs as well as high fidelity neural prosthetics and brain-machine systems.

Textural guidance cues for controlling process outgrowth of mammalian neurons

We explore textural cues as a mechanism for controlling neuronal process outgrowth in primary cultures of mammalian neurons. The work uses a form of decal transfer lithography to generate arrays of PDMS posts of various dimensions and spacings on glass substrates that are rendered growth-compliant by subsequent treatment with a protein activator. Hippocampal neurons plated on these substrates are used to determine how the posts direct process growth by acting as attachment points or guidance cues. Textural features varying over a large range, even as large as 100 microm in diameter, dramatically affect process growth. Indeed, two growth regimes are observed; at the smaller feature sizes considered, process branching strongly aligns (at right angles) along the post mesh, while neuronal outgrowth on the larger feature sizes elicits process wrapping. The latter behavior most strongly manifests in neurons plated initially at approximately 100 cells/mm(2), where the cells were able to form networks, while for isolated neurons, the cells exhibit poorer viability and development. Bag cell neurons from Aplysia californica also display regular growth patterns, but in this case are guided by contact avoidance of the posts, a behavior qualitatively different than that of the hippocampal neurons.

Formation of microtubule-based traps controls the sorting and concentration of vesicles to restricted sites of regenerating neurons after axotomy

Transformation of a transected axonal tip into a growth cone (GC) is a critical step in the cascade leading to neuronal regeneration. Critical to the regrowth is the supply and concentration of vesicles at restricted sites along the cut axon. The mechanisms underlying these processes are largely unknown. Using online confocal imaging of transected, cultured Aplysia californica neurons, we report that axotomy leads to reorientation of the microtubule (MT) polarities and formation of two distinct MT-based vesicle traps at the cut axonal end. Approximately 100 microm proximal to the cut end, a selective trap for anterogradely transported vesicles is formed, which is the plus end trap. Distally, a minus end trap is formed that exclusively captures retrogradely transported vesicles. The concentration of anterogradely transported vesicles in the former trap optimizes the formation of a GC after axotomy.

Experimental and theoretical analysis of neuron–transistor hybrid electrical coupling: The relationships between the electro-anatomy of cultured Aplysia neurons and the recorded field potentials

Understanding the mechanisms that generate field potentials (FPs) by neurons grown on semiconductor chips is essential for implementing neuro-electronic devices. Earlier studies emphasized that FPs are generated by current flow between differentially expressed ion channels on the membranes facing the chip surface, and those facing the culture medium in electrically compact cells. Less is known, however, about the mechanisms that generate FPs by action potentials (APs) that propagate along typical non-isopotential neurons. Using Aplysia neurons cultured on floating gate-transistors, we found that the FPs generated by APs in cultured neurons are produced by current flow along neuronal compartments comprising the axon, cell body, and neurites, rather than by flow between the membrane facing the chip substrate and that facing the culture medium. We demonstrate that the FPs waveform generated by non-isopotential neurons largely depends on the morphology of the neuron.

Ultrastructural Study of Normal and Degenerating Nerve Fibers in the Protocerebral Tract of the Crab Ucides cordatus

Wallerian degeneration is a very well described phenomenon in the vertebrate nervous system. In arthropods, and especially in crustaceans, nerve fiber degeneration has not been described extensively. In addition, literature shows that the events do not follow the same patterns as in vertebrates. In this study we report, by qualitative and quantitative ultrastructural analyses, the features and time course of the protocerebral tract degeneration following extirpation of the optic stalk. No remarkable changes were observed seven days after lesion. After 28 days the protocerebral tracts presented apparently preserved small and large diameter axons and some degenerating medium axons, with irregular contours and empty-looking aspect of the axoplasm. Forty days after the ablation of the optic stalks, both small (type I) and medium (type II and III) axons revealed signs of partial or total degeneration, but large nerve fibers (type IV) were still intact. After 45 days, the tract showed signs of advanced stage of degeneration and, apart from large axons, normal-looking fibers were almost absent. At these 3 last time points, degenerating axons displayed different electron densities and aspects, probably correlating to different onset times of the process. In addition, cells with granules in their cytoplasm, possibly hemocytes, were quite distinct, especially at 40 and 45 days after axotomy. These cells might share with glial cells the function of phagocytosis of cellular debris during the protocerebral tract degeneration. Quantitative analysis showed that the number of degenerating fibers increased significantly from 28 to 40 days after lesion, whereas the number of normal fibers decreased accordingly. Measurements of cross-sectional areas of normal and degenerating axons showed that types II and III (medium) start to degenerate before type I (small). Type IV (large) axons do not degenerate, even after 40 days. Therefore, we can conclude that degeneration in these afferent fibers starts late after axotomy, but proceeds at a faster rate afterwards until the complete degeneration of small and medium axons.

Synapse formation in the absence of cell bodies requires protein synthesis

Protein synthesis at distal synaptic sites is thought to play a critical role in long-term synaptic plasticity at preexisting connections. We tested whether protein synthesis in distal neuritic processes contributes to the formation of new synaptic connections by Aplysia neurons regenerating in cell culture after removing their cell bodies. Removal of either the sensory neuron (SN) or motor cell L7 cell body did not affect the formation of synaptic connections during the next 48--72 hr period. Increases in synaptic efficacy after removal of the SN cell body was accompanied by neurite growth and an increase in the number of SN varicosities contacting L7. The increases in synaptic efficacy and the number of SN varicosities were blocked by anisomycin, a protein synthesis inhibitor. The initial formation of synaptic connections was not affected by the absence of the L7 cell body. In the absence of cell bodies from both presynaptic and postsynaptic cells, synaptic efficacy increased for 48 hr and was blocked reversibly by anisomycin. These results support the idea that distal neuritic processes contain stable mRNAs and the macromolecular machinery for protein synthesis that are required for the formation of new synaptic connections.

c-Jun mediates axotomy-induced dopamine neuron death in vivo

Expression of the transcription factor c-Jun is induced in neurons of the central nervous system (CNS) in response to injury. Mechanical transection of the nigrostriatal pathway at the medial forebrain bundle (MFB) results in the delayed retrograde degeneration of the dopamine neurons in the substantia nigra pars compacta (SNc) and induces protracted expression and phosphorylation of c-Jun. However, the role of c-Jun after axotomy of CNS neurons is unclear. Here, we show that adenovirus-mediated expression of a dominant negative form of c-Jun (Ad.c-JunDN) inhibited axotomy-induced dopamine neuron death and attenuated phosphorylation of c-Jun in nigral neurons. Ad.c-JunDN also delayed the degeneration of dopaminergic nigral axons in the striatum after MFB axotomy. Taken together, these findings suggest that activation of c-Jun mediates the loss of dopamine neurons after axotomy injury.

Protein synthesis in axons and terminals: significance for maintenance, plasticity and regulation of phenotype. With a critique of slow transport theory

This article focuses on local protein synthesis as a basis for maintaining axoplasmic mass, and expression of plasticity in axons and terminals. Recent evidence of discrete ribosomal domains, subjacent to the axolemma, which are distributed at intermittent intervals along axons, are described. Studies of locally synthesized proteins, and proteins encoded by RNA transcripts in axons indicate that the latter comprise constituents of the so-called slow transport rate groups. A comprehensive review and analysis of published data on synaptosomes and identified presynaptic terminals warrants the conclusion that a cytoribosomal machinery is present, and that protein synthesis could play a role in long-term changes of modifiable synapses. The concept that all axonal proteins are supplied by slow transport after synthesis in the perikaryon is challenged because the underlying assumptions of the model are discordant with known metabolic principles. The flawed slow transport model is supplanted by a metabolic model that is supported by evidence of local synthesis and turnover of proteins in axons. A comparison of the relative strengths of the two models shows that, unlike the local synthesis model, the slow transport model fails as a credible theoretical construct to account for axons and terminals as we know them. Evidence for a dynamic anatomy of axons is presented. It is proposed that a distributed "sprouting program," which governs local plasticity of axons, is regulated by environmental cues, and ultimately depends on local synthesis. In this respect, nerve regeneration is treated as a special case of the sprouting program. The term merotrophism is proposed to denote a class of phenomena, in which regional phenotype changes are regulated locally without specific involvement of the neuronal nucleus.

Temperature dependence of membrane sealing following transection in mammalian spinal cord axons

Using an in vitro sucrose-gap recording chamber, sealing of cut axons in isolated strips of white matter from guinea pig spinal cord was measured by recording the "compound membrane potential". This functional sealing was found to correlate well with anatomical resealing, measured by a horseradish peroxidase uptake assay. Near-complete functional and anatomical recovery of the axonal membrane occurred routinely within 60 min following transection at 37 degrees C in regular Krebs' solution. The rate of membrane potential recovery is exponential, with a time-constant of 20+/-5 min. The sealing process at 31 degrees C was similar to that at 37 degrees C, and was effectively blocked at 25 degrees C, under which condition most axons continued to take up horseradish peroxidase for more than 1h, and failed to substantially recover their membrane potential. Seventy-five percent of the cords transected at 40 degrees C had similar sealing behavior to those at 37 degrees C and 31 degrees C. The balance failed to seal the cut end. Two-dimensional morphometric analysis has shown that raising the temperature from 25 degrees C to above 31 degrees C significantly decreases axonal permeabilization to horseradish peroxidase (increases the sealing of transected ends) across all areas of a transverse section of spinal cord. Moreover, this enhancement of sealing exists across all axon calibers. Since severe cooling compromises membrane resealing, caution needs to be taken when hypothermic treatment (below 25 degrees C) is applied within the first 60 min following mechanical injury. In summary, we have found that at normal temperature (37 degrees C), nerve fibers repair their damaged membrane following physical injury with an hour. This is similar at mildly lower (31 degrees C) and relatively higher (40 degrees C) temperature, although some fibers tend to collapse under this febrile temperature. Moreover, severely low temperature (25 degrees C) hindered the repair of damaged membranes. Based on our study, caution is needed in treating spinal cord injury with low temperatures.

Real time imaging of calcium-induced localized proteolytic activity after axotomy and its relation to growth cone formation

The emergence of a neuronal growth cone from a transected axon is a necessary step in the sequence of events that leads to successful regeneration. Yet, the molecular mechanisms underlying its formation after axotomy are unknown. In this study, we show by real time imaging of the free intracellular Ca2+ concentration, of proteolytic activity, and of growth cone formation that the activation of localized and transient Ca2+-dependent proteolysis is a necessary step in the cascade of events that leads to growth cone formation. Inhibition of this proteolytic activity by calpeptin, a calpain inhibitor, abolishes growth cone formation. We suggest that calpain plays a central role in the reorganization of the axon's cytoskeleton during its transition from a stable differentiated structure into a dynamically extending growth cone.

Localized and transient elevations of intracellular Ca2+ induce the dedifferentiation of axonal segments into growth cones

The formation of a growth cone at the tip of a severed axon is a key step in its successful regeneration. This process involves major structural and functional alterations in the formerly differentiated axonal segment. Here we examined the hypothesis that the large, localized, and transient elevation in the free intracellular calcium concentration ([Ca2+]i) that follows axotomy provides a signal sufficient to trigger the dedifferentiation of the axonal segment into a growth cone. Ratiometric fluorescence microscopy and electron microscopy were used to study the relations among spatiotemporal changes in [Ca2+]i, growth cone formation, and ultrastructural alterations in axotomized and intact Aplysia californica neurons in culture. We report that, in neurons primed to grow, a growth cone forms within 10 min of axotomy near the tip of the transected axon. The nascent growth cone extends initially from a region in which peak intracellular Ca2+ concentrations of 300-500 microM are recorded after axotomy. Similar [Ca2+]i transients, produced in intact axons by focal applications of ionomycin, induce the formation of ectopic growth cones and subsequent neuritogenesis. Electron microscopy analysis reveals that the ultrastructural alterations associated with axotomy and ionomycin-induced growth cone formation are practically identical. In both cases, growth cones extend from regions in which sharp transitions are observed between axoplasm with major ultrastructural alterations and axoplasm in which the ultrastructure is unaltered. These findings suggest that transient elevations of [Ca2+]i to 300-500 microM, such as those caused by mechanical injury, may be sufficient to induce the transformation of differentiated axonal segments into growth cones.

Use of Aplysia neurons for the study of cellular alterations and the resealing of transected axons in vitro

The present report describes the experimental advantages offered by the combined use of Aplysia neurons and contemporary techniques to analyze the cellular events associated with nerve injury in the form of axotomy. The experiments were performed by transecting, under visual control, the main axon of identified Aplysia neurons in primary culture while monitoring several related parameters. We found that in cultured Aplysia neurons axotomy leads to the elevation of the [Ca2+]i in both the proximal and distal axonal segments from a resting level of 100 nM up to the millimolar range for a duration of 3-5 min. This increase in [Ca2+]i led to identical alterations in the cytoarchitecture of the proximal and distal segments. The formation of a membrane seal over the transected ends by their constriction and the subsequent fusion of the membrane is a [Ca2+]i-dependent process and is triggered by the elevation of [Ca2+]i to the microM level. Seal formation was followed by down-regulation of the [Ca2+]i to control levels. Following the formation of the membrane seal an increase in membrane retrieval was observed. We hypothesize that the retrieved membrane serves as an immediately available membrane reservoir for growth cone extension.

Acceleration of membrane recycling by axotomy of cultured aplysia neurons

The rapid transition of a stationary axon into a motile growth cone requires the recruitment of membrane and its strategic insertion into the neurolemma. The source of membrane to support the initial rapid growth postaxotomy is not known. Using membrane capacitance measurements, we examined quantitative aspects of membrane dynamics following axotomy of cultured Aplysia neurons. Axotomy activates two processes in parallel: membrane retrieval and exocytosis. Unexpectedly, membrane retrieval is the dominant process in the majority of the experiments. Thus, while a growth cone is vigorously extending, the total neuronal surface area decreases. We suggest that the initial rapid extension phase of the newly formed growth cone postaxotomy is supported by a pool of intracellular membrane that is rapidly retrieved from the neurolemma.

The rate of Wallerian degeneration in cultured neurons from wild type and C57BL/WldS mice depends on time in culture and may be extended in the presence of elevated K+ levels

Wallerian degeneration of severed axons is delayed in C57BL/WldS mice. We have examined this further in cultured sympathetic, sensory and CNS neurons using superior cervical ganglion (SCG), dorsal root ganglion (DRG) and cerebellar granule neurons respectively from neonatal mice. We found that the time taken for the neurites to degenerate depends upon the length of time in culture before cutting, reaching a maximum by approximately 7 days when C57BL/WldS neurites survive for > 6 days after axotomy. The onset of degeneration could also be extended in SCG and DRG neurites from wild type C57BL/6J mice. After 7 days in culture these neurites normally degenerate within approximately 12-16 h of axotomy, but in the presence of raised K+ (50 mM) degeneration often did not begin until a further 2 days had lapsed. Under similar conditions degeneration of neurites from C57BL/WldS mice was also found to be further delayed, extending survival from approximately 5-6 days to > 7 days. The L-type Ca2+ channel blockers nifedipine (5 microM) and verapamil (10 microM) both blocked the effect of raised [K+], although not completely. Thapsigargin, which raises cytoplasmic [Ca2+], and the cAMP analogue 8-(4-chlorophenyl-thio)cAMP were also able to delay degeneration, but only when added 24 h prior to axotomy. These results show that it is possible to influence the course of Wallerian degeneration and that increases in levels of cytoplasmic Ca2+ can protect neurites from its onset.