Repair of traumatic lesions to the plasmalemma of neurons and other cells: Commonalities, conflicts, and controversies

Neuroscientists and Cell Biologists have known for many decades that eukaryotic cells, including neurons, are surrounded by a plasmalemma/axolemma consisting of a phospholipid bilayer that regulates trans-membrane diffusion of ions (including calcium) and other substances. Cells often incur plasmalemmal damage via traumatic injury and various diseases. If the damaged plasmalemma is not rapidly repaired within minutes, activation of apoptotic pathways by calcium influx often results in cell death. We review publications reporting what is less-well known (and not yet covered in neuroscience or cell biology textbooks): that calcium influx at the lesion sites ranging from small nm-sized holes to complete axonal transection activates parallel biochemical pathways that induce vesicles/membrane-bound structures to migrate and interact to restore original barrier properties and eventual reestablishment of the plasmalemma. We assess the reliability of, and problems with, various measures (e.g., membrane voltage, input resistance, current flow, tracer dyes, confocal microscopy, transmission and scanning electron microscopy) used individually and in combination to assess plasmalemmal sealing in various cell types (e.g., invertebrate giant axons, oocytes, hippocampal and other mammalian neurons). We identify controversies such as plug versus patch hypotheses that attempt to account for currently available data on the subcellular mechanisms of plasmalemmal repair/sealing. We describe current research gaps and potential future developments, such as much more extensive correlations of biochemical/biophysical measures with sub-cellular micromorphology. We compare and contrast naturally occurring sealing with recently-discovered artificially-induced plasmalemmal sealing by polyethylene glycol (PEG) that bypasses all natural pathways for membrane repair. We assess other recent developments such as adaptive membrane responses in neighboring cells following injury to an adjacent cell. Finally, we speculate how a better understanding of the mechanisms involved in natural and artificial plasmalemmal sealing is needed to develop better clinical treatments for muscular dystrophies, stroke and other ischemic conditions, and various cancers.

Typical and atypical properties of peripheral nerve allografts enable novel strategies to repair segmental-loss injuries

We review data showing that peripheral nerve injuries (PNIs) that involve the loss of a nerve segment are the most common type of traumatic injury to nervous systems. Segmental-loss PNIs have a poor prognosis compared to other injuries, especially when one or more mixed motor/sensory nerves are involved and are typically the major source of disability associated with extremities that have sustained other injuries. Relatively little progress has been made, since the treatment of segmental loss PNIs with cable autografts that are currently the gold standard for repair has slow and incomplete (often non-existent) functional recovery. Viable peripheral nerve allografts (PNAs) to repair segmental-loss PNIs have not been experimentally or clinically useful due to their immunological rejection, Wallerian degeneration (WD) of anucleate donor graft and distal host axons, and slow regeneration of host axons, leading to delayed re-innervation and producing atrophy or degeneration of distal target tissues. However, two significant advances have recently been made using viable PNAs to repair segmental-loss PNIs: (1) hydrogel release of Treg cells that reduce the immunological response and (2) PEG-fusion of donor PNAs that reduce the immune response, reduce and/or suppress much WD, immediately restore axonal conduction across the donor graft and re-innervate many target tissues, and restore much voluntary behavioral functions within weeks, sometimes to levels approaching that of uninjured nerves. We review the rather sparse cellular/biochemical data for rejection of conventional PNAs and their acceptance following Treg hydrogel and PEG-fusion of PNAs, as well as cellular and systemic data for their acceptance and remarkable behavioral recovery in the absence of tissue matching or immune suppression. We also review typical and atypical characteristics of PNAs compared with other types of tissue or organ allografts, problems and potential solutions for PNA use and storage, clinical implications and commercial availability of PNAs, and future possibilities for PNAs to repair segmental-loss PNIs.

Effect of decorin protein administration on rat sciatic nerve injury: an experimental study

INTRODUCTION

Peripheral nerve traumas are common injuries in young adult population. The myriad of techniques and medications have been defined to obtain better recovery but none of them was proved to have superior effect. This study aims to determine the anti-fibrotic effect of the decorin on sciatic nerve injury in order to enhance functional outcome.

MATERIALS AND METHODS

24 12-week-old male Sprague-Dawley rats (350-400 gr) were divided into four groups. The sciatic nerve was dissected and exposed; a full-thickness laceration was created 1.5 cm proximal to the bifurcation point and 1.5 cm distal to where it originated from the lumbosacral plexus. Motor and sensory tests were conducted before and after the operations for evaluating the nerve healing.

RESULTS

There was a statistically significant difference between DCN bolus and PBS bolus group. (p<0.0001, p&lt;0.05) in neuromotor tests. Increase of the latency was significantly lower in DCN bolus and infusion group when compared with the PBS bolus group. (p&lt;0,001). All operated gastrocnemius muscles were atrophic compared with the contralateral side. The differences between the averages in the sciatic functional index, the improvement of the DCN infusion group was 8.6 units better than the PBS group and 4.4 units better than the DCN bolus group. When the amount of stimulation was 10 mV at the proximal segment in electromyography, there was no significant difference between the DCN bolus and sham groups. (p&gt; 0.05, p = 0.6623).

CONCLUSION

Decorin protein reduces the fibrosis and enhances the motor and sensory recovery both clinically and histologically. Despite the high cost, short half-life and production issues, this protein could be administered after the microsurgical repair but more studies are required to overcome the limitations.

Natural mechanisms and artificial PEG-induced mechanism that repair traumatic damage to the plasmalemma in eukaryotes

Eukaryotic tissues are composed of individual cells surrounded by a plasmalemma that consists of a phospholipid bilayer with hydrophobic heads that bind cell water. Bound-water creates a thermodynamic barrier that impedes the fusion of a plasmalemma with other membrane-bound intracellular structures or with the plasmalemma of adjacent cells. Plasmalemmal damage consisting of small or large holes or complete transections of a cell or axon results in calcium influx at the lesion site. Calcium activates fusogenic pathways that have been phylogenetically conserved and that lower thermodynamic barriers for fusion of membrane-bound structures. Calcium influx also activates phylogenetically conserved sealing mechanisms that mobilize the gradual accumulation and fusion of vesicles/membrane-bound structures that seal the damaged membrane. These naturally occurring sealing mechanisms for different cells vary based on the type of lesion, the type of cell, the proximity of intracellular membranous structures to the lesion and the relation to adjacent cells. The reliability of different measures to assess plasmalemmal sealing need be carefully considered for each cell type. Polyethylene glycol (PEG) bypasses calcium and naturally occurring fusogenic pathways to artificially fuse adjacent cells (PEG-fusion) or artificially seal transected axons (PEG-sealing). PEG-fusion techniques can also be used to rapidly rejoin the closely apposed, open ends of severed axons. PEG-fused axons do not (Wallerian) degenerate and PEG-fused nerve allografts are not immune-rejected, and enable behavioral recoveries not observed for any other clinical treatment. A better understanding of natural and artificial mechanisms that induce membrane fusion should provide better clinical treatment for many disorders involving plasmalemmal damage.

Immediate Enhancement of Nerve Function Using a Novel Axonal Fusion Device After Neurotmesis

Background

The management of peripheral nerve injuries remains a large challenge for plastic surgeons. With the inability to fuse axonal endings, results after microsurgical nerve repair have been inconsistent. Our current nerve repair strategies rely upon the slow and lengthy process of axonal regeneration (~1 mm/d). Polyethylene glycol (PEG) has been investigated as a potential axonal fusion agent; however, the percentage of axonal fusion has been inconsistent. The purpose of this study was to identify a PEG delivery device to standardize outcomes after attempted axonal fusion with PEG.

Materials and Methods

We used a rat sciatic nerve injury model in which we completely transected and repaired the left sciatic nerve to evaluate the efficacy of PEG fusion over a span of 12 weeks. In addition, we evaluated the effectiveness of a delivery device's ability to optimize results after PEG fusion.

Results

We found that PEG rapidly (within minutes) restores axonal continuity as assessed by electrophysiology, fluorescent retrograde tracer, and diffusion tensor imaging. Immunohistochemical analysis shows that motor axon counts are significantly increased at 1 week, 4 weeks, and 12 weeks postoperatively in PEG-treated animals. Furthermore, PEG restored behavioral functions up to 50% compared with animals that received the criterion standard epineurial repair (control animals).

Conclusions

The ability of PEG to rapidly restore nerve function after neurotmesis could have vast implications on the clinical management of traumatic injuries to peripheral nerves.

Polyethylene glycol fusion associated with antioxidants: A new promise in the treatment of traumatic facial paralysis

BACKGROUND

Recent studies in invertebrates have taught us that early cell membrane regeneration is determinant for axonal recovery and survival after trauma. Many authors obtained extraordinary results in neural regeneration using polyethylene glycol fusion protocols, which also involved microsutures and antioxidants.

METHODS

Sixty rats were evaluated with functional and histological protocol after facial nerve neurotmesis. Groups A and B had their stumps coapted with microsuture after 24 hours of neurotmesis and groups C and D after 72 hours. In addition to the microstructure, groups B and D used the polyethylene glycol-fusion protocol for the modulation of the Ca⁺² .

RESULTS

At the sixth week, the latency of group D and duration of group B was lower than groups A and C (P = .011). The axonal diameter of the groups that used polyethylene glycol-fusion was higher than those who did not use polyethylene glycol-fusion (P ≤ .001).

CONCLUSION

Although not providing a functional improvement, polyethylene glycol-fusion slowed down demyelination.

Microarray and qPCR Analyses of Wallerian Degeneration in Rat Sciatic Nerves

Wallerian degeneration occurs immediately following injury to mammal peripheral nerves. To better understand the molecular events occurring during Wallerian degeneration, a rat model of sciatic nerve transection was used to assess differentially expressed genes at 0.5, 1, 6, 12, 24 h, 4 days, 1, 2, 3, and 4 weeks post nerve injury (PNI). Hierarchical clustering, Euclidean distance matrix, and principal component analysis (PCA) collectively suggested three distinct phases within the post-injury period of 4 weeks. Gene ontology (GO) analysis suggested that phase I (0–6 h PNI), phase II (6–24 h PNI), and phase III (4 days to 4 weeks) were associated with acute response to injury, preformation of Wallerian degeneration, and complete execution of Wallerian degeneration, respectively. Critical signaling pathways and transcriptional factor networks responsible for the regulation of Wallerian degeneration were further identified and integrated using Kyoto Enrichment of Genes and Genomes (KEGG) pathway analysis and Ingenuity Pathway Analysis (IPA), respectively. Our results may help to elucidate some molecular mechanisms of gene regulation associated with Wallerian degeneration that occurs after traumatic injury to peripheral nerve axons in mammals.

Repair of traumatic plasmalemmal damage to neurons and other eukaryotic cells

The repair (sealing) of plasmalemmal damage, consisting of small holes to complete transections, is critical for cell survival, especially for neurons that rarely regenerate cell bodies. We first describe and evaluate different measures of cell sealing. Some measures, including morphological/ultra-structural observations, membrane potential, and input resistance, provide very ambiguous assessments of plasmalemmal sealing. In contrast, measures of ionic current flow and dye barriers can, if appropriately used, provide more accurate assessments. We describe the effects of various substances (calcium, calpains, cytoskeletal proteins, ESCRT proteins, mUNC-13, NSF, PEG) and biochemical pathways (PKA, PKC, PLC, Epac, cytosolic oxidation) on plasmalemmal sealing probability, and suggest that substances, pathways, and cellular events associated with plasmalemmal sealing have undergone a very conservative evolution. During sealing, calcium ion influx mobilizes vesicles and other membranous structures (lysosomes, mitochondria, etc.) in a continuous fashion to form a vesicular plug that gradually restricts diffusion of increasingly smaller molecules and ions over a period of seconds to minutes. Furthermore, we find no direct evidence that sealing occurs through the collapse and fusion of severed plasmalemmal leaflets, or in a single step involving the fusion of one large wound vesicle with the nearby, undamaged plasmalemma. We describe how increases in perikaryal calcium levels following axonal transection account for observations that cell body survival decreases the closer an axon is transected to the perikaryon. Finally, we speculate on relationships between plasmalemmal sealing, Wallerian degeneration, and the ability of polyethylene glycol (PEG) to seal cell membranes and rejoin severed axonal ends – an important consideration for the future treatment of trauma to peripheral nerves. A better knowledge of biochemical pathways and cytoplasmic structures involved in plasmalemmal sealing might provide insights to develop treatments for traumatic nerve injuries, stroke, muscular dystrophy, and other pathologies.

Effects of extracellular calcium and surgical techniques on restoration of axonal continuity by polyethylene glycol fusion following complete cut or crush severance of rat sciatic nerves

Complete crush or cut severance of sciatic nerve axons in rats and other mammals produces immediate loss of axonal continuity. Loss of locomotor functions subserved by those axons is restored only after months, if ever, by outgrowths regenerating at ∼1 mm/day from the proximal stumps of severed axonal segments. The distal stump of a severed axon typically begins to degenerate in 1-3 days. We recently developed a polyethylene glycol (PEG) fusion technology, consisting of sequential exposure of severed axonal ends to hypotonic Ca(2+) -free saline, methylene blue, PEG in distilled water, and finally Ca(2+) -containing isotonic saline. This study examines factors that affect the PEG fusion restoration of axonal continuity within minutes, as measured by conduction of action potentials and diffusion of an intracellular fluorescent dye across the lesion site of rat sciatic nerves completely cut or crush severed in the midthigh. Also examined are factors that affect the longer-term PEG fusion restoration of lost behavioral functions within days to weeks, as measured by the sciatic functional index. We report that exposure of cut-severed axonal ends to Ca(2+) -containing saline prior to PEG fusion and stretch/tension of proximal or distal axonal segments of cut-severed axons decrease PEG fusion success. Conversely, trimming cut-severed ends in Ca(2+) -free saline just prior to PEG fusion increases PEG fusion success. PEG fusion prevents or retards the Wallerian degeneration of cut-severed axons, as assessed by measures of axon diameter and G ratio. PEG fusion may produce a paradigm shift in the treatment of peripheral nerve injuries. © 2016 Wiley Periodicals, Inc.

The curious ability of polyethylene glycol fusion technologies to restore lost behaviors after nerve severance

Traumatic injuries to PNS and CNS axons are not uncommon. Restoration of lost behaviors following severance of mammalian peripheral nerve axons (PNAs) relies on regeneration by slow outgrowths and is typically poor or nonexistent when after ablation or injuries close to the soma. Behavioral recovery after severing spinal tract axons (STAs) is poor because STAs do not naturally regenerate. Current techniques to enhance PNA and/or STA regeneration have had limited success and do not prevent the onset of Wallerian degeneration of severed distal segments. This Review describes the use of a recently developed polyethylene glycol (PEG) fusion technology combining concepts from biochemical engineering, cell biology, and clinical microsurgery. Within minutes after microsuturing carefully trimmed cut ends and applying a well-specified sequence of solutions, PEG-fused axons exhibit morphological continuity (assessed by intra-axonal dye diffusion) and electrophysiological continuity (assessed by conduction of action potentials) across the lesion site. Wallerian degeneration of PEG-fused PNAs is greatly reduced as measured by counts of sensory and/or motor axons and maintenance of axonal diameters and neuromuscular synapses. After PEG-fusion repair, cut-severed, crush-severed, or ablated PNAs or crush-severed STAs rapidly (within days to weeks), more completely, and permanently restore PNA- or STA-mediated behaviors compared with nontreated or conventionally treated animals. PEG-fusion success is enhanced or decreased by applying antioxidants or oxidants, trimming cut ends or stretching axons, and exposure to Ca(2+) -free or Ca(2+) -containing solutions, respectively. PEG-fusion technology employs surgical techniques and chemicals already used by clinicians and has the potential to produce a paradigm shift in the treatment of traumatic injuries to PNAs and STAs.

Polyethylene glycol-fused allografts produce rapid behavioral recovery after ablation of sciatic nerve segments

Restoration of neuronal functions by outgrowths regenerating at ∼1 mm/day from the proximal stumps of severed peripheral nerves takes many weeks or months, if it occurs at all, especially after ablation of nerve segments. Distal segments of severed axons typically degenerate in 1-3 days. This study shows that Wallerian degeneration can be prevented or retarded, and lost behavioral function can be restored, following ablation of 0.5-1-cm segments of rat sciatic nerves in host animals. This is achieved by using 0.8-1.1-cm microsutured donor allografts treated with bioengineered solutions varying in ionic and polyethylene glycol (PEG) concentrations (modified PEG-fusion procedure), being careful not to stretch any portion of donor or host sciatic nerves. The data show that PEG fusion permanently restores axonal continuity within minutes, as initially assessed by action potential conduction and intracellular diffusion of dye. Behavioral functions mediated by the sciatic nerve are largely restored within 2-4 weeks, as measured by the sciatic functional index. Increased restoration of sciatic behavioral functions after ablating 0.5-1-cm segments is associated with greater numbers of viable myelinated axons within and distal to PEG-fused allografts. Many such viable myelinated axons are almost certainly spared from Wallerian degeneration by PEG fusion. PEG fusion of donor allografts may produce a paradigm shift in the treatment of peripheral nerve injuries.

Peripheral nerve reconstruction after injury: a review of clinical and experimental therapies

Unlike other tissues in the body, peripheral nerve regeneration is slow and usually incomplete. Less than half of patients who undergo nerve repair after injury regain good to excellent motor or sensory function and current surgical techniques are similar to those described by Sunderland more than 60 years ago. Our increasing knowledge about nerve physiology and regeneration far outweighs our surgical abilities to reconstruct damaged nerves and successfully regenerate motor and sensory function. It is technically possible to reconstruct nerves at the fascicular level but not at the level of individual axons. Recent surgical options including nerve transfers demonstrate promise in improving outcomes for proximal nerve injuries and experimental molecular and bioengineering strategies are being developed to overcome biological roadblocks limiting patient recovery.

Cellular mechanisms of plasmalemmal sealing and axonal repair by polyethylene glycol and methylene blue

Mammalian neurons and all other eukaryotic cells endogenously repair traumatic injury within minutes by a Ca²⁺-induced accumulation of vesicles that interact and fuse with each other and the plasmalemma to seal any openings. We have used uptake or exclusion of extracellular fluorescent dye to measure the ability of rat hippocampal B104 cells or rat sciatic nerves to repair (seal) transected neurites in vitro or transected axons ex vivo. We report that endogenous sealing in both preparations is enhanced by Ca²⁺-containing solutions and is decreased by Ca²⁺-free solutions containing antioxidants such as dithiothreitol (DTT), melatonin (MEL), methylene blue (MB), and various toxins that decrease vesicular interactions. In contrast, the fusogen polyethylene glycol (PEG) at 10-50 mM artificially seals the cut ends of B104 cells and rat sciatic axons within seconds and is not affected by Ca²⁺ or any of the substances that affect endogenous sealing. At higher concentrations, PEG decreases sealing of transected axons and disrupts the plasmalemma of intact cells. These PEG-sealing data are consistent with the hypothesis that lower concentrations of PEG directly seal a damaged plasmalemma. We have considered these and other data to devise a protocol using a well-specified series of solutions that vary in tonicity, Ca²⁺, MB, and PEG content. These protocols rapidly and consistently repair (PEG-fuse) rat sciatic axons in completely cut sciatic nerves in vivo rapidly and dramatically to restore long-lasting morphological continuity, action potential conduction, and behavioral functions.

Polyethylene glycol rapidly restores axonal integrity and improves the rate of motor behavior recovery after sciatic nerve crush injury

The inability to rapidly (within minutes to hours) improve behavioral function after severance of peripheral nervous system axons is an ongoing clinical problem. We have previously reported that polyethylene glycol (PEG) can rapidly restore axonal integrity (PEG-fusion) between proximal and distal segments of cut- and crush-severed rat axons in vitro and in vivo. We now report that PEG-fusion not only reestablishes the integrity of crush-severed rat sciatic axons as measured by the restored conduction of compound action potentials (CAPs) and the intraaxonal diffusion of fluorescent dye across the lesion site, but also produces more rapid recovery of appropriate hindlimb motor behaviors. Improvement in recovery occurred during the first few postoperative weeks for the foot fault (FF) asymmetry test and between week 2 and week 3 for the Sciatic Functional Index (SFI) based on analysis of footprints. That is, the FF test was the more sensitive indicator of early behavioral recovery, showing significant postoperative improvement of motor behavior in PEG-treated animals at 24-48 h. In contrast, the SFI more sensitively measured longer-term postoperative behavioral recovery and deficits at 4-8 wk, perhaps reflecting the development of fine (distal) motor control. These and other data show that PEG-fusion not only rapidly restores physiological and morphological axonal continuity, but also more quickly improves behavioral recovery.

Melatonin enhances the in vitro and in vivo repair of severed rat sciatic axons

This study examines the effects of several experimental compounds [melatonin (MEL), cyclosporin A (CsA), glial-derived neurotrophic factor (GDNF), and methylprednisolone (MP)] on polyethylene glycol (PEG)-induced repair in vitro and/or in vivo by plasmalemmal fusion (PEG-fusion) of sciatic axons severed by crushing. As measured by conduction of compound action potentials (CAPs) through the lesion site, a significantly (p<0.025) higher percentage (75%) of crushed rat sciatic axons can be repaired in vitro by PEG-fusion following exposure to MEL compared to PEG-fusion of severed sciatic axons in control Krebs saline that contains calcium (CTL=20%). In contrast, no other experimental compound (GDNF: 45%; MP: 42%; CsA: 24%) produces a significant improvement in PEG-fusion success compared to CTL. Further, MEL produces significantly (p<0.001) larger peak CAP amplitudes conducted through the lesion site following PEG-fusion compared to CTL or any other experimental compound in vitro. Additionally, MEL significantly (p<0.025) increases the ability to PEG-fuse sciatic axons in vivo, compared to CTL. Finally, PEG-fusion success in vivo is significantly (p<0.01) greater in calcium-free CTL (CTL-Ca) compared to CTL.

Degeneration of mammalian PNS and CNS axons is accelerated by incubation with protein synthesis inhibitors

Protein synthesis inhibitors (PSIs) increase the rate of degeneration, as measured by compound action potential (CAP) conduction, in segments of rat PNS and CNS axons. Sciatic axonal segments maintained in vitro in Krebs at 37-38 degrees C generate CAPs for 24 h compared to 8 h for axons exposed to Krebs containing two PSIs, 100 microM anisomycin and/or 35 microM cycloheximide. Spinal axonal segments at 37-38 degrees C generate CAPs for 3 h compared to 2 h for axons exposed to Krebs containing PSIs. While cooling (6-9 degrees C) slows degeneration rate, cooled sciatic axons exposed to PSIs exhibit lower peak CAPs compared to cooled control segments (P<0.005).

Cooling enhances in vitro survival and fusion-repair of severed axons taken from the peripheral and central nervous systems of rats

Severed segments of rat peripheral (PNS; sciatic) and central nervous system (CNS; spinal) axons continue to conduct action potentials when maintained in vitro at 6-9 degrees C for up to 7 (sciatic axons) and 2 days (spinal axons), compared with only 36 h at 37-38 degrees C for sciatic axons and 6 h for spinal axons. These PNS and CNS axonal segments can be crushed and then treated with polyethylene glycol (PEG), resulting in a rapid reconnection (fusion) of the surviving axons at the crush site, as assessed by conduction of action potentials through the crush site within minutes after PEG administration. Severed PNS or CNS axons maintained in vitro at 6-9 degrees C prior to crushing can be successfully PEG-fused for up to 4 and 1.5 days, respectively, compared with only 24 (sciatic) and 3 h (spinal) at 37-38 degrees C. These data demonstrate that cooling significantly increases both the survival time of severed mammalian PNS and CNS axons and the time that severed axons can still be PEG-fused (rejoined) to rapidly re-establish axonal continuity in vitro.

Death of rat sympathetic ganglion cells in vitro caused by neurite transection: effect of extracellular calcium

Calcium entry into neurons secondary to excitotoxic insults is believed to cause neuronal death after trauma and ischemia, but the role of calcium influx in neuronal death after neurite transection independent of excitotoxicity has not been clearly defined. This study assesses the effect of variations in extracellular calcium concentration ([Ca2+]e) from 50 nM to 5 mM on cell death, in 14-day-old cultures of dissociated sympathetic neurons from the superior cervical ganglia of newborn rats. The neurites were transected with a custom-made injury device, and cell death was assessed with propidium iodide and fluorescence microscopy. We found that neurite transection caused a significant increase (p < 0.05) in cell death at all [Ca2+]e studies, but there was no significant difference in mortality at the various [Ca2+]e. Cell death significantly increased between 2 and 24 h postinjury at all three [Ca2+]e. Cell death increased with decreasing distance between the cell body and the transection site, and there was a significant decrease in mortality at distances greater than 0.66 mm, irrespective of the [Ca2+]e. These results suggest that influx of extracellular calcium is not responsible for posttransection cell death, suggesting that calcium release from internal stores or calcium-independent cell death mechanisms are triggered by neurite transection.

Plasmalemmal repair of severed neurites of PC12 cells requires Ca(2+) and synaptotagmin

Ca(2+) and synaptotagmin (a Ca(2+)-binding protein that regulates axolemmal fusion of synaptic vesicles) play essential roles in the repair of axolemmal damage in invertebrate giant axons. We now report that neurites of a rat pheochromocytoma (PC12) cell line transected and maintained in a serum medium form a dye barrier (exclude an external hydrophilic fluorescent dye) and survive for 24-hr posttransection (based on morphology and retention of another hydrophilic dye internally loaded at 6-hr posttransection). Some (25%) transected neurites that form a dye barrier regrow. Most (83%) neurites transected in a saline solution containing divalent cations (PBS(++)) also exclude entry of an externally placed hydrophilic fluorescent dye at 15-min posttransection. In contrast, only 14 or 17% of neurites maintained in a divalent cation-free solution (PBS(=)) or in PBS(=) + Mg(2+), respectively, form a dye barrier. Neurites that do not form a dye barrier do not survive for 24 hr. When PC12 neurites are loaded with an antibody to squid synaptotagmin, most (81%) antibody-loaded neurites do not form a dye barrier, whereas most (>/=81%) neurites loaded with heat-inactivated antibody or preimmune IgG do form a barrier. These data show that: 1) transected neurites of PC12 cells have mechanism(s) for plasmalemmal repair (dye barrier formation and survival); 2) Ca(2+) is necessary for dye barrier formation, which occurs minutes after transection and is necessary for survival and regrowth; and 3) synaptotagmin is an essential mediator of barrier formation. The similarity in the requirements for plasmalemmal repair in this mammalian cell preparation with those reported previously for invertebrate axons suggests that mechanisms necessary for plasmalemmal repair have been conserved phylogenetically.