Survival of mammalian B104 cells following neurite transection at different locations depends on somal Ca2+ concentration

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.

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.

The Effect of Axon Resealing on Retrograde Neuronal Death after Spinal Cord Injury in Lamprey

Failure of axon regeneration in the central nervous system (CNS) of mammals is due to both extrinsic inhibitory factors and to neuron-intrinsic factors. The importance of intrinsic factors is illustrated in the sea lamprey by the 18 pairs of large, individually identified reticulospinal (RS) neurons, whose axons are located in the same spinal cord tracts but vary greatly in their ability to regenerate after spinal cord transection (TX). The neurons that are bad regenerators also undergo very delayed apoptosis, signaled early by activation of caspases. We noticed that the neurons with a low probability of axon regeneration tend to be larger than the good regenerators. We postulate that the poorly regenerating larger neurons have larger caliber axons, which reseal more slowly, allowing more prolonged entry of toxic signals (e.g., Ca++) into the axon at the injury site. To test this hypothesis, we used a dye-exclusion assay, applying membrane-impermeable dyes to the cut ends of spinal cords at progressively longer post-TX intervals. Axons belonging to the very small neurons (not individually identified) of the medial inferior RS nucleus resealed within 15 min post-TX. Almost 75% of axons belonging to the medium-sized identified RS neurons resealed within 3 h. At this time, only 36% of the largest axons had resealed, often taking more than 24 h to exclude the dye. There was an inverse relationship between an RS neuron's size and the probability that its axon would regenerate (r = -0.92) and that the neuron would undergo delayed apoptosis, as indicated by staining with a fluorescently labeled inhibitor of caspases (FLICA; r = 0.73). The artificial acceleration of resealing with polyethylene glycol (PEG) reduced retrograde neuronal apoptosis by 69.5% at 2 weeks after spinal cord injury (SCI), suggesting that axon resealing is a critical determinant of cell survival. Ca++-free Ringer's solution with EGTA prolonged the sealing time and increased apoptotic signaling, suggesting that factors other than Ca++ diffusion into the injured tip contribute to retrograde death signaling. A longer distance of the lesion from the cell body reduced apoptotic signaling independent of the axon sealing time.

Ca2+/calmodulin-dependent protein kinase II and Dimethyl Sulfoxide affect the sealing frequencies of transected hippocampal neurons

Traumatic injury often results in axonal severance, initiating obligatory Wallerian degeneration of distal segments, whereas proximal segments often survive. Calcium ion (Ca²⁺ ) influx at severed proximal axonal ends activates pathways that can induce apoptosis. However, this same Ca²⁺ -influx also activates multiple parallel pathways that seal the plasmalemma by inducing accumulation and fusion of vesicles at the lesion site that reduce Ca²⁺ -influx and enhance survival. We examined whether various inhibitors of Ca²⁺ /calmodulin-dependent protein kinases (CaMKs), and/or dimethyl sulfoxide (DMSO), a common solvent for biologically active substances, affected the ability of a hippocampal-derived neuronal cell line (B104 cells) to seal membrane damage following axotomy. Axolemmal sealing frequencies were assessed at different transection distances from the axon hillock and at various times after Ca²⁺ -influx (PC times) by observing whether transected cells took-up fluorescent dyes. Inhibition of CaMKII by tatCN21 and KN-93, but not inhibition of CaMKI and CaMKIV by STO-609, affected axonal sealing frequencies. That is, CaMKII is a component of previously reported parallel pathways that induce membrane sealing, whereas CaMKI and CaMKIV are not involved. The effects of these CaMKII inhibitors on plasmalemmal sealing depended on their mechanism of inhibition, transection distance, and PC time. DMSO at low concentrations (90 µM-28 mM or 0.00064%-0.2% v/v) significantly increased membrane-sealing frequencies at most PC times and transection distances, possibly by permeabilizing the plasmalemma to Ca²⁺ . Inhibition of CaMKII, DMSO, PC time, and the transection distance significantly affect plasmalemmal sealing that is critical to somal survival in traumatic lesions.

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.

Analysis of piRNA-Like Small Non-coding RNAs Present in Axons of Adult Sensory Neurons

Small non-coding RNAs (sncRNAs) have been shown to play pivotal roles in spatiotemporal-specific gene regulation that is linked to many different biological functions. PIWI-interacting RNAs (piRNAs), typically 25-34-nucleotide long, are originally identified and thought to be restricted in germline cells. However, recent studies suggest that piRNAs associate with neuronal PIWI proteins, contributing to neuronal development and function. Here, we identify a cohort of piRNA-like sncRNAs (piLRNAs) in rat sciatic nerve axoplasm and directly contrast temporal changes of piLRNA levels in the nerve following injury, as compared with those in an uninjured nerve using deep sequencing. We find that 32 of a total of 53 annotated piLRNAs show significant changes in their levels in the regenerating nerve, suggesting that individual axonal piLRNAs may play important regulatory roles in local messenger RNA (mRNA) translation during regeneration. Bioinformatics and biochemical analyses show that these piLRNAs carry characteristic features of mammalian piRNAs, including sizes, a sequence bias for uracil at the 5'-end and a 2'-O-methylation at the 3'-end. Their axonal expression is directly visualized by fluorescence in situ hybridization in cultured dorsal root ganglion neurons as well as immunoprecipitation with MIWI. Further, depletion of MIWI protein using RNAi from cultured sensory neurons increases axon growth rates, decreases axon retraction after injury, and increases axon regrowth after injury. All these data suggest more general roles for MIWI/piLRNA pathway that could confer a unique advantage for coordinately altering the population of proteins generated in growth cones and axons of neurons by targeting mRNA cohorts.

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.

Sealing frequency of B104 cells declines exponentially with decreasing transection distance from the axon hillock

Transection of nerve axons (axotomy) leads to rapid (Wallerian) degeneration of the distal portion of the severed axon whereas the proximal portion and the soma often survive. Clinicians and neuroscientists have known for decades that somal survival is less likely for cells transected nearer to the soma, compared to further from the soma. Calcium ion (Ca(2+)) influx at the cut axonal end increases somal Ca(2+) concentration, which subsequently activates apoptosis and other pathways that lead to cell death. The same Ca(2+) influx activates parallel pathways that seal the plasmalemma, reduce Ca(2+) influx, and thereby enable the soma to survive. In this study, we have examined the ability of transected B104 axons to seal, as measured by uptake or exclusion of fluorescent dye, and quantified the relationship between sealing frequency and transection distance from the axon hillock. We report that sealing frequency is maximal at about 150μm (μm) from the axon hillock and decreases exponentially with decreasing transection distance with a space constant of about 40μm. We also report that after Ca(2+) influx is initiated, the curve of sealing frequency versus time is well-fit by a one-phase, rising exponential model having a time constant of several milliseconds that is longer nearer to, versus further from, the axon hillock. These results could account for the increased frequency of cell death for axotomies nearer to, versus farther from, the soma of many types of neurons.

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.

Consequences of neurite transection in vitro

In order to quantify degenerative and regenerative changes and analyze the contribution of multiple factors to the outcome after neurite transection, we cultured adult mouse dorsal root ganglion neurons, and with a precise laser beam, we transected the nerve fibers they extended. Cell preparations were continuously visualized for 24 h with time-lapse microscopy. More distal cuts caused a more elongated field of degeneration, while thicker neurites degenerated faster than thinner ones. Transected neurites degenerated more if the uncut neurites of the same neuron simultaneously degenerated. If any of these uncut processes regenerated, the transected neurites underwent less degeneration. Regeneration of neurites was limited to distal cuts. Unipolar neurons had shorter regeneration than multipolar ones. Branching slowed the regenerative process, while simultaneous degeneration of uncut neurites increased it. Proximal lesions, small neuronal size, and extensive and rapid neurite degeneration were predictive of death of an injured neuron, which typically displayed necrotic rather than apoptotic form. In conclusion, this in vitro model proved useful in unmasking many new aspects and correlates of mechanically-induced neurite injury.

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.

A model for sealing plasmalemmal damage in neurons and other eukaryotic cells

Plasmalemmal repair is necessary for survival of damaged eukaryotic cells. Ca(2+) influx through plasmalemmal disruptions activates calpain, vesicle accumulation at lesion sites, and membrane fusion proteins; Ca(2+) influx also initiates competing apoptotic pathways. Using the formation of a dye barrier (seal) to assess plasmalemmal repair, we now report that B104 hippocampal cells with neurites transected nearer (<50 μm) to the soma seal at a lower frequency and slower rate compared to cells with neurites transected farther (>50 μm) from the soma. Analogs of cAMP, including protein kinase A (PKA)-specific and Epac-specific cAMP, each increase the frequency and rate of sealing and can even initiate sealing in the absence of Ca(2+) influx at both transection distances. Furthermore, Epac activates a cAMP-dependent, PKA-independent, pathway involved in plasmalemmal sealing. The frequency and rate of plasmalemmal sealing are decreased by a small molecule inhibitor of PKA targeted to its catalytic subunit (KT5720), a peptide inhibitor targeted to its regulatory subunits (PKI), an inhibitor of a novel PKC (an nPKCη pseudosubstrate fragment), and an antioxidant (melatonin). Given these and other data, we propose a model for redundant parallel pathways of Ca(2+)-dependent plasmalemmal sealing of injured neurons mediated in part by nPKCs, cytosolic oxidation, and cAMP activation of PKA and Epac. We also propose that the evolutionary origin of these pathways and substances was to repair plasmalemmal damage in eukaryotic cells. Greater understanding of vesicle interactions, proteins, and pathways involved in plasmalemmal sealing should suggest novel neuroprotective treatments for traumatic nerve injuries and neurodegenerative disorders.

Oxidized low-density lipoprotein (oxLDL) induces cell death in neuroblastoma and survival autophagy in schwannoma cells

Oxidized low-density lipoprotein (oxLDL) induces apoptosis or autophagy in dependence on the cell type. We here investigated the effect of oxLDL on the B104 neuroblastoma and RN22 schwannoma cells being popular in neuroscience research. Cells were cultivated with and without oxLDL. To generate oxLDL, we added 50 μg/ml nLDL and 50 μM CuSO(4) into the culture medium. After a 24-h-long treatment, oxLDL was detectable in media from both cell culture types and its concentration was approximately 16 μg/ml. In the oxLDL-treated B104 neuroblastoma cell cultures 75% cells died after the 24-h exposure. The intact cells showed impaired mitochondria at the ultrastructural level. Western blot analysis revealed the increased expression of AIF 57 kDa (AIF(57)) protein, as a sign of caspase-independent cell death. In RN22 schwannoma cell cultures, oxLDL did not have any effect on cleaved caspase-3 and AIF(57) protein levels indicating absence of cell death. Treated RN22 schwannoma cells underwent survival autophagy by forming conspicuous autophagosomes and by processing LC3-I into LC3-II protein. Collectively, oxLDL induces AIF-dependent cell death in B104 neuroblastoma cells whereas in RN22 schwannoma cells enhanced signs of survival autophagy are noted.

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.

Critical interval of somal calcium transient after neurite transection determines B 104 cell survival

Nerve cells may survive or die after axonal or dendritic transection. After neurite transection near (<50 mum) the cell body of Fura-2-loaded B 104 neuroblastoma (rat brain-derived) cells, the somal calcium concentration (SCC) undergoes a three-phase transient change: a rapid (0-0.15-min post-transection [PT]) rise phase, followed by an early (0.15--1.5-min PT) rapid decay phase, and succeeded by a late (1.5-60-min PT) slower decay phase that restores SCC to preinjury levels. The SCC in a critical interval (1.5-12.5 min PT) of the third transient phase correlates with cell fate, i.e., most transected cells that exclude dye (restore a barrier) and die have a significantly higher (P<0.005) SCC in this critical interval than do transected cells that exclude dye and survive at 24-hr PT. Loading BAPTA (chelation of somal Ca(2+)) before, but not after, the critical interval increases the percentage of cells that survive compared to that of cells transected without BAPTA loading. Furthermore, most transected cells that die despite successful barrier restoration exhibit characteristics consistent with apoptosis initiated during the critical interval of the SCC, including caspase activation and plasmalemmal phosphatidylserine translocation. These data suggest that decreased cell survival for injuries near the soma is due to Ca(2+)-initiated apoptosis during the critical interval of the third phase of the SCC transient. (c) 2005 Wiley-Liss, Inc.