Calcium-activated proteolysis of neurofilament proteins in goldfish Mauthner axons

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.

Degeneration, Trophic Interactions, and Repair of Severed Axons: A Reconsideration of Some Common Assumptions

We suggest that several interrelated properties of severed axons (degeneration, trophic dependencies, initial repair, and eventual repair) differ in important ways from commonly held assumptions about those properties. Specifically, (1) axotomy does not necessarily produce rapid degeneration of distal axonal segments because (2) the trophic maintenance of nerve axons does not necessarily depend entirely on proteins transported from the perikaryon—but instead axonal proteins can be trophically maintained by slowing their degradation and/or by acquiring new proteins via axonal synthesis or transfer from adjacent cells (e.g., glia). (3) The initial repair of severed distal or proximal segments occurs by barriers (seals) formed amid accumulations of vesicles and/or myelin delaminations induced by calcium influx at cut axonal ends—rather than by collapse and fusion of cut axolemmal leaflets. (4) The eventual repair of severed mammalian CNS axons does not necessarily have to occur by neuritic outgrowths, which slowly extend from cut proximal ends to possibly reestablish lost functions weeks to years after axotomy—but instead complete repair can be induced within minutes by polyethylene glycol to rejoin (fuse) the cut ends of surviving proximal and distal stumps. Strategies to repair CNS lesions based on fusion techniques combined with rehabilitative training and induced axonal outgrowth may soon provide therapies that can at least partially restore lost CNS functions.

Loss of calretinin- and parvalbumin-immunoreactive axons in anterolateral columns beyond the corticospinal tracts of amyotrophic lateral sclerosis spinal cords

In amyotrophic lateral sclerosis (ALS) spinal cords, diffuse myelin pallor (dMP) in the anterolateral columns (ALCs) beyond the corticospinal tracts has been frequently observed; however, its origin still remains to be elucidated. To address this issue, we focused on calretinin (CR) and parvalbumin (PV), since these buffer calcium-binding proteins (CaBP) are predominantly expressed in axons in the ALCs of neurologically normal human spinal white matter. Immunohistochemical methods revealed that numbers of both CR-immunoreactive (ir) and PV-ir axons were significantly lower in ALS patients' spinal cords with dMP compared to those in controls. In ALS patients' spinal cords without dMP, there were also significant reductions in the number of these CaBP-ir axons compared to controls. In contrast, the number of CR-ir neurons in the spinal gray matter did not differ significantly among ALS patients and controls. These findings suggest that a loss of CaBP-ir axons may precede the development of dMP in ALS patients' spinal cords, and the dying back mechanism would underlie this phenomenon.

Critical calpain-dependent ultrastructural alterations underlie the transformation of an axonal segment into a growth cone after axotomy of cultured Aplysia neurons

The transformation of a stable axonal segment into a motile growth cone is a critical step in the regeneration of amputated axons. In earlier studies we found that axotomy of cultured Aplysia neurons leads to a transient and local elevation of the free intracellular Ca2+ concentration, resulting in calpain activation, localized proteolysis of submembranal spectrin, and, eventually, growth cone formation. Moreover, inhibition of calpain by calpeptin prior to axotomy inhibits growth cone formation. Here we investigated the mechanisms by which calpain activation participates in the transformation of an axonal segment into a growth cone. To that end we compared the ultrastructural alterations induced by axotomy performed under control conditions with those caused by axotomy performed in the presence of calpeptin, using cultured Aplysia neurons as a model. We identified the critical calpain-dependent cytoarchitectural alterations that underlie the formation of a growth cone after axotomy. Calpain-dependent processes lead to restructuring of the neurofilaments and microtubules to form an altered cytoskeletal region 50-150 microm proximal to the tip of the transected axon in which vesicles accumulate. The dense pool of vesicles forms in close proximity to a segment of the plasma membrane along which the spectrin membrane skeleton has been proteolyzed by calpain. We suggest that the rearrangement of the cytoskeleton forms a transient cellular compartment that traps transported vesicles and serves as a locus for microtubule polymerization. We propose that this cytoskeletal configuration facilitates the fusion of vesicles with the plasma membrane, promoting the extension of the growth cone's lamellipodium. The growth process is further supported by the radial polymerization of microtubules from the growth cone's center.

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.

Axonal neurofilaments are resistant to calpain-mediated degradation in the WLD(S) mouse

The biological basis for the phenotype of delayed Wallerian degeneration in the WLDs mouse has yet to be elucidated, although it is known that the characteristic is intrinsic to the axon. Previous data suggested that nerves from the WLD(S) are relatively resistant to proteolytic degradation. We investigated the time-course of neurofilament degradation in response to addition of the calcium-activated protease m-calpain, comparing nerves from WLD(S) and wild-type mice. During 10 min of in vitro proteolysis, neurofilaments from the WLD(S) were consistently slower to degrade than were neurofilaments from wild-type mice. Direct comparisons were performed on Western blots, with statistically significant differences in neurofilament immunoreactivity at 2, 4, and 6 min of reaction time (p < 0.01). These findings suggest that the mutation leading to the WLD(S) phenotype may affect the proteolytic interaction between calpain and neurofilaments.

Smooth endoplasmic reticulum in fish Mauthner cells at different functional states

The ultrastructure of Mauthner cells of goldfish fry and adult xenotoca in intact state and after prolonged natural stimulation has been studied qualitatively and quantitatively. Additionally, Mauthner cells of intact adult goldfish and adult rotan Percottus glehni were investigated. In all adult fish the dendroplasm of the two major dendrites was shown to contain a regular network of smooth endoplasmic reticulum, with cisterns and tubules arranged transversally to the dendrite stem. In the Mauthner cells of intact goldfish fry, the reticulum was not clearly expressed, the transversal cisterns occurred occasionally. After stimulation, however, it became more developed probably due to proliferation of additional transversal cisterns. The periodicity of transversally oriented cisterns in the dendrites of Mauthner cells in each fish species studied was nearly the same. However, the number of transversal cisterns per unit of dendrite length, and the total length of cisterns and tubules per unit of cross-section area varied both within and among the species. These parameters increased after stimulation. It is suggested that the proliferation of the transversal cisterns in the endoplasmic reticulum and the extent of their development depend on the functional state of the afferent synapses and the plasticity of the smooth reticulum reflects the involvement of postsynaptic mechanisms in regulation of Mauthner cell stability presumably via the regulation of calcium homeostasis under varying conditions of functioning.

Amyotrophic lateral sclerosis associated with mutations in superoxide dismutase: a putative mechanism of degeneration

Amyotrophic lateral sclerosis (ALS) is a devastating neurologic disease that rapidly progresses from mild motor symptoms to severe motor paralysis and premature death. Until recently, there were few substantive studies conducted on the pathogenesis of the disease. With the genetic linkage of mutations in the superoxide dismutase (SOD-1) gene with familial ALS patients, new avenues for study have become available including transgenic mice and culture models. Although not yet providing a complete picture of the disease mechanism, studies utilizing these model systems have greatly advanced our understanding of the mechanism of degeneration and should eventually lead to putative therapeutic agents. In this review, we will present the important findings from these model systems, provide a framework in which to evaluate these findings, and speculate on the mechanism of degeneration initiated by the mutations in SOD-1.

Co-localization of calretinin and parvalbumin with nicotinamide adenine dinucleotide phosphate-diaphorase in tench Mauthner cells

The co-localization of calretinin (CR) and parvalbumin (PV) immunoreactivity with nicotinamide adenine dinucleotide phosphate-diaphorase (ND) activity was analyzed in the Mauthner cells of the tench. Mauthner cells were ND active, and ND staining was observed in the soma, axon cap region, and axon of these neurons. CR co-localized with ND in the axon of the Mauthner cells but not in the cell body or in the dendrites, whereas PV immunoreactivity co-localized with ND in the soma, axon and dendrites. The presence of two different calcium-binding proteins in the Mauthner cells indicates that these neurons need complex calcium-buffering systems. The co-localization of these calcium-binding proteins with ND might suggests their involvement in nitric oxide-related events.

Calcium ionophore-induced degradation of neurofilament and cell death in MSN neuroblastoma cells

Extensive necrotic death of MSN neuroblastoma cells could be induced after incubation with the calcium ionophore, A23187. The reaction was concentration-dependent and time course-dependent. Levels of the 66 kd/alpha-internexin neurofilament protein (NF-66) and the cognate heat shock protein 70 (Hsc 70) decreased during the Ca2+-activated cell death. Addition of the calcium chelator, ethylene glycol-bis(beta-aminoethyl ether) N,N,N',N'-tetraacetic acid (EGTA) restored the normal level of NF-66 and partially that of the Hsc 70. Use of either calpain I or calpain II inhibitor could alleviate the reduction of 66 kd protein during the ionophore treatment whereas only calpain I inhibitor treatment was effective in restoring the normal level of the Hsc 70. Neither of these calpain inhibitors could block the ionophore triggered cell death. EGTA was toxic to cells in a wide range of concentration suggesting a calcium-independent activation of cell death mechanism.

Delaminating myelin membranes help seal the cut ends of severed earthworm giant axons

Transected axons are often assumed to seal by collapse and fusion of the axolemmal leaflets at their cut ends. Using photomicroscopy and electronmicroscopy of fixed tissues and differential interference contrast and confocal fluorescence imaging of living tissues, we examined the proximal and distal cut ends of the pseudomyelinated medial giant axon of the earthworm, Lumbricus terrestris, at 5-60 min post-transection in physiological salines and Ca2+-free salines. In physiological salines, the axolemmal leaflets at the cut ends do not completely collapse, much less fuse, for at least 60 min post-transection. In fact, the axolemma is disrupted for 20-100 microm from the cut end at 5-60 min post-transection. However, a barrier to dye diffusion is observed when hydrophilic or styryl dyes are placed in the bath at 15-30 min post-transection. At 30-60 min post-transection, this barrier to dye diffusion near the cut end is formed amid an accumulation of some single-layered and many multilayered vesicles and other membranous material, much of which resembles delaminated pseudomyelin of the glial sheath. In Ca2+-free salines, this single and multilayered membranous material does not accumulate, and a dye diffusion barrier is not observed. These and other data are consistent with the hypothesis that plasmalemmal damage in eukaryotic cells is repaired by Ca2+-induced vesicles arising from invaginations or evaginations of membranes of various origin which form junctional contacts or fuse with each other and/or the plasmalemma.

Cyclosporin A retards the wallerian degeneration of peripheral mammalian axons

The distal (anucleate) segments of mammalian peripheral axons typically undergo complete Wallerian degeneration within 1-3 days after severance from their cell bodies, unlike invertebrates and lower vertebrates, where anucleate axons do not degenerate for weeks to months. This rapid Wallerian degeneration in mammals could be due to a more efficient immune system and/or to differences in calcium-dependent pathways relative to invertebrates and lower vertebrates. To suppress the immune system and to inhibit calcium-dependent pathways in axons, we gave daily subcutaneous injections of cyclosporin A (CsA: 10 mg/kg) to Sprague-Dawley rats for 7 days before, and 5 days after, severing their right ventral tail nerves. To confirm that CsA suppressed the immune system, white blood cell density was measured in CsA-treated and in non-treated rats. Our data showed that the number of surviving anucleate myelinated axons at 5 postoperative days in CsA-treated rats was significantly higher than the number in non-treated rats. Anucleate unmyelinated axons in the ventral tail nerve also exhibited better survival in CsA-treated rats than in nontreated rats. These results are consistent with the hypothesis that the immune response and/or calcium-dependent pathways play important roles in the rapid Wallerian degeneration of anucleate mammalian axons.

Calpain activity promotes the sealing of severed giant axons

A barrier (seal) must form at the cut ends of a severed axon if a neuron is to survive and eventually regenerate. Following severance of crayfish medial giant axons in physiological saline, vesicles accumulate at the cut end and form a barrier (seal) to ion and dye diffusion. In contrast, squid giant axons do not seal, even though injury-induced vesicles form after axonal transection and accumulate at cut axonal ends. Neither axon seals in Ca2+-free salines. The addition of calpain to the bath saline induces the sealing of squid giant axons, whereas the addition of inhibitors of calpain activity inhibits the sealing of crayfish medial giant axons. These complementary effects involving calpain in two different axons suggest that endogenous calpain activity promotes plasmalemmal repair by vesicles or other membranes which form a plug or a continuous membrane barrier to seal cut axonal ends.

Repair of plasmalemmal lesions by vesicles

Crayfish medial giant axons (MGAs) transected in physiological saline form vesicles which interact with each other, pre-existing vesicles, and/or with the plasmalemma to form an electrical and a physical barrier that seals a cut axonal end within 60 min. The formation of this barrier (seal) was assessed by measuring the decay of injury current at the cut end; its location at the cut end was determined by the exclusion of fluorescent hydrophilic dye at the cut end. When a membrane-incorporating styryl dye was placed in the bath prior to axonal transection and a hydrophilic dye was placed in the bath just after axonal transection, many vesicles near the barrier at the cut axonal end had their limiting membrane labeled with the styryl dye and their contents labeled with the hydrophilic dye, indicating that these vesicles originated from the axolemma by endocytosis. This barrier does not form in Ca2+-free salines. Similar collections of vesicles have been observed at regions of plasmalemmal damage in many cell types. From these and other data, we propose that plasmalemmal lesions in most eukaryotic cells (including axons) are repaired by vesicles, at least some of which arise by endocytosis induced by Ca2+ inflow resulting from the plasmalemmal damage. We describe several models by which vesicles could interact with each other and/or with intact or damaged regions of the plasmalemma to repair small (1-30 microm) plasmalemmal holes or a complete transection of the plasmalemma.

Ca(2+)-mediated phosphorylation and proteolysis activity associated with the cytoskeletal fraction from cerebral cortex of rats

We describe a Triton-insoluble cytoskeletal fraction extracted from cerebral cortex of young rats retaining an endogenous Ca(2+)-mediated mechanism acting in vitro on Ca2+/calmodulin-dependent protein kinase II (CaM-KII) activity and on phosphorylation and proteolysis of the 150 kDa neurofilament subunit (NF-M), alpha and beta tubulin. Exogenous Ca2+ induced a 70% decrease in the in vitro phosphorylation of the NF-M and tubulins and a 30-50% decrease in the total amount of these proteins. However, when calpastatin was added basal phosphorylation and NF-M and tubulin content were recovered. Furthermore, exogenous Ca2+/calmodulin induced increased in vitro phosphorylation of the cytoskeletal proteins and CaM-KII activity only in the presence of calpastatin, suggesting the presence of Ca(2+)-induced calpain-mediated proteolysis. This fraction could be an interesting model to further studies concerning the in vitro effects of Ca(2+)-mediated protein kinases and proteases associated with the cytoskeletal fraction.