Phosphorylation of neurofilament H subunit as related to arrangement of neurofilaments

Identifying the Phenotypes of Diffuse Axonal Injury Following Traumatic Brain Injury

Diffuse axonal injury (DAI) is a significant feature of traumatic brain injury (TBI) across all injury severities and is driven by the primary mechanical insult and secondary biochemical injury phases. Axons comprise an outer cell membrane, the axolemma which is anchored to the cytoskeletal network with spectrin tetramers and actin rings. Neurofilaments act as space-filling structural polymers that surround the central core of microtubules, which facilitate axonal transport. TBI has differential effects on these cytoskeletal components, with axons in the same white matter tract showing a range of different cytoskeletal and axolemma alterations with different patterns of temporal evolution. These require different antibodies for detection in post-mortem tissue. Here, a comprehensive discussion of the evolution of axonal injury within different cytoskeletal elements is provided, alongside the most appropriate methods of detection and their temporal profiles. Accumulation of amyloid precursor protein (APP) as a result of disruption of axonal transport due to microtubule failure remains the most sensitive marker of axonal injury, both acutely and chronically. However, a subset of injured axons demonstrate different pathology, which cannot be detected via APP immunoreactivity, including degradation of spectrin and alterations in neurofilaments. Furthermore, recent work has highlighted the node of Ranvier and the axon initial segment as particularly vulnerable sites to axonal injury, with loss of sodium channels persisting beyond the acute phase post-injury in axons without APP pathology. Given the heterogenous response of axons to TBI, further characterization is required in the chronic phase to understand how axonal injury evolves temporally, which may help inform pharmacological interventions.

Differentiation of Mesenchymal Stem Cells Derived From Human Adipose Tissue Into Cholinergic-like Cells: An in Vitro Study

Introduction:

Cholinergic-associated diseases currently constitute a significant cause of neurological and neurodegenerative disabilities. As the drugs are not efficient in improving the suffered tissues, stem cell treatment is considered an effective strategy for substituting the lost cells.

Methods:

In the current study, we set out to investigate the differentiation properties of human Adipose-Derived Mesenchymal Stem Cells (AD-MSCs) into cholinergic-like cells by two morphogens of Retinoic Acid (RA) and Sonic Hedgehog (Shh) using a three-step in vitro procedure. The results were evaluated using real-time PCR, flow cytometry, and immunocytochemistry for two weeks.

Results:

Our data showed that the cells could express cholinergic specific markers, including Islet-1, Acetylcholinesterase (AChE), SMI-32, and Nestin, at mRNA and protein levels. We could also quantitatively evaluate the expression of Islet-1, AChE, and Nestin at 14 days post-induction using flow cytometry.

Conclusion:

Human AD-MSCs are potent cells to differentiate into cholinergic-like cells in the presence of RA and Shh through a three-step protocol. Thus, they could be a suitable cell candidate for the regeneration of cholinergic-associated diseases. However, more functional and electrophysiological analyses are needed in this regard.

Neuroprotein Targets of γ-Diketone Metabolites of Aliphatic and Aromatic Solvents That Induce Central-Peripheral Axonopathy

Peripheral neuropathy associated with chronic occupational and deliberate overexposure to neurotoxic organic solvents results from axonal degeneration in the central and peripheral nervous system. Human and experimental studies show that axonopathy is triggered by the action of neuroprotein-reactive γ-diketone metabolites formed from exposure to certain aliphatic solvents (n-hexane, 2-hexanone) and aromatic compounds (1,2-diethylbenzene, 1,2-4-triethylbenzene, 6-acetyl-1,1,4,4-tetramethyl-7-ethyl-1,2,3,4-tetralin). Neuroprotein susceptibility is related primarily to their differential content of lysine, the ∊-amino group of which is targeted by γ-diketones. Specific neuroprotein targets have been identified, and the sequence of molecular mechanisms leading to axonal pathology has been illuminated. While occupational n-hexane neuropathy continues to be reported, lessons learned from its experimental study may have relevance to other causes of peripheral neuropathy, including those associated with aging and diabetes mellitus.

Loss of DDHD2, whose mutation causes spastic paraplegia, promotes reactive oxygen species generation and apoptosis

DDHD2/KIAA0725p is a mammalian intracellular phospholipase A₁ that exhibits phospholipase and lipase activities. Mutation of the DDHD2 gene causes hereditary spastic paraplegia (SPG54), an inherited neurological disorder characterized by lower limb spasticity and weakness. Although previous studies demonstrated lipid droplet accumulation in the brains of SPG54 patients and DDHD2 knockout mice, the cause of SPG54 remains elusive. Here, we show that ablation of DDHD2 in mice induces age-dependent apoptosis of motor neurons in the spinal cord. In vitro, motor neurons and embryonic fibroblasts from DDHD2 knockout mice fail to survive and are susceptible to apoptotic stimuli. Chemical and probe-based analysis revealed a substantial decrease in cardiolipin content and an increase in reactive oxygen species generation in DDHD2 knockout cells. Reactive oxygen species production in DDHD2 knockout cells was reversed by the expression of wild-type DDHD2, but not by an active-site DDHD2 mutant, DDHD2 mutants related to hereditary spastic paraplegia, or DDHD1, another member of the intracellular phospholipase A₁ family whose mutation also causes spastic paraplegia (SPG28). Our results demonstrate the protective role of DDHD2 for mitochondrial integrity and provide a clue to the pathogenic mechanism of SPG54.

Assembly and turnover of neurofilaments in growing axonal neurites

Neurofilaments (NFs) are thought to provide stability to the axon. We examined NF dynamics within axonal neurites of NB2a/d1 neuroblastoma by transient transfection with green fluorescent protein-tagged NF-heavy (GFP-H) under the control of a tetracycline-inducible promoter. Immunofluorescent and biochemical analyses demonstrated that GFP-H expressed early during neurite outgrowth associated with a population of centrally-situated, highly-phosphorylated crosslinked NFs along the length of axonal neurites (‘bundled NFs’). By contrast, GFP-H expressed after considerable neurite outgrowth displayed markedly reduced association with bundled NFs and was instead more evenly distributed throughout the axon. This differential localization was maintained for up to 2 weeks in culture. Once considerable neurite outgrowth had progressed, GFP that had previously associated with the NF bundle during early expression was irreversibly depleted by photobleaching. Cessation of expression allowed monitoring of NF turnover. GFP-H associated bundled NFs underwent slower decay than GFP-H associated with surrounding, less-phosphorylated NFs. Notably, GFP associated with bundled NFs underwent similar decay rates within the core and edges of this bundle. These results are consistent with previous demonstration of a resident NF population within axonal neurites, but suggest that this population is more dynamic than previously considered.

Summary: Immunofluorescent and radiolabel analyses demonstrate that neurofilaments establish a resident population within growing axonal neurites that undergoes exchange with a surrounding, transporting pool.

Compartment-Specific Phosphorylation of Squid Neurofilaments

Studies of the giant axon and synapse of third-order neurons in the squid stellate ganglion have provided a vast literature on neuronal physiology and axon transport. Large neuronal size also lends itself to comparative biochemical studies of cell body versus axon. These have focused on the regulation of synthesis, assembly, posttranslational modification and function of neuronal cytoskeletal proteins (microtubules (MTs) and neurofilaments (NFs)), the predominant proteins in axoplasm. These contribute to axonal organization, stability, transport, and impulse transmission responsible for rapid contractions of mantle muscles underlying jet propulsion. Studies of vertebrate NFs have established an extensive literature on NF structure, organization, and function; studies of squid NFs, however, have made it possible to compare compartment-specific regulation of NF synthesis, assembly, and function in soma versus axoplasm. Since NFs contain over 100 eligible sites for phosphorylation by protein kinases, the compartment-specific patterns of phosphorylation have been a primary focus of biochemical studies. We have learned that NF phosphorylation is tightly compartmentalized; extensive phosphorylation occurs only in the axonal compartment in squid and in vertebrate neurons. This extensive phosphorylation plays a key role in organizing NFs, in association with microtubules (MTs), into a stable, dynamic functional lattice that supports axon growth, diameter, impulse transmission, and synaptic activity. To understand how cytoskeletal phosphorylation is topographically regulated, the kinases and phosphatases, bound to NFs isolated from cell bodies and axoplasm, have also been studied.

Postnatal developmental profile of neurons and glia in motor nuclei of the brainstem and spinal cord, and its comparison with organotypic slice cultures

In vitro preparations of the neonatal rat spinal cord or brainstem are useful to investigate the organization of motor networks and their dysfunction in neurological disease models. Long-term spinal cord organotypic cultures can extend our understanding of such pathophysiological processes over longer times. It is, however, surprising that detailed descriptions of the type (and number) of neurons and glia in such preparations are currently unavailable to evaluate cell-selectivity of experimental damage. The focus of the present immunohistochemical study is the novel characterization of the cell population in the lumbar locomotor region of the rat spinal cord and in the brainstem motor nucleus hypoglossus at 0-4 postnatal days, and its comparison with spinal organotypic cultures at 2-22 days in vitro. In the nucleus hypoglossus, neurons were 40% of all cells and 80% of these were motoneurons. Astrocytes (35% of total cells) were the main glial cells, while microglia was <10%. In the spinal gray matter, the highest neuronal density was in the dorsal horn (>80%) and the lowest in the ventral horn (≤57%) with inverse astroglia numbers and few microglia. The number of neurons (including motoneurons) and astrocytes was stable after birth. Like in the spinal cord, motoneurons in organotypic spinal culture were <10% of ventral horn cells, with neurons <40%, and the rest made up by glia. The present report indicates a comparable degree of neuronal and glial maturation in brainstem and spinal motor nuclei, and that this condition is also observed in 3-week-old organotypic cultures.

SELF-CONSISTENT FIELD MODELING OF THE NEUROFILAMENT NETWORK

We have investigated, on a self-consistent field level, the equilibrium structure of the neurofilament network formed by the NF -H, NF -M and NF -L proteins, using the one-gradient version of the numerical model of Scheutjens and Fleer. We demonstrate a reticulation of NFs in parallel bundles that occurs due to hydrophobic attractions between apolar aminoacid residues in the terminal parts of the M- and H-tails. We elaborate on the feasibility that the stability of the NF network can be enhanced by specific interactions between the projection domains, possibly induced by accessary proteins. We demonstrate that the phosphorylation of KSP repeats in the M- and H-tails promotes the cross-bridging between the NFs and therefore helps form the NF network.

Simultaneous immunohistochemical detection of gangliosides and neuronal markers in paraformaldehyde-fixed nervous tissues by acetone etching

A need for identifying ganglioside-positive cells with neuronal markers prompted us to establish a reliable method for double or triple immunostaining nervous tissues. Perfusion fixation with paraformaldehyde is typically performed for the routine immunostaining of various neuronal markers but is not suitable for immunostaining gangliosides. Acetone fixation of fresh cryosections is frequently used for ganglioside immunodetection; thus, we tested the effect of acetone treatment for unmasking the antigen epitope of gangliosides (acetone etching) on sections of paraformaldehyde-fixed nervous tissue from rats. Acetone etching significantly retrieved ganglioside immunoreactivity while preserving the immunoreactivity of neuronal markers. Various combinations of gangliosides and neuronal markers could be double-stained by the immunoenzyme method or triple-stained by the immunofluorescence method. This new method may provide additional information regarding the relationship between gangliosides and various neuronal markers from routinely paraformaldehyde-fixed nervous tissues, both freshly prepared specimens and those stocked in the laboratory.

Depression of Ca(2+)/calmodulin-dependent protein kinase II in dorsal root ganglion neurons after spinal nerve ligation

The enzyme calcium/calmodulin-dependent protein kinase II (CaMKII) is associated with memory and its alpha isoform is critical for development of activity-induced synaptic changes. Therefore, we hypothesized that CaMKII is involved in altered function of dorsal root ganglion (DRG) neurons after neuronal injury. To test this hypothesis, Sprague-Dawley rats were made hyperalgesic by L5 and L6 spinal nerve ligation (SNL), and changes in total phosphorylated and unphosphorylated CaMKII (tCaMKII) and phosphorylated form of its alpha isoform (pCaMKIIalpha) were analyzed using immunochemistry in different subpopulations of DRG. SNL did not induce any changes in tCaMKII between experimental groups, while the overall percentage of pCaMKIIalpha-positive neurons in injured L5 DRG SNL (24.8%) decreased significantly when compared to control (41.7%). SNL did not change the percentage of pCaMKIIalpha/N52 colabeled neurons but decreased the percentage of N52-negative nonmyelinated neurons that expressed pCaMKIIalpha from 27% in control animals to 11% after axotomy. We also observed a significant decrease in the percentage of small nonpeptidergic neurons labeled with IB4 (37.6% in control vs. 4.0% in L5 SNL DRG), as well as a decrease in the percentage of pCaMKIIalpha/IB4 colabeled neurons in injured L5 DRGs (27% in control vs. 1% in L5 DRG of SNL group). Our results show that reduction in pCaMKIIalpha levels following peripheral injury is due to the loss of IB4-positive neurons. These results indicate that diminished afferent activity after axotomy may lead to decreased phosphorylation of CaMKIIalpha.

Morphological and biochemical signs of age-related neurodegenerative changes in klotho mutant mice

Klotho mutant mice, defective in the klotho gene, develop multiple age-related disorders with very short lifespans. Introduction of the exogenous klotho gene into these mutant mice leads to an improvement in their phenotypes, while overexpression of this gene in wild-type mice significantly extends their lifespan. These observations suggest that the klotho gene/protein has an anti-aging function. Since there have been only a few reports with some disagreement about results on the CNS of the mutant mice, we tried to clarify whether the CNS neurons generate aging-like features, even in premature stages, using biochemical and morphological approaches. Results obtained from the mutant mice, when compared with wild-type mice, were as follows. Neurofilaments (NFs) were increased significantly in axons, with the subunit proteins showing a significant enhancement in phosphorylation or expression of NF-H or NF-L, respectively. Microtubules in Purkinje cell dendrites were closer to each other, and in the CNS tissue tubulin was unaltered, but microtubule-associated protein (MAP) 2 was significantly reduced in expression. Neuronal cellular organelles were morphologically disordered. Lysosomes, cathepsin D and light chain 3 of MAP1A/B (LC3) were augmented with the appearance of putative autophagy-related structures. Antiapoptotic Bcl-xL and proapoptotic Bax were reduced and enhanced, respectively, and mitogen-activated protein kinase was reduced. Synapse-related proteins and structures were decreased. Neuronal degeneration was evident in hippocampal pyramidal cells, and possibly in Purkinje cells. Astrocytic glial filaments and glial fibrillary acidic protein were increased in density and expression, respectively. Together, the CNS neuronal alterations in klotho mutant mice were quite similar to those found in aged animals, including even premature death, so this mouse should be a more appropriate animal model for CNS aging than those previously reported.

NMDA and non-NMDA receptor-mediated differential Ca2+ load and greater vulnerability of motor neurons in spinal cord cultures

Glutamate receptor activated neuronal cell death has been implicated in the pathogenesis of motor neuron disease but the molecular mechanism responsible for neuronal dysfunction needs to be elucidated. In the present study, we examined the contribution of NMDA and non-NMDA sub-types of glutamate receptors in selective vulnerability of motor neurons. Glutamate receptor activated Ca2+ signaling, mitochondrial functions and neurotoxicity in motor neurons and other spinal neurons were studied in mixed spinal cord primary cultures. Exposure of cells to glutamate receptor agonists glutamate, NMDA and AMPA elevated the intracellular Ca2+, mitochondrial Ca2+ and caused mitochondrial depolarization and cytotoxicity in both motor neurons and other spinal neurons but a striking difference was observed in the magnitude and temporal patterns of the [Ca2+]i responses between the two neuronal cell types. The motor neurons elicited higher Ca2+ load than the other spinal neurons and the [Ca2+]i levels were elevated for a longer duration in motor neurons. AMPA receptor stimulation was more effective than NMDA. Both the NMDA and non-NMDA receptor antagonists APV and NBQX inhibited the Ca2+ entry and decreased the cell death significantly; however, NBQX was more potent than APV. Our results demonstrate that both NMDA and non-NMDA sub-types of glutamate receptors contribute to glutamate-mediated motor neuron damage but AMPA receptors play the major role. AMPA receptor-mediated excessive Ca2+ load and differential handling/regulation of Ca2+ buffering by mitochondria in motor neurons could be central in their selective vulnerability to excitotoxicity.

Enprofylline protects motor neurons from in vitro excitotoxic challenge

BACKGROUND

The death of motor neurons in amyotrophic lateral sclerosis (ALS) is believed to result, in part, from unrestrained activation of glutamate receptors (excitotoxicity). In some in vitro models, excitotoxic death only occurs if motor neurons develop in the presence of the growth factor, brain-derived neurotrophic factor (BDNF).

OBJECTIVE

Since the increased vulnerability of motor neurons evoked by BDNF is mediated by activation of TrkB, we sought to identify pharmacological agents that can block this pathway. Adenosine receptors are known to transactivate Trk receptors, leading us to examine the effects of manipulating of adenosine receptor signaling on Trk signaling and excitotoxic sensitivity.

METHODS

Spinal cord cultures were treated with adenosine receptor agonists and antagonists. The biochemical effects on Trk signaling and excitotoxic motor neuron death were examined.

RESULTS

We show here that adenosine A(2a) antagonists can reduce activation of Trk receptors and are neuroprotective. Conversely, activating adenosine A(2a) receptors in the absence of BDNF signaling makes motor neurons vulnerable to excitotoxic challenge.

CONCLUSION

Selective, high-affinity adenosine A(2a) antagonists merit consideration as therapeutic agents for the treatment of ALS.

BMAA selectively injures motor neurons via AMPA/kainate receptor activation

The toxin beta-methylamino-l-alanine (BMAA) has been proposed to contribute to amyotrophic lateral sclerosis-Parkinsonism Dementia Complex of Guam (ALS/PDC) based on its ability to induce a similar disease phenotype in primates and its presence in cycad seeds, which constituted a dietary item in afflicted populations. Concerns about the apparent low potency of this toxin in relation to estimated levels of human ingestion led to a slowing of BMAA research. However, recent reports identifying potential new routes of exposure compel a re-examination of the BMAA/cycad hypothesis. BMAA was found to induce selective motor neuron (MN) loss in dissociated mixed spinal cord cultures at concentrations ( approximately 30 muM) significantly lower than those previously found to induce widespread neuronal degeneration. The glutamate receptor antagonist NBQX prevented BMAA-induced death, implicating excitotoxic activation of AMPA/kainate receptors. Using microfluorimetric techniques, we further found that BMAA induced preferential [Ca(2+)](i) rises and selective reactive oxygen species (ROS) generation in MNs with minimal effect on other spinal neurons. Cycad seed extracts also triggered preferential AMPA/kainate-receptor-dependent MN injury, consistent with the idea that BMAA is a crucial toxic component in this plant. Present findings support the hypothesis that BMAA may contribute to the selective MN loss in ALS/PDC.

p35/p25 is not essential for tau and cytoskeletal pathology or neuronal loss in Niemann-Pick type C disease

Hyperactivation of the cyclin-dependent kinase 5 (cdk5), triggered by proteolytic conversion of its neuronal activator, p35, to a more potent byproduct, p25, has been implicated in Alzheimer's disease (AD), amyotrophic lateral sclerosis, and Niemann-Pick type C disease (NPC). This mechanism is thought to lead to the development of neuropathological hallmarks, i.e., hyperphosphorylated cytoskeletal proteins, neuronal inclusions, and neurodegeneration, that are common to all three diseases. This pathological ensemble is recapitulated in a single model, the npc-1 (npc(-/-)) mutant mouse. Previously, we showed that pharmacological cdk inhibitors dramatically reduced hyperphosphorylation, lesion formation, and locomotor defects in npc(-/-) mice, suggesting that cdk activity is required for NPC pathogenesis. Here, we used genetic ablation of the p35 gene to examine the specific involvement of p35, p25, and hence cdk5 activation in NPC neuropathogenesis. We found that lack of p35/p25 does not slow the onset or progression or improve the neuropathology of NPC. Our results provide direct evidence that p35/p25-mediated cdk5 deregulation is not essential for NPC pathology and suggest that similar pathology in AD may also be cdk5 independent.

Phosphorylation of c-Jun N-terminal kinase (JNK) in sensory neurones of diabetic rats, with possible effects on nerve conduction and neuropathic pain: prevention with an aldose reductase inhibitor

AIMS/HYPOTHESIS

This study was designed to determine whether diabetes in rats is associated with phosphorylation of c-Jun N-terminal kinase (JNK) and one of its transcription factors, c-Jun, in sensory neurones innervating the lower limb. We also sought to determine which neuronal phenotypes are affected and to examine the effect of aldose reductase inhibition on JNK and c-Jun phosphorylation.

METHODS

Diabetes was induced in rats using streptozotocin. Phosphorylation of JNK and c-Jun in lumbar dorsal root ganglia was determined by binding of phospho-specific antibodies using western blots and immunocytochemistry. Neuronal phenotypes were characterised by binding of N52 (an antibody that recognises the heavy neurofilament protein; for large-diameter mechanoceptors) and of calcitonin gene-related peptide and the plant glycoprotein lectin IB4 (for nociceptors). The efficacy of the aldose reductase inhibitor fidarestat was determined by measuring polyol pathway metabolites in sciatic nerve, and functionally by measuring the conduction velocity of motor and sensory nerves.

RESULTS

In control rats, activated JNK and c-Jun were primarily associated with mechanoceptors; in diabetes this was increased, but a greater increase was seen in nociceptors. Phosphorylation was prevented in all cells by fidarestat, which normalised polyol pathway metabolites as well as motor nerve and sensory nerve conduction velocity.

CONCLUSIONS/INTERPRETATION

Fidarestat-sensitive phosphorylation of JNK and c-Jun occurs in fast-conduction mechanoceptors-the same class of neurones that registers the changes in sensory nerve conduction velocity-and in nociceptors. This supports the notion that mitogen-activated protein kinase phosphorylation, via the polyol pathway, may convert the direct effects of raised glucose into impaired nerve conduction and neuropathic pain. For proof of this we await the availability of specific JNK antagonists formulated for in vivo use.

The high and middle molecular weight neurofilament subunits regulate the association of neurofilaments with kinesin: inhibition by phosphorylation of the high molecular weight subunit

Kinesin participates in axonal transport of neurofilaments (NFs), but the mode by which they attach to kinesin is unclear. We compared the association of NFs with kinesin in mice expressing or lacking NF-H or NF-M. In normal and M-/- mice, the leading edge of metabolically labeled NF subunits was selectively co-precipitated with kinesin. By contrast, the entire wave of radiolabeled subunits co-precipitated with kinesin in H-/- mice. Similar bulk levels of NFs co-precipitated with kinesin from normal and H-/- mice, but reduced levels co-precipitated from M-/- mice. These data suggest that both NF-H and NF-M regulate the association of NFs with kinesin. They further indicate that phosphorylation of NF-H dissociates NFs from kinesin and provides a mechanism by which NF-H phosphorylation can contribute to the slowing of NF axonal transport.

Cerebrospinal fluid from amyotrophic lateral sclerosis patients preferentially elevates intracellular calcium and toxicity in motor neurons via AMPA/kainate receptor

Several lines of evidence in the literature purport the contribution of glutamate mediated excitotoxicity in the etiology of amyotrophic lateral sclerosis (ALS) but the cellular mechanisms responsible for selective loss of motor neurons are still obscure. Elevation of intracellular Ca(2+) is considered as the early event in glutamate mediated cell injury. We have studied the changes in [Ca(2+)](i) and cytotoxicity in motor neurons and other spinal neurons in culture upon exposure to cerebrospinal fluid (CSF) from ALS patients. CSFs from 20 ALS patients and 20 disease control patients were examined. Eighteen out of twenty (90%) ALS-CSF samples induced a transient but pronounced elevation of [Ca(2+)](i) in neurons, whereas only 1/20 (5%) sample from disease control patients induced a marginal elevation of [Ca(2+)](i). Strikingly the [Ca(2+)](i) rise was 2-3-fold higher and longer lasting in motor neurons in comparison to the other spinal neurons. Exposure of cells to ALS-CSF drastically decreased the survival rate of motor neurons to 32.26+/-2.06% whereas a moderate decrease was observed in case of other spinal neurons (67.90+/-2.04%). In cultures treated with disease control CSF, a small decrease was observed in the survival rate with 80.14+/-2.00% and 90.07+/-1.37% survival of motor neuron and other spinal neurons respectively. The AMPA/kainate receptor antagonist NBQX rendered significant protection against the ALS-CSF induced Ca(2+) influx and neurotoxicity while the NMDA receptor antagonist APV showed a mild effect. Our data demonstrate that the exposure of spinal cord neurons to ALS-CSF differentially elevates [Ca(2+)](i) and neurotoxicity in motor neurons by activation of glutamate receptors, the AMPA/kainate receptor playing the major role.

2,5-Hexanedione-induced changes in the neurofilament subunit pools of rat peripheral nerve

Axon atrophy is the principle morphological feature of the peripheral neuropathy induced by 2,5-hexanedione (HD). Axon caliber is determined by a stationary neurofilamentous cytoskeleton that is maintained through dynamic interactions with mobile neurofilament (NF) subunits. To determine the effects of HD on the stationary and mobile NF pools, groups of rats were exposed to HD at dosing schedules (175 mg/kg x 101 days or 400 mg/kg x 26 days) that produced moderate levels of neurological deficits and, as assessed by previous studies, prevalent axon atrophy in peripheral nerve. Sciatic and tibial nerves from HD-intoxicated rats and their age-matched controls were triton-extracted and separated by differential centrifugation into a high-speed pellet (P1) of NF polymer and a corresponding supernatant fraction (S1), which presumably contained mobile monomer. Cytoskeletal proteins (NF-L, NF-M, NF-H and beta-tubulin) in each fraction were determined by immunoblot analysis. Results show that regardless of HD dose-rate, triton-soluble NF subunits in the supernatant fractions were significantly reduced, whereas triton-insoluble proteins in the corresponding pellets were inconsistently affected. Beta-tubulin also exhibited inconsistent fractional changes, while abnormal higher molecular weight NF proteins were detected primarily in the triton-insoluble fraction. Studies with antibodies directed against phosphorylated (RT97) and non-phosphorylated (SMI32) epitopes on NF-H did not reveal major changes in subunit phosphorylation. These results suggest that HD intoxication is primarily associated with depletion of soluble NF proteins, which could produce axon atrophy through disruption of cytoskeletal turnover and maintenance.

2,5-Hexanedione-induced changes in the monomeric neurofilament protein content of rat spinal cord fractions

Quantitative morphometric analyses have demonstrated that axon atrophy is the primary neuropathic feature in the CNS and PNS of rats intoxicated with 2,5-hexanedione (HD). Axon caliber is maintained by the exchange of mobile neurofilament (NF) subunits with the stationary polymer and, therefore, HD might produce atrophy by disrupting cytoskeletal turnover. To evaluate this possibility, groups of rats were exposed to HD at dosing schedules (175 mg/kg x 101 days or 400 mg/kg x 26 days) that produced moderate levels of neurological deficits and prevalent axon atrophy in spinal cord white matter tracts. Lumbar spinal cord regions from HD-intoxicated rats and their age-matched controls were Triton-extracted and separated by differential fractionation into a low-speed, insoluble pellet (P1) of NF polymer and a high-speed supernatant fraction (S2), which presumably contained mobile monomer. Cytoskeletal protein contents (NF-L, -M, -H, and beta-tubulin) in each fraction were determined by immunoblot analysis. Results show that regardless of HD dose-rate, the NF polymer in P1 remained unaffected, although soluble monomer in the S2 fraction was depleted significantly (60-80% reduction). Fractional beta-tubulin contents were inconsistently affected and abnormal higher-molecular-weight NF proteins were detected in the P1 fraction only. Studies with antibodies directed against phosphorylated (RT97) and nonphosphorylated (SMI32) epitopes on NF-H and measurements of corresponding isoelectric range suggested that alterations in phosphorylation were not involved. The selective depletion of Triton-soluble protein suggested that HD adduction of NFs interfered with the dynamic interactions of the polymeric and mobile monomeric pools.

Intermediate filaments mediate cytoskeletal crosstalk

Intermediate filaments, actin-containing microfilaments and microtubules are the three main cytoskeletal systems of vertebrate and many invertebrate cells. Although these systems are composed of distinctly different proteins, they are in constant and intimate communication with one another. Understanding the molecular basis of this cytoskeletal crosstalk is essential for determining the mechanisms that underlie many cell-biological phenomena. Recent studies have revealed that intermediate filaments and their associated proteins are important components in mediating this crosstalk.

The dynamic and motile properties of intermediate filaments

For many years, cytoplasmic intermediate filaments (IFs) were considered to be stable cytoskeletal elements contributing primarily to the maintenance of the structural and mechanical integrity of cells. However, recent studies of living cells have revealed that IFs and their precursors possess a remarkably wide array of dynamic and motile properties. These properties are in large part due to interactions with molecular motors such as conventional kinesin, cytoplasmic dynein, and myosin. The association between IFs and motors appears to account for much of the well-documented molecular cross talk between IFs and the other major cytoskeletal elements, microtubules, and actin-containing microfilaments. Furthermore, the associations with molecular motors are also responsible for the high-speed, targeted delivery of nonfilamentous IF protein cargo to specific regions of the cytoplasm where they polymerize into IFs. This review considers the functional implications of the motile properties of IFs and discusses the potential relationships between malfunctions in these motile activities and human diseases.

Regulation of the transition from vimentin to neurofilaments during neuronal differentiation

Vimentin (Vm) is initially expressed by nearly all neuronal precursors in vivo, and is replaced by neurofilaments (NFs) shortly after the immature neurons become post-mitotic. Both Vm and NFs can be transiently detected within the same neurite, and Vm is essential for neuritogenesis at least in culture. How neurons effect the orderly transition from expression of Vm as their predominant intermediate filament to NFs remains unclear. We examined this phenomenon within growing axonal neurites of NB2a/d1 cells. Transfection of cells with a construct expressing Vm conjugated to green fluorescent protein confirmed that axonal transport machinery for Vm persisted following the developmental decrease in Vm, but that the amount undergoing transport decreased in parallel to the observed developmental increase in NF transport. Immunoprecipitation from pulse-chase radiolabeled cells demonstrated transient co-precipitation of newly synthesized NF-H with Vm, followed by increasing co-precipitation with NF-L. Immunofluorescent and immuno-electron microscopic analyses demonstrated that some NF and Vm subunits were incorporated into the same filamentous profiles, but that Vm was excluded from the longitudinally-oriented "bundle" of closely-apposed NFs that accumulates within developing axons and is known to undergo slower turnover than individual NFs. These data collectively suggest that developing neurons are able to replace their Vm-rich cytoskeleton with one rich in NFs simply by down-regulation of Vm expression and upregulation of NFs, coupled with turnover of existing Vm filaments and Vm-NF heteropolymers.

Neuromuscular development after the prevention of naturally occurring neuronal death by Bax deletion

The removal of excess neurons by programmed cell death (PCD) is believed to be critical for the proper development and function of the nervous system. A major role of this neuronal loss is to attain quantitative matching of neurons with their targets and afferents. Because motoneurons (MNs) in Bax knock-out (Bax KO) mice fail to undergo PCD in the face of normal target muscle development, we asked whether the excess rescued neurons in Bax KO mice can develop normally. We observed many small atrophied MNs in postnatal Bax KO mice, and these failed to innervate limb muscle targets. When examined embryonically during the PCD period, however, these excess MNs had initiated target innervation. To examine whether a limitation in trophic factor availability is responsible for postnatal MN atrophy and loss of innervation, we applied glial cell line-derived neurotrophic factor (GDNF) to neonatal mice. GDNF injection for 7-14 d induced the regrowth and reinnervation of muscle targets by atrophic MNs in Bax KO mice and prevented the normal postnatal death of MNs in wild-type mice. These results indicate that, although initially all of the MNs, including those rescued by Bax deletion, are able to project to and innervate targets, because of limited target-derived signals required for maintaining innervation and growth, only a subpopulation can grow and retain target contacts postnatally. Although sensory neurons in the dorsal root ganglia are also rescued from PCD by Bax deletion, their subsequent development is less affected than that of MNs.

A specific ELISA for measuring neurofilament heavy chain phosphoforms

Neurofilaments (Nf) are the major constitutents of the axoskeleton and body fluid Nf levels are an important tool for estimating axonal degeneration in vivo. This paper presents a new sandwich ELISA allowing quantification of the NfH(SMI35) phosphoform from CSF, brain tissue and cell culture homogenates. The sensitivity of the NfH(SMI35) ELISA is 0.2 ng/ml with a recovery of 119% and a mean within- and between-batch precision of 10.6% and 23%, respectively. CSF NfH(SMI35) was stable at 4 degrees C, is not influenced by freeze-thaw cycles, and proteolysis present at room temperature could be prevented by adding protease inhibitors. Aggregate formation was observed for HPLC-purified bovine NfH and could be resolved by sonication. The upper reference value for CSF NfH(SMI35) levels (0.73 ng/ml) was defined as the 95% cumulative frequency from 416 CSF samples. Based on this cutoff, a significantly higher proportion of patients with amyotrophic lateral sclerosis, space-occupying lesions, disc prolapse and subarachnoid haemorrhage had pathologically elevated NfH(SMI35) levels compared to patients with cluster headache or demyelinating disease.A new nomenclature is proposed to facilitate the comparison between ELISA, immunoblotting and immunocytochemistry.

Participation of neurofilament proteins in axonal dark degeneration of rat's optic nerves

Neurofilaments (NF) are neuronal intermediate filaments formed by three different subunits: high (NF-H), medium (NF-M) and light (NF-L). They are responsible for the determination and maintenance of axon caliber. Accumulation of NF or their immunoreactive products are components of several neurodegenerative disease lesions, such as neurofibrillary tangles, Lewy bodies and the spheroids of amyotrophic lateral sclerosis. Also, cytoskeletal breakdown is one of the first ultrastructural changes occurring after nerve crush or section. In the present study, Wistar rats were subjected to bilateral enucleation to induce Wallerian degeneration of optic nerve fibers and perfused 24 h, 48 h and 1 week later. Optic nerve segments were processed for electron microscopy (EM), light microscopy immunofluorescence (LM) and immunoelectronmicroscopy (IEM) for NF subunit detection. LM for NF of control nerves showed a slightly different pattern and intensity for each subunit, with more intense staining of NF-M and NF-H and less intense staining of NF-L. This reaction did not change considerably at 48 h, but was severely reduced 1 week after enucleation. Results of EM showed fibers in: (1) partial cytoskeleton degeneration or (2) watery degeneration or (3) dark degeneration. The number of dark degenerating axons was statistically higher at the latest time-interval studied. Neurofilament clumping areas and dark degenerating axons showed positive immunostaining for the three neurofilaments subunits when examined by IEM. These results suggest that dark degenerating axons develop from areas of neurofilament aggregation. We may also conclude that NF proteins participate in the process of axonal dark degeneration.

BDNF heightens the sensitivity of motor neurons to excitotoxic insults through activation of TrkB

The survival promoting and neuroprotective actions of brain-derived neurotrophic factor (BDNF) are well known but under certain circumstances this growth factor can also exacerbate excitotoxic insults to neurons. Prior exploration of the receptor through which BDNF exerts this action on motor neurons deflects attention away from p75. Here we investigated the possibility that BDNF acts through the receptor tyrosine kinase, TrkB, to confer on motor neurons sensitivity to excitotoxic challenge. We blocked BDNF activation of TrkB using a dominant negative TrkB mutant or a TrkB function blocking antibody, and found that this protected motor neurons against excitotoxic insult in cultures of mixed spinal cord neurons. Addition of a function blocking antibody to BDNF to mixed spinal cord neuron cultures is also neuroprotective indicating that endogenously produced BDNF participates in vulnerability to excitotoxicity. We next examined the intracellular signaling cascades that are engaged upon TrkB activation. Previously we found that inhibition of the phosphatidylinositide-3'-kinase (PI3'K) pathway blocks BDNF-induced excitotoxic sensitivity. Here we show that expression of a constitutively active catalytic subunit of PI3'K, p110, confers excitotoxic sensitivity (ES) upon motor neurons not incubated with BDNF. Parallel studies with purified motor neurons confirm that these events are likely to be occuring specifically within motor neurons. The abrogation of BDNF's capacity to accentuate excitotoxic insults may make it a more attractive neuroprotective agent.

Chronic estrogen deficiency leads to molecular aberrations related to neurodegenerative changes in follitropin receptor knockout female mice

The follitropin receptor knockout (FORKO) mouse undergoes ovarian failure, thereby providing an animal model to investigate the consequences of the depletion of circulating estrogen in females. The estrogen deficiency causes marked defects in the female reproductive system, obesity, and skeletal abnormalities. In light of estrogen's known pleiotropic effects in the nervous system, our study examined the effects of genetically induced estrogen-testosterone imbalance on this system in female FORKO mice. Circulating concentrations of 17-beta-estradiol (E2) in FORKO mice are significantly decreased (FORKO -/-: 1.13+/-0.34 pg/ml; wild-type +/+: 17.6+/-3.5 pg/ml, P<0.0001, n=32-41); in contrast, testosterone levels are increased (-/-: 37.7+/-2.3 pg/ml; wild-type +/+: 3.9+/-1.7 pg/ml, P<0.005, n=25-33). The focus was on the activities of key enzymes in the central cholinergic and peripheral nervous systems, on dorsal root ganglia (DRGs) capacity for neurite outgrowth, and on the phosphorylation state of structural neurofilament (NF) proteins. Choline acetyltransferase activity was decreased in several central cholinergic structures (striatum 50+/-3%, hippocampus 24+/-2%, cortex 12+/-3%) and in DRGs (11+/-6%). Moreover, we observed aberrations in the enzymatic activities of mitogen-activated protein kinases (extracellular-regulated kinase and c-Jun N-terminal kinase) in the hippocampus, DRGs, and sciatic nerves. Hippocampal and sensory ganglia samples from FORKO mice contained hyper-phosphorylated NFs. Finally, explanted ganglia of FORKO mice displayed decreased neurite outgrowth (20-50%) under non-treated conditions and when treated with E2 (10 nM). Our results demonstrate that genetic depletion of circulating estrogen leads to biochemical and morphological changes in central and peripheral neurons, and underlie the importance of estrogen in the normal development and functioning of the nervous system. In particular, the findings suggest that an early and persisting absence of the steroid leads to neurodegenerative changes and identify several key enzymes that may contribute to the process. This model provides a system to explore the consequences of circulating estrogen deprivation and other hormonal imbalances in the nervous system.

Neurofilaments consist of distinct populations that can be distinguished by C-terminal phosphorylation, bundling, and axonal transport rate in growing axonal neurites

We examined the steady-state distribution and axonal transport of neurofilament (NF) subunits within growing axonal neurites of NB2a/d1 cells. Ultrastructural analyses demonstrated a longitudinally oriented "bundle" of closely apposed NFs that was surrounded by more widely spaced individual NFs. NF bundles were recovered during fractionation and could be isolated from individual NFs by sedimentation through sucrose. Immunoreactivity toward the restrictive C-terminal phospho-dependent antibody RT97 was significantly more prominent on bundled than on individual NFs. Microinjected biotinylated NF subunits, GFP-tagged NF subunits expressed after transfection, and radiolabeled endogenous subunits all associated with individual NFs before they associated with bundled NFs. Biotinylated and GFP-tagged NF subunits did not accumulate uniformly along bundled NFs; they initially appeared within the proximal portion of the NF bundle and only subsequently were observed along the entire length of bundled NFs. These findings demonstrate that axonal NFs are not homogeneous but, rather, consist of distinct populations. One of these is characterized by less extensive C-terminal phosphorylation and a relative lack of NF-NF interactions. The other is characterized by more extensive C-terminal NF phosphorylation and increased NF-NF interactions and either undergoes markedly slower axonal transport or does not transport and undergoes turnover via subunit and/or filament exchange with individual NFs. Inhibition of phosphatase activities increased NF-NF interactions within living cells. These findings collectively suggest that C-terminal phosphorylation and NF-NF interactions are responsible for slowing NF axonal transport.

Local control of neurofilament accumulation during radial growth of myelinating axons in vivo. Selective role of site-specific phosphorylation

The accumulation of neurofilaments required for postnatal radial growth of myelinated axons is controlled regionally along axons by oligodendroglia. Developmentally regulated processes previously suspected of modulating neurofilament number, including heavy neurofilament subunit (NFH) expression, attainment of mature neurofilament subunit stoichiometry, and expansion of interneurofilament spacing cannot be primary determinants of regional accumulation as we show each of these factors precede accumulation by days or weeks. Rather, we find that regional neurofilament accumulation is selectively associated with phosphorylation of a subset of Lys-Ser-Pro (KSP) motifs on heavy neurofilament subunits and medium-size neurofilament subunits (NFMs), rising >50-fold selectively in the expanding portions of optic axons. In mice deleted in NFH, substantial preservation of regional neurofilament accumulation was accompanied by increased levels of the same phosphorylated KSP epitope on NFM. Interruption of oligodendroglial signaling to axons in Shiverer mutant mice, which selectively inhibited this site-specific phosphorylation, reduced regional neurofilament accumulation without affecting other neurofilament properties or aspects of NFH phosphorylation. We conclude that phosphorylation of a specific KSP motif triggered by glia is a key aspect of the regulation of neurofilament number in axons during axonal radial growth.

Hypophosphorylated neurofilament subunits undergo axonal transport more rapidly than more extensively phosphorylated subunits in situ

Axonal transport of neurofilaments (NFs) has long been considered to be regulated by phosphorylation. We present evidence that in optic axons of normal mice, the rate of NF axonal transport is inversely correlated with the NF phosphorylation state. In addition to 200 kDa NF-H and 145 kDa NF-M, axonal cytoskeletons from CNS contained a range of phospho-variants of NF-H migrating between 160-200 kDa, and of NF-M migrating at 97-145 kDa. While 160 kDa phospho-variants of NF-H have been well characterized, we confirmed the identity of the previously-described 97 kDa species as a hypophospho-variant of NF-M since (1) pulse-chase metabolic labeling confirmed the 97 kDa species to be a new synthesis product that was converted by phosphorylation over time into a form migrating at 145 kDa, (2) the 97 kDa protein reacted with multiple NF-M antibodies, including one specific for hypophosphorylated NF-M, and (3) dephosphorylation converted NF-M isoforms to 97 kDa. Autoradiographic analyses following metabolic radiolabeling demonstrated that hypophosphorylated NF-H and NF-M isoforms underwent substantially more rapid transport in situ than did extensively phosphorylated isoforms, while NF-H subunits bearing a developmentally delayed C-terminal phospho-epitope transported at a rate slower than that of total 200 kDa NF-H. Differential transport of phospho-variants also highlights that these variants are not homogeneously distributed among NFs, but are segregated to some extent among distinct, although probably overlapping, NF populations, indicating that axonal NFs are not homogeneous with respect to phosphorylation state.

Motor neurons are rich in non-phosphorylated neurofilaments: cross-species comparison and alterations in ALS

The localization and distribution of non-phosphorylated neurofilaments (NP-NF) in the upper and lower motor neurons was investigated in the rat, the common marmoset, the rhesus monkey and man using the SMI-32 antibody. Within the spinal cord of all species studied, the most intense NP-NF immunoreactivity was observed within the ventral horn alpha-motor neurons. Concurrent staining for the cholinergic marker choline acetyltransferase (ChAT) demonstrated that virtually all of the ChAT-positive alpha-motor neurons contain NP-NF immunoreactivity. Although NP-NF staining was also observed in other neurons within the ventral and intermediate horns, these neurons were loosely scattered and contained a considerably lower staining intensity. The only other prominent NP-NF staining in the spinal cord occurred within the neurons of the dorsal nucleus of Clark and the intermediolateral cell column. Phosphorylated neurofilament (P-NF) immunoreactivity was found primarily in neuronal processes. Occasionally, a solitary motor neuron contained weak P-NF immunoreactivity. Within the brainstem, neurons in all cranial nerve motor nuclei contained intense NP-NF immunoreactivity. The distribution and apparent density of NP-NF immunoreactive neurons in these nuclei was virtually identical to that observed for neurons immunoreactive for ChAT. NP-NF immunoreactive neurons of relatively lower intensity were found in many other regions of the brainstem. All of the giant Betz cells of layer (L) V in the motor cortex contained dark NP-NF immunoreactivity. Within the spinal cord of amyotrophic lateral sclerosis (ALS) patients, both Nissl and NP-NF staining demonstrated the dramatic loss of alpha-motor neurons characteristic of this disorder. Some of the remaining motor neurons contained intense P-NF immunoreactivity. These observations suggest that NP-NF immunoreactivity is a good marker for motor neurons in health and disease and may be a useful tool for studies of motor neuron degeneration (MND).

AMPA exposures induce mitochondrial Ca(2+) overload and ROS generation in spinal motor neurons in vitro

The reason for the selective vulnerability of motor neurons in amyotrophic lateral sclerosis (ALS) is primarily unknown. A possible factor is the expression by motor neurons of Ca(2+)-permeable AMPA/kainate channels, which may permit rapid Ca(2+) influx in response to synaptic receptor activation. However, other subpopulations of central neurons, most notably forebrain GABAergic interneurons, consistently express large numbers of these channels but do not degenerate in ALS. Indeed, when subjected to identical excitotoxic exposures, motor neurons were more susceptible than GABAergic neurons to AMPA/kainate receptor-mediated neurotoxicity. Microfluorimetric studies were performed to examine the basis for the difference in vulnerability. First, AMPA or kainate exposures appeared to trigger substantial mitochondrial Ca(2+) loading in motor neurons, as indicated by a sharp increase in intracellular Ca(2+) after addition of the mitochondrial uncoupler carbonyl cyanide p-(trifluoromethoxy)phenyl hydrazone (FCCP) after the agonist exposure. The same exposures caused little mitochondrial Ca(2+) accumulation in GABAergic cortical neurons. Subsequent experiments examined other measures of mitochondrial function to compare sequelae of AMPA/kainate receptor activation between these populations. Brief exposure to either AMPA or kainate caused mitochondrial depolarization, assessed using tetramethylrhodamine ethylester, and reactive oxygen species (ROS) generation, assessed using hydroethidine, in motor neurons. However, these effects were only seen in the GABAergic neurons after exposure to the nondesensitizing AMPA receptor agonist kainate. Finally, addition of either antioxidants or toxins (FCCP or CN(-)) that block mitochondrial Ca(2+) uptake attenuated AMPA/kainate receptor-mediated motor neuron injury, suggesting that the mitochondrial Ca(2+) uptake and consequent ROS generation are central to the injury process.

Light and confocal microscopic studies of evolutionary changes in neurofilament proteins following cortical impact injury in the rat

Previous studies have shown that traumatic brain injury (TBI) produces progressive degradation of cytoskeletal proteins including neurofilaments (e.g., neurofilament 68 [NF68] and neurofilament 200 [NF200]) within the first 24 h after injury. Thus, we employed immunofluorescence (light and confocal microscopy) to study the histopathological correlates of progressive neurofilament protein loss observed at 15 min, 3 h, and 24 h following unilateral cortical injury in rats. TBI produced significant alterations in NF68 and NF200 immunolabeling in dendrites and cell bodies at contusion sites ipsilateral to injury, as well as in the noncontused contralateral cortex. Changes in immunolabeling were associated with, but not exclusively restricted to, regions previously shown to contain dark shrunken neurons labeled by hematoxylin and eosin staining, a morphopathological response to injury suggesting impending cell death. Immunofluorescence microscopic studies of neurofilament proteins in the ipsilateral cerebral cortex detected prominent fragmentation of apical dendrites of pyramidal neurons in layers 3-5 and loss of fine dendritic arborization within layer 1. While modest changes were observed 15 min following injury, more pronounced loss of dendritic neurofilament immunofluorescence was detected 3 and 24 h following injury. Confocal microscopy also revealed progressive alterations in NF68 immunoreactivity in dendrites following TBI. While some evidence of structural alterations was observed 15 min following TBI, dendritic breaks were readily detected in confocal micrographs from 3 to 24 h following injury. However, disturbances in axonal NF68 by immunofluorescence microscopy in the corpus callosum were not detected until 24 h after injury. These studies confirmed that derangements in dendritic neurofilament cytoskeletal proteins are not exclusively restricted to sites of impact contusion. Moreover, changes in dendritic cytoskeletal proteins are progressive and not fully expressed within the first 15 min following impact injury. These progressive dendritic disruptions are characterized by disturbances in the morphology of neurofilament proteins, resulting in fragmentation and focal loss of NF68 immunofluorescence within apical dendrites. In contrast, alterations in axonal cytoskeletal proteins are more restricted and delayed with no pronounced changes until 24 h after injury.

Survival and process regrowth of purified chick retinal ganglion cells cultured in a growth factor lacking medium at low density. Modulation by extracellular matrix proteins

Panning-purified retinal ganglion cells (RGCs) were cultured at low density in a chemical-defined growth factor (GF)-lacking medium on substrate of different extracellular matrix (ECM) proteins. The process regrowth under these severe conditions were evaluated by morphometric measurements and by cell ELISA (CELISA) performed for neurofilaments regardless of their phosphorylated state (NF-CELISA), or for phosphorylated neurofilaments (PNF-CELISA), to respectively assess process regrowth or axonal development. The development obtained in cultures performed on laminin was taken as standard to refer the other substrata. The cellular content of Thy-1 required for panning purification as well as the gangliotetraosylganglioside (GTOG) expression and the lack of the immunolabeling of the RA4 antigen strongly suggest that the purified RGCs were mature neurons. About 80% of the 7-day-old embryo (E7)-RGCs survived 4 days in culture on any substrate, including polylysine. Conversely, E10-RGCs in about 75% of cultures on polylysine did not survive for 4 days. E7-RGCs developed better on thrombospondin and vitronectin. E10-RGCs cultured on vitronectin grew better than on laminin; on thrombospondin and collagen, E10-RGCs grew like on laminin and on fibronectin they had a poor development. The values of PNF-CELISA obtained on vitronectin, collagen and fibronectin on E7-RGC cultures were significantly higher than on laminin, which are in agreement with the longer processes observed. The flavoridin disintegrin caused a dose-response inhibition on E7-RGC cultures on thrombospondin but not on laminin, suggesting on process regrowth, the integrin-thrombospondin interaction(s) are significantly involved, while on laminin, it is the non-integrin receptor(s) which are significant involved.

Abnormal expression of neurofilament proteins in dysmyelinating axons located in the central nervous system of jimpy mutant mice

Myelination in the peripheral nervous system is considered to increase the phosphorylation level of neurofilament proteins in the axon, resulting in an increase in axonal calibre. To understand the relationship between myelination and neurofilament proteins in axons, we examined jimpy mutant mice with a point mutation in the proteolipid protein gene and dysmyelination in the central nervous system. The jimpy mice exhibited a characteristic similarity in neurofilament nature to the myelin-deficient mice in the peripheral nervous system reported previously. The following novel results were obtained in the jimpy mice: dysmyelinated axons, in which the amount of non-phosphorylated neurofilament-H was drastically increased without a significant reduction of the phosphorylated form, compared with the control myelinated axons, did not suffer any decrease in their diameters. Expression levels of all neurofilament subunit proteins and their mRNAs were enhanced in the central nervous system tissue. Because the above biochemical data were obtained from the cytoskeletal fraction, at least some of the increased neurofilament-H and -M proteins appeared to be coassembled into neurofilaments but remained non-phosphorylated. Axonal neurofilaments of the jimpy were, probably due to this abnormal stoichiometry and phosphorylation state in neurofilaments, more compact and random in alignment with less prominent cross-bridges than those of the control, providing possible evidence for disturbing the axonal transport of other organelles. These results suggest that myelination regulates both the expression and phosphorylation of neurofilament proteins, and is essential for the cytoplasmic organization of myelinated axons.

Neurofilament metabolism in sporadic amyotrophic lateral sclerosis

Although the role of intraneuronal neurofilamentous aggregates in the pathogenesis of ALS is unknown, their presence forms a key neuropathological hallmark of the disease process. Conversely, the experimental induction of neurofilamentous aggregates in either neurotoxic or transgenic mice gives rise to motor system degeneration. To determine whether alterations in the physiochemical properties of NF are present in sporadic ALS, we purified NF subunit proteins from cervical spinal cord of ALS and age-matched control patients. The cytoskeleton-enriched, Triton X-100 insoluble fraction was further separated into individual NF subunits using hydroxyapatite HPLC. We observed no differences between control and ALS in the characteristics of NFH, including migration patterns on 2D-IEF, sensitivity to E. coli, alkaline phosphatase mediated dephosphorylation, peptide mapping, or proteolysis (calpain, calpain/calmodulin mediated, phosphorylated or dephosphorylated NFH). NFL showed no differences in 2D-IEF migration patterns, peptide mapping, or the extent of NFL nitrotyrosine immunoreactivity in either the Triton soluble or insoluble fractions. The latter observation demonstrated that NFL nitration is a ubiquitous occurrence in neurons and suggests that NFL might function as a sink for free reactive nitrating species. In contrast to the lack of differences in the post-translational processing of NF in ALS, we did observe a selective suppression of NFL steady state mRNA levels in the limb innervating lateral motor neuron column of ALS. This occurred in the absence of modifications in NFH, NFM or neuronal nitric oxide synthase (Type I NOS; nNOS) steady state mRNA levels. Coupled with previous observations of nNOS immunoreactivity co-localizing with NF aggregates in ALS motor neurons, this suggests activation of the nNOS enzyme complex in ALS, which would be predicted to contribute directly to the generation of reactive nitrating species. Given this, the isolated suppression of NFL steady state mRNA levels in ALS may indicate that ALS motor neurons are at an intrinsic deficit in the ability to buffer free reactive nitrating species.

Motor neuron degeneration is attenuated in bax-deficient neurons in vitro

Apoptosis plays a major role in motor neuron survival during developmental cell death, after axotomy, and in motor neuron diseases. Bax is the first member of the bcl-2 family shown to promote apoptosis. In the present study, we used the bax-deficient mouse model to determine the role of bax in motor neuron survival in vitro by using dissociated spinal cord cultures. This system enables the maturation of individual motor neurons in a controlled in vitro environment. Motor neurons were identified by using the antineurofilament antibody SMI-32 and the antitranscription factor antibody Islet1. Both antibodies labeled large motor neurons in wild-type and bax-null cultures. Differentiated wild-type cultures exhibited a reduction in long-term cultures of two- and fivefold in the number of SMI-32- and Islet1-positive cells, respectively. The reduction in the number of motor neurons was attenuated in bax -/- cultures. Bax deficiency also attenuated serum withdrawal- and kainate-induced apoptosis in motor neurons. For comparison, necrotic cell death led to significant motor neuron cell death in both wild-type and bax -/- cultures. In addition, bax deficiency did not induce proliferation of motor neuron precursors in vitro. This study indicates for the first time that bax has a dominant role in the survival of long-term cultured motor neurons.

Quantification of axonal damage in traumatic brain injury: affinity purification and characterization of cerebrospinal fluid tau proteins

Diffuse axonal injury is a primary feature of head trauma and is one of the most frequent causes of mortality and morbidity. Diffuse axonal injury is microscopic in nature and difficult or impossible to detect with imaging techniques. The objective of the present study was to determine whether axonal injury in head trauma patients could be quantified by measuring levels of CSF tau proteins. Tau proteins are structural microtubule binding proteins primarily localized in the axonal compartment of neurons. Monoclonal antibodies recognizing the form of tau found in the CSF of head trauma patients were developed by differential CSF hybridoma screening using CSF from head trauma and control patients. Clones positive for head trauma CSF tau proteins were used to characterize this form of tau and for ELISA development. Using the developed ELISA, CSF tau levels were elevated >1,000-fold in head trauma patients (mean, 1,519 ng/ml of CSF) when compared with patients with multiple sclerosis (mean, 0.014 ng/ml of CSF; p < 0.001), normal pressure hydrocephalus (nondetectable CSF tau), neurologic controls (mean, 0.031 ng/ml of CSF; p < 0.001), or nonneurologic controls (nondetectable CSF tau; p < 0.001). In head trauma, a relationship between clinical improvement and decreased CSF tau levels was observed. These data suggest that CSF tau levels may prove a clinically useful assay for quantifying the axonal injury associated with head trauma and monitoring efficacy of neuroprotective agents. Affinity purification of CSF tau from head trauma patients indicated a uniform cleavage of approximately 18 kDa from all six tau isoforms, reducing their apparent molecular sizes to 30-50 kDa. These cleaved forms of CSF tau consisted of the interior portion of the tau sequence, including the microtubule binding domain, as judged by cyanogen bromide digestion. Consistent with these data, CSF cleaved tau bound taxol-polymerized microtubules, indicating a functionally intact microtubule binding domain. Furthermore, epitope mapping studies suggested that CSF cleaved tau proteins consist of the interior portion of the tau sequence with cleavage at both N and C terminals.

Mechanism of selective motor neuronal death after exposure of spinal cord to glutamate: involvement of glutamate-induced nitric oxide in motor neuron toxicity and nonmotor neuron protection

In this study, we analyzed the mechanism of selective motor neuronal death, a characteristic of amyotrophic lateral sclerosis, using embryonic rat spinal cord culture. When dissociated cultures were exposed to low-level glutamate (Glu) coadministered with the Glu transporter inhibitor L-trans-pyrrolidine-2,4-decarboxylate (PDC) for 24 hours, motor neurons were selectively injured through N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA)/kainate receptors. Nitric oxide synthase (NOS) inhibitors attenuated this toxicity, and long-acting nitric oxide (NO) donors damaged motor neurons selectively. Nonmotor neurons survived after exposure to low-dose Glu/PDC, but Glu-induced toxicity was potentiated by coadministration of an NO-dependent guanylyl cyclase inhibitor. In addition, 8-bromo-cyclic GMP, a soluble cyclic GMP analogue, rescued nonmotor neurons, but not motor neurons, exposed to high-dose Glu/PDC. Twenty-four hours' incubation with PDC elevated the number of neuronal NOS-immunoreactive neurons by about twofold compared with controls, and a double-staining study, using the motor neuron marker SMI32, revealed that most of them were nonmotor neurons. These findings suggest that selective motor neuronal death caused by chronic low-level exposure to Glu is mediated by the formation of NO in nonmotor neurons, which inversely protects nonmotor neurons through the guanylyl cyclase-cyclic GMP cascade. Induction of neuronal NOS in nonmotor neurons might enhance both the toxicity of motor neurons and the protection of nonmotor neurons, which could explain the pathology of amyotrophic lateral sclerosis.

Regulation of neurofilament interactions in vitro by natural and synthetic polypeptides sharing Lys-Ser-Pro sequences with the heavy neurofilament subunit NF-H: neurofilament crossbridging by antiparallel sidearm overlapping

Neurofilaments are organised into parallel bundles in axons through crossbridges formed by lateral projections of neurofilament subunits. Pure neurofilaments form gels in vitro, consisting of interconnected parallel arrays of filaments regulated by the phosphorylation level of neurofilament subunits. Neurofilament-associated polypeptides sharing phosphorylated epitopes with the repetitive lysine-serine-proline (Lys-Ser-Pro) motifs of the neurofilament heavy subunit sidearm are characterised: they regulate in vitro the neurofilament gelation kinetics in a concentration- and phosphorylation-dependent manner. Studies with synthetic peptides show that interactions between neurofilaments involve both acid and base amino acid residues of neurofilament sidearms and demonstrate the opposite effects of peptides containing either one (inhibition) or two (activation) Lys-Ser-Pro motifs. Electron microscopy reveals an organised network of native neurofilament sidearms, regulated by the phosphorylation level of neurofilament subunits, suggesting a structural transition between intra- and inter-neurofilament sidearm interactions. These results favour the hypothesis of a mechanism of neurofilament crossbridging through the variable antiparallel overlapping of the phosphorylable Lys-Ser-Pro domains of neurofilament sidearms from adjacent filaments, following an equilibrium regulated by neurofilament-associated proteins, bivalent cations and the phosphorylation level of Lys-Ser-Pro motifs from both neurofilament sidearms and neurofilament-associated proteins.

The role of glycoproteins in neural development function, and disease

Glycoproteins play key roles in the development, structuring, and subsequent functioning of the nervous system. However, the complex glycosylation process is a critical component in the biosynthesis of CNS glycoproteins that may be susceptible to the actions of toxicological agents or may be altered by genetic defects. This review will provide an outline of the complexity of this glycosylation process and of some of the key neural glycoproteins that play particular roles in neural development and in synaptic plasticity in the mature CNS. Finally, the potential of glycoproteins as targets for CNS disorders will be discussed.

Contiguous phosphorylated and non-phosphorylated domains along axonal neurofilaments

I have investigated the phosphorylation state of the medium molecular mass neurofilament protein (NF-M) along axonal neurofilaments. Cultured embryonic sensory neurons were treated with non-ionic detergent to cause the cytoskeletal polymers to splay apart from each other. Neurofilaments were visualized by double-label immunofluorescence microscopy and the proportion of their length that stained with various NF-M antibodies was determined using digital image analysis techniques. Monoclonal antibody RMO255, which binds to NF-M independently of phosphorylation state, stained an average of 98% of the neurofilament length. In contrast, monoclonal antibody RMO55, which binds specifically to a phosphorylated epitope on NF-M, stained some neurofilaments completely, some not at all, and some along part of their length. These partly stained neurofilaments exhibited single or multiple discrete segments of staining along their length separated by segments that were unstained. The average proportion of the neurofilament length that stained with this antibody was lowest proximally (12–22%, n=3) and increased along the axon to reach a maximum distally (58–87%, n=3). A converse pattern (77–87% proximally and 2–9% distally, n=3) was observed for neurons stained with monoclonal antibody FNP7, which binds to specifically to a non-phosphorylated epitope in both NF-M and the high molecular mass neurofilament protein, NF-H. Analysis of the staining of individual neurofilaments revealed a bimodal frequency distribution in which neurofilaments were more likely to be phosphorylated along either all or none of their length than along part of their length. These observations indicate that: (a) phosphorylated and non-phosphorylated neurofilaments can coexist side-by-side in these axons, (b) neurofilaments can be composed of single or multiple contiguous phosphorylated and non-phosphorylated epitope domains along their length, (c) the proportion of the neurofilament length that is phosphorylated at these epitopes increases along the axon in a proximal-to-distal manner, and (d) the pattern of phosphorylation is non-random, generating populations of phosphorylated and non-phosphorylated neurofilaments and discrete phosphorylated and non-phosphorylated domains along individual neurofilaments.

A mechanistic analysis of nondisruptive axonal injury: a review

Axons are particularly at risk in human diffuse head injury. Use of immunocytochemical labeling techniques has recently demonstrated that axonal injury (AI) and the ensuing reactive axonal change is, probably, more widespread and occurs over a longer posttraumatic time in the injured brain than had previously been appreciated. But the characterization of morphologic or reactive changes occurring after nondisruptive AI has largely been defined from animal models. The comparability of AI in animal models to human diffuse AI (DAI) is discussed and the conclusion drawn that, although animal models allow the analysis of morphologic changes, the spatial distribution within the brain and the time course of reactive axonal change differs to some extent both between species and with the mode of brain injury. Thus, the majority of animal models do not reproduce exactly the extent and time course of AI that occurs in human DAI. Nonetheless, these studies provide good insight into reactive axonal change. In addition, there is developing in the literature considerable variance in the terminology applied to injured axons or nerve fibers. We explain our current understanding of a number of terms now present in the literature and suggest the adoption of a common terminology. Recent work has provided a consensus that reactive axonal change is linked to pertubation of the axolemma resulting in disruption of ionic homeostatic mechanisms within injured nerve fibers. But quantitative data for changes for different ion species is lacking and is required before a better definition of this homeostatic disruption may be provided. Recent studies of responses by the axonal cytoskeleton after nondisruptive AI have demonstrated loss of axonal microtubules over a period up to 24 h after injury. The biochemical mechanisms resulting in loss of microtubules are, hypothetically, mediated both by posttraumatic influx of calcium and activation of calmodulin. This loss results in focal accumulation of membranous organelles in parts of the length of damaged axons where the axonal diameter is greater than normal to form axonal swellings. We distinguish, on morphologic grounds, between axonal swellings and axonal bulbs. There is also a growing consensus regarding responses by neurofilaments after nondisruptive AI. Initially, and rapidly after injury, there is reduced spacing or compaction of neurofilaments. This compaction is stable over at least 6 h and results from the loss or collapse of neurofilament sidearms but retention of the filamentous form of the neurofilaments. We posit that sidearm loss may be mediated either through proteolysis of sidearms via activation of microM calpain or sidearm dephosphorylation via posttraumatic, altered interaction between protein phosphatases and kinase(s), or a combination of these two, after calcium influx, which occurs, at least in part, as a result of changes in the structure and functional state of the axolemma. Evidence for proteolysis of neurofilaments has been obtained recently in the optic nerve stretch injury model and is correlated with disruption of the axolemma. But the earliest posttraumatic interval at which this was obtained was 4 h. Clearly, therefore, no evidence has been obtained to support the hypothesis that there is rapid, posttraumatic proteolysis of the whole axonal cytoskeleton mediated by calpains. Rather, we hypothesize that such proteolysis occurs only when intra-axonal calcium levels allow activation of mM calpain and suggest that such proteolysis, resulting in the loss of the filamentous structure of neurofilaments occurs either when the amount of deformation of the axolemma is so great at the time of injury to result in primary axotomy or, more commonly, is a terminal degenerative change that results in secondary axotomy or disconnection some hours after injury.