Aluminum alters the electrophoretic properties of neurofilament proteins: role of phosphorylation state

Aluminum induces neurofilament aggregation by stabilizing cross-bridging of phosphorylated c-terminal sidearms

Exposure to neurotoxin aluminum neurotoxicity is accompanied by the perikaryal accumulation of tangles of phosphorylated neurofilaments (NFs). We examined their formation and reversibility under cell-free conditions. AlCl3 induced dose-dependent formation of NF aggregates, ultimately incorporating 100% of detectable NFs. The same concentration of CaCl2 induced approximately 25% of NFs to form longitudinal dimers and did not induce aggregation. AlCl3 induced similar percentages of aggregates in the presence or absence of CaCl2, and CaCl2 could not reduce pre-formed aggregates. CaCl(2)-induced dimers and AlCl(3)-induced aggregates were prevented by prior NF dephosphorylation. While CaCl(2)-induced dimers were dissociated by phosphatase treatment, AlCl(3)-induced aggregates were only reduced by approximately 50%, suggesting that aggregates may sequester phosphorylation sites. Since phosphatases regulate NF phosphorylation within perikarya, inhibition of NF dephosphorylation by aluminum would promote perikaryal NF phosphorylation and foster precocious phospho-dependent NF-NF associations. These findings are consistent with the notion that prolonged interactions induced among phospho-NFs by the trivalent aluminum impairs axonal transport and promotes perikaryal aggregation.

Neurofilament phosphorylation and disruption: A possible mechanism of chronic aluminium toxicity in Wistar rats

The present study was designed to investigate the possible effects of chronic aluminium exposure on neurofilament phosphorylation and its subsequent disruption in various regions of the rat brain. An intra-gastric dose of aluminium (10mg/kg bw for 12 weeks) resulted in a marked enhancement of Ca2+/CaM dependent protein kinase activity as compared to cAMP dependent protein kinase. The levels of phosphoprotein phosphatase were found to be significantly depleted only in the cerebral cortex. After in vitro phosphorylation using [32gamma-P] ATP, various proteins were resolved on one-dimensional 8% SDS-PAGE, stained with Coomassie Blue and autoradiographed. The amount of 32P-incorporated was quantified using ADOPE PHOTOSHOP (7.0). The 200 kDa neurofilament protein was identified using immunoblotting. Finally, the extent of phosphorylation induced neurofilamentous damage was assessed using immunocytochemical studies. The cytoskeletal proteins were found to be aggregated and disrupted in all the three neuronal regions following 12 weeks of aluminium treatment. This study lends further support to the possible role of aluminium as a potent neurotoxic agent and in the etiopathogenisis of various neurodegenerative diseases.

Impairment of anterograde and retrograde neurofilament transport after anti-kinesin and anti-dynein antibody microinjection in chicken dorsal root ganglia

The purpose of the present study was to investigate the participation of the motor proteins kinesin and dynein in axonal transport of neurofilaments (NF) in cultured dorsal root ganglia neurons. Therefore, we performed live-recording studies of the green fluorescent protein-tagged neurofilament M (GFP-NF-M) to assay transport processes in neurons. Co-localization studies revealed that GFP-NF-M was capable to build a functional NF network with other NF subunits, including phosphorylated heavy neurofilaments (NF-H-PH). Time-lapse recordings using confocal laser scanning microscopy exhibited fast transport of NF dots in anterograde and retrograde direction through a photobleached gap. Following microinjection of anti-kinesin antibodies or colchicine treatment an impairment of anterograde as well as retrograde NF transport was observed during live-recording experiments. In contrast, microinjection of anti-dynein antibodies only impaired retrograde transport of NF whereas the anterograde movement of GFP-NF-M was unaffected. Treatment of the cells with unspecific antibodies had no effect.

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.

Defective neurofilament transport in mouse models of amyotrophic lateral sclerosis: a review

Neurofilament proteins synthesized in the cell body of neurons are assembled and transported into axons, where they influence axon radial growth, axonal transport, and nerve conduction velocities. In diseased states, neurofilaments accumulate in cell bodies and proximal axons of affected neurons, and these lesions are characteristic of many neurodegenerative diseases, including amyotrophic lateral sclerosis (ALS), Alzheimer's disease (AD), spinal muscular atrophy (SMA), Charcot-Marie-Tooth disease type 2 (CMT2), and hereditary sensory motor neuropathy. Although the molecular mechanisms that contribute to these accumulations are not yet identified, transgenic mouse models are beginning to provide insight into the role of neurofilament transport in disease-related dysfunction of neurons. This review addresses axonal transport in mouse models of ALS and the special significance of neurofilament transport in this disease.

Kinesin-mediated transport of neurofilament protein oligomers in growing axons

We examined cytoskeleton-associated forms of NF proteins during axonal neuritogenesis in cultured dorsal root ganglion (DRG) neurons and NB2a/d1 neuroblastoma. In addition to filamentous immunoreactivity, we observed punctate NF immunoreactivity throughout perikarya and neurites. Immuno-electron microscopy revealed this punctate immunoreactivity to consist of non-membrane-bound 75 nm round/ovoid structures consisting of amorphous, fibrous material. Endogenous and microinjected NF subunits incorporated into dots prior to their accumulation within filaments. A transfected GFP-conjugated NF-M incorporated into dots and translocated at a rate consistent with slow axonal transport in real-time video analyses. Some dots converted into a filamentous form or exuded filamentous material during transport. Dots contained conventional kinesin immunoreactivity, associated with microtubules, and their transport into axons was blocked by anti-kinesin antibodies and nocodazole. These oligomeric structures apparently represent one form in which NF subunits are transported in growing axons and may utilize kinesin as a transport motor.

Decrease of glial fibrillary acidic protein in rat frontal cortex following aluminum treatment

Aluminum lactate was injected either intraperitoneally or stereotactically into the lateral cerebral ventricles of rats. Rats were killed at various times after treatment, and frontal cortex, hippocampus, and striatum were dissected out. Microtiter plate-based sandwich ELISA and immunohistochemistry were used to measure the glial fibrillary acidic protein (GFAP) concentration. GFAP levels were significantly decreased in frontal cortex 7 days after a single lateral ventricular injection of aluminum lactate and 14 days following systemic treatment. In contrast, neither hippocampus nor striatum exhibited any significant changes in the content of this astrocytic intermediate filament protein after aluminum treatment. Levels of a predominantly astroglial enzyme, glutamine synthetase, were also selectively reduced in the frontal cortex following intraventricular injection of aluminum. This depression exhibited a regional and temporal specificity similar to that found for GFAP. These results suggest a selective and progressive diminution of astrocytic responsivity in frontal cortex following either systemic or intraventricular aluminum dosing. The depression of GFAP levels reported here, which was found in the rat cerebral cortex 7-14 days after aluminum treatment in a species that does not form neurofilamentous aggregates, may reflect extended impairment of astrocytic function and suggests that these cells may be the primary targets of aluminum neurotoxicity.

Ca2+ and Mg2+ selectively induce aggregates of PHF-tau but not normal human tau

The molecular mechanism of pathological aggregation of microtubule-associated protein tau during neurodegeneration is unclear. In the present study, the in vitro effect of various metal ions on the aggregation of tau was examined using paired helical filament tau (PHF-tau) obtained from corticobasal degeneration (CBD) and Alzheimer's disease (AD) brains as well as normal human tau proteins isolated from fetal and adult brains and a recombinant system. Among the metal ions tested, Ca2+ and Mg2+ effectively induced formation of approximately 340 kD aggregates of PHF-tau but not normal tau proteins as determined by sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis and immunoblotting. Al3+ and Fe2+ precipitated both PHF-tau and normal tau protein as SDS-insoluble pellets. The other metal ions examined (Cu2+, Zn2+, and Li+) were inactive and caused neither aggregation nor precipitation of any tau protein. Intermixing experiments using PHF-tau and various normal tau preparations showed that the 340-kD aggregates induced by Ca2+ contained PHF-tau but not normal tau regardless whether unmodified (recombinant) or highly phosphorylated (fetal brain) tau proteins were used. The present results suggest that post-translational modifications other than the fetal-type phosphorylation are required for Ca2+- and Mg2+-dependent aggregation of PHF-tau and that the regional elevation of these ions may trigger pathological deposition of PHF-tau in certain neurodegenerative disorders.

Phosphorylation sensitizes microtubule-associated protein tau to Al(3+)-induced aggregation

In Alzheimer's disease the microtubule-associated protein tau becomes hyperphosphorylated and aggregates into paired helical filaments (PHFs). Although the biochemical basis of the aggregation of tau into PHFs is not very clear, Al3+ has been suggested to play some role. Previous studies have shown that Al3+ alters the phosphorylation state and causes aggregation of tau in experimental animals and cultured neurons. In this study Al3+ inhibited phosphorylation of tau by neuronal cdc2-like kinase and dephosphorylation of phosphorylated tau by phosphatase 2B. These inhibitions are very likely due to Al(3+)-induced aggregations of various proteins present in phosphorylation/dephosphorylation assay mixtures since Al3+ caused aggregations of all proteins examined. Furthermore, compared to other proteins, tau displayed only an average sensitivity towards Al(3+)-induced aggregation. However upon phosphorylation, tau's sensitivity towards Al3+ increased 3.5 fold. In the presence of the metal chelator EDTA, Al(3+)-induced aggregates of tau became soluble, whereas Al(3+)-induced phosphorylated tau aggregates were insoluble in the buffer containing EDTA and remained insensitive to proteolysis. Our data suggest that phosphorylation sensitizes tau to Al3+ and phosphorylated tau transforms irreversibly into a phosphatase and protease resistant aggregate in presence of this metal ion.

Cisplatin induced intermediate filament reorganization and altered mitochondrial function in 3T3 cells and drug-sensitive and -resistant Walker 256 cells

Cisplatin has acute but reversible effects on the organization of the intermediate filament component of the cytoskeleton as well as the mitochondrial function of cultured 3T3 cells. These effects do not involve major changes in total cell or cytoskeletal protein synthesis and appear to be distinct from the long-term cytotoxicity produced by the drug. Cells treated with similar concentrations of second-generation platinum compounds, which have reduced nephrotoxic effects in vivo, do not exhibit alterations in intermediate filament organization nor mitochondrial rhodamine 123 fluorescence. Similar studies with cisplatin-sensitive and -resistant lines of rat Walker 256 cells indicated that the drug-induced intermediate filament collapse and decreased mitochondrial rhodamine 123 fluorescence correlated with the susceptibility of these cells to the lethal effects of cisplatin.

Elevation of cerebral proteases after systemic administration of aluminum

The levels of three proteases in the cerebral cortex of rats following a three week exposure to aluminum, were measured. The activity of apopain (CPP32), an interleukin 1beta converting enzyme (ICE)-like cysteine protease specifically associated with apoptosis, was increased following dosing with aluminum. The activity of calcium-activated neutral protease, calpain, was also increased. However, the enzyme activity of trypsin-like serine protease, known to be elevated by oxidative events, was unchanged. Since aluminum is suspected as a possible factor in the pathogenesis of Alzheimer's disease and other neurological diseases, it is speculated that changed levels in proteolytic enzymes may relate to the neurotoxicity of aluminum.

Aluminum inhibits neurofilament assembly, cytoskeletal incorporation, and axonal transport. Dynamic nature of aluminum-induced perikaryal neurofilament accumulations as revealed by subunit turnover

The mechanism by which aluminum induces formation of perikaryal neurofilament (NF) inclusions remains unclear. Aluminum treatment inhibits: 1. The incorporation of newly synthesized NF subunits into Triton-insoluble cytoskeleton of axonal neurites; 2. Their degradation and dephosphorylation; 3. Their translocation into axonal neurites. It also fosters the accumulation of phosphorylated NFs within perikarya. In the present study, we addressed the relationship among these effects. Aluminum reduced the assembly of newly synthesized NF subunits into NFs. During examination of those subunits that did assemble in the presence of aluminum, it was revealed that aluminum also interfered with transport of newly assembled NFs into axonal neurites. Similarly, a delay in axonal transport of microinjected biotinylated NF-H was observed in aluminum-treated cells. Aluminum also inhibited the incorporation of newly synthesized and microinjected subunits into the Triton-insoluble cytoskeleton within both perikarya and neurites. Once incorporated into Triton-insoluble cytoskeletons, however, biotinylated subunits were retained within perikarya of aluminum-treated cells to a greater extent than within untreated cells. Notably, these subunits were depleted in the presence and absence of aluminum within 48 h, despite the persistence of the aluminum-induced perikaryal accumulation itself, suggesting that individual NF subunits undergo turnover even within aluminum-induced perikaryal accumulations. These findings demonstrate that aluminum interferes with multiple aspects of neurofilament dynamics and furthermore leaves open the possibility that aluminum-induced perikaryal NF whorls may not represent permanent structures, but rather may require continued recruitment of cytoskeletal constituents.

Spinal motor neuron neuroaxonal spheroids in chronic aluminum neurotoxicity contain phosphatase-resistant high molecular weight neurofilament (NFH)

It has previously been shown that a single intracisternal inoculum of AlCl3 in young adult New Zealand white rabbits will induce a dose-dependent phosphatase resistance of high molecular weight neurofilament protein (NFH) that is proportionate to the extent of neurofilamentous inclusion formation (Strong and Jakowec, 1994). To determine if the potential for dissolution of aluminum-induced neurofilamentous inclusions was dependent on the degree of NFH phosphatase resistance, we have examined NFH phosphatase sensitivity in a reversible chronic model of aluminum neurotoxicity. Rabbits receiving repeated intracisternal inoculums of 100 microgram AlCl3 at 28 day intervals until day 267 develop spinal motor neuron perikaryal and neuroaxonal neurofilamentous aggregates in a stereotypic, dose-dependent fashion. In the rabbits receiving inoculums until day 156 with survival until day 267 without further aluminum exposure, neuroaxonal spheroids remained prominent while perikaryal inclusions largely resolved. Immunoreactivity to a monoclonal antibody recognizing phosphorylated NFH (SMI 31) was abolished in perikaryal aggregates at each time interval by dephosphorylation with bovine alkaline phosphatase. However, neuroaxonal spheroids maintained their immunoreactivity. Using time-course dephosphorylation studies of spinal cord homogenates, we observed a significant reduction in the rate of dephosphorylation of NFH following 267 days of AlCl3 exposure (P < 0.05). These observations suggest that neuroaxonal spheroids contain phosphatase-resistant NFH isoforms and that the potential for resolution of intraneuronal neurofilamentous inclusions correlates with the susceptibility of NF within these inclusions to enzymatic dephosphorylation.

Inhibition of proteolysis enhances aluminum-induced perikaryal neurofilament accumulation but does not enhance tau accumulation

As observed for neurons in situ, phosphorylated neurofilament (NF) epitopes are normally segregated within the axonal cytoskeleton of NB2a/d1 cells. However, accumulations of phosphorylated NFs develop in NB2a/d1 perikarya following exposure to aluminum salts and following inhibition of proteolysis. In the present study, we observed that perikarya of cells exposed to both aluminum and the protease inhibitor C1 (also known as "AllNal") were more intensely labeled by monoclonal antibodies directed against both nonphosphorylated and phosphorylated epitopes than were cells treated with either aluminum or protease inhibitor alone. Since these monoclonal antibodies crossreact with tau, we also immunostained cells treated under these conditions with monoclonal antibodies directed against phosphate-insensitive (5E2) and phosphorylated (PHF-1) epitopes of tau. Aluminum treatment, but not C1 treatment, induced accumulation of total tau isoforms as judged by an increase in 5E2 immunoreactivity. Neither treatment, either separately or in combination, induced an increase in PHF-1 immunoreactivity. These findings suggest that alterations in immunoreactivity with SMI antibodies reflected increases in NF epitopes. This was confirmed by immunoblot analyses. Since proteolysis is apparently instrumental in maintaining the normal distribution patterns of phosphorylated NF epitopes, these findings implicate deficiencies in proteolytic mechanisms in the development of neurofibrillary pathology, and underscore the possibility of a multiple etiology in human neuropathological conditions.

Aluminum treatment of intact neuroblastoma cells alters neurofilament subunit phosphorylation, solubility, and proteolysis

Addition of 400 microM AlCl3 to the culture medium for 72 h has been previously shown to induce perikaryal whorls of intermediate-sized filaments in intact mouse NB2a/d1 neuroblastoma cells. Immunoblot analyses demonstrated that in vivo treatment of cells with aluminum induced the de novo appearance of extensively phosphorylated NF-H isoforms in cytoskeletons of undifferentiated cells and increased levels of these isoforms in differentiated cells. Neurofilament subunits isolated from intact cells treated with aluminum were resistant to dephosphorylation in vitro by alkaline phosphatase and to in vitro degradation by endogenous calcium-dependent protease(s). These alterations were accompanied by a greater tendency of neurofilaments to form insoluble aggregates after isolation. These findings demonstrate direct effects of aluminum on neurofilament subunits within intact neuronal cells similar to those previously demonstrated following in vitro exposure of isolated neurofilaments to aluminum.

Brain mRNA from infants of aluminium-exposed lactating rabbits

The neurotoxicity of aluminium (Al) involves bundling of neurofilaments, increased chromatin binding and decreased protein synthesis in Al injected rabbits. Thus, using an amphipathic Al ligand, maltol, experiments were carried out to examine whether or not administration of Al to lactating mother rabbits reduces brain protein synthesis in their offspring. Lactating mother rabbits received s.c. injections 3 times weekly of aluminium (Al) maltolate (1 mg Al/kg body wt) or an equivalent weight of maltol, for 4 weeks post-partum. Polysome preparations were obtained from the brain of their infants in order to assess mRNA translation in cell-free protein synthesizing systems. The brain polysomes showed a statistically significant reduction in the incorporation of [14C]leucine into protein. The poly (A)+ and poly (A)- fractions obtained from these polysomes showed reductions of 44% or more in the incorporation of [35S]methionine into protein. A variety of products separated by SDS-polyacrylamide gel electrophoresis all exhibited decreased labelling. These experiments suggest that infant rabbits exposed to a highly neurotoxic form of Al in milk exhibit changes in brain protein synthesis which resemble those in infants injected directly with Al.

Aluminum binds to the hyperphosphorylated tau in Alzheimer's disease: a hypothesis

The hypterphosphorylation of tau proteins is a well-established step in the expression of the pathological features in Alzheimer's disease (AD). While a primary event that links aluminum and AD, the so-called 'aluminum hypotheses', is far from being demonstrated, however, the metal ion could be an important etiological factor, most likely not the only one, of AD etiopathogenesis acting as an aggregating agent of the cytoskeletal elements forming very stable bridges between phosphate groups of the hyperphosphorylated tau elements.

Calcium modulates aluminum neurotoxicity and interaction with neurofilaments

We examined the influence of calcium on neurotoxicity of AlCl3 and Al-lactate toward differentiated NB2a/d1 cells. Apart from induction of perikaryal neurofibrillary inclusions, AlCl3 at 1 mM induced no obvious additional signs of toxicity when added to culture medium in the presence of the normal medium CaCl2 content of 1.8 mM, nor when extracellular calcium was decreased by the addition to the medium of 0.9 mM EDTA. Increasing the extracellular CaCl2 concentration by fivefold was only marginally toxic, but in the presence of AlCl3, reduced viable cell numbers by well over 50% as compared to control cultures, and by approximately 50% vs fivefold CaCl2 alone. A twofold increase in extracellular CaCl2 did not increase the percentage of cells exhibiting Bielschowsky-positive perikarya, but induced a near doubling in the percentage of cells exhibiting accumulations in the presence of 1 mM Al-lactate. AlCl3 (1 mM) retards the electrophoretic migration of NF subunits on SDS-gels. This effect was eliminated by withholding CaCl2 from the incubation mixture and including 5 mM EDTA during incubation of cytoskeletons with AlCl3. The presence of CaCl2 alone did not alter NF migration. These findings underscore the possibility that multiple factors, including those that compromise general neuronal homeostasis, may contribute to neurofibrillary pathology.

Relative susceptibility of cytoskeleton-associated and soluble neurofilament subunits to aluminum exposure in intact cells. A possible mechanism for reduction of neurofilament axonal transport during aluminum neurotoxicity

Previous studies have demonstrated the appearance of phosphorylated neurofilament (NF) subunits within perikaryal cytoskeletons following aluminum exposure. In order to examine the mechanisms leading to this altered distribution of NF subunits, we carried out biochemical analyses of NF subunits in Triton-insoluble and -soluble fractions derived from aluminum-treated NB2a/d1 cells. In addition to increases in the Triton-insoluble cytoskeleton, increases in all three NF subunits were also detected within the Triton-soluble fraction of aluminum-treated cells. To address the nature of this increase in Triton-soluble subunits, aluminum-treated and untreated cultures were harvested in the absence of Triton and fractionated by established procedures to yield fractions greatly enriched for perikarya and neurites, respectively. Each of these subcellular fractions was then subjected to further homogenization in the presence of 1% Triton and centrifugation to yield Triton-insoluble cytoskeletons and Triton-soluble material derived from perikarya and axonal neurites, respectively. Resulting Triton-soluble fractions were "clarified" by high-speed centrifugation to eliminate oligomeric assemblies or soluble neurofilaments. Immunoblot analysis demonstrated quantitative recovery of the aluminum-induced increase in Triton-soluble NF subunits in the perikaryal fraction. Additional aluminum-treated and untreated cultures were pulse-chase radiolabeled with [35S]methionine and fractionated into Triton-insoluble and soluble fractions from isolated perikarya and axonal neurites. Autoradiographic analysis of immunoprecipitated NF subunits revealed that aluminum treatment delayed the translocation of newly synthesized subunits into neurites and resulted in the accumulation of radiolabeled subunits within the Triton-soluble fraction of perikarya. These findings suggest that aluminum may exert a relatively greater effect on NF subunits that have not yet undergone axonal transport and/or incorporation into Triton-insoluble structures vs those that have already deposited into axons. This possibility was supported by the observation that a higher concentration of aluminum was required to alter the electrophoretic migration of in vitro reassembled neurofilaments vs that required for unassembled NF subunits. These findings provide possible mechanisms for the accumulation of NF subunits in perikarya during aluminum intoxication.

Differential synthesis and cytoskeletal deposition of neurofilament subunits before and during axonal outgrowth in NB2a/d1 cells: evidence that segregation of phosphorylated subunits within the axonal cytoskeleton involves selective deposition

NB2a/d1 cells constitutively express and extensively phosphorylate neurofilament (NF) triplet proteins. However, only hypophosphorylated NFs are observed within the Triton-insoluble perikaryal cytoskeletons of undifferentiated and differentiated cells, while phosphorylated NF isoforms accumulate exclusively within the axonal neurites elaborated following treatment with dbcAMP. We examined NF synthesis and distribution of newly synthesized subunits by immunoprecipitation from 35S-methionine-radiolabeled undifferentiated and dbcAMP-treated differentiated cells. Following a 15 min pulse radiolabeling, NF-H isoforms migrating from approximately 160-200 kDa, NF-M isoforms migrating from approximately 97 k-145 Da, and a single 70 kDa NF-L isoform were readily detectable within Triton-soluble fractions from both undifferentiated and differentiated cells. During chase analyses in the absence of radiolabel, the entire spectrum of isoforms was present in Triton-soluble and -insoluble fractions from both undifferentiated and differentiated cells. However, differentiated cells displayed a significant increase in radiolabel associated with each subunit and isoform. Normalization of their NF synthesis levels to those of undifferentiated cells revealed that differentiated cells deposited 10-fold more radiolabeled subunits into the Triton-insoluble cytoskeleton as compared to undifferentiated cells. Similar levels of radiolabeled subunits were observed throughout the 2 hr period in dbcAMP-treated cells. By contrast, radiolabeled subunits and isoforms increased in undifferentiated cytoskeletons during the chase period, although final levels remained substantially lower than those observed in cytoskeletons of dbcAMP-treated cells. These data were considered with respect to potential mechanisms by which the phosphorylated NFs are normally excluded from perikaryal cytoskeletons. The presence of extensively phosphorylated subunits within perikarya indicates the presence of necessary NF kinases.(ABSTRACT TRUNCATED AT 250 WORDS)

The organization of neurofilaments accumulated in perikaryon following aluminum administration: relationship between structure and phosphorylation of neurofilaments

Neurofilaments accumulated in perikarya and dendrites of anterior horn cells and Purkinje cells of rabbit treated by aluminum chloride were analysed with a variety of techniques. Four different monoclonal antibodies against phosphorylated and nonphosphorylated epitopes on neurofilament H subunit were used to compare phosphorylation state of these accumulated neurofilaments with that of axonal neurofilaments. Although immunoblotting revealed no significant difference in phosphorylation between control and aluminum-treated brains, accumulated neurofilaments were immunocytochemically more phosphorylated than control perikaryal or dendritic neurofilaments. With detailed analysis of cryothin-section immunogold labeling, accumulated neurofilaments were, however, significantly less phosphorylated than axonal neurofilaments. With quick-freeze deep etching, core filaments of accumulated neurofilaments are as dense as axonal neurofilaments but much less regularly aligned. Cross-bridges of accumulated neurofilaments were less frequent and more branched than those of axonal neurofilaments, and when examined with combined immunocytochemistry and deep etching, were less phosphorylated. These results suggest that there is a relationship between the phosphorylation and the structural organization of neurofilaments. The phosphorylation of neurofilament H subunit may be necessary for formation of frequent and straight cross-bridges and resulting regular alignment of core filaments.

Calcium-activated neutral proteinase (calpain) system in aging and Alzheimer's disease

Calpains (CANPs) are a family of calcium-dependent cysteine proteases under complex cellular regulation. By making selective limited proteolytic cleavages, they activate or alter the regulation of certain enzymes, including key protein kinases and phosphatases, and induce specific cytoskeletal rearrangements, accounting for their suspected involvement in intracellular signaling, vesicular trafficking, and structural stabilization. Calpain activity has been implicated in various aging phenomena, including cataract formation and erythrocyte senescence. Abnormal activation of the large stores of latent calpain in neurons induces cell injury and is believed to underlie neurodegeneration in excitotoxicity, Wallerian degeneration, and certain other neuropathologic states involving abnormal calcium influx. In Alzheimer's disease, we found the ratio of activated calpain I to its latent precursor isoform in neocortex to be threefold higher than that in normal individuals and those with Huntington's or Parkinson's disease. Immunoreactivity toward calpastatin, the endogenous inhibitor of calpain, was also markedly reduced in layers II-V of the neocortex in Alzheimer's disease. The excessive calpain system activation suggested by these findings represents a potential molecular basis for synaptic loss and neuronal cell death in the brain in Alzheimer's disease given the known destructive actions of calpain I and its preferential neuronal and synaptic localization. In surviving cells, persistent calpain activation may also contribute to neurofibrillary pathology and abnormal amyloid precursor protein trafficking/processing through its known actions on protein kinases and the membrane skeleton. The degree of abnormal calpain activation in the brain in Alzheimer's disease strongly correlated with the extent of decline in levels of secreted amyloid precursor protein in brain. Cytoskeletal proteins that are normally good calpain substrates become relatively calpain resistant when they are hyperphosphorylated, which may contribute to their accumulation in neurofibrillary tangles. As a major effector of calcium signals, calpain activity may mirror disturbances in calcium homeostasis and mediate important pathologic consequences of such disturbances.

Respective roles of neurofilaments, microtubules, MAP1B, and tau in neurite outgrowth and stabilization

The respective roles of neurofilaments (NFs), microtubules (MTs), and the microtubule-associated proteins (MAPs) MAP 1B and tau on neurite outgrowth and stabilization were probed by the intracellular delivery of specific antisera into transiently permeabilized NB2a/d1 cells during treatment with dbcAMP. Intracellular delivery of antisera specific for the low (NF-L), middle (NF-M), or extensively phosphorylated high (NF-H) molecular weight subunits did not prevent initial neurite elaboration, nor did it induce retraction of existing neurites elaborated by cells that had been previously treated for 1 d with dbcAMP. By contrast, intracellular delivery of antisera directed against tubulin reduced the percentage of cells with neurites at both these time points. Intracellular delivery of anti-NF-L and anti-NF-M antisera did not induce retraction in cells treated with dbcAMP for 3 d. However, intracellular delivery of antisera directed against extensively phosphorylated NF-H, MAP1B, tau, or tubulin induced similar levels of neurite retraction at this time. Intracellular delivery of monoclonal antibodies (RT97 or SMI-31) directed against phosphorylated NF-H induced neurite retraction in cell treated with dbcAMP for 3 d; a monoclonal antibody (SMI-32) directed against nonphosphorylated NF-H did not induce neurite retraction at this time. By contrast, none of the above antisera induced retraction of neurites in cells treated with dbcAMP for 7 d. Neurites develop resistance to retraction by colchicine, first detectable in some neurites after 3 d and in the majority of neurites after 7 d of dbcAMP treatment. We therefore examined whether or not colchicine resistance was compromised by intracellular delivery of the above antisera. Colchicine treatment resulted in rapid neurite retraction after intracellular delivery of antisera directed against extensively phosphorylated NF-H, MAP1B, or tau into cells that had previously been treated with dbcAMP for 7 d. By contrast, colchicine resistance was not compromised by the intracellular delivery of antisera directed against NF-L, NF-M, or tubulin. These findings support previous studies indicating that MT polymerization mediates certain aspects of axonal neurite outgrowth and suggest that NFs do not directly participate in these events. These findings further suggest that NFs function in stabilization of the axonal cytoskeleton, apparently by interactions among NFs and MTs that are mediated by NF-H and MAPs.

Aluminum neurotoxicity: an experimental approach to the induction of neurofilamentous inclusions

Acute or chronic aluminum neurotoxicity experiments in the rabbit suggest that aluminum can induce phosphorylation of neurofilamentous proteins. This may result in abnormal resistance to degradation or transport of neurofilament protein and so to the accumulation of neurofilaments in abnormal cells. The possible importance of this process in ALS is considered in relation to the neurofilamentous abnormalities characteristic of intraneuronal inclusions in ALS and in other neurodegenerative disorders.

Interaction of aluminum ions with phosvitin

We studied the effects of aluminum ions on the dephosphorylation of phosvitin catalyzed by acid phosphatase, and the metachromasia resulting from the interaction of phosvitin with toluidine blue. In both cases the action of Al3+ was inhibitory and the extent of inhibition was dependent on Al3+ concentration and the length of incubation of Al3+/phosvitin mixtures. The inhibition profiles of dephosphorylation of phosvitin (50 micrograms/ml) showed IC50 values of 15 and 2 microM Al3+ at 1 and 48 hr incubation time, respectively. The effect was proved to be substrate directed, while the inhibition was not reversed by EDTA. In contrast, the action of other divalent or trivalent cations on the dephosphorylation process, when inhibitory, was completely reversible by EDTA. Exposure of fluorescein 5-isothiocyanate-labeled phosvitin to Al3+ resulted in: a) the failure of the protein to migrate into sodium dodecyl sulfate containing polyacrylamide gels and b) the decrease of the fluorescence emission of the bound fluorescein. These findings suggest that phosvitin can be used as a model for studying interactions of aluminum with multiphosphorylated proteins and other polyanionic biopolymers.

Multiple interactions of aluminum with neurofilament subunits: regulation by phosphate-dependent interactions between C-terminal extensions of the high and middle molecular weight subunits

Exposure of individual purified neurofilament (NF) proteins to AlCl3 alters their electrophoretic properties in a time- and concentration-dependent manner, as visualized by their failure to migrate into SDS gels. Co-incubation of purified high (NF-H) and middle (NF-M) but not low (NF-L) molecular weight NF subunits prevents this AlCl3-induced alteration in electrophoretic migration. This latter finding suggested that specific interactions between NF-H and NF-M other than filament formation influenced their interaction with AlCl3. Co-incubation of the 160 kDa alpha-chymotryptic cleavage product of NF-H (corresponding to the highly phosphorylated C-terminal sidearm domain) with native NF-M prevented alteration in subunit electrophoretic migration by AlCl3. By contrast, intact, dephosphorylated NF-H subunits were unable to prevent AlCl3-induced alteration of native NF-M electrophoretic migration. Taken together, these findings suggest that phosphate-dependent interactions between the sidearm extensions of NF-H and NF-M diminish the ability of AlCl3 to associate with either subunit in a manner that alters their electrophoretic migration. This interaction of NF-H and NF-M sidearms is SDS-sensitive, while AlCl3-induced alteration in electrophoretic migration of individual subunits is SDS-resistant. Addition of SDS to mixtures of NF-H and NF-M subunits disrupted the protective effect, and promoted AlCl3-induced alterations in subunit electrophoretic migration. These findings support and extend the current hypothesis that the ability of aluminum to interact with NF subunits is a function of subunit phosphorylation, assembly, and extent of neurofilament-neurofilament cross-linking.

Triton-soluble phosphovariants of the high molecular weight neurofilament subunit from NB2a/d1 cells are assembly-competent. Implications for normal and abnormal neurofilament assembly

NB2a/d1 cells incorporate neurofilaments (NFs) containing extensively phosphorylated high (NF-H) molecular weight subunits into the Triton-insoluble cytoskeleton of axonal neurites elaborated during differentiation with dibutyryl cAMP. However, immunocytochemical and biochemical analyses demonstrate the constitutive expression and extensive phosphorylation of a sizeable pool of (200 kDa) NF-H. We examined by cell-free analyses whether or not this Triton-soluble NF-H pool was assembly-competent in cell-free analyses. Triton-soluble fractions from 35S-radiolabeled NB2a/d1 cells were incubated with dissociated mouse CNS Triton-insoluble cytoskeletons that had been dissociated by treatment with 6 M urea. Following overnight dialysis to remove urea, low-speed centrifugation to sediment Triton-insoluble cytoskeletons resulted in the co-sedimentation of radiolabeled NF-H, indicating that Triton-soluble NF-H was capable of association with Triton-insoluble structures. Triton-soluble, extensively phosphorylated NF-H from NB2a/d1 cells was also capable of co-assembling with purified NF-L. Following high-speed centrifugation (100,000 x g for 1 h) to sediment any oligomeric assemblies, the Triton-soluble fraction from NB2a/d1 cells was mixed with purified NF-L that had been solubilized by 6 M urea. Following overnight dialysis to remove urea, high-speed centrifugation sedimented both NF-L and Triton-soluble NF-H from NB2a/d1 cells, demonstrating that Triton-soluble NF-H variants are assembly-competent. These data suggest that NF-H variants represent precursors for NF assembly, and indicate that their assembly within NB2a/d1 cells, demonstrating that Triton-soluble NF-H variants are assembly-competent. These data suggest that NF-H variants represent precursors for NF assembly, and indicate that their assembly within NB2a/d1 cells must be under temporal and spatial regulation.

Oxidative stress: free radical production in neural degeneration

It is not yet established whether oxidative stress is a major cause of cell death or simply a consequence of an unknown pathogenetic factor. Concerning chronic diseases, as Parkinson's and Alzheimer's disease are assumed to be, it is possible that a gradual impairment of cellular defense mechanisms leads to cell damage because of toxic substances being increasingly formed during normal cellular metabolism. This point of view brings into consideration the possibility that, besides exogenous factors, the pathogenetic process of neurodegeration is triggered by endogenous mechanisms, either by an endogenous toxin or by inherited metabolic disorders, which become progressively more evident with aging. In the following review, we focus on the oxidative stress theory of neurodegeneration, on excitotoxin-induced cell damage and on impairment of mitochondrial function as three major noxae being the most likely causes of cell death either independently or in connection with each other. First, having discussed clinical, pathophysiological, pathological and biochemical features of movement and cognitive disorders, we discuss the common features of these biochemical theories of neurodegeneration separately. Second, we attempt to evaluate possible biochemical links between them and third, we discuss experimental findings that confirm or rule out the involvement of any of these theories in neurodegeneration. Finally, we report some therapeutic strategies evolved from each of these theories.

Aggregation of tau protein by aluminum

Aluminum has been detected in Alzheimer neurofibrillary tangles, but the significance of its presence is unknown. The principal component of tangles is the paired helical filament (PHF), comprised of tau protein. We investigated whether aluminum could induce tau protein to form filaments or aggregate. When 10 microM bovine tau or non-phosphorylated recombinant human tau was combined with 400 microM or more aluminum, tau protein appeared to aggregate, observed as a dose-dependent decrease in electrophoretic mobility on SDS-PAGE. Tau appeared as a smear above the region of the expected tau bands and, at higher aluminum doses, failed to enter the gel. A tau fragment encompassing the microtubule binding domains did not show decreased mobility in the presence of aluminum, but did form aggregates that failed to electrophorese. However no fibrillar structures were observed in the aluminum-treated tau samples when observed by electron microscopy. The effect of aluminum on tau mobility was reversed by incubating with 1 mM deferoxamine. In contrast, the morphology of PHF fibrils was unaffected by deferoxamine treatment and the characteristic abnormal mobility of PHF-tau was not reduced by deferoxamine. This suggests that aluminum is not, by itself, a significant factor in maintaining the assembly of PHF-tau as fibrils or in its abnormal mobility on SDS gels. Aluminum treatment of 3T3 fibroblasts transfected with human tau resulted in toxicity, but did not change tau expression levels or induce tau aggregation. In conclusion, aluminum appears to induce isolated tau protein to aggregate in a phosphate-independent way, without the formation of fibrils.(ABSTRACT TRUNCATED AT 250 WORDS)

Deposition of detergent-resistant neurofilaments into Lewy body fibrils

To assess the contribution of neurofilaments (NF) to the detergent-resistant cortical Lewy body (LB) fibril we extracted LBs from Diffuse LB diseased brains and used monoclonal antibodies to probe Western transfers of solubilized LB-derived protein. Antibodies to epitopes located in the COOH-termini of the 200- and 170-kDa NF subunits (NF-H and NF-M) labelled LB proteins corresponding to full length and partially truncated or variably phosphorylated NF-H and NF-M. LB-derived protein at approximately 70-kDa did not contain epitopes detected by monoclonal antibodies to NF-L, tau or the COOH-termini of the NF-H and NF-M. We conclude that NF-H and NF-M are incorporated as integral insoluble components of the cortical LB fibril.

Aluminum inhibits neurofilament protein degradation by multiple cytoskeleton-associated proteases

The environmental neurotoxin aluminum exerts several distinct biochemical effects on neurofilament proteins, including subunit aggregation, disruption of the normal segregation of phosphorylated subunits within axons leading to abnormal perikaryal accumulation, and inhibition of in vitro degradation by the calcium-dependent neutral protease, calpain. In the present study, we demonstrate that exposure of mouse CNS cytoskeletal preparations to aluminum chloride inhibits the degradation of neurofilament proteins by both calcium-dependent and -independent proteases that co-purify with cytoskeletons. Aluminum inhibited both calcium-dependent and calcium-independent proteolysis of the high and middle molecular weight neurofilament subunits, but inhibited only calcium-dependent, and not calcium-independent proteolysis of the low molecular weight neurofilament subunit. These findings demonstrate that aluminum interferes with multiple aspects of neurofilament protein metabolism.