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

Aluminum toxicity and astrocyte dysfunction: a metabolic link to neurological disorders

Aluminum (Al) has been implicated in a variety of neurological diseases. However, the molecular mechanisms that enable Al to be involved in these disorders have yet to be fully delineated. Using astrocytes as a model of the cerebral cellular system, we have uncovered the biochemical networks that are affected by Al toxicity. In this review, we reveal how the inhibitory influence of Al on ATP production and on mitochondrial functions help generate globular astrocytes that are fat producing machines. These biological events may be the contributing factors to Al-triggered brain disorders.

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 dynamics: a tug of war by microtubule motors

Structural support for axons, which can consist of volumes thousands of times larger than the neuronal perikaryon, is provided in part by neurofilaments (NFs), the major fibrous constituent of the axonal cytoskeleton. Most NFs undergo anterograde transport (towards the synapse or growth cone), while a few undergo retrograde transport (back towards the perikaryon). Some NFs exhibit an extended residence time along axons, which allows NFs to provide structural support to the axon yet minimizes NF turnover, which would otherwise impart a prohibitive metabolic burden upon the neuron. Herein, we explore known and hypothesized roles for microtubule motors in transport and distribution of NFs along axons. We present evidence that those NFs that display extended residence along axons are critically dependent upon surrounding microtubules, and that simultaneous interaction with multiple microtubule motors provides the architectural force regulating their distribution.

Organizational dynamics, functions, and pathobiological dysfunctions of neurofilaments

Neurofilament phosphorylation has long been considered to regulate their axonal transport rate, and in doing so it provides stability to mature axons. We evaluate the collective evidence to date regarding how neurofilament C-terminal phosphorylation may regulate axonal transport. We present a few suggestions for further experimentation in this area, and expand upon previous models for axonal NF dynamics. We present evidence that the NFs that display extended residence along axons are critically dependent upon the surrounding microtubules, and that simultaneous interaction with multiple microtubule motors provides the architectural force that regulates their distribution. Finally, we address how C-terminal phosphorylation is regionally and temporally regulated by a balance of kinase and phosphatase activities, and how misregulation of this balance might contribute to motor neuron disease.

Regulation of neurofilament dynamics by phosphorylation

Neurofilament (NF) phosphorylation has long been considered to regulate axonal transport rate and in doing so to provide stability to mature axons. Studies utilizing mice in which the C-terminal region of NF subunits (which contains the vast majority of phosphorylation sites) has been deleted has prompted an ongoing challenge to this hypothesis. We evaluate the collective evidence to date for and against a role for NF C-terminal phosphorylation in regulation of axonal transport and in providing structural support for axons, including some novel studies from our laboratory. We present a few suggestions for further experimentation in this area, and expand upon previous models for axonal NF dynamics. Finally, we address how C-terminal phosphorylation is regionally and temporally regulated by a balance of kinase and phosphatase activities, and how misregulation of this balance can contribute to motor neuron disease.

Inhibition of dynein but not kinesin induces aberrant focal accumulation of neurofilaments within axonal neurites

Studies from several laboratories indicate that the microtubule motors kinesin and dynein respectively participate in anterograde and retrograde axonal transport of neurofilaments. Inhibition of dynein function by transfection with a construct expressing dynamitin or intracellular delivery of anti-dynein antibodies accelerates anterograde transport, which has been interpreted to indicate that the opposing action of both motors mediates the normal distribution of neurofilaments along axons. Herein, we demonstrate that, while expression of relatively low levels of exogenous dynamitin indeed accelerated anterograde neurofilament transport along axonal neurites in culture, expression of progressively increasing levels of dynamitin induced focal accumulation of neurofilaments within axonal neurites and eventually caused neurite retraction. Inhibition of kinesin inhibited anterograde transport, but did not induce similar focal accumulations. These findings are consistent with studies indicating that perturbations in dynein activity can contribute to the aberrant accumulations of neurofilaments that accompany ALS/motor neuron disease.

Traffic jams: dynamic models for neurofilament accumulation in motor neuron disease

The dynamics of axonal transport are often colloquially described using highway traffic as a model system. Examination of the physics of traffic patterns, with emphasis on traffic jams and accidents, provides unique and perhaps counterintuitive insight into the aberrant accumulation of neurofilaments that accompanies amyotrophic lateral sclerosis/motor neuron disease.

Dynein mediates retrograde neurofilament transport within axons and anterograde delivery of NFs from perikarya into axons: Regulation by multiple phosphorylation events

We examined the respective roles of dynein and kinesin in axonal transport of neurofilaments (NFs). Differentiated NB2a/d1 cells were transfected with green fluorescent protein-NF-M (GFP-M) and dynein function was inhibited by co-transfection with a construct expressing myc-tagged dynamitin, or by intracellular delivery of purified dynamitin and two antibodies against dynein's cargo domain. Monitoring of the bulk distribution of GFP signal within axonal neurites, recovery of GFP signal within photobleached regions, and real-time monitoring of individual NFs/punctate structures each revealed that pertubation of dynein function inhibited retrograde transport and accelerated anterograde, confirming that dynein mediated retrograde axonal transport, while intracellular delivery of two anti-kinesin antibodies selectively inhibited NF anterograde transport. In addition, dynamitin overexpression inhibited the initial translocation of newly-expressed NFs out of perikarya and into neurites, indicating that dynein participated in the initial anterograde delivery of NFs into neurites. Delivery of NFs to the axon hillock inner plasma membrane surface, and their subsequent translocation into neurites, was also prevented by vinblastine-mediated inhibition of microtubule assembly. These data collectively suggest that some NFs enter axons as cargo of microtubues that are themselves undergoing transport into axons via dynein-mediated interactions with the actin cortex and/or larger microtubules. C-terminal NF phosphorylation regulates motor association, since anti-dynein selectively coprecipitated extensively phosphorylated NFs, while anti-kinesin selectively coprecipitated less phosphorylated NFs. In addition, however, the MAP kinase inhibitor PD98059 also inhibited transport of a constitutively-phosphorylated NF construct, indicating that one or more additional, non-NF phosphorylation events also regulated NF association with dynein or kinesin.

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.

Aluminum maltolate-induced toxicity in NT2 cells occurs through apoptosis and includes cytochrome c release

Aluminum (Al) compounds are neurotoxic and have been shown to induce experimental neurodegeneration although the mechanism of this effect is unclear. In order to study this neurotoxic effect of Al, we have developed an in vitro model system using Al maltolate and human NT2 cells. Al maltolate at 500 microM caused significant cell death with a 24-h incubation and this toxicity was even more evident after 48 h. Lower doses of Al maltolate were also effective, but required a longer incubation for cell death. Nuclear fragmentation suggestive of apoptosis was observed as early as three hours and increased substantially through 24 h. Chromatin condensation and nuclear fragmentation were confirmed by electron microscopy. In addition, TUNEL positive nuclei were also observed. The release of cytochrome c was demonstrated with Western blot analysis. This in vitro model using human cells adds to our understanding of Al neurotoxicity and could provide insight into the neurodegenerative processes in human disease.

Cyclin-dependent kinase 5 increases perikaryal neurofilament phosphorylation and inhibits neurofilament axonal transport in response to oxidative stress

Cyclin-dependent kinase 5 (cdk5) phosphorylates the high molecular weight neurofilament (NF) protein. Overexpression of cdk5 inhibits NF axonal transport and induces perikaryal accumulation of disordered phospho-NF cables. Experimental and clinical motor neuron disease is characterized by oxidative stress, increased cdk5 activity, and accumulation of phospho-NFs within perikarya or proximal axons. Because oxidative stress increases cdk5 activity in experimental motor neuron disease, we examined whether oxidative stress induced cdk5-mediated NF phosphorylation. Treatment of cultured neuronal cells with hydrogen peroxide inhibited axonal transport of green fluorescent protein-tagged NF subunits and induced perikaryal accumulation of NF phosphoepitopes normally confined to axons. These effects were prevented by treatment with the cdk5 inhibitor roscovitine or transfection with a construct expressing the endogenous cdk5 inhibitor peptide. These findings indicate that oxidative stress can compromise NF dynamics via hyperactivation of cdk5 and suggest that antioxidants may alleviate multiple aspects of neuropathology in motor neuron disease.

A mathematical model for assessing changes in neurofilament protein levels in neurites and cell bodies of differentiating neuroblastoma cells

A mathematical model which allows the calculation of the level of neurofilament protein in the cell body (x) and in the neurites (y) of differentiating SK-N-SH cells is presented. The model considers the changes in cell number (proliferating cells) and the number of cells with neurites (differentiating cells). It takes into account the fact that (i) when cells are cultured in differentiating conditions, an increase in cell number is initially observed and (ii) in a non-synchronized population of differentiating cells, the length of neurite extended by individual cells varies within the population. Total neurofilament protein levels in a population of cells were measured by enzyme-linked immunoabsorbant assay and application of the model to the data allowed values for x and y to be calculated. The validity of the model is supported by the fact that the predicted total neurofilament protein levels are highly correlated with the experimentally derived neurofilament protein levels. The model should be of use in temporal studies of cytoskeletal proteins involved in neuronal growth/differentiation and also in studies in which the system is a target of toxic insult.

Effects of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine on differentiating mouse N2a neuroblastoma cells

The effect of the neurotoxin 1-methyl-4-phenyl-1,2,3, 6-tetrahydropyridine (MPTP) was investigated in mouse N2a neuroblastoma cells, induced to differentiate by serum withdrawal and addition of dibutyryl cyclic AMP, over a 24-h period. Addition of MPTP (10 microM) during differentiation caused a change in cell morphology characterised by an inhibition of axon outgrowth, in the absence of cell death. Biochemical characterisation by western blotting revealed that MPTP had no significant effects on the levels of actin, alpha-tubulin, or total heavy-chain neurofilament (NF-H). However, NF-H phosphorylation appeared to increase following MPTP treatment when blots were probed with the phosphorylation state-specific antibodies RMd09 and Ta51. In addition, indirect immunofluorescence analysis revealed an accumulation of phosphorylated NF-H in the cell perikaryon, suggesting that altered NF-H distribution was associated with the observed effects of MPTP on cell morphology. These changes may represent a useful in vitro marker of MPTP neurotoxicity within a simple differentiating neuronal cell model system.

Involvement of intracellular labile zinc in suppression of DEVD-caspase activity in human neuroblastoma cells

Age-related tissue Zn deficiency may contribute to neuronal and glial cell death by apoptosis in Alzheimer's dementia. To investigate this, we studied the effects of increasing or decreasing the levels of intracellular labile Zn on apoptosis of human neuroblastoma BE(2)-C cells in vitro. BE(2)-C cells were primed for 18 h with butyrate (1 mM) before addition of staurosporine (1 microM), an effector enzyme of apoptosis, for a further 3 h to induce DEVD-caspase activity. An increase in intracellular Zn using Zn ionophore pyrithione suppressed DEVD-caspase activity, while a decrease in intracellular Zn induced by Zn chelator TPEN mimicked staurosporine by activating DEVD-caspase in butyrate-primed cells. The distribution of intracellular Zn in the cells was demonstrated with the UV-excitable Zn-specific fluorophore Zinquin. Confocal images showed distinct cytoplasmic and cytoskeletal fluorescence. We propose that Zn decreases the level of apoptosis in neuronal cells exposed to toxins, possibly by stabilizing their cytoskeleton.