Reversibility of neurofilamentous inclusion formation following repeated sublethal intracisternal inoculums of AlCl3 in New Zealand white rabbits

Molecular mechanism of the carboxyl terminus of Hsc70-interacting protein in TAU hyperphosphorylation induced by AlCl3 in N2a cells

Aluminum (Al) is recognized as a neurotoxin. Studies have confirmed that the neurotoxicity induced by Al may be related to tau hyperphosphorylation. Phosphorylated tau is degraded through the ubiquitin-proteasome pathway (UPP), in which the carboxyl terminus of Hsc70-interacting protein (CHIP) plays an important role. However, whether the CHIP plays a role in regulating tau hyperphosphorylation induced by Al is yet to be determined. The purpose of this study was to explore the molecular mechanism of the CHIP in tau hyperphosphorylation induced by AlCl3 in N2a cells. Mouse neuroblastoma cells (N2a) were exposed to different concentrations of AlCl3 (0, 0.5, 1, and 2 mM) and treated with CHIP/CHIP shRNA/CHIP (ΔU-box)/CHIP (ΔTPR) plasmid transfection. The cell viability was determined by the CCK-8 kit. Protein expression was detected by Western blot. The interaction between CHIP and AlCl3 exposure on the proteins was analyzed by factorial design ANOVA. The results showed that Al can cause tau hyperphosphorylation, mainly affecting the pThr231, pSer262, and pSer396 sites of tau in N2a cells. UPP is involved in the degradation of tau hyperphosphorylation induced by Al in N2a cells, of which CHIP may be the main regulatory target. Both the U-box and TPR domains of CHIP are indispensable and play an important role in the regulation of tau hyperphosphorylation induced by AlCl3 in N2a cells.

Mechanism by Which Aluminum Regulates the Abnormal Phosphorylation of the Tau Protein in Different Cell Lines

Aluminum (Al) is an environmental neurotoxin to which humans are extensively exposed; however, the molecular mechanism of aluminum toxicity is unclear. Several studies have indicated that exposure to aluminum can cause abnormal phosphorylation of the tau protein. The purpose of this study was to investigate respectively the special molecular mechanism of abnormal regulation on synthesis and degradation of the tau protein induced by AlCl₃ in cells of different species. The results of tau protein showed that the sites of abnormal tau phosphorylation induced by AlCl₃ are Thr231, Ser262, and Ser396 in N2a cells. Meanwhile, the expressions of Thr181, Thr231, and Ser262 increased abnormally in SH-SY5Y cells. The result of the study showed that PP2A expression was high in N2a cells, while GSK-3β and PP2A in SH-SY5Y cells were involved in the synthesis process of abnormal tau phosphorylation induced by AlCl₃. In N2a cells, the ubiquitin-proteasome pathway (UPP) mainly regulated tau phosphorylation at Ser262 and Ser396. Meanwhile, in SH-SY5Y cells, the UPP mainly regulated tau phosphorylation at Thr231 and Ser396. In summary, the UPP is involved in the degradation of Tau that is abnormally phosphorylated induced by AlCl₃, but this process is site-specific and differs in cells of different species.

Correcting miR92a-vGAT-Mediated GABAergic Dysfunctions Rescues Human Tau-Induced Anxiety in Mice

Patients with Alzheimer's disease (AD) commonly show anxiety behaviors, but the molecular mechanisms are not clear and no efficient intervention exists. Here, we found that overexpression of human wild-type, full-length tau (termed htau) in hippocampus significantly decreased the extracellular γ-aminobutyric acid (GABA) level with inhibition of γ oscillation and the evoked inhibitory postsynaptic potential (eIPSP). With tau accumulation, the mice show age-dependent anxiety behaviors. Among the factors responsible for GABA synthesis, release, uptake, and transport, we found that accumulation of htau selectively suppressed expression of the intracellular vesicular GABA transporter (vGAT). Tau accumulation increased miR92a, which targeted vGAT mRNA 3' UTR and inhibited vGAT translation. Importantly, we found that upregulating GABA tones by intraperitoneal injection of midazolam (a GABA agonist), ChR2-mediated photostimulating and overexpressing vGAT, or blocking miR92a by using specific antagomir or inhibitor efficiently rescued the htau-induced GABAergic dysfunctions with attenuation of anxiety. Finally, we also demonstrated that vGAT level decreased while the miR92a increased in the AD brains. These findings demonstrate that the AD-like tau accumulation induces anxiety through disrupting miR92a-vGAT-GABA signaling, which reveals molecular mechanisms underlying the anxiety behavior in AD patients and potentially leads to the development of new therapeutics for tauopathies.

ALS-associated peripherin spliced transcripts form distinct protein inclusions that are neuroprotective against oxidative stress

Intracellular proteinaceous inclusions are well-documented hallmarks of the fatal motor neuron disorder amyotrophic lateral sclerosis (ALS). The pathological significance of these inclusions remains unknown. Peripherin, a type III intermediate filament protein, is upregulated in ALS and identified as a component within different types of ALS inclusions. The formation of these inclusions may be associated with abnormal peripherin splicing, whereby an increase in mRNA retaining introns 3 and 4 (Per-3,4) leads to the generation of an aggregation-prone isoform, Per-28. During the course of evaluating peripherin filament assembly in SW-13 cells, we identified that expression of both Per-3,4 and Per-28 transcripts formed inclusions with categorically distinct morphology: Per-3,4 was associated with cytoplasmic condensed/bundled filaments, small inclusions (<10μM), or large inclusions (≥10μM); while Per-28 was associated with punctate inclusions in the nucleus and/or cytoplasm. We found temporal and spatial changes in inclusion morphology between 12 and 48h post-transfected cells, which were accompanied by unique immunofluorescent and biochemical changes of other ALS-relevant proteins, including TDP-43 and ubiquitin. Despite mild cytotoxicity associated with peripherin transfection, Per-3,4 and Per-28 expression increased cell viability during H2O2-mediated oxidative stress in BE(2)-M17 neuroblastoma cells. Taken together, this study shows that ALS-associated peripherin isoforms form dynamic cytoplasmic and intranuclear inclusions, effect changes in local endogenous protein expression, and afford cytoprotection against oxidative stress. These findings may have important relevance to understanding the pathophysiological role of inclusions in ALS.

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.

Innate immunity in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative condition in which motor neurons are selectively targeted. Although the underlying cause remains unclear, evidence suggests a role for innate immunity in disease pathogenesis. Neuroinflammation in areas of motor neuron loss is evident in presymptomatic mouse models of ALS and in human patients. Efforts aimed at attenuating the inflammatory response in ALS animal models have delayed symptom onset and extended survival. Seemingly conversely, attempts to sensitize cells of the innate immune system and modulate their phenotype have also shown efficacy. Effectors of innate immunity in the CNS appear to have ambivalent potential to promote either repair or injury. Because ALS is a syndromic disease in which glutamate excitotoxicity, altered cytoskeletal protein metabolism, oxidative injury, mitochondrial dysfunction and neuroinflammation all contribute to motor neuron degeneration, targeting inflammation via modulation of microglial function therefore holds significant potential as one aspect of therapeutic intervention and could provide insight into the exclusive vulnerability of motor neurons.

Mice with targeted disruption of neurofilament light subunit display formation of protein aggregation in motoneurons and downregulation of complement receptor type 3 alpha subunit in microglia in the spinal cord at their earlier age: a possible feature in pre-clinical development of neurodegenerative diseases

The pathogenesis of neurodegenerative diseases prior to the onset of symptoms is generally not clear. The present study has employed a mouse model with a lack of the low-molecular-weight neurofilament subunit (NFL-/-), in which formation of protein aggregates occurs in neurons, to investigate glial cellular reactions in the lumbar cord segments of NFL-/- mice at ages from 1 to 6 months. Age-matched C57BL/6 mice serve as the control. Apparent neurofilament positive aggregates in the cytoplasm of motoneurons have been observed in NFL-/- mice. However, there were no noticeable changes in microglial numbers and GFAP staining of astrocytes. Unexpectedly, a downregulation in expression of complement receptor type 3 alpha subunit (CD11b) was detected in the spinal cord of NFL-/- mice, while there was no obvious difference between NFL-/- and C57BL/6 mice in the CD11b staining intensity of macrophages from livers and spleens. In addition, retardation in morphological transformation from activated to amoeboid microglia in response to sciatic nerve injury, differential expressions of some cytokines in the lumbar cord segments and induction of Iba-1 (ionized calcium-binding adaptor molecule-1) expression in microglia were observed in NFL-/- mice. Our results suggest not only the existence of an inhibitory niche for CD11b expression in microglia in the lumbar cord segments of NFL-/- mice but also differential microglial reactions between earlier and later stages of neuropathogenesis. Although the real cause for such inhibition is still unknown, this effect might play a particular role in the survival of the abnormal protein aggregate-bearing motoneurons in the early development stage of neurodegeneration in the NFL-/- mice.

The basic aspects of therapeutics in amyotrophic lateral sclerosis

Once thought to be a single pathological disease state, amyotrophic lateral sclerosis (ALS) is now recognized to be the limited phenotypic expression of a complex, heterogeneous group of biological processes, resulting in an unrelenting loss of motor neurons. On average, individuals affected with the disease live <5 years. In this article, the complex nature of the pathogenesis of ALS, including features of age dependency, environmental associations, and genetics, is reviewed. Once held to be uncommon, it is now clear that ALS is associated with a frontotemporal dementia and that this process may reflect disturbances in the microtubule-associated tau protein metabolism. The motor neuron ultimately succumbs in a state where significant disruptions in neurofilament metabolism, mitochondrial function, and management of oxidative stress exist. The microenvironment of the neuron becomes a complex milieu in which high levels of glutamate provide a source of chronic excitatory neurotoxicity, and the contributions of activated microglial cells lead to further cascades of motor neuron death, perhaps serving to propagate the disease once established. The final process of motor neuron death encompasses many features of apoptosis, but it is clear that this alone cannot account for all features of motor neuron loss and that aspects of a necrosis-apoptosis continuum are at play. Designing pharmacological strategies to mitigate against this process thus becomes an increasingly complex issue, which is reviewed in this article.

Progress in clinical neurosciences: the evidence for ALS as a multisystems disorder of limited phenotypic expression

Traditionally, amyotrophic lateral sclerosis (ALS) is considered to be a unique neurodegeneration disorder in which motor neurons are selectively vulnerable to a single disease process. Our current understanding of ALS, however, suggests that this is far too limited an approach. While motor neuron degeneration remains the central component to this process, there is considerable phenotypic variability including broad ranges in survivorship and the presence or absence of cognitive impairment. The number of familial variants of ALS for which unique genetic linkage has been identified is increasing, attesting further to the biological heterogeneity of the disorder. At the cellular level, derangements in cytoskeletal protein and glutamate metabolism, mitochondrial function, and in glial interactions are clearly evident. When considered in this fashion, ALS can be justifiably considered a disorder of multiple biological processes sharing in common the degeneration of motor neurons.

Motor neuronal death in sporadic amyotrophic lateral sclerosis (ALS) is not apoptotic. A comparative study of ALS and chronic aluminium chloride neurotoxicity in New Zealand white rabbits

Whether diseased motor neurones in sporadic amyotrophic lateral sclerosis (ALS) die via apoptosis is unknown. Because this relates primarily to difficulties in utilizing post-mortem tissue from end-stage disease, motor neurone degeneration in ALS spinal cord was compared with that of a model of a chronic motor neurone degeneration. Degenerating motor neurones in ALS, identified by ubiquitin immunoreactivity, did not demonstrate the morphological characteristics of apoptosis and were not c-Jun immunoreactive or TUNEL positive. A temporal analysis of spinal motor neurone death in the chronic AlCl3 neurotoxicity model of motor neurone degeneration was also undertaken. AlCl3 was administered intracisternally every 4 weeks and, at intervals of 51, 107, 156 and 267 days, evidence of apoptosis was sought by morphology, TUNEL hybridization or DNA laddering. Double-labelling immunostudies were also performed with antibodies to either c-Jun, ubiquitin or high molecular weight neurofilament (NFH) with TUNEL hybridization. Although significant neurone loss was evident, apoptosis was not found. These studies demonstrate a lack of apoptosis in ALS spinal motor neurones and suggest that this observation does not relate to the utilization of post-mortem tissue in which apoptotic neurones may have been lost.

A morphological analysis of the motor neuron degeneration and microglial reaction in acute and chronic in vivo aluminum chloride neurotoxicity

The monthly intracisternal inoculation of aluminum chloride (AlCl3) to young adult New Zealand white rabbits induces motor neuron degeneration marked by intraneuronal neurofilamentous aggregates similar to that observed in amyotrophic lateral sclerosis (ALS). However, in contrast to ALS, this process occurs in the experimental paradigm in the absence of a glial response. In addition, whereas ALS is a fatal disorder, the cessation of aluminum exposure leads to both clinical and neuropathological recovery. Because microglia can influence neuronal regeneration, we have examined the effect of both acute and chronic aluminum exposure on microglial activation in vivo. We have studied microglial morphology in young adult New Zealand white rabbits receiving either single (1000 microg) or repeated sublethal (100 microg monthly) intracisternal inoculums of AlCl3. In addition, rabbits receiving 1000 microg AlCl3 inoculums were studied following an unilateral sciatic axotomy 48 h prior to the AlCl3 exposure. Our studies demonstrate that microglial activation in vivo is inhibited by AlCl3 exposure, and that a correlation exists between the extent of microglia suppression and the potential for recovery. This suggests that microglial activation is an important determinant of neuronal injury.

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.

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.

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.