Neurofilaments, free radicals, excitotoxins, and amyotrophic lateral sclerosis

Occurrence of Volcanogenic Inorganic Mercury in Wild Mice Spinal Cord: Potential Health Implications

Mercury accumulation has been proposed as a toxic factor that causes neurodegenerative diseases. However, the hazardous health effects of gaseous elemental mercury exposure on the spinal cord in volcanic areas have not been reported previously in the literature. To evaluate the presence of volcanogenic inorganic mercury in the spinal cord, a study was carried out in São Miguel island (Azores, Portugal) by comparing the spinal cord of mice exposed chronically to an active volcanic environment (Furnas village) with individuals not exposed (Rabo de Peixe village), through the autometallographic silver enhancement histochemical method. Moreover, a morphometric and quantification analysis of the axons was carried out. Results exhibited mercury deposits at the lumbar level of the spinal cord in the specimens captured at the site with volcanic activity (Furnas village). A decrease in axon calibre and axonal atrophy was also observed in these specimens. Given that these are relevant hallmarks in the neurodegenerative pathologies, our results highlight the importance of the surveillance of the health of populations chronically exposed to active volcanic environments.

Emerging Evidence Highlighting the Importance of Redox Dysregulation in the Pathogenesis of Amyotrophic Lateral Sclerosis (ALS)

The cellular redox state, or balance between cellular oxidation and reduction reactions, serves as a vital antioxidant defence system that is linked to all important cellular activities. Redox regulation is therefore a fundamental cellular process for aerobic organisms. Whilst oxidative stress is well described in neurodegenerative disorders including amyotrophic lateral sclerosis (ALS), other aspects of redox dysfunction and their contributions to pathophysiology are only just emerging. ALS is a fatal neurodegenerative disease affecting motor neurons, with few useful treatments. Hence there is an urgent need to develop more effective therapeutics in the future. Here, we discuss the increasing evidence for redox dysregulation as an important and primary contributor to ALS pathogenesis, which is associated with multiple disease mechanisms. Understanding the connection between redox homeostasis, proteins that mediate redox regulation, and disease pathophysiology in ALS, may facilitate a better understanding of disease mechanisms, and lead to the design of better therapeutic strategies.

Dynactin Deficiency in the CNS of Humans with Sporadic ALS and Mice with Genetically Determined Motor Neuron Degeneration

Dynactin is a complex motor protein involved in the retrograde axonal transport disturbances of which may lead to amyotrophic lateral sclerosis (ALS). Mice with hSOD1G93A mutation develop ALS-like symptoms and are used as a model for the disease studies. Similar symptoms demonstrate Cra1 mice, with Dync1h1 mutation. Dynactin heavy (DCTN1) and light (DCTN3) subunits were studied in the CNS of humans with sporadic ALS (SALS), mice with hSOD1G93A (SOD1/+), Dync1h1 (Cra1/+), and double (Cra1/SOD1) mutation at presymptomatic and symptomatic stages. In SALS subjects, in contrast to control cases, expression of DCTN1-mRNA but not DCTN3-mRNA in the motor cortex was higher than in the sensory cortex. However, the mean levels of DCTN1-mRNA and protein were lower in both SALS cortexes and in the spinal cord than in control structures. DCTN3 was unchanged in brain cortexes but decreased in the spinal cord on both mRNA and protein levels. In all SALS tissues immunohistochemical analyses revealed degeneration and loss of neuronal cells, and poor expression of dynactin subunits. In SOD1/+ mice both subunits expression was significantly lower in the frontal cortex, spinal cord and hippocampus than in wild-type controls, especially at presymptomatic stage. Fewer changes occurred in Cra1/SOD1 and Cra1/+ mice.It can be concluded that in sporadic and SOD1-related ALS the impairment of axonal retrograde transport may be due to dynactin subunits deficiency and subsequent disturbances of the whole dynein/dynactin complex structure and function. The Dync1h1 mutation itself has slight negative effect on dynactin expression and it alleviates the changes caused by SOD1G93A mutation.

Atrophy of Large Myelinated Motor Axons and Declining Muscle Grip Strength Following Mercury Vapor Inhalation in Mice

The effects of acute mercury vapor (Hg(0)) exposure on the peripheral motor system have not been previously addressed in the literature. Early case studies report that acute exposure in humans can cause symptoms resembling motor neuron disease (MND). Mercury granules can be histochemically demonstrated in the cytoplasm of murine motor neurons following Hg(0) exposure, suggesting it is transported from the neuromuscular junction (NMJ) to the cell body by retrograde axonal transport mechanisms. We considered the hypothesis that morphological damage to the peripheral motor axonal cytoskeleton possibly involving neurofilaments (NFs) follows Hg(0) exposure. Eight-week-old wild type (Wt) 129S/v mice were exposed to 500 microg/m(3) of Hg(0) for 4 h in an experimental vapor exposure chamber. Forelimb grip strength (FGS) was measured over 4-wk intervals prior to removal of the murine phrenic nerves (MPN) 7 mo postexposure. Autometallography of 7-microm-thick spinal-cord sections from Hg(0)-exposed mice confirmed the presence of mercury deposits in ventral horn motor neurons. The morphology of the myelinated motor axons was assessed by computer-assisted image analysis of 1-microm-thick resin cross sections of the MPN. The group exposed to Hg(0) showed a significant reduction in the mean axon caliber, p < .0001. Gaussian spectral analysis of axon diameter distribution showed atrophy principally to large myelinated fibers, a subpopulation of axons that is also affected in MND. This atrophic change was also accompanied by an increased irregularity in axon shape. FGS initially increased with age until 20 wk and then progressively decreased after 22 wk to 36 wk. In conclusion, Hg(0) exposure appears to reduce axon diameter, suggesting axon caliber-determining cytoskeletal components such as neurofilaments may be damaged by heavy metal-induced oxidative stress mechanisms, resulting in functional changes to motor units.

A study of glutathione S-transferase pi expression in central nervous system of subjects with amyotrophic lateral sclerosis using RNA extraction from formalin-fixed, paraffin-embedded material

The expression of glutathione S-transferase pi (GST pi), an enzyme responsible for inactivation of a large variety of toxic compounds was studied in spinal cord, motor and sensory brain cortex obtained from patients who died in the course of amyotrophic lateral sclerosis (ALS). The studies were performed on formalin-fixed, paraffin-embedded (FFPE) and freshly frozen tissues. The method of RNA isolation from FFPE was modified. A significant decrease of GST pi-mRNA expression was found in cervical spinal cord and motor brain cortex of ALS subjects comparing to analogue control tissues (P<0.01), as well as in motor cortex of ALS subjects comparing to their sensory cortex (P<0.05). In spinal cords the decrease in GST pi-mRNA expression was accompanied by a decrease of GST pi protein level. Results indicated lowered GST pi expression on both mRNA and protein levels in the regions of nervous system affected by ALS. The non-properly inactivated by GST toxic electrophiles and organic peroxides may thus contribute to motor neurons damage.

Early mitochondrial dysfunction occurs in motor cortex and spinal cord at the onset of disease in the Wobbler mouse

The Wobbler mouse is recognized as an animal model for motoneuron disease that exhibits motoneuron pathology. We have recently demonstrated the occurrence of mitochondrial dysfunction in the Wobbler mouse brain. The aim of the present study was to evaluate whether mitochondrial dysfunction occurred at an early age at the time where disease symptoms appear, and whether it was more pronounced in the motor cortex or in the spinal cord. We report here a significant decrease in mitochondrial state 3 and 4 respiration rates at an early age in the Wobbler spinal cord. In addition, there was a pronounced decrease in oxidative phosphorylation in mitochondria isolated from both spinal cord and motor cortex in both age groups. This mitochondrial dysfunction was accompanied by impairment of complex I activity in mitochondria isolated from spinal cord at an early age. Decreases in complex III and IV activities were observed only in mitochondria isolated from the motor cortex at an early age, but impairment of complex III activity prevailed until later in the disease. We conclude that mitochondrial dysfunction ensues at an early stage of the disease and is more pronounced in the spinal cord, which correlates with previous studies that reported degeneration of spinal cord motorneurons.

MRI-FLAIR images of the head show corticospinal tract alterations in ALS patients more frequently than T2-, T1- and proton-density-weighted images

In some patients with amyotrophic lateral sclerosis (ALS), T2-weighted and proton-density-weighted magnetic resonance imaging (MRI) shows hyperintense or hypointense signals at the corticospinal tract. Fluid-attenuated inversion recovery (FLAIR) sequences increase the sensitivity of MRI to detect cortical and subcortical tissue changes. In 31 ALS patients and 33 controls, we studied the frequency and the extent of signal abnormalities in FLAIR images compared to T2-, T1- and proton-density-weighted images. Hyperintense signals at the corticospinal tract were significantly more frequent in FLAIR images than in all other tested sequences. In FLAIR images of ALS patients only, distinct hyperintense signals at the subcortical precentral gyrus (five patients), the centrum semiovale (eight patients), the crus cerebri (nine patients) and the pons (four patients) as well as mild hyperintense signals in the medulla oblongata (three patients) were seen. More frequently, but not exclusively in ALS patients, FLAIR images showed mild hyperintense signals at the subcortical precentral gyrus (15 patients vs. 1 control). Quantitative analysis confirmed the significant difference between ALS patients and controls at the subcortical precentral gyrus in FLAIR images. In T1-weighted images, the corticospinal tract at the capsula interna was hypointense in significantly more controls than ALS patients. Also this difference was confirmed in the quantitative analysis. Similar to previous results, MR image alterations did correlate poorly to clinical data of upper motor neuron affliction.MR images of the head, including FLAIR images, provide additional information regarding corticospinal tract involvement in ALS patients. Because of an overlap with physiological findings, they have to be interpreted cautiously, with the exception of hyperintense signals at the subcortical precentral gyrus.

Localization of the giant axonal neuropathy gene to chromosome 16q24

Giant axonal neuropathy (GAN) is a degenerative disorder of the peripheral nerves that is inherited as an autosomal recessive trait, presenting in early childhood and progressing to death, usually by late adolescence. Diagnosis is made by peripheral nerve biopsy, in which a striking pathological finding is seen--fibers distorted by giant axonal swellings filled with densely packed bundles of neurofilaments (the primary intermediate filament in neurons), with segregation of other axoplasmic organelles. In addition to disorganized neurofilaments in nerve, disorganization of other members of the intermediate filament family of proteins is seen in other tissues; this implies that the underlying defect is one of generalized intermediate filament organization, with neurofilaments predominantly affected. We have pursued a genomewide search for regions of homozygosity of descent in 5 consanguineous families. A 5.3-cM region of homozygosity, shared in all 5 families, was found on chromosome 16q24, and linkage was established to this locus with a LOD score of 4.18 at theta = 0.00 at the most tightly linked marker, D16S3098. Determination of this locus is the first step toward characterizing the gene responsible for a fundamental property of intermediate filament organization and may shed light on other disorders (such as amyotrophic lateral sclerosis) in which neurofilament pathology is prominent.

Autoimmunity and ALS: studies on antibodies to acetylcholinesterase in sera

The involvement of the immune system in the pathogenesis of amyotrophic lateral sclerosis is controversial. It has been suggested that ALS patients develop specific antibodies against acetylcholinesterase (AChE) and that these antibodies by retrograde axonal transport may be the cause of death of the spinal motor neurons. It has also been argued that these antibodies elicit hemolysis of erythrocytes. However, using recombinant human AChE as antigen in ELISA and Western blot analysis, we have been unable to find any evidence for the existence of specific AChE antibodies in ALS patients.

Selective loss of neurofilament proteins after exposure of differentiated human IMR-32 neuroblastoma cells to oxidative stress

Millimolar concentrations of ascorbate in the presence of iron can cause neuronal cell death. This study shows that the human neuronal cell line IMR-32 is sensitive to ascorbate and that cytotoxicity can be blocked by the antioxidant enzymes Cu/Zn-superoxide dismutase and catalase. There was a selective loss of neurofilament proteins after exposure to 5 or 10 mM ascorbate, as assessed by immunostaining and by Western blotting. Loss of actin or tubulin was not seen, suggesting that loss of neurofilaments is a sensitive and selective marker for free radical damage in these cells.

Clinical epidemiology of amyotrophic lateral sclerosis

The introduction of palliative therapies in amyotrophic lateral sclerosis (ALS) will alter the epidemiology of ALS as it is known now. Although incidence rates will remain unchanged in the near future, prevalence rates will likely increase dramatically. Better understanding of the age-specific presentation of motor neuron diseases worldwide will shed light on the vexing questions concerning the variable incidence rates in some countries and apparent incidence gradients in North America and Europe.

Amyotrophic lateral sclerosis is a multifactorial disease

Amyotrophic lateral sclerosis (ALS) is probably biphasic. An initial trigger(s) is followed by a terminal cascade coinciding with the onset of neurological deficits. The terminal cascade involves interactive multifactorial pathogenic mechanisms. Aging must play a crucial role leading to multiple defective or degraded gene products accumulating with progressing years. This in turn leads to failure of receptor integrity and resulting excitotoxicity, free radical accumulation, failure of neurotrophism, and possibly immunological disturbances. These events are predated by months or years by a trigger which is also likely to be multifactorial and cumulative. Evidence suggests that environmental factors may be important triggers. Failure of specific glutamate transporters and calcium binding proteins may account for selective vulnerability of the corticomotoneuronal system. It is postulated that in ALS the primary target cell is the corticomotoneuron or the local circuit interneurons which modulate its activity. Glia cells may play an important role in the demise of the corticomotoneuronal cell. The disordered corticomotoneuron induces excessive excitatory transmitter (glutamate?) release at the corticomotoneuronal-spinal-motoneuronal synapse resulting in the subsequent demise of this neuron.