Immunocytochemical study of the distribution of the free radical scavenging enzymes Cu/Zn superoxide dismutase (SOD1); MN superoxide dismutase (MN SOD) and catalase in the normal human spinal cord and in motor neuron disease

Blood-Brain Barrier Disruption and Its Involvement in Neurodevelopmental and Neurodegenerative Disorders

The blood-brain barrier (BBB) is a highly specialized and dynamic compartment which regulates the uptake of molecules and solutes from the blood. The relevance of the maintenance of a healthy BBB underpinning disease prevention as well as the main pathomechanisms affecting BBB function will be detailed in this review. Barrier disruption is a common aspect in both neurodegenerative diseases, such as amyotrophic lateral sclerosis, and neurodevelopmental diseases, including autism spectrum disorders. Throughout this review, conditions altering the BBB during the earliest and latest stages of life will be discussed, revealing common factors involved. Due to the barrier's role in protecting the brain from exogenous components and xenobiotics, drug delivery across the BBB is challenging. Potential therapies based on the BBB properties as molecular Trojan horses, among others, will be reviewed, as well as innovative treatments such as stem cell therapies. Additionally, due to the microbiome influence on the normal function of the brain, microflora modulation strategies will be discussed. Finally, future research directions are highlighted to address the current gaps in the literature, emphasizing the idea that common therapies for both neurodevelopmental and neurodegenerative pathologies exist.

Structure and Function of the Mammalian Neuromuscular Junction

The mammalian neuromuscular junction (NMJ) comprises a presynaptic terminal, a postsynaptic receptor region on the muscle fiber (endplate), and the perisynaptic (terminal) Schwann cell. As with any synapse, the purpose of the NMJ is to transmit signals from the nervous system to muscle fibers. This neural control of muscle fibers is organized as motor units, which display distinct structural and functional phenotypes including differences in pre- and postsynaptic elements of NMJs. Motor units vary considerably in the frequency of their activation (both motor neuron discharge rate and duration/duty cycle), force generation, and susceptibility to fatigue. For earlier and more frequently recruited motor units, the structure and function of the activated NMJs must have high fidelity to ensure consistent activation and continued contractile response to sustain vital motor behaviors (e.g., breathing and postural balance). Similarly, for higher force less frequent behaviors (e.g., coughing and jumping), the structure and function of recruited NMJs must ensure short-term reliable activation but not activation sustained for a prolonged period in which fatigue may occur. The NMJ is highly plastic, changing structurally and functionally throughout the life span from embryonic development to old age. The NMJ also changes under pathological conditions including acute and chronic disease. Such neuroplasticity often varies across motor unit types. © 2022 American Physiological Society. Compr Physiol 12:1-36, 2022.

Size-Dependent Vulnerability of Lumbar Motor Neuron Dendritic Degeneration in SOD1G93A Mice

The motor neuron (MN) soma surface area is correlated with motor unit type. Larger MNs innervate fast fatigue-intermediate (FInt) or fast-fatiguable (FF) muscle fibers in type FInt and FF motor units, respectively. Smaller MNs innervate slow-twitch fatigue-resistant (S) or fast fatigue-resistant (FR) muscle fibers in type S and FR motor units, respectively. In amyotrophic lateral sclerosis (ALS), FInt and FF motor units are more vulnerable, with denervation and MN death occurring for these units before the more resilient S and FR units. Abnormal MN dendritic arbors have been observed in ALS in humans and rodent models. We used a Golgi-Cox impregnation protocol to examine soma size-dependent changes in the dendritic morphology of lumbar MNs in SOD1^G93A mice, a model of ALS, at pre-symptomatic, onset and mid-disease stages. In wildtype control mice, the relationship between MN soma surface area and dendritic length or dendritic spine number was highly linear (i.e., increased MN soma size correlated with increased dendritic length and spines). By contrast, in SOD1^G93A mice, this linear relationship was lost and dendritic length reduction and spine loss were observed in larger MNs, from pre-symptomatic stages onward. These changes correlated with the neuromotor symptoms of ALS in rodent models. At presymptomatic ages, changes were restricted to the larger MNs, likely to comprise vulnerable FInt and FF motor units. Our results suggest morphological changes of MN dendrites and dendritic spines are likely to contribute ALS pathogenesis, not compensate for it. Anat Rec, 303:1455-1471, 2020. © 2019 American Association for Anatomy.

Ubiquitin and endogenous antioxidant enzymes participate in neuroprotection of the rabbit spinal cord after ischemia and bradykinin postconditioning

The aim of this study was to investigate neuroprotective effect of bradykinin postconditioning on the rabbit spinal cord after 20 min of ischemia and 3 days of reperfusion. Bradykinin was administered by single i.p. application at 1, 6, 12 or 24 h after ischemia. Assessment of neurological function of hind limbs (Tarlov score) was estimated. Quantitative analysis was evaluated by Fluoro Jade B method, NeuN and ubiquitin immunohistochemistry in anterior horn neurons of the spinal cord. Histomorphologically distribution of ubiquitin and endogenous antioxidant enzymes (SOD1, SOD2, catalase) immunoreaction was described. Bradykinin postconditioning showed decreased number of degenerated neurons, increased number of surviving neurons and increase in number of ubiquitin positive neurons in all bradykinin postconditioned groups versus ischemia/reperfusion group. According to our results bradykinin postconditioning applied 24 h after ischemia significantly decreased (p < 0.001) number of degenerated neurons versus ischemia/reperfusion group. The least effective time window for bradykinin postconditioning was at 12 h after ischemia. Tarlov score was significantly improved (p < 0.05) in groups with bradykinin postconditioning applied 1, 6 or 24 h after ischemia versus ischemia/reperfusion group. Tarlov score in group with bradykinin application 12 h after ischemia was significantly decreased (p < 0.05) versus sham control group. Neuronal immunoreaction of ubiquitin, SOD1, SOD2 and catalase influenced by bradykinin postconditioning was dependent on neuronal survival or degeneration. In conclusion, bradykinin postconditioning showed protective effect on neurons in anterior horns of the rabbit spinal cord and improved motor function of hind limbs.

Pathomechanisms of Blood-Brain Barrier Disruption in ALS

The blood-brain barrier (BBB) and the blood-spinal cord barrier (BSCB) are responsible for controlling the microenvironment within neural tissues in humans. These barriers are fundamental to all neurological processes as they provide the extreme nutritional demands of neural tissue, remove wastes, and maintain immune privileged status. Being a semipermeable membrane, both the BBB and BSCB allow the diffusion of certain molecules, whilst restricting others. In amyotrophic lateral sclerosis (ALS) and other neurodegenerative diseases, these barriers become hyperpermeable, allowing a wider variety of molecules to pass through leading to more severe and more rapidly progressing disease. The intention of this review is to discuss evidence that BBB hyperpermeability is potentially a disease driving feature in ALS and other neurodegenerative diseases. The various biochemical, physiological, and genomic factors that can influence BBB permeability in ALS and other neurodegenerative diseases are also discussed, in addition to novel therapeutic strategies centred upon the BBB.

The vulnerability of spinal motoneurons and soma size plasticity in a mouse model of amyotrophic lateral sclerosis

KEY POINTS

Motoneuron soma size is a largely plastic property that is altered during amyotrophic lateral sclerosis (ALS) progression. We report evidence of systematic spinal motoneuron soma size plasticity in mutant SOD1-G93A mice at various disease stages and across sexes, spinal regions and motoneuron types. We show that disease-vulnerable motoneurons exhibit early increased soma sizes. We show via computer simulations that the measured changes in soma size have a profound impact on the excitability of disease-vulnerable motoneurons. This study reveals a novel form of plasticity in ALS and suggests a potential target for altering motoneuron function and survival.

ABSTRACT

α-Motoneuron soma size is correlated with the cell's excitability and function, and has been posited as a plastic property that changes during cellular maturation, injury and disease. This study examined whether α-motoneuron somas change in size over disease progression in the G93A mouse model of amyotrophic lateral sclerosis (ALS), a disease characterized by progressive motoneuron death. We used 2D- and 3D-morphometric analysis of motoneuron size and measures of cell density at four key disease stages: neonatal (P10 - with earliest known disease changes); young adult (P30 - presymptomatic with early motoneuron death); symptom onset (P90 - with death of 70-80% of motoneurons); and end-stage (P120+ - with full paralysis of hindlimbs). We additionally examined differences in lumbar vs. sacral vs. cervical motoneurons; in motoneurons from male vs. female mice; and in fast vs. slow motoneurons. We present the first evidence of plastic changes in the soma size of spinal α-motoneurons occurring throughout different stages of ALS with profound effects on motoneuron excitability. Somatic changes are time dependent and are characterized by early-stage enlargement (P10 and P30); no change around symptom onset; and shrinkage at end-stage. A key finding in the study indicates that disease-vulnerable motoneurons exhibit increased soma sizes (P10 and P30). This pattern was confirmed across spinal cord regions, genders and motoneuron types. This extends the theory of motoneuron size-based vulnerability in ALS: not only are larger motoneurons more vulnerable to death in ALS, but are also enlarged further in the disease. Such information is valuable for identifying ALS pathogenesis mechanisms.

Effect of carnosine, methylprednisolone and their combined application on irisin levels in the plasma and brain of rats with acute spinal cord injury

Spinal cord injury (SCI) might occur to anybody at any time and any age. In its treatment, methylprednisolone (MP) is a first choice worldwide, but there is still no significant breakthrough in truly beneficial treatment due to SCI's complex pathophysiology. We investigated the effect of carnosine, methylprednisolone (MP) and its combination on irisin levels in the plasma, brain and medulla spinalis tissues in SCI using a rat model. The rats were divided into 6 groups: I (Control, saline); II (sham animals with laminectomy without cross-clamping); III (SCI); IV (SCI treated with 150mg/kg carnosine); V (SCI treated with 30mg/kg methylprednisolone); and VI (SCI treated with a combination of carnosine and MP). The animals were given traumatic SCI after laminectomy, using 70-g closing force aneurysm clips (Yasargil FE 721). Irisin concentration was measured by ELISA. The distribution of irisin in brain and spinal cord tissues was examined by immunochemistry. Irisin was mainly expressed in the astrocytes and microglia of brain tissues, and multipolar neurones of the anterior horn of spinal cord tissue in rats of all groups, indicating that irisin is physiologically indispensable. MP and carnosine and the combination of the two, significantly increased irisin in plasma and were accompanied by a significant rise in irisin immunoreactivity of brain and spinal cord tissues of the injured rats compared with control and sham. This finding raises the possibility that methylprednisolone and carnosine regulate the brain and spinal cord tissues in SCI by inducing irisin expression, and may therefore offer a better neurological prognosis.

Biotransformation of nitric oxide in the cerebrospinal fluid of amyotrophic lateral sclerosis patients

Recent findings indicate that nitric oxide (NO*) over-production might be an important factor in the pathogenesis of sporadic amyotrophic lateral sclerosis (SALS). We measured significantly higher concentrations of uric acid and thiol group-containing molecules (R-SH groups) in the cerebrospinal fluid (CSF) from SALS patients compared to controls. The above factors, together with a slightly increased free iron concentration found in the CSF, favour conditions necessary for the formation of the dinitrosyl iron complex, capable of NO* bio-transformation. Thus, we performed ex vivo saturation of CSF (from both SALS patients and controls) with NO*. A decrease in the level of R-SH was found. This was more pronounced in the CSF from SALS patients. In the CSF from SALS patients the production of nitrite and hydroxylamine was greater than that observed in the CSF from controls. Moreover, we also found increased Cu,Zn-SOD activity in the CSF from SALS patients (when compared to control subjects) but no activity corresponding to Mn-SOD in any CSF samples. As Cu,Zn-SOD can react with nitroxyl forming NO*, the conditions for a closed, but continuous, loop of NO* biotransformation are present in the CSF of ALS patients.

Decreased mRNA expression of PGC-1α and PGC-1α-regulated factors in the SOD1G93A ALS mouse model and in human sporadic ALS

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease characterized by selective motoneuron loss. Although the cause of ALS is unknown, oxidative stress, inflammation, and mitochondrial dysfunction have been identified as important components of its pathogenesis. Peroxisome proliferator-activated receptor-γ coactivator 1α (PGC-1α) plays a central role in the regulation of mitochondrial metabolism and biogenesis via activation of transcription factors, such as nuclear respiratory factors 1 and 2 and mitochondrial transcription factor A (Tfam). Alterations in PGC-1α expression and function have previously been described in models of Huntington and Alzheimer diseases. Moreover, the protective effects of PGC-1α have been shown in animal models of ALS. Levels of PGC-1α correlate with the number of acetylcholine receptor clusters in muscle. This is of particular interest because neurodegeneration in ALS may be a dying-back process. We investigated mRNA and protein expressions of PGC-1α and PGC-1α-regulated factors in the spinal cord and muscle tissues of SOD1 ALS mice and in ALS patients. We detected significant alterations in mRNA expression of PGC-1α and downstream factors with their earliest occurrence in muscle tissue. Our data provide evidence for a role of PGC-1α in mitochondrial dysfunction both in the ALS mouse model and in human sporadic ALS that is probably most relevant in the skeletal muscle.

Impact of treadmill running and sex on hippocampal neurogenesis in the mouse model of amyotrophic lateral sclerosis

Hippocampal neurogenesis in the subgranular zone (SGZ) of dentate gyrus (DG) occurs throughout life and is regulated by pathological and physiological processes. The role of oxidative stress in hippocampal neurogenesis and its response to exercise or neurodegenerative diseases remains controversial. The present study was designed to investigate the impact of oxidative stress, treadmill exercise and sex on hippocampal neurogenesis in a murine model of heightened oxidative stress (G93A mice). G93A and wild type (WT) mice were randomized to a treadmill running (EX) or a sedentary (SED) group for 1 or 4 wk. Immunohistochemistry was used to detect bromodeoxyuridine (BrdU) labeled proliferating cells, surviving cells, and their phenotype, as well as for determination of oxidative stress (3-NT; 8-OHdG). BDNF and IGF1 mRNA expression was assessed by in situ hybridization. Results showed that: (1) G93A-SED mice had greater hippocampal neurogenesis, BDNF mRNA, and 3-NT, as compared to WT-SED mice. (2) Treadmill running promoted hippocampal neurogenesis and BDNF mRNA content and lowered DNA oxidative damage (8-OHdG) in WT mice. (3) Male G93A mice showed significantly higher cell proliferation but a lower level of survival vs. female G93A mice. We conclude that G93A mice show higher hippocampal neurogenesis, in association with higher BDNF expression, yet running did not further enhance these phenomena in G93A mice, probably due to a 'ceiling effect' of an already heightened basal levels of hippocampal neurogenesis and BDNF expression.

Comparison of immunoreactivities in 4-HNE and superoxide dismutases in the cervical and the lumbar spinal cord between adult and aged dogs

Aging shows slowly progressive changes and is associated with many functional and morphological changes in the central nervous system. The accumulation of reactive oxygen species causes age-related deterioration in neuronal function and contributes to the increase of disease susceptibility during normal aging. In the present study, we compared the neuronal distribution and immunoreactivities of 4-hydroxy-2E-nonenal (4-HNE, end product of lipid peroxidation), and superoxide dismutase 1 (SOD1) and SOD2 in the cervical and lumbar spinal cord between adult (2-3 years) and aged (10-12 years) dogs. No significant change in neuronal morphology was observed after cresyl violet staining. The number of NeuN (a marker for neurons)-immunoreactive neurons was not significantly changed in the aged group compare to the adult group. In addition, we could not find Fluoro-Jade B (a marker for degenerating neurons) positive cells in both the adult and aged dogs. However, numbers of 4-HNE-, SOD1- and SOD2-immunoreactive cells were significantly increased in both the cervical and lumbar spinal cord of the aged dog: The increase rates of these cells in the aged spinal cord were higher in the lumbar level than the cervical level. In brief, 4-HNE, SOD1 and SOD2 levels are much increased in the aged spinal cord compared to the adult spinal cord.

Novel antibodies reveal inclusions containing non-native SOD1 in sporadic ALS patients

Mutations in CuZn-superoxide dismutase (SOD1) cause amyotrophic lateral sclerosis (ALS) and are found in 6% of ALS patients. Non-native and aggregation-prone forms of mutant SOD1s are thought to trigger the disease. Two sets of novel antibodies, raised in rabbits and chicken, against peptides spaced along the human SOD1 sequence, were by enzyme-linked immunosorbent assay and an immunocapture method shown to be specific for denatured SOD1. These were used to examine SOD1 in spinal cords of ALS patients lacking mutations in the enzyme. Small granular SOD1-immunoreactive inclusions were found in spinal motoneurons of all 37 sporadic and familial ALS patients studied, but only sparsely in 3 of 28 neurodegenerative and 2 of 19 non-neurological control patients. The granular inclusions were by confocal microscopy found to partly colocalize with markers for lysosomes but not with inclusions containing TAR DNA binding protein-43, ubiquitin or markers for endoplasmic reticulum, autophagosomes or mitochondria. Granular inclusions were also found in carriers of SOD1 mutations and in spinobulbar muscular atrophy (SBMA) patients and they were the major type of inclusion detected in ALS patients homozygous for the wild type-like D90A mutation. The findings suggest that SOD1 may be involved in ALS pathogenesis in patients lacking mutations in the enzyme.

Effects of steroid hormones on catalase activity in neuronal and glial cell systems

The aim of the present study was to determine short- (1 and 3 h) and long-term (24 h) effects of glucocorticoids [GCs; corticosterone (CORT), dexamethasone (DEX) and 6-methylprednisolone (6-MP)] and gonadal steroids [GSs; 17beta-estradiol (E2), progesterone (PROG) and testosterone (TEST)] on the activity of the hydrogen-peroxide-detoxifying enzyme catalase (CAT) in neural hippocampal HT22 cells and glial C6 cells because such effects have been described in peripheral organ systems. In HT22 cells, only long-term treatment with glucocorticoids (10(-5) M) induced effects on catalase activity, whereas gonadal steroids (10(-5) M) affected catalase activity after both short- and long-term incubations. At a lower concentration of 10(-7) M, glucocorticoids exerted only short-term treatment effects on catalase activity, while gonadal steroids (10(-7) M) affected the enzyme activity after short- and long-term treatments. In C6 glial cells, both glucocorticoids (10(-7) M) and gonadal steroids (10(-7) M) induced short- and long-term treatment effects. Thereby, our data show that steroid hormones differentially regulate catalase activity in models of the central nervous system (CNS) in a time- and steroid-dependent manner.

Characterization of cellular and neurological damage following unilateral hypoxia/ischemia

Rodent models of stroke are often used to investigate the mechanisms that lead to ischemic neuronal damage. In this study, we used a model of cerebral hypoxia with ischemia to produce unilateral damage in C57Bl/6 mice. Lesion volume, ascertained by TTC staining, increased with longer durations of hypoxia. Additionally, cresyl violet, TUNEL, and FluoroJade staining showed a statistically significant increase in cellular damage in the ipsilateral cortex, CA1 pyramidal layer, and dentate gyrus of the hippocampus of ipsilateral hypoxic/ischemic tissue versus sham tissue. Astrocyte reactivity, determined by GFAP staining, was significantly higher in the ipsilateral H/I cortex and contralateral hippocampus compared to sham cortex and hippocampus, respectively. Increased microglia activation was evident in the H/I-treated cortex and hippocampus versus sham cortex and hippocampus, particularly within areas undergoing degeneration. To examine whether this model produces motor deficits, a battery of tests were administered before and after hypoxia. Following 45 min H/I, locomotor activity, rotarod performance and performance on an inverted wire hang test were all significantly decreased. These data indicate that the histological evidence of neuronal damage is consistent with functional deficits and suggest that this model may be useful for investigating strategies designed to protect neurons from hypoxia/ischemia-induced damage.

Amyotrophic lateral sclerosis: a novel hypothesis involving a gained 'loss of function' in the JNK/SAPK pathway

Amyotrophic lateral sclerosis (ALS) is a progressive and fatal neurodegenerative disease that mainly affects motor neurons. Despite intensive research efforts inspired by the mile-stone discovery linking the Cu/Zn superoxide dismutase 1 (SOD1) gene to a subset of familial cases, the mechanisms underlying disease pathogenesis are still largely unknown. Nonetheless, the recent finding of a second gene associated with familial form of the disease, ALS2, is likely to be of great help in elucidating the key pathways involved in motor neuron degeneration. Here, we provide evidence that the JNK/SAPK pathway plays a critical neuroprotective role in susceptible motor neurons in ALS. The involvement of the JNK/SAPK pathway integrates our knowledge about these two known genetic factors into a single pathogenic pathway involved in both sporadic and familial ALS.

Induction of manganese superoxide dismutase in acute spinal cord injury

Free radical-mediated mechanisms of cellular damage have been implicated in the early stages of spinal cord injury (SCI). Manganese superoxide dismutase (MnSOD) is a potent scavenger of superoxide radicals and likely serves an important cytoprotective role in preventing cellular damage after SCI. We have evaluated the expression of MnSOD to address its role during the early events of SCI using a well-established rat contusion model. Northern analysis showed a rapid induction of MnSOD mRNA between 2 and 6 h post injury. Observed time-dependent increases in MnSOD message was maximal 6 h post injury over that of MnSOD mRNA levels induced by laminectomy alone. Immunoblot and immunohistochemical analysis demonstrated increased expression of MnSOD protein 24 h after SCI with localization primarily within neurons. Interestingly, laminectomy alone also caused an induction of MnSOD gene and protein expression. To evaluate one potential mechanism of MnSOD induction, we microinjected the naive spinal cord with IL-1beta, which caused a similar fold induction of MnSOD mRNA levels by 6 h as observed with SCI, thus implicating it as a potential inducer of MnSOD during SCI. In summary, these results demonstrate that this potent cytoprotective antioxidant enzyme is rapidly and significantly induced as a consequence of SCI.

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.

Molecular factors underlying selective vulnerability of motor neurons to neurodegeneration in amyotrophic lateral sclerosis

Current research evidence suggests that genetic factors, oxidative stress and glutamatergic toxicity, with damage to critical target proteins and organelles, may be important contributory factors to motor neuron injury in amyotrophic lateral sclerosis (ALS). Various molecular and neurochemical features of human motor neurons may render this cell group differentially vulnerable to such insults. Motor neurons are large cells with long axonal processes which lead to requirements for a high level of mitochondrial activity and a high neurofilament content compared to other neuronal groups. The lack of calcium buffering proteins parvalbumin and calbindin D28k and the low expression of the GluR2 AMPA receptor subunit may render human motor neurons particularly vulnerable to calcium toxicity following glutamate receptor activation. Motor neurons also have a high perisomatic expression of the glutamate transporter protein EAAT2 and a very high expression of the cytosolic free radical scavenging enzyme Cu/Zn superoxide dismutase (SOD1) which may render this cell group vulnerable in the face of genetic or post-translational alterations interfering with the function of these proteins. More detailed characterisation of the molecular features of human motor neurons in the future may allow the strategic development of better neuroprotective therapies for the benefit of patients afflicted by ALS.

Oxidative stress and motor neurone disease

The effects of oxidative stress within post mitotic cells such as neurones may be cumulative, and injury by free radical species is a major potential cause of the age-related deterioration in neuronal function seen in several neurodegenerative diseases. There is strong evidence that oxidative stress plays an important role in the pathogenesis of motor neurone disease (MND). Point mutations in the antioxidant enzyme Cu,Zn superoxide dismutase (SOD1) are found in some pedigrees with the familial form of MND. How mutations in this ubiquitous enzyme cause the relatively selective cell death of specific groups of motor neurones is not clear, although a number of hypotheses have been forwarded. These include (1) the formation of hydroxyl radicals, (2) the catalysis of reactions of the nitrogen centred oxidant species peroxynitrite, (3) toxicity of copper or zinc and (4) protein aggregation. Some experimental support for these different hypotheses has been produced by manipulating cells in culture to express the mutant SOD1 proteins and by generating transgenic mice which over-express mutant SOD1. Observations in these model systems are, in some cases at least, supported by observations made on pathological material from patients with similar SOD1 mutations. Furthermore, there are reports of evidence of free radical mediated damage to neurones in the sporadic form of MND. Several lines of evidence suggest that alterations in the glutamatergic neurotransmitter system may also play a key role in the injury to motor neurones in sporadic MND. There are several important subcellular targets, which may be preferentially impaired within motor neurones, including neurofilament proteins and mitochondria. Future research will need to identify the aspects of the molecular and physiological phenotype of human motor neurones that makes them susceptible to degeneration in MND, and to identify those genetic and environmental factors which combine to cause this disease in individuals and in familial pedigrees.

Glutamate, excitotoxicity and amyotrophic lateral sclerosis

The "glutamate hypothesis" is one of three major pathophysiological mechanisms of motor neurone injury towards which current research effort into amyotrophic lateral sclerosis (ALS) is directed. There is great structural and functional diversity in the glutamate receptor family which results from combinations of 14 known gene products and their splice variants, with or without additional RNA editing. It is possible that motor neurones express a unique molecular profile of glutamate receptors. Abnormal activation of glutamate receptors is one of five main candidates as a final common pathway to neuronal death. In classical acute excitotoxicity, there is influx of Na+ and CI-, and destabilisation of intracellular Ca2+ homeostasis, which activates a cascade of harmful biochemical events. The concept of secondary excitotoxicity, where cellular injury by glutamate is triggered by disturbances in neuronal energy status, may be particularly relevant to a chronic neurodegenerative disease such as ALS. Data are now beginning to emerge on the fine molecular structure of the glutamate receptors present on human motor neurones, which have a distinct profile of AMPA receptors. Two important molecular features of motor neurones have been identified that may contribute to their vulnerability to neurodegeneration. The low expression of calcium binding proteins and the low expression of the GluR2 AMPA receptor subunit by vulnerable motor neurone groups may render them unduly susceptible to calcium-mediated toxic events following glutamate receptor activation. Eight lines of evidence that indicate a disturbance of glutamatergic neurotransmission in ALS patients are reviewed. The links between abnormal activation of glutamate receptors and other potential mechanisms of neuronal injury, including activation of calcium-mediated second messenger systems and free radical mechanisms, are emphasised. Riluzole, which modulates the glutamate neurotransmitter system, has been shown to prolong survival in patients with ALS. Further research may allow the development of subunit-specific therapeutic targeting of glutamate receptors and modulation of "downstream" events within motor neurones, aimed at protecting vulnerable molecular targets in specific populations of ALS patients.