The role of intracellular free calcium in motor neuron disease

Neuroprotective gap-junction-mediated bystander transformations in the adult zebrafish spinal cord after injury

The adult zebrafish spinal cord displays an impressive innate ability to regenerate after traumatic insults, yet the underlying adaptive cellular mechanisms remain elusive. Here, we show that while the cellular and tissue responses after injury are largely conserved among vertebrates, the large-size fast spinal zebrafish motoneurons are remarkably resilient by remaining viable and functional. We also reveal the dynamic changes in motoneuron glutamatergic input, excitability, and calcium signaling, and we underscore the critical role of calretinin (CR) in binding and buffering the intracellular calcium after injury. Importantly, we demonstrate the presence and the dynamics of a neuron-to-neuron bystander neuroprotective biochemical cooperation mediated through gap junction channels. Our findings support a model in which the intimate and dynamic interplay between glutamate signaling, calcium buffering, gap junction channels, and intercellular cooperation upholds cell survival and promotes the initiation of regeneration.

Pathological changes in mice with long term cuprizone administration

Multiple sclerosis (MS) is an inflammatory demyelinating disease of the central nervous system (CNS). In MS, a long disease duration is known to be a strong risk factor for converting the clinical course of the disease from relapse remitting MS to secondary progressing MS. There is a hypothesis that long sustained demyelination may exhaust neurons, however, pathological changes induced in neurons following demyelination remain unknown. Cuprizone administration can induce and sustain demyelination in the mouse CNS. We examined pathological changes in mice following long sustained demyelination caused by up to 34-week cuprizone administration. Twelve-week cuprizone administration induced severe demyelination in the cerebral cortex, corpus callosum and deep cerebellar nuclei. Demyelination persisted up to 34 weeks, as shown by myelin basic protein immunohistochemistry. In contrast, cuprizone administration developed demyelination in the striatum by week 34. In these demyelinated regions, no neuronal loss was observed. However, in the striatum and deep cerebellar nuclei, cuprizone-induced demyelination changed the intracellular distribution of parvalbumin (PV). Furthermore, in the striatum, there was an increase in PV in the demyelinated axons and most PV immunoreactivity did not co-localize with SMI32 immunoreactivity in mice with 34-week cuprizone administration. Further, mice with 34-week cuprizone administration showed motor coordination dysfunction in the balance beam test. However, 12-week withdrawal from the cuprizone diet induced remyelination in the regions and motor coordination dysfunction recovered. These results indicate that 34-week cuprizone administration induces and sustains demyelination and results in reversible motor coordination dysfunction. The change of intracellular PV distribution suggests that PV may protect demyelinated axons by Ca²⁺ buffering. This model may be useful to investigate pathological and behavioral changes following demyelination in the CNS.

A repertoire of rhythmic bursting produced by hypoglossal motoneurons in physiological and pathological conditions

The brainstem nucleus hypoglossus contains motoneurons that provide the exclusive motor nerve supply to the tongue. In addition to voluntary tongue movements, tongue muscles rhythmically contract during a wide range of physiological activities, such as respiration, swallowing, chewing and sucking. Hypoglossal motoneurons are destroyed early in amyotrophic lateral sclerosis (ALS), a fatal neurodegenerative disease often associated with a deficit in the transport system of the neurotransmitter glutamate. The present study shows how periodic electrical discharges of motoneurons are mainly produced by a neuronal network that drives them into bursting mode via glutamatergic excitatory synapses. Burst activity is, however, modulated by the intrinsic properties of motoneurons that collectively synchronize their discharges via gap junctions to create 'group bursters'. When glial uptake of glutamate is blocked, a distinct form of pathological bursting spontaneously emerges and leads to motoneuron death. Conversely, H(2)O(2)-induced oxidative stress strongly increases motoneuron excitability without eliciting bursting. Riluzole (the only drug currently licensed for the treatment of ALS) suppresses bursting of hypoglossal motoneurons caused by blockage of glutamate uptake and limits motoneuron death. These findings highlight how different patterns of electrical oscillations of brainstem motoneurons underpin not only certain physiological activities, but also motoneuron death induced by glutamate transporter impairment.

N-methyl-D-aspartate triggers neonatal rat hypoglossal motoneurons in vitro to express rhythmic bursting with unusual Mg2+ sensitivity

The brainstem nucleus hypoglossus innervates the tongue which must contract rhythmically during respiration, chewing and swallowing. Such rhythmic discharges are due to network bursting mediated by AMPA receptor-dependent glutamatergic transmission. The contribution by hypoglossal motoneurons themselves to rhythmicity remains, however, unclear as they might simply express cyclic patterns produced by premotoneurons or, in analogy to spinal motoneurons, might participate to bursting due to activation of their N-methyl-D-aspartate (NMDA) receptors. Using patch clamp recording from hypoglossal motoneurons in slice preparations of neonatal rat brainstem, we observed that NMDA directly depolarized motoneurons to generate various discharge patterns. Most motoneurons produced transient bursts which were consistently restored by repolarizing membrane potential to rest. Fewer motoneurons generated either sustained bursting or random firing. Rhythmic bursts were recorded from XII nerve rootlets even when single motoneuron bursting required hyperpolarization. NMDA evoked bursts were blocked by the Ca2+ antagonist Cd2+, the gap junction blocker carbenoxolone, or Mg2+ free solution, and partially inhibited by tetrodotoxin or nifedipine. Under voltage clamp, NMDA-induced bursting persisted at negative or positive potentials and was resistant to high extracellular Mg2+ in accordance with the observation of widespread motoneuron expression of NMDA 2D receptor subunits that confer poor Mg2+ sensitivity. It is proposed that NMDA depolarized motoneurons with the contribution of Mg2+ insensitive channels, and triggered bursting via cyclic activation/deactivation of voltage-dependent Na+, Ca2+ and K+ currents spread through gap junctions. The NMDA-evoked bursting pattern was similar to the rhythmic discharges previously recorded from the XII nerve during milk sucking by neonatal rats.

Differential expression of group I metabotropic glutamate receptors in human motoneurons at low and high risk of degeneration in amyotrophic lateral sclerosis

Glutamate excitotoxicity has been suggested to play a role in amyotrophic lateral sclerosis, since overstimulation of post-synaptic glutamate receptors by accumulated extracellular glutamate leads to motoneuron cell death. It is however unclear as to why some groups of motoneurons degenerate in this disease while other groups remain relatively intact even during terminal stages of the disease. Our previous studies in the rat showed differential expression of group I metabotropic glutamate receptors in motoneurons at low and high risk of degeneration in amyotrophic lateral sclerosis. Here we have extended this study to normal human brains. In situ hybridization showed that transcripts of both metabotropic glutamate receptor (mGluR) 1 and mGluR5 were expressed in motoneurons in both the resistant motor nucleus III and the vulnerable motor nucleus XII. Immunolabeling of mGluR1alpha and mGluR5 was found in both motoneurons and glia-like cells in all the motor nuclei and the ventral horn of the cervical spinal cord studied. Quantitative analysis of optical density measurements showed higher levels of mGluR1alpha protein expression but lower levels of mGluR5 protein expression in the vulnerable motoneuron pool (VII, XII and spinal cord) than in the resistant motoneuron pool (III, IV and VI). This differential expression of group I metabotropic glutamate receptor proteins within vulnerable motoneuron pools may predispose these neurons to degeneration as seen in amyotrophic lateral sclerosis.

Transgenic mice in the study of ALS: the role of neurofilaments

Amyotrophic lateral sclerosis (ALS) is an adult-onset neurological disorder of multiple etiologies that affects primarily motor neurons in the brain and spinal cord. Abnormal accumulations of neurofilaments (NFs) in motor neurons and a down-regulation of mRNA for the NF light subunit (NF-L) are associated with ALS, but it remains unclear to what extent these NF perturbations contribute to human disease. Transgenic mouse studies demonstrated that overexpression of normal and mutant NF proteins can sometimes provoke a motor neuronopathy characterized by the presence of abnormal NF accumulations resembling those found in ALS. Remarkably, the motor neuronopathy in transgenic mice overexpressing human NF heavy (NF-H) subunits was rescued by the co-expression of a human NF-L transgene at levels that restored a correct stoichiometry of NF-L to NF-H subunits. Transgenic approaches have also been used to investigate the role of NFs in disease caused by Cu/Zn superoxide dismutase (SOD1) mutations, which is responsible for approximately 2% cases of ALS. Studies with transgenic mice expressing low levels of a fusion NF-H/lacZ protein, in which NFs are withheld from the axonal compartment, suggested that axonal NFs are not toxic intermediates required for SOD1-mediated disease. On the contrary, overexpression of human NF-H proteins was found to confer an effective protection against mutant SOD1 toxicity in transgenic mice, a phenomenon that may be due to the ability of NF proteins to chelate calcium. In conclusion, transgenic studies showed that disorganized NFs can sometimes have noxious effects resulting in neuronopathy. However, in the context of motor neuron disease caused by mutant SOD1, there is emerging evidence that NF proteins rather play a protective role.

Parvalbumin and calbindin D-28k immunoreactivity in transgenic mice with a G93A mutant SOD1 gene

Immunohistochemical study was performed to examine if calcium-binding proteins are involved in the degeneration of motor neurons in the brain stems and the spinal cords of transgenic mice carrying a G93A mutant human SOD1 gene. Specimens from age-matched non-transgenic wild-type mice served as controls. In the spinal cord of the controls, the density of parvalbumin-immunoreactive neurons was highest in the large anterior horn neurons and lower in the posterior horn neurons in the spinal cord. On the other hand, calbindin D-28k immunoreactivity was much less apparent than that observed with parvalbumin antisera. Rexed's lamina II was densely immunostained for calbindin D-28k, whereas, in the anterior horn, calbindin-D-28k-positive small neurons were barely dispersed in a scattered pattern. In transgenic mice, parvalbumin-positive anterior horn neurons were severely reduced, even at the presymptomatic stage, whereas calbindin-positive neurons were largely preserved. At the symptomatic stage, both parvalbumin and calbindin D-28k immunoreactivity markedly diminished or disappeared in the anterior horn. Immunoblotting analysis revealed a significant reduction of immunoreactivity to parvalbumin antibody in transgenic mice compared with the controls. In the brain stem, parvalbumin-positive oculomotor and abducens neurons and the calbindin D-28k-positive sixth nucleus were well-preserved in transgenic mice as well as in the controls. Thus, the diffuse and severe loss of parvalbumin immunoreactivity of large motor neurons even at early stages in SOD1-transgenic mice and the absence of calbindin D-28k immunoreactivity of normal large motor neurons suggest that these calcium-binding proteins may contribute to selective vulnerability and an early loss of function of large motor neurons in this SOD1-transgenic mouse model.

Glutamate uptake block triggers deadly rhythmic bursting of neonatal rat hypoglossal motoneurons

In the brain the extracellular concentration of glutamate is controlled by glial transporters that restrict the neurotransmitter action to synaptic sites and avoid excitotoxicity. Impaired transport of glutamate occurs in many cases of amyotrophic lateral sclerosis, a devastating motoneuron disease. Motoneurons of the brainstem nucleus hypoglossus are among the most vulnerable, giving early symptoms like slurred speech and dysphagia. However, the direct consequences of extracellular glutamate build-up, due to uptake block, on synaptic transmission and survival of hypoglossal motoneurons remain unclear and have been studied using the neonatal rat brainstem slice preparation as a model. Patch clamp recording from hypoglossal motoneurons showed that, in about one-third of these cells, inhibition of glutamate transport with the selective blocker dl-threo-beta-benzyloxyaspartate (TBOA; 50 mum) unexpectedly led to the emergence of rhythmic bursting consisting of inward currents of long duration with superimposed fast oscillations and synaptic events. Synaptic inhibition block facilitated bursting. Bursts had a reversal potential near 0 mV, and were blocked by tetrodotoxin, the gap junction blocker carbenoxolone, or antagonists of AMPA, NMDA or mGluR1 glutamate receptors. Intracellular Ca(2+) imaging showed bursts as synchronous discharges among motoneurons. Synergy of activation of distinct classes of glutamate receptor plus gap junctions were therefore essential for bursting. Ablating the lateral reticular formation preserved bursting, suggesting independence from propagated network activity within the brainstem. TBOA significantly increased the number of dead motoneurons, an effect prevented by the same agents that suppressed bursting. Bursting thus represents a novel hallmark of motoneuron dysfunction triggered by glutamate uptake block.

Excitatory amino acid neurotransmission

In recent years great progress has been made in understanding the function of ionotropic and metabotropic glutamate receptors; their pharmacology and potential therapeutic applications. It should be stressed that there are already N-methyl-D-aspartate (NMDA) antagonists in clinical use, such as memantine, which proves the feasibility of their therapeutic potential. It seems unlikely that competitive NMDA receptor antagonists and high-affinity channel blockers will find therapeutic use due to limiting side-effects, whereas agents acting at the glycineB site, NMDA receptor subtype-selective agents and moderate-affinity channel blockers are far more promising. This is supported by the fact that there are several glycineB antagonists, NMDA moderate-affinity channel blockers and NR2B-selective agents under development. Positive and negative modulators of AMPA receptors such as the AMPAkines and 2,3-benzodiazepines also show more promise than e.g. competitive antagonists. Great progress has also been made in the field of metabotropic glutamate receptors since the discovery of novel, allosteric modulatory sites for these receptors. Selective agents acting at these transmembrane sites have been developed that are more drug-like and have a much better access to the central nervous system than their competitive counterparts. The chapter will critically review preclinical and scarce clinical experience in the development of new ionotropic and metabotropic glutamate receptor modulators according to the following scheme: rational, preclinical findings in animal models and finally clinical experience, where available.

Presence and distribution of three calcium binding proteins in projection neurons of the adult rat cochlear nucleus

The presence and distribution of three cytoplasmic calcium binding proteins, calbindin, calretinin, and parvalbumin, have been investigated in the projection neurons of the cochlear nucleus complex in adult rats by using immunohistochemistry in free-floating slices. Identification of the individual cell types was carried out on the basis of their intranuclear localization, morphological characteristics, and (in the cases of pyramidal and bushy neurons) by retrograde labeling with rhodamine-dextran. The most important findings were confirmed by using confocal microscopy. The data obtained in these experiments are the first to demonstrate the presence of parvalbumin in pyramidal neurons and globular and spherical bushy cells of rat cochlear nucleus, whereas octopus and giant cells did not show positivity for parvalbumin. Calretinin was not present in either Purkinje-like cells or giant neurons. According to the double immunolabeling co-localization experiments, the pyramidal neurons, Purkinje-like cells, globular bushy cells, and octopus cells express two different calcium binding proteins in their cytoplasm (although in different combinations) whereas giant cells and spherical bushy cells contain solely calbindin and parvalbumin, respectively. The presence of calretinin in globular bushy cells provides a tool for distinguishing them from spherical bushy cells. The immunolabeling of the fibers and axonal endings of the acoustic nerve in the ventral part of the cochlear nucleus indicated that these structures are also parvalbumin positive. It is concluded that the heterogenous cell composition of the cochlear nucleus is accompanied by a rather complex expression pattern of the cytoplasmic calcium binding proteins.

Decreased expression of calretinin in the cerebral cortex and hippocampus of SOD1G93A transgenic mice

In the present study, we investigated the changes of calretinin (CR) expression in the central nervous system of SOD1G93A transgenic mice as an in vivo model of amyotrophic lateral sclerosis (ALS). In wild-type SOD1 (wtSOD1) transgenic mice, many CR-immunoreactive neurons were found in all cortical regions. In the cerebral cortex of SOD1G93A transgenic mice, the number and staining intensity of CR-positive neurons were decreased. In the hippocampal formation, layer-specific alterations in the staining intensity of CR-immunoreactive neurons were observed in the CA1-3 areas and dentate gyrus. In wtSOD1 transgenic mice, CR-immunoreactive neurons with long processes were found in the stratum oriens and stratum radiatum of CA1-3 areas, and heavily stained band-like molecular layer was prominent in the dentate gyrus. CR immunoreactivity was decreased in each layer of CA1-3 areas and dentate gyrus of SOD1G93A transgenic mice. The first demonstration of decreased immunoreactivity for CR in the cerebral cortex and hippocampus of SOD1G93A transgenic mice may provide insights into the pathogenesis of motor neuron degeneration in human ALS although further quantitative studies are needed.

Recurrent paraplegia after remyelination of the spinal cord

We have conducted a long-term study of spinal cord morphology and motor function recovery in rats that have undergone lumbar spinal demyelination induced by the B-fragment of cholera toxin (CTB)-saporin. We found that, after the initial demyelination and paraplegia, motor function recovered and was stable for up to 9 months, after which there occurred a slow deterioration of motor function accompanied by loss of motoneurons and loss of spinal white matter. A striking morphological feature was the appearance of large spheroids of calcium in the ventral and dorsal horns and occasionally in the white matter. Motor performance deterioration occurred earlier and was more severe in rats that had been exercised on a treadmill, but the same morphological changes occurred in both exercise- and nonexercise-treated animals. Rats given treadmill exercise starting 3 weeks after toxin injection had a mean motor deficit score of 3.0 (i.e., paraplegia) at perfusion, whereas the nontreadmill-treated rats had a mean score of 1.8 (SD 0.38; n = 6; P <.05). These findings suggest that, in addition to the acute effects of the toxin-induced demyelination from which there is recovery of motor function, there are chronic irreversible effects of the toxin, or the initial demyelination, that cause a slow progressive degeneration of the spinal cord. This model might therefore be useful in studying the long-term effects of spinal insult of the type associated with conditions such as postpolio syndrome.

Effects of caffeine on the excitability and intracellular Ca(2+) transients of neonatal rat hypoglossal motoneurons in vitro

Since constitutively-high intracellular Ca(2+) ([Ca(2+)](i)) may confer hypoglossal motoneurons special vulnerability to excitoxic damage, we investigated the spatiotemporal dynamics of [Ca(2+)](i) and its relation to spike firing of rat hypoglossal motoneurons recorded under whole-cell patch clamp coupled with high resolution [Ca(2+)](i) imaging. A rise in [Ca(2+)](i), appearing in conjunction with single action potentials and becoming larger during spike trains, was first detected immediately beneath the cell membrane area, peaked 10-20 ms after each spike, and propagated to the cell core with slow decay time. Depletion of ryanodine-sensitive [Ca(2+)](i) stores by caffeine increased background [Ca(2+)](i), augmented the spike medium afterhyperpolarization, slowed down the action potential firing rate and depolarized cells (after an early hyperpolarization). The decay time constant of [Ca(2+)](i) transients was more than doubled by caffeine, although peak [Ca(2+)](i) remained unchanged. It is suggested that the main role of caffeine-sensitive stores was to buffer [Ca(2+)](i) elevated by sustained firing and to control spike accommodation.

Evaluation of GluR2 subunit involvement in AMPA receptor function of neonatal rat hypoglossal motoneurons

AMPA receptors (AMPAr) mediate fast synaptic responses to glutamate and, when they lack the GluR2 subunit, are strongly Ca2+ permeable and may increase intracellular Ca2+ levels. Because hypoglossal motoneurons possess restricted ability to buffer internal Ca2+ and are vulnerable to Ca2+ excitotoxicity, we wondered if, in these cells, any significant Ca2+ influx could be generated via AMPAr activity. Using whole cell patch-clamp recording from neonatal rat hypoglossal motoneurons, we tested the AMPAr properties conferred by GluR2 subunits, namely Ca2+ permeability, current rectification and sensitivity to pentobarbital or to the subunit-specific channel blockers, IEM-1460 and IEM-1925. We recorded membrane currents generated by the agonist, kainate, and compared them with those obtained from hippocampal pyramidal neurons (expressing GluR2-containing AMPAr) and from striatal giant aspiny or hippocampal interneurons (with GluR2-lacking AMPAr). Ca2+ vs. Na+ permeability of motoneuron AMPAr was relatively low (0.25 +/- 0.05), although higher than that of pyramidal neurons. With intracellularly applied spermine, significant inward rectification was absent from motoneurons. These data indicated the prevalence of functional GluR2 subunits. However, the sensitivity of motoneuron AMPAr to pentobarbital did not differ from that of GluR2-lacking AMPAr on interneurons. Motoneurons possessed sensitivity to IEM-1460 (IC50 = 90 +/- 10 microm) approximately 10-fold lower than striatal interneurons, although 10-fold higher than hippocampal pyramidal cells. IEM-1925 also reduced the amplitude of excitatory synaptic currents in brainstem slice motoneurons. We hypothesize that hypoglossal motoneuron AMPAr (moderately Ca2+ permeable because they contain few GluR2 subunits) may contribute to intracellular Ca2+ rises especially if persistent AMPAr activation (or the pathological GluR2 down-regulation) occurs.

GluR2 AMPA receptor subunit expression in motoneurons at low and high risk for degeneration in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a fatal neurological disorder that results in selective degeneration of most, but not all, groups of motoneurons. The greater susceptibility of vulnerable motoneurons to glutamate excitotoxicity and neurodegeneration has been hypothesized to result from their lower expression of the GluR2 AMPA receptor subunit under control conditions, which renders these receptors permeable to calcium. To address the question of whether there is differential expression of the GluR2 subunit in motoneurons, we compared in normal adult rats expression of GluR2 mRNA and protein within two cranial motor nuclei that are either resistant (III; oculomotor nucleus) or vulnerable (XII; hypoglossal nucleus) to degeneration in ALS. RT-PCR analysis of tissue punched from III and XII motor nuclei detected mRNA for all AMPA subunits (GluR1-R4). In situ hybridization demonstrated no significant difference in GluR2 mRNA expression between III and XII nuclei. Immunohistochemical examination of GluR2 (and GluR4) protein levels demonstrated a similar pattern of the subunit expression in both motor nuclei. This equivalent expression of GluR2 mRNA and protein in motoneurons that differ in their vulnerability to degeneration in ALS suggests that reduced expression of GluR2 is not a factor predisposing motoneurons to degeneration.

Human IgM anti-GM1 autoantibodies modulate intracellular calcium homeostasis in neuroblastoma cells

Increased titers of IgM anti-GM1 antibodies are present in some patients with Lower Motor Neuron Disease (LMND) or Motor Neuropathy (MN), but their pathogenic role and the mechanism of action are unclear. Previous studies have shown that the B subunit of Cholera Toxin (CT), which binds and crosslinks ganglioside GM1, modulate intracellular calcium in murine neuroblastoma cells via the activation of L-type voltage-dependent calcium channels (VGCC). Therefore, using a fluorimetric approach, we have examined the hypothesis that the pentameric IgM anti-GM1 antibodies, could similarly alter calcium concentration in N18 neuroblastoma cells. Sera with human IgM anti-GM1 antibodies were obtained from 5 patients with LMND and 2 patients with MN. Human IgG anti-GM1, IgM anti-Myelin Associated Glycoprotein (MAG), IgM anti-sulfatide antibodies and lectin peanut agglutinin (PNA), that recognizes specifically the Gal(betal-3)GalNAc epitope, were used as control sera. Direct application of either human IgM anti-GM1 antibodies or the B subunit of CT to N18 neuroblastoma cells induced a sustained influx of manganese ions, as indicated by a quench of the intracellular fura-2 fluorescence. Furthermore, the dihydropyridine L-type channel antagonists completely inhibited the manganese influx, suggesting that it is due to activation of an L-type VGCC. The magnitude of the influx was correlated with antibody titers. None of human IgG anti-GM1, IgM anti-MAG, IgM anti-sulfatide antibodies or PNA induce an ion influx, pointing to the selective participation of the pentameric IgM isotype of anti-GM1 in the modulation of L-type calcium channels opening. Given that L-type calcium channels are present on motor neurons, the modulation of L-type calcium channels by IgM GM1 antisera may have important implications in diseases such as LMND and MN.

Calcineurin A and calbindin immunoreactivity in the spinal cord of G93A superoxide dismutase transgenic mice

A qualitative immunohistochemical study was performed on calcineurin A- and calbindin-positive neurons in the spinal cord of transgenic mice, an animal model of amyotrophic lateral sclerosis, carrying the G93A mutation of the Cu/Zn-superoxide dismutase gene. The results show that calcineurin A-immunoreactive motoneurons are affected by the neurodegenerative process; in contrast, calbindin-positive cells are selectively spared. The findings suggest that calcineurin plays a role as an accessory factor responsible for selective vulnerability in the neurodegenerative process of amyotrophic lateral sclerosis.

Mitochondrial carbonic anhydrase in the nervous system: expression in neuronal and glial cells

Carbonic anhydrase (CA) V is a mitochondrial enzyme that has been reported in several tissues of the gastrointestinal tract. In liver, it participates in ureagenesis and gluconeogenesis by providing bicarbonate ions for two other mitochondrial enzymes: carbamyl phosphate synthetase I and pyruvate carboxylase. This study presents evidence of immunohistochemical localization of CA V in the rodent nervous tissue. Polyclonal rabbit antisera against a polypeptide of 17 C-terminal amino acids of rat CA V and against purified recombinant mouse isozyme were used in western blotting and immunoperoxidase stainings. Immunohistochemistry showed that CA V is expressed in astrocytes and neurons but not in oligodendrocytes, which are rich in CA II, or capillary endothelial cells, which express CA IV on their plasma face. The specificity of the immunohistochemical results was confirmed by western blotting, which identified a major 30-kDa polypeptide band of CA V in mouse cerebral cortex, hippocampus, cerebellum, spinal cord, and sciatic nerve. The expression of CA V in astrocytes and neurons suggests that this isozyme has a cell-specific, physiological role in the nervous system. In astrocytes, CA V may play an important role in gluconeogenesis by providing bicarbonate ions for the pyruvate carboxylase. The neuronal CA V could be involved in the regulation of the intramitochondrial calcium level, thus contributing to the stability of the intracellular calcium concentration. CA V may also participate in bicarbonate ion-induced GABA responses by regulating the bicarbonate homeostasis in neurons, and its inhibition could be the basis of some neurotropic effects of carbonic anhydrase inhibitors.

Calcium dynamics and buffering in oculomotor neurones from mouse that are particularly resistant during amyotrophic lateral sclerosis (ALS)-related motoneurone disease

Motoneurones are particularly vulnerable both in human forms of amyotrophic lateral sclerosis (ALS) and corresponding animal models of the disease. While most motoneurone populations are selectively impaired, oculomotor neurones are essentially resistant to ALS-related damage. Motoneurone vulnerability has been closely linked to disruptions of calcium signalling. To investigate underlying events, we performed a quantitative analysis of calcium homeostasis in oculomotor neurones from mice by simultaneous patch-clamp recordings in sliced tissue and microfluorometric-calcium measurements. Somatic calcium dynamics were investigated by using a computer-controlled microfluorometric system. In oculomotor neurones, basal calcium concentrations were around 80 nM and depolarisation-induced calcium responses were observed for membrane voltages positive to -40 u1u1u approximately mV1 approximately . Endogenous calcium homeostasis was quantified by using the 'added buffer' approach. The recovery phase of depolarisation-induced calcium transients was well approximated by a mono-exponential function with a decay time constant that showed a linear dependence on dye concentration. The extrapolated time constant in the absence of indicator dye was 1.7 +/- 0.2 s (n = 11 cells, 21C). Endogenous calcium binding ratios (kappa(s)) were found to be 264 +/- 25 (n = 11 cells), indicating that 99.6 % of cytosolic calcium ions were taken up by endogenous buffers. Recovery of calcium transients was characterised by an 'effective' extrusion rate gamma = 156 +/- 20 s-1 (n = 11 cells, 21 C). Endogenous calcium binding ratios in oculomotor neurones were 5- to 6-fold larger compared with those of more vulnerable motoneurones in the nucleus hypoglossus and spinal cord. In a first order approximation, they reduced the volume of local calcium elevations around open calcium channels, lowered peak amplitudes of global calcium transients for a given influx and prolonged calcium recovery times for a given set of uptake and extrusion mechanisms. With respect to motoneurone degeneration, our measurements suggest that the exceptional stability of oculomotor neurones partially results from a specialised calcium homeostasis based on high buffering capacities. Furthermore, they indicate that cellular adaptations that account for rapid calcium signalling in hypoglossal and spinal motoneurones enhance their vulnerability during ALS-related motoneurone disease.

Activity-related calcium dynamics in motoneurons of the nucleus hypoglossus from mouse

A quantitative analysis of activity-related calcium dynamics was performed in motoneurons of the nucleus hypoglossus in the brain stem slice preparation from mouse by simultaneous patch-clamp and microfluorometric calcium measurements. Motoneurons were analyzed under in vitro conditions that kept them in a functionally intact state represented by rhythmic, inspiratory-related bursts of excitatory postsynaptic currents and associated action potential discharges. Bursts of electrical activity were paralleled by somatic calcium transients resulting from calcium influx through voltage-activated calcium channels, where each action potential accounted for a calcium-mediated charge influx around 2 pC into the somatic compartment. Under in vivo conditions, rhythmic-respiratory activity in young mice occurred at frequencies up to 5 Hz, demonstrating the necessity for rapid calcium elevation and recovery in respiratory-related neurons. The quantitative analysis of hypoglossal calcium homeostasis identified an average extrusion rate, but an exceptionally low endogenous calcium binding capacity as cellular parameters accounting for rapid calcium signaling. Our results suggest that dynamics of somatic calcium transients 1) define an upper limit for the maximum frequency of respiratory-related burst discharges and 2) represent a potentially dangerous determinant of intracellular calcium profiles during pathophysiological and/or excitotoxic conditions.

L-type calcium channels in the hippocampus and cerebellum of Alzheimer's disease brain tissue

There is growing evidence that the selective neuronal cell death observed in Alzheimer's Disease (AD) is the result of dysregulation of intracellular calcium (Ca2+) homeostasis. In the present study, L-type voltage sensitive calcium channels (L-VSCCs) were examined in the cerebellum and hippocampus of AD (n = 6; postmortem interval less than 5 h) and age-matched control (n = 6) tissue by homogenate binding techniques and quantitative in vitro receptor autoradiography using [3H]isradipine (PN200-110). Saturation analyses of the cerebellum revealed unaltered [3H]isradipine binding parameters (Kd and Bmax) between AD and control subjects. Analysis of AD and control hippocampus demonstrated significant differences as [3H]isradipine binding increased (62%) in AD, whereas hippocampal cell density decreased (29%) in AD, relative to control subjects. Moreover, AD differentially affected L-VSCC in area CA1 and dentate gyrus. The dentate gyrus had greatly increased binding (77%) with little cell loss (16%) in AD brains, whereas area CA1 had increased binding (40%) with significant cell loss (42%) in AD brains, relative to controls. The results of the present study suggest that hippocampal area CA1 may experience greater cell loss in response to increased L-VSCCs in AD relative to other brain regions.

Pathogenesis of amyotrophic lateral sclerosis: a critical review

Amyotrophic lateral sclerosis is a neurodegenerative disease with unknown pathogenesis. It is a relatively common disorder of adults (2-4 per 100,000 incidence) and leads to death from respiratory failure. There is no cure at this time, and available treatment and management can at best extend survival to a modest degree. Increasing our understanding of the pathogenesis of this disease is essential to the development of more effective treatments. The level of research interest is very high, and yearly reviews of the literature are helpful in assessing progress.

Amyotrophic lateral sclerosis associated with mutations in superoxide dismutase: a putative mechanism of degeneration

Amyotrophic lateral sclerosis (ALS) is a devastating neurologic disease that rapidly progresses from mild motor symptoms to severe motor paralysis and premature death. Until recently, there were few substantive studies conducted on the pathogenesis of the disease. With the genetic linkage of mutations in the superoxide dismutase (SOD-1) gene with familial ALS patients, new avenues for study have become available including transgenic mice and culture models. Although not yet providing a complete picture of the disease mechanism, studies utilizing these model systems have greatly advanced our understanding of the mechanism of degeneration and should eventually lead to putative therapeutic agents. In this review, we will present the important findings from these model systems, provide a framework in which to evaluate these findings, and speculate on the mechanism of degeneration initiated by the mutations in SOD-1.

Glutamate potentiates the toxicity of mutant Cu/Zn-superoxide dismutase in motor neurons by postsynaptic calcium-dependent mechanisms

Mutations in the Cu/Zn-superoxide dismutase (SOD-1) gene are responsible for a subset of familial cases of amyotrophic lateral sclerosis. Using a primary culture model, we have demonstrated that normally nontoxic glutamatergic input, particularly via calcium-permeable AMPA/kainate receptors, is a major factor in the vulnerability of motor neurons to the toxicity of SOD-1 mutants. Wild-type and mutant (G41R, G93A, or N139K) human SOD-1 were expressed in motor neurons of dissociated cultures of murine spinal cord by intranuclear microinjection of plasmid expression vector. Both a general antagonist of AMPA/kainate receptors (CNQX) and a specific antagonist of calcium-permeable AMPA receptors (joro spider toxin) reduced formation of SOD-1 proteinaceous aggregates and prevented death of motor neurons expressing SOD-1 mutants. Partial protection was obtained by treatment with nifedipine, implicating Ca2+ entry through voltage-gated calcium channels as well as glutamate receptors in potentiating the toxicity of mutant SOD-1 in motor neurons. Dramatic neuroprotection was obtained by coexpressing the calcium-binding protein calbindin-D28k but not by increasing intracellular glutathione levels or treatment with the free radical spin trap agent, N-tert-butyl-alpha-phenylnitrone. Thus, generalized oxidative stress could have contributed in only a minor way to death of motor neurons expressing the mutant SOD-1. These studies demonstrated that the toxicity of these mutants is calcium-dependent and provide direct evidence that calcium entry during neurotransmission, coupled with deficiency of cytosolic calcium-binding proteins, is a major factor in the preferential vulnerability of motor neurons to disease.

Axotomy induces transient calbindin D28K immunoreactivity in hypoglossal motoneurons in vivo

Calbindin D28K, an intracellular calcium-binding protein, acts as Ca2+ buffering system in the cytoplasm. By means of this property, calbindin may protect neurons against large fluctuations in free intracellular Ca2+ and, hence, may prevent cell death. Although axotomy causes a massive influx of calcium into the lesioned neurons, resection of the hypoglossal nerve does not induce extensive neuronal cell death in rats. Even several weeks after axotomy, about 70% of the motoneurons survive despite permanent target deprivation. The mechanisms responsible for this remarkable survival rate are unknown. In this study, we have looked at the modification of calbindin immunoreactivity in axotomized hypoglossal motoneurons. In non-axotomized motoneurons, no calbindin is detectable by immunocytochemistry. Axotomy induced an increase of calbindin immunoreactivity in lesioned motoneurons. This increase, visualised by the number of calbindin-immunoreactive neurons extended from 1 day to 28 days. At this time most, but not all, motoneurons located on the side of the lesion were calbindin-positive as shown by retrograde labeling and immunoquenching. From 14 days post operation, calbindin immunoreactivity decreased and reached its basal value after 35 days post operation. At that time, only fibres were still calbindin immunoreactive. Interestingly, calbindin-immunoreactivity was also increased in almost all cell nuclei, compatible with a nuclear regulation. These data are consistent with the hypothesis that, as a reaction to axotomy, motoneurons trigger an increase in calbindin expression which acts as a compensatory Ca(2+)-buffering system, enabling neurons to maintain Ca2+ homeostasis and the survival of many motoneurons after axotomy.

In vivo and in vitro studies of glycine- and glutamate-evoked acetylcholinesterase release from spinal motor neurones: implications for amyotrophic lateral sclerosis/motor neurone disease pathogenesis

To investigate the spinal cellular structures and molecular mechanisms involved in acetylcholinesterase (AChE) release evoked by both glycine (GLY) and glutamate (GLU)--responses that might play a role in chronic neurotoxicity--we analysed AChE histochemistry and histology upon systemic administration of aspartate (ASP), and conducted in vitro experiments in synaptosomes and slices prepared from mouse spinal ventral horns. Upon superfusion and incubation exposure of these preparations to GLY- and GLU-receptor agonists, we assayed both tissue content and release of AChE, butyrylcholinesterase and lactic dehydrogenase. Histochemical reduction of motor neurone (MN) AChE, calcium dependency, decreases in intracellular AChE and the ratio amongst molecular forms released, suggest that both synaptosomal GLY-evoked AChE release (GLY-EAR) and GLU-receptor-elicited AChE release (GEAR) have release sites located at MN presynaptic terminals. These responses exhibited remarkable postnatal regulation. GEAR seems to be mediated through alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid/kainate receptors after the fourth postnatal week and through both NMDA and non-NMDA receptors at earlier stages. Sustained rises of extracellular AChE might link acute excitotoxic injury with several long-lasting pathways leading to chronic neurotoxicity, since AChE molecular properties include: (1) the ability to block cholinergic mechanisms that protect MN against overactivity; (2) activation of ATP-dependent potassium channels; (3) promotion of neurite and axon outgrowth; and possibly (4) stimulation of brain macrophage migration and activation.

Quantitative immunocytochemical analysis of the spinal cord in G86R superoxide dismutase transgenic mice: neurochemical correlates of selective vulnerability

Transgenic mice with a G86R mutation in the mouse superoxide dismutase (SOD-1) gene, which corresponds to a mutation that has been observed in familial amyotrophic lateral sclerosis (ALS), display progressive loss of motor function and provide a valuable model of ALS. The pathology in the spinal cords of these mice was evaluated to determine whether there are chemically identified populations of neurons that are either highly vulnerable or resistant to degeneration. Qualitatively, there were phosphorylated neurofilament protein (NFP)-immunoreactive inclusions and a pronounced loss of motoneurons in the ventral horn of the spinal cord without the presence of vacuoles that has been reported in other SOD-1 transgenic mice. Neuron counts from SOD-1 and control spinal cords revealed that the percentage loss of NFP-, choline acetyltransferase (ChAT)-, and calretinin (CR)-immunoreactive neurons was greater than the percentage loss of total neurons, suggesting that these neuronal groups are particularly vulnerable in SOD-1 transgenic mice. In contrast, calbindin-containing neurons did not degenerate significantly and represent a protected population of neurons. Quantitative double-labeling experiments suggested that the vulnerability of ChAT- and CR-immunoreactive neurons was due primarily to the presence of NFP within a subset of these neurons, which degenerated preferentially to ChAT- and CR-immunoreactive neurons that did not colocalize with NFP. Our findings suggest that NFP, which has been demonstrated previously to be involved mechanistically in motoneuron degeneration, may also be important in the mechanism of degeneration that is initiated by the SOD-1 mutation.

Amyotrophic lateral sclerosis: the involvement of intracellular Ca2+ and protein kinase C

The neurodegenerative disease, amyotrophic lateral sclerosis (ALS), is characterized by the selective death of motoneurones and corticospinal tract neurones. Abnormalities in excitatory amino acids and their receptors, as well as disordered function of voltage-dependent Ca2+ channels and superoxide dismutase have been reported in ALS patients. Furthermore, the activity of protein kinase C (PKC), a Ca2+, phospholipid-dependent enzyme, is also substantially increased in tissue from ALS patients, suggesting that alterations in intracellular free Ca2+ may be central to many of the diverse pathogenic mechanisms potentially responsible for ALS as discussed here by Charles Krieger and colleagues. Increased PKC activity, in turn, may have direct or indirect effects on neuronal viability and influence the pathogenic process in ALS by modifying the phosphorylation of voltage-dependent Ca2+ channels, neurotransmitter receptors and structural proteins.