Ca2+ and cell death

Ultrastructural evidence for altered calcium in motor nerve terminals in amyotropic lateral sclerosis

Numerous studies of amyotrophic lateral sclerosis have suggested that increased intracellular calcium is a common denominator in motoneuron injury. In experimental models, IgG from patients with amyotrophic lateral sclerosis enhanced calcium entry and induced apoptotic cell death in vitro as well as increased intracellular calcium and induced ultrastructural alterations of the motor nerve terminals in mice in vivo. To determine whether similar increases in intracellular calcium and altered morphology are present in motor nerve terminals of amyotrophic lateral sclerosis patients in vivo, muscle biopsy specimens from 7 patients with amyotrophic lateral sclerosis, 10 nondenervating disease control subjects, and 5 patients with denervating neuropathies were analyzed with ultrastructural techniques, employing oxalate-pyroantimonate fixation to preserve in situ calcium distribution. Motor nerve terminals from amyotrophic lateral sclerosis specimens contained significantly increased calcium, increased mitochondrial volume, and increased numbers of synaptic vesicles compared to any of the disease control groups, without exhibiting excess Schwann envelopment specific to denervating terminals. These results parallel the effect of amyotrophic lateral sclerosis IgG passively transferred to mice, and provide the first demonstration that neuronal calcium is, in fact, increased in amyotrophic lateral sclerosis in vivo.

Traumatically induced axonal damage: evidence for enduring changes in axolemmal permeability with associated cytoskeletal change

Recent studies have demonstrated that delayed or secondary axotomy is a consistent feature of traumatic brain injury in both animals and man. Moreover, these studies have shown that the pathogenesis of this secondary axotomy involves various forms of initiating pathology, with the suggestion that, in some cases, only the axonal cytoskeleton is perturbed, while, in other cases, both the axonal cytoskeleton and related axolemma manifest traumatically induced perturbations. In the current communication, we continue in our investigation of the significance of these traumatically induced alterations in axolemmal permeability and their relation to any related intra-axonal cytoskeletal change. This was accomplished in cats which received intrathecal infusions of peroxidase, an agent normally excluded by the intact axolemma. These animals were subjected to traumatic brain injury, and sites showing altered axolemmal permeability to the peroxidase were assessed at the light and electron microscopic level. Through this approach, we recognized that a traumatic episode of moderate severity evoked changes in axolemmal permeability which surprising endured for up to 5 hrs postinjury. At such focal sites of altered permeability, the related cytoskeleton showed a statistically significantly neurofilament compaction, with the strong suggestion of concomitant neurofilament sidearm loss, microtubular dispersion, and mitochondrial abnormality. Over time, these events led to further disorganization of the axonal cytoskeleton which translated into impaired axoplasmic transport and secondary axotomy. Most likely, these alterations in axolemmal permeability result in either the direct or indirect effects upon the axonal cytoskeleton that precipitate the damaging sequences resulting in delayed axotomy.

Elevation of intracellular calcium levels in spiral ganglion cells by trimethyltin

The neurotoxicant, trimethyltin (TMT) produces cochlear impairment at far lower dose levels and far more rapidly than it does central nervous system effects. The initial effects of TMT in the cochlea, in vivo, are consistent with disruption of the inner hair cell type-1 spiral ganglion cell synapse although it is uncertain whether the effect is on presynaptic and/or postsynaptic units. This synapse is believed to be an excitatory glutamatergic one, providing the possibility that TMT could induce an excitotoxic process resulting in elevations in intracellular calcium ([Ca2+]i). The objective of this study was to determine whether TMT had direct toxic effects on the postsynaptic spiral ganglion cells studied in primary culture and to identify the role of extracellular calcium in such an effect. The marker of interest was the effect of this agent on [Ca2+]i levels as determined using quantitation of the fluorescent calcium dye, Fura-2. TMT did induce a marked and sustained elevation in [Ca2+]i level in the spiral ganglion cells that appeared to have a rapid initial phase and a slower saturating phase. Studies performed using calcium-free medium showed that elevation of [Ca2+]i in spiral ganglion cells by TMT was attenuated but not entirely blocked. Further, the L-type calcium channel blocker, nifedipine, was able to inhibit the initial increase in [Ca2+]i, suggesting that at least this phase of the TMT effect was mediated by calcium channels, although nifedipine had no significant effect on the time to reach the maximal [Ca2+]i level. Parallel control experiments performed using application of exogenous glutamate and depolarizing K+ concentrations also produced elevation in [Ca2+]i levels. The data indicate that TMT elevates [Ca2+]i in isolated spiral ganglion cells both by increasing extracellular uptake via Ca2+ channels and also by releasing Ca2+ from intracellular stores. Thus TMT ototoxicity appears to include a direct postsynaptic toxic event.

Reaction of bromotrichloromethane derived free radicals with uracil in a model system. Structures of products formed

Free radicals generated by benzoyl peroxide-mediated catalytic decomposition of bromotrichloromethane (eg. trichloromethyl) were allowed to react under nitrogen or under air with uracil. Under nitrogen two reaction products were formed, one was identified as 5-chlorouracil and the other as a 5-bromouracil. Under air, besides the above two products other nine were also formed: 5,6-dihydrouracil; 5-hydroxyuracil; a chlorohydroxy adduct of uracil; a bromohydroxy derivative of uracil having the 5,6 bond in the saturated form; other bromohydroxy derivative of uracil having the double bond intact; 5,6-dihydroxyuracil; two dihalogenated hydroxylated uracil derivatives and one peak we were not able to descipher its structure. No single reaction product formed had carbon centered radicals (eg. trichloromethyl) added from CBrCl3 and consequently would be missed in 'in vivo' covalent binding studies where 14C haloalkane (CBrCl3 or carbon tetrachloride) were employed. If similar reaction products resulted during interaction of CBrCl3 reactive metabolites with uracil in RNAs, significant deleterious effects in their function would be expected. That possibility, however, remains to be established.

Phenytoin inhibits expression of microtubule-associated protein 2 and influences cell-viability and neurite growth of cultured cerebellar granule cells

The objective of this study was to show whether an in vitro model for Phenytoin-related cytoskeletal impairment could be helpful to investigate cerebellar side effects of Phenytoin (DPH). DPH dose-and time-dependently resulted in decreasing numbers of vital cells. Cells formed only a rarefied intercellular neuritic network. This effect was already evident 24 h after plating. Western-blot analysis revealed that the expression of the dendritic marker microtubule-associated protein 2 (MAP2) was dramatically decreased in DPH-treated cultures.

Elevation of GRP-78 and loss of HSP-70 following photodynamic treatment of V79 cells: sensitization by nigericin

Chinese hamster V79 cells were treated with photodynamic therapy (PDT) sensitized by aluminum phthalocyanine (AlPc) or with the ionophore nigericin or with combinations of PDT and nigericin. We previously showed that PDT and nigericin interact synergistically in the killing of these cells; i.e. doses of PDT that kill no more than 10% of the cells in combination with nontoxic exposures to nigericin lead to a loss of clonogenicity of three to five orders of magnitude. Photodynamic therapy induces an enhanced rate of expression of the stress gene grp-78 both at the transcriptional and translational levels and causes a decrease in the synthesis of the constitutive heat shock protein HSP-70 as well as in expression of HSP-70 mRNA. When the cells are exposed to PDT in the presence of nigericin, these effects are elicited at three- to four-fold lower PDT doses. Thus, PDT in the presence of nigericin is much more effective in inducing the changes in gene expression than is PDT alone. In the absence of nigericin the PDT dose inducing a two-fold increase in GRP-78 accumulation causes little or no loss of clonogenicity. In the presence of nigericin, however, the PDT dose leading to a similar change in GRP-78 level produces up to a 50% loss of clonogenicity. The fact that nigericin is dose-modifying for both cell killing and stress responses suggests that nigericin either increases the yield of oxidative damage from a given dose of PDT or magnifies the cellular response to a constant level of oxidative stress.

Recessive mutations in a common pathway block thymocyte apoptosis induced by multiple signals

The glucocorticoid receptor (GR) is a ligand-regulated transcription factor that controls genes necessary to initiate glucocorticoid-induced thymocyte apoptosis. We have performed a genetic analysis of thymocyte cell death by isolating and characterizing a panel of GR+ dexamethasone-resistant mutants of the murine WEHI7.2 thymocyte cell line. These apoptosis-defective (Apt-) mutants were used to identify previously unknown early steps in the apoptotic pathway. The Apt- mutants contain nonglucocorticoid receptor, recessive mutations in genes that represent multiple complementation groups. These mutations block apoptosis induced by dexamethasone, gamma irradiation, and c-AMP treatment before the point where Bcl-2 exerts its protective effect. We propose that different signals share a common apoptotic pathway, and that the induction of apoptosis involves multiple precommitment steps that can be blocked by recessive mutations.

Calcium homeostasis in fibroblasts from patients with amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a chronic neurodegenerative disorder of the motor system in the CNS characterized by motor neuron death in the spinal cord, brain stem and cortex. Readily available tissues such as fibroblasts from ALS patients can serve as simple model systems to study the molecular mechanisms leading to degenerative disorders. We have used Fura-2 fluorescence microscopy and single-cell imaging to study the spatiotemporal dynamics of intracellular free calcium ([Ca2+]i) in primary cultures of fibroblasts from skin biopsies from ALS and normal subjects. Increases in [Ca2+]i were induced by stimulation with bradykinin (100 nM); neurotensin (50 nM); N-formyl-Met-Leu-Phe (chemotactic peptide) (1 microM); [Arg8]-vasopressin (1 microM) and histamine (10 microM). The levels of [Ca2+]i in 80-120 individual cells per agonist were monitored for 15 min. No significant differences were found in the resting levels of [Ca2+]i in control (102 +/- 4 nM) and ALS (98 +/- 6 nM) fibroblasts and in the maximal [Ca2+]i levels after stimulation with N-formyl-Met-Leu-Phe, [Arg8]-vasopressin, and histamine. Significantly lower [Ca2+]i transients were found in fibroblasts from ALS donors compared to controls when stimulated with neurotensin (p < 0.002) and bradykinin (p < 0.005). The percentage of individual cells reacting to a given agonist (40-100%) was similar in both groups. The molecular basis of the impaired calcium homeostasis in fibroblasts from ALS patients is not known, but a generalized membrane defect can be excluded since the [Ca2+]i responses are defective only when bradykinin or neurotensin are used as agonists.

Impaired Ca2+ response to glucose in mouse beta-cells infected with coxsackie B or Echo virus

Five strains of Coxsackie B4 virus and one of Echo 11 virus were tested with regard to their ability to replicate in pancreatic mouse beta-cells and interfere with the alterations of the cytoplasmic Ca2+ concentration ([Ca2+]i) induced by glucose. All strains except one both multiplied and caused cytopathic effect. In a control group 68% of the beta-cells responded to 11 mM glucose with large amplitude oscillations of [Ca2+]i. After inoculation with the infectious strains these oscillations appeared in only 5% of the beta-cells, whereas the non-infectious strain did not modify the glucose effect on [Ca2+]i. Despite the virus interference with the glucose response, [Ca2+]i was increased after depolarization with excessive extracellular K+ and the oscillations were induced in most beta-cells when glucose was combined with the insulin-releasing sulfonylurea tolbutamide.

The role of intracellular free calcium in motor neuron disease

The intracellular calcium (Ca2+) concentrations of motoneurons can be altered by the influx of Ca2+ into the cell by the opening of voltage-dependent Ca2+ channels and ligand-gated channels linked to Ca2+ influx, especially by the N-methyl-D-aspartate (NMDA) type of excitatory amino acid receptor. Intracellular Ca2+ concentration is also affected by the release of Ca2+ buffered in mitochondria and endoplasmic reticulum. Evidence that motoneurons may be selectively vulnerable to Ca(2+)-induced cell death include the following observations: (i) the presence of excitatory amino acid receptors on the cell membranes of motoneurons, some of which would permit Ca2+ influx (e.g. NMDA receptors); (ii) the availability of the presynaptic terminal for antibody-mediated effects leading to changes in cell permeability and Ca2+ influx; and (iii) the limited amounts of intracellular Ca(2+)-binding proteins such as calbindin D28K and parvalbumin in motoneurons. Elevation of intracellular free Ca2+ may also be a common event in a number of independent mechanisms leading to motoneuron death in motor neuron disease.

Excitatory amino acid-induced slow biphasic responses of free intracellular calcium in human neuroblastoma cells

Effects of an excitatory amino acid, glutamate, and of ionotropic and metabotropic glutamate receptor agonists on the levels of free intracellular calcium, and their specific receptor binding in human SH-SY5Y neuroblastoma cells were studied. The calcium response was always biphasic, except for AMPA, suggesting both stimulatory and inhibitory effects on free intracellular calcium upon glutamate receptor stimulation, both with ionotropic and metabotropic glutamate receptor agonists. Specific binding of glutamate and other glutamate receptor agonists, together with the biphasic calcium response, suggests that human SH-SY5Y neuroblastoma cells express both ionotropic and metabotropic glutamate receptors. These findings shed new light on the use of human SH-SY5Y neuroblastoma cells as a human neuronal tumor cell model.