Levels and sub-cellular distribution of physiologically important metal ions in neuronal cells cultured from chick embryo cerebral cortex

The molecular basis of memory

We propose a tripartite biochemical mechanism for memory. Three physiologic components are involved, namely, the neuron (individual and circuit), the surrounding neural extracellular matrix, and the various trace metals distributed within the matrix. The binding of a metal cation affects a corresponding nanostructure (shrinking, twisting, expansion) and dielectric sensibility of the chelating node (address) within the matrix lattice, sensed by the neuron. The neural extracellular matrix serves as an electro-elastic lattice, wherein neurons manipulate multiple trace metals (n > 10) to encode, store, and decode coginive information. The proposed mechanism explains brains low energy requirements and high rates of storage capacity described in multiples of Avogadro number (N(A) = 6 × 10(23)). Supportive evidence correlates memory loss to trace metal toxicity or deficiency, or breakdown in the delivery/transport of metals to the matrix, or its degradation. Inherited diseases revolving around dysfunctional trace metal metabolism and memory dysfunction, include Alzheimer's disease (Al, Zn, Fe), Wilson's disease (Cu), thalassemia (Fe), and autism (metallothionein). The tripartite mechanism points to the electro-elastic interactions of neurons with trace metals distributed within the neural extracellular matrix, as the molecular underpinning of "synaptic plasticity" affecting short-term memory, long-term memory, and forgetting.

Altered manganese homeostasis and manganese toxicity in a Huntington's disease striatal cell model are not explained by defects in the iron transport system

Expansion of a polyglutamine tract in Huntingtin (Htt) leads to the degeneration of medium spiny neurons in Huntington's disease (HD). Furthermore, the HTT gene has been functionally linked to iron (Fe) metabolism, and HD patients show alterations in brain and peripheral Fe homeostasis. Recently, we discovered that expression of mutant HTT is associated with impaired manganese (Mn) uptake following overexposure in a striatal neuronal cell line and mouse model of HD. Here we test the hypothesis that the transferrin receptor (TfR)-mediated Fe uptake pathway is responsible for the HD-associated defects in Mn uptake. Western blot analysis showed that TfR levels are reduced in the mutant STHdh(Q111/Q111) striatal cell line, whereas levels of the Fe and Mn transporter, divalent metal transporter 1 (DMT1), are unchanged. To stress the Fe transport system, we exposed mutant and wild-type cells to elevated Fe(III), which revealed a subtle impairment in net Fe uptake only at the highest Fe exposures. In contrast, the HD mutant line exhibited substantial deficits in net Mn uptake, even under basal conditions. Finally, to functionally evaluate a role for Fe transporters in the Mn uptake deficit, we examined Mn toxicity in the presence of saturating Fe(III) levels. Although Fe(III) exposure decreased Mn neurotoxicity, it did so equally for wild-type and mutant cells. Therefore, although Fe transporters contribute to Mn uptake and toxicity in the striatal cell lines, functional alterations in this pathway are insufficient to explain the strong Mn resistance phenotype of this HD cell model.

A Menkes P-type ATPase involved in copper homeostasis in the central nervous system of the rat

We previously reported that copper efflux from C6 rat glioma cells was blocked by a brief exposure to sulfhydryl reagents p-chloromercuribenzoate (PCMB) and iodoacetamide as well as dicyclohexylcarbodiimide, suggesting the possible involvement of a Cu-transporting ATPase in the efflux mechanism. In this report, we show that copper efflux from PC12 cells, a neuron-like cell line established from rat adrenal pheochromocytoma, is also inhibited by PCMB exposure. Furthermore, we show that both C6 and PC12 cells express a homolog of the Menkes gene (MNK) as detected by RT-PCR with primers designed from a mouse cDNA and confirmed by sequence analysis of the amplified product. An expected 760-bp fragment representing the transduction and phosphorylation domains and a 925-bp fragment encoding the heavy metal-binding domain of Atp7a were amplified from a RNA extract of C6 and PC12 cells. Sequence data revealed that 690 bp of the 760-bp fragment from C6 cells were an identical match to a similar fragment from PC12 cells. Both fragments encoded a 229 amino-acid polypeptide that had a 98.7% sequence homology to mouse Atp7a. In addition, 880 bp from the 925-bp fragment of the two cell lines were identical and encoded a 293 amino-acid polypeptide with 94.5% sequence homology to mouse Atp7a. These data establish that a Menkes-type Cu-transporting ATPase is expressed in rat C6 and PC12 cells and strongly support the hypothesis that both neurons and glia are involved in maintaining Cu homeostasis in the central nervous system.

Effect of manganese on the development of glial cells cultured from prenatally alcohol exposed rats

Maternal alcohol abuse is known to produce retardation in brain maturation and brain functions. Using cultured glial cells as a model system to study these effects of alcohol we found an alcohol antagonizing property for manganese (Mn). Mn was added to the alcohol diet (MnCl2 25 mg/l of 20% v/v ethanol) of pregnant rats. Glial cells were cultured during 4 weeks from cortical brain cells of pups born to these mothers. Several biochemical parameters were examined: protein levels, enzymatic markers of glial cell maturation (enolase and glutamine synthetase), superoxide dismutase a scavenger of free radicals produced during alcohol degradation. The results were compared to appropriate controls. A beneficent effect of Mn was observed for the pups weight which was no more significantly different from the control values. Protein levels, enolase and glutamine synthetase activities were increased mainly during the proliferative period when Mn was added to the alcohol diet compared to the only alcohol treated animals. This Mn effect was not found for superoxide dismutase in cultured glial cells but exists in the total brain of the 2 week-old offspring. In the total 2 and 4 week-old brain the alcohol induced decrease of enolase and glutamine synthetase was also antagonized by the Mn supplementation. Our data suggest that Mn may act as a factor overcoming at least partially some aspects of alcohol induced retardation of nerve cell development.

Effects of Ca(II) ions on Mn(II) dynamics in chick glia and rat astrocytes: potential regulation of glutamine synthetase

Previous studies have demonstrated that in glia and astrocytes Mn(II) is distributed with ca. 30-40% in the cytoplasm, 60-70% in mitochondria. Ca(II) ions were observed to alter both the flux rates and distribution of Mn(II) ions in primary cultures of chick glia and rat astrocytes. External (influxing) Ca(II) ions had the greatest effect on Mn(II) uptake and efflux, compared to internal (effluxing) or internal-external equilibrated Ca(II) ions. External (influxing) Ca(II) ions inhibited the net rate and extent of Mn(II) uptake but enhanced Mn(II) efflux from mitochondria. These observations differ from Ca(II)-Mn(II) effects previously reported with "brain" (neuronal) mitochondria. Overall, increased cytoplasmic Ca(II) acts to block Mn(II) uptake and enhance Mn(II) release by mitochondria, which serve to increase the cytoplasmic concentration of free Mn(II). A hypothesis is presented involving external L-glutamate acting through membrane receptors to mobilize cell Ca(II), which in turn causes mitochondrial Mn(II) to be released. Because the concentration of free cytoplasmic Mn(II) is poised near the Kd for Mn(II) with glutamine synthetase, a slight increase in cytoplasmic Mn(II) will directly enhance the activity of glutamine synthetase, which catalyzes removal of neurotoxic glutamate and ammonia.

Developmental profiles of antioxidant enzymes and trace metals in chick embryo

It has been previously well documented that partial pressure of oxygen (PO2) and weight-specific rate of O2 consumption in chick embryo (Gallus gallus domesticus) transiently increase midway through the 21-day in ovo incubation period. The present study found that these oxidative changes were paralleled by the concentrations of glutathione (GSH) and Zn in liver and by the specific activity of superoxide dismutase (SOD) in brain. Levels of antioxidant enzymes and their trace metal cofactors were markedly higher in liver than in brain. Hepatic catalase activity changed in parallel with the concentration of its cofactor, Fe. However, the relative abundance of metal cofactors did not appear to be the determining influence on other antioxidant enzyme activities. Rates of extra-mitochondrial hydrogen peroxide release were also much greater in liver than in brain. Taken together, the results of this initial study of embryonic chick antioxidant systems suggest that certain antioxidants may be regulated by PO2 and rate of oxidative metabolism during fetal development.

Modulation of Mn2+ accumulation in cultured rat neuronal and astroglial cells

The effects of physiological concentrations of K+ on Mn2+ accumulation were compared in rat glial cells and neurons in culture. Increasing the K+ concentration in growth medium increased significantly the Mn2+ level of the cultivated cells, with glial cells more affected than neurons. Ethanol markedly increased the Mn2+ accumulation within glia but not within neurons while ouabain caused inhibition of Mn2+ uptake with neurons and glial cells. A modulation of the total protein synthesis by Mn2+ and ethanol level in the growth medium was observed with glial cells. These data suggest that the mechanisms involved in Mn2+ accumulation in glial cells are different from those present in neurons. Moreover, the results are consistent with the hypothesis that Mn2+ plays a regulatory role in glial cell metabolism.

Manganese(II) dynamics and distribution in glial cells cultured from chick cerebral cortex

The kinetics of manganese(II) ion uptake and efflux have been investigated using tracer 54Mn(II) with glial cells cultured from chick cerebral cortex in chemically defined medium. The initial velocity of Mn(II) uptake versus [Mn(II)] exhibit saturation, with an apparent S0.5 approximately 18(+/- 3) microM. Both the rate and extent of Mn(II) uptake are inhibited by Ca(II), either added externally or preloaded into the glial cells. Preloading of glia with Mn(II) also inhibits the rate of external 54Mn(II) uptake. Zn(II) inhibits but Cu(II) activates Mn(II) uptake. Efflux of Mn(II) from preloaded cells occurs as a biphasic process, with rapid release of 30-40% of total cell Mn(II), then much slower release of the remainder. Permeabilization of cells with dextran sulfate also rapidly released ca. 30% of total cell Mn(II). High external Mn(II) enhanced both the rate and extent of Mn(II) efflux. CCCP, an uncoupler of oxidative phosphorylation, inhibited both Mn(II) uptake and efflux significantly, but addition of cyanide, ouabain, insulin, hydrocortisone, K+, or Nd(III) had no effect on either process. Taken together, these data suggest a model in which Mn(II) is brought across the plasma membrane by facilitated diffusion, binds to cytosolic protein sites, and is partitioned into the mitochondria by an active transport mechanism. The fact that the Mn(II) flux rates observed with cultured glia are much faster than those reported for overall uptake and efflux of brain Mn(II) in vivo suggests that the blood-brain barrier may play a significant role in determining these latter rates in whole animals.