Small amounts of zinc stimulate rapid axonal transport in vitro

The zinc dyshomeostasis hypothesis of Alzheimer's disease

Alzheimer's disease (AD) is the most common form of dementia in the elderly. Hallmark AD neuropathology includes extracellular amyloid plaques composed largely of the amyloid-β protein (Aβ), intracellular neurofibrillary tangles (NFTs) composed of hyper-phosphorylated microtubule-associated protein tau (MAP-tau), and microtubule destabilization. Early-onset autosomal dominant AD genes are associated with excessive Aβ accumulation, however cognitive impairment best correlates with NFTs and disrupted microtubules. The mechanisms linking Aβ and NFT pathologies in AD are unknown. Here, we propose that sequestration of zinc by Aβ-amyloid deposits (Aβ oligomers and plaques) not only drives Aβ aggregation, but also disrupts zinc homeostasis in zinc-enriched brain regions important for memory and vulnerable to AD pathology, resulting in intra-neuronal zinc levels, which are either too low, or excessively high. To evaluate this hypothesis, we 1) used molecular modeling of zinc binding to the microtubule component protein tubulin, identifying specific, high-affinity zinc binding sites that influence side-to-side tubulin interaction, the sensitive link in microtubule polymerization and stability. We also 2) performed kinetic modeling showing zinc distribution in extra-neuronal Aβ deposits can reduce intra-neuronal zinc binding to microtubules, destabilizing microtubules. Finally, we 3) used metallomic imaging mass spectrometry (MIMS) to show anatomically-localized and age-dependent zinc dyshomeostasis in specific brain regions of Tg2576 transgenic, mice, a model for AD. We found excess zinc in brain regions associated with memory processing and NFT pathology. Overall, we present a theoretical framework and support for a new theory of AD linking extra-neuronal Aβ amyloid to intra-neuronal NFTs and cognitive dysfunction. The connection, we propose, is based on β-amyloid-induced alterations in zinc ion concentration inside neurons affecting stability of polymerized microtubules, their binding to MAP-tau, and molecular dynamics involved in cognition. Further, our theory supports novel AD therapeutic strategies targeting intra-neuronal zinc homeostasis and microtubule dynamics to prevent neurodegeneration and cognitive decline.

A subpopulation of hippocampal glial cells specific for the zinc-containing mossy fibre zone in man

The projection of the zinc-containing axons of granule cells of the fascia dentata, e.g. the mossy fibres, is restricted to the hilar region and sector CA3 of the hippocampus. Serial sections of human hippocampi were stained for zinc-containing fibres with a non-perfusion Timm method, while adjacent ones were stained with Darrow red and aldehydefuchsin. GFAP, glutamine synthetase immunocytochemistry and a specific silver stain were employed to label other subtypes of astrocytes. The distribution of Timm-stained areas correlates only with the distribution of aldehydefuchsin-positive glial cells, most probably astrocytes. Since glial cells regulate axonal outgrowth in a region-specific manner, it is temptative to speculate that the aldehydefuchsin-positive glial cell is a candidate for a specific neuron-glia interaction which is somehow involved in the control of outgrowth of mossy fibres.

The mossy fiber terminals in the hippocampal formation of Callithrix jacchus: comparative and evolutionary considerations

The distribution of mossy fiber terminals in the regio inferior of the hippocampus of Callithrix jacchus was studied by means of Timm's method. The topographic distribution of Timm-positive zones in the hilus, in the suprapyramidal and intrapyramidal areas of the CA3 subfield is described. A Timm-positive reaction in intragranular strips and supragranular zones, the presence of Timm-negative zones in the infragranular border of the fascia dentata were found in this species. A comparison between mossy fiber distribution in Callithrix jacchus and that in human was carried out in an attempt to identify interspecies differences in the mossy fiber system in the hippocampus of primates. The hypothesis of a possible functional relevance of the supra- and intrapyramidal mossy fiber terminals on the control of hippocampal pyramidal neurons is expressed.

Effects of marginal zinc deficiency on microtubule polymerization in the developing rat brain

One of the possible mechanisms that has been proposed to underlie the deleterious effects of zinc deficiency on brain development is an impairment in the normal formation of the cytoskeletal network. In the current study, in vivo microtubule polymerization was characterized in brain supernatant fluids, from 20-d-old pups whose dams were fed diets containing control (50 micrograms zinc/g) or marginal levels of zinc (10 micrograms zinc/g) throughout pregnancy and lactation. Pup brain and body weights were similar between the groups; however, plasma zinc concentrations were lower (27%) in pups fed the marginal zinc diet than in controls. Tubulin concentrations in 100,000 g brain supernates were similar between the groups; however, tubulin polymerization in the brain supernates was significantly lower in pups fed the marginal zinc diet compared to controls. Primarily, the early events of polymerization were affected; the lag period of the reaction was doubled, and the initial velocity was slower (26%) in supernates from pups fed the marginal zinc diet than in controls. These findings support the idea that some of the negative effects of marginal zinc deficiency on brain development and function may be mediated by an alteration in microtubule formation.

Modulation of ATPase activities in the central nervous system by the S-100 proteins

The isomeric forms of bovine S-100a and S-100b have been shown to stimulate ATPase activities in fractions enriched in myelin and mitochondria isolated from the Gerbil brain and for S-100b more effectively than for calmodulin in erythrocytes or skeletal muscle. In the presence of Ca2+, S-100a produced a slight increase of ATPase activity in the mitochondrial fraction. However, S-100b in the presence of Zn2+ almost doubled the ATPase activity in brain myelin. S-100a, or S-100b, with or without Ca2+ and Zn2+ respectively, had no effect on the ATPase activity in mitochondria of the Gerbil liver. The observations may indicate a "second messenger" role for S-100b in the presence of Zn2+ in the Schwann cell.

Zinc uptake into synaptosomes

Zinc uptake was studied in synaptosomes, isolated by the Ficoll flotation technique, using the radiotracer 65Zn. True uptake of zinc could be discriminated from binding to the outside of the synaptosomes by the absence of accumulation at 0 degree C and the dependency of the rate of uptake on the medium osmolarity. The zinc uptake, studied in the presence of various zinc-complexing agents, showed saturation kinetics when analyzed in terms of [Zn]free, yielding Km = 0.25 microM. The zinc uptake was independent of both ATP and the Na+ gradient. No efflux of zinc could be demonstrated from preloaded synaptosomes due to the formation of insoluble zinc complexes inside the synaptosomes. The results are discussed in terms of the modulation of diverse neurochemical processes by zinc.

Pattern electroretinogram and visual evoked cortical potential in ethambutol optic neuropathy

Forty-one eyes in 21 cases of ethambutol optic neuropathy were investigated for the pattern visual evoked cortical potential (VECP) and the pattern electroretinogram (ERG). In 21 of the eyes the disappearance VECP responses were not detectable. In the other 20 eyes the peak latency and amplitude of the disappearance VECP were delayed and decreased significantly compared with normal eyes. The recovery of the peak latency of the disappearance VECP in ethambutol optic neuropathy was faster than that of the amplitude. Twelve eyes in six patients were studied by pattern reversal ERG. Though the mean peak latency of the pattern ERG was within normal limits, the mean amplitude was decreased significantly. Our investigations indicated that ethambutol optic neuropathy disturbed not only the optic nerve but also the retina.

The effect of gossypol on fast axonal transport and microtubule assembly

Gossypol at micromolar concentrations (2 microM) was found to inhibit axonal transport and a microsomal ATPase activity in the frog sciatic nerve, although axonal microtubules and the neuronal content of AMP, ADP and ATP were not affected. At slightly higher concentrations (30-40 microM), gossypol also inhibited microtubule assembly and neuronal energy metabolism. Gossypol accumulated in the nerve and the results indicate that gossypol may act as a potent neurotoxin.

Zinc metabolism in normal and zinc-deficient rat brain

Zinc uptake and turnover was measured in nine brain regions, choroid plexus, arachnoid, and cerebrospinal fluid during a 28-day period following a single dose of 65Zn in rats fed Zn-adequate diets. Zinc entry into brain was slow with maximal 65Zn uptake (0.5% of administered dose) occurring between 5 and 14 days in contrast to its rapid metabolism in plasma and nonneural tissues. The brain stem, at the level of the caudal IV ventricle, had the highest rate of initial 65Zn uptake of any brain region. In general, turnover was most rapid in periventricular regions and least in the hippocampus. Relative to plasma, the choroid plexus concentrated 65Zn whereas 65Zn was undetectable in the cerebrospinal fluid after day 1. To determine if specific brain regions were particularly sensitive to changes in Zn status, 65Zn metabolism was measured in Zn-deficient rats and compared with ad libitum- and pair-fed controls. Zinc deficiency was associated with increased 65Zn retention by all brain regions; however, the effect was greatest in optic nerve and choroid plexus. The results of this study suggest that a formidable barrier to Zn entry into brain exists but is under homeostatic control, increasing net Zn uptake during dietary deficiency. Moreover, the choroid plexus may participate in cerebral Zn homeostasis, possibly by transporting Zn out of the cerebrospinal fluid compartment.

Zinc in taste function : A critical review

Zinc has been associated with taste function in humans at several levels of organization-the taste bud, the nerves transmitting taste information, and the brain. Zinc plays specific yet varied roles at each organizational level, although many of these roles have not been clearly identified. They include participation in the structural architecture of the cell, maintenance of cell membrane integrity, and control of activity of several cytoplasmic and membrane enzymes. Early investigators noted that some patients given drugs that altered zinc metabolism or who experienced disease processes associated with abnormalities of zinc metabolism exhibited taste dysfunction. Because of these findings zinc was given to a variety of patients as treatment for taste dysfunction. Initial treatment success was observed, but was quickly tempered by more extensive studies that yielded widely variable results leading to confusion about the role of zinc in both taste function and taste treatment. Further studies revealed that taste disorders were diverse and complex with multiple underlying pathophysiologies that were little understood. Subsequent work by several investigators revealed that patients with zinc deficiency, of any etiology, exhibited taste dysfunction and that treatment of these patients with zinc usually produced improvement of clinical symptoms. These results raised the question of how to define zinc deficiency, for zinc treatment in patients without zinc deficiency was unsuccessful and these patients represent more than three-quarters of all patients with taste dysfunction. New clinical techniques for the definition of human zinc deficiency have been achieved through the use of binding and displacement of(65)Zn on specific sites on erythrocyte membranes; these results offer a guide to the identification of patients (i.e., those with zinc deficiency) who may benefit from zinc treatment.

Lack of correlation between trace metal staining and trace metal content of the rat hippocampus following colchicine microinjection

Following the intrahippocampal injection of colchicine, the trace metal staining with Timm's method is shown to change in the hippocampus. The histochemical examinations were supplemented with atomic absorption spectrophotometric measurement of the trace metals (Zn, Fe, Cu). It was found that intrahippocampal colchicine treatment induces the temporary disappearance of the trace metal staining of the pyramidal cells of the regio superior, while there is a considerable reduction in the staining in the granular cells of the area dentata and in their mossy fibre terminals. Simultaneously, in contrast with the histochemical results, quantitative studies on the trace metal levels showed that colchicine does not lead to evacuation of the trace metals from the hippocampal formation. The combined atomic absorption and trace metal staining investigations prove that there is no correlation between the trace metal staining and the quantitative amounts of the trace metals.

Effects of zinc on the cytoskeletal proteins in the central nervous system of the rat

To test for in vivo zinc neurotoxicity on the cytoskeleton of neurotubules and intermediate filaments, Zn wires were implanted into the brains of adult Lewis rats for periods of 1-35 weeks. After 16 weeks of implant, some neurons showed bundles of intermediate filaments which were often localized in the perinuclear area. At the same time, occasional 200 nm tubular-like structures were seen in swollen dendrites. These structures were morphologically similar to Zn ion-induced aggregates of pure tubulin and structures found in ZnSO4-treated dorsal root ganglion organotypic cultures. The 200 nm structures in dendrites and the intermediate filaments in neurons increased in frequency with time. After 35 weeks of Zn wire implant, few microtubules could be found in the lesion. All the animals showed an astrocyte and glial filament proliferation with axoglial membrane specialization. Other wires studied: Pt, Ni, Co, Mg, demonstrate that Zn wires have a specific effect on cytoskeletal proteins in the CNS of the rat and many of the effects can be explained by an interaction between Zn and tubulin.

Ca2+- or Mg2+-stimulated ATPase activity in bullfrog spinal nerve: relation to Ca2+ requirements for fast axonal transport

Adenosine triphosphatase (ATPase) activity stimulated by Ca2+ or Mg2+ was characterized in spinal nerve and spinal sensory ganglion of bullfrog. Enzyme activity of homogenates from both sources reached a maximum at a 1-2 mM concentration of either cation, although the level of maximal activity in nerve trunks was approximately twice that in ganglia. Enzyme activation was not observed with 2 mM-Sr2+ or Ba2+. Co2+ or Mn2+, at 2 mM, depressed Ca2+ activation of the enzyme by 50-60% in nerve but had no inhibitory effect on ganglia activity. In intact spinal ganglion/spinal nerve preparations, incubated for 20 h in medium containing 0.2 mM-Co2+, no effect was detected on Ca2+/Mg2+ ATPase activity in ganglia or nerve trunks whereas fast axonal transport was inhibited by 80%. Incubation in medium containing 0.02 mM-Hg2+ depressed enzyme activity in ganglia by 64% and in nerve trunks by 44%, whereas fast transport was again inhibited by 80%. When only nerve trunks were exposed to these ions, Hg2+ but not Co2+ was observed to slow the rate of fast axonal transport. The divalent cation specificity of the Ca2+/Mg2+ ATPase activity is distinct from the ion specificities, determined in previous work, of the Ca2+ requirement during initiation of fast axonal transport in the soma, and of the Ca2+ requirement during translocation in the axon. Thus, previous observations of Ca2+-dependent events in fast axonal transport cannot be taken per se to suggest the involvement of Ca2+/Mg+ ATPase in the transport process.

Ca2+- or Mg2+-dependent enzymatic ATP hydrolysis associated with the microsomal fraction of frog sciatic nerves

The microsomal fraction of frog sciatic nerves was found to contain Ca2+- or Mg2+-dependent hydrolytic activity toward different nucleoside di- and triphosphates. In the presence of Ca2+ substrate specificity was in the order CTP > UTP > GTP > ATP. When Mg2+ was used, the triphosphates were approximately equally good substrates. ATP hydrolytic activity was very similar with Ca2+ or Mg2+ as the cofactor, whereas Ca2+ was the more potent activator of hydrolysis of the other triphosphates tested. The preparation showed some activity toward the nucleoside diphosphates but none toward the monophosphates or p-nitrophenylphosphate. The enzymic properties of ATP hydrolysis were more closely studied. The hydrolysis was optimal at 18--24 degrees C in the presence of 1 mM-Ca2+ or 1 mM-Mg2+. Ca2+- and Mg2+-ATP hydrolysis displayed pH maxima around 8.0--8.5 and 7.4--8.0, respectively. Vmax values for Ca2+- and Mg2+-ATP hydrolysis similar: approx. 12 mumol Pi per h per mg protein with a Km value of approx. 0.05 mM. The ATP hydrolysis activity was inhibited by NaF but unaffected by ouabain, vanadate, cytochalasin B, and various drugs known to influence ATPase activity of mitochondria. Zn2+ stimulated the ATP hydrolysis activity at low concentrations (10(-6)-10(-5) M) and inhibited it at higher concentrations. The possibility that these observations account for stimulation and inhibition of axonal transport in frog sciatic nerves exposed to similar concentrations of Zn2+ is discussed.

Metal analysis by energy dispersive x-ray fluorescence of bovine brain tubulin and microtubule-associated proteins prepared by phosphocellulose chromatography

It has been shown by trace metal analysis that tubulin isolated from bovine brain does not contain strongly bound transition metal ions. The traces of zinc and iron found in the fraction of microtubule-associated proteins might originate from previously reported phosphatase activity (Larsson, H., Wallin, M. and Edström, A. (1979) J. Neurochem. 32, 155--161).

The enhancement by dietary zinc deficiency of the susceptibility of the rat duodenum to colchicine

1. The incidence of colchicine-induced lesions in the germinal epithelium oof the rat duodenum was studied in young rats in an early stage of zinc deficiency and in their pair-fed controls. At both dose levels of colchicine used, a marked increase in the amount of cell damage was observed in the duodenum of Zn-deficient rats as compared with the pair-fed, control (Zn-supplemented) rats. 2. No statistical interaction between Zn and colchicine was demonstrable, and no lesions were found in the duodenum of animals that had not been treated with colchicine. 3. The results are discussed in relation to the effects of Zn deficiency in animals and the possible involvement of Zn in the maintenance of the integrity of microtubular structures.

Inhibition and stimulation of rapid axonal transport in vitro by sulfhydryl blockers

The effects of sulfhydryl blocking agents have been studied on the rapid axonal transport in vitro of [3H]leucine-labelled proteins in the frog sciatic nerve. The transport was inhibited in the presence of low concentrations of N-ethylmaleimide (NEM) (greater than or equal to 10(-5) M), p-chloromercuribenzene sulfonic acid (PCMBS) (greater than 10(-5) M) or ions of heavy metals, Cd2+ (greater than or equal to 5 X 10(-5) M), Hg2+ (greater than or equal to 5 X 10(-6) M) and Cu2+(greater than or equal to 10(-4) M). Both the amount and the rate of transported radioactivity were reduced. Transport inhibiting concentrations of these agents also inhibited the binding of colchicine in rat brain or frog nerve supernatants. The amount of transported proteins was increased at an unchanged transport rate by a very low concentration of NEM (10(-6) M), PCMBS (10(-6) M) and Cd2+ (10(-6)M), which did not affect the binding of colchicine. The present results suggest that stimulation of axonal transport can be achieved through an interaction with sulfhydryl groups.