Aluminum-induced conformational changes in calmodulin alter the dynamics of interaction with melittin

Does neurotransmission impairment accompany aluminium neurotoxicity?

Neurobehavioral disorders, except their most overt form, tend to lie beyond the reach of clinicians. Presently, the use of molecular data in the decision-making processes is limited. However, as details of the mechanisms of neurotoxic action of aluminium become clearer, a more complete picture of possible molecular targets of aluminium can be anticipated, which promises better prediction of the neurotoxicological potential of aluminium exposure. In practical terms, a critical analysis of current data on the effects of aluminium on neurotransmission can be of great benefit due to the rapidly expanding knowledge of the neurotoxicological potential of aluminium. This review concludes that impairment of neurotransmission is a strong predictor of outcome in neurobehavioral disorders. Key questions and challenges for future research into aluminium neurotoxicity are also identified.

Al3+ interaction sites of calmodulin and the Al3+ effect on target binding of calmodulin

The interaction between calmodulin (CaM) and Al(3+) was studied by spectroscopic methods. Heteronuclear two-dimensional NMR data indicated that peaks related to the both lobes and middle of the central helix of CaM are largely affected by Al(3+). But chemical shift perturbation suggested that overall conformation of Ca(2+)-loaded CaM is not changed by Al(3+) binding. It is thought that Al(3+) interaction to the middle of the central helix is a key for the property of CaM's target recognition. If the structure and/or flexibility of the central helix are/is changed by Al(3+), target affinity to CaM must be influenced by Al(3+). Thus, we performed surface plasmon resonance experiments to observe the effect of Al(3+) on the target recognition by CaM. The data clearly indicated that target affinity to CaM is reduced by addition of Al(3+). All the results presented here support a hypothesis that Al(3+) may affect on the Ca(2+) signaling pathway in cells.

Aluminium impairs the glutamate-nitric oxide-cGMP pathway in cultured neurons and in rat brain in vivo: molecular mechanisms and implications for neuropathology

Aluminium (Al) is a neurotoxicant and appears as a possible etiological factor in Alzheimer's disease and other neurological disorders. The mechanisms of Al neurotoxicity are presently unclear but evidence has emerged suggesting that Al accumulation in the brain can alter neuronal signal transduction pathways associated with glutamate receptors. In cerebellar neurons in culture, long term-exposure to Al added 'in vitro' impaired the glutamate-nitric oxide (NO)-cyclic GMP (cGMP) pathway, reducing glutamate-induced activation of NO synthase and NO-induced activation of the cGMP generating enzyme, guanylate cyclase. Prenatal exposure to Al also affected strongly the function of the glutamate-NO-cGMP pathway. In cultured neurons from rats prenatally exposed to Al, we found reduced content of NO synthase and of guanylate cyclase, and a dramatic decrease in the ability of glutamate to increase cGMP formation. Activation of the glutamate-NO-cGMP pathway was also strongly impaired in cerebellum of rats chronically treated with Al, as assessed by in vivo brain microdialysis in freely moving rats. These findings suggest that the impairment of the Glu-NO-cGMP pathway in the brain may be responsible for some of the neurological alterations induced by Al.

Probing the role of calmodulin in Al toxicity in maize

The role of calmodulin on Al toxicity was studied in two maize (Zea mays L.) inbred lines, Cat 100-6 (Al-tolerant) and S 1587-17 (Al-sensitive). Increasing levels of Al induced the release of malate at similar rate by roots of both genotypes, while the exudation of citrate, a stronger Al-binding compound, was 3.5 times higher in Cat 100-6 seedlings exposed to 16.2x10(-6) Al(3+) activity. The calmodulin inhibitor trifluoperazine significantly reduced the root growth in both genotypes, mimicking the main effect of Al. However, when Cat 100-6 and S 1587-17 seedlings were challenged with Al in conjunction with trifluoperazine, no further reduction in root growth or any other effect of Al toxicity was observed. The rate of Al-induced citrate exudation by both genotypes was not affected by treatment with trifluoperazine or calmidazolium, another calmodulin inhibitor. The Al(3+) interaction with cytoplasmic CaM was estimated using models for the binding of Al(3+) and Mg(2+) with CaM and physiological concentrations of citrate, CaM, InsP(3), ATP, ADP, Al(3+) and Mg(2+). In this simulation, Al(3+) associated with citrate and InsP(3), but not with CaM. We conclude that calmodulin is not relevant to the physiological processes leading to the Al tolerance in maize, nor is it a primary target for Al toxicity.

Prenatal exposure to aluminum reduces expression of neuronal nitric oxide synthase and of soluble guanylate cyclase and impairs glutamatergic neurotransmission in rat cerebellum

Exposure to aluminum (Al) produces neurotoxic effects in humans. However, the molecular mechanism of Al neurotoxicity remains unknown. Al interferes with glutamatergic neurotransmission and impairs the neuronal glutamate-nitric oxide-cyclic GMP (cGMP) pathway, especially in rats prenatally exposed to Al. The aim of this work was to assess whether Al interferes with processes associated with activation of NMDA receptors and to study the molecular basis for the Al-induced impairment of the glutamate-nitric oxide-cGMP pathway. We used primary cultures of cerebellar neurons prepared from control rats or from rats prenatally exposed to Al. Prenatal exposure to Al prevented glutamate-induced proteolysis of the microtubule-associated protein-2, disaggregation of microtubules, and neuronal death, indicating an impairment of NMDA receptor-associated signal transduction pathways. Prenatal exposure to Al reduced significantly the content of nitric oxide synthase and guanylate cyclase and increased the content of calmodulin both in cultured neurons and in the whole cerebellum. This effect was selective for proteins of the glutamate-nitric oxide-cGMP pathway as the content of mitogen-activated protein kinase and the synthesis of most proteins were not affected by prenatal exposure to Al. The alterations in the expression of proteins of the glutamate-nitric oxide-cGMP pathway could be responsible for some of the neurotoxic effects of Al.

Immuno-detection of aluminium and aluminium induced conformational changes in calmodulin--implications in Alzheimer's disease

Binding of calcium to calmodulin (CAM) induces specific structural rearrangements in the whole protein molecule. Ca2+ organizes and stabilizes the four-domains structure of calmodulin in a helical, active conformation that can bind to its target proteins; the central helix remaining flexible is an essential condition for their bio-recognition. The conformation of calmodulin, and its efficacy to interact with target proteins, is profoundly altered when bound to metal ions other than calcium. As recently reported, the local structural changes of CaM, which occur upon aluminium binding, lead to the impairment of protein flexibility and to the loss of its ability to interact with several other proteins, which may decrease or inhibit the regulatory character of calmodulin. In this study we followed conformational changes occurring in the calmodulin molecule after aluminium binding using highly specific monoclonal antibodies (mAbs) able to differentiate between the conformational states of calmodulin, as well as mAbs which recognize aluminium free or bound to proteins. Under the same experimental conditions, mAb CAM-1, a Ca2+ conformation sensitive antibody raised against calmodulin, fails to recognize the calmodulin-aluminium complex, despite the presence of Ca2+, while the anti-Al antibodies show a maximal binding pattern towards their antigen. These data suggest that Al3+ ions bind to calmodulin in the presence of Ca2+ ions, leading to an inactive, reversible conformation, instead of its physiological active form. Alteration of the conformation of calmodulin imposed by Al binding may have possible implications in the neurotoxicity mechanism related to Alzheimer's disease.

Specificity of an anti-aluminium monoclonal antibody toward free and protein-bound aluminium

Anti-aluminium monoclonal antibodies (mAbs) were prepared using aluminium chloride-bovine serum albumin complex (Al-BSA) as immunogen. Competitive enzyme-linked immunosorbant assay (ELISA), using an Al-BSA coated immunoplate, demonstrated that mice immune sera showed stronger reactivity to AlCl3 than to BSA. Supernatants from hybridomas prepared from cloned anti-Al antibody-producing cells reacted in ELISA assays whether the metal was bound to proteins like calmodulin (CaM) and S100b protein or to immunogen BSA. Moreover, addition of citrate, a potent ligand for trivalent cations, resulted in a significant withdrawal in mAb recognition of aluminium which was previously bound to either CaM or S100b proteins. The anti-Al mAbs also reacted with aluminosilicate complexes formed from aluminium chloride and silicic acid. The results indicate that the monoclonal antibodies recognized aluminium alone, aluminium bound to silicate, or aluminium bound to a protein core and thus may be used as an immunologic tool for identifying aluminium in both in vitro and in vivo systems.

Aluminum neurotoxicity: an experimental approach to the induction of neurofilamentous inclusions

Acute or chronic aluminum neurotoxicity experiments in the rabbit suggest that aluminum can induce phosphorylation of neurofilamentous proteins. This may result in abnormal resistance to degradation or transport of neurofilament protein and so to the accumulation of neurofilaments in abnormal cells. The possible importance of this process in ALS is considered in relation to the neurofilamentous abnormalities characteristic of intraneuronal inclusions in ALS and in other neurodegenerative disorders.

Aluminum alters calcium transport in plasma membrane and endoplasmic reticulum from rat brain

Calcium is actively transported into intracellular organelles and out of the cytoplasm by Ca2+/Mg(2+)-ATPases located in the endoplasmic reticulum and plasma membranes. We studied the effects of aluminum on calcium transport in the adult rat brain. We examined 45Ca-uptake in microsomes and Ca(2+)-ATPase activity in microsomes and synaptosomes isolated from the frontal cortex and cerebellum of adult male Long-Evans rats. ATP-dependent 45Ca-uptake was similar in microsomes from both brain regions. The addition of 50-800 microM AlCl3 resulted in a concentration-dependent inhibition of 45Ca-uptake. Mg(2+)-dependent Ca(2+)-ATPase activity was significantly lower in synaptosomes compared to microsomes in both frontal cortex and cerebellum. In contrast to the uptake studies, AlCl3 stimulated Mg(2+)-dependent Ca(2+)-ATPase activity in both microsomes and synaptosomes from both brain regions. To determine the relationship between aluminum and Mg2+, we measured ATPase activity in the presence of increasing concentrations of Mg2+ or AlCl3. Maximal ATPase activity was obtained between 3 and 6 mM Mg2+. When we substituted AlCl3 for Mg2+, ATPase activity was also stimulated in a concentration-dependent manner, but to a greater extent than with Mg2+. One interpretation of these data is that aluminum acts at multiple sites to displace both Mg2+ and Ca2+, increasing the activity of the Ca(2+)-ATPase, but disrupting transport of calcium.

Differential effects of aluminum ion on smooth muscle calpain I and calpain II activities

1. In millimolar Ca2+, smooth muscle calpains I and II were inhibited by aluminum ion. 2. At sub-millimolar Ca2+, calpain II, but not calpain I, was activated by low millimolar aluminum ion. 3. Calpastatin inhibited aluminum ion-activated calpain II. 4. Aluminum ion-activated and Ca(2+)-activated calpain II gave almost identical patterns of desmin cleavage. 5. Aluminum-activated calpain II, unlike the Ca(2+)-activated enzyme, did not autolyze and retained its proteolytic activity over extended periods of time.

Chronic aluminum-induced motor neuron degeneration: clinical, neuropathological and molecular biological aspects

The monthly intracisternal inoculation of young adult New Zealand white rabbits with low-dose (100 micrograms) aluminum chloride induces aggregates of phosphorylated neurofilament that mimics the intraneuronal inclusions of amyotrophic lateral sclerosis. The chronic progressive myelopathy and topographically-specific motor neuron degeneration that occurs in the absence of suppressions of neurofilament messenger RNA levels in this model contrasts with the acute fulminant encephalomyelopathy and nonspecific gene suppressions that occur subsequent to high-dose (1000 micrograms) aluminum chloride inoculations. Further analysis of this unique model of chronic motor system degeneration can be expected to provide additional insights into the pathogenesis of amyotrophic lateral sclerosis.

Aluminum-altered membrane dynamics in human red blood cell white ghosts

Fluorescence polarization of the hydrophobic membrane probe 1,6-diphenyl hexatriene was used to investigate alterations in membrane dynamics caused by micromolar concentrations of aluminum. Metal titration onto resealed white ghost membranes showed a progressive increase in steady-state fluorescence anisotropy of the probe in the presence of aluminum which was twice the value obtained upon titration of calcium. As calculated from steady-state fluorescence data in the presence and absence of 20 microM aluminum, a temperature dependent lipid order parameter indicated increased lipid packing. Motional dynamics of the probe molecule on the nanosecond time scale showed severe constraints in the presence of 20 microM aluminum as indicated by decreased rotational rate, decreased cone angle, and increased values of time resolved limiting anisotropy. Physiological consequences of altered membrane dynamics are also discussed.

Aluminum-induced neurotoxicity: alterations in membrane function at the blood-brain barrier

Aluminum is established as a neurotoxin, although the basis for its toxicity is unknown. It recently has been shown to alter the function of the blood-brain barrier (BBB), which regulates exchanges between the central nervous system (CNS) and peripheral circulation. The BBB owes its unique properties to the integrity of the cell membranes that comprise it. Aluminum affects some of the membrane-like functions of the BBB. It increases the rate of transmembrane diffusion and selectively changes saturable transport systems without disrupting the integrity of the membranes or altering CNS hemodynamics. Such alterations in the access to the brain of nutrients, hormones, toxins, and drugs could be the basis of CNS dysfunction. Aluminum is capable of altering membrane function at the BBB; many of its effects on the CNS as well as peripheral tissues can be explained by its actions as a membrane toxin.

The influence of dietary calcium reduction on aluminum absorption and kinetics in the rabbit

There is considerable evidence of an aluminum (Al)-calcium (Ca) interaction, including potentiation of Al accumulation and toxicity by Ca deficiency. To elucidate the influence of dietary Ca on Al absorption, rabbits were maintained on a low-Ca (0.024%) or a Ca-replete (0.83%) diet for 2 wk prior to testing. Once weekly, Al hydroxide, nitrate, citrate, or lactate or sucralfate was given orally, or Al lactate was given intravenously (iv). Oral Al bioavailability was determined by comparison of the area under the Al concentration-time curve to that obtained after iv Al. Neither oral Al bioavailability nor the pharmacokinetic parameters of iv Al lactate was significantly affected by dietary Ca concentration. When measured before the weekly Al treatments, total serum Ca of rabbits fed the low-Ca diet averaged 88% of rabbits fed the Ca-replete diet. Total serum Ca 1-72 h after Al treatment decreased from 1% (Al hydroxide) to 15% (Al citrate) below pretreatment concentrations.

Ligand-triggered conformational perturbations elicit changes at the single cysteinyl residue of spinach calmodulin

Following application of stoichiometric amounts of Ca2+ or specific partner peptides to spinach calmodulin, dynamic changes in the nanosecond range could be monitored at a strategically anchored fluorescence or spin probe. For these studies the single cysteinyl residue 26 of spinach calmodulin was labelled with a thiol-specific proxyl (i.e. 2,2,5,5-tetramethyl-1-pyrrolidinyl-oxyl) spin probe or with a bimane fluorescence probe. With Ca2+ and a specific ligand (mastoparan) present, fluorescence studies (anisotropy, lifetime) indicated that the rotational motion of the protein complex becomes slower relative to the motion of calmodulin in the absence of the specific ligand. The probe's attachment site 26 appears to reside in a fairly polar microenvironment as reported by a series of proxyl spin probes varying in label length. The rotational correlation time of the shortest spin probe markedly changed upon binding of a specific peptide to a calmodulin region distant from that of the monitoring spin probe. We interpret these observations as indicating that ligand-triggered conformational perturbations are eliciting specific responses at the cysteinyl residue 26 of spinach calmodulin.

Frictional resistance to motions of bimane-labelled spinach calmodulin in response to ligand binding

The single cysteinyl residue 26 of spinach calmodulin was labelled with the thiol-specific bimane fluorescence probe. Following application of stoichiometric quantities of Ca2+ or aluminum ions to the protein, temperature-dependent fluorescence changes (anisotropy, lifetime) could be monitored via the label. From these data the Y function could be constructed which, as a function of temperature, seems to consist of two linear regions which intersect at the critical temperature, Tc. From the Y function the thermal coefficient, b(T), of the frictional resistance to fluorophore rotation could be determined. b(T) was dependent on the type and stoichiometry of the ligand(s) bound to calmodulin. Changes of the thermal coefficient apparently resulted in part from ligand-triggered structural pertubations transmitted over a considerable distance to calmodulin region I, the site of the fluorophore.