Inhibition of rat brain phosphatidylinositol-specific phospholipase C by aluminum: regional differences, interactions with aluminum salts, and mechanisms

Aluminum exposure decreases dopamine D1 and D2 receptor expression in mouse brain

Aluminum (Al) has been identified as a potential contributing factor in the etiology of several neurodegenerative disorders, but data regarding specific effects on neurotransduction, especially on dopaminergic neurotransduction, are lacking. The objective of this study was to determine the extent of expressional alterations in dopamine receptors (DRs) in two dopaminergic subtypes, D1 and D2, in low and high dose Al-treated mice. After administration of Al (four intraperitoneal injections of 30 or 60 mg/kg AlCl3·6H2O at 2 h intervals), expression of the dopamine D1-like and D2-like receptors (DRD1, DRD2) was examined in the cortex and striatum of mouse brain at bregma levels of 1.10, -0.10 and -1.34 mm. In the cortex, Al treatment decreased densities of DRD1 and DRD2 in a dose-dependent manner at all three bregma levels, especially in the high-dose Al group. Similarly, DRD1 and DRD2 expression in the striatum also exhibited dose dependency and statistically significant decreases were seen in the high-dose group, except in the striatum at bregma level - 1.34. These findings suggest that DR in the caudal striatum is more resistant to the effects of Al exposure than DR in the cortex or rostral striatum. In addition, our results suggest that disturbance of dopaminergic neurotransmission mediated by DRD1 and/or DRD2 may be involved in the pathogenesis of Al neurotoxicity. Human & Experimental Toxicology (2007) 26, 741 — 746

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.

Metals in neurodegeneration: involvement of oxidants and oxidant-sensitive transcription factors

Oxidant-mediated damage and the triggering of oxidant-sensitive transcription factors could be associated with the neurotoxic actions of aluminum, zinc and lead. Aluminum and lead could induce oxidative stress through their capacity to interact with active oxygen species, increasing their oxidant activity, or by affecting membrane rheology. Aluminum-membrane interactions can also affect signaling cascades. Zinc, at high and low concentrations, increases cell oxidant concentrations, affects AP-1 and NF-kappaB transcription factors and induces neuronal cell death. The capacity of lead to promote oxidative stress, affect cell signals and to induce cell death by apoptosis has been mostly attributed to its effect on different calcium-mediated cellular events. The mentioned mechanisms as well as the contribution of these metals to different neurodegenerative disorders are discussed.

Al(3+)-mediated changes on membrane fluidity affects the activity of PI-PLC but not of PLC

We investigated whether Al(3+)-mediated changes in membrane fluidity can affect the activity of prokaryotic enzymes phospholipase C (PLC) and phospholipase C-phosphatidyl inositol specific (PI-PLC) in liposomes of phosphatidyl choline (PC), PC:phosphatidyl inositol (PI), or PC and polyphosphoinositides (PPI). Al(3+) (10-100 microM) promoted membrane rigidification, evaluated with the probes 1,6-diphenyl-1,3,5-hexatriene and Laurdan, and followed the order: PC:PPI>PC:PI>PC. Al(3+) (25 and 50 microM) did not affect PLC-mediated hydrolysis of PC, PI and PIP(2), but stimulated PIP hydrolysis (48.6%). PI-PLC did not affect PC, PI, and PIP concentrations, but caused a 67% decrease in PIP(2). Al(3+) significantly inhibited PIP(2) hydrolysis in a concentration-dependent (25-50 microM) manner. Results suggest that the inhibition of PIP(2) hydrolysis by Al(3+) could be partially due to a higher lipid packing induced by Al(3+) which could affect the interaction between the enzyme and its substrate.

Al(3+)-mediated changes in membrane physical properties participate in the inhibition of polyphosphoinositide hydrolysis

We investigated the possible involvement of Al(3+)-induced alterations in membrane physical properties in Al(3+)-mediated inhibition of polyphosphoinositide (PPI) hydrolysis by the enzyme phosphatidylinositol-specific phospholipase C (PI-PLC). Liposomes composed of brain phosphatidylcholine (PC) or of PC and a mixture of brain PPI (PC:PPI) were incubated in the presence of Al(3+) (1-100 microM). We evaluated: (1) the amount of membrane-bound Al(3+), (2) the effects of Al(3+) on key membrane physical properties (surface potential, lipid fluidity, and lipid arrangement), and (3) the hydrolysis of PPI. Al(3+) binding to PC:PPI (60:40 mol/mol) liposomes was 1.3 times higher than to PC:PPI (90:10 mol/mol) liposomes and did not change after treatment with Triton X-100. Al(3+) increased membrane surface potential, promoted the loss of membrane fluidity, and caused lateral phase separation in PC:PPI liposomes. Phosphatidylinositol and phosphatidylinositol monophosphate hydrolysis in the presence of PI-PLC was not affected by Al(3+), but a significant and concentration-dependent inhibition of PIP(2) hydrolysis was observed, an effect that was prevented by previous bilayer disruption with Triton X-100. The obtained results support the hypothesis that Al(3+) binding to liposomes promotes the formation of rigid clusters enriched in PPI, restricting the accessibility of the enzyme to the substrate and subsequently inhibiting PIP(2) hydrolysis by PI-PLC.

Chronic administration of aluminum-fluoride or sodium-fluoride to rats in drinking water: alterations in neuronal and cerebrovascular integrity

This study describes alterations in the nervous system resulting from chronic administration of the fluoroaluminum complex (AlF3) or equivalent levels of fluoride (F) in the form of sodium-fluoride (NaF). Twenty seven adult male Long-Evans rats were administered one of three treatments for 52 weeks: the control group was administered double distilled deionized drinking water (ddw). The aluminum-treated group received ddw with 0.5 ppm AlF3 and the NaF group received ddw with 2.1 ppm NaF containing the equivalent amount of F as in the AlF3 ddw. Tissue aluminum (Al) levels of brain, liver and kidney were assessed with the Direct Current Plasma (DCP) technique and its distribution assessed with Morin histochemistry. Histological sections of brain were stained with hematoxylin & eosin (H&E), Cresyl violet, Bielschowsky silver stain, or immunohistochemically for beta-amyloid, amyloid A, and IgM. No differences were found between the body weights of rats in the different treatment groups although more rats died in the AlF3 group than in the control group. The Al levels in samples of brain and kidney were higher in both the AlF3 and NaF groups relative to controls. The effects of the two treatments on cerebrovascular and neuronal integrity were qualitatively and quantitatively different. These alterations were greater in animals in the AlF3 group than in the NaF group and greater in the NaF group than in controls.