Accumulation of aluminum by primary cultured astrocytes from aluminum amino acid complex and its apoptotic effect.

Aluminum salts or doses that are unlikely in the human system have been employed in toxicity studies and much attention had been focused on the secondary target (neurons) of its toxicity rather than the primary target (astroglia). In order to address these issues, we have investigated the uptake and apoptotic effects of aluminum amino acid complex on primary cultured astrocytes because these are fundamental in understanding the mechanism of aluminum neurotoxicity. Aluminum solubilized by various amino acids was differentially internalized by astrocytes (glycine>serine>>glutamine>>glutamate), but aluminum was not internalized from citrate complex following 24 h of exposure. Inhibition of glutamine synthetase, by methionine sulfoximine (MSO), enhanced the uptake of aluminum from various amino acid complexes within 8 h except from glutamine complex. Blockade of selective GLT-1 (EAAT2) and GlyT1, as well as nonspecific transporters, did not inhibit or had no effect on uptake of aluminum in complex with the corresponding amino acids. Ouabain also failed to inhibit uptake of aluminum complexed with glycine. Pulse exposure to aluminum glycinate in the absence or presence of MSO caused apoptosis in over 25% of primary cultured astrocytes, and apoptotic features such as chromatin condensation and fragmentation became evident as early as 3 days of culture in normal medium. Lower doses (as low as 0.0125 mM) also caused apoptosis. The present findings demonstrate that aluminum solubilized by amino acids, particularly glycine, could serve as better candidate for neurotoxicity studies. Citrate may be a chelator of aluminum rather than a candidate for its cellular uptake. Amino acid transporters may not participate in the uptake of aluminum solubilized by their substrates. Another pathway of aluminum internalization may be implicated in addition to passive diffusion but may not require energy in form of Na+/K+-ATPase. Impaired astrocyes' metabolism can aggravate their accumulation of aluminum and aluminum can compromise astrocytes via apoptosis. Thus, loss of astrocytic regulatory and supportive roles in the central nervous system (CNS) may be responsible for neurodegeneration observed in Alzheimer's disease.