Neuroprotection by Lowering Cholesterol: a Decrease in Membrane Cholesterol Content Reduces Transporter-mediated Glutamate Release from Brain Nerve Terminals

Background: In our earlier work, a reduction of cholesterol content increased the extracellular glutamate level in rat brain nerve terminals (synaptosomes) that was a result of the lack of transporter-mediated glutamate uptake. The aim of this study was to assess transporter-mediated release of glutamate from cholesterol-deficient synaptosomes. In stroke, cerebral hypoxia/ischemia, and traumatic brain injury, the development of neurotoxicity is provoked by enhanced extracellular glutamate, which is released from nerve cells mainly by glutamate transporter reversal - a distinctive feature of these pathological states.

Methods: Laser scanning confocal microscopy, spectrofluorimetry, radiolabeled assay, and glutamate dehydrogenase assay.

Results: Cholesterol acceptor methyl-β-cyclodextrin (15mM) reduced the cholesterol content in the synaptosomes by one quarter. Transporter-mediated glutamate release from synaptosomes: 1) stimulated by depolarization of the plasma membrane; 2) by means of heteroexchange with competitive transportable inhibitor of glutamate transporters dl-threo-β-hydroxyaspartate; 3) in low [Na(+)] medium; and 4) during dissipation of the proton gradient of synaptic vesicles by the protonophore cyanide-p-trifluoromethoxyphenyl-hydrazon (FCCP); was decreased under conditions of cholesterol deficiency by ~24, 28, 40, and 17%, respectively.

Conclusions: A decrease in the level of membrane cholesterol attenuated transporter-mediated glutamate release from nerve terminals. Therefore, lowering cholesterol may be used in neuroprotection in stroke, ischemia, and traumatic brain injury which are associated with an increase in glutamate uptake reversal. This data may explain the neuroprotective effects of statins in these pathological states and provide one of the mechanisms of their neuroprotective action. However, beside these disorders, lowering cholesterol may cause harmful consequences by decreasing glutamate uptake in nerve terminals.