α2-Macroglobulin Exposure Reduces Calcium Responses to N-Methyl-d-Aspartate via Low Density Lipoprotein Receptor-related Protein in Cultured Hippocampal Neurons

Abstract There is increasing evidence that the low-density lipoprotein receptor-related protein (LRP) can function as a signaling link in the central nervous system. To investigate the pathophysiological role of LRP in the central nervous system, we examined the effects of activated α2-macroglobulin (α2M*), a ligand of LRP, on intracellular calcium signaling in cultured rat hippocampal neurons. Neuronal effects of α2M* (50 nm) were assessed by a comparison of calcium signals produced in control and α2M*-pretreated neurons byN-methyl-d-aspartate (NMDA) and α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid. α2M* pretreatment significantly decreased the calcium signals to NMDA, whereas little change was observed for the signals to α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid. Native α2M, which is not a ligand for LRP, did not affect signals to NMDA. The receptor-associated protein prevented α2M*-induced decrease of calcium responses to NMDA, suggesting that α2M* exerted its effects through an LRP-mediated pathway. Experiments changing calcium sources demonstrated that α2M* pretreatment altered calcium responses to NMDA by primarily changing extracellular calcium influx and subsequently affecting calcium release from intracellular calcium stores. Immunoblot analysis demonstrated that α2M* caused a reduction in the levels of the NMDA receptor subunit, NMDAR1. These results suggest that α2M* can alter the neuronal response to excitatory neurotransmitters and that α2M* pretreatment selectively reduced the calcium responses to NMDA by down-regulating the NMDA receptor.