Lead exposure during synaptogenesis alters vesicular proteins and impairs vesicular release: Potential role of NMDA receptor-dependent BDNF signaling

Lead (Pb²⁺) exposure is known to affect presynaptic neurotransmitter release in both in vivo and cell culture models. However, the precise mechanism by which Pb²⁺ impairs neurotransmitter release remains unknown. In the current study, we show that Pb²⁺ exposure during synaptogenesis in cultured hippocampal neurons produces the loss of synaptophysin (Syn) and synaptobrevin (Syb), two proteins involved in vesicular release. Pb²⁺ exposure also increased the number of presynaptic contact sites. However, many of these putative presynaptic contact sites lack Soluble NSF attachment protein receptor complex proteins involved in vesicular exocytosis. Analysis of vesicular release using FM 1-43 dye confirmed that Pb²⁺ exposure impaired vesicular release and reduced the number of fast-releasing sites. Because Pb²⁺ is a potent N-methyl-D-aspartate receptor (NMDAR) antagonist, we tested the hypothesis that NMDAR inhibition may be producing the presynaptic effects. We show that NMDAR inhibition by aminophosphonovaleric acid mimics the presynaptic effects of Pb²⁺ exposure. NMDAR activity has been linked to the signaling of the transsynaptic neurotrophin brain-derived neurotrophic factor (BDNF), and we observed that both the cellular expression of proBDNF and release of BDNF were decreased during the same period of Pb²⁺ exposure. Furthermore, exogenous addition of BDNF rescued the presynaptic effects of Pb²⁺. We suggest that the presynaptic deficits resulting from Pb²⁺ exposure during synaptogenesis are mediated by disruption of NMDAR-dependent BDNF signaling.