Lead Exposure during Synaptogenesis Alters Vesicular Proteins and Impairs Vesicular Release: Potential Role of NMDA Receptor–Dependent BDNF Signaling

Abstract

Lead (Pb(2+)) exposure is known to affect presynaptic neurotransmitter release in both in vivo and cell culture models. However, the precise mechanism by which Pb(2+) impairs neurotransmitter release remains unknown. In the current study, we show that Pb(2+) exposure during synaptogenesis in cultured hippocampal neurons produces the loss of synaptophysin (Syn) and synaptobrevin (Syb), two proteins involved in vesicular release. Pb(2+) 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 Pb(2+) exposure impaired vesicular release and reduced the number of fast-releasing sites. Because Pb(2+) 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 Pb(2+) 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 Pb(2+) exposure. Furthermore, exogenous addition of BDNF rescued the presynaptic effects of Pb(2+). We suggest that the presynaptic deficits resulting from Pb(2+) exposure during synaptogenesis are mediated by disruption of NMDAR-dependent BDNF signaling.

Mechanisms of lead and manganese neurotoxicity

“…142 The reductions in extracellular levels of BDNF may not only be the result of reduced BDNF gene and protein expression, but may also result from disruption of the transport of BDNF vesicles to dendritic sites. 143 This would support our hypothesis that NMDAR-dependent release of BDNF is disrupted during Pb 2+ exposure, 133,134,143 since the majority of NMDARs are postsynaptically located. 146 …”

“…133 Pb 2+ exposure reduced the expression of key presynaptic proteins involved in vesicular release, such as synaptophysin (Syn) and synaptobrevin (Syb). Reductions of vesicular release proteins were associated with both glutamatergic and GABAergic synapses, consistent with electrophysiological observations regarding EPSC and IPSC generation during Pb 2+ exposure.…”

“…However, the effects of Pb 2+ observed on presynaptic protein expression were dependent on NMDAR activity, based on comparison studies with the specific NMDAR antagonist amino-phosphonovaleric acid (APV, which does not inhibit VGCCs) which resulted in similar effects as Pb 2+ exposure. 133 Thus, while Pb 2+ inhibits VGCCs, which may result in impaired neurotransmission, VGCC inhibition by Pb 2+ is not exclusively responsible for the presynaptic effects of Pb 2+ and long-term NMDAR inhibition plays an important role in these effects.…”

“…142 We have recently shown that BDNF transcript and protein levels are reduced in Pb 2+ -exposed cultures 133,143 and that exogenous BDNF supplementation during Pb 2+ exposure can fully mitigate the effects of Pb 2+ on presynaptic function and protein expression. 133 Furthermore, BDNF expression and release are dependent on Ca 2+ signalling, and both NMDAR- and VGCC-dependent Ca 2+ pathways have been implicated in BDNF neurotransmission. 142,144,145 Interestingly, NMDAR-dependent release of BDNF may play a greater role in dendritic BDNF release rather than axonic BDNF.…”

Avaliação do desenvolvimento do sistema nervoso central de camundongos Balb/c expostos à fumaça do cigarro no início do período pós-natal

“…Esse efeito foi observado tanto em neurônios GABAérgicos como em glutamatérgicos (NEAL et al, 2010).…”