Introduction Lead, cadmium, arsenic, and mercury are widely used in industry and among the leading toxic agents detected in the environment. The deleterious effects encountered after exposure to these individual metals in low doses are well documented. However, human exposure to environmental chemicals is most correctly characterised as an exposure to mixtures, and little is known about the combined impact of these metals.

Methods In our study, the combined impacts of these four metal mixtures (MM) in low doses on synaptic homeostasis as well as the related mechanisms were investigated in cultured hippocampal neurons and NGF-differentiated PC12 cells. F-actin staining was used to emerge the structure of dendrites and spines for synaptic morphology analysis. Immunofluorescence and RT-PCR were applied to examine the expressions of serum-inducible kinase (Snk) and spine-associated Rap guanosinetriphosphatase activating protein (SPAR). The plasmids of shRNA-Snk, SPAR-Wt and SPAR-Mut (S1328) were constructed for transfection Assays.

Results MM exposure declined the density and length of dendritic spines, and dendrite branches in dose-effect relationship in hippocampal neurons. And the mushroom and thin spines were decreased. Simultaneously, the Snk expression were up-regulated accompanying with the down-regulation of SPAR expression. Similar to what observed in the hippocampal neurons, the synaptic morphology analysis of NGF-differentiated PC12 cells showed their neurite length and tip end numbers were declined in dose-effect relationship after MM exposure, which accompanied with the up-regulation of Snk and the down-regulation of SPAR. Snk agonist aggravated these impairment, whereas, Snk knockdown and SPAR overexpression attenuated the changes of neurite outgrowth. In addition, SPAR-Mut (S1328) overexpression performed a better reversion than SPAR-Wt overexpression in the changes of neurite outgrowth induced by MM-exposure.

Conclusion These results indicated that combined exposure to low doses MM disturbed the synaptic homeostasis, and Snk-SPAR pathway might be a novel target to prevent MM induced neurotoxicity.