Role of neurexin2a in lead-induced locomotor defect in developing zebrafish.

Low-dose chronic lead (Pb) exposure interferes with the development of the nervous system, which may lead to learning disabilities, behavioral abnormalities, and mental retardation. Neurexins (Nrxns) are synaptic cell-adhesion molecules associated with neurological disorders. We hypothesized that Pb can affect the expression of nrxns during synapse formation and alter the phenotype behavior. Here, apoptosis, nrxns mRNA expression, and alterations of locomotion were examined after exposure to Pb in zebrafish embryos/larvae. To confirm the function of nrxn2a, rescue experiments were performed using ß-nrxn2a mRNA microinjection. Pb exposure increased apoptosis and altered locomotor behavior in zebrafish larvae. Quantitative PCR showed that among several synaptic adhesion molecules, only nrxn2a were affected by Pb exposure. Moreover, exposure to Pb at 10µmol/L upregulated mRNA expression of nrxn1a and nrxn3a at 24h post fertilization (hpf) and downregulated expression at 48 hpf, whereas the expression remained unchanged at 72 hpf. Only the two isoforms of nrxn2a were downregulated by Pb at 10µmol/L at all three time points. Rescue experiments showed that ß-nrxn2a mRNA injection recovered the decreased locomotor activity and the increased apoptosis induced by Pb. In addition, overexpression of ß-nrxn2a mRNA upregulated α-nrxn2a. These data indicated that Pb inhibited the expression of nrxn2a genes, which play a critical role in neural development, and further altered the behavior of zebrafish embryos/larvae.

Effects of cadmium, manganese, and lead on locomotor activity and neurexin 2a expression in zebrafish.

The synaptic adhesion protein Neurexin 2a (Nrxn2a) plays a key role in neuronal development and is associated with cognitive functioning and locomotor behavior. Although low-level metal exposure poses a potential risk to the human nervous system, especially during the developmental stages, little is known about the effects of metal exposures on nrxn2a expression during embryonic development. We therefore exposed wild-type zebrafish embryos/larvae to cadmium (CdCl2 ), manganese (MnCl2 ), and lead ([CH3 COO]2 Pb), to determine their effect on mortality, malformation, and hatching rate. Concentrations of these metals in zebrafish were detected by inductively coupled plasma mass spectrometry analysis. Locomotor activity of zebrafish larvae was analyzed using a video-track tracking system. Expression of nrxn2a was assessed by in situ hybridization and quantitative polymerase chain reaction. The results showed that mortality, malformation, and bioaccumulation increased as the exposure dosages and duration increased. Developmental exposure to these metals significantly reduced larval swim distance and velocity. The nrxn2aa and nrxn2ab genes were expressed in the central nervous system and downregulated by almost all of the 3 metals, especially Pb. These data demonstrate that exposure to metals downregulates nrxn2a in the zebrafish model system, and this is likely linked to concurrent developmental processes. Environ Toxicol Chem 2017;36:2147-2154. © 2017 SETAC.