Developmental reelin expression and time point-specific alterations from lead exposure in zebrafish.

Reelin (RELN) is a large glycoprotein that plays critical roles during and after neuronal development. In a previous study utilizing developing zebrafish, the RELN ortholog was in a subset of genes associated with neuronal function that was altered following exposure to 100ppb Pb at 72h post-fertilization (hpf). Although RELN is associated with neurodevelopment and neurological diseases, it represents a previously uncharacterized target for developmental Pb neurotoxicity. To further investigate Pb-induced expression decrease of RELN, a more detailed analysis was done using in situ hybridization and quantitative PCR (qPCR) on developing zebrafish at multiple developmental time points and at different exposure concentrations. Time points included in the analysis were 24, 36, 48, 60, 72 and 96hpf. The four exposure conditions tested were 10, 50 or 100ppb Pb as well as a control. In situ hybridization demonstrated concentrated expression of the gene in the telencephalon and in midbrain, hindbrain and spinal cord. Expression was less spatially restricted early in development but became isolated to specific cell populations in the central nervous system by 48hpf. The telencephalon demonstrated a high degree of expression throughout development. Pb exposure did not affect the observed location of expression at any of the time points tested. However, qPCR analysis demonstrated that reln expression statistically decreased in embryos exposed to the highest concentration compared to controls at 60hpf. No changes in brain morphology as assessed by measuring brain length and no significant increase in the number of apoptotic cells in the brain were observed at the highest exposure concentration. These data demonstrate that a decrease in reln expression occurs following Pb exposure at a concentration below one that results in changes in brain length or in the number of apoptotic cells in the zebrafish model system during a short window and is likely linked to concurrent developmental processes.