Effects of age on the response to spinal cord injury: optimizing the larval zebrafish model

Zebrafish are an increasingly popular model to study regeneration after spinal cord injury (SCI). The transparency of larval zebrafish makes them ideal to study cellular processes in real time. Standardized approaches, including age at the time of injury, are not readily available making comparisons of the results with other models challenging. In this study, we systematically examined the response to spinal cord transection of larval zebrafish at three different larval ages (3-, 5-, or 7-days post fertilization (dpf)) to determine whether the developmental complexity of the larvae affects the overall response to SCI. We then used imaging and behavioral analysis to evaluate whether differences existed based on the age of injury. Injury led to increased expression of cytokines associated with the immune response; however, we found that the timing of specific inflammatory markers changed with the age of the injury. We also observed changes in glial and axonal bridging with age. Young larvae (3 dpf) were better able to regenerate axons independent of the glial bridge, unlike older larvae (7 dpf), consistent with results seen in adult zebrafish. Finally, locomotor experiments demonstrated that some swimming behavior occurs independent of glial bridge formation, further highlighting the need for standardization of this model and functional recovery assays. Overall, we found differences based on the age of transection in larval zebrafish, underlining the importance of considering age when designing experiments aimed at understanding regeneration.