Metallothionein-IIA Promotes Initial Neurite Elongation and Postinjury Reactive Neurite Growth and Facilitates Healing after Focal Cortical Brain Injury

Metallothioneins (MTs) are small, cysteine-rich, metal binding proteins. Their function has often been considered as stress-related proteins capable of protecting cells from heavy metal toxicity and oxidative free radicals. However, recent interest has focused on the brain-specific MT-III isoform, which has neurite-inhibitory properties. To investigate the effect of another MT isoform, human MT-IIA, on neurite growth, we used rat cortical neuron cultures. MT-IIA promoted a significant increase in the rate of initial neurite elongation of individually plated neurons. We also investigated the effect of MT-IIA on the neuronal response to axonal transection in vitro. MT-IIA promoted reactive axonal growth after injury, and, by 18 hr after transection, MT-IIA had promoted axonal growth across the injury tract. Exogenous application of MT-IIA after cortical brain injury promoted wound healing, as observed by a significant decrease in cellular degradation at 4 d after injury. Furthermore, MT-IIA-treated rats exhibited numerous SMI-312-immunoreactive axonal processes within the injury tract. This was in contrast to vehicle-treated animals, in which few axonal sprouts were observed. By 7 d after injury, MT-IIA treatment resulted in a total closing over of the injury tract by microglia, astrocytes, and reactive axonal processes. However, although some reactive axonal processes were observed within the injury tract of vehicle-treated rats, the tract itself was almost never entirely enclosed. These results are discussed in relation to a possible physiological role of metallothioneins in the brain, as well as in a therapeutic context.