Basic science; metallothionein I and II attenuate the thalamic microglial response following traumatic axotomy in the immature brain.

The clinical manifestations of inflicted traumatic brain injury in infancy most commonly result from intracranial hemorrhage, axonal stretch and disruption, and cerebral edema. Often hypoxia ischemia is superimposed, leading to early forebrain and later thalamic neurodegeneration. Such acute and delayed cellular injury activates microglia in the CNS. Although activated microglia provide important benefits in response to injury, microglial release of reactive oxygen species can be harmful to axotomized neurons. We have previously shown that the antioxidants metallothionein I and II (MT I & II) promote geniculocortical neuronal survival after visual cortex lesioning. The purpose of this investigation was to determine the influence of MT I & II on the density and rate of thalamic microglial activation and accumulation following in vivo axotomy. We ablated the visual cortex of 10-day-old and adult MT I & II knock out (MT(-/-)) and wild-type mice and then determined the density of microglia in the dorsal lateral geniculate nucleus (dLGN) over time. Compared to the wild-type strain, microglial activation occurred earlier in both young and adult MT(-/-) mice. Similarly, microglial density was significantly greater in young MT(-/-) mice 30, 36, and 48 hours after injury, and 3, 4, and 5 days after injury in MT(-/-) adults. In both younger and older mice, time and MT I & II deficiency each contributed significantly to greater microglial density. Only in younger mice did MT I & II expression significantly slow the rate (density x time) of microglial accumulation. These results suggest that augmentation of MT I & II expression may provide therapeutic benefits to infants with inflicted brain injury.

Metallothionein I and II mitigate age-dependent secondary brain injury.

Both the immediate insult and delayed apoptosis contribute to functional deficits after brain injury. Secondary, delayed apoptotic death is more rapid in immature than in adult CNS neurons, suggesting the presence of age-dependent protective factors. To understand the molecular pathobiology of secondary injury in the context of brain development, we identified changes in expression of oxidative stress response genes during postnatal development and target deprivation-induced neurodegeneration. The antioxidants metallothionein I and II (MT I/II) were increased markedly in the thalamus of adult C57BL/6 mice compared to mice <15 days old. Target deprivation generates reactive oxygen species that mediate neuronal apoptosis in the central nervous system; thus the more rapid apoptosis observed in the immature brain might be due to lower levels of MT I/II. We tested this hypothesis by documenting neuronal loss after target-deprivation injury. MT I/II-deficient adult mice experienced greater thalamic neuron loss at 96 hr after cortical injury compared to that in controls (80 +/- 2% vs. 57 +/- 4%, P < 0.01), but not greater overall neuronal loss (84 +/- 4% vs. 79 +/- 3%, MT I/II-deficient vs. controls). Ten-day-old MT I/II-deficient mice, however, experienced both faster onset of secondary neuronal death (30 vs. 48 hr) and greater overall neuronal loss (88 +/- 2% vs. 69 +/- 4%, P = 0.02). MT I/II are thus inhibitors of age-dependent secondary brain injury, and the low levels of MT I/II in immature brains explains, in part, the enhanced susceptibility of the young brain to neuronal loss after injury. These findings have implications for the development of age-specific therapeutic strategies to enhance recovery after brain injury.