Cellular and molecular correlates to plasticity during recovery from injury in the developing mammalian brain.

In summary, our studies indicate that the perinatal mammalian brain shows considerable plasticity in response to trauma. Studies carried out both in vivo in the perinatal mouse brain and in vitro in cell line culture and organotypic slice cultures of developing brain tissue, indicate that the cytokine, interleukin-1 beta (IL-1 beta) regulates early healing responses that restore the integrity of the damaged structure and create conditions conducive to the sprouting of new connections involved in plasticity. In response to a lesion placed in the cerebral cortex in a late third trimester embryo, astrocytes form a line that delimits damaged tissue being removed by phagocytic macrophages from tissue that will remain part of the neural parenchyma. By six days after birth, this line of delimiting astrocytes (LDA) appears to become the new glial limiting membrane or glial limitans at the lesion site. A gliotic scar covers the new glial limitans, but no gliosis appears within the neural parenchyma itself. The expression of IL-1 beta is upregulated in astrocytes that form the LDA and is also upregulated in the parenchyma internal to the LDA. Experiments done in vivo where the type 1 interleukin-1 receptor was blocked via injection of interleukin-receptor antagonist protein (IL-ra) indicated that both LDA formation and wound closure were dependent upon interleukin type 1 receptor activation. To test the idea that IL-1 beta could directly influence astrocyte shape and orientation, in vitro studies were carried out on astrocytic C6 glioma cells in culture. IL-1 beta induced changes in cell shape and orientation similar to those seen in in vivo formation of the LDA. Addition of IL-1ra blocked IL-1 beta induced changes in C6 cells. IL-1 beta, then, acting upon its type 1 receptor, regulates astrocytic activities that, in vivo, produce successful healing in the perinatal brain. Studies in organotypic slice cultures of early postnatal mouse hippocampus parallel in vivo studies. Phagocytic cells, in this case, "reactive/activated" microglia, reach peak numbers immediately after injury induced by culture preparation. The round microglia were replaced over 10 days in culture by "resting/ramified" microglia. Over the first 2 days of culture, astrocytes appeared thin and elongated, resembling cells that form the LDA in vivo. Over the next 8 days in cultures, astrocytes underwent hypertrophy to form a gliotic scar over the surface of the culture. The scar resembled that seen external to the LDA after healing in in vivo experiments. IL-1 beta was abundantly expressed throughout the culture period by cells showing a variety of morphologies. Finally, neurite sprouting, an indicator of circuit reorganization and plasticity, occurred rapidly in the hippocampal dentate gyrus in both in vivo and in vitro paradigms. A prenatally placed lesion in the entorhinal cortex that partially deafferents the developing dentate gyrus, induced novel sprouting of the axons of dentate granule cells, the mossy fibers, into the dentate molecular layer. Similar sprouting occurred in vitro in organotypic slice culture of deafferented hippocampus. In culture, sprouting was first observed at the time of onset of astrocyte hypertrophy, indicating that astrocyte derived factors may play a role in regulating circuit reorganization. Viewed together, in vivo and in vitro studies indicate that IL-1 beta upregulation in neural tissue correlates with glial activities that underlie rapid healing and repair in the perinatal brain, and that glial activities associated with deafferentation may play a role in inducing compensatory neurite sprouting and cicuit reorganization.