11,95 As a result, cytokines can potentially mediate both neuroprotective and neurotoxic processes at once. For example, ample evidence indicates that IL-iβ may exacerbate neuronal injury both in vivo and in vitro.96-99 In contrast, IL-iβ has also been implicated in neuroprotective processes such as remyelination,100 blood-brain barrier repair,101 ischemic tolerance,102 and neurotrophic factor production.103-106 Importantly, astrocytes can themselves respond to IL-iβ by releasing a number of potentially neuroprotective trophic factors such as nerve growth factor (NGF),

ciliary neurotrophic factor (CNTF), glial cell-line Inhibitors,research,lifescience,medical derived neurotrophic factor GDNF, and fibroblast growth factor (FGF)-2.11,107-109 Taken together, studies such as those mentioned above provide important information about the multiple effects of individual cytokines. However, they also have major limitations, in that they can only take into account a Inhibitors,research,lifescience,medical few pro- and anti-inflammatory pathways at a time. As such, they may only reflect a small fraction of an infinitely more intricate

process in which astrocytes take part. For this reason, the use of genetically manipulated animal models specifically preventing the both proliferation of reactive astrocytes or the activation of their core inflammatory pathways, Inhibitors,research,lifescience,medical has provided important new insight into their overall role in Inhibitors,research,lifescience,medical response to brain injury. For instance, it has been demonstrated that the selective attenuation of astrocytic proinflammatory processes, through genetic inactivation of the transcription factor NF-kB specifically in this cell type, affords substantial neuroprotection following spinal cord

injury.110 By contrast, using a transgenic mouse model in which dividing reactive astrocytes were selectively Inhibitors,research,lifescience,medical ablated, Sofroniew and colleagues have demonstrated that following various types of brain injury, reactive astrocytes play an essential role in temporally and spatially restricting neurointlammation, as well as in promoting blood-brain barrier repair, limiting brain edema, and preserving neuronal viability.94,111-113 Consistent with a role of astrocytes in containing neuroinflammation, it is interesting to note that astrocytes appear to participate in the suppression of microglial Brefeldin_A activation through negative feedback loops. Activated microglial cells release high levels of proinflammatorycytokines and toxic ROS which may negatively impact neuronal survival.114 Several in vitro studies have demonstrated that astrocyte-conditioned medium or the presence of astrocytes attenuates microglial activation in response to various proinflammatory stimuli.115-117 The exact nature of the astrocyte-derived factors involved has not been fully elucidated, but transforming growth factor (TFG)β is thought to contribute to this process.