Transient Neuronal but Persistent Astroglial Activation of ERK/MAP Kinase after Focal Brain Injury in Mice

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Abstract
Astrogliosis is a nearly ubiquitous response to a variety of insults to the central nervous system (CNS). This reaction is triggered rapidly, but can persist for years after the initial trauma. Little is known about the signaling mechanisms responsible for this activation and its chronic maintenance. Extracellular signal-regulated kinase/mitogen-activated protein kinase (ERK/MAPK) activation is implicated in several functions important to the reactive glial phenotype such as cellular proliferation and motility. Here we utilize immunohistochemistry with a phosphorylation state-specific antibody (pERK) to characterize the temporal and spatial pattern of ERK/MAPK activation in neurons and glia following a forebrain stab lesion (FSL) in mice. Early activation (1 h) was primarily in perilesional neuronal elements, particularly of the hippocampus. Occasional perilesional glia were also positive for pERK. Additionally, ependymal cells bilaterally stained prominently for pERK. These patterns of pERK immunoreactivity at 1 h were abolished by pretreatment with the selective MEK inhibitor, SL327. ERK/MAPK activation at later time points between 1 day (d) and 30 d was primarily restricted to perilesional astrocytes with maximum labeling at 3 d. However, pERK-positive astrocytes represented only a subset of total GFAP-positive cells and were found more proximal to the lesion suggesting specific functional activation of these cells. Finally, immunostaining for the phosphorylated form of cAMP response element-binding (CREB) protein, a downstream target of the ERK/MAPK cascade, was increased in perilesional glia 7 d after FSL. Sustained activation of the ERK/MAPK signaling pathway in perilesional reactive glia suggests a critical role for this cascade in astrogliosis.

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