Mechanical trauma induces rapid astroglial activation of ERK/MAP kinase: Evidence for a paracrine signal

Astrogliosis is a prominent and ubiquitous reaction of astrocytes to many f orms of CNS injury, often implicated in the poor regenerative capacity of t he adult mammalian CNS. Transmembrane signals that rapidly trigger and main tain astroglial responses to injury are largely undefined. Several candidat e inducers of astrogliosis, including growth factors and neuropeptides, act via the extracellular signal-regulated kinase (ERK)/mitogen-activated prot ein kinase (MAPK) pathway. We previously observed chronically activated ERK /MAPK in human reactive astrocytes. To investigate mechanisms of pathway ac tivation in a defined in vitro model, primary cultured astroglial monolayer s were subjected to focal mechanical injury. Within 2-10 min, ERK/MAPK was activated, but only in cells near the wound edge. By 30 min, the entire mon olayer showed activation, which persisted for 4 to 8 h. ERK/MAPK activation was specifically blocked by application of the MEK inhibitors, PD98059 and U0126. Cell-cell contact was not necessary for intercellular spread of ERK /MAPK activation, and ERK/MAPK-stimulating activity was found in the injury -conditioned medium. The activating factor was shown to have a native size of 50-100 kD and did not signal through the classical EGF receptor. Injury- induced signaling to ERK/MAPK required Ras, as demonstrated by specific blo ckade after transient transfection with a dominant negative Ha-RasN17 const ruct. Finally, we demonstrated that focal lesioning of adult rat cortex ind uces a rapid activation and spreading of astroglial ERK/MAPK, suggesting th at similar mechanisms may operate in astroglial activation following acute brain injury. (C) 2001 Wiley-Liss, Inc.