Genomic Clues to Secondary Injury Mechanisms in Brain Trauma

Traumatic brain injury (TBI) is a heterogenous disease that can lead to persistent disability or death. The immediate mechanical disruption of brain tissue is followed by a phase of secondary brain injury. During this phase TBI challenges clinical interventions due to its complexity. Increased knowledge is needed about the operating molecules and their time-windows. We used cerebral cortical contusions by weight drop and experimental depolarisation, in order to analyse molecular and genetic responses. Alterations in gene expression were studied by microarrays, RT-PCR, in situ hybridisation and immunohistochemistry. Nestin is an intermediate filament, expressed in CNS progenitor cells during embryogenesis, but restricted to residing stemcells in adults. Depolarisation induced nestin expression in astrocytes along the ipsilateral cortex. NMDA blockade by the MK-801 reduced nestin expression. The nestin expression in reactive astrocytes is interesting since they form the glial scar. Matrix metalloproteinases (MMPs) and their inhibitors TIMPs, determine extracellular matrix turnover and cause overgrowth or disruption when disturbed. We found a time-dependent increase of neuronal TIMP-1 and MMP-9 expression in the ipsilateral cortex after depolarisation and contusion. The early expression of TIMP-1 could be a MMP-9 independent protective response to damage. TIMP-1 and MMP-9 are likely to participate in the tissue reorganisation or neuroprotection. After experimental contusion early and delayed genomic responses were significantly different for genes involved in transcription, cell communication, cell proliferation, cell-death and metabolism. In the early phase genes involved in transcription and cell-death were prominent, while immune response and proteolysis dominated the delayed phase. Osteopontin and the CD44 receptor are involved in inflammation, and were locally upregulated at the impact site. Genes of the calcium signaling pathway i.a. hippocalcin and VILIP-1, were suppressed while genes of the complement and coagulation pathways were upregulated. Depolarisation and experimental contusion largely shared genomic responses. NMDA receptor blockade after depolarisation abolished the regulated genes to a larger extent than after contusion. Genes for nestin, MMP-9, TIMP-1, osteopontin and CD44 were similarly regulated in both models. Almost all depolarisation induced effects were detected in traumatically injured animals, while some post-traumatic changes appeared to be independent of depolarisation. The findings corroborated that depolarisation was a mechanism in experimental trauma. Microarrays could be used to produce a comprehensive image of regulated genes at a specific time or to allow an unbiased search for relevant events.