Laminin chain expression suggests that laminin-10 is a major isoform in the mouse hippocampus and is degraded by the tissue plasminogen activator/plasmin protease cascade during excitotoxic injury

Laminins are important components of the extracellular matrix, and participate in neuronal development, survival and regeneration. The tissue plasminogen activator/plasmin extracellular protease cascade and downstream laminin degradation are implicated in excitotoxin-induced neuronal degeneration. To determine which specific laminin chains are involved, we investigated the expression of laminins in the hippocampus, and the cell types expressing them. Reverse transcription-PCR demonstrated that the messenger RNAs for all laminin chains could be detected in the hippocampus. To determine the localization of laminin chain expression, immunostaining was used. This method showed that α5, β1 and γ1 are most highly expressed in the neuronal cell layers. Immunoblotting confirmed the hippocampal expression of the chains α5, β1 and γ1, and RNA in situ hybridization showed a neuronal expression pattern of α5, β1 and γ1. At early time points following intrahippocampal injection of kainate, α5, β1 and γ1 chain immunoreactivities were lost. In addition, tissue plasminogen activator-deficient mice, which are resistant to kainate-induced neuronal death, show no significant change in laminins α5, β1 and γ1 after intrahippocampal kainate injection. Taken together, these results suggest that laminin-10 (α5-β1-γ1) comprises a major neuronal laminin in the mouse hippocampus, and is degraded before neuronal death during excitotoxic injury by the tissue plasminogen activator/plasmin protease cascade. By identifying a neuronal laminin (laminin-10) that participates in neuronal degeneration after excitotoxic injury, this study clarifies the molecular definition of the extracellular matrix in the hippocampus and further defines a pathway for mechanisms of neuronal death.