Increased calpain expression in experimental demyelinating optic neuritis: an immunocytochemical study

Since calcium activated neutral proteinase (calpain) is present in the central nervous system (CNS) and degrades myelin proteins, this endopeptidase has been suggested to play a role in myelin destruction in demyelinating diseases such as multiple sclerosis (MS). In the present study, calpain immunocytochemical expression was examined in Lewis rats with acute experimental allergic encephalomyelitis (EAE), an animal model for MS and optic neuritis. To identify cells expressing calpain, we labeled rat optic nerve sections for calpain with a polyclonal myelin calpain antibody and with monoclonal antibodies for glial (GFAP, OX42) and inflammatory (CD2, ED2, ED1, IFN- γ) cell-specific markers. The results showed increased calpain expression in microglia (OX42) and infiltrating macrophages (ED1,2) in EAE compared to normal controls. Astrocytes constitutively expressed calpain in controls and acute EAE. Reactive astrocytes in EAE located in or near inflammatory foci, exhibited markedly increased calpain expression. Most T cells in acute EAE showed low level calpain expression while activated IFN- γ-producing lymphocytes in inflammatory foci exhibited elevated levels of calpain expression. Thus, our results demonstrate increased calpain expression (at transcriptional and/or translational levels) in a rat model of optic neuritis. A role for calpain in myelin destruction during optic neuritis may be relevant to the pathogenesis of this disorder.