Astrocytes Upregulate Glial Fibrillary Acidic Protein (GFAP), but not Insulin‐like Growth Factor‐I (IGF‐I) during Experimental Autoimmune Neuritis (EAN)

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Abstract
T cell-mediated autoimmune neuritis produces rapid activation of spinal cord microglia. To determine whether this microglial response upregulates astrocytic expression of IGF-related proteins, we induced EAN and used in situ hybridization and immunocytochemistry to examine the mRNAs and peptides for glial fibrillary acidic protein (GFAP), insulin-like growth factor-I (IGF-I), IGF-I receptor (IGFR-I) and IGF binding protein-2 (IGFBP-2). Relative levels of GFAP mRNA and peptide were highest in the lumbar spinal cord 4–10 d following T cell transfer and significant GFAP elevations were still present after three weeks. The astrocytes expressing GFAP mRNA and peptide were localized around motoneurons which were related topographically to axons in peripheral nerve inflammatory lesions. In the nucleus gracilis, where terminals of dorsal root ganglion neurons are located, astrocytic levels of GFAP mRNA and peptide rose later and did not reach their highest levels until 21 d after T cell transfer. Even though microglia were activated in both locations 2–4 d after transfer, astrocytic levels of IGF-I, IGFR-I and IGFBP-2 mRNA and peptide did not differ significantly from those observed in controls. The dissociation of GFAP and IGF-I expression in EAN suggests that these astrocytic responses may be independently regulated. We also suggest that the type and severity of remote neuronal injury are probably more important inducers and regulators of these astrocytic responses than microglial cell activation.