Effect of optic nerve transection upon myelin protein gene expression by oligodendrocytes: evidence for axonal influences on gene expression

The effect of optic nerve transection on myelin protein gene expression was studied in rats following axotomy at two ages: during active myelination (17 days of age) and after peak expression of the genes (35 days of age). mRNA levels for proteolipid protein, myelin basic protein and myelin-associated glycoprotein were assessed by northern and dot blotting and by in situ hybridization using tissue sections and cultured individual oligodendrocytes. Transection at 17 days caused down-regulation of mRNAs for proteolipid protein, myelin basic protein and myelin-associated glycoprotein by 5 days after axotomy with an increase in GFAP mRNA. A more protracted change followed axotomy at 35 days of age. The abundance of mRNAs for proteolipid protein and myelin basic protein was significantly reduced by 28 days after transection in the affected nerve. Quantification of proteolipid protein mRNA expression in individual oligodendrocytes confirmed the down-regulation. However, in contrast to the effects on the major myelin proteins, the abundance of myelin-associated glycoprotein mRNA increased in the affected nerve for at least the initial month after lesioning at 35 days. The results show that optic nerve transection has significant effects on myelin protein mRNA expression in oligodendrocytes of optic nerve. However, the changes in myelin protein gene activity are relatively small and more protracted than those seen in Schwann cells after peripheral nerve section. Because axotomy also causes marked changes in the glial population of the optic nerve it is not possible unequivocally to ascribe the alteration in gene expression to loss of axons. However, the data may provide evidence that axons do influence myelin protein genes in oligodendrocytes and are necessary for them to develop their full expression.