EVIDENCE FOR CENTRAL-NERVOUS-SYSTEM GLIAL-CELL PLASTICITY IN PHENYLKETONURIA

Phenylketonuria (PKU) is caused by mutation(s) in the phenylalanine hy droxylase (PAH) gene which lead to deficient PAH activity and an accum ulation of phenylalanine in the blood. The primary pathologic finding is hypomyelination and gliosis of central nervous system white matter. Similar white matter pathology is observed in the Pah(enu2) mouse, a genetic model for PKU. We studied this mouse to examine the basis for these neuropathologic changes in PKU and to determine if hypomyelinati on and gliosis occur independently or are interrelated. Although white matter tracts within PKU brains are hypomyelinated, immunostaining an d Western blot analyses revealed that these tracts contain abundant am ounts of myelin markers, i.e. myelin basic protein (MBP), 2',3'-cyclic nucleotide 3'phosphohydrolase, and myelin/oligodendrocyte-specific pr otein (MOSP). However, Western blot analyses also showed that MBP isof orm expression was aberrant. Investigation of individual cells was per formed by extraction of tissue sections with Triton X-100. Most of the MOSP was extracted, with the remaining MOSP clearly visible in dual l abeled cells, i.e. MOSP was colocalized along glial fibrillary acidic protein (GFAP) filaments. Cells expressing both MBP and GFAP were also identified in optic tract. Double labeling with a riboprobe for MBP a nd antibodies specific for GFAP revealed that the majority of GFAP-pos itive cells expressed MBP mRNA. Our in vitro studies examined the resp onse of cultured wild type oligodendrocytes to elevated phenylalanine for 4 weeks (wk). Under these conditions, about 50% of the oligodendro cytes expressed GFAP filaments and failed to elaborate membrane sheets . Proliferation of astrocytes appears not to be the source of gliosis, since the nuclei of GFAP-positive cells in the PKU brains did not imm unostain for proliferating cell nuclear antigen. Dual-labeled cells we re detected in normal mouse brain sections; however, PKU mouse white m atter tracts were found to contain about twice the number of dual-labe led cells compared to normal tissue. Taken together, these data sugges t that both myelinating and nonmyelinating oligodendrocytes are presen t in the normal adult brain, and that in response to a toxic factor su ch as elevated phenylalanine, myelinating oligodendrocytes adopt a non myelinating phenotype that expresses GFAP. Since myelinating Schwann c ells and GFAP-positive nonmyelinating Schwann cells are normally prese nt in adult peripheral nerve, and the myelinating Schwann cells react to pathologic situations by switching to GFAP-positive nonmyelinating cells, it may be that oligodendrocytes and Schwann cells are more simi lar than previously thought.