Abstract
Whether microglia and macrophages are beneficial or harmful in many neurological disorders, including demyelinating diseases such as multiple sclerosis and the leukodystrophies, is currently under debate. Answering this question is of special interest in globoid cell leukodystrophy (GLD), a genetic fatal demyelinating disease, because its rapidly progressive demyelination in the nervous system is accompanied by a characteristic accumulation of numerous globoid macrophages. Therefore, we cross-bred the twitcher (twi) mouse, a bona fide model of GLD, with the macrophage-deficient osteopetrotic mutant and studied the resultant macrophage-deficient twitcher (twi+op) mouse. The twi+op mouse had few microglia and macrophages in the white matter and, interestingly, showed a more severe clinical phenotype compared to the twi mouse. The number of nonmyelinated axons in the spinal cord was significantly higher in twi+op mice than in twi mice at 45 d old. The difference appeared to be due to impaired remyelination in twi+op mice rather than accelerated demyelination. Quantitative reverse transcription PCR and immunohistochemical studies revealed that the recruitment of oligodendrocyte progenitor cells in response to demyelination was compromised in twi+op mice. Increased myelin debris in the white matter parenchyma of twi+op mice suggested that phagocytosis by macrophages may play an important role in promoting remyelination. Macrophage markers for both protective and destructive phenotypes were significantly upregulated in the spinal cord of twi mice but were close to normal in twi+op mice due to the reduced macrophage number. The overall effects of macrophages in GLD appear to be beneficial to myelin by promoting myelin repair.