Bone Morphogenetic Proteins: Inhibitors of Myelination in Development and Disease.

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Myelin, the lipid membrane that surrounds axons, is critical for the propagation of nervous impulses and axonal maintenance. The destruction of myelin or lack of myelin formation due to disease or injury causes severe motor and cognitive disability. Regeneration of myelin is theoretically possible but rarely happens. Myelin is synthesized as the plasma membrane of the oligodendrocyte in the central nervous system. During development, myelin and oligodendrocytes are generated from oligodendrocyte progenitors through a process modulated by extrinsic growth factors signaling to cell-intrinsic proteins. Among the key extrinsic factors are the bone morphogenetic proteins (BMPs), potent inhibitors of oligodendrocyte differentiation and myelin protein expression, likely serving to regulate myelination temporally and spatially. BMPs also promote astrocyte generation. Given the inhibitory role of BMP in oligodendrogliogenesis during development, the expression of BMP during demyelinating disease or injury was investigated, as was whether BMP upregulation could serve to prevent regeneration by both direct inhibition of myelination and increases in astrogliosis. BMPs, predominantly BMP4, were increased in animal models of spinal cord injury, stroke, multiple sclerosis, and perinatal white matter injury. A number of studies inhibited BMP signaling by infusing the injury site with the BMP-specific inhibitor noggin or transplanting stem cells engineered to secrete noggin. In most cases, noggin increased the numbers of mature oligodendrocytes and decreased numbers of astrocytes. Some studies also showed functional improvement. BMP is one of several inhibitory growth factors that now appear to inhibit myelin regeneration. Common downstream mechanisms among these factors are likely to be identified.