Hyperoxia Inhibits the Growth of Mouse Forebrain Oligodendrocyte Progenitors

NG2 chondroitin sulfate proteoglycan positive oligodendrocyte progenitor cells (OPCs) reside throughout the brain. They divide asymmetrically and differentiate into myelinating oligodendrocytes throughout adulthood. OPCs have been successfully isolated from rodents using several techniques including magnetic beads, immunopanning and exploiting differential centripetal adhesion. Whereas rat OPCs are relatively simple to propagate in vitro, it has been difficult to expand mouse OPCs. Therefore, we evaluated the effects of oxygen levels, growth factors and extracellular matrix components to produce a simple and reproducible method to prepare large numbers of nearly homogenous cultures of primary mouse OPCs from postnatal day 0-2 mouse telencephala. Using the McCarthy and de Vellis mechanical separation method OPCs were separated from mixed culture of glial cells. When the OPCs were plated onto fibronectin coated tissue culture plates in a biochemically defined medium that contained fibroblast growth factor-2 (FGF-2) and platelet derived growth factor AA (PDGFAA), and they were maintained in a standard tissue culture incubator, they proliferated very slowly. By contrast, mouse OPCs doubled approximately every 7 days when maintained in a 2% oxygen, nitrogen buffered environment. After 3 passages, greater than 99% of these OPCs were NG2+/PDGFR+. In medium containing only FGF-2, mouse OPCs progressed to late stage OPCs whereupon A2B5 expression decreased and O4 expression increased. When these cells were differentiated between passages 1 and 3, the majority of the OPCs differentiated into MBP+ mature oligodendrocytes However, cells that were repeatedly passaged beyond 4 passages progressed to a late O4+ OPC (even with mitogens present) and when differentiated by mitogen removal a minority of the OPCs differentiated into MBP+ cells. These studies reveal significant differences between mouse and rat OPCs and an inhibitory role for oxygen in mouse OPC proliferation.