NG2-expressing cells in the central nervous system: Are they oligodendroglial progenitors?

Antibodies against the chondroitin sulphate proteoglycan, NG2, are increasingly being used to identify the widespread population of oligodendrocyte progenitor cells in the adult mammalian CNS. However, the specificity of this marker and the role of NG2-expressing cells in CNS function are still open to question. In this review we consider the evidence that NG2(+) cells in the CNS are part of the oligodendrocyte lineage and whether they can give rise to new oligodendrocytes following demyelination. In both the developing and mature rodent CNS, NG2(+) cells express the established oligodendrocyte lineage marker PDGF-alphaR and from P7, the late progenitor antigen O4, which persists in immature oligodendrocytes. They do not express markers of other CNS populations, such as OX42 or GFAP, at any developmental age. NG2(+) cells represent the major cycling cell population in the normal adult rat CNS, suggesting they have stem cell-like properties. NG2 immunoreactivity is upregulated as a result of physical, viral, excitotoxic and inflammatory insults to the CNS. Following demyelination NG2(+) cell number increases in the immediate vicinity of the lesion and rapid remyelination ensues. NG2 expression has also been investigated in human tissue. Multi-process bearing cells, which morphologically resemble those identified with antibodies against O4, persist in chronically demyelinated multiple sclerosis lesions.