A detailed comparison in the developing rat central nervous system between the distribution of the NG2 proteoglycan and the α‐receptor for platelet‐derived growth factor (PDGF) shows that these two molecules are co‐expressed by glial progenitor cells of the O2A lineage and can serve as reliable markers for identification of O2A cells in vivo. Our mapping experiments indicate that NG2‐positive, PDGF α‐receptor positive O2A cells are abundant throughout the developing central nervous system in both white and gray matter. The earliest cells immunoreactive for either of the two markers are found adjacent to the central canal of the embryonic day 15 (E15) spinal cord. These cells express only PDGF α‐receptor and not NG2. By E17, process‐bearing cells expressing both NG2 and PDGF α‐receptor in a highly co‐localized fashion are found throughout the central nervous system. The first postnatal week marks the peak in the number of NG2 and PDGF α‐receptor immunoreactive cells, as well as the peak in the level of expression and the extent of co‐localization of the two molecules. After the first week, the level of expression of both NG2 and PDGF α‐receptor declines, although both molecules continue to be expressed in the adult brain. On O2A cells in the mature brain, NG2 and PDGF α‐receptor are not as well co‐localized at the subcellular level as they are on O2A cells in the younger brain. The functional consequences of co‐localization and subsequent dissociation of NG2 and PDGF α‐receptor on maturing O2A progenitors are investigated in the accompanying paper (Nishiyama et al.: J Neurosci Res 43:315–330, 1996). © 1996 Wiley‐Liss, Inc.

Previous studies on the NG2 chondroitin sulfate proteoglycan have shown that NG2 is expressed on A2B5‐positive O2A progenitor cells, which are known to respond to platelet‐derived growth factor (PDGF). In the accompanying paper (Nishiyama et al.; J Neurosci Res 43:299–314, 1996) we show that on O2A progenitors in the embryonic and newborn rat brain, NG2 and PDGF α‐receptor display an extensive co‐localization which becomes less pronounced as the brain matures past the first postnatal week. The present communication describes the relationship between NG2 and PDGF α‐receptor in vitro. NG2 and PDGF α‐receptor are highly co‐localized on A2B5‐positive O2A cells isolated from neonatal rat cerebrum. Mimicking the situation in vivo, the level of expression of the two molecules and the extent of co‐localization decline as these cells differentiate into O4‐positive pre‐oligodendrocytes. However, maintenance of the cells in a progenitor state by treatment with bFGF results in increased levels of both NG2 and PDGF α‐receptor on the cell surface, suggesting that expression of the two molecules may be coordinately regulated. Furthermore, NG2 can be co‐immunoprecipitated from radiolabeled O2A extracts with a rabbit antibody to PDGF α‐receptor, indicating the presence of a molecular complex that includes NG2 and the receptor. Finally, antibody‐patching and subsequent down‐regulation of NG2 results in reduced expression of PDGF α‐receptor and diminishes the proliferative response of the cells to PDGF. These findings suggest that correct co‐expression of the NG2 proteoglycan and PDGF α‐receptor on the surface of O2A progenitor cells is important for the cells' ability to respond effectively to the mitogenic stimulus of PDGF. © 1996 Wiley‐Liss, Inc.

NG2 is a chondroitin sulfate proteoglycan that is expressed on dividing progenitor cells of several lineages including glia, muscle, and cartilage. It is an integral membrane proteoglycan with a core glycoprotein of 300 kDa. In the present study we have characterized three molecular forms of the NG2 core protein expressed by different cell lines. Many cell lines that express the full length 300-kDa NG2 core protein also release a 290-kDa form into the medium. This species lacks the cytoplasmic domain but contains almost the entire ectodomain. Two core protein species, the intact 300-kDa form and a truncated 275-kDa form, are expressed at the surface of an NG2-transfected cell line U251NG52. The 275-kDa species lacks the cytoplasmic domain and at least 64 amino acids of the ectodomain. Mild trypsinization of B49 cells also generates the 275-kDa species, suggesting that this component is produced by proteolysis of the 300-kDa form. Conversion of the 300-kDa species to the 275-kDa form in U251NG52 cells is stimulated by reagents such as phorbol esters, which activate protein kinase C. Phorbol esters are also known to induce expression of metalloproteinases such as collagenase and stromelysin, which could be responsible for cleavage of the 300-kDa core protein. Although B49 cells do not spontaneously produce the truncated 275-kDa species, use of monoclonal antibodies against NG2 to block the interaction between NG2 and type VI collagen results in the appearance of the 275-kDa component in these cells. Thus the interaction between NG2 and type VI collagen, which contains a Kunitz-type proteinase inhibitor sequence in the alpha 3 chain, may protect the proteoglycan against proteolysis. This is consistent with the observed deficiency of U251NG52 cells in anchoring type VI collagen at the surface.