The expression of NG2 proteoglycan in the developing rat limb

NG2 is a chondroitin sulfate proteoglycan previously found to be expressed by glial progenitor cells of the O2A lineage. We have examined the expression of NG2 in the developing rat limb by immunohistochemistry and northern blot analysis. Staining of embryonic day 14 (E14) rat limb bud sections with polyclonal and monoclonal anti-NG2 antibodies reveals reactivity in the precartilaginous mesenchymal condensation. The staining intensity increases with the differentiation of chondrocytes until E16. NG2 staining is not detected in the mature hypertrophic chondrocytes of E17 and postnatal day 3 (P3) limbs even after treatment of the sections with hyaluronidase or collagenase. Immuno-precipitations with anti-NG2 antibody using 125I-labeled limb cells in culture showed a 400 to 800 x 10(3) Mr proteoglycan species with a core protein size of 300 x 10(3) Mr, comparable to NG2 from O2A cells and neural cell lines. Northern blot analysis reveals the expression of an 8.9 kb mRNA in E16 limbs and at a lower level in P1 cartilage. The northern blot analyses also show that NG2 is distinct from the large aggregating proteoglycan of the cartilage. Our results indicate that in the developing limb cartilage, as in the differentiating oligodendrocytes, NG2 is present on immature cells in the process of differentiating, but its expression is downregulated as terminal differentiation of chondrocytes takes place.