NDRG2 as a marker protein for brain astrocytes

The protein NDRG2 (N-myc downregulated gene 2) is expressed in astrocytes. We show here that NDRG2 is located in the cytosol of protoplasmic and fibrous astrocytes throughout the mammalian brain, including Bergmann glia as observed in mouse, rat, tree shrew, marmoset and human. NDRG2 immunoreactivity is detectable in the astrocytic cell bodies and excrescencies including fine distal processes. Glutamatergic and GABAergic nerve terminals are associated with NDRG2 immunopositive astrocytic processes. Muller glia in the retina displays no NDRG2 immunoreactivity. NDRG2 positive astrocytes are more abundant and more evenly distributed in the brain than GFAP (glial fibrillary acidic protein) immunoreactive cells. Some regions with very little GFAP such as the caudate nucleus show pronounced NDRG2 immunoreactivity. In white matter areas, NDRG2 is less strong than GFAP labeling. Most NDRG2 positive somata are immunoreactive for S100 but not all S100 cells express NDRG2. NDRG2 positive astrocytes do not express nestin and NG2 (chondroitin sulfate proteoglycan 4). The localization of NDRG2 overlaps only partially with that of aquaporin 4, the membrane-bound water channel that is concentrated in the astrocytic endfeet. Reactive astrocytes at a cortical lesion display very little NDRG2, which indicates that expression of the protein is reduced in reactive astrocytes. In conclusion, our data show that NDRG2 is a specific marker for a large population of mature, non-reactive brain astrocytes. Visualization of NDRG2 immunoreactive structures may serve as a reliable tool for quantitative studies on numbers of astrocytes in distinct brain regions and for high-resolution microscopy studies on distal astrocytic processes.