Visualization of proliferating cells in the adult mammalian brain with the aid of ribonucleotide reductase

Proliferating cells are hardly detectable in the adult mammalian brain by microscopy of stained sections, but after pre-labeling with radioactive thymidine or 5'-bromo-2-deoxyuridine (BrdU), either marks the nucleus, as do mitosis-related proteins such as Ki67 and PCNA. Engineered virus may also be used to mark proliferating cells. One alternative approach is to use the enzyme ribonucleotide reductase (RNR), expressed by proliferating cells, but not by quiescent ones. A monoclonal antibody against the M1 subunit of RNR was used to visualize proliferating cells in the brains of adult normal rats, rabbits, pigs and sheep. Stem cells were distinctly outlined. In the subgranular layer in the hippocampal dentate gyrus, most RNR immunoreactive cells were bipolar to multipolar, and had a large cell body and long processes. Two different populations of RNR expressing cells were visualized in the subventricular zone in the forebrain, one dominated by small, bipolar cells extending into the rostral migratory stream, while the other was formed by large multipolar cells, adjacent to the ependyma, with processes extending to the lateral ventricle. Furthermore, rare RNR-expressing cells were recognized throughout the brain. The RNR immunoreactive cells were immature, as they did not express any marker characterizing differentiated neurons and glial cells, except for a fraction that co-expressed the gliofibrillary acidic protein. BrdU and RNR were co-localized in proliferating cells in animals pretreated with BrdU. We conclude that RNR immunohistochemistry can accurately visualize proliferating cells, including stem cells, in adult mammalian brains. The occurrence of processes at cell proliferation is elucidated. Further, the advocated approach does not require any pre-labeling, and can be carried out on fixed tissues.