Abstract
The immortal strand hypothesis (J. Cairns, 1975) proposes that asymmetrically dividing stem cells selectively retain chromosomes that bear the older template DNA strands to avoid accumulation of mutations emerged by replication errors. However, there is lack of evidence to support this hypothesis. Recently, neural stem cells can be isolated and cultured by means of the neurosphere culture. This culture method makes it possible to examine whether the immortal strand hypothesis is correct or not. Therefore, we investigated asymmetrical segregation of chromosomes that bear the older template DNA strands in mouse neural stem cells. Neural stem cells derived from adult and embryo mouse forebrain subventricular zone were isolated and cultured for a week by the neurosphere culture method. Then, neurosphere cells were cultured in the medium containing 5-bromo-2-deoxyuridine (BrdU) for 48 hr to label de novo synthesized DNA. After removal of BrdU, the cells were cultured for an additional week (estimated over 7 cell divisions), fixed and immunostained with anti-BrdU antibodies. Mouse embryo fibroblasts (MEFs) were used for controls. The result revealed that 10% of nuclei derived from both adult and embryo neurosphere cells were BrdU-positive and 3% of them were totally labeled with BrdU over seven cell divisions after removal of BrdU where the random chromosome segregation makes it impossible to detect BrdU positive nuclei. In contrast, only 0.2% of nuclei of MEFs were BrdU-positive. Therefore, the present study demonstrates that neurosphere cells contain subpopulations that selectively retain older template DNA strands, supporting the immortal strand hypothesis.
