Haematopoietic stem cells do not asymmetrically segregate chromosomes or retain BrdU

摘要

1975年，John Cairns提出了“不朽链”(immortal strand)假说，将其 作为成年干细胞可用来减小突变积累的一个机制。这是通过选择性地 保留含有“老”DNA的染色体来做到的，对含有放射性标记或一个核 苷类物质(如bromodeoxyuridine(BrdU))的DNA链所做的分析证明了 这一点。本期Nature发表了关于这一仍在争论的问题的一项新的研究 工作。Kiel等人发现，“不朽链”模型并不是干细胞的一个普遍性质， 因为它并不适用于造血干细胞。这些细胞不能通过保留BrdU标记来识 别，并且在分裂过程中不能保留较老的DNA。%Stem cells are proposed to segregate chromosomes asymmetrically during self-renewing divisions so that older ('immortal') DNA strands are retained in daughter stem cells whereas newly synthesized strands segregate to differentiating cells. Stem cells are also proposed to retain DNA labels, such as 5-bromo-2-deoxyuridine (BrdU), either because they segregate chromosomes asymmetrically or because they divide slowly. However, the purity of stem cells among BrdU-label-retaining cells has not been documented in any tissue, and the 'immortal strand hypothesis' has not been tested in a system with definitive stem cell markers. Here we tested these hypotheses in haematopoietic stem cells (HSCs), which can be highly purified using well characterized markers. We administered BrdU to newborn mice, mice treated with cyclophosphamide and granulocyte colony-stimulating factor, and normal adult mice for 4 to 10 days, followed by 70 days without BrdU. In each case, less than 6% of HSCs retained BrdU and less than 0.5% of all BrdU-retaining haematopoietic cells were HSCs, revealing that BrdU has poor specificity and poor sensitivity as an HSC marker. Sequential administration of 5-chloro-2-deoxyuridine and 5-iodo-2-deoxyuridine indicated that all HSCs segregate their chromosomes randomly. Division of individual HSCs in culture revealed no asymmetric segregation of the label. Thus, HSCs cannot be identified on the basis of BrdU-label retention and do not retain older DNA strands during division, indicating that these are not general properties of stem cells.