The Role Of Gadd45a In Mouse Hematopoietic Stem Cell Function

Gadd45(growth arrest and DNA damage inducible gene) is found by Fornace in1988. The Gadd45family of genes, including Gadd45a (Gadd45a/Gadd45), Gadd45b (Gadd45β/MyD118), Gadd45g (Gadd45γ/CR6), are rapidly induced by a variety of genotoxic stresses as well as by terminal differentiation and apoptotic cytokines in almost all mammalian cells.They play an important role in DNA repair, gene apoptosis and cell cycle. Proteins encoded by Gadd45genes are remarkably similar, sharing55-57%overall identity at the amino acid level. Gadd45proteins modulate signaling in response to physiological and environmental stresses. Expression of Gadd45a genes is rapidly induced by different stresses, including differentiation-inducing cytokines and genotoxic strss. Gadd45a protein is a small spherical acidic protein (molecular weight18,000KD), which is expressed mainly in the nucleus, with a variety of cell cycle-related nuclear protein interactions. There is ample evidence that the functions of Gadd45proteins are mediated via interactions with other cellular proteins implicated in cell cycle regulation and the response of cells to extrinsic stresses, including mitogen-activated protein kinase kinase (the mitogen activated protein kinase kinase, MAPKK/MTK1), Cdc2(cell division control2), proliferating cell nuclear antigen (PCNA), p21and the p38/JNK stress-induced kinase pathways. In the Gadd45a gene intron section, there is a highly conserved p53binding site, which wild-type p53can be combined with to promote transcription of the Gadd45a gene expression. Affecting the p53factors can lead to the Gadd45a gene promoter activity decreasing, such as the the cmyc zinc finger protein ZBRK1. Gadd45a is the only p53target genes in Gadd45family, which play an important role in the life processes of the cell, including the maintenance of the genetic stability, cell cycle, nucleosides acid excision repair and apoptosis etal.Hematopoietic cells, including hematopoietic stem cells (HSCs) and hematopoietic progenitor cells (HPCs), are very sensitive to ionizing radiation. Total body irradiation with5to10Gy doses results in an acute radiation syndrome with possible lethality primarily due to hematopoietic failure. Because Gadd45a is a stress sensor gene, only p53-target genes of the Gadd45family, and p53is an important modulator of the irradiation response pathway regulating cell proliferation, DNA repair and apoptosis, we use Gadd45a knockout mice, bone marrow hematopoietic stem cells, monoclonal culture, competitive transplantation, immunofluorescence, and other experiments to study Gadd45a role in HSC self-renewal capacity and damage repair capacity.Our data showed that Gadd45a gene deletion did not affect the phenotype of HSC/progenitor cell in the normal physiological condition. Functional experiments showed that Gadd45a gene deletion increasd long-term hematopoietic stem cells capacity, while the level of progenitor cell function declines.Under the replicative stress and the irradiation stress conditions, in vitro single-cell clonal assay and the in vivo bone marrow transplantation experiment showed that Gadd45a gene knockout murine HSC has a increase in forming big size colony and reconstituting the recipient hematopoietic system. However, the Gadd45a-/-HSCs did not show increased sensitivity to myelotoxic stress and high dose irradiation. Further expriment showed in the competitive bone marrow transplant recipient mice given twice consecutive4.5Gy radiation, Gadd45a gene deletion increased HSC self-renewal capacity. Thus, our results demonstrate that Gadd45a plays a pivotal role in regulating the HSC maintenance and function.