Pro-apoptotic Bid, a BH3-only Bcl-2 family member, is situated at the interface between the DNA damage response and apoptosis, with roles in death receptor-induced apoptosis as well as cell cycle checkpoints following DNA damage(Kamer et al., 2005; Yin et al., 1999; Zinkel et al., 2005). Here we demonstrate that Bid acts at the level of the sensor complex in the Atm and Rad3-related (Atr)-directed DNA damage response. Bid is found with Replication protein A (RPA) in nuclear foci and associates with the Atr/Atr-interacting protein (Atrip)/RPA complex following replicative stress. Furthermore, Bid-deficient cells demonstrate an impaired response to replicative stress manifest by reduced accumulation of Atr and Atrip on chromatin and at DNA damage foci, reduced recovery of DNA synthesis following replicative stress, and decreased Chk1 activation and RPA phosphorylation. These results establish a direct role for the BH3-only Bcl-2 family member, Bid, acting at the level of the damage sensor complex to amplify the Atr-directed cellular response to replicative DNA damage.

Hematopoietic stem cells (HSCs) possess long-term self-renewal capacity and multipotent differentiative capacity, to maintain the hematopoietic system. Long-term hematopoietic homeostasis requires effective control of genotoxic damage to maintain HSC function and prevent propagation of deleterious mutations. Here we investigate the role of the BH3-only Bcl-2 family member Bid in the response of murine hematopoietic cells to long-term replicative stress induced by hydroxyurea (HU). The PI3-like serine/threonine kinase, ATR, initiates the DNA damage response (DDR) to replicative stress. The pro-apoptotic Bcl-2 family member, Bid, facilitates this response to replicative stress in hematopoietic cells, but the in vivo role of this DDR function of Bid has not been defined. Surprisingly, we demonstrate that long-term HU treatment expands wild-type myeloid progenitor cells (MPCs) and HSC-enriched Lin À Sca1 þ Kit þ (LSK) cells to maintain bone marrow function as measured by long-term competitive repopulating ability. Bid À / À MPCs demonstrate increased sensitivity to HU and are depleted. Bid À / À LSK cells demonstrate increased mobilization manifest by increased Bromodeoxyuridine (BrdU) incorporation. Bid À / À MPCs and LSK cells are relatively depleted, however, and bone marrow from Bid À / À mice demonstrates decreased long-term competitive repopulating ability in both primary and secondary transplants. We thus describe a survival function of Bid in hematopoiesis in the setting of chronic replicative stress.