FACS-Analysegerät

• Subject Area: Medicine
• Term: Funded in 2008
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 68236549

• Final Report Year: 2012

Final Report Abstract

Checkpoints that limit stem cell self-renewal in response to DNA damage could influence cancer and aging but remain yet to be delineated at molecular level. Using in vivo RNAi (shRNA) screens, we show that DNA damage limits self-renewal of hematopoietic stem cells (HSCs) by upregulation of the “Basic leucine zipper transcription factor, ATF-like” (BATF). The study shows that BATF controles lymphoid differentiation of normal, non-damaged HSCs and DNA damage enhances differentiation of HSCs by inducing BATF expression in a G-CSF/STAT3 dependent manner. Inhibition of this pathway ameliorates HSC depletion in response to gamma-irradiation or telomere dysfunction. An analysis of bone marrow biopsies of patients with myelodysplastic syndromes indicates that DNA damage dependent induction of BATF is conserved in human HSCs. Together, these results provide the first experimental evidence that BATF controls lymphoid differentiation of HSCs and this molecular circuit serves as a checkpoint limiting the self-renewal of HSCs in response to DNA damage. There is growing evidence that telomere dysfunction can contribute to human aging. Telomere dysfunction limits lymphopoiesis in aging telomerase knockout (mTerc-/-) mice primarily by the induction of stem cell– extrinsic alterations. The relative contribution of alterations in the stem cell niche and the systemic environment to the impairment of lymphopoiesis in response to telomere dysfunction is currently unknown. This study reveals a minor impact of stem cell– intrinsic defects on the impairment of B and T lymphopoiesis in response to telomere dysfunction. The impairment in B and T lymphopoiesis in aging telomeredysfunctional mice was mainly due to alterations of the systemic environment. Telomere dysfunction had no significant cell-autonomous effects impairing the function of thymic or bone marrow niches in supporting B and T lymphopoiesis. Moreover, age-related alterations in the cellular composition of the thymic epithelium in telomere-dysfunctionalmicewere rescued by transplantation of the thymus into a wild-type environment; these rejuvenated thyme supported normal T lymphopoiesis in recipient mice. Together, these data place alterations in the systemic environment on top of the hierarchy of events limiting lymphopoiesis in response to telomere dysfunction. In a liver specific Trp53 KO mouse model the development of bilineal cancers in Trp53-/- livers suggested that tumors may be derived from liver progenitor cells (LPCs). We purified liver progenitor cells from livers of 2-3 and 8-10 month old mice and sorted single cells into 96-well plates. A subset of these purified single cells exhibited a clonal expansion capacity. Most of the colonies expressed only cholangiocytic marker (K19). But some colonies showed a higher proliferation capacity and a mixed lineage differentiation into albumin and K19 expressing cells. A small subset of cells exhibited co-expression of both markers (K19 and albumin). 8-12% of these freshly purified LPCs had the capacity to form cholangiocytic colonies in cell culture with no significant effect of age or p53 genotype. 0.55-0.82% of the purified LPCs from 2-3 month old mice exhibited bilineal differentiation capacity, which was not affected by the p53 gene status. p53-positive mice exhibited a strong age-dependent decline in LPCs to form bilineal colonies (only 0.045% of the sorted cells formed colonies with bilineal differentiation in 8-10 month old mice). In contrast, isolated LPCs from p53-/- mice did not show a significant age-dependent decline in the capacity to form bilineal colonies. Previous studies have shown that cells from these bilineal colonies can self-renew and exhibit bilineal colony forming capacity at single cell level at later passage. p21, a downstream target of p53, can affect -/- stem cell self-renewal. Therefore, we analyzed LPCs from 8-10 month old p21 mice. The deletion of p21 was associated with a significant increase in colonies with bilineal differentiation (0.22% of sorted cells) compared to age-matched p53-positive mice but the number of bipotent progenitor cells remained reduced compared to age-matched p53-/- mice. Together, these data indicated that p53 induces age-dependent restrictions in bilineal colony forming capacity of LPCs involving p21-dependent and p21-independent mechanisms.

Publications

• Sod2 haploinsufficiency does not accelerate aging of telomere dysfunctional mice. Aging (Albany NY). 2009 Mar 5;1(3):303-15
  Guachalla et al.
  
• CHK2-independent induction of telomere dysfunction checkpoints in stem and progenitor cells EMBO Rep. 2010 August; 11(8): 619–625
  Nalapareddy et al.
  
• Puma and p21 represent cooperating checkpoints limiting self-renewal and chromosomal instability of somatic stem cells in response to telomere dysfunction. Nat Cell Biol. 2011 Dec 4;14(1):73-9
  Sperka et al.
  
• A differentiation checkpoint limits hematopoietic stem cell self-renewal in response to DNA damage. Cell. 2012 Mar 2;148(5):1001-14
  Wang et al.
  
• Disruption of Trp53 in livers of mice induces formation of carcinomas with bilineal differentiation Gastroenterology. 2012 May;142(5):1229-1239
  Katz et al.
  
• Increased reprogramming capacity of mouse liver progenitor cells, compared with differentiated liver cells, requires the BAF complex. Gastroenterology. 2012 Apr;142(4):907-17
  Kleger et al.
  
• Telomere dysfunctional environment induces loss of quiescence and inherent impairments of hematopoietic stem cell function. Aging Cell. 2012 Jun;11(3):449-55
  Song et al.