Iannis AifantisAssociate Professor of Pathology
Early Career Scientist,
Howard Hughes Medical Institute
Ph.D., 1999 University of Paris V/Necker Institute

New York University School of Medicine
Dept. of Pathology, MSB 504
550 First Ave. New York, NY 10016

Tel: (212) 263-5365

Fax: (212) 263-8211

E-mail: Iannis.Aifantis@med.nyu.edu

Lab Website: http://www.aifantislab.com/
Research Theme(s): Cancer Stem Cells, Developmental Immunology, Hematopoiesis, Stem Cell Biology
Keywords: Hematopoiesis, Stem cells, Ubiquitination, Self-Renewal, Cell cycle progression

Research Summary:

Molecular Mechanisms of Stem Cell Self-Renewal, Differentiation and Transformation

  1. The role of the Fbw7 E3 ubiquitin ligase in hematopoietic stem cell function and leukemia
    We would like to understand how the E3 Ubiquitin Ligase, Fbw7, as a regulator of post-transcriptional regulation of protein stability, could regulate hematopoietic stem cell fate decisions. Fbw7 is a potent tumor suppressor as it regulates several proto-oncogenes such as c-Myc, Notch and CyclinE. Work from our lab has concluded that Fbw7 indeed has a physiological role in hematopoiesis. We have confirmed the tumor suppressor role of Fbw7 as conditional ablation of Fbw7 in the hematopoietic compartment leads to T-cell acute lymphoblastic leukemia. However, a significant percentage of mice succumb to anemia before tumor development occurs suggesting an alternative function for Fbw7 in normal hematopoiesis. Indeed, we observed a severe loss of hematopoietic stem cells upon deletion of Fbw7 in hematopoietic compartment. Our lab has addressed the contribution of two well characterized Fbw7 substrates, c-Myc and Notch, to the Fbw7-/- HSC defect. We found that relative c-Myc protein stability (and not Notch) was essential for regulating HSC self-renewal and differentiation. Interestingly, Fbw7 was dispensible for self-renewing embryonic stem cells. This work demonstrated, for the first time, that the ubiquitin proteasome system is a novel regulator of HSC function. Furthermore, it suggested that the Fbw7:substrate interaction can be interpreted differently in stem cells from different ages. At least two interesting questions have emerged from these studies; How do Fbw7:substrate interactions impact stem cell fate decisions over time (i.e during aging)? Do Fbw7:substrate interactions contribute to stem cell-driven malignancies?
  2. Notch signaling in hematopoietic stem cell differentiation.
    Hematopoiesis is a complex process that requires coordination between self-renewal, and differentiation of stem and progenitor cells to generate mature cells in the blood. Notch signaling has been implicated in the regulation of these diverse functions in the hematopoietic system and other tissues. Whereas the importance of Notch1 in lymphocyte development and oncogenic transformation has been well characterized, the relevance of Notch in the specification of other hematopoietic lineages and hematopoietic stem cell (HSC) function remains unclear.
    We performed lineage-tracing experiments in early hematopoietic progenitors to determine the fate of Notch receptor expressing cells within the hematopoietic system using transgenic mice with tamoxifen inducible CreER “knocked in” the locus of each Notch receptor. Crossing these animals to the ROSA26-tdRFP reporter permits the irreversible labeling of hematopoietic cells expressing a given Notch receptor and their progeny. To address whether these receptors were being activated, we analyzed Hes1-eGFP knock-in animals (Hes1GFP/+). Hes1, a bHLH repressor, is a well-characterized transcriptional target of intracellular Notch. We found that GFP expression in these mice faithfully recapitulates Notch signaling and that there are distinct subsets of hematopoietic progenitors that have activation of the Notch pathway. In addition, we are using these and other novel genetic tools to further characterize Notch signaling within bone marrow stem cell niches using in vivo imaging.
    Hes1 is also of special interest because it could be a key mediator of cell fates through its influence on transcription factor networks. For example, the promoters of the myeloid transcription factors PU.1 and CEBP/a genes both contain Hes1 binding sites known as the “ N-box” . Interestingly, their expression is increased upon Notch loss-of-function and decreased with Notch gain-of-function. By combining lineage tracing, Notch reporters, imaging, and deep sequencing we will complete the fragmented picture of Notch receptor expression and signaling during bone-marrow hematopoiesis. This work will provide insights into the normal developmental role of Notch in hematopoietic cells and highlight how Notch-Hes1 may act during oncogenic transformation.
  3. Proteomic landscapes in stem cell differentiation and cancer
    Currently, we are developing proteomic approaches to delineate the role of the ubiquitin proteasome system (UPS) in hematopoiesis, cancer and stem cell function. For that purpose, we mainly focus on the E3 ligase Fbw7, which regulates the degradation of several important oncogenes with central roles in cell division, growth and differentiation. We have recently shown that Fbw7 acts as a tumor suppressor of T-cell acute lymphoblastic leukemia (T-ALL) by targeting Notch1. Furthermore, we have shown that, by targeting c-Myc, Fbw7 regulates the quiescence and self-renewal capacity of hematopoietic stem cells (HSCs); however, it is dispensable for pluripotency of embryonic stem cells (ESC), but appears to acquire a role along with cell differentiation.
    We are now working on the identification of novel Fbw7 substrates by in-vitro tandem purifications combined with mass spectrometry. We then want to understand the role that the newly identified Fbw7:substrate pairs have in the different considered systems. In addition, we are developing strategies to perform global mass spectrometry analysis of various types of cells in which Fbw7 has been deleted by genetic engineering in order to identify additional substrates by comparing them to their controls. For these series of experiments, we include in vivo and in vitro models aiming to define the tissue and function specificity of the landscape of Fbw7 substrates. After these experiments we should be able to answer question such as: Which are the Fbw7 substrates involved in stem cell differentiation? or Which Fbw7 substrates are upregulated after Fbw7 malfunctioning in different types of cancer? Altogether, the ultimate goal of our research is to identify pathways that are new potential therapeutic targets in leukemia and, possibly, in other types of cancer.

Research sponsored by the NIH/NCI, the American Cancer Society and the HHMI


Selected Publications (2009-2011):