**4. Conclusions**

50 Advances in Hematopoietic Stem Cell Research

Necdin is a member of the melanoma antigen family of molecules, whose physiological roles have not been well characterized (Xiao et al., 2004). Necdin acts as a cell cycle regulator in post-mitotic neurons (Yoshikawa, 2000). Intriguingly, recent genetic analyses show that aberrant genomic imprinting of *NDN* on the human 15q11-q13 chromosomal region is, at least in part, responsible for the pathogenesis of Prader-Willi syndrome (MacDonald & Wevrick, 1997; Nakada et al., 1998; Barker et al., 2002), a disorder associated with a mildly increased risk of myeloid leukemia (Davies et al., 2003). Necdin interacts with multiple cell-cycle related proteins, such as SV-40 large T antigen, adenovirus E1A, E2F1, and p53 (Taniura et al., 1998, 1999, 2005; Hu et al., 2003). As shown in Table 2, necdin is one of 32 genes that show higher expression in HSCs than in differentiated hematopoietic cells (Kubota et al., 2009). Other groups also found that necdin is highly expressed in HSCs (Forsberg et al., 2005; Liu et al., 2009). Necdindeficient mice show accelerated recovery of hematopoietic systems after myelosuppressive stress, such as 5-FU treatment and BM transplantation, whereas no overt abnormality is seen under conditions of steady-state hematopoiesis. Considering necdin as a potential negative cell-cycle regulator, it was reasoned that the enhanced hematologic recovery in necdin-null mice could be the result of an increased number of proliferating HSCs and progenitor cells. As expected, after 5-FU treatment, necdindeficient mice had an increased number of HSCs, but this was only transiently observed during the recovery phase (Kubota et al., 2009). These data suggest that the repression of necdin function in HSCs may present a novel strategy for accelerating hematopoietic recovery, thus providing therapeutic benefits after clinical myelosuppressive treatments

Slug belongs to the highly conserved Slug/Snail family of zinc-finger transcriptional repressors found in diverse species ranging from *C. elegans* to humans. SLUG is a target gene for the E2A-HLF chimeric oncoprotein in pro-B cell acute leukemia (Inukai et al., 1999). Slug-deficient mice show normal peripheral blood counts, but they are very sensitive to γirradiation (Inoue et al., 2002). Slug is induced by p53 and protects primitive hematopoietic cells from apoptosis triggered by DNA damage. Slug exerts this function by repressing Puma, a proapoptotic target of p53 (Wu et al., 2005). Sun et al. recently showed that Slug negatively regulates the repopulating ability of HSCs under conditions of stress. Slug deficiency increases HSC proliferation and reconstitution potential *in vivo* after myelosuppressive treatment, and accelerates HSC expansion during *in vitro* culture (Sun et

Accumulating evidence strongly suggests that tumors are organized into cellular hierarchies initiated and maintained by a small pool of self-renewing cancer stem cells (CSCs) (Dick, 2008; Reya et al., 2001). CSCs are thought to be resistant to various cancer treatments because of their relative quiescence (Komarova & Wodarz., 2007). Cancer relapses may occur because the dormancy of CSCs protects them from elimination by various cancer

(e.g., cytoablative chemotherapy or HSC transplantation).

**2.3.2 Necdin** 

**2.3.3 Slug** 

al., 2010).

**3. Cancer stem cells** 

In this review, we have briefly summarized a number of critical regulators involved in the control of HSC self-renewal, quiescence, survival, and responses to external insults. Recent evidence strongly suggests that the BM niche also plays an integral role by providing critical signals that maintain HSCs in a stat of hibernation, thus preventing them from exhausting themselves. However, HSCs are critical for the maintenance and regeneration of an organism after injury/illness. This process must be tightly regulated and coordinated. Intensive studies have uncovered the molecular signatures and key molecules regulating HSC behavior. Moreover, new systems approaches, such as microRNA expression profiling and protein expression profiling, are expected to provide further useful information about HSC biology in the future. However, the overall picture of the molecular mechanisms that govern HSC fate is still unclear. Further understanding of the systems that regulate HSCs will enable the manipulation of stem cells for use in tissue engineering and cell-based therapies.

### **5. Acknowledgments**

This work was supported by a Grant-in-Aid for Young Scientists to Y.K. (no. 23791083) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.

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**3** 

Rasmus Freter

*United Kingdom* 

**Transcriptional Quiescence of** 

*Ludwig Institute for Cancer Research, University of Oxford* 

Haematopoietic stem cells (HSC) have the exceptional capacity to undergo continuous selfrenewal and differentiation into multiple lineages, which is essential for haematopoietic homeostasis and response to injury. To achieve this life long function, these cells have to be protected from cytotoxic and genetic damage. On the other hand, rapid activation of haematopoietic stem cell proliferation in response to stimuli must be ensured. While cellular quiescence is thought to be the key mechanism underlying this paradoxical nature of HSC,

Quiescence is commonly defined as a reversible cell cycle exit. Induction and maintenance of stem cell quiescence has been studied at the level of cell cycle regulation (Orford & Scadden, 2008), cellular metabolism (Tothova & Gilliland, 2007) or interaction with the specific niche (Fuchs *et al.*, 2004). Genome-wide association studies have been performed on a variety of quiescent model systems, such as serum starvation of fibroblasts (Coller *et al.*, 2006), primary lymphocytes (Garriga *et al.*, 1998) or yeast in stationary phase (Patturajan *et al.*, 1998, Radonjic *et al.*, 2005). All of these studies revealed a significant decrease of productive mRNA transcription in these model systems. However, if quiescent adult stem

Due to their relative ease of isolation, cells of the haematopoietic lineage have been extensively studied. Importantly, several assays for hematopoietic stem cell function have been developed, such as colony forming ability and rescue of lethally irradiated mice. These functional tests are lacking in most other adult stem cell models, with the exception of spermatogonia and mammary gland stem cells (Brinster & Nagano, 1998, Shackleton *et al.*, 2006). Functional assays for HSC ability have provided us with the notion that most defined populations of long term repopulating HSC still contain progenitor cells, which can only transiently contribute to repopulation of the haematopoietic system. This heterogeneity is not only evident in defined cell populations, but also in the in vivo niche for HSC, the bone marrow. HSC in the bone marrow are interspersed with transient amplifying cells and differentiated cells, complicating stem cell identification by spatial organization of the tissue. Other stem cell systems, such as spermatogonia, keratinocyte or crypt stem cells have a clearly defined niche architecture, enabling stem cell identification by location only (Fuchs *et al.*, 2004). In this case, resting stem cells and activated progenitors can be separated by

the molecular basis of induction and maintenance of quiescence remains unresolved.

cells share this down regulation of mRNA transcription has never been examined.

**1. Introduction** 

**Hematopoietic Stem Cells** 

(2008) Hematopoietic stem cells reversibly switch from dormancy to self-renewal during homeostasis and repair. *Cell* 135(6):1118-1129.

