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Niche cells produce different molecules (e.g. CXCL12 and SCF) that regulate HSC
numbers, quiescence, self-renewal and trafficking (Asada et al. 2017b; Birbrair and
Frenette 2016 ; Crane et al. 2017 ; Ramalingam et al. 2017 ; Sanchez-Aguilera and
Mendez-Ferrer 2017 ; Yu and Scadden 2016 ). Losses of niche cells, or niche-derived
signals, irrevocably cause alterations in some or all of these functions (Asada et al.
2017b; Birbrair and Frenette 2016 ; Crane et al. 2017 ; Ramalingam et al. 2017 ;
Sanchez-Aguilera and Mendez-Ferrer 2017 ; Yu and Scadden 2016 ). The purpose of
this chapter is to describe our current understanding of the HSC niche and discuss
some of the open questions in the field for future research.
2.2 Identification of Niche Cells
The cellular composition and function of the HSC niche is an area of intense
research and new candidate niche cells and HSC regulators are reported every year.
Different methods have been used to identify niche cells in vivo and it is necessary
to understand the limitations of these approaches to correctly interpret the
literature.
Manipulation of the number of candidate niche cells: although the existence
of HSC niches was proposed in 1978 their existence was not formally proven until
2003 in two seminal studies from the Scadden and Li laboratories (Calvi et al. 2003 ;
Zhang et al. 2003 ). These showed that genetic modifications that caused expansion
of osteoblastic (bone-forming) cells and trabecular bone in the bone marrow also
led to increases in HSC numbers (Calvi et al. 2003 ; Zhang et al. 2003 ). Many other
studies have used genetic approaches to expand or ablate candidate niche cells
in vivo which led to the identification of perivascular and periarteriolar cells, mega-
karyocytes and several other cells as components of BM HSC niches (Asada et al.
2017a; Bruns et al. 2014 ; Kunisaki et al. 2013 ; Mendez-Ferrer et al. 2008 ; Nakamura-
Ishizu et al. 2014 ; Zhao et al. 2014 ). The limitations of this approach are that (1) cell
expansion/ablation frequently cannot distinguish whether the crosstalk between the
niche cell and the HSC is direct or indirect (e.g. between offspring derived from the
ablated cell and the HSC); and (2) that ablation of large numbers of cells in the BM
might lead to non-specific activation of HSC.
Conditional deletion of HSC-supportive factors in candidate niche cells: In
this method the gene encoding a factor known to regulate HSC (e.g. CXCL12 or
SCF) is conditionally deleted via Cre-mediated recombination in candidate niche
populations and the effect of this deletion on HSC (e.g. depletion, proliferation or
mobilization) is then quantified (Ding and Morrison 2013 ; Ding et al. 2012 ). The
big advantage of these methods is that, in contrast to cell ablation, it does not per-
turb the basic cellular architecture of the bone marrow. The main caveat for these
approaches is that, to be successful, it is necessary to achieve almost complete Cre-
mediated deletion of the targeted allele, exclusively, in the candidate cell but no
other niche components. This is frequently not easy because many of the Cre-drivers
(including some that were thought to be lineage-specific) used to target niche cells
D. Lucas