Principles of stem cell biology and cancer: future applications and therapeutics. Edited by T. Regad, T. J. Sayers and R. C. Rees. John Wiley & Sons (2015)
Part I. Stem Cells
Stem cell behaviour is regulated by coordinated environmental signals and intrinsic programmes. Environmental signals are supported by the niche. Currently, proposed niche functions include regulation of stem cell number, cell cycle, fate determination and motility. For example, stem cell number is thought to maintain niche constancy under physiological conditions. The niche provides positive and negative factors controlling proliferation, and the maintenance of stem cell quiescence is a common niche feature. The complex nature of the stem cell niche allows the formation of a distinct and specialized microenvironment for different stem cell types in the same organism. For most stem cell types, the simultaneous activation of several pathways is needed for continuous stem cell self-renewal. For example, the FGF, BDNF and Shh signalling pathways are needed for NSC self-renewal (Zhao et al., 2008). Since specific signals or combinations of signals are needed by different niches, stem cells must stay inside the niche in order to maintain self-renewal. The most consistent method of accomplishing this is to anchor stem cells in their niche using adhesion molecules. Thus, it can be said that niches (i) create a signalling microenvironment that regulates stemness (the capacity for self-renewal and development into more differentiated cell types) and (ii) use adhesive molecules to maintain stem cells within signal-receiving range.
Cadherins, whose main function is cell – cell adhesion, play an important role in cell signalling during critical processes, such as cell migration, gene regulation through catenins (especially the β-catenin/Wnt signalling pathway) and epithelial – mesenchymal transition (EMT). EMT has biological relevance during early embryonic development and later organogenesis, and it can also be activated during wound healing in fibrotic or cancer tissues (Hay, 1995). Cadherins are transmembrane proteins and promote cell adhesion, especially epithelial cadherin (E-cadherin). The family of catenins includes cytoskeletal proteins (a-, β-, ?-, d-catenins) that are important to the formation of a junction between cells, through their ability to link the actin filaments of the cytoskeleton to cadherins.
Integrins are a large family of cell-surface receptors that can bind ECM components, soluble extracellular ligands and other membrane-bound cellsurface molecules (Prowse et al., 2011). Integrins and other adhesion molecules are involved in the regulation of cell – cell interactions and other cellular events, such as cell signalling and cell polarity. Integrins are heterodimers of noncovalently associated aand β-subunits. In vertebrates, there are 18 aand 8 β-subunits, which can assemble into 24 different receptors with different binding properties and different tissue distributions (Hynes, 2002; Campbell and Humphries, 2011). Many stem cell types express integrin molecules and directly contact the ECM or the ECM-rich basal membrane of the niche, such as collagen, fibronectin, laminin and vitronectin, as well as members of the SIBLING family (Small Integrin Binding Ligand, N-Linked Glycoproteins; e.g. osteopontin and bone sialoprotein), by their extracellular domain. Integrins can also associate with receptors on the surfaces of other cells through direct interaction with membrane-bound proteins, such as vascular or intracellular cell adhesion molecules (VCAMs and ICAMs) (Calderwood, 2004; Humphries et al., 2006).
In the SVZ of mouse brain, stem cells attach to the capillary endothelial basement membrane by the laminin receptor a6β1 integrin (Shen et al., 2008). Human stem cells with neurogenic potential express high levels of a6β1 integrin chains and can be used as a marker to isolate SC-enriched populations (Hall et al., 2006). Stem cells in the mouse testis also express high levels of a6β1 integrin (Shinohara et al., 1999). HSCs express high levels of a4, a6, a7, a9 and β1 integrins, which are fundamental for HSC niche homing (Potocnik et al., 2000; Grassinger et al., 2009; Schreiber et al., 2009). In the skin, stem cells attach to the basal membrane via a6, β1 and β4 integrins (Watt, 2002), while in skeletal muscle, a7β1 integrin is localized on the side of satellite cells facing the basal membrane component of the stem cell niche (Kuang et al., 2008).
The ECM exerts control over cells through interaction with the integrins that mediate mechanical and chemical signals. The signals regulate the activities of cytoplasmic kinases, growth factor receptors and ion channels and control the organization of the intracellular actin cytoskeleton. This regulates cell migration, cell survival and growth (Giancotti and Ruoslahti, 1999). Integrins are bidirectional signalling receptors that are involved in outside-in and inside-out signalling. Outside-in signalling of integrin receptors through their interactions with the ECM generates clustering of integrin heterodimers called focal adhesion sites. Inside-out signalling mainly acts to bring the integrin into active conformation (Barczyk et al., 2010).
Interaction of stem cells with niche elements can alter gene expression. Binding with extracellular ligands can induce conformational changes in integrins, which can result in the separation of the aand β-subunits. This changes the cytoplasmic tails, which may contribute to outside-in signalling by favouring recruitment of cytoplasmic proteins, since complex conformational rearrangements governing affinity for ECM proteins will be directed to the cytoplasmic tail associated with cytoskeletal proteins (Cary and Guan, 1999; Arnaout et al., 2007). This can lead to specific tyrosine phosphorylation cascade of a limited number of protein substrates, which will then participate in regulating cytoskeletal organization and gene expression. Osf2 (also called Cbfa1/AML3/PEBP2aA), an osteoblast-specific transcription factor, is a key regulator of the osteoblast phenotype and is necessary for osteoblast-specific expression of the osteocalcin gene. It has been shown that the a2 integrin associated with collagen increases binding of Osf2 to DNA, resulting in activation of the osteocalcin promoter, followed by osteoblast differentiation (Xiao et al., 1998). Interaction of stem cells with niche elements can alter gene expression.
The human salivary gland (HSG) epithelial cell line is an undifferentiated population with a phenotype similar to intercalated duct. The adhesion of HSG cells to fibronectin and collagen rapidly upregulates 32 different genes (Lafrenie and Yamada, 1998), indicating the role of niche – integrin interaction in stem cell regulation and fate. The varied integrin expression of stem cells of different tissue origins, as well as integrin interaction with niche elements, results in integrin-mediated signalling mechanisms that can alter intracellular phosphorylation of target proteins. This results in altered gene expression, which regulates cell motility, as well as proliferation and differentiation.
Stem cells can respond to specific microenvironments and differentiate to a phenotype different from that of their origin. The induction of genes that regulate stem cell differentiation seems to be regulated by specific niche conditions. When placenta-derived MSCs interact with the brain microenvironment, they differentiate to the neural phenotype (Martini et al., 2013). Neurite outgrowth is dependent on contact between MSCs and the astrocyte cell membrane in the neural niche. This reveals the instructive effect of the niche, perhaps sequestering neurogenic growth factors through PGs shed from the cell surface or deposited from ECM (Kerever et al., 2007). Cell surface-shed or ECM-deposited PGs can exert two different biological effects: activation of growth factors or deactivation of biological activity (Alvarez-Silva and Borojevic, 1996; Delehedde et al., 2001; Simons and Horowitz, 2001). This balance in the activity of growth factors may influence the differentiation of local stem cells, resulting in changes in their phenotype. Neuritogenesis is dependent on multiple regulatory molecules, including hormones, growth factors, gangliosides and extracellular molecules, acting as both positive and negative signals (Skaper, 2005; Trentin, 2006). It is possible that the neurogenic growth factors associated with cell membrane molecules and ECM elements are able to properly control neuritogenesis in the course of MSC differentiation. The different types of stem cells are complex systems that often respond differently in different cells, even to the same stimuli.