La ciudad: obstáculo al Absoluto

A defining feature of cell competition is the elimination of aberrant cells only when confronted with more-fit cells. Three broad categories of changes to cells are known to trigger cell competition: those leading to metabolic changes, those causing sharp signalling differences and those disrupting apical-basal polarity (summarised in Table 1.1)

In many studies of cell competition, the two competing populations display different growth rates, for example, Minute lose clones grow slower than wild- type counterparts (Morata and Ripoll, 1975). However, it is now clear that differences in growth rate alone are not sufficient to induce loser cell elimination by cell competition. Overexpression of cell-cycle regulators such as cyclin-D and

cyclin-dependent kinase 4 (Cdk4), are not sufficient for cells to acquire a winner

status and eliminate slower-growing wild-type cells (De La Cova et al., 2004). Conversely, decreasing proliferation of cells in a mosaic fashion via insulin signalling also does not induce cell competition in the slower growing cells (Böhni et al., 1999; Verdu et al., 1999). Furthermore, cell competition can be induced in post-mitotic cells in Drosophila follicular epithelium leading to apoptosis of losers and hypertrophy of winners (Tamori and Deng, 2013).

A number of mutations that alter proliferation and cause cell competition alter protein synthesis. The Minute and Bst mutations disrupt ribosomal proteins and MYC is known to regulate protein synthesis so alterations to this process may be the underlying trigger (Dang, 2012; Eilers and Eisenman, 2008; Meyer and Penn, 2008). In addition to this, metabolic pathways that impact cell proliferation may trigger cell competition. For example, MYC can activate glycolysis and regulate metabolism during cell competition (De La Cova et al., 2014). Difference in energy metabolism have also been shown to be required for elimination of slow-growing cells in immortalised cell models (Penzo-Méndez et al., 2015). Therefore, proliferation may only be a readout of the different properties of winner and loser cells.

1.4.1.1 Signalling Pathways that trigger Cell Competition

The second set of mutations that induce cell competition affect signalling pathways. Sharp differences in bone morphogenetic protein (BMP)/Dpp (Moreno et al., 2002; Sancho et al., 2013), Wnt/Wg (Vincent et al., 2011), JAK-STAT (Rodrigues et al., 2012), Hippo signalling (Mamada et al., 2015; Tyler et al., 2007), Notch signalling (Alcolea and Jones, 2015) and p53 mutations (Watanabe et al., 2018) have all been shown to induce cell competition and are summarised in Table 1.1.

The first signalling pathway found to regulate cell competition was the BMP/Dpp pathway. Minute+/- mutant clones in Drosophila have lower levels of Dpp signalling which causes apoptosis (Moreno et al., 2002). In mammals, pluripotent cells with decreased BMP signalling are also eliminated via an apoptosis-dependent mechanism (Sancho et al., 2013).

Differences in both WNT/Wg and JAK-STAT signalling between cells can also induce cell competition. Work in Drosophila models has demonstrated that cells with low WNT/Wg signalling levels are eliminated when surrounded by normal neighbours. Conversely, cells with higher levels of Wnt/Wg signalling via

Axin or APC mutations, behave as supercompetitors and eliminate surrounding

wild-type cells (Suijkerbuijk et al., 2016; Vincent et al., 2011). Similarly, in JAK- STAT signalling, wild-type cells eliminate JAK-STAT signalling deficient cells with sustained activation of this pathway producing supercompetitors (Rodrigues et al., 2012).

Another pathway that can induce cell competition is the Salvador-Warts- Hippo signalling pathway. In mouse cell cultures, activation of the Hippo signalling pathway in fibroblasts leads to elimination by normal neighbours (Mamada et al., 2015). Furthermore, inhibition of the Hippo pathway by Yorkie or

Tead4 overexpression leads to cells becoming supercompetitors and elimination

of wild-type cells (Mamada et al., 2015; Neto-Silva et al., 2010; Tyler et al., 2007; Ziosi et al., 2010).

Sharp signalling differences can also alter differentiation balance within an epithelium (Alcolea and Jones, 2015). Inactivation of Notch in single murine oesophageal epithelial progenitor cells results in these cells producing more progenitor daughters (Alcolea et al., 2014). Moreover, mutant cells promote the

differentiation of neighbouring wild type cells and hence the elimination of normal cells from the tissue.

Finally, it has recently been demonstrated that in MDCK cells and intestinal organoids expressing mosaic expression of mutant p53 results in elimination of transformed cells (Watanabe et al., 2018). Although p53 is generally considered a sensing mechanism for cell competition, with this role discussed later, cells expressing mutant p53R273H _{undergo necroptosis when}

surrounded by normal epithelial cells.

1.4.1.2 Polarity genes and Disruption of Epithelial Integrity in Cell Competition

Mutation of several different proteins involved in maintenance of apical- basal polarity are also known to induce cell competition. In Drosophila, mutation to Scribble, Lethal giant larvae (Lgl), or Discs Large (Dlg) in all cells leads to neoplastic overgrowth and tumours. However, mutation of these genes in a mosaic fashion in Drosophila or MDCK cells in vitro leads to apoptosis of mutant cells by normal neighbours (Agrawal et al., 1995; Brumby and Richardson, 2003; Chen et al., 2012; Igaki et al., 2006; Norman et al., 2012; Tamori et al., 2010; Woods and Bryant, 1991). This suggests that cell competition does not just function to remove aberrant cells but also removes oncogenic cells. Indeed, overexpression of dMyc in Lgl mutant clones promotes retention of transformed cells and promotes neoplastic outgrowth (Froldi et al., 2010; Menendez et al., 2010). Further indication that cell competition measures relative fitness, not absolute fitness levels, is that polarity deficient cells are not eliminated when surrounded by Minute+/- mutant cells (Froldi et al., 2010)

Factor/Pathway Role in Cell Competition Difference Reference Minute Heterozygous mutants are

eliminated Growth (Morata and Ripoli, 1975)

Myc High expressing cells are _{supercompetitors} Growth (Claveria et al., 2013; de la _{Cova et al., 2004)} BMPs Low levels associated with loser

status Signalling

(Moreno et al., 2002; Sancho et al., 2013,)

Hippo

Activation associated with loser cells whilst nuclear YAP

associated with supercompetitor status

Signalling (Neto-Silva et al., 2010; Ziosi et _{al., 2010)}

JAK/STAT

Low levels associated with loser status and high levels with supercompetitors

Signalling (Rodrigues et al., 2012) Notch

Notch mutant cells promote differentiation and loss of wild type neighbours

Signalling (Watanabe et al., 2018)

p53

p53 mutant cells undergo necroptosis and are basally

extruded Signalling (Alcolea et al., 2014) Wingleess

(Wg)

Low levels associated with loser status and high levels with supercompetitors

Signalling (Vincent et al., 2011) Scribble Scribble mutant cells are

eliminated by normal neighbours Polarity

(Brumby and Richardson, 2003; Igaki et al., 2006; Norman et al., 2012)

Discs large (Dlg)

Dlg mutant cells are eliminated

by normal neighbours Polarity (Igaki et al., 2006) Lethal giant

larvae (Lgl)

Lgl mutant cells are eliminated by

normal neighbours Polarity

(Agrawal et al., 1995; Tamori et al., 2010)

Table 1.1: Summary of cell competition triggering signals