InteractiveFly: GeneBrief

mahjong: Biological Overview | References |

Cell competition, the elimination of cells surrounded by more fit neighbors, is proposed to suppress tumorigenesis. Mahjong (Mahj), a ubiquitin E3 ligase substrate receptor, has been thought to mediate competition of cells mutated for lethal giant larvae (lgl), a neoplastic tumor suppressor that defines apical-basal polarity of epithelial cells. This study shows that Drosophila cells mutated for mahjong, but not for lgl [l(2)gl], are competed because they express the bZip-domain transcription factor Xrp1, already known to eliminate cells heterozygous for ribosomal protein gene mutations (Rp/+ cells). Xrp1 expression in mahj mutant cells results in activation of JNK signaling, autophagosome accumulation, eIF2α phosphorylation and lower translation, just as in Rp/+ cells. Cells mutated for damage DNA binding-protein 1 (ddb1; pic) or cullin 4 (cul4), which encode E3 ligase partners of Mahj, also display Xrp1-dependent phenotypes, as does knockdown of proteasome subunits. These data suggest a new model of mahj-mediated cell competition that is independent of apical-basal polarity and couples Xrp1 to protein turnover (Kumar, 2022).

Cell competition, which is the elimination of, in most cases, slower-growing cells by faster-growing cells in mosaics, is important for precise development, regeneration and physiological maintenance. Cell competition was first recognized in Drosophila in the case of cells lacking one copy of ribosomal protein genes (Rp/+). These mutants, which are known as 'Minutes' because of their thin body bristles, also display slow growth. Minute mutant cells are eliminated from mosaics with wild-type cells by caspase-dependent cell death. Super-competition, the name given to the process of eliminating wild-type cells, happens in mosaics with faster-growing Myc- or Yorkie (Yki)-expressing cells. Considered together, cell competition and super-competition suggest that comparison of cellular fitness leads to cell competition. Because the mammalian homologs of Myc and Yki are proto-oncogenes, it has been suggested that super-competition might contribute to tumor expansion, as several recent studies have confirmed (Kumar, 2022).

Cell competition may also be tumor suppressive. Global loss of apico-basal polarity genes such as lgl [l(2)gl)] or scribble (scrib) leads to polarity-deficient neoplasia of Drosophila imaginal discs, but clones of lgl or scrib cells are eliminated from mosaics. These lgl or scrib mutant clones do not form tumors unless cell competition is blocked and mutant cells remain in the epithelium (Kumar, 2022).

Competition of Rp/+ mutant cells might also serve a tumor-surveillance role. Rp genes are spread throughout the genome, and it has been shown they can serve as sensors for aneuploidy, leading to elimination of aneuploid cells containing monosomies that affect Rp gene dose. Mutants with disrupted cell competition accumulate aneuploid cells. These would be expected to be tumorigenic in mammals, where aneuploidy is associated with tumorigenesis. Competition of Rp/+ cells depends on the Drosophila bZip AT-hook domain transcription factor Xrp1. The Xrp1 expression induced in Rp/+ imaginal discs is also responsible for most of their altered gene expression, their slow growth and their reduced translation, in addition to their propensity to be eliminated by cell competition. Xrp1 is also expressed in the DNA damage response, where its transcription is p53 dependent. Xrp1 induction in Rp/+ cells is independent of p53 but dependent on a particular Rp protein, RpS12, which is thought to play a role in signaling the defect in ribosome biogenesis (Kumar, 2022).

The tumor-suppressive cell competition of polarity gene mutant cells has been proposed to go through Mahjong (Mahj) (Tamori, 2010), a CRL4 E3 ubiquitin ligase (Ly, 2019). mahj physically interacts with Lgl, and its overexpression in lgl mutant clones suppresses their elimination from mosaic tissues (Tamori, 2010). Interestingly, mahj knockdown in MDCK cell cultures also leads to their elimination by co-cultured normal MDCK cells, suggesting a cell competition mechanism that is conserved between Drosophila and mammalian cells (Tamori, 2010). The mammalian homolog of mahj, known as DDB1-Cul4-associated factor 1 (DCAF1) or human immunodeficiency virus type 1 accessory protein Vpr-binding protein (VprBP), is important for G2 cell cycle arrest and virus replication after HIV1 infection. Dcaf1 is required for mouse embryogenesis and its knockdown affects cell proliferation, cell cycle and cell survival in multiple cell types. Dcaf1 interacts with the Hippo pathway and its knockdown also stabilizes p53, but there has been no report of Dcaf1/VprBP affecting epithelial cell polarity in mammals (Kumar, 2022).

In Drosophila, the overall transcriptional signature of mahj mutant wing discs is unexpectedly similar to that of Rp/+ mutants, including upregulation of Xrp1 mRNA. Because mahj and Rp/+ cells were thought to represent distinct mechanisms of cell competition, this finding suggested a gene expression signature common to cells targeted by cell competition. Besides transcription, other similarities have been reported between mahj and Rp/+ mutant cells, including autophagosome accumulation and evidence of proteotoxic stress (Kumar, 2022).

This study shows that mahj mutant cells trigger cell competition through an Xrp1-dependent pathway like that in Rp/+ cells, and distinct from cell competition of lgl or scrib clones, which do not express or depend on Xrp1 function for elimination. Xrp1 expression also makes mahj mutant cells phenotypically like Rp/+ cells, that is, results in 'Minute-like' thin thoracic bristles, slow growth, reduced translation, altered autophagy and increased JNK signaling. Regulation of Xrp1 by mahj likely requires its E3 ligase activity, depending on DNA Damage Binding Protein 1 (Ddb1) and Cullin 4 (Cul4). These results show that mahj mutant cells suffer cell competition because of a transcriptional response to altered ubiquitinylation mediated by Xrp1 and therefore resembling Rp/+ mutant cells. This seems unrelated to elimination of scrib or lgl mutant cells - the polarity-defective cells. Thus, loss of mahj function is an additional genotype triggering elimination by the Xrp1-dependent pathway that also removes Minute cells, not the mechanism for eliminating tumorigenic polarity-deficient cells (Kumar, 2022).

This research explored the cell competition mechanisms of Mahj, a CRL4 ubiquitin ligase (Ly, 2019), the mutation of which triggers similar cellular effects to Rp/+ mutations, including similar changes in gene expression, global translation rates, JNK activity and autophagy, leading mahj cells to be eliminated by competition with wild-type cells, as Rp/+ cells are. The basis of the similarity is that mahj and Rp loss of function both activate expression of Xrp1, the transcription factor that coordinates these effects. Unlike Rp/+ genotypes, which activate Xrp1 through a rpS12-dependent mechanism, mahj regulates Xrp1 most likely through its ubiquitin ligase activity, which depends on DDB1, Cul4 and Roc1a, although the specific ubiquitylated target has not yet been identified. It is suggested that Xrp1 is likely to be activated by a protein, or proteins, that are normally degraded by Mahj-dependent ubiquitylation, because Xrp1 is also activated by inhibition of the proteasome, which is expected to affect the degradation of ubiquitylated proteins, but not other functional consequences of ubiquitylation. The relevant target does not seem to be Warts, despite the fact that levels of Warts and Hippo pathway activity also control cellular growth and global translation levels, and can stimulate cell competition. These studies support the notion that Xrp1 is a sensor of multiple cellular defects that cause cell competition, rather than that of a 'loser signature' common to distinct cell competition mechanisms (Kumar, 2022).

Mahj was previously thought to be responsible for the cell competition of cells mutated for lgl (Tamori, 2010; Baker, 2011; Levayer, 2013), a gene that controls apical basal cell polarity. mahj was originally linked to apical-basal polarity because of a physical interaction with Lgl, and because mahj overexpression can rescue lgl mutant clones for elimination, suggesting that mahj behaves as an intracellular signal transducer of lgl activity in cell competition (Tamori, 2010). As such, it was surprising when similar gene expression changes were observed in mahj mutant and Rp mutant wing discs, because these were assumed to reflect distinct cell competition pathways and suggested a common gene expression signature associated with competed cells. This study shows, however, that neither lgl nor scrib, another related cell polarity gene, affects cell competition by the same mechanism as mahj, because neither lgl nor scrib mutant cells express or require Xrp1. Interestingly, several distinct pathways have recently been described to mediate the elimination of scrib mutant cells in competition with wild-type cells, and none of these pathways are shown to be required for the elimination of Rp/+ mutant cells. In addition, mahj loss by itself does not result in apical-basal polarity defects (Tamori, 2010), and its mammalian homolog is implicated in cell cycle regulation, genome integrity and p53 activity (Hrecka, 2007; Cooper 2014; Lubow, 2020). Drosophila mahj, which is an essential gene, regulates neural stem cell reactivation (Ly, 2019) and may have other roles in non-neuronal tissues, as suggested by defects observed when mahj is depleted in posterior wing compartments. Accordingly, it is concluded that mahj mutants affect cellular growth and cell competition in a manner unrelated to lgl and scrib, and that the functional relationship of mahj to apical-basal polarity pathways, should any exist, is unclear. The functional importance of physical interaction between Mahj and Lgl remains to be explored. It is known that lgl clones are rescued by reduced Hippo signaling, although this study did not detect reduced Hippo signaling after mahj overexpression in the absence of lgl mutations (Kumar, 2022).

These studies provide further evidence for Xrp1 as an integrator of multiple seemingly independent cellular defects that each result in a common spectrum of cellular responses and predispose cells to competitive elimination by wild-type neighbors (Kiparaki, 2022). These functional roles for Xrp1 first became apparent through its role in the slow growth, reduced translation and competitive elimination of Rp/+ cells, in which Xrp1 expression is induced in an rpS12-dependent manner. In the case of mahj, Xrp1 protein expression is induced to confer a very similar spectrum of cellular effects, but independently of rpS12 and perhaps depending on stabilization of a protein normally targeted for proteasomal turnover by mahj-dependent ubiquitylation. Xrp1 expression was first found as a p53-regulated gene, perhaps part of the DNA damage response. Recently, Xrp1 induction has also been found as a response to ER stress, possibly through translational regulation downstream of eIF2α phosphorylation. It has been suggested that eIF2α phosphorylation, and Xrp1 expression, can also be triggered by a global, cytoplasmic proteotoxic stress, which is suggested to occur as a consequence of deficient ribosome assembly in Rp mutant cells. Xrp1 expression in response to proteasome inhibition is one piece of evidence for this model. This study shows, however, that Xrp1 is induced, and cell competition results after loss of mahj, a single E3-ligase adapter protein that probably targets only a moderate number of proteins for degradation. Thus, an alternative explanation of Xrp1 induction after proteasome inhibition is that this could reflect stabilization of one or a few specific proteins. Overall, a picture is emerging of Xrp1 as a stress-responsive transcription factor whose expression can be initiated by multiple distinct pathways, then leading to a common cellular response, including the elimination of the stressed cells by competition with nearby wild-type cells, when such cells are available (Kumar, 2022).

Importantly, cells depleted for DCAF1/VprBP, the mammalian homolog of Mahj, are eliminated by competition with wild-type cells in mammalian cell culture. Thus, cell competition of mahj mutant cells may be a conserved process. Conservation of cell competition has not yet been demonstrated for Rp/+ cells in mammals, although it may very well occur. In mammals, knockdown of either mahj or its binding partner ddb1 results in P53 activation, which is functionally required for the resulting phenotypes. Differences in p53 activity lead to cell competition in many mammalian systems. p53 is not required for mahj-mediated cell competition in Drosophila, but because Xrp1 is a target of Drosophila p53 in irradiated cells, it is possible Xrp1 is a p53 target that has replaced the cell competition role of p53 in Drosophila, as has already been suggested for the competition of Rp/+ cells, which is also p53 independent in Drosophila, although Rp mutations activate p53 in mammals. Thus, mahj-mediated cell competition may provide another example where Xrp1 mediates a process in Drosophila that is dependent on p53 in mammals (Kumar, 2022).

Proteotoxic stress is a driver of the loser status and cell competition

Cell competition allows winner cells to eliminate less fit loser cells in tissues. In Minute cell competition, cells with a heterozygous mutation in ribosome genes, such as RpS3(+/-) cells, are eliminated by wild-type cells. How cells are primed as losers is partially understood and it has been proposed that reduced translation underpins the loser status of ribosome mutant, or Minute, cells. Using Drosophila this study shows that reduced translation does not cause cell competition. Instead, proteotoxic stress was identified as the underlying cause of the loser status for Minute competition and competition induced by mahjong, an unrelated loser gene. RpS3^+/- cells exhibit reduced autophagic and proteasomal flux, accumulate protein aggregates and can be rescued from competition by improving their proteostasis. Conversely, inducing proteotoxic stress is sufficient to turn otherwise wild-type cells into losers. Thus, it is proposed that tissues may preserve their health through a proteostasis-based mechanism of cell competition and cell selection (Baumgartner, 2021).

This work shows that single copy loss of ribosome genes leads to major defects in cellular proteostasis. Heterozygosity of ribosome genes in humans leads to genetic disorders collectively known as ribosomopathies, characterized by severe malformations and pathologies. The mechanisms through which ribosomal mutations lead to these defects are only partially understood. This work suggests that proteotoxic stress may be an underlying cause for some such defects and that they might be improved by drugs that promote proteostasis, such as the FDA-approved compound rapamycin that was used in this study (Baumgartner, 2021).

This work shows that proteotoxic stress is sufficient to confer the loser status. This finding broadens the scope of cell competition and suggests it may be an active mechanism in physiological and pathological contexts characterized by proteotoxic stress. This may help explain the competitive elimination of neurons in Drosophila models of neurodegenerative diseases. It may be especially relevant to cancer, where proteotoxic stress is often observed. These findings suggest that cancer cells might represent concealed losers that have escaped proteotoxic stress-induced cell competition through masking mutations. Understanding how Minute mutations and proteotoxic stress lead to cell competition may help unmask the loser status in cancer cells in ways that could be exploited therapeutically (Baumgartner, 2021).

Healthy proteostasis is a driver of organism fitness and contributes to organism longevity, whereas impaired proteostasis is associated with aging and with age-related pathologies. It is proposed that tissues preserve their health and youth through a proteostasis-based mechanism of cell elimination. By measuring cell fitness on the basis of proteostasis and converting it into the loser status through the activation of the oxidative stress response, proteostasis-based cell competition could act as a general mechanism of cell selection in adult homeostasis. How proteotoxic stress induces the loser status remains to be established (Baumgartner, 2021).

CRL4Mahj E3 ubiquitin ligase promotes neural stem cell reactivation

The ability of neural stem cells (NSCs) to transit between quiescence and proliferation is crucial for brain development and homeostasis. Drosophila Hippo pathway maintains NSC quiescence, but its regulation during brain development remains unknown. This study shows that CRL4Mahj, an evolutionarily conserved E3 ubiquitin ligase, is essential for NSC reactivation (exit from quiescence). Damaged DNA-binding protein 1 (DDB1) and Cullin4, two core components of Cullin4-RING ligase (CRL4), are intrinsically required for NSC reactivation. This study has identified a substrate receptor of CRL4, Mahjong (Mahj), which is necessary and sufficient for NSC reactivation. Moreover, it is shown that CRL4Mahj forms a protein complex with Warts (Wts/large tumor suppressor [Lats]), a kinase of the Hippo signaling pathway, and mahj promotes the ubiquitination of Wts. This genetic analyses further support the conclusion that CRL4Mahj triggers NSC reactivation by inhibition of Wts. Given that Cullin4B mutations cause mental retardation and cerebral malformation, similar regulatory mechanisms may be applied to the human brain (Ly, 2019).

Involvement of Lgl and Mahjong/VprBP in cell competition

During the initial stages of carcinogenesis, transformation events occur in a single cell within an epithelial monolayer. However, it remains unknown what happens at the interface between normal and transformed epithelial cells during this process. In Drosophila, it has been recently shown that normal and transformed cells compete with each other for survival in an epithelial tissue; however the molecular mechanisms whereby "loser cells" undergo apoptosis are not clearly understood. Lgl (lethal giant larvae) is a tumor suppressor protein and plays a crucial role in oncogenesis in flies and mammals. This study has examined the involvement of Lgl in cell competition and shows that a novel Lgl-binding protein is involved in Lgl-mediated cell competition. Using biochemical immunoprecipitati on methods, Mahjong was first identified as a novel binding partner of Lgl in both flies and mammals. In Drosophila, Mahjong is an essential gene, but zygotic mahjong mutants (mahj^-/-) do not have obvious patterning defects during embryonic or larval development. However,mahj^-/- cells undergo apoptosis when surrounded by wild-type cells in the wing disc epithelium. Importantly, comparable phenomena also occur in Mahjong-knockdown mammalian cells; Mahjong-knockdown Madin-Darby canine kidney epithelial cells undergo apoptosis, only when surrounded by non-transformed cells. Similarly, apoptosis of lgl^-/- cells is induced when they are surrounded by wild-type cells in Drosophila wing discs. Phosphorylation of the c-Jun N-terminal kinase (JNK) is increased in mahj^-/-) or lgl^-/- mutant cells, and expression of Puckered (Puc), an inhibitor of the JNK pathway, suppresses apoptosis of these mutant cells surrounded by wild-type cells, suggesting that the JNK pathway is involved in mahj- or lgl-mediated cell competition. Finally, this study has shown that overexpression of mahj in lgl^-/- cells strongly suppresses JNK activation and blocks apoptosis of lgl^-/- cells in the wild-type wing disc epithelium. These data indicate that Mahjong interacts with Lgl biochemically and genetically and that Mahjong and Lgl function in the same pathway to regulate cellular competitiveness. This is the first report that cell competition can occur in a mammalian cell culture system (Tamori, 2010).

Search PubMed for articles about Drosophila Mahjong

Baker, N. E. (2011). Cell competition. Curr Biol 21(1): R11-15. PubMed ID: 21215926

Baumgartner, M. E., Dinan, M. P., Langton, P. F., Kucinski, I. and Piddini, E. (2021). Proteotoxic stress is a driver of the loser status and cell competition. Nat Cell Biol 23(2): 136-146. PubMed ID: 33495633

Levayer, R. and Moreno, E. (2013). Mechanisms of cell competition: themes and variations. J Cell Biol 200(6): 689-698. PubMed ID: 23509066

Lubow, J. and Collins, K. L. (2020). Vpr Is a VIP: HIV Vpr and Infected Macrophages Promote Viral Pathogenesis. Viruses 12(8). PubMed ID: 32726944

Kiparaki, M., Khan, C., Folgado-Marco, V., Chuen, J., Moulos, P. and Baker, N. E. (2022). The transcription factor Xrp1 orchestrates both reduced translation and cell competition upon defective ribosome assembly or function. Elife 11. PubMed ID: 35179490

Kumar, A. and Baker, N. E. (2022). The CRL4 E3 ligase Mahjong/DCAF1 controls cell competition through the transcription factor Xrp1, independently of polarity genes. Development 149(22). PubMed ID: 36278853

Ly, P. T., Tan, Y. S., Koe, C. T., Zhang, Y., Xie, G., Endow, S., Deng, W. M., Yu, F. and Wang, H. (2019). CRL4Mahj E3 ubiquitin ligase promotes neural stem cell reactivation. PLoS Biol 17(6): e3000276. PubMed ID: 31170139

Tamori, Y., Bialucha, C. U., Tian, A. G., Kajita, M., Huang, Y. C., Norman, M., Harrison, N., Poulton, J., Ivanovitch, K., Disch, L., Liu, T., Deng, W. M. and Fujita, Y. (2010). Involvement of Lgl and Mahjong/VprBP in cell competition. PLoS Biol 8(7): e1000422. PubMed ID: 20644714

date revised: 5 August 2023

Home page: The Interactive Fly © 2011 Thomas Brody, Ph.D.