Xrp1 and Irbp18 trigger a feed-forward loop of proteotoxic stress to induce the loser status

Cell competition induces the elimination of less-fit “loser” cells by fitter “winner” cells. In Drosophila, cells heterozygous mutant in ribosome genes, Rp/+, known as Minutes, are outcompeted by wild-type cells. Rp/+ cells display proteotoxic stress and the oxidative stress response, which drive the loser status. Minute cell competition also requires the transcription factors Irbp18 and Xrp1, but how these contribute to the loser status is partially understood. Here we provide evidence that initial proteotoxic stress in RpS3/+ cells is Xrp1-independent. However, Xrp1 is sufficient to induce proteotoxic stress in otherwise wild-type cells and is necessary for the high levels of proteotoxic stress found in RpS3/+ cells. Surprisingly, Xrp1 is also induced downstream of proteotoxic stress, and is required for the competitive elimination of cells suffering from proteotoxic stress or overexpressing Nrf2. Our data suggests that a feed-forward loop between Xrp1, proteotoxic stress, and Nrf2 drives Minute cells to become losers.

Competition between hematopoietic stem and progenitor cells controls hematopoietic stem cell compartment size.

Cellular competition for limiting hematopoietic factors is a physiologically regulated but poorly understood process. Here, we studied this phenomenon by hampering hematopoietic progenitor access to Leptin receptor+ mesenchymal stem/progenitor cells (MSPCs) and endothelial cells (ECs). We show that HSC numbers increased by 2-fold when multipotent and lineage-restricted progenitors failed to respond to CXCL12 produced by MSPCs and ECs. HSCs were qualitatively normal, and HSC expansion only occurred when early hematopoietic progenitors but not differentiated hematopoietic cells lacked CXCR4. Furthermore, the MSPC and EC transcriptomic heterogeneity was remarkably stable, suggesting that it is impervious to dramatic changes in hematopoietic progenitor interactions. Instead, HSC expansion was caused by increased availability of membrane-bound stem cell factor (mSCF) on MSPCs and ECs due to reduced consumption by cKit-expressing hematopoietic progenitors. These studies revealed an intricate homeostatic balance between HSCs and proximal hematopoietic progenitors regulated by cell competition for limited amounts of mSCF.

Cell competition is driven by Xrp1-mediated phosphorylation of eukaryotic initiation factor 2α

Cell competition is a context-dependent cell elimination via cell-cell interaction whereby unfit cells (‘losers’) are eliminated from the tissue when confronted with fitter cells (‘winners’). Despite extensive studies, the mechanism that drives loser’s death and its physiological triggers remained elusive. Here, through a genetic screen in Drosophila, we find that endoplasmic reticulum (ER) stress causes cell competition. Mechanistically, ER stress upregulates the bZIP transcription factor Xrp1, which promotes phosphorylation of the eukaryotic translation initiation factor eIF2α via the kinase PERK, leading to cell elimination. Surprisingly, our genetic data show that different cell competition triggers such as ribosomal protein mutations or RNA helicase Hel25E mutations converge on upregulation of Xrp1, which leads to phosphorylation of eIF2α and thus causes reduction in global protein synthesis and apoptosis when confronted with wild-type cells. These findings not only uncover a core pathway of cell competition but also open the way to understanding the physiological triggers of cell competition.

Learning the rules of cell competition without prior scientific knowledge

Deep learning is now a powerful tool in microscopy data analysis, and is routinely used for image processing applications such as segmentation and denoising. However, it has rarely been used to directly learn mechanistic models of a biological system, owing to the complexity of the internal representations. Here, we develop an end-to-end machine learning model capable of learning the rules of a complex biological phenomenon, cell competition, directly from a large corpus of timelapse microscopy data. Cell competition is a quality control mechanism that eliminates unfit cells from a tissue and during which cell fate is thought to be determined by the local cellular neighborhood over time. To investigate this, we developed a new approach (τ-VAE) by coupling a variational autoencoder to a temporal convolution network to predict the fate of each cell in an epithelium. Using the τ-VAE's latent representation of the local tissue organization and the flow of information in the network, we decode the physical parameters responsible for correct prediction of fate in cell competition. Remarkably, the model autonomously learns that cell density is the single most important factor in predicting cell fate -- a conclusion that has taken over a decade of traditional experimental research to reach. Finally, to test the learned internal representation, we challenge the network with experiments performed in the presence of drugs that block signalling pathways involved in competition. We present a novel discriminator network that, using the predictions of the τ-VAE, can identify conditions which deviate from the normal behaviour, paving the way for automated, mechanism-aware drug screening.

PTP61F Mediates Cell Competition and Mitigates Tumorigenesis

Tissue homeostasis via the elimination of aberrant cells is fundamental for organism survival. Cell competition is a key homeostatic mechanism, contributing to the recognition and elimination of aberrant cells, preventing their malignant progression and the development of tumors. Here, using Drosophila as a model organism, we have defined a role for protein tyrosine phosphatase 61F (PTP61F) (orthologue of mammalian PTP1B and TCPTP) in the initiation and progression of epithelial cancers. We demonstrate that a Ptp61F null mutation confers cells with a competitive advantage relative to neighbouring wild-type cells, while elevating PTP61F levels has the opposite effect. Furthermore, we show that knockdown of Ptp61F affects the survival of clones with impaired cell polarity, and that this occurs through regulation of the JAK–STAT signalling pathway. Importantly, PTP61F plays a robust non-cell-autonomous role in influencing the elimination of adjacent polarity-impaired mutant cells. Moreover, in a neoplastic RAS-driven polarity-impaired tumor model, we show that PTP61F levels determine the aggressiveness of tumors, with Ptp61F knockdown or overexpression, respectively, increasing or reducing tumor size. These effects correlate with the regulation of the RAS–MAPK and JAK–STAT signalling by PTP61F. Thus, PTP61F acts as a tumor suppressor that can function in an autonomous and non-cell-autonomous manner to ensure cellular fitness and attenuate tumorigenesis.

Cell Competition between Wild-Type and JAK2V617F Mutant Cells Prevents Disease Relapse after Stem Cell Transplantation in a Murine Model of Myeloproliferative Neoplasm

Introduction Disease relapse after allogeneic stem cell transplantation is a major cause of treatment-related morbidity and mortality in patients with myeloproliferative neoplasms (MPNs). The cellular and molecular mechanisms for MPN relapse are not well understood. In this study, we investigated the role of cell competition between wild-type and JAK2V617F mutant cells in MPN disease relapse after stem cell transplantation. Methods JAK2V617F Flip-Flop (FF1) mice (which carry a Cre-inducible human JAK2V617F gene driven by the human JAK2 promoter) were crossed with Tie2-cre mice to express JAK2V617F specifically in all hematopoietic cells and vascular endothelial cells (Tie2FF1), so as to model the human diseases in which both the hematopoietic stem cells and endothelial cells harbor the mutation. Results To investigate the underlying mechanisms for MPN disease relapse, we transplanted wild-type CD45.1 marrow directly into lethally irradiated Tie2FF1 mice or age-matched control mice(CD45.2). During a 6-7mo follow up, while all wild-type control recipients displayed full donor engraftment, ~60% Tie2FF1 recipient mice displayed recovery of the JAK2V617Fmutant hematopoiesis (mixed donor/recipient chimerism) 10 weeks after transplantation and developed a MPN phenotype with neutrophilia and thrombocytosis, results consistent with our previous report. Using CD45.1 as a marker for wild-type donor and CD45.2 for JAK2V617F mutant recipient cells, we found that the wild-type HSCs (Lin -cKit +Sca1 +CD150 +CD48 -) were severely suppressed and the JAK2V617F mutant HSCs were significantly expanded in the relapsed mice; in contrast, there was no significant difference between the wild-type and mutant HSC numbers in the remission mice. (Figure 1) Cell competition is an evolutionarily conserved mechanism in which "fitter" cells out-compete their "less-fit" neighbors. We hypothesize that competition between the wild-type donor cells and JAK2V617F mutant recipient cells dictates the outcome of disease relapse versus remission after stem cell transplantation. To support this hypothesis, we found that there was no significant difference in cell proliferation, apoptosis, or senescence between wild-type and JAK2V617F mutant HSPCs in recipient mice who achieved disease remission; in contrast, in recipient mice who relapsed after the transplantation, wild-type HSPC functions were significantly impaired (i.e., decreased proliferation, increased apoptosis, and increased senescence), which could alter the competition between co-existing wild-type and mutant cells and lead to the outgrowth of the JAK2V617F mutant HSPCs and disease relapse. (Figure 2) To understand how wild-type cells prevent the expansion of JAK2V617F mutant HSPCs, we established a murine model of wild-type and JAK2V617F mutant cell competition. In this model, when 100% JAK2V617F mutant marrow cells (from the Tie2FF1 mice) are transplanted alone into lethally irradiated wild-type recipients, the recipient mice develop a MPN phenotype ~4wks after transplantation; in contrast, when a 50-50 mix of mutant and wild-type marrow cells are transplanted together into the wild-type recipient mice, the JAK2V617F mutant donor cells engraft to a similar level as the wild-type donor cells and the recipient mice displayed normal blood counts during more than 4-months of follow up. In this model, compared to wild-type HSPCs, JAK2V617F mutant HSPCs generated significantly more T cells and less B cells in the spleen, and more myeloid-derived suppressor cells (MDSCs) in the marrow; in contrast, there was no difference in T, B, or MDSC numbers between recipients of wild-type HSPCs and recipients of mixed wild-type and JAK2V617F mutant HSPCs. We also found that program death ligand 1 (PD-L1) expression was significantly upregulated on JAK2V617F mutant HSPCs compared to wild-type cells, while PD-L1 expression on mutant HSPCs was significantly decreased when there was co-existing wild-type cell competition. These results indicate that competition between wild-type and JAK2V617F mutant cells can modulate the immune cell composition and PD-L1 expression induced by the JAK2V617F oncogene. (Figure 3) Conclusion Our study provides the important observations and mechanistic insights that cell competition between wild-type donor cells and JAK2V617F mutant recipient cells can prevent MPN disease relapse after stem cell transplantation. Figure 1 Figure 1. Disclosures No relevant conflicts of interest to declare.