Epithelial-mesenchymal transition in tissue repair and degeneration.

Epithelial-mesenchymal transitions (EMTs) are the epitome of cell plasticity in embryonic development and cancer; during EMT, epithelial cells undergo dramatic phenotypic changes and become able to migrate to form different tissues or give rise to metastases, respectively. The importance of EMTs in other contexts, such as tissue repair and fibrosis in the adult, has become increasingly recognized and studied. In this Review, we discuss the function of EMT in the adult after tissue damage and compare features of embryonic and adult EMT. Whereas sustained EMT leads to adult tissue degeneration, fibrosis and organ failure, its transient activation, which confers phenotypic and functional plasticity on somatic cells, promotes tissue repair after damage. Understanding the mechanisms and temporal regulation of different EMTs provides insight into how some tissues heal and has the potential to open new therapeutic avenues to promote repair or regeneration of tissue damage that is currently irreversible. We also discuss therapeutic strategies that modulate EMT that hold clinical promise in ameliorating fibrosis, and how precise EMT activation could be harnessed to enhance tissue repair.

Intron detention tightly regulates the stemness/differentiation switch in the adult neurogenic niche.

The adult mammalian brain retains some capacity to replenish neurons and glia, holding promise for brain regeneration. Thus, understanding the mechanisms controlling adult neural stem cell (NSC) differentiation is crucial. Paradoxically, adult NSCs in the subependymal zone transcribe genes associated with both multipotency maintenance and neural differentiation, but the mechanism that prevents conflicts in fate decisions due to these opposing transcriptional programmes is unknown. Here we describe intron detention as such control mechanism. In NSCs, while multiple mRNAs from stemness genes are spliced and exported to the cytoplasm, transcripts from differentiation genes remain unspliced and detained in the nucleus, and the opposite is true under neural differentiation conditions. We also show that m6A methylation is the mechanism that releases intron detention and triggers nuclear export, enabling rapid and synchronized responses. m6A RNA methylation operates as an on/off switch for transcripts with antagonistic functions, tightly controlling the timing of NSCs commitment to differentiation.

Microenvironmental Snail1-induced immunosuppression promotes melanoma growth.

Melanoma is an aggressive form of skin cancer due to its high metastatic abilities and resistance to therapies. Melanoma cells reside in a heterogeneous tumour microenvironment that acts as a crucial regulator of its progression. Snail1 is an epithelial-to-mesenchymal transition transcription factor expressed during development and reactivated in pathological situations including fibrosis and cancer. In this work, we show that Snail1 is activated in the melanoma microenvironment, particularly in fibroblasts. Analysis of mouse models that allow stromal Snail1 depletion and therapeutic Snail1 blockade indicate that targeting Snail1 in the tumour microenvironment decreases melanoma growth and lung metastatic burden, extending mice survival. Transcriptomic analysis of melanoma-associated fibroblasts and analysis of the tumours indicate that stromal Snail1 induces melanoma growth by promoting an immunosuppressive microenvironment and a decrease in anti-tumour immunity. This study unveils a novel role of Snail1 in melanoma biology and supports its potential as a therapeutic target.

Antifibrotic drugs as therapeutic tools in resistant melanoma.

Melanoma is the most aggressive form of skin cancer. Together with the recent advances in immunotherapy, targeted therapy with inhibitors of the Mitogen Activated Protein Kinase (MAPKi) pathway including BRAF and MEK inhibitors has greatly improved the clinical outcome of these patients. Unfortunately, due to genetic and non-genetic events, many patients develop resistance to MAPKi. Melanoma phenotypic plasticity, understood as the ability of melanoma cells to dynamically transition between different states with varying levels of differentiation/dedifferentiation, is key for melanoma progression. Lineage plasticity has also emerged as an important mechanism of non-genetic adaptive melanoma drug resistance in the clinic (Arozarena & Wellbrock, 2019), highlighting the need for a deeper characterization of the mechanisms that control this process. In this issue of EMBO Molecular Medicine, Diazzi et al (2022) identify a mechanism regulating MAPKi-induced phenotypic plasticity and resistance, providing evidence to support the use of an anti-fibrotic drug as a potential novel combinatorial therapeutic approach.

Are You Interested or Afraid of Working on EMT?

When referring to the epithelial-to-mesenchymal transition (EMT), readers are familiar with sentences alluding to its pivotal role both in embryonic development and in disease. Following that argument, usually there is a point on the importance of studying the process and the impact it has on the design of therapeutic strategies. However, it is also very common to find arguments on how the EMT is very difficult to tackle, being a somehow obscure and complex process, where the field cannot reach universal conclusions, particularly in pathological contexts. Even worse, it is sometimes defined as a process that cannot be described with universal markers, making it therefore very difficult for cancer studies, where there is a need to use optimal animal models and stratify patients for differential therapeutic strategies. In the face of all this, the question is whether you have been frightened off working on pathological EMTs, or even if you are not interested anymore and would prefer waiting till the field reaches a steady state of robust knowledge. Do not be afraid and be interested now. It only involves being more plastic, like the EMT itself.

In primary airway epithelial cells, the unjamming transition is distinct from the epithelial-to-mesenchymal transition.

The epithelial-to-mesenchymal transition (EMT) and the unjamming transition (UJT) each comprises a gateway to cellular migration, plasticity and remodeling, but the extent to which these core programs are distinct, overlapping, or identical has remained undefined. Here, we triggered partial EMT (pEMT) or UJT in differentiated primary human bronchial epithelial cells. After triggering UJT, cell-cell junctions, apico-basal polarity, and barrier function remain intact, cells elongate and align into cooperative migratory packs, and mesenchymal markers of EMT remain unapparent. After triggering pEMT these and other metrics of UJT versus pEMT diverge. A computational model attributes effects of pEMT mainly to diminished junctional tension but attributes those of UJT mainly to augmented cellular propulsion. Through the actions of UJT and pEMT working independently, sequentially, or interactively, those tissues that are subject to development, injury, or disease become endowed with rich mechanisms for cellular migration, plasticity, self-repair, and regeneration.

Genetic Fate Mapping of Transient Cell Fate Reveals N-Cadherin Activity and Function in Tumor Metastasis.

Genetic lineage tracing unravels cell fate and plasticity in development, tissue homeostasis, and diseases. However, it remains technically challenging to trace temporary or transient cell fate, such as epithelial-to-mesenchymal transition (EMT) in tumor metastasis. Here, we generated a genetic fate-mapping system for temporally seamless tracing of transient cell fate. Highlighting its immediate application, we used it to study EMT gene activity from the local primary tumor to a distant metastatic site in vivo. In a spontaneous breast-to-lung metastasis model, we found that primary tumor cells activated vimentin and N-cadherin in situ, but only N-cadherin was activated and functionally required during metastasis. Tumor cells that have ever expressed N-cadherin constituted the majority of metastases in lungs, and functional deletion of N-cad significantly reduced metastasis. The seamless genetic recording system described here provides an alternative way for understanding transient cell fate and plasticity in biological processes.

Glucose Metabolism Takes Center Stage in Epithelial-Mesenchymal Plasticity.

Aerobic glycolysis, or Warburg effect, has been associated with pathologies such as cancer. In this issue of Developmental Cell, Bhattacharya et al. show that aerobic glycolysis is required for neural crest migration in development. These findings point to a link between glucose metabolism and epithelial-mesenchymal plasticity in development and disease.

Guidelines and definitions for research on epithelial-mesenchymal transition.

Epithelial-mesenchymal transition (EMT) encompasses dynamic changes in cellular organization from epithelial to mesenchymal phenotypes, which leads to functional changes in cell migration and invasion. EMT occurs in a diverse range of physiological and pathological conditions and is driven by a conserved set of inducing signals, transcriptional regulators and downstream effectors. With over 5,700 publications indexed by Web of Science in 2019 alone, research on EMT is expanding rapidly. This growing interest warrants the need for a consensus among researchers when referring to and undertaking research on EMT. This Consensus Statement, mediated by 'the EMT International Association' (TEMTIA), is the outcome of a 2-year-long discussion among EMT researchers and aims to both clarify the nomenclature and provide definitions and guidelines for EMT research in future publications. We trust that these guidelines will help to reduce misunderstanding and misinterpretation of research data generated in various experimental models and to promote cross-disciplinary collaboration to identify and address key open questions in this research field. While recognizing the importance of maintaining diversity in experimental approaches and conceptual frameworks, we emphasize that lasting contributions of EMT research to increasing our understanding of developmental processes and combatting cancer and other diseases depend on the adoption of a unified terminology to describe EMT.

The Evolutionary History of Ephs and Ephrins: Toward Multicellular Organisms.

Eph receptor (Eph) and ephrin signaling regulate fundamental developmental processes through both forward and reverse signaling triggered upon cell-cell contact. In vertebrates, they are both classified into classes A and B, and some representatives have been identified in many metazoan groups, where their expression and functions have been well studied. We have extended previous phylogenetic analyses and examined the presence of Eph and ephrins in the tree of life to determine their origin and evolution. We have found that 1) premetazoan choanoflagellates may already have rudimental Eph/ephrin signaling as they have an Eph-/ephrin-like pair and homologs of downstream-signaling genes; 2) both forward- and reverse-downstream signaling might already occur in Porifera since sponges have most genes involved in these types of signaling; 3) the nonvertebrate metazoan Eph is a type-B receptor that can bind ephrins regardless of their membrane-anchoring structure, glycosylphosphatidylinositol, or transmembrane; 4) Eph/ephrin cross-class binding is specific to Gnathostomata; and 5) kinase-dead Eph receptors can be traced back to Gnathostomata. We conclude that Eph/ephrin signaling is of older origin than previously believed. We also examined the presence of protein domains associated with functional characteristics and the appearance and conservation of downstream-signaling pathways to understand the original and derived functions of Ephs and ephrins. We find that the evolutionary history of these gene families points to an ancestral function in cell-cell interactions that could contribute to the emergence of multicellularity and, in particular, to the required segregation of cell populations.

G-protein-coupled receptor kinase 2 safeguards epithelial phenotype in head and neck squamous cell carcinomas.

Head and neck squamous cell carcinoma (HNSCC) arises from the mucosal lining of the upper aerodigestive tract and display few treatment options in advanced stages. Despite increased knowledge of HNSCC molecular biology, the identification of new players involved in triggering HNSCC recurrence and metastatic disease is needed. We uncover that G-protein-coupled receptor kinase-2 (GRK2) expression is reduced in undifferentiated, high-grade human HNSCC tumors, whereas its silencing in model human HNSCC cells is sufficient to trigger epithelial-to-mesenchymal transition (EMT) phenotypic features, an EMT-like transcriptional program and enhanced lymph node colonization from orthotopic tongue tumors in mice. Conversely, enhancing GRK2 expression counteracts mesenchymal cells traits by mechanisms involving phosphorylation and decreased functionality of the key EMT inducer Snail1. Our results suggest that GRK2 safeguards the epithelial phenotype, whereas its downregulation contributes to the activation of EMT programs in HNSCC.

A gene regulatory network to control EMT programs in development and disease.

The Epithelial to Mesenchymal Transition (EMT) regulates cell plasticity during embryonic development and in disease. It is dynamically orchestrated by transcription factors (EMT-TFs), including Snail, Zeb, Twist and Prrx, all activated by TGF-ß among other signals. Here we find that Snail1 and Prrx1, which respectively associate with gain or loss of stem-like properties and with bad or good prognosis in cancer patients, are expressed in complementary patterns during vertebrate development and in cancer. We show that this complementarity is established through a feedback loop in which Snail1 directly represses Prrx1, and Prrx1, through direct activation of the miR-15 family, attenuates the expression of Snail1. We also describe how this gene regulatory network can establish a hierarchical temporal expression of Snail1 and Prrx1 during EMT and validate its existence in vitro and in vivo, providing a mechanism to switch and select different EMT programs with important implications in development and disease.

MicroRNAs Establish the Right-Handed Dominance of the Heart Laterality Pathway in Vertebrates.

Despite their external bilateral symmetry, vertebrates have internal left/right (L/R) asymmetries required for optimal organ function. BMP-induced epithelial to mesenchymal transition (EMT) in the lateral plate mesoderm (LPM) triggers L/R asymmetric cell movements toward the midline, higher from the right, which are crucial for heart laterality in vertebrates. However, how the L/R asymmetric levels of EMT factors are achieved is not known. Here, we show that the posterior-to-anterior Nodal wave upregulates several microRNAs (miRNAs) to transiently attenuate the levels of EMT factors (Prrx1a and Snail1) on the left LPM in a Pitx2-independent manner in the fish and mouse. These data clarify the role of Nodal in heart laterality and explain how Nodal and BMP exert their respective dominance on the left and right sides through the mutual inhibition of their respective targets, ensuring the proper balance of L/R information required for heart laterality and morphogenesis.

Ribosome biogenesis during cell cycle arrest fuels EMT in development and disease.

Ribosome biogenesis is a canonical hallmark of cell growth and proliferation. Here we show that execution of Epithelial-to-Mesenchymal Transition (EMT), a migratory cellular program associated with development and tumor metastasis, is fueled by upregulation of ribosome biogenesis during G1/S arrest. This unexpected EMT feature is independent of species and initiating signal, and is accompanied by release of the repressive nucleolar chromatin remodeling complex (NoRC) from rDNA, together with recruitment of the EMT-driving transcription factor Snai1 (Snail1), RNA Polymerase I (Pol I) and the Upstream Binding Factor (UBF). EMT-associated ribosome biogenesis is also coincident with increased nucleolar recruitment of Rictor, an essential component of the EMT-promoting mammalian target of rapamycin complex 2 (mTORC2). Inhibition of rRNA synthesis in vivo differentiates primary tumors to a benign, Estrogen Receptor-alpha (ERα) positive, Rictor-negative phenotype and reduces metastasis. These findings implicate the EMT-associated ribosome biogenesis program with cellular plasticity, de-differentiation, cancer progression and metastatic disease.

A snail tale and the chicken embryo.

Some 25 years ago, a clone was identified that contained the chicken Slug sequences (now called Snail2 ). How could we anticipate at that time how much the chick embryo would help us to understand the ins and outs of cell migration during development and in disease? Indeed, the chick embryo helped us identify Snail2 as the first transcription factor that could induce the epithelial-mesenchymal transition (EMT), key for the migration of embryonic and cancer cells.