A genome-wide screen identifies YAP/WBP2 interplay conferring growth advantage on human epidermal stem cells

Keratinocyte integrin α3β1 induces expression of the macrophage stimulating factor, CSF-1, through a YAP/TEAD-dependent mechanism

The development of wound therapy targeting integrins is hampered by inadequate understanding of integrin function in cutaneous wound healing and the wound microenvironment. Following cutaneous injury, keratinocytes migrate to restore the skin barrier, and macrophages aid in debris clearance. Thus, both keratinocytes and macrophages are critical to the coordination of tissue repair. Keratinocyte integrins have been shown to participate in this coordinated effort by regulating secreted factors, some of which crosstalk to distinct cells in the wound microenvironment. Epidermal integrin α3β1 is a receptor for laminin-332 in the cutaneous basement membrane. Here we show that wounds deficient in epidermal α3β1 express less epidermal-derived macrophage colony-stimulating factor 1 (CSF-1), the primary macrophage-stimulating growth factor. α3β1-deficient wounds also have fewer wound-proximal macrophages, suggesting that keratinocyte α3β1 may stimulate wound macrophages through the regulation of CSF-1. Indeed, using a set of immortalized keratinocytes, we demonstrate that keratinocyte-derived CSF-1 supports macrophage growth, and that α3β1 regulates Csf1 expression through Src-dependent stimulation of Yes-associated protein (YAP)-Transcriptional enhanced associate domain (TEAD)-mediated transcription. Consistently, α3β1-deficient wounds in vivo display a substantially reduced number of keratinocytes with YAP-positive nuclei. Overall, our current findings identify a novel role for epidermal integrin α3β1 in regulating the cutaneous wound microenvironment by mediating paracrine crosstalk from keratinocytes to wound macrophages, implicating α3β1 as a potential target of wound therapy.

Can Shockwave Treatment Elicit a Molecular Response to Enhance Clinical Outcomes in Pressure Ulcers? The SHOck Waves in wouNds Project

Wound healing requires the coordinated interaction of dermis cells, the proper deposition of extracellular matrix, re-epithelialization, and angiogenesis. Extracorporeal shock wave (ESW) is a promising therapeutic modality for chronic wounds. This study determined the biological mechanisms activated under ESW, facilitating the healing of pressure ulcers (PUs). A group of 10 patients with PUs received two sessions of radial ESW (300 + 100 pulses, 2.5 bars, 0.15 mJ/mm2, 5 Hz). Histomorphological and immunocytochemical assessments were performed on tissue sections obtained from the wound edges before the ESW (M0) and after the first (M1) and second (M2) ESW. The proliferation index of keratinocytes and fibroblasts (Ki-67), the micro-vessels' density (CD31), and the number of myofibroblasts (α-SMA) were evaluated. The involvement of the yes-associated protein (YAP1) in sensing mechanical strain, and whether the nuclear localization of YAP1, was shown. The increased proliferative activity of epidermal cells and skin fibroblasts and the increased number of myofibroblasts, often visible as integrated cell bands, were also demonstrated as an effect of wound exposure to an ESW. The results indicate that the major skin cells, keratinocytes, and fibroblasts are mechanosensitive. They intensify proliferation and extracellular matrix remodeling in response to mechanical stress. A significant improvement in clinical wound parameters was also observed.

UBE2A/B is the trans-acting factor mediating mechanotransduction and contact inhibition

Mechanotransduction and contact inhibition (CI) control gene expression to regulate proliferation, differentiation, and even tumorigenesis of cells. However, their downstream trans-acting factors (TAFs) are not well known due to a lack of a high-throughput method to quantitatively detect them. Here, we developed a method to identify TAFs on the cis-acting sequences that reside in open chromatin or DNaseI-hypersensitive sites (DHSs) and to detect nucleocytoplasmic shuttling TAFs using computational and experimental screening. The DHS-proteomics revealed over 1000 potential mechanosensing TAFs and UBE2A/B (Ubiquitin-conjugating enzyme E2 A) was experimentally identified as a force- and CI-dependent nucleocytoplasmic shuttling TAF. We found that translocation of YAP/TAZ and UBE2A/B are distinctively regulated by inhibition of myosin contraction, actin-polymerization, and CI depending on cell types. Next-generation sequence analysis revealed many downstream genes including YAP are transcriptionally regulated by ubiquitination of histone by UBE2A/B. Our results suggested a YAP-independent mechanotransduction and CI pathway mediated by UBE2A/B.

Histone and Histone Acetylation-Related Alterations of Gene Expression in Uninvolved Psoriatic Skin and Their Effects on Cell Proliferation, Differentiation, and Immune Responses

Psoriasis is a chronic immune-mediated skin disease in which the symptom-free, uninvolved skin carries alterations in gene expression, serving as a basis for lesion formation. Histones and histone acetylation-related processes are key regulators of gene expression, controlling cell proliferation and immune responses. Dysregulation of these processes is likely to play an important role in the pathogenesis of psoriasis. To gain a complete overview of these potential alterations, we performed a meta-analysis of a psoriatic uninvolved skin dataset containing differentially expressed transcripts from nearly 300 individuals and screened for histones and histone acetylation-related molecules. We identified altered expression of the replication-dependent histones HIST2H2AA3 and HIST2H4A and the replication-independent histones H2AFY, H2AFZ, and H3F3A/B. Eight histone chaperones were also identified. Among the histone acetyltransferases, ELP3 and KAT5 and members of the ATAC, NSL, and SAGA acetyltransferase complexes are affected in uninvolved skin. Histone deacetylation-related alterations were found to affect eight HDACs and members of the NCOR/SMRT, NURD, SIN3, and SHIP HDAC complexes. In this article, we discuss how histone and histone acetylation-related expression changes may affect proliferation and differentiation, as well as innate, macrophage-mediated, and T cell-mediated pro- and anti-inflammatory responses, which are known to play a central role in the development of psoriasis.

AR activates YAP/TAZ differentially in prostate cancer

The Hippo signalling pathway is a master regulator of cell growth, proliferation, and cancer. The transcriptional coregulators of the Hippo pathway, YAP and TAZ, are central in various cancers. However, how YAP and TAZ get activated in most types of cancers is not well understood. Here, we show that androgens activate YAP/TAZ via the androgen receptor (AR) in prostate cancer (PCa), and that this activation is differential. AR regulates YAP translation while inducing transcription of the TAZ encoding gene, WWTR1 Furthermore, we show that AR-mediated YAP/TAZ activation is regulated by the RhoA GTPases transcriptional mediator, serum response factor (SRF). Importantly, in prostate cancer patients, SRF expression positively correlates with TAZ and the YAP/TAZ target genes CYR61 and CTGF We demonstrate that YAP/TAZ are not essential for sustaining AR activity, however, targeting YAP/TAZ or SRF sensitize PCa cells to AR inhibition in anchorage-independent growth conditions. Our findings dissect the cellular roles of YAP, TAZ, and SRF in prostate cancer cells. Our data emphasize the interplay between these transcriptional regulators and their roles in prostate tumorigenesis and highlight how these insights might be exploited therapeutically.

The biophysical property of the limbal niche maintains stemness through YAP

The cell fate decisions of stem cells (SCs) largely depend on signals from their microenvironment (niche). However, very little is known about how biochemical niche cues control cell behavior in vivo. To address this question, we focused on the corneal epithelial SC model in which the SC niche, known as the limbus, is spatially segregated from the differentiation compartment. We report that the unique biomechanical property of the limbus supports the nuclear localization and function of Yes-associated protein (YAP), a putative mediator of the mechanotransduction pathway. Perturbation of tissue stiffness or YAP activity affects SC function as well as tissue integrity under homeostasis and significantly inhibited the regeneration of the SC population following SC depletion. In vitro experiments revealed that substrates with the rigidity of the corneal differentiation compartment inhibit nuclear YAP localization and induce differentiation, a mechanism that is mediated by the TGFβ-SMAD2/3 pathway. Taken together, these results indicate that SC sense biomechanical niche signals and that manipulation of mechano-sensory machinery or its downstream biochemical output may bear fruits in SC expansion for regenerative therapy.

Conductive Polymer-Coated 3D Printed Microneedles: Biocompatible Platforms for Minimally Invasive Biosensing Interfaces

Conductive polymeric microneedle (MN) arrays as biointerface materials show promise for the minimally invasive monitoring of analytes in biodevices and wearables. There is increasing interest in microneedles as electrodes for biosensing, but efforts have been limited to metallic substrates, which lack biological stability and are associated with high manufacturing costs and laborious fabrication methods, which create translational barriers. In this work, additive manufacturing, which provides the user with design flexibility and upscale manufacturing, is employed to fabricate acrylic-based microneedle devices. These microneedle devices are used as platforms to produce intrinsically-conductive, polymer-based surfaces based on polypyrrole (PPy) and poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS). These entirely polymer-based solid microneedle arrays act as dry conductive electrodes while omitting the requirement of a metallic seed layer. Two distinct coating methods of 3D-printed solid microneedles, in situ polymerization and drop casting, enable conductive functionality. The microneedle arrays penetrate ex vivo porcine skin grafts without compromising conductivity or microneedle morphology and demonstrate coating durability over multiple penetration cycles. The non-cytotoxic nature of the conductive microneedles is evaluated using human fibroblast cells. The proposed fabrication strategy offers a compelling approach to manufacturing polymer-based conductive microneedle surfaces that can be further exploited as platforms for biosensing.

Inflammation modulates intercellular adhesion and mechanotransduction in human epidermis via ROCK2

Aberrant mechanotransduction and compromised epithelial barrier function are associated with numerous human pathologies including inflammatory skin disorders. However, the cytoskeletal mechanisms regulating inflammatory responses in the epidermis are not well understood. Here we addressed this question by inducing a psoriatic phenotype in human keratinocytes and reconstructed human epidermis using a cytokine stimulation model. We show that the inflammation upregulates the Rho-myosin II pathway and destabilizes adherens junctions (AJs) promoting YAP nuclear entry. The integrity of cell-cell adhesion but not the myosin II contractility per se is the determinative factor for the YAP regulation in epidermal keratinocytes. The inflammation-induced disruption of AJs, increased paracellular permeability, and YAP nuclear translocation are regulated by ROCK2, independently from myosin II activation. Using a specific inhibitor KD025, we show that ROCK2 executes its effects via cytoskeletal and transcription-dependent mechanisms to shape the inflammatory response in the epidermis.

A systematic review on the effects of ROCK inhibitors on proliferation and/or differentiation in human somatic stem cells: A hypothesis that ROCK inhibitors support corneal endothelial healing via acting on the limbal stem cell niche

Rho kinase inhibitors (ROCKi) have attracted growing multidisciplinary interest, particularly in Ophthalmology where the question as to how they promote corneal endothelial healing remains unresolved. Concurrently, stem cell biology has rapidly progressed in unravelling drivers of stem cell (SC) proliferation and differentiation, where mechanical niche factors and the actin cytoskeleton are increasingly recognized as key players. There is mounting evidence from the study of the peripheral corneal endothelium that supports the likelihood of an internal limbal stem cell niche. The possibility that ROCKi stimulate the endothelial SC niche has not been addressed. Furthermore, there is currently a paucity of data that directly evaluates whether ROCKi promotes corneal endothelial healing by acting on this limbal SC niche located near the transition zone. Therefore, we performed a systematic review examining the effects ROCKi on the proliferation and differentiation of human somatic SC, to provide insight into its effects on various human SC populations. An appraisal of electronic searches of four databases identified 1 in vivo and 58 in vitro studies (36 evaluated proliferation while 53 examined differentiation). Types of SC studied included mesenchymal (n = 32), epithelial (n = 11), epidermal (n = 8), hematopoietic and other (n = 8). The ROCK 1/2 selective inhibitor Y-27632 was used in almost all studies (n = 58), while several studies evaluated ≥2 ROCKi (n = 4) including fasudil, H-1152, and KD025. ROCKi significantly influenced human somatic SC proliferation in 81% of studies (29/36) and SC differentiation in 94% of studies (50/53). The present systemic review highlights that ROCKi are influential in regulating human SC proliferation and differentiation, and provides evidence to support the hypothesis that ROCKi promotes corneal endothelial division and maintenance via acting on the inner limbal SC niche.

WW domain binding protein 2 (WBP2) as an oncogene in breast cancer: mechanisms and therapeutic prospects-a narrative review

Background and Objective

WW domain binding protein 2 (WBP2), considered an emerging breast cancer gene, functions as a binding partner for WW domain proteins. The WBP2 gene is involved in mediating the malignant development and clinical drug resistance of breast cancer, but its potential mechanism remains unclear. Therefore, it is necessary to elucidate the mechanism of WBP2 in breast cancer, which will help to provide new methods for clinical diagnosis and treatment of breast cancer.

Methods

The PubMed database was searched using the terms "WW Domain Binding Protein 2" or "WBP2", "breast cancer" or "breast neoplasms" or "human cancer" from January 1997 through August 2022. Through the screening and evaluation of titles and abstracts, about 120 English articles were included in this study.

Key Content and Findings

By describing the multiple regulatory functions of WBP2 at the transcriptional, post-transcriptional, and post-translational levels, and summarizing how WBP2 as a key node crosstalks multiple signaling pathways, we reveal the ability of WBP2 to promote breast cancer malignant progression. In different subtypes of breast cancer, the mechanism of WBP2-mediated drug resistance is related to estrogen receptor and epidermal growth factor receptor (EGFR) 2 status, and hormones may be an essential factor in WBP2-mediated drug resistance. In addition, we discuss the application prospects of WBP2 in targeted therapy and immunotherapy and propose therapeutic strategies to overcome drug resistance in breast cancer by jointly targeting WBP2 and its related molecules. This provides a theoretical basis for the innovation of breast cancer targeted drugs.

Conclusions

WBP2 is a promising target for breast cancer therapy. Nuclear WBP2, as the main functional form of WBP2 after its activation, is a meaningful indicator for the diagnosis and prediction of breast cancer progression.

The Regulation of the Hippo Pathway by Intercellular Junction Proteins

The Hippo pathway is an evolutionarily conserved pathway that serves to promote cell death and differentiation while inhibiting cellular proliferation across species. The downstream effectors of this pathway, yes-associated protein (YAP) and transcriptional co-activator with PDZ-binding motif (TAZ), are considered vital in promoting the output of the Hippo pathway, with activation of upstream kinases negatively regulating YAP/TAZ activity. The upstream regulation of the Hippo pathway is not entirely understood on a molecular level. However, several studies have shown that numerous cellular and non-cellular mechanisms such as cell polarity, contact inhibition, soluble factors, mechanical forces, and metabolism can convey external stimuli to the intracellular kinase cascade, promoting the activation of key components of the Hippo pathway and therefore regulating the subcellular localisation and protein activity of YAP/TAZ. This review will summarise what we have learnt about the role of intercellular junction-associated proteins in the activation of this pathway, including adherens junctions and tight junctions, and in particular our latest findings about the desmosomal components, including desmoglein-3 (DSG3), in the regulation of YAP signalling, phosphorylation, and subcellular translocation.

Targeting the Hippo/YAP/TAZ signalling pathway: Novel opportunities for therapeutic interventions into skin cancers

Skin cancers are by far the most frequently diagnosed human cancers. The closely related transcriptional co-regulator proteins YAP and TAZ (WWTR1) have emerged as important drivers of tumour initiation, progression and metastasis in melanoma and non-melanoma skin cancers. YAP/TAZ serve as an essential signalling hub by integrating signals from multiple upstream pathways. In this review, we summarize the roles of YAP/TAZ in skin physiology and tumorigenesis and discuss recent efforts of therapeutic interventions that target YAP/TAZ in in both preclinical and clinical settings, as well as their prospects for use as skin cancer treatments.

The steep uphill path leading to ex vivo gene therapy for genodermatoses

Cell therapy, gene therapy and tissue engineering have the potential to revolutionize the field of regenerative medicine. In particular, gene therapy is understood as the therapeutical correction of mutated genes by addition of a correct copy of the gene or site-specific gene modifications. Gene correction of somatic stem cells sustaining renewing tissues is critical to ensure long-term clinical success of ex vivo gene therapy. To date, remarkable clinical outcomes arose from combined ex vivo cell and gene therapy of different genetic diseases, such as immunodeficiencies and genodermatoses. Despite the efforts of researchers around the world, only few of these advanced approaches has yet made it to routine therapy. In fact, gene therapy poses one of the greatest technical challenges in modern medicine, spanning safety and efficacy issues, regulatory constraints, registration and market access, all of which need to be addressed to make the therapy available to rare disease patients. In this review, we survey at some of the main challenges in the development of combined cell and gene therapy of genetic skin diseases.

Mechanotransduction in Skin Inflammation

In the process of mechanotransduction, the cells in the body perceive and interpret mechanical stimuli to maintain tissue homeostasis and respond to the environmental changes. Increasing evidence points towards dysregulated mechanotransduction as a pathologically relevant factor in human diseases, including inflammatory conditions. Skin is the organ that constantly undergoes considerable mechanical stresses, and the ability of mechanical factors to provoke inflammatory processes in the skin has long been known, with the Koebner phenomenon being an example. However, the molecular mechanisms and key factors linking mechanotransduction and cutaneous inflammation remain understudied. In this review, we outline the key players in the tissue’s mechanical homeostasis, the available data, and the gaps in our current understanding of their aberrant regulation in chronic cutaneous inflammation. We mainly focus on psoriasis as one of the most studied skin inflammatory diseases; we also discuss mechanotransduction in the context of skin fibrosis as a result of chronic inflammation. Even though the role of mechanotransduction in inflammation of the simple epithelia of internal organs is being actively studied, we conclude that the mechanoregulation in the stratified epidermis of the skin requires more attention in future translational research.

Receptor activity-modifying protein 1 regulates mouse skin fibroblast proliferation via the Gαi3-PKA-CREB-YAP axis

Background

Skin innervation is crucial for normal wound healing. However, the relationship between nerve receptors and wound healing and the intrinsic mechanism remains to be further identified. In this study, we investigated the role of a calcitonin gene-related peptide (CGRP) receptor component, receptor activity‐modifying protein 1 (RAMP1), in mouse skin fibroblast (MSF) proliferation.

Methods

In vivo, Western blotting and immunohistochemical (IHC) staining of mouse skin wounds tissue was used to detect changes in RAMP1 expression. In vitro, RAMP1 was overexpressed in MSF cell lines by infection with Tet-On-Flag-RAMP1 lentivirus and doxycycline (DOX) induction. An IncuCyte S3 Live-Cell Analysis System was used to assess and compare the proliferation rate differences between different treatment groups. Total protein and subcellular extraction Western blot analysis, quantitative real-time-polymerase chain reaction (qPCR) analysis, and immunofluorescence (IF) staining analysis were conducted to detect signalling molecule expression and/or distribution. The CUT & RUN assay and dual-luciferase reporter assay were applied to measure protein-DNA interactions.

Results

RAMP1 expression levels were altered during skin wound healing in mice. RAMP1 overexpression promoted MSF proliferation. Mechanistically, total Yes-associated protein (YAP) and nuclear YAP protein expression was increased in RAMP1-overexpressing MSFs. RAMP1 overexpression increased inhibitory guanine nucleotide-binding protein (G protein) α subunit 3 (Gαi3) expression and activated downstream protein kinase A (PKA), and both elevated the expression of cyclic adenosine monophosphate (cAMP) response element-binding protein (CREB) and activated it, promoting the transcription of YAP, elevating the total YAP level and promoting MSF proliferation.

Conclusions

Based on these data, we report, for the first time, that changes in the total RAMP1 levels during wound healing and RAMP1 overexpression alone can promote MSF proliferation via the Gαi3-PKA-CREB-YAP axis, a finding critical for understanding RAMP1 function, suggesting that this pathway is an attractive and accurate nerve target for skin wound treatment.

Focusing on Mechanoregulation Axis in Fibrosis: Sensing, Transduction and Effecting

Fibrosis, a pathologic process featured by the excessive deposition of connective tissue components, can affect virtually every organ and has no satisfactory therapy yet. Fibrotic diseases are often associated with organ dysfunction which leads to high morbidity and mortality. Biomechanical stmuli and the corresponding cellular response havebeen identified in fibrogenesis, as the fibrotic remodeling could be seen as the incapacity to reestablish mechanical homeostasis: along with extracellular matrix accumulating, the physical property became more “stiff” and could in turn induce fibrosis. In this review, we provide a comprehensive overview of mechanoregulation in fibrosis, from initialing cellular mechanosensing to intracellular mechanotransduction and processing, and ends up in mechanoeffecting. Our contents are not limited to the cellular mechanism, but further expand to the disorders involved and current clinical trials, providing an insight into the disease and hopefully inspiring new approaches for the treatment of tissue fibrosis.

Context-dependent transcriptional regulations of YAP/TAZ in cancer

As the downstream effectors of Hippo pathway, YAP/TAZ are identified to participate in organ growth, regeneration and tumorigenesis. However, owing to lack of a DNA-binding domain, YAP/TAZ usually act as coactivators and cooperate with other transcription factors or partners to mediate their transcriptional outputs. In this article, we first present an overview of the core components and the upstream regulators of Hippo-YAP/TAZ signaling in mammals, and then systematically summarize the identified transcription factors or partners that are responsible for the downstream transcriptional output of YAP/TAZ in various cancers.

Home Away From Home: Bioengineering Advancements to Mimic the Developmental and Adult Stem Cell Niche

The inherent self-organizing capacity of pluripotent and adult stem cell populations has advanced our fundamental understanding of processes that drive human development, homeostasis, regeneration, and disease progression. Translating these principles into in vitro model systems has been achieved with the advent of organoid technology, driving innovation to harness patient-specific, cell-laden regenerative constructs that can be engineered to augment or replace diseased tissue. While developmental organization and regenerative adult stem cell niches are tightly regulated in vivo, in vitro analogs lack defined architecture and presentation of physicochemical cues, leading to the unhindered arrangement of mini-tissues that lack complete physiological mimicry. This review aims to highlight the recent integrative engineering approaches that elicit spatio-temporal control of the extracellular niche to direct the structural and functional maturation of pluripotent and adult stem cell derivatives. While the advances presented here leverage multi-pronged strategies ranging from synthetic biology to microfabrication technologies, the methods converge on recreating the biochemical and biophysical milieu of the native tissue to be modeled or regenerated.

Trehangelins ameliorate inflammation-induced skin senescence by suppressing the epidermal YAP-CCN1 axis

Trehangelins (THG) are newly identified trehalose compounds derived from broth cultures of an endophytic actinomycete, Polymorphospora rubra. THG are known to suppress Cellular Communication Network factor 1 (CCN1), which regulates collagen homeostasis in the dermis. Although the physical properties of THG suggest a high penetration of the stratum corneum, the effect of THG on the epidermis has not been reported. Here we describe a possible mechanism involved in skin aging focusing on the effect of THG on epidermal CCN1. This study shows that: (1) THG suppress epidermal CCN1 expression by inhibiting the translocation of Yes-Associated Protein (YAP) to nuclei. (2) Epidermal CCN1, localized at the basement membrane, regulates the balance between the growth and differentiation of keratinocytes. (3) Keratinocytes secrete more CCN1 than fibroblasts, which leads to disruption of the basement membrane and extracellular matrix components. (4) The secretion of CCN1 from keratinocytes is increased by ultraviolet B exposure, especially in aged keratinocytes, and deteriorates the elastic fiber structures in the underlying dermis. (5) Topical application of THG ameliorates the structure of the basement membrane in ex vivo human skin explants. Taken together, THG might be a promising treatment for aged skin by suppressing the aberrant YAP-CCN1 axis.

Reciprocal Regulation of Hippo and WBP2 Signalling-Implications in Cancer Therapy

Cancer is a global health problem. The delineation of molecular mechanisms pertinent to cancer initiation and development has spurred cancer therapy in the form of precision medicine. The Hippo signalling pathway is a tumour suppressor pathway implicated in a multitude of cancers. Elucidation of the Hippo pathway has revealed an increasing number of regulators that are implicated, some being potential therapeutic targets for cancer interventions. WW domain-binding protein 2 (WBP2) is an oncogenic transcriptional co-factor that interacts, amongst others, with two other transcriptional co-activators, YAP and TAZ, in the Hippo pathway. WBP2 was recently discovered to modulate the upstream Hippo signalling components by associating with LATS2 and WWC3. Exacerbating the complexity of the WBP2/Hippo network, WBP2 itself is reciprocally regulated by Hippo-mediated microRNA biogenesis, contributing to a positive feedback loop that further drives carcinogenesis. Here, we summarise the biological mechanisms of WBP2/Hippo reciprocal regulation and propose therapeutic strategies to overcome Hippo defects in cancers through targeting WBP2.

Fibroblast Memory in Development, Homeostasis and Disease

Fibroblasts are the major cell population in the connective tissue of most organs, where they are essential for their structural integrity. They are best known for their role in remodelling the extracellular matrix, however more recently they have been recognised as a functionally highly diverse cell population that constantly responds and adapts to their environment. Biological memory is the process of a sustained altered cellular state and functions in response to a transient or persistent environmental stimulus. While it is well established that fibroblasts retain a memory of their anatomical location, how other environmental stimuli influence fibroblast behaviour and function is less clear. The ability of fibroblasts to respond and memorise different environmental stimuli is essential for tissue development and homeostasis and may become dysregulated in chronic disease conditions such as fibrosis and cancer. Here we summarise the four emerging key areas of fibroblast adaptation: positional, mechanical, inflammatory, and metabolic memory and highlight the underlying mechanisms and their implications in tissue homeostasis and disease.

Translational control of stem cell function

Stem cells are characterized by their ability to self-renew and differentiate into many different cell types. Research has focused primarily on how these processes are regulated at a transcriptional level. However, recent studies have indicated that stem cell behaviour is strongly coupled to the regulation of protein synthesis by the ribosome. In this Review, we discuss how different translation mechanisms control the function of adult and embryonic stem cells. Stem cells are characterized by low global translation rates despite high levels of ribosome biogenesis. The maintenance of pluripotency, the commitment to a specific cell fate and the switch to cell differentiation depend on the tight regulation of protein synthesis and ribosome biogenesis. Translation regulatory mechanisms that impact on stem cell function include mTOR signalling, ribosome levels, and mRNA and tRNA features and amounts. Understanding these mechanisms important for stem cell self-renewal and differentiation may also guide our understanding of cancer grade and metastasis.

Regulation of tissue architecture and stem cell dynamics to sustain homeostasis and repair in the skin epidermis

Stratified epithelia are made up of several layers of cells, which act as a protective barrier for the organ they cover. To ensure their shielding effect, epithelia are naturally able to cope with constant environmental insults. This ability is enabled by their morphology and architecture, as well as the continuous turnover of stem and progenitor cells that constitute their building blocks. Stem cell fate decisions and dynamics are fundamental key biological processes that allow epithelia to exert their functions. By focusing on the skin epidermis, this review discusses how tissue architecture is generated during development, maintained through adult life, and re-established during regeneration.

Bladder mesenchymal stromal cell-derived exosomal miRNA-217 modulates bladder cancer cell survival through Hippo-YAP pathway

BACKGROUND

Donor cell-derived exosomes regulate recipient cell functions. The aim of this study was to investigate the effect of human normal bladder stromal cell (hBSC) derived exosomal miR-217 on bladder cell cancer proliferation and migration.

METHODS

Human BSCs were transfected with miR-217 mimic or inhibitor and hBSC-derived exosomes were isolated. Human bladder cancer cell lines (T24 and 5367) were co-cultured with hBSC-derived exosomal miR-217 mimic or inhibitor. Proliferation, migration, and apoptosis of the bladder cancer cells were assessed by Edu assay, Transwell migration assay, and Annexin V assay.

RESULTS

Expression of miR-217 was significantly higher in the T24 and 5367 cell lines (P < 0.01). Exosomal miR-217 mimic enhanced proliferation and migration of T24 and 5367 cells, but inhibited apoptosis of the cells (P < 0.01); in contrast, exosomal miR-217 inhibitor suppressed proliferation and migration but stimulated apoptosis of the two cancer cell lines (P < 0.01). Moreover, exosomal miR-217 mimic stimulated YAP and its target proteins including Cyr61, CTGF, and ANKRD1 (P < 0.01), and in contrast, exosomal miR-217 inhibitor suppressed YAP and its target proteins (P < 0.01).

CONCLUSION

These findings suggested that hBSC-derived exosomal miR-217 may act as oncogene in bladder cancer cells, and that Hippo-YAP signaling pathway maybe the target for miR-217 in the bladder cancer cell lines.

Cellular feedback dynamics and multilevel regulation driven by the hippo pathway

The Hippo pathway is a dynamic cellular signalling nexus that regulates differentiation and controls cell proliferation and death. If the Hippo pathway is not precisely regulated, the functionality of the upstream kinase module is impaired, which increases nuclear localisation and activity of the central effectors, the transcriptional co-regulators YAP and TAZ. Pathological YAP and TAZ hyperactivity consequently cause cancer, fibrosis and developmental defects. The Hippo pathway controls an array of fundamental cellular processes, including adhesion, migration, mitosis, polarity and secretion of a range of biologically active components. Recent studies highlight that spatio-temporal regulation of Hippo pathway components are central to precisely controlling its context-dependent dynamic activity. Several levels of feedback are integrated into the Hippo pathway, which is further synergized with interactors outside of the pathway that directly regulate specific Hippo pathway components. Likewise, Hippo core kinases also ‘moonlight’ by phosphorylating multiple substrates beyond the Hippo pathway and thereby integrates further flexibility and robustness in the cellular decision-making process. This topic is still in its infancy but promises to reveal new fundamental insights into the cellular regulation of this therapeutically important pathway. We here highlight recent advances emphasising feedback dynamics and multilevel regulation of the Hippo pathway with a focus on mitosis and cell migration, as well as discuss potential productive future research avenues that might reveal novel insights into the overall dynamics of the pathway.

A dual role of YAP in driving TGFβ-mediated endothelial-to-mesenchymal transition

Endothelial-to-mesenchymal transition (EndMT) is the biological process through which endothelial cells transdifferentiate into mesenchymal cells. During embryo development, EndMT regulates endocardial cushion formation via TGFβ/BMP signaling. In adults, EndMT is mainly activated during pathological conditions. Hence, it is necessary to characterize molecular regulators cooperating with TGFβ signaling in driving EndMT, to identify potential novel therapeutic targets to treat these pathologies. Here, we studied YAP, a transcriptional co-regulator involved in several biological processes, including epithelial-to-mesenchymal transition (EMT). As EndMT is the endothelial-specific form of EMT, and YAP (herein referring to YAP1) and TGFβ signaling cross-talk in other contexts, we hypothesized that YAP contributes to EndMT by modulating TGFβ signaling. We demonstrate that YAP is required to trigger TGFβ-induced EndMT response, specifically contributing to SMAD3-driven EndMT early gene transcription. We provide novel evidence that YAP acts as SMAD3 transcriptional co-factor and prevents GSK3β-mediated SMAD3 phosphorylation, thus protecting SMAD3 from degradation. YAP is therefore emerging as a possible candidate target to inhibit pathological TGFβ-induced EndMT at early stages.

Summary: A new crucial role for YAP as a co-activator of early pathological TGFβ-mediated endothelial-to-mesenchymal transition program and characterization of the underlying molecular mechanism.

WBP2 promotes gastric cancer cell migration via novel targeting of LATS2 kinase in the Hippo tumor suppressor pathway

Dysregulation of signaling pathways is responsible for many human diseases. The lack of understanding of the molecular etiology of gastric cancer (GC) poses a substantial challenge to the development of effective cancer therapy. To better understand the molecular mechanisms underlying the pathogenesis of GC, which will facilitate the identification and development of effective therapeutic approaches to improve patient outcomes, mass spectrometry-based phosphoproteomics analysis was performed to map the global molecular changes in GC. A total of 530 proteins with altered phosphorylation levels were detected across a panel of 15 normal and GC cell lines. WW domain-binding protein 2 (WBP2) was validated to be upregulated in a subset of GC cell lines. WBP2 is overexpressed in 61% cases of GC compared to non-cancer tissues and high WBP2 expression correlates with poor clinical outcomes. WBP2 was found to be required for GC cell migration but is dispensable for cell growth and proliferation. WBP2 knockdown increased p-LATS2 with a concomitant increase in p-YAP, resulting in the cytoplasmic retention of YAP and ultimately the inhibition of YAP/TEAD activity and downregulation of TEAD target genes--CTGF and CYR61. Importantly, the loss of LATS2 reversed the activation of Hippo pathway caused by WBP2 knockdown, indicating that WBP2 acts through LATS2 to exert its function on the Hippo pathway. Moreover, WBP2 interacted with LATS2 to inhibit its phosphorylation and activity. In conclusion, our study established a pivotal role for WBP2 in the promotion of GC cell migration via a novel mechanism that inactivates the Hippo pathway transducer LATS2.

MPDZ as a novel epigenetic silenced tumor suppressor inhibits growth and progression of lung cancer through the Hippo-YAP pathway

MPDZ also named MUPP1 is involved in signal transduction mediated by the formation of protein complexes. However, the expression regulation, clinical significance, potential function, and mechanism of this gene in lung cancer remain unclear. Methylation status of MPDZ was measured by methylation-specific PCR and bisulfite genomic sequencing. Kaplan-Meier and Cox regression analyses were performed to identify the prognostic value of MPDZ. The tumor suppressing effects of MPDZ were determined in vitro and in vivo. The target molecules and signaling pathway that mediated the function of MPDZ were also identified. MPDZ methylation was identified in 61.2% of primary lung cancer tissues and most lung cancer cell lines but not in normal lung tissues. MPDZ expression was significantly downregulated in lung cancer tissues and negatively associated with DNA hypermethylation, and attenuated MPDZ expression predicted a poor outcome. Furthermore, MPDZ overexpression prominently dampened cell growth, migration, and invasion of tumor cells. Conversely, MPDZ knockdown promoted cell proliferation, migration, and invasion in vitro and in vivo. Moreover, MPDZ deficiency promotes tumor metastasis and reduces the survival of MPDZ knockout mice. Importantly, MPDZ promotes tumor suppressor ability that depends on the Hippo pathway-mediated repression of YAP. MPDZ activates the phosphorylation of YAP (Ser127) and inhibits YAP expression through stabilizing MST1 and interaction with LATS1. We first identified and validated that MPDZ methylation and expression could be a good diagnostic marker and independent prognostic factor for lung cancer. MPDZ functions as a tumor suppressor by inhibiting cell proliferation, migration, and invasion through regulating the Hippo-YAP signaling pathway.

Single-keratinocyte transcriptomic analyses identify different clonal types and proliferative potential mediated by FOXM1 in human epidermal stem cells

Autologous epidermal cultures restore a functional epidermis on burned patients. Transgenic epidermal grafts do so also in genetic skin diseases such as Junctional Epidermolysis Bullosa. Clinical success strictly requires an adequate number of epidermal stem cells, detected as holoclone-forming cells, which can be only partially distinguished from the other clonogenic keratinocytes and cannot be prospectively isolated. Here we report that single-cell transcriptome analysis of primary human epidermal cultures identifies categories of genes clearly distinguishing the different keratinocyte clonal types, which are hierarchically organized along a continuous, mainly linear trajectory showing that stem cells sequentially generate progenitors producing terminally differentiated cells. Holoclone-forming cells display stem cell hallmarks as genes regulating DNA repair, chromosome segregation, spindle organization and telomerase activity. Finally, we identify FOXM1 as a YAP-dependent key regulator of epidermal stem cells. These findings improve criteria for measuring stem cells in epidermal cultures, which is an essential feature of the graft.

WBP2 negatively regulates the Hippo pathway by competitively binding to WWC3 with LATS1 to promote non-small cell lung cancer progression

WW domain binding protein-2 (WBP2) can function as a Yes-associated protein/transcriptional co-activator with PDZ-binding motif (YAP/TAZ) co-activator and has a crucial role in promoting breast cancer progression. However, the expression and potential molecular mechanisms of WBP2 in the context of lung cancer are not fully understood. We determined that WBP2 was highly expressed in lung cancer specimens and cell lines and that this expression was closely related to the advanced pTNM stage, lymph node metastasis, and poor prognosis of patients. In addition, gain- and loss-of-function experiments revealed that WBP2 could significantly promote the proliferation and invasion of lung cancer cells both in vivo and in vitro. To elucidate the underlying molecular mechanism, we determined that wild-type WBP2 could competitively bind to the WW domain of WWC3 (WW and C2 domain-containing-3) with LATS1 (Large tumor suppressor-1) through its PPxY motifs, thus inhibiting the formation of the WWC3-LATS1 complex, reducing the phosphorylation level of LATS1, suppressing the activity of the Hippo pathway, and ultimately promoting YAP nuclear translocation. Therefore, from the aspect of upstream molecules of Hippo signaling, WBP2 promotes the malignant phenotype of lung cancer cells in a unique manner that is not directly dependent upon YAP, thus providing a corresponding experimental basis for the development of targeted therapeutic drugs for lung cancer.

Epigenetic and metabolic regulation of epidermal homeostasis

Continuous exposure of the skin to environmental, mechanical and chemical stress necessitates constant self‐renewal of the epidermis to maintain its barrier function. This self‐renewal ability is attributed to epidermal stem cells (EPSCs), which are long‐lived, multipotent cells located in the basal layer of the epidermis. Epidermal homeostasis – coordinated proliferation and differentiation of EPSCs – relies on fine‐tuned adaptations in gene expression which in turn are tightly associated with specific epigenetic signatures and metabolic requirements. In this review, we will briefly summarize basic concepts of EPSC biology and epigenetic regulation with relevance to epidermal homeostasis. We will highlight the intricate interplay between mitochondrial energy metabolism and epigenetic events – including miRNA‐mediated mechanisms – and discuss how the loss of epigenetic regulation and epidermal homeostasis manifests in skin disease. Discussion of inherited epidermolysis bullosa (EB) and disorders of cornification will focus on evidence for epigenetic deregulation and failure in epidermal homeostasis, including stem cell exhaustion and signs of premature ageing. We reason that the epigenetic and metabolic component of epidermal homeostasis is significant and warrants close attention. Charting epigenetic and metabolic complexities also represents an important step in the development of future systemic interventions aimed at restoring epidermal homeostasis and ameliorating disease burden in severe skin conditions.

The keratin network of intermediate filaments regulates keratinocyte rigidity sensing and nuclear mechanotransduction

The keratin network of intermediate filaments provides keratinocytes with essential mechanical strength and resilience, but the contribution to mechanosensing remains poorly understood. Here, we investigated the role of the keratin cytoskeleton in the response to altered matrix rigidity. We found that keratinocytes adapted to increasing matrix stiffness by forming a rigid, interconnected network of keratin bundles, in conjunction with F-actin stress fiber formation and increased cell stiffness. Disruption of keratin stability by overexpression of the dominant keratin 14 mutation R416P inhibited the normal mechanical response to substrate rigidity, reducing F-actin stress fibers and cell stiffness. The R416P mutation also impaired mechanotransduction to the nuclear lamina, which mediated stiffness-dependent chromatin remodeling. By contrast, depletion of the cytolinker plectin had the opposite effect and promoted increased mechanoresponsiveness and up-regulation of lamin A/C. Together, these results demonstrate that the keratin cytoskeleton plays a key role in matrix rigidity sensing and downstream signal transduction.

The role of Hippo-YAP signaling in squamous cell carcinomas

The Hippo‐YAP pathway regulates organ size, tissue homeostasis, and tumorigenesis in mammals. In response to cell density, external mechanical pressure, and/or other stimuli, the Hippo core complex controls the translocation of YAP1/TAZ proteins to the nucleus and thereby regulates cell growth. Abnormal upregulation or nuclear localization of YAP1/TAZ occurs in many human malignancies and promotes their formation, progression, and metastasis. A key example is squamous cell carcinoma (SCC) genesis. Many risk factors and crucial signals associated with SCC development in various tissues accelerate YAP1/TAZ accumulation, and mice possessing constitutively activated YAP1/TAZ show immediate carcinoma in situ (CIS) formation in these tissues. Because CIS onset is so rapid in these mutants, we propose that many SCCs initiate and progress when YAP1 activity is sustained and exceeds a certain oncogenic threshold. In this review, we summarize the latest findings on the roles of YAP1/TAZ in several types of SCCs. We also discuss whether targeting aberrant YAP1/TAZ activation might be a promising strategy for SCC treatment.

Under pressure: Stem cell-niche interactions coordinate tissue adaptation to inflammation

Stem and progenitor cells (SCs) are emerging as key drivers of tissue adaptation to inflammation caused by microbes, injury, noxious agents, and other onslaughts. These pressures are most acutely experienced in epithelial tissues such as the skin and gut that interface with the external environment. Thus, here we review how epithelial SCs of the skin and intestine, along with their supportive niches, sense and respond to inflammation for the sake of preserving tissue integrity. We highlight inflammation-induced plasticity in SCs and their progeny and the lasting memory that forms thereafter. The burgeoning area of SC responses to inflammatory stressors may expand therapeutic perspectives in epithelial inflammatory conditions, wound repair, cancers, and aging.

Role of YAP/TAZ in Cell Lineage Fate Determination and Related Signaling Pathways

The penultimate effectors of the Hippo signaling pathways YAP and TAZ, are transcriptional co-activator proteins that play key roles in many diverse biological processes, ranging from cell proliferation, tumorigenesis, mechanosensing and cell lineage fate determination, to wound healing and regeneration. In this review, we discuss the regulatory mechanisms by which YAP/TAZ control stem/progenitor cell differentiation into the various major lineages that are of interest to tissue engineering and regenerative medicine applications. Of particular interest is the key role of YAP/TAZ in maintaining the delicate balance between quiescence, self-renewal, proliferation and differentiation of endogenous adult stem cells within various tissues/organs during early development, normal homeostasis and regeneration/healing. Finally, we will consider how increasing knowledge of YAP/TAZ signaling might influence the trajectory of future progress in regenerative medicine.

The emerging roles of WBP2 oncogene in human cancers

WW domain-binding protein 2 (WBP2) is an emerging oncoprotein. Over the past decade, WBP2 surfaced as a key node connecting key signaling pathways associated with ER/PR, EGFR, PI3K, Hippo, and Wnt in cancer. In addition to the oncogenic functions of WBP2, this review discusses the latest research regarding the multilevel regulation and modes of action of WBP2 and how they can be exploited for molecular medicine. In translational research, evidence supports the role of WBP2 as a biomarker for early detection, prognosis, and companion diagnostics in breast cancer. Finally, we envision new trends in WBP2 research in the space of molecular etiology of cancer, targeted therapeutics, and precision medicine.

Mechanochemical Feedback Loops in Development and Disease

There is increasing evidence that both mechanical and biochemical signals play important roles in development and disease. The development of complex organisms, in particular, has been proposed to rely on the feedback between mechanical and biochemical patterning events. This feedback occurs at the molecular level via mechanosensation but can also arise as an emergent property of the system at the cellular and tissue level. In recent years, dynamic changes in tissue geometry, flow, rheology, and cell fate specification have emerged as key platforms of mechanochemical feedback loops in multiple processes. Here, we review recent experimental and theoretical advances in understanding how these feedbacks function in development and disease.

YAP1/TAZ-TEAD transcriptional networks maintain skin homeostasis by regulating cell proliferation and limiting KLF4 activity

The Hippo TEAD-transcriptional regulators YAP1 and TAZ are central for cell renewal and cancer growth; however, the specific downstream gene networks involved in their activity are not completely understood. Here we introduce TEADi, a genetically encoded inhibitor of the interaction of YAP1 and TAZ with TEAD, as a tool to characterize the transcriptional networks and biological effects regulated by TEAD transcription factors. Blockage of TEAD activity by TEADi in human keratinocytes and mouse skin leads to reduced proliferation and rapid activation of differentiation programs. Analysis of gene networks affected by TEADi and YAP1/TAZ knockdown identifies KLF4 as a central transcriptional node regulated by YAP1/TAZ-TEAD in keratinocyte differentiation. Moreover, we show that TEAD and KLF4 can regulate the activity of each other, indicating that these factors are part of a transcriptional regulatory loop. Our study establishes TEADi as a resource for studying YAP1/TAZ-TEAD dependent effects.

NUAK2 localization in normal skin and its expression in a variety of skin tumors with YAP

BACKGROUND

NUAK2 is a critical gene that participates in the carcinogenesis of various types of cancers including melanomas. However, the expression patterns of NUAK2 in normal skin and in various types of skin tumors have not been fully elucidated to date.

OBJECTIVES

To elucidate the distribution and localization of NUAK2 expression in normal skin, and characterize the expression patterns of NUAK2 and YAP in various types of skin tumors.

METHODS

In this study, we characterized the expression of NUAK2 in tissues by developing a novel NUAK2-specific monoclonal antibody and using that to determine NUAK2 expression patterns in normal skin and in 155 cases of various types of skin tumors, including extramammary Paget's disease (EMPD), squamous cell carcinoma (SCC), Bowen's disease (BD), actinic keratosis (AK), basal cell carcinoma (BCC) and angiosarcoma (AS). Further, we analyzed the expression patterns of YAP and p-Akt in those tumors.

RESULTS

Our analyses revealed that NUAK2 is expressed at high frequencies in EMPD, SCC, BD, AK, BCC and AS. The expression of p-Akt was positively correlated with tumor size in EMPD (P = 0.001). Importantly, the expression of NUAK2 was significantly correlated with YAP in SCC (P = 0.012) and in BD (P = 0.009).

CONCLUSIONS

Our results suggest that the YAP-NUAK2 axis has critical importance in the tumorigenesis of SCC and BD, and that therapeutic modalities targeting the YAP-NUAK2 axis may be an effective approach against skin tumors including SCC and BD.

Long-Term In Vitro Expansion of Epithelial Stem Cells Enabled by Pharmacological Inhibition of PAK1-ROCK-Myosin II and TGF-β Signaling

Despite substantial self-renewal capability in vivo, epithelial stem and progenitor cells located in various tissues expand for a few passages in vitro in feeder-free condition before they succumb to growth arrest. Here, we describe the EpiX method, which utilizes small molecules that inhibit PAK1-ROCK-Myosin II and TGF-β signaling to achieve over one trillion-fold expansion of human epithelial stem and progenitor cells from skin, airway, mammary, and prostate glands in the absence of feeder cells. Transcriptomic and epigenomic studies show that this condition helps epithelial cells to overcome stresses for continuous proliferation. EpiX-expanded basal epithelial cells differentiate into mature epithelial cells consistent with their tissue origins. Whole-genome sequencing reveals that the cells retain remarkable genome integrity after extensive in vitro expansion without acquiring tumorigenicity. EpiX technology provides a solution to exploit the potential of tissue-resident epithelial stem and progenitor cells for regenerative medicine.

Activation of G-Protein Coupled Receptor-Gαi Signaling Increases Keratinocyte Proliferation and Reduces Differentiation, Leading to Epidermal Hyperplasia

G-protein coupled receptors (GPCRs) and their associated heterotrimeric G proteins impinge on pathways that control epithelial cell self-renewal and differentiation. Although it is known that Gαs protein signaling regulates skin homeostasis in vivo, the role of GPCR-coupled Gαi proteins in the skin is unclear. Here, by using a chemogenetic approach, we demonstrate that GPCR-Gαi activation can regulate keratinocyte proliferation and differentiation and that overactivation of Gαi-signaling in the basal compartment of the mouse skin can lead to epidermal hyperplasia. Our results expand our understanding of the role of GPCR-cAMP signaling in skin homeostasis and reveal overlapping and divergent roles of the cAMP-regulating heterotrimeric Gαs and Gαi proteins in keratinocytes.

The Hippo Pathway Effector TAZ Regulates Ferroptosis in Renal Cell Carcinoma

Despite recent advances, the poor outcomes in renal cell carcinoma (RCC) suggest novel therapeutics are needed. Ferroptosis is a form of regulated cell death, which may have therapeutic potential toward RCC; however, much remains unknown about the determinants of ferroptosis susceptibility. We found that ferroptosis susceptibility is highly influenced by cell density and confluency. Because cell density regulates the Hippo-YAP/TAZ pathway, we investigated the roles of the Hippo pathway effectors in ferroptosis. TAZ is abundantly expressed in RCC and undergoes density-dependent nuclear or cytosolic translocation. TAZ removal confers ferroptosis resistance, whereas overexpression of TAZS89A sensitizes cells to ferroptosis. Furthermore, TAZ regulates the expression of Epithelial Membrane Protein 1 (EMP1), which, in turn, induces the expression of nicotinamide adenine dinucleotide phosphate (NADPH) Oxidase 4 (NOX4), a renal-enriched reactive oxygen species (ROS)-generating enzyme essential for ferroptosis. These findings reveal that cell density-regulated ferroptosis is mediated by TAZ through the regulation of EMP1-NOX4, suggesting its therapeutic potential for RCC and other TAZ-activated tumors.

Delta-like 1-mediated cis-inhibition of Jagged1/2 signalling inhibits differentiation of human epidermal cells in culture

Epidermal homeostasis depends on a balance between self-renewal of stem cells and terminal differentiation of their progeny. Notch signalling is known to play a role in epidermal  stem cell patterning and differentiation. However, the molecular mechanisms are incompletely understood. Here we demonstrate dynamic patterns of Notch ligand and receptor expression in cultured human epidermis. Notch2 and 3 act together to promote differentiation, while Notch1 decreases stem cell proliferation. The Notch ligand Jagged1 triggers differentiation when presented on an adhesive substrate or on polystyrene beads and over-rides the differentiation inhibitory effect of cell spreading. In contrast, Delta-like 1 (Dll1) overexpression abrogates the pro-differentiation effect of Jagged1 in a cell autonomous fashion. We conclude that Dll1 expression by stem cells not only stimulates differentiation of neighbouring cells in trans, but also inhibits differentiation cell autonomously. These results highlight the distinct roles of different Notch receptors and ligands in controlling epidermal homeostasis.

Protein kinases involved in epidermal barrier formation: The AKT family and other animals

Formation of a stratified epidermis is required for the performance of the essential functions of the skin; to act as an outside-in barrier against the access of microorganisms and other external factors, to prevent loss of water and solutes via inside-out barrier functions and to withstand mechanical stresses. Epidermal barrier function is initiated during embryonic development and is then maintained throughout life and restored after injury. A variety of interrelated processes are required for the formation of a stratified epidermis, and how these processes are both temporally and spatially regulated has long been an aspect of dermatological research. In this review, we describe the roles of multiple protein kinases in the regulation of processes required for epidermal barrier formation.

Small Molecule and Pooled CRISPR Screens Investigating IL17 Signaling Identify BRD2 as a Novel Contributor to Keratinocyte Inflammatory Responses

Interleukin-17A (IL17A) plays a critical role in the development of numerous autoimmune diseases, including psoriasis. The clinical success of IL17A neutralizing biologics in psoriasis has underlined its importance as a drug discovery target. While many studies have focused on the differentiation and trafficking of IL17A producing T-helper 17 cells, less is known about IL17A-initiated signaling events in stromal and parenchymal cells leading to psoriatic phenotypes. We sought to discover signaling nodes downstream of IL17A contributing to disease pathogenesis. Using IL17A and tumor necrosis factor α (TNF) to stimulate primary human epidermal keratinocytes, we employed two different phenotypic screening approaches. First, a library of ∼22000 annotated compounds was screened for reduced secretion of the pro-inflammatory chemokine IL8. Second, a library of 729 kinases was screened in a pooled format by utilizing CRISPR-Cas9 and monitoring IL8 intracellular staining. The highest-ranking novel hits identified in both screens were the bromodomain and extra-terminal domain (BET) family proteins and bromodomain-containing protein 2 (BRD2), respectively. Comparison of BRD2, BRD3, and BRD4 silencing with siRNA and CRISPR confirmed that BRD2 was responsible for mediating IL8 production. Pan-BRD inhibitors and BRD2 knockout also reduced IL17A/TNF-mediated CXC motif chemokines 1/2/6 (CXCL1/2/6) and granulocyte colony stimulating factor (G-CSF) production. In RNA-Seq analysis, 438 IL17A/TNF dependent genes were reduced in BRD2-deficient primary keratinocytes. KEGG pathway analysis of these genes showed enrichment in TNF signaling and rheumatoid arthritis relevant genes. Moreover, a number of genes important for keratinocyte homeostasis and cornification were dysregulated in BRD2-deficient keratinocytes. In IL17A/TNF/IL22 stimulated three-dimensional organotypic raft cultures, pan-BRD inhibition reduced inflammatory factor production but elicited aberrant cornification, consistent with RNA-Seq analysis. These studies highlight a novel role for BRDs and BRD2 in particular in IL17A-mediated inflammatory signaling.

Laminin 332-Dependent YAP Dysregulation Depletes Epidermal Stem Cells in Junctional Epidermolysis Bullosa

Laminin 332-deficient junctional epidermolysis bullosa (JEB) is a severe genetic skin disease. JEB is marked by epidermal stem cell depletion, the origin of which is unknown. We show that dysregulation of the YAP and TAZ pathway underpins such stem cell depletion. Laminin 332-mediated YAP activity sustains human epidermal stem cells, detected as holoclones. Ablation of YAP selectively depletes holoclones, while enforced YAP blocks conversion of stem cells into progenitors and indefinitely extends the keratinocyte lifespan. YAP is dramatically decreased in JEB keratinocytes, which contain only phosphorylated, inactive YAP. In normal keratinocytes, laminin 332 and α6β4 ablation abolish YAP activity and recapitulate the JEB phenotype. In JEB keratinocytes, laminin 332-gene therapy rescues YAP activity and epidermal stem cells in vitro and in vivo. In JEB cells, enforced YAP recapitulates laminin 332-gene therapy, thus uncoupling adhesion from proliferation in epidermal stem cells. This work has important clinical implication for ex vivo gene therapy of JEB.

Targeted Therapy Against the Cell of Origin in Cutaneous Squamous Cell Carcinoma

Squamous cell carcinomas (SCC), including cutaneous SCCs, are by far the most frequent cancers in humans, accounting for 80% of all newly diagnosed malignancies worldwide. The old dogma that SCC develops exclusively from stem cells (SC) has now changed to include progenitors, transit-amplifying and differentiated short-lived cells. Accumulation of specific oncogenic mutations is required to induce SCC from each cell population. Whilst as fewer as one genetic hit is sufficient to induce SCC from a SC, multiple events are additionally required in more differentiated cells. Interestingly, the level of differentiation correlates with the number of transforming events required to induce a stem-like phenotype, a long-lived potential and a tumourigenic capacity in a progenitor, a transient amplifying or even in a terminally differentiated cell. Furthermore, it is well described that SCCs originating from different cells of origin differ not only in their squamous differentiation status but also in their malignant characteristics. This review summarises recent findings in cutaneous SCC and highlights transforming oncogenic events in specific cell populations. It underlines oncogenes that are restricted either to stem or differentiated cells, which could provide therapeutic target selectivity against heterogeneous SCC. This strategy may be applicable to SCC from different body locations, such as head and neck SCCs, which are currently still associated with poor survival outcomes.