Fibroblast growth factor receptors 1 and 2 in keratinocytes control the epidermal barrier and cutaneous homeostasis

Dose-dependent effects of Nrf2 on the epidermis in chronic skin inflammation

Atopic dermatitis (AD) is a chronic inflammatory skin disease, characterized by an impaired epidermal barrier and immunological alterations. The activity of the cytoprotective NRF2 transcription factor is reduced in the epidermis of AD patients. To determine the functional relevance of this deficiency, we used mice lacking fibroblast growth factor receptors 1 and 2 in keratinocytes (K5-R1/R2 mice), which exhibit several AD-like symptoms. Proteomics analysis of their epidermis revealed reduced Nrf2 activity. This was accompanied by an increase in DNA damage and in the number of senescent cells. Genetic deletion of Nrf2 in keratinocytes of these mice further promoted DNA damage and senescence, but time-limited pharmacological activation of Nrf2 in the skin had a mild protective effect. Surprisingly, long-term genetic activation of Nrf2 in keratinocytes of K5-R1/R2 mice caused strong hyperkeratosis, keratinocyte hyperproliferation, epidermal thickening, increased keratinocyte apoptosis and DNA damage, and altered immune cell composition. These results reveal a complex role of Nrf2 in the epidermis and show the necessity to optimize the duration and intensity of NRF2 activation for the treatment of epidermal alterations in patients with AD.

FGFR2 in the Development and Progression of Cutaneous Squamous Cell Cancer

Cutaneous squamous cell carcinoma (cSCC) is an increasingly common malignancy of the skin and the leading cause of death from skin cancer in adults over the age of 85. Fibroblast growth factor receptor 2 (FGFR2) has been identified as an important effector of signaling pathways that lead to the growth and development of cSCC. In recent years, there have been numerous studies evaluating the role FGFR2 plays in multiple cancers, its contribution to resistance to anticancer therapy, and new drugs that may be used to inhibit FGFR2. This review will provide an overview of our current understanding of FGFR2 and potential mechanisms in which we can target FGFR2 in cSCC. The goals of this review are the following: (1) to highlight our current knowledge of the role of FGFR2 in healthy skin and contrast this with its role in the development of cancer; (2) to further explain the specific molecular mechanisms that FGFR2 uses to promote tumorigenesis; (3) to describe how FGFR2 contributes to more invasive disease; (4) to describe its immunosuppressive effects in skin; and (5) to evaluate its effect on current anticancer therapy and discuss therapies on the horizon to target FGFR2 related malignancy.

FGF receptor kinase inhibitors exhibit broad antiviral activity by targeting Src family kinases

The development of antiviral strategies is a key task of biomedical research, but broad-spectrum virus inhibitors are scarce. Here we show that fibroblast growth factor receptor (FGFR) tyrosine kinase inhibitors reduce infection of several cell types with DNA and RNA viruses by blocking early stages of infection, but not viral cell association. Unexpectedly, their antiviral activity was largely independent of FGFR kinase inhibition. RNA profiling showed upregulation of interferon response genes by FGFR inhibitors, but their expression did not correlate with the antiviral activity in infected cells. Using bioinformatics analysis of kinome data, targeted kinase assays, siRNA-mediated knock-down and pharmacological inhibition experiments, we show that blockade of Src family kinases, in particular Lyn, is mainly responsible for the antiviral activity of FGFR inhibitors. These results identify FGFR inhibitors as broad-spectrum antiviral agents and suggest the poorly studied Lyn kinase as a promising target for the treatment of viral infections.

Inflammatory Mediators Suppress FGFR2 Expression in Human Keratinocytes to Promote Inflammation

Fibroblast growth factors (FGFs) are key orchestrators of development, tissue homeostasis and repair. FGF receptor (FGFR) deficiency in mouse keratinocytes causes an inflammatory skin phenotype with similarities to atopic dermatitis, but the human relevance is unclear. Therefore, we generated human keratinocytes with a CRISPR/Cas9-induced knockout of FGFR2. Loss of this receptor promoted the expression of interferon-stimulated genes and pro-inflammatory cytokines under homeostatic conditions and in particular in response to different inflammatory mediators. Expression of FGFR2 itself was strongly downregulated in cultured human keratinocytes exposed to various pro-inflammatory stimuli. This is relevant in vivo, because bioinformatics analysis of bulk and single-cell RNA-seq data showed strongly reduced expression of FGFR2 in lesional skin of atopic dermatitis patients, which likely aggravates the inflammatory phenotype. These results reveal a key function of FGFR2 in human keratinocytes in the suppression of inflammation and suggest a role of FGFR2 downregulation in the pathogenesis of atopic dermatitis and possibly other inflammatory diseases.

Self-Assembled Fibroblast Growth Factor Nanoparticles as a Therapeutic for Oxidant-Induced Neuronal and Skin Cell Injury

Traumatic brain injury (TBI) and spinal cord injury (SCI) are neurological conditions that result from immediate mechanical injury, as well as delayed injury caused by local inflammation. Furthermore, TBI and SCI often lead to secondary complications, including pressure wounds of the skin, which can heal slowly and are prone to infection. Pressure wounds are localized areas of damaged tissue caused by prolonged pressure on the skin due to immobility and loss of neurological sensation. With the aim to ameliorate these symptoms, we investigated whether fibroblast growth factors 2 (FGF-2) could contribute to recovery. FGF-2 plays a significant role in both neurogenesis and skin wound healing. We developed a recombinant fusion protein containing FGF-2 linked to elastin-like polypeptides (FGF-ELP) that spontaneously self-assembles into nanoparticles at around 33 °C. The nanoparticle's size was ranging between 220 and 250 nm in diameter at 2 μM. We tested this construct for its ability to address neuronal and skin cell injuries. Hydrogen peroxide was used to induce oxidant-mediated injury on cultured neuronal cells to mimic the impact of reactive oxidants released during the inflammatory response in vivo. We found that FGF-ELP nanoparticles protected against hydrogen peroxide-mediated injury and promoted neurite outgrowth. In the skin cell models, cells were depleted from serum to mimic the reduced levels of nutrients and growth factors in chronic skin wounds. FGF-ELP increased the proliferation and migration of human keratinocytes, fibroblasts, and endothelial cells. FGF-ELP is, therefore, a potentially useful agent to provide both neuroprotection and promotion of cellular processes involved in skin wound healing.

Systemic Inflammatory Proteomic Biomarkers in Atopic Dermatitis: Exploring Potential Indicators for Disease Severity

BACKGROUND

Atopic dermatitis (AD) is a chronic inflammatory cutaneous disorder, that emerges from intricate interplays among genetic predisposition, immune dysregulation, environmental factors, and compromised skin barrier. Understanding the inflammatory pathway in AD is important due to its fundamental role in the pathogenesis of AD. This study aimed to explore the diverse spectrum of proteins linked to the inflammation of AD and the relationship between systemic biomarkers and clinical severity in AD.

METHODS

We examined the blood samples from 48 patients with AD and 48 healthy controls (HCs) using the Proximity Extension Assay (Olink). Differentially expressed proteins (DEPs) were identified and Pearson correlation analysis was conducted to determine systemic proteomic biomarkers associated with severity of AD.

RESULTS

A total of 29 DEPs were significantly up-regulated and 2 DEPs were significantly down-regulated in AD compared with the HC. The MCP-4, IL-18, MCP-3, TNFRSF9, and IL-17C were the top 5 highest DEPs associated with the severity of AD.

CONCLUSION

Our study sheds light on the intricate network of inflammatory proteins in AD and their potential implications for disease severity. Our results indicate that these systemic inflammatory proteins could be valuable for assessing AD severity and enhancing our understanding of the disease's complexity and its potential management strategies.

Targeting FGFR for cancer therapy

The FGFR signaling pathway is integral to cellular activities, including proliferation, differentiation, and survival. Dysregulation of this pathway is implicated in numerous human cancers, positioning FGFR as a prominent therapeutic target. Here, we conduct a comprehensive review of the function, signaling pathways and abnormal alterations of FGFR, as well as its role in tumorigenesis and development. Additionally, we provide an in-depth analysis of pivotal phase 2 and 3 clinical trials evaluating the performance and safety of FGFR inhibitors in oncology, thereby shedding light on the current state of clinical research in this field. Then, we highlight four drugs that have been approved for marketing by the FDA, offering insights into their molecular mechanisms and clinical achievements. Our discussion encompasses the intricate landscape of FGFR-driven tumorigenesis, current techniques for pinpointing FGFR anomalies, and clinical experiences with FGFR inhibitor regimens. Furthermore, we discuss the inherent challenges of targeting the FGFR pathway, encompassing resistance mechanisms such as activation by gatekeeper mutations, alternative pathways, and potential adverse reactions. By synthesizing the current evidence, we underscore the potential of FGFR-centric therapies to enhance patient prognosis, while emphasizing the imperative need for continued research to surmount resistance and optimize treatment modalities.

Ancestry dynamics and trait selection in a designer cat breed

The Bengal cat breed was developed from intercrosses between the Asian leopard cat, Prionailurus bengalensis, and the domestic cat, Felis catus, with a last common ancestor approximately 6 million years ago. Predicted to derive ∼94% of their genome from domestic cats, regions of the leopard cat genome are thought to account for the unique pelage traits and ornate color patterns of the Bengal breed, which are similar to those of ocelots and jaguars. We explore ancestry distribution and selection signatures in the Bengal breed by using reduced representation and whole-genome sequencing from 947 cats. The mean proportion of leopard cat DNA in the Bengal breed is 3.48%, lower than predicted from breed history, and is broadly distributed, covering 93% of the Bengal genome. Overall, leopard cat introgressions do not show strong signatures of selection across the Bengal breed. However, two popular color traits in Bengal cats, charcoal and pheomelanin intensity, are explained by selection of leopard cat genes whose expression is reduced in a domestic cat background, consistent with genetic incompatibility resulting from hybridization. We characterize several selective sweeps in the Bengal genome that harbor candidate genes for pelage and color pattern and that are associated with domestic, rather than leopard, cat haplotypes. We identify the molecular and phenotypic basis of one selective sweep as reduced expression of the Fgfr2 gene, which underlies glitter, a trait desired by breeders that affects hair texture and light reflectivity.

Emerging Role of the IL-36/IL-36R Axis in Multiple Inflammatory Skin Diseases

IL-36 is a most recent member of the IL-1 cytokine family, primarily expressed at barrier sites of the body such as the skin, lungs, and intestine. It plays a vital role in inflammation and is implicated in the development of various cutaneous; intestinal; and pulmonary disorders, including psoriasis, inflammatory bowel disease, and chronic obstructive pulmonary disease. IL-36 comprises 4 isoforms: the proinflammatory IL-36α, IL-36β, and IL-36γ and the anti-inflammatory IL-36R antagonist. An imbalance between proinflammatory and anti-inflammatory IL-36 isoforms can contribute to the inflammatory fate of cells and tissues. IL-36 cytokines signal through an IL-36R heterodimer mediating their function through canonical signaling cacade, including the NF-B pathway. Prominent for its role in psoriasis, IL-36 has recently been associated with disease mechanisms in atopic dermatitis, hidradenitis suppurativa, neutrophilic dermatoses, autoimmune blistering disease, and Netherton syndrome. The major cutaneous source of IL-36 cytokines is keratinocytes, pointing to its role in the communication between the epidermis, innate (neutrophils, dendritic cells) immune system, and adaptive (T helper [Th]1 cells, Th17) immune system. Thus, cutaneous IL-36 signaling is crucial for the immunopathological outcome of various skin diseases. Consequently, the IL-36/IL-36R axis has recently been recognized as a promising drug target for the treatment of inflammatory disorders beyond psoriasis. This review summarizes the current update on IL-36 cytokines in inflammatory skin diseases.

Ocular toxicities of fibroblast growth factor receptor inhibitors: A review

Fibroblast growth factor receptor (FGFR) inhibitors are an emerging class of small molecule targeted cancer drugs with promising therapeutic possibilities for a wide variety of malignancies. While ocular adverse events from FGFR inhibitors are reported in clinical trials, subsequent case studies continue to reveal new toxicities. Disease pathology affecting multiple parts of the eye has been reported, but the ocular surface and the retina are the most commonly encountered areas affected by FGFR inhibitors, manifesting as dry eye and FGFR inhibitor-associated retinopathy, respectively. Corneal thinning and melt is a rare but serious and potentially vision-threatening complication of FGFR inhibitor toxicity. Similarities between toxicities observed from other targeted cancer therapy drugs and FGFR inhibitors may help us understand underlying pathophysiological changes. The management of these adverse events requires close ophthalmologic follow-up and may require discontinuation of the offending agents in some cases.

Adverse events in patients with advanced urothelial carcinoma treated with erdafitinib: a retrospective pharmacovigilance study

Purpose: On 12 April 2019, erdafitinib gained the first FDA approval as the second-line treatment for adult patients with locally advanced or metastatic urothelial cancer following progression during or after at least one previous line of platinum-based chemotherapy. However, the long-term safety profile of erdafitinib in a large patient population remains unexplored. The current study aimed to assess the adverse events (AEs) associated with erdafitinib through data mining of the US Food and Drug Administration Adverse Event Reporting System (FAERS). Method: The reporting odds ratio (ROR), the proportional reporting ratio (PRR), the Bayesian confidence propagation neural network (BCPNN), and the multi-item gamma Poisson shrinker (MGPS) algorithms based on disproportionality were employed to quantify the signals of erdafitinib-associated AEs. Results: A total of 6,322,279 reports of AEs were retrieved from the FAERS database spanning 2019 to 2022, out of which, 700 reports of erdafitinib as the "primary suspected" were identified. These erdafitinib-induced AEs were observed across 24 targeted system organ classes (SOCs). After conforming to the four algorithms at the same time, a total of 441 signals of erdafitinib-induced AEs were detected across 23 SOCs. Notably, signals associated with metabolism and nutrition disorders, eye disorders, and skin and subcutaneous tissue disorders were among the most prevalent. The median onset time for AEs was found to be 54 days [interquartile range (IQR) 17-112 days], with a majority of AEs occurring within the initial 6 months after initiating erdafitinib (37.23% within the first month, 15.53% within the second month, and 16.79% within the third month). Conclusion: The findings of this study align with existing clinical observations, offering a comprehensive long-term post-marketing safety evaluation of erdafitinib. The results provide valuable evidence to enhance the understanding of erdafitinib's safety profile, aiding further research and guiding clinical practice.

FGFR2 is essential for salivary gland duct homeostasis and MAPK-dependent seromucous acinar cell differentiation

Exocrine acinar cells in salivary glands (SG) are critical for oral health and loss of functional acinar cells is a major clinical challenge. Fibroblast growth factor receptors (FGFR) are essential for early development of multiple organs, including SG. However, the role of FGFR signaling in specific populations later in development and during acinar differentiation are unknown. Here, we use scRNAseq and conditional deletion of murine FGFRs in vivo to identify essential roles for FGFRs in craniofacial, early SG development and progenitor function during duct homeostasis. Importantly, we also discover that FGFR2 via MAPK signaling is critical for seromucous acinar differentiation and secretory gene expression, while FGFR1 is dispensable. We show that FGF7, expressed by myoepithelial cells (MEC), activates the FGFR2-dependent seromucous transcriptional program. Here, we propose a model where MEC-derived FGF7 drives seromucous acinar differentiation, providing a rationale for targeting FGFR2 signaling in regenerative therapies to restore acinar function.

Safety analysis of pemigatinib leveraging the US Food and Drug administration adverse event reporting system

Background: Cholangiocarcinoma (CCA) is a highly lethal and aggressive epithelial tumor of the hepatobiliary system. A poor prognosis, propensity for relapse, low chance of cure and survival are some of its hallmarks. Pemigatinib, the first targeted treatment for CCA in the United States, has been demonstrated to have a significant response rate and encouraging survival data in early-phase trials. The adverse events (AEs) of pemigatinib must also be determined. Objective: To understand more deeply the safety of pemigatinib in the real world through data-mining of the US Food and Drug Administration (FDA) Adverse Event Reporting System (FAERS). Methods: Disproportionality analysis was employed in a retrospective pharmacovigilance investigation to identify the AEs linked to pemigatinib use as signals. Data were collected between 1 January 2020 to 30 June 2022. Four data-mining methods (proportional reporting odds ratio; proportional reporting ratio; Bayesian confidence propagation neural networks of information components; empirical Bayes geometric means) were used to calculate disproportionality. Results: A total of 203 cases using pemigatinib as the prime-suspect medication were found in our search, which involved 99 preferred terms (PTs). Thirteen signals of pemigatinib-induced AEs in seven System Organ Classes were detected after confirming the four algorithms simultaneously. Nephrolithiasis was an unexpected significant AE not listed on the drug label found in our data-mining. Comparison of the differences between pemigatinib and platinum drugs in terms of 33 PTs revealed that 13 PTs also met the criteria of the four algorithms. Ten of these PTs were identical to those compared with all other drugs, in which (excluding a reduction in phosphorus in blood) other PT signal values were higher than those of all other drugs tested. However, comparison of the differences between pemigatinib and infigratinib in terms of the 33 PTs revealed no significant signals in each algorithm method. Conclusion: Some significant signals were detected between pemigatinib use and AEs. PTs with apparently strong signals and PTs not mentioned in the label should be taken seriously.

Spatial transcriptomics stratifies psoriatic disease severity by emergent cellular ecosystems

Whereas the cellular and molecular features of human inflammatory skin diseases are well characterized, their tissue context and systemic impact remain poorly understood. We thus profiled human psoriasis (PsO) as a prototypic immune-mediated condition with a high predilection for extracutaneous involvement. Spatial transcriptomics (ST) analyses of 25 healthy, active lesion, and clinically uninvolved skin biopsies and integration with public single-cell transcriptomics data revealed marked differences in immune microniches between healthy and inflamed skin. Tissue-scale cartography further identified core disease features across all active lesions, including the emergence of an inflamed suprabasal epidermal state and the presence of B lymphocytes in lesional skin. Both lesional and distal nonlesional samples were stratified by skin disease severity and not by the presence of systemic disease. This segregation was driven by macrophage-, fibroblast-, and lymphatic-enriched spatial regions with gene signatures associated with metabolic dysfunction. Together, these findings suggest that mild and severe forms of PsO have distinct molecular features and that severe PsO may profoundly alter the cellular and metabolic composition of distal unaffected skin sites. In addition, our study provides a valuable resource for the research community to study spatial gene organization of healthy and inflamed human skin.

FGFR2b is essential for salivary gland duct homeostasis and MAPK-dependent seromucous acinar cell differentiation

Exocrine secretory acinar cells in salivary glands (SG) are critical for oral health and loss of functional acinar cells is a major clinical challenge. Fibroblast growth factor receptors (FGFR) are essential for early development of multiple organs, including SG. However, the role of FGFR signaling in specific epithelial SG populations later in development and during acinar differentiation are unknown. Here, we predicted FGFR dependence in specific populations using scRNAseq data and conditional mouse models to delete FGFRs in vivo. We identifed essential roles for FGFRs in craniofacial and early SG development, as well as progenitor function during duct homeostasis. Importantly, we discovered that FGFR2b was critical for seromucous and serous acinar cell differentiation and secretory gene expression (Bpifa2 and Lpo) via MAPK signaling, while FGFR1b was dispensable. We show that FGF7, expressed by myoepithelial cells (MEC), activated the FGFR2b-dependent seromucous transcriptional program. We propose a model where MEC-derived FGF7 drives seromucous acinar differentiaton, providing a rationale for targeting FGFR2b signaling in regenerative therapies to restore acinar function.

Equine keratinocytes in the pathogenesis of insect bite hypersensitivity: Just another brick in the wall?

Equine insect bite hypersensitivity (IBH) is the most common skin disease affecting horses. It is described as an IgE-mediated, Type I hypersensitivity reaction to salivary gland proteins of Culicoides insects. Together with Th2 cells, epithelial barrier cells play an important role in development of Type I hypersensitivities. In order to elucidate the role of equine keratinocytes in development of IBH, we stimulated keratinocytes derived from IBH-affected (IBH-KER) (n = 9) and healthy horses (H-KER) (n = 9) with Culicoides recombinant allergens and extract, allergic cytokine milieu (ACM) and a Toll like receptor ligand 1/2 (TLR-1/2-L) and investigated their transcriptomes. Stimulation of keratinocytes with Culicoides allergens did not induce transcriptional changes. However, when stimulated with allergic cytokine milieu, their gene expression significantly changed. We found upregulation of genes encoding for CCL5, -11, -20, -27 and interleukins such as IL31. We also found a strong downregulation of genes such as SCEL and KRT16 involved in the formation of epithelial barrier. Following stimulation with TLR-1/2-L, keratinocytes significantly upregulated expression of genes affecting Toll like receptor and NOD-receptor signaling pathway as well as NF-kappa B signaling pathway, among others. The transcriptomes of IBH-KER and H-KER were very similar: without stimulations they only differed in one gene (CTSL); following stimulation with allergic cytokine milieu we found only 23 differentially expressed genes (e.g. CXCL10 and 11) and following stimulation with TLR-1/2-L they only differed by expression of seven genes. Our data suggests that keratinocytes contribute to the innate immune response and are able to elicit responses to different stimuli, possibly playing a role in the pathogenesis of IBH.

New developments in the biology of fibroblast growth factors

The fibroblast growth factor (FGF) family is composed of 18 secreted signaling proteins consisting of canonical FGFs and endocrine FGFs that activate four receptor tyrosine kinases (FGFRs 1-4) and four intracellular proteins (intracellular FGFs or iFGFs) that primarily function to regulate the activity of voltage-gated sodium channels and other molecules. The canonical FGFs, endocrine FGFs, and iFGFs have been reviewed extensively by us and others. In this review, we briefly summarize past reviews and then focus on new developments in the FGF field since our last review in 2015. Some of the highlights in the past 6 years include the use of optogenetic tools, viral vectors, and inducible transgenes to experimentally modulate FGF signaling, the clinical use of small molecule FGFR inhibitors, an expanded understanding of endocrine FGF signaling, functions for FGF signaling in stem cell pluripotency and differentiation, roles for FGF signaling in tissue homeostasis and regeneration, a continuing elaboration of mechanisms of FGF signaling in development, and an expanding appreciation of roles for FGF signaling in neuropsychiatric diseases. This article is categorized under: Cardiovascular Diseases > Molecular and Cellular Physiology Neurological Diseases > Molecular and Cellular Physiology Congenital Diseases > Stem Cells and Development Cancer > Stem Cells and Development.

Fibroblast Growth Factor 7 Suppresses Fibrosis and Promotes Epithelialization during Wound Healing in Mouse Fetuses

Adult mammalian wounds leave visible scars, whereas skin wounds in developing mouse fetuses are scarless until a certain point in development when complete regeneration occurs, including the structure of the dermis and skin appendages. Analysis of the molecular mechanisms at this transition will provide clues for achieving scarless wound healing. The fibroblast growth factor (FGF) family is a key regulator of inflammation and fibrosis during wound healing. We aimed to determine the expression and role of FGF family members in fetal wound healing. ICR mouse fetuses were surgically wounded at embryonic day 13 (E13), E15, and E17. Expression of FGF family members and FGF receptor (FGFR) in tissue samples from these fetuses was evaluated using in situ hybridization and reverse transcription-quantitative polymerase chain reaction. Fgfr1 was downregulated in E15 and E17 wounds, and its ligand Fgf7 was upregulated in E13 and downregulated in E15 and E17. Recombinant FGF7 administration in E15 wounds suppressed fibrosis and promoted epithelialization at the wound site. Therefore, the expression level of Fgf7 may correlate with scar formation in late mouse embryos, and external administration of FGF7 may represent a therapeutic option to suppress fibrosis and reduce scarring.

A review of IL-36: an emerging therapeutic target for inflammatory dermatoses

BACKGROUND

IL-36 cytokines are members of the IL-1 superfamily. Increasing evidence in the IL-36 pathway demonstrates their potential as a therapeutic target for treating inflammatory skin diseases, such as generalized pustular psoriasis (GPP).

OBJECTIVE

A narrative review was written to further study preclinical and clinical evidence for the role of IL-36 in psoriasis, atopic dermatitis (AD), hidradenitis suppurativa (HS), acne, autoimmune blistering diseases, and neutrophilic dermatoses.

RESULTS

IL-36 has important downstream effects such as inducing expression of inflammatory cytokines, antimicrobial peptides, and growth factors. Increased expression of IL-36 cytokines has been observed in the lesional skin of patients with psoriasis. Studies of other inflammatory skin diseases have also noted similar findings, albeit to a lesser extent. IL-36 inhibition has been shown to be effective in GPP and is currently being studied for other inflammatory skin diseases.

CONCLUSIONS

The IL-36 pathway contributes to pathogenesis of many inflammatory skin diseases and is a promising therapeutic target.

Acute and chronic effects of a light-activated FGF receptor in keratinocytes in vitro and in mice

Optogenetic activation of FGFR2 allowed temporally precise induction of signaling and behavioural changes, but counter-regulation at multiple levels prevented a sustained response in keratinocytes.

FGFs and their high-affinity receptors (FGFRs) play key roles in development, tissue repair, and disease. Because FGFRs bind overlapping sets of ligands, their individual functions cannot be determined using ligand stimulation. Here, we generated a light-activated FGFR2 variant (OptoR2) to selectively activate signaling by the major FGFR in keratinocytes. Illumination of OptoR2-expressing HEK 293T cells activated FGFR signaling with remarkable temporal precision and promoted cell migration and proliferation. In murine and human keratinocytes, OptoR2 activation rapidly induced the classical FGFR signaling pathways and expression of FGF target genes. Surprisingly, multi-level counter-regulation occurred in keratinocytes in vitro and in transgenic mice in vivo, including OptoR2 down-regulation and loss of responsiveness to light activation. These results demonstrate unexpected cell type–specific limitations of optogenetic FGFRs in long-term in vitro and in vivo settings and highlight the complex consequences of transferring optogenetic cell signaling tools into their relevant cellular contexts.

IL-36 cytokines in inflammatory and malignant diseases: not the new kid on the block anymore

The IL-36 family of cytokines were first identified in 2000 based on their sequence homology to IL-1 cytokines. Over subsequent years, the ability of these cytokines to either agonise or antagonise an IL-1R homologue, now known as the IL-36 Receptor (IL-36R), was identified and these cytokines went through several cycles of renaming with the current nomenclature being proposed in 2010. Despite being identified over 20 years ago, it is only during the last decade that the function of these cytokines in health and disease has really begun to be appreciated, with both homeostatic functions in wound healing and response to infection, as well as pathological functions now ascribed. In the disease context, over activation of IL-36 has now been associated with many inflammatory diseases including Psoriasis and inflammatory bowel diseases, with roles in cancer also now being investigated. This review summarises the current knowledge of IL-36 biology, its role in inflammatory diseases and focuses on an emerging role for IL-36 in cancer.

The Potential of FGF-2 in Craniofacial Bone Tissue Engineering: A Review

Bone is a hard-vascularized tissue, which renews itself continuously to adapt to the mechanical and metabolic demands of the body. The craniofacial area is prone to trauma and pathologies that often result in large bone damage, these leading to both aesthetic and functional complications for patients. The "gold standard" for treating these large defects is autologous bone grafting, which has some drawbacks including the requirement for a second surgical site with quantity of bone limitations, pain and other surgical complications. Indeed, tissue engineering combining a biomaterial with the appropriate cells and molecules of interest would allow a new therapeutic approach to treat large bone defects while avoiding complications associated with a second surgical site. This review first outlines the current knowledge of bone remodeling and the different signaling pathways involved seeking to improve our understanding of the roles of each to be able to stimulate or inhibit them. Secondly, it highlights the interesting characteristics of one growth factor in particular, FGF-2, and its role in bone homeostasis, before then analyzing its potential usefulness in craniofacial bone tissue engineering because of its proliferative, pro-angiogenic and pro-osteogenic effects depending on its spatial-temporal use, dose and mode of administration.

Polycomb complexes redundantly maintain epidermal stem cell identity during development

In this study, Cohen et al. sought to understand the functional contribution of PRC1 and PRC2, which largely overlap in their genomic binding and cooperate to establish repressive chromatin domains demarcated by H2AK119ub and H3K27me3, to gene repression. By using the developing murine epidermis as a paradigm, they uncovered a previously unappreciated functional redundancy between Polycomb complexes, and their findings show how PRC1 and PRC2 function as two independent counterparts, providing a repressive safety net that protects and preserves lineage identity.

Polycomb repressive complex 1 (PRC1) and PRC2 are critical epigenetic developmental regulators. PRC1 and PRC2 largely overlap in their genomic binding and cooperate to establish repressive chromatin domains demarcated by H2AK119ub and H3K27me3. However, the functional contribution of each complex to gene repression has been a subject of debate, and understanding of its physiological significance requires further studies. Here, using the developing murine epidermis as a paradigm, we uncovered a previously unappreciated functional redundancy between Polycomb complexes. Coablation of PRC1 and PRC2 in embryonic epidermal progenitors resulted in severe defects in epidermal stratification, a phenotype not observed in the single PRC1-null or PRC2-null epidermis. Molecular dissection indicated a loss of epidermal identity that was coupled to a strong derepression of nonlineage transcription factors, otherwise repressed by either PRC1 or PRC2 in the absence of its counterpart. Ectopic expression of subsets of PRC1/2-repressed nonepidermal transcription factors in wild-type epidermal stem cells was sufficient to suppress epidermal identity genes, highlighting the importance of functional redundancy between PRC1 and PRC2. Altogether, our studies show how PRC1 and PRC2 function as two independent counterparts, thereby providing a repressive safety net that protects and preserves lineage identity.

Dermatologic Adverse Events Associated with Selective Fibroblast Growth Factor Receptor Inhibitors: Overview, Prevention, and Management Guidelines

Fibroblast growth factor receptor (FGFR) tyrosine kinases, which are expressed on the cell membrane, are involved in a wide range of biological functions such as cell proliferation, survival, migration, and differentiation. The identification of FGFR fusions and other alterations in a wide range of solid tumors, including cholangiocarcinoma and bladder cancer, has resulted in the development of several selective FGFR inhibitors for use in these indications, for example, infigratinib, erdafitinib, derazantinib, pemigatinib, and futibatinib. In addition to the typical adverse events associated with tyrosine kinases, the FGFR inhibitors appear to give rise to a number of adverse events affecting the skin. Here we describe these skin events, which include the more common nail adverse events (e.g., onycholysis), palmar-plantar erythrodysesthesia syndrome, and stomatitis, as well as less common reactions such as calciphylaxis. This review aims to provide oncologists with an understanding of these dermatologic events and proposes guidelines for the management of treatment-emergent dermatologic adverse events. Awareness of possible adverse events associated with specific drugs should allow physicians to educate patients as to what to expect and implement effective management plans at the earliest possible opportunity, thereby preventing premature discontinuation while maintaining patient quality of life. IMPLICATIONS FOR PRACTICE: Identification of fibroblast growth factor receptor (FGFR) aberrations in cholangiocarcinoma and bladder cancer led to development of selective FGFR inhibitors for these indications, based on clinical benefit and safety profiles. The most frequent adverse events (AEs) include those affecting skin, hair, and nails, a unique class effect of these agents. These are usually mild to moderate in severity. This work reviewed skin AEs reported with FGFR inhibitors and provides management guidelines for physicians, aiming to increase awareness of skin events and provide effective treatment strategies. Early intervention and effective management may improve treatment adherence, optimize outcomes, and improve quality of life.

Transcriptional and signalling regulation of skin epithelial stem cells in homeostasis, wounds and cancer

The epidermis and skin appendages are maintained by their resident epithelial stem cells, which undergo long-term self-renewal and multilineage differentiation. Upon injury, stem cells are activated to mediate re-epithelialization and restore tissue function. During this process, they often mount lineage plasticity and expand their fates in response to damage signals. Stem cell function is tightly controlled by transcription machineries and signalling transductions, many of which derail in degenerative, inflammatory and malignant dermatologic diseases. Here, by describing both well-characterized and newly emerged pathways, we discuss the transcriptional and signalling mechanisms governing skin epithelial homeostasis, wound repair and squamous cancer. Throughout, we highlight common themes underscoring epithelial stem cell plasticity and tissue-level crosstalk in the context of skin physiology and pathology.

Exosomes for Wound Healing: Purification Optimization and Identification of Bioactive Components

Human mesenchymal stem cell exosomes have been shown to promote cutaneous wound healing. Their bioactivity is most often attributed to their protein and nucleic acid components, while the function of exosomal lipids remains comparatively unexplored. This work specifically assesses the involvement of lipids and the transmembrane enzyme CD73 in the exosomes' biological activity in stimulating the cutaneous wound healing process. Since exosome preparation processes are not harmonized yet, certain production and purification parameters are first systematically investigated, enabling the optimization of a standardized protocol delivering high exosome integrity, yield, and purity. An in situ enzymatic assay is introduced to specifically assess the vesicle functionality, and quantitative proteomics is employed to establish the cell culture conditions yielding a stable exosome protein profile. Using a combination of in vitro approaches, CD73 and constitutional lipids of HPV-16 E6/E7 transformed human bone marrow stromal cell-derived exosomes are identified as key bioactive components promoting the exosome-driven acceleration of processes required for wound repair. A pilot wound healing study in mice indeed suggests a role of lipids in the exosomes' biological activity. Strikingly, the extent of the bioactivity of these exosomal components is found to be dependent on the target cell type.

IL-36γ drives skin toxicity induced by EGFR/MEK inhibition and commensal Cutibacterium acnes

Epidermal growth factor receptor (EGFR) and MEK inhibitors (EGFRi/MEKi) are beneficial for the treatment of solid cancers but are frequently associated with severe therapy-limiting acneiform skin toxicities. The underlying molecular mechanisms are poorly understood. Using gene expression profiling we identified IL-36γ and IL-8 as candidate drivers of EGFRi/MEKi skin toxicity. We provide molecular and translational evidence that EGFRi/MEKi in concert with the skin commensal bacterium Cutibacterium acnes act synergistically to induce IL-36γ in keratinocytes and subsequently IL-8, leading to cutaneous neutrophilia. IL-36γ expression was the combined result of C. acnes–induced NF-κB activation and EGFRi/MEKi–mediated expression of the transcription factor Krüppel-like factor 4 (KLF4), due to the presence of both NF-κB and KLF4 binding sites in the human IL-36γ gene promoter. EGFRi/MEKi increased KLF4 expression by blockade of the EGFR/MEK/ERK pathway. These results provide an insight into understanding the pathological mechanism of the acneiform skin toxicities induced by EGFRi/MEKi and identify IL-36γ and the transcription factor KLF4 as potential therapeutic targets.

Antagonism of interferon signaling by fibroblast growth factors promotes viral replication

Fibroblast growth factors (FGFs) play key roles in the pathogenesis of different human diseases, but the cross‐talk between FGFs and other cytokines remains largely unexplored. We identified an unexpected antagonistic effect of FGFs on the interferon (IFN) signaling pathway. Genetic or pharmacological inhibition of FGF receptor signaling in keratinocytes promoted the expression of interferon‐stimulated genes (ISG) and proteins in vitro and in vivo. Conversely, FGF7 or FGF10 treatment of keratinocytes suppressed ISG expression under homeostatic conditions and in response to IFN or poly(I:C) treatment. FGF‐mediated ISG suppression was independent of IFN receptors, occurred at the transcriptional level, and required FGF receptor kinase and proteasomal activity. It is not restricted to keratinocytes and functionally relevant, since FGFs promoted the replication of herpes simplex virus I (HSV‐1), lymphocytic choriomeningitis virus, and Zika virus. Most importantly, inhibition of FGFR signaling blocked HSV‐1 replication in cultured human keratinocytes and in mice. These results suggest the use of FGFR kinase inhibitors for the treatment of viral infections.

Genetic activation of Nrf2 reduces cutaneous symptoms in a murine model of Netherton syndrome

Netherton syndrome is a monogenic autosomal recessive disorder primarily characterized by the detachment of the uppermost layer of the epidermis, the stratum corneum It results from mutations in the SPINK5 gene, which codes for a kallikrein inhibitor. Uncontrolled kallikrein activity leads to premature desquamation, resulting in a severe epidermal barrier defect and subsequent life-threatening systemic infections and chronic cutaneous inflammation. Here, we show that genetic activation of the transcription factor nuclear factor (erythroid-derived 2)-like 2 (Nfe2l2/Nrf2) in keratinocytes of Spink5 knockout mice, a model for Netherton syndrome, significantly alleviates their cutaneous phenotype. Nrf2 activation promoted attachment of the stratum corneum and concomitant epidermal barrier function, and reduced the expression of pro-inflammatory cytokines such as tumor necrosis factor α and thymic stromal lymphopoietin. Mechanistically, we show that Nrf2 activation induces overexpression of secretory leukocyte protease inhibitor (Slpi), a known inhibitor of kallikrein 7 and elastase 2, in mouse and human keratinocytes in vivo and in vitro, respectively. In the Spink5-deficient epidermis, the upregulation of Slpi is likely to promote stabilization of corneodesmosomes, thereby preventing premature desquamation. Our results suggest pharmacological NRF2 activation as a promising treatment modality for Netherton syndrome patients.This article has an associated First Person interview with the first author of the paper.

Investigating the epithelial barrier and immune signatures in the pathogenesis of equine insect bite hypersensitivity

Insect bite hypersensitivity (IBH) is a Th-2, IgE-mediated dermatitis of horses caused by bites of insects of the genus Culicoides that has common features with human atopic dermatitis. Together with Th-2 cells, the epithelial barrier plays an important role in development of type I hypersensitivities. In order to elucidate the role of the epithelial barrier and of the skin immune response in IBH we studied the transcriptome of lesional whole skin of IBH-horses (IBH-LE; n = 9) in comparison to non-lesional skin (IBH-NL; n = 8) as well as to skin of healthy control horses (H; n = 9). To study the "baseline state" of the epithelial barrier, we investigated the transcriptome of non-lesional epidermis in IBH-horses (EPI-IBH-NL; n = 10) in comparison with healthy epidermis from controls (EPI-H; n = 9). IBH-LE skin displayed substantial transcriptomic difference compared to H. IBH-LE was characterized by a downregulation of genes involved in tight junction formation, alterations in keratins and substantial immune signature of both Th-1 and Th-2 types with particular upregulation of IL13, as well as involvement of the hypoxic pathway. IBH-NL shared a number of differentially expressed genes (DEGs) with IBH-LE, but was overall more similar to H skin. In the epidermis, genes involved in metabolism of epidermal lipids, pruritus development, as well as IL25, were significantly differentially expressed between EPI-IBH-NL and EPI-H. Taken together, our data suggests an impairment of the epithelial barrier in IBH-affected horses that may act as a predisposing factor for IBH development. Moreover, these new mechanisms could potentially be used as future therapeutic targets. Importantly, many transcriptional features of equine IBH skin are shared with human atopic dermatitis, confirming equine IBH as a natural model of skin allergy.

Mouse genetics identifies unique and overlapping functions of fibroblast growth factor receptors in keratinocytes

Fibroblast growth factors (FGFs) are key regulators of tissue development, homeostasis and repair, and abnormal FGF signalling is associated with various human diseases. In human and murine epidermis, FGF receptor 3 (FGFR3) activation causes benign skin tumours, but the consequences of FGFR3 deficiency in this tissue have not been determined. Here, we show that FGFR3 in keratinocytes is dispensable for mouse skin development, homeostasis and wound repair. However, the defect in the epidermal barrier and the resulting inflammatory skin disease that develops in mice lacking FGFR1 and FGFR2 in keratinocytes were further aggravated upon additional loss of FGFR3. This caused fibroblast activation and fibrosis in the FGFR1/FGFR2 double‐knockout mice and even more in mice lacking all three FGFRs, revealing functional redundancy of FGFR3 with FGFR1 and FGFR2 for maintaining the epidermal barrier. Taken together, our study demonstrates that FGFR1, FGFR2 and FGFR3 act together to maintain epidermal integrity and cutaneous homeostasis, with FGFR2 being the dominant receptor.

Esrp1-Regulated Splicing of Arhgef11 Isoforms Is Required for Epithelial Tight Junction Integrity

The epithelial-specific splicing regulators Esrp1 and Esrp2 are required for mammalian development, including establishment of epidermal barrier functions. However, the mechanisms by which Esrp ablation causes defects in epithelial barriers remain undefined. We determined that the ablation of Esrp1 and Esrp2 impairs epithelial tight junction (TJ) integrity through loss of the epithelial isoform of Rho GTP exchange factor Arhgef11. Arhgef11 is required for the maintenance of TJs via RhoA activation and myosin light chain (MLC) phosphorylation. Ablation or depletion of Esrp1/2 or Arhgef11 inhibits MLC phosphorylation and only the epithelial Arhgef11 isoform rescues MLC phosphorylation in Arhgef11 KO epithelial cells. Mesenchymal Arhgef11 transcripts contain a C-terminal exon that binds to PAK4 and inhibits RhoA activation byArhgef11. Deletion of the mesenchymal-specific Arhgef11 exon in Esrp1/2 KO epithelial cells using CRISPR/Cas9 restored TJ function, illustrating how splicing alterations can be mechanistically linked to disease phenotypes that result from impaired functions of splicing regulators.

Function and Regulation of IL-36 Signaling in Inflammatory Diseases and Cancer Development

The IL-36 subfamily of cytokines belongs to the IL-1 superfamily and consists of three pro-inflammatory agonists IL-36α, IL-36β, IL-36γ, and an IL-36 receptor (IL-36R) antagonist, IL-36Ra. These IL-36 cytokines function through a common receptor to modulate innate and adaptive immune responses. IL-36 cytokines are expressed as inactive precursors and require proteolytic processing to become fully active. Upon binding to IL-36R, IL-36 agonists augment the expression and production of inflammatory cytokines via activating signaling pathways. IL-36 is mainly expressed in epidermal, bronchial, and intestinal epithelial cells that form the barrier structures of the body and regulates the balance between pro-inflammatory and anti-inflammatory cytokine production at these tissue sites. Dysregulation of IL-36 signaling is a major etiological factor in the development of autoimmune and inflammatory diseases. Besides its critical role in inflammatory skin diseases such as psoriasis, emerging evidence suggests that aberrant IL-36 activities also promote inflammatory diseases in the lung, kidneys, and intestines, underscoring the potential of IL-36 as a therapeutic target for common inflammatory diseases. The role of IL-36 signaling in cancer development is also under investigation, with limited studies suggesting a potential anti-tumor effect. In this comprehensive review, we summarize current knowledge regarding the expression, activation, regulatory mechanisms, and biological functions of IL-36 signaling in immunity, inflammatory diseases, and cancer development.

Biology of IL-36 Signaling and Its Role in Systemic Inflammatory Diseases

Interleukin (IL)-36 is a member of the IL-1 superfamily and includes three agonists (IL-36α, IL-36β, and IL-36γ) and an antagonist (IL-36Ra). IL-36 agonists bind to heterodimeric receptor complexes. Then, the heterotrimer complexes signal via intracellular functional domains, binding to downstream signaling proteins and inducing inflammatory responses. In this review, we summarized the current knowledge about the biological role of IL-36 and its correlation with systemic inflammatory diseases. The information collected will help to increase the understanding of the potential of IL-36 and may give clues for developing novel therapeutic strategies.

Humidity-regulated CLCA2 protects the epidermis from hyperosmotic stress

Low environmental humidity aggravates symptoms of the inflammatory skin disease atopic dermatitis (AD). Using mice that develop AD-like signs, we show that an increase in environmental humidity rescues their cutaneous inflammation and associated epidermal abnormalities. Quantitative proteomics analysis of epidermal lysates of mice kept at low or high humidity identified humidity-regulated proteins, including chloride channel accessory 3A2 (CLCA3A2), a protein with previously unknown function in the skin. The epidermis of patients with AD, organotypic skin cultures under dry conditions, and cultured keratinocytes exposed to hyperosmotic stress showed up-regulation of the nonorthologous human homolog CLCA2. Hyperosmolarity-induced CLCA2 expression occurred via p38/c-Jun N-terminal kinase-activating transcription factor 2 signaling. CLCA2 knockdown promoted keratinocyte apoptosis induced by hyperosmotic stress through impairment of cell-cell adhesion. These findings provide a mechanistic explanation for the beneficial effect of high environmental humidity for AD patients and identify CLCA3A2/CLCA2 up-regulation as a mechanism to protect keratinocytes from damage induced by low humidity.

The Role of Fgf Signaling on Epithelial Cell Differentiation in Mouse Vagina

BACKGROUND/AIM

The mouse vagina exhibits stratified squamous epithelium, which is comprised of multiple cell layers. We previously showed that erbB signaling, induced by epithelial estrogen receptor 1 (ESR1), is required for the initial differentiation of the epithelium. However, the downstream effector that mediates terminal differentiation in the apical layers remains elusive. The contribution of fibroblast growth factor (FGF) to vaginal epithelial cell differentiation was investigated.

MATERIALS AND METHODS

Vaginas from wild-type or epithelium-specific Esr1 conditional knockout (cKO) mice were analyzed using immunohistochemistry and quantitative real-time RT-PCR.

RESULTS

Of the FGF ligands examined, Fgf22 mRNA was significantly induced following estrogen treatment. Furthermore, FGF downstream signaling, phosphorylated FRS2 and ERK1/2 were exclusively expressed in the apical layers of the vaginal epithelium. No changes in such expression were observed in the Esr1 cKO mice.

CONCLUSION

FGF-ERK/MAPK pathway may be a main inducer of terminal differentiation in the mouse vaginal epithelium.

The Aberrant Expression of the Mesenchymal Variant of FGFR2 in the Epithelial Context Inhibits Autophagy

Signaling of the epithelial splice variant of fibroblast growth factor receptor 2 (FGFR2b) triggers both differentiation and autophagy, while the aberrant expression of the mesenchymal FGFR2c isoform in epithelial cells induces impaired differentiation, epithelial mesenchymal transition (EMT) and tumorigenic features. Here we analyzed in the human keratinocyte cell line, as well as in primary cultured cells, the possible impact of FGFR2c forced expression on the autophagic process. Biochemical and quantitative immunofluorescence analysis, coupled to the use of autophagic flux sensors, specific substrate inhibitors or silencing approaches, showed that ectopic expression and the activation of FGFR2c inhibit the autophagosome formation and that AKT/MTOR is the downstream signaling mainly involved. Interestingly, the selective inhibition of AKT or MTOR substrates caused a reversion of the effects of FGFR2c on autophagy, which could also arise from the imbalance of the interplay between AKT/MTOR pathway and JNK1 signaling in favor of JNK1 activation, BCL-2 phosphorylation and possibly phagophore nucleation. Finally, silencing experiments of depletion of ESRP1, responsible for FGFR2 splicing and consequent FGFR2b expression, indicated that the switching from FGFR2b to FGFR2c isoform could represent the key event underlying the inhibition of the autophagic process in the epithelial context. Our results provide the first evidence of a negative impact of the out-of-context expression of FGFR2c on autophagy, suggesting a possible role of this receptor in the modulation of the recently proposed negative loop between autophagy and EMT during carcinogenesis.

Interleukin-36 in Infectious and Inflammatory Skin Diseases

Interleukin-36 (IL-36) comprises to a cytokine family consisting of four isoforms IL-36α, IL-36β, IL-36γ, and IL-36 receptor antagonist (IL-36 Ra). These IL-36 cytokines, in turn, belong to the IL-1 superfamily. The IL-36 receptor (IL-1R6) is functional as a heterodimer formed of IL-1R6 and IL-1 receptor accessory protein (IL-1RAcP). IL-36α, IL-36β, and IL-36γ are regarded as pro-inflammatory ligands and IL-36 Ra as well as IL-38 as anti-inflammatory ligands of IL-1R6. IL-36 cytokines are mainly expressed on the barrier sites of the body e.g., bronchial, intestinal, and dermal epithelium. One of their most important biological functions is the bridging of innate and adaptive immune responses. A disturbed balance between pro-inflammatory and anti-inflammatory branches easily leads to inflammation of the corresponding tissue. The most prominent example for an altered IL-36 expression is the spectrum of psoriasis. In addition to inflammatory dermatoses, IL-36 also seems to play a role in infectious dermatoses. Microbial triggers, especially Staphylococcus aureus infection, increase the production of pro-inflammatory IL-36 cytokines and initiate/promote the inflammation of skin lesions. Due to the discovery of IL-36 as an important immune mediator, it has already been possible to develop important diagnostic tools for dermatitis. Not only in the field of inflammatory skin diseases, but also in pulmonary and intestinal inflammation, there is evidence that IL-36 cytokines might have diagnostic and/or therapeutic relevance.

Nrf2-Mediated Expansion of Pilosebaceous Cells Accelerates Cutaneous Wound Healing

The nuclear factor (erythroid-derived 2)-like 2 (Nrf2) transcription factor is a key regulator of the cellular stress response. Therefore, pharmacologic Nrf2 activation is a promising strategy for skin protection and cancer prevention. This study found that genetic Nrf2 activation in keratinocytes accelerates wound repair. Enhanced proliferation of cells of the pilosebaceous unit peripheral to the wound and a concomitant acceleration of re-epithelialization were identified as the underlying mechanism. Nrf2 specifically promoted the expansion of pilosebaceous cells expressing markers of junctional zone and upper isthmus follicular stem cells. This may result, at least in part, from the up-regulation of the direct Nrf2 target epigen and a concomitant increase in epidermal growth factor receptor signaling. The increase in pilosebaceous cells provided a larger pool of keratinocytes that migrate into the wound, resulting in faster wound closure. These results unravel a novel function of Nrf2 in wound repair and suggest the use of NRF2-activating compounds in patients with impaired healing.

Tight Junction barriers in human hair follicles - role of claudin-1

Barrier function of hair follicles (HFs) is of great interest because they might be an entry port for allergens/pathogens, but could on the other hand be used for drug delivery or vaccination. Therefore we investigated tight junction (TJ) barrier function in human HFs. We show that there is a TJ barrier in the outermost living layer bordering to the environment from the infundibulum to the lower central part and between Henle’s and Huxles layer of anagen HFs. In club hair typical for catagen and telogen HFs a TJ barrier is found surrounding the club. This demonstrates that there is a continuous TJ barrier along interfollicular epidermis and HFs in different phases of HF cycle. However, interestingly, in cell culture experiments we can show that barrier is less tight in HF keratinocytes compared to interfollicular keratinocytes. Knock-down of the TJ protein claudin-1, which we demonstrate here to be less expressed in HFs of lesional atopic dermatitis skin, results in impaired barrier function, decreased proliferation and increased apoptosis of hair keratinocytes. This is in line with a hair growth phenotype in claudin-1 deficient patients (NISCH syndrome) and corresponding knock-out mice and indicates an important role of claudin-1 in HF barrier function and growth.

Role of Fibroblast Growth Factor Receptor 2b in the Cross Talk between Autophagy and Differentiation: Involvement of Jun N-Terminal Protein Kinase Signaling

Fibroblast growth factor receptor 2b (FGFR2b) is a receptor tyrosine kinase expressed exclusively in epithelial cells. We previously demonstrated that FGFR2b induces autophagy and that this process is required for the triggering of FGFR2b-mediated early differentiation of keratinocytes. However, the molecular mechanisms regulating this interplay remain to be elucidated. Since we have also recently shown that Jun N-terminal protein kinase 1 (JNK1) signaling is involved in FGFR2b-induced autophagy and a possible role of the JNK pathway in epidermal differentiation has been suggested (though it is still debated), we investigated here the cross talk between FGFR2b-mediated autophagy and differentiation, focusing on the downstream JNK signaling. Biochemical, molecular, and immunofluorescence approaches in 2-dimensional (2-D) keratinocyte cultures and three-dimensional (3-D) organotypic skin equivalents confirmed that FGFR2b overexpression increased both autophagy and early differentiation. The use of FGFR2b substrate inhibitors and the silencing of JNK1 highlighted that this signaling is required not only for autophagy but also for the triggering of early differentiation. In contrast, the extracellular signal-regulated kinase 1 and 2 (ERK1/2) pathway did not appear to be involved in the two processes, and AKT signaling, whose activation contributes to the FGFR2b-mediated onset of keratinocyte differentiation, was not required for the triggering of autophagy. Overall, our results point to JNK1 as a signaling hub that regulates the interplay between FGFR2b-induced autophagy and differentiation.

Role of FGFR2b expression and signaling in keratinocyte differentiation: sequential involvement of PKCδ and PKCα

The tumor suppressor epithelial isoform of the fibroblast growth factor receptor 2 (FGFR2b) induces human keratinocyte early differentiation. Moreover, protein kinases C (PKCs) are known to regulate the differentiation program in several cellular contexts, including keratinocytes. Therefore, in this paper we propose to clarify if FGFR2b could play a role also in the late steps of keratinocyte differentiation and to assess if this receptor-induced process would sequentially involve PKCδ and PKCα isoforms. Immunofluorescence, biochemical, and molecular approaches, performed on 2D cultures or 3D organotypic rafts of human keratinocytes overexpressing FGFR2b by stable transduction, showed that receptor signaling induced the precocious onset and an accelerated progression of keratinocyte differentiation, indicating that FGFR2b is a crucial regulator of the entire program of keratinocyte differentiation. In addition, the use of specific inhibitors and gene silencing approaches through specific siRNA demonstrated that PKCδ controls the onset of FGFR2b-triggered differentiation, while PKCα plays a role restricted to the terminal stages of the process. Molecular analysis revealed that the two PKC isoforms sequentially act via induction of KLF4 and DLX3, two transcription factors linked by negative loops to p63, suggesting that p63 would represent the hub molecule at the crossroad of an intricate signaling network downstream FGFR2b, involving multiple PKC-induced transcription factors.

Stimulation of hair follicle stem cell proliferation through an IL-1 dependent activation of γδT-cells

The cutaneous wound-healing program is a product of a complex interplay among diverse cell types within the skin. One fundamental process that is mediated by these reciprocal interactions is the mobilization of local stem cell pools to promote tissue regeneration and repair. Using the ablation of epidermal caspase-8 as a model of wound healing in Mus musculus, we analyzed the signaling components responsible for epithelial stem cell proliferation. We found that IL-1α and IL-7 secreted from keratinocytes work in tandem to expand the activated population of resident epidermal γδT-cells. A downstream effect of activated γδT-cells is the preferential proliferation of hair follicle stem cells. By contrast, IL-1α-dependent stimulation of dermal fibroblasts optimally stimulates epidermal stem cell proliferation. These findings provide new mechanistic insights into the regulation and function of epidermal cell–immune cell interactions and into how components that are classically associated with inflammation can differentially influence distinct stem cell niches within a tissue.

The skin is a physical barrier that protects the body from the outside world. If the skin is injured, the body mounts a “wound healing” response to rapidly mend and restore this protective barrier. Wound healing is a complex process and relies on the different types of cells in the skin communicating with each other.

Stem cells provide tissues, like the skin, with new cells. Normally, stem cells are in a resting or inactive state. Yet, during wound healing, stem cells near the injured area are awakened and start producing more cells to repair the wound. Understanding how stem cells become activated in a wound has proved challenging because only a small number of cells near a damaged site will respond, and it is difficult to distinguish their response from that of other cells slightly further away.

Now, Lee et al. overcome this hurdle by analyzing a genetically engineered mouse in which the entire skin displays a wound healing response, even without any injury or trauma. In these mice, most of the stem cells in the skin are awakened from their normal resting state and behave as if there is a wound to heal.

It turns out that a protein called interleukin-1, which is released from damaged skin cells known as keratinocytes, can activate two different groups of stem cells in the skin to help repair the injured tissue. One group lives in the hair follicle and is normally responsible for replacing the hair that falls from the body. Lee et al. found that when the skin is wounded interleukin-1 activates certain immune cells (called γδT-cells). These immune cells then awaken the resting stem cells in the hair follicle to multiply and travel to the wound site to repair the injury. The other group of stem cells resides in the outermost layer of the skin. Interleukin-1 can also activate so-called fibroblast cells, which then stimulate this second group of stem cells to divide and cover the open wound.

Quickly healing wounds has many health benefits such as preventing infection and shortening the time to recover from an injury. These new findings may help to repair injured skin in diseases such as diabetes, where wounds can take months to heal and often leads to permanent tissue damage. The next challenge is to identify the cues that instruct the stem cells to travel to the wound site and turn into the specific cells that are required to replace the damaged cells.

IL-36 cytokines in autoimmunity and inflammatory disease

The inteleukin-36 (IL-36) cytokines include IL-36α, IL-36β, IL-36γ and IL-36Ra, which belong to the IL-1 family and exert pro-inflammatory effects on various target cells such as keratinocytes, synoviocytes, dendritic cells and T cells. Emerging evidence has suggested a role of IL-36 in the pathogenesis of many inflammatory diseases. Here, we provide a brief review on the activation of IL-36 family cytokines and their involvement in autoimmunity and inflammatory diseases, which will provide further insights in understanding the functions of IL-36 family cytokines in the pathophysiology of autoimmunity and inflammatory diseases.

Expression of the FGFR2c mesenchymal splicing variant in human keratinocytes inhibits differentiation and promotes invasion

The altered isoform switching of the fibroblast growth factor receptor 2 (FGFR2) and aberrant expression of the mesenchymal FGFR2c isoform in epithelial cells is involved in cancer progression. We have recently described that the ectopic expression of FGFR2c in normal human keratinocytes induces epithelial‐mesenchymal transition and leads to invasiveness and anchorage‐independent growth. Here, we extended our analysis to the effects of this FGFR2c forced expression on human keratinocyte differentiation and stratification. Our findings demonstrated that, differently from cells overexpressing the epithelial splicing variant FGFR2b, keratinocytes ectopically expressing FGFR2c are not able to form a monolayer and display decreased expression of early differentiation markers. This impaired ability to enter the differentiation program is related to the up‐modulation of the transcription factor ΔNp63. In addition, FGFR2c‐expressing keratinocytes undergo defective stratification and invasion of the collagen matrix in 3D organotypic cultures, further suggesting their tumorigenic potential. Taken together, our results support the hypothesis that the receptor switching and the consequent appearance of the mesenchymal FGFR2c variant in the epithelial context would drive early steps of carcinogenesis, unbalancing the p63/FGFR interplay, and altering the paracrine response to the microenvironment.

Temperature-sensitive heparin-modified poloxamer hydrogel with affinity to KGF facilitate the morphologic and functional recovery of the injured rat uterus

Endometrial injury usually results in intrauterine adhesion (IUA), which is an important cause of infertility and recurrent miscarriage in reproductive women. There is still lack of an effective therapeutic strategy to prevent occurrence of IUA. Keratinocyte growth factor (KGF) is a potent repair factor for epithelial tissues. Here, a temperature-sensitive heparin-modified poloxamer (HP) hydrogel with affinity to KGF (KGF-HP) was used as a support matrix to prevent IUA and deliver KGF. The rheology of KGF-HP hydrogel was carefully characterized. The cold KGF-HP solution was rapidly transited to hydrogel with suitable storage modulus (G') and loss modulus (G″) for the applications of uterus cavity at temperature of 33 °C. In vitro release demonstrated that KGF was released from HP hydrogels in sustained release manner for a long time. In vivo bioluminescence imaging showed that KGF-HP hydrogel was able to prolong the retention of the encapsulated KGF in injured uterus of rat model. Moreover, the morphology and function of the injured uterus were significantly recovered after administration of KGF-HP hydrogel, which were evaluated by two-dimensional ultrasound imaging and receptive fertility. Not only proliferation of endometrial glandular epithelial cells and luminal epithelial cells but also angiogenesis of injured uterus were observed by Ki67 and CD31 staining after 7 d of treatment with KGF-HP hydrogel. Finally, a close relatively relationship between autophagy and proliferation of endometrial epithelial cells (EEC) and angiogenesis was firstly confirmed by detecting expression of LC3-II and P62 after KGF treatment. Overall, KGF-HP may be used as a promising candidate for IUA treatment.

Friends Turned Foes: Angiogenic Growth Factors beyond Angiogenesis

Angiogenesis, the formation of new blood vessels from pre-existing ones is a biological process that ensures an adequate blood flow is maintained to provide the cells with a sufficient supply of nutrients and oxygen within the body. Numerous soluble growth factors and inhibitors, cytokines, proteases as well as extracellular matrix proteins and adhesion molecules stringently regulate the multi-factorial process of angiogenesis. The properties and interactions of key angiogenic molecules such as vascular endothelial growth factors (VEGFs), fibroblast growth factors (FGFs) and angiopoietins have been investigated in great detail with respect to their molecular impact on angiogenesis. Since the discovery of angiogenic growth factors, much research has been focused on their biological actions and their potential use as therapeutic targets for angiogenic or anti-angiogenic strategies in a context-dependent manner depending on the pathologies. It is generally accepted that these factors play an indispensable role in angiogenesis. However, it is becoming increasingly evident that this is not their only role and it is likely that the angiogenic factors have important functions in a wider range of biological and pathological processes. The additional roles played by these molecules in numerous pathologies and biological processes beyond angiogenesis are discussed in this review.