Emanuel Rognoni

Epidermal stem cell maintenance depends on complex niche signalling. In this issue Gaeta, Du et al. (https://doi.org/10.1083/jcb.202507165) show that substantial dermal fibroblast depletion has surprisingly minimal effects on epidermal proliferation and skin function during development and adult homeostasis.

The skin is composed of multiple fibroblast subpopulations with different functions in homeostasis and repair, but their role in skin diseases is largely unknown. Junctional epidermolysis bullosa (JEB) is a hereditary skin disorder characterised by severe skin fragility and aberrant granulation tissue formation, caused by loss-of-function variants in basement membrane proteins, including laminin-332. We developed JEB-like organotypic (OT) cultures with distinct fibroblast subpopulations and explored their role in an inducible JEB in vivo disease model, mimicking key features of the human disease. Mechanistically, papillary fibroblasts are highly increased in the granulation tissue of blistered JEB skin, promoting pathological αvβ6 integrin and TGFβ signalling in JEB keratinocytes. Treatment with the TGFβ receptor inhibitor RepSox not only normalised aberrant cell proliferation, differentiation, and cytokine signalling in JEB OTs but also reduced aberrant granulation tissue formation and skin blistering in laminin-332-depleted mice. Collectively, our study reveals that papillary fibroblasts promote JEB pathogenesis through increasing αvβ6 integrin and TGFβ signalling and disruption of these pathological signalling interactions significantly improved skin health and regeneration in JEB.

Fibroblasts are the major cell population of connective tissue, including the skin dermis, and are best known for their function in depositing and remodelling the extracellular matrix. Besides their role in extracellular matrix homeostasis, fibroblasts have emerged as key players in many biological processes ranging from tissue immunity and wound healing to hair follicle development. Recent advances in single‐cell RNA‐sequencing technologies have revealed an astonishing transcriptional fibroblast heterogeneity in the skin and other organs. A key challenge in the field is to understand the functional relevance and significance of the identified new cell clusters in health and disease. Here, we discuss the functionally distinct fibroblast subtypes identified in skin homeostasis and repair and how they evolve in fibrotic disease conditions, in particular keloid scars and cancer. © 2023 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of The Pathological Society of Great Britain and Ireland.

Skin fibrosis, a common scarring condition that currently has no curative treatments, is caused by persistent activation of fibroblasts after tissue injury or inflammation that results in excessive connective tissue deposition, leading to severe disability, organ malfunction and increased morbidity. This study aims to uncover the early and advanced changes in different fibroblast subsets and molecular pathways that are integral to pathological fibroblast activation using a bleomycin-induced skin fibrosis mouse model that recapitulates well the human disease. We have shown that 2 weeks (early) and 4 weeks (advanced) of subcutaneous bleomycin injections result in an increasing severity of skin fibrosis, characterized by increased dermal thickness, collagen deposition, inflammation, and loss of dermal fibroblasts and adipocytes. We performed single-cell RNA sequencing and successfully isolated 14 944 control, 16 344 early and 6860 advanced fibrotic cells. We identified the changes in fibroblast subclusters and gene signatures that are altered across the time course. Using CellChat, we are dissecting the specific signalling crosstalk between different fibroblast subpopulations and other skin cells, promoting a profibrotic phenotype and locking fibroblasts in an activated state. Furthermore, we are intersecting our novel data set with publicly available wound healing transcriptomic data, allowing us to examine how fibroblast subpopulations, behaviour and key signalling pathways vary in healthy wounding healing and pathological scarring. To understand the impact that changes in extracellular matrix biomechanics elicit on profibrotic fibroblast fate, we are correlating our histological and transcriptomic analysis with nanoindentation atomic force microscopy imaging throughout the fibrotic time course. This novel multidisciplinary approach allows us to compare the cellular and molecular changes in early and advanced fibrotic lesions, identifying the key fibroblast subpopulations, molecular regulators and connective tissue alterations promoting fibrotic tissue repair. Our findings will support the development of new fibroblast-targeted fibrosis therapies in skin and potentially other organs.

Skin fibrosis, a common scarring condition that currently has no curative treatments, is caused by persistent activation of fibroblasts after tis | POSTER

Skin fibrosis describes an excessive accumulation of connective tissue that can manifest in association with systemic diseases such as scleroderm | POSTER