« Non-coding RNAs in skin health and disease »

Contrary to earlier beliefs, research in the past decades has unveiled that most RNAs in human cells do not code for proteins but serve other crucial functions that we are just beginning to understand. These non-coding RNAs (ncRNAs), once deemed « junk, » play vital roles in gene regulation and are altered in disease states. Notably, many ncRNAs are specific to higher organisms, with their numbers correlating with organism complexity. Two major groups of ncRNAs are microRNAs (miRNAs), which suppress gene expression, and long non-coding RNAs (lncRNAs), a structurally and functionally diverse group of ncRNAs.  While their functional characterization is still in its infancy, ncRNAs with roles in epidermal homeostasis and inflammatory skin diseases have been identified.

In this talk, I will discuss the role of ncRNAs in skin health and disease. We and others have implicated ncRNAs in conditions like psoriasis and atopic dermatitis, and showed that their modulation can be beneficial in vitro and in preclinical models. One example is miR-149, a miRNA we recently identified as a regulator of epidermal immune responses, which acts as a break on skin inflammation and whose suppression can contribute to excessive inflammatory responses.

Non-coding RNA-mediated gene regulation represents a new layer of regulatory mechanism indispensable for normal skin functions. With the advent of RNA-based medicine, modulation of non-coding RNAs may become a novel approach to treat skin diseases.

« Linking Epidermal Morphogenesis and Skin Inflammation through Desmosomes »

While classic cadherin-actin connections have ancient origins, desmosomal cadherins and their connections with intermediate filaments (IF) appeared much later in evolution, reaching the height of complexity in terrestrial vertebrates. These connections, which occur at intercellular junctions called desmosomes, provided opportunities create more complex tissues, like the multi-layered epidermis, with barrier functions that allowed us to meet the challenges of life on land.  One of the desmosomal cadherins only expressed in complex tissues of terrestrial vertebrates is the desmosomal cadherin Dsg1.  Dsg1 is first expressed in cells committing to differentiate and transit into the superficial layers. It promotes differentiation by inhibiting EGFR-dependent Erk signaling while working with classic cadherins to remodel cortical actin cytoskeleton and decrease membrane tension to promote cell delamination, a critical contributor to epidermal stratification.  The desmosome-IF scaffold not only creates the architecture required for skin’s physical barrier but also creates an immune barrier that keeps inflammation in check.  Transient acute impairment of the terrestrial cadherin Dsg1 is protective, whereas its chronic loss initiates an IL-17/23 psoriatic-like inflammatory response in Dsg1-deficient animals and patients with a systemic inflammatory disorder SAM syndrome.  Our most recent data have identified the SH2/SH3 adaptor family as molecules that link Dsg1-dependent actin polymerization required for stratification with suppression of IL-23 expression in keratinocytes, highlighting the intimate mechanistic links between Dsg1’s diverse functions in the epidermis.  Collectively, our studies promise not only to elucidate how cytoarchitectural systems are structurally and functionally integrated within a complex tissue, but also how disabling specific nodes in the network contributes to human disease.

« Role of T cells in the skin immune homeostasis »

« Research on the epidermal lamellar granules has revealed that keratinocytes are extremely intelligent cells »

Lamellar granules (LGs), secretory organelles in the epidermis, play a pivotal role in maintaining healthy skin by releasing a diverse array of secretory cargo from the apical plasma membrane of keratinocytes. Aberrations in LG formation, composition, or secretion are implicated in the etiology and pathogenesis of various skin disorders. A comprehensive understanding of LGs is crucial for elucidating the mechanisms underlying healthy skin maintenance and developing effective therapies for skin diseases.

This presentation will delve into our group’s research endeavors on LGs, highlighting recent advances in 3D imaging of LG secretion. These findings have unveiled the remarkable sophistication of epidermal keratinocytes, surpassing our initial expectations. We anticipate that continued research in this field will empower keratinocyte researchers to attain a level of ingenuity akin to that of these remarkable cells.

« Lipids, barrier and atopic dermatitis (AD): A transcriptional view »

The exterior layer of skin, stratum corneum (SC) provides an efficient protective permeability barrier and the composition of SC lipid matrix is crucial for the formation of a competent skin barrier. The integrity of the epidermal permeability barrier (EPB) and maintenance of skin epidermal homeostasis are orchestrated by the concerted action of several transcription factors that regulate both the proliferation and differentiation of epidermal keratinocytes and mediate the intra- and inter-cellular signaling.

We have discovered that transcriptional regulatory proteins COUP-TF-interacting protein 1 and 2 (CTIP1/2), also known as BCL11a and BCL11b, respectively, is highly expressed in both mouse and human skin. Selective ablation of  Ctip2 in epidermal keratinocytes (Ctip2^ep-/-  mice) revealed a novel role of CTIP2 in skin homeostasis (1,2). We have demonstrated that CTIP2 loss in epidermis triggers an atopic dermatitis (AD)-like skin inflammation, characterized by alopecia, pruritus and scaling, as well as extensive infiltration of immune cells including T lymphocytes, mast cells and eosinophils (3). We observed increased expression of T-helper 2 (Th2)-type cytokines/chemokines in the adult skin of mutant mice, as well as systemic immune responses including elevated serum IgE that share striking similarity with human AD patients. CTIP2 regulates expression of genes involved in sphingolipid biosynthesis during skin development  (4).  We also discovered that expression of cytokine thymic stromal lymphopoietin (TSLP), critical in AD-pathogenesis, was significantly induced in the Ctip2-null epidermis as early as postnatal day 1 (P1) and ChIP assay revealed that TSLP is likely a direct transcriptional target of CTIP2 in epidermal keratinocytes.

We also  discovered that germline deletion of CTIP1 (Ctip1^-/- mice) during murine skin development leads to altered epidermal homeostasis, as well as defects in skin barrier (5). We used a targeted lipidomic approach using ultra high-pressure liquid chromatography/ MS/MS and have determined that Ctip1^-/- mice displayed altered composition of major epidermal lipids, such as ceramides and sphingomyelins, compared with wild-type mice. Transcriptome analysis using RNAseq revealed that the expression of several genes encoding various enzymes involved in sphingolipid metabolism was altered in the Ctip1^-/- mice skin (5,6). Altogether, our data underscores a critical cell-autonomous and non-cell autonomous role of CTIP1 and CTIP2 in regulating barrier homeostasis, skin lipid metabolism and in mediating AD pathogenesis.

« The unique Organisation of Stratum Corneum Lipids in Facial Skin »

Data will be presented from a series of exploratory in vivo experiments designed to characterise human stratum corneum (SC) lipid organisation, both intra-molecular and inter-molecular, in healthy face (cheek) skin as a function of SC depth. These studies utilised a combination of sequential tape-stripping, in vivo horizontal attenuated total reflection Fourier transform infrared (HATR-FTIR) spectroscopy, and trans-epidermal water loss (TEWL) measurements. For comparison, and to act as a reference, in vivo measurements were also collected from volar forearms as we, and others, have previously characterised forearm SC lipid order as a function of depth. These data, therefore, served as a comparison and an experimental internal standard for the previously unmeasured in vivo face skin data. In vivo HATR-FTIR spectra were collected after each tape strip, providing a depth profile of spectral data through the SC of both faces and arms.  Spectral data were analysed at discrete SC depths corresponding to locations where TEWL had increased by 25, 50, 75 and 100 percent from baseline.  The mean spectral data revealed clear trends in the face and arm SC lipid behaviour. Increasing intra-molecular lipid order with depth was observed for face skin, as with arm skin, albeit with significant differences at all depths in the absolute lipid order between faces and arms. The in vivo spectroscopy data indicate that SC lipids were significantly more conformationally ordered at all depths in arms versus cheeks. The less ordered lipids of face SC could, in part, be attributed to the presence of fluid sebaceous lipid species observed at all depths in face SC. No evidence of inter-molecular orthorhombic packing was observed at any depth in face SC. This was consistent with the high degree of intra-molecular disorder indicated by the lipid chain fluidity measurements. These in vivo HATR-FTIR studies of face skin highlight significant differences in facial skin barrier lipid organisation compared to the more studied body sites. The implications of these results will be discussed in the context of skin health and the development of topical formulations for face skin.