Atopic Dermatitis: Disease Background and Risk Factors.

Multiple risk factors have been associated with the development of atopic dermatitis (AD). Recent advances in understanding the role of genetics in this disease have been made, with discovery of the filaggrin (FLG) gene as the most notable so far. In addition to FLG gene mutations as a risk factor for AD, a positive family history of atopic or allergic disease in either parent has been shown to confer a greater risk of developing AD. Atopic dermatitis usually presents early in life and is thought to represent the initial step in the "atopic march," which is characterized by the development of other atopic diseases later in life such as asthma, allergic rhinitis, and/or rhinoconjunctivitis, food allergies, and hay fever. Other comorbid diseases that have been associated with AD include increase risk of viral and bacterial skin infections, neuropsychiatric diseases such as attention-deficit hyperactivity disorders (ADHD), and autistic spectrum disorder (ASD). Patients with AD have also been found to have worse sleep quality overall compared to patients without AD. In this chapter, we will discuss the risk factors associated with development of atopic dermatitis as well as the most commonly reported comorbidities in patients with this disease.

The Discovery and Function of Filaggrin.

Keratohyalin granules were discovered in the mid-19th century in cells that terminally differentiate to form the outer, cornified layer of the epidermis. The first indications of the composition of these structures emerged in the 1960s from a histochemical stain for histidine, followed by radioautographic evidence of a high incidence of histidine incorporation into newly synthesized proteins in cells containing the granules. Research during the next three decades revealed the structure and function of a major protein in these granules, which was initially called the 'histidine-rich protein'. Steinert and Dale named the protein 'filaggrin' in 1981 because of its ability to aggregate keratin intermediate filaments. The human gene for the precursor, 'profilaggrin,' was reported in 1991 to encode 10, 11 or 12 nearly identical repeats. Remarkably, the mouse and rat genes encode up to 20 repeats. The lifetime of filaggrin is the time required for keratinocytes in the granular layer to move into the inner cornified layer. During this transition, filaggrin facilitates the collapse of corneocytes into 'building blocks' that become an impermeable surface barrier. The subsequent degradation of filaggrin is as remarkable as its synthesis, and the end-products aid in maintaining moisture in the cornified layer. It was apparent that ichthyosis vulgaris and atopic dermatitis were associated with the absence of this protein. McLean's team in 2006 identified the cause of these diseases by discovering loss-of-function mutations in the profilaggrin gene, which led to dysfunction of the surface barrier. This story illustrates the complexity in maintaining a healthy, functional epidermis.

Filaggrin - revisited.

Profilaggrin (proFLG) and its processing products are critical to the health and appearance of skin. The recent identification of loss-of-function filaggrin (FLG) mutations as a predisposing factor in ichthyosis vulgaris and atopic dermatitis has lead to a resurgent interest in this enigmatic protein. Here, we review the literature on the structure and many functions of proFLG, from its role as a filament-aggregating protein and a source of natural moisturizing factor (NMF), to the more recent discoveries of its role in epidermal barrier formation and its more speculative functions as an antimicrobial and sunscreen. Finally, we discuss the relationship of proFLG with dry skin, the influence of moisturizers on NMF generation and speculate on next generation of FLG research.

Extracellular epimorphin impairs expression and processing of profilaggrin in HaCaT keratinocytes.

The expression and processing of filaggrin, a filament-associated protein in the skin epidermis, is closely associated with keratinocyte cornification. The large precursor profilaggrin (Pro-FLG) is initially detected at the granular layer in keratohyalin granules, subsequently processed into 10 to 12 filaggrin monomers (mFLGs) for keratin assembly, and ultimately degraded into smaller peptides that behave as natural moisturizing factor (NMF) at the outermost epidermis. We previously reported that epimorphin (EPM) extruded upon external stimuli severely perturbs epidermal terminal differentiation. Using HaCaT keratinocytes with inducible expression and recombinant EPM and FLG, we investigated the effect of extracellular EPM on the expression profile of filaggrin. As expression and processing of Pro-FLG in primary keratinocytes are accompanied with apoptotic cell death, we employed HaCaT keratinocytes that grow and express filaggrin mRNA in standard culture medium. In response to ectopic stimulation with extracellular EPM, Pro-FLG expression decreased with elimination of keratohyalin granules in the cells, with filaggrin mRNA remained constant and profilaggrin processing was not accelerated. Additionally, using a recombinant form of mFLG engineered for intracellular localization, we found that extracellular EPM hindered proteolytic cleavage of mFLG for production of NMF. Taken together, extracellularly extruded EPM, an epidermal cornification blocker, not only decreases Pro-FLG expression but also reduces the production of NMF in HaCaT keratinocytes. Supplementary Information: The online version contains supplementary material available at 10.1007/s10616-022-00566-8.

The spatiotemporal control of human matriptase action on its physiological substrates: a case against a direct role for matriptase proteolytic activity in profilaggrin processing and desquamation.

Studies of human genetic disorders and animal models indicate that matriptase plays essential roles in proteolytic processes associated with profilaggrin processing and desquamation at late stages of epidermal differentiation. The tissue distribution profile and zymogen activation status in human skin, however, suggests that matriptase physiological function in the skin more likely lies in the proliferating and differentiating keratinocytes in the basal and spinous layers. Marked acanthosis with expanded spinous layer and lack of significant changes in intensity and expression pattern for several terminal differentiation markers in the skin of ARIH patients support matriptase's role in earlier rather than the later stages of differentiation. In addition to the tissue distribution, differential subcellular localization further limits the ability of extracellular matriptase proteolytic activity to access the cytosolic non-membrane-bound keratohyalin granules, in which profilaggrin processing occurs. The short lifespan of active matriptase, which results from tightly controlled zymogen activation, rapid inhibition by HAI-1, and shedding from cell surface, indicates that active matriptase likely performs physiological functions via limited proteolysis on its substrates, as needed, rather than via a continuous bulk process. We, here, review these spatiotemporal controls of matriptase proteolytic activity at the biochemical, cellular, and tissue level. Based on this in-depth understanding of how matriptase activity is regulated, we argue that there is no direct involvement of matriptase proteolytic activity in profilaggrin processing and desquamation. The defects in epidermal terminal differentiation associated with matriptase deficiency are likely secondary and are due to putative disruption at earlier stages of differentiation.

Immunohistochemical Studies of Cytokeratins and Differentiation Markers in Bovine Ocular Squamous Cell Carcinoma.

Bovine Ocular Squamous Cell Carcinoma is considered the most common bovine tumour, causing significant economic losses, mainly by abattoir condemnations. To obtain a better insight into the genesis and neoplastic transformation, 19 samples collected at slaughter from Holstein Friesian cattle and diagnosed as Ocular Squamous Cell Carcinoma were studied. Tumours were histologically classified into three categories: poorly (26.3%), moderately (26.3%), and well differentiated (47.4%). Expression of keratins (MNF116 and LP34) and of cornified envelope precursors (involucrin and profilaggrin) was studied. Expression of MNF116 was observed in all carcinomas. LP34 immunostaining was seen in all but three carcinomas, one from each degree. Involucrin immunoreaction was observed in all but one poorly differentiated carcinoma. Profilaggrin was present in only two moderately differentiated carcinomas, in all but one well differentiated, and in all but one poorly differentiated. MNF116 is a useful marker to confirm the epithelial origin of the tumour and stain most neoplastic cells in these tumours. The expression of involucrin and LP34 demonstrates that, in all tumours, cells have reached the final program of differentiation, regardless of the grade. The expression of profilaggrin could indicate molecular changes during malignant transformation but their expression does not seem to be of diagnostic value.

Anti-allergic and Profilaggrin (ProFLG)-mRNA expression modulatory effects of sacran.

Atopic dermatitis (AD) is a skin disorder characterized by filaggrin (FLG) defect. We evaluated sacran's effects on dust-mite extracts (DME)-induced AD-like disease and also its effect on profilaggrin (proFLG) in a murine model of 2,4-dinitroflurobenze (DNFB)-induced contact hypersensitivity. In the murine AD-like disease model, allergic NC/Nga mice (N=60) were randomly divided into five treatment groups of 12 animals each: 0.2% and 1%sacran; 0.1% Tacrolimus; Vaseline and buffer-treated controls. Blood samples were drawn and serum levels of representative Th-1, Th-2 and also Th-17 (IL-17A) cytokines were assayed by Cytometric Bead Array (CBA). In the contact hypersensitivity model, diseased NC/Nga mice (N=20) were divided into four groups of five mice each [0.05%sacran, 0.05% chondroitin sulfate (CS), 0.5% prednisolone (PD), non-treated control group] and were treated for 14days. Skin biopsies were performed for the measurement of proFLG-mRNA by real-time PCR. Sacran solutions and 0.1%Tacrolimus reduced disease severity, suppressed histological changes and decreased the serum Th-1 (IFN-γ, TNF-α, IL-2) and Th-2 (IL-4, IL-6, IL-10) cytokines in allergic mice (vs. controls). Additionally, a marked increase of proFLG-mRNA expression was observed in 0.05%sacran group (vs. control 0.05% CS and 0.5% PD groups). Thus, Sacran might be useful as a natural skin barrier enhancer and anti-allergic agent.

Bilateral lower extremity hyperkeratotic plaques: a case report of ichthyosis vulgaris.

Here, we report a case of a middle-aged woman presenting with severe, long-standing, hyperkeratotic plaques of the lower extremities unrelieved by over-the-counter medications. Initial history and clinical findings were suggestive of an inherited ichthyosis. Ichthyoses are genetic disorders characterized by dry scaly skin and altered skin-barrier function. A diagnosis of ichthyosis vulgaris was confirmed by histopathology. Etiology, prevalence, and treatment options are discussed.

Ultraviolet B irradiation induces the expression of hornerin in xenotransplanted human skin.

Ultraviolet (UV) irradiation exerts numerous effects on the skin. Exposure of human skin to UVB at doses that induce mild sunburn reactions causes epidermal hyperproliferation and alterations in the expression of several epidermal differentiation markers. This study investigated the effects of UVB irradiation on the expression of hornerin, a member of the S100 fused-type protein family, using the xenotransplantation of normal human skin onto nude mice. Hornerin mRNA was detected in the UVB-irradiated skin on day 2 using RT-PCR. In accordance with the results of the RT-PCR, the expression of hornerin was induced in the granular layers of the UVB-exposed skin beginning two days after UVB irradiation and occurred in parallel with the expressions of cytokeratin 6 and Ki67. This finding suggests that hornerin induction in UVB-irradiated skin might be associated with epidermal hyperproliferation. This study demonstrated that hornerin is a protein whose expression is changed by UVB irradiation and suggests that the expression of hornerin might be a useful marker of acute UV damage in skin.