Jens-Michael Jensen

The skin: an indispensable barrier

Abstract:  The skin forms an effective barrier between the organism and the environment preventing invasion of pathogens and fending off chemical and physical assaults, as well as the unregulated loss of water and solutes. In this review we provide an overview of several components of the physical barrier, explaining how barrier function is regulated and altered in dermatoses. The physical barrier is mainly localized in the stratum corneum (SC) and consists of protein‐enriched cells (corneocytes with cornified envelope and cytoskeletal elements, as well as corneodesmosomes) and lipid‐enriched intercellular domains. The nucleated epidermis also contributes to the barrier through tight, gap and adherens junctions, as well as through desmosomes and cytoskeletal elements. During epidermal differentiation lipids are synthesized in the keratinocytes and extruded into the extracellular domains, where they form extracellular lipid‐enriched layers. The cornified cell envelope, a tough protein/lipid polymer structure, resides below the cytoplasmic membrane on the exterior of the corneocytes. Ceramides A and B are covalently bound to cornified envelope proteins and form the backbone for the subsequent addition of free ceramides, free fatty acids and cholesterol in the SC. Filaggrin is cross‐linked to the cornified envelope and aggregates keratin filaments into macrofibrils. Formation and maintenance of barrier function is influenced by cytokines, 3′,5′‐cyclic adenosine monophosphate and calcium. Changes in epidermal differentiation and lipid composition lead to a disturbed skin barrier, which allows the entry of environmental allergens, immunological reaction and inflammation in atopic dermatitis. A disturbed skin barrier is important for the pathogenesis of contact dermatitis, ichthyosis, psoriasis and atopic dermatitis.

Roles for tumor necrosis factor receptor p55 and sphingomyelinase in repairing the cutaneous permeability barrier

Epidermal TNF expression increases in response to cutaneous permeability barrier disruption and wound healing. TNF signaling is mediated by acid and neutral sphingomyelinases (A- and N-SMase), which generate ceramide, an important regulator of proliferation, differentiation, and apoptosis. In the epidermis, ceramide is known to be an integral part of the extracellular stratum corneum (SC) lipid bilayers that constitute the permeability barrier of the skin. We show here that topical application of TNF after experimental injury to the SC of hairless mice (hr(-/-)) enhances barrier repair. In TNF receptor p55-deficient (TNF-R55-deficient) mice (hr(+/+)), cutaneous barrier repair was delayed compared with wild-type (hr(+/+)) or TNF-R75-deficient (hr(+/+)) animals. After barrier disruption in hairless (hr(-/-)) and wild-type (hr(+/+)), but not in TNF-R55-deficient (hr(+/+)) mice, the enzymatic activities of both A-SMase and N-SMase were significantly enhanced. Stimulation of SMase activities was accompanied by an increase in C(24)-ceramide levels. Most A-SMase activity in hairless mice (hr(-/-)) was found in the outer epidermal cell layers and colocalized in the lamellar bodies with A-SMase and sphingomyelin. Reduction of epidermal A-SMase activity by the inhibitor imipramine resulted in delayed permeability barrier repair after SC injury. Together, these results suggest that TNF-R55 signaling pathways contribute to cutaneous permeability barrier repair through SMase-mediated generation of ceramide.

Effects of xerosis and ageing on epidermal proliferation and differentiation

The hallmarks of dry skin (xerosis) are scaliness and loss of elasticity. Decreased hydration and a disturbed lipid content of the stratum corneum are also well‐known features. The frequency of dry skin increases with ageing. The aim of this study was to examine if these known features of dry skin are related to changes in epidermal proliferation and differentiation. In addition, age‐related changes in normal and in dry skin were examined; 62 volunteers were divided by clinical grading and biophysical measurements into groups with young/normal, young/dry, aged/normal and aged/dry skin. Biopsy samples from the lower legs (most severe dryness) were examined by two‐dimensional gel electrophoresis and by immunohistochemistry for epidermal proliferation, epidermal keratins and cornified envelope proteins. There was a slight increase in proliferation in both groups with dry skin compared with normal skin of the corresponding age. In aged/normal compared with young/normal skin there was a significant decrease in proliferation. However, epidermal proliferation was the same in aged/dry skin as in young/normal skin. For epidermal differentiation, an age‐independent decrease of keratins K1 and K10 and an associated increase in the basal keratins K5 and K14 was detected in dry skin. There was also an age‐independent premature expression of the cornified envelope protein involucrin. In contrast, loricrin expression was not influenced by dry skin conditions. In summary, epidermal proliferation was significantly decreased in aged/normal compared with young/normal skin. Dry skin showed significant changes in the epidermal expression of basal and differentiation‐related keratins, and a premature expression of involucrin irrespective of age.

Cathepsin D is involved in the regulation of transglutaminase 1 and epidermal differentiation

We previously demonstrated that the aspartate protease cathepsin D is activated by ceramide derived from acid sphingomyelinase. Increased expression of cathepsin D in the skin has been reported in wound healing, psoriasis and skin tumors. We explored specific functions of cathepsin D during epidermal differentiation. Protein expression and enzymatic activity of cathepsin D increased in differentiated keratinocytes in both stratified organotypic cultures and in mouse skin during epidermal barrier repair. Treatment of cultured keratinocytes with exogenous cathepsin D increased the activity of transglutaminase 1, known to cross-link the cornified envelope proteins involucrin and loricrin during epidermal differentiation. Inhibition of cathepsin D by pepstatin A suppressed the activity of transglutaminase 1. Cathepsin D-deficient mice revealed reduced transglutaminase 1 activity and reduced protein levels of the cornified envelope proteins involucrin and loricrin. Also, amount and distribution of cornified envelope proteins involucrin, loricrin, filaggrin, and of the keratins K1 and K5 were significantly altered in cathepsin D-deficient mice. Stratum corneum morphology in cathepsin D-deficient mice was impaired, with increased numbers of corneocyte layers and faint staining of the cornified envelope only, which is similar to the human skin disease lamellar ichthyosis. Our findings suggest a functional link between cathepsin D activation, transglutaminase 1 activity and protein expression of cornified envelope proteins during epidermal differentiation.

Impaired neutral sphingomyelinase activation and cutaneous barrier repair in FAN‐deficient mice

The WD‐40 repeat protein FAN binds to a distinct domain of the p55 receptor for tumor necrosis factor (TNF) and signals the activation of neutral sphingomyelinase (N‐SMase). To analyze the physiological role of FAN in vivo, we generated FAN‐deficient mice by targeted gene disruption. Mice lacking a functional FAN protein do not show any overt phenotypic abnormalities; in particular, the architecture and cellular composition of lymphoid organs appeared to be unaltered. An essential role of FAN in the TNF‐induced activation of N‐SMase was demonstrated using thymocytes from FAN knockout mice. Activation of extracellular signal‐regulated kinases in response to TNF treatment, however, was not impaired by the absence of the FAN protein. FAN‐deficient mice show delayed kinetics of recovery after cutaneous barrier disruption suggesting a physiological role of FAN in epidermal barrier repair. Although FAN exhibits striking structural homologies with the CHS/Beige proteins, FAN‐deficient mice did not reproduce the phenotype of beige mice.

Role of the epidermal barrier in atopic dermatitis.

The skins permeability barrier protects against extensive water loss and prevents the entry into the skin of harmful substances like irritants, allergens and microorganisms. The permeability barrier is mainly located in the stratum corneum and consists of corneocytes and a lipid‐enriched intercellular domain. The barrier is formed during epidermal differentiation. In atopic dermatitis the skin barrier is disturbed already in non‐lesional skin. The disturbed skin barrier allows the entry of environmental allergens from house dust mites, animal dander and grass pollen into the skin. In predisposed individuals these allergens may trigger via immunologic pathways the inflammation of atopy. The causes for the disturbed epidermal skin barrier are changes in skin lipids and in epidermal differentiation, in particular filaggrin mutations. Filaggrin mutations lead to a disturbed skin barrier and dry skin which are hallmarks in atopic dermatitis. Therapeutic agents influence the skin barrier differently; topical therapy with potent corticosteroids does not lead to the repair of the barrier in atopic dermatitis, whereas therapy with the calcineurin inhibitors and lipid‐containing emulsions support barrier repair.

Acid and neutral sphingomyelinase, ceramide synthase, and acid ceramidase activities in cutaneous aging.

Abstract:  In aged skin, decreased levels of stratum corneum ceramides have been described. Epidermal ceramides are generated by sphingomyelin hydrolysis or synthesis from sphingosin and fatty acids and are degraded by ceramidase. We recently showed that epidermal acid sphingomyelinase (A‐SMase) generates ceramides with structural function in the stratum corneum lipid bilayers, which provide for the permeability barrier function of the skin. Here, we examined the activities of epidermal A‐SMase, ceramide synthase, and ceramidase in chronologically aged versus young hairless mouse skin. We found reduced A‐SMase and ceramide synthase activities in the epidermis of aged mice. However, studies on enzyme localization revealed unchanged, ongoing high A‐SMase activity in the outer epidermis, which correlated with reported normal barrier function found in aged skin under basal conditions. Reduced A‐SMase and ceramide synthase activity was noted in the inner epidermis, correlating with reduced capacity for permeability barrier repair in aging. Ceramidase activity was not age dependent. In summary, we found reduced activities of ceramide‐generating SMase and ceramide synthase in the inner epidermis of aged skin, explaining its reduced capacity in barrier repair. In contrast, A‐SMase activity in the outer epidermis was unchanged, indicating that this enzyme is crucially involved in basal permeability barrier homeostasis.

Gene expression is differently affected by pimecrolimus and betamethasone in lesional skin of atopic dermatitis

To cite this article: Jensen JM, Scherer A, Wanke C, Bräutigam M, Bongiovanni S, Letzkus M, Staedtler F, Kehren J, Zuehlsdorf M, Schwarz T, Weichenthal M, Fölster‐Holst R, Proksch E. Gene expression is differently affected by pimecrolimus and betamethasone in lesional skin of atopic dermatitis. Allergy 2012; 67: 413–423.

Differential suppression of epidermal antimicrobial protein expression in atopic dermatitis and in EFAD mice by pimecrolimus compared to corticosteroids

Abstract:  It has been suggested that the increased rate of bacterial infection in atopic dermatitis (AD) may be caused by reduced antimicrobial protein (AMP) expression. We were interested whether common treatments in AD affect antimicrobial defense. We investigated the effects of topically applied corticosteroids betamethasone valerate (BV) and triamacinolone acetonide (TA) and those of the calcineurin inhibitor pimecrolimus for 3 weeks on AMP expression in AD. BV and TA treatment in AD led to a significant reduction in AMP expression; protein expression of human beta‐defensins (hBD)‐2 and hBD‐3, psoriasin, RNase 7 and cathelicidin LL‐37 was below the level in skin of healthy controls. After pimecrolimus treatment, AMP expression was also reduced but less compared to BV and TA; the expression levels of hBD‐2, psoriasin and RNase 7 still remained above the control levels. In essential fatty acid–deficient (EFAD) mice, a model of chronic skin barrier disease with inflammation, expression of the mouse beta‐defensins mBD‐1, mBD‐3 and mBD‐14 (orthologues for hBD‐1, hBD‐2 and hBD‐3, respectively), was reduced by both treatments, again more pronounced by BV compared to pimecrolimus. In summary, we found that treatment for AD with corticosteroids in human skin and EFAD mice caused a strong reduction in AMPs; reduction was less with pimecrolimus. This result may explain the clinical observation that prolonged treatment with topical corticosteroids sometimes leads to bacterial infection.

Treatment of acute radiodermatitis with an oil-in-water emulsion following radiation therapy for breast cancer: a controlled, randomized trial.

Background:A side effect of radiotherapy for breast cancer is acute radiodermatitis. It is a common practice to keep irradiated skin dry on account of data from the 1950s that suggested this regimen limits dermatitis. However, severe dryness of the skin induced by irradiation results in itching and discomfort. Dry skin is characterized by scaliness, epidermal barrier dysfunction, and reduced stratum corneum hydration, and these signs and symptoms are reduced by treatment with an emulsion.Patients and Methods:We performed a randomized, controlled, open-label study with 66 patients (ITT population), treating the irradiated skin in one group (n = 34) with an oil-in-water emulsion (WO1932), while leaving the other group untreated (n = 32). Clinical scoring (ONS radiation skin reaction scoring, pruritus) and biophysical measurements (stratum corneum hydration and transepidermal water loss (TEWL), as a marker of skin barrier function) were determined at day 1 (directly after termination of the radiation therapy), day 8, and day 47 (± 7).Results:Irradiation increased the ONS score and pruritus, whereas skin hydration and TEWL were reduced. The primary hypothesis that the increase in skin hydration was significantly greater in the emulsion-treated compared to the untreated group as early as after 8 days of treatment could not be confirmed. At the end of the study (day 47 ± 7), however, normalization of stratum corneum hydration was more advanced in the treatment group compared to the untreated group and nearly reached the values of the contralateral healthy breast skin. ONS score and pruritus also revealed an advantage for the emulsion-treated group. TEWL did not show significant changes during emulsion treatment. No adverse events were caused by the treatment regimensConclusion:Treatment of radiodermatitis with an oil-in-water emulsion was well tolerated, enhanced stratum corneum hydration, improved clinical indicators, and provided relief from itching.Hintergrund:Eine häufige Nebenwirkung der Strahlentherapie bei Brustkrebs ist die Radiodermatitis. Üblicherweise wird die Haut trocken gehalten, da nach Untersuchungen aus den 50er Jahren so die Haut am wenigsten geschädigt wird. Starke Hauttrockenheit im Rahmen der Bestrahlung führt jedoch zu Missempfindungen und Juckreiz. Trockene Haut weist eine Schuppung, eine Hautbarrierestörung und einen erniedrigten Wassergehalt des Stratum corneum auf.Patienten und Methoden:In einer kontrollierten, randomisierten, unverblindeten Studie behandelten wir 34 von 66 Brustkrebspatientinnen (ITT-Population) mit einer Öl-in-Wasser-Emulsion (WO1932), 32 Patienten blieben unbehandelt. Wir untersuchten den klinischen Befund (ONS-Score), fragten nach dem Juckreiz und führten biophysikalische Messungen des Wassergehalts des Stratum corneum und des transepidermalen Wasserverlusts (TEWL) als Marker der Hautbarrierefunktion am 1., 8. und letzten Tag (Tag 47 ± 7) durch.Ergebnisse:Die Bestrahlung führte zu erhöhten ONS- und Prurituswerten und erniedrigten Werten des Wassergehalts der Haut und des TEWL. Die primäre Hypothese, dass sich die Hauthydration bereits nach 8 Tagen Behandlung gegenüber der unbehandelten Gruppe signifikant verbesserte, konnte nicht bestätigt werden. Jedoch hatte sich nach 47 ± 7 Tagen in der Behandlungsgruppe der erniedrigte Wassergehalt des Stratum corneum weitgehend normalisiert. Im Gegensatz zur unbehandelten Kontrollgruppe wurden in der Behandlungsgruppe auf der bestrahlten Brust fast die Werte der kontralateralen, gesunden Brust erreicht. Der Juckreiz und die ONS-Punktzahl nahmen stärker in der Behandlungsgruppe ab. Klinisch zeigte sich eine stärker ausgeprägte Besserung der Symptome in der Cremetherapiegruppe. Der TEWL als Marker der Barriere änderte sich nicht signifikant. Die Behandlung führte zu keinen unerwünschten Nebenwirkungen.Schlussfolgerung:Die Behandlung mit der Emulsion wurde gut toleriert. Die Ergebnisse zeigen, dass WO1932 das klinische Bild verbesserte, den Wassergehalt des Stratum corneum erhöhte und den Juckreiz reduzierte.