The cystatin M/E-controlled pathway of skin barrier formation: expression of its key components in psoriasis and atopic dermatitis

Background The antiprotease activity of cystatin M/E regulates skin barrier formation, as it inhibits the activity of cathepsin V, cathepsin L and legumain, thereby controlling the processing of transglutaminase 3. Misregulation of this pathway by unrestrained protease activity, as seen in cystatin M/E-deficient mice, leads to abnormal stratum corneum and hair follicle formation, and severe disturbance of skin barrier function. Objectives Our major aim was to make a quantitative analysis of the expression of all players of this pathway in the epidermis of patients with inflammatory skin diseases. A second aim was to determine if reconstructed human skin could be used as an in vitro model system to investigate this pathway. Methods Autopsy material from normal human tissues, biopsies from normal skin of healthy volunteers, and lesional skin from patients with atopic dermatitis and psoriasis were used to study the expression of the above-mentioned molecules at the mRNA level by quantitative real-time polymerase chain reaction. Localization of the protein was performed by immunofluorescence microscopy, and expression was quantitated by image analysis. Results In skin, cystatin M/E is expressed at relatively higher levels than its target proteases, when compared with other tissues, which emphasizes its prominent role in cutaneous biology. We found decreased expression of cystatin M/E and cathepsin V in lesional atopic dermatitis and psoriasis epidermis at the mRNA level as well as the protein level. Cathepsin L and transglutaminase 3 were increased at the transcriptional level; however, this was not reflected by higher protein levels. Interestingly, the expression of all these molecules in reconstructed skin was qualitatively and quantitatively similar to the in vivo situation. Conclusions Disturbance of the cystatin M/E - cathepsin pathway could contribute to the dysregulated skin barrier function observed in inflammatory dermatoses. Human reconstructed skin appears to be a valuable model to study this novel biochemical pathway in vitro.