Down-regulation of peptidylarginine deiminase type 1 in reconstructed human epidermis disturbs nucleophagy in the granular layer and affects barrier function

Prolyl Endopeptidase Is Involved in Filaggrinolysis and Cornification

FLG is a well-known biomarker of atopic dermatitis and skin dryness. Its full proteolysis (or filaggrinolysis) produces the major constituents of the natural moisturizing factor. Some proteases/peptidases remain to be identified in this multistep process. Mining 16 omics analyses, we identified prolyl endopeptidase (PREP) as a candidate peptidase. Indirect immunofluorescence and confocal analysis demonstrated its localization in the granular and deep cornified layers, where it colocalized with FLG. Tandem mass spectroscopy and fluorescent quenching activity assays showed that PREP cleaved several synthetic peptides derived from the FLG sequence, at the carboxyl side of an internal proline. Deimination of these peptides increased PREP enzymatic efficiency. Specific inhibition of PREP in reconstructed human epidermis using benzyloxycarbonyl-pro-prolinal induced the accumulation of FLG monomers. Downregulation of PREP expression in reconstructed human epidermis using RNA interference confirmed the impact of PREP on FLG metabolism and highlighted a more general role of PREP in keratinocyte differentiation. Indeed, quantitative global proteomic, western blotting, and RT-qPCR analyses showed a strong reduction in the expression of bleomycin hydrolase, known to be involved in filaggrinolysis, and of several other actors of cornification such as loricrin. Consequently, at the functional level, the transepidermal electric resistance was drastically reduced.

Neutrophils only die twice

After apoptotic cell death begins neutrophils initiate NETosis, a second cell death program.

Deimination in epidermal barrier and hair formation

Peptidylarginine deiminases (PADs) transform a protein arginine residue into the non-standard amino acid citrulline. This calcium-dependent post-translational modification of proteins is called citrullination or deimination. As described in this special issue, PADs play a role in various physiological processes, and PAD deregulations are involved in many human diseases. Three PADs are expressed in the epidermis, where their roles begin to be deciphered. PAD1 and PAD3 are involved in keratinocyte differentiation, particularly in the epidermal barrier function, keratins, filaggrin and filaggrin-related proteins being the most abundant deiminated epidermal proteins. Reduced amounts of deiminated proteins and PAD1 expression may be involved in the pathogenesis of psoriasis and atopic dermatitis, two very frequent and chronic skin inflammatory diseases. The trichohyalin/PAD3/transglutaminase three pathway is important for hair shaft formation. Mutations of the PADI3 gene, leading to a decreased activity or abnormal localization of the corresponding isotype, are the cause of a rare hair disorder called uncombable hair syndrome, and are associated with the central centrifugal cicatricial alopecia, a frequent alopecia mainly affecting women of African ancestry. This article is part of the Theo Murphy meeting issue 'The virtues and vices of protein citrullination'.