Filaggrin processing is a major biological event in the production of the free amino acid component of Natural Moisturizing Factors (NMF) in skin hydration.  Skin aspartic acid protease (SASPase) is one of the key enzymes involved in the early processing of filaggrin during epidermal terminal differentiation.   Functional mutations in the SASPase gene have resulted in an impaired stratum corneum barrier function and the onset of dry skin condition (xerosis).  In previous studies, yeast two hybrid analyses using SASPase as bait against a human reconstructed skin library identified that the N-terminal domain of filaggrin 2 (Flg2-Nter) strongly bound  to the N-terminal fragment of SASPase.  This study aimed at deepening our understanding of the regulation of SASPase by Flg2-Nter.  Surface Plasmon Resonance analyses confirmed SASPase binding by Flg2-Nter which was unique among related S100 proteins.  Confocal immunohistochemical studies in human skin, using specific antibodies to SASPase and Flg2-Nter showed that the two proteins co-localized within the stratum granulosum.   Their interaction was confirmed by PLA in cultured cells . Western blots confirmed the presence of Flg2-Nter in all layers of the stratum corneum and epidermis.  In vitro enzymatic assays and western blots showed that the Flg2-Nter and a peptide from the EF-Hand of this region enhanced SASPase activity and its auto-activation to its 14 kDa active form.  Thus, the data suggests  that Flg2-Nter  can regulate  the auto-activation of  SASPase, which may be a key event upstream from filaggrin processing to NMF in the human skin.

The N -terminal domain of Filaggrin 2 co-localizes, binds to and activates the Skin aspartic acid protease in the epidermis of human skin