The Amino-Terminal Part of Human Filaggrin-2 is a Component of Cornified Envelopes

Filaggrin-2 (FLG2), a member of the S100-fused type protein family, is a 250 kDa protein mainly expressed in differentiated keratinocytes of the epidermis (Wu et al., 2009; Hsu et al., 2011). Its expression is reduced in atopic dermatitis (Pellerin et al., 2013), and heterozygous non-sense mutations of its gene are associated with a more persistent form of the disease in Afro-American populations (Margolis et al., 2014). Homozygous mutations of its gene are responsible for a rare skin disease called peeling skin syndrome type A (Bolling et al., 2018; Mohamad et al., 2018). However, the exact role of FLG2, a multi-domain protein, is not well known. Its COOH-tail exhibits anti-microbial activity (Hansmann et al., 2015), whereas its B domain is suspected to be involved in the hydration of the upper part of the epidermis, the cornified layer. Here, on the basis of immunological, global proteomics and in vitro enzymatic analysis we demonstrated that its amino-terminal domain is cross-linked to the cornified envelope, a pericellular resistant structure that replaces the plasma membrane during the last step of keratinocyte differentiation. In addition, depletion of FLG2 in reconstructed 3D human epidermis induced a marked fragility of cornified envelopes. These data suggest that FLG2 is important for proper mechanical strength of the cornified layer. They may explain the epidermal fragility observed in peeling skin syndrome type A patients.