| Filaggrin is a highly abundant, histidine‑rich structural protein of the epidermal differentiation complex that is synthesized in granular layer keratinocytes as a large, repetitive polyprotein precursor, profilaggrin, which undergoes calcium‑ and protease‑dependent dephosphorylation and stepwise proteolysis to yield multiple filaggrin monomers that compact keratin intermediate filaments and support assembly of the cornified cell envelope in the stratum corneum. Profilaggrin resides in keratohyalin granules and contains tandem filaggrin repeats flanked by N‑ and C‑terminal domains that regulate its processing and cross‑linking; proteolytic cleavage liberates filaggrin units that bind and aggregate keratin, drive cytoskeletal collapse, and cooperate with transglutaminase‑mediated cross‑linking of loricrin, involucrin, and related proteins to produce a mechanically robust corneocyte shell that provides the scaffold for extracellular lipid lamellae. Filaggrin monomers undergo further enzymatic degradation into hygroscopic amino acids and their derivatives, including urocanic acid and pyrrolidone carboxylic acid, that collectively contribute to natural moisturizing factor and help maintain stratum corneum hydration and an acidic surface pH, which in turn modulates serine proteases and lipid‑processing enzymes that control desquamation, barrier lipid organization, and antimicrobial defense. Filaggrin acts as a central node of epidermal barrier homeostasis, integrating terminal keratinocyte differentiation, cornified envelope formation, and regulation of the microenvironment at the skin surface. The FLG gene shows strong, epidermis‑restricted expression and contains multiple nearly identical filaggrin repeat units, a genomic architecture that is prone to loss‑of‑function variants; truncating mutations and copy‑number changes reduce or abolish profilaggrin production, impair keratin aggregation, and lead to a weaker, more permeable barrier characterized clinically by xerosis and fine scaling, as seen in ichthyosis vulgaris where FLG null alleles represent the primary genetic cause. In atopic dermatitis, FLG loss‑of‑function variants represent the strongest known genetic risk factor, and carriers display increased transepidermal water loss, enhanced penetration of irritants and allergens, and greater susceptibility to colonization, which together favor chronic inflammation of the skin and contribute to atopic multimorbidity including asthma and allergic rhinitis. FLG mutations are enriched in early‑onset and persistent forms of atopic dermatitis and influence disease severity and course, supporting a model in which primary barrier impairment precedes and conditions immune dysregulation at the skin surface. |
