Thibaut S, Bernard BA
L'Oréal Recherche, Clichy, France

Transglutaminases (TGs) are Ca2+-dependent enzymes that catalyze the formation of an isodipeptide cross-link between the epsilon-NH2 of a protein-bound lysine residue and a gamma-amide of a protein-bound glutamine residue, generating an insoluble macromolecular aggregate. Their involvement in terminal differentiation events in the epidermis has been supported by many observations, many studies of co-expression with cornified cell envelope (CCE) structural proteins and in vitro cross-linking. In the human hair follicle, the CCE formation was known to occur in the hair shaft, the hair cuticle, inner root sheath (IRS) and outer root sheath (ORS). We took advantage of new antibodies to study and compare the expressions of TGs 1, 5 and 3 in anagen hair follicle. TG 1 expression was restricted to the IRS cells and the innermost layer of ORS in the distal part of the follicle. TG 5 distribution in the proximal part of the follicle was very close to that of involucrin. The three layers of IRS strongly expressed this TG, and a weak staining was also detected in the hair cuticle and the hair shaft. Then, from the middle ORS to infundibulum, two distinct distributions appeared. TG 5 was uniformly expressed in all ORS cell layers, whereas involucrin expression was restricted to the innermost layer. By contrast, we noted the conspicuous absence of TG 3 from IRS and ORS layers all along the follicle. TG 3 expression thus seemed to be restricted and specific of the hair shaft and the hair cuticle. This result was confirmed by Western Blot, analysis by laser confocal microscopy and detection of transglutaminase activity in the hair shaft. All these data indicate that TG 3 may play a role in hair shaft keratinisation and scaffolding, and have a key function in the rigidity of this structure.