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Transglutaminase 3 Protects against Photodamage Journal of Investigative Dermatology (2017) 137, 1590e1594; doi:10.1016/j.jid.2017.02.982 TO THE EDITOR Transglutaminase (TG) 3 belongs to an important family of protein cross- linking enzymes that includes nine members, three of which (TG1, TG3, and TG5) are expressed in the epidermis (Candi et al., 2005; Pitolli et al., 2017; Terrinoni et al., 2012). TGs are responsible for the formation of a specialized structure: the cornified cell envelope (CE) (Amelio et al., 2012, 2013; Candi et al., 1998, 2002, 2004, 2005; Kalinin et al., 2002). TG3 expression is restricted almost exclu- sively to the differentiated granular layer of the epidermis and to hair folli- cles, and it is important in the rein- forcement step of CE assembly that occurs in the granular layer (Eckert et al., 2005; Hitomi, 2005; Hitomi et al., 2001). Interestingly, ablation of the TG3 enzyme in mice (John et al., 2012) impaired hair development, with minor defects in skin formation and no gross barrier function defects, indicating that TG3 plays a crucial role in hair development. A deeper analysis of the barrier function has been per- formed in further studies, in which transglutaminase 3 knockout (TG3KO) mice epidermis was investigated using the T helper type 1 mouse model of contact hypersensitivity induced by the hapten fluorescein isothiocyanate. Fluorescein isothiocyanate penetration and sensitization at the skin surface was found to be more invasive in TG3KO mice, indicating that TG3 contributes to the functionally intact cutaneous barrier (Bognar et al., 2014). One important function of the skin is to protect organisms against environ- mental stress; in particular, protection against UVB irradiation is of major relevance for life (Kulms and Schwarz, 2000). Because TG3-depleted mice are less efficient in preventing hapten penetration and the TG3 gene has been indicated as a candidate tumor sup- pressor gene in human head and neck cancer (Wu et al., 2013), we sought to determine whether the UVB-filtering capacity of the TG3KO epidermis is compromised. For this reason, we irra- diated newborn TG3KO mice with UVB and analyzed their phenotype. As an experimental model, we used a different TG3 knockout mouse model, obtained by a gene-trap approach (see Supplementary Materials and Methods online), presenting the same pheno- type of the previous generated one (John et al., 2012; Supplementary Figures S1 and S2 online). All the experimental procedures were approved by the Animal Ethics Com- mittee of the University of Rome “Tor Vergata.” Animal care was in compli- ance with Italian (D.L. 116/92) and European Council (O.J. of E.C. L358/1 18/12/86) regulations on the protection of laboratory animals. The back skins of 5.5-day-old neonatal wild-type (WT) and TG3KO mice were irradiated with 250 mJ cm À2 of UVB. Sections of TG3KO skin displayed significantly more tissue damage than WT skin, indicated by the massive enlargement and ballooning of cells in the basal, spinous, and granular layers as well as in derma (Figure 1a). To quantify cellular and tissue damage, we per- formed a TUNEL assay (Figure 1b). TUNEL-positive cells were detected in both WT and TG3KO skins; however, the signal was very intense in TG3KO compared with WT. It was also present deeper in the derma, indicating that in the absence of TG3 the CE is less effi- cient in protecting the organism from UVB-induced damage. Quantification of the epidermal TUNEL-positive cells indicated a three-fold increase of TUNEL-positive cells in TG3KO compared with WT skin (Figure 1c). Accordingly, cleaved caspase 3 is Abbreviations: CE, cornified cell envelope; TG, transglutaminase; TG3KO, transglutaminase 3 knockout mice; WT, wild type Accepted manuscript published online 16 March 2017 ª 2017 The Authors. Published by Elsevier, Inc. on behalf of the Society for Investigative Dermatology. V Frezza et al. TG3 and UVB Radiation Journal of Investigative Dermatology (2017), Volume 137 1590