CLINICAL AND LABORATORY INVESTIGATIONS DOI 10.1111/j.1365-2133.2005.07109.x Plasminogen activator inhibitor-2 is expressed in different types of congenital ichthyosis: in vivo evidence for its cross-linking into the cornified cell envelope by transglutaminase-1 V. Oji, M.E. Oji, N. Adamini, T. Walker, K. Aufenvenne, M. Raghunath* and H. Traupe Department of Dermatology, University of Mu ¨nster, Von-Esmarch-Str. 58, D-48149 Mu ¨nster, Germany *Division of Bioengineering & Department of Biochemistry, National University of Singapore Correspondence Vinzenz Oji MD. E-mail: ojiv@uni-muenster.de, ojiv@mednet.uni-muenster.de Accepted for publication 27 August 2005 Key words cornified cell envelope, genetics, lamellar ichthyosis, Netherton syndrome, serpin, transglutaminase Conflicts of interest None to declare. Summary Background Plasminogen activator inhibitor-2 (PAI-2), a regulatory serpin of the plasminogen activator (PA) system, has been described as a potential component of the cornified cell envelope (CE). Protease inhibitors are essential for skin homeostasis and in particular for the regulation of the desquamation process. Therefore, an aberrant expression of PAI-2 could be involved in the pathogenesis of certain cornification disorders. Objectives Evaluation of the expression of PAI-2 in different types of congenital ichthyosis, especially in lamellar ichthyosis ⁄ nonbullous congenital ichthyosiform erythroderma (LI ⁄ NCIE) and in Netherton syndrome (NTS). Demonstration of the functional relationship between PAI-2 and transglutaminase (TGase)-1. Patients and methods Using immunohistochemistry we evaluated cryosections from individuals suffering from LI ⁄ NCIE (n ¼ 67), NTS (n ¼ 6), ichthyosis– follicularis–atrichia–photophobia syndrome (n ¼ 2) and Harlequin ichthyosis (n ¼ 1) in comparison with psoriasis vulgaris and healthy skin. Moreover, we assessed the respective TGase-1 activity and the presence of TGase-1 protein. A functional assay was developed to elucidate whether PAI-2 is a substrate for TGase-1. Results PAI-2 is expressed in different types of congenital ichthyosis and there is a strong correlation between TGase-1 activity and PAI-2 protein signal. Double staining revealed a strong colocalization of TGase-1 activity and PAI-2 protein. The epidermal incorporation of the specific PAI-2 peptide containing a TGase binding site revealed a strong pericellular staining in the stratum granulosum in healthy skin. In contrast, TGase-1-deficient skin showed only a lamellar staining in the stratum corneum. Conclusions We provide in vivo evidence that PAI-2 is a substrate of TGase-1. The nor- mal expression of PAI-2 in a large group of TGase-1-proficient LI ⁄ NCIE patients makes it rather unlikely that PAI-2 alone is a primary molecular cause of LI ⁄ NCIE. Lamellar ichthyosis (LI) (OMIM 242300, 601277, 604777, 604781, 606545, 609383), also referred to as nonbullous congenital ichthyosiform erythroderma (NCIE), usually pre- sents at birth with a collodion membrane. This clinically and genetically heterogeneous cornification disorder is character- ized by a scaling of the entire integument and a varying expression of erythroderma. 1 So far, four genes underlying LI ⁄ NCIE have been identified; 2–5 a common one is the TGM 1 gene, which encodes transglutaminase (TGase)-1 (OMIM 190195). Mutations in TGM 1 are also involved in the patho- genesis of the self-healing collodion baby. 6 TGases are Ca 2+ - dependent enzymes forming new covalent c-amide bonds between c-carboxamide groups of peptide-bound glutamine residues and various primary amines. 7 Six different types of TGases are known to be expressed in human epithelia, of which TGase-1, TGase-3 and TGase-5 are important partici- pants in the assembly of the cornified cell envelope (CE). 8 This insoluble sheath of -(c-glutamyl)lysine cross-linked 860 Ó 2006 British Association of Dermatologists • British Journal of Dermatology 2006 154, pp860–867