Altered expression of occludin and tight junction formation in psoriasis.

In simple epithelia, tight junctions are well developed and have barrier and fence functions. On the other hand, tight junctions are less developed in stratified epithelia. In the rodent epidermis, only maculae occludentes (i.e. focal strands or spot tight junctions) are observed in the most superficial zone of the granular cell layer. Occludin is an integral membrane protein, and is localized at tight junctions in simple epithelia. In normal epidermis, occludin is expressed at the maculae occludentes in the granular cell layer, indicating that it is associated with keratinocyte differentiation. Thus, we examined occludin expression in psoriasis, in which differentiation of keratinocytes is impaired. In psoriasis, occludin was expressed more broadly in the upper epidermis than in normal epidermis. In addition, immunoelectron microscopy showed occludin to be concentrated on the maculae occludentes in the spinous layer of psoriatic skin. These findings indicate that occludin and the formation of tight junctions are related to the proliferation and differentiation of keratinocytes, and to the pathogenesis of psoriasis.