Cathepsin D is involved in the regulation of transglutaminase 1 and epidermal differentiation.

Friederike Egberts,Jens-Michael Jensen,Stefan SchüTze,Ehrhardt Proksch,Judith Steude,Marc Wickel,Stephan Pfeiffer,Supandi Winoto-Morbach,Paul Saftig,Michael Heinrich,Michael Schunck

doi:10.1242/jcs.01075

Abstract

We previously demonstrated that the aspartate protease cathepsin D is activated by ceramide derived from acid sphingomyelinase. Increased expression of cathepsin D in the skin has been reported in wound healing, psoriasis and skin tumors. We explored specific functions of cathepsin D during epidermal differentiation. Protein expression and enzymatic activity of cathepsin D increased in differentiated keratinocytes in both stratified organotypic cultures and in mouse skin during epidermal barrier repair. Treatment of cultured keratinocytes with exogenous cathepsin D increased the activity of transglutaminase 1, known to cross-link the cornified envelope proteins involucrin and loricrin during epidermal differentiation. Inhibition of cathepsin D by pepstatin A suppressed the activity of transglutaminase 1. Cathepsin D-deficient mice revealed reduced transglutaminase 1 activity and reduced protein levels of the cornified envelope proteins involucrin and loricrin. Also, amount and distribution of cornified envelope proteins involucrin, loricrin, filaggrin, and of the keratins K1 and K5 were significantly altered in cathepsin D-deficient mice. Stratum corneum morphology in cathepsin D-deficient mice was impaired, with increased numbers of corneocyte layers and faint staining of the cornified envelope only, which is similar to the human skin disease lamellar ichthyosis. Our findings suggest a functional link between cathepsin D activation, transglutaminase 1 activity and protein expression of cornified envelope proteins during epidermal differentiation.

Highlights

The goal of epidermal differentiation is to form the epidermal permeability barrier, which prevents excessive water loss and entry of harmful substances into the body
We suggested a functional role of tumor necrosis factor (TNF) and ceramide derived from acid sphingomyelinase in cutaneous permeability barrier repair after experimentally induced injury of the skin
In the present study we explored the functional role of CTSD in the skin during permeability barrier repair and epidermal differentiation using keratinocyte cell culture, wild-type and CTSD-deficient mice

Summary

Introduction

The goal of epidermal differentiation is to form the epidermal permeability barrier, which prevents excessive water loss and entry of harmful substances into the body. Involucrin, an early marker of terminal differentiation, is a soluble protein precursor of the cross-linked cornified envelope (CE) and is synthesized in the keratinocytes of the upper spinous layers (Watt, 1983; Eckert et al, 1993; Kanitakis et al, 1987; Negi et al, 1981; Rice and Green, 1979; Steinert and Marekov, 1997). In hyperproliferative diseases like psoriasis, premature expression of involucrin is found in the lower spinous layers (Thewes et al, 1991). Another main component of the CE is loricrin. It is expressed in a later stage of differentiation (Watt, 1983) and is cross-linked to other epidermal proteins such as cystatin, elafin or filaggrin

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