Corneodesmosin (CDSN), desmoglein 1 (DSG1), and desmocollin 1 (DSC1) are adhesive proteins of the extracellular part of the corneodesmosomes, the junctional structures that mediate corneocyte cohesion. The degradation of these proteins at the epidermis surface is necessary for desquamation. Two serine proteases of the kallikrein family synthesized as inactive precursors have been implicated in this process: the stratum corneum chymotryptic enzyme (SCCE/KLK7/hK7) and the stratum corneum tryptic enzyme (SCTE/KLK5/hK5). Here, we analyzed the capacity of these enzymes to cleave DSG1, DSC1, and epidermal or recombinant forms of CDSN, at an acidic pH close to that of the stratum corneum. SCCE directly cleaved CDSN and DSC1 but was unable to degrade DSG1. But incubation with SCTE induced degradation of the three corneodesmosomal components. Using the recombinant form of CDSN, either with its N-glycan chain or enzymatically deglycosylated, we also demonstrated that oligosaccharide residues do not protect CDSN against proteolysis by SCCE. Moreover, our results suggest that SCTE is able to activate the proform of SCCE. These results strongly suggest that the two kalikreins are involved in desquamation. A model is proposed for desquamation that could be regulated by a precisely controlled protease-protease inhibitor balance.

Serine proteases belonging to the kallikrein group may play a central role in desquamation. We have identified human kallikreins 5, 7, and 14 (hK5, hK7, hK14) in catalytically active form in stratum corneum. All three enzymes are produced as inactive precursors. In this work, we prepared recombinant enzymes and enzyme precursors and characterized the catalytic properties of hK5 and hK14. With peptide substrates hK5 and hK14 both showed trypsin-like specificity and alkaline pH-optima. For the substrates tested, hK14 was superior to hK5 as regards maximum catalytic rate as well as catalytic efficiency. hK5, but not hK14, could activate pro-hK7 in a reaction which was optimal at pH 5-7. hK5 could activate its own precursor as well as pro-hK14. This was in contrast to hK14, which could activate pro-hK5 but not its own precursor. The activation of pro-hK5 either by auto-activation or by hK14 occurred at maximum rate at neutral or weakly alkaline pH, whereas activation of pro-hK14 by hK5 was optimal at pH 6-7. We conclude that the enzymes studied may be part of a protease cascade in the stratum corneum, and that the observed pH effects may have physiological relevance.

LEKTI is a 15-domain serine proteinase inhibitor whose defective expression underlies the severe autosomal recessive ichthyosiform skin disease, Netherton syndrome. Here, we show that LEKTI is produced as a precursor rapidly cleaved by furin, generating a variety of single or multidomain LEKTI fragments secreted in cultured keratinocytes and in the epidermis. The identity of these biological fragments (D1, D5, D6, D8 -D11, and D9 -D15) was inferred from biochemical analysis, using a panel of LEKTI antibodies. The functional inhibitory capacity of each fragment was tested on a panel of serine proteases. All LEKTI fragments, except D1, showed specific and differential inhibition of human kallikreins 5, 7, and 14. The strongest inhibition was observed with D8 -D11, toward KLK5. Kinetics analysis revealed that this interaction is rapid and irreversible, reflecting an extremely tight binding complex. We demonstrated that pH variations govern this interaction, leading to the release of active KLK5 from the complex at acidic pH. These results identify KLK5, a key actor of the desquamation process, as the major target of LEKTI. They disclose a new mechanism of skin homeostasis by which the epidermal pH gradient allows precisely regulated KLK5 activity and corneodesmosomal cleavage in the most superficial layers of the stratum corneum.

A new human 33-kDa serine protease was purified from human epidermis, and its cDNA was cloned from a keratinocyte library, from mRNA from a human keratinocyte line (HaCat) and from mRNA from human skin. Polyclonal antibodies specific for the new protein detected three groups of proteins in partially purified extracts of cornified eptihelium of human plantar skin. The three components are proposed to correspond to proenzyme, active enzyme, and proteolytically modified active enzyme. After N-deglycosylation, there was a decrease in apparent molecular mass of all detected components. Expression of the cloned cDNA in a eukaryotic virus-derived system yielded a recombinant protein that could be converted to an active protease by treatment with trypsin. Polymerase chain reaction analyses of cDNA from a number of human tissues showed high expression of the new enzyme in the skin and low expression in brain, placenta, and kidney. Homology searches yielded the highest score for porcine enamel matrix protease (55% amino acid sequence homology). High scores were also obtained for human and mouse neuropsin and for human stratum corneum chymotryptic enzyme. The function of this new protease, tentatively named stratum corneum tryptic enzyme, may be related to stratum corneum turnover and desquamation in the epidermis.

Mutations in the SPINK5 gene encoding the serine protease (SP) inhibitor, lymphoepithelial-Kazal-type 5 inhibitor (LEKTI), cause Netherton syndrome (NS), a life-threatening disease, owing to proteolysis of the stratum corneum (SC). We assessed here the basis for phenotypic variations in nine patients with "mild", "moderate", and "severe" NS. The magnitude of SP activation correlated with both the barrier defect and clinical severity, and inversely with residual LEKTI expression. LEKTI co-localizes within the SC with kallikreins 5 and 7 and inhibits both SP. The permeability barrier abnormality in NS was further linked to SC thinning and proteolysis of two lipid hydrolases (beta-glucocerebrosidase and acidic sphingomyelinase), with resultant disorganization of extracellular lamellar membranes. SC attenuation correlated with phenotype-dependent, SP activation, and loss of corneodesmosomes, owing to desmoglein (DSG)1 and desmocollin (DSC)1 degradation. Although excess SP activity extended into the nucleated layers in NS, degrading desmosomal mid-line structures with loss of DSG1/DSC1, the integrity of the nucleated epidermis appears to be maintained by compensatory upregulation of DSG3/DSC3. Maintenance of sufficient permeability barrier function for survival correlated with a compensatory acceleration of lamellar body secretion, providing a partial permeability barrier in NS. These studies provide a mechanistic basis for phenotypic variations in NS, and describe compensatory mechanisms that permit survival of NS patients in the face of unrelenting SP attack.