Protease/PAR-2 Signaling in Atopic Dermatitis and it has been proposed that PAR-2 plays an important role in pruritus. During neurogenic inflammation, various endogenous serine proteases such as tryptase from mast cells and trypsins from keratinocytes activate PAR-2 on sensory nerve ending to release calcitonin gene-related pep- tide (CGRP) and substance P (SP). These neuropeptides induce vasodilation, edema, and leukocyte recruitment, resulting in neurogenic inflammation in the local skin lesi- ons.75 PAR-2 is not only expressed in the peripheral nervous system but also in the central nervous system, spinal cord and brain. Therefore, PAR-2 signaling also stimulates the release of neuropeptides from central nerve endings at the spinal cord level, thereby activating CGRP receptor and SP receptor (NK1R) to transmit itch responses to the cen- tral nervous system.76 In addition, skin exposure to exoge- nous microbial proteases could also induce itch and inflam- mation via PAR-2. Recent study reported that the mice that over-expressed epidermal KLK7 displayed massive itchy behavior.77 Another study demonstrated that trypsin-induced scratching behavior in mice was inhibited by a PAR-2 blocking peptide, suggesting the role of serine protease/ PAR-2 signaling in pruritus.78 Moreover, PAR-2 activation is likely to be involved in pruritus of AD.79 PAR-2 and trypsin have been observed to be expressed at high rates in the lesional skin of patients with atopic dermatitis, and PAR-2 agonist peptides induce pruritus in AD patients.41 In addition, the fact that KLK5, -7, -8, and -14 are present in sweat, (sweat being the most common itch-triggering factor in AD) point to the important role of PAR-2 in pruritus of AD patients.7,80 PAR-2 is reported to interact synergisti- cally with transient receptor potential (TRP) vanilloid-type 1 (TRPV1), which belongs to the superfamily of TRP chan- nels, thereby amplifying itch sensation.81 These findings suggest that serine protease inhibitors or PAR-2 antago- nists might be a promising therapeutic tool for the mana- gement of itching and help break the vicious itch-scratch cycle in AD.41,75 ABERRANT PROTEASE/PAR-2 SIGNALING IN THE PATHOGENESIS OF AD Genetic abnormalities in the genes encoding protease/protease inhibitor There is increasing evidence that genes related to protease/ protease inhibitor become deregulated in patients with AD, shifting the balance between proteases and protease inhi- bitors toward increased protease activity (Fig. 4). It has been de-monstrated that there is an association between AD and a four base pair (AACC) insertion in the 3’-untranslated region of KLK7 gene, which increases the half-life of KLK7 mRNA and enzymatic activity of KLK7.82 The rare AACCAACC variant of KLK7 gene showed a more signi- ficant association with AD as compared to the common AACC variant. The genetic variant of KLK7 gene was found to be more significantly relevant in patients who did not have an elevated IgE level. The enzyme KLK7 plays an important role in desquamation by cleaving corneodes- mosomal proteins. In addition, it was reported that transge- nic mice over-expressing KLK7 presented cutaneous manifestations similar to chronic AD.83 This implies that the gain-of-function mutation polymorphism in the KLK7 gene causes a premature breakdown of corneodesmo- somes, leading to excessive desquamation and impairment of the epidermal barrier in the development of AD. In addition to abnormalities in the protease-encoding genes, several genomic defects in genes encoding members of the protease inhibitors have been identified in patients with AD. LEKTI is an inhibitor of multiple serine proteases in skin and tightly regulates the enzymatic activities of serine proteases including KLK5, -6, -7, -13, and -14, thereby controlling epidermal barrier function.31 A premature stop codon mutation in the SPINK5 gene, which encodes LEKTI, is known to be associated with NS, a rare ichthyosiform dermatosis characterized by congeni- tal ichthyosiform erythroderma, severe atopic manifesa- tions, and hair-shaft abnormality.84 The SPINK5 gene consists of 33 exons, encoding 15 LEKTI inhibitory do- mains with selective/specific inhibitory function.85 Previous study attempting to correlate genotype with phenotype in Japanese NS patients has demonstrated that the clinical severity of NS correlates with the residual expression of LEKTI-1.86 SPINK5-deficient mice have been reported to show increased proteolytic activities of KLK5 and KLK7 in the epidermis, abnormal degradation of DSG1, and resultant abnormal corneodesmosome cleavage and premature desquamation, suggesting that LEKTI is a key regulator of KLK5 and KLK7 activity and that defective SC adhesion by epidermal protease hyperactivity is the primary pathogenic event in NS.87 Recently, ELA2, a novel epidermal protease, has been identified in human and mouse skin and conceived as a potential trigger in NS pathogenesis by misprocessing of filaggrin and lipid.88 Being localized to the keratohyalin granule, ELA2 directly degrades (pro-) filaggrin and disrupts the lipid lamellae formation. LEKTI was demonstrated to inhibit ELA2 acti- vity indirectly, by controlling KLK5-mediated cleavage of pro-ELA2, and indeed, ELA2 was observed to be hyperac- tive in the LEKTI-deficient epidermis of NS patients and SPINK5-deficient mice.88 Transgenic mice over-expres- sing ELA2 also mimic the clinical features of NS patients with transient ichthyosiform dermatitis and impaired lipid barrier, implying that ELA2 is a critical epidermal protease that regulates epidermal barrier homeostasis, involving the Yonsei Med J http://www.eymj.org Volume 51 Number 6 November 2010 815