**4.3 Protease inhibitors**

206 Atopic Dermatitis – Disease Etiology and Clinical Management

interactions between the histamine receptors and the barrier repair process have not been

Nitric oxide (NO) is also involved in barrier homeostasis. We first demonstrated that neuronal nitric oxide synthase knockout (nNOS-/-) mice showed a faster barrier recovery rate than did wild-type mice. nNOS is expressed in epidermal keratinocytes [Ormerod 1998]. Thus, NO generated by keratinocytes might delay barrier repair. To examine this possibility, we next evaluated the effects of NO donor and NOS inhibitor on the barrier recovery rate. Topical application of a NO donor, S-nitroso-N-acetyl-D,L-penicillamine delayed barrier recovery. The application of a nNOS inhibitor accelerated barrier recovery, while the application of a inducible nitric oxide synthase (iNOS) inhibitor did not affect it. Moreover, topical application of a guanylyl cyclase inhibitor accelerated barrier recovery. We observed the release of NO from a skin organ culture after barrier disruption. Thus, regulation of nNOS in epidermal keratinocytes might be useful approach to improve barrier homeostasis

Ionotropic receptors Accelerate Barrier

P2X receptor (Denda 2002a) Antagonist Agonist NMDA receptor (Fuziwara 2003) Antagonist Agonist Cholinergic receptor (Denda 2003) Antagonist Agonist GABA(A) receptor (Denda 2002b) Agonist - Glycine receptor (Denda 2003) Agonist -

Adrenergic 2 receptor (Denda 2003b) Antagonist Agonist Dopamine 2-like receptor (Fuziwara 2005) Agonist Antagonist Serotonin receptor (Denda 2005 ) Agonist - Melatonin receptor (Denda 2005) Agonist -

Table 2. Effects of agonists and antagonists of neurotransmitter receptors on skin

Ryanodine receptors (RyR) play an important role as calcium channels in the regulation of intracellular calcium levels in the nervous system and muscles. We investigated the expression of RyR in human epidermis. (Denda 2011) Immunohistochemical studies and RT-PCR indicated the expression of RyR type 1, 2, and 3 proteins in epidermal keratinocytes. The expression level of each RyR subtype was higher in differentiating keratinocytes than in proliferative cells. We also demonstrated the functional expression of RyR by means of calcium imaging. In cultured human keratinocytes, application of the RyR agonist 4-chloro-*m*-cresol (CMC) induced elevation of the intracellular calcium concentration and co-application of the RyR antagonist 1,1'-diheptyl-4,4'-bipyridinium dibromide (DHBP) blocked the elevation. Application of CMC accelerated keratinocyte differentiation *in vitro*. On the other hand, topical application of CMC after tape-stripping of hairless mouse skin delayed barrier recovery, while application of an RyR antagonist,

Recovery

Delay Barrier Recovery

elucidated yet

(Ikeyama 2007).

G-Protein coupled receptors

No effect, or experiment has not been done.

permeability barrier recovery.

Topical application of specific protease inhibitors accelerates barrier recovery after barrier disruption (Denda 1997). Topical application of 4-(aminomethyl)cyclohexane carboxylic acid (tranexamic acid), a well known anti-plasmin reagent, also accelerates barrier recovery. In contrast, inactive analogs of tranexamic acid do not influence barrier recovery. Application of several trypsin-like serine protease inhibitors, e.g., leupeptin, TLCK and PMSF, accelerates barrier recovery, while other protease inhibitors, e.g., EDTA, pepstatin, Nethylmaleimide, chymostatin, and TPCK, have no effect on barrier recovery. Although the mechanism was not clarified, it was shown that protease activated receptor type 2 is associated with barrier homeostasis (Hachem 2006).
