**1. Introduction**

Understanding of human diseases has been enormously expanded by the use of animal models, because they allow for in-depth investigation of pathogenesis and provide invaluable tools for diagnostic and pharmaceutical purposes. Atopic dermatitis (AD) is a chronic, relapsing form of skin inflammation, a disturbance of epidermal-barrier function that culminates in dry skin, pruritus, and IgE-mediated sensitization to food and environmental allergens (Bieber, 2008, Mori, et al., 2010, Tokura, 2010). AD is a common disease with no satisfactory form of therapy; therefore, understanding the mechanism of AD through animal models is an urgent issue to be solved (Jin, et al., 2009, Matsuda, et al., 1997, Shiohara, et al., 2004). The complexity and variability of AD and multiple genetic and environmental factors underlying AD make creating a reproducible, accessible, and relevant animal model of AD particularly challenging (Scharschmidt &Segre, 2008).

Thus far, a number of mouse models have been developed. These models can be categorized into three groups: (1) models induced by epicutaneous application of sensitizers; (2) transgenic mice that either overexpress or lack selective molecules; and (3) mice that spontaneously develop AD-like skin lesions. These models display many features of human AD, and their studies have resulted in a better understanding of the pathogenesis of AD. They allow for an in-depth dissection of the mediators and cells that are critical for the development of allergic responses (Jin, et al., 2009).

Located at the interface between the body and the environment, the epidermis is an elaborate structure that shares few properties with other biological barriers. Key functions include providing physical and biochemical protection (O'Regan &Irvine, 2010), and playing important roles in host defense, inflammation, and regulation of immune responses (Schleimer, et al., 2007). Patients with AD exhibit impaired skin barrier functions and abnormal structure and chemistry of the stratum corneum (SC) (Leung &Bieber, 2003). Alteration of the skin barrier in AD is evidenced by reduction in the water content of the SC and by increased transepidermal water loss (TEWL) (Aioi, et al., 2001). Skin barrier dysfunction has emerged as a critical driving force in the initiation and exacerbation of AD and as "driver" of disease activity (Cork, et al., 2009, Elias, et al., 2008), although it has once been noted as a disease of immunological etiology (Leung &Bieber, 2003).

Elias et al. proposed the outside-to-inside pathogenic mechanisms in AD for the following reasons: (1) the extent of the permeability barrier abnormality parallels the severity of the

Flaky Tail Mouse as a Novel Animal Model of Atopic Dermatitis:

2011) and nickel allergy (Novak, et al., 2008).

**2.2 Filaggrin and altered skin barrier function** 

Possible Roles of Filaggrin in the Development of Atopic Dermatitis 5

Typically atopic manifestations follow a certain sequence, called the atopic march, beginning with AD in early infancy, followed by food allergy, asthma and the development of allergic rhinitis (Illi, et al., 2004). The association of *FLG* mutation with atopic march has been reported in cases involving pediatric asthma (Muller, et al., 2009), peanut allergy (Brown, et al., 2011), atopic asthma (Poninska, et al., 2011), allergic rhinitis (Poninska, et al.,

In addition, epidemiological studies have identified extremely significant statistical association between *FLG* mutation and AD. Intriguingly, these mutations are highly associated with several characteristics in AD patients, such as reduced level of natural moisturizing factor (NMF) in the SC (Kezic, et al., 2008), increased incidence of dry and sensitive skin (Sergeant, et al., 2009), clinical severity and barrier impairment (Nemoto-Hasebe, et al., 2009), allergen sensitization and subsequent development of asthma associated with eczema (Weidinger, et al., 2008), and serum levels of IgE (Wang, et al., 2011). On the other hand, several studies failed to identify an effect of *FLG* mutations on AD, such as skin conditions assessed by clinical scoring of AD and measurement of TEWL in a French population (Hubiche, et al., 2007). A similar lack of association was reported in contact

As the conceptual framework underlying AD moves from solely immunological to epidermal barrier defects, the role of filaggrin and its putative mechanisms in priming AD have come under closer scrutiny. *FLG* mutations are postulated to have wide-ranging downstream biological effects, which include altered pH of SC, cutaneous microflora and

AD is characterized by eczematous skin lesion, dry skin, pruritus, increased TEWL, and enhanced percutaneous penetration of both lipophilic and hydrophilic compounds (Jakasa, et al., 2011, Wollenberg &Bieber, 2000). The skin barrier defect is one of the primary events that initiate disease pathogenesis, allowing the entrance of numerous antigens into the epidermis in patients with AD (Onoue, et al., 2009, Osawa, et al., 2011). The *FLG* mutation carriers have demonstrated elevated TEWL (Jungersted, et al., 2010, Kezic, et al., 2008), basal erythema, skin hydration, increased skin pH (Jungersted, et al., 2010, Nemoto-Hasebe, et al., 2009), SC thickness (Nemoto-Hasebe, et al., 2009), impaired SC integrity upon repeated tape stripping (Angelova-Fischer, et al., 2011), and increased diffusivity of PEG 370 (Jakasa, et al., 2011) compared to healthy donors. Nevertheless, these alterations found in *FLG* mutation carriers are not consistently correlated with AD since AD patients without *FLG* mutation might also share some similar features. (Hubiche, et al., 2007, Jakasa, et al., 2011, Jungersted, et al., 2010, Kezic, et al., 2008). It is, therefore, suggested that other factors besides *FLG* loss-

Since the skin barrier is related to intercellular lipid bilayers of the SC, it might be interesting to examine the composition and the organization of intercellular lipids of the SC in AD patients in relation to *FLG* genotype and disease severity (Jakasa, et al., 2011). Carriers of *FLG* mutations showed significantly reduced levels of NMF in the SC (Kezic, et al., 2008). Similar lipid composition of *FLG* mutation carriers and individuals with normal filaggrin was observed (Angelova-Fischer, et al., 2011, Jungersted, et al., 2010), but a lower cermide/cholesterol ratio was detected in the former group (Angelova-Fischer, et al., 2011). Filaggrins proteolytically degraded into a pool of free amino acids including histidine and glutamine which are further converted to, respectively, urocanic acid (UCA) and 2-

allergy (Carlsen, et al., 2011) and pediatric eczema (O'Regan, et al., 2010).

aberrant innate and adaptive immune responses (O'Regan &Irvine, 2010).

of-function mutations modulate skin barrier integrity, especially in AD.

disease phenotype in AD; (2) both the clinically uninvolved skin sites and the skin cleared of inflammation continue to display significant barrier abnormalities; (3) emollient therapy comprises effective ancillary therapy; and (4) specific replacement therapy which targets the prominent lipid abnormalities that account for barrier abnormality in AD, not only corrects the permeability-barrier abnormality but also comprises an effective anti-inflammatory therapy for AD (Elias, et al., 2008).

The evidence for a primary structural abnormality of the SC in AD is derived from a recently discovered link between the incidence of AD and loss-of-function mutations in the gene encoding filaggrin (*FLG*). Genetic studies have shown a strong association between AD and this mutation (Jin, et al., 2009). Moreover, there is a dose-response relationship between *FLG* deficiency and disease severity, such that patients with double-allele or compound heterozygote mutation in *FLG* display more severe and earlier-onset AD and an increased propensity for AD to persist into adulthood (Brown, et al., 2008, Irvine &McLean, 2006). This rapidly growing body of work has led to a paradigm shift in conception of AD pathogenesis, with increasing weight being placed on the role of a primary barrier abnormality that then precipitates downstream causing immunologic abnormalities as proposed (Elias, et al., 2008).

Based on these findings, it is assumed that mice that have a genetic defect in barrier function will provide a model of AD closer to the human disease than models provided by epidermal sensitization with allergens or haptens or by transgenic overexpression of cytokines in the skin or disruption of immune genes, and that these mice will have an advantage over NC/Nga mice in which the genetic defect is not known. Application of the knowledge gained from existing mouse models of AD to mice with genetic defects in skin barrier function should provide us with AD models that closely mimic human disease (Jin, et al., 2009).
