Epidermal barrier formation and recovery in skin disorders (original) (raw)
The barrier is established during in utero development, maintained throughout life, and reestablished after a breach. The study of congenital epidermal barrier disorders will help to elucidate the pathways necessary to establish the barrier and also hints at the existence of a complex molecular redundancy that is evoked to repair a disrupted barrier. Although these severe congenital defects are rare, impaired epidermal barrier function is a hallmark feature of 2 of the most common inflammatory skin disorders, psoriasis and AD (43–46). Psoriasis is a chronic skin disease in which very distinct erythrosquamous silvery scales cover either areas of inflection, the trunk and proximal extremities, or the entire body. Psoriasis affects 2–3% of the population, with typical age of onset in the third decade (46). As discussed below, the impaired barrier of psoriatic lesions may contribute to keratinocyte hyperproliferation and cytokine activation of lymphocytes. AD is a chronic inflammatory skin disease characterized by pruritus, eczema, and cutaneous hyperreactivity to environmental triggers (45). The onset of AD is typically within the first 6 months of life, and the disease currently affects 10–20% of children in developed countries. This prevalence has risen steadily in the last 30 years, suggesting that environmental factors contribute to pathogenesis (47). The impaired skin barrier function of AD patients may contribute to increased antigen absorption and result in cutaneous hyperreactivity (48). For both of these inflammatory skin disorders, the degree of epidermal barrier disruption correlates with severity of presentation, as measured by degree of inflammation or state of dermatitis (45, 48). However, these disorders are markedly different in skin presentation, gene expression changes, and immune infiltration.
Psoriasis and AD have both genetic and environmental components underlying their pathogenesis. One of the most intriguing findings from the genome-wide screens of AD and psoriasis susceptibility is that multiple loci are coincident. Genetic linkage of both AD and psoriasis susceptibility to the epidermal differentiation complex on chromosome 1q21, a 1.6-Mb locus of at least 30 genes encoding proteins that both build and regulate barrier formation, strongly suggests a role for barrier function or repair in these inflammatory disorders (49, 50). Resident within this epidermal differentiation complex on 1q21 are many excellent candidate genes that may initiate or maintain a skin inflammatory disorder. The clustering of these epidermal-specific genes raises the question of whether they are held together as a result of recent evolution through gene duplication or to enable coordinate gene regulation, as observed in the Globin or Hox gene clusters. A priori, the linkage of both AD and psoriasis susceptibility to the epidermal differentiation complex on chromosome 1q21 may be one of the most difficult to unravel, considering the possibility that more than one of the genes mapping to this region play a significant role in the progression of a skin inflammatory disorder.
Very recently, McLean and colleagues convincingly demonstrated that an impaired barrier contributes to AD with the discovery that common mutations in the gene encoding filaggrin, resident in the epidermal differentiation complex, are a strong predisposing factor for AD (51). Recalling classic studies that AD is particularly common in individuals with severe ichthyosis vulgaris, these scientists determined that AD is inherited as a semidominant trait in the pedigrees they analyzed and thus demonstrated that filaggrin deficiency underlies ichthyosis vulgaris. Recognizing that filaggrin mutations would be in linkage disequilibrium with other genes in the epidermal differentiation complex, they extended the AD studies to 3 additional populations. Notably, in all studies, mutations in filaggrin — in both homozygous and heterozygous states — were highly overrepresented in pediatric AD patients.
Interestingly, 2 of these large AD cohorts were ascertained on the basis of asthma. Approximately half of severe AD patients will develop asthma, and two-thirds will develop allergic rhinitis (also known as hay fever) (52). AD, asthma, and hay fever are epithelial organ–specific allergen responses of the skin, lung, and nose, respectively. The high concordance rate among the atopic disorders raises the question of whether there are shared genetic risks for all 3 disorders or whether antigen exposure in AD is the initiating event. In these recent cohort studies, mutations in filaggrin are selectively associated with individuals that have both asthma and AD, with no association observed with asthma in the absence of AD (51). This finding has important clinical implications for medical management of children with AD, since AD is often the earliest sign of the progression to asthma and hay fever known as the atopic march (52). A classic study in mice demonstrated that epicutaneous sensitization to an allergen can induce dermatitis and augment airway hyperreactivity, characteristics of AD and asthma, respectively (53). Both these animal and human cohort studies suggest that the increased allergen exposure through the skin of the AD infant may itself predispose to asthma. This hypothesis would argue for aggressive means to control the infant’s AD before asthma develops. However, if impaired barrier recovery even in the seemingly unaffected areas underlies the genetic susceptibility to AD and contributes to the development of asthma, then controlling the outbreaks of AD lesions may not be sufficient to curtail the onset of asthma. Teasing apart the complex interaction of environmental challenges to the immunologic and epithelial systems in atopic disorders is a major direction of future research. The incomplete penetrance of heterozygous mutations in filaggrin resulting in either ichthyosis vulgaris or AD may be modified by the variant expression of other epidermal cornification genes also resident in the epidermal differentiation complex (30, 51).
In addition to these recent findings of filaggrin mutations underlying AD, earlier studies showed that a common E420K polymorphism in the lymphoepithelial SPINK5 protein (also known as LEKTI), encoded by the SPINK5 gene, contributes to the risk of developing AD (54). SPINK5, expressed in the outer layers of the epidermis, was discussed previously in this Review as the genetic defect underlying Netherton syndrome, which is characterized by congenital ichthyosis and AD (28). The _Spink5_-deficient animal model may help to unravel whether inadequate barrier development or repair is an initiating event in AD. Initial studies show that transplantation of skin from _Spink5–/–_mice to an immunocompromised mouse elicits an inflammatory infiltration in the dermis (55). Future studies involving allogenic grafting of _Spink5_–/– murine skin to an immunocompetent mouse will reveal the full extent of the immune and epidermal response to Spink5 deficiency. Although some human disorders can be modeled with null alleles in the mouse, the more complex analysis of these multigenic traits will require the creation of hypomorphic alleles. In theory, a more sophisticated animal model of AD would be a mouse with a E420K amino acid change in the Spink5 protein. Unfortunately, the glutamic acid residue at amino acid 420 is not conserved in the mouse Spink5 protein, raising the question of how best to model the common role of SPINK5 in the predisposition to AD. Therefore, future studies need to be directed toward understanding the role of SPINK5 in epidermal differentiation and barrier function. As well, expression of SPINK5 in other epithelial and immune cells needs to be explored in both humans and mouse models to unravel whether the atopic disorders observed in these patients is intrinsic to defects in immune cell activation or in epithelial barriers. Although the most widely recognized feature of the atopic disorders is a misregulation of the immune system, these findings suggest that there may be ways in which an epithelial barrier is built or responds to stress that predispose to these disorders (56).
The genetic linkage of psoriasis to the epidermal differentiation complex has not yet been untangled and may similarly reflect reduced expression of 1 or more genes resident in the locus. Mice with an epidermal-specific deletion of the dimeric transcription factor complex activator protein 1 (AP1), consisting of c-Jun and JunB, demonstrate many of the histological and molecular hallmarks of psoriasis (57). Prior to any skin alterations in these mice, the chemotactic proteins S100A8 and S100A9 — which map to chromosome 1q21 — were found to be highly upregulated, as has also been reported in human psoriatic samples (57, 58). Located immediately proximal to S100A9 are 2 genes encoding peptidoglycan recognition proteins that have also recently been put forth as candidate modifiers of psoriasis (59). Also resident within the epidermal differentiation complex is a family of 10 closely related genes that encode small proline-rich proteins (SPRRs). SPRRs are cross-linked by TGM1 to establish the CE scaffold onto which other proteins are then subsequently cross-linked. SPRR genes are highly upregulated in both human psoriatic plaques and the skin of barrier-deficient mouse models (33, 60, 61). Intriguingly, this upregulation of the SPRR genes is observed as a stress response in a variety of other tissues. Sprr genes are upregulated in bronchial epithelial and mononuclear cells after allergen challenge in an animal model of asthma (62). Similarly, Sprr genes are strongly upregulated in intestinal epithelium both in response to bacterial colonization and with induction of allergic gastrointestinal inflammation (62, 63). Duct ligation results in SPRR gene induction in biliary epithelial cells related to recovery of barrier function (64). These results suggest that there is a common stress response within epidermal, intestinal, bronchial, and biliary epithelium in response to infection. Are the SPRR proteins mediating a restoration of the barrier in multiple epithelial tissues? Alternatively, have the SPRR proteins been subverted as substrates for the CE but play an initial role in a primitive innate immune response? Interestingly, SPRR gene induction in cardiomyocytes responding to either biomechanical or ischemic stress renders a cardioprotective effect (65). This diverse expression data suggests epithelial barriers evoke a common stress response and that inappropriate recovery may contribute to the pathogenesis of inflammatory disorders.
One of the most intriguing findings from the genome-wide screens of AD and psoriasis susceptibility was that multiple identified regions, in addition to the epidermal differentiation complex, are coincident (50). One intriguing hypothesis to explain these common risks to develop either psoriasis or AD is that the underlying genes modulate how the skin repairs a breached barrier and whether the skin mounts an immune response. Breaches of the barrier are common events in our daily lives. For example, topical exposure to organic solvents or detergents that remove lipids from the stratum corneum cause a localized disruption. Scratching or mechanical stress that removes the upper layers of the skin also results in a local breach of this barrier. Using animal models and studies in patients, Elias, Feingold, and colleagues have demonstrated that after an initial rapid repair of this barrier by the lipids released from preformed granules, a slower recovery phase with de novo lipid synthesis follows (66). Two of the most striking results of barrier perturbation are the effects on DNA synthesis and cytokine production (67–69). Specifically, increased DNA synthesis is proportional to the degree of barrier perturbation and is partially restored to normal levels by occlusion of the skin with a water-impermeable membrane. Barrier disruption stimulates immediate production of cytokines, including TNF-α, IFN-γ, IL-1, and GM-CSF (67, 69). This cytokine release acts in an autocrine fashion to induce differentiation and growth of keratinocytes and also functions in a paracrine and endocrine fashion to stimulate both local and systemic inflammatory and immune responses. These studies demonstrate that epidermal hyperplasia and cytokine induction are intrinsic epidermal responses to barrier disruption (67, 69, 70).
Although aberrant lymphocyte activation is considered the root cause of skin inflammatory disorders, the discovery that barrier disruption initiates keratinocyte hyperproliferation and cytokine release suggested that there may be a feedback loop whereby the breach of barrier contributes to the activation of immune cells (69). Recent reports of keratinocyte-specific genetic alterations in animal models for psoriasis, including loss of the dimeric transcription factor complex AP1, consisting of c-Jun and JunB, or ectopic expression of activated STAT3 or TGF-β, expand the links between the keratinocyte and immunologic causes of this complex disorder (57, 71, 72). Of note, these results could have direct translational importance, as activated STAT3 is expressed in the nuclei of human psoriatic lesions (72). As well, JunB maps to chromosome 19p13, 1 of the 6 different psoriasis-susceptibility loci, and has reduced expression in the epidermis of psoriatic plaques (57). It will be interesting to determine whether these animal models have impaired barrier recovery as a component of the psoriatic progress.
To directly test the role of barrier restoration in inflammatory skin disorders, we compared the expression profile of genes misregulated in both barrier-deficient _Klf4_–/– mouse skin and human psoriatic plaques (15, 73, 74). Many similarities were identified, including an increased expression of gap junction proteins. Intriguingly, dominant-acting missense mutations in gap junction proteins underlie several rare ichthyoses including erythrokeratodermia variabilis, keratitis-ichthyosis-deafness syndrome, and Vohwinkel syndrome (75). Mimicking only the increased gap junctional communication observed in both _Klf4_–/– mice skin and human psoriatic plaques in transgenic mice resulted in congenital barrier deficiency (76). As described previously, the barrier is established in utero, maintained throughout life, and reestablished after trauma such as abrasion or wounding. Even when the epidermis has fully re-epithelialized, the ensuing scar at the site of trauma continues to exhibit mild barrier deficiency compared with uninvolved skin for as long as 1 year (77). Wounding is an initiating event in a significant percentage of psoriatic patients, which is clinically referred to as the isomorphic phenomenon. Interestingly, wounds on the mice with the increased gap junctional communication re-epithelialized at a normal rate. However, the inability to restore the barrier arrested the wounds in the hyperproliferative state and resulted in lymphocyte infiltration (76). These studies suggest that the signal to restore the homeostatic balance between proliferation and differentiation after re-epithelialization is restoration of the barrier and that in its absence the skin enters a pathologic state (Figure 2). Untangling some of these complex pathways will be a major area of future research, requiring the examination of tissue-specific genetic changes in both animal models and human patient samples.
Role of barrier acquisition in the epidermal response to wound healing. Under normal conditions the epidermis serves as a barrier to retain water within the body and prevent the entry of infectious agents (e.g., microbes) or chemical agents. Dendritic Langerhans cells, resident in the epidermis, recognize, process, and present antigens to T lymphocytes. In response to trauma to the epidermis, depicted as a full-thickness epidermal wound, keratinocytes increase their proliferation rate and cytokine release. Keratinocytes proliferate and migrate to re-epithelialize the wounded area. T lymphocytes are recruited into the damaged skin. As part of the normal process, keratinocytes initiate the process of terminal differentiation to restore the epidermal barrier. However, if the process of terminal differentiation or barrier recovery is impaired, the skin can enter an inflammatory state.
Even if impaired barrier recovery contributes to psoriasis and AD, there must be additional factors that determine the type of immune response elicited since these disorders are characterized by distinct Th1/Th2 lymphocyte profiles. Beyond identifying the causative polymorphisms that underlie the common susceptibility to both AD and psoriasis, we will need to understand the complexity of how epidermal and immune cells interact to elicit both the keratinocyte- and the T cell–specific manifestations of AD and psoriasis. One obvious possibility is that additional genetic or environmental risk factors modulate and specify the immune response. Much research focuses on the distinct Th1/Th2 lymphocyte infiltrations into psoriatic and AD skin.
The epidermal permeability barrier also has 2 distinct functions: retaining the water within the body and preventing the entry of dangerous substances. An animal model with an epidermal-specific targeted ablation of the adhesion molecule E-cadherin has a selective defect in water loss without any demonstrable defect in the entry of small molecules (41). These E-cadherin–deficient newborn mice have an increased rate of transepidermal water loss, but they demonstrate a normal ability to exclude the penetration of small molecules through their skin. As described above, many of these animal models of congenital barrier deficiencies die perinatally. Future studies of the immune response to a barrier deficiency evoked in adult skin will require either allogenic grafts or postnatally induced gene ablation.
Still, some hints about the type of immune stimulation that might result from the impaired barrier in these genetically distinct animal models can be gleaned from examining their neonatal skin transcriptional profile. For example, barrier-deficient _Klf4_–/– newborn mice already exhibit high levels of the mRNA encoding the cytokine thymic stromal lymphopoietin (Tslp), which activates dendritic cells and is highly upregulated in the epithelial cells of involved skin from patients with AD as well as in bronchial epithelium of asthmatics (78, 79). Recent experiments in animal models have shown that transgenic expression of Tslp in skin and lung epithelium is sufficient to induce AD and allergic airway inflammation, respectively (80, 81). Moreover, TSLP receptor knockout mice do not develop an inflammatory response in the lung to inhaled antigen (82). The high level of expression of TSLP in keratinocytes of AD patients and barrier-deficient mouse skin predicts that the keratinocytes signal to the immune cells to aid in combating the influx of invasive pathogens. This model also predicts that a failure to repair the breach in the barrier would continue to activate the local immune response. Consistent with the role of TSLP in immune cell activation is a possible direct role for this cytokine in epidermal differentiation and barrier acquisition. Altogether, along with understanding the distinct immune cell responses of psoriasis and AD, we need to investigate more specifically the nature of the epidermal barrier deficiency associated with these skin inflammatory disorders.