An integrated view of the epidermal environmental interface

DERMATOLOGICA SINICA 33 (2015) 49e57

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REVIEW ARTICLE

An integrated view of the epidermal environmental interface Peter M. Elias*, Joan S. Wakefield Dermatology Service, VA Medical Center, San Francisco and Department of Dermatology, UC San Francisco, San Francisco, CA, USA

a r t i c l e i n f o

a b s t r a c t

Article history: Received: Feb 16, 2015 Accepted: Mar 15, 2015

The harsh realities of life in a terrestrial environment demand that the epidermis must respond quickly to many types of external stressors that threaten the internal milieu. We review herein how the individual protective functions of the skin (e.g., permeability barrier, antimicrobial barrier, antioxidant defense, integrity, hydration, and mechanical integrity) appear to be integrated into a broad defensive shield. Then, we will review how the epidermis responds to external perturbations through a network of extracellular signaling mechanisms and second messengers, as well as the pathophysiology of these responses in diseases such as atopic dermatitis and psoriasis. Finally, we will briefly discuss the potential therapeutic implications of this new functional paradigm. Copyright © 2015, Taiwanese Dermatological Association. Published by Elsevier Taiwan LLC. All rights reserved.

Keywords: antimicrobial peptide cornified envelope homeostasis stratum corneum tight junction

Introduction Too long viewed as a mere battleground for the immune system, the epidermis is asserting its rightful place at the center of cutaneous biology and pathophysiology. Although immunologists seek ever-finer distinctions between T cell subsets in inflammatory lesions, it is now increasingly clear that the protective requirements of the skin dictate virtually every metabolic process (including adaptive immune responses) in its underlying layers. True, there are “outside-to-inside-back-to-outside” vicious cycles, whereby immune responses further compromise epidermal function, and there are also examples of primary immune disorders, such as autoimmune and bullous diseases, human immunodeficiency virus (HIV) infections, and superantigen-initiated flares of erythrodermic psoriasis, where a primary inflammatory infiltrate can produce downstream abnormalities in epidermal function (e.g., for HIV, see Gunathilake et al1). But, as the example of filaggrin-deficient atopic dermatitis (AD) eloquently demonstrates, most cutaneous immune phenomena occur downstream of primary epidermal insults, whether inherited or acquired, and these responses are recruited

Conflicts of interest: The authors declare that they have no financial or nonfinancial conflicts of interest related to the subject matter or materials discussed in this article. * Corresponding author. Dermatology Service, VA Medical Center, San Francisco and Department of Dermatology, UC San Francisco, 4150 Clement Street, MS 190, San Francisco, CA 94121, USA. E-mail address: [email protected] (P.M. Elias).

only when epidermal homeostatic responses fail to promptly reestablish normal cutaneous function. In this review, we consider: (1) a new “holistic” view of epidermal defense; (2) a concise review of the structural basis for the barrier with an update on tight junctions (TJs) and the corneocyte lipid envelope; (3) intraepidermal metabolic processes that are regulated by barrier requirements; (4) the role of homeostatic signaling mechanisms in regulating these responses; and (5) interrelated processes that impact disease pathogenesis.

Brief review of barrier structure and function Two-compartment model The protective functions of the skin, including the permeability barrier, largely localize to the outer epidermis and stratum corneum (SC; Table 1; Figure 1). The SC is an a nucleate structure, arranged in a “brick-and-mortar” mosaic of flattened corneocytes (“bricks”), embedded in lipid-enriched extracellular matrix (“mortar”) that is organized into parallel stacks of lamellar bilayers, enriched in ceramides, cholesterol, and free fatty acids (FFAs).2 These water-repellent lipids restrict the outward flow of water, while also impeding the inward absorption of toxins, allergens, and microbial pathogens.3 It is the secretion of the contents of multiple, small ovoid lamellar bodies (LBs)2 that delivers both lipid precursors and hydrolytic “processing” enzymes that generate the hydrophobic species, ceramides (Cer), FFAs, and cholesterol, that

http://dx.doi.org/10.1016/j.dsi.2015.03.008 1027-8117/Copyright © 2015, Taiwanese Dermatological Association. Published by Elsevier Taiwan LLC. All rights reserved.

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Table 1 Defensive gradients in outer epidermis. Functions

Outer surface (sebaceous glands)

Stratum corneum

Stratum granulosum

Antimicrobial Permeability barrier Antioxidant UV-B Mechanical Cohesion Cytokine activation Neurosensory Hydration

AMP, FFA (Y pH) d Vitamin E d d d d d Glycerol

FFA (Y pH), AMP, SPI Cholesterol, Cer, FFA in lamellar bilayers Vitamin E, Sprr2d, Sprr2h, Slpi t-UCA (melanin) Cornified envelopes Lipids, corneodesmosomes IL-1a/b release d FLG / NMF; glycerol, urea

AMP, TLR Tight junction (larger xenobiotes) SOD, CoQ, catalase, GluTR Melanin d Desmosomes, adherens junctions TNFa, IL-1a/b, GMCSF, IL-6, NGF, AR, VEGF TRPVs, TRPM8 AQP channels Urea transporters

AMP ¼ antimicrobial peptide; Cer ¼ ceramide; CoQ ¼ coenzyme Q; FFA ¼ free fatty acid; GM-CSF ¼ granulocyte-macrophage colony-stimulating factor; GluTR ¼ glutamyl tRNA reductase; IL ¼ interleukin; NGF ¼ nerve growth factor; NMF ¼ natural moisturizing factor; Slpi ¼ serine leukocyte protease inhibitor; SOD ¼ superoxide dismutase; SPI ¼ serine protease inhibitor; TLR ¼ Toll-like receptor; TNF ¼ tumor necrosis factor; TRPM8 ¼ transient receptor potential melastatin-8; TRPV ¼ transient receptor potential vanilloid; t-UCA ¼ trans-urocanic acid; VEGF ¼ vascular endothelial growth factor.

mediate the permeability barrier (Figure 2).4 These two lipids, along with as-yet-unidentified amphiphilic molecules, are required for the organization of the secreted lipids into mature lamellar bilayers.2 Corneocyte-bound lipid envelope The external surface of the cornified envelope (CE) is coated with a monolayer of u-hydroxyceramides (u-OH-Cer) that are covalently bound to peptides (1
involucrin) within the CE.5e7 Both the origin and the function of this structure are still uncertain. Although most workers believe that it is formed from a pool of secreted acylCer, the corneocyte-bound lipid envelope (CLE) could also derive from the insertion of a myriad of LB limiting membranes during the exocytosis of these organelles.4 We noted that the CLE fails to form in several inherited and acquired disorders that compromise steps that either generate acylCer or oxidize the u-OH-linoleate moiety of acylCer (Figure 2). Because all of these disorders are characterized by a faulty permeability barrier, poor SC hydration, and impaired desquamation, it is a tempting (but still not certain) prospect that the CLE is linked to one or more of these functions.4 TJ controversy How should we interpret an ever-expanding literature that proclaims a potential role for TJ in normal permeability barrier

function (e.g., see Brandner et al8 and Kubo et al9), as well as a potential role for abnormal TJ function in AD?10 We will attempt to navigate this heavily invested subject as follows. First, complex TJ structures, such as those found in the kidney and gastrointestinal tract, do not occur in adult keratinizing epithelia.11 Second, with the exception of highly complex TJs in renal collecting tubules, where they comprise multitiered, overlapping sites of membrane fusion (“zonulae occludentes”), in other tubular epithelia, such as the trachea and gastrointestinal tract, these junctions provide a relatively poor barrier against paracellular water movement.12,13 Much of the confusion in the skin-related literature has occurred because “TJ proteins” are widely equated with “TJ”.8,9,14 Certainly, multiple TJ proteins heavily decorate the apicalelateral plasma membranes of cells in the outer stratum granulosum (SG) of normal adult epidermis, forming “kissing points.” However, these focal attachmentsdi.e., “maculae occludentes”11ddo not comprise true zonulae occludentes (¼ TJ), as occur in tubular epithelia. The most compelling evidence that these putative TJs play no direct role in the paracellular water barrier comes from solvent extraction studies, where removal of SC lipids by repeated, gentle, lipid solvent swabbing completely abrogates the permeability barrier.15 It should be noted that this observation also excludes a possible “backup” role for TJ-like structures in the water barrier, although it remains possible that true TJs eventually could begin to form in response to such repeated solvent wipes. Moreover, these incomplete structures could suffice to interdict the paracellular passage of

Figure 1 Protective (defensive) functions are related, coregulated and interdependent.

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Figure 2 Pathways leading to corneocyte-bound lipid envelope (CLE) formation: insights from inherited and acquired lipid metabolic disorders. Note. From “Formation and functions of the corneocyte lipid envelope (CLE),” by P.M. Elias, R. Gruber, D. Crumrine, et al., 2014, Biochim Biophys Acta, 1814, p. 314e8. Copyright 2014, Elsevier. Adapted with permission.

larger xenobiotes, particularly when the overlying lipid-based barrier becomes defective, as occurs in atopic dermatosis.10 Yet, these structures, although insufficient to contribute directly to the normal water barrier, are nonetheless critical for the development of permeability barrier competence. Transgenic knockout of the key TJ protein, claudin 1, results in a fatal, postnatal permeability barrier abnormality.14 Indeed, our recent studies show that replete TJs are present early in epidermal development, but they become functionally incompetent later in fetal life in parallel with the establishment of the lipid-based barrier.16 Because adult epidermis does not generate the types of complex zonulae occludentes necessary to impede paracellular water movement, attention should be focused instead on the possible functions of these incomplete junctions (maculae occludentes) in normal epidermis, and how acquired defects in such focal connections could contribute to disease pathogenesis. We believe that these structures perform important “fence functions” in adult epidermis, including polarizing the direction of LB secretion toward the apex of the outermost granular layer,17 while also restricting selected membrane transporters, such as the sodiumehydrogen antiporter 1 (NHE1), to the apical plasma membrane of these cells. Although an acquired reduction in the expression of the TJ protein, claudin 1, has been reported in AD,10 treatment of cultured human keratinocytes with the T helper 2 cytokine, interleukin-4, instead upregulates claudin 1 expression, while simultaneously downregulating another TJ protein, occludin (Y. Hatano, personal

communication). Moreover, occludin (but not claudin) protein levels decline in filaggrin-deficient human epidermis.18 Hence, should abnormalities in TJ proteins occur in AD, they likely result from the T helper 2-dominant milieu that downregulates many other epidermal differentiation-linked proteins (e.g., see Howell et al19). Interdependence of, and interrelationships between, epidermal defensive functions Although it is common practice to list the various defensive functions of the skin as discrete processes (Table 1), in most cases, these functions are interrelated, coregulated, and interdependent. As is evident from Figure 1, more and more connections are emerging between these defensive functions, of which we will highlight only a few for consideration here. Best appreciated are the connections between the permeability barrier and antimicrobial defense. Shared structural and biochemical,20 as well as common metabolic processes, unite these two functions (Table 2). Moreover, epidermal LBs provide a common delivery mechanism for components with overlapping functions, such as FFAs and antimicrobial peptides (Table 2), of which at least one, the cathelicidin carboxyterminal peptide, LL-37, is required not only to restrict pathogen invasion, but also as an apparent structural component of lamellar bilayers.21 In multiple clinical situations, in experimental perturbations, and after applications of therapeutic

Table 2 How permeability and antimicrobial barriers are linked. 1. 2. 3. 4. 5. 6. 7.

Colocalization of both functions to extracellular (“mortar”) domains Pathogens attempt to invade through SC extracellular domains Some permeability barrier lipids (e.g., free fatty acids and sphingosine) exhibit potent antimicrobial activity Certain AMPs localize to lamellar bodies (along with lipids) and are codelivered to SC extracellular domains Both AMP expression and secretion accelerate after permeability barrier disruption, paralleling upregulation of lipid synthesis At least one AMP (LL-37) is required for permeability barrier homeostasis Certain serine proteases (e.g., SLPI) that regulate SC cohesion also exhibit potent antimicrobial activity

AMP ¼ antimicrobial peptide; SC ¼ stratum corneum; SLPI ¼ secretory leukocyte protease inhibitor.

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Figure 3 Parallel changes in barrier function and cathelicidin expression. Note. From “Expression of epidermal CAMP changes in parallel with permeability barrier status,” by M. Rodriguez-Martin, G. Martin-Ezquerra, M.Q. Man MQ, et al., 2011, J Invest Dermatol, 131, p. 2263e70. Copyright 2011, Dr. Peter Elias. Adapted with permission.

ingredients that either compromise or improve permeability barrier homeostasis, corresponding alterations occur in LL-37, and to a lesser extent, human beta-defensin 2 (hBD2) expression (Figure 3).21,22 LBs also deliver proteases and antiproteases that initially regulate SC cohesion, and then orchestrate the digestion of corneodesmosomes23e26 (Figures 4 and 5). However, corneodesmosome degradation is only the first in a series of subsequent cellular events that leads to the eventual shedding of corneocytes from the skin surface26 (Figure 5). Finally, as noted above, LBs also secrete at least two antimicrobial peptides, hBD2 and LL-37, into the SC extracellular domains.27e29 Because they appear to be so intertwined, it becomes a matter of semantics as to whether not only these two, but also whether several other functions, should be considered discrete or interrelated processes (Figure 1). There are pathogenic implications of LBs as an integrated delivery system. Not only permeability barrier function, but also antimicrobial defense and SC cohesion, are compromised in AD, and as shown in Figure 6 their altered gene products all converge on the LB secretory system. The multiple functions that are impacted by the epidermal structural protein, filaggrin, serve as another illustrative example of the link between multiple defense functions. First, the full-length

protein becomes a component of the CE,30,31 contributing to epidermal mechanical defense.18 We have shown that an intact CE is required for the supramolecular organization of secreted lipids into lamellar bilayers, as eloquently demonstrated in two disorders of cornification, transglutaminase 1-deficient lamellar ichthyosis32 and loricrin keratoderma.33 But it is the subsequent, humiditydependent proteolysis of FLG above the mid-SC34 that impacts an even broader suite of functions35 (Figure 7). Following FLG hydrolysis, its constituent amino acids are further deiminated, both enzymatically and nonenzymatically, into a suite of polycarboxylic acids (“natural moisturizing factor”) that not only account for much of SC hydration, but also contribute to defense against UV-B and to the acidification of the SC (Figure 7). The reduced pH of the SC, in turn, is critical for multiple functions, including not only antimicrobial defense, but also permeability barrier homeostasis,36 SC cohesion, and proinflammatory cytokine activation. We next highlight another example of linked functions that recently emerged from the laboratory of Sabine Werner (Institute of Cell Biology, Zurich, Switzerland), who showed that a key transcription factor, nuclear factor erythroid 2-related factor 2 (Nrf2), regulates the expression of two CE precursors, small proline-rich proteins (Sprr2b and Sprr2h). This transcription factor also regulates expression of a potent antimicrobial protein, secretory leukocyte protease inhibitor (Slpi), which is also an inhibitor of serine proteases (kallikreins) that regulate SC cohesion37 (Figure 8). The cohesiveness of the SC, in turn, is critical for both permeability barrier function and antimicrobial defense. Together, these examples of functional links illuminate how discrete epidermal protective functions should instead be considered components of a broader, protective “superfunction” of the skin. Metabolic mechanisms that maintain epidermal homeostasis Life in a terrestrial environment requires constant vigilance, accompanied by responses, either draconian or subtle, to external perturbations that potentially threaten the organism with desiccation, microbial invasion, oxidant damage, UV-B-induced apoptosis, and/or impaired mechanical defense. Consider the most dramatic example, i.e., an external thermal burn, with its potentially devastating consequences. The foremost threat to such patients, of course, is rapid desiccation owing to an unrestricted loss of internal fluids and electrolytes, as well as an increased susceptibility to pathogen invasion. Yet, even following such potentially catastrophic injuries, the skin attempts to repair itself. What is the driving force behind the repair of such wounds? Entire generations of surgeons and skin biologists have focused once

Figure 4 Diagram of stratum corneum membrane domains. Note. From “Thematic Review Series: Skin Lipids. Peroxisome proliferator-activated receptors and liver X receptors in epidermal biology,” by M. Schmuth, Y.J. Jiang, S. Dubrac, P.M. Elias, K.R. Feingold KR, 2008, J Lipid Res, 49, p. 499e509. Copyright 2008, Dr. Peter Elias. Adapted with permission.

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Figure 5 Basis for normal exfoliation: summary of observations. aCer'ase ¼ acidic ceramidase; CD ¼ corneodesmosome; CE ¼ cornified envelope; DSC1 ¼ desmocollin 1; DSG1 ¼ desmoglein 1; SC ¼ stratum corneum. Note. From “Cellular changes that accompany shedding of human corneocytes,” by T.K. Lin, D. Crumrine, L.D. Ackerman, et al., 2012, J Invest Dermatol, 132, p. 2430e9. Copyright 2012, Dr. Peter Elias. Adapted with permission.

again on “inside-to-outside” phenomena, related either to the initial inflammatory responses, platelet-derived growth factors, granulation tissue, collagen remodeling, and/or wound contracture as key “drivers” of wound healing. Reepithelialization often is noted only in passing as the inevitable downstream consequence of these earlier events. Few, if any, of these investigators have considered the possibility that it could be the “imperative to reestablish permeability barrier homeostasis” that likely “drives” much of the wound healing sequence, which includes reepithelialization followed by stratification of epidermis into a functional SC. They need only observe that occlusion with vapor-permeable wraps delays wound healing, whereas applications of vapor-permeable wraps

stimulate all of the processes described above, including reepithelialization. We view one of our standard laboratory models, i.e., sequential tape stripping, as a type of superficial wound. Tape stripping (no different than either detergent or solvent wipes) produces a defect in the permeability barrier, and all three of these unrelated, acute perturbations stimulate an identical series of metabolic responses in the underlying epidermis that rapidly re-establishes permeability barrier homeostasis in a predictable sequence, and with characteristic kinetics (Table 3). This approach (which we term the cutaneous stress test or “treadmill of the skin”) can be deployed to identify specific metabolic responses that bring about

Figure 6 How inherited abnormalities in AD produce defective permeability and antimicrobial barriers. AD ¼ atopic dermatitis; CNDS ¼ corneodesmosin; DSG1 ¼ desmoglein 1; FA ¼ fatty acid; Fatp4 ¼ fatty acid transport protein 4; Flg ¼ filaggrin; hBD2 ¼ human beta-defensin 2; Hrn ¼ hornerin; KLK ¼ kallikrein; LB ¼ lamellar body; LEKTI ¼ lymphoepithelial Kazal-type trypsin inhibitor.

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Figure 7 Multiple downstream consequences of filaggrin deficiency in atopic dermatitis. *Trans-urocanic acid (t-UCA) is the most potent endogenous UV-B filter in lightly pigmented skin. Loss of t-UCA could account for the higher incidence of nonmelanoma skin cancers in atopic dermatitis (AD). Casp 14 ¼ caspase 14; KLK ¼ kallikrein; NMF ¼ natural moisturizing factor. Note. From “Ichthyosis vulgaris: the filaggrin mutation disease,” by J.P. Thyssen, E. Godoy-Gijon, P.M., Elias PM, 2013, Br J Dermatol, 168, p. 1155e66. Copyright 2013, Dr. Peter Elias. Adapted with permission.

Figure 8 Nrf2 regulates not only antioxidant defense, but also barrier function and antimicrobial defense (through increased cornified envelopes and Slpi expression). Nrf2 ¼ nuclear factor erythroid 2-related factor 2; Slpi ¼ secretory leukocyte protease inhibitor; Sprr ¼ small proline-rich protein. Note. From “Nrf2 links epidermal barrier function with antioxidant defense,” by M. Schafer, H. Farwanah, A.H. Willrodt, et al., 2012, EMBO Mol Med, 4, p. 364e79. Copyright 2012, Dr. Sabina Werner. Adapted with permission.

reestablishment of barrier homeostasis. The earliest response to acute barrier perturbations is the immediate secretion (within 15e20 minutes) of much of the preformed pool of LBs from cells of the outer SG.17 After exteriorizing their cargo of LB contents, these outermost SG cornify, i.e., they undergo physiologic apoptosis,38 followed immediately by the apical migration of subjacent SG cells17 (Table 3). Yet, barrier perturbations also stimulate injury responses that may be unrelated to the restoration of barrier function. To

distinguish among these two events, one can artificially restore barrier function with a vapor-impermeable wrap, such as a Latex glove or a sheet of Saran wrap. By sending a “message” that the barrier function is now normal, these forms of occlusion shut down metabolic events that are solely directed at restoring barrier function, including virtually all of the changes shown in Figure 939 and Table 3.40 Yet, some responses, such as increased cytokine production (see below), are not blocked by occlusion. These could be dual-purpose, i.e., signals of both barrier homeostasis and an injury response. Finally, it should be noted that the same “stress test” approach has allowed us to identify abnormalities in barrier function in the following: (1) developmental (neonatal and aged skin) settings41,42; (2) human populations, subjected to psychological stress,43 or endowed with different pigment types44,45; and (3) disease settings.46,47 Finally, the stress test led to the development of new generations of “barrier repair” therapeutics48 as well as novel metabolically based, drug delivery technologies.49 Signals of barrier homeostasis It still is only partially understood how perturbations of the outer skin surface signal the underlying nucleated layers to initiate the metabolic responses that restore permeability barrier homeostasis. To date, several extracellular signaling mechanisms have been identified that are known to stimulate a broad array of metabolic responses in the underlying epidermis50 (Figure 10; Table 4). However, it also should be noted that external perturbations “turn on” intracellular signaling mechanisms (second messengers) that also regulate these metabolic responses (Table 5). These include the “liposensor” subclass of nuclear hormone receptors [peroxisome proliferator-activated receptor (PPAR) a,

Table 3 Chronology of metabolic response to acute barrier disruption. Event

Secretion of preformed pool of lamellar bodies

Terminal differentiation (physiologic apoptosis)

[ Lipid synthesis þ secretion

[ Lipid processing

[ DNA synthesis

Known signals Effects of occlusion

Y Ca2þ Blocks

KLK / PAR2 Blocks

SREBPs; [ IL-1a; Y Ca2þ Blocks both lipid synthesis and transport

? Blocks

AR, NGF, IL-1a Blocks DNA synthesis; blocks AR, NGF, VEGF (but not cytokine) production

AR ¼ amphiregulin; IL ¼ interleukin; KLK ¼ kallikrein; NGF ¼ nerve growth factor; PAR2 ¼ proteinase-activated receptor 2; SREBPs ¼ sterol regulatory element binding proteins; VEGF ¼ vascular endothelial growth factor.

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Figure 9 Lipid metabolic events leading to normal barrier formation. b-GlcCer'ase ¼ b-glucocerebrosidase; aSMase ¼ acidic sphingomyelinase; FATPs ¼ fatty acid transport proteins; sPLA2 ¼ secretory phospholipase A2; SSase ¼ steroid sulfatase. Note. From “Role of lipids in the formation and maintenance of the cutaneous permeability barrier,” by K.R. Feingold and P.M. Elias, 2014, Biochim Biophys Acta, 1841, p. 280e94. Copyright 2014, Elsevier. Adapted with permission.

PPARb/d, PPARg, and liver X receptor (LXR)], which regulate the transcription of several genes that are critical for epidermal differentiation and lipid production25,51 (Figure 11). Also carefully studied are sterol regulatory element binding proteins that modulate epidermal sterol and triacylglyceride synthesis.52 Then, barrier

disruption stimulates hyaluronic acid production, whichddepending on fragment sizedregulates epidermal proliferation, differentiation, and cholesterol synthesis,53 nitric oxide production, and endoplasmic reticulum stress responses. It should be noted, however, that egregious external insults result in cell death or apoptosis

Figure 10 Regulation of permeability barrier repair. Examples of how disruption of the permeability barrier results in signals that can either accelerate or delay barrier repair. AO ¼ antioxidants; Ca ¼ calcium; l IL-1a ¼ interleukin-1alpha; K ¼ potassium; Klks ¼ kallikreins; LB ¼ lamellar bodies; NO ¼ nitric oxide; Nrf2 ¼ nuclear factor erythroid 2-related factor 2; PAR2 ¼ proteinase-activated receptor 2; TSLP ¼ thymic stromal lymphopoietin. Note. From “The regulation of permeability barrier homeostasis,” by K.R. Feingold, M. Schmuth, and P.M. Elias, 2007, J Invest Dermatol, 127, p. 1574e6. Copyright 2007, Dr. Peter Elias. Adapted with permission.

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Table 4 Signals that regulate permeability barrier homeostasis. Extracellular modulations

Sensor

Signal 2þ

External humidity

TRPV4, TRPM8

Ca

Osmolar stress (cell volume)

Acidity Barrier disruption

AQPs, UTs, TauT, TonEBP, Na-dependent myoinosital transporter TRPV1 DpH

Ca2þ BGT1, PSLC5A3 Ca2þ Cytokines; Klk/PAR2

Heat Injury (wounding)

TRPV3, Ca2þ TLR3

?Ca2þ ncRNA

Homeostatic responses

Potential pathogenic signala

Lamellar body secretion; Ye[ Epidermal differentiation [ Epidermal differentiation; [ lipid synthesis; [ AMP production, antiapoptotic ([ HSPs) [ NHE1 þ ? others [ Epidermal proliferation; [ lipid synthesis/secretion (IL-1a); terminal differentiation ? [ Lipid synthesis þ secretion; [ innate immunity

No No

Yes (via SP / PAR2) Yes (inflammation, pruritus)

No Yes (inflammation)

AQP ¼ aquaporin; BGT1 ¼ betaine/gamma-amino-n-butyric acid transporter 1; Chol ¼ cholesterol; HSP ¼ heat shock protein; Klk ¼ kallikrein; ncRNA ¼ noncoding RNA; NHE1 ¼ sodiumehydrogen antiporter 1; PAR2 ¼ protease-activator receptor 2; TauT ¼ taurine transporter; TLR3 ¼ Toll-like receptor 3; TonEBP ¼ tonicity enhancer binding protein; TRPM8 ¼ transient receptor potential melastatin-8; TRPV ¼ transient receptor potential vanilloid. a Failed to downregulate with artificial barrier restoration following acute perturbation.

Table 5 Second messengers of permeability barrier homeostasis. External perturbations

Signal

Homeostatic response

Potential pathologic signal

Barrier disruption, UV-B, oxidative stress d

ER stress /[Cer / [S1P

[ Epidermal CAMP (LL-37) production

Cell death, apoptosis, if excessive

[Cholesterol sulfate/ PKGh, AP1 elements SREBPs PPARs, LXR HA / CD44 receptor

[ Epidermal differentiation

Abnormal SC cohesion and barrier function

[ Sterol, triacylglycerol synthesis [ Epidermal differentiation, lipid synthesis [ Epidermal proliferation, differentiation, sterol synthesis Y Barrier repair

No No No

Barrier disruption Barrier disruption Barrier disruption Barrier disruption / oxidative stress

Nitric oxide / [cGMP, [Ca2þ

Inflammation; apoptosis

CAMP ¼ cathelicidin antimicrobial peptide; ER ¼ endoplasmic reticulum; LXR ¼ liver X receptor; PPAR ¼ peroxisome proliferator-activated receptors; SREBPs ¼ SREBPs ¼ sterol regulatory element binding proteins.

(Figure 10), by one or more of these mechanisms.54 In addition, the sulfated sterol, cholesterol sulfate, which is generated late in epidermal differentiation, is a potent transcriptional regulator of epidermal differentiation.55 Yet, new signaling networks, both extra- and intracellular, that link external perturbations to metabolic response in the underlying epidermis continue to be discovered

(Tables 4 and 5). Whereas several of these signals broadly regulate epidermal differentiation and/or lipid production, perhaps in a redundant or overlapping fashion, others modulate more discrete metabolic pathways within the epidermis. It is important to distinguish whether these signaling mechanisms represent purely homeostatic or, in part, injury-mediated

Figure 11 Speculative diagram that illustrates coordinate regulation of epidermal barrier homeostasis by changes in calcium, and activation of the liposensor subclass of class-II nuclear hormone receptors. ABCA12 ¼ ATP-binding cassette transporter A12; FFA ¼ free fatty acid; LXR ¼ liver-x receptor; PPAR ¼ peroxisome proliferator-activated receptor; SREBP ¼ sterol response element binding transcription factor. Note. From “Coordinate regulation of epidermal differentiation and barrier homeostasis,” by P.M. Elias PM and K.R. Feingold, 2001, Skin Pharmacol Appl Skin Physiol, 14, p. 28e34. Copyright 2001, Dr. Peter Elias. Adapted with permission.

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responses. The “gold standard” applies againddo these signals deploy following acute external perturbations, even when the barrier is immediately restored by occlusion (see also above)? Several cytokines, but not the growth factors NGF (nerve growth factor), AR, and VEGF (vascular endothelial growth factor), continue to upregulate, even in the face of barrier restoration by occlusion, indicating that they could represent, at least in part responses to injury, rather than purely homeostatic mechanisms alone (Tables 4 and 5). Acknowledgments This work was supported by the National Institute of Arthritis and Musculoskeletal and Skin Diseases of the National Institutes of Health under Award Numbers AR061106 and AR019098, administered by the Northern California Institute for Research and Education, with additional resources provided by the Veterans Affairs Medical Center, San Francisco, CA, USA. This content is solely the responsibility of the authors and does not necessarily represent the official views of either the National Institutes of Health or the Department of Veterans Affairs. References 1. Gunathilake R, Schmuth M, Scharschmidt TC, et al. Epidermal barrier dysfunction in non-atopic HIV: evidence for an “inside-to-outside” pathogenesis. J Invest Dermatol 2010;130:1185e8. 2. Elias PM, Menon GK. Structural and lipid biochemical correlates of the epidermal permeability barrier. Adv Lipid Res 1991;24:1e26. 3. Prausnitz MR, Elias PM, Franz TJ, et al. Skin barrier and transdermal drug delivery. In: Bolognia J, Jorizzo JL, Schaffer JV, editors. Dermatology. Philadelphia, PA: Elsevier Saunders; 2012. p. 2065e73. 4. Elias PM, Gruber R, Crumrine D, et al. Formation and functions of the corneocyte lipid envelope (CLE). Biochim Biophys Acta 2014;1841:314e8. 5. Rabionet M, Gorgas K, Sandhoff R. Ceramide synthesis in the epidermis. Biochim Biophys Acta 2014;1841:422e34. 6. Breiden B, Sandhoff K. The role of sphingolipid metabolism in cutaneous permeability barrier formation. Biochim Biophys Acta 2014;1841:441e52. 7. Zheng Y, Yin H, Boeglin WE, et al. Lipoxygenases mediate the effect of essential fatty acid in skin barrier formation: a proposed role in releasing omegahydroxyceramide for construction of the corneocyte lipid envelope. J Biol Chem 2011;286:24046e56. 8. Brandner JM, Kief S, Grund C, et al. Organization and formation of the tight junction system in human epidermis and cultured keratinocytes. Eur J Cell Biol 2002;81:253e63. 9. Kubo A, Nagao K, Amagai M. Epidermal barrier dysfunction and cutaneous sensitization in atopic diseases. J Clin Invest 2012;122:440e7. 10. De Benedetto A, Rafaels NM, McGirt LY, et al. Tight junction defects in patients with atopic dermatitis. J Allergy Clin Immunol 2011;127:773e86. e1e7. 11. Elias PM, Goerke J, Friend DS. Mammalian epidermal barrier layer lipids: composition and influence on structure. J Invest Dermatol 1977;69:535e46. 12. Marchiando AM, Graham WV, Turner JR. Epithelial barriers in homeostasis and disease. Annu Rev Pathol 2010;5:119e44. 13. Suzuki T. Regulation of intestinal epithelial permeability by tight junctions. Cell Mol Life Sci 2013;70:631e59. 14. Furuse M, Hata M, Furuse K, et al. Claudin-based tight junctions are crucial for the mammalian epidermal barrier: a lesson from claudin-1-deficient mice. J Cell Biol 2002;156:1099e111. 15. Grubauer G, Feingold KR, Harris RM, Elias PM. Lipid content and lipid type as determinants of the epidermal permeability barrier. J Lipid Res 1989;30:89e96. 16. Celli A, Zhai Y, Jiang YJ, et al. Tight junction properties change during epidermis development. Exp Dermatol 2012;21:798e801. 17. Elias PM, Cullander C, Mauro T, et al. The secretory granular cell: the outermost granular cell as a specialized secretory cell. J Invest Dermatol Symp Proc 1998;3: 87e100. 18. Gruber R, Elias PM, Crumrine D, et al. Filaggrin genotype in ichthyosis vulgaris predicts abnormalities in epidermal structure and function. Am J Pathol 2011;178:2252e63. 19. Howell MD, Fairchild HR, Kim BE, et al. Th2 cytokines act on S100/A11 to downregulate keratinocyte differentiation. J Invest Dermatol 2008;128: 2248e58. 20. Elias PM. The skin barrier as an innate immune element. Sem Immunopath 2007;29:3e14. 21. Aberg KM, Man MQ, Gallo RL, et al. Co-regulation and interdependence of the mammalian epidermal permeability and antimicrobial barriers. J Invest Dermatol 2008;128:917e25. 22. Rodriguez-Martin M, Martin-Ezquerra G, Man MQ, et al. Expression of epidermal CAMP changes in parallel with permeability barrier status. J Invest Dermatol 2011;131:2263e70.

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