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Published in final edited form as:
J Allergy Clin Immunol. 2009 September ; 124(3): 496–506.e6. doi:10.1016/j.jaci.2009.06.046.
FILAGGRIN DEFICIENCY CONFERS A PARACELLULAR BARRIER
ABNORMALITY THAT REDUCES INFLAMMATORY THRESHOLDS
TO IRRITANTS AND HAPTENS
Tiffany C. Scharschmidt, M.D.1,*, Mao-Qiang Man, M.D.1,*, Yutaka Hatano, M.D.1, Debra
Crumrine1, Roshan Gunathilake, M.D.1, John P. Sundberg, D.V.M., Ph.D.2, Kathleen A. Silva,
B.S.2, Theodora M. Mauro, M.D.1, Melanie Hupe, B.S.1, Soyun Cho, M.D., Ph.D.3, Yan Wu,
M.D.4, Anna Celli, Ph.D.1, Matthias Schmuth, M.D.1, Kenneth R. Feingold, M.D.1, and Peter
M. Elias, M.D.1
1Dermatology Service, Veterans Affairs Medical Center, and Department of Dermatology, University
of California, San Francisco, CA
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2Experimental
Dermatology and General Mouse Pathology, The Jackson Laboratory, Bar Harbor,
ME
3Department
of Dermatology, Seoul National University College of Medicine and Boramae Hospital,
Seoul, South Korea
4Department
of Dermatology, First Hospital, Peking University, Beijing, Peoples Republic of China
Abstract
Background—Mutations in filaggrin (FLG) are associated with atopic dermatitis (AD), and are
presumed to provoke a barrier abnormality. Yet, additional acquired stressors may be necessary,
since the same mutations can result in a non-inflammatory disorder, ichthyosis vulgaris.
Objective—We examined here whether FLG deficiency alone suffices to produce a barrier
abnormality; the basis for the putative abnormality; and its pro-inflammatory consequences.
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Methods—Using the flaky-tail (ft/ft) mouse, which lacks processed flg due to a frame-shift mutation
in profilaggrin that mimics some mutations in human AD, we assessed whether FLG deficiency
provokes a barrier abnormality; further localized the defect; identified its subcellular basis; and
assessed thresholds to irritant and hapten-induced dermatitis.
Results—Flaky-tail mice exhibit low-grade inflammation, with increased bidirectional,
paracellular permeability of water-soluble xenobiotes due to impaired lamellar body secretion and
altered stratum corneum extracellular membranes. This barrier abnormality correlates with reduced
inflammatory thresholds to both topical irritants and haptens. Moreover, when exposed repeatedly
to topical haptens, at doses that produce no inflammation in +/+ mice, ft/ft mice develop a severe
AD-like dermatosis, with a further deterioration in barrier function and features of a th2
© 2009 American Academy of Allergy, Asthma and Immunology. Published by Mosby, Inc. All rights reserved.
Corresponding author: Peter M. Elias, MD, Dermatology Service (190), VA Medical Center, 4150 Clement Street, San Francisco, CA
94121, TEL: (415) 750-2091, FAX: (415) 750-2106, eliasp@derm.ucsf.edu.
*contributed equally to this publication
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Scharschmidt et al.
Page 2
immunophenotype (increased CRTH + inflammation, elevated serum IgE levels, and reduced
antimicrobial peptide [mBD3] expression).
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Conclusions—FLG deficiency alone provokes a paracellular barrier abnormality in mice that
reduces inflammatory thresholds to topical irritants/haptens, likely accounting for enhanced antigen
penetration in FLG-associated AD.
Keywords
Atopic dermatitis; barrier function; contact dermatitis; filaggrin; flaky tail mouse; lamellar body
INTRODUCTION
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Atopic dermatitis (AD) is a chronic, relapsing, inflammatory skin condition, that displays an
associated abnormality in permeability barrier function 1–4. Until recently, AD was viewed
as a primary immunologic disorder in which sustained allergen exposure in susceptible hosts
leads to TH2-dominant inflammation, which then secondarily provokes defects in barrier
function (‘inside-outside’ paradigm) [rev in 5]. However, recent genetic studies have shown a
strong association between AD and several loss-of-function mutations in the gene encoding
the stratum corneum structural protein, filaggrin (FLG) 6, 7. Moreover, there is a dose-response
relationship between FLG-deficiency and disease severity, such that patients with double-allele
or compound heterozygote mutations in FLG display more-severe and earlier-onset AD, as
well as an increased propensity for AD to persist into adulthood 8–11. This rapidly-growing
body of work has led to a paradigm shift in conceptions of AD pathogenesis, with increasing
weight being placed on the role of a primary barrier abnormality that then precipitates
downstream immunologic abnormalities (“outside-inside” paradigm) 1,12. According to this
concept, TH2-dominant inflammation results from sustained epicutaneous access of haptens
through a defective skin barrier 13,14. TH2 cytokines then down-regulate expression of TH1
cytokines 5, epidermal antimicrobial peptide expression 15, 16, and epidermal structural
proteins [rev. in 1, 2], completing an “outside-inside-(back to) outside” paradigm for AD 2,
17.
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FLG is an intracellular protein, with important roles in stratum corneum hydration 18 and as a
structural component of the corneocyte cytosol 19. However, normal permeability barrier
function is not regulated by the corneocyte, but rather by the lipid-enriched, extracellular matrix
of the stratum corneum 20, 21. Indeed, in all inflammatory dermatoses studied to date, including
those associated with inherited disorders of corneocyte proteins, the barrier abnormality has
been linked to accelerated paracellular water loss 22, 23. Thus, the new-found link between
FLG and AD leaves unanswered whether FLG-deficiency alone provokes a barrier abnormality
in the epidermis, and if so, the subcellular basis for such an abnormality. While it has been
widely hypothesized that a FLG-associated barrier defect facilitates hapten ingress 12, 13, this
relationship was only recently demonstrated 24. Yet, the mechanism whereby FLG deficiency
compromises barrier function, thereby provoking inflammation, remains unknown.
Pertinently, FLG mutations that occur in AD also occur in the inherited, non-inflammatory
condition, ichthyosis vulgaris 25, 26, raising questions about whether FLG deficiency alone
suffices to provoke inflammation, or whether additional acquired stressors are required to elicit
AD 1, 2.
The above questions are difficult to address within the context of human AD, where multiple
genetic and environmental factors intertwine to provoke disease pathogenesis. Thus, we
employed a pre-existing model, the flaky tail (ft/ft) mouse, which exhibits very reduced levels
of flg 27. Flaky tail mice possess a homozygous frameshift mutation in profilaggrin that
prevents the processing of profilaggrin into flg 24, such that ft/ft mice simulate double allele,
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loss-of-function mutations in certain human AD patients that also result in reduced levels of
processed FLG protein 6, 28. The ft/ft mutation was shown previously to yield a phenotype of
diffuse, mild flaking, analogous to human ichthyosis vulgaris 27. Utilizing the ft/ft model, we
demonstrate that ft/ft mice display: 1) abnormal barrier function and low-grade inflammation
at baseline, confirming recent observations of Fallon, et al. 24; and 2) enhanced bidirectional,
paracellular penetration of water-soluble tracers. We demonstrate further that: 1) Increased
penetration through the stratum corneum can be attributed to structural defects in the lamellar
body secretory system; 2) flg deficiency alone reduces inflammatory thresholds to both irritants
and allergens; and 3) ft/ft mice develop a hapten-induced, AD-like dermatosis at lower
challenge doses than do +/+ mice, mirroring recent work in ft/ft mice exposed to a complete
antigen 24. Thus, flg deficiency yields a paracellular barrier abnormality that likely also favors
elicitation of AD by allowing enhanced/sustained hapten access.
METHODS (SECTION MOVED FROM AFTER DISCUSSION SECTION)
Materials and animals
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Studies were conducted in maft/maft/J(ft/ft) mice (new designation: a/a ma ft/ma ft/JSun
JR#9078), and age/sex-matched C57Bl/6J+/+ mice (The Jackson Lab, Bar Harbor, ME),
housed under pathogen-free conditions. Flaky tail mice are compound mutants at two loci, both
matted (ma/ma) and flaky tail (ft/ft). Matted confers a stable, lifelong, and readily-identifiable
phenotype of subtle hair coarseness that was introduced by Jackson to facilitate identification
of mutants without reliance on genotyping 29. Young ft/ft and +/+ mice were between 6–12
weeks of age, while old mice were 50–52 weeks of age. Ethanol, acetone and lanthanum nitrate
were purchased from Fisher Scientific (Fairlane, NJ). Oxazolone (Ox) and calcium green were
purchased from Sigma Chemical Co. (St Louis, MO). Affinity-purified, rabbit anti-mouse
antibodies to both proflg and flg were purchased from BabCo (Richmond, CA). Rabbit antimouse antibody against the prostaglandin D2 receptor, CRTH2/DP2, which is expressed solely
on th2-bearing lymphocytes and mast cells, was purchased from Cayman Chemical (Ann
Arbor, MI). Rabbit anti-mouse antibody against CD3 was purchased from BD Biosciences
(San Jose, CA).
Experimental protocols and functional studies
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All animal procedures were approved by the Animal Studies Subcommittee (IACUC) of the
San Francisco Veterans Administration Medical Center and performed in accordance with their
guidelines. Basal stratum corneum hydration and surface pH were measured (CM 825/PH 900,
Courage & Khazaka, Germany) on shaved back skin in ft/ft and +/+ mice (n = 4–6 in all
experimental groups, except where noted in figure legends). Transepidermal water loss
(TEWL) measurements were taken under basal conditions, using an electrolytic water analyzer
(Meeco, Warrington, PA), as well as 2 and 4 hours after a tape stripping-induced, 3-fold
increase in TEWL. Barrier recovery was calculated as percent barrier recovery from ‘0’ levels,
immediately after acute disruption 30–37.
Irritant and acute allergic contact dermatitis were induced in ft/ft and +/+ mice using TPA and
Ox, respectively, as described previously 38. TPA (either 0.3% or 0.1% in acetone) was applied
once to the inner and outer surface of the ear. Challenge doses of Ox (either 0.02% or 0.5% in
acetone) were applied once following initial Ox sensitization (2% Ox). In all cases, ear
thickness was measured 2 hrs post-treatment.
To induce an AD-like dermatosis in mice, ft/ft and +/+ mice were sensitized with 2% Ox, as
above. One week later, shaved areas on the flanks of ft/ft and +/+ mice were treated every other
day topically with 60 µl of 0.5% or 0.02% Ox for an additional 3 weeks (10 challenges). Other
groups of ft/ft and +/+ mice were treated with ethanol alone as the vehicle control group (n =
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10–12 in each group). At the end of treatments, basal TEWL and stratum corneum hydration
were measured again.
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Immunohistochemistry
Changes in overall morphology were visualized after hematoxylin and eosin staining of 5 µm
paraffin-embedded tissue samples. Both the number of epidermal nucleated cell layers and the
density of the inflammatory infiltrate were measured in 20–30 randomly-chosen fields from
ft/ft and +/+ mice before and after Ox challenge doses, as above. Immunohistochemical
assessment of changes in profilaggrin/flg, CTRH2, and CD3 were performed, as described
previously 39. Briefly, 5 µm paraffin sections were incubated with the primary antibodies
overnight at 4°C. After washing, sections were incubated with the secondary antibody for 30
min. Staining was detected with ABC-peroxidase kit from Vector Lab. Frozen sections were
examined with a Zeiss fluorescence confocal microscope (Jena, Germany), and digital images
were captured with AxioVision software (Carl Zeiss Vision, Munich, Germany). A similar
sequence was employed to assess two antimicrobial peptides in 5 µm frozen sections [primary
antibodies to mBD3 from Alpha Diagnostics; mouse cathelicidin (CAMP) antibody was a gift
from Dr. Richard Gallo (UCSD)].
Ultrastructural Lipase Detection
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Ultrastructural cytochemistry of lipase activity has been employed as a content marker for
epidermal lamellar bodies, allowing assessment of the efficacy of organelle secretion 40.
Briefly, samples were incubated in 5% Tween 85 (1 ml) in 0.2M HEPES buffer (2.5 ml), 2.5%
sodium taurocholate (2 ml) and 10% calcium chloride (1 ml) in 25 ml distilled water (pH 7.4).
Microwave incubations were carried out twice for 30 sec at 2450 MHz, and the water bath was
changed between the pulses, in order to maintain temperatures below 40°C 41. After incubation
in the same medium for an additional 30 min at 37°C, all samples were incubated further in
0.1M cacodylate buffer containing 0.2% Tween 85 with our without 200 µm of the lipase
inhibitor, tetrahydrolipstatin, as we have described 42.
Tracer Penetration
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To assess the outward permeation of water-soluble markers, freshly-obtained explants of back
skin from untreated ft/ft and +/+ mice were exposed dermis-side-downward (30 min-2 hrs) to
the low-molecular weight tracer, colloidal lanthanum nitrate (4%) in Karnovsky’s solution. To
assess permeation from the outside, we placed 3 +/+ control and 3 ft/ft mice in Petri dishes,
while under general anesthesia, with one side of each mouse immersed in 4% colloidal
lanthanum, in a 37°C incubator for 2 hr. Lanthanum nitrate was prepared in 4% w/v sucrose
in 0.05 M Tris buffer, pH 7.5. After immersion, biopsy samples were taken and processed as
below. In parallel studies, a 20 µM solution of calcium green 5N (Invitrogen) in nanopure water
was applied on the stratum corneum side of ft/ft and wt mice for 10 and 75 min. Ten mm
biopsies were mounted immediately on microscope coverslips, washed in nanopure water, and
imaged in a Zeiss 510 meta NLO (Zeiss Germany) at the Live Cell Analysis Core Facility at
UCSF. An optically-pumped (Coherent) mode-locked Mira 900 Ti:Saph laser (Coherent), was
used as the two-photon excitation source at a wavelength of 800 nm. A 500–550 nm band pass
filter was placed in the detection path in order to minimize sample autofluorescence. Under
these experimental conditions, we estimated the contribution of autofluorescence to the total
signal to be <5%. Z-stacks of 235×235×20 µm3 images were acquired, beginning with the
outer surface of the stratum corneum at z increments of 1 µm using a 40× oil immersion
objective with a 1.3 numerical aperture (Zeiss, Germany). Matlab software (The Math Works)
was used to quantitate image intensity at each z-stack, and to calculate mean penetration depths.
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Electron Microscopy
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Aldehyde-fixed biopsies were post-fixed with either 0.25% ruthenium tetroxide or 1% aqueous
osmium tetroxide, containing 1.5% potassium ferrocyanide, as described previously 43.
Ultrathin sections were examined using an electron microscope (Zeiss 10A, Carl Zeiss,
Thornwood, NY) operated at 60 kV. Images were captured using Digital Micrograph 3.10.0
software from Gatan, Inc. (Pleasanton, CA).
Serum IgE
Blood samples were collected from ft/ft and wt mice under basal conditions, and after prior
Ox sensitization, followed by repeated treatments with either Ox or vehicle. Serum IgE
concentrations were determined with a mouse IgE ELISA quantitation kit from Bethyl
Laboratories (Montgomery, TX), following instructions provided by the manufacturer.
In these, and all other studies, statistical analyses were performed using a Student’s t test to
compare differences between two groups, with a further ANOVA analysis when three or more
groups were compared.
RESULTS
Filaggrin Deficiency Results in Abnormal Barrier Function
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Prior studies have shown that ft/ft mice display markedly-reduced F-type keratohyalin
granules, with an absence of flg 24, due to a recently-identified, frame-shift mutation in
profilaggrin 24. Using a consensus antibody that recognizes both profilaggrin and flg, we found
intense, positive immunostaining restricted to the stratum granulosum (granular layer), with
no residual staining of flg in the stratum corneum in ft/ft mice (suppl Fig. 1, green staining).
In contrast, +/+ epidermis displayed immunostaining in the granular layer, as well as diffuse
staining of the stratum corneum. Thus, the mutation in profilaggrin in ft/ft mice results in
reduced flg in the SC, analogous to some humans with FLG-associated AD 6, 28.
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We next assessed epidermal functional parameters in ft/ft and wild-type (+/+) mice, using
biophysical instrumentation. Permeability barrier homeostasis, assessed as the kinetics of
transepidermal water loss (TEWL) levels after acute disruption 44, but basal barrier function
declines with aging in +/+ aged (52–54 wk) mice, as previously described 44 (Fig. 1A). Though
within the range of normal (≤ 2.5 mg/cm2/hr), basal TEWL levels increase significantly (by
≈20–30%) in 52–54 week old ft/ft mice compared to age-matched +/+ controls (Fig. 1A). As
occurs in human AD [e.g., 30], the kinetics of barrier recovery after acute barrier abrogation
accelerate in old ft/ft vs. +/+ mice (Fig. 1B). Such accelerated barrier recovery also occurs in
other chronic inflammatory dermatoses, and is attributed to ongoing signaling of repair
mechanisms, which are induced by barrier abrogation [e.g., 31, 32]. Finally, stratum corneum
hydration, assessed as changes in electrical capacitance, does not differ in young ft/ft vs. +/+
mice (Fig. 1C), but stratum corneum hydration is significantly lower in young ft/ft mice, with
a further decline in older ft/ft mice (Fig. 1C), again mirroring known changes in human AD
33, 45.
Since basal TEWL levels lie within the normal range in ft/ft mice, we next assessed barrier
function in these mice under basal conditions by an alternate approach (i.e., ultrastructural
visualization of tracer perfusion). The high resolution of this method also allowed us to
determine simultaneously whether the barrier abnormality can be attributed to a defective
corneocyte or to enhanced paracellular transport. In +/+ mice, the water-soluble, low molecular
weight, electron-dense tracer, lanthanum nitrate, does not breach the stratum granulosumstratum corneum interface (Fig. 2A). In contrast, tracer not only breaches this layer, but also
extends several layers outward into the stratum corneum in ft/ft mice (Fig. 2B). Notably, despite
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the intracellular localization of flg, the tracer remains largely confined to the stratum corneum
extracellular spaces in ft/ft mice (Fig. 2B, arrows). Together, these results demonstrate that
flg-deficiency leads to alterations in basal barrier function via a defect in the stratum corneum
extracellular matrix.
Enhanced Uptake of Epicutaneously-Applied, Water-Soluble Tracers via the Extracellular
Route
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Normal stratum corneum does not allow entry of water-soluble xenobiotes 46. Therefore, we
next determined whether epicutaneously-applied, water-soluble tracers enter the stratum
corneum of flg-deficient mice, and by which route. Lanthanum nitrate tracer was applied
topically to the flanks of ft/ft and +/+ mice for 30 min to two hrs, while under general anesthesia.
As shown previously 46, lanthanum fails to breach the stratum corneum of +/+ mice (Fig. 3A).
In contrast, tracer permeates 3–5 layers deep into the stratum corneum of ft/ft mice by 2 hrs
(Fig. 3B&C). Notably, the tracer localizes solely within the extracellular spaces, indicating
again that penetration occurs via the paracellular route (Fig. 3C, arrows). In parallel studies,
we visualized the permeation of the low-molecular weight, water-soluble fluorophore, calcium
green, by dual photon, confocal microscopy. Differences in the extent of penetration can be
seen as early as 10 min after application of tracer to ft/ft skin, with further penetration evident
by 75 min in ft/ft mice (Fig. 3D&E). In contrast, very little of the fluorophore can be detected
beneath the outer stratum corneum of +/+ mice at 75 min. Together, these results demonstrate
that the stratum corneum of flg-deficient mice is more permeable to water-soluble tracers, and
that penetration of an electron-dense, water-soluble tracer occurs via the same paracellular
pathway that is utilized by water itself, exiting the skin.
Structural Basis for Altered Permeability in Flaky Tail Mice
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We next assessed the structural basis for the extracellular barrier abnormality in ft/ft mice. By
standard transmission electron microscopy, ft/ft mice display not only the previouslyreported
paucity of F-type keratohyalin granules in the stratum granulosum [cited in 19], but also a
previously-undescribed abnormality in lamellar body secretion (Fig. 4). Lamellar body
formation appears normal (neither lamellar body density nor organelle contents differ in ft/ft
vs. +/+ mice). Yet, substantial numbers of lamellar bodies appear to be retained within
terminally-differentiating keratinocytes in ft/ft mice (Fig. 4A vs. B). These unsecreted
organelles then become entombed within the cytosol of nascent corneocytes, instead of being
secreted in toto into the extracellular matrix, as occurs in +/+ mice (Fig. 4C vs. D). Partial
entombment (failed secretion) of lamellar bodies could also be demonstrated by an alternate
method: identification of the fate of a lamellar body content marker (acid lipase) by
ultrastructural cytochemistry 32, 40. In +/+ epidermis, enzyme activity localizes both to
lamellar body contents (suppl Fig. 2A), and within the stratum corneum, solely to the
extracellular domains (suppl Fig. 2B). Flaky tail mice reveal a similar localization of enzyme
activity to lamellar bodies, but evidence of reduced secretion; i.e., lower amounts of initiallysecreted enzyme at the stratum granulosum-stratum corneum interface, as well as sparse
(reduced) enzyme product in the stratum corneum interstices (suppl Fig. 2C, arrows). In
contrast to +/+ mice, the cytosol of many ft/ft corneocytes contains abundant enzyme activity,
with enzyme activity localizing near retained lamellar arrays (suppl Fig. 2D), further evidence
for entombment of lamellar body contents.
We next assessed the consequences of impaired lamellar body secretion and enhanced
paracellular transport in ft/ft mice. As a result of impaired lamellar body secretion, the
quantities of extracellular lamellar bilayers correspondingly decline in the stratum corneum of
ft/ft mice (Fig. 4C vs. D). Moreover, the extracellular processing of newly-secreted lamellar
body contents into ‘mature’ lamellar bilayers appears to be impaired or delayed in ft/ft mice
(Fig. 4C vs. D). Together, these results suggest that either profilaggrin accumulation or flg
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deficiency impedes lamellar body secretion, leading to an abnormal stratum corneum
extracellular matrix in ft/ft mice.
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Filaggrin Deficiency Alters Thresholds for Irritant and Acute Allergic Contact Dermatitis
We next assessed whether the barrier abnormality in ft/ft mutant mice results in an increased
propensity to develop cutaneous inflammation. Flaky tail skin displays low-grade
inflammation under basal conditions, indicated by the presence of increased numbers of
inflammatory cells under basal conditions (Figs. 5B, E&F; histologic images that correspond
to these quantitative data are shown in supplementary Fig. 3C vs. A). Moreover, most, but not
all ft/ft mice display elevated serum IgE levels, even under basal conditions (Fig. 5G). Together,
these results suggest that ft/ft mice display low-grade inflammation, even under basal
conditions.
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The phorbol ester, 12-O-tetradecanoylphorbol-13-acetate (TPA), elicits irritant contact
dermatitis when applied topically to mouse ears, with the severity of the inflammatory response
correlating with an increase in ear thickness two (2) hours after topical applications 34, 47. TPA
was first applied at a standard concentration of 0.3% to the ears of ft/ft and +/+ mice. Although
inflammation appeared greater in ft/ft mice, the differences in ear thickness in ft/ft vs. +/+ mice
do not achieve statistical significance (Fig. 6A). However, at a lower dose of TPA (0.1%) that
provokes only marginal inflammation in +/+ mice, ft/ft mice display a marked inflammatory
response, with a significant increase in ear thickness (Fig. 5A;p <0.001; parallel histologic
differences are shown in suppl Fig. 4). Together, these results demonstrate that ft/ft mice
display an enhanced propensity to develop irritant contact dermatitis.
We next assessed whether ft/ft mice display an altered threshold to the development of acute
allergic contact dermatitis. For these studies, we utilized a single challenge dose (2%) of the
universal hapten, oxazolone (Ox), followed one week later by topical application to the ear of
either a single challenge dose at a concentration (0.5%) which produces a prominent dermatitis
after 24–48 hrs 47, 48, or a much lower hapten concentration (0.02%), which does not produce
visible inflammation in +/+ mice. As with the topical irritant (c.f., Fig. 5A), dermatitis appears
more exaggerated in ft/ft mice, after challenges with the standard (0.5%) hapten dose, but again
the results do not achieve statistical significance (Fig. 5B). At the lower challenge dose of Ox
(0.02%), dermatitis does not develop in +/+ mice, while in contrast, substantial inflammation
is evident in similarly-challenged ft/ft mice (Fig. 5B; p<0.0001 vs. +/+; parallel histologic
differences are shown in suppl Fig. 4). These results show that ft/ft mice display a reduced
threshold for the development of hapten-induced, acute allergic contact dermatitis.
Filaggrin Deficiency Predisposes To the Development of an AD-Like Dermatosis in Mice
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We previously showed that Ox sensitization, followed by repeated (10×) hapten challenges
over 20–21 days produces a dermatosis with certain structural, biochemical, and immunologic
features of human AD 49. In the next set of studies, we used reduced challenge doses of Ox
(10 challenges with 0.02% vs. the standard 0.5% concentration) in ft/ft and +/+ mice. Whereas
+/+ mice display minimal signs of inflammation, ft/ft mice reveal erythema, hyperkeratosis,
epidermal hyperplasia, and histologically-prominent inflammation at the lower hapten
concentration (Figs. 6E&F; suppl Fig. 3). Moreover, while virtually no CD3+ lymphocytes are
detected in +/+ mouse skin, a moderately-high density of CD3+ cells can be seen in ft/ft mice
repeatedly challenged with low-dose Ox (Fig. 6D). Flaky tail mice also display numerous
prostaglandin D2 receptor (CRTH2)-positive cells, consistent with a th2-dominant
immunophenotype, as well as much higher serum IGE levels than do comparably-treated +/+
mice (Figs. 7A&B). Finally, ft/ft mice, repeatedly challenged with low-dose Ox, display
reduced immunostaining for mBD3 (mouse homologue of hBD2 [suppl Fig. 5D]), but staining
for cathelicidin antimicrobial peptide (CAMP = mouse homologue of LL37) instead appears
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to increase (suppl Fig. 5C). Together, these results indicate that flg deficiency predisposes to
development of a hapten-stimulated AD-like dermatosis in mice.
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More Severe Barrier Abnormality in Filaggrin-Deficient Mice with AD
In normal mice, repeated challenges with 0.5% Ox result in a progressively more-severe barrier
abnormality 49, consistent with a proposed ‘outside-inside-outside’ pathogenic cycle in AD
1, 2. We next assessed whether repeated, sub-threshold (0.02%) doses of Ox would provoke
functional abnormalities in ft/ft mice that occur only at higher doses in +/+ mice. While stratum
corneum hydration does not decline in such Ox-challenged mice (Fig. 3F), ft/ft mice develop
a severe permeability barrier abnormality at sub-threshold doses of hapten (Fig. 3G), evidenced
by an acceleration of TEWL levels to levels produced by full-challenge hapten doses in +/+
or normal mice (c.f., 49). These results show that development of an AD-like dermatosis in
flg-deficient mice is paralleled by a further exacerbation of the permeability barrier
abnormality.
DISCUSSION
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Human atopic dermatitis (AD) is increasingly viewed as a primary barrier disorder, based upon
recent data from multiple populations that have demonstrated an association with mutations in
FLG, the gene encoding the stratum corneum structural protein, filaggrin (FLG). Yet, while
these molecular genetic studies have shifted views of AD pathogenesis towards a new ‘outsideto-inside’ pathogenic paradigm 1, 2, they leave unanswered mechanistic questions of how
FLG-deficiency leads not only to a barrier abnormality, but also to an inflammatory phenotype
in AD. Because such questions are difficult to address in the complex, multifactorial
environment of human disease, we addressed these issues in an animal model of flg deficiency,
the flaky tail (ft/ft) mouse. Recently, these mice were shown to carry a frameshift mutation
with one base pair deletion (5303delA) in exon3 of flg 24, which mimics two common, distal
mutations in human AD (i.e., R2447X in repeat 7 and 1033del4 in repeat 10) 6, 28.
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In our examination of ft/ft mice, we first confirm that flg deficiency alone suffices to induce a
barrier abnormality and low-grade inflammation under basal conditions, as shown recently for
ichthyosis vulgaris 50. We also provide subcellular pathogenic insights, showing that despite
its intracellular localization, flg deficiency results in enhanced paracellular permeability to both
intradermally and epicutaneously applied water-soluble tracers, which do not penetrate +/+
stratum corneum. Movement of these low-molecular weight, water-soluble tracers through the
stratum corneum interstices likely reflects the pathway for hapten ingress in AD, since
similarly-sized, water-soluble antigens, such as nickel, more frequently induce allergic contact
dermatitis in humans with AD 51. However, since our ft/ft mice were not on a homogenous
C578BL6 background, but also express a hair phenotype (matted), we cannot eliminate the
possibility that some of our observations could have been influenced by the concurrent
matted mutation. Nevertheless, the recent work by Fallon, et al 24 shows enhanced antigen
ingress in mice with the same flg mutation, but no matted in their background. Thus, flg
deficiency alone likely explains the barrier abnormality that we demonstrate here in flaky tail
mice.
Localizing the barrier defect in FLG deficiency to the extracellular matrix is an important first
step in understanding AD pathogenesis. Yet, a key question remains unanswered: how can a
defect in an intracellular protein, such as FLG, yield a more-porous extracellular pathway?
FLG has two putative functions: 1) mediation of stratum corneum hydration through humiditydependent proteolysis of FLG into its constituent amino acids and their deiminated products
(e.g., histidine → trans-urocanic acid)18, 52; and 2) keratin intermediate filament compaction
within the corneocyte cytosol 19, 53. Examination of either of these roles does not immediately
suggest a mechanism by which FLG deficiency could lead to increased extracellular
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permeability. Potential clues are provided by other inherited defects of intracellular corneocyte
proteins that also provoke increased paracellular permeability, but by divergent mechanisms
54. In one example, in K1/10-deficient epidermolytic hyperkeratosis, dominant-negative
pairing of the mutant protein disrupts the cytoskeleton, impairing lamellar body secretion 32.
In a second, alternate mechanism, both loricrin keratoderma and transglutaminase 1-deficient
lamellar ichthyosis yield an abnormal corneocyte scaffold that disrupts the supramolecular
organization of the extracellular lamellar bilayers 36, 37. A third mechanism, mutations in the
V1 subdomain of K1 that produce palmar plantar keratoderma, provokes detachment of keratin
filaments from the cornified envelope 55. This defect could link an abnormally-collapsed
corneocyte to an extracellular defect, yet frozen sections of flg-deficient stratum corneum
display corneocytes of normal shape and dimensions (Elias, unpublished observations). Our
ultrastructural studies on ft/ft mice show a partial failure of lamellar body secretion, as shown
previously for non-genotyped human AD 56. Likewise, our preliminary studies in genotyped
ichthyosis vulgaris +/− AD patients show a similar blockade in lamellar body secretion 50.
Thus, analogous to K1/10-deficient epidermolytic hyperkeratosis 32, unprocessed profilaggrin
might impede lamellar body secretion (op. cit.) (suppl. Fig. 2). This mechanism likely also
pertains to human AD associated with distal FLG mutations, in which residual profilaggrin
expression is seen, but processed FLG is reduced 28. Yet, it is not clear how this mechanism
might apply to those cases of AD caused by more proximal FLG mutations, which instead
decrease both profilaggrin and FLG. A fourth mechanism, which could apply to both categories
of FLG mutations, could be linked to decreased generation of acidic metabolites of FLG 1, 2
suppl. Fig. 6). Deficiency in these polycarboxylic acids could result in a net increase in the pH
of the stratum corneum, which in turn could activate neutral-pH-dependent kallikreins, with a
variety of negative, downstream consequences for the barrier 1,2. Such a pH-dependent
mechanism could also lead to deactivation of two key ceramide-generating enzymes, acidic
sphingomyelinase and β-glucocerebrosidase (suppl. Fig. 6)57. This mechanism may not,
however, be operative in ft/ft mice, because the surface pH of these mice remains normal,
perhaps due to compensatory upregulation of alternate, endogenous acidifying mechanisms
[rev. in 58], such as solute carrier family 9 (sodium-hydrogen exchanger, 1 (slc9a) (old symbol
NHE1) 59 and/or secretory phospholipase A2 60. Nonetheless, surface pH measurements do
not always reflect pH alterations within specific microdomains of the stratum corneum 59. We
did not specifically examine such potential, compensatory mechanisms in this study.
NIH-PA Author Manuscript
While our studies demonstrate the increased movement of topically-applied, water soluble
compounds through the stratum corneum in ft/ft mice, the movement of water from inside to
outside through the stratum corneum (TEWL) is not markedly altered in young ft/ft mice,
becoming only slightly elevated in older ft/ft mice in comparison to older +/+ mice. The
development of a barrier abnormality with aging of ft/ft mice could reflect the added stress
from a steeper water gradient across the SC of older animals reflecting the observed decrease
in SC hydration (Fig. 1C). Alternatively or additionally, it could reflect age-related differences
in permeability barrier homeostasis 44, 61. While the explanation for the discordance in TEWL
rates vs. xenobiote penetration is not clear, it could reflect differences in the thresholds for the
barriers to water loss vs. xenobiote ingress/egress, respectively. Pertinently, to increase TEWL
levels by tape stripping, multiple layers of the stratum corneum must be removed 62. Thus,
there appears to be substantial redundancy, with only a relatively small amount of stratum
corneum required to maintain a normal TEWL. Conversely, the structural basis for inhibiting
water movement from inside to outside (i.e., TEWL) could differ from the structural basis for
inhibiting the egress or ingress of somewhat larger, water-soluble compounds. Since there are
several possible mechanisms that could explain how FLG deficiency leads to a marked
abnormality in xenobiote permeation, without altering water loss, additional studies will be
required to distinguish among these alternatives.
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Having established that FLG deficiency results in an extracellular barrier defect, we next
addressed a question of key functional significance; namely, does this barrier defect suffice to
alter thresholds to the development of inflammation from epicutaneously-applied irritants and
haptens? While FLG mutations are associated with the inflammatory disease, AD (op. cit.),
the relationship between FLG deficiency and inflammation is obscured by the association of
the same FLG mutations with the supposedly noninflammatory disorder of cornification,
ichthyosis vulgaris (IV). Additional stressors to the barrier could be required to provoke human
AD 1, 2, because human skin displays an inherently more competent barrier 63–65 (see below).
Nevertheless, the presumption that IV is non-inflammatory should now be re-examined, in
light of our observations, and those of Fallon, et al. 24, which show that flg deficiency alone
results in epidermal hyperplasia, low-grade inflammation, and elevated IgE levels in otherwise
normal mice.
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In order to ascertain whether FLG deficiency alone leads to increased susceptibility to
inflammatory dermatoses, we employed previously described methods for eliciting irritant and
acute allergic contact dermatitis 38, 47, as well as an AD-like mouse model 49, in ft/ft vs. +/+
mice. Our results show that FLG deficiency alone lowers thresholds to inflammation following
epicutaneous applications of either irritants or haptens. Most importantly, we demonstrate
development of an AD-like phenotype following applications of reduced concentrations of
haptens, which do not produce th2-dominant inflammation in +/+ mice. The AD-like phenotype
included increased CRTH2-positive cells, decreased immunostaining for mBD3 (the murine
homologue of hBD2), and elevated serum IgE levels. Similarly, antigen-challenged mice, with
the same flg mutation as ft/ft mice, also display multiple features of th2 inflammation 24.
Interestingly, these flg-deficient mice reportedly did not develop significantly higher IgE
levels24, likely reflecting a less severe phenotype due to differences in bioavailability of our
hapten vs. the complete antigen employed by Fallon, et al. 24. Finally, the apparent reduction
in mBD3 expression could reflect downstream down-regulation by th2 cytokines 16.
NIH-PA Author Manuscript
Notably, inflammation can be induced in flg-deficient mice, without a requirement for
additional, acquired stressors, such as reduced ambient humidity, high pH surfactants, and/or
increased psychological stress, conditions that are known to both further compromise barrier
function, and to precipitate AD in humans 1–3. This leaves unanswered, however, the question
of why many IV patients, including some with double-allele FLG mutations, do not display a
concomitant AD phenotype 25, 26. Two alternative or perhaps coincident explanations could
explain this apparent paradox. First, it is possible that all patients with FLG mutations have
the potential to develop AD, based upon their inherited barrier defect alone, but repeated
epicutaneous deposition of allergens, sufficient to provoke TH2-dominant inflammation, might
not occur in some IV patients. Second, in mice, unlike humans, development of inflammation
might not require the superimposition of additional acquired stressors, because as noted above,
mouse skin displays reduced barrier competence in comparison to human skin 63–65. This
inherent difference could also explain why hapten-induced, acute allergic contact dermatitis
converts to an AD-like phenotype following repeated challenges in mice, a transition that is
not known to occur in humans.
In summary, we show here that FLG deficiency, as occurs in many cases of human AD, suffices
to provoke a barrier abnormality, which in turn allows enhanced permeation of water-soluble
tracers via the paracellular route. The paracellular defect can be attributed to abnormal
extracellular lamellar bilayers, resulting from compromised lamellar body secretion. These
structural abnormalities allow enhanced irritant/hapten ingress, resulting in reduced
inflammatory thresholds, providing a mechanistic link between FLG deficiency and
development of AD. Finally, these studies substantiate and explain the new, ‘outside-to-inside’
paradigm of AD pathogenesis.
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Scharschmidt et al.
Page 11
KEY MESSAGES
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•
Filaggrin deficiency alone provokes a permeability barrier abnormality (no further
acquired stressors are necessary).
•
The barrier abnormality localizes to the extracellular spaces of the stratum
corneum, where if facilitates bidirectional, paracellular percutaneous transport of
water-soluble xenobiotes.
•
The paracellular abnormality can be ascribed to impaired secretion of epidermal
lamellar bodies.
•
The barrier abnormality correlates with reduced inflammatory thresholds to topical
irritants and haptens.
•
Repeated, low dose hapten applications that fail to elicit inflammation in wild-type
mice, provoke inflammation with certain th2 features in filaggrin-deficient mice,
worsening the barrier abnormality, thereby completing an ‘outside-inside-(back
to) outside’ pathogenic cycle in murine AD.
Short Summary of Clinical Implications
NIH-PA Author Manuscript
FLG deficiency (in human AD) provokes a paracellular barrier abnormality, caused by
abnormal extracellular lamellar bilayers, resulting from compromised lamellar body
secretion. These structural abnormalities reduce inflammatory thresholds to irritants/
haptens, providing a mechanistic link between FLG deficiency and development of AD,
explaining the new, ‘outside-to-inside’ paradigm of AD pathogenesis.
Supplementary Material
Refer to Web version on PubMed Central for supplementary material.
Abbreviations
NIH-PA Author Manuscript
AD
atopic dermatitis
BD
βdefensin
CAMP
cathelicidin antimicrobial peptide
flg (mouse) or FLG (human)
filaggrin
ft/ft
flaky tail
Krt
keratin
Ox
oxazolone
TEWL
transepidermal water loss
TM
targeted mutant
TPA
12-O-tetradecanoylphorbol-13-acetate
+/+
wild-type
Acknowledgments
We are indebted to Ms. Joan Wakefield for superb editorial assistance. These studies were supported by NIH grants
AR019098, AG028492, AI059311, and the Medical Research Service, Department of Veterans Affairs.
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Figure 1. Abnormal Barrier Function and Stratum Corneum Hydration in Flaky Tail Mice
A: Basal transepidermal water loss (TEWL) was assessed with an electrolytic analyzer as mg/
cm2/hr, and expressed as (mean) ± SEM. B: Barrier disruption was achieved with repeated
tape stripping until TEWL levels ≥ 3× normal; then % recovery from an initial ‘0’ value, was
measured 2 and 4 hrs after stripping, and expressed as mean +/−SEM. C: Stratum corneum
hydration was measured as electrical capacitance on shaved back skin in flaky tail and wildtype mice, and shown as changes in arbitrary units (AU) +/− SEM (n=8–13, as indicated).
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Figure 2. Abnormal Paracellular Permeability Barrier to a Subcutaneous, Water-Soluble Tracer
in Flaky Tail Mice
Explants of back skin from sex-matched, 12 wk old, wild-type (WT) and flaky tail (FT) mice
(n = 3 each) were immersed dermis-side-downward on a solution containing 4% colloidal
lanthanum nitrate (pH 7.5) for 30 min to 2 hr, followed by fixation in Karnovsky’s solution
(see Methods). Colloidal lanthanum travels outward through the extracellular spaces, but does
not reach the stratum corneum (SC) in WT skin (A, arrows). In contrast, in FT skin, lanthanum
tracer extends into the lower SC, primarily via the extracellular spaces (B, arrows), suggesting
impaired ‘inside-to-outside’ barrier function. SG = stratum granulosum; A+B, osmium
tetroxide post-fixation; Mag bars = 0.5 µm.
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Figure 3. Epicutaneous Tracer Penetrates Flaky Tail Stratum Corneum via the Extracellular
Spaces
A–C: Flanks of flaky tail (FT) and wild-type (WT) mice (n = 3 each) were immersed in 4%
lanthanum nitrate solution for 30 min to 2 hrs, followed by aldehyde fixation. After 2 hrs, little
or no tracer entered the stratum corneum (SC) of WT mice (A, single small arrow). In contrast,
abundant tracer reaches ≥4 layers into the SC in FT mice (B, arrowheads), which on higher
magnification [C] can be seen to localize to extracellular domains (C, arrowheads). D+E:
Calcium green was applied to freshly-obtained explants from ft/ft and wt mice (n = 3 each),
and penetration was assessed by dual-photon confocal microscopy, along the ‘z’ axis after 10
and 75 min. Note much deeper ‘outside-to-inside’ penetration of calcium green in ft/ft mice at
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both time points. Flaky tail (FT) mice, repeatedly-challenged with low-dose Ox, display a more
severe barrier abnormality (G), but no further decline in stratum corneum (SC) hydration in
comparison to wild-type (WT) mice (F). A–C: Osmium tetroxide post-fixation; mag bar = 0.5
µm.
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Figure 4. Abnormalities in the Lamellar Body Secretory System in Young Flaky Tail Mice
Partial failure of lamellar body exocytosis is evident in flaky tail (FT) epidermis (A, C). Note
lamellar bodies lined up in peripheral cytosol (A, multiple thin arrows); decreased secreted
material at stratum granulosum (SG)-stratum corneum (SC) interface (A&C, short, fat arrows);
decreased numbers of lamellar bilayers (C): delayed maturation of lamellar bilayers (C); and
entombed lamellar body contents within the corneocyte cytosol (C, short, thin arrows). B&D:
Normal lamellar body secretion (B, arrows) and extracellular lamellar bilayers in wild-type
(WT) epidermis (D, thin arrows) A&B, osmium tetroxide post-fixation; C&D, ruthenium
tetroxide post-fixation. Mag bars = A&C, 0.5 µm; B&D, 0.1 µm.
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Figure 5. Reduced Threshold for Development of AD-Like Inflammation in Response to Repeated
Ox Challenges in Flaky Tail Mice
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Flaky tail (FT) and wild-type (WT) mice were sensitized to Ox, and then repeatedly challenged
with a subthreshold concentration of Ox (0.02%) every other day to a shaved area of back skin
for a total of 10 challenges. Under basal conditions, FT mice display low-grade inflammation
(F, see also suppl. Fig. 3C vs. A). FT skin displayed much more prominent erythema, scaling
and excoriations than does WT mouse skin (see text), a change that is associated with a modest
inflammatory infiltrate (B vs. A & suppl. Fig. 3C), that was enriched in CD3+ dermal
lymphocytes (D vs. C). Quantitative changes in epidermal hyperplasia and inflammation are
shown in E&F. G: Serum IgE levels are significantly elevated in most flaky tail mice, even
under basal conditions. Mag bars = 40 µm.
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Figure 6. Altered Sensitivity to Irritant/Hapten-Induced Allergic Contact Dermatitis in Flaky Tail
Mice
In young, wild-type mice, TPA induces irritant contact dermatitis, while Ox induces allergic
contact dermatitis following a single, full-strength challenge dose 38. While higher
concentrations produced comparable changes in young wide-type (WT) and flaky tail mice,
lower doses of either TPA (A) or Ox (B) induce inflammation only in flaky tail mice. Changes
in ear thickness correlate with severity of inflammation (see suppl Fig. 4).
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Figure 7. Flaky Tail Mice Demonstrate Increased Th2 Response to Repeated Low-Dose Ox
Challenges
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Serum was collected from young flaky tail (FT) and wild-type (WT) mice before and after Ox
sensitization, and either ten low-dose Ox (0.02%) or vehicle treatments, followed by
assessment of IgE levels by ELISA. While no difference from baseline was seen between
vehicle and Ox-treated WT mice, a large increase was observed in FT mice (B). Likewise, lowdose Ox-challenged FT mice demonstrate increased staining for CRTH2-bearing cells (A,
arrows) in Ox-treated, FT mice. Mag bars = 40 µm.
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