International Immunology, Vol. 27, No. 6, pp. 269–280
doi:10.1093/intimm/dxv013
Advance Access publication 26 March 2015
© The Japanese Society for Immunology. 2015. All rights reserved.
For permissions, please e-mail: [email protected]
Dissecting the formation, structure and barrier
function of the stratum corneum
Takeshi Matsui1 and Masayuki Amagai1,2
Laboratory for Skin Homeostasis, RIKEN Center for Integrative Medical Sciences (IMS), 1-7-22, Suehiro-cho, Tsurumi-ku,
Yokohama, Kanagawa 230-0045, Japan
2
Department of Dermatology, Keio University School of Medicine, 35 Shinanomachi, Shinjuku-ku, Tokyo 160–8582, Japan
REVIEW
1
Correspondence to: T. Matsui; E-mail: [email protected]
Received 5 March 2015, accepted 19 March 2015
Abstract
The skin is the largest organ of the mammalian body. The outermost layer of mammalian skin, the
stratum corneum (SC) of the epidermis, consists of piles of dead corneocytes that are the endproducts of terminal differentiation of epidermal keratinocytes. The SC performs a crucial barrier
function of epidermis. Langerhans cells, when activated, extend their dendrites through tight
junctions just beneath the SC to capture external antigens. Recently, knowledge of the biology of
corneocytes (‘corneobiology’) has progressed rapidly and many key factors that modulate its barrier
function have been identified and characterized. In this review article on the SC, we summarize its
evolution, formation, structure and function. Cornification is an important step of SC formation at
the conversion of living epithelial cells to dead corneocytes, and consists of three major steps:
formation of the intracellular keratin network, cornified envelopes and intercellular lipids. After
cornification, the SC undergoes chemical reactions to form the mature SC with different functional
layers. Finally, the SC is shed off at the surface (‘desquamation’), mediated by a cascade of several
proteases. This review will be helpful to understand our expanding knowledge of the biology of the
SC, where immunity meets external antigens.
Keywords: barrier, epidermis, evolution, Langerhans, skin, stratum corneum
Introduction
Our body organs are covered with various epithelial cell
sheets. These sheets mainly consist of two kinds of tissues:
simple (mono-layered) and stratified (multi-layered) epithelia. Simple epithelium covers internal organs, such as lungs,
stomach, intestine, colon, liver, kidney and so on. These
organs mediate absorption and secretion. On the other hand,
stratified squamous epithelium is formed in the tissues that
undertake physical protection, such as skin, esophagus,
vagina and so on.
Owing to these two kinds of epithelia, all the organs are
compartmentalized and perform their own characteristic
physiological functions. Epidermis, a major type of stratified
squamous epithelium, is found in the body surface of ‘terrestrial vertebrates’. Because epidermis is located at the outermost side of the body, it is always threatened by the harmful
outside environment. Evolution thus gave epidermis several
strong protective functions that form the ‘epidermal barrier’,
which includes both cell- and molecular-based and molecular barrier functions. Among various skin barrier functions, the
stratum corneum (SC) provides one of the key factors to regulate cutaneous sensitization.
Understanding of the epidermal barrier is important for
understanding self-defense mechanisms of terrestrial vertebrates because its function is closely linked to a part of the
skin-associated innate and adaptive immune system (1–3).
Furthermore, accumulating evidence indicates that enhanced
cutaneous sensitization to external antigens is one of the major
causes of many allergic disorders, including atopic dermatitis
(AD), asthma, food allergy and anaphylaxis (4).
In this review, to allow a better understanding of the cutaneous immune system, especially cutaneous sensitization,
we rather focus on the field of corneobiology (the biology of
the SC) and summarize recent progress in understanding
the barrier mechanism of the SC. Reviews on immunological aspects of skin barrier dysfunction in AD or other allergic
diseases are found elsewhere (4–6).
The evolutionary origin of the air–liquid interface
barrier: the SC
The first terrestrial vertebrates, known as Icthyostega,
appeared 360 million years ago, during the late Devonian
270 Skin: where the immune system meets external antigens
period. Icthyostega are classified as amphibians. Although
there is less information about the surface skin of Icthyostega
from fossils, the surface of many present-day amphibians is
covered with mucus-rich epidermis with SC (7, 8). The unique
feature of amphibian epidermal development is metamorphosis. In response to the thyroid hormone, amphibian larvae
remodel many organs for adaptation to terrestrial life. During
embryonic stages, amphibian tadpoles are covered with
larval epidermis consisting of apical, skein and basal cells,
which does not resemble adult epidermis. During metamorphosis, basal cells differentiate into epidermal stem cells and
form keratinized stratified squamous epithelium (9, 10).
Thus, it is likely that at the moment of adaptation to the air,
these creatures evolved their surface epithelia into a multilayered epithelium that forms a dead but functional air–liquid
interface barrier called the SC. This tissue covering the surface of the adult amphibian body might be the origin of the
skin of terrestrial vertebrates (7, 8).
The first reptiles appeared in the Carboniferous period
(340 million years ago). Then, adaptive radiation formed the
major reptilian groups at the end of Palaeozoic era. In the
Mesozoic era, they become dominant terrestrial vertebrates.
By the end of the Mesozoic era, most of them became extinct,
resulting in three groups—Crocodylians, Chelonians and
Lepidosaurians. The reptiles were the first terrestrial vertebrates able to survive away from an aqueous environment,
for example reproduction became independent of water due
to the appearance of an ‘amniotic’ egg and skin evolved to be
a dehydration-resistant.
The epidermis of present-day lizard scales also has a similar epidermal structure consisting of the stratum germinativum, which is the same as the stratum basale (SB; see later),
an intermediate zone and the SC; note that they have two
main types of SC—a softer region (α-layer) and a stiff region
(β-layer). The β-layer gives a mechanical strength to the scale
epidermis, whereas the α-layer allows stretching and the barrier function. Reptile SC generally possesses a highly impermeable barrier based on the stiff epidermis. The first avians
are thought to appeared around 145 million years ago (11).
Present avian epidermis is more simple but is similar to that of
reptiles, being classified into two layers—the SC and the stratum germinativum. The avian stratum germinativum is further
classified into the stratum transitivum, stratum intermedium
and SB.
After the Permian-Triassic mass extinction (250 million
years ago), the Late Triassic period (about 220 million years
ago) featured the origin and radiation of mammals from nonmammalian synapsids (12). Mammals share many characteristics—the presence of mammalian glands, fur, the single
bone in the lower jaw, the neocortex of the forebrain, the placenta, diaphragm, secondary palate, mammary glands and
auditory ossicles (13). Does our skin have a mammal-specific
character? Even though mammals became evolutionally distant from amphibians, during the embryonic development
of mammals, the dynamic surface changes are somewhat
reminiscent of the ancient evolutionary history of adaptation
to life on the land. Similarly to amphibian tadpoles changing their surface into stratified squamous epithelium during
metamorphosis, mouse embryonic ectoderm (simple epithelial cells that cover the surface of the embryo) begins to
express skin-specific genes and stratify at embryonic day
15.5 (E15.5) to form epidermis. Just before birth (at E18.5), an
almost complete form of epidermis is formed with a functional
SC. Thus, the formation of the mammalian SC in embryonic
development is an ‘intrinsic’ and programed phenomenon.
The mammalian epidermis has highly specialized characteristics that amphibians, reptiles and avians do not have (7,
8). In particular, the SC becomes highly moistened, resulting in the acquisition of a soft epidermis. In concomminant to
the soft epidermis, the frequency of innervation by peripheral
neurons becomes very high, which enables mammals to be
sensitive to the outside environment (14). Therefore, most of
the present mammalian epidermis is thought to have both
an ancestral barrier system [the SC barrier, the tight-junction
(TJ) barrier and the immune barrier; see later] and a soft and
moistened SC, which is a mammal-specific trait.
The innate immune system has been known as a semispecific defense system and the fundamental self-defense
mechanism developed in eukaryotes, fungi, plants, invertebrates and vertebrates (2). Recently, it was also recognized to have specific elements, for example mediated via
various pattern recognition receptors, which are thought
to have evolved before the acquisition of adaptive evolution (15). Mammalian epidermis has several additional elements of the innate immune system: secretion of cytokines
(IL-1, IL-6, TNF-α, etc.), desquamation (shedding off of the
SC), the weakly acidic pH condition in the SC, and the presence of commensal bacteria and of anti-microbial peptides
(defensins, cathelicidins, etc.)
The adaptive immune system is thought to have developed
in addition to the innate immune system in ancestral primitive
vertebrates (fishes) by acquiring the major histocompatibility
complex, T-cell receptors and immunoglobulin superfamily
proteins (1, 16–18). Especially, immunoglobulins have differentially changed their repertoire during the evolution of vertebrates, such as in fishes [which have the isotypes IgM, IgT
(also called IgZ) and IgD], amphibians (IgM, IgX, IgY, IgD
and IgF), reptiles (IgM, IgY, IgA and IgD), birds (IgM, IgY and
IgA) and mammals [IgM, IgG, IgA, IgD and IgE (16, 19)].
Cells similar to mammalian Langerhans cells (LCs) are also
observed in amphibians, reptiles and birds, suggesting that
cutaneous sensitization via LCs was firstly acquired in ancestral amphibians and this coincided with the acquisition of the
SC and epidermis (2).
Compared with reptiles’ epidermis, the mammalian epidermis is composed of a moisturized and soft SC. Thus, our skin
is susceptible to infections and/or allergy with a complicated
crosstalk between innate and adaptive immune systems (20).
In the next section, we describe how the fragile but functional
mammalian SC is formed and how it can be destroyed via
loss of SC-related genes in humans and mice, resulting in
barrier defects and occasionally inflammation.
Epidermal differentiation, keratinization and barrier
function
Epidermal differentiation and keratinization
The epidermis—keratinized stratified squamous epithelium—covers the body surface of terrestrial vertebrates and
serves to protect from entry of pathogens, allergens or toxic
Skin: where the immune system meets external antigens 271
substances and to prevent water loss. The mammalian epidermis mainly consists of four cell layers: the SB, the stratum spinosum (SS), the stratum granulosum (SG) and the SC (Fig. 1).
Keratinocytes proliferate in the SB and they differentiate and
migrate upward into the SS (21, 22). In each layer, keratinocytes express different sets of keratin intermediate filaments.
Keratins are elastic fibrous proteins differentially expressed
during keratinocyte differentiation. Keratins form heterodimers between acidic (type I) and basic (type II) proteins.
Once assembled, they form a three-dimensional cytoskeleton located in the cytoplasm and around the nucleus. These
keratin filaments are anchored to desmosomes, which are
junctional complexes utilized for cell–cell adhesion.
At the SB of the epidermis, keratinocytes express keratin
5 (basic, type II) and keratin 14 (acidic, type I). During differentiation from the SB into the SS, keratinocytes dramatically switch the expression of keratins into keratin 1 (type II,
basic) and keratin 10 (type I, acidic). After the several layers
of SS, numerous keratohyalin granules (KHGs), which are an
amorphous protein complex of keratin and keratin-binding
proteins [such as filaggrin or loricrin] are formed in the SG.
This SG consists of three cell layers, designated as SG1, SG2
and SG3, from the apical to the basal side (3). At the uppermost layer, SG1 cells undergo cell death to form the dead
cell layer—the SC. This layer serves as the air–liquid interface
barrier.
The TJ barrier system of mammalian epidermis
In addition to the air–liquid interface barrier in the SC, the TJ
strand is also important as a ‘liquid–liquid’ interface barrier’
in the epidermis. TJs are commonly observed on the apical
side of simple epithelial cell sheets in vertebrates. They are
responsible for sealing epithelial cells to form a functional
barrier of the paracellular pathway (which otherwise allows
passage between cells) and compartmentalize each organ
for its physiological functions. It was experimentally demonstrated that the SG2 cell layer is sealed together by a functional TJ in epidermis (3, 23, 24). From these morphological
data, SG1 cells are located above the TJ and below the SC
barrier (Fig. 1).
The immune barrier system of mammalian epidermis
Among the several subsets of dendritic cells (DCs), epidermis-specific DCs are called LCs and are thought to be
involved in induction of antigen-specific Th2 responses as
well as maintenance of peripheral tolerance (25, 26). No other
immune cells are resident in the epidermis in the normal state.
Recently, the whole-mount staining method [Fig. 1; (27)] has
shown that, in the resting state, the tips of LC dendrites are
aimed at the apical side below the TJ of the SG2 cell layer.
Once LCs are activated, they extend their dendrites through
the TJ just beneath the SC [Fig. 1; (28)]. When LCs extend
their dendrites, they do not break the TJ, but form a different
TJ between the extended dendrites and adjacent keratinocytes to maintain the TJ barrier (28). It was also observed that
external antigens are taken up by the tips of the extended
dendrites of LCs.
These three elements of the skin barrier (the SC as an
air–liquid interface barrier, the TJ as a liquid–liquid interface
barrier and LCs as a frontline player of the immune barrier)
beautifully coordinate and protect living bodies as the first
line of defense. And these three barrier elements are found
in probably most terrestrial animals, whose life is constantly
threatened by the outside environment in the air. When we
Fig. 1. Structure of mammalian epidermis and its three barrier elements. Mammalian epidermis is composed of three barrier elements: the ‘SC’:
air–liquid interface barrier; the ‘TJ’: liquid–liquid interface barrier; and the ‘LC network’ which is the immunological barrier. Activated LCs extend
their dendrites above the TJ strands to capture external antigens.
272 Skin: where the immune system meets external antigens
think of cutaneous sensitization, external antigens need to
penetrate SC layers to be taken up by the extended dendrites of LCs. Therefore, the function and dysfunction of the
SC determines the nature and quality of cutaneous sensitization. Now, we will discuss how much we know about the
mammalian SC.
‘Cornification’: formation of the SC by a unique cell
death mechanism
The SC is generated from cell-death products of the SG1
layer, and consists of 10–20 piled-up layers of dead cells,
10–30 μm in diameter and 1 μm thick (29, 30). It is composed
of keratin intermediate filaments, water-soluble lipid, proteins
(enzymes) and the humectants [also called ‘natural moisturizing factors’ (NMF)]. The SC is often described as resembling
‘bricks and mortar’ in which corneocytes are the bricks and
intercellular lipid lamellae are the mortar (31, 32).
At the final layer of the SG, SG1 cells undergo a cell death
program that is not a classical ‘apoptosis’ but is classified
as ‘cornification’ (33, 34). Cornification is similar to the cell
death of lens epithelial cells and red blood cells in the respect
that typical apoptosis-related proteins (e.g. caspase-3) are
not activated. However, SG1 cells do not possess a stressinduced cell death program, whereas lens epithelial cells and
red blood cells still retain the ability to undergo cell death by
stresses, such as ultraviolet light (33, 34).
Several mouse and human genes have been reported to
be involved in this formation of the SC in both humans and
mice (Table 1). Most of the knockout mice show an epidermal
barrier defect and some of them induced inflammation. In the
next section, we will describe how the dead bricks-and-mortar layer is formed above the living layers, which is responsible for epidermal barrier function. The major key events of
cornification consist of three events, as detailed below: formation of the intracellular keratin network; formation of cornified envelopes (CE) (crosslinking of lipids and proteins) and
formation of intercellular lipids [secretion from lamellar body
(LB) contents] (Fig. 2).
Formation of the intracellular keratin network
Prior to cornification, many KHGs are gradually formed inside
the cytoplasm of SG3 cells. The KHGs consist of amorphous,
electron-dense materials. KHGs are thought to consist of
a complex of keratin (mainly from keratin 1 and 10) and a
keratin-binding protein, such as profilaggrin (F-granules) or
loricrin [L-granules; only found in rodents (35–37); Table 1].
Profilaggrin is a mammal-specific, insoluble, highly phosphorylated protein [>400 kDa in humans (38, 39)]. Profilaggrin
consists of an amino-terminal Ca2+-binding protein of the
S-100 family, tandemly linked to 10–12 filaggrin monomers
and a carboxy-terminal domain (38, 40, 41) (Fig. 3). Filaggrin
is the major keratin-binding protein in the mammalian SC.
Loss-of-function mutations of filaggrin have been reported
as a major predisposing factor for AD, possibly because of
enhanced penetration of external antigens through the SC in
that disease (42–47) (Table 1).
During cornification, SG1 cells enucleate, lose organelles and change their shapes into a two-dimensional flat
polygonal structure. During this transition of SG1 to SC,
KHGs also gradually disappear. Although the mechanism
of disappearance of KHGs is still unknown, dephosphorylation of profilaggrin is thought to be the initial step of dissociation of the keratin–profilaggrin complex in KHGs (Fig. 2).
Dephosphorylated profilaggrins are cleaved to generate
filaggrin monomers during the SG1-to-SC transition (47).
At this step, several proteases are thought to be involved
in the processing of profilaggrin. SASPase/Taps/ASPRV1
(skin aspartic protease/TPA-inducible aspartic proteinaselike gene/aspartic protease, retroviral-like 1) is exclusively
expressed in SG1 cells and is a candidate protease of profilaggrin linker-cleavage (48) (Table 1).
Other proteases, such as endopeptidase-1 (PEP-1),
μ-calpain, matriptase, prostasin and KLK5 are also reported
to cleave profilaggrin-linker peptides (47, 49–54). The filaggrin
monomers that are produced (37 kDa in human) are thought to
strongly bind and bundle keratin filaments in the lower SC (55).
On the basis of cryo-electron microscopic analysis of
human vitreous skin sections, it has been proposed that
keratin filaments are arranged to form the template for the
membrane in the SC, with cube-like rods packed in the structure [Fig. 2; (56)]. This unique three-dimensional structure is
considered to be important for the hydration of the SC and
gives rigidity to each SC layer. Double-knockout of keratin-1
and keratin-10 in mice causes neonatal lethality due to fragility in the epidermal structure (35). Nevertheless, those mice
still had KHGs and a normal profilaggrin-to-filaggrin processing pathway, suggesting that keratin-1 and keratin-10 are not
directly involved in KHG formation itself but rather involved
in formation of the three-dimensional network of keratin filaments with the aid of filaggrin released from KHGs.
These keratin filament networks are thought to be associated with desmosomes of the SC, and are called ‘corneodesmosomes’; they are slightly modified forms of the desmosomes
in living layers. Corneodesmosomes consist of adhesion
molecules, such as desmoglein 1 (DSG1) and desmocollin 1
(DSC1), and cytoplasmic anchoring proteins, such as plakoglobin, plakophilin and desmoplakin, which are probably also
associated with keratin filaments. Corneodesmosin (CDSN) is
secreted from the LBs and localized in the intracellular space
of the SC and becomes a component of the corneodesmosome to help adhesion functions (57–59) (Fig. 2; Table 1).
Abnormalities of corneodesmosomal proteins are known
to link to several inflammatory diseases in humans and mice.
It has been recently reported that a specific form of DSG1
deficiency in humans results in severe dermatitis, multiple allergies and metabolic wasting (SAM syndrome) with
increased serum IgE (60). CDSN-deficient mice showed
abnormalities in the SC and epidermal barrier function
(58, 59). Furthermore, loss-of-function mutation of CDSN in
humans causes peeling skin syndrome, ichthyosiform erythroderma (61). In this disease, detachment of the SC layer
from epidermis causes chronic dermatitis, asthma, allergic
rhinitis, elevation of serum IgE and food allergy (61). This
evidence suggests the importance of the corneodesmosome in the maintenance of the epidermal barrier and a link
to percutaneous immunization. However, we still don’t know
how these diseases cause SC barrier disruption and lead to
an immune barrier abnormality because of the lack of knowledge of the SC barrier itself.
LEKTI (SPINK 5)
Filaggrin
Involucrin/Envoplakin/ INV/EVPL/
Periplakin
PPL
Loricrin
Tmem79/mattrin
Transglutaminase-1
EDC-related gene
EDC-related gene
EDC-related gene
EDC-related gene
EDC-related gene
TGM1
TMEM79
LOR
FLG
SPINK5
CASP14
Protease inhibitor
Protease
ASPRV1
SASPase/Taps/
ASPRV1
Caspase 14
Protease
KRT10
Cdsn
Keratin-10
Keratin
Phenotype
—
Neonatal lethal. Change in
ceramide population. Aberrant
SC and ceramide formation.
Decreased SC hydration.
Peeling skin
Neonatal lethal due to tearing.
syndrome
Desmosomal break between the
SG and SC, hyperproliferation
of keratinocytes and thick SC.
Degeneration of epidermis and
the hair follicles.
—
Dry skin, aberrant processing
of profilaggrin-to-filaggrin
—
Dry skin, aberrant processing
of profilaggrin-to-filaggrin
(accumulation of filaggrin
intermediates). Shiny and
lichenified SC.
Netherton syndrome, Neonatal lethal. Fragile skin.
AD (polymorphism)
Severe erosion.
Ichthyosis vulgaris
Dry and scaly skin (neonatal)
(IV), AD
Epidermolytic
hyperkeratosis
Human disease
Knock-out mice
—
—
—
—
Inflammation in KO
mice
Spontaneous
dermatitis and atopy
—
Triple knockout of
involucrin/envoplakin/
periplakin caused
infilltration of CD4+ T
cells. Reduction of
resident γδ+ T cells
—
Epidermal barrier Dermatitis, elevated
defect
expression of TSLP
SC barrier defect —
[increased
penetration of
Cr(III) and Calcein
liposome through
the SC]
Epidermal barrier
defect
Normal
Epidermal barrier
defect
Epidermal barrier
defect
Barrier defect
Postnatal hyperkeratosis,
Epidermal
aberrant desquamation,
barrier defect in
abnormal CE, decreased
triple knockout
lipid and mechanical
of involucrin/
integrity. Aberrant profilaggrin envoplakin/
processing.
periplakin
Component of CE
Vohwinkel syndrome Weighed less at birth, congenital —
with ichthyosis
erythroderma with a shiny,
translucent skin. Reduced SC
stability. Susceptibility of SC to
mechanical stress. Compensation
of phenotype in P4–P5 with
increased expression of
SPRRP2D, SPRRP2H and repetin.
Transmembrane protein —
Impaired LB secretory system. Epidermal barrier
Abnormal SC.
defect
Crosslinks lipids and
Ichthyosis,
Neonatal lethal. Abnormal CE Epidermal barrier
proteins of SC
congenital, autosomal and intercelllular lipid lamellae. defect
recessive 1
Component of CE
Bundling keratin in SC.
Production of NMFs
Protease inhibitor
Protease
Protease
Secreted protein.
Adhesion of
corneodesmosome
Cytoskeleton
Gene symbol Protein function
Desmosomal protein Corneodesmosin
Protein
Type of molecule
Table 1. Examples of genes involved in SC formation
Skin: where the immune system meets external antigens 273
274 Skin: where the immune system meets external antigens
—
Epidermal barrier
defect
—
Epidermal barrier
defect
—
Epidermal barrier
defect
ß-glucocerebrosidase GBA
ATP-binding cassette
subfamily A member
12
Lipid
metabolism-related
Lipid
metabolism-related
ABCA12
Hydrolysis of
glucosylceramide into
glucose and ceramide
Energy-dependent
lipid transporter of
glucosylceramide into
LB
Ichthyosis,
Neonatal lethal. Abnormal
congenital, autosomal SG. Fragile CE. Aberrant
recessive 2
lipid composition. Aberrant
profilaggrin processing.
Gaucher disease
Neonatal lethal. Increased
glucosylceramide. Decreased
ceramide.
Harlequin ichthyosis Neonatal lethal. Reduced
amount of total ceramide and
aberrant ceramide composition.
Aberrant profilaggrin
processing. Hyperkeratosis.
Oxygenation of the
linoleic acid residue in
acylceramide
Arachidonate
12-lipoxygenase,
R type
Lipid
metabolism-related
ALOX12B
Phenotype
Protein
Type of molecule
Table 1. Continued
Gene symbol Protein function
Human disease
Knock-out mice
Barrier defect
Inflammation in KO
mice
Formation of the CE: crosslinking of lipids and proteins
During the SG1-to-SC transition, an intracellular Ca2+ increase
induces terminal differentiation. The major target of Ca2+ is transglutaminase (TGase). TGase is a Ca2+-activated enzyme that
crosslinks with ε-(γ-glutamyl)lysine isopeptide bonds. Increased
activity of intracellular membrane-bound TGase I and cytoplasmic TGase III cross-links protein products of the epidermal differentiation complex (EDC) genes, involucrin, loricrin, envoplakin,
periplakin and the small proline-rich protein family (SPRRs), etc.
underneath the plasma membrane (62–64) (Table 1).
Most TGase-crosslinking proteins are products of genes
located in the locus of human chromosome 1q21 called the
EDC. This is a large gene cluster located on chromosome 3
in mice (43, 65, 66). The EDC genes of mammals include the
S100A family, loricrin, involucrin, SPRRs late cornified envelope (LCE) protein family and the S100-fused type proteins
[e.g. filaggrin]. The antimicrobial peptidoglycan-recognition
proteins (PGLYRPs) 3 and 4 are also reported to be localized
in the EDC (67, 68).
Recently, the EDC was reported in non-mammalian vertebrates, such as chickens and green anole lizards (69, 70).
Interestingly, chickens and lizards also have similar EDChomolog genes to mammals that are expressed in an epidermis-specific manner. Comprehensive analysis of EDC genes
in chickens and lizards led to the proposal that epidermal barrier proteins were derived from fusion of ancient S100A and
PGLYRP genes and also the loss of exons and multiple rounds
of fusions, duplications, loss of exons/introns and amino acid
substitutions. It is hypothesized that the predicted origin of
loricrin derives from a common ancestor among amniotes (70).
Many EDC-genes (the LCE family, SPRR family, S100 family, etc.) show highly homologous sequence similarity, suggesting that the SC is formed by a ‘redundant’ mechanism.
Even for single gene products, each EDC-protein shows
functional redundancy: mice with single knockouts for EDCderived involucrin/envoplakin/periplakin did not show any
evident aberrant phenotype in the epidermis (71, 72). Triple
knockout of these three genes resulted in defective epidermal barrier function assessed by transepidermal water loss
measurement, suggesting that the cross-linking of these EDC
gene products is essential for corneocyte permeability barrier function (72).
Recently, the redundant mechanism of SC formation has
been shown to be dependent on the effect of the amniotic
fluid from the uterus. Loricrin-deficient mice have previously shown only a transient abnormality in the neonatal
period, even though loricrin makes up 70% of SC protein
(73) (Table 1). In these mice, epidermal barrier acquisition is
delayed by 24 h at E16.5 and increased expression of Sprr2d
and Sprr2h mRNAs was observed; resulting in cornified cell
envelopes that are composed of the Sprr2 family instead of
loricrin (73, 74). Between E14.5 and E16.5 of mouse embryonic development, the composition of the amniotic fluid
changes (75), which coincides with the formation of the
epidermal barrier, possibly for the protection of the embryo
from the effect from E16.5-amnionic fluid. Genetic blocking
of antioxidant responses in the early embryo by the Nrf2/
Keap1 (NF-E2-related factor 2/Kelch-like ECH-associated
protein 1) pathway resulted in the inhibition of compensatory
Skin: where the immune system meets external antigens 275
Fig. 2. Major events of SC formation. The formation of the SC (cornification) is composed of three major events: (i) the formation of the intracellular keratin network, (ii) the formation of the CE (crosslinking of lipids and proteins) and (iii) the formation of intercellular lipids (secretion from
lamellar body contents). Finally, the surface of the SC is shed off by degradation of corneodesmosomes via the activity of several proteases.
CER, ceramide; CHOL, cholesterol; Evpl, envoplakin; FFA, free fatty acid; Lor, loricrin; Ppl, periplakin; SG, stratum granulosum; SPRR, small
proline-rich protein family; SS, stratum spinosum; TGase, transglutaminase.
phenotypes of loricrin-deficient mice (75) (Table 1). Those
mice showed apparent defects in epidermal barrier function. This mechanism may be a remnant of the preadaptation
mechanism of epidermal barrier evolution.
Unlike the conservation of loricrin across the avians and
reptiles, filaggrin is a newly acquired EDC gene in the mammalian taxa, suggesting that filaggrin is derived from the EDC
of reptiles or mammal-like reptiles (70). Considering that filaggrin is a skin-specific protein and is involved in the formation
of the proper keratin network in the SC, it was suggested to be
acquired for a mammal-specific SC function, such as moisturization or NMF production. The link of filaggrin-dependent
characteristics of the SC in mammals and SC barrier disruption in human AD patients is still a mystery in this field. Other
EDC genes that cause human diseases are listed in Table 1.
Formation of intercellular lipids: secretion from LB
contents
LBs are organelles derived from the Golgi apparatus and
contain phospholipids, glucosylceramides, sphingomyelin
and cholesterol; they begin to form in the SS layers (63, 76).
During the SG1-to-SC transition, at the apical surface of SG1
cells, LBs secrete their contents into the extracellular space
between the SG1 and the lower SC, which includes various
kinds of proteases and protease inhibitors as well as lipids
like glycosylceramide; these components are involved in
the barrier formation by the SC. The ω-hydroxyceramide is
also included in LBs and, once secreted from SG1 cells, it is
cross-linked to the plasma membrane as a 5-nm monolayer
sheet and covers the surface of corneocytes (77). Using this
cross-linked lipid as a template, the extracellular space of
corneocytes is filled with periodic sheets of lipid lamellae,
which serve as an impermeable barrier in the SC.
This process is thought to be performed by
12R-lipoxygenase. Deficiency of the 12R-lipoxygenase
gene in mice results in an epidermal barrier defect and a
decreased amount of ω-hydroxyceramide-bound protein (78,
79) (Table 1). Mutation of the 12R-lipoxygenase in humans
causes non-bullous congenital ichthyosiform erythroderma
(NCIE) [Table 1; (80)]. LB-secreted glucosylceramides
276 Skin: where the immune system meets external antigens
Fig. 3. Summary of functional SC zones. The SC layer is reported to be divided into two or three zones. Isolated cornified cells are classified into two morphologies; stratum compactum (CEf) and stratum disjunctum [CEr] (42). The SC is divided into three zones according to the
profilaggrin processing pathway: in the lower SC, monomer filaggrin bundles keratin filaments; in the middle SC, keratin-bound filaggrins are
citrullinated (Cit) and released from keratin filaments; in the upper SC, filaggrin is degraded into amino acids to produce most of the NMFs
(47). Cryo-electron microscopic observation of swelled SC revealed three distinct zones (98, 99). TOF-SIMS analysis revealed three zones:
sponge-like upper SC (Khigh, Argininehigh), middle SC (Klow, Argininehigh) and lower SC [Klow, Argininelow (100)]. SASPase, skin aspartic protease;
SC, stratum corneum; SG, stratum granulosum.
and sphingomyelins are converted into ceramides by
β-glucocerebrosidase and sphingomyelinase, respectively.
β-glucocerebrosidase-deficient mice showed decreased ceramides in the SC and showed epidermal barrier defects (81,
82) and deficiency is associated with Type 2 Gaucher disease (83, 84). The ATP-binding cassette subfamily A member 12 (ABCA12) is associated with Harlequin ichthyosis and
ABCA12-deficient mice showed neonatal lethality due to epidermal barrier function with hyperkeratosis and accumulation
of lipid droplets, suggesting a defect in incorporation of glucosylceramide into LBs (85, 86) (Table 1).
Lamellar lipids are composed of the same molar ratio of
ceramides, free fatty acids and cholesterol (87). Recent
cryo-electron microscopic analysis has revealed that ceramides are stacked as bilayers of fully extended ceramide
side-chains and cholesterol molecules are associated with
the ceramide sphingoid moiety (88). This report suggested
that the unique arrangement of lipid lamellae in the structure
is important for the skin barrier and robustness of hydration as well as responding to environmental and mechanical
changes. Several mice with knockouts related to lipid metabolism of the SC were reported to have disrupted epidermal
barrier function [reviewed in ref. (76)] (Table 1).
Recently, another EDC gene, transmembrane protein 79/
mattrin (Tmem79/Matt), has been reported to be involved in
secretion of LB contents and identified as a gene responsible
for causing spontaneous dermatitis in mice (89, 90) (Table 1).
Analysis of a human single nucleotide polymorphism of the
Tmem79 gene revealed a low but significant association with
AD (90). Tmem79 is a five-transmembrane protein localized
in LBs of the SG1 layer of epidermis and Tmem79-deficient
mice showed decreased secretion of the contents of lamellar granules, resulting in aberrant SC formation, suggesting a
novel pathway of spontaneous inflammation.
Maturation of the SC: the zone hypothesis
After cornification, dead SC layers are piled up and change
their properties via various chemical reactions, the components of which were already present in SG1 cells. Thus, the
Skin: where the immune system meets external antigens 277
SC zone is not a simple accumulation of homogeneous dead
cornified cells. Various chemical reactions occur during the
upward (lower-to-upper) migration and maturation of the SC.
Several lines of evidence indicated that there are apparent
functionally distinct SC zones.
Morphological analysis of each layer of the SC showed
that, in the lower SC (often called the ‘stratum compactum’),
fragile corneocytes (CEf) are rather small, less hydrophobic
and fragile (CEf) than the cells above. In the upper SC, (the
‘stratum disjunctum’), the SC cells become large, hydrophobic and rigid (rigid corneocytes; CEr) (42). This classification
suggested that at least two kinds of morphologically different
corneocytes are formed.
From the steps of profilaggrin processing, the SC can be
roughly divided into three zones. In the lower SC, filaggrin
monomers are cleaved out from profilaggrin at the SG-to-SC
transitional zone and monomeric filaggrin bundles with
keratin filaments. Probably in the middle SC zone, keratin-bound filaggrins are citrullinated by peptidylimidases
(91–95). This modification may affect the conformation of
filaggrin released from keratin filaments. Released filaggrins
are attacked by several proteases, such as caspase-14,
elastase and further degraded into amino acids by breomycin hydrolases (47, 95, 96). These sequential reactions
occurring in each SC layer were confirmed by data from
immunoelectron-microscopic analysis with an anti-filaggrin
antibody (97) (Fig. 3).
By using cryo-fixation and scanning electron microscopic
(cryo-SEM) technology against human epidermis, three
hydration zones (lower, middle and upper SC) were identified
based on their swelling potential after immersion in 5–20%
salt solutions (98). After swelling, the center zone remains
unchanged, whereas the upper SC swelled massively and
the lower SC swelled twice in size. It was proposed that the
middle SC zone serves as the major permeability SC barrier.
On the contrary, by using isolated human SC, it was demonstrated that middle SC zone swells the most, suggesting that
different sample preparation affects the swelling capacity of
SC hydration zones (99).
The three-hydration-zone hypothesis was also confirmed
by recent observations from time-of-flight secondary ion
mass spectrometry (TOF-SIMS) against the SC of mouse tail
epidermis and revealed that the SC has three functionally distinct layers with different properties (100). Firstly, the lower
SC has a high Na+, low arginine, low K+ concentration and an
impermeable barrier against Cr (VI) ions. Secondly, the middle SC layer has a high arginine, low K+ concentration and
an impermeable barrier against Cr (III) ions. A high arginine
concentration suggests that the zone of filaggrin degradation
results in the production of free amino acids, which make up
almost half of the NMFs. NMFs are hygroscopic substances
and serve as natural humectants in the SC (42, 101). Thirdly,
the upper SC layer has a sponge-like layer (Khigh, Argininehigh),
where external solutes easily flow in and out. In the case
of mice, a high K+ ion concentration was observed due to
smears of urine. In the upper SC, a high concentration of
Na+ ions is also observed within corneocytes but could be
washed out, suggesting the presence of a pathway of transcorneocyte infiltration.
Interestingly, TOF-SIMS analysis of the epidermis of filaggrin-deficient mice demonstrated loss of the arginine-high
layer and increased Cr (III) penetration in the impermeable
barrier present in the lower SC. In support of this observation,
calcein liposomes easily penetrate into the SC in filaggrinknockout epidermis (44). These results suggested that filaggrin deficiency causes a certain abnormality in the formation
of the three SC zones, which may affect the SC barrier and
cutaneous sensitization. Detailed classification of SC zones
and the functional abnormalities of knockout mouse models
or human diseases would be the logical next research field of
corneobiology and cutaneous immune responses.
Desquamation of SC: the end of the SC barrier
At the final uppermost layer, each corneocyte must shed off,
a process called ‘desquamation’. This continuous sheddingoff process is part of the physical innate immune system in
the epidermis, which is useful to remove harmful microorganisms or infectious viruses. Various proteases are involved in
this process. The weakly acidic condition of the upper SC
is the important factor in maintaining the protease activity. Among the proteases, the KLK-related peptidase family
(KLK5/KLK7) is known as a regulator of desquamation (102).
These proteases are tryptic or chymotryptic serine proteases
having a neutral optimum pH and are secreted from LBs of
SG1 cells into the intercellular space between the SG1 and
SC. However, lymphoepithelial Kazal type–related inhibitor
[LEKTI; encoded by the serine peptidase inhibitor, Kazal
type 5 (SPINK5) gene] is also secreted from LBs of the SG1
and inhibits KLK5 and KLK7 (Table 1). In the upper SC, a
decreased pH towards weak acidity induces the release of
KLK5 and KLK7 from LEKTI. Even in the weakly acidic pH,
the activity of KLK5/KLK 7 is thought to be enough to cleave
the extracellular domain of DSG1, DSC1, CDSN, etc. and
finally the uppermost corneocytes are shed off.
Several studies have demonstrated genetic linkage between the SPINK5 gene and dermatitis (Table 1).
Polymorphism of SPINK5 is related to AD, asthma and an
increased level of serum IgE (103–105). Netherton syndrome,
caused by a loss-of-function mutation of SPINK5, is a severe
autosomal recessive ichthyosis with chronic dermatitis,
asthma and allergic rhinitis (106). Electron microscopic analysis of patient skin revealed detachment of the SC from the
SG accompanied by the degradation of corneodesmosomes
(107–109). Consistent with these reports, SPINK5-knockout
mice showed an elevated activity of KLK5 activity in the SC
and increased degradation of CDSN, resulting in defective
epidermal barrier function (110–112). This evidence suggested that the desquamation mechanism itself has some link
to cutaneous sensitization.
Conclusion
Considering that most terrestrial animals have an LC network,
a TJ barrier and an SC barrier in their epidermis, how they
evolved and what is acquired in the mammalian epidermis
may be the key to understand the complex pathophysiology
of cutaneous sensitization, the upstream event of the many
allergic disorders. The link of filaggrin mutation in human AD
278 Skin: where the immune system meets external antigens
patients and the mammal-specific acquisition of the filaggrin
gene still has some missing pieces. A combination of SC-zone
analysis by recent advanced microscopic techniques and
genetically engineered mice, together with the characterization of human disorders with genetic causes, would further
advance the field of corneobiology.
Funding
This work was supported by Grants-in-Aid for Scientific
Research funding from the Ministry of Education, Culture,
Sports, Science and Technology of Japan, by Health Labour
Sciences Research Grants for Research on Allergic Diseases
and Immunology from the Ministry of Health, Labour, and
Welfare of Japan and by the Takeda Science Foundation.
Conflict of interest statement: The authors declared no conflict of
interests.
References
1Buchmann, K. 2014. Evolution of innate immunity: clues from
invertebrates via fish to mammals. Front. Immunol. 5:459.
2Wölfle, U., Martin, S., Emde, M. and Schempp, C. 2009.
Dermatology in the Darwin anniversary. Part 2: evolution of the
skin-associated immune system. J. Dtsch. Dermatol. Ges. 7:862.
3 Kubo, A., Nagao, K. and Amagai, M. 2012. Epidermal barrier dysfunction and cutaneous sensitization in atopic diseases. J. Clin.
Invest. 122:440.
4Irvine, A. D., McLean, W. H. and Leung, D. Y. 2011. Filaggrin
mutations associated with skin and allergic diseases. N. Engl.
J. Med. 365:1315.
5Pasparakis, M., Haase, I. and Nestle, F. O. 2014. Mechanisms
regulating skin immunity and inflammation. Nat. Rev. Immunol.
14:289.
6 Richmond, J. M. and Harris, J. E. 2014. Immunology and skin in
health and disease. Cold Spring Harb. Perspect. Med. 4:a015339.
7Alibardi, L. 2003. Adaptation to the land: the skin of reptiles in
comparison to that of amphibians and endotherm amniotes. J.
Exp. Zool. B. Mol. Dev. Evol. 298:12.
8 Maderson, P. F. 2003. Mammalian skin evolution: a reevaluation.
Exp. Dermatol. 12:233.
9 Suzuki, K., Machiyama, F., Nishino, S. et al. 2009. Molecular features of thyroid hormone-regulated skin remodeling in Xenopus
laevis during metamorphosis. Dev. Growth Differ. 51:411.
10Yoshizato, K. 2007. Molecular mechanism and evolutional significance of epithelial–mesenchymal interactions in the body- and
tail-dependent metamorphic transformation of anuran larval skin.
Int. Rev. Cytol. 260:213.
11Callaway, E. 2014. Rival species recast significance of ‘first bird’.
Nature 516:18.
12Kielan-Jaworowska, Z., Cifelli, R. L. and Luo, Z. 2004. Mammals
from the Age of Dinosaurs. Columbia University Press, New York,
NY, USA.
13Kemp, T. S. 2005. The Origin and Evolution of Mammals. Oxford
University Press, Oxford, UK.
14 Vrontou, S., Wong, A. M., Rau, K. K., Koerber, H. R. and Anderson,
D. J. 2013. Genetic identification of C fibres that detect massagelike stroking of hairy skin in vivo. Nature 493:669.
15 Brubaker, S. W., Bonham, K. S., Zanoni, I. and Kagan, J. C. 2015.
Innate immune pattern recognition: a cell biological perspective.
Annu. Rev. Immunol. Jan 2. [Epub ahead of print].
16 Hirano, M., Das, S., Guo, P. and Cooper, M. D. 2011. The evolution
of adaptive immunity in vertebrates. Adv. Immunol. 109:125.
17Kishishita, N. and Nagawa, F. 2014. Evolution of adaptive immunity: implications of a third lymphocyte lineage in lampreys.
Bioessays 36:244.
18Zimmerman, L. M., Vogel, L. A. and Bowden, R. M. 2010.
Understanding the vertebrate immune system: insights from the
reptilian perspective. J. Exp. Biol. 213:661.
19Kaiser, P. 2010. Advances in avian immunology—prospects for
disease control: a review. Avian Pathol. 39:309.
20 Sugita, K., Kabashima, K., Atarashi, K., Shimauchi, T., Kobayashi,
M. and Tokura, Y. 2007. Innate immunity mediated by epidermal
keratinocytes promotes acquired immunity involving Langerhans
cells and T cells in the skin. Clin. Exp. Immunol. 147:176.
21Eckert, R. L. 1989. Structure, function, and differentiation of the
keratinocyte. Physiol. Rev. 69:1316.
22Watt, F. M. 1989. Terminal differentiation of epidermal keratinocytes. Curr. Opin. Cell Biol. 1:1107.
23Furuse, M., Hata, M., Furuse, K. et al. 2002. Claudin-based tight
junctions are crucial for the mammalian epidermal barrier: a lesson from claudin-1-deficient mice. J. Cell Biol. 156:1099.
24 Hashimoto, K. 1971. Intercellular spaces of the human epidermis
as demonstrated with lanthanum. J. Invest. Dermatol. 57:17.
25Merad, M., Ginhoux, F. and Collin, M. 2008. Origin, homeostasis
and function of Langerhans cells and other langerin-expressing
dendritic cells. Nat. Rev. Immunol. 8:935.
26Malissen, B., Tamoutounour, S. and Henri, S. 2014. The origins
and functions of dendritic cells and macrophages in the skin. Nat.
Rev. Immunol. 14:417.
27Kubo, A., Nagao, K. and Amagai, M. 2013. 3D visualization of
epidermal Langerhans cells. Methods Mol. Biol. 961:119.
28Kubo, A., Nagao, K., Yokouchi, M., Sasaki, H. and Amagai, M.
2009. External antigen uptake by Langerhans cells with reorganization of epidermal tight junction barriers. J. Exp. Med.
206:2937.
29Plewig, G. 1970. Regional differences of cell sizes in the human
stratum corneum. II. Effects of sex and age. J. Invest. Dermatol.
54:19.
30Tagami, H. 2008. Location-related differences in structure and
function of the stratum corneum with special emphasis on those
of the facial skin. Int. J. Cosmet. Sci. 30:413.
31Elias, P. M. 1983. Epidermal lipids, barrier function, and desquamation. J. Invest. Dermatol. 80:44s.
32Nemes, Z. and Steinert, P. M. 1999. Bricks and mortar of the epidermal barrier. Exp. Mol. Med. 31:5.
33Eckhart, L., Lippens, S., Tschachler, E. and Declercq, W. 2013.
Cell death by cornification. Biochim. Biophys. Acta 1833:3471.
34Kroemer, G., Galluzzi, L., Vandenabeele, P. et al. 2009.
Classification of cell death: recommendations of the Nomenclature
Committee on Cell Death 2009. Cell Death Differ. 16:3.
35 Wallace, L., Roberts-Thompson, L. and Reichelt, J. 2012. Deletion
of K1/K10 does not impair epidermal stratification but affects
desmosomal structure and nuclear integrity. J. Cell Sci. 125(Pt
7):1750.
36Jensen, J. M., Schütze, S., Neumann, C. and Proksch, E. 2000.
Impaired cutaneous permeability barrier function, skin hydration,
and sphingomyelinase activity in keratin 10 deficient mice. J.
Invest. Dermatol. 115:708.
37Reichelt, J., Doering, T., Schnetz, E., Fartasch, M., Sandhoff, K.
and Magin, A. M. 1999. Normal ultrastructure, but altered stratum
corneum lipid and protein composition in a mouse model for epidermolytic hyperkeratosis. J. Invest. Dermatol. 113:329.
38Dale, B. A., Resing, K. A. and Lonsdale-Eccles, J. D. 1985.
Filaggrin: a keratin filament associated protein. Ann. N. Y. Acad.
Sci. 455:330.
39Presland, R. B., Rothnagel, J. A. and Lawrence, O. T. 2006.
Profilaggrin and the fused S100 family of calcium-binding proteins. In Elias, P. M. and Feingold, K. R., eds., Skin Barrier, p. 111.
Taylor and Francis, New York, NY, USA.
40 Harding, C. R. and Scott, I. R. 1983. Histidine-rich proteins (filaggrins): structural and functional heterogeneity during epidermal
differentiation. J. Mol. Biol. 170:651.
41McGrath, J. A. and Uitto, J. 2008. The filaggrin story: novel
insights into skin-barrier function and disease. Trends Mol. Med.
14:20.
42 Harding, C. R., Long, S., Richardson, J. et al. 2003. The cornified
cell envelope: an important marker of stratum corneum maturation in healthy and dry skin. Int. J. Cosmet. Sci. 25:157.
43Henry, J., Toulza, E., Hsu, C. Y. et al. 2012. Update on the epidermal differentiation complex. Front. Biosci. (Landmark Ed)
17:1517.
Skin: where the immune system meets external antigens 279
44Kawasaki, H., Nagao, K., Kubo, A. et al. 2012. Altered stratum
corneum barrier and enhanced percutaneous immune responses
in filaggrin-null mice. J. Allergy Clin. Immunol. 129:1538.
45McAleer, M. A. and Irvine, A. D. 2013. The multifunctional role of
filaggrin in allergic skin disease. J. Allergy Clin. Immunol. 131:280.
46Palmer, C. N., Irvine, A. D., Terron-Kwiatkowski, A. et al. 2006.
Common loss-of-function variants of the epidermal barrier protein
filaggrin are a major predisposing factor for atopic dermatitis. Nat.
Genet. 38:441.
47Sandilands, A., Sutherland, C., Irvine, A. D. and McLean, W. H.
2009. Filaggrin in the frontline: role in skin barrier function and
disease. J. Cell Sci. 122(Pt 9):1285.
48 Matsui, T., Miyamoto, K., Kubo, A. et al. 2011. SASPase regulates
stratum corneum hydration through profilaggrin-to-filaggrin processing. EMBO Mol. Med. 3:320.
49Resing, K. A., Thulin, C., Whiting, K., al-Alawi, N. and Mostad, S.
1995. Characterization of profilaggrin endoproteinase 1. A regulated cytoplasmic endoproteinase of epidermis. J. Biol. Chem.
270:28193.
50Yamazaki, M., Ishidoh, K., Suga, Y. et al. 1997. Cytoplasmic processing of human profilaggrin by active mu-calpain. Biochem.
Biophys. Res. Commun. 235:652.
51List, K., Szabo, R., Wertz, P. W. et al. 2003. Loss of proteolytically
processed filaggrin caused by epidermal deletion of Matriptase/
MT-SP1. J. Cell Biol. 163:901.
52Leyvraz, C., Charles, R. P., Rubera, I. et al. 2005. The epidermal
barrier function is dependent on the serine protease CAP1/Prss8.
J. Cell Biol. 170:487.
53Sakabe, J., Yamamoto, M., Hirakawa, S. et al. 2013. Kallikreinrelated peptidase 5 functions in proteolytic processing of profilaggrin in cultured human keratinocytes. J. Biol. Chem. 288:17179.
54de Veer, S. J., Furio, L., Harris, J. M. and Hovnanian, A. 2014.
Proteases: common culprits in human skin disorders. Trends Mol.
Med. 20:166.
55 Dale, B. A., Holbrook, K. A. and Steinert, P. M. 1978. Assembly of
stratum corneum basic protein and keratin filaments in macrofibrils. Nature 276:729.
56Norlén, L. and Al-Amoudi, A. 2004. Stratum corneum keratin
structure, function, and formation: the cubic rod-packing and
membrane templating model. J. Invest. Dermatol. 123:715.
57 Jonca, N., Guerrin, M., Hadjiolova, K. et al. 2002. Corneodesmosin,
a component of epidermal corneocyte desmosomes, displays
homophilic adhesive properties. J. Biol. Chem. 277:5024.
58Leclerc, E. A., Huchenq, A., Mattiuzzo, N. R. et al. 2009.
Corneodesmosin gene ablation induces lethal skin-barrier disruption and hair-follicle degeneration related to desmosome dysfunction. J. Cell Sci. 122:2699.
59 Matsumoto, M., Zhou, Y., Matsuo, S. et al. 2008. Targeted deletion
of the murine corneodesmosin gene delineates its essential role in
skin and hair physiology. Proc. Natl Acad. Sci. U. S. A 105:6720.
60Samuelov, L., Sarig, O., Harmon, R. M. et al. 2013. Desmoglein
1 deficiency results in severe dermatitis, multiple allergies and
metabolic wasting. Nat. Genet. 45:1244.
61 Oji, V., Eckl, K. M., Aufenvenne, K. et al. 2010. Loss of corneodesmosin leads to severe skin barrier defect, pruritus, and atopy:
unraveling the peeling skin disease. Am. J. Hum. Genet. 87:274.
62Candi, E., Tarcsa, E., Digiovanna, J. J. et al. 1998. A highly conserved lysine residue on the head domain of type II keratins is
essential for the attachment of keratin intermediate filaments to
the cornified cell envelope through isopeptide crosslinking by
transglutaminases. Proc. Natl Acad. Sci. U. S. A. 95:2067.
63Candi, E., Schmidt, R. and Melino, G. 2005. The cornified envelope: a model of cell death in the skin. Nat. Rev. Mol. Cell Biol.
6:328.
64Steinert, P. M. and Marekov, L. N. 1995. The proteins elafin, filaggrin, keratin intermediate filaments, loricrin, and small prolinerich proteins 1 and 2 are isodipeptide cross-linked components
of the human epidermal cornified cell envelope. J. Biol. Chem.
270:17702.
65Mischke, D., Korge, B. P., Marenholz, I., Volz, A. and Ziegler, A.
1996. Genes encoding structural proteins of epidermal cornification and S100 calcium-binding proteins form a gene complex
(“epidermal differentiation complex”) on human chromosome
1q21. J. Invest. Dermatol. 106:989.
66 Kalinin, A. E., Kajava, A. V. and Steinert, P. M. 2002. Epithelial barrier function: assembly and structural features of the cornified cell
envelope. Bioessays 24:789.
67 Kashyap, D. R., Wang, M., Liu, L. H., Boons, G. J., Gupta, D. and
Dziarski, R. 2011. Peptidoglycan recognition proteins kill bacteria
by activating protein-sensing two-component systems. Nat. Med.
17:676.
68 Kashyap, D. R., Rompca, A., Gaballa, A. et al. 2014. Peptidoglycan
recognition proteins kill bacteria by inducing oxidative, thiol, and
metal stress. PLoS Pathog. 10:e1004280.
69Mlitz, V., Strasser, B., Jaeger, K. et al. 2014. Trichohyalin-like proteins have evolutionarily conserved roles in the morphogenesis of
skin appendages. J. Invest. Dermatol. 134:2685.
70 Strasser, B., Mlitz, V., Hermann, M. et al. 2014. Evolutionary origin
and diversification of epidermal barrier proteins in amniotes. Mol.
Biol. Evol. 31:3194.
71Djian, P., Easley, K. and Green, H. 2000. Targeted ablation of the
murine involucrin gene. J. Cell Biol. 151:381.
72 Sevilla, L. M., Nachat, R., Groot, K. R. et al. 2007. Mice deficient in
involucrin, envoplakin, and periplakin have a defective epidermal
barrier. J. Cell Biol. 179:1599.
73 Koch, P. J., de Viragh, P. A., Scharer, E. et al. 2000. Lessons from
loricrin-deficient mice: compensatory mechanisms maintaining
skin barrier function in the absence of a major cornified envelope
protein. J. Cell Biol. 151:389.
74Jarnik, M., de Viragh, P. A., Schärer, E. et al. 2002. Quasi-normal
cornified cell envelopes in loricrin knockout mice imply the existence of a loricrin backup system. J. Invest. Dermatol. 118:102.
75Huebner, A. J., Dai, D., Morasso, M. et al. 2012. Amniotic fluid
activates the nrf2/keap1 pathway to repair an epidermal barrier
defect in utero. Dev. Cell 23:1238.
76Breiden, B. and Sandhoff, K. 2014. The role of sphingolipid
metabolism in cutaneous permeability barrier formation. Biochim.
Biophys. Acta 1841:441.
77 Swartzendruber, D. C., Wertz, P. W., Madison, K. C. and Downing,
D. T. 1987. Evidence that the corneocyte has a chemically bound
lipid envelope. J. Invest. Dermatol. 88:709.
78 Epp, N., Fürstenberger, G., Müller, K. et al. 2007. 12R-lipoxygenase
deficiency disrupts epidermal barrier function. J. Cell Biol.
177:173.
79Moran, J. L., Qiu, H., Turbe-Doan, A. et al. 2007. A mouse mutation in the 12R-lipoxygenase, Alox12b, disrupts formation of the
epidermal permeability barrier. J. Invest. Dermatol. 127:1893.
80 Jobard, F., Lefèvre, C., Karaduman, A. et al. 2002. Lipoxygenase-3
(ALOXE3) and 12(R)-lipoxygenase (ALOX12B) are mutated in
non-bullous congenital ichthyosiform erythroderma (NCIE) linked
to chromosome 17p13.1. Hum. Mol. Genet. 11:107.
81Holleran, W. M., Ginns, E. I., Menon, G. K. et al. 1994.
Consequences of beta-glucocerebrosidase deficiency in epidermis. Ultrastructure and permeability barrier alterations in Gaucher
disease. J. Clin. Invest. 93:1756.
82Doering, T., Proia, R. L. and Sandhoff, K. 1999. Accumulation of
protein-bound epidermal glucosylceramides in beta-glucocerebrosidase deficient type 2 Gaucher mice. FEBS Lett. 447:167.
83Brady, R. O., Kanfer, J. N. and Shapiro, D. 1965. Metabolism of
glucocerebrosides. II. Evidence of an enzymatic deficiency in
Gaucher’s disease. Biochem. Biophys. Res. Commun. 18:221.
84Patrick, D. A. 1965. A deficiency of glucocerebrosidase in
Gaucher’s disease. Biochem J. 97:17C.
85Akiyama, M., Sugiyama-Nakagiri, Y., Sakai, K. et al. 2005.
Mutations in lipid transporter ABCA12 in harlequin ichthyosis and
functional recovery by corrective gene transfer. J. Clin. Invest.
115:1777.
86Mitsutake, S., Suzuki, C., Akiyama, M. et al. 2010. ABCA12 dysfunction causes a disorder in glucosylceramide accumulation
during keratinocyte differentiation. J. Dermatol. Sci. 60:128.
87Ponec, M., Weerheim, A., Kempenaar, J., Mommaas, A. M. and
Nugteren, D. H. 1988. Lipid composition of cultured human
keratinocytes in relation to their differentiation. J. Lipid Res.
29:949.
280 Skin: where the immune system meets external antigens
88 Iwai, I., Han, H., den Hollander, L. et al. 2012. The human skin
barrier is organized as stacked bilayers of fully extended ceramides with cholesterol molecules associated with the ceramide
sphingoid moiety. J. Invest. Dermatol. 132:2215.
89 Sasaki, T., Shiohama, A., Kubo, A. et al. 2013. A homozygous
nonsense mutation in the gene for Tmem79, a component for
the lamellar granule secretory system, produces spontaneous
eczema in an experimental model of atopic dermatitis. J. Allergy
Clin. Immunol. 132:1111.
90 Saunders, S. P., Goh, C. S., Brown, S. J. et al. 2013. Tmem79/
Matt is the matted mouse gene and is a predisposing gene for
atopic dermatitis in human subjects. J. Allergy Clin. Immunol.
132:1121.
91 Méchin, M. C., Enji, M., Nachat, R. et al. 2005. The peptidylarginine deiminases expressed in human epidermis differ in their
substrate specificities and subcellular locations. Cell. Mol. Life
Sci. 62:1984.
92 Nachat, R., Méchin, M. C., Takahara, H. et al. 2005.
Peptidylarginine deiminase isoforms 1-3 are expressed in the
epidermis and involved in the deimination of K1 and filaggrin. J.
Invest. Dermatol. 124:384.
93 Tarcsa, E., Marekov, L. N., Mei, G., Melino, G., Lee, S. C. and
Steinert, P. M. 1996. Protein unfolding by peptidylarginine
deiminase. Substrate specificity and structural relationships of
the natural substrates trichohyalin and filaggrin. J. Biol. Chem.
271:30709.
94 Ishida-Yamamoto, A., Senshu, T., Eady, R. A. et al. 2002.
Sequential reorganization of cornified cell keratin filaments
involving filaggrin-mediated compaction and keratin 1 deimination. J. Invest. Dermatol. 118:282.
95 Denecker, G., Hoste, E., Gilbert, B. et al. 2007. Caspase-14 protects against epidermal UVB photodamage and water loss. Nat.
Cell Biol. 9:666.
96 Kamata, Y., Taniguchi, A., Yamamoto, M. et al. 2009. Neutral
cysteine protease bleomycin hydrolase is essential for the
breakdown of deiminated filaggrin into amino acids. J. Biol.
Chem. 284:12829.
97 Manabe, M., Sanchez, M., Sun, T. T. and Dale, B. A. 1991.
Interaction of filaggrin with keratin filaments during advanced
stages of normal human epidermal differentiation and in ichthyosis vulgaris. Differentiation 48:43.
98 Richter, T., Peuckert, C., Sattler, M. et al. 2004. Dead but highly
dynamic—the stratum corneum is divided into three hydration
zones. Skin Pharmacol. Physiol. 17:246.
99 Bouwstra, J. A., de Graaff, A., Gooris, G. S., Nijsse, J., Wiechers,
J. W. and van Aelst, A. C. 2003. Water distribution and related
morphology in human stratum corneum at different hydration
levels. J. Invest. Dermatol. 120:750.
100Kubo, A., Ishizaki, I., Kawasaki, H., Nagao, K., Ohashi, Y. and
Amagai, M. 2013. The stratum corneum comprises three layers
with distinct metal-ion barrier properties. Sci. Rep. 3:1731.
101 Rawlings, A. V. and Harding, C. R. 2004. Moisturization and skin
barrier function. Dermatol. Ther. 17(Suppl 1):43.
102 Lundström, A. and Egelrud, T. 1991. Stratum corneum chymotryptic enzyme: a proteinase which may be generally present in
the stratum corneum and with a possible involvement in desquamation. Acta Derm. Venereol. 71:471.
103 Kato, A., Fukai, K., Oiso, N., Hosomi, N., Murakami, T. and Ishii,
M. 2003. Association of SPINK5 gene polymorphisms with atopic
dermatitis in the Japanese population. Br. J. Dermatol. 148:665.
104Nishio, Y., Noguchi, E., Shibasaki, M. et al. 2003. Association
between polymorphisms in the SPINK5 gene and atopic dermatitis in the Japanese. Genes Immun. 4:515.
105Walley, A. J., Chavanas, S., Moffatt, M. F. et al. 2001. Gene
polymorphism in Netherton and common atopic disease. Nat.
Genet. 29:175.
106 Chavanas, S., Bodemer, C., Rochat, A. et al. 2000. Mutations in
SPINK5, encoding a serine protease inhibitor, cause Netherton
syndrome. Nat. Genet. 25:141.
107 Frenk, E. and Mevorah, B. 1972. Ichthyosis linearis circumflexa
Comèl with Trichorrhexis invaginata (Netherton’s Syndrom):
an ultrastructural study of the skin changes. Arch. Dermatol.
Forsch. 245:42.
108 Fartasch, M., Williams, M. L. and Elias, P. M. 1999. Altered lamellar body secretion and stratum corneum membrane structure in
Netherton syndrome: differentiation from other infantile erythrodermas and pathogenic implications. Arch. Dermatol. 135:823.
109 Müller, F. B., Hausser, I., Berg, D. et al. 2002. Genetic analysis of
a severe case of Netherton syndrome and application for prenatal testing. Br. J. Dermatol. 146:495.
110Descargues, P., Deraison, C., Bonnart, C. et al. 2005. Spink5deficient mice mimic Netherton syndrome through degradation of desmoglein 1 by epidermal protease hyperactivity. Nat.
Genet. 37:56.
111
Descargues, P., Deraison, C., Prost, C. et al. 2006.
Corneodesmosomal cadherins are preferential targets of stratum corneum trypsin- and chymotrypsin-like hyperactivity in
Netherton syndrome. J. Invest. Dermatol. 126:1622.
112Yang, T., Liang, D., Koch, P. J., Hohl, D., Kheradmand, F. and
Overbeek, P. A. 2004. Epidermal detachment, desmosomal dissociation, and destabilization of corneodesmosin in Spink5−/−
mice. Genes Dev. 18:2354.