Keratin and Keratinocyte Differentiation
. / . 11 .. 2557 EPIDERMIS Stratified, cornified epidermis
Continually renewing structure that gives rise to appendages
(pilosebaceous units, nails, and sweat glands) Thickness 0.4 to 1.5
mm (1.5- to 4.0-mm full-thickness skin) Other immigrant resident
cellsmelanocytes, Langerhans cells, and Merkel cells EPIDERMIS
Keratinocyte is an ectodermally derived cell and primary cell type
in the epidermis (80%) Keratinocyte differentiation =
keratinization From basal cells to the terminally keratinized
stratum corneum (corneocyte) Corneocyte contains keratin filaments
matrix protein protein-reinforced plasma membrane
withsurface-associated lipids ultimate fate of these cells is to
contribute the components for the epidermal barrier as the stratum
corneum. Thus, much of the function of the epidermis can be gleaned
from the study of the structure and development of the
keratinocyte. BASAL LAYER Epidermal stem cells
- multipotent epidermal stem cells within the bulge region of the
hair follicle - keratinocytes are organized into vertical columns
of progressively differentiating cells epidermal proliferating
units BASAL LAYER 2. Transit amplifying cells - subset of daughter
cells produced by the infrequent division of stem cells - provide
the bulk of the cell divisions needed for stable self-renewal - are
the most common cells in the basal compartment BASAL LAYER 3.
Postmitotic cells - undergo terminal differentiation - In humans,
the normal transit time basal cell- SC= at least 14 days SC
subsequent desquamation 14 days - these periods of time can be
altered in hyperproliferative or growth-arrested states SPINOUS
LAYER Midepidermis
Spine-like appearance of the cell margins which are abundant
desmosomes (calcium-dependent cell surface modifications) Flatter
and develop lamellar granules
SPINOUS LAYER Polyhedral in shape with a rounded nucleus Flatter
and develop lamellar granules Also contain large bundles of keratin
filaments, organized around the nucleus and inserted into
desmosomes peripherally Differentiate + move upward Lamellar
granules (LG), also known as keratinosomes, lamellar bodies,
membrane-coating granules, and Odland bodies EM: round or oblong,
membrane-delimitated, lamellate organelles
Lamellar granules EM: round or oblong, membrane-delimitated,
lamellate organelles Lamellar granules LG are produced as discrete
granules in the stratum spinosum, probably from the Golgi
apparatus, and then migrate to the cell surface, fuse with the
plasma membrane, extruding their contents in the outer stratum
granulosum Precursors of stratum corneum lipids Genetic diseases
demonstrate the importance of steroid and lipid metabolism for
sloughing of cornified cellsin recessive X-linked ichthyosis, for
example, mutation of steroid sulfatase results in a retention
hyperkeratosis Lamellar granules glucosylceramides (GlcCer) and
other lipids
Components include glucosylceramides (GlcCer) and other lipids
(glycolipids, phospholipids, free sterols) various hydrolytic
enzymes, such as proteases, acid phosphatases, glucosidases, and
lipases other proteins including corneodesmosin (Cdsn),
glycoproteins glucosylceramides; - precursors to ceramides -
dominant component of the stratum corneum lipids SPINOUS LAYER
retain the stable K5/K14 keratins + K1/K10 keratin
Differentiation or keratinization-specific keratins Spinous cells
retain the stable K5/K14 keratins that are produced in the basal
layer and only synthesize new messenger RNA (mRNA) for these
proteins in hyperproliferative disorders. retain the stable K5/K14
keratins + K1/K10 keratin pair occurs in this epidermal layer.
These keratins are characteristic of an epidermal pattern of
differentiation. However, in hyperproliferative conditions such as
psoriasis, actinic keratoses, and wound healing, synthesis of K1
and K10 mRNA and protein is downregulated, and the synthesis and
translation of messages for K6 and K16 are favored. Adherens
junctions Actin microfilaments at cellcell interfaces, via a
distinct set of cadherins (e.g., E-cadherin) and intracellular
catenin adapter molecules GRANULAR LAYER Lamellar granules
Basophilic keratohyalin granules within cells Site of generation of
structural components (epidermal barrier) proteins that process
these components Keratohyalin granulesare composed of profilaggrin,
keratin filaments, and loricrin It is in this layer that the
cornified cell envelope begins to form, with the conversion of
profilaggrin to filaggrin GRANULAR LAYER Keratin aggregation form
macrofilaments
Filaggrin is degraded. Urocanic acid and pyrrolidone carboxylic
acid Contribute to hydration of the stratum corneum and help filter
UV radiation GRANULAR LAYER Loricrin is a cysteine-rich protein
that forms the major protein component of the cornified envelope KH
granules loricrin binds to desmosomal structures and is
subsequently cross-linked to the plasma membrane by tissue
transglutaminases (TGMs, primarily TGMs 3 and 1) to form the
cornified cell envelope. GRANULAR LAYER The final stage of granular
cell differentiation into a corneocyte involves the cells own
programed destruction almost all cellular contents are destroyed
except keratin filaments and filaggrin matrix STRATUM CORNEUM
Anucleate, flattened cornified cells
A two-compartment system of lipid-depleted, protein-enriched
corneocytes surrounded by a continuous extracellular lipid matrix
Barrier activity Provides mechanical protection to the skin and a
barrier to water loss and permeation of soluble substances from the
environment Extracellular lipid matrix
STRATUM CORNEUM Extracellular lipid matrix Corneocytes Regulation
of permeability Mechanical reinforcement Desquamation Hydration AMP
activity Protection from UV damage Toxin exclusion Selective
chemical absorption Cytokine-mediated initiation of inflammation
STRATUM CORNEUM KC terminal differentiation culminates in the
replacement of the plasma membrane with the cornified cell envelope
(CE), a composite of several covalently cross-linked proteins
Examples of CE components include involucrin, small proline-rich
proteins (SPR), XP-5/late envelope proteins (LEP), loricrin,
cystatin, envoplakin, periplakin, elafin, repetin, filaggrin, S100
proteins, keratins and desmosomal proteins Note that mutations in
some of the genes that encode these proteins can lead to skin
disorders. For example, mutations in the loricrin and filaggrin
genes give rise to palmoplantar keratoderma (PPK) and ichthyosis
vulgaris (see below), respectively STRATUM CORNEUM KC terminal
differentiation culminates in the replacement of the plasma
membrane with the cornified cell envelope (CE), a composite of
several covalently cross-linked proteins Examples of CE components
include Proteases processing of CE proteins and the proteolysis of
corneodesmosomes that is required for desquamation A mature,
terminally differentiated cornified cell thus consists of keratin
filaments covalently attached to the CE, which is composed of
protein and lipid envelope components and imbedded in extracellular
lipid lamellae. Defects in transglutaminases, lipid metabolism, CE
structural proteins and proteases lead to a variety of diseases
characterized by ichthyosis and/or keratoderma (13). CHILD,
congenital hemidysplasia with ichthyosiform erythroderma and limb
defects; LI, lamellar ichthyosis; CIE, congenital ichthyosiform
erythroderma Cornified cell envelope (CE) Cornified lipid envelope
(CLE)
The extracellular surface of the CE is covered by lipids, which
form the cornified lipid envelope (CLE) STRATUM CORNEUM CE and the
CLE are required for a cutaneous water barrier If fail increased
transcutaneous water loss + increased susceptibility to infection
CE and the CLE are required for a cutaneous water barrier If fail
increased transcutaneous water loss + increased susceptibility to
infections, a major problem in premature infants, and disorders
such as Netherton syndrome. Cornified cell envelope (CE)
Begins within the upper spinous and granular cell layers Proteins
are chemically cross-linked, primarily by -(-glutamyl) lysine
isopeptide bonds This reaction is catalyzed by enzymes
transglutaminases (TGases). Loss-of-function mutations in the gene
that encodes TGase 1 lead to lamellar ichthyosis and congenital
ichthyosiform erythroderma, generalized skin disorders resulting
from a failure to form proper CEs. Cornified cell envelope
(CE)
TGs are calcium-dependent enzymes TGs also have a role in the
creation of ester bonds between proteins and -hydroxyceramides TG1
(keratinocyte TG; membrane-bound), TG2 (tissue TG; basal layer),
TG3 (epidermal TG; hair follicle and terminally differentiating
KCs) and TG5 (upper epidermis) Such cross-linking is essential for
assembly of the CE. TGs are calcium-dependent enzymes that catalyze
the formation of -glutamyl lysine isopeptide bonds between
proteins. Keratinocytes have keratin.
So, what is keratin ?? Keratins (Cytokeratins)
Structural proteins that belong to the superfamily of intermediate
filament (IF) proteins Heterogeneous in size (4070 kDa), charge (pI
4.78.4), and notoriously insoluble 54 functional keratin genes34
epithelial keratins and 17 hair keratins Keratin They serve a
predominantly structural role in the cells Keratins are a family of
intermediate filaments
Fifty-four different functional keratin genes34 epithelial keratins
and 17 hair keratins The coexpression of specific keratin pairs is
dependent on cell type, tissue type, developmental stage,
differentiation stage, and disease condition (Table 7-2)
Furthermore, the critical role of these molecules is underscored by
the numerous manifestations of disease that arise because of
mutations in these genes (see Table 7-2). Thus, knowledge of
keratin expression, regulation, and structure provides insight into
epidermal differentiation and structure. Keratin They serve a
predominantly structural role in the cells Keratin Characterized by
a chain of amino acids (1 structure of the keratin protein)
Classified as type I (K9K28, K31K40); acidic type II (K1K8,
K71K86); basic Keratin Most type I and II keratin genes are
regulated in a pairwise, tissue type-related, and
differentiation-related fashion. Associated with desmosomes,
hemidesmosomes and protein complexes within the cornified cell
envelope Keratin 54 human keratin genesthree categories:
(1) epithelial keratin genes (2) hair keratin genes (3) keratin
pseudogenes Keratin Soft keratin Hard keratin
Epidermis( palms, soles), ORS, some parts of IRS Keratins in the
Stratum corneum are cross-linked by intermolecular disulfide bonds
Hard keratin Hair cortex/cuticle, IRS, nail plate Intensive
concentration of sulfur through the amino acids cysteine and
methionine Keratin Composed of 3 domains Head Central rod domain
Tail
-helical: four segments (1A, 1B, 2A, 2B) non-helical segments:
linkers Tail In head and tail domains Epithelial keratins: rich of
glycine, serine Hair keratins: rich of cysteine, proline ). The rod
domain is composed of seven-residue amino acid sequence repeats
(a-b-c-d-e-f-g)n termed heptad repeats, where positions a and d
represent hydrophobic residues that are considered crucial for
stabilization of the heterodimer. In the middle of the 2B domain,
the heptad pattern is interrupted, giving rise to the stutter. This
helical segment is highly conserved among intermediate filaments
and does not participate in the formation of the coiled-coil dimer
that forms the basic building block of intermediate filaments The
rod domain is composed of seven-residue amino acid sequence repeats
(a-b-c-d-e-f-g)n termed heptad repeats, where positions a and d
represent hydrophobic residues that are considered crucial for
stabilization of the heterodimer. In the middle of the 2B domain,
the heptad pattern is interrupted, giving rise to the stutter. This
helical segment is highly conserved among intermediate filaments
and does not participate in the formation of the coiled-coil dimer
that forms the basic building block of intermediate filaments (
Keratin intermediate filaments
Intermediate filament proteins Keratins= the largest group provide
resilience to keratinocytes, the most abundant cell type in the
epidermis. Types of intermediate filaments. GFAP, glial fibrillary
acidic protein; L, M and H, low-, medium- and high-molecular
weight. Keratin intermediate filaments
Begins with the heterodimerization of one type I and one type II
keratin protein Can bind signaling proteins controlling the
cytoplasmic + nuclear molecules influence cell cycle progression,
metabolic activity and apoptosis provide resilience to
keratinocytes, the most abundant cell type in the epidermis. Fig
Alignment and assembly of keratin molecules and keratin filament
packing. Intermediate filament assembly takes place in several
stages and begins with the heterodimerization of one type I and one
type II keratin protein in a coiled-coil fashion. Two heterodimers
then associate to form a tetramer. Lateral aggregation of tetramers
yields higher-order polymers which eventually make up the filament
network of the keratinocyte Hair Keratins The medulla: mixture of
epithelial (K17, K75) and hair keratins (K33, K34, K36, K37, K81)
The cortex: type I hair keratins (K31K38) and type II hair keratins
(K81, K83, K85 and K86) In the cuticle: Hair keratins K32, K35,
K82, K85 The three IRS layers: K71, K74, K73. The full thickness of
the ORS: epithelial keratins K5, K14 The isthmus and the lower ORS:
K6, K16 and K17 Additional keratins expressed in the ORS: K15, K19.
Companion layer is located between the IRS and the ORS.
Expression of K6, K16 and K17 is limited to the isthmus and the
lower ORS. Additional keratins expressed in the ORS are K15 and
K19. The epithelial keratins K5 and K14 are found throughout the
full thickness of the ORS, Fig Complex pattern of hair keratin
expression in the human anagen hair follicle. Major type I hair
keratins are in blue, and major type II hair keratins are in green.
Minor hair keratins are in pink. 1This protein is weakly expressed
at this site. 2To date, expression of this protein has only been
detected in single cortex cells. 3To date, this protein has only
been detected in vellus hairs. Autosomal dominant monilethrix is
caused by mutations in K81, K83, and K86. EPIDERMAL
DIFFERENTIATION
keratins that are expressed are highly specific for the state of
differentiation (Fig. 56.7). The mitotically active keratinocytes
in the basal compartment of the epidermis express the keratin pair
K5 and K14. In addition, but less abundantly, K15 is expressed. In
the absence of K14, K15 can assemble with K5, thereby providing
mechanical stability to the keratinocyte. As keratinocytes move
suprabasally to the spinous layer, they withdraw from the cell
cycle. This process is associated with a down-regulation of K5 and
K14 and an induction of the differentiation-specific keratins, K1
and K10. Further maturation of spinous keratinocytes into granular
keratinocytes results in expression of K2, a reinforcement keratin.
With further maturation, filaments containing the suprabasal
keratins are bundled parallel to the surface and, eventually,
keratinocytes lose their cytoplasmic organelles and differentiate
into lifeless corneocytes that are shed into the environment.
Granular KC expresses K2, a reinforcement keratin.
The mitotically active keratinocytes in the basal compartment of
the epidermis express the keratin pair K5 and K14. In addition, but
less abundantly, K15 is expressed. In the absence of K14, K15 can
assemble with K5, thereby providing mechanical stability to the
keratinocyte. e.g., liver, gut, pancreas K9 is specifically
expressed in the suprabasal cells of palmoplantar skin.
The mitotically active keratinocytes in the basal compartment of
the epidermis express the keratin pair K5 and K14. In addition, but
less abundantly, K15 is expressed. In the absence of K14, K15 can
assemble with K5, thereby providing mechanical stability to the
keratinocyte. KC in nail bed, hair follicle, sebaceous and sweat
glands
K6, K16 and K17: - Palmoplantar KC in nail bed, hair follicle,
sebaceous and sweat glands This group of keratins is rapidly
induced by injury, ultraviolet radiation, wounding,
hyperproliferative conditions The mitotically active keratinocytes
in the basal compartment of the epidermis express the keratin pair
K5 and K14. In addition, but less abundantly, K15 is expressed. In
the absence of K14, K15 can assemble with K5, thereby providing
mechanical stability to the keratinocyte. K6a, K6b, K16, K17
recruited KC for restoration of epi barrier following injury.
FUNCTION OF KERATIN IN THE EPIDERMIS AND OTHER SKIN EPITHELIA
Enhance the cells ability to withstand trauma (by IF networks)
Attachment of IFs to adhesion complexes (desmosomes,
hemidesmosomes), and to F-actin and microtubules Loss of this
function leads to fragile cells and unable to sustain mechanical
stress. FUNCTION OF KERATIN IN THE EPIDERMIS AND OTHER SKIN
EPITHELIA
Nonmechanical functions: In hair follicles, K17 promotes the anagen
(growth) phase by attenuating TNF--induced apoptosis in matrix
keratinocytes In the epidermis the suprabasally expressed K10
regulate proliferation in the basal layer of epidermis and in
sebaceous glands while K17 cell autonomously regulates protein
synthesis and cell size in wound-proximal keratinocytes FUNCTION OF
KERATIN IN THE EPIDERMIS AND OTHER SKIN EPITHELIA
Keratins influence the melanin pigment distribution and, thus, skin
pigmentation Ex. - Dowling-Degos dz K5 mutation aberrations in skin
pigmentation - NaegeliFranceschettiJadassohn syndrome and
dermatopathia pigmentosa reticularis - EBS with mottled
pigmentation Regulatory Pathways Involved in Epidermal Development
and Differentiation
The regulatory pathways necessary for normal keratinocyte
differentiation: (1) establish and maintain basal keratinocytes (2)
initiate and execute terminal differentiation (3) form the stratum
corneum Genes required for establishing/maintaining basal KC
Regulatory Pathways Involved in Epidermal Development and
Differentiation Genes required for establishing/maintaining basalKC
The p63 gene encodes six different proteins, each of which can
function as a transcriptional activator or repressor. The p63 is
required for both the initial induction of K5/K14 expression in
embryonic basal keratinocytes and the maintenance of K5/K14
expression in the basal layer of mature epidermis. Also maintain
the proliferative state of basal KC by repress the expression of
cell cycle inhibitors prevent the onset of terminal differentiation
transcription factor p63. In mice, completely p63-deficient fails
to initiate epidermal morphogenesis a single-layered epithelium
covering their bodies rather than a stratified epidermis. Rapid
death due to dehydration.Consistent with this hypothesis, ectopic
p63 expression was shown to induce expression of the epidermal
keratins K5 and K14. Becomimg spinous KC is controlled by
Regulatory Pathways Involved in Epidermal Development and
Differentiation Genes required for terminal differentiation in
mature epidermis Becomimg spinous KC is controlled by an isoform of
p63, Np63 the Notch signaling pathway Np63 synergizes with Notch
signaling to induce K1 expression cell cycle withdrawal terminal
differentiation In addition, Np63 mediates cell cycle exit by
inducing cell cycle inhibitors and by repressing genes required for
cell cycle progression. The importance of p63 for normal epidermal
development and differentiation is further underscored by the
finding that p63 mutations underlie a subset of ectodermal
dysplasias, which are characterized by abnormalities in the skin
and skin appendages (see Ch. 63). Ablation of Notch an extremely
thin spinous layer Active Notch signaling resulted in an expansion
of the spinous layer Ca2+ in epidermal differentiation
Regulatory Pathways Involved in Epidermal Development and
Differentiation Ca2+ in epidermal differentiation An increase in
extracellular Ca2+ trigger of KC differentiation formation of the
granular cell layer Several Ca2+-responsive proteins in the
epidermis that are involved in the formation of the granularlayer
The protein kinase C (PKC) down-regulate K1 and K10 expression as
well as to the induct markers of granular KCs, including loricrin,
filaggrin and transglutaminases The calcium-sensing receptor
(undergo conformational changes upon binding to Ca2+) is expressed
in granular KC. The protein kinase C (PKC) family of proteins is
activated by Ca2+ signaling and functions specifically in the
transition from spinous to granular cells. Interestingly, mice
lacking the full-length form of the calcium-sensing receptor fail
to properly form a granular layer, while overexpression of the
calcium-sensing receptor in basal keratinocytes causes expanded
spinous and granular cell layers17. Genes required for terminal
differentiation in embryonic epidermis
Regulatory Pathways Involved in Epidermal Development and
Differentiation Genes required for terminal differentiation in
embryonicepidermis The molecular mechanisms for development of a
spinous layer during epidermal morphogenesis appear to be different
Basal KCs initially differentiate into intermediate keratinocytes,
which express K1undergo proliferation differentiate into spinous
and granular cells then, terminal differentiation The intermediate
cell layer exists only transiently during epidermal morphogenesis,
and intermediate cells ultimately Genes required for terminal
differentiation in embryonic epidermis
Regulatory Pathways Involved in Epidermal Development and
Differentiation Genes required for terminal differentiation in
embryonicepidermis intermediate cells fail to mature into spinous
and granular cells. Such a block in differentiation occurs in mice
lacking expression of inhibitor of B kinase- (IKK), interferon
regulatory factor 6 (IRF6) or ovo-like 1 (Ovol1), as well as in
mice expressing a mutant form of 143318. The latter mutant protein
was identified in repeated epilation (Er) mutant mice. In all
instances, an expanded intermediate cell layer develops, further
terminal differentiation is disrupted, and the consequent failure
to establish barrier function results in neonatal lethality. The
intermediate cell layer exists only transiently during epidermal
morphogenesis, and intermediate cells ultimately Aggregation of
disorganized keratin bundles
Keratin Disorders Usually with an autosomal dominant; typically
dominant-negative fashion interfering with normal intermediate
filament assembly Aggregation of disorganized keratin bundles Cell
fragility mutations in the helix initiation and termination motifs
are generally associated with relatively severe disease phenotypes,
whereas mutations affecting other keratin domains usually cause
milder disease Keratin Disorders White sponge nevus of Cannon
white plaques involving the oral mucosa +/-other mucosal surfaces
(esophagus, vagina, rectum and nasal cavity) wax and wane over time
suprabasal cytolysis and keratin clumping mutations in K4 and K13,
which are specifically expressed in mucosal keratinocytes mutations
in the helix initiation and termination motifs are generally
associated with relatively severe disease phenotypes, whereas
mutations affecting other keratin domains usually cause milder
disease Keratin Disorders Gastrointestinal disorders
K8 and K18 are the major keratins that are expressed in
gastrointestinal epithelia, including the liver, pancreas and gut..
K8 and K18 mutations risk factors for developing liver and GI
disorders (e.g. cirrhosis, inflammatory bowel disease), with
additional genetic and environmental alterations likely required
for disease development. The mutation-associated predisposition to
tissue injury is likely related to mechanical and non-mechanical
keratin functions, including maintenance of cell integrity and
protection from oxidative injury and apoptosis. Mutations in these
simple keratins are typically located within the head and tail
domains and do not involve the highly conserved helix boundary
regions X-linked dominant CharcotMarieTooth disease
loss of connexin 32 (Cx32) impaired diffusion of nutrients and
signaling molecules into peripheral nerves. recessive mutations
inCx26 the single most important cause of non-syndromic congenital
hearing impairment (with carrier frequencies ranging from 3% to 10%
in the general population) autosomal dominant in Cx26 range from
Vohwinkel syndrome (mutilating PPK with deafness) to
keratitisichthyosisdeafness (KID) syndrome (which also features a
stippled PPK
Top Related