Transcript Biochemistry of skin - Univerzita Karlova v Praze

Biochemistry of skin
Jana Novotná
Skin
• it provides barrier against a range of noxious stress (UV irradiation,
mechanical, chemical and biological insults);
• acts as the periphery´s „sensing“ system;
• system which maintaining body homeostasis.
• 2 m2 in area
• ~ 2.5 mm thick on average
• constitutes 6% of our total body weight (5 – 6 kg)
• barrier to prevent a desiccation and temperature balance
• protection to the UV radiation – absorbing pigmentation system
• complex immuno-regulatory network protection
• normal skin pH is somewhat acidic - the range of 4.2. to 5.6.
Human skin layers
• Mammalian skin is composed of two primary layers:
– the epidermis, which provides waterproofing and serves as a
barrier to infection;
– the dermis is responsible for the tensile strength of skin. Its main
functions are to regulate temperature and to supply the
epidermis with nutrient-saturated blood. Much of the body's
water supply is stored within the dermis.
Epidermis
• An external stratified, non-vasularized
epithelium (75 – 150 mm thick),
continually keratinizing
– Stratum corneum – 15 – 30 sheet
of nonviable, but biochemically
active corneocytes (cornified, with
keratin, lack cytoplasmic
organells). Barrier against physical
and chemical agents, reduces
water loss
– Stratum granulosum – 3 – 5 sheet
of non-dividing keratinocytes,
producing keratino-hyalin
– Stratum spinosum – 8 – 10 sheet
of keratinocytes with limited
dividing capacity, Langerhan´s
cells
– Stratum basale – maturing/aging
keratinocytes, melanocytes, Merkel
cells (receptor cells)
Keratins
• keratinocytes contain filaments of the keratin
intermediate filament (KIF) family (cytoskeleton)
• hair, nails, horny layers of the skin – are formed
from keratin cytoskeleton of dead cells.
• two primary groups of keratins, the a-keratins
and the b-keratins
 a-keratins occur in mammals, b-keratins in
birds, reptiles,
• both form are right handed helical structure
• 2 types
– type I – acidic keratins
– type II – basic keratins
– heterodimer – type I forming a coild coil with type II
Composition and structure of keratin
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Human skin contains ~ 20 genetically different keratins
The most abundant amino acid are glycine and alanine, cysteine can account
for up to 24%
Long stretches a-helix is interrupted by short non-helical segments
Contact between 2 a-helices are formed by hydrophobic amino acid side chain
on 1 edge of each helix
– two polypeptides form a dimeric colid coil
– protofilaments are formed from two antiparralel of head-to-tail associated coils
– protofilaments dimerize to form a protofibril and four of which form a microfibril
Composition and structure of keratin
• Intra- and intermolecular hydrogen bonds, disufid bridges occurre at
all keratins.
• In cells, keratin type I forms pair with keratin type II
• Different keratin types are expressed in different cell types and
different layers of epidermis:
– cytoskeleton of epithelial cells - K14 (type I) & K5 (type II), K18
(type I) & K8 (type II)
– Basal layer – K13 (type I) & K4 (type II)
– Spinus and granular layer – K10 (type I) & K1 (type II)
– Stratum corneum – K3 (type I) & K12 (type II)
– Hairs and nails – various other keratin pairs
The epidermal permeability barrier
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Barrier function in human epidermis depends on
transglutaminase-mediated cross-linking of structural
proteins and lipids („biological glues“).
Transglutaminase reaction = posttranslation modification
of proteins – formation of covalent bond between a free
amine (NH3) group (e.g., protein- or peptide-bound
lysine) and the gamma-carboxamide group of protein or
peptide-bound glutamine free amine group.
Proteins are than highly resistent to mechanical
perturbation and proteolysis.
The quality of the S. corneum barrier depends on the
presence of equimolar concentration of ceramides,
cholesterol and fatty acids.
Changes in the concentration of any of these can affect
barrier quality.
lysine
glutamine
Fatty acids in epidermis
• Arachidonic acid and 20-carbon PUFA can be metabolized by either
cyclooxygenase or lipoxygenase pathways → prostaglandins,
hydroxyeicosatetraenoic acids.
– phospholipids are starting point for the arachidonic acid pathway
during inflammation (allergic reaction)
• Some of these metabolites can interact with signaling system in
proliferating and differentiating epidermal cells → modulation of
protein kinase C, nuclear MAP-kinase
Epidermal cell differentiation and
turnover
• Basal keratinocytes → transformation ~ 30 days to corneocytes.
• Damage cells are removed by normal squamation.
• Genetic damage - (UV-R) → trigger apoptosis (within hours) –
„sunburn“ cells.
• Skin protection against UV-R – concentrating transferred melanin
over vulnerable keratocyte nucleus.
• Other insults can induce keratnocyte apoptosis – chemical,
mechanical, immunological.
• The principal marker for keratinocyte/epidermal differentiation is
expression of particular keratin pairs
Epidermal cell differentiation and
turnover
• Proliferative basal keratinocytes express K5 and K14;
• keratinocytes in the early stages of maturation/differentiation switch
to K1 and K10.
• The „pluri-potent“ stem cells for keratinocytes, sebaceous gland and
epidermis rised from hair folicules.
• Ca2+ plays key role in epidermal differentiation - 4-fold increase of
extracellular Ca2+ in S. corneum.
• Keratinocyte differentiation is regulated by hormones and vitamins D3 and retinol from diet, thyroid hormone and steroid hormones.
• The skin has nuclear receptors for glucocorticoids, estrogen,
androgen and progesterone.
Epidermal cell differentiation and
turnover
• Important factors for keratocyte differentiation are Ca2+-dependent
integrins – the receptors for the extracellular matrix fibronectin
binding.
• Laminin and collagen IV and VII (basemen membrane components)
– regulation of keratinocytes migration on basement membrane
(very important during wound healing).
• Migrating keratinocytes produce many matrix metalloproteinases.
• Mature keratinocytes (in S. graulosum) contain protein-rich,
keratohyalin granules and lipid-rich, lamellar granules.
• Lipids from lamellar granules form the sheets of the lipid
permeability barrier of the epidermis.
Melanocytes
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Melanocytes - melanin-producing cells (S. basale)
Precursor cell - melanoblast
Melanin is stored in the melanosomes.
„Epidermal melanin unit“ - the anatomical relationship between
keratinocytes and melanocytes.
• 1 melanocyte is in contact with ~ 40 keratinocytes
• Melanocytes extend arms to transfer melanosomes into the
keratinocytes
Formation of melanosomes
• Melanosomes - elliptic membranebound organelles (melanin
synthesis).
• Synthesis of matrix proteins and
tyrosinase on the rough endoplasmic
reticulum.
• Tyrosinase undergoes post
translational modification in the form
of glycosylation in the Golgi
apparatus.
• Fusion of premelanosomes with
coated vesicles containing tyrosinase
- formation of the melanosome.
• Melanosome migrates into one of the
dendrites of the melanocyte →
transfer to a neighboring
keratinocyte.
Production of melanin
• Three enzymes in melanosomes whih absolutely required for
different melanin type synthesis
– tyrosinase (TYR) – responsible for critical step of melanogenesis (tyrosine
hydroxylation)
– tyrosinase-related protein 1 (TYR1) and DOPAchrome tautomerase
(DHI = 5,6-dihydroxyindole; DHICA = 5,6-dihydroxyindole-2carboxylic acid)
Melanins
• Melanins are polymorphous and multifunctional polymers of
eumelanin, pheomelanin, mixed melanins (a combination of the
two); and neuromelanin
• Mammalian cells produce black-brown eumelanin and yellow-redish
pheomelanin
• Eumelanin - highly heterogenous polymer consisting of DHI and
DHICA units in reduced or oxidized states.
• Pheomelanin - mainly sulfur-containing benzothiazine derivatives.
• Neuromelanin is produces in dopaminergic neurons of substantia
nigra.
• Melanin absorbs UV light at a wavelength of 280 - 320 nm
• Both eumelanin and pheomelanin play important protective role in
binding to cations, anions, drugs, chemicals, etc.
Factors involve in melanin production
• The melanin granules accumulate above the nuclei of keratinocytes
and absorb harmful UV-R before it can reach the nucleus and
damage the DNA.
• Quick responds of the melanocyte-keratinocyte complex to a wide
range of environmental stimuli (paracrine and/or autocrine) - to UVR, melanocyte-stimulating hormone (MSH), endothelins, growth
factors, cytokines, etc.
• UV-R exposure → melanocytes increase their expression of
proopiomelanocortin (POMC, the precursor of MSH) and its receptor
melanocortin 1 receptor (MC1-R), TYR and TYRP1, protein kinase
C (PKC), and other signaling factors
Factors involve in melanin production
• Fibroblasts (possibly other cells in skin) - produce cytokines, growth
factors, and inflammatory mediators that can increase melanin
production and/or stimulate melanin transfer to keratinocytes by
melanocytes.
• Other factors derived from keratinocytes which can regulate
proliferation and/or differentiation of melanocytes:
– α-MSH, ACTH, basic fibroblast growth factor (bFGF), nerve
growth factor (NGF), endothelins, granulocyte-macrophage
colony-stimulating factor (GM-CSF), leukemia inhibitory factor
(LIF), and hepatocyte growth factor (HGF)
Other epidermal cells
• Langerhans cells - dendritic cells - arise from bone marrow early in
embryonic development, occupy 2 - 8% of epidermis
• important element of the immune system, interacting with T-cells
• resided in suprabasal layer - attracted to keranocytes by Ecadherin receptor
• their motion is regulated by specific integrin receptor and by α –
TNF
• in the stratum germinativum interacts with the allergen and migrates
to the lymphoid gland - then “teach” the T cells about the allergen
• interact specifically with T-lymphocytes and keratinocytes to initiate
host response to antigens (allergens)
• UV B stimulates synthesis and release of TNF-a by keratinocytes
which in turn modifies the behavior and morphology of Langerhans
cells, decreases their total number.
Langehans
cell
Allergen
cytokine
T cell
Activated
T cell
Other epidermal cells
• Merkel cells – location in S. germinativum
• have synaptic contacts with sensory nerve endings
• associated with the sense of „light touch“ discrimination of shapes
and textures.
Dermis
• responsible for the tensile strength of skin
• main functions – regulation of temperature and to supply the
epidermis with nutrient
• much of the body's water supply is stored within the dermis
• components:
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connective tissue
hair follicles
sweat glands
sebaceous or oil glands
apocrine glands
lymph vessels
blood vessels
• The main cell type - fibroblast
Dermal proteins and extracellular matrix
• Collagen – about 90% of total dermal proteins
– predominantly type I (85 – 90%),
– type III (8 -11%),
– minor type V (2 – 4%), (papillary dermis, matrix around vessels,
nerves),
– type VI – associated with fibrils and interfibrillar spaces (responsible for
fine structure in early prenatal development of skin).
• Elastin, proteoglycans, glycoproteins, water and hyaluronic acid
Collagen structure - refer to lecture on Collagens
Elastin, proteoglycans, glycoproteins – refer to lecture on Extracellular
matrix
Skin appendages
• Skin plays in the body homeostasis, therefore is wellequiped with secretory (release of chemicals from cells for
physiological function) and excretory (elimination of weste
products of metabolism) capacity.
– sweat glands [can be sweat secreted with strong odour
(apocrine) or with a faint odour (eccrine)].
– sebaceous glands (secrete sebum onto hair follicle to oil the
hair).
– hair follicle
Sweat glands
• 3 – 4 million eccrine sweat glands are in our
skin – each producing water perspiration
(serves principle to cool us) and maintain core
temperature at 37.5oC.
• At maximum output the eccrine sweat glands
can excrete as much as 3 l/hour, and heat loss
is more than 18 kcal min-1.
• Humans utilize eccrine sweat glands as primary
form of cooling.
• Apocrine sweat glands are larger, have different
mechanism of secretion, and are limited to axila
and perianal area.
Sweat secretion
• Eccrine gland activity is regulated via neural stimulation using
sympathetic nerve fibers distributed around the gland.
• Neurotransmitter is acelylcholine
• Sweating is controlled from hypothalamus (a center in the preoptic
and anterior regions), where thermosensitive neurons are located.
• The stimulus for sweating:
– direct heating alone (39 to 46oC)
– physiological sweating due to nerve reflex arise from sweat
center in brain cortex (emotional), hypothalamus
(thermoregulation)
Eccrine sweat
• contains mainly water (99.0 – 99.5%). It also contains
electrolytes NaCl, K+ and HCO3-, and other simple
molecules - lactate, urea, ammonia, amino acids (serine
ornithine, citruline, aspartic acid) and minerals.
• Mineral composition varies with the individual:
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their acclimatisation to heat,
exercise and sweating,
the particular stress source (exercise, sauna, etc.),
the duration of sweating, and the composition of minerals in the
body
Apocrine sweat
• In lower mammals – secretion of pheromones (trigger
sexual and territorial response)
• In humans – the significance of apocrine secretion of
pheromones is not completely understood.
• Apocrine gland begin secreting at puberty
• Apocrine duct exit to the surface via hair follicle.
• Apocrine sweat – more viscous, with milky consistency
due to high content of fatty acids, cholesterol, squalene,
triglycerides, androgens, ammonia, carbohydrates.
Mineral composition of sweat
sodium
0.9 g/l
potassium
0.2 g/l
calcium
15 mg/l
magnesium
1.3 mg/l
zinc
Microelements
0.4 mg/l
copper
0.3 – 0.8 mg/l
iron
1 mg/l
chromium
0.1 mg/l
nickel
0.05 mg/l
lead
0.05 mg/l
Sebaceous glands
• Glands secrete an oily/waxy
matter, called sebum, to
lubricate the skin and hair
• Composition – 25% wax
monoesters, 41% triglycerides,
16% free fatty acids, 1%
squalene, small amount
cholesterol esters and
cholesterol
Skin metabolism
• Primary source for energy production in epidermis is glucose
from circulation – diffuses into keratinocytes without effect of
insulin. Large proportion of glucose is catabolized up lactate
(even in presence of oxygen)
• citric acid cycle does operate in epidermis – explanation why this
cycle is inefficient due to wide fluctuation of temperature and blood
flow in skin.
• 20% of glucose is metabolized by pentose-phosphate pathway –
production of NADPH and pentose for both FA synthesis and
nucleic acids.
• Secondary source of energy - fatty acids derived from both
epidermal stores and exogenous sources (when glucose flow is
limited, then FA are metabolized).
Skin metabolism
• Glycogen – small amount under physiological conditions, however,
elevation in all manner of injury of epidermis or during hair growth in
follicle – explanation – energy when skin needs to be repaired or to
use glucose immediately, most probably – disequilibrilium in
metabolic processes
• Glucose is a substrate also for synthesis of lipids polysaccharides
and glycoproteins and nucleic acids
• GAG and proteoglycans – highly charged and attract water –
forming gels (see GAG and proteoglycans in „Extracellular matrix“)
Skin metabolism
• Lipid metabolism - components:
a) membranes,
b) major constituents of permeability barrier,
c) energy supply
• Synthesis from both glucose catabolism and AA and circulating FA
- lipogenesis is ongoing in all layers of epidermis - sebum synthesis
– in sebaceous glands no accumulation of lipids from circulation;
even in sexual maturation (higher synthesis of sebum) - increase of
endogenous production and decrease of exogenous
• Degradation - generally - lipases (yields in FA for neutral lipids –
TG, sterol esters) – in outer layers of epidermis,
• specifically (e.g. formation of prostaglandins)
Skin immune system
• Skin not only provides immune protection for itself, but
also for the whole body.
• Cell types containing battery of mediators of immune
response
– Langerhanse cells, monocytes, macrophages, mast cells
(cooperation with T-cells)
• Cell types producing free radicals, anti-bacterial
peptides, cytokines chemokines, pro- and antiinflammatory mediators
– Neutrophils, eosinophils, basophils.
• B-cells secrete immunoglobulins (antibodies)