Epidermis Flashcards
Keratin of stratum basale
K5, K14
(Also K1, K6 in dogs)
What are the 2 types of keratinocytes in stratum basale?
1) Stem cells (slowly dividing)
2) Transit amplifying cells (rapidly proliferating)
What is an Epidermal Proliferative Unit?
Progeny of 1 original stem cell, transit amplifying cells –> terminally differentiated cells in all layers of epidermis
What are the spines in the stratum spinosum?
Desmosomes
What are the keratins of stratum spinosum?
K1, K10
(also K4, K15, K16 in dogs)
In what layer is involucrin made?
Stratum spinosum
In what layer is profilaggrin made?
Stratum spinosum
In what layer are lamellar granules made?
Stratum spinosum
Products of the stratum spinosum (3)
1) Involucrin
2) Profilaggrin
3) Lamellar granules
What is present in keratohyalin granules? (3)
1) Profilaggrin
2) Keratin filaments
3) Loricrin
In which layer does the cornified cell envelope construction begin?
Stratum granulosum
Where are lamellar granules released?
Apical side of stratum granulosum (into base of stratum corneum)
-Plasma membrane fuses to release into intercellular space
What is inside lamellar bodies? (10)
1) Sphingomyelin
2) Glucosylceramides
3) Phospholipids
4) Cholesterol
5) Proteases (steroid sulfatase, phospholipase A2, sphingomyelinase, β-glucocerebrosidase)
6) Protease inhibitors
7) Kallikreins = proteases for desquamation
8) Cathepsins = proteases for desquamation
9) Corneodesmosin
10) AMP’s and beta defensins
11) LEKTI’s
What is in the extracellular lipid matrix? (3)
1) Ceramides
2) Fatty acids
3) Cholesterol
Which cytoskeleton components are used for intracellular transport?
1) Microfilaments (actin)
2) Microtubules (α-tubulin, β-tubulin)
Which cytoskeleton components are used for scaffolding?
Intermediate filaments (keratin)
pH of Type I keratins
Acidic
Size of Type I keratins
Smaller
Numbers of Type I keratins
K9-19
pH of Type II keratins
Basic
Size of Type II keratins
Larger
Numbers of Type II keratins
K1-8
What type of keratins form heterodimers?
A type I keratin (acidic, K9-19) dimerizes with a type II keratin (basic, K1-8)
Parallel alignment
What is the progression of keratin assembly (classic model)?
Type I and Type II keratin line up → antiparallel heterodimers → tetramers → protofilaments → protofibrils → keratin intermediate filaments
How are keratin intermediate filaments bonded?
Crosslinked via disulfide bonds
Who binds KIFs?
Filaggrin
What happens to filaggrin after it successfully assembles KIFs and flattens corneocytes?
Becomes natural moisturizing factors (pyrrolidone carboxylic acid and urocanic acid) via caspase 14.
Where specifically are tight junctions found?
SG2
Responsible for permeability barrier of epidermis
Which is the deepest layer of the stratum granulosum?
SG3 (labeled as though 1-3 from pencil driving into the skin from outside)
What is the role of tight junctions?
Seals intercellular spaces
Holds H2O inside
Prevents antigens from entering from outside
In which layer does keratin intermediate filament assembly occur?
SG1
In which layer does cornified cell envelope construction occur?
SG1
What is the benefit of the cubic rod-packing and membrane templating model of KIF assembly?
Better explains strength and water-holding capacity of epidermis
Which mineral is potentially involved in profilaggrin processing?
Calcium
In which layer is profilaggrin cleaved into filaggrin?
Stratum graulosum
How many filaggrin monomers are cleaved from 1 profilaggrin unit?
4 in dogs
In which layer is filaggrin degraded?
Stratum corneum
What molecules is filaggrin degraded into?
Natural moisturizing factors
-Pyrrolidone carboxylic acid (maintain epidermal hydration) + amino acids
-Urocanic acid (hydration + photoprotection)
*Both are also antimicrobial
What is the job of NMF pyrrolidone carboxylic acid?
Maintain epidermal hydration
What is the job of NMF urocanic acid?
Photoprotection
Which enzyme helps degrade filaggrin into NMF?
Caspase 14
What helps heads and tails of keratins stick together?
Glycine loops- sticky!
In which layer are the lamellar bodies exocytosed?
SG1 (most superficial layer), at junction with stratum corneum
How does a desmosome become a corneodesmosome?
Corneodesmosin (released from lamellar body)
Name 2 families of enzymes important for desquamation
-Kallikreins
-Cathepsins
How do antimicrobial peptides arrive at the epithelial surface?
From lamellar bodies
What are the 2 components of a ceramide?
1) Fatty acid
2) Sphingosine base
What is the corneocyte lipid envelope composed of?
Single layer of ω-hydroxyceramides and ω-hydroxy fatty acids
What is the role of the corneocyte lipid envelope?
-Acts as a scaffold to organize extracellular lamellar bilayers
-Attaches to involucrin on cornified cell envelope
-Linoleic acid (ω-6 OFA) is a component of ω-hydroxyceramides
Where is linoleic acid involved in the skin barrier?
Component of corneocyte lipid envelope; ω-hydroxyceramides
(Also in phospholipids, glucosylceramides, acylceramides)
Which mineral is required for the functioning of transglutaminases?
Calcium-dependent
In which phase of skin development are transglutaminases crucial?
Construction of the cornified cell envelope
What type of bonds do transglutaminase enzymes form? Why do they matter?
-N(ε)-(γ-glutamyl)-lysine isopeptide bonds
-Highly resistant to proteolysis
What are the components of the cornified cell envelope? (6)
1) Involucrin
2) Loricrin
3) Envoplakin
4) Periplakin
5) Small proline-rich peptides (SPRs)
6) + other proteins
Which part of the cornified cell envelope touches the lipid lamellae?
Involucrin
Which part of the cornified cell envelope touches the keratin intermediate filaments+filaggrin?
Loricrin
Which part of the cornified cell envelop is the major structural component?
Loricrin (>70%)
Steps of cornified cell envelope construction (4)
1) OUTER LAYERS OF ENVELOPE
-Periplakin and envoplakin dimerize via Ca++
-Transglutaminase 1 attaches involucrin and periplakin/envoplakin to plasma membrane (Ca++ dependent)
2) LAMELLAR GRANULE FUSION
Lamellar granules fuse with plasma membrane, secreting lipids and enzymes into intercellular space
3) INNER LAYERS OF ENVELOPE
-Transglutaminase 3 links loricrin with small proline-rich peptides (SPRs)
-Transglutaminase 1 links loricrin+SPRs to involucrin (on outer side of CE)
-Transglutaminase 1 links involucrin to ω-hydroxyceramides in extracellular lipid layer
4) COMPLETION OF cornified envelope
-CE replaces cell membrane
-KIFs linked to CE at type II head domains
-Loricrin inside, involucrin outside
If transglutaminase 1 is broken, what step of cornification is affected?
NO CORNIFIED CELL ENVELOPE
-Loricrin/SPRs are not attached to CE
-Involucrin is not attached to ω-hydroxyceramides
What triggers lamellar granules secretion?
Increase in extracellular calcium concentration in SG1
What is this?
Lamellar granule
What is the origin of lamellar bodies?
Trans-Golgi network
Name 2 ceramide precursors (prior to combination w/FFA)
1) Spingomyelin
2) Glucosylceramide
Name 3 lipids in the stratum corneum
1) Free fatty acids
2) Cholesterol
3) Ceramides
What type of bond links fatty acids to sphingoid bases to form ceramides?
Amine- link
What may serve as a scaffold for the intercellular lipid lamellae?
ω-hydroxyceramides
What is the precursor to ω-hydroxyceramides?
Acylceramides
Which structure is key for skin barrier function and ichthyosis pathogenesis?
Cornified lipid envelope
(ω-hydroxyceramides bonded to CE)
What are the 2 components of phospholipids?
1) Free fatty acid
2) Glycerol
What is the function of free fatty acids from phospholipids?
Acidify stratum corneum
(important for enzyme functioning to make ceramides)
What is the function of glycerol from phospholipids?
Hydration support
What is cholesterol transformed into in the extracellular lipid matrix?
-Some is untransformed
-Some is transformed into cholesterol sulfate
What is the function of cholesterol sulfate?
Impairs desquamation
–cholesterol sulfate can be metabolized to cholesterol by steroid sulfatase in ECM, so desquamation can occur
Name 4 keratinocyte-to-keratinocyte adhesion structures
1) Desmosomes
2) Corneodesmosomes
3) Tight junctions
4) Adherens junctions
Which keratinocyte adhesion structure provides structural strength to the epidermis?
Desmosomes
T or F: Desmosomes are fixed
F. Desmosomes are constantly rearranging as keratinocytes migrate suprabasally
What do desmosomes link together?
Neighboring keratinocytes and their KIFs
What is the extracellular portion of the desmosome called?
Desmoglea
What are the intracellular portions of the desmosome called?
-Outer dense plaque
-Inner dense plaque (connects to KIFs
What are the 3 major components of desmosomes?
1) Cadherins (transmembrane, Ca dependent)
2) Armadillo proteins (bind to cadherins)
3) Plakin proteins
What are the cadherins in desmosomes?
1) Desmogleins
2) Desmocollins
What are the armadillo proteins in desmosomes?
1) Plakoglobin
2) Plakophillin
What are the plakin proteins in desmosomes?
1) Desmoplakin
2) Envoplakin
3) Periplakin
In addition to desmosomes (as the plakin proteins), where else in the epidermis are envoplakin/periplakin components?
Cornified cell envelope (next to involucrin)
What are the 3 general regions of the desmosome?
-Desmoglea
-Outer dense plaque
-Inner dense plaque
Role of corneodesmosin in corneodesmosomes?
-Glycine loops are sticky
-Mediate reversible intermolecular adhesion
-In desmoglea
Which cadherins are present in corneodesmosomes?
Desmocollin 1, Desmoglein 1 (also corneodesmosin)
Superficial
Where does corneodesmosin come from?
Lamellar bodies
What is the role of tight junctions?
-Seals intercellular space
-Controls paracellular movement of molecules
-Acts as a semipermeable barrier depending on molecule’s size and ionization –>keeps lipids form CLE out from SG1 and lower
-Performs “fence function” –> helps lamellar bodies know where to dump their materials, where apical surface is
What are the components of tight junctions? (4)
1) Claudins (transmembrane, critical)
2) Occludins (transmembrane)
3) Junctional adhesion molecules, JAMs (transmembrane, Ig-like)
4) Zonula occludens (ZO) proteins (Intracellular, interacts with actin)
Which component of tight junctions is intracellular?
Zonula occludens proteins
Which components of tight junctions are transmembrane?
-Occludins
-Claudins
-Junctional adhesion molecules (JAMs)
Which component of tight junctions interacts with actin?
Zonula occludens proteins
In which layer are there tight junction-related structures?
Stratum corneum
5 functions of adherens junctions?
1) Initiate + maintain cell-cell adhesion
2) Regulate organization of actin cytoskeleton
3) Establish hub for cell signaling and regulation of gene transcription
4) Regulate keratinocyte proliferation and differentiation
5) Role in wound healing
What are the components of adherens junctions?
1) E- Cadherin (transmembrane)
2) Cytoplasmic catenins
3) Actin
What are the Cadherins involved in adherens junctions?
E-cadherin (transmembrane, Ca++ dependent)
What cell is able to BYPASS tight junctions?
Langerhans cells – dendrites can pass, in order to sample superficial AGs
Which 2 protease families are responsible for enzymatic cleavage of corneodesomsomes?
1) Kallikreins (serine proteases)
2) Cathepsins (cystine proteases)
Which protease family can degrade corneodesmosin, desmocollin, and desmoglein?
Kallikreins
What is crucial for proteases to be able to cleave corneodesmosome?
pH! (Free fatty acids in extracellular lipids is crucial for acidifying the stratum corneum)
What is the role of LEKTI (lympho-epithelial kazal type inhibitor)?
Inhibits kallikrein (protease that degrades corneodesmosome)
Where does LEKTI come from?
Lamellar granules
What pH is LEKTI most effective?
Neutral pH
(As Stratum corneum becomes more acidic, LETKI stops inhibiting kallikrein, so desquamation can occur)
What regulates rate of desquamation?
Balance of kallikrein and LEKTI
(Also cholesterol sulfate inhibits kallikriens too!)
What processes are Ca++ responsible for regulating? (4)
1) Transcription of genes (encoding for keratinocyte differentiation proteins)
2) Lamellar granule secretion
3) Transglutaminase activity (cornified cell envelope)
4) Cleavage of profilaggrin to filaggrin
(+adherens junction, needed for E-cadherin)
Calcium level in stratum basale
Low
Calcium level in stratum spinosum
Low
Calcium level in stratum granulosum
High
Calcium level in stratum corneum
Very low
In a disturbed skin barrier, what is the calcium level throughout (including stratum granulosum)
Low
What prevents extracellular calcium from reaching the stratum corneum?
Tight junctions
Why does the basketweave pattern occur in stratum corneum?
Corneodesmosomes are first degraded on top/bottom of cells, then laterally. So the corneocytes are holding hands, and lifting off in sheets together
What are the goals of desquamation (2)?
1) Maintains epidermal homeostasis
2) Removes microorganisms, preventing colonization
Layer affected by hyperkeratosis on histopath
thickening of stratum corneum
Layer affected by lichenification on histopath
thickening of stratum spinosum
In human AD, is expression of filaggrin increased or decreased?
Decreased
In human AD, is expression of loricrin (for CE) increased or decreased?
Decreased
In human AD, is expression of involucrin (for CE) increased or decreased?
Decreased
In human AD, is expression of corneodesmosins increased or decreased?
Decreased
In human AD, is desquamation increased or decreased?
Increased
In human AD, is TEWL increased or decreased?
Increased
In human AD, is percutaneous penetration of allergens/microbes increased or decreased?
Increased
If there is no genetic mutation in filaggrin, why do some AD patients still have decreased filaggrin expression?
Th2 cytokines decrease filaggrin expression
Lamellar body secretion can be abnormal in humans with AD. Where can lamellar bodies be retained?
Retained in stratum corneum
In cAD, is total lipid content in the stratum corneum increased or decreased?
Decreased
-Decreased fatty acids and ceramides too
-Abnormal, disorganized, reduced lipid lamellae
In human AD, is tight junction expression increased or decreased
Decreased
Th2 cytokines downregulate _____, resulting in fewer tight junctions in AD
Claudin 1
3 ways PAR2 (Protease Activated Receptor 2) activation on keratinocytes contributes to inflammation and skin barrier disruption in cAD
1) Disrupts claudin-1 and occludins
2) Reduces tight junction barrier integrity
3) Promotes Th2 inflammation and pruritus
Only 2 general mechanisms by which keratinocytes can respond to a challenge
1) Increase proliferation
2) Stimulate inflammation
How does oral omega fatty acid supplementation help in dogs with cAD?
Normalize stratum corneum lipids - restore skin barrier function
How does scratching impact cAD?
Perpetuates damage to skin barrier; scratching downregulates claudin 1 expression –> worsens tight junction function
(Inhibit Akt-claudin pathway to decrease scratching and rescue claudin expression)
Name that ichthyosis
Norfolk terrier
K10
Mild epidermolytic ichthyosis
(hyperpigmentation, hyperkeratosis)
Which form of ichthyosis has severe adherent “parchment paper” like scale w/ Malassezia overgrowth?
Jack Russel Terrier
TG1
Markedly attenuated/absent CE
Leads to fragmentation of lipid lamellae
Which form of ichthyosis results in generalized large white scale, ventral hyperpigmentation?
Golden retrievers
PNPLA-1
Acyltransferase, which donates linoleic acid to ceramides
CLE attenuated/absent
Which form of ichthyosis results in generalized white scale, ventral adherent scale and erythema, Malassezia overgrowth + pruritus?
American bulldogs
NIPAL-4
Ichthyin (cofactor for enzyme) that synthesizes long chain fatty acids
*Toxic metabolite accumulation in stratum granulosum
*Lamellar body contents inside corneocytes
Which form of ichthyosis results in toxic metabolite accumulation in the stratum granulosum?
American bulldog
NIPAL 4
Treatment for ichthyosis
“Soak and slather” –> manage barrier defects, lifelong
Name that disease: young, scaly dog
Nonepidermolytic ichthyosis
Name that disease: young, scaly dog
Epidermolytic ichthyosis (swelling/lysis in upper spinous and granular cell layers)
What characterizes ichthyosis as “epidermolytic”?
Defect in keratin formation
What gene has been associated with linear hyperkeratotic plaques and follicular fronds in labrador retrievers?
NSDHL
Cholesterol processing
X-Linked semidominant
Pawpad hyperkeratosis
Which gene is responsible for severe pawpad hyperkeratosis with SC papillated proliferations in Rottweilers?
DSG1
Which gene is responsible for hereditary nasal parakeratosis in labrador retrievers (hypopigmentation diffusely, crusting dorsally)?
SUV39H2
What is pathogenesis-based therapy?
Treating the mechanism for the cornification disorder?
ie: in CHILD syndrome/ NSDHL chihuahua (cholesterol metabolism) –> replace cholesterol topically and give oral medication to lower systemic cholesterol levels to prevent toxic metabolic accumulation
3 breeds with palmoplantar keratoderma
1) Dogues de Bordeaux (K16, recessive)
2) Irish Terrier/Kromfohrlander (FAM83G, recessive)
3) Rottweiler (DSG1, spontaneous)
2 breeds with NSDHL disorders (cholesterol metabolism)
1) Labrador retriever (linear hyperkeratotic plaques and papillated projections, X-linked semidominant)
2) Chihuahua (linear epidermal nevi, X linked semidominant)
Name 2 antimicrobial peptides found in lamellar bodies
1) human β-defensin 2
2) cathelicidin LL-37
Epidermal turnover time in dogs
1) Normal
2) w/ seborrhea
3) w/ ichthyosis
1) 22d
2) 7d
3) 3.6d
Epidermal turnover time in horses/cows
17-18d
Which part of the cornified cell envelope is a fundamental promoter of epidermal terminal differentiation?
Loricrin
What is the final shape of the KIF core in mature corneocytes?
Flattened tetrakaidecahedron
Which protein in tight junctions can bind transcription factors, to help regulate cell proliferation?
Zona occludens proteins
Which gene codes of LEKTI?
SPINK5
What are the cytoplasmic catenins involved in adherens junctions?
-β-catenin
-p120 catenin
-α-catenin
