20 - Pigmentation and Melanocyte Biology Flashcards
Specialized cells of the epidermis and hair follicle whose primary function is to synthesize and transfer melanjn to adjacent keratinocytes
Melanocytes
Melanin synthesis occurs in a specialized organelle
Melanosome
Y/N: Melanocyte numbers are different in individuals of different racial backgrounds
No - similar
Location of melanocytes
Basal layer of the epidermis
Hair bulb
Outer root sheath
Melanocytes contain (round/oval) nuclei slightly (larger/smaller) than that of surrounding keratinocytes
Oval
Smaller
Melanocytes are present at about ______ the number of keratinocytes
1/10
Silver stains that rely on the ability of melanin to become impregnated with silver
Fontana-Mason stain
Warthin-Starry stain
Y/N: Silver stains react with melanocytes from eumelanotic, but not pheomelanotic skin tissue
Yes
Immunohistologic identification of melanocytes using antibodies directed against
Tyrosinase
MITF
Melan-A/Mart1
Although less specific for melanocyte lineage, antibodies directed against _____ protein have long been used with great sensitivity to detect melanocytes and melanoma
S100
Immunostaining with antibody _____ is negative in normal adult melanocytes but offers additional specificity for examination of melanocytic neoplasms
HMB-45
Reduced pigmentation of iris and retina Photosensitivity Decreased visual acuity Nystagmus Strabismus Misrouting of optic nerve
Ocular albinism
Cutaneous albinism (pigmentary dilution of skin and hair) Ocular albinsim
Oculocutaneous albinism
OCA: tyrosinase-negative (severe) albinism (complete loss of tyrosinase function)
OCA1A
OCA: tyrosinase-positive (mild to moderate) albinism
OCA1B
OCA: “Brown” albinism
OCA2
OCA: “Rufous albinism”
OCA3
OCA: variable degrees of albinism
OCA4
Piebaldism Congenital deafness Heterochromia irides Synophrys Broad nasal root Dystopia canthorum
Waardenburg syndrome
WS: classic type
WS1
WS: lacks dystopica canthorum
WS2
WS: limb abnormalities (hypoplasia, syndactyly)
WS3
WS: Hirschsprung disease
WS4
Tyrosinase-positive cutaneous albinism
Deafness
Eyebrow hypoplasia
Tietz syndrome
Depigmented forelock and abdominal skin
Piebaldism
Tyrosinase-positive oculocutaneous albinism (melanosome dysfunction) Bleeding diathesis (platelet granule dysfunction)
Hermansky-Pudlak syndrome
HPS: interstitial pulmonary fibrosis
HPS 1, 2, 4
HPS: granulomatous colitis
HPS 1, 4
HPS: neutropenia and cytotoxic T-cell dysfunction
HPS2
Tyrosinase-positive oculocutaneous albinism (melanosome dysfunction) Bleeding diathesis (platelet granule dysfunction) Progressive neurologic dysfunction (mechanism unknown) Severe immunodeficiency (NK cell, cytotoxic T cell, neutrophil dysfunction) Lymphomalike syndrome (overwhelming infiltration of organs by defective white blood cells) Giant lysosome-related organelles (these large granules are visible within leukocytes in PBS)
Chediak-Higashi syndrome
Pigmentary dilution of skin and hair (clumping of melanin within melanocytes)
Griscelli syndrome
GS: neurologic abnormalities
GS1
GS: immunodeficiency
GS2
Generalized patches of hyperpigmentation of skin and mucous membranes without systemic symptoms
Familial progressive hyperpigmentation
Diffuse hyperpigmented and hypopigmented patches without systemic symptoms
Variable penetrance
Familial progressive hyper- and hypopigmentation
Eumalonosome stage: melanosomes are round, approximately 0.3 um in diameter, and contain small intraluminal vesicles reminiscent of multivesicular bodies of the early endosomal compartment
Stage I
Eumalonosome stage: melanosomes are oval in shape and approximately 0.5 um in diameter and exhibit deposition of a parallel fibrillar matrix
Stage II
Eumalonosome stage: melanosomes demonstrate deposition of pigment along the filaments
Stage III
Eumalonosome stage: melanosomes are fully melanized, with the electron-dense melanin completely obscuring the underlying structures
Stage IV
Pheomelanosomes exhibit similar ultrastructural stages of maturation with _____ pheomelansomes exhibiting melanization obscuring underlying structures
Late stage III and IV
(Pheo-/eumelanosomes) are smaller, retain a rounded morphology at all stages, and contain a disorganized and less dense filament network
Pheomelanosomes
Cutaneous interfollicular melanocytes are present at highest density in the _____, and at lower density in the skin of the _____
Facial and genital skin
Trunk and extremities
Characterized by a melanocyte and each of the keratinocytes that it physically contacts
Epidermal melanin unit
Each melanocyte contacts and delivers melanin to approximately _____ keratinocytes
40
Under the pathologic circumstance of nerve injury, _____ may undergo dedifferentiation to generate ectopic melanocytes
Schwann cells
Y/N: Within the hair follicle, the melanocyte population varies by the stage of the hair cycle
Yes
Melanocyte stem cells reside within the
Permanent bulge/secondary hair germ
During the anagen phase of the hair cycle, the McSCs become activated via
Wnt signaling
Differentiated melanocytes of the hair bulb undergo apoptosis during
Late catagen
Y/N: Gradual loss of McSC population occurs through each hair cycle
No - McSC population persists through each hair cycle
Y/N: Gradual loss of McSCs occurs with age
Yes
Has been demonstrated as a niche for McSCs of the acral epidermis
Lower permanent portion of the eccrine gland
Acral melanomas are diagnosed with high sensitivity and specificity as a result of their preferential pigmentation of the eccrine-rich _____ of the dermatoglyphs
Ridges
Synthesize melanin and form melanosomes but these cells are derived from the neuroectoderm and are developmentally unrelated to the neural crest/melanocyte lineage
Retinal pigment epithelium
(Lighter/Darker) eye pigmentation is associated with decreased incidence of age-related macular degeneration
Darker
Autoimmunity directed agains melanocytes can lead to severe CNS inflammation in
Vogt-Koyanagi-Harada syndrome
In the cochlea of the ear, melanocytes are located in and required for the normal development and function of the _____ layer
Stria vascularis
Sensorineural deafness
Pigmentation defects
Waardenburg and Tietz syndromes
Dysfunction of cardiac melanocytes predispose to
Arrhythmias
Melanocytes are exclusively derived from the highly migratiry and multipotent _____ population
Neural crest
Transient cell population that forms at the margin of the early forming neural tube
Neural crest
Neural crest cell migration between the somites and the ectoderm
Dorsolateral migration pathway
Second wave of melanocytes derived from Schwann cell precursors that migrate along developing nerves
Ventromedial migratory pathway
Melanocyte precursors
Melanoblasts
Master melanocyte transcription factor that is first expressed in melanoblasts shortly after exit from neural tube at embryonic day 10.5-11
MITF
Structure between the neural tube and somites where melanoblasts paude before traveling upon the dorsolateral migration pathwayl
Migration staging area
Other markers of melanoblast specification such as the melanogenic enzymes _____ are expressed during the dorsolateral migration
Dct
TYRP1
Melanocytes express the key melanigenic enzyme _____ by embryonic day 14.5
Tyrosinase
Melanocytes display pigmentation by embryonic day
16.5
Y/N: Most premigratory and migratory neural crest cells give rise to multiple cell types and likely retain developmental plasticity
Yes
Factors that regulate melanocyte development, survival and migration: transcription factors
MITF PAX3 SOX10 SOX2 FoxD3
Germline mutations in _____ were identified in Waardenburg syndrome type 2, characterized by pigmentation defects and hearing loss
MITF (microphthalmia-associated transcription factor)
Defects in retinal pigment epithelium, osteoclasts, and mast cells
Loss of melanocytes of the skin and inner ear
Microphthalmia
Most lineage selective in specifying melanocyte differentiation
MITF, in particular the melanocyte-specific isoform m-MITF)
Also regulate expression of other lineages, including certain muscle groups and enteric ganglia/neurons
SOX10
PAX3
Mutation of _____ results in prominent pigment cell-related phenotypes wherease mutations of _____ produce pigment cell defects as well as certain nonpigmentary phenotypes (such as megacolon for _____)
MITF
SOX10 and PAX3
SOX10
Required for the survival, proliferation and differentiation of melanocytes
Acts as an oncogene and is amplifies in a subset of melanoma
MITF
The E318K MITF polymorphism found at low frequency in European, North American, and Australian populations is associated with significantly increased melanoma risk, an effect likely mediated by disruption of a concensus MITF _____ site
Sumoylation
One of the earliest markers of the neural crest
Mutations result in pigmentation defects, abnormalities of neural crest derivatives of the sensory and sympathetic ganglia, Schwann cells, and cardiac neural crest
Positive regulator of the MITF promoter in melanocytes and melanoma
PAX3 (Paired Box 3)
Mutated in human Waardenburg syndromes types 1 and 3 (hearing loss, pigmentation defect, arm and hand developmental defects)
PAX3
Required for the development of neural, glial, and melanocyte neural crest lineages
First expressed in premigratory neural crest cells
Loss leads to complete absence of melanoblasts
Required for survival of postnatal melanocytes, melanocyte stem cells, nevi, and melanoma
SOX10 (SRY-Box 10)
One of 4 transcription factors required for reprogramming of somatic cells to induced pluripotent stem cells
Play a role in specifying the melanocyte lineage during development
Binds and represses the MITF promoter
SOX2
Factors that regulate melanocyte development, survival and migration: growth factor signaling
Ednrb
c-Kit
Wnt/Beta-Catenin
Neuregulin/ErbB2/ErbB3
G-protein-coupled receptor that activates several signaling pathways, including PKC, MAPK and CAMP/PKA
Does not seem to be required for the initial specification of melanoblasts; required mainly for proper melanoblast migration
Induce melanocyte proliferation and skin pigmentation
Ednrb (endothelin receptor type B)
Ednrb agnosists, the endothelins, are encodes by 3 genes:
Edn1
Edn2
Edn3
Ednrb mutation results in
Hirschsprung disease
Edn3 mutation results in
Waardenburg plus megacolon
Upregulated during neural crest development and is important for establishing the neural crest lineage
Loss leads to loss of most neural crest-derived stuctures
Represses MITF to favor the Schwann cell fate
FoxD3 (Forkhead Box D3)
Mutated in Waardenburg type IV (Waardenburg plus megacolon)
SOX10
Also known as Steel factor or stem cell factor
cognate Kit ligand (KITLG)
KITLG exists in transmembrane and soluble forms and is produced by
Endothelial cells and keratinocytes
Mutation of c-Kit results in _____, which is characterized by a depigmentation of the forelock and abdominal skin
Piebaldism
Does not appear to be necessary for the specification of melanoblasts
Deficiency results in melanoblasts soon disappearing after migration from the neural tube
Regulating survival and differentiation/pigmentation of mature melanocytes
c-Kit
Polymorphisms in KITLG has been shown to cause
Familial progressive pigmentation
Familial progressive hyper- and hypopigmentation
Enlarging hyperpigmented patches corresponding to increased melanization without change in melanocyte number
Familial progressive pigmentation
Familial progressive hyper- and hypopigmentation
Activating mutations and amplification of c-Kit drive the progression of a subset of
Acral lentiginous, mucosal, and lentigo maligna melanomas
Consisting of APC, Axin1/2, and GSK3beta
Target beta-catenin for phosphorylation and ubiquitin-dependent degradation
Multiprotein membrane destruction complex
Binding of a Wnt family ligand to a _____ family receptor signals _____ to inhibit the membrane destruction complex, allowing for beta-catenin to enter the nucleus and to activate gene transcription
Frizzled
Dishevelled
Beta-catenin pathway activation also (potentiates/inhibits) melanoma formation and metastasis in mouse models
Potentiates
Controls Schwann cell proliferation, migration, and myelination
ErbB2/ErbB3 heterodimer tyrosine kinase receptor
ErbB2/ErbB3 heterodimer tyrosine kinase receptor ligand
Promotes Schwann cell fate and represses the melanocyte fate
Neuregulin
Factors that regulate melanocyte development, survival and migration: regulation of melanocyte motility and migration
Rac1
Belongs to the Rho superfamily of GTPases
When deleted, melanoblasts migrate less efficiently
Rac1
Mutation of _____, a guanine nucleotide exchange factor, occurs frequently in human melanomas, and these mutations may promote melanoma through activation of Rac1
PREX2
The 2 forms of melanin, eumelanin
and pheomelanin, are both derived from the precursor
Tyrosine
Tyrosinase converts tyrosine to
Dopaquinone
Brown/black melanin
Eumelanin
Yellow/orange melanin
Pheomelanin
Composed of polymers of 5,6-dihydroxyindole and the carboxylate form 5,6-dihydroxyindole-2-carboxylic acid
Eumelanin
Formed by oxidation and polymerization of sulfur-containing cysteinyldopa
Pheomelanin
Eu- vs pheomelanin: may act as a pro-oxidant
Pheomelanin
UVA interacts with pheomelanin, and to a lesser extent, eumelanin, leading to melanin degradation products that induce CPD formation long after UV exposure has ended
Dark CPD
Tyrosinase requires binding of ______ for proper function
Copper
Two tyrosine-related proteins, _____, show significant homology to tyrosinase and play key roles in melanogenesis (proper trafficking and stabilization of tyrosinase)
TYRP1
TYRP2/Dopachrome tautomerase (DCT)
Defects of tyrosinase, TYRP1, and DCT cause
Oculocutaneous albinism
Eumelanogenesis: Dopaquinone spontaneously undergoes intramolecular cyclization to form
Cyclodopa
Eumelanogenesis: Cyclodopa then undergoes redox exchange with a second molecule of dopaquinone to form
Dopachrome
Dopa
Eumelanogenesis: Dopachrome spontaneously decarboxylates to form _____ (less soluble, black) and _____ (more soluble, lighter black/brown)
DHI
DHICA
Eumelanogenesis: Although the uncatalyzed reaction favors DHI formation, the presence of the _____ favors production of DHICA
(DCT) protein
Eumelanogenesis: DHI and DHICA are then oxidized and polymerized to form
Eumelanin
Eumelanogenesis: _____ exhibits DHICA-oxidase activity, promoting incorporation of DHICA into eumelanin
TYRP1
Pheomelanogenesis: Nonenzymatic addition of sulfhydryl groups to dopaquinone results in the formation of
5-S-cysteinaldopa
2-S-cysteinaldopa
Pheomelanogenesis: _____ serves as a major source of sulfhydryl groups, with the other source being ______
Cysteine
Glutathione
Cysteinaldopa synthesis predominates in the presence of ______, whereas high tyrosine and/or low cysteine favor ______
Cysteine
Eumelanogesis
Pheomelanogenesis: Cysteinaldopa then undergoes redox exchange with dopaquinone to produce
Cysteinaldopaquinones
Pheomelanogenesis: Oxidation and cyclization of cysteinaldopaquinones is followed by dehydation, rearrangement, and decarboxylation to form
Benzothiazole intermediates
Pheomelanogenesis: The benzothiazole intermediates then polymerize to form
Pheomelanin
Melanosomes are lysosome-related organelles that exist only in
Melanocytes
Retinal pigment epithelium
As melanosomes mature, the key ultrastructural feature is the appearance of the
Parallel fibrillar network
Pigment cell-specific protein that has been shown to be the main component of the parallel fibrillar network
PMEL (gp100 or silver)
First site of PMEL deposition
Intraluminal vesicles of stage I melanosomes
Increased organization of PMEL amyloid into visible proteinaceous fibrils, which push the intraluminal vesicles outward to fuse with the melanosome membrane
Stage II melanosomes
Become increasingly pigmented as melanogenic enzymes are delivered and melanin synthesis has ceases
Stage III melanosomes
Fully pigmented, and tyrosinase activity and melanin synthesis has ceased
Stage IV melanosome
Mutations of any of the 10 genes encoding components of the BLOC-1, BLOC-2, BLOC-3, and AP-3 protein complexes have been long known to be responsible for
Hermansky-Pudlak syndrome
Hypopigmentation due to melanosome dysfunction in addition to platelet dysfunction, immunodeficiency, and lung fibrosis due to abnormalities of other lysosome-related organelles
Hermansky-Pudlak syndrome
Coordinated the action of microtubule- and actin-dependent machineries to allow proper formation and stabilization of recycling endosomal tubules required for transfer of cargoes to melanosomes
BLOC1
Acts to properly direct recycling endosomal tubular transport intermediates to maturing melanosomes to promote melanogenic cargo delivery
BLOC-2
Also important for sorting cargo proteins bound for melanosomes, and in different scenarios can act in concert with or independently of BLOC complexes
AP-3
The final step in the delivery of cargo to melanosomes is the fusion of cargo-containing vesicle and tubules with the developing melanosome. This involve _____ that are required for membrane fusion
SNARE protein
SNARE responsible for BLOC-1-dependent delivery of TYRP1 to the melanosome
VAMP7
Is required for the subsequent recycling of VAMP7 after delivery of cargo to the melanosome
BLOC-3
Stage IV melanosomes are first transferred to the dendrite tips by _____-mediated microtubule-dependent anterograde transport. _____-mediated retrograde transport melanosomes away from the melanocyte dendrites have been reported
Kinesin
Dynein
GTPase _____ is incorporated into the melanosome membrane by a hydrophobic geranylgeranyl tail
Rab27a
The adapter protein _____ binds Rab27a and serves a linker to the actin-binding motor protein _____, following transport and retention of melanosomes on the actin cytoskeleton
Melanophilin (Mlph)
MyosinVa (MyoVa)
Mutations in Rab27a, Mlph, and MyoVa have been identified in
Griscelli syndrome (partial albinism and immunodeficiency)
Melanosomes are transferred by dynein-mediated retrograde transport to a perinuclear location (“_____”) to protect basal keratinocytes from UV-induced DNA damage
Nuclear capping
In (dark-/light-) skinned individuals, melanosomes are smaller, and are grouped in membrane-enclosed clusters of 4-8 melanosomes
Light
In (darker-/lighter-) skinned individuals, melanosomes are larger, more numerous, and are distributed individually
Darker
Y/N: Melanosomes are thought to be degraded during the keratinocyte terminal differentiation process
Yes
Keratinocytes derived from (darker-/lighter-) skinned individuals degrade melanosomes more slowly
Darker
G-protein-coupled receptor expressed primarily by melanocytes
Critical role in determining if a melanocyte produces predominantly eumelanin or pheomelanin
MC1R (melanocortin 1 receptor)
(Gain-/Loss-) of-function variants of MC1R are associated with the “red hair phenotype”
Loss
Canonical agonist for MC1R
Alpha-melanocyte stimulating hormone
Alpha-MSH is derived from proteolytic processing of its precursor
Proopiomelanocortin (POMC)
_____ gene mutation, in addition to endocrine dysfunction, display the red hair phenotype
POMC
Antagonist of MC1R signaling
Agouti protein
Transcriptionally activated by MITF in melanocytes and serves as a negative feedback regulator that degrades cAMP
Phosphodiesterase D4
Human pigmentation is categorized into
Constitutive and inducible responses
Inducible pigmentation after UV exposure (sun tanning) has 2 components that are wavelength dependent
Immediate tanning
Delayed tanning
Immediate tanning occurs within minutes to hours after sun exposure, and is mainly a response to _____ radiation. It is most noticeable in _____-skinned individuals and is mediated by oxidation and redistribution of existing melanin
UVA
Darker
Delayed tanning is mainly a response to _____ and peaks at about 3 days to 1 week after sun exposure
UVB and shorter-wavelength UVA
Immediate vs delayed tanning: much more protective against subsequent sun exposure
Delayed
Y/N: Pigmentary pathways downstream of a defective MC1R remain intact
Yes
UV radiation of keratinocytes (increases/decreases) TGFbeta1 production
Decreases
