Presentation from the World Vitiligo Symposium 2011. Sponsored by the VR Foundation.

1. Physiology and
Pathophysiology of
Melanocytes
Torello Lotti, MD
Professor of Dermatology and Venereology
Florence, Italy

2. MELANOCYTES
Melanocytes are pigment-producing cells
that originate from the dorsal portion of
the closing neural tube in vertebrate
embryos.

3. MELANOCYTES AND
PIGMENTATION PHYSIOLOGY

4. Pluripotent neural crest cell

5. Melanoblast migration and differentiation into
melanocytes is influenced by a number of signaling
molecules produced by neighboring cells that interact with
their their specific cell surface receptors.
Wnt Bone morphogenetic factor (BMPs)
Endothelin-3 (ET-3) Hepatocyte growth factor (HGF)
Stem Cell Factor (SCF), c-Kit-ligand

6. Wnt Family
16 different secreted glycoproteins;
Directs the maturation of pluripotent neural crest cell
into melanoblasts
Induction and
Wnt Frizzled receptor accumulation of
Β- catenin
Induce the transcription of 3 key
enzyme in melanin synthesis:
Transcription of
- Tyrosinase; Microphthalmia-assocciated
- TRP-1; transcription factor (Mitf)
- TRP-2.

7. MITF is central to
Melanocyte viability and function
Waardenburg syndrome type 2A
- different colored irises;
- white forelock;
- congenital cochlear deafness.
usually MITF activity
normal MITF activity is
is only partially lost
completely lost in these
animals

8. Endothelins Family
ET-1, ET-2, ET-3 EdnrA, EdnrB (receptor)
ET-3 + EdnrB:
- required for survival, prolifeartion and migration of
melanoblasts;
- also affect the development of other neural crest
cells.
- exracutaneous symptomatology in type IV
Waardenburg syndrome and in Hirschsrung
syndrome.

9. Stem Cell Factor
SCF: expressed by keratinocytes
Drive melanoblasts to their final destination
c-Kit (its receptor): expressed on melanoblasts
Mutations of c-Kit or SCF:
melanoblast unable to migrate to the skin and/or survive there
PIEBALDISM

10. Skin and
Hair Follicle
Inner ear Cochlea Leptomeninges
MELANOBLAST
Choroid Ciliary body Iris

11. CUTANEOUS MELANOCYTES
Melanocyte density/mm2:
550-1200 (highest concentration in genitalia and face)
Melanocytes syntesize melanine,
stored in cyosolic organelles
(melanosomes) transferred to
keratinocytes through dendritic
process.
Keratinocytes signals regulate epidermal melanocyte
survival, dendricity, melanogenesis…
Epidermal melanin unit: one melanocyte
surrounded by several keratynocites

12. MELANIZATION
The synthesis and distribution of melanin in the epidermis
(pigmentation) involves several step:
Transcription of proteins required to melanogenesis
Melanosome biogenesis
Sorting of melanogenic proteins into the melanosomes
Transport of melanosomes to the tips of melanocyte dendritic cells
Transfer of melanosomes to keratinocytes
Disruption in any of these events results in Hypopigmentation

13. Melanosome Biogenesis
Unique menbrane bound organelle (modified version of
lysosomes?) in which melanin biosynthesis take place.
Eumelanosomes Pheomelanosomes
Eumelanin Pheomelanin

14. Melanosome
Biogenesis

15. MELANIN BIOSYNTHESIS
Two types of melanin
Eumelanin Pheomelanin
Dark, brown/black Light, red/yellow
• Melanin provide protection against UV (280-400 nm)-
induced DNA damage;
• UV absorbed is converted into heat (less toxic form of
energy).
Melanin and its intermediates can be harmful to
melanocytes:
ROS generation: DNA damage: melanoma

16. MELANOGENIC PROTEINS
Enzymes and proteins involved in melanosomal maturation
Tyrosinase: Chromosome 11
• Synthesized in Endoplasmic reticulus
• Glycosilation in Golgi apparatus
• Packaged in endosomes
• Fuse in melanosome stage II
Mutations (missense, nonsense frameshift, deletion):
OCULOCUTANEOUS
ALBINISM TYPE I

17. Tyrosinase-Related Proteins (TRP)
TRP-1: chromosome 9
- Same Tyrosinase maturation pathway
- Tyrosinase activation/stabilization ?
- Melanosome biogenesis ?
OCULOCUTANEOUS
Mutations: ALBINISM TYPE III
TRP-2: chromosome 13

18. Microphtalmia-Associated Transcription Factor
(Mitf)
• Master gene for melanocyte survival;
• Key factor regulating transcription of melanogenic proteins:
Tyrosinase, TRP-1, TRP-2.
• 9 isoforms:
Mitf-M (specific for melanocytes), -A, -B, -C, -D, -E,
-H, -J and –Mc.
Mitf activity is induced by
binding of SCF to c-Kit
receptor and by cAMP-
elevating agents such α-
MSH.

19. Microphtalmia-Associated Transcription Factor
(Mitf)
Mitf upregulate the expression of anti-apoptotic protein BCl2
Melanocyte survival
Mitf
+
-/+
Cdk2
- p21
+
G1 to S phase Role in melanocyte
(Melanocyte proliferation) proliferation ?

20. Melanocortin Receptor (MCR)
Family of five related receptors (MC1-5R).
MC1R: melanocytes
α-MSH ACTH
MC1R
Mitf Eumelanin
cAMP
transcription synthesis
Polimorphisms within the MC1R
gene are largely responsible for
the different skin/hair color
among different ethnic group.

21. Propiomelancortin (POMC) encodes α-MSH and
other hormones
Both pituitary gland anf epidermal keratinocytes are able to synthesized
POMC

22. UV light activates p53 in keratinocytes,
p53 induces expression of POMC in keratinocytes

23. UV light activates a cascade that results
in elevated melanin synthesis and transport

24. The “THREE ENZYME THEORY” and
the crucial role of 6BH4
3 enzymes, phenylalanine hydroxylase activity (PAH),
tyrosine hydroxylase isoform I (THI) and tyrosinase, are
crucial for the initiation of melanogenesis
6BH4 in turn acts as the
essential electron donor
for PAH to produce L-
tyrosine from L-
phenylalanine and for
THI to convert l-tyrosine
to L-DOPA. 6BH4 is an
allosteric inhibitor of
tyrosinase.
Schallreuter KU et al. Regulation of melanogenesis – controversies and new concepts. Experimental Dermatology 2008;
17: 395–404.

25. MELANIN BIOSYNTHESIS
Dessinioti C et al.A review of genetic disorders of hypopigmentation: Lessons learned from the biology of melanocytes.
Experimental Dermatology 2009; 18: 741–749.

26. MELANOCYTE DENDRITES
• Branching protoplasmatic process that interact with
keratinocytes.
• Actin is a major structural component of dendrites;
• Several keratinocytes-
derived factors (ET-1,
NGF, PGE2, β-endorphin)
play a role in melanocyte
dendricity;
• Integrins also play a
role in dendrite
formation.

27. MELANOSOME TRANSPORT (in Melanocyte)
Microtubules
(arranged parallel to the long axis
+
of the dendrite)
Microtubule-associated motor proteins:
Kinesins (centrifugal movement)
and
Dyneins (Ccentripetal movement)
Other partecipants:
Rab27a
Myosin-Va Mutated in Griscelli syndrome
melanophilin

28. MELANOSOME TRANSPORT (to Keratinocytes)
Several potential ways involved
Exocytosis
Transfer by
Cytophagocitosis
(keratinocytes membrane vesicles
phagocytose the tip of
a melanocyte dendrite)
Fusion of plasma
menbranes

29. REGULATION OF MELANOCYTE FUNCTION
Imokawa G. Autocrine and paracrine regulation of melanocytes in human skin and in
pigmentary disorders. Pigment Cell Res 2004

30. Hypomelanoses: Why ?
1. Loss or reduction of melanocytes;
2. Reduced melanine production from
melanocytes (altered tyrosinase
activity, altered structure/activity
of rough endoplasmic reticulum, lack
of specific melanocyte receptors…);
3. Decreased melanine transfer from
melanocytes to keratinocytes;
4. Primary disorder of keratinocytes.

31. Hypomelanoses
Normal
Albinism
Functional defect in melanine synthesis
Vitiligo
Localized loss / inactivation of melanocytes

32. Disorders of Melanocyte Disorders of Melanin
Development and Migration Synthesis
Dessinioti C et al.A review of genetic disorders of hypopigmentation: Lessons learned from the biology of melanocytes.
Experimental Dermatology 2009; 18: 741–749.

33. Disorders of Melanosome Formation and
Transfer to Keratinocytes
Dessinioti C et al.A review of genetic disorders of hypopigmentation: Lessons learned from the biology of melanocytes.
Experimental Dermatology 2009; 18: 741–749.

34. VITILIGO ETIOPATHOGENESIS
GENETIC PREDISPOSITION
Autoimmune Susceptibility Locus (AIS1)
NEURAL HYPOTHESIS AUTOIMMUNE HYPOTHESIS
MELANOCYTE
DESTRUCTION
AUTOCYTOTOXIC/
RADICALIC HYPOTHESIS ECLECTIC HYPOTHESIS
MELANOCYTORRAGY
SYNERGISTIC THEORY

35. Vitiligo etiopathogenesis
 GENETIC PREDISPOSITION
 Autoimmune Susceptibility Locus (AIS1)
 AUTOIMMUNE
 Umoral mechanism -Autoantibodies
 Citotoxic mechanism – Cell mediated
 METABOLIC
 Hydrogen peroxide accumulation
 Abnormal expression of Tyrosine-Related Protein -1
 OTHERS
 Viral hypothesis
 Neuronal toxicity

36. Autoimmune Pathogenesis
 Presence of “vitiligo antibodies” in patients;
 Vitiligo is associated with several autoimmune
disease (vitiligo is a syndrome, not a disease…):
tyroiditis (up to 40%), diabetes type I (1-7%),
autoimmune gastritis, autoimmune polyglandular
syndromes, alopecia areata…;
 Most effective therapies in inducing
repigmentation have also immunosuppressive
effects (i.e.corticosteroids, ultraviolet,
cytotoxic drugs);
 Immunotherapies for melanoma often cause
vitiligo patches.

37. Autoimmune Pathogenesis
Ongenae K et al. Evidence for an Autoimmune Pathogenesis of Vitiligo. Pigment Cell Res 16: 90–100. 2003

38. Metabolic
Pathogenesis
Altered antioxidant Increased activity
and scavenger of superoxide
mechanism dismutase
High levels of epidermic 7-BH4 and
H2O2
Inhibition of enzyme function (phenylalanine-hydroxilase
and tyrosinase) and abnormal expression of Tyrosinase
Related Protein-1 (TRP-1).
 impaired melanine synthesis

39. Vitiligo: what’s new in 2011
Melanocytes are completely absent in the
depigmented epidermis
 Nordlund JJ and Lerner AB – Arch Dermatol, 1982;118:5-8
 Le Poole IC et Al. J Invest Dermatol, 1993;100:816-822
Vs.
Melanocytes are not completely absent in the
depigmented epidermis
 Bertosi KJ et Al. Eur J Dermatol 1998;8:95-97
 Tobin DJ et Al. J Pathol 2000;191:407-416
 Gottschalk GM, Kidson SH. Int J Dermatol. 2007;46(3):268-72

40. Vitiligo: what’s new in 2011
Melanocytes are not completely absent
in the depigmented epidermis
Normal Skin Perilesional Skin Lesional Skin
Massi D. Histopathological and ultrastructural features
of vitiligo. In: Lotti T & Hercogova J (Eds.) Vitiligo –
Problems and solutions. Marcel Dekker Inc, New York
2004

41. Vitiligo: what’s new in 2011
Melanocytes are not completely absent
in the depigmented epidermis
Comment:
– A subpopulation of “resistant” epidermal
melanocytes can persist independent of
disease duration
– Repigmentation can always
occur independent of
disease duration and with
non-perifollicular pattern

42. VITILIGO:
NOT ONLY A MELANOCYTIC DISEASE?

43. Imokawa G. Autocrine and paracrine regulation of melanocytes in human skin and in
Imokawa G. Autocrine and paracrine regulation of melanocytes in human skin and in
pigmentary disorders. Pigment Cell Res 2004
pigmentary disorders. Pigment Cell Res 2004

44. What’s new in 2011:
A focus on keratinocytes
 Impaired scavenging mechanisms can lead
to ROS increase and subsequent
melanocyte and keratinocyte damaging;
 Altered function of PAR-2 receptor can
impair calcium homeostasis in
keratinocytes and alter melanosome intake
and processing.

45. What’s new in 2011:
the focus on keratinocytes
 The importance in mitochondria in
keratinocytes from perilesional skin and
the role of oxidative stress.
 Prignano F, et al. Ultrastructural and functional alterations
of mitochondria in perilesional vitiligo skin. J Derm Sci
2009;54:157–167

46. Mitochondrial alterations in
perilesional keratinocytes
 Mitochondrial activity plays a crucial role
in normal cell function
 Mitochondrial alterations observed in
perilesional keratinocytes appear to be
very similar to those described in the
same cell types during apoptosis
 The mitochondrial damage is associated
with an increase in ROS production and,
hence, oxidative stress.
Prignano F, et al. J Derm Sci 2009;54:157–167

47. Functional alterations in vitiligo
skin
 High levels of TNF-alpha and FasL in the
depigmented epidermis (role in increasing
apoptosis)
– Kim NH, et al. J Invest Dermatol 2007;127:2612–7.
 mRNA for TNF-α and IL-6, with an inhibitory
effect on pigmentation, was increased in the
epidermis from vitiligo biopsies.
 This could contribute to keratinocyte
apoptosis, which results in reduced release of
melanogenic cytokines and in melanocyte
disappearance.
– Moretti S, et al. Histol Histopathol 2009:24:849-857

48. Functional alterations in vitiligo
skin
 Apoptotic keratinocytes may cause a
decrease in SCF synthesis, which plays an
important role in melanocyte survival and
proliferation
 Keratinocyte apoptosis induces a decrease in
the synthesis of other melanocyte growth
factors, such as bFGF, resulting in
melanocyte disappearance.
– Lee AY, et al. Br J Dermatol
2004;151:995–1003.
– Moretti S, et al. Histol Histopathol
2009:24:849-857

49. Functional alterations in vitiligo
skin
 Endothelin-1 (ET-1) mRNA seems to be
significantly reduced in lesional as compared
to perilesional epidermis
 SCF and ET-1 may contribute to melanocyte
survival
– Moretti S, et al. Histol Histopathol 2009:24:849-857

50. Functional alterations in vitiligo
skin
 Protease-activated receptor (PARs) 2 is
abundantly expressed by keratinocytes, and
seems to contribute to the pigmentation
process
 PAR-2 impairment is seen in vitiligo, and may
contribute to the epidermal pigment deficit
through a reduced melanosome uptake in
keratinocytes.
 To date, a precise cause and effect
relationship between these two conditions
cannot be determined.
– Moretti S, et al. Pigment Cell Melanoma Res 2009;22:335–338

51. OUR CONTRIBUTIONS

52. Berti S, Bellandi S, Bertelli A, Colucci R, Lotti T, Moretti S. Vitiligo
in an Italian outpatient center: a clinical and serologic study of 204
patients in Tuscany. Am J Clin Dermatol. 2011;12(1):43-9.
Prignano F, Ricceri F, Bianchi B, Guasti D, Bonciolini V, Lotti T,
Pimpinelli N. Dendritic cells: ultrastructural and
immunophenotypical changes upon nb-UVB in vitiligo skin. Arch
Dermatol Res. 2010
Arunachalam M, Sanzo M, Lotti T, Colucci R, Berti S, Moretti S.
Common variable immunodeficiency in vitiligo. G Ital Dermatol
Venereol. 2010;145(6):783-8.
Becatti M, Prignano F, Fiorillo C, Pescitelli L, Nassi P, Lotti T, Taddei
N. The involvement of Smac/DIABLO, p53, NF-kB, and MAPK
pathways in apoptosis of keratinocytes from perilesional vitiligo
skin: Protective effects of curcumin and capsaicin. Antioxid Redox
Signal. 2010, 1;13(9):1309-1321.

53. Prignano F, Pescitelli L, Becatti M, Di Gennaro P, Fiorillo C, Taddei
N, Lotti T. Ultrastructural and functional alterations of mitochondria
in perilesional vitiligo skin. J Derm Sci 2009;54:157–167;
Moretti S, Fabbri P, Baroni G, Berti S, Bani D, Berti E, Nassini R,
Lotti T and Massi D. Keratinocyte dysfunction in vitiligo epidermis:
cytokine microenvironment and correlation to keratinocyte
apoptosis. Histol Histopathol 2009;24:849-857;
Moretti S, Nassini R, Prignano F, Pacini A, Materazzi S, Naldini A,
Simoni A, Baroni G, Pellerito S, Filippi I, Lotti T, Geppetti P and Massi
D. Protease-activated receptor-2 downregulation is associated to
vitiligo lesions. Pigment Cell Melanoma Res. 2009;22:335–338.
Prignano F, Pescitelli L, Ricceri F, Lotti T. The importance of genetical link
in immuno-mediated dermatoses: psoriasis and vitiligo. Int J Dermatol
2008;47:1060–1062;
Prignano F, Betts CM, Lotti T. Vogt-Koyanagi-Harada disease and vitiligo:
where does the illness begin? J Electron Microsc (Tokyo). 2008

54. Thank you for your attention