This study analyzed the processing and trafficking of melanogenic proteins in primary melanocytes from mice with the underwhite (uw) mutation, a model for oculocutaneous albinism (OCA) type 4 in humans. The researchers found that uw-mutant melanocytes constantly secrete dark vesicles containing tyrosinase and other melanogenic enzymes into the medium, unlike wild-type melanocytes. Although tyrosinase synthesis was similar between uw-mutant and wild-type cells, tyrosinase activity and melanosome enrichment were reduced in uw-mutant cells. The abnormal tyrosinase secretion from uw-mutant cells, along with other findings, suggests OCA4 disrup

1. Research Article
Tyrosinase processing and intracellular trafficking
is disrupted in mouse primary melanocytes carrying
the underwhite (uw) mutation.
A model for oculocutaneous albinism (OCA) type 4
2013 l 08 l 03 PAPER STUDY
By Eun-Kyung
OJECTIVE
To analyze the intracellular processing and trafficking of melanogenic proteins in primary
uw-mutant melanocyte as a model for OCA type 4, focusing on tyrosinase.
SUMMARY
Oculocutaneous albinism (OCA) type 4 is a newly identified human autosomal recessive hypopigmentary disorder that disrupts pigmentation in the
skin, hair and eyes. Three other forms of OCA have been previously characterized, each resulting from the aberrant processing and/or sorting of
tyrosinase, the enzyme critical to pigment production in mammals. The disruption of tyrosinase trafficking occurs at the level of the endoplasmic
reticulum (ER) in OCA1 and OCA3, but at the post-Golgi level in OCA2. The gene responsible for OCA4 is the human homologue of the mouse
underwhite (uw) gene, which encodes the membraneassociated transporter protein (MATP). To characterize OCA4, we investigated the processing
and sorting of melanogenic proteins in primary melanocytes derived from uw/uw mice and from wild-type mice. OCA4 melanocytes were found to be
constantly secreted into the medium dark vesicles that contain tyrosinase and two other melanogenic enzymes, Tyrp1 (tyrosinase-related protein 1)
and Dct (DOPAchrome tautomerase); this secretory process is not seen in wild-type melanocytes. Although tyrosinase was synthesized at
comparable rates in wild-type and in uwmutant melanocytes, tyrosinase activity in uw-mutant melanocytes was only about 20% of that found in wild-
type melanocytes, and was enriched only about threefold in melanosomes compared with the ninefold enrichment in wild-type melanocytes. OCA4
melanocytes showed a marked difference from wild-type melanocytes in that tyrosinase was abnormally secreted from the cells, a process similar to
that seen in OCA2 melanocytes, which results from a mutation of the pink-eyed dilution (P) gene. The P protein and MATP have 12 transmembrane
regions and are predicted to function as transporters. Ultrastructural analysis shows that the vesicles secreted from OCA4 melanocytes are mostly
early stage melanosomes. Taken together, our results show that in OCA4 melanocytes, tyrosinase processing and intracellular trafficking to the
melanosome is disrupted and the enzyme is abnormally secreted from the cells in immature melanosomes, which disrupts the normal maturation
process of those organelles. This mechanism explains the hypopigmentary phenotype of these cells and provides new insights into the involvement
of transporters in the normal physiology of melanocytes.

2. Introduction
Material and
Methods
Results
Discussion
ALBINISM
Hetero group of genetic hypopigmentary disorders
OA (Ocular Albinism) : pigmentation in the visual system, mild form
OCA (Oculocutaneous Albinism) : reduced/absent pigmenatation
OCA1 Tyrosinase
• Autosomal recessive disorder associated with deficient catalytic
function of tyrosinase
• Endoplasmic reticulum retention disease
OCA2 P protein
• Most common albinism, minimal-to-moderate pigmentation
• Correctly processed through Golgi, but secreted from the cells
• Altered intracellular trafficking of tyrosinase
OCA3 Tyrp1
• Rare form
• Same with OCA1(Endoplasmic reticulum retention disease)
OCA4 MATP
• New distinct type
MATP ?
Membrane-associated transporter protein
Mitf - transcriptional level
12 predicted membrane-spanning region

3. Introduction
Material and
Methods
Results
Discussion
Cell culture
Antibodies and Enzymes
Celullar fractionation
Metabolic labeling
Western blotting
EndoH and PNGase digestions
Melanogenic assays
Immunohistochemical staining
Electron microscopy
Primary melanocyte from wild-type,
uw-mutant, albino C57BL/6J
αPEP1, αPEP7, αPEP8, anti-rabbit
IgG horseradish peroxidase-linked,
KDEL, HMB-45, EndoH, PNGaseF
etc
Extraction buffer,
- Vesicles secreted by melanosome
- Melanosomal –enriched fraction
6 well tissue culture
0.5mCi of[35S]-Methionine and Cysteine
mixture, αPEP1, αPEP7, 8% SDS-PAGE
8% SDS-PAGE, PVDF, first antibody 1:1000,
anti-rabbit IgG horseradish peroxidase-
linked whole antibody 1:1500
Samples to be digested
Tyrosinase enzymatic activity,
melanin contents
Dual labeling; immunofluorescence method and
laser scanning confocal fluorescence
microscopy( green, red, yellow)
2% glutaraldehide and 1% osmium tetroxide fixation,
embedded in PolyBed 812, ultrathin section, uranyl
acetate staining
JEOL JEM-1200 EX electron microscopy

4. Introduction
Material and
Methods
Results
Discussion
Figure 1 Characteristics of wild-type and uw-mutant primary murine melanocytes in culture
• The growth of two type were
quite similar
• Morphologies quite stable
• Uw-mutant melanocyte
: larger, more dendritic
• Wild type
- very pigmented population (C)
- no dark vesicle (E)
• un-mutant
- very lightly pigmented(D)
- dark vesicle detected (F)
into the medium

5. Introduction
Material and
Methods
Results
Discussion
Figure 1 Characteristics of wild-type and uw-mutant primary murine melanocytes in culture

6. Introduction
Material and
Methods
Results
Discussion
(A,B) Phase contrast photomicrographs of wild-type and uw-mutant melanocytes.
(C,D) Bright-field microphotographs emphasizing the pigmentation of wild-type
and uw-mutant melanocytes.
(E,F) Bright-field microphotographs showing the pigmented vesicles secreted by uw-mutant
melanocytes into the medium (arrows) and their absence in the medium of wild-type
melanocytes. All photomicrographs are shown at the same magnification.
(G) Media were collected from wild-type and from uw-mutant melanocytes and were centrifuged
to show the similar appearance of those samples (i.e. no soluble melanin was present).
(H) Pigmented vesicles released into the medium by uw-mutant melanocytes can be readily
sedimented by centrifugation, a process not seen in wild-type melanocytes.
(I) Melanin content of extracts of wild-type and uw-mutant melanocytes.
X 9
Below 1%
X 3
Over 5%

7. Introduction
Material and
Methods
Results
Discussion
Figure 2 Metabolic pulse-chase labeling of wild-type and uw-mutant melanocytes. Wildtype, uw-mutant
and Tyrc albino primary melanocytes were labeled for 1 hour with an [35S]-methionine and cysteine
mixture, harvested at the chase times indicated, solubilized and immunoprecipitated using specific
antibodies for tyrosinase and Tyrp1, respectively.
√ To examine the synthesis and processing of tyrosinase and Tyrp1 as each melanocyte
√ Stability of tyrosinase : wild type-100% remaining, uw-mutant-very little after 24hr
Fully glycosylatied
Early glycosylation
Start polypeptide

8. Introduction
Material and
Methods
Results
Discussion
Figure 3 Western blot analysis of melanogenic protein glycosylation in subcellular fractions of
wild-type and of uw-mutant melanocytes.
Glycosylation of melanosomal proteins was determined following digestion with
buffer (–), EndoH (E) or PNGaseF (P), as indicated.
Fully glycosylatied
Early glycosylation
Start polypeptide
√ Tyrosinase of wild type : none detection of secreted vesicle
√ Tyrp1 processed correctly and found in melanosome of uw-mutant
and also present secreted vesicle fraction

9. Introduction
Material and
Methods
Results
Discussion
Figure 4 Subcellular distribution of melanogenic proteins in wild-type and uw-mutant
primary melanocytes.
Marker for the ER
Marker for the early
(stage Ⅱ) melanosome
√ To investigate the subcellular localization of melanogenic protein in uw-mutant OCA4
√ In wild-type : tyrosinase-perinuclear area(ER, stageⅠ melanosome), little co-staining with HMB-45
√ In uw-mutant : more dendritic and much larger size
• Tyrosianse, Tyrp1, Dct : Red
• KDEL, HMB-45 : green
• Colocalization of Ab : yellow

10. Introduction
Material and
Methods
Results
Discussion
Figure 5 Ultrastructure of primary melanocytes established from wild-type and
from uw-mutant mice.
(A) Wild-type melanocytes : numerous stage III and IV melanosomes
(B) uw-mutant melanocytes : primarily stage I and II melanosomes
swollen, poorly melanized melanosomes
(C) uw-mutant melanocytes : DOPA-reactivity > positive dark staining
in stage I and II
revealed the presence of active tyrosinase
(D) Secreted vesicle fraction from uw-mutant melanocytes where stage I and II
melanosomes, similar to those found in (B)
Bar, 20 mm

11. Introduction
Material and
Methods
Results
Discussion
Primary melanocyte so far available
OCA4 new type of oculocutaneous albinism
mutation of underwhite (MATP) gene
Uw-mutant cells Expression normal level of tyrosinase
Secretion of dark vesicle into medium
> low melanin, 20% reduced tyrosinaes activity
Sorting of tyrosinase from trans-Golgi network
to stage Ⅱ melanosome
Function of MATP(uw-encoded protein)
Melanosome-specific protein
but absence at stage Ⅱ melanosome
> transport proetein
Similarity of OCA2/P protein phenotype, function
What is
this paper
saying ?

12. Introduction
Material and
Methods
Results
Discussion
Figure 6 Melanosomal protein trafficking and abberrant processing seen
in various of OCA.
√ Various type of OCA
√ OCA1, 3 : disruption of tyrosinase trafficking at the level of ER
√ OCA2 : disruption of tyrosinase trafficking at the post-Golgi level
√ OCA4 : cell secretion within vesicles before delivery