©2007 Molecular Vision
Molecular Vision 2007; 13:318-29 <http://www.molvis.org/molvis/v13/a35/>
Received 28 June 2006 | Accepted 13 December 2006 | Published 1 March 2007
Proteomic and phototoxic characterization of melanolipofuscin:
Correlation to disease and model for its origin
Sarah Warburton, Wayne E. Davis, Katie Southwick, Huijun Xin, Adam T. Woolley, Gregory F. Burton, Craig
D. Thulin
Department of Chemistry and Biochemistry, Brigham Young University, Provo, UT
Purpose: Melanolipofuscin (MLF) is a complex granule, exhibiting properties of both melanosomes and lipofuscin (LF)
granules, which accumulates in retinal pigment epithelial (RPE) cells and may contribute to the etiology of age-related
macular degeneration (AMD). MLF accumulation has been reported by Feeney-Burns to more closely reflect the onset of
AMD than the accumulation of lipofuscin. In an effort to assess the possible contribution MLF may have to the onset of
AMD, we analyzed the phototoxicity and protein composition of MLF and compared those results to that of LF.
Methods: Specifically, we observed the accumulation of MLF in human RPE from different decades of life, and assessed
the phototoxicity of these granules. We also employed fluorescence spectroscopy, atomic force microscopy, transmission
and scanning electron microscopy and proteomic analysis to examine the composition of MLF granules in an effort to
ascertain their origin.
Results: Our results show that MLF granules are phototoxic and their accumulation more closely reflects the onset of
AMD than does LF accumulation. Our compositional analysis of MLF has shown that while these granules contain some
similarities to LF granules, MLF is substantially different. Of significant interest is the finding that MLF, in contrast to LF,
does not contain photoreceptor-specific proteins, suggesting that MLF may not originate from the phagocytosis of photoreceptor outer segments. Instead the presence of RPE- and melanosome-specific proteins would suggest that MLF accumulates as a result of the melanosomal autophagocytosis of RPE cells.
Conclusions: Our results provide significant insight into understanding the formation and toxicity of MLF and suggest a
possible contribution to the etiology of retinal diseases.
Several retinal diseases, including age-related macular
degeneration (AMD), have been associated with the accumulation of autofluorescent granules in retinal pigment epithelial (RPE) cells. One such autofluorescent granule, lipofuscin
(LF), may relate to the onset of these ocular diseases because
it has been shown to generate reactive oxygen species via photosensitization with blue light [1-4]; which may cause damage and death of the RPE with subsequent death of the surrounding cells. However, as Feeney-Burns has reported [5],
the accumulation of LF in human RPEs is not consistent with
the onset of AMD. The most dramatic increase of LF in human RPEs, a 95% increase, occurs between young and middleaged groups (defined as ages 1-20 and 21-60, respectively)
while there is only a 21% increase between middle-aged and
old-aged groups (ages 61-100) [5].
Another autofluorescent granule that accumulates in RPE
cells and may contribute to the etiology of AMD is a complex
granule exhibiting properties of both melanosomes and lipofuscin granules called melanolipofuscin (MLF). Although it
is generally accepted that dermal melanin protects the skin
from UV light damage, the biological function of RPE melanin is not completely understood. Melanin is known to absorb
excess light passing through the eye, thereby reducing scatter
and improving image resolution. It has also been suggested to
play a photoprotective role in RPE cells [6,7] by scavenging
reactive oxygen species (ROS) [8-10]. Evidence also exists
for a phototoxic role for melanin in RPE cells, especially in
aged cells, including measurable ROS photoproduction
[6,9,11-13]. Melanosomes have been observed to undergo
morphological and photophysical changes with age, possibly
due to oxidation, which may result in diminished antioxidant
potential. Studies have reported that aged human melanosomes
are highly photoreactive and can result in RPE dysfunction,
while young melanosomes appear to confer photoprotection
[14-16]. With increasing age, a decrease in melanosomes in
the RPE is observed along with an increase in melanolipofuscin
(MLF) [17-19]. In contrast with the accumulation of LF in the
RPE, MLF accumulation has been reported by Feeney-Burns
to more closely reflect the onset of AMD. MLF exhibits a
15% increase between young and middle aged groups and a
50% increase between middle-aged and old-aged groups [5].
Oxidative stress has been suggested to be a major contributing factor for retinal degeneration in AMD. The retinas
constant exposed to light and a relatively high oxygen pressure, which is close to that found in arterial blood, contributes
to light-induced oxidative stress in the retina which may result in oxidative damage to biomolecules in these cells. RPE
cells are post mitotic and therefore must respond to a life time
of oxidative insult. While there are numerous mechanisms for
preventing and combating oxidative injuries, by middle-age
Correspondence to: Dr. Craig D. Thulin, Department of Chemistry
and Biochemistry, Brigham Young University, Benson Science Building C100, Provo, Utah 84602, Phone: 801 422-2795; FAX: 801 4220153; email: craig_thulin@byu.edu
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©2007 Molecular Vision
many of these anti-oxidative mechanisms have begun to break
down, which can increase the susceptibility of RPE cells to
accumulated damage. LF and MLF granules are thought to
result from the accumulation of undegradable material in RPE
cells. Modifications, including oxidation, may render the molecules in these granules undegradable by the cell, contributing to their accumulation.
While the identification of photoreceptor- and lysosomalspecific proteins in LF granules has provided evidence that
LF originates from the accumulation of undigested material
from the phagocytosis of photoreceptor disc in RPE lysosomes [20], little is known about the composition and origin of
MLF granules. Two models for the origin of MLF have been
suggested. The first model involves the autophagy of preexisting melansomes and their incorporation into accreting LF
granules. This model is supported by the observation that
phagosomes containing undegradable material fuse with melanosomes [7]. The second model is that melanin is synthesized de novo in lysosomes, which subsequently fuse with
accreting LF granules. This model is supported by evidence
that synthesis of melanin in depigmented RPE cells is seen in
lysosomal compartments [19,21]. Knowledge of the composition of MLF could provide significant insight into the origin
of these granules, and determining the phototoxicity of these
granules could be useful for ascertaining MLF’s role in the
etiology of AMD and other retinal diseases.
In the present study we observed the accumulation of MLF
in human RPE from different decades of life and assessed the
phototoxicity of these granules. We also employed fluorescence spectroscopy, atomic force microscopy, transmission and
scanning electron microscopy and proteomic analysis-using
1D gel electrophoresis coupled with ESI mass spectrometryto examine the composition of MLF granules in an effort to
ascertain their origin. Collectively these data provide significant insight into understanding the formation and toxicity of
MLF and suggest a possible contribution to the etiology of
retinal diseases. Specifically, these data do not provide direct
support for either previously suggested hypothesis for the origin of MLF, but instead suggest that MLF accumulates as a
result of the melanosomal autophagocytosis of RPE cells. To
our knowledge this is the first report of the phototoxicity and
biochemical analysis of retinal melanolipofuscin.
ules were isolated from the bands at the 1.0 M/1.2 M and 1.2
M/1.4 M interface. The granules were removed from the gradients by inserting a needle through the side of the tube and
extracting the bands sideways so as to minimize contamination of the bands. The material at the border of the LF and
MLF bands was not removed from the gradients but was used
as a buffer zone to keep the two samples separate from each
other during the extraction process. Sucrose gradients were
only briefly exposed to light while photographs of the gradients were taken. Fluorescence spectra of isolated lipofuscin
and melanolipofuscin granules were acquired as described by
Boulton et al. [22] using a Jobin Yvon Fluoromax-3 Spectrofluorometer (Edison, NJ).
Lipofuscin and melanolipofuscin accumulation: To study
the accumulation of LF and MLF over time, sucrose gradient
centrifugation was employed using four groups of RPE, each
consisting of 6 RPE and representing a different decade of
life. The first group had an average age of 33±1.6 yrs; the
second group had an average age of 43±0.9 yrs; the third group
had an average age of 54.3±1.9 yrs; and the fourth group had
an average age of 64±0.0 yrs. To compare the LF and MLF
content in young and old eyes, sucrose gradients were run with
11 RPE from young eyes (average age of 21.2±5.9 yrs) and 14
RPE from old eyes (average age of 66.5±5.9 yrs). Pictures of
the gradients were taken using an Olympus Camedia digital
camera. Image J (National Institutes of Health) was used to
measure the optical density of LF and MLF bands in the sucrose gradients.
All other experiments were performed using LF and MLF
isolated from RPE’s taken from a random donor population
between 40 and 80 years old.
Cell culture: Human retinal pigment epithelial cells
(ARPE-19; ATCC-CRL-2302) were grown in 24-well tissue
culture plates in RPMI 1640 media supplemented with 10%
fetal bovine serum (FBS). Upon reaching confluency the fetal
bovine serum (FBS) in the media was reduced to 1%. Cells
were either maintained in RPMI 1640 media supplemented
with 1% FBS or incubated in the same media which also contained about 300 melanolipofuscin or lipofuscin granules/cell
for 24 h to allow for ingestion of the granules. After the 24 h
incubation, the melanolipofuscin- or lipofuscin-fed RPE cells
were transferred back to RPMI 1640 media supplemented with
1% FBS and maintained for 3 days before bioactivity assay.
Bioactivity assay: To investigate the bioactivity of MLF,
ARPE-19 cells that were fed LF, MLF or neither (control cells)
were either subjected to blue light (390-550 nm) for 48 h at an
intensity of about 2.8 mW/cm2 or maintained in the dark. This
intensity of light, or even higher intensities, have previously
been used by investigators to determine the effect of blue light
exposure on the retina [16,23]. Blue light was introduced into
a 5% CO2 humidified cell incubator using a Mille Luce M1000
Fiber Optic Illuminator with a 150 W quartz halogen bulb, a
25 mm dichroic blue light filter, and a 48 inch fiber optic cable
(Edmund Optics, Barrington, NJ). Photocytotoxicity of the
lipofuscin and melanolipofuscin granules was assessed using
Sulforhodamine B (JNCI 82, p1107) to measure cell viability.
Briefly, cultures were fixed with trichloroacetic acid and
METHODS
Lipofuscin and Melanolipofuscin Isolation and Fluorescence
- Lipofuscin and melanolipofuscin granules were isolated as
previously described [20,22], using a method that has been
widely utilized for this process. Briefly, granules were isolated from human RPE from donor eyes, provided by Dr. Paul
Bernstein of the Moran Eye Institute, University of Utah, Salt
Lake City, UT. The time between donor death and enucleation
was 1-4 h, after which the donor eyes were stored at 4 °C until
dissection. Dissections were carried out by Dr. Bernstein’s lab
at the Moran 6-24 h after donor death in a dim light environment. RPE’s were shipped to BYU on dry ice and stored at 75 °C until use. Lipofuscin granules were isolated from the
band at the 0.3 M/1.0 M interface and melanolipofuscin gran319
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©2007 Molecular Vision
stained with 0.4% sulforhodamine B in 1% acetic acid. The
cultures were washed 4 times with 1% acetic acid to remove
any unbound dye; protein-bound dye was extracted with 10
mM unbuffered Tris base and transferred to a 96 well culture
plate. Absorbance was measured at 570 nm on a CERES
UV900 HDi plate reading spectrophotometer (Bio-Tek Instruments, Winooski, VT).
Microscopy: MLF granules were prepared for scanning
electron microscopy (SEM) analysis by drying the granules
on a silicon wafer and sputter coating them with gold. The
granules were analyzed on a Phillips XL30 ESEM FEG using
a 5 kV accelerating potential. For transmission electron microscopy (TEM) analysis MLF granules were fixed in glutaraldehyde, postfixed in osmic acid, dehydrated and embedded in epoxy resin. 100 nm slices of the sample were imaged
and photographed on a JOEL JEM 2000 FX.
MLF samples for atomic force microscopy (AFM) were
prepared by drying the granules onto a mica slide. Images were
taken with a Multimode IIIa AFM instrument with
microfabricated Si cantilever tips (Nanoscience Instruments,
Phoenix, AZ). Vibrational noise was dampened using an active isolation system (MOD1-M, Halcyonics, Goettingen,
Germany). Typical imaging parameters were (a) tip resonance
frequency, 55-65 kHz; (b) amplitude setpoint, 2.0-2.5 V; (c)
scan rate, 2.0 Hz. Images were processed offline to remove
the background slope using software bundled with the AFM
instrument.
Flow cytometric analysis: To determine the size distribution and concentration (granules/unit volume), suspensions
of MLF granules were diluted 1:100 and 1:1000 with PBS
and 200,000 Flow Check High Intensity Green Alignment
Beads (Polysciences, Inc., Warrington, PA), 5.726±0.375 µm
in diameter, were added to each sample to serve as an internal
standard. The samples were excited with an argon laser at 488
nm on a Beckman Coulter (Beckman, Fullerton, CA) EPICSXL Flow Cytometer with EXPO 32 ADC software for flow
cytometric analysis. The samples were analyzed for forward
light scatter and autofluorescence by collection of data for 300
s, which allowed visualization of at least 10,000 beads and at
least 95,000 MLF granules.
Total protein determination in melanolipofuscin:
Melanolipofuscin granules were pelleted using centrifugation
and lyophilized in an evaporative centrifugal concentrator. The
granules were weighed using a Mettler UMT2 microbalance
(Columbus, OH) to determine their total mass. After weighing the dried granules, the protein in these melanolipofuscin
samples was quantified by solubilizing the granules in 1% SDS
followed by the BCA Protein Assay (Pierce, Rockford, IL).
Three independent measurements were used to calculate the
percent protein and standard deviation.
1D gel electrophoresis and mass spectrometry:
Melanlolipofuscin and lipofuscin granules containing 100 _g
of protein were pelleted by centrifugation, solubilized in 4X
Laemmli buffer (3% SDS, 0.17 M Tris pH 6.8, 35% glycerol,
3.5% 2-mercaptoethanol) and separated on a 10% SDS-PAGE
gel (8.3x6.4x0.1 cm). The gel lanes were sliced into sections
and the proteins were digested in-gel as described by
Shevchenko et al. [24], injected onto a Jupiter C18 reversedphase resin capillary column (150 µm ID, made in-house),
and eluted using a gradient of 5-95% acetonitrile in 0.1% formic acid at a flow rate of 5 µl/min. On-line mass spectrometric analysis was performed on an Applied Biosystems QSTAR
Pulsar i (Foster City, CA) using an API (atmospheric pressure
ionization) source. Automated tandem mass spectrometry using information-dependent acquisition was run, collecting CID
spectra for the three most intense ions from each survey scan
excluding peaks chosen in the preceding 2 min. Fragmentation spectra were submitted to the Mascot (Matrix Science)
website for peptide identification. Proteins in MLF granules
from three independent preparations were examined. Each 1D
gel lane containing MLF or LF proteins was cut into 24 gel
slices for mass spectrometric analysis. Four gel slices (numbers 15, 16, 17, and 19) from two preps of LF granules were
selected for analysis to provide a direct comparison of the differences in LF and MLF proteins. Relative quantization of
proteins was estimated using the method of spectral counting
[25].
Immunoblots: Human retinas were obtained from Dr. Paul
Bernstein from the Moran Eye Institute to make a positive
control for rhodopsin. A retina was gently triturated in 0.75 M
Figure 1. Melanolipofuscin accumulation. A: Comparison of
Melanolipofuscin (MLF) granules from retinal pigment epithelium
(RPE) of young (21.2±5.9 yrs) and old (66.5±5.9 yrs) human donors.
B: Age of RPE donor versus optical density of MLF/LF plotted to
show the accumulation of MLF throughout life. Lipofuscin (LF) granules are observed in human RPE from young individuals, whereas
significant quantities of MLF do not appear to accumulate for decades afterward.
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©2007 Molecular Vision
sucrose, 0.68 mM calcium chloride, 20 mM tris and 1 mM
DTT pH 7.4 to rupture the cells. The suspension was poured
over 4 thicknesses of cheesecloth to remove debris. The sample
was spun at 1475xg for 20 min and the pellet was resuspended
in 1% SDS. Total protein was determined using the BCA assay. An α-rhodopsin antibody (R4, polyclonal antibody, see
Takemoto et al.) [26] was used at 1:1000 dilution in TBS-T.
Oxidized bovine serum albumin (BSA) samples were
made by incubating BSA in hypochlorous acid at 37 °C for 30
min. LF, MLF and BSA samples were derivatized by incubating them in 5% sodium dodecyl sulfate and 10 mM 2,4dinitrophenylhydrazine (DNPH) in 10% (v/v) trifluoroacetic
acid for 30 min at room temperature. The solutions were neutralized by adding 2M Tris and Laemmli buffer (3% SDS, 0.17
M Tris pH 6.8, 35% glycerol, 3.5% 2-mercaptoethanol) and
loaded directly onto a gel. Anti-DNP antibody from rabbit was
purchased from Sigma.
compared to an 80% decrease in cell viability in ARPE-19
cells fed with the same number of granules of LF (Figure 2).
Although this colorimetric assay provides an informative approximation of the phototoxicity of these granules, we are
aware that the nature of ARPE-19 cells makes it difficult to
precisely determine the phototoxicity of these granules. ARPE19 cells migrate up the sides of tissue culture plates as they
proliferate and phagocytose far fewer granules in this sideways position. These cells depress phototoxicity results because their inability to phagocytose granules inhibits them from
undergoing light dependent cell death. Thus, these
phototoxicity measurements are overly conservative. However,
the relative comparison of LF and MLF phototoxicity is not
affected.
Several physical measurements of MLF granules were
made using fluorescence spectroscopy, electron microscopy,
atomic force microscopy, and flow cytometry. Fluorescence
of MLF and LF granules is shown in Figure 3. Both granules
produce similar excitation spectra (data not shown), however,
MLF granules have an emission maximum at 554 nm, whereas
LF granules have an emission maximum at 578 nm. The similarity between the fluorescence spectra of these two granules
is expected because of the A2E fluorophore present in both
granules which dominates the spectra [27]. Apparently, melanin in MLF produces significantly less emission and appears
to be negligible in comparison to the fluorescence of A2E (data
not shown). The shoulder at about 470 nm in the fluorescence
spectrum of MLF increased over time when exposed to light
eventually becoming the maximum in the spectrum (data not
shown). This change may result from the accumulation of
damage on the proteins within MLF granules as a result of
light exposure or from photo-isomerization of A2E or other
lipids. This trend was also observed with LF (data not shown).
RESULTS
Side by side comparison of the LF and MLF content in RPE
from young (21.2±5.9 yr) and old (66.5±5.9 yr) individuals as
seen in sucrose gradients is shown in Figure 1A. This figure
confirms the presence of significant quantities of LF in RPEs
from young individuals where very little if any MLF is present.
In contrast RPEs from older individuals show significant quantities of MLF. This accumulation pattern is further evident
when the optical density of the MLF and LF bands in sucrose
density gradients is plotted versus the age of the RPE donor
(Figure 1B).
Analysis of the phototoxicity of MLF revealed that these
granules cause a 58% decrease in cell viability in ARPE-19
cells fed with MLF and exposed to blue light for 48 h. This is
Figure 2. Bioactivity of lipofuscin and melanolipofuscin granules.
Isolated lipofuscin (LF) and melanolipofuscin (MLF) granules were
fed to ARPE-19 cells. Cells that were not fed LF or MLF were used
as controls. Cells were then subjected to blue light irradiation (solid
bars), or left in the dark (cross-hatched), for 48 h. Cell viability was
determined using the Sulforhodamine B assay. Values represent mean
of at least four independent measurements, error bars represent standard deviation. The phototoxicity of MLF granules in ARPE-19 cells
is at least 72% as potent as that of LF granules, showing that MLF
granules have the potential for deleterious affects on RPE cells in the
retina.
Figure 3. Fluorescence emission spectra of lipofuscin and
melanolipofuscin. Fluorescence emission of lipofuscin (LF) and
melanolipofuscin (MLF) monitored with excitation at 364 nm. Both
granules produce similar excitation spectra (data not shown), however, MLF granules have an emission maximum at 554 nm, whereas
LF granules have an emission maximum at 578 nm.
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SEM and AFM analyses of MLF granules (Figure 4A-C)
show nearly spherical granules with some surface features
which suggest these granules are aggregates of smaller substructures. Transmission electron micrographs of MLF (Figure 4D) show these granules to contain inclusions of higher
density, demonstrating that these granules are complex and
not a mixed population of different granules.
Flow cytometric (FC) analysis allowed a quantitative determination of granule size. Forward light scatter in FC instruments is directly proportional to the size of the objects
passing through the beam. The MLF granules were found to
have a mean diameter of 0.93 µm and a broad standard deviation of 0.60 µm. Flow cytometry also enabled a quantitative
determination of the concentration of granules in our suspensions. Having a count of the MLF granules, we were able to
determine their average weight, which proved to be 2.2±0.1
pg/granule. When compared to LF, MLF is about 35% larger
but weighs about 69% more, again indicating the presence of
a more dense substance.
To determine the percent protein composing MLF granules, the protein in a known quantity of MLF was solubilized
in 1% SDS and quantified by BCA assay. MLF proved to consist of 60.7±6.4% total protein. Compared to LF, MLF contains more protein and therefore less extractable lipids (see
Warburton 2005) [20].
Because of the possibility that the proteins in MLF granules are highly modified exhibiting highly heterogeneous populations and therefore unfocusable on 2D gels, we employed
1D SDS-PAGE coupled with automated LCMSMS to identify the protein constituents of MLF. Figure 5 shows representative 1D lanes of LF and MLF proteins and indicates the gel
slices removed from the lanes. Proteomic analyses of the proteins in the 1D gel slices identified 110 proteins in MLF granules which are listed in Table 1. The proteome of MLF granules was compared to the proteome of other relevant organelles
including RPE melanosomes, macrophage phagosomes, retinal LF, and melanocyte melanosomes. As indicated in Table
2, 23 proteins were previously identified in mature RPE melanosomes [28], 18 proteins were previously identified as part
of the macrophage phagosome proteome [29], 14 proteins were
previously identified in LF granules [20], and 7 proteins were
identified in melanocyte melanosomes [30].
Four gel slices from MLF and LF 1D gels were analyzed
for a direct semi-quantitative comparison of RPE- and photoreceptor-specific proteins in these granules. Spectral counting
of two photoreceptor-specific proteins, rhodopsin and
peripherin, and two RPE-specific proteins, RGR and rpe65,
from these four gel slices is show in Figure 6. The two photoreceptor-specific proteins were identified in LF granules but
absent from MLF granules while RPE-specific proteins were
more abundant in MLF granules. Although RGR was identified in LF granules, it appears to be about 58% less abundant
than in MLF granules. A more comprehensive study of the
proteins in LF granules was previously published by Warburton
et al. [20].
Figure 4. Microscopic structure of melanolipofuscin. A: Scanning
electron micrograph of melanolipofuscin (MLF), showing nearly
spherical granules with some surface features. B, C: Atomic force
micrographs (phase images) showing MLF granules to be aggregates
of about 200 nm and about 50 nm substructures. D: Transmission
electron micrograph of MLF, shows these granules to contain inclusions of higher density, demonstrating that these granules are complexes of lipofuscin and melanin. Each bar represents 0.5 µm.
Figure 5. Electrophoresis of lipofuscin and melanolipofuscin proteins. Representative SDS-PAGE lanes of 50 µg of lipofuscin (LF)
and melanolipofuscin (MLF) proteins. Mobility of molecular weight
markers are indicated to the left. On the right, gel slices taken for
subsequent in-gel digestion are shown. The lack of well-focused bands
in the gel lane indicates microheterogeneous populations of the proteins, probably resulting from extensive modifications.
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TABLE 1. PROTEINS IDENTIFIED IN MELANOLIPOFUSCIN GRANULES
Gel
----1,2
Protein
---------------------------------acid ceramidaseIII
1,2,3
actin, betaIII
2
alpha actinin 4II
1
alpha tubulin, ubiquitous
3
alpha tubulin 2
3
1,2,3
alpha tubulin 4I
alpha tubulin 6III
3
ankyrin 1IV
1,2,3
annexin A2I
III
2
2,3
annexin A5
aspartate aminotransferase
3
2,3
1,2,3
ATP Synthase subunit g
ATP Synthase, H+ transporting,
mitochondrial F0 complex,
subunit 6
ATP Synthase, H+ transporting,
mitochondrial F0 complex,
subunit b
ATP Synthase, H+ transporting,
mitochondrial F0 complex,
subunit d
ATP Synthase, H+ transporting,
mitochondrial F0 complex,
subunit f
ATP Synthase, H+ transporting,
mitochondrial F1 complex,
alpha subunit
ATP Synthase, H+ transporting,
mitochondrial F1 complex, beta
subunitII,III
ATP Synthase, H+ transporting,
mitochondrial F1 complex,
gamma subunit
ATP Synthase, H+ transporting,
mitochondrial F1 complex,
subunit o
beta tubulinII,III
3
beta tubulin polypeptide
1,2,3
calnexinI,II,III
1,2,3
2,3
1,2,3
1,2,3
1,2,3
2,3
1,2,3
2,3
cathepsin DI,II,III
1,2
cell death-regulatory protein
Gel
slice
-------13
Genbank
Accession
number
---------AAC73009
11, 12,
13,
14, 15,
16
6
AAH08633
1, 2, 4,
5, 6, 7,
9, 10,
13, 15,
17, 18,
19, 21
9, 10,
11,
24
10
1, 5,
10,
11, 12,
13, 14,
18, 20,
21, 23,
24
3,4
NP_006073
NP_004915
2,3
1,2,3
3
3
cerebroside sulfate activator
protein
ceroid-lipofuscinosisI,II
lysosomal pepstatin insensitive
protease
chromosome 10 open reading
frame 58
chromosome 10 open reading
frame 70
chromosome 20 open reading
frame 3
Subcellular location
-------------------Lysosomal
Tissue
specificity
-----------Widely
expressed
F-actin cross linking protein
Nuclear and
cytoplasmic
Widely
expressed
Cytoplasmic surface of
erythrocytic plasma
membrane
Erythrocyte
transaminase A, Glutamate
oxaloacetate
transaminase-2
Mitochondrial
matrix
CAA25855
NP_079295
AAH04949
A35049
12, 13,
14, 15
15, 16
14
AAH23990
24
22
AAC61597
AAQ88428
18
AAH05960
22
NP_006347
23
AAH03678
2, 9,
10
AAP35873
10
NP_001677
16
AAH16812
Mitochondrial
20
AAV38639
Mitochondrial
matrix
AAH01429
NP_002071
10, 11,
14, 15,
23
10, 11,
12, 13,
21
6
AAH29529
15, 16,
17
23
AAP35556
23
12, 13
Mitochondrial
Widely
expressed
Ubiquitously
expressed
AAH01938
I53260
NP_057049
GRIM19
2,3
Synonyms
----------------------------------putative heart protein
MHC I antigen binding protein p88
NADH-ubiquinone oxidoreductase
B16.6
subunit, Gene associated with
retinoic-interferon-induced
mortality 19
protein
Type I membrane
protein
Endoplasmic
reticulum
Lysosomal
Mitochondrial inner
Widely
expressed
membrane
AAA36594
AAH14863
tripeptidyl-peptidase I precursor,
Lysosomal
19, 21
AAQ89418
24
AAH59168
12
AAQ89435
Type II
membrane protein
Membraneassociated
3
chromosome 8 open reading
frame 2
13
AAQ88475
2,3
crystallin, alpha BII
22
AAP35416
All tissue
SFLQ611
heat shock protein B5
323
Ubiquitous
Lens as well
as other
tissues
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©2007 Molecular Vision
TABLE 1. CONTINUED.
Gel
slice
-------22
Genbank
Accession
number
---------CAA39187
Gel
----2,3
Protein
---------------------------------cytochrome c oxidase subunit
III
2
cytochrome c oxidase subunitIV
23
AAV38628
1
cytokeratin 9IV
1
CAA82315
1
epidermal cytokeratin 2IV
1
AAC83410
2
erythryocyte membrane protein
band 4.2IV
7, 8
NP_000110
Membraneassociated and
cytoplasmic
2
gamma glutamyltransferase like
activity 1
10, 11
NP_004112
Type II membrane
protein
2
glucose regulated proteinIII
7
CAB71335
1
protein, hsp A5
gp25L2
19
CAA62380
guanine nucleotide binding
protein
guanine-nucleotide binding
protein, alpha transducing
heat shock 70 kDa protein 9BIII
heat shock protein 27
responsive protein
16
AAA52584
13
NP_653082
G protein
8, 9
18, 19
AAH30634
AAA62175
mortalin-2
estrogen-regulated protein, stress
2
heat shock protein 60III
10
AAA36022
chaperonin 60, mitochondrial matrix
protein
2,3
heat shock protein gp96
6
AAK74072
tumor rejection antigen, 94kD
glucose
23
24
24
8
AAD19696
AAN04486
AAD30656
NP_000173
12
NP_000174
20, 21
11
16
6
1
NP_076997
NP_006112.3
KRHUO
NP_000414
AAH34535
2,3
1
1,2
1
1,2,3
3
3
2
2,3
1,2,3
1,2,3
2,3
2
1
regulated protein
hemoglobin beta chainIV
hemoglobin, alpha 2IV
hemoglobin, beta, mutantIV
hydroxyacyl dehydrogenase,
subunit A
hydroxyacyl dehydrogenase,
subunit B
hypothetical protein MGC5508
keratin 1IV
keratin 10IV
keratin 2aIV
keratin 6BIV
Synonyms
-----------------------------------
Subcellular location
-------------------Integral
membrane
protein
Mitochondrial
inner membrane
Mitochondrial
inner membrane
dnaK-type molecular chaperone, BiP
Erythrocyte
Endoplasmic
reticulum
lumen
Type I membrane
protein
Endoplasmic
reticulum
G(o) alpha subunit I
Mitotic spindles in
mitotic cells;
nucleus during
heat shock
Mitochondrial
matrix
Endoplasmic
reticulum
lumen
Red blood cells
Red blood cells
Long chain 3-hydroxyacyl-CoA
dehydrogenase
Mitochondrial
microsomal epoxide hydrolase
1II
12
AAC41694
Membrane-bound
on microsomes
2
microsomal glutathione
S-transferase 3
23
AAQ81301
2
motor protein
6,7
BAA04654
Integral
membrane
protein
Microsomal
Mitochondrial
inner membrane
2
2,3
2,3
15, 19
2
2,4
CAA83513
NP_002465
NP_005955
KIAA0866
2
A61231
MYH9
3
1,2,3
myelin protein zeroI
myosin heavy chain 11II
myosin heavy chain nonmuscle
10II
myosin heavy chain nonmuscle
form AIII
myosin light chain 3
Na+/K+ ATPase alpha chainI,II
2
NAD(P) transhydrogenase
5
CAA90428
3
NADH cytochrome b5 reductase
14, 15
CAA09006
2
NADH dehydrogenase
(ubiquinone) 1 alpha
subcomplex 10
14
NP_004535
24
4, 5,
6
Ubiquitous
Terminal
differentiated
epidermis of
palms and
soles
Epidermal
tissue,
squamous
metaplasias
and carcinomas
2,3
2,3
Tissue
specificity
------------
inner membrane
protein, mitochondrial;
mitofilin,
AAA59853
A26641
Diaphorase 1
324
Integral
membrane
protein
Outside the
mitochondrial
inner membrane
on the matrix side
Membrane bound on ER
and
mitochondrial outer
membrane
Epithelial in
oral
mucosa,
esophagus,
papillae of
tongue
and hair
follicle
Liver
Widely
expressed
Skin and
kidney
Molecular Vision 2007; 13:318-29 <http://www.molvis.org/molvis/v13/a35/>
©2007 Molecular Vision
TABLE 1. CONTINUED.
Gel
----2
1,2,3
1,2
2,3
Protein
---------------------------------NADH dehydrogenase
(ubiquinone) 1 alpha
subcomplex 9
NADH dehydrogenase
(ubiquinone) flavoprotein 2
NADH dehydrogenase
(ubiquinone), Fe-S protein 1I
Gel
slice
-------15, 16
Genbank
Accession
number
---------NP_004993
Synonyms
-----------------------------------
Subcellular location
--------------------
NADH ubiquinone oxidoreductase
Matrix and
cytoplasmic
side of
mitochondrial
inner membrane
19
NP_066552
6, 7, 8
AAH30833
12, 13
CAH72148
17, 18
NP_004542
NADH ubiquinone reductase
19, 20,
21
7, 8
NP_002487
NADH-coenzyme Q reductase
2
NADH dehydrogenase
(ubiquinone), Fe-S
NADH dehydrogenase
(ubiquinone), Fe-S
NADH dehydrogenase
(ubiquinone), Fe-S
NADH dehydrogenase
3
NADH ubiquinone oxidoreductase
4
AAH08146
NDUFV1 protein
2,3
3
1
peptidylprolyl isomerase B
peroxiredoxin 1
peroxiredoxin 2III
thio-specific
antioxidant protein
peroxiredoxin 3II
predicted: similar to RIKEN
cDNA 4732495G21 gene
prenylcysteine oxidase 1I
progesterone membrane binding
protein
progesterone receptor membrane
component 1
prohibitinI,III
protein disulfide isomeraseIII
60
precursor
RAB11BII,III
RAP1BIII
retinal G protein coupled
receptor
22
20
21
CSHUB
CAI13096
CAA80269
cyclophilin B
19
12, 15,
16
10
18
AAV38810
NP_001017992
1,2,3
1,2
2
2,3
3
3
1,2,3
1,2
2,3
2,3
2,3
2,3
Tissue
specificity
------------
protein 2
protein 3
protein 8
precursor
1,2,3
retinal pigment epithelium
specific proteinI,II
2,3
2
retinol binding protein 3
retinol dehydrogenase 11
1,2,3
CAA43412
NP_057381
NP_006311
thioredoxin peroxidase,
Matrix and
cytoplasmic side of
mitochondrial inner
membrane
Matrix side of the
mitochondrial inner
membrane
Cytoplasmic
Cytoplasmic
Mitochondrial
KIAA0908, PCL1
19
NP_006658
17
10
AAS88903
S55507
glucose regulated protein 58 kDa, ER
21
22
13, 15,
17
NP_004209
AAH78173
NP_002912
RAS oncogene family
RAS oncogene family
peropsin, RGR
Lysosomal
Ubiquitous
Microsomal;
membrane-bound
Widely
expressed
Endoplasmic
reticulum
lumen
Retinal
pigment
epithelium
Retinal
pigment
epithelium
2, 6, 7,
8, 9,
10,
11, 12,
13, 15,
17, 24
4
15
NP_000320
retinol dehydrogenase 5
(11-cis and 9-cis)I,II
15, 16
AAH28298
11-cis retinol dehydrogenase
3
ribophorin II
11
AAH02594
KIAA0115,
dolichyl-diphosphooligosaccharide
glycosyltransferase 48 kDa subunit
Type I membrane
protein
3
ribophorin II precursor
9
B26168
Dolichyl-diphosphooligosaccharide
protein
glycosyltransferase 63 kDa subunit
Expressed in
all tissues
tested
2
2,3
serum albuminIV
solute carrier family 2,
member 1II
9
11
CAA23754
NP_006507
Type I membrane
protein
Endoplasmic
reticulum
Secreted
Integral membrane
protein; primarily
at the cell surface
2
solute
member
solute
member
Integral membrane
protein
Mitochondrial
inner membrane
Integral membrane
protein
Mitochondrial
inner membrane
Widely
expressed
2
carrier family 25,
1
carrier family 25,
12
CAH74045
AAH00112
17
NP_005975
9
NP_003696
2
solute carrier family 25,
member 13
9
NP_055066
2
solute
member
solute
member
solute
member
solute
member
25,
12, 13
NP_037518
25,
16
NP_005879
2
2
1
2,3
carrier family
24
carrier family
3
carrier family
3 isoform b
carrier family
4
25,
17
NP_998776
25,
15, 16,
17
NP_001142
solute carrier family 25,
member 4
15, 17,
18
NP_001142
RPE65
androgen-regulated short-chain
dehydrogenase/reductase 1
Glucose transporter type 1
ADP/ADT
325
Type II membrane
protein
Endoplasmic reticulum
Membrane-associated
Integral membrane
protein
Mitochondrial
inner membrane
Retinal
pigment
epithelium
Endoplasmic
reticulum
protein
Plasma
Widely
expressed
Molecular Vision 2007; 13:318-29 <http://www.molvis.org/molvis/v13/a35/>
©2007 Molecular Vision
TABLE 1. CONTINUED.
Gel
----2
Protein
---------------------------------solute carrier family 25,
member 5II
3
solute carrier family 25,
member 6
solute carrier family 25,
member A6
solute carrier family 4, anion
exchanger, member 1
spectrin, alphaII
2,3
1,2,3
1,2,3
Gel
slice
-------10, 14,
18
17
Genbank
Accession
number
---------NP_001143
Synonyms
----------------------------------ADP/ATP carrier protein
AAA36750
ADP.ATP translocase
10, 16
NP_001627
2, 4,
18
1, 2,
4
1, 2
17
NP_000333
spectrin, betaII
succinate dehydrogenase
complex, subunit B, iron sulfur
3
succinate dehydrogenase
complex, subunit AII
ubiquinol-cyctochrome c
reductase, rieske iron-sulfur
protein
ubiquinol-cytochrome-c
reductase
ubiquinol-cytochrome-c
reductase core protein III
ubiquinol-cytochrome-c
reductase core protein II
9
NP_004159
19, 20
AAD38242
18
S00680
12, 13
NP_003356
12
AAH00484
UQCRC2
1,2,3
vimentinI,II,III
13
AAA61279
beta tubulin, polypeptide
1,2,3
voltage dependent anion
channel 1III
15, 16,
18
NP_003365
porin
1,2,3
voltage
channel
voltage
channel
15
CAH73108
porin
16
NP_005653
2
2
3
1,2,3
dependent anion
2II
dependent anion
3
Tissue
specificity
------------
NP_003118
1,2,3
1
1,2,3
Subcellular location
--------------------
NP_003119.2
NP_002991
succinate-ubiquinone oxidoreductase
iron sulfur subunit
Mitochondrial inner
membrane
Mitochondrial inner
membrane
Mitochondrial inner
membrane
Mitochondrial inner
membrane
Mitochondrial inner
membrane; matrix
side
Outer membrane of
mitochondria and
plasma muscle
Highly
expressed in
fibroblasts,
some expression
in T and B
lymphocytes
Heart, liver
and skeletal
membrane
Outer mitochondrial
membrane
Widely
expressed
MLF proteins were fractioned on a 1D gel. The gel lanes were sliced into sections and proteins were digested and analyzed using automated
LC-MSMS and then identified using Mascot. Gel number refers to 3 separate preparations that were analyzed. I represents proteins that were
previously identified as components of lipofuscin granules. II represents proteins that were previously identified as part of the melanosome
proteome. III represents proteins that were previously identified as part of the macrophage phagosome proteome. IV represents proteins that are
preparation contaminants.
Rhodopsin was previously shown to be abundant in LF
granules [20], however; this protein was only identified in a
single gel slice from 1 of the 3 MLF preparations analyzed. In
order to more quantitatively examine this apparent lack of
rhodopsin in MLF granules, we performed an immunoblot of
MLF proteins in which we used an α-RHO antibody to detect
rhodopsin. Figure 7 shows that indeed no significant amount
of rhodopsin was detected in MLF granules.
Because of the extensive modifications on proteins in LF
granules that have been previously reported [20], we used
immunoblot techniques to detect oxidative modifications on
proteins in MLF granules. Dinitrophenylhydrazine (DNPH)
was used to derivatize protein carbonyls, a common product
of protein oxidation, and was detected using an α-DNP antibody. Figure 8 shows that the degree of oxidative modifications on proteins in MLF and LF granules is both extensive
and comparable, though not identical.
with Feeney-Burns results [5], MLF does not begin to accumulate significantly until midlife. This accumulation of MLF
later in life is consistent with the onset of AMD which affects
2% of individuals over 50 and 30% of individuals over 75
[31]. This correlation may suggest that MLF contributes to
the etiology of AMD.
Of significant interest is the fact the MLF is biologically
active, showing a light-dependent decrease in cell viability in
ARPE-19 cells fed MLF and placed in blue light for 48 h. To
our knowledge this is the first report of the phototoxicity of
MLF. The phototoxicity of MLF granules in ARPE-19 cells is
TABLE 2. MELANOLIPOFUSCIN PROTEOME COMPARISON WITH OTHER
ORGANELLES
Organelle
---------------Melanolipofuscin
RPE melanosomes
Phagosomes
Lipofuscin
Melanocyte
melanosomes
DISCUSSION
Sucrose density gradients of human RPE from different decades of life illustrated that MLF is virtually non-existent in
the RPE of younger individuals even though LF granules appear to be abundant in these RPE and has been detected in
RPE as young as 18 years of age (data not shown). Consistent
Total
proteins
-------110
102
140
36
68
No. of
common
-----23
18
14
7
Percent
------22.5
12.9
38.9
10.3
Reference
---------this study
[28]
[29]
[20]
[30]
The proteome of melanolipofuscin (MLF) was compared to the
proteome of several relevant organelles. Organelles are listed in order of decreasing number of proteins in common with MLF.
326
Molecular Vision 2007; 13:318-29 <http://www.molvis.org/molvis/v13/a35/>
©2007 Molecular Vision
at least 72% as potent as that of LF granules. These data show
that MLF granules have the potential for deleterious affects
on RPE cells in the retina.
The physical characteristics of MLF granules support the
description of MLF as a complex granule with characteristics
of both melanosomes and LF. The most compelling characteristic of MLF is the protein complement identified in the granules. Of the 110 proteins identified as components of MLF, 23
were previously identified in mature RPE melanosomes [28],
18 were previously identified as part of the macrophage phagosome proteome [29], 14 were previously identified in LF
granules [20], and 7 were identified in melanocyte melanosomes [30]. As expected, MLF granules appear to be considerably more similar to RPE melanosomes than to melanocyte
melanosomes. While LF and MLF granules contain a significant number of similar proteins, these proteins appear to be
related to the lysosomal processes which these granules both
participate in. However, the lack of similar cell specific proteins would suggest different origins of the material being
degraded.
Of interest is the presence of RPE65, which we previously identified in LF granules where it appeared to be far
less abundant than we observe in MLF granules. RPE65 was
previously identified in 3 of 15 gel slices from a 1D lane of
LF proteins and in 12 of 24 gel slices from a 1D lane of MLF
proteins. RPE65 plays a key role in the isomerization of retinol as part of the visual cycle in the RPE and is therefore
crucial to proper visual acuity. In contrast to RPE65, rhodop-
Figure 6. Semiquantitative analysis of photoreceptor- and retinal pigment epithelium-specific proteins in lipofuscin and melanolipofuscin
granules. Spectral counting was performed on two photoreceptorspecific proteins, rhodopsin and peripherin, and two retinal pigment
epithelium (RPE)-specific proteins, RGR and rpe65, in 4 gel slices
from lipofuscin (LF) and melanolipofuscin (MLF) 1D gels. Photoreceptor-specific proteins were only identified in LF granules, while
RPE-specific proteins were mainly identified in MLF granules. Although RGR was identified in LF granules it appeared to be about
58% less abundant than in MLF granules. This supports the hypothesis that LF granules originate from photoreceptors while MLF granules appear to originate from autophagy of RPE cells.
Figure 8. Dinitrophenyl immunoblot. Lipofuscin (LF) and
melanolipofuscin (MLF) proteins, 1.4 and 4 µg, that had been
derivatized with dinitrophenylhydrazine (DNPH) or not (control) were
run on SDS-PAGE, transferred to nitrocellulose, and probed with an
α-DNP antibody to show the derivatized protein carbonyls, a common product of protein oxidative damage. Shown for comparison
and to demonstrate specificity are lanes of bovine serum albumin
(BSA), BSA treated with DNPH, and BSA oxidized with hypochlorite then treated with DNPH. Oxidative damage on proteins in LF
and MLF granules is both extensive and comparable, though not identical.
Figure 7. Rhodopsin immunoblot. Immunoblot following SDS-PAGE
of 1 and 3 µg of total lipofuscin (LF) protein and 10 and 50 µg of
total melanolipofuscin (MLF) protein. Shown for comparison is 6
µg of protein from photoreceptor cell membranes enriched from human retina. Rhodopsin runs on SDS-PAGE as a mixture of the monomer (about 30 kDa), dimer (about 60 kDa), and trimer (about 90
kDa). Although rhodopsin is seen to be present in abundance in LF
granules, no significant quantity of rhodopsin is detected in MLF
granules.
327
©2007 Molecular Vision
Molecular Vision 2007; 13:318-29 <http://www.molvis.org/molvis/v13/a35/>
4. Rozanowska M, Jarvis-Evans J, Korytowski W, Boulton ME, Burke
JM, Sarna T. Blue light-induced reactivity of retinal age pigment. In vitro generation of oxygen-reactive species. J Biol Chem
1995; 270:18825-30.
5. Feeney-Burns L, Hilderbrand ES, Eldridge S. Aging human RPE:
morphometric analysis of macular, equatorial, and peripheral
cells. Invest Ophthalmol Vis Sci 1984; 25:195-200.
6. Sarna T. Properties and function of the ocular melanin—a
photobiophysical view. J Photochem Photobiol B 1992; 12:21558.
7. Schraermeyer U, Peters S, Thumann G, Kociok N, Heimann K.
Melanin granules of retinal pigment epithelium are connected
with the lysosomal degradation pathway. Exp Eye Res 1999;
68:237-45.
8. Dunford R, Land EJ, Rozanowska M, Sarna T, Truscott TG. Interaction of melanin with carbon- and oxygen-centered radicals
from methanol and ethanol. Free Radic Biol Med 1995; 19:73540.
9. Rozanowska M, Sarna T, Land EJ, Truscott TG. Free radical scavenging properties of melanin interaction of eu- and pheo-melanin models with reducing and oxidising radicals. Free Radic
Biol Med 1999; 26:518-25.
10. Sarna T, Menon IA, Sealy RC. Photosensitization of melanins: a
comparative study. Photochem Photobiol 1985; 42:529-32.
11. Boulton M, Rozanowska M, Rozanowski B. Retinal photodamage.
J Photochem Photobiol B 2001; 64:144-61.
12. Korytowski W, Kalyanaraman B, Menon IA, Sarna T, Sealy RC.
Reaction of superoxide anions with melanins: electron spin resonance and spin trapping studies. Biochim Biophys Acta 1986;
882:145-53.
13. Sarna T, Burke JM, Korytowski W, Rozanowska M, Skumatz
CM, Zareba A, Zareba M. Loss of melanin from human RPE
with aging: possible role of melanin photooxidation. Exp Eye
Res 2003; 76:89-98.
14. Dayhaw-Barker P, Davies S, Shamsi FA, Rozanowska M,
Rozanowska B, Boulton M. The phototoxicity of aged RPE
melanosomes. Invest Ophthalmol Vis Sci 2001; 42: S755
15. Gaillard E, Hill C, Griffiths D. UVC and visible light damage to
re-pigmented RPE cells. ARVO Annual Meeting; 2003 May 49; Fort Lauderdale (FL).
16. Rozanowska M, Korytowski W, Rozanowski B, Skumatz C,
Boulton ME, Burke JM, Sarna T. Photoreactivity of aged human RPE melanosomes: a comparison with lipofuscin. Invest
Ophthalmol Vis Sci 2002; 43:2088-96.
17. Boulton M. Ageing of the retinal pigment epithelium. In: Osborne
NN, Chader G, editors. Progress in retinal research. Vol 11.
Oxford: Pergamon; 1991. p. 125-51.
18. Feeney L. Lipofuscin and melanin of human retinal pigment epithelium. Fluorescence, enzyme cytochemical, and ultrastructural
studies. Invest Ophthalmol Vis Sci 1978; 17:583-600.
19. Schraermeyer U, Stieve H. A newly discovered pathway of melanin formation in cultured retinal pigment epithelium of cattle.
Cell Tissue Res 1994; 276:273-9.
20. Warburton S, Southwick K, Hardman RM, Secrest AM, Grow
RK, Xin H, Woolley AT, Burton GF, Thulin CD. Examining the
proteins of functional retinal lipofuscin using proteomic analysis as a guide for understanding its origin. Mol Vis 2005; 11:112234.
21. Dorey CK, Torres X, Swart T. Evidence of melanogenesis in porcine retinal pigment epithelial cells in vitro. Exp Eye Res 1990;
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22. Boulton M, Docchio F, Dayhaw-Barker P, Ramponi R, Cubeddu
R. Age-related changes in the morphology, absorption and fluo-
sin-which is abundant in LF is practically absent from MLF.
Of significant interest is the finding that MLF, in contrast
to LF, does not contain photoreceptor-specific proteins, suggesting that MLF does not originate from the phagocytosis of
photoreceptor outer segments as does LF, or by the fusion of
melanosomes and lipofuscin. Instead, the presence of RPEand melanosome-specific proteins would suggest that MLF
accumulates as a result of the melanosomal autophagocytosis
of RPE cells. Our results appear to support neither of the two
previously proposed hypotheses for the origin of MLF, because both hypotheses suggest the fusion of LF granules with
additional material to form MLF. Our results instead suggest
a new hypothesis for the origin of MLF which excludes the
involvement of previously existing LF granules. This new
hypothesis for the formation of MLF granules is supported by
recent evidence that melanosomes function as specialized lysosomes. Evidence for this specialized function includes the
related biogenesis of melanosomes and lysosomes [32,33], the
observed fusion between phagosomes and melanosomes [7],
and the presence of lysosomal enzymes in melanosomes [28].
The proteins in MLF granules were shown to be extensively modified by oxidative damage. The degree of oxidative damage is comparable to that found on LF proteins. The
prevalence of oxidative damage on these proteins may render
them undegradable by the cell and therefore lead to their accumulation in MLF granules.
Collectively these data provide significant insight into understanding the formation and toxicity of retinal MLF and suggest a new theory for its formation as well as support a possible contribution to the etiology of retinal diseases. Our findings suggest that MLF might result from the accumulation of
undegradable material perhaps undegradable due to oxidative
damage in the autophagocytic melanosomes of RPE cells.
Furthermore, MLF granules might pose serious risk of photosensitization of these cells allowing blue light to produce cell
death by liberation of reactive oxygen species, perhaps contributing to AMD.
ACKNOWLEDGEMENTS
We gratefully thank and acknowledge: Dr. Paul S. Bernstein
for providing human RPE and retinal samples, Dr. Barry M.
Willardson for providing the R4 antibody, Dr. Paul B. Savage
for the use of the fluorimeter, Dr. Nolan F. Mangelson for the
use of the microbalance, and the staff of the BYU Microscopy
Lab for their assistance. This research was supported by the
Brigham Young University College of Physical and Mathematical Sciences and the Department of Chemistry and Biochemistry.
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The print version of this article was created on 1 Mar 2007. This reflects all typographical corrections and errata to the article through that date.
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