Proteomic and phototoxic characterization of melanolipofuscin: correlation to disease and model for its origin.
OBJECTIVE
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.
Warburton
Sarah
S
Department of Chemistry and Biochemistry, Brigham Young University, Provo, UT 84602, USA. sarah.warburton@gmail.com <sarah.warburton@gmail.com>
Davis
Wayne E
WE
Southwick
Katie
K
Xin
Huijun
H
Woolley
Adam T
AT
Burton
Gregory F
GF
Thulin
Craig D
CD
D000328:Adult; D000368:Aged; D003594:Cytoplasmic Granules / Q000648:ultrastructure; D006801:Humans; D015151:Immunoblotting; D008062:Lipofuscin / Q000378:metabolism / Q000633:toxicity*; D008268:Macular Degeneration / Q000473:pathology*; D020460:Melanosomes / Q000378:metabolism* / Q000648:ultrastructure; D008954:Models, Biological; D010786:Photoreceptor Cells / Q000378:metabolism; D010857:Pigment Epithelium of Eye / Q000166:cytology / Q000378:metabolism; D020543:Proteome / Q000302:isolation & purification; D040901:Proteomics; D012243:Rhodopsin / Q000378:metabolism; D013050:Spectrometry, Fluorescence
D008062:Lipofuscin; D020543:Proteome; D012243:Rhodopsin