Membranous complexes characteristic of melanocytes derived from patients with Hermansky–Pudlak syndrome type 1 are macroautophagosomal entities of the lysosomal compartment

Abstract

Hermansky-Pudlak syndrome (HPS) is an autosomal recessive disorder resulting from mutations in a family of genes required for efficient transport of lysosomal-related proteins from the trans-Golgi network to a target organelle. To date, there are several genetically distinct forms of HPS. Many forms of HPS exhibit aberrant trafficking of melanosome-targeted proteins resulting in incomplete melanosome biogenesis responsible for oculocutaneous albinism observed in patients. In HPS-1, melanosome-targeted proteins are localized to characteristic membranous complexes, which have morphologic similarities to macroautophagosomes. In this report, we evaluated the hypothesis that HPS-1-specific membranous complexes comprise a component of the lysosomal compartment of melanocytes. Using indirect immunofluorescence, an increase in co-localization of misrouted tyrosinase with cathepsin-L, a lysosomal cysteine protease, occurred in HPS-1 melanocytes. In addition, ribophorin II, an integral endoplasmic reticulum protein that is also a component of macroautophagosomes, and LC3, a specific marker of macrophagosomes, demonstrated localization to membranous complexes in HPS-1 melanocytes. At the electron microscopic level, the membranous complexes exhibited acid phosphatase activity and localization of exogenously supplied horseradish peroxidase (HRP)-conjugated gold particles, indicating incorporation of lysosomal and endosomal components to membranous complexes, respectively. These results confirm that membranous complexes of HPS-1 melanocytes are macroautophagosomal representatives of the lysosomal compartment.

Autophagy: In the cROSshairs of cancer

“…In addition to measuring autophagy protein levels by staining intensity, the evaluation of autophagosome formation by transmission electron microscopy (TEM) is considered the “gold standard” for autophagosome measurement. However, technical challenges of TEM are rampant in that clinical samples are often not preserved for subsequent TEM evaluation and pathology expertise is crucial for proper interpretation of TEM images, as melanosomes can be mistaken for double-membraned autophagosomes [90]. Further, melanosomes are degraded by autophagy and are present in the cell as macromelanosomes and autophagic giant melanosome complexes, which can add a significant complication to evaluation of autophagosome formation not owed to melanosome degradation [91].…”

The Ap3b1 gene regulates the ocular melanosome biogenesis and tyrosinase distribution differently from the Hps1 gene

“…The phenomenon is consistent with the different destinies of aberrantly transported tyrosinase in pale ear and pearl mice. In cells deficient in Hps1, the cargo vesicles containing tyrosinase are aberrantly transported to macroautophagosomes, where abnormally transferred tyrosinase may be subsequently degraded, while in the absence of the b3A subunit of AP3, the tyrosinase is aberrantly retained in the multivesicular body (MVB) Richmond et al, 2005;Smith et al, 2005;Nguyen and Wei, 2007). We therefore inferred that the amount of normally transferred tyrosinase in eyes of pearl mice was less than that in pale ear mice, leading to weaker pigmentation in eyes of pearl mice.…”

“…AP-3 facilitates the recruitment of tyrosinasecontained vesicles from the MVBs, while the subsequent recognition, docking, and fusing of these vesicles with stage II melanosomes are associated with the BLOC-3. In the absence of b3A subunit in AP3, tyrosinase is aberrantly retained in the MVBs, while the deficiency of BLOC3 could cause cargo vesicles being aberr-antly transported to macroautophagosomes, where abnormally transferred tyrosinase may be subsequently degraded Richmond et al, 2005;Smith et al, 2005;Nguyen and Wei, 2007).…”

iFly: The eye of the fruit fly as a model to study autophagy and related trafficking pathways

“…Another notable example for an eye color mutant is pink, which encodes the Drosophila ortholog of human HermanskyePudlak syndrome 5 (HPS5) (Syrzycka et al, 2007). HermanskyePudlak syndrome includes a group of inherited diseases associated with abnormal intracellular vesicular trafficking, and in some forms of the disease abnormal autophagy is observed (Huizing et al, 2002;Smith et al, 2005).…”