The Autophagic and Endocytic Pathways Converge at the Nascent Autophagic Vacuoles

We used an improved cryosectioning technique in combination with immunogold cytochemistry and morphometric analysis to study the convergence of the autophagic and endocytic pathways in isolated rat hepatocytes. The endocytic pathway was traced by continuous uptake of gold tracer for various time per...

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Bibliographic Details
Published in:The Journal of cell biology 1997-01, Vol.136 (1), p.61-70
Main Authors: Liou, Willisa, Geuze, Hans J., Math. J. H. Geelen, Slot, Jan W.
Format: Article
Language:English
Subjects:
Animals
Autophagy
Carbonic Anhydrases - analysis
Cathepsin D - analysis
Cell biology
Cells
Cells, Cultured
Cellular biology
Cryopreservation
Cryoultramicrotomy
Endocytosis
Endosomes
Endosomes - ultrastructure
Enzymes
Hepatocytes
Immunohistochemistry
Liver - cytology
Liver cells
Lysosomes
Male
Methylcellulose
Organometallic Compounds
Rats
Rats, Wistar
Rodents
Superoxide Dismutase - analysis
Vacuoles
Vacuoles - ultrastructure
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Summary:We used an improved cryosectioning technique in combination with immunogold cytochemistry and morphometric analysis to study the convergence of the autophagic and endocytic pathways in isolated rat hepatocytes. The endocytic pathway was traced by continuous uptake of gold tracer for various time periods, up to 45 min, while the cells were incubated in serum-free medium to induce autophagy. Endocytic structures involved in fusion with autophagic vacuoles (AV) were categorized into multivesicular endosomes (MVE) and vesicular endosomes (VE). Three types of AV-initial (AVi), intermediate (AVi/d), and degradative (AVd)-were defined by morphological criteria and immunogold labeling characteristics of marker enzymes. The entry of tracer into AV, manifested as either tracer-containing AV profiles (AV+) or fusion profiles (FP+) between AV and tracer-positive endosomal vesicles/vacuoles, was detected as early as 10 min after endocytosis. The number of AV+exhibited an exponential increase with time. FP+between MVE or VE and all three types of AV were observed. Among the 112 FP+scored, 36% involved VE. Of the AV types, AVi and AVi/d were found five to six times more likely to be involved in fusions than AVd. These fusion patterns did not significantly change during the period of endocytosis (15-45 min). We conclude that the autophagic and endocytic pathways converge in a multistage fashion starting within 10 min of endocytosis. The nascent AV is the most upstream and preferred fusion partner for endosomes.
ISSN:0021-9525
1540-8140
DOI:10.1083/jcb.136.1.61