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© The Rockefeller University Press, 0021-9525/1997//61 $5.00
The Journal of Cell Biology, Volume 136, Number 1, , 1997 61-70

Article

The Autophagic and Endocytic Pathways Converge at the Nascent Autophagic Vacuoles



Willisa Liou*,§, Hans J. Geuze*, Math. J.H. Geelen{ddagger}, and Jan W. Slot*

* Department of Cell Biology, Faculty of Medicine, {ddagger} Department of Biochemistry, Faculty of Veterinary Medicine, Institute of Biomembranes, Utrecht University, 3584 CX Utrecht, The Netherlands; and § Department of Anatomy, Chang Gung College of Medicine and Technology, Taiwan, Republic of China

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.

Abbreviations used in this paper: +, indicates an endocytic tracer positive structure; ASGP-R, asialoglycoprotein receptor; AV, autophagic vacuole(s); AVd, degradative AV; AVi, initial AV; AVi/d, intermediate AV; CAIII, carbonic anhydrase type III; DMEM, Dulbecco's minimal essential medium; FP, fusion profile(s); LGP-120, lysosomal glycoprotein; MPR, manose-6-phosphate receptor(s); MU, methyl cellulose–uranyl acetate mixure; MVE, multivesicular endosome(s); PVP, polyvinylpyrrolidone; SOD, CuZn superoxide dismutase; VE, vesicular endosome(s).

The present work was made possible by a three-year research fellowship from the National Science Council, Republic of China in Taiwan, and a study grant from the Chang Gung College of Medicine and Technology to W. Liou.

Address all correspondence to Jan W. Slot, Department of Cell Biology, AZU H02.314, Heidelberglaan 100, 3584 CX Utrecht, The Netherlands. Tel.: (31) 30 2506543. Fax: (31) 30 2541797. E-mail: J.W.Slot@LAB. AZU.NL


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