THE AUTOPHAGIC AND ENDOCYTIC PATHWAYS CONVERGE AT THE NASCENT AUTOPHAGIC VACUOLES

We used an improved cryosectioning technique in combination with immun ogold 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 involv ed in fusion with autophagic vacuoles (AV) were categorized into multi vesicular endosomes (MVE) and vesicular endosomes (VE). Three types of AV-initial (AVi), intermediate (AVi/d), and degradative (AVd)-were de fined by morphological criteria and immunogold labeling characteristic s of marker enzymes. The entry of tracer into AV, manifested as either tracer-containing AV profiles (AV(+)) or fusion profiles (FP+) betwee n AV and tracer-positive endosomal vesicles/vacuoles, was detected as early as 10 min after endocytosis. The number of AV(+) exhibited an ex ponential increase with time. FP+ between MVE or VE and all three type s of AV were observed. Among the 112 FP+ scored, 36% involved VE. Of t he 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 signif icantly change during the period of endocytosis (15-45 min). We conclu de 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.