Endocytic delivery to lysosomes mediated by concurrent fusion and kissing events in living cells

In mammalian cells, macromolecules internalized by endocytosis are transported via endosomes for digestion by lysosomal acid hydrolases [1-3]. The mechanism by which endosomes and lysosomes exchange content remains equivocal [2-9]. However, lysosomes are reusable organelles because they remain accessible to endocytic enzyme replacement therapies [10] and undergo content mixing with late endosomes [5, 6]. The maturation model, which proposes that endosomes mature into lysosomes [9], cannot explain these observations. Three mechanisms for content mixing have been proposed. The first is vesicular transport, best supported by a yeast cell-free assay [11]. The second suggests that endosomes and lysosomes engage in repeated transient fusions termed "kiss-and-run" [4]. The third is that endosomes and lysosomes fuse completely, yielding hybrid compartments from which lysosomes reform [3, 5, 6, 7, 8], termed "fusion-fission" [2]. We utilized time-lapse confocal microscopy to test these hypotheses in living cells. Lysosomes were loaded with rhodamine dextran by pulse-chase, and subsequently late endosomes were loaded with Oregon green 488 dextran. Direct fusions were observed between endosomes and lysosomes, and one such event was captured by correlative electron microscopy. Fluorescence intensity analyses of endosomes that encountered lysosomes revealed a gradual accumulation of lysosomal content. Our data are compatible with a requirement for direct contact between organelles before content is exchanged.