Bright Nicholas A, Gratian Matthew J, Luzio J Paul
Cambridge Institute for Medical Research and Department of Clinical Biochemistry, University of Cambridge, Addenbrooke's Hospital, Hills Road, Cambridge, CB2 2XY, United Kingdom.
Curr Biol. 2005 Feb 22;15(4):360-5. doi: 10.1016/j.cub.2005.01.049.
In mammalian cells, macromolecules internalized by endocytosis are transported via endosomes for digestion by lysosomal acid hydrolases . The mechanism by which endosomes and lysosomes exchange content remains equivocal . However, lysosomes are reusable organelles because they remain accessible to endocytic enzyme replacement therapies and undergo content mixing with late endosomes . The maturation model, which proposes that endosomes mature into lysosomes , 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 . The second suggests that endosomes and lysosomes engage in repeated transient fusions termed "kiss-and-run" . The third is that endosomes and lysosomes fuse completely, yielding hybrid compartments from which lysosomes reform , termed "fusion-fission" . 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.
在哺乳动物细胞中，通过内吞作用内化的大分子通过内体运输，以供溶酶体酸性水解酶进行消化。内体与溶酶体交换内容物的机制仍不明确。然而，溶酶体是可重复使用的细胞器，因为它们对于内吞酶替代疗法仍然是可及的，并且会与晚期内体进行内容物混合。提出内体成熟为溶酶体的成熟模型无法解释这些观察结果。已经提出了三种内容物混合机制。第一种是囊泡运输，这在酵母无细胞测定中得到了最好的支持。第二种认为内体和溶酶体会进行反复的短暂融合，称为“吻-跑”。第三种是内体和溶酶体完全融合，产生混合区室，溶酶体从中重新形成，称为“融合-裂变”。我们利用延时共聚焦显微镜在活细胞中测试这些假设。通过脉冲追踪将罗丹明葡聚糖加载到溶酶体中，随后将俄勒冈绿488葡聚糖加载到晚期内体中。观察到内体和溶酶体之间存在直接融合，并且通过相关电子显微镜捕获了一个这样的事件。对与溶酶体相遇的内体进行荧光强度分析，发现溶酶体内容物逐渐积累。我们的数据与在交换内容物之前细胞器需要直接接触的观点一致。
