Thilo L, Stroud E, Haylett T
Department of Medical Biochemistry, University of Cape Town Medical School, Observatory, South Africa.
J Cell Sci. 1995 Apr;108 ( Pt 4):1791-803. doi: 10.1242/jcs.108.4.1791.
The controversy whether endocytic processing occurs by organellar maturation or by vesicular traffic has not been resolved. It is also not clear whether maturation continues to the stage of lysosomes, to what extent it involves a decrease in organellar fusogenicity, and how it relates to membrane recycling. Maturation and vesicular traffic imply distinct kinetics for the intermingling of endocytic markers after sequential endocytic uptake. We have studied the kinetics of intermingling of fluid-phase markers (fluorescein-labelled dextran and horseradish peroxidase) and cell surface-derived membrane (labelled by galactosylation) in organelles at early and late stages of the endocytic pathway in macrophage-like P388D1 cells. Intermingling declined by sigmoid kinetics, indicating that endosomes matured within about 3 minutes to become non-fusogenic towards early endosomes. During maturation about 60% of internalized membrane was recycled with T1/2 approximately 2 minutes. Whereas matured endosomes were non-fusogenic towards early endosomes and towards each other, a second phase of intermingling was observed upon delivery to lysosomes. This intermingling occurred by a first-order process (T1/2 approximately 4 minutes), concurrent with recycling of the remaining 40% of internalized membrane marker. These kinetic observations suggest a model for endocytic processing which reconciles maturation of early endosomes with the known function of carrier vesicles: Endocytic carrier vesicles do not bud off from permanent early endosomes as proposed for vesicular traffic, but are derived, together with recycling vesicles, from the maturation of early endosomes which are consumed by this process; these carrier vesicles subsequently mediate delivery to lysosomes by vesicular traffic during which the remaining surface-derived membrane is recycled.
关于内吞过程是通过细胞器成熟还是通过囊泡运输发生的争议尚未得到解决。同样不清楚的是，成熟是否会持续到溶酶体阶段，在多大程度上涉及细胞器融合性的降低，以及它与膜循环的关系。成熟和囊泡运输意味着在连续内吞摄取后，内吞标记物混合的动力学不同。我们研究了巨噬细胞样P388D1细胞内吞途径早期和晚期细胞器中液相标记物（荧光素标记的葡聚糖和辣根过氧化物酶）和细胞表面衍生膜（通过半乳糖基化标记）混合的动力学。混合以S形动力学下降，表明内体在约3分钟内成熟，对早期内体变得不具有融合性。在成熟过程中，约60%内化的膜以T1/2约2分钟的时间进行循环。虽然成熟的内体对早期内体和彼此不具有融合性，但在输送到溶酶体时观察到混合的第二阶段。这种混合通过一级过程（T1/2约4分钟）发生，与剩余40%内化膜标记物的循环同时进行。这些动力学观察结果提出了一个内吞过程的模型，该模型将早期内体的成熟与载体囊泡的已知功能协调起来：内吞载体囊泡并非如囊泡运输所提出的那样从永久性早期内体上芽生出来，而是与循环囊泡一起，源自早期内体的成熟，早期内体在此过程中被消耗；这些载体囊泡随后通过囊泡运输介导向溶酶体的输送，在此期间，剩余的表面衍生膜被循环利用。
