Rahner C, Stieger B, Landmann L
Department of Anatomy, University of Basel, Basel, Switzerland.
Gastroenterology. 2000 Dec;119(6):1692-707. doi: 10.1053/gast.2000.20233.
BACKGROUND & AIMS: This study demonstrates and characterizes apical (canalicular) endocytic pathways in hepatocytes in situ.
Endocytic markers were administered by retrograde infusion through the common bile duct. Colocalization with proteins that are specific for various endocytic compartments was performed on stacks of deconvoluted confocal immunofluorescence images. The subcellular distribution of marker proteins was assessed by electron microscopy (EM).
Bulk-phase, as well as membrane-associated markers, were internalized readily at the apical cell pole. At the EM level, marker was found initially in 60-100-nm tubulovesicular structures and 150-200-nm cup-shaped vesicles, whereas multivesicular bodies and lysosomes became labeled after longer time intervals. Apical endocytosis involved clathrin and delivered marker to late endosomes (rab7(+), cathepsin D(+)), as well as lysosomes (rab7(-), cathepsin D(+)). Simultaneous labeling of the basolateral endocytic route resulted in overlap of both pathways in the late endosomal and lysosomal compartments. In addition, apical endocytosis involved a subapical compartment (endolyn-78(+), rab11(+), polymeric IgA receptor [pIgA-R(+)]) that is passed by the transcytotic route, thus constituting a crossroads. pIgA-R immunoreactivity, probably reflecting the cleaved receptor fragment, was associated with apical endocytic marker and colocalized with clathrin and later with cathepsin D.
Apical endocytosis involves coated pits/vesicles, leads to a subapical compartment, and plays a role in the retrieval of canalicular plasma membrane components for lysosomal degradation.
本研究展示并描述了原位肝细胞的顶端（小管状）内吞途径。
通过逆行灌注经胆总管给予内吞标记物。在去卷积共聚焦免疫荧光图像堆栈上进行与各种内吞区室特异性蛋白的共定位。通过电子显微镜（EM）评估标记蛋白的亚细胞分布。
液相以及膜相关标记物在顶端细胞极容易内化。在电子显微镜水平，标记物最初在60 - 100纳米的微管泡状结构和150 - 200纳米的杯状小泡中被发现，而多泡体和溶酶体在更长时间间隔后被标记。顶端内吞涉及网格蛋白，并将标记物递送至晚期内体（rab7(+)，组织蛋白酶D(+)）以及溶酶体（rab7(-)，组织蛋白酶D(+)）。同时标记基底外侧内吞途径导致两条途径在晚期内体和溶酶体区室重叠。此外，顶端内吞涉及一个亚顶端区室（内吞素 - 78(+)，rab11(+)，聚合免疫球蛋白A受体[pIgA - R(+)]），其通过转胞吞途径经过，从而构成一个交叉点。pIgA - R免疫反应性，可能反映裂解的受体片段，与顶端内吞标记物相关，并与网格蛋白共定位，随后与组织蛋白酶D共定位。
顶端内吞涉及被膜小窝/小泡，导致一个亚顶端区室，并在回收小管状质膜成分进行溶酶体降解中发挥作用。
