Internalization and recycling pathways of the thyrotropin receptor

Scant information is available to date on the intracellular trafficking of the TSH receptor. In the present study we have used stably transfected L ce lls that express the TSH receptor, I-125-labeled TSH, and antireceptor anti bodies as well as gold-conjugated antireceptor monoclonal antibodies and ho rmone. The latter allowed us to study, by electron microscopy, the cellular distribution and endocytosis of TSH receptor. The receptor was initially l ocalized on the plasmalemma proper and in clathrin-coated pits but was excl uded from smooth vesicles open to the cell surface. It was internalized thr ough clathrin-coated vesicles. Constitutive endocytosis represented 10% of cell surface receptor molecules. Endocytosis was increased 3-fold by incuba tion with hormone. The majority of internalized receptor molecules (90%) wa s recycled to the cell surface, whereas the hormone was degraded in lysosom es. This recycling of receptor was inhibited by administration of monensin. Electron microscopic and confocal microscopic studies were repeated in pri mary cultures of human thyroid cells and showed a distribution, and endocyt osis pathways, very similar to those observed in transfected L cells. A pre vious study has shown the LH receptor to be endocytosed in high proportion and to be degraded in lysosomes. Confocal microscopy and colocalization stu dies with transferrin receptor confirmed that the highly homologous LH and TSH receptors exhibit, when expressed in the same cells, very different cel lular trafficking properties. The use of LH/TSH receptor chimeras showed th at transmembrane-intracellular domains contain information orienting the pr otein toward recycling or degradative pathways. The extracellular domain se ems to play a role in the extent of internalization. These observations sho uld now allow the identification of the molecular signals involved.