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  • Title: Localization and regulated release of Alzheimer amyloid precursor-like protein in thyrocytes.
    Author: Graebert KS, Lemansky P, Kehle T, Herzog V.
    Journal: Lab Invest; 1995 May; 72(5):513-23. PubMed ID: 7745947.
    Abstract:
    BACKGROUND: Dysregulation in the processing of the Alzheimer precursor protein (APP) is thought to be central to the deposition of the beta-A4 peptide and to the pathogenesis of Alzheimer's disease. Expression and release of APP has also been known to mediate cell-matrix interactions and to participate in the regulation of cell proliferation. It has also been shown that APP is a member of a family of closely related proteins. This family comprises different splice-forms of APP and APP-like proteins (APP/APLP). Because of the specific processing of exportable proteins, thyrocytes represent a particularly useful cell type for the study of the processing of APP/APLP (especially proteolysis and iodination) as an indication of cell surface expression and for the study of the regulation of these functions. EXPERIMENTAL DESIGN: Rats were treated in vivo with propylthiouracil, which is known to cause a rise of serum thyroid-stimulating hormone (TSH) levels and maximum stimulation of thyroid function and growth. The expression of APP/APLP was analyzed in rat thyroid tissue and in a continuous cell line (FRTL-5) by immunofluorescence staining and by sodium dodecyl sulfate polyacrylamide gel electrophoresis and immunoblotting. Using FRTL-5 cells, secretion and turnover were analyzed by biosynthetic radiolabeling and immunoprecipitation of APP/APLP. RESULTS: APP/APLP was detected in follicle cells and in the follicle lumen of resting thyroid glands. In propylthiouracil-treated rats, the complete endocytic removal of the luminal content coincided with the pronounced visualization of APP/APLP in the extrafollicular space, where it was associated with proliferating endothelial cells and fibroblasts. In FRTL-5 cells, APP/APLP was localized mainly in the Golgi complex and in compartments along the endocytic pathway, including lysosomes, where degradation of APP/APLP occurred. Mature and immature forms of APP/APLP became iodinated upon reaching the plasma membrane. Part of the extracellular portion of APP/APLP was released by these cells into the culture medium by TSH-dependent cleavage and secretion mechanisms. CONCLUSIONS: The observations show the expression, maturation, and secretion of APP/APLP in thyrocytes and the up-regulation of these processes by TSH. Part of the immature APP/APLP appeared on the cell surface as indicated by its iodination. Apparently, this portion of immature APP/APLP escaped maturation during its transport to the cell surface.
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