Subcellular distribution, secretion, and posttranslational modifications of clusterin in thyrocytes

Area of Science:

• Endocrinology
• Cell Biology
• Glycobiology

Background:

• Thyrocytes synthesize and secrete various proteins, including glycoproteins.
• Clusterin is a known secretory glycoprotein with diverse cellular functions.
• Thyroid stimulating hormone (TSH) regulates thyroid cell activity.

Purpose of the Study:

• To identify and characterize a prominent secretory glycoprotein in porcine thyrocytes.
• To investigate the regulation of clusterin synthesis and secretion by TSH.
• To elucidate the cellular localization and potential functions of clusterin in thyrocytes.

Main Methods:

• Microsequencing for protein identification.
• TSH treatment to study regulation.
• Metabolic labeling with [(32)P]phosphate.
• Binding assays with cation-independent mannose 6-phosphate receptor.
• Immunolabeling and colocalization studies.
• Biosynthetic iodination.

Main Results:

• A secretory glycoprotein identified as clusterin was detected in porcine thyrocyte culture medium.
• TSH tightly regulates clusterin synthesis and secretion, with cell-associated fractions.
• Clusterin contains N-bound glycans and M6P moieties, suggesting lysosomal targeting.
• Clusterin localizes to apical surfaces, colocalizing with gp330/megalin, and undergoes biosynthetic iodination.
• Basolateral localization suggests involvement in cell adhesion and interactions.

Conclusions:

• Clusterin is a TSH-regulated secretory glycoprotein in porcine thyrocytes.
• Clusterin exhibits complex posttranslational modifications, including glycosylation and phosphorylation.
• Thyroid clusterin plays roles in apical cell surface functions and potentially cell adhesion via basolateral localization.