Intracellular Protein Transport to the Thyrocyte Plasma Membrane: Potential Implications for Thyroid Physiology

We present a snapshot of developments in epithelial biology that may prove helpful in understanding cellular aspects of the machinery designed for the synthesis of thyroid hormones on the thyroglobulin precursor. The functional unit of the thyroid gland is the follicle, delimited by a monolayer of thyrocytes. Like the cells of most simple epithelia, thyrocytes exhibit specialization of the cell surface that confronts two different extracellular environments—apical and basolateral, which are separated by tight junctions. Specifically, the basolateral domain faces the interstitium/bloodstream, while the apical domain is in contact with the lumen that is the primary target for newly synthesized thyroglobulin secretion and also serves as a storage depot for previously secreted protein. Thyrocytes use their polarity in several important ways, such as for maintaining basolaterally located iodide uptake and T₄ deiodination, as well apically located iodide efflux and iodination machinery. The mechanisms by which this organization is established, fall in large part under the more general cell biological problem of intracellular sorting and trafficking of different proteins en route to the cell surface. Nearly all exportable proteins begin their biological life after synthesis in an intracellular compartment known as the endoplasmic reticulum (ER), upon which different degrees of difficulty may be encountered during nascent polypeptide folding and initial export to the Golgi complex. In these initial stages, ER molecular chaperones can assist in monitoring protein folding and export while themselves remaining as resident proteins of the thyroid ER. After export from the ER, most subsequent sorting for protein delivery to apical or basolateral surfaces of thyrocytes occurs within another specialized intracellular compartment known as the trans-Golgi network. Targeting information encoded in secretory proteins and plasma membrane proteins can be exposed or buried at different stages along the export pathway, which is likely to account for sorting and specific delivery of different newly-synthesized proteins. Defects in either burying or exposing these structural signals, and consequent abnormalities in protein transport, may contribute to different thyroid pathologies.