Transthyretin localization in cultured and native human retinal pigment epithelium.

The aim of this study was to determine transthyretin subcellular localization in cultured and native human retinal pigment epithelium. Monoclonal and polyclonal antibodies directed against human plasma transthyretin were used to detect transthyretin-specific immunoreactivity in cultured human retinal pigment epithelium. At the light microscopic level, transthyretin-specific immunoreactivity was observed throughout the cytosol with intense perinuclear staining. Nuclear staining was faint, but detectable. Both monoclonal and polyclonal antibodies exhibited similar staining patterns. An electron microscopic immunogold labeling protocol detected transthyretin-specific immunoreactivity in cultured and native human retinal pigment epithelium. Transthyretin-specific immunogold labeling within mitochondrial, apical, basal, nuclear and cytosolic subcellular compartments was quantitated and statistically analyzed. The pattern of transthyretin labeling was similar for each antibody, and comparable between cultured and native human retinal pigment epithelium. Transthyretin labeling was observed in mitochondrial and nuclear compartments, and in close apposition to both apical and basal membranes. Transthyretin labeling density was highest in the mitochondrial compartment and was significantly greater than labeling in all other compartments. Detection of transthyretin labeling in mitochondrial and nuclear compartments suggests an intracellular role for transthyretin in human retinal pigment epithelium, possibly as a cytoplasmic carrier protein for thyroxine.