Ultrastructural localization of the neurotrophin receptor (TrkA) in cultured rat pheochromocytoma PC12 Cells: Three-dimensional image analysis by high voltage electron microscopy

Nerve growth factor (NGF) is a well-known neurotrophic factor and the NGF signaling through the receptor, TrkA, plays important roles in regulating neuronal differentiation and survival. A recent study has demonstrated that the TrkAs expressed in undifferentiated PC12 cells were associated with caveolae, which were invaginated small pits on the plasma membrane. Caveolae are frequently seen in many cell types such as endothelial cells, fibroblasts and hepatocytes, but few in neurons. In the present study, we performed immunocytochemistry of TrkA in differentiated PC12 cells and analyzed the ultrastructural localization of TrkA by conventional electron microscopy and high-voltage electron microscopic (HVEM) tomography. The TrkA immunoreactivities were mainly associated with the cytoplasmic vesicles (10-30 nm in diameter) and a part of the plasma membrane. The HVEM tomography showed that the TrkA immunoreactivities were often assembled into ring-like structures (400-800 nm in diameter) near the plasma membrane, unlike typical flask-shaped invaginations of caveolae (50-100 nm in diameter). These results suggest that TrkA are not localized in the caveolae, at least in differentiated PC12 cells, but other invaginations are involved in a novel process of internalization of ligand-bound TrkA.