Receptor subtype-induced targeting and subtype-specific internalization of human alpha(2)-adrenoceptors in PC12 cells

The three alpha(2)-adrenergic receptor subtypes have distinct tissue distributions, desensitization properties, and, in some cell types, subtype-specific subcellular localization and trafficking properties. The subtypes also differ in their neuronal physiology. Therefore, we have investigated the localization and targeting of human alpha(2)-adrenoceptors (alpha(2)-AR) in PC12 cells, which were transfected to express the alpha(2)-AR subtypes A, B, and C. Inspection of the receptors by indirect immunofluorescence and confocal microscopy showed that alpha(2A)-AR were mainly targeted to the tips of the neurites, alpha(2B)-AR were evenly distributed in the plasma membrane, and alpha(2C)-AR were mostly located in an intracellular perinuclear compartment. After agonist treatment, alpha(2A)- and alpha(2B)-AR were internalized into partly overlapping populations of intracellular vesicles. Receptor subtype-specific changes in PC12 cell morphology were also discovered: expression of alpha(2A)-AR, but not of alpha(2B)- or alpha(2C)-AR, induced differentiation-like changes in cells not treated with NGF. Also alpha(2B)-AR were targeted to the tips of neurites when they were coexpressed in the same cells with alpha(2A)-AR, indicating that the targeting of receptors to the tips of neurites is a consequence of a change in PC12 cell membrane protein trafficking that the alpha(2A)-subtype induces. The marked agonist-induced internalization of alpha(2A)-AR observed in both nondifferentiated and differentiated PC12 cells contrasts with earlier results from non-neuronal cells and points out the importance of the cellular environment for receptor endocytosis and trafficking. The targeting of alpha(2A)-AR to nerve terminals in PC12 cells is in line with the putative physiological role of this receptor subtype as a presynaptic autoreceptor.