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

The three alpha(2)-adrenergic receptor subtypes have distinct tissue distri butions, desensitization properties, and, in some cell types, subtype-speci fic subcellular localization and trafficking properties. The subtypes also differ in their neuronal physiology. Therefore, we have investigated the lo calization and targeting of human alpha(2)-adrenoceptors (alpha(2)-AR) in P C12 cells, which were transfected to express the alpha(2)-AR subtypes A, B, and C. Inspection of the receptors by indirect immunofluorescence and conf ocal microscopy showed that alpha(2A)-AR were mainly targeted to the tips o f the neurites, alpha(2B)-AR were evenly distributed in the plasma membrane , and alpha(2C)-AR were mostly located in an intracellular perinuclear comp artment. After agonist treatment, alpha(2A)- and alpha(2B)-AR were internal ized into partly overlapping populations of intracellular vesicles. Recepto r subtype-specific changes in PC12 cell morphology were also discovered: ex pression of alpha(2A)-AR, but not of alpha(2B)- or alpha(2C)-AR, induced di fferentiation-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 sa me cells with alpha(2A)-AR, indicating that the targeting of receptors to t he tips of neurites is a consequence of a change in PC12 cell membrane prot ein trafficking that the alpha(2A)-subtype induces. The marked agonist-indu ced internalization of alpha(2A)-AR observed in both nondifferentiated and differentiated PC12 cells contrasts with earlier results from nonneuronal c ells and points out the importance of the cellular environment for receptor endocytosis and trafficking. The targeting of alpha(2A)-AR to nerve termin als in PC12 cells is in line with the putative physiological role of this r eceptor subtype as a presynaptic autoreceptor.