Differential agonist-mediated internalization of the human 5-hydroxytryptamine 7 receptor isoforms

The human 5-hydroxytryptamine 7 (5-HT7) serotonin receptor is a class A G-protein coupled receptor that has three isoforms, 5-HT7(a), 5-HT7(b), and 5-HT7(d), which are produced by alternative splicing. The 5-HT7 receptors are expressed in discrete areas of the brain and in both vascular and gastrointestinal smooth muscle. Central nervous system 5-HT7 receptors may play a role in mood and sleep disorders. 5-HT7 receptors show high affinity for a number of antidepressants and typical and atypical antipsychotics. We report here that the human 5-HT7(d) isoform expressed in human embryonic kidney (HEK) 293 cells exhibits a pattern of receptor trafficking in response to agonist that differ from 5-HT7(a) or 5-HT7(b) isoforms. We employed a modification of a live cell-labeling technique to demonstrate that surface 5-HT7(d) receptors are constitutively internalized in the absence of agonist. This is in contrast to 5-HT7(a) and 5-HT7(b) isoforms, which do not show this profound agonist-independent internalization. Indeed, the 5-HT7(d) isoform displays this internalization in the presence of a 5-HT7-specific antagonist. In addition, the human 5-HT7(d) isoform shows a diminished efficacy in stimulation of cAMP-responsive reporter gene activity in transfected cells compared with 5-HT7(a) or 5-HT7(b) receptors expressed at comparable levels. Thus, the carboxy-terminal tail of 5-HT7(d), which is the longest among known human 5-HT7 isoforms, may contain a motif that interacts with cellular transport mechanisms that is distinct from 5-HT7(a) and 5-HT7(b).