Abstract
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 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).
Footnotes
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doi:10.1124/jpet.104.081919.
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ABBREVIATIONS: 5-HT, 5-hydroxytryptamine; h5-HT, human 5-HT; HA, hemagglutinin; PCR, polymerase chain reaction; DMEM, Dulbecco's modification of Eagle's medium; FBS, fetal bovine serum; PBS, phosphate-buffered saline; 5-CT, 5-carboxamidotryptamine; SB 269970, (R)-3-(2-(2-(4-methylpiperidin-1-yl)ethyl)pyrrolidine-1-sulfonyl) phenol.
- Received December 15, 2004.
- Accepted February 10, 2005.
- The American Society for Pharmacology and Experimental Therapeutics
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