Rat nicotinic ACh receptor α7 and β2 subunits co‐assemble to form functional heteromeric nicotinic receptor channels

Abstract

Rat hippocampal interneurons express diverse subtypes of functional nicotinic acetylcholine receptors (nAChRs), including α7‐containing receptors that have properties unlike those expected for homomeric α7 nAChRs. We previously reported a strong correlation between expression of the α7 and of the β2 subunits in individual neurons. To explore whether co‐assembly of the α7 and β2 subunits might occur, these subunits were co‐expressed in Xenopus oocytes and the functional properties of heterologously expressed nAChRs were characterized by two‐electrode voltage clamp. Co‐expression of the β2 subunit, both wild‐type and mutant forms, with the α7 subunit significantly slowed the rate of nAChR desensitization and altered the pharmacological properties. Whereas ACh, carbachol and choline were full or near‐full agonists for homomeric α7 receptor channels, both carbachol and choline were only partial agonists in oocytes expressing both α7 and β2 subunits. In addition the EC50 values for all three agonists significantly increased when the β2 subunit was co‐expressed with the α7 subunit. Co‐expression with the β2 subunit did not result in any significant change in the current‐voltage curve. Biochemical evidence for the co‐assembly of the α7 and β2 subunits was obtained by co‐immunoprecipitation of these subunits from transiently transfected human embryonic kidney (TSA201) cells. These data provide direct biophysical and molecular evidence that the nAChR α7 and β2 subunits co‐assemble to form a functional heteromeric nAChR with functional and pharmacological properties different from those of homomeric α7 channels. This co‐assembly may help to explain nAChR channel diversity in rat hippocampal interneurons, and perhaps in other areas of the nervous system.

SSR180711, a Novel Selective α7 Nicotinic Receptor Partial Agonist: (1) Binding and Functional Profile

“…Recent data have demonstrated that heteromeric assembly of recombinant a7 and b2 subunits could form functional receptors, displaying some pharmacological differences with homomeric ones (Azam et al, 2003;Khiroug et al, 2002). Given the high expression level of a7 and b2 subunits in the hippocampus, it was therefore of interest to Profile of SSR180711 at a7 nicotinic receptors B Biton et al test the activity and efficacy of SSR180711 at native a7 nAChRs in cultured hippocampal neurons, possibly formed of a mixture of homomeric and heteromeric receptors (Azam et al, 2003).…”

“…Although there is strong evidence that native a7 n-AChRs are homopentamers, some recent data suggest that rat a7 and b2 subunits can form functional heteromeric receptors (Khiroug et al, 2002). In addition, an homomeric a7 nAChR variant called a7-2 has been described to be expressed in some peripheral and central neurons, displaying a-BTX sensitivity and an slower rate of desensitization (Severance and Cuevas, 2004).…”

Desensitization of neuronal nicotinic receptors

“…These receptors desensitize with 10-fold slower time constants ( Ϸ 200ϩ ms), which implies that other factors must regulate this process. Association with other subunits may partially explain this behavior: coexpression of ␤2 and ␣7 in oocytes slows the onset of desensitization twofold (Khiroug et al, 2002). Alternatively, post-translational modification induced by phosphorylation of ␣7 subunits (Moss et al, 1996) could alter desensitization (Huganir and Greengard, 1990;Swope et al, 1992), although there is no evidence at present that phosphorylation alters the kinetics of desensitization of ␣7 subunit-containing receptors (Alkondon et al, 1994;Lui and Berg, 1999).…”

Nicotinic α7 receptors: Synaptic options and downstream signaling in neurons

“…Heteromeric receptors containing the ␣7 and ␤2 gene products can be produced by heterologous coexpression, and the receptors have diminished rates of desensitization (Khiroug et al, 2002). Native receptors containing coassembled ␣7 and ␤2 subunits have not been described, but examples of slowly desensitizing native ␣7-nAChRs have been reported (Cuevas and Berg, 1998;Yu and Role, 1998;Cuevas et al, 2000).…”