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Emerging Concepts in Ion Channel Biophysics
Poster Abstracts
74
26-POS
Board 26
Alpha 7 (Α7) Nicotinic Acetylcholine Receptors (Nachrs) Signal as Both Lgics and Gpcrs
Joseph Farley
, Jayharsh Panchal, Kristi DeBoeuf, Mohammad F. Islam.
n/a, Bloomington, USA.
Α7 nAChRs are widely distributed throughout the nervous system, playing important roles in
learning & memory, and are also implicated in a variety of disease and neurodegenerative
processes including schizophrenia, Alzheimer’s and Parkinson’s disease, inflammation, cancer,
and nicotine addiction. A variety of compounds (agonists, antagonists, PAMs) produce
functional and numerical upregulation of α7 Rs in different cells, implicating multiple signaling
pathways and mechanisms. Prolonged nicotine exposures also upregulate α7 nAChRs, which
may contribute to nicotine dependence/addiction. Previously, we found ~ 2-fold functional and
numerical upregulation of alpha7 nAChRs in
Xenopus
oocytes following 12 hr of 100 µM
nicotine and 7 hr washout. This nicotine-upregulation was dependent upon intracellular Ca
2+
,
being abolished by BAPTA-AM, and involved several Ca
2+
-dependent enzymes (e.g., PP2B,
PKC). But upregulation was independent of Ca
2+
influx, being unaffected by removal of
extracellular Ca
2+
. Similar to glycine receptor 1 channel, the α7 nAChR contains a conserved G
protein-binding cluster (GPBC) in the M3-M4 loop. Mutation of α7 nAChR GPBC (RMKR to
AAAA), to block interaction of Gαq and Gβγ with the GPBC, completely blocked nicotine-
upregulation of α7 nAChRs. Basal receptor expression levels, peak current amplitude, net
charge, and kinetics were all unaffected in the mutants. Conversely, 12 hr exposure to a cell-
permeable, competitive antagonist of α7 nAChRs , methyllycaconitine (MLA; 1 µM); a
calcineurin inhibitor cyclosporine A (CsA) (10 µM); or 2 hr exposure to a dynamin inhibitor -
Dynasore (80 µM), all produced ~2-fold upregulation of both wt and mutant α7 Rs. Collectively,
our results indicate that α7 nAChRs function as both ionotropic and metabotropic receptors.
Moreover, nicotine appears to modulate the upregulation of α7 nAChRs, through GPCR
signaling, via Ca
2+
-dependent inhibition of endocytosis of plasma membrane Rs.