Inside-out neuropharmacology of nicotinic drugs

Abstract

Upregulation of neuronal nicotinic acetylcholine receptors (AChRs) is a venerable result of chronic exposure to nicotine; but it is one of several consequences of pharmacological chaperoning by nicotine and by some other nicotinic ligands, especially agonists. Nicotinic ligands permeate through cell membranes, bind to immature AChR oligomers, elicit incompletely understood conformational reorganizations, increase the interaction between adjacent AChR subunits, and enhance the maturation process toward stable AChR pentamers. These changes and stabilizations in turn lead to increases in both anterograde and retrograde traffic within the early secretory pathway. In addition to the eventual upregulation of AChRs at the plasma membrane, other effects of pharmacological chaperoning include modifications to endoplasmic reticulum stress and to the unfolded protein response. Because these processes depend on pharmacological chaperoning within intracellular organelles, we group them as "inside-out pharmacology". This term contrasts with the better-known, acute, "outside-in" effects of activating and desensitizing plasma membrane AChRs. We review current knowledge concerning the mechanisms and consequences of inside-out pharmacology. This article is part of the Special Issue entitled 'The Nicotinic Acetylcholine Receptor: From Molecular Biology to Cognition'.

Keywords: Chaperoning; Nicotine; Nicotine addiction; Nicotinic receptors; Unfolded protein response; Upregulation.

Figure 1

Structures of agonists (A), competitive antagonists (CA), non-competitive antagonists (NCA), and positive allosteric modulators (PAM) of AChRs. With the exception of d-tubocurarine, all molecules shown have been reported to upregulate AChRs.

Figure 2. Three possible results of nicotinic ligand-AChR binding in the ER

(A) Nicotinic ligand binding eventually favors stable, high-affinity states (a “chaperone”). (B) Nicotine may displace lynx, directing AChRs toward cholesterol-poor domains (an “escort” or “abductor”). (C) Nicotinic ligand binding at subunit interface acts as a maturational enhancer (a “matchmaker”) and results in the increased assembly of stable pentamers.

Figure 3. Evidence Supporting Inside-out pharmacology of nicotine and nicotinic ligands

(1) Nicotine permeates lung epithelium, blood brain barrier and permeates cell membranes to enter intracellular organelles. (2) Nicotine enhances maturation of pentameric AChRs, increasing assembly in the ER (Kuryatov et al., 2005; Sallette et al., 2005). (3) ER retention is necessary for upregulation (Henderson et al., 2014; Srinivasan et al., 2011). (4) Cycling between the Golgi and ER is necessary for upregulation (Henderson et al., 2014). (5) Nicotinic ligands change the area of the peripheral ER (Henderson et al., 2014; Srinivasan et al., 2011). (6) The changes in AChR stoichiometry have occurred by the time AChRs have reached the Golgi (Henderson et al., 2014; Srinivasan et al., 2011). (7) Nicotine enhances the PM insertion rate of vesicles carrying α4β2 and α6β2β3 AChRs (Henderson et al., 2014; Richards et al., 2011). (8) Nicotinic ligands have differential effects on PM stoichiometry (Henderson et al., 2014; Nichols et al., 2014; Richards et al., 2012; Richards et al., 2011; Srinivasan et al., 2011; Srinivasan et al., 2012). (9) Nicotine and cytisine upregulate α4β2 and α6β2β3 AChRs at concentrations that activate ≤0.4% of PM AChRs (Henderson et al., 2014; Richards et al., 2012; Richards et al., 2011; Srinivasan et al., 2011). (10) Quaternary ammonium nicotinic ligands that permeate membranes poorly upregulate AChRs more slowly than nicotine and other tertiary ammonium ligands (Kuryatov et al., 2005). (11) Nicotine increases the number of trans-Golgi network bodies (Henderson et al., 2014; Srinivasan et al., 2011). (12) Nicotine enhances α4β2 AChR glycosylation. (13) β2 AChR subunit mutations that enhance ER exit change stoichiometry, similar to nicotine (Srinivasan et al., 2011). (14) Blocking proteasome activity upregulates AChRs. (15) Nicotine enhances the number of ER exit sites (Henderson et al., 2014; Srinivasan et al., 2011). (16) ER exit sites are increased by β2 AChR subunit M3-M4 loop mutations that introduce ER exit motifs (Srinivasan et al., 2011). (17) ER exit sites are increased by β2 AChR subunit M3-M4 loop mutations that eliminate ER retention motifs (Srinivasan et al., 2011). (18) Nicotinic ligands decrease ATF6 translocation to the nucleus (Srinivasan et al., 2012). (19) Nicotinic ligands decrease eIF2α phosphorylation (Srinivasan et al., 2012). (20) Nicotinic ligands reduce ER stress at concentrations that activate ≤0.4% of PM α4β2 AChRs. Brown font denotes events involved with upregulation of PM AChRs; Blue font denotes events involved with upregulation and the reduced unfolded protein response.