Nicotine up-regulates alpha4beta2 nicotinic receptors and ER exit sites via stoichiometry-dependent chaperoning

J Gen Physiol. 2011 Jan;137(1):59-79. doi: 10.1085/jgp.201010532.

Abstract

The up-regulation of α4β2* nicotinic acetylcholine receptors (nAChRs) by chronic nicotine is a cell-delimited process and may be necessary and sufficient for the initial events of nicotine dependence. Clinical literature documents an inverse relationship between a person's history of tobacco use and his or her susceptibility to Parkinson's disease; this may also result from up-regulation. This study visualizes and quantifies the subcellular mechanisms involved in nicotine-induced nAChR up-regulation by using transfected fluorescent protein (FP)-tagged α4 nAChR subunits and an FP-tagged Sec24D endoplasmic reticulum (ER) exit site marker. Total internal reflection fluorescence microscopy shows that nicotine (0.1 µM for 48 h) up-regulates α4β2 nAChRs at the plasma membrane (PM), despite increasing the fraction of α4β2 nAChRs that remain in near-PM ER. Pixel-resolved normalized Förster resonance energy transfer microscopy between α4-FP subunits shows that nicotine stabilizes the (α4)(2)(β2)(3) stoichiometry before the nAChRs reach the trans-Golgi apparatus. Nicotine also induces the formation of additional ER exit sites (ERES). To aid in the mechanistic analysis of these phenomena, we generated a β2(enhanced-ER-export) mutant subunit that mimics two regions of the β4 subunit sequence: the presence of an ER export motif and the absence of an ER retention/retrieval motif. The α4β2(enhanced-ER-export) nAChR resembles nicotine-exposed nAChRs with regard to stoichiometry, intracellular mobility, ERES enhancement, and PM localization. Nicotine produces only small additional PM up-regulation of α4β2(enhanced-ER-export) receptors. The experimental data are simulated with a model incorporating two mechanisms: (1) nicotine acts as a stabilizing pharmacological chaperone for nascent α4β2 nAChRs in the ER, eventually increasing PM receptors despite a bottleneck(s) in ER export; and (2) removal of the bottleneck (e.g., by expression of the β2(enhanced-ER-export) subunit) is sufficient to increase PM nAChR numbers, even without nicotine. The data also suggest that pharmacological chaperoning of nAChRs by nicotine can alter the physiology of ER processes.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Cell Line
  • Cell Membrane / drug effects
  • Cell Membrane / metabolism
  • Endoplasmic Reticulum / drug effects
  • Endoplasmic Reticulum / genetics
  • Endoplasmic Reticulum / metabolism
  • Fluorescence Resonance Energy Transfer / methods
  • Green Fluorescent Proteins / metabolism
  • Mice
  • Microscopy / methods
  • Microscopy, Fluorescence / methods
  • Molecular Chaperones / genetics
  • Molecular Chaperones / metabolism*
  • Nicotine / pharmacology*
  • Protein Subunits
  • Protein Transport / drug effects
  • Receptors, Nicotinic / biosynthesis
  • Receptors, Nicotinic / genetics*
  • Receptors, Nicotinic / metabolism*
  • Tobacco Use Disorder / genetics
  • Tobacco Use Disorder / metabolism
  • Transfection
  • Up-Regulation / drug effects
  • Vesicular Transport Proteins / genetics
  • Vesicular Transport Proteins / metabolism
  • trans-Golgi Network / metabolism

Substances

  • Molecular Chaperones
  • Protein Subunits
  • Receptors, Nicotinic
  • SEC24D protein, human
  • Vesicular Transport Proteins
  • nicotinic receptor alpha4beta2
  • Green Fluorescent Proteins
  • Nicotine