The AAPS Journal 2006; 7 (4) Article 86 (http://www.aapsj.org). Themed Issue: Drug Addiction - From Basic Research to Therapies Guest Editors - Rao Rapaka and Wolfgang Sadée Neuronal Nicotinic Acetylcholine Receptor Expression and Function on Nonneuronal Cells Submitted: August 24 , 2005 ; Accepted: November 22 , 2005 ; Published: January 13, 2006 Lorise C. Gahring1, 2 and Scott W. Rogers 1, 3 1 Geriatric Research Education and Clinical Center, Salt Lake City VAMC, Salt Lake City, Utah 84132 2 The Department of Internal Medicine, University of Utah School of Medicine, Salt Lake City, Utah 84132 3 The Department of Neurobiology and Anatomy, University of Utah School of Medicine, Salt Lake City, Utah 84132 A BSTRACT otine receptors are among the fi rst (if not the fi rst) neu- Of the thousands of proven carcinogens and toxic agents rotransmitter system whose expression is diminished in 1 2-4 contained within a cigarette, nicotine, while being the addic- Alzheimer ’ s disease. Subsequent studies have suggested tive agent, is often viewed as the least harmful of these com- that chronic nicotine administration might in fact play a pounds. Nicotine is a lipophilic molecule whose effects on benefi cial role in slowing the progression of this disease. neuronal nicotinic acetylcholine receptors (nAChR) have While this fi nding is controversial, there is now ample been primarily focused on its physiologic impact within the evidence supporting a therapeutic benefi t from nicotine in 5 confi nes of the brain and peripheral nervous system. How- Parkinson ’ s disease, and as a neuroprotectant to toxic 6-10 ever, recently, many studies have found neuronal nAChRs insults such as excitotoxins or Beta-amyloid derived 10-12 to be expressed on many different nonneuronal cell types peptides. Understanding the mechanistic basis for these 13 throughout the body, where increasing evidence suggests and other similarly interesting fi ndings, including a cogni- 2 they have important roles in determining the consequences tive benefi t from nicotine, would be of obvious importance. of nicotine use on multiple organs systems and diseases as The name “ neuronal ” was based principally on the tissue diverse as ulcerative colitis, chronic pulmonary obstructive source of the DNA libraries from which these receptors 14 disease, and diabetes, as well as the neurologic disorders of were fi rst cloned, the brain, but growing evidence indi- Parkinson ’ s and Alzheimer ’ s disease. This review highlights cates that cells other than neurons throughout the body 15,16 current evidence for the expression of peripheral nAChRs express these receptors including lymphocytes, macro- in cells other than neurons and how they participate in fun- phages, dendritic cells, adipocytes, keratinocytes, endothe- damental processes, such as infl ammation. Understanding lial cells, and epithelial cells of the intestine and lung. This these processes may offer novel therapeutic strategies to extended expression of nAChRs is of importance because, approach infl ammatory diseases, as well as precautions in in addition to their regulation by endogenous agonists such the design of interventional drugs. as acetylcholine, choline, and the exogenous compound nicotine, their impact upon peripheral processes can be quite diverse as exemplifi ed by their ability to in some cases K EYWORDS: nicotine , infl ammation , nicotinic receptors , enhance (Crohn ’ s disease) disease or in other cases dimin- nonneuronal ish (ulcerative colitis) progression.17-20 These apparent con- tradictions in the effects of nicotine are not uncommon and understanding this complex biology will in turn optimize I NTRODUCTION therapeutic benefi t to ensure that neuroprotective therapy Neuronal nicotinic acetylcholine receptors (nAChR) are for one disease does not promote immune dysfunction and ligand-gated ion channels whose genetics and functional the survival of unwanted cells in other tissues. properties have been studied largely for their role in modu- lating neurotransmission. This receptor system has also Acetylcholine Receptors been recognized as a participant in the progression of severe pathologies of the brain. For example, the high affi nity nic- Acetylcholine receptors (see Lindstrom21 and Hogg et al22 ) consist of 2 major subtypes, the muscarinic-activated meta- botropic receptors (second messenger coupled) and the Corresponding Author: Lorise C. Gahring, Division of fast-ionotropic cationic nicotine-activated channel receptors, Geriatrics, SOM 2C110, Department of Internal Medicine, both of which are activated by the endogenous neurotrans- University of Utah School of Medicine, 50 North Medical mitter, acetylcholine. Receptors of the nicotinic subclass can Drive, Salt Lake City, UT 84132 . Tel: (801) 585-6338 ; Fax: be distinguished further as “ muscle ” or “ neuronal. ” While (801) 585-7327 ; E-mail: [email protected] the muscle and neuronal nicotinic receptors exhibit similar E885 The AAPS Journal 2006; 7 (4) Article 86 (http://www.aapsj.org). sensitivity to gating by acetylcholine, the muscle receptor is bind a -bungarotoxin) exhibit almost a 10:1, Ca+2 :Na + per- much less sensitive to nicotine. Hence, at physiological con- meability ratio, which exceeds that of the glutamate N-methyl- centrations, the majority of nicotine ’ s effects are through D-aspartate (NMDA) receptor and the ~4:1 ratio of most neuronal nicotinic acetylcholine receptors (nAChR), and, in other nAChRs. 21 , 22 This fi nding suggests that the activation fact, when nicotine levels are suffi ciently high to act upon of nAChRa 7-type receptors, which also accumulate extra- the muscle receptor (as might occur when smokers concur- synaptically, including in rafts,36 impact upon free intracel- rently use the transdermal nicotine patch23 ), diffi culties in lular calcium and calcium-dependent mechanisms in a breathing and muscle spasms that can result in death may manner quite distinct from other ligand-activated ion chan- occur. nels as well as other nAChRs. In contrast, the majority of The mammalian nAChR family (for review see Lindstrom21 other nAChRs are composed of various combinations of a and Hogg et al22 and references therein) is composed of and b subunits. Prominent among these are receptors com- multiple members (subunits) including 7 subunits that har- posed of at least nAChRa 4 and b 2 subunits that form the 3 bor the principal components of the ligand binding site (a 2, high-affi nity [ H]nicotine binding receptor. A curiosity of a 3, a 4, a 6, a 7, a 9, and a 10) and 4 structural subunits (a 5, nicotine ’ s effect on this receptor is that when ligand is b 2, b 3, and b 4) that impart unique functional and pharma- in excess and present chronically, as in a smoker, the num- cological properties to the receptors. In general, nAChRs ber of binding sites actually increases in a process termed fall into 3 major subgroups, the high-affi nity nicotine bind- “ up-regulation. ” While the mechanism underlying this 21,22,37 ing receptors harboring nAChRa 4, a -bungarotoxin binding process is controversial ; what is clear is that not 38 proteins composed of nAChRa 7, and the receptors of the all nAChR subtypes undergo upregulation, and it is im - autonomic nervous system composed of nAChRa 3/b 4 sub- minently associated with many of the characteristics of 39 units. A particularly interesting aspect of nAChRs is that nAChR function such as those leading to addiction. despite their being fast-excitatory ion channels, they may be localized in many parts of the cell including aggregates in the cell body (somal), presynaptic terminals (where they Nicotine— Agonist or Antagonist? contribute to modulation of neurotransmitter release), and Although nicotine is most often considered to be an agonist in or adjacent to the postsynaptic density. Because of these or activator of nAChRs, it has several effects on receptor diverse locations, their participation in neurotransmission function that complicate this assignment and require atten- can be somewhat indirect as when they affect the amount of tion in any experimental design that employs this com- neurotransmitter released. This modulatory role directly pound. Unlike the normal ligand such as acetylcholine or contributes to the establishment and maintenance of tone choline, which are either rapidly degraded or removed from between the excitatory and inhibitory systems.24-31 Further, the receptor vicinity, nicotine is not readily degraded or the relatively high calcium permeability of these receptors removed. Further, because it is lipophilic, it accumulates in (especially nAChRa 7) appears to contribute to regulating certain tissues well beyond the concentration suggested by second messenger signaling pathways such as the PI3- measuring the serum.40 For example, concentrations of nic- kinase/AKT pathway,12 , 32 activation of transcriptional sys- otine in the brain, owing to the drug ’ s high hydrophobicity, tems such as CREB,33 and certain proteolytic processes.34 , 35 can reach as high as 10 m M even though its serum concen- Consequently, the placement of relatively small numbers of tration rarely exceeds high nanomolar and is more often in nAChRs at key regulatory sites can lead to multiple out- the low nanogram per milliliter range. Just as nicotine parti- comes in terms of normal cell performance and susceptibil- tions and concentrates in the brain due to this lipophilic ity to exogenous challenges or participation in processes nature, other tissues also have elevated levels of nicotine ranging from neurodegeneration to infl ammation. There- compared with blood. It has been reported, using an animal fore, dysregulation or modifi cation of the function of this model of nicotine infusion,41 that the tissue-to-blood ratios system may be expected to be manifested more in modify- of nicotine in different parts of the body are the following: ing biological or metabolic “ set-points ” rather than having brain 3.0, heart 3.7, muscle 2.0, adipose tissue 0.5, kidney the often dramatic on-off effects seen with other neurotrans- 21.6, liver 3.7, lung 2.0, and gastrointestinal tissue 3.5.