PubMed: 19293145

Nicotinic acetylcholine receptor signalling: roles in Alzheimer's disease and amyloid neuroprotection.
Pharmacological reviews
Brown LA | Buckingham SD | Jones AK | Sattelle DB

Evidence ff46fbb47f

There is, however, a consensus that cholinesterase inhibitors perform measurably, but modestly, in slowing the progression of AD (Raina et al., 2008), one meta-analysis estimating their efficacy to amount to saving 2 months per year in the progression of the disease (Trinh et al., 2003).

Evidence c704600bea

For example, alpha4-specific agonists protect porcine small retinal ganglion cells against L-glutamate toxicity (Thompson et al., 2006), whereas alpha7 nAChRs protect large retinal ganglion cells (Wehrwein et al., 2004) against L-glutamate toxicity.

Evidence fdd72359d8

Genistein, a phytoestrogen, protects SH-SY5Y cells (Bang et al., 2004) as well as cultured hippocampal neurons (Zeng et al., 2004) from Abeta toxicity. However, in addition to its action on estrogen receptors, genistein is also a general tyrosine kinase inhibitor that protects cultured neurons from L-glutamate toxicity (Kajta et al., 2007).

Evidence de53ba1ecd

For example, non-alpha7 nAChRs protect cultured nigral dopaminergic neurons from toxicity induced by 1-methyl-4-phenylpyridinium, a neurotoxin that selectively damages nigrostriatal dopaminergic neurons (Jeyarasasingam et al., 2002), and this effect is not mediated by alpha7 receptors.

Evidence e6da513e53

There is evidence that nicotine’s neuroprotective effects can be mediated through tumor necrosis factor-alpha (TNF-alpha). Application of either nicotine or TNF-alpha protects cultured mouse embryonic cortical neurons from N-methyl-D-aspartate (NMDA) toxicity, but coapplication of both does not.

Evidence b13037fc5e

Furthermore, nicotinic activation of ERK-1/2 promotes survival of cultured murine spinal cord neurons, and the blocking of ERK-1 prevents nicotine’s antiapoptotic action (Toborek et al., 2007). Likewise, the alpha7-specific agonist A-582941 induces phosphorylation of ERK-1/2 in PC12 cells and in mouse brain, and this is completely blocked by the mitogen-activated protein kinase 1 inhibitor SL327 (Bitner et al., 2007).

Evidence c1769a4959

An increasing ratio of the full-length, 1–42 peptide to the 1–40 form is associated with disease (Kumar-Singh et al., 2006), and mutations underlying familial forms of AD either increase this ratio or increase the amount of Abeta secreted.

Evidence d4c32fb728

Abeta1–42 activates heterologously expressed nAChRs (Dineley et al., 2002), and Abeta25–35 has been shown to activate non-alpha7 nAChRs in rat basal forebrain neurons (Fu and Jhamandas, 2003) and to evoke a alpha7- mediated calcium increases in presynaptic terminals isolated from rat hippocampus and neocortex (Dougherty et al., 2003).

Evidence 70e9651cd1

It is generally agreed that the beta-amyloid peptide (Abeta) plays an important role in the development of AD. The brains of patients with AD contain deposits of Abeta, and Abeta is toxic to cultured neurons (Kihara et al., 1997a; Yao et al., 2005). In addition, mice transgenically overexpressing Abeta or with mutations that enhance Abeta aggregation show many of the symptoms of AD (Hsiao et al., 1996; van Groen et al., 2006).

Evidence d305b9e2c4

APP and APP/presenilin-1 (PS-1) mice do not show neurodegeneration (Irizarry et al., 1997) and yet show several features of AD, including accumulation of plaques and defects in learning (Hsiao et al., 1996), suggesting that many features of AD are not the result of neuronal loss. These animals nonetheless have swollen cholinergic nerve terminals at 12 months, suggesting defective nerve sprouting (Hernandez et al., 2001).

Evidence c6e42370e4

In SHSY5Y cells, RNA interference (RNAi) knockdown of alpha7 enhanced Abeta toxicity (Qi et al., 2007), and alpha7 antagonists, but not alpha4beta2 antagonists, block galantamine protection of cultured rat neurons (Kihara et al., 2004). Donepezil protects cultured rat cortical neurons against Abeta toxicity through both alpha7 and non-alpha7 nAChRs (Takada et al., 2003). It is therefore likely that alpha7 nAChRs are the primary mediators of nicotine neuroprotection, but in some cells, non-alpha7 subtypes are also likely to contribute.

Evidence 93f9f01a2c

Thus, it seems that nAChRs may play a role in mediating Abeta toxicity through synergistic mechanisms; in addition to possible direct interactions (binding), nAChRs may also result in accelerated cell death through enhancing intracellular Abeta accumulation.

Evidence 2407dc2e56

There is abundant evidence that Abeta also affects cholinergic signaling in the brain. Recent studies indicate that brain nAChRs are not only affected by Abeta but can also initiate signaling pathways that protect against Abeta toxicity (Kihara et al., 1997b; Takada et al., 2003; Arias et al., 2005; Akaike, 2006; Meunier et al., 2006; Dineley, 2007; Liu et al., 2007).

Evidence 070cd70621

Consequently, there is mounting evidence that Abeta affects cholinergic signaling independent of its cytotoxic action. For example, Abeta blocks long-term potentiation, a cellular correlate of learning, through activation of JNK and p38MAPK (Wang et al., 2004).

Evidence f676dcb515

Recent research interest has focused on the role of calcium dyshomeostasis in AD (Green and LaFerla, 2008); for instance, genetic links with the regulation of cytosolic calcium have been identified (Dreses- Werringloer et al., 2008). Thus nAChRs may provide a link between Abeta and disruption of calcium homeostasis.

Evidence 337f31e98f

Although there is abundant evidence that Abeta can affect nAChR function, studies disagree as to whether Abeta is an antagonist or an agonist at nAChRs (these findings are summarized in Table 1). For example, Abeta has been reported to inhibit single-channel nicotinic receptor currents in rat hippocampal interneurons (Pettit et al., 2001) as well as currents recorded from human alpha7 receptors heterologously expressed in Xenopus laevis oocytes (Tozaki et al., 2002; Grassi et al., 2003; Pym et al., 2005). Abeta, however, activates a mutant (L250T) of the alpha7 receptor—this mutant conducts current in the desensitized state, indicating that Abeta may exert its antagonistic action through receptor desensitization (Grassi et al., 2003).

Evidence 94c3501e50

Similar effects of Abeta on nAChR expression have been confirmed in studies using cultured cells; Abeta causes a reduced expression of nAChRs in PC12 cells (Guan et al., 2001), and alpha4, alpha3, and alpha7 expression are all increased in cultured rat astrocytes (Xiu et al., 2005).

Evidence 32adf8f97a

Tg2576 mice expressing human Abeta show reduced [3H]cytisine binding (a label of nAChRs) in the cortex at 17 months after birth (Apelt et al., 2002). In contrast, however, levels of alpha7 or alpha4 subunits were unchanged in double-mutant Swedish APP/PS-1 mice as determined by radiolabeled cytosine (alpha4beta2) or alpha-bungarotoxin (alpha7) binding (Marutle et al., 2002).

Evidence 6cc250ac61

In addition, not only have alpha7 nAChRs been found colocalized with plaques (Wang et al., 2000b) but alpha7 and alpha4 subunits are also positively correlated with neurons that accumulate Abeta (Wevers et al., 1999).

Evidence 9a580e99c5

Curiously, although most studies are in agreement that nAChRs need to be activated to mediate their protective effects, mouse cortical neurons are protected by the alpha7 antagonist methyllycaconitine (Martin et al., 2004), raising the possibility that neuroprotection by alpha7 agonists may be through desensitization rather than activation of this rapidly desensitizing receptor. This would be consistent with the alpha7- dependent activation of intracellular signaling pathways by Abeta (Bell et al., 2004), but the opposite effects on cell survival exerted by Abeta and nicotine means that other mechanisms must be sought, such as ligand-specific coupling to downstream signaling pathways.

Evidence c4e13a8db2

In contrast, Small et al. (2007) found no displacement of alpha-BTX from SH-SY5Y cells (a cell line very closely related to that used by Wang et al.) by either amyloid or methyllycaconitine. Wang et al. (2000b) also showed similar staining of human AD cortical neurons by alpha7 and Abeta antibodies in double immunofluorescence, suggesting that in human cortical neurons, alpha7 and Abeta are closely associated, although such an approach does not prove direct binding. However another study (Small et al., 2007) showed no displacement of labeled alpha-bungarotoxin from cell lines expressing rat alpha7 nAChRs.

Evidence cba36d446a

Abeta action on nAChRs depends on subunit composition; it has been reported to block alpha7, transiently potentiate alpha4beta2 before blocking, and to have no action on alpha3beta4 (Pym et al., 2005). However, in contrast to its reported transient enhancement when expressed in oocytes, an inhibition of alpha4beta2 has been reported when expressed in human SH-EP1 cells (Wu et al., 2004).

Evidence 2dde416081

Again, despite numerous reports of a block of alpha7, one study indicated that Abeta failed to block alpha7, even though it blocked alpha4beta2, alpha2beta2 and alpha4alpha5beta2 receptors (Lamb et al., 2005).

Evidence a3140e7c08

It has also been observed that although Abeta inhibits recombinant human and mouse alpha7 nAChRs, transgenic mice overexpressing human Abeta express functional alpha7 nAChRs, and the amplitude of alpha7- mediated currents is no different from that of wild-type mice (Spencer et al., 2006).

Evidence aea0cfdeeb

From these findings, it would seem that FYN plays a neuroprotective role. However, FYN may also play a paradoxical role in Abeta toxicity. Indeed, Abeta activates both FYN and the PI3K cascade (Williamson et al., 2002), whereas germline knockout of FYN is neuroprotective in mice (Lambert et al., 1998; Chin et al., 2004). FYN knockout protects mature mouse neurons in organotypic central nervous system cultures (Lambert et al., 1998).

Evidence 8143e20c52

FYN physically interacts with and phosphorylates tau protein, and the affinity of this physical interaction is enhanced in AD-associated mutations in tau protein (Bhaskar et al., 2005). Abeta rapidly induces tyrosine phosphorylation of many proteins (including tau protein) in human and cultured rat cortical neurons (Williamson et al., 2002). This phosphorylation is concomitant with phosphorylation and inactivation of focal adhesion kinase 1 (FADK1, a major downstream target of FYN), is blocked by inhibitors of SRC kinases and PI3K, and involves FYN associating physically with FADK1 (Williamson et al., 2002).

Evidence 217fdca8a6

Calcium signaling pathways are involved both in the toxic action of Abeta and in the protection against that toxicity offered by nicotinic ligands. Given that alpha7 homomeric nAChRs are much more permeable to calcium ions than are most other nAChRs (Bertrand et al., 1993), it is to be expected that nicotinic neuroprotection mediated by nAChRs, notably alpha7, would depend upon the activation of calcium signaling pathways. ABT-418 is a nicotinic agonist that protects primary rat cortical neurons from glutamate toxicity through its activation of alpha7 nAChRs, and this is blocked when calcium is removed from the extracellular medium (Donnelly-Roberts et al., 1996).

Evidence aef1bdb430

It has long been known that cognitive decline in AD correlates well with synaptic loss (Lue et al., 1999), and it has been shown directly that soluble Abeta inhibits synaptic plasticity (Rowan et al., 2004).

Evidence 723664498d

For example, a recent study has shown that alpha7-specific ligands rescue the Abeta-induced decrease in neurite outgrowth of cultured mouse neurons (Hu et al., 2007).

Evidence 804504ab28

ApoE-epsilon4, but not ApoE-epsilon3, disrupts carbachol-stimulated phosphoinositol (PI) hydrolysis and so does Abeta and Abeta/ApoE-epsilon4 complexes in SH-SY5Y cells (Cedazo- Mínguez and Cowburn, 2001). The effect of Abeta and its ApoE complex on PI hydrolysis were blocked by estrogen, and this disruption was itself blocked by wortmannin, suggesting that PI3K mediates estrogen’s effect on PI hydrolysis.

Evidence f88ee25c04

Whatever the mechanism of uptake, it is interesting to note that the signaling pathways evoked by the accumulation of intracellular Abeta resemble those evoked by extracellularly applied Abeta: transgenic rats overexpressing Abeta intraneuronally display elevated levels of phosphorylated ERK2 (Echeverria et al., 2004), as do rat hippocampal slices in response to bath-applied Abeta (Dineley et al., 2001). Again, bath-applied Abeta causes an increase in BAX and a decrease in BCL2 expression in neurons or neuronlike cell lines (Koriyama et al., 2003; Clementi et al., 2006).

Evidence ff15750384

AKT interacts with BAD to regulate apoptosis and, interestingly, also has many interacting partners in the insulin signaling pathway. Abeta increased activity of BAD, lowered the activity of the antiapoptotic protein BCL2, in rat hippocampal neurons in primary culture (Koriyama et al., 2003) and has been shown to be toxic to human neuroblastoma cells by increasing BAX activity and decreasing BCl-2 activity (Clementi et al., 2006).

Evidence 438d1c0f92

It is also noteworthy that Abeta-induced tau protein phosphorylation in PC12 cells is inhibited not only by alpha7 agonists, as would be predicted from the role of alpha7 nAChRs in neuroprotection, but also by alpha-bungarotoxin (Hu et al., 2008), as might be predicted if the competition by alpha-bungarotoxin for the Abeta site blocked a direct action of Abeta on nAChRs. It is therefore possible that the toxicity of Abeta is mediated, at least in part, through a direct physical interaction between Abeta and nAChRs.

Evidence c9f8dc5c52

In neuroblastoma cells, as well as cultured hippocampal neurons, Abeta activates JNK and ERK, and blocking these prevents Abeta hyperphosphorylating tau protein, as does alpha7 antisense oligonucleotides or alpha7 antagonists, suggesting that Abeta may trigger tau protein phosphorylation through ERK and JNK via alpha7 receptors (Wang et al., 2003b). Abeta leads to phosphorylation of AKT in cultured mouse neurons through a mechanism that requires alpha7 nAChRs (Abbott et al., 2008), AKT phosphorylation levels returning to baseline upon prolonged application of Abeta.

Evidence 0b005c992c

For instance, in Abeta-overexpressing mice (PDAPP derived from a heterogeneous background comprising the strains C57BL/6J, DBA/2J, and Swiss-Webster), Abeta seems to target the high-affinity choline transporter (Bales et al., 2006).

Evidence fc0783eb72

The most comprehensive study of the effects of Abeta at different concentrations showed that at 10 pM, Abeta evoked an inward current mediated by rat alpha7 nAChRs expressed in X. laevis oocytes, whereas at 100 nM, Abeta blocked nicotine responses through desensitization (Dineley et al., 2002).

Evidence 83bc14011f

Intracerebral injection of Abeta into rats resulted in a loss of alpha4 and alpha7 subunits as measured by Western blotting but an increase in alpha7 mRNA (Liu et al., 2008), again suggesting that Abeta directly reduces expression of alpha7 nAChRs through mechanisms other than reduced mRNAproduction, although caution should be exercised in interpreting quantitative data from Western blot studies. It is noteworthy that a combined patch-clamp and in situ hybridization study of dissociated human brain tissue (obtained as route-of-access tissue removed during surgery) indicated that neurons near Abeta plaques retained alpha4 and alpha7 mRNA transcripts, whereas these transcripts were absent in neurons burdened with hyperphosphorylated tau protein (Wevers et al., 1999).

Evidence 0914495c2a

For example, nicotine effectively protects wild-type mice, but not alpha4-knockout mice, against methamphetamine-evoked neurodegeneration (Ryan et al., 2001).

Evidence 85515f6902

In addition, APP or APP/PS-1 double-mutant mice have normal or even enhanced levels of ChAT and an unchanged cholinergic cell count (Hernandez et al., 2001). However, a double-mutant mouse expressing the Swedish APP and overexpressing human AChE showed enhanced mRNA levels of alpha7 in brain and adrenal medulla, although in brain tissue this enhancement declined with age. In this same mouse, there was no alteration in mRNA levels for alpha4, and an increase in alpha3 has also been observed in the brain and the adrenal medulla (Mousavi and Nordberg, 2006), a pattern similar to that seen in Abeta single-mutant mice (Bednar et al., 2002), suggesting that it is not attributable to the human AChE.

Evidence 954bdffa59

It has been shown that the alpha7 receptors, but not the alpha3beta2 receptors, specifically trigger calcium release from intracellular stores by activating ryanodine receptors. Such a specific functional coupling of alpha7 receptors and ryanodine-sensitive stores may provide another site of therapeutic intervention. However, the sustained calcium rise seen in these cells upon prolonged nicotine administration, which is more likely to be of relevance to neuroprotection than short-term responses, is more dependent upon the activation of inositol 1,4,5-triphosphate receptors (Dajas-Bailador et al., 2002a), which are also a target for phosphorylation by FYN (Cui et al., 2004).

Evidence ae124fe643

The discovery that nicotine, a ligand acting at nAChRs, and its mimetics can protect neurons against Abeta toxicity (Kihara et al., 1998) is of interest, especially in view of the observation that nicotine also enhances cognition (Rusted et al., 2000). Nicotinic receptors play a particularly prominent role in nicotine protection. The protective effect is blocked by the nicotinic antagonists dihydro-beta-erythroidine and mecamylamine (Kihara et al., 2001; Takada- Takatori et al., 2006).

Evidence 44018ac6db

Nicotine protection of cultured rat cortical neu- rons against Abeta toxicity is blocked by the alpha4beta2 antagonist, dihydro-beta-erythroidine (Kihara et al., 1998).

Evidence 76f549dd9f

Nicotinic neuroprotection against non-Abeta toxicity is also mediated largely through alpha7 nAChRs. alpha7 nAChRs protect PC12 cells against ethanol toxicity (Li et al., 1999a) and from cell death associated with serum depletion (Ren et al., 2005); they protect cultured neurons against glutamate-induced excitotoxicity (Kaneko et al., 1997) and hippocampal slices against oxygen and glucose deprivation (Egea et al., 2007) through the activation of alpha7 nAChRs (Rosa et al., 2006).

Evidence b8dff179f4

Current licensed pharmacological treatments for AD consist largely of three acetylcholinesterase (AChE) inhibitors: rivastigmine, galantamine, and donepezil (Aguglia et al., 2004; Ritchie et al., 2004), although memantine, a blocker of L-glutamate receptors of the Nmethyl- D-aspartate (NMDA) subtype, is also deployed in late stages of the disease

Evidence 6b92827d94

In addition to nicotine, donepezil and rivastigmine, AChE inhibitors currently used as treatments for mild or moderate AD under the brand names of Aricept and Exelon, also protect cultured neuroblastoma cells from the toxic effects of Abeta. Although there is evidence that, in addition to inhibiting AChE, these ligands are also allosteric modulators of nAChRs (Schrattenholz et al., 1996; Coyle et al., 2007), it has not been established whether these AChE inhibitors protect neurons by their actions on alpha7 nAChRs rather than by simply inhibiting AChE, thereby elevating ACh in the medium.

Evidence 40c5813bb6

Donepezil, which protects cultured rat cortical neurons, when applied for 4 days resulted in an up-regulation of alpha4 and alpha7 nAChRs with the result that donepezil was even more potently protective (Kume et al., 2005).

Evidence aecc630fa1

The AD therapeutic AChE inhibitors donepezil, galantamine, and tacrine increase BCL2 expression when applied to cultured neuronal cells (Arias et al., 2004; Takada-Takatori et al., 2006). In these cells, nicotine promotes cell survival and causes the phosphorylation of the proapoptotic protein Bcl2-associated X protein (BAX), through the PI3K/AKT pathway, reducing the movement of BAX from the cytosol to the mitochondria and inhibiting its apoptotic activity (Xin and Deng, 2005).

Evidence 4c90a4a48e

The loss of nAChR subunits, as determined by [3H]- epibatidine binding, seems to take place after the transition from mild cognitive impairment (MCI) to AD (Sabbagh et al., 2006), although the loss of epibatidine binding did not correlate with decline in memory, cognitive performance, or with the development of neurofibrillary tangles or plaques (Sabbagh et al., 2001).

Evidence c6dca8bfaf

In support of this notion, beta-estradiol protects PC12 cells from amyloid toxicity, and this is prevented when alpha7 nAChRs are blocked with methyllycaconitine (Svensson and Nordberg, 1999).

Evidence c05ef59fca

Nicotine stimulates the secretion of betaAPP, which is trophic and neuroprotective against Abeta, from PC12 cells through an alpha7 and calcium-dependent pathway (Kim et al., 1997) as well as increasing the secretion of soluble APP and lowering the Abeta-containing sAPP-gamma in rats (Lahiri et al., 2002), again through nAChR-dependent mechanisms. Galantamine, a nAChR potentiator and AChE inhibitor, also increases the secretion of sAPP from human SH-SY5Y neuroblastoma cells (Lenzken et al., 2007) through the activation of nAChRs. It therefore seems that activation of nAChRs shifts the balance of APP processing away from beta-amyloidogenic to soluble APP production.

Evidence 95379d5c1d

Genistein enhances the amplitude of ACh responses when human alpha7 nAChRs are expressed in Xe- nopus laevis oocytes by inhibiting phosphorylation of the receptor in the intracellular loop (Charpantier et al., 2005; Grønlien et al., 2007) and shows similar actions on alpha7 nAChRs of rat hippocampal and supraoptic nucleus neurons as well as human SH-SY5Y cells (Charpantier et al., 2005).

Evidence 9fec853e69

It is now well established that exposure to nicotine results in increased expression of nAChRs in brain and in cultured cells (for review, see Gentry and Lukas, 2002). Exposure of human neuroblastoma SH-SY5Y cells (which express ganglionic alpha7 and alpha3* nAChRs), human TE671/RD cells, or mouse BC3H-1 cells (which express muscle-type nAChRs) to nicotine for up to 120 h induces a dose- and time-dependent increase in surface ACh and alpha-bungarotoxin (alpha-BTX) binding not attributable to changes in mRNA levels (Ke et al., 1998).

Evidence 869b25d654

The neuroprotective effects of nicotine are blocked by inhibitors of either PI3K or SRC family kinases, and nicotine evokes an increase in levels of phosphorylated AKT, B-cell chronic lymphocytic leukemia/lymphoma (BCL2), and BCL-2-like protein (Shimohama and Kihara, 2001), which are further downstream in the PI3K/AKT pathway (Fig. 3).

Evidence 32cb3d8c04

JAK-2, another early target in the nicotine neuroprotection pathway that may mediate signaling between the nAChR and the PI3K pathway (Shaw et al., 2002), may link nAChR activation with the JAK/signal transducer and activator of transcription 3 (STAT-3) protective pathway. JAK-2 is also activated by nicotine in non-neuronal cells such as nAChR-bearing keratinocytes (Arredondo et al., 2006). In a microarray study, expression of 8 of 33 JAK/STAT pathway genes was altered when human bronchial epithelial cells were exposed to 5 microM nicotine for 4 to 10 h (Tsai et al., 2006). Thus, the JAK-2/STAT-3 pathway is activated by exposure to nicotine.

Evidence ca36c714c8

The nicotine-induced nAChR up-regulation in human SH-EP1 cells heterologously expressing alpha7 nAChRs is mediated by cAMP and protein kinase C (PKC) (Nuutinen et al., 2006). The effects of long-term nicotine treatment on nAChR expression in rat brain differs for receptors of different subtype composition (most pronounced up-regulation being observed for alpha4beta2 receptors) and for different brain regions (Nguyen et al., 2003).

Evidence bd0d225f3c

Nicotine protects SH-SY5Y cells from cell death induced by thapsigargin, an inhibitor of the sarcoplasmic-reticulum calcium pump (Arias et al., 2004).

Evidence 9e803ee98a

Hepatic vagus nerve activity has recently been shown to protect hepatocytes from Fas-induced apoptosis via activation of alpha7 nAChRs (Hiramoto et al., 2008). Thus, nicotine seems to exert a general pro-survival action not only on neurons but also on non-neuronal cells, suggesting that the protection offered by nicotine against Abeta toxicity may therefore simply be the result of a general pro-survival response.

Evidence 4040971b90

Nicotine protects PC12 cells from cell death resulting from serum depletion through a mechanism that depends upon the function of IP3 receptors, L-type calcium channels, ryanodine receptors, and ERK, suggesting that the protective effect of nicotine is mediated by calcium signaling pathways (Ren et al., 2005).

Evidence dbd79d729f

In lung cancer cells, nicotine also exerts an antiapoptotic effect through activating BCL2-antagonist of cell death (BAD), a process that is inhibited by blockers of the extracellular signal-regulated kinase (ERK)/mitogen-activated protein kinase (MAPK) pathway or the PI3K/AKT pathway (Jin et al., 2004).

Evidence 3227f4aceb

For instance, over-expressing PI3K in Drosophila melanogaster neurons in situ results in an increase in functional synapses as well as synaptic sprouting (Martín-Pen˜ a et al., 2006). Thus it is possible that nicotine’s activation of the PI3K pathway results in increased synaptic stability, and it would be of interest to explore this further in vertebrates. Thus, the evidence suggests that activation of nAChRs activates the PI3K/AKT pathway to favor antiapoptotic pathways and possibly induce synaptogenesis.

Evidence fec4000ede

The neuroprotective activation of the PI3K/AKT pathway by nicotine involves the tyrosine kinase FYN, which physically interacts with alpha7 nAChRs, and the p85 subunit of PI3K in rat fetal cortical neurons in culture (Kihara et al., 2001).

Evidence 676c3cc4a9

Nicotine induced phosphorylation of STAT-3 (signal transducer and activator of transcription 3) in peritoneal macrophages is mediated by an alpha7-dependent activation of JAK-2, as part of the anti-inflammatory action of vagal nerve stimulation (de Jonge et al., 2005).

Evidence e37f3b4e40

Shortterm nicotine application also induces phosphorylation of p44/42MAPK, p38MAPK, and STAT-3 and was mediated mostly by alpha7 nAChRs in rat vascular smooth muscle cells (Wada et al., 2007). It is noteworthy that the JAK-2/STAT-3 pathway also mediates the mitogenic effects of insulin, a process recently implicated in AD (Li and Ho¨lscher, 2007).

Evidence 7d2da4b8f7

Microarray studies have shown that 24-h incubation in nicotine causes the up-regulation of several genes in SH-SY5Y cells, including ninein (Dunckley and Lukas, 2006), which is known on the basis of a yeast two-hybrid screen to interact with the AD-implicated gene glycogen synthase kinase 3beta (Hong et al., 2000).

Evidence aaf56a9dd1

Application of nicotine to rat microglia results in the up-regulated expression of cyclooxygenase-2 and prostaglandin E2 (De Simone et al., 2005). Signaling pathways downstream to the MAPK pathway are similarly well placed to effect changes in gene expression. For example, alpha7-dependent activation of the MAPK pathway is known to activate c-Myc (Liu et al., 2007), a protooncogene whose transcription product sensitizes cells to pro-apoptotic stimuli.

Evidence 3e5ce007ce

Nicotine also activates ERK in non-neuronal cells such as pancreatic acinar cells (Chowdhury et al., 2007) and vascular smooth muscle cells (Kanda and Watanabe, 2007), although it is not known in those cases which nAChR subtypes are involved. In the cortex and hippocampus of mice, nicotine’s inhibition of MAPK (shown by RNAi reduction of alpha7 expression to be alpha7-dependent) prevents activation of nuclear factor- kappaB and c-Myc, also thereby reducing the activity of inducible nitric-oxide synthetase and NO production and decreasing Abeta production (Liu et al., 2007).

Evidence c55b67a193

Paradoxically, Abeta also activates the MAPK pathway through an alpha7-dependent pathway (Dineley et al., 2001; Bell et al., 2004). In human oral keratinocytes, the Ras/Raf/mitogen-activated protein kinase kinase 1/ERK pathway cooperates with the nicotine activation of the JAK/STAT-3 pathway (Arredondo et al., 2006); the Ras pathway induces STAT-3 upregulation whereas the JAK/STAT-3 pathway phosphorylates STAT-3.

Evidence 5ae3304174

Nicotine may regulate the neuroprotective secretion of TNFalpha by microglia through enhancement of lowlevel TNF secretion and suppression of lipopolysaccharide- induced TNFalpha secretion (Suzuki et al., 2006; Park et al., 2007) via alpha7-dependent activation of JNK and MAPK pathways.

Evidence a12b518bba

Stevens et al. (2003) showed that calcineurin is involved in nicotine neuroprotection. Abeta, through alpha7 nAChRs, increases Ca2+, which phosphorylates NMDARs via calcineurin and protein tyrosine phosphatase, nonreceptor type 5 (striatum-enriched) (Snyder et al., 2005).

Evidence 7d65b2d442

The APOE-epsilon4 gene-dose effect was also found to correlate with the loss of nAChR binding sites in patients with AD, as well as a reduced respon- siveness to the therapeutic AChE inhibitor tacrine (Poirier et al., 1995). Within an AD cohort, APOE-epsilon4 dose dependently correlates with higher losses of ChAT but not with losses in alpha4beta2 nAChRs (Lai et al., 2006).

Evidence d41180b67e

Considerable interest recently focused upon the discovery that Abeta56*, a form whose molecular weight is consistent with its being a dodecamer, can be isolated from the cerebrospinal fluid of transgenic mice expressing human APP and, when injected into rats, rapidly and reversibly induced impaired maze performance (Lesne´ et al., 2006).

Evidence 30c1ccfbdf

Genetic association studies investigating single nucleotide polymorphisms point to roles for cholinergic signaling components such as the synthetic enzyme ChAT, the inactivating enzyme AChE, and alpha4beta2 nAChRs in AD (Cook et al., 2004, 2005; Vasto et al., 2006). The most vulnerable neurons in AD seem to be those expressing high levels of nAChRs, particularly those containing the alpha7 subunit (D’Andrea and Nagele, 2006), and the numbers of nAChRs as well as some of their associated proteins change in AD (Martin-Ruiz et al., 1999; Gotti et al., 2006; Sabbagh et al., 2006).

Evidence a834d1a2e3

Early work on AD considered the fibrillar form to be the toxic species. However, a lack of correlation between plaque burden and cognitive score contrasted with a strong positive correlation between total soluble amyloid and cognitive decline pointing to soluble, oligomeric forms as the primary toxic factor (Walsh and Selkoe, 2007).

Evidence f0a825d454

However, other naturally occurring oligomeric forms of Abeta are also toxic (Deshpande et al., 2006; Shankar et al., 2008), and evidence is accumulating that the capacity of Abeta, mutant Abeta, or fragments of Abeta to aggregate into oligomers is directly related to toxicity (Luheshi et al., 2007).

Evidence e055e7df9f

Which pathway is activated by Abeta depends upon the time of exposure to the amyloid peptide: chronic applications of oligomeric Abeta to hippocampal slice cultures activate the JNK/MAPK pathway but inhibit the ERK/MAPK pathway, whereas short-term applications of Abeta oligomers do not activate JNK (Bell et al., 2004). This may be one of the routes whereby Abeta impairs memory, because ERK-1 and ERK-2 play key roles in the signaling events central to memory (Satoh et al., 2007).

Evidence 701b425019

In studies on SH-SY5Y cells and cultured rat hippocampal neurons, nicotine, acting through alpha7 nAChRs, results in the activation of ERK-1/2 pathways dependent upon calcium and protein kinase A (Dajas-Bailador et al., 2002b). In addition, the alpha7-specific agonist GTS-21 promotes ERK-1/2 phosphorylation, but not that of c-jun N-terminal kinase (JNK) or p38 (Ren et al., 2005).

Evidence 944b8b1f24

Loss of cholinergic neurons has often been demonstrated as lowered ChAT activity in brains of patients with AD. Early post mortem studies indicated a loss of ChAT activity restricted to the neocortex (Slotkin et al., 1990) and this has been confirmed in more recent studies on frontal lobe and temporal cortex (Lai et al., 2006). It is noteworthy that an increase in ChAT activity in the surviving neurons was interpreted as a possible compensatory mechanism (Slotkin et al., 1990).

Evidence 4abccf4842

Likewise, blocking the PI3K-AKT pathway inhibits the protective effects of AChE inhibitors on neuroblastoma cells or neuronal cells against Abeta (Arias et al., 2005) or L-glutamate neurotoxicity (Takada-Takatori et al., 2006). In all these studies, protection was also inhibited by nAChR blockers, suggesting that these effects are mediated by nAChRs.

Evidence c7860809dd

AD is characterized pathologically by the occurrence of intracellular neurofibrillary tangles rich in tau protein and extracellular plaques containing amyloid peptides (Price et al., 1991).

Evidence 81c76164fd

It is noteworthy that this internalization was blocked by alpha-bungarotoxin, which may indicate that alpha-bungarotoxin either inhibits binding of Abeta to the alpha7 receptor (and therefore that Abeta toxicity results from binding of Abeta to alpha7 nAChRs) or directly inhibits alpha7 nAChR internalization.

Evidence d38003c044

JAK-2, also implicated in the neuroprotective pathway, may play a role in linking nAChR action with calcium signaling, because JAK-2 phosphorylates inositol 1,4,5-triphosphate receptors through its activation of FYN (Wallace et al., 2005).

Evidence 17c10c2152

The apolipoprotein E type 4 allele (APOE-epsilon4) encodes the APOE lipoprotein, which through its lipid transport function plays a role in lipid metabolism. APOE-epsilon4 has been found to be a major risk factor for late familial or sporadic AD, with a strong gene-dosage effect such that the number of APOE-epsilon4 alleles correlated positively with the risk of developing AD and the age of onset (Corder et al., 1993).

Evidence 80399e088c

The loss of alpha4 subunits was suggested to be related to lipid peroxidation, because the loss correlated with the level of peroxidation in the temporal cortex of brains from patients with AD, suggesting that receptor loss may be caused by oxidation of proteins (Yu et al., 2003).

Evidence af88505501

However, it has been shown that Abeta1–42 binds with high affinity to alpha7 nAChRs in several different neuronal tissues (Wang et al., 2000a) and displaces alpha-bungarotoxin binding (Wang et al., 2000a,b) and, rather than inhibiting receptor internalization, alpha-bungarotoxin enhances internalization of heterologously expressed nAChRs (Kumari et al., 2008).

Evidence 57fcd06411

An indication that nAChRs may play a role in Abeta internalization comes from a close inspection of cholinergic neurons in brains from patients with AD, which revealed that neurons with high expression levels of alpha7 also contained large amounts of intracellular Abeta (Nagele et al., 2002). Addition of Abeta to the culture medium of neuroblastoma cells overexpressing alpha7 results in more Abeta internalization than in control cells with lower levels of alpha7 expression (Nagele et al., 2002).

Evidence b53eec2473

Paradoxically, in addition to their neuroprotective action, nAChRs may also partly mediate the toxic action of Abeta. The toxicity of Abeta on SH-SY5Y cells, as measured by a 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay, in which the health of cells is monitored by their ability of cells to reduce 3-(4,5-dimethylthiazol- 2-yl)-2,5-diphenyltetrazolium bromide, was significantly impaired when alpha7 was silenced by RNAi, suggesting that Abeta may exert its toxicity, at least in part, through a pathway that includes alpha7 nAChRs (Qi et al., 2007).

Evidence 0deae4edbe

One study showed high-affinity binding of Abeta at picomolar levels to human alpha7 nAChRs heterologously expressed in cell lines, based both on the ability of Abeta to displace labeled alpha-bungarotoxin and the ability of alpha-bungarotoxin to displace fluorescently labeled Abeta (Wang et al., 2000b).

Evidence 27fb13126e

In addition to Abeta acting upon nAChRs, nAChRs in turn regulate Abeta secretion. Nicotine or epibatidine applied to the human SHEP1 cell line stably transfected with human alpha4beta2 nAChRs and human APP decreases the secretion and intracellular accumulation of Abeta without significantly affecting the APP mRNA, suggesting that these effects are post-translational (Nie et al., 2007).

Evidence f4ede0f6aa

Thus, although other mechanisms are also involved in the development of AD, there is abundant evidence that defects in cholinergic synaptic transmission and, in particular, nAChR-mediated signaling plays a major role in the disease and are hence the subject of attempts to generate new routes to therapy.

Evidence dc6e1fa308

There is evidence that ApoE directly interacts with nAChRs. An APOE-derived peptide blocks nAChRs on rat hippocampal slices with a submicromolar affinity, and this action is dependent on an arginine-rich segment of the APOE peptide (Klein and Yakel, 2004). Block of heterologously expressed alpha7 nAChRs is greater than that for alpha4beta2 or alpha2beta2 nAChRs (Gay et al., 2006). This block of alpha7 receptors is abolished when alpha7 Trp55 is mutated to alanine, providing strong evidence that it results from a direct interaction between the peptide and the receptors (Gay et al., 2007), and the effects of other substitutions of Trp55 suggests that this interaction is hydrophobic

Evidence 943ff17d2e

FYN expression is increased in brains from patients with AD, specifically in a subset of neurons with elevated hyperphosphorylated tau protein (Shirazi and Wood, 1993), but it is not known whether this increase in FYN contributes to hyperphosphorylation of tau or is a protective response to it. In extracts of human brains from patients with AD, soluble FYN increases with cognitive score and synaptophysin levels and inversely with the tangle count, suggestive of a pro-cognitive role for FYN (Ho et al., 2005).

Evidence c9db1dbab9

In a microarray study comparing brains from patients with AD with control brains, FYN was found to be significantly upregulated in AD (Wang et al., 2003a). In this context, it is of interest that FYN has also been shown to activate the PI3K/AKT cascade, thereby inhibiting apoptosis (Tang et al., 2007). Indeed, FYN is required for phos- phorylation of phosphoinositide 3-kinase enhancer (PIKE), which itself regulates AKT (Fig. 3). PIKE binds to AKT and up-regulates its kinase a ctivity, thereby reducing apoptosis. Phosphorylation protects PIKE from caspase cleavage, hence FYN is antiapoptotic (Tang et al., 2007).

Evidence 9fa9f64533

Furthermore, inhibitors of SRC, a closely related tyrosine kinase, also prevent nicotinic protection of differentiated PC12 cells against serum-deprivation-induced cell death (Li et al., 1999b), and inhibitors of FYN or Janus kinase-2 (JAK-2) block the neuroprotection against Abeta toxicity of therapeutic AChE inhibitors (Takada-Takatori et al., 2006).

Evidence 63127335a6

Short-term application of Abeta to the SH-SY5Y human neuroblastoma cell line results in a rapid increase in intracellular calcium ions that is dependent upon both alpha3beta2 and alpha7 nAChRs (Dajas-Bailador et al., 2002a)

Evidence 9240dcfb17

Abeta initiates intracellular signaling cascades via nAChRs (Fig. 3), including the MAPK kinase signaling pathway, resulting in cell death. In hippocampal slices, Abeta activates ERK-2 isoforms of the ERK MAPK. This is blocked by alpha7 antagonists, suggesting that Abeta evokes the cascade through alpha7 nAChRs (Dineley et al., 2001).

Evidence 90634f6ee2

Perfusion of soluble Abeta into mouse prefrontal cortex increases dopamine secretion through a mechanism that is blocked by alpha7 antagonists (Wu et al., 2007).

Evidence 2815bb17e0

The accumulation of plaques consisting of Abeta is one of the histopathological hallmarks of AD. Abeta is the product of serial cleavage of the amyloid precursor protein (APP) first by beta and then by gamma secretases to yield Abeta peptides of varying lengths, predominantly the 37-, 40-, and 42- residue forms.

Evidence bfcf02bf61

Several lines of evidence point to a link between brain nAChRs and the development of AD. Biochemical analysis of brains of patients with AD reveals deficits in nAChRs, an increase in butyrylcholinesterase, reduction in ACh, and attenuated activity of cholinergic synthetic [choline acetyltransferase (ChAT)] and inactivating (AChE) enzymes (Bartus et al., 1982; Francis et al., 1999).Butyrylcholinesterase and AChE help terminate ACh signaling by hydrolyzing the transmitter, thereby inactivating it.

Evidence 83f79f333f

Alzheimer’s disease (AD) is the most common form of dementia in elderly persons. It is a neurodegenerative disease marked by decline in memory and cognitive performance, including deterioration of language as well as defects in visual and motor coordination, and eventual death (for review, see Cummings, 2004).

Evidence 11aa2b442c

AD also involves loss of neurons, beginning in the entorhinal cortex and later spreading to the neocortex (Braak et al., 2006); early in the disease, nicotinic acetylcholine receptors (nAChRs) are lost (Kadir et al., 2006).

Evidence 2e13a0a0a2

It is clear that AD involves loss of cholinergic neurons in the brain as well as an overall reduction in nAChRs, and it seems that different subunits are differentially up- or down-regulated in AD in different brain regions and different cell types.

Evidence 1846974779

Thus, predominantly alpha4 and alpha7 subunits, and to a lesser extent alpha3 subunits, are lost in AD, although there are tissue-specific differences to this pattern, such as the upregulation of nAChRs on astrocytes.

Evidence df7aed1bd2

AD involves loss of cholinergic cells not only in the cortex but also in subcortical nuclei. Up to 50% loss of neurons and of ChAT activity has been reported at autopsy in the locus ceruleus of brains from patients with AD compared with brains from subjects without AD, whereas no change was observed for adrenergic brainstem nuclei (Strong et al., 1991).

Evidence 4637ffed04

A stereological approach, in which specific, identified regions of cortex were excised as a by-product of therapeutic surgery, revealed an approximately 50% decrease in the number of alpha7-containing neurons in the temporal cortices of patients with AD, without overall loss in neuron number (Banerjee et al., 2000). In addition to loss of neurons, there are reports of reduced expression of specific nAChR subtypes, particularly of alpha4beta2 and alpha7 subunits, in many brain areas in AD.

Evidence 10bf81ca21

It is noteworthy that a different pattern of changes in nAChRs is seen in non-neuronal cells; expression levels of alpha7 have been reported to be elevated in astrocytes of brains from patients with AD and in cultured astrocytes (Teaktong et al., 2003; Xiu et al., 2005; Yu et al., 2005). Likewise, studies comparing alpha7 expression in human AD brain and Swedish-mutant mice found enhanced alpha7 expression in astrocytes but decreased expression in neurons compared with controls (Xiao et al., 2006).

Evidence cd4b8f7947

Thus, in brains from patients with AD and in neurons responding to exogenously applied Abeta, there is a reduction in expression of nAChR subunits, especially alpha4, alpha7, beta4, and possibly alpha3. Although AD may also involve changes in expression of other ligand-gated ion channels— for example, the expression of NMDA receptors (Bi and Sze, 2002; Jacob et al., 2007), alpha-amino-3-hydroxy- 5-methyl-4-isoxazolepropionic acid receptors (Jacob et al., 2007), and beta3 GABA receptor subunits are all reduced (Mizukami et al., 1998)—there is abundant evidence of a loss of nAChR subunits in AD possibly caused by the actions of Abeta.

Evidence 01210d27e6

Binding studies using subtypeselective labeled ligands suggest that alpha4beta2 receptors are lost in brains from patients with AD (Warpman and Nordberg, 1995; Martin-Ruiz et al., 1999). Regions showing reduced binding levels include the frontal lobe and the temporal cortex (Lai et al., 2006).

Evidence 68d49bea6e

Similar results have been obtained using subtype-specific antibodies. Binding of monoclonal antibodies raised against the alpha4 or the alpha7 subunit, for example, was significantly reduced in post mortem cortices of five patients with AD compared with five patients without AD of similar age (Burghaus et al., 2000). In one study, Western blots confirmed that the greatest reduction was in alpha4 (Guan et al., 2000). Likewise, subunit-specific antibodies reveal a reduced expression of alpha4 but not alpha3 or alpha7 in brains from patients with AD (Martin-Ruiz et al., 1999)

Evidence 4e6e383af3

However, a reduction of alpha3 subunits in a Western blot analysis of brains from patients with AD has been observed (Guan et al., 2000), although the loss was not as great as that observed for alpha4 or alpha7 subunits. In addition, alpha3 subunit levels were reduced in the temporal cortex and hippocampus of brains from patients with AD, both smokers and nonsmokers, compared with control subjects (Mousavi et al., 2003).

Evidence d1f06e8b71

More recent approaches have confirmed this: RNA profiling of isolated neurons from control brains or brains from patients with AD show no evidence for changes in nAChR RNA (Chow et al., 1998; Ginsberg et al., 2000).


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If you find BEL Commons useful in your work, please consider citing: Hoyt, C. T., Domingo-Fernández, D., & Hofmann-Apitius, M. (2018). BEL Commons: an environment for exploration and analysis of networks encoded in Biological Expression Language. Database, 2018(3), 1–11.