Amyloid beta and amylin fibrils induce increases in proinflammatory cytokine and chemokine production by THP-1 cells and murine microglia.

Activated microglia surrounding amyloid beta-containing senile plaques synthesize interleukin-1, an inflammatory cytokine that has been postulated to contribute to Alzheimer's disease pathology. Studies have demonstrated that amyloid beta treatment causes increased cytokine release in microglia and related cell cultures. The present work evaluates the specificity of this cellular response by comparing the effects of amyloid beta to that of amylin, another amyloidotic peptide. Both lipopolysaccharide-treated THP-1 monocytes and mouse microglia showed significant increases in mature interleukin-1beta release 48 h following amyloid beta or human amylin treatment, whereas nonfibrillar rat amylin had no effect on interleukin-1beta production by THP-1 cells. Lipopolysaccharide-stimulated THP-1 cells treated with amyloid beta or amylin also showed increased release of the proinflammatory cytokines tumor necrosis factor-alpha and interleukin-6, as well as the chemokines interleukin-8 and macrophage inflammatory protein-1alpha and -1beta. THP-1 cells incubated with fibrillar amyloid beta or amylin in the absence of lipopolysaccharide also showed significant increases of both interleukin-1beta and tumor necrosis factor-alpha mRNA. Furthermore, treatment of THP-1 cells with amyloid fibrils resulted in an elevated expression of the immediate-early genes c-fos and junB. These studies provide further evidence that fibrillar amyloid peptides can induce signal transduction pathways that initiate an inflammatory response that is likely to contribute to Alzheimer's disease pathology.

Aß-Induced Proinflammatory Cytokine Release from Differentiated Human THP-1 Monocytes.

As noted in the introductory chapters of this book, neuritic plaques composed of accumulated amyloid ß (Aß) peptide are a hallmark pathological feature of the Alzheimer's disease (AD) brain. Compelling genetic data now implicate these plaques as key causative agents in AD onset, as all known mutations that lead to early onset familial AD (1-6) result in an increased production of the amyloidogenic Aß1-42 isoform (7-11). Although it appears likely that the deposition of multimeric Aß fibrils into plaques is a necessary step in AD onset, there is still uncertainty as to how Aß and neuritic plaques might cause the neuropathology that leads to the dementia that is characteristic of this disease.

Inflammatory responses to amyloid fibrils.

Our laboratory has routinely used the methodologies described here to characterize the effects of fibrillar A beta and amylin on cytokine synthesis and secretion by LPS-differentiated THP-1 cells. Because LPS-treated THP-1 cells resemble macrophage and microglia, this assay system represents an in vitro model of the potential interactions between A beta-containing senile plaques and microglia in the AD brain. As such, these methodologies should prove useful in the identification of compounds that inhibit this A beta-induced inflammatory response.

Identification and pharmacological characterization of a series of new 1H-4-substituted-imidazoyl histamine H3 receptor ligands.

A new series of 1H-4-substituted imidazole compounds were synthesized and identified as potent and selective histamine (HA) H3 receptor ligands. These ligands establish that HA H3 antagonists exhibit stereoselective and conformational preferences in their binding to the HA H3 receptor. Structure-activity relationships were determined in vitro by HA H3 receptor-binding affinities using [3H]Nalpha-methylhistamine and rat cerebral cortical tissue homogenates. Several derivatives containing olefin, amide, and acetylene functional groups were identified as potent HA H3 receptor ligands. In the olefin series, GT-2227 (4-(6-cyclohexylhex-cis-3-enyl)imidazole) was identified as a potent HA H3 receptor ligand with a Ki of 4.2 +/- 0.6 nM, while the trans isomer (GT-2228) displayed a reduced potency (Ki = 15.2 +/- 2.4 nM). GT-2227 was also found to have excellent central nervous system penetration in an ex vivo binding paradigm (ED50 = 0.7 mg/kg i.p.). In the acetylene series, GT-2260 and GT-2286 both exhibited high affinity (Ki = 2.9 +/- 0.2 and 0.95 +/- 0.3 nM) and excellent central nervous system penetration profiles (ED50 = 0.43 and 0.48 mg/kg i.p., respectively). As a prototype for the series, GT-2227 showed high affinity for the human HA H3 receptor (3.2 nM) and minimal affinity for the human HA H1 (Ki = 13,407 +/- 540 nM) and H2 (Ki = 4,469 +/- 564 nM) receptor subtypes. GT-2227 also showed good selectivity for the HA H3 receptor over a broad spectrum of other neurotransmitter receptors (IC50 >/= 1 microM). Furthermore, GT-2227 improved acquisition in a cognitive paradigm without behavioral excitation or effect on spontaneous locomotor activity. In summary, the present studies demonstrate the development of novel HA H3-selective ligands, and lend support for the use of such agents in the treatment of disorders associated with cognitive or attentional deficits.

Development of trans-2-[1H-imidazol-4-yl] cyclopropane derivatives as new high-affinity histamine H3 receptor ligands.

Previously, a novel series of 1H-4-substituted imidazole compounds were described as potent and selective histamine (HA) H3 receptor ligands (Yates et al., 1999). The present studies extend the structure-activity relationships for optimal HA H3 receptor affinity and central nervous system penetration by incorporation of a conformationally restricted cyclopropane nucleus. Moreover, the current studies extend our understanding of ligand-receptor interactions at the HA H3 receptor with the development of high affinity HA H3 receptor antagonists containing a stereochemical presentation. Structure-activity relationships were established from in vitro HA H3 receptor-binding affinities using [3H]Nalpha-methylhistamine and rat cortical tissue homogenates. Systematic optimization of multiple structural features critical for HA H3 receptor affinity provided some of the most potent HA H3 receptor agents described. For example, GT-2331 was determined to bind to a single population of HA H3 receptors with a Ki of 0.125 nM. In vivo, GT-2331 has a favorable central nervous system penetration profile with an ED50 of 0.08 mg/kg (i.p.) in rats and a long duration of action (T1/2 > 4 h). In addition, GT-2331 was extremely selective for the HA H3 receptor versus other HA receptors and a battery of neurotransmitter, neuropeptide, hormone, or enzyme systems. Several compounds were tested in vitro which suggested HA H3 receptor heterogeneity and are discussed in terms of structure-activity relationships for the HA H3 receptor.

Design, synthesis, and structure-activity relationships of acetylene-based histamine H3 receptor antagonists.

New, potent, and selective histamine H3 receptor antagonists have been synthesized by employing the use of (1) an appropriately positioned nonpolar acetylene spacer group, (2) either a two-carbon straight chain linker or a conformationally restricting trans-cyclopropane ring between the C-4 position of an imidazole headgroup and the acetylene spacer, and (3) a Topliss operational scheme for side-chain substitution for optimizing the hydrophobic domain. Compounds 9-18 are examples synthesized with the two-carbon straight chain linker, whereas 26-31 are analogues prepared by incorporation of the trans-(+/-)-cyclopropane at the C-4 position of an imidazole headgroup. Synthesis of both the (1R,2R)- and (1S, 2S)-cyclopropyl enantiomers of the most potent racemic compound 31 (Ki = 0.33 +/- 0.13 nM) demonstrated a stereopreference in H3 receptor binding affinity for the (1R,2R) enantiomer 32 (Ki = 0.18 +/- 0.04 nM) versus the (1S,2S) enantiomer 33 (Ki = 5.3 +/- 0.5 nM). (1R,2R)-4-(2-(5,5-Dimethylhex-1-ynyl)cyclopropyl)imidazole (32) is one of the most potent histamine H3 receptor antagonists reported to date.

Effects of selected histamine H3 receptor antagonists on tele-methylhistamine levels in rat cerebral cortex.

The H3 antagonist thioperamide is thought to act on brain H3 autoreceptors to increase both the release and metabolism of neuronal histamine (HA). Our studies investigated the effects of several new brain-penetrating H3 antagonists on rat cerebral cortical levels of the HA metabolite tele-methylhistamine (t-MH). Animals were pretreated with H3 antagonists (0.3 to 30 mg/kg; 1-4 hr; i.p.) in the presence or absence of the monoamine oxidase inhibitor pargyline to prevent metabolism of t-MH. Cortical t-MH levels were measured by both radioimmunoassay (RIA) and gas chromatography-mass spectrometry (GC-MS). Pargyline (60 mg/kg; 1 hr; i.p.) produced an approximately 2-fold increase in t-MH levels as measured by either GC-MS or RIA. Thioperamide (+/- pargyline) increased t-MH levels as measured by both GC-MS and RIA. In contrast, neither 5-cyclohexyl-1-(4-imidazol-4-ylpiperidyl)pentan-1-one (GT-2016) (+/- pargyline), 4-(6-cyclohexylhex-cis-3-enyl)imidazole (GT-2227) (+/- pargyline), nor clobenpropit (minus pargyline) increased t-MH levels as measured by GC-MS. A good agreement was found between t-MH levels as determined by either RIA or GC-MS except after treatment with GT-2016, which increased apparent t-MH brain levels according to the former but not the latter method. Subsequent studies suggest the in vivo formation of a GT-2016 metabolite, which can cross-react in the t-MH RIA. Although all H3 receptor antagonists studied to date seem capable of enhancing brain HA release, only thioperamide presently was found to enhance cortical t-MH levels. Thus, H3 receptor antagonists may differentially affect HA release and turnover, and brain t-MH levels may not be reliable predictors of H3 agonist, partial agonist, or antagonist in vivo activity.

High antagonist potency of GT-2227 and GT-2331, new histamine H3 receptor antagonists, in two functional models.

GT-2227 (4-(6-cyclohexylhex-cis-3-enyl)imidazole) and GT-2331 ((1R,2R)-4-(2-(5,5-dimethylhex-1-ynyl)cyclopropyl)imidazole) were developed as new potent histamine H3 receptor antagonists. The functional activity of these ligands on the histamine H3 receptor-mediated inhibition of neurogenic contraction of the guinea-pig jejunum and histamine H3 receptor-mediated inhibition of norepinephrine release from guinea-pig heart synaptosomes were investigated. GT-2227 and GT-2331 both antagonized the inhibitory effects of (R)-alpha-methylhistamine on the contraction induced by electrical field stimulation in the guinea-pig jejunum with pA2 values of 7.9+/-0.1 and 8.5+/-0.03, respectively. In addition, GT-2227 and GT-2331 antagonized the inhibition of norepinephrine release in cardiac synaptosomes by GT-2203 ((1R,2R)-trans-2-(1H-imidazol-4-yl)cyclopropylamine), a histamine H3 receptor agonist. The current results demonstrate the antagonist activity for both GT-2227 and GT-2331 in two functional assays for histamine H3 receptors.

New acetylene based histamine H3 receptor antagonists derived from the marine natural product verongamine.

New histamine H3 receptor antagonists were developed using an acetylene moiety as a replacement for the amide-oxime functionality of verongamine 5. Optimization of receptor binding was performed by following aliphatic Topliss tree guidelines. These new H3 ligands demonstrate excellent blood-brain barrier penetration.

Up-regulation of nicotinic acetylcholine receptors following chronic exposure of rats to mainstream cigarette smoke or alpha 4 beta 2 receptors to nicotine.

Smokers are reported to have a higher density of central nicotinic acetylcholine receptors (nAChRs) that non-smokers at autopsy. Whether this increased receptor density is a response to smoking or a result of genetic variability is not known. While sub-chronic treatment of rats and mice with nicotine results in upregulation of central nAChRs, changes in receptor density in response to cigarette smoke have not been studied previously. In this study, male Sprague-Dawley rats were exposed nose-only for 13 weeks to mainstream cigarette smoke followed by assessment of [3H]nicotine binding in five brain regions of smoke- and sham-exposed animals. In smoke-exposed animals, there was a significant increase in nAChR density in the cortex, striatum, and cerebellum (35, 25, and 31% increases, respectively), while there was no significant change in receptor density in the thalamus and hippocampus. Smoke exposure did not alter markedly the affinity of the receptor for nicotine in these brain regions. Furthermore, up-regulation of nAChRs did not alter the biphasic binding properties by which nicotine binds to its receptor. There were no changes in the association (fast phase) or isomerization (slow phase) rate constants, and the percent contribution of slow and fast phase binding to nAChRs was not altered in the up-regulated receptor population compared with control. Similar results were observed following chronic nicotine exposure of cultured cortical cells from fetal rat brain or cells transfected with the alpha 4 beta 2 nAChR subtype. These results show that the up-regulation following smoke exposure in the rat is phenomenologically similar to that observed in vitro. These data provide preliminary evidence for a relationship between cigarette smoking and nAChR up-regulation in vivo and suggest that similar mechanisms of upregulation may underlie chronic smoke exposure of live animals and nicotine exposure of artificially expressed alpha 4 beta 2 receptors in vitro.

Pharmacological characterization of GT-2016, a non-thiourea-containing histamine H3 receptor antagonist: in vitro and in vivo studies.

GT-2016, a non-thiourea-containing imidazole, has been developed as a histamine H3 antagonist. In vitro and in vivo studies in rats were conducted to characterize receptor selectivity and autoreceptor functionality for GT-2016. GT-2016 demonstrated high affinity (43.8 +/- 3.0 nM) and selectivity for the histamine H3 receptor in vitro. In vivo, GT-2016 (3, 10 and 30 mg/kg i.p. and p.o.) was shown to cross the blood-brain barrier and dose-dependently bind to cortical histamine H3 receptors. GT-2016 induced dose-dependent increases in histamine turnover at concentrations that exhibited significant histamine H3 receptor occupancy. Also, in vivo microdialysis experiments were conducted in awake, freely moving rats treated with GT-2016. GT-2016 (10 and 30 mg/kg i.p.) increased histamine release by approximately 75% above baseline within 1 hr, and elevated histamine release was observed for up to 2.5 hr after the higher dose. In contrast, GT-2016 was devoid of activity on histamine methyltransferase in vitro at concentrations up to 3 microM. Taken together, the results show that GT-2016 crosses the blood-brain barrier, binds to H3 receptors and increases the release of histamine in the cerebral cortex, consistent with blockade of presynaptic H3 autoreceptors. In summary, these findings allowed us to identify and characterize the in vitro and in vivo biochemical properties of a novel H3 receptor antagonist, GT-2016.

Advances in the use of the fluorescent probe fura-2 for the estimation of intrasynaptosomal calcium.

Fura-2 has been used to measure intracellular Ca2+ with great success in a variety of cell and subcellular preparations, including synaptosomes. There is, however, a great deal of variability in the reported estimates of resting intrasynaptosomal Ca2+ ([Ca2+]i). Fura-2 AM is highly lipophilic and passes readily across the plasma membrane into the cytoplasm, where it is de-esterified and trapped. The lipophilicity of fura-2, however, promotes the formation of micelles in aqueous media, which may impede the passage of the probe across cell membranes. Our results suggest that some of the variability in the reported [Ca2+]i estimates may be related to fura-2 de-esterification and loading efficiencies. The use of the nonionic detergent pluronic F-127 is recommended to prevent the formation of fura-2 micelles. The use of a detergent is not always an acceptable practice, however, especially in studies in which detergent-lipid interactions may influence membrane parameters. We found that fatty acid free bovine serum albumin (BSA) (0.25%) greatly increases the intrasynaptosomal concentration of the probe, resulting in a significant increase in the signal-to-noise (S/N) ratio. The mechanism appears to be independent of effects of BSA on synaptosomal integrity and directly related to the prevention of fura-2 micelle formation, as evidenced by light spectroscopic scattering measurements. Thus, BSA appears to keep the probe in a form that crosses the synaptic plasma membrane more readily. The effectiveness of BSA in improving the loading of fura-2 into synaptosomes was comparable to the detergent pluronic F-127, making it possible to measure [Ca2+]i without compromising membrane integrity.

Comparative effects of phospholipase A2 neurotoxins and enzymes on membrane potential and Na+/K+ ATPase activity in rat brain synaptosomes.

Beta-Bungarotoxin (beta-BuTX) and notexin are phospholipase A2 (PLA2) neurotoxins which cause an irreversible blockade of neurotransmitter release through specific and potent effects at the presynaptic nerve terminal; however, their mechanism of action is uncertain. We examined the effects of beta-BuTX and notexin on Na+/K+ ATPase activity using Sprague-Dawley rat brain synaptosomes in order to determine if alterations in activity might modulate neurotoxin-induced depolarization. Treatment of synaptosomes with 0.05 to 5 nM beta-BuTX, notexin, and Naja naja atra and Naja nigricollis PLA2 (PLA2 enzymes without selective presynaptic actions) caused a dose-dependent depolarization of synaptosomes with no differences being observed between the effects of the PLA2 neurotoxins and enzymes. N. nigricollis PLA2 (0.5 nM; 20 min) slightly stimulated Na+/K+ ATPase activity while beta-BuTX and notexin (0.5 nM: 10 and 20 min) were without effect. With 50 nM concentrations beta-BuTX and notexin stimulated Na+/K+ ATPase activity, while N. nigricollis and N. n. atra PLA2 inhibited activity. The effects on membrane potential and Na+/K+ ATPase were antagonized or blocked by EDTA (10 mM) and bovine serum albumin (1 mg/ml), suggesting that PLA2 enzymatic activity is essential for their effects on membrane potential and Na+/K+ ATPase activity. Following neurotoxin and enzyme pretreatment, we found a biphasic correlation between synaptosomal free fatty acid (FFA) levels and Na+/K+ ATPase activity, where Na+/K+ ATPase is stimulated by low levels of FFA (0.13 to 0.22 mumol/mg protein) and antagonized by FFA levels in excess of 0.34 mumol/mg protein. In contrast there was a linear correlation between the extent of FFA production and membrane depolarization. We propose that the presynaptic depolarizing actions of beta-BuTX and notexin are not mediated through modulation of Na+/K+ ATPase activity and that the changes observed in ATPase activity and possibly membrane potential are directly due to PLA2 enzymatic activity and the production of FFA.

Inhibitory effect of EDTA.Ca2+ on the hydrolysis of synaptosomal phospholipids by phospholipase A2 toxins and enzymes.

Phospholipases A2 (PLA2) are Ca2(+)-dependent enzymes that are inhibited by EDTA; this inhibition would be expected to be reversed by restoring the Ca2+ concentration. By examining the hydrolysis of synaptosomal phospholipids by PLA2 enzymes, Naja naja atra and Naja nigricollis, and by toxins with PLA2 activity, beta-bungarotoxin (beta-BuTX) and notexin, we demonstrated a novel inhibitory action of EDTA manifested in the presence of excess Ca2+. We postulate the formation of an EDTA.Ca2+ complex which inhibits PLA2 activity in a concentration-dependent manner. Synaptosomes in which phospholipids are hydrolyzed by PLA2 have membranal damage expressed by increased acetylcholine (ACh) release and decreased osmotic activity. Addition of EDTA.Ca2+, which inhibits phospholipid hydrolysis, also reversed the PLA2 effect on ACh release, but not its effect on osmotic activity. The inhibition of PLA2 was observed on membranal phospholipids as well as on an artificial substrate of phospholipid-Triton mixed micelles. Moreover, we found that another enzyme, lactate dehydrogenase, was also inhibited. Our results indicate a non-specific inhibition exerted on the enzyme rather than on the substrate.

Leukotriene and prostaglandin production in rat brain synaptosomes treated with phospholipase A2 neurotoxins and enzymes.

beta-Bungarotoxin (beta-BuTX) and notexin cause an irreversible blockade of neurotransmitter release through specific and potent effects at the presynaptic nerve terminal, however, the mechanism of action is uncertain. We examined the effects of beta-BuTX and notexin on LT and PG production in rat cerebrocortical synaptosomes in order to determine if eicosanoid production might mediate or regulate the pharmacological actions of these phospholipase A2 (PLA2) neurotoxins. The effects of the PLA2 enzymes isolated from Naja naja atra and Naja nigricollis snake venoms (which are not presynaptic selective) on LT and PG production were compared with the effects of beta-BuTX and notexin. N. n. atra PLA2, beta-BuTX, and notexin (all 50 nM) produced a time dependent rise in free fatty acids as measured in synaptic plasma membranes isolated from treated synaptosomes. Both the PLA2 neurotoxins and enzymes stimulated LTC4, LTB4, and PGE2 production, as measured by radioimmunoassay. In all cases, the PLA2 enzymes were more potent than the PLA2 neurotoxins. This observation correlates with their relative enzymatic potencies, as measured by free fatty acid generation. EDTA and BSA antagonized PLA2 induced LTB4 production and BSA also antagonized PLA2 induced PGE2 production. These results suggest that stimulation of eicosanoid production does not mediate the potent and specific presynaptic actions of beta-BuTX and notexin.

Phospholipid hydrolysis and loss of membrane integrity following treatment of rat brain synaptosomes with beta-bungarotoxin, notexin, and Naja naja atra and Naja nigricollis phospholipase A2.

The effects of the phospholipase A2 (PLA2) toxins, beta-bungarotoxin and notexin, and the PLA2 enzymes from Naja naja atra and Naja nigricollis snake venoms on the plasma membrane integrity of synaptosomes were examined. Synaptosomes were isolated from rat brain cerebral cortex, corpus striatum and hippocampus. Osmotic activity, lactate dehydrogenase leakage, and leakage of 2-deoxy-D-(1-3H)-glucose-6-phosphate were monitored (37 degrees C, 10-120 min) following incubation with 0.5, 5 and 50 nM concentrations of toxins and enzymes. Damage to the synaptosomal plasma membrane was time and concentration but not tissue dependent. The potencies of the treatments were as follows: N. n. atra PLA2 greater than or equal to N. nigricollis PLA2 greater than notexin greater than beta-bungarotoxin. Chelation of Ca2+ with 5 mM EDTA completely inhibited plasma membrane disruption caused by beta-bungarotoxin and N. n. atra PLA2. One mg/ml of bovine serum albumin also blocked the disruptive action of N. n. atra PLA2, while 8 mg/ml was required to antagonize beta-bungarotoxin. A correlation between phospholipid hydrolysis and loss of membrane integrity was also observed. The generation of phospholipid hydrolytic products may be critical in the permeabilization of synaptic plasma membranes by these toxins and enzymes, however, they do not explain the presynaptic specificity and potency of beta-bungarotoxin and notexin.

A comparison of the noise sensitivity of nine QRS detection algorithms.

The noise sensitivities for nine different QRS detection algorithms were measured for a normal, single-channel lead II, synthesized ECG corrupted with five different types of synthesized noise. The noise types were electromyographic interference, 60 Hz powerline interference, baseline drift due to respiration, abrupt baseline shift, and a composite noise constructed from all of the other noise types. The percentage of QRS complexes detected, the number of false positives, and the detection delay were measured. None of the algorithms were able to detect all QRS complexes without any false positives for all of the noise types at the highest noise level. Algorithms based on amplitude and slope had the highest performance for EMG-corrupted ECG. An algorithm using a digital filter had the best performance for the composite noise corrupted data.