Alzheimer's disease and the basal forebrain cholinergic system: relations to beta-amyloid peptides, cognition, and treatment strategies

Selective cholinergic denervation, independent from oxidative stress, in a mouse model of Alzheimer’s disease

Alzheimer's disease (AD) is characterized by increases in amyloid-beta (Abeta) peptides, neurofibrillary tangles, oxidative stress and cholinergic deficits. However, the selectivity of these deficits and their relation with the Abeta pathology or oxidative stress remain unclear. We therefore investigated amyloidosis-related changes in acetylcholine (ACh) and serotonin (5-HT) innervations of hippocampus and parietal cortex by quantitative choline acetyltransferase (ChAT) and 5-HT immunocytochemistry, in 6, 12/14 and 18 month-old transgenic mice carrying familial AD-linked mutations (hAPP(Sw,Ind)). Further, using manganese superoxide dismutase (MnSOD) and nitrotyrosine immunoreactivity as markers, we evaluated the relationship between oxidative stress and the ACh deficit in 18 month-old mice. Thioflavin-positive Abeta plaques were seen in both regions at all ages; they were more numerous in hippocampus and increased in number (>15-fold) and size as a function of age. A majority of plaques exhibited or were surrounded by increased MnSOD immunoreactivity, and dystrophic ACh or 5-HT axons were seen in their immediate vicinity. Counts of immunoreactive axon varicosities revealed significant decreases in ACh innervation, with a sparing of the 5-HT, even in aged mice. First apparent in hippocampus, the loss of ACh terminals was in the order of 20% at 12/14 months, and not significantly greater (26%) at 18 months. In parietal cortex, the ACh denervation was significant at 18 months only, averaging 24% across the different layers. Despite increased perivascular MnSOD immunoreactivity, there was no evidence of dystrophic ACh varicosities or their accentuated loss in the perivascular area. Moreover, there was virtually no sign of tyrosine nitration in ChAT nerve terminals or neuronal cell bodies. These data suggest that aggregated Abeta exerts an early, non-selective and focal neurotoxic effect on both ACh and 5-HT axons, but that a selective, plaque- and oxidative stress-independent diffuse cholinotoxicity, most likely caused by soluble Abeta assemblies, is responsible for the hippocampal and cortical ACh denervation.

Biotechnology, the brain and the future

Several new approaches to illness, inspired by recent advances in molecular biology, informatics and nanoscience, are readily applicable to diseases of the central nervous system. Novel classes of drugs will widen the scope of therapeutic action beyond merely modifying transmitter function and stem cell and gene therapies could offer an even more selective mode of targeting. Further into the future, nanotechnology has the potential to allow development of new medicines and novel access routes via miniaturized monitoring and screening devices: these systems, together with increasing use of carbon-silicon interfacing, will challenge traditional neuropharmacology. As the 21(st) century unfolds, the structure and function of the brain, which is incomparable with any other organ, will present unique technological and ethical questions.

Developmental origins of the age-related decline in cortical cholinergic function and associated cognitive abilities

Ontogenetic abnormalities in the regulation of the cortical cholinergic input system are hypothesized to mediate early-life cognitive limitations (ECL) that later escalate, based on reciprocal interactions between a dysregulated cholinergic system and age-related neuronal and vascular processes, to mild cognitive impairment (MCI) and, subsequently, for a majority of subjects, senile dementia. This process is speculated to begin with the disruption of trophic factor support of the basal forebrain ascending cholinergic system early in life, leading to dysregulation of cortical cholinergic transmission during the initial decades of life and associated limitations in cognitive capacities. Results from neurochemical and behavioral experiments support the possibility that aging reveals the vulnerability of an abnormally regulated cortical cholinergic input system. The decline of the cholinergic system is further accelerated as a result of interactions with amyloid precursor protein metabolism and processing, and with cerebral microvascular abnormalities. The determination of the developmental variables that render the cortical cholinergic input system vulnerable to age-related processes represents an important step toward the understanding of the role of this neuronal system in the age-related decline in cognitive functions.

Biochemical and histochemical evidence of nonspecific binding of alpha7nAChR antibodies to mouse brain tissue

Alpha 7 nicotinic acetylcholine receptors are involved in learning and memory, and are implicated in the pathology of Alzheimer's disease and schizophrenia. Detection of alpha7 subunits can be accomplished via immunodetection or alpha-bungarotoxin-binding techniques. Standard protocols for immunohistochemistry and Western blotting were followed using several commercially available antibodies. Various mice were evaluated, including non-transgenics, APP, PS1, APP+PS1, and alpha7 knockouts. Initial results with amyloid-depositing mice revealed alpha7 immunolabeled astrocytes, in addition to expected neuronal staining. Subsequent studies with intrahippocampal injections of lipopolysaccharide (LPS) into alpha7 knockout mice showed that both neuronal and astrocytic labeling by alpha7 antibodies was nonspecific. On Western blots of mouse brain proteins, none of the bands detected with antibodies directed against alpha7 subunits diminished in the alpha7 knockout mice. Although LPS-related changes in the expression of some bands were found, these also were unaffected by the alpha7 genotype of the mice. In general, the Western staining patterns for these antibodies revealed few overlapping bands. These immunodetection data are in contrast to genotyping results and mRNA analyses that confirmed the disruption of the alpha7 allele and lack of alpha7 message in the knockouts. These findings suggest caution in interpreting results when using several commercially available alpha7 nicotinic receptor antibodies.

Profiling proteins related to amyloid deposited brain of Tg2576 mice

Alzheimer's disease (AD) is an age-related neurodegenerative disorder that is characterized by the extracellular deposition of beta-amyloid and intracellular hyperphosphorylation of tau in the cortex and hippocampus of the brain. These characterizations are caused by abnormal expression, modification and deposition of certain proteins. Post-translational modifications of proteins including oxidation and nitration might be involved in the pathogenesis of AD. In this study, AD-related proteins were identified in the cortex of Tg2576 mice used as a model for studying AD. Tg2576 mice express high levels of the Swedish mutated form of human beta-amyloid precursor protein (APP) and generated high levels of beta-amyloid in the brains. Using Western blotting and two-dimensional electrophoresis, proteins with differences in expression, oxidation and nitration in the cortex of Tg2576 mice brains were compared to littermate mice brains used as a control. The proteins with different expression levels were identified using matrix-assisted laser desorption/ionization-time of flight and liquid chromatography-tandem mass spectrometry analyses. As a result, 12 proteins were identified among 37 different proteins using the PDQuest program. Furthermore, two proteins, laminin receptor and alpha-enolase, were more susceptible to oxidative modification in the brains of Tg2576 mice compared to those of littermates. Similarly, alpha-enolase, calpain 12, and Atp5b were more modified by nitration in brains of Tg2576 mice than those of littermates. Taken together, these proteins and their modifications may play an important role in the plaque deposition of Tg2576 mice brains.

Cognitive and neurological deficits induced by early and prolonged basal forebrain cholinergic hypofunction in rats

In the present study we examined the long-term effects of neonatal lesion of basal forebrain cholinergic neurons induced by intracerebroventricular injections of the immunotoxin 192 IgG saporin. Animals were then characterised behaviourally, electrophysiologically and molecularly. Cognitive effects were evaluated in the social transmission of food preferences, a non-spatial associative memory task. Electrophysiological effects were assessed by recording of cortical electroencephalographic (EEG) patterns. In addition, we measured the levels of proteins whose abnormal expression has been associated with neurodegeneration such as amyloid precursor protein (APP), presenilin 1 and 2 (PS-1, PS-2), and cyclooxygenases (COX-1 and COX-2). In animals lesioned on postnatal day 7 and tested 6 months thereafter, memory impairment in the social transmission of food preferences was evident, as well as a significant reduction of choline acetyltransferase activity in hippocampus and neocortex. Furthermore, similar to what observed in Alzheimer-like dementia, EEG cortical patterns in lesioned rats presented changes in alpha, beta and delta activities. Levels of APP protein and mRNA were not affected by the treatment. Levels of hippocampal COX-2 protein and mRNA were significantly decreased whereas COX-1 remained unaltered. PS-1 and PS-2 transcripts were reduced in hippocampus and neocortex. These findings indicate that neonatal and permanent basal forebrain cholinergic hypofunction is sufficient to induce behavioural and neuropathological abnormalities. This animal model could represent a valid tool to evaluate the role played by abnormal cholinergic maturation in later vulnerability to neuropathological processes associated with cognitive decline and, possibly, to Alzheimer-like dementia.

Proteomic approaches in brain research and neuropharmacology

Numerous applications of genomic technologies have enabled the assembly of unprecedented inventories of genes, expressed in cells under specific physiological and pathophysiological conditions. Complementing the valuable information generated through functional genomics with the integrative knowledge of protein expression and function should enable the development of more efficient diagnostic tools and therapeutic agents. Proteomic analyses are particularly suitable to elucidate posttranslational modifications, expression levels and protein-protein interactions of thousands of proteins at a time. In this review, two-dimensional polyacrylamide gel electrophoresis (2D-PAGE) investigations of brain tissues in neurodegenerative diseases such as Alzheimer's disease, Down syndrome and schizophrenia, and the construction of 2D-PAGE proteome maps of the brain are discussed. The role of the Human Proteome Organization (HUPO) as an international coordinating organization for proteomic efforts, as well as challenges for proteomic technologies and data analysis are also addressed. It is expected that the use of proteomic strategies will have significant impact in neuropharmacology over the coming decade.

Alternative therapy of Alzheimer’s disease via supplementation with choline acetyltransferase

Much evidence indicates that the memory and cognitive deficits of patients with Alzheimer's disease are closely associated with dysfunction of central cholinergic system. The degree of reduction of choline acetyltransferase activity in cerebral cholinergic neurons is significantly correlated with the severity of dementia or cognitive impairments observed in Alzheimer's disease. Therefore, Alzheimer's disease may be slowed by supplementation of exogenous choline acetyltransferase. Here we show that choline acetyltransferase mediated by TAT protein transduction domain passes through the blood-brain barrier and enters the neurons in mice, increasing choline acetyltransferase and neurotransmitter acetylcholine contents. The recombination TAT-choline acetyltransferase fusion protein injected intravenously improves the memory and cognitive dysfunction in Alzheimer's disease model mice induced by amyloid-beta peptide. Our results imply a novel and potentially effective way for Alzheimer's disease therapy.

Reduction in acetylcholine release in the hippocampus of dopamine D5 receptor-deficient mice

Activation of the dopamine D1-like receptor stimulates acetylcholine (ACh) release in the hippocampus, apparently through the molecularly defined d5 receptor. In the present study, we used a transgenic mouse completely deprived of functional d5 receptor (d5-/-) to confirm the role and elucidate the possible function of the d5 receptor subtype on hippocampal cholinergic neurotransmission. ACh release was measured using in vivo microdialysis in the mouse dorsal hippocampus of 4 months old homozygous (d5-/-), heterozygous (d5+/-), and the wild-type (d5+/+) littermates. Using the no net flux technique, a significant reduction in basal hippocampal ACh level was found in the d5-/- compared to d5+/- and d5+/+ mice. Moreover, the administration of SKF 38393, a D1-like receptor agonist, systemically (2.0 and 10.0 mg/kg ip), or locally through the dialysis probe (10 and 50 microM), produced a dose-dependent enhancement of ACh release in the d5+/+, a moderate stimulation in the d5+/- but had no effect in the d5-/- mice. Quantitative receptor autoradiography revealed significant increases in M1-like but not in M2-like muscarinic receptor binding sites in the hippocampal formation. These results confirm and extend the role of the d5 receptor in the modulation of hippocampal ACh release and provide evidence for long-term alteration of hippocampal cholinergic neurotransmission resulting from the absence of the d5 receptors including chronically reduced ACh release and change in M1-like receptor levels.

Attenuation of scopolamine-induced learning deficits by LVV-hemorphin-7 in rats in the passive avoidance and water maze paradigms

Central administration of angiotensin IV (Ang IV) analogues attenuates scopolamine-induced amnesia. Ang IV mediates its effects by binding to a high affinity, binding site, AT(4) receptor, that has recently been identified as insulin regulated aminopeptidase (IRAP). The purpose of this study was to examine the effect of the distinct AT(4) ligand, LVV-hemorphin-7 (LVV-H7), on scopolamine-induced learning deficits, one which involves fear-conditioning and the other spatial learning. Rats were pretreated with an intracerebroventricular (ICV) dose of scopolamine hydrobromide followed by treatment with 1 nmol LVV-H7 or artificial cerebrospinal fluid (aCSF). During the acquisition phase of the water maze task, daily ICV infusions of 1 nmol of LVV-H7 25 min after scopolamine treatment produced marked improvement in both the latency and distance swum in order to locate the submerged platform using visual cues compared to animals treated with scopolamine only. In addition, the same dose of LVV-H7 attenuated the learning deficit observed for scopolamine-treated animals in the passive avoidance task. These studies clearly demonstrate that LVV-H7, like Ang IV, is a pharmacologically active AT(4) ligand that attenuates the deleterious effects of scopolamine on learning performance in two different behavioral paradigms.

Real-time imaging of single nerve cell apoptosis in retinal neurodegeneration

Apoptotic nerve cell death is implicated in the pathogenesis of several devastating neurodegenerative conditions, including glaucoma and Alzheimer's and Parkinson's diseases. We have devised a noninvasive real-time imaging technique using confocal laser-scanning ophthalmoscopy to visualize single nerve cell apoptosis in vivo, which allows longitudinal study of disease processes that has not previously been possible. Our method utilizes the unique optical properties of the eye, which allow direct microscopic observation of nerve cells in the retina. We have been able to image changes occurring in nerve cell apoptosis over hours, days, and months and show that effects depend on the magnitude of the initial apoptotic inducer in several models of neurodegenerative disease in rat and primate. This technology enables the direct observation of single nerve cell apoptosis in experimental neurodegeneration, providing the opportunity for detailed investigation of fundamental disease mechanisms and the evaluation of interventions with potential clinical applications, together with the possibility of taking this method through to patients.

Elevation of NMDAR after transplantation of neural stem cells

Cognitive deficits could be alleviated by transplantation of neural stem cells in animals. Grafted cells may differentiate into neurons, thereby improving animal cognition. Alternatively, grafted cells may provide neurotrophic factors to modify neuronal functions and to alleviate cognitive deficits. To test which mechanism is underlying this recovery process, senescence-accelerated mice were transplanted with human neural stem cells into the hippocampus. The effect of cell transplantation was assessed in the Morris water maze. The survival and differentiation of grafted cells and the expression of NMDA receptors were examined. The data suggested that in addition to the neural differentiation of grafted neural stem cells, up-regulation of NMDA receptors after transplantation also contributed to the alleviation of cognitive deficits in this animal model.

Tenuigenin treatment decreases secretion of the Alzheimer’s disease amyloid β-protein in cultured cells

Amyloid beta-protein (A beta) is a pivotal pathological factor in Alzheimer's disease (AD). Tenuigenin, extracted from the Chinese herb Polygala tenuifolia, seems to ameliorate the reduction in cholinergic function on rat models induced by A beta. To examine this therapeutic effect, we tested whether Tenuigenin could inhibit secretion of A beta in neuroblastoma cells stably transfected with two amyloid precursor protein (APP) constructs: the APP695 cDNA (SH-SY5Y APP695) and the C-terminal 99 amino acid residues of APP plus the signal peptide (SH-SY5Y SPA4CT). Tenuigenin inhibited the secretion of A beta and the C-terminal 99 amino acids of APP (C99) in SH-SY5Y APP695 cells, but did not change the A beta and C99 levels in SH-SY5Y SPA4CT cells. Fluorescence Resonance Energy Transfer (FRET) assays showed that Tenuigenin inhibited the proteolytic activities of BACE1 (beta-secretase) on its substrate in vitro. In addition, Tenuigenin did not demonstrate any cytotoxic effects, nor did it affect APP mRNA expression, holoAPP synthesis or sAPP alpha secretion. Our data suggest that Tenuigenin can inhibit the secretion of A beta in SH-SY5Y APP 695 cells via BACE1 inhibition. Taken together, these results suggest that Tenuigenin may be worthy of future study as an anti-AD drug.

Par-4 Inhibits Choline Uptake by Interacting with CHT1 and Reducing Its Incorporation on the Plasma Membrane

CHT1 is a Na(+)- and Cl(-)-dependent, hemicholinium-3 (HC-3)-sensitive, high affinity choline transporter. Par-4 (prostate apoptosis response-4) is a leucine zipper protein involved in neuronal degeneration and cholinergic signaling in Alzheimer's disease. We now report that Par-4 is a negative regulator of CHT1 choline uptake activity. Transfection of neural IMR-32 cells with human CHT1 conferred Na(+)-dependent, HC-3-sensitive choline uptake that was effectively inhibited by cotransfection of Par-4. Mapping studies indicated that the C-terminal half of Par-4 was physically involved in interacting with CHT1, and the absence of Par-4.CHT1 complex formation precluded the loss of CHT1-mediated choline uptake induced by Par-4, indicating that Par-4.CHT1 complex formation is essential. Kinetic and cell-surface biotinylation assays showed that Par-4 inhibited CHT1-mediated choline uptake by reducing CHT1 expression in the plasma membrane without significantly altering the affinity of CHT1 for choline or HC-3. These results suggest that Par-4 is directly involved in regulating choline uptake by interacting with CHT1 and by reducing its incorporation on the cell surface.

Noradrenergic mechanisms in neurodegenerative diseases: a theory

A deficiency in the noradrenergic system of the brain, originating largely from cells in the locus coeruleus (LC), is theorized to play a critical role in the progression of a family of neurodegenerative disorders that includes Parkinson's disease (PD) and Alzheimer's disease (AD). Consideration is given here to evidence that several neurodegenerative diseases and syndromes share common elements, including profound LC cell loss, and may in fact be different manifestations of a common pathophysiological process. Findings in animal models of PD indicate that the modification of LC-noradrenergic activity alters electrophysiological, neurochemical and behavioral indices of neurotransmission in the nigrostriatal dopaminergic system, and influences the response of this system to experimental lesions. In models related to AD, noradrenergic mechanisms appear to play important roles in modulating the activity of the basalocortical cholinergic system and its response to injury, and to modify cognitive functions including memory and attention. Mechanisms by which noradrenaline may protect or promote recovery from neural damage are reviewed, including effects on neuroplasticity, neurotrophic factors, neurogenesis, inflammation, cellular energy metabolism and excitotoxicity, and oxidative stress. Based on evidence for facilitatory effects on transmitter release, motor function, memory, neuroprotection and recovery of function after brain injury, a rationale for the potential of noradrenergic-based approaches, specifically alpha2-adrenoceptor antagonists, in the treatment of central neurodegenerative diseases is presented.

Minocycline protects basal forebrain cholinergic neurons from mu p75-saporin immunotoxic lesioning

Two prominent characteristics of Alzheimer's disease are basal forebrain cholinergic degeneration and neuroinflammation characterized by glial activation and the release of pro-inflammatory cytokines. Mu p75- saporin (SAP) is a novel immunotoxin that mimics the selective loss of basal forebrain cholinergic neurons and induces cognitive impairment in mice. We report that cholinergic cell loss in the medial septal nucleus and ventral diagonal band after i.c.v. injection of mu p75-SAP is accompanied by simultaneous activation of microglia and astrocytes in the basal forebrain region as well as significant memory loss. Consistent with a role of glial cells in the pathology of Alzheimer's disease, minocycline, a second-generation tetracycline with known anti-inflammatory and neuroprotective properties, attenuated mu p75-SAP-induced cholinergic cell loss, glial activation and transcription of downstream pro-inflammatory mediators. In addition to neuroprotection, minocycline treatment mitigated the cognitive impairment that appears to be a functional consequence of mu p75-SAP lesioning. The current study demonstrates that glial-related inflammation plays a significant role in the selective neurotoxicity of mu p75-SAP, and suggests that minocycline may provide a viable therapeutic option for degenerating cholinergic systems.

cDNA microarray and proteomic approaches in the study of brain diseases: focus on schizophrenia and Alzheimer's disease

Recent advances in experimental genomics and proteomics, coupled with the wealth of sequence information available for a variety of organisms, have tremendous implications for how biomedical research is performed. Genomic techniques, such as complementary DNA (cDNA) microarrays, currently allow researchers to quickly and accurately quantify vast numbers of potential gene expression changes simultaneously. Modern proteomic techniques allow for the detection and elucidation of protein-protein interactions on a scale and at a speed never before possible. Although hurdles remain, together, these tools open the possibility of enormous change in our ability to analyze and interpret complex biological processes. The field of neuroscience is particularly well suited to analysis with these new techniques, given the complexity of neuronal signaling and the diversity of cellular responses. This review summarizes the major cDNA microarray and proteomic findings of relevance to schizophrenia and Alzheimer's disease (AD) as 2 representative areas of neuroscience research. The potential for these techniques to help unravel the underlying pathology of complex neurological and neuropsychiatric conditions is considerable and warrants continued investigation.

γ-Secretase Activity of Presenilin 1 Regulates Acetylcholine Muscarinic Receptor-mediated Signal Transduction

Familial Alzheimer's disease (FAD) presenilin 1 (PS1) mutations give enhanced calcium responses upon different stimuli, attenuated capacitative calcium entry, an increased sensitivity of cells to undergo apoptosis, and increased gamma-secretase activity. We previously showed that the FAD mutation causing an exon 9 deletion in PS1 results in enhanced basal phospholipase C (PLC) activity (Cedazo-Minguez, A., Popescu, B. O., Ankarcrona, M., Nishimura, T., and Cowburn, R. F. (2002) J. Biol. Chem. 277, 36646-36655). To further elucidate the mechanisms by which PS1 interferes with PLC-calcium signaling, we studied the effect of two other FAD PS1 mutants (M146V and L250S) and two dominant negative PS1 mutants (D257A and D385N) on basal and carbachol-stimulated phosphoinositide (PI) hydrolysis and intracellular calcium concentrations ([Ca2+]i) in SH-SY5Y neuroblastoma cells. We found a significant increase in basal PI hydrolysis in PS1 M146V cells but not in PS1 L250S cells. Both PS1 M146V and PS1 L250S cells showed a significant increase in carbachol-induced [Ca2+]i as compared with nontransfected or wild type PS1 transfected cells. The elevated carbachol-induced [Ca2+]i signals were reversed by the PLC inhibitor neomycin, the ryanodine receptor antagonist dantrolene, the general aspartyl protease inhibitor pepstatin A, and the specific gamma-secretase inhibitor N-[N-(3,5-difluorophenacetyl)-L-alanyl]-S-phenylglycine t-butyl ester. The cells expressing either PS1 D257A or PS1 D385N had attenuated carbachol-stimulated PI hydrolysis and [Ca2+]i responses. In nontransfected or PS1 wild type transfected cells, N-[N-(3,5-difluorophenacetyl)-L-alanyl]-S-phenylglycine t-butyl ester and pepstatin A also attenuated both carbachol-stimulated PI hydrolysis and [Ca2+]i responses to levels found in PS1 D257A or PS1 D385N dominant negative cells. Our findings suggest that PS1 can regulate PLC activity and that this function is gamma-secretase activity-dependent.

Central cholinergic functions in human amyloid precursor protein knock-in/presenilin-1 transgenic mice

Alzheimer's disease is characterized by amyloid peptide formation and deposition, neurofibrillary tangles, central cholinergic dysfunction, and dementia; however, the relationship between these parameters is not well understood. We studied the effect of amyloid peptide formation and deposition on central cholinergic function in knock-in mice carrying the human amyloid precursor protein (APP) gene with the Swedish/London double mutation (APP-SL mice) which were crossbred with transgenic mice overexpressing normal (PS1wt) or mutated (M146L; PS1mut) human presenilin-1. APP-SLxPS1mut mice had increased levels of Abeta peptides at 10 months of age and amyloid plaques at 14 months of age while APP-SLxPS1wt mice did not have increased peptide levels and did not develop amyloid plaques. We used microdialysis in 15-27 months old mice to compare hippocampal acetylcholine (ACh) levels in the two mouse lines and found that extracellular ACh levels were slightly but significantly reduced in the APP-SLxPS1mut mice (-26%; P=0.044). Exploratory activity in the open field increased hippocampal ACh release by two-fold in both mouse lines; total and relative increases were not significantly different for the two strains under study. Similarly, infusion of scopolamine (1 microM) increased hippocampal ACh release to a similar extent (3-5-fold) in both groups. High-affinity choline uptake, a measure of the ACh turnover rate, was identical in both mouse lines. Neurons expressing choline acetyltransferase were increased in the septum of APP-SLxPS1mut mice (+26%; P=0.046). We conclude that amyloid peptide production causes a small decrease of extracellular ACh levels. The deposition of amyloid plaques, however, does not impair stimulated ACh release and proceeds without major changes of central cholinergic function.

Spatial Learning in the 5-HT1B Receptor Knockout Mouse: Selective Facilitation/Impairment Depending on the Cognitive Demand

Age-related memory decline is associated with a combined dysfunction of the cholinergic and serotonergic systems in the hippocampus and frontal cortex, in particular. The 5-HT1B receptor occupies strategic cellular and subcellular locations in these structures, where it plays a role in the modulation of ACh release. In an attempt to characterize the contribution of this receptor to memory functions, 5-HT1B receptor knockout (KO) mice were submitted to various behavioral paradigms carried out in the same experimental context (water maze), which were aimed at exposing mice to various levels of memory demand. 5-HT1BKO mice exhibited a facilitation in the acquisition of a hippocampal-dependent spatial reference memory task in the Morris water maze. This facilitation was selective of task difficulty, showing thus that the genetic inactivation of the 5-HT1B receptor is associated with facilitation when the complexity of the task is increased, and reveals a protective effect on age-related hippocampal-dependent memory decline. Young-adult and aged KO and wild-type (WT) mice were equally able to learn a delayed spatial matching-to-sample working memory task in a radial-arm water maze with short (0 or 5 min) delays. However, 5-HT1BKO mice, only, exhibited a selective memory impairment at intermediate and long (15, 30, and 60 min) delays. Treatment by scopolamine induced the same pattern of performance in wild type as did the mutation for short (5 min, no impairment) and long (60 min, impairment) delays. Taken together, these studies revealed a beneficial effect of the mutation on the acquisition of a spatial reference memory task, but a deleterious effect on a working memory task for long delays. This 5-HT1BKO mouse story highlights the problem of the potential existence of "global memory enhancers."

Beta-amyloid peptide activates non-alpha7 nicotinic acetylcholine receptors in rat basal forebrain neurons

Alzheimer's disease (AD) is a progressive neurodegenerative condition characterized by profound deficits in memory and cognitive function. Neuropathological hallmarks of the disease include a loss of basal forebrain cholinergic neurons and the deposition of beta-amyloid peptide (Abeta) in neuritic plaques. At a cellular level, considerable attention has focused on a study of Abeta interactions with the neuronal nicotinic acetylcholine receptor (nAChR) subtypes. In this study, using cell-attached and outside-out single channel recordings from acutely dissociated rat basal forebrain neurons, we report that Abeta and nicotine activate nAChRs with two distinct levels of single-channel conductance. Whole cell recordings from these neurons reveal Abeta and nicotine, in a concentration-dependent and reversible manner, evoke brisk depolarizing responses and an inward current. The effects of Abeta on both single channel and whole cell are blocked by the noncompetitive nAChR antagonist mecamylamine and competitive nAChR antagonist dihydro-beta-erythroidine, but not the specific alpha7-selective nAChR antagonist methyllycaconitine, indicating that Abeta activated non-alpha7 nAChRs on basal forebrain neurons. In addition, the non-alpha7 nAChR agonists UB-165, epibatidine, and cytisine, but not the selective alpha7 agonist AR-R17779, induced similar responses as Abeta and nicotine. Thus non-alpha7 nAChRs may also represent a novel target in mediating the effects of Abeta in AD.

Regulated intramembrane proteolysis of the p75 neurotrophin receptor modulates its association with the TrkA receptor

The generation of biologically active proteins by regulated intramembrane proteolysis is a highly conserved mechanism in cell signaling. Presenilin-dependent gamma-secretase activity is responsible for the intramembrane proteolysis of selected type I membrane proteins, including beta-amyloid precursor protein (APP) and Notch. A small fraction of intracellular domains derived from both APP and Notch translocates to and appears to function in the nucleus, suggesting a generic role for gamma-secretase cleavage in nuclear signaling. Here we show that the p75 neurotrophin receptor (p75NTR) undergoes presenilin-dependent intramembrane proteolysis to yield the soluble p75-intracellular domain. The p75NTR is a multifunctional type I membrane protein that promotes neurotrophin-induced neuronal survival and differentiation by forming a heteromeric co-receptor complex with the Trk receptors. Mass spectrometric analysis revealed that gamma-secretase-mediated cleavage of p75NTR occurs at a position located in the middle of the transmembrane (TM) domain, which is reminiscent of the amyloid beta-peptide 40 (Abeta40) cleavage of APP and is topologically distinct from the major TM cleavage site of Notch 1. Size exclusion chromatography and co-immunoprecipitation analyses revealed that TrkA forms a molecular complex together with either full-length p75 or membrane-tethered C-terminal fragments. The p75-ICD was not recruited into the TrkA-containing high molecular weight complex, indicating that gamma-secretase-mediated removal of the p75 TM domain may perturb the interaction with TrkA. Independent of the possible nuclear function, our studies suggest that gamma-secretase-mediated p75NTR proteolysis plays a role in the formation/disassembly of the p75-TrkA receptor complex by regulating the availability of the p75 TM domain that is required for this interaction.

Inositol 1,4,5-trisphosphate receptor ubiquitination is mediated by mammalian Ubc7, a component of the endoplasmic reticulum-associated degradation pathway, and is inhibited by chelation of intracellular Zn2+

In response to activation of certain cell surface receptors, inositol 1,4,5-trisphosphate receptors (InsP3Rs), which are located in the endoplasmic reticulum, can be rapidly ubiquitinated and then degraded by the proteasome. Ubiquitination is mediated by the concerted action of ubiquitin-conjugating enzymes (Ubcs or E2s) and ubiquitin-protein ligases (E3s). In the present study we have examined the enzymology of ubiquitination of endogenous InsP3Rs in muscarinic agonist-stimulated SH-SY5Y human neuroblastoma cells, focusing our attention on two mammalian E2s, MmUbc6 and MmUbc7, that have been implicated in endoplasmic reticulum-associated degradation (ERAD) and are homologous to the yeast ERAD E2s, Ubc6p and Ubc7p. Analysis of SH-SY5Y cells stably expressing these enzymes and their dominant-negative mutants revealed that MmUbc7 mediates InsP3R ubiquitination and down-regulation, but that MmUbc6 does not. These data indicate that InsP3Rs are processed by a component of the ERAD pathway and suggest that MmUbc7 may be employed selectively to ubiquitinate proteins, like InsP3Rs, that are subject to regulated ERAD. Additional studies showed that the Zn2+ chelator N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine blocked InsP3R ubiquitination, suggesting that a RING finger domain-containing E3 is also involved in this process. Finally, muscarinic agonist-induced InsP3R ubiquitination was seen in rat brain slices, indicating that the results obtained from SH-SY5Y cells reflect a physiological process.

Glial Cells and Neurotransmission

Glial cells throughout the nervous system are closely associated with synapses. Accompanying these anatomical couplings are intriguing functional interactions, including the capacity of certain glial cells to respond to and modulate neurotransmission. Glial cells can also help establish, maintain, and reconstitute synapses. In this review, we discuss evidence indicating that glial cells make important contributions to synaptic function.

The cholinergic hypothesis of age and Alzheimer's disease-related cognitive deficits: recent challenges and their implications for novel drug development

The cholinergic hypothesis was initially presented over 20 years ago and suggests that a dysfunction of acetylcholine containing neurons in the brain contributes substantially to the cognitive decline observed in those with advanced age and Alzheimer's disease (AD). This premise has since served as the basis for the majority of treatment strategies and drug development approaches for AD to date. Recent studies of the brains of patients who had mild cognitive impairment or early stage AD in which choline acetyltransferase and/or acetylcholinesterase activity was unaffected (or even up-regulated) have, however, led some to challenge the validity of the hypothesis as well as the rationale for using cholinomimetics to treat the disorder, particularly in the earlier stages. These challenges, primarily based on assays of post mortem enzyme activity, should be taken in perspective and evaluated within the wide range of cholinergic abnormalities known to exist in both aging and AD. The results of both post mortem and antemortem studies in aged humans and AD patients, as well as animal experiments suggest that a host of cholinergic abnormalities including alterations in choline transport, acetylcholine release, nicotinic and muscarinic receptor expression, neurotrophin support, and perhaps axonal transport may all contribute to cognitive abnormalities in aging and AD. Cholinergic abnormalities may also contribute to noncognitive behavioral abnormalities as well as the deposition of toxic neuritic plaques in AD. Therefore, cholinergic-based strategies will likely remain valid as one approach to rational drug development for the treatment of AD other forms of dementia.

Synaptic plasticity in animal models of early Alzheimer's disease

Amyloid beta-protein (Abeta) is believed to be a primary cause of Alzheimer's disease (AD). Recent research has examined the potential importance of soluble species of Abeta in synaptic dysfunction, long before fibrillary Abeta is deposited and neurodegenerative changes occur. Hippocampal excitatory synaptic transmission and plasticity are disrupted in transgenic mice overexpressing human amyloid precursor protein with early onset familial AD mutations, and in rats after exogenous application of synthetic Abeta both in vitro and in vivo. Recently, naturally produced soluble Abeta was shown to block the persistence of long-term potentiation (LTP) in the intact hippocampus. Sub-nanomolar concentrations of oligomeric Abeta were sufficient to inhibit late LTP, pointing to a possible reason for the sensitivity of hippocampus-dependent memory to impairment in the early preclinical stages of AD. Having identified the active species of Abeta that can play havoc with synaptic plasticity, it is hoped that new ways of targeting early AD can be developed.