Effect of a 5-HT(2C) serotonin agonist, dexnorfenfluramine, on amyloid precursor protein metabolism in guinea pigs

Alzheimer Disease-Link With Major Depressive Disorder and Efficacy of Antidepressants in Modifying its Trajectory

Alzheimer disease (AD) is a devastating neurodegenerative disorder that affects millions of individuals worldwide, with no effective cure. The main symptoms include learning and memory loss, and the inability to carry out the simplest tasks, significantly affecting patients' quality of life. Over the past few years, tremendous progress has been made in research demonstrating a link between AD and major depressive disorder (MDD). Evidence suggests that MDD is commonly associated with AD and that it can serve as a precipitating factor for this disease. Antidepressants such as selective serotonin reuptake inhibitors, which are the first line of treatment for MDD, have shown great promise in the treatment of depression in AD, although their effectiveness remains controversial. The goal of this review is to summarize current knowledge regarding the association between AD, MDD, and antidepressant treatment. It first provides an overview of the interaction between AD and MDD at the level of genes, brain regions, neurotransmitter systems, and neuroinflammatory markers. The review then presents current evidence regarding the effectiveness of various antidepressants for AD-related pathophysiology and then finally discusses current limitations, challenges, and future directions.

Serotonin Receptor Gene Polymorphisms Are Associated with Cerebrospinal Fluid, Genetic, and Neuropsychological Biomarkers of Alzheimer's Disease

A decrease in serotonergic transmission throughout the brain is among the earliest pathological changes in Alzheimer's disease (AD). Serotonergic receptors are also affected in AD. Polymorphisms in genes of serotonin (5HT) receptors have been mostly associated with behavioral and psychological symptoms of dementia (BPSD). In this study, we examined if AD patients carrying different genotypes in 5HTR1B rs13212041, 5HTR2A rs6313 (T102C), 5HTR2C rs3813929 (-759C/T), and 5HTR6 rs1805054 (C267T) polymorphisms have a higher risk of faster disease progression (assessed by neuropsychological testing), are more prone to develop AD-related pathology (reflected by levels of cerebrospinal fluid [CSF] AD biomarkers), or have an association with an apolipoprotein E (APOE) haplotype. This study included 115 patients with AD, 53 patients with mild cognitive impairment (MCI), and 2701 healthy controls. AD biomarkers were determined in the CSF of AD and MCI patients using enzyme-linked immunosorbent assays (ELISA), while polymorphisms were determined using either TaqMan SNP Genotyping Assays or Illumina genotyping platforms. We detected a significant decrease in the CSF amyloid β_1-42 (Aβ_1-42) and an increase in p-tau₁₈₁/Aβ_1-42 ratio in carriers of the T allele in the 5HTR2C rs3813929 (-759C/T) polymorphism. A significantly higher number of APOE ε4 allele carriers was observed among individuals carrying a TT genotype within the 5HTR2A T102C polymorphism, a C allele within the 5HTR1B rs13212041 polymorphism, and a T allele within the 5HTR6 rs1805054 (C267T) polymorphism. Additionally, individuals carrying the C allele within the 5HTR1B rs13212041 polymorphism were significantly more represented among AD patients and had poorer performances on the Rey-Osterrieth test. Carriers of the T allele within the 5HTR6 rs1805054 had poorer performances on the MMSE and ADAS-Cog. As all four analyzed polymorphisms of serotonin receptor genes showed an association with either genetic, CSF, or neuropsychological biomarkers of AD, they deserve further investigation as potential early genetic biomarkers of AD.

The Combined Effects of Amyloidosis and Serotonin Deficiency by Tryptophan Hydroxylase-2 Knockout Impacts Viability of the APP/PS1 Mouse Model of Alzheimer’s Disease

Background: A decline of brain serotonin (5-HT) is held responsible for the changes in mood that can be observed in Alzheimer’s disease (AD). However, 5-HT’ergic signaling is also suggested to reduce the production of pathogenic amyloid-4β (Aβ). Objective: To investigate the effect of targeted inactivation of tryptophan hydroxylase-2 (Tph2), which is essential for neuronal 5-HT synthesis, on amyloidosis in amyloid precursor protein (APP)swe/presenilin 1 (PS1) ΔE9 transgenic mice. Methods: Triple-transgenic (3xTg) APP/PS1 mice with partial (+/-) or complete Tph2 knockout (–/–) were allowed to survive until 6 months old with APP/PS1, Tph2–/–, and wildtype mice. Survival and weight were recorded. Levels of Aβ 42/40/38, soluble APPα (sAβPPα) and sAβPPβ, and cytokines were analyzed by mesoscale, neurotransmitters by mass spectrometry, and gene expression by quantitative PCR. Tph2, microglia, and Aβ were visualized histologically. Results: Tph2 inactivation in APP/PS1 mice significantly reduced viability, without impacting soluble and insoluble Aβ 42 and Aβ 40 in neocortex and hippocampus, and with only mild changes of soluble Aβ 42/Aβ 40. However, sAβPPα and sAβPPβ in hippocampus and Aβ 38 and Aβ 40 in cerebrospinal fluid were reduced. 3xTg–/–mice were devoid of Tph2 immunopositive fibers and 5-HT. Cytokines were unaffected by genotype, as were neocortical TNF, HTR2a and HTR2b mRNA levels in Tph2–/– mice. Microglia clustered around Aβ plaques regardless of genotype. Conclusion: The results suggest that Tph2 inactivation influences AβPP processing, at least in the hippocampus, although levels of Aβ are unchanged. The reduced viability of 3xTg–/–mice could indicate that 5-HT protects against the seizures that can impact the viability of APP/PS1 mice.

Trait anxiety, a personality risk factor associated with Alzheimer's Disease

Alzheimer's disease (AD) is the most common neurodegenerative disease in elderly population and the leading cause of dementia worldwide. While senile plaques and neurofibrillary tangles have been proposed as the principal histopathologic hallmarks of AD, the exact etiology of this disease is still far from being clearly understood. AD has been recognized as pathological consequences of complex interactions among genetic, aging, medical, life style and psychosocial factors. Recently, the roles of neuroticism personality traits in AD incidence and progression have come into focus. More specifically, increasing evidence has further shown that the trait anxiety, one major component of neuroticism predicting the individual vulnerability in response to stress, is a risk factor for AD and may correlated with various AD pathologies. In this review, we summarized recent literature on the association of trait anxiety with AD. We also discussed the possible neuroendocrinological and neurochemical mechanisms of this association, which may provide clinical implications for AD diagnosis and therapy.

Effect of escitalopram dose and treatment duration on CSF Aβ levels in healthy older adults: A controlled clinical trial

OBJECTIVE

To determine whether treatment with escitalopram compared with placebo would lower CSF β-amyloid 42 (Aβ₄₂) levels.

RATIONALE

Serotonin signaling suppresses Aβ₄₂ in animal models of Alzheimer disease (AD) and young healthy humans. In a prospective study in older adults, we examined dose and treatment duration effects of escitalopram.

METHODS

Using lumbar punctures to sample CSF levels before and after a course of escitalopram treatment, cognitively normal older adults (n = 114) were assigned to placebo, 20 mg escitalopram × 2 weeks, 20 mg escitalopram × 8 weeks, or 30 mg escitalopram × 8 weeks; CSF sampled pretreatment and posttreatment and within-subject percent change in Aβ₄₂ was used as the primary outcome in subsequent analyses.

RESULTS

An overall 9.4% greater reduction in CSF Aβ₄₂ was found in escitalopram-treated compared with placebo-treated groups (p < 0.001, 95% confidence interval [CI] 4.9%-14.2%, d = 0.81). Positive baseline Aβ status (CSF Aβ₄₂ levels <250 pg/mL) was associated with smaller Aβ₄₂ reduction (p = 0.006, 95% CI -16.7% to 0.5%, d = -0.52) compared with negative baseline amyloid status (CSF Aβ₄₂ levels >250 pg/mL).

CONCLUSIONS

Short-term longitudinal doses of escitalopram decreased CSF Aβ₄₂ in cognitively normal older adults, the target group for AD prevention.

CLINICALTRIALSGOV IDENTIFIER

NCT02161458.

CLASSIFICATION OF EVIDENCE

This study provides Class II evidence that for cognitively normal older adults, escitalopram decreases CSF Aβ₄₂.

Pharmacological Treatment of Depression in Alzheimer's Disease: A Challenging Task

Besides the memory impairment, Alzheimer’s disease (AD) is often complicated by neuropsychiatric symptoms also known as behavioral and psychological symptoms of dementia, which occur in one-third of patients at an early stage of the disease. Although the relationship between depressive disorders and AD is debated, the question if depression is a prodromal symptom preceding cognitive deficits or an independent risk factor for AD is still unclear. Moreover, there is growing evidence reporting that conventional antidepressants are not effective in depression associated with AD and, therefore, there is an urgent need to understand the neurobiological mechanism underlying the resistance to the antidepressants. Another important question that remains to be addressed is whether the antidepressant treatment is able to modulate the levels of amyloid-β peptide (Aβ), which is a key pathological hallmark in AD. The present review summarizes the present knowledge on the link between depression and AD with a focus on the resistance of antidepressant therapies in AD patients. Finally, we have briefly outlined the preclinical and clinical evidences behind the possible mechanisms by which antidepressants modulate Aβ pathology. To our opinion, understanding the cellular processes that regulate Aβ levels may provide greater insight into the disease pathogenesis and might be helpful in designing novel selective and effective therapy against depression in AD.

Serotonergic system, cognition, and BPSD in Alzheimer's disease

Behavioral and Psychological Symptoms of Dementia (BPSD), present in almost 90% of patients with Alzheimer's Disease (AD), cause extensive impairment leading to reduced independence and inability to complete activities of daily living. Though BPSD includes a wide range of symptoms, such as agitation, aggression, disinhibition, anxiety, depression, apathy, delusions, and hallucinations. Certain BPSD in AD co-present and can be clustered into distinct domains based on their frequency of co-occurrence. As these BPSD are so pervasive in any stages of AD, the disease may be better characterized as a disorder of heterogeneous degenerative symptoms across a number of symptom domains, with the most prominent domain comprising memory and cognitive deficits. Importantly, there are no FDA-approved drugs to treat these BPSD, and new approaches must be considered to develop effective treatments for AD patients. The biogenic monoamine 5-hydroxytryptamine (5-HT), or serotonin, works as both a neurotransmitter and neuromodulator, which has been tied to cognitive decline and multiple BPSD domains. This review summarizes the evidence for specific serotonergic system alterations across some of the well-studied cognitive, behavioral, and psychiatric domains. Though differences in overall serotonergic transmission occur in AD, circuit-specific alterations in individual 5-HT receptors (5-HTRs) are likely linked to the heterogeneous presentation of BPSD in AD.

Antidepressant drugs for beta amyloid-induced depression: A new standpoint?

Mounting evidence suggests that depression represents a risk factor and an early manifestation of Alzheimer's disease (AD). Neuropsychiatric symptoms may derive from neurobiological changes in specific brain areas and may be considered prodromal of dementia. We have previously reported the depressive-like profile in rats receiving a single intracerebroventricular injection of soluble amyloid beta protein (ßA). Here, we verified the effect of different classes of antidepressants on the ßA-induced depressive behavior and on cortical monoamine levels. To these purposes, the forced swimming test was performed and cortical levels of serotonin (5-HT) and noradrenaline (NA) were quantified by high performance liquid chromatography (HPLC). We found that acute fluoxetine (20mg/kg, s.c.), reboxetine (10mg/kg, s.c.), and ketamine (15mg/kg, i.p.) significantly reduced the immobility in ßA-treated rats compared to controls. Fluoxetine and reboxetine reversed 5-HT reduction, while βA-induced NA increase was further enhanced by all treatments. Treatments with fluoxetine, reboxetine and ketamine were able to revert soluble ßA-induced decrease of cortical BDNF levels, while only fluoxetine and ketamine, but not reboxetine, had the same effects on cortical NGF expression. Moreover, plasma soluble ßA-levels were lowered by fluoxetine, but not reboxetine and ketamine, treatments. Our data suggest that different classes of antidepressants yield a short-acting effect on rat soluble ßA-induced depressive profile. Thus, we hypothesize a novel common mechanism of action of these drugs also based upon a "ßA lowering" effect. Although further investigations are still needed, our study might open a new scenario for unravelling the molecular antidepressant mechanisms of these drugs.

Redundant Gs-coupled serotonin receptors regulate amyloid-β metabolism in vivo

BACKGROUND

The aggregation of amyloid-β (Aβ) into insoluble plaques is a hallmark pathology of Alzheimer's disease (AD). Previous work has shown increasing serotonin levels with selective serotonin re-uptake inhibitor (SSRI) compounds reduces Aβ in the brain interstitial fluid (ISF) in a mouse model of AD and in the cerebrospinal fluid of humans. We investigated which serotonin receptor (5-HTR) subtypes and downstream effectors were responsible for this reduction.

RESULTS

Agonists of 5-HT4R, 5-HT6R, and 5-HT7R significantly reduced ISF Aβ, but agonists of other receptor subtypes did not. Additionally, inhibition of Protein Kinase A (PKA) blocked the effects of citalopram, an SSRI, on ISF Aβ levels. Serotonin signaling does not appear to change gene expression to reduce Aβ levels in acute timeframes, but likely acts within the cytoplasm to increase α-secretase enzymatic activity. Broad pharmacological inhibition of putative α-secretases increased ISF Aβ and blocked the effects of citalopram.

CONCLUSIONS

In total, these studies map the major signaling components linking serotonin receptors to suppression of brain ISF Aβ. These results suggest the reduction in ISF Aβ is mediated by a select group of 5-HTRs and open future avenues for targeted therapy of AD.

Serotoninergic antidepressants positively affect platelet ADAM10 expression in patients with Alzheimer's disease

BACKGROUND

Studies have demonstrated a decreased platelet ADAM10 expression in patients with Alzheimer's Disease (AD), classifying this protein as a blood-based AD biomarker. About 50% of the patients with AD are diagnosed with depression, which is commonly treated with tricyclic and tetracyclic antidepressants, monoaminoxidade (MAO) inhibitors and, more preferably, with selective serotonin reuptake inhibitors (SSRIs). Considering that a large proportion of patients with AD takes antidepressant medications during the course of the disease we investigated the influence of this medication on the expression of platelet ADAM10, which is considered the main α-secretase preventing beta-amyloid (βA) formation.

METHODS

Blood was collected for protein extraction from platelets. ADAM10 was analyzed by using western blotting and reactive bands were measured using β-actin as endogenous control.

RESULTS

Platelet ADAM10 protein expression in patients with AD was positively influenced by serotoninergic medication.

CONCLUSION

More studies on the positive effects of serotonergic antidepressants on ADAM10 platelet expression should be performed in order to understand its biological mechanisms and to verify whether these effects are reflected in the central nervous system. This work represents an important advance for the study of AD biomarkers, as well as for more effective pharmacological treatment of patients with AD and associated depression.

The serotonergic system and cognitive function

Symptoms of cognitive dysfunction like memory loss, poor concentration, impaired learning and executive functions are characteristic features of both schizophrenia and Alzheimer’s disease (AD). The neurobiological mechanisms underlying cognition in healthy subjects and neuropsychiatric patients are not completely understood. Studies have focused on serotonin (5-hydroxytryptamine, 5-HT) as one of the possible cognitionrelated biomarkers. The aim of this review is to provide a summary of the current literature on the role of the serotonergic (5-HTergic) system in cognitive function, particularly in AD and schizophrenia.

The role of the 5-HTergic system in cognition is modulated by the activity and function of 5-HT receptors (5-HTR) classified into seven groups, which differ in structure, action, and localization. Many 5-HTR are located in the regions linked to various cognitive processes. Preclinical studies using animal models of learning and memory, as well as clinical in vivo (neuroimaging) and in vitro (post-mortem) studies in humans have shown that alterations in 5-HTR activity influence cognitive performance.

The current evidence implies that reduced 5-HT neurotransmission negatively influences cognitive functions and that normalization of 5-HT activity may have beneficial effects, suggesting that 5-HT and 5-HTR represent important pharmacological targets for cognition enhancement and restoration of impaired cognitive performance in neuropsychiatric disorders.

G Protein-Coupled Receptors (GPCRs) in Alzheimer's Disease: A Focus on BACE1 Related GPCRs

The G protein coupled receptors (GPCRs) have been considered as one of the largest families of validated drug targets, which involve in almost overall physiological functions and pathological processes. Meanwhile, Alzheimer’s disease (AD), the most common type of dementia, affects thinking, learning, memory and behavior of elderly people, that has become the hotspot nowadays for its increasing risks and incurability. The above fields have been intensively studied, and the link between the two has been demonstrated, whereas the way how GPCRs perturb AD progress are yet to be further explored given their complexities. In this review, we summarized recent progress regarding the GPCRs interacted with β-site APP cleaving enzyme 1 (BACE1), a key secretase in AD pathogenesis. Then we discussed the current findings on the regulatory roles of GPCRs on BACE1, and the possibility for pharmaceutical treatment of AD patients by the allosteric modulators and biased ligands of GPCRs. We hope this review can provide new insights into the understanding of mechanistic link between GPCRs and BACE1, and highlight the potential of GPCRs as therapeutic target for AD.

Amyloid Hypothesis: Is There a Role for Antiamyloid Treatment in Late-Life Depression?

Antidepressants have modest efficacy in late-life depression (LLD), perhaps because various neurobiologic processes compromise frontolimbic networks required for antidepressant response. We propose that amyloid accumulation is an etiologic factor for frontolimbic compromise that predisposes to depression and increases treatment resistance in a subgroup of older adults. In patients without history of depression, amyloid accumulation during the preclinical phase of Alzheimer disease (AD) may result in the prodromal depression syndrome that precedes cognitive impairment. In patients with early-onset depression, pathophysiologic changes during recurrent episodes may promote amyloid accumulation, further compromise neurocircuitry required for antidepressant response, and increase treatment resistance during successive depressive episodes. The findings that support the amyloid hypothesis of LLD are (1) Depression is a risk factor, a prodrome, and a common behavioral manifestation of AD; (2) amyloid deposition occurs during a long predementia period when depression is prevalent; (3) patients with lifetime history of depression have significant amyloid accumulation in brain regions related to mood regulation; and (4) amyloid deposition leads to neurobiologic processes, including vascular damage, neurodegeneration, neuroinflammation, and disrupted functional connectivity, that impair networks implicated in depression. The amyloid hypothesis of LLD is timely because availability of ligands allows in vivo assessment of amyloid in the human brain, a number of antiamyloid agents are relatively safe, and there is evidence that some antidepressants may reduce amyloid production. A model of LLD introducing the role of amyloid may guide the design of studies aiming to identify novel antidepressant approaches and prevention strategies of AD.

Drug repositioning approaches for the discovery of new therapeutics for Alzheimer's disease

Alzheimer's disease (AD) is the most common cause of dementia and represents one of the highest unmet needs in medicine today. Drug development efforts for AD have been encumbered by largely unsuccessful clinical trials in the last decade. Drug repositioning, a process of discovering a new therapeutic use for existing drugs or drug candidates, is an attractive and timely drug development strategy especially for AD. Compared with traditional de novo drug development, time and cost are reduced as the safety and pharmacokinetic properties of most repositioning candidates have already been determined. A majority of drug repositioning efforts for AD have been based on positive clinical or epidemiological observations or in vivo efficacy found in mouse models of AD. More systematic, multidisciplinary approaches will further facilitate drug repositioning for AD. Some experimental approaches include unbiased phenotypic screening using the library of available drug collections in physiologically relevant model systems (e.g. stem cell-derived neurons or glial cells), computational prediction and selection approaches that leverage the accumulating data resulting from RNA expression profiles, and genome-wide association studies. This review will summarize several notable strategies and representative examples of drug repositioning for AD.

Activation of 5-HT(2C) receptor promotes the expression of neprilysin in U251 human glioma cells

Abeta accumulation, a hallmark of Alzheimer's disease, promotes the disease progress in multiple facets. Abeta is formed through amyloidogenic cleavage pathway of amyloid precursor protein (APP). Production of Abeta can be decreased via activation of 5-HT2C receptor, which enhances alternative APP non-amyloidogenic cleavage. Besides, as one of the best characterized Aβ degrading enzymes, neprilysin (NEP) in AD progress has drawn more and more attention. We investigated whether there exists any connection between 5-HT2C receptor and NEP expression. The mRNA and protein expressions of NEP were increased after treatment of 5-HT2C receptor agonist RO-60-0175 in concentration- and time-dependent manners, and NEP expression was decreased after treatment of 5-HT2C receptor antagonist SB242084 correspondingly. These results suggest that 5-HT2C receptor may inhibit the Abeta formation by promoting NEP expression. The underlying mechanism will be explored in follow-up study and may provide potential target for AD therapy.

An antidepressant decreases CSF Aβ production in healthy individuals and in transgenic AD mice

Serotonin signaling suppresses generation of amyloid-β (Aβ) in vitro and in animal models of Alzheimer's disease (AD). We show that in an aged transgenic AD mouse model (APP/PS1 plaque-bearing mice), the antidepressant citalopram, a selective serotonin reuptake inhibitor, decreased Aβ in brain interstitial fluid in a dose-dependent manner. Growth of individual amyloid plaques was assessed in plaque-bearing mice that were chronically administered citalopram. Citalopram arrested the growth of preexisting plaques and reduced the appearance of new plaques by 78%. In healthy human volunteers, citalopram's effects on Aβ production and Aβ concentrations in cerebrospinal fluid (CSF) were measured prospectively using stable isotope labeling kinetics, with CSF sampling during acute dosing of citalopram. Aβ production in CSF was slowed by 37% in the citalopram group compared to placebo. This change was associated with a 38% decrease in total CSF Aβ concentrations in the drug-treated group. The ability to safely decrease Aβ concentrations is potentially important as a preventive strategy for AD. This study demonstrates key target engagement for future AD prevention trials.

The role of serotonin in memory: interactions with neurotransmitters and downstream signaling

Serotonin, or 5-hydroxytryptamine (5-HT), is found to be involved in many physiological or pathophysiological processes including cognitive function. Seven distinct receptors (5-HT1-7), each with several subpopulations, have been identified for serotonin, which are different in terms of localization and downstream signaling. Because of the development of selective agonists and antagonists for these receptors as well as transgenic animal models of cognitive disorders, our understanding of the role of serotonergic transmission in learning and memory has improved in recent years. A large body of evidence indicates the interplay between serotonergic transmission and other neurotransmitters including acetylcholine, dopamine, γ-aminobutyric acid (GABA) and glutamate, in the neurobiological control of learning and memory. In addition, there has been an alteration in the density of serotonergic receptors in aging and Alzheimer's disease, and serotonin modulators are found to alter the process of amyloidogenesis and exert cognitive-enhancing properties. Here, we discuss the serotonin-induced modulation of various systems involved in mnesic function including cholinergic, dopaminergic, GABAergic, glutamatergic transmissions as well as amyloidogenesis and intracellular pathways.

Role of synaptic activity in the regulation of amyloid beta levels in Alzheimer's disease

Alzheimer's disease (AD) is the most common form of dementia. Accumulation of amyloid-beta (Aβ) peptides is regarded as the critical component associated with AD pathogenesis, which is derived from the amyloid precursor protein (APP) cleavage. Recent studies suggest that synaptic activity is one of the most important factors that regulate Aβ levels. It has been found that synaptic activity facilitates APP internalization and influences APP cleavage. Glutamatergic, cholinergic, serotonergic, leptin, adrenergic, orexin, and gamma-amino butyric acid receptors, as well as the activity-regulated cytoskeleton-associated protein (Arc) are all involved in these processes. The present review summarizes the evidence for synaptic activity-modulated Aβ levels and the mechanisms underlying this regulation. Interestingly, the immediate early gene product Arc may also be the downstream signaling molecule of several receptors in the synaptic activity-modulated Aβ levels. Elucidating how Aβ levels are regulated by synaptic activity may provide new insights in both the understanding of the pathogenesis of AD and in the development of therapies to slow down the progression of AD.

Aging, cortical injury and Alzheimer's disease-like pathology in the guinea pig brain

Alzheimer's disease (AD) is a progressive neurodegenerative disorder characterized histopathologically by the abnormal deposition of the proteins amyloid-beta (Aβ) and tau. A major issue for AD research is the lack of an animal model that accurately replicates the human disease, thus making it difficult to investigate potential risk factors for AD such as head injury. Furthermore, as age remains the strongest risk factor for most of the AD cases, transgenic models in which mutant human genes are expressed throughout the life span of the animal provide only limited insight into age-related factors in disease development. Guinea pigs (Cavia porcellus) are of interest in AD research because they have a similar Aβ sequence to humans and thus may present a useful non-transgenic animal model of AD. Brains from guinea pigs aged 3-48 months were examined to determine the presence of age-associated AD-like pathology. In addition, fluid percussion-induced brain injury was performed to characterize mechanisms underlying the association between AD risk and head injury. No statistically significant changes were detected in the overall response to aging, although we did observe some region-specific changes. Diffuse deposits of Aβ were found in the hippocampal region of the oldest animals and alterations in amyloid precursor protein processing and tau immunoreactivity were observed with age. Brain injury resulted in a strong and sustained increase in amyloid precursor protein and tau immunoreactivity without Aβ deposition, over 7 days. Guinea pigs may therefore provide a useful model for investigating the influence of environmental and non-genetic risk factors on the pathogenesis of AD.

Serotonin signaling is associated with lower amyloid-β levels and plaques in transgenic mice and humans

Aggregation of amyloid-β (Aβ) as toxic oligomers and amyloid plaques within the brain appears to be the pathogenic event that initiates Alzheimer's disease (AD) lesions. One therapeutic strategy has been to reduce Aβ levels to limit its accumulation. Activation of certain neurotransmitter receptors can regulate Aβ metabolism. We assessed the ability of serotonin signaling to alter brain Aβ levels and plaques in a mouse model of AD and in humans. In mice, brain interstitial fluid (ISF) Aβ levels were decreased by 25% following administration of several selective serotonin reuptake inhibitor (SSRI) antidepressant drugs. Similarly, direct infusion of serotonin into the hippocampus reduced ISF Aβ levels. Serotonin-dependent reductions in Aβ were reversed if mice were pretreated with inhibitors of the extracellular regulated kinase (ERK) signaling cascade. Chronic treatment with an SSRI, citalopram, caused a 50% reduction in brain plaque load in mice. To test whether serotonin signaling could impact Aβ plaques in humans, we retrospectively compared brain amyloid load in cognitively normal elderly participants who were exposed to antidepressant drugs within the past 5 y to participants who were not. Antidepressant-treated participants had significantly less amyloid load as quantified by positron emission tomography (PET) imaging with Pittsburgh Compound B (PIB). Cumulative time of antidepressant use within the 5-y period preceding the scan correlated with less plaque load. These data suggest that serotonin signaling was associated with less Aβ accumulation in cognitively normal individuals.

Establishment and characterization of RNA-edited serotonin 2C receptor isoform cell models and alteration of amyloid precursor protein ectodomain secretion in HEK293 APPSwe cells

RNA editing is a mechanism for generating molecular diversity by altering the genetic code at the level of RNA. The 5-HT(2C) receptor is the only G protein-coupled receptor known to be edited. It has been reported that the non-edited 5-HT(2C) receptor stimulates secretion of the APP metabolite APP ectodomain (APPs). However, it remains unknown whether RNA-edited 5-HT(2C) receptors can also affect APPs secretion. In this study, cDNAs of five non-edited or partially/fully edited 5-HT(2C) receptor isoforms (INI, VNI, VNV, VSV and VGV) were stably transfected into HEK293APPSwe cells to detect the cell proliferation and APPs secretion. The results demonstrated that the overexpression of INI and VNI caused increased proliferation of host cells while VNV, VSV and VGV caused inverse effects (P < 0.01). Compared with both control and non-edited isoform INI, APPs levels were significantly increased in the four edited 5-HT(2C) receptor isoforms, VNI (P < 0.05), VNV (P < 0.05), VSV (P < 0.05) and VGV (P < 0.01). These results suggest that the RNA editing of the 5-HT(2C) receptor may affect APPs secretion through different signaling pathways related to cell growth and protein processing, and that these cell models will provide appropriate useful information to study the association between the RNA editing of the serotonin 5-HT(2C) receptor and APP metabolism.

Is there a CSF biomarker profile related to depression in elderly women?

In light of our previous observation of higher levels of cerebrospinal fluid (CSF) amyloid beta-42 (Abeta42) and CSF/serum albumin ratio in major depressive disorder (MDD), we analyzed two additional CSF biomarkers reflecting neurodegeneration-neurofilament protein light (NFL) and glial fibrillary acidic protein (GFAp)-in relationship to prevalent geriatric depression. Neuropsychiatric, physical, and lumbar puncture examinations, with DSM-III-R-based depression diagnoses and measurement of CSF levels of NFL and GFAp, were evaluated among a population-based sample of 78 elderly women (mean age, 73.9+/-3.2 years) without dementia for at least 10 years after CSF collection. Eleven (13.1%) women had MDD, and higher levels of NFL compared with women without depression. A multivariate model including age, NFL, Abeta42 and the CSF/serum albumin ratio showed that each biomarker was independently and positively associated with MDD, and that this biomarker profile explained more variation in the model compared with single or combined biomarkers. A CSF profile with higher levels of NFL, Abeta42, and CSF/serum albumin ratio may indicate neuropathological and vascular events in depression etiology. This contrasts with the well-characterized pattern of low Abeta42, higher CSF/serum albumin ratio, and higher NFL in Alzheimer's disease.

Antidepressants are a rational complementary therapy for the treatment of Alzheimer's disease

There is a high prevalence rate (30-50%) of Alzheimer's disease (AD) and depression comorbidity. Depression can be a risk factor for the development of AD or it can be developed secondary to the neurodegenerative process. There are numerous documented diagnosis and treatment challenges for the patients who suffer comorbidity between these two diseases. Meta analysis studies have provided evidence for the safety and efficacy of antidepressants in treatment of depression in AD patients. Preclinical and clinical studies show the positive role of chronic administration of selective serotonin reuptake inhibitor (SSRI) antidepressants in hindering the progression of the AD and improving patient performance. A number of clinical studies suggest a beneficial role of combinatorial therapies that pair antidepressants with FDA approved AD drugs. Preclinical studies also demonstrate a favorable effect of natural antidepressants for AD patients. Based on the preclinical studies there are a number of plausible antidepressants effects that may modulate the progression of AD. These effects include an increase in neurogenesis, improvement in learning and memory, elevation in the levels of neurotrophic factors and pCREB and a reduction of amyloid peptide burden. Based on this preclinical and clinical evidence, antidepressants represent a rational complimentary strategy for the treatment of AD patients with depression comorbidity.

The relationship between cerebrospinal fluid biomarkers and depression in elderly women

OBJECTIVE

Cerebrospinal fluid (CSF) biomarkers including the 42 amino-acid form of beta-amyloid (Abeta42), total tau protein (T-tau), and the CSF/serum albumin ratio are markers of brain pathology and metabolism. Abeta42 and T-tau are sometimes used to discriminate geriatric depression from mild forms of Alzheimer disease (AD) in clinical studies. However, studies focusing on the relationship between these CSF biomarkers and geriatric depression are lacking.

METHODS

This was a cross-sectional study with a population-based sample of 84 nondemented elderly women in Sweden. Measurements included neuropsychiatric, physical, and lumbar puncture examinations, with Diagnostic and Statistical Manual of Mental Disorders, Third Revision-based depression diagnoses and measurement of CSF levels of Abeta42, T-tau, albumin, and serum albumin.

RESULTS

Fourteen women (mean age: 72.6 years) had any depression (11 with major depressive disorder [MDD]). Compared to women without depression, women with MDD had higher levels of Abeta42 and the CSF/serum albumin ratio. The CSF/serum albumin ratio was also higher in women with any depression. No differences in T-tau were observed; however, T-tau increased with age.

CONCLUSION

Higher levels of CSF Abeta42 were observed among elderly depressed women, in contrast to lower levels usually observed in AD, indicating potential neuropathological differences between the two disorders. Higher CSF/serum albumin ratios observed in depressed women point to potential vascular processes.

Determination of guinea-pig cortical gamma-secretase activity ex vivo following the systemic administration of a gamma-secretase inhibitor

(2S)-2-{[(3,5-Diflurophenyl)acetyl]amino}-N-[(3S)-1-methyl-2-oxo-5-phenyl-2,3-dihydro-1H-1,4-benzodiazepin-3-yl]propanamide (compound E) is a gamma-secretase inhibitor capable of reducing amyloid beta-peptide (1-40) and amyloid beta-peptide (1-42) levels. In this study we investigated the effect of in vivo administration of compound E on guinea-pig plasma, CSF and cortical amyloid beta-peptide (1-40) concentration. Using repeated sampling of CSF, compound E (30 mg/kg p.o.) was shown to cause a time-dependent decrease in CSF amyloid beta-peptide (1-40) levels, which was maximal at 3 h (70% inhibition), compared to baseline controls. After 3 h administration, compound E (3, 10 and 30 mg/kg p.o.), reduced plasma, CSF and DEA-extracted cortical amyloid beta-peptide (1-40) levels by 95, 97 and 99%; 26, 48 and 78%; 32, 33, and 47%, respectively, compared to vehicle control values. In the same animals, compound E (3, 10 and 30 mg/kg p.o.) inhibited cortical gamma-secretase activity, determined ex vivo using the recombinant substrate C100Flag, by 40, 71 and 79% of controls, respectively. These data demonstrate the value of determining not only the extent by which systemic administration of a gamma-secretase inhibitor reduces amyloid beta-peptide, but also the inhibition of brain gamma-secretase activity, as a more direct estimate of enzyme occupancy.

SAR of psilocybin analogs: Discovery of a selective 5-HT2C agonist

An SAR study of psilocybin and psilocin derivatives reveals that 1-methylpsilocin is a selective agonist at the h5-HT(2C) receptor. The corresponding phosphate derivative, 1-methylpsilocybin, shows efficacy in an animal model for obsessive-compulsive disorder, as does 4-fluoro-N,N-dimethyltryptamine. These results suggest a new area for development of novel 5-HT(2C) agonists with applications for drug discovery.

Characterization of serotonin 5-HT2C receptor signaling to extracellular signal-regulated kinases 1 and 2

Serotonin 5-HT2C receptors (5-HT(2C)Rs) are almost exclusively expressed in the CNS, and implicated in disorders such as obesity, depression, and schizophrenia. The present study investigated the mechanisms governing the coupling of the 5-HT(2C)R to the extracellular signal-regulated kinases (ERKs) 1/2, using a Chinese hamster ovary (CHO) cell line stably expressing the receptor at levels comparable to those found in the brain. Using the non-RNA-edited isoform of the 5-HT(2C)R, constitutive ERK1/2 phosphorylation was observed and found to be modulated by full, partial and inverse agonists. Interestingly, agonist-directed trafficking of receptor stimulus was also observed when comparing effects on phosphoinositide accumulation and intracellular Ca2+ elevation to ERK1/2 phosphorylation, whereby the agonists, [+/-]-2,5-dimethoxy-4-iodoamphetamine (DOI) and quipazine, showed reversal of efficacy between the phosphoinositide/Ca2+ pathways, on the one hand, and the ERK1/2 pathway on the other. Subsequent molecular characterization found that 5-HT-stimulated ERK1/2 phosphorylation in this cellular background requires phospholipase D, protein kinase C, and activation of the Raf/MEK/ERK module, but is independent of both receptor- and non-receptor tyrosine kinases, phospholipase C, phosphoinositide 3-kinase, and endocytosis. Our findings underscore the potential for exploiting pathway-selective receptor states in the differential modulation of signaling pathways that play prominent roles in normal and abnormal neuronal signaling.

Enhanced Prospects for Drug Delivery and Brain Targeting by the Choroid Plexus–CSF Route

The choroid plexus (CP), i.e., the blood-cerebrospinal fluid barrier (BCSFB) interface, is an epithelial boundary exploitable for drug delivery to brain. Agents transported from blood to lateral ventricles are convected by CSF volume transmission (bulk flow) to many periventricular targets. These include the caudate, hippocampus, specialized circumventricular organs, hypothalamus, and the downstream pia-glia and arachnoid membranes. The CSF circulatory system normally provides micronutrients, neurotrophins, hormones, neuropeptides, and growth factors extensively to neuronal networks. Therefore, drugs directed to CSF can modulate a variety of endocrine, immunologic, and behavioral phenomema; and can help to restore brain interstitial and cellular homeostasis disrupted by disease and trauma. This review integrates information from animal models that demonstrates marked physiologic effects of substances introduced into the ventricular system. It also recapitulates how pharmacologic agents administered into the CSF system prevent disease or enhance the brain's ability to recover from chemical and physical insults. In regard to drug distribution in the CNS, the BCSFB interaction with the blood-brain barrier is discussed. With a view toward translational CSF pharmacotherapy, there are several promising innovations in progress: bone marrow cell infusions, CP encapsulation and transplants, neural stem cell augmentation, phage display of peptide ligands for CP epithelium, CSF gene transfer, regulation of leukocyte and cytokine trafficking at the BCSFB, and the purification of neurotoxic CSF in degenerative states. The progressively increasing pharmacological significance of the CP-CSF nexus is analyzed in light of treating AIDS, multiple sclerosis, stroke, hydrocephalus, and Alzheimer's disease.

Treatment of dementia with neurotransmission modulation

The prevalence of dementia is growing in developed countries where elderly patients are increasing in numbers. Neurotransmission modulation is one approach to the treatment of dementia. Cholinergic precursors, anticholinesterases, nicotine receptor agonists and muscarinic M(2) receptor antagonists are agents that enhance cholinergic neurotransmission and that depend on having some intact cholinergic innervation to be effective in the treatment of dementia. The cholinergic precursor choline alfoscerate may be emerging as a potential useful drug in the treatment of dementia, with few adverse effects. Of the anticholinesterases, donepezil, in addition to having a similar efficacy to tacrine in mild-to-moderate Alzheimer's disease (AD), appears to have major advantages; its use is associated with lower drop-out rates in clinical trials, a lower incidence of cholinergic-like side effects and no liver toxicity. Rivastigmine is efficacious in the treatment in dementia with Lewy bodies, a condition in which the other anticholinesterases have not been tested extensively to date. Galantamine is an anticholinesterase and also acts as an allosteric potentiating modulator at nicotinic receptors to increase the release of acetylcholine. Pooled data from clinical trials of patients with mild-to-moderate AD suggest that the benefits and safety profile of galantamine are similar to those of the anticholinesterases. Selective nicotine receptor agonists are being developed that enhance cognitive performance without influencing autonomic and skeletal muscle function, but these have not yet entered clinical trial for dementia. Unlike the cholinergic enhancers, the M(1) receptor agonists do not depend upon intact cholinergic nerves but on intact M(1) receptors for their action, which are mainly preserved in AD and dementia with Lewy bodies. The M(1) receptor-selective agonists developed to date have shown limited efficacy in clinical trials and have a high incidence of side effects. A major recent advancement in the treatment of dementia is memantine, a non-competitive antagonist at NMDA receptors. Memantine is beneficial in the treatment of severe and moderate-to-severe AD and may also be of some benefit in the treatment of mild-to-moderate vascular dementia. Drugs that modulate 5-HT, somatostatin and noradrenergic neurotransmission are also being considered for the treatment of dementia.

FDA-preapproved drugs targeted to the translational regulation and processing of the amyloid precursor protein

The 5' untranslated region (5'UTR) of the transcript encoding the Alzheimer's amyloid precursor protein (APP) is a key regulatory sequence that determines the amount of intracellular APP holoprotein present in brain derived cells. Using neuroblastoma cells (SY5Y) we developed a transfection based screen of a library of FDA drugs to identify compounds that limited APP luciferase reporter expression translated from the APP 5'UTR. Paroxetine (Paxil trade mark ), dimercaptopropanol, phenserine, desferrioxamine, tetrathiolmobdylate, and azithromycin were six leads that were subsequently found to also suppress APP holoprotein levels or to alter APP cleavage (azithromycin). Since APP holoprotein levels are proportionate to Abeta peptide output in many systems we tested the efficacy of paroxetine and dimercaptopropanol to limit Abeta secretion as measured by ELISA assays. Paroxetine and dimercaptopropanol limited Abeta peptide secretion from lens epithelial cells (B3 cells). Interestingly, paroxetine changed the steady-state levels of transferrin receptor mRNAs. These data suggested that this serotonin reuptake inhibitor (SSRI) provided extra pharmacological action to chelate interacellular iron or change the intracellular iron distribution. An altered iron distribution would be predicted to indirectly limit APP holoprotein expression and Abeta peptide secretion.

Drug discovery targeted to the Alzheimer's APP mRNA 5'-untranslated region: the action of paroxetine and dimercaptopropanol

We screened for drugs that specifically interact with the 5'-untranslated region of the mRNA encoding the Alzheimer's amyloid precursor protein (APP). Our goal was to use newly discovered APP 5' UTR directed compounds to limit amyloid-beta (Abeta)-peptide output in cell culture systems. The APP 5' UTR folds into a stable RNA secondary structure (Gibbs free energy: DeltaG = -54.9 kcal/mol) and is an important regulator of the amount of APP translated in response to IL-1 (Nilsson et al., 1998; Rogers et al., 1999) and iron (Rogers et al., 2002). Seventeen drug "hits" were identified from a library of 1,200 FDA preapproved drugs (Rogers et al., 2002). Six of the original 17 compounds were validated for their capacity to suppress reporter gene expression in stable neuroblastoma transfectants expressing the dicistronic reporter construct shown in Fig. 2. These six leads suppressed APP 5' UTR driven luciferase translation while causing no effect on the translation of dicistronic GFP gene translated from a viral IRES (negative control to ensure specificity during drug screens). In this report, we show that paroxetine (serotonin reuptake blocker) and dimercaptopropanol (Hg chelator) exerted significant effects on APP expression (steady-state levels of APP), whereas Azithromycin altered APP processing. None of these three compounds altered APLP-1 expression. In the future, we will identify further novel compounds that influence Abeta levels, either via translation inhibition or by changing the activity of proteins coupled between APP translation and APP processing.