Is there a future for cyclo-oxygenase inhibitors in Alzheimer's disease?

Resistance and Resilience to Alzheimer's Disease

Dementia is a significant public health crisis; the most common underlying cause of age-related cognitive decline and dementia is Alzheimer's disease neuropathologic change (ADNC). As such, there is an urgent need to identify novel therapeutic targets for the treatment and prevention of the underlying pathologic processes that contribute to the development of AD dementia. Although age is the top risk factor for dementia in general and AD specifically, these are not inevitable consequences of advanced age. Some individuals are able to live to advanced age without accumulating significant pathology (resistance to ADNC), whereas others are able to maintain cognitive function despite the presence of significant pathology (resilience to ADNC). Understanding mechanisms of resistance and resilience will inform therapeutic strategies to promote these processes to prevent or delay AD dementia. This article will highlight what is currently known about resistance and resilience to AD, including our current understanding of possible underlying mechanisms that may lead to candidate preventive and treatment interventions for this devastating neurodegenerative disease.

The emerging role of brain neuroinflammatory responses in Alzheimer's disease

As the most common cause of dementia, Alzheimer's disease (AD) is characterized by neurodegeneration and synaptic loss with an increasing prevalence in the elderly. Increased inflammatory responses triggers brain cells to produce pro-inflammatory cytokines and accelerates the Aβ accumulation, tau protein hyper-phosphorylation leading to neurodegeneration. Therefore, in this paper, we discuss the current understanding of how inflammation affects brain activity to induce AD pathology, the inflammatory biomarkers and possible therapies that combat inflammation for AD.

The neuroinflammatory role of microglia in Alzheimer's disease and their associated therapeutic targets

INTRODUCTION

Alzheimer's disease (AD), the main cause of dementia, is characterized by synaptic loss and neurodegeneration. Amyloid-β (Aβ) accumulation, hyperphosphorylation of tau protein, and neurofibrillary tangles (NFTs) in the brain are considered to be the initiating factors of AD. However, this hypothesis falls short of explaining many aspects of AD pathogenesis. Recently, there has been mounting evidence that neuroinflammation plays a key role in the pathophysiology of AD and causes neurodegeneration by over-activating microglia and releasing inflammatory mediators.

METHODS

PubMed, Web of Science, EMBASE, and MEDLINE were used for searching and summarizing all the recent publications related to inflammation and its association with Alzheimer's disease.

RESULTS

Our review shows how inflammatory dysregulation influences AD pathology as well as the roles of microglia in neuroinflammation, the possible microglia-associated therapeutic targets, top neuroinflammatory biomarkers, and anti-inflammatory drugs that combat inflammation.

CONCLUSION

In conclusion, microglial inflammatory reactions are important factors in AD pathogenesis and need to be discussed in more detail for promising therapeutic strategies.

Etiology and pathogenesis of late-onset Alzheimer's disease

Alzheimer's disease (AD) is a neurodegenerative condition that occurs in two forms, an early-onset form that is genetically determined and a far more common late-onset form that is not. In both cases, the disease results in severe cognitive dysfunction, among other problems, and the late-onset form of the disease is now considered to be the most common cause of dementia among the elderly. While a good deal of research has been focused on elucidating the etiology of the late-onset form for more than two decades, results to date have been modest and have not yet engendered useful therapeutic strategies for cure of the disease. In this review, we discuss the prevalent ideas that have governed this research for several years, and we challenge these ideas with alternative findings suggesting a multifactorial etiology. We review promising newer ideas that may prove effective as therapeutic interventions for late-onset AD, as well as providing reliable means of earlier and more specific diagnosis of the disease process. In the discussions included here, we reference relevant clinical and basic science literature underlying research into disease etiology and pathogenesis, and we highlight current reviews on the various topics addressed.

1,4-Diaryl-substituted triazoles as cyclooxygenase-2 inhibitors: Synthesis, biological evaluation and molecular modeling studies

A novel group of 1,4-diaryl-substituted triazoles was designed and synthesized by introducing the cyclooxygenase-2 (COX-2) pharmacophore SO2NH2 attached to one aryl ring and various substituents (H, F, Cl, CH3 or OCH3) attached to the other aryl ring. The effects of size and flexibility of the compounds upon COX-1/COX-2 inhibitory potency and selectivity was studied by increasing the size of an alkyl linker chain [(-CH2)n, where n=0, 1, 2]. In vitro COX-1/COX-2 inhibition studies showed that all compounds (14-18, 21-25 and 28-32) are more potent inhibitors of COX-2 isozyme (IC50=0.17-28.0μM range) compared to COX-1 isozyme (IC50=21.0 to >100μM range). Within the group of 1,4 diaryl-substituted triazoles, 4-{2-[4-(4-chloro-phenyl)-[1,2,3]triazol-1-yl]-ethyl}-benzenesulfonamide (compound 30) displayed highest COX-2 inhibitory potency and selectivity (COX-1: IC50=>100μM, COX-2: IC50=0.17μM, SI >588). Molecular docking studies using the catalytic site of COX-1 and COX-2, respectively, provided complementary theoretical support for the obtained experimental biological structure-activity relationship data. Results of molecular docking studies revealed that COX-2 pharmacophore SO2NH2 in compound 30 is positioned in the secondary pocket of COX-2 active site; with the nitrogen atom of the SO2NH2 group being hydrogen bonded to Q192 (N⋯OC=2.85Å), and one of the oxygen atoms of SO2NH2 group forming a hydrogen bond to H90 (SO⋯N=2.38Å).

Angiotensin-(1-7) via the mas receptor alleviates the diabetes-induced decrease in GFAP and GAP-43 immunoreactivity with concomitant reduction in the COX-2 in hippocampal formation: an immunohistochemical study

We have previously shown that chronic treatment with angiotensin-(1-7) [Ang-(1-7)] can prevent diabetes-induced cardiovascular dysfunction. However, effect of Ang-(1-7) treatment on diabetes-induced alterations in the CNS is unknown. The aim of this study was to test the hypothesis that treatment with Ang-(1-7) can produce protection against diabetes-induced CNS changes. We examined the effect of Ang-(1-7) on the number of cyclooxygenase-2 (COX-2) immunoreactive neurons and the glial fibrillary acidic protein (GFAP)-immunoreactive astrocytes and assessed the changes in the neuronal growth-associated protein-43 (GAP-43) of the hippocampal formation in streptozotocin-induced diabetes in rats. Animals were sacrificed 30 days after induction of diabetes and/or treatment with Ang-(1-7). Ang-(1-7) treatment significantly prevented diabetes-induced decrease in the number of GFAP immunoreactive astrocytes and GAP-43 positive neurons in all hippocampal regions. Co-administration of A779, a selective Ang-(1-7) receptor antagonist, inhibited Ang-(1-7)-mediated protective effects indicating that Ang-(1-7) produces its effects through activation of receptor Mas. Further, Ang-(1-7) treatment through activation of Mas significantly prevented diabetes-induced increase in the number of the COX-2 immunolabeled neurons in all sub-regions of the hippocampus examined. These results show that Ang-(1-7) has a protective role against diabetes-induced changes in the CNS.

Neuroprotective Mechanisms of PPARδ: Modulation of Oxidative Stress and Inflammatory Processes

Peroxisome proliferator-activated receptors (PPARα, δ, and γ) are ligand-activated transcription factors that regulate a wide range of cellular processes, including inflammation, proliferation, differentiation, metabolism, and energy homeostasis. All three PPAR subtypes have been identified in the central nervous system (CNS) of rodents. While PPARα and PPARγ are expressed in more restricted areas of the CNS, PPARδ is ubiquitously expressed and is the predominant subtype. Although data regarding PPARδ are limited, studies have demonstrated that administration of PPARδ agonists confers neuroprotection following various acute and chronic injuries to the CNS, such as stroke, multiple sclerosis, and Alzheimer's disease. The antioxidant and anti-inflammatory properties of PPARδ agonists are thought to underly their neuroprotective efficacy. This review will focus on the putative neuroprotective benefits of therapeutically targeting PPARδ in the CNS, and specifically, highlight the antioxidant and anti-inflammatory functions of PPARδ agonists.

Effects of medications on plasma amyloid beta (Abeta) 42: longitudinal data from the VITA cohort

In the course of cognitive deterioration leading to Alzheimer's disease (AD) the increase of amyloid beta (Abeta42) in cerebrospinal fluid or plasma might be an initial event. We previously reported about the associations between concomitant medication and plasma Abeta42 levels in the non-demented population cohort of the Vienna transdanube aging study at baseline. In the present study, the longitudinal influence of insulin, gingko biloba, non-steroidal anti-inflammatory drugs (NSAIDs), oral anti-diabetics (sulfonylurea and biguanides), estrogens, fibrates, and statins on plasma Abeta42 are presented. Associated with medial temporal lobe atrophy (MTA), users of insulin showed significantly increased levels of Abeta42. Long-term users of gingko biloba, independent of their MTA, had significantly decreased plasma Abeta42 and the age-dependent increase of plasma Abeta42 was significantly smaller in long-term gingko biloba treated subjects. The use of fibrates also decreased plasma Abeta42 levels. In multiple testing considering interactions between medications, gender, APOE-epsilon4 presence and creatinine, insulin long-term users again showed significantly increased levels; fibrate and gingko biloba users showed a trend to rather decreased plasma Abeta42 levels compared to the non-users (p=0.05-0.08). Neither statins nor NSAIDs showed a significant effect on plasma Abeta42 in this model. Measuring the effect on cognition, no single medication studied was a significant predictor of conversion to AD or mild cognitive impairment (MCI). Whether the use of gingko biloba might prevent the conversion to MCI or AD needs to be proven in prospective, clinical trials.

Immunomodulation with glatiramer acetate prevents long-term inflammatory pain

The potential application of glatiramer acetate (GA) therapy as a safe pharmacological treatment for the attenuation or prevention of long-term inflammatory pain in a rat model was explored. Peripheral inflammatory pain was induced by an injection of Complete Freund's Adjuvant (CFA) into the plantar surface of the hind paw. Genome-wide DNA microarray studies were used to survey molecular mechanisms involved in long-term GA analgesic responses. Administration of a single or double subcutaneous injection of GA before, or immediately after, intraplantar injection of pro-inflammatory CFA significantly attenuated allodynia and hyperalgesic pain responses up to approximately 3 weeks after CFA treatment. These beneficial effects of GA immunization therapy coincided with the attenuation of expression of the chemotactic fractalkine chemokine (CX3CL1) in the dorsal horn of the lumbar spinal cord (L4-L5) in response to CFA treatment, assessed by DNA microarray and confirmed immunocytochemically (ICC). This study is consistent with the hypothesis that a novel mechanism through which GA immunization therapy may beneficially influence long-term allodynia and hyperalgesia is through central regulation of fractalkine-mediated responses.

Anti-inflammatory and immune therapy for Alzheimer's disease: current status and future directions

From the initial characterizations of inflammatory responses in Alzheimer's disease (AD) affected brains, namely the demonstration of activated microglia and reactive astrocytes, complement system activation, increased production of proinflammatory cytokines, and evidence for microglial-produced neurotoxins, there was hope that reducing inflammation might be a feasible treatment for this memory-robbing disease. This hope was supported by a number of epidemiology studies demonstrating that patients who took non-steroidal anti-inflammatory drugs had significantly lower risk of developing AD. However, clinical trials of anti-inflammatories have not shown effectiveness, and in recent years, the concept of immune therapy has become a treatment option as animal studies and clinical trials with Abeta vaccines have demonstrated enhanced amyloid removal through stimulation of microglial phagocytosis.This review will examine the current status of whether inhibiting inflammation is a valid therapeutic target for treating AD; what lessons have come from the clinical trials; what new pathways and classes of agents are being considered; and how this field of research can progress towards new therapeutics. We will examine a number of agents that have shown effectiveness in reducing inflammation amongst other demonstrated mechanisms of action. The major focus of much AD drug discovery has been in identifying agents that have anti-amyloid properties; however, a number of these agents were first identified for their anti-inflammatory properties. As drug development and clinical testing is a costly and lengthy endeavor, sound justification of new therapeutic targets is required. Possible future directions for AD anti-inflammatory or immune clearance therapy will be discussed based on recent experimental data.

A cyclooxygenase-2 inhibitor ameliorates behavioral impairments induced by striatal administration of epidermal growth factor

Consistent with the hypothesis that neuroinflammatory processes contribute to the neuropathology of schizophrenia, the protein levels of epidermal growth factor (EGF) and its receptor ErbB1 are abnormal in patients with schizophrenia. To evaluate neuropathological significance of this abnormality, we established an animal model for behavioral deficits by administering EGF into the striatum and evaluated the effects of cyclooxygenase-2 (Cox-2) inhibitor celecoxib. Intracranial infusion of EGF into the striatum of adult male rats activated ErbB1 and induced neurobehavioral impairments observed in several schizophrenia models. Unilateral EGF infusion to the striatum lowered prepulse inhibition (PPI) in a dose-dependent manner and impaired latent learning of active shock avoidance without affecting basal learning ability. Bilateral EGF infusion similarly affected PPI. In contrast, EGF infusion to the nucleus accumbens did not induce a behavioral deficit. Intrastriatal EGF infusion also increased Cox-2 expression, elevated tyrosine hydroxylase activity, and upregulated the levels of dopamine and its metabolites. Subchronic administration of celecoxib (10 mg/kg, p.o.) ameliorated the abnormalities in PPI and latent learning as well as normalized dopamine metabolism. We conclude that this EGF-triggered neuroinflammatory process is mediated in part by Cox-2 activity and perturbs dopamine metabolism to generate neurobehavioral abnormalities.

Cellular therapy using microglial cells

Recent insights into the function and dysfunction of microglia may inform future therapies to combat neurodegeneration. We hypothesise how different aspects of microglial activity including migration, activation, oxidative response, phagocytosis, proteolysis, and replenishment could be targeted by novel therapeutic approaches. A combined approach is suggested, encompassing opsonization and anti-inflammatory strategies in conjunction with an engineering of microglial precursors. Xenoproteases for bioremediation could be used to enhance intracellular and extracellular proteolytic capacity. The capacity of microglial precursors to cross the blood-brain barrier and to home in on sites of neural damage and inflammation might prove to be particularly useful for future therapeutic strategies.

Therapeutic approaches to inflammation in neurodegenerative disease

PURPOSE OF REVIEW

According to the neuroinflammatory hypothesis of neurodegenerative diseases, drugs with an anti-inflammatory mode of action should slow the disease progression. Here we review recent advances in our understanding of one such disorder, Parkinson's disease, in which anti-inflammatory drugs are now becoming a new therapeutic focus.

RECENT FINDINGS

The involvement of inflammatory mechanisms in Parkinson's disease has been revealed through in-vitro and in-vivo experimental studies supported by pathological and epidemiological findings. Several of the demonstrated inflammatory mechanisms are shared by other neurodegenerative disorders but some Parkinson's disease-specific mechanisms have also emerged. These include inflammatory stimulation by interaction of alpha-synuclein with microglia and astrocytes and a suppressive action by nonsteroidal anti-inflammatory drugs on dopamine quinone formation.

SUMMARY

It can be anticipated that a more detailed understanding of neuroinflammatory mechanisms in Parkinson's disease will lead to new cellular and molecular targets, which may, in turn, permit design of Parkinson's disease modifying drugs. Future treatment may involve combination therapies with drugs directed at both inflammatory and non-inflammatory mechanisms.

Angiotensin-(1-7)-induced plasticity changes in the lateral amygdala are mediated by COX-2 and NO

It is known from studies outside the brain that upon binding to its receptor, angiotensin-(1-7) elicits the release of prostanoids and nitric oxide (NO). Cyclooxygenase (COX) is a key enzyme that converts arachidonic acid to prostaglandins. Since there are no data available so far on the role of COX-2 in the amygdala, in a first step we demonstrated that the selective COX-2 inhibitor NS-398 significantly reduced the probability of long-term potentiation (LTP) induction in the lateral nucleus of the amygdala. Similarly, in COX-2(-/-) mice, LTP induced by external capsule (EC) stimulation was impaired. Second, we evaluated the action of angiotensin-(1-7) in the amygdala. In wild-type mice, angiotensin-(1-7) increased LTP. This LTP-enhancing effect of Ang-(1-7) was not observed in COX-2(+/-) mice. However, in COX-2(-/-) mice, Ang-(1-7) caused an enhancement of LTP similar to that in wild-type mice. The NO synthetase inhibitor L-NAME blocked this angiotensin-(1-7)-induced increase in LTP in COX-2(-/-) mice. Low-frequency stimulation of external capsule fibers did not cause long-term depression (LTD) in drug-free and angiotensin-(1-7)-treated brain slices in wild-type mice. In contrast, in COX-2(-/-) mice, angiotensin-(1-7) caused stable LTD. Increasing NO concentration by the NO-donor SNAP also caused LTD in wild-type mice. Our study shows for the first time that LTP in the amygdala is dependent on COX-2 activity. Moreover, COX-2 is involved in the mediation of angiotensin-(1-7) effects on LTP. Finally, it is recognized that there is a molecular cross-talk between COX-2 and NO that may regulate synaptic plasticity.

Microglia-mediated neurotoxicity: uncovering the molecular mechanisms

Mounting evidence indicates that microglial activation contributes to neuronal damage in neurodegenerative diseases. Recent studies show that in response to certain environmental toxins and endogenous proteins, microglia can enter an overactivated state and release reactive oxygen species (ROS) that cause neurotoxicity. Pattern recognition receptors expressed on the microglial surface seem to be one of the primary, common pathways by which diverse toxin signals are transduced into ROS production. Overactivated microglia can be detected using imaging techniques and therefore this knowledge offers an opportunity not only for early diagnosis but, importantly, for the development of targeted anti-inflammatory therapies that might slow or halt the progression of neurodegenerative disease.