Accelerated long-term forgetting is a BACE1 inhibitor-reversible incipient cognitive phenotype in Alzheimer's disease model mice

Combination strategy employing BACE1 inhibitor and memantine to boost cognitive benefits in Alzheimer's disease therapy

RATIONALE

The β-secretase BACE1 initiates amyloid-β (Aβ) generation and represents a long-standing prime therapeutic target for the treatment of Alzheimer's disease (AD). However, BACE1 inhibitors tested to date in clinical trials have yielded no beneficial outcomes. In fact, prior BACE1 inhibitor trials targeted at ~ 50-90% Aβ reductions in symptomatic or prodromal AD stages have ended in the discontinuation due to futility and/or side effects, including cognitive worsening rather than expected improvement at the highest dose.

OBJECTIVES

We tested whether a combination strategy with the selective BACE1 inhibitor GRL-8234 and the FDA-approved symptomatic drug memantine may provide synergistic cognitive benefits within their safe dose range.

METHODS

The drug effects were evaluated in the advanced symptomatic stage of 5XFAD mice that developed extensive cerebral Aβ deposition.

RESULTS

Chronic combination treatment with 33.4-mg/kg GRL-8234 and 10-mg/kg memantine, but not either drug alone, rescued cognitive deficits in 5XFAD mice at 12 months of age (the endpoint after 60-day drug treatment), as assessed by the contextual fear conditioning, spontaneous alternation Y-maze and nest building tasks. Intact baseline performances of wild-type control mice on three cognitive paradigms demonstrated that combination treatment did not augment potential cognitive side effects of individual drugs. Biochemical and immunohistochemical examination showed that combination treatment did not synergistically reduce the β-amyloidogenic processing of amyloid precursor protein or Aβ levels in 5XFAD mouse brains.

CONCLUSIONS

A combination strategy with BACE1 inhibitors and memantine may be able to increase the effectiveness of individual drugs within their safe dose range in AD therapy.

A Strategy for Allowing Earlier Diagnosis and Rigorous Evaluation of BACE1 Inhibitors in Preclinical Alzheimer's Disease

Given continued failure of BACE1 inhibitor programs at symptomatic and prodromal stages of Alzheimer's disease (AD), clinical trials need to target the earlier preclinical stage. However, trial design is complex in this population with negative diagnosis of classical hippocampal amnesia on standard memory tests. Besides recent advances in brain imaging, electroencephalogram, and fluid-based biomarkers, new cognitive markers should be established for earlier diagnosis that can optimize recruitment to BACE1 inhibitor trials in presymptomatic AD. Notably, accelerated long-term forgetting (ALF) is emerging as a sensitive cognitive measure that can discriminate between asymptomatic individuals with high risks for developing AD and healthy controls. ALF is a form of declarative memory impairment characterized by increased forgetting rates over longer delays (days to months) despite normal storage within the standard delays of testing (20-60 min). Therefore, ALF may represent a harbinger of preclinical dementia and the impairment of systems memory consolidation, during which memory traces temporarily stored in the hippocampus become gradually integrated into cortical networks. This review provides an overview of the utility of ALF in a rational design of next-generation BACE1 inhibitor trials in preclinical AD. I explore potential mechanisms underlying ALF and relevant early-stage biomarkers useful for BACE1 inhibitor evaluation, including synaptic protein alterations, astrocytic dysregulation and neuron hyperactivity in the hippocampal-cortical network. Furthermore, given the physiological role of the isoform BACE2 as an AD-suppressor gene, I also discuss the possible association between the poor selectivity of BACE1 inhibitors and their side effects (e.g., cognitive worsening) in prior clinical trials.

The Role of Oxidative Stress and Inflammation in Obesity and Its Impact on Cognitive Impairments-A Narrative Review

Obesity is a chronic low-grade inflammatory condition that induces the generation of oxidative stress and inflammation. This oxidative stress and inflammation stimulate brain atrophy and some morphological changes in the brain that eventually result in cognitive impairments. However, there is no exact study that has summarized the role of oxidative stress and inflammation in obesity and its impact on cognitive impairments. Thus, the objective of this review is to recapitulate the current role of oxidative stress and inflammation in cognitive decline based on in vivo evidence. A comprehensive search was performed in Nature, Medline and Ovid, ScienceDirect, and PubMed, and the search was limited to the past 10 years of publication. From the search, we identified 27 articles to be further reviewed. The outcome of this study indicates that a greater amount of fat stored in individual adipocytes in obesity induces the formation of reactive oxygen species and inflammation. This will lead to the generation of oxidative stress, which may cause morphological changes in the brain, suppress the endogenous antioxidant system, and promote neuroinflammation and, eventually, neuronal apoptosis. This will impair the normal function of the brain and specific regions that are involved in learning, as well as memory. This shows that obesity has a strong positive correlation with cognitive impairments. Hence, this review summarizes the mechanism of oxidative stress and inflammation that induce memory loss based on animal model evidence. In conclusion, this review may serve as an insight into therapeutic development focusing on oxidative stress and inflammatory pathways to manage an obesity-induced cognitive decline in the future.

Accelerated long-term forgetting: A sensitive paradigm for detecting subtle cognitive impairment and evaluating BACE1 inhibitor efficacy in preclinical Alzheimer's disease

Given a long preclinical stage of Alzheimer's disease (AD) continuum before the onset of dementia, there is a growing demand for tools capable of detecting the earliest feature of subtle cognitive impairment and optimizing recruitment to clinical trials for potentially disease-modifying therapeutic interventions such as BACE1 inhibitors. Now that all BACE1 inhibitor programs in symptomatic and prodromal AD populations have ended in failure, trials need to shift to target the earlier preclinical stage. However, evaluating cognitive efficacy (if any) in asymptomatic AD individuals is a great challenge. In this context, accelerated long-term forgetting (ALF) is emerging as a sensitive cognitive measure that can discriminate between presymptomatic individuals with high risks for developing AD and healthy controls. ALF is characterized by increased forgetting rates over extended delays (e.g., days, weeks, months) despite normal learning and short-term retention on standard memory assessments that typically use around 30-min delays. This review provides an overview of recent progress in animal model and clinical studies on this topic, focusing on the utility and underlying mechanism of ALF that may be applicable to earlier diagnosis and BACE1 inhibitor efficacy evaluation at a preclinical stage of AD.

Forgetting as a form of adaptive engram cell plasticity

One leading hypothesis suggests that memories are stored in ensembles of neurons (or 'engram cells') and that successful recall involves reactivation of these ensembles. A logical extension of this idea is that forgetting occurs when engram cells cannot be reactivated. Forms of 'natural forgetting' vary considerably in terms of their underlying mechanisms, time course and reversibility. However, we suggest that all forms of forgetting involve circuit remodelling that switches engram cells from an accessible state (where they can be reactivated by natural recall cues) to an inaccessible state (where they cannot). In many cases, forgetting rates are modulated by environmental conditions and we therefore propose that forgetting is a form of neuroplasticity that alters engram cell accessibility in a manner that is sensitive to mismatches between expectations and the environment. Moreover, we hypothesize that disease states associated with forgetting may hijack natural forgetting mechanisms, resulting in reduced engram cell accessibility and memory loss.

Accelerated long-term forgetting is a BACE1 inhibitor-reversible incipient cognitive phenotype in Alzheimer's disease model mice

AIM

After the continued failure of β-secretase (BACE1) inhibitor clinical trials in prodromal as well as mild-to-moderate Alzheimer's disease (AD), they are shifting to further earlier or asymptomatic stages. The aim of this study is to explore a cognitive paradigm that allows us to more sensitively detect beneficial effects of BACE1 inhibitors in presymptomatic AD.

METHODS

GRL-8234 (33.4 mg/kg, ip), a small-molecule BACE1 inhibitor, was administered once daily for 28 days to the 5XAFD transgenic mouse model of AD. The contextual fear conditioning was used to evaluate the effects of GRL-8234 on memory deficits in 5XFAD mice at different ages.

RESULTS

Chronic administration of GRL-8234 to 5XFAD mice rescued their contextual memory deficits, when tested 1 day after training at 6-8 months but not at 12 months of age. Importantly, 4-month-old 5XFAD mice retain the ability to form contextual memory equivalent to wild-type controls, demonstrating that the standard method of 1-day memory assessment is not suitable for evaluating BACE1 inhibitor efficacy in ameliorating cognitive declines during earlier disease stages. Despite normal contextual memory formation, young 5XFAD mice showed faster forgetting when a longer delay (28 days) intervened between training and memory testing. Notably, GRL-8234 administered to 4-month-old 5XFAD mice during the 28-day delay reversed accelerated long-term forgetting almost completely back to wild-type control levels.

CONCLUSION

The results provide experimental evidence that accelerated long-term forgetting represents more sensitive memory testing that can help evaluate BACE1 inhibitor therapy in presymptomatic AD populations.