Conductive Hearing Loss Aggravates Memory Decline in Alzheimer Model Mice

Experimental laboratory models as tools for understanding modifiable dementia risk

Experimental laboratory research has an important role to play in dementia prevention. Mechanisms underlying modifiable risk factors for dementia are promising targets for dementia prevention but are difficult to investigate in human populations due to technological constraints and confounds. Therefore, controlled laboratory experiments in models such as transgenic rodents, invertebrates and in vitro cultured cells are increasingly used to investigate dementia risk factors and test strategies which target them to prevent dementia. This review provides an overview of experimental research into 15 established and putative modifiable dementia risk factors: less early-life education, hearing loss, depression, social isolation, life stress, hypertension, obesity, diabetes, physical inactivity, heavy alcohol use, smoking, air pollution, anesthetic exposure, traumatic brain injury, and disordered sleep. It explores how experimental models have been, and can be, used to address questions about modifiable dementia risk and prevention that cannot readily be addressed in human studies. HIGHLIGHTS: Modifiable dementia risk factors are promising targets for dementia prevention. Interrogation of mechanisms underlying dementia risk is difficult in human populations. Studies using diverse experimental models are revealing modifiable dementia risk mechanisms. We review experimental research into 15 modifiable dementia risk factors. Laboratory science can contribute uniquely to dementia prevention.

Update on modifiable risk factors for Alzheimer's disease and related dementias

PURPOSE OF REVIEW

All human beings undergo a lifelong cumulative exposure to potentially preventable adverse factors such as toxins, infections, traumatisms, and cardiovascular risk factors, collectively termed exposome. The interplay between the individual's genetics and exposome is thought to have a large impact in health outcomes such as cancer and cardiovascular disease. Likewise, a growing body of evidence is supporting the idea that preventable factors explain a sizable proportion of Alzheimer's disease and related dementia (ADRD) cases.

RECENT FINDINGS

Here, we will review the most recent epidemiological, experimental preclinical, and interventional clinical studies examining some of these potentially modifiable risk factors for ADRD. We will focus on new evidence regarding cardiovascular risk factors, air pollution, viral and other infectious agents, traumatic brain injury, and hearing loss.

SUMMARY

While greater and higher quality epidemiological and experimental evidence is needed to unequivocally confirm their causal link with ADRD and/or unravel the underlying mechanisms, these modifiable risk factors may represent a window of opportunity to reduce ADRD incidence and prevalence at the population level via health screenings, and education and health policies.

GDF1 ameliorates cognitive impairment induced by hearing loss

Hearing loss is associated with an increased risk of Alzheimer disease (AD). However, the mechanisms of hearing loss promoting the onset of AD are poorly understood. Here we show that hearing loss aggravates cognitive impairment in both wild-type mice and mouse models of AD. Embryonic growth/differentiation factor 1 (GDF1) is downregulated in the hippocampus of deaf mice. Knockdown of GDF1 mimics the detrimental effect of hearing loss on cognition, while overexpression of GDF1 in the hippocampus attenuates the cognitive impairment induced by deafness. Strikingly, overexpression of GDF1 also attenuates cognitive impairment in APP/PS1 transgenic mice. GDF1 activates Akt, which phosphorylates asparagine endopeptidase and inhibits asparagine endopeptidase-induced synaptic degeneration and amyloid-β production. The expression of GDF1 is downregulated by the transcription factor CCAAT-enhancer binding protein-β. These findings indicate that hearing loss could promote AD pathological changes by inhibiting the GDF1 signaling pathway; thus, GDF1 may represent a therapeutic target for AD.

Comprehensive analysis of N6-methyladenosine-related RNA methylation in the mouse hippocampus after acquired hearing loss

BACKGROUND

The mechanism underlying cognitive impairment after hearing loss (HL) remains unclear. N6-methyladenosine (m6A) is involved in many neurodegenerative diseases; however, its role in cognitive impairment after HL has not yet been investigated. Therefore, we aimed to analyze the m6A modification profile of the mouse hippocampus after HL exposure. A mouse model of neomycin-induced HL was established. An auditory brainstem-response test was utilized for detecting hearing threshold. The passive avoidance test was served as the mean for evaluating cognitive function. The m6A-regulated enzyme expression levels were analyzed by using reverse transcription quantitative real-time polymerase chain reaction and western blot analyses. RNA sequencing (RNA-Seq) and methylated RNA immunoprecipitation sequencing (MeRIP-Seq) were performed with the aim of investigating gene expression differences and m6A modification in the mouse hippocampus.

RESULTS

Neomycin administration induced severe HL in mice. At four months of age, the mice in the HL group showed poorer cognitive performance than the mice in the control group. METTL14, WTAP, and YTHDF2 mRNA levels were downregulated in the hippocampi of HL mice, whereas ALKBH5 and FTO mRNA levels were significantly upregulated. At the protein level, METTL3 and FTO were significantly upregulated. Methylated RNA immunoprecipitation sequencing analysis revealed 387 and 361 m6A hypermethylation and hypomethylation peaks, respectively. Moreover, combined analysis of mRNA expression levels and m6A peaks revealed eight mRNAs with significantly changed expression levels and methylation.

CONCLUSIONS

Our findings revealed the m6A transcriptome-wide profile in the hippocampus of HL mice, which may provide a basis for understanding the association between HL and cognitive impairment from the perspective of epigenetic modifications.

Auditory or Audiovisual Stimulation Ameliorates Cognitive Impairment and Neuropathology in ApoE4 Knock-In Mice

We hypothesized that auditory stimulation could reduce the progression of Alzheimer’s disease (AD), and that audiovisual stimulation could have additional effects through multisensory integration. We exposed 12 month old Apoetm1.1(APOE*4)Adiuj mice (a mouse model of sporadic AD) to auditory (A) or audiovisual stimulation (AV) at 40 Hz for 14 days in a soundproof chamber system (no stimulation, N). Behavioral tests were performed before and after each session, and their brain tissues were assessed for amyloid-beta expression and apoptotic cell death, after 14 days. Furthermore, brain levels of acetylcholine and apoptosis-related proteins were analyzed. In the Y-maze test, the percentage relative alternation was significantly higher in group A than in group N mice. Amyloid-beta and TUNEL positivity in the hippocampal CA3 region was significantly lower in group A and group AV mice than in group N mice (p < 0.05). Acetylcholine levels were significantly higher in group A and group AV mice than in group N mice (p < 0.05). Compared to group N mice, expression of the proapoptotic proteins Bax and caspase-3 was lower in group A, and expression of the antiapoptotic protein Bcl-2 was higher in group AV. In a mouse model of early-stage sporadic AD, auditory or audiovisual stimulation improved cognitive performance and neuropathology.

Demand Coupling Drives Neurodegeneration: A Model of Age-Related Cognitive Decline and Dementia

The societal burden of Alzheimer's Disease (AD) and other major forms of dementia continues to grow, and multiple pharmacological agents directed towards modifying the pathological "hallmarks" of AD have yielded disappointing results. Though efforts continue towards broadening and deepening our knowledge and understanding of the mechanistic and neuropathological underpinnings of AD, our previous failures motivate a re-examination of how we conceptualize AD pathology and progression. In addition to not yielding effective treatments, the phenotypically heterogeneous biological processes that have been the primary area of focus to date have not been adequately shown to be necessary or sufficient to explain the risk and progression of AD. On the other hand, a growing body of evidence indicates that lifestyle and environment represent the ultimate level of causation for AD and age-related cognitive decline. Specifically, the decline in cognitive demands over the lifespan plays a central role in driving the structural and functional deteriorations of the brain. In the absence of adequate cognitive stimulus, physiological demand-function coupling leads to downregulation of growth, repair, and homeostatic processes, resulting in deteriorating brain tissue health, function, and capacity. In this setting, the heterogeneity of associated neuropathological tissue hallmarks then occurs as a consequence of an individual's genetic and environmental background and are best considered downstream markers of the disease process rather than specific targets for direct intervention. In this manuscript we outline the evidence for a demand-driven model of age-related cognitive decline and dementia and why it mandates a holistic approach to dementia treatment and prevention that incorporates the primary upstream role of cognitive demand.

Clinical Features and Potential Mechanisms Relating Neuropathological Biomarkers and Blood-Brain Barrier in Patients With Alzheimer’s Disease and Hearing Loss

Background

The aim of this study was to explore clinical features and potential mechanisms relating neuropathological biomarkers and blood-brain barrier (BBB) in Alzheimer’s disease (AD) and hearing loss (HL).

Materials and Methods

A total of 65 patients with AD were recruited and auditory function was assessed by threshold of pure tone audiometry (PTA). Patients were divided into AD with HL (AD-HL) and AD with no HL (AD-nHL) groups based on the standard of World Health Organization. Clinical symptoms were assessed by multiple rating scales. The levels of neuropathological biomarkers of β amyloid1-42 (Aβ_1–42) and multiple phosphorylated tau (P-tau), and BBB factors of matrix metalloproteinases (MMPs), receptor of advanced glycation end products, glial fibrillary acidic protein, and low-density lipoprotein receptor related protein 1 were measured.

Results

(1) Compared with AD-nHL group, AD-HL group had significantly impaired overall cognitive function and cognitive domains of memory, language, attention, execution, and activities of daily living (ADL) reflected by the scores of rating scales (P < 0.05). PTA threshold was significantly correlated with the impairments of overall cognitive function and cognitive domains of memory and language, and ADL in patients with AD (P < 0.05). (2) P-tau (S199) level was significantly increased in CSF from AD-HL group (P < 0.05), and was significantly and positively correlated with PTA threshold in patients with AD. (3) MMP-3 level was significantly elevated in CSF from AD-HL group (P < 0.05), and was significantly and positively correlated with PTA threshold in patients with AD (P < 0.05). (4) In AD-HL group, P-tau (S199) level was significantly and positively correlated with the levels of MMP-2 and MMP-3 in CSF (P < 0.05).

Conclusion

AD-HL patients have severely compromised overall cognitive function, multiple cognitive domains, and ADL. The potential mechanisms of AD-HL involve elevations of AD neuropathological biomarker of P-tau (S199) and BBB factor of MMP-3, and close correlations between P-tau (S199) and MMP-2/MMP-3 in CSF. Findings from this investigation highly suggest significance of early evaluation of HL for delaying AD progression, and indicate new directions of drug development by inhibiting neuropathological biomarkers of AD and protecting BBB.