Circadian Rhythms Disturbances in Alzheimer Disease: Current Concepts, Diagnosis, and Management

Neuropathology-based approach reveals novel Alzheimer's Disease genes and highlights female-specific pathways and causal links to disrupted lipid metabolism: insights into a vicious cycle

Dementia refers to an umbrella phenotype of many different underlying pathologies with Alzheimer's disease (AD) being the most common type. Neuropathological examination remains the gold standard for accurate AD diagnosis, however, most that we know about AD genetics is based on Genome-Wide Association Studies (GWAS) of clinically defined AD. Such studies have identified multiple AD susceptibility variants with a significant portion of the heritability unexplained and highlighting the phenotypic and genetic heterogeneity of the clinically defined entity. Furthermore, despite women's increased susceptibility to dementia, there is a lack of sex-specific genetic studies and understanding of sex-specific background for the disorder. Here, we aim to tackle the heterogeneity of AD by specifically concentrating on neuropathological features and pursuing sex-specific analysis. We bring together 14 different genomic and neuropathology datasets (6960 individuals) and we integrate our GWAS findings with transcriptomic and phenotypic data aiming to also identify biomarkers for AD progression. We uncover novel genetic associations to AD neuropathology, including BIN1 and OPCML. Our sex-specific analysis points to a role for BIN1 specifically in women as well as novel AD loci including QRFPR and SGCZ. Post-GWAS analyses illuminate the functional and biological mechanisms underlying AD and reveal sex-specific differences. Finally, through PheWAS and Mendelian Randomization analysis, we identify causal links with AD neuropathology pointing to disrupted lipid metabolism, as well as impaired peripheral immune response and liver dysfunction as part of a vicious cycle that fuels neurodegeneration.

Apolipoprotein E Induces Lipid Accumulation Through Dgat2 That Is Prevented with Time-Restricted Feeding in Drosophila

Background: Apolipoprotein E (ApoE) is the leading genetic risk factor for late-onset Alzheimer's disease (AD), which is the leading cause of dementia worldwide. Most people have two ApoE-ε3 (ApoE3) alleles, while ApoE-ε2 (ApoE2) is protective from AD, and ApoE-ε4 (ApoE4) confers AD risk. How these alleles modulate AD risk is not clearly defined, and ApoE's role in lipid metabolism is also not fully known. Lipid droplets increase in AD. However, how ApoE contributes to lipid accumulation in the brain remains unknown. Methods: Here, we use Drosophila to study the effects of ApoE alleles on lipid accumulation in the brain and muscle in a cell-autonomous and non-cell-autonomous manner. Results: We report that pan-neuronal expression of each ApoE allele induces lipid accumulation specifically in the brain, but not in the muscle. However, this was not the case when expressed with muscle-specific drivers. ApoE2- and ApoE3-induced lipid accumulation is dependent on the expression of Dgat2, a key regulator of triacylglycerol production, while ApoE4 still induces lipid accumulation even with knock-down of Dgat2. Additionally, we find that implementation of time-restricted feeding (TRF), a dietary intervention in which food access only occurs in the active period (day), prevents ApoE-induced lipid accumulation in the brain of flies and modulates lipid metabolism genes. Conclusions: Altogether, our results demonstrate that ApoE induces lipid accumulation in the brain, that ApoE4 is unique in causing lipid accumulation independent of Dgat2, and that TRF prevents ApoE-induced lipid accumulation. These results support the idea that lipid metabolism is critical in AD, and that TRF could be a promising therapeutic approach to prevent ApoE-associated dysfunction in lipid metabolism.

Redefining Autoimmune Disorders' Pathoetiology: Implications for Mood and Psychotic Disorders' Association with Neurodegenerative and Classical Autoimmune Disorders

Although previously restricted to a limited number of medical conditions, there is a growing appreciation that 'autoimmune' (or immune-mediated) processes are important aspects of a wide array of diverse medical conditions, including cancers, neurodegenerative diseases and psychiatric disorders. All of these classes of medical conditions are associated with alterations in mitochondrial function across an array of diverse cell types. Accumulating data indicate the presence of the mitochondrial melatonergic pathway in possibly all body cells, with important consequences for pathways crucial in driving CD8⁺ T cell and B-cell 'autoimmune'-linked processes. Melatonin suppression coupled with the upregulation of oxidative stress suppress PTEN-induced kinase 1 (PINK1)/parkin-driven mitophagy, raising the levels of the major histocompatibility complex (MHC)-1, which underpins the chemoattraction of CD8⁺ T cells and the activation of antibody-producing B-cells. Many factors and processes closely associated with autoimmunity, including gut microbiome/permeability, circadian rhythms, aging, the aryl hydrocarbon receptor, brain-derived neurotrophic factor (BDNF) and its receptor tyrosine receptor kinase B (TrkB) all interact with the mitochondrial melatonergic pathway. A number of future research directions and novel treatment implications are indicated for this wide collection of poorly conceptualized and treated medical presentations. It is proposed that the etiology of many 'autoimmune'/'immune-mediated' disorders should be conceptualized as significantly determined by mitochondrial dysregulation, with alterations in the mitochondrial melatonergic pathway being an important aspect of these pathoetiologies.

Sarm1 Regulates Circadian Rhythm Disorder in Alzheimer's Disease in Mice

BACKGROUND

Sarm1 (Sterile alpha and TIR motif-containing 1) is a key protein that regulates neurodegenerative pathologies. Alzheimer's disease (AD) is highly associated with neurodegenerative lesions and biorhythmic disturbances.

OBJECTIVE

This study aims to decipher the role of Sarm1 in AD-induced circadian rhythm disturbances and AD progression.

METHODS

Open field and water maze tests were used to assess the cognitive function of mice. Thioflavin-S staining was used to assess amyloid-β (Aβ) plaque deposition in the hippocampus and cortex. Rhythmic waveform of home cage activity and temperature was recorded to evaluate circadian rhythm. Expression of clock molecules including Bmal1 and Per2 in the hippocampus were analyzed using western blot and real-time PCR. Further, HT22 cells with Sam1 knockout were treated with Aβ 31-35 treatment to initiate circadian rhythm disorder in the cellular level to assess the changes in Bmal1 and Per2.

RESULTS

Our data suggested that Sarm1 deficiency rescued cognitive disorder, decreased Aβ plaque deposition in the hippocampus and cortex, inhibited astrocyte activation, improved circadian rhythm, altered clock molecule expression in the cortex and hippocampus in APP/PS1 mice.

CONCLUSION

Sarm1 attenuates circadian rhythm disturbances and reduces AD progression. These data support the potential use of Sarm1 as a therapeutic target to improve circadian rhythm to impede AD progression.

Melatonin in sleep disorders

Melatonin is the main hormone involved in the control of the sleep-wake cycle. It is easily synthesisable and can be administered orally, which has led to interest in its use as a treatment for insomnia. Moreover, as production of the hormone decreases with age, in inverse correlation with the frequency of poor sleep quality, it has been suggested that melatonin deficit is at least partly responsible for sleep disorders. Treating this age-related deficit would therefore appear to be a natural way of restoring sleep quality, which is lost as patients age. However, despite the undeniable theoretical appeal of this approach to insomnia, little scientific evidence is available that supports any benefit of this substitutive therapy. Furthermore, the most suitable dose ranges and pharmaceutical preparations for melatonin administration are yet to be clearly defined. This review addresses the physiology of melatonin, the different pharmaceutical preparations, and data on its clinical usefulness.

Sleep, neuronal hyperexcitability, inflammation and neurodegeneration: Does early chronic short sleep trigger and is it the key to overcoming Alzheimer's disease?

Evidence links neuroinflammation to Alzheimer's disease (AD); however, its exact contribution to the onset and progression of the disease is poorly understood. Symptoms of AD can be seen as the tip of an iceberg, consisting of a neuropathological build-up in the brain of extracellular amyloid-β (Aβ) plaques and intraneuronal hyperphosphorylated aggregates of Tau (pTau), which are thought to stem from an imbalance between its production and clearance resulting in loss of synaptic health and dysfunctional cortical connectivity. The glymphatic drainage system, which is particularly active during sleep, plays a key role in the clearance of proteinopathies. Poor sleep can cause hyperexcitability and promote Aβ and tau pathology leading to systemic inflammation. The early neuronal hyperexcitability of γ-aminobutyric acid (GABA)-ergic inhibitory interneurons and impaired inhibitory control of cortical pyramidal neurons lie at the crossroads of excitatory/inhibitory imbalance and inflammation. We outline, with a prospective framework, a possible vicious spiral linking early chronic short sleep, neuronal hyperexcitability, inflammation and neurodegeneration. Understanding the early predictors of AD, through an integrative approach, may hold promise for reducing attrition in the late stages of neuroprotective drug development.

Deciphering the Interacting Mechanisms of Circadian Disruption and Alzheimer's Disease

Alzheimer's disease (AD) is one of the crucial causative factors for progressive dementia. Neuropathologically, AD is characterized by the extracellular accumulation of amyloid beta plaques and intracellular neurofibrillary tangles in cortical and limbic regions of the human brain. The circadian system is one of the many affected physiological processes in AD, the dysfunction of which may reflect in the irregularity of the sleep/wake cycle. The interplay of circadian and sleep disturbances inducing AD progression is bidirectional. Sleep-associated pathological alterations are frequently evident in AD. Understanding the interrelation between circadian disruption and AD may allow for earlier identification of AD pathogenesis as well as better suited approaches and potential therapies to combat dementia. In this article, we examine the existing literature related to the molecular mechanisms of the circadian clock and interacting mechanisms of circadian disruption and AD pathogenesis.

Circadian disruption, melatonin rhythm perturbations and their contributions to chaotic physiology

The aim of this report is to summarize the data documenting the vital nature of well-regulated cellular and organismal circadian rhythms, which are also reflected in a stable melatonin cycle, in supporting optimal health. Cellular fluctuations in physiology exist in most cells of multicellular organisms with their stability relying on the prevailing light:dark cycle, since it regulates, via specialized intrinsically-photoreceptive retinal ganglion cells (ipRGC) and the retinohypothalamic tract, the master circadian oscillator, i.e., the suprachiasmatic nuclei (SCN). The output message of the SCN, as determined by the light:dark cycle, is transferred to peripheral oscillators, so-called slave cellular oscillators, directly via the autonomic nervous system with its limited distribution. and indirectly via the pineal-derived circulating melatonin rhythm, which contacts every cell. Via its regulatory effects on the neuroendocrine system, particularly the hypothalamo-pituitary-adrenal axis, the SCN also has a major influence on the adrenal glucocorticoid rhythm which impacts neurological diseases and psychological behaviors. Moreover, the SCN regulates the circadian production and secretion of melatonin. When the central circadian oscillator is disturbed, such as by light at night, it passes misinformation to all organs in the body. When this occurs the physiology of cells becomes altered and normal cellular functions are compromised. This physiological upheaval is a precursor to pathologies. The deterioration of the SCN/pineal network is often a normal consequence of aging and its related diseases, but in today's societies where manufactured light is becoming progressively more common worldwide, the associated pathologies may also be occurring at an earlier age.

Interaction Between Circadian Rhythms, Energy Metabolism, and Cognitive Function

The interaction between meal timing and light regulates circadian rhythms in mammals and not only determines the sleep-wake pattern but also the activity of the endocrine system. Related with that, the necessity to fulfill energy needs is a driving force that requires the participation of cognitive skills whose performance has been shown to undergo circadian variations. These facts have led to the concept that cognition and feeding behaviour can be analysed from a chronobiological perspective. In this context, research carried out during the last two decades has evidenced the link between feeding behaviour/nutritional habits and cognitive processes, and has highlighted the impact of circadian disorders on cognitive decline. All that has allowed hypothesizing a tight relationship between nutritional factors, chronobiology, and cognition. In this connection, experimental diets containing elevated amounts of fat and sugar (high-fat diets; HFDs) have been shown to alter in rodents the circadian distribution of meals, and to have a negative impact on cognition and motivational aspects of behaviour that disappear when animals are forced to adhere to a standard temporal eating pattern. In this review, we will present relevant studies focussing on the effect of HFDs on cognitive aspects of behaviour, paying particular attention to the influence that chronobiological alterations caused by these diets may have on hippocampaldependent cognition.

Interval timing is disrupted in female 5xFAD mice: An indication of altered memory processes

Temporal information processing in the seconds-to-minutes range is disrupted in patients with Alzheimer's disease (AD). In this study, we investigated the timing behavior of the 5xFAD mouse model of AD in the peak interval (PI) procedure. Nine-month-old female mice were trained with sucrose solution reinforcement for their first response after a fixed-interval (FI) and tested in the inter-mixed non-reinforced PI trials that lasted longer than FI. Timing performance indices were estimated from steady-state timed anticipatory nose-poking responses in the PI trials. We found that the time of maximal reward expectancy (peak time) of the 5xFAD mice was significantly earlier than that of the wild-type (WT) controls with no differences in other indices of timing performance. These behavioral differences corroborate the findings of previous studies on the disruption of temporal associative memory abilities of 5xFAD mice and can be accounted for by the scalar timing theory based on altered long-term memory consolidation of temporal information in the 5xFAD mice. This is the first study to directly show an interval timing phenotype in a genetic mouse model of AD.

Mitochondria: Central Organelles for Melatonin's Antioxidant and Anti-Aging Actions

Melatonin, along with its metabolites, have long been known to significantly reduce the oxidative stress burden of aging cells or cells exposed to toxins. Oxidative damage is a result of free radicals produced in cells, especially in mitochondria. When measured, melatonin, a potent antioxidant, was found to be in higher concentrations in mitochondria than in other organelles or subcellular locations. Recent evidence indicates that mitochondrial membranes possess transporters that aid in the rapid uptake of melatonin by these organelles against a gradient. Moreover, we predicted several years ago that, because of their origin from melatonin-producing bacteria, mitochondria likely also synthesize melatonin. Data accumulated within the last year supports this prediction. A high content of melatonin in mitochondria would be fortuitous, since these organelles produce an abundance of free radicals. Thus, melatonin is optimally positioned to scavenge the radicals and reduce the degree of oxidative damage. In light of the "free radical theory of aging", including all of its iterations, high melatonin levels in mitochondria would be expected to protect against age-related organismal decline. Also, there are many age-associated diseases that have, as a contributing factor, free radical damage. These multiple diseases may likely be deferred in their onset or progression if mitochondrial levels of melatonin can be maintained into advanced age.