The Rhythmicity of Clock Genes is Disrupted in the Choroid Plexus of the APP/PS1 Mouse Model of Alzheimer's Disease

Epigenetic events influencing the biological clock: Panacea for neurodegeneration

The human biological clock is the 24-h internal molecular network of circadian genes in synchronization with other cells in response to external stimuli. The rhythmicity of the clock genes is maintained by positive and negative transcriptional feedback loops coordinating the 24-h oscillation in different tissues. The superchiasmatic nucleus, the central pacemaker of the biological clock diminishes with aging causing alterations in the clock rhythmicity leading to the onset of neurodegenerative diseases mainly Alzheimer's disease, Parkinson's disease, and Huntington's disease. Studies have shown that brain and muscle Arnt -like 1 (Bmal1) and Circadian Locomotor Output Cycles Kaput (Clock) gene expression is altered in the onset of neurodegeneration. One of the major symptoms of neurodegeneration is changes in the sleep/wake cycle. Moreover, variations in circadian clock oscillations can happen due to lifestyle changes, addiction to alcohol, cocaine, drugs, smoking, food habits and most importantly eating and sleep/awake cycle patterns which can significantly impact the expression of circadian genes. Recent studies have focused on the molecular function of clock genes affected due to environmental cues. Epigenetic modifications are influenced by the external environmental factors. This review aims to focus on the principal mechanism of epigenetics influencing circadian rhythm disruption leading to neurodegeneration and as well as targeting the epigenetic modulators could be a novel therapeutic approach to combat neurodegenerative disorders.

Circadian timing, melatonin and hippocampal volume in later-life adults

Hippocampal atrophy is a prominent neurodegenerative feature of Alzheimer's disease and related dementias. Alterations in circadian rhythms can exacerbate cognitive aging and neurodegeneration. This study aimed to examine how dim light melatonin onset and melatonin levels are associated with hippocampal volume in cognitively healthy individuals. We studied data from 52 later-life adults (mean age ± SD = 70.0 ± 6.3 years). T1-weighted anatomical images from 3.0 T magnetic resonance imaging data were collected and processed using the BRAINSTools toolbox. Dim light melatonin onset was used to assess circadian timing. The area under the curve was calculated to quantify melatonin concentration levels 6 hr before bedtime, and 14-day wrist actigraphy data were used to assess habitual bedtime. Multiple linear regression modelling with hippocampal volume as the dependent variable was used to analyse the data adjusting for age and sex. The average dim light melatonin onset was 19:45 hours (SD = 84 min), and area under the curve of melatonin levels 6 hr before habitual bedtime was 38.4 pg ml-1 × hr (SD = 29.3). We found that later dim light melatonin onset time (b = 0.16, p = 0.005) and greater area under the curve of melatonin levels 6 hr before habitual bedtime (b = 0.05, p = 0.046) were associated with greater adjusted hippocampal volume. The time between dim light melatonin onset and the midpoint of sleep timing was not associated with hippocampal volume. The findings suggest that earlier circadian timing (dim light melatonin onset) and reduced melatonin may be associated with reduced hippocampal volume in older adults. Future research will help researchers utilize circadian rhythm information to delay brain aging.

Circadian rhythm alterations affecting the pathology of neurodegenerative diseases

The circadian rhythm is a nearly 24-h oscillation found in various physiological processes in the human brain and body that is regulated by environmental and genetic factors. It is responsible for maintaining body homeostasis and it is critical for essential functions, such as metabolic regulation and memory consolidation. Dysregulation in the circadian rhythm can negatively impact human health, resulting in cardiovascular and metabolic diseases, psychiatric disorders, and premature death. Emerging evidence points to a relationship between the dysregulation circadian rhythm and neurodegenerative diseases, suggesting that the alterations in circadian function might play crucial roles in the pathogenesis and progression of neurodegenerative diseases. Better understanding this association is of paramount importance to expand the knowledge on the pathophysiology of neurodegenerative diseases, as well as, to provide potential targets for the development of new interventions based on the dysregulation of circadian rhythm. Here we review the latest findings on dysregulation of circadian rhythm alterations in Parkinson's disease, Alzheimer's disease, Huntington's disease, amyotrophic lateral sclerosis, multiple sclerosis, spinocerebellar ataxia and multiple-system atrophy, focusing on research published in the last 3 years.

Aryl hydrocarbon receptor impairs circadian regulation in Alzheimer's disease: Potential impact on glymphatic system dysfunction

Circadian clocks maintain diurnal rhythms of sleep-wake cycle of 24 h that regulate not only the metabolism of an organism but also many other periodical processes. There is substantial evidence that circadian regulation is impaired in Alzheimer's disease. Circadian clocks regulate many properties known to be disturbed in Alzheimer's patients, such as the integrity of the blood-brain barrier (BBB) as well as the diurnal glymphatic flow that controls waste clearance from the brain. Interestingly, an evolutionarily conserved transcription factor, that is, aryl hydrocarbon receptor (AhR), impairs the function of the core clock proteins and thus could disturb diurnal rhythmicity in the BBB. There is abundant evidence that the activation of AhR signalling inhibits the expression of the major core clock proteins, such as the brain and muscle arnt-like 1 (BMAL1), clock circadian regulator (CLOCK) and period circadian regulator 1 (PER1) in different experimental models. The expression of AhR is robustly increased in the brains of Alzheimer's patients, and protein level is enriched in astrocytes of the BBB. It seems that AhR signalling inhibits glymphatic flow since it is known that (i) activation of AhR impairs the function of the BBB, which is cooperatively interconnected with the glymphatic system in the brain, and (ii) neuroinflammation and dysbiosis of gut microbiota generate potent activators of AhR, which are able to impair glymphatic flow. I will examine current evidence indicating that activation of AhR signalling could disturb circadian functions of the BBB and impair glymphatic flow and thus be involved in the development of Alzheimer's pathology.

Transporters, Ion Channels, and Junctional Proteins in Choroid Plexus Epithelial Cells

The choroid plexus (CP) plays significant roles in secreting cerebrospinal fluid (CSF) and forming circadian rhythms. A monolayer of epithelial cells with tight and adherens junctions of CP forms the blood-CSF barrier to control the movement of substances between the blood and ventricles, as microvessels in the stroma of CP have fenestrations in endothelial cells. CP epithelial cells are equipped with several kinds of transporters and ion channels to transport nutrient substances and secrete CSF. In addition, junctional components also contribute to CSF production as well as blood-CSF barrier formation. However, it remains unclear how junctional components as well as transporters and ion channels contribute to the pathogenesis of neurodegenerative disorders. In this manuscript, recent findings regarding the distribution and significance of transporters, ion channels, and junctional proteins in CP epithelial cells are introduced, and how changes in expression of their epithelial proteins contribute to the pathophysiology of brain disorders are reviewed.

Current progress and challenges in the development of brain tissue models: How to grow up the changeable brain in vitro?

In vitro modeling of brain tissue is a promising but not yet resolved problem in modern neurobiology and neuropharmacology. Complexity of the brain structure and diversity of cell-to-cell communication in (patho)physiological conditions make this task almost unachievable. However, establishment of novel in vitro brain models would ultimately lead to better understanding of development-associated or experience-driven brain plasticity, designing efficient approaches to restore aberrant brain functioning. The main goal of this review is to summarize the available data on methodological approaches that are currently in use, and to identify the most prospective trends in development of neurovascular unit, blood-brain barrier, blood-cerebrospinal fluid barrier, and neurogenic niche in vitro models. The manuscript focuses on the regulation of adult neurogenesis, cerebral microcirculation and fluids dynamics that should be reproduced in the in vitro 4D models to mimic brain development and its alterations in brain pathology. We discuss approaches that are critical for studying brain plasticity, deciphering the individual person-specific trajectory of brain development and aging, and testing new drug candidates in the in vitro models.

Role of Bmal1 and Gut Microbiota in Alzheimer's Disease and Parkinson's Disease Pathophysiology: The Probable Effect of Melatonin on Their Association

In recent years, the role of new factors in the pathophysiology of neurodegenerative diseases has been investigated. Alzheimer's disease (AD) and Parkinson's disease (PD) are the most common neurodegenerative diseases worldwide. Although pathological changes such as the accumulation of aggregated proteins in the brain and inflammatory responses are known as the main factors involved in the development of these diseases, new studies show the role of gut microbiota and circadian rhythm in the occurrence of these changes. However, the association between circadian rhythm and gut microbiota in AD and PD has not yet been investigated. Recent results propose that alterations in circadian rhythm regulators, mainly Bmal1, may regulate the abundance of gut microbiota. This correlation has been linked to the regulation of the expression of immune-related genes and Bmal-1 mediated oscillation of IgA and hydrogen peroxide production. These data seem to provide new insight into the molecular mechanism of melatonin inhibiting the progression of AD and PD. Therefore, this manuscript aims to review the role of the gut microbiota and circadian rhythm in health and AD and PD and also presents a hypothesis on the effect of melatonin on their communication.

The Role of Biological Rhythms in New Drug Formulations to Cross the Brain Barriers

For brain protection, the blood-brain barrier and blood-cerebrospinal fluid barrier limit the traffic of molecules between blood and brain tissue and between blood and cerebrospinal fluid, respectively. Besides their protective function, brain barriers also limit the passage of therapeutic drugs to the brain, which constitutes a great challenge for the development of therapeutic strategies for brain disorders. This problem has led to the emergence of novel strategies to treat neurological disorders, like the development of nanoformulations to deliver therapeutic agents to the brain. Recently, functional molecular clocks have been identified in the blood-brain barrier and in the blood-cerebrospinal fluid barrier. In fact, circadian rhythms in physiological functions related to drug disposition were also described in brain barriers. This opens the possibility for chronobiological approaches that aim to use time to improve drug efficacy and safety. The conjugation of nanoformulations with chronobiology for neurological disorders is still unexplored. Facing this, here, we reviewed the circadian rhythms in brain barriers, the nanoformulations studied to deliver drugs to the brain, and the nanoformulations with the potential to be conjugated with a chronobiological approach to therapeutic strategies for the brain.

The impact of biological clock and sex hormones on the risk of disease

Molecular clocks are responsible for defining 24-h cycles of behaviour and physiology that are called circadian rhythms. Several structures and tissues are responsible for generating these circadian rhythms and are named circadian clocks. The suprachiasmatic nucleus of the hypothalamus is believed to be the master circadian clock receiving light input via the optic nerve and aligning internal rhythms with environmental cues. Studies using both in vivo and in vitro methodologies have reported the relationship between the molecular clock and sex hormones. The circadian system is directly responsible for controlling the synthesis of sex hormones and this synthesis varies according to the time of day and phase of the estrous cycle. Sex hormones also directly interact with the circadian system to regulate circadian gene expression, adjust biological processes, and even adjust their own synthesis. Several diseases have been linked with alterations in either the sex hormone background or the molecular clock. So, in this chapter we aim to summarize the current understanding of the relationship between the circadian system and sex hormones and their combined role in the onset of several related diseases.

Rhythms in barriers and fluids: Circadian clock regulation in the aging neurovascular unit

The neurovascular unit is where two very distinct physiological systems meet: The central nervous system (CNS) and the blood. The permeability of the barriers separating these systems is regulated by time, including both the 24 h circadian clock and the longer processes of aging. An endogenous circadian rhythm regulates the transport of molecules across the blood-brain barrier and the circulation of the cerebrospinal fluid and the glymphatic system. These fluid dynamics change with time of day, and with age, and especially in the context of neurodegeneration. Factors may differ depending on brain region, as can be highlighted by consideration of circadian regulation of the neurovascular niche in white matter. As an example of a potential target for clinical applications, we highlight chaperone-mediated autophagy as one mechanism at the intersection of circadian dysregulation, aging and neurodegenerative disease. In this review we emphasize key areas for future research.

Circadian rhythmicity of amyloid-beta-related molecules is disrupted in the choroid plexus of a female Alzheimer's disease mouse model

The choroid plexus (CP) is part of the blood-cerebrospinal fluid barrier (BCSFB) and was recently described as an important component of the circadian clock system. It is the principal source of cerebrospinal fluid (CSF) and responsible for the synthesis and secretion of various neuroprotective peptides including those involved in amyloid-β (Aβ) transport/degradation, contributing to Aβ homeostasis. Inadequate Aβ metabolic clearance and transport across the BCSFB have been associated with circadian dysfunctions in Alzheimer's disease (AD) patients. To investigate whether AD pathology influences Aβ scavengers circadian expression, we collected CP at different time points from an AD mouse model (APP/PS1) (female and male animals, aged 6- and 12-months-old) and analyzed their mRNA expression by Real-time RT-PCR. Only angiotensin-converting enzyme (Ace) expression in 6-month-old female wild-type mice and transthyretin (Ttr) expression in 12-month-old female wild-type mice presented significant rhythmicity. The circadian rhythmicity of Ace and Ttr, prompt us to analyze the involvement of circadian rhythm in Aβ uptake. A human CP papilloma (HIBCPP) cell line was incubated with Aβ-488 and uptake was evaluated at different time points using flow cytometry. Aβ uptake displayed circadian rhythmicity. Our results suggest that AD might affect Aβ scavengers rhythmicity and that Aβ clearance is a rhythmic process possibly regulated by the rhythmic expression of Aβ scavengers.

Circadian disruption and sleep disorders in neurodegeneration

Disruptions of circadian rhythms and sleep cycles are common among neurodegenerative diseases and can occur at multiple levels. Accumulating evidence reveals a bidirectional relationship between disruptions of circadian rhythms and sleep cycles and neurodegenerative diseases. Circadian disruption and sleep disorders aggravate neurodegeneration and neurodegenerative diseases can in turn disrupt circadian rhythms and sleep. Importantly, circadian disruption and various sleep disorders can increase the risk of neurodegenerative diseases. Thus, harnessing the circadian biology findings from preclinical and translational research in neurodegenerative diseases is of importance for reducing risk of neurodegeneration and improving symptoms and quality of life of individuals with neurodegenerative disorders via approaches that normalize circadian in the context of precision medicine. In this review, we discuss the implications of circadian disruption and sleep disorders in neurodegenerative diseases by summarizing evidence from both human and animal studies, focusing on the bidirectional links of sleep and circadian rhythms with prevalent forms of neurodegeneration. These findings provide valuable insights into the pathogenesis of neurodegenerative diseases and suggest a promising role of circadian-based interventions.

Choroid Plexus Aquaporins in CSF Homeostasis and the Glymphatic System: Their Relevance for Alzheimer's Disease

The glymphatic system, a fluid-clearance pathway involved in brain waste clearance, is known to be impaired in neurological disorders, including Alzheimer's disease (AD). For this reason, it is important to understand the specific mechanisms and factors controlling glymphatic function. This pathway enables the flow of cerebrospinal fluid (CSF) into the brain and subsequently the brain interstitium, supported by aquaporins (AQPs). Continuous CSF transport through the brain parenchyma is critical for the effective transport and drainage of waste solutes, such as toxic proteins, through the glymphatic system. However, a balance between CSF production and secretion from the choroid plexus, through AQP regulation, is also needed. Thus, any condition that affects CSF homeostasis will also interfere with effective waste removal through the clearance glymphatic pathway and the subsequent processes of neurodegeneration. In this review, we highlight the role of AQPs in the choroid plexus in the modulation of CSF homeostasis and, consequently, the glymphatic clearance pathway, with a special focus on AD.

Circadian Clock, Glucocorticoids and NF-κB Signaling in Neuroinflammation- Implicating Glucocorticoid Induced Leucine Zipper as a Molecular Link

Inflammation including neuroinflammation is considered a protective response and is directed to repair, regenerate, and restore damaged tissues in the central nervous system. Persistent inflammation due to chronic stress, age related accrual of free radicals, subclinical infections or other factors lead to reduced survival and increased neuronal death. Circadian abnormalities secondary to altered sleep/wake cycles is one of the earliest signs of neurodegenerative diseases. Brain specific or global deficiency of core circadian trans-activator brain and muscle ARNT (Arylhydrocarbon Receptor Nuclear Translocator)-like protein 1 (BMAL1) or that of the transrepressor REV-ERBα, impaired neural function and cognitive performance in rodents. Consistently, transcripts of inflammatory cytokines and host immune responses have been shown to exhibit diurnal variation, in parallel with the disruption of the circadian rhythm. Glucocorticoids that exhibit both a circadian rhythm similar to that of the core clock transactivator BMAL1 and tissue specific ultradian rhythm are critical in the control of neuroinflammation and re-establishment of homeostasis. It is widely accepted that the glucocorticoids suppress nuclear factor-kappa B (NF-κB) mediated transactivation and suppress inflammation. Recent mechanistic elucidations suggest that the core clock components also modulate NF-κB mediated transactivation in the brain and peripheral tissues. In this review we discuss evidence for interactions between the circadian clock components, glucocorticoids and NF-κB signaling responses in the brain and propose glucocorticoid induced leucine zipper (GILZ) encoded by Tsc22d3, as a molecular link that connect all three pathways in the maintenance of CNS homeostasis as well as in the pathogenesis of neuroinflammation-neurodegeneration.

Choroid plexus function in neurological homeostasis and disorders: The awakening of the circadian clocks and orexins

As research regarding the role of circadian rhythms, sleep, and the orexinergic system in neurodegenerative diseases is growing, it is surprising that the choroid plexus (CP) remains underappreciated in this realm. Despite its extensive role in the regulation of circadian rhythms and orexinergic signalling, as well as acting as the primary conduit between cerebrospinal fluid (CSF) and the circulatory system, providing a mechanism by which toxic waste molecules can be removed from the brain, the CP has been largely unexplored in neurodegeneration. In this review, we explore the role of the CP in maintaining brain homeostasis and circadian rhythms, regulating CSF dynamics, and how these functions change across the lifespan, from development to senescence. In addition, we examine the relationship between the CP, orexinergic signalling, and the glymphatic system, highlighting gaps in the literature and areas that require immediate exploration. Finally, we assess current knowledge, including possible therapeutic strategies, regarding the role of the CP in neurological disorders, such as traumatic brain injury, migraine, Alzheimer's disease, and multiple sclerosis.

Choroid Plexus in Alzheimer's Disease-The Current State of Knowledge

The choroid plexus (CP), located in each of the four ventricles of the brain, is formed by a monolayer of epithelial cells that surrounds a highly vascularized connective tissue with permeable capillaries. These cells are joined by tight junctions forming the blood-cerebrospinal fluid barrier (BCSFB), which strictly regulates the exchange of substances between the blood and cerebrospinal fluid (CSF). The primary purpose of the CP is to secrete CSF, but it also plays a role in the immune surveillance of the central nervous system (CNS) and in the removal of neurotoxic compounds from the CSF. According to recent findings, the CP is also involved in the modulation of the circadian cycle and neurogenesis. In diseases such as Alzheimer's disease (AD), the function of the CP is impaired, resulting in an altered secretory, barrier, transport, and immune function. This review describes the current state of knowledge concerning the roles of the CP and BCSFB in the pathophysiology of AD and summarizes recently proposed therapies that aim to restore CP and BCSFB functions.

Circadian rhythms in neurodegenerative disorders

Endogenous biological clocks, orchestrated by the suprachiasmatic nucleus, time the circadian rhythms that synchronize physiological and behavioural functions in humans. The circadian system influences most physiological processes, including sleep, alertness and cognitive performance. Disruption of circadian homeostasis has deleterious effects on human health. Neurodegenerative disorders involve a wide range of symptoms, many of which exhibit diurnal variations in frequency and intensity. These disorders also disrupt circadian homeostasis, which in turn has negative effects on symptoms and quality of life. Emerging evidence points to a bidirectional relationship between circadian homeostasis and neurodegeneration, suggesting that circadian function might have an important role in the progression of neurodegenerative disorders. Therefore, the circadian system has become an attractive target for research and clinical care innovations. Studying circadian disruption in neurodegenerative disorders could expand our understanding of the pathophysiology of neurodegeneration and facilitate the development of novel, circadian-based interventions for these disabling disorders. In this Review, we discuss the alterations to the circadian system that occur in movement (Parkinson disease and Huntington disease) and cognitive (Alzheimer disease and frontotemporal dementia) neurodegenerative disorders and provide directions for future investigations in this field.

Brain Barriers and brain fluids research in 2020 and the fluids and barriers of the CNS thematic series on advances in in vitro modeling of the blood-brain barrier and neurovascular unit

This editorial discusses advances in brain barrier and brain fluid research in 2020. Topics include: the cerebral endothelium and the neurovascular unit; the choroid plexus; the meninges; cerebrospinal fluid and the glymphatic system; disease states impacting the brain barriers and brain fluids; drug delivery to the brain. This editorial also highlights the recently completed Fluids Barriers CNS thematic series entitled, Advances in in vitro modeling of the bloodbrain barrier and neurovascular unit. Such in vitro modeling is progressing rapidly.

An Optimal Time for Treatment-Predicting Circadian Time by Machine Learning and Mathematical Modelling

Tailoring medical interventions to a particular patient and pathology has been termed personalized medicine. The outcome of cancer treatments is improved when the intervention is timed in accordance with the patient's internal time. Yet, one challenge of personalized medicine is how to consider the biological time of the patient. Prerequisite for this so-called chronotherapy is an accurate characterization of the internal circadian time of the patient. As an alternative to time-consuming measurements in a sleep-laboratory, recent studies in chronobiology predict circadian time by applying machine learning approaches and mathematical modelling to easier accessible observables such as gene expression. Embedding these results into the mathematical dynamics between clock and cancer in mammals, we review the precision of predictions and the potential usage with respect to cancer treatment and discuss whether the patient's internal time and circadian observables, may provide an additional indication for individualized treatment timing. Besides the health improvement, timing treatment may imply financial advantages, by ameliorating side effects of treatments, thus reducing costs. Summarizing the advances of recent years, this review brings together the current clinical standard for measuring biological time, the general assessment of circadian rhythmicity, the usage of rhythmic variables to predict biological time and models of circadian rhythmicity.