Glymphatic system of the brain: structure and practical significance

[Glymphatic dysfunction and sleep disorders: indirect effects on Alzheimer's disease]

Modern research raises the question of the potentially significant role of glymphatic dysfunction in the development of neurodegeneration and pathological aging. The exact molecular mechanisms are not yet fully understood, but there is ample evidence of a link between sleep deprivation and decreased clearance of β-amyloid and other neurotoxin proteins that are associated with the development of neurodegenerative diseases, particularly Alzheimer's disease. The review analyzes current scientific information in this area of research, describes the latest scientific discoveries of the features of the glymphatic system, and also illustrates studies of markers that presumably indicate a deterioration in the glymphatic system. The relationship between sleep deprivation and pathophysiological mechanisms associated with neurodegenerative diseases is considered, and potential targets that can be used to treat or delay the development of these disorders are noted.

[The significance of the glymphatic pathway in the relationship between the sleep-wake cycle and neurodegenerative diseases]

Selective and progressive death of neurons is a characteristic feature of the process of neurodegeneration and leads to corresponding neuronal dysfunctions. Neurodegenerative diseases represent a heterogeneous group of clinically distinct disorders with similar molecular mechanisms of pathogenesis. They are based on the processes of abnormal aggregation of proteins, the formation of fibrillary insoluble structures and their deposition in the form of histopathological inclusions in the tissues of the nervous system. Disturbance of homeostatic functions that regulate neuronal ion and energy metabolism, biosynthesis and degradation of proteins and nucleotides, chronic hypoxia and the penetration of toxic and inflammatory substances into the brain from the bloodstream not only cause metabolic changes associated with age and disorders in the sleep-wake cycle, but also contribute to the development of neurodegenerative diseases. In animal studies, clearance pathways have been identified in which solutes and specific tracers are excreted perivascular into the meningeal lymphatics. The glymphatic pathway promotes the removal of metabolites, including Aβ amyloid and tau protein, from the parenchymal extracellular space of the brain. The glymphatic system is discussed to be more efficient during natural sleep, and fluid dynamics through this pathway exhibit daily fluctuations and are under circadian control. This review systematizes the key aspects and the data of recent research on the role of the glymphatic pathway and astroglial AQP-4 as its main determinant in maintaining homeostatic fluid circulation in the brain in normal and pathological conditions, in particular in relation to the regulatory role of the sleep-wake cycle and in development of neurodegeneration.

[The significance of glymphatic pathway in the relationship between the sleep-wake cycle and neurodegenerative diseases]

Selective and progressive death of neurons is a characteristic feature of the process of neurodegeneration and leads to corresponding neuronal dysfunctions. Neurodegenerative diseases represent a heterogeneous group of clinically distinct disorders with similar molecular mechanisms of pathogenesis. They are based on the processes of abnormal aggregation of proteins, the formation of fibrillary insoluble structures and their deposition in the form of histopathological inclusions in the tissues of the nervous system. Disturbance of homeostatic functions that regulate neuronal ion and energy metabolism, biosynthesis and degradation of proteins and nucleotides, chronic hypoxia and the penetration of toxic and inflammatory substances into the brain from the bloodstream not only cause metabolic changes associated with age and disorders in the sleep-wake cycle, but also contribute to the development of neurodegenerative diseases. In animal studies, clearance pathways have been identified in which solutes and specific tracers are excreted perivascular into the meningeal lymphatics. The glymphatic pathway promotes the removal of metabolites, including Aβ amyloid and tau protein, from the parenchymal extracellular space of the brain. The glymphatic system is discussed to be more efficient during natural sleep, and fluid dynamics through this pathway exhibit daily fluctuations and are under circadian control. This review systematizes the key aspects and scientific data of recent studies on the role of the glymphatic pathway and astroglial AQP-4 as its main determinant in maintaining homeostatic fluid circulation in the brain in normal and pathological conditions, in particular in relation to the regulatory role of the sleep-wake cycle and in development of neurodegeneration.

[Perifocal edema and glymphatic system dysfunction: quantitative assessment based on diffusion tensor magnetic resonance imaging]

BACKGROUND

Pathogenesis of peritumoral cerebral edema is unclear and potentially associated with glymphatic system dysfunction. Diffusion tensor MRI (DT-MRI) with analysis of ALPS (Analysis along the Perivascular Space) index may be valuable for assessment of edema. This approach visualizes fluid flow along perivascular spaces of deep cerebral veins.

OBJECTIVE

To assess glymphatic system function in supratentorial tumors and healthy volunteers using DT-MRI.

MATERIAL AND METHODS

There were 52 patients (59% men) aged 43 (28-64) years with supratentorial tumors (meningioma - 20, grade 3-4 glioma - 15, metastases - 9, lymphoma - 8). Tumors and perifocal edema did not involve deep cerebral veins. The control group included 6 healthy volunteers aged 34-66 years. MRI protocol (Signa HDxt, 3 T) contained standard T1, T2, T2FLAIR, DWI and post-contrast T1 (3D BRAVO). DT-MRI had the following parameters: TR=10 000 ms, TEmin=102 ms, FOV=240 mm, isotropic voxel size 3×3×3 mm3, 60 directions of diffusion gradients. Measurements were carried out at b-factor 0 and 1000 s/mm2. Analysis was carried out in the ReadyView software.

RESULTS

Right- and left-sided ALPS indices were similar in the control group (p=0.917). Perifocal edema (regardless of histological type of tumor) in the ipsilateral hemisphere was accompanied by significantly lower ALPS index (p<0.005), while these values in contralateral (intact) hemisphere were similar in both groups (p=0.7).

CONCLUSION

We found significantly lower ALPS index in deep parts of the affected hemisphere in patients with perifocal edema. These data can indicate the role of glymphatic system dysfunction in pathogenesis of this pathology.

The Blood-Brain Barrier in Neuroimmune Interactions and Pathological Processes

The blood-brain barrier (BBB) is a kind of filter, highly selective in relation to various types of substances. The BBB supports the immune status of the brain and is an important regulator of neuroimmune interactions. Some of the molecular and cellular features of the BBB, as well as the five main pathways of neuroimmune communication mediated by the BBB, are analyzed in this article. The functions of the BBB in neuroimmune interactions in various diseases are discussed: multiple sclerosis and Alzheimer's and Parkinson's diseases. The latest data on BBB dysfunction in COVID-19 coronavirus infection caused by the SARS-CoV-2 virus are considered.

Obstructive sleep apnea and amyloid-β42 in adolescents: The results of a pilot study

Background. Obstructive sleep apnea (OSA) is a chronic condition characterized by repetitive collapse of the upper airway during sleep leading to intermittent hypoxia, excessive arousals and disrupt of sleep homeostasis. OSA is associated with obesity as well as pathological dynamics of cerebral beta-amyloid and dementia.The aim. Comparative assessment of the plasma beta-amyloid 42 levels in adolescents, depending on the presence or absence of OSA and obesity.Materials and methods. Seventy-four obese and non-obese male adolescents were included in the study. All participants underwent polysomnography to verify OSA and measure sleep parameters, and venipuncture for the assessment of betaamyloid 42 by enzyme-linked immunosorbent assay (ELISA).Results. After polysomnography all adolescents have been divided into four study groups: OSA obese (group 1; n = 28); non-OSA obese (group 2; n = 20); OSA non-obese (group 3; n = 11), and non-OSA non-obese (group 4 – control; n = 15). OSA obese adolescents had significant elevations in beta-amyloid 42 plasma levels compared with participants from three other groups (р = 0.007, p = 0.047 and p = 0.0002, correspondingly). Compared to both OSA non-obese adolescents and the control group, non-OSA obese patients had similar beta-amyloid 42 blood concentrations (p = 0.167 and p = 0.155, correspondingly). However, patients from the group 3 showed significantly higher beta-amyloid 42 plasma levels than controls (p = 0.004).Conclusions. Thus, OSA, accompanied by intermittent nocturnal hypoxia, and particularly OSA and obesity comorbidity, in adolescents is associated with increased beta-amyloid 42 circulating concentrations, which has a major role in pathogenesis of dementia. Beta-amyloid 42 plasma level may be recommended for use as early biomarker of cognitive impairment and neurodegenerative diseases, but future researches are needed.