Understanding central nervous system fluid networks: Historical perspectives and a revised model for clinical neurofluid imaging.

The central nervous system (CNS) lacks traditionally defined lymphatic vasculature. However, CNS tissues and barriers compartmentalize the brain, spinal cord, and adjacent spaces, facilitating the transmittal of fluids, metabolic wastes, immune cells, and vital signals, while more conventional lymphatic pathways in the meninges, cervicofacial and paraspinal regions transmit efflux fluid and molecules to peripheral lymph and lymph nodes. Thus, a unique and highly organized fluid circulation network encompassing intraparenchymal, subarachnoid, dural, and extradural segments functions in unison to maintain CNS homeostasis. Pathways involved in this system have been under investigation for centuries and continue to be the source of considerable interest and debate. Modern imaging and microscopy technologies have led to important breakthroughs pertaining to various elements of CNS fluid circuitry and exchange over the past decade, thus enhancing knowledge on mechanisms of mammalian CNS maintenance and disease. Yet, to better understand precise anatomical routes, the physiology and clinical significance of these CNS pathways, and potential therapeutic targets in humans, fluid conduits, flow-regulating factors, and tissue effects must be analyzed systematically and in a global manner in persons across age, demographical factors, and disease states. Here, we illustrate the system-wide nature of intermixing CNS fluid networks, summarize historical and clinical studies, and discuss anatomical and physiological similarities and differences that are relevant for translation of evidence from mice to humans. We also review Cushing's classical model of cerebrospinal fluid flow and present a new framework of this "third circulation" that emphasizes previously unexplained complexities of CNS fluid circulation in humans. Finally, we review future directions in the field, including emerging theranostic techniques and MRI studies required in humans.

Functional Investigation of Meningeal Lymphatic System in Experimental Intracerebral Hemorrhage.

BACKGROUND: Promotion of hematoma resolution in a timely manner reduces intracerebral hemorrhage (ICH) brain injury induced by toxic blood components and subsequent neuroinflammation. The meningeal lymphatic system is responsible for clearance of macromolecules and pathogenic substances from the central nervous system; however, its role in intraparenchymal hematoma clearance and ICH outcomes is unknown. In the present study, we aimed to understand the contribution of the meningeal lymphatic system to ICH pathologies and to test whether pharmacological enhancement of meningeal lymphatic function promotes hematoma resolution and brain recovery after ICH. METHODS: Immunofluorescence of whole-mount meninges was used to measure complexity and coverage level of meningeal lymphatic vasculature following ICH induction. Fluorescent microbeads and PKH-26-labeled erythrocytes were used to evaluate drainage function of the meningeal lymphatic system. Visudyne treatment, deep cervical lymph node ligation, and VEGF (vascular endothelial growth factor)-C injection were performed to manipulate meningeal lymphatic function. Neurobehavioral performance and hematoma volume were assayed by the cylinder test and histological measurements. Iron deposition, residual erythrocytes, neuronal loss, and astrogliosis were assessed by immunohistochemistry and antibody-based fluorescence staining. RESULTS: Meningeal lymphangiogenesis and enhanced lymphatic drainage occurred during the late phase of ICH. Ablation and blockage of meningeal lymphatic vessels impeded hematoma clearance, whereas pharmacological enhancement of their function reduced hematoma volume, improved behavioral performance, and reduced brain residual erythrocytes, iron deposition, neuronal loss, and astroglial activation. CONCLUSIONS: Early enhancement of meningeal lymphatic function is beneficial for ICH recovery. Targeting the meningeal lymphatic system is therefore a potential therapeutic approach for treating ICH.

Corpora amylacea act as containers that remove waste products from the brain.

Corpora amylacea (CA) in the human brain are granular bodies formed by polyglucosan aggregates that amass waste products of different origins. They are generated by astrocytes, mainly during aging and neurodegenerative conditions, and are located predominantly in periventricular and subpial regions. This study shows that CA are released from these regions to the cerebrospinal fluid and are present in the cervical lymph nodes, into which cerebrospinal fluid drains through the meningeal lymphatic system. We also show that CA can be phagocytosed by macrophages. We conclude that CA can act as containers that remove waste products from the brain and may be involved in a mechanism that cleans the brain. Moreover, we postulate that CA may contribute in some autoimmune brain diseases, exporting brain substances that interact with the immune system, and hypothesize that CA may contain brain markers that may aid in the diagnosis of certain brain diseases.

Meningeal immunity: Structure, function and a potential therapeutic target of neurodegenerative diseases.

Meningeal immunity refers to immune surveillance and immune defense in the meningeal immune compartment, which depends on the unique position, structural composition of the meninges and functional characteristics of the meningeal immune cells. Recent research advances in meningeal immunity have demonstrated many new ways in which a sophisticated immune landscape affects central nervous system (CNS) function under physiological or pathological conditions. The proper function of the meningeal compartment might protect the CNS from pathogens or contribute to neurological disorders. Since the concept of meningeal immunity, especially the meningeal lymphatic system and the glymphatic system, is relatively new, we will provide a general review of the meninges' basic structural elements, organization, regulation, and functions with regards to meningeal immunity. At the same time, we will emphasize recent evidence for the role of meningeal immunity in neurodegenerative diseases. More importantly, we will speculate about the feasibility of the meningeal immune region as a drug target to provide some insights for future research of meningeal immunity.

Subarachnoid hemorrhage distinctively disrupts the glymphatic and meningeal lymphatic systems in beagles.

Subarachnoid hemorrhage (SAH) induced acute impairment of the glymphatic system, but few have investigated the dysfunction of the meningeal lymphatic system and their contribution to the pathophysiology of SAH. In addition, most studies were conducted in rodent animals. We aimed to investigate the impact of SAH on glymphatic and meningeal lymphatic function in a large animal model using beagles and to evaluate the effects of intermittent cistern magna CSF drainage on these systems. Methods: The SAH model was created in beagles via endovascular perforation using a digital subtraction angiography machine. Intermittent cistern magna CSF drain was performed daily from 1 d to 3 d after SAH. We examined CSF pressure, neuronal death, enlargement of perivascular space (PVS), hydrocephalus, and neurological and cognitive deficits before and after SAH. The dynamics of glymphatic and meningeal lymphatic functions were analyzed by quantifying the signal intensity of dimeglumine gadopentetate (Gd-DTPA) using T1-weighted magnetic resonance imaging (MRI). Measurements were taken before SAH and at 1 h, 1 week, and 2 weeks post-SAH. Results: SAH in beagles caused significant blood clots, neuronal death, increased CSF pressure, hydrocephalus, and neurological and cognitive deficits. MRI revealed dilated ventricles and enlarged PVS post-SAH. The glymphatic system's function, assessed by Gd-DTPA distribution, showed reduced CSF influx and glymphatic impairment after SAH, particularly in the ipsilateral hemisphere, persisting for a week with partial recovery at 2 weeks. For lymphatic clearance, Gd-DTPA rapidly filled the olfactory bulbs, optic nerves, facial and vestibulocochlear nerves, and spinal nerves under normal conditions. SAH caused delayed and reduced Gd-DTPA efflux outflow in these areas, disrupting lymphatic clearance. Despite initial dysfunction, increased hemoglobin levels in cervical lymph nodes indicated active blood clearance post-SAH, with recovery by 2 weeks. Treatment with intermittent cistern magna CSF drain significantly ameliorated the glymphatic and meningeal lymphatic dysfunction after SAH. Conclusion: SAH impaired both glymphatic and meningeal lymphatic functions in beagles, with better restoration of lymphatic function post-SAH, which may contribute to functional recovery after SAH. External CSF drain is an effective therapeutic approach to facilitate the recovery of glymphatic and meningeal lymphatic function following SAH.

The Regulation of Cerebral Lymphatic Drainage in the Transverse Sinus Region of the Mouse Brain.

Cerebral lymphatic drainage is an important pathway for metabolic waste clearance in the brain, which plays a crucial role in the progression of central nervous system diseases. Recent studies have shown that norepinephrine (NE) is involved in the regulation of cerebral lymphatic drainage function, but the modulation mechanism remains unknown. In this study, we confirmed that NE rapidly reduced glymphatic influx and enhanced meningeal lymphatic clearance. Moreover, the transverse sinus (TS) was the vital region of cerebral lymphatic drainage regulation by NE. Further analysis revealed that NE inhibition could simultaneously enhance glymphatic drainage and dorsal meningeal lymphatic drainage, mainly acting on the TS region. This study demonstrated that the cerebral lymphatic drainage system can be regulated by NE, with the TS region serving as the primary modulating site. The findings provide a potential regulatory target for the amelioration of neurological diseases associated with cerebral lymphatic drainage function.

Age-related changes in meningeal lymphatic function are closely associated with vascular endothelial growth factor-C expression.

Meningeal lymphatic vessels (MLVs) have crucial roles in removing metabolic waste and toxic proteins from the brain and transporting them to the periphery. Aged mice show impaired meningeal lymphatic function. Nevertheless, as the disease progresses, and significant pathological changes manifest in the brain, treating the condition becomes increasingly challenging. Therefore, investigating the alterations in the structure and function of MLVs in the early stages of aging is critical for preventing age-related central nervous system degenerative diseases. We detected the structure and function of MLVs in young, middle-aged, and aged mice. Middle-aged mice, compared with young and aged mice, showed enhanced meningeal lymphatic function along with MLV expansion and performed better in the Y maze test. Moreover, age-related changes in meningeal lymphatic function were closely associated with vascular endothelial growth factor-C (VEGF-C) expression in the brain cortex. Our data suggested that the cerebral cortex may serve as a target for VEGF-C supplementation to ameliorate meningeal lymphatic dysfunction, thus providing a new strategy for preventing age-related central nervous system diseases.

High-frequency ultrasound imaging for monitoring the function of meningeal lymphatic system in mice.

The meningeal lymphatic system drains the cerebrospinal fluid from the subarachnoid space to the cervical lymphatic system, primarily to the deep cervical lymph nodes. Perturbations of the meningeal lymphatic system have been linked to various neurologic disorders. A method to specifically monitor the flow of meningeal lymphatic system in real time is unavailable. In the present study, we adopted the high-frequency ultrasound (HFUS) with 1,1'diocatadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI)-loaded microbubble and FePt@PLGA nanoparticle contrast agents to evaluate the flow of the meningeal lymphatic system in 2-month-old mice. Statistical analysis was performed to identify changes of HFUS signals among the microbubbles, FePt@PLGA nanoparticles, and saline control groups. Approximately 15 min from the start of intracerebroventricular injection of contrast agents, their signals were evident at the deep cervical lymph nodes and lasted for at least 60 min. These signals were validated on the basis of the presence of DiI and Fe signals in the deep cervical lymph nodes. Ligation of afferent lymphatic vessels to the deep cervical lymph nodes eliminated the HFUS signals. Moreover, ablation of lymphatic vessels near the confluence of sinuses decreased the HFUS signals in the deep cervical lymph nodes. Glioma-bearing mice that exhibited reduced lymphatic vessel immunostaining signals near the confluence of sinuses had lowered HFUS signals in the deep cervical lymph nodes within 60 min. The proposed method provides a minimally invasive approach to monitor the qualities of the meningeal lymphatic system in real time as well as the progression of the meningeal lymphatic system in various brain disease animal models.

Assessment of Meningeal Lymphatics in the Parasagittal Dural Space: A Prospective Feasibility Study Using Dynamic Contrast-Enhanced Magnetic Resonance Imaging.

OBJECTIVE: Meningeal lymphatic vessels are predominantly located in the parasagittal dural space (PSD); these vessels drain interstitial fluids out of the brain and contribute to the glymphatic system. We aimed to investigate the ability of dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) in assessing the dynamic changes in the meningeal lymphatic vessels in PSD. MATERIALS AND METHODS: Eighteen participants (26-71 years; male:female, 10:8), without neurological or psychiatric diseases, were prospectively enrolled and underwent DCE-MRI. Three regions of interests (ROIs) were placed on the PSD, superior sagittal sinus (SSS), and cortical vein. Early and delayed enhancement patterns and six kinetic curve-derived parameters were obtained and compared between the three ROIs. Moreover, the participants were grouped into the young (< 65 years; n = 9) or older (≥ 65 years; n = 9) groups. Enhancement patterns and kinetic curve-derived parameters in the PSD were compared between the two groups. RESULTS: The PSD showed different enhancement patterns than the SSS and cortical veins (P < 0.001 and P < 0.001, respectively) in the early and delayed phases. The PSD showed slow early enhancement and a delayed wash-out pattern. The six kinetic curve-derived parameters of PSD was significantly different than that of the SSS and cortical vein. The PSD wash-out rate of older participants was significantly lower (median, 0.09; interquartile range [IQR], 0.01-0.15) than that of younger participants (median, 0.32; IQR, 0.07-0.45) (P = 0.040). CONCLUSION: This study shows that the dynamic changes of meningeal lymphatic vessels in PSD can be assessed with DCE-MRI, and the results are different from those of the venous structures. Our finding that delayed wash-out was more pronounced in the PSD of older participants suggests that aging may disturb the meningeal lymphatic drainage.

The role of the astrocyte in subarachnoid hemorrhage and its therapeutic implications.

Subarachnoid hemorrhage (SAH) is an important public health concern with high morbidity and mortality worldwide. SAH induces cell death, blood-brain barrier (BBB) damage, brain edema and oxidative stress. As the most abundant cell type in the central nervous system, astrocytes play an essential role in brain damage and recovery following SAH. This review describes astrocyte activation and polarization after SAH. Astrocytes mediate BBB disruption, glymphatic-lymphatic system dysfunction, oxidative stress, and cell death after SAH. Furthermore, astrocytes engage in abundant crosstalk with other brain cells, such as endothelial cells, neurons, pericytes, microglia and monocytes, after SAH. In addition, astrocytes also exert protective functions in SAH. Finally, we summarize evidence regarding therapeutic approaches aimed at modulating astrocyte function following SAH, which could provide some new leads for future translational therapy to alleviate damage after SAH.

The Glymphatic System: A Novel Therapeutic Target for Stroke Treatment.

The glymphatic system (GS) is a novel defined brain-wide perivascular transit network between cerebrospinal fluid (CSF) and interstitial solutes that facilitates the clearance of brain metabolic wastes. The complicated network of the GS consists of the periarterial CSF influx pathway, astrocytes-mediated convective transport of fluid and solutes supported by AQP4 water channels, and perivenous efflux pathway. Recent researches indicate that the GS dysfunction is associated with various neurological disorders, including traumatic brain injury, hydrocephalus, epilepsy, migraine, and Alzheimer's disease (AD). Meanwhile, the GS also plays a pivotal role in the pathophysiological process of stroke, including brain edema, blood-brain barrier (BBB) disruption, immune cell infiltration, neuroinflammation, and neuronal apoptosis. In this review, we illustrated the key anatomical structures of the GS, the relationship between the GS and the meningeal lymphatic system, the interaction between the GS and the BBB, and the crosstalk between astrocytes and other GS cellular components. In addition, we contributed to the current knowledge about the role of the GS in the pathology of stroke and the role of AQP4 in stroke. We further discussed the potential use of the GS in early risk assessment, diagnostics, prognostics, and therapeutics of stroke.

Living on the Edge of the CNS: Meninges Cell Diversity in Health and Disease.

The meninges are the fibrous covering of the central nervous system (CNS) which contain vastly heterogeneous cell types within its three layers (dura, arachnoid, and pia). The dural compartment of the meninges, closest to the skull, is predominantly composed of fibroblasts, but also includes fenestrated blood vasculature, an elaborate lymphatic system, as well as immune cells which are distinct from the CNS. Segregating the outer and inner meningeal compartments is the epithelial-like arachnoid barrier cells, connected by tight and adherens junctions, which regulate the movement of pathogens, molecules, and cells into and out of the cerebral spinal fluid (CSF) and brain parenchyma. Most proximate to the brain is the collagen and basement membrane-rich pia matter that abuts the glial limitans and has recently be shown to have regional heterogeneity within the developing mouse brain. While the meninges were historically seen as a purely structural support for the CNS and protection from trauma, the emerging view of the meninges is as an essential interface between the CNS and the periphery, critical to brain development, required for brain homeostasis, and involved in a variety of diseases. In this review, we will summarize what is known regarding the development, specification, and maturation of the meninges during homeostatic conditions and discuss the rapidly emerging evidence that specific meningeal cell compartments play differential and important roles in the pathophysiology of a myriad of diseases including: multiple sclerosis, dementia, stroke, viral/bacterial meningitis, traumatic brain injury, and cancer. We will conclude with a list of major questions and mechanisms that remain unknown, the study of which represent new, future directions for the field of meninges biology.

Corrigendum: Living on the Edge of the CNS: Meninges Cell Diversity in Health and Disease.

[This corrects the article DOI: 10.3389/fncel.2021.703944.].

Cerebral Edema Formation After Stroke: Emphasis on Blood-Brain Barrier and the Lymphatic Drainage System of the Brain.

Brain edema is a severe stroke complication that is associated with prolonged hospitalization and poor outcomes. Swollen tissues in the brain compromise cerebral perfusion and may also result in transtentorial herniation. As a physical and biochemical barrier between the peripheral circulation and the central nervous system (CNS), the blood-brain barrier (BBB) plays a vital role in maintaining the stable microenvironment of the CNS. Under pathological conditions, such as ischemic stroke, the dysfunction of the BBB results in increased paracellular permeability, directly contributing to the extravasation of blood components into the brain and causing cerebral vasogenic edema. Recent studies have led to the discovery of the glymphatic system and meningeal lymphatic vessels, which provide a channel for cerebrospinal fluid (CSF) to enter the brain and drain to nearby lymph nodes and communicate with the peripheral immune system, modulating immune surveillance and brain responses. A deeper understanding of the function of the cerebral lymphatic system calls into question the known mechanisms of cerebral edema after stroke. In this review, we first discuss how BBB disruption after stroke can cause or contribute to cerebral edema from the perspective of molecular and cellular pathophysiology. Finally, we discuss how the cerebral lymphatic system participates in the formation of cerebral edema after stroke and summarize the pathophysiological process of cerebral edema formation after stroke from the two directions of the BBB and cerebral lymphatic system.

Cerebrospinal fluid and the early brain development of autism.

BACKGROUND: There is currently a renaissance of interest in the many functions of cerebrospinal fluid (CSF). Altered flow of CSF, for example, has been shown to impair the clearance of pathogenic inflammatory proteins involved in neurodegenerative diseases, such as amyloid-ß. In addition, the role of CSF in the newly discovered lymphatic system of the brain has become a prominently researched area in clinical neuroscience, as CSF serves as a conduit between the central nervous system and immune system. MAIN BODY: This article will review the importance of CSF in regulating normal brain development and function, from the prenatal period throughout the lifespan, and highlight recent research that CSF abnormalities in autism spectrum disorder (ASD) are present in infancy, are detectable by conventional structural MRI, and could serve as an early indicator of altered neurodevelopment. CONCLUSION: The identification of early CSF abnormalities in children with ASD, along with emerging knowledge of the underlying pathogenic mechanisms, has the potential to serve as early stratification biomarkers that separate children with ASD into biological subtypes that share a common pathophysiology. Such subtypes could help parse the phenotypic heterogeneity of ASD and map on to targeted, biologically based treatments.