Persistent Malfunction of Glymphatic and Meningeal Lymphatic Drainage in a Mouse Model of Subarachnoid Hemorrhage

Glymphatic system: a gateway for neuroinflammation

The glymphatic system is a relatively recently identified fluid exchange and transport system in the brain. Accumulating evidence indicates that glymphatic function is impaired not only in central nervous system disorders but also in systemic diseases. Systemic diseases can trigger the inflammatory responses in the central nervous system, occasionally leading to sustained inflammation and functional disturbance of the central nervous system. This review summarizes the current knowledge on the association between glymphatic dysfunction and central nervous system inflammation. In addition, we discuss the hypothesis that disease conditions initially associated with peripheral inflammation overwhelm the performance of the glymphatic system, thereby triggering central nervous system dysfunction, chronic neuroinflammation, and neurodegeneration. Future research investigating the role of the glymphatic system in neuroinflammation may offer innovative therapeutic approaches for central nervous system disorders.

Enhancing Th17 cells drainage through meningeal lymphatic vessels alleviate neuroinflammation after subarachnoid hemorrhage

BACKGROUND

Subarachnoid hemorrhage (SAH) is a severe cerebrovascular disorder primarily caused by the rupture of aneurysm, which results in a high mortality rate and consequently imposes a significant burden on society. The occurrence of SAH initiates an immune response that further exacerbates brain damage. The acute inflammatory reaction subsequent to SAH plays a crucial role in determining the prognosis. Th17 cells, a subset of T cells, are related to the brain injury following SAH, and it is unclear how Th17 cells are cleared in the brain. Meningeal lymphatic vessels are a newly discovered intracranial fluid transport system that has been shown to drain large molecules and immune cells to deep cervical lymph nodes. There is limited understanding of the role of the meningeal lymphatic system in SAH. The objective of this research is to explore the impact and underlying mechanism of drainage Th17 cells by meningeal lymphatics on SAH.

METHODS

Treatments to manipulate meningeal lymphatic function and the CCR7-CCL21 pathway were administered, including laser ablation, injection of VEGF-C geneknockout, and protein injection. Mouse behavior was assessed using the balance beam experiment and the modified Garcia scoring system. Flow cytometry, enzyme-linked immunosorbent assays (ELISA), and immunofluorescence staining were used to study the impact of meningeal lymphatic on SAH drainage. Select patients with unruptured and ruptured aneurysms in our hospital as the control group and the SAH group, with 7 cases in each group. Peripheral blood and cerebrospinal fluid (CSF) samples were assessed by ELISA and flow cytometry.

RESULTS

Mice with SAH showed substantial behavioral abnormalities and brain damage in which immune cells accumulated in the brain. Laser ablation of the meningeal lymphatic system or knockout of the CCR7 gene leads to Th17 cell aggregation in the meninges, resulting in a decreased neurological function score and increased levels of inflammatory factors. Injection of VEGF-C or CCL21 protein promotes Th17 cell drainage to lymph nodes, an increased neurological function score, and decreased levels of inflammatory factors. Clinical blood and CSF results showed that inflammatory factors in SAH group were significantly increased. The number of Th17 cells in the SAH group was significantly higher than the control group. Clinical results confirmed Th17 cells aggravated the level of neuroinflammation after SAH.

CONCLUSION

This study shows that improving the drainage of Th17 cells by meningeal lymphatics via the CCR7-CCL21 pathway can reduce brain damage and improve behavior in the SAH mouse model. This could lead to new treatment options for SAH.

The Glymphatic System and Subarachnoid Lymphatic-Like Membrane: Recent Developments in Cerebrospinal Fluid Research

BACKGROUND

Cerebrospinal fluid (CSF) circulates throughout the ventricles, cranial and spinal subarachnoid spaces, and central spinal cord canal. CSF protects the central nervous system through mechanical cushioning, regulation of intracranial pressure, regulation of metabolic homeostasis, and provision of nutrients. Recently, investigators have characterized the glial-lymphatic (glymphatic) system, the analog of the lymphatic system in the central nervous system, and described a fourth meningeal layer; the subarachnoid lymphatic-like membrane (SLYM)relevant to the CSF.

METHODS

A narrative review was conducted.

RESULTS

In this review, we summarize these advances. We describe the development of the original model, controversies, a revised model, and a new conceptual framework. We characterize the biological functions, influence of sleep-wake cycles, and effect of aging with relevance to the glymphatic system. We highlight the role of the glymphatic system in Alzheimer's disease, idiopathic normal pressure hydrocephalus, ischemic stroke, subarachnoid hemorrhage, and traumatic brain injury. Next, we characterize the structure and role of the SLYM. Finally, we explore the relevance of the glymphatic system and SLYM to neurosurgery.

CONCLUSIONS

This manuscript will inform clinicians and scientists regarding preclinical and translational advances in the understanding of the structure, dynamics, and function of the CSF.

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.

Therapeutic approaches to CNS diseases via the meningeal lymphatic and glymphatic system: prospects and challenges

The brain has traditionally been considered an "immune-privileged" organ lacking a lymphatic system. However, recent studies have challenged this view by identifying the presence of the glymphatic system and meningeal lymphatic vessels (MLVs). These discoveries offer new opportunities for waste clearance and treatment of central nervous system (CNS) diseases. Various strategies have been developed based on these pathways, including modulation of glymphatic system function, enhancement of meningeal lymphatic drainage, and utilization of these routes for drug delivery. Consequently, this review explores the developmental features and physiological roles of the cerebral lymphatic system as well as its significance in various CNS disorders. Notably, strategies for ameliorating CNS diseases have been discussed with a focus on enhancing glymphatic system and MLVs functionality through modulation of physiological factors along with implementing pharmacological and physical treatments. Additionally, emphasis is placed on the potential use of the CNS lymphatic system in drug delivery while envisioning future directions in terms of mechanisms, applications, and translational research.

β-hydroxybutyrate alleviates neurological deficits by restoring glymphatic and inflammation after subarachnoid hemorrhage in mice

BACKGROUND

Both glymphatic system dysfunction and inflammatory response aggravate neurological dysfunction after subarachnoid hemorrhage (SAH). Studies have shown that β-hydroxybutyrate (BHB) may mitigate painful diabetic neuropathy (PDN) by upregulating SNTA1 expression and reinstating AQP4 polarity. However, the potential of BHB to ameliorate glymphatic system function and inflammatory response in SAH mice remains uncertain.

METHODS

The SAH models were constructed by injection of arterial blood into cisterna Magana. Three groups of C57 mice were randomly assigned: Sham, SAH, and BHB. All mice were subjected to neurological function assessment, western blot, immunofluorescence double staining, and RNA-seq. Glymphatic system function was examined with tracer and immunofluorescence double staining, and the differential genes were examined with RNA-seq. In addition, the expression of related inflammation was detected.

RESULTS

Compared with the SAH group, BHB reinstated AQP4 polarization by upregulating SNTA1 protein to enhance the glymphatic system. According to RNA-seq, the different genes were primarily connected to microglia activation, astrocytes, and inflammation. Western blot and immunofluorescence further confirmed that the related inflammatory protein expression levels were upregulated. BHB attenuated neuroinflammation after SAH. Ultimately, it can mitigate the neurological deficits in SAH mice.

CONCLUSION

The study reveals a novel mechanism that BHB treatment mitigates neurologic impairment in SAH mice. We propose that BHB may play a neuroprotective effect by enhancing glymphatic system function and attenuating neuroinflammatory subarachnoid hemorrhage.

Imaging Interstitial Fluid With MRI: A Narrative Review on the Associations of Altered Interstitial Fluid With Vascular and Neurodegenerative Abnormalities

Interstitial fluid (ISF) refers to the fluid between the parenchymal cells and along the perivascular spaces (PVS). ISF plays a crucial role in delivering nutrients and clearing waste products from the brain. This narrative review focuses on the use of MRI techniques to measure various ISF characteristics in humans. The complementary value of contrast-enhanced and noncontrast-enhanced techniques is highlighted. While contrast-enhanced MRI methods allow measurement of ISF transport and flow, they lack quantitative assessment of ISF properties. Noninvasive MRI techniques, including multi-b-value diffusion imaging, free-water-imaging, T2-decay imaging, and DTI along the PVS, offer promising alternatives to derive ISF measures, such as ISF volume and diffusivity. The emerging role of these MRI techniques in investigating ISF alterations in neurodegenerative diseases (eg, Alzheimer's disease and Parkinson's disease) and cerebrovascular diseases (eg, cerebral small vessel disease and stroke) is discussed. This review also emphasizes current challenges of ISF imaging, such as the microscopic scale at which ISF has to be measured, and discusses potential focus points for future research to overcome these challenges, for example, the use of high-resolution imaging techniques. Noninvasive MRI methods for measuring ISF characteristics hold significant potential and may have a high clinical impact in understanding the pathophysiology of neurodegenerative and cerebrovascular disorders, as well as in evaluating the efficacy of ISF-targeted therapies in clinical trials. LEVEL OF EVIDENCE: 3 TECHNICAL EFFICACY: Stage 2.

Pathogenesis and management of low-pressure hydrocephalus: A narrative review

Patients diagnosed with low-pressure hydrocephalus typically present with enlarged ventricles and unusually low intracranial pressure, often measuring below 5 cmH2O or even below atmospheric pressure. This atypical presentation often leads to low recognition and diagnostic rates. The development of low-pressure hydrocephalus is believed to be associated with a decrease in the viscoelasticity of brain tissue or separation between the ventricular and subarachnoid spaces. Risk factors for low-pressure hydrocephalus include subarachnoid hemorrhage, aqueduct stenosis, prior cranial radiotherapy， ventricular shunting, and cerebrospinal fluid leaks. For potential low-pressure hydrocephalus, diagnostic criteria include neurological symptoms related to hydrocephalus, an Evans index >0.3 on imaging, ICP ≤ 5 cm H2O, symptom improvement with negative pressure drainage, and exclusion of ventriculomegaly caused by neurodegenerative diseases. The pathogenesis and pathophysiological features of low-pressure hydrocephalus differ significantly from other types of hydrocephalus, making it challenging to restore normal ventricular morphology through conventional drainage methods. The primary treatment options for low-pressure hydrocephalus involve negative pressure drainage and third ventriculostomy. With appropriate treatment, most patients can regain their previous neurological function. However, in most cases, permanent shunt surgery is still necessary. Low-pressure hydrocephalus is a rare condition with a high rate of underdiagnosis and mortality. Early identification and appropriate intervention are crucial in reducing complications and improving prognosis.

Extracranial transport of brain lymphatics via cranial nerve in human

Extracranial waste transport from the brain interstitial fluid to the deep cervical lymph node (dCLN) is not extensively understood. The present study aims to show the cranial nerves that have a role in the transport of brain lymphatics vessels (LVs), their localization, diameter, and number using podoplanin (PDPN) and CD31 immunohistochemistry (IHC) and Western blotting. Cranial nerve samples from 6 human cases (3 cadavers, and 3 autopsies) were evaluated for IHC and 3 autopsies for Western blotting. The IHC staining showed LVs along the optic, olfactory, oculomotor, trigeminal, facial, glossopharyngeal, accessory, and vagus nerves. However, no LVs present along the trochlear, abducens, vestibulocochlear, and hypoglossal nerves. The LVs were predominantly localized at the endoneurium of the cranial nerve that has motor components, and LVs in the cranial nerves that had sensory components were present in all 3 layers. The number of LVs accompanying the olfactory, optic, and trigeminal nerves was classified as numerous; oculomotor, glossopharyngeal, vagus, and accessory was moderate; and facial nerves was few. The largest diameter of LVs was in the epineurium and the smallest one was in the endoneurium. The majority of Western blotting results correlated with the IHC. The present findings suggest that specific cranial nerves with variable quantities provide a pathway for the transport of wastes from the brain to dCLN. Thus, the knowledge of the transport of brain lymphatics along cranial nerves may help understand the pathophysiology of various neurological diseases.

The glymphatic system and cerebral small vessel disease

OBJECTIVES

Cerebral small vessel disease is a group of pathologies in which alterations of the brain's blood vessels contribute to stroke and neurocognitive changes. Recently, a neurotoxic waste clearance system composed of perivascular spaces abutting the brain's blood vessels, termed the glymphatic system, has been identified as a key player in brain homeostasis. Given that small vessel disease and the glymphatic system share anatomical structures, this review aims to reexamine small vessel disease in the context of the glymphatic system and highlight novel aspects of small vessel disease physiology.

MATERIALS AND METHODS

This review was conducted with an emphasis on studies that examined aspects of small vessel disease and on works characterizing the glymphatic system. We searched PubMed for relevant articles using the following keywords: glymphatics, cerebral small vessel disease, arterial pulsatility, hypertension, blood-brain barrier, endothelial dysfunction, stroke, diabetes.

RESULTS

Cerebral small vessel disease and glymphatic dysfunction are anatomically connected and significant risk factors are shared between the two. These include hypertension, type 2 diabetes, advanced age, poor sleep, obesity, and neuroinflammation. There is clear evidence that CSVD hinders the effective functioning of glymphatic system.

CONCLUSION

These shared risk factors, as well as the model of cerebral amyloid angiopathy pathogenesis, hint at the possibility that glymphatic dysfunction could independently contribute to the pathogenesis of cerebral small vessel disease. However, the current evidence supports a model of cascading dysfunction, wherein concurrent small vessel and glymphatic injury hinder glymphatic-mediated recovery and promote the progression of subclinical to clinical disease.

Glymphatic and lymphatic communication with systemic responses during physiological and pathological conditions in the central nervous system

Crosstalk between central nervous system (CNS) and systemic responses is important in many pathological conditions, including stroke, neurodegeneration, schizophrenia, epilepsy, etc. Accumulating evidence suggest that signals for central-systemic crosstalk may utilize glymphatic and lymphatic pathways. The glymphatic system is functionally connected to the meningeal lymphatic system, and together these pathways may be involved in the distribution of soluble proteins and clearance of metabolites and waste products from the CNS. Lymphatic vessels in the dura and meninges transport cerebrospinal fluid, in part collected from the glymphatic system, to the cervical lymph nodes, where solutes coming from the brain (i.e., VEGFC, oligomeric α-syn, β-amyloid) might activate a systemic inflammatory response. There is also an element of time since the immune system is strongly regulated by circadian rhythms, and both glymphatic and lymphatic dynamics have been shown to change during the day and night. Understanding the mechanisms regulating the brain-cervical lymph node (CLN) signaling and how it might be affected by diurnal or circadian rhythms is fundamental to find specific targets and timing for therapeutic interventions.

Vascular Endothelial Growth Factor-C Treatment Enhances Cerebrospinal Fluid Outflow during Toxoplasma gondii Brain Infection but Does Not Improve Cerebral Edema

Cerebral edema frequently develops in the setting of brain infection and can contribute to elevated intracranial pressure, a medical emergency. How excess fluid is cleared from the brain is not well understood. Previous studies have shown that interstitial fluid is transported out of the brain along perivascular channels that collect into the cerebrospinal fluid (CSF)-filled subarachnoid space. CSF is then removed from the central nervous system through venous and lymphatic routes. The current study tested the hypothesis that increasing lymphatic drainage of CSF would promote clearance of cerebral edema fluid during infection with the neurotropic parasite Toxoplasma gondii. Fluorescent microscopy and magnetic resonance imaging was used to show that C57BL/6 mice develop vasogenic edema 4 to 5 weeks after infection with T. gondii. Tracer experiments were used to evaluate how brain infection affects meningeal lymphatic function, which demonstrated a decreased rate in CSF outflow in T. gondii-infected mice. Next, mice were treated with a vascular endothelial growth factor (VEGF)-C-expressing viral vector, which induced meningeal lymphangiogenesis and improved CSF outflow in chronically infected mice. No difference in cerebral edema was observed between mice that received VEGF-C and those that rececived sham treatment. Therefore, although VEGF-C treatment can improve lymphatic outflow in mice infected with T. gondii, this effect does not lead to increased clearance of edema fluid from the brains of these mice.

The glymphatic system for neurosurgeons: a scoping review

The discovery of the glymphatic system has revolutionized our understanding of cerebrospinal fluid (CSF) circulation and interstitial waste clearance in the brain. This scoping review aims to synthesize the current literature on the glymphatic system's role in neurosurgical conditions and its potential as a therapeutic target. We conducted a comprehensive search in PubMed and Scopus databases for studies published between January 1, 2012, and October 31, 2023. Studies were selected based on their relevance to neurosurgical conditions and glymphatic function, with both animal and human studies included. Data extraction focused on the methods for quantifying glymphatic function and the main results. A total of 67 articles were included, covering conditions such as idiopathic normal pressure hydrocephalus (iNPH), idiopathic intracranial hypertension (IIH), subarachnoid hemorrhage (SAH), stroke, intracranial tumors, and traumatic brain injury (TBI). Significant glymphatic dysregulation was noted in iNPH and IIH, with evidence of impaired CSF dynamics and delayed clearance. SAH studies indicated glymphatic dysfunction with the potential therapeutic effects of nimodipine and tissue plasminogen activator. In stroke, alterations in glymphatic activity correlated with the extent of edema and neurological recovery. TBI studies highlighted the role of the glymphatic system in post-injury cognitive outcomes. Results indicate that the regulation of aquaporin-4 (AQP4) channels is a critical target for therapeutic intervention. The glymphatic system plays a critical role in the pathophysiology of various neurosurgical conditions, influencing brain edema and CSF dynamics. Targeting the regulation of AQP4 channels presents as a significant therapeutic strategy. Although promising, the translation of these findings into clinical practice requires further human studies. Future research should focus on establishing non-invasive biomarkers for glymphatic function and exploring the long-term effects of glymphatic dysfunction.

Deregulation of the Glymphatic System in Alzheimer's Disease: Genetic and Non-Genetic Factors

Alzheimer's disease (AD) is the most prevalent form of dementia and is characterized by progressive degeneration of brain function. AD gradually affects the parts of the brain that control thoughts, language, behavior and mental function, severely impacting a person's ability to carry out daily activities and ultimately leading to death. The accumulation of extracellular amyloid-β peptide (Aβ) and the aggregation of intracellular hyperphosphorylated tau are the two key pathological hallmarks of AD. AD is a complex condition that involves both non-genetic risk factors (35%) and genetic risk factors (58-79%). The glymphatic system plays an essential role in clearing metabolic waste, transporting tissue fluid, and participating in the immune response. Both non-genetic and genetic risk factors affect the glymphatic system to varying degrees. The main purpose of this review is to summarize the underlying mechanisms involved in the deregulation of the glymphatic system during the progression of AD, especially concerning the diverse contributions of non-genetic and genetic risk factors. In the future, new targets and interventions that modulate these interrelated mechanisms will be beneficial for the prevention and treatment of AD.

The Interplay between Meningeal Lymphatic Vessels and Neuroinflammation in Neurodegenerative Diseases

Meningeal lymphatic vessels (MLVs) are essential for the drainage of cerebrospinal fluid, macromolecules, and immune cells in the central nervous system. They play critical roles in modulating neuroinflammation in neurodegenerative diseases. Dysfunctional MLVs have been demonstrated to increase neuroinflammation by horizontally blocking the drainage of neurotoxic proteins to the peripheral lymph nodes. Conversely, MLVs protect against neuroinflammation by preventing immune cells from becoming fully encephalitogenic. Furthermore, evidence suggests that neuroinflammation affects the structure and function of MLVs, causing vascular anomalies and angiogenesis. Although this field is still in its infancy, the strong link between MLVs and neuroinflammation has emerged as a potential target for slowing the progression of neurodegenerative diseases. This review provides a brief history of the discovery of MLVs, introduces in vivo and in vitro MLV models, highlights the molecular mechanisms through which MLVs contribute to and protect against neuroinflammation, and discusses the potential impact of neuroinflammation on MLVs, focusing on recent progress in neurodegenerative diseases.

Inactivation of ERK1/2 signaling mediates dysfunction of basal meningeal lymphatic vessels in experimental subdural hematoma

Rationale: Meningeal lymphatic vessels (MLVs) are essential for the clearance of subdural hematoma (SDH). However, SDH impairs their drainage function, and the pathogenesis remains unclear. Herein, we aimed to understand the pathological mechanisms of MLV dysfunction following SDH and to test whether atorvastatin, an effective drug for SDH clearance, improves meningeal lymphatic drainage (MLD). Methods: We induced SDH models in rats by injecting autologous blood into the subdural space and evaluated MLD using Gadopentetate D, Evans blue, and CFSE-labeled erythrocytes. Whole-mount immunofluorescence and transmission electron microscopy were utilized to detect the morphology of MLVs. Phosphoproteomics, western blot, flow cytometry, and in vitro experiments were performed to investigate the molecular mechanisms underlying dysfunctional MLVs. Results: The basal MLVs were detected to have abundant valves and play an important role in draining subdural substances. Following SDH, these basal MLVs exhibited disrupted endothelial junctions and dilated lumen, leading to impaired MLD. Subsequent proteomics analysis of the meninges detected numerous dephosphorylated proteins, primarily enriched in the adherens junction, including significant dephosphorylation of ERK1/2 within the meningeal lymphatic endothelial cells (LECs). Subdural injection of the ERK1/2 kinase inhibitor PD98059 resulted in dilated basal MLVs and impaired MLD, resembling the dysfunctional MLVs observed in SDH. Moreover, inhibiting ERK1/2 signaling severely disrupted intercellular junctions between cultured LECs. Finally, atorvastatin was revealed to protect the structure of basal MLVs and accelerate MLD following SDH. However, these beneficial effects of atorvastatin were abolished when combined with PD98059. Conclusion: Our findings demonstrate that SDH induces ERK1/2 dephosphorylation in meningeal LECs, leading to disrupted basal MLVs and impaired MLD. Additionally, we reveal a beneficial effect of atorvastatin in improving MLD.

The state of brain activity modulates cerebrospinal fluid transport

Earlier studies based on 2-photon imaging have shown that glymphatic cerebrospinal fluid (CSF) transport is regulated by the sleep-wake cycle. To examine this association, we used 3DISCO whole-body tissue clearing to map CSF tracer distribution in awake, sleeping and ketamine-xylazine anesthetized mice. The results of our analysis showed that CSF tracers entered the brain to a significantly larger extent in natural sleep or ketamine-xylazine anesthesia than in wakefulness. Furthermore, awake mice showed preferential transport of CSF tracers in the rostro-caudal direction towards the cervical and spinal cord lymphatic vessels, and hence to venous circulation and excretion by the kidneys. The study extends the current literature by showing that CSF dynamics on the whole-body scale is controlled by the state of brain activity.

Clinical Potential of Immunotherapies in Subarachnoid Hemorrhage Treatment: Mechanistic Dissection of Innate and Adaptive Immune Responses

Subarachnoid hemorrhage (SAH), classified as a medical emergency, is a devastating and severe subtype of stroke. SAH induces an immune response, which further triggers brain injury; however, the underlying mechanisms need to be further elucidated. The current research is predominantly focused on the production of specific subtypes of immune cells, especially innate immune cells, post-SAH onset. Increasing evidence suggests the critical role of immune responses in SAH pathophysiology; however, studies on the role and clinical significance of adaptive immunity post-SAH are limited. In this present study, we briefly review the mechanistic dissection of innate and adaptive immune responses post-SAH. Additionally, we summarized the experimental studies and clinical trials of immunotherapies for SAH treatment, which may form the basis for the development of improved therapeutic approaches for the clinical management of SAH in the future.

Pathophysiology, Management, and Therapeutics in Subarachnoid Hemorrhage and Delayed Cerebral Ischemia: An Overview

Subarachnoid hemorrhage (SAH) is a type of hemorrhagic stroke resulting from the rupture of an arterial vessel within the brain. Unlike other stroke types, SAH affects both young adults (mid-40s) and the geriatric population. Patients with SAH often experience significant neurological deficits, leading to a substantial societal burden in terms of lost potential years of life. This review provides a comprehensive overview of SAH, examining its development across different stages (early, intermediate, and late) and highlighting the pathophysiological and pathohistological processes specific to each phase. The clinical management of SAH is also explored, focusing on tailored treatments and interventions to address the unique pathological changes that occur during each stage. Additionally, the paper reviews current treatment modalities and pharmacological interventions based on the evolving guidelines provided by the American Heart Association (AHA). Recent advances in our understanding of SAH will facilitate clinicians' improved management of SAH to reduce the incidence of delayed cerebral ischemia in patients.

Assessment of factors influencing glymphatic activity and implications for clinical medicine

The glymphatic system is a highly specialized fluid transport system in the central nervous system. It enables the exchange of the intercellular fluid of the brain, regulation of the movement of this fluid, clearance of unnecessary metabolic products, and, potentially, brain immunity. In this review, based on the latest scientific reports, we present the mechanism of action and function of the glymphatic system and look at the role of factors influencing its activity. Sleep habits, eating patterns, coexisting stress or hypertension, and physical activity can significantly affect glymphatic activity. Modifying them can help to change lives for the better. In the next section of the review, we discuss the connection between the glymphatic system and neurological disorders. Its association with many disease entities suggests that it plays a major role in the physiology of the whole brain, linking many pathophysiological pathways of individual diseases.

Glymphatic system: an emerging therapeutic approach for neurological disorders

The functions of the glymphatic system include clearance of the metabolic waste and modulation of the water transport in the brain, and it forms a brain-wide fluid network along with cerebrospinal fluid (CSF) and interstitial fluid (ISF). The glymphatic pathway consists of periarterial influx of CSF, astrocyte-mediated interchange between ISF and CSF supported by aquaporin-4 (AQP4) on the endfeet of astrocyte around the periarterioles, and perivenous efflux of CSF. Finally, CSF is absorbed by the arachnoid granules or flows into the cervical lymphatic vessels. There is growing evidence from animal experiments that the glymphatic system dysfunction is involved in many neurological disorders, such as Alzheimer's disease, stroke, epilepsy, traumatic brain injury and meningitis. In this review, we summarize the latest progress on the glymphatic system and its driving factors, as well as changes in the glymphatic pathway in different neurological diseases. We significantly highlight the likely therapeutic approaches for glymphatic pathway in neurological diseases, and the importance of AQP4 and normal sleep architecture in this process.

The glymphatic system: a new perspective on brain diseases

The glymphatic system is a brain-wide perivascular pathway driven by aquaporin-4 on the endfeet of astrocytes, which can deliver nutrients and active substances to the brain parenchyma through periarterial cerebrospinal fluid (CSF) influx pathway and remove metabolic wastes through perivenous clearance routes. This paper summarizes the composition, overall fluid flow, solute transport, related diseases, affecting factors, and preclinical research methods of the glymphatic system. In doing so, we aim to provide direction and reference for more relevant researchers in the future.

The mechanism of ferroptosis in early brain injury after subarachnoid hemorrhage

Subarachnoid hemorrhage (SAH) is a cerebrovascular accident with an acute onset, severe disease characteristics, and poor prognosis. Within 72 hours after the occurrence of SAH, a sequence of pathological changes occur in the body including blood-brain barrier breakdown, cerebral edema, and reduced cerebrovascular flow that are defined as early brain injury (EBI), and it has been demonstrated that EBI exhibits an obvious correlation with poor prognosis. Ferroptosis is a novel programmed cell death mode. Ferroptosis is induced by the iron-dependent accumulation of lipid peroxides and reactive oxygen species (ROS). Ferroptosis involves abnormal iron metabolism, glutathione depletion, and lipid peroxidation. Recent study revealed that ferroptosis is involved in EBI and is significantly correlated with poor prognosis. With the gradual realization of the importance of ferroptosis, an increasing number of studies have been conducted to examine this process. This review summarizes the latest work in this field and tracks current research progress. We focused on iron metabolism, lipid metabolism, reduction systems centered on the GSH/GPX4 system, other newly discovered GSH/GPX4-independent antioxidant systems, and their related targets in the context of early brain injury. Additionally, we examined certain ferroptosis regulatory mechanisms that have been studied in other fields but not in SAH. A link between death and oxidative stress has been described. Additionally, we highlight the future research direction of ferroptosis in EBI of SAH, and this provides new ideas for follow-up research.

rTMS treatment for abrogating intracerebral hemorrhage-induced brain parenchymal metabolite clearance dysfunction in male mice by regulating intracranial lymphatic drainage

BACKGROUND

The discovery of the glymphatic system and meningeal lymphatic vessels challenged the traditional view regarding the lack of a lymphatic system in the central nervous system. It is now known that the intracranial lymphatic system plays an important role in fluid transport, macromolecule uptake, and immune cell trafficking. Studies have also shown that the function of the intracranial lymphatic system is significantly associated with neurological diseases; for example, an impaired intracranial lymphatic system can lead to Tau deposition and an increased lymphocyte count in the brain tissue of mice with subarachnoid hemorrhage.

METHODS

In this study, we assessed the changes in the intracranial lymphatic system after intracerebral hemorrhage and the regulatory effects of repeated transcranial magnetic stimulation on the glymphatic system and meningeal lymphatic vessels in an intracerebral hemorrhage (ICH) model of male mice. Experimental mice were divided into three groups: Sham, ICH, and ICH + repeated transcranial magnetic stimulation (rTMS). Three days after ICH, mice in the ICH+rTMS group were subjected to rTMS daily for 7 days. Thereafter, the function of the intracranial lymphatic system, clearance of RITC-dextran and FITC-dextran, and neurological functions were evaluated.

RESULTS

Compared with the Sham group, the ICH group had an impaired glymphatic system. Importantly, rTMS treatment could improve intracranial lymphatic system function as well as behavioral functions and enhance the clearance of parenchymal RITC-dextran and FITC-dextran after ICH.

CONCLUSION

Our results indicate that rTMS can abrogate ICH-induced brain parenchymal metabolite clearance dysfunction by regulating intracranial lymphatic drainage.

Immune compartments at the brain's borders in health and neurovascular diseases

Recent evidence implicates cranial border immune compartments in the meninges, choroid plexus, circumventricular organs, and skull bone marrow in several neuroinflammatory and neoplastic diseases. Their pathogenic importance has also been described for cardiovascular diseases such as hypertension and stroke. In this review, we will examine the cellular composition of these cranial border immune niches, the potential pathways through which they might interact, and the evidence linking them to cardiovascular disease.

Narrative review of roles of astrocytes in subarachnoid hemorrhage

Background and Objective

Astrocytes play an important role in healthy brain function, including the development and maintenance of blood-brain barrier (BBB), structural support, brain homeostasis, neurovascular coupling and secretion of neuroprotective factors. Reactive astrocytes participate in various pathophysiology after subarachnoid hemorrhage (SAH) including neuroinflammation, glutamate toxicity, brain edema, vasospasm, BBB disruption, cortical spreading depolarization (SD).

Methods

We searched PubMed up to 31 May, 2022 and evaluated the articles for screening and inclusion for subsequent systemic review. We found 198 articles with the searched terms. After exclusion based on the selection criteria, we selected 30 articles to start the systemic review.

Key Content and Findings

We summarized the response of astrocytes induced by SAH. Astrocytes are critical for brain edema formation, BBB reconstruction and neuroprotection in the acute stage of SAH. Astrocytes clear extracellular glutamate by increasing the uptake of glutamate and Na+/K+ ATPase activity after SAH. Neurotrophic factors released by astrocytes contribute to neurological recovery after SAH. Meanwhile, Astrocytes also form glial scars which hinder axon regeneration, produce proinflammatory cytokines, free radicals, and neurotoxic molecules.

Conclusions

Preclinical studies showed that therapeutic targeting the astrocytes response could have a beneficial effect in ameliorating neuronal injury and cognitive impairment after SAH. Clinical trials and preclinical animal studies are still urgently needed in order to determine where astrocytes stand in various pathway of brain damage and repair after SAH and, above all, to develop therapeutic approaches which benefit patient outcomes.

Movement of cerebrospinal fluid tracer into brain parenchyma and outflow to nasal mucosa is reduced at 24 h but not 2 weeks post-stroke in mice

BACKGROUND

Recent data indicates that cerebrospinal fluid (CSF) dynamics are disturbed after stroke. Our lab has previously shown that intracranial pressure rises dramatically 24 h after experimental stroke and that this reduces blood flow to ischaemic tissue. CSF outflow resistance is increased at this time point. We hypothesised that reduced transit of CSF through brain parenchyma and reduced outflow of CSF via the cribriform plate at 24 h after stroke may contribute to the previously identified post-stroke intracranial pressure elevation.

METHODS

Using a photothrombotic permanent occlusion model of stroke in C57BL/6 adult male mice, we examined the movement of an intracisternally infused 0.5% Texas Red dextran throughout the brain and measured tracer efflux into the nasal mucosa via the cribriform plate at 24 h or two weeks after stroke. Brain tissue and nasal mucosa were collected ex vivo and imaged using fluorescent microscopy to determine the change in CSF tracer intensity in these tissues.

RESULTS

At 24 h after stroke, we found that CSF tracer load was significantly reduced in brain tissue from stroke animals in both the ipsilateral and contralateral hemispheres when compared to sham. CSF tracer load was also reduced in the lateral region of the ipsilateral hemisphere when compared to the contralateral hemisphere in stroke brains. In addition, we identified an 81% reduction in CSF tracer load in the nasal mucosa in stroke animals compared to sham. These alterations to the movement of CSF-borne tracer were not present at two weeks after stroke.

CONCLUSIONS

Our data indicates that influx of CSF into the brain tissue and efflux via the cribriform plate are reduced 24 h after stroke. This may contribute to reported increases in intracranial pressure at 24 h after stroke and thus worsen stroke outcomes.

Unpacking the Role of Extracellular Vesicles in Ischemic and Hemorrhagic Stroke: Pathophysiology and Therapeutic Implications

Stroke is a leading cause of death and disability worldwide. Inflammation and microvascular dysfunction have been associated with brain injury and long-term disability after both ischemic and hemorrhagic stroke. Recent studies have suggested a potential role of extracellular vesicles (EVs) as a link underlying these pathogenic processes. EVs are cell-derived particles enveloped by a lipid bilayer, containing proteins, lipids, and nucleic acids. From a functional standpoint, EVs can facilitate intercellular communication, including across the blood-brain barrier (BBB). Recent advances in EV research have shown a preferential release of EVs from specific cell types in the context of stroke, some of which were associated with increased neuroinflammation, microvascular dysfunction, and neuronal cytotoxicity while others offered a degree of neuroprotection. However, one historic challenge in the studies of EVs in stroke is the lack of consistent definitions and methods to analyze EVs, only recently updated in the MISEV2018 guidelines. Given limitations and complexity in the treatment of stroke, particularly delivery of therapeutics across the BBB, increasing attention has been paid towards manipulating EVs as one vehicle that can permit targeted therapeutic delivery to the central nervous system. These discoveries point towards a future where a better understanding of EVs will advance our knowledge of stroke-associated mechanisms of cerebral and systemic injury and contribute to the development of novel treatments. Here, we review the role that EVs play in ischemic and hemorrhagic stroke.

Aquaporin-4 in glymphatic system, and its implication for central nervous system disorders

The clearance function is essential for maintaining brain tissue homeostasis, and the glymphatic system is the main pathway for removing brain interstitial solutes. Aquaporin-4 (AQP4) is the most abundantly expressed aquaporin in the central nervous system (CNS) and is an integral component of the glymphatic system. In recent years, many studies have shown that AQP4 affects the morbidity and recovery process of CNS disorders through the glymphatic system, and AQP4 shows notable variability in CNS disorders and is part of the pathogenesis of these diseases. Therefore, there has been considerable interest in AQP4 as a potential and promising target for regulating and improving neurological impairment. This review aims to summarize the pathophysiological role that AQP4 plays in several CNS disorders by affecting the clearance function of the glymphatic system. The findings can contribute to a better understanding of the self-regulatory functions in CNS disorders that AQP4 were involved in and provide new therapeutic alternatives for incurable debilitating neurodegenerative disorders of CNS in the future.

Effects of Aloe-Emodin on the Expression of Brain Aquaporins and Secretion of Neurotrophic Factors in a Rat Model of Post-Stroke Depression

Post-stroke depression (PSD) is a common complication of stroke that can damage patients' brains. More and more studies have been conducted on PSD in recent years, but the exact mechanism is still not understood. Currently, animal models provide an alternative approach to better understand the pathophysiology of PSD and may also pave the way for the discovery of new treatments for depression. This study investigated the therapeutic effect and mechanism of aloe-emodin (AE) on PSD rats. Previous studies have shown that AE positively affects PSD in rats by improving depression, increasing their activities and curiosities, enhancing the number of neurons, and ameliorating damage to brain tissue. Meanwhile, AE could up-regulate the expression of brain-derived neurotrophic factor (BDNF) and neurotrophic 3 (NTF3), but it could also down-regulate the expression of aquaporins (AQP3, AQP4, and AQP5), glial fibrillary acidic protein (GFAP), and transient receptor potential vanilloid 4 (TRPV4), which is helpful in maintaining homeostasis and alleviating encephaledema. AE may be a prospective solution in the future for the treatment of PSD patients.

Glymphatic system evaluation using diffusion tensor imaging in patients with traumatic brain injury

PURPOSE

Glymphatic system dysfunction has been reported in animal models of traumatic brain injury (TBI). This study aimed to evaluate the activity of the human glymphatic system using the non-invasive Diffusion Tensor Image-Analysis aLong the Perivascular Space (DTI-ALPS) method in patients with TBI.

METHODS

A total of 89 patients with TBI (June 2018 to May 2020) were retrospectively enrolled, and 34 healthy volunteers were included who had no previous medical or neurological disease. Magnetic resonance imaging (MRI) with DTI was performed, and the ALPS index was calculated to evaluate the glymphatic system's activity. Wilcoxon rank-sum test was used to compare the ALPS index between patients with TBI and healthy controls. ANOVA was done to compare the ALPS index among controls and patients with mild/moderate-to-severe TBI. Multivariate logistic regression analyses were used to identify independent clinical and radiological factors associated with the ALPS index. The correlation between Glasgow Coma Scale (GCS) score and the ALPS index was also assessed.

RESULTS

The ALPS index was significantly lower in patients with TBI than in healthy controls (median, 1.317 vs. 1.456, P < 0.0001). There were significant differences in the ALPS index between healthy controls and patients with mild/moderate-to-severe TBI (ANOVA, P < 0.001). The presence of subarachnoid hemorrhage (P = 0.004) and diffuse axonal injury (P = 0.001) was correlated with a lower ALPS index in the multivariate analysis. There was a weak positive correlation between the ALPS index and GCS scores (r = 0.242, P = 0.023).

CONCLUSIONS

The DTI-ALPS method is useful for evaluating glymphatic system impairment and quantifying its activity in patients with TBI.

Brain Waste Removal System and Sleep: Photobiomodulation as an Innovative Strategy for Night Therapy of Brain Diseases

Emerging evidence suggests that an important function of the sleeping brain is the removal of wastes and toxins from the central nervous system (CNS) due to the activation of the brain waste removal system (BWRS). The meningeal lymphatic vessels (MLVs) are an important part of the BWRS. A decrease in MLV function is associated with Alzheimer's and Parkinson's diseases, intracranial hemorrhages, brain tumors and trauma. Since the BWRS is activated during sleep, a new idea is now being actively discussed in the scientific community: night stimulation of the BWRS might be an innovative and promising strategy for neurorehabilitation medicine. This review highlights new trends in photobiomodulation of the BWRS/MLVs during deep sleep as a breakthrough technology for the effective removal of wastes and unnecessary compounds from the brain in order to increase the neuroprotection of the CNS as well as to prevent or delay various brain diseases.

B lymphocytes ameliorate Alzheimer's disease-like neuropathology via interleukin-35

Increasing evidence supports the involvement of the peripheral immune system in the pathogenesis of Alzheimer's disease (AD). In the present study, we found that B lymphocytes could mitigate beta-Amyloid (Aβ) pathology and memory impairments in a transgenic AD mouse model. Specifically, in young 5 × FAD mice, we evidenced increased B cells in the frontal cortex and meningeal tissues; depletion of mature B cells aggravated these mice's Aβ load and memory deficits. The increased B cells produced more interleukin-35 (IL-35) in the front cortex. We further found IL-35 neutralization exacerbated Aβ pathology, while injecting IL-35 mitigated Aβ load and cognitive dysfunction in 5 × FAD mice with or without mature B cell deficiency. Mechanistically, IL-35 inhibited neuronal BACE1 transcription through modulating the SOCS1/STAT1 pathway, and reduced Aβ production accordingly. Reanalysis of the single-cell RNA sequencing data from blood samples of AD patients suggested an increased population of IL-35-producing B cells. Together, the present study revealed a novel effect of B lymphocyte-derived IL-35 on inhibiting Aβ production in the frontal cortex, which may serve as a potential target for future AD treatment.

The meningeal transcriptional response to traumatic brain injury and aging

Emerging evidence suggests that the meningeal compartment plays instrumental roles in various neurological disorders, however, we still lack fundamental knowledge about meningeal biology. Here, we utilized high-throughput RNA sequencing (RNA-seq) techniques to investigate the transcriptional response of the meninges to traumatic brain injury (TBI) and aging in the sub-acute and chronic time frames. Using single-cell RNA sequencing (scRNA-seq), we first explored how mild TBI affects the cellular and transcriptional landscape in the meninges in young mice at one-week post-injury. Then, using bulk RNA-seq, we assessed the differential long-term outcomes between young and aged mice following TBI. In our scRNA-seq studies, we highlight injury-related changes in differential gene expression seen in major meningeal cell populations including macrophages, fibroblasts, and adaptive immune cells. We found that TBI leads to an upregulation of type I interferon (IFN) signature genes in macrophages and a controlled upregulation of inflammatory-related genes in the fibroblast and adaptive immune cell populations. For reasons that remain poorly understood, even mild injuries in the elderly can lead to cognitive decline and devastating neuropathology. To better understand the differential outcomes between the young and the elderly following brain injury, we performed bulk RNA-seq on young and aged meninges 1.5 months after TBI. Notably, we found that aging alone induced upregulation of meningeal genes involved in antibody production by B cells and type I IFN signaling. Following injury, the meningeal transcriptome had largely returned to its pre-injury signature in young mice. In stark contrast, aged TBI mice still exhibited upregulation of immune-related genes and downregulation of genes involved in extracellular matrix remodeling. Overall, these findings illustrate the dynamic transcriptional response of the meninges to mild head trauma in youth and aging.

Overview of the meningeal lymphatic vessels in aging and central nervous system disorders

In the aging process and central nervous system (CNS) diseases, the functions of the meningeal lymphatic vessels (MLVs) are impaired. Alterations in MLVs have been observed in aging-related neurodegenerative diseases, brain tumors, and even cerebrovascular disease. These findings reveal a new perspective on aging and CNS disorders and provide a promising therapeutic target. Additionally, recent neuropathological studies have shown that MLVs exchange soluble components between the cerebrospinal fluid (CSF) and interstitial fluid (ISF) and drain metabolites, cellular debris, misfolded proteins, and immune cells from the CSF into the deep cervical lymph nodes (dCLNs), directly connecting the brain with the peripheral circulation. Impairment and dysfunction of meningeal lymphatics can lead to the accumulation of toxic proteins in the brain, exacerbating the progression of neurological disorders. However, for many CNS diseases, the causal relationship between MLVs and neuropathological changes is not fully clear. Here, after a brief historical retrospection, we review recent discoveries about the hallmarks of MLVs and their roles in the aging and CNS diseases, as well as potential therapeutic targets for the treatment of neurologic diseases.

Effects of increased intracranial pressure on cerebrospinal fluid influx, cerebral vascular hemodynamic indexes, and cerebrospinal fluid lymphatic efflux

The glymphatic-lymphatic fluid transport system (GLFTS) consists of glymphatic pathway and cerebrospinal fluid (CSF) lymphatic outflow routes, allowing biological liquids from the brain parenchyma to access the CSF along with perivascular space and to be cleaned out of the skull through lymphatic vessels. It is known that increased local pressure due to physical compression of tissue improves lymphatic transport in peripheral organs, but little is known about the exact relationship between increased intracranial pressure (IICP) and GLFTS. In this study, we verify our hypothesis that IICP significantly impacts GLFTS, and this effect depends on severity of the IICP. Using a previously developed inflating balloon model to induce IICP and inject fluorescent tracers into the cisterna magna, we found significant impairment of the glymphatic circulation after IICP. We further found that cerebrovascular occlusion occurred, and cerebrovascular pulsation decreased after IICP. IICP also interrupted the drainage of deep cervical lymph nodes and dorsal meningeal lymphatic function, enhancing spinal lymphatic outflow to the sacral lymph nodes. Notably, these effects were associated with the severity of IICP. Thus, our findings proved that the intensity of IICP significantly impacts GLFTS. This may have translational applications for preventing and treating related neurological disorders.

Aβ Deposits in the Neocortex of Adult and Infant Hypoxic Brains, Including in Cases of COVID-19

The brain of a 58-year-old woman was included as a civilian control in an ongoing autopsy study of military traumatic brain injury (TBI). The woman died due to a polysubstance drug overdose, with Coronavirus Disease 2019 (COVID-19) serving as a contributing factor. Immunohistochemical stains for β-amyloid (Aβ), routinely performed for the TBI study, revealed numerous, unusual neocortical Aβ deposits. We investigated the autopsied brains of 10 additional young patients (<60 years old) who died of COVID-19, and found similar Aβ deposits in all, using two different Aβ antibodies across three different medical centers. The deposits failed to stain with Thioflavin-S. To investigate whether or not these deposits formed uniquely to COVID-19, we applied Aβ immunostains to the autopsied brains of COVID-19-negative adults who died with acute respiratory distress syndrome and infants with severe cardiac anomalies, and also biopsy samples from patients with subacute cerebral infarcts. Cortical Aβ deposits were also found in these cases, suggesting a link to hypoxia. The fate of these deposits and their effects on function are unknown, but it is possible that they contribute to the neurocognitive sequelae observed in some COVID-19 patients. Our findings may also have broader implications concerning hypoxia and its role in Aβ deposition in the brain.

Perivascular spaces as a potential biomarker of Alzheimer's disease

Alzheimer's disease (AD) is a highly damaging disease that affects one's cognition and memory and presents an increasing societal and economic burden globally. Considerable research has gone into understanding AD; however, there is still a lack of effective biomarkers that aid in early diagnosis and intervention. The recent discovery of the glymphatic system and associated Perivascular Spaces (PVS) has led to the theory that enlarged PVS (ePVS) may be an indicator of AD progression and act as an early diagnostic marker. Visible on Magnetic Resonance Imaging (MRI), PVS appear to enlarge when known biomarkers of AD, amyloid-β and tau, accumulate. The central goal of ePVS and AD research is to determine when ePVS occurs in AD progression and if ePVS are causal or epiphenomena. Furthermore, if ePVS are indeed causative, interventions promoting glymphatic clearance are an attractive target for research. However, it is necessary first to ascertain where on the pathological progression of AD ePVS occurs. This review aims to examine the knowledge gap that exists in understanding the contribution of ePVS to AD. It is essential to understand whether ePVS in the brain correlate with increased regional tau distribution and global or regional Amyloid-β distribution and to determine if these spaces increase proportionally over time as individuals experience neurodegeneration. This review demonstrates that ePVS are associated with reduced glymphatic clearance and that this reduced clearance is associated with an increase in amyloid-β. However, it is not yet understood if ePVS are the outcome or driver of protein accumulation. Further, it is not yet clear if ePVS volume and number change longitudinally. Ultimately, it is vital to determine early diagnostic criteria and early interventions for AD to ease the burden it presents to the world; ePVS may be able to fulfill this role and therefore merit further research.

The lymphatic system: a therapeutic target for central nervous system disorders

The lymphatic vasculature forms an organized network that covers the whole body and is involved in fluid homeostasis, metabolite clearance, and immune surveillance. The recent identification of functional lymphatic vessels in the meninges of the brain and the spinal cord has provided novel insights into neurophysiology. They emerge as major pathways for fluid exchange. The abundance of immune cells in lymphatic vessels and meninges also suggests that lymphatic vessels are actively involved in neuroimmunity. The lymphatic system, through its role in the clearance of neurotoxic proteins, autoimmune cell infiltration, and the transmission of pro-inflammatory signals, participates in the pathogenesis of a variety of neurological disorders, including neurodegenerative and neuroinflammatory diseases and traumatic injury. Vascular endothelial growth factor C is the master regulator of lymphangiogenesis, a process that is critical for the maintenance of central nervous system homeostasis. In this review, we summarize current knowledge and recent advances relating to the anatomical features and immunological functions of the lymphatic system of the central nervous system and highlight its potential as a therapeutic target for neurological disorders and central nervous system repair.

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.

Molecular, Pathological, Clinical, and Therapeutic Aspects of Perihematomal Edema in Different Stages of Intracerebral Hemorrhage

Acute intracerebral hemorrhage (ICH) is a devastating type of stroke worldwide. Neuronal destruction involved in the brain damage process caused by ICH includes a primary injury formed by the mass effect of the hematoma and a secondary injury induced by the degradation products of a blood clot. Additionally, factors in the coagulation cascade and complement activation process also contribute to secondary brain injury by promoting the disruption of the blood-brain barrier and neuronal cell degeneration by enhancing the inflammatory response, oxidative stress, etc. Although treatment options for direct damage are limited, various strategies have been proposed to treat secondary injury post-ICH. Perihematomal edema (PHE) is a potential surrogate marker for secondary injury and may contribute to poor outcomes after ICH. Therefore, it is essential to investigate the underlying pathological mechanism, evolution, and potential therapeutic strategies to treat PHE. Here, we review the pathophysiology and imaging characteristics of PHE at different stages after acute ICH. As illustrated in preclinical and clinical studies, we discussed the merits and limitations of varying PHE quantification protocols, including absolute PHE volume, relative PHE volume, and extension distance calculated with images and other techniques. Importantly, this review summarizes the factors that affect PHE by focusing on traditional variables, the cerebral venous drainage system, and the brain lymphatic drainage system. Finally, to facilitate translational research, we analyze why the relationship between PHE and the functional outcome of ICH is currently controversial. We also emphasize promising therapeutic approaches that modulate multiple targets to alleviate PHE and promote neurologic recovery after acute ICH.

Ocular Lymphatic and Glymphatic Systems: Implications for Retinal Health and Disease

Clearance of ocular fluid and metabolic waste is a critical function of the eye in health and disease. The eye has distinct fluid outflow pathways in both the anterior and posterior segments. Although the anterior outflow pathway is well characterized, little is known about posterior outflow routes. Recent studies suggest that lymphatic and glymphatic systems play an important role in the clearance of fluid and waste products from the posterior segment of the eye. The lymphatic system is a vascular network that runs parallel to the blood circulatory system. It plays an essential role in maintenance of fluid homeostasis and immune surveillance in the body. Recent studies have reported lymphatics in the cornea (under pathological conditions), ciliary body, choroid, and optic nerve meninges. The evidence of lymphatics in optic nerve meninges is, however, limited. An alternative lymphatic system termed the glymphatic system was recently discovered in the rodent eye and brain. This system is a glial cell-based perivascular network responsible for the clearance of interstitial fluid and metabolic waste. In this review, we will discuss our current knowledge of ocular lymphatic and glymphatic systems and their role in retinal degenerative diseases.

The meningeal lymphatic vessels and the glymphatic system: Potential therapeutic targets in neurological disorders

The recent discovery of the meningeal lymphatic vessels (mLVs) and glymphatic pathways has challenged the long-lasting dogma that the central nervous system (CNS) lacks a lymphatic system and therefore does not interact with peripheral immunity. This discovery has reshaped our understanding of mechanisms underlying CNS drainage. Under normal conditions, a close connection between mLVs and the glymphatic system enables metabolic waste removal, immune cell trafficking, and CNS immune surveillance. Dysfunction of the glymphatic-mLV system can lead to toxic protein accumulation in the brain, and it contributes to the development of a series of neurodegenerative disorders, such as Alzheimer's and Parkinson's diseases. The identification of precise cerebral transport routes is based mainly on indirect, invasive imaging of animals, and the results cannot always be applied to humans. Here we review the functions of the glymphatic-mLV system and evidence for its involvement in some CNS diseases. We focus on emerging noninvasive imaging techniques to evaluate the human glymphatic-mLV system and their potential for preclinical diagnosis and prevention of neurodegenerative diseases. Potential strategies that target the glymphatic-mLV system in order to treat and prevent neurological disorders are also discussed.

Noninvasive ultrasonic induction of cerebrospinal fluid flow enhances intrathecal drug delivery

Intrathecal drug delivery is routinely used in the treatment and prophylaxis of varied central nervous system conditions, as doing so allows drugs to directly bypass the blood-brain barrier. However, the utility of this route of administration is limited by poor brain and spinal cord parenchymal drug uptake from the cerebrospinal fluid. We demonstrate that a simple noninvasive transcranial ultrasound protocol can significantly increase influx of cerebrospinal fluid into the perivascular spaces of the brain, to enhance the uptake of intrathecally administered drugs. Specifically, we administered small (~1 kDa) and large (~155 kDa) molecule agents into the cisterna magna of rats and then applied low, diagnostic-intensity focused ultrasound in a scanning protocol throughout the brain. Using real-time magnetic resonance imaging and ex vivo histologic analyses, we observed significantly increased uptake of small molecule agents into the brain parenchyma, and of both small and large molecule agents into the perivascular space from the cerebrospinal fluid. Notably, there was no evidence of brain parenchymal damage following this intervention. The low intensity and noninvasive approach of transcranial ultrasound in this protocol underscores the ready path to clinical translation of this technique. In this manner, this protocol can be used to directly bypass the blood-brain barrier for whole-brain delivery of a variety of agents. Additionally, this technique can potentially be used as a means to probe the causal role of the glymphatic system in the variety of disease and physiologic processes to which it has been correlated.

Dynamic Evolution of the Glymphatic System at the Early Stages of Subarachnoid Hemorrhage

The early stages of subarachnoid hemorrhage (SAH) are extremely important for the progression and prognosis of this disease. The glymphatic system (GS) has positive implications for the nervous system due to its ability to clearance tau and amyloid-β (Aβ) protein. Previous studies have shown that GS dysfunction will appear after SAH. However, there is no systematic evaluation of the degree of damage and development process of GS function in the early stage after SAH. In this study, we evaluated the GS function and neurobehavioral in the sham, 6 h, 1, 3, and 7 days after SAH, respectively. Our results showed that the function of GS was severely attenuated in mice after SAH with a decreased polarity of Aquaporin-4 (AQP4), increased expression of AQP4, a linear correlation with the dystrophin-associated complex (DAC), the proliferation of reactive astrocytes, increased tau protein accumulation, and decreased neurological function. Collectively, these findings provide a comprehensive understanding of the functional changes of GS after SAH, provide references for subsequent scholars studying SAH, and suggest some potential mechanistic insight that affects AQP4 polarity and GS function.

Drainage of senescent astrocytes from brain via meningeal lymphatic routes

Recent progress on the central lymphatic system has greatly increased our understanding of how the brain maintains its own waste homeostasis. Here, we showed that perivascular spaces and meningeal lymphatic vessels form a functional route for clearance of senescent astrocytes from the aging brain. Blocking meningeal lymphatic drainage by ligation of the deep cervical lymph nodes impaired clearance of senescent astrocytes from brain parenchyma, subsequently increasing neuroinflammation in aged mice. By contrast, enhancing meningeal lymphatic vessel diameter by a recombinant adeno-associated virus encoding mouse vascular endothelial growth factor-C (VEGF-C) improved clearance of senescent astrocytes and mitigated neuroinflammation. Mechanistically, VEGF-C was highly expressed in senescent astrocytes, contributing themselves to migrate across lymphatic vessels along C-C motif chemokine ligand 21 (CCL21) gradient by interacting with VEGF receptor 3. Moreover, intra-cisternal injection of antibody against CCL21 hampered senescent astrocytes into the lymphatic vessels and exacerbated short memory defects of aged mice. Together, these findings reveal a new perspective for the meningeal lymphatics in the removal of senescent astrocytes, thus offering a valuable target for therapeutic intervention.

The glymphatic system and subarachnoid hemorrhage: disruption and recovery

The glymphatic system, or glial-lymphatic system, is a waste clearance system composed of perivascular channels formed by astrocytes that mediate the clearance of proteins and metabolites from the brain. These channels facilitate the movement of cerebrospinal fluid throughout brain parenchyma and are critical for homeostasis. Disruption of the glymphatic system leads to an accumulation of these waste products as well as increased interstitial fluid in the brain. These phenomena are also seen during and after subarachnoid hemorrhages (SAH), contributing to the brain damage seen after rupture of a major blood vessel. Herein this review provides an overview of the glymphatic system, its disruption during SAH, and its function in recovery following SAH. The review also outlines drugs which target the glymphatic system and may have therapeutic applications following SAH.

Aquaporin 4 Depolarization-Enhanced Transferrin Infiltration Leads to Neuronal Ferroptosis after Subarachnoid Hemorrhage in Mice

The infiltration of blood components into the brain parenchyma through the lymphoid system is an important cause of subarachnoid hemorrhage injury. AQP4, a water channel protein located at the astrocyte foot, has been reported to regulate blood-brain barrier integrity, and its polarization is disrupted after SAH. Neuronal ferroptosis is involved in subarachnoid hemorrhage- (SAH-) induced brain injury, but the inducing factors are not completely clear. Transferrin is one of the inducing factors of ferroptosis. This study is aimed at researching the role and mechanism of AQP4 in brain injury after subarachnoid hemorrhage in mice. An experimental mouse SAH model was established by endovascular perforation. An AAV vector encoding AQP4 with a GFAP-specific promoter was administered to mice to achieve specific overexpression of AQP4 in astrocytes. PI staining, Fer-1 intervention, and transmission electron microscopy were used to detect neuronal ferroptosis, and dextran (40 kD) leakage was used to detect BBB integrity. Western blot analysis of perfused brain tissue protein samples was used to detect transferrin infiltration. First, neuronal ferroptosis 24 h after SAH was observed by PI staining and Fer-1 intervention. Second, a significant increase in transferrin infiltration was found in the brain parenchyma 24 h after SAH modeling, while transferrin content was positively correlated with neuronal ferroptosis. Then, we observed that AQP4 overexpression effectively improved AQP depolarization and BBB injury induced by SAH and significantly reduced transferrin infiltration and neuronal ferroptosis after SAH. Finally, we found that AQP4 overexpression could effectively improve the neurobehavioral ability of SAH mice, and the neurobehavioral ability was negatively correlated with transferrin brain content. Taken together, these data indicate that overexpression of AQP4 in the mouse brain can effectively improve post-SAH neuronal ferroptosis and brain injury, at least partly by inhibiting transferrin infiltration into the brain parenchyma in the glymphatic system.