Molecular characterization of perivascular drainage pathways in the murine brain

Advances and controversies in meningeal biology

The dura, arachnoid and pia mater, as the constituent layers of the meninges, along with cerebrospinal fluid in the subarachnoid space and ventricles, are essential protectors of the brain and spinal cord. Complemented by immune cells, blood vessels, lymphatic vessels and nerves, these connective tissue layers have held many secrets that have only recently begun to be revealed. Each meningeal layer is now known to have molecularly distinct types of fibroblasts. Cerebrospinal fluid clearance through peripheral lymphatics and lymph nodes is well documented, but its routes and flow dynamics are debated. Advances made in meningeal immune functions are also debated. This Review considers the cellular and molecular structure and function of the dura, arachnoid and pia mater in the context of conventional views, recent progress, and what is uncertain or unknown. The hallmarks of meningeal pathophysiology are identified toward developing a more complete understanding of the meninges in health and disease.

Commentary on "Structural characterization of SLYM - a 4th meningeal membrane"

For centuries, the meninges have been described as three membranes: the inner pia, middle arachnoid and outer dura. It was therefore sensational when in early 2023 Science magazine published a report of a previously unrecognized - 4th - meningeal membrane located between the pia and arachnoid. Multiple features were claimed for this new membrane: a single cell layer marked by the transcription factor Prox1 that formed a barrier to low molecular weight substances and separated the subarachnoid space (SAS) into two fluid-filled compartments, not one as previously described. These features were further claimed to facilitate unidirectional glymphatic cerebrospinal fluid transport. These claims were immediately questioned by several researchers as misinterpretations of the authors' own data. The critics argued that (i) the 4th meningeal membrane as claimed did not exist as a separate structure but was part of the arachnoid, (ii) the "outer SAS" compartment was likely an artifactual subdural space created by the experimental procedures, and (iii) the 4th membrane barrier property was confused with the arachnoid barrier. Subsequent publications in late 2023 indeed showed that Prox1 + cells are embedded within the arachnoid and located immediately inside of and firmly attached to the arachnoid barrier cells by adherens junctions and gap junctions. In a follow-up study, published in this journal, the lead authors of the Science paper Kjeld Møllgård and Maiken Nedergaard reported additional observations they claim support the existence of a 4th meningeal membrane and the compartmentalization of the SAS into two non-communicating spaces. Their minor modification to the original paper was the 4th meningeal membrane was better observable at the ventral side of the brain than at the dorsal side where it was originally reported. The authors also claimed support for the existence of a 4th meningeal membrane in classical literature. Here, we outline multiple concerns over the new data and interpretation and argue against the claim there is prior support in the literature for a 4th meningeal membrane.

Neuroimmune and neuroinflammation response for traumatic brain injury

Traumatic brain injury (TBI) is one of the major diseases leading to mortality and disability, causing a serious disease burden on individuals' ordinary lives as well as socioeconomics. In primary injury, neuroimmune and neuroinflammation are both responsible for the TBI. Besides, extensive and sustained injury induced by neuroimmune and neuroinflammation also prolongs the course and worsens prognosis of TBI. Therefore, this review aims to explore the role of neuroimmune, neuroinflammation and factors associated them in TBI as well as the therapies for TBI. Thus, we conducted by searching PubMed, Scopus, and Web of Science databases for articles published between 2010 and 2023. Keywords included "traumatic brain injury," "neuroimmune response," "neuroinflammation," "astrocytes," "microglia," and "NLRP3." Articles were selected based on relevance and quality of evidence. On this basis, we provide the cellular and molecular mechanisms of TBI-induced both neuroimmune and neuroinflammation response, as well as the different factors affecting them, are introduced based on physiology of TBI, which supply a clear overview in TBI-induced chain-reacting, for a better understanding of TBI and to offer more thoughts on the future therapies for TBI.

Heterogeneous fibroblasts contribute to fibrotic scar formation after spinal cord injury in mice and monkeys

Spinal cord injury (SCI) leads to fibrotic scar formation at the lesion site, yet the heterogeneity of fibrotic scar remains elusive. Here we show the heterogeneity in distribution, origin, and function of fibroblasts within fibrotic scars after SCI in mice and female monkeys. Utilizing lineage tracing and single-cell RNA sequencing (scRNA-seq), we found that perivascular fibroblasts (PFs), and meningeal fibroblasts (MFs), rather than pericytes/vascular smooth cells (vSMCs), primarily contribute to fibrotic scar in both transection and crush SCI. Crabp2 + /Emb+ fibroblasts (CE-F) derived from meninges primarily localize in the central region of fibrotic scars, demonstrating enhanced cholesterol synthesis and secretion of type I collagen and fibronectin. In contrast, perivascular/pial Lama1 + /Lama2+ fibroblasts (LA-F) are predominantly found at the periphery of the lesion, expressing laminin and type IV collagen and functionally involved in angiogenesis and lipid transport. These findings may provide a comprehensive understanding for remodeling heterogeneous fibrotic scars after SCI.

Perivascular spaces around arteries exceed perivenous spaces in the mouse brain

The perivascular space (PVS) surrounds cerebral blood vessels and plays an important role in clearing waste products from the brain. Their anatomy and function have been described for arteries, but PVS around veins remain poorly characterized. Using in vivo 2-photon imaging in mice, we determined the size of the PVS around arteries and veins, and their connection with the subarachnoid space. After infusion of 70 kD FITC-dextran into the cerebrospinal fluid via the cisterna magna, labeled PVS were evident around arteries, but veins showed less frequent labeling of the PVS. The size of the PVS correlated with blood vessel size for both pial arteries and veins, but not for penetrating vessels. The PVS around pial arteries and veins was separated from the subarachnoid space by a thin meningeal layer, which did not form a barrier for the tracer. In vivo, FITC-dextran signal was observed adjacent to the vessel wall, but minimally within the wall itself. Post-mortem, there was a significant shift in the tracer's location within the arterial wall, extending into the smooth muscle layer. Taken together, these findings suggest that the PVS around veins has a limited role in the exchange of solutes between CSF and brain parenchyma.

A novel histone deacetylase inhibitor W2A-16 improves the barrier integrity in brain vascular endothelial cells

The maturation of brain microvascular endothelial cells leads to the formation of a tightly sealed monolayer, known as the blood-brain barrier (BBB). The BBB damage is associated with the pathogenesis of age-related neurodegenerative diseases including vascular cognitive impairment and Alzheimer's disease. Growing knowledge in the field of epigenetics can enhance the understanding of molecular profile of the BBB and has great potential for the development of novel therapeutic strategies or targets to repair a disrupted BBB. Histone deacetylases (HDACs) inhibitors are epigenetic regulators that can induce acetylation of histones and induce open chromatin conformation, promoting gene expression by enhancing the binding of DNA with transcription factors. We investigated how HDAC inhibition influences the barrier integrity using immortalized human endothelial cells (HCMEC/D3) and the human induced pluripotent stem cell (iPSC)-derived brain vascular endothelial cells. The endothelial cells were treated with or without a novel compound named W2A-16. W2A-16 not only activates Wnt/β-catenin signaling but also functions as a class I HDAC inhibitor. We demonstrated that the administration with W2A-16 sustained barrier properties of the monolayer of endothelial cells, as evidenced by increased trans-endothelial electrical resistance (TEER). The BBB-related genes and protein expression were also increased compared with non-treated controls. Analysis of transcript profiles through RNA-sequencing in hCMEC/D3 cells indicated that W2A-16 potentially enhances BBB integrity by influencing genes associated with the regulation of the extracellular microenvironment. These findings collectively propose that the HDAC inhibition by W2A-16 plays a facilitating role in the formation of the BBB. Pharmacological approaches to inhibit HDAC may be a potential therapeutic strategy to boost and/or restore BBB integrity.

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.

Cerebral amyloid angiopathy and the immune system

Cerebral amyloid angiopathy (CAA) is characterized by the accumulation of amyloid protein in the walls of cerebral blood vessels. This deposition of amyloid causes damage to the cerebral vasculature, resulting in blood-brain barrier disruption, cerebral hemorrhage, cognitive decline, and dementia. The role of the immune system in CAA is complex and not fully understood. While the immune system has a clear role in the rare inflammatory variants of CAA (CAA related inflammation and Abeta related angiitis), the more common variants of CAA also have immune system involvement. In a protective role, immune cells may facilitate the clearance of beta-amyloid from the cerebral vasculature. The immune system can also contribute to CAA pathology, promoting vascular injury, blood-brain barrier breakdown, inflammation, and progression of CAA. In this review, we summarize the role of the immune system in CAA, including the potential of immune based treatment strategies to slow vascular disease in CAA and associated cognitive impairment, white matter disease progression, and reduce the risk of cerebral hemorrhage. HIGHLIGHTS: The immune system has a role in cerebral amyloid angiopathy (CAA) which is summarized in this review. There is an inflammatory response to beta-amyloid that may contribute to brain injury and cognitive impairment. Immune cells may facilitate the clearance of beta-amyloid from the cerebral vasculature. Improved understanding of the immune system in CAA may afford novel treatment to improve outcomes in patients with CAA.

Is CAA a perivascular brain clearance disease? A discussion of the evidence to date and outlook for future studies

The brain's network of perivascular channels for clearance of excess fluids and waste plays a critical role in the pathogenesis of several neurodegenerative diseases including cerebral amyloid angiopathy (CAA). CAA is the main cause of hemorrhagic stroke in the elderly, the most common vascular comorbidity in Alzheimer's disease and also implicated in adverse events related to anti-amyloid immunotherapy. Remarkably, the mechanisms governing perivascular clearance of soluble amyloid β-a key culprit in CAA-from the brain to draining lymphatics and systemic circulation remains poorly understood. This knowledge gap is critically important to bridge for understanding the pathophysiology of CAA and accelerate development of targeted therapeutics. The authors of this review recently converged their diverse expertise in the field of perivascular physiology to specifically address this problem within the framework of a Leducq Foundation Transatlantic Network of Excellence on Brain Clearance. This review discusses the overarching goal of the consortium and explores the evidence supporting or refuting the role of impaired perivascular clearance in the pathophysiology of CAA with a focus on translating observations from rodents to humans. We also discuss the anatomical features of perivascular channels as well as the biophysical characteristics of fluid and solute transport.

The contribution of the meningeal immune interface to neuroinflammation in traumatic brain injury

Traumatic brain injury (TBI) is a major cause of disability and mortality worldwide, particularly among the elderly, yet our mechanistic understanding of what renders the post-traumatic brain vulnerable to poor outcomes, and susceptible to neurological disease, is incomplete. It is well established that dysregulated and sustained immune responses elicit negative consequences after TBI; however, our understanding of the neuroimmune interface that facilitates crosstalk between central and peripheral immune reservoirs is in its infancy. The meninges serve as the interface between the brain and the immune system, facilitating important bi-directional roles in both healthy and disease settings. It has been previously shown that disruption of this system exacerbates neuroinflammation in age-related neurodegenerative disorders such as Alzheimer's disease; however, we have an incomplete understanding of how the meningeal compartment influences immune responses after TBI. In this manuscript, we will offer a detailed overview of the holistic nature of neuroinflammatory responses in TBI, including hallmark features observed across clinical and animal models. We will highlight the structure and function of the meningeal lymphatic system, including its role in immuno-surveillance and immune responses within the meninges and the brain. We will provide a comprehensive update on our current knowledge of meningeal-derived responses across the spectrum of TBI, and identify new avenues for neuroimmune modulation within the neurotrauma field.

A brain-specific angiogenic mechanism enabled by tip cell specialization

Vertebrate organs require locally adapted blood vessels1,2. The gain of such organotypic vessel specializations is often deemed to be molecularly unrelated to the process of organ vascularization. Here, opposing this model, we reveal a molecular mechanism for brain-specific angiogenesis that operates under the control of Wnt7a/b ligands-well-known blood-brain barrier maturation signals3-5. The control mechanism relies on Wnt7a/b-dependent expression of Mmp25, which we find is enriched in brain endothelial cells. CRISPR-Cas9 mutagenesis in zebrafish reveals that this poorly characterized glycosylphosphatidylinositol-anchored matrix metalloproteinase is selectively required in endothelial tip cells to enable their initial migration across the pial basement membrane lining the brain surface. Mechanistically, Mmp25 confers brain invasive competence by cleaving meningeal fibroblast-derived collagen IV α5/6 chains within a short non-collagenous region of the central helical part of the heterotrimer. After genetic interference with the pial basement membrane composition, the Wnt-β-catenin-dependent organotypic control of brain angiogenesis is lost, resulting in properly patterned, yet blood-brain-barrier-defective cerebrovasculatures. We reveal an organ-specific angiogenesis mechanism, shed light on tip cell mechanistic angiodiversity and thereby illustrate how organs, by imposing local constraints on angiogenic tip cells, can select vessels matching their distinctive physiological requirements.

Altered perivascular spaces in subcortical white matter in Parkinson's disease patients with levodopa-induced dyskinesia

Dilated perivascular spaces (PVS) have emerged as a pathological hallmark in various neurological conditions, including Parkinson's disease (PD). Levodopa-induced dyskinesia (LID), an intractable motor complication of PD, remains enigmatic regarding the distribution patterns of PVS. Our objective was to scrutinize the percent PVS (pPVS) changes within PD patients with LID (PD-LID). In total, 132 individuals were enrolled, including PD-LID (n = 42), PD patients without LID (PD-nLID, n = 45), and healthy controls (HCs, n = 45). Employing an automated approach for PVS quantification based on structural magnetic resonance imaging, we comprehensively evaluated total pPVS in subcortical white matter globally and regionally. A significant increase in global pPVS was observed in PD patients versus HCs, particularly evident in PD-LID relative to HCs. Within the PD-LID group, elevated pPVS was discerned in the right inferior frontal gyrus region (rIFG) (pars opercularis), contrasting with PD-nLID and HCs. Moreover, PD patients exhibited increased pPVS in bilateral superior temporal regions compared to HCs. Notably, pPVS in the rIFG positively correlated with dyskinetic symptoms and could well identify LID. Our findings unveiled PVS alternations in subcortical white matter in PD-LID at both global and regional levels, highlighting the increased pPVS in rIFG as a prospective imaging marker for LID.

Circadian Mechanisms in Brain Fluid Biology

The brain is a complex organ, fundamentally changing across the day to perform basic functions like sleep, thought, and regulating whole-body physiology. This requires a complex symphony of nutrients, hormones, ions, neurotransmitters and more to be properly distributed across the brain to maintain homeostasis throughout 24 hours. These solutes are distributed both by the blood and by cerebrospinal fluid. Cerebrospinal fluid contents are distinct from the general circulation because of regulation at brain barriers including the choroid plexus, glymphatic system, and blood-brain barrier. In this review, we discuss the overlapping circadian (≈24-hour) rhythms in brain fluid biology and at the brain barriers. Our goal is for the reader to gain both a fundamental understanding of brain barriers alongside an understanding of the interactions between these fluids and the circadian timing system. Ultimately, this review will provide new insight into how alterations in these finely tuned clocks may lead to pathology.

The role of astrocytes in the glymphatic network: a narrative review

To date, treatment of Central Nervous System (CNS) pathology has largely focused on neuronal structure and function. Yet, revived attention towards fluid circulation within the CNS has exposed the need to further explore the role of glial cells in maintaining homeostasis within neural networks. In the past decade, discovery of the neural glymphatic network has revolutionized traditional understanding of fluid dynamics within the CNS. Advancements in neuroimaging have revealed alternative pathways of cerebrospinal fluid (CSF) generation and efflux. Here, we discuss emerging perspectives on the role of astrocytes in CSF hydrodynamics, with particular focus on the contribution of aquaporin-4 channels to the glymphatic network. Astrocytic structural features and expression patterns are detailed in relation to their function in maintaining integrity of the Blood Brain Barrier (BBB) as part of the neurovascular unit (NVU). This narrative also highlights the potential role of glial dysfunction in pathogenesis of neurodegenerative disease, hydrocephalus, intracranial hemorrhage, ischemic stroke, and traumatic brain injury. The purpose of this literature summary is to provide an update on the changing landscape of scientific theory surrounding production, flow, and absorption of cerebrospinal fluid. The overarching aim of this narrative review is to advance the conception of basic, translational, and clinical research endeavors investigating glia as therapeutic targets for neurological disease.

The niche matters: origin, function and fate of CNS-associated macrophages during health and disease

There are several cellular and acellular structural barriers associated with the brain interfaces, which include the dura, the leptomeninges, the perivascular space and the choroid plexus epithelium. Each structure is enriched by distinct myeloid populations, which mainly originate from erythromyeloid precursors (EMP) in the embryonic yolk sac and seed the CNS during embryogenesis. However, depending on the precise microanatomical environment, resident myeloid cells differ in their marker profile, turnover and the extent to which they can be replenished by blood-derived cells. While some EMP-derived cells seed the parenchyma to become microglia, others engraft the meninges and become CNS-associated macrophages (CAMs), also referred to as border-associated macrophages (BAMs), e.g., leptomeningeal macrophages (MnMΦ). Recent data revealed that MnMΦ migrate into perivascular spaces postnatally where they differentiate into perivascular macrophages (PvMΦ). Under homeostatic conditions in pathogen-free mice, there is virtually no contribution of bone marrow-derived cells to MnMΦ and PvMΦ, but rather to macrophages of the choroid plexus and dura. In neuropathological conditions in which the blood-brain barrier is compromised, however, an influx of bone marrow-derived cells into the CNS can occur, potentially contributing to the pool of CNS myeloid cells. Simultaneously, resident CAMs may also proliferate and undergo transcriptional and proteomic changes, thereby, contributing to the disease outcome. Thus, both resident and infiltrating myeloid cells together act within their microenvironmental niche, but both populations play crucial roles in the overall disease course. Here, we summarize the current understanding of the sources and fates of resident CAMs in health and disease, and the role of the microenvironment in influencing their maintenance and function.

Connecting the Dots: The Cerebral Lymphatic System as a Bridge Between the Central Nervous System and Peripheral System in Health and Disease

As a recently discovered waste removal system in the brain, cerebral lymphatic system is thought to play an important role in regulating the homeostasis of the central nervous system. Currently, more and more attention is being focused on the cerebral lymphatic system. Further understanding of the structural and functional characteristics of cerebral lymphatic system is essential to better understand the pathogenesis of diseases and to explore therapeutic approaches. In this review, we summarize the structural components and functional characteristics of cerebral lymphatic system. More importantly, it is closely associated with peripheral system diseases in the gastrointestinal tract, liver, and kidney. However, there is still a gap in the study of the cerebral lymphatic system. However, we believe that it is a critical mediator of the interactions between the central nervous system and the peripheral system.

Molecular anatomy of adult mouse leptomeninges

Leptomeninges, consisting of the pia mater and arachnoid, form a connective tissue investment and barrier enclosure of the brain. The exact nature of leptomeningeal cells has long been debated. In this study, we identify five molecularly distinct fibroblast-like transcriptomes in cerebral leptomeninges; link them to anatomically distinct cell types of the pia, inner arachnoid, outer arachnoid barrier, and dural border layer; and contrast them to a sixth fibroblast-like transcriptome present in the choroid plexus and median eminence. Newly identified transcriptional markers enabled molecular characterization of cell types responsible for adherence of arachnoid layers to one another and for the arachnoid barrier. These markers also proved useful in identifying the molecular features of leptomeningeal development, injury, and repair that were preserved or changed after traumatic brain injury. Together, the findings highlight the value of identifying fibroblast transcriptional subsets and their cellular locations toward advancing the understanding of leptomeningeal physiology and pathology.

Inorganic phosphate exporter heterozygosity in mice leads to brain vascular calcification, microangiopathy, and microgliosis

Calcification of the cerebral microvessels in the basal ganglia in the absence of systemic calcium and phosphate imbalance is a hallmark of primary familial brain calcification (PFBC), a rare neurodegenerative disorder. Mutation in genes encoding for sodium-dependent phosphate transporter 2 (SLC20A2), xenotropic and polytropic retrovirus receptor 1 (XPR1), platelet-derived growth factor B (PDGFB), platelet-derived growth factor receptor beta (PDGFRB), myogenesis regulating glycosidase (MYORG), and junctional adhesion molecule 2 (JAM2) are known to cause PFBC. Loss-of-function mutations in XPR1, the only known inorganic phosphate exporter in metazoans, causing dominantly inherited PFBC was first reported in 2015 but until now no studies in the brain have addressed whether loss of one functional allele leads to pathological alterations in mice, a commonly used organism to model human diseases. Here we show that mice heterozygous for Xpr1 (Xpr1WT/lacZ ) present with reduced inorganic phosphate levels in the cerebrospinal fluid and age- and sex-dependent growth of vascular calcifications in the thalamus. Vascular calcifications are surrounded by vascular basement membrane and are located at arterioles in the smooth muscle layer. Similar to previously characterized PFBC mouse models, vascular calcifications in Xpr1WT/lacZ mice contain bone matrix proteins and are surrounded by reactive astrocytes and microglia. However, microglial activation is not confined to calcified vessels but shows a widespread presence. In addition to vascular calcifications, we observed vessel tortuosity and transmission electron microscopy analysis revealed microangiopathy-endothelial swelling, phenotypic alterations in vascular smooth muscle cells, and thickening of the basement membrane.

The influence of physiological and pathological perturbations on blood-brain barrier function

The blood-brain barrier (BBB) is located at the interface between the vascular system and the brain parenchyma, and is responsible for communication with systemic circulation and peripheral tissues. During life, the BBB can be subjected to a wide range of perturbations or stresses that may be endogenous or exogenous, pathological or therapeutic, or intended or unintended. The risk factors for many diseases of the brain are multifactorial and involve perturbations that may occur simultaneously (e.g., two-hit model for Alzheimer's disease) and result in different outcomes. Therefore, it is important to understand the influence of individual perturbations on BBB function in isolation. Here we review the effects of eight perturbations: mechanical forces, temperature, electromagnetic radiation, hypoxia, endogenous factors, exogenous factors, chemical factors, and pathogens. While some perturbations may result in acute or chronic BBB disruption, many are also exploited for diagnostic or therapeutic purposes. The resultant outcome on BBB function depends on the dose (or magnitude) and duration of the perturbation. Homeostasis may be restored by self-repair, for example, via processes such as proliferation of affected cells or angiogenesis to create new vasculature. Transient or sustained BBB dysfunction may result in acute or pathological symptoms, for example, microhemorrhages or hypoperfusion. In more extreme cases, perturbations may lead to cytotoxicity and cell death, for example, through exposure to cytotoxic plaques.

Meningeal origins and dynamics of perivascular fibroblast development on the mouse cerebral vasculature

Perivascular fibroblasts (PVFs) are a fibroblast-like cell type that reside on large-diameter blood vessels in the adult meninges and central nervous system (CNS). PVFs contribute to fibrosis following injury but their homeostatic functions are not defined. PVFs were previously shown to be absent from most brain regions at birth and are only detected postnatally within the cerebral cortex. However, the origin, timing and cellular mechanisms of PVF development are not known. We used Col1a1-GFP and Col1a2-CreERT2 transgenic mice to track PVF development postnatally. Using lineage tracing and in vivo imaging we show that brain PVFs originate from the meninges and are first seen on parenchymal cerebrovasculature at postnatal day (P) 5. After P5, PVF coverage of the cerebrovasculature expands via local cell proliferation and migration from the meninges. Finally, we show that PVFs and perivascular macrophages develop concurrently. These findings provide the first complete timeline for PVF development in the brain, enabling future work into how PVF development is coordinated with cell types and structures in and around the perivascular spaces to support normal CNS vascular function.

VE-cadherin in arachnoid and pia mater cells serves as a suitable landmark for in vivo imaging of CNS immune surveillance and inflammation

Meninges cover the surface of the brain and spinal cord and contribute to protection and immune surveillance of the central nervous system (CNS). How the meningeal layers establish CNS compartments with different accessibility to immune cells and immune mediators is, however, not well understood. Here, using 2-photon imaging in female transgenic reporter mice, we describe VE-cadherin at intercellular junctions of arachnoid and pia mater cells that form the leptomeninges and border the subarachnoid space (SAS) filled with cerebrospinal fluid (CSF). VE-cadherin expression also marked a layer of Prox1+ cells located within the arachnoid beneath and separate from E-cadherin+ arachnoid barrier cells. In vivo imaging of the spinal cord and brain in female VE-cadherin-GFP reporter mice allowed for direct observation of accessibility of CSF derived tracers and T cells into the SAS bordered by the arachnoid and pia mater during health and neuroinflammation, and detection of volume changes of the SAS during CNS pathology. Together, the findings identified VE-cadherin as an informative landmark for in vivo imaging of the leptomeninges that can be used to visualize the borders of the SAS and thus potential barrier properties of the leptomeninges in controlling access of immune mediators and immune cells into the CNS during health and neuroinflammation.

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.

New trends in brain tumor immunity with the opportunities of lymph nodes targeted drug delivery

Lymph nodes targeted drug delivery is an attractive approach to improve cancer immunotherapy outcomes. Currently, the depth of understanding of afferent and efferent arms in brain immunity reveals the potential clinical applications of lymph node targeted drug delivery in brain tumors, e.g., glioblastoma. In this work, we systematically reviewed the microenvironment of glioblastoma and its structure as a basis for potential immunotherapy, including the glial-lymphatic pathway for substance exchange, the lymphatic drainage pathway from meningeal lymphatic vessels to deep cervical lymph nodes that communicate intra- and extracranial immunity, and the interaction between the blood-brain barrier and effector T cells. Furthermore, the carriers designed for lymph nodes targeted drug delivery were comprehensively summarized. The challenges and opportunities in developing a lymph nodes targeted delivery strategy for glioblastoma using nanotechnology are included at the end.

Secretomics reveals gelatinase substrates at the blood-brain barrier that are implicated in astroglial barrier function

The gelatinases, matrix metalloproteinase 2 (MMP-2) and MMP-9, are key for leukocyte penetration of the brain parenchymal border in neuroinflammation and the functional integrity of this barrier; however, it is unclear which MMP substrates are involved. Using a tailored, sensitive, label-free mass spectrometry-based secretome approach, not previously applied to nonimmune cells, we identified 119 MMP-9 and 21 MMP-2 potential substrates at the cell surface of primary astrocytes, including known substrates (β-dystroglycan) and a broad spectrum of previously unknown MMP-dependent events involved in cell-cell and cell-matrix interactions. Using neuroinflammation as a model of assessing compromised astroglial barrier function, a selection of the potential MMP substrates were confirmed in vivo and verified in human samples, including vascular cell adhesion molecule-1 and neuronal cell adhesion molecule. We provide a unique resource of potential MMP-2/MMP-9 substrates specific for the astroglia barrier. Our data support a role for the gelatinases in the formation and maintenance of this barrier but also in astrocyte-neuron interactions.

Pathophysiology and probable etiology of cerebral small vessel disease in vascular dementia and Alzheimer's disease

Vascular cognitive impairment and dementia (VCID) is commonly caused by vascular injuries in cerebral large and small vessels and is a key driver of age-related cognitive decline. Severe VCID includes post-stroke dementia, subcortical ischemic vascular dementia, multi-infarct dementia, and mixed dementia. While VCID is acknowledged as the second most common form of dementia after Alzheimer's disease (AD) accounting for 20% of dementia cases, VCID and AD frequently coexist. In VCID, cerebral small vessel disease (cSVD) often affects arterioles, capillaries, and venules, where arteriolosclerosis and cerebral amyloid angiopathy (CAA) are major pathologies. White matter hyperintensities, recent small subcortical infarcts, lacunes of presumed vascular origin, enlarged perivascular space, microbleeds, and brain atrophy are neuroimaging hallmarks of cSVD. The current primary approach to cSVD treatment is to control vascular risk factors such as hypertension, dyslipidemia, diabetes, and smoking. However, causal therapeutic strategies have not been established partly due to the heterogeneous pathogenesis of cSVD. In this review, we summarize the pathophysiology of cSVD and discuss the probable etiological pathways by focusing on hypoperfusion/hypoxia, blood-brain barriers (BBB) dysregulation, brain fluid drainage disturbances, and vascular inflammation to define potential diagnostic and therapeutic targets for cSVD.

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.

Disentangling the impact of cerebrospinal fluid formation and neuronal activity on solute clearance from the brain

BACKGROUND

Despite recent attention, pathways and mechanisms of fluid transposition in the brain are still a matter of intense discussion and driving forces underlying waste clearance in the brain remain elusive. Consensus exists that net solute transport is a prerequisite for efficient clearance. The individual impact of neuronal activity and cerebrospinal fluid (CSF) formation, which both vary with brain state and anesthesia, remain unclear.

METHODS

To separate conditions with high and low neuronal activity and high and low CSF formation, different anesthetic regimens in naive rat were established, using Isoflurane (ISO), Medetomidine (MED), acetazolamide or combinations thereof. With dynamic contrast-enhanced MRI, after application of low molecular weight contrast agent (CA) Gadobutrol to cisterna magna, tracer distribution was monitored as surrogate for solute clearance. Simultaneous fiber-based Ca²⁺-recordings informed about the state of neuronal activity under different anesthetic regimen. T2-weighted MRI and diffusion-weighted MRI (DWI) provided size of subarachnoidal space and aqueductal flow as surrogates for CSF formation. Finally, a pathway and mechanism-independent two-compartment model was introduced to provide a measure of efficiency for solute clearance from the brain.

RESULTS

Anatomical imaging, DWI and Ca²⁺-recordings confirmed that conditions with distinct levels of neuronal activity and CSF formation were achieved. A sleep-resembling condition, with reduced neuronal activity and enhanced CSF formation was achieved using ISO+MED and an awake-like condition with high neuronal activity using MED alone. CA distribution in the brain correlated with the rate of CSF formation. The cortical brain state had major influence on tracer diffusion. Under conditions with low neuronal activity, higher diffusivity suggested enlargement of extracellular space, facilitating a deeper permeation of solutes into brain parenchyma. Under conditions with high neuronal activity, diffusion of solutes into parenchyma was hindered and clearance along paravascular pathways facilitated. Exclusively based on the measured time signal curves, the two-compartment model provided net exchange ratios, which were significantly larger for the sleep-resembling condition than for the awake-like condition.

CONCLUSIONS

Efficiency of solute clearance in brain changes with alterations in both state of neuronal activity and CSF formation. Our clearance pathway and mechanism agnostic kinetic model informs about net solute transport, solely based on the measured time signal curves. This rather simplifying approach largely accords with preclinical and clinical findings.

Ependyma in Neurodegenerative Diseases, Radiation-Induced Brain Injury and as a Therapeutic Target for Neurotrophic Factors

The neuron loss caused by the progressive damage to the nervous system is proposed to be the main pathogenesis of neurodegenerative diseases. Ependyma is a layer of ciliated ependymal cells that participates in the formation of the brain-cerebrospinal fluid barrier (BCB). It functions to promotes the circulation of cerebrospinal fluid (CSF) and the material exchange between CSF and brain interstitial fluid. Radiation-induced brain injury (RIBI) shows obvious impairments of the blood-brain barrier (BBB). In the neuroinflammatory processes after acute brain injury, a large amount of complement proteins and infiltrated immune cells are circulated in the CSF to resist brain damage and promote substance exchange through the BCB. However, as the protective barrier lining the brain ventricles, the ependyma is extremely vulnerable to cytotoxic and cytolytic immune responses. When the ependyma is damaged, the integrity of BCB is destroyed, and the CSF flow and material exchange is affected, leading to brain microenvironment imbalance, which plays a vital role in the pathogenesis of neurodegenerative diseases. Epidermal growth factor (EGF) and other neurotrophic factors promote the differentiation and maturation of ependymal cells to maintain the integrity of the ependyma and the activity of ependymal cilia, and may have therapeutic potential in restoring the homeostasis of the brain microenvironment after RIBI or during the pathogenesis of neurodegenerative diseases.

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.

Meningeal origins and dynamics of perivascular fibroblast development on the mouse cerebral vasculature

Perivascular fibroblasts (PVFs) are a fibroblast-like cell type that reside on large-diameter blood vessels in the adult meninges and central nervous system (CNS). PVFs drive fibrosis following injury but their homeostatic functions are not well detailed. In mice, PVFs were previously shown to be absent from most brain regions at birth and are only detected postnatally within the cerebral cortex. However, the origin, timing, and cellular mechanisms of PVF development are not known. We used Col1a1-GFP and Col1a2-CreERT transgenic mice to track PVF developmental timing and progression in postnatal mice. Using a combination of lineage tracing and in vivo imaging we show that brain PVFs originate from the meninges and are first seen on parenchymal cerebrovasculature at postnatal day (P)5. After P5, PVF coverage of the cerebrovasculature rapidly expands via mechanisms of local cell proliferation and migration from the meninges, reaching adult levels at P14. Finally, we show that PVFs and perivascular macrophages (PVMs) develop concurrently along postnatal cerebral blood vessels, where the location and depth of PVMs and PVFs highly correlate. These findings provide the first complete timeline for PVF development in the brain, enabling future work into how PVF development is coordinated with cell types and structures in and around the perivascular spaces to support normal CNS vascular function.

Summary

Brain perivascular fibroblasts migrate from their origin in the meninges and proliferate locally to fully cover penetrating vessels during postnatal mouse development.

Multi-compartmental model of glymphatic clearance of solutes in brain tissue

The glymphatic system is the subject of numerous pieces of research in biology. Mathematical modelling plays a considerable role in this field since it can indicate the possible physical effects of this system and validate the biologists' hypotheses. The available mathematical models that describe the system at the scale of the brain (i.e. the macroscopic scale) are often solely based on the diffusion equation and do not consider the fine structures formed by the perivascular spaces. We therefore propose a mathematical model representing the time and space evolution of a mixture flowing through multiple compartments of the brain. We adopt a macroscopic point of view in which the compartments are all present at any point in space. The equations system is composed of two coupled equations for each compartment: One equation for the pressure of a fluid and one for the mass concentration of a solute. The fluid and solute can move from one compartment to another according to certain membrane conditions modelled by transfer functions. We propose to apply this new modelling framework to the clearance of 14C-inulin from the rat brain.

Sleep cycle-dependent vascular dynamics in male mice and the predicted effects on perivascular cerebrospinal fluid flow and solute transport

Perivascular spaces are important highways for fluid and solute transport in the brain enabling efficient waste clearance during sleep. However, the underlying mechanisms augmenting perivascular flow in sleep are unknown. Using two-photon imaging of naturally sleeping male mice we demonstrate sleep cycle-dependent vascular dynamics of pial arteries and penetrating arterioles: slow, large-amplitude oscillations in NREM sleep, a vasodilation in REM sleep, and a vasoconstriction upon awakening at the end of a sleep cycle and microarousals in NREM and intermediate sleep. These vascular dynamics are mirrored by changes in the size of the perivascular spaces of the penetrating arterioles: slow fluctuations in NREM sleep, reduction in REM sleep and an enlargement upon awakening after REM sleep and during microarousals in NREM and intermediate sleep. By biomechanical modeling we demonstrate that these sleep cycle-dependent perivascular dynamics likely enhance fluid flow and solute transport in perivascular spaces to levels comparable to cardiac pulsation-driven oscillations.

Update on the current knowledge of lymphatic drainage system and its emerging roles in glioma management

The draining of brain interstitial fluid (ISF) to cerebrospinal fluid (CSF) and the subsequent draining of CSF to meningeal lymphatics is well-known. Nonetheless, its role in the development of glioma is a remarkable finding that has to be extensively understood. The glymphatic system (GS) collects CSF from the subarachnoid space and brain ISF through aquaporin-4 (AQP4) water channels. The glial limiting membrane and the perivascular astrocyte-end-feet membrane both have elevated levels of AQP4. CSF is thought to drain through the nerve sheaths of the olfactory and other cranial nerves as well as spinal meningeal lymphatics via dorsal or basal lymphatic vessels. Meningeal lymphatic vessels (MLVs) exist below the skull in the dorsal and basal regions. In this view, MLVs offer a pathway to drain macromolecules and traffic immunological cells from the CNS into cervical lymph nodes (CLNs), and thus can be used as a candidate curing strategy against glioma and other associated complications, such as neuro-inflammation. Taken together, the lymphatic drainage system could provide a route or approach for drug targeting of glioma and other neurological conditions. Nevertheless, its pathophysiological role in glioma remains elusive, which needs extensive research. The current review aims to explore the lymphatic drainage system, its role in glioma progression, and possible therapeutic techniques that target MLVs in the CNS.

Idiopathic intracranial hypertension in patients with cerebral small vessel disease: A case report

INTRODUCTION

Idiopathic intracranial hypertension (IIH) is a clinical syndrome characterized by increased intracranial pressure (ICP) without any identifiable cause. However, restrictions of cerebrospinal fluid absorption from the cerebral venous system, the glymphatic system overflow, and the cerebrospinal fluid's lymphatic pathways may be involved in the pathophysiology of IIH. Furthermore, an impaired glymphatic system is also implicated in the initiation and progression of cerebral small vessel disease (CSVD). Here, we reported a case of CSVD with concomitant IIH, possibly associated with the brain's glymphatic and lymphatic system dysfunction.

CASE CONCERN

A 39-year-old male presented with worsening headaches over the bilateral parietal areas during the past year and nausea for 2 days. Fundus examination revealed bilateral papilledema and lumbar puncture suggestive of elevated ICP, laboratory results showed hyperhomocysteinemia and mutation of methylenetetrahydrofolate reductase C677T. On magnetic resonance imaging, subcortical small infarct, white matter lesions, lacunes, enlarged perivascular spaces and dilatation of the optic nerve sheaths was detected, and right transverse sinus stenosis and a hypoplastic left sinus were showed on contrast-enhanced magnetic resonance venography.

DIAGNOSIS

The diagnoses of IIH, CSVD, transverse sinus stenosis, and hyperhomocysteinemia were performed.

INTERVISION AND OUTCOMES

The patient received antihypertensive, antiplatelet, anti-atherosclerotic, and homocysteine-lowering therapies. Finally, the patient's symptoms remised, and the increased ICP returned to normal; however, the bilateral TSS persisted after 3 months of follow-up.

CONCLUSIONS

In this case, we speculate that the normal glymphatic outflow pathway may serve as a compensatory mechanism for regulating increased ICP in patients with bilateral venous sinus obstruction, indicating impaired venous outflow pathway, possibly associated with dysfunction of the glymphatic and lymphatic systems in patients with CSVD.

Exogenous laminin exhibits a unique vascular pattern in the brain via binding to dystroglycan and integrins

BACKGROUND

Unlike other proteins that exhibit a diffusion pattern after intracerebral injection, laminin displays a vascular pattern. It remains unclear if this unique vascular pattern is caused by laminin-receptor interaction or laminin self-assembly.

METHODS

We compared the distribution of various wild-type laminin isoforms in the brain after intracerebral injection. To determine what causes the unique vascular pattern of laminin in the brain, laminin mutants with impaired receptor-binding and/or self-assembly activities and function-blocking antibodies to laminin receptors were used. In addition, the dynamics of laminin distribution and elimination were examined at multiple time points after intracerebral injection.

RESULTS

We found that β2-containing laminins had higher affinity for the vessels compared to β1-containing laminins. In addition, laminin mutants lacking receptor-binding domains but not that lacking self-assembly capability showed substantially reduced vascular pattern. Consistent with this finding, dystroglycan (DAG1) function-blocking antibody significantly reduced the vascular pattern of wild-type laminin-111. Although failed to affect the vascular pattern when used alone, integrin-β1 function-blocking antibody further decreased the vascular pattern when combined with DAG1 antibody. EDTA, which impaired laminini-DAG1 interaction by chelating Ca²⁺, also attenuated the vascular pattern. Immunohistochemistry revealed that laminins were predominantly located in the perivascular space in capillaries and venules/veins but not arterioles/arteries. The time-course study showed that laminin mutants with impaired receptor-engaging activity were more efficiently eliminated from the brain compared to their wild-type counterparts. Concordantly, significantly higher levels of mutant laminins were detected in the cerebral-spinal fluid (CSF).

CONCLUSIONS

These findings suggest that intracerebrally injected laminins are enriched in the perivascular space in a receptor (DAG1/integrin)-dependent rather than self-assembly-dependent manner and eliminated from the brain mainly via the perivascular clearance system.

A Microfluidic Model of AQP4 Polarization Dynamics and Fluid Transport in the Healthy and Inflamed Human Brain: The First Step Towards Glymphatics-on-a-Chip

Dysfunction of the aquaporin-4 (AQP4)-dependent glymphatic waste clearance pathway has recently been implicated in the pathogenesis of several neurodegenerative diseases. However, it is difficult to unravel the causative relationship between glymphatic dysfunction, AQP4 depolarization, protein aggregation, and inflammation in neurodegeneration using animal models alone. There is currently a clear, unmet need for in vitro models of the brain's waterscape, and the first steps towards a bona fide "glymphatics-on-a-chip" are taken in the present study. It is demonstrated that chronic exposure to lipopolysaccharide (LPS), amyloid-β(1-42) oligomers, and an AQP4 inhibitor impairs the drainage of fluid and amyloid-β(1-40) tracer in a gliovascular unit (GVU)-on-a-chip model containing human astrocytes and brain microvascular endothelial cells. The LPS-induced drainage impairment is partially retained following cell lysis, indicating that neuroinflammation induces parallel changes in cell-dependent and matrisome-dependent fluid transport pathways in GVU-on-a-chip. Additionally, AQP4 depolarization is observed following LPS treatment, suggesting that LPS-induced drainage impairments on-chip may be driven in part by changes in AQP4-dependent fluid dynamics.

Deciphering the heterogeneity of the Lyve1+ perivascular macrophages in the mouse brain

Perivascular macrophages (pvMs) are associated with cerebral vasculature and mediate brain drainage and immune regulation. Here, using reporter mouse models, whole brain and section immunofluorescence, flow cytometry, and single cell RNA sequencing, besides the Lyve1⁺F4/80⁺CD206⁺CX3CR1⁺ pvMs, we identify a CX3CR1^- pvM population that shares phagocytic functions and location. Furthermore, the brain parenchyma vasculature mostly hosts Lyve1⁺MHCII^- pvMs with low to intermediate CD45 expression. Using the double Cx3cr1^GFP x Cx3cr1-Cre;Rosa^tdT reporter mice for finer mapping of the lineages, we establish that CD45^lowCX3CR1^- pvMs are derived from CX3CR1⁺ precursors and require PU.1 during their ontogeny. In parallel, results from the Cxcr4-CreErt2;Rosa26^tdT lineage tracing model support a bone marrow-independent replenishment of all Lyve1⁺ pvMs in the adult mouse brain. Lastly, flow cytometry and 3D immunofluorescence analysis uncover increased percentage of pvMs following photothrombotic induced stroke. Our results thus show that the parenchymal pvM population is more heterogenous than previously described, and includes a CD45^low and CX3CR1^- pvM population.

Novel localization of folate transport systems in the murine central nervous system

BACKGROUND

Folates are a family of B9 vitamins that serve as one-carbon donors critical to biosynthetic processes required for the development and function of the central nervous system (CNS) in mammals. Folate transport is mediated by three highly specific systems: (1) folate receptor alpha (FRα; FOLR1/Folr1), (2) the reduced folate-carrier (RFC; SLC19A1/Slc19a1) and (3) the proton-coupled folate transporter (PCFT; SLC46A1/Slc46a1). Folate transport into and out of the CNS occurs at the blood-cerebrospinal fluid barrier (BCSFB), mediated by FRα and PCFT. Impairment of folate transport at the BCSFB results in cerebral folate deficiency in infants characterized by severe neurological deficiencies and seizures. In contrast to the BCSFB, CNS folate transport at other brain barriers and brain parenchymal cells has not been extensively investigated. The aim of this study is to characterize folate transport systems in the murine CNS at several known barriers encompassing the BCSFB, arachnoid barrier (AB), blood-brain barrier (BBB) and parenchymal cells (astrocytes, microglia, neurons).

METHODS

Applying immunohistochemistry, localization of folate transport systems (RFC, PCFT, FRα) was examined at CNS barriers and parenchymal sites in wildtype (C57BL6/N) mice. Subcellular localization of the folate transport systems was further assessed in an in vitro model of the mouse AB. Gene and protein expression was analyzed in several in vitro models of brain barriers and parenchyma by qPCR and western blot analysis.

RESULTS

RFC, PCFT, and FRα expression was localized within the BCSFB and BBB consistent with previous reports. Only RFC and PCFT expression was detected at the AB. Varied levels of RFC and PCFT expression were detected in neuronal and glial cells.

CONCLUSIONS

Localization of RFC and PCFT within the AB, described here for the first time, suggest that AB may contribute to folate transport between the peripheral circulation and the CSF. RFC and PCFT expression observed in astrocytes and microglia is consistent with the role that one or both of these transporters may play in delivering folates into cells within brain parenchyma. These studies provide insights into mechanisms of folate transport in the CNS and may enhance our understanding of the critical role folates play in neurodevelopment and in the development of novel treatment strategies for disorders of brain folate deficiency due to impaired transporter function.

Perivascular spaces and their role in neuroinflammation

It is uncontested that perivascular spaces play critical roles in maintaining homeostasis and priming neuroinflammation. However, despite more than a century of intense research on perivascular spaces, many open questions remain about the anatomical compartment surrounding blood vessels within the CNS. The goal of this comprehensive review is to summarize the literature on perivascular spaces in human neuroinflammation and associated animal disease models. We describe the cell types taking part in the morphological and functional aspects of perivascular spaces and how those spaces can be visualized. Based on this, we propose a model of the cascade of events occurring during neuroinflammatory pathology. We also discuss current knowledge gaps and limitations of the available evidence. An improved understanding of perivascular spaces could advance our comprehension of the pathophysiology of neuroinflammation and open a new therapeutic window for neuroinflammatory diseases such as multiple sclerosis.

Cell-specific expression and function of laminin at the neurovascular unit

Laminin, a major component of the basal lamina (BL), is a heterotrimeric protein with many isoforms. In the CNS, laminin is expressed by almost all cell types, yet different cells synthesize distinct laminin isoforms. By binding to its receptors, laminin exerts a wide variety of important functions. However, due to the reciprocal and cell-specific expression of laminin in different cells at the neurovascular unit, its functions in blood-brain barrier (BBB) maintenance and BBB repair after injury are not fully understood. In this review, we focus on the expression and functions of laminin and its receptors in the neurovascular unit under both physiological and pathological conditions. We first briefly introduce the structures of laminin and its receptors. Next, the expression and functions of laminin and its receptors in the CNS are summarized in a cell-specific manner. Finally, we identify the knowledge gap in the field and discuss key questions that need to be answered in the future. Our goal is to provide a comprehensive overview on cell-specific expression of laminin and its receptors in the CNS and their functions on BBB integrity.

The glymphatic system: Current understanding and modeling

We review theoretical and numerical models of the glymphatic system, which circulates cerebrospinal fluid and interstitial fluid around the brain, facilitating solute transport. Models enable hypothesis development and predictions of transport, with clinical applications including drug delivery, stroke, cardiac arrest, and neurodegenerative disorders like Alzheimer's disease. We sort existing models into broad categories by anatomical function: Perivascular flow, transport in brain parenchyma, interfaces to perivascular spaces, efflux routes, and links to neuronal activity. Needs and opportunities for future work are highlighted wherever possible; new models, expanded models, and novel experiments to inform models could all have tremendous value for advancing the field.

Periarteriolar spaces modulate cerebrospinal fluid transport into brain and demonstrate altered morphology in aging and Alzheimer's disease

Perivascular spaces (PVS) drain brain waste metabolites, but their specific flow paths are debated. Meningeal pia mater reportedly forms the outermost boundary that confines flow around blood vessels. Yet, we show that pia is perforated and permissive to PVS fluid flow. Furthermore, we demonstrate that pia is comprised of vascular and cerebral layers that coalesce in variable patterns along leptomeningeal arteries, often merging around penetrating arterioles. Heterogeneous pial architectures form variable sieve-like structures that differentially influence cerebrospinal fluid (CSF) transport along PVS. The degree of pial coverage correlates with macrophage density and phagocytosis of CSF tracer. In vivo imaging confirms transpial influx of CSF tracer, suggesting a role of pia in CSF filtration, but not flow restriction. Additionally, pial layers atrophy with age. Old mice also exhibit areas of pial denudation that are not observed in young animals, but pia is unexpectedly hypertrophied in a mouse model of Alzheimer's disease. Moreover, pial thickness correlates with improved CSF flow and reduced β-amyloid deposits in PVS of old mice. We show that PVS morphology in mice is variable and that the structure and function of pia suggests a previously unrecognized role in regulating CSF transport and amyloid clearance in aging and disease.

Targeting Oxidative Stress and Inflammatory Response for Blood-Brain Barrier Protection in Intracerebral Hemorrhage

Significance: Blood-brain barrier (BBB) disruption is a major pathological change after intracerebral hemorrhage (ICH) and is both the cause and result of oxidative stress and of the immune response post-ICH. These processes contribute to ICH-induced brain injury. Recent Advances: After the breakdown of cerebral vessels, blood components, including erythrocytes and their metabolites, thrombin, and fibrinogen, can access the cerebral parenchyma through the compromised BBB, triggering oxidative stress and inflammatory cascades. These aggravate BBB disruption and contribute to further infiltration of blood components, resulting in a vicious cycle that exacerbates brain edema and neurological injury after ICH. Experimental and clinical studies have highlighted the role of BBB disruption in ICH-induced brain injury. Critical Issues: In this review, we focus on the strategies to protect the BBB in ICH. Specifically, we summarize the evidence and the underlying mechanisms, including the ICH-induced process of oxidative stress and inflammatory response, and we highlight the potential therapeutic targets to protect BBB integrity after ICH. Future Directions: Future studies should probe the mechanism of ferroptosis as well as oxidative stress-inflammation coupling in BBB disruption after ICH and investigate the effects of antioxidants and immunomodulatory agents in more ICH clinical trials. Antioxid. Redox Signal. 37, 115-134.

Distinct features of brain perivascular fibroblasts and mural cells revealed by in vivo two-photon imaging

Perivascular fibroblasts (PVFs) are recognized for their pro-fibrotic role in many central nervous system disorders. Like mural cells, PVFs surround blood vessels and express Pdgfrβ. However, these shared attributes hinder the ability to distinguish PVFs from mural cells. We used in vivo two-photon imaging and transgenic mice with PVF-targeting promoters (Col1a1 or Col1a2) to compare the structure and distribution of PVFs and mural cells in cerebral cortex of healthy, adult mice. We show that PVFs localize to all cortical penetrating arterioles and their offshoots (arteriole-capillary transition zone), as well as the main trunk of only larger ascending venules. However, the capillary zone is devoid of PVF coverage. PVFs display short-range mobility along the vessel wall and exhibit distinct structural features (flattened somata and thin ruffled processes) not seen with smooth muscle cells or pericytes. These findings clarify that PVFs and mural cells are distinct cell types coexisting in a similar perivascular niche.

A network model of glymphatic flow under different experimentally-motivated parametric scenarios

Flow of cerebrospinal fluid (CSF) through perivascular spaces (PVSs) in the brain delivers nutrients, clears metabolic waste, and causes edema formation. Brain-wide imaging cannot resolve PVSs, and high-resolution methods cannot access deep tissue. However, theoretical models provide valuable insight. We model the CSF pathway as a network of hydraulic resistances, using published parameter values. A few parameters (permeability of PVSs and the parenchyma, and dimensions of PVSs and astrocyte endfoot gaps) have wide uncertainties, so we focus on the limits of their ranges by analyzing different parametric scenarios. We identify low-resistance PVSs and high-resistance parenchyma as the only scenario that satisfies three essential criteria: that the flow be driven by a small pressure drop, exhibit good CSF perfusion throughout the cortex, and exhibit a substantial increase in flow during sleep. Our results point to the most important parameters, such as astrocyte endfoot gap dimensions, to be measured in future experiments.

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We model the CSF pathway as a network of hydraulic resistances

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Predictions are bracketed by analyzing parametric scenarios for unknown parameters

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Low-resistance PVSs and high-resistance parenchyma produce realistic flows

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Astrocyte endfoot gap size is among the important parameters to be measured

Theoretical analysis of wake/sleep changes in brain solute transport suggests a flow of interstitial fluid

Clearance of protein waste products from the brain is accomplished by a combination of advection and diffusion in cerebrospinal fluid (CSF) and interstitial fluid (ISF). In the glymphatic model, there is a flow of ISF in the interstitial space, and both advection and diffusion occur there. Such a flow of ISF would be slow and difficult to detect directly, and its existence has proved controversial. Waste clearance has been shown to occur mainly during sleep, during which the volume of the interstitial space increases substantially due to ISF emitted from astrocytes. Here I show that this volume increase of the interstitial space, by itself, should lead to a slight reduction of diffusive transport, due to dilution of the waste solute, but to a significant increase in flow rate and advective transport, due to lowered hydraulic resistance. Thus, a flow of ISF together with the observed volume increase of the interstitial space might provide an important mechanism contributing to the enhanced clearance during sleep.

Techniques for visualizing fibroblast-vessel interactions in the developing and adult CNS

Significance: Fibroblasts are found associated with blood vessels in various locations across the central nervous system (CNS): in the meninges, the choroid plexus, and in the parenchyma within perivascular spaces. CNS fibroblasts have been characterized using transcriptional profiling and a Col1a1-GFP mouse line used to identify CNS fibroblasts in vivo; however, we still know very little regarding their functions and identity. Aim: Current methods for visualizing CNS fibroblasts are lacking and, in particular, prevent adequate assessment of fibroblast-vessel interactions. We aimed to develop new ways to visualize CNS fibroblasts in greater detail. Approach: Here, we describe methods for whole mount visualization of meningeal and choroid plexus fibroblasts, and CUBIC optical tissue clearing methods for visualization of parenchymal vessel-associated fibroblasts. Results: We show that these methods can be used for visualization of vessel-fibroblast interactions in these CNS structures and provide significant improvement over traditional sectioning and staining methods. In addition, we can combine these techniques with immunohistochemistry methods for labeling different cell types in the meninges and blood vasculature as well as EdU-based cell proliferation assays. Conclusions: We expect these methods will advance studies of CNS fibroblast development and functions in homeostasis, injury, and disease.

Fluid transport in the brain

The brain harbors a unique ability to, figuratively speaking, shift its gears. During wakefulness, the brain is geared fully toward processing information and behaving, while homeostatic functions predominate during sleep. The blood-brain barrier establishes a stable environment that is optimal for neuronal function, yet the barrier imposes a physiological problem; transcapillary filtration that forms extracellular fluid in other organs is reduced to a minimum in brain. Consequently, the brain depends on a special fluid [the cerebrospinal fluid (CSF)] that is flushed into brain along the unique perivascular spaces created by astrocytic vascular endfeet. We describe this pathway, coined the term glymphatic system, based on its dependency on astrocytic vascular endfeet and their adluminal expression of aquaporin-4 water channels facing toward CSF-filled perivascular spaces. Glymphatic clearance of potentially harmful metabolic or protein waste products, such as amyloid-β, is primarily active during sleep, when its physiological drivers, the cardiac cycle, respiration, and slow vasomotion, together efficiently propel CSF inflow along periarterial spaces. The brain's extracellular space contains an abundance of proteoglycans and hyaluronan, which provide a low-resistance hydraulic conduit that rapidly can expand and shrink during the sleep-wake cycle. We describe this unique fluid system of the brain, which meets the brain's requisites to maintain homeostasis similar to peripheral organs, considering the blood-brain-barrier and the paths for formation and egress of the CSF.

The gut-meningeal immune axis: Priming brain defense against the most likely invaders

The gastrointestinal tract contains trillions of microorganisms that exist symbiotically with the host due to a tolerant, regulatory cell–rich intestinal immune system. However, this intimate relationship with the microbiome inevitably comes with risks, with intestinal organisms being the most common cause of bacteremia. The vasculature of the brain-lining meninges contains fenestrated endothelium, conferring vulnerability to invasion by circulating microbes. We propose that this has evolutionarily led to close links between gut and meningeal immunity, to prime the central nervous system defense against the most likely invaders. This paradigm is exemplified by the dural venous sinus IgA defense system, where the antibody repertoire mirrors that of the gut.