PDGF-B Is Required for Development of the Glymphatic System

Constructed transferrin receptor-targeted liposome for the delivery of fluvoxamine to improve prognosis in a traumatic brain injury mouse model

The dysregulation of blood-brain barrier (BBB) activates pathological mechanisms such as neuroinflammation after traumatic brain injury (TBI), and glymphatic system dysfunction accelerates toxic waste accumulation after TBI. It is essential to find an effective way to inhibit inflammation and repair BBB and glymphatic system after TBI; however, effective and lasting drug therapy remains challenging because BBB severely prevents drugs from being delivered to central nervous system. Transferrin receptors (TfRs) are mainly expressed on brain capillary endothelial cells. Here, we report a TfR-targeted nanomedicine for TBI treatment by penetrating BBB and delivering fluvoxamine (Flv). The TfR-targeted polypeptide liposome loaded with Flv (TPL-Flv) implements cell targeting ability on human umbilical vein endothelial cells (HUVECs) in vitro detected by flow cytometry, and drug safety was proved through cell viability analysis and blood routine and biochemistry analysis. Afterwards, we established a controlled cortical impact model to explore TPL-Flv administration effects on TBI mice. We confirmed that TPL-Flv could stimulate CXCR4/SDF-1 signaling pathway, activate Treg cells, and inhibit inflammation after TBI. TPL-Flv treatment also alleviated BBB disruption and restored aquaporin-4 (AQP4) polarization, as well as reversed glymphatic dysfunction. Furthermore, TPL-Flv accomplished remarkable improvement of motor and cognitive functions. These findings demonstrate that TPL-Flv can effectively cross BBB and achieve drug delivery to cerebral tissue, validating its potential to improve therapeutic outcomes for TBI.

The glycosaminoglycan chains of perlecan regulate the perivascular fluid transport

BACKGROUND

The perivascular conduct pathway that connects the cerebrospinal fluid spaces with the interstitial fluid in the parenchyma are of importance for solute clearance from the brain. In this pathway, the relatively wide perivascular space (PVS) surrounding the pial arteries provides a low-resistant passage while around the perforating arteries, the solute movement is along the basement membrane (BM), that prevents the free exchange of interstitial fluids and solutes. We hypothesize that this selectivity involves specific components of the vascular BM, which is mainly composed of type IV collagen (Col IV) and laminin networks interconnected by nidogens and heparan sulphate proteoglycans (HSPGs). Perlecan is the major HSPG in the BM that binds to Col IV and laminin via glycosaminoglycan (GAG) chains to form a molecular sieve. GAGs may also provide the charge selectivity required for filtration, and also a scaffold for amyloid-β (Aβ) aggregation. The purpose of this study was the functional characterization of perivascular fluid transport and brain clearance in mice lacking perlecan GAG chains.

METHODS

We generated a novel mouse line (Hspg2∆3∆91) lacking perlecan GAG side chains and investigated perivascular flow and brain clearance in these mice using intravital multiphoton and fluorescence recovery after photobleaching techniques, and functional assays with various tracers. Potentially deleterious effects on brain homeostasis were investigated using transcriptomic, proteomic and immunohistochemical methods. The Hspg2∆3∆91 mice were crossed with a 5xFAD line to examine the importance of GAGs in Aβ aggregation.

RESULTS

We observed a delayed inflow of CSF tracer into the Hspg2∆3∆91 brain with no changes in the clearance of parenchymal injected tracers. Quantification of the Aβ plaques revealed fewer and smaller plaques in the walls of the pial arteries at six months of age, but not in the brain parenchyma. Surprisingly, perlecan GAG deficiency had no severe deleterious effects on brain homeostasis in transcriptomic and proteomic analyses.

CONCLUSIONS

Potential brain clearance mechanisms are dependent on the flow through special ECM structures. BM is mainly known for its barrier function, whereas very little is known about how passage along the perivascular ECM is established. This study shows that the GAG composition of the BM affects the solute dynamics and Aβ deposition in the periarterial space.

Meningeal lymphatic drainage: novel insights into central nervous system disease

In recent years, increasing evidence has suggested that meningeal lymphatic drainage plays a significant role in central nervous system (CNS) diseases. Studies have indicated that CNS diseases and conditions associated with meningeal lymphatic drainage dysfunction include neurodegenerative diseases, stroke, infections, traumatic brain injury, tumors, functional cranial disorders, and hydrocephalus. However, the understanding of the regulatory and damage mechanisms of meningeal lymphatics under physiological and pathological conditions is currently limited. Given the importance of a profound understanding of the interplay between meningeal lymphatic drainage and CNS diseases, this review covers seven key aspects: the development and structure of meningeal lymphatic vessels, methods for observing meningeal lymphatics, the function of meningeal lymphatics, the molecular mechanisms of meningeal lymphatic injury, the relationships between meningeal lymphatic vessels and CNS diseases, potential regulatory mechanisms of meningeal lymphatics, and conclusions and outstanding questions. We will explore the relationship between the development, structure, and function of meningeal lymphatics, review current methods for observing meningeal lymphatic vessels in both animal models and humans, and identify unresolved key points in meningeal lymphatic research. The aim of this review is to provide new directions for future research and therapeutic strategies targeting meningeal lymphatics by critically analyzing recent advancements in the field, identifying gaps in current knowledge, and proposing innovative approaches to address these gaps.

Beyond the cerebellum: perivascular space burden in spinocerebellar ataxia type 3 extends to multiple brain regions

Spinocerebellar ataxia type 3 (SCA3) is an uncommon inherited (autosomal dominant) neurodegenerative disorder caused by abnormal accumulation of ataxin-3 protein. The perivascular space (PVS) burden reflects protein clearance and may worsen in SCA3 disease. This study aimed to quantify the PVS burden and investigate the relationship between the PVS burden and clinical characteristics in individuals with SCA3. This study enrolled 43 SCA3 patients and 43 age- and sex-matched healthy controls (HCs). The cross-sectional study assessed the severity of ataxia in SCA3 patients using the Scale for the Assessment and Rating of Ataxia (SARA) and the International Cooperative Ataxia Rating Scale (ICARS). Various cognitive functions were evaluated in all subjects using the Montreal Cognitive Assessment (MoCA), Rapid Verbal Retrieval (RAR) and Digital Span Test (DST) scales. MRI was used to automatically segment the PVS in all subjects and quantify the PVS burden in 15 brain regions. Compared with the HCs, the SCA3 patients showed a significantly higher PVS burden in the basal ganglia, temporal lobe, right parietal lobe and right cerebellum. There was a positive correlation in motor dysfunction between the PVS volume in the left parietal lobe, right cerebellum and PVS number in the right cerebellum with the SARA and ICARS scores. This study showed that SCA3 patients have an increased PVS burden in many brain regions, leading to motor impairment. The PVS burden could be a new imaging biomarker for disease monitoring and a therapeutic target for SCA3.

Zebrafish glial-vascular interactions progressively expand over the course of brain development

Glial-vascular interactions are critical for the formation and maintenance of brain blood vessels and the blood-brain barrier (BBB) in mammals, but their role in the zebrafish BBB remains unclear. Using three glial gene promoters-gfap, glast, and glastini (a truncated glast)-we explored glial-vascular development in zebrafish. Sparse labeling showed fewer glial-vascular interactions at early stages, with glial coverage and contact area increasing with age. Stable transgenic lines for glast and glastini revealed similar developmental increases, starting at ∼30% coverage at 3 days post-fertilization (dpf) and peaking at ∼60% by 10 dpf, and consistently higher glial coverage in the forebrain and midbrain than in the hindbrain. Electron microscopy analyses showed similar progressive increases in glial-vascular interactions, with maximal coverage of ∼70% in adults-significantly lower than the ∼100% seen in mammals. These findings define the temporal and regional maturation of glial-vascular interactions in zebrafish and highlight differences from mammalian systems.

Genome-wide association study unravels mechanisms of brain glymphatic activity

Brain glymphatic activity, as indicated by diffusion-tensor imaging analysis along the perivascular space (ALPS) index, is involved in developmental neuropsychiatric and neurodegenerative diseases, but its genetic architecture is poorly understood. Here, we identified 17 unique genome-wide significant loci and 161 candidate genes linked to the ALPS-indexes in a discovery sample of 31,021 individuals from the UK Biobank. Seven loci were replicated in two independent datasets. Genetic signals located at the 2p23.3 locus yielded significantly concordant effects in both young and aging cohorts. Genetic correlation and polygenic overlap analyses indicate a common underlying genetic mechanism between the ALPS-index, ventricular volumes, and cerebrospinal fluid tau levels, with GMNC (3q28) and C16orf95 (16q24.2) as the shared genetic basis. Our findings enhance the understanding of the genetics of the ALPS-index and provide insight for further research into the neurobiological mechanisms of glymphatic clearance activity across the lifespan and its relation to neuropsychiatric phenotypes.

Evolutionary changes leading to efficient glymphatic circulation in the mammalian brain

The functional significance of the morphological and genetic changes that occurred in the brain during evolution is not fully understood. Here we show the relationships between evolutionary changes of the brain and glymphatic circulation. We establish a mathematical model to simulate glymphatic circulation in the cerebral hemispheres, and our results show that cortical neurons accumulate in areas of the cerebral hemispheres where glymphatic circulation is highly efficient. We also find that cortical folds markedly enhance the efficiency of glymphatic circulation in the cerebral hemispheres. Furthermore, our in vivo study using ferrets reveals sulcus-dominant cerebrospinal fluid (CSF) influx, which enhances the efficiency of glymphatic circulation in the enlarged cerebral hemispheres of gyrencephalic brains. Sulcus-dominant CSF influx is mediated by preferential expression of aquaporin-4 in sulcal regions, and similar expression patterns of aquaporin-4 are also found in human cerebral hemispheres. These results indicate that evolutionary changes in the cerebral hemispheres are related to improved efficiency of glymphatic circulation. It seems plausible that the efficiency of glymphatic circulation is an important factor determining the evolutionary trajectory of the cerebral hemispheres.

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.

Evaluation of Glymphatic System Development in Neonatal Brain via Diffusion Analysis along the Perivascular Space Index

OBJECTIVE

Glymphatic system is a recently discovered macroscopic waste clearance system associated with numerous neurological diseases. However, little is known about glymphatic system development in neonates. We sought to evaluate diffusion along the perivascular space (ALPS) index, a proxy for glymphatic system function, in neonates and investigate its potential associations with maturation, sex, and preterm birth.

METHODS

Diffusion magnetic resonance imaging (MRI) data in 418 neonates, including 92 preterm neonates (57 males) and 326 term neonates (175 males), from the Developing Human Connectome Project were used for evaluating ALPS index. Linear regression modeling was performed to assess group differences in the ALPS index according to preterm birth and sex. Pearson's and partial correlation analysis were performed to assess the association between the ALPS index and gestational age (GA) as well as postmenstrual age (PMA) at MRI. Moderation analysis was performed to assess the moderation effect of preterm birth on the relationship between the ALPS index and PMA.

RESULTS

Compared to term neonates, preterm neonates exhibited lower ALPS indices (p < 0.001). The ALPS index positively correlated with PMA (p = 0.004) and GA (p < 0.001). Preterm birth (p = 0.013) had a significant moderation effect on the relationship between the ALPS index and PMA. Sex had no significant direct effect (p = 0.639) or moderation effect (p = 0.333) on ALPS index.

INTERPRETATION

Glymphatic system development is a dynamic process in neonates, which can be moderated by preterm birth, the ALPS index could serve as a sensitive biomarker for monitoring this process. ANN NEUROL 2024;96:970-980.

Formaldehyde initiates memory and motor impairments under weightlessness condition

During space flight, prolonged weightlessness stress exerts a range of detrimental impacts on the physiology and psychology of astronauts. These manifestations encompass depressive symptoms, anxiety, and impairments in both short-term memory and motor functions, albeit the precise underlying mechanisms remain elusive. Recent studies have revealed that hindlimb unloading (HU) animal models, which simulate space weightlessness, exhibited a disorder in memory and motor function associated with endogenous formaldehyde (FA) accumulation in the hippocampus and cerebellum, disruption of brain extracellular space (ECS), and blockage of interstitial fluid (ISF) drainage. Notably, the impairment of the blood-brain barrier (BBB) caused by space weightlessness elicits the infiltration of albumin and hemoglobin from the blood vessels into the brain ECS. However, excessive FA has the potential to form cross-links between these two proteins and amyloid-beta (Aβ), thereby obstructing ECS and inducing neuron death. Moreover, FA can inhibit N-methyl-D-aspartate (NMDA) currents by crosslinking NR1 and NR2B subunits, thus impairing memory. Additionally, FA has the ability to modulate the levels of certain microRNAs (miRNAs) such as miRNA-29b, which can affect the expression of aquaporin-4 (AQP4) so as to regulate ECS structure and ISF drainage. Especially, the accumulation of FA may inactivate the ataxia telangiectasia-mutated (ATM) protein kinase by forming cross-linking, a process that is associated with ataxia. Hence, this review presents that weightlessness stress-derived FA may potentially serve as a crucial catalyst in the deterioration of memory and motor abilities in the context of microgravity.

The Cerebrovascular Side of Plasticity: Microvascular Architecture across Health and Neurodegenerative and Vascular Diseases

The delivery of nutrients to the brain is provided by a 600 km network of capillaries and microvessels. Indeed, the brain is highly energy demanding and, among a total amount of 100 billion neurons, each neuron is located just 10-20 μm from a capillary. This vascular network also forms part of the blood-brain barrier (BBB), which maintains the brain's stable environment by regulating chemical balance, immune cell transport, and blocking toxins. Typically, brain microvascular endothelial cells (BMECs) have low turnover, indicating a stable cerebrovascular structure. However, this structure can adapt significantly due to development, aging, injury, or disease. Temporary neural activity changes are managed by the expansion or contraction of arterioles and capillaries. Hypoxia leads to significant remodeling of the cerebrovascular architecture and pathological changes have been documented in aging and in vascular and neurodegenerative conditions. These changes often involve BMEC proliferation and the remodeling of capillary segments, often linked with local neuronal changes and cognitive function. Cerebrovascular plasticity, especially in arterioles, capillaries, and venules, varies over different time scales in development, health, aging, and diseases. Rapid changes in cerebral blood flow (CBF) occur within seconds due to increased neural activity. Prolonged changes in vascular structure, influenced by consistent environmental factors, take weeks. Development and aging bring changes over months to years, with aging-associated plasticity often improved by exercise. Injuries cause rapid damage but can be repaired over weeks to months, while neurodegenerative diseases cause slow, varied changes over months to years. In addition, if animal models may provide useful and dynamic in vivo information about vascular plasticity, humans are more complex to investigate and the hypothesis of glymphatic system together with Magnetic Resonance Imaging (MRI) techniques could provide useful clues in the future.

40 Hz light flickering facilitates the glymphatic flow via adenosine signaling in mice

The glymphatic-lymphatic system is increasingly recognized as fundamental for the homeostasis of the brain milieu since it defines cerebral spinal fluid flow in the brain parenchyma and eliminates metabolic waste. Animal and human studies have uncovered several important physiological factors regulating the glymphatic system including sleep, aquaporin-4, and hemodynamic factors. Yet, our understanding of the modulation of the glymphatic system is limited, which has hindered the development of glymphatic-based treatment for aging and neurodegenerative disorders. Here, we present the evidence from fluorescence tracing, two-photon recording, and dynamic contrast-enhanced magnetic resonance imaging analyses that 40 Hz light flickering enhanced glymphatic influx and efflux independently of anesthesia and sleep, an effect attributed to increased astrocytic aquaporin-4 polarization and enhanced vasomotion. Adenosine-A2A receptor (A2AR) signaling emerged as the neurochemical underpinning of 40 Hz flickering-induced enhancement of glymphatic flow, based on increased cerebrofluid adenosine levels, the abolishment of enhanced glymphatic flow by pharmacological or genetic inactivation of equilibrative nucleotide transporters-2 or of A2AR, and by the physical and functional A2AR-aquaporin-4 interaction in astrocytes. These findings establish 40 Hz light flickering as a novel non-invasive strategy of enhanced glymphatic flow, with translational potential to relieve brain disorders.

Long-term isoflurane anesthesia induces cognitive deficits via AQP4 depolarization mediated blunted glymphatic inflammatory proteins clearance

Perioperative neurocognitive disorders (PND) refer to cognitive deterioration that occurs after surgery or anesthesia. Prolonged isoflurane exposure has potential neurotoxicity and induces PND, but the mechanism is unclear. The glymphatic system clears harmful metabolic waste from the brain. This study sought to unveil the functions of glymphatic system in PND and explore the underlying molecular mechanisms. The PND mice model was established by long term isoflurane anesthesia. The glymphatic function was assessed by multiple in vitro and in vivo methods. An adeno-associated virus was used to overexpress AQP4 and TGN-020 was used to inhibit its function. This research revealed that the glymphatic system was impaired in PND mice and the blunted glymphatic transport was closely associated with the accumulation of inflammatory proteins in the hippocampus. Increasing AQP4 polarization could enhance glymphatic transport and suppresses neuroinflammation, thereby improve cognitive function in the PND model mice. However, a marked impaired glymphatic inflammatory proteins clearance and the more severe cognitive dysfunction were observed when decreasing AQP4 polarization. Therefore, long-term isoflurane anesthesia causes blunted glymphatic system by inducing AQP4 depolarization, enhanced the AQP4 polarization can alleviate the glymphatic system malfunction and reduce the neuroinflammatory response, which may be a potential treatment strategy for PND.

Dexmedetomidine improves the circulatory dysfunction of the glymphatic system induced by sevoflurane through the PI3K/AKT/ΔFosB/AQP4 pathway in young mice

Multiple sevoflurane exposures may damage the developing brain. The neuroprotective function of dexmedetomidine has been widely confirmed in animal experiments and human studies. However, the effect of dexmedetomidine on the glymphatic system has not been clearly studied. We hypothesized that dexmedetomidine could alleviate sevoflurane-induced circulatory dysfunction of the glymphatic system in young mice. Six-day-old C57BL/6 mice were exposed to 3% sevoflurane for 2 h daily, continuously for 3 days. Intraperitoneal injection of either normal saline or dexmedetomidine was administered before every anaesthesia. Meanwhile the circulatory function of glymphatic system was detected by tracer injection at P8 and P32. On P30-P32, behavior tests including open field test, novel object recognition test, and Y-maze test were conducted. Primary astrocyte cultures were established and treated with the PI3K activator 740Y-P, dexmedetomidine, and small interfering RNA (siRNA) to silence ΔFosB. We propose for the first time that multiple exposure to sevoflurane induces circulatory dysfunction of the glymphatic system in young mice. Dexmedetomidine improves the circulatory capacity of the glymphatic system in young mice following repeated exposure to sevoflurane through the PI3K/AKT/ΔFosB/AQP4 signaling pathway, and enhances their long-term learning and working memory abilities.

Glymphatic Imaging in Pediatrics

The glymphatic system, which facilitates cerebrospinal fluid (CSF) flow through the brain parenchyma, is important for brain development and waste clearance. Advances in imaging techniques, particularly magnetic resonance imaging, have make it possible to evaluate glymphatic structures and functions in vivo. Recently, several studies have focused on the development and alterations of the glymphatic system in pediatric disorders. This review discusses the development of the glymphatic system, advances of imaging techniques and their applications in pediatric disorders. First, the results of the reviewed studies indicate that the development of the glymphatic system is a long-lasting process that continues into adulthood. Second, there is a need for improved glymphatic imaging techniques that are non-invasive and fast to improve suitability for pediatric applications, as some of existing methods use contrast injection and are susceptible to motion artifacts from long scanning times. Several novel techniques are potentially feasible for pediatric patients and may be used in the future. Third, the glymphatic dysfunction is associated with a large number of pediatric disorders, although only a few have recently been investigated. In conclusion, research on the pediatric glymphatic system remains an emerging field. The preliminary applications of glymphatic imaging techniques have provided unique insight into the pathological mechanism of pediatric diseases, but mainly limited in visualization of enlarged perivascular spaces and morphological measurements on CSF volumes. More in-depth studies on glymphatic functions are required to improve our understanding of the mechanisms underlying brain development and pediatric diseases. LEVEL OF EVIDENCE: 5 TECHNICAL EFFICACY: Stage 3.

Bone-derived PDGF-BB enhances hippocampal non-specific transcytosis through microglia-endothelial crosstalk in HFD-induced metabolic syndrome

BACKGROUND

It is well known that high-fat diet (HFD)-induced metabolic syndrome plays a crucial role in cognitive decline and brain-blood barrier (BBB) breakdown. However, whether the bone-brain axis participates in this pathological process remains unknown. Here, we report that platelet-derived growth factor-BB (PDGF-BB) secretion by preosteoclasts in the bone accelerates neuroinflammation. The expression of alkaline phosphatase (ALPL), a nonspecific transcytosis marker, was upregulated during HFD challenge.

MAIN BODY

Preosteoclast-specific Pdgfb transgenic mice with high PDGF-BB concentrations in the circulation recapitulated the HFD-induced neuroinflammation and transcytosis shift. Preosteoclast-specific Pdgfb knockout mice were partially rescued from hippocampal neuroinflammation and transcytosis shifts in HFD-challenged mice. HFD-induced PDGF-BB elevation aggravated microglia-associated neuroinflammation and interleukin-1β (IL-1β) secretion, which increased ALPL expression and transcytosis shift through enhancing protein 1 (SP1) translocation in endothelial cells.

CONCLUSION

Our findings confirm the role of bone-secreted PDGF-BB in neuroinflammation and the transcytosis shift in the hippocampal region during HFD challenge and identify a novel mechanism of microglia-endothelial crosstalk in HFD-induced metabolic syndrome.

Repetitive head trauma and apoE4 induce chronic cerebrovascular alterations that impair tau elimination from the brain

Repetitive mild traumatic brain injuries (r-mTBI) sustained in the military or contact sports have been associated with the accumulation of extracellular tau in the brain, which may contribute to the pathogenesis of neurodegenerative tauopathies. The expression of the apolipoprotein E4 (apoE4) isoform has been associated with higher levels of tau in the brain, and worse clinical outcomes after r-mTBI, though the influence of apoE genotype on extracellular tau dynamics in the brain is poorly understood. We recently demonstrated that extracellular tau can be eliminated across blood-brain barrier (BBB), which is progressively impaired following r-mTBI. The current studies investigated the influence of repetitive mild TBI (r-mTBI) and apoE genotype on the elimination of extracellular solutes from the brain. Following intracortical injection of biotin-labeled tau into humanized apoE-Tr mice, the levels of exogenous tau residing in the brain of apoE4 mice were elevated compared to other isoforms, indicating reduced tau elimination. Additionally, we found exposure to r-mTBI increased tau residence in apoE2 mice, similar to our observations in E2FAD animals. Each of these findings may be the result of diminished tau efflux via LRP1 at the BBB, as LRP1 inhibition significantly reduced tau uptake in endothelial cells and decreased tau transit across an in vitro model of the BBB (basolateral-to-apical). Notably, we showed that injury and apoE status, (particularly apoE4) resulted in chronic alterations in BBB integrity, pericyte coverage, and AQP4 polarization. These aberrations coincided with an atypical reactive astrocytic gene signature indicative of diminished CSF-ISF exchange. Our work found that CSF movement was reduced in the chronic phase following r-mTBI (>18 months post injury) across all apoE genotypes. In summary, we show that apoE genotype strongly influences cerebrovascular homeostasis, which can lead to age-dependent deficiencies in the elimination of toxic proteins from the brain, like tau, particularly in the aftermath of head trauma.

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 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.

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.

Yoda1 opens the lymphatic path for craniosynostosis therapy

The rediscovery of meningeal lymphatic vessels (MLVs) has sparked research interest in their function in numerous neurological pathologies. Craniosynostosis (CS) is caused by a premature fusion of cranial sutures during development. In this issue of the JCI, Matrongolo and colleagues show that Twist1-haploinsufficient mice that develop CS exhibit raised intracranial pressure, diminished cerebrospinal fluid (CSF) outflow, and impaired paravascular CSF-brain flow; all features that were associated with MLV defects and exacerbated pathology in mouse models of Alzheimer's disease. Activation of the mechanosensor Piezo1 with Yoda1 restored MLV function and CSF perfusion in CS models and in aged mice, opening an avenue for further development of therapeutics.

The contribution of β-amyloid, Tau and α-synuclein to blood-brain barrier damage in neurodegenerative disorders

Central nervous system (CNS) accumulation of fibrillary deposits made of Amyloid β (Aβ), hyperphosphorylated Tau or α-synuclein (α-syn), present either alone or in the form of mixed pathology, characterizes the most common neurodegenerative diseases (NDDs) as well as the aging brain. Compelling evidence supports that acute neurological disorders, such as traumatic brain injury (TBI) and stroke, are also accompanied by increased deposition of toxic Aβ, Tau and α-syn species. While the contribution of these pathological proteins to neurodegeneration has been experimentally ascertained, the cellular and molecular mechanisms driving Aβ, Tau and α-syn-related brain damage remain to be fully clarified. In the last few years, studies have shown that Aβ, Tau and α-syn may contribute to neurodegeneration also by inducing and/or promoting blood-brain barrier (BBB) disruption. These pathological proteins can affect BBB integrity either directly by affecting key BBB components such as pericytes and endothelial cells (ECs) or indirectly, by promoting brain macrophages activation and dysfunction. Here, we summarize and critically discuss key findings showing how Aβ, Tau and α-syn can contribute to BBB damage in most common NDDs, TBI and stroke. We also highlight the need for a deeper characterization of the role of these pathological proteins in the activation and dysfunction of brain macrophages, pericytes and ECs to improve diagnosis and treatment of acute and chronic neurological disorders.

Neonatal stress disrupts the glymphatic system development and increases the susceptibility to Parkinson's disease in later life

INTRODUCTION

Neonatal stress disrupts brain development and increases the risk of neurological disorders later in life. However, the impact of neonatal stress on the development of the glymphatic system and susceptibility to Parkinson's disease (PD) remains largely unknown.

METHODS

Neonatal maternal deprivation (NMD) was performed on mice for 14 consecutive days to model chronic neonatal stress. Adeno-associated virus expressing A53T-α-synuclein (α-syn) was injected into the substantia nigra to establish PD model mice. Glymphatic activity was determined using in vivo magnetic resonance imaging, ex vivo fluorescence imaging and microplate assay. The transcription and expression of aquaporin-4 (AQP4) and other molecules were evaluated by qPCR, western blotting, and immunofluorescence. Animal's responses to NMD and α-syn overexpression were observed using behavioral tests.

RESULTS

Glymphatic activity was impaired in adult NMD mice. AQP4 polarization and platelet-derived growth factor B (PDGF-B) signaling were reduced in the frontal cortex and hippocampus of both young and adult NMD mice. Furthermore, exogenous α-syn accumulation was increased and PD-like symptoms were aggravated in adult NMD mice.

CONCLUSION

The results demonstrated that NMD could disrupt the development of the glymphatic system through PDGF-B signaling and increase the risk of PD later in life, indicating that alleviating neonatal stress could be beneficial in protecting the glymphatic system and reducing susceptibility to neurodegeneration.

Cerebral glucagon-like peptide-1 receptor activation alleviates traumatic brain injury by glymphatic system regulation in mice

AIM

We aimed to assess the effects of cerebral glucagon-like peptide-1 receptor (GLP-1R) activation on the glymphatic system and whether this effect was therapeutic for traumatic brain injury (TBI).

METHODS

Immunofluorescence was employed to evaluate glymphatic system function. The blood-brain barrier (BBB) permeability, microvascular basement membrane, and tight junction expression were assessed using Evans blue extravasation, immunofluorescence, and western blot. Immunohistochemistry was performed to assess axonal damage. Neuronal apoptosis was evaluated using Nissl staining, terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL) staining, and western blot. Cognitive function was assessed using behavioral tests.

RESULTS

Cerebral GLP-1R activation restored glymphatic transport following TBI, alleviating BBB disruption and neuronal apoptosis, thereby improving cognitive function following TBI. Glymphatic function suppression by treatment using aquaporin 4 inhibitor TGN-020 abolished the protective effect of the GLP-1R agonist against cognitive impairment.

CONCLUSION

Cerebral GLP-1R activation can effectively ameliorate neuropathological changes and cognitive impairment following TBI; the underlying mechanism could involve the repair of the glymphatic system damaged by TBI.

The elusive brain perivascular fibroblast: a potential role in vascular stability and homeostasis

In the brain, perivascular fibroblasts (PVFs) reside within the perivascular spaces (PVSs) of arterioles and large venules, however their physiological and pathophysiological roles remain largely unknown. PVFs express numerous extracellular matrix proteins that are found in the basement membrane and PVS surrounding large diameter vessels. PVFs are sandwiched between the mural cell layer and astrocytic endfeet, where they are poised to interact with mural cells, perivascular macrophages, and astrocytes. We draw connections between the more well-studied PVF pro-fibrotic response in ischemic injury and the less understood thickening of the vascular wall and enlargement of the PVS described in dementia and neurodegenerative diseases. We postulate that PVFs may be responsible for stability and homeostasis of the brain vasculature, and may also contribute to changes within the PVS during disease.

Piezo1 agonist restores meningeal lymphatic vessels, drainage, and brain-CSF perfusion in craniosynostosis and aged mice

Skull development coincides with the onset of cerebrospinal fluid (CSF) circulation, brain-CSF perfusion, and meningeal lymphangiogenesis, processes essential for brain waste clearance. How these processes are affected by craniofacial disorders such as craniosynostosis are poorly understood. We report that raised intracranial pressure and diminished CSF flow in craniosynostosis mouse models associate with pathological changes to meningeal lymphatic vessels that affect their sprouting, expansion, and long-term maintenance. We also show that craniosynostosis affects CSF circulatory pathways and perfusion into the brain. Further, craniosynostosis exacerbates amyloid pathology and plaque buildup in Twist1+/-:5xFAD transgenic Alzheimer's disease models. Treating craniosynostosis mice with Yoda1, a small molecule agonist for Piezo1, reduces intracranial pressure and improves CSF flow, in addition to restoring meningeal lymphangiogenesis, drainage to the deep cervical lymph nodes, and brain-CSF perfusion. Leveraging these findings, we show that Yoda1 treatments in aged mice with reduced CSF flow and turnover improve lymphatic networks, drainage, and brain-CSF perfusion. Our results suggest that CSF provides mechanical force to facilitate meningeal lymphatic growth and maintenance. Additionally, applying Yoda1 agonist in conditions with raised intracranial pressure and/or diminished CSF flow, as seen in craniosynostosis or with ageing, is a possible therapeutic option to help restore meningeal lymphatic networks and brain-CSF perfusion.

Understanding the development, pathogenesis, and injury response of meningeal lymphatic networks through the use of animal models

Meningeal lymphatic vessels (MLVs) help maintain central nervous system (CNS) homeostasis via their ability to facilitate macromolecule waste clearance and neuroimmune trafficking. Although these vessels were overlooked for centuries, they have now been characterized in humans, non-human primates, and rodents. Recent studies in mice have explored the stereotyped growth and expansion of MLVs in dura mater, the various transcriptional, signaling, and environmental factors regulating their development and long-term maintenance, and the pathological changes these vessels undergo in injury, disease, or with aging. Key insights gained from these studies have also been leveraged to develop therapeutic approaches that help augment or restore MLV functions to improve brain health and cognition. Here, we review fundamental processes that control the development of peripheral lymphatic networks and how these might apply to the growth and expansion of MLVs in their unique meningeal environment. We also emphasize key findings in injury and disease models that may reveal additional insights into the plasticity of these vessels throughout the lifespan. Finally, we highlight unanswered questions and future areas of study that can further reveal the exciting therapeutic potential of meningeal lymphatics.

Melatonin alleviates depression-like behaviors and cognitive dysfunction in mice by regulating the circadian rhythm of AQP4 polarization

Depression is a common chronic psychiatric illness, which is resistant to medical treatments. While melatonin may alleviate certain depression symptoms, evidence for its efficacy against core symptoms is lacking. Here, we tested a mechanism whereby melatonin rescues the behavioral outcomes of the chronic unpredictable mild stress (CUMS) mouse model of depression. CUMS mice showed depressive behaviors to tail suspension, open field behavior, and sucrose preference test, and cognitive dysfunction in the Morris water maze. Impairments in these measures were relieved by melatonin treatment. Moreover, CUMS mice had impaired glymphatic function across the sleep-wake cycle due to the astrocytic loss and disturbance of circadian regulation of the polarized expression of aquaporin-4 (AQP4) water channels in perivascular astrocytes. EEG results in CUMS mice showed a reduced total sleep time and non-rapid eye movement (NREM) sleep, due to sleep fragmentation in the light phase. CUMS mice lost the normal rhythmic expressions of circadian proteins Per2, Cry2, Bmal1, Clock, and Per1. However, the melatonin treatment restored glymphatic system function and the polarization of AQP4, while improving sleep structure, and rectifying the abnormal expression of Per2, Bmal1, Clock, and Per1 in CUMS mice. Interestingly, Per2 expression correlated negatively with the polarization of AQP4. Further studies demonstrated that Per2 directed the location of AQP4 expression via interactions with the α-dystrobrevin (Dtna) subunit of AQP4 in primary cultured astrocytes. In conclusion, we report a new mechanism whereby melatonin improves depression outcomes by regulating the expression of the circadian protein Per2, maintaining the circadian rhythm of astrocytic AQP4 polarization, and restoring glymphatic function.

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.

Piezo1 agonist restores meningeal lymphatic vessels, drainage, and brain-CSF perfusion in craniosynostosis and aged mice

Skull development coincides with the onset of cerebrospinal fluid (CSF) circulation, brain-CSF perfusion, and meningeal lymphangiogenesis, processes essential for brain waste clearance. How these processes are affected by craniofacial disorders such as craniosynostosis are poorly understood. We report that raised intracranial pressure and diminished CSF flow in craniosynostosis mouse models associates with pathological changes to meningeal lymphatic vessels that affect their sprouting, expansion, and long-term maintenance. We also show that craniosynostosis affects CSF circulatory pathways and perfusion into the brain. Further, craniosynostosis exacerbates amyloid pathology and plaque buildup in Twist1 +/- :5xFAD transgenic Alzheimer's disease models. Treating craniosynostosis mice with Yoda1, a small molecule agonist for Piezo1, reduces intracranial pressure and improves CSF flow, in addition to restoring meningeal lymphangiogenesis, drainage to the deep cervical lymph nodes, and brain-CSF perfusion. Leveraging these findings, we show Yoda1 treatments in aged mice with reduced CSF flow and turnover improve lymphatic networks, drainage, and brain-CSF perfusion. Our results suggest CSF provides mechanical force to facilitate meningeal lymphatic growth and maintenance. Additionally, applying Yoda1 agonist in conditions with raised intracranial pressure and/or diminished CSF flow, as seen in craniosynostosis or with ageing, is a possible therapeutic option to help restore meningeal lymphatic networks and brain-CSF perfusion.

Low-intensity open-field blast exposure effects on neurovascular unit ultrastructure in mice

Mild traumatic brain injury (mTBI) induced by low-intensity blast (LIB) is a serious health problem affecting military service members and veterans. Our previous reports using a single open-field LIB mouse model showed the absence of gross microscopic damage or necrosis in the brain, while transmission electron microscopy (TEM) identified ultrastructural abnormalities of myelin sheaths, mitochondria, and synapses. The neurovascular unit (NVU), an anatomical and functional system with multiple components, is vital for the regulation of cerebral blood flow and cellular interactions. In this study, we delineated ultrastructural abnormalities affecting the NVU in mice with LIB exposure quantitatively and qualitatively. Luminal constrictive irregularities were identified at 7 days post-injury (DPI) followed by dilation at 30 DPI along with degeneration of pericytes. Quantitative proteomic analysis identified significantly altered vasomotor-related proteins at 24 h post-injury. Endothelial cell, basement membrane and astrocyte end-foot swellings, as well as vacuole formations, occurred in LIB-exposed mice, indicating cellular edema. Structural abnormalities of tight junctions and astrocyte end-foot detachment from basement membranes were also noted. These ultrastructural findings demonstrate that LIB induces multiple-component NVU damage. Prevention of NVU damage may aid in identifying therapeutic targets to mitigate the effects of primary brain blast injury.

Evidence for developmental vascular-associated necroptosis and its contribution to venous-lymphatic endothelial differentiation

During development, apoptosis removes redundant cells and ensures proper organ morphogenesis. Necrosis is long known as an adult-bound inflammatory and pathologic cell death. Whether there exists physiological necrosis during early development has been speculated but yet clearly demonstrated. Here, we report evidence of necroptosis, a type of programmed necrosis, specifically in perivascular cells of cerebral cortex and skin at the early stage of development. Phosphorylated Mixed Lineage Kinase Domain-Like protein (MLKL), a key molecule in executing necroptosis, co-expressed with blood endothelial marker CD31 and venous-lymphatic progenitor marker Sox18. Depletion of Mlkl did not affect the formation of blood vessel network but increased the differentiation of venous-lymphatic lineage cells in postnatal cerebral cortex and skin. Consistently, significant enhancement of cerebrospinal fluid diffusion and lymphatic drainage was found in brain and skin of Mlkl-deficient mice. Under hypobaric hypoxia induced cerebral edema and inflammation induced skin edema, Mlkl mutation significantly attenuated brain-blood-barrier damage and edema formation. Our data, for the first time, demonstrated the presence of physiological vascular-associated necroptosis and its potential involvement in the development of venous-lymphatic vessels.

Choroid plexus-targeted NKCC1 overexpression to treat post-hemorrhagic hydrocephalus

Post-hemorrhagic hydrocephalus (PHH) refers to a life-threatening accumulation of cerebrospinal fluid (CSF) that occurs following intraventricular hemorrhage (IVH). An incomplete understanding of this variably progressive condition has hampered the development of new therapies beyond serial neurosurgical interventions. Here, we show a key role for the bidirectional Na-K-Cl cotransporter, NKCC1, in the choroid plexus (ChP) to mitigate PHH. Mimicking IVH with intraventricular blood led to increased CSF [K+] and triggered cytosolic calcium activity in ChP epithelial cells, which was followed by NKCC1 activation. ChP-targeted adeno-associated viral (AAV)-NKCC1 prevented blood-induced ventriculomegaly and led to persistently increased CSF clearance capacity. These data demonstrate that intraventricular blood triggered a trans-choroidal, NKCC1-dependent CSF clearance mechanism. Inactive, phosphodeficient AAV-NKCC1-NT51 failed to mitigate ventriculomegaly. Excessive CSF [K+] fluctuations correlated with permanent shunting outcome in humans following hemorrhagic stroke, suggesting targeted gene therapy as a potential treatment to mitigate intracranial fluid accumulation following hemorrhage.

Prolonged fluoride exposure induces spatial-memory deficit and hippocampal dysfunction by inhibiting small heat shock protein 22 in mice

Millions of residents in areas with high-fluoride drinking water supply ingest excessive levels of fluoride for long periods. This study investigated the mechanisms and impacts of lifelong exposure to naturally occurring moderate-high-fluoride drinking water on spatial-memory function by studying mice in controlled experiments. Spatial-memory deficits and disorders of hippocampal neuronal electrical activity were observed in mice exposed to 25-ppm or 50-ppm-fluoride drinking water for 56 weeks, but not in adult or old mice exposed to 50 ppm fluoride for 12 weeks. Ultrastructural analysis showed severely damaged hippocampal mitochondria, evidenced by reduced mitochondrial membrane potential and ATP content. Mitochondrial biogenesis was impaired in fluoride-exposed mice, manifesting as a significantly reduced mtDNA content, mtDNA-encoded subunits mtND6 and mtCO1, and respiratory complex activities. Fluoride reduced expression of Hsp22, a beneficial mediator of mitochondrial homeostasis, and decreased levels of signaling for the PGC-1α/TFAM pathway-which regulates mitochondrial biogenesis-and the NF-κβ/STAT3 pathway-which regulates mitochondrial respiratory chain enzyme activity. Hippocampus-specific Hsp22-overexpression improved fluoride-induced spatial-memory deficits by activating the PGC-1α/TFAM and STAT3 signaling pathways, while Hsp22-silencing aggravated the deficits by inhibiting both pathways. Downregulation of Hsp22 plays a vital role in fluoride-induced spatial-memory deficits by impacting mtDNA-encoding subsets and mitochondrial respiratory chain enzyme activity.

Formation and function of the meningeal arachnoid barrier around the developing mouse brain

The arachnoid barrier, a component of the blood-cerebrospinal fluid barrier (B-CSFB) in the meninges, is composed of epithelial-like, tight-junction-expressing cells. Unlike other central nervous system (CNS) barriers, its' developmental mechanisms and timing are largely unknown. Here, we show that mouse arachnoid barrier cell specification requires the repression of Wnt-β-catenin signaling and that constitutively active β-catenin can prevent its formation. We also show that the arachnoid barrier is functional prenatally and, in its absence, a small molecular weight tracer and the bacterium group B Streptococcus can cross into the CNS following peripheral injection. Acquisition of barrier properties prenatally coincides with the junctional localization of Claudin 11, and increased E-cadherin and maturation continues after birth, where postnatal expansion is marked by proliferation and re-organization of junctional domains. This work identifies fundamental mechanisms that drive arachnoid barrier formation, highlights arachnoid barrier fetal functions, and provides novel tools for future studies on CNS barrier development.

Leveraging the glymphatic and meningeal lymphatic systems as therapeutic strategies in Alzheimer's disease: an updated overview of nonpharmacological therapies

Understanding and treating Alzheimer's disease (AD) has been a remarkable challenge for both scientists and physicians. Although the amyloid-beta and tau protein hypothesis have largely explained the key pathological features of the disease, the mechanisms by which such proteins accumulate and lead to disease progression are still unknown. Such lack of understanding disrupts the development of disease-modifying interventions, leaving a therapeutic gap that remains unsolved. Nonetheless, the recent discoveries of the glymphatic pathway and the meningeal lymphatic system as key components driving central solute clearance revealed another mechanism underlying AD pathogenesis. In this regard, this narrative review integrates the glymphatic and meningeal lymphatic systems as essential components involved in AD pathogenesis. Moreover, it discusses the emerging evidence suggesting that nutritional supplementation, non-invasive brain stimulation, and traditional Chinese medicine can improve the pathophysiology of the disease by increasing glymphatic and/or meningeal lymphatic function. Given that physical exercise is a well-regarded preventive and pro-cognitive intervention for dementia, we summarize the evidence suggesting the glymphatic system as a mediating mechanism of the physical exercise therapeutic effects in AD. Targeting these central solute clearance systems holds the promise of more effective treatment strategies.

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.

Stuck on you: Meninges cellular crosstalk in development

The spatial and temporal development of the brain, overlying meninges (fibroblasts, vasculature and immune cells) and calvarium are highly coordinated. In particular, the timing of meningeal fibroblasts into molecularly distinct pia, arachnoid and dura subtypes coincides with key developmental events in the brain and calvarium. Further, the meninges are positioned to influence development of adjacent structures and do so via depositing basement membrane and producing molecular cues to regulate brain and calvarial development. Here, we review the current knowledge of how meninges development aligns with events in the brain and calvarium and meningeal fibroblast "crosstalk" with these structures. We summarize outstanding questions and how the use of non-mammalian models to study the meninges will substantially advance the field of meninges biology.

Cancer Cell-Derived PDGFB Stimulates mTORC1 Activation in Renal Carcinoma

Clear cell renal cell carcinoma (ccRCC) is a hypervascular tumor that is characterized by bi-allelic inactivation of the VHL tumor suppressor gene and mTOR signalling pathway hyperactivation. The pro-angiogenic factor PDGFB, a transcriptional target of super enhancer-driven KLF6, can activate the mTORC1 signalling pathway in ccRCC. However, the detailed mechanisms of PDGFB-mediated mTORC1 activation in ccRCC have remained elusive. Here, we investigated whether ccRCC cells are able to secrete PDGFB into the extracellular milieu and stimulate mTORC1 signalling activity. We found that ccRCC cells secreted PDGFB extracellularly, and by utilizing KLF6- and PDGFB-engineered ccRCC cells, we showed that the level of PDGFB secretion was positively correlated with the expression of intracellular KLF6 and PDGFB. Moreover, the reintroduction of either KLF6 or PDGFB was able to sustain mTORC1 signalling activity in KLF6-targeted ccRCC cells. We further demonstrated that conditioned media of PDGFB-overexpressing ccRCC cells was able to re-activate mTORC1 activity in KLF6-targeted cells. In conclusion, cancer cell-derived PDGFB can mediate mTORC1 signalling pathway activation in ccRCC, further consolidating the link between the KLF6-PDGFB axis and the mTORC1 signalling pathway activity in ccRCC.

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.

Updated Understanding of the Glial-Vascular Unit in Central Nervous System Disorders

The concept of the glial-vascular unit (GVU) was raised recently to emphasize the close associations between brain cells and cerebral vessels, and their coordinated reactions to diverse neurological insults from a "glio-centric" view. GVU is a multicellular structure composed of glial cells, perivascular cells, and perivascular space. Each component is closely linked, collectively forming the GVU. The central roles of glial and perivascular cells and their multi-level interconnections in the GVU under normal conditions and in central nervous system (CNS) disorders have not been elucidated in detail. Here, we comprehensively review the intensive interactions between glial cells and perivascular cells in the niche of perivascular space, which take part in the modulation of cerebral blood flow and angiogenesis, formation of the blood-brain barrier, and clearance of neurotoxic wastes. Next, we discuss dysfunctions of the GVU in various neurological diseases, including ischemic stroke, spinal cord injury, Alzheimer's disease, and major depression disorder. In addition, we highlight the possible therapies targeting the GVU, which may have potential clinical applications.

Gliovascular interface abnormality in mice with endothelial cell senescence

In the brain, neurons, glial cells, vascular endothelial cells (ECs), and mural cells form a functional structure referred to as the neurovascular unit (NVU). The functions of the NVU become impaired with aging. To gain insight into the mechanism underlying the aging-related changes in the NVU, we characterized in the present study the gliovascular interface in transgenic mice expressing a dominant-negative form of the telomeric repeat-binding factor 2 (TERF2) specifically in ECs using the Tie2 promoter. In these transgenic mice, senescence occurred in the cerebral ECs and was accompanied by upregulation of the mRNAs of proinflammatory cell adhesion molecules and cytokines. It is noteworthy that in the deep layers of the cerebral cortex, astrocytes exhibited an increase in the signals for S100β as well as a decrease in the polarization of the water channel aquaporin-4 (AQP4) to the perivascular endfeet of the astrocytes. Mechanistically, the perivascular localization of dystroglycan and its ligand, laminin α2, was decreased, and their localization correlated well with the perivascular localization of AQP4, which supports the notion that their interaction regulates the perivascular localization of AQP4. The diminished perivascular localization of laminin α2 may be attributed to its proteolytic degradation by the matrix metalloproteinase-2 released by senescent ECs. Pericyte coverage was increased and negatively correlated with the decrease in the perivascular localization of AQP4. We propose that aging-related changes in ECs induce a mild morphological alteration of astrocytes and affect the localization of AQP4 at the gliovascular interface.

Gold nanoparticle-enhanced X-ray microtomography of the rodent reveals region-specific cerebrospinal fluid circulation in the brain

Cerebrospinal fluid (CSF) is essential for the development and function of the central nervous system (CNS). However, the brain and its interstitium have largely been thought of as a single entity through which CSF circulates, and it is not known whether specific cell populations within the CNS preferentially interact with the CSF. Here, we develop a technique for CSF tracking, gold nanoparticle-enhanced X-ray microtomography, to achieve micrometer-scale resolution visualization of CSF circulation patterns during development. Using this method and subsequent histological analysis in rodents, we identify previously uncharacterized CSF pathways from the subarachnoid space (particularly the basal cisterns) that mediate CSF-parenchymal interactions involving 24 functional-anatomic cell groupings in the brain and spinal cord. CSF distribution to these areas is largely restricted to early development and is altered in posthemorrhagic hydrocephalus. Our study also presents particle size-dependent CSF circulation patterns through the CNS including interaction between neurons and small CSF tracers, but not large CSF tracers. These findings have implications for understanding the biological basis of normal brain development and the pathogenesis of a broad range of disease states, including hydrocephalus.

Outcomes of the 2019 hydrocephalus association workshop, "Driving common pathways: extending insights from posthemorrhagic hydrocephalus"

The Hydrocephalus Association (HA) workshop, Driving Common Pathways: Extending Insights from Posthemorrhagic Hydrocephalus, was held on November 4 and 5, 2019 at Washington University in St. Louis. The workshop brought together a diverse group of basic, translational, and clinical scientists conducting research on multiple hydrocephalus etiologies with select outside researchers. The main goals of the workshop were to explore areas of potential overlap between hydrocephalus etiologies and identify drug targets that could positively impact various forms of hydrocephalus. This report details the major themes of the workshop and the research presented on three cell types that are targets for new hydrocephalus interventions: choroid plexus epithelial cells, ventricular ependymal cells, and immune cells (macrophages and microglia).

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.

Glymphatic Dysfunction Induced Oxidative Stress and Neuro-Inflammation in Major Depression Disorders

Major Depression disorder (MDD) is a potentially life-threatening mental illness, however, many patients have a poor response to current treatments. Recent studies have suggested that stress- or trauma-induced oxidative stress and inflammation could be important factors involved in the development of MDD, but the mechanisms remain unclear. We showed that the glymphatic system is a recently discovered structure in the brain that may be involved in the clearance of large molecular and cell debris in extracellular space. In addition, the glymphatic system can help with the removal of reactive oxygen species (ROS) and cytokines such as IL-1β and HIF-1α. Glymphatic impairment can lead to ROS accumulation in the microenvironment, inducing cellular injury signaling and activating NLRP3 in microglia to induce inflammation and, thus, many brain diseases, including psychiatric disorders. Therefore, trauma-induced glymphatic impairment could induce oxidative stress and inflammation, and thus MDD. This paper will review recent advances with regard to stress-induced glymphatic system impairment and ROS-mediated inflammation in MDD.

Age dependent contribution of entry via the CSF to the overall brain entry of small and large hydrophilic markers

Background

Apparent permeability of the blood brain barrier to hydrophilic markers has been shown to be higher in the developing brain. Apart from synthesis in situ, any substance detected in the brain parenchyma can originate from two sources: directly through blood vessels of brain vasculature and/or indirectly by entry from the cerebrospinal fluid (CSF) after transfer across the choroid plexuses. The relative quantitative contribution of these two routes to the overall brain entry remains unclear.

Methods

In rats at embryonic day 16, 19 and postnatal day 4 and young adults, a small (sucrose, mw. 342 Da) or a large (dextran, mw. 70 kDa) radiolabelled hydrophilic marker was injected intravenously for very short periods of time (30 s to 5 min) before collection of plasma, cerebrospinal fluid (CSF) and brain samples. Results are presented as concentration ratios between radioactivity measured in CSF or brain and that in plasma (%).

Results

The dextran brain/plasma ratio five minutes post injection was similar (2–4%) from E16 to adulthood whereas the sucrose brain/plasma ratio was significantly higher in fetal brains, but was comparable to dextran values in the adult. Sucrose CSF/plasma ratios were also significantly higher in fetal animals and decreased with age. In very short experiments involving fetal animals, entry of sucrose into the CSF after only 30 s was similar to that of dextran and both markers showed similar brain/plasma ratios.

Conclusions

In the developing brain the apparent higher brain entry of a small hydrophilic marker such as sucrose can be attributed to its higher entry into the CSF and subsequent diffusion into the brain. By contrast, movement of a larger marker like 70 kDa dextran is restricted firstly by choroid plexus epithelial tight junctions and secondly by specialised junctions in the neuroependymal interface between the CSF and brain. Brain/plasma ratios of 70 kDa dextran were similar in fetal and adult rats. Therefore 70 kDa dextran should be considered an appropriate marker if brain residual vascular space is to be measured, especially in younger animals.

Evidence of beta amyloid independent small vessel disease in familial Alzheimer's disease

We studied small vessel disease (SVD) pathology in Familial Alzheimer's disease (FAD) subjects carrying the presenilin 1 (PSEN1) p.Glu280Ala mutation in comparison to those with sporadic Alzheimer's disease (SAD) as a positive control for Alzheimer's pathology and Cerebral Autosomal Dominant Arteriopathy with Subcortical Infarcts and Leukoencephalopathy (CADASIL) bearing different NOTCH3 mutations, as positive controls for SVD pathology. Upon magnetic resonance imaging (MRI) in life, some FAD showed mild white matter hyperintensities and no further radiologic evidence of SVD. In post-mortem studies, total SVD pathology in cortical areas and basal ganglia was similar in PSEN1 FAD and CADASIL subjects, except for the feature of arteriosclerosis which was higher in CADASIL subjects than in PSEN1 FAD subjects. Further only a few SAD subjects showed a similar degree of SVD pathology as observed in CADASIL. Furthermore, we found significantly enlarged perivascular spaces in vessels devoid of cerebral amyloid angiopathy in FAD compared with SAD and CADASIL subjects. As expected, there was greater fibrinogen-positive perivascular reactivity in CADASIL but similar reactivity in PSEN1 FAD and SAD groups. Fibrinogen immunoreactivity correlated with onset age in the PSEN1 FAD cases, suggesting increased vascular permeability may contribute to cognitive decline. Additionally, we found reduced perivascular expression of PDGFRβ AQP4 in microvessels with enlarged PVS in PSEN1 FAD cases. We demonstrate that there is Aβ-independent SVD pathology in PSEN1 FAD, that was marginally lower than that in CADASIL subjects although not evident by MRI. These observations suggest presence of covert SVD even in PSEN1, contributing to disease progression. As is the case in SAD, these consequences may be preventable by early recognition and actively controlling vascular disease risk, even in familial forms of dementia.

High-resolution 3D demonstration of regional heterogeneity in the glymphatic system

Accumulating evidence indicates that the glymphatic system has a critical role in maintaining brain homeostasis. However, the detailed anatomy of the glymphatic pathway is not well understood, mostly due to a lack of high spatial resolution 3D visualization. In this study, a fluorescence micro-optical sectioning tomography (fMOST) was used to characterize the glymphatic architecture in the mouse brain. At 30 and 120 min after intracisternal infusion with fluorescent dextran (Dex-3), lectin was injected to stain the cerebral vasculature. Using fMOST, a high-resolution 3D dataset of the brain-wide distribution of Dex-3 was acquired. Combined with fluorescence microscopy and microplate array, the heterogeneous glymphatic flow and the preferential irrigated regions were identified. These cerebral regions containing large-caliber penetrating arteries and/or adjacent to the subarachnoid space had more robust CSF flow compared to other regions. Moreover, the major glymphatic vessels for CSF influx and fluid efflux in the entire brain were shown in 3D. This study demonstrates the regional heterogeneity in the glymphatic system and provides an anatomical resource for further investigation of the glymphatic function.

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.