Astrocytic endfeet re-cover blood vessels after removal by laser ablation

Satellite glial cell manipulation prior to axotomy enhances developing dorsal root ganglion central branch regrowth into the spinal cord

The central and peripheral nervous systems (CNS and PNS, respectively) exhibit remarkable diversity in the capacity to regenerate following neuronal injury with PNS injuries being much more likely to regenerate than those that occur in the CNS. Glial responses to damage greatly influence the likelihood of regeneration by either promoting or inhibiting axonal regrowth over time. However, despite our understanding of how some glial lineages participate in nerve degeneration and regeneration, less is known about the contributions of peripheral satellite glial cells (SGC) to regeneration failure following central axon branch injury of dorsal root ganglia (DRG) sensory neurons. Here, using in vivo, time-lapse imaging in larval zebrafish coupled with laser axotomy, we investigate the role of SGCs in axonal regeneration. In our studies we show that SGCs respond to injury by relocating their nuclei to the injury site during the same period that DRG neurons produce new central branch neurites. Laser ablation of SGCs prior to axon injury results in more neurite growth attempts and ultimately a higher rate of successful central axon regrowth, implicating SGCs as inhibitors of regeneration. We also demonstrate that this SGC response is mediated in part by ErbB signaling, as chemical inhibition of this receptor results in reduced SGC motility and enhanced central axon regrowth. These findings provide new insights into SGC-neuron interactions under injury conditions and how these interactions influence nervous system repair.

Astrocyte modulation in cerebral ischemia-reperfusion injury: A promising therapeutic strategy

Cerebral ischemia-reperfusion injury (CIRI) poses significant challenges for drug development due to its complex pathogenesis. Astrocyte involvement in CIRI pathogenesis has led to the development of novel astrocyte-targeting drug strategies. To comprehensively review the current literature, we conducted a thorough analysis from January 2012 to December 2023, identifying 82 drugs aimed at preventing and treating CIRI. These drugs target astrocytes to exert potential benefits in CIRI, and their primary actions include modulation of relevant signaling pathways to inhibit neuroinflammation and oxidative stress, reduce cerebral edema, restore blood-brain barrier integrity, suppress excitotoxicity, and regulate autophagy. Notably, active components from traditional Chinese medicines (TCM) such as Salvia miltiorrhiza, Ginkgo, and Ginseng exhibit these important pharmacological properties and show promise in the treatment of CIRI. This review highlights the potential of astrocyte-targeted drugs to ameliorate CIRI and categorizes them based on their mechanisms of action, underscoring their therapeutic potential in targeting astrocytes.

Comprehensive assessment of blood-brain barrier opening and sterile inflammatory response: unraveling the therapeutic window

Microbubbles (MBs) combined with focused ultrasound (FUS) has emerged as a promising noninvasive technique to permeabilize the blood-brain barrier (BBB) for drug delivery into the brain. However, the safety and biological consequences of BBB opening (BBBO) remain incompletely understood. This study aims to investigate the effects of two parameters mediating BBBO: microbubble volume dose (MVD) and mechanical index (MI). High-resolution MRI-guided FUS was employed in mouse brains to assess BBBO by manipulating these two parameters. Afterward, the sterile inflammatory response (SIR) was studied 6 h post-FUS treatment. Results demonstrated that both MVD and MI significantly influenced the extent of BBBO, with higher MVD and MI leading to increased permeability. Moreover, RNA sequencing revealed upregulation of major inflammatory pathways and immune cell infiltration after BBBO, indicating the presence and extent of SIR. Gene set enrichment analysis identified 12 gene sets associated with inflammatory responses that were significantly upregulated at higher MVD or MI. A therapeutic window was established between therapeutically relevant BBBO and the onset of SIR, providing operating regimes to avoid damage from stimulation of the NFκB pathway via TNFɑ signaling to apoptosis. These results contribute to the optimization and standardization of BBB opening parameters for safe and effective drug delivery to the brain and further elucidate the underlying molecular mechanisms driving sterile inflammation.

Structural and molecular characterization of astrocyte and vasculature connectivity in the mouse hippocampus and cortex

The relation of astrocytic endfeet to the vasculature plays a key functional role in the neuro-glia-vasculature unit. We characterize the spatial organization of astrocytes and the structural aspects that facilitate their involvement in molecular exchanges. Using double transgenic mice, we performed co-immunostaining, confocal microscopy, and three-dimensional digital segmentation to investigate the biophysical and molecular organization of astrocytes and their intricate endfoot network at the micrometer level in the isocortex and hippocampus. The results showed that hippocampal astrocytes had smaller territories, reduced endfoot dimensions, and fewer contacts with blood vessels compared with those in the isocortex. Additionally, we found that both connexins 43 and 30 have a higher density in the endfoot and the former is overexpressed relative to the latter. However, due to the limitations of the method, further studies are needed to determine the exact localization on the endfoot. The quantitative information obtained in this study will be useful for modeling the interactions of astrocytes with the vasculature.

A pathologic study of Perivascular pTDP-43 Lin bodies in LATE-NC

BACKGROUND

TAR DNA-Binding Protein 43 (TDP-43) pathological inclusions are a distinctive feature in dozens of neurodegenerative pathologies, including limbic-predominant age-related TDP-43 encephalopathy neuropathologic change (LATE-NC). Prior investigations identified vascular-associated TDP-43-positive micro-lesions, known as "Lin bodies," located on or near the brain capillaries of some individuals with LATE-NC. This study aimed to investigate the relationship between the accumulation of Lin bodies and glial cells in LATE-NC and the potential co-localization with ferritin, a protein associated with iron storage. Using multiplexed immunohistochemistry and digital pathology tools, we conducted pathological analyses to investigate the relationship between Lin bodies and glial markers (GFAP for astrocytes, IBA1 for microglia) and ferritin. Analyses were conducted on post-mortem brain tissues collected from individuals with pathologically confirmed Alzheimer's disease neuropathological changes (ADNC) and LATE-NC.

RESULTS

As shown previously, there was a robust association between Lin bodies and GFAP-positive astrocyte processes. Moreover, we also observed Lin bodies frequently co-localizing with ferritin, suggesting a potential link to compromised vascular integrity. Subsequent analyses demonstrated increased astrocytosis near Lin body-positive vessels compared to those without Lin bodies, particularly in ADNC cases. These results suggest that the accumulation of Lin bodies may elicit an increased glial response, particularly among astrocytes, possibly related to impaired vascular integrity.

CONCLUSIONS

Lin bodies are associated with a local reactive glial response. The strong association of Lin bodies with ferritin suggests that the loss of vascular integrity may be either a cause or a consequence of the pTDP-43 pathology. The reactive glia surrounding the affected vessels could further compromise vascular function.

Spatial organization of astrocyte clones: The role of developmental progenitor timing

Astrocytes represent a diverse and morphologically complex group of glial cells critical for shaping and maintaining nervous system homeostasis, as well as responding to injuries. Understanding the origins of astroglial heterogeneity, originated from a limited number of progenitors, has been the focus of many studies. Most of these investigations have centered on protoplasmic and pial astrocytes, while the clonal relationship of fibrous astrocytes or juxtavascular astrocytes has remained relatively unexplored. In this study, we sought to elucidate the morphological diversity and clonal distribution of astrocytes across adult cortical layers, with particular emphasis on their ontogenetic origins. Using the StarTrack lineage tracing tool, we explored the characteristics of adult astroglial clones derived from single and specific progenitors at various embryonic stages. Our results revealed a heterogeneous spatial distribution of astroglial clones, characterized by variations in location, clonal size, and rostro-caudal dispersion. While a considerable proportion of clones were confined within specific cortical layers, others displayed sibling cells crossing layer boundaries. Notably, we observed a correlation between clone location and developmental stage at earlier embryonic stages, although this relationship diminished in later stages. Fibrous astrocyte clones were exclusively confined to the corpus callosum. In contrast, protoplasmic or juxtavascular clones were located in either the upper or lower cortical layers, with certain clones displayed sibling cells distributed across both regions. Our findings underscore the developmental origins and spatial distribution of astroglial clones within cortical layers, providing new insights into the interplay between their morphology, clonal sizes, and progenitor heterogeneity.

Intermittent Metabolic Switching and Vascular Cognitive Impairment

Intermittent fasting (IF), a dietary pattern alternating between eating and fasting periods within a 24-hour cycle, has garnered recognition for its potential to enhance both healthspan and lifespan in animal models and humans. It also shows promise in alleviating age-related diseases, including neurodegeneration. Vascular cognitive impairment (VCI) spans a severity range from mild cognitive deficits to severe cognitive deficits and loss of function in vascular dementia. Chronic cerebral hypoperfusion has emerged as a significant contributor to VCI, instigating vascular pathologies such as microbleeds, blood-brain barrier dysfunction, neuronal loss, and white matter lesions. Preclinical studies in rodents strongly suggest that IF has the potential to attenuate pathological mechanisms, including excitotoxicity, oxidative stress, inflammation, and cell death pathways in VCI models. Hence, this supports evaluating IF in clinical trials for both existing and at-risk VCI patients. This review compiles existing data supporting IF's potential in treating VCI-related vascular and neuronal pathologies, emphasizing the mechanisms by which IF may mitigate these issues. Hence providing a comprehensive overview of the available data supporting IF's potential in treating VCI by emphasizing the underlying mechanisms that make IF a promising intervention for VCI.

Comprehensive Exploration of the Neuroprotective Mechanisms of Ginkgo biloba Leaves in Treating Neurological Disorders

Neurological disorders (NDs) are diseases that seriously affect the health of individuals worldwide, potentially leading to a significant reduction in the quality of life for patients and their families. Herbal medicines have been widely used in the treatment of NDs due to their multi-target and multi-pathway features. Ginkgo biloba leaves (GBLs), one of the most popular herbal medicines in the world, have been demonstrated to present therapeutic effects on NDs. However, the pharmacological mechanisms of GBLs in the treatment of neurological disorders have not been systematically summarized. This study aimed to summarize the molecular mechanism of GBLs in treating NDs from the cell models, animal models, and clinical trials of studies. Four databases, i.e., PubMed, Google Scholar, CNKI, and Web of Science were searched using the following keywords: "Ginkgo biloba", "Ginkgo biloba extract", "Ginkgo biloba leaves", "Ginkgo biloba leaves extract", "Neurological disorders", "Neurological diseases", and "Neurodegenerative diseases". All items meeting the inclusion criteria on the treatment of NDs with GBLs were extracted and summarized. Additionally, PRISMA 2020 was performed to independently evaluate the screening methods. Out of 1385 records in the database, 52 were screened in relation to the function of GBLs in the treatment of NDs; of these 52 records, 39 were preclinical trials and 13 were clinical studies. Analysis of pharmacological studies revealed that GBLs can improve memory, cognition, behavior, and psychopathology of NDs and that the most frequently associated GBLs are depression, followed by Alzheimer's disease, stroke, Huntington's disease, and Parkinson's disease. Additionally, the clinical studies of depression, AD, and stroke are the most common, and most of the remaining ND data are available from in vitro or in vivo animal studies. Moreover, the possible mechanisms of GBLs in treating NDs are mainly through free radical scavenging, anti-oxidant activity, anti-inflammatory response, mitochondrial protection, neurotransmitter regulation, and antagonism of PAF. This is the first paper to systematically and comprehensively investigate the pharmacological effects and neuroprotective mechanisms of GBLs in the treatment of NDs thus far. All findings contribute to a better understanding of the efficacy and complexity of GBLs in treating NDs, which is of great significance for the further clinical application of this herbal medicine.

Brain Targeting Nanomedicines: Pitfalls and Promise

Brain diseases are the most devastating problem among the world's increasingly aging population, and the number of patients with neurological diseases is expected to increase in the future. Although methods for delivering drugs to the brain have advanced significantly, none of these approaches provide satisfactory results for the treatment of brain diseases. This remains a challenge due to the unique anatomy and physiology of the brain, including tight regulation and limited access of substances across the blood-brain barrier. Nanoparticles are considered an ideal drug delivery system to hard-to-reach organs such as the brain. The development of new drugs and new nanomaterial-based brain treatments has opened various opportunities for scientists to develop brain-specific delivery systems that could improve treatment outcomes for patients with brain disorders such as Alzheimer's disease, Parkinson's disease, stroke and brain tumors. In this review, we discuss noteworthy literature that examines recent developments in brain-targeted nanomedicines used in the treatment of neurological diseases.

Blood-brain barrier transporters: An overview of function, dysfunction in Alzheimer's disease and strategies for treatment

The blood-brain-barrier (BBB) has a major function in maintaining brain homeostasis by regulating the entry of molecules from the blood to the brain. Key players in BBB function are BBB transporters which are highly expressed in brain endothelial cells (BECs) and critical in mediating the exchange of nutrients and waste products. BBB transporters can also influence drug delivery into the brain by inhibiting or facilitating the entry of brain targeting therapeutics for the treatment of brain disorders, such as Alzheimer's disease (AD). Recent studies have shown that AD is associated with a disrupted BBB and transporter dysfunction, although their roles in the development in AD are not fully understand. Modulation of BBB transporter activity may pose a novel approach to enhance the delivery of drugs to the brain for enhanced treatment of AD. In this review, we will give an overview of key functions of BBB transporters and known changes in AD. In addition, we will discuss current strategies for transporter modulation for enhanced drug delivery into the brain.

Astrocytic crosstalk with brain and immune cells in healthy and diseased conditions

Astrocytes interact with various cell types, including neurons, vascular cells, microglia, and peripheral immune cells. These interactions are crucial for regulating normal brain functions as well as modulating neuroinflammation in pathological conditions. Recent transcriptomic and proteomic studies have identified critical molecules involved in astrocytic crosstalk with other cells, shedding light on their roles in maintaining brain homeostasis in both healthy and diseased conditions. Astrocytes perform these various roles through either direct or indirect physical associations with neuronal synapses and vasculature. Furthermore, astrocytes can communicate with other immune cells, such as microglia, T cells, and natural killer cells, through secreted molecules during neuroinflammation. In this review, we discuss the critical molecular basis of this astrocytic crosstalk and the underlying mechanisms of astrocyte communication with other cells. We propose that astrocytes function as a central hub in inter-connecting neurons, vasculatures, and immune cells in healthy and diseased brains.

The Neurovascular Unit as a Locus of Injury in Low-Level Blast-Induced Neurotrauma

Blast-induced neurotrauma has received much attention over the past decade. Vascular injury occurs early following blast exposure. Indeed, in animal models that approximate human mild traumatic brain injury or subclinical blast exposure, vascular pathology can occur in the presence of a normal neuropil, suggesting that the vasculature is particularly vulnerable. Brain endothelial cells and their supporting glial and neuronal elements constitute a neurovascular unit (NVU). Blast injury disrupts gliovascular and neurovascular connections in addition to damaging endothelial cells, basal laminae, smooth muscle cells, and pericytes as well as causing extracellular matrix reorganization. Perivascular pathology becomes associated with phospho-tau accumulation and chronic perivascular inflammation. Disruption of the NVU should impact activity-dependent regulation of cerebral blood flow, blood-brain barrier permeability, and glymphatic flow. Here, we review work in an animal model of low-level blast injury that we have been studying for over a decade. We review work supporting the NVU as a locus of low-level blast injury. We integrate our findings with those from other laboratories studying similar models that collectively suggest that damage to astrocytes and other perivascular cells as well as chronic immune activation play a role in the persistent neurobehavioral changes that follow blast injury.

Alzheimer's disease brain endothelial-like cells reveal differential drug transporter expression and modulation by potentially therapeutic focused ultrasound

The blood-brain barrier (BBB) has a key function in maintaining homeostasis in the brain, partly modulated by transporters, which are highly expressed in brain endothelial cells (BECs). Transporters mediate the uptake or efflux of compounds to and from the brain and they can also challenge the delivery of drugs for the treatment of Alzheimer's disease (AD). Currently there is a limited understanding of changes in BBB transporters in AD. To investigate this, we generated brain endothelial-like cells (iBECs) from induced pluripotent stem cells (iPSCs) with familial AD (FAD) Presenilin 1 (PSEN1) mutation and identified AD-specific differences in transporter expression compared to control (ctrl) iBECs. We first characterized the expression levels of 12 BBB transporters in AD-, Ctrl-, and isogenic (PSEN1 corrected) iBECs to identify any AD specific differences. We then exposed the cells to focused ultrasound (FUS) in the absence (FUSonly) or presence of microbubbles (MB) (FUS+MB), which is a novel therapeutic method that can be used to transiently open the BBB to increase drug delivery into the brain, however its effects on BBB transporter expression are largely unknown. Following FUSonly and FUS+MB, we investigated whether the expression or activity of key transporters could be modulated. Our findings demonstrate that PSEN1 mutant FAD (PSEN1AD) possess phenotypical differences compared to control iBECs in BBB transporter expression and function. Additionally, we show that FUSonly and FUS+MB can modulate BBB transporter expression and functional activity in iBECs, having potential implications on drug penetration and amyloid clearance. These findings highlight the differential responses of patient cells to FUS treatment, with patient-derived models likely providing an important tool for modelling therapeutic effects of FUS.

Astrocyte-Neuron Interactions in Substance Use Disorders

Engagement of astrocytes within the brain's reward circuitry has been apparent for approximately 30 years, when noncontingent drug administration was observed to lead to cytological markers of reactive astrocytes. Since that time, advanced approaches in rodent behavior and astrocyte monitoring have revealed complex interactions between astrocytes with drug type, animal sex, brain region, and dose and duration of drug administration. A number of studies now collectively reveal that rodent drug self-administration followed by prolonged abstinence results in decreased features of structure and synaptic colocalization of astrocytes. In addition, stimulation of astrocytes in the nucleus accumbens with DREADD receptors or pharmacological compounds opposes drug-seeking behavior. These findings provide a clear path for ongoing investigation into astrocytes as mediators of drug action in the brain and underscore the potential therapeutic utility of astrocytes in the regulation of drug craving and relapse vulnerability.

Comprehensive Assessment of Blood-Brain Barrier Opening and Sterile Inflammatory Response: Unraveling the Therapeutic Window

Microbubbles (MBs) combined with focused ultrasound (FUS) have emerged as a promising noninvasive technique to permeabilize the blood-brain barrier (BBB) for drug delivery to the brain. However, the safety and biological consequences of BBB opening remain incompletely understood. This study investigates the effects of varying microbubble volume doses (MVD) and ultrasound mechanical indices (MI) on BBB opening and the sterile inflammatory response (SIR) using high-resolution ultra-high field MRI-guided FUS in mouse brains. The results demonstrate that both MVD and MI significantly influence the extent of BBB opening, with higher doses and mechanical indices leading to increased permeability. Moreover, RNA sequencing reveals upregulated inflammatory pathways and immune cell infiltration after BBB opening, suggesting the presence and extent of SIR. Gene set enrichment analysis identifies 12 gene sets associated with inflammatory responses that are upregulated at higher doses of MVD or MI. A therapeutic window is established between significant BBB opening and the onset of SIR, providing operating regimes for avoiding each three classes of increasing damage from stimulation of the NFκB pathway via TNFL signaling to apoptosis. This study contributes to the optimization and standardization of BBB opening parameters for safe and effective drug delivery to the brain and sheds light on the underlying molecular mechanisms of the sterile inflammatory response.

Significance Statement

The significance of this study lies in its comprehensive investigation of microbubble-facilitated focused ultrasound for blood-brain barrier (BBB) opening. By systematically exploring various combinations of microbubble volume doses and ultrasound mechanical indices, the study reveals their direct impact on the extent of BBB permeability and the induction of sterile inflammatory response (SIR). The establishment of a therapeutic window between significant BBB opening and the onset of SIR provides critical insights for safe and targeted drug delivery to the brain. These findings advance our understanding of the biological consequences of BBB opening and contribute to optimizing parameters for clinical applications, thus minimizing potential health risks, and maximizing the therapeutic potential of this technique.

From static to dynamic: live observation of the support system after ischemic stroke by two photon-excited fluorescence laser-scanning microscopy

Ischemic stroke is one of the most common causes of mortality and disability worldwide. However, treatment efficacy and the progress of research remain unsatisfactory. As the critical support system and essential components in neurovascular units, glial cells and blood vessels (including the blood-brain barrier) together maintain an optimal microenvironment for neuronal function. They provide nutrients, regulate neuronal excitability, and prevent harmful substances from entering brain tissue. The highly dynamic networks of this support system play an essential role in ischemic stroke through processes including brain homeostasis, supporting neuronal function, and reacting to injuries. However, most studies have focused on postmortem animals, which inevitably lack critical information about the dynamic changes that occur after ischemic stroke. Therefore, a high-precision technique for research in living animals is urgently needed. Two-photon fluorescence laser-scanning microscopy is a powerful imaging technique that can facilitate live imaging at high spatiotemporal resolutions. Two-photon fluorescence laser-scanning microscopy can provide images of the whole-cortex vascular 3D structure, information on multicellular component interactions, and provide images of structure and function in the cranial window. This technique shifts the existing research paradigm from static to dynamic, from flat to stereoscopic, and from single-cell function to multicellular intercommunication, thus providing direct and reliable evidence to identify the pathophysiological mechanisms following ischemic stroke in an intact brain. In this review, we discuss exciting findings from research on the support system after ischemic stroke using two-photon fluorescence laser-scanning microscopy, highlighting the importance of dynamic observations of cellular behavior and interactions in the networks of the brain's support systems. We show the excellent application prospects and advantages of two-photon fluorescence laser-scanning microscopy and predict future research developments and directions in the study of ischemic stroke.

Astroglial Hmgb1 regulates postnatal astrocyte morphogenesis and cerebrovascular maturation

Astrocytes are intimately linked with brain blood vessels, an essential relationship for neuronal function. However, astroglial factors driving these physical and functional associations during postnatal brain development have yet to be identified. By characterizing structural and transcriptional changes in mouse cortical astrocytes during the first two postnatal weeks, we find that high-mobility group box 1 (Hmgb1), normally upregulated with injury and involved in adult cerebrovascular repair, is highly expressed in astrocytes at birth and then decreases rapidly. Astrocyte-selective ablation of Hmgb1 at birth affects astrocyte morphology and endfoot placement, alters distribution of endfoot proteins connexin43 and aquaporin-4, induces transcriptional changes in astrocytes related to cytoskeleton remodeling, and profoundly disrupts endothelial ultrastructure. While lack of astroglial Hmgb1 does not affect the blood-brain barrier or angiogenesis postnatally, it impairs neurovascular coupling and behavior in adult mice. These findings identify astroglial Hmgb1 as an important player in postnatal gliovascular maturation.

Biological agents and the aging brain: glial inflammation and neurotoxic signaling

Neuroinflammation is a universal characteristic of brain aging and neurological disorders, irrespective of the disease state. Glial inflammation mediates this signaling, through astrocyte and microglial polarization from neuroprotective to neurotoxic phenotypes. Glial reactivity results in the loss of homeostasis, as these cells no longer provide support to neurons, in addition to the production of chronically toxic pro-inflammatory mediators. These glial changes initiate an inflammatory brain state that injures the central nervous system (CNS) over time. As the brain ages, glia are altered, including increased glial cell numbers, morphological changes, and either a pre-disposition or inability to become reactive. These alterations induce age-related neuropathologies, ultimately leading to neuronal degradation and irreversible damage associated with disorders of the aged brain, including Alzheimer's Disease (AD) and other related diseases. While the complex interactions of these glial cells and the brain are well studied, the role additional stressors, such as infectious agents, play on age-related neuropathology has not been fully elucidated. Both biological agents in the periphery, such as bacterial infections, or in the CNS, including viral infections like SARS-CoV-2, push glia into neuroinflammatory phenotypes that can exacerbate pathology within the aging brain. These biological agents release pattern associated molecular patterns (PAMPs) that bind to pattern recognition receptors (PRRs) on glial cells, beginning an inflammatory cascade. In this review, we will summarize the evidence that biological agents induce reactive glia, which worsens age-related neuropathology.

Retinal astrocyte morphology predicts integration of vascular and neuronal architecture

Astrocytes are important regulators of blood flow and play a key role in the response to injury and disease in the central nervous system (CNS). Despite having an understanding that structural changes to these cells have consequences for local neurovascular physiology, individual astrocyte morphology remains largely unexplored in the retina. Here, we used MORF3 mice to capture full membranous morphology for over fifteen hundred individual astrocytes in the mouse retina, a highly metabolically active component of the CNS. We demonstrate that retinal astrocytes have been misrepresented as stellate in morphology due to marker use like GFAP and S100β which underestimates cell complexity. We also find that astrocytes contain recurring morphological motifs which are predictive of the underlying neurovascular architecture of the inner retina and suggestive of function. These motifs predict fine sampling and integration of retinal ganglion cell electrical activity with consequences for blood flow regulation. Additionally, our data shows that astrocytes participate in neurovascular interactions to a much greater degree than currently reported. 100% of cells contact the vasculature through one of three mutually exclusive classes of connections. Similarly, 100% of cells contact some neuronal element, be it an RGC axon or soma. Finally, we report that astrocyte morphology depends on retinal eccentricity, with cells appearing compressed near the nerve head and in the periphery. These results reveal a large degree of astrocyte morphological complexity that informs their contribution to neurovascular coupling in the retina.

Human hippocampal astrocytes: Computational dissection of their transcriptome, sexual differences and exosomes across ageing and mild-cognitive impairment

The role of astrocytes in Alzheimer's disease is often disregarded. Hence, characterization of astrocytes along their early evolution toward Alzheimer would be greatly beneficial. However, due to their exquisite responsiveness, in vivo studies are difficult. So public microarray data of hippocampal homogenates from (healthy) young, (healthy) elder and elder with mild cognitive impairment (MCI) were subjected to re-analysis by a multi-step computational pipeline. Ontologies and pathway analyses were compared after determining the differential genes that, belonging to astrocytes, have splice forms. Likewise, the subset of molecules exportable to exosomes was also determined. The results showed that astrocyte's phenotypes changed significantly. While already 'activated' astrocytes were found in the younger group, major changes occurred during ageing (increased vascular remodelling and response to mechanical stimulus, diminished long-term potentiation and increased long-term depression). MCI's astrocytes showed some 'rejuvenated' features, but their sensitivity to shear stress was markedly lost. Importantly, most of the changes showed to be sex biassed. Men's astrocytes are enriched in a type 'endfeet-astrocytome', whereas women's astrocytes appear close to the 'scar-forming' type (prone to endothelial dysfunction, hypercholesterolemia, loss of glutamatergic synapses, Ca+2 dysregulation, hypoxia, oxidative stress and 'pro-coagulant' phenotype). In conclusion, the computational dissection of the networks based on the hippocampal gene isoforms provides a relevant proxy to in vivo astrocytes, also revealing the occurrence of sexual differences. Analyses of the astrocytic exosomes did not provide an acceptable approximation to the overall functioning of astrocytes in the hippocampus, probably due to the selective cellular mechanisms which charge the cargo molecules.

Astrocyte Endfeet in Brain Function and Pathology: Open Questions

Astrocyte endfeet enwrap the entire vascular tree within the central nervous system, where they perform important functions in regulating the blood-brain barrier (BBB), cerebral blood flow, nutrient uptake, and waste clearance. Accordingly, astrocyte endfeet contain specialized organelles and proteins, including local protein translation machinery and highly organized scaffold proteins, which anchor channels, transporters, receptors, and enzymes critical for astrocyte-vascular interactions. Many neurological diseases are characterized by the loss of polarization of specific endfoot proteins, vascular dysregulation, BBB disruption, altered waste clearance, or, in extreme cases, loss of endfoot coverage. A role for astrocyte endfeet has been demonstrated or postulated in many of these conditions. This review provides an overview of the development, composition, function, and pathological changes of astrocyte endfeet and highlights the gaps in our knowledge that future research should address.

Microglia and the Blood-Brain Barrier: An External Player in Acute and Chronic Neuroinflammatory Conditions

Microglia are the resident immune cells of the central nervous system that guarantee immune surveillance and exert also a modulating role on neuronal synaptic development and function. Upon injury, microglia get activated and modify their morphology acquiring an ameboid phenotype and pro- or anti-inflammatory features. The active role of microglia in blood-brain barrier (BBB) function and their interaction with different cellular components of the BBB-endothelial cells, astrocytes and pericytes-are described. Here, we report the specific crosstalk of microglia with all the BBB cell types focusing in particular on the involvement of microglia in the modulation of BBB function in neuroinflammatory conditions that occur in conjunction with an acute event, such as a stroke, or in a slow neurodegenerative disease, such as Alzheimer's disease. The potential of microglia to exert a dual role, either protective or detrimental, depending on disease stages and environmental conditioning factors is also discussed.

LPS-Induced Systemic Inflammation Affects the Dynamic Interactions of Astrocytes and Microglia with the Vasculature of the Mouse Brain Cortex

The Neurovascular Unit (NVU), composed of glia (astrocytes, oligodendrocytes, microglia), neurons, pericytes and endothelial cells, is a dynamic interface ensuring the physiological functioning of the central nervous system (CNS), which gets affected and contributes to the pathology of several neurodegenerative diseases. Neuroinflammation is a common feature of neurodegenerative diseases and is primarily related to the activation state of perivascular microglia and astrocytes, which constitute two of its major cellular components. Our studies focus on monitoring in real time the morphological changes of perivascular astrocytes and microglia, as well as their dynamic interactions with the brain vasculature, under physiological conditions and following systemic neuroinflammation triggering both microgliosis and astrogliosis. To this end, we performed 2-photon laser scanning microscopy (2P-LSM) for intravital imaging of the cortex of transgenic mice visualizing the dynamics of microglia and astroglia following neuroinflammation induced by systemic administration of the endotoxin lipopolysaccharide (LPS). Our results indicate that following neuroinflammation the endfeet of activated perivascular astrocytes lose their close proximity and physiological cross-talk with vasculature, an event that most possibly contributes to a loss of blood-brain barrier (BBB) integrity. At the same time, microglial cells become activated and exhibit a higher extent of physical contact with the blood vessels. These dynamic responses of perivascular astrocytes and microglia are peaking at 4 days following LPS administration; however, they still persist at a lower level at 8 days after LPS injection, revealing incomplete reversal of inflammation affecting the glial properties and interactions within the NVU.

Estimates of the permeability of extra-cellular pathways through the astrocyte endfoot sheath

BACKGROUND

Astrocyte endfoot processes are believed to cover all micro-vessels in the brain cortex and may play a significant role in fluid and substance transport into and out of the brain parenchyma. Detailed fluid mechanical models of diffusive and advective transport in the brain are promising tools to investigate theories of transport.

METHODS

We derive theoretical estimates of astrocyte endfoot sheath permeability for advective and diffusive transport and its variation in microvascular networks from mouse brain cortex. The networks are based on recently published experimental data and generated endfoot patterns are based on Voronoi tessellations of the perivascular surface. We estimate corrections for projection errors in previously published data.

RESULTS

We provide structural-functional relationships between vessel radius and resistance that can be directly used in flow and transport simulations. We estimate endfoot sheath filtration coefficients in the range [Formula: see text] to [Formula: see text], diffusion membrane coefficients for small solutes in the range [Formula: see text] to [Formula: see text], and gap area fractions in the range 0.2-0.6%, based on a inter-endfoot gap width of 20 nm.

CONCLUSIONS

The astrocyte endfoot sheath surrounding microvessels forms a secondary barrier to extra-cellular transport, separating the extra-cellular space of the parenchyma and the perivascular space outside the endothelial layer. The filtration and membrane diffusion coefficients of the endfoot sheath are estimated to be an order of magnitude lower than those of the extra-cellular matrix while being two orders of magnitude higher than those of the vessel wall.

Anti-DEspR antibody treatment improves survival and reduces neurologic deficits in a hypertensive, spontaneous intracerebral hemorrhage (hsICH) rat model

Progressive secondary brain injury-induced by dysregulated neuroinflammation in spontaneous intracerebral hemorrhage (sICH)-underlies high sICH-mortality and remains without FDA-approved pharmacotherapy. Clinical insight that hematoma-directed interventions do not improve mortality prioritizes resolving acute secondary brain injury in sICH. As neutrophils are implicated in sICH secondary brain injury, we tested whether inhibition of a rogue neutrophil-subset expressing the dual endothelin-1/signal peptide receptor (DEspR) and associated with secondary tissue injury, DEspR+ CD11b+ immunotype, will attenuate mortality in a hypertensive-sICH (hsICH) rat model. We confirmed sICH-related deaths in hsICH-rats by T2*-weighted 9.4 T MRI and DEspR+ neutrophils in hsICH-rat brain perihematomal areas by immunostaining. At acute sICH, anti-DEspR muIgG1-antibody, mu10a3, treatment increased median survival in hsICH rats vs controls (p < 0.0001). In pre-stroke sICH, weekly 10a3-treatment did not predispose to infection and delayed sICH-onset vs controls (p < 0.0001). As potential sICH-therapeutic, we tested humanized anti-DEspR IgG4S228P-mAb, hu6g8. In vitro, hu6g8 reversed delayed-apoptosis in DEspR+ CD11b+ neutrophils. In vivo, hu6g8 increased median survival and reduced neurologic symptoms in male/female hsICH-rats vs controls (p < 0.0001). Altogether, preclinical efficacy of inhibition of DEspR+ CD11b+ neutrophils in acute sICH-without infection complications, supports the potential of anti-DEspR therapy in sICH. Data provide basis for clinical study of DEspR+ CD11b+ neutrophil-subset in sICH patients.

HIV Nef Expression Down-modulated GFAP Expression and Altered Glutamate Uptake and Release and Proliferation in Astrocytes

HIV infection of astrocytes leads to restricted gene expression and replication but abundant expression of HIV early genes Tat, Nef and Rev. A great deal of neuroHIV research has so far been focused on Tat protein, its effects on astrocytes, and its roles in neuroHIV. In the current study, we aimed to determine effects of Nef expression on astrocytes and their function. Using transfection or infection of VSVG-pseudotyped HIV viruses, we showed that Nef expression down-modulated glial fibrillary acidic protein (GFAP) expression. We then showed that Nef expression also led to decreased GFAP mRNA expression. The transcriptional regulation was further confirmed using a GFAP promoter-driven reporter gene assay. We performed transcription factor profiling array to compare the expression of transcription factors between Nef-intact and Nef-deficient HIV-infected cells and identified eight transcription factors with expression changes of 1.5-fold or higher: three up-regulated by Nef (Stat1, Stat5, and TFIID), and five down-regulated by Nef (AR, GAS/ISRE, HIF, Sp1, and p53). We then demonstrated that removal of the Sp1 binding sites from the GFAP promoter resulted in a much lower level of the promoter activity and reversal of Nef effects on the GFAP promoter, confirming important roles of Sp1 in the GFAP promoter activity and for Nef-induced GFAP expression. Lastly, we showed that Nef expression led to increased glutamate uptake and decreased glutamate release by astrocytes and increased astrocyte proliferation. Taken together, these results indicate that Nef leads to down-modulation of GFAP expression and alteration of glutamate metabolism in astrocytes, and astrocyte proliferation and could be an important contributor to neuroHIV.

Astrocyte endfoot formation controls the termination of oligodendrocyte precursor cell perivascular migration during development

Oligodendrocyte precursor cells (OPCs) undergo an extensive and coordinated migration in the developing CNS, using the pre-formed scaffold of developed blood vessels as their physical substrate for migration. While OPC association with vasculature is critical for dispersal, equally important for permitting differentiation and proper myelination of target axons is their appropriate and timely detachment, but regulation of this process remains unclear. Here we demonstrate a correlation between the developmental formation of astrocytic endfeet on vessels and the termination of OPC perivascular migration. Ex vivo and in vivo live imaging shows that astrocyte endfeet physically displace OPCs from vasculature, and genetic abrogation of endfoot formation hinders both OPC detachment from vessels and subsequent differentiation. Astrocyte-derived semaphorins 3a and 6a act to repel OPCs from blood vessels at the cessation of their perivascular migration and, in so doing, permit subsequent OPC differentiation by insulating them from a maturation inhibitory endothelial niche.

Astrocytic Neuroimmunological Roles Interacting with Microglial Cells in Neurodegenerative Diseases

Both astrocytic and microglial functions have been extensively investigated in healthy subjects and neurodegenerative diseases. For astrocytes, not only various sub-types were identified but phagocytic activity was also clarified recently and is making dramatic progress. In this review paper, we mostly focus on the functional role of astrocytes in the extracellular matrix and on interactions between reactive astrocytes and reactive microglia in normal states and in neurodegenerative diseases, because the authors feel it is necessary to elucidate the mechanisms among activated glial cells in the pathology of neurological diseases in order to pave the way for drug discovery. Finally, we will review cyclic phosphatidic acid (cPA), a naturally occurring phospholipid mediator that induces a variety of biological activities in the brain both in vivo and in vitro. We propose that cPA may serve as a novel therapeutic molecule for the treatment of brain injury and neuroinflammation.

Blood vessel remodeling in the cerebral cortex induced by binge alcohol intake in mice

Ethanol is toxic to the brain and causes various neurological disorders. Although ethanol can directly exert toxicity on neurons, it also acts on other cell types in the central nervous system. Blood vessel endothelial cells interact with, and are affected by blood ethanol. However, the effects of ethanol on the vascular structures of the brain have not been well documented. In this study, we examined the effects of binge levels of ethanol on brain vasculature. Immunostaining analysis indicated structural alterations of blood vessels in the cerebral cortex, which became more tortuous than those in the control mice after ethanol administration. The interaction between the blood vessels and astrocytes decreased, especially in the upper layers of the cerebral cortex. Messenger RNA expression analysis revealed a unique downregulation of Vegfa mRNA encoding vascular endothelial growth factor (VEGF)-A among VEGF, angiopoietin, endothelin family angiogenic and blood vessel remodeling factors. The expression of three proteoglycan core proteins, glypican-5, neurocan, and serglycin, was also altered after ethanol administration. Thus, binge levels of ethanol affect the expression of VEGF-A and blood vessel-supporting proteoglycans, resulting in changes in the vascular structure of the cerebral cortex.

Supplementary Information

The online version contains supplementary material available at 10.1007/s43188-022-00164-y.

Reactive astrocytes transduce inflammation in a blood-brain barrier model through a TNF-STAT3 signaling axis and secretion of alpha 1-antichymotrypsin

Astrocytes are critical components of the neurovascular unit that support blood-brain barrier (BBB) function. Pathological transformation of astrocytes to reactive states can be protective or harmful to BBB function. Here, using a human induced pluripotent stem cell (iPSC)-derived BBB co-culture model, we show that tumor necrosis factor (TNF) transitions astrocytes to an inflammatory reactive state that causes BBB dysfunction through activation of STAT3 and increased expression of SERPINA3, which encodes alpha 1-antichymotrypsin (α1ACT). To contextualize these findings, we correlated astrocytic STAT3 activation to vascular inflammation in postmortem human tissue. Further, in murine brain organotypic cultures, astrocyte-specific silencing of Serpina3n reduced vascular inflammation after TNF challenge. Last, treatment with recombinant Serpina3n in both ex vivo explant cultures and in vivo was sufficient to induce BBB dysfunction-related molecular changes. Overall, our results define the TNF-STAT3-α1ACT signaling axis as a driver of an inflammatory reactive astrocyte signature that contributes to BBB dysfunction.

In vivo spatiotemporal dynamics of astrocyte reactivity following neural electrode implantation

Brain computer interfaces (BCIs), including penetrating microelectrode arrays, enable both recording and stimulation of neural cells. However, device implantation inevitably causes injury to brain tissue and induces a foreign body response, leading to reduced recording performance and stimulation efficacy. Astrocytes in the healthy brain play multiple roles including regulating energy metabolism, homeostatic balance, transmission of neural signals, and neurovascular coupling. Following an insult to the brain, they are activated and gather around the site of injury. These reactive astrocytes have been regarded as one of the main contributors to the formation of a glial scar which affects the performance of microelectrode arrays. This study investigates the dynamics of astrocytes within the first 2 weeks after implantation of an intracortical microelectrode into the mouse brain using two-photon microscopy. From our observation astrocytes are highly dynamic during this period, exhibiting patterns of process extension, soma migration, morphological activation, and device encapsulation that are spatiotemporally distinct from other glial cells, such as microglia or oligodendrocyte precursor cells. This detailed characterization of astrocyte reactivity will help to better understand the tissue response to intracortical devices and lead to the development of more effective intervention strategies to improve the functional performance of neural interfacing technology.

Targetability of the neurovascular unit in inflammatory diseases of the central nervous system

The blood-brain barrier (BBB) is a selectively permeable barrier separating the periphery from the central nervous system (CNS). The BBB restricts the flow of most material into and out of the CNS, including many drugs that could be used as potent therapies. BBB permeability is modulated by several cells that are collectively called the neurovascular unit (NVU). The NVU consists of specialized CNS endothelial cells (ECs), pericytes, astrocytes, microglia, and neurons. CNS ECs maintain a complex "seal" via tight junctions, forming the BBB; breakdown of these tight junctions leads to BBB disruption. Pericytes control the vascular flow within capillaries and help maintain the basal lamina. Astrocytes control much of the flow of material that has moved beyond the CNS EC layer and can form a secondary barrier under inflammatory conditions. Microglia survey the border of the NVU for noxious material. Neuronal activity also plays a role in the maintenance of the BBB. Since astrocytes, pericytes, microglia, and neurons are all able to modulate the permeability of the BBB, understating the complex contributions of each member of the NVU will potentially uncover novel and effective methods for delivery of neurotherapies to the CNS.

Cocaine's cerebrovascular vasoconstriction is associated with astrocytic Ca2+ increase in mice

Human and animal studies have reported widespread reductions in cerebral blood flow associated with chronic cocaine exposures. However, the molecular and cellular mechanisms underlying cerebral blood flow reductions are not well understood. Here, by combining a multimodal imaging platform with a genetically encoded calcium indicator, we simultaneously measured the effects of acute cocaine on neuronal and astrocytic activity, tissue oxygenation, hemodynamics and vascular diameter changes in the mouse cerebral cortex. Our results showed that cocaine constricted blood vessels (measured by vessel diameter Φ changes), decreasing cerebral total blood volume (HbT) and temporally reducing tissue oxygenation. Cellular imaging showed that the mean astrocytic Ca2+ dependent fluorescence [Formula: see text] increase in response to cocaine was weaker but longer lasting than the mean neuronal Ca2+ dependent fluorescence [Formula: see text] changes. Interestingly, while cocaine-induced [Formula: see text] increase was temporally correlated with tissue oxygenation change, the [Formula: see text] elevation after cocaine was in temporal correspondence with the long-lasting decrease in arterial blood volumes. To determine whether the temporal association between astrocytic activation and cocaine induced vasoconstriction reflected a causal association we inhibited astrocytic Ca2+ using GFAP-DREADD(Gi). Inhibition of astrocytes attenuated the vasoconstriction resulting from cocaine, providing evidence that astrocytes play a critical role in cocaine's vasoconstrictive effects in the brain. These results indicate that neurons and astrocytes play different roles in mediating neurovascular coupling in response to cocaine. Our findings implicate neuronal activation as the main driver of the short-lasting reduction in tissue oxygenation and astrocyte long-lasting activation as the driver of the persistent vasoconstriction with cocaine. Understanding the cellular and vascular interaction induced by cocaine will be helpful for future putative treatments to reduce cerebrovascular pathology from cocaine use.

Critical Role of Neuronal Vps35 in Blood Vessel Branching and Maturation in Developing Mouse Brain

Vps35 (vacuolar protein sorting 35), a key component of retromer, plays a crucial role in selective retrieval of transmembrane proteins from endosomes to trans-Golgi networks. Dysfunctional Vps35/retromer is a risk factor for the development of neurodegenerative diseases. Vps35 is highly expressed in developing pyramidal neurons, both in the mouse neocortex and hippocampus, Although embryonic neuronal Vps35’s function in promoting neuronal terminal differentiation and survival is evident, it remains unclear whether and how neuronal Vps35 communicates with other types of brain cells, such as blood vessels (BVs), which are essential for supplying nutrients to neurons. Dysfunctional BVs contribute to the pathogenesis of various neurodegenerative disorders. Here, we provide evidence for embryonic neuronal Vps35 as critical for BV branching and maturation in the developing mouse brain. Selectively knocking out (KO) Vps35 in mouse embryonic, not postnatal, neurons results in reductions in BV branching and density, arteriole diameter, and BV-associated pericytes and microglia but an increase in BV-associated reactive astrocytes. Deletion of microglia by PLX3397 enhances these BV deficits in mutant mice. These results reveal the function of neuronal Vps35 in neurovascular coupling in the developing mouse brain and implicate BV-associated microglia as underlying this event.

Signaling Pathways in Neurovascular Development

During development, the central nervous system (CNS) vasculature grows to precisely meet the metabolic demands of neurons and glia. In addition, the vast majority of the CNS vasculature acquires a unique set of molecular and cellular properties-collectively referred to as the blood-brain barrier-that minimize passive diffusion of molecules between the blood and the CNS parenchyma. Both of these processes are controlled by signals emanating from neurons and glia. In this review, we describe the nature and mechanisms-of-action of these signals, with an emphasis on vascular endothelial growth factor (VEGF) and beta-catenin (canonical Wnt) signaling, the two best-understood systems that regulate CNS vascular development. We highlight foundational discoveries, interactions between different signaling systems, the integration of genetic and cell biological studies, advances that are of clinical relevance, and questions for future research.

Breaking through the barrier: Modelling and exploiting the physical microenvironment to enhance drug transport and efficacy

Pharmaceutical compounds are the main pillar in the treatment of various illnesses. To administer these drugs in the therapeutic setting, multiple routes of administration have been defined, including ingestion, inhalation, and injection. After administration, drugs need to find their way to the intended target for high effectiveness, and this penetration is greatly dependent on obstacles the drugs encounter along their path. Key hurdles include the physical barriers that are present within the body and knowledge of those is indispensable for progress in the development of drugs with increased therapeutic efficacy. In this review, we examine several important physical barriers, such as the blood-brain barrier, the gut-mucosal barrier, and the extracellular matrix barrier, and evaluate their influence on drug transport and efficacy. We explore various in vitro model systems that aid in understanding how parameters within the barrier model affect drug transfer and therapeutic effect. We conclude that physical barriers in the body restrict the quantity of drugs that can pass through, mainly as a consequence of the barrier architecture. In addition, the specific physical properties of the tissue can trigger intracellular changes, altering cell behavior in response to drugs. Though the barriers negatively influence drug distribution, physical stimulation of the surrounding environment may also be exploited as a mechanism to control drug release. This drug delivery approach is explored in this review as a potential alternative to the conventional ways of delivering therapeutics.

Vascularizing the brain in vitro

The brain is arguably the most fascinating and complex organ in the human body. Recreating the brain in vitro is an ambition restricted by our limited understanding of its structure and interacting elements. One of these interacting parts, the brain microvasculature, is distinguished by a highly selective barrier known as the blood-brain barrier (BBB), limiting the transport of substances between the blood and the nervous system. Numerous in vitro models have been used to mimic the BBB and constructed by implementing a variety of microfabrication and microfluidic techniques. However, currently available models still cannot accurately imitate the in vivo characteristics of BBB. In this article, we review recent BBB models by analyzing each parameter affecting the accuracy of these models. Furthermore, we propose an investigation of the synergy between BBB models and neuronal tissue biofabrication, which results in more advanced models, including neurovascular unit microfluidic models and vascularized brain organoid-based models.

Transcranial Direct Current Stimulation Alleviates Neurovascular Unit Dysfunction in Mice With Preclinical Alzheimer’s Disease

Neurons, glial cells and blood vessels are collectively referred to as the neurovascular unit (NVU). In the Alzheimer’s disease (AD) brain, the main components of the NVU undergo pathological changes. Transcranial direct current stimulation (tDCS) can protect neurons, induce changes in glial cells, regulate cerebral blood flow, and exert long-term neuroprotection. However, the mechanism by which tDCS improves NVU function is unclear. In this study, we explored the effect of tDCS on the NVU in mice with preclinical AD and the related mechanisms. 10 sessions of tDCS were given to six-month-old male APP/PS1 mice in the preclinical stage. The model group, sham stimulation group, and control group were made up of APP/PS1 mice and C57 mice of the same age. All mice were histologically evaluated two months after receiving tDCS. Protein content was measured using Western blotting and an enzyme-linked immunosorbent assay (ELISA). The link between glial cells and blood vessels was studied using immunofluorescence staining and lectin staining. The results showed that tDCS affected the metabolism of Aβ; the levels of Aβ, amyloid precursor protein (APP) and BACE1 were significantly reduced, and the levels of ADAM10 were significantly increased in the frontal cortex and hippocampus in the stimulation group. In the stimulation group, tDCS reduced the protein levels of Iba1 and GFAP and increased the protein levels of NeuN, LRP1 and PDGRFβ. This suggests that tDCS can improve NVU function in APP/PS1 mice in the preclinical stage. Increased blood vessel density and blood vessel length, decreased IgG extravasation, and increased the protein levels of occludin and coverage of astrocyte foot processes with blood vessels suggested that tDCS had a protective effect on the blood-brain barrier. Furthermore, the increased numbers of Vimentin, S100 expression and blood vessels (lectin-positive) around Aβ indicated that the effect of tDCS was mediated by astrocytes and blood vessels. There was no significant difference in these parameters between the model group and the sham stimulation group. In conclusion, our results show that tDCS can improve NVU function in APP/PS1 mice in the preclinical stage, providing further support for the use of tDCS as a treatment for AD.

Public Volume Electron Microscopy Data: An Essential Resource to Study the Brain Microvasculature

Electron microscopy is the primary approach to study ultrastructural features of the cerebrovasculature. However, 2D snapshots of a vascular bed capture only a small fraction of its complexity. Recent efforts to synaptically map neuronal circuitry using volume electron microscopy have also sampled the brain microvasculature in 3D. Here, we perform a meta-analysis of 7 data sets spanning different species and brain regions, including two data sets from the MICrONS consortium that have made efforts to segment vasculature in addition to all parenchymal cell types in mouse visual cortex. Exploration of these data have revealed rich information for detailed investigation of the cerebrovasculature. Neurovascular unit cell types (including, but not limited to, endothelial cells, mural cells, perivascular fibroblasts, microglia, and astrocytes) could be discerned across broad microvascular zones. Image contrast was sufficient to identify subcellular details, including endothelial junctions, caveolae, peg-and-socket interactions, mitochondria, Golgi cisternae, microvilli and other cellular protrusions of potential significance to vascular signaling. Additionally, non-cellular structures including the basement membrane and perivascular spaces were visible and could be traced between arterio-venous zones along the vascular wall. These explorations revealed structural features that may be important for vascular functions, such as blood-brain barrier integrity, blood flow control, brain clearance, and bioenergetics. They also identified limitations where accuracy and consistency of segmentation could be further honed by future efforts. The purpose of this article is to introduce these valuable community resources within the framework of cerebrovascular research. We do so by providing an assessment of their vascular contents, identifying features of significance for further study, and discussing next step ideas for refining vascular segmentation and analysis.

Astrocyte plasticity in mice ensures continued endfoot coverage of cerebral blood vessels following injury and declines with age

Astrocytes extend endfeet that enwrap the vasculature, and disruptions to this association which may occur in disease coincide with breaches in blood-brain barrier (BBB) integrity. Here we investigate if focal ablation of astrocytes is sufficient to disrupt the BBB in mice. Targeted two-photon chemical apoptotic ablation of astrocytes induced a plasticity response whereby surrounding astrocytes extended processes to cover vascular vacancies. In young animals, replacement processes occur in advance of endfoot retraction, but this is delayed in aged animals. Stimulation of replacement astrocytes results in constriction of pre-capillary arterioles, suggesting that replacement astrocytes are functional. Pharmacological inhibition of pSTAT3, as well as astrocyte specific deletion of pSTAT3, reduces astrocyte replacement post-ablation, without perturbations to BBB integrity. Similar endfoot replacement occurs following astrocyte cell death due to reperfusion in a stroke model. Together, these studies uncover the ability of astrocytes to maintain cerebrovascular coverage via substitution from nearby cells.

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 Blood-Brain Barrier, an Evolving Concept Based on Technological Advances and Cell-Cell Communications

The construction of the blood–brain barrier (BBB), which is a natural barrier for maintaining brain homeostasis, is the result of a meticulous organisation in space and time of cell–cell communication processes between the endothelial cells that carry the BBB phenotype, the brain pericytes, the glial cells (mainly the astrocytes), and the neurons. The importance of these communications for the establishment, maturation and maintenance of this unique phenotype had already been suggested in the pioneering work to identify and demonstrate the BBB. As for the history of the BBB, the evolution of analytical techniques has allowed knowledge to evolve on the cell–cell communication pathways involved, as well as on the role played by the cells constituting the neurovascular unit in the maintenance of the BBB phenotype, and more particularly the brain pericytes. This review summarises the key points of the history of the BBB, from its origin to the current knowledge of its physiology, as well as the cell–cell communication pathways identified so far during its development, maintenance, and pathophysiological alteration.

Translational Stroke Research Review: Using the Mouse to Model Human Futile Recanalization and Reperfusion Injury in Ischemic Brain Tissue

The approach to reperfusion therapies in stroke patients is rapidly evolving, but there is still no explanation why a substantial proportion of patients have a poor clinical prognosis despite successful flow restoration. This issue of futile recanalization is explained here by three clinical cases, which, despite complete recanalization, have very different outcomes. Preclinical research is particularly suited to characterize the highly dynamic changes in acute ischemic stroke and identify potential treatment targets useful for clinical translation. This review surveys the efforts taken so far to achieve mouse models capable of investigating the neurovascular underpinnings of futile recanalization. We highlight the translational potential of targeting tissue reperfusion in fully recanalized mouse models and of investigating the underlying pathophysiological mechanisms from subcellular to tissue scale. We suggest that stroke preclinical research should increasingly drive forward a continuous and circular dialogue with clinical research. When the preclinical and the clinical stroke research are consistent, translational success will follow.

The blood-brain barrier in systemic infection and inflammation

The vascular blood-brain barrier is a highly regulated interface between the blood and brain. Its primary function is to protect central neurons while signaling the presence of systemic inflammation and infection to the brain to enable a protective sickness behavior response. With increasing degrees and duration of systemic inflammation, the vascular blood-brain barrier becomes more permeable to solutes, undergoes an increase in lymphocyte trafficking, and is infiltrated by innate immune cells; endothelial cell damage may occasionally occur. Perturbation of neuronal function results in the clinical features of encephalopathy. Here, the molecular and cellular anatomy of the vascular blood-brain barrier is reviewed, first in a healthy context and second in a systemic inflammatory context. Distinct from the molecular and cellular mediators of the blood-brain barrier's response to inflammation, several moderators influence the direction and magnitude at genetic, system, cellular and molecular levels. These include sex, genetic background, age, pre-existing brain pathology, systemic comorbidity, and gut dysbiosis. Further progress is required to define and measure mediators and moderators of the blood-brain barrier's response to systemic inflammation in order to explain the heterogeneity observed in animal and human studies.

Glial PAMPering and DAMPening of Adult Hippocampal Neurogenesis

Adult neurogenesis represents a mature brain's capacity to integrate newly generated neurons into functional circuits. Impairment of neurogenesis contributes to the pathophysiology of various mood and cognitive disorders such as depression and Alzheimer's Disease. The hippocampal neurogenic niche hosts neural progenitors, glia, and vasculature, which all respond to intrinsic and environmental cues, helping determine their current state and ultimate fate. In this article we focus on the major immune communication pathways and mechanisms through which glial cells sense, interact with, and modulate the neurogenic niche. We pay particular attention to those related to the sensing of and response to innate immune danger signals. Receptors for danger signals were first discovered as a critical component of the innate immune system response to pathogens but are now also recognized to play a crucial role in modulating non-pathogenic sterile inflammation. In the neurogenic niche, viable, stressed, apoptotic, and dying cells can activate danger responses in neuroimmune cells, resulting in neuroprotection or neurotoxicity. Through these mechanisms glial cells can influence hippocampal stem cell fate, survival, neuronal maturation, and integration. Depending on the context, such responses may be appropriate and on-target, as in the case of learning-associated synaptic pruning, or excessive and off-target, as in neurodegenerative disorders.

Condensed ECM-based nanofilms on highly permeable PET membranes for robust cell-to-cell communications with improved optical clarity

The properties of a semipermeable porous membrane, including pore size, pore density, and thickness, play a crucial role in creating a tissue interface in a microphysiological system (MPS) because it dictates multicellular interactions between different compartments. The small pore-sized membrane has been preferentially used in an MPS for stable cell adhesion and the formation of tissue barriers on the membrane. However, it limited the applicability of the MPS because of the hindered cell transmigration via sparse through-holes and the optical translucence caused by light scattering through pores. Thus, there remain unmet challenges to construct a compartmentalized MPS without those drawbacks. Here we report a submicrometer-thickness (∼500 nm) fibrous extracellular matrix (ECM) film selectively condensed on a large pore-sized track-etched (TE) membrane (10µm-pores) in an MPS device, which enables the generation of functional tissue barriers simultaneously achieving optical transparency, intercellular interactions, and transmigration of cells across the membrane. The condensed ECM fibers uniformly covering the surface and 10µm-pores of the TE membrane permitted sufficient surface areas where a monolayer of the human induced pluripotent stem cell-derived brain endothelial cells is formed in the MPS device. The functional maturation of the blood-brain barrier (BBB) was proficiently achieved due to astrocytic endfeet sheathing the brain endothelial cells through 10µm pores of the condensed-ECM-coated TE (cECMTE) membrane. We also demonstrated the extravasation of human metastatic breast tumor cells through the human BBB on the cECMTE membrane. Thus, the cECMTE membrane integrated with an MPS can be used as a versatile platform for studying various intercellular communications and migration, mimicking the physiological barriers of an organ compartment.

Tumour brain: Pretreatment cognitive and affective disorders caused by peripheral cancers

People that develop extracranial cancers often display co-morbid neurological disorders, such as anxiety, depression and cognitive impairment, even before commencement of chemotherapy. This suggests bidirectional crosstalk between non-CNS tumours and the brain, which can regulate peripheral tumour growth. However, the reciprocal neurological effects of tumour progression on brain homeostasis are not well understood. Here, we review brain regions involved in regulating peripheral tumour development and how they, in turn, are adversely affected by advancing tumour burden. Tumour-induced activation of the immune system, blood-brain barrier breakdown and chronic neuroinflammation can lead to circadian rhythm dysfunction, sleep disturbances, aberrant glucocorticoid production, decreased hippocampal neurogenesis and dysregulation of neural network activity, resulting in depression and memory impairments. Given that cancer-related cognitive impairment diminishes patient quality of life, reduces adherence to chemotherapy and worsens cancer prognosis, it is essential that more research is focused at understanding how peripheral tumours affect brain homeostasis.

Astrocyte-Endotheliocyte Axis in the Regulation of the Blood-Brain Barrier

The evolution of blood-brain barrier paralleled centralisation of the nervous system: emergence of neuronal masses required control over composition of the interstitial fluids. The barriers were initially created by glial cells, which employed septate junctions to restrict paracellular diffusion in the invertebrates and tight junctions in some early vertebrates. The endothelial barrier, secured by tight and adherent junctions emerged in vertebrates and is common in mammals. Astrocytes form the parenchymal part of the blood-brain barrier and commutate with endothelial cells through secretion of growth factors, morphogens and extracellular vesicles. These secreted factors control the integrity of the blood-brain barrier through regulation of expression of tight junction proteins. The astrocyte-endotheliocyte communications are particularly important in various neurological diseases associated with impairments to the blood-brain barrier. Molecular mechanisms supporting astrocyte-endotheliocyte axis in health and disease are in need of detailed characterisation.