Postnatal development of the astrocyte perivascular MLC1/GlialCAM complex defines a temporal window for the gliovascular unit maturation

The Blood-Brain Barrier: Composition, Properties, and Roles in Brain Health

Blood vessels are critical to deliver oxygen and nutrients to tissues and organs throughout the body. The blood vessels that vascularize the central nervous system (CNS) possess unique properties, termed the blood-brain barrier (BBB), which allow these vessels to tightly regulate the movement of ions, molecules, and cells between the blood and the brain. This precise control of CNS homeostasis allows for proper neuronal function and protects the neural tissue from toxins and pathogens, and alterations of this barrier are important components of the pathogenesis and progression of various neurological diseases. The physiological barrier is coordinated by a series of physical, transport, and metabolic properties possessed by the brain endothelial cells (ECs) that form the walls of the blood vessels. These properties are regulated by interactions between different vascular, perivascular, immune, and neural cells. Understanding how these cell populations interact to regulate barrier properties is essential for understanding how the brain functions in both health and disease contexts.

Gene therapy rescues brain edema and motor function in a mouse model of megalencephalic leukoencephalopathy with subcortical cysts

Megalencephalic leukoencephalopathy with subcortical cysts (MLC) is an ultrarare, infantile-onset leukodystrophy characterized by white matter edema for which there is no treatment. More than 75% of diagnosed cases result from biallelic loss-of-function mutations in the astrocyte-specific gene MLC1, leading to early-onset macrocephaly, cerebellar ataxia, epilepsy, and mild cognitive decline. To develop a gene therapy for MLC, we administered an adeno-associated viral vector capable of crossing the murine blood-brain barrier, delivering the human MLC1 cDNA under the control of a human astrocyte-specific promoter, to 10-month-old Mlc1-/- mice. We observed long-term astrocyte-driven expression of MLC1 up to 1 year after viral vector administration in all brain areas analyzed. Despite the late-stage intervention, in vivo magnetic resonance imaging revealed normalization of water accumulation. Notably, our therapy successfully reversed locomotor deficits in Mlc1-/- mice, as evidenced by improved performance in motor tests assessing cerebellar ataxia-like behaviors. Collectively, these findings not only demonstrate the sustained efficacy of our gene therapy but also highlight the reversibility of vacuolation and motor impairments in Mlc1-/- mice, suggesting that MLC patients could benefit from treatment even after symptom onset.

Stress in utero: prenatal dexamethasone exposure causes greater structural gliovascular alterations in female offspring than in males

From early in life, experiences like prenatal stress profoundly affect long-term health and behavior. Fetal exposure to increased levels of glucocorticoids (GC), via maternal stress or through antenatal corticosteroid therapy (commonly used in women at risk of preterm birth), can disrupt brain development and raise the susceptibility to psychiatric disorders. Previous studies on prenatal exposure to synthetic GCs, such as dexamethasone (DEX), revealed impairments in neurogenesis and dendritic spine development. However, the impact of prenatal stress, specifically antenatal DEX exposure, on the gliovascular interface remains unclear. This interface, involving the relationship between astrocytes and blood vessels, is essential for healthy brain development. Astrocytic endfeet coverage and organization are crucial features of the gliovascular interface, and in this study, we evaluated these aspects through aquaporin-4 (AQ4) expression and organization along the lectin labelled-vasculature. At Postnatal Day 14, no differences in AQ4 expression were observed between males and females. However, prenatal stress induced by DEX exposure (50 μg/kg was administered subcutaneously to pregnant mice through gestational days 16, 17 and 18) significantly impacted this structure in females but not in males. Specifically, in female offspring prenatally exposed to DEX, AQ4 expression was significantly upregulated in the hippocampus, and its rearrangement was observed in the prefrontal cortex. A comparison of vascular density between male and female brains showed no significant sex differences in any analyzed regions, though male cerebellar vessel segments were shorter. Interestingly, prenatal stress caused morphological alterations in female brains, including increased vessel tortuosity, while no such changes were seen in males. In the hippocampus, prenatal DEX exposure reduced vessel segment length in males but did not affect females. In the cerebellum, DEX exposure increased vessel segment length in females. This study highlights sex-specific differences in the impact of prenatal stress on the gliovascular structure across various brain regions, suggesting AQ4 as a potential molecular target relevant to depressive-like behaviors in female offspring. Future studies are needed to correlate the gliovascular structural alterations found with functional disturbances and sex-specific mental health issues.

MLC1 alteration in iPSCs give rise to disease-like cellular vacuolation phenotype in the astrocyte lineage

Background

Megalencephalic leukoencephalopathy with subcortical cysts (MLC), a rare and progressive neurodegenerative disorder involving the white matter, is not adequately recapitulated by current disease models. Somatic cell reprogramming, along with advancements in genome engineering, may allow the establishment of in-vitro human models of MLC for disease modeling and drug screening. In this study, we utilized cellular reprogramming and gene-editing techniques to develop induced pluripotent stem cell (iPSC) models of MLC to recapitulate the cellular context of the classical MLC-impacted nervous system.

Methods

Somatic cell reprogramming of peripheral patient-derived blood mononuclear cells (PBMCs) was used to develop iPSC models of MLC. CRISPR-Cas9 system-based genome engineering was also utilized to create the MLC1 knockout model of the disease. Directed differentiation of iPSCs to neural stem cells (NSCs) and astrocytes was performed in a 2D cell culture format, followed by various cellular and molecular biology approaches, to characterize the disease model.

Results

MLC iPSCs established by somatic cell reprogramming and genome engineering were well characterized for pluripotency. iPSCs were subsequently differentiated to disease-relevant cell types: neural stem cells (NSCs) and astrocytes. RNA sequencing profiling of MLC NSCs revealed a set of differentially expressed genes related to neurological disorders and epilepsy, a common clinical finding within MLC disease. This gene set can serve as a target for drug screening for the development of a potential therapeutic for this disease. Upon differentiation to the more disease relevant cell type-astrocytes, MLC-characteristic vacuoles were clearly observed, which were distinctly absent from controls. This emergence recapitulated a distinguishing phenotypic marker of the disease.

Conclusion

Through the creation and analyses of iPSC models of MLC, our work addresses a critical need for relevant cellular models of MLC for use in both disease modeling and drug screening assays. Further investigation can utilize MLC iPSC models, as well as generated transcriptomic data sets and analyses, to identify potential therapeutic interventions for this debilitating disease.

Postnatal Development of Perivascular Astrocytic Processes and Detection of Local mRNA and Translation

Astrocytes' perivascular processes (PvAPs) entirely sheathe the brain's blood vessels and are equipped with an enriched molecular repertoire that sustains astrocytic regulatory functions at the vascular interface. We recently demonstrated that PvAP development in the mouse starts after birth and is essentially complete by postnatal day (P) 15. This time window also corresponds to a progressive molecular maturation of PvAPs with the acquisition of specific proteins. We proposed local translation of mRNAs in PvAPs as a molecular mechanism participating to their development and maturation. Here, we describe methods to quantify the development of PvAPs and detect local translation in developing PvAPs. We also show how to analyze quantitatively astrocytic-specific mRNAs and polysomal mRNAs in developing PvAPs. These experimental approaches allow to characterize important aspects of the dynamic postnatal development and molecular maturation of PvAPs.

The arrangements of the microvasculature and surrounding glial cells are linked to blood-brain barrier formation in the cerebral cortex

The blood-brain barrier (BBB) blocks harmful substances from entering the brain and dictates the central nervous system (CNS)-specific pharmacokinetics. Recent studies have shown that perivascular astrocytes and microglia also control BBB functions, however, information about the formation of BBB glial architecture remains scarce. We investigated the time course of the formation of BBB glial architecture in the rat brain cerebral cortex using Evans blue (EB) and tissue fixable biotin (Sulfo-NHS Biotin). The extent of the leakage into the brain parenchyma showed that the BBB was not formed at postnatal Day 4 (P4). The BBB gradually strengthened and reached a plateau at P15. We then investigated the changes in the configurations of blood vessels, astrocytes, and microglia with age by 3D image reconstruction of the immunohistochemical data. The endfeet of astrocytes covered the blood vessels, and the coverage rate rapidly increased after birth and reached a plateau at P15. Interestingly, microglia were also in contact with the capillaries, and the coverage rate was highest at P15 and stabilized at P30. It was also clarified that the microglial morphology changed from the amoeboid type to the ramified type, while the areas of the respective contact sites became smaller during P4 and P15. These results suggest that the perivascular glial architecture formation of the rat BBB occurs from P4 to P15 because the paracellular transport and the arrangements of perivascular glial cells at P15 are totally the same as those of P30. In addition, the contact style of perivascular microglia dramatically changed during P4-P15.

Dynamic local mRNA localization and translation occurs during the postnatal molecular maturation of perivascular astrocytic processes

Astrocytes are highly ramified and send out perivascular processes (PvAPs) that entirely sheathe the brain's blood vessels. PvAPs are equipped with an enriched molecular repertoire that sustains astrocytic regulatory functions at the vascular interface. In the mouse, PvAP development starts after birth and is essentially complete by postnatal day (P) 15. Progressive molecular maturation also occurs over this period, with the acquisition of proteins enriched in PvAPs. The mechanisms controlling the development and molecular maturation of PvAPs have not been extensively characterized. We reported previously that mRNAs are distributed unequally in mature PvAPs and are locally translated. Since dynamic mRNA localization and local translation influence the cell's polarity, we hypothesized that they might sustain the postnatal maturation of PvAPs. Here, we used a combination of molecular biology and imaging approaches to demonstrate that the development of PvAPs is accompanied by the transport of mRNA and polysomal mRNA into PvAPs, the development of a rough endoplasmic reticulum (RER) network and Golgi cisternae, and local translation. By focusing on genes and proteins that are selectively or specifically expressed in astrocytes, we characterized the developmental profile of mRNAs, polysomal mRNAs and proteins in PvAPs from P5 to P60. We found that some polysomal mRNAs polarized progressively towards the PvAPs. Lastly, we found that expression and localization of mRNAs in developing PvAPs is perturbed in a mouse model of megalencephalic leukoencephalopathy with subcortical cysts. Our results indicate that dynamic mRNA localization and local translation influence the postnatal maturation of PvAPs.

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.

The intersection between toxicology and aging research: A toxic aging coin perspective

We are imminently faced with the challenges of an increasingly aging population and longer lifespans due to improved health care. Concomitantly, we are faced with ubiquitous environmental pollution linked with various health effects and age-related diseases which contribute to increased morbidity with age. Geriatric populations are rarely considered in the development of environmental regulations or in toxicology research. Today, life expectancy is often into one's 80s or beyond, which means multiple decades living as a geriatric individual. Hence, adverse health effects and late-onset diseases might be due to environmental exposures as a geriatric, and we currently have no way of knowing. Considering aging from a different perspective, the term "gerontogen" was coined in 1987 to describe chemicals that accelerate biological aging but has largely been left out of toxicology research. Thus, we are challenged with a two-faced problem that we can describe as a "toxic aging coin"; on one side we consider how age affects the toxic outcome of chemicals, whereas on the other side we consider how chemicals accelerate aging (i.e. how chemicals act as gerontogens). Conveniently, both sides of this coin can be tackled with a single animal study that considers multiple age groups and assesses basic toxicology of the chemical(s) tested and aging-focused endpoints. Here, I introduce the concept of this toxic aging coin and some key considerations for how it applies to toxicology research. My discussion of this concept will focus on the brain, my area of expertise, but could be translated to any organ system.

Clonally expanded B cells in multiple sclerosis bind EBV EBNA1 and GlialCAM

Multiple sclerosis (MS) is a heterogenous autoimmune disease in which autoreactive lymphocytes attack the myelin sheath of the central nervous system. B lymphocytes in the cerebrospinal fluid (CSF) of patients with MS contribute to inflammation and secrete oligoclonal immunoglobulins^1,2. Epstein-Barr virus (EBV) infection has been epidemiologically linked to MS, but its pathological role remains unclear³. Here we demonstrate high-affinity molecular mimicry between the EBV transcription factor EBV nuclear antigen 1 (EBNA1) and the central nervous system protein glial cell adhesion molecule (GlialCAM) and provide structural and in vivo functional evidence for its relevance. A cross-reactive CSF-derived antibody was initially identified by single-cell sequencing of the paired-chain B cell repertoire of MS blood and CSF, followed by protein microarray-based testing of recombinantly expressed CSF-derived antibodies against MS-associated viruses. Sequence analysis, affinity measurements and the crystal structure of the EBNA1-peptide epitope in complex with the autoreactive Fab fragment enabled tracking of the development of the naive EBNA1-restricted antibody to a mature EBNA1-GlialCAM cross-reactive antibody. Molecular mimicry is facilitated by a post-translational modification of GlialCAM. EBNA1 immunization exacerbates disease in a mouse model of MS, and anti-EBNA1 and anti-GlialCAM antibodies are prevalent in patients with MS. Our results provide a mechanistic link for the association between MS and EBV and could guide the development of new MS therapies.

Ubr1-induced selective endophagy/autophagy protects against the endosomal and Ca2+-induced proteostasis disease stress

The cellular defense mechanisms against cumulative endo-lysosomal stress remain incompletely understood. Here, we identify Ubr1 as a protein quality control (QC) E3 ubiquitin-ligase that counteracts proteostasis stresses by facilitating endosomal cargo-selective autophagy for lysosomal degradation. Astrocyte regulatory cluster membrane protein MLC1 mutations cause endosomal compartment stress by fusion and enlargement. Partial lysosomal clearance of mutant endosomal MLC1 is accomplished by the endosomal QC ubiquitin ligases, CHIP and Ubr1 via ESCRT-dependent route. As a consequence of the endosomal stress, a supportive QC mechanism, dependent on both Ubr1 and SQSTM1/p62 activities, targets ubiquitinated and arginylated MLC1 mutants for selective endosomal autophagy (endophagy). This QC pathway is also activated for arginylated Ubr1-SQSTM1/p62 autophagy cargoes during cytosolic Ca²⁺-assault. Conversely, the loss of Ubr1 and/or arginylation elicited endosomal compartment stress. These findings underscore the critical housekeeping role of Ubr1 and arginylation-dependent endophagy/autophagy during endo-lysosomal proteostasis perturbations and suggest a link of Ubr1 to Ca²⁺ homeostasis and proteins implicated in various diseases including cancers and brain disorders.

Molecular diversity of diencephalic astrocytes reveals adult astrogenesis regulated by Smad4

Astrocytes regulate brain‐wide functions and also show region‐specific differences, but little is known about how general and region‐specific functions are aligned at the single‐cell level. To explore this, we isolated adult mouse diencephalic astrocytes by ACSA‐2‐mediated magnetic‐activated cell sorting (MACS). Single‐cell RNA‐seq revealed 7 gene expression clusters of astrocytes, with 4 forming a supercluster. Within the supercluster, cells differed by gene expression related to ion homeostasis or metabolism, with the former sharing gene expression with other regions and the latter being restricted to specific regions. All clusters showed expression of proliferation‐related genes, and proliferation of diencephalic astrocytes was confirmed by immunostaining. Clonal analysis demonstrated low level of astrogenesis in the adult diencephalon, but not in cerebral cortex grey matter. This led to the identification of Smad4 as a key regulator of diencephalic astrocyte in vivo proliferation and in vitro neurosphere formation. Thus, astrocytes show diverse gene expression states related to distinct functions with some subsets being more widespread while others are more regionally restricted. However, all share low‐level proliferation revealing the novel concept of adult astrogenesis in the diencephalon.

Megalencephalic leukoencephalopathy with subcortical cysts is a developmental disorder of the gliovascular unit

Absence of the astrocyte-specific membrane protein MLC1 is responsible for megalencephalic leukoencephalopathy with subcortical cysts (MLC), a rare type of leukodystrophy characterized by early-onset macrocephaly and progressive white matter vacuolation that lead to ataxia, spasticity, and cognitive decline. During postnatal development (from P5 to P15 in the mouse), MLC1 forms a membrane complex with GlialCAM (another astrocytic transmembrane protein) at the junctions between perivascular astrocytic processes. Perivascular astrocytic processes along with blood vessels form the gliovascular unit. It was not previously known how MLC1 influences the physiology of the gliovascular unit. Here, using the Mlc1 knock-out mouse model of MLC, we demonstrated that MLC1 controls the postnatal development and organization of perivascular astrocytic processes, vascular smooth muscle cell contractility, neurovascular coupling, and intraparenchymal interstitial fluid clearance. Our data suggest that MLC is a developmental disorder of the gliovascular unit, and perivascular astrocytic processes and vascular smooth muscle cell maturation defects are primary events in the pathogenesis of MLC and therapeutic targets for this disease.

Control of membrane protein homeostasis by a chaperone-like glial cell adhesion molecule at multiple subcellular locations

The significance of crosstalks among constituents of plasma membrane protein clusters/complexes in cellular proteostasis and protein quality control (PQC) remains incompletely understood. Examining the glial (enriched) cell adhesion molecule (CAM), we demonstrate its chaperone-like role in the biosynthetic processing of the megalencephalic leukoencephalopathy with subcortical cyst 1 (MLC1)-heteromeric regulatory membrane protein complex, as well as the function of the GlialCAM/MLC1 signalling complex. We show that in the absence of GlialCAM, newly synthesized MLC1 molecules remain unfolded and are susceptible to polyubiquitination-dependent proteasomal degradation at the endoplasmic reticulum. At the plasma membrane, GlialCAM regulates the diffusional partitioning and endocytic dynamics of cluster members, including the ClC-2 chloride channel and MLC1. Impaired folding and/or expression of GlialCAM or MLC1 in the presence of diseases causing mutations, as well as plasma membrane tethering compromise the functional expression of the cluster, leading to compromised endo-lysosomal organellar identity. In addition, the enlarged endo-lysosomal compartments display accelerated acidification, ubiquitinated cargo-sorting and impaired endosomal recycling. Jointly, these observations indicate an essential and previously unrecognized role for CAM, where GliaCAM functions as a PQC factor for the MLC1 signalling complex biogenesis and possess a permissive role in the membrane dynamic and cargo sorting functions with implications in modulations of receptor signalling.

The Reproductive Toxicity of Monosodium Glutamate by Damaging GnRH Neurons Cannot Be Relieved Spontaneously Over Time

Objective

The present study aims to evaluate the effect of monosodium glutamate on testicular spermatogenesis in mice from the perspective of the hypothalamic-pituitary-testicular axis and whether this destructive effect is alleviated with time.

Methods

Neonatal mice were randomly divided into a monosodium glutamate (MSG) group and a control group, just below the interscapular region after birth with 10 µL MSG to deliver 4 mg/g (body mass), or with equivalent volumes of 0.9% saline. Samples which involved blood, brains and testicles of mice were collected and measured at puberty at 60 days and adulthood at 90 days.

Results

The results show that the fluorescence intensity of GnRH nerve fibers, the levels of follicle-stimulating hormone (FSH), luteinizing hormone (LH), and testosterone (T) hormones in the reproductive system, the number of spermatocytes and spermatozoa in testicular sections, the body length, body weight, testicular weight, and testicular index in the 60-day-old mice in monosodium glutamate group (MSG60 group) and the MSG90 group were lower than those in the 60-day-old mice in normal control group (NC60 group) (p < 0.05), but the number of apoptotic cells in the testicular section was higher than in the NC60 group (p < 0.05). When the 90-day-old mice in monosodium glutamate group (MSG90 group) was compared with the MSG60 group, except for body weight and testicular weight increase (p < 0.05), there is no significant difference in the other parameters mentioned above (p > 0.05).

Conclusion

Monosodium glutamate can cause reproductive toxicity to male mice by damaging GnRH neurons, and this reproductive toxicity cannot be relieved spontaneously over time. These findings are supported by observed histological changes.

Megalencephalic Leukoencephalopathy: Insights Into Pathophysiology and Perspectives for Therapy

Megalencephalic leukoencephalopathy with subcortical cysts (MLC) is a rare genetic disorder belonging to the group of vacuolating leukodystrophies. It is characterized by megalencephaly, loss of motor functions, epilepsy, and mild mental decline. In brain biopsies of MLC patients, vacuoles were observed in myelin and in astrocytes surrounding blood vessels. It is mainly caused by recessive mutations in MLC1 and HEPACAM (also called GLIALCAM) genes. These disease variants are called MLC1 and MLC2A with both types of patients sharing the same clinical phenotype. Besides, dominant mutations in HEPACAM were also identified in a subtype of MLC patients (MLC2B) with a remitting phenotype. MLC1 and GlialCAM proteins form a complex mainly expressed in brain astrocytes at the gliovascular interface and in Bergmann glia at the cerebellum. Both proteins regulate several ion channels and transporters involved in the control of ion and water fluxes in glial cells, either directly influencing their location and function, or indirectly regulating associated signal transduction pathways. However, the MLC1/GLIALCAM complex function and the related pathological mechanisms leading to MLC are still unknown. It has been hypothesized that, in MLC, the role of glial cells in brain ion homeostasis is altered in both physiological and inflammatory conditions. There is no therapy for MLC patients, only supportive treatment. As MLC2B patients show an MLC reversible phenotype, we speculated that the phenotype of MLC1 and MLC2A patients could also be mitigated by the re-introduction of the correct gene even at later stages. To prove this hypothesis, we injected in the cerebellar subarachnoid space of Mlc1 knockout mice an adeno-associated virus (AAV) coding for human MLC1 under the control of the glial-fibrillary acidic protein promoter. MLC1 expression in the cerebellum extremely reduced myelin vacuolation at all ages in a dose-dependent manner. This study could be considered as the first preclinical approach for MLC. We also suggest other potential therapeutic strategies in this review.

Structural and Functional Features of Developing Brain Capillaries, and Their Alteration in Schizophrenia

Schizophrenia affects more than 1% of the world's population and shows very high heterogeneity in the positive, negative, and cognitive symptoms experienced by patients. The pathogenic mechanisms underlying this neurodevelopmental disorder are largely unknown, although it is proposed to emerge from multiple genetic and environmental risk factors. In this work, we explore the potential alterations in the developing blood vessel network which could contribute to the development of schizophrenia. Specifically, we discuss how the vascular network evolves during early postnatal life and how genetic and environmental risk factors can lead to detrimental changes. Blood vessels, capillaries in particular, constitute a dynamic and complex infrastructure distributing oxygen and nutrients to the brain. During postnatal development, capillaries undergo many structural and anatomical changes in order to form a fully functional, mature vascular network. Advanced technologies like magnetic resonance imaging and near infrared spectroscopy are now enabling to study how the brain vasculature and its supporting features are established in humans from birth until adulthood. Furthermore, the contribution of the different neurovascular unit elements, including pericytes, endothelial cells, astrocytes and microglia, to proper brain function and behavior, can be dissected. This investigation conducted among different brain regions altered in schizophrenia, such as the prefrontal cortex, may provide further evidence that schizophrenia can be considered a neurovascular disorder.

Megalencephalic leukoencephalopathy with subcortical cysts 1 (MLC1) promotes glioblastoma cell invasion in the brain microenvironment

Glioblastoma (GBM), or grade IV astrocytoma, is a malignant brain cancer that contains subpopulations of proliferative and invasive cells that coordinately drive primary tumor growth, progression, and recurrence after therapy. Here, we have analyzed functions for megalencephalic leukoencephalopathy with subcortical cysts 1 (Mlc1), an eight-transmembrane protein normally expressed in perivascular brain astrocyte end feet that is essential for neurovascular development and physiology, in the pathogenesis of GBM. We show that Mlc1 is expressed in human stem-like GBM cells (GSCs) and is linked to the development of primary and recurrent GBM. Genetically inhibiting MLC1 in GSCs using RNAi-mediated gene silencing results in diminished growth and invasion in vitro as well as impaired tumor initiation and progression in vivo. Biochemical assays identify the receptor tyrosine kinase Axl and its intracellular signaling effectors as important for MLC1 control of GSC invasive growth. Collectively, these data reveal key functions for MLC1 in promoting GSC growth and invasion, and suggest that targeting the Mlc1 protein or its associated signaling effectors may be a useful therapy for blocking tumor progression in patients with primary or recurrent GBM.

Astrocytes in the regulation of cerebrovascular functions

Astrocytes are the most numerous type of neuroglia in the brain and have a predominant influence on the cerebrovascular system; they control perivascular homeostasis, the integrity of the blood-brain barrier, the dialogue with the peripheral immune system, the transfer of metabolites from the blood, and blood vessel contractility in response to neuronal activity. These regulatory processes occur in a specialized interface composed of perivascular astrocyte extensions that almost completely cover the cerebral blood vessels. Scientists have only recently started to study how this interface is formed and how it influences cerebrovascular functions. Here, we review the literature on the astrocytes' role in the regulation of the cerebrovascular system. We cover the anatomy and development of the gliovascular interface, the known gliovascular functions, and molecular factors, the latter's implication in certain pathophysiological situations, and recent cutting-edge experimental tools developed to examine the astrocytes' role at the vascular interface. Finally, we highlight some open questions in this field of research.

Megalencephalic Leukoencephalopathy with Subcortical Cysts Disease-Linked MLC1 Protein Favors Gap-Junction Intercellular Communication by Regulating Connexin 43 Trafficking in Astrocytes

Astrocytes, the most numerous cells of the central nervous system, exert critical functions for brain homeostasis. To this purpose, astrocytes generate a highly interconnected intercellular network allowing rapid exchange of ions and metabolites through gap junctions, adjoined channels composed of hexamers of connexin (Cx) proteins, mainly Cx43. Functional alterations of Cxs and gap junctions have been observed in several neuroinflammatory/neurodegenerative diseases. In the rare leukodystrophy megalencephalic leukoencephalopathy with subcortical cysts (MLC), astrocytes show defective control of ion/fluid exchanges causing brain edema, fluid cysts, and astrocyte/myelin vacuolation. MLC is caused by mutations in MLC1, an astrocyte-specific protein of elusive function, and in GlialCAM, a MLC1 chaperon. Both proteins are highly expressed at perivascular astrocyte end-feet and astrocyte-astrocyte contacts where they interact with zonula occludens-1 (ZO-1) and Cx43 junctional proteins. To investigate the possible role of Cx43 in MLC pathogenesis, we studied Cx43 properties in astrocytoma cells overexpressing wild type (WT) MLC1 or MLC1 carrying pathological mutations. Using biochemical and electrophysiological techniques, we found that WT, but not mutated, MLC1 expression favors intercellular communication by inhibiting extracellular-signal-regulated kinase 1/2 (ERK1/2)-mediated Cx43 phosphorylation and increasing Cx43 gap-junction stability. These data indicate MLC1 regulation of Cx43 in astrocytes and Cx43 involvement in MLC pathogenesis, suggesting potential target pathways for therapeutic interventions.

New Insights in the Complexity and Functionality of the Neurovascular Unit

The neurovascular unit (NVU) encompasses all brain cells and underlines that neurons, glia and brain vasculature are in intimate physical and functional association. Brain function is dependent on blood flow and local increases in blood flow in response to neural activity - functional hyperaemia takes place at the NVU. Although this is a vital function of the NVU, many studies have demonstrated that the NVU also performs other tasks. Blood vessels in the brain, which are composed of multiple cell types, are essential for correct brain development. They constitute the niche for brain stem cells, sense the environment and communicate changes to neural tissue, and control the immune quiescence of the CNS. In this brief chapter we will discuss new insights into the biology of NVU, which have further revealed the heterogeneity and complexity of the vascular tree and its neurovascular associations.