BMP signaling through BMPRIA in astrocytes is essential for proper cerebral angiogenesis and formation of the blood-brain-barrier

Blood-brain barrier disruption: a culprit of cognitive decline?

Cognitive decline covers a broad spectrum of disorders, not only resulting from brain diseases but also from systemic diseases, which seriously influence the quality of life and life expectancy of patients. As a highly selective anatomical and functional interface between the brain and systemic circulation, the blood-brain barrier (BBB) plays a pivotal role in maintaining brain homeostasis and normal function. The pathogenesis underlying cognitive decline may vary, nevertheless, accumulating evidences support the role of BBB disruption as the most prevalent contributing factor. This may mainly be attributed to inflammation, metabolic dysfunction, cell senescence, oxidative/nitrosative stress and excitotoxicity. However, direct evidence showing that BBB disruption causes cognitive decline is scarce, and interestingly, manipulation of the BBB opening alone may exert beneficial or detrimental neurological effects. A broad overview of the present literature shows a close relationship between BBB disruption and cognitive decline, the risk factors of BBB disruption, as well as the cellular and molecular mechanisms underlying BBB disruption. Additionally, we discussed the possible causes leading to cognitive decline by BBB disruption and potential therapeutic strategies to prevent BBB disruption or enhance BBB repair. This review aims to foster more investigations on early diagnosis, effective therapeutics, and rapid restoration against BBB disruption, which would yield better cognitive outcomes in patients with dysregulated BBB function, although their causative relationship has not yet been completely established.

Defining the Role of Oral Pathway Inhibitors as Targeted Therapeutics in Arteriovenous Malformation Care

Arteriovenous malformations (AVMs) are vascular malformations that are prone to rupturing and can cause significant morbidity and mortality in relatively young patients. Conventional treatment options such as surgery and endovascular therapy often are insufficient for cure. There is a growing body of knowledge on the genetic and molecular underpinnings of AVM development and maintenance, making the future of precision medicine a real possibility for AVM management. Here, we review the pathophysiology of AVM development across various cell types, with a focus on current and potential druggable targets and their therapeutic potentials in both sporadic and familial AVM populations.

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.

Involvement of Astrocytes in the Formation, Maintenance, and Function of the Blood-Brain Barrier

The blood-brain barrier (BBB) is a fundamental structure that protects the composition of the brain by determining which ions, metabolites, and nutrients are allowed to enter the brain from the blood or to leave it towards the circulation. The BBB is structurally composed of a layer of brain capillary endothelial cells (BCECs) bound to each other through tight junctions (TJs). However, its development as well as maintenance and properties are controlled by the other brain cells that contact the BCECs: pericytes, glial cells, and even neurons themselves. Astrocytes seem, in particular, to have a very important role in determining and controlling most properties of the BBB. Here, we will focus on these latter cells, since the comprehension of their roles in brain physiology has been continuously expanding, even including the ability to participate in neurotransmission and in complex functions such as learning and memory. Accordingly, pathological conditions that alter astrocytic functions can alter the BBB's integrity, thus compromising many brain activities. In this review, we will also refer to different kinds of in vitro BBB models used to study the BBB's properties, evidencing its modifications under pathological conditions.

Reactive astrocytes associated with prion disease impair the blood brain barrier

BACKGROUND

Impairment of the blood-brain barrier (BBB) is considered to be a common feature among neurodegenerative diseases, including Alzheimer's, Parkinson's and prion diseases. In prion disease, increased BBB permeability was reported 40 years ago, yet the mechanisms behind the loss of BBB integrity have never been explored. Recently, we showed that reactive astrocytes associated with prion diseases are neurotoxic. The current work examines the potential link between astrocyte reactivity and BBB breakdown.

RESULTS

In prion-infected mice, the loss of BBB integrity and aberrant localization of aquaporin 4 (AQP4), a sign of retraction of astrocytic endfeet from blood vessels, were noticeable prior to disease onset. Gaps in cell-to-cell junctions along blood vessels, together with downregulation of Occludin, Claudin-5 and VE-cadherin, which constitute tight and adherens junctions, suggested that loss of BBB integrity is linked with degeneration of vascular endothelial cells. In contrast to cells isolated from non-infected adult mice, endothelial cells originating from prion-infected mice displayed disease-associated changes, including lower levels of Occludin, Claudin-5 and VE-cadherin expression, impaired tight and adherens junctions, and reduced trans-endothelial electrical resistance (TEER). Endothelial cells isolated from non-infected mice, when co-cultured with reactive astrocytes isolated from prion-infected animals or treated with media conditioned by the reactive astrocytes, developed the disease-associated phenotype observed in the endothelial cells from prion-infected mice. Reactive astrocytes were found to produce high levels of secreted IL-6, and treatment of endothelial monolayers originating from non-infected animals with recombinant IL-6 alone reduced their TEER. Remarkably, treatment with extracellular vesicles produced by normal astrocytes partially reversed the disease phenotype of endothelial cells isolated from prion-infected animals.

CONCLUSIONS

To our knowledge, the current work is the first to illustrate early BBB breakdown in prion disease and to document that reactive astrocytes associated with prion disease are detrimental to BBB integrity. Moreover, our findings suggest that the harmful effects are linked to proinflammatory factors secreted by reactive astrocytes.

Neuron-astrocyte omnidirectional signaling in neurological health and disease

Astrocytes are an abundantly distributed population of glial cells in the central nervous system (CNS) that perform myriad functions in the normal and injured/diseased brain. Astrocytes exhibit heterogeneous phenotypes in response to various insults, a process known as astrocyte reactivity. The accuracy and precision of brain signaling are primarily based on interactions involving neurons, astrocytes, oligodendrocytes, microglia, pericytes, and dendritic cells within the CNS. Astrocytes have emerged as a critical entity within the brain because of their unique role in recycling neurotransmitters, actively modulating the ionic environment, regulating cholesterol and sphingolipid metabolism, and influencing cellular crosstalk in diverse neural injury conditions and neurodegenerative disorders. However, little is known about how an astrocyte functions in synapse formation, axon specification, neuroplasticity, neural homeostasis, neural network activity following dynamic surveillance, and CNS structure in neurological diseases. Interestingly, the tripartite synapse hypothesis came to light to fill some knowledge gaps that constitute an interaction of a subpopulation of astrocytes, neurons, and synapses. This review highlights astrocytes' role in health and neurological/neurodegenerative diseases arising from the omnidirectional signaling between astrocytes and neurons at the tripartite synapse. The review also recapitulates the disruption of the tripartite synapse with a focus on perturbations of the homeostatic astrocytic function as a key driver to modulate the molecular and physiological processes toward neurodegenerative diseases.

The blood-brain barrier: structure, regulation, and drug delivery

Blood-brain barrier (BBB) is a natural protective membrane that prevents central nervous system (CNS) from toxins and pathogens in blood. However, the presence of BBB complicates the pharmacotherapy for CNS disorders as the most chemical drugs and biopharmaceuticals have been impeded to enter the brain. Insufficient drug delivery into the brain leads to low therapeutic efficacy as well as aggravated side effects due to the accumulation in other organs and tissues. Recent breakthrough in materials science and nanotechnology provides a library of advanced materials with customized structure and property serving as a powerful toolkit for targeted drug delivery. In-depth research in the field of anatomical and pathological study on brain and BBB further facilitates the development of brain-targeted strategies for enhanced BBB crossing. In this review, the physiological structure and different cells contributing to this barrier are summarized. Various emerging strategies for permeability regulation and BBB crossing including passive transcytosis, intranasal administration, ligands conjugation, membrane coating, stimuli-triggered BBB disruption, and other strategies to overcome BBB obstacle are highlighted. Versatile drug delivery systems ranging from organic, inorganic, and biologics-derived materials with their synthesis procedures and unique physio-chemical properties are summarized and analyzed. This review aims to provide an up-to-date and comprehensive guideline for researchers in diverse fields, offering perspectives on further development of brain-targeted drug delivery system.

Reactive astrocytes associated with prion disease impair the blood brain barrier

Background

Impairment of the blood-brain barrier (BBB) is considered to be a common feature among neurodegenerative diseases, including Alzheimer's, Parkinson's and prion diseases. In prion disease, increased BBB permeability was reported 40 years ago, yet the mechanisms behind the loss of BBB integrity have never been explored. Recently, we showed that reactive astrocytes associated with prion diseases are neurotoxic. The current work examines the potential link between astrocyte reactivity and BBB breakdown.

Results

In prion-infected mice, the loss of BBB integrity and aberrant localization of aquaporin 4 (AQP4), a sign of retraction of astrocytic endfeet from blood vessels, were noticeable prior to disease onset. Gaps in cell-to-cell junctions along blood vessels, together with downregulation of Occludin, Claudin-5 and VE-cadherin, which constitute tight and adherens junctions, suggested that loss of BBB integrity is linked with degeneration of vascular endothelial cells. In contrast to cells isolated from non-infected adult mice, endothelial cells originating from prion-infected mice displayed disease-associated changes, including lower levels of Occludin, Claudin-5 and VE-cadherin expression, impaired tight and adherens junctions, and reduced trans-endothelial electrical resistance (TEER). Endothelial cells isolated from non-infected mice, when co-cultured with reactive astrocytes isolated from prion-infected animals or treated with media conditioned by the reactive astrocytes, developed the disease-associated phenotype observed in the endothelial cells from prion-infected mice. Reactive astrocytes were found to produce high levels of secreted IL-6, and treatment of endothelial monolayers originating from non-infected animals with recombinant IL-6 alone reduced their TEER. Remarkably, treatment with extracellular vesicles produced by normal astrocytes partially reversed the disease phenotype of endothelial cells isolated from prion-infected animals.

Conclusions

To our knowledge, the current work is the first to illustrate early BBB breakdown in prion disease and to document that reactive astrocytes associated with prion disease are detrimental to BBB integrity. Moreover, our findings suggest that the harmful effects are linked to proinflammatory factors secreted by reactive astrocytes.

High BMP4 expression in low/intermediate risk BCP-ALL identifies children with poor outcomes

Pediatric B-cell precursor acute lymphoblastic leukemia (BCP-ALL) outcome has improved in the last decades, but leukemic relapses are still one of the main problems of this disease. Bone morphogenetic protein 4 (BMP4) was investigated as a new candidate biomarker with potential prognostic relevance, and its pathogenic role was assessed in the development of disease. A retrospective study was performed with 115 pediatric patients with BCP-ALL, and BMP4 expression was analyzed by quantitative reverse transcription polymerase chain reaction in leukemic blasts at the time of diagnosis. BMP4 mRNA expression levels in the third (upper) quartile were associated with a higher cumulative incidence of relapse as well as a worse 5-year event-free survival and central nervous system (CNS) involvement. Importantly, this association was also evident among children classified as having a nonhigh risk of relapse. A validation cohort of 236 patients with BCP-ALL supported these data. Furthermore, high BMP4 expression promoted engraftment and rapid disease progression in an NSG mouse xenograft model with CNS involvement. Pharmacological blockade of the canonical BMP signaling pathway significantly decreased CNS infiltration and consistently resulted in amelioration of clinical parameters, including neurological score. Mechanistically, BMP4 favored chemoresistance, enhanced adhesion and migration through brain vascular endothelial cells, and promoted a proinflammatory microenvironment and CNS angiogenesis. These data provide evidence that BMP4 expression levels in leukemic cells could be a useful biomarker to identify children with poor outcomes in the low-/intermediate-risk groups of BCP-ALL and that BMP4 could be a new therapeutic target to blockade leukemic CNS disease.

Understanding astrocyte differentiation: Clinical relevance, technical challenges, and new opportunities in the omics era

Astrocytes are a major type of glial cells that have essential functions in development and homeostasis of the central nervous system (CNS). Immature astrocytes in the developing CNS support neuronal maturation and possess neural-stem-cell-like properties. Mature astrocytes partially lose these functions but gain new functions essential for adult CNS homeostasis. In pathological conditions, astrocytes become "reactive", which disrupts their mature homeostatic functions and reactivates some immature astrocyte-like properties, suggesting a partial reversal of astrocyte maturation. The loss of homeostatic astrocyte functions contributes to the pathogenesis of various neurological conditions, and therefore activating maturation-promoting mechanisms may be a promising therapeutic strategy to restore homeostasis. Manipulating the mechanisms underlying astrocyte maturation might also allow to facilitate CNS regeneration by enhancing developmental functions of adult astrocytes. However, such therapeutic strategies are still some distance away because of our limited understanding of astrocyte differentiation and maturation, due to biological and technical challenges, including the high degree of similarity of astrocytes with neural stem cells and the shortcomings of astrocyte markers. Current advances in systems biology have a huge potential to overcome these challenges. Recent transcriptomic analyses have already revealed new astrocyte markers and new regulators of astrocyte differentiation. However, the epigenomic changes that presumably occur during astrocyte differentiation remain an important, largely unexplored area for future research. Emerging technologies such as CRISPR/Cas9-based functional screens will further improve our understanding of the mechanisms underlying astrocyte differentiation. This may open up new clinical approaches to restore homeostasis in neurological disorders and/or promote CNS regeneration. This article is categorized under: Neurological Diseases > Genetics/Genomics/Epigenetics Neurological Diseases > Stem Cells and Development Neurological Diseases > Molecular and Cellular Physiology.

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.

Platelets, Thromboinflammation and Neurovascular Disease

The brain and spinal cord are immune-privileged organs, but in the disease state protection mechanisms such as the blood brain barrier (BBB) are ineffective or overcome by pathological processes. In neuroinflammatory diseases, microglia cells and other resident immune cells contribute to local vascular inflammation and potentially a systemic inflammatory response taking place in parallel. Microglia cells interact with other cells impacting on the integrity of the BBB and propagate the inflammatory response through the release of inflammatory signals. Here, we discuss the activation and response mechanisms of innate and adaptive immune processes in response to neuroinflammation. Furthermore, the clinical importance of neuroinflammatory mediators and a potential translational relevance of involved mechanisms are addressed also with focus on non-classical immune cells including microglia cells or platelets. As illustrative examples, novel agents such as Anfibatide or Revacept, which result in reduced recruitment and activation of platelets, a subsequently blunted activation of the coagulation cascade and further inflammatory process, demonstrating that mechanisms of neuroinflammation and thrombosis are interconnected and should be further subject to in depth clinical and basic research.

Blood-brain barrier models: Rationale for selection

Brain delivery is a broad research area, the outcomes of which are far hindered by the limited permeability of the blood-brain barrier (BBB). Over the last century, research has been revealing the BBB complexity and the crosstalk between its cellular and molecular components. Pathologically, BBB alterations may precede as well as be concomitant or lead to brain diseases. To simulate the BBB and investigate options for drug delivery, several in vitro, in vivo, ex vivo, in situ and in silico models are used. Hundreds of drug delivery vehicles successfully pass preclinical trials but fail in clinical settings. Inadequate selection of BBB models is believed to remarkably impact the data reliability leading to unsatisfactory results in clinical trials. In this review, we suggest a rationale for BBB model selection with respect to the addressed research question and downstream applications. The essential considerations of an optimal BBB model are discussed.

BMP signaling alters aquaporin-4 expression in the mouse cerebral cortex

Aquaporin-4 (AQP4) is a predominant water channel expressed in astrocytes in the mammalian brain. AQP4 is crucial for the regulation of homeostatic water movement across the blood–brain barrier (BBB). Although the molecular mechanisms regulating AQP4 levels in the cerebral cortex under pathological conditions have been intensively investigated, those under normal physiological conditions are not fully understood. Here we demonstrate that AQP4 is selectively expressed in astrocytes in the mouse cerebral cortex during development. BMP signaling was preferentially activated in AQP4-positive astrocytes. Furthermore, activation of BMP signaling by in utero electroporation markedly increased AQP4 levels in the cerebral cortex, and inhibition of BMP signaling strongly suppressed them. These results indicate that BMP signaling alters AQP4 levels in the mouse cerebral cortex during development.

Identification of highest neurotoxic amyloid-β plaque type showing reduced contact with astrocytes

Amyloid-β (Aβ) plaques are strongly associated with the development of Alzheimer's disease (AD). However, it remains unclear how morphological differences in Aβ plaques determine the pathogenesis of Aβ. Here, we categorized Aβ plaques into four types based on the macroscopic features of the dense core, and found that the Aβ-plaque subtype containing a larger dense core showed the strongest association with neuritic dystrophy. Astrocytes dominantly accumulated toward these expanded/dense-core-containing Aβ plaques. Previously, we indicated that deletion of the mitochondrial ubiquitin ligase MITOL/MARCH5 triggers mitochondrial impairments and exacerbates cognitive decline in a mouse model with AD-related Aβ pathology. In this study, MITOL deficiency accelerated the formation of expanded/dense-core-containing Aβ plaques, which showed reduced contacts with astrocytes, but not microglia. Our findings suggest that expanded/dense-core-containing Aβ-plaque formation enhanced by the alteration of mitochondrial function robustly contributes to the exacerbation of Aβ neuropathology, at least in part, through the reduced contacts between Aβ plaques and astrocytes.

Astrocytic neogenin/netrin-1 pathway promotes blood vessel homeostasis and function in mouse cortex

Astrocytes have multiple functions in the brain, including affecting blood vessel (BV) homeostasis and function. However, the underlying mechanisms remain elusive. Here, we provide evidence that astrocytic neogenin (NEO1), a member of deleted in colorectal cancer (DCC) family netrin receptors, is involved in blood vessel homeostasis and function. Mice with Neo1 depletion in astrocytes exhibited clustered astrocyte distribution and increased BVs in their cortices. These BVs were leaky, with reduced blood flow, disrupted vascular basement membranes (vBMs), decreased pericytes, impaired endothelial cell (EC) barrier, and elevated tip EC proliferation. Increased proliferation was also detected in cultured ECs exposed to the conditioned medium (CM) of NEO1-depleted astrocytes. Further screening for angiogenetic factors in the CM identified netrin-1 (NTN1), whose expression was decreased in NEO1-depleted cortical astrocytes. Adding NTN1 into the CM of NEO1-depleted astrocytes attenuated EC proliferation. Expressing NTN1 in NEO1 mutant cortical astrocytes ameliorated phenotypes in blood-brain barrier (BBB), EC, and astrocyte distribution. NTN1 depletion in astrocytes resulted in BV/BBB deficits in the cortex similar to those in Neo1 mutant mice. In aggregate, these results uncovered an unrecognized pathway, astrocytic NEO1 to NTN1, not only regulating astrocyte distribution, but also promoting cortical BV homeostasis and function.

The Roles of TGF-β Signaling in Cerebrovascular Diseases

Cerebrovascular diseases are one of the leading causes of death worldwide, however, little progress has been made in preventing or treating these diseases to date. The transforming growth factor-β (TGF-β) signaling pathway plays crucial and highly complicated roles in cerebrovascular development and homeostasis, and dysregulated TGF-β signaling contributes to cerebrovascular diseases. In this review, we provide an updated overview of the functional role of TGF-β signaling in the cerebrovascular system under physiological and pathological conditions. We discuss the current understanding of TGF-β signaling in cerebral angiogenesis and the maintenance of brain vessel homeostasis. We also review the mechanisms by which disruption of TGF-β signaling triggers or promotes the progression of cerebrovascular diseases. Finally, we briefly discuss the potential of targeting TGF-β signaling to treat cerebrovascular diseases.

Genetic Contributions to Multivariate Data-Driven Brain Networks Constructed via Source-Based Morphometry

Identifying genetic factors underlying neuroanatomical variation has been difficult. Traditional methods have used brain regions from predetermined parcellation schemes as phenotypes for genetic analyses, although these parcellations often do not reflect brain function and/or do not account for covariance between regions. We proposed that network-based phenotypes derived via source-based morphometry (SBM) may provide additional insight into the genetic architecture of neuroanatomy given its data-driven approach and consideration of covariance between voxels. We found that anatomical SBM networks constructed on ~ 20 000 individuals from the UK Biobank were heritable and shared functionally meaningful genetic overlap with each other. We additionally identified 27 unique genetic loci that contributed to one or more SBM networks. Both GWA and genetic correlation results indicated complex patterns of pleiotropy and polygenicity similar to other complex traits. Lastly, we found genetic overlap between a network related to the default mode and schizophrenia, a disorder commonly associated with neuroanatomic alterations.

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.

Astroglia in Leukodystrophies

Leukodystrophies are genetically determined disorders affecting the white matter of the central nervous system. The combination of MRI pattern recognition and next-generation sequencing for the definition of novel disease entities has recently demonstrated that many leukodystrophies are due to the primary involvement and/or mutations in genes selectively expressed by cell types other than the oligodendrocytes, the myelin-forming cells in the brain. This has led to a new definition of leukodystrophies as genetic white matter disorders resulting from the involvement of any white matter structural component. As a result, the research has shifted its main focus from oligodendrocytes to other types of neuroglia. Astrocytes are the housekeeping cells of the nervous system, responsible for maintaining homeostasis and normal brain physiology and to orchestrate repair upon injury. Several lines of evidence show that astrocytic interactions with the other white matter cellular constituents play a primary pathophysiologic role in many leukodystrophies. These are thus now classified as astrocytopathies. This chapter addresses how the crosstalk between astrocytes, other glial cells, axons and non-neural cells are essential for the integrity and maintenance of the white matter in health. It also addresses the current knowledge of the cellular pathomechanisms of astrocytic leukodystrophies, and specifically Alexander disease, vanishing white matter, megalencephalic leukoencephalopathy with subcortical cysts and Aicardi-Goutière Syndrome.

MITOL deletion in the brain impairs mitochondrial structure and ER tethering leading to oxidative stress

MITOL deletion in mouse brain impairs the morphology and ER tethering of mitochondria, resulting in enhanced oxidative stress. This study suggests a relationship between morphological abnormalities of mitochondria and developmental disorder.

Mitochondrial abnormalities are associated with developmental disorders, although a causal relationship remains largely unknown. Here, we report that increased oxidative stress in neurons by deletion of mitochondrial ubiquitin ligase MITOL causes a potential neuroinflammation including aberrant astrogliosis and microglial activation, indicating that mitochondrial abnormalities might confer a risk for inflammatory diseases in brain such as psychiatric disorders. A role of MITOL in both mitochondrial dynamics and ER-mitochondria tethering prompted us to characterize three-dimensional structures of mitochondria in vivo. In MITOL-deficient neurons, we observed a significant reduction in the ER-mitochondria contact sites, which might lead to perturbation of phospholipids transfer, consequently reduce cardiolipin biogenesis. We also found that branched large mitochondria disappeared by deletion of MITOL. These morphological abnormalities of mitochondria resulted in enhanced oxidative stress in brain, which led to astrogliosis and microglial activation partly causing abnormal behavior. In conclusion, the reduced ER-mitochondria tethering and excessive mitochondrial fission may trigger neuroinflammation through oxidative stress.

Inhibiting Bone Morphogenetic Protein 4 Type I Receptor Signaling Promotes Remyelination by Potentiating Oligodendrocyte Differentiation

Blocking inhibitory factors within CNS demyelinating lesions is regarded as a promising strategy to promote remyelination. Bone morphogenetic protein 4 (BMP4) is an inhibitory factor present in demyelinating lesions. Noggin, an endogenous antagonist to BMP, has previously been shown to increase the number of oligodendrocytes and promote remyelination in vivo. However, it remains unclear how BMP4 signaling inhibits remyelination. Here we investigated the downstream signaling pathway that mediates the inhibitory effect that BMP4 exerts upon remyelination through pharmacological and transgenic approaches. Using the cuprizone mouse model of central demyelination, we demonstrate that selectively blocking BMP4 signaling via the pharmacological inhibitor LDN-193189 significantly promotes oligodendroglial differentiation and the extent of remyelination in vivo. This was accompanied by the downregulation of transcriptional targets that suppress oligodendrocyte differentiation. Further, selective deletion of BMP receptor type IA (BMPRIA) within primary mouse oligodendrocyte progenitor cells (OPCs) significantly enhanced their differentiation and subsequent myelination in vitro. Together, the results of this study identify that BMP4 signals via BMPRIA within OPCs to inhibit oligodendroglial differentiation and their capacity to myelinate axons, and suggest that blocking the BMP4/BMPRIA pathway in OPCs is a promising strategy to promote CNS remyelination.

Occurrence and colocalization of surfactant proteins A, B, C and D in the developing and adult rat brain

BACKGROUND

Surfactant proteins (SP's) have been described as inherent proteins of the human central nervous system (CNS). Their distribution pattern in brain tissue and altered cerebrospinal fluid (CSF) - concentrations in different CNS pathologies are indicative of their immunological and rheological importance. The aim of this study has been to investigate when - compared to the lungs - SP's are expressed in the developing rat brain and which functional components in the CNS participate in their production.

MATERIAL AND METHODS

Brain and lung tissue from embryonal (days 10, 12, 14, 16, 17 and 20), newborn, and adult rats were harvested and investigated for expression of SP-A, SP-B, SP-C and SP-D using immunofluorescence microscopy in order to identify and compare the time points of their occurence in the respective tissue. To better identify the location of SP expression in the rat brain, SP's were colocalized with use of an astrocyte marker (GFAP), a neuronal marker (NeuN), an endothelial marker (CD31) and an axonal marker (NF).

RESULTS AND CONCLUSION

SP-A and SP-C are expressed in the CNS of rats during early embryonic age whereas SP-B and SP-D are first present in the adult rat brain. All SP's are expressed in cells adjacent to CSF spaces, probably influencing and maintaining physiological CSF flow. SP's A and C are abundant at the site of the blood brain barrier (BBB).

Understanding the continuum of radionecrosis and vascular disorders in the brain following gamma knife irradiation: An MRI study

PURPOSE

The radiation dose delivered to brain tumors is limited by the possibility to induce vascular damage and necrosis in surrounding healthy tissue. In the present study, we assessed the ability of MRI to monitor the cascade of events occurring in the healthy rat brain after stereotactic radiosurgery, which could be used to optimize the radiation treatment planning.

METHODS

The primary somatosensory forelimb area (S1FL) and the primary motor cortex in the right hemisphere of Fischer rats (n = 6) were irradiated with a single dose of Gamma Knife radiation (Leksell Perfexion, Elekta AG, Stockholm, Sweden). Rats were scanned with a small-animal 7 Tesla MRI scanner before treatment and 16, 21, 54, 82, and 110 days following irradiation. At every imaging session, T₂ -weighted (T₂ w), Gd-DTPA dynamic contrast-enhanced MRI (DCE-MRI), and T2*-weighted ( T2* w) images were acquired to measure changes in fluid content, blood vessel permeability, and structure, respectively. At days 10, 110, and 140, histopathology was performed on brain sections. Locomotion and spatial memory ability were assessed longitudinally by behavioral tests.

RESULTS

No vascular changes were initially observed. After 54 days, a small necrotic volume in the white matter below the S1FL, surrounded by an area presenting significant vascular permeability, was revealed. Between 54 and 110 days, the necrotic volume increased and was accompanied by the formation of a ring-like region, where a mixture of necrosis and permeable blood vessels were observed, as confirmed by histology. Behavioral changes were only observed after day 82.

CONCLUSION

Together, DCE-MRI and T2* w images supported by histology provided a coherent picture of the phenomena involved in the formation of new, leaky blood vessels, which was followed by the detection of radionecrosis in a preclinical model of brain irradiation. Magn Reson Med 78:1420-1431, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Bone mesenchymal stem cell-conditioned medium induces the upregulation of Smad6, which inhibits the BMP-4/Smad1/5/8 signaling pathway

OBJECTIVES

In this study, we aimed to investigate the effect of bone mesenchymal stem cells (BMSCs) conditioned medium on BMP4-induced activation of the Smad signaling pathway during the process of neural stem cells differentiation.

METHODS

We designed four experimental groups: (A) control group, (B) BMP-4 treatment group, (C) BMSC-CM group, and (D) BMSC-CM (BMSC-conditioned medium) combined with BMP-4 group. The expression of microtube-associated protein-2 (MAP-2) detected by immunocytochemical staining and western blot. The expression of glial fibrillary acidic protein (GFAP) and BMP receptors (BMPRII and BMPRIa) was determined by western blot, while SMAD1/5/8 and SMAD6 mRNA expression was detected by quantitative real-time polymerase chain reaction (Q-RT-PCR).

RESULTS

We found that the expression of microtube-associated protein-2 (MAP-2) increased in group B compared to group A. In group C, the expression of GFAP was increased and the expression of MAP-2 was decreased compared to group A. This phenomenon was weaker in group D. We also demonstrated that BMSC-CM could inhibit the activation of BMP signaling by downregulating the expression of BMPRII and BMPRIa proteins. Moreover, BMSC-CM could upregulate the expression of Smad6 mRNA and inhibit the activation of Smad1/5/8 mRNA.

CONCLUSION

These observations suggest that BMSC-CM could neutralize the effect of BMP-4 on the differentiation of NSCs by upregulating the expression of Smad6 and decrease the expression of GFAP by inhibiting the BMP-4-SMAD1/5/8 signaling pathway, conversely increasing the generation of neurons.

Bone Morphogenetic Protein 4 Signalling in Neural Stem and Progenitor Cells during Development and after Injury

Substantial progress has been made in identifying the extracellular signalling pathways that regulate neural stem and precursor cell biology in the central nervous system (CNS). The bone morphogenetic proteins (BMPs), in particular BMP4, are key players regulating neuronal and glial cell development from neural precursor cells in the embryonic, postnatal, and injured CNS. Here we review recent studies on BMP4 signalling in the generation of neurons, astrocytes, and oligodendroglial cells in the CNS. We also discuss putative mechanisms that BMP4 may utilise to influence glial cell development following CNS injury and highlight some questions for further research.

Morphogens and blood-brain barrier function in health and disease

The microvasculature of the brain forms a protective blood-brain barrier (BBB) that ensures a homeostatic environment for the central nervous system (CNS), which is essential for optimal brain functioning. The barrier properties of the brain endothelial cells are maintained by cells surrounding the capillaries, such as astrocytes and pericytes. Together with the endothelium and a basement membrane, these supporting cells form the neurovascular unit (NVU). Accumulating evidence indicates that the supporting cells of the NVU release a wide variety of soluble factors that induce and control barrier properties in a concentration-dependent manner. The current review provides a comprehensive overview of how such factors, called morphogens, influence BBB integrity and functioning. Since impaired BBB function is apparent in numerous CNS disorders and is often associated with disease severity, we also discuss the potential therapeutic value of these morphogens, as they may represent promising therapies for a wide variety of CNS disorders.

Disruption of the blood-brain barrier in pigs naturally infected with Taenia solium, untreated and after anthelmintic treatment

Neurocysticercosis is a widely prevalent disease in the tropics that causes seizures and a variety into of neurological symptoms in most of the world. Experimental models are limited and do not allow assessment of the degree of inflammation around brain cysts. The vital dye Evans Blue (EB) was injected to 11 pigs naturally infected with Taenia solium cysts to visually identify the extent of disruption of the blood-brain barrier. A total of 369 cysts were recovered from the 11 brains and classified according to the staining of their capsules as blue or unstained. The proportion of cysts with blue capsules was significantly higher in brains from pigs that had received anthelmintic treatment 48 and 120h before the EB infusion, indicating a greater compromise of the blood-brain barrier due to treatment. The model could be useful for understanding the pathology of treatment-induced inflammation in neurocysticercosis.

Paracrine regulation of neural stem cells in the subependymal zone

Stem cells maintain their self-renewal and multipotency capacities through a self-organizing network of transcription factors and intracellular pathways activated by extracellular signaling from the microenvironment or "niche" in which they reside in vivo. In the adult mammalian brain new neurons continue to be generated throughout life of the organisms and this lifelong process of neurogenesis is supported by a reservoir of neural stem cells in the germinal regions. The discovery of adult neurogenesis in the mammalian brain has sparked great interest in defining the conditions that guide neural stem cell (NSC) maintenance and differentiation into the great variety of neuronal and glial subtypes. Here we review current knowledge regarding the paracrine regulation provided by the components of the niche and its function, focusing on the main germinal region of the adult central nervous system (CNS), the subependymal zone (SEZ).

Reactive astrogliosis after spinal cord injury-beneficial and detrimental effects

Reactive astrogliosis is a pathologic hallmark of spinal cord injury (SCI). It is characterised by profound morphological, molecular, and functional changes in astrocytes that occur within hours of SCI and evolves as time elapses after injury. Astrogliosis is a defense mechanism to minimize and repair the initial damage but eventually leads to some detrimental effects. Reactive astrocytes secrete a plethora of both growth promoting and inhibitory factors after SCI. However, the production of inhibitory components surpasses the growth stimulating factors, thus, causing inhibitory effects. In severe cases of injury, astrogliosis results in the formation of irreversible glial scarring that acts as regeneration barrier due to the expression of inhibitory components such as chondroitin sulfate proteoglycans. Scar formation was therefore recognized from a negative perspective for many years. Accumulating evidence from pharmacological and genetic studies now signifies the importance of astrogliosis and its timing for spinal cord repair. These studies have advanced our knowledge regarding signaling pathways and molecular mediators, which trigger and modulate reactive astrocytes and scar formation. In this review, we discuss the recent advances in this field. We also review therapeutic strategies that have been developed to target astrocytes reactivity and glial scaring in the environment of SCI. Astrocytes play pivotal roles in governing SCI mechanisms, and it is therefore crucial to understand how their activities can be targeted efficiently to harness their potential for repair and regeneration after SCI.

Connecting vascular and nervous system development: angiogenesis and the blood-brain barrier

The vascular and nervous systems share a common necessity of circuit formation to coordinate nutrient and information transfer, respectively. Shared developmental principles have evolved to orchestrate the formation of both the vascular and the nervous systems. This evolution is highlighted by the identification of specific guidance cues that direct both systems to their target tissues. In addition to sharing cellular and molecular signaling events during development, the vascular and nervous systems also form an intricate interface within the central nervous system called the neurovascular unit. Understanding how the neurovascular unit develops and functions, and more specifically how the blood-brain barrier within this unit is established, is of utmost importance. We explore the history, recent discoveries, and unanswered questions surrounding the relationship between the vascular and nervous systems with a focus on developmental signaling cues that guide network formation and establish the interface between these two systems.

Hypomyelination phenotype caused by impaired differentiation of oligodendrocytes in Emx1-cre mediated Cdk5 conditional knockout mice

Cyclin-dependent kinase 5 (Cdk5) plays a pivotal role in neuronal migration and differentiation, and in axonal elongation. Although many studies have been conducted to analyze neuronal functions of Cdk5, its kinase activity has also been reported during oligodendrocyte differentiation, which suggests Cdk5 may play an important role in oligodendrocytes. Here, we describe a hypomyelination phenotype observed in Emx1-cre mediated Cdk5 conditional knockout (cKO) mice (Emx1-cKO), in which the Cdk5 gene was deleted in neurons, astrocytes and oligodendrocyte -lineage cells. In contrast, the Cdk5 gene in CaMKII cKO mice was deleted only in neurons. Because the development of mature oligodendrocytes from oligodendrocyte precursor cells is a complex process, we performed in situ hybridization using markers for the oligodendrocyte precursor cell and for the differentiated oligodendrocyte. Our results indicate that hypomyelination in Emx1-cKO is due to the impaired differentiation of oligodendrocytes, rather than to the proliferation or migration of their precursors. The present study confirmed the in vivo role of Cdk5 in oligodendrocyte differentiation.

BMP-induced REST regulates the establishment and maintenance of astrocytic identity

Astrocyte differentiation and maintenance is promoted by BMP signaling, which induces REST/NRSF to repress neuronal genes.

Once they have differentiated, cells retain their individual character and repress genes that are specifically expressed in other cell lineages, but how alternative fate choice is restricted during and/or after differentiation remains unclear. In the mammalian central nervous system, neurons, astrocytes, and oligodendrocytes are generated throughout life from common tripotent neural progenitor cells (NPCs). Bone morphogenetic proteins (BMPs) are well-known astrocyte-inducing cytokines. We show here that the expression of a transcriptional repressor, RE1 silencer of transcription (REST)/neuron-restrictive silencer factor (NRSF), is up-regulated and sustained by BMP signal activation in the course of astrocytic differentiation of NPCs, and restricts neuronal differentiation. We further show that, in differentiated astrocytes, endogenous REST/NRSF associates with various neuronal genes and that disruption of its function resulted in their derepression, thereby explaining how ectopic neuronal gene expression is prevented in cells with astrocytic traits. Collectively, our results suggest that REST/NRSF functions as a molecular regulator of the nonneuronal phenotype in astrocytes.

BMPR1a and BMPR1b signaling exert opposing effects on gliosis after spinal cord injury

Astrogliosis following spinal cord injury (SCI) involves an early hypertrophic response that is beneficial and a subsequent formation of a dense scar. We investigated the role of bone morphogenetic protein (BMP) signaling in gliosis after SCI and find that BMPR1a and BMPR1b signaling exerts opposing effects on hypertrophy. Conditional ablation of BMPR1a from glial fibrillary acidic protein (GFAP)-expressing cells leads to defective astrocytic hypertrophy, increased infiltration by inflammatory cells, and reduced axon density. BMPR1b-null mice conversely develop "hyperactive" reactive astrocytes and consequently have smaller lesion volumes. The effects of ablation of either receptor are reversed in the double knock-out animals. These findings indicate that BMPR1a and BMPR1b exert directly opposing effects on the initial reactive astrocytic hypertrophy. Also, BMPR1b knock-out mice have an attenuated glial scar in the chronic stages following injury, suggesting that it has a greater role in glial scar progression. To elucidate the differing roles of the two receptors in astrocytes, we examined the effects of ablation of either receptor in serum-derived astrocytes in vitro. We find that the two receptors exert opposing effects on the posttranscriptional regulation of astrocytic microRNA-21. Further, overexpression of microRNA-21 in wild-type serum-derived astrocytes causes a dramatic reduction in cell size accompanied by reduction in GFAP levels. Hence, regulation of microRNA-21 by BMP signaling provides a novel mechanism for regulation of astrocytic size. Targeting specific BMPR subunits for therapeutic purposes may thus provide an approach for manipulating gliosis and enhancing functional outcomes after SCI.

Astrocytes: biology and pathology

Astrocytes are specialized glial cells that outnumber neurons by over fivefold. They contiguously tile the entire central nervous system (CNS) and exert many essential complex functions in the healthy CNS. Astrocytes respond to all forms of CNS insults through a process referred to as reactive astrogliosis, which has become a pathological hallmark of CNS structural lesions. Substantial progress has been made recently in determining functions and mechanisms of reactive astrogliosis and in identifying roles of astrocytes in CNS disorders and pathologies. A vast molecular arsenal at the disposal of reactive astrocytes is being defined. Transgenic mouse models are dissecting specific aspects of reactive astrocytosis and glial scar formation in vivo. Astrocyte involvement in specific clinicopathological entities is being defined. It is now clear that reactive astrogliosis is not a simple all-or-none phenomenon but is a finely gradated continuum of changes that occur in context-dependent manners regulated by specific signaling events. These changes range from reversible alterations in gene expression and cell hypertrophy with preservation of cellular domains and tissue structure, to long-lasting scar formation with rearrangement of tissue structure. Increasing evidence points towards the potential of reactive astrogliosis to play either primary or contributing roles in CNS disorders via loss of normal astrocyte functions or gain of abnormal effects. This article reviews (1) astrocyte functions in healthy CNS, (2) mechanisms and functions of reactive astrogliosis and glial scar formation, and (3) ways in which reactive astrocytes may cause or contribute to specific CNS disorders and lesions.

BMP signaling induces cell-type-specific changes in gene expression programs of human keratinocytes and fibroblasts

BMP signaling has a crucial role in skin development and homeostasis, whereas molecular mechanisms underlying its involvement in regulating gene expression programs in keratinocytes and fibroblasts remain largely unknown. We show here that several BMP ligands, all BMP receptors, and BMP-associated Smad1/5/8 are expressed in human primary epidermal keratinocytes and dermal fibroblasts. Treatment of both cell types by BMP-4 resulted in the activation of the BMP-Smad, but not BMP-MAPK pathways. Global microarray analysis revealed that BMP-4 treatment induces distinct and cell type-specific changes in gene expression programs in keratinocytes and fibroblasts, which are far more complex than the effects of BMPs on cell proliferation/differentiation described earlier. Furthermore, our data suggest that the potential modulation of cell adhesion, extracellular matrix remodeling, motility, metabolism, signaling, and transcription by BMP-4 in keratinocytes and fibroblasts is likely to be achieved by the distinct and cell-type-specific sets of molecules. Thus, these data provide an important basis for delineating mechanisms that underlie the distinct effects of the BMP pathway on different cell populations in the skin, and will be helpful in further establishing molecular signaling networks regulating skin homeostasis in health and disease.

Retinal astrocyte differentiation mediated by leukemia inhibitory factor in cooperation with bone morphogenetic protein 2

Retinal astrocytes and their precursor cells migrate from the optic nerve. Interleukin 6 family cytokines, whose signal transduction requires gp130, promote astrocyte differentiation in the optic nerve, though the mechanism of astrocyte differentiation in the retina has not been clarified. We found that GFAP-positive astrocytes were significantly decreased in number but that a considerable number of astrocytes were still present in gp130-deficient mouse retina. These findings suggest that gp130-dependent signaling pathways play essential roles in retinal astrocyte differentiation and that retinal astrocyte differentiation can also be promoted by other signaling pathways. We found that leukemia inhibitory factor, bone morphogenetic proteins, and their receptors are expressed in P0 retina. In addition, leukemia inhibitory factor and bone morphogenetic protein 2 synergistically promote astrocyte differentiation of retinal precursor cells isolated from P0 mouse retina. These observations demonstrated that not only gp130-dependent signaling but also bone morphogenetic proteins play essential roles in retinal astrocyte differentiation.

Moxidectin toxicity in senescence-accelerated prone and resistant mice

Moxidectin has been used safely as an antiparasitic in many animal species, including for the eradication of the mouse fur mite, Mycoptes musculinus. Although no side effects of moxidectin have previously been reported to occur in mice, 2 strains of the senescence-accelerated mouse (SAMP8 and SAMR1) sustained considerable mortality after routine prophylactic treatment. To investigate the mechanism underlying this effect, moxidectin toxicosis in these mice was evaluated in a controlled study. Moxidectin was applied topically (0.015 mg), and drug concentrations in both brain and serum were analyzed by using HPLC coupled with mass spectrometry. The moxidectin concentration in brain of SAMP8 mice was 18 times that in controls, and that in brain of SAMR1 mice was 14 times higher than in controls, whereas serum moxidectin concentrations did not differ significantly among the 3 strains. Because deficiency of the blood-brain barrier protein P-glycoprotein leads to sensitivity to this class of drugs in other SAM mice, Pgp immunohistochemistry of brain sections from a subset of mice was performed to determine whether this commercially available analysis could predict sensitivity to this class of drug. The staining analysis showed no difference among the strains of mice, indicating that this test does not correlate with sensitivity. In addition, no gross or histologic evidence of organ toxicity was found in brain, liver, lung, or kidney. This report shows that topically applied moxidectin at a standard dose accumulates in the CNS causing toxicosis in both SAMP8 and SAMR1 mice.