Proteomic dissection of vanishing white matter pathogenesis

Vanishing white matter (VWM) is a leukodystrophy caused by biallelic pathogenic variants in eukaryotic translation initiation factor 2B. To date, it remains unclear which factors contribute to VWM pathogenesis. Here, we investigated the basis of VWM pathogenesis using the 2b5ho mouse model. We first mapped the temporal proteome in the cerebellum, corpus callosum, cortex, and brainstem of 2b5ho and wild-type (WT) mice. Protein changes observed in 2b5ho mice were then cross-referenced with published proteomic datasets from VWM patient brain tissue to define alterations relevant to the human disease. By comparing 2b5ho mice with their region- and age-matched WT counterparts, we showed that the proteome in the cerebellum and cortex of 2b5ho mice was already dysregulated prior to pathology development, whereas proteome changes in the corpus callosum only occurred after pathology onset. Remarkably, protein changes in the brainstem were transient, indicating that a compensatory mechanism might occur in this region. Importantly, 2b5ho mouse brain proteome changes reflect features well-known in VWM. Comparison of the 2b5ho mouse and VWM patient brain proteomes revealed shared changes. These could represent changes that contribute to the disease or even drive its progression in patients. Taken together, we show that the 2b5ho mouse brain proteome is affected in a region- and time-dependent manner. We found that the 2b5ho mouse model partly replicates the human disease at the protein level, providing a resource to study aspects of VWM pathogenesis by highlighting alterations from early to late disease stages, and those that possibly drive disease progression.

Region-specific and age-related differences in astrocytes in the human brain

Astrocyte heterogeneity and its relation to aging in the normal human brain remain poorly understood. We here analyzed astrocytes in gray and white matter brain tissues obtained from donors ranging in age between the neonatal period to over 100 years. We show that astrocytes are differently distributed with higher density in the white matter. This regional difference in cellular density becomes less prominent with age. Additionally, we confirm the presence of morphologically distinct astrocytes, with gray matter astrocytes being morphologically more complex. Notably, gray matter astrocytes morphologically change with age, while white matter astrocytes remain relatively consistent in morphology. Using regional mass spectrometry-based proteomics, we did, however, identify astrocyte specific proteins with regional differences in abundance, reflecting variation in cellular density or expression level. Importantly, the expression of some astrocyte specific proteins region-dependently decreases with age. Taken together, we provide insights into region- and age-related differences in astrocytes in the human brain.

Dominant CST3 variants cause adult onset leukodystrophy without amyloid angiopathy

Leukodystrophies are rare genetic white matter disorders that have been regarded as mainly occurring in childhood. Recent years altered this perception, as a growing number of leukodystrophies was described to have an onset at adult ages. Still, many adult patients presenting with white matter changes remain without a specific molecular diagnosis. We describe a novel adult onset leukodystrophy in 16 patients from eight families carrying one of four different stop-gain or frameshift dominant variants in the CST3 gene. Clinical and radiological features differ markedly from the previously described Icelandic Cerebral Amyloid Angiopathy that was found in patients carrying p.Leu68Asn substitution in CST3. The clinical phenotype consists of recurrent episodes of hemiplegic migraine associated with transient unilateral focal deficits and slowly progressing motor symptoms and cognitive decline in mid-old adult ages. In addition, in some cases acute onset clinical deterioration led to a prolonged episode with reduced consciousness and even early death. Radiologically, pathognomonic changes are found at typical predilection sites involving the deep cerebral white matter sparing a periventricular and directly subcortical rim, the middle blade of corpus callosum, posterior limb of the internal capsule, middle cerebellar peduncles, cerebral peduncles, and specifically the globus pallidus. Histopathologic characterization in two autopsy cases did not reveal angiopathy, but instead micro- to macrocystic degeneration of the white matter. Astrocytes were activated at early stages and later on displayed severe degeneration and loss. In addition, despite loss of myelin, elevated numbers of partly apoptotic oligodendrocytes were observed. A structural comparison of the variants in CST3 suggests that specific truncations of Cystatin C result in an abnormal function, possibly by rendering the protein more prone to aggregation. Future studies are required to confirm the assumed effect on the protein and to determine pathophysiologic downstream events at the cellular level.

Translocator protein is a marker of activated microglia in rodent models but not human neurodegenerative diseases

Microglial activation plays central roles in neuroinflammatory and neurodegenerative diseases. Positron emission tomography (PET) targeting 18 kDa Translocator Protein (TSPO) is widely used for localising inflammation in vivo, but its quantitative interpretation remains uncertain. We show that TSPO expression increases in activated microglia in mouse brain disease models but does not change in a non-human primate disease model or in common neurodegenerative and neuroinflammatory human diseases. We describe genetic divergence in the TSPO gene promoter, consistent with the hypothesis that the increase in TSPO expression in activated myeloid cells depends on the transcription factor AP1 and is unique to a subset of rodent species within the Muroidea superfamily. Finally, we identify LCP2 and TFEC as potential markers of microglial activation in humans. These data emphasise that TSPO expression in human myeloid cells is related to different phenomena than in mice, and that TSPO-PET signals in humans reflect the density of inflammatory cells rather than activation state.

Neurodegenerative disease after hematopoietic stem cell transplantation in metachromatic leukodystrophy

OBJECTIVE

Metachromatic leukodystrophy is a lysosomal storage disease caused by deficient arylsulfatase A. It is characterized by progressive demyelination and thus mainly affects the white matter. Hematopoietic stem cell transplantation may stabilize and improve white matter damage, yet some patients deteriorate despite successfully treated leukodystrophy. We hypothesized that post-treatment decline in metachromatic leukodystrophy might be caused by gray matter pathology.

METHODS

Three metachromatic leukodystrophy patients treated with hematopoietic stem cell transplantation with a progressive clinical course despite stable white matter pathology were clinically and radiologically analyzed. Longitudinal volumetric MRI was used to quantify atrophy. We also examined histopathology in three other patients deceased after treatment and compared them with six untreated patients.

RESULTS

The three clinically progressive patients developed cognitive and motor deterioration after transplantation, despite stable mild white matter abnormalities on MRI. Volumetric MRI identified cerebral and thalamus atrophy in these patients, and cerebellar atrophy in two. Histopathology showed that in brain tissue of transplanted patients, arylsulfatase A expressing macrophages were clearly present in the white matter, but absent in the cortex. Arylsulfatase A expression within patient thalamic neurons was lower than in controls, the same was found in transplanted patients.

INTERPRETATION

Neurological deterioration may occur after hematopoietic stem cell transplantation in metachromatic leukodystrophy despite successfully treated leukodystrophy. MRI shows gray matter atrophy, and histological data demonstrate absence of donor cells in gray matter structures. These findings point to a clinically relevant gray matter component of metachromatic leukodystrophy, which does not seem sufficiently affected by transplantation.

Altered collagen I and premature pulmonary embryonic differentiation in patients with OI type II

Pulmonary hypoplasia and respiratory failure are primary causes of death in patients with osteogenesis imperfecta (OI) type II. OI is a genetic skeletal disorder caused by pathogenic variants in genes encoding collagen type I. It is still unknown if the collagen defect also affects lung development and structure, causing lung hypoplasia in OI type II. The aim of this study was to investigate the intrinsic characteristics of OI embryonic lung parenchyma and to determine whether altered collagen type I may compromise airway development and lung structure. Lung tissue from nine fetuses with OI type II and six control fetuses, matched by gestational age, was analyzed for TTF-1 and collagen type I expression by immunohistochemistry, to evaluate the state of lung development and amount of collagen. The differentiation of epithelium into type 2 pneumocytes during embryonic development was premature in OI type II fetuses compared to controls (p < 0.05). Collagen type I showed no significant differences between the two groups. However, the amount of alpha2(I) chains was higher in fetuses with OI and the ratio of alpha1(I) to alpha2(I) lower in OI compared to controls. Cell differentiation during lung embryonic development in patients with OI type II is premature and impaired. This may be the underlying cause of pulmonary hypoplasia. Altered cell differentiation can be secondary to mechanical chest factors or a consequence of disrupted type I collagen synthesis. Our findings suggest that collagen type I is a biochemical regulator of pulmonary cell differentiation, influencing lung development.

Regional vulnerability of brain white matter in vanishing white matter

Vanishing white matter (VWM) is a leukodystrophy that primarily manifests in young children. In this disease, the brain white matter is differentially affected in a predictable pattern with telencephalic brain areas being most severely affected, while others remain allegedly completely spared. Using high-resolution mass spectrometry-based proteomics, we investigated the proteome patterns of the white matter in the severely affected frontal lobe and normal appearing pons in VWM and control cases to identify molecular bases underlying regional vulnerability. By comparing VWM patients to controls, we identified disease-specific proteome patterns. We showed substantial changes in both the VWM frontal and pons white matter at the protein level. Side-by-side comparison of brain region-specific proteome patterns further revealed regional differences. We found that different cell types were affected in the VWM frontal white matter than in the pons. Gene ontology and pathway analyses identified involvement of region specific biological processes, of which pathways involved in cellular respiratory metabolism were overarching features. In the VWM frontal white matter, proteins involved in glycolysis/gluconeogenesis and metabolism of various amino acids were decreased compared to controls. By contrast, in the VWM pons white matter, we found a decrease in proteins involved in oxidative phosphorylation. Taken together, our data show that brain regions are affected in parallel in VWM, but to different degrees. We found region-specific involvement of different cell types and discovered that cellular respiratory metabolism is likely to be differentially affected across white matter regions in VWM. These region-specific changes help explain regional vulnerability to pathology in VWM.

Focal lesions following intracerebral gene therapy for mucopolysaccharidosis IIIA

OBJECTIVE

Mucopolysaccharidosis type IIIA (MPSIIIA) caused by recessive SGSH variants results in sulfamidase deficiency, leading to neurocognitive decline and death. No disease-modifying therapy is available. The AAVance gene therapy trial investigates AAVrh.10 overexpressing human sulfamidase (LYS-SAF302) delivered by intracerebral injection in children with MPSIIIA. Post-treatment MRI monitoring revealed lesions around injection sites. Investigations were initiated in one patient to determine the cause.

METHODS

Clinical and MRI details were reviewed. Stereotactic needle biopsies of a lesion were performed; blood and CSF were sampled. All samples were used for viral studies. Immunohistochemistry, electron microscopy, and transcriptome analysis were performed on brain tissue of the patient and various controls.

RESULTS

MRI revealed focal lesions around injection sites with onset from 3 months after therapy, progression until 7 months post therapy with subsequent stabilization and some regression. The patient had transient slight neurological signs and is following near-normal development. No evidence of viral or immunological/inflammatory cause was found. Immunohistochemistry showed immature oligodendrocytes and astrocytes, oligodendrocyte apoptosis, strong intracellular and extracellular sulfamidase expression and hardly detectable intracellular or extracellular heparan sulfate. No activation of the unfolded protein response was found.

INTERPRETATION

Results suggest that intracerebral gene therapy with local sulfamidase overexpression leads to dysfunction of transduced cells close to injection sites, with extracellular spilling of lysosomal enzymes. This alters extracellular matrix composition, depletes heparan sulfate, impairs astrocyte and oligodendrocyte function, and causes cystic white matter degeneration at the site of highest gene expression. The AAVance trial results will reveal the potential benefit-risk ratio of this therapy.

Aquaporin-4 and GPRC5B: old and new players in controlling brain oedema

Brain oedema is a life-threatening complication of various neurological conditions. Understanding molecular mechanisms of brain volume regulation is critical for therapy development. Unique insight comes from monogenic diseases characterized by chronic brain oedema, of which megalencephalic leukoencephalopathy with subcortical cysts (MLC) is the prototype. Variants in MLC1 or GLIALCAM, encoding proteins involved in astrocyte volume regulation, are the main causes of MLC. In some patients the genetic cause remains unknown. We performed genetic studies to identify novel gene variants in MLC patients, diagnosed by clinical and MRI features, without MLC1 or GLIALCAM variants. We determined subcellular localization of the related novel proteins in cells and in human brain tissue. We investigated functional consequences of the newly identified variants on volume regulation pathways using cell volume measurements, biochemical analysis and electrophysiology. We identified a novel homozygous variant in AQP4, encoding the water channel aquaporin-4, in two siblings, and two de novo heterozygous variants in GPRC5B, encoding the orphan G protein-coupled receptor GPRC5B, in three unrelated patients. The AQP4 variant disrupts membrane localization and thereby channel function. GPRC5B, like MLC1, GlialCAM and aquaporin-4, is expressed in astrocyte endfeet in human brain. Cell volume regulation is disrupted in GPRC5B patient-derived lymphoblasts. GPRC5B functionally interacts with ion channels involved in astrocyte volume regulation. In conclusion, we identify aquaporin-4 and GPRC5B as old and new players in genetic brain oedema. Our findings shed light on the protein complex involved in astrocyte volume regulation and identify GPRC5B as novel potentially druggable target for treating brain oedema.

Cortical interneuron development is affected in 4H leukodystrophy

4H leukodystrophy is a rare genetic disorder classically characterized by hypomyelination, hypodontia and hypogonadotropic hypogonadism. With the discovery that 4H is caused by mutations that affect RNA polymerase III, mainly involved in the transcription of small non-coding RNAs, also patients with atypical presentations with mainly a neuronal phenotype were identified. Pathomechanisms of 4H brain abnormalities are still unknown and research is hampered by a lack of preclinical models. We aimed to identify cells and pathways that are affected by 4H mutations using induced pluripotent stem cell models. RNA sequencing analysis on induced pluripotent stem cell-derived cerebellar cells revealed several differentially expressed genes between 4H patients and control samples, including reduced ARX expression. As ARX is involved in early brain and interneuron development, we studied and confirmed interneuron changes in primary tissue of 4H patients. Subsequently, we studied interneuron changes in more depth and analyzed induced pluripotent stem cell-derived cortical neuron cultures for changes in neuronal morphology, synaptic balance, network activity and myelination. We showed a decreased percentage of GABAergic synapses in 4H, which correlated to increased neuronal network activity. Treatment of cultures with GABA antagonists led to a significant increase in neuronal network activity in control cells but not in 4H cells, also pointing to lack of inhibitory activity in 4H. Myelination and oligodendrocyte maturation in cultures with 4H neurons was normal, and treatment with sonic hedgehog agonist SAG did not improve 4H related neuronal phenotypes. qPCR analysis revealed increased expression of parvalbumin interneuron marker ERBB4, suggesting that the development rather than generation of interneurons may be affected in 4H. Together, these results indicate that interneurons are involved, possibly parvalbumin interneurons, in disease mechanisms of 4H leukodystrophy.

Cortical Pathology in Vanishing White Matter

Vanishing white matter (VWM) is classified as a leukodystrophy with astrocytes as primary drivers in its pathogenesis. Magnetic resonance imaging has documented the progressive thinning of cortices in long-surviving patients. Routine histopathological analyses, however, have not yet pointed to cortical involvement in VWM. Here, we provide a comprehensive analysis of the VWM cortex. We employed high-resolution-mass-spectrometry-based proteomics and immunohistochemistry to gain insight into possible molecular disease mechanisms in the cortices of VWM patients. The proteome analysis revealed 268 differentially expressed proteins in the VWM cortices compared to the controls. A majority of these proteins formed a major protein interaction network. A subsequent gene ontology analysis identified enrichment for terms such as cellular metabolism, particularly mitochondrial activity. Importantly, some of the proteins with the most prominent changes in expression were found in astrocytes, indicating cortical astrocytic involvement. Indeed, we confirmed that VWM cortical astrocytes exhibit morphological changes and are less complex in structure than control cells. Our findings also suggest that these astrocytes are immature and not reactive. Taken together, we provide insights into cortical involvement in VWM, which has to be taken into account when developing therapeutic strategies.

In vivo targeting of a variant causing vanishing white matter using CRISPR/Cas9

Vanishing white matter (VWM) is a leukodystrophy caused by recessive variants in subunits of eIF2B. At present, no curative treatment is available and patients often die at young age. Due to its monogenic nature, VWM is a promising candidate for the development of CRISPR/Cas9-mediated gene therapy. Here we tested a dual-AAV approach in VWM mice encoding CRISPR/Cas9 and a DNA donor template to correct a pathogenic variant in Eif2b5. We performed sequencing analysis to assess gene correction rates and examined effects on the VWM phenotype, including motor behavior. Sequence analysis demonstrated that over 90% of CRISPR/Cas9-induced edits at the targeted locus are insertion or deletion (indel) mutations, rather than precise corrections from the DNA donor template by homology-directed repair. Around half of the CRISPR/Cas9-treated animals died prematurely. VWM mice showed no improvement in motor skills, weight, or neurological scores at 7 months of age, and CRISPR/Cas9-treated controls displayed an induced VWM phenotype. In conclusion, CRISPR/Cas9-induced DNA double-strand breaks (DSBs) at the Eif2b5 locus did not lead to sufficient correction of the VWM variant. Moreover, indel formation in Eif2b5 induced an exacerbated VWM phenotype. Therefore, DSB-independent strategies like base- or prime editing might better suited for VWM correction.

MODL-16. Generation of immunocompetent syngeneic allograft mouse models for pediatric diffuse midline glioma

Diffuse midline gliomas (DMG) are highly aggressive pediatric brain tumors with a grim prognosis. A lack of effective treatment options highlights the critical need to investigate new therapeutic strategies. This includes the use of immunotherapy, which has shown promise in other hard-to-treat tumors. To facilitate immunotherapeutic research in this field, and to complement the existing immunodeficient patient-derived DMG models, we developed three distinct immunocompetent mouse models representing different DMG subtypes, i.e., histone 3 wildtype and histone 3 K27M mutant DMG (H3.3K27M or H3.1K27M), that can be used for preclinical testing of new therapies. We first established primary tumor cell cultures from murine DMG tumors that were generated by brainstem-targeted intra-uterine electroporation (IUE). This method enabled the introduction of DMG-associated mutations within the intact developing brainstem, thereby generating DMG tumors in a spatially and temporally defined manner, while maintaining a genetically identical (isogenic) background. We then created allograft DMG mouse models by orthotopically implanting the established primary cell cultures into syngeneic (C57BL/6) mice. Herewith, we provide an allograft tool that is better suitable for large-scale therapeutic studies and more accessible to the scientific community. Importantly, we demonstrated that these allograft models recapitulate the histopathologic phenotype of human DMG, including their diffuse infiltrative growth and expression of DMG-associated antigens. Furthermore, CyTOF mass cytometry analysis indicated that these murine pontine tumors exhibit a tumor immune microenvironment (TIME) similar to human DMG, characterized by considerable myeloid cell infiltration and a paucity of T-lymphocytes and NK cells. As such, we provide a representative model to further delineate the immune landscape in DMG and to preclinically investigate novel (immuno)therapies. Currently, we are using these immunocompetent models to study the interaction between DMG cells and microglia, and we are investigating how we can modify the immune microenvironment to improve checkpoint inhibition in DMG.

Generation of Immunocompetent Syngeneic Allograft Mouse Models for Pediatric Diffuse Midline Glioma

Background

Diffuse midline gliomas (DMG) are highly malignant incurable pediatric brain tumors. A lack of effective treatment options highlights the need to investigate novel therapeutic strategies. This includes the use of immunotherapy, which has shown promise in other hard-to-treat tumors. To facilitate preclinical immunotherapeutic research, immunocompetent mouse models that accurately reflect the unique genetic, anatomical, and histological features of DMG patients are warranted.

Methods

We established cell cultures from primary DMG mouse models (C57BL/6) that were generated by brainstem targeted intra-uterine electroporation (IUE). We subsequently created allograft DMG mouse models by orthotopically implanting these tumor cells into syngeneic mice. Immunohistochemistry and -fluorescence, mass cytometry, and cell-viability assays were then used to verify that these murine tumors recapitulated human DMG.

Results

We generated three genetically distinct allograft models representing histone 3 wildtype (H3 WT) and K27M-mutant DMG (H3.3 K27M and H3.1 K27M). These allograft models recapitulated the histopathologic phenotype of their human counterparts, including their diffuse infiltrative growth and expression of DMG-associated antigens. These murine pontine tumors also exhibited an immune microenvironment similar to human DMG, characterized by considerable myeloid cell infiltration and a paucity of T-lymphocytes and NK cells. Finally, we show that these murine DMG cells display similar sensitivity to histone deacetylase (HDAC) inhibition as patient-derived DMG cells.

Conclusions

We created and validated an accessible method to generate immunocompetent allograft models reflecting different subtypes of DMG. These models adequately recapitulated the histopathology, immune microenvironment, and therapeutic response of human DMG, providing useful tools for future preclinical studies.

ACE2 Protein Expression During Childhood, Adolescence, and Early Adulthood

Purpose and context. Angiotensin-converting enzyme 2 is the entry receptor for SARS-CoV and SARS-CoV-2. Variations in ACE2 expression might explain age-related symptomatology of COVID-19, that is, more gastro-intestinal symptoms and less pulmonary complaints. This study qualitatively investigated ACE2 protein expression in various organs from the fetal to the young adolescent stage. Method. Autopsy samples from lung, heart, liver, stomach, small intestine, pancreas, kidney, adrenals, and brain (when available) were obtained from twenty subjects aged 24 weeks gestational age through 28 years. Formalin-fixed paraffin-embedded 4-um-thick tissue sections were stained against ACE2. Key results. We showed that the extent of ACE2 expression is age-related. With age, expression increases in lungs and decreases in intestines. In the other examined organs, ACE2 protein expression did not change with age. In brain tissue, ACE2 was expressed in astrocytes and endothelial cells. Conclusions. Age-related ACE2 expression differences could be one substrate of the selective clinical vulnerability of the respiratory and gastro-intestinal system to SARS-CoV-2 infection during infancy.

Heterogeneity of white matter astrocytes in the human brain

Astrocytes regulate central nervous system development, maintain its homeostasis and orchestrate repair upon injury. Emerging evidence support functional specialization of astroglia, both between and within brain regions. Different subtypes of gray matter astrocytes have been identified, yet molecular and functional diversity of white matter astrocytes remains largely unexplored. Nonetheless, their important and diverse roles in maintaining white matter integrity and function are well recognized. Compelling evidence indicate that impairment of normal astrocytic function and their response to injury contribute to a wide variety of diseases, including white matter disorders. In this review, we highlight our current understanding of astrocyte heterogeneity in the white matter of the mammalian brain and how an interplay between developmental origins and local environmental cues contribute to astroglial diversification. In addition, we discuss whether, and if so, how, heterogeneous astrocytes could contribute to white matter function in health and disease and focus on the sparse human research data available. We highlight four leukodystrophies primarily due to astrocytic dysfunction, the so-called astrocytopathies. Insight into the role of astroglial heterogeneity in both healthy and diseased white matter may provide new avenues for therapies aimed at promoting repair and restoring normal white matter function.

Dominant-acting CSF1R variants cause microglial depletion and altered astrocytic phenotype in zebrafish and adult-onset leukodystrophy

Tissue-resident macrophages of the brain, including microglia, are implicated in the pathogenesis of various CNS disorders and are possible therapeutic targets by their chemical depletion or replenishment by hematopoietic stem cell therapy. Nevertheless, a comprehensive understanding of microglial function and the consequences of microglial depletion in the human brain is lacking. In human disease, heterozygous variants in CSF1R, encoding the Colony-stimulating factor 1 receptor, can lead to adult-onset leukoencephalopathy with axonal spheroids and pigmented glia (ALSP) possibly caused by microglial depletion. Here, we investigate the effects of ALSP-causing CSF1R variants on microglia and explore the consequences of microglial depletion in the brain. In intermediate- and late-stage ALSP post-mortem brain, we establish that there is an overall loss of homeostatic microglia and that this is predominantly seen in the white matter. By introducing ALSP-causing missense variants into the zebrafish genomic csf1ra locus, we show that these variants act dominant negatively on the number of microglia in vertebrate brain development. Transcriptomics and proteomics on relatively spared ALSP brain tissue validated a downregulation of microglia-associated genes and revealed elevated astrocytic proteins, possibly suggesting involvement of astrocytes in early pathogenesis. Indeed, neuropathological analysis and in vivo imaging of csf1r zebrafish models showed an astrocytic phenotype associated with enhanced, possibly compensatory, endocytosis. Together, our findings indicate that microglial depletion in zebrafish and human disease, likely as a consequence of dominant-acting pathogenic CSF1R variants, correlates with altered astrocytes. These findings underscore the unique opportunity CSF1R variants provide to gain insight into the roles of microglia in the human brain, and the need to further investigate how microglia, astrocytes, and their interactions contribute to white matter homeostasis.

Defining tumor-associated vascular heterogeneity in pediatric high-grade and diffuse midline gliomas

The blood–brain barrier (BBB) plays important roles in brain tumor pathogenesis and treatment response, yet our understanding of its function and heterogeneity within or across brain tumor types remains poorly characterized. Here we analyze the neurovascular unit (NVU) of pediatric high-grade glioma (pHGG) and diffuse midline glioma (DMG) using patient derived xenografts and natively forming glioma mouse models. We show tumor-associated vascular differences between these glioma subtypes, and parallels between PDX and mouse model systems, with DMG models maintaining a more normal vascular architecture, BBB function and endothelial transcriptional program relative to pHGG models. Unlike prior work in angiogenic brain tumors, we find that expression of secreted Wnt antagonists do not alter the tumor-associated vascular phenotype in DMG tumor models. Together, these findings highlight vascular heterogeneity between pHGG and DMG and differences in their response to alterations in developmental BBB signals that may participate in driving these pathological differences.

Pathology of the neurovascular unit in leukodystrophies

The blood-brain barrier is a dynamic endothelial cell barrier in the brain microvasculature that separates the blood from the brain parenchyma. Specialized brain endothelial cells, astrocytes, neurons, microglia and pericytes together compose the neurovascular unit and interact to maintain blood-brain barrier function. A disturbed brain barrier function is reported in most common neurological disorders and may play a role in disease pathogenesis. However, a comprehensive overview of how the neurovascular unit is affected in a wide range of rare disorders is lacking. Our aim was to provide further insights into the neuropathology of the neurovascular unit in leukodystrophies to unravel its potential pathogenic role in these diseases. Leukodystrophies are monogenic disorders of the white matter due to defects in any of its structural components. Single leukodystrophies are exceedingly rare, and availability of human tissue is unique. Expression of selective neurovascular unit markers such as claudin-5, zona occludens 1, laminin, PDGFRβ, aquaporin-4 and α-dystroglycan was investigated in eight different leukodystrophies using immunohistochemistry. We observed tight junction rearrangements, indicative of endothelial dysfunction, in five out of eight assessed leukodystrophies of different origin and an altered aquaporin-4 distribution in all. Aquaporin-4 redistribution indicates a general astrocytic dysfunction in leukodystrophies, even in those not directly related to astrocytic pathology or without prominent reactive astrogliosis. These findings provide further evidence for dysfunction in the orchestration of the neurovascular unit in leukodystrophies and contribute to a better understanding of the underlying disease mechanism.

MEK/MELK inhibition and blood-brain barrier deficiencies in atypical teratoid/rhabdoid tumors

Background

Atypical teratoid/rhabdoid tumors (AT/RT) are rare, but highly aggressive. These entities are of embryonal origin occurring in the central nervous system (CNS) of young children. Molecularly these tumors are driven by a single hallmark mutation, resulting in inactivation of SMARCB1 or SMARCA4. Additionally, activation of the MAPK signaling axis and preclinical antitumor efficacy of its inhibition have been described in AT/RT.

Methods

We established and validated a patient-derived neurosphere culture and xenograft model of sonic hedgehog (SHH) subtype AT/RT, at diagnosis and relapse from the same patient. We set out to study the vascular phenotype of these tumors to evaluate the integrity of the blood–brain barrier (BBB) in AT/RT. We also used the model to study combined mitogen-activated protein kinase kinase (MEK) and maternal embryonic leucine zipper kinase (MELK) inhibition as a therapeutic strategy for AT/RT.

Results

We found MELK to be highly overexpressed in both patient samples of AT/RT and our primary cultures and xenografts. We identified a potent antitumor efficacy of the MELK inhibitor OTSSP167, as well as strong synergy with the MEK inhibitor trametinib, against primary AT/RT neurospheres. Additionally, vascular phenotyping of AT/RT patient material and xenografts revealed significant BBB aberrancies in these tumors. Finally, we show in vivo efficacy of the non-BBB penetrable drugs OTSSP167 and trametinib in AT/RT xenografts, demonstrating the therapeutic implications of the observed BBB deficiencies and validating MEK/MELK inhibition as a potential treatment.

Conclusion

Altogether, we developed a combination treatment strategy for AT/RT based on MEK/MELK inhibition and identify therapeutically exploitable BBB deficiencies in these tumors.

Cerebral Microangiopathy in Leukoencephalopathy With Cerebral Calcifications and Cysts: A Pathological Description

Leukoencephalopathy with calcifications and cysts (LCC) is a neurological syndrome recently associated with pathogenic variants in SNORD118. We report autopsy neuropathological findings from an individual with genetically confirmed LCC. Histologic studies included staining of formalin-fixed paraffin-embedded tissue sections by hematoxylin and eosin, elastic van Gieson, and luxol fast blue. Immunohistochemistry stains against glial fibrillary acidic protein, proteolipid protein, phosphorylated neurofilament, CD31, alpha-interferon, LN3, and inflammatory markers were performed. Gross examination revealed dark tan/gray appearing white matter with widespread calcifications. Microscopy revealed a diffuse destructive process due to a vasculopathy with secondary ischemic lesions and mineralization. The vasculopathy involved clustered small vessels, resembling vascular malformations, and sporadic lymphocytic infiltration of vessel walls. The white matter was also diffusely abnormal, with concurrent loss of myelin and axons, tissue rarefaction with multifocal cystic degeneration, and the presence of foamy macrophages, secondary calcifications, and astrogliosis. The midbrain, pons, and cerebellum were diffusely involved. It is not understood why variants in SNORD118 result in a disorder that predominantly causes neurological disease and significantly disrupts the cerebral vasculature. Clinical and radiological benefit was recently reported in an LCC patient treated with Bevacizumab; it is important that these patients are rapidly diagnosed and trial of this treatment modality is considered in appropriate circumstances.

The neurovascular unit in diffuse intrinsic pontine gliomas

Aims: Diffuse intrinsic pontine glioma (DIPG) is a childhood brainstem tumor with a median overall survival of eleven months. Lack of chemotherapy efficacy may be related to an intact blood-brain barrier (BBB). In this study we aim to investigate the neurovascular unit (NVU) in DIPG patients. Methods: DIPG biopsy (n = 4) and autopsy samples (n = 6) and age-matched healthy pons samples (n = 20) were immunohistochemically investigated for plasma protein extravasation, and the expression of tight junction proteins claudin-5 and zonula occludens-1 (ZO-1), basement membrane component laminin, pericyte marker PDGFR-β, and efflux transporters P-gp and BCRP. The mean vascular density and diameter were also assessed. Results: DIPGs show a heterogeneity in cell morphology and evidence of BBB leakage. Both in tumor biopsy and autopsy samples, expression of claudin-5, ZO-1, laminin, PDGFR-β and P-gp was reduced compared to healthy pontine tissues. In DIPG autopsy samples, vascular density was lower compared to healthy pons. The density of small vessels (<10 µm) was significantly lower (P<0.001), whereas the density of large vessels (≥10 µm) did not differ between groups (P = 0.404). The median vascular diameter was not significantly different: 6.21 µm in DIPG autopsy samples (range 2.25-94.85 µm), and 6.26 µm in controls (range 1.17-264.77 µm). Conclusion: Our study demonstrates evidence of structural changes in the NVU in DIPG patients, both in biopsy and autopsy samples, as well as a reduced vascular density in end-stage disease. Adding such a biological perspective may help to better direct future treatment choices for DIPG patients.

DIPG-33. CHARACTERIZING THE NEURO-VASCULAR UNIT IN DIFFUSE INTRINSIC PONTINE GLIOMA

Diffuse intrinsic pontine glioma (DIPG) is a childhood brainstem tumor with a median overall survival of eleven months. Lack of chemotherapy efficacy may be related to an intact blood-brain-barrier (BBB). In this study we aim to compare the neuro-vascular unit (NVU) of DIPG to healthy pons tissue. End-stage DIPG autopsy samples (n=5) and age-matched healthy pons samples (n=22), obtained from the NIH NeuroBioBank, were immunohistochemically stained for tight-junction proteins claudin-5 and zonula occludens-1 (ZO-1), basement membrane component laminin, and pericyte marker PDGFRβ. Claudin-5 stains were also used to determine vascular density and diameters. In DIPG, expression of claudin-5 and ZO-1 was reduced, and claudin-5 was dislocated to the abluminal side of endothelial cells. Laminin expression at the glia limitans was reduced in both pre-existent vessels and neovascular proliferation. In contrast to healthy pons, no PDGFRβ expression was detected. The number of blood vessels in DIPG was significantly reduced compared to healthy pons, 13.9±11.8/mm² versus 26.3±14.2/mm², respectively (P<0.01). Especially the number of smaller blood vessels (<10µm) was significantly lower (P<0.01). Distribution of larger blood vessels (≥10µm) did not differ between groups (P=0.223). Mean vascular diameter was 9.3±9.9µm for DIPG versus 7.7±9.0µm in healthy pons (P=0.016). Our study demonstrates evidence of structural changes in the NVU in end-stage DIPG. Chemotherapeutic inefficacy could be the result of reduced vascular density. However, further research is needed to determine meaning and extent of these changes and to determine whether these observations are caused by the tumor or the result of treatment.

ATRT-18. SHH-SUBTYPE ATYPICAL TERATOID/RHABDOID TUMORS ARE SELECTIVELY SENSITIVE TO GEMCITABINE TREATMENT

Atypical Teratoid Rhabdoid Tumors (ATRT) are highly malignant embryonal tumors of the central nervous system with a dismal prognosis. ATRT can be divided into three molecular subgroups of which the Sonic Hedgehog (SHH) subgroup is most prevalent. In this study, we developed and validated a novel patient-derived ATRT model, which we used along a panel of other primary ATRT models for large scale drug discovery assays. We found that ATRTs are selectively sensitive to the nucleoside analogue gemcitabine, with SHH-subtype ATRTs being the most sensitive subgroup. Gene expression profiles and protein analysis indicated that gemcitabine treatment causes degradation of Sirtuin 1 (SIRT1), which causes ATRT specific cell-death through NF-kB and p53 activation. Furthermore, we found that this gemcitabine induced loss of SIRT1 results in a nucleus-to-cytoplasm shift of the SHH signaling activator Gli, explaining the additional gemcitabine sensitivity in SHH-subtype ATRT. Treatment of SHH-subgroup ATRT xenograft-bearing mice resulted in a &gt;40% increase in median survival (p&lt;0.01, log-rank test) and long-term survivors in two independent models. To prepare translation of our findings to the clinic, we investigated potential gemcitabine induced resistance mechanisms by conducting kinome-wide CRISPR/Cas9 knockout screens in primary ATRT cells. Through these experiments we found that low-dose gemcitabine treatment combined with inhibition of protein kinase C zeta (PKCζ) prevents regrowth of resistant ATRT subclones. Together, these findings show that ATRT are highly sensitive to gemcitabine treatment; and as such we suggest that gemcitabine may be rapidly incorporated into future treatment regimens for SHH-ATRT.

PATH-04. THE BLOOD-BRAIN BARRIER IN DIFFUSE MIDLINE GLIOMA AND ITS IMPLICATIONS FOR DRUG DELIVERY

INTRODUCTION

Chemotherapy has been unsuccessful for pediatric diffuse midline glioma (DMG) most likely due to an intact blood-brain barrier (BBB). However, the BBB has not been characterized in DMG and therefore its implications for drug delivery are unknown. In this study we characterized the BBB in DMG patients and compared this to healthy controls.

METHODS

End-stage DMG pontine samples (n=5) were obtained from the VUmc diffuse intrinsic pontine glioma (DIPG) autopsy study and age-matched healthy pontine samples (n=22) were obtained from the NIH NeuroBioBank. Tissues were stained for BBB markers claudin-5, zonula occludens-1, laminin, and PDGFRβ. Claudin-5 stains were used to determine vascular density and diameter.

RESULTS

In DMG, expression of claudin-5 was reduced and dislocated to the abluminal side of endothelial cells. In addition, the expression of zonula occludens-1 was reduced. The basement membrane protein laminin expression was reduced at the glia limitans in both pre-existent vessels and neovascular proliferation. PDGFRβ expression was not observed in DMG but was present in healthy pons. Furthermore, the number of blood vessels in DMG was significantly (P&lt; 0.01) reduced (13.9 ± 11.8/mm2) compared to healthy pons (26.3 ± 14.2/mm2). Markedly, the number of small blood vessels (&lt; 10µm) was significantly lower (P&lt; 0.01) while larger blood vessels (&gt; 10µm) were not significantly different (P= 0.223). The mean vascular diameter was larger for DMG 9.3 ± 9.9µm compared to 7.7 ± 9.0µm for healthy pons (P= 0.016).

CONCLUSION

Both the BBB and the vasculature are altered at end-stage DMG. The reduced vascular density might have implications for several drug delivery methods such as focused ultrasound and convection enhanced delivery that are being explored for the treatment of DMG. The functional effects of the structurally altered BBB remain unknown and further research is needed to evaluate the BBB integrity at end-stage DMG

Disturbed brain ether lipid metabolism and histology in Sjögren-Larsson syndrome

Sjögren-Larsson syndrome (SLS) is a rare neurometabolic syndrome caused by deficient fatty aldehyde dehydrogenase. Patients exhibit intellectual disability, spastic paraplegia, and ichthyosis. The accumulation of fatty alcohols and fatty aldehydes has been demonstrated in plasma and skin but never in brain. Brain magnetic resonance imaging and spectroscopy studies, however, have shown an abundant lipid peak in the white matter of patients with SLS, suggesting lipid accumulation in the brain as well. Using histopathology, mass spectrometry imaging, and lipidomics, we studied the morphology and the lipidome of a postmortem brain of a 65-year-old female patient with genetically confirmed SLS and compared the results with a matched control brain. Histopathological analyses revealed structural white matter abnormalities with the presence of small lipid droplets, deficient myelin, and astrogliosis. Biochemically, severely disturbed lipid profiles were found in both white and gray matter of the SLS brain, with accumulation of fatty alcohols and ether lipids. Particularly, long-chain unsaturated ether lipid species accumulated, most prominently in white matter. Also, there was a striking accumulation of odd-chain fatty alcohols and odd-chain ether(phospho)lipids. Our results suggest that the central nervous system involvement in SLS is caused by the accumulation of fatty alcohols leading to a disbalance between ether lipid and glycero(phospho)lipid metabolism resulting in a profoundly disrupted brain lipidome. Our data show that SLS is not a pure leukoencephalopathy, but also a gray matter disease. Additionally, the histopathological abnormalities suggest that astrocytes and microglia might play a pivotal role in the underlying disease mechanism, possibly contributing to the impairment of myelin maintenance.

A quantitative neuropathological assessment of translocator protein expression in multiple sclerosis

The 18 kDa translocator protein (TSPO) is increasingly used to study brain and spinal cord inflammation in degenerative diseases of the CNS such as multiple sclerosis. The enhanced TSPO PET signal that arises during disease is widely considered to reflect activated pathogenic microglia, although quantitative neuropathological data to support this interpretation have not been available. With the increasing interest in the role of chronic microglial activation in multiple sclerosis, characterising the cellular neuropathology associated with TSPO expression is of clear importance for understanding the cellular and pathological processes on which TSPO PET imaging is reporting. Here we have studied the cellular expression of TSPO and specific binding of two TSPO targeting radioligands (3H-PK11195 and 3H-PBR28) in tissue sections from 42 multiple sclerosis cases and 12 age-matched controls. Markers of homeostatic and reactive microglia, astrocytes, and lymphocytes were used to investigate the phenotypes of cells expressing TSPO. There was an approximate 20-fold increase in cells double positive for TSPO and HLA-DR in active lesions and in the rim of chronic active lesion, relative to normal appearing white matter. TSPO was uniformly expressed across myeloid cells irrespective of their phenotype, rather than being preferentially associated with pro-inflammatory microglia or macrophages. TSPO+ astrocytes were increased up to 7-fold compared to normal-appearing white matter across all lesion subtypes and accounted for 25% of the TSPO+ cells in these lesions. To relate TSPO protein expression to ligand binding, specific binding of the TSPO ligands 3H-PK11195 and 3H-PBR28 was determined in the same lesions. TSPO radioligand binding was increased up to seven times for 3H-PBR28 and up to two times for 3H-PK11195 in active lesions and the centre of chronic active lesions and a strong correlation was found between the radioligand binding signal for both tracers and the number of TSPO+ cells across all of the tissues examined. In summary, in multiple sclerosis, TSPO expression arises from microglia of different phenotypes, rather than being restricted to microglia which express classical pro-inflammatory markers. While the majority of cells expressing TSPO in active lesions or chronic active rims are microglia/macrophages, our findings also emphasize the significant contribution of activated astrocytes, as well as smaller contributions from endothelial cells. These observations establish a quantitative framework for interpretation of TSPO in multiple sclerosis and highlight the need for neuropathological characterization of TSPO expression for the interpretation of TSPO PET in other neurodegenerative disorders.

Endothelin-1 signaling maintains glial progenitor proliferation in the postnatal subventricular zone

Signaling molecules that regulate neurodevelopmental processes in the early postnatal subventricular zone (SVZ) are critical for proper brain development yet remain poorly characterized. Here, we report that Endothelin-1 (ET-1), a molecular component of the postnatal SVZ, promotes radial glial cell maintenance and proliferation in an autocrine manner via Notch signaling. Loss of ET-1 signaling increases neurogenesis and reduces oligodendrocyte progenitor cell proliferation (OPC) in the developing SVZ, thereby altering cellular output of the stem cell niche. We also show that ET-1 is required for increased neural stem cell and OPC proliferation in the adult mouse SVZ following demyelination. Lastly, high levels of ET-1 in the SVZ of patients with Cathepsin A-related arteriopathy with strokes and leukoencephalopathy correlate with an increased number of SVZ OPCs, suggesting ET-1's role as a regulator of glial progenitor proliferation may be conserved in humans. ET-1 signaling therefore presents a potential new therapeutic target for promoting SVZ-mediated cellular repair.

Combined Therapy of AXL and HDAC Inhibition Reverses Mesenchymal Transition in Diffuse Intrinsic Pontine Glioma

PURPOSE

Diffuse intrinsic pontine glioma (DIPG) is an incurable type of pediatric brain cancer, which in the majority of cases is driven by mutations in genes encoding histone 3 (H3K27M). We here determined the preclinical therapeutic potential of combined AXL and HDAC inhibition in these tumors to reverse their mesenchymal, therapy-resistant, phenotype.

EXPERIMENTAL DESIGN

We used public databases and patient-derived DIPG cells to identify putative drivers of the mesenchymal transition in these tumors. Patient-derived neurospheres, xenografts, and allografts were used to determine the therapeutic potential of combined AXL/HDAC inhibition for the treatment of DIPG.

RESULTS

We identified AXL as a therapeutic target and regulator of the mesenchymal transition in DIPG. Combined AXL and HDAC inhibition had a synergistic and selective antitumor effect on H3K27M DIPG cells. Treatment of DIPG cells with the AXL inhibitor BGB324 and the HDAC inhibitor panobinostat resulted in a decreased expression of mesenchymal and stem cell genes. Moreover, this combination treatment decreased expression of DNA damage repair genes in DIPG cells, strongly sensitizing them to radiation. Pharmacokinetic studies showed that BGB324, like panobinostat, crosses the blood-brain barrier. Consequently, treatment of patient-derived DIPG xenograft and murine DIPG allograft-bearing mice with BGB324 and panobinostat resulted in a synergistic antitumor effect and prolonged survival.

CONCLUSIONS

Combined inhibition of AXL and HDACs in DIPG cells results in a synergistic antitumor effect by reversing their mesenchymal, stem cell-like, therapy-resistant phenotype. As such, this treatment combination may serve as part of a future multimodal therapeutic strategy for DIPG.

Metachromatic leukodystrophy and transplantation: remyelination, no cross-correction

OBJECTIVE

In metachromatic leukodystrophy, a lysosomal storage disorder due to decreased arylsulfatase A activity, hematopoietic stem cell transplantation may stop brain demyelination and allow remyelination, thereby halting white matter degeneration. This is the first study to define the effects and therapeutic mechanisms of hematopoietic stem cell transplantation on brain tissue of transplanted metachromatic leukodystrophy patients.

METHODS

Autopsy brain tissue was obtained from eight (two transplanted and six nontransplanted) metachromatic leukodystrophy patients, and two age-matched controls. We examined the presence of donor cells by immunohistochemistry and microscopy. In addition, we assessed myelin content, oligodendrocyte numbers, and macrophage phenotypes. An unpaired t-test, linear regression or the nonparametric Mann-Whitney U-test was performed to evaluate differences between the transplanted, nontransplanted, and control group.

RESULTS

In brain tissue of transplanted patients, we found metabolically competent donor macrophages expressing arylsulfatase A distributed throughout the entire white matter. Compared to nontransplanted patients, these macrophages preferentially expressed markers of alternatively activated, anti-inflammatory cells that may support oligodendrocyte survival and differentiation. Additionally, transplanted patients showed higher numbers of oligodendrocytes and evidence for remyelination. Contrary to the current hypothesis on therapeutic mechanism of hematopoietic cell transplantation in metachromatic leukodystrophy, we detected no enzymatic cross-correction to resident astrocytes and oligodendrocytes.

INTERPRETATION

In conclusion, donor macrophages are able to digest accumulated sulfatides and may play a neuroprotective role for resident oligodendrocytes, thereby enabling remyelination, albeit without evidence of cross-correction of oligo- and astroglia. These results emphasize the importance of immunomodulation in addition to the metabolic correction, which might be exploited for improved outcomes.

Gastrointestinal Dysmotility in MNGIE: from thymidine phosphorylase enzyme deficiency to altered interstitial cells of Cajal

Background

MNGIE is a rare and fatal disease in which absence of the enzyme thymidine phosphorylase induces systemic accumulation of thymidine and deoxyuridine and secondary mitochondrial DNA alterations. Gastrointestinal (GI) symptoms are frequently reported in MNGIE patients, however, they are not resolved with the current treatment interventions.

Recently, our understanding of the GI pathology has increased, which rationalizes the pursuit of more targeted therapeutic strategies. In particular, interstitial cells of Cajal (ICC) play key roles in GI physiology and are involved in the pathogenesis of the GI dysmotility. However, understanding of the triggers of ICC deficits in MNGIE is lacking. Herein, we review the current knowledge about the pathology of GI dysmotility in MNGIE, discuss potential mechanisms in relation to ICC loss/dysfunction, remark on the limited contribution of the current treatments, and propose intervention strategies to overcome ICC deficits. Finally, we address the advances and new research avenues offered by organoids and tissue engineering technologies, and propose schemes to implement to further our understanding of the GI pathology and utility in regenerative and personalized medicine in MNGIE.

Conclusion

Interstitial cells of Cajal play key roles in the physiology of the gastrointestinal motility. Evaluation of their status in the GI dysmotility related to MNGIE would be valuable for diagnosis of MNGIE. Understanding the underlying pathological and molecular mechanisms affecting ICC is an asset for the development of targeted prevention and treatment strategies for the GI dysmotility related to MNGIE.

Biallelic variants in LARS2 and KARS cause deafness and (ovario)leukodystrophy

OBJECTIVE

To describe the leukodystrophy caused by pathogenic variants in LARS2 and KARS, encoding mitochondrial leucyl transfer RNA (tRNA) synthase and mitochondrial and cytoplasmic lysyl tRNA synthase, respectively.

METHODS

We composed a group of 5 patients with leukodystrophy, in whom whole-genome or whole-exome sequencing revealed pathogenic variants in LARS2 or KARS. Clinical information, brain MRIs, and postmortem brain autopsy data were collected. We assessed aminoacylation activities of purified mutant recombinant mitochondrial leucyl tRNA synthase and performed aminoacylation assays on patients' lymphoblasts and fibroblasts.

RESULTS

Patients had a combination of early-onset deafness and later-onset neurologic deterioration caused by progressive brain white matter abnormalities on MRI. Female patients with LARS2 pathogenic variants had premature ovarian failure. In 2 patients, MRI showed additional signs of early-onset vascular abnormalities. In 2 other patients with LARS2 and KARS pathogenic variants, magnetic resonance spectroscopy revealed elevated white matter lactate, suggesting mitochondrial disease. Pathology in one patient with LARS2 pathogenic variants displayed evidence of primary disease of oligodendrocytes and astrocytes with lack of myelin and deficient astrogliosis. Aminoacylation activities of purified recombinant mutant leucyl tRNA synthase showed a 3-fold loss of catalytic efficiency. Aminoacylation assays on patients' lymphoblasts and fibroblasts showed about 50% reduction of enzyme activity.

CONCLUSION

This study adds LARS2 and KARS pathogenic variants as gene defects that may underlie deafness, ovarian failure, and leukodystrophy with mitochondrial signature. We discuss the specific MRI characteristics shared by leukodystrophies caused by mitochondrial tRNA synthase defects. We propose to add aminoacylation assays as biochemical diagnostic tools for leukodystrophies.

Vanishing white matter: a leukodystrophy due to astrocytic dysfunction

VWM is one of the most prevalent leukodystrophies with unique clinical, pathological and molecular features. It mostly affects children, but may develop at all ages, from birth to senescence. It is dominated by cerebellar ataxia and susceptible to stresses that act as factors provoking disease onset or episodes of rapid neurological deterioration possibly leading to death. VWM is caused by mutations in any of the genes encoding the five subunits of the eukaryotic translation initiation factor 2B (eIF2B). Although eIF2B is ubiquitously expressed, VWM primarily manifests as a leukodystrophy with increasing white matter rarefaction and cystic degeneration, meager astrogliosis with no glial scarring and dysmorphic immature astrocytes and increased numbers of oligodendrocyte progenitor cells that are restrained from maturing into myelin-forming cells. Recent findings point to a central role for astrocytes in driving the brain pathology, with secondary effects on both oligodendroglia and axons. In this, VWM belongs to the growing group of astrocytopathies, in which loss of essential astrocytic functions and gain of detrimental functions drive degeneration of the white matter. Additional disease mechanisms include activation of the unfolded protein response with constitutive predisposition to cellular stress, failure of astrocyte-microglia crosstalk and possibly secondary effects on the oxidative phosphorylation. VWM involves a translation initiation factor. The group of leukodystrophies due to defects in mRNA translation is also growing, suggesting that this may be a common disease mechanism. The combination of all these features makes VWM an intriguing natural model to understand the biology and pathology of the white matter.

Leukodystrophies due to astroyctic dysfunction

Leukodystrophies are genetically determined disorders due to defects in any structural components of the brain white matter. This mini-symposium presents a selection of leukodystrophies due to astrocytic dysfunction, the astrocytopathies. Examples are provided of astrocytopathies due to defects in astrocyte-specific proteins and in which astrocytes play a major role in the pathophysiology. Knowledge on the disease mechanisms underlying these leukodystrophies also provides information how loss of physiologic functions and gain of detrimental functions in astrocytes leads to degeneration of the white matter.

MELK Inhibition in Diffuse Intrinsic Pontine Glioma

Purpose: Diffuse intrinsic pontine glioma (DIPG) is a highly aggressive pediatric brain tumor, for which no effective therapeutic options currently exist. We here determined the potential of inhibition of the maternal embryonic leucine zipper kinase (MELK) for the treatment of DIPG.Experimental Design: We evaluated the antitumor efficacy of the small-molecule MELK inhibitor OTSSP167 in vitro in patient-derived DIPG cultures, and identified the mechanism of action of MELK inhibition in DIPG by RNA sequencing of treated cells. In addition, we determined the blood-brain barrier (BBB) penetration of OTSSP167 and evaluated its translational potential by treating mice bearing patient-derived DIPG xenografts.Results: This study shows that MELK is highly expressed in DIPG cells, both in patient samples and in relevant in vitro and in vivo models, and that treatment with OTSSP167 strongly decreases proliferation of patient-derived DIPG cultures. Inhibition of MELK in DIPG cells functions through reducing inhibitory phosphorylation of PPARγ, resulting in an increase in nuclear translocation and consequent transcriptional activity. Brain pharmacokinetic analyses show that OTSSP167 is a strong substrate for both MDR1 and BCRP, limiting its BBB penetration. Nonetheless, treatment of Mdr1a/b;Bcrp1 knockout mice carrying patient-derived DIPG xenografts with OTSSP167 decreased tumor growth, induced remissions, and resulted in improved survival.Conclusions: We show a strong preclinical effect of the kinase inhibitor OTSSP167 in the treatment of DIPG and identify the MELK-PPARγ signaling axis as a putative therapeutic target in this disease. Clin Cancer Res; 24(22); 5645-57. ©2018 AACR.

Axonal abnormalities in vanishing white matter

Objective

We aimed to study the occurrence and development of axonal pathology and the influence of astrocytes in vanishing white matter.

Methods

Axons and myelin were analyzed using electron microscopy and immunohistochemistry on Eif2b4 and Eif2b5 single‐ and double‐mutant mice and patient brain tissue. In addition, astrocyte‐forebrain co‐culture studies were performed.

Results

In the corpus callosum of Eif2b5‐mutant mice, myelin sheath thickness, axonal diameter, and G‐ratio developed normally up to 4 months. At 7 months, however, axons had become thinner, while in control mice axonal diameters had increased further. Myelin sheath thickness remained close to normal, resulting in an abnormally low G‐ratio in Eif2b5‐mutant mice. In more severely affected Eif2b4‐Eif2b5 double‐mutants, similar abnormalities were already present at 4 months, while in milder affected Eif2b4 mutants, few abnormalities were observed at 7 months. Additionally, from 2 months onward an increased percentage of thin, unmyelinated axons and increased axonal density were present in Eif2b5‐mutant mice. Co‐cultures showed that Eif2b5 mutant astrocytes induced increased axonal density, also in control forebrain tissue, and that control astrocytes induced normal axonal density, also in mutant forebrain tissue. In vanishing white matter patient brains, axons and myelin sheaths were thinner than normal in moderately and severely affected white matter. In mutant mice and patients, signs of axonal transport defects and cytoskeletal abnormalities were minimal.

Interpretation

In vanishing white matter, axons are initially normal and atrophy later. Astrocytes are central in this process. If therapy becomes available, axonal pathology may be prevented with early intervention.

Increased White Matter Inflammation in Aging- and Alzheimer's Disease Brain

Chronic neuroinflammation, which is primarily mediated by microglia, plays an essential role in aging and neurodegeneration. It is still unclear whether this microglia-induced neuroinflammation occurs globally or is confined to distinct brain regions. In this study, we investigated microglia activity in various brain regions upon healthy aging and Alzheimer's disease (AD)-related pathology in both human and mouse samples. In purified microglia isolated from aging mouse brains, we found a profound gene expression pattern related to pro-inflammatory processes, phagocytosis, and lipid homeostasis. Particularly in white matter microglia of 24-month-old mice, abundant expression of phagocytic markers including Mac-2, Axl, CD16/32, Dectin1, CD11c, and CD36 was detected. Interestingly, in white matter of human brain tissue the first signs of inflammatory activity were already detected during middle age. Thus quantification of microglial proteins, such as CD68 (commonly associated with phagocytosis) and HLA-DR (associated with antigen presentation), in postmortem human white matter brain tissue showed an age-dependent increase in immunoreactivity already in middle-aged people (53.2 ± 2.0 years). This early inflammation was also detectable by non-invasive positron emission tomography imaging using [¹¹C]-(R)-PK11195, a ligand that binds to activated microglia. Increased microglia activity was also prominently present in the white matter of human postmortem early-onset AD (EOAD) brain tissue. Interestingly, microglia activity in the white matter of late-onset AD (LOAD) CNS was similar to that of the aged clinically silent AD cases. These data indicate that microglia-induced neuroinflammation is predominant in the white matter of aging mice and humans as well as in EOAD brains. This white matter inflammation may contribute to the progression of neurodegeneration, and have prognostic value for detecting the onset and progression of aging and neurodegeneration.

Megalencephalic leukoencephalopathy with cysts: the Glialcam-null mouse model

Objective

Megalencephalic leukoencephalopathy with cysts (MLC) is a genetic infantile‐onset disease characterized by macrocephaly and white matter edema due to loss of MLC1 function. Recessive mutations in either MLC1 or GLIALCAM cause the disease. MLC1 is involved in astrocytic volume regulation; GlialCAM ensures the correct membrane localization of MLC1. Their exact role in brain ion‐water homeostasis is only partly defined. We characterized Glialcam‐null mice for further studies.

Methods

We investigated the consequences of loss of GlialCAM in Glialcam‐null mice and compared GlialCAM developmental expression in mice and men.

Results

Glialcam‐null mice had early‐onset megalencephaly and increased brain water content. From 3 weeks, astrocytes were abnormal with swollen processes abutting blood vessels. Concomitantly, progressive white matter vacuolization developed due to intramyelinic edema. Glialcam‐null astrocytes showed abolished expression of MLC1, reduced expression of the chloride channel ClC‐2 and increased expression and redistribution of the water channel aquaporin4. Expression of other MLC1‐interacting proteins and the volume regulated anion channel LRRC8A was unchanged. In mice, GlialCAM expression increased until 3 weeks and then stabilized. In humans, GlialCAM expression was highest in the first 3 years to then decrease and stabilize from approximately 5 years.

Interpretation

Glialcam‐null mice replicate the early stages of the human disease with early‐onset intramyelinic edema. The earliest change is astrocytic swelling, further substantiating that a defect in astrocytic volume regulation is the primary cellular defect in MLC. GlialCAM expression affects expression of MLC1, ClC‐2 and aquaporin4, indicating that abnormal interplay between these proteins is a disease mechanism in megalencephalic leukoencephalopathy with cysts.

Ageing and recurrent episodes of neuroinflammation promote progressive experimental autoimmune encephalomyelitis in Biozzi ABH mice

Current therapies for multiple sclerosis (MS) reduce the frequency of relapses by modulating adaptive immune responses but fail to limit the irreversible neurodegeneration driving progressive disability. Experimental autoimmune encephalomyelitis (EAE) in Biozzi ABH mice recapitulates clinical features of MS including relapsing-remitting episodes and secondary-progressive disability. To address the contribution of recurrent inflammatory events and ageing as factors that amplify progressive neurological disease, we examined EAE in 8- to 12-week-old and 12-month-old ABH mice. Compared with the relapsing-remitting (RREAE) and secondary progressive (SPEAE) EAE observed in young mice, old mice developed progressive disease from onset (PEAE) associated with pronounced axonal damage and increased numbers of CD3(+) T cells and microglia/macrophages, but not B cells. Whereas the clinical neurological features of PEAE and SPEAE were comparable, the pathology was distinct. SPEAE was associated with significantly reduced perivascular infiltrates and T-cell numbers in the central nervous system (CNS) compared with PEAE and the acute phase of RREAE. In contrast to perivascular infiltrates that declined during progression from RREAE into SPEAE, the numbers of microglia clusters remained constant. Similar to what is observed during MS, the microglia clusters emerging during EAE were associated with axonal damage and oligodendrocytes expressing heat-shock protein B5, but not lymphocytes. Taken together, our data reveal that the course of EAE is dependent on the age of the mice. Younger mice show a relapsing-remitting phase followed by progressive disease, whereas old mice immediately show progression. This indicates that recurrent episodes of inflammation in the CNS, as well as age, contribute to progressive neurological disease.

Small heat shock proteins are induced during multiple sclerosis lesion development in white but not grey matter

Introduction

The important protective role of small heat-shock proteins (HSPs) in regulating cellular survival and migration, counteracting protein aggregation, preventing apoptosis, and regulating inflammation in the central nervous system is now well-recognized. Yet, their role in the neuroinflammatory disorder multiple sclerosis (MS) is largely undocumented. With the exception of alpha B-crystallin (HSPB5), little is known about the roles of small HSPs in disease.

Results

Here, we examined the expression of four small HSPs during lesion development in MS, focussing on their cellular distribution, and regional differences between white matter (WM) and grey matter (GM). It is well known that MS lesions in these areas differ markedly in their pathology, with substantially more intense blood-brain barrier damage, leukocyte infiltration and microglial activation typifying WM but not GM lesions.

We analysed transcript levels and protein distribution profiles for HSPB1, HSPB6, HSPB8 and HSPB11 in MS lesions at different stages, comparing them with normal-appearing brain tissue from MS patients and non-neurological controls. During active stages of demyelination in WM, and especially the centre of chronic active MS lesions, we found significantly increased expression of HSPB1, HSPB6 and HSPB8, but not HSPB11. When induced, small HSPs were exclusively found in astrocytes but not in oligodendrocytes, microglia or neurons. Surprisingly, while the numbers of astrocytes displaying high expression of small HSPs were markedly increased in actively demyelinating lesions in WM, no such induction was observed in GM lesions. This difference was particularly obvious in leukocortical lesions covering both WM and GM areas.

Conclusions

Since induction of small HSPs in astrocytes is apparently a secondary response to damage, their differential expression between WM and GM likely reflects differences in mediators that accompany demyelination in either WM or GM during MS. Our findings also suggest that during MS, cortical structures fail to benefit from the protective actions of small HSPs.

Electronic supplementary material

The online version of this article (doi:10.1186/s40478-015-0267-2) contains supplementary material, which is available to authorized users.

GM-CSF promotes migration of human monocytes across the blood brain barrier

Multiple sclerosis (MS) is a chronic inflammatory demyelinating disease of the central nervous system (CNS). Infiltration of monocytes into the CNS is crucial for disease onset and progression. Animal studies indicate that granulocyte-macrophages colony-stimulating factor (GM-CSF) may play an essential role in this process, possibly by acting on the migratory capacities of myeloid cells across the blood-brain barrier. This study describes the effect of GM-CSF on human monocytes, macrophages, and microglia. Furthermore, the expression of GM-CSF and its receptor was investigated in the CNS under healthy and pathological conditions. We show that GM-CSF enhances monocyte migration across human blood-brain barrier endothelial cells in vitro. Next, immunohistochemical analysis on human brain tissues revealed that GM-CSF is highly expressed by microglia and macrophages in MS lesions. The GM-CSF receptor is expressed by neurons in the rim of combined gray/white matter lesions and astrocytes. Finally, the effect of GM-CSF on human macrophages was determined, revealing an intermediate activation status, with a phenotype similar to that observed in active MS lesions. Together our data indicate that GM-CSF is a powerful stimulator of monocyte migration, and is abundantly present in the inflamed CNS where it may act as an activator of macrophages and microglia.

Inflammation in neurodegenerative diseases--an update

Neurodegeneration, the progressive dysfunction and loss of neurons in the central nervous system (CNS), is the major cause of cognitive and motor dysfunction. While neuronal degeneration is well-known in Alzheimer's and Parkinson's diseases, it is also observed in neurotrophic infections, traumatic brain and spinal cord injury, stroke, neoplastic disorders, prion diseases, multiple sclerosis and amyotrophic lateral sclerosis, as well as neuropsychiatric disorders and genetic disorders. A common link between these diseases is chronic activation of innate immune responses including those mediated by microglia, the resident CNS macrophages. Such activation can trigger neurotoxic pathways leading to progressive degeneration. Yet, microglia are also crucial for controlling inflammatory processes, and repair and regeneration. The adaptive immune response is implicated in neurodegenerative diseases contributing to tissue damage, but also plays important roles in resolving inflammation and mediating neuroprotection and repair. The growing awareness that the immune system is inextricably involved in mediating damage as well as regeneration and repair in neurodegenerative disorders, has prompted novel approaches to modulate the immune system, although it remains whether these approaches can be used in humans. Additional factors in humans include ageing and exposure to environmental factors such as systemic infections that provide additional clues that may be human specific and therefore difficult to translate from animal models. Nevertheless, a better understanding of how immune responses are involved in neuronal damage and regeneration, as reviewed here, will be essential to develop effective therapies to improve quality of life, and mitigate the personal, economic and social impact of these diseases.

Human macrophage polarization in vitro: maturation and activation methods compared

Macrophages form a heterogeneous cell population displaying multiple functions, and can be polarized into pro- (M1) or anti-inflammatory (M2) macrophages, by environmental factors. Their activation status reflects a beneficial or detrimental role in various diseases. Currently several in vitro maturation and activation protocols are used to induce an M1 or M2 phenotype. Here, the impact of different maturation factors (NHS, M-CSF, or GM-CSF) and activation methods (IFN-γ/LPS, IL-4, dexamethason, IL-10) on the macrophage phenotype was determined. Regarding macrophage morphology, pro-inflammatory (M1) activation stimulated cell elongation, and anti-inflammatory (M2) activation induced a circular appearance. Activation with pro-inflammatory mediators led to increased CD40 and CD64 expression, whereas activation with anti-inflammatory factors resulted in increased levels of MR and CD163. Production of pro-inflammatory cytokines was induced by activation with IFN-γ/LPS, and TGF-β production was enhanced by the maturation factors M-CSF and GM-CSF. Our data demonstrate that macrophage marker expression and cytokine production in vitro is highly dependent on both maturation and activation methods. In vivo macrophage activation is far more complex, since a plethora of stimuli are present. Hence, defining the macrophage activation status ex vivo on a limited number of markers could be indecisive. From this study we conclude that maturation with M-CSF or GM-CSF induces a moderate anti- or pro-inflammatory state respectively, compared to maturation with NHS. CD40 and CD64 are the most distinctive makers for human M1 and CD163 and MR for M2 macrophage activation and therefore can be helpful in determining the activation status of human macrophages ex vivo.

Macrophages migrate in an activation-dependent manner to chemokines involved in neuroinflammation

Background

In neuroinflammatory diseases, macrophages can play a dual role in the process of tissue damage, depending on their activation status (M1 / M2). M1 macrophages are considered to exert damaging effects to neurons, whereas M2 macrophages are reported to aid regeneration and repair of neurons. Their migration within the central nervous system may be of critical importance in the final outcome of neurodegeneration in neuroinflammatory diseases e.g. multiple sclerosis (MS). To provide insight into this process, we examined the migratory capacity of human monocyte-derived M1 and M2 polarised macrophages towards chemoattractants, relevant for neuroinflammatory diseases like MS.

Methods

Primary cultures of human monocyte-derived macrophages were exposed to interferon gamma and lipopolysaccharide (LPS) to evoke proinflammatory (M1) activation or IL-4 to evoke anti-inflammatory (M2) activation. In a TAXIScan assay, migration of M0, M1 and M2 towards chemoattractants was measured and quantified. Furthermore the adhesion capacity and the expression levels of integrins as well as chemokine receptors of M0, M1 and M2 were assessed. Alterations in cell morphology were analysed using fluorescent labelling of the cytoskeleton.

Results

Significant differences were observed between M1 and M2 macrophages in the migration towards chemoattractants. We show that M2 macrophages migrated over longer distances towards CCL2, CCL5, CXCL10, CXCL12 and C1q compared to non-activated (M0) and M1 macrophages. No differences were observed in the adhesion of M0, M1 and M2 macrophages to multiple matrix components, nor in the expression of integrins and chemokine receptors. Significant changes were observed in the cytoskeleton organization upon stimulation with CCL2, M0, M1 and M2 macrophages adopt a spherical morphology and the cytoskeleton is rapidly rearranged. M0 and M2 macrophages are able to form filopodia, whereas M1 macrophages only adapt a spherical morphology.

Conclusions

Together our results indicate that the alternative activation status of macrophages promotes their migratory properties to chemoattractants relevant for neuroinflammatory diseases like MS. Conversely, classically activated, proinflammatory macrophages have reduced migratory properties. Based on our results, we postulate that the activation status of the macrophage influences the capacity of the macrophages to rearrange their cytoskeleton. This is the first step in understanding how modulation of macrophage activation affects macrophage migration in neuroinflammatory diseases like MS.