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Summary: Over half of patients with brain tumors experience debilitating and often progressive cognitive decline after radiotherapy treatment. Microglia, the resident macrophages in the brain, have been implicated in this decline. In response to various insults, microglia can develop innate immune memory (IIM), which can either enhance (priming or training) or repress (tolerance) the response to subsequent inflammatory challenges. Here, we investigate whether radiation affects the IIM of microglia by irradiating the brains of rats and later exposing them to a secondary inflammatory stimulus. Comparative transcriptomic profiling and protein validation of microglia isolated from irradiated rats show a stronger immune response to a secondary inflammatory insult, demonstrating that radiation can lead to long-lasting molecular reprogramming of microglia. Transcriptomic analysis of postmortem normal-appearing non-tumor brain tissue of patients with glioblastoma indicates that radiation-induced microglial priming is likely conserved in humans. Targeting microglial priming or avoiding further inflammatory insults could decrease radiotherapy-induced neurotoxicity.

Background: The increased inflammatory state observed in schizophrenia post-mortem brain and midbrain tissue [1-5] may have detrimental consequences on the brain vasculature of patients with schizophrenia. On the other hand, intrinsic alterations in the brain vasculature of schizophrenia patients may impair toxin efflux and favour the ingression of toxic material and immune cells into the central nervous system, leading to neuroinflammation. Evidence derived from blood and cerebrospinal fluid measurements indicated increased levels of adhesion molecules, such as sP-selectin and sICAM, in schizophrenia [6-8], which may favour the ingression of immune cells into the brain, and higher levels of albumin in CSF as compared to controls, suggesting increased BBB permeability in schizophrenia patients [6, 7, 9]. However, the proper functioning of the BBB involves a variety of cell types, and it is not known which cell type(s) of the BBB may be particularly affected in schizophrenia. Methods: We combined a fluorescence activated sorting isolation strategy with single-nucleus RNA sequencing (snRNAseq) to characterize the cells of the BBB in the midbrain of 15 schizophrenia donors and 14 matched controls, from the well-characterized Stanley Brain Collection. The cases used for the present study were previously grouped into high and low inflammation cases, based on the expression of inflammatory cytokines in their cortical grey matter, and 4 out of the 15 schizophrenia cases were assigned to the cortical high-inflammation group [10]. The different cell types of the BBB were identified with unbiased cluster analysis of nuclear transcriptomic profiles and annotated based on the expression of cell type-specific marker genes. Subclustering analysis, gene set enrichment and comparisons with previously published datasets were carried out to identify sub-populations among the main BBB cell types. We compared the transcriptomic profiles of the major BBB cell types between schizophrenia and controls. In addition, the relative abundance of the main BBB cell types and sub-populations was compared between the diagnoses, using generalized linear modeling. Results: We did not identify changes in the relative abundance of the major BBB cell types, nor in the sub-populations, associated with schizophrenia. However, we identified 14 differentially expressed genes in the cells of the BBB in schizophrenia as compared to controls, including genes that have previously been related to schizophrenia, such as FOXP2 and PDE4D. These transcriptional changes associated with schizophrenia were limited to the ependymal cells and pericytes. Additionally, we detected a sub-population of protoplasmic astrocytes enriched in the high inflammation schizophrenia subgroup. Genes more abundantly expressed in these schizophrenia-related protoplasmic astrocytes were associated with glutamatergic synaptic function rather than with inflammation. Conclusion: Our overall diagnostic results suggest the relative abundance of the BBB cell types and cellular sub-populations remains unaltered in schizophrenia pathology. Nonetheless, transcriptional changes in the midbrain BBB cells associated with schizophrenia are found, but are limited and are specific to two cell types, the ependymal and pericytes. In addition, the high inflammatory status observed in a subgroup of the patients may be interfering with the normal activities of protoplasmic astrocytes, possibly associated with midbrain dopamine and glutamate neurons.

Autophagy is a lysosomal degradative pathway essential for maintaining cellular homeostasis and is also implicated in multiple aspects of both innate and adaptive immunity. Neuroinflammation, along with demyelination and axonal loss, is an important component of multiple sclerosis (MS). Induction of autophagy ameliorated disease progression in experimental autoimmune encephalomyelitis (EAE), a mouse model for MS, underlying a possible link between autophagy and MS pathology. However, it is still unclear how autophagy is affected during different stages of MS. Here, we show a decreased expression of the autophagy-related (ATG) genes during the acute phase of EAE development in mice as well as in mixed active/inactive lesions of post-mortem human MS brain tissues. Using spatial transcriptomics, we observed that this decreased ATG gene expression is most prominent in the core of mixed active/inactive lesions. Furthermore, we observed a hyper-activation of the mammalian target of rapamycin complex 1 (mTORC1) in lesions, which could inhibit both the initiation of autophagy and the transcription factors that regulate the expression of the ATG genes. Thus, based on our data, we propose a negative regulation of autophagy in MS, possibly through persistent mTORC1 activation, which depends on the lesion stage. Our results contribute to the understanding of the role of autophagy in different stages of MS pathology and point to the mTORC1 pathway as a potential modulator that likely regulates central nervous system (CNS) homeostasis and neuroinflammation in MS.

Maternal inflammation during pregnancy can have detrimental effects on embryonic development that persist during adulthood. However, the underlying mechanisms and insights in the responsible cell types are still largely unknown. Here we report the effect of maternal inflammation on fetal microglia, the innate immune cells of the central nervous system (CNS). In mice, a challenge with LPS during late gestation stages (days 15–16–17) induced a pro-inflammatory response in fetal microglia. Adult whole brain microglia of mice that were exposed to LPS during embryonic development displayed a persistent reduction in pro-inflammatory activation in response to a re-challenge with LPS. In contrast, hippocampal microglia of these mice displayed an increased inflammatory response to an LPS re-challenge. In addition, a reduced expression of brain-derived neurotrophic factor (BDNF) was observed in hippocampal microglia of LPS-offspring. Microglia-derived BDNF has been shown to be important for learning and memory processes. In line with these observations, behavioral- and learning tasks with mice that were exposed to maternal inflammation revealed reduced home cage activity, reduced anxiety and reduced learning performance in a T-maze. These data show that exposure to maternal inflammation during late gestation results in long term changes in microglia responsiveness during adulthood, which is different in nature in hippocampus compared to total brain microglia.