CNS macrophages and peripheral myeloid cells in brain tumours

Primary brain tumours (gliomas) initiate a strong host response and can contain large amounts of immune cells (myeloid cells) such as microglia and tumour-infiltrating macrophages. In gliomas the course of pathology is not only controlled by the genetic make-up of the tumour cells, but also depends on the interplay with myeloid cells in the tumour microenvironment. Especially malignant gliomas such as glioblastoma multiforme (GBM) are notoriously immune-suppressive and it is now evident that GBM cells manipulate myeloid cells to support tumour expansion. The protumorigenic effects of glioma-associated myeloid cells comprise a support for angiogenesis as well as tumour cell invasion, proliferation and survival. Different strategies for inhibiting the pathological functions of myeloid cells in gliomas are explored, and blocking the tropism of microglia/macrophages to gliomas or manipulating the signal transduction pathways for immune cell activation has been successful in pre-clinical models. Hence, myeloid cells are now emerging as a promising target for new adjuvant therapies for gliomas. However, it is also becoming evident that some myeloid-directed glioma therapies may only be beneficial for distinct subclasses of gliomas and that a more cell-type-specific manipulation of either microglia or macrophages may improve therapeutic outcomes

Brain-derived neurotrophic factor reduces amyloidogenic processing through control of SORLA gene expression

Sorting protein-related receptor with A-type repeats (SORLA) is a major risk factor in cellular processes leading to Alzheimer's disease (AD). It acts as sorting receptor for the amyloid precursor protein (APP) that regulates intracellular trafficking and processing into amyloidogenic-beta peptides (Abeta). Overexpression of SORLA in neurons reduces while inactivation of gene expression (as in knock-out mouse models) accelerates amyloidogenic processing and senile plaque formation. The current study aimed at identifying molecular pathways that control SORLA gene transcription in vivo and that may contribute to low levels of receptor expression in the brain of patients with AD. Using screening approaches in primary neurons, we identified brain-derived neurotrophic factor (BDNF) as a major inducer of Sorla that activates receptor gene transcription through the ERK (extracellular regulated kinase) pathway. In line with a physiological role as regulator of Sorla, expression of the receptor is significantly impaired in mouse models with genetic (Bdnf(-/-)) or disease-related loss of BDNF activity in the brain (Huntington's disease). Intriguingly, exogenous application of BDNF reduced Abeta production in primary neurons and in the brain of wild-type mice in vivo, but not in animals genetically deficient for Sorla. These findings demonstrate that the beneficial effects ascribed to BDNF in APP metabolism act through induction of Sorla that encodes a negative regulator of neuronal APP processing

O-Vanillin attenuates the TLR2 mediated tumor-promoting phenotype of microglia

Malignant gliomas are primary brain tumors with poor prognoses. These tumors are infiltrated by brain intrinsic microglia and peripheral monocytes which promote glioma cell invasion. In our previous studies, we discovered that the activation of Toll-like receptor 2 (TLR2) on microglia/brain macrophages converts them into a protumorigenic phenotype through the induction of matrix metalloproteinases (MMP) 9 and 14. In the present study, we used in vitro and in situ microglia-glioma interaction experimental models to test the impact of a novel inhibitor of TLR 2, ortho vanillin (O-Vanillin) to block TLR2 mediated microglia protumorigenic phenotype. We demonstrate that O-Vanillin inhibits the TLR2 mediated upregulation of MMP 9, MMP 14, IL 6 and iNOS expression. Similarly, the glioma supernatant induced MMP 9 and MMP 14 expression in murine and human microglia is abrogated by O-Vanillin treatment. O-Vanillin is not toxic for microglia, astrocytes or oligodendrocytes. Glioma growth in murine brain slice cultures is significantly reduced after treatment with O-Vanillin, and this reduced glioma growth depends on the presence of microglia. In addition, we also found that O-Vanillin inhibited the glioma induced proliferation of murine primary microglia. In summary, O-Vanillin attenuates the pro-tumorigenic phenotype of microglia/brain macrophages and thus qualifies as a candidate for glioma therapy

Glioma stem cells but not bulk glioma cells upregulate IL-6 secretion in microglia/brain macrophages via toll-like receptor 4 signaling

Peripheral macrophages and resident microglia constitute the dominant glioma-infiltrating cells. The tumor induces an immunosuppressive and tumor-supportive phenotype in these glioma-associated microglia/brain macrophages (GAMs). A subpopulation of glioma cells acts as glioma stem cells (GSCs). We explored the interaction between GSCs and GAMs. Using CD133 as a marker of stemness, we enriched for or deprived the mouse glioma cell line GL261 of GSCs by fluorescence-activated cell sorting (FACS). Over the same period of time, 100 CD133(+ )GSCs had the capacity to form a tumor of comparable size to the ones formed by 10,000 CD133(-)GL261 cells. In IL-6(-/-)mice, only tumors formed by CD133(+ )cells were smaller compared with wild type. After stimulation of primary cultured microglia with medium from CD133-enriched GL261 glioma cells, we observed an selective upregulation in microglial IL-6 secretion dependent on Toll-like receptor (TLR) 4. Our results show that GSCs, but not the bulk glioma cells, initiate microglial IL-6 secretion via TLR4 signaling and that IL-6 regulates glioma growth by supporting GSCs. Using human glioma tissue, we could confirm the finding that GAMs are the major source of IL-6 in the tumor context

Glioblastoma-induced attraction of endogenous neural precursor cells is associated with improved survival

Neural precursor cells contribute to adult neurogenesis and to limited attempts of brain repair after injury. Here we report that in a murine experimental glioblastoma model, endogenous neural precursors migrate from the subventricular zone toward the tumor and surround it. The association of endogenous precursors with syngenic tumor grafts was observed, after injecting red fluorescent protein-labeled G261 cells into the caudate-putamen of transgenic mice, which express green fluorescent protein under a promoter for nestin (nestin-GFP). Fourteen days after inoculation, the nestin-GFP cells surrounded the tumors in several cell layers and expressed markers of early noncommitted and committed precursors. Nestin-GFP cells were further identified by a characteristic membrane current pattern as recorded in acute brain slices. 5-bromo-2-deoxyuridine labeling and dye tracing experiments revealed that the tumor-associated precursors originated from the subventricular zone. Moreover, in cultured explants from the subventricular zone, the neural precursors showed extensive tropism for glioblastomas. Tumor-induced endogenous precursor cell accumulation decreased with age of the recipient; this correlated with increased tumor size and shorter survival times in aged mice. Coinjection of glioblastoma cells with neural precursors improved the survival time of old mice to a level similar to that in young mice. Coculture experiments showed that neural precursors suppressed the rapid increase in tumor cell number, which is characteristic of glioblastoma, and induced glioblastoma cell apoptosis. Our results indicate that tumor cells attract endogenous precursor cells; the presence of precursor cells is antitumorigenic; and this cellular interaction decreases with aging

CD95 maintains stem cell-like and non-classical EMT programs in primary human glioblastoma cells

Glioblastoma (GBM) is one of the most aggressive types of cancer with limited therapeutic options and unfavorable prognosis. Stemness and non-classical epithelial-to-mesenchymal transition (ncEMT) features underlie the switch from normal to neoplastic states as well as resistance of tumor clones to current therapies. Therefore, identification of ligand/receptor systems maintaining this privileged state is needed to devise efficient cancer therapies. In this study, we show that the expression of CD95 associates with stemness and EMT features in GBM tumors and cells and serves as a prognostic biomarker. CD95 expression increases in tumors and with tumor relapse as compared with non- tumor tissue. Recruitment of the activating PI3K subunit, p85, to CD95 death domain is required for maintenance of EMT-related transcripts. A combination of the current GBM therapy, temozolomide, with a CD95 inhibitor dramatically abrogates tumor sphere formation. This study molecularly dissects the role of CD95 in GBM cells and contributes the rational for CD95 inhibition as a GBM therapy

Bone morphogenetic protein-7 release from endogenous neural precursor cells suppresses the tumourigenicity of stem-like glioblastoma cells

Glioblastoma cells with stem-like properties control brain tumour growth and recurrence. Here, we show that endogenous neural precursor cells perform an anti-tumour response by specifically targeting stem-like brain tumour cells. In vitro, neural precursor cells predominantly express bone morphogenetic protein-7; bone morphogenetic protein-7 is constitutively released from neurospheres and induces canonical bone morphogenetic protein signalling in stem-like glioblastoma cells. Exposure of human and murine stem-like brain tumour cells to neurosphere-derived bone morphogenetic protein-7 induces tumour stem cell differentiation, attenuates stem-like marker expression and reduces self-renewal and the ability for tumour initiation. Neurosphere-derived or recombinant bone morphogenetic protein-7 reduces glioblastoma expansion from stem-like cells by down-regulating the transcription factor Olig2. In vivo, large numbers of bone morphogenetic protein-7-expressing neural precursors encircle brain tumours in young mice, induce canonical bone morphogenetic protein signalling in stem-like glioblastoma cells and can thereby attenuate tumour formation. This anti-tumour response is strongly reduced in older mice. Our results indicate that endogenous neural precursor cells protect the young brain from glioblastoma by releasing bone morphogenetic protein-7, which acts as a paracrine tumour suppressor that represses proliferation, self-renewal and tumour-initiation of stem-like glioblastoma cell

Microglia/macrophage derived human CCL18 promotes glioma progression via CCR8-ACP5 axis analyzed in humanized slice model

Factors released from glioma-associated microglia/macrophages (GAMs) play a crucial role in glioblastoma multiforme (GBM) progression. Here, we study the importance of CCL18, a cytokine expressed in human but not in rodent GAMs, as a modulator of glioma growth. Since CCL18 signaling could not be studied in classical mouse glioma models, we developed an approach by transplanting induced pluripotent stem cell-derived human microglia and human glioma cells into mouse brain slices depleted of their intrinsic microglia. We observe that CCL18 promotes glioma cell growth and invasion. Chemokine (C-C motif) receptor 8 (CCR8) is identified as a functional receptor for CCL18 on glioma cells, and ACP5 (acid phosphatase 5) is revealed as an important part of the downstream signaling cascade for mediating glioma growth. We conclude, based on the results from an in vitro, ex vivo humanized glioma model and an in vivo GBM model that microglia/macrophage-derived CCL18 promotes glioma growth

GABA-A Channel Subunit Expression in Human Glioma Correlates with Tumor Histology and Clinical Outcome

GABA (γ-aminobutyric acid) is the main inhibitory neurotransmitter in the CNS and is present in high concentrations in presynaptic terminals of neuronal cells. More recently, GABA has been ascribed a more widespread role in the control of cell proliferation during development where low concentrations of extrasynaptic GABA induce a tonic activation of GABA receptors. The GABA-A receptor consists of a ligand-gated chloride channel, formed by five subunits that are selected from 19 different subunit isoforms. The functional and pharmacological properties of the GABA-A channels are dictated by their subunit composition. Here we used qRT-PCR to compare mRNA levels of all 19 GABA-A channel subunits in samples of human glioma (n = 29) and peri-tumoral tissue (n = 5). All subunits except the ρ1 and ρ3 subunit were consistently detected. Lowest mRNA levels were found in glioblastoma compared to gliomas of lower malignancy, except for the θ subunit. The expression and cellular distribution of the α1, γ1, ρ2 and θ subunit proteins was investigated by immunohistochemistry on tissue microarrays containing 87 gliomas grade II. We found a strong co-expression of ρ2 and θ subunits in both astrocytomas (r = 0.86, p<0.0001) and oligodendroglial tumors (r = 0.66, p<0.0001). Kaplan-Meier analysis and Cox proportional hazards modeling to estimate the impact of GABA-A channel subunit expression on survival identified the ρ2 subunit (p = 0.043) but not the θ subunit (p = 0.64) as an independent predictor of improved survival in astrocytomas, together with established prognostic factors. Our data give support for the presence of distinct GABA-A channel subtypes in gliomas and provide the first link between specific composition of the A-channel and patient survival

Occupational Radiation Protection in Interventional Radiology: A Joint Guideline of the Cardiovascular and Interventional Radiology Society of Europe and the Society of Interventional Radiology

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