Project description:Gliomas are highly aggressive brain tumors characterized by poor prognosis and composed of diffusely infiltrating tumor cells that intermingle with non-neoplastic cells in the tumor microenvironment, including neurons. Neurons are increasingly appreciated as important reactive components of the glioma microenvironment, due to their role in causing hallmark glioma symptoms, cognitive deficits, and seizures, as well as potentially driving glioma progression. Separately, mTOR signaling has been shown to have pleiotropic effects in the brain tumor microenvironment, including regulation of neuronal hyperexcitability. However, the local cellular level effects of mTOR inhibition on glioma-induced neuronal alterations are not well understood. Here we employed neuron-specific profiling of ribosome-bound mRNA via ‘Ribotag’, morphometric analysis, and intravital imaging, along with pharmacological mTOR inhibition to investigate the impact of glioma burden on excitatory neuronal pathophysiology as well as the impact of mTOR inhibition on these neuronal alterations. The Ribotag analysis of peritumoral excitatory neurons showed an upregulation in transcripts encoding for F-actin binding and other dendritic spine-enriched proteins. Light and electron microscopy analyses revealed marked decreases in dendritic spine density in peritumoral neurons. Intravital two-photon imaging in peritumoral excitatory neurons revealed progressive alterations in neuronal activity, both at the population and single neuron level, throughout tumor growth. Intravital two-photon imaging also revealed altered stimulus-evoked somatic calcium activity, both in rate and temporal alignment, which was most pronounced in neurons with high-tumor burden. A single acute dose of AZD8055, a mTORC1 and mTORC2 inhibitor reversed the translational effect of glioma on neurons, increased dendritic spine density, and functional neuronal alterations. These results point to mTOR-driven pathological plasticity in neurons at the infiltrative margin of glioma – manifested by alterations in ribosome-bound mRNA, dendritic spine morphology, and stimulus-evoked neuronal activity. Collectively, our work identifies the pathological changes that peritumoral neurons experience as both hyperlocal and reversible under the influence of mTOR inhibition, providing foundational knowledge for developing therapies targeting neuronal signaling in glioma.

Project description:Gliomas are highly aggressive brain tumors characterized by poor prognosis and composed of diffusely infiltrating tumor cells that intermingle with non-neoplastic cells in the tumor microenvironment, including neurons. Neurons are increasingly appreciated as important reactive components of the glioma microenvironment, due to their role in causing hallmark glioma symptoms, such as cognitive deficits and seizures, as well as their potential ability to drive glioma progression. Separately, mTOR signaling has been shown to have pleiotropic effects in the brain tumor microenvironment, including regulation of neuronal hyperexcitability. However, the local cellular-level effects of mTOR inhibition on glioma-induced neuronal alterations are not well understood. Here we employed neuron-specific profiling of ribosome-bound mRNA via 'RiboTag,' morphometric analysis of dendritic spines, and in vivo calcium imaging, along with pharmacological mTOR inhibition to investigate the impact of glioma burden and mTOR inhibition on these neuronal alterations. The RiboTag analysis of tumor-associated excitatory neurons showed a downregulation of transcripts encoding excitatory and inhibitory postsynaptic proteins and dendritic spine development, and an upregulation of transcripts encoding cytoskeletal proteins involved in dendritic spine turnover. Light and electron microscopy of tumor-associated excitatory neurons demonstrated marked decreases in dendritic spine density. In vivo two-photon calcium imaging in tumor-associated excitatory neurons revealed progressive alterations in neuronal activity, both at the population and single-neuron level, throughout tumor growth. This in vivo calcium imaging also revealed altered stimulus-evoked somatic calcium events, with changes in event rate, size, and temporal alignment to stimulus, which was most pronounced in neurons with high-tumor burden. A single acute dose of AZD8055, a combined mTORC1/2 inhibitor, reversed the glioma-induced alterations on the excitatory neurons, including the alterations in ribosome-bound transcripts, dendritic spine density, and stimulus evoked responses seen by calcium imaging. These results point to mTOR-driven pathological plasticity in neurons at the infiltrative margin of glioma - manifested by alterations in ribosome-bound mRNA, dendritic spine density, and stimulus-evoked neuronal activity. Collectively, our work identifies the pathological changes that tumor-associated excitatory neurons experience as both hyperlocal and reversible under the influence of mTOR inhibition, providing a foundation for developing therapies targeting neuronal signaling in glioma.

Project description:Although malignant gliomas frequently show aberrant activation of the mammalian target of rapamycin (mTOR), mTOR inhibitors have performed poorly in clinical trials. Besides regulating cell growth and translation, mTOR controls the initiation of autophagy. By recycling cellular components, autophagy can mobilize energy resources, and has thus been attributed cancer-promoting effects. Here, we asked whether the activation of autophagy represents an escape mechanism to pharmacological mTOR inhibition, and explored co-treatment with mTOR and autophagy inhibitors as a therapeutic strategy. Mimicking conditions of the glioma microenvironment, glioma cells were exposed to nutrient starvation and hypoxia. Following treatment with mTOR inhibitor torin2 or rapamycin, and autophagy inhibitors bafilomycin A1 or MRT68921, we analyzed autophagic activity, cell growth, viability and oxygen consumption. Changes in global proteome were quantified by mass spectrometry. In the context of hypoxia and starvation, autophagy was strongly induced in glioma cells, and further increased by mTOR inhibition. While torin2 enhanced glioma cell survival, co-treatment with torin2 and bafilomycin A1 failed to promote cell death. Importantly, treatment with bafilomycin A1 alone also protected glioma cells from cell death. Mechanistically, both compounds significantly reduced glioma growth and oxygen consumption. Mass spectrometry analysis showed that bafilomycin A1 induced broad changes in the cellular proteome. More specifically, proteins downregulated by bafilomycin A1 were associated with the mitochondrial respiratory chain and ATP synthesis. Taken together, our results show that activation of autophagy does not account for the cytoprotective effects of mTOR inhibition in our in vitro model of the glioma microenvironment. Our proteomic findings suggest that the pharmacological inhibition of autophagy induces extensive changes in the cellular proteome that can support glioma cell survival under nutrient-deplete and hypoxic conditions. These findings provide a novel perspective on the complex role of autophagy in gliomas and may have implications for the design of future trials.

Project description:The mitogen-activated protein kinase (MAPK) pathway is one of the most altered pathways in cancer. It is involved in the control of cell proliferation, invasion, metabolism, and resistance to therapy. A number of aggressive malignancies, including melanoma, colon cancer and glioma, are driven by an activating missense mutation (V600E) in one component of the pathway, BRAF. BRAF V600E mutated cancers may respond initially to MEK inhibition, but may develop resistance mediated by increased reliance on mTOR signaling. We have previously demonstrated that the combination of the MEK inhibitor trametinib with the dual mTORC1/2 inhibitor TAK228 improved survival and decreased vascularization in a BRAFV600E mutant glioma model. To elucidate the mechanism of action of, and the changes in response to, MEK and mTOR inhibition, we performed comprehensive unbiased proteomic and phosphoproteomic characterization of BRAFV600E mutant glioma xenografts after short-course treatment with trametinib and TAK228, alone and in combination. We identified distinct response signatures for each monotherapy and combination therapy and validated that combination treatment inhibited activation of the MAPK and mTOR pathways, increased apoptotic signaling and suppressed angiogenesis signaling. Furthermore, we found that trametinib and TAK228 combination treatment broadly suppressed the activity of the cyclin-dependent kinases and increased the levels of proteins (and their activating phosphorylations) involved in glycolysis, the TCA cycle, nucleotide biosynthesis and DNA replication. We also demonstrated activation of both receptor tyrosine kinase and histone deacetylase proteins. This study reports a detailed (phospho)proteomic analysis of the response of BRAFV600E mutant glioma to combined MEK and mTOR pathway inhibition and identifies a number of targetable upregulated proteins and pathways, providing new avenues for the development of additional rational combination therapies for aggressive BRAF-driven tumors.

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Glioma-Induced Alterations in Excitatory Neurons are reversed by mTOR Inhibition

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