Glioma-Induced Alterations in Excitatory Neurons are Reversed by mTOR Inhibition.

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.

High-throughput translational profiling with riboPLATE-seq.

Protein synthesis is dysregulated in many diseases, but we lack a systems-level picture of how signaling molecules and RNA binding proteins interact with the translational machinery, largely due to technological limitations. Here we present riboPLATE-seq, a scalable method for generating paired libraries of ribosome-associated and total mRNA. As an extension of the PLATE-seq protocol, riboPLATE-seq utilizes barcoded primers for pooled library preparation, but additionally leverages anti-rRNA ribosome immunoprecipitation on whole polysomes to measure ribosome association (RA). We compare RA to its analogue in ribosome profiling and RNA sequencing, translation efficiency, and demonstrate both the performance of riboPLATE-seq and its utility in detecting translational alterations induced by specific inhibitors of protein kinases.

Recruitment of the 40S ribosome subunit to the 3'-untranslated region (UTR) of a viral mRNA, via the eIF4 complex, facilitates cap-independent translation.

Barley yellow dwarf virus mRNA, which lacks both cap and poly(A) tail, has a translation element (3'-BTE) in its 3'-UTR essential for efficient translation initiation at the 5'-proximal AUG. This mechanism requires eukaryotic initiation factor 4G (eIF4G), subunit of heterodimer eIF4F (plant eIF4F lacks eIF4A), and 3'-BTE-5'-UTR interaction. Using fluorescence anisotropy, SHAPE (selective 2'-hydroxyl acylation analyzed by primer extension) analysis, and toeprinting, we found that (i) 40S subunits bind to BTE (Kd = 350 ± 30 nm), (ii) the helicase complex eIF4F-eIF4A-eIF4B-ATP increases 40S subunit binding (Kd = 120 ± 10 nm) to the conserved stem-loop I of the 3'-BTE by exposing more unpaired bases, and (iii) long distance base pairing transfers this complex to the 5'-end of the mRNA, where translation initiates. Although 3'-5' interactions have been recognized as important in mRNA translation, barley yellow dwarf virus employs a novel mechanism utilizing the 3'-UTR as the primary site of ribosome recruitment.

Fe2+ binds iron responsive element-RNA, selectively changing protein-binding affinities and regulating mRNA repression and activation.

Iron increases synthesis rates of proteins encoded in iron-responsive element (IRE)-mRNAs; metabolic iron ("free," "labile") is Fe(2+). The noncoding IRE-RNA structure, approximately 30 nt, folds into a stem loop to control synthesis of proteins in iron trafficking, cell cycling, and nervous system function. IRE-RNA riboregulators bind specifically to iron-regulatory proteins (IRP) proteins, inhibiting ribosome binding. Deletion of the IRE-RNA from an mRNA decreases both IRP binding and IRP-independent protein synthesis, indicating effects of other "factors." Current models of IRE-mRNA regulation, emphasizing iron-dependent degradation/modification of IRP, lack answers about how iron increases IRE-RNA/IRP protein dissociation or how IRE-RNA, after IRP dissociation, influences protein synthesis rates. However, we observed Fe(2+) (anaerobic) or Mn(2+) selectively increase the IRE-RNA/IRP K(D). Here we show: (i) Fe(2+) binds to the IRE-RNA, altering its conformation (by 2-aminopurine fluorescence and ethidium bromide displacement); (ii) metal ions increase translation of IRE-mRNA in vitro; (iii) eukaryotic initiation factor (eIF)4F binds specifically with high affinity to IRE-RNA; (iv) Fe(2+) increased eIF4F/IRE-RNA binding, which outcompetes IRP binding; (v) exogenous eIF4F rescued metal-dependent IRE-RNA translation in eIF4F-depeleted extracts. The regulation by metabolic iron binding to IRE-RNA to decrease inhibitor protein (IRP) binding and increase activator protein (eIF4F) binding identifies IRE-RNA as a riboregulator.