Negative allosteric modulators of metabotropic glutamate receptor 3 target the stem-like phenotype of glioblastoma

Hans-Georg Wirsching,Manuela Silginer,Elisa Ventura,Will Macnair,Isabel Burghardt,Manfred Claassen,Silvia Gatti,Jürgen Wichmann,Claus Riemer,Hannah Schneider,Michael Weller

doi:10.1016/j.omto.2020.12.009

Hans-Georg Wirsching, Manuela Silginer + Show 9 more

Open Access

https://doi.org/10.1016/j.omto.2020.12.009

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Abstract

Glioblastoma is an invariably deadly disease. A subpopulation of glioma stem-like cells (GSCs) drives tumor progression and treatment resistance. Two recent studies demonstrated that neurons form oncogenic glutamatergic electrochemical synapses with post-synaptic GSCs. This led us to explore whether glutamate signaling through G protein-coupled metabotropic receptors would also contribute to the malignancy of glioblastoma. We found that glutamate metabotropic receptor (Grm)3 is the predominantly expressed Grm in glioblastoma. Associations of GRM3 gene expression levels with survival are confined to the proneural gene expression subtype, which is associated with enrichment of GSCs. Using multiplexed single-cell qRT-PCR, GSC marker-based cell sorting, database interrogations, and functional assays in GSCs derived from patients’ tumors, we establish Grm3 as a novel marker and potential therapeutic target in GSCs. We confirm that Grm3 inhibits adenylyl cyclase and regulates extracellular signal-regulated kinase. Targeting Grm3 disrupts self-renewal and promotes differentiation of GSCs. Thus, we hypothesize that Grm3 signaling may complement oncogenic functions of glutamatergic ionotropic receptor activity in neuroglial synapses, supporting a link between neuronal activity and the GSC phenotype. The novel class of highly specific Grm3 inhibitors that we characterize herein have been clinically tested as cognitive enhancers in humans with a favorable safety profile.

Highlights

Grm[3] is the predominantly expressed Grm subtype in glioblastoma As a first step to assess a putative role of Grm in glioblastoma, we queried publically available gene expression and clinical data of The Cancer Genome Atlas (TCGA)
GRM3 is located on chromosome 7, which is commonly amplified in IDH wild-type glioblastoma, yet GRM3 copy number gains appear not to be associated with gene expression (Note S2; Figures S1G–S1I)
We focused on CD133 to explore GRM3 expression in glioma stem-like cells (GSCs) versus non-GSCs because the presence of the AC133 epitope of CD133 on the cell surface, but not CD133 gene expression, enriches for GSCs and is associated with the proneural glioblastoma subtype.[20]

Summary

Introduction

Two recent studies identified oncogenic bona fide glutamatergic synapses between neurons and a subset of glioma cells.[6,7] Overlap of post-synaptic and stem-like gene expression patterns on single-cell RNA sequencing analyses suggests a role of glutamate signaling for the regulation of the GSC phenotype that may be exploited therapeutically.[4,7]

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