Abstract
BACKGROUND: Lower grade gliomas with isocitrate dehydrogenase (IDH)1/2 gene mutation produce the oncometabolite (R)-2-hydroxyglutarate (2HG) at a high concentration. 2HG is toxic to glioma cells in vitro, yet has the net result in vivo of promoting gliomagenesis, pointing toward a strong context-specific effect. Here, we investigated the tumor microenvironment of IDH mutant gliomas and uncovered bidirectional neuron-glioma communication that promotes tumor growth. MATERIAL AND METHODS: To investigate how (R)-2-hydroxyglutarate (2HG) influences neuronal activity and subsequently impacts the growth of IDH mutant gliomas, we performed time-lapse imaging of murine primary dissociated neurons engineered to express the fluorescent calcium indicator GCaMP6s. We tested changes in activity resulting from exposure to 2HG. We then used endogenous IDH mutant glioma lines (TS603 and MGG119) to test whether mitogens released by 2HG-activated neurons increased tumor proliferation rates. RESULTS: Consistent with previous reports showing increased neuronal activity with exogenous exposure to 2HG, we triggered an increase in calcium imaging signals of neuronal activity with 2HG application. Furthermore, this activity was similarly provoked by 2HG secreted locally from co-cultured patient-derived IDH mutant cell lines. Activity was completely reversed with IDH inhibitor treatment. In turn, IDH mutant glioma cell proliferation was induced by neuron-conditioned media containing secreted mitogens. Proliferation was dramatically accelerated by conditioned media from 2HG-treated neurons, with concomitant upregulation of MAPK pathway activation in the tumor cells. Conversely, 2HG-stimulated mitogen release was blocked by glutamate receptor inhibition, demonstrating activity-dependence. CONCLUSION: Our data provide evidence of a 2HG-driven neuron-glioma feed-forward mechanism in IDH mutant tumors, similar to what has been observed in IDH wild-type gliomas. These findings can explain how the inherent growth-suppressive effect of 2HG is overcome in vivo by microenvironmental remodeling. Importantly, the contribution of 2HG-driven aberrant neuronal activity to tumor proliferation in IDH mutant gliomas supports the role of mutant IDH inhibitors and neuromodulation as therapeutic modalities for patients with these tumors.