Gliomas are the most common malignant primary brain tumor and are often associated with severe neurological deficits and mortality. Unlike many cancers, gliomas rarely metastasize outside the brain, indicating a possible dependency on unique features of brain microenvironment. Synapses between neurons and glioma cells exist, suggesting that glioma cells rely on neuronal inputs and synaptic signaling for proliferation. Yet, the locations and properties of neurons that innervate gliomas have remained elusive. In this study, we utilized transsynaptic tracing with an EnvA-pseudotyped, glycoprotein-deleted rabies virus to specifically infect TVA and glycoprotein-expressing human glioblastoma cells in an orthotopic xenograft mouse model, allowing us to identify the neurons that form synapses onto the gliomas. Comprehensive whole-brain mapping revealed that these glioma-innervating neurons (GINs) from brain regions, including diverse neuromodulatory centers and specific cortical layers, known to project to the glioma locations. Molecular profiling revealed that long-range cortical GINs are predominantly glutamatergic, and subsets express both glutamatergic and GABAergic markers, whereas local striatal GINs are largely GABAergic. Electrophysiological studies demonstrate that while GINs share passive intrinsic properties with cortex-innervating neurons, their action potential waveforms are altered. Our study introduces a method for identifying and mapping GINs and reveals their consistent integration into existing location-dependent neuronal networks involving diverse neurotransmitters and neuromodulators. The observed intrinsic electrophysiological differences in GINs lay the groundwork for future investigations into how these alterations relate to the postsynaptic characteristics of glioma cells.
Keywords: GABAergic; gliomas; glutamatergic; monosynaptic tracing; neuron-glioma synapse.