Gliomas are the most common type of brain tumor. They develop from glial cells, whose natural role is to support and nourish nerve cells. The glioblastoma group is particularly malignant. These gliomas are believed to originate from glial precursor cells and grow very rapidly. Today, patients with such gliomas have a maximum median survival time of no more than 18 months. Starting from the tumor site, the glial cells spread diffusely through the brain along nerve pathways. In the process, they form electrochemical connections (synapses) with nerve cells. Via these synapses, they intercept electrical impulses that promote their division and accelerate their spread. Varun Venkataramani discovered this eleven years ago as part of his medical doctoral thesis. Together with his mentors and colleagues in Heidelberg, he has since validated, expanded, and further explored this finding over the past decade. In the process, he uncovered the trick tumor cells use to behave like immature neurons during brain development in order to facilitate their own spread. Varun Venkataramani co-founded the field of cancer neuroscience and opened up a new perspective for treating gliomas: disconnecting the tumor from its power supply to stop its growth.

Modern brain tumor research began in the early 1970s with the invention of computed tomography (CT), which, for the first time, enabled non-invasive and clear imaging of brain tumors. Since the 1980s, CT has been replaced by magnetic resonance imaging (MRI) as the preferred imaging technique for diagnosing and monitoring brain tumors. Alongside improvements in imaging, there has been a fundamental shift in the classification of brain tumors. While classification was originally based solely on histology, i.e., the microscopic appearance of fine tissue structures, it has since been expanded through the sequencing of the human genome to include molecular parameters, some of which carry prognostic value. Gliomas with genetic mutations in the enzyme isocitrate dehydrogenase (IDH), for instance, have a significantly better prognosis than those without such mutations. Discoveries like this led to the 2016 WHO classification, which marked a departure from the previously purely morphological classification of brain tumors.

Taking an Incredible Observation Seriously
This did not mean, however, that morphological questions had become irrelevant – especially not for gliomas, whose main mass sends out numerous cells that spread in all directions throughout the brain. These migrating tumor cells usually remain invisible, even in MRI scans. But under the electron microscope they can be visualized – if the tissue sections are properly prepared. This is something Heinz Horstmann succeeded in doing at the Institute of Functional Neuroanatomy at Heidelberg University Hospital. Horstmann had assigned the task of applying this visualization method to doctoral student Varun Venkataramani. But the scope of the project suddenly expanded when a discovery made by Frank Winkler, the head of the neighboring Department of Neurology, shifted the perspective entirely. Winkler had found that glioma cells spin off extremely thin and long projections that, during tumor growth, connect to form a dense network – much like nerve fibers – as if a new brain were forming within the existing one . He asked Horstmann and his team to use the electron microscope to examine the interior of these tumor cell projections. But the doctoral student's curious eye did not linger inside – it wandered outward. And there, alert and astonished, he fixated on a section of the image: what he saw on one of the tumor processes was unmistakably the presynaptic terminal of a neuron. A synapse between the tumor and the nervous system? That sounded so crazy that neither Winkler nor Horstmann nor his department head Thomas Kuner initially wanted to believe it. “We were all very skeptical for years," says Venkataramani. “But it was clear to me that we had to pursue it." He considers it a stroke of luck that he was able to pursue this finding during his doctoral thesis in natural sciences under the aegis of Kuner and Winkler. The leading scientific journal Nature took their results seriously. When Venkataramani submitted the paper for publication in September 2018, Nature assigned four external reviewers, who raised many critical questions. All were answered to their satisfaction. Nearly a year after submission, Nature published Venkataramani's paper.

Glutamate Signals Trigger Gliomas
The publication attracted considerable attention among cancer experts, as it presented a new dimension of contradiction to the traditional research perspective that places the tumor at the center and regards everything around it as mere accessories. Over the years, oncology had increasingly acknowledged the importance of blood vessels and immune activity in the tumor microenvironment. However, the idea that nerve cells might also influence tumor growth had not been on its radar. Had Venkataramani perhaps only seen an artifact in the electron microscope? He was able to refute this electrophysiologically. The synapses on the tumor cells were functional. Signals from nerve cells arrived at their presynaptic terminals, triggering the release of the neurotransmitter glutamate, which docked postsynaptically to AMPA receptors on the tumor cells. This caused calcium ions to flow in, generating an electric current. This sequence of presynaptic nerve fibers and postsynaptic tumor cell fibers might suggest that it is the nerve cells initiating contact with the tumor cells. However, all available evidence suggests the opposite. The tumor cell is characterized by gene expression patterns that cause it to sprout postsynaptic fibers. With these, it entices the nerve cell to grow toward it with presynaptic fibers. The more input glioma cells receive from nerve cells – in other words, the more strongly they are wired into neuronal circuits – the more aggressively they drive their growth and spread. Their integration into neuronal circuits likely also explains the high therapy resistance of glioblastomas.

Technologically Brilliant Tinkerer
But how do glioma cells manage to tap into so many neurons so quickly? Venkataramani and his team answered this question with remarkable technological ingenuity. They combined a series of advanced methods into a workflow that overlays imaging and molecular profiling. Human glioma cells, marked with a green dye, were implanted into the brains of immunodeficient mice to observe their growth in vivo. This was done using a two-photon microscope, whose time-lapse recordings were merged into moving images. The images were denoised and interpreted using AI algorithms. A red dye was also injected, allowing researchers to distinguish glioma cells that were connected either to each other or to other brain cells from those that were not. Non-connected cells wandered randomly through the brain. Using flow cytometry, the different colored tumor cell types were then separated and subjected to single-cell RNA sequencing to study their molecular inner workings. This revealed that the invasive green glioma cells not only closely resembled immature nerve cells internally, but also mimicked their mechanisms of spreading. Their movements resembled the migration of immature nerve cells during brain development.

Does an Epilepsy Drug Slow Down the Tumor?
How exactly neuronal input triggers the tumor's growth signals is still unknown. What is clear, however, is that this input is mediated by glutamate. And there are already medications that inhibit the neurotransmitter glutamate. These are approved for the treatment of epilepsy, which is characterized by excessive activation of glutamatergic signaling. Most of these active substances, however, are not well suited to interrupt the electrical communication between nerve and tumor cells. A different case is perampanel, which has been available in Europe since 2012. It is a selective AMPA receptor antagonist – meaning it blocks precisely the site through which glioma cells receive nerve impulses. This is why Venkataramani and his colleagues selected this epilepsy drug for a repurposing approach, aiming to develop it for the indication of glioblastoma. Since the authorities already have extensive data from its original approval, this development is progressing quickly and has already reached Phase II of clinical trials in a multicenter study. The preclinical results were encouraging. However, perampanel has a narrow therapeutic window, meaning it must be dosed carefully. Still, there is hope that this compound marks the beginning of a new era in the development of improved glioma therapies.

Scientific Council Praises “Exceptional Translational Significance"
With his own research group, which he has been leading at Heidelberg University Hospital since 2022, Venkataramani has already moved beyond this initial stage. In preclinical trials, he has demonstrated proof of concept for a gene therapy-based approach to the diagnosis and treatment of gliomas. . The aim of this approach is to specifically label and eliminate the nerve cells connected to the tumor. In this so-called retrograde virus tracing, tumor cells are engineered to take up modified rabies viruses that carry a fluorescent dye. These viruses spread retrogradely through the postsynaptic extensions of tumor cells into the connected nerve cells via their presynaptic fibers. They go no further. Under the microscope, only the network of neurons directly connected to the tumor lights up. The cells in this network can then be selectively destroyed using the following trick: First, viral gene vectors are used to deliver inactive precursors of a gene into all nerve cells in the tumor's vicinity – a gene that, when activated, induces the cells to self-destruct (apoptosis). The activation signal is built into the modified rabies viruses. As a result, only those neurons connected to the tumor activate the gene and trigger their own death, effectively disconnecting the tumor from its power supply. “This is, of course, a drastic approach," says Venkataramani, who also works as a physician at the Neurological Clinic. “But it could be groundbreaking." After all, a combination of gene therapy and pharmacological measures, together with standard chemotherapy and radiation, has the potential to take the fear out of gliomas. It is for good reason that the Scientific Council, in its statement awarding the prize to the Heidelberg-based clinician scientist, highlighted “the exceptional translational significance of his work in the spirit of Paul Ehrlich".