Deep within our nerve cells, a molecule is at work that has no beginning and no end. Instead of a straight chain, as is it common for most RNA strands, it forms a closed loop. Known as circular RNAs (circRNAs), these molecules are crucial for development, thought, and synaptic function, yet their high prevalence in neurons has long been a scientific mystery. How does the brain produce so many of them?
Now, Max Planck researchers from Freiburg have discovered a crucial mechanism that explains the remarkable abundance of circRNAs in the nervous system. The study reveals that the protein ELAV acts as a global master switch for the production of these molecules.
CircRNAs are found across all life forms. They are expressed in specific patterns during different developmental stages and in different cells – especially abundant in cells of the nervous system. While their roles are not as well-studied as normal, linear RNAs, circRNAs are known to be important in brain development, cognition, and even in conditions like neurodegeneration and addiction.
A stable ring with impact
The ring structure of circRNAs is crucial for their roles because it gives the molecules extreme stability. Unlike linear RNA molecules, circRNAs have no open ends that act as starting points for enzymes, which could rapidly degrade them. This longevity makes circRNAs ideal candidates for long-lasting regulatory tasks, especially in cells that don't divide, such as neurons.
"They can control gene activity, act as sponges for other molecules, or even produce proteins. In our lab, we are fascinated by these RNAs and want to understand how they are regulated," says Mengjin Shi, one of the first-authors of the study.
New research by the lab of Valérie Hilgers conducted on Drosophila embryos, identified the well-known RNA-binding protein ELAV as the key factor. The team found that in developing neurons, ELAV is the central mediator driving the widespread creation of circRNAs.
ELAV protein as a master switch
"When we removed the ELAV protein from the fruit fly embryos, the production of neuronal circRNAs plummeted, with a downregulation by over 75%. Conversely, when we introduced ELAV into cells that normally produce very few circRNAs, it triggered their formation. This confirms ELAV's role as a potent regulator of circRNA expression," says Valérie Hilgers.
The study also provides a clear mechanical insight into how ELAV performs this function: ELAV binds to pre-mRNA, the precursor to mature RNA. By binding there, ELAV slows down the standard process of "linear splicing," which in turn promotes an alternative process called "back-splicing." This action effectively brings the two ends of the future circRNA molecule together, facilitating the creation of the closed loop.
"Our discovery suggests that neuronal circRNAs are not just byproducts of gene expression, but are purposefully generated to fulfill important functions. It enhances our understanding of the molecular basis of how the brain works by revealing how a specific type of molecule in neurons, circRNAs, is regulated. And ELAV is clearly a central part of that story," says Valérie Hilgers
Given that proteins similar to ELAV are conserved from flies to humans, these findings strongly suggest a similar mechanism governs circRNA production also in the human brain. By manipulating ELAV or similar proteins, the researchers could potentially influence circRNA levels providing new strategies to investigate the molecules' role in brain health and neurodegenerative disorders.
