Icing Worms to Prolong Memory

Memories, whether fleeting or long-term, are enigmatic mysteries of the brain. Drawn to understanding memory, Oded Rechavi, a molecular biologist at Tel Aviv University, studies Caenorhabditis elegans, tiny animals with just 302 neurons. C. elegans are an attractive model organism because they form memories—typically smell-related ones that last two to three hours.

“How we encode memory remains a mystery because our brains are enormously complex, but with C. elegans you can really tease apart the location of the memory and how it is encoded,” remarked Rechavi.

Some theories about memory dictate that electrical activity is necessary to retain memories; however, Rechavi wondered if this held true if they stopped electrical activity by exposing the worms to low temperatures.¹ Rechavi’s then graduate student Dana Landschaft Berliner found that worms trained to form a negative association with a smell retained their memories 8-fold longer when placed on ice. This chilling phenomenon, reported in their bioRxiv preprint, sheds light on memory storage and loss in C. elegans.²

To see how chilling the worms affected the forgetting process, Landschaft Berliner grew C. elegans at the standard temperature of 20°C and conditioned them with butanone, a smell that they are innately attracted to, followed by a period of brief starvation, to create a negative association. While placed on ice, these cold-sensitive worms retained their smell-related memories for up to 16 hours. On a plate containing butanone and an unrelated odor, benzaldehyde, worms wriggled away from the butanone side. However, their memories quickly faded when they returned to room temperature.

Rechavi’s team questioned if immediate placement on ice, which prolonged memory retention, would hold true for cold-tolerant worms. Worms raised at 15°C and placed on ice for three hours failed to retain the association and stayed attracted to butanone. When the team transferred worms raised at 15°C to 20°C for two hours before training, the worms became cold-sensitive and exhibited delayed forgetting, suggesting that this process was biologically regulated.

“One of the most remarkable things [in this work] is how shifting the worms’ baseline homeostatic state can rearrange this forgetting machinery,” remarked Rachel Arey, a neuroscientist at Baylor College of Medicine, who was not involved in the study.

Then, Rechavi and his team examined the genes driving these differences between cold-sensitive and cold-tolerant worms. Using RNA sequencing, they identified multiple differentiating genes in the diacylglycerol (DAG) pathway, which is known to regulate cellular processes related to memory and learning. To test DAG's role in delayed forgetting, the team focused on diacylglycerol kinase, which mediates a reduction in DAG levels, and its allele (dgk-1). Mutant worms lacking dgk-1 quickly forgot the negative association on ice and returned to the butanone side of the dish. Similarly, supplementing nonmutant worms on ice with DAG blocked the boost to memory retention.

Next, Rechavi wanted to explore the effect of lithium due to its ability to induce dysfunction in sensory neurons that regulate cold tolerance and inhibit an enzyme that makes a precursor to DAG.³ Worms exposed to lithium overnight before odor training held onto their memories at room temperature, whereas unmedicated worms did not. They also tested whether lithium-delayed forgetting depended on DAG levels; dgk-1 mutants’ elevated DAG levels suppressed the drug’s effects.

Subsequent RNA sequencing revealed a key similarity between ice and lithium treatments: reduced levels of DAG. The researchers referred to DAG as the internal ‘forgetting switch’ because it can be manipulated by cold temperatures and lithium to regulate memory loss. “It was really lovely that they linked this interplay to molecular changes like diacylglycerol signaling and breaking down the layers of this complex process,” remarked Arey.

“Why do worms forget when they are perfectly capable of remembering longer?” Rechavi pondered. “We can see that they can control it to some degree, so perhaps the length of the memory is tuned to something in the environment.” Moving forward, Rechavi’s team plans to investigate the mechanism of this memory phenomenon in other organisms, such as tardigrades, which can survive even colder temperatures.