PNNL

Abstract

How can a single cell learn without a brain? Although learning is often viewed as a unique feature of animals with complex nervous systems, single-celled organisms also demonstrate basic forms of learning and memory. The single cell Stentor coeruleus contracts in response to mechanical taps, but habituates and learns to ignore the taps after repeated stimulation. Here, we explored the molecular changes that occur during the formation of this cellular memory in order to improve our understanding of nonsynaptic learning. We impaired cellular protein synthesis with cycloheximide and puromycin and found that, contrary to the effects of such treatments on metazoa, these drugs accelerate habituation and prolong memory retention in Stentor. Exploratory proteomic and transcriptomic analyses identified candidate proteins and genes that change over the course of learning and forgetting. These candidates, including EF-hand domain-containing proteins, metal-dependent protein phosphatases, and calcium/calmodulin-dependent protein kinase II (CaMKII) homologs, point towards the regulation of Stentor learning by calcium signaling and protein phosphorylation. Building on these results, we found that increased extracellular calcium improved Stentor learning while treatment with kinase and phosphatase inhibitors impaired learning. In particular, KN-93, a drug known to inhibit CaMKII and voltage-gated calcium channels, decreased both the rate and extent of habituation in Stentor, similar to its effects on learning in metazoa. We also discovered that the habituation memory can be maintained in progeny following cell division. Taken together, these results suggest that forgetting in Stentor requires new protein synthesis, and memory formation involves the modification of delocalized mechanoreceptors by phosphorylation and calcium signaling. This is consistent with our previous model of Stentor learning in which habituation occurs through the inactivation of cell surface receptors. Ultimately, these findings shed light on the origins of intelligence independent of complex multicellular circuitry.