Part of what makes us human is our memory. While we’ve done countless studies on memory, one key characteristic is the spacing effect that states that we retain information better if we study something multiple times over a longer period. A new study published by Nature found that this same memory technique is used by other types of cells in our body, like our kidneys. This is incredibly important because memory has never been proven to exist outside the nervous system, making it the first of its kind.
This study looks at how cells respond to repeated stimulation over time, focusing on the massed-spaced effect. This effect means that spreading out learning or stimulation over time (spaced training) leads to better memory than giving the same amount of stimulation all at once (massed training). This pattern has been seen in animals and in brain cells (neurons), both in behavior and at the level of synapses.
What makes this study different is that the researchers tested whether non-neural cells, cells that are not part of the nervous system, can also show this memory-related effect. They used SH-SY5Y cells, a type of human neuroblastoma (cancer) cell line, and exposed them to short, repeated signals using chemicals that activate two important memory-related pathways: the PKA (protein kinase A) and PKC (protein kinase C) signaling cascades. These pathways are usually activated in neurons by serotonin (5HT), but here the researchers used forskolin to activate PKA and TPA (tetradecanoyl phorbol acetate) to activate PKC.
To measure how the cells responded, the scientists created a special reporter system. They inserted a gene for luciferase, a light-producing enzyme, under the control of a CRE (cAMP response element) promoter. This promoter is normally activated by a protein called CREB1 (cAMP response element-binding protein 1), which helps form memory by turning on immediate-early genes (IEGs). These genes are turned on quickly in response to signals and help change how the cell behaves. When CREB1 is activated (phosphorylated) by PKA, PKC, or other kinases like ERK or CaMKII, it switches on these IEGs. The researchers used a version of luciferase that breaks down quickly (PEST-tagged) so they could measure recent activity rather than long-lasting buildup.
They tested the reporter system by treating cells with forskolin and TPA for different lengths of time and measured luciferase activity after 4 hours. They found that TPA’s effects were strong whether given briefly or for a long time, but forskolin needed longer exposure to show stronger results. This showed that PKC responds more to the number of signals (pulses), while PKA responds more to the duration of a signal.
Next, they gave the cells either one or four short pulses of forskolin+TPA and measured luciferase activity again. After 4 hours, the response was slightly stronger in cells that got four pulses, but after 24 hours, the difference was much larger—more than twice as much luciferase in cells that received repeated stimulation. This shows us that repetition helps the cells “remember” the signal longer. Importantly, even though the stimulation lasted less than an hour, the effects were still seen a day later, showing that the response continues even after the signal is gone. This matches how memory works in the brain: repetition not only boosts memory strength right away but also slows down forgetting over time.
Finally, the researchers directly tested the massed-spaced effect in their system. They compared four spaced 3-minute pulses (with breaks of 10, 20, or 30 minutes) to one long 12-minute pulse (massed condition) using forskolin, TPA, or both. In all cases, the spaced pulses caused more luciferase expression after 24 hours than the massed pulse. The best results came from a 10-minute break for TPA and TPA+forskolin, and a 20-minute break for forskolin alone. This suggests that PKA and PKC pathways are “tuned” to respond best at different timing intervals.
This study is important because it shows that non-neural cells, which don’t normally make memories, can still show memory-like behavior using the same molecular tools found in neurons. This opens the door to new ways of studying memory and learning outside the brain and may lead to new approaches for treating memory disorders or enhancing memory through cellular engineering.
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