Total recall – from fiction to fact

When JK Rowling wrote about the pensieve to store memory-threads and to revisit them later - it was pure fiction; but today, almost two decades later, scientists are one step closer to doing something similar. 

A recent paper in Nature by Dr S. Tonegawa’s group at MIT has carried out some very clever genetic experiments to specifically label and activate a memory trace in laboratory mice. A memory is encoded by the activation of a subset of neurons whose ablation or inactivation reduces the strength of the recall. While this establishes their necessity, the sufficiency of these few neurons for the storage and retrieval of a memory was untested till this report came along.

The authors target the dentate gyrus region of the brain (critical for encoding contextual fear memory) and have devised a clever genetic system to specifically label a group of neurons that get activated when the mouse experiences fear in one specific context. To do this, they have used c-fos (a protein that gets activated during memory formation and strong neuronal activation) to drive the expression of Channelrhodopsin (ChR) – a light responsive ion channel. ChR (originally from Chlamydomonas) causes the host neurons to fire upon perceiving blue light. Thus in this system, the neurons that activate c-fos also express ChR and will fire in response to blue light. While this sounds like a simple scheme, the authors have had to juggle a lot of genetics to regulate the specific temporal labeling of neurons. The authors have kept the c-fos function under the control of doxycycline (dox) - a tetracycline analog - in such a way that only neurons that get activated in the absence of dox exhibit c-Fos driven ChR synthesis. This ChR ‘tag’ was then used to selectively activate (by shining blue light) the neurons that originally encoded the particular fear-associated memory trace. They then test their ability to make the mouse experience the same fear - measured as freezing - without being presented with the fear stimulus (freezing in the face of predators is an instinctive reaction of rodents and is good measure of their fear response).

In their key experiment, the authors habituate the mice to a particular context (A) and record their freezing behavior to determine the baseline freezing. They then take the mice off dox and expose them to context B, where the mice experience electric shocks to their footpad paired with a tone stimulus. Thus, the mice were trained to associate the tone with the foot-shock and they freeze in response. Since the mice were off dox during this training period, the neurons that fired in response to the fear conditioning activated c-fos and encoded the downstream ChR. After this training period, the mice were put back on dox to terminate the labeling of neurons and were reintroduced to context A. The big question at this point was – would the activation of these few neurons (by shining blue light on them) in context A (without the foot-shock) activate the fear state in mice and cause them to freeze? It is not hard to guess, that this indeed happened (since the paper was published in Nature). 

Using this elegant approach, the authors were able to ‘remind’ the mice of their bad experience and cause them to freeze in a non-threatening context by merely activating the neurons encoding that memory trace. In other words, the authors show that a neuronal engram or memory trace is sufficient to elicit the appropriate fear response. In addition ot addressing a fundamental question in memory formation, this work also opens up new avenues for fear extinction and conditioning. 

Needless to say, it has been a giant leap in moving from fiction to fact – science is magical too. 

References:

Optogenetic stimulation of a hippocampal engram activates fear memory recall

Nature 484, 381–385 (19 April 2012)

PS - This piece was written for an assignment and I decided to post it here as well. This write up barely conveys the complexity of the experiment or the beauty of it and I do want to do a more detailed follow up sometime soon.