Much More to Explore at the Brain’s Cellular Level
For anyone with an interest in how the cellular inner workings of the brain manifest in our experience of the world, back-to-back morning lectures on Monday, 26 June, from John O’Keefe and Mario Capecchi were the ideal cerebral breakfast. First up was O’Keefe, who was awarded the 2014 Nobel Prize in Physiology alongside May-Britt Moser and Edvard Moser “for their discoveries of cells that constitute a positioning system in the brain”.
O’Keefe’s work in the late 1960s tested individual neurons in rat brains to see how they reacted to environmental stimuli. To his peers at the time, the method went against the grain: “We said, ‘let’s forget what we’re interested in, see what the cells are interested in, and follow the cells as the animal goes about its normal daily living’,” he recalled. “Everybody in physiology said, ‘this is completely crazy, it’ll ruin your career’.”
As it turned out, the naysayers were completely wrong. O’Keefe and his collaborators were the first researchers to successfully transfer the philosophical question of how we perceive our sense of place into the rigorous world of 20th century neurophysiological experimentation. He identified a set of individual nerve cells in a part of the brain called the hippocampus that activated when the lab rat assumed a particular place in its environment. Moreover, he discovered that these ‘place cells’ worked in concert to build a memory of different environments.
Fly in the Ointment
But this did not fully satisfy O’Keefe. He surmised that place cells alone could not provide enough information to build a map of a given location – something more was needed: “You need to have a way from going from one place to another, you need to connect all the parts of the map together,” O’Keefe explained. “And the simplest way of doing that is by creating vectors, mathematical entities which connect in space, the distance and direction between two points.”
The question of whether place cells form a vector-based representation of space sat idle until very recently, when O’Keefe’s team at University College London, UK, used modern technology to record hundreds of rat place cells at the same time while cajoling the rat through a unique ‘honeycomb maze’ consisting of 61 individually raisable platforms. The idea behind the experiment was that a rat is given a goal and placed on a single raised platform a distance away from that goal, and then two adjacent platforms are raised for the rat to choose from. Once the rat has made its choice, the other platforms sink down and two new adjacent platforms pop up. This continues until the rat reaches its goal.
Performing the experiment multiple times, the team discovered that activity forms a vector field concentrated at the goal location. Moreover, cell firing provides information about other directions and ranks them in terms of how good each would be in getting the rat to its goal if the direct approach is blocked.
O’Keefe sees this new discovery, published open access in Nature last year, as a major boost to our understanding of how the hippocampus enables flexible navigation, and good news for those wanting to take this research even further. “We think place cells are a vector processor, so that means we can now tap into all of the 200 years of vector mathematics that is sitting out there to understand it,” he enthused. “It’s going to be a lot of fun.”
Of Mice and Microglia
Continuing the theme of focusing on new, exciting research, next up was Mario Capecchi, who received the 2007 Nobel Prize in Physiology or Medicine alongside Sir Martin Evans and Oliver Smithies “for their discoveries of principles for introducing specific gene modifications in mice by the use of embryonic stem cells”.
Though still embracing mouse gene modification, Capecchi’s research today centres on uncovering the roots of anxiety-related conditions. In lab mice, such conditions manifest in different ways. For instance, mice with obsessive-compulsive spectrum disorder (OCSD)-like behaviours incessantly groom themselves to the point where their fur sloughs off and they develop welts. Other behaviours indicating high anxiety levels include avoiding open spaces. “Freezing is also a sort of extended anxiety response,” explained Capecchi. “They’re so scared, they just freeze.”
A few years ago, Capecchi’s team discovered that a mutation in certain subset of microglia caused mice to exhibit such behaviours. This result was highly unexpected. Neurons were traditionally thought to be the cells that regulate behaviours. In contrast, microglia had a different role, clearing cellular debris and mounting the immune response to pathogens and infections. But in addition to this, the team concluded that Hoxb8 microglia are important for controlling behaviour by communicating with specific neuronal circuits. Yet how this worked was a mystery.
To peel back the curtain, Capecchi’s team turned to optogenetics, using a laser to stimulate specific populations of microglia in the mouse brain. “People said, ‘you’re insane to try to do optogenetics, these aren’t neurons, they can’t fire, and so on’, Capecchi recalled. “So why did we do this experiment? Because it works.”
Specifically stimulating Hoxb8 microglia, work published open access this April in Molecular Biology revealed how Capecchi’s team from the University of Utah, USA, can manipulate anxiety-related behaviours like flipping a switch on and off. And by targeting different parts of the brain they can switch on different specific behaviours. What’s more, when they target both Hoxb and non-Hoxb microglia, it prevents the onset of anxiety-related behaviours.
“Hoxb8 microglia actually function as brakes, they reduce sensitivity. And non-Hoxb8 microglia, they act as an accelerant, increasing the level of grooming and anxiety,” explained Capecchi. “So together, they can tune to find the exact level that you should have.” Further research and a deeper understanding of this relationship in mice could eventually lead to new approaches for targeted therapies for anxiety-related conditions.