Summary: A new miniature brain microscope, dubbed Mini2P, allows researchers to study neural network activity at high resolution in naturally behaving animals.
“Our dream was to invent a window into the brain, so we could see what’s going on inside when we think, plan, feel and remember,” says Professor May-Britt Moser, describing the conversations that she and her longtime collaborator Professor Edvard Moser had as young psychology students in the early 1990s.
Moser is founding director of the Center for Neural Computation and co-director of the Kavli Institute for Systems Neuroscience at the Norwegian University of Science and Technology (NTNU), and a Nobel Laureate, shared with her research partner, Edvard Moser, co-director of the Kavli Institute for systems neuroscience.
Today, Leif Erikson the mouse is the first step in realizing that dream. The mouse is equipped with a window on the head. Above the window are 2.4 grams of pure technological innovation. The portable “Mini2P” can perhaps be described as a tiny cortical observatory, recording live images of neural landscapes like never before.
Live reporting from the brain
In Leif Erikson the mouse’s brain, thousands of neurons work together to solve a very specific task. This activity is visible when a few cells begin to light up. Shortly after, other cells light up.
The Mini2P can record live from the area of the brain that is responsible for Leif’s navigation skills. The flickering brain cells that the researchers see on the screen at the same time allow the mouse to make its way through the ground, up a climbing tower, to the roof of the tower, where delicious vanilla cream cookies await.
“If we want to understand complex behavior, the animal must be free to move and behave in a way that is natural to it,” explains Edvard Moser. “The Mini2P is the first tool that allows us to study neural network activity at high resolution in naturally behaving animals.”
Great inventions come in small packages
The basic mechanics of Mini2P aren’t too different from an optical microscope, or the human eye for that matter. The smallest unit of light is called a photon. Mini2P uses two and two tiny beams of light from a laser to excite and record neurons precisely, at high resolution.
“When developing the Mini2P, we followed two rules that we weren’t willing to compromise on,” says Weijian Zong. Zong is the first author of a new paper describing the technology and is a researcher at the Kavli Institute.
“The first rule was that any improvements to the material should not affect the natural behavior of the animal. So we knew we had to cut some weight to make the microscope and its cable as light and flexible as possible,” he said. “The second rule was that you shouldn’t compromise on the performance of the microscope. If we want researchers to invest time in a new tool, the features of the miniscope must be significantly better than those of its predecessors.
One of the many brilliantly designed features of the Mini2P is a tiny electrically adjustable lens. By using static voltage, Zong was able to manipulate the curvature of the lens without causing a temperature rise. Changing the curvature of the lens will trigger Mini2P to shift the focal plane between the surface and the deeper cell layers of the cortex, also enabling 3D structural recordings of brain tissue.
A gene borrowed from the jellyfish makes brain cells glow when they talk to each other.
Bioluminescence allows researchers to see exactly which cells are participating in different segments of the conversation. They can also observe how the neural conversation gives rise to ideas which the mouse then implements “in the outside world”.
Researchers can also color-code brain cells based on the genes they express and the brain areas with which they communicate. This allows researchers to learn which types of brain cells need to work together to generate different cognitive abilities.
Mini2P records thousands of brain cells simultaneously. It can track the same brain cells for over a month and keep them focused even during the most vigorous activities, like repeated jumps from a 22 centimeter high tower. Mini2P can explore different mental areas and functions throughout the cerebral cortex.
The researchers tested Mini2P in several regions of the brain, such as the navigation system, the memory center and the visual area. Using a kind of patchwork quilt technique, it can map even larger neural landscapes, like 10,000 brain cells across the visual cortex. All measurements were taken while the mouse was moving freely and doing what it normally does. It was simply impossible before Mini2P.
Oppose Mini2P to the closest existing technologies
1-photon miniscopes have been around for a decade. They have several problems. The resolution is insufficient, the imaging may be too slow, you cannot move the focal plane in the Z axis, or they cannot be used for most parts of the cortex with high activity and high cell density .
Mini2P allows recording from multiple planes along the Z axis, at scales ranging from cell substructures such as axonal branches, to topographic maps of ten thousand cells. The current benchtop version of the two-photon microscope weighs half a ton and takes up almost the entire room.
The benchtop microscope requires the mouse’s head to be held in place, which limits the mouse’s natural movement. It also replaces mouse access to the real world with virtual reality. It’s nothing like a mouse normally would, which means its behavior probably isn’t natural either.
Mini2P, on the other hand, weighs 2.4 grams, with a super flexible laser and light-gathering cable that allows the mouse to move as freely and dynamically as it would without wearing Mini2P on its head.
Mini2P is open-source
“We believe the Mini2P is a game-changer and want to share it with neuroscientists and labs around the world,” says May-Britt Moser.
“The amount of research data collected from each recording may also play a role in reducing the number of animals used in research,” she said. The new tool could also play a major role in understanding brain diseases.
“Alzheimer’s disease often begins with lesions of the entorhinal cortex”, explains Edvard Moser. “We know that Alzheimer’s disease causes deficits in the ability to navigate and in memory. These are brain functions that arise from the joint collaboration of thousands of brain cells.
“The Mini2P offers a way to track changes in dynamics between thousands of cells in freely moving mice, using strains of mice that are model organisms for studying Alzheimer’s disease,” he explains. -he. “The ability to label different cell types may also allow us to identify cells that are vulnerable to early changes related to Alzheimer’s disease,” said Edvard Moser.
Mini2P is an open source invention available from NTNU’s Kavli Institute for Systems Neuroscience in Trondheim. The plan, a shopping list and instructional films are available on GitHub. The institute will also offer workshops later in the year.
About this neurotechnology research news
Original research: Free access.
“Large-scale two-photon calcium imaging in freely moving mice” by Edvard I. Moser et al. Cell
Large-scale two-photon calcium imaging in freely moving mice
- We fabricated a lightweight 2-photon miniscope for calcium imaging in free-moving mice
- Stable high-quality imaging has been observed during a wide range of behaviors
- Activity can be monitored in volumes of over 1,000 visual cells or entorhinal cortex
- A tailor-made design z-scanning module enables rapid imaging in multiple planes
We have developed a miniaturized two-photon microscope (MINI2P) for rapid, high-resolution multiplane calcium imaging of more than 1,000 neurons at a time in freely moving mice.
With a microscope weight of less than 3 g and a highly flexible connection cable, MINI2P enabled stable imaging without behavioral hindrance in a variety of assays compared to untethered and unimplanted animals.
Improved cell yield was achieved through an optical system design featuring an expanded field of view (FOV) and a micro-adjustable lens with increased z-Scanning range and speed enabling fast and stable imaging of multiple interlaced planes, as well as 3D functional imaging. Successive imaging over several adjacent FOVs allowed recordings of more than 10,000 neurons in the same animal.
Large-scale proof-of-principle data have been obtained from cell populations in the visual cortex, medial entorhinal cortex, and hippocampus, revealing the spatial tuning of cells in all domains.