A Dartmouth study in Nature Communications reveals that immune cells in the brain use a surprising two-step process to power their search-and-destroy missions.
When immune cells rush to injury sites in the brain, they arrive essentially empty-handed—powered by little more than the cellular equivalent of a candy bar.
By traveling light and calling for backup, the cells can take immediate action and achieve longer-term damage control.
Central nervous system immune cells called microglia respond to threats within minutes by sending out arm-like projections that begin containing the damage. “It’s like troops that are initially responding to some emergency with only a candy bar in their pocket,” says Robert Hill, an associate professor of biological sciences and senior author of the study.
Professor Robert Hill’s team used fluorescent imaging to observe brain immune cells called microglia in action. This video shows microglia cells (magenta) and their mitochondria (green) as microglia travel towards an injury.
“They only carry enough supplies to power that initial invasion, but to really resolve the damage, you need to build a road and pull in the mitochondria,” Hill says.
Surprisingly, the researchers found that these early-responding arms lack mitochondria—the energy-producing “powerhouses” of a cell. They showed that it takes around six hours for mitochondria to travel to the scene, because the cells must first build pathways to transport them. This means that the microglia’s early actions are powered either by glycolysis, a relatively inefficient mode of producing energy from sugar, or from energy produced elsewhere in the cell.
Since microglial metabolism can become dysfunctional during aging and neurodegeneration, understanding how healthy microglia regulate their metabolic needs will help researchers pinpoint what goes wrong.
“This study establishes a foundation for understanding mitochondria and microglia in the healthy brain, and we'll be referring back to for a long time as we start investigating diseased contexts,” says Alicia Pietramale, who led the research as a Guarini PhD candidate in Hill’s research group.
Microglia are one of the main immune cells in the brain and spinal cord. These highly mobile cells are on constant patrol in the brain, searching for invading pathogens and tissue damage. They’re also shapeshifters and multitaskers, rapidly sending out and retracting arms called processes that probe different parts of the surrounding environment. When microglia detect damaged cells, debris, or bacteria, they engulf it with their cell membranes via a process called phagocytosis—essentially eating and digesting the damaged material.
These rapid movements and shapeshifts are very energy intensive, so Pietramale and Hill investigated the cell's mitochondria to understand how microglia allocate energy to different activities.
Previous studies have shown that most types of cells have mitochondria scattered throughout their interiors, though some cells strategically position their mitochondria in areas with high-energy needs. However, little is known about mitochondria in healthy microglia.
“Since microglia need energy to carry out a rapid response to traumatic insult, we expected their mitochondria to move with them,” Hill says.
How mitochondria move to the front lines
The researchers used mice whose microglia and mitochondria were tagged with fluorescent markers, which allowed them to film and photograph the microglia in their natural working environment.
“What’s really powerful about our model is that we can study microglia in living brains,” Pietramale says. “A lot of our knowledge on microglial cells’ mitochondria is from cell culture studies, but microglia function is highly dependent on what's happening in their microenvironment.”
The researchers unexpectedly found that mitochondria are not spread evenly throughout microglia cells. Instead, some of the cells’ arm-like processes contained many mitochondria, while others lacked them entirely, and more active processes had fewer mitochondria than less active processes.
“This discovery reveals a lot about how microglia are able to do many different things at the same time,” Pietramale says. “Why spend the energy transporting mitochondria to all parts of microglia when some of these processes only reside in a position for as little as a minute?”
The researchers then filmed the microglia’s response to damaged brain cells to understand if the uneven distribution of mitochondria plays a functional role. Microglia deployed processes to the damaged tissue within minutes, but these first-responders didn't contain any mitochondria for hours after the injury, the team reports.
Mitochondria only began showing up three hours later and were fully present six hours post-injury.
The team shows that this mitochondrial movement relies on microtubules—like railroad tracks inside cells—rearrangeable proteins that form part of the cytoskeleton (internal scaffolding that gives cells their shape).
“Mitochondria need a road to move along, and that road is made up of microtubules,” Hill says. “It takes time for the microtubules to arrive and additional time for the mitochondria to get transported along them, which is why we see this delay.”
When the team analyzed pre-existing microscopy images of human microglia, they found similar patterns. “The mouse immune system models the human immune system very well, but it's always good to confirm that what we're seeing in the mouse is consistent in humans so that this can be translatable to developing therapeutics down the line,” Pietramale says.
Nature Communications published the Dartmouth paper alongside a complementary study by UCLA researchers that showed that microglial mitochondria behave differently in different parts of the brain, and revealed that mitochondrial genes can impact microglial shape and function. In the spirit of collaborative science, the two research groups coordinated their studies to be published together.
“This was a great collaborative experience, and it shows how scientists can work together instead of competing,” Hill says.
Now that the Dartmouth team has a picture of how microglial mitochondria are partitioned in healthy cells, the researchers are investigating whether this process becomes disrupted during disease and aging.
“Our immediate next step is to examine this process in aging brains and brains with inflammation,” Pietramale says. “I’ve completed those experiments, and I’m excited to share those results in a future publication.”