Careful pruning guides neuron function

The neurons in our brains and bodies do amazing things: sense the outside world, transmit information, guide our behavior. But they don’t do it alone.

Using tiny worms with well-mapped nervous systems, scientists at Fred Hutchinson Cancer Research Center answered a longstanding question about the role of accessory brain cells called glial cells in supporting neuron function. Their study was published recently in the journal eLife.

Each one of our nerve cells is aided by a glial cell. Scientists have known since the 1960s that glial cells nibble at the edges of neurons — but how and why remained unclear. Are glia just cellular janitors that clean up detritus shed by neurons, or are they pilots that carefully steer nerve cell behavior and function?

“Our work allowed us to say that the glial cells are in the driver's seat. They are controlling [this phenomenon],” said Dr. Aakanksha Singhvi, the Fred Hutch brain scientist who led the study. “The neuron is not the one shedding — the glial cell decides. It listens to the neurons and then decides how much it wants to pinch off or not pinch off. And we also figured out how the glial cell is doing that.”

She and graduate student Stephan Raiders, who spearheaded the work, also demonstrated how glial cells shape animal behavior and memory — the first-ever demonstration in any organism that altering pruning by a single glia cell can effect a change in memory, the researchers said.

Changes in glial cell function have been linked to certain diseases, including Parkinson’s and Alzheimer disease. Understanding how glia work in concert with neurons will shed light on how glial cell defects contribute to such diseases, Singhvi said.

Glia: key nervous system players

Neurons were first described in the late 19^th century. Glial cells (also called glia) are a different cell type that are intimately connected to neurons. Though discovered at the same time, over the years they’ve received far less scientific attention. Instead, researchers largely dismissed glia as mere “nurse” cells that support the active, information-transmitting neurons.

Singhvi is one of the researchers correcting this long neglect, shining light on glial cells’ key role in the proper functioning of the nervous system.

The ratio of glia to neurons roughly tracks animals' complexity. The more complex the behaviors an animal’s neurons coordinate, the more help they need from glia. In Caenorhabditis elegans, the minuscule worm Raiders and Singhvi use to study glia, there are six neurons for every glial cell. In humans, there are an equal number of glia as neurons.

It’s been known for decades that glia engulf bits and pieces of nearby neurons, a process termed pruning. In particular, glia appear to prune at sites called neuron receptive endings, or NREs. These are any spot on a neuron that acts as its “antenna” for receiving input, Singhvi said.

“An NRE is a specialized subcellular structure on a neuron where it's receiving input, whether it's from the outside world or whether it's from another neuron,” she explained. “You can imagine very easily that if you change how much a glial cell is coming and eating this NRE, that will change how the neuron is connecting, how it's receiving information.”

This could result in either a neuron receiving too little information, or being overwhelmed by too much.

But which cell decides when, where and how much of the neurons’ NREs are being pruned — the neuron or the glial cell?

“This was the fundamental question, which nobody knew the answer to,” Singhvi said.

Scientists are coming to appreciate just how ubiquitous this process is throughout the nervous system, she noted. Glia have been connected to everything from memory storage to Alzheimer’s disease and vision loss. A better understanding of how they tune nerve cell activity could have wide-ranging health implications.

She and Raiders decided to use the highly detailed nervous system maps of C. elegans to find out. They focused on a nerve cell-glial cell pairing that responds to changes in temperature. This would allow them to not only study the pruning process, but also to see how alterations in pruning affect how worms respond to temperature.

In the driver’s seat

C. elegans worms have a set number of cells, including neurons, connected in a specific way. Because of this, scientists have used C. elegans for decades as a model to study brain circuitry, so both its genes and nervous system are well-mapped. Therefore, it’s an ideal organism for Singhvi and Raiders to study the effects of specific molecules on nervous system function.

First off, they determined that glial cells do prune nerve cells in C. elegans, which had never been shown before.

By genetically manipulating worms to get rid of certain genes, Raiders pinpointed the key molecules involved in glial pruning, including several that had never previously been linked to the phenomenon.

He found that a molecule in the glial cell regulates how much pruning occurs, suggesting that the process is driven by glia, not neurons. And, the glia appear to determine how much to prune by “listening” to neurons and nibbling based on neuronal activity. Higher temperatures, or more activity in the neuron, led to more glial pruning.

Raiders and Singhvi were also able to show that glial pruning determines the shape of the NRE on a temperature-sensing neuron. Genetically modifying the glia to prune the temperature-sensing NRE less resulted in a longer-shaped NRE, while over-pruning shortened it. This influenced worm behavior, the researchers found.

C. elegans can be grown at different temperatures, and normally worms that find themselves in an environment of a different temperature will move toward the temperature at which they were reared. But Raiders and Singhvi found that they could make worms more sensitive to temperature by reducing pruning and enlarging the NRE. These worms preferred surroundings that were even cooler than those they’d been grown in.

“And what we have shown for the first time is that by pruning one neuron, a single glial cell can directly affect animal behavior,” Singhvi said. “The simple memory that this animal has of what it likes and doesn't like is altered by this pruning event. A connection to memory at that single-cell resolution has just not been done in any system.”

In order to perform their NRE-shaping role, glia appear to draw on the same debris-engulfing molecular machinery used by certain white blood cells called macrophages to clean up dead cells — with a twist.

“Instead of [engulfing] being an all-or-nothing event, which is typically how it’s supposed to be for this kind of machinery, the glial cell is actually able to dial it up and down,” Singhvi said. “To my knowledge, it's the first instance where you can have this level of control with this machinery.”

Exploring links to human disease

Glial pruning was first described in the retina, but Raiders’ work is the first time it’s been shown in any other sensory organ, Singhvi said. And while some defects in glial function in, say, the ear, are linked to deafness, little is known about glial function outside of the central nervous system, she said.

But changes in sensation, whether sensory loss or hypersensitivity, are hallmarks of many diseases, suggesting that there may be much to learn about the role glia play in these systems, and how this may go wrong in disease.

“We know that glial tuning happens in normal [neural] maintenance, life, learning and memory — just maintaining your circuitry, everything healthy and happy. But in neurodegeneration, people are starting to implicate glial pruning as a process that might be going awry,” Singhvi said.

She and Raiders aim to use what they’ve learned in worms to help understand how glial defects may contribute to disease. Worms are certainly a far cry from humans, but there may be molecules and pathways that govern inter-cellular interactions that we share. This makes them a potentially powerful model in which to outline fundamental biological processes and even perhaps identify potential drug targets.

Singhvi and Raiders are currently exploring potential connections between glial pruning and human disease by studying what role a gene that’s been linked to Parkinson’s disease may play in the process. They’re also taking a wider look to see how changes in glial function may contribute to the disease.

In Parkinson’s neurodegeneration, “we don’t know what’s cause and what’s effect,” Singhvi said of glial defects, sensory loss, and neurodegeneration. “With this system, we can take out the glia and directly answer that question, which has been an open one in the field.”

This work was supported by the National Institutes of Health; the Anderson Foundation; the Marco J. Heidner Foundation; the Glenn Foundation for Medical Research and the American Federation for Aging Research; and the Simons Foundation for Autism Research.