Too much of a good thing: pruning neurons to control behavior

From the Singhvi Lab, Basic Sciences Division

The purpose of pruning is to improve the quality of the roses, not to hurt the bush. -Florence Littauer

Animal development is generally viewed as a process of growth and construction. Cells divide and connect to build larger, more complex tissues and organs as the animal grows. But in the body, as in any well-tended garden, untamed growth can be a wild and dangerous thing, and sometimes what you remove is just as important as what you allow to remain. Perhaps the most famous example of this concept is the Nobel prize-winning work on programmed cell death, the discovery that some of our cells are born fated to die for the noble cause of tissue balance. So, too, does this process protect us from disease, as our bodies employ mechanisms to detect defective cells, such as those with cancerous mutations, and entice them to die before they can grow into tumors.

As impressive a feat of cellular gardening as any, though, occurs in our brains. During the first couple years of life, billions of tree-like neurons are born and grow trillions of branches, each connecting with other cells to form large networks. This rapid growth results in a toddler brain that resembles a gnarly blackberry bramble, with far more, and more densely-packed, branches than is ultimately needed. So much so, in fact, that our brains spend the next couple decades carefully pruning back those branches to create a well-cultivated, and well-functioning, organ. Errors in pruning are believed to be potential causes of conditions such as autism spectrum disorder (under-pruning) and schizophrenia (over-pruning). What is responsible for neuronal pruning? How is it controlled? And how does it affect animal behavior? These are some of the questions that drive Dr. Aakanksha Singhvi, Assistant Professor in the Basic Sciences Division at Fred Hutch. In a recent paper published in eLife, Dr. Singhvi’s lab identified an active neuronal pruning process that regulates behavior in the nematode worm C. elegans.

Glia are a type of cell in the nervous system that associate with the tips of neuronal branches. “Pruning is a conserved, and disease-relevant glial function where glia eat fragments of a neuron”, said Dr. Singhvi. While this process has been observed in other animals, studying it in C. elegans “lets us focus the powerful genetic toolkit of the worm to understand the underlying molecular machinery.” It also allowed the team to observe this process at high resolution. In this work, the group examined a single neuron-glia pair: the AFD neuron, which extends a dendrite branch with a bush-like tip (see figure) into the nose of the animal and senses temperature, and the AMsh glial cell that surrounds the AFD tip. The authors were surprised to see that, when they labeled the AFD neuron with a green fluorescent protein (GFP) and the AMsh glia with a red fluorescent protein (RFP), they consistently observed small green flecks inside the glial cell. These flecks, they determined, represent small pieces of the AFD neuron that had been eaten by the glia. They then examined a series of genetic mutants to understand how this process happens. They discovered a communication process in which the neuron exposes a phospholipid on its membrane to signal to the glial cell that it should be eaten, and the glia then uses the cell corpse engulfment machinery, dramatically but aptly named for its role in promoting the clearance of dead cells, to nibble at the neuron. Finally, the group examined the consequences of this process.  They found that decreasing the activity of the neuron increased glial pruning, and that blocking pruning causes defects in thermotaxis, the behavioral response to changes in temperatures for which the AFD neuron is responsible. In reflecting on the impact of this work, Dr. Singhvi found the precision and power of the pruning process particularly exciting: “Glia are not just passively cleaning up debris neurons leave behind, but rather have an active control on this process. Finally, this study showed, with single-cell resolution, that pruning of even one neuron by one single glia can profoundly impact how an animal senses its environment and executes behavior.”

Graduate student Stephan Raiders, lead author on this study, is excited to expand this work in C. elegans to better understand how glial pruning affects neuronal behavior in healthy animals: “The next question we are asking is: how is the proper amount of engulfment maintained? Does the regulation of this process change during aging, and if so, how?” as well as in disease states: “Currently, I am looking at mutants that reveal new genes that regulate engulfment, some of which are implicated in neurological diseases,” Raiders explained.

High-resolution image of the bushy tip of the AFD neuron, the branches of which are pruned by the AMsh glia. Image provided by Stephan Raiders

This work was supported by the National Institutes of Health, the Simons Foundation, and the American Federation for Aging Research.

Raiders S, Black EC, Bae A, MacFarlane S, Klein M, Shaham S, Singhvi A. (2021) Glia actively sculpt sensory neurons by controlled phagocytosis to tune animal behavior. Elife.10:e63532. doi: 10.7554/eLife.63532.