The single protein that helps keep motor neurons working smoothly

C. elegans
A transgenic C.elegans worm, where the motor neurons are labeled with two fluorescent reporters (green and red). Motor neurons are located on the under side of the worm, positioned one after the other, and appear as green and red dots. The top three panels are fluorescent images, and the bottom panel shows the actual animal together with the fluorescently labeled motor neurons. (Credit: Paschalis Kratsios)

The DNA of an animal contains the instructions for creating every type of cell in its body. During development, the generation of different cell types (e.g., neurons, blood cells, muscle cells) depends on different sets of genes for each cell type to be expressed at the right time. But what determines which genes are switched on and which genes are turned off or ignored?

A new study by scientists at the University of Chicago in a tiny worm called C. elegans shows that, in motor neurons, a single protein coordinates the decision of which genes will be switched on and which genes will be turned off, determining the ultimate identity of motor neurons and what roles they will play. Once the neuron has developed into its final form, that same protein also regulates expression of the right molecules to keep it functioning properly.

Neurons are different from other types of cells because once they develop into their final state, they stop dividing and don’t renew themselves. Neurons are long-lived and form connections with each other to carry out the vital functions of movement, sensation, and communication between different parts of the body and the brain. And they have to keep doing this for a long time, unlike blood cells, for example, that are constantly replenished and just die off once they start to break down.

Neurobiologist Paschalis Kratsios, PhD, senior author of the new study published in eLife, wanted to understand how different types of neurons maintain their functions over the lifetime of an organism.

“What does it mean for a cell type to be functional?” Kratsios said. “Why do your neurons look like neurons, or your eyes look like eyes, or why does your skin look like skin? They have the same DNA material, but different molecules are expressed in neurons or the eyes or the skin to make those cells different.”

A simple model to understand basic functions

Scientists work with C. elegans because it is a simple organism that is easy to study. It has just 302 neurons to perform different movement and sensory functions, so researchers can single out different genes fairly easily and track their activity over the short lifetime of the worm (about three to four weeks).

Kratsios and his team focused on the subset of motor neurons that are essential for movement, and eventually zeroed in on a protein called UNC-3. UNC-3 is a transcription factor, a protein that controls the expression of multiple genes. His team found that UNC-3 works with other proteins to switch on specific genes needed to produce molecules for building motor neurons, while also shutting down unrelated genes that produce molecules necessary for the function of other types of neurons.

The researchers also used CRISPR genome editing tools to knock out UNC-3, both during development and in adult worms with fully functioning motor neurons. The worms lacking UNC-3 from birth didn’t develop functioning motor neurons, while the adults with UNC-3 knocked out after development began to lose existing motor neuron function. These experiments suggest that the UNC-3 protein must play a sustained regulatory role to keep the motor neurons functioning properly.

“This mechanism seems to be taking place throughout the life of a neuron,” Kratsios said. “Without it, the cells end up kind of confused, because not only do they stop producing their normal function-defining molecules, but they also start producing other unwanted molecules.”

This also seems like an unusual way for this molecule to work. “It’s like UNC-3 acts as a ‘sponge’ to limit the availability of another conserved transcription factor in motor neurons called LIN-39/Hox,” said Weidong Feng, a fourth-year student in the graduate program of Development, Regeneration and Stem Cell Biology (DRSB) and leading author of the study.

Looking for the same function in more complex organisms

These findings may advance our understanding of human diseases that affect motor neurons, such as amyotrophic lateral sclerosis (ALS). In these conditions, motor neurons develop properly and work fine for decades, then start to lose function later in life. While it’s quite a leap to compare the nervous system of C. elegans to that of humans, the gene that produces the UNC-3 protein is highly conserved across species, meaning humans have an equivalent version. Kratsios said the new study could be the beginning to understanding the molecular mechanisms that maintain long-term motor neuron function in humans, and what goes wrong when this function is compromised during disease.

For the next step, he and his colleagues are moving on to look for similar processes in another scientific model organism, the mouse, which has a complex nervous system similar to humans. There is already evidence that mice have a protein similar to UNC-3 in their spinal cord neurons, which gives them a starting point.

“If you think about it, we’re talking about worm motor neurons that express UNC-3, and then millions of years of evolution later we have a much more complex animal like a mouse, in which an equivalent version of UNC-3 is also expressed in spinal motor neurons,” Kratsios said. “That’s already huge, so now the big question is if the function of UNC-3 has been conserved from worms to mice.”

The study, “A terminal selector prevents a hox transcriptional switch to safeguard motor neuron identity throughout life,” was supported by the National Institute of Neurological Disorders and Stroke and the Whitehall Foundation. Additional authors include Weidong Feng, Yinan Li, Pauline Dao, Jihad Aburas, Benayahu Elbaz and Anna Kolarzyk from the University of Chicago; and Priota Islam and Andre ́ EX Brown from MRC London Institute of Medical Sciences and Imperial College London, United Kingdom.