In this figure from a paper produced by Xiaobing Yuan’s team at the Chinese Academy of Sciences (citation below), green indicates neurons in a slice of mouse brain. Each image comes from a mouse with a different version of the PIWIL1 gene. The results helped the team deduce which regions within the gene are critical for neuron migration, which is necessary for typical brain development.
By Sarah Hansen
After extensive experimentation, Dr. Xiaobing Yuan, a Hussman Institute for Autism (HIA) investigator, can say, “This gene really plays a role in the developmental process of neurons. That’s the key conclusion.”
The gene is PIWIL1 (pronounced “pee-wee-el-wun”), and before Yuan’s recent paper, scientists already knew that it helps create cell polarity—the ability of the cell to take on specific shapes and structures so it can do its job. For example, sperm cells are polarized: a round head region stores the genetic material, and a whip-like tail helps the sperm “swim” toward an egg to deliver the goods. It’s been shown that without certain genes in the PIWI family, sperm can’t transition from a simple spherical shape to having a distinct head and tail.
Mature neurons, a type of brain cell, are polarized, too. They consist of a soma (analogous to a sperm head) and a long protruding axon (like a sperm’s tail). The similarity between sperm and brain cells led Yuan to ask, “If this gene plays a role in polarization of [sperm] cells, why can’t it play a similar role in other cells?” He and colleagues at the Chinese Academy of Sciences in Shanghai, China (where he had a lab before coming to HIA) decided to try to answer that question.
First the team demonstrated that the PIWIL1 protein was present in several brain regions of mice. Then, with experiments on rat embryos, they showed it is expressed specifically in newborn neurons. Why the switch from mice to rats? “Both of them are good models, but mouse embryos are much smaller,” said Yuan, which makes rat embryos easier to handle. This finding alone is noteworthy; the fact that PIWIL1 is found both in germline tissue (sperm) and brain tissue is new knowledge.
Next, the team showed that particular regions within the PIWIL1 protein were essential for proper neuron migration. After neurons are born and undergo polarization, they travel a significant distance through several brain layers to reach their final destinations. If they don’t make it, they can’t perform their functions as well. When Yuan’s team silenced mice’s own PIWIL1 genes and added different truncated versions of the gene, only mice that received a PIWIL1 gene version containing two specific regions, PAZ and PIWI, showed normal neuron migration. Neurons in mice that received a version of the gene missing the PAZ and/or PIWI regions did not migrate properly. This showed that the site of a mutation within the gene is critical; if these two regions are unaffected, the function of PIWIL1 may be unaltered.
Yuan and colleagues also showed that PIWIL1 is important for the polarization process of neurons. In control rat embryos, about 70 percent of neurons had become bipolar after the experimental period, but only about 30 percent of neurons in rat embryos with no active PIWIL1 had successfully transitioned from multipolar to bipolar. That transition is required for successful long-distance migration.
These experiments gave Yuan and colleagues evidence that PIWIL1 is involved in polarization and migration of neurons, but discovering the molecular mechanism behind PIWIL1’s role was arduous. The experiments necessitated some difficult and sensitive techniques. “It really brought on a new challenge,” said Yuan.
The team decided to look at the relationship between PIWIL1 and microtubule-associated proteins, or MAPs. MAPs made sense as a target, because microtubules are important for polarization and the extension of axons from neurons. Yuan and colleagues found that when they silenced PIWIL1, MAP levels also plummeted, and migration was stunted. If they silenced PIWIL1 and added a MAP protein, migration was partially restored. The difficulty of the experiment limited them to adding one MAP at a time, but “if we used multiple MAPs to do the experiment, perhaps the rescue would be better,” said Yuan. After at least a year of experiments, they found strong evidence that PIWIL1 may regulate neuron migration by regulating MAPs—a previously undiscovered role for PIWIL1.
Mutations in PIWI gene family members, specifically PIWIL2 and PIWIL4, have been found in a small number of individuals with autism, so there’s a chance that PIWIL1 might be involved, too. Even though the evidence is preliminary, Dr. Gene Blatt, Director of Neuroscience at the HIA, pointed out that any association can be potentially useful. Examining the effects of changes in associated genes in animal models or human post-mortem tissue can help reveal the genes’ functions. If the gene is related to an activity that is more challenging for individuals with autism, such as speech or motor planning, the findings may suggest new places to look for treatments to make life easier for those individuals.
With hundreds of genes already implicated in autism, understanding the condition can feel like looking for a needle in a haystack. Some implicated genes are involved with neuron migration, which lends credibility to the speculation that PIWIL1 might also have a role. But a full understanding of autism and its mechanisms is only emerging, “and that’s why we have the Institute,” said Blatt.
The article is published here:
Zhao P, Yao M, Chang S, Gou L, Liu M, Qiu Z, and Yuan X. (2015). Novel function of PIWIL1 in neuronal polarization and migration via regulation of microtubule-associated proteins. Molecular Brain. DOI: 10.1186/s13041-015-0131-0