More than 70 genes have been linked to autism spectrum disorder (ASD), a developmental condition in which differences in the brain lead to a range of behavioral changes, including problems with language, social communication, hyperactivity and repetitive movements. Scientists try to tease out these specific associations gene by gene, neuron by neuron.
Such a gene is Astrotactin 2 (ASTN2). In 2018, researchers from the Developmental Neurobiology Laboratory at The Rockefeller University discovered how defects in the protein produced by the gene disrupted circuitry in the cerebellum in children with neurodevelopmental disorders.
Now the same lab has discovered that knocking out the gene completely leads to several characteristic behaviors of autism. As they describe in a new paper at PNASmissing mice ASTN2 they showed distinctly different behaviors than their wild-type littermates in four key ways: they vocalized and socialized less, but were more hyperactive and repetitive in their behavior.
All these characteristics have similarities in people with ASD. Alongside these behaviors, we also found structural and physiological changes in the cerebellum.”
Michalina Hanzel, first author of the paper
“It’s a major discovery in the field of neuroscience,” says lab leader Mary E. Hatten, whose work has focused on this area of the brain for decades. “It also underscores this emerging story that the cerebellum has cognitive functions that are completely independent of its motor functions.”
An unexpected role
In 2010, Hatten’s lab discovered that proteins produced by the ASTN2 gene helps guide neurons as they migrate during cerebellum development and form its structure. In the 2018 study, they looked at a family in which three children had both neurodevelopmental disorders and ASTN2 mutations. They found that in a developing brain, the proteins have a similar guiding role: they maintain the chemical conversation between neurons by priming receptors from neural surfaces to make room for new receptors to spin in. In a mutated gene, the proteins fail to act and the receptors accumulate, creating a traffic jam that blocks neuronal connections and communication. This impact could be seen in the children’s afflictions, which included intellectual disability, language delays, ADHD and autism.
The finding was part of a growing body of evidence that the cerebellum — the oldest cortical structure in the brain — is important not only for motor control but also for language, cognition and social behavior.
For the current study, Hanzel wanted to see what effects its complete absence has ASTN2 gene may have on cerebellar structure and behavior. Working with study co-authors Zachi Horn, a former postdoctoral fellow in the Hatten lab, and with the help of Weill Cornell Medicine’s Shiaoching Gong, Hanzel spent two years creating a knockout mouse that lacked ASTN2and then studied the brains and activity of both infant and adult mice.
Behavioral parallels
The knockout mice participated in several non-invasive behavioral experiments to see how they compared to their wild-type littermates. The knockout mice showed distinctly different characteristics in all of them.
In one study, researchers briefly isolated baby mice and then measured how often they called their mothers using ultrasonic vocalizations. These sounds are a key part of a mouse’s social behavior and communication, and are one of the best proxies researchers have for assessing parallels with human language skills.
Wild-type pups quickly called their mothers using pitch-changing complex sounds, while knockout pups gave fewer, shorter calls within a limited pitch range.
Similar communication problems are common in people with ASD, Hanzel says. “It’s one of the more telling traits, but it exists on a spectrum,” he says. “Some autistic people don’t understand metaphor, while others repeat the language they’ve heard, and others don’t speak at all.”
In another experiment, the researchers tested how ASTN2 mice interact with both familiar and unfamiliar mice. They preferred to interact with a mouse they knew rather than one they didn’t. In contrast, wild-type mice always choose the social novelty of a new person.
This, too, has parallels in the behavior of the person with ASD, with the reluctance of unfamiliar environments and people to be common, Hanzel adds. “This is a very important result, because it shows that mice with the knockout mutation do not like social novelty and prefer to spend time with mice they know, which corresponds to people with ASD, who tend to like new social interactions less than known. “
In a third experiment, both types of mice were given free rein to explore an open space for one hour. THE ASTN2 The mice traveled a significantly greater distance than the other mice and engaged in repetitive behaviors, such as circling in place, 40% more. Both hyperactivity and repetitive behaviors are well-known features of ASD.
Poor communication between brain regions
When they analyzed their brains ASTN2 mice, found some small but apparently powerful structural and physiological changes in the cerebellum. One was that large neurons called Purkinje cells had a higher density of dendritic spines, structures that colocalize with the synapses that send nerve signals. But they found this change only in discrete regions of the cerebellum. “For example, we found the biggest difference in the posterior canine region, where repetitive and rigid behaviors are controlled,” says Hanzel.
The scientists also discovered a decrease in the number of immature dendritic spines known as filopodia and in the volume of Bergmann glial fibers, which aid in cell migration.
“The differences are quite subtle, but they clearly affect the way the mice behave,” says Hatten. “The changes probably alter the communication between the cerebellum and the rest of the brain.”
In the future, the researchers plan to study human cerebellar cells, which they have been growing for half a dozen years from stem cells, as well as cells with ASTN2 family-contributed mutations in the 2018 study.
“We would like to see if we can find parallel differences to what we found in mice in human cells,” says Hatten.
He continues, “We also want to look at the detailed biology of other genes associated with autism. There are dozens of them, but there is no common thread that links them together. We are very excited that we were able to show in detail what ASTN2 it does, but there are many more genes to investigate.”
Source:
Journal Reference:
Hanzel, M., et al. (2024) Mice lacking Astn2 have ASD-like behaviors and altered cerebellar circuit properties. PNAS. doi.org/10.1073/pnas.2405901121.
