Using a new approach for studying live embryonic mouse brains at single-cell resolution, researchers have identified an active multi-layer circuit that forms in the cortex during an unexpectedly early stage of development. Perturbing the circuit genetically led to changes similar to those seen in the brains of people with autism. The findings were reported reported in the journal Cell.
“Understanding the detailed development of cell types and circuits in the cortex can provide important insights into autism and other neurodevelopmental diseases,” says the paper’s corresponding author. “This is what our findings confirm.”
Autism has long been associated with faulty circuits in the cortex, which is the part of the brain that governs sensory perception, cognition, and other high-order functions. Most of the cortex is composed of excitatory cells called pyramidal neurons. The team wanted to study when and how these neurons assemble into the first active circuits in the cortex, but that posed a difficult challenge.
Pyramidal neurons measure only a tenth of the width of a human hair, and any movement during experimental procedures might lead to inaccurate recordings of activity. To keep the neurons stable for research, the team devised a surgical solution: Embryos were secured inside of agar-filled 3D holding devices within the mother’s abdominal cavity, so that normal embryonic blood flow and temperature could be maintained.
The prevailing view is that the cortex develops in an “inside out fashion”, with the deepest of its six layers appearing first. Seen this way, pyramidal neurons were thought to slowly become active as they migrate to their final locations in the cortex and form connections with each other.
But during the research, "we actually detected a very different activity pattern,” says one of the paper’s two lead authors. Focusing specifically on pyramidal neurons that develop into layer 5 of the cortex, the team discovered a very early transient circuit that was already highly active and correlated even before the six-layer cortex had formed. This indicates that the neurons were already connected prior to their migration to form layer 5.
The transient circuit initially had 2 layers: a deep layer and a superficial layer. Later, the superficial layer became silent and vanished, while the classical layer-by-layer cortical development resumed, with a third intermediate layer forming layer 5.
"We also wanted to understand how this circuit changes in an autism model," says the paper's other lead author. Working with knock-out mouse lines missing one or both alleles of two autism-associated genes--Chd8 and Grin2b--the team made a key finding. The absence of these genes is known to cause significant autism in children. And in homozygous and heterozygous knockout mice, the superficial layer remained active as a developmental remnant.
"Throughout embryonic development, it never disappeared," the author says. Moreover, the knockout mouse brains contained patchy areas of cortical disorganization similar to those seen in people with autism.
The findings suggest that the spatial organization of pyramidal neurons is regulated by the newly-found circuit, and that "changes to embryonic circuits play a role in dysfunctions associated with neurodevelopmental disorders, including autism spectrum disorder," the author says.
Autism-associated mutations perturb neuronal circuits during embryonic development
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