Connectome-seq Maps Neuronal Connectivity with Single-Synapse Resolution

By tagging neurons with molecular barcodes, researchers have created a faster, higher-resolution way to map connections among thousands of neurons in the mouse brain. The method, called Connectome-seq, could help scientists better understand how brain circuits are organized, how they function, what changes when they fail, and how neurodegenerative diseases progress.

The work was published in Nature Methods. Study leader Boxuan Zhao, from the University of Illinois Urbana-Champaign, said the goal is similar to figuring out the wiring of a computer’s central processing unit. If the circuitry is not understood, he explained, it is difficult to understand function, improve performance, or repair problems when something breaks. He said the same logic applies to the brain. 

Zhao also noted that the new technology can map thousands of neural connections at single-synapse resolution, something he said is not available in current technology.

Brain mapping has traditionally required slicing tissue into very thin sections, imaging the sections with different microscopes, and reconstructing pathways step by step. More recent sequencing-based methods can label many neurons at once, but they usually show where a neuron extends rather than identifying the exact synaptic partner. Connectome-seq was designed to close that gap.

The platform uses RNA barcodes to label each neuron. Specialized proteins move those barcodes from the neuron’s cell body to the synapse, where two neurons meet. The researchers then isolate the synaptic junctions and use high-throughput sequencing to identify which barcodes appear together. That pairing reveals which neurons are connected.

 “We translated the neural connectivity problem into a sequencing problem. Imagine a big bunch of balloons. The main body of each balloon has its unique barcode stickers all over it, and some move down to the end of the string. If two balloons are tied together at the end, the two barcodes meet at the junction,” Zhao said. “Then we snip out the knots and sequence the barcodes in each one. If the same knot has stickers from balloon A and balloon B, we know these two balloons are tied together. We are doing this in the brain, just on the level of thousands of neuron cells. With this information, we can reconstruct a sophisticated map that represents the connections among all these seemingly floaty groups.”

Using this approach, the team mapped more than 1,000 neurons in the pontocerebellar circuit of the mouse brain. That circuit links two different brain regions. The analysis uncovered previously unknown connectivity patterns, including cell types that were not previously known to be directly wired together in the adult brain.

Zhao said the method is still being improved and could eventually be scaled to map the entire mouse brain. Because it is faster and can cover large regions, Connectome-seq may also speed research on neurodegenerative conditions, psychiatric disorders, and other neurological diseases. It could help scientists compare healthy brains with brains at different disease stages, identify where connections change, and locate vulnerable regions before symptoms appear. Zhao also suggested that such information could help pinpoint weak links in Alzheimer’s disease and guide efforts to strengthen those connections so the disease slows or does not progress.