Viewing the neurons of the heart in 3D

Researchers in the US have reported the development of a virtual 3D heart that digitally showcases the unique network of neurons for the first time.

Using the rat heart as a model, the multi-disciplinary team created a map of the intrinsic cardiac nervous system (ICN) at a cellular scale. Gene expression data can be superimposed within it, providing a guide to the functional role that specific neuron clusters play.

A 3D model in rotation, displaying the arrangement of intrinsic cardiac neurons in the rat heart. Credit: Achanta et al. – iScience.

“The only other organ for which such a detailed high-resolution 3D map exists is the brain,” says Raj Vadigepalli from Thomas Jefferson University, lead author of a paper in the journal iScience.

“In effect what we have created is the first comprehensive roadmap of the heart’s nervous system that can be referenced by other researchers for a range of questions about the function, physiology, and connectivity of different neurons in the ICN.”

And things needn’t stop there, the researchers suggest, because their approach and protocols just require everyday lab materials and techniques, and thus can be used for other organ systems to map not just neurons, but other micro-structures.

Nonetheless, the project itself involved a complex mix of imaging, anatomical mapping and gene expression techniques.

First, a diamond knife was used to create fine slices throughout the length of the heart, where microscopic images and tissue samples were taken at each cut. The images were used to create the base of the 3D reconstruction.

In parallel, laser capture microdissection was used to remove individual neurons from the samples, while recording their precise placement within the heart’s anatomy. Researchers then used single-cell transcriptomics to determine the gene expression profiles of each of these collected neurons.

Once all the data were collected, they were fit onto the 3D model.

“With the spatial mapping of the gene expression, we can begin to discuss the precise roles that these neurons play,” says co-author Zixi Jack Cheng from the University of Central Florida.

“Do separate clusters of the ICN neurons have different functions, or do they work into an integrated way to influence heart health? Now we can address these questions in a way that wasn’t possible before.”

The map revealed the complexity of the ICN, the researchers say. Its neurons are found in a coherent band of clusters on the base (top) of the heart, where veins and arteries enter and leave, but also extend down the length of the left atrium on the back of the heart. They’re positioned close to certain key heart structures like the sinoatrial node.

“Seeing the clustering of neurons around it was something we had always suspected but had never known for sure,” says co-author Jonathan Gorky, now at Massachusetts General Hospital.

“It was really interesting to see the physical evidence of the ICN’s function and the precise distribution of the neurons in relationship to the anatomical structures of the heart.”

The researchers also report finding sex-specific differences in the way neurons were organised, both spatially and by their gene expression.