Scientists Create 3D Map of Rat Heart’s Neurons

An international team of researchers has created a comprehensive anatomical 3D map and molecular profile of the rat intracardiac nervous system (ICN).

“Many cardiologists aren’t even aware there are neurons in the heart, let alone that they are critical to heart health,” said senior author Dr. James Schwaber, director of the Daniel Baugh Institute for Functional Genomics and Computational Biology at Thomas Jefferson University.

“The ICN represents a big void in our understanding that falls between neurology and cardiology. Our goal was to bridge that gap by providing an anatomical framework of the ICN.”

“By using this 3D reference space, we can build a comprehensive picture of the heart’s structure which is foundational to address various health concerns.”

The 3D model of the ICN is made possible by interweaving imaging, collection, anatomical mapping, and gene expression techniques.

First, a diamond knife is used to create fine slices throughout the length of the heart, where microscopic images and tissues samples are taken at each cut using 3Scan software. These images are used to create the base of the 3D reconstruction.

In parallel, laser capture microdissection is used to remove individual neurons from the collected samples, while recording their precise placement within the heart’s anatomy.

The researchers then used single-cell transcriptomics to determine the gene expression profiles of each of these collected neurons.

Once all the data are collected, they are fit onto the 3D model to create a comprehensive picture of the heart’s neural network.

“The only other organ for which such a detailed high-resolution 3D map exists is the brain,” said senior author Professor Raj Vadigepalli, also from the Daniel Baugh Institute for Functional Genomics and Computational Biology at Thomas Jefferson University.

“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.”

The new 3D map revealed hitherto unknown complexity of the ICN.

The scientists found that the neurons that make up the ICN are found in a coherent band of clusters on the base of the heart, where the heart’s 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.

“We know the sinoatrial atrial node is important in creating the heart rate or pace,” said co-author Dr. Jonathan Gorky, from the Daniel Baugh Institute for Functional Genomics and Computational Biology at Thomas Jefferson University.

“Seeing the clustering of neurons around it was something we had always suspected but had never known for sure. 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 gene expression analysis of individual neurons also pointed to previously unknown diversity of molecular identities or phenotypes.

“We found that there are several different types of neuromodulators and receptors present,” Dr. Vadigepalli said.

“This means that we don’t just have neurons in the heart that shut on and off activity, but also those that can fine-tune the activity of the ICN.”

“With the spatial mapping of the gene expression, we can begin to discuss the precise roles that these neurons play,” said senior author Dr. Zixi Jack Cheng, a cardiovascular anatomist and physiologist in the University of Central Florida College of Medicine.

“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 way that wasn’t possible before.”

When comparing male and female rat hearts, the authors found sex-specific differences in the way neurons were organized, both spatially and by their gene expression.

“It could help us explain some of the differences in heart disease in men and women,” said co-authors Alison Moss and Shaina Robbins from the Daniel Baugh Institute for Functional Genomics and Computational Biology at Thomas Jefferson University.

“We’re now trying to create a 3D model of the intrinsic nervous system of the pig heart, which is even more anatomically comparable to the human heart, to explore those questions further.”

“Now that we have a comprehensive map of the heart, the way we pursue bioelectronic medicine will significantly change as we have information available at a level of resolution that just wasn’t accessible before this,” Dr. Vadigepalli said.

The results were published in the journal iScience.