How many brains do you have? It may seem like an easy question – but ask a biologist, and you might be surprised by what they say. See, there’s actually a whole bunch of brains in our bodies: specifically, the “mini brains” of our organs – vast neuronal networks that send and receive information to that big wrinkly lump in our skulls.
Of course, there are a few differences between those “mini brains” and our actual brain. For one thing, our actual brain is chock-full of star-shaped multitasking cells called astrocytes. These cells play a crucial role in building and maintaining neural networks in the brain and spine, but aren’t thought to live anywhere else in the body – which is weird, right? If they’re so important, why wouldn’t they be found in other places too?
Well, according to a new study published recently in the journal PLOS BIOLOGY, they are. In a discovery that surprised even the researchers responsible, a new type of cell resembling astrocytes has been found in the heart. Dubbed “nexus glia” because of their net-like appearance around the heart, the team behind the discovery believe these new cells may hold the key to how we understand heart disease and defects in the future.
“For me the definition of great science is something that you discover that opens up even more questions,” said study co-author Cody Smith. “This, I think, is the definition of that.”
Astrocytes belong to a class of cells known as glial cells – the name comes the Greek for “glue,” the only function the 19th-century neurologists who discovered the cells were able to ascertain for them. These days, we know a bit more about glial cells, but not everything – we know they can be found throughout the body, including organs like the pancreas, spleen, lungs, and intestines, for instance, but we don’t know precisely what they do there.
Neither is it clear why astrocytes in particular, so crucial to neuron function in the brain, seem basically non-existent in the peripheral nervous system (PNS) – that is, the parts of the body connected by nerves outside of the brain and spinal cord.
“We were puzzled,” Smith explained, “so we searched for glial like cells in the PNS.”
“Really, [first author] Nina Kikel-Coury searched!” he added. “Nina came to my office every week with more data that supported a glial identity, many of which I was admittedly unconvinced by … Eventually the data became too much to dismiss.”
To begin with, the team looked at zebrafish – an animal that’s rapidly become the guinea pig of choice for scientists modeling diseases in recent years. They discovered a type of cell in the zebrafish heart that seemed to behave like astrocytes – and cross-species analysis revealed the same cells in human and mouse hearts. Formed before birth from the same group of cells that eventually go on to build our faces and smooth muscles, the cells spread out through the heart from the outflow tracts – and that provided an important clue as to the nexus glia’s function.
“’This is fascinating because the outflow tract is defective in 30% congenital heart diseases’ Nina explained … Scoring heart beats she noticed increased heart rates when nexus glia were disrupted,” wrote Smith.
What’s more, when the nexus glia were deprived of a key glial development gene, the heartbeat became irregular. “I thought that if we could find a new cellular piece to the cardiovascular puzzle, it could be foundational for future work,” explained Kikel-Coury.