Joseph Jebelli is a Neuroscience PhD Candidate at University College London (UCL). His research involves studying the cellular and molecular mechanisms of neurodegenerative disorders, such as Alzheimer’s and Parkinson’s disease.
When the English physician William Harvey first described the motion of the heart over 300 years ago, he might have envisioned that by 2013 we would understand the organ in its entirety. But while significant medical advances have been made, the answer to one of the most rudimentary questions about the heart remains deeply enigmatic: can our hearts repair themselves?
The answer has been steeped in controversy for over a century, but recent findings are helping to resolve the issue, with promising implications for our health.
In 2009, a paper in the journal Science gave definitive evidence that the human heart is capable of self-renewal. Somewhat surprisingly, nuclear bomb tests during the Cold War had made this discovery possible; these detonations caused a sharp increase in the levels of radioactive carbon-14 in the atmosphere, which after being absorbed by plants in the form of carbon dioxide, entered the human food chain. By using a technique called radiocarbon dating – where the decay of unstable carbon atoms creates a kind of ‘molecular clock’– the Swedish research team were able to use the elevated levels of carbon-14 in human DNA to retrospectively calculate the age of heart cells. They found that 1% are renewed every year at the age of 25, and that this rate decreases to 0.45% by the age of 75. It doesn’t sound like a lot, but it equates to roughly a 50% total heart cell turnover during an average human lifespan.
This landmark study set the stage for an ongoing worldwide effort to exploit the heart’s innate – albeit small – regenerative capacity to tackle disease. Back in 2011, to promote this research, the British Heart Foundation released evocative television adverts on zebrafish, one of which depicted a digitally animated talking zebrafish who explains how they can already repair their own hearts. Initially, it was thought that these fascinating creatures used a specialised source of stem cells to achieve such a feat. As it turns out though, it is the cells they are already endowed with, their pre-existing heart cells, which divide and repopulate the organ in times of trouble. This raised another question: where do new cells in the human heart come from? Again, stem cells seemed intuitively to be the most reasonable candidates – they are self-renewing, and have the power to become any one of the major cell types in the heart.
Recently however, a group at Harvard Medical School published a paper in the journal Nature giving strong evidence against this idea. They showed that, like the zebrafish, we too generate new heart cells from pre-existing ones, and that this phenomenon increases during a heart attack. This discovery is potentially a game changer for the field at large, for it suggests that our hearts possess zebrafish-like regenerative mechanisms – only on a much more diminished scale. Many researchers now believe that if we can figure out why this is, we may eventually be able to safely boost the regenerative capacity of our hearts for therapeutic gain. Strategies like this are motivating scientists everywhere, and for good reason – heart disease plagues developed nations and kills more people than any other disease. In the UK alone, it accounts for more than 179,000 deaths each year, costing an estimated £19 billion to the economy.
So after centuries of research we may soon have the means to rebuild a heart on the brink of a medical emergency, or after a serious cardiac event. The heart is arguably the most precious organ we own, and no one knew that better than the man who started it all… as Harvey so eloquently put it all those years ago,
‘The heart is the beginning of life, the Sun of the Microcosm, as proportionally the Sun deserves to be call’d the heart of the world’.