Dr. Gabriele D’Uva is finishing up his postdoctoral research at the Weizmann Institute. Here is his account of three years of highly successful research on regenerating heart cells after injury. Among other things, it is the story of the way that different ideas from vastly different research areas can, over the dinner table or in casual conversation, provide the inspiration for outstanding research:
Three years ago, when I joined the lab of Prof. Eldad Tzahor, the emerging field of cardiac regeneration was totally obscure to me. My scientific track at that time was mainly focused on normal and cancer stem cells: cells that build our bodies during development and adulthood. The deregulation of these cells can lead to cancer. I have to admit that I didn’t know even the shape of a cardiac cell when my postdoc journey started…
Eldad’s lab was also switching fields — well, not drastically, like me, but still it was a transition from a basic research on the development of the heart to the challenge of heart regeneration during adult life.
In contrast to most tissues in our body, which renew themselves throughout life using our pools of stem cells, the renewal of heart cells in adulthood is extremely low; it almost doesn’t exist. Just to give an approximate picture of renewal and regeneration processes: Every day we produce billions of new blood cells that completely replace the old ones in a few months. In contrast, heart cells renewal is so low that, many cardiac cells remain with us for our entire life, from birth to death! Consequently, heart injuries cannot be truly repaired, leading to (often lethal) cardiovascular diseases. This might appear somewhat nonsensical, since the heart is our most vital organ: No (heart) “beat” no life.
Hence a challenge for many scientists is to understand how to induce heart regeneration Scientists have been trying different strategies, for example, the injection of stem cells. We decided to adopt a different strategy – one that mimics the natural regenerative process of healing the heart in such “regenerative” organisms as amphibians and fish, and even newly-born mice. In all these cases the regeneration of the heart involves the proliferation of heart muscle cells called cardiomyocytes. Therefore the challenge before us was: “How can we push cardiomyocytes to divide?”
We adopted a team strategy. Cancer turned out to be a somewhat useful model for a “strategy.” After all, the hallmark of this disease is continuous self-renewal and cell proliferation. Starting from this thought, Prof. Yossi Yarden, a leading expert in the cancer field, suggested: “Why don’t you try an oncogene, such as ERBB2, whose deregulation can lead to uncontrolled cellular growth and tumour development?” The idea was that cardiomyocytes could be pushed into a proliferative state by this cancer-promoting agent. To Eldad, this was a nice “life” circle closing, since Eldad, when he was a PhD student in Yossi’s lab, focused exactly on the ERBB2 mechanism of action in cancer progression. I must admit, the idea sounded very intriguing and I really liked it.
Eldad, as a developmental biologist, had a different approach. Based on his field of expertise, his tactic was to apply proliferative (and regenerative) strategies learned from the embryos, when heart cells normally proliferate to form a functional organ. It turned out that a key player in driving embryonic heart growth is again… ERBB2!
So, Yossi and Eldad, from different fields of expertise, had the same idea: Look to ERBB2, which is a receptor on the cell surface that amplifies and transmits growth factor signals. It looked, back then, like a challenging idea; I was very happy to take the dare.
So this is exactly how my three and half years of post doc research started. At that stage, ERBB2 looked like a perfect candidate for cardiac regeneration. The idea to bring together cancer and developmental knowledge doubled the percentage of our success. The odds were on my side!
A first rule to starting a project regarding the role of any protein is to check for its expression. Therefore I started to analyse the kinetic of expression of ERBB2 in a normal heart during post-natal development. Interestingly, I noticed a dramatic reduction in ERBB2 levels in the heart during the first week of post-natal life. I have to mention that mouse cardiomyocytes stop dividing soon after birth, in about a week. It’s probably a residual proliferative ability of their embryonic life. My initial results revealed a strong reduction in ERBB2 expression, exactly coincident with the period in which heart cells lose their proliferative and regenerative capabilities.
I was very intrigued by this result, which immediately opened a very important question: “Is the loss of the regenerative ability of the heart in mice due to the decline of ERBB2 expression after birth?” After hundreds of experiments I can confidently answer: Yes. ERBB2 levels are reduced in cardiomyocytes shortly after birth, and this down-regulation limits the proliferative and regenerative ability of cardiac muscle cells.
To prove that, we first generated mice in which we deleted the Erbb2 gene specifically in heart cells. Loss of the ERBB2 gene (and protein) led to reduced cardiomyocyte proliferation and consequently to a very thin and poorly contracting heart. In the absence of ERBB2, the heart at birth was so weak that it could not tolerate the blood pressure and became dilated, a cardiac disease in humans known as dilated cardiomyopathy. The conclusion was that EBB22 is required for proper proliferation and growth of heart during embryonic development and its expression is physiologically reduced soon after birth to allow maturation of the cardiomyocytes.
Because of its major role in cancer, the only way to study ERBB2 involvement during heart regeneration was to search for a sophisticated system to finely, and transiently, increase its levels in the heart, within defined time windows. For this, we generated mice in which we could switch ERBB2 ON or OFF in cardiomyocytes. The results were amazing. Persistent ERBB2 induction led to a giant heart, two to three times bigger than normal in just a week or two. The analysis of the mechanism demonstrated that ERBB2 gets muscle cell to “rejuvenate” to an earlier stage (a phenomenon called “dedifferentiation”) and to reacquire the ability to proliferate — similar to what happens during embryonic development. In addition, ERBB2 increases the size of the cardiomyocytes (a phenomenon called “hypertrophy”).
Thus far, the project had been proceeding in the right direction. However we soon realized that a bigger team could improve the project’s success. A very talented master’s student, Alla Aharonov, joined me in this effort. Alla’s help was precious in many ways. In particular, she contributed to our resolving the specific molecular pathways that are mediated by ERBB2 activation. Precious help in the analyses of cardiac functions were obtained from the lab of Profs. Jonathan Leor and Michal Neeman. Very important were also the “dinner discussions” with my wife, Mattia Lauriola (who was conducting a parallel postdoc in Yossi Yarden’s lab), in addition to Yossi’s scientific support and help from the beginning of the project. At certain point Eldad also involved Prof. Richard Harvey, a good friend of his and a leading scientist in heart development, whose suggestions turned out to be very effective. The project and the team were blooming.
The main findings, which we are happy to report, are that transient activation of ERBB2 (ranging from 10 days to 3 weeks) can trigger cardiomyocyte dedifferentiation and proliferation. These two processes in turn are critical to achieving cardiac regeneration after the injury that we had induced in mice to mimic human heart attacks. (termed myocardial infarction). Therefore, the activation of ERBB2 is one strategy to promote heart regeneration. It’s important to mention that one of the therapies currently being tested in clinical trials is a growth factor stimulus called Neuregulin1 (NRG1), which activates ERBB2 signalling. However, since we uncovered the fact that that ERBB2 levels are very low in adult mouse cardiomyocytes, we suggest that the efficacy of NRG1 therapy might be limited in adulthood. Further experiments in cardiomyocytes derived from human patients could answer this question.
The good news is that, according to our results, heart patients will definitely improve if we can, in the future, find a way to fine-tune ERBB2 levels. We need to find a way to control the expression of this receptor, or its signalling partners, for a short time to repair the damaged heart. How? That’s the next challenge; but it which could help millions patients worldwide!
Our findings point to a central role of ERBB2 in cardiomyocyte cell division and regeneration.