“In Japan, around 1.2 million people suffer from heart failure. However, there are currently no effective treatments for severe heart failure outside of a heart transplant. And since the number of donors is limited, heart transplants can only save around 50 patients a year in Japan,” Prof. Fukuda explains.
Why is transplantation currently the only treatment for heart failure? The answer has to do with the nature of the myocardium. “The heart muscle, that is to say, the cells of the myocardium, divide in utero but stop after birth. As a result, if some myocardial cells become necrotic due to diseases such as myocardial infarction or myocarditis, they will not regenerate thereafter. A heart that has lost muscle mass has less strength to contract, and its function as a pump is reduced. This condition is known as ‘heart failure.’”
Prof. Fukuda shares the story of an unforgettable patient. “Shortly after I became a doctor, a young patient about my age was admitted to the Keio University Hospital with dilated cardiomyopathy. The best treatments available to us at the time were not effective, and heart transplantation wasn’t an option. It was a harrowing experience, and since then, I have committed myself to developing an alternative treatment for heart failure.”
Convinced that research at the genetic level was necessary to achieve his goal, Prof. Fukuda began a journey that would end up spanning more than a quarter-century.
After working as a young clinician at the Keio University Hospital’s Department of Cardiovascular Medicine, Prof. Fukuda went on to study at the National Cancer Center Research Institute. “In the early ‘90s, oncology was the only area of medicine where genetic-level research was advancing. Cardiovascular medicine at the time hardly gave a second thought to genetics. So I called on Dr. Ken Yamaguchi, a fellow Keio graduate many years my senior who was working at the National Cancer Center Research Institute to ask him to teach me basic techniques in DNA and RNA analysis. I am still grateful to him for accepting a non-specialist like myself and taking me under his wing.”
After acquiring the basics of genetics research, Prof. Fukuda moved to the United States, where he worked on the cell cycle of cardiomyocytes at Harvard University and the University of Michigan. After returning to Japan in 1995, he further immersed himself in research to produce and transplant cardiomyocytes. It would be just four years later, in 1999, when a research paper would launch him into the world’s limelight.
“It was the world’s first successful creation of cardiomyocytes from bone marrow cells (mesenchymal stem cells). At the time, no one thought it was possible to use bone marrow cells to create wiggling, pulsating cardiomyocytes. After my paper was published, it attracted a great deal of attention, and I received offers to give lectures around the world. I was surprised and happy that my first study at Keio proved to be a success.”
Ultimately, it proved challenging to use bone marrow to produce the large numbers of cardiomyocytes needed for clinical application. Still, there is no doubt that this discovery helped drive regenerative medicine worldwide.
While cardiomyocytes can be used to treat heart failure, they are needed in the millions. Around the year 2000, Prof. Fukuda and his colleagues began researching how to produce large numbers of cardiomyocytes from embryonic stem cells (ES cells) and even went so far as to produce cardiomyocytes from human ES cells.
Then, in 2006, Prof. Shinya Yamanaka of Kyoto University announced the creation of the world’s first induced pluripotent stem cells (iPS cells). “While reading through papers in my lab, I came across Dr. Yamanaka’s paper and was thoroughly impressed. I called him straight away and invited him to give a lecture at Keio.”
Prof. Fukuda immediately began research on regenerative medicine using iPS cells. “With iPS cells, we were able to overcome some of the problems we had with ES cells, but there were still many challenges remaining before we could translate our findings into actual regenerative medicine.”
Prof. Fukuda needed to produce safe iPS cells over short periods, and he found a way to generate them without damaging any chromosomes using a gene therapy approach in which the vector—called Sendai virus—that carries the genes does not integrate into the genome, reducing the possibility of iPS cells becoming cancerous. Prof. Fukuda also developed a method to produce iPS cells—previously produced by collected skin tissue—from as little as 0.1 cc of blood, resulting in the more efficient production of a safer iPS cell.
His next challenge was to selectively produce cardiomyocytes from iPS cells. Prof. Fukuda and his colleagues analyzed multiple cell growth factors and discovered three key factors essential for inducing differentiation. He and his team have now succeeded in efficiently producing cardiomyocytes, in particular the ventricular muscle cells necessary for the treatment of heart failure.
“It took ten years, but we were finally able to produce a large enough number of cardiomyocytes using human iPS cells to form the foundation we needed to further explore regenerative medicine.” But as it turned out, an even bigger obstacle lay ahead.
The next hurdle was also the most challenging: the “purification” of cardiomyocytes made from iPS cells. “When iPS cells are used to make cardiac muscle, about 90% become cardiomyocytes, but the remainder inevitably become something else or remain as undifferentiated cells. If transplanted as-is, there was a risk of carcinogenesis, which was a hurdle we had to clear at all costs.”
Prof. Fukuda focused on the difference in cellular metabolism, working on the analysis in collaboration with professors at Keio who specialize in research on cellular energy metabolism. They found that undifferentiated iPS cells require glucose and glutamine to survive and die as soon as they are removed while cardiomyocytes, which also require glucose and glutamine, can survive by taking in lactic acid.
“This discovery allowed us to develop a cell culture where only cardiomyocytes survive. By altering the cell culture, undifferentiated iPS cells are starved out by depleting glucose and glutamine, while cardiomyocytes can procure food (i.e., lactic acid) from a separate source, which is how they survive.”
Purification via this cell culture was the key that enabled Prof. Fukuda to extract only the purest cardiomyocytes. “The idea of altering the cell culture may seem simple at first, but it took us a long time to get there, and I would not have been able to do it without my fellow collaborators. The establishment of this purification technology was a significant step forward in creating safe cardiomyocytes and moving us closer toward real-world applications.”
Having overcome hurdle after hurdle, Prof. Fukuda finally succeeded in transplanting cells into the heart of a mouse.
“When we first tried transplanting myocardial cells with a needle, it initially appeared to work. For a moment, I was overjoyed. I thought to myself, ‘Yes! Now we can finally do a cardiomyocyte transplant!’ However, upon closer inspection, we found that only about 3% of the transplanted myocardium had grown. ‘Well,’ I said to myself, ‘It looks like finding the right transplantation method is also going to be a challenge.’”
After testing various transplant conditions, Prof. Fukuda and his team arrived at the method of making “myocardial spheres,” masses of about 1,000 cardiomyocytes each.
“We found that when we transplanted them separately, their surface would be damaged by a proteolytic enzyme called trypsin, and the cardiomyocytes would die. So we found a company that helped us develop a special Petri dish with numerous holes at the bottom, each one 0.5 millimeters in diameter. When cardiomyocytes are placed in the culture dish, the cardiomyocytes form a ball-shaped mass called a myocardial sphere. When we transplanted these myocardial spheres, we found the implantation rate increased dozens of times over compared to conventional transplantation.”
Following the development of this Petri dish, his work on improving the needle used for implantation continued.
“We found that regular needles, with their scalpel-like tips, bleed easily, allowing the cardiomyocytes to spill out. So we custom-ordered needles with blunt tips, such as those used in acupuncture. We drilled six holes on the side of these 0.51 mm-thin needles and devised a mechanism through which the cardiomyocytes could be implanted.”
As a result of these enhancements, experiments using even large animals such as pigs and monkeys demonstrated that cardiac function is greatly improved after cardiomyocyte transplantation and allows doctors to address concerns about arrhythmia clinically.
“Through my studies in cell embryology and metabolism and the development of Petri dishes and needles for implantation, I’ve come to understand that once you clear one hurdle, there is always another one ahead. Rinse and repeat. That is why I am always preparing myself to move from one hurdle to the next and imagine what kinds of obstacles lie a few years down the road. Yes, it is hard work, but it is a dream come true to be able to work with doctors and companies in overcoming these challenges as we strive to develop new treatments.”
26 years since Prof. Fukuda started working on cardiac regenerative medicine research, his clinical research is finally at hand. “In the next few years, we plan to begin simultaneous clinical trials around the world. Today, there are approximately 65 million heart failure patients worldwide, and I believe that transplantation of cardiomyocytes can improve the condition for around half of them, more than 30 million people.”
And Prof. Fukuda is already looking beyond clinical research.
“We are currently using special iPS cells created by Dr. Yamanaka, and I hope to eventually develop personalized iPS cells so that we can treat most people without needing to use immunosuppressive drugs. We are also working on a catheter for transplantation so that we can forgo surgery. Our work is never over, and research is truly a ‘never-ending story.’”
When asked if he has ever felt like giving up at any point during his quarter-century of research, his answer surprises us: “Every day. I draw inspiration from the Keio School of Medicine’s first dean, Dr. Shibasaburo Kitasato, whose motto was ‘Remain true to your ideals.’ I really like this way of thinking. Instead of continually trying new things to keep up with the current trends, we do consistent research, digging deep into the matter at hand. We believe that this will eventually lead us to develop novel treatment methods, which inspires us to work hard every day.”
Prof. Fukuda’s desk in his lab is adorned with a nameplate that reads ‘BIG BOSS.’ “You probably think it’s funny that I call myself the ‘BIG BOSS,’ huh? This is a reminder to myself. As a researcher and clinician, no matter how good a research paper I write, it will be forgotten after a few years, and providing good clinical care is simply a matter of course. Those things aside, to me, a ‘BIG BOSS’ is someone who has successfully trained a lot of people throughout their career. And so I have that nameplate on my desk to remind myself of the kind of person I want to be.”
As of 2021, 19 of Prof. Fukuda’s students have already gone on to become professors and are contributing to research at universities around the country.
And he wants the next generation of researchers to look beyond their research. “Writing a paper is not important in and of itself. It is the results that matter. Research should lead to the development of new drugs and treatments that save lives. I don't want researchers to forget the underlying purpose of their work, and I hope they will stay motivated in their research. Keio University, in particular, is an environment that values practical learning, so promoting translational research is one of our missions as researchers.
“One last comment—this is something that skiers will understand, but while it’s easy to ski on well-trodden slopes, the tracks you make end up getting lost in those of others. Conversely, tackling a slope blanketed with fresh powder is not easy. There may be bushes or bumpy terrain, and you may fall down. But the tracks you leave behind will be clearly visible to those who come after you. In the same way, I hope that you live your lives so that the paths you pave will endure.”
Professor Fukuda received his Doctor of Medicine (M.D.) from the Keio University School of Medicine in 1983 and his Ph.D. in Clinical Cardiology from the Keio University Graduate School of Medicine in 1987. After working as a resident at Keio University School of Medicine, he went to the U.S. in 1991 to study at the National Cancer Center Research Institute, later studying at Harvard University in 1992 and the University of Michigan in 1994. After returning to Japan and working as an assistant and lecturer at Keio University School of Medicine from 1995 to 2004, he became a professor in the Department of Regenerative Medicine at Keio University in 2005 before assuming his current position in 2010. In 2015, Prof. Fukuda established Heartseed, working to transform treatments for severe heart failure by delivering cardiac regenerative medicine. In 2021, he received numerous awards, including the “Ministry of Education, Culture, Sports, Science and Technology (MEXT) Award” at Academic Startups 2021 and the “Minister of State for Science and Technology Policy Award” at the Japan Venture Awards 2021.