In the mid-1980s, Hajnóczky was a medical student at Semmelweis University in Budapest, Hungary. He remembers colleagues, working in the lab of a well-respected mitochondria researcher, desperately seeking jobs in other areas of biology. But Hajnóczky still had questions about mitochondria, and after graduation and a move to the United States, he spearheaded an effort to visualize and track mitochondria inside living cells, rather than in isolation, as was the convention. Thanks to his work and that of others, the field experienced a resurgence that has yet to wane.
In his 23-year career, Hajnóczky, with the help of colleagues, has invented numerous microscopy techniques and fluorescent probes. Using these tools, he has described novel mechanisms by which mitochondria interact with each other and other organelles in the cell. And he’s not done yet. Today, Hajnóczky directs a brand-new center at Jefferson dedicated to identifying the causes of mitochondrial diseases.
“Here was this beautiful opportunity to visualize and record the activity of mitochondria in tissue. . . . It became my whole career.”
Here, this self-proclaimed “mitochondriac” recounts the lucky break that set him on his career path, how a discussion group led to a multimillion-dollar research center, and why Philadelphia is poised to be the mitochondria capital of the world.
HAJNÓCZKY HOMES IN
Driven by fear. “When I was 10 or so, I became really fearful of our short life span. It had an intense effect on me, and I decided that I needed to make a contribution to medicine and become a physician-scientist. From that point on, my road was straight to medical school.” Born and raised in Budapest, Hajnóczky entered Semmelweis University, Hungary’s oldest medical school, in 1981. In his third year there, he opted to do part-time research and landed in the lab of endocrinologist András Spät, an expert in calcium signaling and steroid hormone production. “I have to admit that, at first, I wasn’t accepted into the lab,” says Hajnóczky. “I only got in because somebody who had been accepted wasn’t performing as expected, and when that person left, Professor Spät called me about the opening.” After receiving his medical degree, Hajnóczky stayed in Spät’s lab for another four years, completing a PhD in physiology. “I got very strong training there, and met my mentor, Tamás Balla, and fellow students György Csordás and Péter Várnai, all of whom became lifelong scientific friends and collaborators. Also at that time, relatively few top scientists visited Hungary, but we were lucky to have some key figures, including Nobel Prize winners, visit our lab.”
Inside look. One of those visitors was Andrew Thomas, a leader in the field of calcium signaling and pioneer of live-cell imaging. Thomas invited Hajnóczky to join his lab at Thomas Jefferson University as a research associate in 1991. “For me, that was a huge breakthrough,” says Hajnóczky. “There, I learned microscopic imaging technology. At first I studied generic cytoplasmic calcium signaling. But during my studies in Hungary, I had become interested in mitochondria, and here was this beautiful opportunity to visualize and record the activity of mitochondria in tissue. I started the mitochondrial work in his lab, and it became my whole career.”
Cell chatter. “The fascinating thing is that these organelles, the mitochondria, come from some bacterial origin—they were free-living bacteria and then invaded cells and somehow made a deal to become the cells’ primary energy producer. They also delegated functions to the rest of the cell: only 13 out of 1,100 mitochondrial proteins are encoded in the mitochondria; the rest are encoded by the nucleus and produced in the cytoplasm. So there is an effective symbiosis between mitochondria and the rest of the cell, which involves a great deal of communication and signaling.” That communication is where Hajnóczky focused his research efforts.
Signals and spikes. Hajnóczky’s chosen area paid off right away, with back-to-back papers in Nature and Cell. First, he characterized the behavior of an intracellular membrane receptor that releases calcium into the cytoplasm upon binding inositol 1,4,5-trisphosphate (IP3). Changes in cytoplasmic calcium concentrations act as signals to initiate certain cellular functions, such as contractions in muscle cells and action potentials in neurons. After describing how IP3 both activates and inactivates the receptor, Hajnóczky looked specifically at calcium spikes in mitochondria. He showed for the first time that calcium oscillations directly affect the activity of the organelles. “Because we were able to measure calcium in the cytoplasm and inside the mitochondria, we could directly demonstrate that calcium spikes stimulate energy metabolism—they cause ATP production.”
Mitochondrial renaissance. Hajnóczky opened his own lab at Jefferson in 1996. “There were a series of discoveries in the 1990s that elevated the field,” he says. “In the early 1990s, mitochondrial involvement in apoptosis generated huge interest. Then there was the study of mitochondrial DNA and the recognition that mitochondria are highly dynamic: they’re mobile, they undergo fusion and fission, and they turn over through autophagy. Then, in 2008, Vamsi Mootha [of Massachusetts General Hospital and the Broad Institute] generated a complete inventory of mitochondrial proteins—the 1,100 proteins that constitute mitochondria. And it turns out that many of them have huge relevance to disease. Since then, mitochondrial disease research has truly taken off.”
On target. All of Hajnóczky’s major discoveries were preceded by an investment in technology. “A key point of our program is to continually develop visualization techniques and molecular tools.” These include the application of two-photon microscopy and multicolored imaging that allows his team to track three or four different functions in mitochondria at the same time. He and his colleagues have also developed numerous genetically targeted probes. “The beauty of this tool is that you can target anywhere in the cell. If you are specifically interested in the space between two mitochondrial membranes, you can target your probe there.”
Close contact. One puzzle to which Hajnóczky applied his probes was the unknown mechanism by which the endoplasmic reticulum (ER) and the mitochondria communicate using calcium as a signal. Some believed calcium traveled unrestricted through the cytoplasm between the two organelles. But in 2006, Hajnóczky and his team proved there is a direct physical coupling between mitochondria and the ER, a tether that allows local calcium flow. “That is useful in a cell because calcium can be dangerous,” says Hajnóczky. “If calcium were traveling through the cytoplasm to the mitochondria, it could trigger cell death. But because the transfer is happening through this tight interface, other parts of the cell are not exposed. These local associations allow the organelles to talk to each other in a very intimate way.” For that work, the team designed probes to directly image the interface between the two organelles. Today, more than 100 laboratories use those targeted probes, says Hajnóczky, and he continually fulfills requests for them, free of charge.
On track. Like other organelles, mitochondria use motor proteins to move around a cell on its cytoskeleton network, but little was known about the signals that incite that movement. In 2004, Hajnóczky and his team showed that mitochondrial movement is inhibited by calcium. In 2008, they discovered that a protein located in the outer mitochondrial membrane, a Miro GTPase, controls this process. “One month after we published in PNAS, two other groups published the same finding in Cell and Neuron. It is clearly a key regulation pathway to provide needed energy to different parts of a cell.”
Fast fusion. In 2009, Hajnóczky’s team used live-cell imaging to observe mitochondrial fusion—when two mitochondria meld together—in real time. They identified two types of fusion events: complete fusion and transient “kiss-and-run” moments. “Mitochondria have two [lipid-bilayer] membranes, and sometimes when they bump into each other, the four membranes fuse, mix contents, and rapidly re-separate. The whole process can be completed in two seconds.” This brief pit stop allows mitochondria to replenish fuels and/or missing or damaged molecules such as DNA and proteins. “It’s likely a mechanism for fixing small problems in an efficient way. It keeps the mitochondria in shape.”
“I hope Philadelphia can grow into a capital for mitochondria research.”
Full circle. Back in 1995, when Hajnóczky first demonstrated that calcium spikes stimulate energy metabolism in mitochondria, he did not have the tools to determine the molecular drivers of the process. After producing the complete list of mitochondrial proteins in 2008, Mootha discovered that one of those proteins, MICU1, is required for calcium uptake. Hajnóczky realized MICU1 was the first molecular piece of the puzzle, and he and Mootha began collaborating to study the protein in greater detail. Last year, they showed that MICU1 senses calcium inputs and allows mitochondria to both ignore low, nonrelevant calcium increases and effectively decode critical calcium spikes. “It’s come full circle. We described a phenomenon in 1995, and 18 years later we have the molecular definition to explain why mitochondria exclude low calcium levels and respond to high calcium levels. We’re lucky. If [Mootha] hadn’t found the protein, we would still be in the dark.”
Deadly effective. Today, Hajnóczky has shifted his sights from calcium to reactive oxygen species (ROS). ROS, natural by-products of mitochondrial metabolism, are useful signaling tools, but, like calcium, they can cause cellular damage. “Many diseases have been linked to dysregulation of reactive oxygen species. We’re trying to understand this dual function—how they can be an effective signal and dangerous.” Like calcium, Hajnóczky suspects ROS are controlled locally and sequestered to certain parts of the cell. “With an understanding of the basic mechanisms, we get closer to human diseases associated with ROS or calcium. The ultimate goal is to improve these diseases by actually manipulating the mitochondria.”
HAJNÓCZKY AT THE HELM
Community building. “A couple years ago, we recognized that there are groups scattered across Jefferson and other local universities with expertise in different aspects of mitochondria. So we started the ‘MitoCircle’ discussion club. We meet each month to discuss papers and listen to presentations. We now have close to 100 members, with people coming even from outside Philadelphia. Then, two years ago, we decided to host a conference on mitochondria and metabolism. To our surprise, 160 people signed up. All this showed there is significant interest in the field.” As a result, in January the university opened the MitoCare Center, a mitochondria research hub. The center currently comprises an inaugural faculty of five, including Hajnóczky, and another 10 collaborators from other Jefferson departments and research centers, he says, and is still recruiting. “We’re a small center at this point, but I think we have big potential. I hope Philadelphia can grow into a capital for mitochondria research.”
Dream big. The day after the MitoCare center opening was announced on local television, Hajnóczky received phone calls from three patients and the United Mitochondrial Disease Foundation, all seeking information about mitochondrial diseases. “I think there is a tremendous unmet need for patients diagnosed with these diseases. If we have the capacity, we’d like to be a group for clinical activities, and though it’s still just a dream, we want to provide a service called ‘iMitochondria,’ an internet site where patients can download information, send questions, and get help.”
Home away from home. “When I came to the U.S. in 1991, I originally wanted to stay here for just two years and then return to Hungary. But I learned to appreciate opportunity in this country; if you really want to do things, you can. Now I’ve spent 23 years here, and all in the same building. I’ve had four different labs on the same floor. Staying at one school for a long time may be unusual here, but it is normal in Europe.”
Like father... “I have three wonderful kids. None of them decided to do exactly the same thing I do, but in a way they each take it further. One is currently doing turtle research in Australia, so in some ways she takes research further than me. The second is a fantastic photographer, so she takes imaging further than me. And my son is very involved in high-end tech, so he takes technology further than me. In each of them, I see a bit of myself.”
On the run. “I’ve run a couple marathons and half-marathons. I usually go out jogging early in the morning. It’s a nice complement to work; I get many of my best ideas when I’m exercising.”
- Visualized mitochondrial energy production and ion metabolism in single live cells.
- Characterized the behavior of IP3 receptors in cells and described how they produce calcium spikes.
- Demonstrated that calcium spikes stimulate ATP production in mitochondria, and identified the molecular mechanism by which this occurs.
- Discovered a physical tether between mitochondria and the endoplasmic reticulum and described its role in signaling.
- Determined how mitochondrial movements are controlled inside a cell.
- Used real-time, live-cell imaging to observe transient mitochondrial fusion events—aka “kiss and run.”