Nature publication: Biochemists clear up a decades-old misconception about a key metabolic pathway
IMAGE:
PROF. DR. MARTIN VAN DER LAAN
view moreCREDIT: THORSTEN MOHR/SAARLAND UNIVERSITY
Imagine that the brightest minds in a particular discipline all agree that the object of their research looks like a triangle. What then happens when someone turns up and says: 'No, it's actually a square.'? 'They'll say he's cuckoo!' said Martin van der Laan, whose unequivocal response directly reflects the experience that his colleague Eunyong Park from the University of California in Berkeley made about two years ago. 'He published a manuscript in which he depicted the structure of the TIM23-TIM17 complex that was quite different from what nearly every expert in the field had assumed. No biochemical experiment of the previous 25 years seemed to fit with this new structure,' explained van der Laan, professor of medical biochemistry at Saarland University whose main area of research is mitochondria – the 'powerhouses' that drive cellular metabolism. The experts were unanimous: Their research colleague in California must be mistaken.
But it turns out he wasn't. Up until recently, scientific doctrine held that the protein complex TIM23 (TIM stands for 'translocase of the inner membrane') forms a tunnel-like structure through which large protein molecules can be transported into the mitochondria from other parts of the cell – something that van der Laan demonstrated by placing his two cupped hands together with their fingertips touching. This hollow channel through the mitochondrial envelope acts like a keyhole that will only accept a molecule that has the right key. When such a molecule approaches, it gets pulled into the channel and transported, together with energy-supplying auxiliary proteins, into the interior of the mitochondrion. That, at least, was the established scientific paradigm for decades.
To help understand why this model is not correct, Martin van der Laan now placed his cupped hands back to back with his fingers now pointing outwards. It was this new structure, which Eunyong Park had proposed on the basis of high-resolution cryo-electron microscopy data, that seemed so utterly surprising. The structure of the transport complex looked completely different from what had been assumed for decades. In this new structure, one of the hands represents TIM23, while the other is its 'fraternal twin' TIM17. But up until then, the TIM17 protein had not really played any significant part in describing the protein transport mechanism in mitochondria. It was thought that TIM17 had more of a supporting or regulatory role in the transport of proteins through the mitochondrial membrane, and that the real star of the team was TIM23.
It turns out that's not the case. Martin van der Laan and his colleague Nils Wiedemann from Freiburg have been collaborating closely for almost two decades. Their research teams recently took another close look at the TIM17-TIM23 complex and have now mapped the functional organization of the complex in great detail using advanced and highly precise biochemical methods. A key feature of this work was the re-evaluation of old data that previously had made little sense, but that now fitted perfectly with this revolutionary new picture of the TIM complex.
The results of the research work carried out in Homburg and Freiburg have completely upturned the previous long-held assumptions about how proteins get inside mitochondria, reinforcing the conclusions drawn from the structural investigations by the research group at UC Berkeley. Martin van der Laan summarized these new findings: 'Taking the new structural and biochemical data together, we can now see that proteins migrate into the mitochondria along a pan-shaped membrane opening formed by TIM17 and not via a TIM23 channel structure.' The supporting actor has suddenly become the star of the show.
According to van der Laan, it was an ingenious experimental trick that has been crucial to this biochemical paradigm shift. 'We made an artificial mitochondrial protein that gets stuck in the TIM17/23 transport pore like a cork getting stuck in the neck of a wine bottle. We then modified the trapped protein complex so that free radicals – highly reactive chemical groups – were released on our artificially engineered protein. The reaction of these free radicals with their molecular surroundings is something that we can observe with extremely high spatial resolution. What we found was that the free radicals were only active in the TIM17 half-channel,' explained Professor van der Laan. This could only mean that the mitochondrial proteins migrate across the envelope membrane in close contact with the TIM17 structure, and not via a TIM23 channel, which is the mechanism presented in practically all the standard textbooks on biochemistry and cell biology.
That alone is a pretty revolutionary discovery. But why is this finding of such importance – and not just to a handful of specialist researchers around the world? As Martin van der Laan puts it: 'Mitochondrial dysfunction can result in severe degenerative and metabolic diseases and is known to be involved in the development of Parkinson's disease, diabetes and certain types of cancer.' Improving our understanding of how proteins actually enter the mitochondria – the 'powerhouses' that maintain cellular energy supply – could facilitate the development of highly effective drugs that are better able to treat those suffering from such serious diseases.
This article is the second paper that Martin van der Laan and his research group have published in Nature, one of the world's top scientific journals (link to first paper)
JOURNAL
Nature
METHOD OF RESEARCH
Experimental study
SUBJECT OF RESEARCH
Cells
ARTICLE TITLE
Central role of Tim17 in mitochondrial presequence protein translocation
Nature article: Scientists at Saarland University decipher a central mechanism of energy production in the human body
IMAGE:
DR. ALEXANDER VON DER MALSBURG
view moreCREDIT: THORSTEN MOHR/SAARLAND UNIVERSITY
Some two and a half thousand years ago, the Chinese general Sun Tzu wrote in the Art of War: 'Know your enemy and know yourself, then you need not fear the outcome of a hundred battles.' And what applied to the battlefields of ancient China also seems to apply in biomedical research. In the case of Martin van der Laan, professor of medical biochemistry at Saarland University, and Alexander von der Malsburg, a research associate at the same institute, the enemies are hereditary defects in the protein OPA1. OPA1 plays a crucial role optimizing energy conversion in the mitochondria, which are often referred to as the 'powerhouses' that drive our cells. Given the importance of OPA1, defects in the protein can have very serious consequences. For instance, if OPA1, which stands for 'Optic Atrophy 1', does not function properly, serious degenerative diseases can result. In many of these cases, it is the optic nerve that is primarily affected and patients with OPA1-related mitochondrial dysfunction often lose their sight.
But until recently studying these defective OPA1 proteins proved extremely challenging, in part because knowledge about the functionality of even healthy OPA1 was still fragmentary. Individual proteins are by their very nature far smaller than the tiny cellular compartments in which they are active and are thus not easy to observe. Recently, however, researchers at the University of California were able to produce the first images of OPA1 using high-resolution cryo-electron microscopy. They showed these images to Professor van der Laan and his team in Homburg, as the group enjoys an excellent global reputation in the field of mitochondrial research. Careful analysis of the new image data provided the first indications of how OPA1 might function. The crucial breakthrough came from Alexander von der Malsburg, who succeeded in establishing the world's first cellular system for studying the function of human OPA1. Martin van der Laan praised his colleague's 'very smart and elegant solution to the problem' and the insights gained are now being published in the prestigious international scientific journal Nature.
Until now, OPA1 has been seen as a 'difficult' protein for scientific study as it occurs in different forms and behaves in a very dynamic way. A few aspects of OPA1 functionality had been elucidated previously by studying specially prepared mouse cells created from embryonic stem cells in a complex procedure. But there were still aspects that seemed inconsistent or contradictory and much that was completely unknown. By cleverly combining and improving a number of genetic and biochemical methods, Alexander von der Malsburg has managed to tame the human OPA1 protein and make it more readily accessible for precise scientific examination. According to van der Laan, von der Malsburg has mastered a 'technically extremely challenging' task with flying colours.
The OPA1 protein strongly influences the efficiency of energy production in cellular mitochondria and thus plays a particularly important role in determining cell performance. OPA1 ensures that healthy mitochondria can fuse with each other and thus concentrate their forces, while defective mitochondria are discarded. Mitochondrial fusion is initiated when the OPA1 protein attaches to the inner membrane of the mitochondria, opening the membrane in a controlled and localized manner. If neighbouring mitochondria are modified in this way, they can fuse with each other and thus optimize mitochondrial functionality in the cell. However, if mitochondrial fusion is inhibited due, for example, to a genetic deficiency that results in the production of defective OPA1, this can seriously impact mitochondrial energy metabolism and, with advancing age, brings with it the risk of severe degenerative diseases. 'There are dozens of different variants of defective OPA1,' explained Martin van der Laan. Precise knowledge of how the OPA1 protein acts and the ability to conduct experimental simulations of malfunctioning OPA1 could potentially help many patients in future.
So just how does OPA1 function? 'Working with our American partners, we found that OPA1 initially attaches to the inner membrane like a foot with a "claw-like" structure and then lifts the "heel" of the foot,' said Alexander von der Malsburg. This mechanism pulls a chunk out of the membrane envelope in a manner not unlike the way a lever corkscrew lifts the cork out of the neck of a bottle of wine. The proof that this mechanism is crucial for OPA1 functionality was ultimately achieved by manipulating the gene that contains the blueprint for producing the OPA1 protein. The research team were able to smuggle modified genetic blueprints into healthy human cells so that they began to create defective variants instead of healthy OPA1. 'After a while, we began to observe that the cells' energy supply mechanism was impaired and that mitochondrial fusion was malfunctioning,' explained Alexander von der Malsburg. He described what they found under the microscope: 'It was apparent that the "claw-like" structure was completely missing in the genetically manipulated version.' The manipulated OPA1 protein was now no longer able to open the membrane, essentially preventing mitochondrial fusion – the protein had effectively switched cellular roles: from crucial ally to dangerous adversary.
'This is a fundamental mechanism that affects numerous variants of defective OPA1,' said Professor Martin van der Laan. 'And we now have the means to study all these variants individually.' These new research findings could help pave the way to customized therapeutic solutions for patients who become ill due to loss of function of OPA1. It is already possible to carry out genetic examinations of patients to determine which of the many known OPA1 defects they have. Van der Laan summarized the scope of the work as follows: 'With this new, much improved understanding of the OPA1 protein, we're hopeful that in future patients will be able to receive treatments that are targeted at the specific underlying protein defect.'
By uncovering the mechanism by which defects in the OPA1 protein lead to mitochondrial dysfunction, Alexander von der Malsburg and Martin van der Laan have complied with the first part of Sun Tzu's famous dictum 'Know your enemy'. And after collaborating so successfully, it's probably fair to say that they also know themselves just as well. Having essentially fulfilled both parts of Sun Tzu's maxim, the researchers look well set for the next hundred research ‘battles’ with dysfunctional OPA1 variants.
JOURNAL
Nature
METHOD OF RESEARCH
Experimental study
SUBJECT OF RESEARCH
Cells
ARTICLE TITLE
Structural mechanism of mitochondrial membrane remodelling by human OPA
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