Muscle myofibrils shape their mitochondria and vice versa

Muscles types can produce medium enduring forces and or high peak forces followed by quick fatigue. The energy fuelling their contractile motors is largely supplied by their mitochondria. By combining genetics in the fruit fly Drosophila with state-of-the-art imaging and deep-learning, researchers at the Developmental Biology Institute of Marseille have found that mitochondria coordinate their formation with myofibril development to match the correct muscle type. To do so, mitochondria and myofibrils undergo highly orchestrated physical interactions that instruct how each of them progresses through development. These findings show that mitochondria and myofibril morphogenesis are tied mechanically to define the correct muscle fate. These results are now published in Nature Communications.

Most animals have different muscle types dedicated for specific tasks. These can twitch slowly and involuntary, such as the ones that ensheath our gut or keep our heart beating, or be fast and quickly exhausted, like the ones we use to generate maximum force for escaping or lifting heavy objects. Mitochondria are the power houses of all muscles and generate most of their energy by converting nutrients from food into ATP, which fuels the muscle motor proteins. Mitochondria adopt different shapes and localisations in different cell types, in particular in different muscle types. How this is achieved is largely unknown. Now, a multidisciplinary project led by Nuno Luis and Frank Schnorrer at the Developmental Biology Institute of Marseille (IBDM) of the CNRS & Aix-Marseille University, has found that the force producing myofibrils coordinate their development closely with their mitochondria by generating a mechanical feedback mechanism that instructs their final morphology and function.

The flight muscle of the fruit fly Drosophila melanogaster is a “stretch-activated” fibrillar muscle type, similar to the mammalian heart. One group of muscles mechanically senses when stretched by the other group, and then contracts. Hence, wings can oscillate 200 times per second to enable flight. These oscillations require an extensive amount of ATP. Thus, the mitochondria are located in intimate contact to and squeezed in between the individual myofibrils, to quickly deliver ATP to the motor proteins. In contrast, the slower leg muscles of the fly used for walking display a different mitochondrial morphology by forming complex shapes above or below the myofibril layer.

By using state-of-the-art 3D imaging and deep-learning algorithms, Jerome Avellaneda, PhD student and first author of this study, discovered that the muscle-type specific mitochondrial morphology is already instructed during the early stages of flight muscle development. Initially, mitochondria are clustered together. However, as soon as the myofibrils assemble mitochondria intercalate in between them to insulate one from its neighbours. In the following steps, myofibrils grow in size and diameter and push their surrounding mitochondria into elongated ellipsoid shapes. The authors found that the mitochondrial fusion and fission machinery is crucial for this dynamic “mitochondrial-myofibril-dance” with fission being necessary for the mitochondria to intercalate between the myofibrils. Surprisingly, a failure in intercalation results in a myofibril organisation resembling the ones in the slow leg muscle. Even more surprisingly, these flight muscles express leg muscle specific contractile proteins suggesting that the mitochondrial morphology directly feeds back on muscle type choice. These results suggest that in force producing muscle tissue the morphogenesis of the different cellular components and organelles is highly coordinated. Interfering with one, may dramatically change the other. This may have important consequences for treatment of muscle and in particular heart disease in human.

The Hippo pathway controls muscle growth

Muscles power animals running, swimming or flying. The forces are produced by chains of actin and myosin motors called myofibrils. During animal development muscles form by fusion of small myoblasts to initially rather small myotubes. These myotubes then build their contractile myofibrils and grow dramatically in size to power future animal movements. How this enormous gain of muscle volume is controlled during development has now been revealed using the fruit fly Drosophila. An interdisciplinary team led by Frank Schnorrer and Bianca Habermann at the Developmental Biology Institute of CNRS & Aix Marseille University, together with Matthias Mann at the Max Planck Institute of Biochemistry in Munich and Bart Deplancke at École Polytechnique Fédérale in Lausanne discovered that a signalling pathway, called the Hippo pathway, is controlling muscle growth during development of Drosophila flight muscles. By combining genetics with proteomics and next generation mRNA sequencing the team found that the Hippo pathway is supervising the production of the contractile proteins actin, myosin and many others that build the contractile myofibrils. Hence, making more of these proteins results in more myofibrils, which directly triggers muscle growth and powerful muscle formation. Similar principles may apply during growth of human muscle, in particular human heart muscle that grows tremendously without the addition of new cells. These new results have just been published in eLife.

The Hippo pathway is a famous conserved gate keeper for tissue growth, discovered in the fruit fly almost 20 years ago. It controls growth also in mammals and is particularly important in cancer development when de-regulated. Most of what we know to date about how the Hippo pathway is regulated stems from studies in epithelial cells, like skin or gut cells. During normal tissue growth the Hippo kinase is inactive, allowing the epithelial cells to grow and divide to fill the available space, matching the growth of the organism. When the available space is filled, tissue mechanics feedbacks and triggers the activation of the Hippo kinase, which stops further growth.

This new study identified an interesting twist how the Hippo pathway is regulated in flight muscle to supervise its growth. In contrast to epithelia, this cell type does not divide anymore but grows by increasing the size of the individual cells, very similar to how human heart is growing during development or exercise. Aynur Kaya-Çopur, first author of the study, found that the Hippo kinase in muscle is regulated by a particular phosphatase complex, called the STRIPAK complex, located at muscle membranes, which are at close contact to the contractile myofibrils. Hence, the authors suggest that a need for growth is sensed mechanically: too small muscles are ‘overstretched’ and hence sense the need to grow. They do so not by cell division but rather by production of more myofibril proteins, which results in more as well as longer and thicker myofibrils. Thus, muscle volume increases. The authors found that Hippo needs to directly bind to the STRIPAK complex in muscle for its normal regulation. How precisely the ‘mechanical signal’ talks to the STRIPAK complex to block Hippo kinase activity remains to be discovered. In the future, it will be exciting to explore if a similar mechanism controls timing and amount of myofibril protein production in the human heart, which also grows tremendously during development or exercise.

To know more :

The Hippo pathway controls myofibril assembly and muscle fiber growth by regulating sarcomeric gene expressioneLife 2021;10:e63726 DOI: 10.7554/eLife.63726

Aynur Kaya-Çopur*, Fabio Marchiano, Marco Y Hein, Daniel Alpern, Julie Russeil, Nuno Miguel Luis, Matthias Mann, Bart Deplancke, Bianca H Habermann, Frank Schnorrer*
* corresponding authors

European Synergy Grant for Muscle Research

The European Research Council (ERC) awarded one of the rare ERC Synergy Grants to an international consortium of scientists, Frank Schnorrer (IBDM, Marseille), Stefan Raunser (Max Planck Institute of Molecular Physiology, Dortmund), Dirk Görlich (Max Planck Institute for Biophysical Chemistry, Göttingen) and Mathias Gautel (King’s College, London). Funds of €11 million during the next six years will enable the research teams to elucidate the molecular details of muscle formation and function in a joint collaboration. The partners in the consortium have already gained groundbreaking insights into the molecular processes of muscle development and function in recent years.

Muscles are composed of muscle fiber bundles with each fiber containing hundreds of long chains, the muscle fibrils. Their smallest repeating functional unit is the sarcomere. This power pack performs the actual muscle work: When the sarcomeres shorten, the muscle contracts. When the sarcomeres lengthen, the muscle extends. During muscle contraction, filamentous strands of the muscle proteins, actin and myosin, slide along each other.

Muscle contraction, however, is also based on the interaction of a plethora of additional proteins. This delicate architecture needs to be correctly set up during muscle development. If the components are misplaced or do not work together properly, severe muscle or heart diseases can develop.

New insights into the structure of muscles

“Using electron microscopy, we have already been able to visualize some muscle building blocks in atomic detail, such as the protein complex of actin and myosin as the central element of the sarcomere. Our ultimate goal is to elucidate the three-dimensional structure of the sarcomere as a whole and gain insights into its formation, maintenance and function,“ explains Stefan Raunser, coordinator of the joint project.

“Our muscles, both skeletal and heart, have to function perfectly for our entire lifespan and therefore must be regularly serviced. We do not yet know how this works exactly. Thus, we want to investigate and compare the composition and the structure of sarcomeres in young and aged muscles,“ Frank Schnorrer and Dirk Görlich refer to one of the goals of this large-scale project.

Combining forces to uncover the nanostructure of sarcomeres

This European consortium intends to use its unparalleled expertise and state-of-the-art technologies to address the fundamental questions of the project. Together, it pursues an innovative, interdisciplinary concept that combines quantitative proteomics and nanoantibody engineering (Görlich) with super-resolution light microscopy (Schnorrer, Gautel), electron cryo-tomography (Raunser), and biochemical as well as functional genetic analyses of sarcomere dynamics in Drosophila flies (Schnorrer), but also in zebrafish and mouse (Gautel).

“Each of us alone would not have been able to achieve what has now become possible through the interactions of our network. The awarding of Synergy Grants is a smart strategy by the European Research Council to enable the collaboration of leading teams that can complement each other’s research endeavors in a joint project,“ comments Mathias Gautel on the role of ERC Synergy Grants for research in Europe.

Coordinator Stefan Raunser predicts that the consortium’s expected research results will provide new insights into the structure of muscle cells at the molecular level and hopefully explain how a muscle cell can generate sarcomeres. This will ultimately contribute to a better understanding of muscle diseases and to the development of innovative agents to mitigate muscle disease and ageing.

About the ERC

The European Research Council, set up by the European Union in 2007, is the premier European funding organization for excellent frontier research. Every year, it selects and funds the most outstanding, creative researchers of any nationality and age to run projects based in Europe. The ERC has three grant schemes for individual principal investigators – Starting Grants, Consolidator Grants, and Advanced Grants – and Synergy Grants for small groups of excellent research teams with up to €14million in funding for up to six years.

In the 2019 competition for these grants, 37 consortia were selected for funding. A total worth of €363 million, these special grants will enable groups of two to four top researchers to bring together complementary skills, knowledge, and resources in one research project. The recipients will be able to tackle some of the most complex research problems, often spanning multiple scientific disciplines. This funding is part of the EU’s research and innovation program, Horizon 2020.

To know more: https://sarcomere2.jimdosite.com

PhD position available in collaboration with the Turing Centre for Living Systems (CENTURI) program

http://centuri-livingsystems.org/phd2018-01/