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Muscle dynamics

We are investigating the mechanobiology of muscle. We are interested in how functional muscles are made during development and how they remain functional through the lifespan of an animal.

The strength of our group lies in its unique combination of systematic genetics and live in vivo cell biology, with mechano- and structural biology. We are using Drosophila in vivo and human iPSC in vitro models to study the mechanisms how muscle assemble their contractile sarcomeres and how these sarcomeres are serviced in the living animal to remain functional throughout life. Sarcomeres are amongst the largest protein assemblies in animals: they produce high forces and mechanically link to the skeleton to power animal movement. Hence, sarcomeres are a fantastic playground to understand basic principles of biology:
How do thousands of large proteins assemble to build a micro-meter large pseudo-crystalline sarcomere?
How do thousands of sarcomeres self-assemble to chains that mechanically connect across centi-meter long muscle fibers?

How are sarcomere development and maintenance coordinated with the other physiological requirements of muscle cells including mitochondria biogenesis, T-tubule formation and proteasomal turn-over of damaged proteins throughout life?
Answering these questions will enable us to better understand how muscle are effectively built during development, how they adapt to their different physiological needs (see heart vs. skeletal muscles) and how they remain functional for our entire life. 

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Section of an adult Drosophila shows body muscles in yellow and blue, and the large flight muscles of the thorax in red. Note that each flight muscle cells spans across the entire thorax.

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Alumni

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Maria Spletter
Assistant Professor at University of Missouri Kansas City, USA
Xu Zhang
Assistant Professor at University of Foshan University, China
Irene Kalchhauser
Full Professor at University of Bern, Switzerland
Sandra Lemke
PostDoc at the Nelson Lab in Princeton University, USA
Aynur Kaya-Copur
Scientific Grants Advisor at DKFZ Heidelberg, Germany
Cornelia Schönbauer
German Center for Diabetes Research, Munich, Germany
Manuela Weitkunat
AstraZeneca, Munich, Germany
Irene Ferreira
Director of Clinical Support, Sirion Biotech, Munich, Germany
Wouter Koolhaas
Helmholtz International Epigenetics Research School, Munich, Germany
Celine Guichard
Immunotech, BeckmannCoulter, Marseille

Funding bodies

They support our research
ARN
Fondation Bettancourt Schueller
HFSP
European Research Council

Gallery

human muscle fibers in vitro early stage
human muscle fibers in vitro later stage
severing flight muscle myofibrils (magenta) results in rounding of mitochondria (green).
Developing muscles are under tension: muscles assemble long chains of sarcomeres into long periodic chains called myofibrils (green). The end of each myofibril is connected to tendons (magenta).
flight muscle myofibrils stained with a Projectin nanobody
flight muscle myofibrils stained with two different Projectin nanobodies

Muscle dynamics

We are investigating the mechanobiology of muscle. We are interested in how functional muscles are made during development and how they remain functional through the lifespan of an animal.

Sarcomeres

Sarcomeres are the universal units that power muscle contractions. They display a pseudo-crystalline regularity with cross-linked actin filaments at the Z-discs that border the sarcomere, which face towards the centrally located myosin filaments.

Both filaments are mechanically linked together by the gigantic titin molecules that are anchored with their N-term to the Z-disc and with the C-term at the middle of the sarcomere (Luis & Schnorrer 2021 Cells & Dev).

Scheme of a sarcomere with some its key protein components. Note their regular arrangement.

Sarcomere assembly

Sarcomeres are the universal units that power muscle contractions. They display a pseudo-crystalline regularity with cross-linked actin filaments at the Z-discs that border the sarcomere, which face towards the centrally located myosin filaments.

Both filaments are mechanically linked together by the gigantic titin molecules that are anchored with their N-term to the Z-disc and with the C-term at the middle of the sarcomere (Luis & Schnorrer 2021 Cells & Dev).

top: tension is low, actin filaments are disordered. bottom: tension is high, periodic myofibrils have assembled mechanically spanning the large muscle cell to the tendon attachments.

Quantifying molecular forces

To get a molecular understanding how proteins may sense tension we are quantifying molecular tension in living muscle cells during development. We have built FRET-based molecular tension sensors that we insert into key proteins, such as the integrin adaptor Talin or the gigantic sarcomere spring Titin. For Talin we found that only a small proportion of molecular feel high forces during muscle development. Increasing tissue forces leads to recruitment of more Talin molecules that can share the load. Such the muscles can adapt to changes in tissue mechanics to prevent mechanical failure in vivo (Lemke et al. PLoS Biology 2019). These sensors provide us with a molecular readout of forces in the assembling sarcomeres.

Fly carrying the Talin tension-sensor. middle: section of the adult flight muscle in magenta, with the tendon attachments in green. bottom: scheme of the Talin tension sensor. Note that high tension will result in low FRET values that are measured in living animals.

Sarcomeres at super-resolution

In collaboration with the Raunser, Görlich and Gautel groups we aim towards a molecular structure of the mature and the developing sarcomere funded by our ERC synergy grant (https://www.studysarcomere.eu). As a first step we determine the precise location of various key sarcomeric proteins, in particular the large titin homologs called Sallimus and Projectin in Drosophila. We are generating small nanobodies against specific domains of these proteins in collaboration with Dirk Görlich in Göttingen. We are using these nanobodies to perform super-resolution DNA PAINT microscopy, which we have set up in our lab with the help of the Jungmann lab in Munich. This enabled us to determine the relative position of Sallimus and Projectin in the flight muscle sarcomere with a current resolution of about 5 nm. Interestingly, we found a staggered organisation of the two linear proteins resulting in an attractive model how these two proteins might mechanically connect the Z-disc with the myosin filament. In the future, we will continue to add additional domains to our super resolution map and work together with the Raunser lab using cryo-electron tomography to solve the molecular structure of the sarcomere. This will for the molecular basis to understand sarcomere formation and function at the molecular level.
Combining nanobodies with DNA-PAINT builds a molecular map of Drosophila titins in flight muscle. A: DNA-PAINT image of a fly sarcomere labelled with two nanobodies revealed four bands around the sarcomeric Z-disc. B: molecular model of staggered organisation of fly titin homologs Sallimus and Projectin. Note the overlap of the N-term of Projectin with the C-term of Sallimus.

Mitochondria and myofibrils

Sarcomeres need substantial amounts of ATP to produce forces. Hence, myofibril development must be tightly coordinated with mitochondria morphogenesis in muscle fibers. We found that both are in such intimate contact that the growing myofibrils are squeezing the growing mitochondria into ellipsoid shapes in the flight muscles. This minimises the distance of myosin motors from the ATP source (Avellaneda et al. 2021 Nature Comm.). This raised the interesting question HOW the mitochondria are so well interspaced with the myofibrils during muscle development.
Myofibrils mechanically shape flight muscle mitochondria. left: Wild-type living flight muscles contain elongated mitochondria (green) in close mechanical contact with the myofibrils (magenta). B: Releasing the mechanical constrain results in a rounding up of the mitochondria

Exercising ageing flies

To guarantee the proper function of muscles, the structural organisation of sarcomeres needs to be maintained throughout life. This is a difficult task, considering that muscles are constantly exposed to high mechanical strain and damaged proteins need to be continuously exchanged without destroying the sarcomeric machine.

Age-related decline in muscle function can be easily studied during the short lifespan of Drosophila (2-3 months). In order to generate a systematic resource of the transcriptional and protein dynamics across the Drosophila lifespan we have established an exercise chamber, we named the ‘FlyDome’. The ‘FlyDome’ allows to stimulate flight in a large population of flies for a given time period per day during their entire lifespan. This many enable us to find a molecular basis for the hypothesis that physical activity may delay the decline of skeletal muscle function at old age and thus may effectively establish a state of ‘healthy ageing’.

Flydome to stimulate flight in a large population of flies in a controlled way.

Human muscle building

In collaboration with the Pourquie lab, we investigate if mechanical tension is also key to instruct myofibrillogenesis in human skeletal muscle. We found that when grown on two-dimensional substrate, developing human myofibers spontaneously align and self-organise into higher-order myofiber bundles. Coinciding with formation of the large myofiber bundles, the individual myofibers stably attach to shared attachment foci. Interestingly, tension levels strongly increase in the bundled myofibers that are in the process of assembling sarcomeres. This suggests that mechanical tension is also key to coordinate the multi-scale self-organisation of human muscle morphogenesis (Mao, Acharya et al., bioRxiv 2022). This new model system will enable us to directly manipulate shapes and forces and test their effects for muscle fiber bundling and sarcomere morphogenesis.
Self-organisation in human iPSC-derived muscle fibers. Note the appearance of few large muscle fiber bundles.