Group leader : F. Schnorrer
We investigate the biomechanics of muscle morphogenesis, particularly focusing on how the contractile apparatus develops its quasi crystalline regularity across the muscle.
Muscles are the major force producing organs in our body. They enable us to climb high peaks, ran marathons or swim across the Channel. All muscles share a similar contractile machine called a sarcomere. Large muscles contain thousands of these sarcomeres arrayed in series to form long chains called myofibrils. These linear myofibrils span across the entire muscle cell.
Muscle morphogenesis is a multi-step process. Myoblasts proliferate, migrate and fuse to myotubes. Myotube tips search for tendon cells, to which they establish stable force-resistant attachments. Finally, myofibrils and sarcomeres assemble to convert myotubes to myofibers. The biomechanics of these myofibrils is tuned to the particular functional needs of each muscle type in the body.
We combine the advantages of the Drosophila genetic toolbox with high resolution in vivo imaging to functionally dissect the biomechanics of muscle morphogenesis. Questions we are particularly fascinated by include:
- How do muscles establish force-resistant attachments to tendons?
- How are contractile myofibrils and sarcomeres built?
- How is functional muscle diversity achieved?
Drosophila adult muscle development
Movie showing the flight muscles (green), which are attaching to tendons (orange); tension is built up and tendons form long extensions.
We use the Drosophila adult muscles, in particular the flight muscles to image muscle-tendon attachment and myofibrillogenesis in intact developing animals. We found that myotubes first attach to tendon cells at both myotube tips and only after being stably attached, myofibrillogenesis is triggered throughout the entire muscle. Using in vivo laser cutting experiments we discovered that mechanical tension is generated after muscles have attached to tendons. Interestingly, this tension build-up is required for ordered myofibrillogenesis (Weitkunat et al. 2014).
Quantifying forces across molecules in vivo
During myofibril and sarcomere assembly a quasi-crystalline pattern of myosin motors (thick filaments), actin tracks (thin filaments) linked by gigantic titin molecules (C-filament) is built in every muscle. Using in vivo imaging we found that myofibrillar pattern organises simultaneously across the entire muscle fiber after tension has been built up. We hypothesize that tension is used as a molecular compass to direct myofibril assembly, confirming that each myofibril spans across the entire muscle fiber. This guarantees effective muscle contractions in the mature muscle. We are testing this hypothesis by applying molecular force sensors, which enable us to quantify tension across individual molecules within developing myofibrils. We are also applying novel high-resolution microscopy techniques to monitor myofibril assembly at the nanoscale.
To walk or to fly
Flight muscles harbour a specialised ‘fibrillar’ contractile apparatus to power fast wing oscillations at 200 Hz. This requires a very stiff muscle fiber type, related to vertebrate heart muscle. In contrast, slowly moving leg muscles display a tubular muscle architecture, closely related to striated vertebrate body muscles. We identified the conserved transcription factor Spalt as myofibril selector gene that instructs fibrillar muscle morphogenesis by inducing expression and alternative splicing of key sarcomeric components (Schönbauer et al. 2011). We found that downstream of Spalt the muscle specific splicing program is regulated by the RNA binding protein Arrest (Bruno) (Spletter et al. 2015). Thus as in vertebrate muscle, the biomechanics of a fiber-type specific contractile apparatus is regulated by transcription and alternative splicing of specific sarcomeric components. Currently, we are investigating the mechanism of fiber-type specific alternative splicing as well as the individual impact of the regulated components on myofibril assembly and muscle biomechanics.
CRISPRing the fly
We have developed an efficient two-step genome engineering protocol to manipulate any Drosophila gene of interest at its endogenous locus (Zhang et al. 2014). In step 1, we apply CRISPR/Cas9-mediated homology directed repair to replace a target region with a selectable marker (red fluorescent eyes), which is flanked by two attP sites. In step 2, we replace the inserted marker with any DNA of choice using ΦC31-mediate cassette exchange (RMCE). This enables flexible and efficient engineering of the locus, for example to generate a tagged allele, or to insert point mutations of choice.
The Drosophila TransgeneOme
In collaboration with the Tomancak, Sarov and VijayRaghavan labs we generated a genome-wide resource for the analysis of protein localisation in Drosophila. We tagged 10,000 proteins by inserting GFP into large genomic FlyFos clones. For about 900 clones we generated transgenic flies, which can be used to assess the in vivo dynamics and subcellular localisation of the tagged protein. For many of the tagged proteins functional antibodies or live imaging tools had not been available before. All transgenic lines are available from VDRC and thus can be used by the fly community. A preprint of the manuscript can be found here.
April 1st, 2017
Mechanical forces during muscle development
April 1st, 2017
Mechanical tension and spontaneous muscle twitching precede the formation of cross-striated muscle in vivo.
October 12th, 2016
A Guide to Genome-Wide In Vivo RNAi Applications in Drosophila.
February 20th, 2016
A genome-wide resource for the analysis of protein localisation in Drosophila.
February 16th, 2015
The RNA-binding protein Arrest (Bruno) regulates alternative splicing to enable myofibril maturation in Drosophila flight muscle.
March 31st, 2014
Tension and force-resistant attachment are essential for myofibrillogenesis in Drosophila flight muscle.
November 16th, 2011
Spalt mediates an evolutionary conserved switch to fibrillar muscle fate in insects
March 11th, 2010
Systematic genetic analysis of muscle morphogenesis and function in Drosophila
July 12th, 2007
A genome-wide transgenic RNAi library for conditional gene inactivation in Drosophila.
May 12th, 2007
The transmembrane protein Kon-tiki couples to Dgrip to mediate myotube targeting in Drosophila
April 5th, 2017
AIDing-targeted protein degradation in Drosophila.
April 15th, 2015
Slit cleavage is essential for producing an active, stable, non-diffusible short-range signal that guides muscle migration.
February 18th, 2014
Ret rescues mitochondrial morphology and muscle degeneration of Drosophila Pink1 mutants
February 5th, 2014
Transcriptional regulation and alternative splicing cooperate in muscle fiber-type specification in flies and mammals
January 15th, 2014
A guide to study Drosophila muscle biology.
January 1st, 2014
A simple protocol to efficiently engineer the Drosophila genome by TALENs
January 24th, 2011
The Drosophila blood brain barrier is maintained by GPCR-dependent dynamic actin structures.
February 8th, 2010
Three-dimensional reconstruction and segmentation of intact Drosophila by ultramicroscopy.
January 28th, 2010
In vivo RNAi rescue in Drosophila melanogaster with genomic transgenes from Drosophila pseudoobscura
April 5th, 2008
High-resolution, high-throughput SNP mapping in Drosophila melanogaster
March 1st, 2008
Ultramicroscopy: 3D reconstruction of large microscopical specimens
January 5th, 2008
Positional cloning by fast-track SNP-mapping in Drosophila melanogaster
October 5th, 2006
The gammaTuRC components Grip75 and Grip128 have an essential microtubule-anchoring function in the Drosophila germline
March 5th, 2005
RhoGEF2 and the formin Dia control the formation of the furrow canal by directed actin assembly during Drosophila cellularisation
July 7th, 2004
Muscle building; mechanisms of myotube guidance and attachment site selection
January 6th, 2004
Axon guidance: morphogens show the way
November 5th, 2002
Gamma-tubulin37C and gamma-tubulin ring complex protein 75 are essential for bicoid RNA localization during drosophila oogenesis
April 5th, 2000
The molecular motor dynein is involved in targeting swallow and bicoid RNA to the anterior pole of Drosophila oocytes.
March 5th, 1999
Oligomerisation of Tube and Pelle leads to nuclear localisation of dorsal
December 18th, 1998