Here, we present an easy-to-use and versatile method to perform live imaging of developmental processes in general and muscle-tendon morphogenesis in particular in living Drosophila pupae.
Muscles together with tendons and the skeleton enable animals including humans to move their body parts. Muscle morphogenesis is highly conserved from animals to humans. Therefore, the powerful Drosophila model system can be used to study concepts of muscle-tendon development that can also be applied to human muscle biology. Here, we describe in detail how morphogenesis of the adult muscle-tendon system can be easily imaged in living, developing Drosophila pupae. Hence, the method allows investigating proteins, cells and tissues in their physiological environment. In addition to a step-by-step protocol with helpful tips, we provide a comprehensive overview of fluorescently tagged marker proteins that are suitable for studying the muscle-tendon system. To highlight the versatile applications of the protocol, we show example movies ranging from visualization of long-term morphogenetic events – occurring on the time scale of hours and days – to visualization of short-term dynamic processes like muscle twitching occurring on time scale of seconds. Taken together, this protocol should enable the reader to design and perform live-imaging experiments for investigating muscle-tendon morphogenesis in the intact organism.
The muscle-tendon apparatus allows animals including humans to move their body parts. The molecular building blocks of the muscle-tendon system are highly conserved. Therefore, concepts of muscle-tendon development relevant for human muscle biology, for example muscle morphogenesis, muscle-tendon attachment and myofibril self-organization, can be studied using Drosophila melanogaster as an easily accessible model system. The Drosophila pupal system has several experimental advantages. First, at the pupal stage – when the adult muscles are formed – the organism is sessile and therefore easy to image on a microscope over a period of hours or even days. Second, many muscles form close enough underneath the pupal surface so that they can be imaged inside the intact, partially translucent organism. Third, the muscles can be investigated in their natural environment, where they are connected to the forming exoskeleton via tendon cells and tissue tension is built up. This is not possible in muscle cell culture systems. And finally, a plethora of genetic tools is available in Drosophila. Among these are many fluorescently tagged markers that allow labeling of specific cell types or subcellular structures for imaging in vivo.
Table 1 summarizes the most important markers used for studying muscle-tendon morphogenesis. It includes markers overexpressed using the GAL4-UAS-system1 and endogenously tagged protein markers2,3,4. The advantage of the GAL4-UAS-system is that the markers are generally expressed at high levels, resulting in a strong signal that can easily be imaged in whole-mount pupae. In addition, tissue specificity can be achieved by choosing GAL4 drivers carefully. The advantage of fusion proteins expressed under endogenous control is that the dynamics of the respective proteins can be studied in vivo, while they can also be used as markers for different cell types or specific subcellular structures, for example, βPS-Integrin-GFP for muscle attachment sites. Together, these markers provide high flexibility in experimental design and choice of research problems that can be solved now and in the future.
Labeled structure | Marker | Expression and localization | Class | Stock number | Comment | Ref. | ||||||
Muscle | Mef2-GAL4 | all myoblasts and all muscles at all stages | GAL4 line | BL 27390 | 5 | |||||||
1151-GAL4 | adult muscle precursors and early myotubes until ≈24 h APF | GAL4 line, enhancer trap | – | 6 | ||||||||
Act79B-GAL4 | jump muscle upon differentiation | GAL4 line | – | 7 | ||||||||
Act88F-GAL4 | indirect flight muscles starting ≈14 h APF | GAL4 line | – | 7 | ||||||||
Act88F-Cameleon 3.1 | indirect flight muscles starting ≈14 h APF | Act88F enhancer/ promoter driving Cameleon 3.1 | – | Ca2+ indicator | 8 | |||||||
Act88F-GFP | indirect flight muscles starting ≈14 h APF | GFP-fusion (fly TransgeneOme line) | fTRG78 and fTRG10028 | 4 | ||||||||
Him-nls-GFP | adult muscle precursors, nuclear, until ≈24 h APF in indirect flight muscles | enhancer/promoter with nls-GFP reporter | – | 1.5 kb enhancer fragment | 9 | |||||||
Mhc-Tau-GFP | microtubules in DLM templates and in differentiating muscles | enhancer/promoter with Tau-GFP reporter | BL 53739 | 10 | ||||||||
βTub60D-GFP | microtubules in myotubes (e.g. in indirect flight muscles from ≈14 h AFP, strongly decreasing after ≈48 h APF) | GFP-fusion (fly TransgeneOme line) | fTRG958 | 4 | ||||||||
Mhc-GFP (weeP26) | sarcomeres (thick filament) in all body muscles (e.g. in indirect flight muscles starting from ≈30 h APF) | GFP-trap | – | use heterozygous, labels an isoform subset | 11 | |||||||
Sls-GFP | sarcomeres (Z-disc) in all body muscles (e.g. in indirect flight muscles starting from ≈30 h APF) | GFP-trap (FlyTrap line) | – | G53, use heterozygous | 2 | |||||||
Zasp66-GFP | Z-disc in all body muscles | GFP-trap (FlyTrap line) | BL 6824 | ZCL0663 | 2,12 | |||||||
Zasp52-GFP | Z-disc in all body muscles | GFP-trap (FlyTrap line) | BL 6838 | G00189 | 2,12 | |||||||
Hts-GFP | actin binding; expressed in epithelium, myoblasts and myotubes | GFP-fusion (fly TransgeneOme line) | fTRG585 | 4 | ||||||||
Dlg1-GFP | epithelial cell junctions, myoblasts and membranes in muscles at all stages | GFP-fusion (fly TransgeneOme line) | fTRG502 | 4 | ||||||||
Muscle attachment site | βPS-Integrin-GFP | muscle attachment sites (e.g., starting ≈18 h AFP in indirect flight muscles) | GFP-knock-in | – | 13 | |||||||
Talin-GFP and -mCherry | muscle attachment sites (e.g., starting ≈18 h AFP in indirect flight muscles) | GFP-trap (MiMIC line) | – | 3 | ||||||||
Talin-GFP | muscle attachment sites (e.g., starting ≈18 h AFP in indirect flight muscles) | GFP-fusion (fly TransgeneOme line) | fTRG587 | 4 | ||||||||
Ilk-GFP | muscle attachment sites (e.g., starting ≈18 h AFP in indirect flight muscles) | GFP-trap (FlyTrap line) | Kyoto 110951 (ZCL3111) | ZCL3111, ZCL3192 | 2 | |||||||
Vinc-GFP and -RFP | muscle attachment sites (e.g., starting ≈18 h AFP in indirect flight muscles) | GFP-fusion (transgene) | – | 13 | ||||||||
Tendon | sr-GAL4 | thorax tendon cells, throughout pupal stage | GAL4 line, enhancer trap | BL 26663 | homozygous lethal | 14 | ||||||
Muscle and Tendon | Duf-GAL4 | muscles and epithelia, early onset | GAL4 line | BL 66682 | kirre-rP298, founder cell marker | 15 | ||||||
UAS-reporters | UAS-GFP-Gma | actin binding | UAS line | BL 31776 | actin binding domain of Moesin fused to GFP | 16 | ||||||
UAS-mCherry-Gma | actin binding | UAS line | – | Gma fused to mCherry | 17 | |||||||
UAS-Lifeact-GFP | actin binding | UAS line | BL 35544 | 18 | ||||||||
UAS-Lifeact-Ruby | actin binding | UAS line | BL 35545 | 18 | ||||||||
UAS-CD8-GFP | membrane binding | UAS line | various stocks, e.g.: BL 32184 | 19 | ||||||||
UAS-CD8-mCherry | membrane binding | UAS line | BL 27391 and 27392 | 20 | ||||||||
UAS-palm-mCherry | membrane binding through palmitoylation | UAS line | BL 34514 | UAS-brainbow | 21 |
Table 1: Fluorescently tagged protein markers suitable for studying muscle-tendon morphogenesis in vivo.
Here, we describe in detail how imaging of muscle-tendon morphogenesis in living pupae can be performed easily and successfully (Figure 1). Alternatively, pupae can be fixed, dissected and immunostained, which allows using antibodies to also label proteins for which no live markers are available22. In this case, the imaging quality is generally higher because there is no movement and the structure of interest can be placed in close proximity to the coverslip. However, dissection and fixation can lead to damage and molecular or tissue dynamics, for example, muscle twitching, can only be studied in the living organism.
The presented protocol describes how to image muscle-tendon morphogenesis in living Drosophila pupae using a variety of fluorescently tagged proteins. This in vivo imaging strategy can be used to study developmental processes in their natural environment of the entire organism.
It is crucial for a successful experiment to find the correct developmental time point to analyze. For example, dorsolongitudinal indirect flight muscles initiate attachment to their tendon targets at ≈16 h APF23 while abdominal muscles develop later and attach on both ends only between 30 and 40 h APF26. Consequently, previously published literature should be used to find the right time points of development to analyze or, if the tissue or structure of interest has not been studied in detail before, the overall development has to be characterized first.
For mounting pupae successfully on the custom-built plastic slides, it is important that the grooves have suitable dimensions: The grooves need to be 1.0 – 1.5 mm wide and 0.3 – 0.4 mm deep. This depth allows adjusting the precise distance to the top coverslip with spacer coverslips as needed. However, at least one spacer coverslip should be used to avoid draining the 50% glycerol away from the sample by capillary forces. The correct positioning of the pupae in the groove requires some experience and should be optimized such that the structure of interest is as close as possible to the coverslip.
If a large number of pupae is supposed to be imaged in one microscope session, they can all be mounted beforehand and then stored in an incubator until imaging to ensure proper developmental timing. The pupae should survive the entire procedure and also at least try to eclose if kept on the slide after imaging. The survival rate can be used as a readout to check whether the imaging conditions harm the pupae.
The imaging settings should be chosen carefully according to the experimental requirements. For short-term movies, a high frame rate versus a high signal-to-noise ratio needs to be balanced, while relatively high laser power can be used without damaging the pupae too much. However, for long-term movies, the laser power has to be kept at a moderate level and the pupae should not be imaged continuously but rather at certain time points, for example, every 20 min. To ensure that the structure of interest does not move out of the field of view, it might be necessary to readjust the positioning of the z-stack between time points. To our knowledge, the opening of the pupal case per se does not affect developmental timing. However, a temperature-controlled stage should be used for long-term movies to ensure proper developmental timing. Keeping these considerations in mind, highly informative movies can be acquired.
The presented protocol can be used to visualize not only muscle-tendon morphogenesis but also other developing tissues, for example, the wing epithelium29. Only three modifications to this protocol are required: (1) opening of the pupal case above the wing instead of the thorax or abdomen, (2) positioning of pupae with the wing towards the top coverslip, and (3) the use of different fluorescent marker proteins. With the advancement of the CRISPR/Cas9-technology, more and more endogenously tagged fluorescent proteins will be available, because it has become more straightforward to target endogenous loci in Drosophila30,31,32. In the future, this will allow elucidating the dynamics of numerous proteins, cells and entire tissues in their physiological environment in detail.
The authors have nothing to disclose.
We thank Manuela Weitkunat for the acquisition of Movie S3. We are grateful to Reinhard Fässler for generous support. This work was supported by the EMBO Young Investigator Program (F.S.), the European Research Council under the European Union's Seventh Framework Programme (FP/2007-2013)/ERC Grant 310939 (F.S.), the Max Planck Society (S.B.L., F.S.), the Centre National de la Recherche Scientifique (CNRS) (F.S.), the excellence initiative Aix-Marseille University AMIDEX (F.S.), the LabEX-INFORM (F.S.) and the Boehringer Ingelheim Fonds (S.B.L.).
Stereomicroscope | Leica | MZ6 | product has been replaced by Leica M60 |
fly food in bottles (or vials) | – | – | standard culture medium |
paint brush | da Vinci | 1526Y | size 1 |
microscope slides | Thermo Scientific | VWR: 631-1303 | 76 x 26 mm |
double-sided tape (optional) | Scotch | 6651263 | 12 mm x 6.3 m |
petri dishes | Greiner Bio-One | 632102 | 94 x 16 mm |
paper tissues | Th.Geyer | 7695251 | |
forceps #5 (Dumont, inox, standard) | Fine Science Tools | 11251-20 | 0.1 mm x 0.06 mm tip |
forceps #5 (Dumont, inox, biology grade) | Fine Science Tools | 11252-20 | 0.05 mm x 0.02 mm tip |
Cohan-Vannas spring scissors | Fine Science Tools | 15000-02 | straight tip |
plastic slides with a groove (reusable) | custom-built | – | 75 x 26 x 4 mm plexi glass slide with 1.0-1.5 mm wide and 0.3-0.4 mm deep groove |
coverslips | Marienfeld | 107032 | 18 x 18 mm, No. 1.5H |
glycerol | Sigma-Aldrich | 49781 | dilute to 50 % in water |
adhesive tape | Tesa | 57370-02 | 1.5 mm x 10 m |