Background

Drosophila melanogaster embryogenesis is a classical model for morphogenesis (Campos-Ortega and Hartenstein, 1985). While many tools have been developed to assess the role of specific proteins in morphogenesis, assessing cellular forces or mechanics still remains challenging. For the last fifteen years, laser dissection has been the most commonly used approach to assess cellular forces (Colombelli and Solon, 2013; Shivakumar and Lenne, 2016). However, laser dissection is invasive, it wounds tissues, and does not facilitate the application of ectopic forces. To overcome these limitations, a few methods have been developed to probe the mechanics of tissues by inducing deformations in droplets of magnetic fluid or through the optical trapping of cellular junctions (Bambardekar et al., 2015; Serwane et al., 2017). These methods have a limited range of force and require specific, complex instrumentation. Here, we present the protocol for an alternative, versatile and low-cost method for applying controlled forces within an epithelium of a living Drosophila embryo (D’Angelo et al., 2019). This method relies on the injection of a magnetic particle within a living embryo and on the application of a magnetic field with an electromagnet. Since the magnetic particle is coated with GBP nanobody, it is possible to target a specific intracellular attachment. Here, we injected the bead in a fly line expressing GFP at the plasma membrane (Resille GFP) to position the bead at the plasma membrane. Our methodology does not impair morphogenesis and repeated force application can be performed without cellular damage.

It is well established that tissue mechanics is an essential component to consider for the control of cellular behavior (differentiation, cell division, migration…) leading to morphogenesis during animal development and to the progression of diseases such as tumor formation and cancer progression (Lecuit et al., 2011; Heisenberg and Bellaiche, 2013; Engler et al., 2006; Frey et al., 2008; Godard and Heisenberg, 2019; Northcott et al., 2018). In these contexts, our method can be used to probe tissue mechanics and its changes or to apply local forces to investigate mechanosensation and mechanotransduction.