Left panel: Cell invasion movie (green, Life-Act; Red, fibronectin). Right panel: Microscopy image showing Myosin-X (GFP) decorating the tips of filopodia (Life-Act RFP).
(A) Functional domains of myosin-X. Myosin-X is composed of a motor domain, three IQ motifs, three PH domains (1a/b, 2 and 3), a myosin tail homology 4 (MyTH4) and a band 4.1, ezrin, radixin, moesin (FERM) domain important for protein binding. (B-D) Myosin-X activation. Full activation of myosin-X is dependent on PH domain-PIP3 interaction (C) and protein dimerisation (D).
Following a comprehensive drug screen, we identified the FDA-approved L-type calcium channel blockers (CCBs) as prominent inhibitors of filopodia formation and cancer cell invasion. This discovery was extremely surprising as L-type calcium channels, the targets of CCBs, are normally restricted to excitable cells including neurons and muscles cells and were not previously thought to be relevant in cancer. Subsequently, L-type calcium channels were found to be expressed by cancer cells and to promote filopodia formation and cancer cell invasion through the calcium target calpain downstream of active integrin signalling. Correspondingly, L-type calcium channels were found to be associated with poor patient survival (see Publications; Jacquemet et al., 2016).
We are continuing to gain a more in-depth understanding of filopodia and their regulators using a combination of mass-spectrometric and drug/siRNA screens and super-resolution microscopy and to investigate the contribution of these filopodia-associated proteins to the metastatic process both in vitro and in vivo. We believe that this multi-pronged approach in studying filopodia and invasive machineries could underpin the development of valuable therapeutic strategies to target cancer cell metastasis. Our work with myosin-X has led to a collaboration with CD3 (Centre for Durg Design and Discovery; non-profit organisation bridging the gap between academia and pharma) to discover novel inhibitors of myosin-X function to disrupt filopodia formation.
Tools: In the course of our filopodia investigations, we developed an open software (FiloQuant) to easily and rapidly quantify filopodia properties such as number, length, from the protruding edge, and dynamics. The manuscript describing FiloQuant was recently published in the Journal of Cell Biology and can also be found on the preprint server bioRxiv. FiloQuant has been deposited on the ImageJ update site and details on how to install the programme can be found in the manual.
Our research: We and others have observed that invasive cell migration is associated with the development of dense filopodia at the cell front and that myosin-X, a key regulator of filopodia formation, is required for cell invasion in 3D microenvironments and in vivo. Through analysis of breast cancer gene expression profiles we revealed that myosin-X is highly expressed in aggressive cancer subtypes. We further demonstrated that myosin-X is required for breast cancer cell invasion and tumour dissemination in multiple cancer cell lines and in
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murine models of cancer metastasis (see Publications; Arjonen et al. 2014). Disrupting the myosin-X integrin-binding domain (in the FERM domain, see diagram below) revealed that the ability of myosin-X to transport β₁ integrins to the filopodia tips is crucial for invasion. Moreover, a p53-mutant resulting in myosin-X overexpression promotes cell invasion.