Contact inhibition of locomotion
The process of contact inhibition of locomotion, whereby migrating cells collide and repel each other, is an intriguing cell biological problem that we know surprisingly little about on the molecular level, and even less regarding its in vivo relevance. This in vitro phenomenon was discovered more than 60 years ago by the pioneering cell biologist, Michael Abercrombie, who spent his career speculating about a role for CIL in development and disease; however to date the study of contact inhibition is largely phenomenological as there are currently few examples of this process within living organisms.
Over the past few years we have developed an in vivo model system using Drosophila macrophages (hemocytes), along with novel analytical tools to dissect the contact inhibition response in cells during development. This work has revealed that CIL is a significant driving force behind embryological movements, and in the absence of external cues, can control the acquisition of embryonic pattern – i.e. it is a developmental ‘cue’ in its own right. Furthermore, preliminary data suggest that on a cell biological level this process is not as simple as it may first appear and entails a series of steps involving complex mechano-chemical signals. We are now elucidating the molecular mechanisms behind this process by exploiting our unique ability to live image and genetically dissect contact inhibition in a living organism.
Understanding the function of the cancer-associated gene, fascin
Expression of the actin-bundling protein, Fascin, is highly correlated with metastatic cancers and is a significant prognostic indicator of poor clinical outcome. It functions by generating filopodia, parallel bundles of actin filaments, which can increase the migratory capacity of cells and thus enhance their metastatic potential. Despite its clinical significance, most of our knowledge of Fascin function and regulation stems from in vitro assays and we have little understanding of its true roles in animal development or disease.
We have been exploiting Drosophila hemocytes to examine Fascin function and regulation within an in vivo setting. Drosophila Fascin, called singed (sn) in the fly, is approximately 41% identical at the amino acid level to human Fascin1. The singed gene has been shown to play a role in generating a number of filopodia-like structures during development, and numerous mutant Drosophila alleles have been isolated with various phenotypic defects. For example, singed was first found to play a role in the formation of bristles on the thorax of the fly, which requires a bundled actin network during their formation. Subsequently, it was discovered that singed played a role in generating actin structures in the nurse cells of egg chambers, and in singed mutants, females are sterile. Pertinent to Fascin’s hypothesized role in cancer cell motility, my laboratory has revealed that singed is also essential for the migration of Drosophila macrophages (hemocytes). Fluorescently tagged Drosophila Fascin localizes to dynamic filopodia and microspikes (actin bundles embedded within lamellae), and singed mutant hemocytes show a reduced capacity to developmentally disperse as a result of actin defects. We are now exploiting this system to dissect the molecular mechanisms regulating Fascin activity; it is becoming increasingly apparent that it is necessary to examine the function and regulation of Fascin within an in vivo setting as we are discovering clear differences with what has been elucidated from in vitro motility models.