PgmNr D1006: Drosophila myosin 7a in Phagocytosis and Eye Development.

Authors:
Amy Hong; James Sellers


Institutes
NIH, Bethesda, MD, USA.


Keyword: cytoskeleton

Abstract:

Drosophila myosin 7a (Dm7a) is an unconventional myosin required for the maintenance of the Johnston’s organ (the auditory center in Drosophila) and the morphology of the bristle structures on the thorax. Both of these structures are formed by bundled arrays of actin. Here we show that Dm7a is involved in the process of engulfment of bacteria particles in the Drosophila larval hemocytes. Using the UAS/GAL4 system, we express GFP-Dm7a in the larval hemocytes where it localizes to the cortical regions of the cell and induces the formation of filopodia that are also enriched in Dm7a. We imaged the phagocytosis of fluorescently-tagged bacteria by hemocytes expressing GFP-Dm7a .  When a bacterium contacts the cell surface of a hemocyte, the latter sends out a filopodium which wraps around the bacterium followed by engulfment. Occasionally an extended filopodia was seen to contact a bacterium and transport it back to the cell surface.  Phagocytosis assays revealed that hemocytes from Dm7a mutant flies are defective in their ability to engulf bacteria compared to wildtype hemocytes, although bacteria do bind to their cell surface. Additionally, a lethality assay whereby larvae were challenged with bacteria show that 21% of wildtype larvae eclosed into adult flies, but less than 10% of Dm7a mutant larvae eclosed under these conditions. These results suggest a role for Dm7a in phagocytosis and in the defense mechanism of the fly.  Additionally, we found that in electron microscopy sections of the adult eyes of Dm7a mutants showed missing pigment granules around the primary pigment cells and at the base of the rhabdomeres of the photoreceptor cells.

We have also identified a binding partner of Dm7a using the MyTH7 as a bait in a yeast two hybrid system and named it Myosin 7a Binding Protein (M7BP). M7BP protein has a potential Rab-binding domain in the N-terminus. Immunoprecipitation experiments using M7BP antibody pulled down several Drosophila Rab proteins. Dm7a, M7BP and Rab11 are expressed in the larval eye disc in the lens-secreting cone cells. We are in the process of investigating the functions of Dm7a and M7BP in the development of the eye.