Characterization of FERM domain proteins, Merlin and Moesin, in Drosophila

• Author / Creator

  Abeysundara, Namal W

• Merlin and the ezrin-radixin-moesin (ERM) proteins are key organizers of the cell cortex through linking membrane-associated proteins to the underlying actin cytoskeleton. Merlin and the ERM proteins have been implicated in the maintenance of cell integrity, adhesion, and motility during tissue development and organization. Functional redundancies between the ERM proteins remain a challenge to further elucidating ERM protein function in mammals. Furthermore, the precise mechanisms underlying the tissue-specific defects associated with the loss of merlin still remain unclear. Thus, we use Drosophila melanogaster as a model organism to further investigate the functional significance of Merlin and Moesin, the single Drosophila orthologues of merlin and the ERM proteins. Biochemical studies have demonstrated that mammalian merlin interacts with ezrin-binding phosphoprotein 50 (EBP50), which is an essential scaffold protein in ERM-mediated membrane organization. However, the functional significance of the merlin and EBP50 interaction still remains unclear. We used Drosophila as a model organism to further characterize the interaction between Merlin and Sip1, the Drosophila orthologue of EBP50. We found that Merlin and Sip1 genetically interact. In vitro binding assays showed that the α-helical domain of Merlin was important for Sip1 binding. Specifically, mutation of two conserved arginine residues within the α-helical domain of Merlin reduced binding to Sip1 and altered Merlin subcellular localization and trafficking in Drosophila wing epithelia and cell culture. When Merlin with reduced binding to Sip1 was expressed in the adult wings, the area of the wing region increased. Furthermore, reduced Merlin and Sip1 binding led to defects in epithelial organization in the follicle cell epithelium surrounding the developing oocyte. These findings suggest that Merlin and Sip1 binding is important for growth inhibition and epithelial organization in Drosophila. As the loss of merlin is associated with the development of central nervous system tumours in humans, the functional significance of Drosophila Merlin was further investigated in a neuronal context. Within the Drosophila optic lobe, neuroepithelial cells differentiate into neural progenitors or neuroblasts, which give rise to the neurons essential for the adult visual system. Multiple signaling pathways have been linked to neuroepithelial cell proliferation and differentiation. We found that both Merlin and Sip1 localized to the neuroepithelial cells and neuroblasts within the developing optic lobe. Loss of Merlin and Sip1 led to defects in optic lobe development. Although the mechanisms still remain largely unknown, these findings suggest that Merlin and Sip1 may regulate neuroepithelial cell proliferation or differentiation. Drosophila neuroblast asymmetric cell division requires an intact actomyosin network for anchoring polarity proteins to the cell cortex and maintaining cell size asymmetry. However, the mechanisms that regulate actomyosin dynamics during neuroblast asymmetric cell division have not been extensively studied. We found that Moesin is essential for neuroblast proliferation and mitotic progression in the developing brain. During metaphase, phosphorylated Moesin (p-Moesin) was enriched at the apical cortex and loss of Moesin led to defects in apical polarity maintenance and cortical stability. This asymmetric distribution of p-Moesin was regulated by components of the apical polarity complex. During early anaphase, p-Moesin remained enriched at the apical cortex, which appeared to contribute to asymmetric cortical extension and myosin basal furrow positioning. Therefore, our findings reveal Moesin as a novel apical polarity protein that drives cortical remodelling of dividing neuroblasts, essential for polarity maintenance and initial establishment of cell size asymmetry. Together, this work provides further insight into the role of Merlin and Moesin in cell or tissue organization during Drosophila development.

• Subjects / Keywords

  • Merlin
  • Drosophila
  • Sip1
  • Moesin

• Graduation date

  Fall 2017

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R3804Z18R

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.