Hicks, Lawrence Joseph; Department of Cellular Biology and Anatomy (5/22/2018)
      Actin remodeling and endocytosis are essential functions for most cells. Defects in these processes present in a variety of diseases. Sorting nexins are known to contribute to endocytic uptake, cytokinesis, the retromer complex, and autophagy. Sorting nexin 9 (Snx9) interacts with major endocytic factors and proteins involved in regulation of actin cytoskeleton dynamics. Nonetheless, Snx9’s exact in vivo roles in these basic cellular processes and disease mechanisms are not known. By examining the roles of Sh3px1, we can better understand the mechanism by which this protein contributes to endocytosis and actin remodeling in vivo. Two additional paralogs, Snx18 and Snx33, complicate studies in mammalian models due to potential redundant mechanisms. Utilizing the single ortholog in Drosophila, sh3px1, this report describes the function of Sh3px1 in membrane organization and actin dynamics. Drosophila S2 cells that are depleted of Sh3px1 fail to form lamellipodia, a process that is also dependent on the actin nucleation factor, Scar. In addition, over-expression of Sh3px1 in S2 cells results in the formation of tubules and also long membrane protrusions, atypical of a classical BAR domain protein. An intact PX-BAR domain is required for these overexpression phenotypes. sh3px1 null flies are viable; however, mutant females have significantly compromised fertility. Female sh3px1 null egg chambers show many morphological defects. The age-dependent degeneration of the null egg chamber is not likely due to compromised endocytosis. Additionally, collective border cell migration is attenuated in the absence of Sh3px1. These cells are known for their reliance on endocytosis and modulation of actin dynamics for migration. We have found that Sh3px1 is essential in efficient lamellipodia production at the start of border cell migration. Our findings also suggest that Scar directly interacts with Sh3px1 and is upregulated in sh3px1 nulls. Mutation of Scar enhances many reproductive defects in sh3px1 nulls. Thus, our work reveals a main in vivo function of Sh3px1 in actin regulation for the production of structures such as lamellipodia.