UC Santa Barbara

Collective cell migration is critical for development, wound healing, and tumor metastasis. Moving cells can sense and respond to physical features of the microenvironment; however, in vivo, the significance of tissue topography is mostly unknown. My research focused on Drosophila border cells in the Drosophila ovary, an established model for in vivo cell migration, to study how chemical and physical information influences path selection. Although chemical cues were thought to be sufficient, live imaging, genetics, modeling, and simulations show that microtopography is also important. Chemoattractants promote predominantly posterior movement, whereas tissue architecture presents orthogonal information, a path of least resistance concentrated near the center of the egg chamber. E-cadherin supplies a permissive haptotactic cue. The results provide insight into how cells integrate and prioritize topographical, adhesive, and chemoattractant cues to choose one path among many.An advantage of the border cell model is that it is amenable to large-scale genetic screens. In a screen for mutations that cause border cell migration defects in mosaic clones, the gene Catsup was identified. The Drosophila ortholog of ZIP7 (SLC39A7), Catsup encodes a multifunctional endoplasmic reticulum (ER) transmembrane protein reported to negatively regulate catecholamine biosynthesis, to be required for Notch and EGFR trafficking, to function as a Zn2+ transporter, and to reduce ER stress. However, the relationship between these functions was unclear. Here we report that Catsup knockdown caused abnormal accumulation of Notch and EGFR proteins and induced ER stress in border cells. Ectopic expression of a folding-defective rhodopsin mutant protein, Rh1G69D, also induced ER stress, inhibited Notch transcriptional responses, and blocked border cell migration, even in the absence of abnormal Notch or EGFR accumulation in the ER. Remarkably, simultaneous overexpression of Catsup and Rh1G69D was sufficient to degrade Rh1G69D, resolve ER stress, and rescue border cell migration. Mutant forms of Catsup predicted to disrupt the Zn2+ transport were nonfunctional, indicating a requirement for Zn2+ transport in resolving ER stress. We propose a model for Catsup/ZIP7 function: local ZIP7-mediated Zn2+ transport at the ER/cytosol interface is rate-limiting for Zn2+-binding ubiquitin ligases that promote ER-associated degradation (ERAD). Accumulation of misfolded proteins in the absence of functional ERAD triggers ER stress, which inhibits Notch transcriptional activity independently of trafficking or proteolytic activation. This proposed mechanism may be evolutionarily ancient, accounting for observations in multiple cell types, tissues, and organisms and suggests a novel treatment strategy for retinitis pigmentosa. Cell movement depends heavily on cytoskeletal dynamics. Intermediate filaments are one of the major cytoskeletal networks involved in cell migration. Until recently, intermediate filaments were unknown in Drosophila. In the last chapter, I identified a new isoform of Tropomyosin1: Tm1-X, which has a domain architecture similar to intermediate filament proteins and forms filaments in vitro. In vivo, Tm1-X promotes border cell migration. Together this work provides new insights into the intracellular and extracellular mechanisms regulating cell migration in vivo.