ORCID ID
Graduation Date
Summer 8-14-2026
Document Type
Dissertation
Degree Name
Doctor of Philosophy (PhD)
Programs
Biochemistry & Molecular Biology
First Advisor
Dr. Steve Caplan
Abstract
After internalization at the plasma membrane (PM), cargos arrive at the early endosome (EE), where they are sorted and trafficked to various destinations. Receptors can be incorporated into MVBs for degradation via the lysosome, trafficked to the TGN, or recycled to the PM. Receptor recycling can occur via two major routes: 1) directly from the EE (fast recycling), or 2) first trafficked to the perinuclear endocytic recycling compartment before arriving at the PM (slow recycling). Efficient receptor recycling requires the coordinated action of membrane remodeling proteins, cargo-sorting machinery, and the actin cytoskeleton, but the molecular mechanisms governing these processes remain incompletely understood. In this dissertation, we studied the mechanisms that regulate receptor recycling and endosome organization, with a particular focus on the role of FCHSD2 and ARP2/3-mediated branched actin.
In Chapter II, we established a previously unrecognized role for FCHSD2 in endosomal trafficking. FCHSD2 was originally characterized as a regulator of clathrin-mediated endocytosis at the plasma membrane, but our findings demonstrated that it also functions at early endosomes to regulate receptor recycling. Using genetic perturbation and confocal imaging, we showed that loss of FCHSD2 impairs endosome fission and delays receptor recycling. Mechanistically, FCHSD2 localizes to endosomes through an interaction with MICAL-L1 and promotes ARP2/3-mediated branched actin polymerization at the endosomal membrane. Together, these studies identify FCHSD2 as a regulator of endosomal recycling and highlight the role of branched actin in endosome fission.
In Chapter III, we investigated how branched actin regulates cargo organization and trafficking at the early endosome. Although hypothesized, evidence for the role of branched actin in endosome organization remains limited. We used constitutively active RAB5 to generate enlarged endosomes to more readily visualize cargo organization at the endosome. Acute inhibition of ARP2/3 activity disrupted this organization, resulting in broader cargo distributions within the endosomal membrane, impaired cargo segregation, and accumulation of receptors at early endosomes. Furthermore, disruption of branched actin by a small molecule ARP2/3 inhibitor or FAM21A siRNA resulted in the coalescence of degradative and recycling subdomains that are normally maintained as distinct regions on the endosomal membrane. These findings support a model in which branched actin functions as a barrier that constrains cargo diffusion and facilitates the generation of cargo-specific transport intermediates.
Collectively, the studies presented in this dissertation establish FCHSD2 and ARP2/3-mediated branched actin as critical regulators of receptor recycling and endosomal organization. Our findings support a model in which branched actin assembly at endosomes organizes cargos into discrete membrane domains and enables efficient sorting and fission. More broadly, this work provides new mechanistic insight into how the actin cytoskeleton coordinates membrane remodeling and cargo organization to govern endocytic trafficking, advancing our understanding of the fundamental principles that maintain intracellular membrane organization.
Rights
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Recommended Citation
Frisby, Devin, "Actin at the Early Endosome: Roles in Cargo Sorting and Endosome Fission" (2026). Theses & Dissertations. 1089.
https://digitalcommons.unmc.edu/etd/1089