UC Irvine

Cell-cell communication (CCC) is essential for coordinating important developmental processes, including tissue patterning. Although cells can communicate through well-characterized mechanisms such as endocrine, synaptic, juxtacrine, and paracrine signaling, these are not the only signaling modalities that exist in nature. Cells can also communicate via specialized filopodia, allowing for direct targeting. There are several subclasses of signaling filopodia, including airinemes, which are the focus of this dissertation. Airinemes are a unique class of specialized filopodia that mediate signaling during zebrafish pigment pattern development through the coordinated activities of xanthoblasts, macrophages, and melanophores. Airinemes originate as surface blebs (airineme blebs) on xanthoblasts that are recognized and extracted by migrating macrophages, which transport airineme vesicles and deposit them onto target melanophores. Despite previous work establishing the importance of airinemes in pigment pattern formation, several aspects of airineme-mediated CCC remained unresolved, including the identity of the macrophage population responsible for airineme transport, the molecular mechanisms governing macrophage-airineme interactions, and basic characterization of airineme blebs.This dissertation addresses these questions through three complementary studies. First, I identified at least two morphologically and behaviorally distinct macrophage populations in the zebrafish skin and demonstrated that amoeboid macrophages, which overlap with the previously described ectoderm-derived macrophage population known as metaphocytes, preferentially localize to the hypodermis where airineme-projecting xanthoblasts reside. I further demonstrated that metaphocyte migration into the hypodermis requires MMP9. Second, I identified the cell-surface glycoprotein CD44 as a mediator of interactions between metaphocytes and airineme bleb-bearing xanthoblasts. Cell-specific disruption of the CD44 extracellular domain reduced airineme frequency, demonstrating that CD44-dependent interactions contribute to airineme-mediated signaling. Finally, I established size- and morphology-based criteria for identifying putative airineme blebs and used these criteria to characterize their developmental distribution and potential maturation. Putative airineme blebs increased in abundance prior to peak airineme production, and a subset exhibited CD44a as well as phosphatidylserine (PS), a previously established recognition signal for macrophage-mediated airineme extraction. Together, these findings support a model in which airineme-mediated CCC is assembled through a series of regulated cellular and molecular interactions involving airineme bleb formation, macrophage recognition and transport, and delivery of signaling cargo to target cells. This work expands our understanding of how immune cells can acquire specialized roles in developmental CCC and provides a framework for investigating the formation and maturation of airineme blebs and other membrane-associated signaling structures.