UBC Theses and Dissertations
Intercellular junction disassembly : assigning a function to apical tubulobulbar complexes in testis Young, J'Nelle Sarah
The focus of this thesis is the characterization of apical tubulobulbar complexes in mammalian testis. These double-membrane, actin-based structures form at sites of attachment between germ cells and Sertoli cells. The location, timing and morphology of these complexes have inspired several proposed functions. It has been proposed that tubulobulbar complexes serve as an anchor to prevent early release of spermatids from the epithelium or that they are a device for elimination of excess cytoplasm. I hypothesize that tubulobulbar complexes are subcellular machines responsible for the internalization of intact intercellular junctions thereby contributing to the process of spermatid release from the seminiferous epithelium. A descriptive approach is taken to determine if key components that are present at similar structures in other systems also are present at tubulobulbar complexes, to determine if integral junction molecules are present at tubulobulbar complexes and to determine the fate of internalized junction material. A functional approach is taken to deplete the expression of an actin-based protein that is localized to tubulobulbar complexes to test the prediction that the structures are involved with spermatid release. Dendritic actin components were localized around the cuff of tubulobulbar complexes and clathrin was localized to coated pits at the ends of the structures. Based on these data, a model of tubulobulbar complex formation was proposed that incorporates clathrin-mediated endocytosis and dendritic actin assembly. Junction molecules known to be present at ectoplasmic specializations were found in vesicles at the ends of tubulobulbar complexes that label positively for early endosome markers. Interestingly, junction protein nectin 2 was colocalized with recycling marker Rab11 at newly forming junctions deeper in the epithelium. This suggests that recycling of junctional proteins may be occurring in Sertoli cells. Finally, depletion of cortactin, a key protein at tubulobulbar complexes, resulted in a short phenotype – an indication that the structures were not able to acquire or maintain their normal length after treatment. Significantly, delay in spermatid release was detected. The data presented here supports the junction internalization hypothesis and introduces a new paradigm for junction internalization generally in cells and links the mechanism to a biologically significant event – sperm release.
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