Cell-Cell Interactions and Cell Junction Dynamics in the Mammalian Testis

This dissertation, "Cell-cell Interactions and Cell Junction Dynamics in the Mammalian Testis" by Ching-hang, Wong, 黃政珩, was obtained from The University of Hong Kong (Pokfulam, Hong Kong) and is being sold pursuant to Creative Commons: Attribution 3.0 Hong Kong License. The content of this dissertation has not been altered in any way. We have altered the formatting in order to facilitate the ease of printing and reading of the dissertation. All rights not granted by the above license are retained by the author. Abstract: Abstract of thesis entitled CELL-CELL INTERACTIONS AND CELL JUNCTION DYNAMICS IN THE MAMMALIAN TESTIS Submitted by Wong Ching Hang for the degree of Doctor of Philosophy at The University of Hong Kong in June 2005 In mammals, spermatogenesis refers to the process in which a diploid spermatogonium develops into haploid spermatozoa in the seminiferous tubule of the testis. The seminiferous epithelium lining the tubule is composed of Sertoli cells that provide the mechanical and nutritional support to developing germ cells. The attachment of germ cells to Sertoli cells in the epithelium relies largely on actin-based adherens junctions (AJs), such as the testis-specific ectoplasmic specialization (ES). Sertoli cells per se are also tightly adhered to each other, forming the blood-testis barrier (BTB) that creates a microenvironment within the tubule to sequester post-meiotic germ cells from the systemic circulation. Interestingly, the BTB is constituted by coexisting AJs and tight junctions (TJs). Developing germ cells must migrate progressively from outside the barrier to the adluminal edge of the epithelium, where they are released at spermiation. This requires rapid restructurings of cell junctions at the BTB and between Sertoli and germ cells. To regulate these events, cross-talks between the two cell types are essential. In this dissertation, junction dynamics at the BTB and the ES between Sertoli and germ cells (apical ES) were studied using different models. Junction restructurings at the BTB were investigated using cadmium, an environmental toxicant, to perturb BTB functionality in vivo. It was shown that after the treatment of rats with cadmium, the BTB was disrupted with a loss in TJ- and AJ-associated proteins, which was mediated by transforming growth factor-β3 via p38 mitogen-activated protein kinase (MAPK). Proteases in the epithelium were also induced during the BTB disruption, indicating their participation in junction disassembly. To counterbalance excessive proteolysis, α -macroglobulin (α -MG), a non-specific protease inhibitor, was induced during 2 2 cadmium-mediated BTB disruption. The production of α -MG in the testis was shown to be regulated by c-Jun N-terminal kinase (JNK). By blocking the activity of JNK with an inhibitor, the induction of α -MG by cadmium was suppressed, and the BTB became more susceptible to cadmium. This illustrates the crucial roles of JNK and α -MG in BTB dynamics. Another model, in which premature release of spermatids from the epithelium was induced by a suppression of testicular testosterone (T) level, was used to study apical ES restructuring. Although the ES is a cell-cell junction type, it is partly composed of an adhesion structure that is usually restricted to the cell-extracellular matrix interface, namely the focal adhesion complex (FAC). Some components of the FAC that are found at the apical ES, including β1-integrin, focal adhesion kinase (FAK) and c-Src, were up-regulated during the T-suppression-induced ES restructuring. FAK and c-Src also formed a functional complex that potentially activated the extracellular signal-regulated kinase (ERK) pathway, mediating a loss of adhesion between Sertoli and germ cells. In summary, findings reported herein have significantly enhanced our understanding of how cells in the seminiferous epithelium interact during sper