Lytic effector function of Natural Killer (NK) cells and CD8+ T cells occurs through discrete and regulated cell biological steps triggered by recognition of diseased cells. Recent studies of the NK cell synapse support the idea that dynamic nanoscale rearrangements in cortical filamentous (F)-actin are a critical cell biological checkpoint for lytic granule access to NK cell membrane. Loss of function mutations in the LYST gene, a well-characterised cause of Chediak- Hegashi syndrome (CHS), result in the formation of giant lysosomal organelles including lytic granules. Here, we report a mismatch between the extent of cortical F-actin remodelling and enlarged lytic granules that limits the functionality of LYST- deficient NK cells in a human model of CHS. Using super-resolution stimulated emission depletion (STED) microscopy we found that LYST-deficient NK cells had nanoscale rearrangements in the organisation of cortical actin filaments that were indistinguishable from control cells- despite a 2.5-fold increase in the size of polarised granules. Importantly, treatment of LYST-deficient NK cells with actin depolymerising drugs increased the formation of small secretory domains at the synapse and restored their ability to lyse target cells. These data establish that sub-synaptic F-actin is the major factor limiting the release of enlarged lytic granules from CHS NK cells, and reveal a novel target for therapeutic interventions. While the importance of cortical actin filaments in NK cell cytotoxicity have been established, its persistence at the early stages of T cell synapse formation is disputed. We studied the organisation of cortical actin filaments in synapses formed by primary human T cells using STED microscopy and detected intact cortical actin filaments in key T cell effector subsets including memory CD8+ T cells as early as 5-minutes post-activation. Quantitative analysis revealed that activation specific rearrangements in cortical actin filaments at both CD4+ and CD8+ T cell synapses serve to increase the space between filaments. Additionally, comparison of cytolytic T cells with freshly isolated and IL-2 activated primary NK cells revealed that rapid maturation of the cortical actin meshwork is a specific feature of CD8+ T cell lytic synapses. Using chemical inhibition of actin nucleators, we show that increased cortical relaxation is mediated primarily by the activity of actin related proteins (Arp) -2/3. Taken together, these data establish the critical requirement for dynamic rearrangements in cortical actin filaments at lytic synapses but underscore cell-specific differences in its regulation.

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Lesions in the human Lyst gene are associated with the lysosomal disorder Chediak Higashi Syndrome. The absence of Lyst causes the formation of enlarged lysosome related compartments in all cells. This defect results in severe immunodeficiency, neurological dysfunction, and ultimately in death. Despite decades of research, the mechanism for how these enlarged compartments arise is not well established. Two opposing models have been proposed for Lyst function. The fission model describes Lyst as a positive regulator of fission from lysosomal compartments, while the fusion model identifies Lyst as a negative regulator of fusion between lysosomes. To date, a consensus on which model is correct has not been reached. This thesis details my investigation of Lyst function using Dictyostelium discoideum. To establish a definitive model for the function of the Dictyostelium Lyst ortholog, LvsB, we used assays that distinguish between defects in vesicle fusion versus fission. We compared the phenotype of cells defective in LvsB with that of two known fission defect mutants ([mu]3 and WASH null mutants). The temporal localization characteristics of the post-lysosomal marker vacuolin, as well as vesicular acidity and fusion dynamics of LvsB null cells are distinct from those of both fission defect mutants. These distinctions are predicted by the fusion defect model and implicate LvsB as a negative regulator of vesicle fusion. This work also presents evidence that LvsB antagonizes the function of two fusion regulatory proteins, Rab14 and dLIP5. The Dictyostelium Rab14 GTPase is known to stimulate lysosome fusion, and here we implicate dLIP5 as a promoter of Rab14 activity. Constitutive activation of Rab14 increases vesicle fusion in wild type cells but not in dLIP5 mutant cells. Thus, Rab14 activity is dependent on dLIP5. Additionally, the aberrant vesicle morphology and fusion phenotypes of LvsB mutant cells are suppressed by expression of dominant inactive Rab14 or disruption of dLIP5. This suppression suggests that LvsB antagonizes Rab14 activity to negatively regulate vesicle fusion. These studies validate the fusion model for LvsB function and provide new insights into the relationships that dictate vesicle fusion regulation. By extension, we propose that Lyst negatively regulates vesicle fusion by antagonizing the activity of a RabGTPase. / text

京都大学 / 新制・論文博士 / 博士(医学) / 乙第13569号 / 論医博第2295号 / 新制||医||1068(附属図書館) / 京都大学大学院医学研究科医学専攻 / (主査)教授 濵﨑 洋子, 教授 生田 宏一, 教授 滝田 順子 / 学位規則第4条第2項該当 / Doctor of Medical Science / Kyoto University / DFAM