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Abstract

Nematocyst, the cnidarian stinging organelle, undergoes a sophisticated and complex morphogenesis, which begins as an unusual intracellular secretory process from the Golgi network. They consist of a capsule body attached to a long thread-like structure, the tubule, which is tightly coiled and invaginated inside the capsule matrix. The tubule is expelled to the outside in response to a mechanical or chemical stimulus during predation or defence. The firing action of nematocysts, driven by a high osmotic pressure of 150 bar, is the fastest reaction in nature and takes about 3 milliseconds. Although the structural assembly of nematocysts has been studied over the last few years, the molecular basis of tubule formation and invagination has so far been elusive. In this thesis work, it is shown that a nonmuscle myosin II homolog (HyNMII) from the lower metazoan Hydra is an essential factor required to drive the morphogenesis of its stinging organelle. Non-muscle myosin II is commonly associated with stress fibres in the leading edge of migrating cells and cleavage furrow of cells in mitosis. More recently it has also been implicated to be associated dynamically with Golgi membranes where it regulates vesicle fission and trafficking. A specific antibody raised against the HyNMII head domain demonstrated an exclusive localization on nematocyst tubules from early to late morphogenesis stages. HyNMII condenses to a collar structure surrounding the apical constriction of the nematocyst vesicle in early nematocysts. HyNMII then facilitates tubule growth by providing a compressive force counteracting the increasing osmotic pressure as evidenced by blebbistatin treatment and genetic knockdown. HyNMII inhibition led to a depletion of early morphogenetic nematocyst stages by tubule disintegration. In addition, the glycoprotein NOWA was redefined in this thesis as a major component of the tubule. NOWA was shown to be essentially involved in the tubule invagination process as evidenced by genetic knockdown and live imaging using a transgenic NOWA::NOWA-GFP line. The data demonstrated in this thesis elucidates a novel function for actomyosin networks in the morphogenesis of the cnidarian nematocyst, one of the most sophisticated organelles in the animal kingdom.