Abstract
During the last years it has become evident that many different cell types can communicate with each other through intercellular transfer of extracellular vesicles. Such cell-derived membrane vesicles function in several physiological processes and also in disease. This thesis describes two subclasses of prostasomes, extracellular vesicles that are released by
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prostate epithelial cells and ultimately enter the seminal plasma. Prostasomes stimulate sperm cell motility and influence sperm cell capacitation and the acrosome reaction, all of which are prerequisites for successful fertilization. Furthermore, prostasomes suppress immune-mediated destruction of sperm cells in the female reproductive tract. However, the exact molecular mechanisms through which prostasomes exert such diverse functions remain largely unknown. In the studies described in this thesis, prostasomes were purified and their protein, lipid and nucleic acid contents characterized. This resulted in the definition of two distinct types of prostasomes, a smaller (50 nm) and a larger (100 nm) type, which may perform different functions. The two prostasome types have distinct protein and lipid compositions. Many other types of extracellular vesicles are known to contain RNA, which may elicit epigenetic effects in their target cells. However, the RNA content of prostasomes had not been explored. In the research described in this thesis, RNA was found associated with the large prostasomes. Deep sequencing of this RNA showed that the majority is composed of a type that had not been described before. Physiological target cells for the larger prostasomes, and hence for their RNA contents, remained unknown. Given its unique composition, prostasomal RNA may serve as a biomarker of prostate cancer. The blood of healthy men is devoid of prostasomes, but in case of prostate cancer prostasomes may reach the blood stream as well. Comparison of prostasomal RNA from prostate cancer patients with that of healthy men, either from blood, urine or seminal plasma, may provide new insights in its potential as cancer biomarker. The interactions between small prostasomes and sperm cells during sperm cell capacitation were studied in an equine model. Small prostasomes were found to bind to sperm cells, but only at conditions that favoured sperm cell capacitation. Moreover, recruited prostasomes reduced the degree of protein tyrosine phosphorylation in sperm cells, which is a hallmark of a late phase in sperm cell capacitation. Prostasomes may therefore help to prevent premature full capacitation of sperm cells and their acrosome reaction. Other studies showed that transfer of progesterone receptors to the sperm cells through fusion of prostasomes with their plasma membrane. Once the sperm cells have reached the progesterone-secreting cumulus cells that surround the oocyte, transfer of progesterone receptors by prostasomes may assist the progesterone-dependent stimulation of sperm cell hypermotility and the acrosome reaction. Thus, prostasomes may bind and accompany sperm cells on their journey to the oocyte, and fuse with the sperm cells only upon their approach of an oocyte, providing them with tools that are required for effective oocyte fertilization
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