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Coming theses from other universities

  • Regulation of docking and priming in pancreatic α- and β-cells Author: Muhmmad Omar-Hmeadi Link: Publication date: 2019-08-20 13:34

    The secretion of islet hormones from endocrine cells of the pancreas plays vital roles in maintaining glucose homeostasis. Dysfunction of these cells leads to diabetes, a devastating metabolic disorder affecting millions worldwide, but underlying mechanisms remain poorly understood. In hyperglycemic conditions, β-cells secrete insulin, whereas α-cells secrete an increased amount of glucagon in hypoglycemic conditions. Both insulin and glucagon are stored in secretory granules preceding their release by regulated exocytosis. This process involves several steps, including tethering, docking, priming, and finally, a fusion of the granules with the plasma membrane. Soluble N-ethylmaleimide–sensitive factor attachment protein receptor (SNARE) proteins and phosphoinositides (PIs) drive pancreatic hormone exocytosis and secretion, which follows a biphasic time course. Biphasic secretion is thought to reflect the vastly different release probabilities of individual granules, but direct evidence for this is still lacking.  Therefore, this thesis investigates exocytosis in the two main pancreatic cell types with a particular focus on preceding steps docking and priming, to identify rate-limiting steps in health and type-2 diabetes (T2D). Our data indicated that granule docking is critical for sustained secretion in α- and β-cells. Glucagon granule exocytosis had a U-shaped sensitivity to glucose in both healthy and T2D α-cells. However, T2D α-cells exhibited a marginal decrease in exocytosis, as well as docking, and they were markedly insensitive to somatostatin and insulin. T2D β-cells reduced exocytosis dramatically, and docking was compromised and no longer responsive to glucose, which correlated with reduced insulin secretion and elevated donor HbA1c. These results were further strengthened by the finding that expression of a group of genes that are involved explicitly in granule docking was reduced (by RNAseq of islets from over 200 human donors), and overexpression of the corresponding proteins increased granule docking in human β-cells.

    We further aimed to study the basis for the recruitment of these proteins to the docking site. Here we tested the hypothesis that highly charged lipids mainly PIs act as a hotspot to interact with SNARE proteins that initiate docking. We showed the homogenous distribution of all PIs markers in the plasma membrane, with no PIs microdomains at the exocytotic site during granule docking. However, rapid and local PI(4,5)P2 signaling at fusion sites was crucial for stabilizing fusion pore by binding to proteins related to the release site. These results suggested a role of PI(4,5)P2 in priming and fusion regulation rather than docking. Overall, this work gives new insights into the mechanisms underlying pancreatic hormone secretion in both healthy and diabetic conditions.

  • Biophysical studies of the actin-myosin motor system and applications in nanoscience Author: Mohammad A. Rahman Link: Publication date: 2019-08-19 15:41

    The actin-myosin motor system plays important roles in cellular processes. In addition, actin and myosin have been used for developments towards nanotechnological applications in recent years. Therefore, fundamental biophysical studies of actin and myosin and the actomyosin force generating cycle are important both in biology and for nanotechnology where the latter applications require methodological insights for optimization. This dual goal is central in the present thesis with major focus on factors that control the function (e.g. velocity) and the effectiveness of transport of filaments (e.g. filament flexural rigidity) through nanoscale channels with supplementation of methodological insights. The thesis thus provides evidence that actin is a dynamic filament whose flexural rigidity is different at different MgATP concentrations as well as in the presence or absence of myosin binding. Furthermore, probing the myosin ATPase cycle with the myosin inhibitor blebbistatin revealed that velocity is easily modified by this drug. Our detailed studies also suggest that actin-myosin force generation is preceded by Pi release and that blebbistatin changes the rate limiting transition in the cycle from the attachment step to a step between weakly attached states. The studies of actin dynamics and of the actomyosin force generating cycle were largely performed using in vitro motility assay (IVMA) where surface adsorbed myosin motor or its proteolytic fragments propel fluorescently labeled actin filaments. The IVMA is often taken as the basis for developments towards different nanotechnological applications. However, in the IVMA, actomyosin motility is often negatively affected by the presence of “dead”, non-functional myosin heads. Therefore, in this thesis, two popular methods, that are often used to remove dead myosin heads, are analyzed and compared. It was found that after affinity purification, the in vitro actin sliding velocity is reduced compared to the control conditions, something that was not seen with the use of blocking actin. Therefore, the effects of the affinity purification method should be considered when interpreting IVMA data. This is important while using IVMA both for fundamental studies and for nanotechnological applications. Another issue in the use of IVMAs in nanotechnological applications is the requirement for expensive and time-consuming fabrication of nanostructured devices. We therefore developed a suitable method for regenerating molecular motor based bionanodevices without a need to disassemble the flow cell. Evidence is presented that, use of proteinase K with a suitable detergent (SDS or Triton X100) lead to successful regeneration of devices where both actin-myosin and microtubule-kinesin motility are used. Lastly, this thesis presents efforts to immobilize engineered light sensitive myosin motors on trimethyl chlorosilane (TMCS) derivatized surfaces for light operated switching of myosin motor in order to control actin movement in nano-networks. This has potential for developing a programmable junction in a biocomputation network. In brief, the described results have contributed both to the fundamental understanding of actin and myosin properties and the actomyosin interaction mechanisms. They have also given technical insights for molecular motor based bionanotechnology.

  • Hemodynamics and physiological effects of Resuscitative Endovascular Balloon Occlusion of the Aorta (REBOA) Author: Mitra Sadeghi Link: Publication date: 2019-08-19 09:53