Browsing by Subject "GFRAL"
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(2020)Growth differentiation factor 15 (GDF15), a member of TGF-β super family is a soluble cytokine that is associated with different pathological conditions including cancer, cardiac and renal failure and obesity. Its high serum levels are linked with symptoms like cachexia/anorexia in cancer patients and can be used as a marker for these diseases. Its crucial role in weight regulation and energy homeostasis has been demonstrated by treating obese mice with GDF15, which results in weight lose along with improved glucose metabolism and increased insulin tolerance. It is now known that GDF15 exerts its metabolic effect by binding to a GDNF receptor -α-Like (GFRAL) receptor along with co-receptor RET. Interestingly, these two receptors co-localize only in the brain stem area of mice and humans indicating involvement of a neuronal circuit in GDF15 mediated effects. Despite its implications in major health disorders, little is known about the interaction of GDF15 with its receptors and how this interaction in turn modulates different cellular signalling and functions. The aim of the thesis was to study the mechanism and factors involved in endocytosis of GDF15. I employed high content imaging and flow cytometry techniques to visualize and analyse the internalization of ligand-receptor complex and investigate the role of actin, dynamin and phosphoinositide 3 kinase in the process. The results suggest that similar to the internalization of other cellular growth factors, the uptake of GDF15 is affected by disruption of the actin cytoskeleton. The role of dynamin is still unclear. I also discovered that the internalization of GDF15 was inefficient even in cells that expressed the receptor GFRAL, with large cell-to-cell variation. By following the intracellular localization of the receptor GFRAL, my results revealed that the receptor GFRAL is not efficiently exported to the plasma membrane and most of the protein is retained in the Golgi compartment of cells. This phenomenon was stronger in murine fibroblast cells, where the receptor was almost exclusively trapped in the secretory compartment, explaining why the uptake of the ligand GDF15 is so inefficient in these cells. The system developed during this project will now be used to analyse different factors involved in the uptake of GDF15 and eventually uncover the possible endocytic pathway. Moreover, the Golgi retention of the receptor opens up new questions to investigate like whether the physiological function of GDF15 is regulated by receptor export signals. This will help deciphering the complex and mysterious interaction of GDF15 with its receptor GFRAL.
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(2021)Growth Differentiation Factor-15 (GDF15) is a neurotrophic factor associated with anorexia and weight loss. It is elevated in obesity and various diseases. It signals by forming a tripartite complex with the coreceptor Glial cell line-derived neurotrophic factor (GDNF) family receptor alpha-like (GFRAL) and the receptor Rearranged during transfection (RET). Targeting this pathway has therapeutic potential for the treatment of obesity and anorexia-cachexia syndrome, but many aspects are still unclear. What is the affinity of binding between these proteins? Does GDF15 induce dose- and time-dependent RET phosphorylation and activate intracellular signaling pathways, and are there differences between GDF15 and GDNF signaling, as the different bend angles of their complexes suggest? Can soluble GDF15-GFRAL mediate the effects of GDF15 outside of the brainstem, and what is the function of the short cytoplasmic domain of GFRAL? Furthermore, how well is the pathway evolutionally conserved between species? Binding affinities were assessed with microscale thermophoresis, whereas RET phosphorylation and intracellular signaling assays were performed utilizing immunoprecipitation and western blotting. GFRAL-RET binding is low-affinity (350 nM ± 223) similarly to GFRα1-RET binding (GDNF family receptor alpha-1), whereas GDF15-RET binding without GFRAL does not occur. GDF15 appears to compete for binding to GFRAL or RET, differing from GDNF mechanisms, but noise in the data may have affected the results. The data provide ideas about the ligand-receptor complex formation. Furthermore, RET phosphorylation by GDF15 is dose- and time-dependent. Firstly, the strongest RET and ERK activation occur at GDF15 concentrations typical of disease states. Secondly, RET activation by GDF15 is rapid and sustained like by GDNF activation, whereas ERK activation by GDF15 is rapid and much more transient than by GDNF. Thirdly, AKT activation by GDF15 is much weaker than by GDNF. The differences may be caused by different conformations of binding surfaces for adaptor proteins being available on RET because the bend angles of the complexes are different. Moreover, soluble GDF15-GFRAL does not activate RET, although soluble GDNF-GFRα1 does. Also, the short cytoplasmic domain of GFRAL is not necessary for activating AKT and ERK pathways, but may be needed to activate RET. Furthermore, GDF15 from cynomolgus monkey, but not rat or mouse, activates RET with human GFRAL, indicating sequence similarity in the active site of GDF15. In conclusion, novel aspects of GDF15 signaling and differences between GDF15 and GDNF signaling were discovered.
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