Browsing Graduate Student Dissertations & Theses by Subject "MUTATIONS"
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ANALYSIS OF INOSITOL 1,4,5-TRISPHOSPHATE RECEPTOR-ERLIN1/2 COMPLEX-RNF170 AXIS MUTATIONS THAT RESULT IN NEURODEGENERATIVE DISEASEInositol 1,4,5-trisphosphate receptors (IP3Rs) are endoplasmic reticulum (ER) proteins that assemble into tetrameric IP3- and Ca2+-gated Ca2+ channels. Activation of IP3Rs begins with stimulation of cell surface receptors that elevate cytosolic IP3 levels. IP3, with its co-agonist Ca2+, binds to IP3Rs and causes a conformational change that results in the opening of the channel aperture, allowing for Ca2+ ions to flow from stores within the ER lumen to the cytosol and thereby promoting a number of Ca2+-dependent cellular events, including secretion, neurotransmitter release and cell division. Intriguingly, it appears that the same conformational change that IP3Rs undergo during activation makes them a target for degradation by the ubiquitin-proteasome pathway. This processing allows the cell to fine-tune its internal Ca2+ responses to extracellular stimuli. In the Wojcikiewicz lab, it was discovered that processing of activated IP3Rs is mediated by the Erlin1/2 complex, a large (~2MDa) complex composed of the proteins Erlin1 and Erlin2. Constitutively-associated with the Erlin1/2 complex is the E3 ubiquitin ligase RNF170. Thus, we employed TALEN and CRISPR/Cas9-mediated gene editing technologies to abrogate expression of these three proteins to define their roles in this process. Remarkably, analysis of cells lacking RNF170 showed that it is required for all ubiquitination of activated IP3Rs. Investigation into the roles of Erlin1 and Erlin2 uncovered that Erlin2 is the “dominant partner” in the Erlin1/2 complex, mediating complex interaction with activated IP3Rs and bringing RNF170 into place to allow for ubiquitination to proceed. Mutations to RNF170 (R199C) and Erlin2 (T65I) have been identified as causative for progressive neurodegenerative diseases. Investigation of the R199C mutation on IP3R processing by RNF170 uncovered that while the mutation did not affect normal RNF170 function, it destabilized the RNF170 protein, resulting in a significantly reduced cellular complement of RNF170 and inhibition of IP3R degradation. Analysis of the Erlin2 T65I mutation showed that the effect of the mutation on Erlin1/2 complex function was two-fold. First, Erlin2 T65I interaction with activated IP3Rs was completely blocked, thereby inhibiting recruitment of RNF170 and subsequent ubiquitination and processing. Second, normal binding of the Erlin1/2 complex to phosphoinositol-3-phosphate (PI(3)P) – the significance and function of which remains undefined – was drastically inhibited. Examination of complex assembly and stability by SDS-PAGE and Native PAGE showed no destabilization of individual Erlin2 proteins nor of overall Erlin1/2 complex assembly. These data demonstrate that proper and tight control of IP3R levels in the cell are critical to overall cellular homeostasis, as disruptive mutations to requisite mediators of IP3R processing - the Erlin1/2 complex and RNF170 – result in the development of progressive neurodegenerative disease.
ANALYSIS OF TEMPERATURE SENSITIVE CYK-1 MUTATIONS EFFECT ON MUSCLE DEVELOPMENT ON CAENORHABDITIS ELEGAN SLARVAESarcomeres are the most basic unit of muscle cells. Formins are actin regulatory proteins that are important for actin filament polymerization and nucleation, and might be responsible for the actin filamentassembly in sarcomeres. In Caenorhabditis elegans, two formins (CYK-1 and FHOD-1) were found in the body wall muscle (BWM), specifically in the Z-line of sarcomeres. Previously, BWM were analyzed in null cyk-1(ok2300) mutant worms, derived from heterozygous parents, and fhod-1(tm2363) mutantworms, andwere found to have smaller muscles than wild-type worms. Yet, there was still functional CYK-1 present in the worms due to inherited maternal CYK-1. To eliminate this potential source of CYK-1, cyk-1(or596ts) temperature sensitive mutant worms were used to allow for all CYK-1 to be non-functional at 26°C. The focus of this study was to understand the importance of cyk-1on muscle development in C. eleganslarvae. Wild-type, fhod-1(tm2363), cyk-1(or596ts), and fhod-1(tm2363);cyk-1(or596ts) double mutant wormsat L1 larval stage were observed for worm shape and muscle abnormalities. Abnormal worm shapes were observed in fhod-1, cyk-1, and more commonly in fhod-1;cyk-1mutant worms at permissive and restrictive temperatures. Abnormal muscle was observed in both permissive and restrictive temperatures forfhod-1, cyk-1, and fhod-1;cyk-1mutant worms that had abnormal worm shapes, while all worms that had normal body shape usually had normal muscle at both temperatures. Worms were alsotested to determine long-termeffects of cyk-1and fhod-1mutations on muscle development. Worms were held at permissive or restrictive temperatures for various times. Fhod-1and cyk-1mutant worms showed reduced muscle size compared to wild-type, while fhod-1;cyk-1mutant worms displayed a more severelyreduced muscle size. Next, cyk-1orfhod-1were tested to see whether they can rescue muscle cell size after mosaic expressionin a cyk-1(-) or fhod-1(-) mutant worm background. Fhod-1(+) muscle cells showeda significant increase in muscle cell size compared to fhod-1(-) cells, while cyk-1(+) muscle cells showeda non-significant increase in muscle size compared to cyk-1(+/-) cells. This shows cell-autonomousexpressionoffhod-1influencesmuscle growth, while cyk-1expression in another organ might influence muscle development.
EFFECTS OF FOCAL SEGMENTAL GLOMERULOSCLEROSIS-ASSOCIATED MUTATIONS ON MYOSIN 1E LOCALIZATION AND ACTIVITYOur lab has discovered that an actin-dependent molecular motor called Myosin 1e (Myo1e) is required for maintaining normal morphology and function in vivo of podocytes, a specialized epithelial cell in the kidney. We have found that Myo1e-null mice develop proteinuria, and mutations in the MYO1E gene, including missense mutations A159P and T119I, and nonsense mutation Y695X, have been identified in focal segmental glomerulosclerosis (FSGS), a primary kidney disease that often leads to end stage renal disease (ESRD). Based on these findings, we have proposed that Myo1e and especially its motor domain, plays a key role in regulating actin cytoskeleton organization in kidney podocytes. To study Myo1e activity at the junctions, we have used cell culture systems. We confirmed that Myo1e is a component of the podocyte slit diaphragm using glomerular fractionation assay and immune-gold labeling electron microscopy. Disruption of Myo1e motor activity by point mutation (A159P) completely disrupted Myo1e cellular localization and led to defective actin assembly at nascent cell-cell contacts. Domain mapping experiments in MDCK cells have suggested that the Myo1e TH2 domain is necessary, but not sufficient for its localization, but addition of the TH1 domain restores its localization to junctions. We have also found that the Myo1e SH3 domain interacts with ZO-1, a slit diaphragm and tight junction protein, in invitro pulldown assays, which might contribute to ZO-1 exchange activity at the junctions. Another FSGS-associated Myo1e motor domain mutation (T119I) also caused mis-localization of Myo1e in the cultured mouse podocytes, suggesting loss-of-function of the motor domain mutants. We have also shown that ZO-1 is not recruited to the nascent cell-cell contacts at the same time with the Myo1e T119I mutants. Finally, by using fission yeast as a model system, we have demonstrated that human kidney disease-associated mutations in fission yeast caused defects in yeast growth and endocytosis processes. Interestingly, after analyzing the colocalization patterns between the FSGS-associated Myo1 mutants and Chaperone Rng3, we have proposed that these two kidney disease-associated mutants likely possess different disease-causing mechanisms.Above all, we have concluded that Myo1e motor domain plays an important role in its localization and activity in podocyte actin cytoskeleton, which might be the link to the disease mechanism of FSGS at the molecular level.