Date of Degree
Access restricted until 01/31/2020
MS (Master of Science)
The synaptonemal complex is a complex of proteins that connect homologous chromosomes, allowing genetic recombination to occur during meiosis in C. elegans. It is made up of lateral element proteins that assemble along each pair of sister chromatids and central region proteins that extend from, and join, each homolog. Here we aimed at understanding how synaptonemal complex activity is regulated through the study of nuclear import and isoform of its structural protein SYP-2. AKIR-1 is a protein with a previously identified meiotic role in the unloading of the Synaptonemal Complex (SC) Central Region (CR) proteins from the short arms of bivalent chromosomes exiting diakinesis. A Yeast-2-hybrid (Y2H) screen for additional proteins that interact with AKIR-1 found a potential interaction with the α-importin IMA-2. The efficient transfer of proteins from the cytoplasm into a meiotic nucleus in C. elegans is dependent on the activity of α- and β-importins. The α-importins associate to both cargo proteins and β-importins forming a complex that transits into the nucleus. Once inside the nucleus, the complex is disassembled and the importin complex is cycled back into the cytoplasm for reuse. Based on this, a double AKIR-1 and IMA-2 mutant was created to determine what role this interaction has in C. elegans meiosis. We found the first evidence of a role for AKIR-1 in the meiotic nuclear import and chromatin loading of meiotic cohesin complex proteins. We identified a meiotic phenotype in the double mutant akir-1(gk528);ima-2(ok256) that was not apparent in either of the akir-1(gk528) or ima-2(ok256) single mutants in which neither the cohesin REC-8 or the lateral element protein HTP-3 did not associate with chromatin normally, instead forming aggregates with which the central region SYP proteins co-localized. Additionally, we found that the pairing center protein HIM-8 was not being efficiently transported into the nucleus, localizing to the nuclear envelope instead of chromatin, as well as in cytoplasmic poly complexes (PCs). This loss of efficient import and localization resulted in morphological changes in the gonad as well. The gonad of akir-1;ima-2 contained significantly fewer nuclei and did not have a transition zone. We saw an increase in recombination intermediaries represented by RAD-51 foci, an ssDNA associating protein, in late pachytene. This increase indicated a lack of synapsis and crossover formation.
We also examined the effect of a hypomorph of syp-2 on the proper assembly of the synaptonemal complex. The hypomorph displayed similar embryonic lethality to that of the syp-2 null mutant. However, unlike the null mutant, the syp-2 hypomorph showed a partial assembly of CR proteins. We also found an increased incidence of recombination intermediaries compared to the syp-2 null mutant, indicating hat DSB repair pathway is altered in this mutant.
Autophagy is a regulator of aging in many organisms, including C. elegans, as well as functioning in the immune response pathway and muscle protein degradation. AKIRIN has been shown to have myriad roles in the soma in other model animals, and can be found in nearly every tissue. It has been shown to coordinate neural development and skeletal muscle differentiation, migration, and repair across multiple species. AKIRIN also acts as a cofactor in chromatin and cytoskeleton remodeling, gene transcription, and mediates interactions between the Twist transcription factor and Brahma. My research found the first evidence for a role of AKIR-1 in autophagosome inhibition after finding a significant increase in autophagosome puncta in the transgenic akir-1(gk529)::mCherry;LGG-1 mutants. We also found increased muscle deterioration and decreased motility in the akir-1(gk528) mutant, suggesting a role for AKIR-1 in actin filament maintenance. To test that association, we created a double mutant with akir-1(gk528);sma-1(ru18), a SMAD protein found in the apical membrane cytoskeleton, and that is required for body elongation in C. elegans. Our research provided evidence of AKIR-1 operating in a parallel pathway to SMA-1 as the double mutant displayed an additive reduction in body length.
xv, 140 pages
Includes bibliographical references (pages 127-140).
Copyright © 2017 Richard Bowman
Available for download on Friday, January 31, 2020