The involvement of cap-independent mRNA translation in cell fate\r\ndecisions

ECU Author/Contributor (non-ECU co-authors, if there are any, appear on document)

J. Kaitlin Morrison (Creator)

Institution

East Carolina University (ECU )

Web Site: http://www.ecu.edu/lib/

Abstract: During cell stress many biochemical processes are shut down. For\r\nexample, global mRNA translation initiation is inhibited due to the\r\ndisruption of the cap-dependent mRNA recruitment mechanism. One\r\nspecific example of stress, apoptosis, results in activated caspases\r\nthat cleave the translation initiation factor eIF4G. This cleavage\r\ndisrupts cap-dependent mRNA translation initiation by removing the\r\ncap-binding domain. However, a specific subset of mRNAs can still be recruited for protein synthesis in a cap-independent manner by the\r\nresidual initiation machinery. This selective recruitment of stress\r\nand apoptosis-related mRNAs promotes stress response and further\r\ninduction of apoptosis. Many of these mRNAs contain internal ribosome entry sites (IRESes) that promote their enhanced translation during these conditions. Still other mRNAs have little dependence on the cap recognition mechanism. The expression of the encoded proteins, both anti- and pro-apoptotic, promote an initial period of attempted cell survival, then commitment to cell death when damage is extensive. This switch in mode of translation initiation and how it allows for selective mRNA translation is not well understood. This study focuses on the utilization of cap-independent protein synthesis in the nematode worm, C. elegans. Due to their relative genetic\r\nsimplicity, while maintaining molecular pathways found in higher\r\norganisms, and semi transparency that permits direct observation of\r\ncell fate decisions, C. elegans are the fitting in vivo model in\r\nwhich to study changes in translational regulation and how they\r\naffect cell fate. In this study we address the translational\r\nregulation of the stress and apoptosis-related mRNAs in C. elegans:\r\nBiP (hsp-3) (hsp-4), Hif-1 (hif-1), p53 (cep-1), Bcl-2 (ced-9) and\r\nApaf-1 (ced-4). Altered translational efficiency of these messages\r\nwas observed upon depletion of cap-dependent translation and\r\ninduction of apoptosis within the C. elegans gonad. Our findings\r\nsuggest a physiological link between the cap-independent mechanism and the enhanced translation of hsp-3 and ced-9. This increase in the efficiency of translation may be integral to the stress response during the induction of physiological apoptosis. Further organism wide RNA-seq studies have begun to identify the entire population of mRNAs that rely highly on cap-dependent and independent translation. Development of this methodology, for detecting changes in translational efficiency on a global level, enables future follow-up studies to confirm additional mRNAs whose translation results in changes in cell fate decisions. Additionally, we carefully and specifically characterized C. elegans ced-4 mRNA, the Apaf-1 homologue, and observed that its structure and mode of translation initiation differed from its mammalian homologue. ced-4 mRNA translation illustrates one example of how selective translation in germ cells may differ from that observed in cultured mammalian cells responding to toxic treatments such as chemotherapy agents and hypoxia. Networks of translational regulation are particularly\r\nimportant during germ cell development. The silencing of\r\ntranscription, associated with chromosome condensation during\r\nmeiosis, results in protein expression patterns that are dependent\r\ntranslational regulation. Genetic analysis shows that loss of key\r\ntranslational regulators leads to the onset of germ cell tumors\r\nwithin the C. elegans gonad. Cell growth and mitotic mRNAs typically\r\nrely highly on cap-dependent translation initiation in mammalian cell\r\nculture. Thus, we predicted that knockdown of cap-dependent\r\ntranslation would decrease expression of growth and mitotic proteins\r\nand result in a reversion of tumor phenotype. However, germ cell\r\ntumor progression was not grossly affected by knockdown of\r\ncap-associated eIF4G. The inability to revert this cell fate is most\r\nlikely due to other regulators of translation at work in the germ\r\nline. Reversion of tumor phenotype require knockdown of multiple\r\nregulators of translation. Overall, these results indicate an\r\nimportant balance between cap-dependent and -independent translation initiation during stress and the affect of this balance on germ cell fate decisions.

Additional Information

Publication

Dissertation

Language: English

Date: 2023

Subjects

Chemistry, Biochemistry;Biology, Molecular;Apaf-1;Bcl-2;BiP;Cap-independent translation;EIF4G;Germ cell apoptosis;Molecular biology;Biochemistry

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