The involvement of cap-independent mRNA translation in cell fate decisions
Loading...
Date
2014
Access
Authors
Morrison, J. Kaitlin
Journal Title
Journal ISSN
Volume Title
Publisher
East Carolina University
Abstract
During cell stress many biochemical processes are shut down. For
example, global mRNA translation initiation is inhibited due to the
disruption of the cap-dependent mRNA recruitment mechanism. One
specific example of stress, apoptosis, results in activated caspases
that cleave the translation initiation factor eIF4G. This cleavage
disrupts cap-dependent mRNA translation initiation by removing the
cap-binding domain. However, a specific subset of mRNAs can still be recruited for protein synthesis in a cap-independent manner by the
residual initiation machinery. This selective recruitment of stress
and apoptosis-related mRNAs promotes stress response and further
induction 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
simplicity, while maintaining molecular pathways found in higher
organisms, and semi transparency that permits direct observation of
cell fate decisions, C. elegans are the fitting in vivo model in
which to study changes in translational regulation and how they
affect cell fate. In this study we address the translational
regulation of the stress and apoptosis-related mRNAs in C. elegans:
BiP (hsp-3) (hsp-4), Hif-1 (hif-1), p53 (cep-1), Bcl-2 (ced-9) and
Apaf-1 (ced-4). Altered translational efficiency of these messages
was observed upon depletion of cap-dependent translation and
induction of apoptosis within the C. elegans gonad. Our findings
suggest 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
important during germ cell development. The silencing of
transcription, associated with chromosome condensation during
meiosis, results in protein expression patterns that are dependent
translational regulation. Genetic analysis shows that loss of key
translational regulators leads to the onset of germ cell tumors
within the C. elegans gonad. Cell growth and mitotic mRNAs typically
rely highly on cap-dependent translation initiation in mammalian cell
culture. Thus, we predicted that knockdown of cap-dependent
translation would decrease expression of growth and mitotic proteins
and result in a reversion of tumor phenotype. However, germ cell
tumor progression was not grossly affected by knockdown of
cap-associated eIF4G. The inability to revert this cell fate is most
likely due to other regulators of translation at work in the germ
line. Reversion of tumor phenotype require knockdown of multiple
regulators of translation. Overall, these results indicate an
important balance between cap-dependent and -independent translation initiation during stress and the affect of this balance on germ cell fate decisions.