Academic Background

Ph. D. (1979) Arizona State University

Translational Control in Meiosis, Mitosis, and Neuronal Synaptic Plasticity

Our laboratory investigates the biochemical basis of regulated mRNA translation, and studies how translational control influences such important biological processes as oocyte development, cell cycle progression, and neuronal synaptic plasticity.  Much of our work is devoted to understanding how the RNA binding protein CPEB controls cytoplasmic polyadenylation and resulting translational activation under a variety of conditions.  In Xenopus (frog) oocytes, CPEB represses the translation of mRNAs that contain a specific 3’ UTR sequence, the CPE (cytoplasmic polyadenylation element).  Translation is inhibited by Maskin, a CPEB-interacting protein that also associates with the cap binding factor eIF4E.  Maskin binding to eIF4E prevents the association of eIF4G with eIF4E, which is necessary for cap-dependent translation.  In response to various signaling events, CPEB becomes activated by phosphorylation, an event leading to polyadenylation and the dissociation for Maskin from eIF4E.  This process is followed by eIF4G binding to eIF4E and resulting translation.

Neuronal synaptic plasticity, the underlying cellular and biochemical basis of long-term memory storage, is regulated at the translational level.  CPEB is probably involved in the process since it is present at synapses of mammalian hippocampal neurons and promotes polyadenylation and translation in response to NMDA receptor activation.  The importance of CPEB in neuronal activity is underscored by the observations that CPEB knockout mice display defects in synaptic plasticity and hippocampal-dependent memories.  Connecting the molecular biology of CPEB with the electrophysiological and behavioral assays is an important undertaking and must include the identification of mRNAs whose translation is altered in the knockout mice.  Assays to identify such mRNAs are under development.

The regulation of translation by CPEB in mammals is important for two other processes.  The first is meiotic progression; oocytes from CPEB knockout mice arrest at the pachytene stage because mRNAs encoding components of the synaptonemal complex are not translated.  When CPEB is knocked down in transgenic mice by RNAi after pachytene, the oocytes again to not develop normally, but extrude polar bodies prematurely, among other defects.  The mRNAs whose translation is under CPEB control during mouse oocyte meiosis is under investigation.

The second process controlled by CPEB in mammals is cellular senescence.  Here, primary cells exit the cell cycle when exposed to various stresses (e.g., DNA damage, mitogenic stimulation); such a limitation of replicative capacity may protect against malignancy.  While fibroblasts derived from wild type mouse embryos (MEFs) become senescent after ~8-10 passages in culture, those derived from CPEB knockout mouse embryos are immortal.  Importantly, ectopic expression of CPEB in the knockout MEFs restores senescence.  The mechanism by which CPEB controls cellular senescence is under investigation, as is the possible relationship between CPEB and malignant transformation.

Figure

Translational Control during the Embryonic Cell Cycle. During S-phase, cyclin B1 RNA is dormant and contains a short poly(A) tail. Within the 3' untranslated region of this RNA are two key cis elements, the CPE (cytoplasmic polyadenylation element) and the AAUAAA hexanucleotide. The CPE is bound by CPEB, which in turn is bound by Maskin, which in turn is bound by eIF4E, the cap-binding factor. The cleavage and polyadenylation specificity factor (CPSF) maybe loosely associated with the AAUAAA. As cells enter mitosis (M-phase), Aurora phosphorylates CPEB serine 174, which causes CPEB to bind and recruit CPSF and poly(A) polymerase (PAP) into an active cytoplasmic polyadenylation complex. PAP catalyzes poly(A) elongation, and as a consequence poly(A) binding protein (PABP) is attracted to the RNA. PABP then associates with eIF4G, which together displace Maskin from eIF4E. eIF4G, through eIF3, brings the 40s ribosomal subunit to the RNA and promotes translation initiation. As cells leave M-phase, a hypothetical phosphatase dephosphorylates CPEB, which causes deadenylation, the reasssociation of Maskin with eIF4E, and translational silencing.