Translational Control in Health and Disease
Summary: Nahum Sonenberg is conducting research on the mechanism and control of translation initiation in eukaryotes in health and disease. He is investigating the role that translational control plays in development; learning and memory; microRNA function; and the dysregulation of translation in cancer, autism, neurodegenerative disease, and virus infections.
The control of eukaryotic protein synthesis occurs predominantly at the initiation level, which under most circumstances is the rate-limiting step in translation. Translation initiation refers to a series of reactions that culminate in the recruitment of the 80S ribosome to the mRNA. This process begins with recognition of the m7GpppN cap structure (5′ cap), which is present at the 5′ end of all nuclear-transcribed mRNAs, by the eukaryotic translation initiation factor 4F (eIF4F). eIF4E is the 5′-cap-binding subunit of the heterotrimeric eIF4F complex. eIF4F also contains the RNA helicase eIF4A, which unwinds the secondary structure in the 5′ UTR of the mRNA, and the scaffolding protein eIF4G, which binds eIF4E, eIF4A, eIF3, and the poly(A)-binding protein (PABP). eIF3 binds the ribosome directly to form the 43S preinitiation complex. This complex scans the mRNA until it encounters an AUG start codon to initiate translation. PABP binds to the 3′ poly (A) tail of the eukaryotic mRNAs, and because of its simultaneous interaction with eIF4G, it brings about the circularization of the mRNA to promote translation.
Because eIF4E is the least abundant initiation factor, the cap-recognition step is rate limiting for translation and a major target for regulation. eIF4E quantity and activity are controlled at many levels, including transcription, mRNA stability, phosphorylation, and interactions with several binding partners, such as the family of 4E binding proteins (4E-BPs). 4E-BPs can directly compete with eIF4G for binding to eIF4E and prevent eIF4F assembly, thus impairing cap-dependent translation initiation. Two major signaling pathways, mTOR and MAPK, regulate eIF4E activity and consequently translation. Thus, translational control via eIF4E constitutes a convergence point for signaling pathways to control important physiological functions such as cell growth and proliferation, development, learning and memory, and metabolic processes. The objectives of my research are to answer fundamental mechanistic questions regarding the translational machinery and its regulation in health and disease.
Translational Control of Cancer
Control of translation plays an important role in the regulation of cell growth and proliferation. Several translation initiation factors are either overexpressed or their activity is dysregulated in human tumors. In addition, overexpression of several initiation factors in cells and mice engenders malignant transformation. The PI3K/Akt/mTOR signaling pathway, which is frequently deregulated in cancer, is a major pathway affecting the activity of translation factors. Key initiation factors, which are controlled by the PI3K/Akt/mTOR pathway, and are implicated in cancer, include the mRNA 5′-cap-binding protein, eIF4E, and its partners eIF4A and eIF4G. The RAS/RAF/MEK/ERK (MAPK) pathway, which is also deregulated in cancer, mediates the phosphorylation of eIF4E on Ser209 that is required for efficient transformation by eIF4E. We generated eIF4E "knockin" mice in which Ser209 was mutated to alanine. These mice are resistant to prostate cancer induced by PTEN deletion, as well as cancers induced by other means. The components of eIF4F have been recently targeted for cancer therapy with either small molecules or antisense approaches.
Translational Control in Learning and Memory
Studies on learning and a variety of forms of synaptic plasticity have revealed an absolute requirement of mRNA translation for learning and memory. The PI3K/Akt/mTOR pathway plays a critical role in synaptic plasticity and memory. To explore the role of translation initiation in hippocampal long-term potentiation (LTP) and learning and memory, we generated knockout (KO) mice for 4E-BP2, which is the predominant 4E-BP isoform in the brain. 4E-BP2 KO mice are impaired in hippocampus-dependent learning and memory tasks. It has been postulated that hyperconnectivity of neuronal circuits resulting from increased synaptic protein synthesis causes autism spectrum disorders (ASDs). In addition, the mammalian target of rapamycin (mTOR) is strongly implicated in ASDs via upstream signaling molecules. Therefore, we are studying the involvement of 4E-BP2 in ASDs.
We are also studying the role that the poly(A)-binding protein (PABP) plays in learning and memory. The translational repressor PABP-interacting protein 2A (PAIP2A), an inhibitor of PABP, is degraded in stimulated neurons. Paip2a KO mice exhibit a lowered threshold for the induction of sustained long-term potentiation and an enhancement of long-term memory in contextual fear conditioning and Morris water maze tasks after weak training. Translation of several mRNAs, whose products are critical for memory consolidation and maintenance, is enhanced in the hippocampus of Paip2a KO mice. Thus, activity-dependent degradation of PAIP2A relieves translational inhibition of memory-related genes and establishes PAIP2A as a pivotal translational regulator of synaptic plasticity and memory.
Mechanism of MicroRNA Action
MicroRNAs (miRNAs) inhibit mRNA expression in general by base pairing to the 3' UTR of target mRNAs and consequently inhibiting translation and/or initiating poly(A) tail deadenylation and mRNA destabilization. We established a mouse Krebs-2 ascites extract that faithfully recapitulates the miRNA action in cells. We demonstrated that the let-7 miRNA inhibits translation of reporter mRNA at the initiation step. Translation inhibition is subsequently consolidated by let-7-mediated deadenylation, which requires both the PABP and the CAF1 deadenylase, which interact with the let-7 miRNA-loaded RNA-induced silencing complex (miRISC). Importantly, we demonstrated that GW182, a core component of the miRISC, directly interacts with PABP via its C-terminus and that this interaction enhances miRNA-mediated deadenylation. The miRISC binds the deadenylation machinery independently of PABP via the CNOT1 subunit of the CNOT-CCR4 deadenylase complex. We are studying how miRISC recruits the deadenylation and translation-suppression machinery in a PABP-independent manner.
As of September 26, 2012