BDNF-induced local translation of GluA1 is regulated by HNRNP A2/B1

The AMPA receptor subunit GluA1 is essential for induction of synaptic plasticity. While various regulatory mechanisms of AMPA receptor expression have been identified, the underlying mechanisms of GluA1 protein synthesis are not fully understood. In neurons, axonal and dendritic mRNAs have been reported to be translated in a cap-independent manner. However, molecular mechanisms of cap-independent translation of synaptic mRNAs remain largely unknown. Here, we show that GluA1 mRNA contains an internal ribosome entry site (IRES) in the 5′UTR. We also demonstrate that heterogeneous nuclear ribonucleoprotein (hnRNP) A2/B1 interacts with GluA1 mRNA and mediates internal initiation of GluA1. Brain-derived neurotrophic factor (BDNF) stimulation increases IRES-mediated GluA1 translation via up-regulation of HNRNP A2/B1. Moreover, BDNF-induced GluA1 expression and dendritic spine density were significantly decreased in neurons lacking hnRNP A2/B1. Together, our data demonstrate that IRES-mediated translation of GluA1 mRNA is a previously unidentified feature of local expression of the AMPA receptor.

A Retrospective on eIF2A—and Not the Alpha Subunit of eIF2

Initiation of protein synthesis in eukaryotes is a complex process requiring more than 12 different initiation factors, comprising over 30 polypeptide chains. The functions of many of these factors have been established in great detail; however, the precise role of some of them and their mechanism of action is still not well understood. Eukaryotic initiation factor 2A (eIF2A) is a single chain 65 kDa protein that was initially believed to serve as the functional homologue of prokaryotic IF2, since eIF2A and IF2 catalyze biochemically similar reactions, i.e., they stimulate initiator Met-tRNAi binding to the small ribosomal subunit. However, subsequent identification of a heterotrimeric 126 kDa factor, eIF2 (α,β,γ) showed that this factor, and not eIF2A, was primarily responsible for the binding of Met-tRNAi to 40S subunit in eukaryotes. It was found however, that eIF2A can promote recruitment of Met-tRNAi to 40S/mRNA complexes under conditions of inhibition of eIF2 activity (eIF2α-phosphorylation), or its absence. eIF2A does not function in major steps in the initiation process, but is suggested to act at some minor/alternative initiation events such as re-initiation, internal initiation, or non-AUG initiation, important for translational control of specific mRNAs. This review summarizes our current understanding of the eIF2A structure and function.

Evidence for Internal Initiation of RNA Synthesis by the Hepatitis C Virus RNA-Dependent RNA Polymerase NS5B In Cellulo

ABSTRACT Initiation of RNA synthesis by the hepatitis C virus (HCV) RNA-dependent RNA polymerase (RdRp) NS5B has been extensively studied in vitro and in cellulo. Intracellular replication is thought to rely exclusively on terminal de novo initiation, as it conserves all genetic information of the genome. In vitro, however, additional modes of initiation have been observed. In this study, we aimed to clarify whether the intracellular environment allows for internal initiation of RNA replication by the HCV replicase. We used a dual luciferase replicon harboring a terminal and an internal copy of the viral genomic 5′ untranslated region, which was anticipated to support noncanonical initiation. Indeed, a shorter RNA species was detected by Northern blotting with low frequency, depending on the length and sequence composition upstream of the internal initiation site. By introducing mutations at either site, we furthermore established that internal and terminal initiation shared identical sequence requirements. Importantly, lethal point mutations at the terminal site resulted exclusively in truncated replicons. In contrast, the same mutations at the internal site abrogated internal initiation, suggesting a competitive selection of initiation sites, rather than recombination or template-switching events. In conclusion, our data indicate that the HCV replicase is capable of internal initiation in its natural environment, although functional replication likely requires only terminal initiation. Since many other positive-strand RNA viruses generate subgenomic messenger RNAs during their replication cycle, we surmise that their capability for internal initiation is a common and conserved feature of viral RdRps. IMPORTANCE Many aspects of viral RNA replication of hepatitis C virus (HCV) are still poorly understood. The process of RNA synthesis is driven by the RNA-dependent RNA polymerase (RdRp) NS5B. Most mechanistic studies on NS5B so far were performed with in vitro systems using isolated recombinant polymerase. In this study, we present a replicon model, which allows the intracellular assessment of noncanonical modes of initiation by the full HCV replicase. Our results add to the understanding of the biochemical processes underlying initiation of RNA synthesis by NS5B by the discovery of internal initiation in cellulo. Moreover, they validate observations made in vitro, showing that the viral polymerase acts very similarly in isolation and in complex with other viral and host proteins. Finally, these observations provide clues about the evolution of RdRps of positive-strand RNA viruses, which might contain the intrinsic ability to initiate internally.

(Not) Baptizing Thecla: Early Interpretive Efforts on 1 Cor 1:17

This piece reconsiders Paul’s deferral of Thecla’s baptism in light of internal initiation themes and in connection with broader historical efforts to understand Paul’s statement in 1 Cor 1:17a (“For Christ did not send me to baptize but to preach the gospel”). To this end, the article surveys previous explanations for this deferral of baptism, proposing to integrate several previous insights into a reading of Thecla as initiate, locates this reading on the spectrum of ancient interpretations of 1 Cor 1:17 from Tertullian to John Chrysostom and finally considers other baptismal material in the Acts of Paul.

Metafizyczne enklawy w prozie Stefana Żeromskiego

The articles presents an interpretation of selected topics from the Stefan Żeromski’s novel which form an articulation of the nineteenth century changes taking place within the metaphysics. The scene in which the protagonists from Żeromski’s novels experienced communication with the dead and lived through the inner enlightenment have been analyzed. The article also describes the poetics of articulation of metaphysical experience in the prose of Żeromski paying particular attention to the theme of light equivalentizing the internal initiation of heroes, leading them mainly to cross the barriers of death. From this perspective, the articles interprets such motives as birch, earth and kiss as functionalizing metaphysical experience of the heroes described in the novels. The analysis of these metaphysical enclaves in the prose of Żeromski allowed to put forward the thesis that the metaphysics of the writer is not inspired by philosophy or theology of the late nineteenth and early twentieth century, but by faith in the strength of family and native ties and by the power of human community which leads man into another dimension of reality.

Regulation of the expression and activity of Unr in mammalian cells

Unr (upstream of N-ras) is a post-transcriptional regulator of gene expression, essential for mammalian development and mutated in many human cancers. The expression of unr is itself regulated at many levels; transcription of unr, which also affects expression of the downstream N-ras gene, is tissue and developmental stage-dependent and is repressed by c-Myc and Max (Myc associated factor X). Alternative splicing gives rise to six transcript variants, which include three different 5′-UTRs. The transcripts are further diversified by the use of three alternative polyadenylation signals, which governs whether AU-rich instability elements are present in the 3′-UTR or not. Translation of at least some unr transcripts can occur by internal initiation and is regulated in a cell-cycle-dependent manner; binding of PTB (polypyrimidine tract-binding protein) and Unr to the 5′-UTR inhibits translation, but these are displaced by heterogeneous nuclear ribonucleoproteins C1/C2 (hnRNPC1/C2) during mitosis to stimulate translation. Finally, Unr is post-translationally modified by phosphorylation and lysine acetylation, although it is not yet known how these modifications affect Unr activity.

Post-transcriptional regulation of gene expression by Unr

Unr (upstream of N-ras) is a eukaryotic RNA-binding protein that has a number of roles in the post-transcriptional regulation of gene expression. Originally identified as an activator of internal initiation of picornavirus translation, it has since been shown to act as an activator and inhibitor of cellular translation and as a positive and negative regulator of mRNA stability, regulating cellular processes such as mitosis and apoptosis. The different post-transcriptional functions of Unr depend on the identity of its mRNA and protein partners and can vary with cell type and changing cellular conditions. Recent high-throughput analyses of RNA–protein interactions indicate that Unr binds to a large subset of cellular mRNAs, suggesting that Unr may play a wider role in translational responses to cellular signals than previously thought.