Rules of engagement promote polarity in RNA trafficking

Structural Insight Into hnRNP A2/B1 Homodimerization and DNA Recognition

Heterogeneous nuclear ribonucleoprotein A2/B1 (hnRNP A2/B1) has been identified as a nuclear DNA sensor. Upon viral infection, hnRNP A2/B1 recognizes pathogen-derived DNA as a homodimer, which is a prerequisite for its translocation to the cytoplasm to activate the interferon response. However, the DNA binding mechanism inducing hnRNP A2/B1 homodimerization is unknown. Here, we show the crystal structure of the RNA recognition motif (RRM) of hnRNP A2/B1 in complex with a U-shaped ssDNA, which mediates the formation of a newly observed protein dimer. Our biochemical assays and mutagenesis studies confirm that the hnRNP A2/B1 homodimer forms in solution by binding to pre-generated ssDNA or dsDNA with a U-shaped bulge. These results depict a potential functional state of hnRNP A2/B1 in antiviral immunity and other cellular processes.

Multivalent Molecules as Modulators of RNA Granule Size and Composition

RNA granules are ensembles of specific RNA and protein molecules that mediate localized translation in eukaryotic cells. The mechanisms for formation and selectivity of RNA granules are unknown. Here we present a model for assembly of one type of RNA granule based on experimentally measured binding interactions among three core multivalent molecular components necessary for such assembly: specific RNA molecules that contain a cis-acting sequence called the A2 response element (A2RE), hnRNP A2 proteins that bind specifically (with high affinity) to A2RE sequences or nonspecifically (with lower affinity) to other RNA sequences, and heptavalent protein cytoskeleton-associated protein 5 (CKAP5, an alternative name for TOG protein) that binds both hnRNP A2 molecules and RNA. Non-A2RE RNA molecules (RNA without the A2RE sequence) that may be recruited to the granules through nonspecific interactions are also considered in the model. Modeling multivalent molecular interactions in granules is challenging because of combinatorial complexity in the number of potential molecular complexes among these core components and dynamic changes in granule composition and structure in response to changes in local intracellular environment. We use a hybrid modeling approach (deterministic-stochastic-statistical) that is appropriate when the overall compositions of multimolecular ensembles are of greater importance than the specific interactions among individual molecular components. Modeling studies titrating the concentrations of various granule components and varying effective site pair affinities and RNA valency demonstrate that interactions between multivalent components (TOG and RNA) are modulated by a bivalent adaptor molecule (hnRNP A2). Formation and disruption of granules, as well as RNA selectivity in granule composition are regulated by distinct concentration regimes of A2. Our results suggest that granule assembly is tightly controlled by multivalent molecular interactions among RNA molecules, adaptor proteins, and scaffold proteins.

Nervous translation, do you get the message? A review of mRNPs, mRNA-protein interactions and translational control within cells of the nervous system

In neurons, translation of a message RNA can occur metres away from its transcriptional origin and in normal cells this is orchestrated with perfection. The life of an mRNA will see it pass through multiple steps of processing in the nucleus and the cytoplasm before it reaches its final destination. Processing of mRNA is determined by a myriad of RNA-binding proteins in multi-protein complexes called messenger ribonucleoproteins; however, incorrect processing and delivery of mRNA can cause several human neurological disorders. This review takes us through the life of mRNA from the nucleus to its point of translation in the cytoplasm. The review looks at the various cis and trans factors that act on the mRNA and discusses their roles in different cells of the nervous system and human disorders.

Molecular basis for the action of a dietary flavonoid revealed by the comprehensive identification of apigenin human targets

Flavonoids constitute the largest class of dietary phytochemicals, adding essential health value to our diet, and are emerging as key nutraceuticals. Cellular targets for dietary phytochemicals remain largely unknown, posing significant challenges for the regulation of dietary supplements and the understanding of how nutraceuticals provide health value. Here, we describe the identification of human cellular targets of apigenin, a flavonoid abundantly present in fruits and vegetables, using an innovative high-throughput approach that combines phage display with second generation sequencing. The 160 identified high-confidence candidate apigenin targets are significantly enriched in three main functional categories: GTPase activation, membrane transport, and mRNA metabolism/alternative splicing. This last category includes the heterogeneous nuclear ribonucleoprotein A2 (hnRNPA2), a factor involved in splicing regulation, mRNA stability, and mRNA transport. Apigenin binds to the C-terminal glycine-rich domain of hnRNPA2, preventing hnRNPA2 from forming homodimers, and therefore, it perturbs the alternative splicing of several human hnRNPA2 targets. Our results provide a framework to understand how dietary phytochemicals exert their actions by binding to many functionally diverse cellular targets. In turn, some of them may modulate the activity of a large number of downstream genes, which is exemplified here by the effects of apigenin on the alternative splicing activity of hnRNPA2. Hence, in contrast to small-molecule pharmaceuticals designed for defined target specificity, dietary phytochemicals affect a large number of cellular targets with varied affinities that, combined, result in their recognized health benefits.

From axon-glial signalling to myelination: the integrating role of oligodendroglial Fyn kinase

Central nervous system myelination requires recognition and signalling processes between neuronal axons and oligodendrocytes. Complex cellular rearrangements occur in myelination-competent oligodendrocytes requiring spatio-temporal control mechanisms. Although the molecular repertoire is becoming increasingly transparent, the signalling mechanisms governing myelination initiation are only poorly understood. The non-receptor tyrosine kinase Fyn has been implicated in axon-glial signal transduction and in several cellular processes required for oligodendrocyte maturation and myelination. Here, we review oligodendroglial Fyn signalling and discuss the role of Fyn in axon-glia interaction mediating myelination.

Heterogeneous nuclear ribonucleoprotein A2 is a common transcriptional coactivator in the nuclear transcription response to mitochondrial respiratory stress

Mitochondrial dysfunction and altered transmembrane potential initiate a mitochondrial respiratory stress response, also known as mitochondrial retrograde response, in a wide spectrum of cells. The mitochondrial stress response activates calcineurin, which regulates transcription factors, including a new nuclear factor-kappaB (NF-kappaB) pathway, different from the canonical and noncanonical pathways. In this study using a combination of small interfering RNA-mediated mRNA knock down, transcriptional analysis, and chromatin immunoprecipitation, we report a common mechanism for the regulation of previously established stress response genes Cathepsin L, RyR1, and Glut4. Stress-regulated transcription involves the cooperative interplay between NF-kappaB (cRel: p50), C/EBPdelta, cAMP response element-binding protein, and nuclear factor of activated T cells. We show that the functional synergy of these factors requires the stress-activated heterogeneous nuclear ribonucleoprotein (hnRNP) A2 as a coactivator. HnRNP A2 associates with the enhanceosome, mostly through protein-protein interactions with DNA-bound factors. Silencing of hnRNP A2 as well as other DNA binding signature factors prevents stress-induced transcriptional activation and reverses the invasiveness of mitochondrial DNA-depleted C2C12 cells. Induction of mitochondrial stress signaling by electron transfer chain inhibitors also involved hnRNPA2 activation. We describe a common mechanism of mitochondrial respiratory stress-induced activation of nuclear target genes that involves hnRNP A2 as a transcription coactivator.

Activation of oligodendroglial Fyn kinase enhances translation of mRNAs transported in hnRNP A2-dependent RNA granules

Central nervous system myelination requires the synthesis of large amounts of myelin basic protein (MBP) at the axon–glia contact site. MBP messenger RNA (mRNA) is transported in RNA granules to oligodendroglial processes in a translationally silenced state. This process is regulated by the trans-acting factor heterogeneous nuclear ribonucleoprotein (hnRNP) A2 binding to the cis-acting A2 response element (A2RE). Release of this repression of MBP mRNA translation is thus essential for myelination. Mice deficient in the Src family tyrosine kinase Fyn are hypomyelinated and contain reduced levels of MBP. Here, we identify hnRNP A2 as a target of activated Fyn in oligodendrocytes. We show that active Fyn phosphorylates hnRNP A2 and stimulates translation of an MBP A2RE–containing reporter construct. Neuronal adhesion molecule L1 binding to oligodendrocytes results in Fyn activation, which leads to an increase in hnRNP A2 phosphorylation. These results suggest that Fyn kinase activation results in the localized translation of MBP mRNA at sites of axon–glia contact and myelin deposition.

Multiplexed RNA trafficking in oligodendrocytes and neurons

In oligodendrocytes and neurons genetic information is transmitted from the nucleus to dendrites in the form of RNA granules. Here we describe how transport of multiple different RNA molecules in individual granules is analogous to the process of multiplexing in telecommunications. In both cases multiple messages are combined into a composite signal for transmission on a single carrier. Multiplexing provides a mechanism to coordinate local expression of ensembles of genes in myelin in oligodendrocytes and at synapses in neurons.

Multiplexed dendritic targeting of alpha calcium calmodulin-dependent protein kinase II, neurogranin, and activity-regulated cytoskeleton-associated protein RNAs by the A2 pathway

In neurons, many different RNAs are targeted to dendrites where local expression of the encoded proteins mediates synaptic plasticity during learning and memory. It is not known whether each RNA follows a separate trafficking pathway or whether multiple RNAs are targeted to dendrites by the same pathway. Here, we show that RNAs encoding alpha calcium calmodulin-dependent protein kinase II, neurogranin, and activity-regulated cytoskeleton-associated protein are coassembled into the same RNA granules and targeted to dendrites by the same cis/trans-determinants (heterogeneous nuclear ribonucleoprotein [hnRNP] A2 response element and hnRNP A2) that mediate dendritic targeting of myelin basic protein RNA by the A2 pathway in oligodendrocytes. Multiplexed dendritic targeting of different RNAs by the same pathway represents a new organizing principle for coordinating gene expression at the synapse.