Regulation of neuronal RNA signatures by ELAV/Hu proteins

The RNA-binding proteins encoded by the highly conserved elav/Hu gene family, found in all metazoans, regulate the expression of a wide range of genes, at both the co-transcriptional and posttranscriptional level. Nervous-system-specific ELAV/Hu proteins are prominent for their essential role in neuron differentiation, and mutations have been associated with human neurodevelopmental and neurodegenerative diseases. Drosophila ELAV, the founding member of the protein family, mediates the synthesis of neuronal RNA signatures by promoting alternative splicing and alternative polyadenylation of hundreds of genes. The recent identification of ELAV's direct RNA targets revealed the protein's central role in shaping the neuronal transcriptome, and highlighted the importance of neuronal transcript signatures for neuron maintenance and organism survival. Animals have evolved multiple cellular mechanisms to ensure robustness of ELAV/Hu function. In Drosophila, elav autoregulates in a 3'UTR-dependent manner to maintain optimal protein levels. A complete absence of ELAV causes the activation and nuclear localization of the normally cytoplasmic paralogue FNE, in a process termed EXon-Activated functional Rescue (EXAR). Other species, including mammals, seem to utilize different strategies, such as protein redundancy, to maintain ELAV protein function and effectively safeguard the identity of the neuronal transcriptome. This article is categorized under: RNA Processing > 3' End Processing RNA in Disease and Development > RNA in Development RNA Interactions with Proteins and Other Molecules > Protein-RNA Interactions: Functional Implications.

Docetaxel-triggered SIDT2/NOX4/JNK/HuR signaling axis is associated with TNF-α-mediated apoptosis of cancer cells

Previous studies have confirmed that docetaxel (DTX) treatment increases TNF-α production in cancer cells, but its mechanism of action remains unclear. Therefore, this study aimed to determine the signaling axis by which DTX induced the expression of TNF-α in U937 leukemia and MCF-7 breast carcinoma cells. DTX treatment promoted Ca²⁺-controlled autophagy and SIDT2 expression, resulting in lysosomal degradation of miR-25 in U937 cells. Downregulation of miR-25 increased NOX4 mRNA stability and protein expression. NOX4-stimulated ROS generation led to JNK-mediated phosphorylation of cytosolic HuR at Ser221, thereby increasing TNF-α protein expression by stabilizing TNF-α mRNA. Consequently, DTX induced TNF-α-dependent death in U937 cells. Depletion of HuR using siRNA or abolishment of JNK activation reduced TNF-α expression and eliminated DTX-mediated cytotoxicity. Knockdown of SIDT2 or pretreatment with chloroquine (a lysosome inhibitor) reduced DTX-induced NOX4 and TNF-α expression and mitigated JNK-mediated HuR phosphorylation. Altogether, our data indicate that DTX triggers HuR-mediated TNF-α mRNA stabilization through the Ca²⁺/SIDT2/NOX4/ROS/JNK axis, thereby inducing TNF-α-dependent apoptosis in U937 cells. In addition, DTX induces apoptosis in MCF-7 cells through SIDT2/NOX4/JNK/HuR axis-mediated TNF-α expression.

Dynamically expressed single ELAV/Hu orthologue elavl2 of bees is required for learning and memory

Changes in gene expression are a hallmark of learning and memory consolidation. Little is known about how alternative mRNA processing, particularly abundant in neuron-specific genes, contributes to these processes. Prototype RNA binding proteins of the neuronally expressed ELAV/Hu family are candidates for roles in learning and memory, but their capacity to cross-regulate and take over each other's functions complicate substantiation of such links. Honey bees Apis mellifera have only one elav/Hu family gene elavl2, that has functionally diversified by increasing alternative splicing including an evolutionary conserved microexon. RNAi knockdown demonstrates that ELAVL2 is required for learning and memory in bees. ELAVL2 is dynamically expressed with altered alternative splicing and subcellular localization in mushroom bodies, but not in other brain regions. Expression and alternative splicing of elavl2 change during memory consolidation illustrating an alternative mRNA processing program as part of a local gene expression response underlying memory consolidation.

Autophagy Stimulus Promotes Early HuR Protein Activation and p62/SQSTM1 Protein Synthesis in ARPE-19 Cells by Triggering Erk1/2, p38MAPK, and JNK Kinase Pathways

RNA-binding protein dysregulation and altered expression of proteins involved in the autophagy/proteasome pathway play a role in many neurodegenerative disease onset/progression, including age-related macular degeneration (AMD). HuR/ELAVL1 is a master regulator of gene expression in human physiopathology. In ARPE-19 cells exposed to the proteasomal inhibitor MG132, HuR positively affects at posttranscriptional level p62 expression, a stress response gene involved in protein aggregate clearance with a role in AMD. Here, we studied the early effects of the proautophagy AICAR + MG132 cotreatment on the HuR-p62 pathway. We treated ARPE-19 cells with Erk1/2, AMPK, p38^MAPK, PKC, and JNK kinase inhibitors in the presence of AICAR + MG132 and evaluated HuR localization/phosphorylation and p62 expression. Two-hour AICAR + MG132 induces both HuR cytoplasmic translocation and threonine phosphorylation via the Erk1/2 pathway. In these conditions, p62 mRNA is loaded on polysomes and its translation in de novo protein is favored. Additionally, for the first time, we report that JNK can phosphorylate HuR, however, without modulating its localization. Our study supports HuR's role as an upstream regulator of p62 expression in ARPE-19 cells, helps to understand better the early events in response to a proautophagy stimulus, and suggests that modulation of the autophagy-regulating kinases as potential therapeutic targets for AMD may be relevant.

Protein Kinase C Activation as a Potential Therapeutic Strategy in Alzheimer's Disease: Is there a Role for Embryonic Lethal Abnormal Vision-like Proteins?

Alzheimer's disease (AD), the most common cause of dementia, is an irreversible and progressive neurodegenerative disorder. It affects predominantly brain areas that are critical for memory and learning and is characterized by two main pathological hallmarks: extracellular amyloid plaques and intracellular neurofibrillary tangles. Protein kinase C (PKC) has been classified as one of the cognitive kinases controlling memory and learning. By regulating several signalling pathways involved in amyloid and tau pathologies, it also plays an inhibitory role in AD pathophysiology. Among downstream targets of PKC are the embryonic lethal abnormal vision (ELAV)-like RNA-binding proteins that modulate the stability and the translation of specific target mRNAs involved in synaptic remodelling linked to cognitive processes. This MiniReview summarizes the current evidence on the role of PKC and ELAV-like proteins in learning and memory, highlighting how their derangement can contribute to AD pathophysiology. This last aspect emphasizes the potential of pharmacological activation of PKC as a promising therapeutic strategy for the treatment of AD.

Concentration and Localization of Coexpressed ELAV/Hu Proteins Control Specificity of mRNA Processing

Neuronally coexpressed ELAV/Hu proteins comprise a family of highly related RNA binding proteins which bind to very similar cognate sequences. How this redundancy is linked to in vivo function and how gene-specific regulation is achieved have not been clear. Analysis of mutants in Drosophila ELAV/Hu family proteins ELAV, FNE, and RBP9 and of genetic interactions among them indicates that they have mostly independent roles in neuronal development and function but have converging roles in the regulation of synaptic plasticity. Conversely, ELAV, FNE, RBP9, and human HuR bind ELAV target RNA in vitro with similar affinities. Likewise, all can regulate alternative splicing of ELAV target genes in nonneuronal wing disc cells and substitute for ELAV in eye development upon artificially increased expression; they can also substantially restore ELAV's biological functions when expressed under the control of the elav gene. Furthermore, ELAV-related Sex-lethal can regulate ELAV targets, and ELAV/Hu proteins can interfere with sexual differentiation. An ancient relationship to Sex-lethal is revealed by gonadal expression of RBP9, providing a maternal fail-safe for dosage compensation. Our results indicate that highly related ELAV/Hu RNA binding proteins select targets for mRNA processing through alteration of their expression levels and subcellular localization but only minimally by altered RNA binding specificity.