The vast majority of eukaryotic messenger RNAs (mRNAs) initiate translation through a canonical, cap-dependent mechanism requiring a free 5' end and 5' cap and several initiation factors to form a translationally active ribosome. Stresses such as hypoxia, apoptosis, starvation, and viral infection down-regulate cap-dependent translation during which alternative mechanisms of translation initiation prevail to express proteins required to cope with the stress, or to produce viral proteins. The diversity of noncanonical initiation mechanisms encompasses a broad range of strategies and cellular cofactors. Herein, we provide an overview and, whenever possible, a mechanistic understanding of the various noncanonical mechanisms of initiation used by cells and viruses. Despite many unanswered questions, recent advances have propelled our understanding of the scope, diversity, and mechanisms of alternative initiation.

Inflammation plays a prominent role in the events following traumatic injury to the central nervous system (CNS). The initial inflammatory response is driven by mediators such as tumor necrosis factor α and interleukin 1β, which are produced by activated astrocytes and microglia at the site of injury. These factors are regulated post-transcriptionally by RNA binding proteins (RBP) that interact with adenylate and uridylate-rich elements (ARE) in the 3'-untranslated region of the messenger RNA (mRNA). Human antigen R (HuR) is one of these RBPs and generally functions as a positive regulator of ARE-containing mRNAs. Here, we hypothesized that HuR plays an important role in the induction of cytokine and chemokines in astrocytes following traumatic injury. Using a mouse model of spinal cord injury, we found HuR to be extensively translocated to the cytoplasm in astrocytes at the level of injury, consistent with its activation. In an in vitro stretch injury model of CNS trauma, we observed a similar cytoplasmic shift of HuR in astrocytes and an attenuation of cytokine induction with HuR knockdown. RNA kinetics and luciferase assays suggested that the effect was more related to transcription than RNA destabilization. A small molecule inhibitor of HuR suppressed cytokine induction of injured astrocytes and reduced chemoattraction for neutrophils and microglia. In summary, HuR is activated in astrocytes in the early stages of CNS trauma and positively regulates the molecular response of key inflammatory mediators in astrocytes. Our findings suggest that HuR may be a therapeutic target in acute CNS trauma for blunting secondary tissue injury triggered by the inflammatory response.

Background: RNA processing abnormalities have been linked to amyotrophic lateral sclerosis (ALS). Results: HuR promotes the expression of the ALS-associated TDP-43 and FUS/TLS RNA-binding proteins. Conclusion: Through its regulation of TDP-43 and FUS/TLS, HuR potentially impacts a wide range of molecular and cellular phenotypes. Significance: A network of proteins exists to maintain RNA processing, and ALS-associated abnormalities may stem from disruption of this network.

Glial activation with the production of pro‐inflammatory mediators is a major driver of disease progression in neurological processes ranging from acute traumatic injury to chronic neurodegenerative diseases such as amyotrophic lateral sclerosis and Alzheimer's disease. Posttranscriptional regulation is a major gateway for glial activation as many mRNAs encoding pro‐inflammatory mediators contain adenine‐ and uridine‐rich elements (ARE) in the 3′ untranslated region which govern their expression. We have previously shown that HuR, an RNA regulator that binds to AREs, plays a major positive role in regulating inflammatory cytokine production in glia. HuR is predominantly nuclear in localization but translocates to the cytoplasm to exert a positive regulatory effect on RNA stability and translational efficiency. Homodimerization of HuR is necessary for translocation and we have developed a small molecule inhibitor, SRI‐42127, that blocks this process. Here we show that SRI‐42127 suppressed HuR translocation in LPS‐activated glia in vitro and in vivo and significantly attenuated the production of pro‐inflammatory mediators including IL1β, IL‐6, TNF‐α, iNOS, CXCL1, and CCL2. Cytokines typically associated with anti‐inflammatory effects including TGF‐β1, IL‐10, YM1, and Arg1 were either unaffected or minimally affected. SRI‐42127 suppressed microglial activation in vivo and attenuated the recruitment/chemotaxis of neutrophils and monocytes. RNA kinetic studies and luciferase studies indicated that SRI‐42127 has inhibitory effects both on mRNA stability and gene promoter activation. In summary, our findings underscore HuR's critical role in promoting glial activation and the potential for SRI‐42127 and other HuR inhibitors for treating neurological diseases driven by this activation.

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