MiR-122 is a liver-specific microRNA (miRNA) that binds to two sites (S1 and S2) on the 5′ untranslated region (UTR) of the hepatitis C virus (HCV) genome and promotes the viral life cycle. It positively affects viral RNA stability, translation, and replication, but the mechanism is not well understood. To unravel the roles of miR-122 binding at each site alone or in combination, we employed miR-122 binding site mutant viral RNAs, Hep3B cells (which lack detectable miR-122), and complementation with wild-type miR-122, an miR-122 with the matching mutation, or both. We found that miR-122 binding at either site alone increased replication equally, while binding at both sites had a cooperative effect. Xrn1 depletion rescued miR-122-unbound full-length RNA replication to detectable levels but not to miR-122-bound levels, confirming that miR-122 protects HCV RNA from Xrn1, a cytoplasmic 5′-to-3′ exoribonuclease, but also has additional functions. In cells depleted of Xrn1, replication levels of S1-bound HCV RNA were slightly higher than S2-bound RNA levels, suggesting that both sites contribute, but their contributions may be unequal when the need for protection from Xrn1 is reduced. miR-122 binding at S1 or S2 also increased translation equally, but the effect was abolished by Xrn1 knockdown, suggesting that the influence of miR-122 on HCV translation reflects protection from Xrn1 degradation. Our results show that occupation of each miR-122 binding site contributes equally and cooperatively to HCV replication but suggest somewhat unequal contributions of each site to Xrn1 protection and additional functions of miR-122. IMPORTANCE The functions of miR-122 in the promotion of the HCV life cycle are not fully understood. Here, we show that binding of miR-122 to each of the two binding sites in the HCV 5′ UTR contributes equally to HCV replication and that binding to both sites can function cooperatively. This suggests that active Ago2–miR-122 complexes assemble at each site and can cooperatively promote the association and/or function of adjacent complexes, similar to what has been proposed for translation suppression by adjacent miRNA binding sites. We also confirm a role for miR-122 in protection from Xrn1 and provide evidence that miR-122 has additional functions in the HCV life cycle unrelated to Xrn1. Finally, we show that each binding site may contribute unequally to Xrn1 protection and other miR-122 functions.

The hnRNP A/B family of proteins is canonically central to cellular RNA metabolism, but due to their highly conserved nature, the functional differences between hnRNP A1, A2/B1, A0, and A3 are often overlooked. In this review, we explore and identify the shared and disparate homeostatic and disease-related functions of the hnRNP A/B family proteins, highlighting areas where the proteins have not been clearly differentiated. Herein, we provide a comprehensive assembly of the literature on these proteins. We find that there are critical gaps in our grasp of A/B proteins’ alternative splice isoforms, structures, regulation, and tissue and cell-type-specific functions, and propose that future mechanistic research integrating multiple A/B proteins will significantly improve our understanding of how this essential protein family contributes to cell homeostasis and disease.

Neurodegeneration is the primary driver of disease progression in multiple sclerosis (MS) resulting in permanent disability, creating an urgent need to discover its underlying mechanisms. Herein, we establish that dysfunction of the RNA binding protein heterogeneous nuclear ribonucleoprotein A1 (hnRNP A1) results in differential of binding to RNA targets causing alternative RNA splicing, which contributes to neurodegeneration in MS and its models. Using RNAseq of MS brains, we discovered differential expression and aberrant splicing of hnRNP A1 target RNAs involved in neuronal function and RNA homeostasis. We confirmed this in vivo in experimental autoimmune encephalomyelitis employing CLIPseq specific for hnRNP A1, where hnRNP A1 differentially binds and regulates RNA, including aberrantly spliced targets identified in human samples. Additionally, dysfunctional hnRNP A1 expression in neurons caused neurite loss and identical changes in splicing, corroborating hnRNP A1 dysfunction as a cause of neurodegeneration. Collectively, these data indicate hnRNP A1 dysfunction causes altered neuronal RNA splicing, resulting in neurodegeneration in MS.

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