Paraspeckles Are Associated with the Activation and Nuclear Localization of Unphosphorylated miR-34a.

Background/Objectives: Canonical microRNAs possess a 5' phosphate required for Argonaute binding and activity. However, prior work identified an unphosphorylated, inactive nuclear pool of the important radiation-responsive microRNA, miR-34, that is rapidly phosphorylated and activated in response to ionizing radiation (IR). Here, we extend this work and investigate the role of paraspeckles, a phase-separated nuclear sub-compartment, and their association with the localization of unphosphorylated miR-34a. Methods: Mass spectrometry was performed to identify interacting partners of unphosphorylated mir-34. CRISPR-mediated deletion of the paraspeckle NEAT1_2 triple helix motif was performed to create an A549 cell line lacking paraspeckles (dTH). Activity and expression of mir-34a post-irradiation were evaluated by qRT-PCR and luciferase assays comparing dTH and wild-type (WT) A549 cell lines. In situ hybridization (ISH) was performed to evaluate mir-34a localization before and after IR, comparing dTH and WT cell lines. Results: Mass spectrometry identified paraspeckle proteins as significantly enriched interacting partners of unphosphorylated mir-34 mimics. By qRT-PCR and luciferase assays, we found that paraspeckle loss prevented radiation-induced early activation of unphosphorylated mir-34a. We found no difference in radiation-induced transcription of pri-miR-34a, but early processing to pre-miR-34a appeared delayed. ISH confirmed that loss of paraspeckles altered the nuclear localization of miR-34a before and after IR. Conclusions: These data suggest that paraspeckles are associated with nuclear localization and early radiation-responsive activation of unphosphorylated miR-34a. This suggests a coordinated nuclear sequestration of this important miR in its unphosphorylated state to enable an enhanced radiation response.