MicroRNAs (miRNAs) are endogenous ~22 nt RNAs. miRNAs are traditionally considered primarily cytoplasmic molecules that regulate gene expression posttranscriptionally by targeting mRNAs for degradation or translational repression (Bartel, 2004 ). In the canonical pathway of biogenesis, pri-miRNAs are transcribed from DNA sequences inside the cell nucleus. Pri-miRNAs are then processed by Drosha into pre-miRNAs and exported to the cytoplasm via the exportin 5 (XPO5)/RanGTP complex. Pre-miRNAs are then processed into mature miRNA duplexes by the RNase III endonuclease Dicer (O’Brien et al., 2018 ). In the intricate world of molecular biology, the discovery of mature microRNA-34a (miR-34a) inside the nucleus of atypical rat kidney epithelial cells (karyomegalic cells) has ignited scientific curiosity, revealing its multifaceted roles in cellular processes. This perspective embarks on a journey, beginning with the groundbreaking work of Dutta et al. (2007 ), and culminating in the recent breakthrough by Ohno et al. (2022 ), shedding light on the association of miR-34a with cell proliferation and its exploration within the nuclear landscape. Other examples of mature miRNAs that have been reported to be present inside the nucleus include miR-21, miR-122, miR-223, and miR-29b (Billi et al., 2024 ).
Dutta et al. (2007 ) laid the foundation for our understanding of miR-34a by revealing its nuanced relationship with cell proliferation. Their pioneering research illuminated how dysregulation of miR-34a profoundly influences cell proliferation, prompting extensive investigations into its regulatory mechanisms and downstream effects.
Through meticulous in situ hybridization experiments, they revealed a striking pattern: while mature miR-34a exhibited robust expression in approximately 50% of human cancers, its presence was nearly negligible in many others. This finding underscores the context-specific nature of the impact of miR-34a, suggesting a sophisticated view of its functional role. The work of Dutta et al. (2007 ) not only expanded our comprehension of miR-34a but also set the stage for subsequent breakthroughs in the field.
Dutta et al. (2007 ) revealed the unexpected presence of mature miR-34a within the nucleus of rat kidney cells (karyomegalic cells) during their in situ microRNA detection experiment with rat kidney cells. Subsequently, Ohno et al. (2022 ) elucidated the involvement of nuclear miR-34a in facilitating the release of paused RNA polymerase II (Pol II) through the DDX21-CDK9 complex. These findings expand our understanding of miR-34a beyond its established role in the cytoplasm, revealing a novel layer of regulatory control within the nucleus. The identification of nuclear miR-34a has challenged the conventional notion that microRNAs predominantly function in the cytoplasm, opening avenues for further exploration.
The journey from Dutta et al. (2007 ) seminal observations of mature miR-34a in the nucleus to Ohno et al. (2022 )'s recent insights underscores the dynamic nature of miR-34a. The nuclear localization of miR-34a prompts intriguing questions about its specific targets and the regulatory networks it modulates within the nucleus. Recent studies suggest that nuclear miR-34a may regulate genes involved in crucial cellular processes, including DNA repair, cell cycle progression, and apoptosis. Its interaction with the DDX21-CDK9 complex may play a role in modulating the transcriptional landscape by facilitating the release of paused RNA Pol II. This was the first experimental demonstration that miR-34a functions in RNA activation (RNAa).
RNAa is a process in which small RNAs such as miRNAs upregulate gene expression rather than suppress it. This newfound dimension adds complexity to the functional implications of miR-34a, urging researchers to decipher the intricacies of its nuclear activities. Furthermore, the presence of miR-34a in the cytoplasmic, mitochondrial (Giuliani et al., 2018 ), and nuclear compartments suggests a sophisticated regulatory mechanism. Understanding the interplay among these locales and deciphering the signals governing the subcellular localization of miR-34a will offer valuable insights into its overall function.
A few noteworthy examples can be highlighted here. Numerous studies have demonstrated that nuclear miRNAs play crucial roles in regulating disorders associated with glycolipid metabolism. Specifically, miR-552-3p exacerbates hepatic glycolipid metabolic disorders by modulating the transcriptional activities of LXRα and FXR within the nucleus. This evidence supports the potential of miR-552-3p as a promising therapeutic target for combating metabolic diseases (Fan et al., 2021 ).
Hypoxia is a critical pathological condition associated with various diseases, such as myocardial infarction and cerebral ischemia, where it disrupts tissue function and triggers a rapid, intricate response at the molecular and cellular levels. miR-210-3p, a well-known hypoxamiR, is notably upregulated in the nuclear compartment following hypoxic stimulation. These findings reveal an additional layer of molecular complexity in the physiological response to ischemic tissue (Turunen et al., 2019 ).
Nuclear miRNAs have emerged as critical transcriptional regulators that influence a wide spectrum of cancer hallmarks, including proliferation, invasion, metastasis, migration, apoptosis, and angiogenesis. By targeting promoters or enhancers, these miRNAs can modulate gene expression and thereby drive tumorigenesis and progression. For example, miRNA-10a suppresses breast cancer development through its interaction with the homeobox D4 promoter, interfering with promoter-associated transcripts (Tan et al., 2009 ). Moreover, miR-339 inhibits breast cancer cell proliferation by upregulating the expression of the tumor suppressor G protein-coupled estrogen receptor 1 via enhancer switching, suggesting its potential as a therapeutic target, especially for triple-negative breast cancer (Liang et al., 2021 ). In lung cancer, miR-1236-3p and miR-370-5p collectively curtail cell proliferation, migration, and invasion by targeting the P21 gene promoter and consequently increasing its expression (Li et al., 2017 ).
As exploration of nuclear miR-34a has progressed, researchers have sought to delve deeper into the molecular mechanisms governing its nuclear functions. Elucidating its specific interactions and downstream effects within the nucleus will not only enrich our understanding of miR-34a but also reveal new therapeutic avenues that target its regulatory pathways. Identifying the direct targets of nuclear miR-34a and investigating the modulated genes and pathways will provide a comprehensive picture of its regulatory impact on cellular processes. Moreover, understanding the factors governing the nuclear and mitochondrial translocation of miR-34a will shed light on the intricate molecular machinery orchestrating its subcellular distribution. The dynamic nature of microRNAs, including miR-34a, presents exciting challenges, inspiring further exploration into their nuclear functions. Deciphering these intricacies will not only deepen our knowledge of gene regulation but also offer novel therapeutic interventions. Targeting nuclear miR-34a could emerge as a strategy for modulating transcriptional processes, presenting potential avenues for precision medicine in various pathological conditions.
The discovery of nuclear miR-34a represents a significant milestone in the evolving narrative of microRNA research. From its initial association with proliferation to its newfound role within the nucleus, miR-34a continues to captivate researchers with its multifaceted functions. This perspective highlights the transformative journey from discovery to functional exploration, emphasizing the need for continued investigation into the complex regulatory landscape of nuclear microRNAs. As we stand on the brink of a new era in microRNA research, the dual nature of miR-34a prompts researchers to unravel the mysteries within the nucleus. The interplay of the cytoplasmic, mitochondrial, and nuclear functions of miR-34a signifies an exciting frontier in molecular biology, which is poised to reshape our understanding of gene regulation and cellular dynamics. The ongoing narrative of miR-34a holds the potential to unlock novel therapeutic strategies and deepen our comprehension of the intricate molecular interactions within the cellular landscape.
Declaration by author
The authors' guidelines were used to generate the manuscript with the assistance of ChatGPT, an artificial intelligence program developed by OpenAI.
Acknowledgements
The author would like to express his sincere gratitude to Professor Shinya Toyokuni, MD, PhD, Kyoto University (present location, Nagoya University), Japan, for supporting, guiding and supervising the first discovery of mature microRNA-34a inside the nucleus of rat kidney cells.
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Conflict of interest
The author declare no competing interests.
Author contributions
Khokon Kumar Dutta contributed to the design and writing of this editorial.