mRNA Export: Molecular Mechanisms, Disease Implications, and Therapeutic Opportunities

Nuclear export of messenger RNA (mRNA) is a vital step in eukaryotic gene expression, as it allows the transport of mRNA molecules from the nucleus to the cytoplasm, where they can be translated into proteins. mRNA export is not a simple passive process, but rather a highly regulated and coordinated one, involving multiple proteins and factors that interact with mRNA and the nuclear pore complex (NPC), the main gateway for nucleocytoplasmic trafficking. The regulation of mRNA export is essential for maintaining cellular function and homeostasis, as it affects mRNA quality, stability, localization, and translation.

Molecular Players and Pathways of mRNA Export

mRNA export is mediated by a complex network of proteins and factors that interact with mRNA molecules and the nuclear pore complex (NPC), the main gateway for nucleocytoplasmic trafficking. The major export receptors for mRNA are NXF1 and CRM1, which belong to the karyopherin family of transport factors. NXF1 is responsible for the bulk export of most mRNAs, while CRM1 exports a subset of mRNAs that contain specific nuclear export signals (NESs) in their coding or untranslated regions. Both NXF1 and CRM1 recognize their mRNA cargoes through adapter proteins that bind to specific RNA elements. For NXF1, the adapter protein is ALYREF, which binds to the exon junction complex (EJC) deposited on spliced mRNAs. For CRM1, the adapter proteins are various RNA-binding proteins that recognize NES-containing mRNAs, such as HuR, hnRNP A1, and NXF3. In addition to the export receptors and adapters, mRNA export also involves other proteins and factors that facilitate or regulate the process, such as TREX, THOC, NMD, and SR proteins. These factors play roles in mRNA processing, quality control, and export coordination, ensuring that only mature and functional mRNAs are exported to the cytoplasm. The mRNA export pathway can be divided into four main steps: mRNP assembly, docking, translocation, and release. In the first step, mRNP assembly, the mRNA molecules are packaged with various proteins and factors in the nucleus, forming export-competent mRNPs. In the second step, docking, the mRNPs are recruited to the NPC by interacting with the nuclear basket proteins, such as Nup98 and Nup153. In the third step, translocation, the mRNPs pass through the central channel of the NPC by interacting with the FG-repeat nucleoporins, such as Nup214 and Nup62. In the fourth step, release, the mRNPs are released from the NPC into the cytoplasm by the action of RanGTP and other factors, such as Gle1 and Dbp5. The molecular mechanisms of mRNA export are still not fully understood, and there are likely to be variations and exceptions to the general model described above. However, it is clear that mRNA export is a highly regulated and coordinated process that involves multiple players and pathways.

Fig.1 A brief overview of mRNA nuclear export. ¹

mRNA Export Dysregulation and Disease Pathogenesis

mRNA export is not only a crucial step for gene expression, but also a potential target for disease intervention. Dysregulation of mRNA export can lead to aberrant gene expression and cellular dysfunction and has been implicated in various diseases, such as cancer, viral infections, and neurodegenerative disorders.

Cancer is a complex and heterogeneous disease characterized by uncontrolled cell proliferation, survival, invasion, and metastasis. Alterations in mRNA export can contribute to cancer development and progression by affecting the expression and function of oncogenes, tumor suppressors, and other genes involved in cell cycle, apoptosis, angiogenesis, and immune response. Several mRNA export factors, such as NXF1, CRM1, ALYREF, and THOC, have been shown to be overexpressed, mutated, or deregulated in various types of cancers, such as breast, prostate, lung, colorectal, and hematological cancers. These factors can modulate the export and expression of specific mRNAs that are relevant for cancer biology, such as p53, c-Myc, Bcl-2, Cyclin D1, and VEGF. For example, NXF1 can promote the export and stability of c-Myc mRNA, a key oncogene that regulates cell growth, metabolism, and differentiation. CRM1 can export mRNAs that contain NESs, such as Bcl-2 mRNA, a pro-survival factor that inhibits apoptosis. ALYREF can facilitate the export of Cyclin D1 mRNA, a cell cycle regulator that promotes cell proliferation. THOC can enhance the export of VEGF mRNA, a growth factor that stimulates angiogenesis and tumor growth. In addition, mRNA export can also be affected by other factors that interact with or regulate mRNA export factors, such as microRNAs, post-translational modifications, and chromatin modifications.

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Viral infections are a major threat to human health and cause various diseases, such as AIDS, hepatitis, influenza, and COVID-19. Viruses are obligate intracellular parasites that depend on the host cell machinery for their replication and propagation. mRNA export is one of the host cell processes that viruses exploit or manipulate for their own benefit. Some viruses, such as retroviruses and herpesviruses, encode their own mRNA export factors that can hijack the host mRNA export pathway and preferentially export viral mRNAs over host mRNAs, resulting in enhanced viral gene expression and reduced host immune response. For example, HIV-1, the causative agent of AIDS, encodes a protein called Rev, which binds to a specific RNA element called RRE on viral mRNAs and recruits CRM1 to export them from the nucleus. HSV-1, the causative agent of herpes simplex, encodes a protein called ICP27, which binds to a specific RNA element called HSE on viral mRNAs and interacts with NXF1 and ALYREF to export them from the nucleus. Other viruses, such as influenza virus and coronavirus, do not encode their own mRNA export factors, but rather interfere with the host mRNA export pathway and inhibit the export of host mRNAs, resulting in reduced host gene expression and increased viral replication. For example, influenza virus, the causative agent of flu, encodes a protein called NS1, which binds to NXF1 and prevents its interaction with ALYREF and mRNA, thereby blocking the export of host mRNAs. SARS-CoV-2, the causative agent of COVID-19, encodes a protein called Nsp1, which binds to the mRNA cap and inhibits its recognition by NXF1 and other mRNA export factors, thereby inhibiting the export of host mRNAs.

Neurodegenerative disorders are a group of diseases that affect the structure and function of neurons, leading to progressive cognitive and motor impairments, such as Alzheimer's disease, Parkinson's disease, Huntington's disease, and amyotrophic lateral sclerosis (ALS). Dysregulation of mRNA export can contribute to neurodegeneration by affecting the expression and localization of neuronal proteins, such as synaptic proteins, cytoskeletal proteins, and RNA-binding proteins, that are essential for neuronal development, maintenance, and plasticity. Several mRNA export factors, such as NXF1, CRM1, ALYREF, and THOC, have been shown to be altered or mislocalized in neurodegenerative disorders, such as ALS, frontotemporal dementia (FTD), and Huntington's disease (HD). These factors can affect the export and expression of specific mRNAs that are relevant for neuronal function and survival, such as FUS, TDP-43, SOD1, and HTT. For example, FUS and TDP-43 are RNA-binding proteins that are involved in mRNA processing and export and are mutated or aggregated in ALS and FTD. FUS and TDP-43 can interact with NXF1 and ALYREF and regulate the export of mRNAs that encode synaptic proteins, such as PSD-95, GluR1, and NR1. SOD1 is an antioxidant enzyme that is mutated in familial ALS and forms toxic aggregates in the cytoplasm. SOD1 mRNA can be exported by CRM1 in a NES-dependent manner, and its export can be modulated by oxidative stress and other factors. HTT is a protein that is involved in various cellular functions, such as vesicle transport, gene transcription, and autophagy, and is mutated in HD. HTT mRNA can be exported by NXF1 and THOC, and its export can be affected by the length of the CAG repeat expansion in the coding region, which causes the production of a polyglutamine-expanded HTT protein that forms aggregates and causes neuronal toxicity.

Table 1. Summary of mRNA Export Dysregulation and Disease Pathogenesis

Therapeutic Opportunities and Challenges Targeting mRNA Export

Modulating mRNA export can affect the expression and function of specific genes that are involved in disease pathogenesis, thus providing therapeutic benefits for various diseases. Several therapeutic strategies have been developed or proposed to target mRNA export for disease treatment, such as small molecules, antisense oligonucleotides, RNA interference, and mRNA therapeutics. These strategies can be classified into two main categories: (1) inhibitors of mRNA export, which block or reduce the export of specific mRNAs or mRNA export factors, and (2) enhancers of mRNA export, which increase or facilitate the export of specific mRNAs or mRNA export factors.

Inhibitors of mRNA export are designed to interfere with the mRNA export pathway and prevent the export of specific mRNAs or mRNA export factors that are associated with disease pathogenesis. For example, inhibitors of CRM1, such as leptomycin B and selinexor, can block the export of CRM1-dependent mRNAs, such as Bcl-2, c-Myc, and HIV-1, and induce their nuclear retention and degradation, resulting in apoptosis, cell cycle arrest, and antiviral effects. Inhibitors of NXF1, such as netosis-inducing peptide and NXF1-targeting siRNA, can inhibit the export of NXF1-dependent mRNAs, such as c-Myc, Cyclin D1, and HTT, and induce their nuclear accumulation and degradation, resulting in cell death, cell cycle arrest, and neuroprotection. Inhibitors of other mRNA export factors, such as ALYREF, THOC, and Nup98, have also been developed or explored, such as ALYREF-targeting antisense oligonucleotides, THOC-targeting shRNA, and Nup98-targeting small molecules, which can modulate the export and expression of specific mRNAs that are relevant for cancer, viral infections, and neurodegeneration.

Enhancers of mRNA export are designed to enhance the mRNA export pathway and promote the export of specific mRNAs or mRNA export factors that are associated with disease treatment. For example, enhancers of FUS and TDP-43 export, such as FUS- and TDP-43-targeting antisense oligonucleotides, can increase the export of FUS and TDP-43 mRNAs and proteins and reduce their nuclear aggregation and toxicity, resulting in neuroprotection and improved motor function in ALS and FTD models. Enhancers of SOD1 export, such as SOD1-targeting antisense oligonucleotides, can increase the export of SOD1 mRNA and protein and reduce their cytoplasmic aggregation and toxicity, resulting in neuroprotection and improved survival in ALS models. Enhancers of mRNA therapeutics, such as lipid nanoparticles and polymeric nanoparticles, can facilitate the delivery and export of exogenous mRNA molecules that encode therapeutic proteins, such as vaccines, antibodies, and enzymes, resulting in enhanced gene expression and therapeutic effects for various diseases, such as COVID-19, cancer, and rare diseases.

Conclusion

mRNA export is a vital step in eukaryotic gene expression that involves multiple molecular players and pathways. Modulating mRNA export can affect the expression and function of specific genes that are involved in disease pathogenesis and thus provide therapeutic benefits for various diseases. Several therapeutic strategies have been developed or proposed to target mRNA export for disease treatment, such as small molecules, antisense oligonucleotides, RNA interference, and mRNA therapeutics. However, targeting mRNA export as a therapeutic approach also poses several challenges and limitations, such as specificity, efficacy, toxicity, delivery, and resistance. Therefore, more research and development are needed to understand the molecular mechanisms, disease implications, and therapeutic opportunities of mRNA export and to overcome the challenges and gaps in this field. mRNA export is a novel and promising target for disease intervention that deserves more attention and exploration.

Reference

1. Katahira, Jun. "Nuclear export of messenger RNA." Genes 6.2 (2015): 163-184.