Poly(A) RNA-binding proteins and polyadenosine RNA: new members and novel functions

PABPN1 Couples the Polyadenylation and Translation of Maternal Transcripts to Mouse Oocyte Meiotic Maturation

During oocyte meiosis, maternal transcript polyadenylation is crucial for regulating mRNA stability and translation, which are essential for oocyte maturation. Polyadenylate-binding protein nuclear 1 (PABPN1) plays a key role in regulating mRNA splicing and polyadenylation in somatic cells and growing oocytes. However, its potential function in regulating the meiotic maturation of fully grown oocytes remains unknown. This study reports that selective Pabpn1 knockout in growing mouse oocytes using Zp3-Cre do not affect folliculogenesis but prevented germinal vesicle breakdown in fully grown oocytes, impaired CDK1 activation, and resulted in abnormal spindle formation and chromosome misalignment. The results of poly(A)-inclusive full-length RNA isoform sequencing (PAIso-seq) and transcriptome sequencing revealed that PABPN1 coordinates meiotic maturation-coupled polyadenylation and degradation of maternal mRNAs, which are key factors of maturation-promoting factor (MPF) and deadenylation mediators, such as B-cell translocation gene-4 (BTG4), ensuring proper meiotic progression. The results of rescue experiments indicate these functions of PABPN1 are mediated by its key domains, which interact with poly(A) polymerase and recruit target mRNAs. This study highlighted the physiological importance of cytoplasmic PABPN1 in mammalian oocyte maturation by integrating maternal transcript polyadenylation, translation, and degradation.

RNA Binding to CCRRM of PABPN1 Induces Conformation Change

Poly(A) Binding Protein Nuclear 1 (PABPN1) is a nuclear poly(A)-binding protein that is highly conserved in eukaryotes. It plays multifaceted roles in RNA processing and metabolism, with its dysregulation closely linked to various diseases. PABPN1 contains an alanine-rich N-terminus, a central coiled-coil domain (CC), a conserved RNA recognition motif (RRM) and a C-terminal extension. PABPN1 influences mRNA splicing and stability through its RNA-binding capabilities, thereby modulating gene expression. While PABPN1 is known to interact with RNA, the molecular mechanism underlying this interaction with RNA awaits further investigation. Here, we designed and purified a PABPN1 fragment encompassing the RNA-binding domain (CCRRM fragment, amino acids 114-254). Using a combination of 3D modeling, small-angle X-ray scattering (SAXS) and selective 2'-hydroxyl acylation analyzed by primer extension (SHAPE) assay, our result indicated that CCRRM exhibits a high affinity for poly(A) RNA, a moderate affinity for GU-rich and CU-rich sequences, and negligible binding to AU-rich and CA-rich sequences. RNA binding induces conformation change in the CC. These results suggest that PABPN1 could potentially be involved in cytoplasmic polyadenylation and may influence the regulation of mRNA translation and degradation, although further investigation is required to confirm this role.

The nuclear poly(A)-binding protein Pab2/PABPN1 promotes heterochromatin assembly through the formation of Pab2 nuclear condensates

The assembly of constitutive heterochromatin is a prerequisite for maintaining genome stability. However, the mechanism of heterochromatin formation has yet to be completely understood. Here, we demonstrate a crucial role of the nuclear poly(A)-binding protein (PABP) Pab2/PABPN1 in promoting constitutive heterochromatin formation in the fission yeast Schizosaccharomyces japonicus. Histone H3 Lys 9 di- and tri-methylation, hallmarks of heterochromatin, are significantly reduced at centromeres in the absence of Pab2. Pab2 forms nuclear condensates through its RNA-recognition motif (RRM) and the intrinsically disordered domain (IDR), both of which bind to centromeric non-coding RNAs. Intriguingly, two key heterochromatin factors, the histone H3 Lys9 methyltransferase Clr4 and the Mi2-type chromatin remodeler Mit1, associate with centromeres in a Pab2-dependent manner. Pab2 interacts with two putative RNA-binding proteins, the ZC3H3 ortholog Red5 and the RBM26·27 ortholog Rmn1, both essential for heterochromatin formation. Deletion of the Pab2 N-terminal region, which disrupts this interaction, largely abolishes Pab2 function, underscoring the importance of this complex. Pab2 also associates and colocalizes with Ppn1 (a PPP1R10 ortholog), a component of the cleavage and polyadenylation specificity factor (CPSF) complex, and ppn1 mutations disrupt constitutive heterochromatin. Notably, both Ppn1 and Rmn1 are able to interact with Clr4. Our findings reveal that Pab2 plays a pivotal role in heterochromatin assembly by forming nuclear condensates through its RRM/IDR, and Pab2 condensates facilitate the recruitment of Clr4 and Mit1 to centromeres, potentially through its binding proteins, Ppn1 and Rmn1. This study provides new insights into the mechanisms underlying heterochromatin formation and highlights the importance of RNA-binding proteins and phase separation in this process.

Antagonistic roles by the conserved nuclear poly(A)-binding proteins PABPN1 and ZC3H14 in nuclear RNA surveillance

Most eukaryotic genomes are transcribed pervasively, thereby producing an array of long non-coding RNAs (lncRNAs) in addition to protein-coding mRNAs. A large fraction of these lncRNAs is targeted by polyadenylation-dependent decay via the poly(A)-binding protein nuclear 1 (PABPN1) and the RNA exosome. Yet, how PABPN1 contributes to nuclear RNA surveillance by facilitating lncRNA turnover by the RNA exosome remains largely unclear. Here, we show that PABPN1 is important for the nuclear retention of polyadenylated lncRNAs, such that PABPN1 loss of function allows target lncRNAs to evade nuclear decay, leading to cytoplasmic accumulation. Interestingly, we found that another nuclear PABP, ZC3H14, functions antagonistically to PABPN1 and the poly(A)-tail exosome targeting (PAXT) connection in the control of nuclear lncRNA turnover. Collectively, our findings disclose the critical interplay between two conserved nuclear PABPs, PABPN1 and ZC3H14, in RNA surveillance via the control of nuclear RNA export.

Comprehensive analysis of liquid-liquid phase separation propensities of HSV-1 proteins and their interaction with host factors

In recent years, it has been shown that the liquid-liquid phase separation (LLPS) of virus proteins plays a crucial role in their life cycle. It promotes the formation of viral replication organelles, concentrating viral components for efficient replication and facilitates the assembly of viral particles. LLPS has emerged as a crucial process in the replication and assembly of herpes simplex virus-1 (HSV-1). Recent studies have identified several HSV-1 proteins involved in LLPS, including the myristylated tegument protein UL11 and infected cell protein 4; however, a complete proteome-level understanding of the LLPS-prone HSV-1 proteins is not available. We provide a comprehensive analysis of the HSV-1 proteome and explore the potential of its proteins to undergo LLPS. By integrating sequence analysis, prediction algorithms and an array of tools and servers, we identified 10 HSV-1 proteins that exhibit high LLPS potential. By analysing the amino acid sequences of the LLPS-prone proteins, we identified specific sequence motifs and enriched amino acid residues commonly found in LLPS-prone regions. Our findings reveal a diverse range of LLPS-prone proteins within the HSV-1, which are involved in critical viral processes such as replication, transcriptional regulation and assembly of viral particles. This suggests that LLPS might play a crucial role in facilitating the formation of specialized viral replication compartments and the assembly of HSV-1 virion. The identification of LLPS-prone proteins in HSV-1 opens up new avenues for understanding the molecular mechanisms underlying viral pathogenesis. Our work provides valuable insights into the LLPS landscape of HSV-1, highlighting potential targets for further experimental validation and enhancing our understanding of viral replication and pathogenesis.

ZC3H14 facilitates backsplicing by binding to exon-intron boundary and 3' UTR

Circular RNAs (circRNAs) are natural outputs of eukaryotic transcription and RNA processing and have emerged as critical regulators in physiology and diseases. Although multiple cis-elements and trans-factors are reported to modulate the backsplicing of circRNA biogenesis, most of these regulations play roles in flanking introns of circRNAs. Here, using a genome-wide CRISPR knockout screen, we have identified an evolutionarily conserved RNA-binding protein ZC3H14 in regulating circRNA biogenesis. ZC3H14 binds to 3' and 5' exon-intron boundaries and 3' UTRs of cognate mRNAs to promote circRNA biogenesis through dimerization and the association with spliceosome. Yeast knockout of the ZC3H14 ortholog Nab2 has significantly lower levels of circRNAs. Zc3h14-/- mice exhibit disrupted spermatogenesis and reduced testicular circRNA levels. Additionally, expression levels of human ZC3H14 are associated with non-obstructive azoospermia. Our findings reveal a conserved requirement for ZC3H14 in the modulation of backsplicing and link ZC3H14 and circRNA biogenesis to male fertility.

Cellular RNA-binding proteins LARP4 and PABPC1 synergistically facilitate viral translation of coronavirus PEDV

Coronaviruses are causing epizootic diseases and thus are a substantial threat for both domestic and wild animals. These viruses depend on the host translation machinery to complete their life cycle. The current paper identified cellular RNA-binding proteins (RBPs), La-related protein 4 (LARP4) and polyadenylate-binding protein cytoplasmic 1 (PABPC1), as critical regulators of efficient translation of the coronavirus porcine epidemic diarrhea virus (PEDV) mRNA. In Vero cells, PEDV infection caused LARP4 to migrate from the nucleus to the cytoplasm in a chromosome region maintenance1 (CRM1)-independent pathway. In the absence of the nuclear export signal of LARP4, viral translation was not promoted by LARP4. A further study unveiled that the cytoplasmic LARP4 binds to the 3'-terminal untranslated region (3'UTR) of PEDV mRNA with the assistance of PABPC1 to facilitate viral translation. LARP4 knockdown reduced the promotion of the PABPC1-induced 3'UTR translation activity. Moreover, the rabbit reticulocyte lysate (RRL) system revealed that the prokaryotic expressed protein LARP4 and PABPC1 enhance PEDV mRNA translation. To our knowledge, this is the first study demonstrating that PEDV induces nucleo-cytoplasmic shuttling of LARP4 to enhance its own replication, which broadens our insights into how viruses use host's RBPs for the efficient translation of viral mRNA.

PAIP1 binds to pre-mRNA and regulates alternative splicing of cancer pathway genes including VEGFA

BACKGROUND

Poly (A) binding protein interacting protein 1 (PAIP1) has been shown to causally contribute to the development and progression of cancer. However, the mechanisms of the PAIP1 regulation in tumor cells remain poorly understood.

RESULTS

Here, we used a recently developed UV cross-linking and RNA immunoprecipitation method (iRIP-seq) to map the direct and indirect interaction sites between PAIP1 and RNA on a transcriptome-wide level in HeLa cells. We found that PAIP1 not only binds to 3'UTRs, but also to pre-mRNAs/mRNAs with a strong bias towards the coding region and intron. PAIP1 binding sites are enriched in splicing enhancer consensus GA-rich motifs. RNA-seq analysis revealed that PAIP1 selectively modulates the alternative splicing of genes in some cancer hallmarks including cell migration, the mTOR signaling pathway and the HIF-1 signaling pathway. PAIP1-regulated alternative splicing events were strongly associated with PAIP1 binding, demonstrating that the binding may promote selection of the nearby splice sites. Deletion of a PAIP1 binding site containing seven repeats of GA motif reduced the PAIP1-mediated suppression of the exon 6 inclusion in a VEGFA mRNA isoform. Proteomic analysis of the PAIP1-interacted proteins revealed the enrichment of the spliceosome components and splicing factors.

CONCLUSIONS

These findings suggest that PAIP1 is both a polyadenylation and alternative splicing regulator, that may play a large role in RNA processing via its role in alternative splicing regulation.

Fluorogenic RNA-based biomaterials for imaging and tracking the cargo of extracellular vesicles

Extracellular vesicles (EVs), or exosomes, play important roles in physiological and pathological cellular communication and have gained substantial traction as biological drug carriers. EVs contain both short and long non-coding RNAs that regulate gene expression and epigenetic processes. To fully capitalize on the potential of EVs as drug carriers, it is important to study and understand the intricacies of EV function and EV RNA-based communication. Here we developed a genetically encodable RNA-based biomaterial, termed EXO-Probe, for tracking EV RNAs. The EXO-Probe comprises an EV-loading RNA sequence (EXO-Code), fused to a fluorogenic RNA Mango aptamer for RNA imaging. This fusion construct allowed the visualization and tracking of EV RNA and colocalization with markers of multivesicular bodies; imaging RNA within EVs, and non-destructive quantification of EVs. Overall, the new RNA-based biomaterial provides a useful and versatile means to interrogate the role of EVs in cellular communication via RNA trafficking to EVs and to study cellular sorting decisions. The system will also help lay the foundation to further improve the therapeutic efficacy of EVs as drug carriers.

From the disruption of RNA metabolism to the targeting of RNA-binding proteins: The case of polyglutamine spinocerebellar ataxias

Polyglutamine spinocerebellar ataxias (PolyQ SCAs) represent a group of monogenetic diseases in which the expanded polyglutamine repeats give rise to a mutated protein. The abnormally expanded polyglutamine protein produces aggregates and toxic species, causing neuronal dysfunction and neuronal death. The main symptoms of these disorders include progressive ataxia, motor dysfunction, oculomotor impairment, and swallowing problems. Nowadays, the current treatments are restricted to symptomatic alleviation, and no existing therapeutic strategies can reduce or stop the disease progression. Even though the origin of these disorders has been associated with polyglutamine-induced toxicity, RNA toxicity has recently gained relevance in polyQ SCAs molecular pathogenesis. Therefore, the research's focus on RNA metabolism has been increasing, especially on RNA-binding proteins (RBPs). The present review summarizes RNA metabolism, exposing the different processes and the main RBPs involved. We also explore the mechanisms by which RBPs are dysregulated in PolyQ SCAs. Finally, possible therapies targeting the RNA metabolism are presented as strategies to reverse neuropathological anomalies and mitigate physical symptoms.

Regulated circRNA nuclear export in neuronal differentiation

Multiple mechanisms have been reported for how circular RNAs (circRNAs) are exported to the cytoplasm. A recent paper by Cao et al. shows that export of a subset of circRNAs with (A)-rich motifs, including one with a clear function, is regulated during neuronal development via a novel mechanism.

The RNA-binding protein Nab2 regulates levels of the RhoGEF Trio to govern axon and dendrite morphology

The Drosophila RNA-binding protein (RBP) Nab2 acts in neurons to regulate neurodevelopment and is orthologous to the human intellectual disability-linked RBP, ZC3H14. Nab2 governs axon projection in mushroom body neurons and limits dendritic arborization of class IV sensory neurons in part by regulating splicing events in ∼150 mRNAs. Analysis of the Sex-lethal (Sxl) mRNA revealed that Nab2 promotes an exon-skipping event and regulates m6A methylation on Sxl pre-mRNA by the Mettl3 methyltransferase. Mettl3 heterozygosity broadly rescues Nab2null phenotypes implying that Nab2 acts through similar mechanisms on other RNAs, including unidentified targets involved in neurodevelopment. Here, we show that Nab2 and Mettl3 regulate the removal of a 5'UTR (untranslated region) intron in the trio pre-mRNA. Trio utilizes two GEF domains to balance Rac and RhoGTPase activity. Intriguingly, an isoform of Trio containing only the RhoGEF domain, GEF2, is depleted in Nab2null nervous tissue. Expression of Trio-GEF2 rescues projection defects in Nab2null axons and dendrites, while the GEF1 Rac1-regulatory domain exacerbates these defects, suggesting Nab2-mediated regulation Trio-GEF activities. Collectively, these data indicate that Nab2-regulated processing of trio is critical for balancing Trio-GEF1 and -GEF2 activity and show that Nab2, Mettl3, and Trio function in a common pathway that shapes axon and dendrite morphology.

Energy stress-induced circDDX21 promotes glycolysis and facilitates hepatocellular carcinogenesis

Cancer cells undergo metabolic reprogramming in response to hostile microenvironments, such as energy stress; however, the underlying mechanisms remain largely unclear. It is also unknown whether energy stress-responsive circular RNA (circRNA) is involved in the regulation of glucose metabolism. Here we report that circDDX21 is upregulated in response to glucose deprivation by the transcription factor c-Myc. Functionally, circDDX21 is shown to promote glycolysis by increasing PGAM1 expression. Mechanistically, circDDX21 interacts with the RNA binding protein PABPC1, disrupting its association with the ubiquitin E3 ligase MKRN3. This disassociation attenuates MKRN3-mediated PABPC1 ubiquitination and enhances the binding of PABPC1 to PGAM1 mRNA, thereby leading to PGAM1 mRNA stabilization. The ability of the circDDX21-PGAM1 axis to promote hepatocellular carcinogenesis is validated in a xenograft mouse model. Additionally, in clinical hepatocellular carcinoma tissues, there is a positive correlation between circDDX21 and PGAM1 expression. These findings establish circDDX21 as an important regulator of glycolysis and suggest circDDX21 as a potential therapeutic target for hepatocellular carcinoma.

Engineered mRNA Delivery Systems for Biomedical Applications

Messenger RNA (mRNA)-based therapeutic strategies have shown remarkable promise in preventing and treating a staggering range of diseases. Optimizing the structure and delivery system of engineered mRNA has greatly improved its stability, immunogenicity, and protein expression levels, which has led to a wider range of uses for mRNA therapeutics. Herein, a thorough analysis of the optimization strategies used in the structure of mRNA is first provided and delivery systems are described in great detail. Furthermore, the latest advancements in biomedical engineering for mRNA technology, including its applications in combatting infectious diseases, treating cancer, providing protein replacement therapy, conducting gene editing, and more, are summarized. Lastly, a perspective on forthcoming challenges and prospects concerning the advancement of mRNA therapeutics is offered. Despite these challenges, mRNA-based therapeutics remain promising, with the potential to revolutionize disease treatment and contribute to significant advancements in the biomedical field.

Poly(A) tale: From A to A; RNA polyadenylation in prokaryotes and eukaryotes

Most eukaryotic mRNAs and different non-coding RNAs undergo a form of 3' end processing known as polyadenylation. Polyadenylation machinery is present in almost all organisms except few species. In bacteria, the machinery has evolved from PNPase, which adds heteropolymeric tails, to a poly(A)-specific polymerase. Differently, a complex machinery for accurate polyadenylation and several non-canonical poly(A) polymerases are developed in eukaryotes. The role of poly(A) tail has also evolved from serving as a degradative signal to a stabilizing modification that also regulates translation. In this review, we discuss poly(A) tail emergence in prokaryotes and its development into a stable, yet dynamic feature at the 3' end of mRNAs in eukaryotes. We also describe how appearance of novel poly(A) polymerases gives cells flexibility to shape poly(A) tail. We explain how poly(A) tail dynamics help regulate cognate RNA metabolism in a context-dependent manner, such as during oocyte maturation. Finally, we describe specific mRNAs in metazoans that bear stem-loops instead of poly(A) tails. We conclude with how recent discoveries about poly(A) tail can be applied to mRNA technology. This article is categorized under: RNA Evolution and Genomics > RNA and Ribonucleoprotein Evolution RNA Processing > 3' End Processing RNA Turnover and Surveillance > Regulation of RNA Stability.

Dysregulation of RNA-Exosome machinery is directly linked to major cancer hallmarks in prostate cancer: Oncogenic role of PABPN1

Novel biomarkers and therapeutic strategies for prostate-cancer (PCa) are required to overcome its lethal progression. The dysregulation/implication of the RNA-Exosome-complex (REC; cellular machinery controlling the 3'-5'processing/degradation of most RNAs) in different cancer-types, including PCa, is poorly known. Herein, different cellular/molecular/preclinical approaches with human PCa-samples (tissues and/or plasma of 7 independent cohorts), and in-vitro/in-vivo PCa-models were used to comprehensively characterize the REC-profile and explore its role in PCa. Moreover, isoginkgetin (REC-inhibitor) effects were evaluated on PCa-cells. We demonstrated a specific dysregulation of the REC-components in PCa-tissues, identifying the Poly(A)-Binding-Protein-Nuclear 1 (PABPN1) factor as a critical regulator of major cancer hallmarks. PABPN1 is consistently overexpressed in different human PCa-cohorts and associated with poor-progression, invasion and metastasis. PABPN1 silencing decreased relevant cancer hallmarks in multiple PCa-models (proliferation/migration/tumourspheres/colonies, etc.) through the modulation of key cancer-related lncRNAs (PCA3/FALEC/DLEU2) and mRNAs (CDK2/CDK6/CDKN1A). Plasma PABPN1 levels were altered in patients with metastatic and tumour-relapse. Finally, pharmacological inhibition of REC-activity drastically inhibited PCa-cell aggressiveness. Altogether, the REC is drastically dysregulated in PCa, wherein this novel molecular event/mechanism, especially PABPN1 alteration, may be potentially exploited as a novel prognostic and therapeutic tool for PCa.

PABPN1 functions as a predictive biomarker in colorectal carcinoma

PURPOSE

PABPN1 acts as a modulator of poly(A) tail length and alternative polyadenylation. This research was aimed to explore the role of PABPN1 in colorectal cancer (CRC).

METHODS

Public databases were performed to analyze expression, location, roles of prognosis and tumor immunity and interaction with RNAs and proteins of PABPN1. To investigate PABPN1 expression in tissues, 78 CRC specimens were collected to conduct IHC, and 30 pairs of frozen CRC and corresponding adjacent normal tissues were used to conduct qRT-PCR and WB. In addition, in vitro experiments were then carried out to identify the role of PABPN1 in CRC.

RESULTS

Compared with normal tissues, PABPN1 expression was significant higher in CRC. Its high level predicted poor outcome of CRC. Th1 and Treg had significant negative relationships not only with PABPN1 expression, but also with six molecules interacting with PABPN1, including IFT172, KIAA0895L, RECQL4, WDR6, PABPC1 and NCBP1. In addition, PABPN1 had negative relationships with quite a few immune markers, such as CSF1R, IL-10, CCL2 and so on. In cellular experiments, silencing PABPN1 inhibited proliferation and promoted apoptosis in HCT-116 CRC cells.

CONCLUSION

In summary, PABPN1 might become a novel biomarker and correlate with tumor immunity in CRC.

Identification of nonfunctional PABPC1L causing oocyte maturation abnormalities and early embryonic arrest in female primary infertility

Oocyte maturation arrest, fertilization failure, and early embryonic arrest are important causes of female infertility, whereas the genetic events that contribute to these processes are largely unknown. Loss-of-function of PABPC1L in mice has been suggested to cause female infertility involved in the absence of mature oocytes or embryos in vivo or in vitro. However, the role of PABPC1L in human female reproduction remains largely elusive. In this study, we identified a homozygous missense mutation (c.536G>A, p.R179Q) and a compound heterozygous mutation (c.793C>T, p.R265W; c.1201C>T, p.Q401*) in PABPC1L in two unrelated infertile females characterized by recurrent oocyte maturation abnormalities and early embryonic arrest. These variants resulted in nonfunctional PABPC1L protein and were associated with impaired chromatin configuration and transcriptional silencing in GV oocytes. Moreover, the binding capacity of mutant PABPC1L to mRNAs related to oocyte maturation and early embryonic development was decreased significantly. Our findings revealed novel PABPC1L mutations causing oocyte maturation abnormalities and early embryonic arrest, confirming the essential role of PABPC1L in human female fertility.

PEDV N protein capture protein translation element PABPC1 and eIF4F to promote viral replication

Porcine epidemic diarrhea (PED) is an acute, highly infectious intestinal disease caused by the porcine epidemic diarrhea virus (PEDV), which seriously endangers the healthy development of the pig industry. PEDV N protein is the most abundant viral structural protein, which can be combined with viral genomic RNA to form ribonucleoprotein complexes, thereby participating in the transcription and replication of the virus. However, how PEDV hijacks the host transcription translation system to promote viral proliferation remains unclear. In this study, we found that there is an interaction between PEDV N, polyadenylate-binding protein cytoplasmic 1 (PABPC1) and eukaryotic initiation factor 4F (eIF4F) proteins through coimmunoprecipitation, GST pulldown and fluorescence microscopy experiments. PABPC1 could bind to the poly(A) tail of the mRNA, and eIF4F could bind to the 5' end cap structure of the mRNA, so the interaction of PABPC1 and eIF4F could facilitate mRNA forming a circular shape to promote translation to the proteins. To further explore the effect of N protein capture protein translation element PABPC1 and eIF4F on PEDV replication, we overexpressed PABPC1, eIF4F (containing eIF4A, eIF4E and eIF4G) separately on Vero cells and LLC-PK1 cells, and we found that the PABPC1 and eIF4F protein could promote PEDV replication. Taken together, our data suggested that PEDV N protein promoted cyclization of viral mRNA carried by N protein through binding with PABPC1 and eIF4F proteins, thus promoting viral transcription and facilitating viral replication.

Role of LncRNA MIR99AHG in breast cancer: Bioinformatic analysis and preliminary verification

Objective

This research was aimed to preliminarily explore the clinical roles and potential molecular mechanisms of MIR99AHG and its significant transcripts in breast cancer (BRCA).

Methods

Public databases were utilized to analyze the expression and prognostic roles of MIR99AHG and its transcripts. Relationships between MIR99AHG expression and immune cells infiltration were analyzed in Xiantao platform. In addition, co-expressed genes and interacting proteins of MIR99AHG were predicted. CancerSEA analyzed its relationship with functional states. Next, CNV status, DNA methylation, interacting transcription factors (TFs) and ceRNA network were analyzed to explore its possible mechanisms. Then, RNA ISH and FISH assays were used to detect its expression and location in BRCA tissues and cell lines, respectively. Finally, qRT-PCR was utilized to investigate MIR99AHG expression in cell lines.

Results

Compared with the corresponding normal tissues, MIR99AHG expression levels were lower in all BRCA subtypes, and luminal B's was the lowest one. And MIR99AHG expression was negatively related to the tumor stage. In addition, 4 transcripts (ENST00000619222.4, ENST00000418813.6, ENST00000602901.5 and ENST00000453910.5) of MIR99AHG showed significant differences in the expression. Databases also suggested that the high MIR99AHG expression levels indicated good prognosis, especially in patients without lymph node metastasis. Xiantao found that MIR99AHG was positively related to 17 immune cells and negatively linked with 2 immune cells. CancerSEA analysis showed no relationships between MIR99AHG and functional states. From GEPIA and BCIP databases, 19 co-expressed genes were highly related to these four significant transcripts of MIR99AHG. StarBase, RNAct and HDOCK showed that several tumor-associated proteins, including U2AF65, hnRNPC, AEBP2, CHIC1 and so on, might interact with MIR99AHG. Genetically, BRCA had a higher proportion of MIR99AHG CNV loss than CNV gain, and the high level of DNA methylation indicated a good prognosis. Furthermore, 19 TFs were predicted to combine with the promoter of MIR99AHG. Then, we screened out 10 miRNAs potentially interacting with the significant transcripts of MIR99AHG, and five were significantly increased in breast tumors compared to normal tissues, including miR-194-5p, miR-320 b and so on, which could combine 14 mRNAs. Through ISH and FISH assays, we verified that MIR99AHG was down-regulated in BRCA samples and cell lines in comparison to non-tumor tissues and mammary epithelial cell line (MCF10A), and MIR99AHG was located both in cytoplasm and nucleus. qRT-PCR assay also showed the lower expression of MIR99AHG in breast cancer cells than that in MCF10A.

Conclusion

These results indicate that MIR99AHG can be a favorable prognostic indicator for BRCA. ENST00000619222.4, ENST00000418813.6, ENST00000602901.5 and ENST00000453910.5 are significant transcripts and their down-regulation may play crucial roles in the progression of BRCA.

PABPN1 aggregation is driven by Ala expansion and poly(A)-RNA binding, leading to CFIm25 sequestration that impairs alternative polyadenylation

Poly(A)-binding protein nuclear 1 (PABPN1) is an RNA-binding protein localized in nuclear speckles, while its alanine (Ala)-expanded variants accumulate as intranuclear aggregates in oculopharyngeal muscular dystrophy. The factors that drive PABPN1 aggregation and its cellular consequences remain largely unknown. Here, we investigated the roles of Ala stretch and poly(A) RNA in the phase transition of PABPN1 using biochemical and molecular cell biology methods. We have revealed that the Ala stretch controls its mobility in nuclear speckles, and Ala expansion leads to aggregation from the dynamic speckles. Poly(A) nucleotide is essential to the early-stage condensation that thereby facilitates speckle formation and transition to solid-like aggregates. Moreover, the PABPN1 aggregates can sequester CFIm25, a component of the pre-mRNA 3'-UTR processing complex, in an mRNA-dependent manner and consequently impair the function of CFIm25 in alternative polyadenylation. In conclusion, our study elucidates a molecular mechanism underlying PABPN1 aggregation and sequestration, which will be beneficial for understanding PABPN1 proteinopathy.

Terminal hairpins improve protein expression in IRES-initiated mRNA in the absence of a cap and polyadenylated tail

Synthesizing mRNA in vitro is a standard and simple procedure. Adding the 5' cap and 3' polyadenylated (poly(A)) tail to make this mRNA functional for use as a vaccine or therapy increases the time and cost of production and usually decreases the yield, however. We designed mRNA that lacked a cap and poly(A) tail but included an internal ribosomal entry site (IRES) to initiate protein translation. To protect the 5' and 3' ends of mRNA from exonucleases, we added stable terminal hairpins. When compared against typical mRNA (i.e., mRNA that contained a cap and poly(A) tail but lacked hairpins), expression of the delivered reporter protein in HEK293 cells was similar. Using a triple instead of a single hairpin at each end increased protein expression even more. This method has the potential to simplify the production and reduce the cost of synthesizing exogenous mRNA for use as biologics or vaccines.

Cytoplasmic Polyadenylation Is an Ancestral Hallmark of Early Development in Animals

Differential regulation of gene expression has produced the astonishing diversity of life on Earth. Understanding the origin and evolution of mechanistic innovations for control of gene expression is therefore integral to evolutionary and developmental biology. Cytoplasmic polyadenylation is the biochemical extension of polyadenosine at the 3'-end of cytoplasmic mRNAs. This process regulates the translation of specific maternal transcripts and is mediated by the Cytoplasmic Polyadenylation Element-Binding Protein family (CPEBs). Genes that code for CPEBs are amongst a very few that are present in animals but missing in nonanimal lineages. Whether cytoplasmic polyadenylation is present in non-bilaterian animals (i.e., sponges, ctenophores, placozoans, and cnidarians) remains unknown. We have conducted phylogenetic analyses of CPEBs, and our results show that CPEB1 and CPEB2 subfamilies originated in the animal stem lineage. Our assessment of expression in the sea anemone, Nematostella vectensis (Cnidaria), and the comb jelly, Mnemiopsis leidyi (Ctenophora), demonstrates that maternal expression of CPEB1 and the catalytic subunit of the cytoplasmic polyadenylation machinery (GLD2) is an ancient feature that is conserved across animals. Furthermore, our measurements of poly(A)-tail elongation reveal that key targets of cytoplasmic polyadenylation are shared between vertebrates, cnidarians, and ctenophores, indicating that this mechanism orchestrates a regulatory network that is conserved throughout animal evolution. We postulate that cytoplasmic polyadenylation through CPEBs was a fundamental innovation that contributed to animal evolution from unicellular life.

An mRNA processing pathway suppresses metastasis by governing translational control from the nucleus

Cancer cells often co-opt post-transcriptional regulatory mechanisms to achieve pathologic expression of gene networks that drive metastasis. Translational control is a major regulatory hub in oncogenesis; however, its effects on cancer progression remain poorly understood. Here, to address this, we used ribosome profiling to compare genome-wide translation efficiencies of poorly and highly metastatic breast cancer cells and patient-derived xenografts. We developed dedicated regression-based methods to analyse ribosome profiling and alternative polyadenylation data, and identified heterogeneous nuclear ribonucleoprotein C (HNRNPC) as a translational controller of a specific mRNA regulon. We found that HNRNPC is downregulated in highly metastatic cells, which causes HNRNPC-bound mRNAs to undergo 3' untranslated region lengthening and, subsequently, translational repression. We showed that modulating HNRNPC expression impacts the metastatic capacity of breast cancer cells in xenograft mouse models. In addition, the reduced expression of HNRNPC and its regulon is associated with the worse prognosis in breast cancer patient cohorts.

PABPN1 prevents the nuclear export of an unspliced RNA with a constitutive transport element and controls human gene expression via intron retention

Intron retention is a type of alternative splicing where one or more introns remain unspliced in a polyadenylated transcript. Although many viral systems are known to translate proteins from mRNAs with retained introns, restriction mechanisms generally prevent export and translation of incompletely spliced mRNAs. Here, we provide evidence that the human nuclear poly(A)-binding protein, PABPN1, functions in such restrictions. Using a reporter construct in which nuclear export of an incompletely spliced mRNA is enhanced by a viral constitutive transport element (CTE), we show that PABPN1 depletion results in a significant increase in export and translation from the unspliced CTE-containing transcript. Unexpectedly, we find that inactivation of poly(A)-tail exosome targeting by depletion of PAXT components had no effect on export and translation of the unspliced reporter mRNA, suggesting a mechanism largely independent of nuclear RNA decay. Interestingly, a PABPN1 mutant selectively defective in stimulating poly(A) polymerase elongation strongly enhanced the expression of the unspliced, but not of intronless, reporter transcripts. Analysis of RNA-seq data also revealed that PABPN1 controls the expression of many human genes via intron retention. Notably, PABPN1-dependent intron retention events mostly affected 3'-terminal introns and were insensitive to PAXT and NEXT deficiencies. Our findings thus disclose a role for PABPN1 in restricting nuclear export of intron-retained transcripts and reinforce the interdependence between terminal intron splicing, 3' end processing, and polyadenylation.

Quantitative proteomics reveals common and unique molecular mechanisms underlying beneficial effects of caffeine and trigonelline on human hepatocytes

Caffeine and trigonelline are the major bioactive compounds in coffee. Caffeine alone or combined with other coffee compounds shows hepatoprotective effects. However, molecular mechanisms underlying such hepatoprotective effects remain unclear. We therefore addressed molecular effects of caffeine and trigonelline on human hepatocytes using quantitative proteomics followed by bioinformatic analyses to obtain topological and functional significance. HepG2 cells were treated with 100 μM caffeine or trigonelline for 24-h and evaluated by quantitative proteomics using nanoLC-ESI-LTQ-Orbitrap MS/MS. A total of 26 and 25 significantly altered proteins were identified in caffeine-treated and trigonelline-treated cells, respectively, compared with control cells. Topological analyses revealed that ribosomal and translation regulatory proteins predominantly served as the hub proteins associated with protein clusters. Functional analyses also revealed that these two bioactive compounds shared some molecular mechanisms via induction of translational processes. There were also other unique molecular functions and biological processes triggered or suppressed by either caffeine or trigonelline. These data highlight common and unique molecular mechanisms underlying the hepatoprotective effects of caffeine and trigonelline that may be useful for future clinical applications.

Comparative transcriptomic analysis of the larval and adult stages of Dibothriocephalus dendriticus (Cestoda: Diphyllobothriidea)

Tapeworms of the genus Dibothriocephalus are widely distributed throughout the world, some of which are agents of human diphyllobothriasis, one of the most important fish-borne zoonoses caused by a cestode parasite. Genomic and transcriptomic data can be used to develop future diagnostic tools and epidemiological studies. The present work focuses on a comparative analysis of the transcriptomes of adult and plerocercoid D. dendriticus and the identification of their differentially expressed genes (DEGs). Transcriptome assembly and analysis yielded and annotated 35,129 unigenes, noting that 16,568 (47%) unigenes were not annotated in known databases, which may indicate a unique set of expressed transcripts for D. dendriticus. A total of 8022 differentially expressed transcripts were identified, including 3225 upregulated and 4797 downregulated differentially expressed transcripts from the plerocercoid and adult animals. The analysis of DEGs has shown that among the most differentially expressed genes, there are important genes characteristic of each stage. Thus, several genes are characteristic of D. dendriticus plerocercoids, including fatty acid-binding protein and ferritin. Among the most highly expressed DEGs of the adult stage of D. dendriticus is the Kunitz-type serine protease inhibitor, in two putative isoforms. The analyses of GO and KEGG metabolic pathways revealed that a large number of the DEGs of D. dendriticus are associated with the biosynthesis of various substances such as arginine and folate, as well as with various metabolic pathways such as galactose metabolism, selenocompound metabolism, and phosphonate and phosphinate metabolism. This will contribute to further research aimed at identifying targets for new generation drugs and the development of specific vaccines.

PABPC1--mRNA stability, protein translation and tumorigenesis

Mammalian poly A-binding proteins (PABPs) are highly conserved multifunctional RNA-binding proteins primarily involved in the regulation of mRNA translation and stability, of which PABPC1 is considered a central regulator of cytoplasmic mRNA homing and is involved in a wide range of physiological and pathological processes by regulating almost every aspect of RNA metabolism. Alterations in its expression and function disrupt intra-tissue homeostasis and contribute to the development of various tumors. There is increasing evidence that PABPC1 is aberrantly expressed in a variety of tumor tissues and cancers such as lung, gastric, breast, liver, and esophageal cancers, and PABPC1 might be used as a potential biomarker for tumor diagnosis, treatment, and clinical application in the future. In this paper, we review the abnormal expression, functional role, and molecular mechanism of PABPC1 in tumorigenesis and provide directions for further understanding the regulatory role of PABPC1 in tumor cells.

PABPN1 functions as a hub in the assembly of nuclear poly(A) domains that are essential for mouse oocyte development

Growing oocytes store a large amount of maternal mRNA to support the subsequent "maternal-zygotic transition" process. At present, it is not clear how the growing oocytes store and process the newly transcribed mRNA under physiological conditions. In this study, we report non-membrane-bound compartments, nuclear poly(A) domains (NPADs), as the hub for newly transcribed mRNA, in developing mouse oocytes. The RNA binding protein PABPN1 promotes the formation of NPAD through its N-terminal disordered domain and RNA-recognized motif by means of liquid phase separation. Pabpn1-null growing oocytes cannot form NPAD normally in vivo and have defects in stability of oocyte growing-related transcripts and formation of long 3' untranslated region isoform transcripts. Ultimately, Pabpn1^fl/fl;Gdf9-Cre mice are completely sterile with primary ovarian insufficiency. These results demonstrate that NPAD formed by the phase separation properties of PABPN1-mRNA are the hub of the newly transcribed mRNA and essential for the development of oocytes and female reproduction.

Elucidation of TRIM25 ubiquitination targets involved in diverse cellular and antiviral processes

The tripartite motif (TRIM) family of E3 ubiquitin ligases is well known for its roles in antiviral restriction and innate immunity regulation, in addition to many other cellular pathways. In particular, TRIM25-mediated ubiquitination affects both carcinogenesis and antiviral response. While individual substrates have been identified for TRIM25, it remains unclear how it regulates diverse processes. Here we characterized a mutation, R54P, critical for TRIM25 catalytic activity, which we successfully utilized to "trap" substrates. We demonstrated that TRIM25 targets proteins implicated in stress granule formation (G3BP1/2), nonsense-mediated mRNA decay (UPF1), nucleoside synthesis (NME1), and mRNA translation and stability (PABPC4). The R54P mutation abolishes TRIM25 inhibition of alphaviruses independently of the host interferon response, suggesting that this antiviral effect is a direct consequence of ubiquitination. Consistent with that, we observed diminished antiviral activity upon knockdown of several TRIM25-R54P specific interactors including NME1 and PABPC4. Our findings highlight that multiple substrates mediate the cellular and antiviral activities of TRIM25, illustrating the multi-faceted role of this ubiquitination network in modulating diverse biological processes.

Pervasive translation of circular RNAs driven by short IRES-like elements

Some circular RNAs (circRNAs) were found to be translated through IRES-driven mechanism, however the scope and functions of circRNA translation are unclear because endogenous IRESs are rare. To determine the prevalence and mechanism of circRNA translation, we develop a cell-based system to screen random sequences and identify 97 overrepresented hexamers that drive cap-independent circRNA translation. These IRES-like short elements are significantly enriched in endogenous circRNAs and sufficient to drive circRNA translation. We further identify multiple trans-acting factors that bind these IRES-like elements to initiate translation. Using mass-spectrometry data, hundreds of circRNA-coded peptides are identified, most of which have low abundance due to rapid degradation. As judged by mass-spectrometry, 50% of translatable endogenous circRNAs undergo rolling circle translation, several of which are experimentally validated. Consistently, mutations of the IRES-like element in one circRNA reduce its translation. Collectively, our findings suggest a pervasive translation of circRNAs, providing profound implications in translation control.

Unbiased screen of random sequences identified many short IRES-like elements to drive circular RNA translation and hundreds of rolling circle translation events, suggesting a pervasive cap-independent translation in human transcriptome.

Nuclear and cytoplasmic poly(A) binding proteins (PABPs) favor distinct transcripts and isoforms

The poly(A)-tail appended to the 3'-end of most eukaryotic transcripts plays a key role in their stability, nuclear transport, and translation. These roles are largely mediated by Poly(A) Binding Proteins (PABPs) that coat poly(A)-tails and interact with various proteins involved in the biogenesis and function of RNA. While it is well-established that the nuclear PABP (PABPN) binds newly synthesized poly(A)-tails and is replaced by the cytoplasmic PABP (PABPC) on transcripts exported to the cytoplasm, the distribution of transcripts for different genes or isoforms of the same gene on these PABPs has not been investigated on a genome-wide scale. Here, we analyzed the identity, splicing status, poly(A)-tail size, and translation status of RNAs co-immunoprecipitated with endogenous PABPN or PABPC in human cells. At steady state, many protein-coding and non-coding RNAs exhibit strong bias for association with PABPN or PABPC. While PABPN-enriched transcripts more often were incompletely spliced and harbored longer poly(A)-tails and PABPC-enriched RNAs had longer half-lives and higher translation efficiency, there are curious outliers. Overall, our study reveals the landscape of RNAs bound by PABPN and PABPC, providing new details that support and advance the current understanding of the roles these proteins play in poly(A)-tail synthesis, maintenance, and function.

Modulation of miRISC-Mediated Gene Silencing in Eukaryotes

Gene expression is regulated at multiple levels in eukaryotic cells. Regulation at the post-transcriptional level is modulated by various trans-acting factors that bind to specific sequences in the messenger RNA (mRNA). The binding of different trans factors influences various aspects of the mRNA such as degradation rate, translation efficiency, splicing, localization, etc. MicroRNAs (miRNAs) are short endogenous ncRNAs that combine with the Argonaute to form the microRNA-induced silencing complex (miRISC), which uses base-pair complementation to silence the target transcript. RNA-binding proteins (RBPs) contribute to post-transcriptional control by influencing the mRNA stability and translation upon binding to cis-elements within the mRNA transcript. RBPs have been shown to impact gene expression through influencing the miRISC biogenesis, composition, or miRISC-mRNA target interaction. While there is clear evidence that those interactions between RBPs, miRNAs, miRISC and target mRNAs influence the efficiency of miRISC-mediated gene silencing, the exact mechanism for most of them remains unclear. This review summarizes our current knowledge on gene expression regulation through interactions of miRNAs and RBPs.

Roles of mRNA poly(A) tails in regulation of eukaryotic gene expression

In eukaryotes, poly(A) tails are present on almost every mRNA. Early experiments led to the hypothesis that poly(A) tails and the cytoplasmic polyadenylate-binding protein (PABPC) promote translation and prevent mRNA degradation, but the details remained unclear. More recent data suggest that the role of poly(A) tails is much more complex: poly(A)-binding protein can stimulate poly(A) tail removal (deadenylation) and the poly(A) tails of stable, highly translated mRNAs at steady state are much shorter than expected. Furthermore, the rate of translation elongation affects deadenylation. Consequently, the interplay between poly(A) tails, PABPC, translation and mRNA decay has a major role in gene regulation. In this Review, we discuss recent work that is revolutionizing our understanding of the roles of poly(A) tails in the cytoplasm. Specifically, we discuss the roles of poly(A) tails in translation and control of mRNA stability and how poly(A) tails are removed by exonucleases (deadenylases), including CCR4-NOT and PAN2-PAN3. We also discuss how deadenylation rate is determined, the integration of deadenylation with other cellular processes and the function of PABPC. We conclude with an outlook for the future of research in this field.

RNA Binding Proteins as Pioneer Determinants of Infection: Protective, Proviral, or Both?

As the first intracellular host factors that directly interact with the genomes of RNA viruses, RNA binding proteins (RBPs) have a profound impact on the outcome of an infection. Recent discoveries brought about by new methodologies have led to an unprecedented ability to peer into the earliest events between viral RNA and the RBPs that act upon them. These discoveries have sparked a re-evaluation of current paradigms surrounding RBPs and post-transcriptional gene regulation. Here, we highlight questions that have bloomed from the implementation of these novel approaches. Canonical RBPs can impact the fates of both cellular and viral RNA during infection, sometimes in conflicting ways. Noncanonical RBPs, some of which were first characterized via interactions with viral RNA, may encompass physiological roles beyond viral pathogenesis. We discuss how these RBPs might discriminate between an RNA of either cellular or viral origin and thus exert either pro- or antiviral effects-which is a particular challenge as viruses contain mechanisms to mimic molecular features of cellular RNA.

Identification of novel proteins and mRNAs differentially bound to the Leishmania Poly(A) Binding Proteins reveals a direct association between PABP1, the RNA-binding protein RBP23 and mRNAs encoding ribosomal proteins

Poly(A) Binding Proteins (PABPs) are major eukaryotic RNA-binding proteins (RBPs) with multiple roles associated with mRNA stability and translation and characterized mainly from multicellular organisms and yeasts. A variable number of PABP homologues are seen in different organisms however the biological reasons for multiple PABPs are generally not well understood. In the unicellular Leishmania, dependent on post-transcriptional mechanisms for the control of its gene expression, three distinct PABPs are found, with yet undefined functional distinctions. Here, using RNA-immunoprecipitation sequencing analysis we show that the Leishmania PABP1 preferentially associates with mRNAs encoding ribosomal proteins, while PABP2 and PABP3 bind to an overlapping set of mRNAs distinct to those enriched in PABP1. Immunoprecipitation studies combined to mass-spectrometry analysis identified RBPs differentially associated with PABP1 or PABP2, including RBP23 and DRBD2, respectively, that were investigated further. Both RBP23 and DRBD2 bind directly to the three PABPs in vitro, but reciprocal experiments confirmed preferential co-immunoprecipitation of PABP1, as well as the EIF4E4/EIF4G3 based translation initiation complex, with RBP23. Other RBP23 binding partners also imply a direct role in translation. DRBD2, in contrast, co-immunoprecipitated with PABP2, PABP3 and with RBPs unrelated to translation. Over 90% of the RBP23-bound mRNAs code for ribosomal proteins, mainly absent from the transcripts co-precipitated with DRBD2. These experiments suggest a novel and specific route for translation of the ribosomal protein mRNAs, mediated by RBP23, PABP1 and the associated EIF4E4/EIF4G3 complex. They also highlight the unique roles that different PABP homologues may have in eukaryotic cells associated with mRNA translation.

The prognostic impacts of PABPC1 expression on gastric cancer patients

Aim: To assess the prognostic impacts of PABPC1 on gastric cancer (GC) patients. Methods: The expression levels of PABPC1 in GC tissues and normal gastric tissues were initially compared via bioinformatics analysis. Immunohistochemical staining was accomplished to assess the expression of PABPC1 in the included GC patients. Then the impacts of PABPC1 expression on survival of GC patients were evaluated by Cox regression and Kaplan-Meier analyses. Results: The expression levels of PABPC1 in gastric tissues were significantly higher than those in normal gastric tissues (paired, p = 0.002; unpaired, p = 3.60e-9). By Kaplan-Meier, it was demonstrated that high expression of PABPC1 was significantly associated with worse overall and disease-free survival. Furthermore, high PABPC1 expression was demonstrated to be an independent predictive factor for both overall (p = 0.013; hazard ratio = 2.058; 95% CI: 1.162-3.644) and disease-free (p = 0.018; hazard ratio = 2.284; 95% CI: 1.153-4.524) survival. Conclusion: PABPC1 is a potential prognostic biomarker for GC patients.

The molecular basis of coupling between poly(A)-tail length and translational efficiency

In animal oocytes and early embryos, mRNA poly(A)-tail length strongly influences translational efficiency (TE), but later in development this coupling between tail length and TE disappears. Here, we elucidate how this coupling is first established and why it disappears. Overexpressing cytoplasmic poly(A)-binding protein (PABPC) in Xenopus oocytes specifically improved translation of short-tailed mRNAs, thereby diminishing coupling between tail length and TE. Thus, strong coupling requires limiting PABPC, implying that in coupled systems longer-tail mRNAs better compete for limiting PABPC. In addition to expressing excess PABPC, post-embryonic mammalian cell lines had two other properties that prevented strong coupling: terminal-uridylation-dependent destabilization of mRNAs lacking bound PABPC, and a regulatory regime wherein PABPC contributes minimally to TE. Thus, these results revealed three fundamental mechanistic requirements for coupling and defined the context-dependent functions for PABPC, which promotes TE but not mRNA stability in coupled systems and mRNA stability but not TE in uncoupled systems.

Cells are microscopic biological factories that are constantly creating new proteins. To do so, a cell must first convert its master genetic blueprint, the DNA, into strands of messenger RNA or mRNA. These strands are subsequently translated to make proteins. Cells have two ways to adjust the number of proteins they generate so they do not produce too many or too few: by changing how many mRNA molecules are available for translation, and by regulating how efficiently they translate these mRNA molecules into proteins.

In animals, both unfertilized eggs and early-stage embryos lack the ability to create or destroy mRNAs, and consequently cannot adjust the number of mRNA molecules available for translation. These cells can therefore only regulate how efficiently each mRNA is translated. They do this by changing the length of the so-called poly(A) tail at the end of each mRNA molecule, which is made up of a long stretch of repeating adenosine nucleotides. The mRNAs with longer poly(A) tails are translated more efficiently than those with shorter poly(A) tails. However, this difference disappears in older embryos, when both long and short poly(A) tails are translated with equal efficiency, and it is largely unknown why.

To find out more, Xiang and Bartel studied frog eggs, and discovered that artificially raising levels of a protein that binds poly(A) tails, also known as PABPC, improved the translation of short-tailed mRNAs to create a situation in which both short- and long-tailed mRNAs were translated with near-equal efficiency. This suggested that short- and long-tailed mRNAs compete for limited amounts of the translation-enhancing PABPC, and that long-tailed mRNAs are better at it than short-tailed mRNAs. Further investigation revealed that eggs also had to establish the right conditions for PABPC to enhance translation and had to protect mRNAs not associated with PABPC from being destroyed before they could be translated.

Overall, Xiang and Bartel found that in eggs and early embryos, PABPC and poly(A) tails enhanced the translation of mRNAs but did not influence their stability, whereas later in development, they enhanced mRNA stability but not translation.

This research provides new insights into how protein production is controlled at different stages of animal development, from unfertilized eggs to older embryos. Understanding how this process is regulated during normal development is crucial for gaining insights into how it can become dysfunctional and cause disease. These findings may therefore have important implications for research into areas such as infertility, reproductive medicine and rare genetic diseases.

Revisiting poly(A)-binding proteins: Multifaceted regulators during gametogenesis and early embryogenesis

Post-transcriptional regulation faces a distinctive challenge in gametes. Transcription is limited when the germ cells enter the division phase due to condensed chromatin, while gene expression during gamete maturation, fertilization, and early cleavage depends on existing mRNA post-transcriptional coordination. The dynamics of the 3'-poly(A) tail play crucial roles in defining mRNA fate. The 3'-poly(A) tail is covered with poly(A)-binding proteins (PABPs) that help to mediate mRNA metabolism and recent work has shed light on the number and function of germ cell-specific expressed PABPs. There are two structurally different PABP groups distinguished by their cytoplasmic and nuclear localization. Both lack catalytic activity but are coupled with various roles through their interaction with multifunctional partners during mRNA metabolism. Here, we present a synopsis of PABP function during gametogenesis and early embryogenesis and describe both conventional and current models of the functions and regulation of PABPs, with an emphasis on the physiological significance of how germ cell-specific PABPs potentially affect human fertility.

Global alternative splicing landscape of skeletal muscle atrophy induced by hindlimb unloading

Background

Long-term exposure to microgravity will cause skeletal muscle atrophy, which can cause serious harm to astronauts in space travel. Therefore, it is important to explore skeletal muscle atrophy’s molecular mechanism for its prevention and treatment. However, as an important regulatory approach of skeletal muscle physiology, the role of alternative splicing in skeletal muscle atrophy, especially skeletal muscle atrophy caused by disuse, is unclear.

Methods

We established a rat hindlimb unloading model and performed RNA sequencing on soleus muscle, which was seriously atrophied during unloading. Several bioinformatics methods were used to identify alternative splicing events and determine their gene functions.

Results

Many alternative splicing events were found to occur at different time points (12 h, 24 h, 36 h, 3 days, and 7 days) after hindlimb unloading. These differential alternative splicing events mainly occurred in the gene's coding domain sequence region, and 59% of the alternative splicing events caused open reading frameshift. Bioinformatics analysis results showed that genes with different alternative splicing events were enriched in multiple pathways related to muscle atrophy, including the insulin signaling pathway, endocytosis, mitophagy, and ubiquitin-proteasome pathway. Moreover, alternative splicing of several deubiquitinase genes persisted during skeletal muscle atrophy induced by unloading. Additionally, we identified 10 differentially expressed RNA binding proteins during skeletal muscle atrophy induced by unloading, mainly containing Xpo4, Eif4e2, P4ha1, Lrrfip1, Zc3h14, Emg1, Hnrnp h1, Mbnl2, RBfox1, and Mbnl1. Hnrnp h1 and Mbnl2 were significantly downregulated, and RBfox1 and Mbnl1 were significantly upregulated during skeletal muscle atrophy caused by unloading.

Conclusions

To the best of our knowledge, the present study is the first to propose alternative splicing alterations related to disuse-induced muscle atrophy, emphasizing that alternative splicing is a new focus of attention in the occurrence of muscle atrophy.

A Two-Layered Targeting Mechanism Underlies Nuclear RNA Sorting by the Human Exosome

Degradation of transcripts in human nuclei is primarily facilitated by the RNA exosome. To obtain substrate specificity, the exosome is aided by adaptors; in the nucleoplasm, those adaptors are the nuclear exosome-targeting (NEXT) complex and the poly(A) (pA) exosome-targeting (PAXT) connection. How these adaptors guide exosome targeting remains enigmatic. Employing high-resolution 3' end sequencing, we demonstrate that NEXT substrates arise from heterogenous and predominantly pA^- 3' ends often covering kilobase-wide genomic regions. In contrast, PAXT targets harbor well-defined pA⁺ 3' ends defined by canonical pA site use. Irrespective of this clear division, NEXT and PAXT act redundantly in two ways: (1) regional redundancy, where the majority of exosome-targeted transcription units produce NEXT- and PAXT-sensitive RNA isoforms, and (2) isoform redundancy, where the PAXT connection ensures fail-safe decay of post-transcriptionally polyadenylated NEXT targets. In conjunction, this provides a two-layered targeting mechanism for efficient nuclear sorting of the human transcriptome.

AlphaScreen Identifies MSUT2 Inhibitors for Tauopathy-Targeting Therapeutic Discovery

Tauopathies are neurological disorders characterized by intracellular tau deposits forming neurofibrillary tangles, neuropil threads, or other disease-specific aggregates composed of the protein tau. Tauopathy disorders include frontotemporal lobar degeneration, corticobasal degeneration, Pick's disease, and the largest cause of dementia, Alzheimer's disease. The lack of disease-modifying therapeutic strategies to address tauopathies remains a critical unmet need in dementia care. Thus, novel broad-spectrum tau-targeted therapeutics could have a profound impact in multiple tauopathy disorders, including Alzheimer's disease. Here we have designed a drug discovery paradigm to identify inhibitors of the pathological tau-enabling protein, MSUT2. We previously showed that activity of the RNA-binding protein MSUT2 drives tauopathy, including tau-mediated neurodegeneration and cognitive dysfunction, in mouse models. Thus, we hypothesized that MSUT2 inhibitors could be therapeutic for tauopathy disorders. Our pipeline for MSUT2 inhibitory compound identification included a primary AlphaScreen, followed by dose-response validation, a secondary fluorescence polarization orthogonal assay, a tertiary specificity screen, and a preliminary toxicity screen. Our work here serves as a proof-of-principle methodology for finding specific inhibitors of the poly(A) RNA-binding protein MSUT2 interaction. Here we identify 4,4'-diisothiocyanostilbene-2,2'-sulfonic acid (DIDS) as a potential tool compound for future work probing the mechanism of MSUT2-induced tau pathology.

Controversies around the function of LARP1

The RNA-binding protein LARP1 has generated interest in recent years for its role in the mTOR signalling cascade and its regulation of terminal oligopyrimidine (TOP) mRNA translation. Paradoxically, some scientists have shown that LARP1 represses TOP translation while others that LARP1 activates it. Here, we present opinions from four leading scientists in the field to discuss these and other contradictory findings.

Global analysis of RNA-protein interactions in TNF-α induced alternative splicing in metabolic disorders

In this report, using the database of RNA-binding protein specificities (RBPDB) and our previously published RNA-seq data, we analyzed the interactions between RNA and RNA-binding proteins to decipher the role of alternative splicing in metabolic disorders induced by TNF-α. We identified 13 395 unique RNA-RBP interactions, including 385 unique RNA motifs and 35 RBPs, some of which (including MBNL-1 and 3, ZFP36, ZRANB2, and SNRPA) are transcriptionally regulated by TNF-α. In addition to some previously reported RBPs, such as RBMX and HuR/ELAVL1, we found a few novel RBPs, such as ZRANB2 and SNRPA, to be involved in the regulation of metabolic syndrome-associated genes that contain an enrichment of tetrameric RNA sequences (AUUU). Taken together, this study paves the way for novel RNA-protein interaction-based therapeutics for treating metabolic syndromes.

RNA stabilization by a poly(A) tail 3'-end binding pocket and other modes of poly(A)-RNA interaction

Polyadenylate [poly(A)] tail addition to the 3' end of a wide range of RNAs is a highly conserved modification that plays a central role in cellular RNA function. Elements for nuclear expression (ENEs) are cis-acting RNA elements that stabilize poly(A) tails by sequestering them in RNA triplex structures. A crystal structure of a double ENE from a rice hAT transposon messenger RNA complexed with poly(A)₂₈ at a resolution of 2.89 angstroms reveals multiple modes of interaction with poly(A), including major-groove triple helices, extended minor-groove interactions with RNA double helices, a quintuple-base motif that transitions poly(A) from minor-groove associations to major-groove triple helices, and a poly(A) 3'-end binding pocket. Our findings both expand the repertoire of motifs involved in long-range RNA interactions and provide insights into how polyadenylation can protect an RNA's extreme 3' end.

Dynamic regulation of CTCF stability and sub-nuclear localization in response to stress

The nuclear protein CCCTC-binding factor (CTCF) has diverse roles in chromatin architecture and gene regulation. Functionally, CTCF associates with thousands of genomic sites and interacts with proteins, such as cohesin, or non-coding RNAs to facilitate specific transcriptional programming. In this study, we examined CTCF during the cellular stress response in human primary cells using immune-blotting, quantitative real time-PCR, chromatin immunoprecipitation-sequence (ChIP-seq) analysis, mass spectrometry, RNA immunoprecipitation-sequence analysis (RIP-seq), and Airyscan confocal microscopy. Unexpectedly, we found that CTCF is exquisitely sensitive to diverse forms of stress in normal patient-derived human mammary epithelial cells (HMECs). In HMECs, a subset of CTCF protein forms complexes that localize to Serine/arginine-rich splicing factor (SC-35)-containing nuclear speckles. Upon stress, this species of CTCF protein is rapidly downregulated by changes in protein stability, resulting in loss of CTCF from SC-35 nuclear speckles and changes in CTCF-RNA interactions. Our ChIP-seq analysis indicated that CTCF binding to genomic DNA is largely unchanged. Restoration of the stress-sensitive pool of CTCF protein abundance and re-localization to nuclear speckles can be achieved by inhibition of proteasome-mediated degradation. Surprisingly, we observed the same characteristics of the stress response during neuronal differentiation of human pluripotent stem cells (hPSCs). CTCF forms stress-sensitive complexes that localize to SC-35 nuclear speckles during a specific stage of neuronal commitment/development but not in differentiated neurons. We speculate that these particular CTCF complexes serve a role in RNA processing that may be intimately linked with specific genes in the vicinity of nuclear speckles, potentially to maintain cells in a certain differentiation state, that is dynamically regulated by environmental signals. The stress-regulated activity of CTCF is uncoupled in persistently stressed, epigenetically re-programmed "variant" HMECs and certain cancer cell lines. These results reveal new insights into CTCF function in cell differentiation and the stress-response with implications for oxidative damage-induced cancer initiation and neuro-degenerative diseases.

Identification of nucleotide sequences and some proteins involved in polyadenylation of RNA transcribed by Pol III from SINEs

We have previously reported that not only transcripts of RNA polymerase II (pol II), but also one type of RNA transcribed by RNA polymerase III (pol III), undergo AAUAAA-dependent polyadenylation. Such an unusual feature is inherent in Short Interspersed Elements (SINEs) from genomes of certain mammals. For polyadenylation of its transcript, SINE should contain, besides an AATAAA hexamer and a transcription terminator, two specific regions: β, located downstream of box B of a promoter, and τ, preceding AATAAA. Here, using nucleotide substitutions in SINEs B2 (mouse) and Ves (bat), we identified nucleotides of β regions necessary for polyadenylation of their transcripts. These sequences (β signals) are the following: ACCACATgg in B2 and GGGCATGT in Ves. Using this approach, we identified τ signal of SINE B2 (GCTACagTGTACTTACAT), where TGTA tetramer is most important for polyadenylation. In Ves, τ region is a long polypyrimidine motif which is able to interact with PTB protein in Ves transcripts. We demonstrated by knockdown that B2 and Ves transcript polyadenylation is performed by canonical poly(A) polymerase with the participation of proteins CSPF-160 and Fip1, the known factors of mRNA polyadenylation. We also showed that a factor CFIm partaking in polyadenylation of many mRNAs, is involved only in polyadenylation of B2 transcripts. CFIm seems to interact with τ signal of В2 RNA and thereby facilitates the recruiting of other proteins engaged in polyadenylation. Thus, SINEs utilize at least some proteins involved in polyadenylation of pol II transcripts to polyadenylate their pol III transcripts.

Targeting Pathological Tau by Small Molecule Inhibition of the Poly(A):MSUT2 RNA-Protein Interaction

Neurofibrillary tangles composed of aberrantly aggregating tau protein are a hallmark of Alzheimer's disease and related dementia disorders. Recent work has shown that mammalian suppressor of tauopathy 2 (MSUT2), also named ZC3H14 (Zinc Finger CCCH-Type Containing 14), controls accumulation of pathological tau in cultured human cells and mice. Knocking out MSUT2 protects neurons from neurodegenerative tauopathy and preserves learning and memory. MSUT2 protein functions to bind polyadenosine [poly(A)] tails of mRNA through its C-terminal CCCH type zinc finger domains, and loss of CCCH domain function suppresses tauopathy in Caenorhabditis elegans and mice. Thus, we hypothesized that inhibiting the poly(A):MSUT2 RNA-protein interaction would ameliorate pathological tau accumulation. Here we present a high-throughput screening method for the identification of small molecules inhibiting the poly(A):MSUT2 RNA-protein interaction. We employed a fluorescent polarization assay for initial small molecule discovery with the intention to repurpose hits identified from the NIH Clinical Collection (NIHCC). Our drug repurposing development workflow included validation of hits by dose-response analysis, specificity testing, orthogonal assays of activity, and cytotoxicity. Validated compounds passing through this screening funnel will be evaluated for translational effectiveness in future studies. This preclinical drug development pipeline identified diverse FDA approved drugs duloxetine, saquinavir, and clofazimine as potential repurposing candidates for reducing pathological tau accumulation.

PABPN1L mediates cytoplasmic mRNA decay as a placeholder during the maternal-to-zygotic transition

Maternal mRNA degradation is a critical event of the maternal-to-zygotic transition (MZT) that determines the developmental potential of early embryos. Nuclear Poly(A)-binding proteins (PABPNs) are extensively involved in mRNA post-transcriptional regulation, but their function in the MZT has not been investigated. In this study, we find that the maternally expressed PABPN1-like (PABPN1L), rather than its ubiquitously expressed homolog PABPN1, acts as an mRNA-binding adapter of the mammalian MZT licensing factor BTG4, which mediates maternal mRNA clearance. Female Pabpn1l null mice produce morphologically normal oocytes but are infertile owing to early developmental arrest of the resultant embryos at the 1- to 2-cell stage. Deletion of Pabpn1l impairs the deadenylation and degradation of a subset of BTG4-targeted maternal mRNAs during the MZT. In addition to recruiting BTG4 to the mRNA 3'-poly(A) tails, PABPN1L is also required for BTG4 protein accumulation in maturing oocytes by protecting BTG4 from SCF-βTrCP1 E3 ubiquitin ligase-mediated polyubiquitination and degradation. This study highlights a noncanonical cytoplasmic function of nuclear poly(A)-binding protein in mRNA turnover, as well as its physiological importance during the MZT.

Host shutoff activity of VHS and SOX-like proteins: role in viral survival and immune evasion

BACKGROUND

Host shutoff refers to the widespread downregulation of host gene expression and has emerged as a key process that facilitates the reallocation of cellular resources for viral replication and evasion of host antiviral immune responses.

MAIN BODY

The Herpesviridae family uses a number of proteins that are responsible for host shutoff by directly targeting messenger RNA (mRNA), including virion host shutoff (VHS) protein and the immediate-early regulatory protein ICP27 of herpes simplex virus types 1 (HSV-1) and the SOX (shutoff and exonuclease) protein and its homologs in Gammaherpesvirinae subfamilies, although these proteins are not homologous. In this review, we highlight evidence that host shutoff is promoted by the VHS, ICP27 and SOX-like proteins and that they also contribute to immune evasion.

CONCLUSIONS

Further studies regarding the host shutoff proteins will not only contribute to provide new insights into the viral replication, expression and host immune evasion process, but also provide new molecular targets for the development of antiviral drugs and therapies.