PTBP1-activated co-transcriptional splicing controls epigenetic status of pluripotent stem cells

Epigenetic Regulation and Therapeutic Targeting of Alternative Splicing Dysregulation in Cancer

Alternative splicing enables a single precursor mRNA to generate multiple mRNA isoforms, leading to protein variants with different structures and functions. Abnormal alternative splicing is frequently associated with cancer development and progression. Recent studies have revealed a complex and dynamic interplay between epigenetic modifications and alternative splicing. On the one hand, dysregulated epigenetic changes can alter splicing patterns; on the other hand, splicing events can influence epigenetic landscapes. The reversibility of epigenetic modifications makes epigenetic drugs, both approved and investigational, attractive therapeutic options. This review provides a comprehensive overview of the bidirectional relationship between epigenetic regulation and alternative splicing in cancer. It also highlights emerging therapeutic approaches aimed at correcting splicing abnormalities, with a special focus on drug-based strategies. These include epigenetic inhibitors, antisense oligonucleotides (ASOs), small-molecule compounds, CRISPR-Cas9 genome editing, and the SMaRT (splice-switching molecule) technology. By integrating recent advances in research and therapeutic strategies, this review provides novel insights into the molecular mechanisms of cancer and supports the development of more precise and effective therapies targeting aberrant splicing.

Characterization of dsRNA binding proteins through solubility analysis identifies ZNF385A as a dsRNA homeostasis regulator

Double-stranded RNA (dsRNA) binding proteins (dsRBPs) play crucial roles in various cellular processes, especially in the innate immune response. Comprehensive characterization of dsRBPs is essential to understand the intricate mechanisms for dsRNA sensing and response. Traditional methods have predominantly relied on affinity purification, favoring the isolation of strong dsRNA binders. Here, we adopt the proteome integral solubility alteration (PISA) workflow for characterizing dsRBPs, resulting in the observation of 18 known dsRBPs and the identification of 200 potential dsRBPs. Next, we focus on zinc finger protein 385 A (ZNF385A) and discover that its knockout activates the transcription of interferon-β in the absence of immunogenic stimuli. The knockout of ZNF385A elevates the level of endogenous dsRNAs, especially transcripts associated with retroelements, such as short interspersed nuclear element (SINE), long interspersed nuclear element (LINE), and long terminal repeat (LTR). Moreover, loss of ZNF385A enhances the bioactivity of 5-Aza-2'-deoxycytidine (5-AZA-CdR) and tumor-killing effect of NK cells. Our findings greatly expand the dsRBP reservoir and contribute to the understanding of cellular dsRNA homeostasis.

PTBP1 Lactylation Promotes Glioma Stem Cell Maintenance through PFKFB4-Driven Glycolysis

Long-standing evidence implicates glioma stem cells (GSC) as the major driver for glioma propagation and recurrence. GSCs have a distinctive metabolic landscape characterized by elevated glycolysis. Lactate accumulation resulting from enhanced glycolytic activity can drive lysine lactylation to regulate protein functions, suggesting that elucidating the lactylation landscape in GSCs could provide insights into glioma biology. Herein, we have demonstrated that global lactylation was significantly elevated in GSCs compared with differentiated glioma cells. Polypyrimidine tract-binding protein 1 (PTBP1), a central regulator of RNA processing, was hyperlactylated in GSCs, and SIRT1 induced PTBP1 delactylation. PTBP1-K436 lactylation supported glioma progression and GSC maintenance. Mechanistically, K436 lactylation inhibited PTBP1 proteasomal degradation by attenuating the interaction with TRIM21. Moreover, PTBP1 lactylation enhanced RNA-binding capacity and facilitated PFKFB4 mRNA stabilization, which further increased glycolysis. Together, these findings uncovered a lactylation-mediated mechanism in GSCs driven by metabolic reprogramming that induces aberrant epigenetic modifications to further stimulate glycolysis, resulting in a vicious cycle to exacerbate tumorigenesis. Significance: Glycolysis-induced lactylation enhances the function of PTBP1 to promote glioma stem cell maintenance, indicating that PTBP1 lactylation could be a potential target for the development of innovative glioma therapies.

Pol II degradation activates cell death independently from the loss of transcription

Pol II-mediated transcription is essential for eukaryotic life. While loss of transcription is thought to be universally lethal, the associated mechanisms promoting cell death are not yet known. Here, we show that death following loss of Pol II is not caused by dysregulated gene expression. Instead, death occurs in response to the loss of Pol II protein itself, specifically loss of the enzymatic subunit, Rbp1. Loss of Pol II exclusively activates apoptosis, and expression of a transcriptionally inactive version of Rpb1 rescues cell viability. Using functional genomics, we identify a previously uncharacterized mechanism that regulates lethality following loss of Pol II, which we call the Pol II Degradation-dependent Apoptotic Response (PDAR). Using the genetic dependencies of PDAR, we identify clinically used drugs that owe their efficacy to a PDAR-dependent mechanism. Our findings unveil a novel apoptotic signaling response that contributes to the efficacy of a wide array of anti-cancer therapies.

RBM4-mediated intron excision of Hsf1 induces BDNF for cerebellar foliation

Brain-derived neurotrophic factor (BDNF) plays important roles in brain development and neural function. Constitutive knockout of the splicing regulator RBM4 reduces BDNF expression in the developing brain and causes cerebellar hypoplasia, an autism-like feature. Here, we show that Rbm4 knockout induced intron 6 retention of Hsf1, leading to downregulation of HSF1 protein and its downstream target BDNF. RBM4-mediated Hsf1 intron excision regulated BDNF expression in cultured granule cells. Ectopic expression of HSF1 restored cerebellar foliation and motor learning of Rbm4-knockout mice, indicating a critical role for RBM4-HSF1-BDNF in cerebellar foliation. Moreover, N-methyl-D-aspartate receptor (NMDAR) signaling promoted the expression and nuclear translocation of RBM4, and hence increased the expression of both HSF and BDNF. A short CU-rich motif was responsible for NMDAR- and RBM4-mediated intron excision. Finally, RBM4 and polypyrimidine tract binding (PTB) proteins play antagonistic roles in intron excision, suggesting a role for splicing regulation in BDNF expression.

LINE1 modulate human T cell function by regulating protein synthesis during the life span

The molecular mechanisms responsible for the heightened reactivity of quiescent T cells in human early life remain largely elusive. Our previous research identified that quiescent adult naïve CD4+ T cells express LINE1 (long interspersed nuclear elements 1) spliced in previously unknown isoforms, and their down-regulation marks the transition to activation. Here, we unveil that neonatal naïve T cell quiescence is characterized by enhanced energy production and protein synthesis. This phenotype is associated with the absence of LINE1 expression attributed to tonic T cell receptor/mTOR complex 1 (mTORC1) signaling and (polypyrimidine tract-binding protein 1 (PTBP1)-mediated LINE1 splicing suppression. The absence of LINE1 expression primes these cells for rapid execution of the activation program by directly regulating protein synthesis. LINE1 expression progressively increases in childhood and adults, peaking in elderly individuals, and, by decreasing protein synthesis, contributes to immune senescence in aging. Our study proposes LINE1 as a critical player of human T cell function across the human life span.

M6A-modified lncRNA FAM83H-AS1 promotes colorectal cancer progression through PTBP1

LncRNA plays a crucial role in cancer progression and targeting, but it has been difficult to identify the critical lncRNAs involved in colorectal cancer (CRC) progression. We identified FAM83H-AS1 as a tumor-promoting associated lncRNA using 21 pairs of stage IV CRC tissues and adjacent normal tissues. In vitro and in vivo experiments revealed that knockdown of FAM83H-AS1 in CRC cells inhibited tumor proliferation and metastasis, and vice versa. M6A modification is critical for FAM83H-AS1 RNA stability through the writer METTL3 and the readers IGF2BP2/IGFBP3. PTBP1-an RNA binding protein-is responsible for the FAM83H-AS1 function in CRC. T4 (1770-2440 nt) and T5 (2440-2743 nt) on exon 4 of FAM83H-AS1 provide a platform for PTBP1 RRM2 interactions. Our results demonstrated that m6A modification dysregulated the FAM83H-AS1 oncogenic role by phosphorylated PTBP1 on its RNA splicing effect. In patient-derived xenograft models, ASO-FAM83H-AS1 significantly suppressed the growth of gastrointestinal (GI) tumors, not only CRC but also GC and ESCC. The combination of ASO-FAM83H-AS1 and oxaliplatin/cisplatin significantly suppressed tumor growth compared with treatment with either agent alone. Notably, there was pathological complete response in all these three GI cancers. Our findings suggest that FAM83H-AS1 targeted therapy would benefit patients primarily receiving platinum-based therapy in GI cancers.

Protocol for auxin-inducible depletion of the RNA-binding protein PTBP1 in mouse embryonic stem cells

Inducible degradation of proteins of interest provides a powerful approach for functional studies. Here, we present a protocol for tightly controlled depletion of the RNA-binding protein PTBP1 in mouse embryonic stem cells (ESCs). We describe steps for establishing an ESC line expressing doxycycline-inducible auxin receptor protein OsTIR1 and tagging endogenous Ptbp1 alleles using CRISPR-Cas9 and homology-directed repair reagents. We then detail procedures for assaying the efficiency of inducible PTBP1 knockdown by immunoblotting. This protocol is adaptable for other protein targets. For complete details on the use and execution of this protocol, please refer to Iannone et al.1.

Regulation of alternative splicing and polyadenylation in neurons

Cell-type-specific gene expression is a fundamental feature of multicellular organisms and is achieved by combinations of regulatory strategies. Although cell-restricted transcription is perhaps the most widely studied mechanism, co-transcriptional and post-transcriptional processes are also central to the spatiotemporal control of gene functions. One general category of expression control involves the generation of multiple transcript isoforms from an individual gene, whose balance and cell specificity are frequently tightly regulated via diverse strategies. The nervous system makes particularly extensive use of cell-specific isoforms, specializing the neural function of genes that are expressed more broadly. Here, we review regulatory strategies and RNA-binding proteins that direct neural-specific isoform processing. These include various classes of alternative splicing and alternative polyadenylation events, both of which broadly diversify the neural transcriptome. Importantly, global alterations of splicing and alternative polyadenylation are characteristic of many neural pathologies, and recent genetic studies demonstrate how misregulation of individual neural isoforms can directly cause mutant phenotypes.

Pre-mRNA splicing and its cotranscriptional connections

Transcription of eukaryotic genes by RNA polymerase II (Pol II) yields RNA precursors containing introns that must be spliced out and the flanking exons ligated together. Splicing is catalyzed by a dynamic ribonucleoprotein complex called the spliceosome. Recent evidence has shown that a large fraction of splicing occurs cotranscriptionally as the RNA chain is extruded from Pol II at speeds of up to 5 kb/minute. Splicing is more efficient when it is tethered to the transcription elongation complex, and this linkage permits functional coupling of splicing with transcription. We discuss recent progress that has uncovered a network of connections that link splicing to transcript elongation and other cotranscriptional RNA processing events.

PTBP1 enforces ATR-CHK1 signaling determining the potency of CDC7 inhibitors

CDC7 kinase is crucial for DNA replication initiation and fork processing. CDC7 inhibition mildly activates the ATR pathway, which further limits origin firing; however, to date the relationship between CDC7 and ATR remains controversial. We show that CDC7 and ATR inhibitors are either synergistic or antagonistic depending on the degree of inhibition of each individual kinase. We find that Polypyrimidine Tract Binding Protein 1 (PTBP1) is important for ATR activity in response to CDC7 inhibition and genotoxic agents. Compromised PTBP1 expression makes cells defective in RPA recruitment, genomically unstable, and resistant to CDC7 inhibitors. PTBP1 deficiency affects the expression and splicing of many genes indicating a multifactorial impact on drug response. We find that an exon skipping event in RAD51AP1 contributes to checkpoint deficiency in PTBP1-deficient cells. These results identify PTBP1 as a key factor in replication stress response and define how ATR activity modulates the activity of CDC7 inhibitors.

A Simplified and Effective Approach for the Isolation of Small Pluripotent Stem Cells Derived from Human Peripheral Blood

Pluripotent stem cells are key players in regenerative medicine. Embryonic pluripotent stem cells, despite their significant advantages, are associated with limitations such as their inadequate availability and the ethical dilemmas in their isolation and clinical use. The discovery of very small embryonic-like (VSEL) stem cells addressed the aforementioned limitations, but their isolation technique remains a challenge due to their small cell size and their efficiency in isolation. Here, we report a simplified and effective approach for the isolation of small pluripotent stem cells derived from human peripheral blood. Our approach results in a high yield of small blood stem cell (SBSC) population, which expresses pluripotent embryonic markers (e.g., Nanog, SSEA-3) and the Yamanaka factors. Further, a fraction of SBSCs also co-express hematopoietic markers (e.g., CD45 and CD90) and/or mesenchymal markers (e.g., CD29, CD105 and PTH1R), suggesting a mixed stem cell population. Finally, quantitative proteomic profiling reveals that SBSCs contain various stem cell markers (CD9, ITGA6, MAPK1, MTHFD1, STAT3, HSPB1, HSPA4), and Transcription reg complex factors (e.g., STAT5B, PDLIM1, ANXA2, ATF6, CAMK1). In conclusion, we present a novel, simplified and effective isolating process that yields an abundant population of small-sized cells with characteristics of pluripotency from human peripheral blood.