Biology subjects

Joshua-Tor, L.

Publications and source records attributed to Joshua-Tor, L..

16 recordsLinked to original sources

bioRxiv2026

Marker-based CRISPR screens identify POU2F1 as a regulator of DLL3 and neuroendocrine identity in small cell lung cancer

Small cell lung cancers (SCLC) often exhibit a neuroendocrine lineage identity marked by high expression of Delta-like Ligand 3 (DLL3). Because DLL3 shows minimal expression in normal adult tissues, it serves as an SCLC-selective tumor antigen and is the basis for clinically efficacious targeted therapies. Understanding the mechanisms that regulate DLL3 expression is therefore critical for advancing therapeutic strategies in this disease. Here, we performed transcription factor-focused and genome-wide CRISPR screens to identify regulators of DLL3 expression in SCLC. Both approaches converged on POU2F1 as a top activator of DLL3 in this tumor context. Despite its ubiquitous expression, we identify an SCLC-specific role for POU2F1 in activating DLL3 and a broader set of neuroendocrine lineage genes. Epigenomic analyses reveal tandem POU2F1-ASCL1 motifs within the DLL3 promoter that underlie the strong codependency between POU2F1 and the neuroendocrine master regulator ASCL1 for high-level DLL3 expression in SCLC. We provide evidence that tandem POU2F1-ASCL1 elements are part of a cis-regulatory code for the lung neuroendocrine cell fate. Together, these findings define a previously unrecognized transcriptional logic controlling DLL3 expression and establish POU2F1 as a context-specific regulator of neuroendocrine lineage in small cell lung cancer.

cancer biology↗

bioRxiv2026

Structural dynamics between Argonaute-2 and CK1α promote target RNA release in microRNA-mediated silencing

Argonaute (Ago) proteins associate with 20-22 nucleotide (nt) long microRNAs (miRNAs) to constitute the functional RISC core and downregulate mRNAs containing complementarity to the seed sequence1-3. Target RNA engagement in RISC stimulates CK1-mediated phosphorylation of the conserved eukaryotic insertion (EI) in Ago, releasing the target and enabling the RISC complex to suppress additional target sites for efficient miRNA-mediated silencing4-6. Here, we provide a complete structural view of miRNA guide and target binding to human Ago2, showing Ago2 holding the double-stranded guide-target RNA in an untwisted conformation at its center. We visualize the dynamic changes that RISC undergoes as the guide supplementary region progressively base pairs with the target, enabling CK1 binding. Following seed-helix assembly, initial supplementary pairing restricts RISC to a "closed" form, while with half-supplementary pairing, the PAZ domain moves to open RISC to become receptive to CK1, exhibiting an initial increase in Ago2 phosphorylation. Complete supplementary pairing supports a full PAZ-CK1 interface, allowing for hierarchical phosphorylation of the EI. The combination of target repulsion by EI phosphorylation with an unwound guide-target enables efficient target release to promote RISC turnover.

biochemistry↗

bioRxiv2026

A developmental timer coordinates organism-wide microRNA transcription

The development of distinct tissues must be precisely coordinated to ensure that growth and cell fate transitions occur in the correct temporal order across the organism, yet the mechanisms that coordinate these timing events remain unclear. In Caenorhabditis elegans, stage-specific cell fate transitions are driven by pulsatile transcription of heterochronic microRNAs, but the source of these rhythms has been unknown. Here, we identify a developmental timer composed of the transcription factor MYRF-1 and the PERIOD-like repressor LIN-42 that operates in all somatic cells. MYRF-1 binds conserved regulatory elements upstream of heterochronic microRNA genes and drives synchronized, once-per-stage transcriptional pulses across tissues, while concurrently activating lin-42 expression. Newly synthesized LIN-42 directly associates with MYRF-1, limiting its nuclear residence and transcriptional activity to constrain the amplitude and duration of each transcriptional burst. This reciprocal transcriptional/translational feedback loop generates organism-wide, phase-locked microRNA expression, coupling tissue-specific development to organismal growth through a shared timing mechanism.

developmental biology↗

bioRxiv2025

NOVEL SMALL-MOLECULE INHIBITORS OF THE PROTEIN KINASE DYRK: POTENTIAL THERAPEUTIC CANDIDATES IN CANCER

Dual-specificity tyrosine-regulated kinase 1A (DYRK1A) is crucial for normal brain development and its disruption has been linked to various cancers. DYRK1A drives glioblastoma (GBM) progression via stabilization of epidermal growth factor receptor (EGFR). Here we describe two, selective, benzothiazole-derived DYRK inhibitors, FC-2 and FC-3, obtained by structure-activity optimization of a natural product lead. Both compounds inhibit DYRK1A with nanomolar potency and display high selectivity across a kinase panel. The co-crystal structure of FC-3 with DYRK1A revealed ATP-competitive binding, with interactions at the hinge region and the DYRK-specific phenylalanine gatekeeper residue explaining target selectivity. Generation of inhibitor-resistant mutants confirmed DYRK1A as the primary cellular target. In GBM cell-models, FC-2 and FC-3 impaired neurosphere self-renewal, cell invasion, and EGFR stability, phenocopying DYRK1A loss. Both compounds crossed the blood-brain barrier and suppressed tumor growth, to prolong survival in intracranial xenografts. These findings identify FC-2 and FC-3 as selective small-molecule inhibitors of DYRK1A with potential therapeutic utility in GBM.

cancer biology↗

bioRxiv2025

Identification and Validation of an inhibitor of the protein kinases PIM and DYRK

Fermented wheat germ extract (FWGE), a nutraceutical with reported anticancer properties, contains numerous biologically active molecules, but its precise therapeutic constituents remain unclear. In this study, we identify and characterize a novel small-molecule inhibitor, CSH-4044, isolated from FWGE. Through preparative HPLC and structural elucidation via X-ray crystallography, CSH-4044 was revealed to be a unique benzothiazole compound. Kinase profiling demonstrated its high specificity toward the PIM and DYRK families of protein kinases. We determined the co-crystal structure of CSH-4044 bound to PIM1, revealing ATP-competitive binding, and critical hydrophobic and hydrogen-bonding interactions. A chemically synthesized version of CSH-4044 mirrored the activity of the natural product, confirming structural integrity and biological equivalence. Functionally, CSH-4044 suppressed PIM3-driven BAD phosphorylation in pancreatic cancer cells and reduced DYRK1A-mediated Tau phosphorylation in neuronal cells. Our findings position CSH-4044 as a promising lead for targeting PIM and DYRK families of kinases and highlight FWGE as a source of potential therapeutic compounds.

biochemistry↗

bioRxiv2025

Structural and mechanistic insights into Dis3L2 mediated degradation of structured RNA

The RNase II/RNB family of exoribonucleases is present in all domains of life and includes three main eukaryotic members, the Dis3-like proteins (Dis3, Dis3L1, Dis3L2). At the cellular level, Dis3L2 is distinguished by the unique preference for uridylated RNA substrates and the highest efficiency in degrading double-stranded RNA. Defects in these enzymes have been linked to some types of cancers and overgrowth disorders in humans. In this work, we used the Dis3L2 protein from the model organism Schizosaccharomyces pombe (SpDis3L2) to better understand the mechanism of action of Dis3-like exoribonucleases, and to elucidate how single amino acid substitutions in these proteins can affect the biochemical properties of the enzymes, potentially contributing to the molecular basis of the related human diseases. We determined the crystal structure of SpDis3L2 bound to a U13 RNA, in which the protein displays a typical vase-like conformation, accommodating 6 nucleotides of the RNA 3-end. Furthermore, we constructed two SpDis3L2 protein variants, harbouring single amino acid substitutions mimicking the ones already found in human patients, to test their catalytic activity in vitro. We highlight the A756R SpDis3L2 variant, which loses the ability to degrade double-stranded RNA substrates and accumulates intermediary degradation products when degrading single-stranded RNA substrates. As such, A756 seems to be a key residue responsible for the normal cellular function of Dis3L2, specifically regarding its important role in the degradation of structured RNA substrates.

biochemistry↗

bioRxiv2024

The molecular basis of Human FN3K mediated phosphorylation of glycated substrate

Glycation, a non-enzymatic post-translational modification occurring on proteins, can be actively reversed via site-specific phosphorylation of the fructose-lysine moiety by FN3K kinase, to impact the cellular function of target protein. A regulatory axis between FN3K and glycated protein targets has been associated with conditions like diabetes and cancer. However the molecular basis of this relationship has not been explored so far. Here, we determined a series of crystal structures of HsFN3K in apo-state, and in complex with different nucleotide analogs together with a sugar substrate mimic to reveal the features important for its kinase activity and substrate recognition. Additionally, the dynamics in sugar substrate binding during the kinase catalytic cycle provide important mechanistic insights into HsFN3K function. Our structural work provides the molecular basis for rationale small molecule design targeting FN3K.

biochemistry↗

bioRxiv2024

The structural landscape of Microprocessor Mediated pri-let-7 miRNAs processing

miRNA biogenesis is initiated upon cleavage of a primary miRNA (pri-miRNA) hairpin by the Microprocessor (MP), composed of the Drosha RNase III enzyme and its partner DGCR8. Multiple pri-miRNA sequence motifs affect MP recognition, fidelity, and efficiency. Here, we performed cryo-EM and biochemical studies of several let-7 family pri-miRNAs in complex with human MP. We show that MP has the structural plasticity to accommodate a range of pri-miRNAs. These structures revealed key features of the 5 UG sequence motif, more comprehensively represented as the "fUN" motif. Our analysis explains how cleavage of class-II pri-let-7 members harboring a bulged nucleotide generates a noncanonical precursor with a 1-nt 3 overhang. Finally, the MP-SRSF3-pri-let-7f1 structure reveals how SRSF3 contributes to MP fidelity by interacting with the CNNC-motif and Droshas PAZ-like domain. Overall, this study sheds light on the mechanisms for flexible recognition, accurate cleavage, and regulated processing of different pri-miRNAs by MP.

biochemistry↗

bioRxiv2023

Chromatin remodeling of histone H3 variants underlies epigenetic inheritance of DNA methylation

Epigenetic inheritance refers to the faithful replication of DNA methylation and histone modification independent of DNA sequence. Nucleosomes block access to DNA methyltransferases, unless they are remodeled by DECREASE IN DNA METHYLATION1 (DDM1Lsh/HELLS), a Snf2-like master regulator of epigenetic inheritance. We show that DDM1 activity results in replacement of the transcriptional histone variant H3.3 for the replicative variant H3.1 during the cell cycle. In ddm1 mutants, DNA methylation can be restored by loss of the H3.3 chaperone HIRA, while the H3.1 chaperone CAF-1 becomes essential. The single-particle cryo-EM structure at 3.2 [A] of DDM1 with a variant nucleosome reveals direct engagement at SHL2 with histone H3.3 at or near variant residues required for assembly, as well as with the deacetylated H4 tail. An N-terminal autoinhibitory domain binds H2A variants to allow remodeling, while a disulfide bond in the helicase domain is essential for activity in vivo and in vitro. We show that differential remodeling of H3 and H2A variants in vitro reflects preferential deposition in vivo. DDM1 co-localizes with H3.1 and H3.3 during the cell cycle, and with the DNA methyltransferase MET1Dnmt1. DDM1 localization to the chromosome is blocked by H4K16 acetylation, which accumulates at DDM1 targets in ddm1 mutants, as does the sperm cell specific H3.3 variant MGH3 in pollen, which acts as a placeholder nucleosome in the germline and contributes to epigenetic inheritance.

molecular biology↗

bioRxiv2022

Circadian rhythm orthologs drive pulses of heterochronic miRNA transcription in C. elegans

Developmental robustness relies on precise control of the timing and order of cellular events. In C. elegans, the invariant sequence of post-embryonic cell fate specification is controlled by oscillatory patterns of heterochronic microRNA transcription that are phase-locked with the larval molting cycle1-4. How these transcriptional patterns are generated and how microRNA dosage is controlled is unknown. Here we show that transcriptional pulses of the lin-4 heterochronic microRNA are produced by two nuclear hormone receptors, NHR-85 and NHR-23, whose mammalian orthologs, Rev-Erb and ROR, function in the circadian clock. While Rev-Erb and ROR play antagonistic roles in regulating once-daily transcription5-7, we find that NHR-85 and NHR-23 bind cooperatively as heterodimers to lin-4 regulatory elements to induce a single brief pulse of expression during each larval stage. We demonstrate that the timing and duration of lin-4 transcriptional pulses are programmed by the phased overlap of NHR-85 and NHR-23 protein expression and that these regulatory interactions are post-transcriptionally controlled by LIN-42, the circadian Period ortholog in C. elegans. These findings suggest that an evolutionary rewiring of the circadian clock machinery is co-opted in nematodes to generate periodic transcriptional patterns that define cell fate progression.

developmental biology↗

bioRxiv2022

Target binding triggers hierarchical phosphorylation of human Argonaute-2 to promote target release

Argonaute (Ago) proteins play a central role in post-transcriptional gene regulation through RNA interference (RNAi). Agos bind small RNAs (sRNAs) including small interfering RNAs (siRNAs) and microRNAs (miRNAs) to form the functional core of the RNA Induced Silencing Complex (RISC). The sRNA is used as a guide to target mRNAs containing either partially or fully complementary sequences, ultimately leading to down regulation of the corresponding proteins. It was previously shown that the kinase CK1 phosphorylates a cluster of residues in the eukaryotic insertion (EI) of Ago, leading to the alleviation of miRNA-mediated repression through an undetermined mechanism. We show that binding of miRNA-loaded human Ago2 to target RNA with complementarity to the seed and 3 supplemental regions of the miRNA primes the EI for hierarchical phosphorylation by CK1. The added negative charges electrostatically promote target release, freeing Ago to seek out additional targets once it is dephosphorylated. The high conservation of potential phosphosites in the EI suggests that such a regulatory strategy may be a shared mechanism for regulating miRNA-mediated repression.

biochemistry↗

bioRxiv2021

A shape-shifting nuclease unravels structured RNA

RNA turnover pathways ensure appropriate gene expression levels by eliminating unwanted transcripts that may otherwise interfere with cellular programs. The enzyme Dis3-like protein 2 (Dis3L2) is a 3-5 exoribonuclease that, through its RNA turnover activity, plays a critical role in human development1. Dis3L2 can independently degrade structured substrates and its targets include many coding and non-coding 3-uridylated RNAs1-5. While the basis for Dis3L2s substrate recognition has been well-characterized6, the mechanism of structured RNA degradation by this family of enzymes is unknown. We characterized the discrete steps of the degradation cycle by determining electron cryo-microscopy structures representing snapshots along the RNA turnover pathway and measuring kinetic parameters for single-stranded (ss) and double-stranded (ds) RNA processing. We discovered a dramatic conformational change that is triggered by the dsRNA, involving repositioning of two cold shock domains by 70 [A]. This movement exposes a trihelix-linker region, which acts as a wedge to separate the two RNA strands. Furthermore, we show that the trihelix linker is critical for dsRNA, but not ssRNA, degradation. These findings reveal the conformational plasticity of this enzyme, and detail a novel mechanism of structured RNA degradation.

biochemistry↗

bioRxiv2020

The dynamic nature of the human Origin Recognition Complex revealed through five cryoEM structures

Genome replication is initiated from specific origin sites established by dynamic events. The Origin Recognition Complex (ORC) is necessary for orchestrating the initiation process by binding to origin DNA, recruiting CDC6, and assembling the MCM replicative helicase on DNA. Here we report five cryoEM structures of the human ORC (HsORC) that illustrate the native flexibility of the complex. The absence of ORC1 revealed a compact, stable complex of ORC2-5. Introduction of ORC1 opens the complex into several dynamic conformations. Two structures revealed dynamic movements of the ORC1 AAA+ and ORC2 winged-helix domains that likely impact DNA incorporation into the ORC core. Additional twist and pinch motions were observed in an open ORC conformation revealing a hinge at the ORC5{middle dot}3 interface that may facilitate ORC binding to DNA. Finally, a structure of ORC was determined with endogenous DNA bound in the core revealing important differences between human and yeast origin recognition.

biochemistry↗

bioRxiv2020

Asterix/Gtsf1 links tRNAs and piRNA silencing of retrotransposons

The piRNA pathway safeguards genomic integrity by silencing transposable elements in the germline. While Piwi is the central piRNA factor, others including Asterix/Gtsf1 have also been demonstrated to be critical for effective silencing. Here, using eCLIP with a custom informatic pipeline, we show that Asterix/Gtsf1 specifically binds tRNAs in cellular contexts. We determined the structure of mouse Gtsf1 by NMR spectroscopy and identified the RNA binding interface on the proteins first zinc finger, which was corroborated by biochemical analysis as well as cryo-EM structures of Gtsf1 in complex with co-purifying tRNA. We further show that LTR retrotransposons are preferentially de-repressed in Asterix mutants. Given the role of tRNAs as LTR retrotransposon primers, our work implicates Asterix/Gtsf1 as exploiting tRNA dependence to identify transposon transcripts and promote piRNA silencing.

biochemistry↗

bioRxiv2020

Evolution of DNA Replication Origin Specification and Gene Silencing Mechanisms

DNA replication in eukaryotic cells initiates from chromosomal locations, called replication origins, that bind the Origin Recognition Complex (ORC) prior to S phase. Origin establishment is guided by well-defined DNA sequence motifs in Saccharomyces cerevisiae and some other budding yeasts, but most eukaryotes lack sequence-specific origins. At present, the mechanistic and evolutionary reasons for this difference are unclear. A 3.9 Å structure of S. cerevisiae ORC-Cdc6-Cdt1-Mcm2-7 (OCCM) bound to origin DNA revealed, among other things, that a loop within Orc2 inserts into a DNA minor groove and an α-helix within Orc4 inserts into a DNA major groove1. We show that this Orc4 α-helix mediates the sequence-specificity of origins in S. cerevisiae. Specifically, mutations were identified within this α-helix that alter the sequence-dependent activity of individual origins as well as change global genomic origin firing patterns. This was accomplished using a massively parallel origin selection assay analyzed using a custom mutual-information-based modeling approach and a separate analysis of whole-genome replication profiling and statistics. Interestingly, the sequence specificity of DNA replication initiation, as mediated by the Orc4 α-helix, has evolved in close conjunction with the gain of ORC-Sir4-mediated gene silencing and the loss of RNA interference.Competing Interest StatementThe authors have declared no competing interest.View Full Text

molecular biology↗

bioRxiv2019

High-resolution cryo-EM structures of outbreak strain human norovirus shells reveal size variations

Noroviruses are a leading cause of food-borne illnesses worldwide. Although GII.4 strains have been responsible for most norovirus outbreaks, the assembled virus shell structures have been available in detail for only a single strain (GI.1). We present high-resolution (2.6-4.1 [A]) cryo-electron microscopy (cryo-EM) structures of GII.4, GII.2, GI.7 and GI.1 human norovirus outbreak strain virus-like particles (VLPs). Although norovirus VLPs have been thought to exist in a single-sized assembly, our structures reveal polymorphism between and within genogroups with small, medium and large particle sizes observed. We developed a new asymmetric reconstruction method and resolved a metal ion adjacent to the co-receptor binding site, which affected the structural stability of the shell. Our structures serve as valuable templates for facilitating vaccine formulations.

biophysics↗