Jean-Philippe Lambert

Development of long-read RNA sequencing technologies has paved the way to the exploration of RNA isoform diversity and its relevance in regulating cell fate. However, identifying new functional isoforms is still very difficult. Here, we leverage long-read RNA sequencing to study changes in isoforms during somatic cell reprogramming and identify novel isoforms occurring throughout cell state transitions. We demonstrate tight regulation of non-coding isoforms and show that isoform switching plays previously overlooked functional roles and outcomes in gene regulation and cell fate changes. We uncover a novel long non-coding RNA, Snhg26 , that undergoes isoform switching during reprogramming to enhance the conversion of differentiated cells towards the pluripotent state. Knock-down of Snhg26 in mouse and human pluripotency models reveals that it is important for pluripotency acquisition. Together, our study provides a resource to study full-length isoform usage during cell fate change. It also demonstrates the power of long-read sequencing to identify functionally relevant gene isoforms in the context of cell plasticity.

INO80 and SWR1 are evolutionarily related ATP-dependent chromatin remodeling complexes that regulate the chromatin occupancy of the histone variant H2A.Z, playing critical roles in transcriptional regulation, genome replication, and DNA repair. While the H2A.Z-related functions of INO80 and SWR1 are well characterized in budding yeast and metazoans, much less is known about their composition and chromatin-targeting mechanisms outside of the Opisthokonts. We previously found that a distinct bromodomain-containing protein, IBD1, is involved in multiple chromatin-related complexes, including the SWR1-complex, in the ciliate protozoan Tetrahymena thermophila. Here, we report that a closely related bromodomain-containing protein, IBD2, functions as an acetyl lysine reader module within a putative INO80 complex. Through iterative proteomic analyses, we show that the Tetrahymena INO80 complex retains several conserved subunits found in its yeast and metazoan counterparts. In vitro binding assays reveal that recombinant IBD2 preferentially recognizes acetylated histone H3 tails. Chromatin immunoprecipitation followed by high-throughput sequencing (ChIP-seq) demonstrates that IBD2 is enriched near transcription start sites and promoter regions. Notably, the IBD1 and IBD2 genomic binding profiles strongly correlate with that of H2A.Z (Hv1), supporting their functional association with the SWRI- and INO80-complexes. Together, our findings support a model in which H2A.Z chromatin dynamics are modulated by SWR1- and INO80-complexes that are differentially recruited to chromatin via distinct bromodomain proteins that recognize specific histone acetylation marks.

Regulating gene expression involves significant changes in the chromatin environment at the locus level, especially at regulatory sequences. However, their modulation following pharmacological treatments or pathological conditions remain mostly undetermined. Here, we report versatile locus-specific proteomics tools to address this knowledge gap, which combine the targeting ability of the CRISPR/Cas9 system and the protein-labeling capability of the highly reactive biotin ligases TurboID (in CasTurbo) and UltraID (in CasUltra). CasTurbo and CasUltra enabled rapid chromatin protein labeling at repetitive sequences like centromeres and telomeres, as well as nonamplified genes. We applied CasUltra to A375 melanoma cell lines to decipher the protein environment of the MYC promoter and characterize the molecular effects of the bromodomain inhibitor JQ1, which targets bromodomain and extra-terminal (BET) proteins that regulate MYC expression. We quantified the consequences of BET protein displacement from the MYC promoter and found that it was associated with a considerable reorganization of the chromatin composition. Additionally, BET protein retention at the MYC promoter was consistent with a model of increased JQ1 resistance. Thus, through the combination of proximity biotinylation and CRISPR/Cas9 genomic targeting, CasTurbo and CasUltra have successfully demonstrated their utility in profiling the proteome associated with a genomic locus in living cells.

Development of long-read RNA sequencing technologies has paved the way to the exploration of RNA isoform diversity and its relevance in regulating cell fate. However, identifying new functional isoforms is still very difficult. Here, we leverage long-read RNA sequencing to study changes in isoforms during somatic cell reprogramming and identify novel isoforms occurring throughout cell state transitions. We demonstrate tight regulation of non-coding isoforms and show that isoform switching plays previously overlooked functional roles and outcomes in gene regulation and cell fate changes. We uncover a novel long non-coding RNA, Snhg26 , that undergoes isoform switching during reprogramming to enhance the conversion of differentiated cells towards the pluripotent state. Knock-down of Snhg26 in mouse and human pluripotency models reveals that it is important for pluripotency acquisition. Together, our study provides a resource to study full-length isoform usage during cell fate change, and demonstrates the power of long-read sequencing to identify functionally relevant gene isoforms in the context of cell plasticity.

Plant homeodomain (PHD) fingers are critical effectors of histone post-translational modifications (PTMs), acting as regulators of gene expression and genome integrity, and frequently presenting in human disease. While most PHD fingers recognize unmodified and methylated states of histone H3 lysine 4 (H3K4), the specific functions of many of the over 100 PHD finger-containing proteins in humans remain poorly understood, despite their significant implications in disease processes. In this study, we undertook a comprehensive analysis of one such poorly characterized PHD finger-containing protein, PHRF1. Using biochemical, molecular, and cellular approaches, we show that PHRF1 robustly binds to histone H3, specifically at its N-terminal region. Through RNA-seq and proteomic analyses, we also find that PHRF1 is intricately involved in transcriptional and RNA splicing regulation and plays a significant role in DNA damage response (DDR). Crucially, mutagenesis of proline 221 to leucine (P221L) in the PHD finger of PHRF1 abolishes histone interaction and fails to rescue defective DDR. These findings underscore the importance of PHRF1-H3 interaction in maintaining genome integrity and provide insight into how PHD fingers contribute to chromatin biology.

Summary: Alpha-synuclein (α-syn) aggregation is a defining feature of Parkinson’s disease (PD) and related synucleinopathies. Despite significant research efforts focused on understanding α-syn aggregation mechanisms, the early stages of this process remain elusive, largely due to limitations in experimental tools that lack the temporal resolution to capture these dynamic events. Here, we introduce UltraID-LIPA, an innovative platform that combines the Light-Inducible Protein Aggregation (LIPA) system with the UltraID proximity-dependent biotinylation assay to identify α-syn-interacting proteins and uncover key mechanisms driving its oligomerization. UltraID-LIPA successfully identified 38 α-syn-interacting proteins, including both established and novel candidates, highlighting the accuracy and robustness of the approach. Notably, a strong interaction with endolysosomal and membrane-associated proteins was observed, supporting the hypothesis that interactions with membrane-bound organelles are pivotal in the early stages of α-syn aggregation. This powerful platform provides new insights into dynamic protein aggregation events, enhancing our understanding of synucleinopathies and other proteinopathies.

The human nucleosome acetyltransferase of histone H4 (NuA4)/Tat-interactive protein, 60 kilodalton (TIP60) coactivator complex, a fusion of the yeast switch/sucrose nonfermentable related 1 (SWR1) and NuA4 complexes, both incorporates the histone variant H2A.Z into nucleosomes and acetylates histones H4, H2A, and H2A.Z to regulate gene expression and maintain genome stability. Our cryo–electron microscopy studies show that, within the NuA4/TIP60 complex, the E1A binding protein P400 (EP400) subunit serves as a scaffold holding the different functional modules in specific positions, creating a distinct arrangement of the actin-related protein (ARP) module. EP400 interacts with the transformation/transcription domain-associated protein (TRRAP) subunit by using a footprint that overlaps with that of the Spt-Ada-Gcn5 acetyltransferase (SAGA) complex, preventing the formation of a hybrid complex. Loss of the TRRAP subunit leads to mislocalization of NuA4/TIP60, resulting in the redistribution of H2A.Z and its acetylation across the genome, emphasizing the dual functionality of NuA4/TIP60 as a single macromolecular assembly.

Regulating gene expression involves significant and frequent changes in the chromatin environment at the locus level, especially at regulatory sequences. However, their modulation in response to pharmacological treatments or pathological conditions remain mostly undetermined. Here, we report versatile locus-specific proteomics tools to address this knowledge gap, which combine the targeting ability of the CRISPR/Cas9 system and the protein-labelling capability of the highly reactive biotin ligases TurboID (in CasTurbo) and UltraID (in CasUltra). CasTurbo and CasUltra enabled rapid chromatin protein labelling under mild conditions at repetitive sequences like centromeres and telomeres, as well as non-amplified genes. We applied CasUltra to A375 melanoma cell lines to decipher the protein environment of the MYC promoter and characterize the molecular effects of the bromodomain inhibitor JQ1, which targets bromodomain and extra-terminal (BET) proteins that regulate MYC expression. We quantified the consequences of BET protein displacement from the MYC promoter and found that it was associated with a considerable reorganisation of the chromatin composition. In addition, BET protein retention at the MYC promoter was consistent with a model of increased JQ1 resistance. Thus, through the combination of proximity biotinylation and CRISPR-Cas9-dependent genomic targeting, CasTurbo and CasUltra have successfully demonstrated their utility in profiling the proteome associated with a genomic locus in living cells. In Brief: Kougnassoukou Tchara et al . report the development and application of CasTurbo and CasUltra, two locus-specific proteomics tools that fuse catalytically dead Cas9 to the engineered biotin ligases TurboID and UltraID. These tools enabled the quantitative mapping of locus-specific chromatin remodelling due to pharmacological inhibition. Highlights: CasTurbo and CasUltra were developed for locus-specific label-free proteomics CasTurbo mapped the proteins localized to the centromeres and telomeres Proteins bound to the MYC promoter were quantified in melanoma cells with CasUltra CasUltra is compatible with investigating pharmacological treatment effects Graphical

Summary: Bromodomain (BRD)-containing proteins are chemically tractable multi-domain scaffolding molecules involved in acetyl lysine (Kac) signaling. BRD inhibitors have shown promise in clinical oncology, including melanomas; however, their narrow therapeutic windows and issues with resistance in pre-clinical models highlight the need to better understand the functions of and interconnection between BRD-containing proteins. Here, we use complementary interaction-mapping techniques (affinity purification and proximity-dependent biotinylation) to map the interactions of 39 of the 42 BRD-containing proteins and 110 additional proteins that physically or functionally associate with them. We uncover 3,892 novel interactions and reveal the intricate connectivity of the Kac machinery. Chemical inhibition of multiple BRD classes revealed that inhibiting BETs—but not mSWI/SNF or CREBBP/EP300 proteins—dramatically rewired the interactome. Finally, we identified MAPKAPK2 activity as a critical determinant of BET inhibitor sensitivity in melanoma through its impact on chromatin composition remodeling. In Brief: Kougnassoukou Tchara et al . generate a static protein interaction map of the human acetyl lysine machinery by coupling two complementary functional proteomics approaches (FLAG affinity purification and proximity-dependent biotinylation) to mass spectrometry. They also investigate network changes upon bromodomain inhibition, and describe a novel resistance mechanism mediated by the p38 stress signaling pathway that causes significant metabolic changes. Highlights: Two complementary interaction proteomics analyses of the human acetyl lysine machinery were performed. Novel target- and compound-specific impacts of bromodomain inhibitors were identified. MAPKAPK2 was identified as a novel resistance gene to BET bromodomain inhibitors in melanoma. BET bromodomain inhibition leads to metabolic adaptation in melanoma.