Alejandra Laguillo-Diego

Mitotic bookmarking enables transmission of regulatory information across cell division, yet how active chromatin states are molecularly preserved during mitosis and how they contribute to postmitotic cell fate decisions remain unresolved. Here, we demonstrate that the acetyltransferases KAT6 and KAT7 remain chromatin-bound and catalytically active throughout mitosis in mouse and human pluripotent stem cells. Acute KAT6/7 depletion disrupts mitotic histone acetylation and the retention of multiple chromatin regulators with minimal effects on chromatin accessibility. Mechanistically, KAT6/7 bookmark both self renewal and developmental gene loci, where they are required for post-mitotic transcriptional reactivation and priming of lineage-specific programs. Consistently, genes impaired during postmitotic reactivation also fail to activate during neuronal differentiation upon continuous KAT6/7 perturbation, compromising the establishment of the neuronal program and identity. Altogether, our findings identify KAT6 and KAT7 as a cooperative mitotic chromatin maintenance system that couples active histone acetylation to preservation of chromatin composition, transcriptional memory, and developmental competence across cell division, extending the bookmarking function beyond transcriptional memory. These findings also provide a new framework for understanding KAT6/7 function in neurodevelopmental disorders and cancer.

The BAF (also known as mammalian SWI/SNF) chromatin remodelling complexes are essential regulators of nucleosome positioning and transcription factor binding ^1,2 . Loss-of-function (LOF) mutations in genes encoding components of the BAF complexes are among the most frequent across cancer types ³ . However, a unifying mechanism that explains their widespread disruption remains unclear. Here we disrupted BAF function through pharmacological inhibition and ARID1A deletion across various mammalian cell types and tissues and observed a conserved chromatin response: increased accessibility at CTCF-binding sites. ARID1A LOF alone was sufficient to drive these changes, which demonstrates that canonical BAF (cBAF) disruption is the key driver of CTCF gain. Genomic profiling showed increased CTCF and SMC3 occupancy, which reveals an antagonistic relationship between cBAF and CTCF–cohesin complexes. Furthermore, cBAF loss restructured 3D genome organization, which led to a strengthening of topologically associating domain boundaries and subdivision of large domains into smaller insulated structures. Long-range enhancer–promoter loops were replaced by shorter, promoter-proximal loops anchored at CTCF-enriched sites. To determine how CTCF function is enabled following cBAF loss, we profiled CTCF-interacting cofactors and found that CTCF binds directly to fully assembled PBAF. After cBAF depletion, PBAF was recruited to sites of increased CTCF occupancy, which demonstrates that there is an antagonistic switch between SWI/SNF subcomplexes. In vivo, conditional Arid1a deletion in primary mouse cells resulted in the expansion of a highly proliferative population marked by elevated accessibility at CTCF sites. This phenotype was fully reversed by either Ctcf or Pbrm1 deletion, which establishes that cBAF and the activity of CTCF and PBAF have contrasting roles.

Summary: Ubiquitously expressed chromatin regulators orchestrate cell-type-specific gene expression programs, yet how they acquire distinct regulatory functions during development remains fundamentally unresolved ^1,2 . Using temporally controlled protein depletion during human cortical neurogenesis in vitro , we show that the autism-associated histone methyltransferase EHMT1 ^3,4 is developmentally repurposed from epiblast to cortical neurons. Broadly expressed zinc finger proteins recruit EHMT1 to distinct cis-regulatory elements within changing chromatin landscapes associated with lineage-specific transcription factor activity, with extensive remodeling of EHMT1 enhancer occupancy during neural induction dependent on ZNF462. This genomic rewiring is accompanied by a fundamental shift from restraining premature developmental gene expression in pluripotent cells to supporting neuronal programs, with distinct transcriptional consequences of EHMT1 loss across neural progenitors and excitatory and inhibitory neuronal subtypes. Timed depletion and recovery experiments identify the neural progenitor stage as the onset of a developmental period during which EHMT1 is required for faithful neuronal maturation. Remarkably, EHMT1 can re-engage stage-specific targets after prolonged deficiency, allowing extensive transcriptional recovery with only limited molecular scarring. These findings reveal how ubiquitous epigenetic machinery acquires developmental specificity while demonstrating the resilience of developmental gene regulatory programs to prolonged disruption, with implications for neurodevelopmental disorders.

Acute cohesin loss causes widespread reorganization of three-dimensional (3D) chromatin architecture but has relatively minor effects on steady-state transcription. It remains unclear whether its role in gene regulation becomes more critical during mitotic exit, when 3D chromatin architecture and transcription are globally re-established. To address this, we acutely depleted RAD21 in mouse embryonic stem cells during mitotic exit under self-renewal or differentiation conditions. Here we show that, although most loops failed to reform without cohesin, the few cohesin-independent loops were linked to active promoters, strong enhancers and H3K27ac mitotic bookmarking. Transcriptional changes were only modest, indicating that gene reactivation largely bypasses cohesin. Sensitive genes showed RAD21 promoter binding, a higher number of structural loops and positioning within well-insulated, gene-poor topologically associating domains. During differentiation, cohesin loss impaired activation of a broader set of developmental genes, partly due to defective de novo regulatory interactions. Together, these findings demonstrate context-specific requirements for cohesin in gene activation.

Prior studies showed that structural loops collapse upon acute cohesin depletion, while regulatory enhancer-promoter (E-P) loops largely persist, consistent with minimal transcriptional changes. However, these studies, conducted in asynchronous cells, could not resolve whether cohesin is required for the establishment of regulatory interactions and transcriptional activation during cell division or cell state transitions. To address this gap, we degraded RAD21, a core cohesin subunit, in naïve mouse embryonic stem cells (ESCs) transitioning from mitosis to G1 either in self-renewal condition or during differentiation toward formative pluripotency. Although most structural loops failed to be re-established without cohesin, about 35% of regulatory loops reformed at normal or higher frequencies. Cohesin-independent loops showed characteristics of strong active enhancers and promoters and a significant association with H3K27ac mitotic bookmarks. However, inhibition of CBP/p300 during mitotic exit did not impact these cohesin-independent interactions, suggesting the presence of complex compensatory mechanisms. At the transcriptional level, cohesin depletion induced only minor changes, supporting that post-mitotic transcriptional reactivation is largely independent of cohesin. The few genes with impaired reactivation were directly bound by RAD21 at their promoters, engaged in many structural loops, and located within strongly insulated TADs with low gene density. Importantly, degrading cohesin during the M-to-G1 transition in the presence of EpiLC differentiation signals revealed a larger group of susceptible genes, including key signature genes and transcription factors. Impaired activation of these genes was partly due to the failure to establish de novo EpiLC-specific interactions in the absence of cohesin. These experiments revealed locus-specific and context-specific dependencies between cohesin, E-P interactions, and transcription.

Microtubule (MT) minus ends in HeLa cell metaphase spindles have open or closed morphologies in electron tomograms. The proportion of open MT minus ends increases upon silencing the minus end–specific stabilizer, MCRS1, suggesting a correlation between their morphology and dynamic state. This points to an unsynchronized behavior of MTs at the spindle poles.

Faithful chromosome segregation requires the assembly of a bipolar spindle, consisting of two antiparallel microtubule (MT) arrays having most of their minus ends focused at the spindle poles and their plus ends overlapping in the spindle midzone. Spindle assembly, chromosome alignment and segregation require highly dynamic MTs. The plus ends of MTs have been extensively investigated; instead, their minus end structure remains poorly characterized. Here, we used large-scale electron tomography to study the morphology of the MT minus ends in 3D-reconstructed metaphase spindles in HeLa cells. In contrast to the homogeneous open morphology of the MT plus ends at the kinetochores, we found that MT minus ends are heterogeneous showing either open or closed morphologies. Silencing the minus-end specific stabilizer, MCRS1 increased the proportion of open MT minus ends. Altogether, these data suggest a correlation between the morphology and the dynamic state of the MT ends. Taking this heterogeneity of the MT minus end morphologies into account, our work indicates an unsynchronized behavior of MTs at the spindle poles, thus laying the ground for further studies on the complexity of MT dynamics regulation.