Biology subjects

Casellas, R.

Publications and source records attributed to Casellas, R..

8 recordsLinked to original sources

bioRxiv2024

Multifaceted roles of cohesin in regulating transcriptional loops

Cohesin is required for chromatin loop formation. However, its precise role in regulating gene transcription remains largely unknown. We investigated the relationship between cohesin and RNA Polymerase II (RNAPII) using single-molecule mapping and live-cell imaging methods in human cells. Cohesin-mediated transcriptional loops were highly correlated with those of RNAPII and followed the direction of gene transcription. Depleting RAD21, a subunit of cohesin, resulted in the loss of long-range (>100 kb) loops between distal (super-)enhancers and promoters of cell-type-specific genes. By contrast, the short-range (<50 kb) loops were insensitive to RAD21 depletion and connected genes that are mostly housekeeping. This result explains why only a small fraction of genes are affected by the loss of long-range chromatin interactions due to cohesin depletion. Remarkably, RAD21 depletion appeared to up-regulate genes located in early initiation zones (EIZ) of DNA replication, and the EIZ signals were amplified drastically without RAD21. Our results revealed new mechanistic insights of cohesins multifaceted roles in establishing transcriptional loops, preserving long-range chromatin interactions for cell-specific genes, and maintaining timely order of DNA replication.

genomics↗

bioRxiv2021

Multimeric nanobodies from camelid engineered mice and llamas potently neutralize SARS-CoV-2 variants

Since the start of the coronavirus disease-2019 (COVID-19) pandemic, severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) has caused more than 2 million deaths worldwide. Multiple vaccines have been deployed to date, but the continual evolution of the viral receptor-binding domain (RBD) has recently challenged their efficacy. In particular, SARS-CoV-2 variants originating in the U.K. (B.1.1.7), South Africa (B.1.351) and New York (B.1.526) have reduced neutralization activity from convalescent sera and compromised the efficacy of antibody cocktails that received emergency use authorization. Whereas vaccines can be updated periodically to account for emerging variants, complementary strategies are urgently needed to avert viral escape. One potential alternative is the use of camelid VHHs (also known as nanobodies), which due to their small size can recognize protein crevices that are inaccessible to conventional antibodies. Here, we isolate anti-RBD nanobodies from llamas and "nanomice" we engineered to produce VHHs cloned from alpacas, dromedaries and camels. Through binding assays and cryo-electron microscopy, we identified two sets of highly neutralizing nanobodies. The first group expresses VHHs that circumvent RBD antigenic drift by recognizing a region outside the ACE2-binding site that is conserved in coronaviruses but is not typically targeted by monoclonal antibodies. The second group is almost exclusively focused to the RBD-ACE2 interface and fails to neutralize pseudoviruses carrying the E484K or N501Y substitutions. Notably however, they do neutralize the RBD variants when expressed as homotrimers, rivaling the most potent antibodies produced to date against SARS-CoV-2. These findings demonstrate that multivalent nanobodies overcome SARS-CoV-2 variant mutations through two separate mechanisms: enhanced avidity for the ACE2 binding domain, and recognition of conserved epitopes largely inaccessible to human antibodies. Therefore, while new SARS-CoV-2 mutants will continue to emerge, nanobodies represent promising tools to prevent COVID-19 mortality when vaccines are compromised.

immunology↗

bioRxiv2021

mRNA vaccine-elicited antibodies to SARS-CoV-2 and circulating variants

To date severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) has infected over 100 million individuals resulting in over two million deaths. Many vaccines are being deployed to prevent coronavirus disease 2019 (COVID-19) including two novel mRNA-based vaccines1,2. These vaccines elicit neutralizing antibodies and appear to be safe and effective, but the precise nature of the elicited antibodies is not known3-6. Here we report on the antibody and memory B cell responses in a cohort of 20 volunteers who received either the Moderna (mRNA-1273) or Pfizer-BioNTech (BNT162b2) vaccines. Consistent with prior reports, 8 weeks after the second vaccine injection volunteers showed high levels of IgM, and IgG anti-SARS-CoV-2 spike protein (S) and receptor binding domain (RBD) binding titers3,5,6. Moreover, the plasma neutralizing activity, and the relative numbers of RBD-specific memory B cells were equivalent to individuals who recovered from natural infection7,8. However, activity against SARS-CoV-2 variants encoding E484K or N501Y or the K417N:E484K:N501Y combination was reduced by a small but significant margin. Consistent with these findings, vaccine-elicited monoclonal antibodies (mAbs) potently neutralize SARS-CoV-2, targeting a number of different RBD epitopes in common with mAbs isolated from infected donors. Structural analyses of mAbs complexed with S trimer suggest that vaccine- and virus-encoded S adopts similar conformations to induce equivalent anti-RBD antibodies. However, neutralization by 14 of the 17 most potent mAbs tested was reduced or abolished by either K417N, or E484K, or N501Y mutations. Notably, the same mutations were selected when recombinant vesicular stomatitis virus (rVSV)/SARS-CoV-2 S was cultured in the presence of the vaccine elicited mAbs. Taken together the results suggest that the monoclonal antibodies in clinical use should be tested against newly arising variants, and that mRNA vaccines may need to be updated periodically to avoid potential loss of clinical efficacy.

immunology↗

bioRxiv2020

Structure of the Mammalian Mediator

The Mediator complex plays an essential and multi-faceted role in regulation of RNA polymerase II transcription in all eukaryotes. Structural analysis of yeast Mediator has provided an understanding of the conserved core of the complex and its interaction with RNA polymerase II but failed to reveal the structure of the Tail module that contains most subunits targeted by activators and repressors. Here we present a molecular model of mammalian (Mus musculus) Mediator, derived from a 4.0 [A] resolution cryo-EM map of the complex. The mammalian Mediator structure reveals that the previously unresolved Tail module, which includes a number of metazoan specific subunits, interacts extensively with core Mediator and has the potential to influence its conformation and interactions.

cell biology↗

bioRxiv2020

Loop Extrusion Mediates Physiological Locus Contraction for V(D)J Recombination

Immunoglobulin heavy chain locus (Igh) VH, D, and JH gene segments are developmentally assembled into V(D)J exons. RAG endonuclease initiates V(D)J recombination by binding a JH-recombination signal sequence (RSS) within a chromatin-based recombination center (RC) and then, in an orientation-dependent process, scans upstream D-containing chromatin presented by cohesin-mediated loop extrusion for convergent D-RSSs to initiate DJH-RC formation1,2. In primary pro-B cells, 100s of upstream VH-associated RSSs, embedded in convergent orientation to the DJH-RC-RSS, gain proximity to the DJH-RC for VH-to-DJH joining via a mechanistically-undefined VH-locus contraction process3-7. Here, we report that a 2.4 mega-base VH locus inversion in primary pro-B cells nearly abrogates rearrangements of normally convergent VH-RSSs and cryptic RSSs, even though locus contraction per se is maintained. Moreover, this inversion activated rearrangement of both cryptic VH-locus RSSs normally in the opposite orientation and, unexpectedly, of normally-oriented cryptic RSSs within multiple, sequential upstream convergent-CBE domains. Primary pro-B cells had significantly reduced transcription of Wapl8, a cohesin-unloading factor, versus levels in v-Abl pro-B lines that lack marked locus contraction or distal VH rearrangements2,9-11. Correspondingly, Wapl depletion in v-Abl lines activated VH-locus contraction and orientation-specific RAG-scanning across the VH-locus. Our findings indicate that locus contraction and physiological VH-to-DJH joining both are regulated via circumvention of CBE scanning impediments.Competing Interest StatementThe authors have declared no competing interest.View Full Text

immunology↗

bioRxiv2020

CTCF orchestrates long-range cohesin-driven V(D)J recombinational scanning

RAG endonuclease initiates V(D)J recombination in progenitor (pro)-B cells1. Upon binding a recombination center (RC)-based JH, RAG scans upstream chromatin via loop extrusion, potentially mediated by cohesin2-10, to locate Ds and assemble a DJH-based RC11. CTCF looping factor-bound elements (CBEs) within IGCR1 upstream of Ds impede RAG-scanning12-15; but their inactivation allows scanning to proximal VHs where additional CBEs activate rearrangement and impede scanning any further upstream15, 16. Distal VH utilization is thought to involve diffusional RC access following large-scale Igh locus contraction17-23. Here, we test the potential of linear RAG-scanning to mediate distal VH usage in G1-arrested, v-Abl-pro-B cell lines24, 25, which undergo robust D-to-JH but little VH-to-DJH rearrangements, presumably due to lack of locus contraction11, 15. Through an auxin-inducible approach26, 27, we degrade the cohesin-component Rad214, 7, 27 or CTCF7, 9 in these G1-arrested lines, which maintain substantial viability throughout four-day experiments. Rad21 degradation eliminated all V(D)J recombination and RAG-scanning-associated interactions, except RC-located DQ52-to-JH joining in which synapsis occurs by diffusion11. Remarkably, while CTCF degradation suppressed most CBE-based chromatin interactions, it promoted robust RC interactions with, and robust VH-to-DJH joining of, distal VHs, with patterns similar to those of "locus-contracted" primary pro-B cells. Thus, down-modulation of CTCF-bound scanning-impediment activity promotes cohesin-driven RAG-scanning across the 2.7Mb Igh locus.

immunology↗

bioRxiv2019

Super-resolution Imaging Reveals 3D Structure and Organizing Mechanism of Accessible Chromatin

Access to cis-regulatory elements packaged in chromatin is essential for directing gene expression and cell viability. Here, we report a super-resolution imaging strategy, 3D ATAC-PALM, that enables direct visualization of the entire accessible genome. We found that active chromosomal segments are organized into spatially-segregated accessible chromatin domains (ACDs). Rapid depletion of CTCF or Cohesin (RAD21 subunit) induced enhanced ACD clustering, reduced physical separation between intrachromosomal ACDs, and differentially regulated ACD compaction. Experimental perturbations and polymer modeling suggest that dynamic protein-protein and protein-DNA interactions within ACDs couple with loop extrusion to organize ACD topology. Dysorganization of ACDs upon CTCF or Cohesin loss alters transcription factor binding and target search dynamics in living cells. These results uncover fundamental mechanisms underpinning the formation of 3D genome architecture and its pivotal function in transcriptional regulation.

molecular biology↗

bioRxiv2017

Cohesin Loss Eliminates All Loop Domains, Leading To Links Among Superenhancers And Downregulation Of Nearby Genes

The human genome folds to create thousands of intervals, called \"contact domains,\" that exhibit enhanced contact frequency within themselves. \"Loop domains\" form because of tethering between two loci - almost always bound by CTCF and cohesin - lying on the same chromosome. \"Compartment domains\" form when genomic intervals with similar histone marks co-segregate. Here, we explore the effects of degrading cohesin. All loop domains are eliminated, but neither compartment domains nor histone marks are affected. Loci in different compartments that had been in the same loop domain become more segregated. Loss of loop domains does not lead to widespread ectopic gene activation, but does affect a significant minority of active genes. In particular, cohesin loss causes superenhancers to co-localize, forming hundreds of links within and across chromosomes, and affecting the regulation of nearby genes. Cohesin restoration quickly reverses these effects, consistent with a model where loop extrusion is rapid.

genomics↗