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RAG endonuclease initiates Igh V(D)J recombination in progenitor B cells by binding a J H -recombination signal sequence (RSS) within a recombination centre (RC) and then linearly scanning upstream chromatin, presented by loop extrusion mediated by cohesin, for convergent D-RSSs 1,2 . The utilization of convergently oriented RSSs and cryptic RSSs is intrinsic to long-range RAG scanning 3 . Scanning of RAG from the DJ H -RC-RSS to upstream convergent V H -RSSs is impeded by D-proximal CTCF-binding elements (CBEs) 2-5 . Primary progenitor B cells undergo a mechanistically undefined contraction of the V H locus that is proposed to provide distal V H s access to the DJ H -RC 6-9 . Here we report that an inversion of the entire 2.4-Mb V H locus in mouse primary progenitor B cells abrogates rearrangement of both V H -RSSs and normally convergent cryptic RSSs, even though locus contraction still occurs. In addition, this inversion activated both the utilization of cryptic V H -RSSs that are normally in opposite orientation and RAG scanning beyond the V H locus through several convergent CBE domains to the telomere. Together, these findings imply that broad deregulation of CBE impediments in primary progenitor B cells promotes RAG scanning of the V H locus mediated by loop extrusion. We further found that the expression of wings apart-like protein homologue (WAPL) 10 , a cohesin-unloading factor, was low in primary progenitor B cells compared with v-Abl-transformed progenitor B cell lines that lacked contraction and RAG scanning of the V H locus. Correspondingly, depletion of WAPL in v-Abl-transformed lines activated both processes, further implicating loop extrusion in the locus contraction mechanism.

IgH class switch recombination (CSR) replaces Cμ constant region (C H ) exons with one of six downstream C H s by joining transcription-targeted double-strand breaks (DSBs) in the Cμ switch (S) region to DSBs in a downstream S region. Chromatin loop extrusion underlies fundamental CSR mechanisms including 3'IgH regulatory region (3'IgHRR)-mediated S region transcription, CSR center formation, and deletional CSR joining. There are 10 consecutive CTCF-binding elements (CBEs) downstream of the 3'IgHRR, termed the "3'IgH CBEs." Prior studies showed that deletion of eight 3'IgH CBEs did not detectably affect CSR. Here, we report that deletion of all 3'IgH CBEs impacts, to varying degrees, germline transcription and CSR of upstream S regions, except that of Sγ1. Moreover, deletion of all 3'IgH CBEs rendered the 6-kb region just downstream highly transcribed and caused sequences within to be aligned with Sμ, broken, and joined to form aberrant CSR rearrangements. These findings implicate the 3'IgH CBEs as critical insulators for focusing loop extrusion-mediated 3'IgHRR transcriptional and CSR activities on upstream C H locus targets., Competing Interests: The authors declare no competing interest., (Copyright © 2021 the Author(s). Published by PNAS.)

We provide a protocol for generating forebrain structures in vivo from mouse embryonic stem cells (ESCs) via neural blastocyst complementation (NBC). We developed this protocol for studies of development and function of specific forebrain regions, including the cerebral cortex and hippocampus. We describe a complete workflow, from methods for modifying a given genomic locus in ESCs via CRISPR-Cas9-mediated editing to the generation of mouse chimeras with ESC-reconstituted forebrain regions that can be directly analyzed. The procedure begins with genetic editing of mouse ESCs via CRISPR-Cas9, which can be accomplished in ~4-8 weeks. We provide protocols to achieve fluorescent labeling of ESCs in ~2-3 weeks, which allows tracing of the injected, ESC-derived donor cells in chimeras generated via NBC. Once modified ESCs are ready, NBC chimeras are generated in ~3 weeks via injection of ESCs into genetically programmed blastocysts that are subsequently transferred into pseudo-pregnant fosters. Our in vivo brain organogenesis platform is efficient, allowing functional and systematic analysis of genes and other genomic factors in as little as 3 months, in the context of a whole organism.

Genetically modified mice are commonly generated by the microinjection of pluripotent embryonic stem (ES) cells into wild-type host blastocysts 1 , producing chimeric progeny that require breeding for germline transmission and homozygosity of modified alleles. As an alternative approach and to facilitate studies of the immune system, we previously developed RAG2-deficient blastocyst complementation 2 . Because RAG2-deficient mice cannot undergo V(D)J recombination, they do not develop B or T lineage cells beyond the progenitor stage 2 : injecting RAG2-sufficient donor ES cells into RAG2-deficient blastocysts generates somatic chimaeras in which all mature lymphocytes derive from donor ES cells. This enables analysis, in mature lymphocytes, of the functions of genes that are required more generally for mouse development 3 . Blastocyst complementation has been extended to pancreas organogenesis 4 , and used to generate several other tissues or organs 5-10 , but an equivalent approach for brain organogenesis has not yet been achieved. Here we describe neural blastocyst complementation (NBC), which can be used to study the development and function of specific forebrain regions. NBC involves targeted ablation, mediated by diphtheria toxin subunit A, of host-derived dorsal telencephalic progenitors during development. This ablation creates a vacant forebrain niche in host embryos that results in agenesis of the cerebral cortex and hippocampus. Injection of donor ES cells into blastocysts with forebrain-specific targeting of diphtheria toxin subunit A enables donor-derived dorsal telencephalic progenitors to populate the vacant niche in the host embryos, giving rise to neocortices and hippocampi that are morphologically and neurologically normal with respect to learning and memory formation. Moreover, doublecortin-deficient ES cells-generated via a CRISPR-Cas9 approach-produced NBC chimaeras that faithfully recapitulated the phenotype of conventional, germline doublecortin-deficient mice. We conclude that NBC is a rapid and efficient approach to generate complex mouse models for studying forebrain functions; this approach could more broadly facilitate organogenesis based on blastocyst complementation.