Nuclear processes, such as V(D)J recombination, are orchestrated by the three-dimensional organization of chromosomes at multiple levels, including compartments1 and topologically associated domains (TADs)2,3 consisting of chromatin loops4. TADs are formed by chromatin-loop extrusion5–7, which depends on the loop-extrusion function of the ring-shaped cohesin complex8–12. Conversely, the cohesin-release factor Wapl13,14 restricts loop extension10,15. The generation of a diverse antibody repertoire, providing humoral immunity to pathogens, requires the participation of all V genes in V(D)J recombination16, which depends on contraction of the 2.8-Mb-long immunoglobulin heavy chain (Igh) locus by Pax517,18. However, how Pax5 controls Igh contraction in pro-B cells remains unknown. Here we demonstrate that locus contraction is caused by loop extrusion across the entire Igh locus. Notably, the expression of Wapl is repressed by Pax5 specifically in pro-B and pre-B cells, facilitating extended loop extrusion by increasing the residence time of cohesin on chromatin. Pax5 mediates the transcriptional repression of Wapl through a single Pax5-binding site by recruiting the polycomb repressive complex 2 to induce bivalent chromatin at the Wapl promoter. Reduced Wapl expression causes global alterations in the chromosome architecture, indicating that the potential to recombine all V genes entails structural changes of the entire genome in pro-B cells. Pax5 regulates contraction of the immunoglobulin heavy chain (Igh) locus—an essential step in V(D)J recombination—by promoting chromatin loop extrusion via repression of Wapl expression.

中文翻译：

---

Pax5 对 Wapl 的抑制通过 Igh 环挤出促进 V 基因重组

核过程，如 V(D)J 重组，是由染色体在多个层面的三维组织精心策划的，包括隔室 1 和由染色质环组成的拓扑相关域 (TAD)2,3。TAD 由染色质环挤出 5-7 形成，这取决于环形粘连蛋白复合物的环挤出功能 8-12。相反，cohesin 释放因子 Wapl13,14 限制循环扩展 10,15。产生多种抗体库，为病原体提供体液免疫，需要所有 V 基因参与 V(D)J 重组，这取决于 Pax517 对 2.8 Mb 长免疫球蛋白重链 (Igh) 基因座的收缩， 18. 然而，Pax5 如何控制 pro-B 细胞中的 Igh 收缩仍然未知。在这里，我们证明了基因座收缩是由整个 Igh 基因座的循环挤压引起的。值得注意的是，Wap1 的表达被 Pax5 特异性地抑制在 pro-B 和 pre-B 细胞中，通过增加 cohesin 在染色质上的停留时间来促进延长的环挤出。Pax5 通过招募多梳抑制复合物 2 在 Wapl 启动子处诱导二价染色质，通过单个 Pax5 结合位点介导 Wapl 的转录抑制。Wapl 表达的减少导致染色体结构的整体改变，表明重组所有 V 基因的潜力需要 pro-B 细胞中整个基因组的结构变化。