Deregulation of KSHV latency conformation by ER-stress and caspase-dependent RAD21-cleavage

Kaposi’s sarcoma (KS)-associated herpesvirus (KSHV) is a human gammaherpesvirus recognized as the principal causative agent of KS and primary effusion lymphoma (PEL). KSHV establishes persistent latent infection in B lymphocytes where viral gene expression is restricted, in part, by a cohesin-dependent chromosome conformation. Here, we show that endoplasmic reticulum (ER) stress induces a rapid, caspase-dependent cleavage of cohesin subunit RAD21. ER stress-induced cleavage of RAD21 correlated with a rapid and strong viral lytic transcriptional activation. This effect was observed in several KSHV positive PEL cells, but not in other B-cells or non-B-cell models of KSHV latency. The cleaved-RAD21 does not dissociate from viral genomes, nor disassemble from other components of the cohesin complex. However, RAD21 cleavage correlated with the disruption of the latency genome conformation as revealed by chromosome conformation capture (3C). Ectopic expression of C-terminal RAD21 cleaved form could partially induce KSHV lytic genes transcription in BCBLI cells, suggesting that ER-stress induced RAD21 cleavage was sufficient to induce KSHV reactivation from latency in PEL cells. Taken together our results reveal a novel aspect for control and maintenance of KSHV genome latency conformation mediated by stress-induced RAD21 cleavage. Our studies also suggest that RAD21 cleavage may be a general regulatory mechanism for rapid alteration of cellular chromosome conformation and cohesin-dependent transcription regulation.

卡波西肉瘤相关疱疹病毒（Kaposi’s sarcoma-associated herpesvirus, KSHV）是一种人类γ疱疹病毒，被认为是卡波西肉瘤（Kaposi’s sarcoma, KS）和原发性渗出性淋巴瘤（primary effusion lymphoma, PEL）的主要致病原。KSHV可在B淋巴细胞中建立持续性潜伏感染，该环境下病毒基因表达的受限过程部分依赖于黏连蛋白（cohesin）介导的染色体构象调控。本研究发现，内质网（endoplasmic reticulum, ER）应激可诱导黏连蛋白亚基RAD21发生快速的半胱天冬酶（caspase）依赖的切割。内质网应激诱导的RAD21切割，与病毒裂解性转录激活的快速且强效诱导显著相关。该效应在数株KSHV阳性的PEL细胞中均有观测到，但在其他B细胞或非B细胞来源的KSHV潜伏感染模型中则未出现。切割后的RAD21并不会从病毒基因组上解离，也不会与黏连蛋白复合物的其他组分发生解聚。然而，通过染色体构象捕获（chromosome conformation capture, 3C）实验证实，RAD21的切割与潜伏感染基因组构象的破坏存在显著相关性。异位表达RAD21的C端切割片段，可在BCBLI细胞中部分诱导KSHV裂解基因的转录，这表明内质网应激诱导的RAD21切割足以触发PEL细胞中KSHV从潜伏状态重新激活。综上，本研究结果揭示了应激诱导的RAD21切割介导的KSHV基因组潜伏构象调控与维持的全新机制。本研究同时表明，RAD21切割可能是一种通用的调控通路，可快速改变细胞染色体构象以及黏连蛋白依赖的转录调控过程。