Stabilizing heterochromatin by ZKSCAN3 safeguards hMSCs from senescence

Zinc finger protein with KRAB and SCAN domain 3 (ZKSCAN3), a transcriptional repressor, involves in multiple cellular functions. However, the functions of ZKSCAN3 in the homeostatic maintenance of human stem cells remains elusive. Here, we demonstrated that ZKSCAN3 was crucial for preventing human mesenchymal stem cells (hMSCs) from senescence in an autophagy-independent manner. Downregulation of ZKSCAN3 was observed in senescent hMSCs, and depletion of ZKSCAN3 led to premature aging in hMSCs. Further study uncovered that ZKSCAN3 maintained heterochromatin (HC) stability by interacting with heterochromatin associated proteins KAP1 and HP1 as well as nuclear envelope proteins Lamin B1, LBR and Emerin. Deficiency of ZKSCAN3 resulted in detachment of Lamina-associated domains (LADs) from the lamina, loss of heterochromatin and more accessible chromatin status within the heterochromatin and aberrant expression of repetitive sequences. Overexpression of ZKSCAN3, KAP1 or HP1 respectively rescued the senescent phenotypes in ZKSCAN3-/- hMSCs. Notably, reintroduction of ZKSCAN3 protein also retarded the cellular senescence in the replicative senescent hMSCs, as well as the pathological or physiological aging hMSCs. Together, our study reveals a novel, autophagy-independent role of ZKSCAN3 in the maintenance of heterochromatin stability and attenuation of hMSC senescence. Thus, ZKSCAN3 may be a candidate for alleviating human aging-related disorders.

含KRAB与SCAN结构域的锌指蛋白3（Zinc finger protein with KRAB and SCAN domain 3，ZKSCAN3）是一种转录抑制因子，参与多种细胞生物学过程。然而，ZKSCAN3在人类干细胞稳态维持中的具体功能仍未阐明。本研究证实，ZKSCAN3可通过不依赖自噬的途径阻止人间充质干细胞（human mesenchymal stem cells，hMSCs）发生衰老。在衰老的hMSCs中可检测到ZKSCAN3的表达下调，而敲除ZKSCAN3会诱导hMSCs提前衰老。进一步研究发现，ZKSCAN3可通过与异染色质相关蛋白KAP1、HP1α以及核被膜蛋白Lamin B1、LBR和Emerin相互作用，维持异染色质（heterochromatin，HC）的稳定性。ZKSCAN3缺失会导致核纤层关联结构域（Lamina-associated domains，LADs）与核纤层分离，引发异染色质丢失、异染色质区域染色质可及性升高以及重复序列的异常表达。分别过表达ZKSCAN3、KAP1或HP1α，均可挽救ZKSCAN3基因敲除hMSCs的衰老表型。值得注意的是，重新导入ZKSCAN3蛋白同样可延缓复制性衰老hMSCs以及病理性或生理性衰老hMSCs的细胞衰老进程。综上，本研究揭示了ZKSCAN3在维持异染色质稳定性以及延缓hMSCs衰老方面的全新不依赖自噬的作用机制。因此，ZKSCAN3或可作为缓解人类衰老相关疾病的潜在候选靶点。