ZZZ3 protects human embryonic stem cells from nucleolar stress by boosting mTOR/ribosome pathway

Embryonic stem cells (ESCs) are defined as stem cells with self-renewing and differentiation capabilities. These unique properties are tightly regulated and controlled by complex genetic and molecular mechanisms whose understanding is essential for both basic and translational research. A large number of existing studies in the broader literature have mostly focused on understanding the molecular mechanisms governing pluripotency and differentiation of ESCs, while the regulation of proliferation has received comparably less attention. Proliferation is a fundamental aspect of stem cell biology and despite its importance, the intricate mechanisms governing proliferation in human ESCs are not fully understood. In mouse ESCs, pluripotency and proliferation can be independent processes meaning that it is possible for mouse ESCs to maintain their pluripotent state without actively proliferating. Ribosome biogenesis and translation have emerged as critical pathways that play essential roles in regulating stem cell homeostasis influencing cell growth and division rates. Here, we aim at investigating the role of ZZZ3 (Zinc Finger ZZ-Type Containing 3) in human ESCs homeostasis. ZZZ3 is expressed in both mouse and human pluripotent stem cells and regulates genes encoding ribosomal proteins (RPGs). However, its precise function in human pluripotent stem cells (PSCs) still remains elusive. Interestingly, we found that knockdown of ZZZ3 strongly decreases ribosome biogenesis, translation, and mTOR signaling leading to nucleolar stress and significant reduction of cell proliferation. This process occurs without affecting pluripotency, suggesting that ZZZ3-depleted ESCs enter a dormant-like state and that proliferation and pluripotency can be uncoupled also in human ESCs. Our study shows that ZZZ3 controls ribosome biogenesis, translation, and proliferation of stem cells and protects hESCs from nucleolar stress.