Graded regulation of cellular quiescence depth between proliferation and senescence by a lysosomal dimmer switch

The reactivation of quiescent cells to proliferate is fundamental to tissue repair and homeostasis in the body. Often referred to as the G0 state, quiescence is however not a uniform state but with graded depth. Shallow quiescent cells exhibit a higher tendency to revert to proliferation than deep quiescent cells, while deep quiescent cells are still fully reversible under physiological conditions, distinct from senescent cells. Cellular mechanisms underlying the control of quiescence depth and the connection between quiescence and senescence are poorly characterized, representing a missing link in our understanding of tissue homeostasis and regeneration. Here we measured transcriptome changes as rat embryonic fibroblasts moved from shallow to deep quiescence over time in the absence of growth signals. We found that lysosomal gene expression was significantly upregulated in deep quiescence, and partially compensated for gradually reduced autophagy flux. Reducing lysosomal function drove cells progressively deeper into quiescence and eventually into a senescence-like irreversibly arrested state; increasing lysosomal function, by lowering oxidative stress, progressively pushed cells into shallower quiescence. That is, lysosomal function modulates graded quiescence depth between proliferation and senescence as a dimmer switch. Lastly, we found that a gene expression signature developed by comparing deep and shallow quiescence in fibroblasts can correctly classify a wide array of senescent and aging cell types in vitro and in vivo, suggesting that while quiescence is generally considered to protect cells from irreversible arrest of senescence, quiescence deepening likely represents a common transition path from cell proliferation to senescence, related to aging.静息细胞重新激活并增殖，是机体组织修复与稳态维持的核心基础。静息状态通常被称为G0期（G0 state），但并非均一的状态，而是存在深度梯度之分。浅静息细胞恢复增殖的倾向远高于深静息细胞；不过在生理条件下，深静息细胞仍可完全逆转，这与衰老细胞截然不同。调控静息深度的细胞机制，以及静息与衰老之间的关联，目前尚未得到充分阐明，这也是我们理解组织稳态与再生过程中的一处缺失环节。本研究在无生长信号的条件下，随时间动态检测了大鼠胚胎成纤维细胞从浅静息状态向深静息状态转变过程中的转录组（transcriptome）变化。我们发现，溶酶体（lysosomal）基因的表达在深静息状态中显著上调，并可部分补偿逐渐减弱的自噬流（autophagy flux）。抑制溶酶体功能会促使细胞逐步加深静息状态，最终进入类似衰老的不可逆阻滞状态；而通过降低氧化应激提升溶酶体功能，则会推动细胞逐步进入更浅的静息状态。换言之，溶酶体功能如同调光开关一般，可调控增殖与衰老之间的静息深度梯度。最后，我们发现通过对比成纤维细胞深、浅静息状态所构建的基因表达特征，能够准确对体外及体内的多种衰老与衰老相关细胞类型进行分类。这表明，尽管通常认为静息状态可保护细胞免于发生不可逆的衰老阻滞，但静息深度的加深或许是细胞从增殖状态转向衰老状态的一条常见通路，且该过程与衰老进程相关。