G₀ phase

The G₀ phase describes a cellular state outside of the replicative cell cycle. Classically^[when?], cells were thought to enter G₀ primarily due to environmental factors, like nutrient deprivation, that limited the resources necessary for proliferation. Thus it was thought of as a resting phase. G₀ is now known to take different forms and occur for multiple reasons. For example, most adult neuronal cells, among the most metabolically active cells in the body, are fully differentiated and reside in a terminal G₀ phase. Neurons reside in this state, not because of stochastic or limited nutrient supply, but as a part of their developmental program.

G₀ was first suggested as a cell state based on early cell cycle studies. When the first studies defined the four phases of the cell cycle using radioactive labeling techniques, it was discovered that not all cells in a population proliferate at similar rates.^[1] A population's "growth fraction" – or the fraction of the population that was growing – was actively proliferating, but other cells existed in a non-proliferative state. Some of these non-proliferating cells could respond to extrinsic stimuli and proliferate by re-entering the cell cycle.^[2] Early contrasting views either considered non-proliferating cells to simply be in an extended G₁ phase or in a cell cycle phase distinct from G₁ – termed G₀.^[3] Subsequent research pointed to a restriction point (R-point) in G₁ where cells can enter G₀ before the R-point but are committed to mitosis after the R-point.^[4] These early studies provided evidence for the existence of a G₀ state to which access is restricted. These cells that do not divide further exit G₁ phase to enter an inactive stage called quiescent stage.

Diversity of G₀ states

Three G₀ states exist and can be categorized as either reversible (quiescent) or irreversible (senescent and differentiated). Each of these three states can be entered from the G₁ phase before the cell commits to the next round of the cell cycle. Quiescence refers to a reversible G₀ state where subpopulations of cells reside in a 'quiescent' state before entering the cell cycle after activation in response to extrinsic signals. Quiescent cells are often identified by low RNA content, lack of cell proliferation markers, and increased label retention indicating low cell turnover.^[5][6] Senescence is distinct from quiescence because senescence is an irreversible state that cells enter in response to DNA damage or degradation that would make a cell's progeny nonviable. Such DNA damage can occur from telomere shortening over many cell divisions as well as reactive oxygen species (ROS) exposure, oncogene activation, and cell-cell fusion. While senescent cells can no longer replicate, they remain able to perform many normal cellular functions.^{[7][8][9][10]} Senescence is often a biochemical alternative to the self-destruction of such a damaged cell by apoptosis. In contrast to cellular senescence, quiescence is not a reactive event but part of the core programming of several different cell types. Finally, differentiated cells are stem cells that have progressed through a differentiation program to reach a mature – terminally differentiated – state. Differentiated cells continue to stay in G₀ and perform their main functions indefinitely.

Characteristics of quiescent stem cells

Transcriptomes

The

Stem cells are cells with the unique ability to produce differentiated daughter cells and to preserve their stem cell identity through self-renewal.^[12] In mammals, most adult tissues contain tissue-specific stem cells that reside in the tissue and proliferate to maintain homeostasis for the lifespan of the organism. These cells can undergo immense proliferation in response to tissue damage before differentiating and engaging in regeneration. Some tissue stem cells exist in a reversible, quiescent state indefinitely until being activated by external stimuli. Many different types of tissue stem cells exist, including muscle stem cells (MuSCs), neural stem cells (NSCs), intestinal stem cells (ISCs), and many others.

Stem cell quiescence has been recently suggested to be composed of two distinct functional phases, G₀ and an 'alert' phase termed G_Alert.^[13] Stem cells are believed to actively and reversibly transition between these phases to respond to injury stimuli and seem to gain enhanced tissue regenerative function in G_Alert. Thus, transition into G_Alert has been proposed as an adaptive response that enables stem cells to rapidly respond to injury or stress by priming them for cell cycle entry. In muscle stem cells,

While a reversible quiescent state is perhaps most important for tissue stem cells to respond quickly to stimuli and maintain proper homeostasis and regeneration, reversible G₀ phases can be found in non-stem cells such as mature hepatocytes.[14] Hepatocytes are typically quiescent in normal livers but undergo limited replication (less than 2 cell divisions) during liver regeneration after partial hepatectomy. However, in certain cases, hepatocytes can experience immense proliferation (more than 70 cell divisions) indicating that their proliferation capacity is not hampered by existing in a reversible quiescent state.^[14]

Examples of irreversible G₀ phase

Senescent cells

Often associated with aging and age-related diseases in vivo, senescent cells can be found in many renewable tissues, including the

Differentiated muscle

During skeletal

Of the four major types of bone cells, osteocytes are the most common and also exist in a terminal G₀ phase. Osteocytes arise from osteoblasts that are trapped within a self-secreted matrix. While osteocytes also have reduced synthetic activity, they still serve bone functions besides generating structure. Osteocytes work through various mechanosensory mechanisms to assist in the routine turnover over bony matrix.

Differentiated nerve

Outside of a few neurogenic niches in the brain, most neurons are fully differentiated and reside in a terminal G₀ phase. These fully differentiated neurons form synapses where electrical signals are transmitted by axons to the dendrites of nearby neurons. In this G₀ state, neurons continue functioning until senescence or apoptosis. Numerous studies have reported accumulation of DNA damage with age, particularly oxidative damage, in the mammalian brain.^[18]

Mechanism of G₀ entry

Role of Rim15

Rim15 was first discovered to play a critical role in initiating

Yeast grows exponentially through fermentation of glucose. When glucose levels drop, yeast shift from fermentation to cellular respiration, metabolizing the fermentative products from their exponential growth phase. This shift is known as the diauxic shift after which yeast enter G₀. When glucose levels in the surroundings are high, the production of cAMP through the RAS-cAMP-PKA pathway (a cAMP-dependent pathway) is elevated, causing protein kinase A (PKA) to inhibit its downstream target Rim15 and allow cell proliferation. When glucose levels drop, cAMP production declines, lifting PKA's inhibition of Rim15 and allowing the yeast cell to enter G₀.^[19]

Nitrogen

In addition to glucose, the presence of nitrogen is crucial for yeast proliferation. Under low nitrogen conditions, Rim15 is activated to promote cell cycle arrest through inactivation of the protein kinases

Yeast cells respond to low extracellular phosphate levels by activating genes that are involved in the production and upregulation of inorganic phosphate. The PHO pathway is involved in the regulation of phosphate levels. Under normal conditions, the yeast cyclin-dependent kinase complex, Pho80-Pho85, inactivates the Pho4 transcription factor through phosphorylation. However, when phosphate levels drop, Pho81 inhibits Pho80-Pho85, allowing Pho4 to be active. When phosphate is abundant, Pho80-Pho85 also inhibits the nuclear pool of Rim 15 by promoting phosphorylation of its Thr1075 Bmh2 binding site. Thus, Pho80-Pho85 acts in concert with Sch9 and TORC1 to promote cytoplasmic retention of Rim15 under normal conditions.^[19]

Mechanism of G₀ exit

Cyclin C/Cdk3 and Rb

The transition from G₁ to