G0 phase

right|thumb|350px|Mitosis in an [[animal cell (phases ordered counter-clockwise), with G₀ labeled at left.]]

thumb|right|200px|Many mammal [[cell (biology)|cells, such as this 9x H neuron, remain permanently or semipermanently in G₀.]]

The G₀ phase describes a cellular state outside of the replicative cell cycle. Classically, 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. 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. 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. 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. 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

thumb|250px|Schematic [[karyogram of the human chromosomes, showing their usual state in the G₀ and G₁ phase of the cell cycle. At top center it also shows the chromosome 3 pair after having undergone DNA synthesis, occurring in the S phase (annotated as S) of the cell cycle. This interval includes the G₂ phase and metaphase (annotated as "Meta.").<br>]]

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. 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. 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.

<span id="Characteristics of Quiescent Stem Cells"></span>Characteristics of quiescent stem cells

Transcriptomes

The transcriptomes of several types of quiescent stem cells, such as hematopoietic, muscle, and hair follicle, have been characterized through high-throughput techniques, such as microarray and RNA sequencing. Although variations exist in their individual transcriptomes, most quiescent tissue stem cells share a common pattern of gene expression that involves downregulation of cell cycle progression genes, such as cyclin A2, cyclin B1, cyclin E2, and survivin, and upregulation of genes involved in the regulation of transcription and stem cell fate, such as FOXO3 and EZH1. Downregulation of mitochondrial cytochrome C also reflects the low metabolic state of quiescent stem cells.

Epigenetic

Many quiescent stem cells, particularly adult stem cells, also share similar epigenetic patterns. For example, H3K4me3 and H3K27me3, are two major histone methylation patterns that form a bivalent domain and are located near transcription initiation sites. These epigenetic markers have been found to regulate lineage decisions in embryonic stem cells as well as control quiescence in hair follicle and muscle stem cells via chromatin modification.

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.

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, mTORC1 activity has been identified to control the transition from G₀ into G_Alert along with signaling through the HGF receptor cMet. 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.

Similarly, senescent pulmonary artery smooth muscle cells caused nearby smooth muscle cells to proliferate and migrate, perhaps contributing to hypertrophy of pulmonary arteries and eventually pulmonary hypertension.

Differentiated muscle

During skeletal myogenesis, cycling progenitor cells known as myoblasts differentiate and fuse together into non-cycling muscle cells called myocytes that remain in a terminal G₀ phase. As a result, the fibers that make up skeletal muscle (myofibers) are cells with multiple nuclei, referred to as myonuclei, since each myonucleus originated from a single myoblast. Skeletal muscle cells continue indefinitely to provide contractile force through simultaneous contractions of cellular structures called sarcomeres. Importantly, these cells are kept in a terminal G₀ phase since disruption of muscle fiber structure after myofiber formation would prevent proper transmission of force through the length of the muscle. Muscle growth can be stimulated by growth or injury and involves the recruitment of muscle stem cells – also known as satellite cells – out of a reversible quiescent state. These stem cells differentiate and fuse to generate new muscle fibers both in parallel and in series to increase force generation capacity.

Cardiac muscle is also formed through myogenesis but instead of recruiting stem cells to fuse and form new cells, heart muscle cells – known as cardiomyocytes – simply increase in size as the heart grows larger. Similarly to skeletal muscle, if cardiomyocytes had to continue dividing to add muscle tissue the contractile structures necessary for heart function would be disrupted.

Differentiated bone

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.

<span id=" Mechanism of G0 Entry"></span>Mechanism of G₀ entry

<span id=" The Role of Rim15"></span>Role of Rim15

Rim15 was first discovered to play a critical role in initiating meiosis in diploid yeast cells. Under conditions of low glucose and nitrogen, which are key nutrients for the survival of yeast, diploid yeast cells initiate meiosis through the activation of early meiotic-specific genes (EMGs). The expression of EMGs is regulated by Ume6. Ume6 recruits the histone deacetylases, Rpd3 and Sin3, to repress EMG expression when glucose and nitrogen levels are high, and it recruits the EMG transcription factor Ime1 when glucose and nitrogen levels are low. Rim15, named for its role in the regulation of an EMG called IME2, displaces Rpd3 and Sin3, thereby allowing Ume6 to bring Ime1 to the promoters of EMGs for meiosis initiation.

In addition to playing a role in meiosis initiation, Rim15 has also been shown to be a critical effector for yeast cell entry into G₀ in the presence of stress. Signals from several different nutrient signaling pathways converge on Rim15, which activates the transcription factors, Gis1, Msn2, and Msn4. Gis1 binds to and activates promoters containing post-diauxic growth shift (PDS) elements while Msn2 and Msn4 bind to and activate promoters containing stress-response elements (STREs). Although it is not clear how Rim15 activates Gis1 and Msn2/4, there is some speculation that it may directly phosphorylate them or be involved in chromatin remodeling. Rim15 has also been found to contain a PAS domain at its N terminal, making it a newly discovered member of the PAS kinase family. The PAS domain is a regulatory unit of the Rim15 protein that may play a role in sensing oxidative stress in yeast. Further observations revealed that levels of cyclin C mRNA are highest when human cells exit G₀, suggesting that cyclin C may be involved in Rb phosphorylation to promote cell cycle re-entry of G₀ arrested cells. Immunoprecipitation kinase assays revealed that cyclin C has Rb kinase activity. Furthermore, unlike cyclins D and E, cyclin C's Rb kinase activity is highest during early G₁ and lowest during late G₁ and S phases, suggesting that it may be involved in the G₀ to G₁ transition. The use of fluorescence-activated cell sorting to identify G₀ cells, which are characterized by a high DNA to RNA ratio relative to G₁ cells, confirmed the suspicion that cyclin C promotes G₀ exit as repression of endogenous cyclin C by RNAi in mammalian cells increased the proportion of cells arrested in G₀. Further experiments involving mutation of Rb at specific phosphorylation sites showed that cyclin C phosphorylation of Rb at S807/811 is necessary for G₀ exit. It remains unclear, however, whether this phosphorylation pattern is sufficient for G₀ exit. Finally, co-immunoprecipitation assays revealed that cyclin-dependent kinase 3 (cdk3) promotes G₀ exit by forming a complex with cyclin C to phosphorylate Rb at S807/811. Interestingly, S807/811 are also targets of cyclin D/cdk4 phosphorylation during the G₁ to S transition. This might suggest a possible compensation of cdk3 activity by cdk4, especially in light of the observation that G₀ exit is only delayed, and not permanently inhibited, in cells lacking cdk3 but functional in cdk4. Despite the overlap of phosphorylation targets, it seems that cdk3 is still necessary for the most effective transition from G₀ to G₁.

Rb and G₀ exit

Studies suggest that Rb repression of the E2F family of transcription factors regulates the G₀ to G₁ transition just as it does the G₁ to S transition. Activating E2F complexes are associated with the recruitment of histone acetyltransferases, which activate gene expression necessary for G₁ entry, while E2F4 complexes recruit histone deacetylases, which repress gene expression. Phosphorylation of Rb by Cdk complexes allows its dissociation from E2F transcription factors and the subsequent expression of genes necessary for G₀ exit. Other members of the Rb pocket protein family, such as p107 and p130, have also been found to be involved in G₀ arrest. p130 levels are elevated in G₀ and have been found to associate with E2F-4 complexes to repress transcription of E2F target genes. Meanwhile, p107 has been found to rescue the cell arrest phenotype after loss of Rb even though p107 is expressed at comparatively low levels in G₀ cells. Taken together, these findings suggest that Rb repression of E2F transcription factors promotes cell arrest while phosphorylation of Rb leads to G₀ exit via derepression of E2F target genes.