Any ideas or comments on the G0 phase of the cell cycle? Are there any definitive molecular markers? I am not talking about post-mitotic, highly differentiated cells but cells that withdraw from the cell cycle and can go back into G1, at least according to the nice little schemes they draw for G0. Who came up with the idea of G0 and does it really exist? For example stem cells are often described with the term quiescent, maybe implicating G0?
The G0 phase (referred to the G zero phase) or resting phase is a period in the cell cycle in which cells exist in a quiescent state. G0 phase is viewed as either an extended G1 phase, where the cell is neither dividing nor preparing to divide, or a distinct quiescent stage that occurs outside of the cell cycle.[1] Some types of cells, such as nerve and heart muscle cells, become quiescent when they reach maturity (i.e., when they are terminally differentiated) but continue to perform their main functions for the rest of the organism's life. Multinucleated muscle cells that do not undergo cytokinesis are also often considered to be in the G0 stage.[1] On occasion, a distinction in terms is made between a G0 cell and a 'quiescent' cell (e.g., heart muscle cells and neurons), which will never enter the G1 phase, whereas other G0 cells may.
Cells enter the G0 phase from a cell cycle checkpoint in the G1 phase, such as the restriction point (animal cells) or the start point (yeast). This usually occurs in response to a lack of growth factors or nutrients. During the G0 phase, the cell cycle machinery is dismantled and cyclins and cyclin-dependent kinases disappear. Cells then remain in the G0 phase until there is a reason for them to divide. Some cell types in mature organisms, such as parenchymal cells of the liver and kidney, enter the G0 phase semi-permanently and can be induced to begin dividing again only under very specific circumstances. Other types of cells, such as epithelial cells, continue to divide throughout an organism's life and rarely enter G0.
Although many cells in the G0 phase may die along with the organism, not all cells that enter the G0 phase are destined to die; this is often simply a consequence of the cell's lacking any stimulation to re-enter in the cell cycle.
Cellular senescence is distinct from quiescence because it is a state that occurs in response to DNA damage or degradation that would make a cell's progeny nonviable. Senescence then, unlike quiescence, is often a biochemical alternative to the self-destruction of such a damaged cell by apoptosis. Furthermore, quiescence is reversible whereas senescence isn't.
Thank you very much! But this is the "standard" definition. The question is: is G0 for real or just an abstract term some scientist came up to describe cells that do not divide for a while. In my opinion it is a highly unscientific term since it is not backed up by any molecular evidence, at least to my knowledge. I hope someone can convince my otherwise.
However, thank you very much for summarizing beautifully the current definition of G0/quiescence!
Upto my knowledge, G0 phase of the cell cycle is repeatedly determined by the lack of expression of the proliferation nuclear protein Ki-67, which has unknown fuction till now. However, it is really a highly controversial area in the stem cell biology, mostly because the lack of specific molecular or functional marker of quiescence.
Exactly, as I mentioned the absence of Ki-67, the low or negative expression of CD71 and the low RNA content. I'll be very gratfull to any one has any idea about what is the marker possibly expressed or increased in expression in dormancy!!
Great question Mazin! No idea. But you could check for interactions between the protein you identified and other proteins using the Interactions category listed for each gene on http://www.ncbi.nlm.nih.gov/gene. You may find additional "markers" that may also behave similarly to your protein. But since most of these interactions are from growing cultures and not dormant/quiescent cultures, there is often little use for the G0 question. There is the DREAM complex, but that suffers from the same conceptional limitations since it is derived from in vitro experiments where mainly S-phase genes seem to be affected by this complex. Makes not too much sense for a true G0/quiescence state.
Some years ago I read a paper which used the reduction of RNA as a marker for G0 using Flow Cytometry to discriminate G0 from G1. Normally cells are said to be in G0/G1 thus avoiding any further discussion. In my opinion the simplest approach would be to have a look around for the paper I mentioned which I think was published in Cytometry and start from there. In the end, you may have to combine different markers in order to precisely confirm that cells are in G0.
Dr. Kitamura's team at the University of Tokyo has recently introduced a fusion protein consisting of mVenus-p27K- probe that is able to effectively discriminate G0 from G1 and studying molecular features of quiescent cells. For further info please look at following paper: http://www.nature.com/srep/2014/140206/srep04012/full/srep04012.html
A very interesting paper Hosein, thanks a lot for your contribution.
What does increase GO/G1 phase ratio in cell cycle mean? Does this have any relation to cancer occurrence and progression?
Iman, an increase in the ratio implies larger population of cells are in G0 which is not a cancer cell. I would guess that with cancer cells the ratio would decrease a lot.
Thanks Peter, But there should be some stimuli that will trigger the cell to leave the G0 phase and enter the the G1 phase. Maybe if could detect the nature of these stimuli, we could recognize what causes tumor recurrence after many years of being dormant.
Dear Thomas
The G0 phase of the cell cycle represents the cells that exit from the cell cycle, the causes for this situation may be natural behaviour of the cells as a result of reaching its limit in number of divisions ( Hayflick limits ) and become senescent cells, and may be the cause of this situation abnormal result from mutation in genes that control the cell division, some of the mutation that thought cause cancer and another may be cause quiescent cells.
Senescent cells are growth arrested in the transition from phase G1 to phase S of the cell cycle.
MY et al. (2016) reports that senescence and quiescence are frequently used as interchangeable terms, and there are common molecules play role in decision of cell cycle arrest.
https://www.ncbi.nlm.nih.gov/pubmed/27558094
Good Luck
There are some marker for cell cycle. You can read more my manuscript entitled "the cell fate: senescense or quiscence".
p53 (TP53; tumor protein p53)
+ G1,G1/S or G2 cell cycle arrests, apoptosis
+ G0 arrest, growth inhibition
mTOR (mechanistic target of rapamycin)
+ cell growth
+ keeping proliferative potential, reduce ROS
pRB (retinoblastoma 1)
+ cell cycle arrest, induce ROS formation/apoptosis
+ cell cycle arrest, reduce ROS formation/apoptosis
p21 (CDK-inhibitor 1A)
+ G1/S phase arrest via p53, apoptosis
+G1/S phase arrest
p16 (CDK-inhibitor 2A)
+ stabilize p53, degrade MDM2
-
p27 (CDK-inhibitor 1B)
+ G1/S arrest
+ G1/S arrest
MDM2 (mouse double minute 2)
+ p53 degradation
+ p53 degradation
p107 (retinoblastoma-like 1)
+ cell cycle arrest
-
p130 (retinoblastoma-like 2)
+ cell cycle arrest
-
ATM (Ataxia telangiectasia mutated) kinase
+ cell cycle arrest, p53 activation
-
IFNs,IFI 16 (Interferons, IFN-inducing protein 16)
+ cell cycle arrest, pRb,p53 activation
-
TIGAR (TP53 induced glycolysis regulatory phosphatase)
+ G1/S arrest via active pRB
+ G1/S arrest via active pRB
ATRX (alpha thalassemia/mental retardation syndrome X-linked)
+ MDM2 degradation
-
+(plus): present, -(minus): absent, G0/1/2: cell cycle phases “Gap 0/1/2”, S: cell cycle phase “synthesis”, ROS: reactive oxygen species
I have a follow up question or extension of the original question. It seems most cells in the adult body are quiescent of some sort: What is the percentage of cells in G0 in the body?
If the majority of cells are quiescent (G0) and most of them can re-enter the cell cycle (e.g., many fibroblast look dormant but once there is a wound, they are activated and shape the wound healing process like few other cells) then why is there so little known about this state. One publication observed: "One recent PubMed search with ‘cell proliferation’ returned over 190 000 articles, while searches for ‘cell quiescence’ and ‘cellular quiescence’ returned about 200 hits each. The lack of study of cellular quiescence is at least partially owing to the fact that, for a long time, quiescence had been simply regarded as a passive and dormant cellular state lacking proliferation activities." (Modelling mammalian cellular quiescence, Yao 2014, Interface Focus. 2014 Jun 6; 4(3): 20130074., doi:10.1098/rsfs.2013.0074).
It also seems as if (deep) quiescence in vitro does not mimic quiescence in vivo very well. A major focus has been on RB1 in regulating quiescence, but RB1 expression is basically absent from most "classical" quiescent cells in vivo, e.g., fibroblasts.
Cells can exit the active cellular cycle in response to regulatory signals and go into a quiescent phase called G0, which correspond to the stable state, and during which the cells do not change in size. They can remain in G0 for variable times, or even indefinitely, or go back in to G1 phase and take back the active cycle. Many cells never enter G0 phase and cycle continuously.
Please go back to the original literature. The cell cycle was discovered by Alma Howard and Steven Pelc when they used tritiated thymidine to identify cells undergoing DNA synthesis (which she called S phase) and found that there were cells that had completed Mitosis (M phase) but not yet entered S (that she called G1 phase for Gap1 in our knowledge, later shown to include synthesis of the RNA and proteins necessary for entry into S phase, i.e., DNA synthesis) and another that had completed S but not yet entered M, which they called G2 phase (in which it subsequently was found that they synthesized the RNA and proteins necessary for entry in M phase). Alma Howard was always most insistent that the Gap phases represented gaps in our knowledge regarding their cell metabolism. I was fortunate to have had my lab immediately between Laszlo Lajtha's and Alma Howard's at the Christie in Manchester. Laszlo Lajtha, a founder of Stem Cell Kinetics (and supervisor for my Oxford DPhil), was concerned that reproductively quiescent cells would have metabolic activities different from those of G1 cells and coined the term G0 to denote such cells. G0 cells are those that have completed M but not yet entered G1, as seen with many liver cells, whose maintenance in G0 is by a concentration of "quorum" protein, the loss or dilution of which releases just that number of cells from G0 into G1 to replace losses such as those induced by a partial hepatectomy (liver) or haemorhage (bone marrow). Lajtha also first suggested the existence of hematological stem cells and stem cells for each tissue. My DPhil thesis showed that survival of irradiated planarians, animals capable of extensive regeneration, possessing a morphologically identifiable population of totipotential stem cells, could be quantitatively understood (LD50 and Probit width) in terms of their number of stem cells, their repopulation probability, and the slope and extrapolated intercept of their radiation survival curve.
I should add that G0 cells are proliferatively quiescent, but are proliferatively capable. Senescent cells are not proliferatively capable (e.g., telomere depleted cells, Hayflick limited). Telomerase expression avoids the latter condition, and is a feature of stem cells. Tumors are driven by their Cancer Stem Cells.
I hope most answers centre in addressing the nuances between the idea distinctively; cells once proliferated and cells that have not initiated cell division. Do the latter have potential to go into G0.....
Thanks Christopher for the wonderful background story. In a week when so few scientists are recognized for their achievements in Stockholm, it may be appropriate to mention the humble "concept" already acknowledged by Newton and before and by many Nobel prize laureates since then: "If I have seen further it is by standing on the shoulders of Giants." Every outstanding finding is based on a long series of curious observations, hard work and serendipity by an army of highly trained scientist. I think, the publication record has now grown to a size that it is often "inconvenient" to find the original contributors, the original group of giants and acknowledge their critical findings. Thanks Christopher for making me aware of the studies of Howard and Pelc.
Getting back to my question: What is the percentage of cells in G0 in the body? Especially, how many are "proliferatively quiescent, but are proliferatively capable" as Christopher has specified. Are we overlooking something by focussing so hard on proliferating cells and ignoring the potentially vast majority of cells in out body?
Perhaps the better question is: "What fraction of normal tissue cells are stem cells?". For most tissues, the rest of the cells are Amplifying Transit Cells (ATCs) differentiating and proliferating to reach the number of divisions of which they are capable before terminal differentiation to limited lifespan nonproliferative functional end cells (e.g., erythrocyte in the synthroid series). This stem cell fraction seems to be on the order of a fraction of a percent. For normal stem cells, most may be in G0 since they proliferate only "as needed" by losses in the functional (terminally differentiated) compartment. For red cells, which live for 120 days, as that cohort dies and is removed, to be replaced by one stem cell dividing, a daughter being pushed out of its niche, and failing to find a vacant niche, is hit by erythropoietin, to become an erythroid precursor cell, i.e., an erythroblast. [Similarly for those hit by one of the 3 myelopoietins or megacaryopoietin to become blast cells of those series.] The erythroblast, an ATC, goes through 8 divisions, differentiating along the way through the recognizable steps in erythroid development, to yield 2^8 = 128 erythrocytes (red cells). NB, This has the effect of reducing the number of immortal stem cell divisions (a mutagenic risk), leaving most of the divisions and errors to take place in cells that will die and be eliminated in relatively short order (ca. 4 months). So the answer to your question, Thomas Andl, is what fraction of the stem cells are in G0? This is likely to depend on the replacement kinetics for each renewable tissue (esp. Bone marrow, gut, lung, skin, bone, etc.), both normal steady state and after injuries (e.g., hemorrhage). These kinetics have been studied in the Radiobiological literature, largely to understand the consequences of ionizing radiation exposures and the recovery of dose-limiting normal tissues.
Not surprising but Interesting that you bring up radiobiology. One of the very few instances when slow-cycling, quiescent stem cells of the oral mucosa (more or less the entire basal cell layer) seem to become "activated" and proliferate, is in response to chemoradiotherapy in head and neck cancer patients! All the TA cells- or committed progenitors- have been killed off and this requires- as you have outlined- the extremely large activation -large in the sense of how many stem cells become activated- of the otherwise resting stem cells (Clin Cancer Res. 1996 Sep; 2(9):1453-60; https://clincancerres.aacrjournals.org/content/2/9/1453.full-text.pdf; FIGURE 5). In this instance, the stem cells function as a "reserve" in case the active duty forces run out of steam or are killed.
Would you know the differences between mouse and human in regard to response to radiation? Shouldn't the mouse be "sloppy" because there is no need to maintain the tissue well for more than a few months while in humans, the time for tissue maintenance is decades? Shouldn't human stem cells be more protected? Basically, is the level of stem cell protection and quiescence a key mechanism to explain Peto's paradox (Trends Ecol Evol. 2011 Apr; 26(4):175-82.) and is the level different in short-lived mice compared to long-lived mammals such as humans?
Another questions; you used this phrase: "immortal stem cell ...". Not sure what you implied. The stem cells are truly immortal or do you refer to the immortal strand hypothesis to protect the genome of stem cells?
In one of my 1969 papers in IntJRadiatBiol (The cellular basis of Radiation lethality, part 4 or 5) I considered the "excess" of stem cells in a planarian. The product N [number of stem cells] times repopulation probability was 5.3. This apparent 5-fold excess would seem to be a significant reserve. However, the mortality probability is given by: P = ln(1.9Nalpha). [alpha is the repopulation probability]. Replacing Nalpha with 5.3 yields a mortality rate of 0.3 to 0.6, i.e., the natural loss rate in the planarian colony. So planarians have just enough stem cells to maintain themselves.
Does this suggest that in each species there has been an adaptation to the average lifespan and this adaptation is reflected within the stem cell compartments of each tissue? And that stem cell compartments that have the poorest adaptation (perhaps "miscalculated") will become the weakest link and a source for trouble close to the end of life? Basically following a sort of "mean time to failure" principle within the adaptation process? E.g., there is no need to make the stem cell compartment or any part of the organism last for 500 years if the expected lifespan is 2 years. On the other hand, if you want to live well for 100 years, the minimum quality of each part, including each stem cell compartment, should have adapted to this lifespan?
Thomas, A very interesting question. It should be investigated. But the definition of stem cells needs to be functionally based, not on the antibody recognized cell surface proteins. For breast cancer, a 3-antibody phenotype corresponds to 1-2% of the tumor, but on limiting dilution assay, 100 such cells injected into the breast fat pad gave 6/10 & 5/8 (60-62.5%) of the mice a tumor. Since this is a Poisson distribution, that means that only 1% of the 1-2%, i.e., 0.01-0.02% of the tumor cells were CSCs. Please email me at [email protected] for further discussions. The initial work could be done by a computer/database literate high school or college student, or perhaps a graduate student for further development if the initial literature data suggest that that is merited.
Sorry, mistake in my previous entry on "is there an excess of stem cells in plznarians". The equation should have been: Nalpha = 1.41x10^5 [N] x 3.76x10^-5 [alpha] = 5.3. So, since Nalpha = -ln(Pm), and therefore, Pm = exp(-5.3) = 0.05% (0.3-0.6)%.
The rest is correct, just a typo in the equation.
Probably an irrelevant aside, but I wonder if plant meristematic cells are the equivalent of stem cells. Totipotency in plant cells is well known. Cell cycle synchrony can be obtained in some plant cell cultures by starvation of nitrogen sources, which accumulates plant cells (such as sycamore cells in large scale suspension culture) in G1 - or would that be G0? Then resupply of a nitrogen source sets off synchronous division.
@Alan, yes, the plant meristem contains a stem cell population, (well described by R Seed) and was the first organized tissue to be studied in terms of radiation effects on stem cell proliferation [Eric Hall and Josie Cavanaugh, in Oxford at the time, early 1960s]. Also, Jack van't Hoff, at Brookhaven Natl Lab in early 80s, showed that he could step pea meristem cells through the cell cycle by sucrose starvation, and causing accumulation in G1, then giving a "sucrose spurt" to enable the cells to enter and complete S, whereupon they accumulated in early G2, and with another spurt, could be triggered to enter G2 and complete M. He was able to run them through a series of stepped cell cycles.
I tried this with mammalian cells (using glucose rather than sucrose) but these cells do not survive glucose starvation, so it did not work.
Christopher: Very interesting, many thanks. I remember Jack van't Hoff and his Principle Control Point hypothesis - met him once at a conference way back. I worked on cell cycle stage effects on radiation sensitivity in HeLa, CHO and other cell lines 1965-1968 at Wantage Research Laboratory (UKAEA). Long out of basic research now, but have always maintained interest, especially in plants.
I found a good article about this topic below.
https://www.sciencedirect.com/topics/immunology-and-microbiology/cell-cycle-g0-phase
G0 normally means cells which do not divide. They have been made and the only way to obtain them is at birth. Brain cells may repair up to some point, but not much. They are made as you were born as stays as it is. Another form are cells from meiosis.
The term G0 was coined by L.G. Lajtha. He designated G0 to differentiate between cells that are NOT committed to enter the cell cycle (G0) and those that are but have not commenced DNA synthesis (G1). He correctly recognized that G1 cells are biochemically different from those in G0. The former are synthesizing the RNA and proteins necessary to enter DNA synthesis (S phase). I know this first hand as Lajtha's DPhil student at Oxford, and subsequently in Manchester at the Christie Hospital & Holt Radium Inst.
I should add that G0 is not a permanent state. Cells can cycle in and out of G0.
Often things become clearer than looked at through the lens of evolution. The saying that "nothing in biology makes sense except in the light of evolution" (Theodosius Dobzhansky), highlights this idea.
Therefore, there must have been evolutions of quiescence in single cell organisms and in multicellular organisms. Not sure how related both are. But looking at G0 phase and quiescence through evolution may produce some meaningful insights.
In unicellular organisms it makes best sense that evolution has driven a quiescence program to protect cells under unfavorable conditions. What about the driver in multicellular organisms? I am sure, there is some slow down in proliferation in multicellular organisms upon starvation. Any publications?
But this would not explain the "fact" that the vast majority of cells in the human body are most likely quiescent (I cannot find a "count" of quiescent versus proliferation cells in mammals. Any data?).
As Dr. Lange has pointed out: G0 does not have to be a permanent state. Even just considering cells in our bodies that can proliferate but do so rarely, raises the question: what is the purpose of this quiescence?
A recent review on the topic (PMID: 34778256: Daignan-Fornier, Laporte & Sagot: "Quiescence Through the Prism of Evolution") explores these questions.
One argument for quiescence in multicellular organisms in this paper is stem cell protection from replication troubles. I think, this is an excellent point. They also argue the idea that “quiescence also permits stem cells to have an inherent low metabolic activity.” I am not sure where this comes from and whether this is a generally true feature of quiescent stem cells. I think, that idea may come from hematopoietic stem cells? Daignan-Fornier, Laporte & Sagot don’t give an example for this hypothesis. I think, this metabolic feature for multicellular stem cells cannot be true for all stem cells. Some stem cells have to have “spatial functions”. What I mean by this is that they are located within a tissue at a place where they often just can’t sit around as completely inert and inactive “rocks”. They then would be a problem for tissue function. To their credit, the authors mention this later in their article, suggesting that “some dedicated metabolic pathways can be active in the total absence of proliferation). Some stem cells may have such functions. Basal cells of squamous epithelial actually build an extracellular barrier and probably also may function as a kind of feeder layer for the overlaying cells.
A wonderful comment in the article that summarizes my interest in stem cells: “However, to date, the cell biology of plant and animal quiescent cells remains largely underexplored.” Despite all the thousands of articles on the topic, we really do not seem to know much about these cells and their quiescence.
One seems clear, the main point of the evolution of quiescence in multicellular organisms is that “the trigger has shifted from nutrient cues to repair or renewing signals” (PMID: 34778256).
The article is filled with surprising ideas. Just here one quote: “Finally, aged quiescent cells may ultimately transition to senescence, a non-proliferative cellular state that is irreversible (Sousa-Victor et al., 2014; Fujimaki and Yao, 2020). Thus, quiescence deepening reveals the limitation of cell replacement relying on stemness and is among the strongest candidates for multicellular organism aging.” Not sure whether all researchers on the topic of aging will agree, but it is one way to looking at it.
However, their main thrust of the article seems to be this idea: “we propose that with the emergence of multicellularity, quiescence became no longer confined to a survival form but may have favored cell specialization thereby launching division of labor.” This may not help much to explain today’s quiescent cells in multicellular organisms, but perhaps to explain the evolution of complex multicellular organisms. In this sense, the G0 phase of the cell cycle has been quintessential for evolution.
Talking about investigative biomarkers: Quiescent cells are usually identified by reduced RNA content and the absence of cell proliferation markers. More specifically, cyclin-dependent kinase inhibitors (CKIs)- p21, p27, and p57, are also crucial for maintaining quiescence. In a previous study using mouse models on hematopoietic stem cells, the knockout of p57 and p27 leads to G0 exit through the nuclear import of cyclin D1 and subsequent phosphorylation of Rb.
All in all, you may choose to particularly investigate the Notch signalling pathway, it's essential in this purview.