The rates of stem cell division determine the cell cycle lengths of its lineage

Adult stem cells and their transit-amplifying progeny alter their proliferation rates to maintain tissue homeostasis. To test how the division rates of stem cells and transit-amplifying progeny affect tissue growth and differentiation, we developed a computation strategy that estimates the average cell-cycle lengths (lifespans) of germline stem cells and their progeny from fixed-tissue demography in the Drosophila testis. Analysis of the wild-type data using this method indicated that during the germline transit-amplification, the cellular lifespans extend by nearly 1.3-fold after the first division and shrink by about 2-folds after the second division. Cell-autonomous perturbations of the stem cell lifespan accordingly altered the lifespans of successive transit-amplifying stages. Remarkably, almost 2-fold alterations in the lifespans of stem cells and their immediate daughters did not affect the subsequent differentiation. The results indicate that the early germline division rates can adjust the following division rates and the onset of differentiation.

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Prediction of cellular lifespan from the demography of transit-amplifying cells

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Lifespans of spermatogonial cells change anomalously during transit-amplification

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Anomalous lifespan extension during transit-amplification precedes the onset of Bam

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Lifespan changes of early TA stages readjust that of the subsequent stages

Cyclin E and Cdk1 regulate the termination of germline transit-amplification process in Drosophila testis

An extension of the G1 is correlated with stem cell differentiation. The role of cell cycle regulation during the subsequent transit amplification (TA) divisions is, however, unclear. Here, we report for the first time that in the Drosophila male germline lineage, the transit amplification divisions accelerate after the second TA division. The cell cycle phases, marked by Cyclin E and Cyclin B, are progressively altered during the TA. Antagonistic functions of the bag-of-marbles and the Transforming-Growth-Factor-β signaling regulate the cell division rates after the second TA division and the extent of the Cyclin E phase during the fourth TA division. Furthermore, loss of Cyclin E during the fourth TA cycle retards the cell division and induces premature meiosis in some cases. A similar reduction of Cdk1 activity during this stage arrests the penultimate division and subsequent differentiation, whereas enhancement of the Cdk1 activity prolongs the TA by one extra round. Altogether, the results suggest that modification of the cell cycle structure and the rates of cell division after the second TA division determine the extent of amplification. Also, the regulation of the Cyclin E and CDK1 functions during the penultimate TA division determines the induction of meiosis and subsequent differentiation.

An interplay between Shugoshin and Spo13 for centromeric cohesin protection and sister kinetochore mono-orientation during meiosis I in Saccharomyces cerevisiae

Meiosis is a specialized cell division process by which haploid gametes are produced from a diploid mother cell. Reductional chromosome segregation during meiosis I (MI) is achieved by two unique and conserved events: centromeric cohesin protection (CCP) and sister kinetochore mono-orientation (SKM). In Saccharomyces cerevisiae, a meiosis-specific protein Spo13 plays a role in both these centromere-specific events. Despite genome-wide association of Spo13, we failed to detect its function in global processes such as cohesin loading, cohesion establishment and homologs pairing. While Shugoshin (Sgo1) and protein phosphatase 2A (PP2ARts1) play a central role in CCP, it is not fully understood whether Spo13 functions in the process through a Sgo1- PP2ARts1-dependent or -independent mechanism. To delineate this and to find the relative contribution of each of these proteins in CCP and SKM, we meticulously observed the sister chromatid segregation pattern in the wild type, sgo1Δ, rts1Δ and spo13Δ single mutants and in their respective double mutants. We found that Spo13 protects centromeric cohesin through a Sgo1- PP2ARts1-independent mechanism. To our surprise, we observed a hitherto unknown role of Sgo1 in SKM. Further investigation revealed that Sgo1-mediated recruitment of aurora kinase Ipl1 to the centromere facilitates monopolin loading at the kinetochore during MI. Hence, this study uncovers the role of Sgo1 in SKM and demonstartes how the regulators (Sgo1, PP2ARts1, Spo13) work in a coordinated manner to achieve faithful chromosome segregation during meiosis, the failure of which leads to aneuploidy and birth defects.