Short cell cycle duration is a phenotype of human epidermal stem cells.

BACKGROUND: A traditional view is that stem cells (SCs) divide slowly. Meanwhile, both embryonic and pluripotent SCs display a shorter cell cycle duration (CCD) in comparison to more committed progenitors (CPs). METHODS: We examined the in vitro proliferation and cycling behavior of somatic adult human cells using live cell imaging of passage zero keratinocytes and single-cell RNA sequencing. RESULTS: We found two populations of keratinocytes: those with short CCD and protracted near exponential growth, and those with long CCD and terminal differentiation. Applying the ergodic principle, the comparative numbers of cycling cells in S phase in an enriched population of SCs confirmed a shorter CCD than CPs. Further, analysis of single-cell RNA sequencing of cycling adult human keratinocyte SCs and CPs indicated a shortening of both G1 and G2M phases in the SC. CONCLUSIONS: Contrary to the pervasive paradigm, SCs progress through cell cycle more quickly than more differentiated dividing CPs. Thus, somatic human adult keratinocyte SCs may divide infrequently, but divide rapidly when they divide. Additionally, it was found that SC-like proliferation persisted in vitro.

The mTOR Target S6 Kinase Arrests Development in Caenorhabditis elegans When the Heat-Shock Transcription Factor Is Impaired.

The widely conserved heat-shock response, regulated by heat-shock transcription factors, is not only essential for cellular stress resistance and adult longevity, but also for proper development. However, the genetic mechanisms by which heat-shock transcription factors regulate development are not well understood. In Caenorhabditis elegans, we conducted an unbiased genetic screen to identify mutations that could ameliorate the developmental-arrest phenotype of a heat-shock factor mutant. Here, we show that loss of the conserved translational activator rsks-1/S6 kinase, a downstream effector of mechanistic Target of Rapamycin (mTOR) kinase, can rescue the developmental-arrest phenotype of hsf-1 partial loss-of-function mutants. Unexpectedly, we show that the rescue is not likely caused by reduced translation, nor by activation of any of a variety of stress-protective genes and pathways. Our findings identify an as-yet unexplained regulatory relationship between the heat-shock transcription factor and the mTOR pathway during C. elegans development.

Functional characterization of Tn1331 gene cassettes.

OBJECTIVES: The transposon Tn1331 possesses a region including three antibiotic resistance genes with the structure aac(6')-Ib-attC-aadA1-attI1*-bla(OXA-9)-attC, which potentially includes four gene cassettes. Experimental data on the mobility of fusion cassettes as well as those on mobility of cassettes in a genetic environment such as Tn1331, which lacks an integrase gene, are limited. Therefore, experiments using pJHCMW1, a plasmid harbouring this transposon, in the presence of IntI1 supplied in trans were carried out to define which cassettes are mobile in vivo. METHODS: In vivo excision of resistance genes was investigated in Escherichia coli cells harbouring pJHCMW1 and in a recombinant clone that included the intI1 gene under the control of the P(tac) promoter. Plasmid DNA was purified and subjected to PCR analysis, and DNA sequencing of PCR products was performed to determine whether excision had occurred. RESULTS AND CONCLUSIONS: In vivo recombination experiments showed that the fused aadA1-attI1*-bla(OXA-9)-attC gene cassette was excised in the presence of IntI1. The excision of a DNA fragment including aadA1-attI1* was also detected but at a lower frequency. The analysis of the latter recombination reaction showed that, although attI1* includes only a small fraction of the complete attI1 sequence, it is still used as a substrate by IntI1, albeit in a very inefficient manner.