CD133 is a marker for long-term repopulating murine epidermal stem cells.

Maintenance, repair, and renewal of the epidermis are thought to depend on a pool of dedicated epidermal stem cells (EpiSCs). Like for many somatic tissues, isolation of a nearly pure population of stem cells is a primary goal in cutaneous biology. We used a quantitative transplantation assay, using injection of keratinocytes into subcutis combined with limiting dilution analysis, to assess the long-term repopulating ability of putative murine EpiSC populations. Putative EpiSC populations were isolated by FACS sorting. The CD133(+) population and the subpopulation of CD133(+) cells that exhibits high mitochondrial membrane potential (DΨm(hi)) were enriched for long-term repopulating EpiSCs versus unfractionated cells (3.9- and 5.2-fold, respectively). Evidence for self-renewal capacity was obtained by serial transplantation of long-term epidermal repopulating units derived from CD133(+) and CD133(+)ΔΨm(hi) keratinocytes. CD133(+) keratinocytes were multipotent and produced significantly more hair follicles than CD133(-) cells. CD133(+) cells were a subset of the previously described integrin α6(+)CD34(+) bulge cell population, and 28.9±8.6% were label-retaining cells. Thus, murine keratinocytes within the CD133(+) and CD133(+)ΔΨm(hi) populations contain EpiSCs that regenerate the epidermis for the long term, are self-renewing, multipotent, and label-retaining cells.

Loss of Blm enhances basal cell carcinoma and rhabdomyosarcoma tumorigenesis in Ptch1+/- mice.

Basal cell carcinomas (BCCs) have relative genomic stability and relatively benign clinical behavior but whether these two are related causally is unknown. To investigate the effects of introducing genomic instability into murine BCCs, we have compared ionizing radiation-induced tumorigenesis in Ptch1(+/-) mice versus that in Ptch1(+/-) mice carrying mutant Blm alleles. We found that BCCs in Ptch1(+/-) Blm(tm3Brd/tm3Brd) mice had a trend toward greater genomic instability as measured by array comprehensive genomic hybridization and that these mice developed significantly more microscopic BCCs than did Ptch1(+/-) Blm(+/tm3Brd) or Ptch1(+/-) Blm(+/+) mice. The mutant Blm alleles also markedly enhanced the formation of rhabdomyosarcomas (RMSs), another cancer to which Ptch1(+/)(-) mice and PTCH1(+/)(-) (basal cell nevus syndrome) patients are susceptible. Highly recurrent but different copy number changes were associated with the two tumor types and included losses of chromosomes 4 and 10 in all BCCs and gain of chromosome 10 in 80% of RMSs. Loss of chromosome 11 and 13, including the Trp53 and Ptch1 loci, respectively, occurred frequently in BCCs, suggesting tissue-specific selection for genes or pathways that collaborate with Ptch deficiency in tumorigenesis. Despite the quantitative differences, there was no dramatic qualititative difference in the BCC or RMS tumors associated with the mutant Blm genotype.

Transit-amplifying cell frequency and cell cycle kinetics are altered in aged epidermis.

Aged epidermis is less proliferative than young, as exemplified by slower wound healing. However, it is not known whether quantitative and/or qualitative alterations in the stem and/or transit-amplifying (TA) compartments are responsible for the decreased proliferation. Earlier studies found a normal or decreased frequency of putative epidermal stem cells (EpiSCs) with aging. We show, using long-term repopulation in vivo and colony formation in vitro, that, although no significant difference was detected in EpiSC frequency with aging, TA cell frequency is increased. Moreover, aged TA cells persist longer, whereas their younger counterparts have already differentiated. Underlying the alteration in TA cell kinetics in the aged is an increase in the proportion of cycling keratinocytes, as well as an increase in cell cycle duration. In summary, although no significant difference in EpiSC frequency was found, TA cell frequency was increased (as measured by in vivo repopulation, growth fraction, and colony formation). Furthermore, the proliferative capacity (cellular output) of individual aged EpiSCs and TA cells was decreased compared to that of young cells. Although longer cell cycle duration contributes to the decreased proliferative output from aged progenitors, the greater number of TA cells may be a compensatory mechanism tending to offset this deficit.