Another asymmetry in stem cells

A nice paper published in the January 26 issue of science by Yukiko M. Yamashita, Anthony P. Mahowald, Julie R. Perlin, and Margaret T. Fuller titled Asymmetric Inheritance of Mother versus Daughter Centrosome in Stem Cell Division adds a new wrinkle to the study of stem cell maintenance. As the title suggests they have identified a new and interesting aspect of stem cell biology. Specifically they use Drosophila male germ line stem cells (GSC) to show that the mother stem cell retains the mother centrosome while the daughter cell takes with it the newly replicated daughter centrosome. This is shown quite eloquently by pulse-chase experiments where centrosomes were labeled by GFP-PACT expression during early embryogenesis with the expression of GFP-PACT being shut off after germband extension (right). After two cell cycles they found that the GSC still retained a labeled centrosome indicating that the newly replicated centrosomes are taken by the daughter cell. Interestingly, the mother centrosome is maintained adjacent to the stem cell niche hub. The authors use EM to show that several microtubules emanate from the mother but not daughter centrosome and speculate that the microtubules might provide a mechanism for maintaining the mother centrosome adjacent to the hub. While much more analysis is needed to build support for this hypothesis, the authors do add one piece of mechanistic data to their intriguing observation. They find that the maintenance of the mother centrosome in the stem cell is dependent on centrin as knocking out centrin makes centrosome inheritance random. While the ultimate importance of centrosome inheritence in stem cell maintenance remains to be seen this observation seems to be too elegant to not be essential.