Asymmetric cell divisions of human hematopoietic stem and progenitor cells meet endosomes.

Hematopoietic stem cells (HSC) are undifferentiated cells, which self-renew over a long period of time and give rise to committed hematopoietic progenitor cells (HPC) containing the capability to replenish the whole blood system. Since both uncontrolled expansion as well as loss of HSC would be fatal, the decision of self-renewal versus differentiation needs to be tightly controlled. There is good evidence that both HSC niches as well as asymmetric cell divisions are involved in controlling whether HSC self-renew or become committed to differentiate. In this context, we recently identified four proteins which frequently segregate asymmetrically in dividing HSC/HPC. Remarkably, three of these proteins, the tetraspanins CD53 and CD63, and the transferrin receptor are endosome-associated proteins. Here, we highlight these observations in conjunction with recent findings in model organisms which show that components of the endosomal machinery are involved in cell-fate specification processes.