Targeting the bone marrow with activin A-overexpressing embryonic multipotent stromal cells specifically modifies B lymphopoiesis.

In vitro and in vivo studies implicate a series of cytokines in regulation of lymphohematopoiesis. However, direct indications for a local role of most of these cytokines within the bone marrow is lacking. In the present study, we aimed to test the contribution of a specific cytokine, activin A, a member of the transforming growth factor-beta (TGF-beta) family, to lymphohematopoiesis in mouse bone marrow. We show that mouse embryonic fibroblasts (MEFs) are indistinguishable from multipotent stromal cells (MSCs). Such MEFs overexpressing activin A, supported in vitro myelopoiesis in long-term bone marrow cultures as effectively as control MEFs. In contrast, activin A-overexpressing MEFs interfered with the in vitro generation of B lineage cells in such cultures. Thus, excessive expression in vitro of activin A, by supportive stromal cells, causes preferential maturation of myeloid rather than lymphoid cells. Moreover, the activin A-overexpressing MEFs caused an increased incidence in vivo of relatively immature B lineage cells; upon transplantation through the spleen route, MEFs engrafted the bone marrow specifically. Activin A-overexpressing MEFs accumulated in the bone marrow compartment and slowed down the progression of B cell precursors along the differentiation pathway, while sparing the myeloid population. The assay system described in this paper provides a means to assess the contribution of a wide range of molecules to hematopoiesis without perturbing the constitution of other organs.

Synchronization of interphase events depends neither on mitosis nor on cdk1.

Human HT2-19 cells with a conditional cdk1 mutation stop dividing upon cdk1 inactivation and undergo multiple rounds of endoreplication. We show herein that major cell cycle events remain synchronized in these endoreplicating cells. DNA replication alternates with gap phases and cell cycle-specific cyclin E expression is maintained. Centrosomes duplicate in synchrony with chromosome replication, giving rise to polyploid cells with multiple centrosomes. Centrosome migration, a typical prophase event, also takes place in endoreplicating cells. The timing of these events is unaffected by cdk1 inactivation compared with normally dividing cells. Nuclear lamina breakdown, in contrast, previously shown to be dependent on cdk1, does not take place in endoreplicating HT2-19 cells. Moreover, breakdown of all other major components of the nuclear lamina, like the inner nuclear membrane proteins and nuclear pore complexes, seems also to depend on cdk1. Interestingly, the APC/C ubiquitin ligase is activated in these endoreplicating cells by fzr but not by fzy. The oscillations of interphase events are thus independent of cdk1 and of mitosis but may depend on APC/Cfzr activity.