No abstract

Defining the molecular identity of stem cells may be critical for formulating a rational strategy for the therapeutic intervention of stem cell dysfunction. We find that high expression of Id1, a dominant-negative helix-loop-helix transcriptional regulator, identifies a rare population GFAP+ astrocytes with stem cell attributes among the subventricular astrocytes in the adult brain. The rare, long-lived, and relatively quiescent Id1high astrocytes with morphology characteristic of B1 type astrocytes self-renew and generate migratory neuroblasts that differentiate into olfactory bulb interneurons. Cultured Id1high neural stem cells can self-renew asymmetrically and generate a stem and a differentiated cell expressing progressively lower levels of Id1, revealing an Id1 gradient in unperturbed cells of subventricular neurogenic lineages. Moreover, Id genes are necessary to confer self-renewal capacity, a characteristic of stem cell identity. We suggest that high expression of a single transcriptional regulator, Id1, molecularly defines the long-sought-after B1 type adult neural stem cells.

Binding of erythropoietin (Epo) to the Epo receptor (EpoR) is crucial for production of mature red cells. Although it is well established that the Epo-bound EpoR is a dimer, it is not clear whether, in the absence of ligand, the intact EpoR is a monomer or oligomer. Using antibody-mediated immunofluorescence copatching (oligomerizing) of epitope-tagged receptors at the surface of live cells, we show herein that a major fraction of the full-length murine EpoR exists as preformed dimers͞oligomers in BOSC cells, which are human embryo kidney 293T-derived cells. This observed oligomerization is specific because, under the same conditions, epitopetagged EpoR did not oligomerize with several other tagged receptors (thrombopoietin receptor, transforming growth factor ␤ receptor type II, or prolactin receptor). Strikingly, the EpoR transmembrane (TM) domain but not the extracellular or intracellular domains enabled the prolactin receptor to copatch with EpoR. Preformed EpoR oligomers are not constitutively active and Epo binding was required to induce signaling. In contrast to tyrosine kinase receptors (e.g., insulin receptor), which cannot signal when their TM domain is replaced by the strongly dimerizing TM domain of glycophorin A, the EpoR could tolerate the replacement of its TM domain with that of glycophorin A and retained signaling. We propose a model in which TM domain-induced dimerization maintains unliganded EpoR in an inactive state that can readily be switched to an active state by physiologic levels of Epo.