Ptf1a/Rbpj complex inhibits ganglion cell fate and drives the specification of all horizontal cell subtypes in the chick retina.

During development, progenitor cells of the retina give rise to six principal classes of neurons and the Müller glial cells found within the adult retina. The pancreas transcription factor 1 subunit a (Ptf1a) encodes a basic-helix-loop-helix transcription factor necessary for the specification of horizontal cells and the majority of amacrine cell subtypes in the mouse retina. The Ptf1a-regulated genes and the regulation of Ptf1a activity by transcription cofactors during retinogenesis have been poorly investigated. Using a retrovirus-mediated gene transfer approach, we reported that Ptf1a was sufficient to promote the fates of amacrine and horizontal cells from retinal progenitors and inhibit retinal ganglion cell and photoreceptor differentiation in the chick retina. Both GABAergic H1 and non-GABAergic H3 horizontal cells were induced following the forced expression of Ptf1a. We describe Ptf1a as a strong, negative regulator of Atoh7 expression. Furthermore, the Rbpj-interacting domains of Ptf1a protein were required for its effects on cell fate specification. Together, these data provide a novel insight into the molecular basis of Ptf1a activity on early cell specification in the chick retina.

Tracing the progeny of the aortic hemangioblast in the avian embryo.

A population of hematopoietic progenitors becomes committed within the embryo proper in the floor of the aorta (P-Sp/AGM in the mouse). In birds, this first aspect of intraembryonic hematopoiesis is prominent during embryonic day 3 (E3) as endothelium-associated "intra-aortic clusters." Between E6 and E8, diffuse hematopoiesis then occurs as "para-aortic foci" located in the dorsal mesentery ventral to the aorta. These foci are not associated with endothelium. Whether these two hematopoietic cell populations arise from distinct or common progenitors is not known. We could recently trace back the origin of intra-aortic clusters in the avian embryo by labeling aortic endothelial cells (EC) in vivo with acetylated low-density lipoproteins. This approach established the derivation of early intraembryonic hemopoietic cells from the endothelium, but did not indicate how long during ontogeny such a relationship may exist, since the progeny of EC labeled at E2 could be traced for 1-2 days at most. Here we report that, when E2 aortic ECs were infected prior to the formation of intra-aortic clusters with a nonreplicative LacZ-bearing retroviral vector, numerous cells were labeled in the para-aortic foci at E6. In contrast, when the retroviral vector was inoculated at E4 rather than E2, that is, after the disappearance of intra-aortic clusters, no cells in the para-aortic foci were labeled. Taken together, our results demonstrate that ECs from the aortic floor seed the two aspects of aorta-associated hemopoiesis and that these ECs with hemangioblastic potential are present only transiently in the aorta.

[Affiliation between endothelial and intra-embryonic hematopoietic stem cells]. / Filiation entre cellules endothéliales et cellules souches hématopoïétiques intraembryonnaires.

We have investigated the developmental relationship of the hemopoietic and endothelial lineages in the floor of the chicken aorta, a site of hemopoietic progenitor emergence in the embryo proper. We show that, prior to the onset of hemopoiesis, the aortic endothelium uniformly expresses the endothelium-specific membrane receptor VEGF-R2. The onset of hemopoiesis can be precisely determined by detecting the common leukocyte antigen CD45. VEGF-R2 and CD45 are expressed in complementary fashion, namely the hemopoietic clusters in the floor of the aorta are CD45+/VEGF-R2-, while the rest of the aortic endothelium is CD45-/VEGF-R2+. To determine if the hemopoietic clusters are derived from EC, we tagged the E2 endothelial tree with a non-replicative retroviral vector and low density lipoproteins. Twenty four-48 hours later, labelled cells in the vascular tree were found to be either endothelial or hemopoietic but exceptionally both. Another 1-2 days later, groups of labelled cells appear in the dorsal mesentery within the hemopoietic "paraortic foci". Since no CD45+ cells were inserted among endothelial cells at the time of vascular labelling, hemopoietic clusters and foci must be concluded to derive from precursors with an endothelial phenotype.