Abstract
Purpose: :
Previous lineage analyses have shown that retinal progenitor cells (RPCs) are multipotent throughout development, and expression profiling studies have shown a great deal of molecular heterogeneity among RPCs. To determine if the molecular heterogeneity predicts competencies or biases to produce particular types of progeny, clonal lineage analysis was used to investigate the progeny of a subset of RPCs, those that express the basic helix-loop-helix (bHLH) transcription factor, Olig2.
Methods: :
In order to address whether the molecular heterogeneity among RPCs correlates with the extensive clonal heterogeneity observed in the previous studies of retrovirally marked clones, we used a novel type of retroviral clonal analysis based upon infection of a defined type of RPC. An Olig2 knock-in line, which expresses Cre, as well as TVA, was used for fate mapping. Olig2-tva-ires-cre+/- mice were crossed with Cre-responsive GFP reporter mice, RC::ePE, in which enhanced GFP (eGFP) is under the control of the constitutively active CAG promoter in the ROSA26 locus. The Cre fate mapping studies suggested that Olig2+ RPCs produced a particular set of daughter cell types. However, since the TVA gene was also in the Olig2 locus, there was an opportunity to examine the descendents of Olig2-expressing RPCs exclusively. Retinas of E13.5, P0, and P3 Olig2-tva-cre+/- mice were infected in vivo with murine retroviruses expressing the marker genes, human placental alkaline phosphatase (AP) (LIA virus) or GFP (pQCXIX-GFP virus).
Results: :
In contrast to the large and complex set of clones generated by viral marking of random embryonic RPCs, the embryonic Olig2+ RPCs underwent terminal divisions, producing small clones with only two of the five cell types being made by the pool of RPCs at that time. The embryonically produced cell types made by Olig2 RPCs were cone photoreceptors and horizontal cell (HC) interneurons. Moreover, the embryonic Olig2+ RPC did not make the later Olig2+ RPC. The later, postnatal Olig2+ RPCs also made terminal divisions, which were biased towards production of rod photoreceptors and amacrine cell (AC) interneurons.
Conclusions: :
These data indicate that the multipotent progenitor pool is made up of distinctive types of RPCs, which have biases towards producing subsets of retinal neurons in a terminal division, with the types of neurons produced varying over time. This strategy is similar to that of the developing Drosophila melanogaster ventral nerve cord, with the Olig2+ cells behaving as ganglion mother cells.
Keywords: retinal development • transcription factors • regeneration