Notch activity permits retinal cells to progress through multiple progenitor states and acquire a stem cell property Ashutosh P. Jadhav*, Seo-Hee Cho†, and Constance L. Cepko†‡ *Harvard–Massachusetts Institute of Technology Division of Health Sciences and Technology and †Department of Genetics and Howard Hughes Medical Institute, Harvard Medical School, 77 Avenue Louis Pasteur, Boston, MA 02115 Contributed by Constance L. Cepko, October 10, 2006 (sent for review November 9, 2005) Signaling through the Notch pathway regulates multiple aspects of Here, we show that activation of Notch in mitotic retinal pro- development. The vertebrate retina allows an investigation of the genitor cells allows the normal temporal progression of progenitor basis for these various effects, because the major cell types of the cells, leading eventually to cells that express features of glial cells, retina arise from a common progenitor that expresses Notch1. The in keeping with previous results (4). However, a comprehensive Notch pathway was constitutively activated in distinct populations molecular characterization revealed that these cells also express of retinal cells during development. Prolonged Notch activity in progenitor genes not expressed by normal Mu¨ller glia. A functional progenitor cells maintained cells in the progenitor state without assay showed that these cells could form neurospheres, as can stem perturbing temporal identity, promoting early progenitor charac- cells, whereas normal retinal cells were unable to form such spheres. teristics early in development and late progenitor characteristics In contrast, when Notch was activated in postmitotic cells, glial cells later in development. Eventually, constitutive Notch activation led without progenitor or stem cell features were produced. these cells to acquire characteristics of glial and stem cells. In contrast, reactivating the Notch pathway in newly postmitotic Results retinal cells promoted mature glial cell formation in a subset of Constitutive Notch Activity in Embryonic Progenitor Cells Promotes cells. These data suggest that prolonged Notch activity does not Formation of Cells with Glial and Progenitor Features in the Postnatal disrupt the normal progression of progenitor temporal states, and Retina. To determine the role of Notch signaling in embryonic that down-regulating or overcoming Notch activity is required for mouse retinal development, the Notch pathway was constitutively proper formation of both neuronal and glial cell fates. activated by crossing RosaN1-IC (17) mice to mice expressing Cre recombinase under the Chx10 promoter (Chx10-CRE; ref. 18). The development ͉ glia ͉ retina RosaN1-IC mice contain, in the ubiquitously expressed Rosa26 locus, a STOP sequence flanked by loxP sites followed by an NIC (Fig. 6A, he generation of cellular diversity during development requires which is published as supporting information on the PNAS web Tthat progenitor cells dynamically interpret both intrinsic and site). Upon exposure to Cre recombinase, the STOP sequence is extrinsic cues (1, 2). Notch is a transmembrane receptor that removed, and NIC is expressed. In addition, an internal ribosomal transduces an extrinsic cue, that of the binding of its ligand, e.g., entry sequence and the gene for nuclear-localized enhanced GFP Delta or Serrate, to directly activate transcription of as-yet ill- leads to GFP expression. This allows for expression of NIC and ϩ defined target genes (3). Studies in both the vertebrate and inver- nuclear GFP in Cre cells and their descendants. Expression of Cre tebrate nervous systems have established a critical role for Notch in Chx10-CRE mice begins at embryonic day (E)11–E12 in retinal signaling in preserving a pool of undifferentiated progenitor cells progenitor cells and is relatively specific to the retina (18). (3). More recent studies have suggested an additional role for The eventual fate of NIC-expressing progenitor cells was assayed at postnatal day 10 (P10), when development is nearly complete. activated Notch in specifying or promoting a particular cell fate, the ϩ glial cell fate (4–6). Furthermore, Notch activity also has been The retinae from NIC-overexpressing mice (Chx10-CRE/ ; N1-IC ϩ N1-IC shown to induce stem cell features and participate in neuronal Rosa / or Chx10 ) were folded and smaller (Fig. 6C) relative to WT retinae (Fig. 6B). Cryosections revealed that the subtype specification (7–9). These studies raise the question of how N1-IC Notch activity can influence the generation of multiple cell fates, Chx10 retina had a slightly smaller outer nuclear layer, ex- while also acting in a more generic fashion to preserve a progenitor panded inner nuclear layer (INL), and reduced inner plexiform pool. layer, causing the ganglion cell layer to compress against the INL (compare Fig. 1 E and F). RNA in situ hybridization (ISH) was The vertebrate retina is a paradigm for studying the mechanisms N1-IC of cell fate determination (1, 10, 11). All major cell types are known performed on Chx10 vs. WT retinae for Notch1, Delta1 (Dll1), to arise sequentially in a conserved order from multipotent pro- Hes1, and Hey1. At this stage, expression of Notch1 and Hes1 is localized to Mu¨ller glia, Hey1 is localized to progenitors (4, 19), and genitors. Studies in frog, rat, chick, and mouse have shown that N1-IC Notch1 is expressed by proliferating and undifferentiated cells Delta1 ISH signal is no longer detectable (13). In Chx10 mice, Notch1 A B during development (12–14). Among the last cells generated in the itself was up-regulated (Fig. 1 vs. ), as were its canonical targets, Hes1 and Hey1, whereas Dll1 was unchanged (data not retina are Mu¨ller glial cells, which retain expression of Notch1 (4, 12). Constitutive activation of the Notch pathway by overexpression of the intracellular domain of Notch (NIC) in fish, frog, chick and Author contributions: A.P.J. and C.L.C. designed research; A.P.J. and S.-H.C. performed rat retina has been shown to inhibit neurogenesis (12, 13, 15, 16). research; A.P.J. contributed new reagents/analytic tools; A.P.J., S.-H.C., and C.L.C. analyzed Furthermore, NIC-overexpressing cells in the fish and rat acquire data; and A.P.J. and C.L.C. wrote the paper. glial features (4, 16), suggesting that Notch may instruct gliogenesis The authors declare no conflict of interest. at the expense of neuronal production. In contrast, activation of the Freely available online through the PNAS open access option. Notch pathway in the early frog retina did not seem to promote Abbreviations: ISH, in situ hybridization; INL, inner nuclear layer; NIC, intracellular domain gliogenesis but resulted in nondividing cells with neuroepithelial of Notch; Pn, postnatal day n;En, embryonic day n. morphology (12). It is unclear whether these differences were due †To whom correspondence should be addressed. E-mail: [email protected]. to the timing and/or dose of prolonged Notch activity. © 2006 by The National Academy of Sciences of the USA 18998–19003 ͉ PNAS ͉ December 12, 2006 ͉ vol. 103 ͉ no. 50 www.pnas.org͞cgi͞doi͞10.1073͞pnas.0608155103 Downloaded by guest on September 28, 2021 However, progenitor genes, such as fibroblast growth factor 15 (FGF15) and cyclin D1 (20, 22), also had higher signals in the Chx10N1-IC retinae (Table 1). To confirm these profiling results, ISH was performed for FGF15 and cyclin D1. Both of these genes are weakly or not expressed at P10, when much of development is complete (20), but were expressed in the NIC-expressing retinae (Fig. 2 A–D). Furthermore, WT and Chx10N1-IC retinae were immunostained for Pax6 and Chx10. At this age, anti-Pax6 antibody marks amacrine, ganglion, and horizontal cells, anti-Chx10 marks bipolar cells and, very weakly, a minority of Mu¨ller glia. During development, both of these genes are expressed in most, if not all, retinal progenitor cells (18, 23). In the WT retina, Pax6 expression can be detected in the lower half of the INL and ganglion cell layer (Fig. 2E), consistent with expression in amacrine and ganglion cells. Interestingly in the Chx10N1-IC retinae, Pax6 was expressed at a high level throughout the INL and found to be colocalized with the GFP-positive cells (Fig. 2F). Chx10 was also found to be expressed at a higher level and colocalized with the NIC-expressing cells (data not shown). Notch Activity Promotes Early Progenitor but Not Late Progenitor or Glial Features in the Embryonic Retina. It was of interest to determine whether Notch activity was sufficient for precocious expression of late progenitor and/or glial genes in early progenitor cells, which normally do not express them. Gene expression profiling was performed on the E13.5 WT and Chx10N1-IC retinae. RNA was isolated from retinae pooled from two WT or mutant mice, and the experiment was performed as a color-swap duplicate. Many of the genes with lower signals in the Chx10N1-IC retinae are markers of retinal ganglion cells, such as GAP43 and neurofilament light polypeptide (NF-L). Because this is the major neuronal cell type Fig. 1. Directed misexpression of NIC in Chxlo expression cells. Section ISH on present at this age, neurogenesis is most likely inhibited in the WT (A and C) and Chx10N1-IC (B and D) retinae at P10. (A and B) Notch1.(C and NIC-overexpressing cells. Genes with higher signals in the N1-IC D) clusterin. Immunostaining on WT (E and I) and Chx10N1-IC (F–H and J–L) Chx10 retinae included Notch1; known Notch targets, such as retinae at P10. (E and F) Merge of DAPI, nGFP, and ␤ tubulin III (␤ tub III). (I and Nrarp (24); and progenitor genes, such as Chx10, FGF15, and Sox9 J) merge of DAPI, nGFP, and glutamine synthetase (glut syn). (G and K) nGFP. (Table 3; ref. 20). Hey1 appeared to have both higher (Table 7) and (H) ␤ tubulin III.