The Proneural Basic Helix-Loop-Helix GeneAscl1aIs Required for Retina Regeneration

Abstract

Unlike mammals, teleost fish can regenerate an injured retina, restoring lost visual function. Little is known of the molecular events that underlie retina regeneration. We previously found that in zebrafish, retinal injury stimulates Müller glia to generate multipotent ␣1-tubulin (␣1T) and pax6-expressing progenitors for retinal repair. Here, we report the identification of a critical E-box in the ␣1T promoter that mediates transactivation by achaete-scute complex-like 1a (ascl1a) during retina regeneration. More importantly, we show that ascl1a is essential for retina regeneration. Within 4 h after retinal injury, ascl1a is induced in Müller glia. Knockdown of ascl1a blocks the induction of ␣1T and pax6 as well as Müller glial proliferation, consequently preventing the generation of retinal progenitors and their differentiated progeny. These data suggest ascl1a is required to convert quiescent Müller glia into actively dividing retinal progenitors, and that ascl1a is a key regulator in initiating retina regeneration.

Genetic evidence for shared mechanisms of epimorphic regeneration in zebrafish

“…For example, the maximum fold change of hspd1 reported previously was 2.0 (13) or 1.7 (14) compared with 3.6 in our study, and changes in mps1 were not reported in one of these studies (14). A recent analysis of mechanically injured zebrafish retina in which ascl1a (achaete-scute complex-like 1a) function was knocked down with morpholino antisense oligonucleotides verified that it is required for the regenerative response (25), which validates the utility of our data set as a tool for discovering genes that induce a neurogenic program in differentiated glial cells. Consistent with the increased expression of hspd1 that we observed in the ONL by in situ hybridization, the microarray data from laser-captured ONL tissue also showed up-regulation of hspd1 (15).…”

“…S4C) are transcription factors expressed in retinal progenitors in the ciliary marginal zone of postembryonic zebrafish retina (23). The proneural basic helix-loop-helix (bHLH) gene ascl1a is induced in activated Müller glia and their neurogenic progeny following retinal lesions (25). The homeobox transcription factor six3 plays a crucial role in early eye development and interacts with bHLH proteins (26,27).…”

Ascl1a/Dkk/β-catenin signaling pathway is necessary and glycogen synthase kinase-3β inhibition is sufficient for zebrafish retina regeneration

“…1 B and C). This pan-retinal decline was unusual in a model of focal retinal injury where all previously studied injury-responsive genes were confined to MG residing close to the injury site (4,15,16). Interestingly, Ascl1 expression is associated with DKK repression in human lung cancer (21), and in the injured retina ascl1a induction is correlated with dkk suppression (Figs.…”

“…To test this idea, we knocked down Ascl1a with previously validated ascl1a-targeting morpholino-modified antisense oligonucleotides (MOs) (15,16) and assayed dkk expression at 8 hpi ( Fig. 1 E and F).…”

“…One of these genes, lin-28, participates in an Ascl1a/Lin-28/let-7 microRNA signaling pathway that contributes to MG dedifferentiation (15). Ascl1a may also regulate the proliferation of dedifferentiated MG (16). In addition, injury-dependent induction of Pax6 appears to control the expansion of MG-derived progenitors, but not their initial entry into the cell cycle (17).…”