Dedifferentiation is the reversion of mature cells to a stem cell‐like fate, whereby gene expression programs are altered and genes associated with multipotency are (re)expressed. Misexpression of multipotency factors and pathways causes the formation of ectopic neural stem cells (NSCs). Whether dedifferentiated NSCs faithfully produce the correct number and types of progeny, or undergo timely terminal differentiation, has not been assessed. Here, we show that ectopic NSCs induced via bHLH transcription factor Deadpan (Dpn) expression fail to undergo appropriate temporal progression by constantly expressing mid‐temporal transcription factor(tTF), Sloppy‐paired 1/2 (Slp). Consequently, this resulted in impaired terminal differenation and generated an excess of Twin of eyeless (Toy)‐positive neurons at the expense of Reversed polarity (Repo)‐positive glial cells. Preference for a mid‐temporal fate in these ectopic NSCs is concordant with an enriched binding of Dpn at mid‐tTF loci and a depletion of Dpn binding at early‐ and late‐tTF loci. Retriggering the temporal series via manipulation of the temporal series or cell cycle is sufficient to reinstate neuronal diversity and timely termination.

IntroductionLoss of neurons in the neural retina is a leading cause of vision loss. While humans do not possess the capacity for retinal regeneration, zebrafish can achieve this through activation of resident Müller glia. Remarkably, despite the presence of Müller glia in humans and other mammalian vertebrates, these cells lack an intrinsic ability to contribute to regeneration. Upon activation, zebrafish Müller glia can adopt a stem cell-like state, undergo proliferation and generate new neurons. However, the underlying molecular mechanisms of this activation subsequent retinal regeneration remains unclear.Methods/ResultsTo address this, we performed single-cell RNA sequencing (scRNA-seq) and report remarkable heterogeneity in gene expression within quiescent Müller glia across distinct dorsal, central and ventral retina pools of such cells. Next, we utilized a genetically driven, chemically inducible nitroreductase approach to study Müller glia activation following selective ablation of three distinct photoreceptor subtypes: long wavelength sensitive cones, short wavelength sensitive cones, and rods. There, our data revealed that a region-specific bias in activation of Müller glia exists in the zebrafish retina, and this is independent of the distribution of the ablated cell type across retinal regions. Notably, gene ontology analysis revealed that injury-responsive dorsal and central Müller glia express genes related to dorsal/ventral pattern formation, growth factor activity, and regulation of developmental process. Through scRNA-seq analysis, we identify a shared genetic program underlying initial Müller glia activation and cell cycle entry, followed by differences that drive the fate of regenerating neurons. We observed an initial expression of AP-1 and injury-responsive transcription factors, followed by genes involved in Notch signaling, ribosome biogenesis and gliogenesis, and finally expression of cell cycle, chromatin remodeling and microtubule-associated genes.DiscussionTaken together, our findings document the regional specificity of gene expression within quiescent Müller glia and demonstrate unique Müller glia activation and regeneration features following neural ablation. These findings will improve our understanding of the molecular pathways relevant to neural regeneration in the retina.

Dedifferentiation is the reversion of differentiated cells to a stem cell like fate, whereby, the gene expression program of mature cells is altered and genes associated with multipotency are expressed. Appropriate terminal differentiation of NSCs is essential for restricting the overall number of neurons produced; in addition, faithful production of neuronal subtypes that populate the brain is important for NSC function. Both characteristics of NSCs are specified through temporal patterning of the NSCs driven by the successive expression of temporal transcription factors (tTFs). In this study, we found that ectopic NSCs induced via bHLH transcription factor Deadpan (Dpn) expression fail to undergo timely expression of temporal transcription factors (tTFs), where they express mid-tTF, Sloppy-paired 1 (Slp-1) and fail to express late-tTF Tailless (Tll); consequently generating an excess of Twin of eyeless (Toy) positive neurons at the expense of Reversed polarity (Repo) positive glial cells. In addition to disrupted production of neuronal/glial progeny, Dpn overexpression also resulted in stalled progression through the cell cycle, and a failure to undergo timely terminal differentiation. Mechanistically, DamID studies demonstrated that Dpn directly binds to both Dichaete (D), a Sox-box transcription factor known to repress Slp-1, as well as a number of cell cycle genes. Promoting cell cycle progression or overexpression of D were able to re-trigger the progression of the temporal series in dedifferentiated NBs, restoring both neuronal diversity and timely NB terminal differentiation.

Damage to light-sensing photoreceptors (PRs) occurs in highly prevalent retinal diseases. As humans cannot regenerate new PRs, these diseases often lead to irreversible blindness. Intriguingly, animals, such as the zebrafish, have the ability to regenerate PRs efficiently and restore functional vision. Upon injury, mature Müller glia (MG) undergo reprogramming to adopt a stem cell-like state. This process is similar to cellular dedifferentiation, and results in the generation of progenitor cells, which, in turn, proliferate and differentiate to replace lost retinal neurons. In this study, we tested whether factors involved in dedifferentiation of Drosophila CNS are implicated in the regenerative response in the zebrafish retina. We found that hairy-related 6 (her6) negatively regulates of PR production by regulating the rate of cell divisions in the MG-derived progenitors. prospero homeobox 1 (prox1) is expressed in differentiated PRs, and likely promotes PR differentiation through phase separation. Interestingly, upon Her6 downregulation, Prox1 is precociously upregulated in the PRs, to promote PR differentiation; conversely, loss of Prox1 also induces a downregulation of Her6. Together, we identified two novel candidates of PR regeneration that cross regulate each other, and may be exploited to promote human retinal regeneration and vision recovery.