Yumi Ueki

ASCL1 reprograms mouse Müller glia into neurogenic retinal progenitors

Non-mammalian vertebrates have a robust ability to regenerate injured retinal neurons from Müller glia (MG) that activate the gene encoding the proneural factor Achaete-scute homolog 1 (Ascl1; also known as Mash1 in mammals) and de-differentiate into progenitor cells. By contrast, mammalian MG have a limited regenerative response and fail to upregulate Ascl1 after injury. To test whether ASCL1 could restore neurogenic potential to mammalian MG, we overexpressed ASCL1 in dissociated mouse MG cultures and intact retinal explants. ASCL1-infected MG upregulated retinal progenitor-specific genes and downregulated glial genes. Furthermore, ASCL1 remodeled the chromatin at its targets from a repressive to an active configuration. MG-derived progenitors differentiated into cells that exhibited neuronal morphologies, expressed retinal subtype-specific neuronal markers and displayed neuron-like physiological responses. These results indicate that a single transcription factor, ASCL1, can induce a neurogenic state in mature MG.

Transgenic expression of the proneural transcription factor Ascl1 in Müller glia stimulates retinal regeneration in young mice

Significance The retina is subject to a variety of insults that lead to degeneration of one or more types of neurons and ultimate visual impairment and blindness. Although the retinas of nonmammalian vertebrates can regenerate new neurons after injury, mammalian retinas largely lack this potential. We have tested whether the expression of the proneural transcription factor Ascl1 may be a key difference between the fish and mouse by targeting this factor to the cells that provide new retinal progenitors in mature retina, the Müller glia. Our results show that at least one of the differences between mammal and fish Müller glia that bears on their difference in regenerative potential is the proneural transcription factor Ascl1. Müller glial cells are the source of retinal regeneration in fish and birds; although this process is efficient in fish, it is less so in birds and very limited in mammals. It has been proposed that factors necessary for providing neurogenic competence to Müller glia in fish and birds after retinal injury are not expressed in mammals. One such factor, the proneural transcription factor Ascl1, is necessary for retinal regeneration in fish but is not expressed after retinal damage in mice. We previously reported that forced expression of Ascl1 in vitro reprograms Müller glia to a neurogenic state. We now test whether forced expression of Ascl1 in mouse Müller glia in vivo stimulates their capacity for retinal regeneration. We find that transgenic expression of Ascl1 in adult Müller glia in undamaged retina does not overtly affect their phenotype; however, when the retina is damaged, the Ascl1-expressing glia initiate a response that resembles the early stages of retinal regeneration in zebrafish. The reaction to injury is even more pronounced in Müller glia in young mice, where the Ascl1-expressing Müller glia give rise to amacrine and bipolar cells and photoreceptors. DNaseI-seq analysis of the retina and Müller glia shows progressive reduction in accessibility of progenitor gene cis-regulatory regions consistent with the reduction in their reprogramming. These results show that at least one of the differences between mammal and fish Müller glia that bears on their difference in regenerative potential is the proneural transcription factor Ascl1.

Genome-wide analysis of Müller glial differentiation reveals a requirement for Notch signaling in postmitotic cells to maintain the glial fate.

Previous studies have shown that Müller glia are closely related to retinal progenitors; these two cell types express many of the same genes and after damage to the retina, Müller glia can serve as a source for new neurons, particularly in non-mammalian vertebrates. We investigated the period of postnatal retinal development when progenitors are differentiating into Müller glia to better understand this transition. FACS purified retinal progenitors and Müller glia from various ages of Hes5-GFP mice were analyzed by Affymetrix cDNA microarrays. We found that genes known to be enriched/expressed by Müller glia steadily increase over the first three postnatal weeks, while genes associated with the mitotic cell cycle are rapidly downregulated from P0 to P7. Interestingly, progenitor genes not directly associated with the mitotic cell cycle, like the proneural genes Ascl1 and Neurog2, decline more slowly over the first 10–14 days of postnatal development, and there is a peak in Notch signaling several days after the presumptive Müller glia have been generated. To confirm that Notch signaling continues in the postmitotic Müller glia, we performed in situ hybridization, immunolocalization for the active form of Notch, and immunofluorescence for BrdU. Using genetic and pharmacological approaches, we found that sustained Notch signaling in the postmitotic Müller glia is necessary for their maturation and the stabilization of the glial identity for almost a week after the cells have exited the mitotic cell cycle.

P53 is required for the developmental restriction in Müller glial proliferation in mouse retina

Müller glia are normally mitotically quiescent cells, but in certain pathological states they can re‐enter the mitotic cell cycle. While several cell cycle regulators have been shown to be important in this process, a role for the tumor suppressor, p53, has not been demonstrated. Here, we investigated a role for p53 in limiting the ability of Müller glia to proliferate in the mature mouse retina. Our data demonstrate that Müller glia undergo a developmental restriction in their potential to proliferate. Retinal explants or dissociated cultures treated with EGF become mitotically quiescent by the end of the second postnatal week. In contrast, Müller glia from adult trp53−/+ or trp53−/− mice displayed a greater ability to proliferate in response to EGF stimulation in vitro. The enhanced proliferative ability of trp53 deficient mice correlates with a decreased expression of the mitotic inhibitor Cdkn1a/p21cip and an increase in c‐myc, a transcription factor that promotes cell cycle progression. These data show that p53 plays an essential role in limiting the potential of Müller glia to re‐enter the mitotic cycle as the retina matures during postnatal development.

EGF stimulates Muller glial proliferation via a BMP-dependent mechanism.

Müller glia, the major type of glia in the retina, are mitotically quiescent under normal conditions, though they can be stimulated to proliferate in some pathological states. Among these stimuli, EGF is known to be a potent mitogen for Müller glia. However, the signaling pathways required for EGF‐mediated proliferation of Müller glia are not clearly understood. In this study, postnatal day 12 (P12) or adult trp53−/− mouse retinas were explanted and cultured in the presence of EGF to stimulate Müller glial proliferation. Treatment with signaling inhibitors showed that activation of both MEK/ERK1/2 and PI3K/AKT pathways is required for EGF‐induced proliferation of Müller glia. Interestingly, BMP/Smad1/5/8 activation downstream of PI3K/AKT signaling was also necessary for robust Müller glial proliferation, though activation of BMP/Smad1/5/8 signaling alone failed to stimulate their proliferation. In dissociated Müller glial culture, treatment with EGF induced the upregulation of Bmp7, and this upregulation was blocked significantly by co‐treatment with the BMP inhibitor dorsomorphin, suggesting that BMP/Smad1/5/8 activation is mediated at least in part by an autocrine mechanism in Müller glia. A better understanding of how BMP/Smad1/5/8 signaling is involved in glial proliferation may have important implications for proliferative disorders, as well as for retinal regeneration in mammalian retinas.

Activation of BMP-Smad1/5/8 Signaling Promotes Survival of Retinal Ganglion Cells after Damage In Vivo

While the essential role of bone morphogenetic protein (BMP) signaling in nervous system development is well established, its function in the adult CNS is poorly understood. We investigated the role of BMP signaling in the adult mouse retina following damage in vivo. Intravitreal injection of N-Methyl-D-aspartic acid (NMDA) induced extensive retinal ganglion cell death by 2 days. During this period, BMP2, -4 and -7 were upregulated, leading to phosphorylation of the downstream effector, Smad1/5/8 in the inner retina, including in retinal ganglion cells. Expression of Inhibitor of differentiation 1 (Id1; a known BMP-Smad1/5/8 target) was also upregulated in the retina. This activation of BMP-Smad1/5/8 signaling was also observed following light damage, suggesting that it is a general response to retinal injuries. Co-injection of BMP inhibitors with NMDA effectively blocked the damage-induced BMP-Smad1/5/8 activation and led to further cell death of retinal ganglion cells, when compared with NMDA injection alone. Moreover, treatment of the retina with exogenous BMP4 along with NMDA damage led to a significant rescue of retinal ganglion cells. These data demonstrate that BMP-Smad1/5/8 signaling is neuroprotective for retinal ganglion cells after damage, and suggest that stimulation of this pathway can serve as a potential target for neuroprotective therapies in retinal ganglion cell diseases, such as glaucoma.

A transient wave of BMP signaling in the retina is necessary for Müller glial differentiation

The primary glial cells in the retina, the Müller glia, differentiate from retinal progenitors in the first postnatal week. CNTF/LIF/STAT3 signaling has been shown to promote their differentiation; however, another key glial differentiation signal, BMP, has not been examined during this period of Müller glial differentiation. In the course of our analysis of the BMP signaling pathway, we observed a transient wave of Smad1/5/8 signaling in the inner nuclear layer at the end of the first postnatal week, from postnatal day (P) 5 to P9, after the end of neurogenesis. To determine the function of this transient wave, we blocked BMP signaling during this period in vitro or in vivo, using either a BMP receptor antagonist or noggin (Nog). Either treatment leads to a reduction in expression of the Müller glia-specific genes Rlbp1 and Glul, and the failure of many of the Müller glia to repress the bipolar/photoreceptor gene Otx2. These changes in normal Müller glial differentiation result in permanent disruption of the retina, including defects in the outer limiting membrane, rosette formation and a reduction in functional acuity. Our results thus show that Müller glia require a transient BMP signal at the end of neurogenesis to fully repress the neural gene expression program and to promote glial gene expression. Summary: BMP signalling is transiently activated in the postnatal mouse retina to terminate the neurogenic program and promote the expression of glial-specific genes.