Retinal degeneration are blinding eye diseases that impact millions of lives around the world. Age-related macular degeneration alone is predicted to affect over 250 million people around 2040 as the population ages. The most prevalent form of inherited retinal degeneration, retinitis pigmentosa, has been found to have more than 200 different causative mutations, showing that treatments for retinal degeneration may not have a one-size-fit-all solution and require different approaches and concerted efforts from scientists around the world.
Gene therapy is an emerging and effective treatment option in restoring the vision and the quality of life of a subset of retinal degenerative patients. However, the success of gene therapy faces the limitations of low vector transduction efficiency, low vector carrying capacity, and immutable disease progression. Stem cell therapies face similar problems of immunogenicity and low cell integration efficiency. We show here that, through a combination of these two treatment modalities, endogenous regeneration through the genetic reprogramming of glia may be another effective and more broadly applicable approach in treating retinal degeneration.
Müller cells, the primary glia of the retina, have been demonstrated to possess neuroprotective as well as retinal progenitor cell-like regenerative properties in cases of retinal damage in zebrafish. The helix-loop-helix transcription factor Ascl1 is a key factor in determining neuronal cell fate in the nervous system, and it has also been shown to be capable of transdifferentiating astrocytes as well as Müller glia into neurons in vitro. The Let7 siRNA regulator Lin28 likewise seems to influence the zebrafish Müller glia cell fate through an Ascl1-dependent manner. While damage-dependent regeneration of the retina in zebrafish relies on the expression of Ascl1 and Lin28, no large scale proliferation and differentiation of the Müller glia have been observed in the mammalian retina, and as such, the role of Ascl1 and Lin28 in determining the mammalian Müller glia cell fate is currently unknown.
In chapter 2, we demonstrate that Müller glia in zebrafish and mice are capable of proliferation and fate change in vivo through the approach of forced expression of both Ascl1 and Lin28. Lin28 enhances the Ascl1-dependent proliferation of the Müller glia in zebrafish and mice when retinal damage is induced via NMDA. Notch suppression increases this proliferation in zebrafish further, but curiously not in mice. In chapter 3 we look at the different conditions involved in this Ascl1/Lin28 dependent proliferation through the exploration of different retinal damage models such as cobalt chloride induced hypoxic response and UV light induced photoreceptor apoptosis. We also attempt to induce Müller glia proliferation in retinal degenerative mouse models. Interestingly, while we find some success in inducing Müller glia proliferation in the retinal damage models, we find that genetic mouse models of retinal degeneration do not seem to respond to Ascl1 and Lin28 manipulation.