Zebrafish have the remarkable ability to regenerate their retinas upon damage, which makes them ideal candidates to study possible ways to restore vision following retinal injury. A new study by researchers at IISER, Mohali, has found a new mechanism via which epigenetic factors regulate retinal regeneration in zebrafish.
Retinal damage is a common cause of blindness. However, we are still far away from an efficient strategy to treat it. Retinal regeneration is a process by which vision can be restored, especially in vertebrates that have suffered retinal degeneration. Ray-finned fishes like zebrafish have an innate ability for this, due to which, they serve as ideal model organisms to study this phenomenon.
A new study from a group of scientists from the Indian Institute of Science Education and Research (IISER), Mohali, sheds light on the mechanism of retinal regeneration in zebrafish. Headed by Rajesh Ramachandran, the researchers tried to understand the role played by a group of enzymes called Histone deacetylases (HDACs) in Zebrafish retinal regeneration. HDACs are enzymes that remove acetyl groups from histone proteins, which package and condense DNA in cells. This modification allows histones to bind DNA more tightly, preventing gene transcription in that region of DNA.
“We wanted to explore the potential roles played by HDACs during retina regeneration, especially in the control of cell proliferation,” says Ramachandran. His group believes that the molecular pathways of retina regeneration can be understood by deciphering epigenome modifiers like the HDACs, which may, in turn, lead to treatment of mammalian retinal damage.
The vertebrate retina is a thin sheet of neural tissue at the back of the eye. Among its many layers is a layer of rod and cone photoreceptors, which is further subdivided into outer and inner segments. A layer of Müller glia (MG) cells, the most common type of glial cells found in the vertebrate retina; separate the inner segments and the photoreceptor cell bodies. The MG cells support retinal cells by taking up neurotransmitters, removing debris, regulating potassium ion levels, storing glycogen, etc.
In zebrafish, following retinal injury, MG cells dedifferentiate to form MG-derived progenitor cells (MGPCs), which have stem cell-like properties. During retina regeneration in zebrafish, these stem cell-like MGPCs produce retinal neurons and MG cells. This process also requires exposure to cytokines, growth factors, and transcription factors.
In order to observe retinal regeneration in action, the researchers performed retinal injury on zebrafish. Both uninjured (control) and injured fish were treated with Valproic acid (VPA), a drug that inhibits HDAC enzymes. RNA was isolated from the retinas of both injured and uninjured fish post-injury. The isolated RNA was sequenced to understand gene expression.
There are different classes of HDAC proteins in the cell. On retinal injury, the genes for different HDAC proteins are expressed to varying degrees. The researchers observed that while hdac3, hdac5, and hdac6 are induced rapidly after retinal injury, hdac1, hdac4, hdac7, and hdac9 get downregulated instead. They also learnt that following injury, there was a decrease in the mRNA levels of hdac1 specifically throughout the retina.
To understand the exact role of hdac1, the scientists allowed the retina to regenerate for either 2 or 4 days post-injury and then blocked the expression of hdac1 using a Morpholino oligomer. Morpholino oligomers are synthetic stretches of nucleic acids that can bind to a target sequence within an mRNA and block gene expression. An appropriate control to this was to treat the injured retina with a morpholino that does not bind to the hdac1 mRNA.
The researchers observed that the number of MGPCs (the stem cell-like progenitors) decreased in the retina of those fish that were treated with hdac1-blocking morpholino as compared to the control group. This strongly suggested that hdac1-mediated gene regulation is involved in retina regeneration.
The days to come are exciting for this team. Ramachandran plans to compare if fish fin and heart regeneration are regulated by similar mechanisms. He adds, “One unsolved question is how specific gene regulatory chromatin is selectively targeted by HDACs, causing gene repression. We plan to study this by gene tagging and conditional gene knockdown approaches for better clarity.”
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