Regeneration of Optic Nerve through Gene Therapy: A new approach towards Glaucoma Treatment

In a breakthrough that could aid the development of new therapies for glaucoma, one of the leading causes of blindness worldwide, researchers used gene therapy to regenerate damaged axons in the eye.

Scientists tested, in a study published in Nature Communications, whether the gene responsible for producing the protein Protrudin could promote nerve cell regeneration and protect nerve cells from cell death after injury.

Axons of the central nervous system of adult mammals have inherently low regenerative ability, so axonal damage has lasting implications. Increasing the axonal supply of growth molecules and organelles is one approach to improving regeneration. "We accomplished this by expressing the adaptor molecule Protrudin, which is typically present in non-regenerative neurons at low levels," the researchers wrote.

Elevated expression of Protrudin allowed vigorous regeneration of the central nervous system both in vitro in primary cortical neurons and in vivo in the optic nerve of the injured adult. Protrudin overexpression allowed the accumulation of endoplasmic reticulum, integrins, and Rab11 endosomes in the distal axon, while the regenerative effects were abrogated by the removal of Protrudin endoplasmic reticulum localization, kinesin-binding or phosphoinositide-binding properties.

"These findings show that Protrudin promotes regeneration by acting as a scaffold to connect axonal organelles, motors and membranes, and by establishing important roles for these cellular components in the adult central nervous system in mediating regeneration."

A cell culture method was used by the team to cultivate brain cells in a dish. They then, using a laser, injured their axons and examined the reaction using live-cell microscopy to this injury. Increasing the amount or behaviour of Protrudin in these nerve cells significantly improved their capacity to regenerate, the researchers found.

In order to transmit and process visual information, retinal ganglion cells spread their axons from the eye to the brain via the optic nerve. The researchers relied on a gene therapy technique to increase the amount and function of Protrudin in the eye and optic nerve to investigate whether Protrudin could promote repair in the damaged CNS of an intact organism.

The team discovered that Protrudin had allowed the axons to regenerate over wide distances when they measured the amount of regeneration a few weeks after a crush injury to the optic nerve. They also discovered that the cells of the retinal ganglion were shielded from cell death.

The researchers showed that this procedure, a common eye disease, would help protect against glaucoma. The optic nerve connecting the eye to the brain is gradually weakened in glaucoma, often in combination with elevated pressure inside the eye.

Glaucoma can lead to loss of vision if not detected early enough. In the U.K., glaucoma affects around one in 50 people over the age of 40 and one in ten people over the age of 75.

The researchers used a full retina from a mouse eye to demonstrate this protective effect of Protrudin against glaucoma and developed it in a cell culture dish. Typically, within three days of retinal removal, about half of retinal neurons die, but scientists found that growing or activating Protrudin resulted in almost full defence of retinal neurons.

"The causes of glaucoma are not well known, but the main focus is on discovering potential therapies through preventing the death of nerve cells in the retina and attempting to restore vision loss by regenerating diseased axons via the optic nerve," said Veselina Petrova, PhD, of the Department of Clinical Neuroscience at the University of Cambridge.

It is possible that our [gene therapy] therapy may be further evolved to protect retinal neurons from death, as well as to induce the regrowth of their axons. It is important to point out that more study will be required for these results to see whether they could be translated into successful human treatments.

Within the endoplasmic reticulum, Protrudin usually exists. In this study , the team showed that in order to promote the regeneration process after injury, the endoplasmic reticulum contained in axons appears to provide materials and other cellular structures necessary for growth and survival. Protrudin enhances the transport to the site of injury of these products.

Nerve cells in the central nervous system, as they age, lose the ability to regenerate their axons, so they have very little regrowth potential. "This suggests that brain, spinal cord, and optic nerve injuries have life-altering effects," Petrova added. The model of optic nerve injury is often used to test new therapies to stimulate the regeneration of the CNS axon, and therapies found in this manner often show promise in the injured spinal cord. To boost regeneration in the damaged spinal cord, increased or activated Protrudin can be used.