Gene therapies that act by replacing a missing or defective gene with a healthy version have shown great promise in the treatment of rare, monogenic diseases (those caused by a single defective gene). However, neurodegenerative diseases are complex polygenic conditions (caused by a combination of defective genes) and gene therapy approaches to treating these diseases have been limited.

A hallmark of neurodegenerative disease is the disruption of axonal transport, a cellular process responsible for the movement of molecules (such as mitochondria, lipids, and regulatory proteins) from one end of an axon to the other. Axons transmit electrical signals and allow nerve cells to communicate with each other and with muscle. Previous research has suggested that stimulating axonal transport with the use of intrinsic neuronal processes in a diseased central nervous system might be an effective way to repair damaged nerve cells.

"The axons of nerve cells function a bit like a railway system, where the cargo is essential components required for the cells to survive and function," explained Dr. Tasneem Khatib, first author and a clinical research fellow at the University of Cambridge, in a statement. "In neurodegenerative diseases, this railway system can get damaged or blocked."

To this end, brain-derived neurotrophic factor (BDNF) and its receptor, tropomyosin receptor kinase B (TrkB), have the potential to promote axon growth and improve axonal function through the extracellular signal-regulated kinase signaling pathway.

The research team at the University of Cambridge suggested that the combined use of both factors would be more successful in treating neurodegenerative diseases compared to the receptor or ligand alone. The team tested their theory in two models of neurodegenerative disease – glaucoma (eye condition caused by high pressure that damages the optic nerve) and tauopathy (abnormal accumulation of tau proteins associated with Alzheimer's disease, frontotemporal dementia, and other neurodegenerative diseases).

Simple single-vector design

To begin, the team had to overcome a hurdle associated with combination gene therapy, which has traditionally used multiple vectors administered in a single formulation. These dual promoter techniques have only selective expression and reduced efficacy. Conversely, the team administered the receptor and ligand via a single self-cleaving 2A peptide-based gene therapy vector that is under the control of a single promoter. They hoped that this would promote the longer-term expression of both proteins in target cells.

"We reckoned that replacing two molecules that we know work effectively together would help to repair this transport network more effectively than delivering either one alone, and that is exactly what we found," Khatib explained.

Testing the combination therapy in preclinical disease models

In uninjured optic nerves, the team demonstrated that the administration of the vector with the receptor and ligand resulted in enhanced axonal transport compared to either TrkB or BDNF alone.

Next, the researchers used a transgenic tauopathy mouse model, characterized by the buildup of "tangles" of tau protein in the brain and reduced axonal transport in optic nerves, to explore the efficacy of their combination gene therapy. They found that relative to controls, the treatment significantly increased optic nerve axonal transport. Notably, the treatment was more effective before the onset of tau pathology.

In an experimental glaucoma model, which is known to cause retinal ganglion cell degeneration and disruption of axonal transport due to increased intraocular pressure, the team showed that intravitreal administration of the combination therapy restores axonal transport with functional recovery of vision after one month. A tracer dye was used to measure the changes in axonal transport and cell function.

"This combined approach also leads to a much more sustained therapeutic effect, which is very important for a treatment aimed at a chronic degenerative disease," Khatib said. "Rather than using the standard gene therapy approach of replacing or repairing damaged genes, we used the technique to supplement these molecules in the brain."

Interestingly, administration of the combination gene therapy was associated with improved short-term memory in the disease models. Prior to treatment, the researchers tested the mice on an object recognition task. The team measured the amount of the time a mouse spent exploring two objects -- one new and one old -- to see whether it had remembered the object from a previous task. This task was repeated after treatment and the researchers found a small improvement in short-term memory. They are planning a larger study to confirm the effect.

"While this is currently early stage research, we believe it shows promise for helping to treat neurodegenerative diseases that have so far proved intractable," said Dr. Keith Martin, a professor at the University of Melbourne. Martin is the study's lead author and conducted it while at Cambridge.

"Gene therapy has already proved effective for some rare monogenic conditions, and we hope it will be similarly useful for these more complex diseases which are much more common," said Martin.

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