Treating neurodegenerative diseases has been a challenge that neuroscientists have been trying to defeat for decades now. CRISPR is an emerging technology that seems to have new applications every day and its potential to treat neurodegenerative diseases is finally being explored. Zhou and colleagues have published their study in the journal Cell and have shown evidence for the use of CRISPR CasRx as a treatment for a Parkinson’s disease mouse model as well as restoring vision in mice who underwent retinal ganglion cell death by means of converting glial cells into functioning neurons. These findings could pave the way for future treatment in other neurodegenerative diseases like Alzheimer’s disease and Huntington’s disease as well as sensory loss such as hearing and vision loss.

Background

The fight against neurodegenerative diseases is one that really began around 1970 with emerging techniques in immunostaining and an advancing of our understanding of neurophysiology (Young, 2009). No neurodegenerative disease has been cured, but plenty of treatments have been made available to reduce symptoms or halt the spread of disease (Duraes et al. 2018). Treatment for Alzheimer’s disease is mainly used to treat symptoms by balancing neurotransmitter disturbances caused by the disease (Yiannopoulou et al. 2020) and Parkinson’s disease is often times treated with the use of dopamine agonists (Rizek et al. 2016) with emerging techniques looking at using stem cell therapies to treat Parkinson’s disease (Barker et al. 2017). In 1987, the CRISPR system was discovered in E. Coli and continues to be one of the most useful technologies in biomedical research today (Ishino et al. 2018). The use of CRISPR to treat neurodegenerative diseases has only been suggested in the past, with scientists suggesting it could be used to repair damaged DNA that is leading to a neurological disease (Kolli et al. 2018) or target genes that are a risk factor for developing diseases like Alzheimer’s disease (Rohn et al. 2018). It was not until 2020 that Zhou and colleagues used the CRISPR system to effectively treat neurodegeneration and specifically Parkinson’s disease (Zhou et al. 2020). 

Methods

Researchers injected oxidopamine (6-OHDA) into the mice’s brains, which is a neurotoxin that kills dopaminergic neurons (Simola et al. 2007) to simulate Parkinson’s disease as the loss of dopaminergic neurons is a common feature of the disease (Naoi et al. 1999). After 3 weeks, they injected the brains of the same mice with the CRISPR CasRx system which aimed to knockout the ptbp1 gene from glia cells and after 3 more weeks, they tested to see what improvements the mice saw compared to mice that only received the 6-OHDA injection and not the CRISPR CasRx injection. They put mice through a rotarod test to test physical and cognitive abilities like balance, coordination, and motor memory and learning. Next, they looked at the individual neurons that were converted from glia to see that they functioned normally by sending a current through them and measuring the response. Finally, they looked inside the neuron to individual vesicles to look for VMAT2, which is a transporter that suggests dopamine can be released from and packaged into that vesicle. 

They also looked at converting glia into neurons within the mice’s visual system. They tested this by injecting NMDA into the mice’s brains which destroys retinal ganglion cells (Lambuk et al. 2019). These cells are neurons and receive visual information from photoreceptors in the retina. 3 weeks after the NMDA injection, mice were placed in a light/dark preference test. Next, the mice were injected with the CRISPR CasRx system and then stained to see where retinal ganglion cells appeared and where they projected to. Finally, they stimulated the retinal ganglion cells with light to check that they appropriately responded to light. 

Results

The researchers found that mice injected with the CRISPR CasRx, after being injected with the 6-OHDA, performed better on the rotarod test than mice only injected with the 6-OHDA. They found that their astrocytes were the specific glia that converted to neurons in the striatum of the brain, and these new neurons responded normally to being injected with current. Furthermore, VMAT2 was present on the neuron’s vesicles, suggesting that these neurons were dopaminergic ones, explaining the gain of motor function compared to the Parkinson’s disease mouse model who were only injected with the 6-OHDA. Mice injected with the NMDA and CRISPR CasRx had their Muller glial cells converted to retinal ganglion cells which projected all the way to the dorsal lateral geniculate nucleus and the superior colliculus which are the places the retina normally sends information to (Ellis et al. 2016). These mice spent more time in the dark in the light/dark preference test, which is what they found in the control mice. Finally, they found that most of the converted retinal ganglion cells responded normally to light. 

These findings open the door for new treatments for a number of neurodegenerative diseases like the tested Parkinson’s disease as well as Alzheimer’s and Huntington’s disease. Aside from neurodegenerative diseases, this treatment could be used in sensory loss such as hearing loss and the tested vision loss. What was shown here is that glia can be converted into neurons and new neurons can be made in the brain without the need of stem cell transplant or other invasive treatment. The CRISPR system has great potential and could be used in many different areas of research within biomedicine. More research needs be done on the long-term effects this treatment has on individuals and weigh these effects against living with a neurodegenerative disease, but what Zhou and colleagues have shown here is a proof of concept that has long been speculated within the field and is sure to be the foundation for a new era of neurodegenerative disease research and subsequent treatment.

[+] References

[+] Other Work By Derek Schreiner