Written by Frida Niss Edited by Dr. Siddharth Nath
Can neurodegeneration in SCA7 in part be due to faulty calcium homeostasis in the cerebellum?
Polyglutamine diseases are caused by an increase in the length of CAG repeats within a specific gene. The mutation for spinocerebellar ataxia type 7 (SCA7) was discovered more than two decades ago, but many of the details surrounding how the mutation actually causes disease remain fuzzy. We know that the increased repeat length in the gene makes it difficult for the resulting protein to arrange or fold itself properly. We also know that the mutated protein binds to itself and to other proteins in an unusual way. It building up large deposits of seemingly useless debris in the cell, called ‘aggregates’. However, the exact pathways this leads to cell death, and subsequently neurodegeneration, is not completely clear.
There is currently research underway to directly target and inhibit the repeat proteins themselves. However, finding other pathways in the cell that are easier to target with medication is also a priority. In this research, Stoyas and her colleagues wanted to find out more about which cellular pathways are disturbed in the polyglutamine disease SCA7.
SCA7 mice have disordered productions of proteins that help balance ions concentrations
In SCA7, the protein that carries the mutation is Ataxin-7. Ataxin-7 participates in transcription through complexes of proteins that together can change some signalling particles on the DNA. Depending on what signalling particles are attached to a certain gene, the gene is either transcribed and made into a protein, or “silenced” and skipped over. In the case of Ataxin-7 and its complex, they work together to cause transcription of genes. One of the main theories of how a polyglutamine mutation can be toxic in Ataxin-7 is that the mutation disturbs Ataxin-7’s normal function within this transcription activating complex. Instead of being regular and orderly, ataxin-7 starts acting unpredictably. Some things that should be transcribed are not, some that shouldn’t be transcribed are.
Thus, Stoyas and colleagues started by checking how certain genes were transcribed in a SCA7 mouse model. They measured which genes were being transcribed, and to what degree, in the cerebellum of mice carrying the SCA7 mutation. They compared this to healthy mice and found that many genes in calcium ion homeostasis pathways were not being transcribed as much as they should be.
In neurons, the concentration of different ions in specific compartments of the cell is very important. This regulates how the neuron can communicate with other neurons. To communicate, neurons use an electrical signal called an action potential, which is causes by the fast change of where ions are in the cell. Since ions are charged, the change in where they are located creates an electrical pulse. However, if the ion homeostasis (the balancing of different types of ions) is off-kilter, this can prevent neurons being able to communicate efficiently with other neurons.
An imbalance of ions makes of mess of electric signals
One of the main symptoms of SCA7 is ataxia, or trouble with movement and coordination. This is mainly caused by the death of Purkinje cells in the cerebellum. Purkinje cells are very large, with many connections to surrounding cells. It is important that they have efficient signalling in this cells for humans to be able to balance and move. The next step of Stoyas and colleagues’ study was to characterize how the SCA7 Purkinje cells differ in electrical capacity to healthy Purkinje cells. They again used their SCA7 mouse model, this time to measure how much electric charge could be stored in Purkinje cells and how regularly they sent out action potentials.
They found that both the electric charge and the regularity of signals was decreased in Purkinje cells of the SCA7 mouse model. However, if they made the mice produce extra amounts of a protein which helps balance the amount of potassium ions in the cell, both of these things went back to normal. This specific protein acts like a channel and is turned on and off by calcium ions. It was one of the proteins that was downregulated in the genetic screen.
After having seen that one protein in the calcium homeostasis pathway made a difference in the previous experiment, Stoyas and colleagues wanted to investigate this more. They asked if the previously identified genes that were transcribed less had anything else in common, besides being part of calcium ion homeostasis. It turned out that many of the genes were regulated by proteins that were in turn regulated by Sirt1.
Increasing Sirt1 activity improved SCA7 symptoms in mice
Sirt1 is a protein that removes a specific signalling particle from other proteins: acetyl groups. The presence of acetyl groups on some proteins changes how they interact with other proteins in the cell. To remove acetyl groups from proteins, Sirt1 must have a co-factor, called NAD+. NAD+ is a very common molecule that is used in hundreds of metabolic processes in the cell. Sometimes when the cell is stressed, several pathways and proteins can compete with each other for the supply of NAD+. Stoyas and colleagues believe that this is what might be happening in SCA7 mice. They could see DNA damage in cells taken from the mice. When the DNA is damaged, this takes precedence over other activity in the cell, taking NAD+ away from the calcium homeostasis pathway.
To confirm this, they tried two things. First, they made the mice produce excess Sirt1 protein. Second, they gave the mice a supplement in their food that would increase the about of NAD+ in their body. Both of these two methods caused the SCA7 mice to perform better in all of the tests applied. This included a slightly better survival rate.
What does this mean for human patients?
Could this also be relevant to humans? Well, Stoyas and colleagues decided to test this by sampling some cells from SCA7 patients. It is possible to take skin cells from patients and “reprogram” them to become a type of stem cell. These cells can they be reprogramed again to cells that resemble neurons. Although this take time, obtaining neuron-like cells this way is far less invasive for the person donating them
The researchers repeated the same types of experiments they did in the mice in these new human cells. Making the cells produce excess Sirt1 or increasing the amount of NAD+ had the same trendin humans as for the mice. Both the survival rate increased and calcium homeostasis was improved. When the researchers looked at cerebella tissue samples of diseased SCA7 patients, they also saw signs of DNA damage in their Purkinje cells, just like in the mice.
With this work, Stoyas and colleagues were able to identify which pathways of proteins were altered in SCA7 mice. They also identified Sirt1 as a key protein that is impacted by SCA7. Sirt1 has previously been implicated in other neurodegenerative diseases. But exact mechanisms or ways that this happens is not well understood. Most importantly, the researchers identified a range of new proteins to target for SCA7 therapy research. As there is currently neither a cure nor a treatment for SCA7, this work provides important hope and insight for the field.
Molecular pathways: Networks of proteins and molecules that are connected through their functions, depending on each other to perform at the right time and in the correct place. One protein often depends on another protein to be activated, which in turn is activated by a third protein and so forth.
Action potential: Electrical pulses created by the flow of ions across cell membranes, which travel along neurons as communication signals.
Mouse model: A line of mice that carries genetic traits that closely mimic a human disease. Normally used for experiments that investigate how the disease works and/or can be treated.
Ion homeostasis: The balance of different concentrations of ions in different parts of the cell. The concentration differences between the different compartments creates a driving force that the cell can use to send messages within the cell and to other cells.
Conflict of Interest Statement
Colleen Stoyas and David Bushart, who completed the work described in the summary, are previous and current contributors to SCAsource, respectively. Neither Colleen Stoyas nor David Bushart had any contribution to the writing or editing of this summary.
The authors and editor declare no conflict of interest.
Citation of Article Reviewed
Stoyas C.A. et al. Nicotinamide Pathway-Dependent Sirt1 Activation Restores Calcium Homeostasis to Achieve Neuroprotection in Spinocerebellar Ataxia Type 7. Neuron, 2020. 4(105): p. 630-644 e9. (https://www.ncbi.nlm.nih.gov/pubmed/31859031)