What has been achieved and where to from here
A report to the HSP community by Prof. Alan Mackay-Sim, Director of the National Centre for Adult Stem Cell Research and Co-Principal Investigator of the HSP research study program.
It was a satisfying achievement to finally publish the results of the last 3 years work that was supported by funding from the HSP Research Foundation. It is now available in a journal that is accessible to everyone and is featured in this edition of Research Highlights on this website.
The study concentrated on understanding how cells from HSPers and non-HSPers differed, in order to learn how the genetic mutations in the spastin gene lead to the disease. We found some key differences in HSPers’ cells that suggest how the nerves in the spinal cord are affected. These differences concern the microtubules, part of the internal skeleton of every cell, that are involved in transporting “energy cargoes” around cells and especially along nerves. Microtubules are normally being formed and reformed constantly in cells.
Knowing what we know about spastin, we expected that spastin mutations (lower levels of spastin) would lead to more stable microtubules (not a good thing), but found an unexpected drop in the levels of stable microtubules. On closer inspection we found that another protein, stathmin, was in higher levels than expected, effectively compensating for the reduced spastin, and the reason for the level of microtubule stability measured. Why is this important? Mainly because it gives us a better idea of how the disease affects cells and what cell processes to investigate when screening for potential drugs. So we studied the effects of two drugs whose effects on microtubules are well known. This research showed that very, very low doses of the drugs could restore the microtubules in HSP cells to normal.
What are we doing now? Fan is now experimenting on several potential drugs of the same family, which have greater ability getting into the brain and spinal cord, as this is where they will need to finish up to be effective in treating HSP. Some drugs in this family are not able to get into the brain and spinal cord, and all are toxic to healthy cells at the concentrations in which they are used to treat cancer, so the challenge now is to identify ones that:
- restore HSP cell functions to normal
- can successfully get into the brain and spinal cord to do their work
- are effective in very low concentrations, and
- are safe to use on an ongoing basis.
The next stage of drug discovery is to validate candidate drugs by testing them on different cell functions, especially on cell functions associated with the disease. The team is following a number of paths here.
Fan is working on methods to turn the olfactory stem cells into nerve cells (neurons) because these will have long axons, like those that make up the nerve tracts of the spinal cord that are affected in HSP.
Gautam and Simon are studying how the spastin mutations affect the dynamics of cargo transport inside HSP stem cells and “neuron-like” cells with long axon-like processes.
Gautam has been using time-lapse videos to follow how far and how fast the ‘cargos’ travel inside cells. Already we know that one type of cargo travels slower. Why are we doing this? The goal is to find disease-associated differences in cell functions related to what happens inside the long axons in the spinal cord nerves of HSPers. Levels of these altered cell functions are then used as part of the screening process to find drug candidates that may be effective.
Simon has developed ways to see the microtubules being formed inside the cells. Time-lapse videos show the new microtubules as little comets. He is seeing if HSP and healthy cells have different numbers of newly forming microtubules and whether their speeds of formation differ.
Where are we going in the next year?
Fan will be:
- completing the work to make HSP neurons
- completing the analysis of the tubulin-binding drugs, and
- starting on the screening of our library of 800 drugs already in use for other diseases.
Gautam will be:
- completing his analyses of cargo transport in HSP stem cells and neuron-like HSP cells
- assaying the neurons made by Fan
- assisting Fan in the drug screening work.
Simon will be comparing and contrasting stem cells with and without the spastin mutation. This will be important in learning if drugs for people with mutations in spastin might be effective for those with other HSP mutations.
Gautam and Simon will together be doing some deeper biological analyses aimed at finding out more about exactly how the spastin genetic mutations cause the effects that have been observed in HSP stem cells. This is important for understanding how drugs that are effective might be working.
The next stages of research will be to test any candidate drugs in mice with spastin mutations. We are planning for this eventuality and negotiating to collaborate with international researchers who have genetically engineered mice suitable for this purpose. We are also in discussion with Dr. Carolyn Sue, who has collaborated on the stem cell research to this point, to work together on “induced pluripotent stem cells” from HSPers. These are skin cells that have been genetically engineered to turn them into stem cells. This type of stem cell is all the rage at the moment and they can be turned into many types of neurons, including neurons like those in the affected part of the HSP brain. These will also be used to validate any potential drug candidates that we discover and to test for their toxicity on brain cells.