Cell therapy for Huntington's disease: addressing critical knowledge gaps

Cell therapies aim to replace cells that have been lost or damaged through disease, by transplanting cells into the area of loss. Such potential therapies are under consideration for a range of diseases of the brain, such as Parkinson's (PD) and Huntington's (HD) diseases. We are particularly interested in cell therapy for HD, a currently untreatable condition that is passed down through families and leads to progressive movement, thinking, and psychiatric deterioration, typically between the ages of 30-50.

After transplantation into the brain, donor cells can survive and send out cellular processes to connect with particular areas of the host brain cells (their 'target' areas). We are particularly interested in cell transplantation for HD as the transplanted donor cells are placed directly into the area of cell loss and so have the opportunity to restore normal connections with the host brain. This is in contrast to conditions such as PD in which the donor cells have to be transplanted into their target regions (rather than the area of loss) and so the connections they make can never be completely normal. Thus, as well as being an area of unmet need, HD is a good 'model' in which to test whether transplanted cells can indeed restore normal connections in the brain.

One of the primary changes seen in the brains of people with HD is death of a specific type of cell, called the medium spiny neuron (MSNs). Our goal is to "repair the brain" by replacing degenerated MSNs. We have shown in animal models and in early clinical trials that transplanting new MSNs into the brain results in cell grafts that survive and improve behavioural symptoms. However, the original cell source studied is very scarce and difficult to access, so it is essential to generate MSNs from new donor cell sources to allow for more widespread clinical application. We, and others, have identified stem cells as an optimal source of cells for this purpose and, over the last few years, have developed robust and promising methods to make functional MSNs from stem cells.

In this project, our goal is to test these new stem cell-derived MSNs in rodents with HD. The aim is to determine whether the stem cell-derived MSNs transplants improve the same range of movement, thinking, and other behaviours that we see with transplants of the original cell source. We will use different rat models of HD to determine how robust the improvements are and which symptoms are improved, and we will compare these new stem cell-derived MSNs directly to the original 'authentic' cells that we have used in other studies. We also want to know how the cells improve these behaviours, and have a number of tools that will allow us to look at the whether the transplanted cells make direct physical contact with the host cells, whether they are electrically connected to host cells, and whether functional improvements disappear when we use special tools to temporarily switch off the grafts. We will also collect detailed molecular data that will allow us to thoroughly characterise the donor cells and grafts, and so provide the opportunity to further improve the cell therapy in the future.

The information gathered in this application will be essential for us to move cell therapy for HD forward towards clinical application in a way that will be safe and continues to provide for therapeutic improvements.

Cell therapy provides a strategy for treating diseases in which predominant loss of a specific neural cell type is responsible for a substantial element of the functional decline. Huntington's disease (HD) is a devastating, incurable condition, and is a good model for testing cell therapy as loss of striatal medium spiny neurons (MSNs) is a prominent and early feature, and strongly related to functional decline. We have worked on cell therapy for HD for many years. We have developed considerable preclinical knowledge and have started to prepare for clinical translation. The most robust preclinical knowledge is based on rodent foetal allografts, although human foetal cells have shown proof of concept improvements in HD patients. Foetal cells are 'properly' specified through normal development and so provide essential benchmarking, but widespread clinical application will require MSN to be derived from renewable donor sources, such as human embryonic stem cells (hESC-MSNs). Here we address critical questions of hESC-MSNs: (i) Can hESC-MSNs improve function in rodent HD models? We use a sophisticated battery of motor/cognitive tests, first in a widely-used lesion model of HD to comprehensively compare hESC-MSN grafts against the benchmark foetal cells, and second in a transgenic HD rat model; this is a more challenging, but more representative, model of the disease process. (ii) We use a range of approaches to test whether hESC-MSNs make functional synapses with host neurons: slice electrophysiology, DREADDS and rabies transsynaptic tracing. (iii) We will perform scRNAseq to compare hESC-MSNs with their foetal counterparts to assess their degree of heterogeneity, and assess how closely hESC-MSNs and foetal-derived MSNs resemble each other. We will also perform scRNAseq on dissociated mature hESC-MSN and foetal grafts to ask similar questions. This information will guide the route to the clinic as well as guiding the optimisation of next generation donor cells.

There are significant potential economic and societal benefits to this research, including reducing the financial burden of neurodegenerative diseases on health systems to engaging people to become interested and involved in research.

HEALTH AND WELL-BEING
The data generated in this project are ultimately geared towards impacting upon the health, well-being and quality of life for people with Huntington's disease (HD) and their families. We anticipate using this project to lay the groundwork for a first-in-man clinical trial using our novel stem cell derived therapy. This therapy aims to replace the degenerated medium spiny neurons that are lost early in the disease process and, to this end, has the potential to be a stand-alone intervention for HD. Importantly, however, this treatment is also anticipated to be suitable for combined use with other disease-modifying therapies, for example any of the novel antisense-oligonucleotide or genetic modification therapy currently in trial or being developed. These manipulations of host neural cells may slow disease progression, but will not have the ability to stimulate regeneration of cells to replace those lost to the disease process, so it is anticipated that cell therapies will be required to overcome the impact of the early degeneration. Successfully improving patients' well-being and quality of life has the potential to impact considerably on the economic burden of this disease. The longer people are able to work, to live independently, to engage socially, to care for dependents - the less intervention and support is required from family members and ultimately from the state.
Importantly, we anticipate that the biological principles established in this program of work will have impact beyond Huntington's disease, as it is likely that similar processes will govern the capacity of donor cells to survive and connect in most other parts of the brain.
Furthermore, although the emphasis in this program application has been on cell therapy, we are also interested in using transplantation paradigms to model disease using mutation carrying human iPSCs. This is potentially useful as current 2D and 3D culture systems, even organoids, are limited in the length of time that neurons can survive in a healthy state. Culture systems are also limited in the extent to which the diseased cells can integrate and form networks, and ultimately any attempt to understand the impact of the mutated cell on animal behaviour requires that the cells exist within a living animal. This is likely to be especially important for psychiatric conditions. Thus, a better understanding of the biological principles important for circuit reconstruction by stem cell derived donor cells, is likely to be important for using grafts for disease-modelling.

INCREASING PUBLIC ENGAGEMENT WITH RESEARCH
Developing therapies for degenerative diseases and exploring the mechanisms underlying brain function and repair provides a unique opportunity to engage the public and young people in science. The hypothesis is straightforward (i.e. cells die in the brain and we can replace those cells to repair the brain) but the implementation is somewhat more complex. Developing clinically-relevant interventions is of interest to non-scientists. We have used this topic to engage people through interactive demonstrations of the process of 'repairing a brain', which has been remarkably impactful for both young people and adults. Ultimately this public engagement can further manifest in many ways, including more people entering scientific careers, stimulating fundraising to support scientific development, greater awareness of neurological conditions and engendering enhanced public perception of scientific endeavours.