Transplantation in HD: Are We Transplanting the Right Cells?

Cell replacement therapy is a viable option for the treatment of Huntington's disease (HD), where the aim is to replace the lost medium spiny projection neurons of the stria‐ tum. The intra‐striatal engraftment of developing striatal precursors harvested from the foetal brain has provided proof of concept in both rodent models and human patients that these primary foetal tissue grafts can bring about a degree of functional recovery in a HD‐ degenerated brain. With the advent of pluripotent stem cell technologies, novel, potential alternative donor cell sources have become available. Ongoing studies are assessing the capacity of these cells to differentiate towards striatal precursors for transplantation in HD. Here, we review the characteristics of potential donor cells for HD with respect to available cell markers, functional properties and maturity of cells upon transplantation. We consider the optimal composition of the donor cell population, that is, whether a heterogeneous population containing all cell types from the developing striatum (the whole ganglionic eminence) is preferable to a more homogeneous population of striatal projection neurons, as directed by differentiation protocols applied to pluripotent stem cells. Furthermore, we consider what might be required to improve transplant efficacy and success, with respect to striatal differentiation of transplanted cells and functional improvement.