The objective of this project is to develop technologies and approaches that will improve differentiation of stem cells into midbrain dopaminergic (DA) neurons. DA neurons are of central importance in the project, because they are the cells that are impaired in patients with Parkinson’s disease (PD). It appears that midbrain dopaminergic neurons have an enormous energy requirement, which might help explain their vulnerability to degeneration in PD. Current differentiation methods typically produce low yields of DA neurons. The methods also give variable results, and cell populations contain many types of cells. These impediments have hampered the study of disease mechanisms for PD, as well as other uses for the cells, such as drug screening and cell replacement therapy. Our strategy is to develop a novel method to introduce genes into the genome at a specific place, so we can rapidly add genes that might help in the differentiation of DA neurons. The genes we would like to add are called transcription factors, which are proteins involved in differentiation of stem cells into DA neurons. We have placed the genes for three transcription factors into a safe, active position on human chromosome 22 in the cell lines we are studying. These cells, called pluripotent stem cells, have the ability to differentiate into almost any type of cell. We are using embryonic stem cells in our study, as well as induced pluripotent stem cells (iPSC), which are similar, but are derived from adult cells, rather than an embryo. We are using iPSC derived from a PD patient, as well as iPSC from a normal person, for comparison. By forced expression of neuronal transcription factors, we may achieve more efficient and reproducible generation of DA neurons. We want to evaluate the effects of expressing different combinations of three transcription factors called Lmx1a, FoxA2, and Otx2 on DA neuronal differentiation in the context of embryonic stem cells (ESC) as the gold standard, as well as in iPSC derived from a PD patient with a severe mutation in alpha-synuclein, and in iPSC derived from a normal person without PD. Comparative functional assays of the resulting DA neurons will complete the analysis.
To date, this project created a novel technology for modifying the genome. The strategy developed out of the one that we originally proposed, but contains several innovations that make it more powerful and useful. The new methodology, called DICE for Dual Integrase Cassette Exchange, allowed us to generate “master” or recipient cell lines for ESC, normal iPSC, and PD iPSC. These recipient cell lines contain a “landing pad” placed into a newly-identified actively-expressed location on human chromosome 22 called H11 that permits robust expression of genes placed into it. We then generated a series of cell lines by “cassette exchange” at the H11 locus. In cassette exchange, the new genes we want to add take the place of the landing pad we originally put into the cells. Cassette exchange is a good way to introduce various genes into the same place in the chromosomes.
We created cell lines expressing three neuronal transcription factors suspected to be involved in DA neuronal differentiation, in all pair-wise combinations, including lines with expression of all three factors, and negative control lines with no transcription factors added. This collection of modified human pluripotent stem cell lines is being used to study neural differentiation. The modified ESC were used to form embryoid bodies, which are spherical aggregations of stem cells similar to an embryo that are favorable for producing differentiated cells. We found that the embyroid bodies underwent a rapid process of spontaneous differentiation into DA neurons. The differentiation was stimulated in the cells that expressed inserted transcription factors, and some combinations of transcription factors were better than others in bringing about DA neuronal differentiation. We obtained the best differentiation in the lines that expressed the LMX1A and OTX2 transcription factors. In continuing studies, we will analyze functional properties of the differentiated DA neurons, with special emphasis on disease-related features of the cells derived from PD iPSC.