Public Summary of Scientific Progress
The discovery of induced pluripotent stem (iPS) cells has generated much excitement, because, like embryonic stem (ES) cells, they have the potential to differentiate into every cell type in the body, yet their derivation is straightforward technically and does not require the use of human embryos. There are three related challenges that must be met before iPS cells can be applied to regenerative therapy: 1) Identify methods that improve the efficiency of iPS cell generation and the quality of derived iPS cells, 2) determine whether iPS cells can function identically to ES cells, which remain the gold standard, and 3) identify methods that can control the differentiation of iPS cells into the cell types desired for regenerative therapy. Current methods for iPS cell derivation are highly inefficient: only a small percentage of any human adult cell population can be successfully de-differentiated to an ES-like state. Directing the differentiation of iPS or ES cells is also highly inefficient: in every directed-differentiation method, only a small percentage of cells differentiate into the desired cell type. Therefore, understanding the behavior of individual cells within a mixed population is the level of detail required to understand the processes that will ultimately improve the methodologies utilized for their application to regenerative medicine.
A new proteomic technology called mass cytometry, developed by Dr. Scott Tanner at the University of Toronto, was recently applied in the Nolan lab to measure over 40 (eventually 100) parameters simultaneously at the single cell level. Mass cytometry uses rare earth metal-tagged antibodies to label cells both on the surface and inside the cell, then vaporizes those cells at 13,000 degrees Fahrenheit and counts how many of each of the different tags a cell had at the time of its demise. During this year a critical aspect of this study was the development of new software tools to organize and interpret the high-dimensional data sets generated by mass cytometry. An effective and informative means for displaying this multi-dimensional data was the application of Spanning-tree Progression Analysis of Density normalized Event (SPADE analysis), which creates an intuitive 2D representation of multi-dimensional data. SPADE recovers the underlying branching and continual structure of high-dimensional cytometry data by recognizing the subtle differences between cellular phenotypes in n-dimensional space (where n is the number of markers acquired) and tracing them in a straightforward manner. The entire n-dimensional hierarchy is visualized in two dimensions as SPADE trees. Furthermore, in this past year we developed “Time-SPADE” analysis in which trees are built sequentially by time point, allowing for time course analysis of the changing cell populations that occur during the highly dynamic processes of iPS cell derivation and ES/iPS cell differentiation.
In our continuing collaboration with Dr. Marius Wernig at Stanford, a recognized leader in the biology and methodologies of iPS cell reprogramming, we applied mass cytometry and SPADE analysis to study the behavior of single cells during reprogramming. Using specific metal-tagged antibody panels that we developed, we were able to trace the molecular path of de-differentiation to the ES-like state, and we showed that the co-expression of the four Yamanaka factors at both the mRNA and protein level is critical for successful reprogramming. To study differentiation in addition to de-differentiation, we applied mass cytometry and Time/SPADE methodologies to trace the behaviors of single cells within an entire sample as they undergo differentiation into defined cell types. For these experiments we were able to trace ES cells as they underwent changes into mesoderm, ectoderm and endoderm, the three cell lineages that give rise to all of our organs. These experiments set the stage for using protocols to induce differentiation into a specific cell type such as neuronal, hematopoietic, cardiac, and pancreatic as just some of the possibilities. Studying these processes at the single cell level with unprecedented detail will facilitate our optimization of iPS cell derivation and differentiation for human regenerative therapy.