Stem Cells: Getting to the heart of the matter
Every complex organism begins its existence as a single cell, which through countless rounds of replication gives rise to the many billions of cells making up the adult body. During this process of development, different lineages of cells undergo profound changes in form and function as they shape themselves into diverse organs and tissues according to intricate biological programming. The nature of this programming – just how cells “know” to change into, say, a nerve cell versus a muscle cell in the correct place and time – remains poorly understood, but new research continually adds pieces to the puzzle. Within mammalian embryos, one of the first organs to develop is the heart. A recent publication employs the latest tools of molecular biology – using the mouse embryo as a model system – to examine in a new level of detail the process by which initially amorphous cells shape themselves into this structure and begin to beat.
Gene expression – cell by cell
The stem cells which give rise to the tissues of the heart are termed cardiovascular progenitor (CP) cells, and in mice, they are first identifiable during the process of gastrulation, beginning around day 6 of embryonic development (the embryo consisting of perhaps a few hundred cells in total at this point). From previous lineage tracing, it has been shown that a gene called Mesp1 is critical for directing cells to follow the cardiac development pathway, and is actively expressed in all heart progenitors at this stage. The researchers dissected 6 and 7 day-old mouse embryos to single cells, and by using a fluorescent expression marker attached to Mesp1, were able to use fluorescence-activated cell sorting to isolate the CPs from the rest of the cell population. The researchers then carried out total RNA sequencing from each individual cell (while DNA constitutes the genome or long-term hereditary copy of an organism’s blueprint, RNA provides a readout of which genes are actively switched on within that cell at that time, often called the transcriptome), generating unique expression profiles combining the activity levels of thousands of genes simultaneously, for thousands of cells. This bewilderingly complex multidimensional data can then be “number-crunched” by sophisticated algorithms to detect commonalities and patterns, allowing subpopulations of cells within the noise to be teased apart.
This analysis identified five distinct “destination cell types” (DCTs), expressing sets of genes associated with specific end-states such as muscle cells and the epicardium, as well as many genes previously unrecognised as involved in cardiovascular development. This shows that progenitor cells segregate themselves into distinct lineages within a short period of time; in embryos as early as 6 days old, cells form heterogeneous subpopulations as measured by their gene activity profiles, already committed to different lineages and regions of the heart, well before such structures have actually taken shape. Different populations also appear at different timepoints and migrate through specific areas of the developing embryo.
How to mend a broken heart
Around 9 in every 1000 children born in the UK suffer from some form of congenital heart defects, many of which are undoubtedly consequences of errors in these early developmental steps, and can result in lifelong health difficulties. Understanding the details of heart development is a necessary step toward early detection of such problems, and possible therapies (while this particular study was done in mice and not humans, the two species share broad similarities in the way their hearts develop, other than the obvious difference in size). The ability to read transcriptomes on a cell-by-cell basis, and thus to assemble lists of genes which characterise tissue-specific developmental pathways, brings us closer to decoding the programming which drives development. When combined with technologies such as CRISPR which allow both editing of genes and manipulation of their activity, there is clear potential for therapies based both on reprogramming cells within patients to correct defects, and for lab-grown stem cells which could be engineered to produce synthetic tissues for transplant.
By Robin Floyd