How to study specific cell types

Biomedical research can be approached from many different angles: different physiological and pathophysiological processes, evolutionary factors, and/or focusing on specific cell types. Especially when it comes to diseases, there are usually many different cell types involved, which fulfil (or fail to fulfil) different functions. Hence, to find out how and why a disease develop, it is of uttermost importance to dissect the contribution of specific cell types.

Study specific cell types

In the last episode of this blog series I already discussed fluorescence microscopy, which helps to study specific cells. However, there are substantial limitations as to what and how much information we can obtain from microscopy. For example, microscopy is a great way to resolve spatial information in tissue and expression of specific proteins, however, there is a limit as to how many proteins you can assess, and quantification is a difficult and tedious process. Therefore, it is difficult to enitrely characterize the properties of a specific cell. One way to resolve some of these issues is to use a different but yet very similar technique: flow-activated cell sorting (short: FACS).

Sorting cells

FACS allows us to literally sort specific cells based on expression of proteins (e.g. proteins that are only expressed by one specific cell type), and collect them. We mash up fresh tissue (blood, biopsies, post-mortem tissue), use fluorescently-labelled antibodies — the same antibodies that are used for fluorescence microscopy — and run this suspension through the FACS machine. Each cell will pass through a laser and if the cell was labelled by the fluorescently-labelled antibody, it will emit a signal that is detected by the machine. This signal is proportional to the amount of antibodies that are bound to a cell, meaning proportional to the amount of protein that was expressed by the cell.

The machine is able to apply an electric charge to labelled cells and as soon as the cell passes by an electrically charged surface, it will be redirected. It works a bit like throwing a metallic ball alongside a giant magnet: the ball will be pulled to the magnet which shifts it’s path to a different direction. Just place a tube or any other collection container in the right position and you will be able to collect the redirected cells.

And next?

Now that we have a tube full of cells, what’s next? There are actually many subsequent steps you can take in order to quantify RNA levels, protein levels, assess DNA alterations, etc. Commonly, these cells undergo RNA sequencing to determine which mRNA is expressed. This can give an idea about the state that the cell is in, since it provides an unbiased and extensive snapshot of what genes a cell expresses. Comparing a snapshot of cells from healthy individuals to diseased patients may identify disease-associated genes potentially targetable by drugs.

FACS enabled researchers to easily focus on specific cell types. In combination with other techniques such a RNA sequencing, it allows science to dissect what goes wrong in which cell type during disease.