How do we track cellular heterogeneity?
Cellular heterogeneity essentially observes all dimensions of a single cell. Heterogeneity tracking helps us to measure the functionality of each cell. An example of its common use, is to track cancer patients’ cell or tumour growth or decrease over time.
Investigating various types of cells and their complex physiological functions has led us to some important insights. In order to decipher the mechanisms and functions of heterogeneous cell types, monitoring them is key.
By monitoring cells in real time (known as the cell tracking method), we are tracking the cell’s progress, in order to be able to treat it successfully.
How has cell imaging changed over time?
The first ‘in vivo imaging’ (non-invasive imaging) was reported way back in 18391. Skip forward over 180 years, and imaging such as microscopy has evolved vastly and it has never been more high-resolution.
Both confocal and multi-photon microscopy are widely used for imaging and cell tracking. The evolution and development of these forms of imaging have meant that light is no longer limiting our results. This enables us to focus on important areas of cells and is known as diffraction.
Using cell tracking in cancer treatment
Cell tracking and live imaging gives us the possibility to obtain real-time dynamic data to capture the evolution of cell-to-cell differences. This technique then enables us to see the living cell’s cellular process over time.
The imaging of live cells and tissues is now a common technique used in research. Measurements of viable cells over time ensures that we can provide a much more complete analysis of the biological process of cells2-5.
During cell tracking cells are:
- Kept in focus
- Continually measured for several days
- Maintained at 37°C and 5% CO26
- Drug response
- Drug metabolism
- Mesenchymal mode of single-cell migration
- Collective migration8,10,11
- New bio-markers of disease
- Drug targets
- Drug mechanism of activity at the cellular to the molecular level12
- Endogenous reporters - representing fluorescent proteins that are produced by the cells that have to be followed
- Exogenous probes - interact with the targeted cells14
Where cancerous cells are present, we can monitor behaviour or cell population and reconstruct trajectories, in order to treat it in the best way.
In this instance, cell tracking has a key role in investigating the potential and invasiveness of cancer cells. Once these are identified, we then have the opportunity to control the disease. It also allows us to monitor whether environmental conditions affect the cell migration.
The tracking of cancer cells can also give us more accurate answers about molecular mechanisms of:
The benefits of monitoring single cell behaviour
Monitoring the behaviour of a single cancer cell allows us to follow and analyse individual cells. This gives us the possibility to determine the sub-population of the cells with different migratory or proliferative potential. This, in turn allows us to determine different responses to any given treatment.
Bolgioni et al. developed the live cell imaging protocol for monitoring the mitotically arrested cells after treatment with an anti-mitotic drug called Paclitaxel. Their method now allows us to decide whether cells die or slip back into interphase.
It is important to detect if the cells evade mitotic death by slipping into interphase, because interphase cells can execute apoptosis, undergo cell cycle arrest, and even re-enter the cell cycle. Since mitotic cells are tetraploid, if they slip into interphase, they can drive tumour relapse because of gained chromosome instability.
With live cell imaging, we can detect factors and mechanisms, or even control the cell fate post-treatment, which is critical for optimising the therapies available for the patient.
Investigating cell changes with cell tracking
By tracking cells, we are able to monitor the changes in morphological features, which happen during different processes or conditions.
During metastasis, cancer cells experience morphological changes that are characteristic for different types of migration patterns7. These include:
By using both two-dimensional (2D) and three-dimensional (3D) spheroids and organoids, we are able to investigate changes that are associated with the cell’s environment.
This tracking of 2D and 3D cultures can give us important information about the mechanism of drug activity in order to measure its success or failures8.
The importance of labelling in cell tracking
When tracking live imagine, cells can be targeted by direct labelling, or transfection with a conjugated (the transfer of DNA between cells, usually bacteria, by cell-to-cell contact) marker gene.
High-resolution imaging and genetically encoded fluorescent probes allow us to monitor and understand the interactions of cell metabolism and signalling. It also enables us to visualise single molecules in living cells9.
As well as this, we can identify:
The combination of the molecule-specific contrast and live-cell imaging technique makes fluorescent microscopy the most popular imaging approach in the field of cell biology research13.
Fluorophores that can be used for fluorescence-based microscopy are divided into two categories:
What the two fluorophores have in common, is that they should be non-toxic, photostable and sufficiently bright.
Long-term and short-term cell tracking
Experimental cells can also be labelled with dyes that provide long-term or short-term cell tracking.
Long-term cell tracking allows for prolonged monitoring of the same cell and its behaviour. The main limitation of long-term tracking is that fluorescent proteins or reporters often fail to give a strong signal.
Long-term cell tracking also often has non-efficient detection, which can be overcome using other techniques, such as transfection or transduction techniques and ex vivo manipulations14,15.
For short-term cell tracking, cytoplasmic and nuclear dyes are usually used. These dyes then penetrate cells, metabolising to the products that bind components from the cytoplasm16,17.
The downfalls of short-term dyes mean they usually have high cytotoxicity and their fluorescence also decays rapidly18.
Labelled cell monitoring can sometimes limit the length of time you can conduct cell tracking19,20. With these findings, there was an increased need to develop a label-free quantitative method of cell tracking21.
This is what inspired Digital Holographic Microscopy (DHM) - a method which represents the non-phytotoxic technique, which allows long-term imaging22. Images obtained as a result of DHM have pixel intensities proportional to the absolute phase shifts of the cell sample23,24,25,26.
Processing your image analysis results
As we know, image analysis is a multi-step process that begins with low-level pre-processing. A large number of images can be summarised into a few numerical descriptors.
Although cell tracking is an extremely powerful method for the investigation of cell behaviour on different levels, it must be combined with other methods and approaches to characterise and investigate the process thoroughly and successfully.