Colocalization

In fluorescence microscopy, colocalization refers to different data analysis methods to characterize the degree of overlap between two different fluorescent labels, each having a separate emission wavelength, to see if two different cellular "targets" are located in the same area or very near to one another.

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This technique is an important one to many cellular physiologists, and it is often used to support data concerning the location of cellular components or to (interestingly) test the actions of novel fluorescent labels.

The purpose of a colocalization coefficient is to globally characterize the degree of overlap between two images, usually two channels in a multidimensional microscopy image recorded at different emission wavelenghts. On the other hand, a colocalization map parametrizes the colocalization locally. In a map, a single colocalization value is calculated per voxel resulting in a distribution with the same number of dimensions than the original images (typically 3D).

Different coefficients and maps are widely used in literature and lend themselves in principle for comparison of results obtained in different studies. (Some examples are Pearson's or Manders' coefficients). Still, the specific properties of the coefficients, especially those related to the image background levels and resolution, make cross-study comparison problematic.

Techniques

• Förster resonance energy transfer (FRET): 10 nm proximity
• (Light microscopy: only 250 nm resolution; no certainty of effective interaction)

Example

It was shown that some impermeable fluorescent zinc dyes can detectably label the cytosol and nuclei of apoptizing and necrotizing cells among each of four different tissue types examined. Namely: the cerebral cortex, the hippocampus, the cerebellum, and it was also demonstrated that colocalized detection of zinc increase and the well accepted cell death indicator propidium iodide also occurred in kidney cells. Using the principles of fluorescent colocalization. Coincident detection of zinc accumulation and propidium iodide (a traditional cell death indicator) uptake in multiple cell types could be demonstrated. (& Li, The Journal of Neuroscience Methods, 2006)

Scientific applications

• CoLocalizer Pro [1]