Quantification of Exosomes

Abstract

Exosomes are released by cells as self-contained vesicles with an intact lipid bilayer that encapsulates a small portion of the parent cell. Exosomes have been studied widely as information-rich sources of potential biomarkers that can reveal cellular physiology. We suggest that quantification is essential to understand basic biological relationships between exosomes and their parent cells and hence the underlying interpretation of exosome signals. The number of methods for quantifying exosomes has expanded as interest in exosomes has increased. However, a consensus on proper quantification has not developed, making each study difficult to compare to another. Overcoming this ad hoc approach will require widely available standards that have been adequately characterized, and multiple comparative studies across platforms. We outline the current status of these technical approaches and our view of how they can become more coherent. J. Cell. Physiol. 232: 1587-1590, 2017. © 2016 Wiley Periodicals, Inc.

Figure 1

Exosomes are formed via the endosomal pathway. Endocytic vesicles result from invagination of the plasma membrane and can fuse to form early endosomes. During maturation from early to late endosomes, also called multivesicular bodies (MVBs), invaginations in the endosomal membrane form intraluminal vesicles (ILVs). The late endosome/MVB then either fuses with a lysosome to degrade its contents, or fuses with the plasma membrane and releases the ILVs into the extracellular space as exosomes.

Figure 2

Schematic representation of the Nanosight NTA system. A laser beam refracts at the glass/liquid interface illuminating a plane within the flow chamber. Particles in the solution scatter light that is detected by a microscope objective and video camera focused on the illuminated plane. Due to Brownian motion the position of each particle changes between frames. Small particles will move further than large particles, enabling the distance moved to be used to calculate the particle size. A size distribution can be developed by studying multiple particle tracks.

Figure 3

Schematic representation of the TRPS instrument. A voltage is applied across a stretchable membrane containing a pore (A). The dynamic range of the instrument can be tuned to match the particle properties by stretching the membrane to change the pore diameter (B). The current across the membrane is monitored and will drop as particles pass through the pore, momentarily increasing its electrical resistance (C). The number of events can be used to calculate the concentration and the magnitude of the event used to develop a size distribution.