Particle delivery using bead-loading gives insight into heterogeneity of the cellular cytoplasm*

Microrheology is an important technique for probing the properties of the cytoplasm, but requires particle delivery inside cells. Here we report a particle delivery method based on a method of protein delivery called bead-loading, requiring no specialized instruments. Bead loading is able to deliver 100nm fluorescent particles that are widely dispersed into the cellular cytoplasm. Particle tracking reveals that the fluorescent particles probe two very different regions of the cytoplasm. One set of beads are significantly confined, and treatment by drugs indicate that the confinement is due to the actin cytoskeleton. The second set of beads seemingly display free diffusion, but ATP depletion experiments show that the motion has an actively driven component. Most strikingly, the main effect of most drug treatments is through changing the relative distribution of these two populations. Our analysis suggests that the specific mode of particle delivery may be strongly affecting the measured properties of cells, underlining the importance of intracellular heterogeneity. The general applicability of our particle delivery technique and the distribution of intracellular movement were confirmed in three different cell lines.