Circulating microparticles: square the circle

Abstract

Background: The present review summarizes current knowledge about microparticles (MPs) and provides a systematic overview of last 20 years of research on circulating MPs, with particular focus on their clinical relevance.

Results: MPs are a heterogeneous population of cell-derived vesicles, with sizes ranging between 50 and 1000 nm. MPs are capable of transferring peptides, proteins, lipid components, microRNA, mRNA, and DNA from one cell to another without direct cell-to-cell contact. Growing evidence suggests that MPs present in peripheral blood and body fluids contribute to the development and progression of cancer, and are of pathophysiological relevance for autoimmune, inflammatory, infectious, cardiovascular, hematological, and other diseases. MPs have large diagnostic potential as biomarkers; however, due to current technological limitations in purification of MPs and an absence of standardized methods of MP detection, challenges remain in validating the potential of MPs as a non-invasive and early diagnostic platform.

Conclusions: Improvements in the effective deciphering of MP molecular signatures will be critical not only for diagnostics, but also for the evaluation of treatment regimens and predicting disease outcomes.

Figure 1

Distribution of Dragon Green-conjugated beads in sizes of 190 nm, 510 nm, and 730 nm (images acquired with a SORP Aria 2 cytometer, Flow and Imaging Cytometry Resource, PCMM, Boston Children’s Hospital). This figure was included in an advanced abstract as part of the Proceedings of the International Workshop on Applied Cytometry (2012).

Figure 2

MP images (erythrocyte-derived MPs) acquired with an Imagestream 100 (40x objective) (Amnis Inc, Seattle, USA). A dotblot showing a mixture of erythrocytes and erythrocyte-derived microparticles (X-axis-brightfield area; Y-axis-brightfield aspect ratio of intensity). a.) Multiple erythrocytes (region R3 on dotblot); b.) Single erythrocytes (region R2 on dotblot); c.) Microparticles (brightfield) (region R1 on dotblot); d.) Microparticles stained with calcein AM (images from calcein AM-channel are particles taken from R1-region).

Figure 3

Basic scheme of MP formation via reverse budding. A. Activated cells release MPs in response to Ca⁺⁺ agonists. Increased concentration of Ca⁺⁺ alter the asymmetric PS distribution of the plasma membrane, activate kinases, inhibit phosphatases and activate calpain, which leads to reorganization of cytoskeleton and increased MPs production. B. MP formation during the early stages of apoptosis is associated with GTP-bound Rho proteins, which activate the ROCK-I kinase. This kinase is involved in cortical myosin-II contraction, detachment of the plasma membrane from the cytoskeleton, and release of MPs that have hijacked cytoplasmic components, nucleic acids, and membrane antigens.

Figure 4

An epifluorescence microscopy image shows that hCMEC/D3 cells have internalized small MPs (arrowheads), which had been purified from human glioma cells treated with TRAIL (100 units/ml) and stained with PKH-67 (Sigma, USA) before addition to the hCMEC/D3 cells. MPs that are attached to the cell surface are out of focus (representative photo from Z-stack collection). Objective Plan Apo x60/1.4. Bar 5 μm.

Figure 5

Potential mechanisms of MP action.