Circulating Microparticles

Composition of Microparticles

MPs are composed of a phospholipid bilayer, which carries several bioactive molecules, including transmembrane proteins, receptors, counterreceptors, and adhesion molecules, as well as cytoplasmic products such as nucleic acids, chemokines, cytokines, enzymes, growth factors, and signaling proteins. The membrane shows externalization of phosphatidylserine (PS) that provides a platform for the assembly of coagulation factors. The precise content of lipids and proteins depends on their cellular origin and the type of stimulus involved in their formation.

Classification of Microparticles

Living cells are capable of releasing different types of membrane vesicles that can be classified based on their size, sedimentation, and secretion mechanisms into exosomes, MPs, and apoptotic vesicles. The term MP encompasses a heterogeneous group of poorly characterized vesicles ranging in size from 100 to 1000 nm that are released by budding of the plasma membrane (ectocytosis). Based on their size, these MPs have been divided into microvesicles (100–1000 nm), ectosomes (50–200 nm), membrane particles (50–80 nm), and exosome-like vesicles (20–50 nm) ( Fig. 162.1 ; Table 162.1 ). These large vesicles should be distinguished from exosomes (<100 nm) that are membrane vesicles that are stored intracellularly in multivesicular compartments and are secreted following the fusion of internal compartments with the cell plasma membrane ( Fig. 162.1 ). Dying or apoptotic cells are also able to secrete membrane vesicles with different features to those from living cells.

Mechanisms of Microparticle Release

It is thought that MPs form when the asymmetric distribution of plasma membrane lipids is lost. As highlighted by Hugel et al., under resting conditions, phospholipids are asymmetrically distributed in the plasma membrane, with PS almost exclusively located in the inner membrane layer, and phosphatidylcholine and sphingomyelin located on the external one. Increased concentrations of cytosolic calcium, like those seen in response to cellular activation, may lead to lipid redistribution and surface exposure of PS, with membrane blebbing and subsequent MP shedding. Once exposed, PS promotes blood coagulation by serving as a scaffold for the assembly of the prothrombinase and thrombinase complexes. Of interest, some investigators have shown that platelet-derived MP surfaces have 50- to 100-fold higher procoagulant activity when compared with the normal platelet surface. The physiological importance of MP shedding is exemplified in a rare disorder known as Scott syndrome, which is characterized by moderate to severe bleeding episodes, usually provoked hemorrhages due to an enzymatic deficit that leads to decreased surface exposure of PS and decreased MP formation.

Circulating vesicles originating from apoptotic cells are thought to be produced by less-controlled mechanisms, possibly secondary to loss of membrane integrity or mechanical destruction. Although these apoptotic vesicles display PS on their surface, in contrast to the MPs, they show no or weak procoagulant activity. Surface exposure of PS also serves as a signal for the clearance of senescent cells by the reticuloendothelial system.

Role of Microparticles

MPs play an important role in homeostasis and intercellular communication through several mechanisms including

• 1.

  Transfer of surface receptors,

• 2.

  Transfer of mRNA,

• 3.

  Release of proteins or active lipids, and

• 4.

  Induction of adaptive immune response ( Table 162.2 ).

  Table 162.2

  Biological Function of Microparticles

  Biological Effect	Proposed Mechanisms
  Procoagulant	Surface exposure of phosphatidylserine
  	Expression of Tissue Factor (mostly monocyte-derived MPs)
  	Transfer of GPIIb/IIIa
  	Presence of membrane functional effectors (integrins, P-selectin, VWF)
  Anticoagulant (in vitro)	Proteolytic inactivation of FVa by activated protein C
  Fibrinolytic	Expression of u-PA and u-PAR
  Proteolytic	Expression of matrix metalloproteinases
  Vascular	Membrane expression of thromboxane A2
  	Impairment of endothelium-dependent relaxation through eNOS downregulation
  Proinflammatory	Release of proinflammatory endothelial cytokines (IL-6 or MCP-1)
  	Induction of expression of ICAM-1, VCAM-1 and e-selectin
  	Serve as a substrate for production of lysophosphatidic acid
  Antiinflammatory	Secretion of TGFβ, potent inhibitor of macrophage activation
  	Expression of Annexin A1, endogenous antiinflammatory protein
  Immunity	Expression of major histocompatibility complex molecules
  	Display of autoantigens, such as RNA and DNA, which may act as potent autoadjuvants, inducing B cell tolerance
  	Expression of Fas-L leading to apoptosis, leading to immune evasion

Microparticles in Human Disease

Abnormal levels of circulating MPs have been associated with a variety of disorders ( Table 162.3 ).