Platelet microparticles are not created equal

In this issue of Blood, Boudreau et al report that activated platelets release respiratory-competent mitochondria either as free organelles or packed in platelet microparticles.¹ 

Secreted phospholipase A2 IIA hydrolyzes the membrane of these free mitochondria to generate proinflammatory mediators. Results from this study are important in 3 key aspects. First, the authors show that activated platelets produced 3 types of particles: free mitochondria, microparticles containing mitochondria, and mitochondrion-free microparticles. They further show that both free mitochondria and mitochondria packed in microparticles remained active in the respiratory reaction, but the latter required the plasma membrane to be permeabilized by a detergent. The presence and biological activities of cellular microparticles have been recognized for decades, but morphological, structural, and biological characteristics of these subcellular vesicles remain poorly defined and, in some incidences, controversial. Microparticles are currently defined as membrane-bound cellular fragments produced from the plasma membrane of cells that are activated or undergo apoptosis.²  They are primarily composed of the plasma membrane along with a limited amount of cytoplasm, varying from 0.1 to 1 μm in diameter. A subclass of microparticles is the exosome, which is smaller (50-100 nm) and is derived from granule-containing vesicular bodies. Free mitochondria structurally fit in the definition of exosomes. Microparticles have long been considered as the source and carrier of active molecules that are inflammatory, proproliferation, and proangiogenesis. They also have a unique ability to deliver these active molecules to specific targets through receptors and counterreceptors selectively expressed on them. The current study provides direct evidence that platelet microparticles are heterogeneous not only in size and, perhaps, targeting, but also in biological activities depending on the cargo they carry. Free mitochondria reported in this study can be particularly active in inducing oxidative reactions and, as discussed in the next section, catalytically releasing inflammatory lipid mediators and mitochondrial DNA. Similarly, there may also be platelet microparticles that are enriched in α-granules and contain adhesive molecules and cytokines that are proangiogenesis and proinflammation. The finding of this study, therefore, adds a level of complexity in classifying (platelet) microparticles in subtypes that have different activities and/or action targets. The study also raises questions such as whether (1) platelets randomly release free mitochondria and mitochondria packed in microparticles on activation or selectively one over the other in response to different agonists, and (2) mitochondria packed in microparticles are as active as free mitochondria in promoting inflammation.

Second, the authors demonstrate for the first time that free mitochondria released from platelets were the substrate of secreted phospholipase A2 group IIA, which is produced and secreted by cells constitutionally or induced during an acute phase reaction.³  This phospholipase hydrolyzes the sn-2 acyl bond of glycerophospholipids to release free fatty acids and lysophospholipids. The membrane hydrolysis by the enzyme also compromises the structural integrity of mitochondria to release mitochondrial DNA. Consistent with these in vitro results, mitochondria intravenously injected into a mouse rapidly associated with and were internalized by neutrophils, resulting in neutrophil activation and interaction with the endothelium. This inflammatory activity was significantly enhanced in the presence of exogenous secreted phospholipase A2 group IIA. The finding has filled a critical knowledge gap in a well-documented, but poorly understood, selective inflammatory activity of secreted phospholipase A2 group IIA and its targets. This phospholipase is known to act poorly on the plasma membrane of eukaryotic cells, including platelets, but is highly active toward the membrane of gram-positive bacteria.³  The mitochondrion shares its ancestral root with bacteria because the mitochondrial genome structure is closely related to that of the alphaproteobacterium Rickettsia prowazekii, which multiplies only inside eukaryotic cells.^4,5  It is also interesting that platelets are enriched in secreted phospholipase A2 group IIA,⁶  so that the force driving activated platelets to produce free mitochondria could also promote them to release secreted phospholipase A2 group IIA. The enzyme and the substrate, once compartmentalized inside platelets, can now catalytically interact with each other, likely in a localized environment. Because of its preference for negatively charged phosphatidylethanolamine and phosphatidylserine, one may also ask whether secreted phospholipase A2 group IIA hydrolyzes the membrane of activated platelets that express these negatively charged phospholipids.

Finally, the authors made several attempts to model these in vitro results in human diseases. For example, a significant level of free mitochondria was detected in bronchoalveolar lavage fluids from mice subjected to experimental transfusion-related acute lung injury. Consistent with results from the mouse experiments, a high level of free mitochondria was also detected in platelet concentrates that have been associated with adverse transfusion reactions in human recipients. These correlative studies need to be further validated in large patient cohorts but can further define pathological conditions where mitochondria, free or packed in microparticles, induce and propagate inflammation. Together, results from this study demonstrate that platelet microparticles are not created equal in their biological activities and identify a new target for secreted phospholipase A2 group IIA to propagate inflammation (see figure).