Henrik J. Johansson

Cells release nano-sized membrane vesicles that are involved in intercellular communication by transferring biological information between cells. It is generally accepted that cells release at least three types of extracellular vesicles (EVs): apoptotic bodies, microvesicles and exosomes. While a wide range of putative biological functions have been attributed to exosomes, they are assumed to represent a homogenous population of EVs. We hypothesized the existence of subpopulations of exosomes with defined molecular compositions and biological properties. Density gradient centrifugation of isolated exosomes revealed the presence of two distinct subpopulations, differing in biophysical properties and their proteomic and RNA repertoires. Interestingly, the subpopulations mediated differential effects on the gene expression programmes in recipient cells. In conclusion, we demonstrate that cells release distinct exosome subpopulations with unique compositions that elicit differential effects on recipient cells. Further dissection of exosome heterogeneity will advance our understanding of exosomal biology in health and disease and accelerate the development of exosome-based diagnostics and therapeutics.

Extracellular vesicles (EVs) are natural nanoparticles that mediate intercellular transfer of RNA and proteins and are of great medical interest; serving as novel biomarkers and potential therapeutic agents. However, there is little consensus on the most appropriate method to isolate high-yield and high-purity EVs from various biological fluids. Here, we describe a systematic comparison between two protocols for EV purification: ultrafiltration with subsequent liquid chromatography (UF-LC) and differential ultracentrifugation (UC). A significantly higher EV yield resulted from UF-LC as compared to UC, without affecting vesicle protein composition. Importantly, we provide novel evidence that, in contrast to UC-purified EVs, the biophysical properties of UF-LC-purified EVs are preserved, leading to a different in vivo biodistribution, with less accumulation in lungs. Finally, we show that UF-LC is scalable and adaptable for EV isolation from complex media types such as stem cell media, which is of huge significance for future clinical applications involving EVs. FROM THE CLINICAL EDITOR: Recent evidence suggests extracellular vesicles (EVs) as another route of cellular communication. These EVs may be utilized for future therapeutics. In this article, the authors compared ultrafiltration with size-exclusion liquid chromatography (UF-LC) and ultra-centrifugation (UC) for EV recovery.

Mesenchymal stem cells (MSC) are known to facilitate healing of ischemic tissue related diseases through proangiogenic secretory proteins. Recent studies further show that MSC derived exosomes function as paracrine effectors of angiogenesis, however, the identity of which components of the exosome proteome responsible for this effect remains elusive. To address this we used high-resolution isoelectric focusing coupled liquid chromatography tandem mass spectrometry, an unbiased high throughput proteomics approach to comprehensively characterize the proteinaceous contents of MSCs and MSC derived exosomes. We probed the proteome of MSCs and MSC derived exosomes from cells cultured under expansion conditions and under ischemic tissue simulated conditions to elucidate key angiogenic paracrine effectors present and potentially differentially expressed in these conditions. In total, 6,342 proteins were identified in MSCs and 1,927 proteins in MSC derived exosomes, representing to our knowledge the first time these proteomes have been probed comprehensively. Multilayered analyses identified several putative paracrine effectors of angiogenesis present in MSC exosomes and increased in expression in MSCs exposed to ischemic tissue-simulated conditions; these include platelet derived growth factor, epidermal growth factor, fibroblast growth factor, and most notably nuclear factor-kappaB (NFkB) signaling pathway proteins. NFkB signaling was identified as a key mediator of MSC exosome induced angiogenesis in endothelial cells by functional in vitro validation using a specific inhibitor. Collectively, the results of our proteomic analysis show that MSC derived exosomes contain a robust profile of angiogenic paracrine effectors, which have potential for the treatment of ischemic tissue-related diseases.

Cell-penetrating peptides (CPPs) are peptides able to promote uptake of various cargos, including proteins and plasmids. Advances in recent years imply the uptake to be endocytic, where the current hurdle for efficient intracellular delivery is material being retained in the endosomes. In this study we wanted to compare the ability of various established CPPs to deliver siRNA and induce gene silencing of luciferase, with a novel designed penetratin analog having endosomolytic properties, using a noncovalent strategy. In principal, the penetratin analog EB1 will, upon protonation in the early-late endosomes, be able to form an amphipathic alpha helix resulting in permeabilization of the endosomal membrane. We demonstrate that even though all CPPs evaluated in this study can form complexes with siRNA, there is not a direct relationship between the complex formation ability and delivery efficacy. More important, although all CPPs significantly promote siRNA uptake, in some cases no gene silencing effect can be observed unless endosomal escape is induced. We find the designed endosomolytic peptide EB1 to be far more effective both in forming complexes and transporting biologically active siRNA than its parent peptide penetratin. We believe that developing CPPs with increased endosomolytical properties is a necessary step toward achieving biological effects at low concentrations for future in vivo applications.