Apoptotic exosome-like vesicles transfer specific and functional mRNAs to endothelial cells by phosphatidylserine-dependent macropinocytosis.

Apoptosis of endothelial cells prompts the release of apoptotic exosome-like vesicles (ApoExos), subtype extracellular vesicles secreted by apoptotic cells after caspase-3 activation. ApoExos are different from both apoptotic bodies and classical exosomes in their protein and nucleic acid contents and functions. In contrast to classical apoptotic bodies, ApoExos induce immunogenic responses that can be maladaptive when not tightly regulated. In the present study, we elucidated the mechanisms by which ApoExos are internalized by endothelial cells, which leads to shared specific and functional mRNAs of importance to endothelial function. Using flow cytometry and confocal microscopy, we revealed that ApoExos were actively internalized by endothelial cells. SiRNA-induced inhibition of classical endocytosis pathways with pharmacological inhibitors showed that ApoExos were internalized via phosphatidylserine-dependent macropinocytosis independently of classical endocytosis pathways. An electron microscopy analysis revealed that ApoExos increased the macropinocytosis rate in endothelial cells, setting in motion a positive feedback loop that increased the amount of internalized ApoExos. Deep sequencing of total RNA revealed that ApoExos possessed a unique protein-coding RNA profile, with PCSK5 being the most abundant mRNA. Internalization of ApoExos by cells led to the transfer of this RNA content from the ApoExos to cells. Specifically, PCSK5 mRNA was transferred to cells that had taken up ApoExos, and these cells subsequently expressed PCSK5. Collectively, our findings suggest that macropinocytosis is an effective entry pathway for the delivery of RNAs carried by ApoExos and that these RNAs are functionally expressed by the endothelial cells that internalize them. As ApoExos express a specific mRNA signature, these results suggest new avenues to understand how ApoExos produced at sites of vascular injury impact vascular function.

Autolysosomes and caspase-3 control the biogenesis and release of immunogenic apoptotic exosomes.

Apoptotic exosome-like vesicles (ApoExos) are a novel type of extracellular vesicle that contribute to the propagation of inflammation at sites of vascular injury when released by dying cells. ApoExos are characterized by the presence of the C-terminal perlecan LG3 fragment and 20S proteasome, and they are produced downstream of caspase-3 activation. In the present study, we assessed the relative roles of autophagy and caspase-3-mediated pathways in controlling the biogenesis and secretion of immunogenic ApoExos. Using electron microscopy and confocal immunofluorescence microscopy in serum-starved endothelial cells, we identified large autolysosomes resulting from the fusion of lysosomes, multivesicular bodies, and autophagosomes as a site of ApoExo biogenesis. Inhibition of autophagy with ATG7 siRNA or biochemical inhibitors (wortmannin and bafilomycin) coupled with proteomics analysis showed that autophagy regulated the processing of perlecan into LG3 and its loading onto ApoExos but was dispensable for ApoExo biogenesis. Caspase-3 activation was identified using caspase-3-deficient endothelial cells or caspase inhibitors as a pivotal regulator of fusion events between autolysosomes and the cell membrane, therefore regulating the release of immunogenic ApoExos. Collectively, these findings identified autolysosomes as a site of ApoExo biogenesis and caspase-3 as a crucial regulator of autolysosome cell membrane interactions involved in the secretion of immunogenic ApoExos.

Caspase-3-dependent peritubular capillary dysfunction is pivotal for the transition from acute to chronic kidney disease after acute ischemia-reperfusion injury.

Ischemia-reperfusion injury (IRI) is a major risk factor for chronic renal failure. Caspase-3, an effector responsible for apoptosis execution, is activated within the peritubular capillary (PTC) in the early stage of IRI-induced acute kidney injury (AKI). Recently, we showed that caspase-3-dependent microvascular rarefaction plays a key role in fibrosis development after mild renal IRI. Here, we further characterized the role of caspase-3 in microvascular dysfunction and progressive renal failure in both mild and severe AKI, by performing unilateral renal artery clamping for 30/60 min with contralateral nephrectomy in wild-type (C57BL/6) or caspase-3-/- mice. In both forms of AKI, caspase-3-/- mice showed better long-term outcomes despite worse initial tubular injury. After 3 wk, they showed reduced PTC injury, decreased PTC collagen deposition and α-smooth muscle actin expression, and lower tubular injury scores compared with wild-type animals. Caspase-3-/- mice with severe IRI also showed better preservation of long-term renal function. Intravital imaging and microcomputed tomography revealed preserved PTC permeability and better terminal capillary density in caspase-3-/- mice. Collectively, these results demonstrate the pivotal importance of caspase-3 in regulating long-term renal function after IRI and establish the predominant role of PTC dysfunction as a major contributor to progressive renal dysfunction.NEW & NOTEWORTHY Our findings demonstrate the pivotal importance of caspase-3 in regulating renal microvascular dysfunction, fibrogenesis, and long-term renal impairment after acute kidney injury induced by ischemia-reperfusion injury. Furthermore, this study establishes the predominant role of peritubular capillary integrity as a major contributor to progressive renal dysfunction after ischemia-reperfusion injury.

Autophagy, tissue repair, and fibrosis: a delicate balance.

Tissue repair and fibrosis, an abnormal form of repair, occur in most human organs in response to injury or inflammation. Fibroblasts play a major role in the normal repair process by differentiating into myofibroblasts that synthesize extracellular matrix (ECM) components and favor tissue remodeling to reestablish normal function and integrity. However, their persistent accumulation at the site of injury is a hallmark of fibrosis. Autophagy is a catabolic process that occurs in eukaryotic cells as a stress response to allow cell survival and maintenance of cellular homeostasis by degrading and recycling intracellular components. Recent advances identify autophagy as an important regulator of myofibroblast differentiation, tissue remodeling, and fibrogenesis. In this mini-review, we provide an overview of the interactions between autophagy, ECM, and fibrosis, and emphasize the molecular mechanisms involved in myofibroblast differentiation. We also describe the emerging concept of secretory autophagy as a new avenue for intercellular communication at the site of tissue injury and repair.

Apoptotic exosome-like vesicles regulate endothelial gene expression, inflammatory signaling, and function through the NF-κB signaling pathway.

Persistent endothelial injury promotes maladaptive responses by favoring the release of factors leading to perturbation in vascular homeostasis and tissue architecture. Caspase-3 dependent death of microvascular endothelial cells leads to the release of unique apoptotic exosome-like vesicles (ApoExo). Here, we evaluate the impact of ApoExo on endothelial gene expression and function in the context of a pro-apoptotic stimulus. Endothelial cells exposed to ApoExo differentially express genes involved in cell death, inflammation, differentiation, and cell movement. Endothelial cells exposed to ApoExo showed inhibition of apoptosis, improved wound closure along with reduced angiogenic activity and reduced expression of endothelial markers consistent with the first phase of endothelial-to-mesenchymal transition (endoMT). ApoExo interaction with endothelial cells also led to NF-κB activation. NF-κB is known to participate in endothelial dysfunction in numerous diseases. Silencing NF-κB reversed the anti-apoptotic effect and the pro-migratory state and prevented angiostatic properties and CD31 downregulation in endothelial cells exposed to ApoExo. This study identifies vascular injury-derived extracellular vesicles (ApoExo) as novel drivers of NF-κB activation in endothelial cells and demonstrates the pivotal role of this signaling pathway in coordinating ApoExo-induced functional changes in endothelial cells. Hence, targeting ApoExo-mediated NF-κB activation in endothelial cells opens new avenues to prevent endothelial dysfunction.

Efficient Transcriptionally Controlled Plasmid Expression System for Investigation of the Stability of mRNA Transcripts in Primary Alveolar Epithelial Cells.

Studying posttranscriptional regulation is fundamental to understanding the modulation of a given messenger RNA (mRNA) and its impact on cell homeostasis and metabolism. Indeed, fluctuations in transcript expression could modify the translation efficiency and ultimately the cellular activity of a transcript. Several experimental approaches have been developed to investigate the half-life of mRNA although some of these methods have limitations that prevent the proper study of posttranscriptional modulation. A promoter induction system can express a gene of interest under the control of a synthetic tetracycline-regulated promoter. This method allows the half-life estimation of a given mRNA under any experimental condition without disturbing cell homeostasis. One major drawback of this method is the necessity to transfect cells, which limits the use of this technique in isolated primary cells that are highly resistant to conventional transfection techniques. Alveolar epithelial cells in primary culture have been used extensively to study the cellular and molecular biology of the alveolar epithelium. The unique characteristics and phenotype of primary alveolar cells make it essential to study the posttranscriptional modulations of genes of interest in these cells. Therefore, our aim was to develop a novel tool to investigate the posttranscriptional modulations of mRNAs of interest in alveolar epithelial cells in primary culture. We designed a fast and efficient transient transfection protocol to insert a transcriptionally controlled plasmid expression system into primary alveolar epithelial cells. This cloning strategy, using a viral epitope to tag the construct, allows for the easy discrimination of construct expression from that of endogenous mRNAs. Using a modified ΔΔ quantification cycle (Cq) method, the expression of the transcript can then be quantified at different time intervals to measure its half-life. Our data demonstrate the efficiency of this novel approach in studying posttranscriptional regulation in various pathophysiological conditions in primary alveolar epithelial cells.

Autophagy drives fibroblast senescence through MTORC2 regulation.

Sustained macroautophagy/autophagy favors the differentiation of fibroblasts into myofibroblasts. Cellular senescence, another means of responding to long-term cellular stress, has also been linked to myofibroblast differentiation and fibrosis. Here, we evaluate the relationship between senescence and myofibroblast differentiation in the context of sustained autophagy. We analyzed markers of cell cycle arrest/senescence in fibroblasts in vitro, where autophagy was triggered by serum starvation (SS). Autophagic fibroblasts expressed the senescence biomarkers CDKN1A/p21 and CDKN2A/p16 and exhibited increased senescence-associated GLB1/beta-galactosidase activity. Inhibition of autophagy in serum-starved fibroblasts with 3-methyladenine, LY294002, or ATG7 (autophagy related 7) silencing prevented the expression of senescence-associated markers. Similarly, suppressing MTORC2 activation using rapamycin or by silencing RICTOR also prevented senescence hallmarks. Immunofluorescence microscopy showed that senescence and myofibroblast differentiation were induced in different cells, suggesting mutually exclusive activation of senescence and myofibroblast differentiation. Reactive oxygen species (ROS) are known inducers of senescence and exposing fibroblasts to ROS scavengers decreased ROS production during SS, inhibited autophagy, and significantly reduced the expression of senescence and myofibroblast differentiation markers. ROS scavengers also curbed the AKT1 phosphorylation at Ser473, an MTORC2 target, establishing the importance of ROS in fueling MTORC2 activation. Inhibition of senescence by shRNA to TP53/p53 and shRNA CDKN2A/p16 increased myofibroblast differentiation, suggesting a negative feedback loop of senescence on autophagy-induced myofibroblast differentiation. Collectively, our results identify ROS as central inducers of MTORC2 activation during chronic autophagy, which in turn fuels senescence activation and myofibroblast differentiation in distinct cellular subpopulations. Abbreviations: 3-MA: 3-methyladenine; ACTA2: actin, alpha 2, smooth muscle, aorta; AKT1: AKT serine/threonine kinase 1; p-AKT1: AKT1 Ser473 phosphorylation; t-AKT1: total AKT serine/threonine kinase 1; ATG4A: autophagy related 4A cysteine peptidase; ATG7: autophagy gene 7; C12FDG: 5-dodecanoylaminofluorescein Di-ß-D-Galactopyranoside; CDKN1A: cyclin dependent kinase inhibitor 1A; CDKN2A: cyclin dependent kinase inhibitor 2A; Ctl: control; DAPI: 4',6-diamidino-2-phenylindole, dilactate; ECM: extracellular matrix; GSH: L-glutathione reduced; H2O2: hydrogen peroxide; HLF: adult human lung fibroblasts; Ho: Hoechst 33342 (2'-[4-ethoxyphenyl]-5-[4-methyl-1-piperazinyl]-2.5'-bi-1H-benzimidazole); HSC: hepatic stellate cells; LY: LY294002; MAP1LC3B/LC3B: microtubule-associated protein 1 light chain 3 beta; MTORC1/2: mechanistic target of rapamycin kinase complex 1/2; N: normal growth medium; NAC: N-acetyl-L-cysteine; PBS: phosphate-buffered saline; PDGFA: platelet derived growth factor subunit A; PRKCA/PKCα: protein kinase C alpha; PtdIns3K: class III phosphatidylinositol 3-kinase; PTEN: phosphatase and tensin homolog; R: rapamycin; RICTOR: RPTOR independent companion of MTOR complex 2; ROS: reactive oxygen species; RPTOR: regulatory associated protein of MTOR complex 1; SA-GLB1/ß-gal: senescence-associated galactosidase beta 1; SGK1: serum/glucocorticoid regulated kinase 1; shRNA: short hairpin RNA; siCtl: control siRNA; siRNA: small interfering RNA; SQSTM1: sequestosome 1; SS: serum-free (serum starvation) medium; TP53: tumor protein p53; TUBA: tubulin alpha; V: vehicle.

Apoptotic endothelial cells release small extracellular vesicles loaded with immunostimulatory viral-like RNAs.

Endothelial cells have multifaceted interactions with the immune system, both as initiators and targets of immune responses. In vivo, apoptotic endothelial cells release two types of extracellular vesicles upon caspase-3 activation: apoptotic bodies and exosome-like nanovesicles (ApoExos). Only ApoExos are immunogenic: their injection causes inflammation and autoimmunity in mice. Based on deep sequencing of total RNA, we report that apoptotic bodies and ApoExos are loaded with divergent RNA cargos that are not released by healthy endothelial cells. Apoptotic bodies, like endothelial cells, contain mainly ribosomal RNA whereas ApoExos essentially contain non-ribosomal non-coding RNAs. Endogenous retroelements, bearing viral-like features, represented half of total ApoExos RNA content. ApoExos also contained several copies of unedited Alu repeats and large amounts of non-coding RNAs with a demonstrated role in autoimmunity such as U1 RNA and Y RNA. Moreover, ApoExos RNAs had a unique nucleotide composition and secondary structure characterized by strong enrichment in U-rich motifs and unstably folded RNAs. Globally, ApoExos were therefore loaded with RNAs that can stimulate a variety of RIG-I-like receptors and endosomal TLRs. Hence, apoptotic endothelial cells selectively sort in ApoExos a diversified repertoire of immunostimulatory "self RNAs" that are tailor-made for initiation of innate immune responses and autoimmunity.

Post-Transcriptional Modulation of aENaC mRNA in Alveolar Epithelial Cells: Involvement of its 3' Untranslated Region.

BACKGROUND/AIMS: The epithelial sodium channel (ENaC) expressed in alveolar epithelial cells plays a major role in lung liquid clearance at birth and lung edema resorption in adulthood. We showed previously that αENaC mRNA expression is downregulated in part via posttranscriptional regulation of mRNA stability. In the present work, the role of the αENaC 3' untranslated region (3'UTR) in the regulation of mRNA stability was studied further. METHODS: Quantitative reverse transcription PCR (qRT-PCR) was performed to investigate the expression of αENaC in alveolar epithelial cells. The role of the αENaC 3'UTR was evaluated through sequential deletions. RNA affinity chromatography and mass spectrometry were achieved to investigate the nature of the proteins that could bind this sequence. The function of these proteins was assessed through knockdown and overexpression in vitro. RESULTS: First, we found that αENaC mRNA half-life was much shorter than expected when using a transcriptionally controlled plasmid expression system compared to Actinomycin D treatment. Sequential deletions of the αENaC 3'UTR revealed that the αENaC 3'UTR plays an important role in the modulation of αENaC mRNA stability, and that there is a complex stabilizing and destabilizing interplay between different regions of the 3'UTR that modulate this process. Finally, we identified RNA-binding proteins that interact with the αENaC 3'UTR and showed that Dhx36 and Tial1 are involved in the decrease in αENaC mRNA stability via the proximal region of its 3'UTR. CONCLUSION: Taken together, these findings indicate that the αENaC 3'UTR plays an important role in modulating transcript levels, and Dhx36 and Tial1 seem to be involved in posttranscriptional regulation of αENaC expression in alveolar epithelial cells.

Cycloheximide and lipopolysaccharide downregulate αENaC mRNA via different mechanisms in alveolar epithelial cells.

Active Na(+) transport mediated by epithelial Na(+) channel (ENaC) is vital for fetal lung fluid reabsorption at birth and pulmonary edema resolution. Previously, we demonstrated that αENaC expression and activity are downregulated in alveolar epithelial cells by cycloheximide (Chx) and Pseudomonas aeruginosa. The regulatory mechanisms of αENaC mRNA expression by Chx and lipopolysaccharide (LPS) from P. aeruginosa were further studied in the present work. Both agents decreased αENaC mRNA expression to 50% of control values after 4 h. Chx repressed αENaC expression in a dose-dependent manner independently of protein synthesis. Although extracellular signal-regulated kinases 1 and 2 (ERK1/2) and p38 mitogen-activated protein kinase (MAPK) pathways were activated by the two treatments, their mechanisms of ENaC mRNA modulation were different. First, activation of the signaling pathways was sustained by Chx but only transiently by LPS. Second, ERK1/2 or p38 MAPK inhibition attenuated the effects of Chx on αENaC mRNA, whereas suppression of both signaling pathways was necessary to alleviate the outcome of LPS on αENaC mRNA. The molecular mechanisms involved in the decrease of αENaC expression were investigated in both conditions. LPS, but not Chx, significantly reduced αENaC promoter activity via the ERK1/2 and p38 MAPK pathways. These results suggest that LPS attenuates αENaC mRNA expression via diminution of transcription, whereas Chx could trigger some posttranscriptional mechanisms. Although LPS and Chx downregulate αENaC mRNA expression similarly and with similar signaling pathways, the mechanisms modulating ENaC expression are different depending on the nature of the cellular stress.

Induction of nitric oxide synthase expression by lipopolysaccharide is mediated by calcium-dependent PKCα-ß1 in alveolar epithelial cells.

Nitric oxide (NO) plays an important role in innate host defense and inflammation. In response to infection, NO is generated by inducible nitric oxide synthase (iNOS), a gene product whose expression is highly modulated by different stimuli, including lipopolysaccharide (LPS) from gram-negative bacteria. We reported recently that LPS from Pseudomonas aeruginosa altered Na⁺ transport in alveolar epithelial cells via a suramin-dependent process, indicating that LPS activated a purinergic response in these cells. To further study this question, in the present work, we tested whether iNOS mRNA and protein expression were modulated in response to LPS in alveolar epithelial cells. We found that LPS induced a 12-fold increase in iNOS mRNA expression via a transcription-dependent process in these cells. iNOS protein, NO, and nitrotyrosine were also significantly elevated in LPS-treated cells. Ca²âº chelation and protein kinase C (PKCα-ß1) inhibition suppressed iNOS mRNA induction by LPS, implicating Ca²âº-dependent PKC signaling in this process. LPS evoked a significant increase of extracellular ATP. Because PKC activation is one of the signaling pathways known to mediate purinergic signaling, we evaluated the hypothesis that iNOS induction was ATP dependent. Although high suramin concentration inhibited iNOS mRNA induction, the process was not ATP dependent, since specific purinergic receptor antagonists could not inhibit the process. Altogether, these findings demonstrate that iNOS expression is highly modulated in alveolar epithelial cells by LPS via a Ca²âº/PKCα-ß1 pathway independent of ATP signaling.

K+ channels regulate ENaC expression via changes in promoter activity and control fluid clearance in alveolar epithelial cells.

Active Na+ absorption by alveolar ENaC is the main driving force of liquid clearance at birth and lung edema resorption in adulthood. We have demonstrated previously that long-term modulation of KvLQT1 and KATP K+ channel activities exerts sustained control in Na+ transport through the regulation of ENaC expression in primary alveolar type II (ATII) cells. The goal of the present study was: 1) to investigate the role of the alpha-ENaC promoter, transfected in the A549 alveolar cell line, in the regulation of ENaC expression by K+ channels, and 2) to determine the physiological impact of K+ channels and ENaC modulation on fluid clearance in ATII cells. KvLQT1 and KATP channels were first identified in A549 cells by PCR and Western blotting. We showed, for the first time, that KvLQT1 activation by R-L3 (applied for 24 h) increased alpha-ENaC expression, similarly to KATP activation by pinacidil. Conversely, pharmacological KvLQT1 and KATP inhibition or silencing with siRNAs down-regulated alpha-ENaC expression. Furthermore, K+ channel blockers significantly decreased alpha-ENaC promoter activity. Our results indicated that this decrease in promoter activity could be mediated, at least in part, by the repressor activity of ERK1/2. Conversely, KvLQT1 and KATP activation dose-dependently enhanced alpha-ENaC promoter activity. Finally, we noted a physiological impact of changes in K+ channel functions on ERK activity, alpha-, beta-, gamma-ENaC subunit expression and fluid absorption through polarized ATII cells. In summary, our results disclose that K+ channels regulate alpha-ENaC expression by controlling its promoter activity and thus affect the alveolar function of fluid clearance.