Protein phosphatase 2A-mediated flotillin-1 dephosphorylation up-regulates endothelial cell migration and angiogenesis regulation

Enhanced therapeutic potential of Flotillins-modified MenSCs by improve the survival, proliferation and migration

Menstrual blood-derived endometrial stem cells (MenSCs) have attracted increasing interest due to their excellent safety, and lack of ethical dilemma as well as their ability to be periodically obtained in a noninvasive manner. However, although preclinical research as shown the therapeutic potential of MenSCs in several diseases, their poor cell survival and low engraftment at disease sites reduce their clinical efficacy. Flotillins (including Flot1 and Flot2) are implicated in various cellular processes, such as vesicular trafficking, signal transduction, cell proliferation, migration and apoptosis. In this study, we aimed to determine the effects of Flotillins on MenSCs survival, proliferation and migration. Our experimental results show that MenSCs were modified to overexpress Flot1 and/or Flot2 without altering their intrinsic characteristics. Flot1 and Flot2 co-overexpression promoted MenSC viability and proliferation capacity. Moreover, Flot1 or Flot2 overexpression significantly promoted the migration and inhibited the apoptosis of MenSCs compared with the negative control group, and these effects were stronger in the Flot1 and Flot2 gene co-overexpression group. However, these effects were significantly reversed after Flot1 and/or Flot2 knockdown. In conclusion, our results indicate that Flot1 and Flot2 overexpression in MenSCs improved their proliferation and migration and inhibited their apoptosis, and this might be an effective approach to improve the efficiency of cell-based therapies.

PP2A Affects Angiogenesis via Its Interaction with a Novel Phosphorylation Site of TSP1

Alterations in angiogenic properties play a pivotal role in the manifestation and onset of various pathologies, including vascular diseases and cancer. Thrombospondin-1 (TSP1) protein is one of the master regulators of angiogenesis. This study unveils a novel aspect of TSP1 regulation through reversible phosphorylation. The silencing of the B55α regulatory subunit of protein phosphatase 2A (PP2A) in endothelial cells led to a significant decrease in TSP1 expression. Direct interaction between TSP1 and PP2A-B55α was confirmed via various methods. Truncated TSP1 constructs were employed to identify the phosphorylation site and the responsible kinase, ultimately pinpointing PKC as the enzyme phosphorylating TSP1 on Ser93. The biological effects of B55α-TSP1 interaction were also analyzed. B55α silencing not only counteracted the increase in TSP1 expression during wound closure but also prolonged wound closure time. Although B55α silenced cells initiated tube-like structures earlier than control cells, their spheroid formation was disrupted, leading to disintegration. Cells transfected with phosphomimic TSP1 S93D exhibited smaller spheroids and reduced effectiveness in tube formation, revealing insights into the effects of TSP1 phosphorylation on angiogenic properties. In this paper, we introduce a new regulatory mechanism of angiogenesis by reversible phosphorylation on TSP1 S93 by PKC and PP2A B55α.

NGR-bearing human adenovirus type 5 infects cells in flotillin- or caveolin-mediated manner depending on the NGR insertion site

Human adenoviruses represent attractive candidates for the design of cancer gene therapy vectors. Modification of adenovirus tropism by incorporating a targeting ligand into the adenovirus capsid proteins allows retargeting of adenovirus towards the cells of interest. Human adenovirus type 5 (HAdV-C5) bearing NGR containing peptide (CNGRCVSGCAGRC) inserted into the fiber (AdFNGR) or the hexon (AdHNGR) protein demonstrated an increased transduction of endothelial cells showing expression of aminopeptidase N, also known as CD13, and αvβ3 integrin both present on tumor vasculature, indicating that NGR-bearing adenoviruses could be used as tools for anti-angiogenic cancer therapy. Here we investigated how AdFNGR and AdHNGR infect cells lacking HAdV-C5 primary receptor, coxsackie and adenovirus receptor, and we showed that both AFNGR and AdHNGR enter cells by dynamin- and lipid raft-mediated endocytosis, while clathrin is not required for endocytosis of these viruses. We present evidence that productive infection of both AdFNGR and AdHNGR involves lipid rafts, with usage of flotillin-mediated cell entry for AdFNGR and limited role of caveolin in AdHNGR transduction efficiency. Lipid rafts play important role in angiogenesis and process of metastasis. Therefore, the ability of AdFNGR and AdHNGR to use lipid raft-dependent endocytosis, involving respectively flotillin- or caveolin-mediated pathway, could give them an advantage in targeting tumor cells lacking HAdV-C5 primary receptor.

Generalized Bioluminescent Platform To Observe and Track Cellular Interactions

Cell-to-cell communications are critical to biological processes ranging from embryonic development to cancer progression. Several imaging strategies have been developed to capture such interactions, but many are challenging to deploy in thick tissues and other complex environments. Here, we report a platform termed Luminescence to Observe and Track Intercellular Interactions (LOTIIS). The approach features split fragments of a luciferase enzyme that reassemble when target cells come into proximity. One fragment is secreted by "sender" cells, and the complementary piece is secreted by "receiver" cells. Split reporter assembly is facilitated by a single chain variable fragment (scFv)-peptide interaction on the receiver cell, resulting in localized light production. We demonstrate that LOTIIS can rapidly label cells in close proximity in a time- and distance-dependent fashion. The platform is also compatible with bioluminescence resonance energy transfer probes for multiplexed imaging. Collectively, these data suggest that LOTIIS will enable a variety of cellular interactions to be tracked in biological settings.

Molecular characterization and function of the lipid raft protein Lvflotillin-1A from Litopenaeus vannamei

White spot syndrome virus (WSSV) can cause a contagious, high virulent and pandemic disease for crustaceans, especially shrimps. However, the molecular mechanism of WSSV pathogenesis remains unclear. Flotillins are lipid raft-associated proteins, which mainly include flotillin-1 and flotillin-2. They are involved in the formation of large heteromeric protein complexes engaged in diverse signalling pathways at the membrane-cytosol interface. They defined a clathrin-independent endocytic pathway in mammalian cells. Our previous studies suggested that shrimp flotillin-2 might mediate endocytosis involved in WSSV infection. To further explore the function of shrimp flotillin, a flotillin-1 homologous, Lvflotillin-1A was identified and characterized in Litopenaeus vanamei. The transcription of Lvflotillin-1A showed a significant decline at 12h post-infection, followed by complete recovery and a slight up-regulation after the WSSV challenge. Gene silencing revealed that inhibition of Lvflotillin-1A raised the virus infection, suggesting Lvflotillin-1A might play an important role in shrimp immunity. Furthermore, co-immunoprecipitation and immunofluorescence illustrated that Lvflotillin-1A and Lvflotillin-2 could form hetero-oligomers, and co-expression promoted the accumulation of intracellular vesicles. The study revealed that WSSV might up-regulate Lvflotillin-2 expression and alter the subcellular location of Lvflotillin-1 protein to facilitate virus infection. These results will provide information for understanding the interaction between WSSV and shrimp.

High fructose induces dysfunctional vasodilatation via PP2A-mediated eNOS Ser1177 dephosphorylation

Background

Processed foods are popular and contain large amounts of industrial fructose, which changes people’s diet and exacerbates the negative health effects of high fructose. Several studies have shown that excessive intake of fructose has a major impact on vascular disease. However, the mechanism of the effect of high fructose on blood vessels is currently unclear.

Methods

The effect of fructose on the vasodilatation of isolated thoracic aortic rings was observed by using wire myography in wild-type (WT) mice. Cell viability and nitric oxide (NO) production were assessed by the corresponding kits in mouse vascular endothelial cells. The effect of fructose on endothelial nitric oxide synthase (eNOS) and protein phosphatase 2A (PP2A) and their changes in phosphorylation were detected by using Western blots. Moreover, a PP2A inhibitor (okadaic acid, OA) was used to evaluate the relationship between fructose and PP2A. Furthermore, PP2ACα endothelial-specific knockout (PP2A cKO) mice were used to detect the vasodilatation of in vitro fructose-incubated thoracic aortic rings by using wire myography.

Results

High fructose induced endothelium-dependent dysfunctional vasodilatation. High fructose reduced acetylcholine (Ach)-induced vasodilation but did not affect sodium nitroprusside (SNP)-induced vasodilation. Accordingly, NO production and the phosphorylation level of eNOS at serine (Ser) 1177 (P-eNOS) in vascular endothelial cells were remarkably reduced without changes in cell viability. The expression of protein phosphatase 2A catalytic subunit (PP2AC) was increased and the expression of phosphorylated PP2AC (P-PP2A, tyrosine [Tyr] 307) was significantly decreased. Nevertheless, these effects were reversed by OA. Moreover, knockout of the PP2A gene could recover the response of vessels to Ach under high fructose stimulation.

Conclusions

Our observations demonstrate an underlying mechanism of fructose-induced dysfunctional vasodilatation. Fructose could activate PP2A, which leads to decrease in the phosphorylation of eNOS at Ser1177 and the reduction of NO release, thus leading to the occurrence of endothelium-dependent dysfunctional vasodilatation.

A Bioluminescent Sensor for Rapid Detection of PPEP-1, a Clostridioides difficile Biomarker

Current assays for Clostridioides difficile in nonhospital settings are outsourced and time-intensive, resulting in both delayed diagnosis and quarantining of infected individuals. We designed a more rapid point-of-care assay featuring a “turn-on” bioluminescent readout of a C. difficile-specific protease, PPEP-1. NanoLuc, a bright and stable luciferase, was “caged” with a PPEP-1-responsive peptide tail that inhibited luminescence. Upon proteolytic cleavage, the peptide was released and NanoLuc activity was restored, providing a visible readout. The bioluminescent sensor detected PPEP-1 concentrations as low as 10 nM. Sensor uncaging was achieved within minutes, and signal was captured using a digital camera. Importantly, the sensor was also functional at ambient temperature and compatible with fecal material, suggesting that it can be readily deployed in a variety of settings.

Ser69 phosphorylation of TIMAP affects endothelial cell migration

PURPOSE/AIM

TIMAP (TGF-β-inhibited membrane-associated protein) is a regulatory subunit of protein phosphatase 1 (PP1). The N-terminal region contains a binding motif for the catalytic subunit of PP1 (PP1c) and a nuclear localization signal (NLS). Phosphorylation of TIMAP on Ser331, Ser333 and Ser337 side chains was shown to regulate the activity of the TIMAP-PP1c complex. Several studies, however, reported an additional side chain of TIMAP. Ser69 is located near to the PP1c binding motif and NLS, therefore, we hypothesized that the phosphorylation of this side chain perhaps may regulate the interaction between TIMAP and PP1c, or may affect the nuclear transport of TIMAP. Materials and Methods: To study the significance of Ser69 phosphorylation, GST-tagged or c-myc-tagged wild type, phosphomimic S69D and phosphonull S69A recombinant TIMAP proteins were expressed in bacteria or endothelial cells, respectively. Protein-protein interactions of the wild type or mutant forms of TIMAP were studied by pull-down and Western blot. Localization of TIMAP S69 mutants in pulmonary artery endothelial cells was detected by immunofluorescent staining and expression and localization of the recombinants were investigated by subcellular fractionation and Western blot. Results: Modifications of Ser69 of TIMAP had no effect on binding of PP1c, ERM or RACK1. However, S69D TIMAP showed enhanced membrane localization and an increased number of membrane protrusions were observed in the cells overexpressing this phosphomimic mutant. Furthermore, significantly faster wound healing and migration rate of the S69D mutant overexpressing cells were detected by endothelial barrier resistance measurements (ECIS). Specific interaction was shown between TIMAP and polo-like kinase 4 (PLK4), a potential kinase to phosphorylate Ser69. Conclusions: Altogether, our results indicate that Ser69 phosphorylation by PLK4 may evoke an enrichment of TIMAP in the plasma membrane region and may play an important role in endothelial cell migration without affecting the PP1c binding ability of TIMAP.

Dephosphorylation of annexin A2 by protein phosphatase 1 regulates endothelial cell barrier

Annexin A2 (ANXA2) is a multifunctional protein expressed in nearly all human tissues and cell types, playing a role in various signaling pathways. It is subjected to phosphorylation, but no specific protein phosphatase has been identified in its posttranslational regulation yet. Using pull-down assay followed by liquid chromatography-mass spectrometry analysis we found that ANXA2 interacts with TIMAP (TGF-beta-inhibited membrane-associated protein) in pulmonary artery endothelial cells. TIMAP is highly expressed in endothelial cells, where it acts as a regulatory and targeting subunit of protein phosphatase 1 (PP1). TIMAP plays an important role in the regulation of the endothelial barrier maintenance through the dephosphorylation of its several substrate proteins. In the present work, phosphorylation of Ser25 side chain in ANXA2 by protein kinase C (PKC) was shown both in vivo and in vitro. Phosphorylation level of ANXA2 at Ser25 increased greatly by inhibition of PP1 and by depletion of its regulatory subunit, TIMAP, implying a role of this PP1 holoenzyme in the dephosphorylation of ANXA2. Immunofluorescence staining and subcellular fractionations revealed a diffuse subcellular localization for the endogenous ANXA2, but phospho-Ser25 ANXA2 was mainly detected in the membrane. ANXA2 depletion lowered the basal endothelial barrier and inhibited cell migration, but had no significant effect on cell proliferation or viability. ANXA2 depleted cells failed to respond to PMA treatment, indicating an intimately involvement of phospho-ANXA2 in PKC signaling. Moreover, phosphorylation of ANXA2 disrupted its interaction with S100A10 suggesting a phosphorylation dependent multiple regulatory role of ANXA2 in endothelial cells. Our results demonstrate the pivotal role of PKC-ANXA2-PP1 pathway in endothelial cell signaling, especially in barrier function and cell migration.

The role of lipid species in membranes and cancer-related changes

Several studies have demonstrated interactions between the two leaflets in membrane bilayers and the importance of specific lipid species for such interaction and membrane function. We here discuss these investigations with a focus on the sphingolipid and cholesterol-rich lipid membrane domains called lipid rafts, including the small flask-shaped invaginations called caveolae, and the importance of such membrane structures in cell biology and cancer. We discuss the possible interactions between the very long-chain sphingolipids in the outer leaflet of the plasma membrane and the phosphatidylserine species PS 18:0/18:1 in the inner leaflet and the importance of cholesterol for such interactions. We challenge the view that lipid rafts contain a large fraction of lipids with two saturated fatty acyl groups and argue that it is important in future studies of membrane models to use asymmetric membrane bilayers with lipid species commonly found in cellular membranes. We also discuss the need for more quantitative lipidomic studies in order to understand membrane function and structure in general, and the importance of lipid rafts in biological systems. Finally, we discuss cancer-related changes in lipid rafts and lipid composition, with a special focus on changes in glycosphingolipids and the possibility of using lipid therapy for cancer treatment.

Flotillins: At the Intersection of Protein S-Palmitoylation and Lipid-Mediated Signaling

Flotillin-1 and flotillin-2 are ubiquitously expressed, membrane-associated proteins involved in multifarious cellular events from cell signaling, endocytosis, and protein trafficking to gene expression. They also contribute to oncogenic signaling. Flotillins bind the cytosolic leaflet of the plasma membrane and endomembranes and, upon hetero-oligomerization, serve as scaffolds facilitating the assembly of multiprotein complexes at the membrane-cytosol interface. Additional functions unique to flotillin-1 have been discovered recently. The membrane-binding of flotillins is regulated by S-palmitoylation and N-myristoylation, hydrophobic interactions involving specific regions of the polypeptide chain and, to some extent, also by their oligomerization. All these factors endow flotillins with an ability to associate with the sphingolipid/cholesterol-rich plasma membrane domains called rafts. In this review, we focus on the critical input of lipids to the regulation of the flotillin association with rafts and thereby to their functioning. In particular, we discuss how the recent developments in the field of protein S-palmitoylation have contributed to the understanding of flotillin1/2-mediated processes, including endocytosis, and of those dependent exclusively on flotillin-1. We also emphasize that flotillins affect directly or indirectly the cellular levels of lipids involved in diverse signaling cascades, including sphingosine-1-phosphate and PI(4,5)P₂. The mutual relations between flotillins and distinct lipids are key to the regulation of their involvement in numerous cellular processes.