Endothelial cell senescence is associated with disrupted cell-cell junctions and increased monolayer permeability.

BACKGROUND: Cellular senescence is associated with cellular dysfunction and has been shown to occur in vivo in age-related cardiovascular diseases such as atherosclerosis. Atherogenesis is accompanied by intimal accumulation of LDL and increased extravasation of monocytes towards accumulated and oxidized LDL, suggesting an affected barrier function of vascular endothelial cells. Our objective was to study the effect of cellular senescence on the barrier function of non-senescent endothelial cells. METHODS: Human umbilical vein endothelial cells were cultured until senescence. Senescent cells were compared with non-senescent cells and with co-cultures of non-senescent and senescent cells. Adherens junctions and tight junctions were studied. To assess the barrier function of various monolayers, assays to measure permeability for Lucifer Yellow (LY) and horseradish peroxidase (PO) were performed. RESULTS: The barrier function of monolayers comprising of senescent cells was compromised and coincided with a change in the distribution of junction proteins and a down-regulation of occludin and claudin-5 expression. Furthermore, a decreased expression of occludin and claudin-5 was observed in co-cultures of non-senescent and senescent cells, not only between senescent cells but also along the entire periphery of non-senescent cells lining a senescent cell. CONCLUSIONS: Our findings show that the presence of senescent endothelial cells in a non-senescent monolayer disrupts tight junction morphology of surrounding young cells and increases the permeability of the monolayer for LY and PO.

Cobblestone HUVECs: a human model system for studying primary ciliogenesis.

Primary cilia are microtubule based sensory organelles that play an important role in maintaining cellular homeostasis. Malfunctioning results in a number of abnormalities, diseases (ciliopathies) and certain types of cancer. Morphological and biochemical knowledge on cilia/flagella, (early) ciliogenesis and intraflagellar transport is often obtained from model systems (e.g. Chlamydomonas) or from multi ciliary cells like lung or kidney epithelium. In this study endothelial cells in isolated human umbilical veins (HUVs) and cultured human umbilical vein endothelial cells (HUVECs) are compared and used to study primary ciliogenesis. By combining fluorescence microscopy, SEM, 2D and 3D TEM techniques we found that under the tested culturing conditions 60% of cobblestone endothelial cells form a primary cilium. Only a few of these cilia are present (protruding) on the endothelial cell surface, meaning that most primary cilia are in the cytoplasm (non-protruding). This was also observed in situ in the endothelial cells in the umbilical vein. The exact function(s?) of these non-protruding cilia remains unclear. Ultra-structural analysis of cultured HUVECs and the endothelial layer of the human umbilical veins reveal that there are: vesicles inside the ciliary pocket during the early stages of ciliogenesis; tubules/vesicles from the cytoplasm fuse with the ciliary sheath; irregular axoneme patterns, and two round, membranous vesicles inside the basal body. We conclude that cobblestone cultured HUVECs are comparable to the in vivo epithelial lining of the umbilical veins and therefore provide a well defined, relatively simple human model system with a reproducible number of non-protruding primary cilia for studying ciliogenesis.

Novel role of cPLA(2)alpha in membrane and actin dynamics.

Actin-directed processes such as membrane ruffling and cell migration are regulated by specific signal transduction pathways that become activated by growth factor receptors. The same signaling pathways that lead to modifications in actin dynamics also activate cPLA(2)alpha. Moreover, arachidonic acid, the product of cPLA(2)alpha activity, is involved in regulation of actin dynamics. Therefore, it was investigated whether cPLA(2)alpha plays a role in actin dynamics, more specifically during growth factor-induced membrane ruffling and cell migration. Upon stimulation of ruffling and cell migration by growth factors, endogenous cPLA(2)alpha and its active phosphorylated form were shown to relocate at protrusions of the cell membrane involved in actin and membrane dynamics. Inhibition of cPLA(2)alpha activity with specific inhibitors blocked growth factor-induced membrane and actin dynamics, suggesting an important role for cPLA(2)alpha in these processes.

Golgi-associated cPLA2alpha regulates endothelial cell-cell junction integrity by controlling the trafficking of transmembrane junction proteins.

In endothelial cells specifically, cPLA2alpha translocates from the cytoplasm to the Golgi complex in response to cell confluence. Considering the link between confluence and cell-cell junction formation, and the emerging role of cPLA2alpha in intracellular trafficking, we tested whether Golgi-associated cPLA2alpha is involved in the trafficking of junction proteins. Here, we show that the redistribution of cPLA2alpha from the cytoplasm to the Golgi correlates with adherens junction maturation and occurs before tight junction formation. Disruption of adherens junctions using a blocking anti-VE-cadherin antibody reverses the association of cPLA2alpha with the Golgi. Silencing of cPLA2alpha and inhibition of cPLA2alpha enzymatic activity using various inhibitors result in the diminished presence of the transmembrane junction proteins VE-cadherin, occludin, and claudin-5 at cell-cell contacts, and in their accumulation at the Golgi. Altogether, our data support the idea that VE-cadherin triggers the relocation of cPLA2alpha to the Golgi and that in turn, Golgi-associated cPLA2alpha regulates the transport of transmembrane junction proteins through or from the Golgi, thereby controlling the integrity of endothelial cell-cell junctions.

A regulated interaction with the UIM protein Eps15 implicates parkin in EGF receptor trafficking and PI(3)K-Akt signalling.

Mutations in the parkin gene are responsible for a common familial form of Parkinson's disease. As parkin encodes an E3 ubiquitin ligase, defects in proteasome-mediated protein degradation are believed to have a central role in the pathogenesis of Parkinson's disease. Here, we report a novel role for parkin in a proteasome-independent ubiquitination pathway. We have identified a regulated interaction between parkin and Eps15, an adaptor protein that is involved in epidermal growth factor (EGF) receptor (EGFR) endocytosis and trafficking. Treatment of cells with EGF stimulates parkin binding to both Eps15 and the EGFR and promotes parkin-mediated ubiquitination of Eps15. Binding of the parkin ubiquitin-like (Ubl) domain to the Eps15 ubiquitin-interacting motifs (UIMs) is required for parkin-mediated Eps15 ubiquitination. Furthermore, EGFR endocytosis and degradation are accelerated in parkin-deficient cells, and EGFR signalling via the phosphoinositide 3-kinase (PI(3)K)-Akt pathway is reduced in parkin knockout mouse brain. We propose that by ubiquitinating Eps15, parkin interferes with the ability of the Eps15 UIMs to bind ubiquitinated EGFR, thereby delaying EGFR internalization and degradation, and promoting PI(3)K-Akt signalling. Considering the role of Akt in neuronal survival, our results have broad new implications for understanding the pathogenesis of Parkinson's disease.

Protein expression dynamics during replicative senescence of endothelial cells studied by 2-D difference in-gel electrophoresis.

Endothelial senescence contributes to endothelium dysfunctionality and is thereby linked to vascular aging. A dynamic proteomic study on human umbilical vein endothelial cells, isolated from three umbilical cords, was performed. The cells were cultured towards replicative senescence and whole cell lysates were subjected to 2-D difference gel electrophoresis (DIGE). Despite the biological variability of the three independent isolations, a set of proteins was found that showed senescence-dependent expression patterns in all isolations. We focused on those proteins that showed significant changes, with a paired analysis of variance (RM-ANOVA) p-value of < or =0.05. Thirty-five proteins were identified with LC-Fourier transform MS, and functional annotation revealed that endothelial replicative senescence is accompanied by increased cellular stress, protein biosynthesis and reduction in DNA repair and maintenance. Nuclear integrity becomes affected and cytoskeletal structure is also changed. Such important changes in the cell infrastructure might accelerate endothelium dysfunctionality. This study provides biological information that will initiate studies to further unravel endothelial senescence and gain more knowledge about the consequences of this process in the in vivo situation.

Ubiquilin recruits Eps15 into ubiquitin-rich cytoplasmic aggregates via a UIM-UBL interaction.

Eps15 and its related protein Eps15R are key components of the clathrin-mediated endocytic pathway. We searched for new binding partners of Eps15 using a yeast two-hybrid screen. We report here that ubiquilin (hPLIC1), a type-2 ubiquitin-like protein containing a ubiquitin-like domain (UBL) and a ubiquitin-associated domain (UBA), interacts with both Eps15 and Eps15R. Using glutathione-S-transferase pull-down experiments, we show that the first ubiquitin-interacting motif of Eps15 (UIM1) interacts directly with the UBL domain of ubiquilin, whereas it does not bind to ubiquitinated proteins. The second UIM of Eps15 (UIM2) binds poorly to the UBL domain but does bind to ubiquitinated proteins. Two other UIM-containing endocytic proteins, Hrs and Hbp, also interact with ubiquilin in a UIM-dependent manner, whereas epsin does not. Immunofluorescence analysis showed that endogenous Eps15 and Hrs, but not epsin, colocalize with green-fluorescent-protein-fused ubiquilin in cytoplasmic aggregates that are not endocytic compartments. We have characterized these green-fluorescent-protein-fused-ubiquilin aggregates as ubiquitin-rich intracytoplasmic inclusions that are recruited to aggresomes upon proteasome inhibition. Moreover, we show that endogenous Eps15 and endogenous ubiquilin colocalize to cytoplasmic aggregates and aggresomes. Finally, we show that the recruitment of Eps15 into ubiquilin-positive aggregates is UIM dependent. Altogether, our data identify ubiquilin as the first common UIM-binding partner of a subset of UIM-containing endocytic proteins. We propose that this UIM/UBL-based interaction is responsible for the sequestration of certain UIM-containing endocytic proteins into cytoplasmic ubiquitin-rich protein aggregates.