Enhanced receptor–clathrin interactions induced by N-glycan–mediated membrane micropatterning

Glycan–protein interactions are emerging as important modulators of membrane protein organization and dynamics, regulating multiple cellular functions. In particular, it has been postulated that glycan-mediated interactions regulate surface residence time of glycoproteins and endocytosis. How this precisely occurs is poorly understood. Here we applied single-molecule-based approaches to directly visualize the impact of glycan-based interactions on the spatiotemporal organization and interaction with clathrin of the glycosylated pathogen recognition receptor dendritic cell-specific intercellular adhesion molecule-3-grabbing nonintegrin (DC-SIGN). We find that cell surface glycan-mediated interactions do not influence the nanoscale lateral organization of DC-SIGN but restrict the mobility of the receptor to distinct micrometer-size membrane regions. Remarkably, these regions are enriched in clathrin, thereby increasing the probability of DC-SIGN–clathrin interactions beyond random encountering. N-glycan removal or neutralization leads to larger membrane exploration and reduced interaction with clathrin, compromising clathrin-dependent internalization of virus-like particles by DC-SIGN. Therefore, our data reveal that cell surface glycan-mediated interactions add another organization layer to the cell membrane at the microscale and establish a novel mechanism of extracellular membrane organization based on the compartments of the membrane that a receptor is able to explore. Our work underscores the important and complex role of surface glycans regulating cell membrane organization and interaction with downstream partners.Glycans are fundamental cellular components ubiquitously present in the extracellular matrix and cell membrane as glycoproteins or glycolipids. Glycan-binding proteins such as galectins, siglecs, and selectins are mostly multivalent and thus thought to cross-link glycoproteins into higher-order aggregates, creating a cell surface glycan-based connectivity also called glycan lattice or network (1–3). By concentrating specific glycoproteins or glycolipids while excluding other cell surface molecules, surface glycan-based connectivity can organize the plasma membrane into specialized domains that perform unique functions (1, 3–6). Nevertheless, direct observation of glycan-mediated ligand cross-linking in living cells remains challenging (7). Notwithstanding, there is no doubt that surface glycan-based connectivity is essential in the control of multiple biological processes including immune cell activation and homeostasis, cell proliferation and differentiation, and receptor turnover and endocytosis (1, 5, 6, 8).Clathrin-mediated endocytosis (CME) constitutes the primary pathway of cargo internalization in mammalian cells regulating the surface expression of receptors (9). Formation of clathrin-coated pits (CCPs) starts by nucleation of coat assembly at distributed positions in the inner surface of the plasma membrane, where it continues to grow or dissolve rapidly unless coat stabilization occurs (10, 11). One event that clearly correlates with successful CCP stabilization is cargo loading (11). Recent studies show that cargo molecules diffuse randomly on the cell membrane until they meet growing CCPs, with the extent of cargo interactions regulating CCP maturation (12). As such, factors that affect cargo mobility within/at the cell surface will inevitably impact on CCP maturation and successful internalization. In the context of surface glycan–protein interactions, it has been shown that glycoproteins with an intact glycan-based connectivity exhibit reduced lateral mobility and this correlates with compromised endocytosis (3, 13–17). How this precisely occurs is poorly defined, although fluorescence recovery after photobleaching on the EGF receptor (EGFR) suggested that cell surface glycan-based interactions restrict EGFR dynamics and localization into membrane regions away from endocytic platforms (14, 17). Whether this is a general mechanism for glycosylated proteins or specific to EGFR is not known. Moreover, visualization of receptor interactions with the endocytic machinery under the influence of the glycan network has not yet been attained.In this work we applied superresolution nanoscopy and developed a dedicated dual-color single-molecule spatio-dynamic exploration approach to visualize the impact of glycan-based interactions on the spatiotemporal organization and clathrin interaction of a glycosylated membrane receptor involved in pathogen recognition and uptake. We focused on the transmembrane glycoprotein dendritic cell-specific intercellular adhesion molecule-3-grabbing nonintegrin (DC-SIGN) given its importance in supporting primary immune responses such as pathogen recognition and uptake on immature dendritic cells (imDCs), signaling, and cell adhesion (6, 18–20). Moreover, DC-SIGN contains a single N-glycosylation site, organizes in nanoclusters at the cell membrane (19, 21–23), and internalizes bound antigens via CPPs for subsequent processing and presentation to T cells (20, 24–26). Our work provides insights on how surface glycan-mediated interactions tune spatiotemporal micropatterning of receptors on the cell membrane, potentially regulating interactions with the endocytic machinery and underscoring the importance and complex role of surface glycans on cell membrane organization and function.     

The difference in gliosis induced by β-amyloid and Tau treatments in astrocyte cultures derived from senescence accelerated and normal mouse strains

Astrocytes react to various neurodegenerative insults rapidly and undergo changes known as gliosis or astrogliosis. In Alzheimer’s disease (AD), a wall of reactive astrocytes surrounds senile plaques of β-amyloid (Aβ) and might play an important role in clearing of Aβ. AD is neuropathologically characterized by the co-existence of two pathological structures, senile plaques and neurofibrillary tangles composed of Aβ and Tau protein respectively. However, the molecular mechanisms underlie astrogliosis and increased expressions of GFAP and other astrogliosis markers are poorly understood. Since AD is age related, the aim of this study is to compare the gliosis of aging prone astrocytes cultured from senescence-accelerated mice and astrocytes from normal mice in response to Aβ and Tau treatment. Our results demonstrated that the aging prone astrocytes have showed larger degree of gliosis than normal astrocytes. Since reactive astrocytes had less ability to support co-cultured neurons as compared with control astrocytes. Therefore, it is likely that aging prone astrocytes might contribute to cell loss or dysfunction associated with insults in AD. In other words, aging prone astrocytes might have decreased ability than normal astrocytes to protect or prevent neuronal dysfunction in AD pathology. In addition, further AD related studies should use aging prone astrocytes instead of normal astrocytes.     

RANKL–OPG and RAGE modulation in vascular calcification and diabetes: novel targets for therapy

Type 2 diabetes is associated with increased cardiovascular morbidity and mortality and early vascular ageing. This takes the form of atherosclerosis, with progressive vascular calcification being a major complication in the pathogenesis of this disease. Current research and drug targets in diabetes have hitherto focused on atherosclerosis, but vascular calcification is now recognised as an independent predictor of cardiovascular morbidity and mortality. An emerging regulatory pathway for vascular calcification in diabetes involves the receptor activator for nuclear factor κB (RANK), RANK ligand (RANKL) and osteoprotegerin (OPG). Important novel biomarkers of calcification are related to levels of glycation and inflammation in diabetes. Several therapeutic strategies could have advantageous effects on the vasculature in patients with diabetes, including targeting the RANKL and receptor for AGE (RAGE) signalling pathways, since there has been little success—at least in macrovascular outcomes—with conventional glucose-lowering therapy. There is substantial and relevant clinical and basic science evidence to suggest that modulating RANKL–RANK–OPG signalling, RAGE signalling and the associated proinflammatory milieu alters the natural course of cardiovascular complications and outcomes in people with diabetes. However, further research is critically needed to understand the precise mechanisms underpinning these pathways, in order to translate the anti-calcification strategies into patient benefit.     

Angiotensin II receptor blocker irbesartan attenuates sleep apnea–induced cardiac apoptosis and enhances cardiac survival and Sirtuin 1 upregulation

Background

The purpose of this study was to investigate whether or not angiotensin II type 1 receptor blocker irbesartan (ARB) with a partial agonist of PPAR-γ could protect against chronic nocturnal intermittent hypoxia (CIH)–induced cardiac Fas/FasL-mediated to mitochondria-mediated apoptosis.

Methods

Sprague–Dawley rats were in a normoxic control group (CON-G), or rats were in a chronic nocturnal intermittent hypoxia group (HP-G, from 3 to 7% oxygen versus 21% oxygen per forty seconds cycle, nocturnally 8 h per day for 1 month), or rats were in a chronic nocturnal intermittent hypoxia group pretreated with ARB (50 mg/kg/day, S.C.) (ARB-HP-G). Echocardiography, H&E staining, TUNEL staining, and Western blotting were measured in the left ventricle.

Results

Hypoxia-induced SIRT1 degradation, Fas receptors, FADD, active caspase-8 and caspase-3 (Fas/FasL apoptotic pathway) and Bax, tBid, active caspase-9 and -3 (mitochondrial apoptotic pathway) and TUNEL-positive apoptosis were reduced in ARB-HP-G when compared with HP-G. IGF-I, IGF1 receptor, p-PI3k, p-Akt, Bcl2, and Bcl-XL (IGF1/PI3K/AKT pro-survival pathway) were increased in ARB-HP-G compared to HP-G.

Conclusions

Our findings suggest that the ARB may prevent cardiac Fas/FasL to mitochondrial apoptotic pathways and enhance cardiac IGF1/PI3K/AKT pro-survival pathway in the sleep apnea model associated with JNK de-activation and SIRT1 upregulation. ARB prevents chronic sleep apnea–enhanced cardiac apoptosis via enhancing survival pathways.

     

Expression of vascular endothelial growth factor C and chemokine receptor CCR7 in gastric carcinoma and their values in predicting lymph node metastasis

AIM:To study the expression of vascular endothelial growthfactor C (VEGF-C) and chemokine receptor CCR7 in gastriccarcinoma and to investigate their associations with lymphnode metastasis of gastric carcinoma and their values inpredicting lymph node metastasis.METHODS:The expression of VEGF-C and CCR7 in gastriccarcinoma tissues obtained from 118 patients who underwentcurative gastrectomy was examined by immunohistochemistry.Among these patients,39 patients underwent multi-slicespiral CT (MSCT) examination.RESULTS:VEGF-C and CCR7 were positively expressed in52.5 and 53.4% of patients.VEGF-C expression was morefrequently found in tumors with lymph node metastasis thanthose without it (P<0.001).VEGF-C expression was alsoclosely related to lymphatic invasion (P<0.001),vascularinvasion (P<0.01),and TNM stage (P<0.001).However,there was no significant correlation between VEGF-Cexpression and age at surgery,gender,tumor size,tumorlocation,Lauren classification,and depth of invasion.CCR7expression was significantly higher in patients with lymphnode metastasis compared with those without lymph nodemetastasis (P<0.001) and was also associated with tumorsize (P<0.01),depth of invasion (P<0.001),lymphaticinvasion (P<0.001),and TNM stage (P<0.001).However,the presence of CCR7 had no correlation to age at surgery,gender,tumor location,Lauren classification,and vascularinvasion.Among the 39 patients who underwent MSCTexamination,only CCR7 expression was related to lymphnode metastasis determined by MSCT (P<0.05).In thecurrent retrospective study,the sensitivity,specificity,positivepredictive value (PPV),negative predictive value (NPV),andaccuracy of VEGF-C and CCR7 expression in the diagnosisof lymph node metastasis for patients with gastric carcinomawere 73.8%,70.2%,72.6%,71.4% and 72.0%,and 82.0%,77.2%,79.4%,80.0% and 79.7%,respectively.Aftersubdivision according to the combination of VEGF-C andCCR7 expression,receiver operating characteristic (ROC)analysis showed that the accuracy of the combinedexamination of VEGF-C and CCR7 expression in predicting lymph node metastasis was relatively high (area under ROCcurve [Az]=0.83).CONCLUSION:The expression of VEGF-C and CCR7 isrelated to lymph node metastasis of gastric carcinoma andboth of them may become new targets for the treatment ofgastric carcinoma.Furthermore,the combined examinationof VEGF-C and CCR7 expression in endoscopic biopsyspecimens may be useful in predicting lymph node metastasisof gastric carcinoma and deciding the extent of surgical lymphnode resection.     

Induction of macrophage inflammatory protein-1α and vascular endothelial growth factor during inflammatory neovascularization in the mouse cornea

We predicted that the appearance of macrophages in inflammatory areas is necessary for angiogenic responses in various inflammatory diseases. Using a mouse inflammatory corneal model in which model mouse corneas were cauterized with silver nitrate, we examined the infiltration of macrophages immunohistochemically and the total area of neovascularization quantitively. The expression of macrophage inflammatory protein-1α (MIP-1α) and vascular endothelial growth factor (VEGF) levels were also examined. A day after cauterization, short capillaries began to develop into the corneal stroma, and after 4 or 5 days the neovascularization became maximal and then began to regress. The number of macrophages within the cauterized cornea increased to a maximum at day 3 and began to decrease at day 5. The number of infiltrated macrophages reached maximum at day 3. Both MIP-1α and VEGF protein levels increased markedly immediately after the chemical cauterization, and production of MIP-1α (85.8 pg/4 corneas) and VEGF (206.5 pg/4 corneas) was maximal at 1 day and 0.5 day after cauterization, respectively. MIP-1α and VEGF mRNA levels also increased at 0.5 day after cauterization. In situ hybridization showed that MIP-1α was localized in corneal epithelial cells, and VEGF was localized in corneal epithelial cells and infiltrating inflammatory cells. MIP-1α and VEGF may have an important role in recruiting macrophages and neovascularization. This revised version was published online in June 2006 with corrections to the Cover Date.