Presynaptic α2δ subunits are key organizers of glutamatergic synapses

In nerve cells the genes encoding for α2δ subunits of voltage-gated calcium channels have been linked to synaptic functions and neurological disease. Here we show that α2δ subunits are essential for the formation and organization of glutamatergic synapses. Using a cellular α2δ subunit triple-knockout/knockdown model, we demonstrate a failure in presynaptic differentiation evidenced by defective presynaptic calcium channel clustering and calcium influx, smaller presynaptic active zones, and a strongly reduced accumulation of presynaptic vesicle-associated proteins (synapsin and vGLUT). The presynaptic defect is associated with the downscaling of postsynaptic AMPA receptors and the postsynaptic density. The role of α2δ isoforms as synaptic organizers is highly redundant, as each individual α2δ isoform can rescue presynaptic calcium channel trafficking and expression of synaptic proteins. Moreover, α2δ-2 and α2δ-3 with mutated metal ion-dependent adhesion sites can fully rescue presynaptic synapsin expression but only partially calcium channel trafficking, suggesting that the regulatory role of α2δ subunits is independent from its role as a calcium channel subunit. Our findings influence the current view on excitatory synapse formation. First, our study suggests that postsynaptic differentiation is secondary to presynaptic differentiation. Second, the dependence of presynaptic differentiation on α2δ implicates α2δ subunits as potential nucleation points for the organization of synapses. Finally, our results suggest that α2δ subunits act as transsynaptic organizers of glutamatergic synapses, thereby aligning the synaptic active zone with the postsynaptic density

Cell surface localised Hsp70 is a cancer specific regulator of clathrin-independent endocytosis.

The stress inducible heat shock protein 70 (Hsp70) is present specifically on the tumour cell surface yet without a pro-tumour function revealed. We show here that cell surface localised Hsp70 (sHsp70) supports clathrin-independent endocytosis (CIE) in melanoma models. Remarkably, ability of Hsp70 to cluster on lipid rafts in vitro correlated with larger nano-domain sizes of sHsp70 in high sHsp70 expressing cell membranes. Interfering with Hsp70 oligomerisation impaired sHsp70-mediated facilitation of endocytosis. Altogether our findings suggest that a sub-fraction of sHsp70 co-localising with lipid rafts enhances CIE through oligomerisation and clustering. Targeting or utilising this tumour specific mechanism may represent an additional benefit for anti-cancer therapy

The Complex Function of Hsp70 in Metastatic Cancer

Elevated expression of the inducible heat shock protein 70 (Hsp70) is known to correlate with poor prognosis in many cancers. Hsp70 confers survival advantage as well as resistance to chemotherapeutic agents, and promotes tumor cell invasion. At the same time, tumor-derived extracellular Hsp70 has been recognized as a “chaperokine”, activating antitumor immunity. In this review we discuss localization dependent functions of Hsp70 in the context of invasive cancer. Understanding the molecular principles of metastasis formation steps, as well as interactions of the tumor cells with the microenvironment and the immune system is essential for fighting metastatic cancer. Although Hsp70 has been implicated in different steps of the metastatic process, the exact mechanisms of its action remain to be explored. Known and potential functions of Hsp70 in controlling or modulating of invasion and metastasis are discussed

Lck mediates signal transmission from CD59 to the TCR/CD3 pathway in Jurkat T cells.

The glycosylphosphatidylinositol (GPI)-anchored molecule CD59 has been implicated in the modulation of T cell responses, but the underlying molecular mechanism of CD59 influencing T cell signaling remained unclear. Here we analyzed Jurkat T cells stimulated via anti-CD3ε- or anti-CD59-coated surfaces, using time-resolved single-cell Ca(2+) imaging as a read-out for stimulation. This analysis revealed a heterogeneous Ca(2+) response of the cell population in a stimulus-dependent manner. Further analysis of T cell receptor (TCR)/CD3 deficient or overexpressing cells showed that CD59-mediated signaling is strongly dependent on TCR/CD3 surface expression. In protein co-patterning and fluorescence recovery after photobleaching experiments no direct physical interaction was observed between CD59 and CD3 at the plasma membrane upon anti-CD59 stimulation. However, siRNA-mediated protein knock-downs of downstream signaling molecules revealed that the Src family kinase Lck and the adaptor molecule linker of activated T cells (LAT) are essential for both signaling pathways. Furthermore, flow cytometry measurements showed that knock-down of Lck accelerates CD3 re-expression at the cell surface after anti-CD59 stimulation similar to what has been observed upon direct TCR/CD3 stimulation. Finally, physically linking Lck to CD3ζ completely abolished CD59-triggered Ca(2+) signaling, while signaling was still functional upon direct TCR/CD3 stimulation. Altogether, we demonstrate that Lck mediates signal transmission from CD59 to the TCR/CD3 pathway in Jurkat T cells, and propose that CD59 may act via Lck to modulate T cell responses

TCR/CD3- and CD59-mediated Ca²⁺ signaling are dependent on Lck and LAT.

<p>Cluster distribution of Ca²⁺ time traces in differently treated cells upon anti-CD3 and anti-CD59 stimulation. Each color represents the percentage of a certain Ca²⁺ time trace cluster in the cell population. (A) Ca²⁺ measurements were performed with WT cells transiently transfected with negative control siRNA (siNeg), Lck-specific siRNA (siLck), WT cells treated with 10 µM PP2 (PP2), and Lck-deficient J.CaM1.6 cells. Clusters representing Ca²⁺ release patterns are framed in black (89.4±10.1%, 76.5±10.4%, 28.3±39.1%, and 23.4±9.9% upon anti-CD3 stimulation, 36.9±13.2%, 12.7±6.1%, 2.6±2.0%, and 1.2±1.2% upon anti-CD59 stimulation for siNeg, siLck, PP2-treated, and J.CaM1.6 cells, respectively). Mean values from at least two independent experiments, each with three technical replicates, are shown (n ≥ 204 per cell type and condition). (B) Cluster analysis of Ca²⁺ time traces in WT cells transiently transfected with negative control siRNA (siNeg), LAT-specific siRNA (siLAT), and LAT-deficient J.CaM2.5 cells. Clusters representing Ca²⁺ release patterns are framed in black (89.4±10.1%, 73.5±5.8%, and 3.0±2.3% upon anti-CD3 stimulation, 36.9±13.2%, 13.0±6.6%, and 1.7±1.6% upon anti-CD59 stimulation for siNeg, siLAT, and J.CaM2.5 cells, respectively). Mean values from at least three independent experiments, each with three technical replicates, are shown (n ≥ 208 per cell type and condition). Multiple comparison tests for the fractions showing Ca²⁺ release patterns in (A) and (B) were assessed by one-way ANOVA, significances are shown where applicable, ** p < 0.01, *** p < 0.001. (C) Knock-down of target proteins was tested by Western blotting. 48 h after transfection cell lysates from siRNA treated cells were probed with anti-Lck, anti-LAT, and anti-β-actin. J.CaM1.6 cells, J.CaM2.5 cells, and WT cells served as controls.</p

Single-cell Ca²⁺ measurements reveal differential heterogeneity upon anti-CD3 and anti-CD59 stimulation.

<p>Jurkat cells (WT) were loaded with Indo-1/AM followed by stimulation with anti-CD3-, anti-CD59-, or anti-CD71-coated surfaces. Individual Ca²⁺ time traces were measured for 200 s after identification of initial cell-surface contact as described in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0085934#pone-0085934-g001" target="_blank">Figure 1S</a>. (A) Box plots of individual Ca²⁺ time traces generated by analysis of the whole cell population upon anti-CD3, anti-CD59, and anti-CD71 stimulation (black line  =  median, grey area  = 50% of Ca²⁺ time traces, dotted lines  =  75% of Ca²⁺ time traces). (B) Individual Ca²⁺ time traces from single-cell measurements were grouped into 11 clusters by affinity propagation clustering as described in Materials and Methods. Each plot shows the respective Ca²⁺ time traces for a cluster, an exemplar trace for each cluster is shown in black. Clusters representing Ca²⁺ release patterns are framed in black. (C) Stimulus-dependent cluster distribution upon anti-CD3, anti-CD59, and anti-CD71 stimulation in WT cells is shown by stacked bar plots. Each color represents the percentage of a certain Ca²⁺ time trace cluster in the cell population. Clusters representing Ca²⁺ release patterns are framed in black (88.5±4.9%, 31.8±12.0%, and 5.5±5.3% for anti-CD3, anti-CD59, and anti-CD71 stimulation, respectively). Mean values from at least three independent experiments, each with three technical replicates, are shown (n ≥ 249 per stimulatory condition). (D) CD3ε and CD59 surface expression levels in WT cells. WT cells were surface stained with Alexa Fluor 647-conjugated anti-CD3ε and FITC-conjugated anti-CD59 or isotype controls and analyzed by flow cytometry. Live cells were gated based on the Forward Scatter and Side Scatter profiles and propidium iodide exclusion. A representative dot plot of four technical replicates is shown.</p

Lck expression and anti-CD59 stimulation influence CD3 surface expression.

<p>WT cells were transfected with negative control siRNA (siNeg) or Lck-specific siRNA (siLck) and experiments were performed 48 h after transfection. (A) Testing of Lck knock-down efficiency. Cell lysates were probed by Western blotting for Lck expression and β-actin as a control. (B) Efficiency of Lck knock-down tested by ensemble Ca²⁺ measurements. siLck and siNeg cells were loaded with Indo-1/AM. Cells were incubated with anti-CD59 mAb or anti-IgG2a for 5 min at 37 °C. For antibody cross-linking, goat anti-mouse F(ab’)₂ was added to samples and Ca²⁺ mobilization of the whole cell population was measured by a microplate reader. For testing TCR surface expression levels, cells were stimulated with anti-CD59 or anti-IgG2a, as isotype control, followed by incubation with goat anti-mouse F(ab’)₂ for (C) 1 h or (D) 15 h at 37°C. Cells were surface stained at 4 °C with FITC-conjugated anti-CD3ε or isotype control and analyzed by flow cytometry. Live cells were gated based on the Forward Scatter and Side Scatter profiles and propidium iodide exclusion. Fluorescence values displayed are isotype control corrected. Representative results of two separate experiments are shown (mean ±SD, n = 4). Multiple comparison tests were assessed by one-way ANOVA, significances are shown where applicable, ***p < 0.001.</p

CD59-mediated Ca²⁺ signaling requires CD3ζ expression.

<p>(A) Cluster distribution of Ca²⁺ time traces in WT and TCR^high cells is shown upon anti-CD3 and anti-CD59 stimulation. Each color represents the percentage of a certain Ca²⁺ time trace cluster in the cell population. Clusters representing Ca²⁺ release patterns are framed in black (91.4±2.1% and 92.9±3.9% upon anti-CD3 stimulation, 34.4±16.3% and 72.0±16.6% upon anti-CD59 stimulation in WT and TCR^high cells, respectively). Mean values from two independent experiments, each with three technical replicates are shown (n ≥ 167 per cell type and condition). (B) Cluster distribution of Ca²⁺ time traces in WT, TCR^-, and cells expressing CD8-ζ fusion protein is shown upon anti-CD3 and anti-CD59 stimulation. Each color represents the percentage of a certain Ca²⁺ time trace cluster in the cell population. Clusters representing Ca²⁺ release patterns are framed in black (90.1±1.4%, 1.9±0.7% and 7.4±2.3% upon anti-CD3 stimulation, 25.2±6.8%, 1.6±0.4% and 13.4±1.6% upon anti-CD59 stimulation for WT, TCR^-, and CD8-ζ cells, respectively). Mean values from four independent experiments, each with three technical replicates, are shown (n ≥ 343 per cell type and condition). Multiple comparison tests for the fractions showing Ca²⁺ release patterns in (A) and (B) were assessed by one-way ANOVA, significances are shown where applicable, * p < 0.05, ** p < 0.01, *** p < 0.001.</p

Reconstitution of Lck by forced interaction of CD3ζ and Lck facilitates TCR/CD3- but not CD59-mediated Ca²⁺ signaling.

<p>(A) Lck expression levels in WT cells, J.CaM1.6 cells and J.CaM1.6 cells expressing mEGFP-tagged Lck fused to CD3ζ (CD3ζ-Lck) were tested by Western blotting, the same blot was reprobed using anti-β-actin as a control. (B) Plasma membrane localization of CD3ζ-Lck-mEGFP in J.CaM1.6 cells was imaged by fluorescence microscopy. CD3ζ-Lck-mEGFP fluorescence and a bright field (BF) image of a transfected J.CaM1.6 cell are shown (scale bars  =  10 µm). (C) Cluster distribution of Ca²⁺ time traces in Lck-deficient J.CaM1.6 cells and J.CaM1.6 cells stably expressing mEGFP-tagged Lck fused to CD3ζ (CD3ζ-Lck) is shown upon anti-CD3 or anti-CD59 stimulation. Each color represents the percentage of a certain Ca²⁺ time trace cluster in the cell population. Clusters representing Ca²⁺ release patterns are framed in black (35.7±4.9% and 71.8±2.3% upon anti-CD3 stimulation, 1.4±2.3% and 1.0±1.3% upon anti-CD59 stimulation for J.CaM1.6 and CD3ζ-Lck cells, respectively). Mean values from three independent experiments, each with three technical replicates, are shown (n ≥ 323 per cell type and condition). Multiple comparison tests for the fractions showing Ca²⁺ release patterns were assessed by one-way ANOVA, significances are shown where applicable, ***p < 0.001.</p