Phosphodiesterases 4B and 4D Differentially Regulate cAMP Signaling in Calcium Handling Microdomains of Mouse Hearts

The ubiquitous second messenger 3',5'-cyclic adenosine monophosphate (cAMP) regulates cardiac excitation-contraction coupling (ECC) by signaling in discrete subcellular microdomains. Phosphodiesterase subfamilies 4B and 4D are critically involved in the regulation of cAMP signaling in mammalian cardiomyocytes. Alterations of PDE4 activity in human hearts has been shown to result in arrhythmias and heart failure. Here, we sought to systematically investigate specific roles of PDE4B and PDE4D in the regulation of cAMP dynamics in three distinct subcellular microdomains, one of them located at the caveolin-rich plasma membrane which harbors the L-type calcium channels (LTCCs), as well as at two sarco/endoplasmic reticulum (SR) microdomains centered around SR Ca2+-ATPase (SERCA2a) and cardiac ryanodine receptor type 2 (RyR2). Transgenic mice expressing Förster Resonance Energy Transfer (FRET)-based cAMP-specific biosensors targeted to caveolin-rich plasma membrane, SERCA2a and RyR2 microdomains were crossed to PDE4B-KO and PDE4D-KO mice. Direct analysis of the specific effects of both PDE4 subfamilies on local cAMP dynamics was performed using FRET imaging. Our data demonstrate that all three microdomains are differentially regulated by these PDE4 subfamilies. Whereas both are involved in cAMP regulation at the caveolin-rich plasma membrane, there are clearly two distinct cAMP microdomains at the SR formed around RyR2 and SERCA2a, which are preferentially controlled by PDE4B and PDE4D, respectively. This correlates with local cAMP-dependent protein kinase (PKA) substrate phosphorylation and arrhythmia susceptibility. Immunoprecipitation assays confirmed that PDE4B is associated with RyR2 along with PDE4D. Stimulated Emission Depletion (STED) microscopy of immunostained cardiomyocytes suggested possible co-localization of PDE4B with both sarcolemmal and RyR2 microdomains. In conclusion, our functional approach could show that both PDE4B and PDE4D can differentially regulate cardiac cAMP microdomains associated with calcium homeostasis. PDE4B controls cAMP dynamics in both caveolin-rich plasma membrane and RyR2 vicinity. Interestingly, PDE4B is the major regulator of the RyR2 microdomain, as opposed to SERCA2a vicinity, which is predominantly under PDE4D control, suggesting a more complex regulatory pattern than previously thought, with multiple PDEs acting at the same location.

Differences of the tumour cell glycocalyx affect binding of capsaicin-loaded chitosan nanocapsules

The glycocalyx regulates the interaction of mammalian cells with extracellular molecules, such as cytokines. However, it is unknown to which extend the glycocalyx of distinct cancer cells control the binding and uptake of nanoparticles. In the present study, exome sequencing data of cancer patients and analysis of distinct melanoma and bladder cancer cell lines suggested differences in cancer cell-exposed glycocalyx components such as heparan sulphate. Our data indicate that glycocalyx differences affected the binding of cationic chitosan nanocapsules (Chi-NCs). The pronounced glycocalyx of bladder cancer cells enhanced the internalisation of nanoencapsulated capsaicin. Consequently, capsaicin induced apoptosis in the cancer cells, but not in the less glycosylated benign urothelial cells. Moreover, we measured counterion condensation on highly negatively charged heparan sulphate chains. Counterion condensation triggered a cooperative binding of Chi-NCs, characterised by a weak binding rate at low Chi-NC doses and a strongly increased binding rate at high Chi-NC concentrations. Our results indicate that the glycocalyx of tumour cells controls the binding and biological activity of nanoparticles. This has to be considered for the design of tumour cell directed nanocarriers to improve the delivery of cytotoxic drugs. Differential nanoparticle binding may also be useful to discriminate tumour cells from healthy cells.

Visualizing Adhesion Formation in Cells by Means of Advanced Spinning Disk-Total Internal Reflection Fluorescence Microscopy

In living cells, processes such as adhesion formation involve extensive structural changes in the plasma membrane and the cell interior. In order to visualize these highly dynamic events, two complementary light microscopy techniques that allow fast imaging of live samples were combined: spinning disk microscopy (SD) for fast and high-resolution volume recording and total internal reflection fluorescence (TIRF) microscopy for precise localization and visualization of the plasma membrane. A comprehensive and complete imaging protocol will be shown for guiding through sample preparation, microscope calibration, image formation and acquisition, resulting in multi-color SD-TIRF live imaging series with high spatio-temporal resolution. All necessary image post-processing steps to generate multi-dimensional live imaging datasets, i.e. registration and combination of the individual channels, are provided in a self-written macro for the open source software ImageJ. The imaging of fluorescent proteins during initiation and maturation of adhesion complexes, as well as the formation of the actin cytoskeletal network, was used as a proof of principle for this novel approach. The combination of high resolution 3D microscopy and TIRF provided a detailed description of these complex processes within the cellular environment and, at the same time, precise localization of the membrane-associated molecules detected with a high signal-to-background ratio.

ORAI1, STIM1/2, and RYR1 shape subsecond Ca2+ microdomains upon T cell activation

The earliest intracellular signals that occur after T cell activation are local, subsecond Ca²⁺ microdomains. Here, we identified a Ca²⁺ entry component involved in Ca²⁺ microdomain formation in both unstimulated and stimulated T cells. In unstimulated T cells, spontaneously generated small Ca²⁺ microdomains required ORAI1, STIM1, and STIM2. Super-resolution microscopy of unstimulated T cells identified a circular subplasmalemmal region with a diameter of about 300 nm with preformed patches of colocalized ORAI1, ryanodine receptors (RYRs), and STIM1. Preformed complexes of STIM1 and ORAI1 in unstimulated cells were confirmed by coimmunoprecipitation and Förster resonance energy transfer studies. Furthermore, within the first second after T cell receptor (TCR) stimulation, the number of Ca²⁺ microdomains increased in the subplasmalemmal space, an effect that required ORAI1, STIM2, RYR1, and the Ca²⁺ mobilizing second messenger NAADP (nicotinic acid adenine dinucleotide phosphate). These results indicate that preformed clusters of STIM and ORAI1 enable local Ca²⁺ entry events in unstimulated cells. Upon TCR activation, NAADP-evoked Ca²⁺ release through RYR1, in coordination with Ca²⁺ entry through ORAI1 and STIM, rapidly increases the number of Ca²⁺ microdomains, thereby initiating spread of Ca²⁺ signals deeper into the cytoplasm to promote full T cell activation.

Abstract 3364: RAI2 maintains genome stability of breast cancer cells

Genome instability is considered as an enabling hallmark of cancer formation and progression as well as a source of intratumoral heterogeneity. Retinoid acid-induced protein 2 (RAI2) was initially identified as a new metastasis-associated protein especially related with the presence of disseminated tumor cells in the bone marrow of breast cancer patients. In primary breast tumors low RAI2 gene expression correlates with mutant TP53 status and moreover, RAI2 depleted luminal breast cancer cell lines show decreased expression of key regulators of G2/M transition. We hypothesize that RAI2 has an influence on mitotic fidelity and is involved in maintaining genomic stability in breast cancer cell lines. To analyze consequences of RAI2 depletion on mitotic fidelity in luminal breast cancer cells, we followed mitotic cells by live cell imaging and analyzed chromosomal segregation by immune fluorescence staining and karyotyped RAI2 depleted cells. For analysis of cell cycle dependent RAI2 protein expression we executed Western blot analysis after cell synchronization and treatment with genotoxic agents. Localization of ectopically expressed HA-tagged RAI2 protein was analyzed in breast cancer cells treated with genotoxic agents using immune fluorescence staining. Finally, we performed in silico validation in different breast cancer data sets. Our findings show that RAI2 depletion in luminal breast cancer cell lines leads to increased chromosomal instability, associated with loss of acentric chromosomal fragments during ana- and metaphase as well as a prolonged mitosis. Furthermore, karyotyping revealed a significant reduction of chromosome number from 78 to 73 in RAI2 depleted cells. In synchronized parental breast cancer cells we could show an increased RAI2 expression during S/G2 phase compared to unsynchronized cells. Interestingly, genotoxic treatment resulted in a significant upregulation of RAI2 protein, which can also be seen as an increased formation of nuclear speckles (p<0.05). Finally, in silico analysis of breast cancer data sets revealed a significant correlation between low RAI2 expression and an expression signature of chromosomal instability (p>0,001). Our findings show that RAI2 depletion in luminal breast cancer cell lines leads to increased chromosomal instability. Our results show that RAI2 depletion causes pre-mitotic DNA lesions. Moreover, in breast cancer patients a low RAI2 expression was significantly associated with chromosomal instability. We conclude that the RAI2 protein preserves mitotic fidelity and chromosomal integrity in luminal breast cancer cell lines by affecting either the DNA damage response or DNA replication during S- and/or G2-phase.

Citation Format: Lena Boettcher, Bernd Zobiak, Kerstin Borgmann, Alexandra Zielinski, Antonio Virgilio Failla, Klaus Pantel, Harriet Wikman, Stefan Werner. RAI2 maintains genome stability of breast cancer cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 3364.

Advanced spinning disk-TIRF microscopy for faster imaging of the cell interior and the plasma membrane

Understanding the cellular processes that occur between the cytosol and the plasma membrane is an important task for biological research. Till now, however, it was not possible to combine fast and high-resolution imaging of both the isolated plasma membrane and the surrounding intracellular volume. Here, we demonstrate the combination of fast high-resolution spinning disk (SD) and total internal reflection fluorescence (TIRF) microscopy for specific imaging of the plasma membrane. A customised SD-TIRF microscope was used with specific design of the light paths that allowed, for the first time, live SD-TIRF experiments at high acquisition rates. A series of experiments is shown to demonstrate the feasibility and performance of our setup.

Reelin and cofilin cooperate during the migration of cortical neurons: a quantitative morphological analysis

In reeler mutant mice, which are deficient in reelin (Reln), the lamination of the cerebral cortex is disrupted. Reelin signaling induces phosphorylation of LIM kinase 1, which phosphorylates the actin-depolymerizing protein cofilin in migrating neurons. Conditional cofilin mutants show neuronal migration defects. Thus, both reelin and cofilin are indispensable during cortical development. To analyze the effects of cofilin phosphorylation on neuronal migration we used in utero electroporation to transfect E14.5 wild-type cortical neurons with pCAG-EGFP plasmids encoding either a nonphosphorylatable form of cofilin 1 (cofilin(S3A)), a pseudophosphorylated form (cofilin(S3E)) or wild-type cofilin 1 (cofilin(WT)). Wild-type controls and reeler neurons were transfected with pCAG-EGFP. Real-time microscopy and histological analyses revealed that overexpression of cofilin(WT) and both phosphomutants induced migration defects and morphological abnormalities of cortical neurons. Of note, reeler neurons and cofilin(S3A)- and cofilin(S3E)-transfected neurons showed aberrant backward migration towards the ventricular zone. Overexpression of cofilin(S3E), the pseudophosphorylated form, partially rescued the migration defect of reeler neurons, as did overexpression of Limk1. Collectively, the results indicate that reelin and cofilin cooperate in controlling cytoskeletal dynamics during neuronal migration.

Analysis of Yersinia enterocolitica Effector Translocation into Host Cells Using Beta-lactamase Effector Fusions

Many gram-negative bacteria including pathogenic Yersinia spp. employ type III secretion systems to translocate effector proteins into eukaryotic target cells. Inside the host cell the effector proteins manipulate cellular functions to the benefit of the bacteria. To better understand the control of type III secretion during host cell interaction, sensitive and accurate assays to measure translocation are required. We here describe the application of an assay based on the fusion of a Yersinia enterocolitica effector protein fragment (Yersinia outer protein; YopE) with TEM-1 beta-lactamase for quantitative analysis of translocation. The assay relies on cleavage of a cell permeant FRET dye (CCF4/AM) by translocated beta-lactamase fusion. After cleavage of the cephalosporin core of CCF4 by the beta-lactamase, FRET from coumarin to fluorescein is disrupted and excitation of the coumarin moiety leads to blue fluorescence emission. Different applications of this method have been described in the literature highlighting its versatility. The method allows for analysis of translocation in vitro and also in in vivo, e.g., in a mouse model. Detection of the fluorescence signals can be performed using plate readers, FACS analysis or fluorescence microscopy. In the setup described here, in vitro translocation of effector fusions into HeLa cells by different Yersinia mutants is monitored by laser scanning microscopy. Recording intracellular conversion of the FRET reporter by the beta-lactamase effector fusion in real-time provides robust quantitative results. We here show exemplary data, demonstrating increased translocation by a Y. enterocolitica YopE mutant compared to the wild type strain.

An 18 kDa scaffold protein is critical for Staphylococcus epidermidis biofilm formation

Virulence of the nosocomial pathogen Staphylococcus epidermidis is crucially linked to formation of adherent biofilms on artificial surfaces. Biofilm assembly is significantly fostered by production of a bacteria derived extracellular matrix. However, the matrix composition, spatial organization, and relevance of specific molecular interactions for integration of bacterial cells into the multilayered biofilm community are not fully understood. Here we report on the function of novel 18 kDa Small basic protein (Sbp) that was isolated from S. epidermidis biofilm matrix preparations by an affinity chromatographic approach. Sbp accumulates within the biofilm matrix, being preferentially deposited at the biofilm–substratum interface. Analysis of Sbp-negative S. epidermidis mutants demonstrated the importance of Sbp for sustained colonization of abiotic surfaces, but also epithelial cells. In addition, Sbp promotes assembly of S. epidermidis cell aggregates and establishment of multilayered biofilms by influencing polysaccharide intercellular-adhesin (PIA) and accumulation associated protein (Aap) mediated intercellular aggregation. While inactivation of Sbp indirectly resulted in reduced PIA-synthesis and biofilm formation, Sbp serves as an essential ligand during Aap domain-B mediated biofilm accumulation. Our data support the conclusion that Sbp serves as an S. epidermidis biofilm scaffold protein that significantly contributes to key steps of surface colonization. Sbp-negative S. epidermidis mutants showed no attenuated virulence in a mouse catheter infection model. Nevertheless, the high prevalence of sbp in commensal and invasive S. epidermidis populations suggests that Sbp plays a significant role as a co-factor during both multi-factorial commensal colonization and infection of artificial surfaces.

Biofilm formation is a key phenotype allowing the otherwise harmless skin commensal S. epidermidis to establish chronic implant-associated infections, affecting millions of patients worldwide. S. epidermidis biofilm assembly relies on the production of an extracellular matrix that serves as glue to stabilize the multilayered bacterial architecture. Here we identified novel 18 kDa Small basic protein (Sbp) as a key component of the extracellular matrix that promotes pivotal steps of bacterial biofilm formation in vitro. Importantly, Sbp is deposited specifically at the interface between biofilm and substrate, as well as in larger humps interspersed within the bacterial cell architecture, thereby forming a proteinaceous biofilm scaffold. This localization enables Sbp to foster stable S. epidermidis interactions with an artificial surface and also contributes to S. epidermidis cell aggregation mechanisms, i.e., polysaccharide intercellular adhesin (PIA) and accumulation associated protein (Aap). In fact, by demonstrating direct Sbp-Aap interactions we provide the first evidence supporting the idea that specific molecular interactions between S. epidermidis and matrix components are involved in S. epidermidis biofilm accumulation. In conclusion, we here show that Sbp promotes key phenotypic features important for S. epidermidis to evolve as an opportunistic pathogen.

Organelle segregation into Plasmodium liver stage merozoites

The liver stage of the Plasmodium parasite remains one of the most promising targets for intervention against malaria as it is clinically silent, precedes the symptomatic blood stage and represents a bottleneck in the parasite life cycle. However, many aspects of the development of the parasite during this stage are far from understood. During the liver stage, the parasite undergoes extensive replication, forming tens of thousands of infectious merozoites from each invading sporozoite. This implies a very efficient and accurate process of cytokinesis and thus also of organelle development and segregation. We have generated for the first time Plasmodium berghei double-fluorescent parasite lines, allowing visualization of the apicoplast, mitochondria and nuclei in live liver stage parasites. Using these we have seen that in parallel with nuclear division, the apicoplast and mitochondrion become two extensively branched and intertwining structures. The organelles then undergo impressive morphological and positional changes prior to cell division. To form merozoites, the parasite undergoes cytokinesis and the complex process of organelle development and segregation into the forming daughter merozoites could be analysed in detail using the newly generated transgenic parasites.

Differentially expressed cortical genes contribute to perivascular deposition in transgenic mice with inducible neuron‐specific expression of TGF‐β1

In the brain the expression of transforming growth factor beta1 (TGF-beta1) is involved both in neuroprotective and neurodegenerative processes. Recently, we have established a transgenic mouse model with inducible neuron-specific expression of TGF-beta1 based on the tetracycline-regulated gene expression system. A long-term expression of TGF-beta1 results in persisting perivascular thioflavin-positive depositions, which did not disappear even though the transgene synthesis was repressed completely by administration of doxycycline. Formation and composition of these depositions are hardly elucidated. The aim of this study was to identify TGF-beta1 responding genes potentially participating in forming these depositions. To address this problem we have compared the cortical mRNA expression pattern of TGF-beta1 expressing mice with mice impeded to express the transgenic protein using oligonucleotide microarray analysis. Differential gene expression was further characterized by quantitative real-time reverse transcription-polymerase chain reaction including animals, where the long-lasting TGF-beta1 expression was repressed. While no change of amyloid precursor protein RNA expression level was detected, various genes strongly involved in calcium homeostasis, tissue mineralization or vascular calcification were identified differentially expressed. It is suggested, that these genes might contribute to the perivascular depositions in the TGF-beta1 expressing mice.