This work has been supported by grants of the Deutsche Forschungsgemeinschaft (DFG) BL567/3-1 and the Friedrich-Baur-Stiftung. We would like to thank Roger Y. Tsien, Howard Hughes Medical Institute Laboratories at the University of California, San Diego for providing us with Tag-RFP-T2. We thankfully acknowledge David Baltimore, California Institute of Technology, Pasadena, and Didier Trono, University of Geneva, Geneva, for providing us the lentiviral plasmids FUGW, and psPAX2/pMD2.G.

<ol>
	<li>Berridge, M. J., Bootman, M. D., Roderick, H. L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Calcium+signalling:+dynamics,+homeostasis+and+remodelling.">Calcium signalling: dynamics, homeostasis and remodelling.</a> Nature reviews. Molecular Cell Biology. 4, 517-529 (2003).</li><li>Berridge, M. J., Lipp, P., Bootman, M. D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+versatility+and+universality+of+calcium+signalling.">The versatility and universality of calcium signalling.</a> Nature reviews. Molecular Cell Biology. 1, 11-21 (2000).</li><li>Agulhon, C., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=What+is+the+role+of+astrocyte+calcium+in+neurophysiology.">What is the role of astrocyte calcium in neurophysiology.</a> Neuron. 59, 932-946 (2008).</li><li>Augustine, G. J., Santamaria, F., Tanaka, K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Local+calcium+signaling+in+neurons.">Local calcium signaling in neurons.</a> Neuron. 40, 331-346 (2003).</li><li>Berridge, M. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Neuronal+calcium+signaling.">Neuronal calcium signaling.</a> Neuron. 21, 13-26 (1998).</li><li>Grienberger, C., Konnerth, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Imaging+calcium+in+neurons.">Imaging calcium in neurons.</a> Neuron. 73, 862-885 (2012).</li><li>Neher, E., Sakaba, T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Multiple+roles+of+calcium+ions+in+the+regulation+of+neurotransmitter+release.">Multiple roles of calcium ions in the regulation of neurotransmitter release.</a> Neuron. 59, 861-872 (2008).</li><li>Jaepel, J., Blum, R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Capturing+ER+calcium+dynamics.">Capturing ER calcium dynamics.</a> European Journal of Cell Biology. 90, 613-619 (2011).</li><li>Verkhratsky, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Physiology+and+pathophysiology+of+the+calcium+store+in+the+endoplasmic+reticulum+of+neurons.">Physiology and pathophysiology of the calcium store in the endoplasmic reticulum of neurons.</a> Physiological Reviews. 85, 201-279 (2005).</li><li>Burdakov, D., Petersen, O. H., Verkhratsky, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Intraluminal+calcium+as+a+primary+regulator+of+endoplasmic+reticulum+function.">Intraluminal calcium as a primary regulator of endoplasmic reticulum function.</a> Cell Calcium. 38, 303-310 (2005).</li><li>Streb, H., Irvine, R. F., Berridge, M. J., Schulz, I. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Release+of+Ca2++from+a+nonmitochondrial+intracellular+store+in+pancreatic+acinar+cells+by+inositol-1,4,5-trisphosphate.">Release of Ca2+ from a nonmitochondrial intracellular store in pancreatic acinar cells by inositol-1,4,5-trisphosphate.</a> Nature. 306, 67-69 (1983).</li><li>Choi, J. H., Ryu, S. H., Suh, P. G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=On/off-regulation+of+phospholipase+C-gamma+1-mediated+signal+transduction.">On/off-regulation of phospholipase C-gamma 1-mediated signal transduction.</a> Advances in Enzyme Regulation. 47, 104-116 (2007).</li><li>Clapham, D. E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=TRP+channels+as+cellular+sensors.">TRP channels as cellular sensors.</a> Nature. 426, 517-524 (2003).</li><li>Cahalan, M. D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=STIMulating+store-operated+Ca(2+)+entry.">STIMulating store-operated Ca(2+) entry.</a> Nature Cell Biology. 11, 669-677 (2009).</li><li>Soboloff, J., Madesh, M., Gill, D. L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Sensing+cellular+stress+through+STIM+proteins.">Sensing cellular stress through STIM proteins.</a> Nature Chemical Biology. 7, 488-492 (2011).</li><li>Erdmann, F., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Interaction+of+calmodulin+with+Sec61alpha+limits+Ca2++leakage+from+the+endoplasmic+reticulum.">Interaction of calmodulin with Sec61alpha limits Ca2+ leakage from the endoplasmic reticulum.</a> The EMBO Journal. 30, 17-31 (2011).</li><li>Sch&#228;uble, N., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=BiP-mediated+closing+of+the+Sec61+channel+limits+Ca(2+)+leakage+from+the+ER.">BiP-mediated closing of the Sec61 channel limits Ca(2+) leakage from the ER.</a> The EMBO Journal. , (2012).</li><li>Park, M. K., Choi, Y. M., Kang, Y. K., Petersen, O. H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+endoplasmic+reticulum+as+an+integrator+of+multiple+dendritic+events.">The endoplasmic reticulum as an integrator of multiple dendritic events.</a> The Neuroscientist : a review journal bringing neurobiology, neurology and psychiatry. 14, 68-77 (2008).</li><li>Cooney, J. R., Hurlburt, J. L., Selig, D. K., Harris, K. M., Fiala, J. C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Endosomal+compartments+serve+multiple+hippocampal+dendritic+spines+from+a+widespread+rather+than+a+local+store+of+recycling+membrane.">Endosomal compartments serve multiple hippocampal dendritic spines from a widespread rather than a local store of recycling membrane.</a> The Journal of Neuroscience : the official journal of the Society for Neuroscience. 22, 2215-2224 (2002).</li><li>Holbro, N., Grunditz, A., Oertner, T. G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Differential+distribution+of+endoplasmic+reticulum+controls+metabotropic+signaling+and+plasticity+at+hippocampal+synapses.">Differential distribution of endoplasmic reticulum controls metabotropic signaling and plasticity at hippocampal synapses.</a> Proceedings of the National Academy of Sciences of the United States of America. 106, 15055-15060 (2009).</li><li>Terasaki, M., Slater, N. T., Fein, A., Schmidek, A., Reese, T. S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Continuous+network+of+endoplasmic+reticulum+in+cerebellar+Purkinje+neurons.">Continuous network of endoplasmic reticulum in cerebellar Purkinje neurons.</a> Proceedings of the National Academy of Sciences of the United States of America. 91, 7510-7514 (1994).</li><li>Barreto-Chang, O. L., Dolmetsch, R. E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Calcium+imaging+of+cortical+neurons+using+Fura-2+AM.">Calcium imaging of cortical neurons using Fura-2 AM.</a> J. Vis. Exp. (23), e1067 (2009).</li><li>Lang, S., Sch&#228;uble, N., Cavalie, A., Zimmermann, R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Live+cell+calcium+imaging+combined+with+siRNA+mediated+gene+silencing+identifies+Ca(2)(+)+leak+channels+in+the+ER+membrane+and+their+regulatory+mechanisms.">Live cell calcium imaging combined with siRNA mediated gene silencing identifies Ca(2)(+) leak channels in the ER membrane and their regulatory mechanisms.</a> J. Vis. Exp. (53), e2730 (2011).</li><li>Rudolf, R., Mongillo, M., Rizzuto, R., Pozzan, T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Looking+forward+to+seeing+calcium.">Looking forward to seeing calcium.</a> Nature reviews. Molecular Cell Biology. 4, 579-586 (2003).</li><li>Mank, M., Griesbeck, O. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Genetically+encoded+calcium+indicators.">Genetically encoded calcium indicators.</a> Chemical Reviews. 108, 1550-1564 (2008).</li><li>Miyawaki, A., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Fluorescent+indicators+for+Ca2++based+on+green+fluorescent+proteins+and+calmodulin.">Fluorescent indicators for Ca2+ based on green fluorescent proteins and calmodulin.</a> Nature. 388, 882-887 (1997).</li><li>Montero, M., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Monitoring+dynamic+changes+in+free+Ca2++concentration+in+the+endoplasmic+reticulum+of+intact+cells.">Monitoring dynamic changes in free Ca2+ concentration in the endoplasmic reticulum of intact cells.</a> The EMBO Journal. 14, 5467-5475 (1995).</li><li>Griesbeck, O., Baird, G. S., Campbell, R. E., Zacharias, D. A., Tsien, R. Y. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Reducing+the+environmental+sensitivity+of+yellow+fluorescent+protein.+Mechanism+and+applications.">Reducing the environmental sensitivity of yellow fluorescent protein. Mechanism and applications.</a> The Journal of Biological Chemistry. 276, 29188-29194 (2001).</li><li>Ishii, K., Hirose, K., Iino, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Ca2++shuttling+between+endoplasmic+reticulum+and+mitochondria+underlying+Ca2++oscillations.">Ca2+ shuttling between endoplasmic reticulum and mitochondria underlying Ca2+ oscillations.</a> EMBO Reports. 7, 390-396 (2006).</li><li>Palmer, A. E., Jin, C., Reed, J. C., Tsien, R. Y. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Bcl-2-mediated+alterations+in+endoplasmic+reticulum+Ca2++analyzed+with+an+improved+genetically+encoded+fluorescent+sensor.">Bcl-2-mediated alterations in endoplasmic reticulum Ca2+ analyzed with an improved genetically encoded fluorescent sensor.</a> Proceedings of the National Academy of Sciences of the United States of America. 101, 17404-17409 (2004).</li><li>Hofer, A. M., Machen, T. E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Technique+for+in+situ+measurement+of+calcium+in+intracellular+inositol+1,4,5-trisphosphate-sensitive+stores+using+the+fluorescent+indicator+mag-fura-2.">Technique for in situ measurement of calcium in intracellular inositol 1,4,5-trisphosphate-sensitive stores using the fluorescent indicator mag-fura-2.</a> Proceedings of the National Academy of Sciences of the United States of America. 90, 2598-2602 (1993).</li><li>Solovyova, N., Verkhratsky, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Monitoring+of+free+calcium+in+the+neuronal+endoplasmic+reticulum:+an+overview+of+modern+approaches.">Monitoring of free calcium in the neuronal endoplasmic reticulum: an overview of modern approaches.</a> Journal of Neuroscience Methods. 122, 1-12 (2002).</li><li>Solovyova, N., Veselovsky, N., Toescu, E. C., Verkhratsky, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Ca(2+)+dynamics+in+the+lumen+of+the+endoplasmic+reticulum+in+sensory+neurons:+direct+visualization+of+Ca(2+)-induced+Ca(2+)+release+triggered+by+physiological+Ca(2+)+entry.">Ca(2+) dynamics in the lumen of the endoplasmic reticulum in sensory neurons: direct visualization of Ca(2+)-induced Ca(2+) release triggered by physiological Ca(2+) entry.</a> The EMBO Journal. 21, 622-630 (2002).</li><li>Rehberg, M., Lepier, A., Solchenberger, B., Osten, P., Blum, R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+new+non-disruptive+strategy+to+target+calcium+indicator+dyes+to+the+endoplasmic+reticulum.">A new non-disruptive strategy to target calcium indicator dyes to the endoplasmic reticulum.</a> Cell Calcium. 44, 386-399 (2008).</li><li>Blum, R., Verkhratsky, A., Petersen, O. H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Calcium imaging of intracellular organelles: endoplasmic reticulum. 43, (2010).</li><li>Shaner, N. C., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Improving+the+photostability+of+bright+monomeric+orange+and+red+fluorescent+proteins.">Improving the photostability of bright monomeric orange and red fluorescent proteins.</a> Nature Methods. 5, 545-551 (2008).</li><li>Lois, C., Hong, E. J., Pease, S., Brown, E. J., Baltimore, D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Germline+transmission+and+tissue-specific+expression+of+transgenes+delivered+by+lentiviral+vectors.">Germline transmission and tissue-specific expression of transgenes delivered by lentiviral vectors.</a> Science. 295, 868-872 (2002).</li><li>Zufferey, R., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Self-inactivating+lentivirus+vector+for+safe+and+efficient+in+vivo+gene+delivery.">Self-inactivating lentivirus vector for safe and efficient in vivo gene delivery.</a> Journal of Virology. 72, 9873-9880 (1998).</li><li>Zufferey, R., Nagy, D., Mandel, R. J., Naldini, L., Trono, D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Multiply+attenuated+lentiviral+vector+achieves+efficient+gene+delivery+in+vivo.">Multiply attenuated lentiviral vector achieves efficient gene delivery in vivo.</a> Nature Biotechnology. 15, 871-875 (1997).</li><li>Li, D., Herault, K., Oheim, M., Ropert, N. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=FM+dyes+enter+via+a+store-operated+calcium+channel+and+modify+calcium+signaling+of+cultured+astrocytes.">FM dyes enter via a store-operated calcium channel and modify calcium signaling of cultured astrocytes.</a> Proceedings of the National Academy of Sciences of the United States of America. 106, 21960-21965 (2009).</li><li>Blum, R., Petersen, O. H., Verkhratsky, A., Verkhratsky, A., Petersen, O. H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Calcium measurement methods. Vol. Neuromethods 43 (Springer protocols), 147-167 (2010).</li><li>Tian, L., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Selective+esterase-ester+pair+for+targeting+small+molecules+with+cellular+specificity.">Selective esterase-ester pair for targeting small molecules with cellular specificity.</a> Proceedings of the National Academy of Sciences of the United States of America. 109, 4756-4761 (2012).</li><li>Samtleben, S., Blum, R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Improved+vectors+for+direct+ER+calcium+imaging+with+targeted-estersase+induced+dye+loading.">Improved vectors for direct ER calcium imaging with targeted-estersase induced dye loading.</a> , (2012).</li></ol>

The authors declare that they have no competing financial interests.

The advantages and disadvantages of the TED method have been discussed extensively in recent publications 34,35. Compared to other methods described above, TED circumvents the problem of disrupting and perfusing the cells. Furthermore, two aspects need to be emphasized. The principle of TED for ER calcium imaging requires (1.) the targeted expression of an active carboxylesterase in the ER lumen with the help of TED vector constructs and (2.) the efficient and preferential release of low-affinity synthe...

In order to resolve physiological calcium responses of the ER, we developed a new strategy to improve the trapping of synthetic calcium sensitive dyes into the ER. The method enables the direct, non-disruptive real-time monitoring of free ER calcium in presence of extracellular calcium.	
	Function and signaling of ER Calcium
	Calcium signals are found in different cell types, e.g. muscle-cells, neurons and glial cells and their functions range from mediating muscle contraction to an involvement in synaptic transmission in learning and memory 1,2. Changes in free calcium concentration are of high scientific interest because calcium is involved in the regulation of gene transcription, cell proliferation, neuronal excitability, cell death and other cell signaling events 1-7. All these cellular calcium signals are functionally connected and contribute to intracellular calcium store dynamics 8-10. 
	A common feature among all calcium signals is the flow of calcium between the extracellular space, the cytosol and organelles, mainly the ER and mitochondria. This causes dynamic changes in calcium concentration within these organelles, which are sensed by different signaling components. In general, the calcium concentration in the ER ranges between 100 to 800 &mu;M, in the cytosol the calcium concentration is close to 100 nM, and in the extracellular space the concentration is around 1-2 mM. Accordingly, there is a high chemical driving force for calcium flow towards the cytosol 2,9,10.
	The most commonly investigated ER-derived calcium signals depend on the stimulation of G-protein coupled receptors (GpcR), which then activate phospholipase C (PLC). PLC in turn produces inositol 1,4,5-trisphosphate (IP3) 1. Upon binding of IP3 to its receptor (IP3-Rec, Figure 1) in the ER-membrane, calcium ions are released from the ER lumen. Historically, IP3-mediated calcium release from the ER was first - even though indirectly - measured in acinar pancreatic cells by Streb et al. in 1983 11. This publication suggested for the first time a signaling cascade involving acetylcholine, phospholipase C, and IP3. This way of calcium release is generally termed IP3-induced calcium release (IiCR) (Figure 1). Kinase-dependent activation of phospholipase C&gamma; by receptor tyrosin kinases links the action of growth factors and neurotrophic factors to ER calcium signaling after IP3 elevation 12. In addition to IiCR, calcium elevation may be mediated by ionotropic calcium entry, for instance via voltage-gated calcium channels (CaV), and subsequent calcium-induced calcium release (CiCR) by ryanodine receptors (RyR). IiCR and CiCR are physiologically linked to store-operated calcium entry (SOCE). SOCE includes the action of STIM (stroma interacting molecule), which is a sensor for ER calcium release. STIM has been shown to stimulate extracellular calcium entry through transient receptor potential channels (Trp) 13, Orai calcium channels 14 and even voltage-gated calcium channels 15 (Figure 1) . Loss of ER calcium is dynamically rescued by the action of the sarco-endoplasmic reticulum calcium ATPase (SERCA), which actively pumps calcium back into the ER. Blocking the SERCA with drugs such as thapsigargin unveils a continuous loss of ER calcium to the cytosolic compartment. This ER calcium "leak" is caused by ER intramembrane pore complexes such as the Sec61 protein complex 16,17 (Figure 1).
	In 1998, Berridge published a model, the "neuron within a neuron model", which suggests a principle physiological role of the ER in integrating neuronal calcium 5. This model considers the existence of a continuous ER membrane system forming an intracellular "image" of the neuronal plasma membrane 5. This binary eukaryotic membrane system was claimed to be a basic prerequisite for temporal and spatial integration of fast and slow calcium signals in neurons. Calcium signals that occur either concomitantly or subsequently in different spines or dendrites of the same neuron are conferred to the cell's soma or nucleus via the ER, where they are summed up 5,18. Then, their sum may have effects on the excitability of the neuron, regulation of gene transcription or integration of signaling cascades. Thus, the ER supports the integration of calcium signals. One prerequisite for this concept is the continuity of the ER in a single cell, which has been claimed by several studies and which has been proven at least for somato-dendritic areas and short distance axonal projections 19-21. Whether there is ER continuity within long axonal projections is a matter of debate.
	Strategies to measure the flow of free calcium over the ER membrane
	Calcium signals are most frequently monitored in the cytosol 22,23. Therefore, it cannot easily be distinguished whether Ca2+ is flowing into the cytosol from extracellular or from intracellular stores 6,24. To overcome this limitation, methodological strategies for direct ER calcium imaging have been developed. In summary, the following strategies are used: (1) ER-targeted genetically engineered protein-indicators 25-27. Protein-based low-affinity Ca2+ indicators use the bioluminescent protein aequorin or GFP in combination with a calcium sensing protein. These genetically engineered Ca2+ indicators (GECIs) can be targeted to the ER with the help of a signal peptide and are actively kept in the ER using a retention and retrieval motif. Common ER Ca2+ indicators base on the Cameleon principle and are the Cameleon YC4.3 26,28; Cameleon split YC7.3ER 29, and Cameleon D1 30. (2) Direct esterase-based dye loading of AM-ester low-affinity Ca2+ indicators 31,32. AM-derivatives of the indicator dyes (Mag-Fura2-AM, Mag-Fluo4-AM or Fluo5N-AM) pass the biological membranes in a lipophilic, calcium-insensitive state. Then, in the cytosol as well as in the ER, endogenous esterases cleave of the AM-ester group and release the Ca2+ indicator, leaving behind a certain amount of active dye in the cytosol and in the ER. Therefore, this approach is useful under conditions of a high calcium concentration in the ER as long as the cytosolic calcium concentration stays well below the detection limit of the low-affinity indicators, particularly during characteristic calcium signals (e.g. nM to low &mu;M). (3) AM-ester loading in combination with plasma membrane permeabilization 32. Any remaining cytosolic Ca2+ indicator is removed by plasma membrane permeabilization with small amounts of a "mild" detergent (e.g. saponin) in an artificial intracellular buffer. Thus, the intracellular membranes may be stimulated, e.g. with IP3 in the intracellular buffer, directly through "pores" in the plasma membrane. (4) Dialysis of the cytosol under whole-cell configuration and simultaneous measurements of Ca2+ in the ER lumen and the cytosol 32,33. A cell is first loaded with a low-affinity Ca2+ indicator (e.g. Mag-Fura2-AM, ratiometric, UV-light). Afterwards, with the help of a patch pipette, any remaining cytosolic low-affinity Ca2+ indicator is dialysed out of the cytosol with a buffer containing a high-affinity Ca2+ indicator (e.g. Fluo-3, visible light). This strategy enables the simultaneous recording of cytosolic and ER derived signals. (5) Targeted-esterase-induced dye loading 8,34. A Carboxylesterase (CES) is targeted to the lumen of the ER and provides a high esterase activity for efficient trapping of the AM-ester form of low-affinity Ca2+ indicators.
	Targeted-esterase induced dye loading (TED)
	To improve targeting of low-affinity Ca2+ indicators to the ER lumen, TED was developed. TED requires the overexpression of an ER targeted mouse carboxylesterase (CES2) (Figure 2), which is achieved via expression constructs. Cells expressing a recombinant CES-construct are incubated with the AM-ester form of a calcium indicator dye (Fluo5N-AM, Figure 2). Then, in the ER, the dye is converted to the Ca2+ sensitive, membrane impermeable Ca2+ indicator complex (Fluo5N/Ca2+) by the high esterase activity, therefore trapping the dye in a high concentration in the ER lumen 8,34. The method is especially useful to investigate ER calcium-release via the IiCR-pathways for instance via metabotropic, purinergic- or glutamate receptors 8,34 and to visualize ER calcium depletion via "leak channels" directly, for instance after blockade of the SERCA 17,34. To our experience the low-affinity Ca2+ indicator Fluo5N-AM is currently the best available indicator to use with the TED dye loading strategy. Fluo5N-AM has a low-affinity for Ca2+ (dissociation constant KD ~90 &mu;M, Figure 2), is almost non-fluorescent in its AM-form, but provides high fluorescence emission upon calcium binding 8,34. The Fluo5N/Ca2+ complex can be excited with a standard light source of ~490 nm, which corresponds to standard dyes such as FITC, Alexa 488, or eGFP. Cytosolic calcium signals scarcely reach a concentration in the low &mu;M range and are therefore barely detected by Fluo5N in the cytosol 35.
	To improve the TED performance, several recombinant vector constructs were developed (Figure 3). Originally, TED vectors based on the coding sequence of CES2 (Refseq accession number NM_145603, CES2c) and best TED performance is observed with stable expression of CES constructs. New TED vectors express a core element of CES2 fused to the red fluorescence protein TagRFP-T2 36. These vectors have the advantage that they can be used to identify transduced cells and to use the red fluorescence as an internal control for the normalization of changes in Fluo5N/Ca2+ fluorescence. The red fluorescence also offers the possibility to visualize the structural distribution of the ER and changes in ER dynamics under stimulation conditions.

<table border="1">
	 <tbody>
 <tr>
 <td>Name of Reagent/Material</td>
 <td>Company</td>
 <td>Catalogue Number</td>
 <td>Comments</td>
 </tr>
 <tr>
 <td>(S)-3,5-DHPG (Dihydroxyphenylglycine)</td>
 <td>Tocris</td>
 <td>0805</td>
 <td></td>
 </tr>
 <tr>
 <td>5';-ATP-Na2, ATP disodium salt hydrate</td>
 <td>Sigma</td>
 <td>A26209-1G</td>
 <td></td>
 </tr>
 <tr>
 <td>B-27 supplement,50x</td>
 <td>Invitrogen</td>
 <td>17504-044</td>
 <td></td>
 </tr>
 <tr>
 <td>Carbamoylcholine chloride (Charbachol)</td>
 <td>Sigma</td>
 <td>C4382-1g</td>
 <td></td>
 </tr>
 <tr>
 <td>Cyclopiazonic acid (CPA)</td>
 <td>Ascent scientific</td>
 <td>Asc-300</td>
 <td></td>
 </tr>
 <tr>
 <td>Dimethylsulfoxide (DMSO)</td>
 <td>Sigma</td>
 <td>D2650</td>
 <td></td>
 </tr>
 <tr>
 <td>DMEM with Glutamax</td>
 <td>Invitrogen-Gibco</td>
 <td>31966-047</td>
 <td></td>
 </tr>
 <tr>
 <td>Dulbecco's PBS without Ca/Mg 1x</td>
 <td>PAA</td>
 <td>H15-002</td>
 <td></td>
 </tr>
 <tr>
 <td>Fetal calf serum</td>
 <td>Invitrogen-Gibco</td>
 <td>10270-106</td>
 <td></td>
 </tr>
 <tr>
 <td>Fluo-5N-AM, 10 x 50 &mu;g</td>
 <td>Invitrogen</td>
 <td>F14204</td>
 <td></td>
 </tr>
 <tr>
 <td>Glutamate</td>
 <td>Ascent scientific</td>
 <td>Asc-049</td>
 <td></td>
 </tr>
 <tr>
 <td>Glutamax</td>
 <td>Invitrogen</td>
 <td>35050-038</td>
 <td></td>
 </tr>
 <tr>
 <td>Ham's F12 nutrients mixture</td>
 <td>Invitrogen-Gibco</td>
 <td>21765-029</td>
 <td></td>
 </tr>
 <tr>
 <td>Hank's BSS(1x) without Ca,without Mg, with Phenol Red, HBSS</td>
 <td>PAA Laboratories</td>
 <td>H15-010</td>
 <td></td>
 </tr>
 <tr>
 <td>Horse serum</td>
 <td>SHD3250YK</td>
 <td>Linearis</td>
 <td></td>
 </tr>
 <tr>
 <td>Ionomycin Ca2+ salt</td>
 <td>Ascent scientific</td>
 <td>Asc-116</td>
 <td></td>
 </tr>
 <tr>
 <td>murine EGF </td>
 <td>PreproTech</td>
 <td>315-09</td>
 <td></td>
 </tr>
 <tr>
 <td>N2 supplement 100x</td>
 <td>Invitrogen</td>
 <td>17502-048</td>
 <td></td>
 </tr>
 <tr>
 <td>Neurobasal medium</td>
 <td>Invitrogen-Gibco</td>
 <td>21103-049</td>
 <td></td>
 </tr>
 <tr>
 <td>Pluronic F127, low UV absorbance,2g</td>
 <td>Invitrogen</td>
 <td>P6867</td>
 <td></td>
 </tr>
 <tr>
 <td>Poly-DL-ornithine hydrobromide PORN</td>
 <td>Sigma</td>
 <td>P8638</td>
 <td></td>
 </tr>
 <tr>
 <td>Poly-D-lysine hydrobromide</td>
 <td>Sigma</td>
 <td>P7280</td>
 <td></td>
 </tr>
 <tr>
 <td>Poly-L-Lysin</td>
 <td>Sigma</td>
 <td>P2636</td>
 <td></td>
 </tr>
 <tr>
 <td>Thapsigargin</td>
 <td>Calbiochem</td>
 <td>586005-1mg</td>
 <td></td>
 </tr>
 <tr>
 <td>TryLE Express</td>
 <td>Invitrogen</td>
 <td>12605-028</td>
 <td></td>
 </tr>
 <tr>
 <td>Trypsin</td>
 <td>Worthington</td>
 <td>LS-003707</td>
 <td></td>
 </tr>
 <tr>
 <td>Trypsininhibitor from Glycine MAX (soybean)</td>
 <td>Sigma</td>
 <td>T6522</td>
 <td></td>
 </tr>
 <tr>
 <td></td>
 <td></td>
 <td></td>
 <td>Materials</td>
 </tr>
 <tr>
 <td>Confocal system: inverted laser scanning microscope: FV1000 with IX81</td>
 <td>Olympus</td>
 <td></td>
 <td>user-specific configuration</td>
 </tr>
 <tr>
 <td>Confocal system: laser combiner FV10 and diode lasers (405, 473, 559, 635)</td>
 <td>Olympus</td>
 <td></td>
 <td>user-specific configuration</td>
 </tr>
 <tr>
 <td>Confocal system: scan head FV10-SPD with spectraldetector</td>
 <td>Olympus</td>
 <td></td>
 <td>user-specific configuration</td>
 </tr>
 <tr>
 <td>Heater controller TC-344B</td>
 <td>Warner Instruments</td>
 <td>64-0101</td>
 <td>to control in line solution heater</td>
 </tr>
 <tr>
 <td>NIH ImageJ software</td>
 <td>WS Rasband, ImageJ, US National Institutes of Health, <a href="http://rsb.info.nih.gov/ij/" target="_blank">http://rsb.info.nih.gov/ij/</a>, 1997--2006</td>
 <td></td>
 <td></td>
 </tr>
 <tr>
 <td>Objective: UAPON20XW340 NA 0,7 WD 0,35</td>
 <td>Olympus</td>
 <td>N2709100</td>
 <td></td>
 </tr>
 <tr>
 <td>Objective: UPLFLN40XO</td>
 <td>Olympus</td>
 <td>N1478700</td>
 <td></td>
 </tr>
 <tr>
 <td>Objective: UPLSAPO60XO/1.35, WD=0.15</td>
 <td>Olympus</td>
 <td>N1480700</td>
 <td></td>
 </tr>
 <tr>
 <td>Perfusion chamber, inverse</td>
 <td>custom-made</td>
 <td></td>
 <td></td>
 </tr>
 <tr>
 <td>Perfusion chamber, RC-49FS</td>
 <td>Warner Instruments</td>
 <td>W4 64-1709 </td>
 <td></td>
 </tr>
 <tr>
 <td>Perfusion tube: PT-49</td>
 <td>Warner Instruments</td>
 <td>64-1730</td>
 <td>for perfusion</td>
 </tr>
 <tr>
 <td>Peristaltic pump MiniPlus 3 (4 channels)</td>
 <td>Gilson</td>
 <td>n.a. </td>
 <td>perfusion pump</td>
 </tr>
 <tr>
 <td>Single inline solution heater SH-27B</td>
 <td>Warner Instruments</td>
 <td>64-0102</td>
 <td>controlled by heater controller</td>
 </tr>
 <tr>
 <td>Suction tubes: ST3R</td>
 <td>Warner Instruments</td>
 <td>64-1407</td>
 <td>for perfusion</td>
 </tr>
 <tr>
 <td>Heating insert: Tempocontrol 37-2 digital 2-channel</td>
 <td>Pecon</td>
 <td>n.a. </td>
 <td></td>
 </tr>
	 </tbody>
 </table>

direct imaging of er calcium with targeted-esterase induced dye loading (ted)

Visualization of calcium dynamics is important to understand the role of calcium in cell physiology. To examine calcium dynamics, synthetic fluorescent Ca2+ indictors have become popular. Here we demonstrate TED (= targeted-esterase induced dye loading), a method to improve the release of Ca2+ indicator dyes in the ER lumen of different cell types. To date, TED was used in cell lines, glial cells, and neurons in vitro. TED bases on efficient, recombinant targeting of a high carboxylesterase activity to the ER lumen using vector-constructs that express Carboxylesterases (CES). The latest TED vectors contain a core element of CES2 fused to a red fluorescent protein, thus enabling simultaneous two-color imaging. The dynamics of free calcium in the ER are imaged in one color, while the corresponding ER structure appears in red. At the beginning of the procedure, cells are transduced with a lentivirus. Subsequently, the infected cells are seeded on coverslips to finally enable live cell imaging. Then, living cells are incubated with the acetoxymethyl ester (AM-ester) form of low-affinity Ca2+ indicators, for instance Fluo5N-AM, Mag-Fluo4-AM, or Mag-Fura2-AM. The esterase activity in the ER cleaves off hydrophobic side chains from the AM form of the Ca2+ indicator and a hydrophilic fluorescent dye/Ca2+ complex is formed and trapped in the ER lumen. After dye loading, the cells are analyzed at an inverted confocal laser scanning microscope. Cells are continuously perfused with Ringer-like solutions and the ER calcium dynamics are directly visualized by time-lapse imaging. Calcium release from the ER is identified by a decrease in fluorescence intensity in regions of interest, whereas the refilling of the ER calcium store produces an increase in fluorescence intensity. Finally, the change in fluorescent intensity over time is determined by calculation of &Delta;F/F0.

This protocol provides a non-disruptive approach for direct imaging of free ER calcium. Low-affinity synthetic Ca2+ indicators are released and trapped in the ER lumen with the help of an ER targeted, recombinant esterase enzyme activity. Improved loading of the Ca2+ indicator dyes to the ER lumen enables a direct and fast imaging of ER calcium store dynamics.
Cell types for TED
The feasibility of the method has been shown in the cell lines BHK21,...

Watch this Scientific Journal Video about Direct Imaging of ER Calcium with Targeted-Esterase Induced Dye Loading TED at JoVE.com

Direct Imaging of ER Calcium with Targeted-Esterase Induced Dye Loading TED

Targeted-esterase induced dye loading (TED) supports the analysis of intracellular calcium store dynamics by fluorescence imaging. The method bases on targeting of a recombinant Carboxylesterase to the endoplasmic reticulum (ER), where it improves the local unmasking of synthetic low-affinity Ca2+ indicator dyes in the ER lumen.

Direct Imaging of ER Calcium with Targeted-Esterase Induced Dye Loading (TED)

Visualization of calcium dynamics is important to understand the role of calcium in cell physiology. To examine calcium dynamics, synthetic fluorescent ...

direct-imaging-er-calcium-with-targeted-esterase-induced-dye-loading

Research

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Biology

19.1K Views.  University of Wuerzburg. Targeted-esterase induced dye loading (TED) supports the analysis of intracellular calcium store dynamics by fluorescence imaging. The method bases on targeting of a recombinant Carboxylesterase to the endoplasmic reticulum (ER), where it improves the local unmasking of synthetic low-affinity Ca2+ indicator dyes in the ER lumen.

Video: Direct Imaging of ER Calcium with Targeted-Esterase Induced Dye Loading TED

Loading Drosophila Nerve Terminals with Calcium Indicators

Calcium plays many roles in the nervous system but none more impressive than as the trigger for neurotransmitter release, and none more profound than as the messenger essential for the synaptic plasticity that supports learning and memory. To further elucidate the molecular underpinnings of Ca2+-dependent synaptic mechanisms, a model system is required that is both genetically malleable and physiologically accessible. Drosophila melanogaster provides such a model. In this system, genetically-encoded fluorescent indicators are available to detect Ca2+ changes in nerve terminals. However, these indicators have limited sensitivity to Ca2+ and often show a non-linear response. Synthetic fluorescent indicators are better suited for measuring the rapid Ca2+ changes associated with nerve activity. Here we demonstrate a technique for loading dextran-conjugated synthetic Ca2+ indicators into live nerve terminals in Drosophila larvae. Particular emphasis is placed on those aspects of the protocol most critical to the technique's success, such as how to avoid static electricity discharges along the isolated nerves, maintaining the health of the preparation during extended loading periods, and ensuring axon survival by providing Ca2+ to promote sealing of severed axon endings. Low affinity dextran-conjugated Ca2+-indicators, such as fluo-4 and rhod, are available which show a high signal-to-noise ratio while minimally disrupting presynaptic Ca2+ dynamics. Dextran-conjugation helps prevent Ca2+ indicators being sequestered into organelles such as mitochondria. The loading technique can be applied equally to larvae, embryos and adults.

Calcium is a ubiquitous messenger in the nervous system, essential for triggering neurotransmitter release and changes in synaptic strength. Here we demonstrate a technique for loading Ca2+-indicators into Drosophila nerve terminals. We also demonstrate fabrication of the required apparatus and emphasize points critical for the technique's success.

Calcium plays many roles in the nervous system but none more impressive than as the trigger for neurotransmitter release, and none more profound than as ...

In Vivo 2-Photon Calcium Imaging in Layer 2/3 of Mice

To understand network dynamics of microcircuits in the neocortex, it is essential to simultaneously record the activity of a large number of neurons . In-vivo two-photon calcium imaging is the only method that allows one to record the activity of a dense neuronal population with single-cell resolution . The method consists in implanting a cranial imaging window, injecting a fluorescent calcium indicator dye that can be taken up by large numbers of neurons and finally recording the activity of neurons with time lapse calcium imaging using an in-vivo two photon microscope. Co-injection of astrocyte-specific dyes allows one to differentiate neurons from astrocytes. The technique can be performed in mice expressing fluorescent molecules in specific subpopulations of neurons to better understand the network interactions of different groups of cells.

To understand network dynamics of microcircuits in the neocortex, it is essential to simultaneously record the activity of a large number of neurons . In-vivo two-photon calcium imaging is the only method that allows one to record the activity of a dense neuronal population with single-cell resolution .

To understand network dynamics of microcircuits in the neocortex, it is essential to simultaneously record the activity of a large number of neurons .  ...

Calcium Imaging of Cortical Neurons using Fura-2 AM

Calcium imaging is a common technique that is useful for measuring calcium signals in cultured cells. Calcium imaging techniques take advantage of calcium indicator dyes, which are BAPTA-based organic molecules that change their spectral properties in response to the binding of Ca2+ ions. Calcium indicator dyes fall into two categories, ratio-metric dyes like Fura-2 and Indo-1 and single-wavelength dyes like Fluo-4. Ratio-metric dyes change either their excitation or their emission spectra in response to calcium, allowing the concentration of intracellular calcium to be determined from the ratio of fluorescence emission or excitation at distinct wavelengths.  The main advantage of using ratio-metric dyes over single wavelength probes is that the ratio signal is independent of the dye concentration, illumination intensity, and optical path length allowing the concentration of intracellular calcium to be determined independently of these artifacts. One of the most common calcium indicators is Fura-2, which has an emission peak at 505 nM and changes its excitation peak from 340 nm to 380 nm in response to calcium binding.  Here we describe the use of Fura-2 to measure intracellular calcium elevations in neurons and other excitable cells.

Calcium signals play a key role in many cellular processes including gene expression, survival and differentiation. Here we demonstrate how to perform calcium imaging using Fura-2 AM. Calcium imaging is a valuable tool to study the regulation of intracellular calcium in real time and its regulation of signaling cascades.

Calcium imaging is a common technique that is useful for measuring calcium signals in cultured cells. Calcium imaging techniques take advantage of calcium ...

Assembly, Loading, and Alignment of an Analytical Ultracentrifuge Sample Cell

The analytical ultracentrifuge (AUC) is a powerful biophysical tool that allows us to record macromolecular sedimentation profiles during high speed centrifugation. When properly planned and executed, an AUC sedimentation velocity or sedimentation equilibrium experiment can reveal a great deal about a protein in regards to size and shape, sample purity, sedimentation coefficient, oligomerization states and protein-protein interactions.
This technique, however, requires a rigorous level of technical attention. Sample cells hold a sectored center piece sandwiched between two window assemblies. They are sealed with a torque pressure of around 120-140 in/lbs. Reference buffer and sample are loaded into the centerpiece sectors and then after sealing, the cells are precisely aligned into a titanium rotor so that the optical detection systems scan both sample and reference buffer in the same radial path midline through each centerpiece sector while rotating at speeds of up to 60, 000 rpm and under very high vacuum
Not only is proper sample cell assembly critical, sample cell components are very expensive and must be properly cared for to ensure they are in optimum working condition in order to avoid leaks and breakage during experiments. Handle windows carefully, for even the slightest crack or scratch can lead to breakage in the centrifuge. The contact between centerpiece and windows must be as tight as possible; i.e. no Newton s rings should be visible after torque pressure is applied. Dust, lint, scratches and oils on either the windows or the centerpiece all compromise this contact and can very easily lead to leaking of solutions from one sector to another or leaking out of the centerpiece all together. Not only are precious samples lost, leaking of solutions during an experiment will cause an imbalance of pressure in the cell that often leads to broken windows and centerpieces. In addition, plug gaskets and housing plugs must be securely in place to avoid solutions being pulled out of the centerpiece sector through the loading holes by the high vacuum in the centrifuge chamber. Window liners and gaskets must be free of breaks and cracks that could cause movement resulting in broken windows.
This video will demonstrate our procedures of sample cell assembly, torque, loading and rotor alignment to help minimize component damage, solution leaking and breakage during the perfect AUC experiment.

The analytical ultracentrifuge (AUC) sample cell holds sample and reference buffer and during experiments and is exposed to high vacuum and rotor speeds up to 60,000 rpm. This video will demonstrate the rigorous attention to detail necessary for assembly, loading and alignment of this very important component of an AUC experiment.

The analytical ultracentrifuge (AUC) is a powerful biophysical tool that allows us to record macromolecular sedimentation profiles during high speed ...

Live Cell Calcium Imaging Combined with siRNA Mediated Gene Silencing Identifies Ca2+ Leak Channels in the ER Membrane and their Regulatory Mechanisms

In mammalian cells, the endoplasmic reticulum (ER) plays a key role in protein biogenesis as well as in calcium signalling1. The heterotrimeric Sec61 complex in the ER membrane provides an aqueous path for newly-synthesized polypeptides into the lumen of the ER. Recent work from various laboratories suggested that this heterotrimeric complex may also form transient Ca2+ leak channels2-8. The key observation for this notion was that release of nascent polypeptides from the ribosome and Sec61 complex by puromycin leads to transient release of Ca2+ from the ER. Furthermore, it had been observed in vitro that the ER luminal protein BiP is involved in preventing ion permeability at the level of the Sec61 complex9,10. We have established an experimental system that allows us to directly address the role of the Sec61 complex as potential Ca2+ leak channel and to characterize its putative regulatory mechanisms11-13. This system combines siRNA mediated gene silencing and live cell Ca2+ imaging13. Cells are treated with siRNAs that are directed against the coding and untranslated region (UTR), respectively, of the SEC61A1 gene or a negative control siRNA. In complementation analysis, the cells are co-transfected with an IRES-GFP vector that allows the siRNA-resistant expression of the wildtype SEC61A1 gene. Then the cells are loaded with the ratiometric Ca2+-indicator FURA-2 to monitor simultaneously changes in the cytosolic Ca2+ concentration in a number of cells via a fluorescence microscope. The continuous measurement of cytosolic Ca2+ also allows the evaluation of the impact of various agents, such as puromycin, small molecule inhibitors, and thapsigargin on Ca2+ leakage. This experimental system gives us the unique opportunities to i) evaluate the contribution of different ER membrane proteins to passive Ca2+ efflux from the ER in various cell types, ii) characterize the proteins and mechanisms that limit this passive Ca2+ efflux, and iii) study the effects of disease linked mutations in the relevant components.

Live Cell Calcium Imaging Combined with siRNA Mediated Gene Silencing Identifies Ca2+ Leak Channels in the ER Membrane and their Regulatory Mechanisms

The endoplasmic reticulum plays a key role in protein biogenesis and in calcium homeostasis. We have established an experimental system that allows us to address the role of Ca2+ leak channels and to characterize their putative regulatory mechanisms. This system involves siRNA mediated gene silencing and live cell Ca2+ imaging.

In mammalian cells, the endoplasmic reticulum (ER) plays a key role in protein biogenesis as well as in calcium signalling1. The heterotrimeric Sec61 ...

Expression and Purification of Virus-like Particles for Vaccination

Virus-like particles (VLPs) and subviral particles (SVPs) are an alternative approach to viral vaccine design that offers the advantages of increased biosafety and stability over use of live pathogens. Non-infectious and self-assembling, VLPs are used to present structural proteins as immunogens, bypassing the need for live pathogens or recombinant viral vectors for antigen delivery. In this article, we demonstrate the different stages of VLP design and development for future applications in preclinical animal testing. The procedure includes the following stages: selection of antigen, expression of antigen in cell line of choice, purification of VLPs/SVPs, and quantification for antigen dosing. We demonstrate use of both mammalian and insect cell lines for expression of our antigens and demonstrate how methodologies differ in yield. The methodology presented may apply to a variety of pathogens and can be achieved by substituting the antigens with immunogenic structural proteins of the user&#39;s microorganism of interest. VLPs and SVPs assist with antigen characterization and selection of the best vaccine candidates.

Here, we present a protocol for synthesizing virus-like particles using either baculovirus or mammalian expression systems, and ultracentrifugation purification. This highly customizable approach is used to identify viral antigens as vaccine targets in a safe and flexible manner.

Virus-like particles (VLPs) and subviral particles (SVPs) are an alternative approach to viral vaccine design that offers the advantages of increased ...

Monitoring ER/SR Calcium Release with the Targeted Ca2+ Sensor CatchER+

Intracellular calcium (Ca2+) transients evoked by extracellular stimuli initiate a multitude of biological processes in living organisms. At the center of intracellular calcium release are the major intracellular calcium storage organelles, the endoplasmic reticulum (ER) and the more specialized sarcoplasmic reticulum (SR) in muscle cells. The dynamic release of calcium from these organelles is mediated by the ryanodine receptor (RyR) and the inositol 1,4,5-triphosphate receptor (IP3R) with refilling occurring through the sarco/endoplasmic reticulum calcium ATPase (SERCA) pump. A genetically encoded calcium sensor (GECI) called CatchER was created to monitor the rapid calcium release from the ER/SR. Here, the detailed protocols for the transfection and expression of the improved, ER/SR-targeted GECI CatchER+ in HEK293 and C2C12 cells and its application in monitoring IP3R, RyR, and SERCA pump-mediated calcium transients in HEK293 cells using fluorescence microscopy is outlined. The receptor agonist or inhibitor of choice is dispersed in the chamber solution and the intensity changes are recorded in real time. With this method, a decrease in ER calcium is seen with RyR activation with 4-chloro-m-cresol (4-cmc), the indirect activation of IP3R with adenosine triphosphate (ATP), and inhibition of the SERCA pump with cyclopiazonic acid (CPA). We also discuss protocols for determining the in situ Kd and quantifying basal [Ca2+] in C2C12 cells. In summary, these protocols, used in conjunction with CatchER+, can elicit receptor mediated calcium release from the ER with future application in studying ER/SR calcium related pathologies.

Monitoring ER/SR Calcium Release with the Targeted Ca2+ Sensor CatchER+

We present protocols for the application of our targeted genetically-encoded calcium indicator (GECI) CatchER+ for monitoring rapid calcium transients in the endoplasmic/sarcoplasmic reticulum (ER/SR) of HEK293 and skeletal muscle C2C12 cells using real-time fluorescence microscopy. A protocol for the in situ Kd measurement and calibration is also discussed.

Intracellular calcium (Ca2+) transients evoked by extracellular stimuli initiate a multitude of biological processes in living organisms. At the center of ...

Imaging Intermediate Filaments and Microtubules with 2-dimensional Direct Stochastic Optical Reconstruction Microscopy

The cytoskeleton, composed of actin microfilaments, microtubules, and intermediate filaments (IF), plays a key role in the control of cell shape, polarity, and motility. The organization of the actin and microtubule networks has been extensively studied but that of IFs is not yet fully characterized. IFs have an average diameter of 10 nm and form a network extending throughout the cell cytoplasm. They are physically associated with actin and microtubules through molecular motors and cytoskeletal linkers. This tight association is at the heart of the regulatory mechanisms that ensure the coordinated regulation of the three cytoskeletal networks required for most cell functions. It is therefore crucial to visualize IFs alone and also together with each of the other cytoskeletal networks. However, IF networks are extremely dense in most cell types, especially in glial cells, which makes its resolution very difficult to achieve with standard fluorescence microscopy (lateral resolution of ~250 nm). Direct STochastic Optical Reconstruction Microscopy (dSTORM) is a technique allowing a gain in lateral resolution of one order of magnitude. Here, we show that lateral dSTORM resolution is sufficient to resolve the dense organization of the IF networks and, in particular, of IF bundles surrounding microtubules. Such tight association is likely to participate in the coordinated regulation of these two networks and may, explain how vimentin IFs template and stabilize microtubule organization as well as could influence microtubule dependent vesicular trafficking. More generally, we show how the observation of two cytoskeletal components with dual-color dSTORM technique brings new insight into their mutual interaction.

The overall goal of this methodology is to give the optimal experimental conditions from sample preparation to image acquisition and reconstruction in order to perform 2D dual color dSTORM images of microtubules and intermediate filaments in fixed cells

The cytoskeleton, composed of actin microfilaments, microtubules, and intermediate filaments (IF), plays a key role in the control of cell shape, ...

Fluorescence Recovery after Photobleaching of Yellow Fluorescent Protein Tagged p62 in Aggresome-like Induced Structures

Fluorescence recovery after photobleaching (FRAP)&#160;is a microscopy technique that can be used to quantify protein mobility in live cells. In a typical FRAP experiment, steady-state fluorescence is observed by repeated imaging with low-intensity laser light. Subsequently, the fluorescent molecules are rapidly and irreversibly impaired via brief exposure to high-intensity laser light. Information about protein mobility is obtained by monitoring the recovery of fluorescence. We used FRAP to determine the mobility of p62 in aggresome-like induced structures (ALIS) in murine macrophages after stimulation with lipopolysaccharide (LPS). Because many existing FRAP protocols are either incomplete or complex, our goal was to provide a comprehensive, practical, and straightforward step-by-step protocol for FRAP experiments with live cells. Here, we describe RAW264.7 macrophage transfection with yellow fluorescent protein-p62 (YFP-p62), induction of ALIS by exposing the cells to LPS, and a step-by-step method for collecting prebleach and postbleach FRAP images and data analysis. Finally, we discuss important factors to consider when conducting a FRAP experiment.

We describe a comprehensive and practical protocol for fluorescence recovery after photobleaching experiments with live cells. Although the protocol was used to measure the mobility of yellow fluorescent protein-tagged p62 in aggresome-like induced structures, it can be applied to a variety of microscopy systems and fluorescent proteins.

Fluorescence recovery after photobleaching (FRAP)&#160;is a microscopy technique that can be used to quantify protein mobility in live cells. In a typical ...

Imaging Calcium Dynamics in Subpopulations of Mouse Pancreatic Islet Cells

Pancreatic islet hormones regulate blood glucose homeostasis. Changes in blood glucose induce oscillations of cytosolic calcium in pancreatic islet cells that trigger secretion of three main hormones: insulin (from &#946;-cells), glucagon (&#945;-cells) and somatostatin (&#948;-cells). &#946;-Cells, which make up the majority of islet cells and are electrically coupled to each other, respond to the glucose stimulus as one single entity. The excitability of the minor subpopulations, &#945;-cells and &#948;-cells (making up around 20% (30%) and 4% (10%) of the total rodent1 (human2) islet cell numbers, respectively) is less predictable and is therefore of special interest.
Calcium sensors are delivered into the peripheral layer of cells within the isolated islet. The islet or a group of islets is then immobilized and imaged using a fluorescence microscope. The choice of the imaging mode is between higher throughput (wide-field) and better spatial resolution (confocal). Conventionally, laser scanning confocal microscopy is used for imaging tissue, as it provides the best separation of the signal between the neighboring cells. A wide-field system can be utilized too, if the contaminating signal from the dominating population of &#946;-cells is minimized.
Once calcium dynamics in response to specific stimuli have been recorded, data are expressed in numerical form as fluorescence intensity vs. time, normalized to the initial fluorescence and baseline-corrected, to remove the effects linked to bleaching of the fluorophore. Changes in the spike frequency or partial area under the curve (pAUC) are computed vs. time, to quantify the observed effects. pAUC is more sensitive and quite robust whereas spiking frequency provides more information on the mechanism of calcium increase.
Minor cell subpopulations can be identified using functional responses to marker compounds, such as adrenaline and ghrelin, that induce changes in cytosolic calcium in a specific populations of islet cells.

Here, we present a protocol for imaging and quantifying calcium dynamics in heterogeneous cell populations, such as pancreatic islet cells. Fluorescent reporters are delivered into the peripheral layer of cells within the islet, which is then immobilized and imaged, and per-cell analysis of the dynamics of fluorescence intensity is performed.

Pancreatic islet hormones regulate blood glucose homeostasis. Changes in blood glucose induce oscillations of cytosolic calcium in pancreatic islet cells ...