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

Summary

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.

Abstract

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 ΔF/F₀.

Introduction

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 h....

Protocol

This protocol introduces TED applied to cell lines, hippocampal neurons and cortical glial cells. TED performance is best when TED vectors are stably expressed, for instance by lentiviral vectors. A schematic overview of the TED method is shown in Figure 4.

1. Preparation of Solutions

The following solutions should be prepared before starting and can be stored as indicated. We generally use sterilized glass and plastic material and autoclaved water.

1. Prepare HEPES-Ringer (in mM): 125 NaCl, 3 KCl, 25 HEPES, 2 MgSO₄.7H₂O, 2 CaCl₂, 1.25 NaH₂PO

Results

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,.......

Discussion

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.......

Materials

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