Development of a far-red to near-infrared fluorescence probe for calcium ion and its application to multicolor neuronal imaging

Article abstract of DOI:10.1021/ja205809h

To improve optical imaging of Ca²⁺ and to make available a distinct color window for multicolor imaging, we designed and synthesized CaSiR-1, a far-red to near-infrared fluorescence probe for Ca²⁺, using Si-rhodamine (SiR) as the flu

Full text of DOI:10.1021/ja205809h

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Development of a Far-Red to Near-Infrared Fluorescence Probe for
Calcium Ion and its Application to Multicolor Neuronal Imaging
Takahiro Egawa, Kenjiro Hanaoka, Yuichiro Koide, Sakiko Ujita, Naoya Takahashi, Yuji Ikegaya,
Norio Matsuki, Takuya Terai, Tasuku Ueno, Toru Komatsu, and Tetsuo Nagano*
Graduate School of Pharmaceutical Sciences, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
S Supporting Information
b
suitable for investigating intracellular Ca2+ dynamics and for
multicolor imaging.
ABSTRACT: To improve optical imaging of Ca2+ and to
make available a distinct color window for multicolor
imaging, we designed and synthesized CaSiR-1, a far-red
to near-infrared fluorescence probe for Ca2+, using Si-
rhodamine (SiR) as the fluorophore and the well-known
Ca2+ chelator BAPTA. This wavelength region is advanta-
geous, affording higher tissue penetration, lower back-
ground autofluorescence, and lower phototoxicity in
comparison with the UV to visible range. CaSiR-1 has a
high fluorescence off/on ratio of over 1000. We demon-
strate its usefulness for multicolor fluorescence imaging of
action potentials (visualized as increases in intracellular Ca2+)
in brain slices loaded with sulforhodamine 101 (red color;
specific for astrocytes) that were prepared from transgenic
mice in which some neurons expressed green fluorescent
protein.
This is a challenging task because almost all far-red to NIR
fluorescence dyes, such as cyanine dyes, have relatively high-lying
highest occupied molecular orbital (HOMO) energy levels,
which means that fluorescence of these dyes is unlikely to be
quenched efficiently via the photoinduced electron transfer
(PeT) mechanism,24 which is the fluorescence-controlling me-
chanism generally used for off/on-type Ca2+ probes.25 As a result,
previously reported far-red to NIR fluorescence probes for Ca2+
have high background fluorescence and low values of the
activation ratio,25,26 making them inadequate for biological re-
search applications.
We recently reported a unique far-red to NIR fluorophore,
Si-rhodamine (SiR),27,28 so we chose this dye as a scaffold for the
development of a novel Ca2+ probe. The unique feature of SiR is
that it has relatively low-lying lowest unoccupied molecular
orbital (LUMO) and HOMO energy levels,29 and consequently,
the PeT threshold of SiR differs greatly from those of other long-
wavelength fluorophores. As a Ca2+ chelator, we chose the well-
known 1,2-bis(o-aminophenoxy)ethane-N,N,N0,N0-tetraacetic
acid (BAPTA) moiety. The fluorophore and chelator were
coupled via a short linker to afford the new probe, CaSiR-1
(Figure 1a).3
First we examined the photophysical properties of CaSiR-1.
The fluorescence intensity increased with increasing concentra-
tion of free Ca2+, while the absorption spectrum scarcely changed
(Figure 1b,c). The fluorescence intensity of 0 μM free Ca2+
solution was too small to be distinguished from the baseline, and
the maximum activation ratio of the fluorescence intensity
(39 μM free Ca2+/0 μM free Ca2+, averaged intensity from 660
to 670 nm) reached 1370. In the design of this probe, we
considered that a short distance between SiR and BAPTA would
be preferable because a larger separation of the chelator from the
fluorophore would be expected to result in a smaller change in
PeT efficiency upon chelation or release of Ca2+. To confirm
this, we also synthesized another Ca2+ indicator (compound 12;
see Scheme 2 in the Supporting Information) in which the two
elements are relatively distant, and indeed, it showed a rather
small activation ratio (Figures S2ꢀS4 in the Supporting In-
formation). Furthermore, the fluorescence quantum yield of
CaSiR-1 was 0.20 in the presence of 39 μM free Ca2+, and the
Kd value was 0.58 μM. These values are sufficient for detection of
alcium ion (Ca2+) is a pivotal second messenger inside
C
cells,1 and visualization of the intracellular dynamics of Ca2+
has yielded substantial biological knowledge.2ꢀ8 To understand
information processing by neurons, optical imaging of neuronal
action potentials in terms of somatic Ca2+ increase is currently
being used in the field of neuroscience.9ꢀ14 Although electro-
physiological recording techniques are superior in terms of tem-
poral resolution, fluorescence imaging of Ca2+ provides a unique
opportunity to analyze the profile of multineuronal activities
because bulk-loading methods using acetoxymethyl (AM) ester
derivatives of Ca2+ probes have been established for simultaneous
monitoring of more than 1000 individual neurons,15 which is
impossible with other methods such as patch-clamp recording and
functional magnetic resonance imaging.16ꢀ21
Since the first introduction of a fluorescence probe for Ca2+,
many probes with various characteristics have been developed to
meet the demands of researchers.22 UV to visible fluorescence
probes such as Fura-22 have been most widely utilized in almost
all research areas; however, far-red to near-infrared (NIR)
fluorescence probes are expected to be superior because the
light in this wavelength region shows greater tissue penetration,
has less overlap with the spectrum of background autofluores-
cence, and exhibits less phototoxicity.23 Furthermore, far-red to
NIR fluorescence is likely to be separable from the fluorescence
windows of other fluorescence indicators and markers, including
genetically expressed fluorescent proteins,22 and thus has poten-
tial for multicolor imaging. Therefore, we sought to develop a
novel far-red to NIR fluorescence probe for Ca2+ that would be
Received: June 22, 2011
Published: August 09, 2011
r
2011 American Chemical Society
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Figure 3. (a) Mouse cerebral cortical neurons loaded with CaSiR-1
AM. (b) Fluorescence traces of individual cells identified in (a).
Spontaneous action potentials were visualized in each cell, and some
of the activities were synchronized across neurons. The frame rate
was 10 Hz.
Figure 1. (a) Chemical structures of the novel Ca2+ fluorescence probe
CaSiR-1 and its cell-permeable derivative CaSiR-1 AM. (b) Absorption
and (c) emission spectra of 1 μM CaSiR-1 in the presence of various
concentrations of free Ca2+ (0, 0.017, 0.038, 0.065, 0.100, 0.150, 0.225,
0.351, 0.602, 1.35, and 39 μM) in 30 mM 3-(N-morpholino)propanesulfo-
nic acid (MOPS) buffer containing 100 mM KCl and 10 mM ethylene
glycol tetraacetic acid (EGTA) (pH 7.2) at 22 °C. The excitation
wavelength was 620 nm. The fluorescence quantum yield in the presence
of 39 μM Ca2+ was 0.20. The fluorescence spectrum of the Ca2+-free
solution was indistinguishable from the baseline.
Figure 4. (a) Triple-color imaging and the merged image of an acute
hippocampal slice prepared from a Thy1-mGFP mouse. The CA1
pyramidal cell layer, which contains a small fraction of GFP-positive
neurons (green) was loaded with CaSiR-1 AM (blue) and sulforhoda-
mine 101 (red), an astrocyte-specific fluorescent marker. (b) Electrical
stimulation of Schaffer collateral-induced calcium responses of the cells
numbered in (a). The frame rate was 10 Hz.
Figure 2. (a) Mouse hippocampal CA1 pyramidal cell loaded with
CaSiR-1 through a patch pipet. (b) Action potential (AP)-induced
increases in the CaSiR-1 fluorescence intensity of the neuronal cell body.
APs were evoked by current injection (stim) into the neuron through a
patch-clamp pipet. Each stimulus consisted of 1ꢀ3 current pulses, which
evoked a burst of 1ꢀ3 APs. (c) Increased in the amplitude of the
fluorescence change as a function of the number of APs. Error bars:
mean ( SD. The frame rate was 20 Hz.
readily cleaved by intracellular esterase, resulting in intracellular
release of the probe in the cell-impermeable form. Thus, high
intracellular accumulation can be achieved nearly noninvasively.
We bulk-loaded acute cerebral cortex slices with CaSiR-1 AM
and observed spontaneous neuronal firing expressed in terms of
somatic changes in the CaSiR-1 fluorescence intensity (Figure 3).
The simultaneous activation of plural neurons was identifiable
(Figure 3b; also see the movie in the Supporting Information),
demonstrating synchronous action potentials across neurons, which
are thought to play an important role in brain function.
Finally, we examined the potential for use of the fluorescence
wavelength region of CaSiR-1 as a new color window for multicolor
imaging of Ca2+. Acute brain slices were prepared from transgenic
mice in which a fraction of neurons expressed green fluorescent
protein (GFP; green color) and loaded with both CaSiR-1 AM and
sulforhodamine 101 (SR101, red color), a fluorescent marker
specific for astroglial cells (Figure 4).30 Electrical stimulation was
applied to evoke action potentials in a small population of neurons
of the CA1 pyramidal cell layer. We succeeded in optically separat-
ing the CaSiR-1 signal from the fluorescence of GFP and SR101,
allowing us to visualize the neuronal Ca2+ dynamics.
changes in the intracellular Ca2+ concentration over the usual
range, which is generally on the order of 0.1 μM.
Next, to examine the utility of CaSiR-1, we employed this
probe to visualize the action-potential-mediated Ca2+ increase in
the neuronal cell body. Neurons were loaded with CaSiR-1
through whole-cell recording pipettes (Figure 2a), and action
potentials were evoked with brief current injections. Action
potentials were detectable as time-dependent fluorescence in-
tensity changes (Figure 2b). Furthermore, the amplitude of the
fluorescence change was positively correlated with the number of
action potentials, allowing the number of action potentials to be
estimated (Figure 2c).
We further synthesized CaSiR-1 AM, the cell-membrane-
permeable AM ester of CaSiR-1, for network analyses of neuron
populations (Figure 1a). AM ester formation is a widely used
method for increasing permeability; although CaSiR-1 itself is
almost cell-impermeable because of its high water solubility,
CaSiR-1 AM can diffuse across cell membranes. The AM ester is
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dx.doi.org/10.1021/ja205809h |J. Am. Chem. Soc. 2011, 133, 14157–14159

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In conclusion, we have synthesized CaSiR-1 as the first
biologically applicable far-red to NIR fluorescence probe for
Ca2+. Some far-red to NIR fluorescence probes have already been
reported, but they all have significant drawbacks. For example,
Fura Red shows a decrease in its fluorescence intensity in
response to increment of free Ca2+, which often makes it difficult
to use in biological systems. Moreover, its excitation wavelength
lies in a shorter wavelength region (400ꢀ500 nm), which, in
contrast to the far-red to NIR region, does not offer the
advantages of high tissue penetration, low autofluorescence,
and low phototoxicity.23 Other probes show low off/on ratios
and in fact are not used in biological research.25,26 Our results
indicate that CaSiR-1 and its AM ester offer great advantages over
existing probes for fluorescence imaging of Ca2+ in neuronal
systems, and we have also confirmed that this probe can be used
for multicolor imaging of neuronal action potentials in brain
slices. We believe that CaSiR-1 could become the first choice of a
fluorescence probe for Ca2+ in the far-red to NIR window for
optical imaging in wide range of biological areas such as neuro-
science.
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’ ASSOCIATED CONTENT
S
Supporting Information. Synthesis, experimental de-
b
tails, characterization of CaSiR-1 and another fluorescence probe
for Ca2+, complete ref 5, and a movie (AVI) showing the optical
imaging of spontaneous neuronal firing in mouse cerebral cortex
loaded with CaSiR-1 AM (speeded up by a factor of 10). This
material is available free of charge via the Internet at http://pubs.
acs.org.
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’ AUTHOR INFORMATION
(27) Koide, Y.; Urano, Y.; Hanaoka, K.; Terai, T.; Nagano, T. ACS
Chem. Biol. 2011, 6, 600–608.
(28) Koide, Y.; Urano, Y.; Hanaoka, K.; Terai, T.; Nagano, T. J. Am.
Chem. Soc. 2011, 133, 5680–5682.
Corresponding Author
tlong@mol.f.u-tokyo.ac.jp
(29) Fu, M.; Xiao, Y.; Qian, X.; Zhao, D.; Xu, Y. Chem. Commun.
2008, 1780–1782.
’ ACKNOWLEDGMENT
(30) Nimmerjahn, A.; Kirchhoff, F.; Kerr, J. N. D.; Helmchen, F. Nat.
Methods 2004, 1, 1–7.
This research was supported in part by the Ministry of
Education, Culture, Sports, Science and Technology of Japan
(Specially Promoted Research Grants 22000006 to T.N. and
21659204 to K.H.), the Industrial Technology Development
Organization (NEDO) of Japan (to T.T.), and the Funding
Program for Next Generation World-Leading Researchers
(LS023) to Y.I. K.H. was also supported by the Inoue Founda-
tion for Science, the Konica Minolta Science and Technology
Foundation, and The Asahi Glass Foundation.
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