TRPA1 channels are regulators of astrocyte basal calcium levels and long-term potentiation via constitutive D-serine release - PubMed (original) (raw)

TRPA1 channels are regulators of astrocyte basal calcium levels and long-term potentiation via constitutive D-serine release

Eiji Shigetomi et al. J Neurosci. 2013.

Abstract

Astrocytes are found throughout the brain where they make extensive contacts with neurons and synapses. Astrocytes are known to display intracellular Ca(2+) signals and release signaling molecules such as D-serine into the extracellular space. However, the role(s) of astrocyte Ca(2+) signals in hippocampal long-term potentiation (LTP), a form of synaptic plasticity involved in learning and memory, remains unclear. Here, we explored a recently discovered novel TRPA1 channel-mediated transmembrane Ca(2+) flux pathway in astrocytes. Specifically, we determined whether block or genetic deletion of TRPA1 channels affected LTP of Schaffer collateral to CA1 pyramidal neuron synapses. Using pharmacology, TRPA1(-/-) mice, imaging, electrophysiology, and D-serine biosensors, our data indicate that astrocyte TRPA1 channels contribute to basal Ca(2+) levels and are required for constitutive D-serine release into the extracellular space, which contributes to NMDA receptor-dependent LTP. The findings have broad relevance for the study of astrocyte-neuron interactions by demonstrating how TRPA1 channel-mediated fluxes contribute to astrocyte basal Ca(2+) levels and neuronal function via constitutive D-serine release.

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Figures

Figure 1.

Figure 1.

RT-PCR evidence for TRPA1 mRNAs in hippocampal astrocytes. The gel shows the results of RT-PCR experiments for TRPA1 and GFAP (control) with and without reverse transcriptase for mRNA harvested from purified hippocampal astrocytes. The lower bar graph quantifies the results from multiple experiments (n = 3).

Figure 2.

Figure 2.

Functional evidence that astrocyte branches express TRPA1 channels that mediate spotty Ca2+ signals. A, Representative traces of spontaneous spotty Ca2+ signals recorded from branches of astrocytes expressing Lck-GCaMP3 (Shigetomi et al., 2013). B, As in A, but in the presence of HC 030031 (80 μ

m

). C, Cumulative probability plots for Ca2+ signal peak heights (dF/F), duration (_T_0.5), and frequency. The cumulative probability plots are shifted to smaller amplitudes and lower frequencies in the presence of HC 030031. Note the two distributions are significantly different on the basis of a Kolmogorov–Smirnov test. Average data are presented in the text. D, Scatter plot shows how the number of spotty Ca2+ events measured per astrocyte with Lck-GCaMP3 decreases in the presence of HC 030031. E, Plots the number of spotty Ca2+ events measured as a function of distance from the soma. Note there is a consistent decrease in the number of Ca2+ events measured in the presence of HC 030031 throughout astrocyte territories, implying that the significant decrease shown in E does not occur at any specific distance from the soma. Rather, the HC 030031-sensitive Ca2+ events appear to be drawn from entire astrocytes.

Figure 3.

Figure 3.

Blocking TRPA1 channels reduces astrocyte basal Ca2+ levels. A, Graphs plot dF/F over time from astrocytes loaded with Fluo-4 in hippocampal slices from adult mice. Application of HC 030031 (80 μ

m

) reduced the baseline fluorescence of astrocyte somata. Note that in these traces, some of the peaky Ca2+ transients have been clipped to more clearly show the drop in baseline Ca2+. B, Cumulative probability plot of dF/F evoked by HC 030031 applications to astrocytes in acute hippocampal slices from WT and _TRPA1_−/− mice. Note the two distributions are significantly different on the basis of a Kolmogorov–Smirnov analysis. The bar graph summarizes the finding that HC 030031 reduces resting Ca2+ levels in astrocytes from WT hippocampal slices.

Figure 4.

Figure 4.

Blocking TRPA1 channels reduces LTP. A, fEPSP traces were recorded from the stratum radiatum before and after high-frequency stimulation (HFS; 2 × 100 Hz, intertrain interval 10 s). Traces are shown for several experiments at various time points before and after HFS. B, HFS-induced robust LTP in control experiments (n = 7). HC 030031 (80 μ

m

, n = 6) suppressed LTP in a manner that was rescued by 10 μ

m d

-serine (n = 6). C,

d

-Serine (10 μ

m

) did not affect LTP (n = 6, p = 0.876). D, The “glycine site” antagonist DCKA (10 μ

m

; n = 6) suppressed LTP E, AITC (100 μ

m

) did not affect LTP (n = 7). F, G, HC 030031 did not affect AMPAR-mediated fEPSPs (F) or presynaptic fiber volleys (G; n = 7).

Figure 5.

Figure 5.

Reduced LTP in _TRPA1_−/− mice. A, Traces for fEPSPs recorded from hippocampal slices harvested from WT and _TRPA1_−/− mice. The relationship between fEPSP slope and presynaptic fiber volley for WT and _TRPA1_−/− mice is also shown. The inset shows an average fEPSP. B, LTP was suppressed in _TRPA1_−/− mice relative to WT controls. C, Suppressed LTP in _TRPA1_−/− mice was restored by bath applications of 10 μ

m d

-serine. D, Summary bar graph showing the degree of LTP in control recordings and under the various conditions indicated. HC 030031 (80 μ

m

) failed to suppress LTP in _TRPA1_−/− mice and LTP was equivalent in _IP3R2_−/− and WT littermate mice.

Figure 6.

Figure 6.

LTP was not altered when GAT-3 was blocked or when GABA was applied in the bath. A–C, LTP was normal and indiscernible from WT controls in slices treated with (A) the GAT-3 blocker β-alanine (100 μ

m

), (B) GABA (10 μ

m

), and (C) both β-alanine and GABA together. D, Summary bar graph of the findings shown in A–C.

Figure 7.

Figure 7.

Extracellular

d

-serine levels are regulated by TRPA1 channel function. A, Photograph of a hippocampal slice from a 2-month-old mouse with biosensor electrodes placed in the stratum radiatum. This diagram illustrates how the biosensors work. B, Representative traces and average data for biosensor electrode calibration. C, Representative traces showing measurement of [

d

-serine] with biosensor electrodes when the biosensor was positioned above the slice, moved into the slice, and then withdrawn from the slice. This protocol results in two measurements of [

d

-serine] indicated in blue and red text. D, As in C, but for recordings from a _TRPA1_−/− mouse. E, The bar graph summarizes the peak [

d

-serine] for three experimental conditions. There were no significant differences. F, The bar graph summarizes the steady-state [

d

-serine] within the slice for three experimental conditions. Note that [

d

-serine] levels are reduced in WT slices treated with HC 030031 and in slices from _TRPA1_−/− mice.

Figure 8.

Figure 8.

Blocking TRPA1 channels regulates the NMDAR to AMPAR ratio for evoked EPSCs. A, Traces showing the AMPA and NMDA components of evoked EPSCs measured from CA1 pyramidal neurons (average of 5 sweeps). B, NMDAR/AMPAR ratios (recorded at +40 mV) in CA1 pyramidal cells from 17 _TRPA1_−/− neurons (n = 4 mice) and 15 WT neurons cells (n = 4 mice). The NMDAR-mediated component of the EPSC was significantly reduced in _TRPA1_−/− neurons (p < 0.01). The same experiment was done in the presence of 10 μ

m d

-serine (pink bars). Results are from 10 neurons from three WT mice and 11 neurons from three _TRPA1_−/− mice are shown. In the presence of

d

-serine the NMDAR/AMPAR ratios in _TRPA1_−/− cells were not significantly different from WT (p = 0.886). In _TRPA1_−/− neurons, the ratios are significantly higher in the presence of

d

-serine (p < 0.01) while

d

-serine has no effect on the NMDAR component of EPSCs in WT neurons cells (p = 0.215). C, A 10 min bath application of 80 μ

m

HC 030031 inhibits the NMDAR-mediated component of the EPSCs in WT neurons (paired t test comparison, p < 0.01, n = 9 cells) but has no effect on the NMDAR-mediated component of EPSCs from _TRPA1_−/− mice (paired t test comparison, p = 0.488, n = 8 cells). D, HC 030031 has no effect on the NMDAR-mediated EPSCs in WT (8 cells) or _TRPA1_−/− neurons (8 cells) when applied in the presence of 10 μ

m d

-serine.

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References

    1. Agulhon C, Petravicz J, McMullen AB, Sweger EJ, Minton SK, Taves SR, Casper KB, Fiacco TA, McCarthy KD. What is the role of astrocyte calcium in neurophysiology? Neuron. 2008;59:932–946. doi: 10.1016/j.neuron.2008.09.004. - DOI - PMC - PubMed
    1. Agulhon C, Fiacco TA, McCarthy KD. Hippocampal short- and long-term plasticity are not modulated by astrocyte Ca2+ signaling. Science. 2010;327:1250–1254. doi: 10.1126/science.1184821. - DOI - PubMed
    1. Akerboom J, Chen TW, Wardill TJ, Tian L, Marvin JS, Mutlu S, Calderón NC, Esposti F, Borghuis BG, Sun XR, Gordus A, Orger MB, Portugues R, Engert F, Macklin JJ, Filosa A, Aggarwal A, Kerr RA, Takagi R, Kracun S, et al. Optimisation of a GCaMP calcium indicator for neural activity imaging. J Neurosci. 2012;32:13819–13840. doi: 10.1523/JNEUROSCI.2601-12.2012. - DOI - PMC - PubMed
    1. Barres BA. The mystery and magic of glia: a perspective on their roles in health and disease. Neuron. 2008;60:430–440. doi: 10.1016/j.neuron.2008.10.013. - DOI - PubMed
    1. Basu AC, Tsai GE, Ma CL, Ehmsen JT, Mustafa AK, Han L, Jiang ZI, Benneyworth MA, Froimowitz MP, Lange N, Snyder SH, Bergeron R, Coyle JT. Targeted disruption of serine racemase affects glutamatergic neurotransmission and behavior. Mol Psychiatry. 2009;14:719–727. doi: 10.1038/mp.2008.130. - DOI - PMC - PubMed

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