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. 2009 Sep 1;106(35):15055-60.
doi: 10.1073/pnas.0905110106. Epub 2009 Aug 18.

Differential distribution of endoplasmic reticulum controls metabotropic signaling and plasticity at hippocampal synapses

Niklaus Holbro  1 , Asa Grunditz, Thomas G Oertner
Affiliations

Differential distribution of endoplasmic reticulum controls metabotropic signaling and plasticity at hippocampal synapses

Niklaus Holbro et al. Proc Natl Acad Sci U S A. .

Abstract

Synaptic plasticity is considered essential for learning and storage of new memories. Whether all synapses on a given neuron have the same ability to express long-term plasticity is not well understood. Synaptic microanatomy could affect the function of local signaling cascades and thus differentially regulate the potential for plasticity at individual synapses. Here, we investigate how the presence of endoplasmic reticulum (ER) in dendritic spines of CA1 pyramidal neurons affects postsynaptic signaling. We show that the ER is targeted selectively to large spines containing strong synapses. In ER-containing spines, we frequently observed synaptically triggered calcium release events of very large amplitudes. Low-frequency stimulation of these spines induced a permanent depression of synaptic potency that was independent of NMDA receptor activation and specific to the stimulated synapses. In contrast, no functional changes were induced in the majority of spines lacking ER. Both calcium release events and long-term depression depended on the activation of metabotropic glutamate receptors and inositol trisphosphate receptors. In summary, spine microanatomy is a reliable indicator for the presence of specific signaling cascades that govern plasticity on a micrometer scale.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Fig. 1.
Fig. 1.
ER labeling and spine properties. (A) Organotypic hippocampal cultures were transfected biolistically with a cytoplasmic RFP (red) and an ER-targeted EGFP (GFP-ER, green). (B) Two-photon image (maximum intensity projection) of transfected CA1 pyramidal cell. White box indicates region of analyzed oblique dendrites. (Scale bar: 50 μm.) (C) Dendrite of a transfected CA1 pyramidal cell with one large ER-containing spine (arrow). Overlay of red (RFP) and green (GFP-ER) fluorescence results in yellow color. (Scale bar: 1 μm.) (D) FRAP of the GFP-ER label was performed on ER+ spines, and GFP diffusion inside the ER lumen was monitored (n = 33 spines). (E) Spine head volume of ER+ spines was significantly larger compared with that of ER− spines (ER+, n = 49; ER−, n = 91) (F) Cytoplasmic FRAP time constants were not different in ER+ and ER− spines (ER+, n = 26; ER−, n = 35). Values in D–F represent mean ± SEM.
Fig. 2.
Fig. 2.
ER-containing spines bear strong synapses. (A) Stimulation of ER-containing spine by two-photon glutamate uncaging (arrowhead). (Scale bar: 1 μm.) (B) Average uEPSCs for ER-containing (n = 30) and other spines (n = 44). Colored region represents SEM. (C) Peak amplitude of uEPSC was significantly larger in ER+ spines (n = 30) compared with that in ER− spines (n = 44). Values in C represent mean ± SEM. (D) Relationship between spine volume and uEPSC amplitude (n = 62).
Fig. 3.
Fig. 3.
Uncaging-evoked spine calcium transients. (A) Individual spines were stimulated by two-photon glutamate uncaging (yellow bar, arrowheads). Fluorescence of the calcium-sensitive dye (Fluo5F, green) was monitored in line scan mode and normalized by the fluorescence of the calcium-insensitive dye (Alexa Fluor 594, red). (Scale bars: 200 ms.) (B) Average time course of NMDAR-mediated calcium transients in ER-containing (n = 19) and other spines (n = 26). Colored region represents SEM. (C) Delayed calcium transients (CaT2) in an ER-containing spine (eight consecutive stimulations). (D) In ER+ spines (yellow), amplitudes of delayed calcium transients (CaT2) were large compared with NMDAR-mediated transients (CaT1) (n = 19). No CaT2 were observed in ER− spines (n = 26). Values represent mean ± SEM. (E) Group I mGluR block abolished delayed calcium transients (ER+ spine). (F) Pharmacological profile of delayed calcium transients in ER+ spines (n = 19, 9, 12, 9, and 5).
Fig. 4.
Fig. 4.
Optical induction of synaptic depression. (A) uEPSCs before and after 15 min of glutamate LFU (0.2 Hz) on ER+ spine. Thick trace: Average of five uEPSCs. The LFU-induced depression was long-lasting (n = 6 ER+ spines; each time point represents the average response to five test pulses at 0.03 Hz). (B) The LFU-induced depression in ER-containing spines (n = 20, yellow circles mark the average uEPSC amplitude elicited by five test pulses before and immediately after 15 min of LFU), but not in other spines (n = 11, red circles mark mean uEPSCs). (C) Pharmacological profile of LFU-induced depression (metabotropic glutamate receptor antagonists, n = 7; heparin, n = 9). (D) The LFU-induced depression is homosynaptic (n = 8 spine pairs). Values in A, C, and D represent mean ± SEM.
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