Formation and Maintenance of Functional Spines in the Absence of Presynaptic Glutamate Release

Abstract

Dendritic spines are the major transmitter reception compartments of glutamatergic synapses in most principal neurons of the mammalian brain and play a key role in the function of nerve cell circuits. The formation of functional spine synapses is thought to be critically dependent on presynaptic glutamatergic signaling. By analyzing CA1 pyramidal neurons in mutant hippocampal slice cultures that are essentially devoid of presynaptic transmitter release, we demonstrate that the formation and maintenance of dendrites and functional spines are independent of synaptic glutamate release.

Keywords: activity dependence; dendrite formation; glutamate release; glutamate uncaging; spine formation; synaptogenesis.

Figure 1

Functional Synaptic Inputs to M13-DKO and CTR Neurons (A) Example traces of sPSCs in M13-DKO (red) and CTR neurons (black). (B and C) sPSC frequencies (B) and amplitudes (C) in M13-DKO (red) and CTR neurons (black). (D) Examples traces of mEPSCs in M13-DKO (red) and CTR neurons (black). (E and F) mEPSC frequencies (E) and amplitudes (F) in M13-DKO (red) and CTR neurons (black). (G) Examples traces of mIPSCs in M13-DKO (red) and CTR neurons (black). (H and I) mIPSC frequencies (H) and amplitudes (I) in M13-DKO (red) and CTR neurons (black). 1/2/3 W, WIV1/2/3. Data are shown as mean ± SEM. n Values of cells are given in/above bars. ^∗p < 0.05, ^∗∗p < 0.01, ^∗∗∗p < 0.001 (Student’s t test). See also Table S1.

Figure 2

Dendrite Development in M13-DKO and CTR Neurons (A and B) Confocal fluorescence image of representative M13-DKO (A) and CTR neurons (B) at WIV3, after filling with biocytin and subsequent staining with Alexa Fluor 555-labeled streptavidin. The concentric circles in (A) indicate spheres at 50 μm distance intervals relative to the soma (dashed line, apical dendrite; dotted line, basal dendrite). The white boxes indicate the dendrite subsections shown in Figure 3. Scale bars, 50 μm. (C–E) Sholl analysis of dendrite complexity in M13-DKO and CTR neurons at WIV1 (C), WIV2 (D), and WIV3 (E). Linear Sholl plots for the number of intersections of dendrites (mean ± SEM) with spheres at 50-μm distance intervals relative to the soma are shown. Basal and apical dendrites were analyzed separately, and the data for basal dendrite intersections are plotted for negative Sholl radius values, with the soma center at 0. Data are shown as mean ± SEM. n Values of cells are given in the insets. ^∗p < 0.05 (Student’s t test).

Figure 3

Spine Number and Distribution in M13-DKO and CTR Neurons (A and B) Representative dendrite segments (first-order branches from main dendrites; see white boxes in Figures 2A and 2B) in an M13-DKO (A) and a CTR (B) neuron at WIV3, after filling with biocytin and staining with Alexa Fluor 555-labeled streptavidin. Identified spines were classified as thin (upright rectangles), stubby (squares), and mushroom-type (triangles). Filopodia, which are rarely seen at this developmental stage, were not found in the dendrite sections shown. White boxes indicate the regions shown at higher magnification in (C) and (D) and (E) and (F), respectively. Scale bars, 5 μm. (C and E) Angular projections of spines in three different angles within ± 60°, in the regions indicated by the white boxes in (A) and (B), respectively. Scale bars, 2 μm. (D and F) Schematic drawings for (C) and (E), respectively. (G–R) Quantitative analysis of the numbers and distributions along dendrites of dendritic protrusions (i.e., thin, stubby, and mushroom-type spines and filopodia) in M13-DKO and CTR neurons at WIV1, WIV2, and WIV3. Dendritic spines were counted for each entire neuron, separating data for basal and apical dendrites (Figures 3A and 3B). Each data point represents the mean ± SEM for a 50 μm bin along the dendrite, measured from the soma center. Positive values on the abscissa represent data from apical dendrites, and negative values on the abscissa represent data from basal dendrites. Shown are stubby spines (G–I), mushroom type spines (J–L), thin spines (M–O), and filopodia (P–R) at WIV1, WIV2, and WIV3. Data are shown as mean ± SEM. n Values of cells are given in the insets. ^∗p < 0.05 (Student’s t test). See also Figure S1.

Figure 4

Activity-Dependent Spinogenesis and Spine Responsiveness in M13-DKO and CTR Neurons (A) Images of dendrites from M13-DKO (top) and CTR (bottom) CA1-PNs expressing tdTomato and exposed to HFU (yellow crosses, 60 pulses at 10 Hz) at WIV1. Yellow arrowheads indicate de novo spine formation following HFU. Scale bars, 1 μm. (B) Success rate of de novo spine formation by HFU at WIV1, WIV2, or WIV3 in M13-DKO (red bars; WIV1, n = 19 trials, 8 cells; WIV2, n = 20 trials, 9 cells; WIV3, n = 21 trials, 10 cells) and CTR CA1-PNs (black bars; WIV1, n = 18 trials, 8 cells; WIV2, n = 25 trials, 10 cells; WIV3, n = 22 trials, 9 cells). (C) Summary plots of the distance-dependent success rate of de novo spine formation from M13-DKO (red) and CTR CA1-PNs (black). (D) Two-photon image of a dendritic segment from a CA1-PN recorded in whole-cell voltage-clamp mode in organotypic slice culture. A target spine was exposed to glutamate uncaging test pulses (green cross, eight to ten trials at 0.1 Hz). Scale bars, 10 μm (left) and 1 μm (right). (E) uEPSCs evoked by glutamate uncaging from M13-DKO (red traces) and CTR CA1-PNs (black traces) measured in whole-cell voltage-clamp mode at −65 mV for AMPAR-uEPSCs (filled circles) and +40 mV for NMDAR-uEPSCs (open circles). The green dotted line (70 ms after the uncaging pulse) indicates measuring points for NMDAR-uEPSC amplitudes. (F and G) The amplitude of AMPAR-uEPSCs (F) and NMDAR-uEPSCs (G) plotted against relative spine size from M13-DKO (red circles, n = 76 spines, 21 cells) and CTR CA1-PNs (black circles, n = 66 spines, 22 cells). (H) Left: mean AMPAR-uEPSC amplitudes plotted against the mean estimated size of small, medium, medium-large, and large spines for M13-DKO (red circles) and CTR CA1-PNs (black circles). Right: summary graph of AMPAR-uEPSC amplitudes from M13-DKO (red bars; small spines, n = 22; medium spines, n = 17; medium-large spines, n = 20; large spines, n = 17; 21 cells) and CTR CA1-PNs (black bars; small spines, n = 27; medium spines, n = 20; medium-large spines, n = 12; large spines, n = 7; 22 cells). (I) Summary graph of NMDAR-uEPSC amplitudes from M13-DKO (red bar; n = 76 spines, 22 cells) and CTR CA1-PNs (black bar; n = 66 spines, 21 cells). Where relevant, data are shown as mean ± SEM. ^∗p < 0.05, Student’s t test. See also Table S2.