Fig 1.
Dendritic spine reconstruction and Ca2+ modeling.
(A) Spine morphologies are based on serial reconstructions of transmission electron micrographs obtained from human cortical tissue. (B) An example of a reconstructed dendritic spine (dendrite, red; spine compartment, yellow) carrying a spine apparatus organelle (SA, green). Upon release of Ca2+ at the reconstructed postsynaptic density (cyan), changes in [Ca2+] are determined in the head, neck and dendritic region, respectively. Scale bar, 200 nm. (C) Immunogold staining for the actin-binding protein synaptopodin [22], which is a marker and essential component of the SA. Asterisk indicates presynaptic, circle the postsynaptic compartment. Scale bar, 200 nm. (D) In all reconstructed human spines the SA was attached to a single standardized dendritic ER tubule in a dendritic compartment with identical dimensions. Scale bar, 500 nm. (E) The model accounts for Ca2+ exchange mechanisms on the plasma membrane (Na+/Ca2+ exchangers (NCX), plasma membrane Ca2+-ATPases (PMCA)) and on the ER (green; ryanodine-receptors (RyR)). Calcium buffering capacity (Calbindin, CalB) is kept constant. Further details on physiological values are provided in the main text and in Table 1. Tables 2 and 3 contain morphological measurements such as surface area. See also S1 and S2 Videos, these videos demonstrate the propagation of calcium to the dendritic region.
Table 1.
Numerical Values for Simulation Parameters.
Table 2.
RyR Critical Density, Surface Area Measurements, and Number of RyRs.
Table 3.
Neck Cross-Sectional Areas.
Fig 2.
Morphological parameter of reconstructed spines.
(A) Three-dimensional models based on reconstructed human dendritic spines. Spine apparatus organelle, green. Postsynaptic density, cyan. Scale bar, 200 nm (B) Dendritic spines with large volumes have large postsynaptic densities. (C) Positive correlation between spine neck diameter and postsynaptic density sizes. (D) Postsynaptic densities in large spines are not further away from spine base. (E) Large spines contain large spine apparatus organelles, which are found in spines with wider spine necks. n = 9 reconstructed dendritic spines from 7 independent samples.
Fig 3.
Effects of passive spine SA on spine-to-dendrite Ca2+ signaling, with fixed Ca2+ influx. Row A, without SA, and row B, with SA, show [Ca2+] profiles for 10 ms initial Ca2+ release into the spine head. Consistent with experimental data, spine-to-dendrite Ca2+ signaling does not occur for all 9 simulated spines, and this also holds true for simulations where Ca2+ influx is adapted to the postsynaptic area, see additional plots. Row C (fixed Ca2+ influx) demonstrates correlations between maximum [Ca2+] in the head region versus spine volume, ratio of SA area/Spine area, and SA volume/Spine volume. Presence of a passive SA, i.e., no ryanodine receptors, has no major effect on [Ca2+] dynamics in the spine head and neck.
Table 4.
Peak calcium concentrations in measuring zones.
Fig 4.
Passive spine SA spine-to-dendrite signaling.
Effects of passive spine SA on spine-to-dendrite Ca2+ signaling, with PSD adjusted Ca2+ influx. Plots in row A are without SA and plots in row B are with SA, showing [Ca2+] profiles for 10 ms initial Ca2+ release into the spine head. Consistent with experimental data, spine-to-dendrite Ca2+ signaling does not occur for all 9 simulated spines. Presence of a passive SA, i.e., no ryanodine receptors, has no major effect on Ca2+ dynamics in the spine head and neck.
Fig 5.
Effects of active SA with RyR calcium exchange mechanisms only.
These plots were generated with active SA with only RyR. Plots in row A show the calcium concentration traces in the head and dendrite at the critical RyR density. Plots in row B show the calcium concentration traces at sub-critical (5% below critical) RyR density. It is worth observing that Ca2+ coupling is significantly decreased. In Fig 5C, 5D, and 5E, we demonstrate for spine 8 that there is a critical RyR density such that Ca2+ coupling occurs in the dendritic region. The RyR density parameter is incremented in 0.01 μm−2 steps. The maximum [Ca2+] in the dendritic region has a significant jump at ≈ 3.50 μm−2, please also see the supplemental video showing an accumulation of calcium in the spine region; however, at critical RyR density calcium is propagated to the dendritic region. For plot D we performed the same RyR density increment experiments but with different Ca2+ influx at the synapse. For all spine simulations (rows A and B) we used a fixed Ca2+ influx at the synapse.
Fig 6.
All plots were generated with PSD adjusted calcium influx. In row A, these are the calcium concentration profiles in the head and dendrite region at critical RyR density. In row B, these calcium concentration profiles correspond to sub-critical RyR density.
Fig 7.
In A we plot the critical RyR density against the ratio of Spine Apparatus Neck area to Spine Neck area. In Table 2 we show the values that correspond to A. In C and D we demonstrate that widening the neck diameter of a spine affects the coupling of Ca2+ to the dendritic region. In particular E indicates that neck widening causes the critical RyR density to increase.
Fig 8.
“All or-nothing” spine-to-dendrite calcium communication.
Schematic illustration summarizing our major findings. Ca2+ communication between spine head and dendrite is controlled by the interplay between RyR density, and morphological changes of dendritic spines and spine apparatus organelles (SA) in the neck region. RyR, ryanodine receptors.