The ultrastructural properties of the endoplasmic reticulum govern microdomain signaling in perisynaptic astrocytic processes

Abstract

Astrocytes are now widely accepted as key regulators of brain function and behavior. Calcium (Ca2+) signals in perisynaptic astrocytic processes (PAPs) enable astrocytes to fine-tune neurotransmission at tripartite synapses. As most PAPs are below the diffraction limit, their content in Ca2+ stores and the contribution of the latter to astrocytic Ca2+ activity is unclear. Here, we reconstruct hippocampal tripartite synapses in 3D from a high resolution electron microscopy (EM) dataset and find that 75% of PAPs contain some endoplasmic reticulum (ER), a major calcium store in astrocytes. The ER in PAPs displays strikingly diverse shapes and intracellular spatial distributions. To investigate the causal relationship between each of these geometrical properties and the spatio-temporal characteristics of Ca2+ signals, we implemented an algorithm that generates 3D PAP meshes by altering the distribution of the ER independently from ER and cell shape. Reaction-diffusion simulations in these meshes reveal that astrocyte activity is governed by a complex interplay between the location of Ca2+ channels, ER surface-volume ratio and spatial distribution. In particular, our results suggest that ER-PM contact sites can act as local signal amplifiers if equipped with IP3R clusters but attenuate PAP Ca2+ activity in the absence of clustering. This study sheds new light on the ultrastructural basis of the diverse astrocytic Ca2+ microdomain signals and on the mechanisms that regulate neuron-astrocyte signal transmission at tripartite synapses. Main pointsO_LI6 nm isotropic volume EM reveals that 75% of perisynaptic astrocytic processes (PAPs) contain some ER C_LIO_LIPAPs & ER of the same cell display diverse geometric properties C_LIO_LISimulations in EM-derived meshes hint that ER geometry governs Ca2+ signals in PAPs C_LI TOC O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=199 SRC="FIGDIR/small/482292v4_ufig1.gif" ALT="Figure 1"> View larger version (64K): org.highwire.dtl.DTLVardef@e2387aorg.highwire.dtl.DTLVardef@1e9db45org.highwire.dtl.DTLVardef@3ab0d1org.highwire.dtl.DTLVardef@12e9ab2_HPS_FORMAT_FIGEXP M_FIG C_FIG