A Uniform and Isotropic Cytoskeletal Tiling Fills Dendritic Spines

Abstract

Dendritic spines are submicron, subcellular compartments whose shape is defined by actin filaments and associated proteins. Accurately mapping the cytoskeleton is a challenge, given the small size of its components. It remains unclear whether the actin-associated structures analyzed in dendritic spines of neurons in vitro apply to dendritic spines of intact, mature neurons in situ. Here, we combined advanced preparative methods with multitilt serial section electron microscopy (EM) tomography and computational analysis to reveal the full three-dimensional (3D) internal architecture of spines in the intact brains of male mice at nanometer resolution. We compared hippocampal (CA1) pyramidal cells and cerebellar Purkinje cells in terms of the length distribution and connectivity of filaments, their branching-angles and absolute orientations, and the elementary loops formed by the network. Despite differences in shape and size across spines and between spine heads and necks, the internal organization was remarkably similar in both neuron types and largely homogeneous throughout the spine volume. In the tortuous mesh of highly branched and interconnected filaments, branches exhibited no preferred orientation except in the immediate vicinity of the cell membrane. We found that new filaments preferentially split off from the convex side of a bending filament, consistent with the behavior of Arp2/3-mediated branching of actin under mechanical deformation. Based on the quantitative analysis, the spine cytoskeleton is likely subject to considerable mechanical force in situ .

Ion-concentration gradients induced by synaptic input increase the voltage depolarization in dendritic spines

“…( 3 ). The head volume is set to , to match the typical volume of the intracellular space of spines from hippocampal pyramidal cells, excluding organelles and cytoskeleton (Eberhardt et al, 2022 ). Then a constant influx of sodium ions that leads to an electric current of 25 pA for a time of 10 ms is injected.…”

“…Due to the small and restricted volume of spines, their intracellular ion concentrations can easily change (Lagache et al, 2019 ). A typical pyramidal cell spine head, with a volume of (Eberhardt et al, 2022 ), is estimated to contain approximately sodium ions 1 at a concentration of 10 mM. During an excitatory postsynaptic potential (EPSP) with a synaptic current of 23 pA (Cornejo et al, 2022 ) that lasts for 12 ms (Acker et al., 2016 ), the estimated number of sodium ions entering the spine 2 is roughly .…”

Postmortem changes in brain cell structure: a review

“…To integrate diverse empirical findings of postmortem changes into a unified understanding, it is essential to have a model of how cell membranes decompose after death. Brain cell membrane shape is largely maintained through the cytoskeleton, a gel-like network of proteins that exhibits both passive elastic and active viscous behavior (Ananthakrishnan et al, 2006;Eberhardt et al, 2022;Mogilner and Manhart, 2018). In particular, cell membrane biomolecules are tethered to the underlying actin cortex through interacting proteins (Chugh and Paluch, 2018;Svitkina, 2020).…”