Structural organization of the actin-spectrin–based membrane skeleton in dendrites and soma of neurons

Abstract

Actin, spectrin, and associated molecules form a membraneassociated periodic skeleton (MPS) in neurons. In the MPS, short actin filaments, capped by actin-capping proteins, form ring-like structures that wrap around the circumference of neurites, and these rings are periodically spaced along the neurite by spectrin tetramers, forming a quasi-1D lattice structure. This 1D MPS structure was initially observed in axons and exists extensively in axons, spanning nearly the entire axonal shaft of mature neurons. Such 1D MPS was also observed in dendrites, but the extent to which it exists and how it develops in dendrites remain unclear. It is also unclear whether other structural forms of the membrane skeleton are present in neurons. Here, we investigated the spatial organizations of spectrin, actin, and adducin, an actin-capping protein, in the dendrites and soma of cultured hippocampal neurons at different developmental stages, and compared results with those obtained in axons, using superresolution imaging. We observed that the 1D MPS exists in a substantial fraction of dendritic regions in relatively mature neurons, but this structure develops slower and forms with a lower propensity in dendrites than in axons. In addition, we observed that spectrin, actin, and adducin also form a 2D polygonal lattice structure, resembling the expanded erythrocyte membrane skeleton structure, in the somatodendritic compartment. This 2D lattice structure also develops substantially more slowly in the soma and dendrites than the development of the 1D MPS in axons. These results suggest membrane skeleton structures are differentially regulated across different subcompartments of neurons.actin | spectrin | adducin | super-resolution imaging | STORM I t was recently discovered that actin, spectrin, and associated molecules form a membrane-associated periodic skeleton (MPS) structure in neurons (1). As revealed by superresolution stochastic optical reconstruction microscopy (STORM) (1), this structure contains molecules homologous to those present in the erythrocyte membrane skeleton, including spectrin (2-4), actin, and actincapping proteins such as adducin (5, 6), but adopts a structural organization that is distinct from the polygonal lattice structure of the erythrocyte membrane skeleton (7,8). In this structure, short actin filaments capped by adducin are organized into repetitive, ring-like structures that wrap around the circumference of the axon underneath the axonal membrane, and adjacent actin rings are connected by spectrin tetramers, forming a long-range quasi-1D periodic structure with a periodicity of ∼190 nm (1). This periodic structure is formed extensively throughout the axonal shaft, including the axon initial segment and the unmyelinated distal axons, as well as the nodes of Ranvier and internodal segments in myelinated axons (1, 9-14), but appears to be perturbed at synaptic sites (13,15,16). It is a highly prevalent structure observed in many different types of neurons, including both excitatory and inhibitory neuro...

The Spectrin-Actin-Based Periodic Cytoskeleton as a Conserved Nanoscale Scaffold and Ruler of the Neural Stem Cell Lineage

“…Similar to the extensively characterized primary hippocampal neurons (D’Este et al, 2015; Ganguly et al, 2015; Han et al, 2017; Xu et al, 2013; Zhong et al, 2014), NSC-derived neurons exhibited a 1D periodic membrane cytoskeleton (Figure S2). …”

“…Although initially noted in neuronal axons as adducin-capped actin rings connected by spectrin tetramers to form a periodic, one-dimensional (1D) lattice of well-defined, ~180- to 190-nm periodicity (Xu et al, 2013), related periodic or quasi-periodic cytoskeletal structures have also been observed in dendrites (D’Este et al, 2015; Han et al, 2017) and certain glial cell types (D’Este et al, 2016, 2017; He et al, 2016). Such periodic nanostructures are markedly different from the traditional view of the actin-based cytoskeleton in common mammalian cell types (e.g., dense filament networks and bundles in fibroblasts and epithelial cells) (Chhabra and Higgs, 2007; Pollard and Cooper, 2009; Xu et al, 2012) as well as the spectrin-actin-based cytoskeleton in erythrocytes (2D triangular lattices of short actin filaments connected by spectrin tetramers) (Baines, 2010; Bennett and Baines, 2001; Bennett and Gilligan, 1993; Fowler, 2013; Pan et al, 2018).…”

Cytoskeletal makeup of the synapse: Shaft versus spine

“…Interestingly, in contrast to longitudinal and branched actin filaments in the spines, periodic F‐actin structures are very stable and resistant to depolymerization induced by treatment with Latrunculin A and B (Abouelezz, Micinski, Lipponen, & Hotulainen, ). Furthermore, F‐actin forms a membrane‐associated periodic lattice together with βII, βIII, and βIV spectrins, where actin filaments are spaced by spectrin tetramers with a periodicity of approximately 190 nm (Bär et al, ; D'Este et al, ; Han, Zhou, Xia, & Zhuang, ). A very recent correlative dSTORM and electron microscopy study indicated that on an ultrastructural level, this periodically organized F‐actin forms a braid‐like structure containing two long, entwined actin filaments (Vassilopoulos, Gibaud, Jimenez, Caillol, & Leterrier, ).…”

“…It has been suggested by Han et al, that spectrin isoforms partially exhibit a compartmentally distinct expression. While βII spectrin seems to be expressed uniformly within axons and dendrites, βIII spectrin is particularly enriched in dendrites (Han et al, ). Indeed, only a fraction of dendritic spines had a periodic lattice formed by F‐actin/βII spectrin (Bär et al, ).…”

Cargo hold and delivery: Ankyrins, spectrins, and their functional patterning of neurons

“…For instance, in addition to crosslinking actin rings to form the periodic membrane structure (MPS) in axon and dendrites, a cytosolic pool of βII‐spectrin independently facilitates bidirectional axonal transport of synaptic vesicles and other cargo (Lorenzo et al, ). Likewise, αII‐spectrin and βIII‐spectrin have been shown to both help assemble the MPS and to associate with motor protein complexes in neurons (Han, Zhou, Xia, & Zhuang, ; Holleran et al, ; Huang et al, ; Lorenzo et al, ; Takeda et al, ).…”