Ultrastructure of the axonal periodic scaffold reveals a braid-like organization of actin rings

Abstract

Recent super-resolution microscopy studies have unveiled a periodic scaffold of actin rings regularly spaced by spectrins under the plasma membrane of axons. However, ultrastructural details are unknown, limiting a molecular and mechanistic understanding of these enigmatic structures. Here, we combine platinum-replica electron and optical super-resolution microscopy to investigate the cortical cytoskeleton of axons at the ultrastructural level. Immunogold labeling and correlative super-resolution/ electron microscopy allow us to unambiguously resolve actin rings as braids made of two long, intertwined actin filaments connected by a dense mesh of aligned spectrins. This molecular arrangement contrasts with the currently assumed model of actin rings made of short, capped actin filaments. Along the proximal axon, we resolved the presence of phospho-myosin light chain and the scaffold connection with microtubules via ankyrin G. We propose that braided rings explain the observed stability of the actin-spectrin scaffold and ultimately participate in preserving the axon integrity.

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

“…New improvements in superresolution (Culley, Tosheva, Matos Pereira, & Henriques, ; Wang, Taki, et al, ) and correlative microscopies (Vassilopoulos, Gibaud, Jimenez, Caillol, & Leterrier, ) should soon enable us to capture the dynamic itineraries of ankyrins and spectrins at nanoscale resolution, which will in turn help us more precisely pinpoint their roles in, and individual commitments to, known and novel pathways. This will represent a significant step in deciphering the code behind their modes of functional partitioning and long‐range and local organization.…”

Cytoskeletal makeup of the synapse: Shaft versus spine

“…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.…”

Role of actin in organelle trafficking in neurons

“…Actin is present in the neuron in various pools like the actinspectrin membrane periodic skeleton (D'Este et al, 2015;Xu et al, 2013), waves (Flynn et al, 2009), trails (Ganguly et al, 2015) and actin-rich regions (Sood et al, 2018;van Bommel et al, 2019). The MPS shows the presence of braided filaments that may arise from complex actin arrangements (Vassilopoulos et al, 2019). However, a detailed description for other structures like actin trails or branched actin at dendritic spines or the actin enrichment seen at the AIS is not well known.…”

“…Actin and associated proteins help the neurons withstand compressive forces and maintain neuronal structure through the formation of membrane associated periodic structures (MPS) (Hammarlund, Jorgensen, & Bastiani, 2007;Krieg, Dunn, & Goodman, 2014;Xu et al, 2013). MPS, is a quasione-dimensional periodic lattice structure present under the plasma membrane, comprising of braided ring like actin structures connected by αand β-spectrin heterotetramers (D'Este, Kamin, Gottfert, El-Hady, & Hell, 2015;He et al, 2019;Vassilopoulos, Gibaud, Jimenez, Caillol, & Leterrier, 2019;Zhong et al, 2014). Actin waves form growth-cone-like structures along the shaft of neuronal processes (Flynn, Pak, Shaw, Bradke, & Bamburg, 2009).…”