Myosin V regulates synaptopodin clustering and localization in the dendrites of hippocampal neurons.

Anja Konietzny,Alberto Perez-Alvarez,Dick H W Dekkers,Matthias Kneussel,Wolfgang Wagner,Michael Frotscher,Marina Mikhaylova,Thomas G Oertner,Jeroen A A Demmers,John A Hammer,Julia Bär,Judit González-Gallego,Alexander Drakew

doi:10.1242/jcs.230177

Abstract

ABSTRACTThe spine apparatus (SA) is an endoplasmic reticulum-related organelle that is present in a subset of dendritic spines in cortical and pyramidal neurons, and plays an important role in Ca2+ homeostasis and dendritic spine plasticity. The protein synaptopodin is essential for the formation of the SA and is widely used as a maker for this organelle. However, it is still unclear which factors contribute to its localization at selected synapses, and how it triggers local SA formation. In this study, we characterized development, localization and mobility of synaptopodin clusters in hippocampal primary neurons, as well as the molecular dynamics within these clusters. Interestingly, synaptopodin at the shaft-associated clusters is less dynamic than at spinous clusters. We identify the actin-based motor proteins myosin V (herein referring to both the myosin Va and Vb forms) and VI as novel interaction partners of synaptopodin, and demonstrate that myosin V is important for the formation and/or maintenance of the SA. We found no evidence of active microtubule-based transport of synaptopodin. Instead, new clusters emerge inside spines, which we interpret as the SA being assembled on-site.

Highlights

As an adaptation to their enormous size, neurons have developed a highly sophisticated trafficking system that mediates long-distance transport and local control of membrane and protein turnover (Hanus and Ehlers, 2016)
Asanuma et al showed that this short splice isoform contains two α-actinin-2- and α-actinin-4-binding sites of its own, but they did not study synaptopodin in brain (Asanuma et al, 2005). α-Actinin is non-selectively enriched in all types of spines, its role in localizing synpatopodin and the spine apparatus (SA) to selected spines is questionable (Hodges et al, 2014; Matt et al, 2018; Nakagawa et al, 2004)
We hypothesized that the processive myosins V and VI could transport and anchor synaptopodin to dendritic spines, thereby mediating the synaptopodin-dependent formation of the spine apparatus

Summary

Introduction

As an adaptation to their enormous size, neurons have developed a highly sophisticated trafficking system that mediates long-distance transport and local control of membrane and protein turnover (Hanus and Ehlers, 2016). The majority of dendritic spines of pyramidal neurons in the cortex and hippocampus do not contain ER (Toresson and Grant, 2005) In those cells a subset of spines contain a complex ER-based organelle called the spine apparatus (SA; Deller et al, 2003). The SA is usually localized in the spine neck or at the base of the spine head and consists of laminar ER stacks with intervening electron-dense plates, connected to the main ER network. This intriguing structure serves as a Ca2+ store and is important for synaptic plasticity (Deller et al, 2003)

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