Synapses: Changing the size of dendritic spines

Interactions between an enzyme kinase, an ion channel and cytoskeletal proteins maintain the structure of synapses involved in memory formation.
  1. Takeo Saneyoshi  Is a corresponding author
  1. Department of Pharmacology, Kyoto University Graduate School of Medicine, Japan

Neurons connect to one another at sites called synapses, which are essential for creating and storing memories. Synapses are also dynamic and can undergo structural changes, such as increasing in size, that modify their strength. Structures called dendritic spines – which protrude from the dendrites that extend from the cell body of a neuron – have an important role in synapses as they contain an actin cytoskeleton that allows them to change shape and size.

An enzyme called CaMKII (which is short for Ca2+/calmodulin-dependent protein kinase II) also has a central role in the functional and structural changes that take place in dendritic spines during memory formation. During synapse strengthening, an influx of calcium ions through a receptor known as NMDAR, triggers the activation of CaMKII: the calcium ions form a complex with a messenger protein called calmodulin, and this complex binds to the regulatory segment of CaMKII. This influences the dynamics of the actin cytoskeleton and hence determines the size of the synapses. However, the details of this process – especially the link between CaMKII and the actin cytoskeleton at the molecular level – are not fully understood.

One candidate for linking CaMKII to the actin cytoskeleton is alpha-actinin-2, a structural protein that crosslinks actin filaments (Walikonis et al., 2001). This protein can anchor and organize other proteins in the dendritic spine, such as CaMKII (Robison et al., 2005), the receptor NMDAR (Wyszynski et al., 1997), and certain channel proteins that allow calcium ions to enter cells (Hall et al., 2013). Now, in eLife, Matthew Gold from University College London and colleagues – including Ashton Curtis and Jian Zhu as joint first authors – report new insights into the molecular interactions between CaMKII, alpha-actinin-2, and a subunit of NMDAR called GluN2B (Curtis et al., 2023; Figure 1).

Molecular interactions and synapses.

(A) In the naïve synapse, the only protein interactions are between actin filaments (blue) and alpha-actinin-2, a structural protein that crosslinks actin filaments. In particular, the enzyme CaMKII (hexagon shape) is inactive, and a messenger protein called calmodulin (CaM) does not interact with anything else. (B) When the NMDAR receptor in the cell membrane is activated by the neurotransmitter glutamate, calcium ions (Ca2+; solid pink circles) enter the cell. The ions form a complex with CaM, and this complex activates the enzyme CaMKII, which subsequently interacts with the GluN2B subunits in the intracellular region of the NMDAR receptor. Crosslinking of the actin filaments also causes the dendritic spine to increase in size. (C) In the enlarged synapse, the interaction between GluN2B, CaMKII and alpha-actinin-2 continues, even in the absence of Ca2+/CaM, and the enzymatic activity of CaMKII is maintained by binding to GluN2B. The complex formed by GluN2B, CaMKII and alpha-actinin-2 also associates with actin filaments to maintain the synaptic structure.

Using a proximity labeling assay method – which enables molecular interactions to be detected in situ – the researchers showed that CaMKII interacts with alpha-actinin-2 when the NMDAR receptor is stimulated, leading to an enlargement of the dendritic spine. The interaction takes place at a domain in alpha-actinin-2 called EF hands 1–4, which is known to bind calcium ions. Furthermore, when this domain was overexpressed, stimulation of the NMDAR receptor did not result in dendritic spine enlargement, suggesting that interaction via the EF1-4 domain has a critical role in structural changes to the dendritic spine.

Affinity and structural experiments showed that alpha-actinin-2 binds to the CaMKII enzyme at the same regulatory domain segment as calmodulin (which also contains EF hands) does. However, Curtis et al. found that the EF hands of alpha-actinin-2 clashed with one of the domains in the enzyme. Consistent with this, the inactive form of the enzyme cannot bind to alpha-actinin-2 via its regulatory domain segment. However, the interaction between the CaMKII enzyme and alpha-actinin-2 in the dendritic spine increased four hours after stimulation of the NMDAR receptor: this led Curtis et al. to hypothesize that, following stimulation, the configuration of the enzyme changes when it interacts with a subunit of the NMDAR receptor called GluN2B, and these changes enable alpha-actinin-2 to fully access the enzyme.

A pull-down assay – which detects interactions between proteins – revealed that GluN2B enhances the interaction between the CaMKII enzyme and alpha-actinin-2, independently of the concentration of calcium ions (Ca2+). This indicates that GluN2B enhances the binding of CaMKII with alpha-actinin-2 even in the presence of Ca2+/calmodulin, although the EF hands of alpha-actinin-2 have a lower affinity for CaMKII than the EF hands of Ca2+/calmodulin. This provides important insights into a reorganization process for the molecular architecture of synapses.

Taken together, the findings suggest that the tripartite interactions of CaMKII, GluN2B, and alpha-actinin-2 crosslink a membrane channel to the cytoskeleton beneath the synapse, and there are plenty of empty binding sites that other proteins can bind to. Moreover, the fact that certain interactions would generate autonomous enzymatic activity in CaMKII (Bayer et al., 2001; Saneyoshi et al., 2019) suggest a possible role in biochemical signalling for synaptic memory.

Recent evidence indicates that the CaMKII and GluN2B in synapses can undergo liquid–liquid phase separation, a process that involves biomolecules separating into distinct components within a cell (Hosokawa et al., 2021; Yasuda et al., 2022). Actin bundling can also be modulated by liquid–liquid phase separation in synapses (Chen et al., 2023), so it would be interesting to explore if this process has a role in changing and/or maintaining the structure of dendritic spines.

References

Article and author information

Author details

  1. Takeo Saneyoshi

    Takeo Saneyoshi is in the Department of Pharmacology, Kyoto University Graduate School of Medicine, Kyoto, Japan

    For correspondence
    saneyoshi.takeo.3v@kyoto-u.ac.jp
    Competing interests
    No competing interests declared
    ORCID icon "This ORCID iD identifies the author of this article:" 0000-0003-2361-3339

Publication history

  1. Version of Record published: September 7, 2023 (version 1)

Copyright

© 2023, Saneyoshi

This article is distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use and redistribution provided that the original author and source are credited.

Metrics

  • 384
    Page views
  • 44
    Downloads
  • 0
    Citations

Article citation count generated by polling the highest count across the following sources: Crossref, PubMed Central, Scopus.

Download links

A two-part list of links to download the article, or parts of the article, in various formats.

Downloads (link to download the article as PDF)

Open citations (links to open the citations from this article in various online reference manager services)

Cite this article (links to download the citations from this article in formats compatible with various reference manager tools)

  1. Takeo Saneyoshi
(2023)
Synapses: Changing the size of dendritic spines
eLife 12:e91566.
https://doi.org/10.7554/eLife.91566

Further reading

    1. Evolutionary Biology
    2. Neuroscience
    Katja Heuer, Nicolas Traut ... Roberto Toro
    Research Article

    The process of brain folding is thought to play an important role in the development and organisation of the cerebrum and the cerebellum. The study of cerebellar folding is challenging due to the small size and abundance of its folia. In consequence, little is known about its anatomical diversity and evolution. We constituted an open collection of histological data from 56 mammalian species and manually segmented the cerebrum and the cerebellum. We developed methods to measure the geometry of cerebellar folia and to estimate the thickness of the molecular layer. We used phylogenetic comparative methods to study the diversity and evolution of cerebellar folding and its relationship with the anatomy of the cerebrum. Our results show that the evolution of cerebellar and cerebral anatomy follows a stabilising selection process. We observed 2 groups of phenotypes changing concertedly through evolution: a group of 'diverse' phenotypes - varying over several orders of magnitude together with body size, and a group of 'stable' phenotypes varying over less than 1 order of magnitude across species. Our analyses confirmed the strong correlation between cerebral and cerebellar volumes across species, and showed in addition that large cerebella are disproportionately more folded than smaller ones. Compared with the extreme variations in cerebellar surface area, folial anatomy and molecular layer thickness varied only slightly, showing a much smaller increase in the larger cerebella. We discuss how these findings could provide new insights into the diversity and evolution of cerebellar folding, the mechanisms of cerebellar and cerebral folding, and their potential influence on the organisation of the brain across species.

    1. Neuroscience
    Amanda J González Segarra, Gina Pontes ... Kristin Scott
    Research Article

    Consumption of food and water is tightly regulated by the nervous system to maintain internal nutrient homeostasis. Although generally considered independently, interactions between hunger and thirst drives are important to coordinate competing needs. In Drosophila, four neurons called the interoceptive subesophageal zone neurons (ISNs) respond to intrinsic hunger and thirst signals to oppositely regulate sucrose and water ingestion. Here, we investigate the neural circuit downstream of the ISNs to examine how ingestion is regulated based on internal needs. Utilizing the recently available fly brain connectome, we find that the ISNs synapse with a novel cell-type bilateral T-shaped neuron (BiT) that projects to neuroendocrine centers. In vivo neural manipulations revealed that BiT oppositely regulates sugar and water ingestion. Neuroendocrine cells downstream of ISNs include several peptide-releasing and peptide-sensing neurons, including insulin producing cells (IPCs), crustacean cardioactive peptide (CCAP) neurons, and CCHamide-2 receptor isoform RA (CCHa2R-RA) neurons. These neurons contribute differentially to ingestion of sugar and water, with IPCs and CCAP neurons oppositely regulating sugar and water ingestion, and CCHa2R-RA neurons modulating only water ingestion. Thus, the decision to consume sugar or water occurs via regulation of a broad peptidergic network that integrates internal signals of nutritional state to generate nutrient-specific ingestion.