Liquid-liquid phase separation (LLPS) is emerging as a major mechanism for the organization of macromolecules in compartments non-limited by a membrane or scaffold (Banani et al., 2017). During the last few years, a surge of studies demonstrated that the spatial and temporal organization of many structures in neurons is based on LLPS. This ever growing array of structures includes the cluster of synaptic vesicles (SVs) (Milovanovic and De Camilli, 2017; Milovanovic et al., 2018), RNA-containing granules (Lin et al., 2015; Molliex et al., 2015), active zones (Wu et al., 2019; McDonald et al., 2020), postsynaptic densities (both excitatory and inhibitory) (Zeng et al., 2016; Bai et al., 2021). At the same time, several observations led to the conclusion that the dysregulation of LLPS in neurons causes protein and organelle aggregation (Shin and Brangwynne, 2017), which is a hallmark of many neurodegenerative diseases.
A collection of papers within this Research Topic discusses the effects of LLPS on the organization of synapses, RNA-binding proteins and the assembly of RNA granules, and the consequences that the aberrant phase separation has on protein aggregation.
The synaptic bouton is one of the best characterized neuronal compartments (Wilhelm et al., 2014; Reshetniak et al., 2020), with numerous proteins (and protein families) combining here to achieve rapid neurotransmitter release and well-regulated SV recycling. SVs form liquid condensates, able to recruit synaptic proteins such as intersectin and alpha-synuclein (Milovanovic et al., 2018; Pechstein et al., 2020; Hoffmann et al., 2021). (Fouke et al.) capitalize on the large reticulospinal synapse of the lamprey to demonstrate that the acute depletion of alpha-synuclein disrupts the SV cluster in a piecemeal fashion into smaller SV clumps. This corroborates with the studies that indicate that the abundance of alpha-synuclein alters the mesoscale organization of SV condensates. Furthermore, at pathologically high concentrations, alpha-synuclein undergoes LLPS on its path to aggregation (Ray et al., 2020). (Brodin et al.) argue that a dual relation exists between alpha-synuclein and the SV phase: alpha-synuclein presumably helps assemble large SV condensates, and, in return, the biochemical milieu within these condensates ensures that alpha-synuclein remains soluble, and is prevented from pathological aggregate formation.
AMPA receptors are known to form nanoclusters at the postsynapse (Choquet and Hosy, 2020), which are further stabilized during prolonged synaptic activity (Opazo et al., 2010). Interestingly, stargazin, an auxiliary subunit of the AMPA receptor, undergoes LLPS with the proteins of the postsynaptic density, which affects the dynamics of both AMPA and NMDA receptors in excitatory synapses (Zeng et al., 2018; Hosokawa et al., 2021). In a review, (Hosokawa and Liu) discuss the relationship between synaptic plasticity, LLPS, and the AMPA receptors at the postsynaptic plasma membrane.
Neurons are highly polarized cells, with the axons of motor neurons being particularly clear examples of the cellular polarization. They can reach lengths of over 1 m, leading to substantial pressure on the transport of organelles and molecular complexes, such as the RNA-containing transport granules. TDP-43 is a nucleic acid-binding protein that is often associated with RNA transport granules in the neuronal cytoplasm and is able to undergo LLPS (Conicella et al., 2016). (Vishal et al.) investigate how the mutations that disrupt LLPS of TDP-43 affect the trafficking of TDP 43-containing granules in axons. The authors show that the conserved alpha-helical domain at the C-terminal region, surrounding FG motifs, tryptophan residues, and RGG motifs all affect the transport of TDP-43 granules in axons, indicating their relevance for the recruitment of motors and adaptor proteins.
The aberrant phase separation of RNA-binding proteins (RNPs) leads to the formation of aggregates, which are a hallmark of Amyotrophic Lateral Sclerosis (ALS) and Frontotemporal Dementia (Blokhuis et al., 2013). (Carey and Guo) review two specific RNPs, TDP-43, and FUS, two molecules that have been implicated in disease, and are known to undergo phase separation. In fact, (Milicevic et al.) argue that LLPS-disrupting mutations are a common feature of many RNPs implicated in the pathology of ALS, such annexin 11, ataxin 2, hnRNPA1, hnRNPA2, and TIA-1. The cause-effect relationship between RNA granule assembly, their association with membrane-bound compartments such as ER, lysosomes or mitochondria, and their response to (oxidative) stress, are all important for the appropriate neuronal physiology (Liao et al., 2019; Amen and Kaganovich, 2021; Trnka et al., 2021). The aberrant phase separation of RNPs induces the loss of their cellular function and also triggers the formation of inclusions that may trap folded proteins and membranes.
Misfolded host prion protein (PrP) forms amyloid fibrils, a hallmark of diseases referred to as Transmissible Spongiform Encephalopathies (TSEs) (Heumüller et al., 2022). (Aguilar et al.) demonstrate that differentiating neuronal cells exposed to an infectious TSE agent can induce a dramatic increase in interferon-beta mRNA as well as a reduction of PrP mRNA and protein levels, implying the interplay between the innate immune response and PrP dynamics in TSE. This exciting finding should be followed by studies that will assess the extent to which LLPS is involved in intracellular agent sequestration, and the potential surveillance mechanisms that can affect the ensuing neurodegeneration.
Overall, the papers within this Research Topic showcase the numerous roles LLPS has in neuronal function, in health and disease, from synaptic organization and dynamics, to RNA trafficking in neurites, or to neuronal pathophysiology. The years ahead promise to shed light on the interactions between biomolecular condensates and the better-studied membrane trafficking processes, as well as how to capitalize on these interactions while tackling neurological and neurodegenerative diseases.
Statements
Author contributions
DM and SR prepared this editorial together.
Conflict of interest
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
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All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.
References
1
AmenT.KaganovichD. (2021). Stress Granules Inhibit Fatty Acid Oxidation by Modulating Mitochondrial Permeability. Cell Rep.35, 109237. 10.1016/j.celrep.2021.109237
2
BaiG.WangY.ZhangM. (2021). Gephyrin-mediated Formation of Inhibitory Postsynaptic Density Sheet via Phase Separation. Cell Res.31, 312–325. 10.1038/s41422-020-00433-1
3
BananiS. F.LeeH. O.HymanA. A.RosenM. K. (2017). Biomolecular Condensates: Organizers of Cellular Biochemistry. Nat. Rev. Mol. Cell Biol.18, 285–298. 10.1038/nrm.2017.7
4
BlokhuisA. M.GroenE. J. N.KoppersM.van den BergL. H.PasterkampR. J. (2013). Protein Aggregation in Amyotrophic Lateral Sclerosis. Acta Neuropathol.125, 777–794. 10.1007/s00401-013-1125-6
5
ChoquetD.HosyE. (2020). AMPA Receptor Nanoscale Dynamic Organization and Synaptic Plasticities. Curr. Opin. Neurobiology63, 137–145. 10.1016/j.conb.2020.04.003
6
ConicellaA. E.ZerzeG. H.MittalJ.FawziN. L. (2016). ALS Mutations Disrupt Phase Separation Mediated by α-Helical Structure in the TDP-43 Low-Complexity C-Terminal Domain. Structure24, 1537–1549. 10.1016/j.str.2016.07.007
7
HeumüllerS.-E.HornbergerA. C.HebestreitA. S.HossingerA.VorbergI. M. (2022). Propagation and Dissemination Strategies of Transmissible Spongiform Encephalopathy Agents in Mammalian Cells. Int. J. Mol. Sci.23, 2909. 10.3390/ijms23062909
8
HoffmannC.SansevrinoR.MorabitoG.LoganC.VabulasR. M.UlusoyA.et al (2021). Synapsin Condensates Recruit Alpha-Synuclein. J. Mol. Biol.433, 166961. 10.1016/j.jmb.2021.166961
9
HosokawaT.LiuP.-W.CaiQ.FerreiraJ. S.LevetF.ButlerC.et al (2021). CaMKII Activation Persistently Segregates Postsynaptic Proteins via Liquid Phase Separation. Nat. Neurosci.24, 777–785. 10.1038/s41593-021-00843-3
10
LiaoY.-C.FernandopulleM. S.WangG.ChoiH.HaoL.DrerupC. M.et al (2019). RNA Granules Hitchhike on Lysosomes for Long-Distance Transport, Using Annexin A11 as a Molecular Tether. Cell179, 147–164. e20. 10.1016/j.cell.2019.08.050
11
LinY.ProtterD. S. W.RosenM. K.ParkerR. (2015). Formation and Maturation of Phase-Separated Liquid Droplets by RNA-Binding Proteins. Mol. Cell60, 208–219. 10.1016/j.molcel.2015.08.018
12
McDonaldN. A.FetterR. D.ShenK. (2020). Assembly of Synaptic Active Zones Requires Phase Separation of Scaffold Molecules. Nature588, 454–458. 10.1038/s41586-020-2942-0
13
MilovanovicD.De CamilliP. (2017). Synaptic Vesicle Clusters at Synapses: A Distinct Liquid Phase?Neuron93, 995–1002. 10.1016/j.neuron.2017.02.013
14
MilovanovicD.WuY.BianX.De CamilliP. (2018). A Liquid Phase of Synapsin and Lipid Vesicles. Science361, 604–607. 10.1126/science.aat5671
15
MolliexA.TemirovJ.LeeJ.CoughlinM.KanagarajA. P.KimH. J.et al (2015). Phase Separation by Low Complexity Domains Promotes Stress Granule Assembly and Drives Pathological Fibrillization. Cell163, 123–133. 10.1016/j.cell.2015.09.015
16
OpazoP.LabrecqueS.TigaretC. M.FrouinA.WisemanP. W.De KoninckP.et al (2010). CaMKII Triggers the Diffusional Trapping of Surface AMPARs through Phosphorylation of Stargazin. Neuron67, 239–252. 10.1016/j.neuron.2010.06.007
17
PechsteinA.TomilinN.FredrichK.VorontsovaO.SopovaE.EvergrenE.et al (2020). Vesicle Clustering in a Living Synapse Depends on a Synapsin Region that Mediates Phase Separation. Cell Rep.30, 2594–2602. e3. 10.1016/j.celrep.2020.01.092
18
RayS.SinghN.KumarR.PatelK.PandeyS.DattaD.et al (2020). α-Synuclein Aggregation Nucleates through Liquid-Liquid Phase Separation. Nat. Chem.12, 705–716. 10.1038/s41557-020-0465-9
19
ReshetniakS.UßlingJ. E.PeregoE.RammnerB.SchikorskiT.FornasieroE. F.et al (2020). A Comparative Analysis of the Mobility of 45 Proteins in the Synaptic Bouton. Embo J.39, e104596. 10.15252/embj.2020104596
20
ShinY.BrangwynneC. P. (2017). Liquid Phase Condensation in Cell Physiology and Disease. Science357, eaaf4382. 10.1126/science.aaf4382
21
TrnkaF.HoffmannC.WangH.SansevrinoR.RankovicB.RostB. R.et al (2021). Aberrant Phase Separation of FUS Leads to Lysosome Sequestering and Acidification. Front. Cell Dev. Biol.9, 716919. 10.3389/fcell.2021.716919
22
WilhelmB. G.MandadS.TruckenbrodtS.KröhnertK.SchäferC.RammnerB.et al (2014). Composition of Isolated Synaptic Boutons Reveals the Amounts of Vesicle Trafficking Proteins. Science344, 1023–1028. 10.1126/science.1252884
23
WuX.CaiQ.ShenZ.ChenX.ZengM.DuS.et al (2019). RIM and RIM-BP Form Presynaptic Active-zone-like Condensates via Phase Separation. Mol. Cell73, 971–984. 10.1016/j.molcel.2018.12.007
24
ZengM.ChenX.GuanD.XuJ.WuH.TongP.et al (2018). Reconstituted Postsynaptic Density as a Molecular Platform for Understanding Synapse Formation and Plasticity. Cell174, 1172–1187. e16. 10.1016/j.cell.2018.06.047
25
ZengM.ShangY.ArakiY.GuoT.HuganirR. L.ZhangM. (2016). Phase Transition in Postsynaptic Densities Underlies Formation of Synaptic Complexes and Synaptic Plasticity. Cell166, 1163–1175. e12. 10.1016/j.cell.2016.07.008
Summary
Keywords
phase separation, neuron, neurodegenerative diseases, synapse, AMPA and NMDA-type receptors, alpha-synuclein, synaptic vesicles, TDP43
Citation
Milovanovic D and Rizzoli SO (2022) Editorial: Protein Phase Separation and Aggregation in (Patho)Physiology of Neurons. Front. Physiol. 13:959570. doi: 10.3389/fphys.2022.959570
Received
01 June 2022
Accepted
13 June 2022
Published
04 July 2022
Volume
13 - 2022
Edited and reviewed by
Christoph Fahlke, Helmholtz Association of German Research Centres (HZ), Germany
Updates
Copyright
© 2022 Milovanovic and Rizzoli.
This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
*Correspondence: Dragomir Milovanovic, dragomir.milovanovic@dzne.de; Silvio O. Rizzoli, srizzol@gwdg.de
This article was submitted to Membrane Physiology and Membrane Biophysics, a section of the journal Frontiers in Physiology
Disclaimer
All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article or claim that may be made by its manufacturer is not guaranteed or endorsed by the publisher.