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• W2103705255 abstract "•Dendrites exhibit periodic actin organization•Cytoskeletal proteins show the same periodicity at nodes of Ranvier•Cytosolic actin organization is developmentally and spatially regulated•Actin patches in the axon initial segment co-localize with synaptic markers In the axons of cultured hippocampal neurons, actin forms various structures, including bundles, patches (involved in the preservation of neuronal polarity), and a recently reported periodic ring-like structure. Nevertheless, the overlaying organization of actin in neurons and in the axon initial segment (AIS) is still unclear, due mainly to a lack of adequate imaging methods. By harnessing live-cell stimulated emission depletion (STED) nanoscopy and the fluorescent probe SiR-Actin, we show that the periodic subcortical actin structure is in fact present in both axons and dendrites. The periodic cytoskeleton organization is also found in the peripheral nervous system, specifically at the nodes of Ranvier. The actin patches in the AIS co-localize with pre-synaptic markers. Cytosolic actin organization strongly depends on the developmental stage and subcellular localization. Altogether, the results of this study reveal unique neuronal cytoskeletal features. In the axons of cultured hippocampal neurons, actin forms various structures, including bundles, patches (involved in the preservation of neuronal polarity), and a recently reported periodic ring-like structure. Nevertheless, the overlaying organization of actin in neurons and in the axon initial segment (AIS) is still unclear, due mainly to a lack of adequate imaging methods. By harnessing live-cell stimulated emission depletion (STED) nanoscopy and the fluorescent probe SiR-Actin, we show that the periodic subcortical actin structure is in fact present in both axons and dendrites. The periodic cytoskeleton organization is also found in the peripheral nervous system, specifically at the nodes of Ranvier. The actin patches in the AIS co-localize with pre-synaptic markers. Cytosolic actin organization strongly depends on the developmental stage and subcellular localization. Altogether, the results of this study reveal unique neuronal cytoskeletal features. IntroductionInvestigations of neuronal actin organization have so far focused on structural aspects of dendritic spines (Izeddin et al., 2011Izeddin I. Specht C.G. Lelek M. Darzacq X. Triller A. Zimmer C. Dahan M. Super-resolution dynamic imaging of dendritic spines using a low-affinity photoconvertible actin probe.PLoS ONE. 2011; 6: e15611Crossref PubMed Scopus (119) Google Scholar, Korobova and Svitkina, 2010Korobova F. Svitkina T. Molecular architecture of synaptic actin cytoskeleton in hippocampal neurons reveals a mechanism of dendritic spine morphogenesis.Mol. Biol. Cell. 2010; 21: 165-176Crossref PubMed Scopus (278) Google Scholar, Tatavarty et al., 2009Tatavarty V. Kim E.J. Rodionov V. Yu J. Investigating sub-spine actin dynamics in rat hippocampal neurons with super-resolution optical imaging.PLoS ONE. 2009; 4: e7724Crossref PubMed Scopus (77) Google Scholar, Testa et al., 2012Testa I. Urban N.T. Jakobs S. Eggeling C. Willig K.I. Hell S.W. Nanoscopy of living brain slices with low light levels.Neuron. 2012; 75: 992-1000Abstract Full Text Full Text PDF PubMed Scopus (104) Google Scholar, Urban et al., 2011Urban N.T. Willig K.I. Hell S.W. Nägerl U.V. STED nanoscopy of actin dynamics in synapses deep inside living brain slices.Biophys. J. 2011; 101: 1277-1284Abstract Full Text Full Text PDF PubMed Scopus (222) Google Scholar, Willig et al., 2014Willig K.I. Steffens H. Gregor C. Herholt A. Rossner M.J. Hell S.W. Nanoscopy of filamentous actin in cortical dendrites of a living mouse.Biophys. J. 2014; 106: L01-L03Abstract Full Text Full Text PDF PubMed Scopus (61) Google Scholar). At the same time, very little is known about the actin architecture in axons. Recently, super-resolution fluorescence microscopy revealed a striking ring-like actin organization with a periodicity of ∼180 nm in axons of cultured hippocampal neurons (Lukinavičius et al., 2014Lukinavičius G. Reymond L. D’Este E. Masharina A. Göttfert F. Ta H. Güther A. Fournier M. Rizzo S. Waldmann H. et al.Fluorogenic probes for live-cell imaging of the cytoskeleton.Nat. Methods. 2014; 11: 731-733Crossref PubMed Scopus (482) Google Scholar, Xu et al., 2013Xu K. Zhong G. Zhuang X. Actin, spectrin, and associated proteins form a periodic cytoskeletal structure in axons.Science. 2013; 339: 452-456Crossref PubMed Scopus (805) Google Scholar). Interestingly, electron microscopy failed to reveal such structural features altogether (Jones et al., 2014Jones S.L. Korobova F. Svitkina T. Axon initial segment cytoskeleton comprises a multiprotein submembranous coat containing sparse actin filaments.J. Cell Biol. 2014; 205: 67-81Crossref PubMed Scopus (81) Google Scholar, Watanabe et al., 2012Watanabe K. Al-Bassam S. Miyazaki Y. Wandless T.J. Webster P. Arnold D.B. Networks of polarized actin filaments in the axon initial segment provide a mechanism for sorting axonal and dendritic proteins.Cell Rep. 2012; 2: 1546-1553Abstract Full Text Full Text PDF PubMed Scopus (86) Google Scholar) and instead highlighted the presence of actin-patch features. While actin patches in axons have been identified as critical for maintaining neuronal polarity, the role of the highly periodic lattice is still unclear (Arnold and Gallo, 2014Arnold D.B. Gallo G. Structure meets function: actin filaments and myosin motors in the axon.J. Neurochem. 2014; 129: 213-220Crossref PubMed Scopus (28) Google Scholar).Disagreements regarding actin ultrastructure trace back to the fact that actin is inherently difficult to image with nanoscale resolution. This is because the actin network is too fine and too dense to be resolved by diffraction-limited optical microscopy, while the preparation required for electron microscopy is rather harsh, altering the overall structure of the network (Fifková, 1985Fifková E. Actin in the nervous system.Brain Res. 1985; 356: 187-215Crossref PubMed Scopus (121) Google Scholar, Schnell et al., 2012Schnell U. Dijk F. Sjollema K.A. Giepmans B.N. Immunolabeling artifacts and the need for live-cell imaging.Nat. Methods. 2012; 9: 152-158Crossref PubMed Scopus (316) Google Scholar). Optical nanoscopy of phalloidin-stained actin is a good compromise between high-resolution imaging and the aggressiveness of sample preparation (Xu et al., 2012Xu K. Babcock H.P. Zhuang X. Dual-objective STORM reveals three-dimensional filament organization in the actin cytoskeleton.Nat. Methods. 2012; 9: 185-188Crossref PubMed Scopus (363) Google Scholar). However, it typically requires chemical fixation, which may itself cause artifacts (Schnell et al., 2012Schnell U. Dijk F. Sjollema K.A. Giepmans B.N. Immunolabeling artifacts and the need for live-cell imaging.Nat. Methods. 2012; 9: 152-158Crossref PubMed Scopus (316) Google Scholar). Therefore, fluorescence nanoscopy or super-resolution of actin in living cells under relevant normal growth conditions is the method of choice for overcoming these observational limitations.Being membrane permeable, the recently introduced silicon rhodamine actin label (SiR-Actin) allows labeling of actin inside living cells, without the need to overexpress fluorescently tagged proteins such as LifeAct, actin monomers, or actin-binding proteins. Its fluorogenicity provides a high signal-to-noise ratio, facilitating the visualization of finest details. SiR-Actin was already successfully tested for stimulated emission depletion (STED) nanoscopy, confirming the presence of a periodic actin pattern also in axons of living neurons (Lukinavičius et al., 2014Lukinavičius G. Reymond L. D’Este E. Masharina A. Göttfert F. Ta H. Güther A. Fournier M. Rizzo S. Waldmann H. et al.Fluorogenic probes for live-cell imaging of the cytoskeleton.Nat. Methods. 2014; 11: 731-733Crossref PubMed Scopus (482) Google Scholar). Here, we report several novel observations pertaining to subcortical actin ultrastructure, enabled by the use of SiR-Actin and two-color STED nanoscopy in living primary cultured hippocampal neurons. We discovered that subcortical actin forms a periodic structure in the dendritic compartments in addition to axonal compartments, where it is shown to appear shortly after axon specification. A similar periodic pattern of cytoskeletal proteins was also found in myelinated sciatic nerve fibers at the nodes of Ranvier. Regarding cytosolic actin in living cells, SiR-Actin reveals that its organization in patches and bundles is both evolving in time (corresponding to developmental stage) and spatially regulated (depending on subcellular compartment). Actin bundles in the somatodendritic compartment end abruptly at the beginning of the axon initial segment (AIS), while actin patches in the AIS play a role in the organization of synaptic boutons (Sankaranarayanan et al., 2003Sankaranarayanan S. Atluri P.P. Ryan T.A. Actin has a molecular scaffolding, not propulsive, role in presynaptic function.Nat. Neurosci. 2003; 6: 127-135Crossref PubMed Scopus (252) Google Scholar, Waites et al., 2011Waites C.L. Leal-Ortiz S.A. Andlauer T.F. Sigrist S.J. Garner C.C. Piccolo regulates the dynamic assembly of presynaptic F-actin.J. Neurosci. 2011; 31: 14250-14263Crossref PubMed Scopus (57) Google Scholar), since they co-localize with pre-synaptic markers. Our work reveals the ubiquity of the periodic structure of cytoskeleton in neurons and elucidates the involvement of actin in the maintenance of neuronal polarity. By the same token, our work also emphasizes the utility of live-cell nanoscopy for the study of neuronal ultrastructures, where these unprecedented imaging capabilities will undoubtedly impact the search for mechanisms that underlie basic neuronal functions.ResultsSubcortical Actin Forms a Periodic Lattice Also in Dendrites of Living NeuronsWe exploited the membrane permeability of SiR-Actin to study the organization of filamentous actin (F-actin) in living neurons at different developmental stages. After 1.5 days in vitro (DIV), corresponding to developmental stage 3, cultured neurons first specify their axon, which becomes biochemically recognizable starting from 5 DIV (Dotti et al., 1988Dotti C.G. Sullivan C.A. Banker G.A. The establishment of polarity by hippocampal neurons in culture.J. Neurosci. 1988; 8: 1454-1468Crossref PubMed Google Scholar). Indeed, at this stage, the AIS assembles, and its characteristic proteins accumulate (Boiko et al., 2007Boiko T. Vakulenko M. Ewers H. Yap C.C. Norden C. Winckler B. Ankyrin-dependent and -independent mechanisms orchestrate axonal compartmentalization of L1 family members neurofascin and L1/neuron-glia cell adhesion molecule.J. Neurosci. 2007; 27: 590-603Crossref PubMed Scopus (92) Google Scholar) (Figure 1A). To obtain sub-diffraction resolution, we employed STED nanoscopy (Göttfert et al., 2013Göttfert F. Wurm C.A. Mueller V. Berning S. Cordes V.C. Honigmann A. Hell S.W. Coaligned dual-channel STED nanoscopy and molecular diffusion analysis at 20 nm resolution.Biophys. J. 2013; 105: L01-L03Abstract Full Text Full Text PDF PubMed Scopus (207) Google Scholar). Living cultured rat hippocampal neurons were first stained with SiR-Actin under growth conditions. Subsequently, the axon was identified by live labeling with an antibody directed against the extracellular domain of the AIS marker neurofascin 186 (Davis et al., 1996Davis J.Q. Lambert S. Bennett V. Molecular composition of the node of Ranvier: identification of ankyrin-binding cell adhesion molecules neurofascin (mucin+/third FNIII domain-) and NrCAM at nodal axon segments.J. Cell Biol. 1996; 135: 1355-1367Crossref PubMed Scopus (304) Google Scholar) (Figure 1B). This approach allowed a straightforward distinction between axons and dendrites (Figures 1C and 2). Live two-color STED imaging revealed a clear subcortical actin periodic pattern in the axons.Figure 2Actin Rings Form before AIS Specification and Intercalate with NeurofascinShow full caption(A) Representative STED images of axons of living hippocampal neurons at different days in vitro (DIV). Insets represent the specific neurofascin labeling to highlight the axon. SiR-Actin reveals that actin rings appear already at 2 DIV.(B) Percentage of axons in which actin rings can be observed at different developmental stages, both with SiR-Actin and with phalloidin. Phalloidin has a weaker sensitivity, and actin rings could not be detected at 2–3 DIV (SiR-Actin, three independent experiments: 2 DIV, n = 28 axons; 3 DIV, n = 24; 5 DIV, n = 28; 8–9 DIV, n = 42; 16–17 DIV, n = 26; phalloidin, two independent experiments: 2 DIV, n = 20 axons; 3 DIV, n = 20; 5 DIV, n = 19; 8–9 DIV, n = 29; 16–17 DIV, n = 36).(C) STED image of a living hippocampal neuron at 5 DIV stained with SiR-Actin (upper panel, green) and anti-neurofascin antibody (lower panel, red), and the merged image.(D) Line profile along the dashed line in (C) showing alternating intensity peaks of actin and neurofascin.Scale bars, 1 μm. All images depict raw STED data.View Large Image Figure ViewerDownload (PPT)To our surprise, the periodic organization was readily recognizable also in neurofascin-negative processes representing dendrites (Figure 1C). The spacing of the actin lattice in dendrites was 192 ± 37 nm (n = 50) (Figure 1D) and is therefore in good agreement with the 181 ± 20 nm previously measured with STED in living axons labeled with SiR-Actin (Lukinavičius et al., 2014Lukinavičius G. Reymond L. D’Este E. Masharina A. Göttfert F. Ta H. Güther A. Fournier M. Rizzo S. Waldmann H. et al.Fluorogenic probes for live-cell imaging of the cytoskeleton.Nat. Methods. 2014; 11: 731-733Crossref PubMed Scopus (482) Google Scholar). This suggests that the structural arrangement of the actin scaffold is similar and likely the same in both axons and dendrites.Next, we investigated the presence of the actin lattice in dendrites of neurons at different developmental stages in vitro, starting at 5 DIV, in order to ensure reliable distinction between axons and dendrites (Figure 1A). Overall, 90 cells between 5 and 17 DIV were analyzed using SiR-Actin labeling. An actin lattice in dendrites was visible in 21 neurons (23.3%). More specifically, the actin lattice was identified in 33.3% of neurons at 5 DIV (9 cells out of 27 imaged), 25.6% at 8–9 DIV (10 out of 39), and 8.3% at 16–17 DIV (2 out of 24) (Figure 1E). Note, however, that in neurons at >16 DIV, the presence of spines highly enriched in actin rendered the identification of the fine actin pattern much more difficult (compare Figure 1C, 17 DIV), and their presence may thus be underestimated at 16–17 DIV. Interestingly, phalloidin staining failed to identify the actin lattice in dendrites at 5 DIV, while it confirmed the results obtained with SiR-Actin labeling in cultures older than 8–9 DIV (Figures 1E and S1). While highlighting deficiencies of detection with phalloidin at early time points, this result rules out the possibility that the actin lattice is induced by the SiR-Actin dye and, at the same time, demonstrates that periodicity is not an artifact of the fixation procedure.We conclude that the subcortical periodic organization of actin is not a hallmark of axons as previously reported (Xu et al., 2013Xu K. Zhong G. Zhuang X. Actin, spectrin, and associated proteins form a periodic cytoskeletal structure in axons.Science. 2013; 339: 452-456Crossref PubMed Scopus (805) Google Scholar) but is also present in dendrites.The Actin Periodic Pattern Arises in Concert with Axon SpecificationIn fixed cells stained with phalloidin, actin rings were reported to arise in axons at around 5 DIV (Xu et al., 2013Xu K. Zhong G. Zhuang X. Actin, spectrin, and associated proteins form a periodic cytoskeletal structure in axons.Science. 2013; 339: 452-456Crossref PubMed Scopus (805) Google Scholar), at the time point when the AIS assembles (Figure 1A). Indeed, also in our hands, virtually all axons from 5 DIV on clearly show actin rings both with live SiR-Actin and fixed phalloidin staining (Figure 2). However, live SiR-Actin STED imaging clearly revealed the presence of actin rings in at least one of the neurites (which may develop into the axon) as early as 2–3 days before, in 35.7% of the analyzed neurons at 2 DIV, and 62.5% at 3 DIV (Figures 2A, 2B, and S2). Simultaneous two-color STED imaging revealed that neurofascin intercalates with the actin rings (Figures 2C and 2D). Note that we were not able to identify actin rings in fixed phalloidin stained neurons of 2–3 DIV (Figures 2B and S2) (Xu et al., 2013Xu K. Zhong G. Zhuang X. Actin, spectrin, and associated proteins form a periodic cytoskeletal structure in axons.Science. 2013; 339: 452-456Crossref PubMed Scopus (805) Google Scholar). Moreover, in some axons, both SiR-Actin and phalloidin showed a more complex lattice (Figure S2), which may provide the required rigidity when the neurite exceeds a given size.Importantly, these results, together with the observation in dendrites, rule out the possibility that the formation of the actin pattern is dependent on AIS assembly.The Periodic Structure of the Cytoskeleton Is Replicated at Nodes of RanvierSo far, the periodicity of the subcortical cytoskeleton was proven only in unmyelinated cultured neurons. Yet most of the axons in the central and peripheral nervous system exhibit a myelin coat and a heterogeneous cytoskeleton with highly specialized compartments. Prominent examples include the nodes of Ranvier and the AIS. Both share a similar molecular composition of ion channels, cell-adhesion molecules, and cytoskeletal proteins (Peles and Salzer, 2000Peles E. Salzer J.L. Molecular domains of myelinated axons.Curr. Opin. Neurobiol. 2000; 10: 558-565Crossref PubMed Scopus (198) Google Scholar, Susuki and Rasband, 2008Susuki K. Rasband M.N. Spectrin and ankyrin-based cytoskeletons at polarized domains in myelinated axons.Exp. Biol. Med. (Maywood). 2008; 233: 394-400Crossref PubMed Scopus (46) Google Scholar). One of the common cytoskeletal molecules is βIVspectrin, which was shown to exhibit ∼180 nm periodicity in the AIS and is supposed to connect the actin rings (Xu et al., 2013Xu K. Zhong G. Zhuang X. Actin, spectrin, and associated proteins form a periodic cytoskeletal structure in axons.Science. 2013; 339: 452-456Crossref PubMed Scopus (805) Google Scholar). Therefore, we investigated the ultrastructural organization of βIVspectrin at nodes of Ranvier of mouse sciatic nerve fibers. Optical nanoscopy of myelinated axons is challenging, since the myelin sheets coating the axon generate significant aberrations. To overcome this problem, the nerve fibers were sliced into thin sections of 250–500 nm following melamine embedding (Punge et al., 2008Punge A. Rizzoli S.O. Jahn R. Wildanger J.D. Meyer L. Schönle A. Kastrup L. Hell S.W. 3D reconstruction of high-resolution STED microscope images.Microsc. Res. Tech. 2008; 71: 644-650Crossref PubMed Scopus (71) Google Scholar). Unfortunately, phalloidin and SiR-Actin stainings do not persist during this procedure. The nodes were identified by co-staining CASPR (contactin-associated protein), a protein residing in the paranodal region (Einheber et al., 1997Einheber S. Zanazzi G. Ching W. Scherer S. Milner T.A. Peles E. Salzer J.L. The axonal membrane protein Caspr, a homologue of neurexin IV, is a component of the septate-like paranodal junctions that assemble during myelination.J. Cell Biol. 1997; 139: 1495-1506Crossref PubMed Scopus (312) Google Scholar). STED nanoscopy of the sciatic nerve revealed different βIVspectrin patterns according to the relative axial position of the slice (Figure 3). A section of the middle part of the nerve (red plane in Figure 3A) shows a spotty organization of βIVspectrin only at the sides of the node (Figure 3B), with typically six to eight evenly spaced dots (Figure 3C). When a nerve was sliced at the lower or upper side (blue plane in Figure 3A), including the subcortical cytoskeletal layer, a more complex βIVspectrin pattern was observed (Figure 3D), which still exhibits ∼180 nm periodicity. In total, the separation of highly resolved βIVspectrin spots at the nodes of Ranvier was on average 180 ± 35 nm (n = 49) (Figure 3E), which is in excellent agreement with the periodicity shown for the AIS of unmyelinated cultured axons. This indicates that the ultrastructural organization of the AIS is replicated at the nodes of Ranvier and demonstrates the presence of a periodicity also in sciatic nerves. Hence, the cytosolic periodic pattern may be regarded as a general, ubiquitous feature of both the central and peripheral nervous systems.Figure 3βIVspectrin Replicates AIS Organization at the Nodes of RanvierShow full caption(A, B, and D) Sciatic nerves were embedded in melamine and sliced in 250–500 nm thin sections (A). Depending on the relative position of the slice to the nerve, different staining patterns can be observed. STED images of nodes of Ranvier from nerves sliced in the middle (B) or at the bottom (D). In both cases, nodes and paranodes are stained with βIVspectrin (green) and CASPR (red), respectively.(C) Line profile along the dashed line in (B) shows ∼180 nm periodicity of βIVspectrin.(E) Distribution histogram of measured interpeak distances (n = 49 from eight nodes), indicating average spacing ± 1 SD.Scale bars, 1 μm. Images were smoothed with a low-pass Gaussian filter.View Large Image Figure ViewerDownload (PPT)Cytosolic Actin Is Developmentally RegulatedSiR-Actin, of course, does not exclusively stain subcortical F-actin. It also stains the cytosolic F-actin pool, revealing interesting features. Focusing on axons, we noticed that the abundance of actin bundles running along the main axis varies over time, being high at 2 DIV (92.9%), decreasing until 8–9 DIV (54.2%, 39.3%, and 21.4% at 3, 5, and 8–9 DIV, respectively), and subsequently increasing again at 16–17 DIV (57.7%) (Figures 4A and 4B ). In younger cultures, the filaments appear as short, not-resolvable aggregate-like objects, while in older cultures, they appear long and largely continuous and are separated well by nanoscopy (Figures 2A, 4B, and S2). In mature cultures (>16 DIV), the organization of these filaments changes dramatically at the proximal side of the AIS, forming what can be considered an actin organizational border (Figures 4B and S3). In contrast, continuity in the actin organization across somata and dendrites was observed (Figure S3). Such ultrastructural differences indeed make it possible to distinguish between axons and dendrites by using their actin organization.Figure 4Cytosolic Actin Organization in Living NeuronsShow full caption(A) Percentage of axons at different developmental stages in which SiR-Actin labeling shows longitudinal bundles, rings or both (same dataset as Figure 2).(B) AIS-presenting actin filaments along the axon in living cells (24 DIV, inset shows neurofascin staining).(C) Co-localization of actin patches (phalloidin staining, green) with bassoon (red) in fixed neurons at 17 DIV. The axon was identified by staining NrCAM (inset, white; confocal image using an Alexa-488-coupled secondary antibody). Scale bars, 1 μm. All images are raw STED data.(D) Model of actin organization in cultured neurons at different developmental stages. The periodicity of subcortical actin in the axon is present already at 2 DIV. The cytosolic actin arrangement varies, consisting of short filaments in younger cultures (2–3 DIV) (see also Figure S2), which disappear at ∼8 DIV. In mature cultures (17 DIV), long actin fibers are present, but they stop mainly at the beginning of the AIS. The red spot indicates a synaptic bouton co-localizing with an actin patch. In dendrites, the subcortical actin periodicity is not visible at 2 DIV but becomes prominent by 8 DIV, when only few actin filaments populate the dendrites. In mature cultures, the presence of spines, in which actin is highly enriched, and long filaments in the neurite make the identification of the actin periodicity less straightforward (compare Figure 1C, box 4). Note also that actin organization differs in the soma.View Large Image Figure ViewerDownload (PPT)Previous electron microscopy studies showed the presence of actin patches along the AIS, considering them responsible for maintaining the neuronal polarity (Arnold and Gallo, 2014Arnold D.B. Gallo G. Structure meets function: actin filaments and myosin motors in the axon.J. Neurochem. 2014; 129: 213-220Crossref PubMed Scopus (28) Google Scholar, Song et al., 2009Song A.H. Wang D. Chen G. Li Y. Luo J. Duan S. Poo M.M. A selective filter for cytoplasmic transport at the axon initial segment.Cell. 2009; 136: 1148-1160Abstract Full Text Full Text PDF PubMed Scopus (247) Google Scholar, Watanabe et al., 2012Watanabe K. Al-Bassam S. Miyazaki Y. Wandless T.J. Webster P. Arnold D.B. Networks of polarized actin filaments in the axon initial segment provide a mechanism for sorting axonal and dendritic proteins.Cell Rep. 2012; 2: 1546-1553Abstract Full Text Full Text PDF PubMed Scopus (86) Google Scholar). SiR-Actin staining of living neurons highlights the presence of these patches along the AIS as well (bright localized locations in Figures 1C and 2A). Patches were seen in 52.9%, 64.5%, and 75% of axons at 5, 8–9, and 16–17 DIV, respectively, indicating a developmental increase in number (Figure S3). In order to better understand the functional significance of these patches, we analyzed whether they co-localize with the pre-synaptic active zone marker bassoon or with the synaptic vesicle marker synaptotagmin-1. In mature neuronal cultures, in which synaptic connections are already strengthened and patches are clearly visible, they co-localized with both bassoon and synaptotagmin-1 (Figures 4C and S3), indicating that they represent the scaffolds which support pre-synaptic boutons (Sankaranarayanan et al., 2003Sankaranarayanan S. Atluri P.P. Ryan T.A. Actin has a molecular scaffolding, not propulsive, role in presynaptic function.Nat. Neurosci. 2003; 6: 127-135Crossref PubMed Scopus (252) Google Scholar, Waites et al., 2011Waites C.L. Leal-Ortiz S.A. Andlauer T.F. Sigrist S.J. Garner C.C. Piccolo regulates the dynamic assembly of presynaptic F-actin.J. Neurosci. 2011; 31: 14250-14263Crossref PubMed Scopus (57) Google Scholar) rather than preventing the dendritic cargos from entering into the axon.In conclusion, cytosolic actin organization is highly dependent on both the developmental stage of the neurons and the subcellular compartment (axonal versus somatodendritic compartment).DiscussionIn this work, we used two-color STED live imaging to investigate the ultrastructure of endogenous F-actin in living cultured hippocampal neurons. Previous work showed the presence of a cytoskeletal periodic lattice as a hallmark of axons (Xu et al., 2013Xu K. Zhong G. Zhuang X. Actin, spectrin, and associated proteins form a periodic cytoskeletal structure in axons.Science. 2013; 339: 452-456Crossref PubMed Scopus (805) Google Scholar). SiR-Actin nanoscopy revealed the existence of the subcortical actin lattice not only in axons but also in dendrites of cultured neurons. The cytosolic actin organization is both developmentally and spatially regulated, showing diverse features at different DIV and in different subcellular compartments. We also identified a similar cytoskeletal periodic organization in nodes of Ranvier, alluding to the existence of this ultrastructural arrangement also in myelinated neurons. The use of SiR-Actin in combination with STED nanoscopy showed enhanced sensitivity compared to phalloidin staining and is thus, at present, the best way to detect fragile and fine actin features. The presented results are in agreement with an early electron microscopy study on hippocampal slices, where actin filaments were decorated with myosin S-1. There, an actin organization either in bundles or in a periodic lattice-like form was described in both dendrites and axons (Fifková and Delay, 1982Fifková E. Delay R.J. Cytoplasmic actin in neuronal processes as a possible mediator of synaptic plasticity.J. Cell Biol. 1982; 95: 345-350Crossref PubMed Scopus (244) Google Scholar). The presence of a similar cortical actin organization in both axons and dendrites may also explain how axons can acquire dendritic features when ankyrinG (the AIS master regulator) is impaired and how dendrites can turn into axons upon AIS disruption (reviewed in Rasband, 2010Rasband M.N. The axon initial segment and the maintenance of neuronal polarity.Nat. Rev. Neurosci. 2010; 11: 552-562Crossref PubMed Scopus (302) Google Scholar). Indeed, this process would require the rearrangement of cytosolic actin, but not of the subcortical actin structures. The universality of the cytoskeleton periodicity is stressed by the fact that it can be seen also at the nodes of Ranvier of sciatic nerves and hence, for the first time, in the peripheral nervous system. The organization of the cytoskeleton underneath the myelin sheet, however, still needs to be investigated.The functional relevance of the detected reorganizations of cytosolic actin is unclear (Figures 4A and 4D). It may link the morphological staging of neuronal development with such cytoskeletal rearrangements (Dotti et al., 1988Dotti C.G. Sullivan C.A. Banker G.A. The establishment of polarity by hippocampal neurons in culture.J. Neurosci. 1988; 8: 1454-1468Crossref PubMed Google Scholar) (Figure 1A). Actin dynamics have been implicated in long-term potentiation and can be modulated by neuronal activity (Fonseca, 2012Fonseca R. Activity-dependent ac" @default.
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• W2103705255 date "2015-03-01" @default.
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• W2103705255 title "STED Nanoscopy Reveals the Ubiquity of Subcortical Cytoskeleton Periodicity in Living Neurons" @default.
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• W2103705255 doi "https://doi.org/10.1016/j.celrep.2015.02.007" @default.
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