neurofilament subunits are integral components of …cdr.rfmh.org/publications/pdfs_nixon/yuan 2015...

ORIGINAL ARTICLE

Neurofilament subunits are integral components of synapsesand modulate neurotransmission and behavior in vivoA Yuan1,2,9, H Sershen2,3,9, Veeranna1,2,9, BS Basavarajappa4,5, A Kumar1,2, A Hashim3, M Berg1, J-H Lee1,2, Y Sato1,10, MV Rao1,2,PS Mohan1, V Dyakin1, J-P Julien6, VM-Y Lee7 and RA Nixon1,2,8

Synaptic roles for neurofilament (NF) proteins have rarely been considered. Here, we establish all four NF subunits as integralresident proteins of synapses. Compared with the population in axons, NF subunits isolated from synapses have distinctivestoichiometry and phosphorylation state, and respond differently to perturbations in vivo. Completely eliminating NF proteins frombrain by genetically deleting three subunits (α-internexin, NFH and NFL) markedly depresses hippocampal long-term potentiationinduction without detectably altering synapse morphology. Deletion of NFM in mice, but not the deletion of any other NF subunit,amplifies dopamine D1-receptor-mediated motor responses to cocaine while redistributing postsynaptic D1-receptors fromendosomes to plasma membrane, consistent with a specific modulatory role of NFM in D1-receptor recycling. These results identifya distinct pool of synaptic NF subunits and establish their key role in neurotransmission in vivo, suggesting potential novelinfluences of NF proteins in psychiatric as well as neurological states.

Molecular Psychiatry (2015) 20, 986–994; doi:10.1038/mp.2015.45; published online 14 April 2015

INTRODUCTIONNeurofilaments (NFs), the intermediate filaments of matureneurons, are among the most abundant proteins in brain. NFsexpand the calibers of large myelinated axons to support nerveconduction1,2 and support dendrites of large motoneurons.3,4

Unlike the intermediate filaments of other cell types, centralnervous system (CNS) NFs are hetero-polymers composed of NFL,NFM, NFH and α-internexin.5,6 NFM, NFL and NFH subunits alsodiffer from most other intermediate filament proteins in beingregulated by phosphorylation at many sites by multiple proteinkinases.7 The complex hetero-polymeric structure and dynamicallychanging phosphate topography of NF proteins suggest that theymight serve additional biological roles beyond static structuralsupport of axon caliber.8,9 Notably, NF proteins have beenreported to be altered in certain neuropsychiatric states althougha mechanistic understanding of their involvement is lacking.Synapses have long been considered to be degradative sites for

NF reaching terminals by axonal transport. A functional role for NFproteins within synapses, however, has rarely been consideredalthough scattered observations hint at this possibility.8,9 Forexample, NF subunits can be transported to synaptic terminals inoligomeric form, including hetero-dimers.10 Individual NF subunitsor their fragments have been detected in synaptic fractions andbound to specific isolated synaptic proteins7 although contamina-tion by axonal NF has not been excluded.11 A splice variant of theNMDA receptor subunit NR1 has been reported to bind specificallythrough its cytoplasmic C-terminal domain to the NFL subunit

in vitro and in non-neuronal cells transfected with NF proteins.8

Evidence is lacking, however, for a direct interaction between NFLand NR1 in brain. NFM was shown to bind via its C-terminus to thethird cytoplasmic loop of the dopamine D1 receptor (D1R) in vitrowhen both proteins were co-transfected in non-neuronal cells,which lowered D1R cell surface occupancy and sensitivity of thecells to D1R agonists.9 D1R is a primary site of action of stimulantdrugs like cocaine and amphetamine and, interestingly, chronicexposure to drugs of abuse in humans12 and in animal models ofdrug addiction13,14 selectively lowers levels of NF proteins andalters their phosphorylation states. Cocaine administration alsoactivates extracellular signal-regulated kinase (ERK), a known NFprotein kinase,15 by promoting its phosphorylation,16–18 which iscoupled to increased NF phosphorylation.12,16 The functionalsignificance of these cytoskeletal changes and their implicationsfor synaptic function remain unclear.In this study, we used multiple independent approaches to

establish that all four NF subunits of the mature CNS are integralproteins of synapses, particularly the postsynaptic area. Thissynaptic population can be distinguished from NFs within theaxonal cytoskeleton, thus excluding the possibility of artifactual NFcontamination as a basis for their presence in synapses. We furtherobserved that eliminating NF proteins from the CNS profoundlydisrupts synaptic plasticity without altering the structural integrityof synapses. Finally, using a large panel of genetic mouse modelsin which individual NF subunits were selectively deleted, weestablish in vivo a specific functional role of postsynaptic NFM

1Center for Dementia Research, Nathan Kline Institute, Orangeburg, NY, USA; 2Department of Psychiatry, New York University School of Medicine, New York, NY, USA;3Neurochemistry Division, Nathan Kline Institute, Orangeburg, NY, USA; 4Analytical Psychopharmacology Division, Nathan Kline Institute, Orangeburg, NY, USA; 5Department ofPsychiatry, College of Physicians & Surgeons, Columbia University, New York, NY, USA; 6Centre de Recherche du Centre Hospitalier de l'Université Laval, Département d'anatomieet physiologie de l'Université Laval, Québec, QC, Canada; 7Department of Pathology & Laboratory Medicine, University of Pennsylvania, Philadelphia, PA, USA and 8Department ofCell Biology, New York University School of Medicine, New York, NY, USA. Correspondence: Dr A Yuan or Dr RA Nixon, Center for Dementia Research, Nathan Kline Institute, 140Old Orangeburg Road, Orangeburg, NY 10962, USA.E-mail: [email protected] or [email protected] authors contributed equally to this work.10Current address: Laboratory of Molecular Neuroscience, Department of Biological Sciences, Tokyo Metropolitan University, Building 9, 4th Floor, Room 493, 1-1 Minami-osawa,Hachioji, Tokyo 192-0397, Japan.Received 22 September 2014; revised 4 March 2015; accepted 9 March 2015; published online 14 April 2015

Molecular Psychiatry (2015) 20, 986–994© 2015 Macmillan Publishers Limited All rights reserved 1359-4184/15

www.nature.com/mp

http://dx.doi.org/10.1038/mp.2015.45

mailto:[email protected]

mailto:[email protected]

http://www.nature.com/mp

subunits in modulating D1 receptor-mediated behavior. Ourfindings indicate that distinctive pools of NF proteins withinsynapses perform roles in neurotransmission quite different fromthe conventional role of NF in axons. Thus, they provide insightinto the functional significance of selective NF subunit alterationsobserved in certain neuropsychiatric disorders, in addition to thebetter known alterations associated with neurodegenerativediseases.19–23

MATERIALS AND METHODSGeneration of transgenic animals, drugs and antibodiesPlease see Supplemental Information.

Analytical methodsOur published methods were used for all the procedures. Please seeSupplemental Information.

RESULTSIdentification of a unique population of NF proteins withinsynapsesAlthough NF subunits have diverse ligands, including severalstructural and receptor proteins enriched in synapticterminals,8,9,24,25 it is not known whether or not NF proteins areactually stable constituents of synapses or simply axonal NFcontaminants. To resolve this issue, we applied multiple com-plementary analytical approaches, beginning with investigationsof NF subunits in synapses using immunogold electron micro-scopy. We first performed quantitative immunogold analyses onNFM in synaptic terminals of mouse striatum with monoclonalanti-NFM RMO44 using a post-embedding labeling procedure. Inanalyses of 4100 digitized micrographs using Bioquant software,we quantified the densities of gold particles contained within thecytoplasm of presynaptic or postsynaptic bouton areas, respec-tively, identified by clear synaptic vesicles or a postsynapticdensity, and separately analyzed the dendrites adjacent tosynaptic terminals. To assess non-specific labeling, we analyzedgrids incubated without primary antibodies or with primaryantibody pre-absorbed as a negative control (Figures 1e and f)and, as a second negative control, we also compared immunogoldlabeling over synaptic mitochondria, which should lack specificlabeling. Comparisons of gold particles per unit area for thesemorphological structures (Figures 1a–d) revealed a significantNFM enrichment in postsynaptic bouton areas compared withpreterminal dendritic areas or presynaptic terminals. Immunogoldparticle density for NFM in postsynaptic boutons was eightfoldhigher (Po0.0001, Mann-Whitney test) than that associated withmitochondria and fourfold greater than that in adjacent pre-terminal regions of dendrites (Po0.0001, Mann-Whitney test).NFM immunogold density in presynaptic terminals, while fourfoldhigher than non-specific mitochondrial labeling (Po0.0001,Mann-Whitney test) was much lower than that in adjacent axonpreterminal regions, contrasting with the NFM enrichment inpostsynaptic boutons relative to preterminal dendritic areas.We then confirmed NFM enrichment in postsynaptic boutons

by double-immunofluorescence analysis of primary corticalneurons derived from wild-type C57BL/6J mice at E17.5 days.Neurons (21 DIV) immunostained with mouse mAb RMO-44 oranti-NFM rabbit polyclonal antibody displayed concentrations ofNFM immunofluorescence that colocalized with the postsynapticmarker, PSD95, identified by the monoclonal antibody fromSigma, and were apposed to presynaptic terminals labeled withantibodies to synaptophysin (monoclonal, Sigma) or synapsin I(polyclonal, Sigma) (Figures 1g and i). NFM immunofluorescencein the postsynaptic terminal was stronger than in the preterminaldendrite. We could observe short 10 nm filaments in occasional

synapses, which in some cases, were linearly decorated by goldparticles after reaction with anti-NFM antibody in immunoelectronmicroscopy analyses (Figures 1j and p). The images indicate that atleast some synaptic NF subunits exist in polymerized form. Finally,we also confirmed that the immuno-gold labeling of NFM was a150 kDa intact subunit and not the degradation products of thissubunit by immunoblot analysis, which showed that full-lengthNFM is the major NFM-related protein species of synaptosomes(Supplementary Figure S1).We also performed pre-embedding immunogold EM labeling to

investigate the presence of all four NF subunits in synapses of WTmouse striatum (Supplementary Figures S2a and e) and hippo-campus (Supplementary Figures S2f and h). With each antibody,we analyzed, as negative controls, brain sections from micelacking all NF proteins through knockout of α-internexin, NFH andNFL (triple IHL-TKO mice, described in detail in Figure 3) to assessthe specific binding of antibodies to each NF subunit (Supple-mentary Figure S2c and d). Under the conditions used in ourlabeling studies, morphometric analyses showed that each NFsubunit antibody negligibly labeled IHL-TKO striatum (Figures 2cand d) or hippocampus (not shown) in contrast to the strongdecoration of the abundant NFs in myelinated axons of WT mice,which served as a positive control (Supplementary Figure S2a andb). Morphometric analysis of gold particles associated with each ofthe four NF subunits, NFM (Supplementary Figure S2e), NFL(Supplementary Figure S2f), NFH (Supplementary Figure S2g) andα-internexin (Supplementary Figure S2h) showed significantlyhigher labeling density in wild-type than in IHL-TKO mice(Supplementary Figure S2i) (Po0.0001, Mann-Whitney test, mean±s.e.m., n=10–26). Similar to NFM labeling, more labeling of NFL,NFH and α-internexin was seen in postsynaptic areas than inpresynaptic areas. To further establish the identity of NF assembliesin synapses, we performed immuno-gold double-labeling of NFLwith NFM or NFL with NFH. The labeling of two subunits was clearlyaligned along single filaments or associated with a loosefilamentous web within the postsynaptic bouton (Figures 2a and g).In further confirmation of the immunoelectron microscopy

results, western blot profiles of fractions from 30% Optiprepgradients demonstrated that all four NF subunits co-fractionate insignificant quantities with vesicular fractions containing thesynaptosomal marker PSD95 (Figure 2h, fractions 9–14). Support-ing the uniqueness of the NF subunit assemblies in synaptosomes,the ratio of α-internexin to NFL in synaptosomes (SupplementaryFigure S3) from striatal tissue was significantly higher than that inTriton-insoluble NF fractions (Po0.05, n= 3, Student’s t-test)(Figure 2i). Moreover, we observed a significant difference inphosphorylation states of the long C-terminal tails of synapticNFM and NFH compared with their counterparts in the total tissue(mainly axonal) NF population, as evidenced by significantlyless phosphorylation of NFM (Po0.01, n= 3, Student’s t-test)(Figure 2j) and greater phosphorylation of NFH (Po0.01, n= 3,Student’s t-test) (Figure 2k). We confirmed that the same patternexists in hippocampal synaptosomes (Figures 2l and n).When NFM was deleted from mice, the proportion of NFH

relative to NFL increased 107% in the synaptic pool comparedwith the total pool (P= 0.05, n= 3, Student’s t-test), while the NFHto α-internexin ratio showed a similar though non-significanttrend (P= 0.16, Student’s t-test, n= 3)(Figures 2o and q). Moreover,in the absence of NFM, phosphorylation of synaptic NFH increasedrelative to that of total (mainly axonal) NF (P= 0.02, n= 3, Student’st-test). These results indicate a differential effect of NFM loss onaxonal and synaptic NF pools.

Elimination of NFs impairs hippocampal long-term potentiation(LTP) inductionIHL-TKO mice were further characterized and used to investigatethe importance of NF proteins for synaptic function. In the

Function of neurofilament subunit in synapseA Yuan et al

987

© 2015 Macmillan Publishers Limited Molecular Psychiatry (2015), 986 – 994

*

*

*

Figure 1. Presence of NFM proteins in synapses and 10 nm NF polymers in synapses. Post-embedding EM using anti-NFM antibody (RMO-44)(a) shows immunogold decoration of presynaptic (a, 'Pre', arrows) and postsynaptic regions (a, 'Post', arrowheads). Mitochondria (Mito), as abackground control, exhibit little or no labeling. (b) Morphometry of NFM-gold particles shows eightfold more immunogold in postsynapticregions (Po0.0001) per unit area than in mitochondria and fourfold more than in preterminal dendrites where the presence of NFM isestablished (Po0.0001). Gold particles in the presynaptic region are fourfold more numerous than in mitochondria. Stars indicate astatistically significant difference compared with mitochondria. (c, d) Two examples illustrating the greater immunogold labeling inpostsynaptic regions than in presynaptic regions ('Pre', arrows and 'Post', arrowheads). Negative controls without primary antibody (e) or withprimary antibody pre-absorbed (f) show negligible labeling. Arrow points to a dendrite in (e) and arrowheads to synapses in (e, f). Double-immunolabeled primary neurons show colocalization of NFM and the postsynaptic marker, PSD95 (g). A focal region of strong NFMimmunofluorescence is apposed to a region containing strong immunofluorescence for presynaptic markers synaptophysin (h) or synapsin I(i). Although not abundant, NF structures could be visualized in synapses of human (j) and mouse brain (k). The presence of 9–10 nmfilaments was previously reported in dendritic spines of mouse brain prepared by rapid-freezing technique (Landis and Reese,29 reproducedby permission) (l). Linear structures in the synapse were also decorated by anti-NFM antibody visualized by immunogold labeling (m–p).Arrows point to 10nm filaments. n and p show higher magnifications of m and o, respectively. Scale bar, 100 nm in a; 50 nm in c, d; 200 nm ine, f and 5 μm in g–i; 100 nm in j, k, l, m and o; 50 nm in n and p.


988

Molecular Psychiatry (2015), 986 – 994 © 2015 Macmillan Publishers Limited

absence of the three subunits from these mice, NFM was alsoundetectable in the CNS (Figures 3a and b), including in synapto-somal preparations from hippocampus (Figure 3c). Despite thecomplete absence of NF, CNS neurons maintain normal structural

appearance with no evident neurodegenerative changes (Figures3d and g). In electrophysiological studies, we determined theinput/output responses of field excitatory postsynaptic potential(FEPSP) in the Schaffer collateral pathway of the hippocampal

Figure 2. Distinct NF populations in synapses and axons. Ultrastructural colocalization of NFL and NFH (a, b, c) or NFL with NFM (d, e, f) on thesame NF or on a filamentous web within the postsynaptic terminal by pre-embedding (a) or post-embedding immunoelectron microscopy(b–g). Paraformaldehyde-fixed samples were incubated with mouse anti-NFL or rabbit anti-NFH or NFM antibodies and probed with goat anti-mouse IgG and goat anti-rabbit IgG conjugated to 15 and 10 nm gold beads. a shows an enlarged spine containing NF decorated by both NFL(arrowheads) and NFH (arrows). b shows a filament decorated by only NFH (arrows). d shows a filament decorated by both NFL (arrowheads)and NFM (arrows). f shows two linear structures decorated by NFM (arrows) and one of them is in close contact with a vesicle (arrow). d showsa NF decorated by both NFL (arrowhead) and NFM (arrows) in an axon. Scale bars, 200nm in a; 100nm in b, c, d, e, g; 50nm in f. Pre,presynaptic terminal; Post, postsynaptic terminal. Western blot profiles of fractions 1–18 from 0 to 30% Optiprep gradient separation ofhippocampal post-nuclear supernatant showing NF proteins are associated with synaptosomes (j). (j) Ponceau S stain and immunoblotsstained with corresponding antibodies. Total NF (mainly axonal) and synaptosomal NF were isolated from striatum (i, j, k) or hippocampus (l,m, n) and quantified after immunoblotting with corresponding antibodies. (i) Relative ratios of α-internexin to NFL in fractions of total NF andsynaptosomal NF. (j) Relative ratio of phosphorylated NFM (RMO55) to total NFM(RMO44) in total NF and synaptosomal NF. (k) Relative ratio ofphosphorylated NFH (SMI31) to non-phosphorylated NFH (SMI33) in total NF and synaptosomal NF. (l) Relative ratios of α-internexin to NFL infractions of total NF and synaptosomal NF. (m) Relative ratio of phosphorylated NFM (RMO55) to total NFM(RMO44) in total NF andsynaptosomal NF. (n) Relative ratio of phosphorylated NFH (SMI31) to non-phosphorylated NFH (SMI33) in total NF and synaptosomal NF. (o)Proportion of NFH relative to NFL increased in the synaptic pool compared with the total pool in striatum of MKO mice. (p) Proportion of NFHrelative to α-internexin increased in the synaptic pool compared with the total pool in striatum of MKO mice. (q) Relative ratio ofphosphorylated NFH (SMI31) to non-phosphorylated NFH (SMI33) in total NF and synaptosomal NF from striatum of MKO mice. ANT, α-internexin; PSD95, postsynaptic density protein 95; WT, wild-type; TKO, α-internexin, NFH and NFL triple knockout mice. IHL-TKO, α-internexin,NFH and NFL triple knockout mice. t-NF, total NF; s-NF, synaptic NF; MKO, NFM knockout mice.


989


slices prepared from control and IHL-TKO animals. Increasingstimulus intensity evoked robust input/output responses of fEPSPin wild-type (WT) mice, whereas the fEPSP slope was significantlyreduced in IHL-TKO mice (Figure 3h). Before tetanic stimulations,the baseline fEPSP was recorded in 60-s intervals for 10 min withstimulation at an intensity equivalent to about 35% of themaximum evoked response. The tetanic stimulation26,27 evoked atypical LTP of fEPSP in slices from WT mice. These responses werestable over 120min. However, tetanic stimulation evoked asignificantly reduced fEPSP slopes in IHL-TKO slices (n=12 slicesper 5 mice per group, Po0.001, two-way analysis of variance with

Bonferroni’s post hoc test) (Figures 3i and j). NF proteins are,therefore, required for synaptic plasticity. Consistent with this result,IHL-TKO mice showed social interaction deficits using the 5-trialsocial memory test (Supplementary Figure S4), indicating that micelacking NF proteins failed to develop normal social memory.

NFM specifically influences dopamine D1R-mediated behaviorin vivoWe evaluated the effect of genetically deleting the NFM subuniton a classic D1 receptor-mediated behavior—locomotor activity

Figure 3. Construction and characterization of NF triple knockout homozygous mice and impaired CA1 long-term potentiation (LTP) in IHL-TKOmice. NF triple knockout homozygous mice were generated by cross-breeding α-internexin knockout mice with NFH knockout mice and thenwith NFL knockout mice. (a) Southern blot analysis of genomic tail DNA digested with enzymes and hybridized with 32P-labeled probes.Targeted disruption of all three genes was confirmed. (b) Western blot analysis of CNS and PNS nerve extracts of the triple knockout mice.Protein extracts prepared from optic nerves, corpus callosum, spinal cord and sciatic nerve were separated on a 10% SDS polyacrylamide geland transferred to nitrocellulose membrane. Membranes were probed with different antibodies. Absence of the three proteins was confirmedwith corresponding antibodies. Note that NFM was undetectable in optic nerves, corpus callosum, and spinal cord and limited (3% of normallevel) in sciatic nerves. (c) NFM was also undetectable in isolated hippocampal synaptosomes from TKO mice compared with wild-type mice.There was no neurodegeneration of synapses in the brain of TKO mice (e, g) compared with wild-type mice (d, f). OPN, optic nerve; CC, corpuscallosum; SC, spinal cord; SN, sciatic nerve. TKO, α-internexin, NFH and NFL triple knockout mice. (h) A summary graph showing the field input/output relationship for WT (light blue) and IHL-TKO (pink) mice. (i) A time course of the average of the field excitatory postsynaptic potentials(FEPSP) slopes from slices obtained from WT and IHL-TKO mice. The fEPSP slopes were normalized to the average value during the 10min priorto stimulation in each experiment. An arrow shows the time of tetanic stimulation (4 pulses at 100 Hz with bursts repeated at 5 Hz, and eachtetanus including 3 10-burst trains separated by 15 s). (j) A combined plot of the averages of fEPSP slopes at several time points. Each point isthe mean± s.e.m. (n= 5 mice per group, 12 slices per group). Two-way analysis of variance with Bonferroni’s post hoc test; ***Po0.001. Scalebars, 500 nm in d, e, f, and 200 nm in g.


990


response to cocaine administration. Baseline locomotor activity,calculated as total ambulatory counts over 14 h of nighttimeactivity in 30- min segments, was 33% higher in MKO mice thanin WT controls (25217 ± 3058 vs 18861 ± 3525, n= 10, P= 0.19,

Student’s t-test). To achieve sensitization to cocaine, we theninjected the mice with saline on day 1 followed by daily cocaineinjections (25 mg kg− 1) for 3 days. Activity measurements over thefirst 90 min after each administration showed that the repeated

2000

8

6

4

2

0

1500

1000

Day 1 Day 2 Day 3 Day 4

500

0

Loco

mot

or A

ctiv

ity (A

U)

Figure 4. Enhancement of cocaine-induced locomotor activity in MKO mice. Mice given a saline injection on day 1 followed by cocaine(25mg kg− 1, s.c.) on days 2–4 were analyzed for locomotor activity during 90min after each injection. Sensitization occurred in both MKO andwild-type controls after repeated injection as evidenced by higher activity after the second cocaine injection compared with the first cocaineinjection (a). Statistical analyses of the pooled data from 24 MKO and 66 wild-type controls indicate faster onset of activity (the first 10min)with the second compared with the first cocaine injection both in MKO (582± 132 vs 1636± 349, n= 24, P= 0.0024, Mann-Whitney Test) and inwild-type controls (237± 29 vs 536± 78, n= 66, P= 0.0002, Mann-Whitney test). Data are presented as mean± s.e.m., n= 24-66. Wild-type,open square; MKO, filled square. Locomotor activity was two to threefold higher than wild-type control levels (Po0.0001, Mann-Whitney test)in MKO mice. Dependence of increased locomotor responses in MKO mice on dopamine D1 receptor activation (b). Mice injected with4mg kg− 1 amphetamine on days 1–3 were divided into two groups: one treated with 0.5 mg kg− 1 D1 receptor antagonist SCH23390 30minbefore amphetamine and the second given saline vehicle. On day 9 (6- day abstinence), each group of mice was challenged with 1mg kg− 1

amphetamine (no SCH23390 given) and locomotor activity was measured during a 90-min interval immediately after injection. MKO mice (noSCH23390, filled square) show more sensitization compared with wild-type (no SCH23390, open diamond). These sensitization responses werecompletely blocked by SCH23390-treatment in MKO (open square) and wild-type mice (filled diamond). Measurements are mean± s.e.m. fromat least 8 animals for each group. (c) Enhanced cocaine-induced locomotor activity after deletion of NFM, but not NFL, NFH or α-internexin,from mice. Mice given a saline injection on day 1 followed by cocaine (25mg kg− 1, s.c.) on days 2–4 were analyzed for locomotor activityduring 90min after each injection. Locomotor activity was threefold higher than wild-type control levels (Po0.0001, Mann-Whitney test) inmice lacking NFM but not significantly altered in mice lacking NFL, NFH or α-internexin or containing one gene copy of NFM. Stars indicate astatistically significant difference compared with WT controls. Measurements are mean± s.e.m. from at least 8 animals for each group. NFMdepletion leads to recycling of the D1 receptor from endosomes (ES) to plasma membrane (PM) in response to cocaine administration (d).PM-enriched and ES-enriched fractions of mouse striata were isolated following subcutaneous injections of cocaine (25mg kg− 1 per day) orsaline for 7 days. Analysis of SCH23390 binding to synaptic membranes or ESs showed increased D1R localization in striatal synaptosomal PMvs ES in response to cocaine in MKO mice (d). Measurements are mean± s.d. (n= 5, P= 0.0448, Student’s t-test). sal, saline; coc, cocaine.Ultrastructural colocalization of D1R and NFM in the postsynaptic terminal by post-embedding immunogold electron microscopy (e–h).Paraformaldehyde-fixed samples were incubated with mouse anti-D1R and rabbit anti-NFM antibodies and probed with goat anti-mouse IgGand goat anti-rabbit IgG conjugated to 10 and 15 nm gold beads. D1R (arrows) was located both on the PM and inside postsynaptic terminalwhereas NFM (arrowheads) was mainly inside the postsynaptic terminal and some in close contact with D1R (e, f). Co-immunoprecipitation ofNFM and endosome markers (EEA1 and Rho B) with D1R from mouse synaptosomal preparations (i). EEA1, early endosome antigen 1; RhoB,Rho-related GTP-binding protein RhoB; B, bound; UB, unbound.


991


injections induced sensitization in both mouse groups, asevidenced by higher activity after the second cocaine injectioncompared with the first cocaine injection (Figure 4a). We obtainedsimilar results after injecting amphetamine, another dopamine-releasing drug (Figure 4b). In analyses of the pooled data from 24MKO and 66 WT controls, onset of a motor response was faster(within the first 10min) after the second cocaine injection thanafter the first injection in both MKO (582 ± 132 vs 1636 ± 349,n= 24, P= 0.0024, Mann-Whitney test) and WT controls (237 ± 29vs 536 ± 78, n= 66, P= 0.0002, Mann-Whitney test). Additional WTand MKO mice were later studied and the pooled data from 59MKO and 157 WT mice showed a significantly higher locomotoractivity in MKO mice than WT controls after cocaine injection ondays 2, 3 and 4 (Po0.0001, Mann-Whitney test) (Figure 4a). Weshowed that these enhanced responses to stimulant drugs in MKOmice were D1R-mediated by measuring stimulant drug responsesin the presence of the specific D1R antagonist SCH-23390. On days1–3, MKO and WT mice received 4mg kg− 1 amphetamine; half ofeach group was also treated with 0.5 mg kg− 1 SCH-23390 for30min before amphetamine. On day 9 (6-day abstinence), micewere challenged with 1 mg kg− 1 amphetamine (no SCH-23390given). In the absence of SCH-23390, MKO mice exhibited a higherdegree of sensitization than WT controls; however, in SCH-23390-treated MKO and WT mice, the sensitization responses weremarkedly attenuated to the same low level (Figure 4b).To determine whether or not these enhanced cocaine

responses were selective for NFM among the four CNS NFsubunits, we also analyzed mice lacking either NFH, NFL orα-internexin.5 In contrast to MKO mice, these three mouse linesexhibited normal responses to cocaine (Figure 4c). Replacing halfthe normal level of NFM in MKO (MKO± ) mice by cross-breedingthem with wild-type mice restored normal cocaine responses(Figure 4c), indicating that the effect on D1R-mediated locomotorbehavior was related to NFM (Supplementary Figure S5) and couldbe achieved at half the normal NFM gene dosage.In mice receiving daily cocaine injections for 7 days, p-ERK

immunoreactivity was stronger in neurons of MKO mice than inWT controls (Supplementary Figure S6). High-performance liquidchromatography-electrochemical detection of dopamine, seroto-nin and their metabolites revealed no differences in levelsbetween WT and MKO mice (Supplementary Figure S7), thus

excluding a change in biogenic amine levels as an explanation forthe enhancement of cocaine-induced locomotor activity in MKOmice. In addition, total D1R levels in striatum, determined byquantitative immunoblot analysis, were not significantly affectedby NFM deletion (Supplementary Figure S8). To investigate thisrelationship in neurons in vivo in the context of cocainesensitization, we isolated striata from 12 MKO and 12 WT miceinjected daily for 7 days with saline or cocaine to induce sensiti-zation and assesses D1R distribution within plasma membrane(PM)-enriched and endosome (ES)-enriched subcellular fractions.Western blot analyses of the PM marker ATPase or the ES markerEEA1 confirmed the substantial enrichment of PMs and ESs,respectively (Supplementary Figure S9). As predicted, cocainetreatment caused more D1R to shift from ESs to PMs in MKO miceas demonstrated by SCH ligand binding (Po0.05, Student’s t-test)(Figure 4d). To further establish the relationship of D1R-positiveESs to NFM, we performed immuno-gold double labeling of D1Rand NFM, which demonstrated co-localization of D1R and NFM inpostsynaptic boutons (Figures 4e and h). Moreover, in synapto-somal fractions, we could show that co-immunoprecipitation ofNFM and endosomal markers also pulls down D1R furtherindicating in vivo interaction between NFM and D1R-containingESs (Figure 4i).

DISCUSSIONAlthough NF proteins have been detected in synaptic fractionsand bound to synaptic proteins in vitro,7 they are usuallyconsidered to be axonal NF contaminants and their presence,let alone their possible function, in synapses has beencontroversial.11,28 Using multiple independent approaches, wehave shown unequivocally for the first time that a uniquepopulation of NF proteins is present and functional in synapsesand relatively abundant in the postsynaptic area relative toterminal dendritic branches. NF proteins within the synapses notonly exhibit different relative subunit ratios than the ones inaxons, but they are also differently phosphorylated, indicating thatNF proteins in synapses may serve different functions from thosein axons. The relatively high proportions of α-internexin andlowered phosphorylation state of NFM in synaptic NF assemblies

Figure 5. Model of D1R-containing endosomes anchored on NFM-containing cytoskeletal assemblies. On the basis of collective findings on NFscaffolding functions and our D1R data on NF subunit null mice, we propose a model by which NFM acts in synaptic terminals to anchor D1R-containing endosomes formed after agonist-induced internalization of membrane D1R. Retention of D1R in a readily available internal poolwithin the synapse would favor desensitization to D1R stimulation: in the absence of NFM, the greater recycling back to the plasmamembrane surface would favor hypersensitivity to D1R agonists, as observed in our in vivo studies.


992


would be expected to confer greater plasticity to the cytoskeletonand allow increased interaction with binding partners.Although NF proteins are present in both pre- and postsynaptic

areas, 10nm NF polymers are not abundant in synaptic areascompared with myelinated axons, estimated by conventionalelectron microscopy. Using a rapid-freezing technique, Landis andReese29 described infrequent 9–10 nm filaments in dendriticspines of mouse brain (Figure 1l) but interpreted these structuresas actin filaments because of their periodicity. NF also co-isolatedwith PSD30 but have generally been regarded as contaminants.11

The infrequency of conventional long intermediate-sized polymersin synapses indicates that most NF subunits in synapses areprobably in the form of oligomeric structures (protofilaments orprotofibrils). Consistent with this interpretation, transport of NFMin non-filamentous, but oligomeric form, has been reportedin vivo.10,31 It is also possible that NF polymers in synapses aremore susceptible than axonal NF to disassembly and/or degrada-tion. Our findings that NF proteins within the synapses containmore α-internexin and a less phosphorylated form of NFM supportthe notion that NF proteins within the synaptic cytoskeletonnetwork are in a relatively immature state as in developingneurons, making the cytoskeletal structure more dynamic andsusceptible to disassembly. Because NFL and NFM monomers areinefficiently transported along axons,5,10 NF proteins in synapsesare at least oligomeric in form and can be transported as such atleast in axons, providing one possible basis for their postsynapticlocation. It is also possible that NF proteins in postsynapticboutons are synthesized locally because mRNAs of NFL32,33 and α-internexin34 together with mRNAs of β-actin35 and tubulin34 werereported to be present in dendrites and polyribosomes arepreferentially localized under the base of dendritic spines.36

Furthermore, brain-derived neurotrophic factor-induced synapticplasticity from hippocampal slices is blocked by inhibitors ofprotein synthesis and Schaffer-collateral CA1 synapses that wereisolated from their pre- and postsynaptic cell bodies still exhibitedprotein synthesis-dependent plasticity, suggesting a local den-dritic source of protein synthesis.37 In this regard, evidenceindicates that NF proteins identified in nerve terminals isolatedfrom squid brain are generated by local protein synthesis.38

Therefore, by either mechanism, assembly states and ratios of NFsubunits could be regulated by the local concentration andphosphorylation of individual subunits. In our triple knockout micelacking α-internexin, NFH and NFL, or a second mouse line lackingα-internexin, NFH and NFM (not shown), the content of NFM orNFL proteins in the CNS, respectively, is negligible suggesting thatsingle subunits are unlikely to be a functional form of NF proteinsin synapses. Collectively, our data suggest that a platformminimally of oligomeric NF assemblies or short 10 nm NFpolymers may be needed for function at synapses.Our gene deletion studies demonstrated for the first time that

lack of NFM leads to changes of D1R-mediated LTP and D1R-mediated behavior. Our findings are consistent with a model(Figure 5) in which NFM, within a synaptic cytoskeletal structure,anchors D1R-containing ESs within synapses, thereby establishinga reservoir of receptors available for rapid recycling to the synapticPM following dopamine agonist stimulation. Without NFM,recycling back to the surface is accentuated favoring hypersensi-tivity to D1R agonists. NFM is one of two NF subunits withexceptionally long carboxyl-terminal tail domains, but it seems toplay a much more important role than its larger counterpart, NFH,in regulating the structure and function of NFs in axons.5,10 Thediversity of roles for NFM in neuronal function is consistent with itscomplex regulation by phosphorylation, which involves second-messenger-regulated kinase regulation of up to 36 sites on thehead domain and 12 sites on the C-terminal tail of human NFM,which are regulated by at least four potential proline-directedkinases and multiple protein phosphatases.39–41 The D1 receptorhas been shown to interact with the C-terminal tail of NFM and it

will be important for future studies to address the role ofphosphorylation of this domain in regulating D1R binding.Our demonstrations of NFM involvement in D1R-mediated LTP

and D1R-mediated behavior provide evidence that a uniquepopulation of NF assemblies is functional in synapses and that theobserved effects on synaptic function are not a result of abnormaltransport or abnormal effects on nerve conduction or axonaldiameter. Only deletion of NFM but not other NF subunitsmarkedly exaggerated D1R-mediated response to cocaine, eventhough deletions of NFL or NFH markedly alter caliber andconduction or conduction, respectively. Besides NFM, it might bepredicted that α-internexin, NFH and NFL also have specificfunctions in synapses because deletion of all NFs has profoundeffects on synaptic plasticity and social memory. Supporting thisview, our further observations also indicate that NFL selectivelyinfluences glutamate receptor function (Veeranna/Basavarajappaet al. unpublished data). Moreover, depressed hippocampal LTPinduction is also NF subunit-selective: maintenance of LTP isdeficient in NFH-null mice while basal neurotransmission andinduction of LTP are normal in NFM-null mice (not shown).Binding of NFM to D1R has been previously shown in non-

neuronal cells of epithelial origin, although both NFM and D1Rwere overexpressed and foreign to these cultured humanembryonic kidney cells.8,9 Our study demonstrates in vivo func-tional significance of this molecular interaction at synapses. Thesefindings strongly support the view that NF proteins are integralcomponents of synapses, providing insight into the causes andfunctional significance of NF subunit alterations observed inneuropsychiatric diseases.21–23,42,43

CONFLICT OF INTERESTThe authors declare no conflict of interest.

ACKNOWLEDGMENTSWe thank Corrine Peterhoff for assistance with figures and Nicole Gogel formanuscript preparation. This work was supported by Grant 5R01AG005604 (RAN)from the National Institutes on Aging. BSB is supported by NIH grant (R01 AA019443).

AUTHOR CONTRIBUTIONSAY, HS, V, BSB and RAN designed the research; AY, HS, V, BSB, AK, AH, MB, J-HLand YS performed the research; AY, HS, V, BSB, MB, AK, AH, MVR, VD, J-PJ andRAN analyzed the data; PSM provided reagents; VM-YL provided reagents andcritical advice; AY, HS and RAN wrote the paper.

REFERENCES1 Friede RL, Samorajski T. Axon caliber related to neurofilaments and microtubules

in sciatic nerve fibers of rats and mice. Anat Rec 1970; 167: 379–387.2 Hoffman PN, Cleveland DW, Griffin JW, Landes PW, Cowan NJ, Price DL.

Neurofilament gene expression: a major determinant of axonal caliber. Proc NatlAcad Sci USA 1987; 84: 3472–3476.

3 Kong J, Tung VW, Aghajanian J, Xu Z. Antagonistic roles of neurofilament subunitsNF-H and NF-M against NF-L in shaping dendritic arborization in spinal motorneurons. J Cell Biol 1998; 140: 1167–1176.

4 Zhang Z, Casey DM, Julien JP, Xu Z. Normal dendritic arborization in spinalmotoneurons requires neurofilament subunit L. J Comp Neurol 2002; 450:144–152.

5 Yuan A, Rao MV, Sasaki T, Chen Y, Kumar A, Veeranna et al. Alpha-internexin isstructurally and functionally associated with the neurofilament triplet proteins inthe mature CNS. J Neurosci 2006; 26: 10006–10019.

6 Yuan A, Sasaki T, Kumar A, Peterhoff CM, Rao MV, Liem RK et al. Peripherin is asubunit of peripheral nerve neurofilaments: implications for differential vulner-ability of CNS and peripheral nervous system axons. J Neurosci 2012; 32:8501–8508.

7 Yuan A, Rao MV, Veeranna, Nixon RA. Neurofilaments at a glance. J Cell Sci 2012;125: 3257–3263.


993


8 Ehlers MD, Fung ET, O'Brien RJ, Huganir RL. Splice variant-specific interaction ofthe NMDA receptor subunit NR1 with neuronal intermediate filaments. J Neurosci1998; 18: 720–730.

9 Kim OJ, Ariano MA, Lazzarini RA, Levine MS, Sibley DR. Neurofilament-M interactswith the D1 dopamine receptor to regulate cell surface expression and desensi-tization. J Neurosci 2002; 22: 5920–5930.

10 Yuan A, Rao MV, Kumar A, Julien JP, Nixon RA. Neurofilament transport in vivominimally requires hetero-oligomer formation. J Neurosci 2003; 23: 9452–9458.

11 Matus A, Pehling G, Ackermann M, Maeder J. Brain postsynaptic densities: therelationship to glial and neuronal filaments. J Cell Biol 1980; 87: 346–359.

12 Beitner-Johnson D, Guitart X, Nestler EJ. Neurofilament proteins and the meso-limbic dopamine system: common regulation by chronic morphine and chroniccocaine in the rat ventral tegmental area. J Neurosci 1992; 12: 2165–2176.

13 Garcia-Sevilla JA, Ventayol P, Busquets X, La Harpe R, Walzer C, Guimon J. Markeddecrease of immunolabelled 68 kDa neurofilament (NF-L) proteins in brains ofopiate addicts. Neuroreport 1997; 8: 1561–1565.

14 Sbarbati A, Bunnemann B, Cristofori P, Terron A, Chiamulera C, Merigo F et al.Chronic nicotine treatment changes the axonal distribution of 68 kDa neurofila-ments in the rat ventral tegmental area. Eur J Neurosci 2002; 16: 877–882.

15 Veeranna, Amin ND, Ahn NG, Jaffe H, Winters CA, Grant P et al. Mitogen-activatedprotein kinases (Erk1,2) phosphorylate Lys-Ser-Pro (KSP) repeats in neurofilamentproteins NF-H and NF-M. J Neurosci 1998; 18: 4008–4021.

16 Berhow MT, Hiroi N, Nestler EJ. Regulation of ERK (extracellular signal regulatedkinase), part of the neurotrophin signal transduction cascade, in the rat meso-limbic dopamine system by chronic exposure to morphine or cocaine. J Neurosci1996; 16: 4707–4715.

17 Valjent E, Corvol JC, Pages C, Besson MJ, Maldonado R, Caboche J. Involvement ofthe extracellular signal-regulated kinase cascade for cocaine-rewarding proper-ties. J Neurosci 2000; 20: 8701–8709.

18 Lu L, Hope BT, Dempsey J, Liu SY, Bossert JM, Shaham Y. Central amygdala ERKsignaling pathway is critical to incubation of cocaine craving. Nat Neurosci 2005; 8:212–219.

19 Rosengren LE, Karlsson JE, Sjogren M, Blennow K, Wallin A. Neurofilament proteinlevels in CSF are increased in dementia. Neurology 1999; 52: 1090–1093.

20 Bajo M, Yoo BC, Cairns N, Gratzer M, Lubec G. Neurofilament proteins NF-L, NF-Mand NF-H in brain of patients with Down syndrome and Alzheimer's disease.Amino Acids 2001; 21: 293–301.

21 Cairns NJ, Zhukareva V, Uryu K, Zhang B, Bigio E, Mackenzie IR et al. alpha-internexin is present in the pathological inclusions of neuronal intermediatefilament inclusion disease. Am J Pathol 2004; 164: 2153–2161.

22 English JA, Dicker P, Focking M, Dunn MJ, Cotter DR. 2-D DIGE analysis implicatescytoskeletal abnormalities in psychiatric disease. Proteomics 2009; 9: 3368–3382.

23 Reines A, Cereseto M, Ferrero A, Bonavita C, Wikinski S. Neuronal cytoskeletalalterations in an experimental model of depression. Neuroscience 2004; 129:529–538.

24 Hirao K, Hata Y, Deguchi M, Yao I, Ogura M, Rokukawa C et al. Association ofsynapse-associated protein 90/ postsynaptic density-95-associated protein(SAPAP) with neurofilaments. Genes Cells 2000; 5: 203–210.

25 Frappier T, Stetzkowski-Marden F, Pradel LA. Interaction domains of neuro-filament light chain and brain spectrin. Biochem J 1991; 275: 521–527.

26 Vitolo OV, Sant'Angelo A, Costanzo V, Battaglia F, Arancio O, Shelanski M. Amyloidbeta -peptide inhibition of the PKA/CREB pathway and long-term potentiation:

reversibility by drugs that enhance cAMP signaling. Proc Natl Acad Sci USA 2002;99: 13217–13221.

27 Sadrian B, Subbanna S, Wilson DA, Basavarajappa BS, Saito M. Lithium preventslong-term neural and behavioral pathology induced by early alcohol exposure.Neuroscience 2012; 206: 122–135.

28 Ouimet CC, da Cruz e Silva EF, Greengard P. The alpha and gamma 1 isoforms ofprotein phosphatase 1 are highly and specifically concentrated indendritic spines. Proc Natl Acad Sci USA 1995; 92: 3396–3400.

29 Landis DM, Reese TS. Cytoplasmic organization in cerebellar dendritic spines.J Cell Biol 1983; 97: 1169–1178.

30 DeGiorgis JA, Galbraith JA, Dosemeci A, Chen X, Reese TS. Distribution of thescaffolding proteins PSD-95, PSD-93, and SAP97 in isolated PSDs. Brain Cell Biol2006; 35: 239–250.

31 Terada S, Nakata T, Peterson AC, Hirokawa N. Visualization of slow axonal trans-port in vivo. Science 1996; 273: 784–788.

32 Paradies MA, Steward O. Multiple subcellular mRNA distribution patterns inneurons: a nonisotopic in situ hybridization analysis. J Neurobiol 1997; 33:473–493.

33 Crino PB, Eberwine J. Molecular characterization of the dendritic growth cone:regulated mRNA transport and local protein synthesis. Neuron 1996; 17:1173–1187.

34 Villace P, Marion RM, Ortin J. The composition of Staufen-containing RNA granulesfrom human cells indicates their role in the regulated transport and translation ofmessenger RNAs. Nucleic Acids Res 2004; 32: 2411–2420.

35 Tiruchinapalli DM, Oleynikov Y, Kelic S, Shenoy SM, Hartley A, Stanton PK et al.Activity-dependent trafficking and dynamic localization of zipcode binding pro-tein 1 and beta-actin mRNA in dendrites and spines of hippocampal neurons.J Neurosci 2003; 23: 3251–3261.

36 Steward O, Levy WB. Preferential localization of polyribosomes under the base ofdendritic spines in granule cells of the dentate gyrus. J Neurosci 1982; 2: 284–291.

37 Kang H, Schuman EM. A requirement for local protein synthesis in neurotrophin-induced hippocampal synaptic plasticity. Science 1996; 273: 1402–1406.

38 Crispino M, Capano CP, Kaplan BB, Giuditta A. Neurofilament proteins are syn-thesized in nerve endings from squid brain. J Neurochem 1993; 61: 1144–1146.

39 Sihag RK, Nixon RA. Phosphorylation of the amino-terminal head domain of themiddle molecular mass 145-kDa subunit of neurofilaments. Evidence for regula-tion by second messenger-dependent protein kinases. J Biol Chem 1990; 265:4166–4171.

40 Sihag RK, Jaffe H, Nixon RA, Rong X. Serine-23 is a major protein kinase Aphosphorylation site on the amino-terminal head domain of the middle mole-cular mass subunit of neurofilament proteins. J Neurochem 1999; 72: 491–499.

41 Pant HC, Rudrabhatla P. Topographic regulation of neuronal intermediate fila-ment proteins by phosphorylation: in health and disease. In: Nixon RA, Yuan A(eds) Cytoskeleton of the Nervous System vol. 3. Springer: New York: New York, NY,2011 pp 627–656.

42 Clark D, Dedova I, Cordwell S, Matsumoto I. Altered proteins of the anteriorcingulate cortex white matter proteome in schizophrenia. Proteomics Clin Appl2007; 1: 157–166.

43 Pennington K, Beasley CL, Dicker P, Fagan A, English J, Pariante CM et al. Pro-minent synaptic and metabolic abnormalities revealed by proteomic analysisof the dorsolateral prefrontal cortex in schizophrenia and bipolar disorder.Mol Psychiatry 2008; 13: 1102–1117.

Supplementary Information accompanies the paper on the Molecular Psychiatry website (http://www.nature.com/mp)


994


neurofilament subunits are integral components of …cdr.rfmh.org/publications/pdfs_nixon/yuan 2015...

Documents