Samp1 is a component of TAN lines and is required fornuclear movement

Joana Borrego-Pinto1,2,*, Thibaud Jegou1,2,*, Daniel S. Osorio1,2,3,*, Frederic Aurade1,2, Matyas Gorjanacz4,Birgit Koch4, Iain W. Mattaj4 and Edgar R. Gomes1,2,`

1UPMC Univ. Paris 06, INSERM UMR-S 787, 75013 Paris, France2Groupe Hospitalier Pitie-Salpetriere, Institut de Myologie, 75013, Paris, France3PhD Program in Biomedicine, Faculty of Medicine, University of Porto, Porto, Portugal4European Molecular Biology Laboratory, Meyerhofstr. 1, 69117, Heidelberg, Germany

*These authors contributed equally to this work`Author for correspondence (edgar.gomes@upmc.fr)

Accepted 24 October 2011Journal of Cell Science 125, 1099–1105� 2012. Published by The Company of Biologists Ltddoi: 10.1242/jcs.087049

SummaryThe position of the nucleus is regulated in different developmental stages and cellular events. During polarization, the nucleus movesaway from the future leading edge and this movement is required for proper cell migration. Nuclear movement requires the LINCcomplex components nesprin-2G and SUN2, which form transmembrane actin-associated nuclear (TAN) lines at the nuclear envelope.

Here we show that the nuclear envelope protein Samp1 (NET5) is involved in nuclear movement during fibroblast polarization andmigration. Moreover, we demonstrate that Samp1 is a component of TAN lines that contain nesprin-2G and SUN2. Finally, Samp1associates with SUN2 and lamin A/C, and the presence of Samp1 at the nuclear envelope requires lamin A/C. These results support a

role for Samp1 in the association between the LINC complex and lamins during nuclear movement.

Key words: LINC complex, Lamina, Nuclear movement, SUN2, Samp1, Cell polarization

IntroductionProper positioning of the nucleus is required for different cellular

and developmental processes (Reinsch and Gonczy, 1998; Burke

and Roux, 2009). Actin-dependent nuclear movement has been

described in migrating fibroblasts, astrocytes and neurons

(Gomes et al., 2005; Dupin et al., 2009; Bellion et al., 2005;

Luxton et al., 2010) and involves nuclear envelope proteins that

tether the nucleus to the actin cytoskeleton. In the outer nuclear

membrane, KASH domain proteins connect the actin

cytoskeleton to the nucleus (Starr and Han, 2002). In the inner

nuclear membrane, SUN domain proteins connect KASH domain

proteins to A-type lamins (Libotte et al., 2005; Padmakumar et al.,

2005; Crisp et al., 2006). Thus, the KASH–SUN protein complex,

also known as the linker of nucleoskeleton and cytoskeleton

(LINC) complex, provides a direct connection from the

cytoskeleton to the nuclear lamina (Razafsky and Hodzic,

2009; Folker et al., 2011). The LINC complex is also involved

in centrosome–nucleus connection and chromosome movement

during meiosis (Hiraoka and Dernburg, 2009; Razafsky and

Hodzic, 2009).

During fibroblast polarization before cell migration, the

nucleus moves away from the leading edge whereas the

centrosome remains at the cell centroid. This nuclear

movement results in the orientation of the centrosome towards

the leading edge (Gomes et al., 2005; Shen et al., 2008). The

KASH domain protein nesprin-2G and SUN2 are LINC complex

members that are necessary for nuclear movement in migrating

fibroblasts (Luxton et al., 2010). At the dorsal surface of the

nucleus, these proteins form linear arrays that colocalize with F-

actin. These structures, named TAN (transmembrane actin-

associated nuclear) lines, move with the nucleus, presumably

transmitting the forces from the retrograde actin flow to the

nucleus. Interestingly, although A-type lamins are also required

for nuclear movement and centrosome orientation, they do not

accumulate at the TAN lines. A-type lamins are required to

couple TAN lines to the nucleus (Folker et al., 2011; Luxton et al.,

2010).

Ima1, a Schizosaccharomyces pombe inner nuclear envelope

protein, is required for the stability of Kms2 (KASH domain

protein) and Sad1 (SUN domain protein) at the nuclear envelope

(King et al., 2008). Therefore, the function of Ima1 has been

proposed to be similar to lamins, which is to couple the LINC

complex to the nucleus (King et al., 2008; Hutchison and

Worman, 2004). Ima1 is conserved in all metazoa and the

mammalian homolog is Samp1 (NET5), encoded by the

TMEM201 gene. There are three predicted Samp1 isoforms and

in human cells, the shorter isoform is an inner nuclear membrane

protein that accumulates at the spindle matrix and is functionally

associated with the LINC complex and lamins (Buch et al., 2009;

Schirmer et al., 2003; Gudise et al., 2011).

We investigated the role of Samp1 in migrating fibroblasts. We

describe a role for Samp1 in centrosome orientation and nuclear

movement prior to cell migration. We found that Samp1 is also

involved in cell migration. Furthermore, we show that Samp1

localizes to TAN lines, along with nesprin-2G and SUN2.

Finally, we identified an association of Samp1 with both SUN2

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Fig. 1. Samp1 is involved in nuclear positioning and cell migration. (A) Quantification of centrosome orientation in NIH3T3 cells in untreated (control) or cells

treated with the indicated siRNAs. Confluent cell monolayers were serum starved, wounded and centrosome reorientation was induced with LPA. SiRNA-resistant

Samp1b (rSamp1b–GFP) was microinjected in wound edge cells 2 hours before addition of LPA. (B) Average positions of the nucleus (red) and centrosome (blue)

relative to the cell centroid, measured in cells treated as in A. (C,D) Representative images of wound-edge serum-starved NIH3T3 cells from A stained for

microtubules (green), centrosomes (c-tubulin, red) and DNA (DAPI, blue). In D, a cell expressing rSamp1–GFP (arrow and inset) is shown. The wound edge is at the

top of the panels and the centrosome is the bright dot at the MT focus indicated by white circles. (E,F) Sequential phase-contrast images from supplementary material

Movie 1 of LPA-stimulated wound-edge NIH3T3 cells untreated (control) (D) or treated with Samp1 siRNA (E). Images to the left show the starting frame from

which the kymographs to the right were constructed (1 frame per 5 minutes). The white boxes indicate the region used to generate the kymographs. Time in

hours:minutes. (G) Quantification of closed wound area relative to control, 16 hours after wound. (H) Representative images for the wound-closure assay quantified

in G. Error bars represent s.e.m. ***P,0.001; **P,0.01; *P,0.05; n.s., P.0.05. Scale bars: 10 mm (C–F), 50 mm (H).

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and lamin A/C, and a requirement of lamin A/C for the

maintenance of Samp1 at the nuclear envelope. This workprovides evidence for a role of Samp1 on the regulation of theLINC complex during nuclear movement and cell migration.

Results and DiscussionTo study the involvement of Samp1 in nuclear movement,we used serum-starved monolayers of NIH-3T3 cells. Upon

wounding and stimulation with lysophosphatidic acid (LPA),wound-edge cells move their nucleus rearwards to reorient theircentrosome towards the leading edge (Gomes et al., 2005). Wetransfected cells with three different Samp1 siRNAs and found

that LPA-induced centrosome orientation and nuclear positioningwas reduced when compared with that in control non-treatedcells, or cells treated with siRNA against Gapdh, although not to

the same extent as in nesprin-2G-depleted cells (Fig. 1A–C)(Luxton et al., 2010). In cells that were not stimulated with LPA,centrosome orientation and nuclear position were not affected

(supplementary material Fig. S1C,D). Expression of the siRNA-resistant Samp1 isoform (rSamp1b–GFP) reverted the inhibitionof centrosome orientation and nuclear positioning in Samp1

siRNA cells (Fig. 1A,B,D). Expression of SUN2 and nesprin-2G

was not altered in Samp1 siRNA cells (supplementary materialFig. S1A,B and Fig. S3G). In addition, time-lapse microscopyalso revealed the inhibition of nuclear movement in Samp1

siRNA cells (Fig. 1E,F; supplementary material Movie 1).Thus, Samp1 is involved in centrosome orientation and nuclearpositioning.

Nuclear positioning is implicated in cell migration (Luke et al.,2008; Bellion et al., 2005; Luxton et al., 2010). We measuredwound closure in Samp1 siRNA monolayers and observed less-efficient wound closure when compared with non-transfected

monolayers, to similar levels as in cells in which nesprin-2G wasknocked down (Fig. 1G,H). Therefore, Samp1 is involved in cellmigration. In addition, we measured the migrational persistence

of individual wound edge cells and found no difference betweencells grown in control or siRNA conditions (supplementarymaterial Fig. S1E), whereas migrational velocity was reduced in

siRNA conditions when compared with that in the control(supplementary material Fig. S1F), suggesting that major changesin directionality are not the cause of the observed reduced cellmigration in nesprin-2G- and Samp1-depleted cells.

Three different isoforms have been predicted for the mouseTmem201 gene (Gene ID: 230917). We named them Samp1a,Samp1b and Samp1c. Samp1a, which is the shortest one,

corresponds to the Samp1 isoform previously described inhuman cells (Buch et al., 2009). These isoforms share the sameN-terminus but differ in their C-terminus (supplementarymaterial Fig. S2A). We raised an antibody against Samp1 by

injecting rabbits with a purified MBP-tagged Samp1 fragment (aa39–209) located in the N-terminal region that is common to allisoforms. The post-immune serum (Samp1-91) detected two

different bands at 69 kDa and 72 kDa, which correspond to thepredicted sizes of Samp1b and Samp1c, respectively, whereas nobands were detected by the pre-immune serum (supplementary

material Fig. S2F,G). However, using RT-PCR, we found thatall three Samp1 isoforms are present in NIH3T3 cells(supplementary material Fig. S2B). Furthermore, we confirmed

that Samp1 siRNAs specifically decreased the levels of Samp1protein (supplementary material Fig. S2G,H). Using thisantibody, we confirmed that Samp1 is a nuclear envelope

protein (supplementary material Fig. S2E) (Buch et al., 2009).However, the antibody also labeled the centrosome. We raised a

second antibody (Samp1-404) in rabbits against a purified His–Samp1 fragment (aa 1–213) and affinity purified it. The Samp1-404 antibody labeled the nuclear envelope in control cells but notin Samp1 siRNA cells. No centrosomal staining was observed

(supplementary material Fig. S2H and data not shown).Furthermore, expressed Samp1 was not found at thecentrosome (supplementary material Fig. S2C,D). Thus,

Samp1-91 centrosomal immunoreactivity is probably notspecific.

Samp1 distribution appeared punctate, resembling the

distribution of the nuclear pore complex (NPC); however, wedid not observe colocalization between Samp1 and NPC(supplementary material Fig. S3A,B). Careful observation ofthe dorsal surface of the nuclei of wound-edge cells expressing

Samp1b–GFP revealed linear structures of Samp1 puncta thatcolocalized with actin cables (Fig. 2A). This organization isreminiscent of the recently described TAN lines that are

implicated in nuclear movement (Luxton et al., 2010).Although less noticeable, endogenous Samp1 also formedlinear structures of puncta that colocalized with actin cables

and GFP–mini–nesprin-2G (a probe that accumulates at TANlines) (Fig. 2B,C). Endogenous Samp1 and Samp1b–GFP alsoformed linear structures in the absence of actin staining, stronglysuggesting that the colocalization is not a bleed-through artifact

(supplementary material Fig. S3C,D). In addition, soluble GFPdid not form actin-associated linear structures (supplementarymaterial Fig. S3E). Therefore, Samp1 localizes to the TAN lines.

To determine whether Samp1 TAN lines are involved in nuclearmovement, we simultaneously imaged Samp1b–GFP and actincables labeled with Lifeact–mCherry on the dorsal surface of the

nucleus. We found that Samp1b–GFP TAN lines moved withdorsal actin cables during nuclear movement in wound edge cells(Fig. 2D). SUN2, another component of the LINC complex, also

forms TAN lines during nuclear movement (Luxton et al., 2010).We also found that Samp1b–GFP colocalized with SUN2 andactin cables at TAN lines (Fig. 3A). To test whetherSamp1 associates with SUN2, we expressed Samp1b–GFP

and found that endogenous SUN2 co-immunoprecipitatedwith Samp1b–GFP (Fig. 3B). GFP–mini–nesprin-2G also co-immunoprecipitated SUN2 confirming the formation of

the LINC complex in our cells. Conversely, GFP–SUN2co-immunoprecipitated with HA-Samp1b (Fig. 3C). In addition,GFP–SUN2 also co-immunoprecipitated lamin A/C as previously

showed (Crisp et al., 2006). Overall, our data demonstrate thatSamp1 is a component of the TAN lines and that it associateswith SUN2.

Ima1, the Samp1 homolog in S. pombe is involved in

the regulation of LINC complex and is required for forcetransduction from the cytoskeleton into the nucleus (King et al.,2008). Samp1 also functionally associates with LINC complex

proteins (Gudise et al., 2011). Therefore, we hypothesized thatduring nuclear movement, Samp1 might be involved in thelocalization of LINC complex proteins at the nuclear envelope.

Immunostaining of LINC complex proteins (SUN2 andnesprin-2G) and other nuclear-envelope-associated proteins(lamin A/C, lamin B2 and emerin) in Samp1-depleted cells did

not reveal any defects on nuclear envelope localization of theseproteins (supplementary material Fig. S3G). Therefore, this datasuggests that Samp1 is not required for the localization of LINC

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complex proteins at the nuclear envelope. A-type lamins are

required for nuclear movement, although they are not always

required for the localization of the LINC complex proteins at the

nuclear envelope (Crisp et al., 2006; Haque et al., 2006; Folker

et al., 2011). Instead, A-type lamins are proposed to stabilize

the LINC complex at the nuclear envelope to allow force

transmission from the retrograde actin flow to the nucleus

(Folker et al., 2011; Ostlund et al., 2009). Because we found that

Samp1 colocalizes with the LINC complex members (SUN2 and

nesprin-2G) at the TAN lines and it is involved in nuclear

movement, we tested whether the recruitment of Samp1 to the

nuclear envelope is dependent on lamin A/C. We found that

Samp1 accumulation at the nuclear envelope was reduced in cells

depleted of lamin A/C (Fig. 3D; supplementary material

Fig. S3H). Furthermore, Samp1b–GFP co-immunoprecipitated

with A-type lamins (Fig. 3E). GFP–mini–nesprin-2G also co-

immunoprecipitated lamin A/C, suggesting that the LINC complex

is associated with A-type lamins in our cells. Therefore, A-type

lamins associate with Samp1 and are required for the localization

of Samp1 to the nuclear envelope.

In this report, we show that Samp1 is a nuclear envelope

protein involved in nuclear movement during fibroblast

Fig. 2. Samp1 is a component of the TAN lines. (A) Fluorescence images of a nucleus dorsal plane from a wound edge cell expressing Samp1b–GFP (green),

stained with phalloidin (F-actin, magenta). Samp1 forms punctate linear arrays that colocalize with dorsal actin cables at the nuclear surface (arrows). Line-scan

analysis (bottom) in regions indicated by arrows 1–5 reveals a strong correlation between Samp1b–GFP and F-actin fluorescence. (B) Fluorescence images of a

nucleus dorsal plane from a wound edge cell expressing the TAN line marker GFP–mini–nesprin-2G (GFP-MiniN2G) and stained with Samp1-91 antibody and

phalloidin (F-actin). GFP–mini–nesprin-2G linear arrays and dorsal actin cables colocalize with Samp1 linear arrays. Line-scan analysis (bottom panel) in regions

indicated by arrows 1–5 reveals a strong correlation between Samp1, F-actin and GFP–mini–nesprin-2G fluorescence. (C) Fluorescence images of a nucleus

dorsal plane from a wound edge cell stained with Samp1-91 antibody (green) and phalloidin (F-actin, magenta). Endogenous Samp1 forms punctate linear arrays

that colocalize with dorsal actin cables at the nuclear surface (arrows). Line-scan analysis (bottom) in regions indicated by arrows 1–3 reveals a strong correlation

between Samp1 and F-actin fluorescence. (D) Fluorescence kymograph of a cell expressing Lifeact–mCherry (F-actin, magenta) and Samp1b–GFP (green)

from supplementary material Movie 2, during nuclear movement (wound edge on the top). Fluorescence images were acquired every 5 minutes from dorsal planes

of the nucleus. Arrows show dorsal cables of actin that move with Samp1 TAN lines as the nucleus moves away from the wound edge upon stimulation with

LPA. Scale bars: 10 mm.

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polarization prior to cell migration. We also show that Samp1 is

involved in cell migration, is a component of the TAN lines and

associates with SUN2 and lamin A/C. Finally, we show that the

maintenance of Samp1 at the nuclear envelope is dependent on

A-type lamins. Ima1, the Samp1 homolog in S. pombe has a

similar function of lamins in metazoa, which is to stabilize the

LINC complex at the nuclear envelope during nuclear positioning

(King et al., 2008). Our data suggest that this function of Ima1/

Samp1 is conserved in metazoa and also provides evidence that

the role of Samp1 during nuclear movement in metazoa is to

stabilize the interaction between the LINC complex and lamins.

During nuclear movement, nesprin-2G and SUN2 form TAN

lines that are involved in the transmission of force from the

cytoskeleton into the nucleus (Luxton et al., 2010). A-type lamins

are also required for nuclear movement; however, they do

not accumulate at the TAN lines. Instead, A-type lamins are

proposed to provide the stable scaffold for the anchoring of the

TAN lines at the nucleoplasm to harness the force of actin

Fig. 3. Samp1 interacts with SUN2 and lamin A/C. (A) Fluorescence images of a nucleus from a wound edge cell expressing Samp1b–GFP, stained with

phalloidin (F-actin) and SUN2. Samp1 linear arrays colocalize with dorsal actin cables and SUN2 linear arrays at the nuclear surface (arrows). Line-scan analysis

(bottom) in regions indicated by arrows 1–3 reveals a strong correlation between Samp1b–GFP, F-actin and SUN2 fluorescence. (B) Co-immunoprecipitation (IP)

assays demonstrating the association between Samp1b and SUN2. Samp1b-GFP co-immunoprecipitates with endogenous SUN2. GFP–mini–nesprin-2G also co-

immunoprecipitates with SUN2. GFP western blots (bottom) are from the same blot and correspond to the same exposure time. (C) Co-immunoprecipitation

assays demonstrating the association between SUN2 and Samp1. GFP–Sun2 co-immunoprecipitates with HA–Samp1b and lamin A/C. GFP western blots

(bottom) are from the same blot and correspond to the same exposure time. (D) Fluorescence images of nuclei in untransfected (Control) cells or cells transfected

with siRNA to knock down Lamin A/C, immunostained for Lamin A/C and Samp1-91. Note that lamin-A/C-depleted cells exhibit a strong disruption of Samp1

from the nuclear envelope (arrows). (E) Co-immunoprecipitation assays demonstrating the association between Samp1b and Lamin A/C. Samp1b–GFP

co-immunoprecipitates with lamin A/C. GFP western blots (bottom) are from the same blot and correspond to the same exposure time. Scale bars: 10 mm.

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movement for productive nuclear movement (Folker et al., 2011).

Our data suggest that Samp1 might stabilize the association

between the LINC complex and nuclear lamina at the TAN lines

and this function is required for nuclear movement (Fig. 4).

The LINC complex is involved in nuclear migration, nuclear

positioning, centrosome–nucleus connection and chromosome

movement during meiosis in different species (Hiraoka and

Dernburg, 2009; Razafsky and Hodzic, 2009). Our results

implicating Samp1 in the regulation of the LINC complex

during nuclear movement and cell migration, together with the

described role of Samp1 on centrosome–nucleus connection and

nuclear organization (Buch et al., 2009; Gudise et al., 2011)

support a role for Samp1 in the numerous processes dependent on

the LINC complex.

Materials and MethodsConstructs

Mouse Samp1b was amplified from cDNA clone 30357476 (GeneService) usingthe forward primer 59-TACTCGAGCCATGGAGGGAGTGAGCGC-39 and thereverse primer 59-ATAAGCTTCAGACCTTTCCAGGGAGTGG-39 and clonedinto pEGFP-N1 vector (Clontech). For cloning of HA–Samp1b, Gatewaytechnology (Invitrogen) was used. attB-containing primers were used to amplifySamp1b from the original clone and recombined with pDONR221 (Invitrogen)entry vector. pDONR221-Samp1b was recombined with the destination vectorpHA GW (gift from Geri Kreitzer, Weill Cornell Medical College of CornellUniversity, New York, NY). For siRNA-resistant constructs (siR), a syntheticDNA (GeneArt, Life Technologies) corresponding to the Samp1b ORF betweenBssHII and BspEI restriction sites and containing silent point mutations at thesiRNAs target sequences (#93 and #94) was exchanged in the original pEGFP-N1Samp1b vector.

Mouse Samp1 loop1 (bp 115–627) was amplified from the aforementionedcDNA clone with attB-containing primers. The amplified fragment wasrecombined with the entry vector pDONR221. pDONR211-Samp1-loop1 wasrecombined with pMBP-GW (gift from Renata Basto, Institut Curie, Paris, France)

to obtain MBP–Samp1-loop1. Human Samp1 N-terminal fragment aa 1–213 wasamplified by PCR and cloned into pET28a and pGEX-KG.

GFP–SUN2 vector was kindly provided by Howard Worman (ColumbiaUniversity, New York, NY), GFP–mini–nesprin-2G by Gregg Gundersen(Columbia University, New York, NY), LifeAct–mCherry by Roland Wedlich-Soldner (Max-Planck-Institut fur Biochemie, Martinsried, Germany).

Cell culture, cDNA and siRNA transfections and microinjection

NIH 3T3 cells were cultured in DMEM without sodium pyruvate with 10%calf serum. Plasmid DNA was transfected with Lipofectamine LTX with plusreagent (Invitrogen). siRNA were transfected using Lipofectamine RNAiMAX(Invitrogen), 72 hours before treatment with the following siRNAs: (1) Samp1 #93,59-ACGAUACACUGGUGCCCUAtt-39, Samp1 #94, 59-CUAUGGGAAUCGGC-ACUGUtt-39; Samp1 #18, 59-CAGUACAUGGAACACCUGAtt-39 containingSilencer Select modifications (Life Technologies). (2) lamin A/C, SASI_Mm01_00084266 (Sigma). (3) Nesprin-2G: 59-CCAUCAUCCUGCACUUU-CAtt-39 (Genecust Europe). (4) GAPDH, AM4624 (Life Technologies). cDNAMicroinjection for siRNA rescue and dual color imaging were performed aspreviously described (Gomes et al., 2005), using a Xenoworks microinjectionsystem (Sutter Instruments).

Centrosome reorientation, nuclear movement and wound-healingmigration assays

Confluent cell monolayers were serum-starved for 24 hours, scratch-wounded witha pipette tip and stimulated with 20 mM LPA. Centrosome reorientation andquantification of the relative position of nucleus and centrosome was conducted aspreviously described (Gomes et al. 2005). For each independent experiment, morethan 200 (centrosome reorientation) or 30 (nucleus and centrosome position) cellswere quantified.

For wound-healing assays, wounded monolayers were stimulated post starvingwith complete medium and imaged at multiple positions (n§10) for a period of 16hours, with one phase-contrast image every 30 minutes. Wound closure wasquantified by measuring the percentage wound closure area after 16 hours (n§3independent experiments) using the TScratch software (Geback et al., 2009).

Immunofluorescence

Cells were grown on coverslips and were fixed in either methanol at 220 C or 4%paraformaldehyde and permeabilized with 0.5% Triton X-100 in PBS for 5minutes at room temperature; primary antibodies were diluted in PBS containing10% goat serum. The following primary antibodies were used: mouse anti-Emerin(Novacastra), anti-c-tubulin (Sigma), anti-lamin A/C (gift from Glen Morris,Wolfson Center for Inherited Neuromuscular Disease, Wrexham, UK), anti-NPC(mAb414, Covance), anti-lamin B2; Guinea pig anti-LBR (gift from HaraldHerrmann, German Cancer Research Center (DKFZ), Heidelberg, Germany);Rabbit anti-nesprin-2 (gift from Gregg Gundersen), anti-Sun2 (gift from DidierHodzic, Washington University School of Medicine, St Louis, MO andImmuquest, Seamer, UK); Rat monoclonal anti-tyrosinated a-tubulin (YL1/2)(ECACC, UK). Secondary antibodies used were conjugated with Cy2, Cy3(Jackson ImmunoResearch) or Alexa Fluor 488 and Alexa Fluor 555 dyes(Invitrogen). Line-scan colocalization analysis was performed in linesperpendicular to TAN lines using NIH ImageJ and Metamorph (MolecularDevices).

Cell imaging

All the images were acquired in a Nikon TE2000 or Nikon Ti microscope equippedwith a Coolsnap HQ2 (Roper) CCD camera, using 406 NA 1.30, 606 NA 1.40or 1006 NA 1.40 plan apo oil-immersion objectives (Nikon) controlled byMetamorph (Molecular Devices). For time-lapse imaging, a heated (37 C)chamber with 5% CO2 (Oko-lab) was used together with the Nikon PerfectFocus system.

Antibody production

MBP–Samp1–loop1 was expressed in E. coli BL21 (NEB), purified by standardprotocols and used for the production of Samp1-91 rabbit polyclonal antibody(Eurogentec). Human His–Samp1 residues 1–213 was expressed in the E.coliRosetta strain (Novagen), purified under denaturing conditions using Talon MetalAffinity Resin (Clontech) and used for the production of rabbit polyclonalantibody Samp1-404. For antibody affinity purification from the rabbit sera,Human GST–Samp1 was expressed in the E. coli Rosetta strain, purified undernative conditions using Glutathione Sepharose 4B (GE) and covalently coupled toAffiGel10 Gel (Bio-Rad).

Immunoprecipitation

Cells were transfected with the indicated constructs. After 24 hours, cells werescraped in cold PBS, pelleted and lysates were prepared using lysis buffer (10 mMTris-HCl, 100 mM NaCl, 1mM EDTA, 1% Triton X-100, 0.5% NP40, pH 7.4)containing a protease inhibitor cocktail (Roche). A GFP-Trap kit (Chromotek) was

Fig. 4. Model for the role of Samp1 in nuclear movement. Samp1

stabilizes the LINC complex at the TAN lines on the nuclear envelope by

participating in its anchoring to the nuclear lamina through SUN2. Forces

applied by the actin cytoskeleton are transduced through nesprin-2G, SUN2

and Samp1 (at the TAN lines) to the nuclear lamina resulting in nuclear

movement. In Samp1 depleted cells, the LINC complex is less stable at the

nuclear envelope and actin-driven forces are less efficiently transmitted to the

nucleus, thus impairing nuclear movement.

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used for immunoprecipitation according to the manufacturer’s instructions. Totaland bound fractions were run in a 4–12% Bis–Tris gradient gel and transferred tonitrocellulose membranes. Western blots were probed using chicken anti-GFP(Aves labs), rat anti-HA (Roche), rabbit anti-SUN2 (Immuquest) and rabbit anti-lamin A/C (Santa Cruz) antibodies.

Statistical analysis

Statistical analysis was performed using Excel. Results are expressed as mean 6

s.e.m and statistical significance was assessed using Student’s t-tests.

AcknowledgementsWe thank H. Worman, G. Gundersen, G. Kreitzer, R. Basto, G.Morris, H. Herrmann, D. Hodzic, R. Wedlich-Soldner for reagents;E. Folker, C. Almeida, B. Cadot and J. Rowell for reading themanuscript; B. Cadot for confocal expertise.

FundingThis work was supported by a PhD fellowship from Fundacao para aCiencia e Tecnologia, Portugal [SFRH /BD/36770/2007] (to D.S.O.)and grants for the Association pour la Recherche sur le Cancer,France and La Ligue Nacionale contre le Cancer, France (to E.R.G.).

Supplementary material available online at

http://jcs.biologists.org/lookup/suppl/doi:10.1242/jcs.087049/-/DC1

ReferencesBellion, A., Baudoin, J. P., Alvarez, C., Bornens, M. and Metin, C. (2005).

Nucleokinesis in tangentially migrating neurons comprises two alternating phases:

forward migration of the golgi/centrosome associated with centrosome splitting and

myosin contraction at the rear. J. Neurosci. 25, 5691-5699.

Buch, C., Lindberg, R., Figueroa, R., Gudise, S., Onischenko, E. and Hallberg, E.

(2009). An integral protein of the inner nuclear membrane localizes to the mitotic

spindle in mammalian cells. J. Cell. Sci. 122, 2100-2107.

Burke, B. and Roux, K. J. (2009). Nuclei take a position: managing nuclear location.

Dev. Cell 17, 587-597.

Crisp, M., Liu, Q., Roux, K., Rattner, J. B., Shanahan, C., Burke, B., Stahl, P. and

Hodzic, D. (2006). Coupling of the nucleus and cytoplasm: role of the LINC

complex. J. Cell Biol. 172, 41-53.

Dupin, I., Camand, E. and Etienne-Manneville, S. (2009). Classical cadherins control

nucleus and centrosome position and cell polarity. J. Cell Biol. 185, 779-786.

Folker, E. S., Ostlund, C., Luxton, G. W. G., Worman, H. J. and Gundersen, G. G.

(2011). Lamin A variants that cause striated muscle disease are defective in anchoring

transmembrane actin-associated nuclear lines for nuclear movement. Proc. Natl.

Acad. Sci. USA 108, 131-136.

Geback, T., Schulz, M. M. P., Koumoutsakos, P. and Detmar, M. (2009). TScratch: anovel and simple software tool for automated analysis of monolayer wound healingassays. BioTechniques 46, 265-274.

Gomes, E. R., Jani, S. and Gundersen, G. G. (2005). Nuclear movement regulated byCdc42, MRCK, myosin, and actin flow establishes MTOC polarization in migratingcells. Cell 121, 451-63.

Gudise, S., Figueroa, R. A., Lindberg, R., Larsson, V. and Hallberg, E. (2011).Samp1 is functionally associated with the LINC complex and A-type laminanetworks. J. Cell Sci.124, 2077-2085.

Haque, F., Lloyd, D. J., Smallwood, D. T., Dent, C. L., Shanahan, C. M., Fry, A. M.,

Trembath, R. C. and Shackleton, S. (2006). SUN1 interacts with nuclear Lamin Aand cytoplasmic nesprins to provide a physical connection between the nuclear laminaand the cytoskeleton. Mol. Cell Biol. 26, 3738-3751.

Hiraoka, Y. and Dernburg, A. F. (2009). The SUN rises on meiotic chromosomedynamics. Dev. Cell 17, 598-605.

Hutchison, C. J. and Worman, H. J. (2004). A-type lamins: guardians of the soma?Nat. Cell Biol. 6, 1062-1067.

King, M. C., Drivas, T. G. and Blobel, G. (2008). A network of nuclear envelopemembrane proteins linking centromeres to microtubules. Cell 134, 427-438.

Libotte, T., Zaim, H., Abraham, S., Padmakumar, V. C., Schneider, M., Lu, W.,

Munck, M., Hutchison, C., Wehnert, M., Fahrenkrog, B. et al. (2005). Lamin A/Cdependent localization of Nesprin-2, a giant scaffolder at the nuclear envelope. Mol.

Biol. Cell 16, 3411-3424.Luke, Y., Zaim, Hafida., Karakesisoglou, Iakowos., Jaeger, V. M., Sellin, L., Lu,

Wenshu., Schneider, M., Neumann, S., Beijer, A., Munck, M. et al. (2008).Nesprin-2 Giant (NUANCE) maintains nuclear envelope architecture and composi-tion in skin. J. Cell Sci. 121, 1887-1898.

Luxton, G. W. G., Gomes, E. R., Folker, E. S., Vintinner, E. and Gundersen, G. G.

(2010). Linear arrays of nuclear envelope proteins harness retrograde actin flow fornuclear movement. Science 329, 956-959.

Ostlund, C., Folker, E. S., Choi, J. C., Gomes, E. R., Gundersen, G. G. andWorman, H. J. (2009). Dynamics and molecular interactions of linker ofnucleoskeleton and cytoskeleton (LINC) complex proteins. J. Cell Sci. 122, 4099-4108.

Padmakumar, V. C., Libotte, T., Lu, W., Zaim, H., Abraham, S., Noegel, A. A.,

Gotzmann, J., Foisner, R. and Karakesisoglou, I. (2005). The inner nuclearmembrane protein Sun1 mediates the anchorage of Nesprin-2 to the nuclear envelope.J. Cell Sci. 118, 3419-3430.

Razafsky, D. and Hodzic, D. (2009). Bringing KASH under the SUN: the many faces ofnucleo-cytoskeletal connections. J. Cell Biol. 186, 461-472.

Reinsch, S. and Gonczy, P. (1998). Mechanisms of nuclear positioning. J. Cell Sci. 111,2283-2295.

Schirmer, E. C., Florens, L., Guan, T., Yates, J. R. and Gerace, L. (2003). Nuclearmembrane proteins with potential disease links found by subtractive proteomics.Science 301, 1380-1382.

Shen, Y., Li, N., Wu, S., Zhou, Y., Shan, Y., Zhang, Q., Ding, C., Yuan, Q., Zhao, F.,Zeng, R. et al. (2008). Nudel binds Cdc42GAP to modulate Cdc42 activity at theleading edge of migrating Cells. Dev. Cell 14, 342-353.

Starr, D. A. and Han, M. (2002). Role of ANC-1 in tethering nuclei to the actincytoskeleton. Science 298, 406-409.

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