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. 2017 Oct 9;27(19):2999-3009.e9.
doi: 10.1016/j.cub.2017.08.031. Epub 2017 Sep 28.

Nesprin-1α-Dependent Microtubule Nucleation from the Nuclear Envelope via Akap450 Is Necessary for Nuclear Positioning in Muscle Cells

Petra Gimpel  1 , Yin Loon Lee  2 , Radoslaw M Sobota  3 , Alessandra Calvi  2 , Victoria Koullourou  4 , Rutti Patel  4 , Kamel Mamchaoui  1 , François Nédélec  5 , Sue Shackleton  4 , Jan Schmoranzer  6 , Brian Burke  2 , Bruno Cadot  7 , Edgar R Gomes  8
Affiliations

Nesprin-1α-Dependent Microtubule Nucleation from the Nuclear Envelope via Akap450 Is Necessary for Nuclear Positioning in Muscle Cells

Petra Gimpel et al. Curr Biol. .

Abstract

The nucleus is the main microtubule-organizing center (MTOC) in muscle cells due to the accumulation of centrosomal proteins and microtubule (MT) nucleation activity at the nuclear envelope (NE) [1-4]. The relocalization of centrosomal proteins, including Pericentrin, Pcm1, and γ-tubulin, depends on Nesprin-1, an outer nuclear membrane (ONM) protein that connects the nucleus to the cytoskeleton via its N-terminal region [5-7]. Nesprins are also involved in the recruitment of kinesin to the NE and play a role in nuclear positioning in skeletal muscle cells [8-12]. However, a function for MT nucleation from the NE in nuclear positioning has not been established. Using the proximity-dependent biotin identification (BioID) method [13, 14], we found several centrosomal proteins, including Akap450, Pcm1, and Pericentrin, whose association with Nesprin-1α is increased in differentiated myotubes. We show that Nesprin-1α recruits Akap450 to the NE independently of kinesin and that Akap450, but not other centrosomal proteins, is required for MT nucleation from the NE. Furthermore, we demonstrate that this mechanism is disrupted in congenital muscular dystrophy patient myotubes carrying a nonsense mutation within the SYNE1 gene (23560 G>T) encoding Nesprin-1 [15, 16]. Finally, using computer simulation and cell culture systems, we provide evidence for a role of MT nucleation from the NE on nuclear spreading in myotubes. Our data thus reveal a novel function for Nesprin-1α/Nesprin-1 in nuclear positioning through recruitment of Akap450-mediated MT nucleation activity to the NE.

Keywords: Akap450; Nesprin-1; Nesprin-1α; centrosome; cytosim computer simulation; microtubules; non-centrosomal MTOC; nuclear envelope; nuclear positioning; skeletal muscle.

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Figures

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Graphical abstract
Figure 1
Figure 1
LINC Complex Comprising Nesprin-1 and Sun1/Sun2 Is Required for NE Localization of Centrosomal Proteins during Myogenic Differentiation (A) C2C12 myoblasts were differentiated by serum starvation for the indicated time points (hours of differentiation). Cell lysates were analyzed by western blot using antibodies against myosin heavy chain (MHC), Nesprin-1α (MANNES1E monoclonal antibody [mAb]), and GAPDH. (B) Schematic of Nesprin-1α fused to myc-BirA∗ (BioID-Nesprin-1α) for BioID. (C) Non-differentiated C2C12 cells stably expressing doxycycline-inducible myc-BirA∗-Nesprin-1α were treated with (+DOX) or without (−DOX) doxycycline, fixed and stained for Nesprin-1 (green, clone 9F10), myc (red), or nuclei (DAPI, blue). The scale bar represents 20 μm. (D) Depicted are normalized quantities of proteins purified on streptavidin beads in BioID-Nesprin-1ɑ-expressing C2C12 myotubes (y axis) and myoblasts (x axis) treated with biotin. Each protein quantity is the ratio of the amount of a protein in cells treated with doxycycline (+DOX) to the amount from untreated cells (−DOX), as determined by tandem mass tag mass spectrometry following streptavidin affinity purification. Proteins in green are previously described binding partners of Nesprin-1, whereas proteins in red are known centrosomal proteins investigated in this study. See also Figure S1 and Data S1. (E) Dual-color SD-dSTORM image of Pericentrin (Pcnt, green) and Nesprin-1 (red, MANNES1E) [15, 18] at the nuclear surface of a differentiated C2C12 myoblast (left) and the same SD-dSTORM image rendered to the resolution of a conventional wide-field microscope (right). Insets show higher magnifications of colocalization regions (arrows). The scale bar represents 1 μm. The scale bar of insets represents 100 nm. (F) Representative epi-fluorescence images of 48 hr differentiated C2C12 myotubes, transfected with the indicated siRNAs. Cells were stained for Pericentrin (Pcnt, red), nuclei (DAPI, blue), and myosin heavy chain (MHC, green) to identify myotubes. The scale bar represents 20 μm. See also Figures S2A–S2F. (G) Quantification of Pericentrin recruitment to the NE in myotube nuclei after treatment with the indicated siRNAs. Error bars ± SD; n represents total number of nuclei from at least three independent experiments. ∗∗∗p < 0.001; ∗p < 0.05; n.s., not statistically significant; t test. (H) Representative epi-fluorescence images of 72 hr differentiated primary myotubes from wild-type, Sun1−/−, Sun2−/−, or Sun1−/−;Sun2−/− knockout mice, stained for Pericentrin (Pcnt, red), MHC (green), and nuclei (DAPI, blue). The scale bar represents 20 μm. (I) Quantification of Pericentrin recruitment to the NE as shown in (H). Error bars ± SD; n represents total number of nuclei from two independent experiments. ∗∗p < 0.01; n.s., not statistically significant, t test.
Figure 2
Figure 2
The Muscle-Specific Nesprin-1α Isoform Is Required for Recruiting Centrosomal Proteins to the Nucleus (A and B) Representative epi-fluorescence images of differentiated human immortalized myotubes from a healthy control (wild-type) or from a patient carrying a nonsense mutation within the SYNE1 (23560 G>T) gene immunostained for Pericentrin (Pcnt, red), Akap450 (red), or PCM1 (red) and (A) Myogenin (MYOG, gray) as differentiation marker or (B) the cis-Golgi marker GM130 (green) and nuclei (DAPI, blue). The scale bar represents 10 μm. See also Figure S2G. (C) Representative epi-fluorescence images of C2C12 myoblasts transfected with dsRed-PACT and GFP or GFP-Nesprin-1α (GFP-N1α). Cells were stained for nuclei (DAPI, blue) and Myogenin (not shown). The scale bar represents 10 μm. (D) Quantification of dsRed-PACT recruitment to the NE in non-differentiated, Myogenin-negative C2C12 cells expressing GFP or GFP-Nesprin-1α. Error bars ± SD; n represents total number of nuclei from three independent experiments. (E) C2C12 wild-type or Nesprin-1 CRISPR mutant cells transduced with mycBirA∗-Nesprin-1α without and with 1 μg/mL doxycycline (−/+DOX) were differentiated for 48 hr, fixed, and stained for Nesprin-1 (green, clone 9F10), Pericentrin (Pcnt, red), and Myogenin (MYOG, gray). The scale bar represents 10 μm. See also Figures S2H–S2J. (F) Quantification of Pericentrin recruitment to the NE in Myogenin-(MYOG)-positive nuclei as described in (E). Error bars ± SEM; n represents total number of nuclei from three independent experiments. ∗∗∗p < 0.001; n.s., not statistically significant, Tukey’s multiple comparisons test following one-way ANOVA. (G) Schematic representation of the different myc-BirA∗-Nesprin constructs used for the experiments shown in (H). (H) C2C12 wild-type, untransduced Nesprin-1 CRISPR mutant cells or CRISPR mutant cells transduced with mycBirA∗-Nesprin-1α (N1α), mycBirA∗-Nesprin-1α with the LEWD motif mutated to LEAA (N1α [WD/AA]), or mycBirA∗-Nesprin-2β (N2β) were incubated with doxycycline and differentiated for 48 hr, fixed, and stained for myosin heavy chain (MHC, green), Akap450 (red), and nuclei (DAPI, blue). The scale bar represents 10 μm. See also Figure S2K.
Figure 3
Figure 3
MT Nucleation from the NE Requires Nesprin-1, Sun1/2, and Akap450 (A) 3D-SIM fluorescent image of the nucleus of a differentiated C2C12 cell, stained for Nesprin-1 (white, MANNES1E), Pericentrin (Pcnt, red), and microtubules (MTs, green) after 5 min nocodazole washout to allow MT regrowth. The scale bar represents 5 μm. (B) 48 hr differentiated C2C12 cells, treated with the indicated siRNAs, were immunostained for Pericentrin (Pcnt, red), microtubules (MTs, green) and Myogenin (MYOG, gray) after nocodazole washout. The scale bar represents 10 μm. See also Figure S3A. (C) Quantification of the mean percentage (%) of Myogenin-positive cells with MT nucleation from the NE as described in (B). Error bars ± SD; n represents total number of nuclei from at least three independent experiments. ∗∗∗p < 0.001; ∗∗p < 0.01; n.s., not statistically significant, t test. (D) Differentiated human immortalized myotubes from a healthy control (wild-type) or from a patient carrying a nonsense mutation within the SYNE1 gene (23560 G>T) were immunostained for PCM1 (red), microtubules (MTs, green) and Myogenin (MYOG, gray) after nocodazole washout. Images represent maximum projections of confocal z sections. The scale bar represents 10 μm. See also Figures S3B and S3C. (E) Quantification of the mean percentage (%) of Myogenin-positive cells with MT nucleation from the NE as described in (D). Error bars ± SD; n represents total number of nuclei from two independent experiments. ∗∗p < 0.01; t test. (F) 48 hr differentiated C2C12 cells, treated with the indicated siRNAs were immunostained for Akap450 (red), microtubules (MTs, green) and Myogenin (MYOG, gray) after nocodazole washout. The scale bar represents 10 μm. (G) Graph shows the mean percentage (%) of Myogenin-positive cells with MT nucleation from the NE as described in (F). Error bars ± SEM; n represents total number of nuclei from two independent experiments. ∗p < 0.05; n.s., not statistically significant, one-way ANOVA with Dunnett’s multiple comparisons test.
Figure 4
Figure 4
Microtubule Nucleation from the Nucleus Is Required for Proper Nuclear Positioning (A) Differentiated human immortalized myotubes from a healthy control (wild-type) or from a patient carrying a nonsense mutation within the SYNE1 (23560 G>T) gene were immunostained with Akap450 (red), GM130 (gray), microtubules (MTs, green), and nuclei (DAPI, blue) following nocodazole washout. The scale bar represents 10 μm. See also Figure S4A. (B) C2C12 cells were transfected with the indicated siRNAs, differentiated for 48 hr, and stained for nuclei (DAPI, blue) and myosin heavy chain (MHC, white). Myotube outlines are marked by dashed lines, and nuclei are encircled. (C) Spreading factor analysis of nuclei in C2C12 myotubes as shown in (B) and for cells transfected with Nesprin-1 siRNA no. 2. Results are depicted as mean (blue line) with interquartile range (black bars) from three independent experiments. ∗∗∗p < 0.001; ∗∗p < 0.01; Mann-Whitney test. (D) Spreading factor analysis of nuclei in differentiated human immortalized myotubes from a healthy control (wild-type) or SYNE1 (23560 G>T) patient cells. Results are depicted as mean (blue line) with interquartile range (black bars) from two independent experiments. ∗∗∗p < 0.001; Mann-Whitney test. See also Movie S1. (E) C2C12 cells were transfected with the indicated siRNAs, differentiated for 48 hr, and stained for nuclei (DAPI, blue) and myosin heavy chain (MHC, white). Myotube outlines are marked by dashed lines, and nuclei are encircled. (F) Spreading factor analysis of nuclei in C2C12 myotubes shown in (E) and for myotubes transfected with Akap450 siRNA no. 2 and Pcnt (Pericentrin) siRNA no. 2. Results are depicted as mean (blue line) with interquartile range (black bars) from four independent experiments. ∗∗∗p < 0.001; n.s., not statistically significant, Mann-Whitney test. (G) Snapshots of myotubes simulated with Cytosim. Nuclei are blue, and MTs in white. Three conditions are shown from top to bottom, with (+) and without (−) MTs nucleated from the NE and ±Kif5b anchored at the NE, as indicated. ∗∗∗p < 0.001; Mann-Whitney test. See also Movie S2 and Figures S4B–S4D. (H) Spreading factor analysis of nuclei in simulated myotubes as shown in (G). Results are depicted as mean (blue line) with interquartile range (black bars). ∗∗∗p < 0.001; ∗∗p < 0.01; Mann-Whitney test. See also Figure S4E.
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