Moving and positioning the nucleus in skeletal muscle – one step at a time

Abstract

Nuclear movement and positioning within cells has become an area of great interest in the past few years due to the identification of different molecular mechanisms and functions in distinct organisms and contexts. One extreme example occurs during skeletal muscle development and regeneration. Skeletal muscles are composed of individual multinucleated myofibers with nuclei positioned at their periphery. Myofibers are formed by fusion of mononucleated myoblasts and during their development, successive nuclear movements and positioning events have been described. The position of the nuclei in myofibers is important for muscle function. Interestingly, during muscle regeneration and in some muscular diseases, nuclei are positioned in the center of the myofiber. In this review, we discuss the multiple mechanisms of nuclear positioning that occur during myofiber formation and regeneration. We also discuss the role of nuclear positioning for skeletal muscle function.

Abbreviations

AchR

Acetylcholine receptor

BIN1

Amphiphysin-2

CNM

CentroNuclear Myopathies

DNM2

Dynamin-2

ER

Endoplasmic Reticulum

KASH

Klarsicht, Anc1, and Syne Homology

KLC

Kinesin Light Chain

LINC

LInker of Nucleoskeleton and Cytoskeleton

Lrp4

Low-Density Lipoprotein Receptor-Related Protein 4

Map7

Microtubule Associated Protein 7

MTJ

Myotendinous Junction

MTM1

Myotubularin

MTOC

MT organizing center

MTs

MicroTubules

MUSK

Muscle-Specific receptor tyrosine Kinase

NE

Nuclear Envelope

NMJ

Neuro-Muscular Junction

SUN

Sad1 and Unc-83

TAN

Transmembrane Actin-associated Nuclear.

Introduction

Nucleus is the biggest organelle in eukaryotic cells. Its main function is to compartmentalize and protect DNA integrity. To this extent, the nucleus possesses 2 membranes, jointly termed the nuclear envelope (NE), which separate nucleoplasm from cytoplasm. The outer nuclear membrane is continuous with both the endoplasmic reticulum (ER) and the inner nuclear membrane at the nuclear pores.¹ Although traditional illustrations of a cell represent the nucleus at the center, the position of the nucleus is much more diverse. This versatility of the nucleus to be positioned in different regions of the cell is of vital importance and a multitude of mechanisms are involved in nuclear movement and positioning in different contexts (Fig. 1). One of the first examples of nuclear positioning was described in 1935 in the neuroepithelium of the chick neural tube.² Sauer described the movement of the nucleus during interphase in the chick neuroepithelium, a process named later interkinetic nuclear movement. Following these pioneer studies, new examples and roles for nuclear positioning have been described during cell polarization, migration, differentiation and lineage specification.³ The most well-known nuclear movement is probably the movement of female and male pronucleus after fertilization to form the zygote nucleus.⁴ Other examples were described in several types of organisms, from yeast to humans.³ Disruption of nuclear movement in these systems has a profound impact in cell behavior, organ development and function.^5-12

Figure 1.

The nuclear movement multiplicity. Representative drawings of nuclear movements occurring in several cell types and organisms. Nuclei are depicted in shaded purple. In yeast, nucleus has to migrate toward the cleavage site to allow proper division. In starved fibroblasts, nuclei are positioned at the middle of the cells and move backward upon serum exposure before cell migration can proceed.¹² After fertilization of an egg, the 2 pro-nuclei have to come in close vicinity to mix their DNAs and trigger cell division. In the vertebrate neuro-epithelium, nuclear movement is associated with cell cycle; In G1, nuclei move toward the basal side, and toward the apical side in G2. Mitosis occurs apically.¹¹² During the formation of the hypodermis in C. elegans, hyp7 precursors cells has to positioned their nuclei in a face-to-face manner.¹¹³ Nuclear movement occurs during drosophila oogenesis, between stages6 and 8, from the posterior to the anterior edge of the oocyte.²² Finally, light can induce nuclear movement in leaf cells.¹¹⁴

Nuclear movement and positioning is driven by cytoskeletal networks of microtubules (MTs), actin and/or intermediate filaments. In most cases these movements involve a connection between the cytoskeleton and the nuclear envelope.³ Each cytoskeletal family exerts force on the nucleus in various ways. MT-dependent nuclear movements involve: a) pushing and pulling forces through an MT organizing center (MTOC) tightly bound to the NE^13-16; b) forces exerted by MT motors attached to the NE^17-21 c) forces exerted by MT polymerization against the nucleus.²² Actin-dependent nuclear movements act by: a) tethering of the nucleus to moving actin filaments forming linear assemblies at the NE surface called Transmembrane Actin-associated Nuclear (TAN) lines^12,23; b) actomyosin contraction,^17,24,25 c) pushing forces exerted by actin polymerization.²⁶ Cytoplasmic intermediate filaments (vimentin and desmin) have also been involved in nuclear positioning, although the mechanisms are poorly defined.^27,28 Additionally, nuclear intermediate filaments (lamins) play an important role in nuclear architecture, maintenance of NE integrity as well as in transmitting cytoskeletal forces to chromatin and the nucleoskeleton²⁹which is required for nuclear movement.

The connection of the nucleus with the cytoskeleton is mediated by transmembrane nuclear envelope proteins and/or nuclear pore complexes. The LInker of Nucleoskeleton and Cytoskeleton (LINC) complex,³⁰ conserved from yeast to human, is composed of KASH (Klarsicht, Anc1, and Syne Homology) proteins at the outer NE and SUN (Sad1 and Unc-83) proteins at the inner NE.³¹ KASH and SUN proteins bind together and form a complex that spans the space between inner and outer membrane.³² In mammals, KASH proteins (Nesprins) can bind actin,^23,33 MT motors^20,21 or intermediate filaments.³⁴ These interactions are crucial for nuclear movement or positioning. On the nucleoplasm side, SUN proteins bind to nuclear lamins^30,35 thereby anchoring the entire LINC complex to the nuclear frame. Another site of interaction of the nucleus with the MTs can occur at the nuclear pore complexes using 2 different tethers: The nucleoporin RANBP2 recruits Dynein/Dynactin complex to the NE via BICD2,³⁶ and NUP133 recruits Dynein/LIS1 complex to the NE via NUDE and NUDEL.^37-41

Several nuclear speeds have been quantified so far in several systems without any correlation between the type of cytoskeleton and nuclear speed. Speed can vary between 0.05 and 16μm/min (Table 1). The range of observed speeds illustrates the diversity of mechanisms involved in moving this organelle. In some cases, only one cytoskeletal element drives nuclear movement while in others, a collaboration exists between MTs and actin or even intermediate filaments.²⁷

Table 1.

Kinetics of nuclear movements

System	Organism	Speed (m/min)	Cytoskeleton	Ref
Myotube, spreading	G gallus	0.02–0.09	—	54
Myofiber, spreading	R. norvegicus	0.03–0.14	Microtubules	54
Polarizing astrocytes	M. musculus	0.05	Actin/Intermediate filaments	27
Interkinetic movement, apical directed	M. musculus	0.06	Microtubules / Actin	17,18
Oocytes	D. melanogaster	0.07	Microtubules	22
Oocytes	M musculus	0.08	Actin	25
Retinal progenitors	D. Rerio	0.13–0.52	microtubules	112,115
Interkinetic movement, basal directed	M. musculus	0.14	Microtubules	18
Leaf cells	A. capillus-veneris	0.2	Actin	116
Myotube, spreading	M. musculus	0.2	Microtubules	61
Female pronucleus	X. laevis	0.2–1.5	Microtubules	117
Hyp7 cells	C. elegans	0.23	Microtubules	31
Polarizing fibroblasts	M. musculus	0.28–0.35	Actin	12,23
Cortical root cells	A. thaliana	0.3–2	Actin	118
Interkinetic movement	D. Rerio	0.3	Actin	115,119
Cortical neurons	M. musculus	0.33	Microtubules	17
Neuroepithelium	O. latipes	0.7	Microtubules	120
Myotube, centration	M. musculus	0.76–0.88	Microtubules	53
Granular neurons	M. musculus	1	Microtubules	24,121
Infected BHK21-F cells	M. auratus	1–2	Microtubules	63
Budding yeast	S. cerevisiae	1.18	Microtubules/actin	13
Subventricular explanted neurons	M. musculus	1.2–5	Actin/microtubules	122
Nuclear centration	C. elegans	2.9	Actin	123
Hair root cells	A. thaliana	4–5	Actin	118
Mature hyphae	N. crassa	6.6–11.4	Actin	124
Male pronucleus	X. laevis	16	Microtubules	117

The mammalian skeletal muscle fiber (myofiber) is a peculiar syncytia as it possesses hundreds of nuclei. In a fully matured myofiber, most nuclei are positioned and spaced regularly at the periphery, just below the plasma membrane.^42,43 The spatial organization of these nuclei is not random. An agglomeration of nuclei can be found clustered under the neuro-muscular junction (NMJ) and express specific mRNAs involved in synapse communication.^44-46 Others are found in increased density at the myotendinous junction (MTJ).⁴⁷ In contrast with fully matured myofibers, regenerating myofibers are characterized by the presence of non-peripheral nuclei. Historically, these shift of the nuclei from a peripheral to a central position suggested that nuclei actively move. However, it is not until 1969 that this theory was confirmed. Myofibers from E20 mouse embryos were found to display more peripheral nuclei than E18 myofibers, and a correlation was made between nuclei positioning and acetylcholine receptors clustering in in vitro innervated myofibers.^44-46 The interest increased later during the era of molecular genetics when several muscular diseases were associated with mutations in several nuclear envelope proteins,⁴⁸ known to link the nucleus to the cytoskeletons as well as in altering nuclear positioning in other systems.^3,23 In addition, a specific muscle disorder has been described, coined centronuclear myopathy (CNM) due to the abnormal localization of nuclei in the center of the myofiber, even in the absence of regeneration.⁴⁹ All these observations led to the formulation of 2 opposite hypothesis: 1) Position of the nucleus is important for muscle formation and function thereby stipulating that nuclear mis-positioning has a role in muscle dysfunction 2) Positioning of the nucleus is a consequence of muscle formation thereby tending toward the idea that nuclear mis-positioning is a consequence of muscle dysfunction. In this review we will resume multiple mechanisms that drive nuclear movement and position in skeletal muscle and discuss the role of nuclear positioning in skeletal muscle function.

Nuclear Movement and Positioning in Skeletal Muscle Fiber

Each skeletal muscle fiber (myofiber) is a syncitium that forms as a result of the fusion of hundreds of myoblasts.⁵⁰ Prior to fusion, myoblasts are highly proliferative, but exit the cell cycle at the onset of myotube differentiation and acquire the potential to fuse with each other.⁵¹ As myoblasts fuse, they form multinucleated syncytia called myotubes (Movie S1) that mature into myofibers possessing contractile units (sarcomeres) and transversal triads. A fully developed muscle is composed of an association of myofibers into bundles.⁵² When muscles undergo an injury in vivo, adult muscle progenitor cells (satellite cells) are activated, proliferate, and differentiate into myoblasts. The new myoblasts then contribute to the formation of the repaired myofibers.

At least 4 types of nuclear movements and positioning events occur during myofiber formation (Fig. 2A): 1) After fusion, the nucleus from the fusing myoblast rapidly moves toward the center of myotube (nuclear centration),⁵³ 2) in myotubes, myonuclei align along the longer axis and become evenly spaced (nuclear spreading),⁵⁴ 3) as myotube matures into myofiber, nuclei migrate from the center of the fiber to the periphery and become uniformly distributed (nuclear dispersion),^42,47 4) some nuclei cluster under the NMJ and MTJ (nuclear clustering).^47,55

Figure 2.

Nuclear movements during muscle formation. (A) After fusion of a myoblast with a myotube, the new nucleus migrates toward the center of the myotube (centration). Then myotube nuclei get dispatched along the longest axis (spreading), followed by a migration toward the cell periphery where they become anchored (dispersion). Finally, few nuclei aggregate under the neuromuscular junction (clustering). (B) The nuclear movement after fusion involves MTs emanating from nuclei. They can bind NE through Dynein anchored by Par6. Dynein, through its ability to walk toward the minus end of MTs, will bring nuclei together. (C) The spreading movement requires MTs and can be achieved by 3 different mechanisms: 1, Anti-parallel MTs, bound to the NE through their minus end, are cross-linked by an evolutionary bridge made by the complex Map7-Kinesin-1. Kinesin-1, by walking toward the plus end of MTs will push apart nuclei. 2, Kinesin Heavy Chain bound to the nuclear envelope through Kinesin Light Chain and Nesprin can generate nuclear movements and rotations. In the same way, Dynein, bound to the nuclear envelope through Nesprin, Clip190 or Dynactin, can also generate nuclear movements. 3, Microtubules and Dynein, cortically-anchored at myotube poles through Clip190 and Raps/Pins respectively, can produce forces on plus end of microtubules. These forces will be transmitted to the NE where microtubules minus ends are anchored and then transformed in nuclear movements.

Nuclear Centration

A major reorganization of the cytoskeleton occurs during myotube formation. In contrast to myoblasts that possess a centrosome as the MT organizing center, post-mitotic myoblasts undergo a transfer of their MT organizing center from the centrosome to the NE.^56-59 Therefore, microtubules are nucleated at the NE of nuclei in myotubes or myofibers instead of being nucleated at the centrosome in myoblasts. Actin on the other hand becomes organized in parallel filaments in post-mitotic myoblasts and myotubes, and participates in the elongated shape of the myotubes.^59,60 Recent work demonstrated that after fusion of a myoblast with a myotube, the myoblast nucleus moves immediately to the center of the myotube at an average speed of 0.88 um/min. Centration movement is driven by MTs and is regulated by Cdc42 as well as the polarity proteins Par6 and Par3. Furthermore, the dynein/dynactin complex, a MT motor, is also required for the centration nuclear movement. To this extent, a strong accumulation of Par6 and dynein/dynactin complex components is observed at the nuclear envelope of differentiated myoblasts and myotube. The accumulation of dynein/dynactin at the nuclear envelope was dependent on Par6β but not on MTs, suggesting that Par6β is the anchoring partner for dynein/dynactin at the NE. How Par6β is maintained at the nuclear envelope is unknown, but this process is also independent of MTs. Therefore, the centration movement probably results from nuclei pulling the MTs that are nucleated from the NE of other nuclei. The MT motors that are found at the NE are the driving force for nuclear movement. It is also possible that the myoblast nucleus moves toward the center utilizing the MTs that emanate from myotube nuclei, similar to vesicle movement on MTs (Fig. 2B).^53,61 Differentiated myoblasts and early myotubes share similar characteristics, such as MT nucleation from the NE, parallel organization of the cytoskeletons and elongated shape. It suggests that Cdc42 and Par proteins might function in the same manner in both cells, by tethering molecular motors at the NE. This mechanism is similar to nuclear movement that brings together the male and female pro-nucleus after fertilization.⁶² Curiously, MT-dependent nuclear movement toward the center of a syncytia also occurs after fusion induced by virus infection,⁶³ suggesting a common mechanism for nuclear positioning in the center of a syncytia.

Nuclear Spreading

The spreading of nuclei along the axis of myotubes also involves the MT network but the behavior of nuclei is strikingly different (Fig. 2C). Nuclei move along myotubes, but pauses are commonly observed. Nuclei speed, when in motion, is approximately 0.2 um/min (unpublished data,^54,61), which is 4 times slower than nuclear movement after fusion (nuclear centration).⁵³ Three types of non-mutually exclusive mechanisms have been proposed to drive nuclei spreading within the myotube. The first one involves an interaction between Kif5b/Kinesin-1 and a Microtubule Associated Protein, Map7. This complex maintains an anti-parallel network of MTs and, through the forces exerted by the motor Kinesin-1 toward the plus-ends, allows nuclei to align and evenly spread within the myotube^47,64,65 (Fig. 2C,1). In Drosophila, Ensconsin, the Map7 ortholog, and khc, the Kinesin-1 ortholog, were found responsible for nuclear positioning.⁶⁴ In a mouse model of a muscle conditional knock-out for Kinesin-1, newborn mice die due to a severe dystrophy of skeletal muscles⁶⁶ with myofibers showing a strong phenotype of non-dispersed nuclei. Interestingly, the translocation movement along myotube axis is accompanied by nuclear rotations, leading to the elaboration of a second proposed mechanism.⁶¹ In fact, a subset of Kinesin-1 is localized at the NE through the binding of its partner, KLC-2 (Kinesin Light Chain 2), itself bound to Nesprin-2, a member of the LINC complex (Fig. 2C,2). This indirect interaction was described to be responsible for rotations and movement along MTs.^61,67 The third type of mechanism, only described in Drosophila, involves Dynein. Dynein is cortically-anchored at myotube poles through Raps/Pins, which can pull on microtubules anchored at the cortex by Clip190 (Fig. 2C,3).⁶⁸ The cortical localization of Dynein has been found to be regulated by a Jnk signaling cascade.⁶⁹ It implicates the action of SYD/JIP3, an adaptor protein which promotes Dynein localization at myotube poles through a Khc-dependent transport. Finally, Dynein also has a role in nuclear movement and positioning through its localization at the NE and its interaction with Clip190 and p150Glued (Fig. 2C,2).^61,68 Interfering with Dynein function leads to aberrant myonuclear positioning, less nuclear rotations and function defects in mammalian cell and drosophila larvae muscles.^61,68 However, through its binding to LIS1, Dynein can also regulate muscle length⁶⁸ and is therefore involved in 2 separate pathways regulating muscle growth. Overall, these studies reveal the complexity of moving nuclei inside myotubes to obtain an even distribution and how this distribution is required for proper muscle function.

Nuclear Dispersion

The movement of nuclei toward the periphery of myofibers (nuclear dispersion) may occur at 2 different periods of life in mammals: a) during mouse and human development, nuclei are centrally located and go to the periphery after birth,^70-72 b) after injury, centrally located nuclei move to the periphery which is part of the repair mechanism.^73,74 Nuclear dispersion is still under investigation but it is established that non-synaptic nuclei are not randomly distributed⁴⁷ suggesting the presence of specific factors involved in this process. Recently, it was demonstrated that peripheral nuclear positioning requires 2 consecutive events driven by different cytoskeleton elements: first, spreading of nuclei along the myofiber (MTs, Map7 and Kinesin-1 dependent) and second, an actin and Nesprin- dependent movement toward the periphery.⁷⁵ It is noteworthy that striated muscles, such as cardiac or skeletal muscle cells, express shorter isoforms of Nesprin 1 and 2,⁷⁶ which may have other functions than the full length proteins.⁷⁷

N-Wasp, an actin nucleation promoting factor, is required for nuclear movement to the periphery. N-Wasp is activated downstream of Amphiphysin-2/Bin1, a gene mutated in centronuclear myopathy.^75,78 Interestingly, this discovery implies a consecutive switch from an MT-driven to an actin-driven nuclear positioning during myofiber formation. Therefore, nesprins and SUN proteins might be involved not only in anchoring the nuclei at the periphery of the fibers as previously described,^79-82 but are probably also required for the movement of the nuclei to the periphery of the myofiber (nuclear dispersion). Also in drosophila, absence of one or both of the KASH containing proteins, MSP300 and KLAR, leads to aberrant nuclear positioning and anchoring to the acto-myosin apparatus.^80,83 Other factors have also been described to regulate peripheral positioning. One of the intermediate filaments, Desmin, has been found to play a role in positioning non-synaptic nuclei^28,84-86 alongside an extracellular cue, the presence of blood vessels in the myofiber vicinity.²⁸ Blood vessels are supposed to act as a nuclei attractant toward myofiber membrane and Desmin as a repellant between nuclei, leading to an even distribution of nuclei at the fiber periphery. Actually, Desmin can be found associated with the nuclear envelope, and its absence enhance the Nesprin1 KO phenotype of nuclear mis-positioning.^86,87

Nuclear Clustering

Clustering of nuclei under the NMJ may be correlated with the formation of the synapse itself as these nuclei express NMJ-specific mRNAs.^88,89 Interestingly, mutation or depletion of LmnA leads to aberrant and fragile NMJs with nuclei not clustered under the NMJ.⁹⁰ In the mouse embryo, at E11, fusion between muscle progenitors leads to myotube formation, which will become the future skeletal muscles. Shortly after, motor neurons invade muscles and form intramuscular nerves which are usually restricted to the central region of the muscle.⁹¹ To ensure a sufficient depolarization of the muscle membrane and trigger action potentials, acetylcholine receptors (AchRs) are highly concentrated at the post-synaptic region. To this extent, muscles express MuSK (Muscle-Specific receptor tyrosine Kinase) and LRP4 (Low-Density Lipoprotein Receptor-Related Protein 4) which form a complex that clusters AchRs under the action of the motor neuron-derived Agrin.⁹² This creates a pattern in the middle of fibers where axons will specifically connect. Enhanced expression of MuSK leads to the formation of functional synapses far from the middle region. Therefore, NMJ formation can occur out of the fiber central region and is tightly correlated with the presence of axons.⁹³ Even if the presence of axons is not required for AChRs clustering, they are required for their maintenance and fine tuning through the expression of Agrin and Acetylcholine.^94-96 All these observations point to a strong interplay between the axon endplate and myonuclei clustering. Whether clustering of synaptic nuclei is concomitant to AchRs clustering remains an open question. However, pieces of evidence indicate that nuclei clustering might be dependent on MuSK patterning as Nesprins have been first discovered as partners of MuSK and localized at the NMJ.⁹⁷ Interestingly, Nesprin and SUN KO mice show severe synaptic nuclei anchoring defects. Furthermore, patients with mutations in SUN genes also have defects of synaptic nuclei anchoring.^79,98 These results strongly suggest a close connection between plasma membrane and NE at the NMJ through a MuSK-Nesprin-SUN bridge.

Nuclei Localization and Muscle Disease

Several muscle diseases are characterized by abnormal nuclear positioning and are due to mutations of nuclear envelope proteins.^99,100 CNM describe a heterogeneous group of inherited muscle diseases pathologically defined by the abnormal localization of nuclei in the center of muscle fibers.⁴⁹ As with most congenital myopathies, the symptoms of CNM are detected at birth or within the first year of children's life; symptoms include reduced fetal movements or delay in motor milestones, as well as muscle weakness and hypotonia. Although these health conditions present great similarities between distinct CNM, their clinical features and genetic background are heterogeneous.⁴⁹ Over the recent years, there have been important advances in understanding the genetic bases of CNM. Unlike many myopathies, centrally positioned nuclei in CNMs are not linked to excessive degeneration/regeneration processes. Several forms of CNM have been described in humans including the X-linked recessive form, which exhibits the most severe phenotype and affects newborns due to mutations in Myotubularin (MTM1), a phosphoinositide phosphatase,¹⁰¹ the autosomal dominant form due to mutations in Dynamin-2 (DNM2), a large GTPase,¹⁰² and the autosomal recessive form due to mutations in Amphiphysin-2 (BIN1), a Bar-domain containing protein.¹⁰³ All these genes encode proteins that participate in different aspect of membrane remodeling and trafficking. Three additional genes lead to CNM phenotypes when mutated or absent: the skeletal muscle ryanodine receptor (RYR1) gene, encoding the principal sarcoplasmic reticulum calcium release channel, the muscle-specific protein kinase (SRPK3) and a phosphoinositide phosphatase (JUMPY) functionally similar to MTM1.^104-106 How those genes can affect nuclear positioning is still unclear but we could suppose that mutations affect membrane properties and can therefore affect the anchoring of nuclei at the periphery.

Conclusions

The importance of nuclear positioning in muscle fiber function is still a matter of debate. However, multiple observations and reports provide evidence for a role of nuclear positioning in muscle function. Following injury, regenerated myofibers are characterized by centrally located nuclei several weeks after damage. Although sarcomeric ultra-structure, myofiber size and contractile properties are almost completely restored, these myofibers fatigue much faster than non-regenerated muscle.^74,107,108 Nesprin-1 −/− mice that have mis-positioned nuclei have a reduced exercise capacity, without any defects on heart function.⁸¹ Finally, Ensconsin mutant fly larvae, where nuclei are mis-positioned due to a mutation in the Map7 ortholog, exhibit motility defects. Detailed analysis of muscle structure showed that nuclear number, neuromuscular junctions, t-tubules, sarcomere structures and mitochondria localization were intact, strongly suggesting a role for nuclear positioning in muscle function.⁶⁴

More than 2 decades ago it was proposed that each myofiber nucleus “influences” the region surrounding it, thus creating a virtual compartmentalization within the myofiber. This compartmentalization is achieved by the localization of mRNA and different proteins in the vicinity of the nuclei where mRNAs are transcribed.^109,110 These regions clearly observed at the NMJ and the MTJ were named nuclear domains.^88,111 Some myofiber nuclei cluster at these junctions and specifically express mRNAs encoding multiple proteins important for the function of NMJ or MTJ. Undifferentiated myotubes do form nuclear domains,^109,110 suggesting peripheral nuclei along the myofibers might also form nuclear domains. In situations where the nuclei are mis-positioned, nuclear domain coverage of the myofibers might be disrupted resulting in muscle dysfunction. The structures and muscle functions that are disrupted under these situations remain to be discovered.

Muscle cells undergo dramatic rearrangements of their structure and reorganize completely the intracellular distribution of organelles during the course of their differentiation. Therefore, they represent a powerful system to study intracellular plasticity and their effect on cell function. Mutations of nuclear envelope proteins that result in muscle diseases and recent work showing an association between mis-positioning of nuclei and muscle function reinforce the importance of understanding nuclear movement and positioning in muscle.

Disclosure of Potential Conflicts of Interest

No potential conflicts of interest were disclosed.

Acknowledgments

We thank Vanessa Ribes for comments on the manuscript, Gomes laboratory for discussions and William Roman for correcting the manuscript.

Supplemental Material

Supplemental Material may be downloaded here: publisher's website

Authors' Contributions

Article was written by BC, VG and EG. All authors have read and approved the final version of the document.

Funding

This work was supported by Muscular Dystrophy Association (MDA), INSERM Avenir program and Agence Nationale de la Recherche (ANR) grants to ERG. BC was supported initially by a Fondation pour la Recherche Médicale (FRM) fellowship. VG was supported initially by a Region Ile-de-France fellowship.