RESUMEN
The nucleus is connected to the cytoskeleton, and these connections are involved in multiple functions such as nuclear positioning, shape and stiffness, cytoskeleton organization, mechanotransduction, gene expression, chromosome positioning, DNA repair, and cell migration.
Asunto(s)
Núcleo Celular/metabolismo , Citoesqueleto/metabolismo , Animales , Membrana Nuclear/metabolismo , Proteínas Nucleares/metabolismoRESUMEN
Neutrophil recruitment to sites of infection and inflammation is an essential process in the early innate immune response. Upon activation, a subset of neutrophils rapidly assembles the multiprotein complex known as the NLRP3 inflammasome. The NLRP3 inflammasome forms at the microtubule organizing center, which promotes the formation of interleukin (IL)-1ß and IL-18, essential cytokines in the immune response. We recently showed that mice deficient in NLRP3 (NLRP3-/-) have reduced neutrophil recruitment to the peritoneum in a model of thioglycolate-induced peritonitis. Here, we tested the hypothesis that this diminished recruitment could be, in part, the result of defects in neutrophil chemotaxis. We find that NLRP3-/- neutrophils show loss of cell polarization, as well as reduced directionality and velocity of migration toward increasing concentrations of leukotriene B4 (LTB4) in a chemotaxis assay in vitro, which was confirmed through intravital microscopy of neutrophil migration toward a laser-induced burn injury of the liver. Furthermore, pharmacologically blocking NLRP3 inflammasome assembly with MCC950 in vitro reduced directionality but preserved nondirectional movement, indicating that inflammasome assembly is specifically required for polarization and directional chemotaxis, but not cell motility per se. In support of this, pharmacological breakdown of the microtubule cytoskeleton via nocodazole treatment induced cell polarization and restored nondirectional cell migration in NLRP3-deficient neutrophils in the LTB4 gradient. Therefore, NLRP3 inflammasome assembly is required for establishment of cell polarity to guide the directional chemotactic migration of neutrophils.
Asunto(s)
Quimiotaxis , Leucotrieno B4 , Proteína con Dominio Pirina 3 de la Familia NLR , Animales , Ratones , Inflamasomas , Leucotrieno B4/metabolismo , Neutrófilos , Proteína con Dominio Pirina 3 de la Familia NLR/metabolismoRESUMEN
Skeletal muscle myofibers are large and elongated cells with multiple and evenly distributed nuclei. Nuclear distribution suggests that each nucleus influences a specific compartment within the myofiber and implies a functional role for nuclear positioning. Compartmentalization of specific mRNAs and proteins has been reported at the neuromuscular and myotendinous junctions, but mRNA distribution in non-specialized regions of the myofibers remains largely unexplored. We report that the bulk of mRNAs are enriched around the nucleus of origin and that this perinuclear accumulation depends on recently transcribed mRNAs. Surprisingly, mRNAs encoding large proteins - giant mRNAs - are spread throughout the cell and do not exhibit perinuclear accumulation. Furthermore, by expressing exogenous transcripts with different sizes we found that size contributes to mRNA spreading independently of mRNA sequence. Both these mRNA distribution patterns depend on microtubules and are independent of nuclear dispersion, mRNA expression level and stability, and the characteristics of the encoded protein. Thus, we propose that mRNA distribution in non-specialized regions of skeletal muscle is size selective to ensure cellular compartmentalization and simultaneous long-range distribution of giant mRNAs.
Asunto(s)
Fibras Musculares Esqueléticas , Músculo Esquelético , Núcleo Celular/genética , ARN Mensajero/genética , TendonesRESUMEN
Skeletal muscle cells possess a unique cellular architecture designed to fulfill their contractile function. Muscle cells (also known as myofibers) result from the fusion of hundreds of myoblasts and grow into a fiber of several centimeters in length. Cellular structures gradually become organized during muscle development to raise a mature contractile cell. A hallmark of this singular cell architecture is the position of nuclei at the periphery of the myofiber, below the plasma membrane. Nuclei in myofibers are evenly distributed except in specialized regions like the neuromuscular or myotendinous junctions. Disruption of nuclear positioning results in hindered muscle contraction and occurs in a multitude of muscle disorders as well as in regenerative myofibers. We will explore in this review the step by step nuclear migrations during myogenesis for nuclei to reach their evenly distributed anchored position at the periphery.
Asunto(s)
Núcleo Celular/metabolismo , Músculo Esquelético/metabolismo , HumanosRESUMEN
The neuromuscular junction (NMJ), a cellular synapse between a motor neuron and a skeletal muscle fiber, enables the translation of chemical cues into physical activity. The development of this special structure has been subject to numerous investigations, but its complexity renders in vivo studies particularly difficult to perform. In vitro modeling of the neuromuscular junction represents a powerful tool to delineate fully the fine tuning of events that lead to subcellular specialization at the pre-synaptic and post-synaptic sites. Here, we describe a novel heterologous co-culture in vitro method using rat spinal cord explants with dorsal root ganglia and murine primary myoblasts to study neuromuscular junctions. This system allows the formation and long-term survival of highly differentiated myofibers, motor neurons, supporting glial cells and functional neuromuscular junctions with post-synaptic specialization. Therefore, fundamental aspects of NMJ formation and maintenance can be studied using the described system, which can be adapted to model multiple NMJ-associated disorders.
Asunto(s)
Unión Neuromuscular/crecimiento & desarrollo , Neurofisiología/métodos , Animales , Forma de la Célula , Técnicas de Cocultivo , Femenino , Espacio Intracelular/metabolismo , Potenciales de la Membrana , Ratones , Contracción Muscular , Fibras Musculares Esqueléticas/metabolismo , Unión Neuromuscular/metabolismo , Neuronas/citología , Ratas Sprague-Dawley , Médula Espinal/metabolismo , Sinapsis/metabolismoRESUMEN
The basic unit of skeletal muscle in all metazoans is the multinucleate myofibre, within which individual nuclei are regularly positioned. The molecular machinery responsible for myonuclear positioning is not known. Improperly positioned nuclei are a hallmark of numerous diseases of muscle, including centronuclear myopathies, but it is unclear whether correct nuclear positioning is necessary for muscle function. Here we identify the microtubule-associated protein ensconsin (Ens)/microtubule-associated protein 7 (MAP7) and kinesin heavy chain (Khc)/Kif5b as essential, evolutionarily conserved regulators of myonuclear positioning in Drosophila and cultured mammalian myotubes. We find that these proteins interact physically and that expression of the Kif5b motor domain fused to the MAP7 microtubule-binding domain rescues nuclear positioning defects in MAP7-depleted cells. This suggests that MAP7 links Kif5b to the microtubule cytoskeleton to promote nuclear positioning. Finally, we show that myonuclear positioning is physiologically important. Drosophila ens mutant larvae have decreased locomotion and incorrect myonuclear positioning, and these phenotypes are rescued by muscle-specific expression of Ens. We conclude that improper nuclear positioning contributes to muscle dysfunction in a cell-autonomous fashion.
Asunto(s)
Núcleo Celular/metabolismo , Cinesinas/metabolismo , Proteínas Asociadas a Microtúbulos/metabolismo , Músculo Esquelético/citología , Músculo Esquelético/fisiología , Animales , Compartimento Celular/genética , Línea Celular , Polaridad Celular/genética , Células Cultivadas , Drosophila melanogaster/citología , Drosophila melanogaster/genética , Drosophila melanogaster/crecimiento & desarrollo , Cinesinas/química , Cinesinas/deficiencia , Cinesinas/genética , Larva/citología , Larva/genética , Larva/metabolismo , Locomoción/genética , Locomoción/fisiología , Ratones , Proteínas Asociadas a Microtúbulos/química , Proteínas Asociadas a Microtúbulos/genética , Microtúbulos/metabolismo , Fibras Musculares Esqueléticas/citología , Fibras Musculares Esqueléticas/metabolismo , Especificidad de Órganos , Fenotipo , Unión Proteica , Estructura Terciaria de ProteínaRESUMEN
The position of the nucleus is regulated in different developmental stages and cellular events. During polarization, the nucleus moves away from the future leading edge and this movement is required for proper cell migration. Nuclear movement requires the LINC complex 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 and migration. Moreover, we demonstrate that Samp1 is a component of TAN lines that contain nesprin-2G and SUN2. Finally, Samp1 associates 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.
Asunto(s)
Núcleo Celular/fisiología , Proteínas de la Membrana/metabolismo , Membrana Nuclear/metabolismo , Proteínas Nucleares/metabolismo , Animales , Línea Celular , Movimiento Celular/fisiología , Núcleo Celular/metabolismo , Lamina Tipo A/genética , Lamina Tipo A/metabolismo , Proteínas de la Membrana/genética , Ratones , Proteínas del Tejido Nervioso/metabolismo , Proteínas Nucleares/genética , Interferencia de ARN , ARN Interferente Pequeño , Proteínas de Schizosaccharomyces pombe/metabolismo , Proteínas de Unión a Telómeros/metabolismoRESUMEN
Cells actively position their nucleus within the cytoplasm. One striking example is observed during skeletal myogenesis. Differentiated myoblasts fuse to form a multinucleated myotube with nuclei positioned in the centre of the syncytium by an unknown mechanism. Here, we describe that the nucleus of a myoblast moves rapidly after fusion towards the central myotube nuclei. This movement is driven by microtubules and dynein/dynactin complex, and requires Cdc42, Par6 and Par3. We found that Par6ß and dynactin accumulate at the nuclear envelope of differentiated myoblasts and myotubes, and this accumulation is dependent on Par6 and Par3 proteins but not on microtubules. These results suggest a mechanism where nuclear movement after fusion is driven by microtubules that emanate from one nucleus that are pulled by dynein/dynactin complex anchored to the nuclear envelope of another nucleus.
Asunto(s)
Proteínas Adaptadoras Transductoras de Señales/metabolismo , Moléculas de Adhesión Celular/metabolismo , Núcleo Celular/metabolismo , Dineínas/metabolismo , Microtúbulos/metabolismo , Fibras Musculares Esqueléticas/metabolismo , Proteína de Unión al GTP cdc42/metabolismo , Animales , Proteínas de Ciclo Celular , Fusión Celular , Línea Celular , Complejo Dinactina , Ratones , Proteínas Asociadas a Microtúbulos/metabolismo , Modelos Biológicos , Fibras Musculares Esqueléticas/citología , Mioblastos/citología , Mioblastos/metabolismo , Membrana Nuclear/metabolismo , Transporte de ProteínasRESUMEN
The nucleus is one of the hallmarks of eukaryotic cells. The history of its discovery and characterisation is intimately entangled with that of cell biology as a discipline. Here, we provide a broad historical perspective of the nucleus, from its initial descriptions until the present. We describe the key events that led to the formulation of the chromosomal theory, the discovery of the nuclear pore complex, nucleo-cytoplasmic transport and the structure of chromatin. We also focus on the rising importance of the nuclear periphery as a key subject in nuclear research, with the characterisation of the multiple roles of nuclear lamina and the proteins involved in connecting the nuclear envelope and the cytoskeleton. Over the last decades, critical technical advancements from electron microscopy to protein structural characterisation have allowed us to gain in-depth knowledge of nuclear substructure and components, from its core to the envelope. This knowledge has set the stage for a rising challenge: understanding specialised nuclear configurations and their role in different tissues, developmental stages and disease.
Asunto(s)
Biología Celular/historia , Núcleo Celular/metabolismo , Células Eucariotas/fisiología , Microscopía/historia , Transporte Activo de Núcleo Celular/fisiología , Animales , Compartimento Celular , Núcleo Celular/genética , Núcleo Celular/ultraestructura , Cromatina/metabolismo , Cromatina/ultraestructura , Cromosomas/metabolismo , Cromosomas/ultraestructura , Citoesqueleto/genética , Citoesqueleto/metabolismo , Citoesqueleto/ultraestructura , Citosol/metabolismo , Citosol/ultraestructura , Células Eucariotas/ultraestructura , Historia del Siglo XVIII , Historia del Siglo XIX , Historia del Siglo XX , Historia del Siglo XXI , Humanos , Microtúbulos/genética , Microtúbulos/metabolismo , Microtúbulos/ultraestructuraRESUMEN
The position of the nucleus in the cytoplasm is a highly regulated process and is required for multiple cellular and developmental processes. Defects on different nuclear positioning events are associated with several pathologies such as muscle and nervous system disorders. In this chapter we describe the current knowledge on the mechanism of nuclear positioning. We discuss how the nucleus connects to the cytoskeleton by nesprins and SUN proteins, how this connection is regulated by Samp1, and how this connection is required for proper nuclear positioning. Furthermore, we discuss how nesprins, SUN, and Samp1 form transmembrane actin-associated nuclear (TAN) lines, novel nuclear envelope structures involved in force transduction during nuclear movement. Finally, we describe the recent evidences suggesting a role for the connection between the nucleus and the cytoskeleton in cancer.
Asunto(s)
Movimiento Celular/fisiología , Núcleo Celular/fisiología , Citoesqueleto/fisiología , Actinas/metabolismo , Núcleo Celular/metabolismo , Citoesqueleto/metabolismo , HumanosRESUMEN
Methodologies for culturing muscle tissue are currently lacking in terms of quality and quantity of mature cells produced. We analyse images from in vitro experiments to quantify the effects of culture media composition on mouse-derived myoblast behaviour and myotube quality. Metrics of early indicators of cell quality were defined. Images of muscle cell differentiation reveal that altering culture media significantly affects quality indicators and myoblast migratory behaviours. To study the effects of early-stage cell behaviours on mature cell quality, metrics drawn from experimental images or inferred by approximate Bayesian computation (ABC) were applied as inputs to an agent-based model (ABM) of skeletal muscle cell differentiation with quality indicator metrics as outputs. Computational modelling was used to inform further in vitro experiments to predict the optimum media composition for culturing muscle cells. Our results suggest that myonuclei production in myotubes is inversely related to early-stage nuclei fusion index and that myonuclei density and spatial distribution are correlated with residence time of fusing myoblasts, the age at which myotube-myotube fusion ends and the repulsion force between myonuclei. Culture media with 5% serum was found to produce the optimum cell quality and to make muscle cells cultured in a neuron differentiation medium viable.
Asunto(s)
Fibras Musculares Esqueléticas , Mioblastos , Ratones , Animales , Teorema de Bayes , Fibras Musculares Esqueléticas/fisiología , Diferenciación Celular , Medios de Cultivo/farmacología , Músculo Esquelético/fisiología , Células CultivadasRESUMEN
In the skeletal muscle tissue, cells are organized following an anisotropic architecture, which is both required during myogenesis when muscle precursor cells fuse to generate myotubes and for its contractile function. To build an in vitro skeletal muscle tissue, it is therefore essential to develop methods to organize cells in an anisotropic fashion, which can be particularly challenging, especially in 3D. In this study, we present a versatile muscle-on-chip system with adjustable collagen hollow tubes that can be seeded with muscle precursor cells. The collagen acts both as a tube-shaped hollow mold and as an extracellular matrix scaffold that can house other cell types for co-culture. We found that the diameter of the channel affects the organization of the muscle cells and that proper myogenesis was obtained at a diameter of 75 µm. In these conditions, muscle precursor cells fused into long myotubes aligned along these collagen channels, resulting in a fascicle-like structure. These myotubes exhibited actin striations and upregulation of multiple myogenic genes, reflecting their maturation. Moreover, we showed that our chip allowed muscle tissue culture and maturation over a month, with the possibility of fibroblast co-culture embedding in the collagen matrix.
Asunto(s)
Técnicas de Cocultivo , Desarrollo de Músculos , Músculo Esquelético , Animales , Ratones , Técnicas de Cocultivo/instrumentación , Músculo Esquelético/citología , Músculo Esquelético/metabolismo , Fibras Musculares Esqueléticas/citología , Fibras Musculares Esqueléticas/metabolismo , Dispositivos Laboratorio en un Chip , Colágeno/química , Colágeno/metabolismo , Diferenciación Celular , Células CultivadasRESUMEN
Wired neurons form new presynaptic boutons in response to increased synaptic activity, however the mechanism(s) by which this occurs remains uncertain. Drosophila motor neurons (MNs) have clearly discernible boutons that display robust structural plasticity, being therefore an ideal system in which to study activity-dependent bouton genesis. Here, we show that in response to depolarization and in resting conditions, MNs form new boutons by membrane blebbing, a pressure-driven mechanism that occurs in 3-D cell migration, but to our knowledge not previously described to occur in neurons. Accordingly, F-actin is decreased in boutons during outgrowth, and non-muscle myosin-II is dynamically recruited to newly formed boutons. Furthermore, muscle contraction plays a mechanical role, which we hypothesize promotes bouton addition by increasing MN confinement. Overall, we identified a mechanism by which established circuits form new boutons allowing their structural expansion and plasticity, using trans-synaptic physical forces as the main driving force.
Asunto(s)
Proteínas de Drosophila , Drosophila , Animales , Neuronas Motoras/metabolismo , Terminales Presinápticos/fisiología , Proteínas de Drosophila/metabolismo , Contracción Muscular , SinapsisRESUMEN
The composition of fiber types within skeletal muscle impacts the tissue's physiological characteristics and susceptibility to disease and ageing. In vitro systems should therefore account for fiber-type composition when modelling muscle conditions. To induce fiber specification in vitro, we designed a quantitative contractility assay based on optogenetics and particle image velocimetry. We submitted cultured myotubes to long-term intermittent light-stimulation patterns and characterized their structural and functional adaptations. After several days of in vitro exercise, myotubes contract faster and are more resistant to fatigue. The enhanced contractile functionality was accompanied by advanced maturation such as increased width and up-regulation of neuron receptor genes. We observed an up-regulation in the expression of fast myosin heavy-chain isoforms, which induced a shift towards a fast-twitch phenotype. This long-term in vitro exercise strategy can be used to study fiber specification and refine muscle disease modelling.
Asunto(s)
Fibras Musculares de Contracción Rápida , Fibras Musculares de Contracción Lenta , Fibras Musculares de Contracción Rápida/química , Fibras Musculares de Contracción Rápida/metabolismo , Fibras Musculares de Contracción Lenta/química , Fibras Musculares de Contracción Lenta/metabolismo , Optogenética , Fibras Musculares Esqueléticas , Músculo Esquelético/metabolismoRESUMEN
DNA double-strand breaks (DSBs) trigger specialized cellular mechanisms that collectively form the DNA damage response (DDR). In proliferating cells, the DDR serves the function of mending DNA breaks and satisfying the cell-cycle checkpoints. Distinct goals exist in differentiated cells that are postmitotic and do not face cell-cycle checkpoints. Nonetheless, the distinctive requirements and mechanistic details of the DDR in differentiated cells are still poorly understood. In this study, we set an in vitro differentiation model of human skeletal muscle myoblasts into multinucleated myotubes that allowed monitoring DDR dynamics during cell differentiation. Our results demonstrate that myotubes have a prolonged DDR, which is nonetheless competent to repair DSBs and render them significantly more resistant to cell death than their progenitors. Using live-cell microscopy and single-molecule kinetic measurements of transcriptional activity, we observed that myotubes respond to DNA damage by rapidly and transiently suppressing global gene expression and rewiring the epigenetic landscape of the damaged nucleus. Our findings provide novel insights into the DDR dynamics during cellular differentiation and shed light on the strategy employed by human skeletal muscle to preserve the integrity of the genetic information and sustain long-term organ function after DNA damage.
RESUMEN
Nuclear position is central to cell polarization, and its disruption is associated with various pathologies. The nucleus is moved away from the leading edge of migrating cells through its connection to moving dorsal actin cables, and the absence of connections to immobile ventral stress fibers. It is unclear how these asymmetric nucleo-cytoskeleton connections are established. Here, using an in vitro wound assay, we find that remodeling of endoplasmic reticulum (ER) impacts nuclear positioning through the formation of a barrier that shields immobile ventral stress fibers. The remodeling of ER and perinuclear ER accumulation is mediated by the ER shaping protein Climp-63. Furthermore, ectopic recruitment of the ER to stress fibers restores nuclear positioning in the absence of Climp-63. Our findings suggest that the ER mediates asymmetric nucleo-cytoskeleton connections to position the nucleus.
Asunto(s)
Actinas , Retículo Endoplásmico , Actinas/metabolismo , Núcleo Celular/metabolismo , Citoesqueleto/metabolismo , Retículo Endoplásmico/metabolismo , Fibras de Estrés/metabolismoRESUMEN
The linker of nucleoskeleton and cytoskeleton (LINC) complex is situated in the nuclear envelope and forms a connection between the lamina and cytoskeletal elements. Sun1, Sun2 and nesprin-2 are important components of the LINC complex. We expressed these proteins fused to green fluorescent protein in embryonic fibroblasts and studied their diffusional mobilities using fluorescence recovery after photobleaching. We show that they all are more mobile in embryonic fibroblasts from mice lacking A-type lamins than in cells from wild-type mice. Knockdown of Sun2 also increased the mobility of a short, chimeric form of nesprin-2 giant (mini-nesprin-2G), whereas the lack of emerin did not affect the mobility of Sun1, Sun2 or mini-nesprin-2G. Fluorescence resonance energy transfer experiments showed Sun1 to be more closely associated with lamin A than is Sun2. Sun1 and Sun2 had similar affinity for the nesprin-2 KASH domain in plasmon surface resonance (Biacore) experiments. This affinity was ten times higher than that previously reported between nesprin-2 and actin. Deletion of the actin-binding domain had no effect on mini-nesprin-2G mobility. Our data support a model in which A-type lamins and Sun2 anchor nesprin-2 in the outer nuclear membrane, whereas emerin, Sun1 and actin are dispensable for this anchoring.
Asunto(s)
Actinas/metabolismo , Lamina Tipo A/metabolismo , Proteínas de la Membrana/metabolismo , Proteínas Asociadas a Microtúbulos/metabolismo , Complejos Multiproteicos/metabolismo , Proteínas del Tejido Nervioso/metabolismo , Proteínas Nucleares/metabolismo , Proteínas de Unión a Telómeros/metabolismo , Animales , Sitios de Unión , Células Cultivadas , Citoesqueleto/metabolismo , Recuperación de Fluorescencia tras Fotoblanqueo , Transferencia Resonante de Energía de Fluorescencia , Técnicas de Silenciamiento del Gen , Lamina Tipo A/genética , Proteínas de la Membrana/genética , Ratones , Ratones Noqueados , Proteínas del Tejido Nervioso/genética , Lámina Nuclear/metabolismo , Proteínas Nucleares/genética , Dominios y Motivos de Interacción de Proteínas/genética , Proteínas Recombinantes de Fusión/genética , Proteínas Recombinantes de Fusión/metabolismo , Resonancia por Plasmón de Superficie , Proteínas de Unión a Telómeros/genéticaRESUMEN
In both dividing and interphase cells, microtubules are remodeled in response to signal transduction pathways triggered by a variety of stimuli. Members of the Rho family of small GTPases have emerged as key intermediates in transmitting signals to cortical factors that mediate capture of dynamic microtubules at specific sites. The specificity of cortical capture appears to be controlled by microtubule tip proteins and cortical receptors that bind these proteins. Recent studies suggest that some of the proteins interacting with microtubule tips behave as bridging proteins between the microtubule tip proteins and their cortical receptors. Such bridging proteins may enhance cortical capture of microtubules directly or indirectly through interactions with the actin cytoskeleton.
Asunto(s)
Microtúbulos/metabolismo , Proteínas de Unión al GTP rho/fisiología , Animales , División Celular , Microtúbulos/química , Microtúbulos/ultraestructura , Transducción de SeñalRESUMEN
Loss of nuclear integrity correlates with increased DNA damage in different tissues. In a recent issue of Cell, Nader et al. reveal that nuclear envelope ruptures in dense tissue microenvironments cause TREX1-dependent DNA damage and promote the transition from in situ to invasive carcinomas.
Asunto(s)
Núcleo Celular , Daño del ADN , Daño del ADN/genética , Membrana Nuclear , Fosfoproteínas/genéticaRESUMEN
Regeneration of skeletal muscle is a highly synchronized process that requires muscle stem cells (satellite cells). We found that localized injuries, as experienced through exercise, activate a myofiber self-repair mechanism that is independent of satellite cells in mice and humans. Mouse muscle injury triggers a signaling cascade involving calcium, Cdc42, and phosphokinase C that attracts myonuclei to the damaged site via microtubules and dynein. These nuclear movements accelerate sarcomere repair and locally deliver messenger RNA (mRNA) for cellular reconstruction. Myofiber self-repair is a cell-autonomous protective mechanism and represents an alternative model for understanding the restoration of muscle architecture in health and disease.