Microtubular organization in elongating myogenic cells

Filamin-A-interacting protein 1 (FILIP1) is a dual compartment protein linking myofibrils and microtubules during myogenic differentiation and upon mechanical stress

Variations in the gene encoding filamin-A-interacting protein 1 (FILIP1) were identified to be associated with a combination of neurological and muscular symptoms. While FILIP1 was shown to regulate motility of brain ventricular zone cells, a process important for corticogenesis, the function of the protein in muscle cells has been less well characterized. The expression of FILIP1 in regenerating muscle fibres predicted a role in early muscle differentiation. Here we analysed expression and localization of FILIP1 and its binding partners filamin-C (FLNc) and microtubule plus-end-binding protein EB3 in differentiating cultured myotubes and adult skeletal muscle. Prior to the development of cross-striated myofibrils, FILIP1 is associated with microtubules and colocalizes with EB3. During further myofibril maturation its localization changes, and FILIP1 localizes to myofibrillar Z-discs together with the actin-binding protein FLNc. Forced contractions of myotubes by electrical pulse stimulation (EPS) induce focal disruptions in myofibrils and translocation of both proteins from Z-discs to these lesions, suggesting a role in induction and/or repair of these structures. The immediate proximity of tyrosylated, dynamic microtubules and EB3 to lesions implies that also these play a role in these processes. This implication is supported by the fact that in nocodazole-treated myotubes that lack functional microtubules, the number of lesions induced by EPS is significantly reduced. In summary, we here show that FILIP1 is a cytolinker protein that is associated with both microtubules and actin filaments, and might play a role in the assembly of myofibrils and their stabilization upon mechanical stress to protect them from damage.

Insights into Cell-Specific Functions of Microtubules in Skeletal Muscle Development and Homeostasis

The contractile cells of skeletal muscles, called myofibers, are elongated multinucleated syncytia formed and maintained by the fusion of proliferative myoblasts. Human myofibers can be hundreds of microns in diameter and millimeters in length. Myofibers are non-mitotic, obviating the need for microtubules in cell division. However, microtubules have been adapted to the unique needs of these cells and are critical for myofiber development and function. Microtubules in mature myofibers are highly dynamic, and studies in several experimental systems have demonstrated the requirements for microtubules in the unique features of muscle biology including myoblast fusion, peripheral localization of nuclei, assembly of the sarcomere, transport and signaling. Microtubule-binding proteins have also been adapted to the needs of the skeletal muscle including the expression of skeletal muscle-specific protein isoforms generated by alternative splicing. Here, we will outline the different roles microtubules play in skeletal muscle cells, describe how microtubule abnormalities can lead to muscle disease and discuss the broader implications for microtubule function.

Reconstitution of muscle cell microtubule organization in vitro

Skeletal muscle differentiation occurs as muscle precursor cells (myoblasts) elongate and fuse to form multinucleated syncytial myotubes in which the highly-organized actomyosin sarcomeres of muscle fibers assemble. Although less well characterized, the microtubule cytoskeleton also undergoes dramatic rearrangement during myogenesis. The centrosome-nucleated microtubule array found in myoblasts is lost as the nuclear membrane acquires microtubule nucleating activity and microtubules emerge from multiple sites in the cell, eventually rearranging into a grid-like pattern in myotubes. In order to characterize perinuclear microtubule organization using a biochemically tractable system, we isolated nuclei from mouse C2C12 skeletal muscle cells during the course of differentiation and incubated them in cytoplasmic extracts prepared from eggs of the frog Xenopus laevis. Whereas centrosomes associated with myoblast nuclei gave rise to radial microtubule arrays in extracts, myotube nuclei produced a sun-like pattern with microtubules transiently nucleating from the entire nuclear envelope. Perinuclear microtubule growth was suppressed by inhibition of Aurora A kinase or by degradation of RNA, treatments that also inhibited microtubule growth from sperm centrosomes. Myotube nuclei displayed microtubule motor-based movements leading to their separation, as occurs in myotubes. This in vitro assay therefore recapitulates key features of microtubule organization and nuclear movement observed during muscle cell differentiation. This article is protected by copyright. All rights reserved.

Nuclear Mechanotransduction in Skeletal Muscle

Skeletal muscle is composed of multinucleated, mature muscle cells (myofibers) responsible for contraction, and a resident pool of mononucleated muscle cell precursors (MCPs), that are maintained in a quiescent state in homeostatic conditions. Skeletal muscle is remarkable in its ability to adapt to mechanical constraints, a property referred as muscle plasticity and mediated by both MCPs and myofibers. An emerging body of literature supports the notion that muscle plasticity is critically dependent upon nuclear mechanotransduction, which is transduction of exterior physical forces into the nucleus to generate a biological response. Mechanical loading induces nuclear deformation, changes in the nuclear lamina organization, chromatin condensation state, and cell signaling, which ultimately impacts myogenic cell fate decisions. This review summarizes contemporary insights into the mechanisms underlying nuclear force transmission in MCPs and myofibers. We discuss how the cytoskeleton and nuclear reorganizations during myogenic differentiation may affect force transmission and nuclear mechanotransduction. We also discuss how to apply these findings in the context of muscular disorders. Finally, we highlight current gaps in knowledge and opportunities for further research in the field.

Microtubules provide guidance cues for myofibril and sarcomere assembly and growth

BACKGROUND

Muscle myofibrils and sarcomeres present exceptional examples of highly ordered cytoskeletal filament arrays, whose distinct spatial organization is an essential aspect of muscle cell functionality. We utilized ultra-structural analysis to investigate the assembly of myofibrils and sarcomeres within developing myotubes of the indirect flight musculature of Drosophila.

RESULTS

A temporal sequence composed of three major processes was identified: subdivision of the unorganized cytoplasm of nascent, multi-nucleated myotubes into distinct organelle-rich and filament-rich domains; initial organization of the filament-rich domains into myofibrils harboring nascent sarcomeric units; and finally, maturation of the highly-ordered pattern of sarcomeric thick (myosin-based) and thin (microfilament-based) filament arrays in parallel to myofibril radial growth. Significantly, organized microtubule arrays were present throughout these stages and exhibited dynamic changes in their spatial patterns consistent with instructive roles. Genetic manipulations confirm these notions, and imply specific and critical guidance activities of the microtubule-based cytoskeleton, as well as structural interdependence between the myosin- and actin-based filament arrays.

CONCLUSIONS

Our observations highlight a surprisingly significant, behind-the-scenes role for microtubules in establishment of myofibril and sarcomere spatial patterns and size, and provide a detailed account of the interplay between major cytoskeletal elements in generating these essential contractile myogenic units.

Cachexia does not induce loss of myonuclei or muscle fibres during xenografted prostate cancer in mice

AIM

Cachexia is a severe wasting disorder involving loss of body- and muscle mass reducing survival and quality of life in cancer patients. We aim at determining if cachexia is a mere perturbation of the protein balance or if the condition also involves a degenerative loss of myonuclei within the fibre syncytia or loss of whole muscle fibres.

METHODS

We induced cachexia by xenografting PC3 prostate cancer cells in nu/nu mice. Six weeks later, we counted myonuclei by in vivo microscopic imaging of single live fibres in the extensor digitorum longus muscle (EDL), and the EDL, soleus and tibialis anterior muscles were also harvested for ex vivo histology.

RESULTS

The mice lost on average 15% of the whole-body wt. The muscle wet weight of the glycolytic, fast EDL was reduced by 14%, the tibialis anterior by 17%, and the slow, oxidative soleus by 6%. The fibre cross-sectional area in the EDL was reduced by 21% with no loss of myonuclei or any significant reduction in the number of muscle fibres. TUNEL-positive nuclei or fibres with embryonic myosin were rare both in cachectic and control muscles, and haematoxylin-eosin staining revealed no clear signs of muscle pathology.

CONCLUSION

The data suggest that the cachexia induced by xenografted prostate tumours induces a pronounced atrophy not accompanied by a loss of myonuclei or a loss of muscle fibres. Thus, stem cell related treatment might be redundant, and the quest for treatment options should rather focus on intervening with intracellular pathways regulating muscle fibre size.

The thickness of poly-phenoxyethyl methacrylate brush interferes with cellular behavior and function of myofibers

Introducing or grafting molecules onto biomaterial surfaces to regulate muscle cell destination via biophysical cues is one of the important steps for biomaterial design in muscle tissue engineering. Therefore, it is important to understand the interaction between myoblasts and myofibers with substrates modified by biomimetic layer with different thicknesses. In this study, we used a surface-induced atom transfer radical polymerization method to synthetize and graft poly-phenoxyethyl methacrylate (PHEMA) brushes having different lengths on the glass substrates. C2C12 myoblasts were seeded on the PHEMA brushes and differentiated using horse serum, for analyzing the sensibility of muscle cells to feel environment changing, and further investigating whether the depths of grafting layer on the biomaterial surface are important factors in regulating muscle cell behaviors. Our results demonstrated that on the thicker PHEMA brushes surface (200 and 450 nm), C2C12 myoblasts showed a better survival and proliferation and were favorable for cell fusion and myotube formation. Furthermore, myofibers survived on the thicker brushes were more functional and upregulated cytoskeleton proteins (tubulin, vimentin, and vinculin) and FAK levels, and enhanced the expression levels for mechanical stress molecules (HGF, NOS-1, and c-Met). These results suggest that grafting thickness of PHEMA layer on the substrate led to the myoblasts/myofiber behavior change, which would be valuable for the design and preparation of the modified layer on muscle tissue engineering scaffolds. © 2019 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 107A: 1264-1272, 2019.

Small molecule screen in embryonic zebrafish using modular variations to target segmentation

Small molecule in vivo phenotypic screening is used to identify drugs or biological activities by directly assessing effects in intact organisms. However, current screening designs may not exploit the full potential of chemical libraries due to false negatives. Here, we demonstrate a modular small molecule screen in embryonic zebrafish that varies concentration, genotype and timing to target segmentation disorders, birth defects that affect the spinal column. By testing each small molecule in multiple interrelated ways, this screen recovers compounds that a standard screening design would have missed, increasing the hit frequency from the chemical library three-fold. We identify molecular pathways and segmentation phenotypes, which we share in an open-access annotated database. These hits provide insight into human vertebral segmentation disorders and myopathies. This modular screening strategy is applicable to other developmental questions and disease models, highlighting the power of relatively small chemical libraries to accelerate gene discovery and disease study.

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

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, 2, 3, 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, 6, 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, 9, 10, 11, 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.

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BioID of Nesprin-1α identifies centrosomal proteins at myotube nuclear envelope

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Nesprin-1α-containing LINC complexes recruit Akap450 to myotube nuclear envelope

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Akap450 is required for microtubule nucleation at the nuclear envelope

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Microtubule nucleation at the nuclear envelope is involved in nuclear positioning

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

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.

A mechanism for the elimination of the female gamete centrosome in Drosophila melanogaster

An important feature of fertilization is the asymmetric inheritance of centrioles. In most species it is the sperm that contributes the initial centriole, which builds the first centrosome that is essential for early development. However, given that centrioles are thought to be exceptionally stable structures, the mechanism behind centriole disappearance in the female germ line remains elusive and paradoxical. We elucidated a program for centriole maintenance in fruit flies, led by Polo kinase and the pericentriolar matrix (PCM): The PCM is down-regulated in the female germ line during oogenesis, which results in centriole loss. Perturbing this program prevents centriole loss, leading to abnormal meiotic and mitotic divisions, and thus to female sterility. This mechanism challenges the view that centrioles are intrinsically stable structures and reveals general functions for Polo kinase and the PCM in centriole maintenance. We propose that regulation of this maintenance program is essential for successful sexual reproduction and defines centriole life span in different tissues in homeostasis and disease, thereby shaping the cytoskeleton.

NOCA-1 functions with γ-tubulin and in parallel to Patronin to assemble non-centrosomal microtubule arrays in C. elegans

Non-centrosomal microtubule arrays assemble in differentiated tissues to perform mechanical and transport-based functions. In this study, we identify Caenorhabditis elegans NOCA-1 as a protein with homology to vertebrate ninein. NOCA-1 contributes to the assembly of non-centrosomal microtubule arrays in multiple tissues. In the larval epidermis, NOCA-1 functions redundantly with the minus end protection factor Patronin/PTRN-1 to assemble a circumferential microtubule array essential for worm growth and morphogenesis. Controlled degradation of a γ-tubulin complex subunit in this tissue revealed that γ-tubulin acts with NOCA-1 in parallel to Patronin/PTRN-1. In the germline, NOCA-1 and γ-tubulin co-localize at the cell surface, and inhibiting either leads to a microtubule assembly defect. γ-tubulin targets independently of NOCA-1, but NOCA-1 targeting requires γ-tubulin when a non-essential putatively palmitoylated cysteine is mutated. These results show that NOCA-1 acts with γ-tubulin to assemble non-centrosomal arrays in multiple tissues and highlight functional overlap between the ninein and Patronin protein families.

DOI:http://dx.doi.org/10.7554/eLife.08649.001

Microtubules are hollow, rigid filaments that are found in the cells of animals and other eukaryotes. These filaments are built from smaller building blocks called tubulin heterodimers; and in dividing animal cells, they mainly emerge from structures called centrosomes. When a cell is dividing, arrays of microtubules that originate from centrosomes help assemble the spindle-like structure that segregates the chromosomes.

Many non-dividing or specialized cells—including neurons, skin cells and muscle fibers—assemble other arrays of microtubules that do not emerge from centrosomes, but nevertheless perform a variety of structural, mechanical and transport-based roles. Compared to the centrosomal arrays, much less is known about how these non-centrosomal microtubules are assembled.

A vertebrate protein called ‘ninein’ had previously been shown to be involved in anchoring microtubules at centrosomes. Ninein can change its localization from centrosomes to the cell surface in mammalian skin cells, suggesting that it might also have a role in assembling the peripheral microtubule arrays that are found in these cells. Now, Wang et al. have identified a protein from worms called NOCA-1, which contains a region similar to the part of ninein that was previously shown to be needed to anchor microtubules at centrosomes.

The experiments show that NOCA-1 guides the assembly of non-centrosomal microtubule arrays in multiple tissues in C. elegans worms. This includes in the outer layer of the worm's larvae, which is similar to mammalian skin. The results also highlight that NOCA-1 performs many of the same roles as a member of the Patronin family of proteins called PTRN-1, which interacts with the ‘minus’ end of a microtubule to prevent the microtubule from breaking apart.

Wang et al. also found that NOCA-1 works with another protein called γ-tubulin, which helps new microtubules to form and also interacts with microtubule minus ends. In contrast, PTRN-1 works independently of γ-tubulin. This suggests that NOCA-1 works together with γ-tubulin to protect new microtubule ends or promote their assembly, a role similar to what has been proposed for Patronin family proteins. Overall, Wang et al.'s results highlight the importance of ninein-related proteins in the assembly of non-centrosomal microtubule arrays and suggest overlapping roles for the ninein and Patronin families of proteins.

DOI:http://dx.doi.org/10.7554/eLife.08649.002

A novel isoform of MAP4 organises the paraxial microtubule array required for muscle cell differentiation

The microtubule cytoskeleton is critical for muscle cell differentiation and undergoes reorganisation into an array of paraxial microtubules, which serves as template for contractile sarcomere formation. In this study, we identify a previously uncharacterised isoform of microtubule-associated protein MAP4, oMAP4, as a microtubule organising factor that is crucial for myogenesis. We show that oMAP4 is expressed upon muscle cell differentiation and is the only MAP4 isoform essential for normal progression of the myogenic differentiation programme. Depletion of oMAP4 impairs cell elongation and cell–cell fusion. Most notably, oMAP4 is required for paraxial microtubule organisation in muscle cells and prevents dynein- and kinesin-driven microtubule–microtubule sliding. Purified oMAP4 aligns dynamic microtubules into antiparallel bundles that withstand motor forces in vitro. We propose a model in which the cooperation of dynein-mediated microtubule transport and oMAP4-mediated zippering of microtubules drives formation of a paraxial microtubule array that provides critical support for the polarisation and elongation of myotubes.

DOI:http://dx.doi.org/10.7554/eLife.05697.001

Skeletal muscles—which enable animals to move—are made up of large elongated muscle cells that span the entire length of the muscle. These cells contain stacks of structures called sarcomeres that enable the cells to contract and generate the force required for movement.

Cells called myoblasts elongate and fuse together at their tips to make the muscle cells. Within the myoblasts, long filaments called microtubules are arranged in an overlapping linear pattern. The filaments act as a template that helps the sarcomeres to align as the muscle cells form. A family of microtubule-associated proteins (or ‘MAPs’ for short) bind to microtubules and assist in organising the filaments, but it is not clear how they work.

Mogessie et al. used microscopy to observe the formation of the microtubule filaments in living myoblasts. The experiments show that the filaments progressively become more ordered as the myoblasts develop into muscle cells. Mogessie et al. identified a new member of the MAP family that is produced in myoblasts as soon as they start to form muscle fibres, and named it oMAP4. The microtubules in cells that make smaller amounts of this protein were more disorganised, and these cells were unable to fuse with each other to form muscle cells.

The experiments also found that oMAP4 can create links between different microtubules and act as a brake to prevent the filaments being moved excessively by motor proteins. Therefore, Mogessie et al. suggest that oMAP4 contributes to the formation of a strong and stable arrangement of filaments. This, in turn, allows the muscle cells to become very long.

Making more oMAP4 alone is not sufficient to form the elongated muscle cells. Therefore, the next challenge is to understand how other processes—such as the selective stabilisation of some microtubules and the movement of cell materials along the microtubules—cooperate to control muscle fibre formation.

DOI:http://dx.doi.org/10.7554/eLife.05697.002

A new directionality tool for assessing microtubule pattern alterations

The cytoskeleton (microtubules, actin and intermediate filaments) has a cell type-specific spatial organization that is essential and reflects cell health. We are interested in understanding how changes in the organization of microtubules contribute to muscle diseases such as Duchenne muscular dystrophy (DMD). The grid-like immunofluorescence microtubule pattern of fast-twitch muscle fibers lends itself well to visual assessment. The more complicated pattern of other fibers does not. Furthermore, visual assessment is not quantitative. Therefore we have developed a robust software program for detecting and quantitating microtubule directionality. Such a tool was necessary because existing methods focus mainly on local image features and are not well suited for microtubules. Our tool, texture detection technique (TeDT), is based on the Haralick texture method and takes into account both local and global features with more weight on the latter. The results are expressed in a graphic form responsive to subtle variations in microtubule distribution, while a numerical score allows quantitation of directionality. Furthermore, the results are not affected by imaging conditions or post-imaging procedures. TeDT successfully assesses test images and microtubules in fast-twitch fibers of wild-type and mdx mice (a model for DMD); TeDT also identifies and quantitates microtubule directionality in slow-twitch fibers, in the fibers of young animals, and in other mouse models which could not be assessed visually. TeDT might also contribute to directionality assessments of other cytoskeletal components.

Mesoporous silica nanoparticle-based substrates for cell directed delivery of Notch signalling modulators to control myoblast differentiation

Biochemical cues are critical to control stem cell function and can be utilized to develop smart biomaterials for stem cell engineering. The challenge is to deliver these cues in a restricted manner with spatial and temporal control. Here we have developed bilayer films of mesoporous silica nanoparticles for delayed cellular delivery of Notch modulators to promote muscle stem cell differentiation. We demonstrate that drug-loaded particles are internalized from the particle-covered surface, which allows for direct delivery of the drug into the cell and a delayed and confined drug release. Substrates of particles loaded with γ-secretase-inhibitors, which block the Notch signalling pathway, promoted efficient differentiation of myoblasts. The particle substrates were fully biocompatible and did not interfere with the inherent differentiation process. We further demonstrate that impregnating commercially available, biocompatible polymer scaffolds with MSNs allows for a free standing substrate for cell directed drug delivery.

Nuclear movement during myotube formation is microtubule and dynein dependent and is regulated by Cdc42, Par6 and Par3

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 Par6β 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.

Opposing microtubule motors drive robust nuclear dynamics in developing muscle cells

Dynamic interactions with the cytoskeleton drive the movement and positioning of nuclei in many cell types. During muscle cell development, myoblasts fuse to form syncytial myofibers with nuclei positioned regularly along the length of the cell. Nuclear translocation in developing myotubes requires microtubules, but the mechanisms involved have not been elucidated. We find that as nuclei actively translocate through the cell, they rotate in three dimensions. The nuclear envelope, nucleoli and chromocenters within the nucleus rotate together as a unit. Both translocation and rotation require an intact microtubule cytoskeleton, which forms a dynamic bipolar network around nuclei. The plus- and minus-end-directed microtubule motor proteins, kinesin-1 and dynein, localize to the nuclear envelope in myotubes. Kinesin-1 localization is mediated at least in part by interaction with klarsicht/ANC-1/Syne homology (KASH) proteins. Depletion of kinesin-1 abolishes nuclear rotation and significantly inhibits nuclear translocation, resulting in the abnormal aggregation of nuclei at the midline of the myotube. Dynein depletion also inhibits nuclear dynamics, but to a lesser extent, leading to altered spacing between adjacent nuclei. Thus, oppositely directed motors acting from the surface of the nucleus drive nuclear motility in myotubes. The variable dynamics observed for individual nuclei within a single myotube are likely to result from the stochastic activity of competing motors interacting with a complex bipolar microtubule cytoskeleton that is also continuously remodeled as the nuclei move. The three-dimensional rotation of myotube nuclei may facilitate their motility through the complex and crowded cellular environment of the developing muscle cell, allowing for proper myonuclear positioning.

LKB1 destabilizes microtubules in myoblasts and contributes to myoblast differentiation

BACKGROUND

Skeletal muscle myoblast differentiation and fusion into multinucleate myotubes is associated with dramatic cytoskeletal changes. We find that microtubules in differentiated myotubes are highly stabilized, but premature microtubule stabilization blocks differentiation. Factors responsible for microtubule destabilization in myoblasts have not been identified.

FINDINGS

We find that a transient decrease in microtubule stabilization early during myoblast differentiation precedes the ultimate microtubule stabilization seen in differentiated myotubes. We report a role for the serine-threonine kinase LKB1 in both microtubule destabilization and myoblast differentiation. LKB1 overexpression reduced microtubule elongation in a Nocodazole washout assay, and LKB1 RNAi increased it, showing LKB1 destabilizes microtubule assembly in myoblasts. LKB1 levels and activity increased during myoblast differentiation, along with activation of the known LKB1 substrates AMP-activated protein kinase (AMPK) and microtubule affinity regulating kinases (MARKs). LKB1 overexpression accelerated differentiation, whereas RNAi impaired it.

CONCLUSIONS

Reduced microtubule stability precedes myoblast differentiation and the associated ultimate microtubule stabilization seen in myotubes. LKB1 plays a positive role in microtubule destabilization in myoblasts and in myoblast differentiation. This work suggests a model by which LKB1-induced microtubule destabilization facilitates the cytoskeletal changes required for differentiation. Transient destabilization of microtubules might be a useful strategy for enhancing and/or synchronizing myoblast differentiation.

Nuclei of non-muscle cells bind centrosome proteins upon fusion with differentiating myoblasts

BACKGROUND

In differentiating myoblasts, the microtubule network is reorganized from a centrosome-bound, radial array into parallel fibres, aligned along the long axis of the cell. Concomitantly, proteins of the centrosome relocalize from the pericentriolar material to the outer surface of the nucleus. The mechanisms that govern this relocalization are largely unknown.

METHODOLOGY

In this study, we perform experiments in vitro and in cell culture indicating that microtubule nucleation at the centrosome is reduced during myoblast differentiation, while nucleation at the nuclear surface increases. We show in heterologous cell fusion experiments, between cultures of differentiating mouse myoblasts and human cells of non-muscular origin, that nuclei from non-muscle cells recruit centrosome proteins once fused with the differentiating myoblasts. This recruitment still occurs in the presence of cycloheximide and thus appears to be independent of new protein biosynthesis.

CONCLUSIONS

Altogether, our data suggest that nuclei of undifferentiated cells have the dormant potential to bind centrosome proteins, and that this potential becomes activated during myoblast differentiation.

Centrosome proteins form an insoluble perinuclear matrix during muscle cell differentiation

BACKGROUND

Muscle fibres are formed by elongation and fusion of myoblasts into myotubes. During this differentiation process, the cytoskeleton is reorganized, and proteins of the centrosome re-localize to the surface of the nucleus. The exact timing of this event, and the underlying molecular mechanisms are still poorly understood.

RESULTS

We performed studies on mouse myoblast cell lines that were induced to differentiate in culture, to characterize the early events of centrosome protein re-localization. We demonstrate that this re-localization occurs already at the single cell stage, prior to fusion into myotubes. Centrosome proteins that accumulate at the nuclear surface form an insoluble matrix that can be reversibly disassembled if isolated nuclei are exposed to mitotic cytoplasm from Xenopus egg extract. Our microscopy data suggest that this perinuclear matrix of centrosome proteins consists of a system of interconnected fibrils.

CONCLUSION

Our data provide new insights into the reorganization of centrosome proteins during muscular differentiation, at the structural and biochemical level. Because we observe that centrosome protein re-localization occurs early during differentiation, we believe that it is of functional importance for the reorganization of the cytoskeleton in the differentiation process.

Myoseverin disrupts sarcomeric organization in myocytes: an effect independent of microtubule assembly inhibition

Although disruption of the microtubule (MT) array inhibits myogenesis in myocytes, the relationship between the assembly of microtubules (MT) and the organization of the contractile filaments is not clearly defined. We now report that the assembly of mature myofibrils in hypertrophic cardiac myocytes is disrupted by myoseverin, a compound previously shown to perturb the MT array in skeletal muscle cells. Myoseverin treated cardiac myocytes showed disruptions of the striated Z-bands containing alpha-actinin and desmin and the localization of tropomyosin, titin and myosin on mature sarcomeric filaments. In contrast, MT depolymerization by nocodazole did not perturb sarcomeric filaments. Similarly, expression of constitutively active stathmin as a non-chemical molecular method of MT depolymerization did not prevent sarcomere assembly. The extent of MT destabilization by myoseverin and nocodazole were comparable. Thus, the effect of myoseverin on sarcomere assembly was independent of its capacity for MT inhibition. Furthermore, we found that upon removal of myoseverin, sarcomeres reformed in the absence of an intact MT network. Sarcomere formation in cardiac myocytes therefore, does not appear to require an intact MT network and thus we conclude that a functional MT array appears to be dispensable for myofibrillogenesis.

EB3 regulates microtubule dynamics at the cell cortex and is required for myoblast elongation and fusion

During muscle differentiation, myoblasts elongate and fuse into syncytial myotubes [1]. An early event during this process is the remodeling of the microtubule cytoskeleton, involving disassembly of the centrosome and, crucially, the alignment of microtubules into a parallel array along the long axis of the cell [2–5]. To further our understanding on how microtubules support myogenic differentiation, we analyzed the role of EB1-related microtubule-plus-end-binding proteins. We demonstrate that EB3 [6] is specifically upregulated upon myogenic differentiation and that knockdown of EB3, but not that of EB1, prevents myoblast elongation and fusion into myotubes. EB3-depleted cells show disorganized microtubules and fail to stabilize polarized membrane protrusions. Using live-cell imaging, we show that EB3 is necessary for the regulation of microtubule dynamics and microtubule capture at the cell cortex. Expression of EB1/EB3 chimeras on an EB3-depletion background revealed that myoblast fusion depends on two specific amino acids in the calponin-like domain of EB3, whereas the interaction sites with Clip-170 and CLASPs are dispensable. Our results suggest that EB3-mediated microtubule regulation at the cell cortex is a crucial step during myogenic differentiation and might be a general mechanism in polarized cell elongation.

How to build a myofibril

Building a myofibril from its component proteins requires the interactions of many different proteins in a process whose details are not understood. Several models have been proposed to provide a framework for understanding the increasing data on new myofibrillar proteins and their localizations during muscle development. In this article we discuss four current models that seek to explain how the assembly occurs in vertebrate cross-striated muscles. The models hypothesize: (a) stress fiber-like structures as templates for the assembly of myofibrils, (b) assembly in which the actin filaments and Z-bands form subunits independently from A-band subunits, with the two subsequently joined together to form a myofibril, (c) premyofibrils as precursors of myofibrils, or (d) assembly occurring without any intermediary structures. The premyofibril model, proposed by the authors, is discussed in more detail as it could explain myofibrillogenesis under a variety of different conditions: in ovo, in explants, and in tissue culture studies on cardiac and skeletal muscles.

Myofibrillogenesis in skeletal muscle cells in the presence of taxol

We address the controversy of whether mature myofibrils can form in the presence of taxol, a microtubule-stabilizing compound. Previous electron microscopic studies reported the absence of actin filaments and Z-bands in taxol-treated myocytes [Antin et al., 1981: J Cell Biol 90:300-308; Toyoma et al., 1982: Proc Natl Acad Sci USA 79:6556-6560]. Quail skeletal myoblasts were isolated from 10-day-old embryos and grown in the presence or absence of taxol. Taxol inhibited the formation of multinucleated elongated myotubes. Myocytes cultured in the continual presence of taxol progressed from rounded to stellate shapes. Groups of myocytes that were clustered together after the isolation procedure fused in the presence of taxol but did not form elongated myotubes. Actin filaments and actin-binding proteins were detected with several different fluorescent probes in all myofibrils that formed in the presence of taxol. The Z-bands contained both alpha-actinin and titin, and the typical arrays of A-Bands were always associated with actin filaments in the myofibrils. Myofibril formation was followed by fixing cells each day in culture and staining with probes for actin, muscle-specific alpha-actinin, myosin II, nebulin, troponin, tropomyosin, and non-muscle myosin II. Small linear aggregates of alpha-actinin or Z-bodies, premyofibrils, were detected at the edges of the myocytes and in the arms of the taxol-treated cells and were always associated with actin filaments. Non-muscle myosin II was detected at the edges of the taxol-treated cells. Removal of the taxol drug led to the cells assuming a normal compact elongated shape. During the recovery process, additional myofibrils formed at the spreading edges of these elongated and thicker myotubes. Staining of these taxol-recovering cells with specific fluorescent reagents reveals three different classes of actin fibers. These results are consistent with a model of myofibrillogenesis that involves the transition of premyofibrils to mature myofibrils.

Changes in cell shape, cytoskeletal proteins and adhesion sites of cultured cells after extracellular Ca2+ chelation

Although much is known about the molecules involved in extracellular Ca2+ regulation, the relationship of the ion with overall cell morphology is not understood. The objective of the present study was to determine the effect of the Ca2+ chelator EGTA on the major cytoskeleton components, at integrin-containing adhesion sites, and their consequences on cell shape. Control mouse cell line C2C12 has a well-spread morphology with long stress fibers running in many different directions, as detected by fluorescence microscopy using rhodamine-phalloidin. In contrast, cells treated with EGTA (1.75 mM in culture medium) for 24 h became bipolar and showed less stress fibers running in one major direction. The adhesion plaque protein alpha 5-integrin was detected by immunofluorescence microscopy at fibrillar adhesion sites in both control and treated cells, whereas a dense labeling was seen only inside treated cells. Microtubules shifted from a radial arrangement in control cells to a longitudinal distribution in EGTA-treated cells, as analyzed by immunofluorescence microscopy. Desmin intermediate filaments were detected by immunofluorescence microscopy in a fragmented network dispersed within the entire cytoplasm in EGTA-treated cells, whereas a dense network was seen in the whole cytoplasm of control cells. The present results suggest that the role of extracellular Ca2+ in the regulation of C2C12 cell shape can be mediated by actin-containing stress fibers and microtubules and by intermediate filament reorganization, which may involve integrin adhesion sites.

The architecture of microtubular network and Golgi orientation in osteoclasts--major differences between avian and mammalian species

In the present study, we analyze multinuclear osteoclasts obtained from several avian and mammalian species and describe the reorganization of their microtubular architecture and Golgi complex orientation during osteoclast differentiation and activation for bone resorption. In nonresorbing quail and chicken multinuclear osteoclasts, microtubules radiate from multiple centrosomal microtubule-organizing centers (MTOCs), whose number is equal to the number of nuclei. However, centrosomal MTOCs disappear at the time of cell activation for bone resorption and the Golgi membranes redistribute to circumscribe nuclei. In contrast to avian osteoclasts, both resorbing and nonresorbing rat, rabbit, and human osteoclasts have no or few centrosomal MTOCs. Instead, after cold-induced depolymerization, regrowing microtubules nucleate from the perinuclear area where immunofluoresce and immunoelectron scanning microscopy reveal pericentriolar matrix protein pericentrin associated with vimentin filaments. Furthermore, the circumnuclear reorganization of MTOCs and the Golgi is a result of mammalian osteoclast maturation and occur before any resorptive activity of the mononuclear osteoclasts and their fusion into multinucleated cells. Our results show that unlike previously suggested, the nuclear surfaces of mammalian osteoclasts act as the microtubule anchoring sites similarly to nuclear surfaces in multinucleated myotubes and suggest the role of perinuclear intermediate filament network in orchestrating the microtubular cytoskeleton.

Muscle-specific microtubule-associated protein 4 is expressed early in myogenesis and is not sufficient to induce microtubule reorganization

The expression of a muscle-specific variant of microtubule-associated protein 4 (mMAP4) has been analyzed during myogenesis of C(2)C(12) cells using an isoform-specific antibody. MMAP4 localizes to microtubules (MTs) and is expressed prior to a very early morphogenetic event, the formation of mononucleate spindle-shaped cells. MMAP4 protein appears at about the same time as titin and coincident with Golgi reorganization, but antedates myosin expression. Misexpression of EGFP-mMAP4 in non-muscle and proliferating C(2)C(12) cells does not induce dramatic changes in MT organization or stability, nor in Golgi organization. Expression of full-length mMAP4 or of a truncated form lacking the MT-binding domain does not disrupt myotube formation or myofibrillogenesis. While previous antisense studies indicated that mMAP4 is necessary for normal myotube formation [Mangan and Olmsted, 1996: Development 122:771-781], these data indicate mMAP4 is not sufficient to induce the reorganization of MTs or the Golgi into patterns typical of muscle cells. Thus, with respect to MT organizing properties, this tissue-specific variant differs from related neuronal MAPs, MAP2, and tau, which induce neural-like changes in MT organization.

Transient association of titin and myosin with microtubules in nascent myofibrils directed by the MURF2 RING-finger protein

Assembly of muscle sarcomeres is a complex dynamic process and involves a large number of proteins. A growing number of these have regulatory functions and are transiently present in the myofibril. We show here that the novel tubulin-associated RING/B-box protein MURF2 associates transiently with microtubules, myosin and titin during sarcomere assembly. During sarcomere assembly, MURF2 first associates with microtubules at the exclusion of tyrosinated tubulin. Then, MURF2-labelled microtubules associate transiently with sarcomeric myosin and later with A-band titin when non-striated myofibrils differentiate into mature sarcomeres. Finally, MURF2 labelled microtubules disappear from the sarcomere after the incorporation of myosin filaments and the elongation of titin. This suggests that the incorporation of myosin into nascent sarcomeres and the elongation of titin require an active, microtubule-dependent transport process and that MURF2-associated microtubules play a role in the alignment and extension of nascent sarcomeres. MURF2 is expressed in at least four isoforms, of which a 27 kDa isoform is cardiac specific. A C-terminal isoform is generated by alternative reading frame use, a novelty in muscle proteins. In mature cardiac sarcomeres, endogenous MURF2 can associate with the M-band, and is translocated to the nucleus. MURF2 can therefore act as a transient adaptor between microtubules, titin and nascent myosin filaments, as well as being involved in signalling from the sarcomere to the nucleus.

Tau expression in denervated rat muscles

We studied whether denervation affects the expression of tau, in particular phosphorylated tau, and how it is degraded in rat soleus muscles. Immunoblot analysis showed a high molecular weight, approximately 110 kDa (big tau), in normal muscle. Tau levels increased significantly in denervated muscles treated with chloroquine (a lysosomotrophic agent) and in untreated ones, as compared to levels of similarly treated contralateral, innervated muscles. Most of the tau in the innervated and denervated muscles was phosphorylated. Immunohistochemically, tau and beta-tubulin colocated in the sarcoplasm of innervated, saline-treated (intact) muscle, but the staining intensities were very weak. Both proteins, however, were expressed extensively in these areas in the denervated muscles from saline-treated rats. In the denervated muscle of chloroquine-treated rats there were numerous autophagic vacuoles in the sarcoplasm, and phosphorylated-tau accumulation was marked within these vacuoles, indicative that tau first was taken into autophagic, vacuoles by nonselective autophagy then degraded via the lysosomal as well as the nonlysosomal calpain system. Our findings suggest that phosphorylated big tau accumulates with beta-tubulin in denervated muscular atrophy, possibly in order to maintain or preserve the integrity of the muscle fiber during progressive atrophy or regeneration.

The M-cadherin catenin complex interacts with microtubules in skeletal muscle cells: implications for the fusion of myoblasts

M-cadherin, a calcium-dependent intercellular adhesion molecule, is expressed in skeletal muscle cells. Its pattern of expression, both in vivo and in cell culture as well as functional studies, have implied that M-cadherin is important for skeletal muscle development, in particular the fusion of myoblasts into myotubes. M-cadherin formed complexes with the catenins in skeletal muscle cells similar to E-cadherin in epithelial cells. This suggested that the muscle-specific function of the M-cadherin catenin complex might be mediated by additional interactions with yet unidentified cellular components, especially cytoskeletal elements. These include the microtubules which also have been implicated in the fusion process of myoblasts. Here we present evidence that the M-cadherin catenin complex interacts with microtubules in myogenic cells by using three independent experimental approaches. (1) Analysis by laser scan microscopy revealed that the destruction of microtubules by nocodazole leads to an altered cell surface distribution of M-cadherin in differentiating myogenic cells. In contrast, disruption of actin filaments had little effect on the surface distribution of M-cadherin. (2) M-cadherin antibodies coimmunoprecipitated tubulin from extracts of nocodazole-treated myogenic cells but not of nocodazole-treated epithelial cells ectopically expressing M-cadherin. Vice versa, tubulin antibodies coimmunoprecipitated M-cadherin from extracts of nocodazole-treated myogenic cells but not of nocodazole-treated M-cadherin-expressing epithelial cells. (3) M-cadherin and the catenins, but not a panel of control proteins, were copolymerized with tubulin from myogenic cell extracts even after repeated cycles of assembly and disassemly of tubulin. Moreover, neither M-cadherin nor E-cadherin could be found in a complex with microtubules in epithelial cells ectopically expressing M-cadherin. Our data are consistent with the idea that the interaction of M-cadherin with microtubules might be essential to keep the myoblasts aligned during fusion, a process in which both M-cadherin and microtubules have been implicated.

Identification of kinesin-like molecules in myogenic cells

Numerous organelles are repositioned during myogenic differentiation and are maintained in an asymmetric distribution throughout the life span of a myotube. It is likely that members of the kinesin superfamily may be responsible for some or all of these microtubule-dependent movements. Consequently, we have attempted to identify kinesin-like molecules expressed throughout myogenesis. Using a standard PCR-based strategy, we cloned two kinesin-like molecules from a rat myogenic cell line, L6. Sequence analysis of the first of these, KIF3C, defines it as a novel member of the KIF3 subfamily of kinesin-like proteins. KIF3C is expressed throughout myogenesis as well as in numerous rat tissues. Like other members of the KIF3 subfamily, KIF3C has an N-terminal motor domain. The second molecule identified is a rat homolog of murine KIF1B, a putative mitochondrial transporter. KIF1B is also expressed ubiquitously both in myogenic cells at all stages and in a variety of rat tissues.

Planar polarization of Drosophila and vertebrate epithelia

Our understanding of the actin and microtubule rearrangements that generate planar polarity in Drosophila and in vertebrate epithelia has been extended by recent discoveries. Three different Rho family proteins have been shown to mediate polarization in the wing and the eye of Drosophila. In vertebrates, the importance of myosin VIIa has been uncovered by mutations that cause defects in planar polarization in the ear. Advances in our understanding of the Frizzled pathway, which coordinates planar polarization in Drosophila, are moving the field closer to understanding the links between signal transduction and polarized cytoskeletal reorganization.

Remodeling of the cytoskeletal lattice in denervated skeletal muscle

The effect of denervation-induced atrophy on the cytoskeletal lattice in rat fast- and slow-twitch skeletal muscle has been investigated. Immunochemical analyses and immunofluorescence microscopy experiments employing monospecific antibodies to dystrophin, desmin, and alpha-tubulin were carried out on intact and denervated muscles. The relative cellular content of dystrophin and desmin were reduced in the soleus muscle (slow-twitch), while significant increases were shown in the gastrocnemius muscle (fast-twitch). In both muscles, alpha-tubulin levels increased up to 12-fold as a function of time compared to control values. Immunofluorescence microscopy revealed a distinct rearrangement of the microtubule network toward a predominantly longitudinal alignment, which was accompanied by an increase in the density of the fluorescence. It is concluded that the relative increase of the three structural proteins in the fast-twitch gastrocnemius muscle may be related to the apparent resistance of this muscle type to denervation-induced atrophy. The increased alpha-tubulin content in denervated slow- and fast-twitch muscles could be indicative of an adaptive mechanism designed to maintain the integrity of the muscle fiber in view of eventual regenerative activities.

A muscle-specific variant of microtubule-associated protein 4 (MAP4) is required in myogenesis

Microtubule-associated protein 4 (MAP4) transcripts vary in different mouse tissues, with striated muscle (skeletal and cardiac) expressing 8- and 9-kb transcripts preferentially to the more widely distributed 5.5- and 6.5-kb transcripts (West, R. W., Tenbarge, K. M. and Olmsted, J. B. (1991). J. Biol. Chem. 266, 21886–21896). Cloning of the sequence unique to the muscle transcripts demonstrated that these mRNAs vary from the more ubiquitous ones by a single 3.2-kb coding region insertion within the projection domain of MAP4. During differentiation of the myogenic cell line, C2C12, muscle-specific MAP4 transcripts appear within 24 hours of growth in differentiation medium, and a larger MAP4 isotype (350 X 10(3) Mr) accumulates to high levels by 48 hours of differentiation. In situ hybridization analyses of transcript distribution in mouse embryos demonstrated that muscle-specific transcripts appear early in myogenesis. To block the expression of the muscle-specific MAP4, stable lines of C2C12 were generated bearing an antisense construct with the muscle-specific MAP4 sequence. Myoblast growth was unaffected whereas myotube formation was severely perturbed. Fusion occurred in the absence of the muscle MAP4 isotype, but the multinucleate syncytia were short and apolar, microtubules were disorganized and normal anisotropic myofibrils were absent. The patterns of expression of the muscle-specific transcripts and the antisense experiments indicated that this unique structural form of MAP4 plays a critical role in the formation and maintenance of muscle.

Tau protein immunoreactivity in muscle fibers with rimmed vacuoles differs from that in regenerating muscle fibers

To determine whether tau protein found in muscle fibers with rimmed vacuoles and in regenerating fibers was phosphorylated, we examined eight muscle biopsy samples containing rimmed vacuoles (from five patients with distal myopathy with rimmed vacuole formation and three patients with inclusion body myositis) and three muscle biopsy samples from patients with Duchenne muscular dystrophy containing numerous regenerating fibers. Although both rimmed vacuolated and regenerating fibers had increased immunoreactivity against tubulin and tau protein, tau protein in the former was more highly phosphorylated than that in the latter. While very few microtubules in muscle fibers with rimmed vacuoles were recognizable by electron microscopy, regenerating fibers, especially immature ones, contained numerous microtubules. Since tau protein found in vacuolated fibers is hyperphosphorylated, it can be considered to have reduced ability to bind tubulin molecules. Thus, the tau protein cannot stabilize microtubules, resulting in their depolymerization even in the presence of tubulin molecules.

Microtubule-associated protein tau epitopes are present in fiber lesions in diverse muscle disorders

The microtubule-associated protein tau is a major cytoskeletal protein involved in the neurofibrillary tangles of Alzheimer's disease. Although tau is predominantly a neuronal protein, it has been demonstrated in glia and other nonneuronal cells. We describe the presence of microtubule-associated protein tau epitopes in various muscle fiber lesions in oculopharyngeal and Becker muscular dystrophy, dermatomyositis, central core disease, neurogenic atrophy, and in the recovery phase of an attack of malignant hyperthermia. Western blot demonstrated a 100- to 110-kd tau-immunoreactive protein probably corresponding to 'big tau' as described in peripheral nerves. Tau immunoreactivity in muscle fiber lesions usually co-localized with tubulin, although electron microscopy failed to show an increase in microtubules. Tau and tubulin reactivity also correlated with the presence of desmin and vimentin epitopes. Possible explanations for the presence of tau are briefly discussed.

Assembly of body wall muscle and muscle cell attachment structures in Caenorhabditis elegans

C. Elegans has four muscle quadrants that are used for locomotion. Contraction is converted to locomotion because muscle cells are anchored to the cuticle (the outer covering of the worm) by a specialized basement membrane and hemidesmosome structures in the hypodermis (a cellular syncytium that covers the worm and secretes the cuticle). To study muscle assembly, we have used antibodies to determine the spatial and temporal distribution of muscle and attachment structure components in wild-type and mutant C. elegans embryos. Myofibrillar components are first observed diffusely distributed in the muscle cells, and are expressed in some dividing cells. Later, the components accumulate at the membrane adjacent to the hypodermis where the sarcomeres will form, showing that the cells have become polarized. Assembly of muscle attachment structures is spatially and temporally coordinated with muscle assembly suggesting that important developmental signals may be passed between muscle and hypodermal cells. Analysis of embryos homozygous for mutations that affect muscle assembly show that muscle components closer to the membrane than the affected protein assemble quite well, while those further from the membrane do not. Our results suggest a model where lattice assembly is initiated at the membrane and the spatial organization of the structural elements of the muscle is dictated by membrane proximal events, not by the filament components themselves.

Colchicine inhibits human trophoblast differentiation in vitro

When cultured in keratinocyte growth medium, mononuclear cytotrophoblast cells aggregate into multicellular colonies which then fuse to form multinucleated syncytiotrophoblast. In an attempt to characterize better the mechanism of human trophoblast differentiation and to obtain information about the role of the cytoskeleton, experiments were performed using cytoskeletal-disrupting drugs and primary cultures of cytotrophoblast cells from term placentae. Addition of colchicine 6 h after plating permitted aggregation but the cells did not form syncytiotrophoblast, as revealed by staining for desmosomes and nuclei. Staining with an anti-tubulin antibody showed that microtubules were present in untreated control cells but absent in colchicine-treated cultures. If colchicine was added 24 h after plating, the cells also failed to differentiate. When cells were exposed to colchicine for the first 24 h after plating and then cultured in the absence of the drug, differentiation proceeded normally. Cells exposed to colchicine for 48 h and then incubated in the absence of the drug failed to form syncytiotrophoblast. The results suggest that a decision to differentiate is made between 24 and 48 h after plating. The effects of colchicine were observed between 2.5 and 250 microM. Beta-lumicolchicine blocked differentiation at 250 microM but was ineffective at lower concentrations. Colchicine also inhibited HCG secretion in a dose-dependent manner; beta-lumicolchicine only caused inhibition at 250 microM. Staining with antitubulin antibody revealed that lumicolchicine-treated cells had intact microtubules. These results suggest a role for microtubules in trophoblast differentiation.

Organization of microtubules in cochlear hair cells

The organization of microtubules in hair cells of the guinea-pig cochlea has been investigated using transmission electron microscopy and correlated with the location of tubulin-associated immunofluorescence in surface preparations of the organ of Corti. Results from both techniques reveal consistent distributions of microtubules in inner and outer hair cells. In the inner hair cells, microtubules are most concentrated in the apex. Reconstruction from serial sections shows three main groups: firstly, in channels through the cuticular plate and in a discontinuous belt around its upper perimeter; secondly, forming a ring inside a rim extending down from the lower perimeter of the plate; and thirdly, in a meshwork underlying the main body of the plate. In the cell body, microtubules line the inner face of the subsurface cistern and extend longitudinally through a tubulo-vesicular track between the apex and base. In outer hair cells, the pattern of microtubules associated with the cuticular plate is similar, although there are fewer present than in inner hair cells. In outer hair cells from the apex of the cochlea, microtubules occur around an infracuticular protrusion of cuticular plate material. In the cell body, many more microtubules occur in the region below the nucleus compared with inner hair cells. The possible functions of microtubules in hair cells are discussed by comparison with those found in other systems. These include morphogenesis and maintenance of cell shape; intracellular transport, e.g., of neurotransmitter vesicles; providing a possible substrate for motility; mechanical support of structures associated with sensory transduction.

Early development and myogenesis of the posterior anuran lymph hearts

The early development of anuran posterior lymph hearts studied by light- and electron-microscopy in frog larval stages 25-29 (Gosner 1960) can be subdivided into three phases. In phase I, mesenchymal myogenic cells are found, each possessing a single 9 + 0 cilium and numerous filopode-like processes aggregated near the vena caudalis lateralis, forming up to three metameric organ anlagen arranged like a cuff around the preexisting lymphatics (stages 26/27). In phase II, cell proliferation starts at stage 28 within the lymph heart wall as does the formation of primarily polynuclear myofibres by fusion of several myoblasts. At this stage immature myofibres show a vast sarcoplasm, a poorly developed SR and only few myofibrils with not yet distinguishable A- and I-bands. In phase III, the afferent and efferent valves are formed at the onset of pulsation in stage 29. Contractile myofibres contain large glycogen fields and a considerable amount of myofibrils which frequently branch and show distinct Z-lines, A-, I-, H- and M-bands; 1-3 cilia were found lying within a channel-like cell invagination. The peculiarities of organogenesis and myofibre development are discussed.

Generation of a stable, posttranslationally modified microtubule array is an early event in myogenic differentiation

Microtubules (MTs) have been implicated to function in the change of cell shape and intracellular organization that occurs during myogenesis. However, the mechanism by which MTs are involved in these morphogenetic events is unclear. As a first step in elucidating the role of MTs in myogenesis, we have examined the accumulation and subcellular distribution of posttranslationally modified forms of tubulin in differentiating rat L6 muscle cells, using antibodies specific for tyrosinated (Tyr), detyrosinated (Glu), and acetylated (Ac) tubulin. Both Glu and Ac tubulin are components of stable MTs, whereas Tyr tubulin is the predominant constituent of dynamic MTs. In proliferating L6 myoblasts, as in other types of proliferating cells, the level of Glu tubulin was very low when compared with the level of Tyr tubulin. However, when we shifted proliferating L6 cells to differentiation media, we observed a rapid accumulation of Glu tubulin in cellular MTs. By immunofluorescence, the increase in Glu tubulin was first detected in MTs of prefusion myoblasts and was specifically localized to MTs that were associated with elongating portions of the cell. MTs in the multinucleated myotubes observed at later stages of differentiation maintained the elevated level of Glu tubulin that was observed in the prefusion myoblasts. When cells at early stages of differentiation (less than 1 d after switching the culture medium) were immunostained for Glu tubulin and the muscle-specific marker, muscle myosin, we found that the increase in Glu tubulin preceded the accumulation of muscle myosin. Thus, the elaboration of Glu MTs is one of the very early events in myogenesis. Ac tubulin also increased during L6 myogenesis; however, the increase in acetylation occurred later in myogenesis, after fusion had already occurred. Because detyrosination was temporally correlated with early events of myogenesis, we examined the mechanism responsible for the accumulation of Glu tubulin in the MTs of prefusion myoblasts. We found that an increase in the stability of L6 cell MTs occurred at the onset of differentiation, suggesting that the early increase in detyrosination that we observed is a manifestation of a decrease in MT dynamics in elongating myoblasts. We conclude that the establishment of an oriented array of microtubules heightened in its stability and its level of posttranslationally modified subunits may be involved in the subcellular remodeling that occurs during myogenesis.

A variant beta-tubulin isoform of Drosophila melanogaster (beta 3) is expressed primarily in tissues of mesodermal origin in embryos and pupae, and is utilized in populations of transient microtubules

The beta 3-tubulin gene of Drosophila melanogaster codes for a variant tubulin isoform which is expressed at two distinct times during development: (1) during midembryogenesis from 8-16 hr postfertilization, and (2) during the 4 days of pupal development. We have determined the spatial pattern of beta 3-tubulin expression by localizing the beta 3 mRNA in paraffin sections using a 3' message-specific RNA probe and by localizing the beta 3 protein using a polyclonal antibody specific for Drosophila beta 3-tubulin. During embryogenesis beta 3 is restricted to and is expressed in all of the developing muscles. During pupal development beta 3 is also expressed at high levels in developing adult muscles. In addition, early in pupal development beta 3 is expressed in the imaginal discs, while at later times beta 3 is expressed in the epidermal cells of the wing blade, the optic lobe, the ovaries, and the testes. The expression of beta 3 tubulin ceases by the end of pupal development in all of these tissues except the ovaries and testes where expression persists into the adult. In both developing muscles and wings our results indicate that beta 3-tubulin is utilized in populations of specialized but transient cytoskeletal microtubules which are involved in establishing the final form of the tissue.

Centrosome splitting during nuclear elongation in the Drosophila embryo

In the early Drosophila embryo, nuclear elongation occurs during cellularization of the syncytial blastoderm. This process is closely related to the presence of microtubular bundles forming a basket-like structure surrounding the nuclei. In immunofluorescence observations with antibodies against alpha-tubulin, the microtubules appear to radiate from two bright foci widely separated from each other. We used electron microscopy to show that these foci are true centrosomes constituted by daughter and parent centrioles orthogonally disposed and surrounded by pericentriolar electrondense material. The centrosomes may be observed in the apical region of the blastoderm cells from the beginning of cellularization until the reestablishment of the first postblastodermic mitosis, when they organize the spindle poles. Until this time the dimensions of the procentrioles remain unchanged. The significance of these results is discussed in relation to the known behavior of centrioles in the cell cycle.

The cytoskeletal mechanics of brain morphogenesis. Cell state splitters cause primary neural induction

There is a functional device in embryonic ectodermal cells that we propose causes them to differentiate into either neuroepithelial or epidermal tissue during the process called primary neural induction. We call this apparatus the "cell state splitter." Its main components are the apical microfilament ring and the coplanar apical mat of microtubules, which exert forces in opposite radial directions. We analyze the mechanical interaction between these cytoskeletal components and show that they are in an unstable mechanical equilibrium. The role of the cell state splitter is thus to create a mechanical instability corresponding to the embryonic state of "competence" in an otherwise mechanically stable cell. When the equilibrium of the cell state splitter is disturbed so as to produce a slight contraction of the apical end, apical contraction continues and the distinctive columnar neuroepithelial cells are produced. A slight expansion from the equilibrium state, on the other hand, results in flattened epidermal cells. The calculated forces are consistent with the known constitutive and force-generating properties and morphology of microfilaments and microtubules, and with free tubulin concentrations. There are no free parameters in the analysis. The first cells to assume the neuroepithelial state lie over the notochord. Propagation of the neuroepithelial state (homoiogenetic induction) then proceeds via stretch-induced constriction of the apical microfilament rings, until a hemisphere is covered, at which point the high rate of change of the meridional stress component necessary for further propagation vanishes. The remaining cells are stretched somewhat by this process and become epidermis. A sharp boundary between the tissues is thus formed (explaining "compartmentalization" and the binary nature of differentiation in general). Normal induction apparently involves setup of the cell state splitters in all of the ectoderm cells, perhaps synchronously timed by global embryo tension. The initial transition of cells from the ectodermal to the neuroepithelial state begins at the notoplate, where cell attachments to the notochord may both cause basal actin deposition and significantly reduce the stress induced in the ectoderm by the global tension, biasing the notoplate cell state splitters toward the neuroepithelial state. Introduction of an organizer or other solid substrate (artificial inducer) elsewhere, to which ectodermal cells can adhere, may likewise have both of these effects. Differentiation to either epidermis or neuroepithelium is thus a mechanical event followed by the synthesis of specific proteins.(ABSTRACT TRUNCATED AT 400 WORDS)

Control of microtubule nucleation and stability in Madin-Darby canine kidney cells: the occurrence of noncentrosomal, stable detyrosinated microtubules

The microtubule-nucleating activity of centrosomes was analyzed in fibroblastic (Vero) and in epithelial cells (PtK2, Madin-Darby canine kidney [MDCK]) by double-immunofluorescence labeling with anti-centrosome and antitubulin antibodies. Most of the microtubules emanated from the centrosomes in Vero cells, whereas the microtubule network of MDCK cells appeared to be noncentrosome nucleated and randomly organized. The pattern of microtubule organization in PtK2 cells was intermediate to the patterns observed in the typical fibroblastic and epithelial cells. The two centriole cylinders were tightly associated and located close to the nucleus in Vero and PtK2 cells. In MDCK cells, however, they were clearly separated and electron microscopy revealed that they nucleated only a few microtubules. The stability of centrosomal and noncentrosomal microtubules was examined by treatment of these different cell lines with various concentrations of nocodazole. 1.6 microM nocodazole induced an almost complete depolymerization of microtubules in Vero cells; some centrosome nucleated microtubules remained in PtK2 cells, while many noncentrosomal microtubules resisted that treatment in MDCK cells. Centrosomal and noncentrosomal microtubules regrew in MDCK cells with similar kinetics after release from complete disassembly by high concentrations of nocodazole (33 microM). During regrowth, centrosomal microtubules became resistant to 1.6 microM nocodazole before the noncentrosomal ones, although the latter eventually predominate. We suggest that in MDCK cells, microtubules grow and shrink as proposed by the dynamic instability model but the presence of factors prevents them from complete depolymerization. This creates seeds for reelongation that compete with nucleation off the centrosome. By using specific antibodies, we have shown that the abundant subset of nocodazole-resistant microtubules in MDCK cells contained detyrosinated alpha-tubulin (glu tubulin). On the other hand, the first microtubules to regrow after nocodazole removal contained only tyrosinated tubulin. Glu-tubulin became detectable only after 30 min of microtubule regrowth. This strongly supports the hypothesis that alpha-tubulin detyrosination occurs primarily on "long lived" microtubules and is not the cause of the stabilization process. This is also supported by the increased amount of glu-tubulin that we found in taxol-treated cells.

Fate of microtubule-organizing centers during myogenesis in vitro

Microtubule organization and nucleation were studied during in vitro human myogenesis by immunocytology that used monoclonal and polyclonal antitubulin antibodies and a rabbit nonimmune serum that reacts with human centrosomes. In myoblasts, we observed a classical microtubule network centered on juxtanuclear centrosomes. Myotubes possessed numerous microtubules organized in parallel without any apparent nucleation centers. Centrosomes in these cells were not associated one to each nucleus but were often clustered in the vicinity of nuclei groups. They were significantly smaller than those of the mononucleated cells. The periphery of each nucleus in myotubes was labeled with the serum that labels centrosomes suggesting a profound reorganization of microtubule-nucleating material. Regrowth experiments after Nocodazole treatment established that microtubules were growing from the periphery of the nuclei. The redistribution of nucleating material was shown to take place early after myoblast fusion. Such a phenomenon appears to be specific to myogenic differentiation in that artificially induced polykaryons behaved differently: the centrosomes aggregated to form only one or a few giant nucleating centers and the nuclei did not participate directly in the nucleation of microtubules. The significance of these results is discussed in relation to the possible role of the centrosome in establishing cell polarity.