Molecular structure of sarcomere-to-membrane attachment at M-Lines in C. elegans muscle

UNC-82/NUAK kinase is required by myosin A, but not myosin B, to assemble and function in the thick filament arms of C. elegans striated muscle

The mechanisms that ensure proper assembly, activity, and turnover of myosin II filaments are fundamental to a diverse range of cellular processes. In Caenorhabditis elegans striated muscle, thick filaments contain two myosins that are functionally distinct and spatially segregated. Using transgenic double mutants, we demonstrate that the ability of increased myosin A expression to restore muscle structure and movement in myosin B mutants requires UNC-82/NUAK kinase activity. Myosin B function appears unaffected in the kinase-impaired unc-82(e1220) mutant: the recessive antimorphic effects on early assembly of paramyosin and myosin A in this mutant are counteracted by increased myosin B expression and exacerbated by loss of myosin B. Using chimeric myosins and motility assays, we mapped the region of myosin A that requires UNC-82 activity to a 531-amino-acid region of the coiled-coil rod. This region includes the 264-amino-acid Region 1, which is sufficient in chimeric myosins to rescue the essential filament-initiation function of myosin A, as well as two sites that interact with myosin head domains in the Interacting Heads Motif. A specific physical interaction between myosin A and UNC-82::GFP is supported by GFP labeling of ectopic myosin A filaments but not thin filaments. We hypothesize that UNC-82 regulates assembly competence of myosin A during parallel assembly in the filament arms.

Sequences in the myosin A rod interact with UNC-89/obscurin and the zinc-finger protein UNC-98 during thick filament assembly and M-line formation in C. elegans striated muscle

The M-line of striated muscle is a complex structure that anchors myosin-containing thick filaments and also participates in signaling and proteostasis. While the physical associations among many M-line components have been defined, the mechanism of thick filament attachment is not completely understood. In Caenorhabditis elegans, myosin A is essential for viability and forms the site of M-line attachment at the center of the filament, whereas myosin B forms the filament arms. Using a mutant myosin A that forms ectopic filaments, we examined interactions between myosin A and M-line proteins in intact muscle cells. Ectopic myosin A recruits the giant kinase UNC-89/obscurin, a presumed scaffolding protein, in an interaction that requires the zinc-finger protein UNC-98, but not UNC-82/NUAK, UNC-97/PINCH, or UNC-96. In myosin A mutants, UNC-89/obscurin patterning is highly defective in embryos and adults. A chimeric myosin containing 169 residues of the myosin A C-terminal rod, coincident with the UNC-98/ZnF binding site, is sufficient for colocalization of UNC-89/obscurin and UNC-98/ZnF in M-line structures whereas a myosin chimera lacking these residues colocalizes with UNC-89/obscurin in M-lines that lack UNC-98. Thus, at least two myosin A rod regions contribute independently to M-line organization. We hypothesize that these M-line-organizing functions correspond to the essential "filament initiation function" performed by this isoform.

Role of Muscle LIM Protein in Mechanotransduction Process

The induction of protein synthesis is crucial to counteract the deconditioning of neuromuscular system and its atrophy. In the past, hormones and cytokines acting as growth factors involved in the intracellular events of these processes have been identified, while the implications of signaling pathways associated with the anabolism/catabolism ratio in reference to the molecular mechanism of skeletal muscle hypertrophy have been recently identified. Among them, the mechanotransduction resulting from a mechanical stress applied to the cell appears increasingly interesting as a potential pathway for therapeutic intervention. At present, there is an open question regarding the type of stress to apply in order to induce anabolic events or the type of mechanical strain with respect to the possible mechanosensing and mechanotransduction processes involved in muscle cells protein synthesis. This review is focused on the muscle LIM protein (MLP), a structural and mechanosensing protein with a LIM domain, which is expressed in the sarcomere and costamere of striated muscle cells. It acts as a transcriptional cofactor during cell proliferation after its nuclear translocation during the anabolic process of differentiation and rebuilding. Moreover, we discuss the possible opportunity of stimulating this mechanotransduction process to counteract the muscle atrophy induced by anabolic versus catabolic disorders coming from the environment, aging or myopathies.

Eggshell membrane modulates gut microbiota to prevent murine pre-cachexia through suppression of T helper cell differentiation

BACKGROUND

Cachexia is a life-threatening condition observed in several pathologies, such as cancer or chronic diseases. Interleukin 10 (Il10) gene transfer is known to improve cachexia by downregulating Il6. Here, we used an IL10-knockout mouse model to simulate cachexia and investigate the effects of eggshell membrane (ESM), a resistant protein, on general pre-cachexia symptoms, which is particularly important for the development of cachexia therapeutics.

METHODS

Five-week-old male C57BL6/J mice were fed an AIN-93G powdered diet (WT), and 5-week-old male B6.129P2-Il10 < tm1Cgn>/J (IL10^-/- ) mice were fed either the AIN-93G diet (KO) or an 8% ESM-containing diet (KOE) for 28 weeks. The tissue weight and levels of anaemia-, blood glucose-, lipid metabolism-, and muscular and colonic inflammation-related biochemical markers were measured. Transcriptomic analysis on liver and colon mucus and proteomic analysis on skeletal muscle were performed. Ingenuity Pathway Analysis was used to identify molecular pathways and networks. Caecal short-chain fatty acids (SCFAs) were identified using HPLC, and caecal bacteria DNA were subjected to metagenomic analysis. Flow cytometry analysis was performed to measure the CD4⁺ IL17⁺ T cells in mesenteric lymph nodes.

RESULTS

The body weight, weight of gastrocnemius muscle and fat tissues, colon weight/length ratio, plasma HDL and NEFA, muscular PECAM-1 levels (P < 0.01), plasma glucose and colonic mucosal myeloperoxidase activity (P < 0.05) and T helper (Th) 17 cell abundance (P = 0.071) were improved in KOE mice over KO mice. Proteomic analysis indicated the protective role of ESM in muscle weakness and maintenance of muscle formation (>1.5-fold). Transcriptomic analysis revealed that ESM supplementation suppressed the LPS/IL1-mediated inhibition of RXR function pathway in the liver and downregulated the colonic mucosal expression of chemokines and Th cell differentiation-related markers (P < 0.01) by suppressing the upstream BATF pathway. Analysis of the intestinal microenvironment revealed that ESM supplementation ameliorated the microbial alpha diversity and the abundance of microbiota associated with the degree of inflammation (P < 0.05) and increased the level of total organic acids, particularly of SCFAs such as butyrate (2.3-fold), which could inhibit Th1 and Th17 production.

CONCLUSIONS

ESM supplementation ameliorated the chief symptoms of cachexia, including anorexia, lean fat tissue mass, skeletal muscle wasting and reduced physical function. ESM also improved colon and skeletal muscle inflammation, lipid metabolism and microbial dysbiosis. These results along with the suppressed differentiation of Th cells could be associated with the beneficial intestinal microenvironment and, subsequently, attenuation of pre-cachexia. Our findings provide insights into the potential of ESM in complementary interventions for pre-cachexia prevention.

Phosphofructokinase relocalizes into subcellular compartments with liquid-like properties in vivo

Although much is known about the biochemical regulation of glycolytic enzymes, less is understood about how they are organized inside cells. We systematically examine the dynamic subcellular localization of glycolytic protein phosphofructokinase-1/PFK-1.1 in Caenorhabditis elegans. We determine that endogenous PFK-1.1 localizes to subcellular compartments in vivo. In neurons, PFK-1.1 forms phase-separated condensates near synapses in response to energy stress from transient hypoxia. Restoring animals to normoxic conditions results in cytosolic dispersion of PFK-1.1. PFK-1.1 condensates exhibit liquid-like properties, including spheroid shapes due to surface tension, fluidity due to deformations, and fast internal molecular rearrangements. Heterologous self-association domain cryptochrome 2 promotes formation of PFK-1.1 condensates and recruitment of aldolase/ALDO-1. PFK-1.1 condensates do not correspond to stress granules and might represent novel metabolic subcompartments. Our studies indicate that glycolytic protein PFK-1.1 can dynamically form condensates in vivo.

Caenorhabditis elegans SORB-1 localizes to integrin adhesion sites and is required for organization of sarcomeres and mitochondria in myocytes

We have identified and characterized sorb-1, the only sorbin and SH3 domain-containing protein family member in Caenorhabditis elegans SORB-1 is strongly localized to integrin adhesion complexes in larvae and adults, including adhesion plaques and dense bodies (Z-disks) of striated muscles and attachment plaques of smooth muscles. SORB-1 is recruited to the actin-binding, membrane-distal regions of dense bodies via its C-terminal SH3 domains in an ATN-1(α-actinin)- and ALP-1(ALP/Enigma)-dependent manner, where it contributes to the organization of sarcomeres. SORB-1 is also found in other tissues known to be under mechanical stress, including stress fibers in migratory distal tip cells and the proximal gonad sheath, where it becomes enriched in response to tissue distention. We provide evidence for a novel role for sorbin family proteins: SORB-1 is required for normal positioning of the mitochondrial network in muscle cells. Finally, we demonstrate that SORB-1 interacts directly with two other dense body components, DEB-1(vinculin) and ZYX-1(zyxin). This work establishes SORB-1 as a bona fide sorbin family protein-one of the late additions to the dense body complex and a conserved regulator of body wall muscle sarcomere organization and organelle positioning.

High-resolution imaging of muscle attachment structures in Caenorhabditis elegans

We used structured illumination microscopy (SIM) to obtain super-resolution images of muscle attachment structures in Caenorhabditis elegans striated muscle. SIM imaging of M-line components revealed two patterns: PAT-3 (β-integrin) and proteins that interact in a complex with the cytoplasmic tail of β-integrin and localize to the basal muscle cell membrane [UNC-112 (kindlin), PAT-4 (ILK), UNC-97 (PINCH), PAT-6 (α-parvin), and UNC-95], are found in discrete, angled segments with gaps. In contrast, proteins localized throughout the depth of the M-line (UNC-89 (obscurin) and UNC-98) are imaged as continuous lines. Systematic immunostaining of muscle cell boundaries revealed that dense body components close to the basal muscle cell membrane also localize at cell boundaries. SIM imaging of muscle cell boundaries reveal "zipper-like" structures. Electron micrographs reveal electron dense material similar in appearance to dense bodies located adjacent to the basolateral cell membranes of adjacent muscle cells separated by ECM. Moreover, by EM, there are a variety of features of the muscle cell boundaries that help explain the zipper-like pattern of muscle protein localization observed by SIM. Short dense bodies in atn-1 mutants that are null for α-actinin and lack the deeper extensions of dense bodies, showed "zipper-like" structures by SIM similar to cell boundary structures, further indicating that the surface-proximal components of dense bodies form the "zipper-like" structures at cell boundaries. Moreover, mutants in thin and thick filament components do not have "dot-like" dense bodies, suggesting that myofilament tension is required for assembly or maintenance of proper dense body shape.

The Role of the UNC-82 Protein Kinase in Organizing Myosin Filaments in Striated Muscle of Caenorhabditis elegans

We study the mechanisms that guide the formation and maintenance of the highly ordered actin-myosin cytoskeleton in striated muscle. The UNC-82 kinase of Caenorhabditis elegans is orthologous to mammalian kinases ARK5/NUAK1 and SNARK/NUAK2. UNC-82 localizes to the M-line, and is required for proper organization of thick filaments, but its substrate and mechanism of action are unknown. Antibody staining of three mutants with missense mutations in the UNC-82 catalytic domain revealed muscle structure that is less disorganized than in the null unc-82(0), but contained distinctive ectopic accumulations not found in unc-82(0) These accumulations contain paramyosin and myosin B, but lack myosin A and myosin A-associated proteins, as well as proteins of the integrin-associated complex. Fluorescently tagged missense mutant protein UNC-82 E424K localized normally in wild type; however, in unc-82(0), the tagged protein was found in the ectopic accumulations, which we also show to label with recently synthesized paramyosin. Recruitment of wild-type UNC-82::GFP to aggregates of differing protein composition in five muscle-affecting mutants revealed that colocalization of UNC-82 and paramyosin does not require UNC-96, UNC-98/ZnF, UNC-89/obscurin, CSN-5, myosin A, or myosin B individually. Dosage effects in paramyosin mutants suggest that UNC-82 acts as part of a complex, in which its stoichiometric relationship with paramyosin is critical. UNC-82 dosage affects muscle organization in the absence of paramyosin, perhaps through myosin B. We present evidence that the interaction of UNC-98/ZnF with myosin A is independent of UNC-82, and that UNC-82 acts upstream of UNC-98/ZnF in a pathway that organizes paramyosin during thick filament assembly.

A Common Suite of Coagulation Proteins Function in Drosophila Muscle Attachment

The organization and stability of higher order structures that form in the extracellular matrix (ECM) to mediate the attachment of muscles are poorly understood. We have made the surprising discovery that a subset of clotting factor proteins are also essential for muscle attachment in the model organism Drosophila melanogaster One such coagulation protein, Fondue (Fon), was identified as a novel muscle mutant in a pupal lethal genetic screen. Fon accumulates at muscle attachment sites and removal of this protein results in decreased locomotor behavior and detached larval muscles. A sensitized genetic background assay reveals that fon functions with the known muscle attachment genes Thrombospondin (Tsp) and Tiggrin (Tig). Interestingly, Tig is also a component of the hemolymph clot. We further demonstrate that an additional clotting protein, Larval serum protein 1γ (Lsp1γ), is also required for muscle attachment stability and accumulates where muscles attach to tendons. While the local biomechanical and organizational properties of the ECM vary greatly depending on the tissue microenvironment, we propose that shared extracellular protein-protein interactions influence the strength and elasticity of ECM proteins in both coagulation and muscle attachment.

The SH3 domain of UNC-89 (obscurin) interacts with paramyosin, a coiled-coil protein, in Caenorhabditis elegans muscle

UNC-89 is a giant polypeptide located at the sarcomeric M-line of Caenorhabditis elegans muscle. The human homologue is obscurin. To understand how UNC-89 is localized and functions, we have been identifying its binding partners. Screening a yeast two-hybrid library revealed that UNC-89 interacts with paramyosin. Paramyosin is an invertebrate-specific coiled-coil dimer protein that is homologous to the rod portion of myosin heavy chains and resides in thick filament cores. Minimally, this interaction requires UNC-89's SH3 domain and residues 294-376 of paramyosin and has a KD of ∼1.1 μM. In unc-89 loss-of-function mutants that lack the SH3 domain, paramyosin is found in accumulations. When the SH3 domain is overexpressed, paramyosin is mislocalized. SH3 domains usually interact with a proline-rich consensus sequence, but the region of paramyosin that interacts with UNC-89's SH3 is α-helical and lacks prolines. Homology modeling of UNC-89's SH3 suggests structural features that might be responsible for this interaction. The SH3-binding region of paramyosin contains a "skip residue," which is likely to locally unwind the coiled-coil and perhaps contributes to the binding specificity.

Ultra-structural time-course study in the C. elegans model for Duchenne muscular dystrophy highlights a crucial role for sarcomere-anchoring structures and sarcolemma integrity in the earliest steps of the muscle degeneration process

Duchenne muscular dystrophy (DMD) is a genetic disease characterized by progressive muscle degeneration due to mutations in the dystrophin gene. In spite of great advances in the design of curative treatments, most patients currently receive palliative therapies with steroid molecules such as prednisone or deflazacort thought to act through their immunosuppressive properties. These molecules only slightly slow down the progression of the disease and lead to severe side effects. Fundamental research is still needed to reveal the mechanisms involved in the disease that could be exploited as therapeutic targets. By studying a Caenorhabditis elegans model for DMD, we show here that dystrophin-dependent muscle degeneration is likely to be cell autonomous and affects the muscle cells the most involved in locomotion. We demonstrate that muscle degeneration is dependent on exercise and force production. Exhaustive studies by electron microscopy allowed establishing for the first time the chronology of subcellular events occurring during the entire process of muscle degeneration. This chronology highlighted the crucial role for dystrophin in stabilizing sarcomeric anchoring structures and the sarcolemma. Our results suggest that the disruption of sarcomeric anchoring structures and sarcolemma integrity, observed at the onset of the muscle degeneration process, triggers subcellular consequences that lead to muscle cell death. An ultra-structural analysis of muscle biopsies from DMD patients suggested that the chronology of subcellular events established in C. elegans models the pathogenesis in human. Finally, we found that the loss of sarcolemma integrity was greatly reduced after prednisone treatment suggesting a role for this molecule in plasma membrane stabilization.

Suppressor mutations suggest a surface on PAT-4 (Integrin-linked Kinase) that interacts with UNC-112 (Kindlin)

Caenorhabditis elegans striated muscle cells attach to basement membrane and transmit the force of muscle contraction through integrin adhesion complexes. The cytoplasmic tail of β-integrin (PAT-3) is associated with a conserved four-protein complex that includes UNC-112 (kindlin), PAT-4 (integrin-linked kinase), PAT-6 (α-parvin/actopaxin), and UNC-97 (PINCH). The proper localization of UNC-112 to muscle integrin adhesion sites requires PAT-4. A recent report (Qadota, H., Moerman, D. G., and Benian, G. M. (2012) A molecular mechanism for the requirement of PAT-4 (integrin-linked kinase (ILK)) for the localization of UNC-112 (kindlin) to integrin adhesion sites. J. Biol. Chem. 287, 28537-28551) suggests a possible molecular mechanism for this requirement: that UNC-112 exists in closed inactive and open active conformations, and conversion to the open active form is promoted by binding to PAT-4 (ILK). Previously, we also reported identification of a single missense mutation in UNC-112, D382V, which abolishes both binding to PAT-4 and normal localization to integrin adhesion sites in vivo. In this report, we describe isolation and characterization of PAT-4 missense mutations that permit binding with UNC-112 D382V and place nine affected residues on a homology model of PAT-4. These nine residues cluster in two regions on the surface of PAT-4, do not overlap the likely binding surface for PAT-6 (α-parvin), and therefore may reside along the interaction surface of PAT-4 for UNC-112 (kindlin). We also show that one of these PAT-4 mutations restores the ability of UNC-112 D382V to localize to integrin adhesions and participate in complex formation.

An approach for exploring interaction between two proteins in vivo

We describe a strategy for exploring the function of protein-protein interactions in striated muscle in vivo. We describe our experience using this strategy to study the interaction of UNC-112 (kindlin) with PAT-4 (integrin linked kinase). Random mutagenesis is used to generate a collection of mutants that are screened for lack of binding or gain of binding using a yeast 2-hybrid assay. The mutant proteins are then expressed in transgenic C. elegans to determine their ability to localize in the sarcomere. We emphasize two advantages of this strategy: (1) for studying the interaction of protein A with protein B, when protein A can interact with multiple proteins, and (2) it explores the function of an interaction rather than the absence of, or reduced level of, a protein as can be obtained with null mutants or knockdown by RNAi. We propose that this method can be generalized for studying the meaning of a protein-protein interaction in muscle for any system in which transgenic animals can be generated and their muscles can be imaged.

Integrins and integrin-associated proteins in the cardiac myocyte

Integrins are heterodimeric, transmembrane receptors that are expressed in all cells, including those in the heart. They participate in multiple critical cellular processes including adhesion, extracellular matrix organization, signaling, survival, and proliferation. Particularly relevant for a contracting muscle cell, integrins are mechanotransducers, translating mechanical to biochemical information. Although it is likely that cardiovascular clinicians and scientists have the highest recognition of integrins in the cardiovascular system from drugs used to inhibit platelet aggregation, the focus of this article will be on the role of integrins specifically in the cardiac myocyte. After a general introduction to integrin biology, the article will discuss important work on integrin signaling, mechanotransduction, and lessons learned about integrin function from a range of model organisms. Then we will detail work on integrin-related proteins in the myocyte, how integrins may interact with ion channels and mediate viral uptake into cells, and also play a role in stem cell biology. Finally, we will discuss directions for future study.

Knockdown of the C. elegans kinome identifies kinases required for normal protein homeostasis, mitochondrial network structure, and sarcomere structure in muscle

BACKGROUND

Kinases are important signalling molecules for modulating cellular processes and major targets of drug discovery programs. However, functional information for roughly half the human kinome is lacking. We conducted three kinome wide, >90%, RNAi screens and epistasis testing of some identified kinases against known intramuscular signalling systems to increase the functional annotation of the C. elegans kinome and expand our understanding of kinome influence upon muscle protein degradation.

RESULTS

96 kinases were identified as required for normal protein homeostasis, 74 for normal mitochondrial networks and 50 for normal sarcomere structure. Knockdown of kinases required only for normal protein homeostasis and/or mitochondrial structure was significantly less likely to produce a developmental or behavioural phenotype than knockdown of kinases required for normal sarcomere structure and/or other sub-cellular processes. Lastly, assessment of kinases for which knockdown produced muscle protein degradation against the known regulatory pathways in C. elegans muscle revealed that close to half of kinase knockdowns activated autophagy in a MAPK dependent fashion.

CONCLUSIONS

Roughly 40% of kinases studied, 159 of 397, are important in establishing or maintaining muscle cell health, with most required for both. For kinases where decreased expression triggers protein degradation, autophagy is most commonly activated. These results increase the annotation of the C. elegans kinome to roughly 75% and enable future kinome research. As 33% of kinases identified have orthologues expressed in human muscle, our results also enable testing of whether identified kinases function similarly in maintaining human muscle homeostasis.

ZYX-1, the unique zyxin protein of Caenorhabditis elegans, is involved in dystrophin-dependent muscle degeneration

In vertebrates, zyxin is a LIM-domain protein belonging to a family composed of seven members. We show that the nematode Caenorhabditis elegans has a unique zyxin-like protein, ZYX-1, which is the orthologue of the vertebrate zyxin subfamily composed of zyxin, migfilin, TRIP6, and LPP. The ZYX-1 protein is expressed in the striated body-wall muscles and localizes at dense bodies/Z-discs and M-lines, as well as in the nucleus. In yeast two-hybrid assays ZYX-1 interacts with several known dense body and M-line proteins, including DEB-1 (vinculin) and ATN-1 (α-actinin). ZYX-1 is mainly localized in the middle region of the dense body/Z-disk, overlapping the apical and basal regions containing, respectively, ATN-1 and DEB-1. The localization and dynamics of ZYX-1 at dense bodies depend on the presence of ATN-1. Fluorescence recovery after photobleaching experiments revealed a high mobility of the ZYX-1 protein within muscle cells, in particular at dense bodies and M-lines, indicating a peripheral and dynamic association of ZYX-1 at these muscle adhesion structures. A portion of the ZYX-1 protein shuttles from the cytoplasm into the nucleus, suggesting a role for ZYX-1 in signal transduction. We provide evidence that the zyx-1 gene encodes two different isoforms, ZYX-1a and ZYX-1b, which exhibit different roles in dystrophin-dependent muscle degeneration occurring in a C. elegans model of Duchenne muscular dystrophy.

A molecular mechanism for the requirement of PAT-4 (integrin-linked kinase (ILK)) for the localization of UNC-112 (Kindlin) to integrin adhesion sites

Caenorhabditis elegans muscle cells attach to basement membrane through adhesion plaques. PAT-3 (β-integrin), UNC-112 (kindlin), and PAT-4 (integrin-linked kinase) are associated with these structures. Genetic analysis indicated that PAT-4 is required for UNC-112 to be properly localized. We investigated the molecular basis of this requirement. We show that the cytoplasmic tail of PAT-3 binds to full-length UNC-112 and that the N- and C-terminal halves of UNC-112 bind to each other. We demonstrate competition between the UNC-112 C-terminal half and PAT-4 for binding to the UNC-112 N-terminal half. The D382V mutation results in lack of binding to PAT-4 and lack of localization to adhesion structures. T346A or E349K mutations, which abolish interaction of the N- and C-terminal halves, permit D382V UNC-112 to localize to adhesion structures. The following model is proposed. UNC-112 exists in closed inactive and open active conformations, and upon binding of PAT-4 to the UNC-112 N-terminal half, UNC-112 is converted into the open state, able to bind to PAT-3.

A novel mutation in β integrin reveals an integrin-mediated interaction between the extracellular matrix and cki-1/p27KIP1

The cell-extracellular matrix (ECM) interaction plays an essential role in maintaining tissue shapes and regulates cell behaviors such as cell adhesion, differentiation and proliferation. The mechanism by which the ECM influences the cell cycle in vivo is poorly understood. Here we demonstrate that the β integrin PAT-3 regulates the localization and expression of CKI-1, a C. elegans homologue of the cyclin dependent kinase inhibitor p27(KIP1). In nematodes expressing wild type PAT-3, CKI-1::GFP localizes primarily to nucleoli in hypodermal cells, whereas in animals expressing mutant pat-3 with a defective splice junction, CKI-1::GFP appears clumped and disorganized in nucleoplasm. RNAi analysis links cell adhesion genes to the regulation of CKI-1. RNAi of unc-52/perlecan, ina-1/α integrin, pat-4/ILK, and unc-97/PINCH resulted in abnormal CKI-1::GFP localization. Additional RNAi experiments revealed that the SCF E3 ubiquitin-ligase complex genes, skpt-1/SKP2, cul-1/CUL1 and lin-23/F-box, are required for the proper localization and expression of CKI-1, suggesting that integrin signaling and SCF E3 ligase work together to regulate the cellular distribution of CKI-1. These data also suggest that integrin plays a major role in maintaining proper CKI-1/p27(KIP1) levels in the cell. Perturbed integrin signaling may lead to the inhibition of SCF ligase activity, mislocalization and elevation of CKI-1/p27(KIP1). These results suggest that adhesion signaling is crucial for cell cycle regulation in vivo.

Integrated control of protein degradation in C. elegans muscle

Protein degradation is a fundamental cellular process, the genomic control of which is incompletely understood. The advent of transgene-coded reporter proteins has enabled the development of C. elegans into a model for studying this problem. The regulation of muscle protein degradation is surprisingly complex, integrating multiple signals from hypodermis, intestine, neurons and muscle itself. Within the muscle, degradation is executed by separately regulated autophagy-lysosomal, ubiquitin-proteasome and calpain-mediated systems. The signal-transduction mechanisms, in some instances, involve modules previously identified for their roles in developmental processes, repurposed in terminally differentiated muscle to regulate the activities of pre-formed proteins. Here we review the genes, and mechanisms, which appear to coordinately control protein degradation within C. elegans muscle. We also consider these mechanisms in the context of development, physiology, pathophysiology and disease models.

UNC-89 (obscurin) binds to MEL-26, a BTB-domain protein, and affects the function of MEI-1 (katanin) in striated muscle of Caenorhabditis elegans

The ubiquitin proteasome system is involved in degradation of old or damaged sarcomeric proteins. Most E3 ubiquitin ligases are associated with cullins, which function as scaffolds for assembly of the protein degradation machinery. Cullin 3 uses an adaptor to link to substrates; in Caenorhabditis elegans, one of these adaptors is the BTB-domain protein MEL-26 (maternal effect lethal). Here we show that MEL-26 interacts with the giant sarcomeric protein UNC-89 (obscurin). MEL-26 and UNC-89 partially colocalize at sarcomeric M-lines. Loss of function or gain of function of mel-26 results in disorganization of myosin thick filaments similar to that found in unc-89 mutants. It had been reported that in early C. elegans embryos, a target of the CUL-3/MEL-26 ubiquitylation complex is the microtubule-severing enzyme katanin (MEI-1). Loss of function or gain of function of mei-1 also results in disorganization of thick filaments similar to unc-89 mutants. Genetic data indicate that at least some of the mel-26 loss-of-function phenotype in muscle can be attributed to increased microtubule-severing activity of MEI-1. The level of MEI-1 protein is reduced in an unc-89 mutant, suggesting that the normal role of UNC-89 is to inhibit the CUL-3/MEL-26 complex toward MEI-1.

C. elegans PAT-9 is a nuclear zinc finger protein critical for the assembly of muscle attachments

Background

Caenorhabditis elegans sarcomeres have been studied extensively utilizing both forward and reverse genetic techniques to provide insight into muscle development and the mechanisms behind muscle contraction. A previous genetic screen investigating early muscle development produced 13 independent mutant genes exhibiting a Pat (paralyzed and arrested elongation at the two-fold length of embryonic development) muscle phenotype. This study reports the identification and characterization of one of those genes, pat-9.

Results

Positional cloning, reverse genetics, and plasmid rescue experiments were used to identify the predicted C. elegans gene T27B1.2 (recently named ztf-19) as the pat-9 gene. Analysis of pat-9 showed it is expressed early in development and within body wall muscle lineages, consistent with a role in muscle development and producing a Pat phenotype. However, unlike most of the other known Pat gene family members, which encode structural components of muscle attachment sites, PAT-9 is an exclusively nuclear protein. Analysis of the predicted PAT-9 amino acid sequence identified one putative nuclear localization domain and three C2H2 zinc finger domains. Both immunocytochemistry and PAT-9::GFP fusion expression confirm that PAT-9 is primarily a nuclear protein and chromatin immunoprecipitation (ChIP) experiments showed that PAT-9 is present on certain gene promoters.

Conclusions

We have shown that the T27B1.2 gene is pat-9. Considering the Pat-9 mutant phenotype shows severely disrupted muscle attachment sites despite PAT-9 being a nuclear zinc finger protein and not a structural component of muscle attachment sites, we propose that PAT-9 likely functions in the regulation of gene expression for some necessary structural or regulatory component(s) of the muscle attachment sites.

Calpains mediate integrin attachment complex maintenance of adult muscle in Caenorhabditis elegans

Two components of integrin containing attachment complexes, UNC-97/PINCH and UNC-112/MIG-2/Kindlin-2, were recently identified as negative regulators of muscle protein degradation and as having decreased mRNA levels in response to spaceflight. Integrin complexes transmit force between the inside and outside of muscle cells and signal changes in muscle size in response to force and, perhaps, disuse. We therefore investigated the effects of acute decreases in expression of the genes encoding these multi-protein complexes. We find that in fully developed adult Caenorhabditis elegans muscle, RNAi against genes encoding core, and peripheral, members of these complexes induces protein degradation, myofibrillar and mitochondrial dystrophies, and a movement defect. Genetic disruption of Z-line– or M-line–specific complex members is sufficient to induce these defects. We confirmed that defects occur in temperature-sensitive mutants for two of the genes: unc-52, which encodes the extra-cellular ligand Perlecan, and unc-112, which encodes the intracellular component Kindlin-2. These results demonstrate that integrin containing attachment complexes, as a whole, are required for proper maintenance of adult muscle. These defects, and collapse of arrayed attachment complexes into ball like structures, are blocked when DIM-1 levels are reduced. Degradation is also blocked by RNAi or drugs targeting calpains, implying that disruption of integrin containing complexes results in calpain activation. In wild-type animals, either during development or in adults, RNAi against calpain genes results in integrin muscle attachment disruptions and consequent sub-cellular defects. These results demonstrate that calpains are required for proper assembly and maintenance of integrin attachment complexes. Taken together our data provide in vivo evidence that a calpain-based molecular repair mechanism exists for dealing with attachment complex disruption in adult muscle. Since C. elegans lacks satellite cells, this mechanism is intrinsic to the muscles and raises the question if such a mechanism also exists in higher metazoans.

Muscle is a dynamic tissue that grows in response to use and nutrition and shrinks in response to lack of use, poor nutrition, or disease. Loss of muscle mass is an important public health problem, but we understand little of the genes that regulate muscle shrinkage. We have found that, in adult worm muscle, attachment to the basement membrane is continuously required to prevent catastrophic sub-cellular defects that result in impaired ability of muscle to function. We have also identified a group of proteases that are activated when the attachment fails to be properly maintained. Conversely, when these proteases are lacking in adult muscle, the muscles fail to maintain attachment to the basement membrane. Thus, we have discovered a group of proteases that appear to act to maintain attachment to the basement membrane and therefore to maintain muscle itself. Because these worms lack satellite cells, this maintenance system is intrinsic to muscle, thus raising the question whether a similar or identical system also works in humans.

Bending amplitude - a new quantitative assay of C. elegans locomotion: identification of phenotypes for mutants in genes encoding muscle focal adhesion components

The nematode Caenorhabditis elegans uses striated muscle in its body wall for locomotion. The myofilament lattice is organized such that all the thin filament attachment structures (dense bodies, analogous to Z-disks) and thick filament organizing centers (M-lines) are attached to the muscle cell membrane. Thus, the force of muscle contraction is transmitted through these structures and allows locomotion of the worm. Dense bodies and M-lines are compositionally similar to focal adhesions and costameres, and are based on integrin and associated proteins. Null mutants for many of the newly discovered dense body and M-line proteins do not have obvious locomotion defects when observed casually, or when assayed by counting the number of times a worm moves back and forth in liquid. We hypothesized that many of these proteins, located as they are in muscle focal adhesions, function in force transmission, but we had not used an appropriate or sufficiently sensitive assay to reveal this function. Recently, we have developed a new quantitative assay of C. elegans locomotion that measures the maximum bending amplitude of an adult worm as it moves backwards. The assay had been used to reveal locomotion defects for null mutants of genes encoding ATN-1 (α-actinin) and PKN-1 (protein kinase N). Here, we describe the details of this method, and apply it to 21 loss of function mutants in 17 additional genes, most of which encode components of muscle attachment structures. As compared to wild type, mutants in 11 genes were found to have less ability to bend, and mutants in one gene were found to have greater ability to bend. Loss of function mutants for eight proteins had been reported to have normal locomotion (ZYX-1 (zyxin), ALP-1 (Enigma), DIM-1, SCPL-1), or locomotion that was not previously investigated (FRG-1 (FRG1), KIN-32 (focal adhesion kinase), LIM-8), or had only slightly decreased locomotion (PFN-3 (profilin)).

Obscurin depletion impairs organization of skeletal muscle in developing zebrafish embryos

During development, skeletal myoblasts differentiate into myocytes and skeletal myotubes with mature contractile structures that are precisely oriented with respect to surrounding cells and tissues. Establishment of this highly ordered structure requires reciprocal interactions between the differentiating myocytes and the surrounding extracellular matrix to form correctly positioned and well-organized attachments from the skeletal muscle to the bony skeleton. Using the developing zebrafish embryo as a model, we examined the relationship between new myofibril assembly and the organization of the membrane domains involved in cell-extracellular matrix interactions. We determined that depletion of obscurin, a giant muscle protein, resulted in irregular cell morphology and disturbed extracellular matrix organization during skeletal muscle development. The resulting impairment of myocyte organization was associated with disturbance of the internal architecture of the myocyte suggesting that obscurin participates in organizing the internal structure of the myocyte and translating those structural cues to surrounding cells and tissues.

The Caenorhabditis elegans paxillin orthologue, PXL-1, is required for pharyngeal muscle contraction and for viability

Caenorhabditis elegans pxl-1 is the orthologue of vertebrate paxillin and is expressed in body wall and pharyngeal muscle. In body wall muscle PXL-1 localizes to dense bodies, M-lines, and adhesion plaques, and in pharyngeal muscle PXL-1 localizes to podosome-like actin attachment complexes. PXL-1 is required in the pharynx for muscle contraction and viability.

We have identified the gene C28H8.6 (pxl-1) as the Caenorhabditis elegans orthologue of vertebrate paxillin. PXL-1 contains the four C-terminal LIM domains conserved in paxillin across all species and three of the five LD motifs found in the N-terminal half of most paxillins. In body wall muscle, PXL-1 antibodies and a full-length green fluorescent protein translational fusion localize to adhesion sites in the sarcomere, the functional repeat unit in muscle responsible for contraction. PXL-1 also localizes to ring-shaped structures near the sarcolemma in pharyngeal muscle corresponding to podosome-like sites of actin attachment. Our analysis of a loss-of-function allele of pxl-1, ok1483, shows that loss of paxillin leads to early larval arrested animals with paralyzed pharyngeal muscles and eventual lethality, presumably due to an inability to feed. We rescued the mutant phenotype by expressing paxillin solely in the pharynx and found that these animals survived and are essentially wild type in movement and body wall muscle structure. This indicates a differential requirement for paxillin in these two types of muscle. In pharyngeal muscle it is essential for contraction, whereas in body wall muscle it is dispensable for filament assembly, sarcomere stability, and ultimately movement.

PKN-1, a homologue of mammalian PKN, is involved in the regulation of muscle contraction and force transmission in C. elegans

To examine the in vivo functions of protein kinase N (PKN), one of the effectors of Rho small guanosine triphosphatases (GTPases), we used the nematode Caenorhabditis elegans as a genetic model system. We identified a C. elegans homologue (pkn-1) of mammalian PKN and confirmed direct binding to C. elegans Rho small GTPases. Using a green fluorescent protein reporter, we showed that pkn-1 is mainly expressed in various muscles and is localized at dense bodies and M lines. Overexpression of the PKN-1 kinase domain and loss-of-function mutations by genomic deletion of pkn-1 resulted in a loopy Unc phenotype, which has been reported in many mutants of neuronal genes. The results of mosaic analysis and body wall muscle-specific expression of the PKN-1 kinase domain suggests that this loopy phenotype is due to the expression of PKN-1 in body wall muscle. The genomic deletion of pkn-1 also showed a defect in force transmission. These results suggest that PKN-1 functions as a regulator of muscle contraction-relaxation and as a component of the force transmission mechanism.