Links in the chain: the contribution of kettin to the elasticity of insect muscles

Mechanisms of Frank-Starling law of the heart and stretch activation in striated muscles may have a common molecular origin

Vertebrate cardiac muscle generates progressively larger systolic force when the end diastolic chamber volume is increased, a property called the "Frank-Starling Law", or "length dependent activation (LDA)". In this mechanism a larger force develops when the sarcomere length (SL) increased, and the overlap between thick and thin filament decreases, indicating increased production of force per unit length of the overlap. To account for this phenomenon at the molecular level, we examined several hypotheses: as the muscle length is increased, (1) lattice spacing decreases, (2) Ca2+ sensitivity increases, (3) titin mediated rearrangement of myosin heads to facilitate actomyosin interaction, (4) increased SL activates cross-bridges (CBs) in the super relaxed state, (5) increased series stiffness at longer SL promotes larger elementary force/CB to account for LDA, and (6) stretch activation (SA) observed in insect muscles and LDA in vertebrate muscles may have similar mechanisms. SA is also known as delayed tension or oscillatory work, and universally observed among insect flight muscles, as well as in vertebrate skeletal and cardiac muscles. The sarcomere stiffness observed in relaxed muscles may significantly contributes to the mechanisms of LDA. In vertebrate striated muscles, the sarcomere stiffness is mainly caused by titin, a single filamentary protein spanning from Z-line to M-line and tightly associated with the myosin thick filament. In insect flight muscles, kettin connects Z-line and the thick filament to stabilize the sarcomere structure. In vertebrate cardiac muscles, titin plays a similar role, and may account for LDA and may constitute a molecular mechanism of Frank-Starling response.

Molecular analysis of the muscle protein projectin in Lepidoptera

Striated muscles of both vertebrates and insects contain a third filament composed of the giant proteins, namely kettin and projectin (insects) and titin (vertebrates). All three proteins have been shown to contain several domains implicated in conferring elasticity, in particular a PEVK segment. In this study, the characterization of the projectin protein in the silkmoth, Bombyx mori L. (Lepidoptera: Bombycidae), and the monarch butterfly, Danaus plexippus L. (Lepidoptera: Nymphalidae), as well as a partial characterization in the Carolina sphinx, Manduca sexta L. (Lepidoptera: Sphingidae), are presented. This study showed that, similar to other insects, projectin's overall modular organization was conserved, but in contrast, the PEVK region had a highly divergent sequence. The analysis of alternative splicing in the PEVK region revealed a small number of possible isoforms and the lack of a flight-muscle specific variant, both characteristics being in sharp contrast with findings from other insects. The possible correlation with difference in flight muscle stiffness and physiology between Lepidoptera and other insect orders is discussed.

Projectin PEVK domain, splicing variants and domain structure in basal and derived insects

The third elastic filament of striated muscles consists of giant proteins: titin (in vertebrates) and kettin/projectin (in insects). In all three proteins, elasticity is at least partly associated with the so-called PEVK domain. The projectin PEVK domains of diverse insects are highly divergent compared with an otherwise conserved protein organization. We present the characterization of the PEVK domain in two dragonflies and in human lice. A conserved segment at the end of the PEVK, the NH(2)-terminal conserved segment-1 (NTCS-1), may serve as an anchor point for projectin to either myosin or actin, providing a mechanical link. The analysis of alternative splicing variants identifies the shortest PEVK isoform as the predominant form in the flight muscles of several insects, possibly contributing to myofibrillar stiffness.

The myofibrillar protein, projectin, is highly conserved across insect evolution except for its PEVK domain

All striated muscles respond to stretch by a delayed increase in tension. This physiological response, known as stretch activation, is, however, predominantly found in vertebrate cardiac muscle and insect asynchronous flight muscles. Stretch activation relies on an elastic third filament system composed of giant proteins known as titin in vertebrates or kettin and projectin in insects. The projectin insect protein functions jointly as a "scaffold and ruler" system during myofibril assembly and as an elastic protein during stretch activation. An evolutionary analysis of the projectin molecule could potentially provide insight into how distinct protein regions may have evolved in response to different evolutionary constraints. We mined candidate genes in representative insect species from Hemiptera to Diptera, from published and novel genome sequence data, and carried out a detailed molecular and phylogenetic analysis. The general domain organization of projectin is highly conserved, as are the protein sequences of its two repeated regions-the immunoglobulin type C and fibronectin type III domains. The conservation in structure and sequence is consistent with the proposed function of projectin as a scaffold and ruler. In contrast, the amino acid sequences of the elastic PEVK domains are noticeably divergent, although their length and overall unusual amino acid makeup are conserved. These patterns suggest that the PEVK region working as an unstructured domain can still maintain its dynamic, and even its three-dimensional, properties, without the need for strict amino acid conservation. Phylogenetic analysis of the projectin proteins also supports a reclassification of the Hymenoptera in relation to Diptera and Coleoptera.

Invertebrate muscles: thin and thick filament structure; molecular basis of contraction and its regulation, catch and asynchronous muscle

This is the second in a series of canonical reviews on invertebrate muscle. We cover here thin and thick filament structure, the molecular basis of force generation and its regulation, and two special properties of some invertebrate muscle, catch and asynchronous muscle. Invertebrate thin filaments resemble vertebrate thin filaments, although helix structure and tropomyosin arrangement show small differences. Invertebrate thick filaments, alternatively, are very different from vertebrate striated thick filaments and show great variation within invertebrates. Part of this diversity stems from variation in paramyosin content, which is greatly increased in very large diameter invertebrate thick filaments. Other of it arises from relatively small changes in filament backbone structure, which results in filaments with grossly similar myosin head placements (rotating crowns of heads every 14.5 nm) but large changes in detail (distances between heads in azimuthal registration varying from three to thousands of crowns). The lever arm basis of force generation is common to both vertebrates and invertebrates, and in some invertebrates this process is understood on the near atomic level. Invertebrate actomyosin is both thin (tropomyosin:troponin) and thick (primarily via direct Ca(++) binding to myosin) filament regulated, and most invertebrate muscles are dually regulated. These mechanisms are well understood on the molecular level, but the behavioral utility of dual regulation is less so. The phosphorylation state of the thick filament associated giant protein, twitchin, has been recently shown to be the molecular basis of catch. The molecular basis of the stretch activation underlying asynchronous muscle activity, however, remains unresolved.

The function of elastic proteins in the oscillatory contraction of insect flight muscle

Oscillatory contraction of asynchronous insect flight muscle is activated by periodic stretches at constant low concentrations of Ca2+. The fibres must be relatively stiff to respond to small length changes occurring at high frequency. Several proteins in the flight muscle may determine the overall stiffness of the fibres. The Drosophila sallimus (sls) gene codes for multiple isoforms with a modular structure made up of immunoglobulin (Ig) and elastic PEVK domains, unique sequence, and a few fibronectin (Fn) domains at the end of the molecule. Kettin, derived from the sls gene, has Ig domains separated by linker sequences and is bound to actin near the Z-disc; the C-terminus is associated with the end of the A-band. Flight muscle also has longer isoforms of Sls, with extensible PEVK sequence, and C-terminal Fn domains; all extend from the Z-disc to the end of the A-band. Projectin, from a different gene, has repeating modules of Fn and Ig domains, and is associated with the end of thick filaments; tandem Ig and PEVK domains at the N-terminus are in the I-band. Projectin, kettin and other Sls isoforms form a mechanical link between thick and thin filaments; all are probably part of the connecting filaments, which branch from the thick filaments and are linked to actin near the Z-disc. The elasticity of fibres may depend on the relative amounts of those isoforms with extensible PEVK sequence. Flightin is bound on the outside of thick filaments and maintains the stiffness necessary for the transmission of stress along the filaments. Insect flight muscle has multiple elastic proteins to give the sarcomere the optimum compliance necessary for high frequency oscillatory contraction.

Molecular and biochemical characterization of kettin in Caenorhabditis elegans

Kettin is a unique member of the connectin/titin family of muscle elastic proteins, which has repetitive immunoglobulin-like domains that are separated by weakly conserved linker sequences. In striated muscles of insects and crayfish, kettin binds to actin filaments and localizes to the Z-disc and its adjacent region in the I-band. Recent sequence analysis of invertebrate connectin/titin (also known as SLS proteins) has revealed that kettin is a splice variant of connectin/titin. In contrast, in the nematode Caenorhabditis elegans, the kettin gene is independent of the genes for other connectin/titin-related proteins. Immunofluorescent localization of kettin shows that it localizes to the I-bands in the obliquely striated body wall muscle. Therefore, C. elegans is an attractive model system to study specific functions of kettin in muscle cells.

In indirect flight muscles Drosophila projectin has a short PEVK domain, and its NH2-terminus is embedded at the Z-band

Insect indirect flight muscles (IFM) contain a third filament system made up of elastic connecting or C-filaments. The giant protein projectin is the main, if not the only, component of these structures. In this study we found that projectin is oriented within the IFM sarcomere with its NH2-terminus embedded in the Z-bands. We demonstrate that this protein has an elastic region that can be detected by the movement of specific epitopes following stretch. One possible elastic region is the PEVK-like domain located close to the NH2-terminus. The amino acid length of this region is short, and 52% of its residues are P, E, V or K. We propose a model in which projectin extends from the Z-band to the lateral borders of the A-band. The PEVK-like domain and a series of Ig domains spanning the intervening I-band may provide the elastic properties of projectin.

Caenorhabditis elegans kettin, a large immunoglobulin-like repeat protein, binds to filamentous actin and provides mechanical stability to the contractile apparatuses in body wall muscle

Kettin is a large actin-binding protein with immunoglobulin-like (Ig) repeats, which is associated with the thin filaments in arthropod muscles. Here, we report identification and functional characterization of kettin in the nematode Caenorhabditis elegans. We found that one of the monoclonal antibodies that were raised against C. elegans muscle proteins specifically reacts with kettin (Ce-kettin). We determined the entire cDNA sequence of Ce-kettin that encodes a protein of 472 kDa with 31 Ig repeats. Arthropod kettins are splice variants of much larger connectin/titin-related proteins. However, the gene for Ce-kettin is independent of other connectin/titin-related genes. Ce-kettin localizes to the thin filaments near the dense bodies in both striated and nonstriated muscles. The C-terminal four Ig repeats and the adjacent non-Ig region synergistically bind to actin filaments in vitro. RNA interference of Ce-kettin caused weak disorganization of the actin filaments in body wall muscle. This phenotype was suppressed by inhibiting muscle contraction by a myosin mutation, but it was enhanced by tetramisole-induced hypercontraction. Furthermore, Ce-kettin was involved in organizing the cytoplasmic portion of the dense bodies in cooperation with alpha-actinin. These results suggest that kettin is an important regulator of myofibrillar organization and provides mechanical stability to the myofibrils during contraction.

Stretchin-klp, a novel Drosophila indirect flight muscle protein, has both myosin dependent and independent isoforms

Stretchin-klp is a newly described protein in Drosophila indirect flight muscles (IFM) that migrates on SDS gels as two distinct components of approximately 225 and 231 kD. Although the larger isoform is IFM specific, the smaller stretchin-klp isoform is expressed not only in IFM, but also in wild-type tissues of the adult head, abdomen and thorax from which the IFM has been removed. It is not detected, however, in jump or leg muscles. Probes derived from a cDNA encoding part of stretchin-klp hybridize with a 6.7 kb mRNA. Stretchin-klp is one of several putative products of the Stretchin-Myosin light chain kinase gene and is predicted to have multiple immunoglobulin domains arranged in tandem pairs separated by variable length spacers. Polyclonal antibodies directed against the expressed peptide of the stretchin-klp cDNA label the IFM myofibril A-band, though not its central and lateral regions. Analyses of IFM mutants indicate that the larger stretchin-klp isoform is myosin dependent. Although the normal adult myosin filament or the 'headless' myosin rod is sufficient for accumulation of both the large and small stretchin-klp isoforms, loss of myosin, or substitution of the adult rod with an embryonic one in IFM prevents the larger isoform from being formed or stabilized. During development stretchin-klp is first detected at pupal stage p8, when myofibrils are being constructed. These studies suggest that this newly identified protein is a major component of the Drosophila IFM thick filament.

Invertebrate Muscles: Muscle Specific Genes and Proteins

This is the first of a projected series of canonic reviews covering all invertebrate muscle literature prior to 2005 and covers muscle genes and proteins except those involved in excitation-contraction coupling (e.g., the ryanodine receptor) and those forming ligand- and voltage-dependent channels. Two themes are of primary importance. The first is the evolutionary antiquity of muscle proteins. Actin, myosin, and tropomyosin (at least, the presence of other muscle proteins in these organisms has not been examined) exist in muscle-like cells in Radiata, and almost all muscle proteins are present across Bilateria, implying that the first Bilaterian had a complete, or near-complete, complement of present-day muscle proteins. The second is the extraordinary diversity of protein isoforms and genetic mechanisms for producing them. This rich diversity suggests that studying invertebrate muscle proteins and genes can be usefully applied to resolve phylogenetic relationships and to understand protein assembly coevolution. Fully achieving these goals, however, will require examination of a much broader range of species than has been heretofore performed.

Expression of cardiac myosin-binding protein-C (cMyBP-C) in Drosophila as a model for the study of human cardiomyopathies

Mutations in the MYBPC3 gene encoding human cardiac myosin-binding protein-C (cMyBP-C) are associated with familial hypertrophic cardiomyopathy (FHC), but the molecular mechanisms involved are not fully understood. In addition, development of FHC is sensitive to genetic background, and the search for candidate modifier genes is crucial with a view to proposing diagnosis and exploring new therapies. We used Drosophila as the model to investigate the in vivo consequences of human cMyBP-C mutations. We first produced transgenic flies that specifically express human wild-type or two C-terminal truncated cMyBP-Cs in indirect flight muscles (IFM), a tissue particularly amenable to genetic and molecular analyses. First, incorporation of human cMyBP-C into the IFM led to sarcomeric structural abnormalities and to a flightless phenotype aggravated by age and human gene dosage. Second, transcriptome analysis of transgenic IFM using nylon microarrays showed the remodelling of a transcriptional program involving 97 out of 3570 Drosophila genes. Among them, the Calmodulin gene encoding a key component of muscle contraction, found up-regulated in transgenic IFM, was evaluated as a potential modifier gene. Calmodulin mutant alleles rescued the flightless phenotype, and therefore behave as dominant suppressors of the flightless phenotype suggesting that Calmodulin might be a modifier gene in the context of human FHC. In conclusion, we suggest that the combination of heterologous transgenesis and transcriptome analysis in Drosophila could be of great value as a way to glean insights into the molecular mechanisms underlying FHC and to propose potential candidate modifier genes.

Kettin, a major source of myofibrillar stiffness in Drosophila indirect flight muscle

Kettin is a high molecular mass protein of insect muscle that in the sarcomeres binds to actin and alpha-actinin. To investigate kettin's functional role, we combined immunolabeling experiments with mechanical and biochemical studies on indirect flight muscle (IFM) myofibrils of Drosophila melanogaster. Micrographs of stretched IFM sarcomeres labeled with kettin antibodies revealed staining of the Z-disc periphery. After extraction of the kettin-associated actin, the A-band edges were also stained. In contrast, the staining pattern of projectin, another IFM-I-band protein, was not altered by actin removal. Force measurements were performed on single IFM myofibrils to establish the passive length-tension relationship and record passive stiffness. Stiffness decreased within seconds during gelsolin incubation and to a similar degree upon kettin digestion with mu-calpain. Immunoblotting demonstrated the presence of kettin isoforms in normal Drosophila IFM myofibrils and in myofibrils from an actin-null mutant. Dotblot analysis revealed binding of COOH-terminal kettin domains to myosin. We conclude that kettin is attached not only to actin but also to the end of the thick filament. Kettin along with projectin may constitute the elastic filament system of insect IFM and determine the muscle's high stiffness necessary for stretch activation. Possibly, the two proteins modulate myofibrillar stiffness by expressing different size isoforms.