Binding of connectin to myosin filaments

Titin: roles in cardiac function and diseases

The giant protein titin is an essential component of muscle sarcomeres. A single titin molecule spans half a sarcomere and mediates diverse functions along its length by virtue of its unique domains. The A-band of titin functions as a molecular blueprint that defines the length of the thick filaments, the I-band constitutes a molecular spring that determines cell-based passive stiffness, and various domains, including the Z-disk, I-band, and M-line, serve as scaffolds for stretch-sensing signaling pathways that mediate mechanotransduction. This review aims to discuss recent insights into titin's functional roles and their relationship to cardiac function. The role of titin in heart diseases, such as dilated cardiomyopathy and heart failure with preserved ejection fraction, as well as its potential as a therapeutic target, is also discussed.

Stretching the story of titin and muscle function

The discovery of the giant protein titin, also known as connectin, dates almost half a century back. In this review, I recapitulate major advances in the discovery of the titin filaments and the recognition of their properties and function until today. I briefly discuss how our understanding of the layout and interactions of titin in muscle sarcomeres has evolved and review key facts about the titin sequence at the gene (TTN) and protein levels. I also touch upon properties of titin important for the stability of the contractile units and the assembly and maintenance of sarcomeric proteins. The greater part of my discussion centers around the mechanical function of titin in skeletal muscle. I cover milestones of research on titin's role in stretch-dependent passive tension development, recollect the reasons behind the enormous elastic diversity of titin, and provide an update on the molecular mechanisms of titin elasticity, details of which are emerging even now. I reflect on current knowledge of how muscle fibers behave mechanically if titin stiffness is removed and how titin stiffness can be dynamically regulated, such as by posttranslational modifications or calcium binding. Finally, I highlight novel and exciting, but still controversially discussed, insight into the role titin plays in active tension development, such as length-dependent activation and contraction from longer muscle lengths.

miR-486 is modulated by stretch and increases ventricular growth

Perturbations in biomechanical stimuli during cardiac development contribute to congenital cardiac defects such as hypoplastic left heart syndrome (HLHS). This study sought to identify stretch-responsive pathways involved in cardiac development. miRNA-Seq identified miR-486 as being increased in cardiomyocytes exposed to cyclic stretch in vitro. The right ventricles (RVs) of patients with HLHS experienced increased stretch and had a trend toward higher miR-486 levels. Sheep RVs dilated from excessive pulmonary blood flow had 60% more miR-486 compared with control RVs. The left ventricles of newborn mice treated with miR-486 mimic were 16.9%-24.6% larger and displayed a 2.48-fold increase in cardiomyocyte proliferation. miR-486 treatment decreased FoxO1 and Smad signaling while increasing the protein levels of Stat1. Stat1 associated with Gata-4 and serum response factor (Srf), 2 key cardiac transcription factors with protein levels that increase in response to miR-486. This is the first report to our knowledge of a stretch-responsive miRNA that increases the growth of the ventricle in vivo.

Calcium-dependent titin-thin filament interactions in muscle: observations and theory

Gaps in our understanding of muscle mechanics demonstrate that the current model is incomplete. Increasingly, it appears that a role for titin in active muscle contraction might help to fill these gaps. While such a role for titin is increasingly accepted, the underlying molecular mechanisms remain unclear. The goals of this paper are to review recent studies demonstrating Ca²⁺-dependent interactions between N2A titin and actin in vitro, to explore theoretical predictions of muscle behavior based on this interaction, and to review experimental data related to the predictions. In a recent study, we demonstrated that Ca²⁺ increases the association constant between N2A titin and F-actin; that Ca²⁺ increases rupture forces between N2A titin and F-actin; and that Ca²⁺ and N2A titin reduce sliding velocity of F-actin and reconstituted thin filaments in motility assays. Preliminary data support a role for Ig83, but other Ig domains in the N2A region may also be involved. Two mechanical consequences are inescapable if N2A titin binds to thin filaments in active muscle sarcomeres: (1) the length of titin's freely extensible I-band should decrease upon muscle activation; and (2) binding between N2A titin and thin filaments should increase titin stiffness in active muscle. Experimental observations demonstrate that these properties characterize wild type muscles, but not muscles from mdm mice with a small deletion in N2A titin, including part of Ig83. Given the new in vitro evidence for Ca²⁺-dependent binding between N2A titin and actin, it is time for skepticism to give way to further investigation.

Topology of interaction between titin and myosin thick filaments

Titin is a giant protein spanning between the Z- and M-lines of the sarcomere. In the A-band titin is associated with the myosin thick filament. It has been speculated that titin may serve as a blueprint for thick-filament formation due to the super-repeat structure of its A-band domains. Accordingly, titin might provide a template that determines the length and structural periodicity of the thick filament. Here we tested the titin ruler hypothesis by mixing titin and myosin at in situ stoichiometric ratios (300 myosins per 12 titins) in buffers of different ionic strength (KCl concentration range 100-300 mM). The topology of the filamentous complexes was investigated with atomic force microscopy. We found that the samples contained distinct, segregated populations of titin molecules and myosin thick filaments. We were unable to identify complexes in which myosin molecules were regularly associated to either mono- or oligomeric titin in either relaxed or stretched states of the titin filaments. Thus, the electrostatically driven self-association is stronger in both myosin and titin than their binding to each other, and it is unlikely that titin functions as a geometrical template for thick-filament formation. However, when allowed to equilibrate configurationally, long myosin thick filaments appeared with titin oligomers attached to their surface. The titin meshwork formed on the thick-filament surface may play a role in controlling thick-filament length by regulating the structural dynamics of myosin molecules and placing a mechanical limit on the filament length.

Differences in titin segmental elongation between passive and active stretch in skeletal muscle

Since the 1950s, muscle contraction has been explained using a two-filament system in which actin and myosin exclusively dictate active force in muscle sarcomeres. Decades later, a third filament called titin was discovered. This titin filament has recently been identified as an important regulator of active force, but has yet to be incorporated into contemporary theories of muscle contraction. When sarcomeres are actively stretched, a substantial and rapid increase in force occurs, which has been suggested to arise in part from titin-actin binding that is absent in passively stretched sarcomeres. However, there is currently no direct evidence for such binding within muscle sarcomeres. Therefore, we aimed to determine whether titin binds to actin in actively but not in passively stretched sarcomeres by observing length changes of proximal and distal titin segments in the presence and absence of calcium. We labeled I-band titin with fluorescent F146 antibody in rabbit psoas myofibrils and tracked segmental elongations during passive (no calcium) and active (high calcium) stretch. Without calcium, proximal and distal segments of titin elongated as expected based on their free spring properties. In contrast, active stretch differed statistically from passive stretch, demonstrating that calcium activation increases titin segment stiffness, but not in an actin-dependent manner. The consistent elongation of the proximal segment was contrary to what was expected if titin's proximal segment was attached to actin. This rapid calcium-dependent change in titin stiffness likely contributes to active muscle force regulation in addition to actin and myosin.

Method for isolation of intact titin (connectin) molecules from mammalian cardiac muscle

Cardiac titin was isolated from rabbit and ground squirrel ventricular muscles by a method that was used earlier to obtain myofibrils with intact minor proteins located in A-bands of sarcomeres (Podlubnaya, Z. A., et al. (1989) J. Mol. Biol., 210, 655-658). Small pieces of cardiac muscle were incubated for 2-3 weeks at 4°C in Ca²⁺-depleting solution before their homogenization to decrease activity of Ca²⁺-dependent proteases. Then the muscle was homogenized, and titin was isolated by the method of Soteriou, A., et al. (1993) J. Cell Sci., 14, 119-123. In control experiments, titin was isolated from cardiac muscle without its preincubation in Ca²⁺-depleting solution. Sometimes control titin preparations contained only T2-fragment, but generally they contained ~5-20% N2B-isoform of titin along with its T2-fragment. Preparations of titin obtained from rabbit cardiac muscle by our method contained ~30-50% of N2BA- and N2B-titin isoforms along with its T2-fragment. The content of α-structures in titin isolated by our method was increased. Actomyosin ATPase activity in vitro increased in the presence of titin preparations containing more intact molecules. This result confirms the significant role of titin in the regulation of actin-myosin interaction in muscles. The method used by us to preserve titin might be used for isolation of other proteins that are substrates of Ca²⁺-dependent proteases.

Multiple antibodies to titin immunoreact with AHNAK and localize to the mitotic spindle machinery

Recently, the large filamentous striated-muscle protein titin has been observed in non-muscle cells, and, in one instance, has been proposed to have a nuclear function as a chromosomal component contributing to structure and elasticity. In this study, we sought to further characterize the presumptive nuclear isoform of titin. Immunofluorescence microscopy with multiple titin-specific monoclonal antibodies shows localization to the nucleus in interphase cells and to the spindle machinery in mitotic cells in all cell types examined; localization to condensed chromosomes is not observed. An abundant 700-kDa phosphoprotein is the predominant species immunoprecipitated with these antibodies. Sequencing of peptide fragments of the immunopurified protein reveals identity to AHNAK, a nuclear phosphoprotein, an identification that was confirmed by Western blot analysis with antibodies to AHNAK and peptide fragmentation patterns. Sequence comparison suggests similarities between the repetitive heptad phi+/-phiP+/-phi+/- motif in AHNAK and the PEVK region of titin, potentially explaining the cross-reactivity observed between AHNAK antibodies and titin antibodies. Interestingly, although some AHNAK antibodies stain interphase nuclei, no evidence of mitotic spindle localization is seen, suggesting that the identity of the protein at the latter location is more closely related to titin than AHNAK. This concept is further supported by observations that cell lines not expressing AHNAK have similar antititin antibody localization to the mitotic spindle. We conclude that (1) multiple titin antibodies, particularly those recognizing the PEVK region, cross-react with AHNAK, and (2) the mitotic spindle staining observed with antititin antibodies is most likely due to the association of titin or a titin-like molecule with this structure.

Calcium-dependent inhibition of in vitro thin-filament motility by native titin

Titin ( also known as connectin) is a giant filamentous protein that spans the distance between the Z- and M-lines of the vertebrate muscle sarcomere and plays a fundamental role in the generation of passive tension. Titin has been shown to bind strongly to myosin, making it tightly associated to the thick filament in the sarcomere. Recent observations have suggested the possibility that titin also interacts with actin, implying further functions of titin in muscle contraction. We show -- using in vitro motility and binding assays -- that native titin interacts with both filamentous actin and reconstituted thin filaments. The interaction results in the inhibition of the filaments' in vitro motility. Furthermore, the titin-thin filament interaction occurs in a calcium-dependent manner: increased calcium results in enhanced binding of thin filaments to titin and greater suppression of in vitro motility.

Cellular titin localization in stress fibers and interaction with myosin II filaments in vitro

We previously discovered a cellular isoform of titin (originally named T-protein) colocalized with myosin II in the terminal web domain of the chicken intestinal epithelial cell brush border cytoskeleton (Eilertsen, K.J., and T.C.S. Keller. 1992. J. Cell Biol. 119:549-557). Here, we demonstrate that cellular titin also colocalizes with myosin II filaments in stress fibers and organizes a similar array of myosin II filaments in vitro. To investigate interactions between cellular titin and myosin in vitro, we purified both proteins from isolated intestinal epithelial cell brush borders by a combination of gel filtration and hydroxyapatite column chromatography. Electron microscopy of brush border myosin bipolar filaments assembled in the presence and absence of cellular titin revealed a cellular titin-dependent side-by-side and end-to-end alignment of the filaments into highly ordered arrays. Immunogold labeling confirmed cellular titin association with the filament arrays. Under similar assembly conditions, purified chicken pectoralis muscle titin formed much less regular aggregates of muscle myosin bipolar filaments. Sucrose density gradient analyses of both cellular and muscle titin-myosin supramolecular arrays demonstrated that the cellular titin and myosin isoforms coassembled with a myosin/titin ratio of approximately 25:1, whereas the muscle isoforms coassembled with a myosin:titin ratio of approximately 38:1. No coassembly aggregates were found when cellular myosin was assembled in the presence of muscle titin or when muscle myosin was assembled in the presence of cellular titin. Our results demonstrate that cellular titin can organize an isoform-specific association of myosin II bipolar filaments and support the possibility that cellular titin is a key organizing component of the brush border and other myosin II-containing cytoskeletal structures including stress fibers.

Connectin, an elastic protein of striated muscle

Connectin, also called titin, a giant elastic protein of striated muscle (approximately 3000 kDa) mainly consists of fibronectin type III and immunoglobulin C2 globular domains, the beta-sheets of which are parallel to the main axis of the molecule. One connectin molecule runs through the I band and binds onto the myosin filament up to the M line starting from the Z line. It positions the myosin filament at the center of a sarcomere. Connectin is also responsible for resting tension generation. Biodiversity of the connectin family exists in invertebrate muscle.

A survey of interactions made by the giant protein titin

A simple solid-phase binding assay was used to screen for interactions that the giant myofibrillar protein titin makes with other sarcomeric proteins. The titin used in the tests was purified by a modified procedure that results in isolation of approximately 20 mg relatively undegraded protein in < 24 h. In addition to the approximately 3 MDa polypeptide, bands at approximately 160 kDa and approximately 100 kDa were also consistently seen on gels. Binding of titin to myosin, C-protein, X-protein and AMP-deaminase was observed. The interaction with myosin appears to be with the light meromyosin part of the molecule.

Understanding the functions of titin and nebulin

Individual molecules of the giant muscle proteins titin and nebulin span large distances in the sarcomere. Approximately one-third of the titin molecule forms elastic filaments linking the ends of thick filaments to the Z-line. The remainder of the molecule is probably bound to the thick filament where it may regulate assembly of myosin and the other thick filament proteins. This region also contains a sequence similar to catalytic domains in protein kinases. Nebulin appears to be associated with thin filaments and may regulate actin assembly.

Characterization and localization of alpha-connectin (titin 1): an elastic protein isolated from rabbit skeletal muscle

A simplified procedure to isolate alpha-connectin (titin 1, TI), a gigantic elastic protein, from rabbit skeletal muscle is described. A rapid column chromatography step to concentrate alpha-connectin is introduced. Separation of alpha-connectin from beta-connectin is introduced. Separation of alpha-connectin from beta-connectin (titin 2, TII) in the presence of 4 M urea at pH 7.0 did not cause any change in the secondary structure of alpha-connectin as judged by circular dichroic spectra. Ultraviolet absorption spectra and the amino acid composition of alpha-connectin (MW, approximately 3 x 10(6)) were similar to those of its proteolytic product, beta-connectin (MW, approximately 2 x 10(6)). Circular dichroic spectra suggested that both alpha- and beta-connectin consist of 60% beta-sheet and 30% beta-turn. It thus appears that the whole elastic filament of connectin has a folded beta-strand structure. Proteolysis of alpha-connectin by calpain resulted in formation of beta-connectin and smaller peptides. The alpha-connectin interacted with both myosin and actin filaments similarly to beta-connectin. Polyclonal antibodies raised against 1200 kDa peptides obtained from aged rabbit skeletal myofibrils reacted with alpha-connectin (titin 1, TI) but only weakly with beta-connectin (titin 2, TII) in rabbit skeletal muscle. Immunoelectron microscopy and indirect immunofluorescence microscopy revealed that the antibodies bound at the Z-line and at the epitope regions in the I-band near the binding site of a monoclonal antibody SM1 whose position depends on sarcomere length. It thus appears that beta-connectin extends from the edge of M-line to the above epitope region in the I-band.

Projectin is an invertebrate connectin (titin): isolation from crayfish claw muscle and localization in crayfish claw muscle and insect flight muscle

A filamentous protein was isolated from crayfish claw muscle. This protein had physiochemical properties very similar to vertebrate skeletal muscle connectin (titin), although its apparent molecular mass (approximately 1200 kDa) was considerably lower than that of connectin (approximately 3000 kDa). Polyclonal as well as monoclonal antibodies against chicken skeletal muscle connectin reacted with the 1200 kDa protein from crayfish claw muscle. Conversely, polyclonal antibodies against crayfish 1200 kDa protein cross-reacted with chicken connectin. Circular dichroic spectra indicated the abundance of beta-sheet structure (approximately 60%). Low-angle shadowed images showed filamentous structures (0.2-0.5 microns) by electron microscopy. Proteolysis of the 1200 kDa protein by alpha-chymotrypsin or V8 protease rapidly resulted in formation of 1000 kDa or 1100 and 800 kDa peptides. The amino acid composition was very similar to those of vertebrate connectins and of honeybee flight muscle projectin. Based on the molecular weight and amino acid composition, the 1200 kDa protein is regarded to be crayfish projectin. Immunofluorescence and immunoelectron microscopy revealed that crayfish projectin was localized in the A/I junction area and A-band except for its centre region in crayfish claw muscles. Polyclonal antibodies against crayfish claw muscle projectin reacted with 1200 kDa projectin of honeybee and beetle flight muscle. A monoclonal antibody against chicken skeletal muscle connectin also reacted with honeybee and beetle projectin. Immunoelectron microscopic observations revealed that anti-crayfish projectin antibodies bound the connecting filaments linking the Z-line and the thick filaments up to the M-line of honeybee muscle sarcomere. Anti-crayfish projectin antibodies bound the I-band region near the Z-line of beetle flight muscle. It is concluded that the 1200 kDa projectin from crayfish claw muscle is an invertebrate connectin (titin). Recent work with locust flight muscle mini-titin (Nave & Weber, 1990) is in good agreement with the present study, except that the isolated mini-titin estimated as 600 kDa appears to be a proteolytic product (approximately 1100 kDa) of the parent molecule (approximately 1200 kDa).