From connecting filaments to co-expression of titin isoforms

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.

Titin visualization in real time reveals an unexpected level of mobility within and between sarcomeres

Contrary to prior models in which titin serves as a stable scaffold in sarcomeres, sarcomeric and soluble titin exchange dynamically in myofibers when calcium levels are low.

The giant muscle protein titin is an essential structural component of the sarcomere. It forms a continuous periodic backbone along the myofiber that provides resistance to mechanical strain. Thus, the titin filament has been regarded as a blueprint for sarcomere assembly and a prerequisite for stability. Here, a novel titin-eGFP knockin mouse provided evidence that sarcomeric titin is more dynamic than previously suggested. To study the mobility of titin in embryonic and neonatal cardiomyocytes, we used fluorescence recovery after photobleaching and investigated the contribution of protein synthesis, contractility, and calcium load to titin motility. Overall, the kinetics of lateral and longitudinal movement of titin-eGFP were similar. Whereas protein synthesis and developmental stage did not alter titin dynamics, there was a strong, inhibitory effect of calcium on titin mobility. Our results suggest a model in which the largely unrestricted movement of titin within and between sarcomeres primarily depends on calcium, suggesting that fortification of the titin filament system is activity dependent.

Mouse intact cardiac myocyte mechanics: cross-bridge and titin-based stress in unactivated cells

A carbon fiber-based cell attachment and force measurement system was used to measure the diastolic stress-sarcomere length (SL) relation of mouse intact cardiomyocytes, before and after the addition of actomyosin inhibitors (2,3-butanedione monoxime [BDM] or blebbistatin). Stress was measured during the diastolic interval of twitching myocytes that were stretched at 100% base length/second. Diastolic stress increased close to linear from 0 at SL 1.85 µm to 4.2 mN/mm(2) at SL 2.1 µm. The actomyosin inhibitors BDM and blebbistatin significantly lowered diastolic stress by ∼1.5 mN/mm(2) (at SL 2.1 µm, ∼30% of total), suggesting that during diastole actomyosin interaction is not fully switched off. To test this further, calcium sensitivity of skinned myocytes was studied under conditions that simulate diastole: 37°C, presence of Dextran T500 to compress the myofilament lattice to the physiological level, and [Ca(2+)] from below to above 100 nM. Mean active stress was significantly increased at [Ca(2+)] > 55 nM (pCa 7.25) and was ∼0.7 mN/mm(2) at 100 nM [Ca(2+)] (pCa 7.0) and ∼1.3 mN/mm(2) at 175 nM Ca(2+) (pCa 6.75). Inhibiting active stress in intact cells attached to carbon fibers at their resting SL and stretching the cells while first measuring restoring stress (pushing outward) and then passive stress (pulling inward) made it possible to determine the passive cell's mechanical slack SL as ∼1.95 µm and the restoring stiffness and passive stiffness of the cells around the slack SL each as ∼17 mN/mm(2)/µm/SL. Comparison between the results of intact and skinned cells shows that titin is the main contributor to restoring stress and passive stress of intact cells, but that under physiological conditions, calcium sensitivity is sufficiently high for actomyosin interaction to contribute to diastolic stress. These findings are relevant for understanding diastolic function and for future studies of diastolic heart failure.

Functional genomics of chicken, mouse, and human titin supports splice diversity as an important mechanism for regulating biomechanics of striated muscle

Titin is a giant filamentous elastic protein that spans from the Z-disk to M-band regions of the sarcomere. The I-band region of titin is extensible and develops passive force in stretched sarcomeres. This force has been implicated as a factor involved in regulating cardiac contraction. To better understand the adaptation in the extensible region of titin, we report the sequence and annotation of the chicken and mouse titin genes and compare them to the human titin gene. Our results reveal a high degree of conservation within the genomic region encoding the A-band segment of titin, consistent with the structural similarity of vertebrate A-bands. In contrast, the genomic region encoding the Z-disk and I-band segments is highly divergent. This is most prominent within the central I-band segment, where chicken titin has fewer but larger PEVK exons (up to 1,992 bp). Furthermore, in mouse titin we found two LINE repeats that are inserted in the Z-disk and I-band regions, the regions that account for most of the splice isoform diversity. Transcript studies show that a group of 55 I-band exons is differentially expressed in chicken titin. Consistent with a large degree of titin isoform plasticity and variation in PEVK content, chicken skeletal titins range in size from approximately 3,000 to approximately 3,700 kDa and vary greatly in passive mechanical properties. Low-angle X-ray diffraction experiments reveal significant differences in myofilament lattice spacing that correlate with titin isoform expression. We conclude that titin splice diversity regulates structure and biomechanics of the sarcomere.

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.

M line-deficient titin causes cardiac lethality through impaired maturation of the sarcomere

Titin, the largest protein known to date, has been linked to sarcomere assembly and function through its elastic adaptor and signaling domains. Titin's M-line region contains a unique kinase domain that has been proposed to regulate sarcomere assembly via its substrate titin cap (T-cap). In this study, we use a titin M line-deficient mouse to show that the initial assembly of the sarcomere does not depend on titin's M-line region or the phosphorylation of T-cap by the titin kinase. Rather, titin's M-line region is required to form a continuous titin filament and to provide mechanical stability of the embryonic sarcomere. Even without titin integrating into the M band, sarcomeres show proper spacing and alignment of Z discs and M bands but fail to grow laterally and ultimately disassemble. The comparison of disassembly in the developing and mature knockout sarcomere suggests diverse functions for titin's M line in embryonic development and the adult heart that not only involve the differential expression of titin isoforms but also of titin-binding proteins.

Obscurin is a ligand for small ankyrin 1 in skeletal muscle

The factors that organize the internal membranes of cells are still poorly understood. We have been addressing this question using striated muscle cells, which have regular arrays of membranes that associate with the contractile apparatus in stereotypic patterns. Here we examine links between contractile structures and the sarcoplasmic reticulum (SR) established by small ankyrin 1 (sAnk1), a approximately 17.5-kDa integral protein of network SR. We used yeast two-hybrid to identify obscurin, a giant Rho-GEF protein, as the major cytoplasmic ligand for sAnk1. The binding of obscurin to the cytoplasmic sequence of sAnk1 is mediated by a sequence of obscurin that is C-terminal to its last Ig-like domain. Binding was confirmed in two in vitro assays. In one, GST-obscurin, bound to glutathione-matrix, specifically adsorbed native sAnk1 from muscle homogenates. In the second, MBP-obscurin bound recombinant GST-sAnk1 in nitrocellulose blots. Kinetic studies using surface plasmon resonance yielded a K(D) = 130 nM. On subcellular fractionation, obscurin was concentrated in the myofibrillar fraction, consistent with its identification as sarcomeric protein. Nevertheless, obscurin, like sAnk1, concentrated around Z-disks and M-lines of striated muscle. Our findings suggest that obscurin binds sAnk1, and are the first to document a specific and direct interaction between proteins of the sarcomere and the SR.

The cardiac mechanical stretch sensor machinery involves a Z disc complex that is defective in a subset of human dilated cardiomyopathy

Muscle cells respond to mechanical stretch stimuli by triggering downstream signals for myocyte growth and survival. The molecular components of the muscle stretch sensor are unknown, and their role in muscle disease is unclear. Here, we present biophysical/biochemical studies in muscle LIM protein (MLP) deficient cardiac muscle that support a selective role for this Z disc protein in mechanical stretch sensing. MLP interacts with and colocalizes with telethonin (T-cap), a titin interacting protein. Further, a human MLP mutation (W4R) associated with dilated cardiomyopathy (DCM) results in a marked defect in T-cap interaction/localization. We propose that a Z disc MLP/T-cap complex is a key component of the in vivo cardiomyocyte stretch sensor machinery, and that defects in the complex can lead to human DCM and associated heart failure.