Disease relevance of Ttn

• During ischemia the cardiac stress protein, alpha B-crystallin, was shown by immunoelectron microscopy to translocate to the N(2)-line area of myofibrillar I-bands of rat cardiomyocytes where alpha B-crystallin resisted extraction with 1 m NaSCN and 2 m urea as did titin [1].
• RESULTS: Regression analysis of titin isoform ratios, myosin heavy chain isoform ratios, and heart weight to body weight ratios all suggest a smaller proportion of N2BA titin (longer isoform) was expressed in rat left ventricles with increased hypertrophy [2].
• These findings suggest that changes in titin play a role in the change in myocardial functional characteristics and may be one of the causes of diabetic cardiomyopathy [3].

High impact information on Ttn

• An interaction between the C-terminal domain of alpha-actinin and titin Z-repeat motifs targets alpha-actinin to the Z-disk [4].
• A unique sequence within the giant titin molecule, the PEVK domain, has been suggested to greatly contribute to passive force development of relaxed skeletal muscle during stretch [5].
• Enthalpic contributions are likely to arise from electrostatic stiffening, as evidenced by the ionic-strength dependency of titin-based myofibril stiffness; at high stretch, hydrophobic effects also might become relevant [5].
• Gel electrophoresis showed that this treatment had degraded titin without clearly affecting other proteins, and immunoelectron microscopy revealed that the elastic segment of titin in the I band was missing from the sarcomere [6].
• Restoring force of myocytes was found to be depressed after titin had been degraded with trypsin [6].

Biological context of Ttn

• Proteolysis of the spring-like domain of titin may predispose cardiomyocytes to diastolic dysfunction, myofilament instability, and cell death by necrosis [7].
• Co-treatment with calpain inhibitors resulted in preservation of titin, reduction in myofibrillar disarray, and attenuation of cardiomyocyte necrosis but not apoptosis [7].
• The extra length of the fetal titin isoforms appeared to be due to both a greater number of middle Ig domains expressed plus the inclusion of more PEVK exons [8].
• Developmental changes in the alternative splicing patterns of titin were observed in rat cardiac muscle [8].
• Our work suggests that at the level of the whole heart, part of the actomyosin-based active force that is developed during systole is harnessed by titin, allowing for elastic diastolic recoil and aiding in ventricular filling [6].

Anatomical context of Ttn

• Titin, the largest myofilament protein, serves as a template for sarcomere assembly and acts as a molecular spring to contribute to diastolic function [7].
• Antibodies raised against the genetically expressed titin motifs specifically recognized intact rat cardiac and skeletal muscle titins in Western blotting and immunofluorescence microscopy, confirming the authenticity of the cloned fragments [9].
• Treatment of cultured adult rat cardiomyocytes with 1 or 3 micromol/liter doxorubicin for 24 h resulted in degradation of titin in myocyte lysates, which was confirmed by a reduction in immunostaining of an antibody to the spring-like (PEVK) domain of titin at the I-band of the sarcomere [7].
• Binding of alpha B-crystallin to titin during cardiac ischemia could serve to stabilize titin against denaturation and might provide an endogenous mechanism to delay ischemic damage of this important elastic component of myofibrils [1].
• Titin isoform changes in rat myocardium during development [8].

Associations of Ttn with chemical compounds

• We hypothesized that titin degradation is an early event in doxorubicin-induced cardiac injury and that titin degradation occurs by activation of the calcium-dependent proteases, the calpains [7].
• This represents the basis for experiments interfering with sarcomere formation using three different antisense phosphorothioate oligonucleotides (S-ODN) at a dosage of 10 microM specific for titin mRNA [10].
• However, expression of an exogenously supplied MyoD gene in aortic smooth muscle cells cultured from newborn and adult animals converts these cells to elongated myoblasts and myotubes expressing the skeletal muscle genes for titin, nebulin, myosin, and skeletal alpha-actin [11].
• The assembly of myosin, titin, and actin into sarcomeric bands, as well as X22, is inhibited by chloroquine [12].
• The elastic portion of titin comprises two distinct structural motifs, immunoglobulin (Ig) domains and the PEVK titin, which is a novel motif family rich in proline, glutamate, valine and lysine residues [13].

Other interactions of Ttn

• We conclude that calpain activation is an early event after doxorubicin treatment in cardiomyocytes and appears to target the degradation of titin [7].
• The sequence of titin consists mainly of repeats of two types of approximately 100-amino acid motifs (class I and class II that show homology to the fibronectin type III and immunoglobulin-C2 domains, respectively) [9].
• Co-treatment with a caspase inhibitor did not prevent the degradation of titin, which precludes caspase-3 as an early mechanism of titin proteolysis [7].
• In muscle, titin acts as a molecular ruler organizing the actin cytoskeleton via interactions with many sarcomeric proteins, including the crosslinking protein alpha-actinin [4].
• Using double immunofluorescent labeling, the localization of these two triadins was studied in comparison to well-characterized proteins such as ryanodine receptor, calsequestrin, desmin, Ca(2+)-ATPase, and titin [14].

Analytical, diagnostic and therapeutic context of Ttn

• At days 12 and 16 in culture, antisense S-ODN treatment resulted in reduced expression of titin and disturbance of myosin incorporation into sarcomeres, evident by diffuse myosin labeling and a significantly decreased area of regular myosin cross-striation (control 75%, day 12 S-ODN 20%, day 16 14%) shown by laser scanning confocal microscopy [10].
• Agarose gel electrophoresis demonstrated two different N2BA titin isoforms in all rat ventricles [2].
• Gel electrophoresis and scanning densitometry were performed to determine ratios of myosin heavy chain and titin isoforms expressed [2].
• In situ hybridization using a cRNA probe specific for N2BA titin and a positive control in the N2B unique region was used to demonstrate tissue location of the titin message [2].
• With increasing denervation time, we found a greater loss of titin than myosin heavy chain (MHC) and actin contents in atrophic TA muscle [15].

References