Disease relevance of Ttn

• Chromosomal localization of the mouse titin gene and its relation to "muscular dystrophy with myositis" and nebulin genes on chromosome 2 [1].
• Thus, the altered composition of the titin N2A complex is specific for the titin-based skeletal muscular dystrophy in MDM [2].
• Muscle atrophy in Titin M-line deficient mice [3].
• Our data demonstrate an important role for MEx1 and MEx2 in early cardiac development (embryonic lethality) as well as postnatally when disruption of M-line titin leads to muscle weakness and death at approximately 5 weeks of age [4].
• Thus scaffold formation, axon pathfinding and fasciculation involve specific contacts between connectin-positive cells [5].
• Functionally, p94-N2A interactions suppress p94 autolysis and protected connectin from proteolysis [6].

High impact information on Ttn

• Troponin and tropomyosin isoforms determine the variable sensitivity to calcium, whereas titin isoforms dictate the elastic properties of muscle fibers at rest [7].
• Additional isoforms, including products of tropomyosin, myosin light chain 1 fast, troponin T, titin, and nebulin genes, can be generated from the same gene through alternative splicing or use of alternative promoters [7].
• 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 [8].
• 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 [8].
• These reveal a periodic pattern of myosin, nebulin, and those titin epitopes known to occur at and close to the Z line [9].

Biological context of Ttn

• The gene order (distances in cM) cenVim-16.9 +/- 4.7-Neb-7.6 +/- 3.0-Ttn, Acra-18.0 +/- 4.9-Pax-6-17.7 +/- 4.9-a ... has been determined [1].
• Myogenesis in the mouse embryo: differential onset of expression of myogenic proteins and the involvement of titin in myofibril assembly [9].
• 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 [8].
• We investigated the response to deletion of the titin M-line region in striated muscle, using a titin knockout model and a range of techniques that include histology, in situ hybridization, electron microscopy, and 2D gel analysis [3].
• The mutant protein was designed to have a similar size as the skeletal MyBP-C isoforms, whereas known myosin and titin binding sites as well as the phosphorylatable MyBP-C motif were not altered [10].

Anatomical context of Ttn

• Our results indicate that the putative elastic titin filaments act as integrators during skeletal muscle development [9].
• In contrast titin epitopes, which are present in mature myofibrils along the A band and at the A-I junction, are still randomly distributed [9].
• After fusion of myoblasts to myotubes at g.d. 13 and 14 all titin epitopes show the myofibrillar banding pattern [9].
• This alternative spliced form of MyBP-C(+) has a markedly decreased binding affinity to myosin filaments and connectin/titin in vitro and does not localize to A-bands in cardiac myocytes [11].
• We investigated the effects of diabetes, physical training, and their combination on the gene expression of proteins of putative titin stretch-sensing complexes in skeletal and cardiac muscle [12].

Associations of Ttn with chemical compounds

• Another unique feature of titin is the presence of a serine/threonine kinase-like domain at the edge of the M-line region of the sarcomere, for which no physiological catalytic function has yet been shown [4].
• Effects of streptozotocin-induced diabetes and physical training on gene expression of titin-based stretch-sensing complexes in mouse striated muscle [12].
• The gene product, connectin, is a member of the leucine-rich repeat protein family and we show that it is attached to the cell surface via a glycosylphosphatidylinositol linkage and that it can mediate homotypic cell-cell adhesion in vitro [5].
• Here we show by low-percentage polyacrylamide-gel electrophoresis that developmentally regulated switching of cardiac titin/connectin size occurs in the hearts of mouse, rat, pig, and chicken [13].

Physical interactions of Ttn

• Thus, the composition of the titin N2A protein complex is altered in MDM by incorporation of CARP and loss of p94/calpain-3 [2].

Regulatory relationships of Ttn

• To investigate the role of IGHMBP2 in the pathogenesis of DCM, we generated transgenic mice expressing the full-length Ighmbp2 cDNA specifically in myocytes under the control of the mouse titin promoter [14].
• We tested whether titin-based passive tension influences the interfilament lattice spacing by measuring the width of the myocyte and by using small-angle x-ray diffraction of mouse left ventricular wall muscle [15].

Other interactions of Ttn

• Passive stiffness changes in soleus muscles from desmin knockout mice are not due to titin modifications [16].
• Induction and myofibrillar targeting of CARP, and suppression of the Nkx2.5 pathway in the MDM mouse with impaired titin-based signaling [2].
• In addition to the structural importance of the titin M-line region in any striated muscle, our data show how differences in M-line composition between heart and skeletal muscle affect sarcomere stability and function [3].
• Herein, we are reporting a novel alternative spliced form of cardiac MyBP-C, MyBP-C(+), which includes an extra 30 nucleotides, encoding 10 amino acids in the carboxyl-terminal connectin/titin binding region [11].
• One of these mutants carries a stop codon mutation in dystrophin, and we have recently identified another carrying a mutation in titin [17].

Analytical, diagnostic and therapeutic context of Ttn

• This is the first demonstration that a mutation in Ttn is associated with muscular dystrophy and provides a novel animal model to test for functional interactions between TTN and CAPN3 [18].
• We used a gene-targeting approach in cultured myoblasts to truncate the titin gene so that the kinase domain and other domains downstream of the kinase were not expressed [19].
• Immunofluorescence staining of postmitotic mononuclear myoblasts indicate that the investigated epitopes of the titin molecule are displayed in a punctate pattern with neighboring, but clearly separate spots in the cytoplasm of the cells [20].
• In this work, immunoelectron microscopy was used to study the molecular origin of the in vivo extensibility of the I-band region of cardiac titin [21].
• This difference from wildtype controls (+/+ or +/mdm) is also reflected in the yield of mononucleate myogenic cells upon dissociation of skeletal muscle and subsequent cell culture [22].

References