Titin, the largest known protein in the body expressed in three isoforms (N2A, N2BA and N2B), is essential for muscle structure, force generation, conduction and regulation. Since the 1950s, muscle contraction mechanisms have been explained by the sliding filament theory involving thin and thick muscle filaments, while the contribution of cytoskeleton in force generation and conduction was ignored. With the discovery of insoluble protein residues and large molecular weight proteins in muscle fibers, the third myofilament, titin, has been identified and attracted a lot of interests. The development of single molecule mechanics and gene sequencing technology further contributed to the extensive studies on the arrangement, structure, elastic properties and components of titin in sarcomere. Therefore, this paper reviews the structure, isforms classification, elastic function and regulatory factors of titin, to provide better understanding of titin.
Connectin/genetics* ; Muscle Proteins/metabolism* ; Protein Isoforms/genetics* ; Sarcomeres/metabolism* ; Muscle Fibers, Skeletal/metabolism*

AIMTo investigate the molecular mechanisms of smooth muscle 22 alpha (SM22alpha) whereby cytoskeleton remodeling of vascular smooth muscle cells (VSMCs) is regulated.

METHODSSynthetic (dedifferentiated) VSMCs were converted to contractile (differentiated) VSMCs by serum deprivation. Cells were transfected with pEGFP-SM22alpha, and localization of SM22alpha and its relationship with F-actin were observed through fluorescence microscopy. Fractional extraction of proteins and Western blotting were used to detect polymerization of SM alpha-actin in antisense-pcD2-SM22alpha-transfected VSMCs.Furthermore, effect of SM22alpha on F-actin cross-linking was observed by F-actin polymerization experiment.

RESULTSFluorescence microscopy showed that SM22alpha co-localized with F-actin in contractile VSMCs. Western blotting of protein extracts from F-/G-actin fractions revealed that polymerization of SM alpha-actin was lower in antisense-pcD2-SM22alpha-transfected VSMCs, in which SM alpha-actin mostly existed as soluble G-actin. Moreover, F-actin polymerization in vitro also showed that GST-SM22alpha could promote cross-linking of F-actin to form thick and bundled stress fibres,while extracts from VSMCs transfected with antisense-pcD2-SM22alpha could not effectively induce the polymerization of F-actin.

CONCLUSIONSM22alpha acts as a modulator to participate in VSMC cytoskeleton remodeling. It can not only induce polymerization of G-actin to F-actin, but also promote cross-linking of F-actin to bundled stress fibres, indicating its vital role in cytoskeleton remodeling.

Actins ; metabolism ; Animals ; Cytoskeleton ; metabolism ; Male ; Microfilament Proteins ; metabolism ; Muscle Proteins ; metabolism ; Muscle, Smooth, Vascular ; cytology ; metabolism ; Rats ; Rats, Sprague-Dawley

OBJECTIVETo study the effects of insulin on the proteolysis of cultured rabbit skeletal muscular myotubes in vitro, and their possible mechanisms.

METHODSMuscles of lower limbs of juvenile rabbits were isolated for tissue-block culture. After passage, myoblasts were formed and fused into myotubes. Then the protein in myotubes was radiolabelled with L-[ 3,5-3H] tyrosine. The myotubes were cultured in DMEM medium containing 100 nmol/L insulin (n = 24, group B) , 100 nmol/L dexamethasone (n = 24, group C) , 100 nmol/L insulin and 100 nmol/L dexamethasone (n = 24, group D) , no insulin or dexamethasone (n =24, group A), respectively. Twenty-four hours after culture, the L-[3,5-3H] tyrosine content in culture medium and cells were determined, and the degradation rates of protein were calculated. The mRNA expressions of ubiquitin and protease C2 subunit were determined by Northern blot.

RESULTSThe degradation rates of myotube protein in group A(0. 38+/-0.04) was obviously lower than that in group C (0.50+/-0.03, P <0.01), but it was obviously higher than that in group B(0. 35+/-0.03, P <0.05). Though the degradation rates of myotube protein in group D (0.41+/-0. 03) was evidently lower than that in group C ( P < 0.01) , it was still higher than that in group A( P < 0.05 ). The mRNA expressions of ubiquitin and protease C2 subunit in group A ( the scale: 2. 4 kb ubiquitin was 0. 82+/-0. 15, 1. 2 kb ubiquitin was 0. 60+/-0. 10, C2 subunit was 0. 75+/-0. 16) was obviously lower than that in group C ( the scale: 2.4 kb ubiquitin was 2. 15+/-0. 23, 1.2 kb ubiquitin was 1.50+/-0. 14,C2 subunit was 1.50+/-0. 13 , P <0. 01) , but it in group D was lower than that in group C (the scale: 2. 4 kb ubiquitin was 1. 25+/-0. 17, 1. 2 kb ubiquitin was 0. 85+/-0. 09, C2 subunit was 0. 90+/-0. 15, P <0. 01) , and it was similar to that in group B (the scale: 2.4 kb ubiquitin was 0. 85+/-0.07, 1.2 kb ubiquitin was 0. 65+/- 0. 12, C2 subunit was 0. 76 +/-0. 09, P > 0. 05).

CONCLUSIONThe effects of insulin on the activity of ubiquitin-proteasome pathway and the proteolytic rate in normal myotubes were relatively weak. However, insulin can significantly inhibit the effects of dexamethasone on the gene expressions of ubiquitin system and the proteolytic rate in myotubes, but the mechanism needs further research.

Animals ; Cells, Cultured ; In Vitro Techniques ; Insulin ; pharmacology ; Male ; Muscle Fibers, Skeletal ; drug effects ; metabolism ; Muscle Proteins ; metabolism ; Rabbits ; Ubiquitin ; metabolism

Kindlin-2 belongs to a subfamily of FERM domain containing proteins, which plays key roles in activating integrin transmembrane receptors and mediating cell adhesion. Compared to conventional FERM domains, kindlin-2 FERM contains an inserted pleckstrin homology (PH) domain that specifically binds to phosphatidylinositol (3,4,5) trisphosphate (PIP3) and regulates the kindlin-2 function. We have determined the crystal structure of kindlin-2 PH domain at 1.9 Å resolution, which reveals a conserved PH domain fold with a highly charged and open binding pocket for PIP3 head group. Structural comparison with a previously reported solution structure of kindlin-2 PH domain bound to PIP3 head group reveals that upon PIP3 insertion, there is a significant conformational change of both the highly positively charged loop at the entry of the PIP3 binding pocket and the entire β barrel of the PH domain. We propose that such "induced-fit" type change is crucial for the tight binding of PIP3 to anchor kindlin-2 onto the membrane surface, thereby promoting its binding to integrins. Our results provide important structural insight into kindlin-2-mediated membrane anchoring and integrin activation.
Animals ; Crystallography, X-Ray ; Cytoskeletal Proteins ; chemistry ; metabolism ; Humans ; Integrins ; metabolism ; Membrane Proteins ; chemistry ; metabolism ; Mice ; Models, Molecular ; Muscle Proteins ; chemistry ; metabolism ; Neoplasm Proteins ; chemistry ; metabolism ; Protein Conformation

The progressive destruction of condylar cartilage is a hallmark of the temporomandibular joint (TMJ) osteoarthritis (OA); however, its mechanism is incompletely understood. Here, we show that Kindlin-2, a key focal adhesion protein, is strongly detected in cells of mandibular condylar cartilage in mice. We find that genetic ablation of Kindlin-2 in aggrecan-expressing condylar chondrocytes induces multiple spontaneous osteoarthritic lesions, including progressive cartilage loss and deformation, surface fissures, and ectopic cartilage and bone formation in TMJ. Kindlin-2 loss significantly downregulates the expression of aggrecan, Col2a1 and Proteoglycan 4 (Prg4), all anabolic extracellular matrix proteins, and promotes catabolic metabolism in TMJ cartilage by inducing expression of Runx2 and Mmp13 in condylar chondrocytes. Kindlin-2 loss decreases TMJ chondrocyte proliferation in condylar cartilages. Furthermore, Kindlin-2 loss promotes the release of cytochrome c as well as caspase 3 activation, and accelerates chondrocyte apoptosis in vitro and TMJ. Collectively, these findings reveal a crucial role of Kindlin-2 in condylar chondrocytes to maintain TMJ homeostasis.
Aggrecans/metabolism* ; Animals ; Cartilage, Articular/metabolism* ; Chondrocytes/pathology* ; Cytoskeletal Proteins/metabolism* ; Mice ; Muscle Proteins/metabolism* ; Osteoarthritis/pathology* ; Temporomandibular Joint/pathology*

OBJECTIVE: To study the relationship between postmortem interval and the myofibril fragmentation index of skeletal muscle. METHODS: Rabbit skeletal muscle were left in the room temperature for different postmortem intervals, and the protein concentration of each sample was detected by using biuret method. Furthermore, the myofibril fragmentation index of each sample was measured under the protein concentration level of 0.5 mg/mL. RESULTS: The myofibril fragmentation index increased obviously according to the postmortem interval prolongation. CONCLUSION The myofibril fragmentation index may be used on the estimation of early postmortem interval.
Animals ; Calcium-Binding Proteins/metabolism* ; Female ; Male ; Muscle, Skeletal/metabolism* ; Myofibrils/metabolism* ; Postmortem Changes ; Proteins/metabolism* ; Rabbits ; Spectrophotometry ; Time Factors

Disuse atrophy of skeletal muscle is a common clinical problem and its exact mechanisms have not been fully understood. Previous studies suggested that disuse muscle atrophy is realized through the activation of one or more cell signaling pathways, but studies have shown that disuse atrophy is the activation of the ubiquitin-proteasome caused extensive decomposition of the protein. The present researches for disuse atrophy mainly focus on regulatory role in the upstream signaling molecules MuRF1 and Atroginl/MAFbx by NF-kappaB, IGF-1/PI3K/Akt, TGF-beta/Smad and MAPK signal pathway and a plurality of signal pathway activation or inhibition and interaction,and then through the ubiquitin--proteasome to influence the metabolism of protein. But regulation of expression of MuRF1 and Atroginl/MAFbxs still to be studied. Participate in disuse atrophy also needs to be further studied with atrophy confirmation and functional gene verification. The paper summarized recent original articles about the researches of skeletal muscle disuse atrophy and reviewed the various signal pathways and related u-biquitin-proteasome protein metabolism of disuse muscle atrophy.
Animals ; Humans ; Muscle, Skeletal ; metabolism ; Muscular Disorders, Atrophic ; metabolism ; Proteins ; metabolism ; Signal Transduction

OBJECTIVE: To investigate the expression of Calponin-1 and Transgenlin in the uterine smooth muscles during normal labor on-sets, and to evaluate their effect on initiating the normal labor. METHODS: A total of 14 uterine bodies and lower segments of human pregnancy were divided to a non-labor group (NIL) and a labor group(IL). Immunohistochemical technology and Western blot were used to determine the expression of Calponin-1 and Transgelin in the 2 groups. RESULTS: Immunohistochemical detection and Western blot showed that Calponin-1 protein in the uterine smooth muscle tissue of the body and the lower uterine segment of smooth muscle tissues had significant difference (P<0.05). The expression of Transgelin in the uterine body smooth muscle tissue in the IL was higher than that in the NIL(P<0.05). In the lower uterine segments of the smooth muscle, the expression of Transgelin was not significantly different in the 2 groups (P>0.05). CONCLUSION Calponin-1 of the uterine smooth muscle and Transgelin of the uterine body smooth muscle may involve in the regulation of uterine smooth muscle contractility, which is closely related to child birth launch.
Adult ; Calcium-Binding Proteins ; genetics ; metabolism ; Female ; Humans ; Labor Onset ; metabolism ; Microfilament Proteins ; genetics ; metabolism ; Muscle Proteins ; genetics ; metabolism ; Myometrium ; metabolism ; Pregnancy ; Uterine Contraction ; metabolism

OBJECTIVETo explore the relationships between the expression of transgelin in dendritic cells (DCs) pulsed with hepatocellular carcinoma lysates and the functions of the DCs.

METHODSDCs derived from healthy human white blood cells were divided into 3 groups: one was pulsed with high metastatic potential hepatocellular carcinoma cell line (MHCC97H) lysates, one with lysates of a low metastatic potential cell line (MHCC97L), and one un-pulsed DCs served as the control. The morphology of the DCs was observed by confocal microscopy and scanning electron microscopy. The phenotypes of the DCs were detected by flowcytometric analysis. The mixed leucocyte reaction (MLR) test and IL-12 secretion of DCs in the supernatants of MLR were employed to determine the functions of the DCs; the expression of transgelin was detected by Western blot.

RESULTSThere were no morphological changes in the different DCs, but the levels of HLA-DR, CD80, CD83, CD86, MLR and IL-12 and transgelin were significantly higher in the two pulsed groups than those in the control group (P less than 0.01). In MHCC97H pulsed DCs, their CD80, CD83, CD86, and the expression of transgelin were also higher than those in the control group (P less than 0.05). The expression of transgelin was significantly higher in the MHCC97H pulsed group than in the MHCC97L loaded group, but CD80, CD83, CD86 and the level of IL-12 were all lower in the MHCC97H loaded DC group in comparison with those in the MHCC97 pulsed group (P less than 0.05).

CONCLUSIONThe expression of transgelin in DCs pulsed with HCC lysates is related to the functions of the DCs.

Carcinoma, Hepatocellular ; metabolism ; Cell Line, Tumor ; Dendritic Cells ; metabolism ; Humans ; Liver Neoplasms ; metabolism ; Microfilament Proteins ; biosynthesis ; Muscle Proteins ; biosynthesis

AIMTo investigate the interaction between C-terminal domains of SM22alpha and cytoskeleton F-actin.

METHODSProkaryotic expression vector containing SM22alpha cDNA and GST sequence was constructed. The induction conditions were optimized to increase the product of soluble GST-SM22alpha fusion protein in E coli. Expression products were purified and rabbit anti-GST-SM22alpha polyclonal antibody was produced by the purified fusion protein. In order to explore the effect of SM22alpha on cytoskeleton reorganization, VSMCs were treated with serum withdrawal and then serum stimulation to induce contractile/synthetic phenotypic modulation. SM22alpha protein distribution in F-actin/G-actin fractions was detected by Western blotting. The interaction between SM22alpha and actin was examined by GST pull down assay and coimmunoprecipitation. Colocalization of endogenous SM22alpha with F-actin was observed by immunofluorescence.

RESULTSThe results showed that the expression of soluble GST-SM22alpha protein was the highest under condition induced by 30 degrees C, 0.5 mmol/L IPTG for 6 h. Immunofluorescence and Western blotting of protein extracts from F-actin/G-actin fractions revealed that SM22alpha colocalized with F-actin during VSMC redifferentiation. GST pull down assay and coimmunoprecipitation showed that SM22alpha interacted with F-actin by C-terminal domains to participate in cytoskeleton reorganization.

CONCLUSIONThe recombinant SM22alpha C-terminal domains have the ability to bind F-actin, by which SM22alpha interacts with actin and participates in cytoskeleton reorganization.

Actins ; metabolism ; Amino Acid Sequence ; Animals ; Cell Line ; Cytoskeleton ; metabolism ; physiology ; Microfilament Proteins ; genetics ; Molecular Sequence Data ; Muscle Proteins ; genetics ; Muscle, Smooth, Vascular ; cytology ; metabolism ; Myocytes, Smooth Muscle ; metabolism ; Protein Binding ; Rats ; Rats, Sprague-Dawley ; Recombinant Proteins ; genetics