An actin filament population defined by the tropomyosin Tpm3.1 regulates glucose uptake.

Actin has an ill-defined role in the trafficking of GLUT4 glucose transporter vesicles to the plasma membrane (PM). We have identified novel actin filaments defined by the tropomyosin Tpm3.1 at glucose uptake sites in white adipose tissue (WAT) and skeletal muscle. In Tpm 3.1-overexpressing mice, insulin-stimulated glucose uptake was increased; while Tpm3.1-null mice they were more sensitive to the impact of high-fat diet on glucose uptake. Inhibition of Tpm3.1 function in 3T3-L1 adipocytes abrogates insulin-stimulated GLUT4 translocation and glucose uptake. In WAT, the amount of filamentous actin is determined by Tpm3.1 levels and is paralleled by changes in exocyst component (sec8) and Myo1c levels. In adipocytes, Tpm3.1 localizes with MyoIIA, but not Myo1c, and it inhibits Myo1c binding to actin. We propose that Tpm3.1 determines the amount of cortical actin that can engage MyoIIA and generate contractile force, and in parallel limits the interaction of Myo1c with actin filaments. The balance between these actin filament populations may determine the efficiency of movement and/or fusion of GLUT4 vesicles with the PM.

Hypertrophy and dietary tyrosine ameliorate the phenotypes of a mouse model of severe nemaline myopathy.

Nemaline myopathy, the most common congenital myopathy, is caused by mutations in genes encoding thin filament and thin filament-associated proteins in skeletal muscles. Severely affected patients fail to survive beyond the first year of life due to severe muscle weakness. There are no specific therapies to combat this muscle weakness. We have generated the first knock-in mouse model for severe nemaline myopathy by replacing a normal allele of the α-skeletal actin gene with a mutated form (H40Y), which causes severe nemaline myopathy in humans. The Acta1(H40Y) mouse has severe muscle weakness manifested as shortened lifespan, significant forearm and isolated muscle weakness and decreased mobility. Muscle pathologies present in the human patients (e.g. nemaline rods, fibre atrophy and increase in slow fibres) were detected in the Acta1(H40Y) mouse, indicating that it is an excellent model for severe nemaline myopathy. Mating of the Acta1(H40Y) mouse with hypertrophic four and a half LIM domains protein 1 and insulin-like growth factor-1 transgenic mice models increased forearm strength and mobility, and decreased nemaline pathologies. Dietary L-tyrosine supplements also alleviated the mobility deficit and decreased the chronic repair and nemaline rod pathologies. These results suggest that L-tyrosine may be an effective treatment for muscle weakness and immobility in nemaline myopathy.

An Actn3 knockout mouse provides mechanistic insights into the association between alpha-actinin-3 deficiency and human athletic performance.

A common nonsense polymorphism (R577X) in the ACTN3 gene results in complete deficiency of the fast skeletal muscle fiber protein alpha-actinin-3 in an estimated one billion humans worldwide. The XX null genotype is under-represented in elite sprint athletes, associated with reduced muscle strength and sprint performance in non-athletes, and is over-represented in endurance athletes, suggesting that alpha-actinin-3 deficiency increases muscle endurance at the cost of power generation. Here we report that muscle from Actn3 knockout mice displays reduced force generation, consistent with results from human association studies. Detailed analysis of knockout mouse muscle reveals reduced fast fiber diameter, increased activity of multiple enzymes in the aerobic metabolic pathway, altered contractile properties, and enhanced recovery from fatigue, suggesting a shift in the properties of fast fibers towards those characteristic of slow fibers. These findings provide the first mechanistic explanation for the reported associations between R577X and human athletic performance and muscle function.

Tropomyosin isoforms: divining rods for actin cytoskeleton function.

Actin filament functional diversity is paralleled by variation in the composition of isoforms of tropomyosin in these filaments. Although the role of tropomyosin is well understood in skeletal muscle, where it regulates the actin-myosin interaction, its role in the cytoskeleton has been obscure. The intracellular sorting of tropomyosin isoforms indicated a role in spatial specialization of actin filament function. Genetic manipulation and protein chemistry studies have confirmed that these isoforms are functionally distinct. Tropomyosins differ in their recruitment of myosin motors and their interaction with actin filament regulators such as ADF-cofilin. Tropomyosin isoforms have therefore provided a powerful mechanism to diversify actin filament function in different intracellular compartments.

A cytoskeletal tropomyosin can compromise the structural integrity of skeletal muscle.

We have identified a number of extra-sarcomeric actin filaments defined by cytoskeletal tropomyosin (Tm) isoforms. Expression of a cytoskeletal Tm (Tm3) not normally present in skeletal muscle in a transgenic mouse resulted in muscular dystrophy. In the present report we show that muscle pathology in this mouse is late onset (between 2 and 6 months of age) and is predominately in the back and paraspinal muscles. In the Tm3 mice, Evans blue dye uptake in muscle and serum levels of creatine kinase were markedly increased following downhill exercise, and the force drop following a series of lengthening contractions in isolated muscles (extensor digitorum longus) was also significantly increased in these mice. These results demonstrate that expression of an inappropriate Tm in skeletal muscle results in increased susceptibility to contraction-induced damage. The extra-sarcomeric actin cytoskeleton therefore may have an important role in protecting the muscle from contractile stress.

Diverse roles of the actin cytoskeleton in striated muscle.

In addition to the highly specialized contractile apparatus, it is becoming increasingly clear that there is an extensive actin cytoskeleton which underpins a wide range of functions in striated muscle. Isoforms of cytoskeletal actin and actin-associated proteins (non-muscle myosins, cytoskeletal tropomyosins, and cytoskeletal alpha-actinins) have been detected in a number of regions of striated muscle: the sub-sarcolemmal costamere, the Z-disc and the T-tubule/sarcoplasmic reticulum membranes. As the only known function of these proteins is through association with actin filaments, their presence in striated muscles indicates that there are spatially and functionally distinct cytoskeletal actin filament systems in these tissues. These filaments are likely to have important roles in mechanical support, ion channel function, myofibrillogenenous and vesicle trafficking.

Sorting of a nonmuscle tropomyosin to a novel cytoskeletal compartment in skeletal muscle results in muscular dystrophy.

Tropomyosin (Tm) is a key component of the actin cytoskeleton and >40 isoforms have been described in mammals. In addition to the isoforms in the sarcomere, we now report the existence of two nonsarcomeric (NS) isoforms in skeletal muscle. These isoforms are excluded from the thin filament of the sarcomere and are localized to a novel Z-line adjacent structure. Immunostained cross sections indicate that one Tm defines a Z-line adjacent structure common to all myofibers, whereas the second Tm defines a spatially distinct structure unique to muscles that undergo chronic or repetitive contractions. When a Tm (Tm3) that is normally absent from muscle was expressed in mice it became associated with the Z-line adjacent structure. These mice display a muscular dystrophy and ragged-red fiber phenotype, suggestive of disruption of the membrane-associated cytoskeletal network. Our findings raise the possibility that mutations in these tropomyosin and these structures may underpin these types of myopathies.

Tropomyosins in skeletal muscle diseases.

A number of congenital muscle diseases and disorders are caused by mutations in genes that encode the proteins present in or associated with the thin filaments of the muscle sarcomere. These genes include alpha-skeletal actin (ACTA1), beta-tropomyosin (TPM2), alpha-tropomyosin slow (TPM3), nebulin (NEB), troponin I fast (TNNI2), troponin T slow (TNNT1), troponin T fast (TNNT3) and cofilin (CFL2). Mutations in two of the four tropomyosin (Tm) genes, TPM2 and TPM3, result in at least three different skeletal muscle diseases and one disorder as distinguished by the presence of specific clinical features and/or structural abnormalities--nemaline myopathy (TPM2 and TPM3), distal arthrogryposis (TPM2), cap disease (TPM2) and congenital fiber type disproportion (TPM3). These diseases have overlapping clinical features and pathologies and there are cases of family members who have the same mutation, but different diseases (Table 1). The relatively recent discovery of nonmuscle or cytoskeletal Tms in skeletal muscle adds to this complexity since it is now possible that a disease-causing mutation could be in a striated isoform and a cytoskeletal isoform both present in muscle.

On-target action of anti-tropomyosin drugs regulates glucose metabolism.

The development of novel small molecule inhibitors of the cancer-associated tropomyosin 3.1 (Tpm3.1) provides the ability to examine the metabolic function of specific actin filament populations. We have determined the ability of these anti-Tpm (ATM) compounds to regulate glucose metabolism in mice. Acute treatment (1 h) of wild-type (WT) mice with the compounds (TR100 and ATM1001) led to a decrease in glucose clearance due mainly to suppression of glucose-stimulated insulin secretion (GSIS) from the pancreatic islets. The impact of the drugs on GSIS was significantly less in Tpm3.1 knock out (KO) mice indicating that the drug action is on-target. Experiments in MIN6 ß-cells indicated that the inhibition of GSIS by the drugs was due to disruption to the cortical actin cytoskeleton. The impact of the drugs on insulin-stimulated glucose uptake (ISGU) was also examined in skeletal muscle ex vivo. In the absence of drug, ISGU was decreased in KO compared to WT muscle, confirming a role of Tpm3.1 in glucose uptake. Both compounds suppressed ISGU in WT muscle, but in the KO muscle there was little impact of the drugs. Collectively, this data indicates that the ATM drugs affect glucose metabolism in vivo by inhibiting Tpm3.1's function with few off-target effects.

Parenteral versus enteral nutrition: effect on serum cytokines and the hepatic expression of mRNA of suppressor of cytokine signaling proteins, insulin-like growth factor-1 and the growth hormone receptor in rodent sepsis.

INTRODUCTION: Early nutrition is recommended for patients with sepsis, but data are conflicting regarding the optimum route of delivery. Enteral nutrition (EN), compared with parenteral nutrition (PN), results in poorer achievement of nutritional goals but may be associated with fewer infections. Mechanisms underlying differential effects of the feeding route on patient outcomes are not understood, but probably involve the immune system and the anabolic response to nutrients. We studied the effect of nutrition and the route of delivery of nutrition on cytokine profiles, the growth hormone-insulin-like growth factor-1 (IGF-I) axis and a potential mechanism for immune and anabolic system interaction, the suppressors of cytokine signaling (SOCS), in rodents with and without sepsis. METHODS: Male Sprague-Dawley rats were randomized to laparotomy (Sham) or to cecal ligation and puncture (CLP), with postoperative saline infusion (Starve), with EN or with PN for 72 hours. Serum levels of IL-6 and IL-10 were measured by immunoassay, and hepatic expressions of cytokine-inducible SH2-containing protein, SOCS-2, SOCS-3, IGF-I and the growth hormone receptor (GHR) were measured by real-time quantitative PCR. RESULTS: IL-6 was detectable in all groups, but was only present in all animals receiving CLP-PN. IL-10 was detectable in all but one CLP-PN rat, one CLP-EN rat, approximately 50% of the CLP-Starve rats and no sham-operated rats. Cytokine-inducible SH2-containing protein mRNA was increased in the CLP-EN group compared with the Sham-EN group and the other CLP groups (P < 0.05). SOCS-2 mRNA was decreased in CLP-PN rats compared with Sham-PN rats (P = 0.07). SOCS-3 mRNA was increased with CLP compared with sham operation (P < 0.03). IGF-I mRNA (P < 0.05) and GHR mRNA (P < 0.03) were greater in the fed CLP animals and in the Sham-PN group compared with the starved rats. CONCLUSION: In established sepsis, nutrition and the route of administration of nutrition influences the circulating cytokine patterns and expression of mRNA of SOCS proteins, GHR and IGF-I. The choice of the administration route of nutrition may influence cellular mechanisms that govern the response to hormones and mediators, which further influence the response to nutrients. These findings may be important in the design and analysis of clinical trials of nutritional interventions in sepsis in man.

ER/Golgi trafficking is facilitated by unbranched actin filaments containing Tpm4.2.

We have identified novel actin filaments defined by tropomyosin Tpm4.2 at the ER. EM analysis of mouse embryo fibroblasts (MEFs) isolated from mice expressing a mutant Tpm4.2 (Tpm4Plt53/Plt53 ), incapable of incorporating into actin filaments, revealed swollen ER structures compared with wild-type (WT) MEFs (Tpm4+/+ ). ER-to-Golgi, but not Golgi-to-ER trafficking was altered in the Tpm4Plt53/Plt53 MEFs following the transfection of the temperature sensitive ER-associated ts045-VSVg construct. Exogenous Tpm4.2 was able to rescue the ER-to-Golgi trafficking defect in the Tpm4Plt53/Plt53 cells. The treatment of WT MEFs with the myosin II inhibitor, blebbistatin, blocked the Tpm4.2-dependent ER-to-Golgi trafficking. The lack of an effect on ER-to-Golgi trafficking following treatment of MEFs with CK666 indicates that branched Arp2/3-containing actin filaments are not involved in anterograde vesicle trafficking. We propose that unbranched, Tpm4.2-containing filaments have an important role in maintaining ER/Golgi structure and that these structures, in conjunction with myosin II motors, mediate ER-to-Golgi trafficking.

Male Presence can Increase Body Mass and Induce a Stress-Response in Female Mice Independent of Costs of Offspring Production.

Sexual reproduction in animals requires close interactions with the opposite sex. These interactions may generate costs of reproduction, because mates can induce detrimental physiological or physical effects on one another, due to their interest in maximising their own fitness. To understand how a male's presence influences aspects of female physiology implicated in reproductive costs in mice, independent of offspring production, we paired females with vasectomised, castrated or intact males, or other females. Being paired with a male, irrespective of his gonadal status, increased female weight. This effect was transient in females paired with castrated males but more persistent in those with vasectomised males. Those paired with males also showed an increase in corticosterone, suggesting an increased stress response. However, this was dependent on the gonadal status of the male housing partner, since those housed with vasectomised males had lower corticosterone than those with castrated males. Altered energy metabolism was only detectable in pregnant females, and oxidative stress was not consistently affected by a female's housing partner. These results suggest that a male's presence alters female weight, and stresses associated with reproduction could be induced by simply the presence of a male, but reduced by mating and/or being solicited to mate.

Does growth hormone allow more efficient nitrogen sparing in postoperative patients requiring parenteral nutrition? A double-blind, placebo-controlled randomised trial.

BACKGROUND & AIMS: Growth hormone (GH) has a strong anabolic effect and is thought to be useful in improving the efficacy of parenteral nutrition (PN) to preserve muscle mass (MM) in the postoperative setting. Unfortunately, the negative clinical outcome of GH treatment in intensive care patients limits its use in this setting, but demands answers to the mechanism behind the action of this therapy. METHOD: In a double-blind randomised controlled study consecutive patients after major abdominal surgery were divided into four groups of either 1/2-PN (0.13 g N/kg/day and 52% of calories as lipid) or full-strength PN (Full-PN) (0.3 g N/kg/day and 65% of calories as lipid) receiving daily injections of either GH (8-16 IU) or placebo for a period of 14 days postoperative. Outcome measures included MM derived from measures of total body potassium (40K counting) and total body nitrogen (TBN) (in vivo neutron capture technique); Fat mass from skin folds; serum insulin like growth factor-I (IGF-I) and its binding proteins (IGFBP). RESULTS: From 43 major upper GI surgical patients randomised 35 completed the study (one patient died from sepsis in the half-strength PN (1/2-PN)+GH group). 1/2-PN (n=11) lost TBN (P=0.001), MM (P=0.005) but not fat. Full-PN (n=9) maintained TBN, MM (P=0.056) and fat. 1/2-PN+GH (n=8) maintained TBN and fat but lost MM (P=0.038). Full-PN+GH (n=7) maintained TBN and MM but lost fat (P=0.018). Two-way ANOVA indicated that PN input (P=0.031) and not GH had a significant effect on MM. GH caused a significant rise in IGF-I levels (290+/-67 and 454+/-71 microg/l for 1/2-PN+GH and Full-PN+GH, respectively) and restored serum IGFBP3 and the acid labile subunit to normal, by the postoperative day 9. CONCLUSION: After major gastrointestinal surgery, GH causes a marked hepatic IGF-I response and nitrogen retention but its effect on body composition was more significant with a high PN input. Further, Full-PN alone was sufficient to prevent nitrogen loss and preserved MM and addition of GH does not provide further metabolic advantage.

Regulation of cell proliferation by ERK and signal-dependent nuclear translocation of ERK is dependent on Tm5NM1-containing actin filaments.

ERK-regulated cell proliferation requires multiple phosphorylation events catalyzed first by MEK and then by casein kinase 2 (CK2), followed by interaction with importin7 and subsequent nuclear translocation of pERK. We report that genetic manipulation of a core component of the actin filaments of cancer cells, the tropomyosin Tm5NM1, regulates the proliferation of normal cells both in vitro and in vivo. Mouse embryo fibroblasts (MEFs) lacking Tm5NM1, which have reduced proliferative capacity, are insensitive to inhibition of ERK by peptide and small-molecule inhibitors, indicating that ERK is unable to regulate proliferation of these knockout (KO) cells. Treatment of wild-type MEFs with a CK2 inhibitor to block phosphorylation of the nuclear translocation signal in pERK resulted in greatly decreased cell proliferation and a significant reduction in the nuclear translocation of pERK. In contrast, Tm5NM1 KO MEFs, which show reduced nuclear translocation of pERK, were unaffected by inhibition of CK2. This suggested that it is nuclear translocation of CK2-phosphorylated pERK that regulates cell proliferation and this capacity is absent in Tm5NM1 KO cells. Proximity ligation assays confirmed a growth factor-stimulated interaction of pERK with Tm5NM1 and that the interaction of pERK with importin7 is greatly reduced in the Tm5NM1 KO cells.

IGF-I has no effect on postexercise suppression of the ubiquitin-proteasome system in rat skeletal muscle.

Both exercise and insulin-like growth factor I (IGF-I) are known to have major hypertrophic effects in skeletal muscle; however, the interactive effect of exogenous IGF-I and exercise on muscle protein turnover or the ubiquitin-proteasome pathway has not been reported. In the present study, we have examined the interaction between endurance exercise training and IGF-I treatment on muscle protein turnover and the ubiquitin-proteasome pathway in the postexercise period. Adult male rats (270-280 g) were randomized to receive 5 consecutive days of progressive treadmill exercise and/or IGF-I treatment (1 mg. kg body wt(-1). day(-1)). Twenty-four hours after the last bout of exercise, the rate of protein breakdown in incubated muscles was significantly reduced compared with that in unexercised rats. This was associated with a significant reduction in the chymotrypsin-like activity of the proteasome and the rate of ubiquitin-proteasome-dependent casein hydrolysis in muscle extracts from exercised compared with unexercised rats. In contrast, the muscle expression of the 20S proteasome subunit beta-1, ubiquitin, and the 14-kDa E2 ubiquitin-conjugating enzyme was not altered by exercise or IGF-I treatment 24 h postexercise. Exercise had no effect on the rates of total mixed muscle protein synthesis in incubated muscles 24 h postexercise. IGF-I treatment had no effect on muscle weights or the rates of protein turnover 24 h after endurance exercise. These results suggest that a suppression of the ubiquitin-proteasome proteolytic pathway after endurance exercise may contribute to the acute postexercise net protein gain.

A study of parenteral versus enteral nutrition following caecal ligation and puncture in the rat: Influence on survival and tissue protein turnover.

BACKGROUND & AIMS: Methods of nutritional management in abdominal sepsis remain controversial. METHODS: Sprague Dawley rats were either fed via a central line in the right internal jugular vein or duodenally via a gastrostomy tube, and were randomised to undergo either caecal ligation and puncture (CLP) or laparotomy only. Post-operatively, animals received either parenteral nutrition, enteral nutrition or saline only (parenteral and enteral nutrition protocols were isocaloric and isonitrogenous). After 72 h, fractional rate of protein synthesis (Ks, %/day) was measured in gastrocnemius muscle and liver, and protein breakdown was measured in incubated epitrochlearis muscles. Serum insulin-like growth factor-I (IGF-I), acid-labile subunit (ALS) and IGF binding protein-1 (IGFBP-1) levels were determined by specific radioimmunoassay methods. RESULTS: After CLP, when compared with starved animals, only enteral nutrition resulted in a significant decrease in survival to 72 h (P < 0.001). Parenteral nutrition, but not enteral nutrition, increased muscle (P = 0.02) and liver (P < 0.001) Ks, IGF-I (P < 0.001) and ALS levels (P < 0.001), whereas both parenteral and enteral nutrition reduced IGFBP-1 levels (P < 0.001). Neither enteral nor parenteral nutrition reduced protein breakdown in septic animals. CONCLUSIONS: In this model of severe abdominal sepsis where gut function cannot be assessed, enteral nutrition was associated with increased mortality and was less effective than parenteral nutrition in augmenting muscle and liver protein synthesis.

Multimodal analgesia and intravenous nutrition preserves total body protein following major upper gastrointestinal surgery.

BACKGROUND AND OBJECTIVES: This study examined whether perioperative multimodal analgesia (MMA) improves the effectiveness of intravenous nutrition (IVN) as a means of preventing protein wasting following major upper abdominal surgery (UAS). The MMA regimen utilized combined epidural opioid/local anesthetic and the systemic nonsteroidal anti-inflammatory drug (NSAID) ketorolac for 48 hours. METHODS: In a prospective, randomized, nonblinded study, 47 patients scheduled for major UAS were allocated to receive the following: MMA +/- intravenous lipid-based nutrition (IVN) or patient-controlled analgesia with opioids (PCA) +/- IVN. Pain scores, nitrogen balance, total body protein (TBP), arterial blood gases, and various hormones were measured. RESULTS: Pain control was significantly better in the MMA patients at rest and coughing. Only the MMA + IVN group maintained TBP, mean (+/-95% confidence interval) preoperative day 1, 10.5 (+/-1.0) kg; day 14, 10.7 (+/-1.2) kg, whereas TBP decreased in the other groups (P =.04). Nitrogen balance was significantly greater in patients receiving IVN on day 7 (P =.01), but there was no effect related to the analgetic regimen. Decreased PaO(2) seen on postoperative day 2 was not prevented by MMA. The hormonal response to surgery was not influenced by treatment modality, apart from a return to postprandial insulin levels on postoperative day 7 in those patients receiving IVN (P =.002). CONCLUSIONS: In conclusion, we have shown that the combination of MMA and IVN prevents protein loss and improves pain control after major UAS. Our results suggest that after UAS, MMA significantly reduced pain and, in combination with IVN, preserves total body protein and fat. This is the first direct evidence of such effects associated with a commonly used multimodal regimen.

Tropomyosin isoforms support actomyosin biogenesis to generate contractile tension at the epithelial zonula adherens.

Epithelial cells generate contractile forces at their cell-cell contacts. These are concentrated at the specialized apical junction of the zonula adherens (ZA), where a ring of stabilized E-cadherin lies adjacent to prominent actomyosin bundles. Coupling of adhesion and actomyosin contractility yields tension in the junction. The biogenesis of junctional contractility requires actin assembly at the ZA as well as the recruitment of nonmuscle myosin II, but the molecular regulators of these processes are not yet fully understood. We now report a role for tropomyosins 5NM1 (Tm5NM1) and 5NM2 (Tm5NM2) in their generation. Both these tropomyosin isoforms were found at the ZA and their depletion by RNAi or pharmacological inhibition reduced both F-actin and myosin II content at the junction. Photoactivation analysis revealed that the loss of F-actin was attributable to a decrease in filament stability. These changes were accompanied by a decrease in E-cadherin content at junctions. Ultimately, both long-term depletion of Tm5NM1/2 and acute inhibition with drugs caused junctional tension to be reduced. Thus these tropomyosin isoforms are novel contributors to junctional contractility and integrity.

Tropomyosin regulates cell migration during skin wound healing.

Precise orchestration of actin polymer into filaments with distinct characteristics of stability, bundling, and branching underpins cell migration. A key regulator of actin filament specialization is the tropomyosin family of actin-associating proteins. This multi-isoform family of proteins assemble into polymers that lie in the major groove of polymerized actin filaments, which in turn determine the association of molecules that control actin filament organization. This suggests that tropomyosins may be important regulators of actin function during physiological processes dependent on cell migration, such as wound healing. We have therefore analyzed the requirement for tropomyosin isoform expression in a mouse model of cutaneous wound healing. We find that mice in which the 9D exon from the TPM3/γTm tropomyosin gene is deleted (γ9D -/-) exhibit a more rapid wound-healing response 7 days after wounding compared with wild-type mice. Accelerated wound healing was not associated with increased cell proliferation, matrix remodeling, or epidermal abnormalities, but with increased cell migration. Rac GTPase activity and paxillin phosphorylation are elevated in cells from γ9D -/- mice, suggesting the activation of paxillin/Rac signaling. Collectively, our data reveal that tropomyosin isoform expression has an important role in temporal regulation of cell migration during wound healing.