Cancer cachexia is regulated by selective targeting of skeletal muscle gene products

Cachexia is a syndrome characterized by wasting of skeletal muscle and contributes to nearly one-third of all cancer deaths. Cytokines and tumor factors mediate wasting by suppressing muscle gene products, but exactly which products are targeted by these cachectic factors is not well understood. Because of their functional relevance to muscle architecture, such targets are presumed to represent myofibrillar proteins, but whether these proteins are regulated in a general or a selective manner is also unclear. Here we demonstrate, using in vitro and in vivo models of muscle wasting, that cachectic factors are remarkably selective in targeting myosin heavy chain. In myotubes and mouse muscles, TNF-alpha plus IFN-gamma strongly reduced myosin expression through an RNA-dependent mechanism. Likewise, colon-26 tumors in mice caused the selective reduction of this myofibrillar protein, and this reduction correlated with wasting. Under these conditions, however, loss of myosin was associated with the ubiquitin-dependent proteasome pathway, which suggests that mechanisms used to regulate the expression of muscle proteins may be cachectic factor specific. These results shed new light on cancer cachexia by revealing that wasting does not result from a general downregulation of muscle proteins but rather is highly selective as to which proteins are targeted during the wasting state.

Electrophoretic variants of cardiac myosin heavy chain-α in Sprague Dawley rats

Analysis of cardiac myosin revealed differences in gel electrophoretic migration patterns of the alpha-isoform of myosin heavy chain, but not the beta-isoform, in Sprague Dawley rats. No differences in the migration patterns of the alpha-or beta-isoforms were observed in other rat strains. Three electrophoretic migration patterns of the alpha-isoforms were observed in individual rats: a slower migrating isoform alone (4% of all rats tested), a faster migrating isoform alone (55%), and both isoforms (41%). The isoform expression pattern was identical in all myocardial regions in each rat. Frequency of expression patterns suggests multiple gene sequences for alpha-cardiac myosin heavy chain in Sprague Dawley rats. Sequence analysis of amplified regions of the Sprague Dawley and Brown Norway rat alpha-myosin genes, specifically the 5'-untranslated region, exons 1-3, and associated introns, showed numerous single nucleotide polymorphisms in coding and noncoding regions, including putative regulatory sites in Sprague Dawley rats, but not in Brown Norway rats. All Sprague Dawley rats varied from Brown Norway rats and no heterogeneity was observed in Brown Norway rats. Several deletions and dimorphic positions were also observed. Dimorphic positions were evident on automated sequencing comparisons. The data indicate that at least two alpha-myosin heavy chain isoforms exist in Sprague Dawley rats and these rats exhibit sequence diversity within that portion of the alpha-myosin heavy chain gene reported in this study.

Myosin Light Chain 1 Isoforms in Slow Fibers from Global and Orbital Layers of Canine Rectus Muscles

PURPOSE

Initial results of an examination of the low molecular mass (< or =45 kDa) protein composition of canine rectus muscle homogenates, based on gel electrophoresis, revealed a distinct difference between the global and orbital layers in the myosin light chain (MLC)-1 region. The objectives of the present study were, therefore, to identify isoforms of MLC1 in homogenates of the global and orbital layers of adult canine rectus muscles and to determine the MLC1 isoform expression pattern among single muscle fibers isolated from both layers.

METHODS

Muscle homogenates and single fibers from the global and orbital layers of canine rectus muscles were analyzed, using sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). Matrix-assisted laser desorption/ionization-time-of-flight mass spectrometry (MALDI-TOF-MS) was used to identify a protein band in the orbital layer that comigrated with MLC1 in the adult canine atrium.

RESULTS

Adult canine extraocular rectus muscles expressed embryonic skeletal/atrial MLC1 (MLC1(E/A)), in addition to the fast-type MLC1 (MLC1F) and slow-type MLC1 (MLC1S) isoforms expressed in limb skeletal muscles. MLC1(E/A) was detected in slow fibers of the orbital but not the global layer, and MLC1S was detected in slow fibers in only the global but not the orbital layer. Densitometric analysis of gel bands from homogenates supported these results, with significantly greater amounts of MLC1S in the global layer and of MLC1(E/A) in the orbital layer.

CONCLUSIONS

MLC1(E/A) is expressed in rectus muscles of adult dogs. Furthermore, two types of slow fibers, distinguished on the basis of MLC1 isoform expression, exist in separate layers of canine rectus muscles.

Electrophoretic separation of ventricular myosin isoenzymes using a native polyacrylamide minigel system

A method is presented to separate rabbit cardiac ventricular myosin isoenzymes (V(1), V(2), V(3)), which are large and important contractile proteins. This polyacrylamide gel electrophoresis--using a slab minigel format--does not involve preparation of an acrylamide gradient or denaturing conditions. The isoenzyme migration order was confirmed through identification with an anti beta-myosin heavy chain in cardiac ventricles (i.e., V(3)) antibody. Extracts from atrial and soleus muscle were used as positive control for V(1) and V(3), respectively. The relative quantification was obtained densitometrically and analyzed via TINA/Software. The reproducibility of method was additionally tested. The procedure employs Coomassie blue staining and is rapid and reproducible. Thus, the method permits easy and economic analysis of myosin isoenzymes under native conditions.

Gender differences in myosin heavy chain-beta and phosphorylated phospholamban in diabetic rat hearts

The objective of this study was to determine whether a gender difference exists in myosin heavy chain (MHC) isoform or sarcoplasmic reticulum protein levels in diabetic rat hearts. As is the case with normal rodent hearts, all four chambers of the control rat hearts expressed almost 100% MHC-alpha. In 6-wk diabetic rats, MHC-beta expression in ventricles of males was significantly greater (78 +/- 7%) than in females (50 +/- 5%). The cardiac sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA2a) protein level was decreased and the phospholamban (PLB) protein level was increased in the left ventricle of diabetic rats, but there was no difference between male and female diabetic rats. The phosphorylated PLB level was decreased more in male than in female diabetic rats. Insulin treatment completely normalized blood glucose level, cardiac SERCA2a and PLB protein levels, and the decrease in MHC-beta levels in both male and female diabetic rats. Insulin treatment completely normalized serum insulin and almost completely normalized phosphorylation of PLB at serine 16 in male diabetic rats. Although insulin treatment completely normalized serum insulin levels in male diabetic rats, in females it only partially normalized serum insulin levels. Also, insulin treatment almost completely normalized phosphorylation of PLB at threonine 17 in female diabetic rats; however, the increase was significantly greater than that identified for insulin-treated male diabetic rats. We conclude that higher levels of MHC-beta and dephosphorylated PLB may contribute to more contractile dysfunction in male than in female diabetic rat hearts, and that phosphorylation of PLB at threonine 17 is more responsive to insulin in female diabetic rat hearts.

Effects of peroxynitrite on isolated cardiac trabeculae: selective impact on myofibrillar energetic controllers

Formation of peroxynitrite and cardiac protein nitration have been implicated in multiple cardiac disease states, but their contributions to disease initiation remain undefined. We have previously observed nitration of myofibrillar regions of cardiac myocytes in several experimental and clinical settings of cardiac myocyte dysfunction and postulated that oxidative insult to key components of the contractile apparatus may be initiating events. Here we tested the hypothesis that peroxynitrite alters myofibrillar contractile function, and investigated a mechanistic role for nitration in this process. Isolated rat ventricular trabeculae were exposed to physiologically relevant concentrations of peroxynitrite and ATP-dependent contractile responses were measured. Maximal trabecular force generation was significantly impaired following 300 nM peroxynitrite exposures. Several myofibrillar proteins demonstrated increased tyrosine nitration, the most significant increases occurred in the myosin heavy chain and the myofibrillar isoform of creatine kinase. Additional functional experiments were conducted using phosphocreatine (high energy phosphate substrate for myofibrillar creatine kinase) as the primary energy substrate. Myofibrillar creatine kinase-dependent force generation was impaired at peroxynitrite concentrations as low as 50 nM, suggesting potent inactivation of the enzyme. Extent of tyrosine nitration of myofibrillar creatine kinase was negatively correlated to myofibrillar creatine kinase-dependent force generation. These data demonstrate that the cardiac contractile apparatus is highly sensitive to peroxynitrite, and that MM-CK may be a uniquely vulnerable target.

A simplified method for identification of human cardiac myosin heavy-chain isoforms

Cardiac myosin is a central participant in the cross-bridge cycling that mediates myocyte contraction and consists of multiple subunits that mediate both hydrolysis of ATP and mechanical production of contractile force Two isoforms of myosin heavy chain (MHC- alpha and MHC- beta ) are known to exist in mammalian cardiac tissue, and it is within this myosin subunit that ATPase activity resides. These isoforms differ by less than 0.2% in total molecular mass and amino acid sequence, but, strikingly, influence the rate and efficiency of energy utilization for generation of contractile force. Changes in the MHC- alpha /MHC- beta ratio has been classically viewed as an adaptation of a failing myocyte in both animal models and humans; however, their measurement has traditionally required specialized preparations and materials for sufficient resolution. Here we describe a greatly simplified method for routine assessments of myosin isoform composition in human cardiac tissues. The primary advantages of our approach include higher throughput and reduced supply costs with no apparent loss of statistical power, reproducibility or achieved results. Use of this more convenient method may provide enhanced access to an otherwise specialized technique and could provide additional opportunity for investigation of cardiac myocyte adaptive changes.

Engineering competitive magnesium binding into the first EF-hand of skeletal troponin C

The goal of this study was to examine the mechanism of magnesium binding to the regulatory domain of skeletal troponin C (TnC). The fluorescence of Trp(29), immediately preceding the first calcium-binding loop in TnC(F29W), was unchanged by addition of magnesium, but increased upon calcium binding with an affinity of 3.3 microm. However, the calcium-dependent increase in TnC(F29W) fluorescence could be reversed by addition of magnesium, with a calculated competitive magnesium affinity of 2.2 mm. When a Z acid pair was introduced into the first EF-hand of TnC(F29W), the fluorescence of G34DTnC(F29W) increased upon addition of magnesium or calcium with affinities of 295 and 1.9 microm, respectively. Addition of 3 mm magnesium decreased the calcium sensitivity of TnC(F29W) and G34DTnC(F29W) approximately 2- and 6-fold, respectively. Exchange of G34DTnC(F29W) into skinned psoas muscle fibers decreased fiber calcium sensitivity approximately 1.7-fold compared with TnC(F29W) at 1 mm [magnesium](free) and approximately 3.2-fold at 3 mm [magnesium](free). Thus, incorporation of a Z acid pair into the first EF-hand allows it to bind magnesium with high affinity. Furthermore, the data suggests that the second EF-hand, but not the first, of TnC is responsible for the competitive magnesium binding to the regulatory domain.

TGF-beta1 mediates the hypertrophic cardiomyocyte growth induced by angiotensin II

Angiotensin II (Ang II), a potent hypertrophic stimulus, causes significant increases in TGFb1 gene expression. However, it is not known whether there is a causal relationship between increased levels of TGF-beta1 and cardiac hypertrophy. Echocardiographic analysis revealed that TGF-beta1-deficient mice subjected to chronic subpressor doses of Ang II had no significant change in left ventricular (LV) mass and percent fractional shortening during Ang II treatment. In contrast, Ang II-treated wild-type mice showed a >20% increase in LV mass and impaired cardiac function. Cardiomyocyte cross-sectional area was also markedly increased in Ang II-treated wild-type mice but unchanged in Ang II-treated TGF-beta1-deficient mice. No significant levels of fibrosis, mitotic growth, or cytokine infiltration were detected in Ang II-treated mice. Atrial natriuretic factor expression was approximately 6-fold elevated in Ang II-treated wild-type, but not TGF-beta1-deficient mice. However, the alpha- to beta-myosin heavy chain switch did not occur in Ang II-treated mice, indicating that isoform switching is not obligatorily coupled with hypertrophy or TGF-beta1. The Ang II effect on hypertrophy was shown not to result from stimulation of the endogenous renin-angiotensis system. These results indicate that TGF-beta1 is an important mediator of the hypertrophic growth response of the heart to Ang II.

Intracellular distribution of peroxynitrite during doxorubicin cardiomyopathy: evidence for selective impairment of myofibrillar creatine kinase

Cardiac peroxynitrite and protein nitration are increased during doxorubicin cardiotoxicity, but the intracellular targets and functional consequences have not been defined. We investigated the intracellular distribution of protein nitration during doxorubicin cardiotoxicity in mice. Following in vivo cardiac function assessments by echocardiography, cardiac tissues were prepared for immunohistochemistry and electron microscopy 5 days after doxorubicin (20 mg kg(-1)) or vehicle control. Increased cardiac 3-nitrotyrosine was observed using light microscopy in doxorubicin treated animals. Immunogold electron microscopy (55,000x) revealed increased myofibrillar and mitochondrial 3-nitrotyrosine levels following doxorubicin, but cellular 3-nitrotyrosine density was 2 fold higher in myofibrils. We therefore investigated the actions of peroxynitrite on intact cardiac contractile apparatus. Skinned ventricular trabeculae were exposed to physiologically relevant peroxynitrite concentrations (50 or 300 nM) for 1 h, then Ca(2+) induced contractile responses were measured in the presence of ATP (4 mM) or phosphocreatine (12 mM) as high energy phosphate supplier. ATP maximal force generation was unaltered after 50 nM peroxynitrite, but phosphocreatine/ATP response was reduced (0.99+/-0.63 vs 1.59+/-0.11), suggesting selective inactivation of myofibrillar creatine kinase (MM-CK). Reduction of ATP maximal force was observed at 300 nM peroxynitrite and phosphocreatine/ATP response was further reduced (0.64+/-0.30). Western blotting showed concentration dependent nitration of MM-CK in treated trabeculae. Similarly, cardiac tissues from doxorubicin treated mice demonstrated increased nitration and inactivation of MM-CK compared to controls. These results demonstrate that peroxynitrite-related protein nitration are mechanistic events in doxorubicin cardiomyopathy and that the cardiac myofibril is an important oxidative target in this setting. Furthermore, MM-CK may be a uniquely vulnerable target to peroxynitrite in vivo.

Selected Contribution: Improved anoxic tolerance in rat diaphragm following intermittent hypoxia

Intermittent hypoxia (IH), associated with obstructive sleep apnea, initiates adaptive physiological responses in a variety of organs. Little is known about its influence on diaphragm. IH was simulated by exposing rats to alternating 15-s cycles of 5% O2 and 21% O2 for 5 min, 9 sets/h, 8 h/day, for 10 days. Controls did not experience IH. Diaphragms were excised 20-36 h after IH. Diaphragm bundles were studied in vitro or analyzed for myosin heavy chain isoform composition. No differences in maximum tetanic stress were observed between groups. However, peak twitch stress (P < 0.005), twitch half-relaxation time (P < 0.02), and tetanic stress at 20 or 30 Hz (P < 0.05) were elevated in IH. No differences in expression of myosin heavy chain isoforms or susceptibility to fatigue were seen. Contractile function after 30 min of anoxia (95% N2-5% CO2) was markedly preserved at all stimulation frequencies during IH and at low frequencies after 15 min of reoxygenation. Anoxia-induced increases in passive muscle force were eliminated in the IH animals (P < 0.01). These results demonstrate that IH induces adaptive responses in the diaphragm that preserve its function in anoxia.

Impaired myofibrillar energetics and oxidative injury during human atrial fibrillation

BACKGROUND

Atrial fibrillation (AF) is associated with severe contractile dysfunction and structural and electrophysiological remodeling. Mechanisms responsible for impaired contractility are undefined, and current therapies do not address this dysfunction. We have found that myofibrillar creatine kinase (MM-CK), an important controller of myocyte contractility, is highly sensitive to oxidative injury, and we hypothesized that increased oxidative stress and energetic impairment during AF could contribute to contractile dysfunction. Methods and Results-- Right atrial appendages were obtained from AF patients undergoing the Maze procedure and from control patients who were in normal sinus rhythm and undergoing cardiac surgery. MM-CK activity was reduced in AF patients compared with controls (25.4+/-3.4 versus 18.2+/-3.8 micromol/mg of myofibrillar protein per minute; control versus AF; P<0.05). No reduction in total CK activity or myosin ATPase activity was detected. This selective reduction in MM-CK activity was associated with increased relative expression of the beta-myosin isoform (25+/-6 versus 63+/-5%beta, CTRL versus AF; P<0.05). Western blotting of AF myofibrillar isolates demonstrated no changes in protein composition but showed increased prevalence of protein oxidation as detected by Western blotting for 3-nitrotyrosine (peroxynitrite biomarker) and protein carbonyls (hydroxyl radical biomarker; P<0.05). Patterns of these oxidative markers were distinct, which suggests discrete chemical events and differential protein vulnerabilities in vivo. MM-CK inhibition was statistically correlated to extent of nitration (P<0.01) but not to carbonyl presence.

CONCLUSIONS

The present results provide novel evidence of oxidative damage in human AF that altered myofibrillar energetics may contribute to atrial contractile dysfunction and that protein nitration may be an important participant in this condition.

Human cardiac myosin heavy chain isoforms in fetal and failing adult atria and ventricles

The goal of this study was to test the hypothesis that the relative amounts of the cardiac myosin heavy chain (MHC) isoforms MHC-alpha and MHC-beta change during development and transition to heart failure in the human myocardium. The relative amounts of MHC-alpha and MHC-beta in ventricular and atrial samples from fetal (gestational days 47--110) and nonfailing and failing adult hearts were determined. The majority of the fetal right and left ventricular samples contained small relative amounts of MHC-alpha (mean < 5% of total MHC). There was a small significant decrease in the level of MHC-alpha in the ventricles between 7 and 12 wk of gestation. Fetal atria expressed predominantly MHC-alpha (mean > 95%), with MHC-beta being detected in most samples. The majority of adult nonfailing right and left ventricular samples had detectable levels of MHC-alpha ranging from 1 to 10%. Failing right and left ventricles expressed a significantly lower level of MHC-alpha. MHC-alpha comprised approximately 90% of the total MHC in adult nonfailing left atria, whereas the relative amount of MHC-alpha in the left atria of individuals with dilated or ischemic cardiomyopathy was approximately 50%. The differences in MHC isoform composition between fetal and nonfailing adult atria and between fetal and nonfailing adult ventricles were not statistically significant. We concluded that the MHC isoform compositions of fetal human atria are the same as those of nonfailing adult atria and that the ventricular MHC isoform composition is different between adult nonfailing and failing hearts. Furthermore, the marked alteration in atrial MHC isoform composition, associated with cardiomyopathy, does not represent a regression to a pattern that is uniquely characteristic of the fetal stage.

Fibroblast growth factor-2 mediates pressure-induced hypertrophic response

In vitro, fibroblast growth factor-2 (FGF2) has been implicated in cardiomyocyte growth and reexpression of fetal contractile genes, both markers of hypertrophy. However, its in vivo role in cardiac hypertrophy during pressure overload is not well characterized. Mice with or without FGF2 (Fgf2(+/+) and Fgf2(-/-), respectively) were subjected to transverse aortic coarctation (AC). Left ventricular (LV) mass and wall thickness were assessed by echocardiography preoperatively and once a week postoperatively for 10 weeks. In vivo LV function during dobutamine stimulation, cardiomyocyte cross-sectional area, and recapitulation of fetal cardiac genes were also measured. AC Fgf2(-/-) mice develop significantly less hypertrophy (4-24% increase) compared with AC Fgf2(+/+) mice (41-52% increase). Cardiomyocyte cross-sectional area is significantly reduced in AC Fgf2(-/-) mice. Noncoarcted (NC) and AC Fgf2(-/-) mice have similar beta-adrenergic responses, but those of AC Fgf2(+/+) mice are blunted. A lack of mitotic growth in both AC Fgf2(+/+) and Fgf2(-/-) hearts indicates a hypertrophic response of cardiomyocytes. Consequently, FGF2 plays a major role in cardiac hypertrophy. Comparison of alpha- and beta-cardiac myosin heavy chain mRNA and protein levels in NC and AC Fgf2(+/+) and Fgf2(-/-) mice indicates that myosin heavy chain composition depends on hemodynamic stress rather than on FGF2 or hypertrophy, and that isoform switching is transcriptionally, not posttranscriptionally, regulated.

Differential effects of ovariectomy on calcium activation of cardiac and soleus myofilaments

The hypothesis that ovarian sex hormone deficiency affects cardiac myofilament activation was tested. Chemically skinned ventricular trabeculae and single soleus muscle fibers were prepared from 10- and 14-wk ovariectomized and control rats. Tension-pCa (-log [Ca(2+)]) relations of left ventricular trabeculae and soleus fibers were compared to test whether thin filament proteins are potential sites of modulated activation. Trabeculae from ovariectomized rats exhibited a significant increase in Ca(2+) sensitivity with no change in maximal tension-generating ability. In contrast, soleus fibers demonstrated no shift in Ca(2+) sensitivity but generated significantly less maximal tension. No changes in thin filament protein isoform expression or loss of thin filament proteins were apparent in the trabeculae or soleus fibers from ovariectomized rats. Although not directly tested, our results are consistent with a possible modulation of regulatory proteins (e.g., cardiac troponin I) to account for the observed change in myofilament responsiveness of hearts from ovariectomized rats. Other possible mechanisms for the altered myocardial Ca(2+) sensitivity after ovariectomy are discussed.

Electrophoretic separation and quantitation of cardiac myosin heavy chain isoforms in eight mammalian species

A protocol for sample preparation and gel electrophoresis is described that reliably results in the separation of the alpha- and beta-isoforms of cardiac myosin heavy chain (MHC-alpha and MHC-beta) in eight mammalian species. The protocol is based on a simple, nongradient denaturing gel. The magnitude of separation of MHC-alpha and MHC-beta achieved with this protocol is sufficient for quantitative determination of the relative amounts of these two isoforms in mouse, rat, guinea pig, rabbit, canine, pig, baboon, and human myocardial samples. The sensitivity of the protocol is sufficient for the detection of MHC isoforms in samples at least as small as 1 microgram. The glycerol concentration in the separating gel is an important factor for successfully separating MHC-alpha and MHC-beta in myocardial samples from different species. The effect of sample load on MHC-alpha and MHC-beta band resolution is illustrated. The results also indicate that inclusion of a homogenization step during sample preparation increases the amount of MHC detected on the gel for cardiac samples to a much greater extent than for skeletal muscle samples. Although the protocol described in this study is excellent for analyzing cardiac samples, it should be noted that the same protocol is not optimal for separating MHC isoforms expressed in skeletal muscle, as is illustrated.

Induction of neuronal type nitric oxide synthase in skeletal muscle by chronic electrical stimulation in vivo

Fast-twitch skeletal muscles contain more neuronal-type nitric oxide synthase (nNOS) than slow-twitch muscles because nNOS is present only in fast (type II) muscle fibers. Chronic in vivo electrical stimulation of tibialis anterior and extensor digitorum longus muscles of rabbits was used as a method of inducing fast-to-slow fiber type transformation. We have studied whether an increase in muscle contractile activity induced by electrical stimulation alters nNOS expression, and if so, whether the nNOS expression decreases to the levels present in slow muscles. Changes in the expression of myosin heavy chain isoforms and maximum velocity of shortening of skinned fibers indicated characteristic fast-to-slow fiber type transformation after 3 wk of stimulation. At the same time, activity of NOS doubled in the stimulated muscles, and this correlated with an increase in the expression of nNOS shown by immunoblot analysis. These data suggest that nNOS expression in skeletal muscle is regulated by muscle activity and that this regulation does not necessarily follow the fast-twitch and slow-twitch pattern during the dynamic phase of phenotype transformation.

Contractile properties and protein isoforms of single fibres from the chicken pectoralis red strip muscle

1. The contractile properties of single muscle fibres of the red strip region of adult chicken pectoralis major (PM) muscle, some of which are known to express an embryonic isoform of myosin heavy chain (MHC), were determined and compared with the properties of the fast white fibres of the PM and the slow tonic fibres of the anterior latissimus dorsi (ALD) muscle. 2. The red strip fibres could be classified into two groups, fast and slow. The mean velocity of unloaded shortening (Vmax) in fast red strip fibres was approximately half the Vmax of fast white fibres. Vmax of slow red strip fibres was less than 20% of the value for fast red strip fibres and was not different from Vmax of ALD fibres. 3. The tension-generating ability, i.e. the maximal isometric tension/fibre cross-sectional area (P0/CSA), was the same in fast red strip fibres and fast white fibres. P0/CSA was approximately 30% lower in slow red strip fibres compared with fast red strip fibres but was 70% greater in slow red strip fibres compared with ALD fibres. 4. The tension-pCa relation of fast red strip fibres was shifted to lower pCa values, indicating a lower calcium sensitivity compared with fast white fibres, and this difference was associated with a difference in troponin T isoform composition. The tension-pCa relation of slow red strip fibres was not different from that in ALD fibres. 5. The difference in Vmax between fast red strip fibres and fast white fibres was associated with different MHC compositions of these fibres. 6. The myofibrillar protein isoform composition of slow red strip fibres was identical to that of the slow tonic fibres of ALD muscle and these two groups of fibres had very similar contractile properties.

Alterations in neuromuscular junction morphology during fast-to-slow transformation of rabbit skeletal muscles

Chronic low frequency stimulation of motor nerves results in transformation of muscle fibre phenotype from fast- to slow-twitch. We examined the light and electron microscopic structure of neuromuscular junctions in normally fast twitch muscles, tibialis anterior and extensor digitorum longus of rabbit after 3 weeks of stimulation to determine whether synaptic structure is also modified during fibre type transformation. Neuromuscular junctions of stimulated and unstimulated (control) tibialis anterior and extensor digitorum longus muscles and unstimulated slow twitch soleus muscle were visualized with rhodamine-conjugated alpha-bungarotoxin. Video light microscopic images of neuromuscular junctions were digitized to allow quantification of their surface areas, perimeters, lengths and widths. Three weeks of stimulation resulted in a decrease in the maximal velocity of muscle fibre shortening and augmentation of mitochondrial volume in fast muscles, demonstrating the efficacy of the stimulation protocol employed in altering muscle fibre phenotype. Neuromuscular junctions of control tibialis anterior and extensor digitorum longus are thin, compact, and continuous, with complex branching patterns. In contrast, those of slow-twitch soleus are thicker and discontinuous. Neuromuscular junctions in control tibialis anterior and extensor digitorum longus are larger than those in soleus. Three weeks of stimulation causes a marked decrease in the size of neuromuscular junctions in tibialis anterior and extensor digitorum longus, as reflected in the significant reduction in neuromuscular junction surface area, length and width. Electron microscopy of these junctions suggests that secondary postsynaptic folds in stimulated muscles are more closely spaced. Also, axon terminals of stimulated muscles appear to contain more densely packed synaptic vesicles and mitochondria than controls. Decreases in neuromuscular junction dimensions can be partly explained by muscle fibre atrophy. However, the decrease in neuromuscular junction size is proportionately greater than that of muscle fibre diameter in both muscles, indicating that factors other than fibre atrophy may contribute to the reduced neuromuscular junction size in stimulated muscles. Neuromuscular junctions of stimulated tibialis anterior and extensor digitorum longus muscles exhibit some features characteristic of normal soleus neuromuscular junctions, indicating structural adaptations consistent with the altered muscle fibre phenotype. On the other hand, neuromuscular junctions of 3 week stimulated tibialis anterior and extensor digitorum longus and their synaptic branches remain as thin and continuous as those of unstimulated controls, suggesting that the transformation of neuromuscular junctions towards a morphology characteristic of slow muscle, is only partial. These results demonstrate that an altered pattern of impulse activity cause significant synaptic remodelling in adult rabbit skeletal muscles.

Enhanced electrophoretic separation and resolution of myosin heavy chains in mammalian and avian skeletal muscles

We report a sodium dodecyl sulfate-polyacrylamide gel electrophoresis protocol for the reliable separation, with high resolution, of myosin heavy chain isoforms in adult avian (chicken) and mammalian (mouse) skeletal muscles. The sample preparation time can be relatively short, thereby minimizing endogenous proteolytic activity which may otherwise result in dispersed and spurious bands. Inclusion of 2-mercaptoethanol in the upper electrode buffer greatly improves band resolution. Glycerol is commonly included in the reported protocols for myosin heavy chain separation and our results demonstrate that the concentration of glycerol employed can have a marked effect on the relative order of migration among myosin heavy chain isoforms.

Intermediate filament proteins increase during chronic stimulation of skeletal muscle

Chronic low-frequency electrical stimulation of rabbit fast-twitch skeletal muscle induces increased levels of two intermediate filament proteins, desmin and vimentin, during the first 3 weeks of stimulation. These increases occur over the same timecourse as reported shifts in alpha-actinin expression and increased Z-disc width, but precede the fast-to-slow shifts in contractile proteins, which have been described by others. Desmin and vimentin levels increase during the first 2 weeks of stimulation, at which time the increase in desmin appears to plateau while vimentin continues to increase significantly through 3 weeks of stimulation. Absolute amounts of vimentin are lower than desmin at all time points, however increases in desmin and vimentin levels are strongly correlated during the stimulation period, suggesting that the two proteins are coordinately increased during the initial phases of muscle transformation. We suggest that rapid increases in the expression of intermediate filament proteins, which coincide with alterations in Z-disc structure, may indicate a fortification of the force-bearing ultrastructure of the muscle fibre in response to the increased activity that is induced by stimulation. The presence of vimentin and elevated levels of desmin expression suggest that mature skeletal muscle reverts toward a developmental program of intermediate filament protein expression during fast-to-slow transformation.

Troponin I isoforms and differential effects of acidic pH on soleus and cardiac myofilaments

Differences in pH sensitivity of tension generation between developing and adult cardiac myofilaments, which contain the same isoform of troponin C (TnC), have been proposed to be due to troponin I (TnI) isoform switching from the slow skeletal (ss) to cardiac (c) TnI isoforms (21). We investigated the effects of acidic pH on Ca(2+)-activation of force in chemically skinned preparations of adult rat trabeculae and single soleus fibers that also share the same TnC isoform. Compared with the soleus fibers, trabeculae demonstrated a greater suppression of tension and a rightward shift in pCa50 (-log half-maximally activating molar Ca2+ concentration) when pH was decreased from 7.0 to 6.2. The pH-induced shift in pCa50 in soleus fibers did not change with sarcomere length. Troponin subunit interactions were also investigated, using cardiac troponin C (cTnCIA) labeled with a fluorescent probe, 2-(4'-iodoacetamidoanilino)-naphthalene-6-sulfonic acid. Under acidic conditions, cTnCIA demonstrated a decrease in Ca(2+)-affinity. This decrease was amplified both in the binary complex cTnCIA-cTnI and in the complex cTnCIA-cTnI-cTnT-tropomyosin to the same extent. In contrast, substitution of ssTnI for cTnI in these complexes produced the same decrease in Ca2+ affinity in response to acidic pH as cTnCIA alone. These results support our hypothesis that differential effects of pH on tension generation and Ca2+ sensitivity between soleus fibers and trabeculae are due to the presence of different isoforms of TnI.

Tension production and thin-filament protein isoforms in developing rat myocardium

The calcium sensitivity of tension production and the expression of troponin I (TnI) and troponin T (TnT) isoforms in skinned neonatal (7 days after birth) and adult rat myocardium were determined. Isometric tension was measured after activation at a known resting sarcomere length in ventricular trabeculae at adult and, for the first time, neonatal ages. Analysis of the tension-pCa relationships indicates a greater calcium sensitivity (approximately 0.3 pCa units) in neonatal ventricular trabeculae compared with adult trabeculae. The maximal isometric tension-generating ability (i.e., tension-tissue cross-sectional area) is threefold greater in adult compared with neonatal trabeculae. Developmental transitions in TnI and TnT isoform expression in atrial and ventricular tissue were examined simultaneously and were found to be dissimilar. Shifts in the expression of TnT isoforms precede shifts in TnI isoforms in ventricular tissue. The opposite pattern occurs in atrial tissue, with shifts in TnI preceding those in TnT. The results show that the greater calcium sensitivity of neonatal compared with adult rat ventricular tissue is associated with developmental changes in both TnT and TnI isoform expressions. These isoform expression patterns may facilitate myocardial tension production at the neonatal stage, when the tension-generating ability of individual trabeculae is much lower than that in the adult.

Developmental changes in troponin T isoform expression and tension production in chicken single skeletal muscle fibres

1. The Ca2+ sensitivity of tension development was characterized in single skinned fibres from the slow anterior latissimus dorsi (ALD), fast posterior latissimus dorsi (PLD), and fast pectoralis major (PM) muscles of the chicken at adult and neonatal (2 weeks post-hatch) stages of development. In the adult, the PM was most sensitive, the ALD intermediate, and the PLD least sensitive to Ca2+. 2. PM and PLD fibres were less sensitive to Ca2+ at the neonatal stage of development than in the adult. However, ALD fibres exhibited no age-dependent changes in Ca2+ sensitivity. 3. Characterization of regulatory protein composition indicated that the PM and PLD fibres had identical fast isoforms of troponin C and troponin I at each developmental stage examined, but there were muscle-specific and age-dependent expressions of troponin T isoforms in these fibres. 4. In the ALD fibres, identical slow isoforms of troponin C, troponin I and tropomyosin were found at each stage. In addition, the troponin T isoform that was present did not change with age. 5. The results suggest a relationship between the specific troponin T isoform composition of individual muscle fibres and their calcium sensitivities of tension development.

Activation in frog atrial trabeculae: dependence on temperature and length

Isolated frog atrial trabeculae were activated using the method of Na+ withdrawal to induce contractures of relatively steady tension. External Na+ concentration [( Na+]o) during contractures was varied between 0.25 and 45 mM. Isometric contracture tension was measured at cold (4 degrees C) and warm (20 degrees C) temperatures. In addition, rapid temperature jumps (complete in approximately 400 ms) were imposed during cold contractures, resulting in tension transients that consisted of an initial increase in tension followed by a decrease, the latter phase being greater at small and moderate reductions in [Na+]o. Peak contracture tension varied with relative muscle length. The trabeculae became more sensitive with stretch to Na+ withdrawal at 20 degrees C and generated relatively greater tensions at a given [Na+]o. The initial tension increase after a temperature jump was directly proportional to the peak contracture tension immediately preceding the increase in temperature and was therefore interpreted as reflecting an effect of the higher temperature on the attached force-generating cross bridges. The effects of cold and warm steady temperatures and temperature jumps during isometric twitches were also studied. Peak twitch tension varied inversely with temperature (stimulus frequency = 0.2 Hz). In contrast, temperature jumps imposed during the rising phase of twitches at a steady cold temperature (approximately 4 degrees C) resulted in a large initial increase in tension followed by relaxation at a rate that was characteristic of the elevated temperature. The results suggest that, at the warmer temperature (approximately 20 degrees C), activation (i.e., number of attached cross bridges) of the myocardium is significantly less than maximal during the twitch response. The dependence of the tension vs. [Na+]o curves and the tension transients resulting from the temperature jumps on relative muscle length provide evidence for a length dependency of contractile activation in intact atrial trabeculae under conditions of steady-state tension development.

Variations in contractile properties of rabbit single muscle fibres in relation to troponin T isoforms and myosin light chains

1. The maximal velocity of shortening (Vmax), tension-pCa relationships and the contractile and regulatory protein composition were determined in single, chemically skinned fibres from adult rabbit plantaris muscles. 2. Three groups of fibres were identified based on their protein compositions. One group had exclusively the slow-type myosin heavy chain (MHC) and myosin light chains (LC) and had low velocities. Another group of fibres had mixtures of fast-type and slow-type MHCs and LCs and had intermediate shortening velocities. The third group of fibres had fast-type myosin heavy and light chains and high velocities. 3. The low-velocity fibres had a mean velocity (+/- S.E.M.) of 0.86 +/- 0.03 muscle lengths/s (ML/s) at 15 degrees C. The remaining fibres formed a continuum with respect to Vmax from 1.37 to 3.94 ML/s. These results indicate that a much greater diversity exists among single fibres from adult mammalian skeletal muscle than previously recognized. The intermediate- and high-velocity fibres formed a continuum (from slow to fast) with respect to the amount of myosin light chain 3 (LC3). That is, Vmax increased with the relative LC3 content in single fibres in the intermediate- and high-velocity groups in a quantitative, statistically significant manner. 4. Three isoforms of fast-type troponin T were identified among the intermediate- and high-velocity fibres. These fibres also contained fast-type troponin C and troponin I. As was the case with the relative LC3 content, these fibres also formed a continuum with respect to the relative proportions of the three isoforms of fast-type troponin T. It appears that different isoforms of troponin T are responsible for a slightly higher Ca2+ sensitivity of tension development in the high-velocity fibres compared to the intermediate fibres. The continuum in troponin T isoform composition paralleled an increase in Vmax among these fibres. 5. The low-velocity fibres had the highest Ca2+ sensitivity of the three groups and had exclusively the slow-type isoforms of the regulatory proteins in the troponin complex. 6. The co-ordinated variations in troponin T and LC3 compositions among the intermediate- and high-velocity fibres are discussed as a possible means for the further differentiation of the contractile properties of the fibres in these two groups, beyond that provided by myosin heavy chain isoforms alone.

Myosin heavy chain composition of single cells from avian slow skeletal muscle is strongly correlated with velocity of shortening during development

We have determined the myosin heavy chain (MHC) composition (using a sensitive sodium dodecyl sulfate-polyacrylamide gel electrophoresis system) and the maximal velocity of shortening (Vmax) of single cells from neonatal and adult chicken anterior latissimus dorsi (ALD) muscles. In addition, the MHC, myosin light chain, and regulatory protein (i.e., troponin and tropomyosin subunits) compositions of bundles of ALD fibers were determined at late embryonic, neonatal, and adult ages. At young ages, there are two MHCs in ALD muscle, SM1 and SM2, with SM1 decreasing in relative amount with increasing age, as shown previously by others. The mean Vmax of single fibers also decreases from neonatal to adult ages. A strong quantitative correlation is demonstrated between the specific MHC composition and Vmax among individual cells of the ALD muscle at several ages. Since virtually no changes occur in the regulatory protein and myosin light chain compositions of the ALD muscle between late embryonic and adult ages, it appears that the MHC composition of an individual cell in this muscle is the primary determinant of the maximal shortening velocity. These results are the first to illustrate the functional significance of the developmental transition in myosin heavy chain composition of an avian slow skeletal muscle, consistent with our previous findings on mammalian muscle.

Functional significance of myosin transitions in single fibers of developing soleus muscle

The maximal velocity of shortening and myosin heavy chain (MHC) composition of single, chemically skinned fibers from neonatal and adult rat soleus muscles were examined to determine the relationship between these parameters during slow muscle development in the rat. In addition, the MHC composition of bundles of fibers from soleus muscles at the same ages was studied. The MHC compositions were examined using sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis. The results from the bundles of fibers indicate that from 3 days to 5 mo postnatal, the rat soleus contains predominantly MHCs that migrate in the vicinity of the MHC from adult slow muscle. From 14 days to 2 mo postnatal, there are also significant amounts of additional MHCs that comigrate on SDS gels with those characteristic of adult rat fast muscle. All the fibers studied at 3 and 7 days postnatal and at 5 mo and the majority of fibers from 14 days to 2 mo postnatal had relatively low shortening velocities. A few fibers from the latter group had significantly higher velocities. The faster fibers at each age had greater amounts of the MHCs that comigrate with the adult fast-type MHC on SDS gels. Thus the velocity of shortening of single fibers from the rat soleus muscle appears to be related to MHC composition during postnatal development.

Effects of immobilization on the isometric contractile properties of embryonic avian skeletal muscle

Chicken embryos were chronically immobilized by applying a neuromuscular blocking agent, curare, to the chorioallantoic membrane from day 8 through day 16 of incubation to study the effects of a deficit in motor activity on the development of contractile properties of skeletal muscle. Compared with control embryos, spontaneous embryonic motor activity was depressed by 60 to 90% in the curare-treated animals during the treatment period. Growth of the posterior latissimus dorsi muscle, a fast-twitch muscle in the adult, was greatly affected by immobilization. The average blotted mass of the muscles from curare-treated 18- to 19-day embryos was approximately 20% of that from control embryos. The isometric contractile properties of posterior latissimus dorsi muscles isolated from control and curare-treated embryos were compared at 18 to 19 days of incubation. The times to peak tension and to one-half relaxation of the twitch and tetanic responses were significantly greater for the muscles from the immobilized embryos. The peak twitch and tetanic tensions, normalized for muscle cross-sectional area, were significantly less than control values for the muscles from curare-treated embryos. The maximal rate of tetanic tension production was, however, unaffected by immobilization. The results of this study demonstrate that the development of isometric contractile properties of embryonic skeletal muscle is significantly altered by an experimentally induced reduction of spontaneous motor activity. A disruption in the functional development of the sarcoplasmic reticulum following a similar decrease in motor activity, as reported by others, is discussed as a potential mechanism for the altered contractile properties of muscles from the curare-treated embryos.

Myosin subunits and contractile properties of single fibers from hypokinetic rat muscles

The effects of prolonged hypokinesia on the contractile properties and myosin isozymes of single fibers from the synergistic fast-twitch plantaris (PL) and slow-twitch soleus (SOL) skeletal muscles of adult rats were studied after 28 days of hindlimb suspension. There was a 31% increase in the mean maximal velocity of unloaded shortening (Vmax) among fibers from SOL with no change in the mean Vmax of fibers from PL after suspension. The myosin heavy and light chain (MHC and MLC) composition of bundles and the MHC composition of single fibers from control and suspended muscles were examined using sodium dodecyl sulfate-polyacrylamide gel electrophoresis. There was a marked increase in the relative amount of fast-type MHC's in hypokinetic SOL and a smaller increase in the amount of fast-type MHC's in the PL. Relatively minor changes occurred in the MLC's during hypokinesia. As Vmax increased among individual fibers from control and suspended muscles, the relative amount of fast-type MHC's increased. The results demonstrate that the myosin isozyme composition of skeletal muscle, especially the heavy chains, is altered during hypokinesia, and this finding provides an explanation for changes in Vmax of rat single muscle fibers under the same conditions.

Shortening velocity and myosin heavy chains of developing rabbit muscle fibers

The regulation of vertebrate muscle contraction with respect to the role of the different subunits of myosin remains somewhat uncertain. One approach to gaining a better understanding of the molecular basis of contraction is to study developing muscle which undergoes changes in myosin isozyme composition and contractile properties during the normal course of maturation. The present study utilizes single fibers from psoas muscles of rabbits at several ages as a model system for fast-twitch muscle development. This approach eliminates the inherent problems of interpreting results from studies on whole muscles which usually contain heterogeneous fiber types with respect to contractile properties and isoenzyme composition. Maximum velocity of shortening and tension-generating ability of individual fibers were measured and the myosin heavy chain composition of the same fibers was examined using an ultrasensitive sodium dodecyl sulfate-polyacrylamide gel system. The results indicate that 1) with regard to contractile properties, there is a transitional period from slow to fast shortening velocities within the first postnatal month; 2) a strong, positive correlation exists between the speed of shortening and tension-generating ability of individual postnatal day 7 fibers, suggesting that as more myosin is incorporated in these developing fibers it is of the fast type; and 3) there is a wide variation in maximum velocity of shortening among postnatal day 7 psoas fibers which is also a time when a mixture of heavy chain isoforms characterizes the myosin composition of single muscle fibers.

Shortening velocity in single fibers from adult rabbit soleus muscles is correlated with myosin heavy chain composition

Extensive variations exist in the heavy and light chain components of myosin in vertebrate striated muscles. In the present study, we have characterized a specific contractile property, velocity of shortening, and protein subunit composition of single fibers from adult rabbit soleus muscles. Maximum velocity of shortening (Vmax) was measured using the slack test method, and the myosin composition of these same fibers was determined using an ultrasensitive sodium dodecyl sulfate-polyacrylamide gel electrophoresis system. While most fibers were found to have velocities between 0.5 and 1.0 muscle length/s, several had velocities distributed between 1.33 and 2.99 muscle length/s. The fibers in the slower group had myosin subunits that were solely of the slow type; however, those in the faster group contained both fast and slow heavy chains and light chains. The velocity of shortening measured in fibers having both myosin types was highly correlated with the myosin heavy chain composition, with velocity increasing as the proportion of fast-type heavy chain increased. Variations in light chain composition, particularly fast and slow myosin light chain 1, appeared to occur independently of the variations in heavy chain composition, suggesting that some myosin molecules consist of mixtures of slow- and fast-type subunits.

Isometric contractile properties and velocity of shortening during avian myogenesis

Isometric twitch and tetanic contractile properties and velocity of unloaded shortening (V0) of whole avian posterior latissimus dorsi muscle (PLD) were examined between embryonic day 15 and the first 2 wk after hatching. The time to peak twitch force, time to half-relaxation of the twitch response, and time to half-peak tetanic force all change significantly during the final week in ovo but do not change during the first 2 wk ex ovo. Comparisons with previously published reports by others indicate that the twitch half-relaxation time at hatching is approximately the same as that of the adult PLD. The velocity of unloaded shortening increases 2.3-fold during the period studied. It has previously been shown by other that the velocity of shortening is well correlated with a muscle's myosin ATPase activity. Therefore, the observed changes in V0 suggest that the myosin ATPase activity of the avian PLD increases between embryonic day 15 and the first 2 wk posthatching, and this change could account, at least in part, for some of the changes in the isometric properties that were measured.

Development of contractile properties in avian embryonic skeletal muscle

The development of the twitch and tetanic responses of the embryonic chick posterior latissimus dorsi muscle has been studied during the last week in ovo. Normalized twitch and tetanic forces increased 3- and 12-fold, respectively, during this period. The changes in the kinetics of the twitch and tetanic responses differed during this developmental period. The time to peak twitch force progressively decreased. The decrease in time to half-peak tetanic force and the increase in the time differential of force production of the tetanic response did not continue after day 18. A prolonged tonic contractile component was described for both the twitch and tetanic responses, particularly in muscles from the younger embryos (days 14-18). A large decrease in the time to one-half relaxation of the twitch response also takes place during the final week in ovo. This detailed description of the development of the contractile properties provides a model system of fast-twitch muscle development in which neurogenic and myogenic components of muscular differentiation can be studied from several approaches.