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Department of Nutrition and Dietetics, Kings College, London, UK,
* University of Heidelberg, Institute of Physiology and Pathophysiology, Heidelberg, Germany,
Alcohol Research Unit, National Public Health Institute, Helsinki, Finland and
Department of Health Sciences, Alcuin College, University of York, Heslington, York, UK
2To whom correspondence should be addressed. E-mail: ross.hunter{at}kcl.ac.uk
| ABSTRACT |
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KEY WORDS: alcohol titin nebulin protein muscle
Skeletal muscle myopathy attributed to alcoholism occurs in between one third and two thirds of all chronic alcohol abusers, and is a major cause of morbidity (1 ,2 ). Alcoholic myopathy is characterized, in both humans and rats, by a reduction in skeletal muscle protein synthesis, total RNA and protein content, and more specifically, the myofibrillary protein contents (3 8 ). These changes are confined to muscles rich in type II muscle fibers such as plantaris and gastrocnemius, with soleus (type I fiber predominant) being relatively unaffected (5 ). Light microscopy with staining for type II fibers confirms that these changes are fiber type specific (9 ).
The muscular weakness accompanying alcoholic myopathy may therefore be ascribed to a reduction in the contractile apparatus and/or disruption of the contractile process. However, aside from the actual contractile proteins, there are many other proteins intimately involved in the contractile process. Titin (connectin) is a giant filamentous protein that spans from the Z disc to the M line, thereby forming a third filament system of the sarcomere, contributing to passive force development of relaxed muscle fibers, and maintaining the structural integrity of the myofibril during contraction (10 ). The A band section of titin provides regular binding sites for other sarcomeric proteins, and so may contribute to sarcomere assembly (11 ,12 ). The I band section consists of immunoglobulin-like domains, and the PEVK segment, both of which contribute to the extensibility and passive force development of relaxed skeletal muscle fibers when stretched (10 ). The titin molecule also contains phosphorylation sites, a serine/threonine kinase domain and binding sites for muscle-specific calpain proteases, indicating that it may also play a role in myofibrillar signal transduction pathways (11 ). Nebulin is an inextensible filamentous giant protein associated with the Z disc, thought to act as a molecular ruler to specify the length of the actin thin filament (13 ). Nebulin may also act as scaffolding to which actin monomers must join during sarcomere assembly, therefore playing a role in myofibrillary synthesis (14 ).
This study was designed to quantify changes in the cytoskeletal proteins nebulin and titin attributed to alcohol. We used the new high fat, alcohol feeding regimen, previously shown to cause histological liver lesions exceeding those caused by traditional high carbohydrate diets (15 ). Clinically, women are more susceptible than men to alcoholic myopathy, cardiomyopathy and liver disease (16 ,17 ), so we investigated whether there was any interaction between the effects of alcohol and gender on skeletal muscle biochemistry, to see whether a biochemical basis for this clinical observation could be found.
| MATERIALS AND METHODS |
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All rats were treated in accordance with the ethical guidelines provided by the university at which the study was conducted. Wistar rats (Mollegaard, Ejby, Denmark) were individually housed in stainless steel wire cages. There were four groups: 1) male rats consumed the alcohol containing diet ad libitum, 2) male rats pair-fed the control diet, 3) female rats consumed the alcohol containing diet ad libitum, 4) female rats pair-fed the control diet. In alcohol feeding studies, the control rats should always be pair-fed to the alcohol group to control for the anorexia and reduced food consumption of rats fed alcohol (3 8 ). The mean daily intake of the rats fed alcohol was recorded, and weight-matched controls were pair-fed the same amount of the control diet the subsequent day.
The high fat/low carbohydrate liquid feeding regimen was used as described previously (15 ). This involved modification of the LieberDeCarli diet (LD101 and LD101A; Purina Mills, Richmond, IN) described previously (18 ). A 50% level of energy was provided by the LD101A diet (LD101 in controls), and corn oil was added to increase the fat content from 35 to 44%, with a corresponding decrease in the content of carbohydrate from 11 to 5.5% (or from 47 to 40% in controls). The percentage of energy supplied as ethanol was 34.5% (compared to 36% in the LieberDeCarli diet). Casein, vitamins and minerals (technical grade; Sigma, St. Louis, MO) were added to equal the composition of the LieberDeCarli diet. In addition, carboxymethylcellulose (CMC, 4 g/L; Metsa Specialty Chemicals Oy, Aanekoski, Finland) was added as a stabilizer to increase viscosity. The metabolically inert CMC is widely used as a thickener in a variety of dairy and other food industry products. There was no difference between the male and female rats in the daily consumption of alcohol per unit body weight (12.9 g/kg), with consistent elevation of diurnal blood ethanol levels (40.3 ± 14.9 mmol/L) that did not differ between genders (15 ).
Body weights are not presented because it has been previously shown that in glucose fed control and alcohol fed rats, body weights can change markedly over a day because of episodic engorgement of the liquid diets, so that body weights are not meaningful (19 ). Alcohol has been previously shown to reduce muscle weight (6 ,20 ).
After 10 wk, soleus, plantaris and gastrocnemius were removed, weighed and frozen in liquid nitrogen. Muscles were homogenized and assayed for protein and RNA. The myofibrillary protein fraction was isolated from gastrocnemius muscle by differential solubility and high speed centrifugation as described previously (21 ). Gastrocnemius protein was then analyzed for titin, nebulin and myosin heavy chain (MHC).
Tissue processing.
All steps, including homogenization, were carried out at 04°C, and all centrifugations were at 2000 x g for 10 min, unless otherwise stated. Muscles were homogenized in ice-cold water, and portions of homogenate containing 200400 mg tissue were either immediately precipitated with perchloric acid to a final concentration of 0.2 mmol/L or used for protein fractionation (described later).
Mixed protein and RNA.
After the first precipitation, the acid supernatant was discarded after centrifugation and the protein pellet was then washed twice with perchloric acid (0.2 mmol/L). The pellet was then subjected to alkali digestion in NaOH (0.3 mol/L) before protein estimation by the Biuret reaction (22 ). The protein was then reprecipitated, and RNA was measured in the supernatant (3 5 ).
Protein fractionation.
Homogenates from the above procedures (containing 200 mg tissue) were immediately transferred to 20 mL of low ionic strength buffer, pH 7.0 [imidazole, 10 mmol; KCl, 60 mmol; EGTA, 0.5 mmol; MgCl2, 4.0 mmol; sodium azide, 1.0 mmol; and dithiothreitol, 1.0 mmol/L plus 0.5% (v/v) Triton X-100] (22
). After centrifugation at 33,000 x g, the supernatant was discarded. The protein pellet was then disrupted in a ground-glass homogenizer with 10 mL of high ionic strength [potassium dihydrogen orthophosphate, 100 mmol; dipotassium hydrogen orthophosphate, 50 mmol; KCl, 300 mmol; EDTA, 1.0 mmol; and ATP, 5.0 mmol/L, pH 6.3 (22
)]. After centrifugation at 33,000 x g, supernatants containing solubilized "myofibrillar" proteins were decanted. The protein fractions were then processed as described above. Measurement of myofibrillary protein is quantitative, with a fractional recovery of
100% (3
,21
).
SDS-gel electrophoresis.
Low percentage SDSpolyacrylamide gel electrophoresis (SDSPAGE) was employed to investigate high-molecular-weight proteins in rat gastrocnemius muscles. Procedures were carried out essentially as described previously (23 ). Briefly, agarose-strengthened 2.8% polyacrylamide gels with a Laemmli buffer system (24 ) were prepared to detect titin, nebulin and myosin. Equal amounts of solubilized tissue were loaded in each lane, and the protein contents were then measured spectrophotometrically, by use of a protein assay, to ensure equal loading between lanes (Bio-Rad protein assay; Bio-Rad Laboratories GmbH, Munich, Germany). Approximately 100 µg of protein was loaded in each lane, but greater emphasis was placed on equal loading of the lanes. Protein bands were visualized with Coomassie brilliant blue R. Titin and nebulin were previously sequenced for rabbit soleus muscle (molecular weights of 3700 and 800 kDa, respectively), so rabbit soleus was used as a standard to verify that the bands obtained were in fact titin and nebulin (24 ). Gel images were scanned at a resolution of 600 dpi. Protein bands on scanned gels were analyzed for their optical volume (integrated optical density), using TotalLab software (Phoretix, Newcastle-upon-Tyne, UK). This ensured that gels were assessed objectively, given that ratios were used to express titin and nebulin relative to MHC. Data were expressed as ratios to account for variation between gels. Density ratios between proteins of interest were calculated after constructing calibration curves for myosin, nebulin and titin. The optical volume/mg of total protein was then calculated.
Statistics.
Data are expressed as means ± SEM, n = 710. Data were compared using two-way ANOVA, with differences of P < 0.05 considered significant.
| RESULTS |
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| DISCUSSION |
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The effect of alcohol on skeletal muscle biochemistry.
The ethanol feeding regimen used resulted in a myopathy of type II fibers, as evidenced by the reduced protein contents in gastrocnemius and plantaris muscles (type II fiber rich), with no effect on soleus (type I fiber predominant). This is well documented in other alcohol feeding studies (6 ). The myofibrillary protein contents and MHC specifically were also reduced, indicating that the contractile proteins in particular were affected by alcohol, as has been demonstrated in conventional alcohol feeding studies (8 ). A reduction in tissue synthetic potential was also demonstrated by the decreased RNA/protein ratio in the plantaris muscle.
The effect of alcohol on titin and nebulin.
Titin contributes to the passive force development of relaxed muscle fibers, maintains the structural integrity of the myofibril during contraction and may also be involved in sarcomere assembly and turnover (10 ). Titin has been implicated in several disease processes such as limb girdle muscular dystrophy type 2a and cardiac failure (25 ,26 ). Nebulin binds tightly to actin and may control the length of actin filaments (13 ). Genetic mutations in the nebulin gene are associated with nemaline myopathy (27 ,28 ). Therefore, titin and nebulin play important roles in normal muscle physiology and in disease.
Titin and nebulin contents were expressed relative to MHC. The titin/MHC and nebulin/MHC ratios were unaffected by alcohol, which indicates that titin and nebulin are reduced proportionately to the decrease in MHC. This reduction in titin and nebulin may decrease the stability of the sarcomere and myofibrils at rest and during contraction, which may impair muscle tension generation and function.
The effect of gender.
Because women are clinically more susceptible than men to the pathological effects of alcohol (16 ,17 ), male and female groups were included to explore any effect of gender on alcohol-induced perturbations in skeletal muscle biochemistry. The RNA/protein ratio in plantaris muscle showed an alcohol/gender interaction, with a greater reduction in females. The RNA/protein ratio is a measure of the tissues capacity for protein synthesis (29 ). The protein content is a static measure that gives little insight into rates of synthesis and proteolysis. The decreased protein content in alcoholic myopathy is attributed to reduced rates of protein synthesis. This is thought to be under both translational and transcriptional control, with a reduction in ribosomal RNA, attributed at least in part to increased RNase activity (30 ). The interaction between gender and alcohol for this variable may indicate differences between the sexes in the effects of alcohol on RNase activity.
This is the first study to explore changes in skeletal muscle expression of titin and nebulin in alcoholic myopathy. Titin and nebulin were reduced in proportion to the reduction in MHC. The decrease in titin and nebulin contents may affect muscle function. The data also showed for the first time that female rats are more susceptible than males to the adverse effects of alcohol on skeletal muscle biochemistry.
| FOOTNOTES |
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3 Abbreviations used: CMC, carboxymethylcellulose; MHC, myosin heavy chain. ![]()
Manuscript received 4 July 2002. Initial review completed 24 July 2002. Revision accepted 1 October 2002.
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