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MUSCLE CELL BIOLOGY AND CELL MOTILITY
Departments of 1Anatomy and Physiology and 2Experimental Veterinary Sciences, University of Padova, Padova, Italy
Submitted 10 February 2008 ; accepted in final form 26 September 2008
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ABSTRACT |
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 TOP ABSTRACT MATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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force; shortening velocity
In mammals, at least nine distinct genes coding for MyHC isoforms are expressed in skeletal muscles (1). They are grouped in clusters located in different chromosomes and forming three distinct subfamilies. 1) The subfamily of fast isoforms comprises genes coding for three isoforms (2A, 2X, and 2B) expressed in adult fast fibers of limb and trunk muscles and genes coding for extraocular, embryonic, and neonatal isoforms. 2) The subfamily of cardiac isoforms is composed of two genes, coding for β/slow MyHC, expressed in cardiac muscle and in slow or type 1 fibers of skeletal muscles, and for 
-MyHC, expressed in cardiomyocytes and in specialized skeletal muscles (masticatory, extraocular, and laryngeal), respectively. 3) Only the gene coding for the masticatory or M MyHC belongs to the third subfamily. This isoform represents a subfamily by itself not only because of the distinct chromosomal localization but also because sequence analysis carried out in cat, dog, and human shows a large diversity compared with all other MyHC isoforms (12, 15, 19); also phylogenetic comparisons point to its close link with MyHC isoforms present in distant animals as shark or crocodile and with nonmuscle isoforms (12, 19).
Masticatory MyHC has been first identified in cat temporalis muscle, more precisely in posterior temporalis bundle where it is expressed in all fibers (23) and has been defined as "superfast" because contraction time parameters of the motor units of the cat temporalis muscle were faster than those recorded in fast motor units of the limb muscles (29). A well-developed sarcoplasmic reticulum and a high (2–3 times higher than fast fibers in limb muscles) ATPase activity have been also observed in temporalis fibers (23). The most accepted name "M" derives from the exclusive localization of the expression in the masticatory muscles (12, 22). M MyHC is expressed in muscles derived from the first branchial arch, i.e., jaw-closer muscles as masseter, temporalis, pterygoideus medialis and lateralis, and also tensor veli palatini and tensor tympani, although with pronounced interspecies differences (12, 22). It is not expressed in jaw-depressor muscles despite the common embryological origin. In carnivores, jaw-closer muscles are virtually composed only by M fibers (i.e., fibers expressing M MyHC) possibly associated with few slow fibers, whereas, in primates, slow fibers and 2A fast fibers are also present, and, in marsupials, M fibers coexist with fibers expressing - and β-MyHC (11).
No comprehensive studies on structure and function of fibers expressing M MyHC are presently available. There are indications that M MyHC combines with specific masticatory myosin light subunits (MyLC) (18) and integrates in the sarcomeric structure together with other specific masticatory isoforms as masticatory -tropomyosin (22) and masticatory myosin-binding protein C (10, 31, 33). Although ATPase activity has been measured and found high both in M fibers (24) and in M myosin preparations (23), more limited information is available on the mechanical properties. Preliminary results based on the determination of fmin (the frequency at which the dynamic stiffness of the active fiber reaches a minimum) and reported by Hoh in his review (12) suggest that cat fibers expressing M MyHC are not faster than fibers expressing 2A or 2X MyHC. In a recent presentation at the Biophysical Society Annual Meeting, Reiser and Bicer (21) have confirmed that the raccoon M fibers are not faster but develop a significantly higher force compared with slow and fast 2A fibers.
In view of the uncertainty still existing about the physiology of the fibers expressing M MyHC, we sought to extend our work on the contractile properties of dog muscle fibers (30) to include fibers expressing M MyHC. In addition, we decided to repeat a similar analysis also in cat muscle fibers, with the aim to confirm or disprove the conclusion reached in dog using a second species of common domestic carnivore. MyHC isoforms of the cat have been electrophoretically separated (14, 28), and three isoforms, i.e., one slow and two fast, 2A and 2X, respectively, have been identified in trunk and limb muscle fibers, whereas no information is available on force and shortening velocity of the corresponding fiber types.
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MATERIALS AND METHODS |
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TOPABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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The following muscles were dissected to obtain samples with different composition in myosin isoforms: 1) longissimus dorsi, diaphragm, soleus (only in cats), tibialis anterior (only in dogs), and semimembranosus taken as examples of trunk and limb muscles, 2) temporalis and masseter sampled as specific sources of M fibers.
Dissected samples were divided in small bundles immersed in ice-cold skinning solution (see Single fiber mechanics.) with 50% glycerol and used for muscle fiber mechanics or frozen in isopentane cooled with liquid nitrogen for protein electrophoresis.
Gel electrophoresis. Muscle samples for protein electrophoresis were solubilized in Laemmli solution (Tris 62.5 mM pH 6.8, glycerol 10%, SDS 2.3%, β-mercaptoethanol 5%, with E-64 0.1% and leupeptin 0.1% as antiproteolytic factors). After being heated for 5 min at 80°C, appropriate amounts of the protein suspension were loaded onto polyacrylamide gels (about 1 µg of total protein/lane). For isoform identification in single fibers, segments 1–2 mm long were solubilized in 20 µl of Laemmli solution, and 2–3 µl were loaded onto gels. MyHC isoforms were separated on 8% polyacrylamide slab gels according to the protocol described previously (30).
MyHC concentration determination. In a subset of fibers, MyHC concentration was measured with a method based on electrophoretic separation and densitometry, derived from those described by Geiger et al. (8) and by D'Antona et al. (6). Briefly, the MyHC concentration in individual fiber segment was obtained as the ratio between the protein content and the volume. The volume was determined as the product of the segment length by the cross-sectional area (CSA), which, to increase the precision of the measurement for this particular set of fibers, was calculated assuming an elliptical shape. Both length and CSA were measured while the fiber segment was mounted by means of small aluminum clips in the experimental set up, as described below. The two diameters of the ellipsis were measured on a horizontal plane and on a vertical plane, respectively. The diameters on the horizontal plane were calculated as described below for CSA determination for mechanical studies, i.e., from video images taken at 400x magnification. The vertical diameter or depth was measured by carefully focusing on the top of the fiber and then on the bottom and measuring the distance between the two focus planes as proposed by Larsson and Moss (13). When the fiber segments were removed from the set up, care was taken to cut them along the edges of the clips and immerse them in precisely controlled volumes of Laemmli solution (10 µl) enriched with antiproteolytic factors (see above). After solubilization (5 min at 80°C), a known fraction of such volume (2–3 µl) was run on 10–20% polyacrylamide gradient gels together with known amounts of purified myosin (M-3889; Sigma, St. Louis, MO). Gels were then silver stained (Silver stain plus; Bio-Rad, Hercules, CA), and the brightness-area product (BAP, i.e., the product of the area of the band by the average brightness, subtracted local background after black-white inversion) of the MyHC band was determined with the accuracy of 1,200 dpi. The choice of the gradient gels was aimed to visualize all MyHC as a sharp single band, whereas the choice of silver staining was aimed to optimize the sensitivity of the method. The loading of known amounts of purified myosin provided the data to obtain, for each gel, a calibration curve that allowed the determination of the amount of myosin present in the fiber segment under investigation by interpolation. An example of calibration curve is depicted in Figure 6, A and B. As can be seen, the calibration curve showed a nonlinear trend over the whole range, likely due to saturation of the silver staining. Importantly, the amounts of MyHC from individual fiber segments fell in all cases in the initial steep part of the curve, where the sensitivity is higher. MyHC isoform composition of each fiber used for MyHC quantification was determined by 8% polyacrylamide gel electrophoresis on a second fraction of the sample.
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Skinning, relaxing, preactivating, and activating solutions employed in single fiber experiments were prepared as previously described (3). The pH of all solutions was adjusted to 7.0 at the temperature at which solutions were used (12°C or 24°C). Protease inhibitors (E64 10 µM and leupeptin 40 µM) were added to all solutions.
The fiber segments were transferred to the experimental set up and, with the help of the aluminum clips, mounted between the force transducer (AME-801 SensorOne, Sausalito, CA) and the electromagnetic puller (SI, Heidelberg, Germany) equipped with a displacement transducer. Each fiber segment was mounted in relaxing solution at slack length and then stretched by 
20% to reach a sarcomere length close to 2.8 µm and at the same time avoid any increase in resting tension that might alter the results of the slack test protocol. A sarcomere length of 2.8 µm ensures that, during maximal isometric contraction, tension is developed at the upper edge of the plateau of the tension-length relation, taking into account that series compliance is extended by 5% (see Ref. 30), and that shortening during slack test takes place over the plateau. Video images were collected at high (400x) magnification, and on such images sarcomere length and three diameters on the horizontal plane equally spaced along the segment were measured. CSA was calculated from the average of the three diameters assuming a circular shape without corrections for fiber swelling. As recently shown (5), the assumption of a circular shape leads to an overestimation of CSA by about 20%, without differences among fiber types. From relaxing solution, each segment was transferred into the preactivating solution for at least 2 min and then maximally activated by immersion in the activating solution (pCa 4.6). Relaxation was induced by returning the segment to relaxing solution, and the cycle activation-relaxation was repeated five or six times. During activation after maximal tension was achieved, a quick release (amplitude 7–15% of segment length) was performed to make the fiber slack and measure the time to tension redevelopment. From data of displacement and time to tension redevelopment, unloaded shortening velocity (vo) and series elasticity were calculated according Edman's procedure [slack test protocol (7)]. In a separate maximal activation, the quick release was followed within 15 ms by a relengthening to initial length, and the rate constant of the subsequent tension redevelopment (KTR) was determined as described by Brenner and Eisenberg (4). For KTR calculation, the curve of tension redevelopment was fitted with a single exponential. Isometric tension (Po, i.e., isometric force normalized to CSA) was calculated from the drop of force, which followed a quick release of amplitude sufficient to make the fiber slack. At the end of the mechanical experiment, all fibers were stored in Laemmli solution for electrophoretic identification of MyHC isoforms.
Statistical analysis. Data were expressed as means and standard errors. One-way ANOVA, followed by Newman-Keuls test, was used for comparison among fiber types. Statistical significance was set at P < 0.05. The Prism software (GraphPad, San Diego, CA) was utilized for linear and nonlinear fitting and statistical analysis.
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RESULTS |
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TOPABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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DISCUSSION |
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TOPABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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A second element of novelty in this study is given by the data on the contractile parameters of the fibers of cat trunk and limb muscles. Interesting comparisons can be made with muscle fibers of other species and in particular of the dog. In feline fibers, vo increases regularly from slow to fast 2A to fast 2X as expected from previous observations in several species (17, 30). Only minor diversity in vo values exists between corresponding fibers of the dog and the cat. According to the scaling principle (17, 30), similar values of kinetic parameters should imply an approximately similar body size. Actually, whereas the body mass of adult cats is rather consistent in all breeds, a large diversity in body mass is present among various dog breeds [see discussion in Toniolo et al. 2007 (30)]. It is thus uncertain which body size of the dog must be considered for interspecies comparison. Furthermore, although present results are far to be conclusive in this respect, cat muscles are expected to be faster because they are more rich in 2X fibers, whereas 2A fibers are predominant in dog muscles.
Muscles and muscle fibers are classified as fast or slow in relation to a number of distinct functional parameters: rate of tension rise and decline during isometric twitch, speed of shortening at low or zero load, and rate of ATP hydrolysis, just to mention a few. Jaw-closer muscles of carnivores have been defined fast or superfast muscles on the basis of the isometric twitch kinetics (29). They have also been considered fast in relation to the rate at which ATP is hydrolyzed by myosin alone (23) or by myosin integrated in skinned fibers (24). The parameters that characterize the contraction kinetics in maximally activated skinned fibers (vo and KTR) determined in the present study show, in substantial agreement with the available preliminary evidence on raccoon fibers (21) and on cat fibers (12), that M fibers are as fast as 2A fibers. A peculiar feature of M fibers is their ability to generate high specific tension (i.e., force normalized to CSA, Po). The present results show that M fibers can achieve Po values higher than any other fiber type at 12°C and also at 24°C. Skinned fibers become unreliable if activated at higher temperatures unless specific methods as temperature jumps after activation at low temperature are applied (20). It will be of interest to assess whether the difference in tension development is still present at physiological temperature.
The possibility that the difference in specific tension was due to a different density of the force generators, i.e., the myosin heads, was explored by determining myosin concentration in feline muscle fibers. The values obtained of myosin concentration are around 50 µg/µl, corresponding to a head concentration of 200 µM in good agreement with previously published data (8, 32). The lack of any significant difference in myosin concentration among the four groups of fiber examined gives support to an explanation on the basis of kinetic diversity. Such an explanation leaves open the question whether the difference in specific tension still exists at physiological temperature as the kinetic parameters of the cross-bridge cycle are characterized by distinct temperature sensitivity (16).
The explanation of the higher tension developed by M fibers in terms of cross-bridge kinetics can be based on the assumptions that unitary force exerted by an attached cross bridge is constant and that isometric tension is determined by the fraction of strongly bound cross bridges. Huxley's 1957 cross-bridge model predicts that the fraction of cross bridges attached is given by f/(f + g), whereas KTR is given by (f + g). The direct comparison between M fibers and 2A fibers shows that in jaw muscles the specific isometric force is 40% greater in M than that in 2A fibers while the KTR is unchanged. To fit with Huxley's model, the elevated isometric force must result from an increase in cross-bridge attachment rate (f) and an equivalent diminution in the rate of cross-bridge detachment (g) so that f/(f + g) rises, but KTR does not change. It is worth underlining that difference in g values might be restricted to the detachment rate in isometric conditions since vo, which is proportional to the detachment rate constant during filament sliding at zero load, displays similar values in M fibers and in 2A fibers.
An important caveat must be considered before entering in any further discussion on the explanation of the specific features of M fibers: in M fibers, not only MyHC but also other proteins are present with specific masticatory isoforms, among them MyLC, tropomyosin, and C protein (10, 18, 22). Available evidence confirms, however, that MyHC isoforms dictate both vo and ATP consumption rate with only a minor modulatory effect of MyLC isoforms (see Refs. 17 and 25). It is, therefore, tempting to find the basis of the specific contractile parameters of M fibers in the amino acid sequence of M MyHC. In a recently published study (15), we have compared the primary structure of M MyHC with other sarcomeric MyHC isoforms in the dog, paying particular attention to those regions where the interisoform diversity is greater. Loop 1 has been shown to modulate filament sliding velocity and ADP release rate (27) and is very short in M MyHC with only 8 residues compared with 12 in slow MyHC and 14 in 2B MyHC (15). The origin of the low detachment rate constant suggested by the above discussion on the actomyosin interactions might be found in the short length of loop 1, which controls ADP release. On the actin-binding surface, loop 2 has been proposed as determinant not only for the diversity of ATPase rate between isoforms but also for the specific sensitivity of ATPase to alkali or acid preincubation (15). M MyHC exhibits a very short loop 2 (17 residues instead of 23 present in most fast MyHC), whereas loop 3 is 1 residue longer in M MyHC than in other sarcomeric MyHC isoforms.
In his review article, Hoh (12) suggests, "Early during mammalian radiation, some taxa (carnivores, chiropterans, primates, most marsupial orders) retained masticatory myosin expression where high force and power in jaw closers remained functionally advantageous to their life style. Others (rodents, ungulates, rabbits) replaced masticatory myosin with functionally more appropriate isoforms normally expressed in limb muscles or the heart." It is, however, possible that the explanation is more complex since, as shown by the present results, the combination of high force and moderate velocity values should generate a peak power not much different in M fiber compared with other fast fibers. It is possible that presence or the absence of M MyHC expression depends on the combination of the evolutionary pressure related with feeding habits, the presence of mutations in the coding or regulatory part of the gene, and the specific programs of gene expression based on embryological origin (first branchial arch). In this respect, the observation that M MyHC is expressed also in muscles without any masticatory function (tensor veli palatini) (22) and in satellite cells of masticatory muscles transplanted in limb muscles (9) plays in favor of a major role of the embryological origin. The mutations in human M MyHC gene documented by Stedman (26) also bring support to the view of a complex interaction of several factors, and it will be of interest to study the mechanisms that, in species as the rodents, have led the complete disappearance of M MyHC expression.
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FOOTNOTES |
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The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
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REFERENCES |
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