doi:10.1369/jhc.6A7039.2006
Volume 55 (3): 247-254, 2007 Copyright ©The Histochemical Society, Inc. Differential Expression of Calcineurin and SR Ca2+ Handling Proteins in Equine Muscle Fibers During Early Postnatal Growth
Department of Pathobiology, Division of Anatomy and Physiology, Faculty of Veterinary Medicine, Utrecht University, Utrecht, The Netherlands Correspondence to: Karin Eizema, Division of Anatomy and Physiology, Faculty of Veterinary Medicine, Utrecht University, P.O. Box 80.158, NL-3508 TD, Utrecht, The Netherlands. E-mail: c.g.h.eizema{at}vet.uu.nl
During early postnatal development, the myosin heavy chain (MyHC) expression pattern in equine gluteus medius muscle shows adaptation to movement and load,resulting in a decrease in the number of fast MyHC fibers and an increase in the number of slow MyHC fibers. In the present study we correlated the expression of MyHC isoforms to the expression of sarcoplasmic(endo)reticulum Ca2+-ATPase 1 and 2a (SERCA), phospholamban (PLB), calcineurin A (CnA), and calcineurin B (CnB). Gluteus medius muscle biopsies were taken at 0, 2, 4, and 48 weeks and analyzed using immunofluorescence. Both SERCA isoforms and PLB were expressed in almost all fiber types at birth. From 4 weeks of age onward, SERCA1 was exclusively expressed in fast MyHC fibers and SERCA2a and PLB in slow MyHC fibers. At all time points, CnA and CnB proteins were expressed at a basal level in all fibers, but with a higher expression level in MyHC type 1 fibers. From 4 weeks onward, expression of only CnA was also higher in MyHC type 2a and 2ad fibers. We propose a double function of calcineurin in calcium homeostasis and maintenance of slow MyHC fiber type identity. Although equine muscle is already functional at birth, expression patterns of the monitored proteins still show adaptation, depending on the MyHC fiber type. (J Histochem Cytochem 55:247254, 2007)
Key Words: myosin heavy chain immunofluorescence signal transduction differentiation
MATURITY OF THE MAMMALIAN skeletal muscle system at birth is dependent on the species. Muscles are largely undifferentiated in small mammals (Arai et al. 1992
In previous reports we chose the gluteus medius muscle of the horse because of its important role in locomotion to investigate postnatal changes in MyHC protein expression (Linnane et al. 1999
The major Ca2+-sequestering protein important for relaxation, SERCA, exists in adult skeletal muscle as two isoforms, the fast SERCA1 and the slow SERCA2a isoform (Hamalainen and Pette 1997
Phenotypic differentiation leading to the mature MyHC fiber identity is assumed to be partly influenced by innervation (Gunning and Hardeman 1991
The calcium-activated protein phosphatase calcineurin (Cn) has been proposed as an important regulator of muscle fiber type to translate activity at the neuromuscular junction into a specific transcriptional response driving fiber type remodeling (Oh et al. 2005
Monitoring the expression levels of Cn in different MyHC muscle fiber types using serial sections provides clues about the fiber type-specific expression of these proteins (Torgan and Daniels 2006 In this study we monitored expression patterns of MyHC isoforms together with expression of SERCA1/2a, PLB, CnA, and CnB on a fiber-to-fiber basis during early postnatal growth. Because the muscle is already functional at birth, our hypothesis is that the expression of SERCA isoforms and PLB would at all ages be identical to the adult expression pattern. Given the role in regulating slow fiber-type expression, we speculate that at birth the amount of Cn subunits will already be higher in slow MyHC type 1 fibers as compared with fast MyHC type 2 fibers. We also expect higher expression levels of Cn subunits in fibers changing from MyHC type 2a to type 1.
All chemicals were obtained from Merck (Amsterdam, NL) unless otherwise indicated.
Foals
Muscle Biopsies
Immunofluorescence Staining
Ten-µm transverse serial sections were obtained as described previously (Eizema et al. 2003
Analyses
We analyzed expression of the Ca2+ handling proteins SERCA1, SERCA2a, and the inhibitor of SERCA2a, PLB, in biopsies from the gluteus medius muscle of young foals. Expression patterns of the Cn subunits CnA and CnB were also analyzed.
Results obtained by immunofluorescence are shown in Figure 1
. Figure 1A shows the results obtained in a biopsy taken at 0 weeks of age. Figure 1B shows the results at 48 weeks of age from the same foal (#27). Some fibers are labeled according to the MyHC protein(s) they express. Table 1
presents all the data obtained at the different ages: 0, 2, 4, and 48 weeks. Expression patterns are related to the delineated MyHC fiber type; the amount (%) of each fiber type is also indicated. As previously demonstrated, the gluteus muscle of foals consists of a high percentage of fast 2ad and 2d MyHC fibers at birth, but during the first year of life the muscle becomes slower, indicated by an increase in type 1 and 2a MyHC fiber types (Table 1) (Dingboom et al. 2002
Expression Patterns of SERCA1, SERCA2a, and PLB At birth, SERCA isoforms and PLB expression were not restricted to either MyHC fast type 2 or slow type 1 MyHC fibers (Figure 1; Table 1) as in adult muscle. At birth and at 2 weeks, a low number of slow fibers express SERCA1, whereas a substantial amount of fast fibers express SERCA2a and PLB especially at birth, albeit at a lower level. From 4 weeks of age onward, expression of SERCA1 was restricted to fast MyHC fibers and SERCA2a and PLB to slow fibers (Table 1). Expression patterns of SERCA2a and PLB were almost identical at all time points.
Expression Patterns of CnA and CnB
In this paper we compared the expression pattern of the MyHC proteins with the expression pattern of SR Ca2+ handling proteins and the two subunits of Cn during the first year of life in a large mammal. The major findings are that, in contrast to our expectations, at birth until 4 weeks of age the SERCA2a and 1 isoforms are not yet fully coordinated with the expression of slow type 1 and fast type 2 MyHC, respectively. Expression of PLB is as expected almost identical to the expression of SERCA2a. We hypothesized that the amount of Cn subunits would be higher in slow MyHC type 1 fibers. We show here that the expression level of CnA and CnB is complex and changes within the fiber types during the first 4 weeks of life. From then on, CnA has a higher expression level in MyHC type 1, 2a, and 2ad fibers; in contrast CnB has a higher expression level only in type 1 fibers.
Figure 2
displays a schematic representation of the expression patterns seen at birth (left row) and from 4 weeks onward (adult, right row). Each circle represents a MyHC fiber type; expression level of the Ca2+handling proteins and of the Cn subunits is indicated. Bold indicates a high expression level; normal indicates a low expression level. As demonstrated in the present study and from our previous studies, we found an increase in the amount of slow type 1 MyHC fibers together with a decrease in the fast type 2 MyHC fibers in these foals during the first 5 months after birth (Dingboom et al. 2002
Expression Patterns of SERCA1, SERCA2a, and PLB This study presents data on SERCA isoforms and PLB expression during early postnatal life in a large mammal. At the earliest ages, small differences between the expression of SERCA2a and PLB were seen. In fetal mice and pig, an uncoordinated expression of these two proteins was also observed (Arai et al. 1992
The general process of myogenesis is similar in different species. However, the kinetics of muscle fiber development with respect to MyHC isoform expression appears to be different among species, depending on their maturity at birth (Picard et al. 2002
Expression of MyHC and SERCA is normally highly correlated, even in experimental models in which muscle undergoes phenotypic transitions such as chronic low-frequency stimulation (Hamalainen and Pette 1997
This was also suggested by the observation that the slower muscular development of dystrophic muscle of mdx mice resulted in an increase of the discordance between the MyHC and SERCA expression early in the postnatal period (Divet et al. 2005
Previously it was shown that the number of type 1 MyHC fibers increases during the first year of life of a foal at the expense of the fast isoforms of MyHC (Dingboom et al. 1999
Expression Patterns of CnA and CnB
Our findings corroborate with those of Delling et al. (2000)
Using the myogenic C2C12 cell line, it was demonstrated that Cn influences in vitro the promoter activity of MyHC 1 and 2a genes, the same fiber types in which we observed higher expression levels of CnA (Chin et al. 1998
Additionally, we also observed basal expression of CnA and CnB in the other fiber types. This could point to a general role for Cn in maintaining calcium homeostasis in muscle fibers. Whether this role is important in all fibers or only in type 2 MyHC cannot be concluded from our data. A role in the maintenance of fiber gene programs was also suggested from studies with regeneration and transgenics (Serrano et al. 2001 We propose that low expression seen in muscle fibers is necessary for maintenance of calcium homeostasis (Figure 2); high expression is necessary for maintenance of fiber type-specific gene programs (Figure 2, bold). As also seen for the SERCA expression, the expression patterns of the Cn subunits changed until the foal was 4 weeks old, again pointing toward a not yet fully matured innervation status of the muscle at birth.
Cn is a key component of the transduction pathway relaying nerve electrical activity and transcriptional regulation of slow muscle fiber-type specificity (Serrano et al. 2001
Interestingly, it was shown in rats that using cain, a Cn inhibitor, or partial tenotomy, expression of MyHC 1 was prevented, but SERCA2a expression was unaffected in fibers of regenerating soleus muscles (Zador et al. 2005 We conclude that although equine muscle is already functional at birth, expression patterns of the monitored proteins still show adaptation, depending on the MyHC fiber type.
We thank Dr. A.F.M. Moorman and Dr. J.A.M. Korfage, University of Amsterdam, Amsterdam, The Netherlands for the generous gift of monoclonal antibodies. We are grateful for the skillful technical assistance of C.W. van der Wiel. We thank Ing. Anko de Graaff and Dr. Richard Wubbolts of the Center for Cell Imaging, Faculty of Veterinary Medicine, Utrecht University, Utrecht, The Netherlands for technical advice.
Received for publication June 23, 2006; accepted October 30, 2006
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