Originally published as JHC exPRESS on August 4, 2008. doi:10.1369/jhc.2008.951632
Volume 56 (11): 1013-1022, 2008 Copyright ©The Histochemical Society, Inc. The Effect of Passive Movement on Denervated Soleus Highlights a Differential Nerve Control on SERCA and MyHC Isoforms
Institute of Biochemistry, Faculty of General Medicine, University of Szeged, Szeged, Hungary (AS,EZ), and Laboratory of Ca2+-transport ATPases, Department of Molecular Cell Biology, K.U.Leuven, Leuven, Belgium (FW)
Correspondence to: Dr. Ern
The sarco-endoplasmic reticulum Ca2+ ATP-ase (SERCA) and myosin heavy chain (MyHC) levels were measured in hindlimb-denervated and selectively denervated rat soleus muscles. Selective denervation allowed passive movement of the soleus, whereas hindlimb denervation rendered it to passivity. To minimize chronic effects, we followed the changes only for 2 weeks. Selective denervation resulted in less muscle atrophy, a faster slow-to-fast transition of MyHC isoforms, and less coordinated expressions of the slow vs fast isoforms of MyHC and SERCA. Generally, expression of the slow-twitch type SERCA2a was found to be less dependent, whereas the slow-twitch type MyHC1 was the most dependent on innervation. Our study shows that passive movement is able to ameliorate denervation-induced atrophy of the soleus and that it also accentuates the dyscoordination in the expression of the corresponding slow and fast isoforms of MyHC and SERCA. (J Histochem Cytochem 56:1013–1022, 2008)
Key Words: SERCA MyHC selective denervation passive movement
IT IS WIDELY ACCEPTED that the pattern of neuronal activity largely defines the phenotype of skeletal muscle, whereas neurotrophic factors might exert only a supplementary role (Midrio et al. 1998
In this study, we hypothesized that, in the normal soleus muscle, the impact of nerve activity on the expression of SERCAs differs from that on the corresponding MyHC isoforms. To test this hypothesis, we denervated the hindlimb of the rat. To further refine our study, we compared this intervention with selectively denervating the soleus, thereby leaving the other hindlimb muscles intact (Zádor and Wuytack 2003
Animals and Treatments Experiments with animals were approved by the Ethics Committee of Animal Treatment of the Medical Faculty of the University of Szeged.
Male Wistar rats 3 months of age and weighing 280–350 g were used. The rats were anesthetized with an intraperitoneal injection of 1 ml of 4% chloral hydrate/100 g body weight. We treated the left hindlimbs of the animals. In the HD group,
RT-PCR
Protein Determination Total protein levels were measured by the bicinconinic acid method using the NanoDrop spectrophotometer. The homogenate made in the first step of the SERCA extraction was used to determine the total protein level.
Immunoblotting
IHC
Fiber Cross-sectional Areas
Statistics
Fresh Weight, Fiber CSA, Total RNA, and Protein Levels The muscle weight, fiber CSA, and protein and RNA content were measured in HD and SD soleus muscles. In both conditions of denervation, the fresh weight of the muscles decreased from Day 7 onward (Table 1); however, this decrease was less pronounced in SD than in HD muscles on Day 14 (p<0.05). Also, the fiber CSA was smaller compared with normal muscle at all stages in both types of denervation (Table 1). On SD, this decrease was larger after Day 3, whereas on HD, it was more pronounced after Days 7 and 14. The RNA content decreased in HD soleus at Days 7 and 14, whereas in SD muscles, it decreased only after 2 weeks. Interestingly, in SD muscles, the RNA level increased first at Days 3 and 7 compared with the level in HD muscle (Table 1). The total protein levels also decreased after 14 days in both types of denervation but more dramatically on HD than SD. All together, the fresh weight, fiber CSA, and total RNA and protein contents decreased more after 2 weeks in HD than in SD soleus.
mRNA Levels The level of the slow-twitch muscle-specific SERCA2a mRNA did not respond much to either form of denervation. Only on Day 7 of SD (Figure 1B) was it significantly elevated above normal and the HD counterpart (p<0.01). The mRNA level of the fast oxidative MyHC2a (Figure 1C) changed differently in both types of denervation: it did not deviate significantly from the normal level in the HD group, whereas in the SD muscles, it increased on Days 3 and 7 compared with the HD muscles on the same days.
The level of the fast SERCA1 mRNA (Figure 1D) was significantly increased at Day 3 in HD soleus and at Days 3 and 7 in SD. Because SERCA1 exists in two splice variants, a neonatal SERCA1b isoform (lacking a 40-bp alternative exon) and an adult SERCA1a form, we also analyzed the level of both mRNAs separately, using ratio RT-PCR (Zádor et al. 1996 The MyHC2x mRNA levels (Figure 1E) showed a pronounced increase in both types of denervations, most remarkably on Days 3 and 7 in SD, when it was significantly higher than in the HD muscles. The levels of GAPDH mRNA did not change during the denervations (Figure 1F), so it could be used as an adequate reference control level for total mRNA.
Protein Levels
The SERCA2a protein levels (Figure 2B) were not significantly altered in either type of denervation. Of note, similar results were obtained when the SERCA2a protein levels were compared between normal and 14-day HD soleus of female rats of the same weight (Figure 3 ). The MyHC2a protein levels (Figure 2C) did not seem to change significantly compared with the normal level in any of the stages of both types of denervation. However, at Day 3, the protein density showed a tendency to rise in both denervation types and showed a significant decrease at Day 14 in HD and at Days 7 and 14 in SD compared with the previous stages of the same denervation.
The total SERCA1 (SERCA1a + SERCA1b) protein levels (Figure 2D) dropped significantly only at Day 14 of HD, but it did not change in SD soleus muscles. In 200-g female rats, the SERCA1 protein level was also significantly lower (41.81%; p=0.0361) after 14 days in HD than in normal muscles (Figure 3).
IHC Results
In HD muscles, the ratio of MyHC2a-expressing Type IIA fibers (Figure 5A) increased on Days 7 and 14 in comparison with normal soleus muscles. However, in SD muscles, the percentage of IIA fibers did not change compared with normal muscles. On Day 7, the ratio of Type IIA fibers differed significantly between HD and SD.
Hybrid Fibers
SERCA Fiber Types
Coexpression of MyHCs With the Corresponding SERCAs
We used the effect of passive movement in selective denervation to highlight the difference in nerve control of expression of MyHC and SERCA isoforms. It is another novelty of our study that we showed this difference in the same non-regenerating muscles. It is clear that the relative expression levels of slow myosin isoform MyHC1 and slow Ca2+ pump SERCA2a are less coordinated in denervated soleus than in the normal muscle. Particularly, the mRNA and protein levels of MyHC1 decreased on denervation, whereas those of SERCA2a did not change or increased. This shows that SERCA2a is not directly dependent on the nerve in the normal soleus. Our results do not agree with the previous report of Schulte et al. (1994) 200 g, whereas we used male rats of 300 g. To make the comparison more relevant, we also measured the SERCA1 and SERCA2a protein levels in 200-g female Wistar rats after 14 days of HD, and we obtained similar results as from 300-g male rats (Figure 3). Therefore, the difference in the results of Schulte et al. (1994)
The analysis of SERCA protein levels in Schulte et al. (1994)
Unlike in Schulte et al. (1994)
The lack of decrease in SERCA2a expression (mRNA and protein) on denervation that we observed is in agreement with a corresponding absence of changes in SERCA2a levels in spinal cord isolation (Zhong et al. 2002 In contrast to SERCA2a, the level of SERCA1 clearly decreased in the soleus after 2 weeks of denervation. This means that the changes in the level of the fast muscle–specific Ca2+ pump were also not coordinated with those of the fast-type MyHC2a, because the latter remained unchanged on denervation. However, at the level of the fibers, the coordination of expression of SERCA1 and MyHC2a was stronger than that of SERCA2a and MyHC1.
It is worthwhile to note that, independently from the protein, both SERCA1 mRNA isoforms were increased at 3 days of denervation. A similar increase of these mRNAs can be induced by stretch (Zádor et al. 1999 The largest discrepancy in coordinated protein expression was found when, on SD, the MyHC1 protein level dramatically decreased, whereas the SERCA2a did not change. This drop in the level of MyHC1 protein was more than could be expected from a lack of maintenance by innervation; it indicated that a selective degradation of MyHC1 protein accompanied the slow-to-fast transformation in the muscle adapting to passive movement. A parallel drop in the level of SERCA2a was not observed, again showing that myosin and Ca2+ pumps respond differently to passive movement. On denervation, the number of MyHC1-positive and SERCA2a-positive fibers decreased more or less together, but, for both types of denervation, after 14 days, the coordination in expression of these isoforms was practically lost. The loss of coordination was more pronounced in selective denervation. A similar, but much less pronounced, loss of coordination was observed in both denervations for the fast isoforms: MyHC2a and SERCA1. The loss of coordinated expression resulted in the appearance of hybrid fibers. The ratio of MyHC1-MyHC2a hybrid fibers, SERCA1-SERCA2a hybrid fibers (Figure 5), and mismatch fibers (expressing MyHC1, SERCA1, MyHC2a, and SERCA2a) increased after 7 or 14 days in both types of denervation. More SERCA-hybrid fibers than MyHC-hybrid fibers were found in SD soleus than in HD muscles, and SERCA-hybrids appeared earlier. This also shows that the expressions of MyHC and SERCA are differentially regulated.
At longer term (i.e., denervation of >72 days), the coexpression of the corresponding myosin and SERCA isoforms again match (Hämäläinen and Pette 2001 In conclusion, the expression of SERCA and myosin isoforms seems to be separately controlled by a unique set of regulatory factors in the soleus muscle. The slow innervation is a major controller for the slow type myosin, whereas the fast myosin (MyHC2a) is expressed more when the slow myosin is decreased. Disappearance of the slow myosin isoform on denervation is, however, not balanced by an equimolar replacement of the fast isoform. Therefore, the switch of SERCA2a to SERCA1a isoform does not coincide with the slow-to-fast switch of the myosin isoforms. As a consequence, the slow-type SERCA is not expressed in coordination with the slow type myosin in many of the fibers, and there are more SERCA-hybrid fibers than myosin-hybrid fibers.
This work was supported by the TéT B-20/04 in Hungary and the BIL 02/18 from the Ministerie Vlaamse Gemeenschap, Belgium.
Received for publication April 15, 2008; accepted July 22, 2008
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