Changes in myosin mRNA and protein expression in denervated rat soleus and tibialis anterior

Abstract

Denervation differs from other models of reduced neuromuscular activation due to the absence of a nerve-muscle connection and limited data exists regarding the effects of denervation on myosin heavy chain (MHC) expression. Thus, adult MHC expression (I, IIa, IIx, IIb) was studied in the rat soleus and tibialis anterior (TA) at the mRNA and protein levels 2, 4, 7, 10, 14, and 30 days following sciatic nerve transection. MHC protein content was quantified with SDS/PAGE and mRNA levels with the RNaseprotection assay. Control soleus consisted predominately of type I MHC mRNA and protein, however, 4 days after denervation type I MHC mRNA was significantly decreased to 41Ϯ 8% of control and continued to remain below control values. Soleus IIa mRNA was significantly elevated 7 and 10 days after denervation while IIx mRNA remained relatively constant until 30 days when it increased to 197Ϯ 23% of control. At the protein level, soleus I MHC significantly decreased to 80% of the total while IIa MHC significantly increased to 20% of the total. At 30 days, IIx MHC protein accounted for 9.4Ϯ 1.6 % of the total soleus MHC protein. In the TA, IIb mRNA was significantly decreased to 57% of control by day 4 and remained significantly decreased for up to a month. TA IIx mRNA was also significantly decreased at 10 and 30 days after denervation. Similar to the soleus, TA IIa mRNA was significantly increased over control 7Ϫ14 days after denervation. There were no significant changes in TA MHC protein profile during one month of denervation. In both the soleus and TA, denervation significantly shifted the MHC mRNA profile as early as 4 days following denervation without any corresponding changes at the protein level. Significant mRNA changes without large changes in MHC protein composition continued throughout the denervation period suggesting that the muscle may be prevented from premature functional transitions by mechanisms such as decreased mRNA stability, translational block, or increased turnover of newly synthesized proteins.Keywords : myosin heavy chain ; denervation; ribonuclease-protection assay; adaptation.The importance of the nerve in the determination and maintenance of myosin heavy chain (MHC) isoforms has been studied using experimental models including cross-reinnervation, electrical stimulation, and denervation [1]. In contrast to cross-reinnervation or electrical stimulation, denervation provides a model in which the muscle is deprived of both electrical activity and trophic factors. The majority of denervation studies have been performed in newborn animals and suggest that continuous innervation is necessary for maturation of slow muscles but not an absolute requirement for maturation of fast muscles [2,3]. In contrast, D'Albis et al. [4] reported that denervation of the soleus and gastrocnemius in 8-day-old rabbits caused both muscles to mature to the slow-type phenotype. Those studies that have looked at adult mammals have studied the later stages of denervation (2 weeks to 5 months; [5Ϫ8]) and have provided con...

Atrophy, but not necrosis, in rabbit skeletal muscle denervated for periods up to one year

“…The fiber type transformation seen in rabbit muscles did not differ from that in rat muscles examined at an earlier stage of denervation (5,23,25,34,56,59). In one of those studies, coexpression of the different isoforms showed that the switch occurred within individual fibers and not by replacement with fibers of another type (34).…”

“…There is also some suggestion that during this final stage muscle fibers are replaced altogether by fibrous and adipose tissue. As a result of the changes during the first phase the muscle declines to 20 -30% of its original mass by 3-4 wk (5,8,25,44,52,60), and by 1-2 mo it develops only 25% of its original isometric tetanic tension (8,47). In the denervated rat EDL muscle, tetanic force is reported to decline even further over this period, to 1-3% of control values (1,14,30); at this stage mean fiber area is only 20 -30% of control (1,46), falling to 2.5% of control over subsequent months (1,13).…”

“…The mass of the affected muscles falls rapidly within 5-7 days of axotomy (11,18,22,25,55,57,58) and declines further to 30 -50% of control weight in succeeding weeks (1,5,11,14,23,41,60). After several months, muscle weight stabilizes at ϳ5-20% of control (1,13,45).…”

Denervation produces different single fiber phenotypes in fast- and slow-twitch hindlimb muscles of the rat

“…mechanically skinned fibers; myosin heavy chain isoforms; lineage; sarcoplasmic reticulum; Ca 2ϩ and Sr 2ϩ sensitivity; Long-Evans hooded rat MAMMALIAN SKELETAL MUSCLE fibers display a broad spectrum of structural and functional characteristics determined by the complement of homologous, but not identical, molecular structures involved in the excitation-contraction-relaxation cycle (33,34,41). Cross-innervation (4, 6), denervation (16,17,26,28,37), and chronic low-frequency stimulation (7,20,33,34) experiments have produced compelling evidence that the pattern of neural stimulation plays a crucial role in determining the functional and/or structural properties of skeletal muscle (38). However, neural control of fiber phenotype does not extend to the entire complement of cellular structures responsible for muscle fiber function, as demonstrated by crossinnervation studies.…”

Uncoordinated expression of the main contractile and relaxation proteins in pathological skeletal muscles

“…This implies that there exist layers of regulation of gene expression at the mRNA and the protein level that might act differently and independently (Leberer et al, 1986;Huey and Bodine, 1998). …”