Regulation of dihydropyridine receptor gene expression in mouse skeletal muscles by stretch and disuse

Denervation-Induced Activation of the Ubiquitin-Proteasome System Reduces Skeletal Muscle Quantity Not Quality

It is well known that the ubiquitin-proteasome system is activated in response to skeletal muscle wasting and functions to degrade contractile proteins. The loss of these proteins inevitably reduces skeletal muscle size (i.e., quantity). However, it is currently unknown whether activation of this pathway also affects function by impairing the muscle’s intrinsic ability to produce force (i.e., quality). Therefore, the purpose of this study was twofold, (1) document how the ubiquitin-proteasome system responds to denervation and (2) identify the physiological consequences of these changes. To induce soleus muscle atrophy, C57BL6 mice underwent tibial nerve transection of the left hindlimb for 7 or 14 days (n = 6–8 per group). At these time points, content of several proteins within the ubiquitin-proteasome system were determined via Western blot, while ex vivo whole muscle contractility was specifically analyzed at day 14. Denervation temporarily increased several key proteins within the ubiquitin-proteasome system, including the E3 ligase MuRF1 and the proteasome subunits 19S, α7 and β5. These changes were accompanied by reductions in absolute peak force and power, which were offset when expressed relative to physiological cross-sectional area. Contrary to peak force, absolute and relative forces at submaximal stimulation frequencies were significantly greater following 14 days of denervation. Taken together, these data represent two keys findings. First, activation of the ubiquitin-proteasome system is associated with reductions in skeletal muscle quantity rather than quality. Second, shortly after denervation, it appears the muscle remodels to compensate for the loss of neural activity via changes in Ca²⁺ handling.

Abnormalities of calcium handling proteins in skeletal muscle mirror those of the heart in humans with heart failure: a shared mechanism?

BACKGROUND

In the failing human heart, abnormalities of Ca(2+) cycling have been described, but there is scant knowledge about Ca(2+) handling in the skeletal muscle of humans with heart failure (HF). We tested the hypothesis that in humans with HF, Ca(2+) cycling proteins in skeletal muscle are abnormal.

METHODS AND RESULTS

Ten advanced HF patients (50.4 ± 3.7 years), and 9 age-matched controls underwent vastus lateralis biopsy. Western blot analysis showed that sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA)2a, which is responsible for Ca(2+) sequestration into the sarcoplasmic reticulum(SR), was lower in HF versus controls (4.8 ± 0.5 vs 7.5 ± 0.8 AU, P = .01). Although phospholamban (PLN), which inhibits SERCA2a, was not different in HF versus controls, phosphorylation (SER16 site) of PLN, which relieves this inhibition, was reduced (0.8 ± 0.1 vs 3.9 ± 0.9 AU, P = .004). Dihydropyridine receptors were reduced in HF, (2.1 ± 0.4 vs 3.6 ± 0.5 AU, P = .04). We tested the hypothesis that these abnormalities of Ca(2+) handling protein content and regulation were due to increased oxidative stress, but oxygen radical scavenger proteins were not elevated in the skeletal muscle of HF patients.

CONCLUSION

In chronic HF, marked abnormalities of Ca(2+) handling proteins are present in skeletal muscle, which mirror those in failing heart tissue. This suggests a common mechanism, such as chronic augmentation of sympathetic activity and autophosphorylation of Ca(2+)-calmodulin-dependent-protein kinase II.

Functional classification of skeletal muscle networks. II. Applications to pathophysiology

In our preceding companion paper (Wang Y, Winters J, Subramaniam S. J Appl Physiol. doi: 10.1152/japplphysiol.01514.2011), we used extensive expression profile data on normal human subjects, in combination with legacy knowledge to classify skeletal muscle function into four models, namely excitation-activation, mechanical, metabolic, and signaling-production model families. In this paper, we demonstrate how this classification can be applied to study two well-characterized myopathies: amyotrophic lateral sclerosis (ALS) and Duchenne muscular dystrophy (DMD). Using skeletal muscle profile data from ALS and DMD patients compared with that from normal subjects, normal young in the case of DMD, we delineate molecular mechanisms that are causative and consequential to skeletal muscle dysfunction. In ALS, our analysis establishes the metabolic role and specifically identifies the mechanisms of calcium dysregulation and defects in mitochondrial transport of materials as important for muscle dysfunction. In DMD, we illustrate how impaired mechanical function is strongly coordinated with other three functional networks, resulting in transformation of the skeletal muscle into hybrid forms as a compensatory mechanism. Our functional models also provide, in exquisite detail, the mechanistic role of myriad proteins in these four families in normal and disease function.

Altered myoplasmic Ca(2+) handling in rat fast-twitch skeletal muscle fibres during disuse atrophy

Calcium-dependent signalling pathways are believed to play an important role in skeletal muscle atrophy, but whether intracellular Ca(2+) homeostasis is affected in that situation remains obscure. We show here that there is a 20% atrophy of the fast-type flexor digitorum brevis (FDB) muscle in rats hind limb unloaded (HU) for 2 weeks, with no change in fibre type distribution. In voltage-clamp experiments, the amplitude of the slow Ca(2+) current was found similar in fibres from control and HU animals. In fibres loaded with the Ca(2+) dye indo-1, the value for the rate of [Ca(2+)] decay after the end of 5-100-ms-long voltage-clamp depolarisations from -80 to +10 mV was found to be 30-50% lower in fibres from HU animals. This effect was consistent with a reduced contribution of both saturable and non-saturable components of myoplasmic Ca(2+) removal. However, there was no change in the relative amount of parvalbumin, and type 1 sarco-endoplasmic reticulum Ca(2+)-ATPase was increased by a factor of three in the atrophied muscles. Confocal imaging of mitochondrial membrane potential showed that atrophied FDB fibres had significantly depolarized mitochondria as compared to control fibres. Depolarization of mitochondria in control fibres with carbonyl cyanide-p-trifluoromethoxyphenylhydrazone induced a slowing of the decay of [Ca(2+)] transients accompanied by an increase in resting [Ca(2+)] and a reduction of the peak amplitude of the transients. Overall results provide the first functional evidence for severely altered intracellular Ca(2+) removal capabilities in atrophied fast-type muscle fibres and highlight the possible contribution of reduced mitochondrial polarisation.

Differentially expressed genes and morphological changes during lengthened immobilization in rat soleus muscle

To examine the effect of lengthened immobilization on the expression of genes and concomitant morphological changes in soleus muscle, rat hindlimbs were immobilized at the ankle in full dorsiflexion by plaster cast. After removing the muscle (after 1 hr, 1, 4, and 7 days of immobilization), morphology and differential gene expression were analyzed through electron microscopy and differential display reverse transcription-polymerase chain reaction (DDRT-PCR), respectively. At the myotendinous junction (MTJ), a large cytoplasmic space appeared after 1 hr of immobilization and became enlarged over time, together with damaged Z lines. Interfibrillar space was detected after 1 day of immobilization, but diminished after 7 days. At the muscle belly, Z-line streaming and widening were observed following 1 hr of immobilization. Disorganization of myofilaments (misalignment of adjacent sarcomeres, distortion, or absence of Z lines) was detected after 4 days. Furthermore, mitochondrial swelling and cristae disruption were observed after 1 day of stretching. A set of 15 differentially expressed candidate genes was identified through DDRT-PCR. Of 11 known genes, seven (Atp5g3, TOM22, INrf2, Slc25a4, Hdac6, Tpm1, and Sv2b) were up and three (Podxl, Myh1, and Surf1) were down-regulated following immobilization. In the case of Acyp2, 1-day stretching-specific expression was observed. Atp5g3, Slc25a4, TOM22, and Surf1 are mitochondrial proteins related to energy metabolism, except TOM22, which has a chaperone-like activity located in the mitochondrial outer membrane. Together with these, INrf2, Hdac6, Podxl, and Acyp2 are related more or less to stress-induced apoptosis, indicating the responses to apoptotic changes in mitochondria caused by stretching. The expression of both Tpm1 and Myh1, fast twitch isoforms, suggests adaption to the immobilization. These results altogether indicate that lengthened immobilization regulates the expression of several stress/apoptosis-related and muscle-specific genes responsible for the slow-to-fast transition in soleus muscle despite profound muscle atrophy.

Denervation in murine fast-twitch muscle: short-term physiological changes and temporal expression profiling

Denervation deeply affects muscle structure and function, the alterations being different in slow and fast muscles. Because the effects of denervation on fast muscles are still controversial, and high-throughput studies on gene expression in denervated muscles are lacking, we studied gene expression during atrophy progression following denervation in mouse tibialis anterior (TA). The sciatic nerve was cut close to trochanter in adult CD1 mice. One, three, seven, and fourteen days after denervation, animals were killed and TA muscles were dissected out and utilized for physiological experiments and gene expression studies. Target cDNAs from TA muscles were hybridized on a dedicated cDNA microarray of muscle genes. Seventy-one genes were found differentially expressed. Microarray results were validated, and the expression of relevant genes not probed on our array was monitored by real-time quantitative PCR (RQ-PCR). Nuclear- and mitochondrial-encoded genes implicated in energy metabolism were consistently downregulated. Among genes implicated in muscle contraction (myofibrillar and sarcoplasmic reticulum), genes typical of fast fibers were downregulated, whereas those typical of slow fibers were upregulated. Electrophoresis and Western blot showed less pronounced changes in myofibrillar protein expression, partially confirming changes in gene expression. Isometric tension of skinned fibers was little affected by denervation, whereas calcium sensitivity decreased. Functional studies in mouse extensor digitorum longus muscle showed prolongation in twitch time parameters and shift to the left in force-frequency curves after denervation. We conclude that, if studied at the mRNA level, fast muscles appear not less responsive than slow muscles to the interruption of neural stimulation.