Resting Potential–dependent Regulation of the Voltage Sensitivity of Sodium Channel Gating in Rat Skeletal Muscle In Vivo

Normal muscle has a resting potential of −85 mV, but in a number of situations there is depolarization of the resting potential that alters excitability. To better understand the effect of resting potential on muscle excitability we attempted to accurately simulate excitability at both normal and depolarized resting potentials. To accurately simulate excitability we found that it was necessary to include a resting potential–dependent shift in the voltage dependence of sodium channel activation and fast inactivation. We recorded sodium currents from muscle fibers in vivo and found that prolonged changes in holding potential cause shifts in the voltage dependence of both activation and fast inactivation of sodium currents. We also found that altering the amplitude of the prepulse or test pulse produced differences in the voltage dependence of activation and inactivation respectively. Since only the Nav1.4 sodium channel isoform is present in significant quantity in adult skeletal muscle, this suggests that either there are multiple states of Nav1.4 that differ in their voltage dependence of gating or there is a distribution in the voltage dependence of gating of Nav1.4. Taken together, our data suggest that changes in resting potential toward more positive potentials favor states of Nav1.4 with depolarized voltage dependence of gating and thus shift voltage dependence of the sodium current. We propose that resting potential–induced shifts in the voltage dependence of sodium channel gating are essential to properly regulate muscle excitability in vivo.

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Exercise-enhanced satellite cell proliferation and new myonuclear accretion in rat skeletal muscle

Journal of Applied Physiology ◽

10.1152/jappl.2001.90.4.1407 ◽

2001 ◽

Vol 90 (4) ◽

pp. 1407-1414 ◽

Cited By ~ 59

Author(s):

Heather K. Smith ◽

Linda Maxwell ◽

Carol D. Rodgers ◽

Nancy H. McKee ◽

Michael J. Plyley

Keyword(s):

Skeletal Muscle ◽

Cell Proliferation ◽

Satellite Cell ◽

Normal Activity ◽

Adult Skeletal Muscle ◽

Vastus Intermedius ◽

Muscle Loading ◽

Satellite Cell Proliferation ◽

Exercise Induced

The effects of increased functional loading on early cellular regenerative events after exercise-induced injury in adult skeletal muscle were examined with the use of in vivo labeling of replicating myofiber nuclei and immunocyto- and histochemical techniques. Satellite cell proliferation in the soleus (Sol) of nonexercised rats (0.4 ± 0.2% of fibers) was unchanged after an initial bout of declined treadmill exercise but was elevated after two (1.0 ± 0.2%, P ≤ 0.01), but not four or seven, daily bouts of the same task. Myonuclei produced over the 7-day period comprised 0.9–1.9% of myonuclei in isolated fibers of Sol, tibialis anterior, and vastus intermedius of nonexercised rats. The accretion of new myonuclei was enhanced ( P ≤ 0.05) in Sol and vastus intermedius by the initial exercise followed by normal activity (to 3.1–3.4% of myonuclei) and more so by continued daily exercise (4.2–5.3%). Observed coincident with a lower incidence of histological fiber injury and unchanged fiber diameter and myonuclei per millimeter, the greater new myonuclear accretion induced by continued muscle loading may contribute to an enhanced fiber repair and regeneration after exercise-induced injury.

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A Nonerythroid Isoform of Protein 4.1R Interacts with Components of the Contractile Apparatus in Skeletal Myofibers

Molecular Biology of the Cell ◽

10.1091/mbc.11.11.3805 ◽

2000 ◽

Vol 11 (11) ◽

pp. 3805-3817 ◽

Cited By ~ 29

Author(s):

Aikaterini Kontrogianni-Konstantopoulos ◽

Shu-Ching Huang ◽

Edward J. Benz

Keyword(s):

Skeletal Muscle ◽

Protein S ◽

Binding Assays ◽

Adult Skeletal Muscle ◽

Exon 2 ◽

Alternatively Spliced ◽

Translation Initiation Codon ◽

Protein 4.1R

The ∼80-kDa erythroid 4.1R protein is a major component of the erythrocyte cytoskeleton, where it links transmembrane proteins to the underlying spectrin/actin complexes. A diverse collection of 4.1R isoforms has been described in nonerythroid cells, ranging from ∼30 to ∼210 kDa. In the current study, we identified the number and primary structure of 4.1R isoforms expressed in adult skeletal muscle and characterized the localization patterns of 4.1R message and protein. Skeletal muscle 4.1R appears to originate solely from the upstream translation initiation codon (AUG-1) residing in exon 2′. Combinations of alternatively spliced downstream exons generate an array of distinct 4.1R spliceoforms. Two major isoform classes of ∼105/110 and ∼135 kDa are present in muscle homogenates. 4.1R transcripts are distributed in highly ordered signal stripes, whereas 4.1R protein(s) decorate the sarcoplasm in transverse striations that are in register with A-bands. An ∼105/110-kDa 4.1R isoform appears to occur in vivo in a supramolecular complex with major sarcomeric proteins, including myosin, α-actin, and α-tropomyosin. In vitro binding assays showed that 4.1R may interact directly with the aforementioned contractile proteins through its 10-kDa domain. All of these observations suggest a topological model whereby 4.1R may play a scaffolding role by anchoring the actomyosin myofilaments and possibly modulating their displacements during contraction/relaxation.

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Molecular Targets of Androgen Signaling that Characterize Skeletal Muscle Recovery and Regeneration

Nuclear Receptor Signaling ◽

10.1621/nrs.13005 ◽

2015 ◽

Vol 13 (1) ◽

pp. nrs.13005 ◽

Cited By ~ 11

Author(s):

James G. MacKrell ◽

Benjamin C. Yaden ◽

Heather Bullock ◽

Keyue Chen ◽

Pamela Shetler ◽

...

Keyword(s):

Skeletal Muscle ◽

Molecular Targets ◽

Gene Signature ◽

Muscle Recovery ◽

In Vivo Models ◽

Adult Skeletal Muscle ◽

Injury Model ◽

Androgen Regulation

The high regenerative capacity of adult skeletal muscle relies on a self-renewing depot of adult stem cells, termed muscle satellite cells (MSCs). Androgens, known mediators of overall body composition and specifically skeletal muscle mass, have been shown to regulate MSCs. The possible overlapping function of androgen regulation of muscle growth and MSC activation has not been carefully investigated with regards to muscle regeneration. Therefore, the aim of this study was to examine coinciding androgen-mediated genetic changes in an in vitro MSC model and clinically relevant in vivo models. A gene signature was established via microarray analysis for androgen-mediated MSC engagement and highlighted several markers including follistatin (FST), IGF-1, C-X-C chemokine receptor 4 (CXCR4), hepatocyte growth factor (HGF) and glucocorticoid receptor (GR/Nr3c1). In an in vivo muscle atrophy model, androgen re-supplementation significantly increased muscle size and expression of IGF-1, FST, and HGF, while significantly decreasing expression of GR. Biphasic gene expression profiles over the 7-day re-supplementation period identifed temporal androgen regulation of molecular targets involved in satellite cell engagement into myogenesis. In a muscle injury model, removal of androgens resulted in delayed muscle recovery and regeneration. Modifications in the androgen signaling gene signature, along with reduced Pax7 and MyoD expression, suggested that limited MSC activation and increased inflammation contributed to the delayed regeneration. However, enhanced MSC activation in the androgen-deplete mouse injury model was driven by an androgen receptor (AR) agonist. These results provide novel in vitro and in vivo evidence describing molecular targets of androgen signaling, while also increasing support for translational use of AR agonists in skeletal muscle recovery and regeneration.

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FGF-2–dependent signaling activated in aged human skeletal muscle promotes intramuscular adipogenesis

Proceedings of the National Academy of Sciences ◽

10.1073/pnas.2021013118 ◽

2021 ◽

Vol 118 (37) ◽

pp. e2021013118 ◽

Cited By ~ 1

Author(s):

Sebastian Mathes ◽

Alexandra Fahrner ◽

Umesh Ghoshdastider ◽

Hannes A. Rüdiger ◽

Michael Leunig ◽

...

Keyword(s):

Skeletal Muscle ◽

Transcriptional Activation ◽

Human Skeletal Muscle ◽

Muscle Growth ◽

Muscle Cells ◽

Adeno Associated Virus ◽

Adult Skeletal Muscle

Aged skeletal muscle is markedly affected by fatty muscle infiltration, and strategies to reduce the occurrence of intramuscular adipocytes are urgently needed. Here, we show that fibroblast growth factor-2 (FGF-2) not only stimulates muscle growth but also promotes intramuscular adipogenesis. Using multiple screening assays upstream and downstream of microRNA (miR)-29a signaling, we located the secreted protein and adipogenic inhibitor SPARC to an FGF-2 signaling pathway that is conserved between skeletal muscle cells from mice and humans and that is activated in skeletal muscle of aged mice and humans. FGF-2 induces the miR-29a/SPARC axis through transcriptional activation of FRA-1, which binds and activates an evolutionary conserved AP-1 site element proximal in the miR-29a promoter. Genetic deletions in muscle cells and adeno-associated virus–mediated overexpression of FGF-2 or SPARC in mouse skeletal muscle revealed that this axis regulates differentiation of fibro/adipogenic progenitors in vitro and intramuscular adipose tissue (IMAT) formation in vivo. Skeletal muscle from human donors aged >75 y versus <55 y showed activation of FGF-2–dependent signaling and increased IMAT. Thus, our data highlights a disparate role of FGF-2 in adult skeletal muscle and reveals a pathway to combat fat accumulation in aged human skeletal muscle.

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Comparison of Na+ currents from type IIa and IIb human intercostal muscle fibers

AJP Cell Physiology ◽

10.1152/ajpcell.1993.265.1.c171 ◽

1993 ◽

Vol 265 (1) ◽

pp. C171-C177 ◽

Cited By ~ 44

Author(s):

R. L. Ruff ◽

D. Whittlesey

Keyword(s):

Skeletal Muscle ◽

Voltage Dependence ◽

Muscle Fibers ◽

Fiber Types ◽

Slow Inactivation ◽

Intercostal Muscle ◽

Fast Inactivation ◽

Na Currents ◽

The Difference ◽

Fast Twitch

The voltage dependence and amplitude of Na+ currents (INa) were studied with the loose-patch voltage-clamp technique on 19 fast-twitch human intercostal skeletal muscle fibers at the endplate border and > 200 microns from the endplate (extrajunctional). The fibers were histochemically classified as fast-twitch oxidative-glycolytic (type IIa, n = 9) or fast-twitch glycolytic (type IIb, n = 10). The voltage dependence of activation and fast and slow inactivation of INa were similar for membrane patches recorded on the endplate border and on extrajunctional membrane for both fiber types. INa was about fivefold larger on the endplate border compared with extrajunctional membrane for both fiber types. Type IIb fibers had larger values of INa and manifest fast inactivation of INa at more negative potentials than type IIa fibers. The difference between type IIa and IIb fibers may enable IIb fibers to operate at higher firing frequencies for brief periods.

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Thrombospondin-4, an extracellular matrix protein expressed in the developing and adult nervous system promotes neurite outgrowth.

The Journal of Cell Biology ◽

10.1083/jcb.131.4.1083 ◽

1995 ◽

Vol 131 (4) ◽

pp. 1083-1094 ◽

Cited By ~ 102

Author(s):

S Arber ◽

P Caroni

Keyword(s):

Skeletal Muscle ◽

Extracellular Matrix ◽

Nervous System ◽

Neuromuscular Junction ◽

Neurite Outgrowth ◽

Adult Skeletal Muscle ◽

Wide Range ◽

Thrombospondin 4

Extracellular matrix (ECM) molecules are involved in multiple aspects of cell-to-cell signaling during development and in the adult. In nervous system development, specific recognition processes, e.g., during axonal pathfinding and synaptogenesis involve modulation and signaling by ECM components. Much less is known about their presence and possible roles in the adult nervous system. We now report that thrombospondin-4 (TSP-4), a recently discovered member of the TSP gene family is expressed by neurons, promotes neurite outgrowth, and accumulates at the neuromuscular junction and at certain synapse-rich structures in the adult. To search for muscle genes that may be involved in neuromuscular signaling, we isolated cDNAs induced in adult skeletal muscle by denervation. One of these cDNAs coded for the rat homologue of TSP-4. In skeletal muscle, it was expressed by muscle interstitial cells. The transcript was further detected in heart and in the developing and adult nervous system, where it was expressed by a wide range of neurons. An antiserum to the unique carboxyl-terminal end of the protein allowed to specifically detect TSP-4 in transfected cells in vitro and on cryostat sections in situ. TSP-4 associated with ECM structures in vitro and in vivo. In the adult, it accumulated at the neuromuscular junction and at synapse-rich structures in the cerebellum and retina. To analyze possible activities of TSP-4 towards neurons, we carried out coculture experiments with stably transfected COS cells and motor, sensory, or retina neurons. These experiments revealed that TSP-4 was a preferred substrate for these neurons, and promoted neurite outgrowth. The results establish TSP-4 as a neuronal ECM protein associated with certain synapse-rich structures in the adult. Its activity towards embryonic neurons in vitro and its distribution in vivo suggest that it may be involved in local signaling in the developing and adult nervous system.

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