Mechanical stretch activates signaling events for protein translation initiation and elongation in C2C12 myoblasts

Secretome from myoblasts statically loaded at low intensity promotes tenocyte proliferation via the IGF-1 receptor pathway

Exercise is widely recognized as beneficial for tendon healing. Recently, it has been described that muscle-derived molecules secreted in response to static exercise influence tendon healing. In this study, the optimal static loading intensity for tendon healing and the composition of secretome released by myoblasts in response to different intensities of static strain were investigated. In an in vitro coculture model, myoblasts were mechanically loaded using a Flexcell Tension System. Tenocytes were seeded on transwell inserts that allowed communication between the tenocytes and myoblasts without direct contact. Proliferation and migration assays, together with RNA sequencing, were used to determine potential cellular signaling pathways. The secretome from myoblasts exposed to 2% static loading increased the proliferation and migration of the cocultured tenocytes. RNA-seq analysis revealed that this loading condition upregulated the expression of numerous genes encoding secretory proteins, including insulin-like growth factor-1 (IGF-1). Confirmation of IGF-1 expression and secretion was carried out using qPCR and enzyme-linked immunosorbt assay (ELISA), revealing a statistically significant upregulation in response to 2% static loading in comparison to both control conditions and higher loading intensities of 5% and 10%. Addition of an inhibitor of the IGF-1 receptor (PQ401) to the tenocytes significantly reduced myoblast secretome-induced tenocyte proliferation. In conclusion, IGF-1 may be an important molecule in the statically loaded myoblast secretome, which is responsible for influencing tenocytes during exercise-induced healing.

Inter-set stretch: A potential time-efficient strategy for enhancing skeletal muscle adaptations

Time is considered a primary barrier to exercise adherence. Therefore, developing time-efficient resistance training (RT) strategies that optimize muscular adaptations is of primary interest to practitioners. A novel approach to the problem involves combining intensive stretch protocols with RT. Conceivably, integrating stretch into the inter-set period may provide an added stimulus for muscle growth without increasing session duration. Mechanistically, stretch can regulate anabolic signaling via both active and passive force sensors. Emerging evidence indicates that both lengthening contractions against a high load as well as passive stretch can acutely activate anabolic intracellular signaling pathways involved in muscle hypertrophy. Although longitudinal research investigating the effects of stretching between RT sets is limited, some evidence suggests it may in fact enhance hypertrophic adaptations. Accordingly, the purpose of this paper is threefold: (1) to review how the active force of a muscle contraction and the force of a passive stretched are sensed; (2) to present evidence for the effectiveness of RT with inter-set stretch for muscle hypertrophy (3) to provide practical recommendations for application of inter-set stretch in program design as well as directions for future research.

Insights Into the Pathologic Roles and Regulation of Eukaryotic Elongation Factor-2 Kinase

Eukaryotic Elongation Factor-2 Kinase (eEF2K) acts as a negative regulator of protein synthesis, translation, and cell growth. As a structurally unique member of the alpha-kinase family, eEF2K is essential to cell survival under stressful conditions, as it contributes to both cell viability and proliferation. Known as the modulator of the global rate of protein translation, eEF2K inhibits eEF2 (eukaryotic Elongation Factor 2) and decreases translation elongation when active. eEF2K is regulated by various mechanisms, including phosphorylation through residues and autophosphorylation. Specifically, this protein kinase is downregulated through the phosphorylation of multiple sites via mTOR signaling and upregulated via the AMPK pathway. eEF2K plays important roles in numerous biological systems, including neurology, cardiology, myology, and immunology. This review provides further insights into the current roles of eEF2K and its potential to be explored as a therapeutic target for drug development.

Preparation of Stretchable Nanofibrous Sheets with Sacrificial Coaxial Electrospinning for Treatment of Traumatic Muscle Injury

Traumatic muscle injury with massive loss of muscle volume requires intramuscular implantation of proper scaffolds for fast and successful recovery. Although many artificial scaffolds effectively accelerate formation and maturation of myotubes, limited studies are showing the therapeutic effect of artificial scaffolds in animal models with massive muscle injury. In this study, improved myotube differentiation is approved on stepwise stretched gelatin nanofibers and applied to damaged muscle recovery in an animal model. The gelatin nanofibers are fabricated by a two-step process composed of co-axial electrospinning of poly(ɛ-caprolactone) and gelatin and subsequent removal of the outer shells. When stepwise stretching is applied to the myoblasts on gelatin nanofibers for five days, enhanced myotube formation and polarized elongation are observed. Animal models with volumetric loss at quadriceps femoris muscles (>50%) are transplanted with the myotubes cultivated on thin and flexible gelatin nanofiber. Treated animals more efficiently recover exercising functions of the leg when myotubes and the gelatin nanofiber are co-implanted at the injury sites. This result suggests that mechanically stimulated myotubes on gelatin nanofiber is therapeutically feasible for the robust recovery of volumetric muscle loss.

Analysis of mechanotransduction dynamics during combined mechanical stimulation and modulation of the extracellular-regulated kinase cascade uncovers hidden information within the signalling noise

Osteoporosis is a bone disease characterized by brittle bone and increased fracture incidence. With ageing societies worldwide, the disease presents a high burden on health systems. Furthermore, there are limited treatments for osteoporosis with just two anabolic pharmacological agents approved by the US Food and Drug Administration. Healthy bones are believed to be maintained via an intricate relationship between dual biochemical and mechanical (bio-mechanical) stimulations. It is widely considered that osteoporosis emerges as a result of disturbances to said relationship. The mechanotransduction process is key to this balance, and disruption of its dynamics in bone cells plays a role in osteoporosis development. Nonetheless, the exact details and mechanisms that drive and secure the health of bones are still elusive at the cellular and molecular scales. This study examined the dual modulation of mechanical stimulation and mechanotransduction activation dynamics in an osteoblast (OB). The aim was to find patterns of mechanotransduction dynamics demonstrating a significant change that can be mapped to alterations in the OB responses, specifically at the level of gene expression and osteogenic markers such as alkaline phosphatase. This was achieved using a three-dimensional hybrid multiscale computational model simulating mechanotransduction in the OB and its interaction with the extracellular matrix, combined with a numerical analytical technique. The model and the analysis method predict that within the noise of mechanotransduction, owing to modulation of the bio-mechanical stimulus and consequent gene expression, there are unique events that provide signatures for a shift in the system's dynamics. Furthermore, the study uncovered molecular interactions that can be potential drug targets.

Autophagy under glucose starvation enhances protein translation initiation in response to re-addition of glucose in C2C12 myotubes

Proteolysis is known to play a crucial role in maintaining skeletal muscle mass and function. Autophagy is a conserved intracellular process for the bulk degradation of proteins in lysosomes. Although nutrient starvation is known to induce autophagy, the effect of nutrient repletion following starvation on the mTOR pathway‐mediated protein translation remains unclear. In the present study, we examined the effect of glucose starvation on the initiation of protein translation in response to glucose re‐addition in C2C12 myotubes. Glucose starvation decreased the phosphorylation of p70 S6 kinase (p70S6K), a bonafide marker for protein translation initiation. Following re‐addition of glucose, phosphorylation of p70S6K markedly increased only in glucose‐starved cells. Inhibiting autophagy using pharmacological inhibitors diminished the effect of glucose re‐addition on the phosphorylation of p70S6K, whereas inhibition of the ubiquitin‐proteasome system did not exert any effect. In conclusion, autophagy under glucose starvation partially accounts for the activation of translation initiation by re‐addition of glucose.

Mechanical stress promotes biological functions of C2C12 myoblasts by activating PI3K/AKT/mTOR signaling pathway

The PI3K/AKT signaling pathway regulates cell proliferation and differentiation in multiple types of cells. The present study aimed to investigate the effects of mechanical stress on C2C12 cell proliferation and to explore the associated mechanisms. A cyclic mechanical stress model of C2C12 myoblasts was established. Reverse transcription‑quantitative PCR and western blotting assay were used to examine the PI3K signaling pathways involved in the progress of cell differentiation. Cell counting kit‑8 (CCK‑8) assay was used to evaluate the proliferation of C2C12 cells. Flow cytometry was employed to evaluate apoptosis following mechanical stress. The results demonstrated that mechanical stress activated the PI3K signaling pathway in C2C12 myoblasts. Mechanical stress significantly promoted phosphorylation (p‑) of AKT and expression of mammalian target of rapamycin (mTOR) compared with the normal group. Mechanical stress significantly promoted 4E‑binding protein 1 (4EBP1) expression in C2C12 cells compared with the normal group. The PI3K specific inhibitor LY294002 significantly decreased 4EBP1 expression and reduced p‑AKT and p‑mTOR expression compared with the mechanical stress group. Mechanical stress promoted C2C12 cell proliferation. Apoptosis of C2C12 significantly decreased in the mechanical stress group compared with the normal group. Cyclin D levels significantly increased in the mechanical stress group compared with the normal group. In conclusion, mechanical stress promoted biological functions of C2C12 cells by activating the PI3K/AKT signaling pathway. These results may contribute to a better understanding of the effects of mechanical stress on cells.

A new in vitro muscle contraction model and its application for analysis of mTORC1 signaling in combination with contraction and beta-hydroxy-beta-methylbutyrate administration

Several food constituents augment exercise-induced muscle strength improvement; however, the detailed mechanism underlying these combined effects is unknown because of the lack of a cultured cell model for evaluating the contraction-induced muscle protein synthesis level. Here, we aimed to establish a new in vitro muscle contraction model for analyzing the activation of mammalian target of rapamycin complex 1 (mTORC1) signaling. We adopted the tetanic electric stimulation of 50 V at 100 Hz for 10 min in L6.C11 myotubes. Akt, ERK1/2, and p70S6K phosphorylation increased significantly after electrical pulse stimulation (EPS), compared to untreated cells. Next, we used this model to analyze mTORC1 signaling in combination with exercise and beta-hydroxy-beta-methylbutyrate (HMB), an l-leucine metabolite. p70S6K phosphorylation increased significantly in the EPS+HMB group compared to that in the EPS-alone group. These findings show that our model could be used to analyze mTORC1 signaling and that HMB enhances muscle contraction-activated mTORC1 signaling.

A Novel Microfluidic Platform for Biomechano-Stimulations on a Chip

Mechanical stress has been proven to be an important factor interfering with many biological functions through mechano-sensitive elements within the cells. Despite the current interest in mechano-transduction, the development of suitable experimental tools is still characterized by the strife to design a compact device that allows high-magnification real-time imaging of the stretched cells, thus enabling to follow the dynamics of cellular response to mechanical stimulations. Here we present a microfluidic multi-layered chip that allows mechanical deformation of adherent cells maintaining a fixed focal plane, while allowing independent control of the soluble microenvironment. The device was optimized with the aid of FEM simulation and fully characterized in terms of mechanical deformation. Different cell lines were exposed to tunable mechanical strain, which results in continuous area deformation up to 20%. Thanks to the coupling of chemical glass etching, 2-dimensional deformation of a thin elastomeric membrane and microfluidic cell culture, the developed device allows a unique combination of cell mechanical stimulation, in line imaging and accurate control of cell culture microenvironment.

Effects of Cyclic Mechanical Stretch on the Proliferation of L6 Myoblasts and Its Mechanisms: PI3K/Akt and MAPK Signal Pathways Regulated by IGF-1 Receptor

Myoblast proliferation is crucial to skeletal muscle hypertrophy and regeneration. Our previous study indicated that mechanical stretch altered the proliferation of C2C12 myoblasts, associated with insulin growth factor 1 (IGF-1)-mediated phosphoinositide 3-kinase (PI3K)/Akt (also known as protein kinase B) and mitogen-activated protein kinase (MAPK) pathways through IGF-1 receptor (IGF-1R). The purpose of this study was to explore the same stretches on the proliferation of L6 myoblasts and its association with IGF-1-regulated PI3K/Akt and MAPK activations. L6 myoblasts were divided into three groups: control, 15% stretch, and 20% stretch. Stretches were achieved using FlexCell Strain Unit. Cell proliferation and IGF-1 concentration were detected by CCK8 and ELISA, respectively. IGF-1R expression, and expressions and activities of PI3K, Akt, and MAPKs (including extracellular signal-regulated kinases 1 and 2 (ERK1/2) and p38) were determined by Western blot. We found that 15% stretch promoted, while 20% stretch inhibited L6 myoblast proliferation. A 15% stretch increased IGF-1R level, although had no effect on IGF-1 secretion of L6 myoblasts, and PI3K/Akt and ERK1/2 (not p38) inhibitors attenuated 15% stretch-induced pro-proliferation. Exogenous IGF-1 reversed 20% stretch-induced anti-proliferation, accompanied with increases in IGF-1R level as well as PI3K/Akt and MAPK (ERK1/2 and p38) activations. In conclusion, stretch regulated L6 myoblasts proliferation, which may be mediated by the changes in PI3K/Akt and MAPK activations regulated by IGF-1R, despite no detectable IGF-1 from stretched L6 myoblasts.

The Role of IGF-1 Signaling in Skeletal Muscle Atrophy

Insulin-like growth factor 1 (IGF-1) is a key anabolic growth factor stimulating phosphatidylinositol 3-kinase (PI3K)/Akt signaling which is well known for regulating muscle hypertrophy. However, the role of IGF-1 in muscle atrophy is less clear. This review provides an overview of the mechanisms via which IGF-1 signaling is implicated in several conditions of muscle atrophy and via which mechanisms protein turnover is altered. IGF-1/PI3K/Akt signaling stimulates the rate of protein synthesis via p70S6Kinase and p90 ribosomal S6 kinase and negatively regulates protein degradation, predominantly by its inhibiting effect on proteasomal and lysosomal protein degradation. Caspase-dependent protein degradation is also attenuated by IGF/PI3K/Akt signaling, whereas evidence for an effect on calpain-dependent protein degradation is inconclusive. IGF-1/PI3K/Akt signaling reduces during denervation-, unloading-, and joint immobilization-induced muscle atrophy, whereas IGF-1/PI3K/Akt signaling seems unaltered during aging-associated muscle atrophy. During denervation and aging, IGF-1 overexpression or injection counteracts denervation- and aging-associated muscle atrophy, despite enhanced anabolic resistance with regard to IGF-1 signaling with aging. It remains unclear whether pharmacological stimulation of IGF-1/PI3K/Akt signaling attenuates immobilization- or unloading-induced muscle atrophy. Exploration of the possibilities to interfere with IGF-1/PI3K/Akt signaling reveals that microRNAs targeting IGF-1 signaling components are promising targets to counterbalance muscle atrophy. Overall, the findings summarized in this review show that in disuse conditions, but not with aging, IGF-1/PI3K/Akt signaling is attenuated and that in some conditions stimulation of this pathway may alleviate skeletal muscle atrophy.

Leucine supplementation after mechanical stimulation activates protein synthesis via L-type amino acid transporter 1 in vitro

Branched-chain amino acid supplements consumed following exercise are widely used to increase muscle mass. Although both exercise (ie, mechanical stimulation) and branched-chain amino acid leucine supplementation have been reported to stimulate muscle protein synthesis by activating the mammalian target of rapamycin (mTOR) signaling pathway independently, the mechanisms underlying their synergistic effects are largely unknown. Utilizing cultured differentiated C2C12 myotubes, we established a combination treatment model in which the cells were subjected to cyclic uniaxial mechanical stretching (4 h, 15%, 1 Hz) followed by stimulation with 2 mM leucine for 45 min. Phosphorylation of p70 S6 kinase (p70S6K), an mTOR-regulated marker of protein translation initiation, was significantly increased following mechanical stretching alone but returned to the baseline after 4 h. Leucine supplementation further increased p70S6K phosphorylation, with a greater increase observed in the stretched cells than in the non-stretched cells. Notably, the expression of L-type amino acid transporter 1 (LAT1), a stimulator of the mTOR pathway, was also increased by mechanical stretching, and siRNA-mediated knockdown partially attenuated leucine-induced p70S6K phosphorylation. These results suggest that mechanical stretching promotes LAT1 expression and, consequently, amino acid uptake, leading to enhanced leucine-induced activation of protein synthesis. LAT1 has been demonstrated to be a point of crosstalk between exercise- and nutrition-induced skeletal muscle growth.

Role of androgen receptor on cyclic mechanical stretch-regulated proliferation of C2C12 myoblasts and its upstream signals: IGF-1-mediated PI3K/Akt and MAPKs pathways

OBJECTS

To detect the effects of androgen receptor (AR) on cyclic mechanical stretch-modulated proliferation of C2C12 myoblasts and its pathways: roles of IGF-1, PI3K and MAPK.

METHODS

C2C12 were randomly divided into five groups: un-stretched control, six or 8 h of fifteen percent stretch, and six or 8 h of twenty percent stretch. Cyclic mechanical stretch of C2C12 were completed using a computer-controlled FlexCell Strain Unit. Cell proliferation and IGF-1 concentration in medium were detected by CCK8 and ELISA, respectively. Expressions of AR and IGF-1R, and expressions and activities of PI3K, p38 and ERK1/2 in stretched C2C12 cells were determined by Western blot.

RESULTS

①The proliferation of C2C12 cells, IGF-1 concentration in medium, expressions of AR and IGF-1R, and activities of PI3K, p38 and ERK1/2 were increased by 6 h of fifteen percent stretch, while decreased by twenty percent stretch for six or 8 h ②The fifteen percent stretch-increased proliferation of C2C12 cells was reversed by AR inhibitor, Flutamide. ③The increases of AR expression, activities of PI3K, p38 and ERK1/2 resulted from fifteen percent stretch were attenuated by IGF-1 neutralizing antibody, while twenty percent stretch-induced decreases of the above indicators were enhanced by recombinant IGF-1. ④Specific inhibitors of p38, ERK1/2 and PI3K all decreased the expression of AR in fifteen percent and twenty percent of stretched C2C12 cells.

CONCLUSIONS

Cyclic mechanical stretch modulated the proliferation of C2C12 cells, which may be attributed to the alterations of AR via IGF-1-PI3K/Akt and IGF-1-MAPK (p38, ERK1/2) pathways in C2C12 cells.

Tension Regulation at the Suture Lines for Repair of Neglected Achilles Tendon Laceration

BACKGROUND

Operative intervention is the preferred option for management of the neglected laceration of the Achilles tendon. However, the commonly used techniques rarely follow the principles of the regenerative medicine for the restoration of the lost tissue. This study postulated that incorporation of the autogenous tendon graft would properly progress when the interplay between mechanical loading and healing phases was correctly applied.

METHODS

A prospective study included 15 patients who were treated for neglected Achilles tendon laceration using the technique of lengthening of the proximal tendon stump. An absorbable reinforcement suture was used for control of the mechanical environment at the suture lines.

RESULTS

By an average 5 years of the prospective follow-up, all the repaired tendons had restored continuity and length. The calf circumference equalized to the uninjured side in 12 patients. However, 3 patients had calf atrophy but they improved compared to the preoperative measurements. Sonogram confirmed the restoration of the normal thickness and the gliding characteristics of the repaired tendon.

CONCLUSION

The technique restored continuity and tension of the repaired tendon, preserved the calf circumference, and prevented peritendinous adhesions. The absorbable reinforcement suture spontaneously allowed for the mechanical loading of the grafted tendon.

LEVEL OF EVIDENCE

Level IV, case series.

Mechanical stretch activates mammalian target of rapamycin and AMP-activated protein kinase pathways in skeletal muscle cells

Cellular protein synthesis is believed to be antagonistically regulated by mammalian target of rapamycin (mTOR) and AMP-activated protein kinase (AMPK) signaling pathways. In the present study, we examined the relationship between mTOR/p70 S6 kinase (p70S6K) and AMPK in response to mechanical stretch. C2C12 myoblasts were grown on a silicone elastomer chamber to confluence and further cultured in differentiation medium for 4 days to form multinucleated myotubes. Cells were subjected to 15% cyclic uniaxial stretch for 4 h at a frequency of 1 Hz. Phosphorylation of p70S6K at threonine 389 and AMPK at threonine 172 of the catalytic α subunit were concomitantly increased by mechanical stretch. Stimulation of the mTOR pathway by adding leucine and insulin increased the phosphorylation of p70S6K without inactivation of AMPK. In contrast, addition of compound C, a pharmacological inhibitor of AMPK, increased the phosphorylation of p70S6K in stretched cells. Activation of AMPK by the addition of 5-amino-4-imidazolecarboxamide ribonucleoside reduced the phosphorylation of p70S6K in response to mechanical stretch. In conclusion, crosstalk between mTOR and AMPK signaling was not tightly regulated in response to physiological stimuli, such as mechanical stress and/or nutrients. However, pharmacological modulation of AMPK influenced the mTOR/p70S6K signaling pathway.

Low-Frequency Mechanical Stimulation Modulates Osteogenic Differentiation of C2C12 Cells

Mechanical stimulation influences stem cell differentiation and may therefore provide improved lineage specification control for clinical applications. Low-frequency oscillatory mechanical stimulation (0.01 Hz) has recently been shown to suppress adipogenic differentiation of mesenchymal stem cells, indicating that the range of effective stimulation frequencies is not limited to those associated with locomotion, circulation, and respiration. We hypothesized that low-frequency mechanical stimulation (0.01 Hz) can also promote osteogenic cell differentiation of myoblastic C2C12 cells in combination with BMP-2. Results indicate that low-frequency mechanical stimulation can significantly enhance osteogenic gene expression, provided that differentiation is initiated by a priming period involving BMP-2 alone. Subsequent application of low-frequency mechanical stimulation appears to act synergistically with continued BMP-2 exposure to promote osteogenic differentiation of C2C12 cells and can even partially compensate for the removal of BMP-2. These effects may be mediated by the ERK and Wnt signalling pathways. Osteogenic induction of C2C12 cells by low-frequency mechanical stimulation is therefore critically dependent upon previous exposure to growth factors, and the timing of superimposed BMP-2 and mechanical stimuli can sensitively influence osteogenesis. These insights may provide a technically simple means for control of stem cell differentiation in cell-based therapies, particularly for the enhancement of differentiation toward desired lineages.

Ahsg-fetuin blocks the metabolic arm of insulin action through its interaction with the 95-kD β-subunit of the insulin receptor

We previously have shown that Ahsg, a liver glycoprotein, inhibits insulin receptor (InsR) tyrosine kinase (TK) activity and the ERK1/2 mitogenic signaling arm of insulin signaling. Here we show that Ahsg blocks insulin-stimulated GLUT4 translocation and Akt activation in intact cells (mouse myoblasts). Furthermore, Ahsg inhibits InsR autophosphorylation of highly-purified insulin holoreceptors in a cell-free, ATP-dependent system, with an IC50 within the range of single-chain Ahsg concentrations in human serum. Binding of (125)I-insulin to living cells overexpressing the InsR shows a dissociation constant (KD) of 250pM, unaltered in the presence of 300 nM Ahsg. A mutant InsR cDNA encoding the signal peptide, the β-subunit and the furin processing site, but deleting the α-subunit, was stably expressed in HEK293 cells. Treatment with peroxovanadate, but not insulin, dramatically increased the 95 kD β-subunit tyrosine phosphoryation. The level of tyrosine phosphorylation of the 95-kD β-subunit can be driven down sharply by treatment of living HEK293 transfectant cells with physiological doses of Ahsg. Treatment of myogenic cells with Ahsg blunts insulin-stimulated InsR autophosphorylation and AKT phosphorylation. Taken together, we show that Ahsg antagonizes the metabolic functions initiated by InsR activation without interference in insulin binding. The experiments suggest a direct interaction of Ahsg with the InsR ectodomain β-subunit in a mode that does not significantly alter the high-affinity binding of insulin to the holoreceptor's two complementing α-subunits.

The role of mTORC1 in regulating protein synthesis and skeletal muscle mass in response to various mechanical stimuli

Skeletal muscle plays a fundamental role in mobility, disease prevention, and quality of life. Skeletal muscle mass is, in part, determined by the rates of protein synthesis, and mechanical loading is a major regulator of protein synthesis and skeletal muscle mass. The mammalian/mechanistic target of rapamycin (mTOR), found in the multi-protein complex, mTORC1, is proposed to play an essential role in the regulation of protein synthesis and skeletal muscle mass. The purpose of this review is to examine the function of mTORC1 in relation to protein synthesis and cell growth, the current evidence from rodent and human studies for the activation of mTORC1 signaling by different types of mechanical stimuli, whether mTORC1 signaling is necessary for changes in protein synthesis and skeletal muscle mass that occur in response to different types of mechanical stimuli, and the proposed molecular signaling mechanisms that may be responsible for the mechanical activation of mTORC1 signaling.

β-hydroxy-β-methylbutyrate did not enhance high intensity resistance training-induced improvements in myofiber dimensions and myogenic capacity in aged female rats

Older women exhibit blunted skeletal muscle hypertrophy following resistance training (RT) compared to other age and gender cohorts that is partially due to an impaired regenerative capacity. In the present study, we examined whether β-hydroxy-β-methylbutyrate (HMB) provision to aged female rodents would enhance regenerative mechanisms and facilitate RT-induced myofiber growth. Nineteen-month old female Sprague-Dawley rats were randomly divided into three groups: HMB (0.48 g/kg/d; n = 6), non-HMB (n = 6), and control (n = 4). HMB and non-HMB groups underwent RT every third day for 10 weeks using a ladder climbing apparatus. Whole body strength, grip strength, and body composition was evaluated before and after RT. The gastrocnemius and soleus muscles were analyzed using magnetic resonance diffusion tensor imaging, RT-PCR, and immunohistochemistry to determine myofiber dimensions, transcript expression, and satellite cells/myonuclei, respectively. ANOVAs were used with significance set at p < 0.05. There were significant time effects (pre vs. post) for whole body strength (+262%), grip strength (+17%), lean mass (+20%), and fat mass (-19%). Both RT groups exhibited significant increases in the mean myofiber cross-sectional area (CSA) in the gastrocnemius and soleus (+8-22%) compared to control. Moreover, both groups demonstrated significant increases in the numbers of satellite cells (+100-108%) and myonuclei (+32%) in the soleus but not the gastrocnemius. A significant IGF-I mRNA elevation was only observed in soleus of the HMB group (+33%) whereas MGF and myogenin increased significantly in both groups (+32-40%). Our findings suggest that HMB did not further enhance intense RT-mediated myogenic mechanisms and myofiber CSA in aged female rats.

Engineering muscle tissues on microstructured polyelectrolyte multilayer films

The use of surface coating on biomaterials can render the original substratum with new functionalities that can improve the chemical, physical, and mechanical properties as well as enhance cellular cues such as attachment, proliferation, and differentiation. In this work, we combined biocompatible polydimethylsiloxane (PDMS) with a biomimetic polyelectrolyte multilayer (PEM) film made of poly(L-lysine) and hyaluronic acid (PLL/HA) for skeletal muscle tissue engineering. By microstructuring PDMS in grooves of a different width (5, 10, 30, and 100 μm) and by modulating the stiffness of the (PLL/HA) films, we guided skeletal muscle cell differentiation into myotubes. We found optimal conditions for both the formation of parallel-oriented myotubes and their maturation. Significantly, the myoblasts were collectively prealigned to the grooves before their differentiation. Before fusion, the highest aspect ratio and orientation of nuclei were observed for the 5 and 10 μm wide micropatterns. The formation of myotubes was observed regardless of the size of the micropatterns, and we found that their typical width was 10-12 μm. Their maturation was characterized by the immunolabeling of type II isomyosin. The amount of myosin striation was not affected by the topography, except for the 5 μm wide micropatterns. We highlighted the spatial constraints that led to an important nuclei deformation and further impairment of maturation within the 5 μm grooves. Altogether, our results show that the PEM film combined with PDMS is a powerful tool that is used for skeletal muscle engineering. This work opens perspectives for the development of skeletal muscle tissue in contact with films containing bioactive peptides or growth factors as well as for the study of pathogenic myotubes.