Cellular alignment and fusion: Quantifying the effect of macrophages and fibroblasts on myoblast terminal differentiation

Impact of Uniaxial Static Strain on Myoblast Differentiation in Collagen-Coated PCL Microfilament Scaffolds: Role of Onset Time of Mechanical Stimulation

Tissue engineering endeavors to create in vitro constructs that replicate the properties of native tissue, such as skeletal muscle. This study investigated the use of mechanical stimulation to promote myogenic differentiation and enhance the functionality of bioengineered tissues. Specifically, it aimed to facilitate the differentiation of myoblasts within a three-dimensional scaffold using a defined pattern of mechanical stimulation. C2C12 cells were cultured on a collagen-coated PCL microfilament scaffold and subjected to 24 h of uniaxial static strain using a biomechanical stimulation system. Two onset times of stimulation, 72 h and 120 h post-seeding, were evaluated. Cell proliferation, myogenic marker expression, and alterations in cell morphology and orientation were assessed. Results indicate that static strain on the scaffold promoted myoblast differentiation, evidenced by morphological and molecular changes. Notably, strain initiated at 72 h induced an early differentiation stage marked by MyoD expression, whereas stimulation beginning at 120 h led to a mid-stage differentiation characterized by the co-expression of MyoD and Myogenin, culminating in myotube formation. These results highlight the critical influence of myoblast maturity at the time of strain application on the differentiation outcome. This study provides insights that could guide the optimization of mechanical stimulation protocols in tissue engineering applications.

Induced Pluripotent Stem Cells for Modeling Physiological and Pathological Striated Muscle Complexity

Neuromuscular disorders (NMDs) are a large group of diseases associated with either alterations of skeletal muscle fibers, motor neurons or neuromuscular junctions. Most of these diseases is characterized with muscle weakness or wasting and greatly alter the life of patients. Animal models do not always recapitulate the phenotype of patients. The development of innovative and representative human preclinical models is thus strongly needed for modeling the wide diversity of NMDs, characterization of disease-associated variants, investigation of novel genes function, or the development of therapies. Over the last decade, the use of patient's derived induced pluripotent stem cells (hiPSC) has resulted in tremendous progress in biomedical research, including for NMDs. Skeletal muscle is a complex tissue with multinucleated muscle fibers supported by a dense extracellular matrix and multiple cell types including motor neurons required for the contractile activity. Major challenges need now to be tackled by the scientific community to increase maturation of muscle fibers in vitro, in particular for modeling adult-onset diseases affecting this tissue (neuromuscular disorders, cachexia, sarcopenia) and the evaluation of therapeutic strategies. In the near future, rapidly evolving bioengineering approaches applied to hiPSC will undoubtedly become highly instrumental for investigating muscle pathophysiology and the development of therapeutic strategies.

Evaluating the role of nitric oxide in myogenesis in vitro

Skeletal muscle injury activates satellite cells to proliferate as myoblasts and migrate, differentiate and fuse with existing fibres at the site of injury. Nitric oxide (NO), a free radical produced by NO synthase, is elevated and supports healing after in vivo injury. NOS-independent elevation of NO levels in vitro is possible via donors such as molsidomine (SIN-1). We hypothesized that alterations in NO levels may directly influence myogenic processes critical for skeletal muscle wound healing. This study aimed to clarify the role of NO in myoblast proliferation, migration and differentiation. Baseline NO levels were established in vitro, whereafter NO levels were manipulated during myogenesis using l-NAME (NOS inhibitor) or SIN-1. Baseline NO levels generated by myoblasts in proliferation media did not change 1 h after stimulation. Addition of a pro-proliferative dose of HGF slightly elevated NO levels 1 h post-stimulation, whereas cell numbers assessed 24 h later increased significantly; l-NAME reduced the HGF-driven increase in NO and proliferation, reducing wound closure over 16 h. In differentiation media, NO levels increased significantly within 24 h, returning to baseline over several days. Regular addition of l-NAME to differentiating cells significantly reduced NO levels and fusion. SIN-1 increased NO levels in a dose-dependent manner, reaching maximal levels 16 h post-treatment. SIN-1, added at 0, 2 and 4 days, significantly increased myofiber area (26 ± 1.8% vs 18.6 ± 3.4% in control at 5 day, p < 0.0001), without affecting proliferation or migration. In conclusion, this study demonstrates that, during skeletal muscle regeneration, increased NO specifically stimulates myoblast differentiation.

Co-culture of pro-inflammatory macrophages and myofibroblasts: Evaluating morphological phenotypes and screening the effects of signaling pathway inhibitors

Skeletal muscle regeneration is a complex process influenced by non-myogenic macrophages and fibroblasts, which acquire different phenotypes in response to changes in the injury milieu or changes in experimental conditions. In vitro, serum stimulates the differentiation of fibroblasts into myofibroblasts, while lipopolysaccharide (LPS) stimulates the polarization of unstimulated (M0) macrophages to acquire an M1 pro-inflammatory phenotype. We characterized these phenotypes using morphology (with circularity as shape descriptor; perfect circularity = 1.0) and phenotype-specific markers. Myofibroblasts (high α-smooth muscle actin [SMA] expression) had high circularity (mean 0.60 ± 0.03). Their de-differentiation to fibroblasts (low α-SMA expression) significantly lessened circularity (0.47 ± 0.01 and 0.35 ± 0.02 in 2% or 0% serum culture media respectively (p < 0.05). Unstimulated (M0) macrophages (no CD86 expression) had high circularity (0.72 ± 0.02) which decreased when stimulated to M1 macrophages (CD86 expression) (LPS; 0.61 ± 0.02; p < 0.05). Utilizing these established conditions, we then co-cultured M1 macrophages with myofibroblasts or myoblasts. M1 macrophages significantly decreased relative myofibroblast numbers (from 223 ± 22% to 64 ± 7%), but not myoblast numbers. This pro-inflammatory co-culture model was used to rapidly screen the following four compounds for ability to prevent M1 macrophage-mediated decrease in myofibroblast numbers: L-NAME (inducible nitric oxide synthase inhibitor), SB203580 (p38 mitogen-activated protein kinase inhibitor), SP600125 (c-Jun N-terminal kinase inhibitor) and LY294002 (phosphoinositide 3-kinase [PI3K] inhibitor). We found that LY294002 rescued myofibroblasts and decreased macrophage numbers. Myofibroblast rescue did not occur with L-NAME, SB203580 or SP600125 incubation. In conclusion, these data suggest a PI3K-associated mechanism whereby myofibroblasts can be rescued, despite simulated pro-inflammatory conditions.

Cardiac and lung endothelial cells in response to fluid shear stress on physiological matrix stiffness and composition

OBJECTIVE

Preconditioning of endothelial cells from different vascular beds has potential value for re-endothelialization and implantation of engineered tissues. Understanding how substrate stiffness and composition affects tissue-specific cell response to shear stress will aid in successful endothelialization of engineered tissues. We developed a platform to test biomechanical and biochemical stimuli.

METHODS

A novel polydimethylsiloxane-based parallel plate flow chamber enabled application of laminar fluid shear stress of 2 dynes/cm² for 12 hours to microvascular cardiac and lung endothelial cells cultured on cardiac and lung-derived extracellular matrix. Optical imaging of cells was used to quantify cell changes in cell alignment. Analysis of integrin expression was performed using flow cytometry.

RESULTS

Application of fluid shear stress caused the greatest cell alignment in cardiac endothelial cells seeded on polystyrene and lung endothelial cells on polydimethylsiloxane. This resulted in elongation of the lung endothelial cells. α_v and β₃ integrin expression decreased after application of shear stress in both cell types.

CONCLUSION

Substrate stiffness plays an important role in regulating tissue-specific endothelial response to shear stress, which may be due to differences in their native microenvironments. Furthermore, cardiac and lung endothelial cell response to shear stress was significantly regulated by the type of coating used.

Prostate Transition Zone Fibrosis is Associated with Clinical Progression in the MTOPS Study

PURPOSE

Medications targeting androgen receptor activity (eg finasteride) or smooth muscle contractility (eg doxazosin) do not resolve lower urinary tract symptoms indicative of lower urinary tract dysfunction in an important subgroup of men. Recently fibrosis has been implicated as another pathobiology contributing to male lower urinary tract symptoms but to our knowledge no systematic studies have been done to assess fibrosis in the context of medical treatment. We determine whether fibrotic changes in the prostate transition zone are associated with an increased risk of clinical progression in participants treated with doxazosin, finasteride or finasteride plus doxazosin in the MTOPS (Medical Therapy of Prostatic Symptoms) study.

MATERIALS AND METHODS

Transition zone biopsy tissues from men who did or did not experience clinical progression on placebo, doxazosin, finasteride or combination therapy were assessed for collagen content and architectural changes using picrosirius red birefringence and CT-FIRE (Curvelet Transform-Fiber Extraction) analysis. Correlations were made with annotated demographic and clinical data. Statistical analyses were done with the Pearson correlation coefficient, ANOVA and the t-test.

RESULTS

High levels of wavy, aligned prostate transition zone collagen significantly correlated with an increased risk of clinical progression among MTOPS trial participants treated with doxazosin plus finasteride, particularly those with a high body mass index.

CONCLUSIONS

Fibrotic changes in the prostate transition zone are associated with an increased risk of clinical progression in men treated with doxazosin plus finasteride. Antifibrotic therapeutics might provide a new treatment approach in men with lower urinary tract dysfunction who do not respond to current medical treatment approaches.