This protocol describes a robust, reproducible and simple method of isolation and culture of myoblast progenitor cells from the skeletal muscle of adult and aged people. The muscles used here include foot and leg muscles. This approach enables the isolation of an enriched population of primary myoblasts for functional studies.
Skeletal muscle homeostasis depends on muscle growth (hypertrophy), atrophy and regeneration. During ageing and in several diseases, muscle wasting occurs. Loss of muscle mass and function is associated with muscle fiber type atrophy, fiber type switching, defective muscle regeneration associated with dysfunction of satellite cells, muscle stem cells, and other pathophysiological processes. These changes are associated with changes in intracellular as well as local and systemic niches. In addition to most commonly used rodent models of muscle ageing, there is a need to study muscle homeostasis and wasting using human models, which due to ethical implications, consist predominantly of in vitro cultures. Despite the wide use of human Myogenic Progenitor Cells (MPCs) and primary myoblasts in myogenesis, there is limited data on using human primary myoblast and myotube cultures to study molecular mechanisms regulating different aspects of age-associated muscle wasting, aiding in the validation of mechanisms of ageing proposed in rodent muscle. The use of human MPCs, primary myoblasts and myotubes isolated from adult and aged people, provides a physiologically relevant model of molecular mechanisms of processes associated with muscle growth, atrophy and regeneration. Here we describe in detail a robust, inexpensive, reproducible and efficient protocol for the isolation and maintenance of human MPCs and their progeny — myoblasts and myotubes from human muscle samples using enzymatic digestion. Furthermore, we have determined the passage number at which primary myoblasts from adult and aged people undergo senescence in an in vitro culture. Finally, we show the ability to transfect these myoblasts and the ability to characterize their proliferative and differentiation capacity and propose their suitability for performing functional studies of molecular mechanisms of myogenesis and muscle wasting in vitro.
Disease- and age-related progressive loss of skeletal muscle mass and function results in frailty, decline in strength and decrease in quality of life of aged people. Skeletal muscle accounts for approximately 40% body mass1. During ageing and disease, progressive atrophy of individual myofibers and reduction of muscle quality due to the infiltration of fat and fibrosis occurs1,2,3,4,5,6. Recently, it has been proposed that species-specific differences in ageing of skeletal muscle occur, specifically that muscle fiber loss occurring in rodents, may not occur in humans7. Nevertheless, the remaining muscle fibers of aged mammals are characterized by increased susceptibility to damage and impaired regeneration8. Adult muscle repair and maintenance is mediated by satellite cells9,10. Upon muscle injury and other relevant cues, satellite cells become activated and proliferate. A subset of the cells returns to the quiescent state and the remainder progresses into myoblasts (Myogenic Progenitor Cells - MPCs). These contribute to repair of the existing myofiber11. The functionality of satellite cells determines the success of muscle regeneration and the changes in satellite cell availability with ageing have been demonstrated12,13,14,15. Moreover, satellite cells from the muscle of old humans and rodents show a transcriptional profile switch and reduced regenerative potential16,17,18,19. Satellite cells of muscle from old mice and humans have also been shown to undergo senescence resulting in their reduced functionality20.
The most established cell line enabling the study of muscle homeostasis is murine C2C12 cell line21. A significant amount of studies have also used murine primary myoblasts22. These cultures have led to a significant understanding of murine and vertebrate myogenesis as well as muscle regeneration, myotube/myofiber atrophy, and hypertrophy processes occurring during muscle disease and ageing23,24,25,26. More recently, several groups have described using human primary myoblasts to study myogenesis and muscle ageing. However, there is lack of consensus with regards to differences between primary myoblasts isolated from the muscle of adult and aged humans27,28,29,30,31. Despite differences characterized in the systemic and local environment occurring during development, ageing and disease 6,32,33,34, in vitro myoblast and myotube cultures remain the most accessible tools for studying molecular mechanisms associated with muscle development, growth and atrophy. Additionally, these studies provide not only a robust, but also a relatively quick, inexpensive and high-throughput in vitro tool. Moreover, ethical implications associated with studies of human muscles mean that for functional experiments involving manipulations of gene expression, in vitro human myoblast and myotube cultures remain the only alternative available to vertebrate model organisms.
Here, we show a simple experimental protocol for robust, inexpensive, and reproducible isolation of primary myoblasts, or MPCs, from the muscle of adult and aged people and describe standardized conditions of in vitro culture (Figure 1). As primary cultures from muscle usually contain fibroblasts in addition to myoblasts, we recommend a preplating step aiming at improved purity and quality of primary myoblasts. To summarize, we have established a protocol allowing for efficient and reproducible isolation, culture and functional studies of enriched and functional MPCs/primary myoblasts from skeletal muscle of adult and aged people.
Here, we present a simple, robust, inexpensive, reproducible and efficient method of isolating muscle progenitor cells/primary myoblasts from adult and aged humans from extensor digitorium brevis, tibialis anterior or abductor halluces muscles. This protocol aims to allow studies using human primary myoblasts from adult and aged humans, especially when more sophisticated methods, such as FACS- or MACS-sorting, are not possible or not practical.
The isolation method presented in this manuscript takes approximately 2 hours. During muscle isolation, muscle was washed in 70% ethanol in order to avoid contamination. Prior to enzymatic dissociation of the muscle, it is important to cut the muscle into small but visible pieces, and avoid cell damage from too much mincing. The digestion results in the dissociation of myofibers and the release of satellite cells and myogenic precursor cells. In our case for ~20 mg of skeletal muscle, one 60 mm (20 cm2) Petri dish was the most appropriate surface area for harvesting the cells. Cells plated onto a larger surface showed reduced proliferation, whereas cells plated onto a smaller surface showed increased cell death and agglutination.
Upon isolation, the cells were cultured and expanded on laminin-covered plates. The use of non-coated surfaces tended to decrease the success of the isolation. For this reason, cells can be preferably harvested on a pre-coated surface directly after isolation. Fibroblasts-enriched cultures will predominate rather than myoblasts-derived cells if the cells are harvested on a non-coated surface directly after isolation. Apart from laminin, the use of other cell attachment solutions such as Matrigel and collagen-based reagents can be used. Coating solutions may include growth factors and other compounds that will promote cell growth, but these could alter the cell behavior and therefore the experimental results. In our experience, 10 µg/mL laminin is the optimum concentration and appropriate coating reagent for satellite cells and myoblast attachment and proliferation as it lacks any growth factor or other complements. Moreover, laminin is naturally present in the basal lamina, directly linked to the sarcolemma, which plays a key function in satellite cell attachment and migration through the skeletal muscle fiber.
The supplements of the culture media may also have a detrimental influence on the behavior of the primary myoblast. For example growth factor groups, such as FGFs or IGFs, have pleiotropic effects on primary myoblast cultures, with FGF-2 controlling both mitogenic and programmed cell death response31. It is therefore necessary to rigorously control the culture conditions, especially because the differences in the behavior of primary myoblasts isolated from muscle of adult and aged people are very likely to be due to the purity of the cultures and the likelihood of fibroblasts overrunning the myoblasts in culture during long-term cultures35. We have used 1-hour pre-plating of the cells during the first splitting onto a non-coated surface in order to decrease the contamination of the cultures with fibroblasts.
The method we describe is appropriate for isolating myogenic progenitor cells from the muscles of both adult and aged humans. The isolated cell consists of a representative myogenic population of cells as indicated by a high percentage of myogenic cells (MyoD expression and myogenic properties visualized by MF20 immunostaining in Figures 1 and 2) and can be used as an in vitro model for functional studies of processes associated with muscle homeostasis.
Previous studies have characterized the isolation and differences in properties, or the lack thereof, of human primary myoblasts from adult and aged people6,20,27,28,29,30,31,35,36,37,38. The existence of geriatric and/or non-functional human MPCs has been demonstrated6,20,22. However, no difference in the behavior of freshly isolated human MPCs has also been shown27. Our protocol allows for the isolation of primary myoblasts that at least partially retain their phenotype, such as reduced proliferative potential or senescence of primary myoblasts isolated from muscle of aged people and permits the use of these cells for functional studies of molecular mechanisms of muscle homeostasis during ageing22.
The primary myoblasts isolated using the method described here can be used not only for myogenic differentiation studies but also to investigate intracellular changes, such as changes in gene expression occurring in human myogenic precursor cells during ageing. However, changes that occur in cells during prolonged ex vivo culture need to be considered when analyzing phenotypic and genotypic changes occurring during ageing. We recommend using freshly isolated cells for this purpose.
Moreover, the primary myoblast culture method described here allows for expansion and relatively long-term culture of human primary myoblasts, allowing for robust in vitro functional studies. We have previously shown that myogenic progenitor cells isolated using our method can be used for both expression profiling and functional studies of processes associated with muscle ageing22. This method is also applicable to the muscles of adult and old rodents and allows for isolation of an enriched culture of myoblasts that can be used for profiling genetic and epigenetic changes during ageing and functional studies22. The limitations of this method include the use of, to some degree, mixed population of cells rather than a pure population of satellite cells, which can be obtained using more sophisticated published methods6,28,29,39,40,41,42,43.
We present a simplified, affordable, and reproducible protocol for the isolation of primary myoblasts cells from adult and aged humans. In our experience, the available, more sophisticated methods of isolation and culture of human primary myoblasts (such as MACS- or FACS-sorted satellite cells) are ideal for some types of studies, such as profiling transcriptomic or proteomic changes in the cells. However, these methods are expensive, require at least some level of expertise, and may prove difficult due to the low proliferative rate of pure primary myoblast cultures and fibroblasts overgrowing myoblasts.
We present a reproducible protocol that permits the simple isolation and culture of human primary myoblasts for use in functional studies. Additionally, we propose the use of laminin42 and the limited use of bFGF as key factors for a successful culture44. We also propose avoiding the stress generated by centrifugation when splitting the cells and one pre-plating step at the first passage45. To summarize, we have optimized an efficient protocol for isolation and culture of primary myoblasts/MPCs from the muscles of adult and aged humans that is also applicable to the muscles of rodents and enables expression and functional studies of muscle homeostasis.
The authors have nothing to disclose.
This work is supported by the Biotechnology and Biological Sciences Research Council (BBSRC; BB/L021668/1), the MRC and Arthritis Research UK as part of the MRC – Arthritis Research UK Centre for Integrated Research into Musculoskeletal Ageing (CIMA) and the Wellcome Trust Institutional Strategic Support Fund (097826/Z/11/A). The authors would like to thank Dr Dada Pisconti (University of Liverpool) for her expertise and advice in the isolation of muscle progenitor cells.
60mm Petri dishes | Greiner Bio One | 628160 | Cellstar Cell culture dish, PS, 60/15 MM, VENTS. |
Cell culture plates (6 wells) | Sigma-Aldrich | CLS3516 | Corning Costar cell culture plates. 6 well, flat bottom (Individually wrapped) . |
Cell culture plates (12 wells) | Greiner bio-one | 657 160 | Cellstar Cell culture Multiwell Plates. |
Culture flasks | Greiner Bio One | 690175 (25cm2); 658175 (75cm2). | Cellstar Filter Cap Cell Culture Flasks. |
Standard Disposable Scalpel | Granton | 91310 | Sterile stainless steel blade, pattern: 10. |
Pipettes | Greiner bio-one | 606 180 (5 ml); 607 180 (10 ml); 760 180 (25 ml) | Cellstar Serological Pipettes. |
Pasteur plastic pipettes | Starlab | E1414-0311 | 3.0ml Graduated Pasteur Pipette (Sterile), Ind. Wrapped. |
Syringe | BD | 300613 | 20 mL BD eccentric tip syringe. |
0.2 µm filters | Gilson | ALG422A | Sterile Syringe Filters CA 0.2um 33mm Pk50. |
Cell strainers | Fisher Scientific | 11597522 | Cell culture strainer sterile individually packed 70µm polypropylene. |
Collagenase D | Roche | 11088882001 | Collagenase D; ctivity: >0.15 U/mg |
Dispase II | Sigma-Aldrich | D4693 | Dispase II Protease from Bacillus polymyx. Activity: ≥0.5 units/mg solid. |
CaCl2 | Sigma-Aldrich | 449709 | Calcium chloride, anhydrous, beads, −10 mesh, ≥99.9% trace metals basis |
Laminin | Sigma-Aldrich | 114956-81-9 | Laminin from Engelbreth-Holm-Swarm murine sarcoma basement membrane. 1mg/mL. |
DMEM-high glucose | Sigma-Aldrich | D5671 | Dulbecco’s Modified Eagle’s Medium – high glucose. With 4500 mg/L glucose and sodium bicarbonate, without L-glutamine and sodium pyruvate. |
F-12 media | Gibco | 21765029 | Ham's F-12 Nutrient Mix. 1x + L-glutamine. |
FGF-b | PetroTech | 100-18B | Recombinant Human Fibroblast Growth Factor-basic. |
Fetal Bovine Serum (FBS) | Gibco | 10270-106 | Fetal Bovine Serum. |
Horse serum (HS) | Sigma-Aldrich | H1270 | Horse Serum. Donor herd, USA origin, sterile-filtered. |
Penicillin-Streptomycin | Sigma-Aldrich | P0781 | Penicillin-Streptomycin with 10,000 units penicillin and 10 mg streptomycin per mL in 0.9% NaCl, sterile-filtered. |
L-Glutamine | Sigma-Aldrich | G7513 | L-Glutamine solution. 200 mM, solution, sterile-filtered. |
Trypsin-EDTA | Sigma-Aldrich | T4049 | Trypsin-EDTA solution. 0.25%, sterile-filtered. |
TrypLE Express | Gibco | 12604-013 | TrypLE Express Enzyme (1X), no phenol red. |
DPBS (cell culture) | Sigma-Aldrich | D8537 | Dulbecco’s Phosphate Buffered Saline. Modified, without calcium chloride and magnesium chloride. |
PBS (immunostaining) | Sigma-Aldrich | P4417-50TAB | Phosphate buffered saline tablet. One tablet per 200 mL of deionized water (0.01 M phosphate buffer, 0.0027 M potassium chloride and 0.137 M sodium chloride, pH 7.4). |
Methanol | Fisher | M/4000/PC17 | Methanol Analytical Reagent Grade |
Triton X-100 | Sigma-Aldrich | T8787 | Triton X-100 for molecular biology. |
anti-MF 20 antibody | DSHB | MF20-c 2ea 211 µg/ml. | MYH1E (MF 20) Mouse mAb. |
anti-MyoD antibody | Cell Signaling Technology | 13812P | MyoD1 (D8G3) XP Rabbit mAb. |
anti-Ki67 antibody | Abcam | ab16667 | Rabbit mAb to Ki67 [SP6]. |
Anti-mouse 488 secondary antibody | Invitrogen | A-11029 | Goat anti-Mouse IgG (H+L) Secondary Antibody, Alexa Fluor 488 conjugate. |
Anti-rabbit 488 secondary antibody | ThermoFisher Scientific | A-11034 | Goat anti-Rabbit IgG (H+L) Secondary Antibody, Alexa Fluor 488 conjugate. |
DAPI | Sigma-Aldrich | Sigma-Aldrich | DAPI (4',6-Diamidino-2-Phenylindole, Dihydrochloride) |
Senescence β-Galactosidase Staining Kit | Cell Signaling Technology | 9860 | Senescence β-Galactosidase Staining kit. |
DMSO | Sigma-Aldrich | 41639 | Dimethyl sulfoxide. BioUltra, for molecular biology, ≥99.5% (GC). |
Acridine Orange | Sigma-Aldrich | A8097 | Acridine Orange hydrochloride solution, 10 mg/mL in H2O. |
Ethidium bromide | Sigma-Aldrich | E1510 | Ethidium bromide solution. BioReagent, for molecular biology, 10 mg/mL in H2O. |
Lipofectamine 2000 | ThermoFisher Scientific | 11668019 | Lipofectamine 2000 Transfection Reagent |
Scramble control for transfections | Qiagen | 1027271 | miScript Inhibitor Neg. Control (5 nmol) |
Hsa-miR-378a-3p miScript Primer Assay | Qiagen | 218300 | Hs_miR-422b_1 miScript Primer Assay (targets mature miRNA: hsa-miR-378a-3p). MIMAT0000732: 5'ACUGGACUUGGAGUCAGAAGGC |
Anti-hsa-miR-378a-3p miScript miRNA Inhibitor | Qiagen | 219300 | Anti-hsa-miR-378a-3p miScript miRNA Inhibitor (targets mature miRNA: hsa-miR-378a-3p). MIMAT0000732: 5'ACUGGACUUGGAGUCAGAAGGC |
Megafuge 2.0 R Centrifuge | Heraeus | 75003085 | n/a |
Centrifuge rotor | Heraeus | 3360 | Heraeus Sepatech Megafuge Centrifuge Rotor BS4402/A. Max. radius: 15.5 cm. |
Eclipse Ti-E Inverted Microscope System | Nikon | n/a | Eyepieces: CFI 10x/22; Total magnification: 100x (MF20, Live/dead and Ki67). |
Axiovert 200 inverted microscope | Carl Zeiss | n/a | Eyepieces: Carl Zeiss 1016-758 W-PI 10x/25; Total magnification: 100x (Senescence β-Galactosidase Staining). |
Axiovert 25 inverted microscope | Carl Zeiss | n/a | Eyepieces: E-PL 10x/20. Total magnification: 100x (bright field). |
Diaphot Inverted Tissue Culture Microscope | Nikon | n/a | Eyepiece: CFWN 10x/20. Total magnification: 100x (bright field). |
Hydromount | National Diagnostics | HS-106 | Hydromount |