In vitro experiments: cell culture and staining
To explore muscle differentiation in vitro, we used a clone of satellite cells (C1F) derived from the H-2kb-tsA58 mouse (Morgan et al., 1994; Richardson et al., 2022). These cells proliferate at 33°C in the presence of IFNɣ in growth medium (Dulbecco’s minimal essential medium (DMEM), high glucose, containing Glutamax (Gibco), supplemented with 20% FCS, 1% Penicillin/Streptomycin (v/v) (Gibco), 2% Chick Embryo Extract (E.G.G. Technologies)) and are switched to differentiate by changing the medium to DMEM supplemented with 4% Horse Serum and 1% penicillin/streptomycin, and increasing the temperature to 37°C. Proliferation ceases within 24 hours, and cells start to fuse at this time point. RNASeq experiments suggested that Megf10 is expressed in these cells (Richardson et al., 2022).
For immunostaining, cells were plated onto washed 13 mm diameter glass coverslips, coated with 0.1% gelatin. Cells were fixed with 4% paraformaldehyde (PFA) in phosphate buffered saline (PBS) for 20 minutes, washed in PBS, permeabilised with 0.5% Triton X-100 diluted in PBS containing 1% (w/v) bovine serum albumin (BSA) for 30 minutes prior to incubating with primary antibodies (Supplementary Table 1) diluted into PBS with 1% (w/v) BSA for 60 minutes at room temperature. Coverslips were washed x5 with PBS, and then appropriate anti-mouse or anti-rabbit secondary antibodies (Alex-Fluor conjugated, Invitrogen) diluted 1/400 in PBS with 1% (w/v) BSA were added. Fluorescent phalloidin was used to stain filamentous actin and DAPI (4′,6-diamidino-2-phenylindole) was used to stain nuclei (Sigma). Of note, we were unable to validate any commercial antibody to MEGF10, and thus could not stain or blot for endogenous protein. Once stained, coverslips were mounted onto glass microscope slides using Prolong gold antifade mountant (Invitrogen). Cells were imaged using a Zeiss LSM Airyscan confocal microscope, using a x40 objective lens (NA 1.4) or an Olympus widefield microscope using a x63 objective lens (N.A. 1.4) followed by deconvolution. The resulting images were assembled into figures using Adobe photoshop.
GFP-MEGF10 adenoviral construct generation
Adenoviral expression constructs for GFP-MEGF10 (full length human MEGF10, 3423 bp was provided by Colin Johnson) and for GFP were generated by PCR-based cloning into a pDC315 vector (Addgene). For GFP-MEGF10, eGFP was placed at the C-terminus of MEGF10, separated from MEGF10 by a short 3x glycine linker, and a 6xHis tag was incorporated at the C-terminus after the eGFP (Fig. 1B). Fully sequenced constructs, with no sequence errors, were used to generate adenovirus using Ad293 cells (Microbix). Purified DNA plasmids (pDC315 alongside pBHGloxΔE1,3Cre) were transfected into Ad293 cells, using Fugene and the resulting adenovirus was amplified as described in the manufacturer’s protocol (Wolny et al., 2013). After the final round of amplification, virus was purified using the Vivapure Adenopack 100 kit (Sartorius Stedim Biotech), purified virus was stored in storage buffer (20 mM Tris/HCl, 25 mM NaCl, 2.5% glycerol (w/v), pH8) at -80°C. Viral titre was determined by tissue culture infectious dose 50 (TCID50) method. Final titres for GFP-MEGF10 and the GFP virus were 3 x 108 and 6 x 108 PFU per ml respectively. This was used to estimate the MOI of infection, when infecting cultured myoblasts. Western blots confirmed the expected sizes for GFP-MEGF10 and GFP (Supplemental Fig. 1).
Expression and purification of extracellular domain constructs of MEGF10.
Extracellular MEGF10 constructs (human) were generated for mammalian protein expression (Fig. 1B). The cDNA for the extracellular domain (ECD: residues Leu35 to Gly861) or the ECD lacking the EMI domain (ECF, His 108 to Gly861) were cloned into the pSecTag2A plasmid (Invitrogen), sequences confirmed, and constructs were transfected into HEK-293 cells using calcium chloride transfections. pSecTag2a contains an N-terminal secretion signal from Ig-κ for efficient protein secretion in the media, a cytomegalovirus (CMV) promoter for high-level expression and C-terminal 6xHis and c-myc tags for nickel column purification and antibody detection, respectively, as well as a Zeocin resistance gene for the selection of stably expressing mammalian cell lines. 48 hours after transfection, cells were harvested, diluted to 1 x 105 cells per ml, and seeded at 2 x 104 cell per ml in selection medium (DMEM, 10% FCS, 1% FCS, 200 µg/ml Zeocin). After 10 days, individual clones, were picked into 24 well plates and allowed to grow. Samples of media from each colony were analysed by dot blot, to isolate clones for which expression was highest.
For expression and purification of the MEGF10 extracellular domain constructs, stable cell lines with high levels of expression, were seeded into five 75 cm3 flasks coated with 20 µg ml− 1 poly-L-lysine, grown to 80% confluence in normal growth medium and then the medium was exchanged for OptiMEM low serum medium (GIBCO) to reduce contaminants during purification, and cells cultured for a further 3 days. The medium was then removed, centrifuged at 1000 x g rcf, and the supernatant incubated with 1 ml Complete His-Tag Purification Resin slurry (Roche) and Complete EDTA-free protease inhibitor cocktail tablet (Roche) for 30 min on a roller. The mixture was then applied to a 5 ml column, and the flow through collected. The resin was washed 5x with column wash buffer (300 mM NaCl, 50 mM NaHPO4) and eluted with elution buffer (300 mM NaCl, 50 mM NaHPO4, 200 mM Imidazole). Eluted protein (in 2 mL) was dialysed into PBS overnight using a Gebaflex Maxi Dialysis Tube (MWCO = 3.5kDa) (Generon). Purified protein was stored in 200 µL aliquots at -80°C. Protein concentration was measured using a Cary 50 Bio-UV visible spectrophotometer (Varian) at a wavelength of 280 nm. Typical concentrations were 60–100 ng/µl for ECD and 150–200 ng/µl for ECF. Protein identity was confirmed by mass spectrometry. A lectin blot (Biotinylated Lectin Kit I (Vector Laboratories)) was used to confirm that the ECD and EGF constructs had been glycosylated.
Analysis of protein expression
Samples of cells used for western blotting were either prepared by scraping cells directly into 2x Laemlli buffer prior to boiling at 100°C for 10 min and freezing aliquots at -80°C, or freshly pelleted cells were resuspended into 50 µl ice-cold lysis buffer (150 mM NaCl, 50 mM Tris (pH8), 1% Triton X-100, 1 mM EDTA (pH8)) containing Halt Protease Inhibitor, single-use cocktail (ThermoScientific), incubated on ice for 30 mins with regular vortexing, centrifuged at 17000 x g rcf for 20 mins at 4°C, and lysates stored at -20°C. Protein concentration was then quantified by a Pierce micro BCA Protein Assay kit (Thermo Scientific) following the manufacturer’s instructions. Absorbance at a wavelength of 544nm was measured using a Polstar Optima plate reader.
Analysis of myoblast fusion and cell motility
To analyse cell motility, cells were imaged for 14 hours, capturing images every 10 minutes, using differential interference contrast (DIC) microscopy (Olympus widefield microscope), x10 objective lens at 37°C. For eGFP-MEGF10 expressing cells, individual wells of a 96 well plate with a borosilicate glass bottom (Iwaki) were seeded with 50 µl of C1F cells at 1x105 cells/ml and infected overnight using an MOI of 100 for each adenoviral construct in 500 µl culture media. For each condition, 5 fields of view were imaged and the experiment was repeated three times (3 biological replicates). Cell motility was analysed using ImageJ software (MTrackJ plugin), tracking 10 cells per field of view. To determine the effect of the purified ECD and ECF domains on cell motility, individual wells of the borosilicate glass were coated with 1.25 µg of purified protein for 20 min at 37°C. Excess coating was aspirated and wells seeded with 50 µl C1F cells at 1x105 cells/ml. Cells were incubated with 50 µl of culture media at 33°C, 10% CO2 for 24 hrs, medium made up to 500 µl and then cells were filmed and motility analysed as above.
To estimate the fusion index, C1F cells were seeded onto pre-washed 13 mm diameter coverslips, coated with 0.1% gelatin and differentiated for seven days. Cells were fixed with pre-warmed 4% PFA for 20 minutes, washed with PBS and then permeabilised with 0.1% Triton X-100 in PBS. The nuclei were stained for nuclei using DAPI, filamentous actin using fluorescently labelled phalloidin and striated muscle myosin using the A4.1025 antibody (Cho et al., 1994; Maggs et al., 2000). The fusion index is calculated from the percentage of nuclei found in skeletal myosin positive myotubes as a percentage of the total number of nuclei. Only skeletal myosin positive myotubes with three or more nuclei were classed as myotubes. For each condition, five fields of view were imaged and cells counted from three biological replicates.
Cell attachment and cell motility assays
To determine the ability of the ECD and ECF proteins to mediate C1F myoblast attachment to a surface a cell attachment assay was performed. Briefly, the wells of a non-adhesive 24 well plate (Greiner), were coated with 2.5 µg protein diluted in 150 µl PBS for 20 min at 37°C. Harvested myoblast cells (C1F clone) were prepared at 1x104 cells ml− 1 in growth medium and 100 µl added to each well. Cells were incubated at 33°C, 10% CO2 for 30 min before adding 500 µl of fresh medium. After 24 hr incubation, cells were imaged using a Cytomate instrument, taking 5 different fields of view and counting the number of nuclei from each field. The experiment was repeated three times. Alternatively, a 96 well borosilicate glass plate (Iwaki) was coated for 20 minutes at 37°C with 0.1% gelatin, or with 1.25 µg of the EGF or the ECD domains of MEGF10, or left uncoated, coating was aspirated and wells seeded with 50 µl of CIF cells at 1 x 105 cells per ml. Cells were allowed to attach, supplemented with additional medium (50 µl), and filmed overnight to analyse their motility.
Megf10 knockout mice
Megf10tm1(KOMP)Vlcg mice (RRID: MMRRC_048576-UCD, MGI ID: 4454190, background: C57BL/6Tac, intragenic targeted knockout deletion, gene ID: 70417) were obtained from the Mary Lyon Centre at the MRC Harwell Institute and exported to our laboratory to establish breeding colonies. Briefly, the model was generated by replacing exons 1–24 of mouse Megf10 by homologous recombination with an expression selection cassette as detailed by the Knockout Mouse Project (KOMP, University of California Davis, Davis, CA: https://www.komp.org/geneinfo.php?geneid=68051). The Megf10tm1(KOMP)Vlcg mice harbour the Velocigene cassette ZEN-Ub1 inserted into the Megf10 gene between positions 57340143 and 57372060 on chromosome 18, generating a 31918bp deletion that deletes exons 1–24 of MEGF10. The mouse line was rederived in the Harwell facility after its original generation at Regeneron Pharmaceuticals. qPCR has shown no mRNA for MEGF10 is expressed in the cerebellum in homozygote animals (Mouse Genomics Informatics) (Iram et al., 2016) and knockouts, hereafter designated Megf10−/−, lack MEGF10 protein (Fig. 1f in (Chung et al., 2013)), confirming that the tm1 allele is null. Megf10−/− (homozygous knockout) mice were generated on a C57BL/6NTac background. Megf10−/−, Megf10+/− (heterozygous), and wild type animals used in the experiments were generated by crossing Megf10+/− heterozygotes, and all comparisons between genotypes are between age-matched littermates. Male and female adult C57BL/6Tac (bred in-house at the University of Leeds) and Megf10tm1(KOMP)Vlcg mice (final body mass approx. 25g) were used in this study.
All experimental procedures and sacrifice were conducted with approval of the local animal welfare and ethics committee, under Home Office project licences 70/8674 and PP1775021. Mice were housed in groups in a temperature-controlled environment with access to food and water ad libitum. Cages underwent 12/12 light/dark cycles. Breeding was carried out under service licence PP0237211, with two breeding cages of C57BL/6Tac x heterozygous Megf10tm1(KOMP)Vlcg, and one breeding cage of heterozygous Megf10tm1(KOMP)Vlcg x heterozygous Megf10tm1(KOMP)Vlcg. Mice were stunned by concussion and killed by cervical dislocation.
Genotyping
To accurately determine the genotype for each mouse, ear biopsies were collected by unit staff at Central Biomedical Services (CBS), University of Leeds. Biopsies were placed into 96-well plates, sealed, and shipped to Transnetyx for genotyping (Transnetyx Inc. Cordova, TN) via courier. A bespoke PCR assay to determine genotype was designed by the Genetic Services team at Transnetyx based on information provided by KOMP (Knockout Mouse Project) mouse repository (Supplementary Fig. 1). Results were obtained within 72 hours of sending the samples. Of note, we did not obtain the expected Mendelian ratio of 1:2:1 (Supplemental Table 2).
Hypertrophy model surgery
Unilateral extirpation (removal) of tibialis anterior (TA) muscle was performed under aseptic conditions and inhalation anaesthesia. All instruments were sterilised and work carried out under a dissection microscope. Mice were first anaesthetised with 5% isoflurane in 2Lmin− 1 O2. The left leg was then shaved and wiped with ethanol to sterilize the area and remove surface bacteria. For the remainder of the operation, mice were maintained under anaesthetic at 2% isoflurane in 2L/min O2. All possible steps were taken to avoid animals suffering at each stage of the experiment.
A single incision was made on the hindleg to expose the TA, tweezers used to lift the superficial distal tendon, and the TA removed by making incisions at proximal and distal points of attachment using a scalpel. The TA cut end was then held over the wound area for approx. 10s to allow the release of chemokines to aid repair and blood clotting. The TA was then discarded and 1–2 drops of 2.5% Baytril (Bayer AG) was applied to the wound for antiseptic protection. The area was swabbed with a cotton bud to remove blood and then the incision was sutured with MERSILK™ (Ethicon Inc.) braided silk suture, size 5.0. Sutures were intermittent and double-knotted to reduce the chance of mice unravelling them post-operatively. 1–2 more drops of Baytril® were applied to the closed wound and swabbed with sterile cotton buds, to remove dried blood that may lead to irritation. 0.1ml 10% Vetergesic (Ceva Animal Health, Ltd) was administered to the scruff of the neck to provide post-operative analgesia. Mice were placed in a heated cage without sawdust for approximately 10 minutes to recover from anaesthetic, before being placed back into their original cage. Mice were observed to be normally ambulant, thus overloading the extensor digitorum longus (EDL), for a pre-determined length of time before sampling.
EDL isolation
Changes in EDL muscle phenotype were assessed in control animals (no overload), as well as animals overloaded for 10 days, to observe changes in the muscle. This interval was chosen as we observed that the overload response had peaked at this time point in test experiments. Mice were killed by Schedule 1 (concussion followed by cervical dislocation). Muscle was removed as quickly as possible to minimise post-mortem biochemical changes. The leg of a freshly killed mouse was shaven and dabbed with ethanol to promote cutaneous vasoconstriction. A small incision from just lateral to the knee to the beginning of the hindfoot aided blunt dissection using scissors and forceps to break through the layer of fascia atop the muscle. On the unoperated (contralateral) leg, the TA was first removed to access the EDL underneath. The EDL was then accessed and removed in the same way (forceps used to hold the tendon and scalpel used to release it at the base). On the ipsilateral (overloaded) leg, the EDL was simply removed as described. The EDL, and the whole mouse, were both weighed to determine the EDL mass as a percentage of total body mass.
Preparation of muscle samples for imaging.
Samples of skeletal muscle were additionally harvested for single fibre isolation (as described above), or for cryo-sectioning. For accretion measurements, single fibres were fixed and permeabilised, incubated with DAPI (1/500) for 90 minutes at room temperature before washing with TBST and finally with PBS. Samples were mounted on cleaned glass microscope slides using ProLong Gold and covered by a 20 x 40 mm glass coverslip. For cryosectioning, intact muscles were trimmed, mounted onto a cork disk with optimum cutting temperature compound (OCT) (Agar Scientific) and immediately snap frozen in isopentane-liquid nitrogen and stored at a temperature of -80°C. Diaphragm muscle was also prepared for cryo-sectioning, using a similar approach. Samples were sectioned (30 µm and 10 µm sections) using a cryostat (Leica) pre-cooled to -20°C and sections placed onto labelled glass slides and stored at -20°C until ready to fix and stain.
Slides were left at room temperature for ~ 10 mins to dissipate condensation. A hydrophobic barrier pen was then used to draw around each segment of 3–4 sections joined together. Within the confines of the hydrophobic barriers, tissue was fixed by applying 100–200 µl ice-cold 100% methanol and incubating at room temperature for 10 mins. Sections were then washed three times with PBS. Non-specific antibody binding was reduced by incubating with 5% BSA diluted in PBS at room temperature for 30 min. Primary antibodies were diluted in PBS and applied to tissue sections following removal of the blocking solution. Slides were incubated with the primary antibody overnight at 4°C. Primary antibody was removed and sections washed three times with wash buffer (1% BSA in PBS). Secondary antibodies were diluted in PBS, applied to tissue sections, and incubated for 1hr at room temperature. Secondary antibody was removed, and sections washed three times with wash buffer, before a final wash with PBS. Two drops of ProLong Gold antifade mountant was then added to the slide, and a 20 mm x 40 mm glass coverslip was placed on top. Slides were left overnight at room temperature in the dark overnight, and then stored at 4°C.
Quantification of transcription factor expression and fibre cross-sectional area
Stained fibres were imaged using a widefield Olympus IX-70 microscope, using a 40x, N.A. 1.4 objective lens. For measuring myonuclear accretion, 15 x DAPI stained fibres were imaged per condition (three fibres per EDL sample). Numbers of nuclei were counted from 1 mm sections using ImageJ (Fiji).
For transcription factor staining, the number of nuclei positively stained for a transcription factor (Pax7, MyoD or myogenin) per 50 myonuclei along the fibre was measured from three fibres per EDL sample, stained for DAPI and the transcription factor. The numbers of positive nuclei per fibre were expressed as a percentage of 50 myonuclei, and the number per fibre was averaged.
Cross-sectional area of individual muscle fibres was also determined using ImageJ (FIJI). Outlines of each individual fibre were drawn around on each image using the freehand selection tool, and area in µm2 was automatically calculated.
Measurement of laminin thickness
The width of the basement membrane between fibres in stained diaphragm cross sections was measured using ImageJ processing software (NIH). The ‘straight line’ tool was used to take orthogonal measurements of laminin (visualised with anti-rabbit IgG Alexa Fluor 488 conjugate) normal to the sarcolemma, and this value recorded. Five measurements were taken per fibre, and five fibres were measured for 3 wild-type and 3 Megf10+/− mice.
Statistical analyses
Statistical tests were performed, and graphs generated, using GraphPad Prism for Mac (GraphPad Software, La Jolla California, USA, www.graphpad.com). Graphs show mean ± standard deviation (S.D.) for each observation. Unpaired t-tests with Welch’s correction and one-way and two-way ANOVAs were carried out to test for any statistically significant differences between conditions. The level of statistical significance is indicated by the number of asterisks displayed above graphs: **** represents a P value < 0.0001. *** represents a P value < 0.001. ** represents a P value < 0.01. * represents a P value < 0.05.