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In This Article

  • Summary
  • Abstract
  • Introduction
  • Protocol
  • Results
  • Discussion
  • Disclosures
  • Acknowledgements
  • Materials
  • References
  • Reprints and Permissions

Summary

We describe a reproducible, automated, and unbiased imaging system for characterizing neuromuscular junction function using human engineered skeletal muscle tissue and optogenetic motoneurons. This system allows for the functional quantification of neuromuscular connectivity over time and detects diminished neuromuscular function caused by neurotoxins and myasthenia gravis patient serum.

Abstract

Many neuromuscular diseases, such as myasthenia gravis (MG), are associated with dysfunction of the neuromuscular junction (NMJ), which is difficult to characterize in animal models due to physiological differences between animals and humans. Tissue engineering offers opportunities to provide in vitro models of functional human NMJs that can be used to diagnose and investigate NMJ pathologies and test potential therapeutics. By incorporating optogenetic proteins into induced pluripotent stem cells (iPSCs), we generated neurons that can be stimulated with specific wavelengths of light. If the NMJ is healthy and functional, a neurochemical signal from the motoneuron results in muscle contraction. Through the integration of optogenetics and microfabrication with tissue engineering, we established an unbiased and automated methodology for characterizing NMJ function using video analysis. A standardized protocol was developed for NMJ formation, optical stimulation with simultaneous video recording, and video analysis of tissue contractility. Stimulation of optogenetic motoneurons by light to induce skeletal muscle contractions recapitulates human NMJ physiology and allows for repeated functional measurements of NMJ over time and in response to various inputs. We demonstrate this platform's ability to show functional improvements in neuromuscular connectivity over time and characterize the damaging effects of patient MG antibodies or neurotoxins on NMJ function.

Introduction

The neuromuscular junction (NMJ) is the chemical synapse between motoneurons (MNs) and skeletal muscle cells (SkM) that allows for muscle contraction. Toxins, such as neurotoxin α-bungarotoxin (BTX), or neuromuscular diseases (NMD) like myasthenia gravis (MG) can lead to degeneration of the NMJ and reductions in muscle control1. Bioengineered human tissue models better recapitulate the functional and physiological mechanisms of human NMJs and offer greater translational potential than animal models.

While animal models have advanced the understanding of the formation and function of the NMJ, there are significant differences between human and animal synapses that limit the translation of results to humans and make in vivo characterization of the NMJ challenging2,3,4. Studies have shown distinct physiological differences between mouse and human NMJs. Mice have larger NMJs and smaller active zone densities when compared to human NMJs4. Additionally, drug studies conducted in animal models do not always reflect the effects found in human clinical trials. Engineered human tissue models provide the opportunity to study the healthy development of the NMJ and the pathology of neuromuscular diseases and allow for drug screenings. Human induced pluripotent stem cells (hiPSCs)5 can be differentiated into a variety of cell types, including skeletal muscle cells6,7 and motoneurons8,9. hiPSCs can be generated easily from patient cells, allowing for better disease modeling10 and drug screening11,12 through patient-specific tissue models.

Two-dimensional (2D) monolayer co-cultures of SkMs and MNs lack the morphology, phenotype, organization, and functional behavior of physiological NMJs. NMJs randomly form in 2D culture, which inhibits the isolation of motor units for analysis, limits accurate functional measurements, and prevents their use for repeated, systematic experiments13. Three-dimensional (3D) tissue models of NMJs overcome many of these limitations, recapitulating the morphological and functional characteristics of physiological NMJs7,14,15,16,17. Using this model, the two tissue types are developed separately and then integrated by directing axonal growth, allowing for more organized NMJs to develop compared to 2D culture systems.

Our previous study demonstrated that combining optogenetics with tissue engineering can allow for accurate non-invasive stimulation and evaluation of NMJ function18,19. Through genetic engineering, light-sensitive proteins can be integrated into the genome of hiPSCs. Integrating channelrhodopsin-2 (ChR2), an ion channel that opens in response to blue light, into the membrane of excitable cells such as neurons allows for non-contact spatiotemporal control over cell activation20,21,22. hiPSCs carrying ChR2 can be differentiated into optogenetic motoneurons sensitive to blue light, removing the need for typical invasive electrodes that stimulate neurons and avoiding unwanted stimulation of the muscle cells by electrodes23. This system uses optogenetic motoneurons to stimulate contractions in non-optogenetic skeletal muscle cells. Combining video acquisition and controlled blue light illumination allows for the co-cultured tissues to be simultaneously stimulated and recorded for NMJ function.

MG is caused by autoantibodies targeting nicotinic acetylcholine receptors (AChR), which results in decreased NMJ function and muscle weakness24. It is diagnosed based on presented symptoms, electrodiagnosis, and detection of autoantibodies via serological blood tests. However, not all autoantibodies involved in MG have been identified, and some seronegative patients are diagnosed with MG but with no recognized antibodies25,26. Our system allows for repeated functional assessment of the NMJ before and after the addition of serum from MG patients, providing invaluable insight into the functional and biochemical changes caused by the MG antibodies18. Our protocol illustrates how to produce 3D in vitro models of functional human NMJ that can be used to diagnose and investigate NMJ pathologies and test potential therapeutics. We demonstrate the versatility of the system in two platforms, a microfluidic device, and a larger open-well bioreactor platform.

Protocol

All cell lines for this work were created and used in compliance with the institutional guidelines of Columbia University, NY, USA.

1. Bioreactor preparation

  1. Make bioreactor molds
    1. Download a bioreactor CAD file from the Supplementary CAD File or create a custom own design.
    2. Generate a CNC toolpath from the 3D model using CAM software.
    3. Machine acetal molds using a CNC milling machine.
  2. Fabrication of bioreactors
    1. Mix a 10:1 base to curing agent mixture of polydimethylsiloxane (PDMS, 77 g of mixture per 4 platforms/molds).
    2. Place the mixture into a vacuum chamber, close all the valves, turn on the vacuum, and de-gas the mixture for at least 30 min until no air bubbles remain. Pour the mixture into molds and de-gas the molds in the vacuum chamber for 1 h.
    3. Close the molds with the top half of the mold in the correct orientation. Place a steel hexagonal rod over the center and clamp on both sides.
    4. Refill the top with PDMS.
    5. Cure the molds in a 65 °C oven for at least 4 h.
    6. Remove the platforms from the molds after cooling down to room temperature.
    7. Clean the devices in an ultrasonic bath using 1 h cycles of dish soap, 400 mL of 100% isopropanol, and distilled water.
    8. Dry overnight at 65 °C.
    9. Soak glass coverslips in 1% nonionic surfactant polyol for 30 min, and ensure that the coverslips do not stack so that they are all properly coated.
    10. Rinse well with distilled water and dry overnight at 65 °C.
    11. Use a plasma cleaner on high with 6 L/min of oxygen for 1-2 min to the treat glass coverslips and PDMS platform. Once treated, bond the two together by pressing the PDMS down onto the coverslip for at least 30 s.
    12. Autoclave the bonded devices before adding cells.

2. Building an optical stimulation setup

  1. Using a 30 mm cage cube system, attach a 573 nm dichroic mirror in the center. Couple a red 627 nm LED with a 594 nm long-pass excitation filter and attach it to the top side of the cube, with the LED facing the mirror. Then attach a blue 488 nm LED with a 546 nm short-pass excitation filter to the adjacent side of the cube, with the LED facing the mirror as shown in the schematic in Figure 3A.
  2. Attach a ring-actuated iris diaphragm to the bottom of the cage cube to control the size of the illuminated area.
  3. Power each LED by a T-Cube LED driver and control via an Arduino Uno Rev3 board. Connect the side input of the LED driver to a power source plug, connect the middle output to the Arduino, and connect the side output directly to the LEDs.
  4. Connect the Arduino Uno to a PC by a USB cable.
  5. Download files from GitHub link (https://github.com/ofvila/NMJ-function-analysis).
  6. Open the "Optical_Stimulation_Ramp_Arduino.ino" file from GitHub folder.
  7. Set starting parameters: Steps = 30; startFrequency = 0.5; endFrequency = 3; pulseLength = 100.
  8. Ensure that pin output 4 is assigned to the Blue LED driver and pin output 2 is assigned to the Red LED driver. Check the middle wires of the LED driver are connected to the ground and to the respective pin output.
  9. Compile and load the "Optical_Stimulation_Ramp_Arduino.ino" program to Arduino.
  10. Connect each LED driver to its corresponding channel.

3. Cell culture setup (day -21-0)

  1. Primary skeletal muscle cells
    1. Thaw and expand primary skeletal myoblasts (obtained from Cook Myosite) for a maximum of six passages using Myotonic growth medium (+ supplement). Maintain cells in an incubator set to 37 °C and 5% CO2.
    2. Change the media every 2 days.
    3. Once cells are about 60% confluent, passage them using 0.05% Trypsin-EDTA (1x, for 5 min at 37 °C). Collect the dissociated cells and add fresh media to neutralize the trypsin.
    4. Spin down the cells at 300 x g and aspirate the supernatant above the cell pellet.
    5. Resuspend the cells with MGM and seed 1:3 with 60 mL per triple-layer flask.
  2. ChR2-hiPSC
    NOTE: All cell lines for this work were created and used in compliance with the institutional guidelines of Columbia University, NY, USA. In this protocol, ChR2-expressing hiPSCs were generated via CRISPR-Cas9 genome editing using previously described methods18, but any stable optogenetic cell lines (lentiviral, piggyBac, etc.) can be used in the same way. Constitutive promoters expressed in both iPSCs and motoneurons were chosen (CAG). The cells were found to have healthy karyotype, as noted in previous publications18, 19.
    1. Coat 6-well plates with 1 mL/well of solubilized basement membrane matrix diluted in DMEM/F12 (1:80) and incubate the plates at room temperature for 1 h.
    2. Seed iPSCs on coated 6-well plates with 2 mL of feeder-free cell culture medium (iPSC media), exchanging 2 mL of media every other day and passaging every 5-7 days. Maintain cells in an incubator set to 37°C and 5% CO2.
    3. Passaging
      1. Dissociate the stem cells by incubating with 1 mL of enzyme-free stem cell releasing reagent for 4 min and mechanically shearing with a wide tip P1000 pipette.
      2. Seed cells at a ratio of 1:24 or 1:48 in iPSC media with 2 μM of Y-27632 dihydrochloride in 2 mL/well onto coated 6-well plates.

4. Skeletal muscle tissue seeding (day -3)

  1. Isolate and count 30 x 106 myoblasts using an automated cell counter.
  2. Resuspend the myoblasts in a 4:1 mix of 3 mg/mL collagen I and solubilized basement membrane matrix (800 μL of collagen mixture + 200 μL of basement membrane matrix to reach a final volume of 1 mL). For the collagen component, add the accompanying neutralizing agent (9:1 mix of collagen to neutralizing agent) and dilute the mixture with 1x PBS to achieve a volume of 800 μL.
  3. Add 15 μL of cell-collagen suspension to each muscle chamber of the bioreactor, being sure to spread the suspension across both pillars using the pipette tip (Figure 2).
  4. Let the cell-gel mixture polymerize at 37 °C for 30 min and then fill each bioreactor well with 450 μL of Myotonic growth media.
  5. After 3 days (once the gel has compacted), begin myotube differentiation.

5. Myotube differentiation (days 0-14)

  1. Begin myotube infusion by switching tissues to 450 μL of fusion inducing media (FS, Table 1) for 7 days, changing the media every other day.
    NOTE: Try to begin myotube differentiation on the same day as motoneuron differentiation from hiPSCs in order to seed motoneurons at the end of both cell types' differentiation schedules.
  2. On day 7 change the media to 450 μL of maturation media Ia (MMIa, Table 1).
  3. On day 9, change to 450 μL of maturation media Ib (MMIb, Table 1).
  4. On day 11, change the media to 450 μL of NbActiv4. Keep changing the media every 2 days until the motoneurons are seeded (Step 7).

6. Motoneuron differentiation (days 0-14)

NOTE: Our motoneuron differentiation protocol was adapted from Maury et al8.

  1. On day 0, transfer 4 x 106 ChR2- hiPSCs to an ultra-low attachment Petri dish with 15 mL of motoneuron suspension culture medium (MSCM, Table 1). Supplement MSCM with 3 μM CHIR99021, 0.2 μM LDN193189, 40 μM SB431542 hydrate and 5 μM Y-27632 dihydrochloride.
  2. On day 2, isolate the neurospheres (NS) with a 37 μM reversible strainer and replate in 15 mL of MSCM with 3 μM CHIR99021, 0.2 μM LDN193189, 40 μM SB431542 hydrate, and 0.1 μM retinoic acid. NS should be visible in the Petri dish without using a microscope after day 2.
  3. On day 4, transfer the cells and media to a 50 mL tube and allow the neurospheres to settle to the bottom (5 min). Aspirate the supernatant and resuspend the cells in 15 mL of MSCM supplemented with 0.5 μM SAG, 0.2 μM LDN193189, 40 μM SB431541, and 0.1 μM retinoic acid.
  4. On day 7, repeat Step 6.3. but resuspend the cells in 15 mL of MSCM supplemented with 0.5 μM SAG and 0.1μM retinoic acid.
  5. On day 9, repeat Step 6.3. but resuspend cells in 15 mL of MSCM supplemented with 10 μM DAPT.
  6. On day 11, repeat Step 6.3. but resuspend the cells in 15 mL of MSCM supplemented with 20 ng/mL BDNF and 10 ng/mL GDNF.
  7. On day 14, seed the motoneurons into the platform.

7. Seeding motoneurons aggregates in bioreactor (day 14)

  1. Prepare a 4:1 gel mixture of 2 mg/mL collagen I and Matrigel (800 μL of collagen mixture + 200 μL of Matrigel to reach a final volume of 1 mL). For the collagen component, add the accompanying neutralizing agent (9:1 mix of collagen to neutralizing agent) and dilute the mixture with 1x PBS to achieve a final volume of 800 μL.
  2. Use a 400 nm cell strainer to select large neurospheres and resuspend them in the gel mixture.
  3. Aspirate media from the reservoir and carefully from the neurosphere well (Figure 2).
  4. Add 15 μL of gel mixture into the neurosphere channel.
  5. Load a 10 μL pipette with 10 μL of gel and then pick one neurosphere.
  6. Deposit the NS into the neurosphere channel and ensure that the NS is in the chamber. Slowly raise the pipette while releasing the remaining gel once the NS is deposited. If unsure that the NS was correctly deposited, check its location using a microscope.
  7. Allow the gel to polymerize for 30 min at 37 °C.
  8. Add 450 μL of NbActiv4 supplemented with 20 ng/mL BDNF and 10 ng/mL GDNF to the reservoirs.
  9. Change the media every other day to allow for axonal growth from NS to muscle tissue.

8. Simultaneous optical stimulation and video recording of NMJ function (day 24+)

  1. For imaging, use an inverted fluorescent microscope with a scientific complementary metal-oxide-semiconductor (sCMOS) camera.
  2. Set the camera software binning to 2x2, exposure to 20 ms, rolling shutter ON, readout rate to 540 MHz, dynamic range: 12-bit & gain 1, and sensor mode: overlap.
  3. Use 2x objective on the microscope to image the microtissues.
  4. Attach a live-cell chamber (37 °C, 5% CO2) to the microscope stage.
  5. Select the region of interest (ROI) that contains the innervated skeletal tissues tissue to minimize the file size and processing time.
  6. Place a 594 nm long-pass emission filter between the sample and the imaging objective to filter out blue light pulses from the camera.
  7. Place a rectangular 4-well plate containing 4 bioreactors (24 tissues) into the live-cell chamber.
  8. Click Live View. Center and focus the image with the desired ROI.
  9. Upload the custom macro code from the GitHub folder (https://github.com/ofvila/NMJ-function-analysis) to control the stage position, the Arduino board, and video acquisition.
  10. Set output movie as: day_tissue group_tissue name_experiment.nd2.
  11. Run the macro code with the desired X,Y coordinates set on the stage and acquire a fast time lapse with 1700 frames at 50 frames/s.
  12. Replace the media after imaging and return the samples to the incubator. Allow at least 24 h between image acquisition sessions to avoid tissue fatigue.

9. Batch processing and analysis (day 24+)

  1. Movie processing
    1. Use the custom MATLAB code to process the videos in batch analysis. The files can be downloaded from the GitHub folder (https://github.com/ofvila/NMJ-function-analysis). The functions are listed and explained in Table 2.
      NOTE: The code is compatible with .nd2 and .czi formats. It requires parallel processing to be activated in MATLAB and needs the bioformats package.
    2. Run the recursiveOSAnalysis script to analyze the movies through parallel processing. Have all of the acquired videos in the same folder and select that folder when running the code.
    3. Adjust the post-analysis parameters as needed.
      1. Change baselineTime (option 1) if spontaneous contraction is picked up at beginning of recording. This will show an initial reading way above 0 and will need the frame to be shifted to compensate.
      2. Change peakThreshold (option 2) if the contractions are not being registered. The code will detect contractions that are above 25% of the highest peak by default so that this value can be changed.
      3. Change minMinProminence and minMinWidth to adjust the sensitivity when detecting the start of each peak.
    4. Generate video and graph output.
      1. Run recursiveOSMovie to generate a video file for each tissue with its respective contractility trace.

10. Perturbation of NMJ function (day 24+)

  1. Prepare treatment media by adding external effector to NbActiv4 basal media supplemented with 20 ng/mL BDNF and 10 ng/mL GDNF.
    1. For the MG sera experiment, use 20% MG patient sera in NbActiv4 + BDNF + GDNF.
    2. For the BTX experiment, use 5 μg/mL BTX in NbActiv4 + BDNF + GDNF.
  2. Stimulate/image using Step 3.2. to record the baseline.
  3. Replace the media in tissues with treatment media (450 μL/tissue).
  4. Incubate for the desired time (48 h for patient sera, 20 min for BTX).
  5. Stimulate/image again using Step 3.2. to record function after treatment.
  6. Remove the treatment media and allow the tissues to rest for the desired time (48 h after sera removal)
    NOTE: Allow at least 24 h between stimulation and imaging to avoid tissue fatigue

Results

Neuromuscular junctions were generated by co-culturing optogenetic hiPSC-derived motoneurons with non-optogenetic skeletal muscle tissue. Human primary skeletal myoblasts (SkM) were seeded into the platforms and differentiated into multinucleated myotubes using the 2-week protocol. The optogenetic motoneurons were differentiated separately, but in parallel with the myotube differentiation, and then seeded into the platform (Figure 1). The tissues began contracting in response to blue light s...

Discussion

This system is an engineered 3D human tissue model that combines optogenetics and video processing to enable automated and unbiased evaluation of NMJ function. Using a standardized protocol, we have demonstrated the ability to measure changes in NMJ function during physiological development and characterize the damaging effects of pathologies such as neurotoxin exposure and myasthenia gravis patient sera.

Previous studies have reported the ability to model MG with optogenetic hPSC-derived moto...

Disclosures

The authors declare no conflict of interest.

Acknowledgements

We gratefully acknowledge funding support by the NIH [grant numbers EB025765 and EB027062], DOD [award number W81XWH-18-1-0095], and the UCSF Health Innovation via Engineering (HIVE Fellowship). We gratefully acknowledge the Columbia University Stem Cell Core for their help and guidance with cell reprogramming.

Materials

NameCompanyCatalog NumberComments
Cells
SkMDCCook MyositeP01059-14M
Media and Supplements
Advanced DMEM/F12ThermoFisher Scientific12634-020
Bovine Serum Albumin solutionMillipore SigmaA9576-50ML
G-5 Supplement (100X)ThermoFisher Scientific17503-012
Geneticin Selective Antibiotic (G418 Sulfate) (50 mg/mL)ThermoFisher Scientific10131-035
Insulin, Recombinant HumanMillipore Sigma91077C-100MG
MatrigelCorning354277
mTeSR PlusStem Cell Technologies100-0276
MyoTonic Growth Media KitCook MyositeMK-4444
N-2 SupplementThermoFisher Scientific17502-048
NBactiv4 500 mLBrainBits LLCNb4-500
Neurobasal MediumThermoFisher Scientific21103-049
Neurobasal-A MediumThermoFisher ScientificA13710-01
Pluronic F-127Sigma AldrichP2443
ReLeSRStem Cell Technologies5872
Plasticware
30 mm cage cube systemThorLabsCM1-DCH, CP33, ER1-P4 and ER2-P4
37 µm Reversible Strainer, largeStem Cell Technologies27250
546 nm short-pass excitation filterSemrockFF01-546/SP-25
573 nm dichroic mirrorSemrockFF573-Di01–25x36
594 nm long- pass emission filterSemrockBLP01-594R-25
594 nm long-pass excitation filterSemrockBLP01-594R-25
Blue (470nm) Rebel LED on a SinkPAD-II 10mm Square Base - 65 lm @ 700mALuxeonStarLEDsSP-05-B4
Carclo 29.8° Frosted 10 mm Circular Beam Optic - Integrated LegsLuxeonStarLEDs10413
Corning 60 mm Ultra-Low Attachment Culture DishCorning3261
Heat sinkLuxeonStarLEDsLPD-19-10B
Optics
pluriStrainer 400 µm, 25 pack, sterilePluriSelect43-50400-03
pluriStrainer 500 µm, 25 pack, sterilePluriSelect43-50500-03
Red (627nm) Rebel LED on a SinkPAD-II 10mm Square Base - 65 lm @ 700mALuxeonStarLEDsSP-05-R5
ring-actuated iris diaphragmThorLabsSM1D12D
T-Cube LED driversThorLabsLEDD1B, KPS101
Molds
Female Threaded Hex Standoffs,  3 1/2" 10-32, Partially Threaded 1/2"McMaster91920A046
Low-Profile C-ClampMcMaster1705A12
Growth Factors
Adenosine 3′,5′-cyclic monophosphateMillipore SigmaA9501-1G
CHIR 99021, 10 mgTocris4423/10
DAPT 10 mgR&D Systems2634/10
Human CNTF, research grade, 5 µgMiltenyl Biotec130-096-336
Human Vitronectin Protein, CFR&D Systems2349-VN-100
Human Vitronectin Protein, CFR&D Systems2349-VN-100
IGF1 Recombinant Human ProteinThermoFisher ScientificPHG0078
Laminin mouse protein, naturalThermoFisher Scientific23017015
Recombinant Human Agrin ProteinR&D Systems6624-AG-050
Recombinant Human GDNF Protein, CF 50ugR&D Systems212-GD-050/CF
Recombinant Human Neurotrophin 3 100 ugCell SciencesCRN500D
Recombinant Human Neurotrophin-4Cell SciencesCRN501B
Recombinant Human Sonic Hedgehog/Shh (C24II) N-TerminusR&D Systems1845-SH-100
Recombinant Human/Murine/Rat BDNF 50 ugPeprotech450-02
Retinoic Acid, 50 mgMillipore SigmaR2625-50
SAG Smoothened AgonistMillipore Sigma566660
SB431542 10 mgStem Cell Technologies72234
StemMACS LDN-193189Miltenyl Biotec130-103-925
Vitronectin from human plasmaMillipore SigmaV8379-50UG
Y-27632 dihydrochlorideTocris1254
Antibodies
α-actinin mAb (Mouse IgG1)Abcamab9465
Choline Acetyltransferase (ChAT) (Goat)MilliporeAB144P
Desmin mAb (Mouse IgG1)DakoM076029-2
Myosin Heavy Chain (MHC) (Mouse IgG2b)DSHBMF20
Equipment
Arduino Uno R3ArduinoA000066
Automated stageApplied scientific instrumentationMS- 2000 XYZ
Expanded plasma cleanerHarrick PlasmaPDC-001 (115V)
Invitrogen Countess Automated Cell CounterMarshal ScientificI-CACC
IX-81 Inverted fluorescence microscopeOlympusIX-ILL100LH
Series Stage Top Incubator SystemTokai Hit STXTOKAI-HIT-STXG
Zyla 4.2 sCOMS CameraAndor TechnologyZYLA-4.2P-CL10
Software
Arduino Software (IDE)ArduinoIDE 1.8.19
MastercamMastercamMastercam for Solidworks
MatlabMatlabR2021b
NIS elementsNikonBasic Research
Solidworks 3D CADSolidworksSolidworks Standard

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