Functional neuromuscular organoids have been developed by researchers in the Gouti lab at the Max Delbrück Center for Molecular Medicine (MDC; Berlin, Germany). The team reported that these organoids can direct muscle tissue to contract by their ability to form complex neuronal networks, as they self-organize into spinal cord neurons and muscle tissue.
The study, which has been published in Cell Stem Cell, paves the way forward for the study of human neuromuscular system development and disease.
These neuromuscular organoids are an attractive model for studying neuromuscular disorders because they include functional neuromuscular junctions. In addition to this, these models provide the first example where spinal cord neurons, skeletal muscles and Schwann cells co-develop from the same progenitor cells to form these junctions. Furthermore, complex circuitries were also developed within these systems that closely resemble central pattern generator (CPG) circuits.
“Our initial goal was to develop functional neuromuscular junctions, but the findings exceeded our expectations because the additional development of the CPG-like networks was an exciting but unexpected finding,” commented study author Mina Gouti (MDC). “This has not been shown in a human in vitro model before, and offers entirely novel possibilities, including the study of CPG involvement in neurodegenerative diseases.”
Previously, to study human neuromuscular junctions in organoids, researchers had to grow spinal cord neurons and muscle tissue separately. These were then combined later and allowed to interact. Although this approach did produce neuromuscular junctions, they were functionally limited as there were no Schwann cells present.
To overcome this obstacle, the investigators followed up on previous findings that permitted them to convert human pluripotent stem cells into axial stem cells, particularly neuromesodermal progenitor cells. During normal embryonic development, axial stem cells are known to give rise to both the spinal cord and skeletal muscle.
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Once the axial stem cells were placed into a 3D cell culture, the researchers noted that they differentiated and self-organized into structures containing both spinal cord neurons and skeletal muscle tissue. This subsequently resulted in the development of functional neuromuscular junctions containing Schwann cells, including the formation of complex networks similar to CPGs.
Gouti explained that: “These organoids started contracted after 40 days in culture. This activity was driven by the release of neurotransmitter acetylcholine from the resident motor neurons in the organoid and was not due to spontaneous muscle activity seen in other systems. We were able to show that, because pharmacological blocking of the acetylcholine receptors was sufficient to abolish muscle contraction.”
To evaluate their potential application in studying neuromuscular disorders, the researchers modelled a disease known as myasthenia gravis. For 72 hours, serum from two individuals with myasthenia gravis was applied to the organoids. The researchers revealed that this resulted in fewer muscle contractions, which could reflect the muscle weakness experienced by patients.
To conclude, Gouti stated: “The derivation of neuromuscular organoids using patient-derived induced pluripotent stem cells will allow us to investigate their precise origin and progression. These organoids are better suited to study the contributions of specific cell types, such as terminal Schwann cells, at different stages of neuromuscular junction development and maturation that may contribute to neuromuscular diseases.”
Looking to the future, the team anticipate that further studies will utilize patient-derived neuromuscular organoids to investigate the effectiveness of different drugs and for personalized medicine approaches.
Sources: Faustino Martins J-M, Fischer C, Urzi A et al. Self-organizing 3D human trunk neuromuscular organoids. Cell Stem Cell doi:10.1016/j.stem.2019.12.007 (2019) (Epub ahead of print); www.mdc-berlin.de/news/press/neuromuscular-organoid-its-contracting