Researchers at Ulm University and Eberhard Karls University in Germany recently developed a new in vitro model for the study of motor neuron diseases including amyotrophic lateral sclerosis (ALS). The study was published in the journal Stem Cell Research and is entitled “Formation and characterisation of neuromuscular junctions between hiPSC derived motoneurons and myotubes”.
Neuromuscular junctions (NMJ) are formed through the interaction between motor neurons and the skeletal muscle. NMJs are important for signal transmission with disorders affecting such junctions usually leading to muscle weakness. Growing evidence suggests that changes in NMJs and/or the skeletal muscle itself can have an impact on the pathogenesis of ALS.
ALS is a progressive neurodegenerative disease characterized by the gradual degeneration and atrophy of motor neurons in the brain and spinal cord that are responsible for controlling essential voluntary muscles, such as the ones related to movement, speaking, eating, and even breathing. ALS patients may become totally paralyzed and the majority dies due to respiratory failure within two to five years after diagnosis. It is estimated that more than 300,000 Americans suffer from the disease and there is currently no cure or life-prolonging treatments.
In vitro models like skeletal muscle cells are an important tools in the study of motor neuron disorders and for testing therapeutic options. Human induced pluripotent stem cell (hiPSC) technology is an example of a valuable resource for research on ALS and other motor neuron disorders. Stem cells are undifferentiated cells capable of differentiating into several specialized cell types, like muscle, bone, skin, etc. Neurological disease specific hiPSCs can differentiate into the type of cells that are crucial for disease development and be successfully used as disease models. In addition, the current ability to repair gene mutations in specific cells from patients through innovative gene editing systems offers the possibility of autologous (from the same person) cell transplantation.
Myoblasts are primordial muscle cells that have the potential to develop into muscle fibers. In the laboratory, myoblasts can be easily grown from human muscle tissue; otherwise, myoblasts can also be generated from hiPSCs, preferably without the involvement of genetic engineering. The goal of the study was to optimize the generation of hiPSCs-derived myoblasts.
In order to select myoblast progenitors, the team used a marker expressed on these cells – CD34. These CD34 positive cells were selected and expanded under proper cell culture conditions that stimulated myogenesis (formation of muscle tissue). The cells fused and formed multinucleated striated muscle fibers called myotubes, which in turn expressed a set of key markers for muscle differentiation. The myotubes formed were able to respond to environmental stimuli, contract upon electrical stimulation, and generate action potentials, similar to normal, function muscle.
Researchers then co-cultured these myotubes with motor neurons, both generated from identical hiPSCs cell lines, and found that these co-cultures allowed the generation of mature NMJs, indicating the formation of motor units (motor neuron and contacting muscle cells).
In conclusion, the research team developed a successful method to generate a functional muscular system derived from stem cells and comprising two distinct communicating cells types – motor neurons and skeletal muscle. The authors believe that this new in vitro co-culture system could be a valuable tool for research on diseases in which motor neurons and NMJ are predominantly affected like in ALS.