Amyotrophic lateral sclerosis (ALS) is a progressive neurodegenerative disease that affects nerve cells in the brain and the spinal cord. Based on U.S. population studies, a little over 5,000 people in the United States are diagnosed with ALS each year. There are several research studies investigating possible risk factors that may be associated with ALS, however, more work is needed to conclusively determine what genetic and/or environmental factors contribute to developing ALS. Neuromuscular junction (NMJ) disruption is an early pathogenic event in ALS. It is through NMJs that skeletal muscles receive signals making them contract or relax. In ALS, NMJs are destroyed, leading to progressive muscle weakness and ultimately death. Researchers require accurate lab-based models for ALS to understand its causes and to develop and test new therapies. Now researchers from the VIB, Center for Brain & Disease Research, Laboratory of Neurobiology in Leuven, Belgium, report they have developed a new miniaturized model to provide further insights into ALS.

Their findings are published in the journal Stem Cell Reports in a paper titled, “Human motor units in microfluidic devices are impaired by FUS mutations and improved by HDAC6 inhibition.”

“NMJs ensure communication between motor neurons (MNs) and muscle; however, in MN disorders, such as ALS, NMJs degenerate resulting in muscle atrophy,” wrote the researchers. “The aim of this study was to establish a versatile and reproducible in vitro model of a human motor unit to investigate the effects of ALS-causing mutations. Therefore, we generated a co-culture of human-induced pluripotent stem cell (iPSC)-derived MNs and human primary mesoangioblast-derived myotubes in microfluidic devices.”

Scanning electron microscopy image of a human neuromuscular junction generated in microfluidic devices. Human-induced pluripotent stem cell-derived motor neuron axons fan out and embed themselves into the human muscle tissue, which creates a large connection surface.[Pieter Baatsen, PhD, LiMoNe, Research Group Molecular Neurobiology and Katarina Stoklund Dittlau, Laboratory of Neurobiology at VIB-KU Leuven Center for Brain and Disease Research]
In the sophisticated model, human motor neurons, which were derived from ALS patients, via induced pluripotent stem cells engineered from their own skin cells, and human skeletal muscle cells from healthy donors grew in separate mini-chambers on opposite sides of the device, whereby tiny channels connected the two chambers.

In their model, the researchers investigated the effects of mutant FUS on the formation of human NMJs. More than 20 genes have been linked to familial ALS, and many of them encode RNA-binding proteins, including FUS.

“Interestingly, we saw a reduction in total NMJs with an almost 50% decrease in the FUS-P525L system in comparison with the control P525P. The FUS mutations did not affect the amount of single contact point NMJs, but had a large impact on the number of multiple contact point NMJs. This is in line with the dying-back mechanism where mature NMJs are lost in disease, while newly formed immature NMJs are made simultaneously to compensate for the lack of muscle innervation,” noted the researchers.

“Overall, we established a versatile and relatively easy to use motor unit system to study the functionality of human NMJs in culture. This co-culture model has broad application potential and is suited to test therapeutic strategies focusing on reinnervation and/or the stabilization of NMJs,” concluded the researchers.

A human NMJ model eliminates the challenge of interspecies variability and reduces the need for overexpression of mutated genes in animal models, which might not fully recapitulate human disease etiology or pathology. Their model and findings may lead to new strategies and therapies for ALS.

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