Understanding ALS requires studying the disease in multiple models, from individual cells to whole organisms. Among these, one of the most important is the study of individual motor neurons. The difficulty of growing large numbers of motor neurons has historically hindered such research. But within the past decade, and especially the past few years, researchers have developed methods to create an unlimited supply of motor neurons from stem cells. Those stem cells can originate from embryonic stem cells or can be derived from skin cells, including skin cells of people with ALS. These developments have accelerated ALS research significantly and have shed new light on the subtle and important differences among different classes of motor neurons.
In a recently published state-of-the-art review, Brandi Davis-Dusenberry, Ph.D., Kevin Eggan, Ph.D., and colleagues describe in detail how basic research in neuronal development provided the key insights that allow scientists to convert non-neuronal cells into neurons. They also outline the importance of quality control and use of cross-laboratory best practices in standardizing the motor neuron supply. Dr. Davis-Dusenberry is an ALS Association Milton Safenowitz Fellow. The abstract of the review can be found here.
Stem cells can be converted to neurons by subjecting them to various growth factors that also act during neural development, including retinoic acid and fibroblast growth factor. The concentration and timing of application are critical, especially for increasing efficiency of cell conversion. More recently, motor neurons have been created directly from skin cells, skipping the stem cell stage altogether, in a process called direct lineage conversion, saving weeks in the lab.
Not every cell made is a motor neuron, and not every motor neuron is the same. The authors propose four characteristics for evaluating motor neurons created through these methods. Cells should display motor neuron-specific markers, or even better, a full suite of gene changes evaluated with genome-wide transcription profiling. Cells should be electrically active, mimicking the firing activity of bona fide motor neurons in contact with a muscle. When cultured with muscle, they should form neuromuscular junctions, the special synapses at which the neuron contacts the muscle and controls contraction. Finally, they should be able to survive grafting into the spinal cord of a model animal, although this is so technically challenging that it may not be testable in all situations.
The authors conclude, “We are optimistic that continued efforts to collaboratively establish best practices for motor neuron production, culture and evaluation in vitro will provide the keys to unlock novel therapeutic strategies for devastating neurological disorders, including ALS.”