Neuromuscular diseases encompass a variety of clinically and genetically distinct pathologies that are characterized by the alteration of muscle and/or the nervous system. Because of advances in sequencing technologies, hundreds of genes responsible for these rare disorders have been identified during the last decade (list available at the Neuromuscular Disease Center, http://neuromuscular.wustl.edu/index.html). The variety of identified mutations indicates that different mutations in a single gene can cause different phenotypes and diseases1,2,3 and that mutations in different genes can produce similar phenotypes4,5. In this context, there are efforts to develop cellular models that can become powerful tools for analyzing mutation consequences and pathological mechanisms.
Spinal MNs have large somas located in the ventral horns of the spinal cord, form long axons to target skeletal muscle fibers, and allow for voluntary movements through the release of acetylcholine at neuromuscular junctions. Since MNs are affected by neuromuscular diseases such as amyotrophic lateral sclerosis, Charcot-Marie-Tooth disease (CMT), and spinal muscular atrophy, Dr. Henderson and colleagues developed the first protocol6 that allowed for cultivation of in vitro spinal MNs and the discovery of neurotrophic factor GDNF7 (glial cell derived neurotrophic factor). Technical refinements since then have allowed for more accurate purification of spinal MNs and their subtypes using FACS8, but enrichment by density gradient remains powerful and widely used in laboratories currently working with primary spinal MNs9,10,11,12,13,14. Subsequently, it is also possible to obtain a higher MN purification grade through immunopanning by taking advantage of surface marker p75(NTR) 15,16,17.
The spinal cord contains different subtypes of cervical, thoracic, and lumbar MNs, as well as median and lateral motor columns that differ among their location in the anterior horn on a dorso-ventral axis and among the targets they innervate8,18. Primary MN cultures can recover all of these MN subtypes in physiological proportions. The main limitation of this technique is the low number of MNs obtained at the end of the procedure; in fact, it can be expected to obtain around 105 MNs from six embryos, which is suitable for microscopy but limiting for biochemistry experiments. To perform experiments with more standardized subtypes and abundant MNs (>106 cells), embryonic stem cell-derived MNs should be considered18.
Transfection of wild-type/mutant transgenes or knockdown endogenous genes into primary MNs is a rapid and helpful tool for deciphering physiopathological pathways, especially when mouse models are unavailable. Magnetofection is one technique for transfecting primary neurons, similar to lipofection without the related neurotoxicity. Furthermore, transfection can be performed on mature neurons after several days in vitro, unlike techniques based on electroporation9. However, one disadvantage of this technique is that the beads bind nucleic acids in the culture, causing noise in DAPI labeling. Viral infection is likely the most efficient technique for transfecting MNs; however, magnetofection does not require certain safety procedures needed for viral production and cellular infection.