Synthetic myelination substrates for high-throughput screening of multiple sclerosis therapeutics
In multiple sclerosis (MS), neurons degenerate because the electrical insulation surrounding them, myelin, is gradually lost. Myelin is formed by oligodendrocytes, another cell type in the central nervous system, and damage to myelin can be repaired by their precursors, which are present in adults and are recruited to sites of damage. Remyelination prevents neuronal damage, but cannot keep pace with the damage in MS; drugs that promote it would be an obvious means of slowing progression. However, no such drugs have yet been identified.
A possible reason for this lack of disease-modifying drugs in MS is that screening candidates for promotion of such a complex process would be highly time-intensive, as quantifying myelination in neuronal cultures is laborious. However, the recent observation that oligodendrocytes can myelinate electrospun fibers led a team at UC San Francisco supervised by Jonah Chan to design a substrate for high-throughput screens. As an assay that yields a simple, countable output would be most efficient, postdoc Feng Mei and colleagues examined whether vertical silica micropillars could act as myelination substrates, as these would yield rings detectable by staining for a myelin surface marker.
After showing that oligodendrocytes do indeed extend myelin around micropillars, they then modified the assay to identify compounds that promote myelination by oligodendrocyte precursors, since these are the true drug target. Staining for both a precursor marker and a mature oligodendrocyte marker identified compounds that promoted differentiation and myelination (indicated by a high ratio of mature marker-stained to precursor marker-stained rings). Demonstration that one of the active compounds promotes remyelination in mice treated with an agent that causes demyelination indicates that this rapid assay yields valid candidates.
Micropillar assay provides easily analyzed results: green rings (positive for PDGFRα) indicate wrapping by oligodendrocyte precursors, while red rings (positive for myelin basic protein, MBP) indicate wrapping by mature oligodendrocytes. Ideal drug candidates would promote both myelination and differentiation; clemastine was the most effective compound among those screened in promoting both outcomes. (Thyroid hormone T3 is known to promote myelination.) Used with permission from Nature Publishing Group.
While the family of compounds that promote remyelination in this assay are not likely drug candidates given their sedative and psychoactive properties, they do indicate a direction for future drug screening, as all effective compounds inhibit a particular class of acetylcholine receptor. However, an important caveat to these results is that drugs were screened in healthy oligodendrocyte precursors; whether they would have the same effect in MS-affected cells is unknown. Employing patient-derived cells (perhaps via induction of pluripotency in skin or other accessible cells, followed by differentiation into oligodendrocyte precursors) in future studies would provide more directly relevant candidates.
This work suggests that micropatterned surfaces could be an important tool in drug screening. While micropillars are ideal substrates for myelination assays, other patterns may simplify image analysis for other cell fates or outcomes; for example, wavy or ridged surfaces could be used in assays related to skeletal muscle differentiation.
Mei F et al., Micropillar arrays as a high-throughput screening platform for therapeutics in multiple sclerosis, Nature Medicine 2014; published online July 6.