History

Synthetic oligos were conceived by Summerton (Gene Tools) at AntiVirals Inc. (now Sarepta Therapeutics) and originally developed in collaboration with Weller.^{[citation needed]}

Structure

Morpholinos are synthetic molecules that are the product of a redesign of natural nucleic acid structure.^[8] Usually 25 bases in length, they bind to complementary sequences of RNA or single-stranded DNA by standard nucleic acid base-pairing. In terms of structure, the difference between Morpholinos and DNA is that, while Morpholinos have standard nucleic acid bases, those bases are bound to morpholine rings linked through phosphorodiamidate groups instead of phosphates.^[8] The figure compares the structures of the two strands depicted there, one of RNA and the other of a Morpholino. Replacement of anionic phosphates with the uncharged phosphorodiamidate groups eliminates ionization in the usual physiological pH range, so Morpholinos in organisms or cells are uncharged molecules. The entire backbone of a Morpholino is made from these modified subunits.

Function

Morpholinos do not degrade their target RNA molecules, unlike many antisense structural types (e.g., phosphorothioates, siRNA). Instead, Morpholinos act by "steric blocking", binding to a target sequence within an RNA, inhibiting molecules that might otherwise interact with the RNA.^[1] Morpholino oligos are often used to investigate the role of a specific mRNA transcript in an embryo. Developmental biologists inject Morpholino oligos into eggs or embryos of zebrafish,^[9] African clawed frog (Xenopus),^[10] sea urchin^[11] and killifish (F. heteroclitus) producing morphant embryos, or electroporate Morpholinos into chick^[12] embryos at later development stages. With appropriate cytosolic delivery systems, Morpholinos are effective in cell culture.^[13][14] Vivo-Morpholinos, in which the oligo is covalently linked to a delivery dendrimer, enter cells when administered systemically in adult animals.^[15]