Glycosphingolipid biosynthesis may not be necessary for vertebrate brain development

Antisense oligonucleotides delivered to the amniotic cavity in utero modulate gene expression in the postnatal mouse

Congenital diseases account for a large portion of pediatric illness. Prenatal screening and diagnosis permit early detection of many genetic diseases. Fetal therapeutic strategies to manage disease processes in utero represent a powerful new approach for clinical care. A safe and effective fetal pharmacotherapy designed to modulate gene expression ideally would avoid direct mechanical engagement of the fetus and present an external reservoir of drug. The amniotic cavity surrounding the fetus could serve as an ideal drug reservoir. Antisense oligonucleotides (ASOs) are an established tool for the therapeutic modulation of gene expression. We hypothesize that ASOs administered to the amniotic cavity will gain entry to the fetus and modulate gene expression. Here, we show that an ASO targeting MALAT1 RNA, delivered by transuterine microinjection into the mouse amniotic cavity at embryonic day 13-13.5, reduces target RNA expression for up to 4 weeks after birth. A similarly delivered ASO targeting a causal splice site mutation for Usher syndrome corrects gene expression in the inner ear, a therapeutically relevant target tissue. We conclude that intra-amniotic delivery of ASOs is well tolerated and produces a sustained effect on postnatal gene expression. Transuterine delivery of ASOs is an innovative platform for developing fetal therapeutics to efficaciously treat congenital disease.

Why Is N-Glycolylneuraminic Acid Rare in the Vertebrate Brain?

The sialic acids N-acetylneuraminic acid (Neu5Ac) and N-glycolylneuraminic acid (Neu5Gc) differ by a single oxygen atom and are widely found at the terminal position of glycans on vertebrate cell surfaces. In animals capable of synthesizing Neu5Gc, most tissues and cell types express both sialic acids, in proportions that vary between species. However, it has long been noted that Neu5Gc is consistently expressed at trace to absent levels in the brains of all vertebrates studied to date. Although several reports have claimed to find low levels of Neu5Gc-containing glycans in neural tissue, no study definitively excludes the possibility of contamination with glycans from non-neural cell types. This distribution of a molecule - prominently but variably expressed in extraneural tissues but very low or absent in the brain - is, to our knowledge, unique. The evolutionarily conserved brain-specific suppression of Neu5Gc may indicate that its presence is toxic to this organ; however, no studies to date have directly addressed this very interesting question. Here we provide a historical background to this issue and discuss potential mechanisms causing the suppression of Neu5Gc expression in brain tissue, as well as mechanisms by which Neu5Gc may exert the presumed toxicity. Finally, we discuss future approaches towards understanding the mechanisms and implications of this unusual finding.

The role of glycosphingolipid metabolism in the developing brain

Glycosphingolipids (GSLs) are amphipathic lipids ubiquitously expressed in all vertebrate cells and body fluids, but they are especially abundant in the nervous system. The synthesis of GSLs generally is initiated in the endoplasmic reticulum and completed in the Golgi apparatus, followed by transportation to the plasma membrane surface as an integral component. The amount and expression patterns of GSLs change drastically in brains during the embryonic to postnatal stages. Recent studies have revealed that GSLs are highly localized in cell surface microdomains and function as important components that mediate signal transduction and cell adhesion. Also in developing brains, GSLs are suggested to play important roles in nervous system formation. Disturbance of GSL expression and metabolism affects brain function, resulting in a variety of diseases, particularly lysosomal storage diseases. In this review, we describe some aspects of the roles of GSLs, especially of gangliosides, in brain development.