Glycan selection in the brain

Published April 03 2017

Humans always have had an interest in understanding how we evolved to be what we are today. One of the ways that humans are different from most other mammals is loss of an enzyme called CMP-Neu5Ac hydroxylase, or Cmah for short. In a paper in the Journal of Biological Chemistry that was selected as one of the Editors’ Picks, Yuko Naito-Matsui and others in Ajit Varki’s lab at the University of California, San Diego, explored how the suppression of Cmah in the brain may confer subtle evolutionary advantages. “This is a complicated evolutionary story — it is difficult to prove what really happened during millions of years of evolution,” says Varki. “However, this complexity may also attract readers, because all of us are the results of such evolution.”

Neu5Ac is a sialic acid found on the surface of all vertebrate cells. Most other mammals use Cmah to convert Neu5Ac into a similar sialic acid known as Neu5Gc in all tissues except in the brain, where Cmah is not present in neural cells. The tissue-specific repression of Cmah in the neural cells of mammals and the complete loss of Cmah in humans led to the idea that the loss of this enzyme may hold evolutionary advantages.

To begin to understand what selection processes led to the suppression of Neu5Gc production in the vertebrate brain, Varki and colleagues created a mouse model called NCmahTg that expressed Cmah in the brain. These mice showed a dramatic increase in the Neu5Gc/Neu5Ac ratio as well as incorporation of Neu5Gc into neural cell-surface structures, such as gangliosides and polysialic acid.

Once they had achieved a functional model system, Varki and colleagues wanted to test whether ganglioside activity was affected by the incorporation of Neu5Gc. Gangliosides are glycosphingolipids that contain sialic acids and, in their Neu5Ac state, interact with myelin-associated glycoprotein, known as MAG, which is thought to mediate the preservation of myelination and axonal outgrowth.

Varki’s team collaborated with the team of Ronald Schnaar at Johns Hopkins University. By staining with an antibody containing the ganglioside binding domain of MAG, the investigators found that Neu5Gc incorporation disrupted the interaction between gangliosides and MAG. Images of the major axon tract in the central nervous system also showed a significant reduction in myelination, with some large axons showing complete loss of myelination.

Next, Varki and colleagues conducted a series of neuronal and behavioral tests to determine if the NCmahTg mice had any neurological impairments. Neu5Ac is a component of several neural cell-surface structures; myelination is important for motor coordination and balance. NCmahTg mice displayed slight defects in hind-limb extension and a shorter stride compared with wild-type mice. The NCmahTg mice also were shown to have impaired memory. While the phenotypes were mild, they did show a negative impact on neuronal function of Neu5Gc production in the brain.

The mild neural phenotypes did not seem to explain fully the evolutionary selection for suppression of Cmah expression in the vertebrate brain, so Varki and colleagues decided to explore whether Neu5Gc production in the brain increased susceptibility to microbial toxins. Sialic acids are known to be recognized by virulence factors, such as bacterial adhesins or viral agglutinins, and previous work by James and Adrienne Paton of the University of Adelaide in Australia identified subtilase cytotoxin as a bacterial toxin that preferentially recognized Neu5Gc. The investigators administered subtilase cytotoxin to wild-type and NCmahTg mice and monitored their survival rate. Indeed, NCmahTg mice showed higher susceptibility to subtilase cytotoxin, indicating that loss of Neu5Gc in the brain may have evolved as a protective measure against such virulence factors targeting the brain.

Protection of brain function is paramount to survival and reproductive fitness. It makes sense that, over millions of years, vertebrates evolved mechanisms that provide even a small protective advantage. “Humans completely lack Neu5Gc because of a mutation in the Neu5Gc-synthesizing enzyme,” says Varki. “The present work could potentially contribute to understanding how human brains became different from those of other related species.”

Amber Lucas Amber Lucas is a graduate student at Carnegie Mellon University.