Glucosidase I: Could targeting N-linked glycosylation of proteins help us fight viral infection?

Viruses are sneaky.  As you probably already know from your last bout with the common cold or the flu, for most viral infections doctors will simply prescribe plenty of water and bed rest.  But what if we could stop the virus from replicating, or block it from entering the host cells in the first place?
Recent research on two siblings with a rare mutation in a key glycosylation enzyme (alpha-glucosidase I) sparked our interest as these children were found to be resistant to a number of bacterial and viral infections.  While they themselves had a lot of other health issues, what if we could target glycosylation temporarily to help patients fight off infection?

Glycosylation – the addition of sugars, such as glucose, to proteins during their synthesis – is an important protein modification associated with their correct folding, substrate binding, solubility and stability (among other properties!).  It is especially important in determining blood type, something we’ve discussed in a previous article.  In this recent study, it was found that while defects in N-linked glycosylation caused the children to have severe hypogammaglobulinaemia (an immune deficiency caused by a reduction in immunoglobulin levels, the basis of your immune system) amongst other mainly neurological symptoms.  Despite this, they had less bacterial infections than “normal” children as well as being more resistant to a number of viruses such as HIV and influenza.  But how is this possible?  To explain, we first have to understand the process of glycosylation as a whole.

 

What is glucosidase I and how is it involved in glycosylation?

An overview of the process of N-Glycosylation. Click for more detail (c) Amy Swanston 2014

The gene MOGS codes for the protein/enzyme alpha-glucosidase I  (also known as mannosyl-oligosaccharide glucosidase), and is the protein in which the siblings in this study had a mutation.  When proteins are being transcribed from their template RNA by the ribosome, specific modifications are carried out at defined sequences of amino acids on the protein.

One such modification is N-linked glycosylation, in which proteins are modified by the addition of an oligosaccharide precursor (a chain of sugars) to the NH2 of Asparagine at a particular sequence of amino acids.  This occurs at the same time as protein translation, before it has had a chance to become fully folded.  This pattern of sugars is then modified further in the ER and Golgi.  Glycosidase I is a very important protein whose role is to trim the first glucose from the larger N-linked precursor allowing other enzymes to further modify this branch of sugars.  See image 2 for an overview of the pathway.

This precursor eventually reaches a state where it exists as a “trimmed oligosaccharide”, which can be recognised by a number of ER chaperones designed to assist the protein in folding correctly.  Chaperones often recognise monoglucosylated proteins but these proteins still have 3 glucose that would normally be trimmed as glycosidase I has been unable to remove them.  Normally the presence of a single glucose acts quality control mechanism, and is added and removed to the trimmed oligosaccharide to signal that the protein has not folded correctly. If this occurs then the N-glycosylated protein will be bound by chaperone proteins which recruit further additional proteins (PDIs) to “shuffle” the disulphide bonds holding the protein together. This will occur repeatedly, and if correct folding cannot be achieved the protein is degraded.

In this study, it was shown that the unfolded protein response, a coping mechanism for misbehaving unfolded proteins, and ER associated degradation machinery were intact and operating at normal levels, so the disease symptoms shown by the siblings couldn’t be explained by defects in these pathways.  High mannose glycans were found throughout the patients in high levels, showing a lack of sugar trimming.  We suspect that because the N-linked oligosaccharide doesn’t reach the final trimmed sugar stage it is not captured by the quality control mechanism in the ER and is thus able to evade the cellular machinery.

 

Reduced susceptibility to viral infection

Strangely, despite having a reduced IgG half-life and being technically “immunocompromised”, both siblings were less susceptible to a number of bacterial and viral infections.  In experiments using cell lines and mouse models, HIV could enter the host cells but was unable to replicate further, and EL16 (Epstein-barr virus) was also shown to be less infectious, as was influenza.  All of these viruses rely on glycosylation either for entry into the cell or to reproduce their viral coat.  Other viruses such as adenovirus and vaccinia were just as infective as in controls, but these viruses don’t rely glycosylation to proliferate.

 

This research gives some interesting questions for the future.  Could targeting MOGS with drugs in new anti-viral medications in the future be useful for infection with viruses known to require glycosylation to enter host cells or to reproduce?  Would these drugs have too many side effects to be viable?  And why did these patients despite having deficiencies in IgG have less bacterial infections in comparison to control children?

I hope you’ve enjoyed today’s dose of science!

Amy

 

References

Sadat, M., Moir, S., Chun, T., Lusso, P., Kaplan, G., Wolfe, L., Memoli, M., He, M., Vega, H., Kim, L., Huang, Y., Hussein, N., Nievas, E., Mitchell, R., Garofalo, M., Louie, A., Ireland, D., Grunes, C., Cimbro, R., Patel, V., Holzapfel, G., Salahuddin, D., Bristol, T., Adams, D., Marciano, B., Hegde, M., Li, Y., Calvo, K., Stoddard, J., Justement, J., Jacques, J., Priel, D., Murray, D., Sun, P., Kuhns, D., Boerkoel, C., Chiorini, J., Di Pasquale, G., Verthelyi, D., & Rosenzweig, S. (2014). Glycosylation, Hypogammaglobulinemia, and Resistance to Viral Infections New England Journal of Medicine DOI: 10.1056/NEJMoa1302846

Herscovics, A. (1999). Importance of glycosidases in mammalian glycoprotein biosynthesis Biochimica et Biophysica Acta (BBA) – General Subjects, 1473 (1), 96-107 DOI: 10.1016/S0304-4165(99)00171-3

Image 1: A 3D structure of HIV, from http://www.flickr.com/photos/microbeworld/6217704321/

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