MSc by Research: Heart failure and fatal epilepsy caused by unbranched glycogen

About the Project

The accumulation and precipitation of unbranched glycogen in human tissues as toxic polyglucosan bodies (PB) is the cause of cardiomyopathy and heart failure and/or a fatal epilepsy called Lafora’s disease in young adults. Mutations encoding functionally inactive mutants of the glycogen priming protein glycogenin-1 (GYG1) or the enzyme HOIL-1 cause cardiomyopathy and heart failure, while functionally inactive mutants of laforin or malin cause Lafora’s diease, a fatal myoclonus epilepsy. However, some laforin and malin deficient patients also develop cardiomyopathy and some HOIl-1 deficient patients develop neurological disorders.  The deficiency of glycogen branching enzyme (GBE1) itself can cause a variety of disorders depending on the extent to which its activity is lost.

Interestingly, HOIL-1 and Malin are both E3 ubiquitin ligases and we discovered that HOIL-1 is a most unusual E3 ligase that attaches ubiquitin to the hydroxyl side chains of serine and threonine residues in proteins and to the C6 hydroxyl group of glucose residues in unbranched glycogen in vitro  https://doi.org/10.15252/embj.2021109700 . In contrast Malin, like nearly all other E3 ligases, form isopeptide bonds between ubiquitin and lysine residues in proteins, including other ubiquitin molecules

We recently discovered that unbranched glycogen formed in GBE1 knockout (KO) human cells is polyubiquitylated, but normally branched glycogen formed in normal cells is not ubiquitylated. We also found that two proteins are attached covalently to unbranched glycogen in human cells, namely GYG1 and ubiquitin. These studies are revealed that the GYG1 attached to unbranched glycogen is ubiquitylated at two lysine residues.  These findings suggested that the role of Malin might be to ubiquitylate GYG1 while the role of HOIL-1 may be to polyubiquitylate GYG1 after it has been monoubiquitylated by HOIL-1. The aim of the project is to investigate this hypothesis.

Our working hypothesis is that the polyubiquitylation of unbranched glycogen formed by errors of metabolism facilitates it removal by glycophagy. Similar to autophagy, glycophagy may involve the uptake of unbranched glycogen into endosomes, which then fuse with lysosomes leading to hydrolysis of the unbranched glycogen by the lysosomal acid alpha1:4 glucosidase (GAA) and recycling of the glucose for other purposes. Consistent with this hypothesis humans with GAA deficiency gradually accumulate vast amounts of glycogen another serious disorder of glycogen storage called Pompe’s disease.

The project will involve generating Malin/GBE1 and HOIL-1/GBE1 knockout cell lines by CRISPR technology, isolating the unbranched glycogen from these cells, digesting it with amylase to hydrolyse the glycogen and so release GYG1 and polyubiquitin. The GYG1 and polyubiquitin will be digested with trypsin and the digests analysed by immunoblotting and by mass spectrometry. The aim is to find out whether the ubiquitylation of GYG1 is impaired and the ubiquitin chains are abnormal in these Malin or HOIL-1 deficient cells. The project will therefore use a number of state of the art molecular techniques used in cell and molecular biology

These studies aim to establish whether different fatal human diseases caused by the accumulation and precipitation of unbranched glycogen can be explained by a common underlying mechanism i.e. abnormal ubiquitylation of the GYG1 attached to unbranched glycogen causing a failure of these molecules to be taken up into lysosomes and hydrolysed.

Our research community thrives on the diversity of students and staff which helps to make the University of Dundee a UK university of choice for postgraduate research. We welcome applications from all talented individuals and are committed to widening access to those who have the ability and potential to benefit from higher education.

Please see our website for further details on the programme and how to apply:

Life Sciences MSc by Research MSc by Research (Postgraduate) : Study : University of Dundee

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