Cystathionine γ-lyase deficiency mediates neurodegeneration in Huntington’s disease
Bindu D. Paul, Juan I. Sbodio, Risheng Xu, M. Scott Vandiver, Jiyoung Y. Cha, Adele M. Snowman & Solomon H. Snyder.
Nature, March 2014
Huntingdon's disease is in many ways one of the most devastating genetic conditions to affect individuals. Oddly, despite the fact that we know what the genetic defect associated with this disease is, how this defect translates into a phenotype remains unclear. In Huntingdon's disease the, disease causing agent is a massive protein called Huntingtin.
Proteins and protein structure
Huntingtin, like all proteins, is just made of a bunch of different amino acids is a very specific order. You can think about amino acids as the building blocks for proteins - the lego pieces used to construct a lego model. There are twenty distinct amino acids, from which all proteins are constructed. The building blocks are stuck together to make a long chain (a polymer) in a specific order. This order is incredibly precise. Every protein of a certain type has exactly the same order, and this order is called the primary sequence.
So, proteins are these super-complex heteropolymers (hetero = different, poly = many, mer = unit – so lots of different units stuck together) which fold up in a specific way such that a single primary sequence will consistently form the same final structure in 3D. For more information on protein folding see my summary on protein folding from my denaturant paper
So, generally speaking, across all different proteins (of which there are tens of thousands just in humans) their lengths are highly variable, as are their linear sequences, both between different proteins and along the primary sequence. Let's explain this with an analogy, for which English (as a language) serves nicely – words are proteins and letters are amino acids. Take twenty random words in English. These words have different lengths – some, (“cat”) are short, while some (“antidisestablishmentarianism”) are long. They also have heterogenous (variable) sequences when compared to one another – if words had the same sequence, they'd be the same word! But, the final point I want to make here is that within a primary sequence there's great heterogeneity along the sequence – i.e. you (relatively) rarely see double letters and never see triple letters. This is also true in proteins– normally you don't expect to see more than four or five identical amino acids next to one another in the primary sequence.
So – back to Huntington's disease.
Huntingtin and polyglutamine
In the protein Huntingtin, there's a region of the protein which has a region of repeated amino acids - so the same amino acid is repeated many times. The culprit amino acid here is glutamine. In healthy people this isn't a problem. However, in some people this region becomes expanded, meaning that instead of 10-20 consecutive glutamine residues you have, thirty, fourty, fifty or even more. This expanded glutamine region is referred to as an expanded polyglutamine stretch. Why this expansion happens is unclear, but what is clear is that in individuals where you have more than thirtyfive to forty consecutive glutamine amino acids you're going to develop Huntington's disease.
People have proposed various possible mechanism by which this expansion might cause the disease, but even now none of have been confirmed. In fact, one of the things my lab does is look at how this polyglutamine regions behaves, how it aggregate, and why you see different behavior as the number of glutamine residues in the polyglutamine region changes.
In this paper, the authors propose a specific mechanism by which the polyglutamine region may disrupt normal neuronal function.
The general idea here is that when you have the expanded polyglutamine stretch inside Huntingtin, this polyglutamine strech can sequester away another protein, SP1. SP1 is involved in generating another protein, CSE, and this CSE protein seems to plays a role in protecting the cell from oxidative damage. Huntington's disease seems to have a link with oxidative damage, so this provides a direct mechanism by which the disease's hallmark polyglutamine expansion connects to this oxidative damage. When the researchers artificially created more SP1, this created more CSE, which in turn seemed to reduce the amount of damage being done to cells.
All this comes together to suggest that the polyglutamine region may be disrupting the cells normal ability to protect themselves from this oxidative damage through an indirect mechanism. This generates a range of possible additional experiments and follow up studies, but if the authors are able to both reproduce and build on this work, it could end up being one of the more important Huntington's disease discoveries in recent years.
Huntington's disease is characterized by an expanded polyglutamine stretch in the N-terminal region of the Huntingtin protein. In normal individuals there's a short glutamine stretch in this N-terminal domain. However, in disease-affected individuals, this region expands to become Q35-40 or more. While this genetic defect is highly predictive of disease severity, the link between this expansion and the actual disease remains unclear.
In this paper, the authors identify an enzyme - cystathione-gamma-lyase (CSE) which had previously been thought to only occur in the peripheral tissue – which when deleted lead to behavior typical of a Huntington's disease mouse model. As a result of this observation, the state of CSE was investigated in the context of a Huntington's mouse models.
CSE is involved in cysteine production, which itself is subsequently used in creating glutathione, an important antioxidant. It also creates H2S, which is used as a signalling marker for a range of different proteins.
It was noticed that in Huntington's mouse models and human patient samples, CSE levels were markedly reduced when compared to controls. Further investigation found that this reduction in CSE doesn't seem to happen at the protien level, but there is a clear reduction of CSE mRNA transcripts. Huntingtin with expanded polyQ (mHtt) is known to bind to SP1, which itself is a transcription factor for CSE. Considering this, the implication is that mHtt binds SP1 and reduces the expression of CSE by sequestering away the SP1 transcription factor.
By over-expressing SP1, the authors were able to return both mRNA and protein levels of CSE in cells with Q111 expansions back to normal levels, and these cells became competent in a cysteine free medium.
CSE is implicated in resolving mitochondrial dysfunction, and Huntington's disease has as-of-yet unclear connections with mitochondrial damage and oxidative stress. When cells lack CSE, they are much more susceptible to oxidative damage than control cells. The authors suggest that SP1 and CSE may be the link between oxidative stress, mitochondrial damage and the polyglutamine expansion observed consistently in Huntington's disease.
In fact – in the original mHtt-SP1 binding paper the authors there note that over-expression of SP1 reduces cellular toxicity (as also observed here) and that it's specifically monomeric mHtt which binds SP1. This is consistent with a growing body of work in general neurodegeneration which suggests that the pathological species in diseases where you get aggregation (e.g. Alzheimer's, Parkinson's etc.) may not be the aggregates themselves. One question firmly remains with Huntington's – why does this only become phenotypically relevant in middle age? An obvious approach here is to examine CSE related parameters in the context of Juvenile onset Huntington's disease.