How human can a worm be?

Nevertheless, despite this shared mechanism, each of these diseases is unique, as are the brain areas affected, and to understand them is crucial to identify the biological function of the mutated protein behind the disease, something not always easy. But now, an elegant study just published in the advance online edition of the Faseb journal shows how the study of a simpler genetic model – the worm Caenorhabditis elegans (C.elegans) – was able to give insight into the function of the human protein ataxin-3, which when mutated is linked to Machado-Joseph disease (MJD), a fatal genetic neurodegenerative disorder.

The research relies on the fact that vital molecules tend to be conserved throughout evolution, and shows that ataxin-3 seems to be involved in protein degradation, structural and motility components and in several development pathways. Even more interestingly, the study also shows that C.elegans without ataxin-3 have apparently no signs of disease or loss of any of the neural functions characteristic of MJD, raising the possibility that ataxin-3 elimination could treat MJD patients without producing major side effects

Machado-Joseph disease (MJD), also called spinocerebellar ataxia type 3, belongs to the Polyglutamine (polyQ) group of disorders, which is characterised by an abnormal repetition of three nucleotides (DNA building blocks) within a gene, that leads to proteins with long stretches of glutamine (thus being called Poly Q disorders), and that include, among others, Huntington’s disease. These mutated proteins are incapable of folding and working properly (proper protein folding is critical for its biological function) and, instead, accumulate in the brain of patients where they are linked to the neurons’ death characteristic of these disorders. In the case of MJD the pathology results from a mutation in the ATX-3 gene that encodes a protein of unknown function called ataxin-3, and is associated with increasing limb weakness (ataxia means lack of muscle control) and widespread clumsiness, difficulty with speech and swallowing, vision problems and a general loss of motor control that eventually confines the patient to a wheelchair.

MJD affects specifically the spinocerebellar neurons even if ataxin-3 is widely produced in the brain and this, together with the fact that patients producing both mutated and normal ataxin-3 suffer less disease than patients with only the abnormal protein, emphasises the importance of understand the biological functions of ataxin-3 to understand the mechanism of MJD.

In order to try and identify these functions and due to all the difficulties of working in humans Ana-João Rodrigues, Patricia Maciel and colleagues in Portugal and the USA decided to used instead the worm C.elegans, which is a species widely used as model to understand human diseases due to their common conserved metabolic and developmental pathways, as well as genetic material. The team of researchers started by identifying in C.elegans the gene and the protein equivalent to ATX-3 and ataxin-3 in humans, to then find that ataxin-3 in the two species shared functional groups, reacted with the same molecules, had similar patterns of expression and showed the same activity when tested in laboratory. These results confirmed that ataxin-3 study in C.elegans was a valid model to understand the functions of its human counterpart.

The next step was the study of C.elegans that had been manipulated into losing the ATX-3 gene, looking for alterations in these animals. To the researchers surprise, C.elegans without ataxin-3 looked and behaved normally showing no changes in locomotion or other characteristics typical of MJD patients. However, when a molecular analysis was done in these specimens, it was found that about 1.4% of their genome (290 genes) was altered, including genes involved in the conversion of signals from outside the cell into functional changes within the cell (also called signal transduction), the ubiquitinproteasome pathway (a mechanism for protein degradation), structural and motility components and development pathways. Interestingly, previous work by other researchers have suggested that many of the biochemical pathways found altered in C.elegans mutants are also disturbed in MJD patients, again supporting the validity of this model to study human ataxin-3.

Based on their results, Rodrigues, Maciel and colleagues were able to propose a model where ataxin-3 is associated to the regulation of multiple cellular processes through protein degradation (by the ubiquitin proteasome pathway which we know is central to the regulation of almost all cellular processes), and transcriptional regulation, which is the process by which different genes are expressed in different tissues and organs or at different times of development. The involvement of ataxin-3 in cellular regulation explains why such multitude of different genes is affected when ATX-3 is deregulated. These functions, the authors propose, might be specific for some cells only, what would help explaining why just a particular subset of neurons is affected in MJD, despite the fact that ataxin-3 is widely present in neurons. The observation that ataxin-3 loss does not seem to affect the general state of the animals, is another important result from the research as it raises new therapeutic possibilities for MJD.

Rodrigues, Maciel and colleagues’ work shows how the use of simple genetic models, like C.elegans, which have the advantage of possessing a fully sequenced genome and many of their genes’ functions already identified, can help to understand better complex human diseases by allowing a manipulation impossible to exercise in human subjects. The fact that ataxin-3 in humans and C.elegans not only share functional molecular groups and react to the same molecules, but also that ATX3 disruption affects the same genes in humans and C.elegans strongly supports the importance and validity of this study.

Piece researched and written by: Catarina Amorim (catarina.amorim@linacre.ox.ac.uk)