Wednesday March 29 2006
A new potential therapeutic target for Huntington’s disease
Mol. Biol. Cell. (2006). EOP
Researchers at the Frédéric Joliot Hospital Service (DRM, CEA Orsay) jointly with others have successfully characterised a specific impairment of a mitochondrial membrane protein in Huntington’s disease.
A mutation of huntingtin
Huntington’s disease is characterised in particular by the degeneration of a specific part of the brain, the striatum. To date, no effective therapy is known that will slow the progression of this disorder. The disease is caused by an abnormal expansion of the triplet CAG in the coding part of the huntingtin protein. This mutation results in an abnormal number of glutamine moieties, lending the huntingtin protein a new function that is toxic for the cells of the striatum.
How does mutant huntingtin cause the death of striatal neurones? Finding an answer to this question is essential for the effective targeting of new drugs.
Among the possible mechanisms, mitochondrial dysfunction may play a role in this disorder, as it does in many neurodegenerative diseases. The mitochondria are essential for cell survival because they control the key cell death pathways (apoptosis and necrosis), and the production of energy needed by the cell through the transfer of electrons along the respiratory chain.
Previous work in experimental models showed that the chronic administration of an irreversible inhibitor of complex II in the mitochondrial respiratory chain (3‑nitropropionate) produced neurological symptoms and a degeneration of the striatum similar to those observed in patients with Huntington’s disease.
Mitochondrial complex II is implicated
Post-mortem analysis of the components of the respiratory chain in brain samples from Huntington patients revealed an appreciable reduction in the expression of mitochondrial complex II subunits. In particular, subunit Ip (iron-sulphur protein) seemed to be affected more than the others. The construction of an in vitro model of Huntington’s disease confirmed the initial loss of this subunit due to the mutant huntingtin. This model consists in infecting cultured striatal neurones with lentiviral vectors coding for the N-terminal part of wild huntingtin (19 glutamine moieties), mutant huntingtin (82 glutamine moieties), and reporter genes (e.g., GFP). Mutant huntingtin produces very gradual degeneration; the extent of cell death is maximal at between six and eight weeks in vitro. In this model, mutant huntingtin produces a loss of expression of the components of mitochondrial complex II starting with the subunit Ip and accompanied by loss of enzyme activity.
This depressed expression of complex II is not merely an outcome of degeneration; it plays an active role in the cell death induced by mutant huntingtin. The over-expression of complex II subunits, achieved by the lentiviral vectors, inhibits the neurone death triggered by mutant huntingtin. Thus the loss of complex II plays a major role in the striatal degeneration in Huntington’s disease.
There remains the task of verifying that impairment of complex II in vivo is an important event in Huntington’s disease. In parallel, the possibility of enhancing or stabilising the expression of complex II in Huntington’s disease by administering pharmacological agents is being studied in projects funded by the HighQ Foundation and the Hereditary Disease Foundation. If candidate agents can be found, their therapeutic efficacy can be tested in genetic models of Huntington’s disease using the MIRCEN facility that will soon be available at CEA Fontenay-aux-Roses.
Mol. Biol. Cell.(2006). 17(4):1652-1663
Contact: Emmanuel Brouillet
Caption: New cell model for the study of neurone death caused by mutant huntingtin, the protein responsible for Huntington’s disease. In A, the neurones have been infected with a lentiviral vector coding for green fluorescent protein (GFP). The green fluorescence observed a few weeks after the infection indicates that most of the neurones are infected and that the expression of the protein is stable. In B, the neurones infected by the lentivirus coding for mutant huntingtin present aggregates that are detected (red) by a specific antibody (EM48). Note that the whole culture presents this pathological appearance characteristic of the disease. In C and D, the neurones expressing the striatum marker DARPP32 are visualised by immunofluorescence. In C, infection by wild huntingtin (non-mutant) does not affect the survival of neurones after one month of culture. In contrast, infection by a lentivirus coding for mutant huntingtin (in D) leads to a gradual degeneration of the striatal neurones. A biochemical analysis of this model has shown that complex II of the mitochondrial respiratory chain is implicated in the toxicity of mutant huntingtin.