Research has shown that mitochondrial dysfunction is associated with neuronal loss in Huntington’s disease (HD). However, it is unclear how mutant huntingtin (Htt) may cause such dysfunction. Researchers at University of Pittsburg and Washington University have discovered evidence of a direct relationship between mutant Htt and the mitochondrial protein import machinery1. This study has many implications for HD research including the development of mitochondrial protein import-based therapies.
Mitochondria are specialized subunits within a cell often referred to as the ‘energy powerhouse’ as they are responsible for conversion of nutrients into energy for the cell. Mitochondria can be found in every cell of the human body (with the exception of red blood cells) and they are also responsible for calcium homeostasis and the regulation of apoptosis (programmed cell death) 2.
Unfortunately, mitochondria may not function properly due to the obstruction of important pathways, which can compromise the energy production of the cell. When mitochondria in the brain malfunction, less energy is generated within the neuronal cells, which can lead to cell injury, and eventually cell death. Therefore, neuronal cells need healthy mitochondria to survive. There is evidence that mitochondrial dysfunction may be a critical driver of HD pathophysiology1.
The mitochondrial protein import pathway is crucial for healthy mitochondrial function since proteins are often created in the cytoplasm before traveling through several complexes to make their way to the mitochondria. To stress the importance of this pathway, mitochondria contain approximately 1,500 different proteins, yet 99% of which are encoded by the nuclear genome.
Initially, the scientists observed that mutant Htt protein, unlike normal Htt protein, was present within brain mitochondria of HD patients. They hypothesized the existence of an interaction between mHtt and some mitochondrial proteins. The researchers conducted a series of experiments using mice models and cell lines to determine what that interaction is exactly and if that interaction negatively affects the mitochondria function.
In order to further understand how this deficiency in mitochondrial protein import is occurring, the scientists further investigated the role of Htt protein using an in vitro protein import assay with a radiolabeled precursor matrix protein (pOTC) which allows the measurement of the import activity for many mitochondrial proteins. They performed this assay in normal mitochondria in the presence of either normal (23Q) or mutant (97Q) recombinant Htt fusion proteins (shorter N-terminal Htt fragments that have been tagged to allow for purification and increased expression3) and observed that, if the Htt fragment carries a polyQ expansion, it will inhibit the uptake of preproteins into the mitochondria through direct association with the TIM23 mitochondrial protein import complex1. (The TIM23 complex assists in the movements of certain proteins across the inner mitochondrial membrane into the mitochondrial matrix.) The mutant Htt does, in fact, cause mitochondrial dysfunction by interfering with the mitochondria’s protein import function.
To understand how to prevent such dysfunction, the scientists overexpressed the TIM23 complex of the mitochondria. This resulted in more pre-proteins being able to travel through the membrane and into the mitochondria, and consequently, successfully limited the problematic cell death caused by the mutant Htt. This observation confirms that the purposeful overexpression of the TIM23 complex may actually be a key therapeutic target as cell death could be avoided despite a presence of mutant Htt.
It is important to note that there is a brain-specific reduction in the activity of protein import in neuronal mitochondria, rather than an increased sensitivity to import dysfunction, as previously hypothesized. Because of the energetic demands of synaptic transmission, synaptosomal mitochondria might be more sensitive to changes in protein import.
Finally, since HD is an age-dependent progressive neurodegenerative disease, the researchers decide to investigate how age-related insults like oxidative stress (link to definition) might compound the progression of the disease. For this experiment, mitochondrial protein import activity was observed in the presence of a sub lethal dose of hydrogen peroxide. This dosage did not impact the wild type neurons. However, it did significantly decrease import activity in the 195CAG HD neurons.
These results suggest that mutant Htt inhibits mitochondrial protein import via a direct interaction with the import machinery and have important implications in respect to the development of future HD therapies. Scientists can now apply these findings to target mechanisms that prevent the mutant Htt from blocking mitochondrial protein import in an attempt to prevent or delay neuronal cell death due to mitochondrial dysfunction. Such therapeutic approach could have the potential to slow down the progression of the disease and reduce the need to focus on the more difficult task of neurogenesis and network repair.
For Further Reading^
3. “GST-tagged Proteins – Production and Purification.” GST-tagged Proteins. N.p., n.d. Web. 02 July 2014.
4. . Johri, A. & Beal, M.F. Antioxidants in Huntington’s disease. Biochem. Biophys. Acta 1822, 664–674 (2012).
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