Mitochondrial Respiration and ATP Production Are Significantly Impaired in Striatal Cells Expressing Mutant Huntingtin

Tamara Milakovic,Gail V.W Johnson

doi:10.1074/jbc.m504749200

Tamara Milakovic, Gail V.W Johnson

Open Access

https://doi.org/10.1074/jbc.m504749200

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Abstract

There is significant evidence that energy production impairment and mitochondrial dysfunction play a role in the pathogenesis of Huntington disease. Nonetheless, the specific mitochondrial defects due to the presence of mutant huntingtin have not been fully elucidated. To determine the effects of mutant huntingtin on mitochondrial energy production, a thorough analysis of respiration, ATP production, and functioning of the respiratory complexes was carried out in clonal striatal cells established from Hdh(Q7) (wild-type) and Hdh(Q111) (mutant huntingtin knock-in) mouse embryos. Mitochondrial respiration and ATP production were significantly reduced in the mutant striatal cells compared with the wild-type cells when either glutamate/malate or succinate was used as the substrate. However, mitochondrial respiration was similar in the two cell lines when the artificial electron donor TMPD/ascorbate, which feeds into complex IV, was used as the substrate. The attenuation of mitochondrial respiration and ATP production when either glutamate/malate or succinate was used as the substrate was not due to impairment of the respiratory complexes, because their activities were equivalent in both cell lines. Intriguingly, in the striatum of presymptomatic and pathological grade 1 Huntington disease cases there is also no impairment of mitochondrial complexes I-IV (Guidetti, P., Charles, V., Chen, E. Y., Reddy, P. H., Kordower, J. H., Whetsell, W. O., Jr., Schwarcz, R., and Tagle, D. A. (2001) Exp. Neurol. 169, 340-350). To our knowledge, this is the first comprehensive analysis of the effects of physiological levels of mutant huntingtin on mitochondrial respiratory function within an appropriate cellular context. These findings demonstrate that the presence of mutant huntingtin impairs mitochondrial ATP production through one or more mechanisms that do not directly affect the function of the respiration complexes.

Highlights

Huntington disease (HD)1 is an autosomal dominant neurodegenerative disorder caused by a pathological expansion of CAG repeats in the gene encoding for a protein called huntingtin
The attenuation of mitochondrial respiration and ATP production when either glutamate/ malate or succinate was used as the substrate was not due to impairment of the respiratory complexes, because their activities were equivalent in both cell lines
Treatment of the STHdhQ7/Q7 cells with 3-nitropropionic acid (3-NP) resulted in the initiation of apoptotic cell death, whereas cells expressing mutant huntingtin died by a nonapoptotic process when treated with 3-NP [17]

Summary

Introduction

Huntington disease (HD) is an autosomal dominant neurodegenerative disorder caused by a pathological expansion of CAG repeats in the gene encoding for a protein called huntingtin. Previous studies using the STHdhQ111/Q111 and STHdhQ7/Q7 cells have provided indirect evidence that there is mitochondrial dysfunction in the mutant huntingtin-expressing striatal cells. Treatment of the STHdhQ7/Q7 cells with 3-NP resulted in the initiation of apoptotic cell death, whereas cells expressing mutant huntingtin died by a nonapoptotic process when treated with 3-NP [17]. Overall these and other studies clearly demonstrate that the STHdhQ111/Q111 and STHdhQ7/Q7 cell lines are appropriate models for studying HD pathogenesis. We used these clonal striatal cell lines expressing mutant (STHdhQ111/Q111) or wild-type (STHdhQ7/Q7) huntingtin [14] to examine the effects of mutant huntingtin on. A thorough analysis of O2 consumption, ATP production, and respiratory complex activities was carried out to delineate the effects of mutant huntingtin on mitochondrial bioenergetic processes

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