Mutant Huntingtin Expression Induces Mitochondrial Calcium Handling Defects in Clonal Striatal Cells

Rodrigo A. Quintanilla,Gail V.W. Johnson,Tamara Milakovic

doi:10.1074/jbc.m603845200

Rodrigo A. Quintanilla, Gail V.W. Johnson + Show 1 more

Open Access

https://doi.org/10.1074/jbc.m603845200

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Abstract

Huntington disease (HD) is caused by a pathological elongation of CAG repeats in the huntingtin protein gene and is characterized by atrophy and neuronal loss primarily in the striatum. Mitochondrial dysfunction and impaired Ca2+ homeostasis in HD have been suggested previously. Here, we elucidate the effects of Ca2+ on mitochondria from the wild type (STHdhQ7/Q7) and mutant (STHdhQ111/Q111) huntingtin-expressing cells of striatal origin. When treated with increasing Ca2+ concentrations, mitochondria from mutant huntingtin-expressing cells showed enhanced sensitivity to Ca2+, as they were more sensitive to Ca2+-induced decreases in state 3 respiration and DeltaPsim, than mitochondria from wild type cells. Further, mutant huntingtin-expressing cells had a reduced mitochondrial Ca2+ uptake capacity in comparison with wild type cells. Decreases in state 3 respiration were associated with increased mitochondrial membrane permeability. The DeltaPsim defect was attenuated in the presence of ADP and the decreases in Ca2+ uptake capacity were abolished in the presence of Permeability Transition Pore (PTP) inhibitors. These findings clearly indicate that mutant huntingtin-expressing cells have mitochondrial Ca2+ handling defects that result in respiratory deficits and that the increased sensitivity to Ca2+ induced mitochondrial permeabilization maybe a contributing mechanism to the mitochondrial dysfunction in HD.

Highlights

Huntington disease (HD)2 is a neurodegenerative disease that is inherited in an autosomal dominant manner
To determine the effects of Ca2ϩ on oxidative phosphorylation in mitochondria isolated from the cells expressing endogenous levels of wild type (STHdhQ7/Q7) or mutant (STHdhQ111/Q111) huntingtin, we measured state 4 and state 3 respiration rates in respiration buffers containing increasing free ␮M Ca2ϩ concentrations
These results indicate that mitochondria from STHdhQ111/Q111 cells are more sensitive to Ca2ϩ-induced changes in oxidative phosphorylation than mitochondria from STHdhQ7/Q7 cells

Summary

FUNCTIONAL CONSEQUENCES*

We elucidate the effects of Ca2؉ on mitochondria from the wild type (STHdhQ7/Q7) and mutant (STHdhQ111/Q111) huntingtin-expressing cells of striatal origin. The ⌬⌿m defect was attenuated in the presence of ADP and the decreases in Ca2؉ uptake capacity were abolished in the presence of Permeability Transition Pore (PTP) inhibitors These findings clearly indicate that mutant huntingtin-expressing cells have mitochondrial Ca2؉ handling defects that result in respiratory deficits and that the increased sensitivity to Ca2؉ induced mitochondrial permeabilization maybe a contributing mechanism to the mitochondrial dysfunction in HD. To study the effects of mutant huntingtin on mitochondrial function, conditionally immortalized cells of striatal origin that express endogenous, comparable levels of either wild type (STHdhQ7/Q7) or mutant (STHdhQ111/Q111) huntingtin were used [17]. This study clearly demonstrates that mitochondrial Ca2ϩ buffering capacity in STHdhQ111/Q111 cells is compromised, and suggests increased sensitivity to Ca2ϩ-induced mitochondrial permeabilization as a mechanism of mitochondrial dysfunction in HD

EXPERIMENTAL PROCEDURES

RESULTS

DISCUSSION