Oxidative metabolism and Ca 2+ handling in striatal mitochondria from YAC128 mice, a model of Huntington's disease

Abstract

The mechanisms implicated in the pathology of Huntington’s disease (HD) remain not completely understood, although dysfunction of mitochondrial oxidative metabolism and Ca2+ handling have been suggested as contributing factors. However, in our previous studies with mitochondria isolated from the whole brains of HD mice, we found no evidence for defects in mitochondrial respiration and Ca2+ handling. In the present study, we used the YAC128 mouse model of HD to evaluate the effect of mHtt on respiratory activity and Ca2+ uptake capacity of mitochondria isolated from the striatum, the most vulnerable brain region in HD. Isolated, Percoll-gradient purified striatal mitochondria from YAC128 mice were free of cytosolic and ER contaminations, but retained attached mHtt. Both nonsynaptic and synaptic striatal mitochondria isolated from early symptomatic 2-month-old YAC128 mice had similar respiratory rates and Ca2+ uptake capacities compared with mitochondria from wild-type FVB/NJ mice. Consistent with the lack of difference in mitochondrial respiration, we found that the expression of several nuclear-encoded proteins in striatal mitochondria was similar between wild-type and YAC128 mice. Taken together, our data demonstrate that mHtt does not alter respiration and Ca2+ uptake capacity in striatal mitochondria isolated from YAC128 mice, suggesting that respiratory defect and Ca2+ uptake deficiency most likely do not contribute to striatal pathology associated with HD.

Mutant huntingtin fails to directly impair brain mitochondria

“…In our study, we treated isolated brain mitochondria with cytosolic fraction prepared from the brains of YAC128 mice and containing natural human full‐length mHtt, which is more physiologically relevant. The lack of direct and acute mHtt effect on brain mitochondria, described in the current paper, is consistent with our previous reports, indicating the lack of defects both in brain mitochondria isolated from HD mice (R6/2 and YAC128) and in mitochondria of cultured neurons from these animals (Pellman et al ; Hamilton et al ; Hamilton et al ; Hamilton et al ).…”

“…Overall, our current data with mHtt acutely applied to isolated brain mitochondria and previously published data with whole‐brain and striatal mitochondria do not provide evidence for mHtt‐induced mitochondrial respiratory deficiency, abnormal ROS production, and defects in mitochondrial Ca 2+ handling (Pellman et al ; Hamilton et al ; Hamilton et al ; Hamilton et al ). The lack of alterations in respiration, membrane potential, ROS production, and Ca 2+ uptake by brain mitochondria acutely exposed to mHtt suggests that mHtt does not directly impair mitochondria.…”

“…Differences in experimental models of HD used in these studies as well as the wide array of experimental techniques applied by different investigators may account for some of the discrepancies in experimental results. In our previous work, we assessed mitochondrial functions in different mouse models of HD and demonstrated that non‐synaptic and synaptic mitochondria isolated from either whole brains of YAC128 and R6/2 mice or from striata of YAC128 mice had similar respiratory rates, ROS production, and Ca 2+ uptake capacity compared to mitochondria from WT animals (Pellman et al ; Hamilton et al ; Hamilton et al ; Hamilton et al ). Consistent with our results produced with isolated mitochondria, experiments with cultured striatal neurons derived from YAC128 and R6/2 mice and corresponding WT mice indicated that oxidative metabolism and mitochondrial Ca 2+ handling were not altered in neurons expressing mHtt compared to WT neurons (Pellman et al ; Hamilton et al ; Hamilton et al ).…”

Cause or compensation?—Altered neuronal Ca2+ handling in Huntington's disease

“…Despite the above evidence, an exhaustive set of studies by Nickolay Brustovetsky's group, using multiple mouse HD models, failed to observe any metabolic or Ca 2+ handling abnormalities in SPN‐derived mitochondria . These studies mainly relied on isolated mitochondria, suggesting other cellular constituents might be required to reveal mHTT‐mediated mitochondrial dysfunction.…”

“…Beyond simply responding to increased cytosolic Ca 2+ , mitochondria themselves might be dysfunctional in HD. There is a general consensus that mHTT strongly associates with mitochondria and mitochondria‐related proteins . In humans, functional PET studies indicate metabolic changes in striatum, while HD patients show persistent weight loss, despite appropriate caloric intake, suggesting global metabolic dysfunction .…”