Impaired Regulation of Brain Mitochondria by Extramitochondrial Ca2+ in Transgenic Huntington Disease Rats

Huntington disease ( HD) is characterized by polyglutamine expansions of huntingtin (htt), but the underlying pathomechanisms have remained unclear. We studied brain mitochondria of transgenic HD rats with 51 glutamine repeats (htt(51Q)), modeling the adult form of HD. Ca-free(2+) up to 2 mu M activated state 3 respiration of wild type mitochondria with glutamate/malate or pyruvate/malate as substrates. Ca-free(2+) above 2 mu M inhibited respiration via cyclosporin A-dependent permeability transition (PT). Ruthenium red, an inhibitor of the mitochondrial Ca2+ uniporter, did not affect the Ca2+ -dependent activation of respiration but reduced Ca2+ -induced inhibition. Thus, Ca2+ activation was mediated exclusively by extramitochondrial Ca2+, whereas inhibition was promoted also by intramitochondrial Ca2+. In contrast, htt(51Q) mitochondria showed a deficient state 3 respiration, a lower sensitivity to Ca2+ activation, and a higher susceptibility to Ca2+ -dependent inhibition. Furthermore htt(51Q) mitochondria exhibited a diminished membrane potential stability in response to Ca2+, lower capacities and rates of Ca2+ accumulation, and a decreased Ca2+ threshold for PT in a substrate-independent but cyclosporin A-sensitive manner. Compared with wild type, Ca2+ -induced inhibition of respiration of htt(51Q) mitochondria was less sensitive to ruthenium red, indicating the involvement of extramitochondrial Ca2+. In conclusion, we demonstrate a novel mechanism of mitochondrial regulation by extramitochondrial Ca2+. We suggest that specific regulatory Ca2+ binding sites on the mitochondrial surface, e. g. the glutamate/aspartate carrier (aralar), mediate this regulation. Interactions between htt(51Q) and distinct targets such as aralar and/or the PT pore may underlie mitochondrial dysregulation leading to energetic depression, cell death, and tissue atrophy in HD.