BRAIN CYTOCHROME-OXIDASE SUBUNIT COMPLEMENTARY DNAS - ISOLATION, SUBCLONING, SEQUENCING, LIGHT AND ELECTRON-MICROSCOPIC IN-SITU HYBRIDIZATION OF TRANSCRIPTS, AND REGULATION BY NEURONAL-ACTIVITY

The goal of the present study was to isolate, for the first time, cyto chrome oxidase subunit genes from murine brain complementary DNA libra ry and to characterize the expression of these genes from mitochondria l and nuclear sources at both light and electron microscopic levels. B rain subunit III (mitochondrial) shared 100% identity with that of mur ine L cells. Subunit VIa (nuclear) was known to have tissue-specific i soforms in other species: the ubiquitous liver isoform and the heart/m uscle isoform. Our brain subunit VIa shared 93% homology with that of the rat liver and 100% identity with the recently reported murine live r isoform, which is only 62% identical to that of the rat heart isofor m. In situ hybridization with riboprobes revealed messenger RNA labell ing that was similar, though not identical, to that of cytochrome oxid ase histochemistry. Monocular enucleation in adult. mice induced a sig nificant down-regulation of both subunit messages in the contralateral lateral geniculate nucleus. However, the decrease in subunit III mess enger RNAs surpassed that of subunit VIa at all time periods examined, suggesting that mitochondrial gene expression is more tightly regulat ed by neuronal activity than that of nuclear ones. At the electron mic roscopic level, subunit III messenger RNA was localized to the mitocho ndrial compartment in both cell bodies and processes, while that of nu clear-encoded subunit VIa was present exclusively in the extramitochon drial compartment of somata and not of dendrites or axons. Surprisingl y, the message was primarily associated with the rough endoplasmic ret iculum, suggesting a novel pathway for its synthesis and trafficking. Our results indicate that the unique properties of neurons impose spec ial requirements for subunits of a single mitochondrial enzyme with du al genomic origins. At sites of high energy demands (such as postsynap tic dendrites and some axon terminals), mitochondrial-encoded cytochro me oxidase subunits can be locally transcribed and translated, and the y provide the framework for the subsequent importation and incorporati on of nuclear-encoded subunits, which are strictly synthesized in the cell bodies. Dynamic local energy needs are met when subunits from the two genomic sources are assembled to form functional holoenzymes. Cop yright (C) 1996 IBRO. Published by Elsevier Science Ltd.