Control of Cytochrome c Oxidase Biosynthesis in the Thermal Remodeling of White Muscle of Two Cyprinid Minnows
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Many fish species respond to cold temperatures by inducing mitochondrial biogenesis, reflected in an increase in the activity of the mitochondrial enzyme cytochrome c oxidase (COX). COX is composed of 13 subunits, 3 encoded by mtDNA and 10 encoded by nuclear genes. I used thermal acclimation/winter acclimatization to explore how fish muscle controls the synthesis of COX. In this study, I used real-time PCR to measure mRNA levels for the 10 nuclear-encoded COX genes and several transcriptional regulators. I compared the thermal response of two cyprinid species, the tropical zebrafish (Danio rerio, acclimated to 11 and 30°C) and the temperate redbelly dace (Phoxinus eos, winter and summer acclimatized). I hypothesized that (i) there would be an increase in COX activity in the cold- versus warm-acclimated fish and (ii) changes in COX activity would be paralleled in the transcript levels of the nuclear-encoded COX subunits as well as the master-regulators and transcription factors of mitochondrial biogenesis. Zebrafish COX activity did not change in the cold but the transcript levels of some subunits decreased up to 70%. Redbelly dace COX activity was 2.9-fold higher in winter fish and though nuclear-encoded subunits had higher transcript levels the increases did not parallel enzyme activity, ranging from 1.7- to 21-fold higher in winter. There also did not appear to be parallel patterns in mRNA for the transcriptional regulators. In zebrafish, when COX activity did not change, there was no significant change in PGC-1α mRNA. In redbelly dace, when COX activity was 2.9-fold higher, PGC-1α mRNA was 6.3-fold higher. These observations suggest that coordination of COX subunit expression is imperfect, implying that subsets of these genes are more important in determining the COX activity. I assert that those genes that are most likely the candidates for regulating COX activity are COX4 and COX5A as they are the first regulatory subunits incorporated into the holoenzyme. Though arguments can also be made for COX5B, 6A and 7B based on the parallels between changes in enzyme activity and transcript abundance as well as the position in which they are assembled into the enzyme complex.