Functional characterization of an uncoupling protein in goldfish white skeletal muscle

Uncoupling protein 1 in snakehead (Channa argus): Cloning, tissue distribution, and its expression in response to fasting and refeeding

In mammals, uncoupling protein 1 (UCP1) is well known for its thermogenic role in brown adipose tissue (BAT). However, the UCP1 physiological roles are still unclear in fish, although several teleost ucp1 genes have been identified. The aim of this study is to investigate the potential roles of fish UCP1 involved in food intake regulation and energy homeostasis. We herein report on the molecular cloning, tissue distribution and the effect of fasting and refeeding on the expression of ucp1 in Channa argus. UCP1 consisted of a 921 bp open reading frame predicted to encode 306 amino acids. Sequence analysis revealed that snakehead UCP1 was highly conserved (>80%) with teleost UCP1, but shared a lower identity (60-72%) with mammals. Phylogenetic analysis supported that snakehead UCP1 was closely related to piscine UCP1. In addition, ucp1 was found to extensively expressed in all detected tissues, with the highest level in liver. Futhermore, the hepatic ucp1 was found to significantly increased after short-term and long-term food deprivation, and dramatically increased following refeeding. These findings suggested that snakehead UCP1 might play important roles in food intake regulation and fatty acid metabolism in snakehead fish, and it could be as a potential target locus to improve commercial production of this kind of fish.

Subspecies differences in thermal acclimation of mitochondrial function and the role of uncoupling proteins in killifish

Thermal effects on mitochondrial efficiency and ATP production can influence whole-animal thermal tolerance and performance. Thus, organisms may have the capacity to alter mitochondrial processes through acclimation or adaptation to mitigate these effects. One possible mechanism is through the action of uncoupling proteins (UCPs) which can decrease the proton motive force independent of the production of ATP. To test this hypothesis, we examined the mRNA expression patterns of UCP isoforms and characterized the effects of thermal acclimation and putative local thermal adaptation on mitochondrial capacity, proton leak, and P/O ratios in two subspecies of Atlantic killifish (Fundulus heteroclitus). Ucp1 was the dominant isoform in liver and was more highly expressed in northern killifish. We found that cold acclimation increased mitochondrial capacity (state III and maximum substrate oxidation capacity), state II membrane potential, proton leak, and P/O ratios in northern, but not southern killifish liver mitochondria. Palmitate-induced mitochondrial uncoupling was detected in northern, but not southern, killifish liver mitochondria, consistent with the differences in mRNA expression between the subspecies. Taken together, our data suggest that mitochondrial function is more plastic in response to thermal acclimation in northern killifish than southern killifish and that UCP1 may play a role in regulating the proton motive force in northern, but not southern killifish in response to thermal acclimation. These data demonstrate the potential for adaptive variation in mitochondrial plasticity in response to cold.

The conserved regulation of mitochondrial uncoupling proteins: From unicellular eukaryotes to mammals

Uncoupling proteins (UCPs) belong to the mitochondrial anion carrier protein family and mediate regulated proton leak across the inner mitochondrial membrane. Free fatty acids, aldehydes such as hydroxynonenal, and retinoids activate UCPs. However, there are some controversies about the effective action of retinoids and aldehydes alone; thus, only free fatty acids are commonly accepted positive effectors of UCPs. Purine nucleotides such as GTP inhibit UCP-mediated mitochondrial proton leak. In turn, membranous coenzyme Q may play a role as a redox state-dependent metabolic sensor that modulates the complete activation/inhibition of UCPs. Such regulation has been observed for UCPs in microorganisms, plant and animal UCP1 homologues, and UCP1 in mammalian brown adipose tissue. The origin of UCPs is still under debate, but UCP homologues have been identified in all systematic groups of eukaryotes. Despite the differing levels of amino acid/DNA sequence similarities, functional studies in unicellular and multicellular organisms, from amoebae to mammals, suggest that the mechanistic regulation of UCP activity is evolutionarily well conserved. This review focuses on the regulatory feedback loops of UCPs involving free fatty acids, aldehydes, retinoids, purine nucleotides, and coenzyme Q (particularly its reduction level), which may derive from the early stages of evolution as UCP first emerged.

Molecular cloning and tissue expression of uncoupling protein 1, 2 and 3 genes in Chinese perch (Siniperca chuatsi)

Uncoupling proteins (UCPs) are mitochondrial anion carrier proteins, which play important roles in several physiological processes, including thermogenesis, reactive oxygen species generation, growth, lipid metabolism and insulin secretion. Although the roles of UCPs are well understood in mammals, little is known in fish. To investigate the thermogenesis roles in Chinese perch (Siniperca chuatsi), we cloned the UCP1, 2 and 3. The UCP1 consisted of six exons and five introns, and the UCP2 consisted of eight exons and seven introns. The UCP1 was primarily expressed in liver, UCP2 was ubiquitously expressed, and UCP3 was primarily expressed in muscle. The mRNA levels of UCP1 and UCP2 in liver, and UCP3 in muscle were significantly increased after prolonged cold exposure, but did not change after prolonged heat exposure, suggesting that Chinese perch might have a mechanism of response to cold environment, but not to hot environment. The intestinal UCP1 mRNA level was significantly up-regulated after prolonged heat exposure, while the UCP2 mRNA level was significantly up-regulated after prolonged cold exposure, suggesting that the two paralogs might play different roles in intestine of Chinese perch. In addition, the phylogenetic analysis could shed new light on the evolutionary diversification of UCP gene family.

Identification of uncoupling protein 4 from the blood-sucking insect Rhodnius prolixus and its possible role on protection against oxidative stress

Uncoupling proteins (UCPs) play a critical role in the control of the mitochondrial membrane potential (ΔΨm) due to their ability to dissipate the proton gradient, which results in the uncoupling of mitochondrial respiration from ATP production. Most reactive oxygen species generation in mitochondria occurs in complex III, due to an increase of semiquinone (Q(-)) half-life. When active, UCPs can account as a potential antioxidant system by decreasing ΔΨm and increasing mitochondrial respiration, thus reducing Q(-) life time. The hematophagous insect Rhodnius prolixus, a vector of Chagas disease, is exposed to a huge increase in oxidative stress after a blood meal because of the hydrolysis of hemoglobin and the release of the cytotoxic heme molecule. Although some protective mechanisms were already described for this insect and other hematophagous arthropods, the putative role of UCP proteins as antioxidants in this context has not been explored. In this report, two genes encoding UCP proteins (RpUcp4 and RpUcp5) were identified in the R. prolixus genome. RpUcp4 is the predominant transcript in most analyzed organs, and both mRNA and protein expression are upregulated (13- and 3-fold increase, respectively) in enterocytes the first day after the blood feeding. The increase in UCP4 expression is coincident with the decrease in hydrogen peroxide (H2O2) generation by midgut cells. Furthermore, in mitochondria isolated from enterocytes, the modulation of UCP activity by palmitic acid and GDP resulted in altered ΔΨm, as well as modulation of H2O2 generation rates. These results indicate that R. prolixus UCP4 may function in an antioxidation mechanism to protect the midgut cells against oxidative damage caused by blood digestion.