UCP1-KO Mice Research Articles

Skeletal Muscle Fibers of Cold-Acclimated Mice Display Increases in Basal Calcium, Mitochondrial Content and Fatigue Resistance

Mammals initially generate heat by repetitive muscle activity (shivering) when exposed to a cold environment. Shivering can later be replaced by heat generated in brown adipose tissue by activation of uncoupling protein-1 (UCP1). Interestingly, adaptations in skeletal muscles of cold exposed animals are similar to those obtained with endurance training. We studied the function of non-shivering flexor digitorum brevis (FDB) muscles of wild-type (WT) and UCP1-KO mice kept at room temperature (24°C) or cold-acclimated (4°C) for 4-5 weeks. Myoplasmic free [Ca2+] ([Ca2+]i; measured with indo-1) and force were measured under resting conditions and during fatigue induced by repeated tetanic stimulation in intact single fibers. We observed no differences between fibers from WT and UCP1-KO mice. On the other hand, muscle fibers from cold-acclimated mice showed increases in basal [Ca2+]i (∼50%), tetanic [Ca2+]i (∼40%), and SR Ca2+ leak (∼four-fold) as compared to fibers from room-temperature mice. Muscles of cold-acclimated mice also showed increases in expression of peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1α), citrate synthase activity (reflecting increased mitochondrial content), and fatigue resistance. In conclusion, cold exposure induces changes in FDB muscles similar to those observed with endurance training and we propose that increased basal [Ca2+]i has a key role in these adaptations.

Nonshivering thermogenesis protects against defective calcium handling in muscle

When acutely exposed to a cold environment, mammals shiver to generate heat. During prolonged cold exposure, shivering is replaced by adaptive adrenergic nonshivering thermogenesis with increased heat production in brown adipose tissue due to activation of uncoupling protein-1 (UCP1). This cold acclimation is associated with chronically increased sympathetic stimulation of skeletal muscle, which may increase the sarcoplasmic reticulum (SR) Ca(2+) leak via destabilized ryanodine receptor 1 (RyR1) channel complexes. Here, we use genetically engineered UCP1-deficient (UCP1-KO) mice that rely completely on shivering in the cold. We examine soleus muscle, which participates in shivering, and flexor digitorum brevis (FDB) muscle, a distal and superficial muscle that does not shiver. Soleus muscles of cold-acclimated UCP1-KO mice exhibited severe RyR1 PKA hyperphosphorylation and calstabin1 depletion, as well as markedly decreased SR Ca(2+) release and force during contractions. In stark contrast, the RyR1 channel complexes were little affected, and Ca(2+) and force were not decreased in FDB muscles of cold-acclimated UCP1-KO mice. These results indicate that activation of UCP1-mediated heat production in brown adipose tissue during cold exposure reduces the necessity for shivering and thus prevents the development of severe dysfunction in shivering muscles.

Uncoupling protein 1 contributes to fat-reducing effect of leptin

Leptin is proposed to reduce body fat by increasing energy expenditure, in addition to decreasing food intake, through the activation of brown adipose tissue (BAT) thermogenesis. To confirm this, we investigated the effects of leptin on whole body energy expenditure, BAT functions and adiposity in wild-type (WT) mice, and compared with those in mice deficient in uncoupling protein 1 (UCP1), a key molecule for BAT thermogenesis. Chronic hyperleptinemia induced by adenovirus gene transfer reduced food intake in both WT and UCP1-KO mice. WT mice with hyperleptinemia, compared to pair-fed controls, showed increased oxygen consumption, elevated UCP1 expression in BAT, ectopic UCP1 induction in white adipose tissue (WAT), and reduced body fat content. These effects of chronic hyperleptinemia were not observed in UCP1-KO mice. It was concluded that the fat-reducing effect of leptin is due to not only decreased food intake, but also increased UCP1-dependent energy expenditure.:

Indispensable role of mitochondrial UCP1 for antiobesity effect of β3-adrenergic stimulation

Mitochondrial uncoupling protein-1 (UCP1) has been thought to be a key molecule for thermogenesis during cold exposure and spontaneous hyperphagia and thereby in the autonomic regulation of energy expenditure and adiposity. However, UCP1 knockout (KO) mice were reported to be cold intolerant but unexpectedly did not get obese even after hyperphagia, implying that UCP1 may not be involved in the regulation of adiposity. Treatment of obese animals with beta3-adrenergic agonists is known to increase lipid mobilization, induce UCP1, and, finally, reduce body fat content. To obtain direct evidence for the role of UCP1 in the anti-obesity effect of beta3-adrenergic stimulation, in the present study, UCP1-KO and wild-type (WT) mice were fed on cafeteria diets for 8 wk and then given a beta3-adrenergic agonist, CL-316,243 (CL), or saline for 2 wk. A single injection of CL increased whole body oxygen consumption and brown fat temperature in WT mice but not in KO mice, and it elicited almost the same plasma free fatty acid response in WT and KO mice. WT and KO mice increased similarly their body and white fat pad weights on cafeteria diets compared with those on laboratory chow. Daily treatment with CL resulted in a marked reduction of white fat pad weight and the size of adipocytes in WT mice, but not in KO mice. Compared with WT mice, KO mice expressed increased levels of UCP2 in brown fat but decreased levels in white fat and comparable levels of UCP3. It was concluded that the anti-obesity effect of beta3-adrenergic stimulation is largely attributable to UCP1, but less to UCP2 and UCP3, and thereby to UCP1-dependent degradation of fatty acids released from white adipose tissue.

Uncoupling Protein 1 Is Necessary for Norepinephrine-Induced Glucose Utilization in Brown Adipose Tissue

Sympathetic stimulation activates glucose utilization in parallel with fatty acid oxidation and thermogenesis in brown adipose tissue (BAT) through the beta-adrenergic receptors. To clarify the roles of the principal thermogenic molecule mitochondrial uncoupling protein 1 (UCP1) in the sympathetically stimulated glucose utilization, we investigated the uptake of 2-deoxyglucose (2-DG) into BAT and some other tissues of UCP1-knockout (KO) mice in vivo. In wild-type (WT) mice, administration of norepinephrine (NE) accelerated the disappearance of plasma 2-DG and increased 2-DG uptake into BAT and heart without any rise of plasma insulin level. In UCP1-KO mice, the stimulatory effect of NE on 2-DG uptake into BAT, but not into heart, disappeared completely. Insulin administration increased 2-DG uptake into BAT and also heart similarly in WT and UCP1-KO mice. NE also increased the activity of AMP-activated protein kinase (AMP kinase) in BAT of WT but not UCP1-KO mice. Our results, together with reports that the activation of AMP kinase increases glucose transport in myocytes, suggest that the sympathetically stimulated glucose utilization in BAT is due to the serial activation of UCP1 and AMP kinase.

The Relationship between Summated Tissue Respiration and Metabolic Rate in the Mouse and Dog

The Relationship between Summated Tissue Respiration and Metabolic Rate in the Mouse and Dog