Beyond day and night: The importance of ultradian rhythms in mouse physiology.

Our circadian world shapes much of metabolic physiology. In mice ∼40% of the light and ∼80% of the dark phase time is characterized by bouts of increased energy expenditure (EE). These ultradian bouts have a higher body temperature (Tb) and thermal conductance and contain virtually all of the physical activity and awake time. Bout status is a better classifier of mouse physiology than photoperiod, with ultradian bouts superimposed on top of the circadian light/dark cycle. We suggest that the primary driver of ultradian bouts is a brain-initiated transition to a higher defended Tb of the active/awake state. Increased energy expenditure from brown adipose tissue, physical activity, and cardiac work combine to raise Tb from the lower defended Tb of the resting/sleeping state. Thus, unlike humans, much of mouse metabolic physiology is episodic with large ultradian increases in EE and Tb that correlate with the active/awake state and are poorly aligned with circadian cycling.

A human obesity-associated MC4R mutation with defective Gq/11α signaling leads to hyperphagia in mice.

Melanocortin 4 receptor (MC4R) mutations are the most common cause of human monogenic obesity and are associated with hyperphagia and increased linear growth. While MC4R is known to activate Gsα/cAMP signaling, a substantial proportion of obesity-associated MC4R mutations do not affect MC4R/Gsα signaling. To further explore the role of specific MC4R signaling pathways in the regulation of energy balance, we examined the signaling properties of one such mutant, MC4R (F51L), as well as the metabolic consequences of MC4RF51L mutation in mice. The MC4RF51L mutation produced a specific defect in MC4R/Gq/11α signaling and led to obesity, hyperphagia, and increased linear growth in mice. The ability of a melanocortin agonist to acutely inhibit food intake when delivered to the paraventricular nucleus (PVN) was lost in MC4RF51L mice, as well as in WT mice in which a specific Gq/11α inhibitor was delivered to the PVN; this provided evidence that a Gsα-independent signaling pathway, namely Gq/11α, significantly contributes to the actions of MC4R on food intake and linear growth. These results suggest that a biased MC4R agonist that primarily activates Gq/11α may be a potential agent to treat obesity with limited untoward cardiovascular and other side effects.

2-Amino-5-arylethynyl-thiophen-3-yl-(phenyl)methanones as A1 Adenosine Receptor Positive Allosteric Modulators.

A1 adenosine receptor (A1AR) agonists have cerebroprotective, cardioprotective, antinociceptive, and other pharmaceutical applications. We explored the structure-activity relationship of 5-arylethynyl aminothiophenes as A1AR positive allosteric modulators (PAMs). The derivatives were compared in binding and functional assays at the human A1AR, indicating that some fluoro-substituted analogues have enhanced PAM activity. We identified substitution of the terminal phenyl ring in 12 (2-F-Ph), 15 (3,4-F2-Ph, MRS7935), and 21 (2-CF3-Ph) as particularly enhancing the PAM activity. 15 was also shown to act as an A1 ago-PAM with EC50 ≈ 2 µM, without activity (30 µM) at other ARs. Molecular modeling indicated that both the 5-arylethynyl and the 4-neopentyl groups are located in a region outside the receptor transmembrane helix bundle that is in contact with the phospholipid bilayer, consistent with the preference for nonpolar substitution of the aryl moiety. Although they are hydrophobic, these PAMs could provide potential drug candidate molecules for engaging protective A1ARs.

Loss of Otopetrin 1 affects thermoregulation during fasting in mice.

OBJECTIVE: Otopetrin 1 (OTOP1) is a proton channel that is highly expressed in brown adipose tissue. We examined the physiology of Otop1-/- mice, which lack functional OTOP1. METHODS: Mice were studied by indirect calorimetry and telemetric ambulatory body temperature monitoring. Mitochondrial function was measured as oxygen consumption and extracellular acidification. RESULTS: Otop1-/- mice had similar body temperatures as control mice at baseline and in response to cold and hot ambient temperatures. However, in response to fasting the Otop1-/- mice exhibited an exaggerated hypothermia and hypometabolism. Similarly, in ex vivo tests of Otop1-/- brown adipose tissue mitochondrial function, there was no change in baseline oxygen consumption, but the oxygen consumption was reduced after maximal uncoupling with FCCP and increased upon stimulation with the ß3-adrenergic agonist CL316243. Mast cells also express Otop1, and Otop1-/- mice had intact, possibly greater hypothermia in response to mast cell activation by the adenosine A3 receptor agonist MRS5698. No increase in insulin resistance was observed in the Otop1-/- mice. CONCLUSIONS: Loss of OTOP1 does not change basal function of brown adipose tissue but affects stimulated responses.

Constitutive and conditional deletion reveals distinct phenotypes driven by developmental versus neurotransmitter actions of the neuropeptide PACAP.

Neuropeptides may exert trophic effects during development, and then neurotransmitter roles in the developed nervous system. One way to associate peptide-deficiency phenotypes with either role is first to assess potential phenotypes in so-called constitutive knockout mice, and then proceed to specify, regionally and temporally, where and when neuropeptide expression is required to prevent these phenotypes. We have previously demonstrated that the well-known constellation of behavioral and metabolic phenotypes associated with constitutive pituitary adenylate cyclase-activating peptide (PACAP) knockout mice are accompanied by transcriptomic alterations of two types: those that distinguish the PACAP-null phenotype from wild-type (WT) in otherwise quiescent mice (cPRGs), and gene induction that occurs in response to acute environmental perturbation in WT mice that do not occur in knockout mice (aPRGs). Comparing constitutive PACAP knockout mice to a variety of temporally and regionally specific PACAP knockouts, we show that the prominent hyperlocomotor phenotype is a consequence of early loss of PACAP expression, is associated with Fos overexpression in hippocampus and basal ganglia, and that a thermoregulatory effect previously shown to be mediated by PACAP-expressing neurons of medial preoptic hypothalamus is independent of PACAP expression in those neurons in adult mice. In contrast, PACAP dependence of weight loss/hypophagia triggered by restraint stress, seen in constitutive PACAP knockout mice, is phenocopied in mice in which PACAP is deleted after neuronal differentiation. Our results imply that PACAP has a prominent role as a trophic factor early in development determining global central nervous system characteristics, and in addition a second, discrete set of functions as a neurotransmitter in the fully developed nervous system that support physiological and psychological responses to stress.

The metabolic cost of physical activity in mice using a physiology-based model of energy expenditure.

OBJECTIVE: Physical activity is a major component of total energy expenditure (TEE) that exhibits extreme variability in mice. Our objective was to construct a general, physiology-based model of TEE to accurately quantify the energy cost of physical activity. METHODS: Spontaneous home cage physical activity, body temperature, TEE, and energy intake were measured with frequent sampling. The energy cost of activity was modeled considering six contributors to TEE (basal metabolic rate, thermic effect of food, body temperature, cold induced thermogenesis, physical activity, and body weight). An ambient temperature of 35 °C was required to remove the contribution from cold induced thermogenesis. Basal metabolic rate was adjusted for body temperature using a Q10 temperature coefficient. RESULTS: We developed a TEE model that robustly explains 70-80% of the variance in TEE at 35 °C while fitting only two parameters, the basal metabolic rate and the mass-specific energy cost per unit of physical activity, which averaged 60 cal/km/g body weight. In Ucp1-/- mice the activity cost was elevated by 60%, indicating inefficiency and increased muscle thermogenesis. The diurnal rhythm in TEE was quantitatively explained by the combined diurnal differences in physical activity, body temperature, and energy intake. Incorporating body temperature into human basal metabolic rate measurements significantly reduced the inter-individual variation. CONCLUSIONS: The physiology-based model of TEE allows quantifying the energy cost of physical activity. While applied here to mice, the model should be generally valid across species. Due to the effect of body temperature, we suggest that basal metabolic rate measurements be corrected to a reference body temperature, including in humans. Having an accurate cost of physical activity allows mechanistic dissection of disorders of energy homeostasis, including obesity.

In vivo phenotypic validation of adenosine receptor-dependent activity of non-adenosine drugs.

Some non-adenosinergic drugs are reported to also act through adenosine receptors (ARs). We used mouse hypothermia, which can be induced by agonism at any of the four ARs, as an in vivo screen for adenosinergic effects. An AR contribution was identified when a drug caused hypothermia in wild type mice that was diminished in mice lacking all four ARs (quadruple knockout, QKO). Alternatively, an adenosinergic effect was identified if a drug potentiated adenosine-induced hypothermia. Four drugs (dipyridamole, nimodipine, cilostazol, cyclosporin A) increased the hypothermia caused by adenosine. Dipyridamole and nimodipine probably achieved this by inhibition of adenosine clearance via ENT1. Two drugs (cannabidiol, canrenoate) did not cause hypothermia in wild type mice. Four other drugs (nifedipine, ranolazine, ketamine, ethanol) caused hypothermia, but the hypothermia was unchanged in QKO mice indicating non-adenosinergic mechanisms. Zinc chloride caused hypothermia and hypoactivity; the hypoactivity was blunted in the QKO mice. Interestingly, the antidepressant amitriptyline caused hypothermia in wild type mice that was amplified in the QKO mice. Thus, we have identified adenosine-related effects for some drugs, while other candidates do not affect adenosine signaling by this in vivo assay. The adenosine-modulating drugs could be considered for repurposing based on predicted effects on AR activation.

A3 adenosine receptor agonists containing dopamine moieties for enhanced interspecies affinity.

Following our study of 4'-truncated (N)-methanocarba-adenosine derivatives that displayed unusually high mouse (m) A3AR affinity, we incorporated dopamine-related N6 substituents in the full agonist 5'-methylamide series. N6-(2-(4-Hydroxy-3-methoxy-phenyl)ethyl) derivative MRS7618 11 displayed Ki (nM) 0.563 at hA3AR (∼20,000-fold selective) and 1.54 at mA3AR. 2-Alkyl ethers maintained A3 affinity, but with less selectivity than 2-alkynes. Parallel functional assays of G protein-dependent and ß-arrestin 2 (ßarr2)-dependent pathways indicate these are full agonists but not biased. Through use of computational modeling, we hypothesized that phenyl OH/OMe groups interact with polar residues, particularly Gln261, on the mA3AR extracellular loops as the basis for the affinity enhancement. Although the pharmacokinetics indicated facile clearance of parent O-methyl catechol nucleosides 21 and 31, prolonged mA3AR activation in vivo was observed in a hypothermia model, suggested potential formation of active metabolites through demethylation. Selected analogues induced mouse hypothermia following i.p. injection, indicative of peripheral A3AR agonism in vivo.

Combining a ß3 adrenergic receptor agonist with alpha-lipoic acid reduces inflammation in male mice with diet-induced obesity.

OBJECTIVES: Beta-3 adrenergic receptors (ß3-AR) stimulate lipolysis and thermogenesis in white and brown adipose tissue (WAT and BAT). Obesity increases oxidative stress and inflammation that attenuate AT ß3-AR signaling. The objective of this study was to test the hypothesis that the combination of the ß3-AR agonist CL-316,243 (CL) and the antioxidant alpha-lipoic acid (ALA) would lower inflammation in diet-induced obesity (DIO) and improve ß3-AR function. METHODS: A total of 40 DIO mice were separated into four groups: Control (per os and intraperitoneal [IP] vehicle); CL alone (0.01 mg/kg IP daily); ALA alone (250 mg/kg in drinking water); or ALA+CL combination, all for 5 weeks. RESULTS: Food intake was similar in all groups; however, mice receiving ALA+CL showed improved body composition and inflammation as well as lower body weight (+1.7 g Control vs. -2.5 g ALA+CL [-7%]; p < 0.01) and percentage of body fat (-9%, p < 0.001). Systemic and epididymal WAT inflammation was lower with ALA+CL than all other groups, with enhanced recruitment of epididymal WAT anti-inflammatory CD206+ M2 macrophages. ß3-AR signaling in WAT was enhanced in the combination-treatment group, with higher mRNA and protein levels of thermogenic uncoupling protein 1 and AT lipases. CONCLUSIONS: Chronic treatment with ALA and a ß3-AR agonist reduces DIO-induced inflammation. AT immune modulation could be a therapeutic target in patients with obesity.

The effects of housing density on mouse thermal physiology depend on sex and ambient temperature.

OBJECTIVE: To improve understanding of mouse energy homeostasis and its applicability to humans, we quantitated the effects of housing density on mouse thermal physiology in both sexes. METHODS: Littermate wild type and Brs3-null mice were single- or group- (three per cage) housed and studied by indirect calorimetry with continuous measurement of core body temperature, energy expenditure, physical activity, and food intake. RESULTS: At 23 °C, below thermoneutrality, single-housed males had a lower body temperature and unchanged metabolic rate compared to group-housed controls. In contrast, single-housed females maintained a similar body temperature to group-housed controls by increasing their metabolic rate. With decreasing ambient temperature below 27 °C, only group-housed mice decreased their heat conductance, likely due to huddling, thus interfering with the energy expenditure vs ambient temperature relationship described by Scholander. In a hot environment (35 °C), the single-housed mice were less heat stressed. Upon fasting, single-housed mice had larger reductions in body temperature, with male mice having more torpor episodes of similar duration and female mice having a similar number of torpor episodes that lasted longer. Qualitatively, the effects of housing density on thermal physiology of Brs3-null mice generally mimicked the effects in controls. CONCLUSIONS: Single housing is more sensitive than group housing for detecting thermal physiology phenotypes. Single housing increases heat loss and amplifies the effects of fasting or a cold environment. Male and female mice utilize different thermoregulatory strategies to respond to single housing.

CaMKII oxidation is a critical performance/disease trade-off acquired at the dawn of vertebrate evolution.

Antagonistic pleiotropy is a foundational theory that predicts aging-related diseases are the result of evolved genetic traits conferring advantages early in life. Here we examine CaMKII, a pluripotent signaling molecule that contributes to common aging-related diseases, and find that its activation by reactive oxygen species (ROS) was acquired more than half-a-billion years ago along the vertebrate stem lineage. Functional experiments using genetically engineered mice and flies reveal ancestral vertebrates were poised to benefit from the union of ROS and CaMKII, which conferred physiological advantage by allowing ROS to increase intracellular Ca2+ and activate transcriptional programs important for exercise and immunity. Enhanced sensitivity to the adverse effects of ROS in diseases and aging is thus a trade-off for positive traits that facilitated the early and continued evolutionary success of vertebrates.

The contribution of the mouse tail to thermoregulation is modest.

Understanding mouse thermal physiology informs the usefulness of mice as models of human disease. It is widely assumed that the mouse tail contributes greatly to heat loss (as it does in rat), but this has not been quantitated. We studied C57BL/6J mice after tail amputation. Tailless mice housed at 22°C did not differ from littermate controls in body weight, lean or fat content, or energy expenditure. With acute changes in ambient temperature from 19 to 39°C, tailless and control mice demonstrated similar body temperatures (Tb), metabolic rates, and heat conductances and no difference in thermoneutral point. Treatment with prazosin, an α1-adrenergic antagonist and vasodilator, increased tail temperature in control mice by up to 4.8 ± 0.8°C. Comparing prazosin treatment in tailless and control mice suggested that the tail's contribution to total heat loss was a nonsignificant 3.4%. Major heat stress produced by treatment at 30°C with CL316243, a ß3-adrenergic agonist, increased metabolic rate and Tb and, at a matched increase in metabolic rate, the tailless mice showed a 0.72 ± 0.14°C greater Tb increase and 7.6% lower whole body heat conductance. Thus, the mouse tail is a useful biomarker of vasodilation and thermoregulation, but in our experiments contributes only 5-8% of whole body heat dissipation, less than the 17% reported for rat. Heat dissipation through the tail is important under extreme scenarios such as pharmacological activation of brown adipose tissue; however, non-tail contributions to heat loss may have been underestimated in the mouse.

BRS3 in both MC4R- and SIM1-expressing neurons regulates energy homeostasis in mice.

OBJECTIVE: Bombesin-like receptor 3 (BRS3) is an orphan receptor and Brs3 knockout mice develop obesity with increased food intake and reduced resting metabolic rate and body temperature. The neuronal populations contributing to these effects were examined. METHODS: We studied energy metabolism in mice with Cre-mediated recombination causing 1) loss of BRS3 selectively in SIM1- or MC4R-expressing neurons or 2) selective re-expression of BRS3 from a null background in these neurons. RESULTS: The deletion of BRS3 in MC4R neurons increased body weight/adiposity, metabolic efficiency, and food intake, and reduced insulin sensitivity. BRS3 re-expression in these neurons caused partial or no reversal of these traits. However, these observations were confounded by an obesity phenotype caused by the Mc4r-Cre allele, independent of its recombinase activity. The deletion of BRS3 in SIM1 neurons increased body weight/adiposity and food intake, but not to the levels of the global null. The re-expression of BRS3 in SIM1 neurons reduced body weight/adiposity and food intake, but not to wild type levels. The deletion of BRS3 in either MC4R- or SIM1-expressing neurons affected body temperature, with re-expression in either population reversing the null phenotype. MK-5046, a BRS3 agonist, increases light phase body temperature in wild type, but not Brs3 null, mice and BRS3 re-expression in either population restored response to MK-5046. CONCLUSIONS: BRS3 in both MC4R- and SIM1-expressing neurons contributes to regulation of body weight/adiposity, insulin sensitivity, food intake, and body temperature.

Mouse Thermoregulation: Introducing the Concept of the Thermoneutral Point.

Human and mouse thermal physiology differ due to dissimilar body sizes. Unexpectedly, in mice we found no ambient temperature zone where both metabolic rate and body temperature were constant. Body temperature began increasing once cold-induced thermogenesis was no longer required. This result reproduced in male, female, C57BL/6J, 129, chow-fed, diet-induced obese, and ob/ob mice as well as Trpv1-/-;Trpm8-/-;Trpa1-/- mice lacking thermal sensory channels. During the resting-light phase, the energy expenditure minimum spanned ∼4°C of ambient temperature, whereas in the active-dark phase it approximated a point. We propose the concept of a thermoneutral point (TNP), a discrete ambient temperature below which energy expenditure increases and above which body temperature increases. Humans do not have a TNP. As studied, the mouse TNP is ∼29°C in light phase and ∼33°C in dark phase. These observations inform how thermoneutrality is defined and how mice are used to model human energy physiology and drug development.

Physiology and effects of nucleosides in mice lacking all four adenosine receptors.

Adenosine is a constituent of many molecules of life; increased free extracellular adenosine indicates cell damage or metabolic stress. The importance of adenosine signaling in basal physiology, as opposed to adaptive responses to danger/damage situations, is unclear. We generated mice lacking all four adenosine receptors (ARs), Adora1-/-;Adora2a-/-;Adora2b-/-;Adora3-/- (quad knockout [QKO]), to enable investigation of the AR dependence of physiologic processes, focusing on body temperature. The QKO mice demonstrate that ARs are not required for growth, metabolism, breeding, and body temperature regulation (diurnal variation, response to stress, and torpor). However, the mice showed decreased survival starting at about 15 weeks of age. While adenosine agonists cause profound hypothermia via each AR, adenosine did not cause hypothermia (or bradycardia or hypotension) in QKO mice, indicating that AR-independent signals do not contribute to adenosine-induced hypothermia. The hypothermia elicited by adenosine kinase inhibition (with A134974), inosine, or uridine also required ARs, as each was abolished in the QKO mice. The proposed mechanism for uridine-induced hypothermia is inhibition of adenosine transport by uridine, increasing local extracellular adenosine levels. In contrast, adenosine 5'-monophosphate (AMP)-induced hypothermia was attenuated in QKO mice, demonstrating roles for both AR-dependent and AR-independent mechanisms in this process. The physiology of the QKO mice appears to be the sum of the individual knockout mice, without clear evidence for synergy, indicating that the actions of the four ARs are generally complementary. The phenotype of the QKO mice suggests that, while extracellular adenosine is a signal of stress, damage, and/or danger, it is less important for baseline regulation of body temperature.

Design and in Vivo Characterization of A1 Adenosine Receptor Agonists in the Native Ribose and Conformationally Constrained (N)-Methanocarba Series.

(N)-Methanocarba ([3.1.0]bicyclohexyl) adenosines and corresponding ribosides were synthesized to identify novel A1 adenosine receptor (A1AR) agonists for CNS or peripheral applications. Human and mouse AR binding was determined to assess the constrained ring system's A1AR compatibility. N6-Dicyclobutylmethyl ribose agonist (9, MRS7469, >2000-fold selective for A1AR) and known truncated N6-dicyclopropylmethyl methanocarba 7 (MRS5474) were drug-like. The pure diastereoisomer of known riboside 4 displayed high hA1AR selectivity. Methanocarba modification reduced A1AR selectivity of N6-dicyclopropylmethyl and endo-norbornyladenosines but increased ribavirin selectivity. Most analogues tested (ip) were inactive or weak in inducing mouse hypothermia, despite mA1AR full agonism and variable mA3AR efficacy, but strong hypothermia by 9 depended on A1AR, which reflects CNS activity (determined using A1AR or A3AR null mice). Conserved hA1AR interactions were preserved in modeling of 9 and methanocarba equivalent 24 (∼400-fold A1AR-selective). Thus, we identified, and characterized in vivo, ribose and methanocarba nucleosides, including with A1AR-enhancing N6-dicyclobutylmethyl-adenine and 1,2,4-triazole-3-carboxamide (40, MRS7451) nucleobases.

Melanotan II causes hypothermia in mice by activation of mast cells and stimulation of histamine 1 receptors.

Intraperitoneal administration of the melanocortin agonist melanotan II (MTII) to mice causes a profound, transient hypometabolism/hypothermia. It is preserved in mice lacking any one of melanocortin receptors 1, 3, 4, or 5, suggesting a mechanism independent of the canonical melanocortin receptors. Here we show that MTII-induced hypothermia was abolished in KitW-sh/W-sh mice, which lack mast cells, demonstrating that mast cells are required. MRGPRB2 is a receptor that detects many cationic molecules and activates mast cells in an antigen-independent manner. In vitro, MTII stimulated mast cells by both MRGPRB2-dependent and -independent mechanisms, and MTII-induced hypothermia was intact in MRGPRB2-null mice. Confirming that MTII activated mast cells, MTII treatment increased plasma histamine levels in both wild-type and MRGPRB2-null, but not in KitW-sh/W-sh, mice. The released histamine produced hypothermia via histamine H1 receptors because either a selective antagonist, pyrilamine, or ablation of H1 receptors greatly diminished the hypothermia. Other drugs, including compound 48/80, a commonly used mast cell activator, also produced hypothermia by both mast cell-dependent and -independent mechanisms. These results suggest that mast cell activation should be considered when investigating the mechanism of drug-induced hypothermia in mice.

Dystrophin-deficient dogs with reduced myostatin have unequal muscle growth and greater joint contractures.

BACKGROUND: Myostatin (Mstn) is a negative regulator of muscle growth whose inhibition promotes muscle growth and regeneration. Dystrophin-deficient mdx mice in which myostatin is knocked out or inhibited postnatally have a less severe phenotype with greater total mass and strength and less fibrosis and fatty replacement of muscles than mdx mice with wild-type myostatin expression. Dogs with golden retriever muscular dystrophy (GRMD) have previously been noted to have increased muscle mass and reduced fibrosis after systemic postnatal myostatin inhibition. Based partly on these results, myostatin inhibitors are in development for use in human muscular dystrophies. However, persisting concerns regarding the effects of long-term and profound myostatin inhibition will not be easily or imminently answered in clinical trials. METHODS: To address these concerns, we developed a canine (GRippet) model by crossbreeding dystrophin-deficient GRMD dogs with Mstn-heterozygous (Mstn (+/-)) whippets. A total of four GRippets (dystrophic and Mstn (+/-)), three GRMD (dystrophic and Mstn wild-type) dogs, and three non-dystrophic controls from two litters were evaluated. RESULTS: Myostatin messenger ribonucleic acid (mRNA) and protein levels were downregulated in both GRMD and GRippet dogs. GRippets had more severe postural changes and larger (more restricted) maximal joint flexion angles, apparently due to further exaggeration of disproportionate effects on muscle size. Flexors such as the cranial sartorius were more hypertrophied on magnetic resonance imaging (MRI) in the GRippets, while extensors, including the quadriceps femoris, underwent greater atrophy. Myostatin protein levels negatively correlated with relative cranial sartorius muscle cross-sectional area on MRI, supporting a role in disproportionate muscle size. Activin receptor type IIB (ActRIIB) expression was higher in dystrophic versus control dogs, consistent with physiologic feedback between myostatin and ActRIIB. However, there was no differential expression between GRMD and GRippet dogs. Satellite cell exhaustion was not observed in GRippets up to 3 years of age. CONCLUSIONS: Partial myostatin loss may exaggerate selective muscle hypertrophy or atrophy/hypoplasia in GRMD dogs and worsen contractures. While muscle imbalance is not a feature of myostatin inhibition in mdx mice, findings in a larger animal model could translate to human experience with myostatin inhibitors.

Genetic disruption of Smad7 impairs skeletal muscle growth and regeneration.

KEY POINTS: Smad7 is an intracellular antagonist of transforming growth factor-ß signalling pathways and modulates muscle growth in vivo. Loss of Smad7 results in decreased muscle mass, reduced force generation, fibre type switching from glycolytic towards oxidative type and delayed recovery from injury. Upregulated Smad2/3 signalling in Smad7(-/-) muscle results in reduced myoblast proliferation and differentiation. Smad7 is an important regulator of muscle growth and may be a potential intracellular therapeutic target for muscle disorders. ABSTRACT: The transforming growth factor-ß (TGF-ß) family of growth factors plays an essential role in mediating cellular growth and differentiation. Myostatin is a muscle-specific member of the TGF-ß superfamily and a negative regulator of muscle growth. Myostatin inhibitors are currently being pursued as therapeutic options for muscle disorders. Smad7 inhibits intracellular myostatin signalling via Smad2/3, and thus presents a means of regulating myostatin and potentiating muscle growth. We investigated the functional loss of Smad7 on muscle in vivo by examining muscle growth and differentiation in mice deficient in Smad7 (Smad7(-/-) ). Smad7(-/-) mice showed reduced muscle mass, hypotrophy and hypoplasia of muscle fibres, as well as an increase in oxidative fibre types. Examination of muscle strength showed reduced force generation in vivo and ex vivo compared to wild-type controls. Analysis of muscle regeneration showed a delay in recovery, probably as a result of decreased activation, proliferation and differentiation of satellite cells, as confirmed in vitro. Additionally, myostatin expression was upregulated in Smad7(-/-) muscle. Our findings suggest that increased Smad2/3 signalling in the absence of Smad7 inhibition impedes muscle growth and regeneration. Taken together, our experiments demonstrate that Smad7 is an important mediator of muscle growth in vivo. Our studies enhance our understanding of in vivo TGF-ß pathway modulation and suggest that Smad7 may be an important therapeutic target for muscle disorders.

A cytoplasmic helix is required for pentamer formation of the Escherichia coli MscL mechanosensitive channel.

Many membrane proteins such as ion channels are oligomers, but the determinants of the degree of oligomerization are not fully understood. Mechanosensitive channel with large conductance (MscL), which is ubiquitous in bacteria, is a homopentamer with two transmembrane helices and a cytoplasmic helix in each subunit. The carboxyl-terminal cytoplasmic helices assemble into a pentameric bundle that resembles cartilage oligomeric matrix protein. To address the role of cytoplasmic helices in the pentamer formation of Escherichia coli MscL, we generated MscL constructs with various deletions at the carboxyl terminus and translated them in a cell-free system. Deletions of Leu-129 and the downstream sequence resulted in formation of various oligomers without preference to pentamers, suggesting that nearly the whole cytoplasmic helix is required for MscL pentamer formation.