The Beta-adrenergic agonist, Ractopamine, increases skeletal muscle expression of Asparagine Synthetase as part of an integrated stress response gene program

Integrated transcriptomics and proteomics analysis reveals muscle metabolism effects of dietary Ulva lactuca and ulvan lyase supplementation in weaned piglets

Seaweeds, including the green Ulva lactuca, can potentially reduce competition between feed, food, and fuel. They can also contribute to the improved development of weaned piglets. However, their indigestible polysaccharides of the cell wall pose a challenge. This can be addressed through carbohydrase supplementation, such as the recombinant ulvan lyase. The objective of our study was to assess the muscle metabolism of weaned piglets fed with 7% U. lactuca and 0.01% ulvan lyase supplementation, using an integrated transcriptomics (RNA-seq) and proteomics (LC-MS) approach. Feeding piglets with seaweed and enzyme supplementation resulted in reduced macronutrient availability, leading to protein degradation through the proteasome (PSMD2), with resulting amino acids being utilized as an energy source (GOT2, IDH3B). Moreover, mineral element accumulation may have contributed to increased oxidative stress, evident from elevated levels of antioxidant proteins like catalase, as a response to maintaining tissue homeostasis. The upregulation of the gene AQP7, associated with the osmotic stress response, further supports these findings. Consequently, an increase in chaperone activity, including HSP90, was required to repair damaged proteins. Our results suggest that enzymatic supplementation may exacerbate the effects observed from feeding U. lactuca alone, potentially due to side effects of cell wall degradation during digestion.

A cross-talk between sestrins, chronic inflammation and cellular senescence governs the development of age-associated sarcopenia and obesity

The rapid increase in both the lifespan and proportion of older adults is accompanied by the unprecedented rise in age-associated chronic diseases, including sarcopenia and obesity. Aging is also manifested by increased susceptibility to multiple endogenous and exogenous stresses enabling such chronic conditions to develop. Among the main physiological regulators of cellular adaption to various stress stimuli, such as DNA damage, hypoxia, and oxidative stress, are sestrins (Sesns), a family of three evolutionarily conserved proteins, Sesn1, 2, and 3. Age-associated sarcopenia and obesity are characterized by two key processes: (i) accumulation of senescent cells in the skeletal muscle and adipose tissue and (ii) creation of a systemic, chronic, low-grade inflammation (SCLGI). Presumably, failed SCLGI resolution governs the development of these chronic conditions. Noteworthy, Sesns activate senolytics, which are agents that selectively eliminate senescent cells, as well as specialized pro-resolving mediators, which are factors that physiologically provide inflammation resolution. Sesns reveal clear beneficial effects in pre-clinical models of sarcopenia and obesity. Based on these observations, we propose a novel treatment strategy for age-associated sarcopenia and obesity, complementary to the conventional therapeutic modalities: Sesn activation, SCLGI resolution, and senescent cell elimination.

ATF5 is a regulator of exercise-induced mitochondrial quality control in skeletal muscle

OBJECTIVES

The Mitochondrial Unfolded Protein Response (UPR^mt) is a compartment-specific mitochondrial quality control (MQC) mechanism that uses the transcription factor ATF5 to induce the expression of protective enzymes to restore mitochondrial function. Acute exercise is a stressor that has the potential to temporarily disrupt organellar protein homeostasis, however, the roles of ATF5 and the UPR^mt in maintaining basal mitochondrial content, function and exercise-induced MQC mechanisms in skeletal muscle are not known.

METHODS

ATF5 KO and WT mice were examined at rest or after a bout of acute endurance exercise. We measured protein content in whole muscle, nuclear, cytosolic and mitochondrial fractions, in addition to mRNA transcript levels in whole muscle. Using isolated mitochondria, we quantified rates of oxygen consumption and ROS emission to observe the effects of the absence of ATF5 on organelle function.

RESULTS

ATF5 KO mice exhibited a larger and less functional muscle mitochondrial pool, most likely a culmination of enhanced biogenesis via increased PGC-1α expression, and attenuated mitophagy. The absence of ATF5 resulted in a reduction in antioxidant proteins and increases in mitochondrial ROS emission, cytosolic cytochrome c, and the expression of mitochondrial chaperones. KO muscle also displayed enhanced exercise-induced stress kinase signaling, but a blunted mitophagic and UPR^mt gene expression response, complemented by significant increases in the basal mRNA abundance and nuclear localization of ATF4. Instead of promoting its nuclear translocation, acute exercise caused the enrichment of ATF5 in mitochondrial fractions. We also identified PGC-1α as an additional regulator of the basal expression of UPR^mt genes.

CONCLUSION

The transcription factor ATF5 retains a critical role in the maintenance of mitochondrial homeostasis and the appropriate response of muscle to acute exercise for the optimization of mitochondrial quality control.

Sex differences in α-adrenergic receptor function contribute to impaired hypothalamic metaplasticity following chronic intermittent ethanol exposure

BACKGROUND

Individuals with alcohol use disorder (AUD) exhibit maladaptive responses of the hypothalamic-pituitary-adrenal (HPA) axis to stress, which has been linked to high rates of relapse to drinking among abstinent individuals. Corticotropin-releasing factor (CRF) parvocellular neuroendocrine cells (PNCs) within the paraventricular nucleus of the hypothalamus (PVN) are critical to stress-induced HPA axis activation. Here, we investigate sex differences in synaptic transmission and plasticity in PNCs following the application of the stress-associated neurotransmitter norepinephrine (NE) in a rat model of AUD.

METHODS

Adult Sprague-Dawley rats were exposed to 40 days of chronic intermittent ethanol (CIE) vapor and 30 to 108 days of protracted withdrawal. We measured changes in holding current, evoked synaptic currents, and short-term glutamatergic plasticity (STP) in putative PNCs following the application of NE (10 μM) with and without the selective α1 adrenergic receptor (AR) antagonist prazosin (10 μM) or the α2AR antagonist atipamezole (10 μM). The experiments were performed using whole-cell patch clamp recordings in slices from CIE rats and air-exposed controls.

RESULTS

NE application caused two distinct effects: a depolarizing, inward, postsynaptic current and a reduction in amplitude of an evoked glutamatergic excitatory postsynaptic current (eEPSC). Both effects were sex- and CIE-specific. Prazosin blocked the postsynaptic inward current, while atipamezole blocked the NE-mediated suppression of eEPSCs. Additionally, STP formation was facilitated following NE application only in stress-naïve males and this response was lost in stressed animals exposed to a 30-min restraint stress following CIE exposure. Furthermore, NE + prazosin restored STP formation in stressed CIE males.

CONCLUSIONS

NE exerts excitatory and inhibitory effects on CRF PVN PNCs, and both effects are influenced by sex and CIE. Behavioral and hormonal responses to stress are influenced by STP formation within the PVN, which is lost following CIE and restored with the preapplication of prazosin. The selective blockade of α1AR may, therefore, ameliorate CIE-induced deficits in HPA responses to stress in a sex-specific manner.

Colorectal Cancer Chemotherapy Drug Bevacizumab May Induce Muscle Atrophy Through CDKN1A and TIMP4

The muscle in the organism has the function of regulating metabolism. Long-term muscle inactivity or the occurrence of chronic inflammatory diseases are easy to induce muscle atrophy. Bevacizumab is an antiangiogenic drug that prevents the formation of neovascularization by inhibiting the activation of VEGF signaling pathway. It is used in the first-line treatment of many cancers in clinic. Studies have shown that the use of bevacizumab in the treatment of tumors can cause muscle mass loss and may induce muscle atrophy. Based on bioinformatics analysis, this study sought the relationship and influence mechanism between bevacizumab and muscle atrophy. The differences of gene and sample expression between bevacizumab treated group and control group were studied by RNA sequencing. WGCNA is used to find gene modules related to bevacizumab administration and explore biological functions through metascape. Differential analysis was used to analyze the difference of gene expression between the administration group and the control group in different muscle tissues. The key genes timp4 and CDKN1A were obtained through Venn diagram, and then GSEA was used to explore their biological functions in RNA sequencing data and geo chip data. This study studied the role of bevacizumab in muscle through the above methods, preliminarily determined that timp4 and CDKN1A may be related to muscle atrophy, and further explored their functional mechanism in bevacizumab myotoxicity.

Consumption of meat containing ractopamine might enhance tumor growth through induction of asparagine synthetase

There is currently no evidence of the carcinogenic effect of the β-adrenergic agonist ractopamine added in finishing swine and cattle feed for promoting leanness. Nonetheless, it has the capability of stimulating expression of asparagine synthetase (ASNS) through activating transcription factor 5, and many other genes involved in the stress reaction in the skeletal muscle of pigs according to published scientific articles. Because overexpression of ASNS has been detected as a key player in amino acid response and unfolded protein response during the development of not a few malignant diseases, especially those with KRAS mutations, and found to be closely related to tumor proliferation, invasion and metastasis, it seems reasonable to hypothesize that intake of ractopamine residue in meat might bring negative effects to cancer patients.

Effect of AAV-mediated overexpression of ATF5 and downstream targets of an integrated stress response in murine skeletal muscle

We previously reported that growth promoter-induced skeletal muscle hypertrophy co-ordinately upregulated expression of genes associated with an integrated stress response (ISR), as well as potential ISR regulators. We therefore used Adeno-Associated Virus (AAV)-mediated overexpression of these genes, individually or in combination, in mouse skeletal muscle to test whether they induced muscle hypertrophy. AAV of each target gene was injected into mouse Tibialis anterior (TA) and effects on skeletal muscle growth determined 28 days later. Individually, AAV constructs for Arginase-2 (Arg2) and Activating transcription factor-5 (Atf5) reduced hindlimb muscle weights and upregulated expression of genes associated with an ISR. AAV-Atf5 also decreased Myosin heavy chain (MyHC)-IIB mRNA, but increased MyHC-IIA and isocitrate dehydrogenase-2 (Idh2) mRNA, suggesting ATF5 is a novel transcriptional regulator of Idh2. AAV-Atf5 reduced the size of both TA oxidative and glycolytic fibres, without affecting fibre-type proportions, whereas Atf5 combined with Cebpg (CCAAT enhancer binding protein-gamma) only reduced the size of glycolytic fibres and tended to increase the proportion of oxidative fibres. It is likely that persistent Atf5 overexpression maintains activation of the ISR, thereby reducing protein synthesis and/or increasing protein degradation and possibly apoptosis, resulting in inhibition of muscle growth, with overexpression of Arg2 having a similar effect.

Evaluation of Effects of Ractopamine on Cardiovascular, Respiratory, and Locomotory Physiology in Animal Model Zebrafish Larvae

Ractopamine (RAC) is a beta-adrenoceptor agonist that is used to promote lean and increased food conversion efficiency in livestock. This compound has been considered to be causing behavioral and physiological alterations in livestock like pig. Few studies have addressed the potential non-target effect of RAC in aquatic animals. In this study, we aimed to explore the potential physiological response after acute RAC exposure in zebrafish by evaluating multiple endpoints like locomotor activity, oxygen consumption, and cardiovascular performance. Zebrafish larvae were subjected to waterborne RAC exposure at 0.1, 1, 2, 4, or 8 ppm for 24 h, and the corresponding cardiovascular, respiratory, and locomotion activities were monitored and quantified. In addition, we also performed in silico molecular docking for RAC with 10 zebrafish endogenous β-adrenergic receptors to elucidate the potential acting mechanism of RAC. Results show RAC administration can significantly boost locomotor activity, cardiac performance, oxygen consumption, and blood flow rate, but without affecting the cardiac rhythm regularity in zebrafish embryos. Based on structure-based flexible molecular docking, RAC display similar binding affinity to all ten subtypes of endogenous β-adrenergic receptors, from adra1aa to adra2db, which are equivalent to the human one. This result suggests RAC might act as high potency and broad spectrum β-adrenergic receptors agonist on boosting the locomotor activity, cardiac performance, and oxygen consumption in zebrafish. To validate our results, we co-incubated a well-known β-blocker of propranolol (PROP) with RAC. PROP exposure tends to minimize the locomotor hyperactivity, high oxygen consumption, and cardiac rate in zebrafish larvae. In silico structure-based molecular simulation and binding affinity tests show PROP has an overall lower binding affinity than RAC. Taken together, our studies provide solid in vivo evidence to support that RAC plays crucial roles on modulating cardiovascular, respiratory, and locomotory physiology in zebrafish for the first time. In addition, the versatile functions of RAC as β-agonist possibly mediated via receptor competition with PROP as β-antagonist.

The phosphoenolpyruvate carboxykinase (PEPCK) inhibitor, 3-mercaptopicolinic acid (3-MPA), induces myogenic differentiation in C2C12 cells

Phosphoenolpyruvate carboxykinase (PEPCK) is a gluconeogenic enzyme with a cytosolic (Pck1/PEPCK-C) and mitochondrial (Pck2/PEPCK-M) isoform. Here we investigate the effect of 3-mercaptopicolinic acid (3-MPA), a PEPCK inhibitor, on C2C12 muscle cells. We report that Pck2 mRNA is 50–5000-fold higher than Pck1 during C2C12 myogenesis, indicating Pck2 is the predominant PEPCK isoform. C2C12 cell proliferation was inhibited in a dose-dependent manner following 48 h 3-MPA treatment (0.01–1 mM). C2C12 myogenic differentiation was significantly induced following 3-MPA treatment (0.25, 0.5, 1 mM) from day 0 of differentiation, demonstrated by increased creatine kinase activity, fusion index and myotube diameter; likewise, the myosin heavy chain (MyHC)-IIB isoform (encoded by Myh4) is an indicator of hypertrophy, and both porcine MYH4-promoter activity and endogenous Myh4 mRNA were also significantly induced. High doses (0.5 and/or 1 mM) of 3-MPA reduced mRNA expression of Pck2 and genes associated with serine biosynthesis (Phosphoglycerate dehydrogenase, Phgdh; phosphoserine aminotransferase-1, Psat1) following treatment from days 0 and 4. To conclude, as Pck2/PEPCK-M is the predominant isoform in C2C12 cells, we postulate that 3-MPA promoted myogenic differentiation through the inhibition of PEPCK-M. However, we were unable to confirm that 3-MPA inhibited PEPCK-M enzyme activity as 3-MPA interfered with the PEPCK enzyme assay, particularly at 0.5 and 1 mM.

Changes in expression of serine biosynthesis and integrated stress response genes during myogenic differentiation of C2C12 cells

Skeletal muscle is a highly metabolic and dynamic tissue that is formed through the complex and well-organised process of myogenesis. Although there is a good understanding about the role of the Muscle Regulatory Factors during myogenesis, little is known about the potential interplay of other metabolic proteins. The aim of this study was to determine the endogenous mRNA expression profile for a novel group of genes, recently associated with β2-adrenergic agonist (BA) induced muscle hypertrophy in pigs [1], during myogenic differentiation in C2C12 cells and their response to dibutyryl cyclic-AMP (dbcAMP). These genes included mitochondrial phosphoenolpyruvate carboxykinase (PCK2/PEPCK-M), genes involved in serine biosynthesis (Phosphoglycerate dehydrogenase, PHGDH; Phosphoserine aminotransferase-1, PSAT1; Phosphoserine phosphatase, PSPH) and those involved in an integrated stress response (Asparagine synthetase, ASNS; Sestrin-2, SESN2; and Activating transcription factor-5, ATF5). A coordinated peak in endogenous PCK2, PHGDH, PSAT1, PSPH, ASNS, ATF5 and SESN2 mRNA expression was observed at day 2 of differentiation (P < 0.001) in C2C12 cells, which coincided with the peak in myogenin mRNA. Myotube hypertrophy was induced with dbcAMP (1 mM) treatment from day 0, thereby mimicking the in vivo BA response. Although dbcAMP treatment from day 0 induced larger myotubes and increased both myosin heavy chain-IIB (MyHC-IIB) and pyruvate carboxylase (PC) mRNA, the expression of PCK2, PHGDH, PSAT1 and ASNS mRNA were all unaffected. Treatment with dbcAMP from day 4 increased MyHC-IIB mRNA, however this was less dramatic compared to the response observed following treatment from day 0, but there was no effect on PC mRNA. There was also no effect of dbcAMP treatment from day 4 on PCK2, PHGDH, PSAT1 and ASNS mRNA. To conclude, the coordinated day 2 peak in endogenous expression of PCK2, PHGDH, PSAT1, PSPH, ASNS, ATF5 and SESN2 mRNA may relate to a shift in biosynthetic demand required to initiate myogenic differentiation. However, dbcAMP had no effect on the expression of these genes in vitro suggesting that the effects observed in BA-treated pigs might be via other signalling pathways from the activation of the β2-adrenergic receptor, but independent of cAMP, or that there are species differences in the response.

Effect of adeno-associated virus (AAV)-mediated overexpression of PEPCK-M (Pck2) on Clenbuterol-induced muscle growth

We previously identified PEPCK-M (encoded by the Pck2 gene) to be highly up-regulated in skeletal muscle of pigs treated with Ractopamine, an anabolic beta-adrenergic receptor agonist. To determine whether PEPCK-M had a causative role in modulating the skeletal muscle growth response to Ractopamine, we used adeno-associated virus 1 (AAV1) to over-express Pck2 (AAV-Pck2) in murine skeletal muscle. A contralateral limb design was employed, such that each mouse served as its own control (injected with a GFP-only expressing AAV1, labelled AAV-GFP). Daily injections of Clenbuterol (1 mg/kg for 21 days) or vehicle control were also carried out to assess the effects of AAV-Pck2 overexpression on the anabolic response to a beta-adrenergic agonist. AAV-Pck2 overexpression in leg muscles of male C57BL6/J mice for 4 weeks (6–10 weeks of age) increased Pck2 mRNA (~100-fold), protein (not quantifiable) and enzyme activity (~3-fold). There was a trend (p = 0.0798) for AAV-Pck2 overexpression to reduce TA muscle weights, but there was no significant effect on muscle fibre diameters or myosin heavy chain isoform (MyHC) mRNA expression. When skeletal muscle growth was induced by daily administration of Clenbuterol (for 21 days), overexpression of AAV-Pck2 had no effect on the growth response, nor did it alter the expression of Phosphoserine Aminotransferase-1 (Psat1) or Asparagine Synthetase (Asns) mRNA or the Clenbuterol-induced decreases in MyHC IIa and IIx mRNA expression (p = 0.0065 and p = 0.0267 respectively). However AAV-Pck2 overexpression reduced TA muscle weights (p = 0.0434), particularly in the Control (vehicle treated) mice (p = 0.059 for AAV x Clenbuterol interaction) and increased the expression of Seryl-tRNA Synthetase (Sars) mRNA (p = 0.0477). Hence, contrary to the original hypothesis, AAV-Pck2 overexpression reduced TA muscle weights and did not mimic or alter the muscle hypertrophic effects of the beta-adrenergic agonist, Clenbuterol.