Ghrelin knockout mice display defective skeletal muscle regeneration and impaired satellite cell self-renewal

Growth hormone secretagogues modulate inflammation and fibrosis in mdx mouse model of Duchenne muscular dystrophy

Introduction

Growth hormone secretagogues (GHSs) exert multiple actions, being able to activate GHS-receptor 1a, control inflammation and metabolism, to enhance GH/insulin-like growth factor-1 (IGF-1)-mediated myogenesis, and to inhibit angiotensin-converting enzyme. These mechanisms are of interest for potentially targeting multiple steps of pathogenic cascade in Duchenne muscular dystrophy (DMD).

Methods

Here, we aimed to provide preclinical evidence for potential benefits of GHSs in DMD, via a multidisciplinary in vivo and ex vivo comparison in mdx mice, of two ad hoc synthesized compounds (EP80317 and JMV2894), with a wide but different profile. 4-week-old mdx mice were treated for 8 weeks with EP80317 or JMV2894 (320 µg/kg/d, s.c.).

Results

In vivo, both GHSs increased mice forelimb force (recovery score, RS towards WT: 20% for EP80317 and 32% for JMV2894 at week 8). In parallel, GHSs also reduced diaphragm (DIA) and gastrocnemius (GC) ultrasound echodensity, a fibrosis-related parameter (RS: ranging between 26% and 75%). Ex vivo, both drugs ameliorated DIA isometric force and calcium-related indices (e.g., RS: 40% for tetanic force). Histological analysis highlighted a relevant reduction of fibrosis in GC and DIA muscles of treated mice, paralleled by a decrease in gene expression of TGF-β1 and Col1a1. Also, decreased levels of pro-inflammatory genes (IL-6, CD68), accompanied by an increment in Sirt-1, PGC-1α and MEF2c expression, were observed in response to treatments, suggesting an overall improvement of myofiber metabolism. No detectable transcript levels of GHS receptor-1a, nor an increase of circulating IGF-1 were found, suggesting the presence of a novel receptor-independent mechanism in skeletal muscle. Preliminary docking studies revealed a potential binding capability of JMV2894 on metalloproteases involved in extracellular matrix remodeling and cytokine production, such as ADAMTS-5 and MMP-9, overactivated in DMD.

Discussion

Our results support the interest of GHSs as modulators of pathology progression in mdx mice, disclosing a direct anti-fibrotic action that may prove beneficial to contrast pathological remodeling.

Association between fat-free mass loss, changes in appetite and weight regain in individuals with obesity

BACKGROUND

The role of fat-free mass loss (FFML) in modulating weight regain, in individuals with obesity, as well as the potential mechanisms involved, remain inconsistent.

AIMS

To determine if % FFML following weight loss (WL) is a predictor of weight regain, and to investigate the association between %FFML and changes in appetite markers.

METHODS

Seventy individuals with obesity (BMI: 36±4kg/m²; age: 44±9 years; 29 males) underwent 8 weeks of a very low-energy diet (550-660 kcal/day), followed by 4 weeks of gradual refeeding and weight stabilization, and a 9-month maintenance program (eucaloric diet). Body weight and body composition (fat mass (FM) and FFM) (primary outcomes), as well as ß-hydroxybutyrate (ßHB) plasma concentration (a marker of ketosis) in fasting and appetite-related hormones (ghrelin, glucagon-like peptide 1, peptide YY, and cholecystokinin) and subjective appetite feelings, in fasting and every 30 minutes after a fixed breakfast for 2.5h (secondary outcomes), were measured at baseline, week 9 and 1 year (and week 13 in 35 subjects (25 males)). The association between FFML, weight regain and changes in appetite was assessed by linear regression.

RESULTS

WL at week 9 was 17.5±4.3kg and %FFML 20.4±10.6%. Weight regain at 1 year was 1.7±8.2kg (8.8±45.0%). After adjusting for WL and FM at baseline, %FFML at week 9 was not a significant predictor of weight regain. Similar results were seen at week 13. The greater the %FFML at week 9, but not 13, the smaller the reduction, or greater the increase in basal ghrelin concentration (ß:-3.2; 95% CI: -5.0, -1.1; P=0.003), even after adjusting for WL and ß-hydroxybutyrate.

CONCLUSION

%FFML was not a significant predictor of weight regain at 1-year in individuals with obesity. However, a greater %FFML was accompanied by a greater increase in ghrelin secretion under ketogenic conditions, suggesting a link between FFM and appetite regulation.

CLINICAL TRIAL REGISTRATION

ClinicalTrials.gov identifier NCT01834859.

Diverse and Complementary Effects of Ghrelin and Obestatin

Ghrelin and obestatin are two “sibling proteins” encoded by the same preproghrelin gene but possess an array of diverse and complex functions. While there are ample literature documenting ghrelin’s functions, the roles of obestatin are less clear and controversial. Ghrelin and obestatin have been perceived to be antagonistic initially; however, recent studies challenge this dogma. While they have opposing effects in some systems, they function synergistically in other systems, with many functions remaining debatable. In this review, we discuss their functional relationship under three “C” categories, namely complex, complementary, and contradictory. Their functions in food intake, weight regulation, hydration, gastrointestinal motility, inflammation, and insulin secretion are complex. Their functions in pancreatic beta cells, cardiovascular, muscle, neuroprotection, cancer, and digestive system are complementary. Their functions in white adipose tissue, thermogenesis, and sleep regulation are contradictory. Overall, this review accumulates the multifaceted functions of ghrelin and obestatin under both physiological and pathological conditions, with the intent of contributing to a better understanding of these two important gut hormones.

An insight into the multifunctional role of ghrelin and structure activity relationship studies of ghrelin receptor ligands with clinical trials

Ghrelin is a multifunctional gastrointestinal acylated peptide, primarily synthesized in the stomach and regulates the secretion of growth hormone and energy homeostasis. It plays a central role in modulating the diverse biological, physiological and pathological functions in vertebrates. The synthesis of ghrelin receptor ligands after the finding of growth hormone secretagogue developed from Met-enkephalin led to reveal the endogenous ligand ghrelin and the receptors. Subsequently, many peptides, small molecules and peptidomimetics focusing on the ghrelin receptor, GHS-R1a, were derived. In this review, the key features of ghrelin's structure, forms, its bio-physiological functions, pathological roles and therapeutic potential have been highlighted. A few peptidomimetics and pseudo peptide synthetic perspectives have also been discussed to make ghrelin receptor ligands, clinical trials and their success.

Enteral nutrition alleviated lipopolysaccharides-induced hypercatabolism through ghrelin/GHS-R1α-POMC

Sustained hypercatabolism induced by sepsis contributed to serious complications and mortality in the intensive care unit. Enteral nutrition (EN) was required to maintain the energy balance during sepsis. Ghrelin, which was stimulated secretion by EN, had been shown to regulate energy homeostasis. Therefore, we tested whether EN alleviated hypercatabolism through ghrelin/GH secretagogue receptor 1α (GHS-R1α)-proopiomelanocortin (POMC) in endotoxemic rats. Rats in the Sham and lipopolysaccharide (LPS) groups were free access to rodent chow diet and water. Rats in the EN, EN + Lys and EN + 3-MA groups were intracerebroventricularly injected with saline, D-Lys3-GHRP-6 or 3-MA and then received EN for three days. Hypercatabolism was measured by the change of body weight, insulin resistance, leptin, corticosterone, muscle protein synthesis and atrophy. Serum and hypothalamic total ghrelin, acylated ghrelin, GHS-R1α and AMP-activated protein kinase (AMPK)-autophagy-POMC pathway were also detected. The results showed that EN increased serum and hypothalamic total ghrelin, acylated ghrelin and GHS-R1α, effectively activated the hypothalamic AMPK-autophagy-POMC pathway and alleviated hypercatabolism in endotoxemic rats. The improving effects of EN on hypercatabolism and hypothalamic AMPK-autophagy-POMC pathway were abolished with the central administration of D-Lys3-GHRP-6 to inhibited hypothalamic GHS-R1α. And with the central administration of 3-MA to inhibited hypothalamic autophagy, the improving effect of EN on hypercatabolism was also abolished in endotoxemic rats. In conclusions, EN could significantly alleviate hypercatabolism through ghrelin/GHS-R1α-POMC in endotoxemic rats.

Effect of unacylated ghrelin on peripheral nerve regeneration

Ghrelin is a circulating peptide hormone released by enteroendocrine cells of the gastrointestinal tract as two forms, acylated and unacylated. Acylated ghrelin (AG) binds to the growth hormone secretagogue receptor 1a (GHSR1a), thus stimulating food intake, growth hormone release, and gastrointestinal motility. Conversely, unacylated GHR (UnAG), through binding to a yet unidentified receptor, protects the skeletal muscle from atrophy, stimulates muscle regeneration, and protects cardiomyocytes from ischemic damage. Recently, interest about ghrelin has raised also among neuroscientists because of its effect on the nervous system, especially the stimulation of neurogenesis in spinal cord, brain stem, and hippocampus. However, few information is still available about its effectiveness on peripheral nerve regeneration. To partially fill this gap, the aim of this study was to assess the effect of UnAG on peripheral nerve regeneration after median nerve crush injury and after nerve transection immediately repaired by means of an end-to-end suture. To this end, we exploited FVB1 Myh6/Ghrl transgenic mice in which overexpression of the ghrelin gene (Ghrl) results in selective up-regulation of circulating UnAG levels, but not of AG. Regeneration was assessed by both functional evaluation (grasping test) and morphometrical analysis of regenerated myelinated axons. Results obtained lead to conclude that UnAG could have a role in development of peripheral nerves and during more severe lesions.

A skeleton in the cupboard in ghrelin research: Where are the skinny dwarfs?

Based on studies delivering ghrelin or ghrelin receptor agonists, we have learned a great deal about the importance of the brain ghrelin signalling system for a wide range of physiological processes that include feeding behaviours, growth hormone secretion and glucose homeostasis. Because these processes can be considered as essential to life, the question arises as to why mouse models of depleted ghrelin signalling are not all skinny dwarfs with a host of behavioural and metabolic problems. Here, we provide a systematic detailed review of the phenotype of mice with deficient ghrelin signalling to help better understand the relevance and importance of the brain ghrelin signalling system, with a particular emphasis on those questions that remain unanswered.

Rbm24 displays dynamic functions required for myogenic differentiation during muscle regeneration

Skeletal muscle has a remarkable capacity of regeneration after injury, but the regulatory network underlying this repair process remains elusive. RNA-binding proteins play key roles in the post-transcriptional regulation of gene expression and the maintenance of tissue homeostasis and plasticity. Rbm24 regulates myogenic differentiation during early development, but its implication in adult muscle is poorly understood. Here we show that it exerts multiple functions in muscle regeneration. Consistent with its dynamic subcellular localization during embryonic muscle development, Rbm24 also displays cytoplasm to nucleus translocation during C2C12 myoblast differentiation. In adult mice, Rbm24 mRNA is enriched in slow-twitch muscles along with myogenin mRNA. The protein displays nuclear localization in both slow and fast myofibers. Upon injury, Rbm24 is rapidly upregulated in regenerating myofibers and accumulates in the myonucleus of nascent myofibers. Through satellite cell transplantation, we demonstrate that Rbm24 functions sequentially to regulate myogenic differentiation and muscle regeneration. It is required for myogenin expression at early stages of muscle injury and for muscle-specific pre-mRNA alternative splicing at late stages of regeneration. These results identify Rbm24 as a multifaceted regulator of myoblast differentiation. They provide insights into the molecular pathway orchestrating the expression of myogenic factors and muscle functional proteins during regeneration.

Both ghrelin deletion and unacylated ghrelin overexpression preserve muscles in aging mice

Sarcopenia, the decline in muscle mass and functionality during aging, might arise from age-associated endocrine dysfunction. Ghrelin is a hormone circulating in both acylated (AG) and unacylated (UnAG) forms with anti-atrophic activity on skeletal muscle. Here, we show that not only lifelong overexpression of UnAG (Tg) in mice, but also the deletion of ghrelin gene (Ghrl KO) attenuated the age-associated muscle atrophy and functionality decline, as well as systemic inflammation. Yet, the aging of Tg and Ghrl KO mice occurs with different dynamics: while old Tg mice seem to preserve the characteristics of young animals, Ghrl KO mice features deteriorate with aging. However, young Ghrl KO mice show more favorable traits compared to WT animals that result, on the whole, in better performances in aged Ghrl KO animals. Treatment with pharmacological doses of UnAG improved muscle performance in old mice without modifying the feeding behavior, body weight, and adipose tissue mass. The antiatrophic effect on muscle mass did not correlate with modifications of protein catabolism. However, UnAG treatment induced a strong shift towards oxidative metabolism in muscle. Altogether, these data confirmed and expanded some of the previously reported findings and advocate for the design of UnAG analogs to treat sarcopenia.

Physiological Effect of Ghrelin on Body Systems

Ghrelin is a relatively novel multifaceted hormone that has been found to exert a plethora of physiological effects. In this review, we found/confirmed that ghrelin has effect on all body systems. It induces appetite; promotes the use of carbohydrates as a source of fuel while sparing fat; inhibits lipid oxidation and promotes lipogenesis; stimulates the gastric acid secretion and motility; improves cardiac performance; decreases blood pressure; and protects the kidneys, heart, and brain. Ghrelin is important for learning, memory, cognition, reward, sleep, taste sensation, olfaction, and sniffing. It has sympatholytic, analgesic, antimicrobial, antifibrotic, and osteogenic effects. Moreover, ghrelin makes the skeletal muscle more excitable and stimulates its regeneration following injury; delays puberty; promotes fetal lung development; decreases thyroid hormone and testosterone; stimulates release of growth hormone, prolactin, glucagon, adrenocorticotropic hormone, cortisol, vasopressin, and oxytocin; inhibits insulin release; and promotes wound healing. Ghrelin protects the body by different mechanisms including inhibition of unwanted inflammation and induction of autophagy. Having a clear understanding of the ghrelin effect in each system has therapeutic implications. Future studies are necessary to elucidate the molecular mechanisms of ghrelin actions as well as its application as a GHSR agonist to treat most common diseases in each system without any paradoxical outcomes on the other systems.

The influence of skeletal muscle on appetite regulation

INTRODUCTION

Fat-free mass, of which skeletal muscle is amajor component, correlates positively with energy intake at energy balance. This is due to the effects of metabolically active tissue on energy expenditure which in itself appears to signal to the brain adrive to eat to ensure cellular energy homeostasis. The mechanisms responsible for this drive to eat are unknown but are likely to be related to energy utilization. Here muscle imparts an indirect influence on hunger. The drive to eat is also enhanced after muscle loss secondary to intentional weight loss. The evidence suggests loss of both fat mass and skeletal muscle mass directly influences the trajectory and magnitude of weight regain highlighting their potential role in long-termappetite control. The mechanisms responsible for the potential direct drive to eat stemming from muscle loss are unknown.

AREAS COVERED

The literature pertaining to muscle and appetite at energy balance and after weight loss was examined. Aliterature search was conducted to identify studies related to appetite, muscle, exercise, and weight loss.

EXPERT OPINION

Understanding the mechanisms which link energy expenditure and muscle loss to hunger has the potential to positively impact both the prevention and the treatment of obesity.

The DNA Methylation Status of Wnt and Tgfβ Signals Is a Key Factor on Functional Regulation of Skeletal Muscle Satellite Cell Development

DNA methylation is an important form of epigenetic regulation that can regulate the expression of genes and the development of tissues. Muscle satellite cells play an important role in skeletal muscle development and regeneration. Therefore, the DNA methylation status of genes in satellite cells is important in the regulation of the development of skeletal muscle. This study systematically investigated the changes of genome-wide DNA methylation in satellite cells during skeletal muscle development. According to the MeDIP-Seq data, 52,809-123,317 peaks were obtained for each sample, covering 0.70-1.79% of the genome. The number of reads and peaks was highest in the intron regions followed by the CDS regions. A total of 96,609 DMRs were identified between any two time points. Among them 6198 DMRs were annotated into the gene promoter regions, corresponding to 4726 DMGs. By combining the MeDIP-Seq and RNA-Seq data, a total of 202 overlap genes were obtained between DMGs and DEGs. GO and Pathway analysis revealed that the overlap genes were mainly involved in 128 biological processes and 23 pathways. Among the biological processes, terms related to regulation of cell proliferation and Wnt signaling pathway were significantly different. Gene-gene interaction analysis showed that Wnt5a, Wnt9a, and Tgfβ1 were the key nodes in the network. Furthermore, the expression level of Wnt5a, Wnt9a, and Tgfβ1 genes could be influenced by the methylation status of promoter region during skeletal muscle development. These results indicated that the Wnt and Tgfβ signaling pathways may play an important role in functional regulation of satellite cells, and the DNA methylation status of Wnt and Tgfβ signals is a key regulatory factor during skeletal muscle development. This study provided new insights into the effects of genome-wide methylation on the function of satellite cells.