PPARs and Microbiota in Skeletal Muscle Health and Wasting.

Skeletal muscle is a major metabolic organ that uses mostly glucose and lipids for energy production and has the capacity to remodel itself in response to exercise and fasting. Skeletal muscle wasting occurs in many diseases and during aging. Muscle wasting is often accompanied by chronic low-grade inflammation associated to inter- and intra-muscular fat deposition. During aging, muscle wasting is advanced due to increased movement disorders, as a result of restricted physical exercise, frailty, and the pain associated with arthritis. Muscle atrophy is characterized by increased protein degradation, where the ubiquitin-proteasomal and autophagy-lysosomal pathways, atrogenes, and growth factor signaling all play an important role. Peroxisome proliferator-activated receptors (PPARs) are members of the nuclear receptor family of transcription factors, which are activated by fatty acids and their derivatives. PPARs regulate genes that are involved in development, metabolism, inflammation, and many cellular processes in different organs. PPARs are also expressed in muscle and exert pleiotropic specialized responses upon activation by their ligands. There are three PPAR isotypes, viz., PPARα, -ß/δ, and -γ. The expression of PPARα is high in tissues with effective fatty acid catabolism, including skeletal muscle. PPARß/δ is expressed more ubiquitously and is the predominant isotype in skeletal muscle. It is involved in energy metabolism, mitochondrial biogenesis, and fiber-type switching. The expression of PPARγ is high in adipocytes, but it is also implicated in lipid deposition in muscle and other organs. Collectively, all three PPAR isotypes have a major impact on muscle homeostasis either directly or indirectly. Furthermore, reciprocal interactions have been found between PPARs and the gut microbiota along the gut-muscle axis in both health and disease. Herein, we review functions of PPARs in skeletal muscle and their interaction with the gut microbiota in the context of muscle wasting.

Mycobacterium ulcerans infections cause progressive muscle atrophy and dysfunction, and mycolactone impairs satellite cell proliferation.

Clinical observations from Buruli ulcer (BU) patients in West Africa suggest that severe Mycobacterium ulcerans infections can cause skeletal muscle contracture and atrophy leading to significant impairment in function. In the present study, male mice C57BL/6 were subcutaneously injected with M. ulcerans in proximity to the right biceps muscle, avoiding direct physical contact between the infectious agent and the skeletal muscle. The histological, morphological, and functional properties of the muscles were assessed at different times after the injection. On day 42 postinjection, the isometric tetanic force and the cross-sectional area of the myofibers were reduced by 31% and 29%, respectively, in the proximate-infected muscles relative to the control muscles. The necrotic areas of the proximate-infected muscles had spread to 7% of the total area by day 42 postinjection. However, the number of central nucleated fibers and myogenic regulatory factors (MyoD and myogenin) remained stable and low. Furthermore, Pax-7 expression did not increase significantly in mycolactone-injected muscles, indicating that the satellite cell proliferation is abrogated by the toxin. In addition, the fibrotic area increased progressively during the infection. Lastly, muscle-specific RING finger protein 1 (MuRF-1) and atrogin-1/muscle atrophy F-box protein (atrogin-1/MAFbx), two muscle-specific E3 ubiquitin ligases, were upregulated in the presence of M. ulcerans. These findings confirmed that skeletal muscle is affected in our model of subcutaneous infection with M. ulcerans and that a better understanding of muscle contractures and weakness is essential to develop a therapy to minimize loss of function and promote the autonomy of BU patients.

Gut microbiota composition influences outcomes of skeletal muscle nutritional intervention via blended protein supplementation in posttransplant patients with hematological malignancies.

BACKGROUND: Skeletal muscle atrophy is an important and independent predictor of survival after hematopoietic stem cell transplantation (HSCT). Our previous study found that soy-whey blended protein (SWP) can improve muscle mass in acute leukemia patients. OBJECTIVE: We aimed to explore potential factors that influence muscle outcomes after nutritional intervention. METHODS: In this case-control study, 13 patients who received HSCT and failed to improve muscle function within half a year were included. After two months of SWP intervention, the subjects were divided into two groups (MSI: muscle status improved; MNI: muscle status not improved). 16S rDNA sequencing, principal coordinate analysis (PCoA) and the PICRUSt algorithm were used to analyze the composition, structure and function of the intestinal microbiota between the groups. This study was registered in the Chinese Clinical Trial Registry (ChiCTR 1800017765). RESULTS: SWP significantly improved muscle status (muscle area: from 330.4 mm2 to 384.8 mm2, p = 0.02; muscle strength: from 19.2 kg to 21.3 kg, p = 0.04). However, there were a small number of subjects whose muscle status was not effectively improved. After SWP intervention, the diversity (Shannon: from 1.7 to 3.8, p = 0.01; Simpson: from 0.6 to 0.8, p = 0.015) of the intestinal microbiota in the MSI group increased significantly, whereas that in the MNI group did not. Principal component analysis (PCA) revealed separate groupings of the microbiota of the Baseline-MSI and Endpoint-MSI time points in the MSI group. Opposite patterns of microbial abundance change were found between the MSI group (75% of changed genera were increased) and the MNI group (80% of changed genera were decreased). Three bacterial taxa (negative correlation: Streptococcus; positive correlations: Ruminococcus and Veillonella) were significantly related to muscle improvement outcomes. Both pentose phosphate (p = 0.048) and amino acid biosynthesis (p = 0.039), which are related to muscle metabolism, were found to be significantly changed in the MSI group through PICRUSt algorithm prediction. CONCLUSIONS: Our results suggest that the intestinal microbiota plays important roles in the regulation of muscle metabolism.

Depletion of gut microbiota induces skeletal muscle atrophy by FXR-FGF15/19 signalling.

Background: Recent evidence indicates that host-gut microbiota crosstalk has nonnegligible effects on host skeletal muscle, yet gut microbiota-regulating mechanisms remain obscure.Methods: C57BL/6 mice were treated with a cocktail of antibiotics (Abx) to depress gut microbiota for 4 weeks. The profiles of gut microbiota and microbial bile acids were measured by 16S rRNA sequencing and ultra-performance liquid chromatography (UPLC), respectively. We performed qPCR, western blot and ELISA assays in different tissue samples to evaluate FXR-FGF15/19 signaling.Results: Abx treatment induced skeletal muscle atrophy in mice. These effects were associated with microbial dysbiosis and aberrant bile acid (BA) metabolism in intestine. Ileal farnesoid X receptor (FXR)-fibroblast growth factor 15 (FGF15) signaling was inhibited in response to microbial BA disturbance. Mechanistically, circulating FGF15 was decreased, which downregulated skeletal muscle protein synthesis through the extracellular-signal-regulated protein kinase 1/2 (ERK1/2) signaling pathway. Treating Abx mice with FGF19 (human FGF15 ortholog) partly reversed skeletal muscle loss.Conclusions: These findings indicate that the BA-FXR-FGF15/19 axis acts as a regulator of gut microbiota to mediate host skeletal muscle.

Systemic IGF-I administration attenuates the inhibitory effect of chronic arthritis on gastrocnemius mass and decreases atrogin-1 and IGFBP-3.

Adjuvant arthritis is an animal model of rheumatoid arthritis that decreases liver and circulating IGF-I as well as skeletal muscle mass. The aim of this work was to elucidate whether IGF-I administration was able to prevent the effect of arthritis on body weight and on two skeletal muscles, gastrocnemius and soleus. On day 4 after adjuvant injection, control and arthritic rats were treated with IGF-I (100 microg/kg s.c.) two times a day, until day 15 when all rats were killed. Arthritis decreased body weight gain and gastrocnemius weight. In arthritic rats, IGF-I treatment increased body weight gain and gastrocnemius weight, without modifying food intake or the external signs of arthritis. Arthritis increased atrogin-1 and muscle ring finger 1 (MuRF1) gene expression in the gastrocnemius and to a lesser extent in the soleus muscle. IGF-I attenuated the arthritis-induced increase in atrogin-1 and MuRF1 expression in the gastrocnemius, whereas it did not modify the expression of these genes in the soleus muscle. Arthritis also increased IGF-binding protein (IGBP)-3 and IGFBP-5 gene expression in gastrocnemius and soleus, whereas IGF-I administration decreased IGFBP-3, but not IGFBP-5, gene expression in both muscles. In both groups of arthritic rats and in control rats treated with IGF-I, proliferating cell nuclear antigen and myogenic differentiation proteins were increased in the gastrocnemius. These data suggest that the inhibitory effect of chronic arthritis on skeletal muscle is higher in fast glycolytic than in slow oxidative muscle and that IGF-I administration attenuates this effect and decreases atrogin-1 and IGFBP-3 gene expression.

Downregulation of muscle protein degradation in sepsis by eicosapentaenoic acid (EPA).

Eicosapentaenoic acid (EPA) has been shown to attenuate muscle atrophy in cancer, starvation and hyperthermia by downregulating the increased expression of the ubiquitin-proteasome proteolytic pathway leading to a reduction in protein degradation. In the current study EPA (0.5 g/kg) administered to septic mice completely attenuated the increased protein degradation in skeletal muscle by preventing the increase in both gene expression and protein concentration of the alpha- and beta-subunits of the 20S proteasome, as well as functional activity of the proteasome, as measured by the 'chymotrypsin-like' enzyme activity. These results suggest that muscle protein catabolism in sepsis is mediated by the same intracellular signalling pathways as found in other catabolic conditions.

Chronic Escherichia coli infection induces muscle wasting without changing acetylcholine receptor numbers.

OBJECTIVE: Muscle weakness in septic patients is a serious problem as it complicates and lengthens hospital stays, prolongs rehabilitation and increases costs. We examined the effects of a chronic infection with Escherichia coli on muscle function, muscle mass, and the expression of nicotinic acetylcholine receptors (AChRs). DESIGN AND SETTING: Prospective, randomized animal study in an animal laboratory, university hospital. SUBJECTS: Sprague-Dawley rats. INTERVENTIONS: A catheter was implanted into the external jugular vein of anesthetized rats, and a dose of 3.2x10(8) CFU of E. coli bacteria was injected; the catheter was then sealed and tunneled subcutaneously. MEASUREMENTS AND RESULTS: Animals injected with E. coli bacteria showed a significant decrease in body and muscle mass over the 14-day experimental period. Neuromuscular function was tested by mechanomyography on days 3, 7, and 14 following injection. Tetanic tension decreased over the time course of sepsis, without effecting tetanic fade. Serum levels of acute-phase protein, alpha1-acid glycoprotein, were increased by day 3, and remained significantly higher until day 14. AChRs were quantitated using 125I-labeled bungarotoxin and revealed no differences between groups. CONCLUSIONS: Central venous injection of E. coli bacteria induces systemic inflammation evidenced as loss of body weight, muscle mass and increased alpha1-acid glycoprotein levels. The inflammation-induced muscle weakness is due to muscle atrophy and not to upregulated AChRs. This model may prove useful for studying maneuvers to prevent muscle wasting with inflammation.

[Experimental amyotrophic leukospongiosis]. / Eksperimental'nyi amiotroficheskii leikospongioz.

Experimental amyotrophic leukospongiosis (ALSP) has first been reproduced in 2 squirrel monkeys 16 and 23 months after inoculation of the animals with a brain suspension from the patient who had died of this disease. From the brain cell cultures of the monkeys a hemadsorbing agent was isolated which induced cell proliferation and was inhibited by the antisera from patients with ALSP.

Muscle wasting: the gut microbiota as a new therapeutic target?

Muscle wasting is characterized by a loss of muscle mass and strength, and occurs in several pathological conditions such as cancer, chronic heart failure, chronic infection and malnutrition. Muscle wasting can be caused by inflammation and inappropriate nutritional status. Interestingly, gut microbiota has recently been proposed as an environmental factor involved, among others, in energy sparing from the diet, and in the regulation of host immunity and metabolism. This review presents evidence supporting the existence of a gut microbiota-muscle axis and discusses the potential role and therapeutic interest of gut microbiota in muscle wasting, specifically in the context of cancer and malnutrition. This review also proposes possible molecular mechanisms underlying the gut microbiota-muscle axis. This article is part of a Directed Issue entitled: Molecular basis of muscle wasting.

Skeletal Muscle wasting and contractile performance in septic rats.

We investigated the temporal effects of sepsis on muscle wasting and function in order to study the contribution of wasting to the decline in muscle function; we also studied the fiber-type specificity of this muscle wasting. Sepsis was induced by injecting rats intraperitoneally with a zymosan suspension. At 2 h and at 2, 6, and 11 days after injection, muscle function was measured using in situ electrical stimulation, Zymosan injection induced severe muscle wasting compared to pair-fed and ad libitum fed controls. At 6 days, isometric force-generating capacity was drastically reduced in zymosan-treated rats. We conclude that this was fully accounted fo by the reduction of muscle mas. At day 6, we also observed increased activity of the 20S proteasome in gastrocnemius but not soleus muscle from septic rats. In tibialis anterior but not in soleus, muscle wasting occurred in a fiber-type specific fashion, i.e., the reduction in cross-sectional area was significantly smaller in type 1 than type 2A and 2B/X fibers. These findings suggest that both the inherent function of a muscle and the muscle fiber-type distribution affect the responsiveness to catabolic signals.

Isolation of a new strain of cytomegalovirus from explanted normal skin.

Forty-three fibroblast cell line were initiated from normal skin biopsies. One cell line from a patient with Charcot--Marie--Tooth disease (CMT) spontaneously developed c.p.e. suggestive of cytomegalovirus (CMV). Characterization of the virus showed it to be a new strain of CMV and the results suggested that skin fibroblasts from CMT patients may carry latent CMV.