Irisin: good or bad for the bone? A new path forward after the reported discovery of irisin receptor?

In patients with anorexia nervosa, myokine levels are altered but are not associated with bone mineral density loss and bone turnover alteration

Objectives

The two-fold aim of this study was: (i) to determine the effects of undernutrition on the myokines in patients with restrictive anorexia nervosa (AN) and (ii) to examine the potential link between myokines and bone parameters.

Methods

In this study, 42 young women with restrictive AN and 42 age-matched controls (CON) (mean age, 18.5 ± 4.2 years and 18.6 ± 4.2 years, respectively) were enrolled. aBMD and body composition were determined with DXA. Resting energy expenditure (REEm), a marker of energy status, was indirectly assessed by calorimetry. Bone turnover markers and myokines (follistatin, myostatin and irisin) were concomitantly evaluated.

Results

AN patients presented low aBMD at all bone sites. REEm, bone formation markers, myostatin and IGF-1 were significantly lower, whereas the bone resorption marker and follistatin were higher in AN compared with controls. No difference was observed between groups for irisin levels. When the whole population was studied, among myokines, only myostatin was positively correlated with aBMD at all bone sites. However, multiple regression analyses showed that in the AN group, the independent variables for aBMD were principally amenorrhoea duration, lean tissue mass (LTM) and procollagen type I N-terminal propeptide (PINP). For CON, the independent variables for aBMD were principally LTM, age and PINP. Whatever the group analysed, none of the myokines appeared as explicative independent variables of aBMD.

Conclusion

This study demonstrated that despite the altered myokine levels in patients with AN, their direct effect on aBMD loss and bone turnover alteration seems limited in comparison with other well-known disease-related factors such as oestrogen deprivation.

Evaluation of the Effects of Systemic Irisin Hormone Application on Osseointegration of Titanium Implants: An Experimental Study

ABSTRACT

This study aimed to conduct a biomechanical investigation of the effects of systemic irisin hormone application on the osseointegration of titanium implants in rat tibias. After surgical implementation of titanium implants in the metaphyseal part of the tibiae of rats, the rats were randomly divided into 2 equal groups: control group (n = 10) and irisin group (n = 10). After surgery in the control group, the rats received no further treatment during the 4-week experimental period. The rats in the irisin group were given 100 ng/kg irisin every day for the 4-week experimental period after surgery. At the end of the experimental period, the rats were euthanized. Implants and surrounding bone tissues were collected for biomechanical (Newton) bone implant connection analysis. The Student t test was used for statistical analysis. There were no significant differences in the biomechanical osseointeration values (Newton) of the groups (P > 0.05, P = 0.59). Also, in the irisin group, there was numerically but not statistically more bone implant connection than in the controls. Within the limitations of this study, irisin did not affect the osseointegration of titanium implants.

The production, detection, and origin of irisin and its effect on bone cells

Irisin is a muscle factor discovered in 2012 that plays an important role in many tissues, including bone. Eight years since its discovery, there are still many controversies regarding its molecular biology, detection, and effects on bone. This article summarizes the points raised to date, and discusses the mechanisms by which irisin regulates bone cells. The information reviewed here provides a useful foundation for future research.

Plasma Concentration of Irisin and Brain-Derived-Neurotrophic Factor and Their Association With the Level of Erythrocyte Adenine Nucleotides in Response to Long-Term Endurance Training at Rest and After a Single Bout of Exercise

The study aimed to assess the effect of a single bout of incremental exercise on irisin and BDNF plasma concentrations as related to erythrocyte purine nucleotides concentration at rest and after exercise. Master endurance master athletes (training experience 38 ± 6 years) and a group of untrained participants completed a single bout of progressive incremental exercise test until exhaustion. The dual-energy x-ray absorptiometry and blood collection were performed. Blood was taken twice at rest and 10 min after exercise. Concentrations of ATP, ADP, and AMP were assessed in the erythrocytes. Hypoxanthine and uric acid were determined in plasma using the high-performance liquid chromatography. Plasma concentrations of irisin and BDNF were assessed through the immunoenzymatic method. The ATP level, ATP/ADP ratio and AEC value were significantly higher in the athletic group. A significantly higher concentration of BDNF was it also noted in the trained group that correlated with the erythrocyte energy status at rest. The single session of exercise induced a significant increase in ATP erythrocyte levels in both groups. Both exerkines significantly correlated at rest with red blood cell adenine nucleotides and degradation products (BDNF positively and irisin negatively). The blood concentration of BDNF and irisin, in response to exercise, was not significantly different between groups. Obtained data revealed a higher erythrocyte energy status and lower purine degradation products concentration in master athletes. Also resting plasma exerkines differed substantially between groups. In conclusion, long-term training resulted in exercise adaptation reflected by a higher erythrocyte energy status, lower purine degradation products concentration and modified concentration of exerkines (higher BDNF and lower irisin blood concentrations). Therefore, we consider the training-induced adaptations in master athletes to be beneficial and significant. The moderate level of physical activity in the untrained group, even if sufficient in terms of general health, did not cause any discernible changes.

Cleavage of FNDC5 and insights into its maturation process

FNDC5 corresponds to an irisin precursor that increases with exercise. Studies suggest that irisin mediates beneficial effects in adipose tissues, skeletal muscle, bone, and brain. However, the cleavage and maturation processes of FNDC5 have not been clearly identified. This study aimed to show that the signal peptide and transmembrane domain of FNDC5 were associated with the secretion of its ectodomain. Localization studies identified the signal peptide that was responsible for endoplasmic reticulum targeting activity of nascent FNDC5 and showed that the FNDC5 ectodomain corresponding to irisin could be transported across the membrane by a transmembrane domain. Analysis of cleavage constructs revealed that the ectodomain of FNDC5 could be cleaved from its signal peptide and transmembrane attachment. Genetic ablation of the signal peptide cleavage site blocked N-glycosylation of FNDC5. Identification of the FNDC5 maturation process should facilitate our understanding of irisin secretion.

Irisin increases the expression of anorexigenic and neurotrophic genes in mouse brain

BACKGROUND

Irisin, a newly discovered muscle-derived hormone, acts in different organs and tissues, improving energy homeostasis. In this study, we assessed, for the first time, the effects of intraperitoneal irisin injections on circulating levels of leptin and ghrelin, mRNA expression of the major hypothalamic appetite regulators and brain neurotrophic factors, as well as feeding behaviour in healthy mice.

METHODS

Twelve male 6-week-old C57BL/6 mice were randomized into two groups and intraperitoneally injected daily with irisin (0.5 μg/g body weight) or vehicle (phosphate-buffered saline [PBS]) for 14 days. On the last day of observation, leptin and ghrelin levels were measured with an enzyme-linked immunosorbent assay (ELISA). mRNA levels of genes of interest were analysed by quantitative reverse transcription polymerase chain reaction (qRT-PCR) in brain extracts.

RESULTS

Irisin administration did not change leptin or ghrelin serum concentrations. However, irisin injection increased CART, POMC, NPY, and BDNF mRNA levels, without affecting the mRNA expression of AgRP, orexin, PMCH, and UCP2. Finally, over the time frame of irisin treatment, body weight and feeding behaviour were unaltered.

CONCLUSIONS

These results suggest that intraperitoneal injection of irisin, although without effects on feeding behaviour and body weight, can increase the expression of anorexigenic and neurotrophic genes in mouse brain.