Irisin Serum Levels and Skeletal Muscle Assessment in a Cohort of Charcot-Marie-Tooth Patients

Background: Charcot-Marie-Tooth (CMT) indicates a group of inherited polyneuropathies whose clinical phenotypes primarily include progressive distal weakness and muscle atrophy. Compelling evidence showed that the exercise-mimetic myokine irisin protects against muscle wasting in an autocrine manner, thus possibly preventing the onset of musculoskeletal atrophy. Therefore, we sought to determine if irisin serum levels correlate with biochemical and muscle parameters in a cohort of CMT patients. Methods: This cohort study included individuals (N=20) diagnosed with CMT disease. Irisin and biochemical markers were quantified in sera. Skeletal muscle mass (SMM) was evaluated by bioelectric impedance analysis, muscle strength by handgrip, and muscle quality was derived from muscle strength and muscle mass ratio. Results: CMT patients (m/f, 12/8) had lower irisin levels than age and sex matched healthy subjects (N=20) (6.51 ± 2.26 vs 9.34 ± 3.23 μg/ml; p=0.003). SMM in CMT patients was always lower compared to SMM reference values reported in healthy Caucasian population matched for age and sex. Almost the totality of CMT patients (19/20) showed low muscle quality and therefore patients were evaluated on the basis of muscle strength. Irisin was lower in presence of pathological compared to normal muscle strength (5.56 ± 1.26 vs 7.67 ± 2.72 μg/ml; p=0.03), and directly correlated with the marker of bone formation P1PN (r= 0.669; 95%CI 0.295 to 0.865; p=0.002), but inversely correlated with Vitamin D (r=-0.526; 95%CI -0,791 to -0,095; p=0.017). Surprisingly, in women, irisin levels were higher than in men (7.31 ± 2.53 vs 5.31 ± 1.02 μg/ml, p=0.05), and correlated with both muscle strength (r=0.759; 95%CI 0.329 to 0.929; p=0.004) and muscle quality (r=0.797; 95%CI 0.337 to 0.950; p=0.006). Conclusion: Our data demonstrate lower irisin levels in CMT patients compared to healthy subjects. Moreover, among patients, we observed, significantly higher irisin levels in women than in men, despite the higher SMM in the latter. Future studies are necessary to establish whether, in this clinical contest, irisin could represent a marker of the loss of muscle mass and strength and/or bone loss

LIGHT/TNFSF14 regulates estrogen deficiency-induced bone loss

Bone loss induced by ovariectomy is due to the direct activity on bone cells and mesenchymal cells and to the dysregulated activity of bone marrow cells, including immune cells and stromal cells, but the underlying mechanisms are not completely known. Here, we demonstrate that ovariectomy induces the T-cell co-stimulatory cytokine LIGHT, which stimulates both osteoblastogenesis and osteoclastogenesis by modulating osteoclastogenic cytokine expression, including TNF, osteoprotegerin, and the receptor activator of nuclear factor-κB ligand (RANKL). Predictably, LIGHT-deficient (Tnfsf14−/−) mice are protected from ovariectomy-dependent bone loss, whereas trabecular bone mass increases in mice deficient in both LIGHT and T and B lymphocytes (Rag−/−Tnfsf14−/−) and is associated with an inversion of the TNF and RANKL/OPG ratio. Furthermore, women with postmenopausal osteoporosis display high levels of LIGHT in circulating T cells and monocytes. Taken together, these results indicate that LIGHT mediates bone loss induced by ovariectomy, suggesting that patients with postmenopausal osteoporosis may benefit from LIGHT antagonism. © 2020 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd. © 2020 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd

Myokines and Osteokines in the Pathogenesis of Muscle and Bone Diseases

Purpose of Review: In this review we aim to summarize the latest findings on the network of molecules produced by muscle and bone under physiological and pathological conditions. Recent Findings: The concomitant onset of osteoporosis and sarcopenia is currently one of the main threats that can increase the risk of falling fractures during aging, generating high health care costs due to hospitalization for bone fracture surgery. With the growing emergence of developing innovative therapies to treat these two age-related conditions that often have common onset, a broader understanding of molecular messengers regulating the communication between muscle and bone tissue became imperative. Summary: Recently it has been highlighted that two muscle-derived signals, such as the myokines Irisin and L-BAIBA, positively affect bone tissue. In parallel, there are signals derived from bone that affect either positively the skeletal muscle, such as osteocalcin, or negatively, such as RANKL

Irisin Prevents Disuse-Induced Osteocyte Apoptosis

Previous results showed that intermittently administered irisin improves bone mass in normal mice and prevents the development of disuse-induced osteoporosis and muscular atrophy in hindlimb-suspended mice, a murine model able to mimic the absence of mechanical loading. A recent study showed that irisin increases survival of osteocytes acting through integrin αV/β5 receptors. To better understand the action of irisin on these cells, we investigated the downstream signaling cascades in osteocyte-like cells (MLO-Y4) treated with recombinant irisin (rec-irisin) in vitro and we analyzed survival of osteocytes and caspase activation in cortical bone of osteoporotic mice treated with rec-irisin in vivo. Our results revealed that rec-irisin activated the MAP kinases Erk1 and Erk2 and increased the expression of the transcription factor Atf4 (2.5-fold, p <.05) through an Erk-dependent pathway in osteocytes. Some key genes expressed by MLO-Y4 cells were modulated by long-term irisin treatment, either continuously administered or given with intermittent short pulses. Interestingly, Sost mRNA was severely downregulated only upon intermittent irisin administration (10-fold, p <.001). Furthermore, rec-irisin upregulated Tfam mRNA (fourfold, p <.05) and Bcl2/Bax ratio (twofold, p <.05) in MLO-Y4 cells. By detecting caspase-9 and caspase-3, we also found that rec-irisin inhibited apoptosis induced by hydrogen peroxide and dexamethasone, respectively. In cortical bone of unloading C57BL6 mice treated with vehicle (unload-veh), irisin prevented disuse-induced reduction of viable osteocytes (+30% versus unload-veh, p <.05) and increase of empty lacunae (+110% versus unload-veh, p <.05), as well as caspase-9 (threefold, p <.05) and caspase-3 (twofold, p <.05) activations. Our findings revealed underlying mechanisms of irisin action on osteocytes, which increases their functions and exerts anti-apoptotic effects, confirming that mechanosensor cells of bone are sensitive to the exercise-mimetic myokine irisin. © 2019 American Society for Bone and Mineral Research

Irisin Protects against Loss of Trabecular Bone Mass and Strength in Adult Ovariectomized Mice by Stimulating Osteoblast Activity

Irisin is a peptide secreted by skeletal muscle that plays a major role in bone metabolism. Experiments in mouse models have shown that administration of recombinant irisin prevents disuse-induced bone loss. In this study, we aimed to evaluate the effects of irisin treatment for the prevention of bone loss in the ovariectomized (Ovx) mouse, the animal model commonly used to investigate osteoporosis caused by estrogen deficiency. Micro-Ct analysis conducted on Sham mice (Sham-veh) and Ovx mice treated with vehicle (Ovx-veh) or recombinant irisin (Ovx-irisn) showed bone volume fraction (BV/TV) decreases in femurs (Ovx-veh 1.39± 0.71 vs. Sham-veh 2.84 ± 1.23; p = 0.02) and tibia at both proximal condyles (Ovx-veh 1.97 ± 0.68 vs. Sham-veh 3.48 ± 1.26; p = 0.03) and the subchondral plate (Ovx-veh 6.33 ± 0.36 vs. Sham-veh 8.18 ± 0.41; p = 0.01), which were prevented by treatment with a weekly dose of irisin for 4 weeks. Moreover, histological analysis of trabecular bone showed that irisin increased the number of active osteoblasts per bone perimeter (Ovx-irisin 32.3 ± 3.9 vs. Ovx-veh 23.5 ± 3.6; p = 0.01), while decreasing osteoclasts (Ovx-irisin 7.6 ± 2.4 vs. Ovx-veh 12.9 ± 3.04; p = 0.05). The possible mechanism by which irisin enhances osteoblast activity in Ovx mice is upregulation of the transcription factor Atf4, one of the key markers of osteoblast differentiation, and osteoprotegerin, thereby inhibiting osteoclast formation

Antidepressant Effect of Intermittent Long‐Term Systemic Administration of Irisin in Mice

Depression is a psychiatric disorder increasingly diffused worldwide. Evidence suggests that irisin, a myokine secreted by contracting muscle, mediates beneficial effects on several targets, including the brain. Here, the potential antidepressant properties of long‐term intermittent systemic irisin administration (100 μg/kg/weekly for 1 month) were evaluated in mice by the Tail Suspension Test (TST), Forced Swim Test (FST), and Open Field Test (OFT). Furthermore, to deepen the molecular pathways underlying irisin treatment, the expression of irisin precursor, neurotrophic/growth factors, and cytokines was analyzed. Irisin treatment significantly decreased the immobility time in the TST and FST, suggesting an antidepressant effect. Additionally, irisin seemed to display an an-xiolytic‐like effect increasing the time spent in the OFT arena center. These findings were probably due to the modulation of endogenous brain factors as the gene expression of some neurotrophins, such as brain‐derived neurotrophic factor (BDNF) and insulin‐like growth factor (IGF‐1), was up-regulated only in irisin‐treated mouse brain. Moreover, irisin modulated the expression of some cytokines (IL‐1β, IL‐4, IL‐6, and IL‐10). To the best of our knowledge, this is the first study demon-strating that the irisin antidepressant effect may be observed even with a systemic administration in mice. This could pave the way toward intriguing preclinical research in humans

Irisin Correlates Positively With BMD in a Cohort of Older Adult Patients and Downregulates the Senescent Marker p21 in Osteoblasts

Irisin is a myokine produced by skeletal muscle during exercise in both mice and humans. We previously showed that irisin treatment ameliorates immobility-induced osteoporosis and muscular atrophy in mice. Data in humans showed a positive association between irisin and bone mineral density (BMD) in athletes and a population of healthy children. However, the role of this myokine regarding the state of muscle and bone in the same population remained to be determined. For this purpose, 62 patients (age 68.71 ± 12.31 years) undergoing total hip or knee replacement were recruited. Our results showed that irisin serum levels negatively correlated with age (R = −0.515; p =.000018) and positively correlated with femoral BMD (R = 0.619; p =.001) and vertebral BMD (R = 0.201; p =.0001). Irisin was also positively associated with Fndc5 mRNA in muscle biopsies (R = 0.248; p =.016), as well as with Osteocalcin (Ocn) mRNA in bone biopsies (R = 0.708; p =.006). In skeletal muscle, FNDC5 positive fibers positively correlate with BMD of total femur (R = 0.765; p =.0014) and BMD of femoral neck (R = 0.575; p =.031), Interestingly, by analyzing patients divided by their T-score, we found lower irisin levels (p =.0011) in patients with osteopenia/osteoporosis (OP) compared to healthy controls matched for age and sex. By analyzing the senescence marker p21, we found a significant increase of its mRNA expression in the bone biopsies of OP patients compared to control ones. Therefore, we investigated in vitro whether rec-irisin had a direct effect on this senescence marker, showing that p21 mRNA expression was significantly downregulated in osteoblasts by the treatment with irisin. Overall, these results indicate that higher irisin levels are associated with a lower rate of age-related osteoporosis and that irisin could be effective in delaying the osteoblast aging process, suggesting a potential senolytic action of this myokine. © 2020 American Society for Bone and Mineral Research (ASBMR)

Short-Term Irisin Treatment Enhanced Neurotrophin Expression Differently in the Hippocampus and the Prefrontal Cortex of Young Mice

As a result of physical exercise, muscle releases multiple exerkines, such as “irisin”, which is thought to induce pro-cognitive and antidepressant effects. We recently demonstrated in young healthy mice the mitigation of depressive behaviors induced by consecutive 5 day irisin administration. To understand which molecular mechanisms might be involved in such effect, we here studied, in a group of mice previously submitted to a behavioral test of depression, the gene expression of neurotrophins and cytokines in the hippocampus and prefrontal cortex (PFC), two brain areas frequently investigated in the depression pathogenesis. We found significantly increased mRNA levels of nerve growth factor (NGF) and fibroblast growth factor 2 (FGF-2) in the hippocampus and brain-derived growth factor (BDNF) in the PFC. We did not detect a difference in the mRNA levels of interleukin 6 (IL-6) and IL-1β in both brain regions. Except for BDNF in the PFC, two-way ANOVA analysis did not reveal sex differences in the expression of the tested genes. Overall, our data evidenced a site-specific cerebral modulation of neurotrophins induced by irisin treatment in the hippocampus and the PFC, contributing to the search for new antidepressant treatments targeted at single depressive events with short-term protocols

LIGHT/TNFSF14 regulates estrogen deficiency-induced bone loss

Bone loss induced by ovariectomy is due to the direct activity on bone cells and mesenchymal cells and to the dysregulated activity of bone marrow cells, including immune cells and stromal cells, but the underlying mechanisms are not completely known. Here, we demonstrate that ovariectomy induces the T-cell co-stimulatory cytokine LIGHT, which stimulates both osteoblastogenesis and osteoclastogenesis by modulating osteoclastogenic cytokine expression, including TNF, osteoprotegerin, and the receptor activator of nuclear factor-κB ligand (RANKL). Predictably, LIGHT-deficient (Tnfsf14-/- ) mice are protected from ovariectomy-dependent bone loss, whereas trabecular bone mass increases in mice deficient in both LIGHT and T and B lymphocytes (Rag -/- Tnfsf14 -/- ) and is associated with an inversion of the TNF and RANKL/OPG ratio. Furthermore, women with postmenopausal osteoporosis display high levels of LIGHT in circulating T cells and monocytes. Taken together, these results indicate that LIGHT mediates bone loss induced by ovariectomy, suggesting that patients with postmenopausal osteoporosis may benefit from LIGHT antagonism. © 2020 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd

In Reply to the Letter to the Editor: Involvement of Irisin in Age-Related Osteoporosis and Its Inhibitory Effect on the Senescent Marker p21 in Osteoblasts

We appreciate the thoughtful letter from Professor Naharciregarding our recently published article showing that iri-sin correlates positively with bone mineral density (BMD) in acohort of older adult patients and downregulates the senescentmarker p21 in osteoblasts.(1)We thank him for the interest in ourrecent work and welcome the opportunity to respond to hiscomments. As noted by Naharci, several chronic diseases couldhave inuenced the levels of circulating irisin measured in ourcohort of older adult patients. In agreement with this premise,in our study, we enrolled patients by excluding all subjects withat least one chronic disease. Because our primary outcome wasto investigate possible links between circulating irisin, FNDC5expression in skeletal muscle, and BMD, the main exclusion cri-teria were taking anti-osteoporosis therapy and physical impair-ments affecting the performance of daily activities. However,subjects were also excluded if they had renal failure with glomer-ular ltration rate <30 mL/min, diastolic blood pressure>90 mmHg, systolic blood pressure >140 mmHg, any impair-ment of glucose metabolism such as diabetes or pre-diabetes,glucocorticoid use within the past 2 years, and thyroid, parathy-roid, and liver diseases