Fndc5/irisin-enriched extracellular vesicles: a new hormonal relay in the regular race against vascular ageing

Graphical Abstract

Extracellular vesicles (EV), bilayer-delimited lipid particles that are released naturally by all cell types are emerging as crucial cellular messengers in endocrinology. Skeletal muscle, which is the largest secretory organ, releases exerkines enclosed in EV that improve their selective release and absorption by target cells in accordance with frequency, intensity, time and type of exercise without corruption by circulating proteases, thus delaying age-related organ damage. FNDC5/Irisin = fibronectin type III domain-containing protein 5. Sirtuin-6 = Sirtuin family of NAD⁺ -dependent enzymes.

This editorial refers to ‘Exerkine fibronectin type-III domain-containing protein 5/irisin-enriched extracellular vesicles delay vascular ageing by increasing SIRT6 stability’, by C. Chi et al., https://doi.org/10.1093/eurheartj/ehac431.

Do those who exercise regularly age in the same way as those who do not, despite being the same age? It is fair to ask this question since the number of elderly people is expected to double to 1.5 billion in 2050, and the major risk factor for cardiovascular disease (CVD) is advancing age. It is well established that regular aerobic exercise produces positive changes in the human body and these are likely to be due to its vascular health-enhancing influence.¹ Despite regular aerobic physical activity being one of the most evidence-based and cost-effective strategies for reducing CVD risk with ageing in both men and women,¹ scientists continue to be drawn to understanding key cellular mechanisms that are still unknown. Common aerobic exercise combination interventions improve stiffness of the large arteries in previously sedentary middle-aged and older men and post-menopausal women¹ by reducing oxidative stress, chronic low-grade inflammation, elastin degradation, and collagen accumulation, and preserving endothelial nitric oxide (NO) bioavailability. However, understanding the multiple mechanisms underlying chronic vascular adaptations to regular aerobic exercise with advancing age is not as straightforward as we once thought. More information is needed on the factors responsible for remote conditioning leading to improvement of age-related vascular stiffness and prevention of CVD risk, including the epigenetic role of circulating messengers of the ‘stimulus’ of aerobic exercise.

The exerkines are metabolically active proteins/peptides and RNAs released in the blood by skeletal muscle (myokines), liver (hepatokines), adipose tissue (adipokines), and an unknown source [growth differentiation factor-15 (GDF-15)] in response to exercise.² Although the exerkine field is still in its infancy, we cannot ignore the fact that these bioactive substances are attracting great interest as signalling factors involved in the remote regulation of cross-talk among many organs and cells during exercise.² Moreover, the homeostatic relevance of exerkine-mediated signals is highlighted by their systemic transport into extracellular vesicles (EVs; a heterogeneous population of membrane-bound vesicles of varying size and content)³ that improve their selective release and uptake by target cells in accordance with frequency, intensity, time, and type of exercise without corruption by circulating proteases or ribonucleases. It is conceivable that skeletal muscle remains the largest secretory organ of exerkines preventing age-related arterial stiffness, even if age-related muscle loss is a natural part of ageing. Indeed, muscle mass decreases with age until it reaches ∼25% of total body weight at age 75–80 years, but we do not know how much this may interfere with the dose–effect relationship of circulating EVs. In fact, previous pre-clinical and clinical studies have demonstrated that cardiovascular benefits of EVs may depend on a threshold effect.^4,5 In their study published in this issue of the European Heart Journal, Chi et al. focused on the role against vascular ageing of a myokine, fibronectin type III domain-containing protein 5 (FNDC5) and its soluble form, the circulating glycosylated 112 amino acid hormone named ‘irisin’,⁶ a metabolic myokine discovered in 2012.⁷ FNDC5/irisin levels were reduced in both skeletal muscle and the bloodstream of naturally aged mice and also following a 4-week infusion of angiotensin II (AngII), the major effector peptide of the renin–angiotensin system (RAS)-associated with vascular ageing that increases blood pressure, arterial stiffness, and cellular senescence.⁸ Interestingly, arterial stiffness and senescence were more pronounced in naturally aged mice with FNDC5 deletion than in wild-type mice of a similar age, but young FNDC5-knockout (KO) mice showed a normal aorta.⁶ No less interesting was the detection of a greater increase in vascular senescence in the aortas of FNDC5-KO mice chronically exposed to AngII. Indeed, these findings define FNDC5/irisin deficiency as a molecular event that worsens vascular ageing triggered by sustained low-grade inflammation even regardless of chronological age. It was like having demonstrated the existence of a switch on which our eternal youth may depend. Indeed, obesity,⁹ sedentariness,¹⁰ sleep deprivation,¹¹ and social stress¹² may increase AngII and angiotensin II type 1 receptor activation in young subjects, contributing to vascular ageing. Yet, high levels of irisin, because of peroxisome proliferator-activated receptor gamma co-activator 1-alpha (PGC-1α) expression in fast-twitch skeletal muscle simultaneously induced by cAMP and Sirtuin,^1,7 may counteract AngII-induced vascular ageing through reduction of proinflammatory cytokines, oxidative stress-related NADPH oxidase (NOX) type 1 and 4, and reactive oxygen species. Moreover, the discovery that the exercise-induced hormone irisin even stimulates the browning of white adipose tissue, which improves insulin sensitivity preventing age-related diseases such as CVD and extends life span in a metabolic manner,⁷ may deserve a new look in a hospital setting. Since many drugs widely used for heterogeneous clinical purposes, a Mediterranean diet, and nutraceuticals induce FNDC5/irisin expression,¹³ understanding of remote peripheral regulation across the life span of this protein duet delivered by EVs will be helpful to enhance the clinical effectiveness of therapeutics and prophylactics in older patients through combination with a regular exercise programme.

The work published by Chi et al.⁶ further supports the view of FNDC5/irisin-enriched EVs as critical rejuvenating agents through long-distance communication between hormone-producing cells and cells that express hormone receptors, beyond the well-known role as autocrine and paracrine mediators.¹⁴ Indeed, the additional proof-of-concept population study reveals a negative relationship between the plasma levels of FNDC5/irisin and vascular ageing in humans.⁶ Beyond their role as biomarkers of vascular ageing, the infusion of allogenic FNDC5/irisin-enriched EVs, recapitulating benefits of exercise, could be the first step toward the development of a precision antiageing therapy in patients struggling with injury, sarcopenia, or severe exercise limitations.

Although many cell types in response to different exogenous stimuli release several vesicular macromolecules,¹⁵ EVs are the mediators that most effectively influence the phenotype of vascular cells throughout the life span. Significant changes in the concentration of circulating procoagulant EVs were associated with post-menopausal and type 2 diabetes mellitus metabolic syndromes in the onset of CVD, as well as correlated with severity of COVID-19 (reviewed in Salomon et al.¹⁴). Recently, stimulation of human endothelial cells with EVs isolated from the blood of primary aldosteronism patients resulted in the overexpression of specific genes previously identified as implicated in endothelial dysfunction, vascular inflammation, and remodelling. Such effects were abolished by removal of EV surface antigens, suggesting the interaction of cells with specific epitopes expressed on the membrane of circulating EVs.¹⁶ Alternatively, Chi et al.⁶ revealed a new signalling pathway involved in the antiageing effect of the phenotype-specific EV subpopulation. Indeed, they have demonstrated that exercise-induced FNDC5/irisin-enriched circulating EVs stabilize expression of Sirtuin-6 (SIRT6), a longevity NAD⁺-dependent histone deacetylase that is downstream of DnaJb3, a co-chaperone protein working synergistically with heat-shock protein 70 in preventing AngII-induced senescence of vascular smooth muscle cells through reducing SIRT6 ubiquitination.⁶ Taken together, these findings indicate that the intensity of the exercise programme designed to target SIRT6 will also make an important contribution to the fight against vascular ageing without excess post-exercise oxygen consumption.

Another noteworthy technical aspect of the present paper is that the authors based western blot analyses for the detection of EV-enclosed FNDC5/irisin on the amount of the ∼22 kDa band, using commercially available monoclonal antibodies.⁶ This band roughly matched the size of irisin reported by others but does not add much to the ongoing debate on whether commercial antibodies used to detect irisin are specific for FNDC5/irisin or not.¹⁷ However, the fact that FNDC5/irisin enclosed in EVs (either exposed on the vesicles surface or encapsulated within their lumen) may partially explain the reason why previous studies using antibodies binding to the C-terminal region of the FNDC5 protein recognized cleaved irisin composed only of the extracellular N-terminal residue (reviewed in Albrecht et al.¹⁷ and Raschke et al.¹⁸). It is plausible that muscle cells induce post-translational modification, having a transmembrane domain C-terminus enclosed in EVs and released into the extracellular space. This can only be ascertained by sequencing EV-associated FNDC5/irisin. The latter would also be an important aspect to unravel because the authors provided the evidence that EVs released from skeletal muscle, but not those released by the liver, critically contribute to exercise-conferred protection against vascular senescence and ageing in vivo and in vitro. These findings strongly indicate that the isoform of FNDC5/irisin in liver-derived EVs differs from that enclosed in ‘muscular’ EVs, thus has a cell-specific selective packaging mechanism that may confer on FNDC5/irisin the role of a tool to enrich muscle-specific EVs from the blood. Purifying single subpopulations of endogenous tissue-specific EVs remains challenging, requiring highly selective antigens/binders. Biomarkers inferred by profiling tissue-specific circulating EVs will be precious, complementary diagnostic tools for several diseases since EVs mirror specificities of the corresponding parental cells and their perturbations occurring in pathological conditions.

Data availability

No new data were generated or analysed in support of this research.

Author notes

The opinions expressed in this article are not necessarily those of the Editors of the European Heart Journal or of the European Society of Cardiology.

Vincenzo Lionetti and Lucio Barile contributed equally to this work.

Conflict of interest: None declared.