Myokines: Muscle-Derived Signaling Molecules

Myokines are cytokines, peptides, and other small proteins produced and released by skeletal muscle fibers during contraction. The term was coined by Bente Klarlund Pedersen in 2003, reflecting the recognition that skeletal muscle functions as an endocrine organ — not merely a mechanical system for locomotion but an active secretory tissue that communicates with the brain, liver, adipose tissue, bone, pancreas, and immune system through hundreds of signaling molecules.

This concept fundamentally reshaped exercise physiology. The systemic health benefits of physical activity — reduced inflammation, improved insulin sensitivity, enhanced cognition, lower cancer risk — are not simply byproducts of calorie expenditure. They are mediated, at least in part, by the specific molecular signals that contracting muscle releases into the circulation.

The muscle-as-endocrine-organ concept

Skeletal muscle constitutes approximately 40% of total body mass, making it the largest organ in the human body by weight. Proteomic studies have identified over 600 proteins secreted by muscle cells (the "muscle secretome"), though only a fraction of these have been functionally characterized as myokines with confirmed endocrine or paracrine roles.

The secretion of myokines is exercise-dependent: resting muscle produces low basal levels, while contractile activity dramatically increases production and release. This exercise-dependence distinguishes myokines from adipokines (adipose-derived) or hepatokines (liver-derived), though some molecules (such as IL-6) are produced by multiple tissues.

Key myokines

Interleukin-6 (IL-6): the prototypical myokine

IL-6 was the first identified myokine and remains the most thoroughly studied. Plasma IL-6 levels can increase up to 100-fold during prolonged exercise, with peak levels occurring immediately at exercise cessation. The magnitude of increase depends on exercise duration, intensity, and muscle mass involved.

The exercise-derived IL-6 response is fundamentally different from the chronic low-grade IL-6 elevation seen in obesity and metabolic disease:

• Acute exercise IL-6 — released transiently from muscle fibers without prior TNF-alpha activation. It stimulates hepatic glucose output during exercise (maintaining blood glucose), enhances lipolysis in adipose tissue, promotes anti-inflammatory cytokine release (IL-10, IL-1ra), and improves insulin sensitivity in the post-exercise period.
• Chronic inflammatory IL-6 — produced by macrophages and adipocytes in the context of sustained inflammation. It contributes to insulin resistance, is associated with TNF-alpha co-elevation, and drives pathological signaling.

This dual role illustrates a recurring principle in biology: context determines function. The same molecule can be protective or harmful depending on the source, duration, and accompanying signals.

Irisin: linking exercise to fat metabolism

Irisin was identified in 2012 by Bruce Spiegelman's group as a cleavage product of fibronectin type III domain-containing protein 5 (FNDC5), a transmembrane protein expressed on muscle cell surfaces. During exercise, the transcriptional coactivator PGC-1alpha is upregulated in muscle, which increases FNDC5 expression. The extracellular domain of FNDC5 is then cleaved and released into the circulation as irisin.

Irisin's most notable reported function is the "browning" of white adipose tissue — it stimulates the expression of uncoupling protein 1 (UCP1) in white fat cells, converting them into beige/brite adipocytes that dissipate energy as heat rather than storing it. This mechanism theoretically links exercise to improved metabolic efficiency and thermogenesis.

However, irisin research has been controversial. Early studies used antibodies with questionable specificity, and some groups failed to detect meaningful circulating irisin in humans. A 2015 mass spectrometry study confirmed that irisin does circulate in human plasma but at very low concentrations (approximately 3.6 ng/mL), and that exercise increases levels modestly. The physiological significance of these low concentrations in humans remains debated, and clinical translation has not materialized as initially hoped.

BDNF: the exercise-brain connection

BDNF is produced by contracting skeletal muscle, though the majority of exercise-induced circulating BDNF increase likely originates from the brain itself (particularly the hippocampus) rather than from muscle. Nevertheless, muscle-derived BDNF acts in an autocrine/paracrine fashion to enhance fatty acid oxidation within muscle tissue via AMPK activation.

The exercise-BDNF-cognition axis is one of the most robust findings in neuroscience: regular aerobic exercise increases hippocampal BDNF, promotes neurogenesis, and improves cognitive function across the lifespan. Whether muscle-derived BDNF crosses the blood-brain barrier in meaningful quantities or whether the brain autonomously upregulates BDNF in response to exercise-related signals (lactate, irisin, cathepsin B) remains an area of active investigation.

Myostatin: the negative regulator

Myostatin (GDF-8), a member of the TGF-beta superfamily, is a unique myokine in that it acts as a negative regulator of muscle growth. Secreted by muscle fibers, myostatin signals through activin type II receptors to inhibit myoblast proliferation and differentiation. Loss-of-function mutations in myostatin produce dramatic muscle hypertrophy in animals (the "double-muscled" cattle phenotype) and, in one documented human case, exceptional muscularity from birth.

Exercise reduces myostatin expression, removing the brake on muscle growth and contributing to exercise-induced hypertrophy. This mechanism is particularly relevant to the peptide field because myostatin-inhibiting biologics (such as follistatin and activin receptor decoys) have been investigated as therapeutic approaches for sarcopenia and muscular dystrophies, with mixed clinical results to date.

Meteorin-like (Metrnl): immune-metabolic bridge

Meteorin-like (Metrnl) is a myokine induced by exercise (particularly in the context of PGC-1alpha4 activation) and by cold exposure. It stimulates eosinophil activity and alternatively activated macrophage polarization in adipose tissue, promoting an anti-inflammatory immune environment that enhances thermogenesis and glucose tolerance. Metrnl represents the growing recognition that exercise-immune-metabolism crosstalk is mediated by specific molecular intermediaries.

The exercise-myokine-health axis

The myokine concept provides a mechanistic framework for understanding why physical inactivity is a major risk factor for chronic disease. When muscles are inactive, myokine secretion drops, removing the anti-inflammatory, insulin-sensitizing, neuroprotective, and metabolic signals that contracting muscle provides. Pedersen has proposed the term "diseasome of physical inactivity" to describe the cluster of chronic diseases (type 2 diabetes, cardiovascular disease, cancer, depression, dementia) that share physical inactivity as a common risk factor and reduced myokine signaling as a potential common mechanism.

This framework also explains exercise's pleiotropic benefits — a single intervention (exercise) produces diverse organ-level effects because muscle secretes a cocktail of signaling molecules that reach multiple target tissues simultaneously.

Connection to peptide biology

The myokine field intersects with peptide biology in several ways:

• Endogenous precedent — myokines demonstrate that endogenous peptides and small proteins, released in response to physiological stimuli, can produce systemic health effects at nanomolar concentrations. This provides a biological framework for understanding how exogenous peptide administration might produce therapeutic effects.
• Target overlap — several exogenous peptides under investigation target the same pathways that myokines engage. For example, peptides that activate AMPK share a downstream mechanism with exercise-induced IL-6; BDNF-enhancing peptides (Semax, Cerebrolysin) augment the same neurotrophin that exercise upregulates.
• Myostatin inhibition — the development of peptide and protein therapeutics aimed at blocking myostatin signaling (follistatin, ACE-031) directly targets a myokine pathway.

However, it is important to recognize that exercise produces a coordinated, temporally regulated release of hundreds of myokines simultaneously, along with metabolic changes (lactate production, glycogen depletion, redox shifts) that modulate their effects. No single exogenous peptide replicates this coordinated response, and the health benefits of exercise almost certainly depend on the integrated action of multiple signaling molecules rather than any single myokine.

Clinical relevance and research gaps

Myokine research is still maturing. Key unresolved questions include: which myokines are necessary and sufficient for specific exercise benefits; whether myokine-based therapeutics can replicate exercise effects in individuals unable to exercise (the "exercise mimetic" concept); and how muscle mass, fiber type composition, and training status affect the myokine secretome. The identification of new myokines continues, and translating secretome discoveries into validated clinical biomarkers or therapeutic targets remains a significant challenge. Large-scale human studies with standardized exercise protocols and proteomic methods are needed to move beyond the current landscape of preclinical observations and small cohort studies.