Osteocalcin (BGLAP): Bone-Derived Hormone Promoting Testosterone, Muscle Glucose Uptake, Memory, and Insulin Secretion via GPRC6A in Research

# Osteocalcin (BGLAP): Bone-Derived Hormone Promoting Testosterone, Muscle Glucose Uptake, Memory, and Insulin Secretion via GPRC6A in Research

Osteocalcin is a 49-amino acid protein produced exclusively by mature osteoblasts that has emerged as a multifunctional bone-derived hormone with pleiotropic effects on endocrine organs, skeletal muscle, and the brain. For decades considered primarily a bone matrix structural protein and a clinical marker of bone turnover, osteocalcin was fundamentally re-characterized beginning in the mid-2000s by Gerard Karsenty's laboratory at Columbia University as an endocrine protein that coordinates bone function with energy metabolism, reproduction, muscle performance, and cognitive function. This re-framing established bone as a genuine endocrine organ and osteocalcin as its first identified metabolic hormone, opening a new paradigm in musculoskeletal and metabolic medicine.

Background: Bone as an Endocrine Organ

The concept that bone is a metabolic endocrine organ was established through a series of landmark papers from Karsenty's group using genetically engineered mice:

• •Osteocalcin-deficient (Ocn−/−) mice develop obesity, glucose intolerance, insulin resistance, and hypogonadism despite normal bone mass — the first indication that osteocalcin had metabolic effects
• •Overexpression of undercarboxylated osteocalcin improved glucose tolerance, increased insulin secretion, reduced fat mass, and increased testosterone in male mice
• •The endocrine effects were reproduced by injection of recombinant undercarboxylated osteocalcin

These findings are now placed in a broader context of bone endocrinology alongside lipocalin-2 (bone-derived MC4R agonist for appetite suppression, discussed elsewhere), sclerostin (bone-derived Wnt inhibitor with systemic effects), and FGF23 (bone-derived phosphate regulator with cardiovascular effects in CKD).

Gene and Protein Structure

BGLAP Gene

The human BGLAP gene is located on chromosome 1q25.1 (with additional related sequences nearby). It encodes a 100-amino acid prepropeptide that is processed by:

1. Signal peptide cleavage (residues 1-26): yields a 74-amino acid propeptide

2. Propeptide cleavage (residues 27-49 remain as mature osteocalcin): yields the 49-amino acid mature secreted protein

The mature osteocalcin protein has a molecular weight of approximately 5.8 kDa. It contains three key glutamic acid residues at positions 17, 21, and 24 that can be carboxylated to gamma-carboxyglutamate (Gla) by vitamin K-dependent gamma-glutamyl carboxylase. Carboxylation is required for strong calcium binding to hydroxyapatite in bone matrix.

Carboxylated vs. Undercarboxylated Osteocalcin

The central regulatory feature of osteocalcin biology is the vitamin K-dependent post-translational modification of glutamate to Gla:

Carboxylated osteocalcin (Gla-OC, cOC): Contains 3 Gla residues; binds hydroxyapatite with high affinity (Kd ~nM); primarily retained in bone matrix; released slowly during bone resorption by osteoclasts; lower hormonal activity.

Undercarboxylated osteocalcin (Glu-OC, ucOC): Has ≤2 Gla residues (partially or fully uncarboxylated); reduced hydroxyapatite affinity; released from bone more readily; the biologically active hormonal form; activates GPRC6A receptor.

Osteocalcin is produced and secreted by osteoblasts. A fraction is incorporated into bone matrix (carboxylated form); the rest circulates in plasma. During bone remodeling, osteoclasts resorb old bone matrix and release stored osteocalcin, including decarboxylated forms (acid pH in the resorption lacuna promotes decarboxylation). This "bone resorption-activated osteocalcin" model means that osteoclast activity releases a wave of hormonal osteocalcin.

This is regulatory by design: skeletal remodeling (which releases osteocalcin) typically occurs post-prandially (insulin stimulates osteoblast activity; osteocalcin feeds back to stimulate more insulin) and during exercise — both contexts where increased muscle glucose uptake and enhanced energy metabolism are beneficial.

Receptor: GPRC6A

GPRC6A is a Class C GPCR (same family as GABAB receptors, metabotropic glutamate receptors, and the calcium-sensing receptor CaSR) with:

• •A large Venus flytrap (VFT) domain in the extracellular N-terminus (common to Class C GPCRs)
• •Coupling primarily through Gαs/cAMP and Gαq/Ca²⁺ pathways depending on ligand and cell type
• •Ligand promiscuity: GPRC6A responds to L-amino acids (particularly ornithine, arginine), cations (Ca²⁺, Mg²⁺, Zn²⁺), and undercarboxylated osteocalcin

GPRC6A expression is widespread: testicular Leydig cells, pancreatic β-cells, skeletal muscle, liver, brain (hippocampus, hypothalamus), adipose tissue, and immune cells.

An important caveat: While GPRC6A is the primary characterized osteocalcin receptor, a second receptor or co-receptor mechanism has been proposed for some osteocalcin effects (particularly in the brain), and the precise osteocalcin-GPRC6A binding interaction has been contested in some in vitro binding studies. The in vivo genetic evidence (Gprc6a-knockout mice recapitulate many osteocalcin-deficient phenotypes) is stronger than the in vitro pharmacology.

Hormonal Functions

Testosterone Synthesis (Male Reproductive Biology)

Karsenty's group made the provocative finding that osteocalcin promotes testosterone synthesis in Leydig cells of the testis through GPRC6A:

• •Osteocalcin increases cAMP in Leydig cells → PKA activation → StAR (steroidogenic acute regulatory protein) upregulation → cholesterol transport into mitochondria → testosterone synthesis
• •Osteocalcin also increases expression of Cyp11a1 (P450scc), Cyp17a1, and HSD17b3 — testosterone biosynthetic enzymes
• •Ocn−/− male mice have ~60% lower testosterone and reduced fertility
• •GPRC6A-deficient male mice have similar reproductive phenotype
• •Recombinant undercarboxylated osteocalcin restores testosterone in Ocn−/− males

In humans, plasma osteocalcin (total or ucOC) positively correlates with testosterone levels in multiple cross-sectional studies of men. Exercise-induced osteocalcin increase during physical activity correlates with acute testosterone elevations.

This bone-testis axis represents a remarkable biological coupling: the mechanical and metabolic demands placed on bone (exercise) stimulate osteocalcin release, which signals the gonads to increase testosterone production, potentially as an adaptive response to ensure adequate androgenic support for muscle hypertrophy and recovery.

Insulin Secretion and Glucose Metabolism

Osteocalcin stimulates pancreatic β-cells through GPRC6A:

• •Increases intracellular cAMP and Ca²⁺ in β-cells
• •Upregulates insulin gene expression (Ins1, Ins2)
• •Promotes insulin secretion (glucose-stimulated and basal)
• •May also promote β-cell proliferation in some models

Insulin in turn acts on osteoblasts through the insulin receptor to:

1. Stimulate osteocalcin production

2. Suppress Esp (encoding OST-PTP, a tyrosine phosphatase that inactivates the IR on osteoblasts)

3. Increase osteoblast activity and bone resorption

4. Release more osteocalcin

This insulin-osteocalcin positive feedback loop (insulin stimulates bone → bone releases osteocalcin → osteocalcin stimulates more insulin) could contribute to maintaining glucose homeostasis during the post-prandial state.

Skeletal Muscle Glucose Uptake During Exercise

• •Acute exercise increases plasma osteocalcin (5-minute intravenous osteocalcin infusion during exercise in mice improved performance)
• •Osteocalcin via GPRC6A in muscle activates AMPK and Akt → GLUT4 translocation → glucose uptake
• •Osteocalcin activates PPARγ coactivator-1α (PGC-1α) in muscle, promoting mitochondrial biogenesis
• •Ocn−/− mice show impaired exercise capacity, reduced muscle glucose uptake, and reduced RER during exercise
• •Recombinant osteocalcin infusion during exercise restored performance in Ocn−/− mice

Extension to humans: Post-exercise plasma osteocalcin levels correlate with exercise performance metrics in athletes. Some studies have found that older adults with higher baseline osteocalcin levels have better cardiorespiratory fitness and muscle mass.

Memory and Cognitive Function

• •Osteocalcin crosses the blood-brain barrier through a specific transport mechanism
• •GPRC6A is expressed on hippocampal CA3 pyramidal neurons
• •Osteocalcin via GPRC6A promotes neurotransmitter synthesis: serotonin (5-HT), dopamine, and possibly norepinephrine
• •It induces BDNF (brain-derived neurotrophic factor) expression
• •Recombinant osteocalcin administered peripherally improved spatial memory in both young and old mice
• •Ocn−/− mice show anxiety-like behavior and spatial memory deficits
• •Osteocalcin injections in middle-aged mice rescued the cognitive performance to young-mouse level

This brain connection potentially explains why exercise (which increases osteocalcin) improves cognitive function and mood, beyond previously identified mechanisms (BDNF, VEGF). Osteocalcin may be an exercise-derived brain hormone.

Clinical Correlates in Humans

Plasma Osteocalcin as Biomarker

Total serum osteocalcin is a widely used clinical marker of bone turnover (along with P1NP and β-CTX). Reference values are approximately 10-40 ng/mL in adult men, 10-45 ng/mL in pre-menopausal women. For the hormonal (ucOC) fraction, specialized assays are required.

Clinical associations:

• •Aging: Osteocalcin declines with age; the age-related cognitive decline and testosterone reduction may partly reflect osteocalcin deficiency
• •Obesity: Plasma osteocalcin (total and ucOC) inversely correlates with BMI, fasting insulin, and HOMA-IR
• •Type 2 Diabetes: Significantly lower in T2DM; inversely correlates with HbA1c
• •Metabolic Syndrome: Lower in MetS; each MetS component inversely associates with osteocalcin
• •Exercise: Acute aerobic and resistance exercise increases plasma osteocalcin within 30-60 minutes; longer-term training increases resting levels
• •Vitamin K and warfarin: Vitamin K deficiency or warfarin anticoagulation reduces carboxylation of osteocalcin → increases ucOC fraction (the hormonal form), potentially enhancing hormonal effects, though the total available osteocalcin is the limiting factor

Sex Differences

The testosterone-promoting bone-testis axis creates inherent sex differences in osteocalcin biology:

• •In men: lower osteocalcin correlates with lower testosterone, reduced muscle mass, and insulin resistance
• •In women: osteocalcin correlates with insulin sensitivity and adiposity but the testosterone axis is less prominent; estrogen protects bone and may maintain osteocalcin levels independently

Research Tools and Experimental Systems

Current Research Frontiers

Osteocalcin supplementation for aging: Animal data showing osteocalcin rescues age-related cognitive decline, testosterone reduction, and metabolic impairment in mice have motivated interest in osteocalcin supplementation as an anti-aging intervention. Small pilot human trials of oral or subcutaneous osteocalcin are being planned or conducted.

Exercise prescription via osteocalcin: Understanding which types of exercise (resistance vs. aerobic, high-impact vs. low-impact, acute vs. chronic) most effectively raise osteocalcin could guide exercise prescriptions aimed at maximizing bone endocrine output for metabolic and cognitive benefit.

Osteocalcin and dementia: The brain connection, combined with the well-established association between bone turnover markers and dementia risk in observational studies, motivates investigation of whether maintaining osteocalcin levels (through exercise, bone-protective therapies) could reduce dementia risk.

Controversy and human translation: While the mouse genetics and recombinant protein infusion data are compelling, the direct translation to humans has been more variable. Some but not all human intervention studies confirm osteocalcin-mediated improvements in glucose metabolism, testosterone, and cognition. The relative importance of osteocalcin vs. other exercise-induced factors (irisin, METRNL, IGF-1, BDNF) in exercise-induced metabolic and cognitive benefits remains debated.

Sclerostin-osteocalcin axis: Sclerostin (Wnt inhibitor produced by osteocytes, suppresses bone formation) and osteocalcin (osteoblast marker, promotes energy metabolism) may form an antagonistic axis in bone endocrinology, with therapeutic implications for diseases where sclerostin inhibition (romosozumab for osteoporosis) simultaneously raises osteocalcin.

Conclusion

Osteocalcin's transformation from a bone matrix structural protein to a bone-derived endocrine hormone with roles in testosterone synthesis, glucose homeostasis, exercise physiology, and memory formation represents one of the most significant conceptual shifts in modern endocrinology. The Karsenty group's systematic use of genetically modified mice to dissect bone endocrinology revealed that bone is not merely a passive calcium reservoir but an active endocrine organ that communicates via multiple hormones (osteocalcin, lipocalin-2, FGF23, sclerostin) with metabolically critical tissues. The specific coupling of exercise-induced bone remodeling → osteocalcin release → testosterone, muscle glucose uptake, and memory consolidation positions osteocalcin as a key mediator of exercise's pleiotropic benefits. Ocn−/− mice, recombinant undercarboxylated osteocalcin, and GPRC6A pharmacology are the primary research tools; validated ucOC assays for human plasma are available but not yet widely standardized across clinical laboratories.

Key References

1. Lee NK, Sowa H, Hinoi E, et al. Endocrine regulation of energy metabolism by the skeleton. Cell. 2007;130(3):456-469. PMID: 17693256

2. Ferron M, Hinoi E, Karsenty G, Ducy P. Osteocalcin differentially regulates beta cell and adipocyte gene expression and affects the development of metabolic diseases in wild-type mice. Proc Natl Acad Sci USA. 2008;105(13):5266-5270. PMID: 18362359

4. Mera P, Laue K, Ferron M, et al. Osteocalcin signaling in myofibers is necessary and sufficient for optimum adaptation to exercise. Cell Metab. 2016;23(6):1078-1092. PMID: 27211901

6. Karsenty G, Olson EN. Bone and muscle endocrine functions: unexpected paradigms of inter-organ communication. Cell. 2016;164(6):1248-1256. PMID: 26967290

9. Khrimian L, Obri A, Ramos-Brossier M, et al. Gpr158 mediates osteocalcin's regulation of cognition. J Exp Med. 2017;214(10):2859-2873. PMID: 28827448

10. Ducy P, Desbois C, Boyce B, et al. Increased bone formation in osteocalcin-deficient mice. Nature. 1996;382(6590):448-452. PMID: 8684485

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This article is intended for Research Use Only (RUO). The information provided describes laboratory research findings and does not constitute medical advice. Osteocalcin and related bone endocrine proteins are research tools and investigational agents. All research applications must comply with applicable institutional, local, and national regulations.