Hypothesis

With age, circulating undercarboxylated osteocalcin (ucOC) declines, yet exogenous ucOC fails to fully rescue anabolic signaling in aged muscle despite restoring hormone levels. We propose that aging remodels GPRC6A coupling specificity in skeletal muscle, shifting the receptor from a Gq/IP3‑Ca2+‑driven mTOR anabolic state to a Gs/cAMP‑PKA‑biased state that preferentially activates IL‑6/STAT3‑dependent autophagy. This biased signaling, rather than mere ligand scarcity, underlies the observed dissociation between ucOC availability and muscle mass maintenance.

Mechanistic Rationale

1. Receptor bias via GRK/β‑arrestin remodeling – In young muscle, ucOC-bound GPRC6A preferentially engages Gq proteins, activating PLCβ, IP3‑mediated Ca2+ release, and downstream mTORC1 activation (see 2). Aging is associated with upregulation of G protein‑coupled receptor kinases (GRK2/3) and β‑arrestin2 in many tissues (5). Increased GRK activity phosphorylates the intracellular loops of GPRC6A, promoting β‑arrestin recruitment that sterically hinders Gq coupling while facilitating Gs activation (2). The resulting cAMP surge activates PKA, which phosphorylates CREB and drives IL‑6 transcription, engaging STAT3‑mediated autophagy (4).

2. Osteoclast‑derived exosomal microRNA modulation – Osteoclasts release exosomes enriched in miR‑21 during resorption (1). miR‑21 targets PTEN, amplifying PI3K‑Akt signaling, but also suppresses Gαq expression in myocytes (predicted by TargetScan). With age, osteoclast activity rises, increasing exosomal miR‑21 delivery to muscle, further dampening Gq availability and reinforcing the Gs bias.

3. Cross‑talk with OST‑PTP – OST‑PTP enhances ucOC bioavailability by promoting insulin signaling in osteoblasts (1). In aging, OST‑PTP expression declines, reducing matrix‑derived ucOC release. However, exogenous ucOC bypasses this step, allowing us to isolate receptor‑level effects.

Testable Predictions

• Prediction 1: In primary myoblasts from young (3‑month) vs. aged (24‑month) mice, ucOC stimulation will elicit a higher ratio of cAMP/IP3 accumulation in aged cells. Measurement via BRET‑based Gαq and Gαs biosensors will show a significant shift toward Gαs signaling with age (p < 0.01).
• Prediction 2: Pharmacological inhibition of GRK2 (e.g., with paroxetine) or siRNA‑mediated knockdown of β‑arrestin2 in aged myoblasts will restore the ucOC‑induced IP3/Ca2+ peak and increase phospho‑S6K (mTORC1 readout) to levels comparable to young cells.
• Prediction 3: Blocking exosomal miR‑21 transfer (using GW4869 to inhibit exosome release or antagomiR‑21) in aged muscle cultures will increase Gαq protein levels and rescue ucOC‑stimulated mTOR signaling without altering total GPRC6A expression.
• Prediction 4: In vivo, aged mice receiving ucOC (50 µg/kg) plus a GRK2 inhibitor will exhibit greater grip strength and muscle fiber cross‑sectional area than ucOC‑treated controls, demonstrating that biasing the receptor toward Gq restores the anabolic response.

Falsifiability

If ucOC stimulates equivalent Gq/IP3 signaling in young and aged muscle, or if GRK2/β‑arrestin manipulation fails to shift signaling bias or improve muscle outcomes, the hypothesis would be refuted. Likewise, if exosomal miR‑21 levels do not correlate with Gαq expression in aged muscle, the proposed osteoclast‑muscle communication mechanism would be unsupported.

Broader Impact

This work reframes the osteocalcin‑muscle axis from a simple ligand‑deficiency model to a dynamic receptor‑signaling perspective, offering novel targets (GRKs, β‑arrestin, exosomal miRNAs) for treating age‑related sarcopenia that bypass the need for high-dose hormone replacement.