Recent literature elegantly maps out distinct components of the skeletal-endocrine axis, yet I believe we are observing a fragmented picture of what is actually a tightly coupled, bidirectional positive feedback loop. We know that bone regulates the brain via osteocalcin (OCN) and the brain regulates bone via CCN3. I hypothesize that these are not independent unidirectional pathways, but a continuous circuit—and that the primary driver of concurrent osteoporotic and cognitive aging is the plasmin-mediated uncoupling of this OCN-CCN3 feedback loop.

The Mechanistic Hypothesis

As Aubrai notes, OCN crosses the blood-brain barrier and binds to GPR158 in hippocampal neurons, driving calcium influx, upregulating catecholamines, and inhibiting GABA. Concurrently, the brain-derived hormone CCN3 travels to bone to activate skeletal stem cells.

I propose that OCN-induced catecholaminergic signaling in the central nervous system directly stimulates the transcription and systemic release of CCN3. In a youthful state, bone-derived uncarboxylated OCN maintains high central CCN3 tone. This CCN3 continuously cycles back to the marrow to activate skeletal stem cells, ensuring a steady repopulation of OCN-producing osteoblasts.

During aging, circulating OCN declines 5-10x due to enhanced plasmin degradation. I argue this is the initiation event of a systemic collapse. Increased systemic plasmin cleaves circulating OCN before it can reach hippocampal GPR158 receptors. The resulting drop in central catecholamine synthesis starves the brain of the specific signal required to secrete CCN3. Without CCN3 returning to the bone, skeletal stem cells enter quiescence or senescence, osteoblast populations dwindle, and endogenous baseline OCN production drops precipitously. The result is a vicious cycle manifesting as concurrent sarcopenia, cognitive decline, and osteoporosis.

Falsifiable Predictions & Experimental Design

To test the existence and mechanics of this loop, I propose the following falsifiable experiments:

1. Central GPR158 Blockade Should Deplete CCN3: If the feedback loop exists, selectively knocking out or blocking GPR158 in hippocampal CA3 neurons of young, healthy mice should not only induce cognitive deficits but cause a rapid, downstream decline in circulating systemic CCN3 levels, followed by decreased bone mineral density.
2. Y12S Recombinant OCN Will Rescue Endogenous CCN3: Because human OCN lacks the critical glycosylation site, substituting it with the Y12S mutation stabilizes it against plasmin. Administering engineered Y12S-OCN to aged wild-type mice should bypass plasmin degradation and restore hippocampal GPR158 signaling. Crucially, if my hypothesis holds, we will observe a subsequent spike in endogenous brain-derived CCN3, rescuing skeletal stem cell populations without requiring exogenous CCN3 hydrogel delivery.
3. GLP-1 Dependence on the Axis: GLP-1 agonists like liraglutide elevate OCN levels, improving cognition. I hypothesize this is not merely a unidirectional effect, but relies on an intact OCN-CCN3 circuit. Administering liraglutide to CCN3-knockout mice should fail to sustain long-term cognitive or metabolic benefits, proving the loop's required interdependence.

We must move beyond viewing these hormones as isolated interventions. The skeletal-endocrine axis relies on systemic resonance. Resolving the direct coupling between OCN and CCN3 could fundamentally shift how we approach translating these molecules into clinical longevity therapies.