Introduction
A research team from Pennsylvania State University (USA) and Westlake University jointly developed a material called CitraBoneQMg, which achieves the synergistic activation of mTORC1 and AMPK to support bone regeneration. In cellular metabolism regulation, these two classic pathways have long been regarded as antagonistic[1]. What makes this citrate-based material so special that it can resolve the long-standing "conflict" between the two pathways?
Research Article Information
- Journal: Science Advances (Science sub-journal)
- Title: Metabotissugenic citrate biomaterials orchestrate bone regeneration via citrate-mediated signaling pathways
- Authors: Hui Xu, Xinyu Tan, Su Yan, Jian Yang, Ethan Gerhard, Hao Zhang, Rohitraj Ray, Yuqi Wang, Sri-Rajasekhar Kothapalli, Elias B. Rizk, April D. Armstrong
- Publication Date: July 23, 2025
- Volume/Issue: Science Advances, Vol 11, Issue 30
1. What Does Bone Repair Require? How Do mTORC1 and AMPK Drive Repair?
CitraBoneQMg is a bone repair material designed around a core concept: integrating three natural metabolic substances—citric acid, glutamine, and magnesium—into a "fuel package for cellular energy factories," which is then embedded in a biodegradable scaffold.
This scaffold provides a foothold and living space for bone cells, slowly releases active substances to promote bone growth, and gradually degrades over time, eventually being replaced by new bone. In a rat cranial defect model, 12 weeks of observation after CitraBoneQMg implantation showed via CT imaging that the defect area was almost completely filled with new bone[2].
Figure Notes:
- (A, B, C, E) In the rat cranial defect experiment, CitraBoneQMg (labeled as BPLP-Gln-Mg/HA) showed significantly better bone regeneration effects compared to control materials (PLGA/HA, POC/HA) at 6 and 12 weeks.
How the Body’s Self-Repair System Works for Bone Defects
Before the advent of such advanced materials, when bone defects occur, the body’s self-repair system relies on "mesenchymal stem cells"—a reserve force that responds to injury signals by differentiating into osteoblasts (cells specialized in building new bone) to fill and repair the defect[2].
This repair process requires extensive support from both anabolic and catabolic metabolism. Two key metabolic sensors regulate this process: mTORC1 and AMPK. Ideally, both pathways need to be activated for efficient bone repair.
| Pathway | Activation Condition | Main Function |
|---|---|---|
| mTORC1 | Nutrient sufficiency | Promotes protein synthesis and cell proliferation (anabolism) |
| AMPK | Energy deprivation | Enhances mitochondrial function and catabolism to increase ATP production; inhibits mTORC1 to maintain cell survival via catabolism |
These two pathways also play important roles in anti-aging. For example, the classic anti-aging drug metformin[4] exerts its effects primarily through these pathways: it inhibits mitochondrial electron transport chain Complex I to activate AMPK (optimizing mitochondrial function and suppressing oxidative stress), and the activated AMPK indirectly inhibits mTORC1 (reducing excessive protein synthesis to delay cellular senescence).
2. Activation Pathways of mTORC1 and AMPK, and Their Antagonism
Traditional theory holds that the activation of these two pathways depends on two distinct energy states of the body—making them like "switches" that cannot be turned on simultaneously. Below is an overview of their respective activation pathways and classic antagonistic relationship.
AMPK Activation Under Energy Deprivation
The most common activation pathway of AMPK during energy shortage involves the phosphorylation of the Thr172 (threonine) site—the "activation switch" of AMPK[1].
Figure Note: AMPK activation pathway—AMP binds to the CBS domains (CBS1, CBS3) of AMPK’s γ subunit, triggering conformational changes that expose the Thr172 site for phosphorylation.
AMPK can also be activated through both canonical and non-canonical mechanisms:
- Canonical mechanism: Triggered by energy stress (e.g., high AMP/ADP ratio), glucose starvation, or DNA damage, involving kinases like LKB1.
- Non-canonical mechanism: Activated by calcium signals (e.g., increased intracellular Ca²⁺) via the Ca²⁺/calmodulin-dependent kinase CaMKK2[3].
Once activated, AMPK upregulates catabolism (e.g., via targets like ULK1, ACC1) and downregulates anabolism (e.g., by inhibiting mTORC1).
mTORC1 Activation Under Nutrient Sufficiency
mTORC1 activation primarily occurs on the surface of lysosomes and is regulated by amino acids and growth factors when nutrients are abundant[1]:
- Growth factor-mediated activation: Insulin or other growth factors bind to cell surface receptors, increasing the activity of GTP (guanosine triphosphate) in lysosomal complexes to activate mTORC1.
- Amino acid-mediated activation: Key amino acids (e.g., leucine, glutamine) promote the binding of cytoplasmic proteins to the lysosomal surface, where these proteins interact to activate mTORC1.
Figure Note: Mechanism of mTORC1 activation—growth factors and amino acids drive mTORC1 recruitment to the lysosomal surface, where it is activated by Rheb-GTP.
Antagonism Between mTORC1 and AMPK
The two pathways counteract each other under different energy states:
-
AMPK inhibits mTORC1 (energy deprivation):
- AMPK activates the TSC1/2 protein complex (enhancing its inhibitory effect on mTORC1).
- AMPK phosphorylates RAPTOR (a scaffold protein of mTORC1), suppressing RAPTOR function and thus inhibiting mTORC1[1].
-
mTORC1 inhibits AMPK (energy sufficiency):
- When glucose levels are high (no need for AMPK-mediated energy production), mTORC1 downregulates the insulin signaling pathway, inhibiting the phosphorylation of AMPK’s Thr172 site and thus suppressing (but not completely eliminating) AMPK signaling[1].
3. How CitraBoneQMg Synergistically Activates AMPK and mTORC1
The magic of CitraBoneQMg lies in its ability to resolve the "conflict" between these two opposing pathways, enabling them to work together to guide bone cells in efficiently producing new bone.
Simultaneous Activation of AMPK and mTORC1
CitraBoneQMg’s three main components (citric acid, glutamine, magnesium) synergistically enhance mitochondrial oxidative phosphorylation efficiency, increasing intracellular ATP levels by approximately 50% compared to the control group and boosting overall energy production.
1. Overcoming Energy Dependence: Calcium Signals Activate AMPK
Traditionally, AMPK is only activated under energy deprivation. However, researchers found that the upstream kinase CaMKK2 can activate AMPK even under nutrient sufficiency[3].
When cells receive signals from CitraBoneQMg’s three components (citric acid, glutamine, magnesium), intracellular calcium ion concentration increases. Elevated Ca²⁺ then activates CaMKK2, which directly phosphorylates AMPK’s Thr172 site to activate the pathway—independent of energy status.
2. Resolving Pathway Antagonism: AKT Pathway Activates mTORC1
Under energy sufficiency, mTORC1 is normally activated—but traditional theory suggests AMPK (now activated by calcium signals) would inhibit it. CitraBoneQMg solves this by:
- Activating the AKT pathway: Citric acid and glutamine released by the material activate the AKT pathway.
- Inhibiting TSC1/2: Activated AKT suppresses the TSC1/2 protein complex—directly counteracting AMPK’s activation of TSC1/2.
- Unlocking mTORC1: This suppression of TSC1/2 relieves mTORC1 from AMPK’s inhibition, allowing mTORC1 to remain active and sustain anabolism.
In summary, CitraBoneQMg uses a two-pronged strategy—calcium-signal-mediated AMPK activation and AKT-mediated TSC1/2 isolation—to break the traditional antagonism between AMPK and mTORC1, supporting bone regeneration.
4. Differences from Traditional Bone Regeneration Materials
Compared to traditional bone regeneration materials (e.g., POC/HA, a citrate-based composite), CitraBoneQMg’s three active components offer additional benefits:
- Anti-inflammatory effect: Significantly increases the expression of the anti-inflammatory factor IL-10 in macrophages.
- Neuroregenerative effect: Promotes the expression of the neuroregeneration marker TUBB3.
Key Advantages of CitraBoneQMg
- Traditional materials: Act as passive "fillers," relying solely on physical support for bone ingrowth.
- CitraBoneQMg: Functions as an active "cellular metabolism commander," synchronously driving bone regeneration, immune regulation, and neuroreconstruction.
Notably, the novel degradable citrate material developed by Professor Jian Yang (a core member of the research team) is currently the only FDA-approved biodegradable thermosetting polyester polymer for medical implant devices worldwide[5]—laying a critical foundation for the clinical translation of next-generation bone regeneration materials like CitraBoneQMg.
Conclusion
Researchers note that details of mechanisms (e.g., calcium-mediated AMPK regulation) require further investigation. However, as biomaterials learn to "communicate" with cells, we draw closer to the vision of intelligent regenerative medicine.
References
[1] González, A., Hall, M. N., Lin, S. C., & Hardie, D. G. (2020). AMPK and TOR: The Yin and Yang of Cellular Nutrient Sensing and Growth Control. Cell Metabolism, 31(3), 472–492. https://doi.org/10.1016/j.cmet.2020.01.015[2] Xu, H., Tan, X., Gerhard, E., Zhang, H., Ray, R., Wang, Y., Kothapalli, S. R., Rizk, E. B., Armstrong, A. D., Yan, S., & Yang, J. (2025). Metabotissugenic citrate biomaterials orchestrate bone regeneration via citrate-mediated signaling pathways. Science Advances, 11(30), eady2862. https://doi.org/10.1126/sciadv.ady2862[3] Hawley, S. A., Pan, D. A., Mustard, K. J., Ross, L., Bain, J., Edelman, A. M., Frenguelli, B. G., & Hardie, D. G. (2005). Calmodulin-dependent protein kinase kinase-β is an alternative upstream kinase for AMP-activated protein kinase. Cell Metabolism, 2(1), 9–19. https://doi.org/10.1016/j.cmet.2005.05.009[4] Zhang, T., Zhou, L., Makarczyk, M. J., Feng, P., & Zhang, J. (2025). The Anti-Aging Mechanism of Metformin: From Molecular Insights to Clinical Applications. Molecules (Basel, Switzerland), 30(4), 816. https://doi.org/10.3390/molecules30040816[5] Xu, H., Yan, S., Gerhard, E., Xie, D., Liu, X., Zhang, B., Shi, D., Ameer, G. A., & Yang, J. (2024). Citric Acid: A Nexus Between Cellular Mechanisms and Biomaterial Innovations. Advanced Materials (Deerfield Beach, Fla.), 36(32), e2402871. https://doi.org/10.1002/adma.202402871