Jian Yang1
Westlake University1
Background: Although significant progress has been made in the development of orthopedic biomaterials, the currently available materials are limited by their inabilities to mimic the native tissue composition, weak mechanical strength, minimal osteoinductivity, significant inflammatory responses, poor bone integration, and slow bone regeneration. Metabolic factors such as glucose were shown to regulate the energy needs of stem cells during osteoblast differentiation. However, a full picture of the metabolic processes guiding or supporting osteogenic differentiation is far from complete. Citrate is a small molecule participating as the starting metabolite in the TCA cycle, is also a key component of bone. However, very limited information is available on what role the citrate plays in bone development and bone biomaterial design despite of its well-known unusual high abundance in bone.<br/><br/>Methods: In this study, a new class of highly versatile and functional citrate-based biomaterials has been developed by reacting citric acid with various carefully selected compounds including amino acids, diols and other functional molecules. We seek to answer several unexplored questions: 1) Can exogenous soluble citrate be uptaken by MSCs? 2) What is the role of extracellular citrate on the energy metabolism of bone forming cells? 3) Can the citrate-regulated cell energy metabolism crosstalk with MSCs signaling pathways involved in osteogenic differentiation? 4) Can the citrate-presenting materials mediate MSCs differentiation through the same mechanism as the soluble citrate? We also explore how these understandings can be applied to novel citrate-presenting biomaterial design and validate if the use of citrate-presenting anatomically similar and mechanically compliant scaffolds can enhance tissue integration and bone regeneration in a critically sized bone defect model.<br/><br/>Results: Our studies showed that extracellular citrate uptake through solute carrier family 13, member 5 (SLC13a5) supported osteogenic differentiation via regulation of energy-producing metabolic pathways, which led to elevated cell energy status to fuel osteo-differentiation of hMSCs with high metabolic demands. We next identified citrate and phosphoserine (PSer) as a synergistic pair in polymeric design, exhibiting concerted action not only in metabonegenic potential for orthopedic regeneration, but also in facile reactivity into a fluorescent system for materials tracking and imaging. We herein designed a novel, citrate/phosphoserine-based photoluminescent biodegradable polymer (BPLP-PSer), which was lastly fabricated into BPLP-PSer/hydroxyapatite composite microparticulate scaffolds, demonstrating significant improvements in bone regeneration and tissue response in rat femoral condyle and cranial defect models.<br/><br/>Discussion and Conclusion: We have revealed a previously unexplored expression pattern of SLC13a5 citrate transporter along osteo-differentiation, and a mechanism focusing on the metabolic regulation of citrate to elevate cell energy status for bone formation, referred to as citrate metabonegenic regulation. These findings not only identify citrate as a new metabolic factor in the stem cell micro-environment favorable for osteo-differentiation, but also suggest that citrate should be considered in bone biomaterials design, providing guidance to develop biomimetic BPLP-PSer/HA for prolonged citrate metabonegenic effect well into late stage of differentiation, which demonstrated therapeutic potential for bone injuries [1-3]. The unprecedented knowledge on the citrate mechanism enables us to design the next generation of biomimetic dynamic orthopedic implants that may present citrate signals in demand during cellular and tissue development.<br/><br/>References:<br/>Xinyu Tan, et.al. Small 2022 Sep;18(36):e2203003.<br/>Chuying Ma, et.al. Advanced Science 2019, 1900819.<br/>Chuying Ma, et. al. PNAS 2018, 115 (50): E11741-E11750