Small RNA-Modulated Exosome Mimetics For Craniofacial Regeneration

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Abstract Large craniofacial defects remain an extraordinary challenge to clinical surgical reconstruction. Conventional approaches of auto/allografting for clinical craniofacial reconstruction are significantly compromised by availability and donor-site morbidity. Mesenchymal stem cells (MSCs) with multipotency are increasingly employed as a cell-based approach for skeletal regeneration. Nonetheless, accumulative evidences suggest that contribution of MSCs to regenerated tissues is limited while stimulation of local healing processes through paracrine secretion exerts more important roles. MSC-derived extracellular vesicles (EVs) have shown regenerative potency in varying animal models and displayed therapeutic advantages like intrinsic homing effect, stability, low immunogenicity and effective signaling stimulation. However, the widespread use of exosome-mediated treatment still requires the significant improvement of production yield and regenerative ability. Developing a scalable approach like generation of exosome mimetics (EMs) with substantial yields has been investigated in previous studies. Additionally, exosome-mediated cargo of exogenous therapeutic factors like siRNAs has been conducted to improve its regenerative capability. The exogenous transport of these factors is also growing concerns of high cost, poor pharmacokinetics and inefficiency. Herein, the augmentation of intrinsic inductive/therapeutic molecules within exosomes exhibits a promising therapeutic strategy. Skeletal cells secrete important growth factors like BMP2, which incite osteoblastic commitment of skeletal progenitor cells and subsequent mineral deposition. In response to BMP stimulus, MSCs or osteoblasts elevate BMP antagonist like noggin, suggesting a negative feedback to prevent overexposure of BMP signaling. Introduction of exogenous noggin was revealed to impair cranial formation while inhibition of endogenous noggin promoted cranial regeneration by activating endogenous BMP signaling. These observations highlight the potency of noggin suppression on up-regulation of endogenous BMP activity and subsequent osseous deposition. In our preliminary studies, EMs were generated from MSCs via a distinctive extrusion approach, demonstrating high yields of exosomes with apparent osteogenic induction. Moreover, EMs obtained from noggin-suppression MSCs (EM-NG) revealed the elevated noggin siRNA and osteogenic potency. Together, we hypothesize that EM-NG could enhance osteoblastic commitment of endogenous skeletal stem cells (SSCs) and craniofacial regeneration. Two specific aims are proposed to investigate this hypothesis: 1) To investigate the effect of EM-NG on endogenous cranial SSCs and bone healing; 2) To examine the implant of EM-NG-laden scaffold for restoration of segmental mandibular defects. The completion of this proposal will offer significant foundation to further develop effective cell-free approaches for clinical craniofacial defect repair. The additional roles of EMs on craniofacial restoration will be further explored in large animal model in future. These future studies will be proposed in the subsequent R01 application.