Molecular and Cellular Mechanisms of Delayed Fracture Healing in Mmp10 (Stromelysin 2) Knockout Mice

The remodeling of the extracellular matrix is a central function in endochondral ossification and bone homeostasis. During secondary fracture healing, vascular invasion and bone growth requires the removal of the cartilage intermediate and the coordinate action of the collagenase matrix metalloproteinase (MMP)-13, produced by hypertrophic chondrocytes, and the gelatinase MMP-9, produced by cells of hematopoietic lineage. Interfering with these MMP activities results in impaired fracture healing characterized by cartilage accumulation and delayed vascularization. MMP-10, Stromelysin 2, a matrix metalloproteinase with high homology to MMP-3 (Stromelysin 1), presents a wide range of putative substrates identified in vitro, but its targets and functions in vivo and especially during fracture healing and bone homeostasis are not well defined. Here, we investigated the role of MMP-10 through bone regeneration in C57BL/6 mice. During secondary fracture healing, MMP-10 is expressed by hematopoietic cells and its maximum expression peak is associated with cartilage resorption at 14 days post fracture (dpf). In accordance with this expression pattern, when Mmp10 is globally silenced, we observed an impaired fracture-healing phenotype at 14 dpf, characterized by delayed cartilage resorption and TRAP-positive cell accumulation. This phenotype can be rescued by a non-competitive transplant of wild-type bone marrow, indicating that MMP-10 functions are required only in cells of hematopoietic linage. In addition, we found that this phenotype is a consequence of reduced gelatinase activity and the lack of proMMP-9 processing in macrophages. Our data provide evidence of the in vivo function of MMP-10 during endochondral ossification and defines the macrophages as the lead cell population in cartilage removal and vascular invasio

Optimization of mimetic periosteum autografts for the treatment of nonunions

Bone presents truly regenerative capacity being able to regenerate into a native state in response to injuries. Despite this self-renewal potential, bone healing is not absent of complications and different conditions can interfere with the regenerative process, leading to delayed fracture and in some cases fracture nonunion. Fracture nonunion is a major cause of chronic pain and disability and, despite the low incidence of nonunion and delayed union fractures (5-10%), the numerous fractures that take place globally (~180 million every year) emphasizes the huge economic burden that fracture nonunion represents. Once detected, fracture nonunion requires a surgical approach, and the use of bone autografts that provide and osteoinductive, osteogenic and osteoconductive environment for a successful repair. However, the availability of bone grafts is limited. The scarcity of bone tissue that can be used for autografts have consolidated the need for novel tissue engineering approaches as potential candidates for the treatment of nonunion and for long bone defects, prone to evolve to nonunions. Tissue engineering strategies allow for the combination of novel tunable materials along with different biological adjuvants, including growth factors and cells. During the bone regenerative response, the periosteum, a fibrous layer surrounding the bone, plays a key role delivering osteochondroprogenitor cells and crucial growth factors into the injured tissue. Thus, we developed a tissue engineering strategy where biocompatible, 3D melt-electro-written polycaprolactone membrane would act as a mimetic periosteum. The engineered mimetic periosteum allows vascularization of the construct either when implanted ectopically or orthotopically. Additionally, we demonstrated its capacity to be functionalized with rhBMP-2, the most important morphogen for bone regeneration, both exposed on the membrane surface attached through PEA-hFN or encapsulated in microparticles covalently bound to the PCL membrane. When functionalized with low doses of rhBMP-2 the mimetic periosteum demonstrated great osteogenic potential in vitro, inducing human MSCs differentiation into osteoblasts. More importantly, in vivo results indicate that the functionalization of the mimetic periosteum with rhBMP-2 allows regenerative properties able to heal critical size femoral defects in SD rats with high efficiency and reproducibility using unpreceded low doses of rhBMP-2. Ultimately, the mimetic periosteum demonstrated its ability to deliver key mesenchymal progenitor cells into the injured site. All these results indicate that our engineered mimetic periosteum represents an efficient system for rhBMP-2 and progenitor cells delivery with important translational potential

An engineered periosteum for efficient delivery of rhBMP-2 and mesenchymal progenitor cells during bone regeneration

During bone regeneration, the periosteum acts as a carrier for key regenerative cues, delivering osteochondroprogenitor cells and crucial growth factors to the injured bone. We developed a biocompatible, 3D polycaprolactone (PCL) melt electro-written membrane to act as a mimetic periosteum. Poly (ethyl acrylate) coating of the PCL membrane allowed functionalization, mediated by fibronectin and low dose recombinant human BMP-2 (rhBMP-2) (10-25 mu g/ml), resulting in efficient, sustained osteoinduction in vitro. In vivo, rhBMP-2 functionalized mimetic periosteum demonstrated regenerative potential in the treatment of rat critical-size femoral defects with highly efficient healing and functional recovery (80%-93%). Mimetic periosteum has also proven to be efficient for cell delivery, as observed through the migration of transplanted periosteum-derived mesenchymal cells to the bone defect and their survival. Ultimately, mimetic periosteum demonstrated its ability to deliver key stem cells and morphogens to an injured site, exposing a therapeutic and translational potential in vivo when combined with unprecedentedly low rhBMP-2 doses