The Potential of FGF-2 in Craniofacial Bone Tissue Engineering: A Review

Abstract

Bone is a hard-vascularized tissue, which renews itself continuously to adapt to the mechanical and metabolic demands of the body. The craniofacial area is prone to trauma and pathologies that often result in large bone damage, these leading to both aesthetic and functional complications for patients. The "gold standard" for treating these large defects is autologous bone grafting, which has some drawbacks including the requirement for a second surgical site with quantity of bone limitations, pain and other surgical complications. Indeed, tissue engineering combining a biomaterial with the appropriate cells and molecules of interest would allow a new therapeutic approach to treat large bone defects while avoiding complications associated with a second surgical site. This review first outlines the current knowledge of bone remodeling and the different signaling pathways involved seeking to improve our understanding of the roles of each to be able to stimulate or inhibit them. Secondly, it highlights the interesting characteristics of one growth factor in particular, FGF-2, and its role in bone homeostasis, before then analyzing its potential usefulness in craniofacial bone tissue engineering because of its proliferative, pro-angiogenic and pro-osteogenic effects depending on its spatial-temporal use, dose and mode of administration.

Keywords: FGF-2; angiogenesis; bone tissue engineering; mineralization; signaling pathways.

Figure A1

Morphology of cortical and trabecular bone—The cortical bone is dense and compact with penetrating vascular canals, that has slow turnover rate and high resistance to torsional and bending forces. It constitutes 80% of the skeleton, and it makes up the outer part of skeletal structures. This bone has an outer periosteal surface containing blood vessels, nerve endings, osteoblasts, and osteoclasts and an inner endosteal surface adjacent to the marrow. This tissue is arranged in osteons (A), which are concentric layers, composed of collagen fibers. These are made up of three helical chains and combine to form fibrils, which are interwoven and bound by crosslinks, providing bone elasticity, flexibility, and tensile strength. Cortical bone provides mechanical strength and protection, and it may also participate in metabolic responses, particularly when there is a long-lasting mineral deficit. In contrast, trabecular bone represents just 20% of the skeletal mass, but 80% of the bone surface. This type of bone, which is less dense and more elastic, has a higher turnover rate than cortical bone and has high resistance to compression. It provides mechanical support, helping to maintain skeletal strength and integrity with its rods and plates aligned in a pattern that provides maximal strength. It also exhibits greater metabolic activity than cortical bone, having a larger surface area for mineral exchange. These properties explain it being found inside the long bones, throughout the bodies of the vertebrae, and in the inner portions of the pelvis and other flat bones.

Figure 1

Bone remodeling regulation can be paracrine or endocrine. Several factors participate in paracrine regulation including cytokines (IL-1, IL-6, TNF-alpha, IL-4 and interferon-gamma), PGE2, VEGF, and hypoxia, as well as bone cells. There are three main cell types involved: osteoblasts, osteocytes and osteoclasts. Osteoblasts, which differentiate from mesenchymal progenitors thanks to certain proteins (Runx2, Osx and Wnt) and FGF signaling pathways, are responsible for bone formation. They can also become osteocytes able to regulate osteoblastogenesis through production of inhibitors (DKK-1 and SOST), that inhibit Wnt signaling. Lastly, osteoclasts, involved in bone resorption are activated through RANK-RANKL-OPG signaling pathway cross-talk. Whenever there is a need for bone resorption, osteoblasts and osteocytes express RANKL on their surface, and this then binds to RANK in osteoclast precursors, activating their differentiation. OPG is then secreted to stop bone resorption binding to RANKL blocking the possibility of RANK-RANKL binding and preventing bone resorption. Once activated, mature osteoclasts bind to the bone matrix, becoming polarized. Their cytoskeleton organizes into actin rings forming the sealing zone, which provides an isolated acidic microenvironment, to dissolve minerals and digest the selected bone matrix thanks to the ruffle border (RB). After resorption, the osteoclasts endocytose the degraded collagen fragments, and the calcium and phosphate released are then transported through the cell and liberated at the functional secretory domain before being released into the bloodstream. Bone formation and resorption are also influenced by endocrine regulation. Various factors may be involved, for example, PTH, 1,25(OH) Vitamin D, calcitonin and thyroid hormone.

Figure 2

Bone metabolism: Modeling vs. Remodeling—While bone modeling implies a change in bone shape or size since resorption and formation occur independently at distinct sites: osteoblasts secrete osteoid matrix in the opposite site where osteoclasts resorb bone. Bone remodeling involves the resorption and formation of bone, one after the other, at the same site to replace old and/or damaged bone by newly formed bone. An initiating remodeling signal, such as hormonal or mechanical signal, is detected by the bone, inducing the release of paracrine factors that lead to retraction of the bone lining cells which exposes the bone surface, allowing recruitment of osteoclast precursors from the capillaries directly into the basic multicellular unit. MSC-F and RANKL, secreted by osteocytes, induce recruitment of precursor cells of hematopoietic lineage, initiating their differentiation to multinucleated osteoclasts. The differentiated attached osteoclasts rearrange their cytoskeleton to adhere to the bone surface, decreasing the pH to as low as 4.5, this dissolving the bone mineral. Once resorption is finished, the osteoclasts go through apoptosis. After resorption, mononuclear cells are recruited to remove collagen fragments from the surface, and then new osteoblasts begin collagen deposition, forming what is known as osteoid matrix, until the cavities are filled. Osteoblasts produce new bone, and some of them become buried within the newly formed bone matrix turning into osteocytes with their extensive canalicular network connecting them to the bone surface lining cells, osteoblasts and other osteocytes. The osteoid mineralizes, and the bone enters into a quiescent phase.

Figure 3

FGF/FGFR signaling—FGFs can bind to FGFRs with the help of heparan sulfate, a co-factor, and thereby induce their biological effects through activation of four major signaling pathways: RAS-MAPK-ERK1/2, PI3K-AKT-GSK3, PLCγ-PKC, and STAT-Jak.

Figure 4

FGF/FGFR signaling in bone—FGF-2 is highly expressed in bone tissues. There is a high molecular weight (HMW) form, located in the nucleus, that acts as a transcriptional factor, and upregulates the expression of SOST and FGF-23 responsible for inducing mineralization. The low molecular weight (LMW) form is cytoplasmic or membrane associated. The latter can promote osteoblast differentiation and mineralization through the Wnt pathway, BMP-2 signaling, or synergistic action with BMP-2. By activation of FGFR signaling, LMW FGF-2 also activates MAPK-ERK/2, which acts as a transcriptional factor that upregulates mineralization genes such as RUNX2.