Delivery of bone morphogenetic proteins for orthopedic tissue regeneration

Carriers for bone morphogenetic proteins (BMPs) are used to increase retention of these factors at orthopedic treatment sites for a sufficient period of time to allow regenerative tissue forming cells to migrate to the area of injury and to proliferate and differentiate. Carriers can also serve as a matrix for cell infiltration while maintaining the volume in which repair tissue can form. Carriers have to be biocompatible and are often required to be bioresorbable. Carriers also have to be easily, and cost-effectively, manufactured for large-scale production, conveniently sterilized and have appropriate storage requirements and stability. All of these processes have to be approvable by regulatory agencies. The four major categories of BMP carrier materials include natural polymers, inorganic materials, synthetic polymers, composites of these materials. Autograft or allograft carriers have also used. Carrier configurations range from simple depot delivery systems to more complex systems mimicking the extracellular matrix structure and function. Bone regenerative carriers include depot delivery systems for fracture repair, three-dimensional polymer or ceramic composites for segmental repairs and spine fusion and metal or metal/ceramic composites for augmenting implant integration. Tendon/ligament regenerative carriers range from depot delivery systems to three-dimensional carriers that are either randomly oriented or linearly oriented to improve regenerative tissue alignment. Cartilage regenerative systems generally require three-dimensional matrices and often incorporate cells in addition to factors to augment the repair. Alternative BMP delivery systems include viral vectors, genetically altered cells, conjugated factors and small molecules.     

The application of bone morphogenetic proteins to dental tissue engineering

Progress in understanding the role of bone morphogenetic proteins (BMPs) in craniofacial and tooth development, the demonstration of stem cells in dental pulp and accumulating knowledge on biomaterial scaffolds have set the stage for tissue engineering and regenerative therapy of the craniofacial complex. Furthermore, the recent approval by the US Food and Drug Administration (FDA; Rockville, MD, USA) of recombinant human BMPs for accelerating bone fusion in slow-healing fractures indicates that this protein family may prove useful in designing regenerative treatments in dental applications. In the near term, these advances are likely to be applied to endodontics and periodontal surgery; ultimately, they may facilitate approaches to regenerating whole teeth for use in tooth replacement.     

Bone grafts and bone morphogenetic proteins in spine fusion

Spinal fusions are being performed for various pathologies of the spine. Stabilizing vertebral segments by eliminating motion across those segments becomes critical in dealing with pathologies of the spine that lead to instability. The use of autograft has been the gold standard for spine fusion. However, due to complications such as donor site morbidity, increased operating time, and limited supply, the use of allograft as a graft extender has become an acceptable practice especially in fusions spanning multiple segments. The discovery and isolation of novel proteins (i.e., bone morphogenetic proteins, BMPs), which initiate the molecular cascade of bone formation, have experimentally been shown in numerous animal studies to be as effective as autografts. Although the use of BMPs has exciting applications in spine surgery, long-term clinical studies must be evaluated for its efficacy in various applications in humans. The use of biomimetic materials such as hydroxyapatite (HA), or tricalcium phosphate (TCP) has also been examined in several animal models as bone graft substitutes or carriers. Although these materials have shown some promise in specific site applications, more work remains in elucidating an efficacious combination of these materials and BMPs that can be as effective as autografts. This review will present the status of bone grafts, bone morphogenetic proteins, gene therapy, and work that has been done to facilitate spinal fusion and simultaneously eliminate the need for bone graft. This revised version was published online in July 2006 with corrections to the Cover Date.     

Bone morphogenetic proteins in dentin regeneration for potential use in endodontic therapy

The human dentition is indispensable for nutrition and physiology. The teeth have evolved for mastication of food. Caries is a common dental problem in which the dentin matrix is damaged. When the caries is deep and the dental pulp is exposed, the pulp has to be removed in many cases, resulting ultimately in loss of the tooth. Therefore, the regeneration of dentin-pulp complex is the long-term goal of operative dentistry and endodontics. The key elements of dentin regeneration are stem cells, morphogens such as bone morphogenetic proteins (BMPs) and a scaffold of extracellular matrix. The dental pulp has stem/progenitor cells that have the potential to differentiate into dentin-forming odontoblasts in response to BMPs. Pulpal wound healing consists of stem/progenitor cells release from dental pulp niche after noxious stimuli such as caries, migration to the injured site, proliferation and differentiation into odontoblasts. There are two main strategies for pulp therapy to regenerate dentin: (1) in vivo method of enhancing the natural healing potential of pulp tissue by application of BMP proteins or BMP genes, (2) ex vivo method of isolation of stem/progenitor cells, differentiation with BMP proteins or BMP genes and transplantation to the tooth. This review summarizes recent advances in application of BMPs for dentin regeneration and possible use in endodotic therapy.     

Delivery of recombinant bone morphogenetic proteins for bone regeneration and repair. Part B: Delivery systems for BMPs in orthopaedic and craniofacial tissue engineering

Localized and release-controlled delivery systems for the sustained expression of the biologic potency of rhBMPs are essential. A substantial number of biomaterials have been investigated thus far. Most fail after implantation or administration mainly due to either being too soft, difficult to control and/or stabilize mechanically. In the second part of this review, we review a representative selection of rhBMP-2 and rhBMP-7 carrier materials and delivery systems ranging from simple nano/microparticles to complex 3-D scaffolds in sites of orthopaedic and craniofacial bone regeneration and repair.     

Bone morphogenetic proteins: a critical review

Bone Morphogenetic Proteins (BMPs) are potent growth factors belonging to the Transforming Growth Factor Beta superfamily. To date over 20 members have been identified in humans with varying functions during processes such as embryogenesis, skeletal formation, hematopoesis and neurogenesis. Though their functions have been identified, less is known regarding levels of regulation at the extracellular matrix, membrane surface, and receptor activation. Further, current models of activation lack the integration of these regulatory mechanisms. This review focuses on the different levels of regulation, ranging from the release of BMPs into the extracellular components to receptor activation for different BMPs. It also highlights areas in research that is lacking or contradictory.     

Bone morphogenetic proteins inhibit adipocyte differentiation by bone marrow stromal cells

The bone morphogenetic proteins were originally identified based on their ability to induce ectopic bone formation in vivo and have since been identified as members of the transforming growth factor-β gene superfamily. It has been well established that the bone morphogenetic cytokines enhance osteogenic activity in bone marrow stromal cells in vitro. Recent reports have described how bone morphogenetic proteins inhibited myogenic differentiation of bone marrow stromal cells in vitro. In vivo, bone marrow stromal cells differentiate along the related adipogenic pathway with advancing age. The current work reports the inhibitory effects of the bone morphorphogenetic proteins on adipogenesis in a multipotent murine bone marrow stromal cell line, BMS2. When exposed to bone morphogenetic protein-2, the pre-adipocyte BMS2 cells exhibited the expected induction of the osteogenic-related enzyme, alkaline phosphatase. Following induction of the BMS2 cells with adipogenic agonists, adipocyte differentiation was assessed by morphologic, enzymatic, and mRNA markers. Flow cytometric analysis combined with staining by the lipophilic fluorescent dye, Nile red, was used to quantitate the extent of lipid accumulation within the BMS2 cells. By this morphologic criteria, the bone morphogenetic proteins inhibited adipogenesis at concentrations of 50 to 500 ng/ml. This correlated with decreased levels of adipocyte specific enzymes and mRNAs. The BMS2 pre-adipocytes constitutively expressed mRNA encoding bone morphogenetic protein-4 and this was inhibited by adipogenic agonists. Together, these findings demonstrate that bone morphogenetic proteins act as adipogenic antagonists. This supports the hypothesis that adipogenesis and osteogenesis in the bone marrow microenvironment are reciprocally regulated.     

Delivery of recombinant bone morphogenetic proteins for bone regeneration and repair. Part A: Current challenges in BMP delivery

Recombinant human bone morphogenetic proteins (rhBMPs) have been extensively investigated for developing therapeutic strategies aimed at the restoration and treatment of orthopaedic as well as craniofacial conditions. In this first part of the review, we discuss the rationale for the necessary use of carrier systems to deliver rhBMP-2 and rhBMP-7 to sites of bone tissue regeneration and repair. General requirements for growth factor delivery systems emphasizing the distinction between localized and release-controlled delivery strategies are presented highlighting the current limitations in the development of an effective rhBMP delivery system applicable in clinical bone tissue engineering.     

Delivering on the promise of bone morphogenetic proteins.

The advent of bone growth factors has been widely anticipated since their successful production using recombinant DNA technology. Bone morphogenetic proteins (BMPs) are an important class of bone growth factors and will be the focus of this article. In the near future these therapeutics might revolutionize how clinicians treat such diverse orthopedic applications as the healing of broken bones, increasing bone density lost through aging, and strengthening the spine. These potent proteins require application directly at the site of repair via a delivery system. The choice of delivery system has a profound effect on the clinical outcome. In the past decade, researchers have focused on developing efficient delivery systems and advancing these factors from the bench to the clinic.     

Bone morphogenetic protein 1 prodomain specifically binds and regulates signaling by bone morphogenetic proteins 2 and 4

Highly purified fractions of bone extracts capable of inducing ectopic bone formation have been reported to contain peptides corresponding to the mature active regions of the TGF-beta-like bone morphogenetic proteins (BMPs) 2-7, and to the prodomain region of the metalloproteinase BMP1. Co-purification of BMPs 2-7 with BMP1 prodomain sequences through the multiple biochemical steps used in these previous reports has suggested the possibility of interactions between the BMP1 prodomain and BMPs 2-7. Here we demonstrate that the BMP1 prodomain binds BMPs 2 and 4 with high specificity and with a KD of approximately 11 nM, in the physiological range. It is further demonstrated that the BMP1 prodomain is capable of modulating signaling by BMPs 2 and 4 in vitro and in vivo, and that endogenous BMP1 prodomain-BMP4 complexes exist in cell culture media and in tissues.     

Bone morphogenetic proteins in inflammation,glucose homeostasis and adipose tissue energy metabolism

Bore morphogenetic proteins (BMPs) are members of the transforming growth factor (TGF)-β superfamily, a group of secreted proteins that regulate embryonic development. This review summarizes the effects of BMPs on physiological processes not exclusively linked to the musculoskeletal system. Specifically, we focus on the involvement of BMPs in inflammatory disorders, e.g. fibrosis, inflammatory bowel disease, anchylosing spondylitis, rheumatoid arthritis. Moreover, we discuss the role of BMPs in the context of vascular disorders, and explore the role of these signalling proteins in iron homeostasis (anaemia, hemochromatosis) and oxidative damage. The second and third parts of this review focus on BMPs in the development of metabolic pathologies such as type-2 diabetes mellitus and obesity. The pancreatic beta cells are the sole source of the hormone insulin and BMPs have recently been implicated in pancreas development as well as control of adult glucose homeostasis. Lastly, we review the recently recognized role of BMPs in brown adipose tissue formation and their consequences for energy expenditure and adiposity. In summary, BMPs play a pivotal role in metabolism beyond their role in skeletal homeostasis. However, increased understanding of these pleiotropic functions also highlights the necessity of tissue-specific strategies when harnessing BMP action as a therapeutic target.     

Targeting intercellular signals for bone regeneration from bone marrow mesenchymal progenitors

There are tremendous unmet clinical needs for effective strategies to enhance bone regeneration in vivo. The sustained presence of multipotent mesenchymal progenitors in the bone marrow in aged and osteoporotic individuals offers the potential for therapeutic interventions to induce osteoblast production from the resident progenitors. Recent advances in understanding the intercellular signals governing osteogenic decisions may provide targets for developing novel bone-enhancing therapeutics.     

Bone morphogenetic protein-2 enhances bone regeneration mediated by transplantation of osteogenically undifferentiated bone marrow-derived mesenchymal stem cells

We have hypothesized that human bone marrow-derived mesenchymal stem cells (BMMSCs), that are not osteogenically differentiated prior to implantation, would regenerate bone extensively in vivo once exogenous bone morphogenetic protein-2 (BMP-2) was delivered to the implantation site. BMP-2 released from heparin-conjugated poly(lactic-co-glycolic acid) (HCPLGA) scaffolds stimulates osteogenic differentiation of cultured BMMSCs. Upon implantation, undifferentiated BMMSCs on BMP-2-loaded HCPLGA scaffolds induce far more extensive bone formation than either undifferentiated BMMSCs or osteogenically differentiated BMMSCs on HCPLGA scaffolds. These BMP-2-loaded HCPLGA scaffolds could prove invaluable for in vivo regeneration of bone from undifferentiated human BMMSCs.     

Expression of bone morphogenetic proteins in stromal cells from human bone marrow long-term culture.

Highly purified primitive hemopoietic stem cells express BMP receptors but do not synthesize bone morphogenetic proteins (BMPs). However, exogenously added BMPs regulate their proliferation, differentiation, and survival. To further explore the mechanism by which BMPs might be involved in hemopoietic differentiation, we tested whether stromal cells from long-term culture (LTC) of normal human bone marrow produce BMPs, BMP receptors, and SMAD signaling molecules. Stromal cells were immunohistochemically characterized by the presence of lyzozyme, CD 31, factor VIII, CD 68, S100, alkaline phosphatase, and vimentin. Gene expression was analyzed by RT-PCR and the presence of BMP protein was confirmed by immunohistochemistry (IHC). The supportive role of the stromal cell layer in hemopoiesis in vitro was confirmed by a colony assay of clonogenic progenitors. Bone marrow stromal cells express mRNA and protein for BMP-3, -4, and -7 but not for BMP-2, -5, and -6 from the first to the eighth week of culture. Furthermore, stromal cells express the BMP type I receptors, activin-like kinase-3 (ALK-3), ALK-6, and the downstream transducers SMAD-1, -4, and -5. Thus, human bone marrow stromal cells synthesize BMPs, which might exert their effects on hemopoietic stem cells in a paracrine manner through specific BMP receptors.     

Regulation of cartilage and bone differentiation by bone morphogenetic proteins.

Quantum advances have recently been made in the understanding of the regulation of cartilage and bone differentiation through the identification, purification, genetic cloning and expression of recombinant bone morphogenetic proteins. Bone morphogenetic proteins are a family of pleiotropic differentiation factors with actions on chemotaxis, mitosis, initiation and promotion of chondrogenic and osteogenic phenotypes. They bind extracellular matrix components, heparin and type IV collagen and initiate bone repair. The cascade of cartilage and bone differentiation consists of several continuous phases: initiation, promotion, maintenance and termination.     

Recent progress in bone induction by osteogenin and bone morphogenetic proteins: challenges for biomechanical and tissue engineering

Implantation of demineralized bone matrix results in local bone induction. Bone induction is a sequential biological chain reaction that consists of chemotaxis and proliferation of mesenchymal cells and differentiation of bone. Osteogenin, a bone morphogenetic protein has been purified and the amino acid sequence determined. Recently a family of bone morphogenetic proteins have been cloned and expressed by recombinant DNA technology. The availability of growth and morphogenetic factors will permit the rational design of new bone. The challenge for the biomechanical engineer is to attain mechanically optimal and functionally adaptive new bone for various skeletal prostheses. We are on the threshold for fabrication of new bone based on sound architectural design principles of tissue engineering based on cellular and molecular biology of growth and differentiation factors.     

Effects of bone morphogenetic proteins on epithelial repair

Epithelial tissue has important functions such as protection, secretion, and sensation. Epithelial damage is involved in various pathological processes. Bone morphogenetic proteins (BMPs) are a class of growth factors with multiple functions. They play important roles in epithelial cells, including in differentiation, proliferation, and migration during the repair of the epithelium. This article reviews the functions and mechanisms of the most profoundly studied BMPs in the process of epithelial damage repair and their clinical significance.