Bone morphogenetic proteins (BMPs) are multi-functional growth factors belonging to the Transforming Growth Factor-Beta (TGF-β) superfamily. These proteins are essential to many developmental processes, including cardiogenesis, neurogenesis, and osteogenesis. Specifically, within the BMP family, Bone Morphogenetic Protein-2 (BMP-2) was the first BMP to be characterized and has been well-studied. BMP-2 has important roles during embryonic development, as well as bone remodeling and homeostasis in adulthood. Some of its specific functions include digit formation and activating osteogenic genes, such as Runt-Related Transcription Factor 2 (RUNX2). Because of its diverse functions and osteogenic potential, the Food and Drug Administration (FDA) approved usage of recombinant human BMP-2 (rhBMP-2) during spinal fusion surgery, tibial shaft repair, and maxillary sinus reconstructive surgery. However, shortly after initial injections of rhBMP-2, several adverse complications were reported, and alternative therapeutics have been developed to limit these side-effects. As the clinical application of BMP-2 is largely implicated in bone, we focus primarily on its role in bone. However, we also describe briefly the role of BMP-2 in development. We then focus on the structure of BMP-2, its activation and regulation signaling pathways, BMP-2 clinical applications, and limitations of using BMP-2 as a therapeutic. Further, this review explores other potential treatments that may be useful in treating bone disorders.

The most common bone disease in humans is osteoporosis (OP). Current therapeutics targeting OP have several negative side effects. Bone morphogenetic protein 2 (BMP2) is a potent growth factor that is known to activate both osteoblasts and osteoclasts. It completes these actions through both SMAD-dependent and SMAD-independent signaling. A novel interaction between the BMP type Ia receptor (BMPRIa) and casein kinase II (CK2) was discovered, and several CK2 phosphorylation sites were identified. A corresponding blocking peptide (named CK2.3) was designed to further elucidate the phosphorylation site’s function. Previously, CK2.3 demonstrated an increased osteoblast activity and decreased osteoclast activity in a variety of animal models, cell lines, and isolated human osteoblasts. It is hypothesized that CK2.3 completes these actions through the BMP signaling pathway. Furthermore, it was recently discovered that BMP2 did not elicit an osteogenic response in osteoblasts from patients diagnosed with OP, while CK2.3 did. In this study, we explore where in the BMP pathway the signaling disparity or defect lies in those diagnosed with OP. We found that osteoblasts isolated from patients diagnosed with OP did not activate SMAD or ERK signaling after BMP2 stimulation. When OP osteoblasts were stimulated with BMP2, both BMPRIa and CK2 expression significantly decreased. This indicates a major disparity within the BMP signaling pathway in patients diagnosed with osteoporosis.

Bone is one of the most important organs in the human body. It provides structure, function, and protection for other vital organs; therefore, bone maintenance and homeostasis are critical processes. As humans age, their bone mineral density decreases, which leads to diseases like osteoporosis. This disease affects one in two women and one in five men aged 50 and over. As the aging population increases, the interest and significance of studying this debilitating bone disease becomes more relevant. Current therapeutic products for osteoporosis have many side effects and can be taken for a limited number of years. Most therapeutic products only focus on decreasing bone resorption, not increasing bone formation. Bone morphogenetic protein 2 is an essential growth factor that drives osteoblast differentiation and activity and is essential for bone formation. However, usage in the clinic is unsuccessful due to several side effects. Recently, a signaling disparity in bone marrow stromal cells within the bone morphogenetic protein pathway that led to decreased bone morphogenetic protein 2 responsiveness was identified in patients diagnosed with osteoporosis. However, it is unclear how other cell populations, especially osteoblasts, which are key players in bone remodeling, are affected and whether the bone morphogenetic protein pathway is affected during osteoporosis. Our research group designed a novel peptide, casein kinase 2.3, that acts downstream of the bone morphogenetic receptor type Ia and increases bone mineralization in murine cells and primary bovine osteoblasts. The aim of the study presented here was to compare the responsiveness of osteoblasts to bone morphogenetic protein 2 and casein kinase 2.3, especially in patients diagnosed with osteoporosis. Mature osteoblasts were extracted from patients diagnosed with osteoporosis or osteoarthritis from Christiana Care Hospital in Newark, Delaware. They were stimulated with either bone morphogenetic protein 2 or casein kinase 2.3, and their effect on osteoblast activity was determined. The osteoporotic patients showed no mineralization response to bone morphogenetic protein 2 stimulation, while the osteoarthritis patients significantly responded to bone morphogenetic protein 2 stimulation. Furthermore, markers for osteoblast activity were increased by casein kinase 2.3, which was in sharp contrast to bone morphogenetic protein 2. This further supports a major bone morphogenetic protein signaling disparity in both the elderly and those suffering with osteoporosis. Both patient types did significantly respond to casein kinase 2.3. Further analysis of the bone morphogenetic protein pathway could lead to new therapeutic products for osteoporosis.

Background: Osteoporosis is a degenerative skeletal disease with a limited number of treatment options. CK2.3, a novel peptide, may be a potential therapeutic. It induces osteogenesis and bone formation in vitro and in vivo by acting downstream of BMPRIA through releasing CK2 from the receptor. However, the detailed signaling pathways, the time frame of signaling, and genes activated remain largely unknown. Methods: Using a newly developed fluorescent CK2.3 analog, specific inhibitors for the BMP signaling pathways, Western blot, and RT-qPCR, we determined the mechanism of CK2.3 in C2C12 cells. We then confirmed the results in primary BMSCs. Results: Using these methods, we showed that CK2.3 stimulation activated OSX, ALP, and OCN. CK2.3 stimulation induced time dependent release of CK2β from BMPRIA and concurrently CK2.3 colocalized with CK2α. Furthermore, CK2.3 induced BMP signaling depends on ERK1/2 and Smad1/5/8 signaling pathways. Conclusion: CK2.3 is a novel peptide that drives osteogenesis, and we detailed the molecular sequence of events that are triggered from the stimulation of CK2.3 until the induction of mineralization. This knowledge can be applied in the development of future therapeutics for osteoporosis.

Quantum Dot^®s (QDot^®s) are novel, semi-conductive nanostructures that emit a certain fluorescence when excited by specific wavelengths. QDot^®s are more photostable, brighter, and photobleach less than other fluorescent dyes. These characteristics give them the potential to be used in many biological applications. The shells of QDot^®s are coated with functional groups, such as carboxylate and organic groups, allowing them to couple to peptides/proteins and be used for real-time imaging and high-resolution microscopy. Here, we utilize Quantum Dot^®s and Bone Morphogenetic Protein-2 (BMP-2) to create a BMP-2-QDot^®s conjugate. BMP-2 is a growth factor that drives many processes such as cardiogenesis, neural growth, and osteogenesis. Despite its numerous roles, the trafficking and uptake of BMP-2 into cells is not well-established, especially during progression of diseases. The results presented here demonstrate for the first time a fluorescent BMP-2 analog that binds to the BMP-receptors (BMPRs), remains biologically active, and is stable for long time periods. Previous attempts to develop a biological BMP-2 analog with Fluorescein isothiocyanate (FITC) or nanodiamonds lacked data on the analog’s stability. Furthermore, these analogs did not address whether they can signal within the cell by binding to the BMPRs or were mediated by non-stable conjugates.

Current methods for drug development and discovery involve pre-clinical analyses that are extremely expensive and time consuming. Animal models are not the best precedent to use, when comparing to human models as they are not synonymous with the human response, thus, alternative methods for drug development are needed. One of which could be the use of an ex vivo human organ where drugs could be tested and the effects of those drugs could be observed. Finding a viable human organ to use in these preliminary ex vivo studies is difficult due to the availability, cost, and viability. Bone tissue and marrow contain a plethora of both bone and stem cells, however, these cells need constant perfusion to be viable over a longer time range. Here we maintain bone cell sustainability in an ex vivo model, through the use of human femoral heads in a novel bioreactor. This bioreactor was designed to directly perfuse cell culture media (DMEM) through the vasculature of a femoral head, providing ideal nutrients and conditions required for maintaining organ viability. We show, for the first time, that cells within a femoral head can stay alive up to 12 h. Further development could be used to determine the effects of drugs on a human organ system and could aid in the understanding of the progression of bone diseases and pathologies.