Bone Morphogenetic Proteins (BMP’s)

Introduction:

Bone morphogenetic proteins (BMP’s), composed of 400–525 amino acids, comprise one subset of the transforming growth factor- β superfamily. BMP’s stimulate proliferation and migration of undifferentiated bone cell precursors with little to no effect on mature osteoprogenitor cells 1.  Thus, the main action of BMP’s is to commit undifferentiated puripotential cells to differentiate in to cartilage and bone forming cells 2-4BMP’s are abundant in bone and are produced by several cell types including osteoblasts 5-6.

Properties of Bone morphogenetic proteins (BMP’s):

  1. They act as mitogens on undifferentiated mesenchimal cells and osteoblast precursors.
  2. Structurally they are related members of TGF-β superfamily.
  3. BMP 2-12 singly initiate de novo endochondral bone formation 7-10.
  4. BMP’s induce bone formation where as other growth factors such as TGF-β1 or PDGF do not.  
  5. BMPs have an anabolic effect on periodontal tissue through stimulation of osteoblastic differentiation in human periodontal ligament cells 11-12.
  6. Bone allograft materials contain varying amount of BMP’s, such as BMP- 2,-4 and -7  13. A deficiency of BMP like proteins retards bone cell differentiation and may account for a failure of fracture to heal 14.
  7. Recombinant BMP’s (rh BMP’s) have been shown to promote bone formation 15-16.
  8. They induce the expression of osteoblast phenotype (i.e. increase in alkaline phosphatise activity in bone cells).
  9. Act as chemoattractants for mesenchymal cells and monocytes as well as binding to extracellular marix type IV collagen 17.

Historical background:

In 1889 Senn first found that decalcified ox bone promoted healing of osteomyelitic defects. He proposed that there was something in decalcified bone that was responsible for bone formation.

In 1938 Lavender 18 implanted living bone fragments 1-1.5 cm in length either subcutaneously or intramuscularly. These fragments were first treated by scraping away the periosteum, and in some specimens a superficial layer of bone was also removed. Upon obtaining regenerated bone, he was able to show that it was neither the periosteum nor the cells on the surface or within the graft that was responsible for new bone growth. He proposed that there must be something which originated from the graft itself, possibly a substance which was soluble in the lymph tissue.

In early 1960’s several researchers were investigating the process of calcification. In a series of experiments designed to test the triphasic theory of calcification, Urist et al discovered that control samples of untreated, decalcified bone implanted into muscle pouches of rabbits and rats resulted in new cartilage and bone formation by auto-induction 19-20.

The purification of BMPs was completed using the rat ectopic bone formation assay. This assay involves combining the sample containing the unknown protein to be assayed with demineralized rat bone matrix, which has been treated with dissociative agents such as guanidine and urea to remove all of the endogenous BMP activity. This combination is then implanted subcutaneously in rats, and after 1–2 weeks, formation of new cartilage and bone is detected histologically. Using this bioassay, BMPs were purified and sequenced. Johnson et al 21 in 1992 successfully purified human bone morphogenetic proteins. 13 BMP sequences have been cloned so far. BMP-1 which does not belong to the TGF-β family of proteins, was identified as a type I procollagen carboxy-terminal proteinase 22-23.

Recombinant human BMP-2 through BMP- 6 and osteogenic protein-l and 2 (OP-1 and OP-2, also known as BMP-7 and BMP-8, respectively), in conjunction with the collagenous matrix, singly induce de novo bone formation when implanted into extraskeletal sites of a variety of animal models 24-26. These recombinant BMPs have been produced using both a mammalian cell expression system and an Escherichia coli expression system.

Structure of BMP’s:

The BMP’S are 30- to 38 kDa homodimers , glycosylated proteins. As individual BMP proteins are synthesized by a cell, they dimerise and become glycosylated. They are synthesized as prepropeptides of approximately 400–525 amino acids. The mature C-terminal region of 100–140 amino acid residues is released from a propeptide region by cleavage at an Arg-X-X-Arg sequence. Cleavage event occurs upon secretion that results in the formation of mature active protein, a dimer of carboxy-terminal region of the precursor peptide. Because of the dimeric nature of the BMP’s, it is possible that both homodimeric and heterodimeric forms exist. The BMPs have been grouped into subsets based on amino acid sequence homology. The groupings are suggested to be as follows:

  1. BMP-2 and BMP-4,
  2. BMP-3 and BMP-3b,
  3. BMP-5, BMP-6, BMP-7, and BMP-8,
  4. BMP-9 and BMP-10,
  5. BMP-12, BMP-13, and BMP-14, and
  6. BMP-11 and growth/differentiation factor 8 (GDF-8).

Bone Morphogenetic Proteins (BMP’s) & their alternate names

BMP

ALTERNATIVE NAME

BMP-2

BMP-2a

BMP-3

Osteogenin

BMP-4

BMP-2b

BMP-5

-

BMP-6

Vgr- 1

BMP-7

OP-1

BMP-8

OP-2

BMP-9

GDF-2

BMP-10

-

BMP-11

-

BMP-12

GDF-11, CDMP-3

BMP-13

GDF-6, CDMP-2

BMP-14

GDF-5, CDMP-2

BMP-15

CDMP-1

OP: Osteogenic proteinGDF: Growth and differentiation factorCDMP: Cartilage derived morphogenetic proteinVgr: Vegetal related growth factor .

BMP signalling system:

BMPs signal through serine/threonine kinase receptors that are composed of type I and type II subtypes. Three type I receptors have been shown to bind BMP ligands, including type IA and IB BMP receptors [BMPR-IA (ALK-3) and BMPR-IB (ALK-6)] and type IA activin receptor (ActRIA or ALK-2).

The type I BMP receptor substrates include the Smad proteins, which play a central role in the relay of BMP signals from the receptor to target genes in the nucleus. Smad1, Smad5 and Smad8 are phosphorylated by BMP receptors in a ligand-dependent manner. After release from the receptor, Smad proteins associate with the related protein Smad4, which acts as a shared partner. This complex translocates into the nucleus and participates in gene transcription with other transcription factors.

Brief description of some well investigated BMP’s:

BMP-1 : Genes for BMP 1 are located on chromosome 8. It does not belong to the TGF-β family of proteins. It is a metalloprotease that acts on procollagen I, II and III. It is involved in cartilage development.

BMP-2 :  It is a major inducer of osteoblast differentiation. The genes are located on chromosome 20.

BMP-3 : Induces bone formation. The genes are located on chromosome 14.

BMP-4 : Regulates the formation of teeth, limbs and bone from mesoderm. It also plays a role in fracture repair. Genes are located on chromosome 14.

BMP-5 : Performs functions in cartilage development. Genes are located on chromosome 6.

BMP-6 : Plays a role in joint integrity in adults. Genes are located on chromosome 6.

BMP-7 : Plays a key role in osteoblast differentiation. It also induces the production of SMAD1. Also key in renal development and repair. Genes are located on chromosome 20. It is also known as osteogenic protein-1.

Role of BMP’s in periodontal regeneration:

BMPs have been used extensively by researchers to induce periodontal tissue regeneration in a variety of animal models 27-31 as well as in human studies 32-33 with varying degrees of success. Researchers 34 have proposed that BMPs possess a structure/activity profile with BMP-2 exhibiting mainly osteogenic properties while osteogenic protein-1 (OP-1), also known as BMP-7, was mainly cementogenic in its activities.

Recombinant human bone morphogenetic protein-2 (rhBMP-2) has been used to investigate periodontal regeneration. Researchers 32,35 successfully achieved periodontal regeneration in dogs using recombinant human bone morphogenetic protein-2 (rhBMP-2) and a synthetic carrier.

Clinical trials using rhBMP-2 in an absorbable collagen sponge carrier 32-33 have yielded encouraging results with the protein and carrier being well tolerated locally and systemically.

Challenges in clinical application of BMP’s:

The major challenges in clinical application of BMP’s are 36-37:

  • Need for high doses.
  • Non-specific activity on different cell lineages in time and space.
  • Rapid loss of topically applied growth factors.

The greatest challenge of all in clinical application of BMP’s is the formulation of appropriate delivery systems for BMPs suitable for periodontal regeneration. The operational reconstitution and the implantation of the BMP’s with the collagenous matrix as carrier resulted in the induction of cementogenesis and the morphogenesis of a functionally oriented periodontal ligament. An important function for an osteogenic delivery system is the initiation of optimal osteoinductivity with relatively low doses of recombinant BMPs.

Recent research:

As the expression of both BMP-2 and BMP-7 has been found during periodontal tissue morphogenesis, it suggests that optimal therapeutic regeneration may require the combined use of the two BMP’s. Recent research has been concentrated on the application of BMP’s in regenerative periodontal therapy to aid in the healing of bone. In one study recombinant human (rh) osteogenic protein-1 (also referred to as BMP-7) and BMP-2 (rhBMP-2) have been shown to be safe and effective in improving and accelerating bone healing in orthotropic animal models and fibrous nonunion fracture healing 38. To deliver BMP’s in an active form in a periodontal defect through a carries is one of the most important step in its clinical application.

Application of Ca-P-coated porous titanium fiber mesh loaded with rhBMP-2 in subcutaneous implants using a rat model has been used to find out the osteoinductive property of recombinant BMP 39. Ectopic bone formation with a cartilaginous phase was observed within 7–9 days, and bone formation was observed to be similar to endochondral ossification. Another study done on rat bone marrow stromal cell showed that rhBMP-2 incorporated into Ca-P coatings had a greater potential to stimulate alkaline phosphatase activity which is indicative of bone formation 40.

Other members of the BMP family, such as growth and differentiation factor-5, 6, and 7 have been detected on the surfaces of alveolar bone, cementum and periodontal ligament fiber bundles during the process of periodontal development 41. Their therapeutic applications require further investigations.

Summary of biological activities of BMP’s:

Embryo genesis

  • BMPs play an important role in early embryonic early development.

Apoptotic activity

  • BMPs appear to mediate programmed cell death at advanced stages of development.

Mitogenic for cells:

  • BMPs act as mitogens on undifferentiated mesenchymal cells and osteoblast precusors, inducing the expression of osteoblast phenotype (e.g. increasing alkaline phosphatase activity in bone cells). Also acts as chemoattractants for mesenchymal cells and osteocytes as well as binding to extracellular matrix and collagen.

Periodontal regeneration

  • Many invivo studies have shown the ability of BMPs in inducing new bone and cementum formation.

WARNING

Any unauthorized use or reproduction of periobasics.com content for commercial or any purposes is strictly prohibited and constitutes copyright infringement liable to legal action.

References:

Please contact author for references

References:

Leave a Reply

You must be logged in to post a comment.