Biomedical Engineering Reference
In-Depth Information
grafts have been utilized by orthopaedic surgeons for over a century. Urist (1965) 40 made the
key discovery that intermolecular implantation of demineralized, lyophilized segments of allo-
geneic rabbit bone induced new bone formation. Bone induction is a sequential multistep
cascade and the three key steps are chemotaxis, mitosis and differentiation. 25,29,30 Chemotaxis
is the directed migration of cells in response to a chemical gradient of signals released from the
demineralized bone matrix. 4 The migration and attachment of progenitor cells to the collag-
enous matrix is mediated by fibronectin. On day 3 there is a peak in proliferation of cells in
response to growth factors released from the insoluble demineralized matrix. 24 Chondrogen-
esis is maximal on days 7-8 and is followed by vascular invasion and osteogenesis on day 9.
Bone formation is maximal on days 10-12 as indicated by alkaline phosphatase activity and is
followed by increases in osteocalcin, the bone γ -carboxyglutamic acid containing protein (BGP).
The newly formed ossicle is filled with hematopoietic marrow on day 21. 25 The demineralized
bone matrix-induced bone morphogenesis system led to the isolation of bone morphogenetic
proteins (BMPs) the primordial signals for morphogenesis of bone and a variety of organ sys-
tems beyond bone such as brain, heart, kidney, lungs, liver, skin and teeth. Hence, one can refer
to BMPs as body morphogenetic proteins.
Bone Morphogenetic Proteins
The demineralized bone matrix was extracted by dissociative agents such as 4 M guanidine
HCL, 8 M urea or 1% sodium dodecyl sulfate at pH 7.4. 35,36 Approximately three percent of
the proteins were solubilized and the residue was predominantly type I insoluble bone col-
lagen. Although the soluble extract or insoluble collagen were not osteoinductive singly when
recombined and reconstituted together it restored bone induction. Thus, there is a collabora-
tion between a soluble signal and an insoluble substratum of collagen to initiate new bone
formation. The soluble signal was purified by heparin affinity chromatography, hydroxyapatite
columns, and molecular sieve chromatography. The final purification was accomplished by
preparative gel electrophoresis and novel BMPs were isolated, cloned and expressed. 15,21,27,45
Table 1 summarizes the 15 BMPs in the mammals and are related to members of the TGF β
superfamily. BMPs are dimeric due to a critical intermolecular disulfate linkage. The dimeric
conformation is critical for bone induction and morphogenesis. Each of the two monomers is
biosynthesized as a molecule of over 400 amino acids. However, the mature BMP monomer
derived by proteolytic processing is about 120 amino acid polypeptide. BMPs are pleiotropic
signals. Pleiotropy is the property of a gene or protein to act in a multiplicity of steps. BMPs act
on the three key steps in the sequential cascade of bone morphogenesis such as chemotaxis,
mitosis and differentiation of transient stage of cartilage and the permanent induction of bone.
Thus, all BMPs are chondrogenic, therefore can be considered as cartilage morphogenetic pro-
teins.
Cartilage morhogenesis is a key rate-limiting step in the bone morphogenetic cascade, and
is critical for bone and joint development. In this reference it is noteworthy that articular
cartilage served as a starting point for the isolation of novel cartilage-derived morphogenetic
proteins (CDMPs) using a chondrogenesis bioassay based on dissociative extraction and recon-
stitution of chondrogenic proteins. 27,28
Although BMPs were first isolated, cloned and expressed from bone, they have actions
beyond bone. Genetic evidence based on gene knockouts have implicated BMPs in develop-
ment and morphogenesis of brain, eye, heart, kidney, liver, lung, ovary, skin, teeth, testis and in
a variety of tissues during various steps of epithelial-mesenchymal interactions during embryo-
genesis. It is indeed gratifying to note that BMPs is the basis of key developments in morpho-
genesis of many tissues.
BMPs elicit their biological actions by their interaction with types I and II BMP receptors.
There are two kinds of type I BMP Receptors, types IA and IB. 9,37 BMPs receptors are protein
kinases that phosphorylate cytoplasmic substrates called Smads 1, 5 and 8. The phosphory-
lated Smads 1, 5 and 8 partner with a coSmad called Smad 4 and enter the nucleus to turn on
 
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