Transcription Factors in Bone: Developmental and Pathological Aspects - Engineered Bone: Tissue Engineering Intelligence Unit - page 6

mutant mice. 19,20 This observation suggests Cbfa1 is necessary for chondrocyte hypertrophy
and that this function is independent of its osteoblast differentiation ability (see below).
Transcriptional Control of Osteoblast Differentiation
In addition to being a hypertrophic chondrocyte differentiation factor, Cbfa1/Runx2, a
mouse homolog of the drosophila Runt protein, 24 is the only transcription factor reported to
date to act as an inducer of osteoblast differentiation. Runt proteins are a group of transcrip-
tion factors that are conserved from C. elegans to human. 25 They all share a typical DNA
binding domain called Runt domain of 128 amino acids long. 26 Cbfa1 was initially identified
as one of the two key regulators for the osteoblast-specific expression of osteocalcin gene. 27,28
Cbfa1 has all characteristics of a differentiation regulator in the osteoblast lineage. First, its
expression correlates with osteogenesis during development. The onset of its expression is as
early as 9.5 dpc in notochord 22 and starting at 10.5 dpc Cbfa1 is expressed in all mesenchymal
condensations. 28 Later during development Cbfa1 is expressed at high levels in osteoblasts an
at lower levels in prehypertrophic chondrocytes but not in any other cell types. 20 Second,
Cbfa1 is necessary for osteoblast differentiation. Targeted inactivation of Cbfa1 in mice gener-
ated mice that have no osteoblasts. 29 Inactivation of only one Cbfa1 allele causes cleidocranial
dysplasia syndrome, a disease characterized by a delay in osteoblast differentiation in bones
forming through intramembraneous ossification in mice and human. 30 Third, Cbfa1 is also
sufficient to induce osteoblast differentiation in vivo and in vitro. In vivo, ectopic expression of
Cbfa1 in transgenic animals leads to ectopic endochondral ossification. 20 In vitro, forced ex-
pression of Cbfa1 in non-osteoblast cells induces osteoblast-specific expression of all bone marker
genes, including osteocalcin and bone sialoproteins. 28
Besides Cbfa1, two broadly expressed AP1 family members, ∆ FosB and Fra-1, have been
shown to act as positive regulators for osteoblast differentiation by gain-of-function studies. 31,32
∆ FosB is an alternative spliced product of FosB, an oncogene and a transcription repressor of
c-Fos. 33 ∆ FosB lacks the last 101 amino acids of FosB and functionally it differs from FosB
because it lacks oncogenic ability and fails to suppress c-Fos transcription. FosB is expressed at
the highest levels in bones and cartilages. 33 Mice lacking FosB develop normally. 34,35 However
in agreement with its distribution, generalized overexpression of ∆ FosB in transgenic mice
leads to a postnatal osteosclerosis secondary to an increase in osteoblast differentiation. 32 Mir-
roring that of ∆ FosB, forced ubiquitously expression of Fra-1 caused a similar bone pheno-
type. 31 Targeted deletion of Fra-1 in mice led to embryonic lethality around 10 dpc and mice
rescued to birth through blastocyst injection contained mature osteoblasts, 36 indicating that
Fra-1 is not essential for osteoblast differentiation.
Several other transcription factors are being characterized in several laboratories. They may
control osteoblast differentiation in Cbfa1-dependent or independent pathways. A zinc-finger
protein AJ18 has been recently identified to modulate Cbfa1's activity by competing with the
same consensus sequence. 37 Another example is Osf1, the protein that binds and activates one
of the two osteoblast-specific cis-elements (OSE1) in osteocalcin gene. 27 Unlike Cbfa1, Osf1 is
only present in non-mineralized osteoblasts. 38 These factors are not the only ones are being
studied. The description of the function of each of these factors in vivo will greatly increase our
understanding of osteoblast differentiation.
Transcription Control of Osteoclast Differentiation
Unlike the two other cell types of the skeleton, the osteoclasts are derived from myeloid
origin which gives rise also to macrophages and monocytes. Hence, it is not surprising that
many growth factors such as macrophage colony-stimulating factor and receptor activator of
NF κ B ligand, and transcription factors such as Pu.1 and NF κ B that are involved in
haematopoietic cell differentiation also affect osteoclast differentiation. Pu.1 is an ETS (a winged
helix-turn-helix wing motif ) domain-containing protein whose absence causse multilineage
defects in the generation of B lymphocytes, monocytes, and granulocytes. 39 Accordingly, its
expression pattern is lineage- and stage-restricted in the myeloid, B-lymphoid origin and early