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For osteoblasts derived from fetal and neonatal nor-
mal donors, the versican-like chondroitin sulfate pro-
teoglycan was initially the most abundant proteoglycan.
With increasing age, this proteoglycan's levels decreased
while biglycan and decorin increased in abundance.
135
On average, OI osteoblasts derived from donors of any
age exhibited a “fetal-like” proteoglycan fingerprint,
with the versican-like chondroitin sulfate proteoglycan
staying with the major component regardless of donor
age. The absolute levels of all four proteoglycans were
consistently low at all OI donor ages and similar to the
absolute levels observed in normal cells from donors >60
years of age. Thus, OI-derived proteoglycans exhibited a
fetal distribution pattern, but levels produced were simi-
lar to those from aged normal donors.
Normal human osteoblasts exhibit a temporal pattern
of expression of extracellular matrix components
55,131,136
and it is possible that the reduction observed in
OI-derived cells arises from a delay in the temporal
sequence. When following over 5 weeks the metabolism
of collagen, hyaluronan and proteoglycans by osteo-
blasts derived from OI and normal subjects, OI-derived
osteoblasts failed to reach normal biosynthetic levels of
collagen and proteoglycan, suggesting that the reduced
levels do not arise from a delay in the temporal expres-
sion pattern.
133
A possible interpretation of the observed alterations
in cellular proliferation and extracellular matrix stoichio-
metry is that OI osteoblasts fail to parallel the normal
bone cell developmental expression pattern and are
“arrested” in a fetal-like state (characterized by low col-
lagen and decorin levels and elevated thrombospondin,
fibronectin and versican-like chondroitin sulfate proteo-
glycan levels). Alternatively, the reduction in maximal
cell growth rates and levels of some extracellular matrix
components observed in OI-derived osteoblast-like cells,
while being abnormally low compared to age-matched
controls, were equivalent to values observed in cells
derived from elderly donors >60 years of age.
125,131,132
Thus, OI cells could be exhibiting facets of accelerated
aging and the observed phenotype of osteopenia and
brittle bones would then share certain common features
and mechanisms with age-related osteoporosis. The lev-
els and post-translational modifications of proteoglycans
change with age,
137,138
and in normal osteoblasts exhibit
distinct patterns of increased sulfation and glucuronate
epimerization, and reduced N- and O-lined oligosac-
charide additions with increasing age.
139
In OI osteo-
blasts, proteoglycan post-translational modifications did
not change with donor age and maintained a fetal-like
structure. These observations suggested that a deficit in
maturation of osteoblasts occurs as a consequence of a
mutation in type I collagen.
139
The role of osteoblast lineage maturation in the patho-
physiology of OI was elegantly investigated by crossing
the
Oim
mouse with a transgenic model developed to
identify preosteoblast, osteoblast and osteocyte stages of
differentiation.
140,141
In the resultant model, osteoprogeni-
tor cell differentiation was intact, though only a propor-
tion of cells attained the mature osteoblast stage. The high
proportion of immature osteoblasts resulted in a lower
per cell production of bone matrix.
142
Additionally, these
immature osteoblasts exhibited a stronger potential to
support osteoclast formation and differentiation, though
this osteoclastogenesis was not stimulated by osteoblast-
produced
Rankl
and
Opg
but by increased TNF-α expres-
sion.
142
In another mouse model of OI, the
Brtl
mice,
mesenchymal progenitor cells exhibited a deficit in dif-
ferentiation as they expressed reduced levels of early
(
Runx2
and
Sp7
) and late (
Col1a1
and
Ibsp
) osteoblastic
markers with respect to wild-type.
143
Mutant mesenchy-
mal progenitor cells favored adipogenesis and those that
did differentiate toward the osteoblastic lineage exhibited
upregulated autophagy as a consequence of endoplasmic
reticulum stress due to mutant collagen retention.
143
Osteoclasts in OI
OI may be considered to arise primarily from defects
in osteoblast functional capacity. As described ear-
lier, a state of high bone turnover in OI coupled with
an increased number of osteoblasts suggested that
bone formation rate per osteoblast is reduced in OI.
144
Though qualitative assessment of the osteoclast number
in OI revealed no difference from normal bone,
122
quan-
titative analysis of 70 children with OI demonstrated
higher osteoclast surface and number in children with
OI.
144
The impact of OI on osteoclast number has also
been observed in mouse models of OI. The numbers of
osteoclasts in bone were increased relative to wild-type
for both
Oim
145
and
Brtl
146
mice. Osteoclasts in bone
from the
Oim
model exhibited cellular changes and
greater resorptive activity compared to osteoclasts in
wild-type mice.
147
Perturbation of osteoclasts in OI fol-
lows from the tight coupling and crosstalk of osteoblasts
and osteoclasts and the generalized effect OI mutations
have on osteoblast differentiation and function.
PATHWAYS LEADING TO ALTERED
OST
EOBLAST DIFFERENTIATI
ON
The basic phenotype of OI - bone fragility - is pres-
ent from the mild to most severe forms. In the mild
form of the disease (OI type I) mutations in type I col-
lagen give rise to a silent allele, no mutant protein is
made and normal type I collagen levels are reduced,
thus there is a haploinsufficiency that gives rise to
disease phenotype. The fact that a 50% reduction in
normal collagen is sufficient to cause the disease pheno-
type suggests a couple of pathways whereby an altered
osteoblast differentiation state could arise. In the first
