what-when-how
In Depth Tutorials and Information
a missense mutation in cyclophilin B
5
. The secretion
rate of type I collagen in the CypB null mouse was not
studied; however, it is likely that the rate of folding is
slower and therefore the rate of secretion is also slower.
If combined together the collagen folding rates in
OI cases resulting from mutations in P3H1, CRTAP
and CypB are decreased. Type I collagen overmodifica-
tion is observed in most cases and that can generally be
explained by the longer times that collagen spends in the
ER in an unfolded state and therefore can be attributed to
the chaperone function of the complex. However, some
mutations in CypB might alter the activity of the lysyl
hydroxylase 1 (LH1), resulting in near normal collagen
modifications regardless of the delayed folding rate.
IS OI DUE TO THE ABSENCE OF THE
3-HYP?
What are the Possible Functions for the
3-Hydroxyl Group of Pro986 in the
α
1 Chain of
Type I Collagen?
Knowing that 4-Hyp is required for the structural
stability of the triple helix
48
it is logical to suspect that
3-Hyp might serve a similar function. The crystal struc-
ture of a model collagen peptide demonstrated that the
presence of 3-Hyp does not lead to the structural altera-
tions in the collagen triple helix.
82
It was also rather
stabilizing than destabilizing the triple helix formed by
synthetic peptides.
83
Type I collagen extracted from the
tail tendon of the P3H1 null mouse also showed a slight
increase in stability compared to the wild-type (the
melting temperature was 1°C higher than control).
11
Therefore, the stability and structure of the collagen tri-
ple helix are not substantially affected in the absence of
the 3-hydroxyl group. According to the crystal structure
of the synthetic peptide, the 3-OH group sticks out of
the triple helix and might be available for collagen pro-
tein interactions.
82
Based on this finding and on obser-
vations discussed below the most widely discussed
hypothesis was proposed by Weis et al.
84
These authors
suggested that 3-hydroxylation plays a role in fibril
supramolecular assembly. The work published by this
group identified additional 3-hydroxyprolines in type
I collagen of different species which were spaced with
approximately the D-period.
85
Furthermore, they sug-
gested a fundamental role for the 3-hydroxyl group in
assembly of the collagen fibrils by providing hydrogen
bonding between triple helices and therefore dimer-
izing and predisposing them for registering in further
fibril packing. The most recent publication from this
group by Hudson et al. suggested some potency for
the self-association of collagen triple helices in the pres-
ence of 3-Hyp and lack of such an association in 3-Hyp-
deficient OI patients.
61
Our data (unpublished results),
however, demonstrate a normal D-periodicity in P3H1
null mice tendon collagen fibrils which completely lack
the Pro986 3-hydroxyproline and show large distur-
bances in the fibril structure.
Another possible function for the 3-hydroxyproline
could be to serve as a binding site for collagen chaper-
ones, such as HSP47 or FKBP65 or interactions with
minor collagens. Similarities in the phenotypes result-
ing from mutations in HSP47 and FKBP65 (see next
paragraphs) with those resulting from alterations in the
complex components represent the basis for such specu-
lations. An approach taken by Brendan Lee's group in
which a transgenic mouse with altered P3H1 enzymatic
activity but preserved chaperone activity of the complex
is being created will provide a clear answer.
CONSEQUENCES OF OVERMODIFIED
TYPE I COLLAGEN FOR COLLAGEN
FIBRILS
How does the overmodified collagen influence fibril
formation? Tendon or skin collagen fibrils were ana-
lyzed in each mouse model
11,15,18
and in some patients.
7
Collagen fibrils from cultured dermal fibroblasts of the
CRTAP knockout mouse showed an increased diame-
ter.
18
Consistently, an increased diameter of the skin col-
lagen fibrils was observed in a patient with a mutation in
CRTAP.
7
In the CypB null mouse, the subcutaneous tis-
sue collagen fibrils were abnormally thick in the mutant
mice in comparison to the control mice.
15
The most severe effect was observed in the P3H1 null
mouse tendon collagen fibrils. While skin collagen fibrils
showed some disorganization and an increase in diam-
eter, a complete disorganization was evident in tendon.
The average tendon fibril diameter was decreased in
P3H1 null mice compared to control, but occasionally
fibrils with larger than normal diameter were observed.
Most strikingly the tendon collagen fibrils in null mice
were branching and somewhat disoriented.
11
Our most
recent data (unpublished) suggests that this disorganiza-
tion is a postnatal process since newborn tendon fibrils
exhibit only a slight increase in diameter but otherwise
look normal. We speculate that overmodification of type
I collagen originally results in slightly bulkier triple
helices which influences their packing ability and leads
to slightly larger but well-organized newborn fibrils.
However, the most dramatic effect is seen in the postna-
tal maturation when smaller fibrils are unable to laterally
fuse together. This is causing incompetent tail tendon as
seen in the P3H1 and the CypB null mice.
11,15
The altera-
tion of the lateral fibril fusion could be due to the over-
modified collagen alone or, alternatively, an interaction
with some binding protein might be affected. This brings
us back to the question about the function of 3-Hyp in
type I collagen.
