Biology Reference
In-Depth Information
have essential zinc and calcium ions in their proteolytic domains, are synthesized as
inactive zymogens, and are inhibited by tissue inhibitors of metalloproteinases
(TIMPs) (Birkedal-Hansen et al.
1993
; McCawley and Matrisian
2000
; Werb
1997
). MMP zymogens are activated by breaking an intramolecular contact
between an unpaired cysteine residue in the propeptide domain and the zinc ion
in the active site, which usually is achieved by regulated auto- or heterocatalytic
removal of the propeptide by proteolysis, which can be triggered by a variety of
external factors (Ra and Parks
2007
; Van Wart and Birkedal-Hansen
1990
). Only a
subset of the MMPs have the capacity to cleave native collagens in vitro or in cell-
based assays at physiological pH and, thus, have the potential to participate in
collagen turnover in vivo (Fig.
3.1
). This subset includes the membrane-type
metalloproteinases, MMP-14, -15, and -16 (MT1-MMP, MT2-MMP, and MT3-
MMP) (Hotary et al.
2006
; Shi et al.
2008
); the soluble collagenases, MMP-1,
MMP-8, and MMP-13 (collagenase 1-3) (Krane and Inada
2008
); and the gelati-
nase, MMP-2 (gelatinase A) (Aimes and Quigley
1995
). MMP collagenases all
cleave fibrillar collagens at a single conserved Gly-Ile or Gly-Leu bond that is
located three quarters of the distance from the N-terminus toward the C-terminus of
the triple helix, thereby generating the telltale ¼ and
fragments after gel electro-
phoresis. Although other Gly-Ile or Gly-Leu peptide bonds occur in several places
in fibrillar collagen, only this site is susceptible to cleavage by MMP collagenases
for reasons that are not yet clear (Krane and Inada
2008
). It has been proposed
that peptide bond hydrolysis is preceded by the local unwinding of the triple helix
by the hemopexin-like domain of the MMPs, providing a general mechanism for
the action of MMP collagenases (Chung et al.
2004
). This initial MMP collagenase
cleavage leads to the rapid denaturation of collagen, which then makes the triple
helix in principle susceptible to enzymatic attack by a number of other proteolytic
enzymes, such as MMPs with gelatinolytic activity and non-MMP proteases. This
action also generates soluble or partially soluble forms of collagen that are suscep-
tible to complete degradation by the endocytic pathways of collagen turnover (see
Sect.
3.3
). As a potential “fly in the ointment” regarding this otherwise comprehen-
sive model for the turnover of fibrillar collagens, it needs to be mentioned that mice
with a point mutation introduced in type-I collagen, designed to prevent collage-
nase cleavage (Wu et al.
1990
) and which leads to the production of type-I collagen
by these mice that is completely resistant to MMP collagenase cleavage in vitro, are
viable and display a phenotype that is milder than that of mice with individual or
combined mutations in MMP collagenases (Holmbeck et al.
1999
; Shi et al.
2008
;
Zhou et al.
2000
), suggesting that other mechanisms for initial proteolytic attack of
native interstitial collagens may exist.
Delineation of the specific roles of each MMP collagenase in physiological colla-
gen turnover has been greatly advanced by loss of function studies in mice and by the
development of cell-based assays of native collagen fibril degradation (Windsor et al.
2002
). Caution should be exerted as to the direct translation of results from these
studies, as mice and humans may not use the equivalent complement of MMP
collagenases for the same purposes, and because collagen degradation ex vivo may
not accurately reflect in vivo conditions. For example, congenital MMP-2 deficiency
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