Download MMP inhibition as a potential method to augment the healing of

J Appl Physiol 115: 884–891, 2013.
First published May 2, 2013; doi:10.1152/japplphysiol.00137.2013.
Review
Role of Inflammation in Skeletal Muscle, Connective Tissue,
and Exertional Injuries: To Block or Not to Block?
HIGHLIGHTED TOPIC
MMP inhibition as a potential method to augment the healing of skeletal
muscle and tendon extracellular matrix
Max E. Davis,1 Jonathan P. Gumucio,1,2 Kristoffer B. Sugg,1,2,3 Asheesh Bedi,1
and Christopher L. Mendias1,2
1
Department of Orthopaedic Surgery, University of Michigan Medical School, Ann Arbor, Michigan; 2Department of
Molecular and Integrative Physiology, University of Michigan Medical School, Ann Arbor, Michigan; and 3Department of
Surgery, Section of Plastic Surgery, University of Michigan Medical School, Ann Arbor, Michigan
Submitted 31 January 2013; accepted in final form 25 April 2013
MMP; TIMP; collagen; tendinopathy; muscle injury; extracellular matrix
THE EXTRACELLULAR MATRIX (ECM) of the musculoskeletal system is a dynamic tissue that plays an important role in force
transmission, protection of other tissues from injury, and regeneration of tissues if they become injured. The ECM is
composed of many different molecular components, consisting
mostly of collagens, proteoglycans, and glycosaminoglycans.
The specific molecules that constitute the ECM vary, depending on the types of cells that are embedded within that matrix
and the mechanical demands of the tissue, and this regional
specialization helps to optimize efficient force transmission
throughout the kinetic chain (1, 21, 35, 41, 44). While there is
tremendous variation in the minor molecular components of
ECM, the essential structural and functional building block of
musculoskeletal ECM is collagen. The molecular diversity
between collagen subtypes is striking, with almost 30 unique
members belonging to this protein superfamily (11). Having
many types of collagen molecules allows for the ECM to
Address for reprint requests and other correspondence: C. L. Mendias, Dept.
of Orthopaedic Surgery, Univ. of Michigan Medical School, 109 Zina Pitcher
Place, BSRB 2017, Ann Arbor, MI 48109-2200 (e-mail: cmendias@umich.
edu).
884
become highly specialized to specific mechanical loading regimes. The ability to break down and synthesize new collagen
molecules in a tightly controlled and organized fashion is
critical to maintain optimized force transmission and reflects
the high degree of adaptability in ECM tissue.
Matrix metalloproteinases (MMPs) are zinc-dependent
endopeptidases that digest collagen and other structural
molecules. MMPs can be organized into four subgroups,
based on their primary substrate: collagenases, gelatinases,
membrane-bound MMPs, and stromelysins (16, 59). While
MMPs are required for baseline ECM homeostasis, injury or
disease can disregulate MMP activity, which can lead to
alterations in normal ECM architecture and disruptions in
normal force transmission (1, 58). Disruption in the normal
architecture of ECM tissue can be clinically very challenging to treat, and the targeted manipulation of MMP activity
may have the potential to enhance the healing of diseased or
injured musculoskeletal tissue. The objective of this review
is to discuss the mechanobiology of ECM and MMP regulation and the potential of targeted inhibition of MMP
activity to enhance the healing of diseased and injured
skeletal muscle and tendon tissue.
8750-7587/13 Copyright © 2013 the American Physiological Society
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Davis ME, Gumucio JP, Sugg KB, Bedi A, Mendias CL. MMP inhibition as a
potential method to augment the healing of skeletal muscle and tendon extracellular
matrix. J Appl Physiol 115: 884 – 891, 2013. First published May 2, 2013;
doi:10.1152/japplphysiol.00137.2013.—The extracellular matrix (ECM) of skeletal
muscle and tendon is composed of different types of collagen molecules that play
important roles in the transmission of forces throughout the body, and in the repair and
regeneration of injured tissues. Fibroblasts are the primary cells in muscle and tendon
that maintain, repair, and modify the ECM in response to mechanical loading, injury,
and inactivity. Matrix metalloproteinases (MMPs) are enzymes that digest collagen and
other structural molecules, which are synthesized and excreted by fibroblasts. MMPs
are required for baseline ECM homeostasis, but disruption of MMP regulation due to
injury or disease can alter the normal ECM architecture and prevent proper force
transmission. Chronic injuries and diseases of muscles and tendons can be severely
debilitating, and current therapeutic modalities to enhance healing are quite limited.
This review will discuss the mechanobiology of MMPs, and the potential use of MMP
inhibitors to improve the treatment of injured and diseased skeletal muscle and tendon
tissue.
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MMP Inhibition and ECM Regeneration
Perimysium
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Davis ME et al.
Muscle Fiber
Satellite Cell
Epimysium
Endomysium
Fibrillar Collagen (I, III)
Network Collagen (IV, VI)
Endotenon
Fibroblast
Tendon Fascicle
Epitenon
Collagen Fiber
Fig. 1. Overview of the ultrastructure of skeletal muscle and tendon, demonstrating the interaction between different cell types and their surrounding matrixes.
STRUCTURE AND COMPOSITION OF SKELETAL MUSCLE
AND TENDON ECM
An overview of the ultrastructure of skeletal muscle and
tendon is shown in Fig. 1. Skeletal muscle consists of hundreds
to thousands, sometimes millions, of long, multinucleated
fibers organized and held together by an ECM. There are four
general layers of ECM in muscle. The outermost layer is the
epimysium, which covers the surface of the muscle and has
important roles in force transmission and insulation (40).
Processes from the epimysium extend into muscle tissue and
form the second layer of connective tissue, the perimysium,
which structurally divides muscle fibers into functional groups
called fascicles. The variation in the number of fascicles allows
muscles to adopt a complex geometry and facilitate complicated movements at joints. The epimysium and perimysium are
primarily composed of the fibrillar collagens, types I and III
(38). These collagens act as molecular springs and exist in
parallel with the muscle fibers (11). The endomysium is composed of two layers of mostly type I and type III collagen that
surround individual muscle fibers. These layers fuse with the
perimysium to form a sheetlike structure that inserts into
the tendon and allows for the longitudinal transmission of force
(40). The endomysium is connected to the basement membrane
that is attached to the sarcolemma itself. The basement membrane is composed mostly of type IV and type VI collagen and
is important in transmitting forces generated within the muscle
to the tendon, as well as laterally to other muscle fibers (38,
67). Unlike the fibrillar type I and type III collagens, type IV
and type VI collagens form meshlike networks that surround
the muscle fiber and allow for the lateral transmission of forces
generated within activated muscle fiber sarcomeres to the
overall ECM (11).
Tendon tissue is arranged in a hierarchical order, similar to
skeletal muscle. Tendons are linked to muscle tissue by myotendinous junctions located at the ends of the tendon (84).
Many muscles have an aponeurosis, or internal tendon, that is
an extension of the tendon into the muscle tissue. At the other
end, tendons are connected to bone by strong fibrous structures
called entheses (8). The fundamental anatomical structure in
tendons is the tendon fiber (28, 84). The tendon fiber is
composed of mostly type I and type III collagen. Individual
tendon fibers coalesce to form tendon fascicles that are organized by the endotenon, which is a basement membrane enriched in network type IV and type VI collagens. Superficial to
the endotenon is the epitenon, which is a looser layer of
connective tissue that covers the tendon along its entire length.
The epitenon provides a smooth gliding surface for tendon
fascicles and is also the source of vascular nervous and lymphatic supply for the tendon. The paratenon or a synovial
sheath surrounds the epitenon to provide lubrication that helps
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Paratenon
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MMP Inhibition and ECM Regeneration
REGULATION OF COLLAGEN AND MMP EXPRESSION
AND ACTIVITY
Type I collagen is the most abundant collagen found in the
ECM of muscle and tendon (34, 35, 43, 44). Type I collagen is
synthesized as procollagen in the lumen of the rough endoplasmic reticulum and secreted for final assembly outside of the
cell (11). In tendon, type I collagen is deposited into the ECM
by long, thin plasma membrane projections called fibripositors
(13). Mature type I collagen is produced from two different
genes, Col1a1 and Col1a2. Three peptides, typically two ␣1
and one ␣2, coalesce into a triple-helix procollagen molecule.
Procollagen is then transported outside of the cell via the Golgi
apparatus, and a final extracellular step cleaves extension
peptides at the amino- and carboxyterminal ends, resulting in
the formation of mature type I collagen (11, 34). The process
is similar for type III collagen, except there is only one gene,
Col3a1, and three ␣1 peptides are used to make the mature
protein (11). There are several signal transduction pathways
that regulate fibrillar collagen expression, but the transforming
growth factor (TGF)-␤ pathway appears to be central to regulating the expression of type I and type III collagen. TGF-␤
binds to both type I and type II TGF-␤ transmembrane receptors, which then activate intracellular Smad2/3 and TGF-␤-
Davis ME et al.
activated kinase 1 pathways (82). Myostatin, a cytokine that is
a closely related member of the TGF-␤ superfamily, activates
similar signaling pathways to increase type I collagen gene
expression (47).
Type IV collagen is formed from six distinct genes, Col4a1
through Col4a6, that give rise to six corresponding peptides,
␣1 through ␣6. Mature type IV collagen molecules are trimeric
proteins formed first into protomers of three collagen IV
peptides that are further arranged into larger, mechanically
flexible networks that traverse the basement membrane (11).
Type VI collagen is composed of three peptides, ␣1 through
␣3, transcribed from three distinct genes, Col6a1 through
Col6a3. Mature type VI collagen molecules also coalesce into
a variety of structural motifs that span the basement membrane
(11). These systems of fibrillar and network collagens work in
concert to efficiently transmit forces throughout muscle and
tendon ECM, with the fibrillar collagens primarily transferring
forces longitudinally and the network collagens transmitting
forces laterally between cells (68). Type VI collagen also plays
an important role in organizing the overall matrix, as mice
deficient in Col6a1 display mechanically weak tendons with
disrupted fibrillar collagen organization (26). Fibroblasts are
intimately linked to network collagens via several transmembrane receptors that provide an important basis for mechanotransduction within the cell (15, 26, 75). The signaling pathways that regulate network collagen gene expression are not as
well understood as fibrillar collagens, but TGF-␤ appears to
induce the expression of both type IV and VI collagen gene
transcripts, either directly or by downregulating miR-29 gene
expression (33, 73, 83).
Measuring the collagen content of tissue can be difficult.
Collagen molecules are largely insoluble and cross-linked into
large structures that can be several megadaltons, which limits
the ability to quantify these proteins via immunoblot or ELISA
(11). Collagen molecules are relatively stable in tendons,
which generally demonstrate low levels of protein synthesis
throughout most of the core of the tendon (25). The expression
levels of collagen genes are commonly reported in the literature, but, due to the long half-life of collagen molecules, this
technique is primarily useful to estimate acute changes in
collagen production. More commonly, to quantify collagen
content, samples are enzymatically or chemically digested into
soluble peptides or amino acids, and the specific peptides and
amino acids found in collagen molecules can then be analyzed
with colorimetric assays, chromatography, or mass spectroscopy (11, 17, 85). While the use of aggressive digestion and
extraction techniques can be helpful in quantifying the collagen
content of tissues, this approach limits the ability to quantify
other proteins that may be of interest in the same samples.
MMPs are the main proteinases that break down collagen in
the ECM. MMPs are typically synthesized and released as
proenzymes and must then be activated by other proteinases,
including other MMPs, once they are outside of the cell (59,
65). The regulation of MMP activity in connective tissue
biology is an area of intense study, including the control of
MMP synthesis and activation by growth factors, chemical
agents, and other soluble factors, as well as mechanical loading
and cell-cell interactions (1, 44, 59, 86). Inflammation is a
common trigger for the induction of MMP expression and
activation of MMP enzymes (14). For acute injuries, the
elevation in MMP activity that occurs as part of the inflam-
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to cushion the tendon and reduce friction from adjacent tissues
as the tendon is stretched and relaxed. The epitenon and
paratenon together compose the peritenon.
Fibroblasts are the major cellular component of muscle and
tendon ECM and consequently are responsible for the maintenance, repair, and modification of the matrix (34). While
muscle and tendon fibroblasts are thought to perform similar
functions, they appear to arise from different populations of
progenitor cells during development and are regulated by
different sets of transcription factors. The transcription factor T
cell factor-4 is required for the proper formation of muscle
ECM during development, while the basic helix-loop-helix
transcription factor scleraxis is required for limb tendon formation during development (30, 55). Both transcription factors
are required for the initial development of limb tissue and also
appear to play important roles in the adaptation of muscles and
tendons of adult animals to mechanical loading and regeneration following injury (42, 48, 49, 57, 74).
Satellite cells, which are the main myogenic stem cell
population responsible for the repair and replacement of injured muscle fibers, are found within the basement membrane
surrounding muscle fibers (23). In response to injury, satellite
cells awaken from quiescence, migrate to the site of injury, and
fuse with damaged muscle fibers to promote fiber regeneration
(23). The interaction between satellite cells and fibroblasts has
also been shown to be critical to promote the regeneration of
injured muscles (57). While progenitor cells in tendon have
been identified in cell culture from segments of tendon digested
in vitro, the exact location of tendon progenitor cells in vivo
remains unknown, although the epitenon and paratenon are
attractive candidates (9, 48, 50). The migration of both fibroblasts and satellite cells through the ECM requires the activity
of various MMPs, and although the expression of various
MMPs has been evaluated during cell proliferation in vitro, the
contribution of the major MMPs to cell migration in muscles
and tendons remains unknown (2, 54, 61, 81, 87).
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can be measured using reverse zymography, which uses a
similar approach, but instead measures the activity of MMPs
that are incorporated into the gel (78). While there are some
limitations in the ability to connect changes in MMP and TIMP
gene expression to the in vivo activity of these enzymes, the
extraction of proteins from tissues with a dense ECM-like
tendon can be difficult, which limits the use of zymography for
certain types of tissue.
MMPs AND TIMPs IN SKELETAL MUSCLE AND TENDON
INJURIES AND DISEASES
A pathological accumulation of collagen is a common phenotype among many different types of musculoskeletal disease
states, in which the balance between collagen production and
degradation becomes dysregulated. This failure in ECM homeostasis can be distilled down to one of three proposed
mechanisms: overproduction of collagens, failure to breakdown damaged ECM, and improper reorganization of complex
supramolecular collagen networks. Many of these conditions
are directly attributed to the dysregulation of MMP activity.
This section of the review will discuss ECM synthesis and
MMP regulation in the context of physiological and pathophysiological processes involving skeletal muscle and tendon,
along with the potential for targeted MMP inhibition to enhance the healing of connective tissue.
Skeletal muscle and tendon adaption in response to exercise
training requires the precise coordination between muscle fibers and muscle and tendon ECM to optimize athletic performance (37). This necessitates a balance between the synthesis
and proper alignment of new collagen molecules, and the
breakdown of existing collagen molecules by MMPs. Disruption of this balance between collagen synthesis and the activity
of MMPs and TIMPs may result in chronic injuries, which
currently have limited treatment options.
In skeletal muscle, MMP-2, -9, and -14 are key enzymatic
factors involved in the ECM adaption to mechanical loading.
Chronic endurance training in human subjects resulted in
increases in the expression of MMP-2, -9, and -14 and TIMP-1,
although changes in collagen expression were not examined
(72). In a study of isometric, concentric, and eccentric training
in rats, for all three types of training increases in type I and III
collagen, MMP-2 and TIMP-1 and -2 were observed (24). In a
mouse model of plantaris muscle hypertrophy caused by ablation of the synergist gastrocnemius and soleus muscles, 2 days
after induction of hypertrophy, MMP-2 and TIMP-2 were downregulated, while elevations in MMP-9 and -14 and TIMP-1 were
observed. By 7 and 14 days after overload, MMP-2 and -14 and
TIMP-1 and TIMP-2 were upregulated, while MMP-9 was downregulated at these time points (12). MMPs also play a role in
muscle regeneration and injury-induced fibrosis. Following cardiotoxin injury, MMP-2 and -9 levels are upregulated and
return to baseline by 7 days after injury (32). There is also
differential expression of MMPs by fast- and slow-fibered
muscles during regeneration, with fast-fibered muscles displaying increased levels of MMP-2 and decreased MMP-9 during
regeneration, while slow-fibered muscles displayed increased
MMP-9 during regeneration (88). While these studies have
provided important descriptive information regarding changes
in MMP expression or activity during mechanical loading and
regeneration, there are a limited number of studies evaluating
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matory phase of healing is likely beneficial in the repair and
regeneration of damaged ECM. However, in chronic injuries, a
persistent increase in MMP activity likely leads to disrupted
ECM ultrastructure and tissue function. MMP gene expression
is regulated by various cytokines and signaling molecules,
including TGF-␤, IL-1␤, TNF-␣, and Wnt, but the precise
components of the signal transduction pathways that regulate
MMP expression are not well understood (14, 60). Gaining
further insight into which signaling molecules downstream of
growth factor and cytokines receptors, as well as the specific
transcription factors that regulate MMP expression, is an area
of research that warrants further study.
There are four canonical groupings of MMP enzymes, organized by their ability to degrade various ECM proteins: collagenases, gelatinases, membrane-bound MMPs, and stromelysins.
Because of the homogeneity of protein structure between each
group, substrate specificity is important in the understanding of
how MMP inhibition may change tissue structure and function.
Collagenases degrade whole collagen molecules, especially
fibrillar collagens, and include MMP-1, -8, and -13 (10, 65).
Gelatinases include MMP-2 and -9 and function to degrade
smaller network collagens and the pieces of fibrillar collagens
left over from collagenase activity (10, 65). Membrane-bound
MMPs have variable levels of endogenous collagenase and
gelatinase activity and are also critical for activating other
MMPs. MMP-14 (MT1-MMP) is one of the most widely
expressed membrane-bound MMPs (10, 65). MMP-14 plays a
central role in cell autonomous migration of pulmonary fibroblasts through type I collagen-rich ECM (71), but the role of
MMP-14 in satellite cell and muscle and tendon fibroblast
migration is not known. Stromelysins, which include MMP-3
and -10, degrade many different ECM structural proteins, but
are incapable of degrading fibrillar collagens (10).
Tissue inhibitor of metalloproteinases (TIMPs) are a class of
proteins that act as endogenous inhibitors of MMPs by binding
to the active site of the MMP catalytic domain. Four TIMPs
have been identified, TIMP-1 through -4, and all TIMPs share
a highly conserved amino-terminal domain, and all appear to
be expressed in skeletal muscle and tendon tissue (1, 10, 86).
Similar to MMPs, multiple signaling pathways appear to regulate the expression of TIMPs (56). While in vitro studies have
demonstrated that all four TIMPs can inhibit all known MMPs,
the ability of specific TIMPs to inhibit specific MMPs in vivo
is an area of continued study (56). In addition to regulating the
biological activity of MMPs, the carboxy-terminal region of
TIMPs can act as signaling molecules (53), and this particular
role of TIMPs as signaling molecules warrants further study in
musculoskeletal diseases.
There are several ways to measure MMP activity. The
expression of MMP and TIMP mRNA is commonly reported in
the literature; however, MMPs are subject to posttranslational
regulation, which limits the strength of gene expression in
estimating the actual activity of MMPs. Immunoblots or ELISAs are
other techniques used to quantify MMP and TIMP protein
abundance, but these methods have similar limitations as gene
expression in interpreting MMP activity. The most common
method to measure MMP activity is zymography, in which
tissue or cell lysates are subjected to electrophoresis in polyacrylamide gels containing specific MMP substrates that can be
quantified by staining the gels with specific dyes, or through
the use of fluorescent detection reagents (78). TIMP activity
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information about the basic molecular mechanisms that control
tendon growth and remodeling.
Tendinopathy is one of the more common chronic musculoskeletal conditions that can result in severe disability, pain,
and tendon rupture. Tendinopathies often arise due to the
failure of tendons to properly adapt to mechanical loading (43,
44, 86). Tendinopathy typically manifests with the overexpression of fibrillar collagens, disorganization of collagen fibril
orientation, a reduction in network collagen content, and
grossly altered fibroblast morphology (43, 44, 69, 86). This
increase in fibrillar collagens is accompanied by an increase in
the expression of MMP-1, -2, -8, -9, and -13 (62, 69). The
elevation of MMP-1, -8, and -13 may contribute to the development of tears, as these MMPs digest the primary loadbearing collagen fibers within the tendon. Since fibroblast
activity is required to maintain and remodel the ECM, and
fibroblasts are surrounded by network collagens that eventually
interact with load-bearing fibrillar collagens, increased levels
of MMP-2 and -9, which degrade network collagens, may be
responsible for the altered fibroblast morphology and inhibition
of tendon regeneration that is the hallmark of tendinopathies.
While the use of MMP inhibitors has some potential promise in
the treatment of chronic tendinopathies, with the exception of
the rotator cuff overuse tendinopathy model (5), the general
lack of physiologically relevant small-animal models of
chronic tendinopathy has prevented preclinical studies in the
area. In a case series of patients receiving injections of the
broad spectrum MMP inhibitor aprotinin for the treatment of
Achilles or patellar tendinopathies, most patients reported
subjective improvements in pain and function and believed that
aprotinin therapy assisted in their recovery (62). While the
local inhibition of MMP activity has some early but encouraging results, systemic MMP inhibition may be detrimental to
normal tendon function. In a clinical trial evaluating the effi-
B
MMP
C
DYSREGULATION OF MMP9
MECHANICAL LOADING
MMP
TIMP
TIMP
Col I,III
Col I,III
MMP9 INHIBITION?
NORMAL ECM HOMEOSTASIS
MMP, TIMP and collagen balanced
Fibrillar Collagen (I, III)
Network Collagen (IV, VI)
Increased
MMP-9 Activity
POSITIVE ADAPTATIONS
Coordinated regulation of
MMPs, TIMPs and collagen to
synthesize and remodel ECM
Basement Membrane
Failure
MALADAPTATIONS OF ECM
Fibroblasts unable to accurately
sense force transmission in
order to repair and remodel ECM
Fig. 2. Proposed mechanism of matrix metalloproteinase (MMP)-9 dysregulation and the development of tendinopathy. A: during normal extracellular matrix
(ECM) homeostasis, the activities of MMPs and tissue inhibitor of metalloproteinases (TIMPs) are balanced with collagen production. B: in response to increased
physiological loading, fibroblasts sense the increased load through interaction with the basement membrane collagens and adjust the activity of MMPs and TIMPs
and collagen expression and orientation, which results in an increase in ECM volume and improved ECM organization. C: in cases of chronic injury or unloading,
elevated MMP-9 activity leads to degradation of the basement membrane network collagens and the inability of fibroblasts to correctly sense force transmission.
Targeted inhibition of MMP-9 could prevent the further progression of chronic disease and may allow for the restoration of a well-organized basement membrane
collagen network and allow the fibroblast to properly respond to its environment.
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MMP inhibition as a method to improve muscle repair after
injury. In a rat ischemia reperfusion injury model, nonspecific
inhibition of MMPs using doxycycline protected the muscle
from reperfusion injury (70). Using a crush injury model in
rats, the use of doxycycline or an MMP-9-specific inhibitor
reduced fibrosis and promoted regeneration, while specific
inhibition of MMP-2 did not impact regeneration (89). While
these studies have been informative, the use of targeted, temporally controlled, genetically modified mouse models would
also allow for further insight into the molecular mechanisms of
MMP-mediated muscle ECM adaptation and regeneration.
Adaptation of tendon ECM to mechanical loading follows a
similar pattern seen in muscle ECM. While there are similarities between muscle and tendon in the biological mechanisms
of ECM adaptation, muscle injuries are often responsive
to rehabilitation and other conservative treatments, whereas
chronic tendon injuries generally have slower rates of healing
and appear to have poorer clinical outcomes than muscle (3,
27). Regional differences in collagen, MMP, and TIMP expression occur throughout the length of the tendon, likely due
to different mechanical demands placed on the tendon (45). In
response to a single bout of uphill treadmill training, increases
in MMP-2 and -9 and TIMP-1 and -2 were observed in
Achilles tendon dialysate (35). Similar to what was observed in
muscles, the tendons of rats that underwent isometric, concentric, and eccentric training had increases in type I and III
collagen, MMP-2, and TIMP-1 and -2 expression for all
training types (24). In various studies of cultured tendon
fibroblasts, mechanical stretching increased type I collagen and
MMP-1 and -3 expression, with no change in MMP-2 or -9 (4,
22, 87). With recent developments in identifying promoters
that are generally specific to tendon fibroblasts like scleraxis
(66), the use of targeted, temporally controlled, genetically
modified mouse models would also provide much greater
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EVALUATION AND FUTURE DIRECTION OF MMP
INHIBITION IN CONNECTIVE TISSUE HEALING
It is clear that MMPs play a central role in the adaptation of
skeletal muscle and tendon tissues to mechanical loading,
injury, and disease. There is strong potential for the application
of MMP biology in the treatment of connective tissue disorders, and also for the use of MMPs as biomarkers of disease
progression. While much attention has focused on factors that
regulate type I collagen synthesis and degradation, in part due
to the overwhelming abundance of type I collagen in the ECM,
the role that type IV and type VI collagen play in muscle and
tendon injuries and diseases has been overlooked. Even though
these network collagens only make up a small fraction of the
total mass of muscle and tendon ECM, they serve as a critical
link between fibroblasts and their surrounding environment. A
frequent observation in many musculoskeletal injury and disease states is an upregulation of MMP-9, a downregulation of
TIMPs, and disordered collagen expression. We hypothesize
that an MMP-9-mediated degradation of network collagens
impairs the ability of fibroblasts to properly sense forces
transmitted through the ECM, disrupts their potential to respond to mechanical loading, and leads to the failure of
fibroblasts to repair sites of injury (Fig. 2). Targeted, temporal
inactivation or overexpression of various MMPs and TIMPs
using fibroblast-specific promoters would allow for the testing
of this hypothesis. Gaining a greater understanding of MMP
biology will help in the design and selection of specific MMP
inhibitors that could substantially advance the treatment of
skeletal muscle and tendon injuries and diseases.
GRANTS
This work was supported by National Institute of General Medical Sciences
Grant GM-008322 and a fellowship from the Alpha Omega Alpha Honor
Medical Society.
DISCLOSURES
No conflicts of interest, financial or otherwise, are declared by the author(s).
AUTHOR CONTRIBUTIONS
Author contributions: M.E.D., J.P.G., K.B.S., A.B., and C.L.M. prepared
figures; M.E.D., J.P.G., K.B.S., A.B., and C.L.M. drafted manuscript; M.E.D.,
J.P.G., K.B.S., A.B., and C.L.M. edited and revised manuscript; M.E.D.,
J.P.G., K.B.S., A.B., and C.L.M. approved final version of manuscript.
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prevent metastasis in cancer patients, 30% of subjects reported
tendon inflammation and pain that subsided within 2 wk of
discontinuing marimastat treatment (18). Combined, these results suggest a potential role for local and specific MMP
inhibition in the treatment of chronic tendinopathies, and that
there is a clear need for randomized, double-blind, placebocontrolled studies in this area.
Platelet-rich plasma (PRP) is a therapeutic modality that is
gaining in popularity in the sports medicine field for the
treatment of patients with muscle injuries and tendinopathies
(51, 63). A detailed analysis of the results of clinical trials of
PRP for the treatment of tendinopathies is reviewed elsewhere
(76, 80), but, in general, PRP has shown some promise and
improved clinical outcomes with proper use. PRP is readily
administered percutaneously, and exposure to the fibrillar collagens induces platelet activation and subsequent degranulation
and release of various growth factors, cytokines, and MMPs.
Injected locally into the site of injury, PRP is thought to
stimulate cell proliferation, migration, and differentiation, as
well as collagen synthesis and angiogenesis (19, 52). There is
a lack of consensus on the ability of PRP to regulate collagen
degradation and the expression and activation of specific
MMPs, but PRP contains MMPs, and some of the growth
factors concentrated within PRP have roles in regulating MMP
expression (29, 46, 64, 77, 79). Both the basic science and
clinical literature regarding PRP are complicated by a lack of
standardization of PRP preparation, study models, assessment
techniques, and subject selection. While there is some encouraging early work, further investigation into regulation of
MMPs by PRP and appropriately designed prospective clinical
studies is warranted.
Tendon rupture, either from an acute traumatic event or
following chronic tendinopathy, is a more severe injury that, in
most cases, requires surgical repair. MMPs play a central role
in the susceptibility of the tendon to tear, as well as in the
recovery of tendons following surgical repair. In ruptured
Achilles tendons, there is an increase in the activity of MMP-2
and -9 and TIMP-1 and -2 (31). While Achilles tendon tears
occur relatively infrequently, the most common site of tendon
tear following chronic tendinopathy is the rotator cuff (6, 7). In
ruptured rotator cuff tendons, MMP-1, -9, and -13 expression
is elevated and positively correlated with size of the tear, while
TIMP-2, -3, and -4 are downregulated (36, 39, 86). Additionally, torn rotator cuff tendons contain a higher proportion of
type III collagen compared with that in healthy controls (16).
In a preclinical rat model of rotator cuff tendon tear followed
by acute repair, the use of the nonspecific MMP inhibitors
doxycycline and ␣2-macroglobulin improved repair by enhancing the orientation and organization of the tendon ECM,
thereby developing greater amounts of mature fibrocartilage
and subsequently increasing the tendon load to failure (6, 7).
The simultaneous decrease in TIMP expression with an increase in MMP expression likely inhibits the repair of torn
rotator cuff tendons and contributes to the generally poor
outcomes in patients with chronic tears (20). Preclinical models of MMP inhibition have shown promise in improving
rotator cuff repair (6, 7, 20), and future studies evaluating
targeted, specific, and local MMP inhibition in large-animal
models or human subjects are warranted.
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