Multifunctional roles of MT1-MMP in myofiber formation and morphostatic maintenance of skeletal muscle

ECM roles in the function of metabolic tissues

All metazoan cells produce and/or interact with tissue-specific extracellular matrices (ECMs). Such ECMs play important structural roles not only in connective tissues, but in all tissues in which they provide support and anchorage for cells. However, in addition to such structural roles it has become increasingly clear that the tissue-specific microenvironments formed by the ECM play instructional roles that inform the proper phenotypes and functional behaviors of specialized cell types, and recent in vivo and in vitro studies suggest that ECM components also affect metabolic function. This review summarizes data that provide insights into the roles of the ECM in informing the proper development and functioning of highly specialized cells of metabolic tissues, such as adipocytes and islet β cells.

Adenosine A3 receptor stimulation reduces muscle injury following physical trauma and is associated with alterations in the MMP/TIMP response

We have previously demonstrated that in response to traumatic injury in skeletal muscle, there is a dysregulation of the matrix metalloproteases (MMPs) and their inhibitors (TIMPs), a response hypothesized to interfere with proper skeletal muscle regeneration. Moreover, we have shown that pharmacological activation of the adenosine A(3) receptor by Cl-IBMECA in skeletal muscle can protect against ischemia-reperfusion and eccentric exercise injury. However, the mechanism by which Cl-IBMECA protects muscle tissue is poorly defined. This study evaluated the effects of Cl-IBMECA on MMP/TIMP expression in skeletal muscle and tested the hypothesis that adenosine A(3) receptor-stimulated protection of skeletal muscle following traumatic injury is associated with a blunting of MMPs involved in inflammatory processes and collagen degradation, and an increase in MMPs associated with extracellular matrix remodeling. Sixty C57BL/6J male mice were injected with Cl-IBMECA (n = 30) or a vehicle (n = 30), and Evans blue dye. Injury was induced by applying a cold steel probe (-79°C) to the tibialis anterior (TA) muscle for 10 s. TA muscles from uninjured and injured legs were collected 3, 10, and 24 h postinjury for analysis of muscle injury and MMP/TIMP mRNA and protein levels. Twenty-four hours postinjury, 56.8% of the fibers were damaged in vehicle-treated mice vs. 35.4% in Cl-IBMECA-treated mice (P = 0.02). Cl-IBMECA treatment reduced membrane type 1 (MT1)-MMP, MMP-3, MMP-9, and TIMP-1 mRNA expression 2- to 20-fold compared with vehicle-treated mice (P < 0.05). Cl-IBMECA decreased protein levels of latent/shed MT1-MMP 23-2,000%, respectively, 3-10 h postinjury. In Cl-IBMECA-treated mice, latent MMP-2 was decreased 20% 3 h postinjury, active MMP-3 was decreased 64% 3 h postinjury, and latent/active MMP-9 was decreased 417,631% 3 h postinjury and 20% 10 h postinjury. Protein levels of active MMP-2 and latent MMP-3 were increased 25% and 74% 3 h postinjury, respectively. The present study elucidates a new protective role of adenosine A(3) receptor stimulation in posttraumatic skeletal muscle injury.

Lack of tissue inhibitor of metalloproteinases 2 leads to exacerbated left ventricular dysfunction and adverse extracellular matrix remodeling in response to biomechanical stress

BACKGROUND

Remodeling of the extracellular matrix (ECM) is a key aspect of myocardial response to biomechanical stress and heart failure. Tissue inhibitors of metalloproteinases (TIMPs) regulate the ECM turnover through negative regulation of matrix metalloproteinases (MMPs), which degrade the ECM structural proteins. Tissue inhibitor of metalloproteinases 2 is unique among TIMPs in activating pro-MMP2 in addition to inhibiting a number of MMPs. Given this dual role of TIMP2, we investigated whether TIMP2 serves a critical role in heart disease.

METHODS AND RESULTS

Pressure overload by transverse aortic constriction (TAC) in 8-week-old male mice resulted in greater left ventricular hypertrophy, fibrosis, dilation, and dysfunction in TIMP2-deficient (TIMP2(-/-)) compared with wild-type mice at 2 weeks and 5 weeks post-TAC. Despite lack of MMP2 activation, total collagenase activity and specific membrane type MMP activity were greater in TIMP2(-/-)-TAC hearts. Loss of TIMP2 resulted in a marked reduction of integrin β1D levels and compromised focal adhesion kinase phosphorylation, resulting in impaired adhesion of cardiomyocytes to ECM proteins, laminin, and fibronectin. Nonuniform ECM remodeling in TIMP2(-/-)-TAC hearts revealed degraded network structure as well as excess fibrillar deposition. Greater fibrosis in TIMP2(-/-)-TAC compared with wild-type TAC hearts was due to higher levels of SPARC (secreted protein acidic and rich in cysteine) and posttranslational stabilization of collagen fibers rather than increased collagen synthesis. Inhibition of MMPs including membrane type MMP significantly reduced left ventricular dilation and dysfunction, hypertrophy, and fibrosis in TIMP2(-/-)-TAC mice.

CONCLUSIONS

Lack of TIMP2 leads to exacerbated cardiac dysfunction and remodeling after pressure overload because of excess activity of membrane type MMP and loss of integrin β1D, leading to nonuniform ECM remodeling and impaired myocyte-ECM interaction.

Cellular and molecular mechanisms regulating fibrosis in skeletal muscle repair and disease

The repair of an injured tissue is a complex biological process involving the coordinated activities of tissue-resident and infiltrating cells in response to local and systemic signals. Following acute tissue injury, inflammatory cell infiltration and activation/proliferation of resident stem cells is the first line of defense to restore tissue homeostasis. However, in the setting of chronic tissue damage, such as in Duchenne Muscular Dystrophy, inflammatory infiltrates persist, the ability of stem cells (satellite cells) is blocked and fibrogenic cells are continuously activated, eventually leading to the conversion of muscle into nonfunctional fibrotic tissue. This review explores our current understanding of the cellular and molecular mechanisms underlying efficient muscle repair that are dysregulated in muscular dystrophy-associated fibrosis and in aging-related muscle dysfunction.

MT1-MMP: A novel component of the macrophage cell fusion machinery

Mice deficient in the matrix metalloproteinase MT1-MMP display defects in tissue development and angiogenesis, together with a complex bone phenotype characterized by several skeletal abnormalities and osteopenia. OCs and giant cells are multinucleated cells arising from the fusion of myeloid progenitors/macrophages that specialize respectively in bone resorption and engulfment of pathogens and foreign bodies. Our work identifies MT1-MMP as a novel component of the macrophage fusion machinery during OC and giant cell formation in vitro and in vivo. MT1-MMP is required for the proper lamellipodia formation and motility required to achieve proximity between fusioncompetent myeloid cells; and roles of MT1-MMP in subsequent steps of the fusion process cannot be ruled out. For example, MT1-MMP might exert additional functions at fusion sites by forming molecular complexes with CD44 or tetraspanin proteins. Interestingly, the contribution of MT1-MMP to macrophage motility and fusion does not involve its catalytic activity. Instead, the MT1-MMP-cytosolic tail, in particular Tyr(573), is required to bind the adaptor protein p130Cas and regulate localized Rac1 activity in myeloid progenitors. Modulation of this novel MT1-MMPp130Cas- Rac1 signaling pathway in macrophages might have potential in the treatment of disorders involving increased OC activity or uncontrolled giant cell formation.

Hes6 is required for actin cytoskeletal organization in differentiating C2C12 myoblasts

Hes6 is a member of the hairy-enhancer-of-split family of transcription factors that regulate proliferating cell fate in development and is known to be expressed in developing muscle. Here we investigate its function in myogenesis in vitro. We show that Hes6 is a direct transcriptional target of the myogenic transcription factors MyoD and Myf5, indicating that it is integral to the myogenic transcriptional program. The localization of Hes6 protein changes during differentiation, becoming predominantly nuclear. Knockdown of Hes6 mRNA levels by siRNA has no effect on cell cycle exit or induction of myosin heavy chain expression in differentiating C2C12 myoblasts, but F-actin filament formation is disrupted and both cell motility and myoblast fusion are reduced. The knockdown phenotype is rescued by expression of Hes6 cDNA resistant to siRNA. These results define a novel role for Hes6 in actin cytoskeletal dynamics in post mitotic myoblasts.

Aberrant repair and fibrosis development in skeletal muscle

The repair process of damaged tissue involves the coordinated activities of several cell types in response to local and systemic signals. Following acute tissue injury, infiltrating inflammatory cells and resident stem cells orchestrate their activities to restore tissue homeostasis. However, during chronic tissue damage, such as in muscular dystrophies, the inflammatory-cell infiltration and fibroblast activation persists, while the reparative capacity of stem cells (satellite cells) is attenuated. Abnormal dystrophic muscle repair and its end stage, fibrosis, represent the final common pathway of virtually all chronic neurodegenerative muscular diseases. As our understanding of the pathogenesis of muscle fibrosis has progressed, it has become evident that the muscle provides a useful model for the regulation of tissue repair by the local microenvironment, showing interplay among muscle-specific stem cells, inflammatory cells, fibroblasts and extracellular matrix components of the mammalian wound-healing response. This article reviews the emerging findings of the mechanisms that underlie normal versus aberrant muscle-tissue repair.

Molecular mechanisms of myoblast fusion across species

Skeletal muscle development, growth and regeneration depend on the ability of progenitor myoblasts to fuse to one another in a series of ordered steps. Whereas the cellular steps leading to the formation of a multinucleated myofiber are conserved in several model organisms, the molecular regulatory factors may vary. Understanding the common and divergent mechanisms regulating myoblast fusion in Drosophila melanogaster (fruit fly), Danio rerio (zebrafish) and Mus musculus (mouse) provides a better insight into the process of myoblast fusion than any of these models could provide alone. Deciphering the mechanisms of myoblast fusion from simpler to more complex organisms is of fundamental interest to skeletal muscle biology and may provide therapeutic avenues for various diseases that affect muscle.

Relaxin regulates MMP expression and promotes satellite cell mobilization during muscle healing in both young and aged mice

The polypeptide hormone relaxin has been proven to be effective in promoting both the remodeling and regeneration of various tissues, including cardiac muscle. In addition, our previous study demonstrated that relaxin is beneficial to skeletal muscle healing by both promoting muscle regeneration and preventing fibrosis formation. However, the molecular and cellular mechanisms of relaxin in regulating both myogenic cell differentiation and muscle healing process are still unclear. In this study, C2C12 mouse myoblasts and primary human myoblasts were treated with relaxin to investigate its potential effect in vitro; relaxin was also injected intramuscularly into the injured site of the mouse on the second day after injury to observe its function in vivo, especially in the aged muscle. Results showed that relaxin promoted myogenic differentiation, migration, and activation of matrix metalloproteinases (MMPs) of cultured myoblasts in vitro. In the injured muscle, relaxin administration promoted the activation of Pax7-positive skeletal muscle satellite cells and increased its local population compared with nontreated control muscles. Meanwhile, both angiogenesis and revascularization were increased, while the extended inflammatory reaction was repressed in the relaxin-treated injured muscle. Moreover, relaxin similarly promoted muscle healing in mice with aged muscle. These results revealed the multiple effects of relaxin in systematically improving muscle healing as well as its potential for clinical applications in patients with skeletal muscle injuries and diseases.

MMP-9 overexpression improves myogenic cell migration and engraftment

Myoblast migration requires matrix metalloproteinase (MMP) activity but the contribution of individual MMPs or tissue inhibitors of matrix metalloproteinase (TIMPs), particularly MMP-9 and TIMP-1, is lacking. Using two clones derived for differential regulation of MMP-2, MMP-9, and TIMP-1, we correlated protein expression with cell migration. MMP/TIMP regulation was determined by zymography, western blots, and quantitative reverse transcriptase-polymerase chain reaction (qRT-PCR). Cell migration was compared in vitro and after grafting into nude-mdx mouse muscles. C2M9 clones produced high MMP-9 and low MMP-2, and migrated better than C2F clones, which secreted low MMP-9, but overexpressed MMP-2 and TIMP-1. Improvement of C2F invasion by MMP-9 and inhibition of C2M9 migration by MMP-9 inhibitor I confirmed the role of MMP-9 and pointed to potential inhibition by TIMP-1. Higher complementation achieved by C2M9 grafts corroborated the beneficial effect of MMP-9 overexpression. Modulation of MMP-9 expression opens perspectives for improved efficacy of cell therapy for muscular dystrophies.

Loss of PI3Kγ enhances cAMP-dependent MMP remodeling of the myocardial N-cadherin adhesion complexes and extracellular matrix in response to early biomechanical stress

RATIONALE

Mechanotransduction and the response to biomechanical stress is a fundamental response in heart disease. Loss of phosphoinositide 3-kinase (PI3K)γ, the isoform linked to G protein-coupled receptor signaling, results in increased myocardial contractility, but the response to pressure overload is controversial.

OBJECTIVE

To characterize molecular and cellular responses of the PI3Kγ knockout (KO) mice to biomechanical stress.

METHODS AND RESULTS

In response to pressure overload, PI3KγKO mice deteriorated at an accelerated rate compared with wild-type mice despite increased basal myocardial contractility. These functional responses were associated with compromised phosphorylation of Akt and GSK-3α. In contrast, isolated single cardiomyocytes from banded PI3KγKO mice maintained their hypercontractility, suggesting compromised interaction with the extracellular matrix as the primary defect in the banded PI3KγKO mice. β-Adrenergic stimulation increased cAMP levels with increased phosphorylation of CREB, leading to increased expression of cAMP-responsive matrix metalloproteinases (MMPs), MMP2, MT1-MMP, and MMP13 in cardiomyocytes and cardiofibroblasts. Loss of PI3Kγ resulted in increased cAMP levels with increased expression of MMP2, MT1-MMP, and MMP13 and increased MMP2 activation and collagenase activity in response to biomechanical stress. Selective loss of N-cadherin from the adhesion complexes in the PI3KγKO mice resulted in reduced cell adhesion. The β-blocker propranolol prevented the upregulation of MMPs, whereas MMP inhibition prevented the adverse remodeling with both therapies, preventing the functional deterioration in banded PI3KγKO mice. In banded wild-type mice, long-term propranolol prevented the adverse remodeling and systolic dysfunction with preservation of the N-cadherin levels.

CONCLUSIONS

The enhanced propensity to develop heart failure in the PI3KγKO mice is attributable to a cAMP-dependent upregulation of MMP expression and activity and disorganization of the N-cadherin/β-catenin cell adhesion complex. β-Blocker therapy prevents these changes thereby providing a novel mechanism of action for these drugs.

Extrinsic regulation of satellite cell specification

Cellular commitment during vertebrate embryogenesis is controlled by an interplay of intrinsic regulators and morphogenetic signals. These mechanisms recruit a subset of cells in the developing organism to become the ancestors of skeletal muscle. Signals that control progression through the myogenic lineage converge on a battery of hierarchically organized transcription factors which modulate the cells to either remain in a primitive state or allow their commitment and differentiation into skeletal muscle fibers. A small population of cells will retain a largely unspecified state throughout development. Such stem cells, in conjunction with more committed myogenic progenitors, form a heterogeneous population that colonizes adult skeletal muscle as satellite cells. The satellite cell pool is responsible for the remarkable regenerative capacity of skeletal muscle. Similar to their counterparts during embryonic development, satellite cells are capable of self-renewal and can give rise to myogenic progeny. Impaired satellite cell homeostasis has been associated with numerous muscular disorders. Due to intense research efforts in the past two decades, the complex biology of muscle stem cells has now revealed some of its secrets and new avenues for the development of therapeutic molecules have emerged. In the present review we focus on the extrinsic mechanisms that control self-renewal, specification and differentiation of satellite cells and their significance for the development of biologic drugs.

A membrane protease regulates energy production in macrophages by activating hypoxia-inducible factor-1 via a non-proteolytic mechanism

Most cells produce ATP in the mitochondria by oxidative phosphorylation. However, macrophages, which are major players in the innate immune system, use aerobic glycolysis to produce ATP. HIF-1 (hypoxia-inducible factor-1) regulates expression of glycolysis-related genes and maintains macrophage glycolytic activity. However, it is unclear how HIF-1 activity is maintained in macrophages during normoxia. In this study, we found that macrophages lacking membrane type 1 matrix metalloproteinase (MT1-MMP/MMP-14), a potent invasion-promoting protease, exhibited considerably lower ATP levels than wild-type cells. HIF-1 was activated by an unanticipated function of MT1-MMP, which led to the stimulation of ATP production via glycolysis. The cytoplasmic tail of MT1-MMP bound to FIH-1 (factor inhibiting HIF-1), which led to the inhibition of the latter by its recently identified inhibitor, Mint3/APBA3. We have thus identified a new function of MT1-MMP to mediate production of ATP so as to support energy-dependent macrophage functions by a previously unknown non-proteolytic mechanism.

MT1-MMP and integrins: Hand-to-hand in cell communication

Integrins are transmembrane adhesion receptors essential for cell communication with the environment and in particular with the extracellular matrix (ECM). ECM components can be processed by several enzymes; one of the largest families involved in this task being matrix metalloproteinases (MMPs). MT1-MMP (membrane type 1-matrix metalloproteinase) is a membrane-anchored MMP with important roles in processes such as tissue development, tumor invasion, and angiogenesis. In addition to its catalytic-dependent functions, MT1-MMP can interact, via its cytosolic tail, with intracellular components, and trigger signaling pathways that impact cell decisions. These features make MT1-MMP similar to integrins, because both are able to integrate events in the extracellular and intracellular milieus. Accordingly, it is probably no coincidence that MT1-MMP often associates and functionally cooperates with distinct integrins at specific cellular compartments. In this review, we discuss aspects of the molecular and functional interplay between MT1-MMP and integrins in distinct cellular and biological contexts.

TIMP2 Deficiency Accelerates Adverse Post–Myocardial Infarction Remodeling Because of Enhanced MT1-MMP Activity Despite Lack of MMP2 Activation

Rationale : Myocardial infarction (MI) results in remodeling of the myocardium and the extracellular matrix (ECM). Tissue inhibitors of metalloproteinases (TIMPs) are critical regulators of ECM integrity via inhibiting matrix metalloproteinases (MMPs). TIMP2 is highly expressed in the heart and is the only TIMP that, in addition to inhibiting MMPs, is required for cell surface activation of pro-MMP2. Hence, it is difficult to predict the function of TIMP2 as protective (MMP-inhibiting) or harmful (MMP-activating) in heart disease.

Objective : We examined the role of TIMP2 in the cardiac response to MI.

Methods and Results : MI was induced in 11- to 12-week-old male TIMP2 −/− and age-matched wild-type mice. Cardiac function was monitored by echocardiography at 1 and 4 weeks post-MI. ECM fibrillar structure was visualized using second harmonic generation and multiphoton imaging of unfixed/unstained hearts. Molecular analyses were performed at 3 days and 1 week post-MI on flash-frozen infarct, periinfarct, and noninfarct tissue. Membrane type 1 (MT1)-MMP levels and activity were measured in membrane protein fractions. TIMP2 −/− -MI mice exhibited a 25% greater infarct expansion, markedly exacerbated left ventricular dilation (by 12%) and dysfunction (by 30%), and more severe inflammation compared to wild-type MI mice. Adverse ECM remodeling was detected by reduced density and enhanced disarray of fibrillar collagen in TIMP2 −/− -MI compared to wild-type MI hearts. TIMP2 deficiency completely abrogated MMP2 activation but markedly increased collagenase activity, particularly MT1-MMP activity post-MI.

Conclusions : The MMP-inhibitory function of TIMP2 is a key determinant of post-MI myocardial remodeling primarily because of its inhibitory action on MT1-MMP. TIMP2 replenishment in diseased myocardium could provide a potential therapy in reducing or preventing disease progression.

MT1-MMP is required for myeloid cell fusion via regulation of Rac1 signaling

Cell fusion is essential for fertilization, myotube formation, and inflammation. Macrophages fuse under various circumstances, but the molecular signals involved in the distinct steps of their fusion are not fully characterized. Using null mice and derived cells, we show that the protease MT1-MMP is necessary for macrophage fusion during osteoclast and giant-cell formation in vitro and in vivo. Specifically, MT1-MMP is required for lamellipodia formation and for proper cell morphology and motility of bone marrow myeloid progenitors prior to membrane fusion. These functions of MT1-MMP do not depend on MT1-MMP catalytic activity or downstream pro-MMP-2 activation. Instead, MT1-MMP null cells show a decreased Rac1 activity and reduced membrane targeting of Rac1 and the adaptor protein p130Cas. Retroviral rescue experiments and protein binding assays delineate a signaling pathway in which MT1-MMP, via its cytosolic tail, contributes to macrophage migration and fusion by regulating Rac1 activity through an association with p130Cas.

Scarless wound healing

Scarring results from injuries and disease in mammalian adults and can cause pain and loss of function in the afflicted tissues. This negative aspect of wound repair is not always true for certain amphibians and during fetal development of mammals. Based on this knowledge, scientists and clinicians are investigating the mechanisms and growth factors that contribute to or deter a suitable environment for wound healing. This review summarizes these aspects and challenges for scarless repair.

Navigating ECM barriers at the invasive front: the cancer cell-stroma interface

A seminal event in cancer progression is the ability of the neoplastic cell to mobilize the necessary machinery to breach surrounding extracellular matrix barriers while orchestrating a host stromal response that ultimately supports tissue-invasive and metastatic processes. With over 500 proteolytic enzymes identified in the human genome, interconnecting webs of protease-dependent and protease-independent processes have been postulated to drive the cancer cell invasion program via schemes of daunting complexity. Increasingly, however, a body of evidence has begun to emerge that supports a unifying model wherein a small group of membrane-tethered enzymes, termed the membrane-type matrix metalloproteinases (MT-MMPs), plays a dominant role in regulating cancer cell, as well as stromal cell, traffic through the extracellular matrix barriers assembled by host tissues in vivo. Understanding the mechanisms that underlie the regulation and function of these metalloenzymes as host cell populations traverse the dynamic extracellular matrix assembled during neoplastic states should provide new and testable theories regarding cancer invasion and metastasis.

Myoblast fusion: when it takes more to make one

Cell-cell fusion is a crucial and highly regulated event in the genesis of both form and function of many tissues. One particular type of cell fusion, myoblast fusion, is a key cellular process that shapes the formation and repair of muscle. Despite its importance for human health, the mechanisms underlying this process are still not well understood. The purpose of this review is to highlight the recent literature pertaining to myoblast fusion and to focus on a comparison of these studies across several model systems, particularly the fly, zebrafish and mouse. Advances in technical analysis and imaging have allowed identification of new fusion genes and propelled further characterization of previously identified genes in each of these systems. Among the cellular steps identified as critical for myoblast fusion are migration, recognition, adhesion, membrane alignment and membrane pore formation and resolution. Importantly, striking new evidence indicates that orthologous genes govern several of these steps across these species. Taken together, comparisons across three model systems are illuminating a once elusive process, providing exciting new insights and a useful framework of genes and mechanisms.

Role of matrix metalloproteinases in skeletal muscle: migration, differentiation, regeneration and fibrosis

Matrix metalloproteases (MMPs) are key regulatory molecules in the formation, remodeling and degradation of extracellular matrix (ECM) components in both physiological and pathological processes in many tissues. In skeletal muscle, MMPs play an important role in the homeostasis and maintenance of myofiber functional integrity by breaking down ECM and regulating skeletal muscle cell migration, differentiation and regeneration. Skeletal muscle satellite cells, a group of quiescent stem cells located between the basement membrane and the plasmalemma of myofibers, are responsible for lifelong maintenance and repairing, which can be activated and as a result migrate underneath the basement membrane to promote regeneration at the injured site. MMPs are able to degrade ECM components, thereby facilitating satellite cell migration and differentiation. This current review will focus on the critical roles of MMPs in skeletal muscle injury and repair, which include satellite cell activation with migration and differentiation. The effect of MMPs on muscle regeneration and fibrous scar tissue formation, as well as therapeutic insights for the future will be explored.

Alterations in mRNA and protein levels of metalloproteinases-2, -9, and -14 and tissue inhibitor of metalloproteinase-2 responses to traumatic skeletal muscle injury

This study characterizes the temporal relationship of membrane type-1 matrix metalloproteinase (MT1-MMP) and tissue inhibitor of metalloproteinase-2 (TIMP-2) expression in skeletal muscle following injury. Tibialis anterior (TA) muscles from 60 mice were exposed and injured by applying a cold steel probe (-79 degrees C) to the muscle for 10 s. Thereafter, TA muscles from uninjured and injured legs were collected at 3, 10, 24, 48, and 72 h postinjury for analysis of local MT1-MMP, TIMP-2, and matrix metalloproteinases-2 and -9 (MMP-2 and MMP-9) mRNA and protein content via quantitative RT-PCR, immunoblotting, zymography, and immunofluorescence. All data are expressed as fold change of injured leg vs. uninjured leg. MT1-MMP mRNA levels were decreased significantly at 48 and 72 h postinjury by approximately 9- and 21-fold, respectively (P < 0.01). Both TIMP-2 and MMP-2 mRNA expression significantly decreased in the injured leg by approximately 4- to 10-fold at 10-72 h postinjury (P < 0.01). MMP-9 mRNA expression was significantly increased at 10, 24, and 48 h postinjury by 6- (P < 0.05), 25-, and 12-fold (P < 0.01), respectively. Protein content of latent (63 kDa) MT1-MMP was decreased at 48 and 72 h postinjury by approximately 2-fold (P < 0.01). Content of the soluble (50 kDa) fragment of MT1-MMP was significantly increased by approximately 17-, 25-, and 67-fold at 24 (P < 0.05), 48, and 72 h (P < 0.01) postinjury, respectively. TIMP-2 protein levels diminished from 3 to 48 h postinjury by 1.5-fold to 1.8-fold (P < 0.01), before returning to baseline levels at 72 h postinjury. Zymography revealed visual increases in gelatinase activity in molecular weight regions corresponding to MMP-9 and MMP-2. In conclusion, skeletal muscle injury initiates a sequence of events in the MT1-MMP proteolytic cascade resulting in elevated levels of the soluble (50 kDa) fragment of MT1-MMP, which could enhance pericellular extracellular matrix remodeling.

Biochemical insights into the role of matrix metalloproteinases in regeneration: challenges and recent developments

Matrix metalloproteinases (MMPs) are a group of proteases that belong to the metazincin family. These proteins consist of similar structures featuring a signaling peptide, a propeptide domain, a catalytic domain where the notable zinc ion binding site is found and a hinge region that binds to the C-terminal hemoplexin domain. MMPs can be produced by numerous cell types through secretion or localization to the cell membrane. While certain chemical compounds have been known to generally inhibit MMPs, naturally occurring proteins known as tissue inhibitors of metalloproteinases (TIMPs) effectively interact with MMPs to modify their biological roles. MMPs are very important enzymes that actively participate in remodeling the extracellular matrix by degrading certain constituents, along with promoting cell proliferation, migration, differentiation, apoptosis and angiogenesis. In normal adult tissue, they are almost undetectable; however, when perturbed through injury, disease or pregnancy, they have elevated expression. The goal of this review is to identify new experimental findings that have provided further insight into the role of MMPs in skeletal muscle, nerve and dermal tissue, as well as in the liver, heart and kidneys. Increased expression of MMPs can improve the regeneration potential of wounds; however, an imbalance between MMP and TIMP expression can prove to be destructive for afflicted tissues.

Nap1-mediated actin remodeling is essential for mammalian myoblast fusion

Myoblast fusion is crucial for the formation, growth, maintenance and regeneration of healthy skeletal muscle. Unfortunately, the molecular machinery, cell behaviors, and membrane and cytoskeletal remodeling events that govern fusion and myofiber formation remain poorly understood. Using time-lapse imaging approaches on mouse C2C12 myoblasts, we identify discrete and specific molecular events at myoblast membranes during fusion and myotube formation. These events include rearrangement of cell shape from fibroblast to spindle-like morphologies, changes in lamellipodial and filopodial extensions during different periods of differentiation, and changes in membrane alignment and organization during fusion. We find that actin-cytoskeleton remodeling is crucial for these events: pharmacological inhibition of F-actin polymerization leads to decreased lamellipodial and filopodial extensions and to reduced myoblast fusion. Additionally, shRNA-mediated inhibition of Nap1, a member of the WAVE actin-remodeling complex, results in accumulations of F-actin structures at the plasma membrane that are concomitant with a decrease in myoblast fusion. Our data highlight distinct and essential roles for actin cytoskeleton remodeling during mammalian myoblast fusion, provide a platform for cellular and molecular dissection of the fusion process, and suggest a functional conservation of Nap1-regulated actin-cytoskeleton remodeling during myoblast fusion between mammals and Drosophila.

Identification and characterization of Lutheran blood group glycoprotein as a new substrate of membrane-type 1 matrix metalloproteinase 1 (MT1-MMP): a systemic whole cell analysis of MT1-MMP-associating proteins in A431 cells

Membrane-type 1 matrix metalloproteinase 1 (MT1-MMP) is a potent modulator of the pericellular microenvironment and regulates cellular functions in physiological and pathological settings in mammals. MT1-MMP mediates its biological effects through cleavage of specific substrate proteins. However, our knowledge of MT1-MMP substrates remains limited. To identify new substrates of MT1-MMP, we purified proteins associating with MT1-MMP in human epidermoid carcinoma A431 cells and analyzed them by mass spectrometry. We identified 163 proteins, including membrane proteins, cytoplasmic proteins, and functionally unknown proteins. Sixty-four membrane proteins were identified, and they included known MT1-MMP substrates. Of these, eighteen membrane proteins were selected, and we confirmed their association with MT1-MMP using an immunoprecipitation assay. Co-expression of each protein together with MT1-MMP revealed that nine proteins were cleaved by MT1-MMP. Lutheran blood group glycoprotein (Lu) is one of the proteins cleaved by MT1-MMP, and we confirmed the cleavage of the endogenous Lu protein by endogenous MT1-MMP in A431 cells. Mutation of the cleavage site of Lu abrogated processing by MT1-MMP. Lu protein expressed in A431 cells bound to laminin-511, and knockdown of MT1-MMP in these cells increased both their binding to laminin-511 and the amount of Lu protein on the cell surface. Thus, the identified membrane proteins associated with MT1-MMP are an enriched source of physiological MT1-MMP substrates.

Subtilisin-like proprotein convertase PACE4 is required for skeletal muscle differentiation

Most growth factors stimulate myoblast proliferation and prevent differentiation, whereas insulin-like growth factors (IGFs) promote myoblast differentiation through the phosphatidylinositol 3-kinase (PI3K) pathway. Subtilisin-like proprotein convertases (SPCs) are involved in cell growth and differentiation via activation of pro-growth factors. However, the role of SPCs in myogenesis remains poorly understood. Here we show that PACE4, a member of the SPC family, plays a critical role in myogenic differentiation of C2C12 cells. PACE4 mRNA levels increased markedly during myogenesis, whereas the expression of other member of SPC family, furin and PC6, remained unchanged. The expression pattern of pro-IGF-II, which is processed extracellularly by SPCs, was similar to that of PACE4. The expression of shRNA targeting PACE4, but not furin, suppressed the expression of the muscle-specific myosin light chain (MLC). Interestingly, reduced expression of MLC was restored following treatment with recombinant mature IGF-II. Finally, we demonstrated that the PI3K inhibitor LY294002 blocked the induction of PACE4 mRNA, a result not observed when another myogenic differentiation inhibitor, SB203580 (p38 MAP kinase inhibitor), was employed, indicating the presence of a positive feedback loop regulating PACE4 expression. These results suggest that PACE4 plays an important role in myogenic differentiation through its association with the IGF-II pathway.

Muscle expression of genes associated with inflammation, growth, and remodeling is strongly correlated in older adults with resistance training outcomes

A group (n = 8) of healthy older (68 +/- 6 yr) adults participated in a 36-session progressive resistance exercise training program targeting the thigh muscles to determine the relationship between muscle gene expression and gains in muscle size and strength. Biopsies were obtained from the vastus lateralis at baseline 72 h after an acute bout of exercise and 72 h after completion of the training program. Training increased thigh muscle size (7%) and strength for the three exercises performed: knee extension (30%) and curl (28%) and leg press (20%). We quantified 18 transcripts encoding factors that function in inflammation, growth, and muscle remodeling that were demonstrated previously to be regulated by aging and acute exercise. The gain in extension strength and muscle size showed a high number of significant correlations with gene expression. These gains were most strongly correlated (P < or = 0.003, R > or = 0.89) with the baseline mRNA levels for insulin-like growth factor-1, matrix metalloproteinase-2 and its inhibitor TIMP1, and ciliary neurotrophic factor. Moreover, strength gains were inversely correlated with the change in these mRNA levels after training (P < or = 0.002 and R < or = -0.90). Changes in gene expression after acute exercise were not associated with training outcomes. These results suggest that higher baseline expression for key genes in muscle conveys an adaptive advantage for certain older adults. Individuals with lower baseline expression of these genes show less adaptation to exercise despite increased gene expression in response to training. These genes hold promise as useful predictors of training outcomes that could be used to design more effective exercise regimens for maintaining muscle function in older adults.

Cytoplasmic tail of MT1-MMP regulates macrophage motility independently from its protease activity

Membrane type-1 matrix metalloproteinase (MT1-MMP) is a proinvasive protease that regulates various cellular functions as evidenced by myriad defects in different types of cells and tissues in MT1-MMP-deficient (MT1(-/-)) mice. Here we demonstrate that MT1(-/-) mice exhibit fewer infiltrating macrophages into sites of inflammation. MT1(-/-)macrophages exhibited a reduced ability to invade reconstituted basement membrane (Matrigel) and invasion by wild type (WT) macrophages was inhibited by a synthetic MMP inhibitor (BB94) to a level similar to that of MT1(-/-) cells. The rate of migration of MT1(-/-) macrophages was also low compared to that of the WT cells and re-expression of MT1-MMP in MT1(-/-) macrophages reconstituted their migratory activity. Unexpectedly, however, BB94 did not inhibit the migration of WT macrophages. The migration-boosting activity of MT1-MMP is retained in a mutant that lacks most of the extracellular portion including the catalytic and hemopexin-like domains. In contrast, deletion of the cytoplasmic (CP) tail abolished the activity completely. Thus, we have demonstrated that MT1-MMP regulates macrophages via its invasion-promoting protease activity as well as its CP-dependent non-proteolytic activity to boost cell migration.

Matrix metalloproteinase-1 promotes muscle cell migration and differentiation

Injured skeletal muscle has the capacity to regenerate through a highly coordinated sequence of events that involves both myoblast migration and differentiation into myofibers. Fibrosis may impede muscle regeneration by posing as a mechanical barrier to cell migration and fusion, providing inappropriate signals for cell differentiation, and limiting vascular perfusion of the injury site, subsequently leading to incomplete functional recovery. Our previous studies demonstrated that matrix metalloproteinase-1 (MMP-1) is able to digest fibrous scar tissue and improve muscle healing after injury. The goal of this study is to investigate whether MMP-1 could further enhance muscle regeneration by improving myoblast migration and differentiation. In vitro wound healing assays, flow cytometry, reverse transcriptase-polymerase chain reaction (RT-PCR), and Western blot analyses demonstrated that MMP-1 enhances myoblast migration but is not chemoattractive. We discovered that MMP-1 also enhances myoblast differentiation, which is a critical step in the sequence of muscle regeneration. In addition, RT-PCR and Western blot analyses demonstrated the up-regulation of myogenic factors after MMP-1 treatment. In vivo, we observed that myoblast transplantation was greatly improved after MMP-1 treatment within the dystrophic skeletal muscles of MDX mice. MMP-1 may therefore be able to improve muscle function recovery after injury or disease by increasing both the number of myofibers that are generated by activated myoblasts and the size of myoblast coverage area by promoting migration, thus fostering a greater degree of engraftment.

Dynamic formation of microenvironments at the myotendinous junction correlates with muscle fiber morphogenesis in zebrafish

Muscle development involves the specification and morphogenesis of muscle fibers that attach to tendons. After attachment, muscles and tendons then function as an integrated unit to transduce force to the skeletal system and stabilize joints. The attachment site is the myotendinous junction, or MTJ, and is the primary site of force transmission. We find that attachment of fast-twitch myofibers to the MTJ correlates with the formation of novel microenvironments within the MTJ. The expression or activation of two proteins involved in anchoring the intracellular cytoskeleton to the extracellular matrix, Focal adhesion kinase (Fak) and beta-dystroglycan is up-regulated. Conversely, the extracellular matrix protein Fibronectin (Fn) is down-regulated. This degradation of Fn as fast-twitch fibers attach to the MTJ results in Fn protein defining a novel microenvironment within the MTJ adjacent to slow-twitch, but not fast-twitch, muscle. Interestingly, however, Fak, laminin, Fn and beta-dystroglycan concentrate at the MTJ in mutants that do not have slow-twitch fibers. Taken together, these data elucidate novel and dynamic microenvironments within the MTJ and indicate that MTJ morphogenesis is spatially and temporally complex.

Time-lapse analysis and mathematical characterization elucidate novel mechanisms underlying muscle morphogenesis

Skeletal muscle morphogenesis transforms short muscle precursor cells into long, multinucleate myotubes that anchor to tendons via the myotendinous junction (MTJ). In vertebrates, a great deal is known about muscle specification as well as how somitic cells, as a cohort, generate the early myotome. However, the cellular mechanisms that generate long muscle fibers from short cells and the molecular factors that limit elongation are unknown. We show that zebrafish fast muscle fiber morphogenesis consists of three discrete phases: short precursor cells, intercalation/elongation, and boundary capture/myotube formation. In the first phase, cells exhibit randomly directed protrusive activity. The second phase, intercalation/elongation, proceeds via a two-step process: protrusion extension and filling. This repetition of protrusion extension and filling continues until both the anterior and posterior ends of the muscle fiber reach the MTJ. Finally, both ends of the muscle fiber anchor to the MTJ (boundary capture) and undergo further morphogenetic changes as they adopt the stereotypical, cylindrical shape of myotubes. We find that the basement membrane protein laminin is required for efficient elongation, proper fiber orientation, and boundary capture. These early muscle defects in the absence of either lamininβ1 or lamininγ1 contrast with later dystrophic phenotypes in lamininα2 mutant embryos, indicating discrete roles for different laminin chains during early muscle development. Surprisingly, genetic mosaic analysis suggests that boundary capture is a cell-autonomous phenomenon. Taken together, our results define three phases of muscle fiber morphogenesis and show that the critical second phase of elongation proceeds by a repetitive process of protrusion extension and protrusion filling. Furthermore, we show that laminin is a novel and critical molecular cue mediating fiber orientation and limiting muscle cell length.

Despite the importance of muscle fiber development and tendon attachment, this process is incompletely understood in vertebrates. One critical step is muscle fiber elongation; muscle precursor cells are short and subsequent elongation/fusion generates long, multinucleate muscle fibers. Using a vertebrate model organism, the zebrafish, we find that single round myoblasts elongate to span the entire width of the myotome prior to fusion. Using rigorous and objective mathematical characterization techniques, we can further divide muscle development into three stages: short precursor cells, intercalation/elongation, and boundary capture/myotube formation. The second phase, elongation, occurs via a two-step mechanism of protrusion extension and filling. Myotube formation involves boundary capture, where the ends of muscle fibers anchor themselves to the myotome boundary and stop elongating. We show that the protein laminin is required for boundary capture, normal fiber length, and proper fiber orientation. Genetic mosaic experiments in laminin-deficient embryos reveal that boundary capture is a cell autonomous phenomenon. Wild-type (normal) cells capture the boundary appropriately and stop elongating in laminin-deficient embryos. Although adhesion to laminin has been implicated in muscular dystrophies where the attachment between muscle cells and tendons fails, no early developmental requirements for laminin in fast muscle morphogenesis have been shown until now.

Muscle CD31(-) CD45(-) side population cells promote muscle regeneration by stimulating proliferation and migration of myoblasts

CD31(-) CD45(-) side population (SP) cells are a minor SP subfraction that have mesenchymal stem cell-like properties in uninjured skeletal muscle but that can expand on muscle injury. To clarify the role of these SP cells in muscle regeneration, we injected green fluorescent protein (GFP)-positive myoblasts with or without CD31(-) CD45(-) SP cells into the tibialis anterior muscles of immunodeficient NOD/scid mice or dystrophin-deficient mdx mice. More GFP-positive fibers were formed after co-transplantation than after transplantation of GFP-positive myoblasts alone in both mdx and NOD/scid muscles. Moreover, grafted myoblasts were more widely distributed after co-transplantation than after transplantation of myoblasts alone. Immunohistochemistry with anti-phosphorylated histone H3 antibody revealed that CD31(-) CD45(-) SP cells stimulated cell division of co-grafted myoblasts. Genome-wide gene expression analyses showed that these SP cells specifically express a variety of extracellular matrix proteins, membrane proteins, and cytokines. We also found that they express high levels of matrix metalloproteinase-2 mRNA and gelatinase activity. Furthermore, matrix metalloproteinase-2 derived from CD31(-) CD45(-) SP cells promoted migration of myoblasts in vivo. Our results suggest that CD31(-) CD45(-) SP cells support muscle regeneration by promoting proliferation and migration of myoblasts. Future studies to further define the molecular and cellular mechanisms of muscle regeneration will aid in the development of cell therapies for muscular dystrophy.

Kindlin-2 is required for myocyte elongation and is essential for myogenesis

Background

Integrins are required for normal muscle differentiation and disruptions in integrin signaling result in human muscle disease. The intracellular components that regulate integrin function during myogenesis are poorly understood. Unc-112 is an integrin-associated protein required for muscle development in C. elegans. To better understand the intracellular effectors of integrin signaling in muscle, we examined the mammalian homolog of Unc-112, kindlin-2.

Results

Kindlin-2 expression is upregulated during differentiation and highly enriched at sites of integrin localization. RNAi knockdown of kindlin-2 in C2C12 cells results in significant abnormalities during the early stages of myogenesis. Specifically, differentiating myocytes lacking kindlin-2 are unable to elongate and fail to fuse into multinucleated myotubes. These changes are correlated with decreased cell substratum adhesion and increased cell motility. They are also associated with redistribution of a known kindlin-2 binding partner, integrin linked kinase (ILK), to the membrane insoluble subcellular fraction.

Conclusion

In all, our study reveals kindlin-2 as a novel integrin adaptor protein important for muscle differentiation, and identifies it particularly as a critical regulator of myocyte elongation.

Membrane type 1-matrix metalloproteinase: substrate diversity in pericellular proteolysis

Enzymes in the matrix metalloproteinase (MMP) family have been linked to key events in developmental biology for almost 50 years. Biochemical, cellular and in vivo analyses have established that pericellular proteolysis contributes to numerous aspects of ontogeny including ovulation, fertilization, implantation, cellular migration, tissue remodeling and repair. Surface anchoring of proteinase activity provides spatial restrictions on substrate targeting. This review will utilize membrane type 1 MMP (MT1-MMP) as an example to highlight substrate diversity in pericellular proteolysis catalyzed by a membrane anchored MMP.

Molecular control of mammalian myoblast fusion

The fusion of postmitotic mononucleated myoblasts to form syncytial myofibers is a critical step in the formation of skeletal muscle. Myoblast fusion occurs both during development and throughout adulthood, as skeletal muscle growth and regeneration require the accumulation of additional nuclei within myofibers. Myoblasts must undergo a complex series of molecular and morphological changes prior to fusing with one another. Although many molecules regulating myoblast fusion have been identified, the precise mechanism by which these molecules act in concert to control fusion remains to be elucidated. A comprehensive understanding of how myo-blast fusion is controlled may contribute to the treatment of various disorders associated with loss of muscle mass. In this chapter, we examine progress made toward elucidating the cellular and molecular pathways involved in mammalian myoblast fusion. Special emphasis is placed on the molecules that regulate myofiber formation without discernibly affecting biochemical differentiation.

Tissue inhibitor of metalloproteinase-2 (TIMP-2) regulates myogenesis and beta1 integrin expression in vitro

Myogenesis in vitro involves myoblast cell cycle arrest, migration, and fusion to form multinucleated myotubes. Extracellular matrix (ECM) integrity during these processes is maintained by the opposing actions of matrix metalloproteinase (MMP) proteases and their inhibitors, the tissue inhibitor of metalloproteinases (TIMPs). Here, we report that TIMP-2, MMP-2, and MT1-MMP are differentially expressed during mouse myoblast differentiation in vitro. A specific role for TIMP-2 in myogenesis is demonstrated by altered TIMP-2(-/-) myotube formation. When differentiated in horse serum-containing medium, TIMP-2(-/-) myotubes are larger than wild-type myotubes. In contrast, when serum-free medium is used, TIMP-2(-/-) myotubes are smaller than wild-type myotubes. Regardless of culture condition, myotube size is directly correlated with MMP activity and inversely correlated with beta1 integrin expression. Treatment with recombinant TIMP-2 rescues reduced TIMP-2(-/-) myotube size and induces increased MMP-9 activation and decreased beta1 integrin expression. Treatment with either MMP-2 or MMP-9 similarly rescues reduced myotube size, but has no effect on beta1 integrin expression. These data suggest a specific regulatory relationship between TIMP-2 and beta1 integrin during myogenesis. Elucidating the role of TIMP-2 in myogenesis in vitro may lead to new therapeutic options for the use of TIMP-2 in myopathies and muscular dystrophies in vivo.

Activation and localization of matrix metalloproteinase-2 and -9 in the skeletal muscle of the muscular dystrophy dog (CXMDJ)

Background

Matrix metalloproteinases (MMPs) are key regulatory molecules in the formation, remodeling and degradation of all extracellular matrix (ECM) components in both physiological and pathological processes in various tissues. The aim of this study was to examine the involvement of gelatinase MMP family members, MMP-2 and MMP-9, in dystrophin-deficient skeletal muscle. Towards this aim, we made use of the canine X-linked muscular dystrophy in Japan (CXMD_J) model, a suitable animal model for Duchenne muscular dystrophy.

Methods

We used surgically biopsied tibialis cranialis muscles of normal male dogs (n = 3) and CXMD_J dogs (n = 3) at 4, 5 and 6 months of age. Muscle sections were analyzed by conventional morphological methods and in situ zymography to identify the localization of MMP-2 and MMP-9. MMP-2 and MMP-9 activity was examined by gelatin zymography and the levels of the respective mRNAs in addition to those of regulatory molecules, including MT1-MMP, TIMP-1, TIMP-2, and RECK, were analyzed by semi-quantitative RT-PCR.

Results

In CXMD_J skeletal muscle, multiple foci of both degenerating and regenerating muscle fibers were associated with gelatinolytic MMP activity derived from MMP-2 and/or MMP-9. In CXMD_J muscle, MMP-9 immunoreactivity localized to degenerated fibers with inflammatory cells. Weak and disconnected immunoreactivity of basal lamina components was seen in MMP-9-immunoreactive necrotic fibers of CXMD_J muscle. Gelatinolytic MMP activity observed in the endomysium of groups of regenerating fibers in CXMD_J did not co-localize with MMP-9 immunoreactivity, suggesting that it was due to the presence of MMP-2. We observed increased activities of pro MMP-2, MMP-2 and pro MMP-9, and levels of the mRNAs encoding MMP-2, MMP-9 and the regulatory molecules, MT1-MMP, TIMP-1, TIMP-2, and RECK in the skeletal muscle of CXMD_J dogs compared to the levels observed in normal controls.

Conclusion

MMP-2 and MMP-9 are likely involved in the pathology of dystrophin-deficient skeletal muscle. MMP-9 may be involved predominantly in the inflammatory process during muscle degeneration. In contrast, MMP-2, which was activated in the endomysium of groups of regenerating fibers, may be associated with ECM remodeling during muscle regeneration and fiber growth.

Stroma-derived matrix metalloproteinase (MMP)-2 promotes membrane type 1-MMP-dependent tumor growth in mice

Matrix metalloproteinase-2 (MMP-2) is a stroma-derived MMP belonging to the type IV collagenase family. It is believed to mediate tumor cell behavior by degrading deposits of type IV collagen, a major component of the basement membrane. The membrane type 1-MMP (MT1-MMP) is a highly potent activator of MMP-2 and is expressed in many tumor and stromal cells. However, the roles played by stromal MMP-2 in tumor progression in vivo remain poorly understood. We established a colon epithelial cell line from an Mt1-mmp(-/-) mouse strain and transfected these cells with an inducible expression system for MT1-MMP (MT1rev cells). Following s.c. implantation into Mmp-2(+/+) mice and induction of MT1-MMP expression, MT1rev cells grew rapidly, whereas they grew very slowly in Mmp-2(-/-) mice, even in the presence of MT1-MMP. This MT1-MMP-dependent tumor growth of MT1rev cells was enhanced in Mmp-2(-/-) mice as long as MMP-2 was supplied via transfection or coimplantation of MMP-2-positive fibroblasts. MT1rev cells cultured in vitro in a three-dimensional collagen gel matrix also required the MT1-MMP/MMP-2 axis for rapid proliferation. MT1rev cells deposit type IV collagen primarily at the cell-collagen interface, and these deposits seem scarce at sites of invasion and proliferation. These data suggest that cooperation between stroma-derived MMP-2 and tumor-derived MT1-MMP may play a role in tumor invasion and proliferation via remodeling of the tumor-associated basement membrane. To our knowledge, this is the first study demonstrating that MT1-MMP-dependent tumor growth in vivo requires stromal-derived MMP-2. It also suggests that MMP-2 represents a potential target for tumor therapeutics.

Skeletal muscle extracellular matrix remodelling after aestivation in the green striped burrowing frog, Cyclorana alboguttata

Connective tissue has recently been found to play a role in mediating mammalian skeletal muscle atrophy. We investigated connective tissue remodelling in the skeletal muscle of a species of the Australian burrowing frog, Cyclorana alboguttata. Despite being inactive whilst aestivating, the frog shows an inhibition of muscle atrophy. Connective tissue size and distribution was measured in histological sections of the cruralis muscle of control and aestivating C. alboguttata. Using a custom written software application we could detect no significant difference in any connective tissue morphological parameter between the two treatment groups. Biochemical measurements of gelatinase activity showed 2-fold higher activity in aestivating gastrocnemius muscle than in controls (p<0.001). We measured the messenger RNA transcript levels for C. alboguttata metalloproteinase 2 (MMP2) and tissue inhibitor of metalloproteinase 2 (TIMP2) in cruralis skeletal muscle using quantitative real-time PCR. The trend of reduced expression of the two genes in the aestivators did not meet statistical significance. This work indicates that aestivation in C. alboguttata leads to subtle and specific changes in some extracellular matrix remodelling factors. Their main impact is to maintain proportional representation of extracellular matrix components of skeletal muscle and therefore preserve the active frog phenotype.

Negative regulation of osteoclastogenesis by ectodomain shedding of receptor activator of NF-kappaB ligand

Receptor activator of NF-kappaB ligand (RANKL) is a transmembrane glycoprotein that has an essential role in the development of osteoclasts. The extracellular portion of RANKL is cleaved proteolytically to produce soluble RANKL, but definite RANKL sheddase(s) and the physiologic function of RANKL shedding have not yet been determined. In the present study, we found that matrix metalloproteinase (MMP) 14 and a disintegrin and metalloproteinase (ADAM) 10 have strong RANKL shedding activity. In Western blot analysis, soluble RANKL was detected as two different molecular weight products, and RNA interference of MMP14 and ADAM10 resulted in a reduction of both the lower and higher molecular weight products. Suppression of MMP14 in primary osteoblasts increased membrane-bound RANKL and promoted osteoclastogenesis in cocultures with macrophages. Soluble RANKL produced by osteoblasts from MMP14-deficient mice was markedly reduced, and their osteoclastogenic activity was promoted, consistent with the findings of increased osteoclastogenesis in vivo. RANKL shedding is an important process that down-regulates local osteoclastogenesis.