LIM mineralization protein-1 potentiates bone morphogenetic protein responsiveness via a novel interaction with Smurf1 resulting in decreased ubiquitination of Smads

Research Progress in Function and Regulation of E3 Ubiquitin Ligase SMURF1

Smad ubiquitylation regulatory factor 1 (Smurf1) is an important homologous member of E6-AP C-terminus type E3 ubiquitin ligase. Initially, Smurf1 was reportedly involved in the negative regulation of the bone morphogenesis protein (BMP) pathway. After further research, several studies have confirmed that Smurf1 is widely involved in various biological processes, such as bone homeostasis regulation, cell migration, apoptosis, and planar cell polarity. At the same time, recent studies have provided a deeper understanding of the regulatory mechanisms of Smurf1's expression, activity, and substrate selectivity. In our review, a brief summary of recent important biological functions and regulatory mechanisms of E3 ubiquitin ligase Smurf1 is proposed.

The unexpected versatility of ALP/Enigma family proteins

One of the most intriguing features of multicellular animals is their ability to move. On a cellular level, this is accomplished by the rearrangement and reorganization of the cytoskeleton, a dynamic network of filamentous proteins which provides stability and structure in a stationary context, but also facilitates directed movement by contracting. The ALP/Enigma family proteins are a diverse group of docking proteins found in numerous cellular milieus and facilitate these processes among others. In vertebrates, they are characterized by having a PDZ domain in combination with one or three LIM domains. The family is comprised of CLP-36 (PDLIM1), Mystique (PDLIM2), ALP (PDLIM3), RIL (PDLIM4), ENH (PDLIM5), ZASP (PDLIM6), and Enigma (PDLIM7). In this review, we will outline the evolution and function of their protein domains which confers their versatility. Additionally, we highlight their role in different cellular environments, focusing specifically on recent advances in muscle research using Drosophila as a model organism. Finally, we show the relevance of this protein family to human myopathies and the development of muscle-related diseases.

NEDD4 E3 Ligases: Functions and Mechanisms in Bone and Tooth

Protein ubiquitination is a precisely controlled enzymatic cascade reaction belonging to the post-translational modification of proteins. In this process, E3 ligases catalyze the binding of ubiquitin (Ub) to protein substrates and define specificity. The neuronally expressed developmentally down-regulated 4 (NEDD4) subfamily, belonging to the homology to E6APC terminus (HECT) class of E3 ligases, has recently emerged as an essential determinant of multiple cellular processes in different tissues, including bone and tooth. Here, we place special emphasis on the regulatory role of the NEDD4 subfamily in the molecular and cell biology of osteogenesis. We elucidate in detail the specific roles, downstream substrates, and upstream regulatory mechanisms of the NEDD4 subfamily. Further, we provide an overview of the involvement of E3 ligases and deubiquitinases in the development, repair, and regeneration of another mineralized tissue—tooth.

SMURF1, a promoter of tumor cell progression?

Overexpression of HECT-type E3 ubiquitin ligase SMURF1 is correlated with poor prognosis in patients with various cancers, such as glioblastoma, colon cancer, and clear cell renal cell carcinoma. SMURF1 acts as a tumor promoter by ubiquitination modification and/or degradation of tumor-suppressing proteins. Combined treatment of Smurf1 knockdown with rapamycin showed collaborative antitumor effects in mice. This review described the role of HECT, WW, and C2 domains in regulating SMURF1 substrate selection. We summarized up to date SMURF1 substrates regulating different type cell signaling, thus, accelerating tumor progression, invasion, and metastasis. Furthermore, the downregulation of SMURF1 expression, inhibition of its E3 activity and regulation of its specificity to substrates prevent tumor progression. The potential application of SMURF1 regulators, specifically, wisely choose certain drugs by blocking SMURF1 selectivity in tumor suppressors, to develop novel anticancer treatments.

The functions and regulation of Smurfs in cancers

Smad ubiquitination regulatory factor 1 (Smurf1) and Smurf2 are HECT-type E3 ubiquitin ligases, and both Smurfs were initially identified to regulate Smad protein stability in the TGF-β/BMP signaling pathway. In recent years, Smurfs have exhibited E3 ligase-dependent and -independent activities in various kinds of cells. Smurfs act as either potent tumor promoters or tumor suppressors in different tumors by regulating biological processes, including metastasis, apoptosis, cell cycle, senescence and genomic stability. The regulation of Smurfs activity and expression has therefore emerged as a hot spot in tumor biology research. Further, the Smurf1- or Smurf2-deficient mice provide more in vivo clues for the functional study of Smurfs in tumorigenesis and development. In this review, we summarize these milestone findings and, in turn, reveal new avenues for the prevention and treatment of cancer by regulating Smurfs.

Bone regeneration strategies: Engineered scaffolds, bioactive molecules and stem cells current stage and future perspectives

Bone fractures are the most common traumatic injuries in humans. The repair of bone fractures is a regenerative process that recapitulates many of the biological events of embryonic skeletal development. Most of the time it leads to successful healing and the recovery of the damaged bone. Unfortunately, about 5-10% of fractures will lead to delayed healing or non-union, more so in the case of co-morbidities such as diabetes. In this article, we review the different strategies to heal bone defects using synthetic bone graft substitutes, biologically active substances and stem cells. The majority of currently available reviews focus on strategies that are still at the early stages of development and use mostly in vitro experiments with cell lines or stem cells. Here, we focus on what is already implemented in the clinics, what is currently in clinical trials, and what has been tested in animal models. Treatment approaches can be classified in three major categories: i) synthetic bone graft substitutes (BGS) whose architecture and surface can be optimized; ii) BGS combined with bioactive molecules such as growth factors, peptides or small molecules targeting bone precursor cells, bone formation and metabolism; iii) cell-based strategies with progenitor cells combined or not with active molecules that can be injected or seeded on BGS for improved delivery. We review the major types of adult stromal cells (bone marrow, adipose and periosteum derived) that have been used and compare their properties. Finally, we discuss the remaining challenges that need to be addressed to significantly improve the healing of bone defects.

Network modules uncover mechanisms of skeletal muscle dysfunction in COPD patients

Background

Chronic obstructive pulmonary disease (COPD) patients often show skeletal muscle dysfunction that has a prominent negative impact on prognosis. The study aims to further explore underlying mechanisms of skeletal muscle dysfunction as a characteristic systemic effect of COPD, potentially modifiable with preventive interventions (i.e. muscle training). The research analyzes network module associated pathways and evaluates the findings using independent measurements.

Methods

We characterized the transcriptionally active network modules of interacting proteins in the vastus lateralis of COPD patients (n = 15, FEV₁ 46 ± 12% pred, age 68 ± 7 years) and healthy sedentary controls (n = 12, age 65 ± 9  years), at rest and after an 8-week endurance training program. Network modules were functionally evaluated using experimental data derived from the same study groups.

Results

At baseline, we identified four COPD specific network modules indicating abnormalities in creatinine metabolism, calcium homeostasis, oxidative stress and inflammatory responses, showing statistically significant associations with exercise capacity (VO₂ peak, Watts peak, BODE index and blood lactate levels) (P < 0.05 each), but not with lung function (FEV₁). Training-induced network modules displayed marked differences between COPD and controls. Healthy subjects specific training adaptations were significantly associated with cell bioenergetics (P < 0.05) which, in turn, showed strong relationships with training-induced plasma metabolomic changes; whereas, effects of training in COPD were constrained to muscle remodeling.

Conclusion

In summary, altered muscle bioenergetics appears as the most striking finding, potentially driving other abnormal skeletal muscle responses.

Trial registration The study was based on a retrospectively registered trial (May 2017), ClinicalTrials.gov identifier: NCT03169270

Electronic supplementary material

The online version of this article (10.1186/s12967-018-1405-y) contains supplementary material, which is available to authorized users.

BMP and Beyond: A 25-Year Historical Review of Translational Spine Research at Emory University

A high rate of symptomatic spinal pseudoarthrosis and a wide range of complications associated with the use of iliac crest bone graft (the gold standard) have prompted the spine surgery community to seek alternative options to promote spinal fusion. Emory University has been one of the global leaders in this endeavor. This invited review covers the last 25 years of Emory's contributions to translational spine research, focusing specifically on our work with bone morphogenetic proteins (BMP) and the BMP signaling pathway. As a result of this work, recombinant human BMP-2 is the only Food and Drug Administration approved biologic bone graft substitute. It has been shown to significantly increase spinal fusion rates across the spinal column because of its potent ability to stimulate local bone formation through the recruitment of mesenchymal stem cells. This review covers our development of animal models of spinal fusion, our body of work regarding the translation of BMP from the benchtop to the clinic, the discovery of LMP-1 and strategies to enhance cellular responsiveness to BMPs, and the design of various small molecule drugs that can enhance local bone formation.

FHL2 interacts with EGFR to promote glioblastoma growth

Four-and-a-half LIM protein2 (FHL2) is a member of the LIM-only protein family, which plays a critical role in tumorigenesis. We previously reported that FHL2 is upregulated and plays an oncogenic role in glioblastoma (GBM), the most common and aggressive brain tumor. GBM is also marked by amplification of the epidermal growth factor receptor (EGFR) gene and its mutations, of which EGFRvIII is the most common and functionally significant. Here we report that FHL2 physically interacts with the wild-type EGFR and its mutated EGFRvIII form in GBM cells. Expression of FHL2 caused increased EGFR and EGFRvIII protein levels and this was due to an increase in protein stability rather than an increase in EGFR mRNA expression. In contrast, FHL2 knockdown using RNA interference reduced EGFR and EGFRvIII protein expression and the phosphorylation levels of EGFR and AKT. Consistent with these features, EGFR expression was significantly lower in mouse FHL2-null astrocytes, where reintroduction of FHL2 was able to restore EGFR levels. Using established GBM cell lines and patient-derived neurosphere lines, FHL2 silencing markedly induced cell apoptosis in EGFRvIII-positive cells. Targeting FHL2 significantly prevented EGFRvIII-positive GBM tumor growth in vivo. FHL2 expression also positively correlated with EGFR expression in GBM samples from patients. Taken together, our results demonstrate that FHL2 interacts with EGFR and EGFRvIII to increase their levels and this promotes glioma growth, representing a novel mechanism that may be therapeutically targetable.

Bone morphogenetic protein signaling in bone homeostasis

Bone morphogenetic proteins (BMPs) are cytokines belonging to the transforming growth factor-β (TGF-β) superfamily. They play multiple functions during development and tissue homeostasis, including regulation of the bone homeostasis. The BMP signaling pathway consists in a well-orchestrated manner of ligands, membrane receptors, co-receptors and intracellular mediators, that regulate the expression of genes controlling the normal functioning of the bone tissues. Interestingly, BMP signaling perturbation is associated to a variety of low and high bone mass diseases, including osteoporosis, bone fracture disorders and heterotopic ossification. Consistent with these findings, in vitro and in vivo studies have shown that BMPs have potent effects on the activity of cells regulating bone function, suggesting that manipulation of the BMP signaling pathway may be employed as a therapeutic approach to treat bone diseases. Here we review the recent advances on BMP signaling and bone homeostasis, and how this knowledge may be used towards improved diagnosis and development of novel treatment modalities. This article is part of a Special Issue entitled "Muscle Bone Interactions".

LIM Mineralization Protein-1 Enhances Bone Morphogenetic Protein-2-Mediated Osteogenesis Through Activation of ERK1/2 MAPK Pathway and Upregulation of Runx2 Transactivity

LIM mineralization protein-1 (LMP-1) is an intracellular regulator of bone formation. Upregulation of bone morphogenetic proteins (BMPs) and stabilization of BMP/Smad signaling have been proven to be the key mechanisms through which LMP-1 enhances osteogenesis. However, how LMP-1 regulates BMPs expression and related bone formation remains unclear. In this study, a LMP-1-induced osteogenesis cell model was used to study the molecular action of LMP-1 on BMP-2 expression and bone formation. The results show that overexpression of LMP-1 significantly increases, whereas downregulation of endogenous LMP-1 decreases BMP-2 expression and bone formation. Antagonism of BMP-2 with noggin or short hairpin BMP-2 significantly attenuates the osteoinductive effect of LMP-1, suggesting that the osteoinductive effect of LMP-1 is mediated by BMP-2. LMP-1 regulation of BMP-2 is found to occur at the transcription level using a luciferase reporter assay with a reporter construct containing a BMP-2 promoter. A promoter deletion assay reveals that -1000/-500 bp is the key regulated region by LMP-1. A Runx2-binding site is then located at -934/-920 bp and confirmed by luciferase assay using a reporter construct containing repeats of this Runx2-binding site and the site-directed mutagenesis analysis. Overexpression of LMP-1 significantly increases Runx2 expression. Downregulation of Runx2 expression significantly decreases BMP-2 promoter activity and BMP-2 expression. A ChIP assay demonstrates that LMP-1 increases the interaction between Runx2 and BMP-2 promoter. A luciferase reporter assay using the OSE2 promoter containing a Runx2-binding site confirms that Runx2 transactivity can be upregulated by LMP-1. Moreover, inhibiting the activation of different pathways with specific pathway inhibitors reveals that ERK1/2 MAPK activation is essential for LMP-1-induced upregulation of Runx2 transactivity and subsequent BMP-2 expression. In conclusion, our novel findings describe a positive regulatory effect of LMP-1 on BMP-2 expression and BMP-2-mediated osteogenesis. This effect occurs through activation of ERK1/2 pathway and subsequent upregulation of Runx2 transactivity.

LIM mineralization protein-1 suppresses TNF-α induced intervertebral disc degeneration by maintaining nucleus pulposus extracellular matrix production and inhibiting matrix metalloproteinases expression

Imbalanced metabolism of Nucleus pulposus (NP) extracellular matrix (ECM) is closely correlated to Intervertebral Disc Degenerative Disease. LIM mineralization protein-1 (LMP-1) has been proven to induce sulfated glycosaminoglycan (sGAG) production in NP and have an anti-inflammatory effect in pre-osteoclast. However, whether it has any effect on the NP ECM production and degradation under inflammatory stimulation has not been studied. In the current study, a TNF-α induced cell model was established in vitro. Lentivirus encoding LMP-1 (LV-LMP-1) and short heparin LMP-1 (LV-shLMP-1) were constructed to overexpress and knockdown LMP-1 expression in NP cells. LMP-1 mRNA level was regulated in a dose-dependent manner after transfection. LV-LMP-1 increased whereas LV-shLMP-1 decreased collagen II, aggrecan, versican expression, and sGAG production. LV-LMP-1 abolished while LV-shLMP-1 aggravated TNF-α mediated down-regulation of the above matrix genes via ERK1/2 activation. Moreover, LV-LMP-1 abrogated TNF-α induced MMP-3 and MMP-13 expression via inhibiting p65 translocation and MMP-3 and MMP-13 promoter activity. These results indicated that LMP-1 had an ECM production maintenance effect under inflammatory stimulation. This effect was via up-regulation of matrix genes expression at least partially through ERK1/2 activation, and down-regulation of MMPs expression through NF-κB inhibition.

Characterization of a unique motif in LIM mineralization protein-1 that interacts with jun activation-domain-binding protein 1

Development and repair of the skeletal system and other organs are highly dependent on precise regulation of the bone morphogenetic protein (BMP) pathway. The use of BMPs clinically to induce bone formation has been limited in part by the requirement of much higher doses of recombinant proteins in primates than were needed in cell culture or rodents. Therefore, increasing cellular responsiveness to BMPs has become our focus. We determined that an osteogenic LIM mineralization protein, LMP-1 interacts with Smurf1 (Smad ubiquitin regulatory factor 1) and prevents ubiquitination of Smads resulting in potentiation of BMP activity. In the region of LMP-1 responsible for bone formation, there is a motif that directly interacts with the Smurf1 WW2 domain and thus effectively competes for binding with Smad1 and Smad5, key signaling proteins in the BMP pathway. Here we show that the same region also contains a motif that interacts with Jun activation-domain-binding protein 1 (Jab1) which targets a common Smad, Smad4, shared by both the BMP and transforming growth factor-β (TGF-β) pathways, for proteasomal degradation. Jab1 was first identified as a coactivator of the transcription factor c-Jun. Jab1 binds to Smad4, Smad5, and Smad7, key intracellular signaling molecules of the TGF-β superfamily, and causes ubiquitination and/or degradation of these Smads. We confirmed a direct interaction of Jab1 with LMP-1 using recombinantly expressed wild-type and mutant proteins in slot-blot-binding assays. We hypothesized that LMP-1 binding to Jab1 prevents the binding and subsequent degradation of these Smads causing increased accumulation of osteogenic Smads in cells. We identified a sequence motif in LMP-1 that was predicted to interact with Jab1 based on the MAME/MAST sequence analysis of several cellular signaling molecules that are known to interact with Jab-1. We further mutated the potential key interacting residues in LMP-1 and showed loss of binding to Jab1 in binding assays in vitro. The activities of various wild-type and mutant LMP-1 proteins were evaluated using a BMP-responsive luciferase reporter and alkaline phosphatase assay in mouse myoblastic cells that were differentiated toward the osteoblastic phenotype. Finally, to strengthen physiological relevance of LMP-1 and Jab1 interaction, we showed that overexpression of LMP-1 caused nuclear accumulation of Smad4 upon BMP treatment which is reflective of increased Smad signaling in cells.

The role of small molecules in bone regeneration

Although several methods have been used in bone regeneration medicine, current methods still have many limitations. The tissue used for autogenous bone graft is limited and allograft has weak osteoinductive activity. Tissue engineering provides a good choice for bone regeneration. However, the growth factors needed have a high price and short half-life. Recently, a number of small molecules have been confirmed to have osteoinductive activity and some have been clinically used. Natural small molecules including decalpenic acid, flavonoids, quinones can be extracted from plants and others can be synthesized according to the structure designed or mimicking the structure of natural small molecules. Small molecules can act as co-activator of BMP2 pathway or activate Wnt pathway; others can be the inhibitors of NF-κB signaling pathway. This review gives an overview on the small molecules with osteoinductive activity and discusses the mechanism of the small molecules.

Smurf-mediated differential proteolysis generates dynamic BMP signaling in germline stem cells during Drosophila testis development

Germline stem cells (GSCs) produce gametes throughout the reproductive life of many animals, and intensive studies have revealed critical roles of BMP signaling to maintain GSC self-renewal in Drospophila adult gonads. Here, we show that BMP signaling is downregulated as testes develop and this regulation controls testis growth, stem cell number, and the number of spermatogonia divisions. Phosphorylated Mad (pMad), the activated Drosophila Smad in germ cells, was restricted from anterior germ cells to GSCs and hub-proximal cells during early larval development. pMad levels in GSCs were then dramatically downregulated from early third larval instar (L3) to late L3, and maintained at low levels in pupal and adult GSCs. The spatial restriction and temporal down-regulation of pMad, reflecting the germ cell response to BMP signaling activity, required action in germ cells of E3 ligase activity of HECT domain protein Smurf. Analyses of Smurf mutant testes and dosage-dependent genetic interaction between Smurf and mad indicated that pMad downregulation was required for both the normal decrease in stem cell number during testis maturation in the pupal stage, and for normal limit of four rounds of spermatogonia cell division for control of germ cell numbers and testis size. Smurf protein was expressed at a constant low level in GSCs and spermatogonia during development. Rescue experiments showed that expression of exogenous Smurf protein in early germ cells promoted pMad downregulation in GSCs in a stage-dependent but concentration-independent manner, suggesting that the competence of Smurf to attenuate response to BMP signaling may be regulated during development. Taken together, our work reveals a critical role for differential attenuation of the response to BMP signaling in GSCs and early germ cells for control of germ cell number and gonad growth during development.

Regulation of the transforming growth factor β pathway by reversible ubiquitylation

The transforming growth factor β (TGFβ) signalling pathway plays a central role during embryonic development and in adult tissue homeostasis. It regulates gene transcription through a signalling cascade from cell surface receptors to intracellular SMAD transcription factors and their nuclear cofactors. The extent, duration and potency of signalling in response to TGFβ cytokines are intricately regulated by complex biochemical processes. The corruption of these regulatory processes results in aberrant TGFβ signalling and leads to numerous human diseases, including cancer. Reversible ubiquitylation of pathway components is a key regulatory process that plays a critical role in ensuring a balanced response to TGFβ signals. Many studies have investigated the mechanisms by which various E3 ubiquitin ligases regulate the turnover and activity of TGFβ pathway components by ubiquitylation. Moreover, recent studies have shed new light into their regulation by deubiquitylating enzymes. In this report, we provide an overview of current understanding of the regulation of TGFβ signalling by E3 ubiquitin ligases and deubiquitylases.

A Smurf1 tale: function and regulation of an ubiquitin ligase in multiple cellular networks

Since being discovered and intensively studied for over a decade, Smad ubiquitylation regulatory factor-1 (Smurf1) has been linked with several important biological pathways, including the bone morphogenetic protein pathway, the non-canonical Wnt pathway, and the mitogen-activated protein kinase pathway. Multiple functions of this ubiquitin ligase have been discovered in cell growth and morphogenesis, cell migration, cell polarity, and autophagy. Smurf1 is related to physiological manifestations in terms of age-dependent deficiency in bone formation and invasion of tumor cells. Smurf1-knockout mice have a significant phenotype in the skeletal system and considerable manifestations during embryonic development and neural outgrowth. In depth studying of Smurf1 will help us to understand the etiopathological mechanisms of related disorders. Here, we will summarize historical and recent studies on Smurf1, and discuss the E3 ligase-dependent and -independent functions of Smurf1. Moreover, intracellular regulations of Smurf1 and related physiological phenotypes will be described in this review.

Nonviral gene transfer strategies to promote bone regeneration

Despite the inherent ability of bone to regenerate itself, there are a number of clinical situations in which complete bone regeneration fails to occur. In view of shortcomings of conventional treatment, gene therapy may have a place in cases of critical-size bone loss that cannot be properly treated with current medical or surgical treatment. The purpose of this review is to provide an overview of gene therapy in general, nonviral techniques of gene transfer including physical and chemical methods, RNA-based therapy, therapeutic genes to be transferred for bone regeneration, route of application including ex vivo application, and direct gene therapy approaches to regenerate bone.

A novel low-molecular-weight compound enhances ectopic bone formation and fracture repair

BACKGROUND

Use of recombinant human bone morphogenetic protein-2 (rhBMP-2) is expensive and may cause local side effects. A small synthetic molecule, SVAK-12, has recently been shown in vitro to potentiate rhBMP-2-induced transdifferentiation of myoblasts into the osteoblastic phenotype. The aims of this study were to test the ability of SVAK-12 to enhance bone formation in a rodent ectopic model and to test whether a single percutaneous injection of SVAK-12 can accelerate callus formation in a rodent femoral fracture model.

METHODS

Collagen disks with rhBMP-2 alone or with rhBMP-2 and SVAK-12 were implanted in a standard athymic rat chest ectopic model, and radiographic analysis was performed at four weeks. In a second set of rats (Sprague-Dawley), SVAK-12 was percutaneously injected into the site of a closed femoral fracture. The fractures were analyzed radiographically and biomechanically (with torsional testing) five weeks after surgery.

RESULTS

In the ectopic model, there was dose-dependent enhancement of rhBMP-2 activity with use of SVAK-12 at doses of 100 to 500 μg. In the fracture model, the SVAK-12-treated group had significantly higher radiographic healing scores than the untreated group (p = 0.028). Biomechanical testing revealed that the fractured femora in the 200 to 250-μg SVAK-12 group were 43% stronger (p = 0.008) and 93% stiffer (p = 0.014) than those in the control group. In summary, at five weeks the femoral fracture group injected with SVAK-12 showed significantly improved radiographic and biomechanical evidence of healing compared with the controls.

CONCLUSIONS

A single local dose of a low-molecular-weight compound, SVAK-12, enhanced bone-healing in the presence of low-dose exogenous rhBMP-2 (in the ectopic model) and endogenous rhBMPs (in the femoral fracture model).

CLINICAL RELEVANCE

This study demonstrates that rhBMP-2 responsiveness can be enhanced by a novel small molecule, SVAK-12. Local application of anabolic small molecules has the potential for potentiating and accelerating fracture-healing. Use of this small molecule to lower required doses of rhBMPs might both decrease their cost and improve their safety profile.

Deubiquitinase FAM/USP9X interacts with the E3 ubiquitin ligase SMURF1 protein and protects it from ligase activity-dependent self-degradation

Ubiquitination is an essential post-translational modification that mediates diverse cellular functions. SMAD-specific E3 ubiquitin protein ligase 1 (SMURF1) belongs to the Nedd4 family of HECT ubiquitin ligases that directly catalyzes ubiquitin conjugation onto diverse substrates. As a result, SMURF1 regulates a great variety of cellular physiologies including bone morphogenetic protein (BMP) signaling, cell migration, and planar cell polarity. Structurally, SMURF1 consists of a C2 domain, two WW domain repeats, and a catalytic HECT domain essential for its E3 ubiquitin ligase activity. This modular architecture allows for interactions with other proteins, which are either substrates or adaptors of SMURF1. Despite the increasing number of SMURF1 substrates identified, current knowledge regarding regulatory proteins and their modes of action on controlling SMURF1 activity is still limited. In this study, we employed quantitative mass spectrometry to analyze SMURF1-associated cellular complexes, and identified the deubiquitinase FAM/USP9X as a novel interacting protein for SMURF1. Through domain mapping study, we found the second WW domain of SMURF1 and the carboxyl terminus of USP9X critical for this interaction. SMURF1 is autoubiquitinated through its intrinsic HECT E3 ligase activity, and is degraded by the proteasome. USP9X association antagonizes this activity, resulting in deubiquitination and stabilization of SMURF1. In MDA-MB-231 breast cancer cells, SMURF1 expression is elevated and is required for cellular motility. USP9X stabilizes endogenous SMURF1 in MDA-MB-231 cells. Depletion of USP9X led to down-regulation of SMURF1 and significantly impaired cellular migration. Taken together, our data reveal USP9X as an important regulatory protein of SMURF1 and suggest that the association between deubiquitinase and E3 ligase may serve as a common strategy to control the cellular protein dynamics through modulating E3 ligase stability.

Protein interactions of the transcription factor Hoxa1

Background

Hox proteins are transcription factors involved in crucial processes during animal development. Their mode of action remains scantily documented. While other families of transcription factors, like Smad or Stat, are known cell signaling transducers, such a function has never been squarely addressed for Hox proteins.

Results

To investigate the mode of action of mammalian Hoxa1, we characterized its interactome by a systematic yeast two-hybrid screening against ~12,200 ORF-derived polypeptides. Fifty nine interactors were identified of which 45 could be confirmed by affinity co-purification in animal cell lines. Many Hoxa1 interactors are proteins involved in cell-signaling transduction, cell adhesion and vesicular trafficking. Forty-one interactions were detectable in live cells by Bimolecular Fluorescence Complementation which revealed distinctive intracellular patterns for these interactions consistent with the selective recruitment of Hoxa1 by subgroups of partner proteins at vesicular, cytoplasmic or nuclear compartments.

Conclusions

The characterization of the Hoxa1 interactome presented here suggests unexplored roles for Hox proteins in cell-to-cell communication and cell physiology.

Lim mineralization protein 3 induces the osteogenic differentiation of human amniotic fluid stromal cells through Kruppel-like factor-4 downregulation and further bone-specific gene expression

Multipotent mesenchymal stem cells with extensive self-renewal properties can be easily isolated and rapidly expanded in culture from small volumes of amniotic fluid. These cells, namely, amniotic fluid-stromal cells (AFSCs), can be regarded as an attractive source for tissue engineering purposes, being phenotypically and genetically stable, plus overcoming all the safety and ethical issues related to the use of embryonic/fetal cells. LMP3 is a novel osteoinductive molecule acting upstream to the main osteogenic pathways. This study is aimed at delineating the basic molecular events underlying LMP3-induced osteogenesis, using AFSCs as a cellular model to focus on the molecular features underlying the multipotency/differentiation switch. For this purpose, AFSCs were isolated and characterized in vitro and transfected with a defective adenoviral vector expressing the human LMP3. LMP3 induced the successful osteogenic differentiation of AFSC by inducing the expression of osteogenic markers and osteospecific transcription factors. Moreover, LMP3 induced an early repression of the kruppel-like factor-4, implicated in MSC stemness maintenance. KLF4 repression was released upon LMP3 silencing, indicating that this event could be reasonably considered among the basic molecular events that govern the proliferation/differentiation switch during LMP3-induced osteogenic differentiation of AFSC.

Key role for ubiquitin protein modification in TGFβ signal transduction

The transforming growth factor β (TGFβ) superfamily of signal transduction molecules plays crucial roles in the regulation of cell behavior. TGFβ regulates gene transcription through Smad proteins and signals via non-Smad pathways. The TGFβ pathway is strictly regulated, and perturbations lead to tumorigenesis. Several pathway components are known to be targeted for proteasomal degradation via ubiquitination by E3 ligases. Smurfs are well known negative regulators of TGFβ, which function as E3 ligases recruited by adaptors such as I-Smads. TGFβ signaling can also be enhanced by E3 ligases, such as Arkadia, that target repressors for degradation. It is becoming clear that E3 ligases often target multiple pathways, thereby acting as mediators of signaling cross-talk. Regulation via ubiquitination involves a complex network of E3 ligases, adaptor proteins, and deubiquitinating enzymes (DUBs), the last-mentioned acting by removing ubiquitin from its targets. Interestingly, also non-degradative ubiquitin modifications are known to play important roles in TGFβ signaling. Ubiquitin modifications thus play a key role in TGFβ signal transduction, and in this review we provide an overview of known players, focusing on recent advances.

LIM domain protein-3 (LMP3) cooperates with BMP7 to promote tissue regeneration by ligament progenitor cells

Gene transfer of key regulators of osteogenesis for mesenchymal stem cells represents a promising strategy to regenerate bone. It has been reported that LMP3, a transcription variant of LIM domain mineralization protein (LMP) lacking LIM domains, can induce osteogenesis in vitro and in vivo. As little is known about the effects of LMP3 gene therapy on periodontal ligament (PDL) cell osteogenic differentiation, this study sought to explore whether gene delivery of LMP3 can promote PDL cell mineralization and bone formation. Our results showed that adenoviral mediated gene transfer of LMP3 (AdLMP3) significantly upregulated ALP (Alkaline Phosphatase), BSP (Bone Sialoprotein) and BMP2 gene expression and increased in vitro matrix mineralization in human PDL. Although AdLMP3 gene delivery to PDL cells did not induce ectopic bone formation in vivo, we found that AdLMP3 augments new bone formation, which co-delivered with AdBMP7 gene transfer. Our study provides the evidence that there is a synergistic effect between LMP3 and BMP-7 in vivo, suggesting that LMP3 delivery may be used to augment BMP-mediated osteogenesis. LMP3 and BMP-7 combinatory gene therapy may also have specific applications for oral and periodontal regenerative medicine.

Progress and outlook of inorganic nanoparticles for delivery of nucleic acid sequences related to orthopedic pathologies: a review

The anticipated growth in the aging population will drastically increase medical needs of society; of which, one of the largest components will undoubtedly be from orthopedic-related pathologies. There are several proposed solutions being investigated to cost-effectively prepare for the future--pharmaceuticals, implant devices, cell and gene therapies, or some combination thereof. Gene therapy is one of the more promising possibilities because it seeks to correct the root of the problem, thereby minimizing treatment duration and cost. Currently, viral vectors have shown the highest efficacies, but immunological concerns remain. Nonviral methods show reduced immune responses but are regarded as less efficient. The nonviral paradigms consist of mechanical and chemical approaches. While organic-based materials have been used more frequently in particle-based methods, inorganic materials capable of delivery have distinct advantages, especially advantageous in orthopedic applications. The inorganic gene therapy field is highly interdisciplinary in nature, and requires assimilation of knowledge across the broad fields of cell biology, biochemistry, molecular genetics, materials science, and clinical medicine. This review provides an overview of the role each area plays in orthopedic gene therapy as well as possible future directions for the field.

Combined VEGF and LMP-1 delivery enhances osteoprogenitor cell differentiation and ectopic bone formation

A novel strategy to enhance bone repair is to combine angiogenic factors and osteogenic factors. We combined vascular endothelial growth factor (VEGF) and LIM mineralization protein-1 (LMP-1) by using an internal ribosome entry site to link the genes within a single plasmid. We then evaluated the effects on osteoblastic differentiation in vitro and ectopic bone formation in vivo with a subcutaneously placed PLAGA scaffold loaded with a cloned mouse osteoprogenitor cell line, D1, transfected with plasmids containing VEGF and LMP-1 genes. The cells expressing both genes elevated mRNA expression of RunX2 and β-catenin and alkaline phosphatase activity compared to cells from other groups. In vivo, X-ray and micro-CT analysis of the retrieved implants revealed more ectopic bone formation at 2 and 3 weeks but not at 4 weeks compared to other groups. The results indicate that the combination of the therapeutic growth factors potentiates cell differentiation and may promote osteogenesis.

A synthetic compound that potentiates bone morphogenetic protein-2-induced transdifferentiation of myoblasts into the osteoblastic phenotype

There is an urgent need to develop methods that lower costs of using recombinant human bone morphogenetic proteins (BMPs) to promote bone induction. In this study, we demonstrate the osteogenic effect of a low-molecular weight compound, SVAK-12, that potentiated the effects of BMP-2 in inducing transdifferentiation of C2C12 myoblasts into the osteoblastic phenotype. Here, we report a specific compound, SVAK-12, which was selected based on in silico screenings of small-molecule databases using the homology modeled interaction motif of Smurf1-WW2 domain. The enhancement of BMP-2 activity by SVAK-12 was characterized by evaluating a BMP-specific reporter activity and by monitoring the BMP-2-induced expression of mRNA for osteocalcin and alkaline phosphatase (ALP), which are widely accepted marker genes of osteoblast differentiation. Finally, we confirmed these results by also measuring the enhancement of BMP-2-induced activity of ALP. Smurf1 is an E3 ligase that targets osteogenic Smads for ubiquitin-mediated proteasomal degradation. Smurf1 is an interesting potential target to enhance bone formation based on the positive effects on bone of proteins that block Smurf1-binding to Smad targets or in Smurf1-/- knockout mice. Since Smads bind Smurf1 via its WW2 domain, we performed in silico screening to identify compounds that might interact with the Smurf1-WW2 domain. We recently reported the activity of a compound, SVAK-3. However, SVAK-3, while exhibiting BMP-potentiating activity, was not stable and thus warranted a new search for a more stable and efficacious compound among a selected group of candidates. In addition to being more stable, SVAK-12 exhibited a dose-dependent activity in inducing osteoblastic differentiation of myoblastic C2C12 cells even when multiple markers of the osteoblastic phenotype were parallelly monitored.

Enigma negatively regulates p53 through MDM2 and promotes tumor cell survival in mice

The human E3 ubiquitin ligase murine double minute 2 (MDM2) targets the tumor suppressor p53 for ubiquitination and degradation but also promotes its own ubiquitination and subsequent degradation. As the balance between MDM2 and p53 levels plays a crucial role in regulating cell proliferation and apoptosis, we sought to identify factors selectively inhibiting MDM2 self-ubiquitination. Here we have shown that the LIM domain protein Enigma directly interacts with MDM2 to form a ternary complex with p53 in vitro and in human hepatoma and colon carcinoma cell lines and mouse embryonic fibroblasts. We found that Enigma elicited p53 degradation by inhibiting MDM2 self-ubiquitination and increasing its ubiquitin ligase activity toward p53 in cells. Moreover, mitogenic stimuli such as serum, FGF, and HGF increased Enigma transcription via induction of serum response factor (SRF), leading to MDM2 stabilization and subsequent p53 degradation. We observed similar results in the livers of mice treated with HGF. In humans, we found SRF and Enigma coexpressed with MDM2 but not p53 in several liver and stomach tumors. Finally, we showed that Enigma promoted cell survival and chemoresistance by suppressing p53-mediated apoptosis in both cell lines and a mouse xenograft model. Our findings suggest a role for Enigma in tumorigenesis and uncover a mechanism whereby mitogens attenuate p53 antiproliferative activity through an SRF/Enigma/MDM2 pathway.

Smurf control in bone cells

The homologous to the E6-associated protein carboxyl terminus (HECT) domain E3 ubiquitin ligase Smurf1 is the first E3 ligase to be implicated in regulating bone cell function. The involvement of Smurf1 in multiple signaling pathways and pathological conditions is presently an area of extensive scientific interest. This review highlights recent works exploring Smurf-regulated biological processes in bone cells and highlights recent discoveries surrounding the regulatory mechanisms modulating its catalytic activity and substrate recognition capability. Moreover, we discuss the relevance of targeting the HECT E3s through the development of small-molecule inhibitors as an anticancer therapeutic strategy.

Gene therapy for bone healing

Clinical problems in bone healing include large segmental defects, spinal fusions, and the nonunion and delayed union of fractures. Gene-transfer technologies have the potential to aid healing by permitting the local delivery and sustained expression of osteogenic gene products within osseous lesions. Key questions for such an approach include the choice of transgene, vector and gene-transfer strategy. Most experimental data have been obtained using cDNAs encoding osteogenic growth factors such as bone morphogenetic protein-2 (BMP-2), BMP-4 and BMP-7, in conjunction with both nonviral and viral vectors using in vivo and ex vivo delivery strategies. Proof of principle has been convincingly demonstrated in small-animal models. Relatively few studies have used large animals, but the results so far are encouraging. Once a reliable method has been developed, it will be necessary to perform detailed pharmacological and toxicological studies, as well as satisfy other demands of the regulatory bodies, before human clinical trials can be initiated. Such studies are very expensive and often protracted. Thus, progress in developing a clinically useful gene therapy for bone healing is determined not only by scientific considerations, but also by financial constraints and the ambient regulatory environment.

Nucleocytoplasmic functions of the PDZ-LIM protein family: new insights into organ development

Recent work on the PDZ-LIM protein family has revealed that it has important activities at the cellular level, mediating signals between the nucleus and the cytoskeleton, with significant impact on organ development. We review and integrate current knowledge about the PDZ-LIM protein family and propose a new functional role, sequestering nuclear factors in the cytoplasm. Characterized by their PDZ and LIM domains, the PDZ-LIM family is comprised of evolutionarily conserved proteins found throughout the animal kingdom, from worms to humans. Combining two functional domains in one protein, PDZ-LIM proteins have wide-ranging and multi-compartmental cell functions during development and homeostasis. In contrast, misregulation can lead to cancer formation and progression. New emerging roles include interactions with integrins, T-box transcription factors, and receptor tyrosine kinases. Facilitating the assembly of protein complexes, PDZ-LIM proteins can act as signal modulators, influence actin dynamics, regulate cell architecture, and control gene transcription.

Osteoinductive LIM mineralization protein-1 suppresses activation of NF-kappaB and selectively regulates MAPK pathways in pre-osteoclasts

LIM mineralization protein-1 (LMP-1) is an intracellular regulator of bone formation and has been shown to be osteoinductive in vitro and in vivo. The effect of LMP-1 on other aspects of bone homeostasis has not been previously studied. In a pilot study we observed that LMP-1 decreased nitric oxide (NO) production in pre-osteoclasts. Here we report a new anti-inflammatory effect of LMP-1 and define its mechanism of action in lipopolysaccharide (LPS)-stimulated RAW 264.7 pre-osteoclasts. We found that LMP-1 significantly inhibited LPS-induced NO production. LMP-1 also effectively inhibited the expression of inducible nitric oxide synthase (iNOS), potently suppressed the transcriptional activity and nuclear translocation of nuclear factor kappa B (NF-kappaB), and prevented the phosphorylation of inhibitor of kappa B (IkappaB). Interestingly, LMP-1 had no effect on Receptor-Activator of Nuclear Factor B Ligand (RANKL)-induced activation of NF-kappaB. Furthermore, LMP-1 had no effect on the LPS-induced phosphorylation of extracellular signal-regulated kinase 1/2 (ERK1/2), whereas it did attenuate the phosphorylation of c-Jun NH2-terminal kinase (JNK) while enhancing phosphorylation of p38 mitogen-activated protein kinases (p38 MAPK). These results suggest that LMP-1 has an anti-inflammatory effect, and this effect is, at least in part, due to the inhibition of NO production by the suppression of NF-kappaB activation and selective regulation of mitogen-activated protein kinase (MAPK) pathways.

Development and optimization of a cell-based assay for the selection of synthetic compounds that potentiate bone morphogenetic protein-2 activity

The requirement of large amounts of the recombinant human bone morphogenetic protein-2 (BMP-2) produces a huge translational barrier for its routine clinical use due to high cost. This leads to an urgent need to develop alternative methods to lower costs and/or increase efficacies for using BMP-2. In this study, we describe the development and optimization of a cell-based assay that is sensitive, reproducible, and reliable in identifying reagents that potentiate the effects of BMP-2 in inducing transdifferentiation of C2C12 myoblasts into the osteoblastic phenotype. The assay is based on a BMP-responsive Smad1-driven luciferase reporter gene. LIM mineralization protein-1 (LMP-1) is a novel intracellular LIM domain protein that has been shown by our group to enhance cellular responsiveness to BMP-2. Our previous report elucidated that the binding of LMP-1 with the WW2 domain in Smad ubiquitin regulatory factor-1 (Smurf1) rescues the osteogenic Smads from degradation. Here, using the optimized cell-based assay, we first evaluated the activity of the recombinantly prepared proteins, LMP-1, and its mutant (LMP-1DeltaSmurf1) that lacks the Smurf1-WW2 domain-binding motif. Both the wild type and the mutant proteins were engineered to contain an 11-amino acid HIV-TAT protein derived membrane transduction domain to aid the cellular delivery of recombinant proteins. The cell-based reporter assay confirmed that LMP-1 potentiates the BMP-induced stimulation of C2C12 cells towards the osteoblastic phenotype. The potentiating effect of LMP-1 was significantly reduced when a specific-motif known to interact with Smurf1 was mutated. We validated the results obtained in the reporter assay by also monitoring the expression of mRNA for osteocalcin and alkaline phosphatase (ALP) which is widely accepted osteoblast differentiation marker genes. Finally, we provide further confirmation of our results by measuring the activity of alkaline phosphatase in support of the accuracy and reliability of our cell-based assay. Direct delivery of synthesized protein can be limited by high cost, instability or inadequate post-translational modifications. Thus, there would be a clear benefit for a low cost, cell penetrable chemical compound. We successfully used our gene expression-based assay to choose an active compound from a select group of compounds that were identified by computational screenings as the most likely candidates for mimicking the function of LMP-1. Among them, we selected SVAK-3, a compound that showed a dose-dependent potentiation of BMP-2 activity in inducing osteoblastic differentiation of C2C12 cells. We show that either the full length LMP-1 protein or its potential mimetic compound consistently exhibit similar potentiation of BMP-2 activity even when multiple markers of the osteoblastic phenotype were parallely monitored.

Early transcriptional events during osteogenic differentiation of human bone marrow stromal cells induced by Lim mineralization protein 3

Lim mineralization protein-3 (LMP3) induces osteoblast differentiation by regulating the expression and activity of certain molecules involved in the osteogenic cascade, including those belonging to the bone morphogenetic protein (BMP) family. The complete network of molecular events involved in LMP3-mediated osteogenesis is still unknown. The aim of this study was to analyze the genome-wide gene expression profiles in human mesenchymal stem cells (hMSC) induced by exogenous LMP3 to mediate osteogenesis. For this purpose hMSC were transduced with a defective adenoviral vector expressing the human LMP3 gene and microarray analysis was performed 1 day post-adenoviral transduction. Cells transduced with the vector backbone and untransduced cells were used as independent controls in the experiments. Microarray data were independently validated by means of real-time PCR on selected transcripts. The statistical analysis of microarray data produced a list of 263 significantly (p < 0.01) differentially expressed transcripts. The biological interpretation of the results indicated, among the most noteworthy effects, the modulation of genes involved in the TGF-beta1 pathway: 88 genes coding for key regulators of the cell cycle regulatory machinery and 28 genes implicated in the regulation of cell proliferation along with the development of connective, muscular, and skeletal tissues. These results suggested that LMP3 could affect the fine balance between cell proliferation/differentiation of mesenchymal cells mostly by modulating the TGF-beta1 signaling pathway.

Engineering, cloning, and functional characterization of recombinant LIM mineralization protein-1 containing an N-terminal HIV-derived membrane transduction domain

Short peptide sequences known as protein transduction domains have become increasingly prevalent as tools to internalize molecules that would otherwise remain extracellular. Here, we determine whether a purified recombinant mammalian intracellular osteogenic factor delivered by a HIV-derived TAT-peptide tag is indeed capable of intracellular localization in a form accessible to interaction with other proteins. We engineered and bacterially expressed a TAT-fusion-cDNA construct of a known osteogenic factor, LIM mineralization protein-1 (LMP-1) involved in the bone morphogenetic protein (BMP) pathway that has the potential to serve as an enhancer of BMP-2 efficacy. The expressed recombinant protein contains an N-terminal (His)(6)-tag, a hemagglutinin(HA)-tag, and an 11-amino acid HIV-derived TAT-membrane transduction domain and was purified to homogeneity by Sephacryl S-100 molecular exclusion and Ni(2+)-affinity chromatography. The purified TAT-LMP-1 protein was chemically labeled with fluorescein, and its time and concentration dependent entry into rabbit blood cells was monitored by flow cytometry. We demonstrate the accumulation of TAT-tagged LMP-1 both in cytoplasmic and nuclear compartments. By performing affinity pull-down assays, we confirm our earlier findings that the recombinant TAT-LMP-1, when used as molecular bait to identify the intracellular binding proteins, interacts with Smurf1, a known binding partner of LMP-1. We also show potentiation of BMP-2 activity using the purified TAT-LMP-1 in mouse muscle C2C12 cells by assaying a heterologous luciferase-reporter construct containing multiple copies of a BMP-responsive sequence motif. Finally, we also confirm the biological activity of the purified TAT-LMP-1 by showing enhancement of BMP-2 induced increase of alkaline phosphatase mRNA and protein by RT-PCR and enzyme activity, respectively.

Expression, purification and mass spectrometric analysis of LIM mineralization protein-1 in human lung epithelial cells

LIM mineralization protein-1 (LMP-1) is a novel osteoinductive protein that has been cloned and shown to induce bone formation both in vitro and in vivo. Detection and evaluation of the possible presence of carbohydrate structures in LMP-1 is an important regulatory consideration for the therapeutic use of recombinantly expressed protein. The sequence of LMP-1 contains a highly conserved N-terminal PDZ domain and three C-terminal LIM domains. The sequence analysis of LMP-1 predicts two potential N-glycosylation sites and several O-glycosylation sites. Here, we report the cloning and overexpression of LMP-1 in human lung carcinoma (A549) cells. Even though our group already reported the sequence of LMP-1 cDNA, we undertook this work to clarify whether or not the overexpressed protein undergoes any glycosylation in vivo. The expressed full-length recombinant protein was purified and subjected to chemical analysis and internal sequencing. The absence of any hexosamines (N-acetyl glucosamine or N-acetyl galactosamine) in chemical composition analysis of LMP-1 protein revealed that there is little or no post-translational glycosylation of the LMP-1 polypeptide in lung carcinoma cells (A549). We performed in-gel trypsin digestion on purified LMP-1, and the resulting peptide digests were analyzed further using matrix-assisted laser desorption and ionization mass spectrometry for peptide mass finger printing, which produced several exact matches with the corresponding LMP-1 peptides. Separation by high performance liquid chromatography and purification of the desired peptides followed by N-terminal sequencing resulted in many exact LMP-1 matches for several purified peptides, thus establishing the identity of the purified protein as LMP-1.

LMP-1 retroviral gene therapy influences osteoblast differentiation and fracture repair: a preliminary study

LIM mineralization protein-1 (LMP-1) is a novel intracellular osteogenic factor associated with bone development that has been implicated in the bone morphogenetic protein (BMP) pathway. This preliminary study evaluated the possibility of LMP-1-based retroviral gene therapy to stimulate osteoblast differentiation in vitro and fracture repair in vivo. A Moloney leukemia virus (MLV)-based retroviral vector to express LMP-1 with a hemagglutinin (HA) tag was developed, and its effects were evaluated on MC3T3-E1 cell differentiation and in the rat femur fracture model. MC3T3-E1 osteoblasts transduced with the MLV-HA-LMP-1 vector demonstrated significantly increased osteoblast marker gene expression (P < 0.05) and mineral deposition compared to control transduced cells. Femoral midshaft fractures were produced in Fischer 344 rats by the three-point bending technique. The MLV-HA-LMP-1 or control vector was applied at the fracture site through percutaneous injections 1 day postfracture. Analysis of fracture healing of 10 MLV-HA-LMP-1-treated and 10 control MLV-beta-galactosidase (beta-gal)-treated animals was completed at 3 weeks by X-ray, peripheral quantitative computed tomography, and histology. MLV-HA-LMP-1-treated animals had 63% more bone mineral content at the fracture site (P < 0.01), 34% greater total hard callus area (P < 0.05), and 45% less cartilage in the fracture callus (P < 0.05) compared to MLV-beta-gal-treated animals. There was no effect of LMP-1 treatment on the density of the hard callus. Immunohistochemistry revealed expression of the LMP-1 transgene in the fracture callus at 21 days postfracture. Immunohistochemistry also revealed that LMP-1 transgene expression did not result in an increase in BMP-4 expression in the fracture callus. Compared to MLV-BMP-4 gene therapy studies, MLV-HA-LMP-1 gene therapy improved bony union of the fracture gap to a greater extent and did not cause heterotopic bone formation. This suggests that LMP-1 may be a better potential candidate for gene therapy for fracture repair than BMP-4. These exciting, albeit preliminary, findings indicate that LMP-1-based gene therapy may potentially be a simple and effective means to enhance fracture repair that warrants further investigation.

Targeting WW domains linker of HECT-type ubiquitin ligase Smurf1 for activation by CKIP-1

E3 ubiquitin ligases are final effectors of the enzyme cascade controlling ubiquitylation. A central issue in understanding their regulation is to decipher mechanisms of their assembly and activity. In contrast with RING-type E3s, fewer mechanisms are known for regulation of HECT-type E3s. Smad ubiquitylation regulatory factor 1 (Smurf1), a C2-WW-HECT-domain E3, is crucial for bone homeostasis, in which it suppresses osteoblast activity. However, whether and how its activity is regulated remains unclear. Here we show that Smurf1, but not Smurf2, interacts with casein kinase-2 interacting protein-1 (CKIP-1), resulting in an increase in its E3 ligase activity. Surprisingly, CKIP-1 targets specifically the linker region between the WW domains of Smurf1, thereby augmenting its affinity for and promoting ubiquitylation of the substrate. Moreover, CKIP-1-deficient mice undergo an age-dependent increase in bone mass as a result of accelerated osteogenesis and decreased Smurf1 activity. These findings provide evidence that the WW domains linker is important in complex assembly and in regulating activity of HECT-type E3s and that CKIP-1 functions as the first auxiliary factor to enhance the activation of Smurf1.

Ex vivo-transduced autologous skin fibroblasts expressing human Lim mineralization protein-3 efficiently form new bone in animal models

Local gene transfer of the human Lim mineralization protein (LMP), a novel intracellular positive regulator of the osteoblast differentiation program, can induce efficient bone formation in rodents. To develop a clinically relevant gene therapy approach to facilitate bone healing, we have used primary dermal fibroblasts transduced ex vivo with Ad.LMP-3 and seeded on a hydroxyapatite/collagen matrix prior to autologous implantation. Here, we demonstrate that genetically modified autologous dermal fibroblasts expressing Ad.LMP-3 are able to induce ectopic bone formation following implantation of the matrix into mouse triceps and paravertebral muscles. Moreover, implantation of the Ad.LMP-3-modified dermal fibroblasts into a rat mandibular bone critical size defect model results in efficient healing, as determined by X-rays, histology and three-dimensional microcomputed tomography (3DmuCT). These results demonstrate the effectiveness of the non-secreted intracellular osteogenic factor LMP-3 in inducing bone formation in vivo. Moreover, the utilization of autologous dermal fibroblasts implanted on a biomaterial represents a promising approach for possible future clinical applications aimed at inducing new bone formation.

Regulation of myocardin factor protein stability by the LIM-only protein FHL2

Extensive evidence indicates that serum response factor (SRF) regulates muscle-specific gene expression and that myocardin family SRF cofactors are critical for smooth muscle cell differentiation. In a yeast two hybrid screen for novel SRF binding partners expressed in aortic SMC, we identified four and a half LIM domain protein 2 (FHL2) and confirmed this interaction by GST pull-down and coimmunoprecipitation assays. FHL2 also interacted with all three myocardin factors and enhanced myocardin and myocardin-related transcription factor (MRTF)-A-dependent transactivation of smooth muscle alpha-actin, SM22, and cardiac atrial natriuretic factor promoters in 10T1/2 cells. The expression of FHL2 increased myocardin and MRTF-A protein levels, and, importantly, this effect was due to an increase in protein stability not due to an increase in myocardin factor mRNA expression. Treatment of cells with proteasome inhibitors MG-132 and lactacystin strongly upregulated endogenous MRTF-A protein levels and resulted in a substantial increase in ubiquitin immunoreactivity in MRTF-A immunoprecipitants. Interestingly, the expression of FHL2 attenuated the effects of RhoA and MRTF-B on promoter activity, perhaps through decreased MRTF-B nuclear localization or decreased SRF-CArG binding. Taken together, these data indicate that myocardin factors are regulated by proteasome-mediated degradation and that FHL2 regulates SRF-dependent transcription by multiple mechanisms, including stabilization of myocardin and MRTF-A.

Ubiquitin ligase Smurf1 mediates tumor necrosis factor-induced systemic bone loss by promoting proteasomal degradation of bone morphogenetic signaling proteins

Chronic inflammatory disorders, such as rheumatoid arthritis, are often accompanied by systemic bone loss, which is thought to occur through inflammatory cytokine-mediated stimulation of osteoclast resorption and inhibition of osteoblast function. However, the mechanisms involved in osteoblast inhibition remain poorly understood. Here we test the hypothesis that increased Smad ubiquitin regulatory factor 1 (Smurf1)-mediated degradation of the bone morphogenetic protein pathway signaling proteins mediates reduced bone formation in inflammatory disorders. Osteoblasts derived from bone marrow or long bone samples of adult tumor necrosis factor (TNF) transgenic (TNF-Tg) mice were used in this study. TNF decreased the steady-state levels of Smad1 and Runx2 protein similarly to those in long bones of TNF-Tg mice. In the presence of the proteasome inhibitor MG132, TNF increased accumulation of ubiquitinated Smad1 protein. TNF administration over calvarial bones caused decreases in Smad1 and Runx2 protein levels and mRNA expression of osteoblast marker genes in wild-type, but not in Smurf1(-/-) mice. Vertebral bone volume and strength of TNF-Tg/Smurf1(-/-) mice were examined by a combination of micro-CT, bone histomorphometry, and biomechanical testing and compared with those from TNF-Tg littermates. TNF-Tg mice had significantly decreased bone volume and biomechanical properties, which were partially rescued in TNF-Tg/Smurf1(-/-) mice. We conclude that in chronic inflammatory disorders where TNF is increased, TNF induces the expression of ubiquitin ligase Smurf1 and promotes ubiquitination and proteasomal degradation of Smad1 and Runx2, leading to systemic bone loss. Inhibition of ubiquitin-mediated Smad1 and Runx2 degradation in osteoblasts could help to treat inflammation-induced osteoporosis.

Transforming growth factor-beta signaling and ubiquitinators in cancer

Transforming growth factor-beta (TGF-beta) represents a large family of growth and differentiation factors that mobilize complex signaling networks to regulate cellular differentiation, proliferation, motility, adhesion, and apoptosis. TGF-beta signaling is tightly regulated by multiple complex mechanisms, and its deregulation plays a key role in the progression of many forms of cancer. Upon ligand binding, TGF-beta signals are transduced by Smad proteins, which in turn are tightly dependent on modulation by adaptor proteins such as embryonic liver fodrin, Smad anchor for receptor activation, filamin, and crkl. A further layer of regulation is imposed by ubiquitin-mediated targeting and proteasomal degradation of specific components of the TGF-beta signaling pathway. This review focuses on the ubiquitinators that regulate TGF-beta signaling and the association of these ubiquitin ligases with various forms of cancer. Delineating the role of ubiquitinators in the TGF-beta signaling pathway could yield powerful novel therapeutic targets for designing new cancer treatments.

Genetic engineering for skeletal regenerative medicine

The clinical challenges of skeletal regenerative medicine have motivated significant advances in cellular and tissue engineering in recent years. In particular, advances in molecular biology have provided the tools necessary for the design of gene-based strategies for skeletal tissue repair. Consequently, genetic engineering has emerged as a promising method to address the need for sustained and robust cellular differentiation and extracellular matrix production. As a result, gene therapy has been established as a conventional approach to enhance cellular activities for skeletal tissue repair. Recent literature clearly demonstrates that genetic engineering is a principal factor in constructing effective methods for tissue engineering approaches to bone, cartilage, and connective tissue regeneration. This review highlights this literature, including advances in the development of efficacious gene carriers, novel cell sources, successful delivery strategies, and optimal target genes. The current status of the field and the challenges impeding the clinical realization of these approaches are also discussed.

Concise Review: Role and Function of the Ubiquitin-Proteasome System in Mammalian Stem and Progenitor Cells

Highly ordered degradation of cell proteins by the ubiquitin-proteasome system, a sophisticated cellular proteolytic machinery, has been identified as a key regulatory mechanism in many eukaryotic cells. Accumulating evidence reveals that the ubiquitin-proteasome system is involved in the regulation of fundamental processes in mammalian stem and progenitor cells of embryonic, neural, hematopoietic, and mesenchymal origin. Such processes, including development, survival, differentiation, lineage commitment, migration, and homing, are directly controlled by the ubiquitin-proteasome system, either via proteolytic degradation of key regulatory proteins of signaling and gene expression pathways or via nonproteolytic mechanisms involving the proteasome itself or posttranslational modifications of target proteins by ubiquitin or other ubiquitin-like modifiers. Future characterization of the precise roles and functions of the ubiquitin-proteasome system in mammalian stem and early progenitor cells will improve our understanding of stem cell biology and may provide an experimental basis for the development of novel therapeutic strategies in regenerative medicine. Disclosure of potential conflicts of interest is found at the end of this article.

Modeling and analysis of molecularinteraction between Smurf1-WW2 domain and various isoforms of LIM mineralization protein

LIM Mineralization Protein-1 (LMP-1) has been cloned and shown to be osteoinductive. Our efforts to understand the mode of action of LMP-1 led to the determination that LMP-1 interacts with Smad Ubiquitin Regulatory Factor-1 (Smurf1). Smurf1 targets osteogenic Smads, Smad1/5, for ubiquitin-mediated proteasomal degradation. Smurf1 interaction with LMP-1 or Smads is based on the presence of unique WW-domain interacting motif in these target molecules. By performing site-directed mutagenesis and binding studies in vitro on purified recombinant proteins, we identified a specific motif within the osteogenic region of several LMP isoforms that is necessary for Smurf1 interaction. Similarly, we have identified that the WW2 domain of Smurf1 is necessary for target protein interaction. Here, we present a homology-based modeling of the Smurf1 WW2 domain and its interacting motif of LMP-1. We performed computational docking of the interacting domains in Smurf1 and LMPs to identify the key amino acid residues involved in their binding regions. In support of the computational predictions, we also present biochemical evidence supporting the hypothesis that the physical interaction of Smurf1 and osteoinductive forms of LMP may prevent Smurf1 from targeting osteogenic Smads by ubiquitin-mediated proteasomal degradation.

Truncated human LMP-1 triggers differentiation of C2C12 cells to an osteoblastic phenotype in vitro

LIM mineralization protein-1 (LMP-1) is a novel intracellular osteoinductive protein that has been shown to induce bone formation both in vitro and in vivo. LMP-1 contains an N-terminal PDZ domain and three C-terminal LIM domains. In this study, we investigated whether a truncated form of human LMP-1 (hLMP-1[t]), lacking the three C-terminal LIM domains, triggers the differentiation of pluripotent myoblastic C(2)C(12) cells to the osteoblast lineage. C(2)C(12) cells were transiently transduced with Ad5-hLMP-1(t)-green fluorescent protein or viral vector control. The expression of hLMP-1(t) RNA and the truncated protein were examined. The results showed that hLMP-1(t) blocked myotube formation in C(2)C(12) cultures and significantly enhanced the alkaline phosphatase (ALP) activity. In addition, the expressions of ALP, osteocalcin, and bone morphogenetic protein (BMP)-2 and BMP-7 genes were also increased. The induction of these key osteogenic markers suggests that hLMP-1(t) can trigger the pluripotent myoblastic C2C12 cells to differentiate into osteoblastic lineage, thus extending our previous observation that LMP-1 and LMP-1(t) enhances the osteoblastic phenotype in cultures of cells already committed to the osteoblastic lineage. Therefore, C(2)C(12) cells are an appropriate model system for the examination of LMP-1 induction of the osteoblastic phenotype and the study of mechanisms of LMP-1 action.