Structural and functional analysis of two small leucine-rich repeat proteoglycans, fibromodulin and chondroadherin

Fibromodulin-overexpressing fibroblast cells increase wound contraction, improve scar quality and enhance angiogenesis: an in-vivo study

INTRODUCTION

Fibromodulin, a small leucine rich proteoglycan has been suggested to have prominent role in wound healing. On the other hand, fibroblast cells, due to their ability to secrete growth factors and control inflammation in the wound area, have been proposed as effective approaches in cell therapy for wounds. In the current study we attempted to improve treatment results using a combination of fibroblast and fibromodulin features.

METHOD

Fibroblast cells were isolated from the skin and transfected with a vector carrying the fibromodulin gene. Following the assessment of fibromodulin protein production, the effect of transfected fibroblast cells was studied in an animal wound model.

RESULTS

Flow cytometry analysis showed high expression of the CD90 marker (97.2%) and very low expression of the CD34 marker (0.47%). Additionally, enzyme-linked immunosorbent assay (ELISA) findings confirmed high expression of the fibromodulin gene in the transfected fibroblast cells. In vivo studies demonstrated that the animals treated with fibroblast cells transfected with fibromodulin (V + G+) exhibited significantly improved wound contraction on day 7 (i.e., contraction percentage: 21.79 ± 9.96%, compared with 7.23 ± 2.30% in the PBS-treated group). Histopathological studies also indicated improvements in angiogenesis score and collagen density score in the animals treated with the V + G + group.

CONCLUSION

The results of this study showed that fibroblast cells expressing the fibromodulin gene improve wound contraction and some histological parameters in the deep wound model of the rat.

Versican binds collagen via its G3 domain and regulates the organization and mechanics of collagenous matrices

Type I collagen is the most abundant structural protein in the body and, with other fibrillar collagens, forms the fibrous network of the extracellular matrix. Another group of extracellular matrix polymers, the glycosaminoglycans, and glycosaminoglycan-modified proteoglycans, play important roles in regulating collagen behaviors and contribute to the compositional, structural, and mechanical complexity of the extracellular matrix. While the binding between collagen and small leucine-rich proteoglycans has been studied in detail, the interactions between collagen and the large bottlebrush proteoglycan versican are not well understood. Here, we report that versican binds collagen directly and regulates collagen structure and mechanics. Versican colocalizes with collagen fibers in vivo and binds to collagen via its C-terminal G3 domain (a non-GAG-modified domain present in all known versican isoforms) in vitro; it promotes the deposition of a highly aligned collagen-rich matrix by fibroblasts. Versican also shows an unexpected effect on the rheology of collagen gels in vitro, causing decreased stiffness and attenuated shear strain stiffening, and the cleavage of versican in the liver results in reduced tissue compression stiffening. Thus, versican is an important collagen-binding partner and plays a role in modulating collagen organization and mechanics.

Fibromodulin facilitates the osteogenic effect of Masquelet's induced membrane by inhibiting the TGF-β/SMAD signaling pathway

Masquelet's induced membrane (IM) technique is a promising treatment strategy for the repair of substantial bone defects. The formation of an IM around polymethylmethacrylate bone cement plays a crucial role in this technique. Several studies have indicated that IMs have bioactivity because they contain abundant blood vessels, a variety of cells, and biological factors. The bioactivity of an IM increases during the initial stages of formation, thereby facilitating bone regeneration and remodeling. Nevertheless, the precise mechanisms underlying the enhancement of IM bioactivity and the promotion of bone regeneration necessitate further investigation. In this study, we successfully developed a Masquelet IM model of critical femur defects in rats. By employing proteomics analysis and biological detection techniques, we identified fibromodulin (FMOD) as a pivotal factor contributing to angiogenesis and the enhanced bioactivity of the IM. A significant increase in angiogenesis and the expression of bioactive factors in the IM was also observed with the upregulation of FMOD expression. Furthermore, this effect is mediated through the inhibition of the transforming growth factor beta (TGF-β)/SMAD signaling pathway. We also demonstrated that administering recombinant human FMOD enhanced osteogenesis in rat bone marrow mesenchymal stem cells and angiogenesis in human umbilical vein endothelial cells in vitro. Furthermore, the negative regulatory effect of the TGF-β signaling pathway was verified. In conclusion, this study provides a novel theoretical basis for the application of IMs in bone-defect reconstruction and explores possible new mechanisms that may play an important role in promoting the bioactivity and osteogenic potential of IMs.

Spatial gene expression in the adult rat patellar tendon

Tendons are dense connective tissues with relatively few cells which makes studying the molecular profile of the tissue challenging. There is not a consensus on the spatial location of various cell types within a tendon, nor the accompanying transcriptional profile. In the present study, we used two male rat patellar tendon samples for sequencing-based spatial transcriptomics to determine the gene expression profile. We integrated our data with a mouse Achilles single cell dataset to predict the cell type composition of the patellar tendon as a function of location within the tissue. The spatial location of the predicated cell types suggested that there were two populations of tendon fibroblasts, one located in the tendon midsubstance, while the other localized with red blood cells, pericytes, and immune cells to the tendon peripheral connective tissue. Of the highest expressed spatially variable genes, there were multiple genes with known function in tendon: Col1a1, Col1a2, Dcn, Fmod, Sparc, and Comp. Further, a novel spatially regulated gene (AABR07000398.1) with no known function was identified. The spatial gene expression of tendon associated genes (Scx, Thbs4, Tnmd, Can, Bgn, Lum, Adamts2, Lox, Ppib, Col2a1, Col3a1, Col6a2) was also visualized. Both patellar tendon samples had similar expression patterns for all these genes. This dataset provides new spatial insights into gene expression in a healthy tendon.

The role of fibromodulin in inflammatory responses and diseases associated with inflammation

Inflammation is an immune response that the host organism eliminates threats from foreign objects or endogenous signals. It plays a key role in the progression, prognosis as well as therapy of diseases. Chronic inflammatory diseases have been regarded as the main cause of death worldwide at present, which greatly affect a vast number of individuals, producing economic and social burdens. Thus, developing drugs targeting inflammation has become necessary and attractive in the world. Currently, accumulating evidence suggests that small leucine-rich proteoglycans (SLRPs) exhibit essential roles in various inflammatory responses by acting as an anti-inflammatory or pro-inflammatory role in different scenarios of diseases. Of particular interest was a well-studied member, termed fibromodulin (FMOD), which has been largely explored in the role of inflammatory responses in inflammatory-related diseases. In this review, particular focus is given to the role of FMOD in inflammatory response including the relationship of FMOD with the complement system and immune cells, as well as the role of FMOD in the diseases associated with inflammation, such as skin wounding healing, osteoarthritis (OA), tendinopathy, atherosclerosis, and heart failure (HF). By conducting this review, we intend to gain insight into the role of FMOD in inflammation, which may open the way for the development of new anti-inflammation drugs in the scenarios of different inflammatory-related diseases.

Small leucine rich proteoglycans: Biology, function and their therapeutic potential in the ocular surface

Small leucine rich proteoglycans (SLRPs) are the largest family of proteoglycans, with 18 members that are subdivided into five classes. SLRPs are small in size and can be present in tissues as glycosylated and non-glycosylated proteins, and the most studied SLRPs include decorin, biglycan, lumican, keratocan and fibromodulin. SLRPs specifically bind to collagen fibrils, regulating collagen fibrillogenesis and the biomechanical properties of tissues, and are expressed at particularly high levels in fibrous tissues, such as the cornea. However, SLRPs are also very active components of the ECM, interacting with numerous growth factors, cytokines and cell surface receptors. Therefore, SLRPs regulate major cellular processes and have a central role in major fundamental biological processes, such as maintaining corneal homeostasis and transparency and regulating corneal wound healing. Over the years, mutations and/or altered expression of SLRPs have been associated with various corneal diseases, such as congenital stromal corneal dystrophy and cornea plana. Recently, there has been great interest in harnessing the various functions of SLRPs for therapeutic purposes. In this comprehensive review, we describe the structural features and the related functions of SLRPs, and how these affect the therapeutic potential of SLRPs, with special emphasis on the use of SLRPs for treating ocular surface pathologies.

Fibromodulin, a Multifunctional Matricellular Modulator

Fibromodulin (FMOD) is an archetypal member of the class II small leucine-rich proteoglycan family. By directly binding to extracellular matrix structural components, such as collagen and lysyl oxidase, FMOD regulates collagen cross-linking, packing, assembly, and fibril architecture via a multivalent interaction. Meanwhile, as a pluripotent molecule, FMOD acts as a ligand of various cytokines and growth factors, especially those belonging to the transforming growth factor (TGF) β superfamily, by interacting with the corresponding signaling molecules involved in cell adhesion, spreading, proliferation, migration, invasion, differentiation, and metastasis. Consequently, FMOD exhibits promigratory, proangiogenic, anti-inflammatory, and antifibrogenic properties and plays essential roles in cell fate determination and maturation, progenitor cell recruitment, and tissue regeneration. The multifunctional nature of FMOD thus enables it to be a promising therapeutic agent for a broad repertoire of diseases, including but not limited to arthritis, temporomandibular joint disorders, caries, and fibrotic diseases among different organs, as well as to be a regenerative medicine candidate for skin, muscle, and tendon injuries. Moreover, FMOD is also considered a marker for tumor diagnosis and prognosis prediction and a potential target for cancer treatment. Furthermore, FMOD itself is sufficient to reprogram somatic cells into a multipotent state, creating a safe and efficient cell source for various tissue reconstructions and thus opening a new avenue for regenerative medicine. This review focuses on the recent preclinical efforts bringing FMOD research and therapies to the forefront. In addition, a contemporary understanding of the mechanism underlying FMOD's function, particularly its interaction with TGFβ superfamily members, is also discussed at the molecular level to aid the discovery of novel FMOD-based treatments.

Differential MMP-14 targeting by biglycan, decorin, fibromodulin, and lumican unraveled by in silico approach

Small leucine-rich proteoglycans (SLRPs) are major regulators of extracellular matrix assembly and cell signaling. Lumican, a member of the SLRPs family, and its derived peptides were shown to possess antitumor activity by interacting directly with the catalytic domain of MMP-14 leading to the inhibition of its activity. The aim of the present report was to characterize by in silico three-dimensional (3D) modeling the structure and the dynamics of four SLRPs including their core protein and their specific polysaccharide chains to assess their capacity to bind to MMP-14 and to regulate its activity. Molecular docking experiments were performed to identify the specific amino acids of MMP-14 interacting with each of the four SLRPs. The inhibition of each SLRP (100 nM) on MMP-14 activity was measured and the constants of inhibition (K_i) were evaluated. The impact of the number of glycan chains, structures, and dynamics of lumican on the interaction with MMP-14 was assessed by molecular dynamics simulations. Molecular docking analysis showed that all SLRPs bind to MMP-14 through their concave face, but in different regions of the catalytic domain of MMP-14. Each SLRPs inhibited significantly the MMP-14 activity. Finally, molecular dynamics showed the role of glycan chains in interaction with MMP-14 and shielding effect of SLRPs. Altogether, the results demonstrated that each SLRP exhibited inhibition of MMP-14 activity. However, the differential targeting of MMP-14 by the SLRPs was shown to be related not only to the core protein conformation but also to the glycan chain structures and dynamics.

Crystal structures of FNIP/FGxxFN motif-containing leucine-rich repeat proteins

The Cafeteria roenbergensis virus (Crov), Dictyostelium, and other species encode a large family of leucine-rich repeat (LRR) proteins with FGxxFN motifs. We determined the structures of two of them and observed several unique structural features that set them aside from previously characterized LRR family members. Crov588 comprises 25 regular repeats with a LxxLxFGxxFNQxIxENVLPxx consensus, forming a unique closed circular repeat structure. Novel features include a repositioning of a conserved asparagine at the middle of the repeat, a double phenylalanine spine that generates an alternate core packing arrangement, and a histidine/tyrosine ladder on the concave surface. Crov539 is smaller, comprising 12 repeats of a similar LxxLxFGxxFNQPIExVxW/LPxx consensus and forming an unusual cap-swapped dimer structure. The phenylalanine spine of Crov539 is supplemented with a tryptophan spine, while a hydrophobic isoleucine-rich patch is found on the central concave surface. We present a detailed analysis of the structures of Crov588 and Crov539 and compare them to related repeat proteins and other LRR classes.

Numerous variants of leucine rich repeats in proteins from nucleo-cytoplasmic large DNA viruses

Leucine rich repeats (LRRs) occurring in tandem are 20-29 amino acids long. Eleven LRR types have been recognized. Sequence features of LRRs from viruses were investigated using over 600 LRR proteins from 89 species. Directly before, metagenome data of nucleo-cytoplasmic large dsDNA viruses (NCLDVs) have been published; the 2,074 NCLDVs encode 199,021 proteins. From the NCLDVs 547 LRR proteins were identified and 502 were used for analysis. Various variants of known LRR types were identified in viral LRRs. A comprehensive analysis of TpLRR and FNIP that belong to an LRR type was first performed. The repeating unit lengths (RULs) in five types are 19 residues which is the shortest among all LRRs. The RULs of eight LRR types including FNIP are one to five residues shorter than those of the known, corresponding LRR types. The conserved hydrophobic residues such as Leu, Val or Ile in the consensus sequences are frequently substituted by cysteine at one or two positions. Four unique LRR motifs that are different from those identified previously are observed. The present study enhances the previous result. An evolutionary scenario of short or unique LRR was discussed.

Gold micro-particles for knee osteoarthritis

Background

This exploratory study investigates if intra‐articular injected gold microparticles in knee osteoarthritis (KOA) reduce immunomodulatory‐based pain via proteomic changes in the synovial fluid (SF) and serum.

Methods

Thirty patients with moderate KOA were included. Intraarticular injections with 20 mg gold microparticles (72.000 particles, 20–40 µm in diameter) using the patient's synovial fluid (SF) as carrier were performed. Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) subscores for pain, stiffness, and function were assessed at inclusion, 8 weeks and 2 years The PainDetect questionnaire, pain pressure threshold (PPT), temporal summation (TS), and conditioned pain modulation (CPM), and pain diary were assessed at inclusion and 8 weeks. Proteome analysis was performed on SF and blood samples before and after 8 weeks of treatment.

Results

A decrease in WOMAC scores (pain (p = 0.0001), stiffness (p = 0.0088), activity (p = 0.0001)), PainDetect (p = 0.0002) and increase in PPT (p = 0.001) and CPM (p = 0.021) and a decrease in TS (p = 0.03) were found after 8 weeks compared to inclusion assessments. At 2 years follow‐up compared to baseline there was a decrease in WOMAC scores (pain (p = 0.0001), stiffness (p = 0.007), activity (p = 0.0001)) and PainDetect (p = 0.0001). In SF, 28 different proteins were downregulated and 11 upregulated (p < 0.05) mainly associated immune response. Similarly, 31 proteins were downregulated and 1 upregulated in serum (p < 0.05) reflecting key immune response and anatomical structure development processes. No adverse effects related to the treatment were recorded.

Conclusions

Gold microparticles injected intra‐articular in KOA joints may provide pain relief and an inflammatory modulatory effect based on proteome changes found in SF and serum. A randomized, controlled, double‐blind study is needed to infer a conclusion.

Significance

This study indicates that intra‐articular gold may provide advantages in clinical practice for managing knee osteoarthritic pain. The use of intraarticular gold can add new knowledge to the treatment of inflammation and pain.

The effects of maturation and aging on the rotator cuff tendon-to-bone interface

Rotator cuff tendon injuries often occur at the tendon-to-bone interface (i.e., enthesis) area, with a high prevalence for the elderly population, but the underlying reason for this phenomenon is still unknown. The objective of this study is to identify the histological, molecular, and biomechanical alterations of the rotator cuff enthesis with maturation and aging in a mouse model. Four different age groups of mice (newborn, young, adult, and old) were studied. Striking variations of the entheses were observed between the newborn and other matured groups, with collagen content, proteoglycan deposition, collagen fiber dispersion was significantly higher in the newborn group. The compositional and histological features of young, adult, and old groups did not show significant differences, except having increased proteoglycan deposition and thinner collagen fibers at the insertion sites in the old group. Nanoindentation testing showed that the old group had a smaller compressive modulus at the insertion site when compared with other groups. However, tensile mechanical testing reported that the old group demonstrated a significantly higher failure stress when compared with the young and adult groups. The proteomics analysis detected dramatic differences in protein content between newborn and young groups but minor changes among young, adult, and old groups. These results demonstrated: (1) the significant alterations of the enthesis composition and structure occur from the newborn to the young time period; (2) the increased risk of rotator cuff tendon injuries in the elderly population is not solely because of old age alone in the rodent model.

Novel cell sources for bone regeneration

A plethora of both acute and chronic conditions, including traumatic, degenerative, malignant, or congenital disorders, commonly induce bone disorders often associated with severe persisting pain and limited mobility. Over 1 million surgical procedures involving bone excision, bone grafting, and fracture repair are performed each year in the U.S. alone, resulting in immense levels of public health challenges and corresponding financial burdens. Unfortunately, the innate self-healing capacity of bone is often inadequate for larger defects over a critical size. Moreover, as direct transplantation of committed osteoblasts is hindered by deficient cell availability, limited cell spreading, and poor survivability, an urgent need for novel cell sources for bone regeneration is concurrent. Thanks to the development in stem cell biology and cell reprogramming technology, many multipotent and pluripotent cells that manifest promising osteogenic potential are considered the regenerative remedy for bone defects. Considering these cells' investigation is still in its relative infancy, each of them offers their own particular challenges that must be conquered before the large-scale clinical application.

Quantitative proteomic analysis of scleras in guinea pig exposed to wavelength defocus

Myopia is the most common optical disorder in the world, and wavelength defocus induced ametropia and myopia have attracted great attention. The objective was to identify and quantify scleral proteins involved in the response to the wavelength defocus. Guinea pigs were randomly divided into 3 groups that received different lighting conditions for 8 weeks: white light, short wavelength light, and long wavelength light. Refraction and axial length were measured, Hematoxylin-Eosin staining and transmission electron microscope were adopted to observe the scleral structure, and scleral proteome was also detected to analyze protein abundance by employing TMT labeling method. After light stimulation, the long- and short -wavelength light induced myopic and hyperopic effect on the guinea pig's eye and induced distinct protein signature, respectively. 186 dyregulated proteins between the short- and long-wavelength group were identified, which were mainly located in extracellular region and involved in metabolic process. We also found that 5 proteins in the guinea pigs scleras in response to wavelength defocus were also human myopic candidate targets, suggesting functional overlap between dyregulated proteins in scleral upon exposure to wavelength defocus and genes causing myopia in humans. SIGNIFICANCE: Wavelength defocus induces refractive errors and leads to myopia or hyperopia. However, sclera proteomics respond to wavelength defocus is lacking, which is crucial to understanding how wavelength defocus influences refractive development and induces myopia. In this proteome analysis, we identified unique protein signatures response to wavelength defocus in sclera of guinea pigs, identified potential mechanisms contributing to myopia formation, and found that several human myopia-related genes may involve in response to wavelength defocus. The results of this study provide a foundation to understand the mechanisms of myopia and wavelength defocus induced ametropia.

TGF beta -1, -2 and -3 in the modulation of fibrosis in the cornea and other organs

The TGF beta-1, -2 and -3 isoforms are transcribed from different genes but bind to the same receptors and signal through the same canonical and non-canonical signal transduction pathways. There are numerous regulatory mechanisms controlling the action of each isoform that include the organ-specific cells producing latent TGF beta growth factors, multiple effectors that activate the isoforms, ECM-associated SLRPs and basement membrane components that modulate the activity and localization of the isoforms, other interactive cytokine-growth factor receptor systems, such as PDGF and CTGF, TGF beta receptor expression on target cells, including myofibroblast precursors, receptor binding competition, positive and negative signal transduction effectors, and transcription and translational regulatory mechanisms. While there has long been the view that TGF beta-1and TGF beta-2 are pro-fibrotic, while TGF beta-3 is anti-fibrotic, this review suggests that view is too simplistic, at least in adult tissues, since TGF beta-3 shares far more similarities in its modulation of fibrotic gene expression with TGF beta-1 and TGF beta-2, than it does differences, and often the differences are subtle. Rather, TGF beta-3 should be seen as a fibro-modulatory partner to the other two isoforms that modulates a nuanced and better controlled response to injury. The complex interplay between the three isoforms and numerous interactive proteins, in the context of the cellular milieu, controls regenerative non-fibrotic vs. fibrotic healing in a response to injury in a particular organ, as well as the resolution of fibrosis, when that occurs.

Sequence features, structure, ligand interaction, and diseases in small leucine rich repeat proteoglycans

Small leucine rich repeat proteoglycans (SLRPs) are a group of active components of the extracellular matrix in all tissues. SLRPs bind to collagens and regulate collagen fibril growth and fibril organization. SLRPs also interact with various cytokines and extracellular compounds, which lead to various biological functions such cell adhesion and signaling, proliferation, and differentiation. Mutations in SLRP genes are associated with human diseases. Now crystal structures of five SLRPs are available. We describe some features of amino acid sequence and structures of SLRPs. We also review ligand interactions and then discuss the interaction surfaces. Furthermore, we map mutations associated with human diseases and discuss possible effects on structures by the mutations.

Constructing 3-Dimensional Atomic-Resolution Models of Nonsulfated Glycosaminoglycans with Arbitrary Lengths Using Conformations from Molecular Dynamics

Glycosaminoglycans (GAGs) are the linear carbohydrate components of proteoglycans (PGs) and are key mediators in the bioactivity of PGs in animal tissue. GAGs are heterogeneous, conformationally complex, and polydisperse, containing up to 200 monosaccharide units. These complexities make studying GAG conformation a challenge for existing experimental and computational methods. We previously described an algorithm we developed that applies conformational parameters (i.e., all bond lengths, bond angles, and dihedral angles) from molecular dynamics (MD) simulations of nonsulfated chondroitin GAG 20-mers to construct 3-D atomic-resolution models of nonsulfated chondroitin GAGs of arbitrary length. In the current study, we applied our algorithm to other GAGs, including hyaluronan and nonsulfated forms of dermatan, keratan, and heparan and expanded our database of MD-generated GAG conformations. Here, we show that individual glycosidic linkages and monosaccharide rings in 10- and 20-mers of hyaluronan and nonsulfated dermatan, keratan, and heparan behave randomly and independently in MD simulation and, therefore, using a database of MD-generated 20-mer conformations, that our algorithm can construct conformational ensembles of 10- and 20-mers of various GAG types that accurately represent the backbone flexibility seen in MD simulations. Furthermore, our algorithm efficiently constructs conformational ensembles of GAG 200-mers that we would reasonably expect from MD simulations.

The origins and developments of sulfation-prone tyrosine-rich and acidic N- and C-terminal extensions of class ll and lll small leucine-rich repeat proteins shed light on connective tissue evolution in vertebrates

Background

Small leucine-rich repeat protein (SLRP) family members contain conserved leucine-rich repeat motifs flanked by highly variable N- and C-terminal regions. Most class II and III SLRPs have tyrosine-rich N-terminal regions and some of these are sulfated. However, the evolutionary origin and conservation of the tyrosine-rich and acidic terminal regions remain undetermined. In this study, we present the most comprehensive multiple sequence alignment (MSA) analyses of all eight class II and III SLRPs to date. Based on the level of conservation of tyrosine residues and adjacent sequences, we predict which tyrosine residues are most likely to be sulfated in the terminal regions of human class II and III SLRPs.

Results

Using this novel approach, we predict a total of 22 tyrosine sulfation sites in human SLRPs, of which only 8 sites had been experimentally identified in mammals. Our analyses suggest that sulfation-prone, tyrosine-rich and acidic terminal regions of the class II and III SLRPs emerged via convergent evolution at different stages of vertebrate evolution, coinciding with significant evolutionary events including the development of endochondral bones and articular cartilage, the aquatic to terrestrial transition, and the formation of an amnion.

Conclusions

Our study suggests that selective pressures due to changes in life conditions led to the formation of sulfotyrosine-rich and acidic terminal regions. We believe the independent emergence and evolution of sulfotyrosine-rich and acidic N- and C-terminal regions have provided each class II and III SLRP member with novel vital functions required to develop new specialized extracellular matrices and tissues in vertebrate species.

Structural basis of the leukocyte integrin Mac-1 I-domain interactions with the platelet glycoprotein Ib

Cell-surface receptor interactions between leukocyte integrin macrophage-1 antigen (Mac-1, also known as CR3, αMβ2, CD11b/CD18) and platelet glycoprotein Ibα (GPIbα) are critical to vascular inflammation. To define the key residues at the binding interface, we used nuclear magnetic resonance (NMR) to assign the spectra of the mouse Mac-1 I-domain and mapped the residues contacting the mouse GPIbα N-terminal domain (GPIbαN) to the locality of the integrin metal ion-dependant adhesion site (MIDAS) surface. We next determined the crystal structures of the mouse GPIbαN and Mac-1 I-domain to 2 Å and 2.5 Å resolution, respectively. The mouse Mac-1 I-domain crystal structure reveals an active conformation that is stabilized by a crystal contact from the α7-helix with a glutamate side chain completing the octahedral coordination sphere of the MIDAS Mg²⁺ ion. The amino acid sequence of the α7-helix and disposition of the glutamic acid matches the C-terminal capping region α-helix of GPIbα effectively acting as a ligand mimetic. Using these crystal structures in combination with NMR measurements and docking analysis, we developed a model whereby an acidic residue from the GPIbα leucine-rich repeat (LRR) capping α-helix coordinates directly to the Mac-1 MIDAS Mg²⁺ ion. The Mac-1:GPIbαN complex involves additional interactions consolidated by an elongated pocket flanking the GPIbαN LRR capping α-helix. The GPIbαN α-helix has an HxxxE motif, which is equivalent by homology to RxxxD from the human GPIbαN. Subsequent mutagenesis of residues at this interface, coupled with surface plasmon resonance studies, confirmed the importance of GPIbαN residues H218, E222, and the Mac-1 MIDAS residue T209 to formation of the complex.

Exploring the role of extracellular matrix proteins to develop biomarkers of plaque vulnerability and outcome

Cardiovascular disease (CVD) is the most common cause of death in industrialized countries. One underlying cause is atherosclerosis, which is a systemic disease characterized by plaques of retained lipids, inflammatory cells, apoptotic cells, calcium and extracellular matrix (ECM) proteins in the arterial wall. The biologic composition of an atherosclerotic plaque determines whether the plaque is more or less vulnerable, that is prone to rupture or erosion. Here, the ECM and tissue repair play an important role in plaque stability, vulnerability and progression. This review will focus on ECM remodelling in atherosclerotic plaques, with focus on how ECM biomarkers might predict plaque vulnerability and outcome.

Leucine Rich Repeat Proteins: Sequences, Mutations, Structures and Diseases

Mutations in the genes encoding Leucine Rich Repeat (LRR) containing proteins are associated with over sixty human diseases; these include high myopia, mitochondrial encephalomyopathy, and Crohn's disease. These mutations occur frequently within the LRR domains and within the regions that shield the hydrophobic core of the LRR domain. The amino acid sequences of fifty-five LRR proteins have been published. They include Nod-Like Receptors (NLRs) such as NLRP1, NLRP3, NLRP14, and Nod-2, Small Leucine Rich Repeat Proteoglycans (SLRPs) such as keratocan, lumican, fibromodulin, PRELP, biglycan, and nyctalopin, and F-box/LRR-repeat proteins such as FBXL2, FBXL4, and FBXL12. For example, 363 missense mutations have been identified. Replacement of arginine, proline, or cysteine by another amino acid, or the reverse, is frequently observed. The diverse effects of the mutations are discussed based on the known structures of LRR proteins. These mutations influence protein folding, aggregation, oligomerization, stability, protein-ligand interactions, disulfide bond formation, and glycosylation. Most of the mutations cause loss of function and a few, gain of function.

Collagen-binding proteins: insights from the Collagen Toolkits

The Collagen Toolkits are libraries of 56 and 57 triple-helical synthetic peptides spanning the length of the collagen II and collagen III helices. These have been used in solid-phase binding assays to locate sites where collagen receptors and extracellular matrix components bind to collagens. Truncation and substitution allowed exact binding sites to be identified, and corresponding minimal peptides to be synthesised for use in structural and functional studies. 170 sites where over 30 proteins bind to collagen II have been mapped, providing firm conclusions about the amino acid distribution within such binding sites. Protein binding to collagen II is not random, but displays a periodicity of approximately 28 nm, with several prominent nodes where multiple proteins bind. Notably, the vicinity of the collagenase-cleavage site in Toolkit peptide II-44 is highly promiscuous, binding over 20 different proteins. This may reflect either the diverse chemistry of that locus or its diverse function, together with the interplay between regulatory binding partners. Peptides derived from Toolkit studies have been used to determine atomic level resolution of interactions between collagen and several of its binding partners and are finding practical application in tissue engineering.

Super Secondary Structure Consisting of a Polyproline II Helix and a β-Turn in Leucine Rich Repeats in Bacterial Type III Secretion System Effectors

Leucine rich repeats (LRRs) are present in over 100,000 proteins from viruses to eukaryotes. The LRRs are 20–30 residues long and occur in tandem. LRRs form parallel stacks of short β-strands and then assume a super helical arrangement called a solenoid structure. Individual LRRs are separated into highly conserved segment (HCS) with the consensus of LxxLxLxxNxL and variable segment (VS). Eight classes have been recognized. Bacterial LRRs are short and characterized by two prolines in the VS; the consensus is xxLPxLPxx with Nine residues (N-subtype) and xxLPxxLPxx with Ten residues (T-subtype). Bacterial LRRs are contained in type III secretion system effectors such as YopM, IpaH3/9.8, SspH1/2, and SlrP from bacteria. Some LRRs in decorin, fribromodulin, TLR8/9, and FLRT2/3 from vertebrate also contain the motifs. In order to understand structural features of bacterial LRRs, we performed both secondary structures assignments using four programs—DSSP-PPII, PROSS, SEGNO, and XTLSSTR—and HELFIT analyses (calculating helix axis, pitch, radius, residues per turn, and handedness), based on the atomic coordinates of their crystal structures. The N-subtype VS adopts a left handed polyproline II helix (PPII) with four, five or six residues and a type I β-turn at the C-terminal side. Thus, the N-subtype is characterized by a super secondary structure consisting of a PPII and a β-turn. In contrast, the T-subtype VS prefers two separate PPIIs with two or three and two residues. The HELFIT analysis indicates that the type I β-turn is a right handed helix. The HELFIT analysis determines three unit vectors of the helix axes of PPII (P), β-turn (B), and LRR domain (A). Three structural parameters using these three helix axes are suggested to characterize the super secondary structure and the LRR domain.

Monoclonal and Polyclonal Antibodies Specific to Human Fibromodulin

Background

The unique expression of fibromodulin (FMOD) in patients with chronic lymphocytic leukemia (CLL) has been previously reported. Detecting FMOD in CLL patients using specific anti-FMOD mAbs might provide a promising method in detection, monitoring, and prognosis of CLL.

Objectives

In this study, we aimed for producing specific antibodies against FMOD to facilitate further cohort study of CLL, thus addressing FMOD as a potential target of detection.

Materials and Methods

Human FMOD gene (1087 bp) was extracted from genome of the CLL patients, and was cloned into the expression vector of pET-22b (+). The recombinant FMOD protein (rFMOD) was expressed in Escherichia coli. The purified rFMOD protein was used as an immunogen in rabbit and mice. Hybridoma technology was used to develop the monoclonal antibodies (mAbs). Polyclonal antibody (pAb) was purified from the rabbit sera using affinity column. The reactivity of anti-FMOD antibodies was assessed in ELISA, immunocytochemistry (ICC) and Western blot.

Results

ICC results showed that the anti-FMOD antibodies specifically detected FMOD in CLL PBMCs and cell lines. The developed anti-FMOD pAb detected FMOD in CLL lysates, compared to healthy PBMCs, in Western blot and ELISA.

Conclusions

The developed anti-FMOD mAbs, and pAb specifically detect FMOD in CLL samples and might be used as research tools for further investigations in CLL.

Cartilage diseases

Hyaline cartilages, fibrocartilages and elastic cartilages play multiple roles in the human body including bearing loads in articular joints and intervertebral discs, providing joint lubrication, forming the external ears and nose, supporting the trachea, and forming the long bones during development and growth. The structure and organization of cartilage's extracellular matrix (ECM) are the primary determinants of normal function. Most diseases involving cartilage lead to dramatic changes in the ECM which can govern disease progression (e.g., in osteoarthritis), cause the main symptoms of the disease (e.g., dwarfism caused by genetically inherited mutations) or occur as collateral damage in pathological processes occurring in other nearby tissues (e.g., osteochondritis dissecans and inflammatory arthropathies). Challenges associated with cartilage diseases include poor understanding of the etiology and pathogenesis, delayed diagnoses due to the aneural nature of the tissue and drug delivery challenges due to the avascular nature of adult cartilages. This narrative review provides an overview of the clinical and pathological features as well as current treatment options available for various cartilage diseases. Late breaking advances are also described in the quest for development and delivery of effective disease modifying drugs for cartilage diseases including osteoarthritis, the most common form of arthritis that affects hundreds of millions of people worldwide.

Glycosaminoglycan synthesis in the nucleus pulposus: Dysregulation and the pathogenesis of disc degeneration

Few human tissues have functions as closely linked to the composition of their extracellular matrices as the intervertebral disc. In fact, the hallmark of intervertebral disc degeneration, commonly accompanying low back and neck pain, is the progressive loss of extracellular matrix molecules - specifically the GAG-substituted proteoglycans. While this loss is often associated with increased extracellular catabolism via metalloproteinases and pro-inflammatory cytokines, there is strong evidence that disc degeneration is related to dysregulation of the enzymes involved in GAG biosynthesis. In this review, we discuss those environmental factors, unique to the disc, that control expression and function of XT-1, GlcAT-I, and ChSy/ChPF in the healthy and degenerative state. Additionally, we address the pathophysiology of aberrant GAG biosynthesis and highlight therapeutic strategies designed to augment the loss of extracellular matrix molecules that afflict the degenerative state.

A novel mouse model of intervertebral disc degeneration shows altered cell fate and matrix homeostasis

Intervertebral disc degeneration and associated low back and neck pain is a ubiquitous health condition that affects millions of people world-wide, and causes high incidence of disability and enormous medical/societal costs. However, lack of appropriate small animal models with spontaneous disease onset has impeded our ability to understand the pathogenetic mechanisms that characterize and drive the degenerative process. We report, for the first time, early onset spontaneous disc degeneration in SM/J mice known for their poor regenerative capacities compared to "super-healer" LG/J mice. In SM/J mice, degenerative process was marked by decreased nucleus pulposus (NP) cellularity and changes in matrix composition at P7, 4, and 8 weeks with increased severity by 17 weeks. Distinctions between NP and annulus fibrosus (AF) or endplate cartilage were lost, and NP and AF of SM/J mice showed higher histological grades. There was increased NP cell death in SM/J mice with decreased phenotypic marker expression. Polarized microscopy and FTIR spectroscopy demonstrated replacement of glycosaminoglycan-rich NP matrix with collagenous fibrous tissue. The levels of ARGxx were increased in, indicating higher aggrecan turnover. Furthermore, an aberrant expression of collagen X and MMP13 was observed in the NP of SM/J mice, along with elevated expression of Col10a1, Ctgf, and Runx2, markers of chondrocyte hypertrophy. Likewise, expression of Enpp1 as well as Alpl was higher, suggesting NP cells of SM/J mice promote dystrophic mineralization. There was also a decrease in several pathways necessary for NP cell survival and function including Wnt and VEGF signaling. Importantly, SM/J discs were stiffer, had decreased height, and poor vertebral bone quality, suggesting compromised motion segment mechanical functionality. Taken together, our results clearly demonstrate that SM/J mouse strain recapitulates many salient features of human disc degeneration, and serves as a novel small animal model.

Molecular basis for governing the morphology of type-I collagen fibrils by Osteomodulin

Small leucine-rich repeat proteoglycan (SLRP) proteins have an important role in the organization of the extracellular matrix, especially in the formation of collagen fibrils. However, the mechanism governing the shape of collagen fibrils is poorly understood. Here, we report that the protein Osteomodulin (OMD) of the SLRP family is a monomeric protein in solution that interacts with type-I collagen. This interaction is dominated by weak electrostatic forces employing negatively charged residues of OMD, in particular Glu284 and Glu303, and controlled by entropic factors. The protein OMD establishes a fast-binding equilibrium with collagen, where OMD may engage not only with individual collagen molecules, but also with the growing fibrils. This weak electrostatic interaction is carefully balanced so it modulates the shape of the fibrils without compromising their viability.

Takumi Tashima and colleagues provide structural insights into how collagen fibrils are shaped by Osteomodulin. Osteomodulin keeps a fast-binding equilibrium with the collagen fibrils to slow down its growth, promoting the formation of uniform, intact collagen fibrils.

Structural analysis of glycoproteins: building N-linked glycans with Coot

Coot is a graphics application that is used to build or manipulate macromolecular models; its particular forte is manipulation of the model at the residue level. The model-building tools of Coot have been combined and extended to assist or automate the building of N-linked glycans. The model is built by the addition of monosaccharides, placed by variation of internal coordinates. The subsequent model is refined by real-space refinement, which is stabilized with modified and additional restraints. It is hoped that these enhanced building tools will help to reduce building errors of N-linked glycans and improve our knowledge of the structures of glycoproteins.