Dwarfism in mice lacking collagen-binding integrins α2β1 and α11β1 is caused by severely diminished IGF-1 levels

Osteoclast-specific Plastin 3 knockout in mice fail to develop osteoporosis despite dramatic increased osteoclast resorption activity

PLS3 loss-of-function mutations in humans and mice cause X-linked primary osteoporosis. However, it remains largely unknown how PLS3 mutations cause osteoporosis and which function PLS3 plays in bone homeostasis. A recent study showed that ubiquitous Pls3 KO in mice results in osteoporosis. Mainly osteoclasts were impacted in their function However, it has not been proven if osteoclasts are the major cell type affected and responsible for osteoporosis development in ubiquitous Pls3 KO mice. Here, we generated osteoclast-specific Pls3 KO mice. Additionally, we developed a novel polyclonal PLS3 antibody that showed specific PLS3 loss in immunofluorescence staining of osteoclasts in contrast to previously available antibodies against PLS3, which failed to show PLS3 specificity in mouse cells. Moreover, we demonstrate that osteoclast-specific Pls3 KO causes dramatic increase in resorptive activity of osteoclasts in vitro. Despite these findings, osteoclast-specific Pls3 KO in vivo failed to cause any osteoporotic phenotype in mice as proven by micro-CT and three-point bending test. This demonstrates that the pathomechanism of PLS3-associated osteoporosis is highly complex and cannot be reproduced in a system singularly focused on one cell type. Thus, the loss of PLS3 in alternative bone cell types might contributes to the osteoporosis phenotype in ubiquitous Pls3 KO mice.

Novel roles for cooperating collagen receptor families in fibrotic niches

Recent data indicate that integrin and non-integrin collagen receptors cooperate in the fibrosis-specific microenvironment (i.e., the fibrotic niche). In certain tumor types, DDR1 can regulate the interaction with collagen III to regulate dormancy and metastasis, whereas in other tumor types, DDR1 can be shed and used to reorganize collagen. DDR1 expressed on tumor cells, together with DDR2 and α11β1 integrin expressed on cancer-associated fibroblasts, can increase tumor tissue stiffness. Integrin α1β1 and α2β1 are present on immune cells where they together with the immunosuppressive collagen receptor LAIR-1 can mediate binding to intratumor collagens. In summary, collagen-binding integrins together with DDRs, can create fibrillar collagen niches that act as traps to hinder immune cell trafficking into the tumor cell mass. Binding of collagens via LAIR-1 on immune cells in turn results in CD8+T-cell exhaustion. Continued studies of these complex interactions are needed for successful new stroma-based therapeutic interventions. In the current review, we will summarize recent data on collagen receptors with a special focus on their potential role in tumor fibrosis and highlight their collaborative roles in tumor fibrotic niches.

Ablation of integrin-mediated cell-collagen communication alleviates fibrosis

OBJECTIVES

Activation of fibroblasts is a hallmark of fibrotic processes. Besides cytokines and growth factors, fibroblasts are regulated by the extracellular matrix environment through receptors such as integrins, which transduce biochemical and mechanical signals enabling cells to mount appropriate responses according to biological demands. The aim of this work was to investigate the in vivo role of collagen-fibroblast interactions for regulating fibroblast functions and fibrosis.

METHODS

Triple knockout (tKO) mice with a combined ablation of integrins α1β1, α2β1 and α11β1 were created to address the significance of integrin-mediated cell-collagen communication. Properties of primary dermal fibroblasts lacking collagen-binding integrins were delineated in vitro. Response of the tKO mice skin to bleomycin induced fibrotic challenge was assessed.

RESULTS

Triple integrin-deficient mice develop normally, are transiently smaller and reveal mild alterations in mechanoresilience of the skin. Fibroblasts from these mice in culture show defects in cytoskeletal architecture, traction stress generation, matrix production and organisation. Ablation of the three integrins leads to increased levels of discoidin domain receptor 2, an alternative receptor recognising collagens in vivo and in vitro. However, this overexpression fails to compensate adhesion and spreading defects on collagen substrates in vitro. Mice lacking collagen-binding integrins show a severely attenuated fibrotic response with impaired mechanotransduction, reduced collagen production and matrix organisation.

CONCLUSIONS

The data provide evidence for a crucial role of collagen-binding integrins in fibroblast force generation and differentiation in vitro and for matrix deposition and tissue remodelling in vivo. Targeting fibroblast-collagen interactions might represent a promising therapeutic approach to regulate connective tissue deposition in fibrotic diseases.

EMILIN1 deficiency causes arterial tortuosity with osteopenia and connects impaired elastogenesis with defective collagen fibrillogenesis

EMILIN1 (elastin-microfibril-interface-located-protein-1) is a structural component of the elastic fiber network and localizes to the interface between the fibrillin microfibril scaffold and the elastin core. How EMILIN1 contributes to connective tissue integrity is not fully understood. Here, we report bi-allelic EMILIN1 loss-of-function variants causative for an entity combining cutis laxa, arterial tortuosity, aneurysm formation, and bone fragility, resembling autosomal-recessive cutis laxa type 1B, due to EFEMP2 (FBLN4) deficiency. In both humans and mice, absence of EMILIN1 impairs EFEMP2 extracellular matrix deposition and LOX activity resulting in impaired elastogenesis, reduced collagen crosslinking, and aberrant growth factor signaling. Collagen fiber ultrastructure and histopathology in EMILIN1- or EFEMP2-deficient skin and aorta corroborate these findings and murine Emilin1^-/- femora show abnormal trabecular bone formation and strength. Altogether, EMILIN1 connects elastic fiber network with collagen fibril formation, relevant for both bone and vascular tissue homeostasis.

Integrin α11β1 and syndecan-4 dual receptor ablation attenuates cardiac hypertrophy in the pressure overloaded heart

Pathological myocardial hypertrophy in response to an increase in left ventricular (LV) afterload may ultimately lead to heart failure. Cell surface receptors bridge the interface between the cell and the ECM in cardiac myocytes and cardiac fibroblasts, and have been suggested to be important mediators of pathological myocardial hypertrophy. We identify for the first time that integrin α11 (α11) is preferentially upregulated amongst integrin beta 1 heterodimer-forming α subunits in response to increased afterload induced by aortic banding (AB) in wild type mice (WT). Mice were anesthetized in a chamber with 4% isoflurane and 95% oxygen before being intubated and ventilated with 2.5% isoflurane and 97% oxygen. For pre- and post-operative analgesia, animals were administered 0.02 mL buprenorphine (0.3 mg/mL) subcutaneously. Surprisingly, mice lacking α11 develop myocardial hypertrophy following AB comparable to WT. In the mice lacking α11, we further show a compensatory increase in the expression of another mechanoreceptor, syndecan-4, following AB compared to WT AB mice, indicating that syndecan-4 compensated for lack of α11. Intriguingly, mice lacking mechanoreceptors α11 and syndecan-4 show ablated myocardial hypertrophy following AB compared to WT mice. Expression of the main cardiac collagen isoforms col1a2 and col3a1 was significantly reduced in AB mice lacking mechanoreceptors α11 and syndecan-4 compared to WT AB.

Integrin α11β1 is a receptor for collagen XIII

Collagen XIII is a conserved transmembrane collagen mainly expressed in mesenchymal tissues. Previously, we have shown that collagen XIII modulates tissue development and homeostasis. Integrins are a family of receptors that mediate signals from the environment into the cells and vice versa. Integrin α11β1 is a collagen receptor known to recognize the GFOGER (O=hydroxyproline) sequence in collagens. Interestingly, collagen XIII and integrin α11β1 both have a role in the regulation of bone homeostasis. To study whether α11β1 is a receptor for collagen XIII, we utilized C2C12 cells transfected to express α11β1 as their only collagen receptor. The interaction between collagen XIII and integrin α11β1 was also confirmed by surface plasmon resonance and pull-down assays. We discovered that integrin α11β1 mediates cell adhesion to two collagenous motifs, namely GPKGER and GF(S)QGEK, that were shown to act as the recognition sites for the integrin α11-I domain. Furthermore, we studied the in vivo significance of the α11β1-collagen XIII interaction by crossbreeding α11 null mice (Itga11-/-) with mice overexpressing Col13a1 (Col13a1oe). When we evaluated the bone morphology by microcomputed tomography, Col13a1oe mice had a drastic bone overgrowth followed by severe osteoporosis, whereas the double mutant mouse line showed a much milder bone phenotype. To conclude, our data identifies integrin α11β1 as a new collagen XIII receptor and demonstrates that this ligand-receptor pair has a role in the maintenance of bone homeostasis.

Signaling pathways affected by mutations causing osteogenesis imperfecta

Osteogenesis imperfecta (OI) is a clinically and genetically heterogeneous connective tissue disorder characterized by bone fragility and skeletal deformity. To maintain skeletal strength and integrity, bone undergoes constant remodeling of its extracellular matrix (ECM) tightly controlled by osteoclast-mediated bone resorption and osteoblast-mediated bone formation. There are at least 20 recognized OI-forms caused by mutations in the two collagen type I-encoding genes or genes implicated in collagen folding, posttranslational modifications or secretion of collagen, osteoblast differentiation and function, or bone mineralization. The underlying disease mechanisms of non-classical forms of OI that are not caused by collagen type I mutations are not yet completely understood, but an altered ECM structure as well as disturbed intracellular homeostasis seem to be the main defects. The ECM orchestrates local cell behavior in part by regulating bioavailability of signaling molecules through sequestration, release and activation during the constant bone remodeling process. Here, we provide an overview of signaling pathways that are associated with known OI-causing genes and discuss the impact of these genes on signal transduction. These pathways include WNT-, RANK/RANKL-, TGFβ-, MAPK- and integrin-mediated signaling as well as the unfolded protein response.

Comparative proteomic analysis of osteogenic differentiated human adipose tissue and bone marrow-derived stromal cells

Mesenchymal stromal cells are promising candidates for regenerative applications upon treatment of bone defects. Bone marrow‐derived stromal cells (BMSCs) are limited by yield and donor morbidity but show superior osteogenic capacity compared to adipose‐derived stromal cells (ASCs), which are highly abundant and easy to harvest. The underlying reasons for this difference on a proteomic level have not been studied yet. Human ASCs and BMSCs were characterized by FACS analysis and tri‐lineage differentiation, followed by an intraindividual comparative proteomic analysis upon osteogenic differentiation. Results of the proteomic analysis were followed by functional pathway analysis. 29 patients were included with a total of 58 specimen analysed. In these, out of 5148 identified proteins 2095 could be quantified in >80% of samples of both cell types, 427 in >80% of ASCs only and 102 in >80% of BMSCs only. 281 proteins were differentially regulated with a fold change of >1.5 of which 204 were higher abundant in BMSCs and 77 in ASCs. Integrin cell surface interactions were the most overrepresented pathway with 5 integrins being among the proteins with highest fold change. Integrin 11a, a known key protein for osteogenesis, could be identified as strongly up‐regulated in BMSC confirmed by Western blotting. The integrin expression profile is one of the key distinctive features of osteogenic differentiated BMSCs and ASCs. Thus, they represent a promising target for modifications of ASCs aiming to improve their osteogenic capacity and approximate them to that of BMSCs.

Reciprocal Interplay Between Fibrillar Collagens and Collagen-Binding Integrins: Implications in Cancer Progression and Metastasis

Cancers are complex ecosystems composed of malignant cells embedded in an intricate microenvironment made of different non-transformed cell types and extracellular matrix (ECM) components. The tumor microenvironment is governed by constantly evolving cell-cell and cell-ECM interactions, which are now recognized as key actors in the genesis, progression and treatment of cancer lesions. The ECM is composed of a multitude of fibrous proteins, matricellular-associated proteins, and proteoglycans. This complex structure plays critical roles in cancer progression: it functions as the scaffold for tissues organization and provides biochemical and biomechanical signals that regulate key cancer hallmarks including cell growth, survival, migration, differentiation, angiogenesis, and immune response. Cells sense the biochemical and mechanical properties of the ECM through specialized transmembrane receptors that include integrins, discoidin domain receptors, and syndecans. Advanced stages of several carcinomas are characterized by a desmoplastic reaction characterized by an extensive deposition of fibrillar collagens in the microenvironment. This compact network of fibrillar collagens promotes cancer progression and metastasis, and is associated with low survival rates for cancer patients. In this review, we highlight how fibrillar collagens and their corresponding integrin receptors are modulated during cancer progression. We describe how the deposition and alignment of collagen fibers influence the tumor microenvironment and how fibrillar collagen-binding integrins expressed by cancer and stromal cells critically contribute in cancer hallmarks.

Cell Type- and Sex-Dependent Transcriptome Profiles of Rat Anterior Pituitary Cells

Understanding the physiology and pathology of an organ composed of a variety of cell populations depends critically on genome-wide information on each cell type. Here, we report single-cell transcriptome profiling of over 6,800 freshly dispersed anterior pituitary cells from postpubertal male and female rats. Six pituitary-specific cell types were identified based on known marker genes and characterized: folliculostellate cells and hormone-producing corticotrophs, gonadotrophs, thyrotrophs, somatotrophs, and lactotrophs. Also identified were endothelial and blood cells from the pituitary capillary network. The expression of numerous developmental and neuroendocrine marker genes in both folliculostellate and hormone-producing cells supports that they have a common origin. For several genes, the validity of transcriptome analysis was confirmed by qRT-PCR and single cell immunocytochemistry. Folliculostellate cells exhibit impressive transcriptome diversity, indicating their major roles in production of endogenous ligands and detoxification enzymes, and organization of extracellular matrix. Transcriptome profiles of hormone-producing cells also indicate contributions toward those functions, while also clearly demonstrating their endocrine function. This survey highlights many novel genetic markers contributing to pituitary cell type identity, sexual dimorphism, and function, and points to relationships between hormone-producing and folliculostellate cells.

Role of prolyl hydroxylation in the molecular interactions of collagens

Co- and post-translational hydroxylation of proline residues is critical for the stability of the triple helical collagen structure. In this review, we summarise the biology of collagen prolyl 4-hydroxylases and collagen prolyl 3-hydroxylases, the enzymes responsible for proline hydroxylation. Furthermore, we describe the potential roles of hydroxyproline residues in the complex interplay between collagens and other proteins, especially integrin and discoidin domain receptor type cell adhesion receptors. Qualitative and quantitative regulation of collagen hydroxylation may have remarkable effects on the properties of the extracellular matrix and consequently on the cell behaviour.

A distal super enhancer mediates estrogen-dependent mouse uterine-specific gene transcription of Igf1 (insulin-like growth factor 1)

Insulin-like growth factor 1 (IGF1) is primarily synthesized in and secreted from the liver; however, estrogen (E2), through E2 receptor α (ERα), increases uterine Igf1 mRNA levels. Previous ChIP-seq analyses of the murine uterus have revealed a potential enhancer region distal from the Igf1 transcription start site (TSS) with multiple E2-dependent ERα-binding regions. Here, we show E2-dependent super enhancer-associated characteristics and suggest contact between the distal enhancer and the Igf1 TSS. We hypothesized that this distal super-enhancer region controls E2-responsive induction of uterine Igf1 transcripts. We deleted 430 bp, encompassing one of the ERα-binding sites, thereby disrupting interactions of the enhancer with gene-regulatory factors. As a result, E2-mediated induction of mouse uterine Igf1 mRNA is completely eliminated, whereas hepatic Igf1 expression remains unaffected. This highlights the central role of a distal enhancer in the assembly of the factors necessary for E2-dependent interaction with the Igf1 TSS and induction of uterus-specific Igf1 transcription. Of note, loss of the enhancer did not affect fertility or uterine growth responses. Deletion of uterine Igf1 in a PgrCre;Igf1f/f model decreased female fertility but did not impact the E2-induced uterine growth response. Moreover, E2-dependent activation of uterine IGF1 signaling was not impaired by disrupting the distal enhancer or by deleting the coding transcript. This indicated a role for systemic IGF1, suggested that other growth mediators drive uterine response to E2, and suggested that uterine-derived IGF1 is essential for reproductive success. Our findings elucidate the role of a super enhancer in Igf1 regulation and uterine growth.

Plastin 3 influences bone homeostasis through regulation of osteoclast activity

Over 200 million people suffer from osteoporosis worldwide, one third of which will develop osteoporotic bone fractures. Unfortunately, no effective cure exists. Mutations in plastin 3 (PLS3), an F-actin binding and bundling protein, cause X-linked primary osteoporosis in men and predisposition to osteoporosis in postmenopausal women. Moreover, the strongest association so far for osteoporosis in elderly women after menopause was connected to a rare SNP in PLS3, indicating a possible role of PLS3 in complex osteoporosis as well. Interestingly, 5% of the general population are overexpressing PLS3, with yet unknown consequences. Here, we studied ubiquitous Pls3 knockout and PLS3 overexpression in mice and demonstrate that both conditions influence bone remodeling and structure: while Pls3 knockout mice exhibit osteoporosis, PLS3 overexpressing mice show thickening of cortical bone and increased bone strength. We show that unbalanced PLS3 levels affect osteoclast development and function, by misregulating the NFκB pathway. We found upregulation of RELA (NFκB subunit p65) in PLS3 overexpressing mice-known to stimulate osteoclastogenesis-but strikingly reduced osteoclast resorption. We identify NFκB repressing factor (NKRF) as a novel PLS3 interactor, which increasingly translocates to the nucleus when PLS3 is overexpressed. We show that NKRF binds to the NFκB downstream target and master regulator of osteoclastogenesis nuclear factor of activated T cells 1 (Nfatc1), thereby reducing its transcription and suppressing osteoclast function. We found the opposite in Pls3 knockout osteoclasts, where decreased nuclear NKRF augmented Nfatc1 transcription, causing osteoporosis. Regulation of osteoclastogenesis and bone remodeling via the PLS3-NKRF-NFκB-NFATC1 axis unveils a novel possibility to counteract osteoporosis.

Proline hydroxylation in collagen supports integrin binding by two distinct mechanisms

Collagens are the most abundant extracellular matrix proteins in vertebrates and have a characteristic triple-helix structure. Hydroxylation of proline residues is critical for helix stability, and diminished prolyl hydroxylase activity causes wide-spread defects in connective tissues. Still, the role of proline hydroxylation in the binding of collagen receptors such as integrins is unclear. Here, we isolated skin collagen from genetically modified mice having reduced prolyl 4-hydroxylase activity. At room temperature, the reduced proline hydroxylation did not affect interactions with the recombinant integrin α2I domain, but at 37 °C, collagen hydroxylation correlated with the avidity of α2I domain binding. Of note, LC-MS/MS analysis of isolated skin collagens revealed no major changes in the hydroxyproline content of the main integrin-binding sites. Thus, the disrupted α2I domain binding at physiological temperatures was most likely due to structural destabilization of the collagenous helix. Integrin α2I binding to the triple-helical GFPGER motif was slightly weaker than to GFOGER (O = hydroxyproline). This phenomenon was more prominent when α1 integrin was tested. Integrin α1β1 expressed on CHO cells and recombinant α1I domain showed remarkably slower binding velocity and weaker avidity to GFPGER when compared with GFOGER. Structural modeling revealed the critical interaction between Arg-218 in α1I and the hydroxyproline residue in the integrin-binding motif. The role of Arg-218 was further validated by testing a variant R218D α1I domain in solid-phase binding assays. Thus, our results show that the lack of proline hydroxylation in collagen can affect integrin binding by a direct mechanism and via structural destabilization of the triple helix.

Control of the Osteoblast Lineage by Mitogen-Activated Protein Kinase Signaling

PURPOSE OF THE REVIEW

This review will provide a timely assessment of MAP kinase actions in bone development and homeostasis with particular emphasis on transcriptional control of the osteoblast lineage.

RECENT FINDINGS

ERK and p38 MAP kinases function as transducers of signals initiated by the extracellular matrix, mechanical loading, TGF-β, BMPs and FGF2. MAPK signals may also affect and/or interact with other important pathways such as WNT and HIPPO. ERK and p38 MAP kinase pathways phosphorylate specific osteogenic transcription factors including RUNX2, Osterix, ATF4 and DLX5. For RUNX2, phosphorylation at specific serine residues initiates epigenetic changes in chromatin necessary for decondensation and increased transcription. MAPK also suppresses marrow adipogenesis by phosphorylating and inhibiting PPARγ, which may explain the well-known relationship between reduced skeletal loading and marrow fat accumulation.

SUMMARY

MAPKs transduce signals from the extracellular environment to the nucleus allowing bone cells to respond to changes in hormonal/growth factor signaling and mechanical loading thereby optimizing bone structure to meet physiological and mechanical needs of the body.

The integrin-collagen connection--a glue for tissue repair?

The α1β1, α2β1, α10β1 and α11β1 integrins constitute a subset of the integrin family with affinity for GFOGER-like sequences in collagens. Integrins α1β1 and α2β1 were originally identified on a subset of activated T-cells, and have since been found to be expressed on a number of cell types including platelets (α2β1), vascular cells (α1β1, α2β1), epithelial cells (α1β1, α2β1) and fibroblasts (α1β1, α2β1). Integrin α10β1 shows a distribution that is restricted to mesenchymal stem cells and chondrocytes, whereas integrin α11β1 appears restricted to mesenchymal stem cells and subsets of fibroblasts. The bulk of the current literature suggests that collagen-binding integrins only have a limited role in adult connective tissue homeostasis, partly due to a limited availability of cell-binding sites in the mature fibrillar collagen matrices. However, some recent data suggest that, instead, they are more crucial for dynamic connective tissue remodeling events--such as wound healing--where they might act specifically to remodel and restore the tissue architecture. This Commentary discusses the recent development in the field of collagen-binding integrins, their roles in physiological and pathological settings with special emphasis on wound healing, fibrosis and tumor-stroma interactions, and include a discussion of the most recently identified newcomers to this subfamily--integrins α10β1 and α11β1.

Integrin α11β1 regulates cancer stromal stiffness and promotes tumorigenicity and metastasis in non-small cell lung cancer

Integrin α11β1 is a stromal cell-specific receptor for fibrillar collagens and is overexpressed in carcinoma-associated fibroblasts (CAFs). We have investigated its direct role in cancer progression by generating severe combined immune deficient (SCID) mice deficient in integrin α11 (α11) expression. The growth of A549 lung adenocarcinoma cells and two patient-derived non-small cell lung carcinoma (NSCLC) xenografts in these α11 knockout (α11^−/−) mice was significantly impeded, as compared with wild-type (α11^+/+) SCID mice. Orthotopic implantation of a spontaneously metastatic NCI-H460SM cell line into the lungs of α11^−/− and α11^+/+ mice showed significant reduction in the metastatic potential of these cells in the α11^−/− mice. We identified that collagen cross-linking is associated with stromal α11 expression, and the loss of tumor stromal α11 expression was correlated with decreased collagen reorganization and stiffness. This study shows the role of integrin α11β1, a receptor for fibrillar collagen in differentiation of fibroblasts into CAFs. Furthermore, our data support an important role for α11 signaling pathway in CAFs, promoting tumor growth and metastatic potential of NSCLC cells and being closely associated with collagen cross-linking and the organization and stiffness of fibrillar collagen matrices.

Nanotopology potentiates growth hormone signalling and osteogenesis of mesenchymal stem cells

Custom engineered materials can influence the differentiation of human mesenchymal stem cells (MSCs) towards osteoblasts, chondrocytes and adipocytes, through the control of chemistry, stiffness and nanoscale topography. Here we demonstrate that polycaprolactone growth surfaces engineered with disordered (but controlled) 120 nm diameter dots (NSQ50), but not flat surfaces, promote osteogenic conversion of MSCs in the absence of other osteogenic stimuli. Differentiating MSCs on NSQ50 were found to express growth hormone receptors (GH) and stimulation with recombinant human GH (rhGH) further enhanced NSQ50-driven osteogenic conversion of MSCs. This increased osteogenesis coincided with an enhanced ability of GH to activate ERK MAP kinase on NSQ50, but not on flat topology. The importance of ERK for MSC differentiation was demonstrated by using the inhibitor of ERK activation, U0126, which completely suppressed osteogenesis of GH-stimulated MSCs on NSQ50. The ability of GH to activate ERK in MSCs may therefore be a central control mechanism underlying bone development and growth.

The effect of conditional inactivation of beta 1 integrins using twist 2 Cre, Osterix Cre and osteocalcin Cre lines on skeletal phenotype

Skeletal development and growth are complex processes regulated by multiple microenvironmental cues, including integrin-ECM interactions. The β1 sub-family of integrins is the largest integrin sub-family and constitutes the main integrin binding partners of collagen I, the major ECM component of bone. As complete β1 integrin knockout results in embryonic lethality, studies of β1 integrin function in vivo rely on tissue-specific gene deletions. While multiple in vitro studies indicate that β1 integrins are crucial regulators of osteogenesis and mineralization, in vivo osteoblast-specific perturbations of β1 integrins have resulted in mild and sometimes contradictory skeletal phenotypes. To further investigate the role of β1 integrins on skeletal phenotype, we used the Twist2-Cre, Osterix-Cre and osteocalcin-Cre lines to generate conditional β1 integrin deletions, where Cre is expressed primarily in mesenchymal condensation, pre-osteoblast, and mature osteoblast lineage cells respectively within these lines. Mice with Twist2-specific β1 integrin disruption were smaller, had impaired skeletal development, especially in the craniofacial and vertebral tissues at E19.5, and did not survive beyond birth. Osterix-specific β1 integrin deficiency resulted in viable mice which were normal at birth but displayed early defects in calvarial ossification, incisor eruption and growth as well as femoral bone mineral density, structure, and mechanical properties. Although these defects persisted into adulthood, they became milder with age. Finally, a lack of β1 integrins in mature osteoblasts and osteocytes resulted in minor alterations to femur structure but had no effect on mineral density, biomechanics or fracture healing. Taken together, our data indicate that β1 integrin expression in early mesenchymal condensations play an important role in skeletal ossification, while β1 integrin-ECM interactions in pre-osteoblast, odontoblast- and hypertrophic chondryocyte-lineage cells regulate incisor eruption and perinatal bone formation in both intramembranously and endochondrally formed bones in young, rapidly growing mice. In contrast, the osteocalcin-specific β1 integrin deletion had only minor effects on skeletal phenotype.

A 3D matrix platform for the rapid generation of therapeutic anti-human carcinoma monoclonal antibodies

Efforts to develop unbiased screens for identifying novel function-blocking monoclonal antibodies (mAbs) in human carcinomatous states have been hampered by the limited ability to design in vitro models that recapitulate tumor cell behavior in vivo. Given that only invasive carcinoma cells gain permanent access to type I collagen-rich interstitial tissues, an experimental platform was established in which human breast cancer cells were embedded in 3D aldimine cross-linked collagen matrices and used as an immunogen to generate mAb libraries. In turn, cancer-cell-reactive antibodies were screened for their ability to block carcinoma cell proliferation within collagen hydrogels that mimic the in vivo environment. As a proof of principle, a single function-blocking mAb out of 15 identified was selected for further analysis and found to be capable of halting carcinoma cell proliferation, inducing apoptosis, and exerting global changes in gene expression in vitro. The ability of this mAb to block carcinoma cell proliferation and metastatic activity was confirmed in vivo, and the target antigen was identified by mass spectroscopy as the α2 subunit of the α2β1 integrin, one of the major type I collagen-binding receptors in mammalian cells. Validating the ability of the in vitro model to predict patterns of antigen expression in the disease setting, immunohistochemical analyses of tissues from patients with breast cancer verified markedly increased expression of the α2 subunit in vivo. These results not only highlight the utility of this discovery platform for rapidly selecting and characterizing function-blocking, anticancer mAbs in an unbiased fashion, but also identify α2β1 as a potential target in human carcinomatous states.

Collagen advanced glycation inhibits its Discoidin Domain Receptor 2 (DDR2)-mediated induction of lysyl oxidase in osteoblasts

Diabetes increases the risk of bone fracture. Organic and inorganic bone extracellular matrix components determine bone strength. Previous studies indicate that in diabetes, glycation of collagen causes abnormal arrangements of collagen molecules and fragile bones. Diabetic bone fragility is additionally attributed to reduced levels of lysyl oxidase enzyme-dependent collagen cross-links. The mechanism underlying the presence of lower enzymatic collagen cross-links in diabetic bone has not been directly investigated. Here we determine in primary osteoblast cultures the regulation of lysyl oxidase protein by type I collagen and collagen modified by carboxymethylation (CML-collagen), a form of advanced glycation endproducts. Data indicate that non-glycated collagen up-regulates lysyl oxidase levels both in primary non-differentiated and in differentiating mouse and rat osteoblast cultures, while CML-collagen fails to regulate lysyl oxidase in these cells. Collagen binding to Discoidin Domain Receptor-2 (DDR2) mediates lysyl oxidase increases, determined in DDR2 shRNA knockdown studies. DDR2 binding and activation were disrupted by collagen glycation, pointing to a mechanism for the diminished levels of lysyl oxidase and consequently low lysyl oxidase-derived cross-links in diabetic bone. Our studies indicate that collagen-integrin interactions may not play a major role in up-regulating lysyl oxidase. Furthermore, non-collagenous ligands for the receptor for advanced glycation end products (RAGE) failed to alter lysyl oxidase levels. Taken together with published studies a new understanding emerges in which diabetes- and age-dependent inhibition of normal collagen-stimulated DDR2- and integrin-signaling, and independent advanced glycation-stimulated RAGE-signaling, each contributes to different aspects of diabetic osteopenia.

Molecular composition and function of integrin-based collagen glues-introducing COLINBRIs

BACKGROUND

Despite detailed knowledge about the structure and signaling properties of individual collagen receptors, much remains to be learned about how these receptors participate in linking cells to fibrillar collagen matrices in tissues. In addition to collagen-binding integrins, a group of proteins with affinity both for fibrillar collagens and integrins link these two protein families together. We have introduced the name COLINBRI (COLlagen INtegrin BRIdging) for this set of molecules. Whereas collagens are the major building blocks in tissues and defects in these structural proteins have severe consequences for tissue integrity, the mild phenotypes of the integrin type of collagen receptors have raised questions about their importance in tissue biology and pathology.

SCOPE OF REVIEW

We will discuss the two types of cell linkages to fibrillar collagen (direct- versus indirect COLINBRI-mediated) and discuss how the parallel existence of direct and indirect linkages to collagens may ensure tissue integrity.

MAJOR CONCLUSIONS

The observed mild phenotypes of mice deficient in collagen-binding integrins and the relatively restricted availability of integrin-binding sequences in mature fibrillar collagen matrices support the existence of indirect collagen-binding mechanisms in parallel with direct collagen binding in vivo.

GENERAL SIGNIFICANCE

A continued focus on understanding the molecular details of cell adhesion mechanisms to collagens will be important and will benefit our understanding of diseases like tissue- and tumor fibrosis where collagen dynamics are disturbed. This article is part of a Special Issue entitled Matrix-mediated cell behaviour and properties.

Inhibition of a novel specific neuroglial integrin signaling pathway increases STAT3-mediated CNTF expression

Background

Ciliary neurotrophic factor (CNTF) expression is repressed in astrocytes by neuronal contact in the CNS and is rapidly induced by injury. Here, we defined an inhibitory integrin signaling pathway.

Results

The integrin substrates laminin, fibronectin and vitronectin, but not collagen, thrombospondin or fibrinogen, reduced CNTF expression in C6 astroglioma cells. Antibodies against αv and β5, but not α6 or β1, integrin induced CNTF. Together, the ligand and antibody specificity suggests that CNTF is repressed by αvβ5 integrin. Antibodies against Thy1, an abundant neuronal surface protein whose function is unclear, induced CNTF in neuron-astrocyte co-cultures indicating that it is a neuroglial CNTF repressor. Inhibition of the integrin signaling molecule Focal Adhesion Kinase (FAK) or the downstream c-Jun N-terminal kinase (JNK), but not extracellular regulated kinase (ERK) or p38 MAPK, greatly induced CNTF mRNA and protein expression within 4 hours. This selective inhibitory pathway phosphorylated STAT3 on its inhibitory ser-727 residue interfering with activity of the pro-transcription Tyr-705 residue. STAT3 can activate CNTF transcription because it bound to its promoter and FAK antagonist-induced CNTF was reduced by blocking STAT3. Microinjection of FAK inhibitor directly into the brain or spinal cord in adult mice rapidly induced CNTF mRNA and protein expression. Importantly, systemic treatment with FAK inhibitors over 3 days induced CNTF in the subventricular zone and increased neurogenesis.

Conclusions

Neuron-astroglia contact mediated by integrins serves as a sensor to enable rapid neurotrophic responses and provides a new pharmacological avenue to exploit the neuroprotective properties of endogenous CNTF.

Targeting integrins to promote bone formation and repair

The ageing skeleton experiences a progressive decline in the rate of bone formation, which can eventually result in osteoporosis--a common disease characterized by reduced bone mass and altered bone microarchitecture which can result in fractures. One emerging therapy involves the identification of molecules that target bone-marrow mesenchymal stromal cells (MSCs) and promote their differentiation into osteoblasts, thereby counteracting bone loss. This Review highlights the discovery that some integrins, a family of heterodimeric transmembrane proteins that can interact with matrix proteins and generate intracellular signals, can be targeted to promote homing of MSCs to bone, osteogenic differentiation and bone formation. Specifically, priming of the α(5)β(1) integrin, which is required for osteoblastic differentiation of MSCs, leads to increased bone formation and improved bone repair in mice. Additionally, treatment with a peptidomimetic ligand of the α(4)β(1) integrin coupled to an agent with a high affinity for bone improves the homing of MSCs to bone and promotes osteoblast differentiation and bone formation, leading to increased bone mass in osteopenic mice. Strategies that target key integrins expressed by MSCs might, therefore, translate into improved therapies for age-related bone loss and possibly other disorders.