Mechanical strain increases expression of type XII collagen in murine osteoblastic MC3T3-E1 cells

The coordinated activities of collagen VI and XII in maintenance of tissue structure, function and repair: evidence for a physical interaction

Collagen VI and collagen XII are structurally complex collagens of the extracellular matrix (ECM). Like all collagens, type VI and XII both possess triple-helical components that facilitate participation in the ECM network, but collagen VI and XII are distinct from the more abundant fibrillar collagens in that they also possess arrays of structurally globular modules with the capacity to propagate signaling to attached cells. Cell attachment to collagen VI and XII is known to regulate protective, proliferative or developmental processes through a variety of mechanisms, but a growing body of genetic and biochemical evidence suggests that at least some of these phenomena may be potentiated through mechanisms that require coordinated interaction between the two collagens. For example, genetic studies in humans have identified forms of myopathic Ehlers-Danlos syndrome with overlapping phenotypes that result from mutations in either collagen VI or XII, and biochemical and cell-based studies have identified accessory molecules that could form bridging interactions between the two collagens. However, the demonstration of a direct or ternary structural interaction between collagen VI or XII has not yet been reported. This Hypothesis and Theory review article examines the evidence that supports the existence of a functional complex between type VI and XII collagen in the ECM and discusses potential biological implications.

A defective mechanosensing pathway affects fibroblast-to-myofibroblast transition in the old male mouse heart

The cardiac fibroblast interacts with an extracellular matrix (ECM), enabling myofibroblast maturation via a process called mechanosensing. Although in the aging male heart, ECM is stiffer than in the young mouse, myofibroblast development is impaired, as demonstrated in 2-D and 3-D experiments. In old male cardiac fibroblasts, we found a decrease in actin polymerization, α-smooth muscle actin (α-SMA), and Kindlin-2 expressions, the latter an effector of the mechanosensing. When Kindlin-2 levels were manipulated via siRNA interference, young fibroblasts developed an old-like fibroblast phenotype, whereas Kindlin-2 overexpression in old fibroblasts reversed the defective phenotype. Finally, inhibition of overactivated extracellular regulated kinases 1 and 2 (ERK1/2) in the old male fibroblasts rescued actin polymerization and α-SMA expression. Pathological ERK1/2 overactivation was also attenuated by Kindlin-2 overexpression. In contrast, old female cardiac fibroblasts retained an operant mechanosensing pathway. In conclusion, we identified defective components of the Kindlin/ERK/actin/α-SMA mechanosensing axis in aged male fibroblasts.

Collagen XII mediated cellular and extracellular mechanisms in development, regeneration, and disease

Collagen XII, a fibril-associated collagen with interrupted triple helices (FACIT), influences fibrillogenesis in numerous tissues. In addition to this extracellular function, collagen XII also directly regulates cellular function. Collagen XII is widely expressed in connective tissues, particularly tendons, ligaments, and the periodontium and periosteum, where it is enriched in the pericellular regions. Mutations in the collagen XII gene cause myopathic Ehlers-Danlos syndrome (mEDS), an early-onset disease characterized by overlapping connective tissue abnormalities and muscle weakness. Patients with mEDS exhibit delayed motor development, muscle weakness, joint laxity, hypermobility, joint contractures, and abnormal wound healing. A mEDS mouse model was generated by deletion of the Col12a1 gene, resulting in skeletal and muscle abnormalities with disorganized tissue structures and altered mechanical properties. Extracellularly, collagen XII interacts with collagen I fibrils and regulates collagen fibril spacing and assembly during fibrillogenesis. Evidence for the binding of collagen XII to other EDS-related molecules (e.g., decorin and tenascin X) suggests that disruption of ECM molecular interactions is one of the causes of connective tissue pathology in mEDS. Collagen XII also has been shown to influence cell behavior, such as cell shape and cell-cell communication, by providing physical connection between adjacent cells during tissue development and regeneration. The focus of this review is on the functions of collagen XII in development, regeneration, and disease.

Downregulation of COL12A1 and COL13A1 by a selective EP2 receptor agonist, omidenepag, in human trabecular meshwork cells

Omidenepag isopropyl (OMDI) is an intraocular pressure (IOP)-lowering drug used to treat glaucoma. The active form of OMDI, omidenepag (OMD), lowers elevated IOP, the main risk factor for glaucoma, by increasing the aqueous humor outflow; however, a detailed understanding of this mechanism is lacking. To clarify the IOP-lowering mechanism of OMDI, the effects of OMD on the mRNA expression of the extracellular matrix, matrix metalloproteinases (MMPs), and tissue inhibitors of metalloproteinases (TIMPs) were evaluated in human trabecular meshwork cells. Under 2D culture conditions, the mRNA expression of FN1, COL1A1, COL1A2, COL12A1, and COL13A1 decreased in a concentration-dependent manner after 6 or 24 h treatment with 10 nM, 100 nM, and 1 μM OMD, while that of COL18A1 decreased after 6 h treatment with 1 μM OMD. Significant changes in expression were observed for many MMP and TIMP genes. Under 3D culture conditions, the extracellular matrix-related genes COL12A1 and COL13A1 were downregulated by OMD treatment at all three concentrations. Under both 2D and 3D culture conditions, COL12A1 and COL13A1 were downregulated following OMD treatment. Reduction in the extracellular matrix contributes to the decrease in outflow resistance, suggesting that the downregulation of the two related genes may be one of the factors influencing the IOP-lowering effect of OMDI. Our findings provide insights for the use of OMDI in clinical practice.

Protection of primary cilia is an effective countermeasure against the impairment of osteoblast function induced by simulated microgravity

The molecular mechanism for the microgravity-induced decrease in bone formation remains unclear and there is a lack of effective specific preventative therapies. We recently reported that primary cilia of osteoblasts became shorter and even disappeared when the cells were exposed to random positioning machine (RPM)-simulated microgravity and that the microgravity-induced loss of osteogenic potential of osteoblasts could be attenuated when the resorption of primary cilia was prevented by treatment with 0.1 μM cytochalasin D. In the current study, it was further found that the loss of the osteogenic capacity of rat calvarial osteoblasts (ROBs) was associated with the inhibition of the BMP-2/Smad1/5/8 signalling pathway, of which most of the signalling proteins including BMP-2, BMPRII, Smad1/5/8 and p-Smad1/5/8 were found localized to primary cilia. Accompanying the resorption of primary cilia following the cells being exposed to simulated microgravity, the expression levels of these signalling proteins were reduced significantly. Furthermore, the expression of miRNA-129-3p, a microRNA previously reported to control cilium biogenesis, was found to be reduced quickly and changed in a similar tendency with the length of primary cilia. Moreover, overexpression of miRNA-129-3p in ROBs significantly attenuated microgravity-induced inhibition of BMP-2 signalling and loss of osteogenic differentiation and mineralization. These results indicated the important role of miRNA-129-3p in microgravity-induced resorption of primary cilia of osteoblasts and the potential of replenishing the miRNA-129-3p as an effective countermeasure against microgravity-induced loss of primary cilia and impairment of osteoblast function.

Multiple Optical Elastography Techniques Reveal the Regulation of Corneal Stiffness by Collagen XII

Purpose

Collagen XII plays a role in regulating the structure and mechanical properties of the cornea. In this work, several optical elastography techniques were used to investigate the effect of collagen XII deficiency on the stiffness of the murine cornea.

Methods

A three-prong optical elastography approach was used to investigate the mechanical properties of the cornea. Brillouin microscopy, air-coupled ultrasonic optical coherence elastography (OCE) and heartbeat OCE were used to assess the mechanical properties of wild type (WT) and collagen XII-deficient (Col12a1-/-) murine corneas. The Brillouin frequency shift, elastic wave speed, and compressive strain were all measured as a function of intraocular pressure (IOP).

Results

All three optical elastography modalities measured a significantly decreased stiffness in the Col12a1-/- compared to the WT (P < 0.01 for all three modalities). The optical coherence elastography techniques showed that mean stiffness increased as a function of IOP; however, Brillouin microscopy showed no discernable trend in Brillouin frequency shift as a function of IOP.

Conclusions

Our approach suggests that the absence of collagen XII significantly softens the cornea. Although both optical coherence elastography techniques showed an expected increase in corneal stiffness as a function of IOP, Brillouin microscopy did not show such a relationship, suggesting that the Brillouin longitudinal modulus may not be affected by changes in IOP. Future work will focus on multimodal biomechanical models, evaluating the effects of other collagen types on corneal stiffness, and in vivo measurements.

Mechanical Pressure Driving Proteoglycan Expression in Mammographic Density: a Self-perpetuating Cycle?

Regions of high mammographic density (MD) in the breast are characterised by a proteoglycan (PG)-rich fibrous stroma, where PGs mediate aligned collagen fibrils to control tissue stiffness and hence the response to mechanical forces. Literature is accumulating to support the notion that mechanical stiffness may drive PG synthesis in the breast contributing to MD. We review emerging patterns in MD and other biological settings, of a positive feedback cycle of force promoting PG synthesis, such as in articular cartilage, due to increased pressure on weight bearing joints. Furthermore, we present evidence to suggest a pro-tumorigenic effect of increased mechanical force on epithelial cells in contexts where PG-mediated, aligned collagen fibrous tissue abounds, with implications for breast cancer development attributable to high MD. Finally, we summarise means through which this positive feedback mechanism of PG synthesis may be intercepted to reduce mechanical force within tissues and thus reduce disease burden.

Collagen XII mediated cellular and extracellular mechanisms regulate establishment of tendon structure and function

Tendons have a uniaxially aligned structure with a hierarchical organization of collagen fibrils crucial for tendon function. Collagen XII is expressed in tendons and has been implicated in the regulation of fibrillogenesis. It is a non-fibrillar collagen belonging to the Fibril-Associated Collagens with Interrupted Triple Helices (FACIT) family. Mutations in COL12A1 cause myopathic Ehlers Danlos Syndrome with a clinical phenotype involving both joints and tendons supporting critical role(s) for collagen XII in tendon development and function. Here we demonstrate the molecular function of collagen XII during tendon development using a Col12a1 null mouse model. Col12a1 deficiency altered tenocyte shape, formation of interacting cell processes, and organization resulting in impaired cell-cell communication and disruption of hierarchal structure as well as decreased tissue stiffness. Immuno-localization revealed that collagen XII accumulated on the tenocyte surface and connected adjacent tenocytes by building matrix bridges between the cells, suggesting that collagen XII regulates intercellular communication. In addition, there was a decrease in fibrillar collagen I in collagen XII deficient tenocyte cultures compared with controls suggesting collagen XII signaling specifically alters tenocyte biosynthesis. This suggests that collagen XII provides feedback to tenocytes regulating extracellular collagen I. Together, the data indicate dual roles for collagen XII in determination of tendon structure and function. Through association with fibrils it functions in fibril packing, fiber assembly and stability. In addition, collagen XII influences tenocyte organization required for assembly of higher order structure; intercellular communication necessary to coordinate long range order and feedback on tenocytes influencing collagen synthesis. Integration of both regulatory roles is required for the acquisition of hierarchal structure and mechanical properties.

Collagen XII Is a Regulator of Corneal Stroma Structure and Function

Purpose

The aim of this study was to determine the roles of collagen XII in the regulation of stromal hierarchical organization, keratocyte organization, and corneal mechanics.

Methods

The temporal and spatial expression of collagen XII at postnatal days 4, 10, 30, 90, and 150 were evaluated in wild-type (WT) mice. The role of collagen XII in hierarchical organization was analyzed by measuring fibril diameter and density, as well as stromal lamellar structure, within ultrastructural micrographs obtained from WT and collagen XII-deficient mice (Col12a1^–/–). Keratocyte morphology and networks were assessed using actin staining with phalloidin and in vivo confocal microscopy. The effects of collagen XII on corneal biomechanics were evaluated with atomic force microscopy.

Results

Collagen XII was localized homogeneously in the stroma from postnatal day 4 to day 150, and protein accumulation was shown to increase during this period using semiquantitative immunoblots. Higher fibril density (P < 0.001) and disruption of lamellar organization were found in the collagen XII null mice stroma when compared to WT mice. Keratocyte networks and organization were altered in the absence of collagen XII, as demonstrated using fluorescent microscopy after phalloidin staining and in vivo confocal microscopy. Corneal stiffness was increased in the absence of collagen XII. Young's modulus was 16.2 ± 5.6 kPa in WT and 32.8 ± 6.4 kPa in Col12a1^–/– corneas. The difference between these two groups was significant (P < 0.001, t-test).

Conclusions

Collagen XII plays a major role in establishing and maintaining stromal structure and function. In the absence of collagen XII, the corneal stroma showed significant abnormalities, including decreased interfibrillar space, disrupted lamellar organization, abnormal keratocyte organization, and increased corneal stiffness.

Composition, structure and function of the corneal stroma

No other tissue in the body depends more on the composition and organization of the extracellular matrix (ECM) for normal structure and function than the corneal stroma. The precise arrangement and orientation of collagen fibrils, lamellae and keratocytes that occurs during development and is needed in adults to maintain stromal function is dependent on the regulated interaction of multiple ECM components that contribute to attain the unique properties of the cornea: transparency, shape, mechanical strength, and avascularity. This review summarizes the contribution of different ECM components, their structure, regulation and function in modulating the properties of the corneal stroma. Fibril forming collagens (I, III, V), fibril associated collagens with interrupted triple helices (XII and XIV), network forming collagens (IV, VI and VIII) as well as small leucine-rich proteoglycans (SLRP) expressed in the stroma: decorin, biglycan, lumican, keratocan, and fibromodulin are some of the ECM components reviewed in this manuscript. There are spatial and temporal differences in the expression of these ECM components, as well as interactions among them that contribute to stromal function. Unique regions within the stroma like Bowman's layer and Descemet's layer are discussed. To define the complexity of corneal stroma composition and structure as well as the relationship to function is a daunting task. Our knowledge is expanding, and we expect that this review provides a comprehensive overview of current knowledge, definition of gaps and suggests future research directions.

Mimicking the Organic and Inorganic Composition of Anabolic Bone Enhances Human Mesenchymal Stem Cell Osteoinduction and Scaffold Mechanical Properties

Engineered bone graft designs have been largely inspired by adult bone despite functionally significant differences from the composition of anabolic bone in both the mineralized and non-mineralized fractions. Specifically, anabolic bone contains hydroxyapatite with ionic substitutions that facilitate bone turnover and relatively rare collagens type VI and XII that are important for normal bone development. In this work, human mesenchymal stem cells (hMSCs) were cultured in lyophilized collagen type I scaffolds mineralized with hydroxyapatite containing Mg²⁺ substitutions, then induced to deposit an extracellular matrix (ECM) containing collagens VI and XII by exposure to GW9662, a PPARγ inhibitor. Delivery of GW9662 was accomplished through either Supplemented Media or via composite microspheres embedded in the scaffolds for localized delivery. Furthermore, hMSCs and scaffolds were cultured in both static and perfuse conditions to investigate the interaction between GW9662 treatment and perfusion and their effects on ECM deposition trends. Perfusion culture enhanced cell infiltration into the scaffold, deposition of collagen VI and XII, as well as osteogenic differentiation, as determined by gene expression of osteopontin, BMP2, and ALP. Furthermore, scaffold mineral density and compressive modulus were increased in response to both GW9662 treatment and perfusion after 3 weeks of culture. Local delivery of GW9662 with drug-eluting microspheres had comparable effects to systemic delivery in the perfusate. Together, these results demonstrate a strategy to create a scaffold mimicking both organic and inorganic characteristics of anabolic bone and its potential as a bone graft.

The minor collagens in articular cartilage

Articular cartilage is a connective tissue consisting of a specialized extracellular matrix (ECM) that dominates the bulk of its wet and dry weight. Type II collagen and aggrecan are the main ECM proteins in cartilage. However, little attention has been paid to less abundant molecular components, especially minor collagens, including type IV, VI, IX, X, XI, XII, XIII, and XIV, etc. Although accounting for only a small fraction of the mature matrix, these minor collagens not only play essential structural roles in the mechanical properties, organization, and shape of articular cartilage, but also fulfil specific biological functions. Genetic studies of these minor collagens have revealed that they are associated with multiple connective tissue diseases, especially degenerative joint disease. The progressive destruction of cartilage involves the degradation of matrix constituents including these minor collagens. The generation and release of fragmented molecules could generate novel biochemical markers with the capacity to monitor disease progression, facilitate drug development and add to the existing toolbox for in vitro studies, preclinical research and clinical trials.

Mechano-regulation of collagen biosynthesis in periodontal ligament

Periodontal ligament (PDL) plays critical roles in the development and maintenance of periodontium such as tooth eruption and dissipation of masticatory force. The mechanical properties of PDL are mainly derived from fibrillar type I collagen, the most abundant extracellular component. The biosynthesis of type I collagen is a long, complex process including a number of intra- and extracellular post-translational modifications. The final modification step is the formation of covalent intra- and intermolecular cross-links that provide collagen fibrils with stability and connectivity. It is now clear that collagen post-translational modifications are regulated by groups of specific enzymes and associated molecules in a tissue-specific manner; and these modifications appear to change in response to mechanical force. This review focuses on the effect of mechanical loading on collagen biosynthesis and fibrillogenesis in PDL with emphasis on the post-translational modifications of collagens, which is an important molecular aspect to understand in the field of prosthetic dentistry.

Collagen XII: Protecting bone and muscle integrity by organizing collagen fibrils

Collagen XII, largest member of the fibril-associated collagens with interrupted triple helix (FACIT) family, assembles from three identical α-chains encoded by the COL12A1 gene. The molecule consists of three threadlike N-terminal noncollagenous NC3 domains, joined by disulfide bonds and a short interrupted collagen triple helix toward the C-terminus. Splice variants differ considerably in size and properties: "small" collagen XIIB (220 kDa subunit) is similar to collagen XIV, whereas collagen XIIA (350 kDa) has a much larger NC3 domain carrying glycosaminoglycan chains. Collagen XII binds to collagen I-containing fibrils via its collagenous domain, whereas its large noncollagenous arms interact with other matrix proteins such as tenascin-X. In dense connective tissues and bone, collagen XII is thought to regulate organization and mechanical properties of collagen fibril bundles. Accordingly, recent findings show that collagen XII mutations cause Ehlers-Danlos/myopathy overlap syndrome associated with skeletal abnormalities and muscle weakness in mice and humans.

Mutations in the collagen XII gene define a new form of extracellular matrix-related myopathy

Bethlem myopathy (BM) [MIM 158810] is a slowly progressive muscle disease characterized by contractures and proximal weakness, which can be caused by mutations in one of the collagen VI genes (COL6A1, COL6A2 and COL6A3). However, there may be additional causal genes to identify as in ∼50% of BM cases no mutations in the COL6 genes are identified. In a cohort of -24 patients with a BM-like phenotype, we first sequenced 12 candidate genes based on their function, including genes for known binding partners of collagen VI, and those enzymes involved in its correct post-translational modification, assembly and secretion. Proceeding to whole-exome sequencing (WES), we identified mutations in the COL12A1 gene, a member of the FACIT collagens (fibril-associated collagens with interrupted triple helices) in five individuals from two families. Both families showed dominant inheritance with a clinical phenotype resembling classical BM. Family 1 had a single-base substitution that led to the replacement of one glycine residue in the triple-helical domain, breaking the Gly-X-Y repeating pattern, and Family 2 had a missense mutation, which created a mutant protein with an unpaired cysteine residue. Abnormality at the protein level was confirmed in both families by the intracellular retention of collagen XII in patient dermal fibroblasts. The mutation in Family 2 leads to the up-regulation of genes associated with the unfolded protein response (UPR) pathway and swollen, dysmorphic rough-ER. We conclude that the spectrum of causative genes in extracellular matrix (ECM)-related myopathies be extended to include COL12A1.

Recessive and dominant mutations in COL12A1 cause a novel EDS/myopathy overlap syndrome in humans and mice

Collagen VI-related myopathies are disorders of connective tissue presenting with an overlap phenotype combining clinical involvement from the muscle and from the connective tissue. Not all patients displaying related overlap phenotypes between muscle and connective tissue have mutations in collagen VI. Here, we report a homozygous recessive loss of function mutation and a de novo dominant mutation in collagen XII (COL12A1) as underlying a novel overlap syndrome involving muscle and connective tissue. Two siblings homozygous for a loss of function mutation showed widespread joint hyperlaxity combined with weakness precluding independent ambulation, while the patient with the de novo missense mutation was more mildly affected, showing improvement including the acquisition of walking. A mouse model with inactivation of the Col12a1 gene showed decreased grip strength, a delay in fiber-type transition and a deficiency in passive force generation while the muscle seems more resistant to eccentric contraction induced force drop, indicating a role for a matrix-based passive force-transducing elastic element in the generation of the weakness. This new muscle connective tissue overlap syndrome expands on the emerging importance of the muscle extracellular matrix in the pathogenesis of muscle disease.

Abnormal corneal endothelial maturation in collagen XII and XIV null mice

PURPOSE

Maturation of the endothelium and the adjacent matrix was characterized in wild-type (WT) mice. The influence of FACIT collagen XII and XIV deficiency on the morphology, maturation, and function of the corneal endothelium was examined.

METHODS

Analysis of the endothelium and Descemet's membrane (DM) was performed using transmission electron microscopy at postnatal day (P)4, P14, and P30 in WT, Col12a1(-/-), Col14a1(-/-), and Col12a1(-/-)/Col14a1(-/-) mice. Endothelial junctions were analyzed using ZO-1. The presence of endothelial-stromal communications was evaluated with phalloidin staining as well as electron microscopy. Finally, corneal thickness was assessed.

RESULTS

A thin DM, clefts between endothelial cells and DM, and large "vacuole-like" structures were present in the endothelial cells of WT mice at P4 but not noted at P30. The endothelia of Col12a1(-/-), Col14a1(-/-), and compound Col12a1(-/-)/Col14a1(-/-) in the P30 cornea maintained the vacuole-like structures seen at P4. A mature endothelial junction pattern was delayed in the null corneas. Expression of ZO-1 in WT endothelia at P14 was diffuse and localized to the basolateral and apical cell membrane. At P30, staining was localized to intercellular junctions. ZO-1 reactivity was patchy in Col12a1(-/-), Col14a1(-/-), and compound Col12a1(-/-)/Col14a1(-/-) corneas at P14 and P30. Stromal thickness was increased in P30 null corneas. Endothelial cell processes were demonstrated penetrating the DM and into the underlying stroma, throughout the entire endothelial layer in the P4 cornea.

CONCLUSIONS

Collagen XII and XIV null mice demonstrate delayed endothelial maturation. The structural alterations suggest functional changes in endothelial function resulting in increased corneal thickness. Endothelial-stromal interactions suggest a pathway for signal transduction.

Cell differentiation and osseointegration influenced by nanoscale anodized titanium surfaces

Aims: We aimed to study the interactions between human mesenchymal stem cells and the bone integration of nanostructured titanium implants. Materials & methods: Nanopores of 20, 30 and 50 nm were prepared by anodization of titanium at 5, 10 and 20 V in a mixture of fluorhydric and acetic acid. Ti 30 and 50 nanostructures promoted early osteoblastic gene differentiation of the human mesenchymal stem cells without osteogenic supplements. The osseointegration of nanostructured and control titanium implants was compared by implantation in rat tibias for 1 and 3 weeks. Results: The nanostructures significantly accelerated bone apposition and bone bonding strength in vivo in correlation with in vitro results. Conclusion: These findings demonstrate that specific nanostructures controlled the differentiation of cells and, thus, the integration of implants in tissues. These nanoporous titanium surfaces may be of considerable interest for dental and orthopedic implants.

Original submitted 10 September 2011; Revised submitted 11 November 2011; Published online 6 March 2012

Type XII collagen regulates osteoblast polarity and communication during bone formation

Type XII collagen–null mice have fragile bones with disorganized collagen fiber arrangement, decreased bone matrix formation, and delayed osteoblast differentiation.

Differentiated osteoblasts are polarized in regions of bone deposition, demonstrate extensive cell interaction and communication, and are responsible for bone formation and quality. Type XII collagen is a fibril-associated collagen with interrupted triple helices and has been implicated in the osteoblast response to mechanical forces. Type XII collagen is expressed by osteoblasts and localizes to areas of bone formation. A transgenic mouse null for type XII collagen exhibits skeletal abnormalities including shorter, more slender long bones with decreased mechanical strength as well as altered vertebrae structure compared with wild-type mice. Col12a^−/− osteoblasts have decreased bone matrix deposition with delayed maturation indicated by decreased bone matrix protein expression. Compared with controls, Col12a^−/− osteoblasts are disorganized and less polarized with disrupted cell–cell interactions, decreased connexin43 expression, and impaired gap junction function. The data demonstrate important regulatory roles for type XII collagen in osteoblast differentiation and bone matrix formation.