Cyclophilin B Deficiency Causes Abnormal Dentin Collagen Matrix

Generation of bone-specific lysyl hydroxylase 2 knockout mice and their phenotypes

Lysyl hydroxylase 2 (LH2) catalyzes the hydroxylation of lysine residues in the telopeptides of type I collagen. This modification is critical for the formation of stable hydroxylysine-aldehyde derived collagen cross-links, thus, for the stability of collagen fibrils. Though dysfunction of LH2 causes Bruck syndrome, recessive osteogenesis imperfecta with joint contracture, the molecular mechanisms by which LH2 affects bone formation are still not well understood. Since the Plod2 knockout mice are embryonically lethal, we generated bone-specific LH2 conditional knockout mice (bsLH2-cKO) using the osteocalcin-Cre/loxP system, and evaluated phenotypes of femurs. LH2 mRNA and protein levels assessed by qPCR, immunohistochemistry and Data Independent Acquisition proteomics were all markedly low in bsLH2-cKO femurs when compared to controls. Lysine hydroxylation of both carboxy- and amino-terminal telopeptides of an α1(I) chain were significantly diminished resulting in reduction of the hydroxylysine-aldehyde derived cross-links. The collagen fibrils in bsLH2-cKO appeared to be thicker, often fused and irregular when compared to controls. In addition, bone mineral density and mechanical properties of bsLH2-cKO femurs were significantly impaired. Taken together, these data demonstrate that LH2-catalyzed modification and consequent cross-linking of collagen are critical for proper bone formation and mechanical strength.

The Versatility of Collagen in Pharmacology: Targeting Collagen, Targeting with Collagen

Collagen, a versatile family of proteins with 28 members and 44 genes, is pivotal in maintaining tissue integrity and function. It plays a crucial role in physiological processes like wound healing, hemostasis, and pathological conditions such as fibrosis and cancer. Collagen is a target in these processes. Direct methods for collagen modulation include enzymatic breakdown and molecular binding approaches. For instance, Clostridium histolyticum collagenase is effective in treating localized fibrosis. Polypeptides like collagen-binding domains offer promising avenues for tumor-specific immunotherapy and drug delivery. Indirect targeting of collagen involves regulating cellular processes essential for its synthesis and maturation, such as translation regulation and microRNA activity. Enzymes involved in collagen modification, such as prolyl-hydroxylases or lysyl-oxidases, are also indirect therapeutic targets. From another perspective, collagen is also a natural source of drugs. Enzymatic degradation of collagen generates bioactive fragments known as matrikines and matricryptins, which exhibit diverse pharmacological activities. Overall, collagen-derived peptides present significant therapeutic potential beyond tissue repair, offering various strategies for treating fibrosis, cancer, and genetic disorders. Continued research into specific collagen targeting and the application of collagen and its derivatives may lead to the development of novel treatments for a range of pathological conditions.

Role of the mitochondrial protein cyclophilin D in skin wound healing and collagen secretion

Central for wound healing is the formation of granulation tissue, which largely consists of collagen and whose importance stretches past wound healing, including being implicated in both fibrosis and skin aging. Cyclophilin D (CyD) is a mitochondrial protein that regulates the permeability transition pore, known for its role in apoptosis and ischemia-reperfusion. To date, the role of CyD in human wound healing and collagen generation has been largely unexplored. Here, we show that CyD was upregulated in normal wounds and venous ulcers, likely adaptive as CyD inhibition impaired reepithelialization, granulation tissue formation, and wound closure in both human and pig models. Overexpression of CyD increased keratinocyte migration and fibroblast proliferation, while its inhibition reduced migration. Independent of wound healing, CyD inhibition in fibroblasts reduced collagen secretion and caused endoplasmic reticulum collagen accumulation, while its overexpression increased collagen secretion. This was confirmed in a Ppif-KO mouse model, which showed a reduction in skin collagen. Overall, this study revealed previously unreported roles of CyD in skin, with implications for wound healing and beyond.

Calreticulin: Endoplasmic reticulum Ca2+ gatekeeper

Endoplasmic reticulum (ER) luminal Ca2+ is vital for the function of the ER and regulates many cellular processes. Calreticulin is a highly conserved, ER-resident Ca2+ binding protein and lectin-like chaperone. Over four decades of studying calreticulin demonstrate that this protein plays a crucial role in maintaining Ca2+ supply under different physiological conditions, in managing access to Ca2+ and how Ca2+ is used depending on the environmental events and in making sure that Ca2+ is not misused. Calreticulin plays a role of ER luminal Ca2+ sensor to manage Ca2+-dependent ER luminal events including maintaining interaction with its partners, Ca2+ handling molecules, substrates and stress sensors. The protein is strategically positioned in the lumen of the ER from where the protein manages access to and distribution of Ca2+ for many cellular Ca2+-signalling events. The importance of calreticulin Ca2+ pool extends beyond the ER and includes influence of cellular processes involved in many aspects of cellular pathophysiology. Abnormal handling of the ER Ca2+ contributes to many pathologies from heart failure to neurodegeneration and metabolic diseases.

Cyclophilins modify their profile depending on the organ or tissue in a murine inflammatory model

Inflammation is the leading subjacent cause of many chronic diseases. Despite several studies in the last decades, the molecular mechanism involving its pathophysiology is not fully known. Recently, the implication of cyclophilins in inflammatory-based diseases has been demonstrated. However, the main role of cyclophilins in these processes remains elusive. Hence, a mouse model of systemic inflammation was used to better understand the relationship between cyclophilins and their tissue distribution. To induce inflammation, mice were fed with high-fat diet for 10 weeks. In these conditions, serum levels of interleukins 2 and 6, tumour necrosis factor-α, interferon-ϒ, and the monocyte chemoattractant protein 1 were elevated, evidencing a systemic inflammatory state. Then, in this inflammatory model, cyclophilins and CD147 profiles in the aorta, liver, and kidney were studied. The results demonstrate that, upon inflammatory conditions, cyclophilins A and C expression levels were increased in the aorta. Cyclophilins A and D were augmented in the liver, meanwhile, cyclophilins B and C were diminished. In the kidney, cyclophilins B and C levels were elevated. Furthermore, CD147 receptor was also increased in the aorta, liver, and kidney. In addition, when cyclophilin A was modulated, serum levels of inflammatory mediators were decreased, indicating a reduction in systemic inflammation. Besides, the expression levels of cyclophilin A and CD147 were also reduced in the aorta and liver, when cyclophilin A was modulated. Therefore, these results suggest that each cyclophilin has a different profile depending on the tissue, under inflammatory conditions.

Lysyl hydroxylase 2 mediated collagen post-translational modifications and functional outcomes

Lysyl hydroxylase 2 (LH2) is a member of LH family that catalyzes the hydroxylation of lysine (Lys) residues on collagen, and this particular isozyme has been implicated in various diseases. While its function as a telopeptidyl LH is generally accepted, several fundamental questions remain unanswered: 1. Does LH2 catalyze the hydroxylation of all telopeptidyl Lys residues of collagen? 2. Is LH2 involved in the helical Lys hydroxylation? 3. What are the functional consequences when LH2 is completely absent? To answer these questions, we generated LH2-null MC3T3 cells (LH2KO), and extensively characterized the type I collagen phenotypes in comparison with controls. Cross-link analysis demonstrated that the hydroxylysine-aldehyde (Hyl^ald)-derived cross-links were completely absent from LH2KO collagen with concomitant increases in the Lys^ald-derived cross-links. Mass spectrometric analysis revealed that, in LH2KO type I collagen, telopeptidyl Lys hydroxylation was completely abolished at all sites while helical Lys hydroxylation was slightly diminished in a site-specific manner. Moreover, di-glycosylated Hyl was diminished at the expense of mono-glycosylated Hyl. LH2KO collagen was highly soluble and digestible, fibril diameters were diminished, and mineralization impaired when compared to controls. Together, these data underscore the critical role of LH2-catalyzed collagen modifications in collagen stability, organization and mineralization in MC3T3 cells.

Decrease of lysyl hydroxylase 2 activity causes abnormal collagen molecular phenotypes, defective mineralization and compromised mechanical properties of bone

Lysyl hydroxylase 2 (LH2) is an enzyme that catalyzes the hydroxylation of lysine (Lys) residues in fibrillar collagen telopeptides, a critical post-translational modification for the stability of intermolecular cross-links. Though abnormal LH2 activities have been implicated in various diseases including Bruck syndrome, the molecular basis of the pathologies is still not well understood. Since LH2 null mice die at early embryonic stage, we generated LH2 heterozygous (LH2^+/-) mice in which LH2 level is significantly diminished, and characterized collagen and bone phenotypes using femurs. Compared to the wild-type (WT), LH2^+/- collagen showed a significant decrease in the ratio of hydroxylysine (Hyl)- to the Lys-aldehyde-derived collagen cross-links without affecting the total number of aldehydes involved in cross-links. Mass spectrometric analysis revealed that, in LH2^+/- type I collagen, the extent of hydroxylation of all telopeptidyl Lys residues was significantly decreased. In the helical domain, Lys hydroxylation at the cross-linking sites was either unaffected or slightly lower, but other sites were significantly diminished compared to WT. In LH2^+/- femurs, mineral densities of cortical and cancellous bones were significantly decreased and the mechanical properties of cortical bones evaluated by nanoindentation analysis were compromised. When cultured, LH2^+/- osteoblasts poorly produced mineralized nodules compared to WT osteoblasts. These data provide insight into the functionality of LH2 in collagen molecular phenotype and its critical role in bone matrix mineralization and mechanical properties.

Collagen molecular phenotypic switch between non-neoplastic and neoplastic canine mammary tissues

In spite of major advances over the past several decades in diagnosis and treatment, breast cancer remains a global cause of morbidity and premature death for both human and veterinary patients. Due to multiple shared clinicopathological features, dogs provide an excellent model of human breast cancer, thus, a comparative oncology approach may advance our understanding of breast cancer biology and improve patient outcomes. Despite an increasing awareness of the critical role of fibrillar collagens in breast cancer biology, tumor-permissive collagen features are still ill-defined. Here, we characterize the molecular and morphological phenotypes of type I collagen in canine mammary gland tumors. Canine mammary carcinoma samples contained longer collagen fibers as well as a greater population of wider fibers compared to non-neoplastic and adenoma samples. Furthermore, the total number of collagen cross-links enriched in the stable hydroxylysine-aldehyde derived cross-links was significantly increased in neoplastic mammary gland samples compared to non-neoplastic mammary gland tissue. The mass spectrometric analyses of type I collagen revealed that in malignant mammary tumor samples, lysine residues, in particular those in the telopeptides, were markedly over-hydroxylated in comparison to non-neoplastic mammary tissue. The extent of glycosylation of hydroxylysine residues was comparable among the groups. Consistent with these data, expression levels of genes encoding lysyl hydroxylase 2 (LH2) and its molecular chaperone FK506-binding protein 65 were both significantly increased in neoplastic samples. These alterations likely lead to an increase in the LH2-mediated stable collagen cross-links in mammary carcinoma that may promote tumor cell metastasis in these patients.

The Role of Endoplasmic Reticulum Chaperones in Protein Folding and Quality Control

Molecular chaperones assist the folding of nascent chains in the cell. Chaperones also aid in quality control decisions as persistent chaperone binding can help to sort terminal misfolded proteins for degradation. There are two major molecular chaperone families in the endoplasmic reticulum (ER) that assist proteins in reaching their native structure and evaluating the fidelity of the maturation process. The ER Hsp70 chaperone, BiP, supports adenine nucleotide-regulated binding to non-native proteins that possess exposed hydrophobic regions. In contrast, the carbohydrate-dependent chaperone system involving the membrane protein calnexin and its soluble paralogue calreticulin recognize a specific glycoform of an exposed hydrophilic protein modification for which the composition is controlled by a series of glycosidases and transferases. Here, we compare and contrast the properties, mechanisms of action and functions of these different chaperones systems that work in parallel, as well as together, to assist a large variety of substrates that traverse the eukaryotic secretory pathway.

IFT20 is critical for collagen biosynthesis in craniofacial bone formation

Intraflagellar transport (IFT) is essential for assembling primary cilia required for bone formation. Disruption of IFT frequently leads to bone defects in humans. While it has been well studied about the function of IFT in osteogenic cell proliferation and differentiation, little is known about its role in collagen biosynthesis during bone formation. Here we show that IFT20, the smallest IFT protein in the IFT-B complex, is important for collagen biosynthesis in mice. Deletion of Ift20 in craniofacial osteoblasts displayed bone defects in the face. While collagen protein levels are unaffected by loss of Ift20, collagen cross-linking was significantly altered. In both Ift20:Wnt1-Cre and Ift20:Ocn-Cre mice the bones exhibit increased hydroxylysine-aldehyde deived cross-linking, and decreased lysine-aldehyde derived cross-linking. To obtain insight into the molecular mechanisms, we examined the expression levels of telopeptidyl lysyl hydroxylase 2 (LH2), and associated chaperone complexes. The results demonstrated that, while LH2 levels were unaffected by loss of Ift20, its chaperone, FKBP65, was significantly increased in Ift20:Wnt1-Cre and Ift20:Ocn-Cre mouse calvaria as well as femurs. These results suggest that IFT20 plays a pivotal role in collagen biosynthesis by regulating, in part, telopeptidyl lysine hydroxylation and cross-linking in bone. To the best of our knowledge, this is the first to demonstrate that the IFT components control collagen post-translational modifications. This provides a novel insight into the craniofacial bone defects associated with craniofacial skeletal ciliopathies.

Dental and craniofacial defects in the Crtap-/- mouse model of osteogenesis imperfecta type VII

BACKGROUND

Inactivating mutations in the gene for cartilage-associated protein (CRTAP) cause osteogenesis imperfecta type VII in humans, with a phenotype that can include craniofacial defects. Dental and craniofacial manifestations have not been a focus of case reports to date. We analyzed the craniofacial and dental phenotype of Crtap^-/- mice by skull measurements, micro-computed tomography (micro-CT), histology, and immunohistochemistry.

RESULTS

Crtap^-/- mice exhibited a brachycephalic skull shape with fusion of the nasofrontal suture and facial bones, resulting in mid-face retrusion and a class III dental malocclusion. Loss of CRTAP also resulted in decreased dentin volume and decreased cellular cementum volume, though acellular cementum thickness was increased. Periodontal dysfunction was revealed by decreased alveolar bone volume and mineral density, increased periodontal ligament (PDL) space, ectopic calcification within the PDL, bone-tooth ankylosis, altered immunostaining of extracellular matrix proteins in bone and PDL, increased pSMAD5, and more numerous osteoclasts on alveolar bone surfaces.

CONCLUSIONS

Crtap^-/- mice serve as a useful model of the dental and craniofacial abnormalities seen in individuals with osteogenesis imperfecta type VII.

Type III collagen is a key regulator of the collagen fibrillar structure and biomechanics of articular cartilage and meniscus

Despite the fact that type III collagen is the second most abundant collagen type in the body, its contribution to the physiologic maintenance and repair of skeletal tissues remains poorly understood. This study queried the role of type III collagen in the structure and biomechanical functions of two structurally distinctive tissues in the knee joint, type II collagen-rich articular cartilage and type I collagen-dominated meniscus. Integrating outcomes from atomic force microscopy-based nanomechanical tests, collagen fibril nanostructural analysis, collagen cross-link analysis and histology, we elucidated the impact of type III collagen haplodeficiency on the morphology, nanostructure and biomechanical properties of articular cartilage and meniscus in Col3a1^+/- mice. Reduction of type III collagen leads to increased heterogeneity and mean thickness of collagen fibril diameter, as well as reduced modulus in both tissues, and these effects became more pronounced with skeletal maturation. These data suggest a crucial role of type III collagen in mediating fibril assembly and biomechanical functions of both articular cartilage and meniscus during post-natal growth. In articular cartilage, type III collagen has a marked contribution to the micromechanics of the pericellular matrix, indicating a potential role in mediating the early stage of type II collagen fibrillogenesis and chondrocyte mechanotransduction. In both tissues, reduction of type III collagen leads to decrease in tissue modulus despite the increase in collagen cross-linking. This suggests that the disruption of matrix structure due to type III collagen deficiency outweighs the stiffening of collagen fibrils by increased cross-linking, leading to a net negative impact on tissue modulus. Collectively, this study is the first to highlight the crucial structural role of type III collagen in both articular cartilage and meniscus extracellular matrices. We expect these results to expand our understanding of type III collagen across various tissue types, and to uncover critical molecular components of the microniche for regenerative strategies targeting articular cartilage and meniscus repair.

Cyclophilin B control of lysine post-translational modifications of skin type I collagen

Covalent intermolecular cross-linking of collagen is essential for tissue stability. Recent studies have demonstrated that cyclophilin B (CypB), an endoplasmic reticulum (ER)-resident peptidyl-prolyl cis-trans isomerase, modulates lysine (Lys) hydroxylation of type I collagen impacting cross-linking chemistry. However, the extent of modulation, the molecular mechanism and the functional outcome in tissues are not well understood. Here, we report that, in CypB null (KO) mouse skin, two unusual collagen cross-links lacking Lys hydroxylation are formed while neither was detected in wild type (WT) or heterozygous (Het) mice. Mass spectrometric analysis of type I collagen showed that none of the telopeptidyl Lys was hydroxylated in KO or WT/Het mice. Hydroxylation of the helical cross-linking Lys residues was almost complete in WT/Het but was markedly diminished in KO. Lys hydroxylation at other sites was also lower in KO but to a lesser extent. A key glycosylation site, α1(I) Lys-87, was underglycosylated while other sites were mostly overglycosylated in KO. Despite these findings, lysyl hydroxylases and glycosyltransferase 25 domain 1 levels were significantly higher in KO than WT/Het. However, the components of ER chaperone complex that positively or negatively regulates lysyl hydroxylase activities were severely reduced or slightly increased, respectively, in KO. The atomic force microscopy-based nanoindentation modulus were significantly lower in KO skin than WT. These data demonstrate that CypB deficiency profoundly affects Lys post-translational modifications of collagen likely by modulating LH chaperone complexes. Together, our study underscores the critical role of CypB in Lys modifications of collagen, cross-linking and mechanical properties of skin.

Deficiency of cyclophilin B (CypB), an endoplasmic reticulum-resident peptidyl-prolyl cis-trans isomerase, causes recessive osteogenesis imperfecta type IX, resulting in defective connective tissues. Recent studies using CypB null mice revealed that CypB modulates lysine hydroxylation of type I collagen impacting collagen cross-linking. However, the extent of modulation, the molecular mechanism and the effect on tissue properties are not well understood. In the present study, we show that CypB deficiency in mouse skin results in the formation of unusual collagen cross-links, aberrant tissue formation, altered levels of lysine modifying enzymes and their chaperones, and impaired mechanical property. These findings highlight an essential role of CypB in collagen post-translational modifications which are critical in maintaining the structure and function of connective tissues.

Aberrant Collagen Cross-linking in Human Oral Squamous Cell Carcinoma

Tumor progression is a complex process involving extracellular matrix (ECM) remodeling and stiffening. However, the mechanisms that govern these processes and their roles in tumor progression are still poorly understood. In this study, we performed bioinformatics, immunohistochemical, and biochemical analyses to examine if collagen cross-linking is associated with tumor stage and regional lymph node metastasis (RLNM) in oral squamous cell carcinoma (OSCC). We found that the genes encoding key enzymes for cross-linking are frequently overexpressed in oral, head, and neck cancers. Specifically, the enzymes lysyl hydroxylase 2 (LH2) or lysyl oxidase (LOX) and LOX-like 2 (LOXL2) were significantly upregulated in late-stage tumors and associated with poor patient prognosis. The protein levels of these enzymes in the primary human OSCC were also significantly increased in late-stage tumors and markedly elevated in the RLNM-positive tumors. Notably, while overall LOX/LOXL2-catalyzed collagen cross-links were enriched in late-stage and RLNM-positive tumors, LH2-mediated stable cross-links were significantly increased. To our knowledge, this is the first study to investigate the association of collagen cross-linking and expression of key enzymes regulating this process with OSCC stage. The data indicate a critical role for collagen cross-linking in OSCC tumor progression and metastasis, which may provide insights into development of novel therapeutic strategies to prevent OSCC progression.