Targeted induction of endoplasmic reticulum stress induces cartilage pathology

Endoplasmic reticulum stress causes long bone shortening in P4hbC402R/+ mice: A mouse model exhibiting significant features of cole-carpenter syndrome driven by P4HB mutations

Cole-Carpenter syndrome (CCS) is a rare autosomal-dominant genetic disease characterized by craniosynostosis, ocular proptosis, hydrocephalus, distinctive facial features, and bone fragility. Previous cases of CCS are associated with genetic variations in P4HB, which encodes the protein disulfide isomerase (PDI), a key enzyme in protein folding. Patients with CCS caused by P4HB mutations often present with short stature, limb deformities, and abnormal epiphyseal plates. However, the underlying mechanisms are largely unknown. To investigate this, a mouse model expressing the P4hbC402R mutation (corresponding to P4HBC400R in humans) was generated. Although the mouse model did not exhibit craniofacial bone defects or brittle bone phenotypes, it did show significantly shortened long bones-a prominent characteristic of P4HB-induced CCS. This was due to impaired proliferation and delayed hypertrophy of growth plate chondrocytes. Mutant PDI was found to accumulate abnormally in the endoplasmic reticulum (ER), and in vitro experiments revealed defects in both the catalytic and chaperone activities of mutant PDI. In addition, we observed enhanced ER stress and activation of the PKR-like ER kinase (PERK) pathway in P4hbC402R/+ chondrocytes. Inhibition of ER stress mitigated PERK activation, alleviated defective chondrocyte proliferation and differentiation, thereby rescuing bone length. Taken together, enhanced ER stress and the activation of the PERK, potentially initiated by the malfunctioning of PDIC402R or its abnormal accumulation within the ER, or both, lead to compromised chondrocyte proliferation and differentiation in mice, and ultimately stunts mice growth. This provides new insights into the pathogenesis of P4HB-dominated CCS and offers potential therapeutic targets.

A Collagen10a1 mutation disrupts cell polarity in a medaka model for metaphyseal chondrodysplasia type Schmid

Heterozygous mutations in COL10A1 lead to metaphyseal chondrodysplasia type Schmid (MCDS), a skeletal disorder characterized by epiphyseal abnormalities. Prior analysis revealed impaired trimerization and intracellular retention of mutant collagen type X alpha 1 chains as cause for elevated endoplasmic reticulum (ER) stress. However, how ER stress translates into structural defects remained unclear. We generated a medaka (Oryzias latipes) MCDS model harboring a 5 base pair deletion in col10a1, which led to a frameshift and disruption of 11 amino acids in the conserved trimerization domain. col10a1Δ633a heterozygotes recapitulated key features of MCDS and revealed early cell polarity defects as cause for dysregulated matrix secretion and deformed skeletal structures. Carbamazepine, an ER stress-reducing drug, rescued this polarity impairment and alleviated skeletal defects in col10a1Δ633a heterozygotes. Our data imply cell polarity dysregulation as a potential contributor to MCDS and suggest the col10a1Δ633a medaka mutant as an attractive MCDS animal model for drug screening.

The Radiological and Histological Phenotype of Skeletal Abnormalities in Fetal ARCN1-Related Syndrome

Mutations in ARCN1 give rise to a syndromic disorder with rhizomelic short stature with microretrognathia and developmental delay. ARCN1 encodes the delta subunit of the coat protein I complex, which is required for intracellular trafficking of collagen 1 and which may also be involved in the endoplasmic reticulum (ER) stress response. In this paper we describe for the first time the skeletal histological abnormalities in an 18-week-old fetus with an ARCN1 mutation, and we suggest that the skeletal phenotype in ARCN1-related syndrome has more resemblance with ER stress than with a defect in collagen 1 metabolism.

Hypertrophic chondrocytes at the junction of musculoskeletal structures

Hypertrophic chondrocytes are found at unique locations at the junction of skeletal tissues, cartilage growth plate, articular cartilage, enthesis and intervertebral discs. Their role in the skeleton is best understood in the process of endochondral ossification in development and bone fracture healing. Chondrocyte hypertrophy occurs in degenerative conditions such as osteoarthritis. Thus, the role of hypertrophic chondrocytes in skeletal biology and pathology is context dependent. This review will focus on hypertrophic chondrocytes in endochondral ossification, in which they exist in a transient state, but acting as a central regulator of differentiation, mineralization, vascularization and conversion to bone. The amazing journey of a chondrocyte from being entrapped in the extracellular matrix environment to becoming proliferative then hypertrophic will be discussed. Recent studies on the dynamic changes and plasticity of hypertrophic chondrocytes have provided new insights into how we view these cells, not as terminally differentiated but as cells that can dedifferentiate to more progenitor-like cells in a transition to osteoblasts and adipocytes, as well as a source of skeletal stem and progenitor cells residing in the bone marrow. This will provide a foundation for studies of hypertrophic chondrocytes at other skeletal sites in development, tissue maintenance, pathology and therapy.

Growth plate extracellular matrix defects and short stature in children

Many etiological factors causing short stature have already been identified in humans. In the last few years, the advent of new techniques for the detection of chromosomal and molecular abnormalities has made it possible to better identify patients with genetic causes of growth failure. Some of these factors directly affect the development and growth of the skeleton, since they damage the epiphyseal growth plate, where linear growth occurs, influencing chondrogenesis. In particular, defects in genes involved in the organization and function of the growth plate are responsible for several well-known conditions with short stature. These genes play a pivotal role in various mechanisms involving the extracellular matrix, intracellular signaling, paracrine signaling, endocrine signaling, and epigenetic regulation. In this review, we will discuss the genes involved in extracellular matrix disorders. The identification of genetic defects in linear growth failure is important for clinicians and researchers in order to improve the care of children affected by growth disorders.

Valosin-containing protein (VCP/p97) inhibition reduces viral clearance and induces toxicity associated with muscular damage

Valosin-containing protein (VCP)/p97 has emerged as a central regulator of the ubiquitin-proteasome system by connecting ubiquitylation and degradation. The development of CB-5083, an ATPase D2-domain-selective and orally bioavailable inhibitor of VCP/p97, allows targeting of the ubiquitin-proteasome system in human diseases. In this study, we evaluated the effect of CB-5083 on the immune response in mice by using the lymphocytic choriomeningitis virus (LCMV) as an infection model. We demonstrate that LCMV infection increased the susceptibility to CB-5083 treatment in a CD8-independent manner. Administration of CB-5083 to mice reduced the cytotoxic T cell response and impaired viral clearance. Compared to uninfected cells, CB-5083 treatment enhanced the unfolded protein response in LCMV-infected cells. Administration of CB-5083 during the expansion of CD8⁺ T cells led to strong toxicity in mice within hours, which resulted in enhanced IL-6 levels in the serum and accumulation of poly-ubiquitinated proteins. Furthermore, we linked the observed toxicity to the specific formation of aggregates in the skeletal muscle tissue and the upregulation of both lactate dehydrogenase and creatine kinase in the serum.

The natural product salicin alleviates osteoarthritis progression by binding to IRE1α and inhibiting endoplasmic reticulum stress through the IRE1α-IκBα-p65 signaling pathway

Despite the high prevalence of osteoarthritis (OA) in older populations, disease-modifying OA drugs (DMOADs) are still lacking. This study was performed to investigate the effects and mechanisms of the small molecular drug salicin (SA) on OA progression. Primary rat chondrocytes were stimulated with TNF-α and treated with or without SA. Inflammatory factors, cartilage matrix degeneration markers, and cell proliferation and apoptosis markers were detected at the mRNA and protein levels. Cell proliferation and apoptosis were evaluated by EdU assays or flow cytometric analysis. RNA sequencing, molecular docking and drug affinity-responsive target stability analyses were used to clarify the mechanisms. The rat OA model was used to evaluate the effect of intra-articular injection of SA on OA progression. We found that SA rescued TNF-α-induced degeneration of the cartilage matrix, inhibition of chondrocyte proliferation, and promotion of chondrocyte apoptosis. Mechanistically, SA directly binds to IRE1α and occupies the IRE1α phosphorylation site, preventing IRE1α phosphorylation and regulating IRE1α-mediated endoplasmic reticulum (ER) stress by IRE1α-IκBα-p65 signaling. Finally, intra-articular injection of SA-loaded lactic-co-glycolic acid (PLGA) ameliorated OA progression by inhibiting IRE1α-mediated ER stress in the OA model. In conclusion, SA alleviates OA by directly binding to the ER stress regulator IRE1α and inhibits IRE1α-mediated ER stress via IRE1α-IκBα-p65 signaling. Topical use of the small molecular drug SA shows potential to modify OA progression.

The effect of carbamazepine on bone structure and strength in control and osteogenesis imperfecta (Col1a2 +/p.G610C ) mice

The inherited brittle bone disease osteogenesis imperfecta (OI) is commonly caused by COL1A1 and COL1A2 mutations that disrupt the collagen I triple helix. This causes intracellular endoplasmic reticulum (ER) retention of the misfolded collagen and can result in a pathological ER stress response. A therapeutic approach to reduce this toxic mutant load could be to stimulate mutant collagen degradation by manipulating autophagy and/or ER‐associated degradation. Since carbamazepine (CBZ) both stimulates autophagy of misfolded collagen X and improves skeletal pathology in a metaphyseal chondrodysplasia model, we tested the effect of CBZ on bone structure and strength in 3‐week‐old male OI Col1a2^+/p.G610C and control mice. Treatment for 3 or 6 weeks with CBZ, at the dose effective in metaphyseal chondrodysplasia, provided no therapeutic benefit to Col1a2^+/p.G610C mouse bone structure, strength or composition, measured by micro‐computed tomography, three point bending tests and Fourier‐transform infrared microspectroscopy. In control mice, however, CBZ treatment for 6 weeks impaired femur growth and led to lower femoral cortical and trabecular bone mass. These data, showing the negative impact of CBZ treatment on the developing mouse bones, raise important issues which must be considered in any human clinical applications of CBZ in growing individuals.

Collagen misfolding mutations: the contribution of the unfolded protein response to the molecular pathology

Mutations in collagen genes cause a broad range of connective tissue pathologies. Structural mutations that impact procollagen assembly or triple helix formation and stability are a common and important mutation class. How misfolded procollagens engage with the cellular proteostasis machinery and whether they can elicit a cytotoxic unfolded protein response (UPR) is a topic of considerable research interest. Such interest is well justified since modulating the UPR could offer a new approach to treat collagenopathies for which there are no current disease mechanism-targeting therapies. This review scrutinizes the evidence underpinning the view that endoplasmic reticulum stress and chronic UPR activation contributes significantly to the pathophysiology of the collagenopathies. While there is strong evidence that the UPR contributes to the pathology for collagen X misfolding mutations, the evidence that misfolding mutations in other collagen types induce a canonical, cytotoxic UPR is incomplete. To gain a more comprehensive understanding about how the UPR amplifies to pathology, and thus what types of manipulations of the UPR might have therapeutic relevance, much more information is needed about how specific misfolding mutation types engage differentially with the UPR and downstream signaling responses. Most importantly, since the capacity of the proteostasis machinery to respond to collagen misfolding is likely to vary between cell types, reflecting their functional roles in collagen and extracellular matrix biosynthesis, detailed studies on the UPR should focus as much as possible on the actual target cells involved in the collagen pathologies.

4PBA reduces growth deficiency in osteogenesis imperfecta by enhancing transition of hypertrophic chondrocytes to osteoblasts

Short stature is a major skeletal phenotype in osteogenesis imperfecta (OI), a genetic disorder mainly caused by mutations in genes encoding type I collagen. However, the underlying mechanism is poorly understood, and no effective treatment is available. In OI mice that carry a G610C mutation in COL1A2, we previously found that mature hypertrophic chondrocytes (HCs) are exposed to cell stress due to accumulation of misfolded mutant type I procollagen in the endoplasmic reticulum (ER). By fate mapping analysis of HCs in G610C OI mice, we found that HCs stagnate in the growth plate, inhibiting translocation of HC descendants to the trabecular area and their differentiation to osteoblasts. Treatment with 4-phenylbutyric acid (4PBA), a chemical chaperone, restored HC ER structure and rescued this inhibition, resulting in enhanced longitudinal bone growth in G610C OI mice. Interestingly, the effects of 4PBA on ER dilation were limited in osteoblasts, and the bone fragility was not ameliorated. These results highlight the importance of targeting HCs to treat growth deficiency in OI. Our findings demonstrate that HC dysfunction induced by ER disruption plays a critical role in the pathogenesis of OI growth deficiency, which lays the foundation for developing new therapies for OI.

Clinical Characteristics of Short-Stature Patients With Collagen Gene Mutation and the Therapeutic Response to rhGH

CONTEXT

Clinical genetic evaluation has been demonstrated as an important tool to elucidate the causes of growth disorders. Genetic defects of collagen formation (the collagenopathies) have been reported to be associated with short stature and skeletal dysplasias. Etiological diagnosis of skeletal abnormality-related short stature is challenging, and less is known about recombinant human growth hormone (rhGH) therapy.

OBJECTIVE

This is a single-center cohort study which aims at exploring the genetic architecture of short-stature children with skeletal abnormalities and evaluating the frequency of collagenopathies to determine their phenotype, including the rhGH treatment response.

PATIENTS AND METHODS

One hundred and six children with short stature and skeletal abnormalities were enrolled who were evaluated by next-generation sequencing (NGS) to detect variants in the skeletal collagen genes including COL1A1, COL1A2, COL2A1, COL9A1, COL9A2, COL9A3, COL10A1, COL11A1, and COL11A2. The results were evaluated using American College of Medical Genetics and Genomics (ACMG) guidelines. Clinical characteristics and rhGH treatment response were summarized.

RESULTS

Twenty-four pathogenic or likely pathogenic variants of collagen genes were found in 26 of 106 (24.5%) short-stature patients with skeletal abnormalities, of which COL2A1 mutations were the most common, accounting for about 57.7%. Other frequent mutations associated with skeletal development include FGFR3, ACAN, NPR2, COMP, and FBN1 in 12.2%, 0.9%, 0.8%, 0.4%, and 0.4%, respectively, resulting in significantly different degrees of short stature. An overview of clinical features of collagenopathies showed growth retardation, skeletal abnormalities, and heterogeneous syndromic abnormalities involving facial, eye, hearing, and cardiac abnormalities. The average height of 9 patients who received rhGH treatment improved from a median of -3.2 ± 0.9 SDS to -2.2 ± 1.3 SDS after 2.8 ± 2.1 years. The most significant height improvement of 2.3 SDS and 1.7 SDS was also seen in two patients who had been treated for more than 6 years.

CONCLUSIONS

A proband-based NGS revealed that distinct genetic architecture underlies short stature in varying degrees and clinical features. Skeletal abnormality-related short stature involving multiple systems should be tested for skeletal collagen gene mutation. Limited rhGH treatment data indicate an improved growth rate and height, and close monitoring of adverse reactions such as scoliosis is required.

Four decades in the making: Collagen III and mechanisms of vascular Ehlers Danlos Syndrome

Vascular Ehlers Danlos (vEDS) syndrome is a severe multi-systemic connective tissue disorder characterized by risk of dissection and rupture of the arteries, gastro-intestinal tract and gravid uterus. vEDS is caused by mutations in COL3A1, that encodes the alpha 1 chain of type III collagen, which is a major extracellular matrix component of the vasculature and hollow organs. The first causal mutations were identified in the 1980s but progress in our understanding of the pathomolecular mechanisms has been limited. Recently, the application of more refined animal models combined with global omics approaches has yielded important new insights both in terms of disease mechanisms and potential for therapeutic intervention. However, it is also becoming apparent that vEDS is a complex disorder in terms of its molecular disease mechanisms with a poorly understood allelic and mechanistic heterogeneity. In this brief review we will focus our attention on the disease mechanisms of COL3A1 mutations and vEDS, and recent progress in therapeutic approaches using animal models.

Tetramethylpyrazine alleviates endoplasmic reticulum stress‑activated apoptosis and related inflammation in chondrocytes

Excessive apoptosis of chondrocytes and degradation of the extracellular matrix (ECM) contribute to the typical pathological characteristics of osteoarthritis (OA). Various studies have reported that tetramethylpyrazine (TMP) protects against multiple disorders by inhibiting inflammation and oxidative stress. The present study investigated the effects of TMP on chondrocytes and evaluated the associated mechanisms. To determine the effect of TMP on OA and the underlying mechanisms, chondrocytes were incubated with TMP and IL-1β or thapsigargin (TG) Western blotting assays were performed to examine the expression levels of endoplasmic reticulum (ER) stress proteins, and TUNEL staining, fluorescence immunostaining and reverse transcription-quantitative PCR were used to determine the apoptosis levels, and catabolic and inflammatory factors. It was found that TMP protected chondrocytes by suppressing IL-1β-induced expression of glucose-regulated protein 78 (GRP78) and CHOP (an apoptotic protein). TMP regulated the TG-mediated upregulated expression of GRP78 and CHOP in the chondrocytes of rats, as well as markedly suppressed levels of ER stress-triggered inflammatory cytokines (TNF-α and IL-6). Furthermore, TMP modulated TG-induced changes in ECM catabolic metabolism in rat chondrocytes. Collectively, TMP alleviated ER-stress-activated apoptosis and related inflammation in chondrocytes, indicating that it has therapeutic potential for the treatment of OA.

Diazinon exposure reduces bone mineral density in adult and immature rats: A histomorphometric and radiographic study

Diazinon has been widely used as a domestic and agricultural pesticide. This study examined the effects of diazinon on bone mineral density (BMD) of mature and immature rats. For this purpose, 24 adult Wistar rats (male; 8 weeks old) were initially divided into four groups (n = 6). Corn oil was used as the control while diazinon at 15, 30, and 45 mg/kg in corn oil was given to mature rats via gavage per day. Since these dosages were lethal for the immature rats, 12 immature Wistar rats (male; 4 weeks old) (n = 6) were gavaged with corn oil as control and 5 mg/kg of diazinon in corn oil. The animals were sacrificed on day 28 with their left femur bones removed for histomorphometric studies. BMD was measured in the right femur, using standardized radiographs in the femoral head, femoral neck, greater trochanter, and shaft. The Image J Program was used for measuring the bone lamellae and epiphyseal growth plates. The results of this study for the first time revealed that diazinon reduced BMD in both adults and immature rats. Diazinon exposure was associated with diminished trabecular and cortical bone density. Correspondingly, our results indicated that in immature rats, DZN led to the reduction in the epiphyseal growth plate width, both in the proliferation and hypertrophic zones. These results suggested that diazinon might be associated with impaired bone longitudinal growth as well as bone metabolism in adults.

Amelioration of osteogenesis in iPSC-derived mesenchymal stem cells from osteogenesis imperfecta patients by endoplasmic reticulum stress inhibitor

AIM

Osteogenesis imperfecta (OI) is a hereditary connective tissue disorder primarily caused by mutations in COL1A1 or COL1A2, which encode type I collagen. These mutations affect the quantity and/or quality of collagen composition in bones, leading to bone fragility. Currently, there is still a lack of treatment that addresses disease-causing factors due to an insufficient understanding of the pathological mechanisms involved.

MAIN METHODS

Induced pluripotent stem cells (iPSCs) were generated from OI patients with glycine substitution mutations in COL1A1 and COL1A2 and developed into mesenchymal stem cells (iPS-MSCs). OI-derived iPS-MSCs underwent in vitro osteogenic induction to study cell growth, osteogenic differentiation capacity, mRNA expression of osteogenic and unfolded protein response (UPR) markers and apoptosis. The effects of 4-phenylbutyric acid (4-PBA) were examined after treatment of OI iPS-MSCs during osteogenesis.

KEY FINDINGS

OI-derived iPS-MSCs exhibited decreased cell growth and impaired osteogenic differentiation and collagen expression. Expression of UPR genes was increased, which led to an increase in apoptotic cell death. 4-PBA treatment decreased apoptotic cells and reduced expression of UPR genes, including HSPA5, XBP1, ATF4, DDIT3, and ATF6. Osteogenic phenotypes, including RUNX2, SPP1, BGLAP, and IBPS expression, as well as calcium mineralization, were also improved.

SIGNIFICANCE

MSCs differentiated from disease-specific iPSCs have utility as a disease model for identifying disease-specific treatments. In addition, the ER stress-associated UPR could be a pathogenic mechanism associated with OI. Treatment with 4-PBA alleviated OI pathogenesis by attenuating UPR markers and apoptotic cell death.

Targeting cellular stress in vitro improves osteoblast homeostasis, matrix collagen content and mineralization in two murine models of osteogenesis imperfecta

Most cases of dominantly inherited osteogenesis imperfecta (OI) are caused by glycine substitutions in the triple helical domain of type I collagen α chains, which delay collagen folding, and cause the synthesis of collagen triple helical molecules with abnormal structure and post-translational modification. A variable extent of mutant collagen ER retention and other secondary mutation effects perturb osteoblast homeostasis and impair bone matrix quality. Amelioration of OI osteoblast homeostasis could be beneficial both to osteoblast anabolic activity and to the content of the extracellular matrix they deposit. Therefore, the effect of the chemical chaperone 4-phenylbutyrate (4-PBA) on cell homeostasis, collagen trafficking, matrix production and mineralization was investigated in primary osteoblasts from two murine models of moderate OI, Col1a1+/G349C and Col1a2+/G610C. At the cellular level, 4-PBA prevented intracellular accumulation of collagen and increased protein secretion, reducing aggregates within the mutant cells and normalizing ER morphology. At the extracellular level, increased collagen incorporation into matrix, associated with more mature collagen fibrils, was observed in osteoblasts from both models. 4-PBA also promoted OI osteoblast mineral deposition by increasing alkaline phosphatase expression and activity. Targeting osteoblast stress with 4-PBA improved both cellular and matrix abnormalities in culture, supporting further in vivo studies of its effect on bone tissue composition, strength and mineralization as a potential treatment for classical OI.

Next-generation sequencing in patients with familial FSGS: first report of collagen gene mutations in Tunisian patients

Focal segmental glomerulosclerosis (FSGS) is a histological lesion with many causes, including inherited genetic defects, with significant proteinuria being the predominant clinical finding at presentation. FSGS is considered as a podocyte disease due to the fact that in the majority of patients with FSGS, the lesion results from defects in the podocyte structure. However, FSGS does not result exclusively from podocyte-associated genes. In this study, we used a genetic approach based on targeted next-generation sequencing (NGS) of 242 genes to identify the genetic cause of FSGS in seven Tunisian families. The sequencing results revealed the presence of eight distinct mutations including seven newly discovered ones: the c.538G>A (p.V180M) in NPHS2, c.5186G>A (p.R1729Q) in PLCE1 and c.232A>C (p.I78L) in PAX2 and five novel mutations in COL4A3 and COL4A4 genes. Four mutations (c.209G>A (p.G70D), c.725G>A (p.G242E), c.2225G>A (p.G742E), and c. 1681_1698del) were detected in COL4A3 gene and one mutation (c.1424G>A (p.G475D)) was found in COL4A4. In summary, NGS of a targeted gene panel is an ideal approach for the genetic testing of FSGS with multiple possible underlying etiologies. We have demonstrated that not only podocyte genes but also COL4A3/4 mutations should be considered in patients with FSGS.

Differentiation of Hypertrophic Chondrocytes from Human iPSCs for the In Vitro Modeling of Chondrodysplasias

Chondrodysplasias are hereditary diseases caused by mutations in the components of growth cartilage. Although the unfolded protein response (UPR) has been identified as a key disease mechanism in mouse models, no suitable in vitro system has been reported to analyze the pathology in humans. Here, we developed a three-dimensional culture protocol to differentiate hypertrophic chondrocytes from induced pluripotent stem cells (iPSCs) and examine the phenotype caused by MATN3 and COL10A1 mutations. Intracellular MATN3 or COL10 retention resulted in increased ER stress markers and ER size in most mutants, but activation of the UPR was dependent on the mutation. Transcriptome analysis confirmed a UPR with wide-ranging changes in bone homeostasis, extracellular matrix composition, and lipid metabolism in the MATN3 T120M mutant, which further showed altered cellular morphology in iPSC-derived growth-plate-like structures in vivo. We then applied our in vitro model to drug testing, whereby trimethylamine N-oxide led to a reduction of ER stress and intracellular MATN3.

Review: ER stress-induced cell death in osteoarthritic cartilage

In cartilage, chondrocytes are responsible for the biogenesis and maintenance of the extracellular matrix (ECM) composed of proteins, glycoproteins and proteoglycans. Various cellular stresses, such as hypoxia, nutrient deprivation, oxidative stress or the accumulation of advanced glycation end products (AGEs) during aging, but also translational errors or mutations in cartilage components or chaperone proteins affect the synthesis and secretion of ECM proteins, causing protein aggregates to accumulate in the endoplasmic reticulum (ER). This condition, referred to as ER stress, interferes with cartilage cell homeostasis and initiates the unfolded protein response (UPR), a rescue mechanism to regain cell viability and function. Chronic or irreversible ER stress, however, triggers UPR-initiated cell death. Due to unresolved ER stress in chondrocytes, diseases of the skeletal system, such as chondrodysplasias, arise. ER stress has also been identified as a contributing factor to the pathogenesis of cartilage degeneration processes such as osteoarthritis (OA). This review provides current knowledge about the biogenesis of ECM components in chondrocytes, describes possible causes for the impairment of involved processes and focuses on the ER stress-induced cell death in articular cartilage during OA. Targeting of the ER stress itself or intervention in UPR signaling to reduce death of chondrocytes may be promising for future osteoarthritis therapy.

Role of autophagy in intervertebral disc and cartilage function: implications in health and disease

The intervertebral disc and cartilage are specialized, extracellular matrix-rich tissues critical for absorbing mechanical loads, providing flexibility to the joints, and longitudinal growth in the case of growth plate cartilage. Specialized niche conditions in these tissues, such as hypoxia, are critical in regulating cellular activities including autophagy, a lysosomal degradation pathway that promotes cell survival. Mounting evidence suggests that dysregulation of autophagic pathways underscores many skeletal pathologies affecting the spinal column, articular and growth plate cartilages. Many lysosomal storage disorders characterized by the accumulation of partially degraded glycosaminoglycans (GAGs) due to the lysosomal dysfunction thus affect skeletal tissues and result in altered ECM structure. Likewise, pathologies that arise from mutations in genes encoding ECM proteins and ECM processing, folding, and post-translational modifications, result in accumulation of misfolded proteins in the ER, ER stress and autophagy dysregulation. These conditions evidence reduced secretion of ECM proteins and/or increased secretion of mutant proteins, thereby impairing matrix quality and the integrity of affected skeletal tissues and causing a lack of growth and degeneration. In this review, we discuss the role of autophagy and mechanisms of its regulation in the intervertebral disc and cartilages, as well as how dysregulation of autophagic pathways affects these skeletal tissues.

COL4A3 mutation is an independent risk factor for poor prognosis in children with Alport syndrome

BACKGROUND

Alport syndrome (AS) is an inherited glomerular disease caused by mutations in COL4A3, COL4A4, or COL4A5. Associations between clinical manifestations and genotype are not yet well defined. Our study aimed to define clinical and genetic characteristics, establish genotype-phenotype correlations, and determine prognosis of AS in children.

METHODS

A total of 87 children with AS from 10 pediatric nephrology centers, whom had genetic analyses performed at the Hacettepe University Nephrogenetics Laboratory between February 2017 and February 2019, were included. Data regarding demographics, family history, clinical and laboratory characteristics, histopathological and genetic test results, treatments, and yearly follow-up results were retrospectively analyzed.

RESULTS

Of 87 patients, 16% presented with nephrotic syndrome. In patients with nephrotic syndrome, kidney biopsy findings showed focal segmental glomerulosclerosis (FSGS) in 79%, and COL4A3 mutations were the leading genetic abnormality (50%). Twenty-four percent of all patients progressed to chronic kidney disease (CKD). The rate of progression to CKD and the decline in the glomerular filtration rate of the patients with COL4A3 mutation were higher than other mutation groups (p < 0.001 and p = 0.04, respectively). In kidney survival analysis, nephrotic syndrome presentation, histopathology of FSGS, COL4A3 mutations, and autosomal recessive inheritance were found as independent risk factors for earlier progression to CKD. Cyclosporin A treatment did not improve kidney survival.

CONCLUSIONS

We emphasize that genetic testing is important for patients suspected as having AS. Furthermore, COL4A mutations should be considered in patients with FSGS and steroid-resistant nephrotic syndrome. This approach will shed light on the prognosis of patients and help with definitive diagnosis, preventing unnecessary and potentially harmful medications. Graphical abstract.

RGS12 is required for the maintenance of mitochondrial function during skeletal development

Mitochondrial morphology and function are crucial for tissue homeostasis, such as for skeletal development, but the cellular and molecular mechanisms remain unclear. Here, we provide evidence that regulator of G-protein signaling 12 (RGS12) is present in the mitochondria of primary chondrocytes and cartilage tissues. Deletion of RGS12 in type II collagen-positive cells led to a significant decrease in mitochondrial number, membrane potential, and oxidative phosphorylation function. Mechanistically, RGS12 promoted the function of ATP5A as an enhancer of tyrosine phosphorylation. Mice with RGS12 deficiency in the chondrocyte lineage showed serious body retardation, decreased bone mass, and chondrocyte apoptosis due to the defective activity of ATP synthase. To our knowledge, this is the first report that RGS12 is required for maintaining the function of mitochondria, which may allow it to orchestrate responses to cellular homeostasis.

Multiple epiphyseal dysplasia and related disorders: Molecular genetics, disease mechanisms, and therapeutic avenues

For the vast majority of the 6000 known rare disease the pathogenic mechanisms are poorly defined and there is little treatment, leading to poor quality of life and high healthcare costs. Genetic skeletal diseases (skeletal dysplasias) are archetypal examples of rare diseases that are chronically debilitating, often life-threatening and for which no treatments are currently available. There are more than 450 unique phenotypes that, although individually rare, have an overall prevalence of at least 1 per 4000 children. Multiple epiphyseal dysplasia (MED) is a clinically and genetically heterogeneous disorder characterized by disproportionate short stature, joint pain, and early-onset osteoarthritis. MED is caused by mutations in the genes encoding important cartilage extracellular matrix proteins, enzymes, and transporter proteins. Recently, through the use of various cell and mouse models, disease mechanisms underlying this diverse phenotypic spectrum are starting to be elucidated. For example, ER stress induced as a consequence of retained misfolded mutant proteins has emerged as a unifying disease mechanisms for several forms of MED in particular and skeletal dysplasia in general. Moreover, targeting ER stress through drug repurposing has become an attractive therapeutic avenue.

Chondrodysplasias and Aneurysmal Thoracic Aortopathy: An Emerging Tale of Molecular Intersection

Although at first glance chondrodysplasia and aneurysmal thoracic aortopathy seem oddly dissimilar, recent lines of evidences indicate that they share molecular similarities. Chondrodysplasias are a group of skeletal disorders characterized by genetic defects in hyaline cartilage. Aneurysmal thoracic aortopathy is the pathological enlargement of the thoracic aorta due to wall weakness, along with its ensuing life-threatening complications (i.e., aortic dissection and/or rupture). Extracellular matrix dysregulation, abnormal TGF-β signaling, and, to a more limited extent, endoplasmic reticulum stress emerge as common disease processes. In this review we provide a comprehensive overview of the genetic and pathomechanistic overlap as well as of how these commonalities can guide treatment strategies for both disease entities.

Variations of type IV collagen-encoding genes in patients with histological diagnosis of focal segmental glomerulosclerosis

Focal segmental glomerulosclerosis (FSGS), an important cause of end-stage kidney disease (ESKD), covers a spectrum of clinicopathological syndromes sharing a common glomerular lesion, based on an injury of podocytes caused by diverse insults to glomeruli. Although it is well expressed in many reports that the term FSGS is not useful and applicable to a single disease, particularly in genetic studies, FSGS continues to be used as a single clinical diagnosis. Distinguishing genetic forms of FSGS is important for the treatment and overall prognosis because secondary forms of FSGS, produced by rare pathogenic variations in podocyte genes, are not good candidates for immunosuppressive treatment. Over the past decade, several next generation sequencing (NGS) methods have been used to investigate the patients with steroid resistance nephrotic syndrome (SRNS) or FSGS. Pathogenic variants in COL4A3, COL4A4, or COL4A5 genes have been frequently identified in patients with histologic diagnosis of FSGS. The contribution of these mostly heterozygous genetic variations in FSGS pathogenesis and the clinical course of patients with these variations have not been well characterized. This review emphasizes the importance of appropriate approach in selection and diagnosis of cases and interpretation of the genetic data in these studies and suggests a detailed review of existing clinical variant databases using newly available population genetic data.

Cellular Plasticity in Musculoskeletal Development, Regeneration, and Disease

In this review, we highlight themes from a recent workshop focused on "Plasticity of Cell Fate in Musculoskeletal Tissues" held at the Orthopaedic Research Society's 2019 annual meeting. Experts in the field provided examples of mesenchymal cell plasticity during normal musculoskeletal development, regeneration, and disease. A thorough understanding of the biology underpinning mesenchymal cell plasticity may offer a roadmap for promoting regeneration while attenuating pathologic differentiation. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 38:708-718, 2020.

New perspectives on the treatment of skeletal dysplasia

The last few decades have been marked by the identification of numerous genes implicated in genetic disorders, helping in the elucidation of the underlying pathophysiology of these conditions. This has allowed new therapeutic approaches to emerge such as cellular therapy, gene therapy, or pharmacological therapy for various conditions. Skeletal dysplasias are good models to illustrate these scientific advances. Indeed, several therapeutic strategies are currently being investigated in osteogenesis imperfecta; there are ongoing clinical trials based on pharmacological approaches, targeting signaling pathways in achondroplasia and fibrodysplasia ossificans progressiva or the endoplasmic reticulum stress in metaphyseal dysplasia type Schmid or pseudoachondroplasia. Moreover, the treatment of hypophosphatasia or Morquio A disease illustrates the efficacy of enzyme drug replacement. To provide a highly specialized multidisciplinary approach, these treatments are managed by reference centers. The emergence of treatments in skeletal dysplasia provides new perspectives on the prognosis of these severe conditions and may change prenatal counseling in these diseases over the coming years.

The Pervasive Effects of ER Stress on a Typical Endocrine Cell: Dedifferentiation, Mesenchymal Shift and Antioxidant Response in the Thyrocyte

The endoplasmic reticulum stress and the unfolded protein response are triggered following an imbalance between protein load and protein folding. Until recently, two possible outcomes of the unfolded protein response have been considered: life or death. We sought to substantiate a third alternative, dedifferentiation, mesenchymal shift, and activation of the antioxidant response by using typical endocrine cells, i.e. thyroid cells. The thyroid is a unique system both of endoplasmic reticulum stress (a single protein, thyroglobulin represents the majority of proteins synthesized in the endoplasmic reticulum by the thyrocyte) and of polarized epithelium (the single layer of thyrocytes delimiting the follicle). Following endoplasmic reticulum stress, in thyroid cells the folding of thyroglobulin was disrupted. The mRNAs of unfolded protein response were induced or spliced (X-box binding protein-1). Differentiation was inhibited: mRNA levels of thyroid specific genes, and of thyroid transcription factors were dramatically downregulated, at least in part, transcriptionally. The dedifferentiating response was accompanied by an upregulation of mRNAs of antioxidant genes. Moreover, cadherin-1, and the thyroid (and kidney)-specific cadherin-16 mRNAs were downregulated, vimentin, and SNAI1 mRNAs were upregulated. In addition, loss of cortical actin and stress fibers formation were observed. Together, these data indicate that ER stress in thyroid cells induces dedifferentiation, loss of epithelial organization, shift towards a mesenchymal phenotype, and activation of the antioxidant response, highlighting, at the same time, a new and wide strategy to achieve survival following ER stress, and, as a sort of the other side of the coin, a possible new molecular mechanism of decline/loss of function leading to a deficit of thyroid hormones formation.

4-Phenylbutyric Acid Reduces Endoplasmic Reticulum Stress in Chondrocytes That Is Caused by Loss of the Protein Disulfide Isomerase ERp57

Objective

The integrity of cartilage depends on the correct synthesis of extracellular matrix (ECM) components. In case of insufficient folding of proteins in the endoplasmic reticulum (ER) of chondrocytes, ECM proteins aggregate, ER stress evolves, and the unfolded protein response (UPR) is initiated. By this mechanism, chondrocytes relieve the stress condition or initiate cell death by apoptosis. Especially persistent ER stress has emerged as a pathogenic mechanism in cartilage diseases, such as chondrodysplasias and osteoarthritis. As pharmacological intervention is not available yet, it is of great interest to understand cartilage ER stress in detail and to develop therapeutics to intervene.

Methods

ERp57-deficient chondrocytes were generated by CRISPR/Cas9-induced KO. ER stress and autophagy were studied on mRNA and protein level as well as by transmission electron microscopy (TEM) in chondrocyte micromass or cartilage explant cultures of ERp57 KO mice. Thapsigargin (Tg), an inhibitor of the ER-residing Ca²⁺-ATPase, and 4-Phenylbutyric acid (4-PBA), a small molecular chemical chaperone, were applied to induce or inhibit ER stress.

Results

Our data reveal that the loss of the protein disulfide isomerase ERp57 is sufficient to induce ER stress in chondrocytes. 4-PBA efficiently diffuses into cartilage explant cultures and diminishes excessive ER stress in chondrocytes dose dependently, no matter if it is induced by ERp57 KO or stimulation with Tg.

Conclusion

ER-stress-related diseases have different sources; therefore, various targets for therapeutic treatment exist. In the future, 4-PBA may be used alone or in combination with other drugs for the treatment of ER-stress-related skeletal disorders in patients.

Cartilage endoplasmic reticulum stress may influence the onset but not the progression of experimental osteoarthritis

BACKGROUND

Osteoarthritis has been associated with a plethora of pathological factors and one which has recently emerged is chondrocyte endoplasmic reticulum (ER) stress. ER stress is sensed by key ER-resident stress sensors, one of which is activating transcription factor 6 (ATF6). The purpose of this study is to determine whether increased ER stress plays a role in OA.

METHODS

OA was induced in male wild-type (+/+), ColIITgcog (c/c) and Atf6α-/- mice by destabilisation of the medial meniscus (DMM). c/c mice have increased ER stress in chondrocytes via the collagen II promoter-driven expression of ER stress-inducing Tgcog. Knee joints were scored histologically for OA severity. RNA-seq was performed on laser-micro-dissected RNA from cartilage of +/+ and c/c DMM-operated mice.

RESULTS

In situ hybridisation demonstrated a correlation between the upregulation of ER stress marker, BiP, and early signs of proteoglycan loss and cartilage damage in DMM-operated +/+ mice. Histological analysis revealed a significant reduction in OA severity in c/c mice compared with +/+ at 2 weeks post-DMM. This chondroprotective effect in c/c mice was associated with a higher ambient level of BiP protein prior to DMM and a delay in chondrocyte apoptosis. RNA-seq analysis suggested Xbp1-regulated networks to be significantly enriched in c/c mice at 2 weeks post-DMM. Compromising the ER through genetically ablating Atf6α, a key ER stress sensor, had no effect on DMM-induced OA severity.

CONCLUSION

Our studies indicate that an increased capacity to effectively manage increases in ER stress in articular cartilage due either to pre-conditioning as a result of prior exposure to ER stress or to genetic pre-disposition may be beneficial in delaying the onset of OA, but once established, ER stress plays no significant role in disease progression.

XBP1 signalling is essential for alleviating mutant protein aggregation in ER-stress related skeletal disease

The unfolded protein response (UPR) is a conserved cellular response to the accumulation of proteinaceous material in endoplasmic reticulum (ER), active both in health and disease to alleviate cellular stress and improve protein folding. Multiple epiphyseal dysplasia (EDM5) is a genetic skeletal condition and a classic example of an intracellular protein aggregation disease, whereby mutant matrilin-3 forms large insoluble aggregates in the ER lumen, resulting in a specific 'disease signature' of increased expression of chaperones and foldases, and alternative splicing of the UPR effector XBP1. Matrilin-3 is expressed exclusively by chondrocytes thereby making EDM5 a perfect model system to study the role of protein aggregation in disease. In order to dissect the role of XBP1 signalling in aggregation-related conditions we crossed a p.V194D Matn3 knock-in mouse model of EDM5 with a mouse line carrying a cartilage specific deletion of XBP1 and analysed the resulting phenotype. Interestingly, the growth of mice carrying the Matn3 p.V194D mutation compounded with the cartilage specific deletion of XBP1 was severely retarded. Further phenotyping revealed increased intracellular retention of amyloid-like aggregates of mutant matrilin-3 coupled with dramatically decreased cell proliferation and increased apoptosis, suggesting a role of XBP1 signalling in protein accumulation and/or degradation. Transcriptomic analysis of chondrocytes extracted from wild type, EDM5, Xbp1-null and compound mutant lines revealed that the alternative splicing of Xbp1 is crucial in modulating levels of protein aggregation. Moreover, through detailed transcriptomic comparison with a model of metaphyseal chondrodysplasia type Schmid (MCDS), an UPR-related skeletal condition in which XBP1 was removed without overt consequences, we show for the first time that the differentiation-state of cells within the cartilage growth plate influences the UPR resulting from retention of a misfolded mutant protein and postulate that modulation of XBP1 signalling pathway presents a therapeutic target for aggregation related conditions in cells undergoing proliferation.

Respiratory chain inactivation links cartilage-mediated growth retardation to mitochondrial diseases

Children with mitochondrial diseases often present with slow growth and short stature, but the underlying mechanism remains unclear. In this study, Holzer et al. provide in vivo evidence that mitochondrial respiratory chain dysfunction induces cartilage degeneration coincident with altered metabolism, impaired extracellular matrix formation, and cell death at the cartilage–bone junction.

In childhood, skeletal growth is driven by transient expansion of cartilage in the growth plate. The common belief is that energy production in this hypoxic tissue mainly relies on anaerobic glycolysis and not on mitochondrial respiratory chain (RC) activity. However, children with mitochondrial diseases causing RC dysfunction often present with short stature, which indicates that RC activity may be essential for cartilage-mediated skeletal growth. To elucidate the role of the mitochondrial RC in cartilage growth and pathology, we generated mice with impaired RC function in cartilage. These mice develop normally until birth, but their later growth is retarded. A detailed molecular analysis revealed that metabolic signaling and extracellular matrix formation is disturbed and induces cell death at the cartilage–bone junction to cause a chondrodysplasia-like phenotype. Hence, the results demonstrate the overall importance of the metabolic switch from fetal glycolysis to postnatal RC activation in growth plate cartilage and explain why RC dysfunction can cause short stature in children with mitochondrial diseases.

Carbamazepine reduces disease severity in a mouse model of metaphyseal chondrodysplasia type Schmid caused by a premature stop codon (Y632X) in the Col10a1 gene

Mutations, mostly in the region of the COL10A1 gene encoding the C-terminal non-collagenous domain, cause the dwarfism metaphyseal chondrodysplasia type Schmid (MCDS). In most cases, the disease mechanism involves the misfolding of the mutant protein causing increased endoplasmic reticulum (ER) stress and an unfolded protein response (UPR). However, in an iliac crest biopsy, the COL10A1 p.Y632X mutation was found to produce instability of the mutant mRNA such that little mutant protein may be produced. To investigate the disease mechanism further, a gene-targeted mouse model of the Col10a1 p.Y632X mutation was generated. In this model, the mutant mRNA showed no instability, and in mice heterozygous for the mutation, mutant and wild-type mRNAs were present at equal concentrations. The protein was translated from the mutant allele and retained within the cell, triggering increased ER stress and a UPR. The mutation produced a relatively severe form of MCDS. Nevertheless, treatment of the mice with carbamazepine (CBZ), a drug which stimulates intracellular proteolysis and alleviates ER stress, effectively reduced the disease severity in this model of MCDS caused by a premature stop codon in the Col10a1 gene. Specifically, the drug reduced ER stress in the growth plate, restored growth plate architecture toward the wild-type state, significantly increased bone growth and within 2 weeks of treatment corrected the MCDS-induced hip distortion. These results indicate that CBZ is likely to be effective in ongoing clinical trials against all forms of MCDS whether caused by premature stop codons or substitutions.

Gene cloning to clinical trials-the trials and tribulations of a life with collagen

This review, based on the BSMB Fell-Muir Lecture I presented in July 2018 at the Matrix Biology Europe Conference in Manchester, gives a personal perspective of my own laboratory's contributions to research into type X collagen, metaphyseal chondrodysplasia type Schmid and potential treatments for this disorder that are currently entering clinical trial. I have tried to set the advances made in the context of the scientific technologies available at the time and how these have changed over the more than three decades of this research.

Hemiepiphysiodesis stapling induces ER stress apoptosis and autophagy in rat growth plates

Angular deformities of adolescents can be treated with temporary hemiepiphysiodesis. It is confirmed that mechanical staples leading to apoptosis of chondrocyte in the growth plate. In addition, clinical evidences revealed that release from growth-inhibition condition resulted in catch-up growth, which caused damage to the patients. Thus, the current study aimed to investigate the mechanisms underlying the cell growth inhibition and the rebound growth during the temporary hemiepiphysiodesis on the growth plate. Rats with knee stapling were housed for indicated weeks, then were separated into control group, hemiepiphysiodesis groups and removal of staple groups. The tissue samples were analyzed by histopathological staining or western blotting. The results indicated there was significant growth arrest and cell apoptosis in rats treated with mechanical stress loaded (hemiepiphysiodesis group). Additionally, immunohistochemistry staining and western blotting revealed the ER-stress induced cell apoptosis was involved in growth inhibition. In removal of staple group, growth-inhibition, apoptotic cells, ER stress and autophagy-related markers were all decreased when the staples were removed from mice. Moreover, IκB/NF-κB pathway were activated in the growth plate of rats when the loads were released. In conclusion, mechanical load leaded to growth inhibition in the growth plate. ER-stress induced apoptosis and autophagy might be responsible for this process. In contrast, the possible reason for the rebound growth of growth plate may be due to the elevated IκB/NF-κB activity.

Genetic Disorders of the Extracellular Matrix

Mutations in the genes for extracellular matrix (ECM) components cause a wide range of genetic connective tissues disorders throughout the body. The elucidation of mutations and their correlation with pathology has been instrumental in understanding the roles of many ECM components. The pathological consequences of ECM protein mutations depend on its tissue distribution, tissue function, and on the nature of the mutation. The prevalent paradigm for the molecular pathology has been that there are two global mechanisms. First, mutations that reduce the production of ECM proteins impair matrix integrity largely due to quantitative ECM defects. Second, mutations altering protein structure may reduce protein secretion but also introduce dominant negative effects in ECM formation, structure and/or stability. Recent studies show that endoplasmic reticulum (ER) stress, caused by mutant misfolded ECM proteins, makes a significant contribution to the pathophysiology. This suggests that targeting ER‐stress may offer a new therapeutic strategy in a range of ECM disorders caused by protein misfolding mutations. Anat Rec, 2019. © 2019 The Authors. The Anatomical Record published by Wiley Periodicals, Inc. on behalf of American Association of Anatomists.

Mesencephalic astrocyte-derived neurotropic factor is an important factor in chondrocyte ER homeostasis

Mesencephalic astrocyte-derived neurotrophic factor (MANF) is an endoplasmic reticulum (ER) resident protein that can be secreted due to an imperfect KDEL motif. MANF plays a cytoprotective role in several soft tissues and is upregulated in conditions resulting from intracellular retention of mutant protein, including two skeletal diseases, metaphyseal chondrodysplasia, Schmid type (MCDS) and multiple epiphyseal dysplasia (MED). The role of MANF in skeletal tissue homeostasis is currently unknown. Interestingly, cartilage-specific deletion of Manf in a mouse model of MED resulted in increased disease severity, suggesting its upregulation may be chondroprotective. Treatment of MED chondrocytes with exogenous MANF led to a decrease in the cellular levels of BiP (GRP78), confirming MANF's potential to modulate ER stress responses. However, it did not alleviate the intracellular retention of mutant matrilin-3, suggesting that it is the intracellular MANF that is of importance in the pathobiology of skeletal dysplasias. The Col2Cre-driven deletion of Manf from mouse cartilage resulted in a chondrodysplasia-like phenotype. Interestingly, ablation of MANF in cartilage did not have extracellular consequences but led to an upregulation of several ER-resident chaperones including BiP. This apparent induction of ER stress in turn led to dysregulated chondrocyte apoptosis and decreased proliferation, resulting in reduced long bone growth. We have previously shown that ER stress is an underlying disease mechanism for several skeletal dysplasias. The cartilage-specific deletion of Manf described in this study phenocopies our previously published chondrodysplasia models, further confirming that ER stress itself is sufficient to disrupt skeletal growth and thus represents a potential therapeutic target.

Effect of Hemiepiphysiodesis on the Growth Plate: The Histopathological Changes and Mechanism Exploration of Recurrence in Mini Pig Model

PURPOSE

Hemiepiphysiodesis has been widely used to correct angular deformity of long bone in immature patients. However, there is a limited knowledge about the biomechanical effect of this technique on the histopathological changes of the growth plate and the mechanism of recurrence of malformation after implant removal. We aimed to evaluate the biomechanical effect of hemiepiphysiodesis on the histopathological changes of the growth plate and the mechanism of recurrence of malformation after implant removal in Bama miniature pigs, and to explore the role of asymmetric stress during this procedure.

METHODS

Eight 3-month-old male Bama miniature pigs sustained surgeries on the bilateral medial hind leg proximal tibia as the intervention group (n=16), and four pigs sustained bilateral sham surgeries as the control (n=8). In the 18th week after surgeries, hardware was removed in the unilateral leg of each animal in the intervention group. In the 24th week of the study, all animals were euthanized. A total of 24 samples were obtained and stained with H&E, TUNEL, and immunohistochemistry. Sixteen samples in the intervention group were divided into two subgroups. The tibias without an implant were included in the implant removal group (IR group), while the tibias with an implant were included in the implant persist group (IP group). The proximal tibia specimens were divided into 3 equidistant parts from medial to lateral, named as area A, area B, and area C, respectively. The change of thickness of growth plates, chondral apoptosis index, and the expression of Caspase-3, Caspase-9, CHOP, and P65 were compared.

RESULTS

H&E staining showed the thickness of growth plate to be varied in different areas. In the IP group, the thickness of growth plate in areas A and B was statistically significantly thinner than that in area C (p<0.05). In the IR group, the thickness of growth plate in areas A and B was statistically significantly thicker than that in area C (p<0.05). TUNEL staining showed that the apoptosis rate increased significantly after hemiepiphysiodesis and declined after implant removal (p<0.05). Immunohistochemical staining suggested that the expression of Caspase-3, Caspase-9, P65, and CHOP protein was upregulated in the experimental group and downregulated after implant removal.

CONCLUSION

The thickness parameter of the growth plate changes with asymmetric pressure. When the pressure is relieved, the recurrence of malformation is related to the thickening of the growth plate.

Endoplasmic reticulum stress is induced in growth plate hypertrophic chondrocytes in G610C mouse model of osteogenesis imperfecta

Osteogenesis imperfecta (OI) is a hereditary bone disorder most commonly caused by autosomal dominant mutations in genes encoding type I collagen. In addition to bone fragility, patients suffer from impaired longitudinal bone growth. It has been demonstrated that in OI, an accumulation of mutated type I collagen in the endoplasmic reticulum (ER) induces ER stress in osteoblasts, causing osteoblast dysfunction leading to bone fragility. We hypothesize that ER stress is also induced in the growth plate where bone growth is initiated, and examined a mouse model of dominant OI that carries a G610C mutation in the procollagen α2 chain. The results demonstrated that G610C OI mice had significantly shorter long bones with growth plate abnormalities including elongated total height and hypertrophic zone. Moreover, we found that mature hypertrophic chondrocytes expressed type I collagen and ER dilation was more pronounced compared to wild type littermates. The results from in vitro chondrocyte cultures demonstrated that the maturation of G610C OI hypertrophic chondrocytes was significantly suppressed and ER stress related genes were upregulated. Given that the alteration of hypertrophic chondrocyte activity often causes dwarfism, our findings suggest that hypertrophic chondrocyte dysfunction induced by ER stress may be an underlying cause of growth deficiency in G610C OI mice.

Different Forms of ER Stress in Chondrocytes Result in Short Stature Disorders and Degenerative Cartilage Diseases: New Insights by Cartilage-Specific ERp57 Knockout Mice

Cartilage is essential for skeletal development by endochondral ossification. The only cell type within the tissue, the chondrocyte, is responsible for the production of macromolecules for the extracellular matrix (ECM). Before proteins and proteoglycans are secreted, they undergo posttranslational modification and folding in the endoplasmic reticulum (ER). However, the ER folding capacity in the chondrocytes has to be balanced with physiological parameters like energy and oxygen levels. Specific cellular conditions, e.g., a high protein demand, or pathologic situations disrupt ER homeostasis and lead to the accumulation of poorly folded or misfolded proteins. This state is called ER stress and induces a cellular quality control system, the unfolded protein response (UPR), to restore homeostasis. Different mouse models with ER stress in chondrocytes display comparable skeletal phenotypes representing chondrodysplasias. Therefore, ER stress itself seems to be involved in the pathogenesis of these diseases. It is remarkable that chondrodysplasias with a comparable phenotype arise independent from the sources of ER stress, which are as follows: (1) mutations in ECM proteins leading to aggregation, (2) deficiencies in ER chaperones, (3) mutations in UPR signaling factors, or (4) deficiencies in the degradation of aggregated proteins. In any case, the resulting UPR substantially impairs ECM protein synthesis, chondrocyte proliferation, and/or differentiation or regulation of autophagy and apoptosis. Notably, chondrodysplasias arise no matter if single or multiple events are affected. We analyzed cartilage-specific ERp57 knockout mice and demonstrated that the deficiency of this single protein disulfide isomerase, which is responsible for formation of disulfide bridges in ECM glycoproteins, is sufficient to induce ER stress and to cause an ER stress-related bone phenotype. These mice therefore qualify as a novel model for the analysis of ER stress in chondrocytes. They give new insights in ER stress-related short stature disorders and enable the analysis of ER stress in other cartilage diseases, such as osteoarthritis.

ADAMTS10-mediated tissue disruption in Weill-Marchesani syndrome

Fibrillin microfibrils are extracellular matrix assemblies that form the template for elastic fibres, endow blood vessels, skin and other elastic tissues with extensible properties. They also regulate the bioavailability of potent growth factors of the TGF-β superfamily. A disintegrin and metalloproteinase with thrombospondin motifs (ADAMTS)10 is an essential factor in fibrillin microfibril function. Mutations in fibrillin-1 or ADAMTS10 cause Weill-Marchesani syndrome (WMS) characterized by short stature, eye defects, hypermuscularity and thickened skin. Despite its importance, there is poor understanding of the role of ADAMTS10 and its function in fibrillin microfibril assembly. We have generated an ADAMTS10 WMS mouse model using Clustered Regularly Spaced Interspaced Short Palindromic Repeats and CRISPR associated protein 9 (CRISPR-Cas9) to introduce a truncation mutation seen in WMS patients. Homozygous WMS mice are smaller and have shorter long bones with perturbation to the zones of the developing growth plate and changes in cell proliferation. Furthermore, there are abnormalities in the ciliary apparatus of the eye with decreased ciliary processes and abundant fibrillin-2 microfibrils suggesting perturbation of a developmental expression switch. WMS mice have increased skeletal muscle mass and more myofibres, which is likely a consequence of an altered skeletal myogenesis. These results correlated with expression data showing down regulation of Growth differentiation factor (GDF8) and Bone Morphogenetic Protein (BMP) growth factor genes. In addition, the mitochondria in skeletal muscle are larger with irregular shape coupled with increased phospho-p38 mitogen-activated protein kinase (MAPK) suggesting muscle remodelling. Our data indicate that decreased SMAD1/5/8 and increased p38/MAPK signalling are associated with ADAMTS10-induced WMS. This model will allow further studies of the disease mechanism to facilitate the development of therapeutic interventions.

Paradoxical roles of ATF6α and ATF6β in modulating disease severity caused by mutations in collagen X

Whilst the role of ATF6α in modulating the unfolded protein response (UPR) has been well documented, the function of its paralogue ATF6β is less well understood. Using knockdown in cell culture and gene ablation in mice we have directly compared the roles of ATF6α & β in responding to the increased ER stress induced by mutant forms of type X collagen that cause the ER stress-associated metaphyseal chondrodysplasia type Schmid (MCDS). ATF6α more efficiently deals with the disease-associated ER stress in the absence of ATF6β and conversely, ATF6β is less effective in the absence of ATF6α. Furthermore, disease severity in vivo is increased by ATF6α ablation and decreased by ATF6β ablation. In addition, novel functions for each paralogue are described including an ATF6β-specific role in controlling growth plate chondrocyte proliferation. The clear demonstration of the intimate relationship of the two ATF6 isoforms and how ATF6β can moderate the activity of ATF6α and vice versa is of great significance for understanding the UPR mechanism. The activities of both ATF6 isoforms and their separate roles need consideration when deciding how to target increased ER stress as a means of treating MCDS and other ER stress-associated diseases.

Inhibiting the integrated stress response pathway prevents aberrant chondrocyte differentiation thereby alleviating chondrodysplasia

The integrated stress response (ISR) is activated by diverse forms of cellular stress, including endoplasmic reticulum (ER) stress, and is associated with diseases. However, the molecular mechanism(s) whereby the ISR impacts on differentiation is incompletely understood. Here, we exploited a mouse model of Metaphyseal Chondrodysplasia type Schmid (MCDS) to provide insight into the impact of the ISR on cell fate. We show the protein kinase RNA-like ER kinase (PERK) pathway that mediates preferential synthesis of ATF4 and CHOP, dominates in causing dysplasia by reverting chondrocyte differentiation via ATF4-directed transactivation of Sox9. Chondrocyte survival is enabled, cell autonomously, by CHOP and dual CHOP-ATF4 transactivation of Fgf21. Treatment of mutant mice with a chemical inhibitor of PERK signaling prevents the differentiation defects and ameliorates chondrodysplasia. By preventing aberrant differentiation, titrated inhibition of the ISR emerges as a rationale therapeutic strategy for stress-induced skeletal disorders.

Impact of Arginine to Cysteine Mutations in Collagen II on Protein Secretion and Cell Survival

Inherited point mutations in collagen II in humans affecting mainly cartilage are broadly classified as chondrodysplasias. Most mutations occur in the glycine (Gly) of the Gly-X-Y repeats leading to destabilization of the triple helix. Arginine to cysteine substitutions that occur at either the X or Y position within the Gly-X-Y cause different phenotypes like Stickler syndrome and congenital spondyloepiphyseal dysplasia (SEDC). We investigated the consequences of arginine to cysteine substitutions (X or Y position within the Gly-X-Y) towards the N and C terminus of the triple helix. Protein expression and its secretion trafficking were analyzed. Substitutions R75C, R134C and R704C did not alter the thermal stability with respect to wild type; R740C and R789C proteins displayed significantly reduced melting temperatures (T_m) affecting thermal stability. Additionally, R740C and R789C were susceptible to proteases; in cell culture, R789C protein was further cleaved by matrix metalloproteinases (MMPs) resulting in expression of only a truncated fragment affecting its secretion and intracellular retention. Retention of misfolded R740C and R789C proteins triggered an ER stress response leading to apoptosis of the expressing cells. Arginine to cysteine mutations towards the C-terminus of the triple helix had a deleterious effect, whereas mutations towards the N-terminus of the triple helix (R75C and R134C) and R704C had less impact.

Increased intracellular proteolysis reduces disease severity in an ER stress-associated dwarfism

The short-limbed dwarfism metaphyseal chondrodysplasia type Schmid (MCDS) is linked to mutations in type X collagen, which increase ER stress by inducing misfolding of the mutant protein and subsequently disrupting hypertrophic chondrocyte differentiation. Here, we show that carbamazepine (CBZ), an autophagy-stimulating drug that is clinically approved for the treatment of seizures and bipolar disease, reduced the ER stress induced by 4 different MCDS-causing mutant forms of collagen X in human cell culture. Depending on the nature of the mutation, CBZ application stimulated proteolysis of misfolded collagen X by either autophagy or proteasomal degradation, thereby reducing intracellular accumulation of mutant collagen. In MCDS mice expressing the Col10a1.pN617K mutation, CBZ reduced the MCDS-associated expansion of the growth plate hypertrophic zone, attenuated enhanced expression of ER stress markers such as Bip and Atf4, increased bone growth, and reduced skeletal dysplasia. CBZ produced these beneficial effects by reducing the MCDS-associated abnormalities in hypertrophic chondrocyte differentiation. Stimulation of intracellular proteolysis using CBZ treatment may therefore be a clinically viable way of treating the ER stress–associated dwarfism MCDS.

A human case of SLC35A3-related skeletal dysplasia

Researchers have identified a subset of Holstein having a range of skeletal deformities, including vertebral anomalies, referred to as complex vertebral malformation due to mutations in the SLC35A3 gene. Here, we report the first case in humans of SLC35A3-related vertebral anomalies. Our patient had prenatally diagnosed anomalous vertebrae, including butterfly, and hemivertebrae throughout the spine, as well as cleft palate, micrognathia, patent foramen ovale, patent ductus arteriosus, posterior embryotoxon, short limbs, camptodactyly, talipes valgus, rocker bottom feet, and facial dysmorphism including proptosis, nevus flammeus, and a cupped left ear. Clinical exome sequencing revealed a novel missense homozygous mutation in SLC35A3. Follow-up biochemical analysis confirmed abnormal protein glycosylation, consistent with a defective Golgi UDP-GlcNAc transporter, validating the mutations. Congenital disorders of glycosylation, including SLC35A3-CDG, can present as a wide phenotypic spectrum, including skeletal dysplasia. Previously reported patients with SLC35A3-CDG have been described with syndromic autism, epilepsy, and arthrogryposis.

Silencing of both ATF4 and PERK inhibits cell cycle progression and promotes the apoptosis of differentiating chondrocytes

In the current study, we demonstrate that the silencing of protein kinase R (PKR)-like endoplasmic reticulum (ER) kinase (PERK) and activating transcription factor 6 (ATF4) (using small interfering RNA expression constructs) inhibits the chondrocyte cell cycle and proliferation in vitro and ex vivo. The silencing of PERK alone using siRNA against PERK (siPERK) led to arrest in the G1 phase, it decreased the number of cells in the S phase, and delayed progressoin to the G2-M phase. Co-transfection with siRNA against ATF (siATF4) led to a more profound inhibitory effect on cell cycle progression. Moreover, transfection with siPERK was associated with enhanced endoplasmic reticulum (ER) stress-induced apoptosis during bone morphogenetic protein 2 (BMP2)-induced chondrogenesis, and transfection with siATF4 exacerbated ER stress-related cell death. Data from flow cytometry (FCM), immunohistochemistry and TUNEL assays supported these findings in vitro and ex vivo. As shown by our results, the combined effect of the silencing of ATF4 and PERK led to the activation of an ER stress-specific caspase cascade in the cartilage tissue. On the whole, these findings reveal a new crucial combined effect of the silencing of PERK and ATF4 in modulating ER stress-mediated apoptosis during chondrocyte differentiation and proliferation.

Identification of key genes associated with Schmid-type metaphyseal chondrodysplasia based on microarray data

This study aimed to gain a better understanding of the molecular circuitry of Schmid-type metaphyseal chondrodysplasia (SMCD), and to identify more potential genes associated with the pathogenesis of SMCD. Microarray data from GSE72261 were downloaded from the NCBI GEO database, including collagen X p.Asn617Lys knock-in mutation (ColX^N617K), ablated XBP1 activity (Xbp1^CartΔEx2), compound mutant (C/X), and wild-type (WT) specimens. Differentially expressed genes (DEGs) were screened in Xbp1 vs. WT, Col vs. WT and CX vs. WT, respectively. Pathway enrichment analysis of these DEGs was performed. Transcription factors (TFs) of the overlapping DEGs were identified. Weighted correlation network analysis (WGCNA) was performed to find modules of DEGs with high correlations, followed by gene function analysis and a protein-protein interaction network construction. In total, 481, 1,530 and 1,214 DEGs were identified in Xbp1 vs. WT, Col vs. WT and CX vs. WT, respectively. These DEGs were enriched in different pathways, such as extracellular matrix (ECM)-receptor interaction and metabolism-related pathways. A total of 7 TFs were found to regulate 19 common upregulated genes, and 4 TFs were identified to regulate 21 common downregulated genes. Two significant gene co-expression modules were enriched and DEGs in the 2 modules were mainly enriched in different biological processes, such as ribosome biogenesis. Moreover, Kras (downregulated), Col5a1 (upregulated) and Furin (upregulated) were both identified in the regulatory networks and protein-protein interaction (PPI) network. On the whole, our findings indicate that the Kras, Col5a1 and Furin genes may play essential roles in the molecular mechanisms of SMCD, which warrants further investigation.

The Chemical Chaperone, PBA, Reduces ER Stress and Autophagy and Increases Collagen IV α5 Expression in Cultured Fibroblasts From Men With X-Linked Alport Syndrome and Missense Mutations

Introduction

X-linked Alport syndrome (OMIM 301050) is caused by COL4A5 missense variants in 40% of families. This study examined the effects of chemical chaperone treatment (sodium 4-phenylbutyrate) on fibroblast cell lines derived from men with missense mutations.

Methods

Dermal fibroblast cultures were established from 2 affected men and 3 normals. Proliferation rates were examined, the collagen IV α5 chain localized with immunostaining, and levels of the intra- and extracellular chains quantitated with an in-house enzyme-linked immunosorbent assay. COL4A5 mRNA was measured using quantitative reverse transcriptase polymerase chain reaction. Endoplasmic reticulum (ER) size was measured on electron micrographs and after HSP47 immunostaining. Markers of ER stress (ATF6, HSPA5, DDIT3), autophagy (ATG5, BECN1, ATG7), and apoptosis (CASP3, BAD, BCL₂) were also quantitated by quantitative reverse transcriptase polymerase chain reaction. Measurements were repeated after 48 hours of incubation with 10 mM sodium 4-phenylbutyrate acid.

Results

Both COL4A5 missense variants were associated with reduced proliferation rates on day 6 (P = 0.01 and P = 0.03), ER enlargement, and increased mRNA for ER stress and autophagy (all P values < 0.05) when compared with normal. Sodium 4-phenylbutyrate treatment increased COL4A5 transcript levels (P < 0.01), and reduced ER size (P < 0.01 by EM and P < 0.001 by immunostaining), ER stress (p HSPA5 and DDIT3, all P values < 0.01) and autophagy (ATG7, P < 0.01). Extracellular collagen IV α5 chain was increased in the M1 line only (P = 0.06).

Discussion

Sodium 4-phenylbutyrate increases collagen IV α5 mRNA levels, reduces ER stress and autophagy, and possibly facilitates collagen IV α5 extracellular transport. Whether these actions delay end-stage renal failure in men with X-linked Alport syndrome and missense mutations will only be determined with clinical trials.