Bone biology: insights from osteogenesis imperfecta and related rare fragility syndromes

A recurrent de novo missense mutation in COL1A1 causes osteogenesis imperfecta type II and preterm delivery in Normande cattle

BACKGROUND

Nine male and eight female calves born to a Normande artificial insemination bull named "Ly" were referred to the French National Observatory of Bovine Abnormalities for multiple fractures, shortened gestation, and stillbirth or perinatal mortality.

RESULTS

Using Illumina BovineSNP50 array genotypes from affected calves and 84 half-sib controls, the associated locus was mapped to a 6.5-Mb interval on chromosome 19, assuming autosomal inheritance with germline mosaicism. Subsequent comparison of the whole-genome sequences of one case and 5116 control genomes, followed by genotyping in the affected pedigree, identified a de novo missense substitution within the NC1 domain of the COL1A1 gene (Chr19 g.36,473,965G > A; p.D1412N) as unique candidate variant. Interestingly, the affected residue was completely conserved among 243 vertebrate orthologs, and the same substitution in humans has been reported to cause type II osteogenesis imperfecta (OI), a connective tissue disorder that is characterized primarily by bone deformity and fragility. Moreover, three COL1A1 mutations have been described to cause the same syndrome in cattle. Necropsy, computed tomography, radiology, and histology confirmed the diagnosis of type II OI, further supporting the causality of this variant. In addition, a detailed analysis of gestation length and perinatal mortality in 1387 offspring of Ly and more than 160,000 progeny of 63 control bulls allowed us to statistically confirm in a large pedigree the association between type II OI and preterm delivery, which is probably due to premature rupture of fetal membranes and has been reported in several isolated cases of type II OI in humans and cattle. Finally, analysis of perinatal mortality rates and segregation distortion supported a low level of germ cell mosaicism in Ly, with an estimate of 4.5% to 7.7% of mutant sperm and thus 63 to 107 affected calves born. These numbers contrast with the 17 cases reported and raise concerns about the underreporting of congenital defects to heredo-surveillance platforms, even for textbook genetic syndromes.

CONCLUSIONS

In conclusion, we describe a large animal model for a recurrent substitution in COL1A1 that is responsible for type II OI in humans. More generally, this study highlights the utility of such datasets and large half-sib families available in livestock species to characterize sporadic genetic defects.

Local Net Charge State of Collagen Triple Helix Is a Determinant of FKBP22 Binding to Collagen III

Mutations in the FKBP14 gene encoding the endoplasmic reticulum resident collagen-related proline isomerase FK506 binding protein 22 kDa (FKBP22) result in kyphoscoliotic Ehlers-Danlos Syndrome (EDS), which is characterized by a broad phenotypic outcome. A plausible explanation for this outcome is that FKBP22 participates in the biosynthesis of subsets of collagen types: FKBP22 selectively binds to collagens III, IV, VI, and X, but not to collagens I, II, V, and XI. However, these binding mechanisms have never been explored, and they may underpin EDS subtype heterogeneity. Here, we used collagen Toolkit peptide libraries to investigate binding specificity. We observed that FKBP22 binding was distributed along the collagen helix. Further, it (1) was higher on collagen III than collagen II peptides and it (2) was correlated with a positive peptide charge. These findings begin to elucidate the mechanism by which FKBP22 interacts with collagen.

Deep coverage and quantification of the bone proteome provides enhanced opportunities for new discoveries in skeletal biology and disease

Dysregulation of cell signaling in chondrocytes and in bone cells, such as osteocytes, osteoblasts, osteoclasts, and an elevated burden of senescent cells in cartilage and bone, are implicated in osteoarthritis (OA). Mass spectrometric analyses provides a crucial molecular tool-kit to understand complex signaling relationships in age-related diseases, such as OA. Here we introduce a novel mass spectrometric workflow to promote proteomic studies of bone. This workflow uses highly specialized steps, including extensive overnight demineralization, pulverization, and incubation for 72 h in 6 M guanidine hydrochloride and EDTA, followed by proteolytic digestion. Analysis on a high-resolution Orbitrap Eclipse and Orbitrap Exploris 480 mass spectrometer using Data-Independent Acquisition (DIA) provides deep coverage of the bone proteome, and preserves post-translational modifications, such as hydroxyproline. A spectral library-free quantification strategy, directDIA, identified and quantified over 2,000 protein groups (with ≥ 2 unique peptides) from calcium-rich bone matrices. Key components identified were proteins of the extracellular matrix (ECM), bone-specific proteins (e.g., secreted protein acidic and cysteine rich, SPARC, and bone sialoprotein 2, IBSP), and signaling proteins (e.g., transforming growth factor beta-2, TGFB2), and lysyl oxidase homolog 2 (LOXL2), an important protein in collagen crosslinking. Post-translational modifications (PTMs) were identified without the need for specific enrichment. This includes collagen hydroxyproline modifications, chemical modifications for collagen self-assembly and network formation. Multiple senescence factors were identified, such as complement component 3 (C3) protein of the complement system and many matrix metalloproteinases, that might be monitored during age-related bone disease progression. Our innovative workflow yields in-depth protein coverage and quantification strategies to discover underlying biological mechanisms of bone aging and to provide tools to monitor therapeutic interventions. These novel tools to monitor the bone proteome open novel horizons to investigate bone-specific diseases, many of which are age-related.

Radiofrequency Echographic Multispectrometry (REMS): A New Option in the Assessment Bone Status in Adults with Osteogenesis Imperfecta

This study aimed to estimate the utility of the Radiofrequency Echographic Multispectrometry (REMS) approach in the assessment of bone mineral density (BMD) in subjects with osteogenesis imperfecta (OI). In 41 subjects (40.5 ± 18.7 years) with OI and in 36 healthy controls, we measured BMD at the lumbar spine (LS-BMD), femoral neck (FN-BMD) and total hip (TH-BMD), employing a dual-energy X-ray absorptiometry tool. Additionally, REMS scans were also performed at the lumbar and femoral sites. The presence and number of reported fractures were assessed in the study population. Patients characterized by a history of fragility fractures represented 84.5% of the study population. OI subjects showed significantly reduced BMD values both at the level of the lumbar spine and the femoral subregions (p < 0.01) compared to healthy controls when performed using both the DXA and the REMS method. Dividing OI patients on the basis of the Sillence classification, no differences were found between the LS-BMD values carried out using the DXA technique between the OI type I group and OI Type III and IV groups. On the contrary, the OI Type III and IV groups presented significantly lower values of both Trabecular Bone Score (TBS) and LS-BMD through REMS with respect to OI type I patients (p < 0.05). Based on the data of this study, it is possible to conclude that even the new REMS assessment, which does not use ionizing radiation, represents an excellent method for studying the bone status in subjects affected by OI.

FAM46C Is an Interferon-Stimulated Gene That Inhibits Lentiviral Particle Production by Modulating Autophagy

FAM46C is a multiple myeloma (MM) tumor suppressor whose function is only starting to be elucidated. We recently showed that in MM cells FAM46C triggers apoptosis by inhibiting autophagy and altering intracellular trafficking and protein secretion. To date, both a physiological characterization of FAM46C role and an assessment of FAM46C-induced phenotypes outside of MM are lacking. Preliminary reports suggested an involvement of FAM46C with regulation of viral replication, but this was never confirmed. Here, we show that FAM46C is an interferon-stimulated gene and that the expression of wild-type FAM46C in HEK-293T cells, but not of its most frequently found mutant variants, inhibits the production of both HIV-1-derived and HIV-1 lentiviruses. We demonstrate that this effect does not require transcriptional regulation and does not depend on inhibition of either global or virus-specific translation but rather mostly relies on FAM46C-induced deregulation of autophagy, a pathway that we show to be required for efficient lentiviral particle production. These studies not only provide new insights on the physiological role of the FAM46C protein but also could help in implementing more efficient antiviral strategies on one side and lentiviral particle production approaches on the other. IMPORTANCE FAM46C role has been thoroughly investigated in MM, but studies characterizing its role outside of the tumoral environment are still lacking. Despite the success of antiretroviral therapy in suppressing HIV load to undetectable levels, there is currently no HIV cure, and treatment is lifelong. Indeed, HIV continues to be a major global public health issue. Here, we show that FAM46C expression in HEK-293T cells inhibits the production of both HIV and HIV-derived lentiviruses. We also demonstrate that such inhibitory effect relies, at least in part, on the well-established regulatory role that FAM46C exerts on autophagy. Deciphering the molecular mechanism underlying this regulation will not only facilitate the understanding of FAM46C physiological role but also give new insights on the interplay between HIV and the cellular environment.

Vitamin D Alleviates Cadmium-Induced Inhibition of Chicken Bone Marrow Stromal Cells' Osteogenic Differentiation In Vitro

Vitamin D is a lipid soluble vitamin that is mostly used to treat bone metabolism-related diseases. In this study, the effect of Cd toxicity in vitro on osteogenic differentiation derived from BMSCs and the alleviating effect of lα, 25-(OH)2D3 were investigated. Cell index in real time was monitored using a Real-time cell analyzer (RTCA) system. The activity of alkaline phosphatase (ALP), and the calcified nodules and the distribution of Runx2 protein were detected using ALP staining, alizarin red staining, and immunofluorescence, respectively. Furthermore, the mitochondrial membrane potential and the apoptotic rate of BMSCs, the mRNA levels of RUNX2 and type Ⅰ collagen alpha2 (COL1A2) genes, and the protein expression of Col1 and Runx2 were detected using flow cytometry, qRT-PCR and western blot, respectively. The proliferation of BMSCs and osteogenic differentiation were enhanced after treatment with different concentrations of lα, 25-(OH)2D3 compared with the control group. However, 5 μmol/L Cd inhibited the proliferation of BMSCs. In addition, 10 nmol/L lα,25-(OH)2D3 attenuated the toxicity and the apoptosis of BMSCs treated by Cd, and also promoted the osteogenic differentiation including the activity of ALP, and the protein expression of Col1 and Runx2. lα, 25-(OH)2D3 can alleviate cadmium-induced osteogenic toxicity in White Leghorn chickens in vitro.

Absence of TRIC-B from type XIV Osteogenesis Imperfecta osteoblasts alters cell adhesion and mitochondrial function - A multi-omics study

Osteogenesis Imperfecta (OI) is a heritable collagen-related bone dysplasia characterized by bone fractures, growth deficiency and skeletal deformity. Type XIV OI is a recessive OI form caused by null mutations in TMEM38B, which encodes the ER membrane intracellular cation channel TRIC-B. Previously, we showed that absence of TMEM38B alters calcium flux in the ER of OI patient osteoblasts and fibroblasts, which further disrupts collagen synthesis and secretion. How the absence of TMEM38B affects osteoblast function is still poorly understood. Here we further investigated the role of TMEM38B in human osteoblast differentiation and mineralization. TMEM38B-null osteoblasts showed altered expression of osteoblast marker genes and decreased mineralization. RNA-Seq analysis revealed that cell-cell adhesion was one of the most downregulated pathways in TMEM38B-null osteoblasts, with further validation by real-time PCR and Western blot. Gap and tight junction proteins were also decreased by TRIC-B absence, both in patient osteoblasts and in calvarial osteoblasts of Tmem38b-null mice. Disrupted cell adhesion decreased mutant cell proliferation and cell cycle progression. An important novel finding was that TMEM38B-null osteoblasts had elongated mitochondria with altered fusion and fission markers, MFN2 and DRP1. In addition, TMEM38B-null osteoblasts exhibited a significant increase in superoxide production in mitochondria, further supporting mitochondrial dysfunction. Together these results emphasize the novel role of TMEM38B/TRIC-B in osteoblast differentiation, affecting cell-cell adhesion processes, gap and tight junction, proliferation, cell cycle, and mitochondrial function.

CaMKII inhibition due to TRIC-B loss-of-function dysregulates SMAD signaling in osteogenesis imperfecta

Ca2+ is a second messenger that regulates a variety of cellular responses in bone, including osteoblast differentiation. Mutations in trimeric intracellular cation channel B (TRIC-B), an endoplasmic reticulum channel specific for K+, a counter ion for Ca2+flux, affect bone and cause a recessive form of osteogenesis imperfecta (OI) with a still puzzling mechanism. Using a conditional Tmem38b knock out mouse, we demonstrated that lack of TRIC-B in osteoblasts strongly impairs skeleton growth and structure, leading to bone fractures. At the cellular level, delayed osteoblast differentiation and decreased collagen synthesis were found consequent to the Ca2+ imbalance and associated with reduced collagen incorporation in the extracellular matrix and poor mineralization. The impaired SMAD signaling detected in mutant mice, and validated in OI patient osteoblasts, explained the osteoblast malfunction. The reduced SMAD phosphorylation and nuclear translocation were mainly caused by alteration in Ca2+ calmodulin kinase II (CaMKII)-mediated signaling and to a less extend by a lower TGF-β reservoir. SMAD signaling, osteoblast differentiation and matrix mineralization were only partially rescued by TGF-β treatment, strengthening the impact of CaMKII-SMAD axes on osteoblast function. Our data established the TRIC-B role in osteoblasts and deepened the contribution of the CaMKII-SMAD signaling in bone.

Genetic analysis of osteogenesis imperfecta in a large Brazilian cohort

INTRODUCTION

Osteogenesis imperfecta (OI) is a genetically and clinically heterogeneous disorder caused by disruption of type I collagen synthesis. Previous Brazilian molecular OI studies have been restricted to case reports or small cohorts. The Brazilian OI Network (BOIN) is a multicenter study collecting clinical OI treatment data from five reference centers in three regions of Brazil.

OBJECTIVE

To describe the molecular analysis of a large cohort of OI registered at BOIN.

METHODS

Targeted next-generation sequencing (NGS) was performed at a centralized laboratory with the Ion Torrent platform, covering 99.6 % of the coding regions of 18 OI-associated genes. Clinical information was obtained from a clinical database.

RESULTS

We included 156 subjects in the molecular analyses. Variants were detected in 121 subjects: 65 (53.7 %) in COL1A1, 42 (34.7 %) in COL1A2, 2 (1.7 %) in IFITM5, one (0.8 %) in CRTAP, three (2.5 %) in P3H1, two (1.7 %) in PPIB, four (3.3 %) FKBP10, one (0.8 %) in SERPINH1, and one (0.8 %) in TMEM38B. Ninety-one distinct variants were identified, of which 26 were novel. Of the 107 variants identified in COL1A1 and COL1A2, 24.5 % cause mild OI, while the remaining 75.5 % cause moderate, severe, or lethal OI, of which 49.3 % are glycine to serine substitutions. A single variant in FKBP10 (c.179A>C; p.Gln60Pro) was found in three unrelated and non-consanguineous participants living in the same geographic area in Northeast Brazil, suggesting a possible founder effect.

CONCLUSION

Consistent with the literature, 88.4 % of the subjects had a variant in the COL1A1 and COL1A2 genes, with 10 % inherited in an autosomal recessive manner. Notably, one variant in FKBP10 with a potential founder effect requires further investigation. Data from this large cohort improves our understanding of genotype-phenotype correlations for OI in Brazil.

Biallelic frameshift variants inPHLDB1cause mild-type osteogenesis imperfecta with regressive spondylometaphyseal changes

Background

Osteogenesis imperfecta (OI) is a heterogeneous group of inherited disorders characterised by susceptibility to fractures, primarily due to defects in type 1 collagen. The aim of this study is to present a novel OI phenotype and its causative candidate gene.

Methods

Whole-exome sequencing and clinical evaluation were performed in five patients from two unrelated families.PHLDB1mRNA expression in blood and fibroblasts was investigated by real-time PCR, and western blot analysis was further performed on skin fibroblasts.

Results

The common findings among the five affected children were recurrent fractures and/or osteopaenia, platyspondyly, short and bowed long bones, and widened metaphyses. Metaphyseal and vertebral changes regressed after early childhood, and no fractures occurred under bisphosphonate treatment. We identified biallelic NM_001144758.3:c.2392dup and NM_001144758.3:c.2690_2693del pathogenic variants inPHLDB1in the affected patients, respectively, in the families; parents were heterozygous for these variants.PHLDB1encodes pleckstrin homology-like domain family B member-1 (PHLDB1) protein, which has a role in insulin-dependent Akt phosphorylation. Compared with controls, a decrease in the expression levels ofPHLDB1in the blood and skin fibroblast samples was detected. Western blot analysis of cultured fibroblasts further confirmed the loss of PHLDB1.

Conclusion

Two biallelic frameshift variants in the candidate genePHLDB1were identified in independent families with a novel, mild-type, autosomal recessive OI. The demonstration of decreasedPHLDB1mRNA expression levels in blood and fibroblast samples supports the hypothesis thatPHLDB1pathogenic variants are causative for the observed phenotype.

Lysyl hydroxylase 3-mediated post-translational modifications are required for proper biosynthesis of collagen α1α1α2(IV)

Collagens are the most abundant proteins in the body and among the most biosynthetically complex. A molecular ensemble of over 20 endoplasmic reticulum resident proteins participates in collagen biosynthesis and contributes to heterogeneous post-translational modifications. Pathogenic variants in genes encoding collagens cause connective tissue disorders, including osteogenesis imperfecta, Ehlers-Danlos syndrome, and Gould syndrome (caused by mutations in COL4A1 and COL4A2), and pathogenic variants in genes encoding proteins required for collagen biosynthesis can cause similar but overlapping clinical phenotypes. Notably, pathogenic variants in lysyl hydroxylase 3 (LH3) cause a multisystem connective tissue disorder that exhibits pathophysiological features of collagen-related disorders. LH3 is a multifunctional collagen-modifying enzyme; however, its precise role(s) and substrate specificity during collagen biosynthesis has not been defined. To address this critical gap in knowledge, we generated LH3 KO cells and performed detailed quantitative and molecular analyses of collagen substrates. We found that LH3 deficiency severely impaired secretion of collagen α1α1α2(IV) but not collagens α1α1α2(I) or α1α1α1(III). Amino acid analysis revealed that LH3 is a selective LH for collagen α1α1α2(IV) but a general glucosyltransferase for collagens α1α1α2(IV), α1α1α2(I), and α1α1α1(III). Importantly, we identified rare variants that are predicted to be pathogenic in the gene encoding LH3 in two of 113 fetuses with intracranial hemorrhage-a cardinal feature of Gould syndrome. Collectively, our findings highlight a critical role of LH3 in α1α1α2(IV) biosynthesis and suggest that LH3 pathogenic variants might contribute to Gould syndrome.

Early-Onset Osteoporosis: Rare Monogenic Forms Elucidate the Complexity of Disease Pathogenesis Beyond Type I Collagen

Early-onset osteoporosis (EOOP), characterized by low bone mineral density (BMD) and fractures, affects children, premenopausal women and men aged <50 years. EOOP may be secondary to a chronic illness, long-term medication, nutritional deficiencies, etc. If no such cause is identified, EOOP is regarded primary and may then be related to rare variants in genes playing a pivotal role in bone homeostasis. If the cause remains unknown, EOOP is considered idiopathic. The scope of this review is to guide through clinical and genetic diagnostics of EOOP, summarize the present knowledge on rare monogenic forms of EOOP, and describe how analysis of bone biopsy samples can lead to a better understanding of the disease pathogenesis. The diagnostic pathway of EOOP is often complicated and extensive assessments may be needed to reliably exclude secondary causes. Due to the genetic heterogeneity and overlapping features in the various genetic forms of EOOP and other bone fragility disorders, the genetic diagnosis usually requires the use of next-generation sequencing to investigate several genes simultaneously. Recent discoveries have elucidated the complexity of disease pathogenesis both regarding genetic architecture and bone tissue-level pathology. Two rare monogenic forms of EOOP are due to defects in genes partaking in the canonical WNT pathway: LRP5 and WNT1. Variants in the genes encoding plastin-3 (PLS3) and sphingomyelin synthase 2 (SGMS2) have also been found in children and young adults with skeletal fragility. The molecular mechanisms leading from gene defects to clinical manifestations are often not fully understood. Detailed analysis of patient-derived transiliac bone biopsies gives valuable information to understand disease pathogenesis, distinguishes EOOP from other bone fragility disorders, and guides in patient management, but is not widely available in clinical settings. Despite the great advances in this field, EOOP remains an insufficiently explored entity and further research is needed to optimize diagnostic and therapeutic approaches. © 2022 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Dissecting the phenotypic variability of osteogenesis imperfecta

Osteogenesis imperfecta (OI) is a heterogeneous family of collagen type I-related diseases characterized by bone fragility. OI is most commonly caused by single-nucleotide substitutions that replace glycine residues or exon splicing defects in the COL1A1 and COL1A2 genes that encode the α1(I) and α2(I) collagen chains. Mutant collagen is partially retained intracellularly, impairing cell homeostasis. Upon secretion, it assembles in disorganized fibrils, altering mineralization. OI is characterized by a wide range of clinical outcomes, even in the presence of identical sequence variants. Given the heterotrimeric nature of collagen I, its amino acid composition and the peculiarity of its folding, several causes may underlie the phenotypic variability of OI. A deep analysis of entries regarding glycine and splice site collagen substitution of the largest publicly available patient database reveals a higher risk of lethal phenotype for carriers of variants in α1(I) than in α2(I) chain. However, splice site variants are predominantly associated with lethal phenotype when they occur in COL1A2. In addition, lethality is increased when mutations occur in regions of importance for extracellular matrix interactions. Both extracellular and intracellular determinants of OI clinical severity are discussed in light of the findings from in vitro and in vivo OI models. Combined with meticulous tracking of clinical cases via a publicly available database, the available OI animal models have proven to be a unique tool to shed light on new modulators of phenotype determination for this rare heterogeneous disease.

Early-Onset Osteoporosis

Osteoporosis is a skeletal disorder with enhanced bone fragility, usually affecting the elderly. It is very rare in children and young adults and the definition is not only based on a low BMD (a Z-score < - 2.0 in growing children and a Z-score ≤ - 2.0 or a T-score ≤ - 2.5 in young adults) but also on the occurrence of fragility fractures and/or the existence of underlying chronic diseases or secondary factors such as use of glucocorticoids. In the absence of a known chronic disease, fragility fractures and low BMD should prompt extensive screening for secondary causes, which can be found in up to 90% of cases. When fragility fractures occur in childhood or young adulthood without an evident secondary cause, investigations should explore the possibility of an underlying monogenetic bone disease, where bone fragility is caused by a single variant in a gene that has a major role in the skeleton. Several monogenic forms relate to type I collagen, but other forms also exist. Loss-of-function variants in LRP5 and WNT1 may lead to early-onset osteoporosis. The X-chromosomal osteoporosis caused by PLS3 gene mutations affects especially males. Another recently discovered form relates to disturbed sphingolipid metabolism due to SGMS2 mutations, underscoring the complexity of molecular pathology in monogenic early-onset osteoporosis. Management of young patients consists of treatment of secondary factors, optimizing lifestyle factors including calcium and vitamin D and physical exercise. Treatment with bone-active medication should be discussed on a personalized basis, considering the severity of osteoporosis and underlying disease versus the absence of evidence on anti-fracture efficacy and potential harmful effects in pregnancy.

The Polygenic and Monogenic Basis of Paediatric Fractures

PURPOSE OF REVIEW

Fractures are frequently encountered in paediatric practice. Although recurrent fractures in children usually unveil a monogenic syndrome, paediatric fracture risk could be shaped by the individual genetic background influencing the acquisition of bone mineral density, and therefore, the skeletal fragility as shown in adults. Here, we examine paediatric fractures from the perspective of monogenic and complex trait genetics.

RECENT FINDINGS

Large-scale genome-wide studies in children have identified ~44 genetic loci associated with fracture or bone traits whereas ~35 monogenic diseases characterized by paediatric fractures have been described. Genetic variation can predispose to paediatric fractures through monogenic risk variants with a large effect and polygenic risk involving many variants of small effects. Studying genetic factors influencing peak bone attainment might help in identifying individuals at higher risk of developing early-onset osteoporosis and discovering drug targets to be used as bone restorative pharmacotherapies to prevent, or even reverse, bone loss later in life.

Zebrafish Models for Human Skeletal Disorders

In 2019, the Nosology Committee of the International Skeletal Dysplasia Society provided an updated version of the Nosology and Classification of Genetic Skeletal Disorders. This is a reference list of recognized diseases in humans and their causal genes published to help clinician diagnosis and scientific research advances. Complementary to mammalian models, zebrafish has emerged as an interesting species to evaluate chemical treatments against these human skeletal disorders. Due to its versatility and the low cost of experiments, more than 80 models are currently available. In this article, we review the state-of-art of this “aquarium to bedside” approach describing the models according to the list provided by the Nosology Committee. With this, we intend to stimulate research in the appropriate direction to efficiently meet the actual needs of clinicians under the scope of the Nosology Committee.

Plastin 3 in health and disease: a matter of balance

For a long time, PLS3 (plastin 3, also known as T-plastin or fimbrin) has been considered a rather inconspicuous protein, involved in F-actin-binding and -bundling. However, in recent years, a plethora of discoveries have turned PLS3 into a highly interesting protein involved in many cellular processes, signaling pathways, and diseases. PLS3 is localized on the X-chromosome, but shows sex-specific, inter-individual and tissue-specific expression variability pointing towards skewed X-inactivation. PLS3 is expressed in all solid tissues but usually not in hematopoietic cells. When escaping X-inactivation, PLS3 triggers a plethora of different types of cancers. Elevated PLS3 levels are considered a prognostic biomarker for cancer and refractory response to therapies. When it is knocked out or mutated in humans and mice, it causes osteoporosis with bone fractures; it is the only protein involved in actin dynamics responsible for osteoporosis. Instead, when PLS3 is upregulated, it acts as a highly protective SMN-independent modifier in spinal muscular atrophy (SMA). Here, it seems to counteract reduced F-actin levels by restoring impaired endocytosis and disturbed calcium homeostasis caused by reduced SMN levels. In contrast, an upregulation of PLS3 on wild-type level might cause osteoarthritis. This emphasizes that the amount of PLS3 in our cells must be precisely balanced; both too much and too little can be detrimental. Actin-dynamics, regulated by PLS3 among others, are crucial in a lot of cellular processes including endocytosis, cell migration, axonal growth, neurotransmission, translation, and others. Also, PLS3 levels influence the infection with different bacteria, mycosis, and other pathogens.

Cytoplasmic polyadenylation by TENT5A is required for proper bone formation

Osteoblasts orchestrate bone formation through the secretion of type I collagen and other constituents of the matrix on which hydroxyapatite crystals mineralize. Here, we show that TENT5A, whose mutations were found in congenital bone disease osteogenesis imperfecta patients, is a cytoplasmic poly(A) polymerase playing a crucial role in regulating bone mineralization. Direct RNA sequencing revealed that TENT5A is induced during osteoblast differentiation and polyadenylates mRNAs encoding Col1α1, Col1α2, and other secreted proteins involved in osteogenesis, increasing their expression. We postulate that TENT5A, possibly together with its paralog TENT5C, is responsible for the wave of cytoplasmic polyadenylation of mRNAs encoding secreted proteins occurring during bone mineralization. Importantly, the Tent5a knockout (KO) mouse line displays bone fragility and skeletal hypomineralization phenotype resulting from quantitative and qualitative collagen defects. Thus, we report a biologically relevant posttranscriptional regulator of collagen production and, more generally, bone formation.

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.

Network-driven discovery yields new insight into Shox2-dependent cardiac rhythm control

The homeodomain transcription factor SHOX2 is involved in the development and function of the heart's primary pacemaker, the sinoatrial node (SAN), and has been associated with cardiac conduction-related diseases such as atrial fibrillation and sinus node dysfunction. To shed light on Shox2-dependent genetic processes involved in these diseases, we established a murine embryonic stem cell (ESC) cardiac differentiation model to investigate Shox2 pathways in SAN-like cardiomyocytes. Differential RNA-seq-based expression profiling of Shox2^+/+ and Shox2^-/- ESCs revealed 94 dysregulated transcripts in Shox2^-/- ESC-derived SAN-like cells. Of these, 15 putative Shox2 target genes were selected for further validation based on comparative expression analysis with SAN- and right atria-enriched genes. Network-based analyses, integrating data from the Mouse Organogenesis Cell Atlas and the Ingenuity pathways, as well as validation in mouse and zebrafish models confirmed a regulatory role for the novel identified Shox2 target genes including Cav1, Fkbp10, Igfbp5, Mcf2l and Nr2f2. Our results indicate that genetic networks involving SHOX2 may contribute to conduction traits through the regulation of these genes.

Positive airway pressure therapy for obstructive sleep apnea in patients with Osteogenesis imperfecta: a prospective pilot study

BACKGROUND

Obstructive sleep apnea (OSA) is prevalent in individuals with Osteogenesis imperfecta (OI). To date, no study has investigated treatment of OSA in adult individuals with OI using positive airway pressure (PAP). This observational pilot study examined the adherence of adults with OI to treatment of OSA with PAP therapy, and the evolution of self-experienced sleepiness and depression symptoms before and after treatment.

METHODS

We included 20 patients, with a mean age of 51 years, who represented varying severity of OI and displayed an apnea and hypopnea index ≥ 5 /sleeping hour as recorded by an overnight polysomnography. PAP therapy was proposed to all patients. Epworth Sleepiness Scale (ESS) questionnaire to evaluate daytime sleepiness, and a validated self-rating depression questionnaire to identify possible depression, were completed prior to PAP therapy and repeated after a minimum of one year. The datasets supporting the conclusions of this article are included within the article.

RESULTS

From the 20 patients, 15 initiated PAP therapy, and two patients later interrupted it. The mean PAP follow-up period was 1230 days. At baseline, an abnormally high ESS score was reported by 29% of the respondents, and an abnormally high number of symptoms suggesting depression by 29%. Follow-up questionnaires were completed by 60% of the patients, of whom 83% were adherent to PAP treatment. ESS score and depression symptoms did not decrease significantly with PAP therapy.

CONCLUSIONS

Patients with OI accepted well PAP therapy and remained compliant. Sleepiness and depression persisted unaltered despite good PAP adherence. These unexpectedly poor improvements in symptoms by PAP therapy may be due to subjective depression symptoms and the complexity of factors underlying persisting sleepiness in OI. Further research is needed to confirm this novel finding.

A novel mutation in PLS3 causes extremely rare X-linked osteogenesis imperfecta

BACKGROUND

Osteogenesis imperfecta (OI) is a phenotypically and genetically heterogeneous bone disease characterized by bone fragility and recurrent fractures. X-linked inherited OI with mutation in PLS3 is so rare that its genotype-phenotype characteristics are not available.

METHODS

We designed a novel targeted next-generation sequencing (NGS) panel with the candidate genes of OI to detect pathogenic mutations and confirmed them by Sanger sequencing. The phenotypes of the patients were also investigated.

RESULTS

The proband, a 12-year-old boy from a nonconsanguineous family, experienced multiple fractures of long bones and vertebrae and had low bone mineral density (BMD Z-score of -3.2 to -2.0). His younger brother also had extremity fractures. A novel frameshift mutation (c.1106_1107insGAAA; p.Phe369Leufs*5) in exon 10 of PLS3 was identified in the two patients, which was inherited from their mother who had normal BMD. Blue sclerae were the only extraskeletal symptom in all affected individuals. Zoledronic acid was beneficial for increasing BMD and reshaping the compressed vertebral bodies of the proband.

CONCLUSION

We first identify a novel mutation in PLS3 that led to rare X-linked OI and provide practical information for the diagnosis and treatment of this disease.

Interaction between KDELR2 and HSP47 as a Key Determinant in Osteogenesis Imperfecta Caused by Bi-allelic Variants in KDELR2

Osteogenesis imperfecta (OI) is characterized primarily by susceptibility to fractures with or without bone deformation. OI is genetically heterogeneous: over 20 genetic causes are recognized. We identified bi-allelic pathogenic KDELR2 variants as a cause of OI in four families. KDELR2 encodes KDEL endoplasmic reticulum protein retention receptor 2, which recycles ER-resident proteins with a KDEL-like peptide from the cis-Golgi to the ER through COPI retrograde transport. Analysis of patient primary fibroblasts showed intracellular decrease of HSP47 and FKBP65 along with reduced procollagen type I in culture media. Electron microscopy identified an abnormal quality of secreted collagen fibrils with increased amount of HSP47 bound to monomeric and multimeric collagen molecules. Mapping the identified KDELR2 variants onto the crystal structure of G. gallus KDELR2 indicated that these lead to an inactive receptor resulting in impaired KDELR2-mediated Golgi-ER transport. Therefore, in KDELR2-deficient individuals, OI most likely occurs because of the inability of HSP47 to bind KDELR2 and dissociate from collagen type I. Instead, HSP47 remains bound to collagen molecules extracellularly, disrupting fiber formation. This highlights the importance of intracellular recycling of ER-resident molecular chaperones for collagen type I and bone metabolism and a crucial role of HSP47 in the KDELR2-associated pathogenic mechanism leading to OI.

Extracellular matrix: key structural and functional meshwork in health and disease

The extracellular matrix (ECM) is a multifunctional 3D network of interconnected macromolecules that mediate cellular responses in normal and pathological conditions, including wound healing, tissue homeostasis, skeletal and cardiovascular disease, cancer and drug resistance. The potential of targeting ECM components, such as proteoglycans and glycosaminoglycans, matrix-degrading enzymes and miRNAs will serve as a novel tool for innovative diagnostic methods and therapeutic strategies. With this Special Issue on Extracellular Matrix in Health and Disease, The FEBS Journal offers an updated overview of the functional properties and biological roles of several ECM components, highlighting the significance of ECM targeting in disease management.

Diversity and bioactivity of fungi associated with the marine sea cucumber Holothuria poli: disclosing the strains potential for biomedical applications

AIMS

Identification of the mycobiota associated to the marine echinoderm Holothuria poli and investigation of cytotoxic and pro-osteogenic potential of isolated strains.

METHODS AND RESULTS

Fungal strains were isolated from the animal's body-wall, intestine and faeces. The species identification was based on DNA barcoding and morphophysiological observations. Forty-seven species were identified, all are Ascomycota and mainly belonging to Aspergillus and Penicillium genera. Sixteen strains were grown on three media for chemical extraction. Cytotoxic activity was tested on a hepatic cancer cell line (HepG2), the cells viability was evaluated after treatment using a resazurin based assay (AlamarBlue). Pro-osteogenic activity was tested on human Mesenchymal stem cell, differentiation was measured as the alkaline phosphatase production through reaction with p-nitrophenylphosphate or as the cells ability to mineralize calcium using a colorimetric kit (StanBio). Cytotoxic activity was recorded for four fungal species while five of 48 extracts highlighted bioactivity towards human mesenchymal stem cells.

CONCLUSIONS

The presence of relevant animal-associated mycobiota was observed in H. poli and selected strains showed cytotoxic potential and pro-osteogenic activity.

SIGNIFICANCE AND IMPACT OF THE STUDY

Our work represents the first report of a Mediterranean Sea cucumber mycobiota and highlights the isolates potential to synthetize compounds of pharmaceutical interest for regenerative medicine.

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.

Osteogenesis imperfecta – Pathophysiologie und aktuelle Behandlungsstrategien

Die Osteogenesis imperfecta (OI) ist eine angeborene Erkrankung des Knochens und Bindegewebes. Sie geht mit einer erhöhten Frakturneigung, Deformierung der Extremität, aber auch mit extraskelettalen Symptomen einher. Nach einer kurzen Darstellung von Klinik, Diagnostik und aktueller Therapie folgt ein umfassender Überblick über die genetischen und pathophysiologischen Grundlagen der Erkrankung und die daraus abgeleiteten zukünftigen therapeutischen Möglichkeiten.

Ungefähr 80 % der Patienten haben eine Mutation in den Kollagen-Genen COL1A1 und COL1A2. Bei diesen Patienten ist für das Kollektiv keine klare Genotyp-Phänotyp-Korrelation beschrieben. Stoppmutationen führen in der Regel zu einem quantitativen Kollagendefekt, wodurch weniger normales Kollagen gebildet wird und ein eher leichter Phänotyp entsteht. Missense-Mutationen führen zu strukturell verändertem Kollagen (qualitativer Defekt) und zu einem schwereren Phänotyp. Trotzdem gibt es Unterschiede und Vorhersagen über den individuellen Verlauf sind nur sehr eingeschränkt möglich. Neben Veränderungen in den Kollagen-Genen gibt es Mutationen, welche die Kollagenmodifikation und die Kollagensekretion betreffen. Eine eigene Gruppe bilden Gene, welche an der Osteoblastendifferenzierung beteiligt sind. Wie auch bei den weiteren, nicht näher zugeordneten Genen sind dies häufig übergeordnete Gene, deren Funktion in der Osteogenese nicht völlig verstanden ist.

Abgeleitet aus den pathophysiologischen Grundlagen, können vorhandene Medikamente zukünftig womöglich zielgerichtet eingesetzt werden. So ist der „Receptor-Activator-of-Nuclear-Factor-Kappa B-Ligand“ (RANKL)-Antikörper Denosumab spezifischer als Bisphosphonate und wird schon heute bei OI-Typ VI (SERPINF1) verwendet. Weitere Medikamente wie Anti-Sklerostin oder Stammzelltherapien werden unter Berücksichtigung der Pathophysiologie aktuell entwickelt.

Osteoblasts mineralization and collagen matrix are conserved upon specific Col1a2 silencing

Classical osteogenesis imperfecta (OI) is an inherited rare brittle bone disease caused by dominant mutations in the COL1A1 or COL1A2 genes, encoding for the α chains of collagen type I. The definitive cure for the disease will require a gene therapy approach, aimed to correct or suppress the mutant allele. Interestingly, individuals lacking α2(I) chain and synthetizing collagen α1(I)₃ homotrimers do not show bone phenotype, making appealing a bone specific COL1A2 silencing approach for OI therapy. To this aim, three different Col1a2-silencing RNAs (siRNAs), −3554, −3825 and −4125, selected at the 3′-end of the murine Col1a2 transcript were tested in vitro and in vivo. In murine embryonic fibroblasts Col1a2-siRNA-3554 was able to efficiently and specifically target the Col1a2 mRNA and to strongly reduce α2(I) chain expression. Its efficiency and specificity were also demonstrated in primary murine osteoblasts, whose mineralization was preserved. The efficiency of Col1a2-siRNA-3554 was proved also in vivo. Biphasic calcium phosphate implants loaded with murine mesenchymal stem cells were intramuscularly transplanted in nude mice and injected with Col1a2-siRNA-3554 three times a week for three weeks. Collagen α2 silencing was demonstrated both at mRNA and protein level and Masson's Trichrome staining confirmed the presence of newly formed collagen matrix. Our data pave the way for further investigation of Col1a2 silencing and siRNA delivery to the bone tissue as a possible strategy for OI therapy.

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Identification of a specific and efficient Col1a2 siRNA

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Silencing of Col1a2 allows osteoblasts mineralization.

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Col1a2 silencing is not impairing matrix deposition in vivo.

Zebrafish: A Resourceful Vertebrate Model to Investigate Skeletal Disorders

Animal models are essential tools for addressing fundamental scientific questions about skeletal diseases and for the development of new therapeutic approaches. Traditionally, mice have been the most common model organism in biomedical research, but their use is hampered by several limitations including complex generation, demanding investigation of early developmental stages, regulatory restrictions on breeding, and high maintenance cost. The zebrafish has been used as an efficient alternative vertebrate model for the study of human skeletal diseases, thanks to its easy genetic manipulation, high fecundity, external fertilization, transparency of rapidly developing embryos, and low maintenance cost. Furthermore, zebrafish share similar skeletal cells and ossification types with mammals. In the last decades, the use of both forward and new reverse genetics techniques has resulted in the generation of many mutant lines carrying skeletal phenotypes associated with human diseases. In addition, transgenic lines expressing fluorescent proteins under bone cell- or pathway- specific promoters enable in vivo imaging of differentiation and signaling at the cellular level. Despite the small size of the zebrafish, many traditional techniques for skeletal phenotyping, such as x-ray and microCT imaging and histological approaches, can be applied using the appropriate equipment and custom protocols. The ability of adult zebrafish to remodel skeletal tissues can be exploited as a unique tool to investigate bone formation and repair. Finally, the permeability of embryos to chemicals dissolved in water, together with the availability of large numbers of small-sized animals makes zebrafish a perfect model for high-throughput bone anabolic drug screening. This review aims to discuss the techniques that make zebrafish a powerful model to investigate the molecular and physiological basis of skeletal disorders.