Mutations in the carboxyl-terminal propeptide of the pro alpha 1(I) chain of type I collagen result in defective chain association and produce lethal osteogenesis imperfecta

Collagen processing with mesoscale aggregates as templates and building blocks

Collagen presents a well-organized hierarchical multilevel structure. Microfibers, fibers, and fiber bundles are the aggregates of natural collagen; which achieve an ideal balance of mechanical strength and toughness at the mesoscopic scale for biological tissue. These mesostructured aggregates of collagen isolated from biological tissues retain these inherent organizational features to enable their use as building blocks for constructing new collagen materials with ideal mechanical performance, thermal and dimensional stability. This strategy is distinct from the more common bottom-up or molecular-level design and assembly approach to generating collagen materials. The present review introduces the hierarchical structure of biological collagen with a focus on mesostructural features. Isolation strategies for these collagen aggregates (CAs) are summarized. Recent progress in the use of these mesostructural components for the construction of new collagen materials with emerging applications is reviewed, including in catalysis, environmental applications, biomedicine, food packaging, electrical energy storage, and flexible sensors. Finally, challenges and prospects are assessed for controllable production of CAs as well as material designs.

Non-canonical DNA structures: Diversity and disease association

A complete understanding of DNA double-helical structure discovered by James Watson and Francis Crick in 1953, unveil the importance and significance of DNA. For the last seven decades, this has been a leading light in the course of the development of modern biology and biomedical science. Apart from the predominant B-form, experimental shreds of evidence have revealed the existence of a sequence-dependent structural diversity, unusual non-canonical structures like hairpin, cruciform, Z-DNA, multistranded structures such as DNA triplex, G-quadruplex, i-motif forms, etc. The diversity in the DNA structure depends on various factors such as base sequence, ions, superhelical stress, and ligands. In response to these various factors, the polymorphism of DNA regulates various genes via different processes like replication, transcription, translation, and recombination. However, altered levels of gene expression are associated with many human genetic diseases including neurological disorders and cancer. These non-B-DNA structures are expected to play a key role in determining genetic stability, DNA damage and repair etc. The present review is a modest attempt to summarize the available literature, illustrating the occurrence of non-canonical structures at the molecular level in response to the environment and interaction with ligands and proteins. This would provide an insight to understand the biological functions of these unusual DNA structures and their recognition as potential therapeutic targets for diverse genetic diseases.

Visualized procollagen Iα1 demonstrates the intracellular processing of propeptides

Procollagen Iα1 with two tags reveals the different fates of processed propeptides, the rate-limiting step in collagen secretion, and a link between defects in intracellular processing and diseases.

The processing of type I procollagen is essential for fibril formation; however, the steps involved remain controversial. We constructed a live cell imaging system by inserting fluorescent proteins into type I pre-procollagen α1. Based on live imaging and immunostaining, the C-propeptide is intracellularly cleaved at the perinuclear region, including the endoplasmic reticulum, and subsequently accumulates at the upside of the cell. The N-propeptide is also intracellularly cleaved, but is transported with the repeating structure domain of collagen into the extracellular region. This system makes it possible to detect relative increases and decreases in collagen secretion in a high-throughput manner by assaying fluorescence in the culture medium, and revealed that the rate-limiting step for collagen secretion occurs after the synthesis of procollagen. In the present study, we identified a defect in procollagen processing in activated hepatic stellate cells, which secrete aberrant collagen fibrils. The results obtained demonstrated the intracellular processing of type I procollagen, and revealed a link between dysfunctional processing and diseases such as hepatic fibrosis.

Effectively remove p-arsanilic acid from water over amphiphilic amino modified collagen fiber

Efficient and rapid removal of p-arsanilic acid (p-ASA) in water is very important in environmental protection and human health, however it is still a severe challenge in actual engineering. Herein, a novel sorbent (CF-PEI) was successfully fabricated by simply modifying the amphiphilic skin collagen fiber (CF) substrate with Polyethylenimine (PEI). The as-prepared CF-PEI exhibits high-efficiency adsorption for negatively charged p-ASA with aromatic rings due to the introduction of amino groups and the existence of hydrophobic bands, and the maximum adsorption capacity of CF-PEI for p-ASA was high up to 285.71 mg g^-1. In addition, the adsorption mechanism of CF-PEI on p-ASA mainly includes electrostatic interaction, hydrogen bond and amphiphilicity. The multi-level all-fiber structure of CF makes it mainly focus on surface mass transfer with short mass transfer distance, and its capillary drainage effect can realize large flow and rapid separation. CF-PEI based on CF can realize the ability to separate low-concentration p-ASA with high flow rate and high efficiency. The effective processing volume was 12.5 L g^-1 when the separation flux reached as high as 9931.27 L m^-2 h^-1. Notably, the p-ASA adsorbed on CF-PEI was almost completely eluted by NaOH (0.5 mol L^-1). The adsorbent is convenient to prepare, recyclable, high in efficiency, and has a great application prospect in removing organic micro-pollutants.

Collagen's enigmatic, highly conserved N-glycan has an essential proteostatic function

Intracellular procollagen folding begins at the protein's C-terminal propeptide (C-Pro) domain, which initiates triple-helix assembly and defines the composition and chain register of fibrillar collagen trimers. The C-Pro domain is later proteolytically cleaved and excreted from the body, while the mature triple helix is incorporated into the extracellular matrix. The procollagen C-Pro domain possesses a single N-glycosylation site that is widely conserved in all the fibrillar procollagens across humans and diverse other species. Given that the C-Pro domain is removed once procollagen folding is complete, the N-glycan might be presumed to be important for folding. Surprisingly, however, there is no difference in the folding and secretion of N-glycosylated versus non-N-glycosylated collagen type-I, leaving the function of the N-glycan unclear. We hypothesized that the collagen N-glycan might have a context-dependent function, specifically, that it could be required to promote procollagen folding only when proteostasis is challenged. We show that removal of the N-glycan from misfolding-prone C-Pro domain variants does indeed cause serious procollagen and ER proteostasis defects. The N-glycan promotes folding and secretion of destabilized C-Pro variants by providing access to the ER's lectin-based chaperone machinery. Finally, we show that the C-Pro N-glycan is actually critical for the folding and secretion of even wild-type procollagen under ER stress conditions. Such stress is commonly incurred during development, wound healing, and other processes in which collagen production plays a key role. Collectively, these results establish an essential, context-dependent function for procollagen's previously enigmatic N-glycan, wherein the carbohydrate moiety buffers procollagen folding against proteostatic challenge.

Insights into Regional Wetting Behaviors of Amphiphilic Collagen for Dual Separation of Emulsions

Industrial manufacture generates a huge quantity of emulsion wastewater, which causes serious threats to the aquatic ecosystems. Water-in-oil (W/O) and oil-in-water (O/W) emulsions are two major types of emulsions discharged by industries. However, dual separation of W/O and O/W emulsions remains a challenging issue due to the contradictory permselectivity for separating the two emulsions. In the present investigation, the amphiphilicity-derived regional wetting mechanism of water and oil on the amphiphilic collagen fibers was revealed based on the combination of numerous experiments and molecular dynamics (MD) simulations. Electrostatic interactions and van der Waals force were manifested to be the driving forces of regional wetting in the hydrophilic and hydrophobic regions, respectively. The regional wetting endowed amphiphilic collagen fibers with underwater oleophobicity and underoil hydrophilicity, which enabled dual separation of emulsions by selectively retaining the dispersed water phase of W/O emulsions in the hydrophilic regions while the dispersed oil phase of O/W emulsions in the hydrophobic regions. The achieved separation efficiency was higher than 99.98%, and the flux reached 3337.6 L m^-2 h^-1. Initial wetting status significantly affects the regional wetting-enabled dual separation. Based on the MD simulations, amphiphilic intramolecular conformations of tropocollagen were suggested to be the origins of regional wetting on collagen fibers. Our findings may pave the way for developing high-performance dual separation materials that are promising to be utilized for the practical treatment of emulsion wastewater.

COL1A1 C-propeptide mutations cause ER mislocalization of procollagen and impair C-terminal procollagen processing

Mutations in the type I procollagen C-propeptide occur in ~6.5% of Osteogenesis Imperfecta (OI) patients. They are of special interest because this region of procollagen is involved in α chain selection and folding, but is processed prior to fibril assembly and is absent in mature collagen fibrils in tissue. We investigated the consequences of seven COL1A1 C-propeptide mutations for collagen biochemistry in comparison to three probands with classical glycine substitutions in the collagen helix near the C-propeptide and a normal control. Procollagens with C-propeptide defects showed the expected delayed chain incorporation, slow folding and overmodification. Immunofluorescence microscopy indicated that procollagen with C-propeptide defects was mislocalized to the ER lumen, in contrast to the ER membrane localization of normal procollagen and procollagen with helical substitutions. Notably, pericellular processing of procollagen with C-propeptide mutations was defective, with accumulation of pC-collagen and/or reduced production of mature collagen. In vitro cleavage assays with BMP-1 ± PCPE-1 confirmed impaired C-propeptide processing of procollagens containing mutant proα1(I) chains. Overmodified collagens were incorporated into the matrix in culture. Dermal fibrils showed alterations in average diameter and diameter variability and bone fibrils were disorganized. Altered ER-localization and reduced pericellular processing of defective C-propeptides are expected to contribute to abnormal osteoblast differentiation and matrix function, respectively.

A cysteine-based molecular code informs collagen C-propeptide assembly

Fundamental questions regarding collagen biosynthesis, especially with respect to the molecular origins of homotrimeric versus heterotrimeric assembly, remain unanswered. Here, we demonstrate that the presence or absence of a single cysteine in type-I collagen’s C-propeptide domain is a key factor governing the ability of a given collagen polypeptide to stably homotrimerize. We also identify a critical role for Ca²⁺ in non-covalent collagen C-propeptide trimerization, thereby priming the protein for disulfide-mediated covalent immortalization. The resulting cysteine-based code for stable assembly provides a molecular model that can be used to predict, a priori, the identity of not just collagen homotrimers, but also naturally occurring 2:1 and 1:1:1 heterotrimers. Moreover, the code applies across all of the sequence-diverse fibrillar collagens. These results provide new insight into how evolution leverages disulfide networks to fine-tune protein assembly, and will inform the ongoing development of designer proteins that assemble into specific oligomeric forms.

Collagen proteins assemble into trimers from distinct monomers with high specificity, yet the molecular basis for this specificity remains unclear. Here the authors demonstrate the crucial role of conserved C-terminal domain cysteine residues and calcium in homotrimeric procollagen assembly.

Osteogenesis imperfecta and therapeutics

Osteogenesis imperfecta, or brittle bone disease, is a congenital disease that primarily causes low bone mass and bone fractures but it can negatively affect other organs. It is usually inherited in an autosomal dominant fashion, although rarer recessive and X-chromosome-linked forms of the disease have been identified. In addition to type I collagen, mutations in a number of other genes, often involved in type I collagen synthesis or in the differentiation and function of osteoblasts, have been identified in the last several years. Seldom, the study of a rare disease has delivered such a wealth of new information that have helped our understanding of multiple processes involved in collagen synthesis and bone formation. In this short review I will describe the clinical features and the molecular genetics of the disease, but then focus on how OI dysregulates all aspects of extracellular matrix biology. I will conclude with a discussion about OI therapeutics.

The unfolded protein response in skeletal development and homeostasis

Osteoblasts and chondrocytes produce a large number of extracellular matrix proteins to generate and maintain the skeletal system. To cope with their functions as secretory cells, these cells must acquire a considerable capacity for protein synthesis and also the machinery for the quality-control and transport of newly synthesized secreted proteins. The unfolded protein response (UPR) plays a crucial role during the differentiation of these cells to achieve this goal. Unexpectedly, however, studies in the past several years have revealed that the UPR has more extensive functions in skeletal development than was initially assumed, and the UPR critically orchestrates many facets of skeletal development and homeostasis. This review focuses on recent findings on the functions of the UPR in the differentiation of osteoblasts, chondrocytes, and osteoclasts. These findings may have a substantial impact on our understanding of bone metabolism and also on establishing treatments for congenital and acquired skeletal disorders.

Absence of the ER Cation Channel TMEM38B/TRIC-B Disrupts Intracellular Calcium Homeostasis and Dysregulates Collagen Synthesis in Recessive Osteogenesis Imperfecta

Recessive osteogenesis imperfecta (OI) is caused by defects in proteins involved in post-translational interactions with type I collagen. Recently, a novel form of moderately severe OI caused by null mutations in TMEM38B was identified. TMEM38B encodes the ER membrane monovalent cation channel, TRIC-B, proposed to counterbalance IP₃R-mediated Ca²⁺ release from intracellular stores. The molecular mechanisms by which TMEM38B mutations cause OI are unknown. We identified 3 probands with recessive defects in TMEM38B. TRIC-B protein is undetectable in proband fibroblasts and osteoblasts, although reduced TMEM38B transcripts are present. TRIC-B deficiency causes impaired release of ER luminal Ca²⁺, associated with deficient store-operated calcium entry, although SERCA and IP₃R have normal stability. Notably, steady state ER Ca²⁺ is unchanged in TRIC-B deficiency, supporting a role for TRIC-B in the kinetics of ER calcium depletion and recovery. The disturbed Ca²⁺ flux causes ER stress and increased BiP, and dysregulates synthesis of proband type I collagen at multiple steps. Collagen helical lysine hydroxylation is reduced, while telopeptide hydroxylation is increased, despite increased LH1 and decreased Ca²⁺-dependent FKBP65, respectively. Although PDI levels are maintained, procollagen chain assembly is delayed in proband cells. The resulting misfolded collagen is substantially retained in TRIC-B null cells, consistent with a 50–70% reduction in secreted collagen. Lower-stability forms of collagen that elude proteasomal degradation are not incorporated into extracellular matrix, which contains only normal stability collagen, resulting in matrix insufficiency. These data support a role for TRIC-B in intracellular Ca²⁺ homeostasis, and demonstrate that absence of TMEM38B causes OI by dysregulation of calcium flux kinetics in the ER, impacting multiple collagen-specific chaperones and modifying enzymes.

Osteogenesis imperfecta (OI) is a heritable disorder of connective tissues characterized by fracture susceptibility and growth deficiency. Most OI cases are caused by autosomal dominant mutations in the genes encoding type I collagen, COL1A1 and COL1A2. Delineation of novel gene defects causing dominant and recessive forms of OI has led to the understanding that the bone pathology results not only from abnormalities in type I collagen quantity and primary structure, but also from defects in post-translational modification, folding, intracellular transport and extracellular matrix incorporation. Recently, mutations in TMEM38B, which encodes the integral ER membrane K⁺ channel TRIC-B, have been identified as causative for the OI phenotype. However, the mechanism by which absence of TRIC-B causes OI has not been reported. Using cell lines established from three independent probands, we have demonstrated that absence of TRIC-B leads to abnormal ER Ca²⁺ flux and store-operated calcium entry (SOCE), although ER steady state Ca²⁺ is normal. Disruption of intracellular calcium dynamics alters the expression and activity of multiple collagen interacting chaperones and modifying enzymes within the ER. Thus TRIC-B deficiency causes OI by dysregulation of collagen synthesis, through the impairment of calcium-dependent gene expression and protein-protein interactions within the ER.

Extracellular matrix structure

Extracellular matrix (ECM) is a non-cellular three-dimensional macromolecular network composed of collagens, proteoglycans/glycosaminoglycans, elastin, fibronectin, laminins, and several other glycoproteins. Matrix components bind each other as well as cell adhesion receptors forming a complex network into which cells reside in all tissues and organs. Cell surface receptors transduce signals into cells from ECM, which regulate diverse cellular functions, such as survival, growth, migration, and differentiation, and are vital for maintaining normal homeostasis. ECM is a highly dynamic structural network that continuously undergoes remodeling mediated by several matrix-degrading enzymes during normal and pathological conditions. Deregulation of ECM composition and structure is associated with the development and progression of several pathologic conditions. This article emphasizes in the complex ECM structure as to provide a better understanding of its dynamic structural and functional multipotency. Where relevant, the implication of the various families of ECM macromolecules in health and disease is also presented.

Type I procollagen C-propeptide defects: study of genotype-phenotype correlation and predictive role of crystal structure

The type I procollagen carboxyterminal(C-)propeptides are crucial in directing correct assembly of the procollagen heterotrimers. Defects in these domains have anecdotally been reported in patients with Osteogenesis Imperfecta (OI) and few genotype-phenotype correlations have been described. To gain insight in the functional consequences of C-propeptide defects, we performed a systematic review of clinical, molecular, and biochemical findings in all patients in whom we identified a type I procollagen C-propeptide defect, and compared this with literature data. We report 30 unique type I procollagen C-propeptide variants, 24 of which are novel. The outcome of COL1A1 nonsense and frameshift variants depends on the location of the premature termination codon. Those located prior to 50-55 nucleotides upstream of the most 3' exon-exon junction lead to nonsense-mediated mRNA decay (NMD) and cause mild OI. Those located beyond this boundary escape NMD, generally lead to production of stable, overmodified procollagen chains, which may partly be retained intracellularly, and are usually associated with severe-to-lethal OI. Proα1(I)-C-propeptide defects that permit chain association result in more severe phenotypes than those inhibiting chain association. We demonstrate that the crystal structure of the proα1(III)-C-propeptide is a reliable tool to predict phenotypic severity for most COL1A1-C-propeptide missense variants, whereas for COL1A2-C-propeptide variants, the phenotypic outcome is milder than predicted.

Mutational and structural characteristics of four novel heterozygous C-propeptide mutations in the proα1(I) collagen gene in Chinese osteogenesis imperfecta patients

OBJECTIVE

Osteogenesis imperfecta (OI) with C-propeptide mutations in proα1(I) collagen gene are rarely reported. We report four novel C-propeptide mutations in COL1A1 gene from Chinese OI patients.

METHODS

Clinical characteristics and radiographic findings were described for four OI patients with C-propeptide mutations in proα1(I) collagen gene. Mutations were identified by traditional DNA sequencing based on PCR. The locations of mutations were mapped, and in silico prediction was conducted to analyse their effects on protein structure. Histology studies of skin, bone and muscle tissues were performed.

RESULTS

All four C-propeptide heterozygous mutations identified were in the COL1A1 gene. Heterozygous mutation of c.4021C>T (p.Q1341X) disrupted the chain recognition sequences and was found in patients with type IV OI. Mutations of c.3893C>A (p.T1298N) and c.3897C>A (p.C1299X) impeded the formation of disulphide bonds and were associated with type IV OI phenotype. Missense mutation of c.3835A>C (p.N1279H) disrupted Ca(2+) binding and led to a severe type III OI phenotype. In silico programs predicted damaging effects for the patients with type III OI and the creation of an exonic splicing enhancer hexamer sequence for the type IV patients. Expansion of the bone marrow cavity and disorganization of osteocyte alignment was evident in bone specimens; and muscle atrophy and enlargement of intramuscular connective tissue were found in muscle specimens.

CONCLUSIONS

Four novel C-propeptide mutations in proα1(I) collagen gene were identified in Chinese OI patients, and their clinical severity ranged from moderate type IV to severe type III. In silico prediction of the mutation effect and histological characteristics of tissue specimens was in accordance with the OI phenotypes.

Osteogenesis imperfecta IIC caused by a novel heterozygous mutation in the C-propeptide region of COL1A1

Osteogenesis imperfecta IIC (OI IIC), which is a rare variant of lethal OI that has been considered to be an autosomal recessive trait, is characterized by twisted, slender long bones with dense metaphyseal margins. Here, we report a typical case of OI IIC caused by a novel heterozygous mutation in the C-propeptide region of COL1A1. OI IIC seems to be caused by a dominant mutation of COL1A1.

Identification of a novel heterozygous mutation in exon 50 of the COL1A1 gene causing osteogenesis imperfecta

A 19-year-old woman was diagnosed with osteogenesis imperfecta (OI). She had sustained numerous low-trauma fractures throughout her childhood, including a recent pelvic fracture (superior and inferior ramus) following a low-impact fall. She had the classical blue sclerae, and dual energy X-ray absorptiometry (DEXA) bone scanning confirmed low bone mass for her age in the lumbar spine (Z-score was −2.6). However, despite these classical clinical features, the diagnosis of OI had not been entertained throughout the whole of her childhood. Sequencing of her genomic DNA revealed that she was heterozygous for the c.3880_3883dup mutation in exon 50 of the COL1A1 gene. This mutation is predicted to result in a frameshift at p.Thr1295, and truncating stop codon 3 amino acids downstream. To our knowledge, this mutation has not previously been reported in OI.

Absence of FKBP10 in recessive type XI osteogenesis imperfecta leads to diminished collagen cross-linking and reduced collagen deposition in extracellular matrix

Recessive osteogenesis imperfecta (OI) is caused by defects in genes whose products interact with type I collagen for modification and/or folding. We identified a Palestinian pedigree with moderate and lethal forms of recessive OI caused by mutations in FKBP10 or PPIB, which encode endoplasmic reticulum resident chaperone/isomerases FKBP65 and CyPB, respectively. In one pedigree branch, both parents carry a deletion in PPIB (c.563_566delACAG), causing lethal type IX OI in their two children. In another branch, a child with moderate type XI OI has a homozygous FKBP10 mutation (c.1271_1272delCCinsA). Proband FKBP10 transcripts are 4% of control and FKBP65 protein is absent from proband cells. Proband collagen electrophoresis reveals slight band broadening, compatible with ≈10% over-modification. Normal chain incorporation, helix folding, and collagen T(m) support a minimal general collagen chaperone role for FKBP65. However, there is a dramatic decrease in collagen deposited in culture despite normal collagen secretion. Mass spectrometry reveals absence of hydroxylation of the collagen telopeptide lysine involved in cross-linking, suggesting that FKBP65 is required for lysyl hydroxylase activity or access to type I collagen telopeptide lysines, perhaps through its function as a peptidylprolyl isomerase. Proband collagen to organics ratio in matrix is approximately 30% of normal in Raman spectra. Immunofluorescence shows sparse, disorganized collagen fibrils in proband matrix.

Structural basis of fibrillar collagen trimerization and related genetic disorders

The C propeptides of fibrillar procollagens have crucial roles in tissue growth and repair by controlling both the intracellular assembly of procollagen molecules and the extracellular assembly of collagen fibrils. Mutations in C propeptides are associated with several, often lethal, genetic disorders affecting bone, cartilage, blood vessels and skin. Here we report the crystal structure of a C-propeptide domain from human procollagen III. It reveals an exquisite structural mechanism of chain recognition during intracellular trimerization of the procollagen molecule. It also gives insights into why some types of collagen consist of three identical polypeptide chains, whereas others do not. Finally, the data show striking correlations between the sites of numerous disease-related mutations in different C-propeptide domains and the degree of phenotype severity. The results have broad implications for understanding genetic disorders of connective tissues and designing new therapeutic strategies.

Production and crystallization of the C-propeptide trimer from human procollagen III

The C-propeptide domains of the fibrillar procollagens, which are present throughout the Metazoa in the form of ∼90 kDa trimers, play crucial roles in both intracellular molecular assembly and extracellular formation of collagen fibrils. The first crystallization of a C-propeptide domain, that from human procollagen III, is described. Following transient expression in mammalian 293T cells of both the native protein and a selenomethionine derivative, two crystal forms of the homotrimer were obtained: an orthorhombic form (P2(1)2(1)2(1)) that diffracted to 1.7 Å resolution and a trigonal form (P321) that diffracted to 3.5 Å resolution. Characterization by MALDI-TOF mass spectrometry allowed the efficiency of selenomethionine incorporation to be determined.

Identification of a mutation causing deficient BMP1/mTLD proteolytic activity in autosomal recessive osteogenesis imperfecta

Herein, we have studied a consanguineous Egyptian family with two children diagnosed with severe autosomal recessive osteogenesis imperfecta (AR-OI) and a large umbilical hernia. Homozygosity mapping in this family showed lack of linkage to any of the previously known AR-OI genes, but revealed a 10.27 MB homozygous region on chromosome 8p in the two affected sibs, which comprised the procollagen I C-terminal propeptide (PICP) endopeptidase gene BMP1. Mutation analysis identified both patients with a Phe249Leu homozygous missense change within the BMP1 protease domain involving a residue, which is conserved in all members of the astacin group of metalloproteases. Type I procollagen analysis in supernatants from cultured fibroblasts demonstrated abnormal PICP processing in patient-derived cells consistent with the mutation causing decreased BMP1 function. This was further confirmed by overexpressing wild type and mutant BMP1 longer isoform (mammalian Tolloid protein [mTLD]) in NIH3T3 fibroblasts and human primary fibroblasts. While overproduction of normal mTLD resulted in a large proportion of proα1(I) in the culture media being C-terminally processed, proα1(I) cleavage was not enhanced by an excess of the mutant protein, proving that the Phe249Leu mutation leads to a BMP1/mTLD protein with deficient PICP proteolytic activity. We conclude that BMP1 is an additional gene mutated in AR-OI.

New perspectives on osteogenesis imperfecta

A new paradigm has emerged for osteogenesis imperfecta as a collagen-related disorder. The more prevalent autosomal dominant forms of osteogenesis imperfecta are caused by primary defects in type I collagen, whereas autosomal recessive forms are caused by deficiency of proteins which interact with type I procollagen for post-translational modification and/or folding. Factors that contribute to the mechanism of dominant osteogenesis imperfecta include intracellular stress, disruption of interactions between collagen and noncollagenous proteins, compromised matrix structure, abnormal cell-cell and cell-matrix interactions and tissue mineralization. Recessive osteogenesis imperfecta is caused by deficiency of any of the three components of the collagen prolyl 3-hydroxylation complex. Absence of 3-hydroxylation is associated with increased modification of the collagen helix, consistent with delayed collagen folding. Other causes of recessive osteogenesis imperfecta include deficiency of the collagen chaperones FKBP10 or Serpin H1. Murine models are crucial to uncovering the common pathways in dominant and recessive osteogenesis imperfecta bone dysplasia. Clinical management of osteogenesis imperfecta is multidisciplinary, encompassing substantial progress in physical rehabilitation and surgical procedures, management of hearing, dental and pulmonary abnormalities, as well as drugs, such as bisphosphonates and recombinant human growth hormone. Novel treatments using cell therapy or new drug regimens hold promise for the future.

COL1 C-propeptide cleavage site mutations cause high bone mass osteogenesis imperfecta

Osteogenesis imperfecta (OI) is most often caused by mutations in the type I procollagen genes (COL1A1/COL1A2). We identified two children with substitutions in the type I procollagen C-propeptide cleavage site, which disrupt a unique processing step in collagen maturation and define a novel phenotype within OI. The patients have mild OI caused by mutations in COL1A1 (Patient 1: p.Asp1219Asn) or COL1A2 (Patient 2: p.Ala1119Thr), respectively. Patient 1 L1-L4 DXA Z-score was +3.9 and pQCT vBMD was+3.1; Patient 2 had L1-L4 DXA Z-score of 0.0 and pQCT vBMD of -1.8. Patient BMD contrasts with radiographic osteopenia and histomorphometry without osteosclerosis. Mutant procollagen processing is impaired in pericellular and in vitro assays. Patient dermal collagen fibrils have irregular borders. Incorporation of pC-collagen into matrix leads to increased bone mineralization. FTIR imaging confirms elevated mineral/matrix ratios in both patients, along with increased collagen maturation in trabecular bone, compared to normal or OI controls. Bone mineralization density distribution revealed a marked shift toward increased mineralization density for both patients. Patient 1 has areas of higher and lower bone mineralization than controls; Patient 2's bone matrix has a mineral content exceeding even classical OI bone. These patients define a new phenotype of high BMD OI and demonstrate that procollagen C-propeptide cleavage is crucial to normal bone mineralization.

Mutation characteristics in type I collagen genes in Chinese patients with osteogenesis imperfecta

Osteogenesis imperfecta is normally caused by an autosomal dominant mutation in the type I collagen genes COL1A1 and COL1A2. The severity of osteogenesis imperfecta varies, ranging from perinatal lethality to a very mild phenotype. Although there have been many reports of COL1A1 and COL1A2 mutations, few cases have been reported in Chinese people. We report on five unrelated families and three sporadic cases. The mutations were detected by PCR and direct sequencing. Four mutations in COL1A1 and one in COL1A2 were found, among which three mutations were previously unreported. The mutation rates of G>C at base 128 in intron 31 of the COL1A1 gene and G>A at base 162 in intron 30 of the COL1A2 gene were higher than normal. The patients' clinical characteristics with the same mutation were variable even in the same family. We conclude that mutations in COL1A1 and COL1A2 also have an important role in osteogenesis imperfecta in the Chinese population. As the Han Chinese people account for a quarter of the world's population, these new data contribute to the type I collagen mutation map.

Mutations in PPIB (cyclophilin B) delay type I procollagen chain association and result in perinatal lethal to moderate osteogenesis imperfecta phenotypes

Recessive mutations in the cartilage-associated protein (CRTAP), leucine proline-enriched proteoglycan 1 (LEPRE1) and peptidyl prolyl cis-trans isomerase B (PPIB) genes result in phenotypes that range from lethal in the perinatal period to severe deforming osteogenesis imperfecta (OI). These genes encode CRTAP (encoded by CRTAP), prolyl 3-hydroxylase 1 (P3H1; encoded by LEPRE1) and cyclophilin B (CYPB; encoded by PPIB), which reside in the rough endoplasmic reticulum (RER) and can form a complex involved in prolyl 3-hydroxylation in type I procollagen. CYPB, a prolyl cis-trans isomerase, has been thought to drive the prolyl-containing peptide bonds to the trans configuration needed for triple helix formation. Here, we describe mutations in PPIB identified in cells from three individuals with OI. Cultured dermal fibroblasts from the most severely affected infant make some overmodified type I procollagen molecules. Proα1(I) chains are slow to assemble into trimers, and abnormal procollagen molecules concentrate in the RER, and bind to protein disulfide isomerase (PDI) and prolyl 4-hydroxylase 1 (P4H1). These findings suggest that although CYPB plays a role in helix formation another effect is on folding of the C-terminal propeptide and trimer formation. The extent of procollagen accumulation and PDI/P4H1 binding differs among cells with mutations in PPIB, CRTAP and LEPRE1 with the greatest amount in PPIB-deficient cells and the least in LEPRE1-deficient cells. These findings suggest that prolyl cis-trans isomerase may be required to effectively fold the proline-rich regions of the C-terminal propeptide to allow proα chain association and suggest an order of action for CRTAP, P3H1 and CYPB in procollagen biosynthesis and pathogenesis of OI.

Mutation and polymorphism spectrum in osteogenesis imperfecta type II: implications for genotype-phenotype relationships

Osteogenesis imperfecta (OI), also known as brittle bone disease, is a clinically and genetically heterogeneous disorder primarily characterized by susceptibility to fracture. Although OI generally results from mutations in the type I collagen genes, COL1A1 and COL1A2, the relationship between genotype and phenotype is not yet well understood. To provide additional data for genotype–phenotype analyses and to determine the proportion of mutations in the type I collagen genes among subjects with lethal forms of OI, we sequenced the coding and exon-flanking regions of COL1A1 and COL1A2 in a cohort of 63 subjects with OI type II, the perinatal lethal form of the disease. We identified 61 distinct heterozygous mutations in type I collagen, including five non-synonymous rare variants of unknown significance, of which 43 had not been seen previously. In addition, we found 60 SNPs in COL1A1, of which 17 were not reported previously, and 82 in COL1A2, of which 18 are novel. In three samples without collagen mutations, we found inactivating mutations in CRTAP and LEPRE1, suggesting a frequency of these recessive mutations of ∼5% in OI type II. A computational model that predicts the outcome of substitutions for glycine within the triple helical domain of collagen α1(I) chains predicted lethality with ∼90% accuracy. The results contribute to the understanding of the etiology of OI by providing data to evaluate and refine current models relating genotype to phenotype and by providing an unbiased indication of the relative frequency of mutations in OI-associated genes.

ER stress-mediated apoptosis in a new mouse model of osteogenesis imperfecta

Osteogenesis imperfecta is an inherited disorder characterized by increased bone fragility, fractures, and osteoporosis, and most cases are caused by mutations affecting the type I collagen genes. Here, we describe a new mouse model for Osteogenesis imperfecta termed Aga2 (abnormal gait 2) that was isolated from the Munich N-ethyl-N-nitrosourea mutagenesis program and exhibited phenotypic variability, including reduced bone mass, multiple fractures, and early lethality. The causal gene was mapped to Chromosome 11 by linkage analysis, and a C-terminal frameshift mutation was identified in the Col1a1 (procollagen type I, alpha 1) gene as the cause of the disorder. Aga2 heterozygous animals had markedly increased bone turnover and a disrupted native collagen network. Further studies showed that abnormal proα1(I) chains accumulated intracellularly in Aga2/+ dermal fibroblasts and were poorly secreted extracellularly. This was associated with the induction of an endoplasmic reticulum stress-specific unfolded protein response involving upregulation of BiP, Hsp47, and Gadd153 with caspases-12 and −3 activation and apoptosis of osteoblasts both in vitro and in vivo. These studies resulted in the identification of a new model for Osteogenesis imperfecta, and identified a role for intracellular modulation of the endoplasmic reticulum stress-associated unfolded protein response machinery toward osteoblast apoptosis during the pathogenesis of disease.

Osteogenesis imperfecta (OI) is a heterogeneous collection of connective tissue disorders typically caused by mutations in the COL1A1/2 genes that encode the chains of type I collagen, the principle structural protein of bone. Phenotypic expression in OI depends on the nature of the mutation, causing a clinical heterogeneity ranging from a mild risk of fractures to perinatal lethality. Here, we describe a new OI mouse model with a dominant mutation in the terminal C-propeptide domain of Col1a1 generated using the N-ethyl-N-nitrosourea (ENU) mutagenesis strategy. Heterozygous animals developed severe-to-lethal phenotypes that were associated with endoplasmic reticulum stress, and caspases-12 and −3 activation within calvarial osteoblasts. We provide evidence for endoplasmic reticulum stress–associated apoptosis as a key component in the pathogenesis of disease.

Defective C-propeptides of the proalpha2(I) chain of type I procollagen impede molecular assembly and result in osteogenesis imperfecta

Type I procollagen is a heterotrimer composed of two proalpha1(I) chains and one proalpha2(I) chain, encoded by the COL1A1 and COL1A2 genes, respectively. Mutations in these genes usually lead to dominantly inherited forms of osteogenesis imperfecta (OI) by altering the triple helical domains, but a few affect sequences in the proalpha1(I) C-terminal propeptide (C-propeptide), and one, which has a phenotype only in homozygotes, alters the proalpha2(I) C-propeptide. Here we describe four dominant mutations in the COL1A2 gene that alter sequences of the proalpha2(I) C-propeptide in individuals with clinical features of a milder form of the disease, OI type IV. Three of the four appear to interfere with disulfide bonds that stabilize the C-propeptide conformation and its interaction with other chains in the trimer. Cultured cells synthesized proalpha2(I) chains that were slow to assemble with proalpha1(I) chains to form heterotrimers and that were retained intracellularly. Some alterations led to the uncharacteristic formation of proalpha1(I) homotrimers. These findings show that the C-propeptide of proalpha2(I), like that of the proalpha1(I) C-propeptide, is essential for efficient assembly of type I procollagen heterotrimers. The milder OI phenotypes likely reflect a diminished amount of normal type I procollagen, small populations of overmodified heterotrimers, and proalpha1(I) homotrimers that are compatible with normal skeletal growth.

Selective retention and degradation of molecules with a single mutant alpha1(I) chain in the Brtl IV mouse model of OI

We investigated the secretion, matrix incorporation and interactions of molecules with one and two mutant alpha1(I) collagen chains in the Brtl IV murine model for Osteogenesis Imperfecta, carrying a Gly-349 to Cys substitution in one col1a1 allele. We detected a significant deviation from the expected 25 and 50% content of the molecules with no (37-46%) and one (26-40%) mutant chains in skin and bone as well as in fibroblast and osteoblast cell culture media. Steady-state labeling with (35)S-Cys demonstrated incomplete secretion of the mutant collagen in cell culture, particularly molecules containing one mutant chain. Pulse and pulse-chase experiments revealed slower secretion of the latter. An enlargement of endoplasmic reticulum in skin fibroblasts from Brtl IV mice, clearly visible by electron microscopy, supported the abnormal secretion identified by biochemical studies. We observed increased susceptibility of molecules with one mutant chain to proteolytic degradation in vitro, but we did not detect significant selective degradation in cell culture media. Mutant collagen molecules incorporated from the media into newly deposited fibers and into fully crosslinked and mature matrix in the same ratio as they were secreted. Specific labeling of reactive -SH demonstrated that about half of the Cys349-SH groups in the mutant molecules were exposed and potentially available for aberrant interactions with other molecules inside or outside the cells. Based on these and our previous findings, we argue that the outcome in Brtl IV may be significantly affected by cellular stress and malfunction caused by the retention and degradation of newly synthesized mutant collagen.

Endoplasmic reticulum stress induces hyaluronan deposition and leukocyte adhesion

There is mounting evidence that perturbations in endoplasmic reticulum (ER) function play a key role in the pathogenesis of a broad range of diseases. We have examined the ability of ER stress to modulate leukocyte binding to colonic and aortic smooth muscle cells. In vitro, control smooth muscle cells bind few leukocytes, but treatment with compounds that induce ER stress, including tunicamycin, A23187, and thapsigargin, promotes leukocyte binding. Likewise, dextran sulfate, another agent capable of inducing ER stress and promoting inflammation in vivo, strongly induces leukocyte adhesion. The bound leukocytes are released by hyaluronidase treatment, indicating a critical role for hyaluronan-containing structures in mediating leukocyte binding. Affinity histochemistry demonstrated that hyaluronan accumulates and is present in cable-like structures in the treated, but not the untreated, cultures and that these structures serve as attachment sites for leukocytes. Hyaluronan-rich regions of both murine and human inflamed colon contain numerous cells that stain intensely for ER-resident chaperones containing the KDEL sequence, demonstrating a relationship between ER stress and hyaluronan deposition in vivo. These results indicate that ER stress may contribute to chronic inflammation by forming a hyaluronan-rich extracellular matrix that is conducive to leukocyte binding.

HSP47 as a collagen-specific molecular chaperone: function and expression in normal mouse development

A large family of molecular chaperones can be divided into two major groups: general chaperone and substrate-specific chaperone. HSP47 is a collagen-specific molecular chaperone residing in the endoplasmic reticulum (ER). Recent studies revealed that HSP47 is essential molecular chaperone for mouse development and is essential for collagen molecular maturation in the ER. In the absence of HSP47, collagen microfibril formation and basement membrane formation are impaired in mouse embryos because the failure in the molecular maturation of types I and IV collagens, respectively. The tissue-specific expression of HSP47 is always correlated with that of various types of collagens and closely related with the collagen-related diseases including fibrosis in various organs. The importance of HSP47 in the therapeutic strategy for fibrotic diseases as well as for a marker of collagen-related autoimmune diseases will also be discussed.

Trimerization of the amino propeptide of type IIA procollagen using a 14-amino acid sequence derived from the coiled-coil neck domain of surfactant protein D

The folding of a collagen triple helix usually requires the presence of additional sequences that contribute to the association and correct alignment of the collagen chains. We recently reported that the C-terminal neck and lectin domains of a collagenous C-type lectin, rat pulmonary surfactant protein D (SP-D), are sufficient to drive the trimerization of a heterologous type IIA procollagen amino propeptide sequence. However, the conformation of the resulting trimeric IIA propeptide and the specific contributions of the SP-D sequence to trimerization were not elucidated. In the present study, we show that trimerization of the fusion protein is associated with correct folding of the collagen helix within the IIA propeptide domain (as assessed by circular dichroism) and that the constituent chains are hydroxylated. Chemical cross-linking and analytical ultracentrifugation showed that the IIA amino-propeptide retains its trimeric configuration even after proteolytic removal of the SP-D domains. By contrast, IIA amino-propeptides synthesized without fusion to the neck or lectin domains are assembled exclusively as monomers. To localize the trimerization sequence, mutant chimeric cDNA constructs were designed containing premature termination codons within the coiled-coil neck domain. A short, 14-amino acid sequence corresponding to the first two heptad repeats of the neck domain was sufficient to drive the trimeric association of the IIA amino-propeptide alpha-chains. However, deletion of the collagen domain resulted in the secretion of monomers. These studies demonstrate that two heptad repeats are sufficient for trimeric association of the propeptide but indicate that cooperative interactions between the coiled-coil and collagen domains are required for the formation of a stable helix.

Deletions and duplications of Gly-Xaa-Yaa triplet repeats in the triple helical domains of type I collagen chains disrupt helix formation and result in several types of osteogenesis imperfecta

Triple helix formation is a prerequisite for the passage of type I procollagen from the endoplasmic reticulum and secretion from the cell to form extracellular fibrils that will support mineral deposition in bone. Analysis of cDNA from 11 unrelated individuals with osteogenesis imperfecta (OI) revealed the presence of 11 novel, short in-frame deletions or duplications of three, nine, or 18 nucleotides in the helical coding regions of the COL1A1 and COL1A2 collagen genes. Triple helix formation was impaired, type I collagen alpha chains were post-translationally overmodified, and extracellular secretion was markedly reduced. With one exception, the obligate Gly-Xaa-Yaa repeat pattern of amino acids in the helical domains was not altered, but the Xaa- and Yaa position residues were out of register relative to the amino acid sequences of adjacent chains in the triple helix. Thus, the identity of these amino acids, in addition to third position glycines, is important for normal helix formation. These findings expand the known repertoire of uncommon in-frame deletions and duplications in OI, and provide insight into normal collagen biosynthesis and collagen triple helix formation.

The amino-terminal heptad repeats of the coiled-coil neck domain of pulmonary surfactant protein d are necessary for the assembly of trimeric subunits and dodecamers

Pulmonary surfactant protein D (SP-D), a lung host defense protein, is assembled as multimers of trimeric subunits. Trimerization of SP-D monomers is required for high affinity saccharide binding, and the oligomerization of trimers is required for many of its functions. A peptide containing the alpha-helical neck region can spontaneously trimerize in vitro. However, it is not known whether this sequence is necessary for the complete cellular assembly of disulfide-cross-linked, trimeric subunits and dodecamers. For the present studies, we synthesized mutant cDNAs with deletions or site-directed substitutions in the neck domain of rat SP-D, and examined the assembly of the newly synthesized proteins after transfection of CHO-K1 cells. The neck domain contains three "classical" heptad repeat motifs with leucine residues at the "d position," and a distinctive C-terminal repeat previously suggested to drive trimeric chain association. Deletion of the highly conserved core of the latter repeat (FSRYLKK) did not interfere with the secretion of dodecamers with lectin activity. By contrast, deletion of the entire neck domain or deletion of one or two amino-terminal repeats resulted in defective molecular assembly. The secreted proteins eluted in the position of monomers by gel filtration under nondenaturing conditions. In addition, the neck + carbohydrate recognition domain of SP-D was necessary and sufficient for the trimerization of a heterologous collagen sequence located amino-terminal to the trimeric coiled-coil. These studies provide strong evidence that the amino-terminal heptad repeats of the neck domain are necessary for the intracellular, trimeric association of SP-D monomers and for the assembly and secretion of functional dodecamers.

Canine COL1A2 mutation resulting in C-terminal truncation of pro-alpha2(I) and severe osteogenesis imperfecta

RNA and type I collagen were analyzed from cultured skin fibroblasts of a Beagle puppy with fractures consistent with type III osteogenesis imperfecta (OI). In a nonisotopic RNAse cleavage assay (NIRCA), the proband's RNA had a unique cleavage pattern in the region of COL1A2 encoding the C-propeptide. DNA sequence analyses identified a mutation in which nucleotides 3991-3994 ("CTAG") were replaced with "TGTCATTGG." The first seven bases of the inserted sequence were identical to nucleotides 4002-4008 of the normal canine COL1A2 sequence. The resulting frameshift changed 30 amino acids and introduced a premature stop codon. Reverse-transcription polymerase chain reaction (RT-PCR) with primers flanking the mutation site amplified two complementary DNA (cDNA) fragments for the proband and a single product for the control. Restriction enzyme digestions also were consistent with a heterozygous mutation in the proband. Type I procollagen labeled with [3H]proline was analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). Increased density of pC-alpha2(I) suggested comigration with the similarly sized pro-alpha2(I) derived from the mutant allele. Furthermore, a-chains were overhydroxylated and the ratio of alpha1(I):alpha2(I) was 3.2:1, consistent with the presence of alpha1(I) homotrimers. Analyses of COL1A2 and type I collagen were both consistent with the described heterozygous mutation affecting the pro-alpha2(I) C-propeptide and confirmed a diagnosis of OI.

Collagens: building blocks at the end of the development line

Collagens are some of the major building blocks of the vertebrate body. In addition to their structural role, they are important for cell guidance during development and for maintaining tissue integrity. In their absence, phenotypes range from lethal to mild. These studies demonstrate that collagens,in their rich array, play important roles in development and are significant elements in reading the developmental code.

Procollagen folding and assembly: the role of endoplasmic reticulum enzymes and molecular chaperones

Procollagen assembly occurs within the endoplasmic reticulum, where the C-propeptide domains of three polypeptide alpha-chains fold individually, and then interact and trimerise to initiate folding of the triple helical region. This highly complex folding and assembly pathway requires the co-ordinated action of a large number of endoplasmic reticulum-resident enzymes and molecular chaperones. Disease-causing mutations in the procollagens disturb folding and assembly and lead to prolonged interactions with molecular chaperones, retention in the endoplasmic reticulum, and intracellular degradation. This review focuses predominantly on prolyl 1-hydroxylase, an essential collagen modifying enzyme, and HSP47, a collagen-specific binding protein, and their proposed roles as molecular chaperones involved in fibrillar procollagen folding and assembly, quality control, and secretion.

Proteasomal degradation of unassembled mutant type I collagen pro-alpha1(I) chains

We have previously shown that type I procollagen pro-alpha1(I) chains from an osteogenesis imperfecta patient (OI26) with a frameshift mutation resulting in a truncated C-propeptide, have impaired assembly, and are degraded by an endoplasmic reticulum-associated pathway (Lamandé, S. R., Chessler, S. D., Golub, S. B., Byers, P. H., Chan, D., Cole, W. G., Sillence, D. O. and Bateman, J. F. (1995) J. Biol. Chem. 270, 8642-8649). To further explore the degradation of procollagen chains with mutant C-propeptides, mouse Mov13 cells, which produce no endogenous pro-alpha1(I), were stably transfected with a pro-alpha1(I) expression construct containing a frameshift mutation that predicts the synthesis of a protein 85 residues longer than normal. Despite high levels of mutant mRNA in transfected Mov13 cells, only minute amounts of mutant pro-alpha1(I) could be detected indicating that the majority of the mutant pro-alpha1(I) chains synthesized are targeted for rapid intracellular degradation. Degradation was not prevented by brefeldin A, monensin, or NH(4)Cl, agents that interfere with intracellular transport or lysosomal function. However, mutant pro-alpha1(I) chains in both transfected Mov13 cells and OI26 cells were protected from proteolysis by specific proteasome inhibitors. Together these data demonstrate for the first time that procollagen chains containing C-propeptide mutations that impair assembly are degraded by the cytoplasmic proteasome complex, and that the previously identified endoplasmic reticulum-associated degradation of mutant pro-alpha1(I) in OI26 is mediated by proteasomes.

Expression of collagen alpha1(I) mRNA variants during tooth and bone formation in the rat

Collagen alpha1(I) mRNA is composed of two variants of 5 and 6 kb, differing in the length of the 3' untranslated region. In this work, the nucleotide sequences of the two rat mRNA variants were compared, and their expression pattern in cells forming bone, dentin, and cementum were analyzed. The sequences were determined from cDNA inserts of tooth and bone libraries plus directly from PCR fragments, obtained from bone. A total of 5721 bases of the rat collagen alpha1(I) sequence from cDNA of tooth and bone was determined. All sequences of the short variant were represented in the long variant. Only the alternatively poly-A additions gave rise to the variants in hard tissue. Two oligonucleotides were chosen as probes, one of which recognized, on Northern blots, the two bands of 5 and 6 kb, and the other the 6-kb variant only. The oligonucleotides were used in in situ hybridization experiments, for study of the distribution of the variants in different extracellular matrix-forming cells. Osteoblasts, odontoblasts, and cementum-associated cells were closely examined in sections from rat maxillae from 2 to 25 days of age. A similar or identical pattern of mRNA expression was observed with both oligonucleotides, indicating that the two mRNA variants were co-expressed in all cases.

In vitro expression analysis of collagen biosynthesis and assembly

While the generalised pathway of collagen biosynthesis is well understood, the specific molecular interactions that drive chain recognition and assembly and the formation of tissue-specific extracellular supramolecular structures have not been elucidated. This review focuses on the use of in vitro collagen expression systems to explore some of these fundamental questions on the molecular basis of normal and mutant collagen assembly. Three in vitro expression/assembly systems are discussed. Firstly, a simple cell-free transcription/translation system to study the initial stages of collagen chain assembly. Secondly, a novel T7-driven high level expression system, using a recombinant vaccinia virus expressing T7 RNA polymerase, in transiently transfected cells which allows appropriate postranslational modification and collagen folding. Thirdly, the more complex questions of normal and mutant collagen extracellular matrix assembly are addressed by stable transfection and expression in cells which allow the formation of a 'tissue equivalent' matrix during long-term culture.

Mutation in the carboxy-terminal propeptide of the Pro alpha 1(I) chain of type I collagen in a child with severe osteogenesis imperfecta (OI type III): possible implications for protein folding

A young girl presented with severe type III osteogenesis imperfecta; her otherwise healthy mother also had a mild connective tissue disorder with blue sclerae and recurrent joint dislocations. Skin fibroblast cultures from the child produced both normal and post-translationally over-modified type I collagen. The mutant collagen was poorly secreted but had normal thermal stability. Cyanogen bromide peptide maps of the abnormal protein indicated a C-terminal mutation. The mother's cells produced only normal-appearing collagens. Mismatch analysis and extensive sequencing of cDNAs covering the suspect region did not reveal any potentially causal changes in the triple helical domains of either the alpha 1(I) or alpha 2(I) chains. However, examination of the C-propeptide sequences revealed two heterozygous single base changes in the child. One, an A->C changing threonine to proline at residue 29 of the alpha 2(I) C-propeptide was also present in the mother and maternal grandfather but not in 50 unrelated control individuals. The second, a T->C altered the last amino acid residue of the alpha 1(I) C-propeptide from leucine to proline and had occurred de novo in the affected child. This mutation highlights the importance of the C-propeptides in molecular assembly but it is not clear how such an extreme mutation causes the delay in triple helix formation indicated by the extensive over-modification and reduced secretion of the mutant type I collagen. It may inhibit intrachain disulfide bonding or possibly affect the association of the procollagen chain with an intracellular "chaperone" protein that normally assists the assembly of trimeric procollagen molecules.

The type I collagen pro alpha 1(I) COOH-terminal propeptide N-linked oligosaccharide. Functional analysis by site-directed mutagenesis

The C-propeptides of the pro alpha 1(I) and pro alpha 2(I) chains of type I collagen are each substituted with a single high-mannose N-linked oligosaccharide. Conservation of this motif among the fibrillar collagens has led to the proposal that the oligosaccharide has structural or functional importance, but a role in collagen biosynthesis has not been unambiguously defined. To examine directly the function of the pro alpha 1(I) C-propeptide N-linked oligosaccharide, the acceptor Asn residue was changed to Gln by site-directed mutagenesis. In transfected mouse Mov13 and 3T6 cells, unglycosylated mutant pro alpha 1(I) folded and assembled normally into trimeric molecules with pro alpha 2(I). In biosynthetic pulse-chase experiments mutant pro alpha 1(I) were secreted at the same rate as wild-type chains; however, following secretion, the chains were partitioned differently between the cell layer and medium, with a greater proportion of the mutant pro alpha 1(I) being released into the medium. This distribution difference was not eliminated by the inclusion of yeast mannan indicating that the high-mannose oligosaccharide itself was not binding to the matrix or the fibroblast surface after secretion. Subtle alterations in the tertiary structure of unglycosylated C-propeptides may have decreased their affinity for a cell-surface component. Further support for a small conformational change in the mutant C-propeptides came from experiments suggesting that unglycosylated pro alpha 1(I) chains were cleaved in vitro by the purified C-proteinase slightly less efficiently than wild-type chains. Mutant and normal pro alpha 1(I) were deposited with equal efficiency into the 3T6 cell accumulated matrix, thus the reduced cleavage by C-proteinase and altered distribution in the short pulse-chase experiments were not functionally significant in this in vitro extracellular matrix model system.

Endoplasmic reticulum-mediated quality control of type I collagen production by cells from osteogenesis imperfecta patients with mutations in the pro alpha 1 (I) chain carboxyl-terminal propeptide which impair subunit assembly

A heterozygous single base change in exon 49 of COL1A1, which converted the codon for pro alpha 1(I) carboxyl-terminal propeptide residue 94 from tryptophan (TGG) to cysteine (TGT) was identified in a baby with lethal osteogenesis imperfecta (OI64). The C-propeptide mutations in OI64 and in another lethal osteogenesis imperfecta cell strain (OI26), which has a frameshift mutation altering the sequence of the carboxyl-terminal half of the propeptide (Bateman, J. F., Lamande, S. R., Dahl, H.-H. M., Chan, D., Mascara, T. and Cole, W. G. (1989) J. Biol. Chem. 264, 10960-10964), disturbed procollagen folding and retarded the formation of disulfide-linked trimers. Although assembly was delayed, the presence of slowly migrating, overmodified alpha 1(I) and alpha 2(I) chains indicated that mutant pro alpha 1(I) could associate with normal pro alpha 1(I) and pro alpha 2(I) to form pepsin-resistant triple-helical molecules, a proportion of which were secreted. Further evidence of the aberrant folding of mutant procollagen in OI64 and OI26 was provided by experiments demonstrating that the endoplasmic reticulum resident molecular chaperone BiP, which binds to malfolded proteins, was specifically bound to type I procollagen and was coimmunoprecipitated in the osteogenesis imperfecta cells but not control cells. Experiments with brefeldin A, which inhibits protein export from the endoplasmic reticulum, demonstrated that unassembled mutant pro alpha 1(I) chains were selectively degraded within the endoplasmic reticulum resulting in reduced collagen production by the osteogenesis imperfecta cells. This biosynthetic deficiency was reflected in the inability of OI64 and OI26 cells to produce a substantial in vitro collagenous matrix when grown in the continuous presence of ascorbic acid to allow collagen matrix formation. Both these carboxyl-terminal propeptide mutants showed a marked reduction in collagen accumulation to 20% (or less) of control cultures, comparable to the reduced collagen content of tissues from OI26.

Perinatal lethal osteogenesis imperfecta

Perinatal lethal osteogenesis imperfecta is the result of heterozygous mutations of the COL1A1 and COL1A2 genes that encode the alpha 1(I) and alpha 2(I) chains of type I collagen, respectively. Point mutations resulting in the substitution of Gly residues in Gly-X-Y amino acid triplets of the triple helical domain of the alpha 1(I) or alpha 2(I) chains are the most frequent mutations. They interrupt the repetitive Gly-X-Y structure that is mandatory for the formation of a stable triple helix. Most babies have their own private de novo mutation. However, the recurrence rate is about 7% owing to germline mosaicism in one parent. The mutations act in a dominant negative manner as the mutant pro alpha chains are incorporated into type I procollagen molecules that also contain normal pro alpha chains. The abnormal molecules are poorly secreted, more susceptible to degradation, and impair the formation of the extracellular matrix. The collagen fibres are abnormally organised and mineralisation is impaired. The severity of the clinical phenotype appears to be related to the type of mutation, its location in the alpha chain, the surrounding amino acid sequences, and the level of expression of the mutant allele.