Zebrafish Collagen Type I: Molecular and Biochemical Characterization of the Major Structural Protein in Bone and Skin.

Over the last years the zebrafish imposed itself as a powerful model to study skeletal diseases, but a limit to its use is the poor characterization of collagen type I, the most abundant protein in bone and skin. In tetrapods collagen type I is a trimer mainly composed of two α1 chains and one α2 chain, encoded by COL1A1 and COL1A2 genes, respectively. In contrast, in zebrafish three type I collagen genes exist, col1a1a, col1a1b and col1a2 coding for α1(I), α3(I) and α2(I) chains. During embryonic and larval development the three collagen type I genes showed a similar spatio-temporal expression pattern, indicating their co-regulation and interdependence at these stages. In both embryonic and adult tissues, the presence of the three α(I) chains was demonstrated, although in embryos α1(I) was present in two distinct glycosylated states, suggesting a developmental-specific collagen composition. Even though in adult bone, skin and scales equal amounts of α1(I), α3(I) and α2(I) chains are present, the presented data suggest a tissue-specific stoichiometry and/or post-translational modification status for collagen type I. In conclusion, this data will be useful to properly interpret results and insights gained from zebrafish models of skeletal diseases.

Human prolidase and prolidase deficiency: an overview on the characterization of the enzyme involved in proline recycling and on the effects of its mutations.

Here we summarized what is known at the present about function, structure and effect of mutations in the human prolidase. Among the peptidases, prolidase is the only metalloenzyme that cleaves the iminodipeptides containing a proline or hydroxyproline residue at the C-terminal end. It is relevant in the latest stage of protein catabolism, particularly of those molecules rich in imino acids such as collagens, thus being involved in matrix remodelling. Beside its intracellular functions, prolidase has an antitoxic effect against some organophosphorus molecules, can be used in dietary industry as bitterness reducing agent and recently has been used as target enzyme for specific melanoma prodrug activation. Recombinant human prolidase was produced in prokaryotic and eukaryotic hosts with biochemical properties similar to the endogenous enzyme and represents a valid tool both to better understand the structure and biological function of the enzyme and to develop an enzyme replacement therapy for the prolidase deficiency (PD). Prolidase deficiency is a rare recessive disorder caused by mutations in the prolidase gene and characterized by severe skin lesions. Single amino acid substitutions, exon splicing, deletions and a duplication were described as causative for the disease and are mainly located at highly conserved amino acids in the sequence of prolidase from different species. The pathophysiology of PD is still poorly understood; we offer here a review of the molecular mechanisms so far hypothesized.

Stability and networks of hydrogen bonds of the collagen triple helical structure: influence of pH and chaotropic nature of three anions.

The thermal stability of the trimeric species formed by seven type I collagen CNBr peptides was determined at neutral and acidic pH. Melting temperature of peptide trimers and free energy change for monomer to trimer transition were used as indices of trimer stability. A greater stability at neutral pH than at acidic pH was found for all peptides analysed because in most conditions an entropic gain overwhelms an enthalpic cost. Enthalpic reasons are prevailing only in some conditions of the more acidic peptides. The overlap zone of type I collagen fibrils is more basic than the gap zone and is therefore more sensitive to variations of pH from neutral to acidic, e.g. in bone degradation when osteoclasts acidify the lacuna lying between cell and bone. Peptide trimer stability in neutral conditions is influenced also by the chaotropic nature and the concentration of three anions: chloride, sulfate and phosphate. This was more evident for sulfate at the highest concentration used (0.5 M) when a greater stability is caused by entropic reasons. The contribution of hydroxyproline to the stability of peptide trimers is greater at neutral than at acidic pH, particularly at the highest concentration of sulfate. All our data indicate that pH, chaotropic nature and concentration of three anions influence the networks of hydrogen bonds present in the collagen triple helical structure.

Conformational analysis and stability of collagen peptides by CD and by 1H- and 13C-NMR spectroscopies.

Four small type I collagen CNBr peptides containing complete natural sequences were purified from bovine skin and investigated by CD and 1H- and 13C-nmr spectroscopies to obtain information concerning their conformation and thermal stability. CD showed that a triple helix was formed at 10 degrees C in acidic aqueous solution by peptide alpha l(I) CB2 only, and to lesser extent, by alpha 1(I) CB4, whereas peptides alpha 1(I) CB5 and alpha 2(I) CB2 remained unstructured. Analytical gel filtration confirmed that peptides alpha 1(I) CB2 and alpha 1(I) CB4 only were able to form trimeric species at temperature between 14 and 20 degrees C, and indicated that the monomer = trimer equilibrium was influenced by the chaotropic nature of the salt present in the eluent, by its concentration, and by temperature variations. CD measurements at increasing temperatures showed that alpha 1(I) CB2 was less stable than its synthetic counterpart due to incomplete prolyl hydroxylation of the preparation from the natural source. 1H- and 13C-nmr spectra acquired in the temperature range 0-47 and 0-27 degrees C, respectively, indicated that with decreasing temperature the most abundant from of alpha 1(I) CB2 was in slow exchange with an assembled form, characterized by broad lines, as expected for the triple-helical conformation. A large number of trimer cross peaks was observed both in the proton and carbon spectra, and these were most likely due to the nonequivalence of the environments of the three chains in the triple helix. This nonequivalence may have implications for the aggregation of collagen molecules and for collagen binding to other molecules. The thermal transition from trimer to monomer was also monitored by 1H-nmr following the change in area of the signal belonging to one of the two beta protons of the C-terminal homoserine. The unfolding process was found to be fully reversible with a melting temperature of 13.4 degrees C, in agreement with CD results. The qualitative superposition of the melting curves obtained by CD for the peptide bond characteristics and by nmr for a side chain suggests that triple-helical backbone and side chains constitute a single unit.

Conformational study of a collagen peptide by 1H NMR spectroscopy: observation of the 14N-1H spin-spin coupling of the Arg guanidinium moiety in the triple-helix structure.

CB2, a CNBr peptide of 36 residues from type I collagen alpha1(I) chain has been studied by NMR spectroscopy as a function of temperature. At low temperature, the guanidinium protons of Arg9 showed sharp 1:1:1 NMR triplets around 6.95 ppm, characteristic of 14N coupled protons (1J(NH)=52 Hz) when the quadrupolar relaxation rate is drastically reduced. These spectral characteristics and the low temperature coefficient of the 1:1:1 triplets (delta delta/delta T of -3.6 ppb/degrees C) suggest that the H atoms of the protonated guanidinium moiety of Arg9 in the triple helix are slowly exchanging with bulk water, most likely involved in hydrogen bonds. On the basis of conformational energy computations on a model segment of type I collagen (Vitagliano, L., Némethy, G., Zagari, A. and Scheraga, H.A. (1993) Biochemistry 32, 7354-7359), similar to CB2, our data could indicate that the guanidinium group of Arg9 form hydrogen bonds with a backbone carbonyl of an adjacent chain probably by using the N(epsilon) hydrogen, leaving the four N(eta) hydrogens bound to water molecules that must be in slow exchange with bulk water and that could therefore be considered structural elements of the trimeric alpha1(I) CB2 triple helix. The behaviour of Arg9 has been investigated also in terms of equilibrium between random monomer and helical trimer conformations controlled by temperature. The thermal unfolding process was found to be reversible and the melting point resulted to be 17 degrees C.

Stability of type I collagen CNBr peptide trimers.

As indices of triple helix stability of type I collagen CNBr peptide homotrimers, deltaG degrees for monomer-trimer transitions and melting temperatures were obtained from circular dichroism measurements at increasing temperatures. The data were compared with the stability of the parent native molecule. Peptides were found to have a lower stability than the whole collagen molecule. The general implication is that the coordinated water molecules play a key role in determining collagen triple helical stability and high cooperativity at melting. Other factors (monomer stability, ionic and hydrophobic factors, variations of composition, specific sequences) could also contribute towards peptide stability; these factors could explain the data obtained in the case of peptide alpha1(I) CB3.

A comparative study on biochemical markers of bone collagen breakdown in post-menopausal women.

The aim of this study was to compare urinary galactosylhydroxylysine (GHyl) and deoxypyridinoline (d-Pyr) as biochemical markers of bone resorption in post-menopausal women treated and untreated with estrogen and cyclic etidronate. Fasting urinary GHyl, D-Pyr, pyridinoline, serum osteocalcin and total alkaline phosphatase were measured in three subgroups, i.e. post-menopausal women undergoing hormone replacement therapy, untreated post-menopausal women and post-menopausal women with low BMD treated with disodium etidronate. The results indicated that GHyl did not significantly discriminate between untreated post-menopausal women and estrogen replated ones unless an osteoporotic untreated group was selected. d-Pyr and GHyl showed similar performances when their values after bisphosphonate treatment were compared to those found in untreated post-menopausal women, thus suggesting that both markers were equal in their ability to detect the bone response to cyclic etidronate administration. This observation further proves the statement that GHyl is prone to confounding factors under estrogen therapy but it is adequate as is d-Pyr in monitoring the bone response to bisphosphonate treatment.

Pre-column derivatisation method for the measurement of glycosylated hydroxylysines of collagenous proteins.

Measurement of the glycosylated hydroxylysines galactosyl- and glucosylgalactosylhydroxylysine (GH and GGH) in combination with other amino acids has been based on ion-exchange chromatography followed by reaction with ninhydrin. Here, a rapid and sensitive high-performance liquid chromatographic method with fluorimetric detection has been developed and employed to determine the glycosylated hydroxylysine residues in alkaline collagen hydrolysates. After hydrolysis, amino acids were derivatised with 9-fluorenylmethyl chloroformate and separated on a Micropak ODS-80TM reversed-phase column (150x4.6 mm). With a multistep gradient system all amino acids were separated in less than 30 min, including the collagen-specific hydroxylysine, hydroxyproline and the glycosylated hydroxylysines. The method was used to evaluate the glycosylation levels of human articular cartilage derived from femoral head, femoral condyle, tibial plateau and ankle. GGH was highest in cartilage from femoral head and ankle; GH showed no differences between the different sources of cartilage.

Osteogenesis imperfecta mutations may probe vital functional domains (e.g. proteoglycan binding sites) of type 1 collagen fibrils.

Osteogenesis imperfecta (OI) is a disease characterized by bone malformations caused by mutations in type 1 collagen. Since many of the 338 possible glycine mutations have not been observed in clinical practice, is this due to chance alone? Because only 83 mutations have been reported in 126 patients, we conclude that many mutations are absent from clinical data for non-random causes. Mutations affecting vital intermolecular interactions in the extracellular matrix (e.g. potential collagen binding sites for proteoglycans) may result in non-viable fetuses that do not progress to clinical status. Some mutations may be silent because they do not significantly affect normal function. The total number of clinically active mutations that will be observed may be far fewer than the potential 338 maximum.

Effect of an oral calcium load on urinary markers of collagen breakdown.

Aim of this study was to investigate whether osteoclast activity changes as a consequence of even mild physiological perturbation of plasma calcium as such induced by an oral calcium load. Osteoclast activity was determined indirectly by measuring, in spot urines at two and four hours after oral calcium load, the urinary excretion of hydroxylysylpyridinoline (Pyr), deoxylysylpyridinoline (D-Pyr), hydroxyproline (Hyp) and galactosyl-hydroxylysine (GHyl). The occurrence of the metabolic perturbation of plasma calcium homeostasis was assessed by measuring three indexes: i.e. calcemic response, PTH reduction and calciuric response at times following oral calcium loading. A significant fall of urinary D-Pyr and Pyr followed the perturbation of calcium homeostasis induced by the oral calcium load in two groups of healthy young adult and postmenopausal women. The highest mean percent reduction was observed for D-Pyr and was quantitatively similar in the two groups. Since urinary D-Pyr is the most specific bone resorption marker, it may be inferred that the perturbation of plasma calcium homeostasis induced by an oral calcium load is able to acutely inhibit osteoclast activity. This supports the view that osteoclasts are involved in the short-term error correction of plasma calcium.

Comparison between urinary pyridinium cross-links and hydroxylysine glycosides in monitoring the effects of ovariectomy and 17 beta-estradiol replacement in aged rats.

This study was undertaken to assess the sensitivity of hydroxylysylpyridinoline (HP), lysylpyridinoline (LP), galactosylhydroxylysine (GHyl) and glucosylgalactosylhydroxylysine (GGHyl) to monitor bone response to estrogen deficiency and replacement by comparing their excretory patterns in ovariectomized aged (11-14 months old) rats. The ovariectomized (OVX) rats were randomized into two groups: (1) OVX plus vehicle; (2) OVX plus 17 beta-estradiol (17-beta E, 10 micrograms/kg, s.c., 4 days/week). Treatment with 17-beta E started immediately after OVX and continued for 60 days. The collagen catabolites were measured in urine for 1 month before OVX and thereafter for 60 days. In temporal coincidence with urine collection, bone area and bone mineral density (BMD) of lumbar vertebrae, femoral diaphysis and distal metaphysis were measured by dual-energy X-ray absorptiometry. In the untreated rats, BMD of the femoral metaphysis and lumbar vertebrae decreased significantly and the urinary excretion of LP, HP, GHyl and GGHyl increased with different patterns. In the treated rats, 17-beta E replacement prevented the increment in LP excretion, partially prevented the increase in HP excretion, but had no effect on the excretion of GHyl and GGHyl. In conclusion pyridinolines and glycosides have different sensitivities to the bone response to OVX. Glycoside excretion after OVX also reflects metabolic processes not strictly related to bone loss and, in contrast with LP, is not sensitive to estrogen replacement.

Type I collagen CNBr peptides: species and behavior in solution.

The properties of type I collagen CNBr peptides in solution were studied to investigate the molecular species formed, their conformation, and factors influencing equilibria between peptide species. Peptides formed homologous trimers, even though the native parent protein is heterotrimeric, [alpha 1(I)]2 alpha 2-(I). Their triple-helical content was found to be high (> 75% for most peptides). Full helical content was not reached mainly because of the presence of monomer species; chain misalignment, if present, and trimer unraveling at terminal ends appeared to play a minor role in reducing helicity. Circular dichroism spectra and resistance to trypsin digestion at 4 and 20 degrees C demonstrated that the conformation of trimers was very similar to the collagen triple-helical conformation. Rotary shadowing of peptide alpha 1(I) CB7 supported this finding. Analytical gel filtration in nondenaturing conditions showed that the trimers of some peptides have the ability to autoaggregate. In the case of peptides alpha 1(I) CB8 and alpha 2(I) CB4, most of the intermolecular interactions between trimeric molecules were disrupted by 0.5 M NaCl, demonstrating that their ionic character is important. Changes in ionic strength also altered the hydrodynamic size of single- and triple-stranded molecules. The different molecular species are in equilibrium. The kinetics of the conversion of trimer to monomer species was determined in a time course experiment using trypsin digestion and found to be a relatively slow process (trimer half-life is a few days at 4 degrees C, about one order of magnitude lower at 20 degrees C) with an activation energy of roughly 4-9 kcal/mol. The circular dichroism profile at increasing temperatures showed that the melting temperature for triple-helical peptides is about 6-10 degrees C lower than that of the parent native type I collagen. The folding of peptides is a spontaneous process (exothermic but with unfavourable entropy change), and the triple-helical conformation originates solely as the result of the collagen sequence because it forms from heat-denatured samples.

Gly85 to Val substitution in pro alpha 1(I) chain causes mild osteogenesis imperfecta and introduces a susceptibility to protease digestion.

In this paper we describe a mild moderate form of osteogenesis imperfecta caused by a point mutation in COL1A1 which converted glycine 85 to valine. The valine substitution introduced into the triple-helical domain of type-I collagen a conformational perturbation causing susceptibility to digestive proteases. In fact, SDS/PAGE of pepsin-treated collagen showed the presence of a faint band, migrating between alpha 1(I) and alpha 2(I), both in the medium and in the cell layer. On trypsin digestion the band, a shortened form of alpha 1(I), had a melting temperature of 39.5 degrees C. If the triple-helical collagen was obtained after trypsin or chymotrypsin digestion of procollagen, two shortened bands were identified; the enzymes cleaved about 40% of the trimers. The mutant procollagen was normally secreted and processed in the extracellular matrix at a normal rate. When native type-I collagen was formed after dextran-sulfate incubation, only chains of normal length were found, suggesting that the fibroblast proteases did not recognize the alteration introduced by the mutation. The effects of glycine 85 to valine substitution are compared with those produced by a previously described arginine substitution of the same residue (Deak et al., 1991).

Extracellular matrix deposition in cultured dermal fibroblasts from four probands affected by osteogenesis imperfecta.

Type I procollagen biosynthesis and matrix deposition were studied in cultured fibroblasts of four probands affected by Osteogenesis Imperfecta and in whom the mutations have been characterized. The mutations along the triple helix altered all biochemical parameters considered, i.e. thermal stability, kinetics of procollagen secretion and rate of maturation from procollagen to collagen. The biochemical findings were peculiar for each case considered, but there was no correlation between biochemical parameters and clinical phenotype. In all our probands, regardless of the clinical severity, mutant chains appeared in the insoluble matrix formed by fibroblasts cultured in the presence of dextran sulfate. The densitometric scanning revealed a relative increased amount of fibronectin, suggesting that the matrix contained a lower quantity of type I collagen. Furthermore, the amount of mutant chains found in the insoluble fraction was clearly less than expected, considering that 75% of new synthesized trimers are abnormal. Therefore, in the presence of a mutation, the protein available for extracellular matrix formation is reduced and the mutant trimers incorporated in the matrix probably interfere with normal fibril performance. The abnormal fibril morphology has a dramatic effect in bone, interfering presumably with a correct mineral deposition and interactions with non/collagenous bone proteins.

The structural characterization and bilirubin-binding properties of albumin Herborn, a [Lys240-->Glu] albumin mutant.

We report the molecular defect of albumin Herborn, a new genetic variant of human serum albumin which has been found in Germany. Isoelectric focusing analysis of CNBr fragments from the purified variant allowed us to localize the mutation in fragment CNBr 3 (residues 124-298). This fragment was isolated on a preparative scale and subjected to tryptic and V8 protease digestion. Sequence determination of the abnormal tryptic and V8 peptides revealed that the variant arises from the substitution Lys240-->Glu. The -2 charge change of albumin Herborn, which is probably due to a A-->G transition in the first position of the corresponding codon in the structural gene, has no significant effect on its electrophoretic mobility under non-denaturating conditions. Therefore we have assumed that residue 240, which has been implicated in the bilirubin primary binding site (Jacobsen, C. (1978) Biochem. J. 171, 453-459), is buried. The binding of bilirubin and biliverdin by albumin Herborn was quantified using the fluorescence quenching method. The apparent equilibrium association constants (Ka +/- SD) and the number of high-affinity binding sites (n) of the defatted variant for bilirubin and biliverdin were Ka = 1.03 +/- 0.18 x 10(8) M-1, n = 1.07; and Ka = 7.48 +/- 1.10 x 10(6) M-1, n = 1.01, respectively. The Ka values are about 93.3% and 99.1% of the values found for the normal protein under the same conditions. These results strongly suggest that Lys240 of human serum albumin is not the basic residue involved in ion pairing with one of the carboxylate groups of bilirubin at its high-affinity site.

Osteogenesis imperfecta and type-I collagen mutations. A lethal variant caused by a Gly910-->Ala substitution in the alpha 1 (I) chain.

In this study we describe a new dominant point mutation in COL1A1 causing a lethal form of Osteogenesis imperfecta (type II B). Dermal cultured fibroblasts from the proband were shown to produce both normal and heavily overmodified type-I collagen. The mutation introduced a local conformational perturbation, which causes abnormal exposure of arginine residues; the triple helical domain was susceptible to trypsin digestion even at 30 degrees C. The chains bearing the point mutation were poorly secreted and short-term pulse experiments showed that the extensive intracellular retention of mutant trimers also impaired the secretion of normal chains. The molecular defect was localized in a COL1A1 allele by cloning and sequencing a cDNA region corresponding to the CB6 peptide. A G to C transversion which causes the substitution in the triple helical region of Gly910 with alanine was found. The mutation also causes the disappearance of a MspI-recognition site at nucleotide 3263 of the pro alpha 1 (I) coding sequence. Restriction analysis, along with the biochemical screening of collagens, allowed us to perform prenatal diagnosis on cells from chorionic-villus sampling and to exclude the recurrence of the mutation in the sibling.

Possible role of overglycosylation in the type I collagen triple helical domain in the molecular pathogenesis of osteogenesis imperfecta.

The underlying defect in patients affected by a form of osteogenesis imperfecta (OI) clarified at the molecular level regards the amount or the structure of type I collagen synthesized. This leads to a decreased and/or abnormal mineral deposition in bone and affects bone mass and/or strength. Abnormal interactions between collagen molecules in the presence of mutant trimers could give rise to abnormal fibrils, which, in turn, can determine incorrect interactions with noncollagenous matrix macromolecules. The interactions can be disturbed or modulated by an abnormal distribution on the collagen fibril surface of electrically charged or hydrophobic groups, or by an increased presence of sugar moieties linked to hydroxylysyl residues due to chain post-translational overmodifications (lysyl overhydroxylation and hydroxylysyl overglycosylation) of the portion of the triple helical domain of abnormal type I collagen molecules N-terminal with respect to the defect localization.

Deposition of mutant type I collagen in the extracellular matrix of cultured dermal fibroblasts in osteogenesis imperfecta.

To study how mutant type I collagen interferes with matrix deposition we investigated the extracellular matrix produced by cultured skin fibroblasts in thirteen patients affected by different forms of Osteogenesis Imperfecta. Two different approaches were used: a) the pericellular matrix produced during 24 h label was analyzed by SDS-PAGE; b) type I collagen present in the insoluble cell-layer fraction in long-term cultures was studied. Results showed that a very small amount of abnormal type I trimers were present regardless of the clinical phenotype. In only two cases mutant chains were clearly incorporated. These data indicate a selective deposition of normal collagen trimers over abnormal ones. Moreover, in long-term cultures a decreased amount of type I collagen was deposited as indicated by the relative increase in type V collagen. These data are discussed in light of results found in bone by other authors and suggest that decreased deposition of type I collagen could be a general feature in OI and not limited to null-allele OI probands.

Mild dominant osteogenesis imperfecta with intrafamilial variability: the cause is a serine for glycine alpha 1(I) 901 substitution in a type-I collagen gene.

The molecular defect responsible for a case of mild osteogenesis imperfecta (OI) with repeated femoral fractures was investigated. The proband and his mother, who presented minor OI signs but no bone fractures, were shown to produce normal and abnormal type-I procollagen molecules in their dermal fibroblasts. The molecular defect was localized in about half of the proband's pro alpha 1(I) mRNA molecules by chemical cleavage with piperidine of hydroxylamine-reacted mRNA:cDNA heteroduplexes. The corresponding region was reverse-transcribed and amplified by polymerase chain reaction (PCR). Cloning and sequencing of the amplified products revealed in both subjects a G-to-A transition in the first base of codon 901 of the alpha 1(I) triple helical domain, which led to a serine for glycine substitution. Allele-specific oligonucleotide hybridization to amplified genomic DNA from fibroblasts and leukocytes confirmed the heterozygous nature of both patients and proved the absence of mosaicism. The presence of the mutation was excluded in other healthy family members, who were reported to have bluish selerae. The mild phenotypic outcome of this newly characterized mutation contradicts previous findings on glycine substitutions in the C-terminal region of collagen triple helix, most of which caused lethal OI.

Phenotypic variability and abnormal type I collagen unstable at body temperature in a family with mild dominant osteogenesis imperfecta.

Autosomal dominant inheritance of a mild form of osteogenesis imperfecta (osteogenesis imperfecta type I) with different phenotypic expression was found in a family. Phenotypic expression was different for the affected mother and son, in the presence of the same biochemical results. Dermal fibroblast cultures synthesized normal and mutant type I collagen alpha chains. Collagen heterotrimers containing abnormal chains were overmodified along the entire triple helical domain and showed an unusually low denaturation temperature, so far found only in lethal cases. The mild phenotype in the family is probably due to the fact that abnormal type I collagen molecules are more likely to be degraded than utilized in the extracellular matrix.