Osteogenesis Imperfecta Collagen-Like Peptides: Self-Assembly and

Ayad , S. ; Boot-Handford , R. P. ; Humphries , M. J. ; Kadler , K. E. ; Shuttleworth , C. A. The extracellular matrix facts book; Academic Press: Lon...
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Biomacromolecules 2008, 9, 1551–1557

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Osteogenesis Imperfecta Collagen-Like Peptides: Self-Assembly and Mineralization on Surfaces Peng Xu,† Jia Huang,† Peggy Cebe,‡ and David L. Kaplan*,† Departments of Chemical and Biological Engineering and Biomedical Engineering, and Department of Physics and Astronomy, Tufts University, Medford, Massachusetts 02155 Received December 10, 2007; Revised Manuscript Received March 24, 2008

Collagens constitute a large family of extracellar matrix proteins in humans and mammals that provide a structural framework for tissues. A number of hereditary connective tissue diseases are associated with mutations in collagens, including Osteogenesis imperfecta (OI). Collagen-like peptides were synthesized with alterations in primary sequence to represent native and mutation states related to this disease. The peptides were self-assembled in solution and on surfaces to investigate the influence of sequence chemistry on self-organization and, subsequently, how changes in structural organization impact templating for hydroxyapatite (HA) crystallization. Bulkier and more hydrophilic amino acid side chains in the peptide sequence, representing increasing severity of the disease state, resulted in a progressive disruption of the triple helix and supramolecular assembly. These changes also resulted in alterations in the nature of the mineralization pattern and composition of the calcium phosphate deposited on assembled templates.

Introduction Collagen is the most abundant protein in the human body, accounting for about one-quarter of all proteins in mammals. The basic unit of collagen is a polypeptide chain consisting of the repeating sequence Gly-X-Y, where X and Y are often proline and hydroxyproline, respectively. The repetitive triplet enables triple-helical folding of collagens as a critical structural building block in the formation of complex fibrillar collagen extracellular matrices.1 Intramolecular interactions, attributed to hydrogen bonding, stabilize the collagen triple helices. Hydrophobic and polar residues in the accessible proline positions help intermolecular associations of the rod-like triple helices and the binding of ligands. Collagens provide tensile strength to tissues while also helping to form critical components of the extracellular matrix. The self-assembly of collagen triple helices into fibrillar structures in these matrices plays a central role in the structural framework of most tissues in the body, including skin, tendon, cartilage, and bone.2–4 Collagen is also widely used as a biocompatible biomaterial for medical devices, such as sutures, sponges, and scaffolds for tissue engineering. There are more than 20 different types of collagen and they form right-handed triple helices, which are rarely found in other proteins.1,5 Healthy tissues contain optimum amounts of well-organized extracellular matrices,2 and the collagen type directly affects the supramolecular assembly of the matrices, including orientation, packing density, the selective deposition of other matrix components, including mineral components, and cell attachment. Thus, collagens and their correct structural hierarchical self-assembly are at the core of native tissue formation and function. Therefore, mutations in native collagen primary sequence can disrupt normal collagen assembly and thus function, resulting in a variety of pathologies.6 A number of hereditary connective * To whom correspondence should be addressed. Telephone: 617-6273251. Fax: 617-627-3231. E-mail: [email protected]. † Departments of Chemical and Biological Engineering and Biomedical Engineering. ‡ Department of Physics and Astronomy.

tissue diseases have been associated with mutations in the triple helical domains of collagens.7 Type-I collagen is the major extracellular matrix protein in bone tissue and consists of two R1(I) and one R2(I) polypeptide chains. Many mutations in the gene coding for type-I procollagen chains have been identified in the disease.8–11 Most of these mutations are single nucleotide changes, resulting in a glycine substitution in at least one R1(I) chain in the collagen triple helix.12–14 OI, also called “brittle bone disease”, is a heritable connective tissue disorder characterized by a general decrease in bone mass and bone fractures. The substitution of glycine in the triplet repeat with other bulkier side chain amino acids results in the disordering or distortion of the collagen triple helical structures.15–19 OI has been divided into four clinical phenotypes. Type I is the mildest and most common form of the disease, characterized by prepubertal fractures and slight growth retardation. Type III and IV OI patients suffer from progressive deformities and severe growth deficiencies. Type II OI is the most severe form, which is usually prenatal lethal.20,21 The severity of OI is also dependent on the mutation site and local environment.22,23 The mechanism of how a single glycine substitution results in pathological conditions in OI is not clear.1 However, defective chain folding plays a role in collagen diseases resulting from glycine substitutions. Mutations that substitute bulkier amino acids for glycine residues prevent proper assembly of the protein into triple helices, because the collagen triple helices require a glycine at every third amino acid to allow proper packing at the center of the helix due to steric constraints.10,24 In many mutations in the R1(I) chain, sites near the carboxyl terminus result in more severe clinical phenotypes than those near the amino terminus, possibly because assembly of the triple helix is initiated near the carboxyl terminus and zippers up to the amino terminus.15,25 Bone is a connective tissue composed primarily of collagen and minerals, which combine to provide mechanical and transport functions in the body. Around 30% of bone is composed of organic components, of which 90-95% is type I collagen. The other 70% percent of bone is made up of the inorganic

10.1021/bm701365x CCC: $40.75  2008 American Chemical Society Published on Web 05/23/2008

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mineral HA, which includes calcium phosphate, calcium carbonate, calcium fluoride, calcium hydroxide, and citrate. Once the collagen molecules are secreted, they are cross-linked by the lysyl oxidase through the conversion of lysine or hydroxylysine residues to aldehydes.26 Collagen performs a regulatory role in bone mineral formation.27 The mineralization of fibrils is promoted by specific structures and orientations of the molecules in the fibrils.28,29 The formation of collagen fibrils also improved the mechanical properties and reduced bioresorbability of the collagen/minerals complex.30 The surface deposition of collagen fibrils affected the molecular patterns, which were in direct control of the surface mineralization.31 Collagen-like peptides are widely used in studies of collagen folding and packing. All types of native-like and mutant collagen molecules are simplified to several triplets to facilitate the study of assembly in vitro. These peptides allow characterization of the effect of glycine substitutions within a defined sequence and controlled environment in a homogeneous population of triple helical molecules.20 Collagen-like peptides exhibit strong affinity for both native and gelatinized type I collagen based on their ability to associate into a triple-helical architecture.5 In the present study, collagen-like peptides were synthesized and utilized to understand how changes in the polypeptide primary sequence influence folding to triple helices, supramolecular assembly to fibrils, and subsequently mineralization. The understanding of relationships between primary sequence and functional mineralization related to the formation of bone-like mineralized composite formation is an important step in developing a fundamental perspective of structure-function relationships with these peptides as in vitro models to understand disease states. The collagen-like peptides were designed with one glycine in the middle of the polypeptide sequence substituted by alanine, valine, or aspartic acid, to mimic the normal to severe states of the OI disease based on known mutations.

Xu et al. Mineralization. Mineralization was carried out by immersing the collagen-like peptide films in 200 mM aqueous calcium chloride solution with the Tris buffer for 2 h at 37 °C. The peptide films were rinsed thoroughly with distilled water to remove any unbound calcium ions before immersing into a 120 mM aqueous disodium hydrogenphosphate in Tris buffer for another 2 h at 37 °C, followed by rinsing with distilled water. This process represented one round of mineralization. Additional rounds of mineralization were conducted as needed to increase the content of mineral growth on the assembled collagen templates. Atomic Force Microscopy (AFM) Conditions. All imaging and dip-pen nanolithography (DPN) were performed in tapping mode on a Dimension 3100 Scanning Probe Microscope with Nanoscope III and IV controllers (Digital Instruments, Santa Barbara, CA) and equipped with rotated tapping-mode etched silicon probes (RTESP; Nanodevices, Santa Barbara, CA). RETSP probe cantilever length is 125 µm with a spring constant of 40 N/m and resonant frequency of 300 ( 50 KHz developed to measure high-aspect ratio features. All DPN experiments were carried out on gold coated silicon surfaces. Parallel lines were made in the indent mode at the tip trig threshold varying from 0.35 V and step point of 0.05 V, with a constant scratching rate of 0.1 Hz. Analysis. Solution state circular dichroism (CD) spectra were obtained on a Jasco (Easton, MD) J-710 spectropolarimeter equipped with a 0.1 cm path length quartz cell. The peptide solutions were preequilibrated at 4 °C for at least 24 h. CD spectra were obtained by scanning 250-180 nm at 4 °C. The melting temperatures (Tm) of collagen-like peptides were calculated according to the thermal transition curve.32 The fraction unfolded Fu was defined as

Fu(T) )

Y(T) - Yf(T) Yu(T) - Yf(T)

(1)

where Y is the observed ellipticity and Yf and Yu represented folded and unfolded regions in the thermal transition curve and was obtained by linear fitting the top and bottom plateau of the curve. The Fu curve obtained by eq 1 can be fitted and correlated to Tm according to the following equation33

Experimental Section Materials. The collagen-like peptides were synthesized by Fmoc chemistry at the core protein chemistry laboratory (Tufts University Medical School) with molecular weights of native-like 4270.52 Da, alanine mutant (OI-A) 4284.96 Da, valine mutant (OI-V) 4312.68 Da, and aspartic acid mutant (OI-D) 4328.58 Da, which were determined by MALDI-TOF mass spectroscopy. Collagen-like peptide aqueous solutions were prepared with concentrations of 1.0 mg/mL and 0.1 mg/ mL. They were stored at 4 °C at least 24 h for preassembly of tripe helix in solution. The 0.1 M Tris buffer was prepared by dissolving 1.214 g of Tris base (Tris-hydroxymethyl aminomethane) in 100 mL ultrapure water at room temperature. The pH was adjusted to 7.4 with 1N HCl. The calcium chloride and disodium hydrogenphosphate aqueous solution were prepared in Tris buffer with the concentrations of 200 mM and 120 mM, respectively. All other chemicals and solvents used were commercially available, of analytical grade or better, and used as received. Gold surfaces were prepared by coating silicon chips with a thin layer of titanium followed by ∼200 nm of 99.99% gold (Electron Microscopy Sciences, Fort Washington, PA) by thermal evaporation. The whole process was auto completed on an Edwards Auto 306 vacuum evaporator. Collagen Peptide Assembly on Surfaces. The collagen-like peptide solutions were kept at 4 °C for at least 12 h allowing the peptides to self-assemble. Self-assembled monolayers (SAMs) of collagen-like peptides were prepared by immersing the gold coated silicon chips into collagen-like peptide stock solutions (0.1 mg/mL) for 1 h, followed by thoroughly rinsing with ultrapure water. Thicker collagen-like peptide films were obtained by drop casting 10 µL of 0.1 mg/mL peptide stock solutions onto silicon or gold surfaces, followed by either air drying or slowly drying, keeping the samples in a closed container overnight until dry.

Fu )

1 1 + e(Tm-T)/a

(2)

where a is a constant to let eq 2 best fit the Fu curve. When T ) Tm, Fu ) 1/2. If Fu ) 1/2 in the Fu ∼ T curve, then the corresponding T should be Tm. Surface morphologies of the minerals grown on collagen peptide thin films were characterized by a LEO 982 field emission scanning electron microscope (SEM) at 3.0 kV. The molar ratios of calcium to phosphorus were investigated by energy dispersive X-ray spectroscopy (EDS) at 10.0 kV. The overlapped peaks were automatically deconvolved and the peak area was integrated by the EDS software to calculate the Ca/P ratio.

Results and Discussion The substitution (mutation) of glycine in specific collagen primary sequences results in disordered collagen triple helical structures. There are many different types of OI collagen, most of which are derived from a single glycine substitution.1 Based on these known mutations and subsequent macroscopic failures or disease states in vivo, collagen mimetic peptides were prepared and used for studies of self-assembly in vitro to screen changes in primary sequence chemistry related to changes in assembly, supramolecular morphology and function based on mineralization. The design criteria are based on the sequence of R1(I) chain, in which GPO triplets are the key units to form triple helical structure. Because native collagen only dissolves in acetic acid at low pH, to eliminate the influence of buffer, avoid acidic environment and improve water solubility (50 mg/

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Table 1. Sequence Chemistry of Native-Like and OI Collagen Peptides, where O Denotes 4-Hydroxyprolinea peptides native-like OI-A OI-V OI-D a

sequence CGKO (GPO)2 GDO (GPO)5 CGKO (GPO)2 GDO (GPO)5 CGKO (GPO)2 GDO (GPO)5 CGKO (GPO)2 GDO (GPO)5

GEO (GPO)2 GRO GPO GPO GEO (GPO)2 GRO APO GPO GEO (GPO)2 GRO VPO GPO GEO (GPO)2 GRO DPO GPO

Change in primary sequence is shown by an underline.

Figure 2. Thermal denaturation curves for collagen-like peptides, 1.0 mg/mL in ultra pure water, measured by CD at fixed wavelength of 225 nm with variable temperature.

Figure 1. CD measurements of collagen-like peptide solutions, 1.0 mg/mL, with a quartz path length of 0.1 cm. Solutions pre-equilibrated at 4 °C for at least 24 h.

mL is required in our future study of collagen peptide liquid crystalline mesophase), hydrophilic amino acids were introduced into GPO repeating units without interfere with normal selfassembly, as we have previously reported.34–36 To understand the influence of mutant amino acids on the collagen peptide assembly and subsequent mineralization, alanine (small hydrophobic residual), valine (bulky hydrophobic residual), and aspartic acid (bulky hydrophilic residual) were selected to substitute glycine. The amino acids selected for mutation were distinct for their stereo hindrance and hydrophilicity. The sequences of native-like and OI collagen peptides are shown in Table 1. The substitution position was selected in the middle of the collagen peptide sequence to eliminate the influence of mutation site. Cysteine was also introduced at the N terminus to anchor collagen peptides on gold substrate to facilitate DPN and self-assembly on surfaces. CD was used to determine changes in secondary structure related to sequence chemistry of the four peptides. All collagenlike peptide solutions were stored at 4 °C for at least 24 h before CD measurement to allow polypeptide chains to self-assemble into triple helices. Figure 1 shows the far-UV (250-217.5 nm) CD spectra, which were recorded at 4 °C. The maxima at 225 nm, a characteristic peak for native collagen and representative of triple helical structure, was observed for all collagen-like peptides. The mean residue ellipticity (deg · cm2 · dmol-1) was independent of analyte concentration and light path length37 and a value of [θ]225 ) 4440 deg · cm2 · dmol-1 for the native-like peptide was found. This value is indicative of a full triple helical conformation, as found in native collagen and other collagen model peptides. The corresponding values of [θ]225 for the mutant collagen-peptides, OI-A (3780 deg · cm2 · dmol-1), OI-V

(2700 deg · cm2 · dmol-1), and OI-D (2360 deg · cm2 · dmol-1), were lower than for the native-like peptide but still in the range expected for triple helices, which indicated OI peptides still have the capability to partially form triple helical structure.20 The content of triple helices in OI peptides were OI-A 86.1%, OI-V 61.9%, and OI-D 53.5%, compared to that of full triple helix in native-like peptides. The mutant collagen-like peptides failed to assemble fully into triple helices, presumably due to the disruption of the interchain packing required to form stabilized triple helices due to the bulkier amino acid side chains present. The severity of the disruption of triple helix formation was directly dependent on the size and polarity of the residues used to replace the glycine in the three mutant collagen-like peptides. The thermal stability and melting temperatures of the collagen-like peptides were assessed by CD by monitoring absorption at 225 nm with temperatures varying from 4 to 100 °C, increased at 1 °C/min. Thermal denaturation curves are shown in Figure 2. The native-like peptide exhibited the highest triple helical content and the highest denaturation temperature. The melting temperatures (Tm) of collagen-like peptides were calculated according to the thermal transition curves as nativelike 62.85 °C, OI-A 56.48 °C, OI-V 46.69 °C, and OI-D 44.39 °C. The Tm decreased with respect to the size of substituent residues and their hydrophilicity, which was consistent with the trend found for triple helical content. The thermal denaturation curve for the native-like peptide is not very sharp due to the high heating rate and a small amount of short peptides, based on MALDI-TOF analysis of the synthesized peptides.38 According to the CD analysis, the glycine substitutions in mutant collagen-like peptides prevented zipper-like folding, resulting in incomplete triple helices.24,25 The mutations may result in knobs in the triple helices or distortion of the rod-like collagen molecules.10 This structural distortion prevents further packing into fibrils and suprafibrillar structures.39,40 In the present study, based on this initial structural analysis, the influence of sequence chemistry on assembly and mineralization of native-like and mutant collagen peptides was investigated at different length scales to assess relationships. The DPN process facilitates molecular alignment and assembly along the direction that the SPM probe travels during surface patterning as we have previously demonstrated for collagen and collagen-like peptides.41,42 Because the collagen-

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Figure 3. AFM topography (left) and phase (right) images of DPN patterned collagen-like peptides with glycine substitutions: (A) native-like, (B) OI-A, (C) OI-V, and (D) OI-D.

Figure 4. AFM topography (left) and phase (right) images of collagen-like peptides drop-cast on gold surfaces (scan size 1 µm × 1 µm).

like peptides were terminated with cysteines, they are expected to anchor on a gold surface and align along the path of deposition as they assemble. In the present study, the nativelike and mutant collagen peptides were dissolved in ultra pure water (0.1 mg/mL) and coated on a RTESP tip. The DPN process conditions were identical for all collagen peptides, including the preparation of peptide solutions, the initial and step point voltages applied on tip bias, tip scratching rate, and environmental humidity. A series of lines were patterned with a tip scratching rate of 0.1 Hz. Topography and phase-lag images were recorded simultaneously for each peptide (Figure 3). Uneven line patterns generated with the mutant collagen peptides would be expected based on the CD results. Triple helical collagens pack into fibrils with an axial periodicity of 67 nm and lateral spacing of approximately 1.5 nm.40,43 The disordered conformation of the triple helix and distortion of rodlike molecules alters the distance between collagen peptide rodlike molecules when packing into fibrils. The AFM tip had little effect on alignment of the mutant collagen peptides. Nativelike collagen peptides exhibited smooth and continuous lines as a result of the DPN patterning, reflecting “normal” assembly. The alanine mutant exhibited similar patterns to the native-like peptide, while the pattern of the valine mutant was thicker and less continuous. The phase-lag images were less defined for the

alanine and valine peptide mutants, which may be a result of the looser packing of molecules. In the case of the aspartic acid collagen-peptide mutant, no continuous lines were obtained. The severity in disruption of morphology of the patterned lines was related to the size of the amino acid residue side chain substituted for glycine in the collagen peptides according to the order: alanine < valine < aspartic acid. The morphology of self-assembled collagen-like peptides was studied by direct drop casting on gold and polished silicon surfaces. The topography and phase images of the collagenlike peptide films are shown in Figure 4. The native-like peptide and alanine variants exhibited relatively larger and uniform clusters, while the aspartic acid variant was loose patterned and valine variant showed smaller clusters. This was possibly due to the partially assembled triple helices and distortion of the molecules, which resulted in less optimal packing. Because the peptides were anchored on gold by the terminal cysteines, the assembly was strongly affected by conformation. Distorted peptide molecules would be less likely to associate with each other. The thiol-Au bonding may influence peptide assembly. To observe assembly free of this potential interference, silicon surfaces were used to physically adsorb the collagen-like peptides. The silicon chips were immersed into 0.1 mg/mL

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Figure 5. AFM phase-lag images of collagen-like peptide monolayers adsorbed on polished silicon surfaces (scan size 1 µm × 1 µm).

peptide solution for 30 min followed by thorough rinsing with ultra pure water to remove unadsorbed molecules. The peptide monolayers were imaged (Figure 5). The native-like collagen peptide adsorbed film exhibited a relatively uniformly patterned surface. The alanine mutant was less uniform in morphology and almost no material was detected on the surface for the valine and aspartic acid mutants. Assembly on such surfaces showed the packing ability in the order of native-like > OI-A > OI-V, OI-D and was again consistent with the disease severity order related to the OI collagen mutations. The mineral, mostly HA, accounts for about 70% by weight in bone. This inorganic component typically in the form of [Ca3(PO4)2]3 · Ca(OH)2 is predominantly microcrystalline, though may be present in amorphous forms.44 The microcrystallite forms are platelets or rods, about 8-15 Å thick, 20-40 Å wide, and 200-400 Å long. Collagen performs a regulatory role in bone mineral formation, while gelatin, the heat-denatured collagen (without triple helical structure), has a negligible effect on the formation kinetics and HA microstructure.27,45,46 These results indicate the structure other than sequence chemistry determines the HA formation. The biomechanical properties of OI bones are significantly reduced compared to native bone and the collagen content is decreased and the mineral crystalline size and orientation appear altered.47–49 To understand the effect of the collagen matrices on HA crystallization and deposition, mineralization was carried out by alternatively dipping collagen peptide monolayers into calcium chloride and disodium hydrogen phosphate solutions. Mineralization was monitored by AFM. Phase images were recorded for the first, second, and third rounds of mineralization (Figure 6). Minerals grew on all collagen peptide films. The increase of mineral size and density were remarkable on the native-like collagen peptide SAMs; the mineral growth was larger and thicker than on the OI peptide SAMs. This may be attributed to the ordered assembly of the peptides, facilitating mineral growth via a more organized organic template, akin to native bone. It was difficult for minerals to nucleate and grow on the collagen peptide films formed from the mutant collagen peptides, including the valine and aspartic acid mutants. On the alanine variants, some large clusters of minerals composed of many small crystallites formed.

Figure 6. AFM phase images of mineralization on collagen peptide monolayers. For each peptide sample, left column represents 5 µm × 5 µm and right column represents 1 µm × 1 µm scan size; the top, middle, and bottom rows are first, second, and third round of mineralization, respectively.

Larger minerals were obtained after 10 rounds of mineralization on native-like and OI collagen peptide films. The morphology of the minerals was imaged by SEM and their composition was analyzed by EDS (Figure 7). The calcium to phosphorus molar ratio is shown in Table 2. The calcium phosphate minerals on native-like and alanine mutant collagen peptide films were thicker than those on the valine and aspartic acid variants. The EDS analysis revealed a Ca/P molar ratio of 1.66 for minerals obtained on native-like collagen peptide films, consistent with HA grown on native collagen matrices (Ca/P ) 1.67).50 Lower Ca/P molar ratios are found in OI bones compared with normal bone.51 The Ca/P molar ratio of calcium phosphates on OI-A films was 1.56, as a consequence of the alanine mutation. In the cases of valine and aspartic acid mutants, higher Ca/P molar ratios (valine mutant, Ca/P ) 1.87; aspartic acid mutant, Ca/P ) 1.81) were observed. This is likely due to the lower triple helical content, ∼60% or less in OI-V and OI-D mutants because calcium phosphates prepared in solution and grown on gelatin membranes, no triple helical content, had a higher Ca/P molar ratio 2.33.52 The Ca/P ratios differing from that of HA indicate the calcium salts obtained from OI peptide mineralization do not agree with the HA formular structure or at least a mixture of HA with other types of calcium phosphates. The SEM and EDS results indicate that the morphology and composition of the calcium phosphates are regulated by collagen peptide

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Figure 7. SEM images of calcium phosphate grown on native-like and OI collagen peptide films. Elemental analysis of the minerals was by EDS at 10 KeV. Table 2. Ca/P Molar Ratio of Calcium Phosphates Obtained by Mineralization on Native-Like and OI Collagen Peptide Films sample

Ca/P (molar ratio)

native-like collagen peptide film alanine mutant collagen peptide film valine mutant collagen peptide film aspartic acid mutant collagen peptide film

1.66 1.56 1.87 1.81

chemistry and the corresponding features of the assembled structural template.

Conclusions Collagen-like peptides, designed to represent native to OI mutants, were successfully self-assembled and patterned on surfaces. Their assembly behavior demonstrated the influence of primary sequence chemistry on self-organization. Further, changes in the peptide folding and packing impacted function. The surface morphologies of collagen peptide films formed on gold and silicon surfaces found a relationship between the nature of the glycine substitutions to the severity of OI phenotypes with the trend native-like > OI-A > OI-V, OI-D. Mineralization of HA on collagen peptide films demonstrated a direct correlation between alteration in mineralization patterns and amino acid sequence. The insight from sequence-dependent changes in selfassembly and mineralization, patterned after native mutations

of OI, may be useful in gaining further insight into these disease states as well as in probing in vitro modes to influence these outcomes. Acknowledgment. Support from NASA is gratefully acknowledged. We thank Barbara Brodsky for helpful comments on the manuscript.

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