The ion-binding characteristics of reconstituted collagen

Smart/stimuli-responsive chitosan/gelatin and other polymeric macromolecules natural hydrogels vs. synthetic hydrogels systems for brain tissue engineering: A state-of-the-art review

Currently, there are no viable curative treatments that can enhance the central nervous system's (CNS) recovery from trauma or illness. Bioengineered injectable smart/stimuli-responsive hydrogels (SSRHs) that mirror the intricacy of the CNS milieu and architecture have been suggested as a way to get around these restrictions in combination with medication and cell therapy. Additionally, the right biophysical and pharmacological stimuli are required to boost meaningful CNS regeneration. Recent research has focused heavily on developing SSRHs as cutting-edge delivery systems that can direct the regeneration of brain tissue. In the present article, we have discussed the pathology of brain injuries, and the applicable strategies employed to regenerate the brain tissues. Moreover, the most promising SSRHs for neural tissue engineering (TE) including alginate (Alg.), hyaluronic acid (HA), chitosan (CH), gelatin, and collagen are used in natural polymer-based hydrogels and thoroughly discussed in this review. The ability of these hydrogels to distribute bioactive substances or cells in response to internal and external stimuli is highlighted with particular attention. In addition, this article provides a summary of the most cutting-edge techniques for CNS recovery employing SSRHs for several neurodegenerative diseases.

Tailoring the assembly of collagen fibers in alginate microspheres

The application of microspheres instead of bulk hydrogels in cell-laden biomaterials offers multiple advantages such as a high surface-to-volume-ratio and, consequently, a better nutrition and oxygen transfer to and from cells. The preparation of inert alginate microspheres is facile, quick, and well-established and the fabrication of alginate-collagen microspheres has been previously reported. However, no detailed characterization of the collagen fibrillogenesis in the alginate matrix is available. We use second-harmonic imaging microscopy reflection confocal microscopy and turbidity assay to study the assembly of collagen in alginate microspheres. We show that the assembly of collagen fibers in a gelled alginate matrix is a complex process that can be aided by addition of small polar molecules, such as glycine and by a careful selection of the gelling buffer used to prepare alginate hydrogels.

Bundling of Collagen Fibrils Using Sodium Sulfate for Biomimetic Cell Culturing

Collagen is the most abundant extracellular matrix protein. The concentrations, structural arrangement, and directionality of collagen depend on the type of tissue. Thick fibril bundles of collagen are observed in most collagenous tissues, including connective tissues, bones, and tendons, indicating that they play a critical role in many cell functions. In this study, we developed a new method to regulate collagen bundling without altering the protein concentration, temperature, or pH by using sodium sulfate to replicate bundled collagen fibrils found in vivo. Microstructure analysis revealed that both the thickness of the fibril bundles and the pore size of the matrix increased with the amount of sodium sulfate. In contrast, there was no significant change in the bulk mechanical stiffness of the collagen matrix. The modified collagen bundle matrix was used to investigate the responses of human cervical cancer cells by mimicking the extracellular environments of a tumor. Compared to the normal collagen matrix, cells on the collagen bundle matrix exhibited significant changes in morphology, with a reduced cell perimeter and aspect ratio. The cell motility, which was analyzed in terms of the speed of migration and mean squared displacement, decreased for the collagen bundle matrix. Additionally, the critical time taken for the peak turning angle to converge to 90° decreased, indicating that the migration direction was regulated by geometric cues provided by collagen bundles rather than by the intrinsic cell persistence. The experimental results imply that collagen bundles play an important role in determining the magnitude and direction in cancer cell migration. The proposed method of extracellular matrix modification can be applied to investigate various cellular behaviors in both physiological and pathological environments.

Electrostatic effects in collagen fibril formation

Using light scattering and Atomic Force Microscopy techniques, we have studied the kinetics and equilibrium scattering intensity of collagen association, which is pertinent to the vitreous of the human eye. Specifically, we have characterized fibrillization dependence on pH, temperature, and ionic strength. At higher and lower pH, collagen triple helices remain stable in solution without fibrillization. At physiological pH, fibrillization occurs and the fibril growth is slowed upon either an increase in ionic strength or a decrease in temperature. The total light scattering with respect to ionic strength is non-monotonic in these conditions as a result of a competing dependence of fibril concentration and size on ionic strength. Fibril concentration is the highest at lower ionic strengths and rapidly decays for higher ionic strengths. On the other hand, fibril size is larger in solutions with higher ionic strength. We present a theoretical model, based on dipolar interactions in solutions, to describe the observed electrostatic nature of collagen assembly. At extreme pH values, either very low or very high, collagen triple helices carry a large net charge of the same sign preventing their assembly into fibrils. At intermediate pH values, fluctuations in the charge distribution of the collagen triple helices around roughly zero net charge lead to fibrillization. The growth kinetics of fibrils in this regime can be adequately described by dipolar interactions arising from charge fluctuations.

Bioactivity evaluation of collagen-based scaffolds containing a series of Sr-doped melt-quench derived phosphate-based glasses

Phosphate-based glasses have been attracting attention due to their possible medical applications arising from unique dissolution characteristics in the human body leading to the possibility of new tissue regeneration. In this study, the leaching kinetics of a series of melt-quenched Sr-doped phosphate glasses are presented. Regardless of the presence of Sr, all the glasses have an initial linear and sustained release of the ions followed by a plateau. To guarantee proper nutritional support to the growing tissue during regeneration and to mimic the 3-dimensional architecture of tissues, organic scaffold systems have been developed. However, their poor mechanical strength has limited their application. To overcome this problem, cross-linkers can be used although this then limits the solubility of the materials. To succeed in dealing with such a limitation, in this paper, by freeze-drying, the aforementioned soluble melt-quenched phosphate glasses were combined as powders with collagen fibres from bovine achilles tendon to make degradable scaffolds. The scaffolds were characterized by SEM, EDX and BET. Changes to the dissolution behaviour of the glasses arising from the presence of collagen interacting with the ions leached were reported. Furthermore, the ability of the scaffolds to induce hydroxyapatite (HA) formation was evaluated: one the elaborated scaffold could grow an HA-like layer after a week in SBF. Based on the results obtained, a possible application in restorative dentistry is proposed for one or more materials.

Self-assembly of collagen-based biomaterials: preparation, characterizations and biomedical applications

By combining regulatory parameters with characterization methods, researchers can selectively fabricate collagenous biomaterials with various functional responses for biomedical applications.

Collagen intrafibrillar mineralization as a result of the balance between osmotic equilibrium and electroneutrality

Mineralization of fibrillar collagen with biomimetic process-directing agents has enabled scientists to gain insight into the potential mechanisms involved in intrafibrillar mineralization. Here, by using polycation- and polyanion-directed intrafibrillar mineralization, we challenge the popular paradigm that electrostatic attraction is solely responsible for polyelectrolyte-directed intrafibrillar mineralization. As there is no difference when a polycationic or a polyanionic electrolyte is used to direct collagen mineralization, we argue that additional types of long-range non-electrostatic interaction are responsible for intrafibrillar mineralization. Molecular dynamics simulations of collagen structures in the presence of extrafibrillar polyelectrolytes show that the outward movement of ions and intrafibrillar water through the collagen surface occurs irrespective of the charges of polyelectrolytes, resulting in the experimentally verifiable contraction of the collagen structures. The need to balance electroneutrality and osmotic equilibrium simultaneously to establish Gibbs-Donnan equilibrium in a polyelectrolyte-directed mineralization system establishes a new model for collagen intrafibrillar mineralization that supplements existing collagen mineralization mechanisms.

The collagen type I segment long spacing (SLS) and fibrillar forms: Formation by ATP and sulphonated diazo dyes

The collagen type I segment long spacing (SLS) crystallite is a well-ordered rod-like molecular aggregate, ∼300nm in length, which is produced in vitro under mildly acidic conditions (pH 2.5-3.5) in the presence of 1mM ATP. The formation of the SLS crystallite amplifies the inherent linear structural features of individual collagen heterotrimers, due to the punctate linear distribution and summation of the bulkier amino acid side chains along the length of individual collagen heterotrimers. This can be correlated structurally with the 67nm D-banded collagen fibril that is found in vivo, and formed in vitro. Although first described many years ago, the range of conditions required for ATP-induced SLS crystallite formation from acid-soluble collagen have not been explored extensively. Consequently, we have addressed biochemical parameters such as the ATP concentration, pH, speed of formation and stability so as to provide a more complete structural understanding of the SLS crystallite. Treatment of collagen type I with 1mM ATP at neutral and higher pH (6.0-9.0) also induced the formation of D-banded fibrils. Contrary to previous studies, we have shown that the polysulphonated diazo dyes Direct red (Sirius red) and Evans blue, but not Congo red and Methyl blue, can also induce the formation of SLS-like aggregates of collagen, but under markedly different ionic conditions to those employed in the presence of ATP. Specifically, pre-formed D-banded collagen fibrils, prepared in a higher than the usual physiological NaCl concentration (e.g. 500mM NaCl, 20mM Tris-HCl pH7.4 or x3 PBS), readily form SLS aggregates when treated with 0.1mM Direct red and Evans blue, but this did not occur at lower NaCl concentrations. These new data are discussed in relation to the anion (Cl(-)) and polyanion (phosphate and sulphonate) binding by the collagen heterotrimer and their likely role in collagen fibrillogenesis and SLS formation.

Effects of various salts on structural polymorphism of reconstituted type I collagen fibrils

Even though the behavior of collagen monomers self-assembling into fibrils is commonly understood in terms of hydrophobic and electrostatic interactions, the mechanisms that drive their ordered, longitudinal alignment to form a characteristic periodicity are still unclear. By introducing various salts into the collagen fibrillogenesis system, the intermolecular interactions of fibril formation were studied. We found that the pH and ion species play a critical role in forming native fibrils. Turbidity and electron microscopy revealed that collagen self-assembled into fibrils with a native banding pattern in the presence of multivalent ions. The isoelectric point of collagen in 12mM of NaCl is pH 8.9; it shifted to pH 9.4 and pH 7.0 after adding 10mM CaCl2 and Na2SO4, respectively. Native fibrils were reconstituted at pH 7.4 in salts with divalent anions and at pH 9.0 in salts with divalent cations. Circular dichroism spectroscopy showed a loss of helicity in the conditions where fibrillogenesis was unable to be achieved. The multivalent ions not only change the surface charge of collagen, but also facilitate the formation of fibrils with the native D-periodic banding pattern. It is likely that the binding multivalent ions induce the like-charge attraction and facilitate monomers' longitudinal registration to form fibrils with the native banding.

The vWFA2 domain of type VII collagen is responsible for collagen binding

Type VII collagen (Col7) is the major component of anchoring fibrils and very important for skin integrity. This is emphasized by the Col7 related skin blistering diseases dystrophic epidermolysis bullosa and epidermolysis bullosa acquisita. Structural data that provides insights into the interaction network of Col7 and thus providing a basis for a better understanding of the pathogenesis of the diseases is missing. We proved that the von-Willebrand-factor A like domain 2 (vWFA2) of Col7 is responsible for type I collagen binding. The interaction has a K(D) value of 90 μM as determined by SPR and is enthalpy driven as derived from the van't Hoff equation. Furthermore, a hitherto unknown interaction of this domain with type IV collagen was identified. The interaction of vWFA2 with type I collagen is sensitive to the presence of magnesium ions, however, vWFA2 does not contain a magnesium binding site thus magnesium must bind to type I collagen. A lysine residue has been identified to be crucial for type I collagen binding. This allowed localization of the binding site. Mutational analysis suggests different interaction mechanisms in different species and that these interactions might be of covalent nature.

Formation of calcium phosphate/collagen composites through mineralization of collagen matrix

Several types of calcium phosphate/collagen composites, including noncrystalline calcium phosphate/collagen, poorly crystalline carbonate-apatite (PCCA)/collagen, and PCCA + tetracalcium phosphate/collagen composites, were prepared through the mineralization of collagen matrix. The type I collagen was presoaked with a PO(3-)(4) containing solution and then immersed in a Ca(2+) containing solution to allow mineral deposition. The solution of 0.56 M sodium dibasic phosphate (Na(2)HPO(4)) with a pH of nearly 14 was metastable and its crystallization produced Na(2)HPO(4) and sodium tripolyphosphate hexahydrate (Na(5)P(3)O(10)). 6H(2)O), leading to a controlled release of orthophosphate ions during the subsequent mineral precipitation. The development of the composites was investigated in detail. The mineral contributed up to 60-70% of the weight of the final composites. The strength and Young's modulus of the composites in tensile tests overlapped the lower range of values reported for bone. When implanted in muscle tissue, the composite showed biodegradability that was partly through a multinucleated giant cell mediated process. In a bone explant culture model it was observed that bone-derived cells deposited mineralizing collagenous matrix on the composite.

Effect of solution pH on the calcium phosphates formation and ionic diffusion on and through the collagenous matrix

The effect of pH on the calcification and the permselectivity for the Ca2+ and PO4(3-) ions of a collagenous matrix were studied at pH6.5, 7.0 and 7.4 and at 37 degrees C, using slices of Achilles tendon of a cow, which were cut into disks, in an experimental system where Ca2+ and PO4(3-) ions diffused into the collagen disk from the mutually opposite sides of the disk. The products on and inside the disk were a mixture of octacalcium phosphate (OCP) and apatite regardless of the solution pH. At pH6.5, few crystals grew on the PO4-side of the disk, while many crystals grew on the Ca-side. Crystal growth on the PO4-side was enhanced at higher pH, while that on the Ca-side was suppressed. With an increase in pH, the diffusion of Ca2+ ion through the disk increased, while that of PO4(3-) ion decreased. The mode of crystal growth on the collagen disk was consistent with the mode of ionic diffusion through the matrix. Inside the disk, the influence of pH on the crystal growth was not obvious. It was suggested that the electrochemical property of the collagenous matrix regulated the ionic inflow into the matrix and thereby affected its calcification.

Oriented growth of octacalcium phosphate on and inside the collagenous matrix in vitro

To investigate the factors which regulate the growth of apatite in a collagenous matrix, a calcification experiment was carried out in a model system, where Ca2+ and PO43- ions diffused into a slice of Achilles tendon from mutually opposite grew on the Ca-side of the collagenous matrix, while small plate-like OCP crystals (< 1 micron) grew inside the matrix. The major part of crystals grew with the c-axis parallel to the collagen fibers. Others grew with the c-axis parallel to the collagen bands or perpendicular to the fibrils. Crystals grew with orientation on collagen fibrils from the beginning. It was concluded that collagen fibrils and property of the collagenous matrix played a regulatory role in the deposition and growth of OCP in the collagenous matrix.

Vectorial sequence of mineralization in the turkey leg tendon determined by electron microscopic imaging

Turkey leg tendons were used as a model tissue to study the spatial and temporal relationships of mineral deposition between matrix vesicles and collagen fibrils by various electron microscopic techniques--bright field, selected-area dark field (SADF), and electron spectroscopic imaging (ESI). These latter imaging techniques enabled the direct localization and spatial distributions of both apatite crystals and atomic elements (Ca, P) within matrix vesicles and collagen. In longitudinal planes of section, a consistent vectorial gradient of mineralization was observed which started with the first localization of apatite mineral in matrix vesicles; with further development, the mineral spread from the vesicle to the extravesicular interstices and then into the adjacent collagen fibrils. Once intrafibrillar, the mineral was observed to advance both laterally and axially. The association of vesicle/collagen mineral was examined by ESI analysis of Ca and P elemental maps and appeared as a continuum between the vesicles and the adjacent collagen fibrils. Similarly, an intimate spatial relationship was observed between the mineral of vesicles and collagen in transversely cut sections of tendon. The sequential development of this mineralized matrix is discussed in light of matrix vesicle/collagen interactions.

Cation movement in rat articular and non-articular cartilage and in isolated chondrocytes: calcium influx and efflux

1. Calcium ion influx varies between different types of young adult rat cartilage. Sternal cartilage accumulates significantly less Ca2+ than other cartilage types. 2. Influxes of Ca2+ into young adult and ageing tibial cartilage display no significant differences. 3. Efflux of Ca2+ from sternal and tibial cartilage resolves into exponential phases indicative of three compartments. Tracheal cartilage displays two compartment behaviour only. 4. Efflux of Ca2+ from isolated chondrocytes has different characteristics to cartilage efflux with the third slow compartment reduced. 5. Modification of Ca2+ efflux by lanthanum and barium is suggestive of an exchange of strongly bound extracellular calcium during the slow phase of the efflux from young adult tibial cartilage. 6. The metabolic inhibitor 2,4-dinitrophenol is without effect on the efflux of Ca2+ from tibial articular cartilage. 7. The degree of calcium binding exhibited during efflux depends upon cartilage type. Non-articular sternal cartilage binds calcium more strongly than articular tibial, both binding more strongly than non-articular tracheal cartilage. 8. In articular cartilage calcium binding shows an age-related increase.

The promotion of collagen polymerization by lanthanide and calcium ions

Ca2+ (1-5 mM) and lanthanide (20-250 microM) ions enhance the rate of polymerization of purified calf skin collagen (1.5 mg/ml) at pH 7.0 in the presence of 30mM-Tris/HCl and 0.2 M-NaCl. Both the nucleation phase and the growth phase of polymerization are accelerated. The activation energy of the growth phase, 239.3 +/- 24.3 kJ/mol (57.2 +/- 5.8 kcal/mol), is decreased to 145.6 +/- 9.6 kJ/mol (34.8 +/- 2.3 kcal/mol) by 5 mM-Ca2+ and to 75.3 +/- 4.6 kJ/mol (18.0 +/- 1.1 kcal/mol) by 25 microM-Sm3+. In contrast, the activation energy of the nucleation phase, 191.6 +/- 23.4 kJ/mol (45.8 +/- 5.6 kcal/mol), is only slightly decreased by Ca2+ or Sm3+. Collagen fibrils formed in the presence of Sm3+ are thinner than control fibrils, and more thermoresistant.

The effect of inorganic pyrophosphate (PPi) anions on the in vitro collagen fibril formation

This paper presents a study on the effect of sodium salt of pyrophosphate (Na-PPi) in different concentrations (1-10 mMol/l) on the in vitro collagen fibril formation. Collagen was mixture of type I and III collagen dissolved in neutral buffer containing sodium chloride of 1.0 mol/l. Na-PPi added to collagen delayed the fibrillary precipitation of collagen from the solution. Electronmicroscopically the fibrils are of native collagen type, but they are thicker and show a pattern of fine substriation. Calcium and other divalent cations showed differences in their influence on the effect of Na-PPi. This experimental model was developed in order to interpret the supposed role of PPi in the inhibition of apatite crystal formations as well as the fibrogenic effect of its calcium salt in PPi-arthropathy. The effect of PPi on the collagen fibril formation should also be taken into consideration.

[Origin and fixation of pyrophosphates in the parietal bones of young rats]

Calvaria from 6 to 13-day-old rats mineralize and bind pyrophosphates which are transferred from nucleotides, as shown by : 1 -- the increase of pyrophosphates in young rat calvaria incubated with nucleoside triphosphate, especially ATP ; 2 -- the more important increase of pyrophosphate content when ATP is renewed in the incubation medium ; and 3 -- binding of [32P] pyrophosphate [beta 32P] ATP. The effect of preheating of calvaria on 32P binding from [beta32P]ATP led us to assume that two systems might be involved in pyrophosphate fixation : a heat-labile, non-specific, phosphatase system, and a heat-stable pyrophosphotransferase system. cAMP increases the pyrophosphate content of calvaria incubated with ATP : that effect may result from either an inhibition of the phosphatase system, or an activation of the pyrophosphotransferase system.

The influence of inorganic phosphate and citrate anions on the effect of glycosaminoglycans during collagen fibril-formation

Some anions like phosphate, sulfate and citrate delay the fibril-formation. The interaction of phosphate and citrate with chondroitin sulfate-A (CSA) in binding to collagen was investigated in different environmental conditions. By changing the concentration of phosphate from 5 to 50 mM/l and that of CSA from 0.5 to 16 mM/l some kind of competition of the anions was discovered. When different equilibration, systems were used the affinity of CSA to collagen was found to be 10 times greater than that of phosphate at pH 7.2 and I=0.15. As phosphate anions bind to collagen at physiological concentration. phosphate anions may be supposed to influence the biological fibril-formation. On the other hand the CSA exchanges phosphate for collagen. Therefore, glycosamino-glycans (GAG) in connective tissue ground substance in fibrillogenesis may regulate the ion binding and through this the tendency of aggregation of the collagen molecules. The binding of citrate to collagen and its effect and possible role in fibril-formation was also evaluated.

The composition of the cyst wall of the beet cyst-nematode Heterodera schachtii

1. Cyst walls of the beet cyst-nematode (Heterodera schachtii Schmidt) were obtained by sieving a suspension of crushed cysts; about 15mg of dried cyst walls was obtained from 1000 cysts. 2. The cyst walls contained 68% protein calculated from nitrogen content. Glutamic acid, glycine, proline and hydroxyproline made up about 54% by weight of the amino acids obtained on acid hydrolysis. 3. Minor constituents of the cyst wall were hexosamine (3.3%), lipid (6%), carbohydrate (2%) and phenols (2%). The hexosamine was identified as galactosamine. 4. The cyst walls contained inorganic material (ash 17%), most of which was extractable with EDTA, but not with water. Major inorganic components were calcium and phosphorus (1.7% and 1.5% respectively, by weight). Carbon dioxide (about 1% by weight) was liberated from the cyst walls on acidification. 5. The cyst walls of H. schachtii and the potato cyst-nematode (Heterodera rostochiensis) contained different amounts of the same amino acids. They also differed in their inorganic content and in the nature of the hexosamine present.

The connective tissue fiber-water interface: physiological and pathological implications

Water has an important role in the maintenance of the structure and strength of collagen fibers. Lathyrism and certain corticoid hormones alter the degree of hydration of the fibers and consequently affect their strength.

The effect of acid mucopolysaccharides and acid mucopolysaccharide-proteins on fibril formation from collagen solutions

1. The effects of acid mucopolysaccharides and acid mucopolysaccharide-proteins on the size and rate of formation of fibril aggregates from collagen solutions in pH7.6 buffers were studied by turbidimetric and light-scattering methods. 2. Serum albumin, orosomucoid, methylated cellulose, chondroitin sulphate A and chondroitin sulphate C of molecular weight less than 20000, and hyaluronate of molecular weight less than 40000 did not influence rates of fibril formation. Chondroitin sulphate A, chondroitin sulphate C and hyaluronate of high molecular weight retarded the rate of fibril formation. This effect of high-molecular-weight chondroitin sulphate C decreased with increasing ionic strength. Heparin, though of low molecular weight (13000), was highly effective, as was also heparitin sulphate. The chondroitin sulphate-proteins of very high molecular weight were highly effective, despite the fact that for some preparations the component chondroitin sulphate chains had molecular weights much less than 20000. 3. Agents that had delayed fibril formation were also effective in producing an increase in degree of aggregation of fibrillar collagen, as indicated by dissymmetry changes observed in light-scattering experiments at low collagen concentrations. Methylated cellulose and heparin at 2.5mug./ml. were unusual in decreasing aggregation, but heparin at 0.25mug./ml. increased aggregation. Electron microscopy of gels showed fibrils and fibril aggregates with ;normal' collagen spacing and dimensions consistent with the light-scattering results. 4. The rates of electrical transport of agents and of solvent (electro-osmosis) through collagen gels indicated a contribution of molecular entanglement that increased with increase in molecular size of the agents. Electrostatic binding of heparin to collagen was noted. Binding to collagen during fibril formation was also found for heparitin sulphate and a chondroitin sulphate with extra sulphate groups. 5. Electrostatic binding of acid mucopolysaccharide-proteins to collagen may be an important factor in the organization and functioning of connective tissues at all stages of growth and development. Excluded-volume (molecular-entanglement) effects may also be important. These factors operate simultaneously and interact mutually so that precise assessment of their relative importance is difficult.