Morphological Evolution of Calcite Grown in Zwitterionic Hydrogels: Charge Effects Enhanced by Gel-Incorporation

The incorporation of charged biomacromolecules is widely found in biomineralization. To investigate the significance of this biological strategy for mineralization control, gelatin-incorporated calcite crystals grown from gelatin hydrogels with different charge concentrations along the gel networks are examined. It is found that the bound charged groups on gelatin networks (amino cations, gelatin-NH₃ ⁺ and carboxylic anions, gelatin-COO^- ) play crucial roles in controlling the single-crystallinity and the crystal morphology. And the charge effects are greatly enhanced by the gel-incorporation because the incorporated gel networks force the bound charged groups on them to attach to crystallization fronts. In contrast, ammonium ions (NH₄ ⁺ ) and acetate ions (Ac^- ) dissolve in the crystallization media do not exhibit the similar charge effects because the balance of attachment/detachment make them more difficult to be incorporated. Employing the revealed charge effects, the calcite crystal composites with different morphologies can be flexibly prepared.

Second harmonic generation microscopy of otoconia

The origin of second harmonic generation (SHG) signal in otoconia was investigated. SHG signal intensity from otoconia was compared to pure calcite crystals, given calcite is the primary component of otoconia and is known to emit surface SHG. The SHG intensity from calcite was found to be ∼41× weaker than the SHG intensity from otoconia signifying that the SHG signal from otoconia is likely generated from the organic matrix. Furthermore, the SHG intensity from otoconia increased when treated with a chelating agent known to dissolve calcite which confirms that calcite is not the source of SHG. Additionally, polarization-resolved SHG microscopy imaging revealed that the arrangement of the SHG emitters is radial and can form highly ordered domains.

Molecular mechanism of calcium induced trimerization of C1q-like domain of otolin-1 from human and zebrafish

The C1q superfamily includes proteins involved in innate immunity, insulin sensitivity, biomineralization and more. Among these proteins is otolin-1, which is a collagen-like protein that forms a scaffold for the biomineralization of inner ear stones in vertebrates. The globular C1q-like domain (gC1q), which is the most conserved part of otolin-1, binds Ca²⁺ and stabilizes its collagen-like triple helix. The molecular details of the assembly of gC1q otolin-1 trimers are not known. Here, we substituted putative Ca²⁺-binding acidic residues of gC1q otolin-1 with alanine to analyse how alanine influences the formation of gC1q trimers. We used human and zebrafish gC1q otolin-1 to assess how evolutionary changes affected the function of the protein. Surprisingly, the mutated forms of gC1q otolin-1 trimerized even in the absence of Ca²⁺, although they were less stable than native proteins saturated with Ca²⁺. We also found that the zebrafish gC1q domain was less stable than the human homologue under all tested conditions and became stabilized at higher concentrations of Ca²⁺, which showed that specific interactions leading to the neutralization of the negative charge at the axis of a gC1q trimer by Ca²⁺ are required for the trimers to form. Moreover, human gC1q otolin-1 seems to be optimized to function at lower concentrations of Ca²⁺, which is consistent with reported Ca²⁺ concentrations in the endolymphs of fish and mammals. Our results allow us to explain the molecular mechanism of assembly of proteins from the C1q superfamily, the modulating role of Ca²⁺ and expand the knowledge of biomineralization of vertebrate inner ear stones: otoliths and otoconia.

Nanostructure of mouse otoconia

Mammalian otoconia of the inner ear vestibular apparatus are calcium carbonate-containing mineralized structures critical for maintaining balance and detecting linear acceleration. The mineral phase of otoconia is calcite, which coherently diffracts X-rays much like a single-crystal. Otoconia contain osteopontin (OPN), a mineral-binding protein influencing mineralization processes in bones, teeth and avian eggshells, for example, and in pathologic mineral deposits. Here we describe mineral nanostructure and the distribution of OPN in mouse otoconia. Scanning electron microscopy and atomic force microscopy of intact and cleaved mouse otoconia revealed an internal nanostructure (~50 nm). Transmission electron microscopy and electron tomography of focused ion beam-prepared sections of otoconia confirmed this mineral nanostructure, and identified even smaller (~10 nm) nanograin dimensions. X-ray diffraction of mature otoconia (8-day-old mice) showed crystallite size in a similar range (73 nm and smaller). Raman and X-ray absorption spectroscopy - both methods being sensitive to the detection of crystalline and amorphous forms in the sample - showed no evidence of amorphous calcium carbonate in these mature otoconia. Scanning and transmission electron microscopy combined with colloidal-gold immunolabeling for OPN revealed that this protein was located at the surface of the otoconia, correlating with a site where surface nanostructure was observed. OPN addition to calcite growing in vitro produced similar surface nanostructure. These findings provide details on the composition and nanostructure of mammalian otoconia, and suggest that while OPN may influence surface rounding and surface nanostructure in otoconia, other incorporated proteins (also possibly including OPN) likely participate in creating internal nanostructure.

Magnesium as an intrinsic component of human otoconia

OBJECTIVES

To investigate morphology changes of artificial otoconia (CGC) in the presence of magnesium during growth under in vitro conditions.

METHODS

Investigating human otoconia by environmental scanning electron microscope and determining their magnesium content by energy-dispersive X-ray microanalysis (EDX). Comparing structural and morphological data of human and artificial otoconia (CGC, Ca₁Mg₀) without and with magnesium substitution (Ca_1-xMg_x).

RESULTS

EDX- and X-ray data reveals that the inorganic component in human otoconia consists of calcite containing a minor amount of magnesium substitution (Ca_1-xMg_x). CGC containing magnesium (length 397.0 ± 146.4 µm, diameter 325.6 ± 100.1 µm) are slimmer and significantly smaller (p < .01) than pure CGC (length 548.6 ± 160 µm, diameter 373.0 ± 110.4 µm) and reveal a significant influence on the final morphology. The length/diameter ratio is significantly higher by incorporation of magnesium into CGC (1.84 ± 0.25 µm versus 1.48 ± 0.11 µm in pure CGC, p < .01), which brings the overall shape to a close relationship with human otoconia (1.98 ± 0.08 µm).

CONCLUSIONS

Magnesium is an intrinsic component of human otoconia by partial substitution of calcium in the calcite crystal structure (Ca_1-xMg_x) and affects the development of the shape of artificial otoconia (calcite gelatin composites, CGC).

The sense of balance in humans: Structural features of otoconia and their response to linear acceleration

We explored the functional role of individual otoconia within the otolith system of mammalians responsible for the detection of linear accelerations and head tilts in relation to the gravity vector. Details of the inner structure and the shape of intact human and artificial otoconia were studied using environmental scanning electron microscopy (ESEM), including decalcification by ethylenediaminetetraacetic acid (EDTA) to discriminate local calcium carbonate density. Considerable differences between the rhombohedral faces of human and artificial otoconia already indicate that the inner architecture of otoconia is not consistent with the point group -3m. This is clearly confirmed by decalcified otoconia specimen which are characterized by a non-centrosymmetric volume distribution of the compact 3+3 branches. This structural evidence for asymmetric mass distribution was further supported by light microscopy in combination with a high speed camera showing the movement of single otoconia specimen (artificial specimen) under gravitational influence within a viscous medium (artificial endolymph). Moreover, the response of otoconia to linear acceleration forces was investigated by particle dynamics simulations. Both, time-resolved microscopy and computer simulations of otoconia acceleration show that the dislocation of otoconia include significant rotational movement stemming from density asymmetry. Based on these findings, we suggest an otolith membrane expansion/stiffening mechanism for enhanced response to linear acceleration transmitted to the vestibular hair cells.

[Otoconia : Current aspects of research]

Otoconia are calcite-based nanocomposites containing >90 % calcite and <10 % organic material. The mean size is approximately 10 µm. The external structure of all otoconia in the utricle and saccule is similar, with a cylindrical bulbous body with a slightly hexagonal contour. The internal structure consists of a composite with varying volume thickness, dense branching structures (branches) and less dense surrounding areas (bellies). Intact otoconia can be clearly identified only by scanning electron microscopy. In the case of morphological changes (e.g. due to "degeneration") the origin of even very small particles of otoconia can be assigned using physical and chemical analytical methods. The inorganic component of otoconia (calcite) is extremely sensitive to chemical influences, which leads to morphological alterations. A "degeneration" of otoconia can be objectively accomplished in vitro by alterations in pH, electrolyte imbalance and by the influence of complex formation. These three main processes then lead to irreversible morphological alterations. Artificial (biomimetic) otoconia serve as a suitable model system for detailed investigation of growth and degenerative processes.

Otoconia: Mimicking a calcite-based functional material of the human body. From basic research to medical aspects

Otoconia (calcite-based biominerals) are part of the sensory system in the inner ear of vertebrates, acting as gravity receptors responding to linear accelerations. Biomimetic otoconia are grown by double-diffusion into gelatine-gel matrices, and represent the first example of successful imitation of a biomineral, not only in outer shape but also in composite structure and hierarchical inner architecture. Biomimetic and biogenic (human) otoconia are investigated by X-ray methods, chemical analytics, ESEM, and TEM. Shape development (morphogenesis) as well as (partial) dissolution of the calcite component of the composite underline the hierarchical inner architecture built of more dense rhombohedral branches (with plane end-faces) and a rounded, more porous belly area. Atomistic simulations are performed in order to get insight into very first nucleation steps. Based on the detailed observations made up to now, first assumptions for the function of otoconia are developed, including the questions of density distribution within the volume of the specimen, the surrounding endolymph, as well as anchoring and interconnections of otoconia. A final point concerns the degeneration of otoconia which is caused by complexing agents and/or changes in ion concentrations (and pH) of the endolymph.

Principles of calcite dissolution in human and artificial otoconia

Human otoconia provide mechanical stimuli to deflect hair cells of the vestibular sensory epithelium for purposes of detecting linear acceleration and head tilts. During lifetime, the volume and number of otoconia are gradually reduced. In a process of degeneration morphological changes occur. Structural changes in human otoconia are assumed to cause vertigo and balance disorders such as benign paroxysmal positional vertigo (BPPV). The aim of this study was to investigate the main principles of morphological changes in human otoconia in dissolution experiments by exposure to hydrochloric acid, EDTA, demineralized water and completely purified water respectively. For comparison reasons artificial (biomimetic) otoconia (calcite gelatin nanocomposits) and natural calcite were used. Morphological changes were detected in time steps by the use of environmental scanning electron microscopy (ESEM). Under in vitro conditions three main dissolution mechanisms were identified as causing characteristic morphological changes of the specimen under consideration: pH drops in the acidic range, complex formation with calcium ions and changes of ion concentrations in the vicinity of otoconia. Shifts in pH cause a more uniform reduction of otoconia size (isotropic dissolution) whereas complexation reactions and changes of the ionic concentrations within the surrounding medium bring about preferred attacks at specific areas (anisotropic dissolution) of human and artificial otoconia. Owing to successive reduction of material, all the dissolution mechanisms finally produce fragments and remnants of otoconia. It can be assumed that the organic component of otoconia is not significantly attacked under the given conditions. Artificial otoconia serve as a suitable model system mimicking chemical attacks on biogenic specimens. The underlying principles of calcite dissolution under in vitro conditions may play a role in otoconia degeneration processes such as BPPV.

Gentamicin-induced structural damage of human and artificial (biomimetic) otoconia

CONCLUSIONS

Gentamicin causes irreversible structural damage of human and artificial otoconia by progressive dissolution of calcite. The inner architecture of otoconia is strongly affected by degradation scenarios during gentamicin exposure. Artificial otoconia can be used as a model system mimicking the chemical attacks for detailed investigations.

OBJECTIVES

To investigate the chemical interactions of gentamicin with natural calcite and human and artificial otoconia under in vivo conditions.

METHODS

Pure calcite crystals and artificial and human otoconia were exposed to gentamicin injection solutions at various concentrations. Morphological changes were observed in time steps by the use of environmental scanning electron microscopy (ESEM).

RESULTS

Dissolution of pure calcite crystals results in the formation of well oriented nanoshoots indicating an irreversible chemical reaction with gentamicin. Human and artificial otoconia reveal irreversible structural changes of their surface areas as well as of their inner structure, resulting in characteristic changes at different gentamicin concentrations. Minor changes are first observed by surface alterations and dissolution of calcite in the belly region. Major changes result in further reduction of the belly area reaching the center of symmetry. Finally, a complete dissolution of the branches takes place. Artificial otoconia provide detailed insight into surface alterations.