Christopher E. Killian

Transformation and crystallization energetics of synthetic and biogenic amorphous calcium carbonate

Amorphous calcium carbonate (ACC) is a metastable phase often observed during low temperature inorganic synthesis and biomineralization. ACC transforms with aging or heating into a less hydrated form, and with time crystallizes to calcite or aragonite. The energetics of transformation and crystallization of synthetic and biogenic (extracted from California purple sea urchin larval spicules, Strongylocentrotus purpuratus) ACC were studied using isothermal acid solution calorimetry and differential scanning calorimetry. Transformation and crystallization of ACC can follow an energetically downhill sequence: more metastable hydrated ACC → less metastable hydrated ACC⇒anhydrous ACC ∼ biogenic anhydrous ACC⇒vaterite → aragonite → calcite. In a given reaction sequence, not all these phases need to occur. The transformations involve a series of ordering, dehydration, and crystallization processes, each lowering the enthalpy (and free energy) of the system, with crystallization of the dehydrated amorphous material lowering the enthalpy the most. ACC is much more metastable with respect to calcite than the crystalline polymorphs vaterite or aragonite. The anhydrous ACC is less metastable than the hydrated, implying that the structural reorganization during dehydration is exothermic and irreversible. Dehydrated synthetic and anhydrous biogenic ACC are similar in enthalpy. The transformation sequence observed in biomineralization could be mainly energetically driven; the first phase deposited is hydrated ACC, which then converts to anhydrous ACC, and finally crystallizes to calcite. The initial formation of ACC may be a first step in the precipitation of calcite under a wide variety of conditions, including geological CO2 sequestration.

Mechanism of calcite co-orientation in the sea urchin tooth.

Sea urchin teeth are remarkable and complex calcite structures, continuously growing at the forming end and self-sharpening at the mature grinding tip. The calcite (CaCO(3)) crystals of tooth components, plates, fibers, and a high-Mg polycrystalline matrix, have highly co-oriented crystallographic axes. This ability to co-orient calcite in a mineralized structure is shared by all echinoderms. However, the physico-chemical mechanism by which calcite crystals become co-oriented in echinoderms remains enigmatic. Here, we show differences in calcite c-axis orientations in the tooth of the purple sea urchin ( Strongylocentrotus purpuratus ), using high-resolution X-ray photoelectron emission spectromicroscopy (X-PEEM) and microbeam X-ray diffraction (muXRD). All plates share one crystal orientation, propagated through pillar bridges, while fibers and polycrystalline matrix share another orientation. Furthermore, in the forming end of the tooth, we observe that CaCO(3) is present as amorphous calcium carbonate (ACC). We demonstrate that co-orientation of the nanoparticles in the polycrystalline matrix occurs via solid-state secondary nucleation, propagating out from the previously formed fibers and plates, into the amorphous precursor nanoparticles. Because amorphous precursors were observed in diverse biominerals, solid-state secondary nucleation is likely to be a general mechanism for the co-orientation of biomineral components in organisms from different phyla.

Phase transitions in biogenic amorphous calcium carbonate

Crystalline biominerals do not resemble faceted crystals. Current explanations for this property involve formation via amorphous phases. Using X-ray absorption near-edge structure (XANES) spectroscopy and photoelectron emission microscopy (PEEM), here we examine forming spicules in embryos of Strongylocentrotus purpuratus sea urchins, and observe a sequence of three mineral phases: hydrated amorphous calcium carbonate (ACC·H2O) → dehydrated amorphous calcium carbonate (ACC) → calcite. Unexpectedly, we find ACC·H2O-rich nanoparticles that persist after the surrounding mineral has dehydrated and crystallized. Protein matrix components occluded within the mineral must inhibit ACC·H2O dehydration. We devised an in vitro, also using XANES-PEEM, assay to identify spicule proteins that may play a role in stabilizing various mineral phases, and found that the most abundant occluded matrix protein in the sea urchin spicules, SM50, stabilizes ACC·H2O in vitro.

Characterization of the Proteins Comprising the Integral Matrix of Strongylocentrotus purpuratus Embryonic Spicules

In the present study, we enumerate and characterize the proteins that comprise the integral spicule matrix of the Strongylocentrotus purpuratus embryo. Two-dimensional gel electrophoresis of [S]methionine radiolabeled spicule matrix proteins reveals that there are 12 strongly radiolabeled spicule matrix proteins and approximately three dozen less strongly radiolabeled spicule matrix proteins. The majority of the proteins have acidic isoelectric points; however, there are several spicule matrix proteins that have more alkaline isoelectric points. Western blotting analysis indicates that SM50 is the spicule matrix protein with the most alkaline isoelectric point. In addition, two distinct SM30 proteins are identified in embryonic spicules, and they have apparent molecular masses of approximately 43 and 46 kDa. Comparisons between embryonic spicule matrix proteins and adult spine integral matrix proteins suggest that the embryonic 43-kDa SM30 protein is an embryonic isoform of SM30. An adult 49-kDa spine matrix protein is also identified as a possible adult isoform of SM30. Analysis of the SM30 amino acid sequences indicates that a portion of SM30 proteins is very similar to the carbohydrate recognition domain of C-type lectin proteins.

Nacre Protein Fragment Templates Lamellar Aragonite Growth

Proteins play a major role in the formation of all biominerals. In mollusk shell nacre, complex mixtures and assemblies of proteins and polysaccharides were shown to induce aragonite formation, rather than the thermodynamically favored calcite (both aragonite and calcite are CaCO(3) polymorphs). Here we used N16N, a single 30 amino acid-protein fragment originally inspired by the mineral binding site of N16, a protein in the nacre layer of the Japanese pearl oysters (Pinctada fucata). In a calcite growth solution this short peptide induces in vitro biomineralization. This model biomineral was analyzed using X-ray PhotoElectron Emission spectroMicroscopy (X-PEEM) and found to be strikingly similar to natural nacre: lamellar aragonite with interspersed N16N layers. This and other findings combined suggest a hypothetical scenario in which in vivo three proteins (N16, Pif80, and Pif97) and a polysaccharide (chitin) work in concert to form lamellar nacre.

Characterization and expression of a gene encoding a 30.6-kDa Strongylocentrotus purpuratus spicule matrix protein

We describe here the isolation and characterization of several cDNA clones that encode a single 30.6-kDa Strongylocentrotus purpuratus spicule matrix protein designated SM30. The clones were isolated by screening a lambda gt11 cDNA library with a rabbit polyclonal antiserum raised against S. purpuratus total spicule matrix proteins. DNA sequencing reveals that the SM30 protein is acidic. RNA blot analysis shows that the cDNAs hybridize to a single 1.8-kb transcript and that there is a sharp increase in the SM30 transcript levels at middle to late mesenchyme blastula stage. SM30 transcript levels remain high through the 3-day pluteus stage. In situ hybridization analysis indicates that, within the embryo, SM30 transcript accumulation is restricted to the primary mesenchyme cells. Quantitations of SM30 transcript levels show that by the prism stage there are about 29,000 SM30 transcripts present per embryo, which averages to approximately 480 transcripts per primary mesenchyme cell. Additionally, RNA blot analysis of total RNA isolated from adult tissues shows that SM30 mRNA accumulates exclusively in mineralized tissues. These findings taken together strongly suggest that the gene corresponding to the SM30 cDNAs does in fact encode a spicule matrix protein.

Ultrastructural localization of proteins involved in sea urchin biomineralization.

Three skeletal tissues of the adult echinoid Paracentrotus lividus (the pedicellaria primordium, the test, and the tooth) were immunolabeled with three sera raised against the total mineralization organic matrix and two specific matrix proteins (SM30 and SM50) from the embryo of the echinoid Strongylocentrotus purpuratus. Two conventional chemical fixation protocols and two high-pressure freezing/freeze-substitution protocols were tested. One conventional protocol is recommended for its good preservation of the ultrastructure, and one high-pressure freezing/freeze-substitution protocol is recommended for its good retention of antigenicity. Immunolabeling was obtained in the three adult tissues. It was confined to the active skeleton-forming cells and to the structured organic matrix. The results indicate that the matrix proteins follow the classical routes of secretory protein assembly and export and suggest that SM30 and SM50 are a part of the tridimensional network formed by the organic matrix before the onset of mineralization. They show that the genetic program of part of skeletogenesis is conserved among different calcification models and developmental stages.

The dynamics of secretion during sea urchin embryonic skeleton formation

Skeleton formation involves secretion of massive amounts of mineral precursor, usually a calcium salt, and matrix proteins, many of which are deposited on, or even occluded within, the mineral. The cell biological underpinnings of this secretion and subsequent assembly of the biomineralized skeletal element is not well understood. We ask here what is the relationship of the trafficking and secretion of the mineral and matrix within the primary mesenchyme cells of the sea urchin embryo, cells that deposit the endoskeletal spicule. Fluorescent labeling of intracellular calcium deposits show mineral precursors are present in granules visible by light microscopy, from whence they are deposited in the endoskeletal spicule, especially at its tip. In contrast, two different matrix proteins tagged with GFP are present in smaller post-Golgi vesicles only seen by electron microscopy, and the secreted protein are only incorporated into the spicule in the vicinity of the cell of origin. The matrix protein, SpSM30B, is post-translationally modified during secretion, and this processing continues after its incorporation into the spicule. Our findings also indicate that the mineral precursor and two well characterized matrix proteins are trafficked by different cellular routes.

Crystal lattice tilting in prismatic calcite

We analyzed the calcitic prismatic layers in Atrina rigida (Ar), Haliotis iris (Hi), Haliotis laevigata (HL), Haliotis rufescens (Hrf), Mytilus californianus (Mc), Pinctada fucata (Pf), Pinctada margaritifera (Pm) shells, and the aragonitic prismatic layer in the Nautilus pompilius (Np) shell. Dramatic structural differences were observed across species, with 100-μm wide single-crystalline prisms in Hi, HL and Hrf, 1-μm wide needle-shaped calcite prisms in Mc, 1-μm wide spherulitic aragonite prisms in Np, 20-μm wide single-crystalline calcite prisms in Ar, and 20-μm wide polycrystalline calcite prisms in Pf and Pm. The calcite prisms in Pf and Pm are subdivided into sub-prismatic domains of orientations, and within each of these domains the calcite crystal lattice tilts gradually over long distances, on the order of 100 μm, with an angle spread of crystal orientation of 10-20°. Furthermore, prisms in Pf and Pm are harder than in any other calcite prisms analyzed, their nanoparticles are smaller, and the angle spread is strongly correlated with hardness in all shells that form calcitic prismatic layers. One can hypothesize a causal relationship of these correlated parameters: greater angle spread may confer greater hardness and resistance to wear, thus providing Pf and Pm with a structural advantage in their environment. This is the first structure-property relationship thus far hypothesized in mollusk shell prisms.

The accumulation and translation of a spicule matrix protein mRNA during sea urchin embryo development

In this report we further characterize the expression of the gene that encodes the 50-kDa spicule matrix protein (SM50) during development of the sea urchin Strongylocentrotus purpuratus. Quantitative measurements of SM50 mRNA levels using the single-stranded probe excess titration technique indicate that SM50 transcript levels attain a maximum level of 8000 to 10,000 transcripts per embryo by the gastrula stage, representing 120 to 200 SM50 mRNAs per primary mesenchyme cell. Experiments analyzing run-on transcription in nuclei isolated at different stages of development indicate that the sharp increase in SM50 mRNA levels occurring at the time of primary mesenchyme ingression is concomitant with an increase in transcription of the SM50 gene. We have also analyzed the RNA sequences present on polyribosomes at different stages of development. These studies indicate that SM50 mRNA is present on polyribosomes as soon as it begins to accumulate (which is well in advance of overt spicule formation) and SM50 mRNA remains on polyribosomes through subsequent development. From estimates of the rate of SM50 protein synthesis based on these data, we calculated that the maximum amount of SM50 accumulated during development through the 4-day pluteus stage is approximately 7.4 pg/embryo. This approximation is concordant with the amount of SM50 actually found in the sea urchin embryo.