Nature creates 3-D calcium carbonate (CaCO3) inorganic/organic-based materials to form seashells, invertebrate exoskeletons, vertebrate bone, and, most importantly to the New York University College of Dentistry, dentine and enamel. Researchers there are now focusing on the proteins that modulate the formation of biominerals that, in turn, create new composite materials with unique properties such as increased fracture and puncture resistances—with an eye on better dental composites.
Currently, these researchers are exploring how the CaCO3 matrix is organized inside sea urchin spicules. At first, these spicules are nothing more than chalk. But when combined with sea urchin proteins, they form tiny stacks of “bricks,” creating a structure that provides some of the toughest defense against predators and harsh ocean conditions.
“Primary mesenchyme cells (PMCs) inside a sea urchin embryo deposits amorphous CaCO3 within the matrix of spicule proteins where these bricks are shaped into layers of CaCO3 crystals,” said Gaurav Jain, PhD, and coauthor of the study. “However, the functional and assembly capabilities of individual spicule matrix proteins aren’t clear. We are currently investigating one such protein found inside the spicules of a sea urchin embryo to understand what makes these proteins such efficient ‘brick organizers.’”
The researchers examined SM50, one of the most abundant and well-studied proteins inside these spicules. They found that a recombinant version of SM50, rSpSM50, is a highly aggregation-prone protein that forms tiny jelly-like structures called hydrogels in solution. These “jellies” capture tiny mineral nanoparticles and organize them into crystalline “bricks.” Moreover, rSpSM50 causes surface texturing and forms randomly interconnected porous channels within these crystals.
“What is unique about rSpSM50 is that it fosters the formation and organization of 2 different forms of CaCO3, calcite and vaterite, within the ‘jellies’ themselves, including fracture resistance to the overall structure,” said Jain.
Using a specific type of titration method that revealed the details about very early events in the spicule formation, the researchers found that rSpSM50 is a vital part of the process, as it slows the formation kinetics but neither stabilizes nor destabilizes the extremely tiny mineral particles that eventually form these bricks.
“Our ultimate goal is to determine the molecular properties of these proteins that allow matrices to assemble, mineralize, and participate in the formation of naturally occurring organic/inorganic skeletal structures,” said study coauthor John Evans, DMD, PhD. “The hope is that the comprehensive understanding of spicule proteins will enable the development of tunable fracture resistant materials that one day will find its use in developing lightweight ‘armor’ and ‘sturdier’ dental composites.”
The study, “A Model Sea Urchin Spicule Matrix Protein, rSpSM50, Is a Hydrogelator That Modifies and Organizes the Mineralization Process,” was published by Biochemistry.
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