Development of biomimetic models to investigate the role of strontium on bone mineralization

Author(s)

Camila Bussola Tovani

• Document type: Doctoral Thesis
• Press: Ribeirão Preto.
• Institution: Universidade de São Paulo (USP). Faculdade de Filosofia, Ciências e Letras de Ribeirão Preto (PCARP/BC)
• Defense date: 2020-06-01

Examining board members

Ana Paula Ramos; Celso Aparecido Bertran; Vera Regina Leopoldo Constantino; Rogéria Rocha Gonçalves

• Advisor: Ana Paula Ramos

Abstract

Strontium ions (Sr2+) are the active component of strontium ranelate, a drug which reduces bone fractures in osteoporotic patients. This finding has in turn encouraged the incorporation of Sr2+ into biomaterials and organic molecules aiming at bone healing. Although the widespread interest in Sr2+-based biomaterials has emerged over the last years, the mechanisms underlying its involvement as well as the possible effects resulting from its accumulation in bone tissue have received by far less attention. Clinical studies have reported the development of pathological mineralization due to the excess of Sr2+ reinforcing the need of deeper investigations on the action of this ion at the molecular level of bone. To address this shortcoming, this thesis presents a comprehensive investigation of the impacts of Sr2+ on the structural properties of bone comprising its organic and inorganic parts. Therefore, three biomimetic models were stablished: (i) carbonated apatite displaying the main structural features found in bone mineral i.e. crystals with plate-like morphology and preferential crystallographic orientation along the c axis, (ii) dense and anisotropic mineralized type-I collagen matrices which reproduce the hierarchy of bone at the tissue level and (iii) polycarbonate membranes with cylindrical pores mimicking the confined spaces where biomineralization takes place. While the substitution of Ca2+ by Sr2+ in synthetic apatite is commonly described as isomorphic in the whole range of concentration, we unexpectedly observed a secondary phase. Such phase was characterized as a Sr2+-rich amorphous calcium phosphate [Sr(ACP)] and was formed simultaneously with Sr2+-substituted apatite. This finding opens questions on the current knowledge concerning the substitution of Ca2+ by Sr2+ in synthetic apatite. Moreover, the detection of Sr(ACP) is an evidence of the physicochemical interference of Sr2+ on the formation of bone apatite and therefore its adverse effects at the regime of high doses observed in vivo. Interestingly, the self-assembly of collagen fibrils, the basic building blocks of bone extracellular matrix, was also impaired at high concentrations of Sr2+. The use of confinement provided information regarding the early stages of bone formation, showing that Sr2+ and the physical environment act in synergism to stabilize kinetic intermediates of apatite. All these results were obtained relying on a panel of techniques, i.e., transmission electron microscopy, selected area electron diffraction, energy electron loss spectroscopy, solid state nuclear magnetic resonance and Raman spectroscopy. Overall, by using biomimetic models, this thesis gives clues from the physicochemical standpoint to understand the multiple effects of Sr2+ on bone tissue. (AU)

• FAPESP's process: 14/24249-0 - Association of osteogenic proteins in biominerals and metallic oxides doped with rare-earth: interaction with membrane models systems
• Grantee: Camila Bussola Tovani
• Support Opportunities: Scholarships in Brazil - Doctorate (Direct)