Hydroxyapatite Formation Coexists with Amyloid-like Self-Assembly of Human Amelogenin.

Jing Zhang,Chengwei Ma,Junxia Lu,Jian Wang

doi:10.3390/ijms21082946

Abstract

Tooth enamel is formed in an extracellular environment. Amelogenin, the major component in the protein matrix of tooth enamel during the developing stage, could assemble into high molecular weight structures, regulating enamel formation. However, the molecular structure of amelogenin protein assembly at the functional state is still elusive. In this work, we found that amelogenin is able to induce calcium phosphate minerals into hydroxyapatite (HAP) structure in vitro at pH 6.0. Assessed using X-ray diffraction (XRD) and 31P solid-state NMR (SSNMR) evidence, the formed HAP mimics natural enamel closely. The structure of amelogenin protein assembly coexisting with the HAP was also studied using atomic force microscopy (AFM), transmission electron microscopy (TEM) and XRD, indicating the β-amyloid structure of the protein. SSNMR was proven to be an important tool in detecting both the rigid and dynamic components of the protein assembly in the sample, and the core sequence 18EVLTPLKWYQSI29 was identified as the major segment contributing to the β-sheet secondary structure. Our research suggests an amyloid structure may be an important factor in controlling HAP formation at the right pH conditions with the help of other structural components in the protein assembly.

Highlights

Enamel, the material covering and protecting the teeth, is composed of hydroxyapatite (HAP) crystals [1]
The final mixtures were subjected to atomic force microscopy (AFM) examination (Figure 1a–c), showing smaller curvy aggregates at pH 4.5 and straight rod structures at pH 5.0 and 5.5
Our results indicate that the amyloid formation could happen in a big pH range, the precipitated mineral components could be very different

Summary

Introduction

The material covering and protecting the teeth, is composed of hydroxyapatite (HAP) crystals [1]. Different from bone or dentin, the elongated hydroxyapatite crystals are interwoven into a highly organized microstructure in enamel, bestowing enamel with extraordinary strength and resistance to fracture [2]. Amelogenin, as the major component of enamel protein, plays a key role in regulating enamel mineral morphology [3]. Mutation of amelogenin causes amelogenesis imperfect disorders in human [4]. Knockout of amelogenin in mice results in defective enamel formation [5]. The biomimetic approach has utilized amelogenin to regenerate enamel-like materials on teeth in gentle and close to physiological conditions [6]. Full-length amelogenin [7], leucine-rich amelogenin peptide (LRAP) [8] and other amelogenin-derived peptides [9] have all been tested and proved to have great potential in dentistry or biomaterials

Methods

Results

Conclusion