Regional contribution of proteoglycans to the fracture toughness of the dentin extracellular matrix.
This study investigated the contribution of small leucine rich proteoglycans (SLRPs) to the fracture toughness of the dentin extracellular matrix (ECM) by enzymatically-assisted selective removal of glycosaminoglycan chains (GAGs) and proteoglycans (PGs) core protein. We adapted the Mode III trouser tear test to evaluate the energy required to tear the dentin ECM. Trouser-shaped dentin specimens from crown and root were demineralized. Depletion of GAGs and PGs followed enzymatic digestion using chondroitinase ABC (c-ABC) and matrix metalloproteinase 3 (MMP-3), respectively. The legs from specimen were stretched under tensile force and the load at tear propagation was determined to calculate the tear energy (T, kJ/m2). SLRPs decorin and biglycan were visualized by immunohistochemistry and ECM tear pattern was analyzed in SEM. Results showed T of crown ECM was not affected by PGs/GAGs depletion (p = 0.799), whereas the removal of PGs significantly reduced T in root dentin ECM (p = 0.001). Root dentin ECM exhibited higher T than crown (p < 0.03), however no regional difference are present after PG depletion (p = 0.480). Immunohistochemistry confirmed removal of GAGs and PGs. SEM images showed structural modifications after PGs/GAGs removal such as enlargement of dentinal tubules, increased interfibrillar spaces and presence of untwisted fibrils with increased diameter. Findings indicate that the capacity of the PGs to unfold and untwist contribute to the dentin ECM resistance to tear, possibly influencing crack growth propagation. The regional differences are likely an evolutionary design to increase tooth survival, that undergoes repetitive mechanical loading and load stress dissipation over a lifetime of an individual.