The contribution of proteoglycans to the mechanical behavior of mineralized tissues

It has been widely shown that proteoglycans (PG) and their glycosaminoglycan (GAG) side-chains form supramolecular aggregates that interconnect the collagenous network in connective tissues and play a significant role in regulating the mechanical behavior of the extracellular matrix, particularly in soft tissues. However, collective evidence of the mechanical participation of PGs and GAGs in mineralized tissues remains poorly explored in the literature. Here, we address this knowledge gap and discuss the participation of PGs on the biomechanics of mineralized tissues including dentine, cementum and bone. We review evidence suggesting that, on a microscale, PGs regulate the hydrostatic and osmotic pressure, as well as the poroelastic behavior of dentine and bone. On the nanoscale, we review the so-called sliding filament theory and intramolecular stretching of GAGs. We also discuss recent interpretations whereby folding and unfolding of the PG protein core, potentially in association with SIBLING proteins, may be a contributing factor to the mechanical behavior of mineralized tissues. Finally, we review in vitro and in vivo studies of mineralized tissues with targeted disruption or digestion of specific PG family members, which provide further insights into their relevance to the mechanical properties of load bearing hard tissues. In summary, this review brings forth collective evidence suggesting that PGs and GAGs, although less than 5% of the tissue matrix, may play a role in the mechanical behavior and durability of mineralized tissues.