New aspects detached from the molecular landscape of dental caries process

Dental caries are considered an irreversible pathological process of teeth calcified tissue, characterized by demineralization and further tooth organic substance destruction, finally triggering the cavity formation. The dynamic demineralization process occurs several times during 24 h, but, usually, is balanced by the salivary buffering activity. However, this balance disturbance will trigger caries progression, which will induce several dentin modifications: mineral content reduction, increased microand nano-porosity, due to dentin collagen structure modifications, and redistribution of non-collagenous proteins. During carious lesion evolution, after dentin demineralization, the next step is the collagenous organic matrix destruction. Dentinal and salivary proteases (mainly MMPs, together with cathepsins) should be regarded as important targets for the therapeutic strategies of carious lesions.


INTRODUCTION
Dentin should be regarded as a natural composite consisting of a collagen organic matrix mineralized with nanosized hydroxyapatite crystals [1].
Dental caries are considered an irreversible pathological process of teeth calcified tissue, characterized by demineralization and further tooth organic substance destruction, finally triggering the cavity formation [1]. Dentin carious lesion progression requires, as a first step, a bacterial attack along the dentin-enamel junction [1].
Then, during the demineralization phase of the cariogenic process, hydroxyapatite crystals are dissolved by the organic acids formed in the bacterial metabolism. When the local pH falls below 5.5, these bacterial organic acids are able diffuse into the mineralized dental tissues [1].
The dynamic demineralization process occurs several times during 24 h, but, usually, is balanced by the salivary buffering activity [1]. Normally, saliva is able to supports the remineralization processes. However, this balance disturbance will trigger caries progression, which will induce several dentin modifications: mineral content reduction, increased micro-and nano-porosity, due to dentin collagen structure modifications, and redistribution of non-collagenous proteins [1,2].
During carious lesion evolution, after dentin demineralization, the next step is the collagenous organic matrix destruction, first supposed to be the result of the bacterial proteases' activity [1,2].
Physiological aging and disease usually induce dentin modifications, affecting its biochemical and biomechanical properties [14].
Despite the fact that mineralized dentin does not suffer significant modifications, however, inside the dentinal tubules, pre-dentin and non-mineralized dentin may be altered in response to various functional necessities [18]. It has been suggested that the alterations in the inter-tubular collagen matrix, orchestrated by MMPs, are main contributors to these structural responses [18]. Arola et al. pointed that dentin's important mechanical properties, like fatigue resistance and flexural strength, significantly change with age [18].
MMPs, as well as cysteine cathepsins have been identified in saliva, mineralized dentin, and dentinal fluid. Currently, these enzymes are considered main ac-tors of the dentin cariogenic process, from the early phases of demineralization to the cavity formation [1,25].
Shimada et al. highlighted that cariogenic bacteria can't degrade dentin collagenous matrix after demineralization [26]. Moreover, the bacterial cells from in situ created dentinal lesions were unable to induce collagen degradation in vitro [26]. It also has been reported that in acidic environments the purified bacterial collagenases had low catalytic activity [26]. All these data have led to the idea that the degradation of the organic matrix within dentin carious lesions may be caused especially by the host MMPs [4,16].
It also has been suggested that cysteine cathepsins may be involved in dentinal carious lesions initiation and progression [20,25]. Nascimento et al. and Vidal et al. highlighted that odontoblast or pulp-derived cysteine cathepsin activity level varies depending on the different compartments of the carious lesion [20,25]. Nascimento et al. reported that within carious dentin the cysteine cathepsin activity significantly increased with increasing depth toward the pulp [20].
Once the carious lesions become active, they begin to demineralize and remineralize cyclically. Their pH will cycle from 5.0 (inappropriate for MMPs, but optimum for cathepsin activity), to 7.0 (inappropriate for the cathepsin activity, but the optimum for MMPs). In this way, a significant proteolytic activity (due to cathepsins or MMPs activity) is insured for long periods of time.
The organic matrix degradation may be also caused by the salivary proteases' activity [4,27,28]. Several salivary MMPs have been identified in both gingival crevicular fluid and total saliva [29]. MMP-8 and MMP-9 are the most abundant salivary MMPs and predominate in dentin caries lesions [4,27,28]. Salivary MMPs are also able to efficiently degrade the exposed dentinal collagen fibrils [4]. Experimental data revealed higher MMP activities in active carious lesions, compared with the chronic ones [20].
It is considered that in caries-affected dentin the collagen matrix may remain mostly unaffected, until become seriously demineralized. It is most probably that the dentinal collagen fibers still retain the capacity to re-mineralize, even after almost half of the mineral content has been lost. However, an efficient remineralization impose the re-growing of nanometer-sized apatite crystals in the gap zones of the collagen network [30]. Experimental evidence indicated that in carious lesions dentin collagen molecules may undergo structural changes, like characteristic periodicity loss, even after relatively mild demineralization [24,25,30,31].
Given that at least MMP-2, MMP -9 and cysteine cathepsin K are able to cleave off the C-terminal end of the collagen molecule, dentinal MMPs and cysteine cathepsins should be regarded as important players in the CTX (carboxyterminal telopeptides, cleaved by cathepsins) and telopeptide fragments -ICTP (carboxy-terminal telopeptides of type I collagen, cleaved by MMPs) releasing process [24,25,30,31].

CONCLUSIONS
All those presented above outline the idea that dentinal and salivary MMPs, together with cathepsins, should be regarded as important targets for the therapeutic strategies of carious lesions.
Due to MMPs and cathepsins involvement in caries progression and bond stability, these enzymes' inhibitors may play a crucial role in the new preventive and therapeutic protocols' elaboration.
However, more experimental and clinical studies are needed on new compounds able to inhibit these proteases' activity inhibition and, thus, to contribute effectively to caries prevention and improved stability of the adhesive interface.