Introduction: Caries Disease and the Evolution of Dental Restorative Materials

As one of the most prevalent non-communicable diseases, dental caries largely affects the global population. Current estimates indicate that 2 billion people have permanent teeth with caries lesions, and more than 500 million children present caries in primary teeth [1]. Over the last decades, scientific evidence led to a shift on our understanding of caries disease’s mechanisms. These changes fundamentally impact how caries is diagnosed and managed (i.e., by assessment of patient’s risks for development and/or progression of lesions) [2]. The concept of minimally invasive dentistry plays a central role in this novel approach, in which minimum intervention sets the basis for disease management [3, 4]. Accordingly, the operative parameters that guide cavity preparation and teeth restoration changed from a model of “extension for prevention,” established by G.V. Black in 1891, to the selective carious tissue removal. This new model aims at preserving the tissue that can be still repaired/regenerated and only removing the necessary tissue to provide support for the final restoration [5]. Such an approach becomes feasible only because in parallel, the dental materials have evolved from being non-adhesive materials (i.e., dental amalgam), which require extensive tissue removal to ensure proper retention, to materials that can be bonded micromechanically to the tooth structure. Thus, other than proposing an esthetic revolution, the advent of adhesive dentistry allowed for more conservative management of tooth decay.

Interestingly, the advances in Cariology research that provided a better appreciation of the disease also inspired material research and development. For decades now, the dental materials science has been diligently focused on the design of materials that are not only capable of replacing the tissue lost by caries with mechanical and esthetic competence, but also of eliciting host-tissue biological responses. Novel and next generation of dental materials and restorative therapies are expected to promote tissue repair and/or trigger endogenous reactions that include the remineralization of dental hard tissues [6, 7], the strengthening of dentin organic matrix [8, 9], the control of biofilm formation and activity [10], the induction of pulp responses to promote dentin repair and resolve inflammation [11], etc. While a tremendous advancement in the development of dental restorative materials is already in course, the future of dental materials science and regenerative dentistry still requires clear understanding of the complexity of the tissue’s responses to caries progression, particularly related to dental tissues’ destruction, pulp inflammation, and reparative mechanisms.

As caries lesions progress into dentin leading to the destruction of its organic components and pulp inflammation, a significant constraint is posed to the more conservative (i.e., non-surgical) management of the disease. For example, carious dentin is mechanically compromised due to several modifications in its proteins’ (including collagen and non-collagenous proteins) structure and organization [12]. Moreover, dentin is also dramatically affected by the mineral loss, which causes a gradient in its relative content and composition at the different zones of the lesion [13]. As we have deepened our comprehension on the responses of the dentin-pulp complex to bacterial invasion, less invasive techniques of tissue handling and removal associated with the use of materials that can trigger pulp repair and tertiary dentin formation have been proposed. Yet, more predictable results using the existing materials as well as the development of novel minimally invasive treatment options for dentin caries require a clear understanding of tissue degradation and reparative mechanisms.

Research from the past decades supports the hypothesis of an endogenous mechanism to explain dentin breakdown in caries, which contests the exclusive involvement of bacteria in caries disease/lesion development and progression. This review focuses on discussing the scientific evidence that show the host and bacterial contributions to dentin organic matrix destruction in caries progression as well as alludes to their role also in inflammation resolution and tissue repair. Interestingly, these three mechanisms — tissue destruction, inflammation, and repair — are intricately connected and should be taken into consideration in the design of new dental materials. Advancing our knowledge of exogenous and endogenous mechanisms taking place in caries progression will lead to exciting new venues in caries management and development of novel reparative and restorative dental materials to promote oral health.

Dentin Degradation in Caries: Enzymes and Mechanisms Driving Tissue Breakdown

The Role of Host and Bacterial Enzymes in Dentin Degradation

The breakdown of the dentin substrate in caries has been traditionally attributed to the metabolic activity of bacterial enzymes/by-products of dysbiotic biofilms. However, evidence from the last 25 years has challenged the concept of bacterial exclusivity in guiding caries progression. The exact contribution of host and bacterial enzymes and their roles in carious dentin degradation remain unclear, but there is growing evidence of their interplay. As early as in 1983, Dayan et al. proposed that a collagenase with latent activity would be present in the dentin, suggesting its involvement with a slow rate of degradation of collagen fibrils within the demineralized tissue [14]. In addition, the lack of correlation between levels of dentin degradation and gelatinolytic activity of bacteria in studies in vitro and in situ resulted in the first speculations that collagen could be initially cleaved by endogenous collagenases from macrophages or polymorphonuclear leukocytes [15]. The presence and activity of dentin proteinases were later confirmed and further characterized, with the identification of several matrix metalloproteinases (MMPs) in carious soft dentin and increased gelatinolytic activity in acidic conditions [16]. In addition, salivary MMPs showed potential gelatinolytic activity but only after acid activation. Those findings served to ground the hypothesis of a bacterial acid-activation of dentin MMPs followed by subsequent hydrolysis of dentin collagen when the pH of oral fluids returns to neutral [16]. While most of the evidence from studies published in the 2000s point out the role of MMPs in caries progression came from in vitro investigations, a patent reduction in the rate of caries lesions’ progression in rats that received MMP inhibitors firmly corroborates with such a hypothesis [17]. More recently, a decrease in MMP activity in both active and chronic lesions with an increase in the age of the subjects was reported for carious dentin excavated from human teeth, regardless of the extension of the lesions [18]. These in vivo findings confirmed the contribution of dentin-bound MMPs to caries progression. In addition, human studies showed changes in the expression of MMPs, type I collagen, and bone sialoprotein in carious dentin after restoration that strongly support the hypothesis of host enzymes’ involvement in the progression of caries [19, 20].

In parallel, Cariology studies have associated dentin degradation with bacterial invasion. Nonetheless, the use of dentin samples (which likely contain endogenous enzymes) or single species of bacteria in many of these in vitro studies raises questions about the exclusive role of bacterial collagenases in promoting the dentin collagen proteolysis [21, 22]. For example, bacteria showed ability to degrade gelatin (denatured collagen), but did not exhibit collagenolytic activity against sound demineralized dentin [23]. A recent study demonstrated degradation of type I collagen mainly by secreted proteins from S. mutans, with an increased degradative potential in the late growth stage of this bacteria [24]. Another in vitro investigation reported that specific mutant strains of S. mutans can express proteins with affinity to collagen type I, but other studies failed to either exclude the contribution of host MMPs or demonstrate the role of S. mutans proteins in collagen binding and hydrolysis in animal caries models [25].

While these findings raise questions about the actual role of bacteria and host proteinases in dentin caries progression, they highlight the gap in the knowledge regarding the specific contribution of the different exogenous and endogenous enzymes in tissue breakdown. While controversial results exist in the literature, based on recent reports, it is wise to state that both contribute for tissue breakdown. Therefore, unanswered questions include when and how these proteinases work together to result in dentin proteolysis. Possibly, as previously speculated, endogenous enzymes could initiate collagen degradation after removal of extrafibrillar minerals due to their telopeptidase activity, but salivary or bacterial collagenases would contribute to complete destruction of the collagen triple-helix only after demineralization has advanced [26]. Therefore, future studies focused on the interplay between bacterial and host enzymes in carious dentin breakdown rather than attributing tissue degradation exclusively to one source of enzymes are encouraged.

Different Host Enzymes, Their Release and Activation in Caries Progression

The first family of host enzymes to be studied in the progression of dentin caries is the MMPs. MMPs comprise a family of 28 calcium- and zinc-dependent endopeptidases able to degrade a variety of proteins in the extracellular matrix and regulate different cellular and signaling pathways [27]. MMPs participate in physiological and pathological processes to promote cell proliferation, migration, differentiation, and apoptosis as well as are involved in tissue immune response, repair, and angiogenesis. Synthesized and secreted in an inactive form, MMP activity is finely regulated at the expression level, by their activation from zymogen to active form, and by the action of endogenous tissue inhibitors of MMPs (TIMPs) [27]. In addition, MMP activity is highly dependent on the tissular concentration of metal ions such as zinc and calcium [28].

Several MMPs were identified in dentin, pulp, and odontoblasts including mainly MMP-2, MMP-3, MMP-7, MMP-8, MMP-9, MMP-13, and MMP-20 [29,30,31,32,33,34]. Evidence points out that MMPs are synthesized by the odontoblasts, released, and incorporated into the dentin mineralized matrix [33, 35]. MMPs are also present in the dentinal fluid and dentinal tubules, possibly a result of more recent odontoblast response to injuries or other stimuli and are considered odontoblast-derived instead of dentinal MMPs (immobilized into the mineralized matrix) [33]. The differential location of MMPs in mineral-bound and mineral-unbound protein extracts [35] as well as in the dentinal fluid [36] suggests specific roles for these enzymes, which is discussed in detail as follows. The specific proportion of these enzymes in dentinal fluid and dentin tissue bound and unbound to the minerals is still unknown. Most studies reported increased presence of some of these enzymes in carious than in sound tissue [18, 37,38,39,40]. Interestingly, there are very few studies reporting decreased presence or activity of MMPs in caries when compared to sound condition [36]. Table 1 compiles a number of in vitro, in situ, and in vivo studies that reported the presence, levels, and activity of MMPs in coronal dentin in caries.

Table 1 Summary of in vitro, in situ, and in vivo studies reported herein that evaluated presence, levels, activity, and/or other mechanisms of MMPs in coronal dentin and saliva in caries and in pulp inflammation. The literature presented in the table shows MMPs involved in tissue degradation, remodeling, repair, and inflammation
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Tjäderhane et al. [16] were the first to propose the acid-activation mechanism to explain the release and activation of MMPs in carious dentin. Accordingly, that classic study hypothesized that as dentin is demineralized by the solubilization of hydroxyapatite due to the release of organic acids by the bacteria, the organic components of dentin extracellular matrix (ECM), including matrix-bound MMPs, become exposed and prone to contribute for the tissue breakdown. Building upon this hypothesis, it has been further suggested that the decrease in the carious dentin milieu pH, caused by increased concentration of organic acids, could lead to a chemical activation of MMPs by disturbance of the cysteine-switch present in their active site [42]. Thus, once activated in an exposed demineralized dentin ECM, MMPs will be available not only to mediate collagen’s degradation, but also contribute with the breakdown of other organic components of dentin (Fig. 1). While the acidic environment is not ideal for MMP proteolytic activity, the dentin buffering mechanism, which returns the pH to neutral, is thought to allow MMPs to act cleaving dentin organic components. The acid-activation mechanism of MMPs in dentin is supported by other in vitro studies [29, 41], and it is considered to be key for the activity of cysteine cathepsins as well.

Fig. 1
figure 1

Changes in the levels/activity of MMPs and cysteine cathepsins in dentin-pulp complex and the associated degradative (in red) and reparative (in blue) mechanisms. Caries progression into dentin exposes MMPs from the dentin matrix resulting in higher levels of these enzymes in carious tissue [37,38,39,40, 83]. Following demineralization, bacterial acids can activate MMPs to start degradation of the dentin organic matrix [16]. The acidic environment can also activate cysteine cathepsins present in the dentin [18], which in turn have the potential to both degrade collagen as well as activate other dentinal MMPs. In addition, the acids from bacteria, by promoting dentin demineralization, release bioactive components from the matrix that can stimulate pulp response and repair [57]. Other bioactive molecules can also be released from the dentin matrix after its digestion by some MMPs [75••]. In pulp, as bacteria invasion progresses, by-products stimulate different cells in the pulp tissue to express higher levels of MMPs, which are involved in pulp extracellular matrix degradation and tissue inflammation [69, 70]

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Cathepsins comprise another clan of enzymes shown to be highly expressed in carious dentin [18, 37]. Similar to MMPs, cathepsins regulate several physiological and pathological processes, participating in tissue degradation in cancer, cardiovascular and bone diseases, and arthritis [42]. Cathepsin functions seem to go beyond tissue remodeling as they appeared to be prevalently involved in processing of inflammatory markers and signaling pathways that contribute to disease progression. Cathepsins are also synthesized as proenzymes, their activity is regulated by pH and endogenous inhibitors, and a mildly acidic pH is required for the activity of most cathepsins [42]. Identified in dentin, pulp, saliva, and odontoblasts [18, 37, 43, 44], cathepsins were shown to have their expression and activity remarkably augmented in carious dentin in comparison to healthy teeth [18, 37].

Evidence shows that MMPs can also be activated by other proteinases [45]. It has been suggested that cathepsins work together with MMPs in dentin destruction in caries in a mechanism where cathepsins can activate MMPs, cleave and inactive TIMPs, and/or directly degrade the extracellular matrix [18]. Therefore, one of the roles of cathepsins in caries lesion progression is amplifying the degradative process. Interestingly, evidence suggests that, even though cathepsins are present in the dentin, the main source of these enzymes is the pulp tissue in response to bacterial invasion in caries. Important data from Nascimento et al. [18] shows the greatest activity of cathepsins in deep caries lesions in teeth from young patients, suggesting that most of these enzymes are synthesized by the odontoblast in response to the bacteria invasion (Fig. 1).

Besides interaction between different families of host enzymes, the potential of bacterial proteinases to activate MMPs, which is reported in other diseases and conditions [46], is still underestimated in caries. A cysteine (thiol-)proteinase from P. gingivalis showed potential to activate MMP-1, MMP-3, and MMP-9 [47], while fibroblasts cultured in the presence of bacterial products from P. gingivalis exhibited increased ability to degrade both collagen and activated MMPs (i.e., MMP-1, MMP-2, MMP-3, and MMP-14) [48]. In addition, reduced levels of TIMP-1 expressed by fibroblast treated with by-products of P. gingivalis were reported [48], which indicated that MMP activity may be regulated at the level of TIMP-MMPs complexes by such bacteria metabolite. More recently, the effects of bacteria endotoxin on the activity of cathepsin K in dentin were also described. In effect, the contamination of dentin with bacterial lipopolysaccharides (LPS) presented an increased telopeptidase activity [49]. According to Bafail et al. [49], negatively charged LPS could interact with the positively charged residues of cathepsin K, modulating the activity of this enzyme in vitro. Therefore, additional studies are required to fully distinguish the interactions between MMPs and cathepsins in caries progression. In addition, a comprehensive characterization of the interactions between host and bacterial proteinases to degrade the dentin in caries is still missing and will be fundamental for the development of predictable clinical therapies to arrest lesion progression by reducing tissue degradation.

Mechanisms of Dentin Proteolytic Degradation

Interestingly, collagen structure, organization, and cross-linkage make it resistant to degradation by many proteinases. Specifically looking at collagen breakdown by proteinases (not necessarily dentinal collagen), only some MMPs and bacterial collagenases are able to degrade it in its helical structure and are considered true collagenases (MMP-1, MMP-8, MMP-13), while other enzymes (MMP-2 and MMP-9) can only hydrolyze collagen in its denatured form [50]. The main difference between bacterial collagenolytic enzymes and MMPs is that, while MMPs degrade collagen at a specific site to release three-quarter N-terminal and one-quarter C-terminal fragments, bacterial enzymes hydrolyze the collagen molecule into a mix of multiple peptides. However, some bacterial collagenases present a cleavage pattern of collagen that is similar to that of MMPs, at least in the first step of the degradation [50]. Among cathepsins, cathepsin K can cleave collagen at multiple sites, making this enzyme a potent collagenase [51]. Some MMPs and cathepsins also have the ability to cleave collagen at the terminal telopeptides.

As already mentioned in the previous section, once demineralized by the bacteria-derived acid, the dentin organic matrix becomes exposed, displaying collagen and MMPs, including acid-activated proteinases. Modifications in the organic components also take place in carious dentin, including molecular changes or possible denaturation of collagen, reduction in the levels of proteoglycans, and loss of collagen periodicity [37, 52]. In fact, loss of collagen periodicity [53] combined with reduction of auto-fluorescence from molecularly intact collagen in caries-affected dentin [37] suggests that collagen modifications occur even at early stages of caries development. As already discussed in the literature, changes in collagen organization and mineralization (presence of extra- and intrafibrillar minerals) could dictate the action of different proteinases [26]. It has been speculated that demineralization-remineralization cycles in caries initially remove extrafibrillar minerals and might expose some intrafibrillar minerals [26]. Possibly, the removal of extrafibrillar minerals from collagen exposes the C-terminal telopeptides to the action of endogenous telopeptidases (such as MMP-2, MMP-9, and cathepsin K) resulting in loss of collagen periodicity. Then, as demineralization advances and more telopeptides are removed, intrafibrillar minerals are lost, facilitating the collagen breakdown by more telopeptidases (cleaving the N-terminal telopeptides) and resulting in gradual degradation of collagen. Bacterial collagenases would then be able to continue the collagen destruction at this stage, but only after disassembling or initial degradation of the helical part of collagen [26]. However, these mechanisms have yet to be proven.

Lastly, it is important to highlight that some MMPs and cathepsins cleave non-collagenous proteins or interact with other proteins, still contributing to lesion progression but either regulating other mechanisms and/or destroying extracellular components other than collagen. For example, the degradation of decorin, byglican, and other SIBLING (small integrin-binding ligand, N-linked glycoprotein) proteins by MMP-3 releases dentinal molecules capable of activating other MMPs [52, 54] (Fig. 1). Regarding activity of cathepsins in caries, specific functional mechanisms of tissue degradation by these enzymes remain elusive and rely on the description of the function of these proteinases in other tissues. While both cathepsin B and K can contribute to extracellular matrix destruction, including dentin in caries, cathepsin K is referred as the most potent collagenase in promoting bone resorption [55]. Cathepsins could have several roles in dentin breakdown by regulating MMP-TIMP complexes, activating MMPs, and interacting with other non-collagenous proteins in dentin, as proposed by Nascimento et al. [18]. Cathepsin K activity is regulated by proteoglycans possibly by binding to glycosaminoglycans attached to collagen and then recruiting another enzyme that will unwind the triple helical collagen to facilitate its cleavage [56]. As already mentioned, novel research in dentin degradation in caries should consider all these potential interactions between different enzymes from the same or different families and sources as well as their interactions with other proteinases to facilitate tissue breakdown in caries.

Contribution of Dentin Matrix Components and Host Enzymes to the Dentin-Pulp Degradation-Repair Interplay

Dentin shares similar composition with bone, while key differences in morphology and behavior of its cells, the odontoblasts, make dentin a unique tissue. Apart from the inorganic content, mainly hydroxyapatite crystals, dentin extracellular matrix components are mainly made of type I collagen (i.e., ~ 90 wt%) intermingled with non-collagenous proteins, which include phosphorylated proteins (i.e., SIBLINGs), non-phosphorylated proteins (i.e., osteocalcin), and proteoglycans, growth factors, cytokines, neuropeptides and neurotrophic factors, and serum or plasma proteins [57]. Many of these components of the dentin organic matrix are involved in tooth formation but also possess potential to trigger specific reparative mechanisms in the dentin-pulp complex. For example, SIBLINGs, osteocalcin, and proteoglycans regulate or participate in mineralization events, and dentin matrix protein-1 (DMP-1) is also involved in dentin mineralization by regulating nucleation and formation of hydroxyapatite crystals [58,59,60]. Growth factors also have key roles in stimulating mineralization and dentin-pulp regeneration, mainly promoted by transforming growth factor-β1 (TGF-β1) and bone morphogenetic proteins (BMPs) [61,62,63].

As caries lesions initiate and progress, bacteria and their components will trigger a variety of innate, immune, and inflammatory responses from the dentin-pulp complex [64•]. From morphological changes in odontoblasts seen in early-stage enamel lesions, continuous demineralization and breakdown will lead to cavitation. Once the dentin is exposed, demineralization following bacterial invasion and diffusion of their products into the dentin tubules results in hypermineralized dentin and tertiary dentin formation to stop the progression of the insult to the pulp and resolution of inflammation [65]. If left untreated, chronic inflammation can progress to pulp necrosis and loss of the tissue function and potential to respond to diverse stimuli.

Changes in MMPs levels have been also reported in inflamed pulp tissue in caries-affected teeth [66,67,68] (Table 1). Together with increase in interleukins (IL), increased levels of MMP-1, MMP-8, MMP-9, and MMP-13 have been reported in inflamed pulps in comparison to healthy tissue [69], with data suggesting that the nuclear factor kappa light chain enhancer of activated B cells (NF-KB) signaling pathway controls the expression of some of these MMPs by odontoblasts [70]. However, MMP presence and activity have also been attributed to the pulp response to caries and bacteria invasion by triggering reparative mechanisms. For example, MMP-3 showed potential to act reducing inflammation in LPS-treated pulps in vivo [71]. In addition, in a mild pulpitis animal model, pulp treatment with MMP-3 resulted in absence of inflammatory cells, low levels of IL-6, and increased vascularization, confirming the role of this enzyme in pulp repair/regeneration [72, 73]. These studies shine light on the fact that MMPs can regulate both host defense and pathological processes in caries like in other inflammatory diseases, and their functions depend on the type of cell, substrate, and disease [74].

Interestingly, as proposed by Smith et al. in 1995, the traditional views of odontoblast stimulation by acid released by bacteria metabolism are likely to be overlooked by the idea of dentin solubilization by the acids from bacteria [65]. Following dentin demineralization, growth factors and other molecules are exposed and released and will in turn stimulate odontoblasts to secrete reactionary dentin [65]. Since then, several studies have proved the complexity of cellular and molecular events in tissue degradation and repair in caries progression, which are intricately interconnected [57, 75••, 76, 77]. In fact, inflammation and regeneration responses are linked in a way that many pro-inflammatory molecules are also implicated in signaling reparative processes [77]. In this context, studies showed that dentin demineralization by bacterial acid releases a complex cocktail of bioactive molecules capable of inducing different processes including cell migration and tissue mineralization [57, 78, 79]. A variety of in vitro and animal studies support the concept of the dentin as a reservoir of several bioactive molecules that can trigger reparative mechanisms of the odontoblasts (Table 1). These bioactive molecules released from the dentin showed potential to increase pulp and apical papilla stem cell proliferation and differentiation, and expression of genes related to odontogenic lineage [79, 80]. Therefore, dentin degradation and repair in caries progress should always be investigated as coordinating mechanisms that elicit the host response to bacterial invasion. The same concept should be applied to the investigation of the role of bacteria and MMPs in such responses.

Investigating in more detail the reparative potential of dentin matrix components, Okamoto et al. hypothesized that MMP-digested dentin proteins could stimulate reparative mechanisms in the pulp, without any benefits of using MMPs only or undigested dentin proteins [75••]. More specifically, to prove the reparative potential of MMP-digested dentin proteins, the authors used dentin-extracted protein mixtures embedded in gelatin sponges for pulp capping in an animal model and reported tertiary dentin formation when dentin proteins were digested by MMP-1, MMP-9, MMP-13, and MMP-20 (Table 1). In the same study, MMPs alone did not affect the cells response in vitro or the pulp repair in vivo, confirming the key role of MMP digestion of dentin components to stimulate tissue repair. Later on, using proteomic analysis, the same research group identified protein S100-A7 as the major bioactive molecule released from MMP-20 digestion of dentin matrix proteins with potential to induce tertiary dentinogenesis in a direct pulp capping model [78]. These results support the role of MMPs in the wound healing of the dentin-pulp complex and demonstrate that even though tissue degradation is a result of MMP activity in carious dentin, the by-products generated by some of these proteinases can have a positive impact on the tooth response to caries progression and inflammation. Similarly, cathepsins could also be involved in dentin-pulp complex repair; however, there is little evidence in the literature to support that. It has been suggested that cathepsins could release bioactive molecules from the dentin after the tissue degradation. For example, TGF-β1, which is known to be involved in the activity of the odontoblasts in tissue repair, upregulated the expression of cathepsin B in pulp cells, suggesting that this proteinase could be involved in dentin-pulp response in caries [81].

Besides the release of specific bioactive molecules from the degraded dentin, the activity of MMPs can also result in activation of some key proteins for tissue repair. For example, membrane-type 1 matrix metalloproteinase (MT1-MMP) can activate latent TGF-β complexes from the subendothelial extracellular matrix in a process that requires activities of other signaling pathways [82]. Higher levels of MT1-MMP were reported in response to caries, so it is possible that this mechanism takes place in the dentin-pulp complex. MMP-2 also seems to be greatly involved in tertiary dentin formation. Charadram et al. proposed that MT1-MMP could activate MMP-2 and, considering the high levels of MMP-2 and dentin sialoprotein (DSP) in reactionary dentin in carious teeth and other findings reported in the literature, it is possible that MMP-2 facilitates the release of dentin sialophosphoprotein DSPP into DSP to favor tissue mineralization [83].

This interesting knowledge of inflammation-repair interplay and the dual role of MMPs in these processes have inspired the development of novel restorative materials and should continue to open new venues for dental materials research, mainly bioactive materials to treat dentin and pulp in caries lesions. An already explored approach is to stimulate pulp wound healing and dentin repair by pulp capping materials that can modulate tissue’s response to caries progression. Mineral trioxide aggregate (MTA) promotes release of bioactive molecules sequestered in dentin to stimulate cellular response in the pulp favoring dentin-pulp complex repair [84]. For tissue regeneration purposes, the use of scaffolds containing MMP-2 cleavage sites has been proposed recently and showed that MMP-2 facilitated binding of growth factors into the scaffold to promote their sustained release both in vitro and in vivo [85]. In terms of diagnostic purposes, measuring the levels of MMP-9 in dentinal fluid can be used to aid in the diagnosis of pulp vitality and level of inflammation for clinical decision-making [86].

Conclusion

Combined with the multifactorial etiology of caries, the specific contributions of host and bacterial proteinases and their by-products to dentin breakdown and lesion progress remain unclear. Therefore, further research is still needed to better understand the specific roles of proteinases from different sources (bacteria, dentin, pulp, and saliva) in tissue breakdown. In addition, dentin-pulp inflammation and regeneration are intertwined in caries. These complex and intricate mechanisms linking inflammation and regeneration open new venues to explore therapeutic possibilities to accelerate natural tissue healing and resolve inflammation. While the ultimate goal would be to promote tissue repair and regeneration, these therapies are far from being translated into clinical practice for treating dentin caries. However, dental materials scientists should continue to focus on shifting from restoration after tissue removal to the stimulation of innate and immune dentin-pulp complex responses based on the modulation of key pathways that favor repair. This concept departs from the status quo as it pertains to targeting broad MMP inhibition to prevent dentin degradation to the idea of taking advantage of the properties of these enzymes to stimulate dentin repair while avoiding further tissue breakdown.