Introduction

Apical periodontitis (AP) a common infectious disease caused by endodontic etiological agents that leads to inflammation and periradicular tissue loss. The most frequent inflammatory lesion affecting teeth in the jaws is AP, which has a complex inflammatory and non-inflammatory cellular profile that helps regulate extremely complex disease processes. Bacterial infection of the tooth pulp, caries, trauma, or an iatrogenic injury can induce periapical inflammatory reactions, leading to the formation of granulomas and cysts and bone loss. Endogenous mediators such as prostanoids, kinins, and neuropeptides can mediate immediate-type responses such as vasodilation, increased vascular permeability, and leukocyte extravasation in these inflammatory reactions (P. Stashenkoet al.,1998).

The host responds to the inflammatory response by developing an array of cells that includes intercellular messengers, antibodies, and effector molecules. Microbial factors and host defense forces collide with and destroy much of the periapical tissue, resulting in the creation of numerous AP lesions, the most frequent of which are reactive granulomas and cysts, as well as bone resorption around the roots of impacted teeth (I. Graunaite et al.,2011). Increased production of cytokines, proteases, and other pro-inflammatory mediators is a hallmark of inflammatory/infectious disorders. The goal of host modulatory therapy is to restore the balance of pro-inflammatory and anti-inflammatory mediators in the body. There are several ways for controlling the host's response with AP. All cellular events, regardless of the host's innate or acquired immunity response, depend on the activation of several signal transduction pathways, which are affected by various microbial and host-derived factors such as lipopolysaccharide (LPS), proteases, cytokines, and other enzymes [ M. E. Ryan,2003]. Phosphorylation of certain amino acid residues by kinases, which causes a conformational change in the protein's tridimensional structure, is the most prevalent modification linked with signal transduction. Since it does not involve de novo gene expression, phosphorylation is a very efficient method of transferring energy and regulating biological activity (J. A. C. de Souza,et al.,2012).

Targeting cell signaling pathways are essential for gene expression and are a modern strategy in regulating the levels of inflammatory mediators involved in the immune response, which can be done by activating a limited number of these signaling pathways. Indeed, the short-term modification of cell signaling pathways could affect cytokine gene expression without negatively influencing other fundamental cellular functions. As a result, it is crucial to consider individual signaling pathways in periodontal disease inflammation and tissue degradation (J. A. C. de Souza,et al.,20124). Therefore, this review provides current information on the essential signal transduction pathways and genetic basis in inflammatory periodontal disease.

Signaling pathways in periapical lesions

WNT signaling pathway

The WNT/-catenin (Wingless-Integrated/Beta-catenin) signaling system is one of the most critical intercellular signaling pathways in humans, and it is involved in cellular proliferation, differentiation, and regeneration, along with other functions (I. Ejaz S. Ghafoor 2019) . The pathway has been divided into two types: non-canonical (beta-catenin-independent) and canonical (beta-catenin-dependent). WNT pathway ligands include a broad family of 350-400 amino acid glycolipoproteins such as WNT1, WNT2, and WNT3, etc. Wnt will bind to the Frizzled (Fzd) and low-density lipoprotein receptor-related protein 5/6 (LRP5/6) receptors, enabling ß-catenin to be translocated from the cytoplasm to the nucleus, where it will bind to T-cell factor/lymphoid enhancer factor (TCF/LEF) to regulate target gene transcription ( L. C. de Souza 2019]. Various skeletal abnormalities, fibrotic diseases, inflammatory disorders, and malignant transformations of numerous malignancies are caused by misregulation of the WNT/-catenin system. Periapical lesions form due to bacterial infection of the dental pulp, which leads to the production of inflammatory substances, which disrupts the triad of RANK-ligand (RANK), RANKL, and OPG receptors. WNT/b-catenin signaling regulation may provide better therapeutic aids for controlling bone loss in endodontic disease. Bone loss in periapical and periodontal lesions is affected by agonists and antagonists of the WNT signaling pathway. Active Wnt/-catenin signaling is important in bone, especially for osteoblast differentiation. The activation of the canonical Wnt pathway in osteoblasts has been found to prevent bone resorption by upregulating OPG expression while down-regulating RANKL expression. Variations in WNT3 have also been linked to changes in bone mineral density and have been linked to osteoporosis.

Differential expression of WNT proteins in AP tissues supports the evidence that these molecules play a role in AP development.WNT3, WNT3A, and WNT5A expression were shown to be considerably greater in AP lesions and WNT3A and WNT5A expression in chronic periodontal disease tissues as compared to healthy tissues (L. C. de Souza 2019).

Table 1. Signalling pathway gene polymorphism in Apical periodontitis (AP)

Mitogen activated protein kinase (MAPK)

MAPKs are an evolutionarily conserved family of protein kinases that use diverse receptors to mediate basic biological processes and cellular responses to various external stimuli. MAPKs, or proline-directed serine/threonine kinases, play a significant role in signaling extracellular hormones, growth factors, cytokines, bacterial antigens, and environmental stressors (such as lipopolysaccharide) from gram-negative bacteria), as well as a variety of immune-mediated inflammatory responses.

The MAPK family is divided into three groups: ERK, JNK, and p38 MAPK, which control the activation of several transcription factors. MAPK is primarily active in inflammatory cells, which causes the manufacture of major inflammatory mediators such as tumor necrosis factor (TNF-), interleukin (IL)-1, IL-6, IL-8, and cyclooxygenase-2, either through direct gene transcription activation or messenger RNA stabilization(M. Suzuki et al.,2000, R. Beyaert et al1996. R. Winzen et al.,1999).  Periapical lesions are caused by bacterial infection of the tooth pulp, which leads to inflammatory bone resorption. All three MAPK families, ERK, JNK, and p38, are activated by RANK in osteoclasts or osteoclast precursors. p38 MAPK also plays a key role in controlling IL-1 and TNF signaling networks, as well as NF-?B ligand (RANKL)-induced osteoclastic bone resorption(C. Peifer et al.,2006).  

p38 MAPK regulates osteoclast development by various factors, including prostaglandin E2, TNF-, and RANKL, according to recent research (G. L. Schieven2005),. Furthermore, studies using p38 inhibitors have confirmed that p38 MAPK is involved in osteoclastogenesis (R. Zhang,et al.,2008, T. Kaneko 2019). Studies have reported that p38 MAPK is important in osteoclast development and that its activation is linked to bone resorption in periapical lesions.

Nuclear factor kappa B (NF-?B)

The NF-?B transcription factor family has been involved in several pathways, and it plays an important role in regulating the expression of several genes that control innate and adaptive immune responses. REL-a (p65), NF-B1 (p50; p105), NF-B2 (p52; p100), c-REL, and REL-b24 are the five members of the NF-?B family [4]. Bacterial LPS, TNF-, IL-1, MMPs, COX2, and inducible nitric oxide synthase (iNOS) can activate the NF-?B pathway, resulting in the production of a variety of pro-inflammatory mediators. When NF-?B is activated in the presence of a wide range of biologically active molecules, it can activate other signaling pathways such as MAPKs and TLRs. A recent study showed that NF-B (p50/p65) activation is significant in periodontally diseased tissues, suggesting that NF-?B inhibitors could be useful in treating periodontitis ( E. Karl 2005).

Angiogenesis is assisted by NF-?B, which regulates the expression of pro-angiogenic chemokines CXCL1 and CXCL8 [ S. P. Tabruyn, A. W. Griffioen2008]. Therefore, inhibiting NF-?B in the angiogenic pathway may reduce periapical lesions and suppress angiogenesis (J. N. Ihle and I. M. Kerr, 1995). The effects of NF-?B decoy ODN treatment on experimentally created periapical lesions and the suppression of NF-?B inhibits periapical lesion formation via blockage of angiogenesis are investigated in a study. The formation of experimentally produced periapical lesions was decreased with a reduction in angiogenic responses in endothelial cells when NF-?B activity was inhibited by decoy ODN administration.

Janus tyrosine kinase-signal transducer and activator of transcription (JAK/STAT)

The JAK2-STAT3 (Janus kinase 2 / signal transducer and activator of transcription 3) pathway is a signaling pathway that involves activated enzymes known as Janus kinases (JAK1, JAK2, JAK3, and Tyk2) that are found in the cytoplasm of transmembrane receptors (C. Zhang et al.,2019). Its biological function is mainly related to cell proliferation, differentiation, apoptosis, and immune regulation, and it has been found to have a crucial role in the occurrence and development of a variety of diseases, including cardiovascular disease, stroke, and malignancies. Activated JAKs will phosphorylate the receptor's cytoplasmic domain, resulting in the activation of its substrate protein, STATs (STAT1-4, 5a, 5b, and 6). STATs can form homo- or heterodimers after being phosphorylated, allowing them to enter the nucleus to regulate gene transcription. Although various ligands can activate individual STAT proteins, certain cytokines selectively activate specific STATs. For example, IFN- activates STAT1 predominantly via JAK1/JAK2, whereas IL-6 activates STAT3 via JAK1 (R. Starr and D. J. Hilton,1999).

Downregulation of the receptor/ligand complex, degradation of signaling intermediates, dephosphorylation of positive regulators (receptor, JAK, or STAT), and activation of particular suppressors [ T. Berglundh and M. Donati 2005] are some of the regulatory mechanisms that control signal pathways. Many cytokines that are expected to play biologically important roles in rheumatoid arthritis (IFN-, IL-2, IL-4, IL-6, IL-7, IL-10, IL-12, and IL-15) and periodontal disease (INF-, TNF-, IL-1, IL-4, IL-6, and IL-10) signal through the JAK-STAT pathway. (E. A. Roberts et al.,1995, J. G. Walker 2006, S. Artavanis-Tsakonas et al.,1999)

Notch Signaling pathway

The Notch signaling pathway is a highly conserved ligand-receptor cell signaling system involved in a wide range of cell functions, including cell proliferation, survival, differentiation, and apoptosis (P. F. Christopoulos, et al., 2021). In vertebrates, four genes encoding Notch receptors (Notch 1-4) and five genes encoding Notch receptor ligands (Jagged 1 and 2 and also Delta 1, 3, and 4) have been discovered. Its role in bone homeostasis has also been identified (D. del Álamo et al.,2011).

The signaling is activated by directly binding the Notch receptor's extracellular domain on one cell to the relevant ligand on an adjacent cell. The Notch receptor is proteolytically cleaved by the gamma-secretase, releasing the Notch receptor intracellular domain that translocates to the nucleus and activates transcription of its target genes, such as the Heygene family of genes [ R. A. Backer 2014]. Notch ligands are transmembrane proteins with large extracellular domains, just like their receptors. As a result, Notch signaling is a short-range signaling mechanism that allows the direct cell to cell communication. The Notch signaling pathway is a central regulator of inflammation, controlling the differentiation and activity of many cells in the innate and adaptive immune response (dendritic cells, natural killer cells, macrophages, B lymphocytes, and various T-cell types) (J. López-López 2012). Proinflammatory cytokines found near the site of inflammation stimulate the activity of the Notch signaling pathway in these cells. TNF-a and IL-1b act as Notch signaling pathway activators, inducing the expression of Notch ligands and receptors and promoting nuclear translocation of Notch receptor intracellular domains.

Genetics of periapical lesions

A complex signaling network is involved in determining the nature and extent of AP progression and the associated bone destruction process. Genetic predisposition can contribute to an individual's susceptibility to developing AP. Environmental and acquired risk factors such as diabetes, psychological stress, smoking, and genetic factors (such as gene polymorphisms) can increase the host inflammatory response and, as a result, periodontal disease susceptibility. The link between periodontal disease and higher levels of cytokines and pro-inflammatory mediators shows that endogenous pathways for negative regulation of these genes may be faulty or malfunctioning, which could be one factor contributing to increased periodontal disease susceptibility (A. Aminoshariae and J. C. Kulild 2015).

For years, several kinds of research have been carried out to assess host risk factors such as age, gender, and systemic disorders (M. J. B. Silva 2011, J. F. Siqueira Jr,et al.,2009). The impact of the host's genetic background on apical periodontitis (AP) development, on the other hand, is a hot topic in endodontic research. Many genes are involved in AP pathogenesis, according to research involving knockout mice (KO) as an animal model [ A. ElAyouti et al.,2011] and studies with human samples (E. C. Küchler et al.,2019) analyzing the complexity of endodontic biology( A. R. De Sá 2007). These investigations highlight candidate genes that could serve as genetic markers for AP establishment and development variations between individuals. Genes are considered candidate genes for AP etiology if a genetic variant is associated with the disease.  A genome-wide association study approach, which uses a large sample size and a high density of genetic markers across the entire human genome, could identify new genetic variations. Furthermore, patients with AP may have one or more genetic variations in various genes, requiring the conversion of new genetic findings into clinical or public health applications.   

Table 2. Signaling pathway gene expression in AP

Conclusion

Gene expression and variation in signaling pathways lead to the identification of new therapeutic targets for inflammatory diseases. Since the WNT, MAPK, NF-B, JAK/STAT, and Notch signalling pathways are shared by a variety of inflammatory mediators, blocking them may be more effective than targeting specific cytokines. However, because these pathways are involved in a variety of other physiological processes, inhibiting them can have undesirable side effects. Understanding the genetic basis of diseases will eventually lead to genetic tests to identify disease risk and the development of modifying factors for treatments. Determining the genetic basis for apical periodontitis in signalling pathways could help researchers to understand how genetic variation and expression influence host response. More systematic genetic research, such as  association study design with a larger sample size, twin studies, segregation studies, and, ultimately, linkage analysis, are required to completely understand genetic components implicated in signalling pathways connected to AP.

Author contribution

Athira Ajith, Usha Subbiah  conceived of the presented idea. Deepika. P and Minthami Sharon P encouraged and supervised the findings of this work. All authors discussed the results and contributed to the final manuscript.

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