Apoptotic signaling in mouse odontogenesis

Epigallocatechin-3-gallate attenuates fluoride induced apoptosis via PI3K/FoxO1 pathway in ameloblast-like cells

Fluoride is a double-edged sword. It was widely used for early caries prevention while excessive intake caused a toxicology effect, affected enamel development, and resulted in dental fluorosis. The study aimed to evaluate the protective effect and mechanism of Epigallocatechin-3-gallate (EGCG) on the apoptosis induced by fluoride in ameloblast-like cells. We observed that NaF triggered apoptotic alterations in cell morphology, excessive NaF arrested cell cycle at the G1, and induced apoptosis by up-regulating Bax and down-regulating Bcl-2. NaF activated the insulin-like growth factor receptor (IGFR), and phosphatidylinositol-3-hydroxylase (p-PI3K), while dose-dependently down-regulating the expression of Forkhead box O1 (FoxO1). EGCG supplements reversed the changes in LS8 morphology, the cell cycle, and apoptosis induced by fluoride. These results indicated that EGCG possesses a protective effect against fluoride toxicity. Furthermore, EGCG suppressed the activation of p-PI3K and the down-regulation of FoxO1 caused by fluoride. Collectively, our findings suggested that EGCG attenuated fluoride-induced apoptosis by inhibiting the PI3K/FoxO1 signaling pathway. EGCG may serve as a new alternative method for dental fluorosis prevention, control, and treatment.

The role of autophagy in odontogenesis, dental implant surgery, periapical and periodontal diseases

Autophagy is a cellular process that is evolutionarily conserved, involving the sequestration of damaged organelles and proteins into autophagic vesicles, which subsequently fuse with lysosomes for degradation. Autophagy controls the development of many diseases by influencing apoptosis, inflammation, the immune response and different cellular processes. Autophagy plays a significant role in the aetiology of disorders associated with dentistry. Autophagy controls odontogenesis. Furthermore, it is implicated in the pathophysiology of pulpitis and periapical disorders. It enhances the survival, penetration and colonization of periodontal pathogenic bacteria into the host periodontal tissues and facilitates their escape from host defences. Autophagy plays a crucial role in mitigating exaggerated inflammatory reactions within the host's system during instances of infection and inflammation. Autophagy also plays a role in the relationship between periodontal disease and systemic diseases. Autophagy promotes wound healing and may enhance implant osseointegration. This study reviews autophagy's dento-alveolar effects, focusing on its role in odontogenesis, periapical diseases, periodontal diseases and dental implant surgery, providing valuable insights for dentists on tooth development and dental applications. A thorough examination of autophagy has the potential to discover novel and efficacious treatment targets within the field of dentistry.

Autophagy in tooth: Physiology, disease and therapeutic implication

Autophagy is an evolutionarily conserved cellular process, in which damaged organelles and proteins are engulfed in autophagic vesicles and subsequently fuse with lysosomes for degradation. Autophagy is widely involved in different physiologic or pathologic processes in human. Accumulating evidence indicates that autophagy operates as a critical quality control mechanism to maintain pulp homeostasis and structural integrity of the dentin-pulp complex. Autophagy is activated during stresses and is involved in the pathogenesis of pulpitis and periapical infection. Recent discoveries have also provided intriguing insights into the roles of autophagy in tooth development, pulp aging and stress adaptation. In this review, we provide an update on the multifaceted functions of autophagy in physiology and pathophysiology of tooth. We also discuss the therapeutic implications of autophagy modulation in diseases and the regeneration of dentin-pulp complex.

FasL Is Required for Osseous Healing in Extraction Sockets in Mice

Fas ligand (FasL) is a member of the tumor necrosis factor (TNF) superfamily involved in the activation of apoptosis. Assuming that apoptosis is initiated after tooth extraction it is reasonable to suggest that FasL may play a pivotal role in the healing of extraction sockets. Herein, we tested the hypothesis of whether the lack of FasL impairs the healing of extraction sockets. To this end, we extracted upper right incisors of FasL knockout (KO) mice and their wildtype (WT) littermates. After a healing period of two weeks, bone volume over total volume (BV/TV) via µCT and descriptive histological analyses were performed. µCT revealed that BV/TV in the coronal region of the socket amounted to 39.4% in WT and 21.8% in KO, with a significant difference between the groups (p=0.002). Likewise, in the middle region of the socket, BV/TV amounted to 50.3% in WT and 40.8% in KO (p<0.001). In the apical part, however, no difference was noticed. Consistently, WT mice displayed a significantly higher median trabecular thickness and a lower trabecular separation when compared to the KO group at the coronal and central region of the socket. There was the overall tendency that in both, female and male mice, FasL affects bone regeneration. Taken together, these findings suggest that FasL deficiency may reduce bone regeneration during the healing process of extraction sockets.

Novel variants in critical domains of ATP8A2 and expansion of clinical spectrum

ATP8A2 is a P4-ATPase that flips phosphatidylserine across membranes to generate and maintain transmembrane phospholipid asymmetry. Loss-of-function variants cause severe neurodegenerative and developmental disorders. We have identified three ATP8A2 variants in unrelated Iranian families that cause intellectual disability, dystonia, below-average head circumference, mild optic atrophy, and developmental delay. Additionally, all the affected individuals displayed tooth abnormalities associated with defects in teeth development. Two variants (p.Asp825His and p.Met438Val) reside in critical functional domains of ATP8A2. These variants express at very low levels and lack ATPase activity. Inhibitor studies indicate that these variants are misfolded and degraded by the cellular proteasome. We conclude that Asp825, which coordinates with the Mg²⁺ ion within the ATP binding site, and Met438 are essential for the proper folding of ATP8A2 into a functional flippase. We also provide evidence on the association of tooth abnormalities with defects in ATP8A2, thereby expanding the clinical spectrum of the associated disease.

Squamate egg tooth development revisited using three-dimensional reconstructions of brown anole (Anolis sagrei, Squamata, Dactyloidae) dentition

The egg tooth is a hatching adaptation, characteristic of all squamates. In brown anole embryos, the first tooth that starts differentiating is the egg tooth. It develops from a single tooth germ and, similar to the regular dentition of all the other vertebrates, the differentiating egg tooth of the brown anole passes through classic morphological and developmental stages named according to the shape of the dental epithelium: epithelial thickening, dental lamina, tooth bud, cap and bell stages. The differentiating egg tooth consists of three parts: the enamel organ, hard tissues and dental pulp. Shortly before hatching, the egg tooth connects with the premaxilla. Attachment tissue of the egg tooth does not undergo mineralization, which makes it different from the other teeth of most squamates. After hatching, odontoclasts are involved in resorption of the egg tooth's remains. This study shows that the brown anole egg tooth does not completely conform to previous reports describing iguanomorph egg teeth and reveals a need to investigate its development in the context of squamate phylogeny.

Expression of new genes in vertebrate tooth development and p63 signaling

BACKGROUND

p63 is an evolutionarily ancient transcription factor essential to vertebrate tooth development. Our recent gene expression screen comparing wild-type and "toothless" p63^-/- mouse embryos implicated in tooth development several new genes that we hypothesized act downstream of p63 in dental epithelium, where p63 is also expressed.

RESULTS

Via in situ hybridization and immunohistochemistry, we probed mouse embryos (embryonic days 10.5-14.5) and spotted gar fish embryos (14 days postfertilization) for these newly linked genes, Cbln1, Cldn23, Fermt1, Krt15, Pltp and Prss8, which were expressed in mouse and gar dental epithelium. Loss of p63 altered expression levels but not domains. Expression was comparable between murine upper and lower tooth organs, implying conserved gene functions in maxillary and mandibular dentitions. Our meta-analysis of gene expression databases supported that these genes act within a p63-driven gene regulatory network important to tooth development in mammals and more evolutionary ancient vertebrates (fish, amphibians).

CONCLUSIONS

Cbln1, Cldn23, Fermt1, Krt15, Pltp, and Prss8 were expressed in mouse and fish dental epithelium at placode, bud, and/or cap stages. We theorize that these genes participate in cell-cell adhesion, cell polarity, and extracellular matrix signaling to support dental epithelium integrity, folding, and epithelial-mesenchymal cross talk during tooth development.

Activation of Pro-apoptotic Caspases in Non-apoptotic Cells During Odontogenesis and Related Osteogenesis

Caspases are well known proteases in the context of inflammation and apoptosis. Recently, novel roles of pro-apoptotic caspases have been reported, including findings related to the development of hard tissues. To further investigate these emerging functions of pro-apoptotic caspases, the in vivo localisation of key pro-apoptotic caspases (-3,-6,-7,-8, and -9) was assessed, concentrating on the development of two neighbouring hard tissues, cells participating in odontogenesis (represented by the first mouse molar) and intramembranous osteogenesis (mandibular/alveolar bone). The expression of the different caspases within the developing tissues was correlated with the apoptotic status of the cells, to produce a picture of whether different caspases have potentially distinct, or overlapping non-apoptotic functions. The in vivo investigation was additionally supported by examination of caspases in an osteoblast-like cell line in vitro. Caspases-3,-7, and -9 were activated in apoptotic cells of the primary enamel knot of the first molar; however, caspase-7 and -8 activation was also associated with the non-apoptotic enamel epithelium at the same stage and later with differentiating/differentiated odontoblasts and ameloblasts. In the adjacent bone, active caspases-7 and -8 were present abundantly in the prenatal period, while the appearance of caspases-3,-6, and -9 was marginal. Perinatally, caspases-3 and -7 were evident in some osteoclasts and osteoblastic cells, and caspase-8 was abundant mostly in osteoclasts. In addition, postnatal activation of caspases-7 and -8 was retained in osteocytes. The results provide a comprehensive temporo-spatial pattern of pro-apoptotic caspase activation, and demonstrate both unique and overlapping activation in non-apoptotic cells during development of the molar tooth and mandibular/alveolar bone. The importance of caspases in osteogenic pathways is highlighted by caspase inhibition in osteoblast-like cells, which led to a significant decrease in osteocalcin expression, supporting a role in hard tissue cell differentiation.

Bcl-2 expression is essential for development and normal physiological properties of tooth hard tissue and saliva production

BACKGROUND

Apoptosis plays a fundamental role in appropriate tissue development and function. Although expression of Bcl-2 has been reported during tooth and submandibular gland (SMG) development, the physiological role Bcl-2 plays during these processes has not been addressed. This study was performed to evaluate the impact of Bcl-2 expression on the formation and properties of tooth hard tissue, and saliva production.

METHODS

Twenty-four mice (12 males and 12 females) were divided into three groups of eight (n=8): group A (Bcl-2 +/+), group B (Bcl-2 +/-), and group C (Bcl-2 -/-) and subjected to micro-CT analyses. The mineral content of first molars was analyzed by X-Ray diffraction (XRD) and scanning electron microscopy (SEM) color dot map. The surface microhardness was determined by Vickers test on labial surfaces of incisors. Saliva was collected from different groups of mice after subcutaneous injection of pilocarpine.

RESULTS

Samples from Bcl-2 -/- mice showed significantly smaller micro-CT values, lower and poor crystallinity of hydroxyapatite (HA), and lowest surface micro hardness. SMG from Bcl-2 -/- mice showed remarkable reduction in size, consistent with reduced saliva accumulation.

CONCLUSIONS

The absence of Bcl-2 expression in SMG did not affect the expression of other Bcl-2 family members. Thus, Bcl-2 expression influence on the formation and properties of tooth hard tissue, and saliva accumulation.

SIGNIFICANCE

Bcl-2 expression has a significant impact on the mineralogical content of enamel crystals of tooth structure. Lack of Bcl-2 expression led to impaired production of enamel ACP crystals.

A wide temporal window for conjunctival papillae development ensures the formation of a complete sclerotic ring

BACKGROUND

The conjunctival papillae are epithelial thickenings of the conjunctiva that are required for the induction of underlying bones (the scleral ossicles). These transient papillae develop and become inductively active over an extended temporal period (HH 30-36, 6.5-10 dpf). While their inductive capacity was discovered in the mid-1900s, little is known about their development.

RESULTS

Through a series of timed surgical ablations followed by in situ hybridization for Bmp2, we show that the ring of conjunctival papillae is not altered if the conjunctival epithelium is ablated either prior to or shortly after papillae induction (i.e., HH 29-30, 6.5-7 dpf). A conjunctival papilla ablated at or prior to HH 34 (8 dpf), when the complete ring is present, regenerates and quickly becomes inductively active, inducing an underlying scleral condensation with only a slight delay. This regenerative capacity extends until HH 35.5, a full 36 hours beyond the normal timeline of papillae induction. As such, the period of epithelial competency for papilla induction is longer than previously identified.

CONCLUSIONS

Papilla regeneration is a mechanism that ensures the formation of a complete sclerotic ring and provides another level of redundancy for the induction of a complete sclerotic ring during the normal inductive period. Developmental Dynamics 246:381-391, 2017. © 2017 Wiley Periodicals, Inc.

Apoptosis-associated protein expression in human salivary gland morphogenesis

OBJECTIVE

Salivary gland (SG) development is based on branching morphogenesis, in which programmed cell death has been proposed to play a role in cell signalling and organ shaping. In the mouse salivary gland apoptosis has been suggested to play a key role in lumen formation, removing the central cells of the epithelial stalks. Here we analyse the expression of several anti- and pro-regulators of apoptosis during human SG development in a range of developmental stages.

DESIGN

Foetal SGs obtained from the University of São Paulo were analysed by immunohistochemistry to assess the expression of apoptosis-associated proteins: caspases (caspase-6, -7, -9 and cleaved caspase-3), Bcl-2 family members (Bax, Bak, Bad, Bid, Bcl-2, Bcl-x and Bcl-xL), Survivin (BIRC5), Cytochrome C and Apaf-1.

RESULTS

Nuclear expression of Bax and Bak was identified in presumptive luminal areas at initial stages, while Bcl-xL showed the most relevant anti-apoptotic activity. Caspase-6, -7 and -9 were expressed during all stages, while interestingly cleaved caspase-3 showed no prominent expression, indicating that caspase-7 is the main effector. Apoptosome complex components Apaf-1 and Cytochrome C, as well as survivin were all positive in developing glands.

CONCLUSIONS

The particular expression pattern of several apoptotic regulators in human SG development suggests the existence of a fundamental role for apoptosis during duct formation. The absence of Bad and Bid expressions indicates that the instrinsic pathway is more active then the extrinsic during human gland formation. The subcellular localisation of intrinsic-apoptosis proteins correlated with apoptotic activity, but also suggested additional non-apoptotic functions.

Developmental mechanisms underlying differential claw expression in the autopodia of geckos

BACKGROUND

The limb and autopodium are frequently employed to study pattern formation during embryonic development, providing insights into how cells give rise to complex anatomical structures. With regard to the differentiation of structures at the distal tips of digits, geckos constitute an attractive clade, because within their ranks they exhibit multiple independent occurrences of claw loss and reduction, these being linked to the development of adhesive pads. The developmental patterns that lead to claw loss, however, remain undescribed. Among geckos, Tarentola is a genus characterized by large claws on digits III and IV of the manus and pes, with digits I, II, and V bearing only vestigial claws, or lacking them entirely. The variable expression of claws on different digits provides the opportunity to investigate the processes leading to claw reduction and loss within a single species.

RESULTS

Here, we document the embryonic developmental dynamics that lead to this intraspecifically variable pattern, focusing on the cellular processes of proliferation and cell death. We find that claws initially develop on all digits of all autopodia, but, later in development, those of digits I, II, and V regress, leading to the adult condition in which robust claws are evident only on digits III and IV. Early apoptotic activity at the digit tips, followed by apoptosis of the claw primordium, premature ossification of the terminal phalanges, and later differential proliferative activity are collectively responsible for claw regression in particular digits.

CONCLUSIONS

Claw reduction and loss in Tarentola result from differential intensities of apoptosis and cellular proliferation in different digits, and these processes have already had some effect before visible signs of claw development are evident. The differential processes persist through later developmental stages. Variable expression of iteratively homologous structures between digits within autopodia makes claw reduction and loss in Tarentola an excellent vehicle for exploring the developmental mechanisms that lead to evolutionary reduction and loss of structures.

Hyperbaric oxygen suppresses hypoxic-ischemic brain damage in newborn rats

The optimal therapeutic time-window and protective mechanism of hyperbaric oxygen in hypoxic-ischemic brain damage remain unclear. This study aimed to determine the neuroprotective effects of hyperbaric oxygen. Following hypoxic-ischemic brain damage modeling in neonatal rats, hyperbaric oxygen was administered at 6, 24, 48, and 72 hours and 1 week after hypoxia, respectively, once daily for 1 week. Fourteen days after hypoxic-ischemic brain damage, cell density and apoptosis rate, number of Fas-L+, caspase-8+, and caspase-3+ neuronal cells, levels of nitric oxide, malondialdehyde, and superoxide dismutase in hippocampus were examined. Morris water maze test was conducted 28 days after insult. Significant improvements were found in cell density, rate of apoptosis, oxidative stress markers, FasL, and caspases in rats treated with hyperbaric oxygen within 72 hours compared to hypoxic-ischemic injury. Similarly, time-dependent behavioral amelioration was observed in pups treated with hyperbaric oxygen. Our findings suggest that hyperbaric oxygen protects against hypoxic-ischemic brain damage by inhibiting oxidative stress and FasL-induced apoptosis, and optimal therapeutic time window is within 72 hours after hypoxic-ischemic brain damage.

The intramembrane protease SPPL2A is critical for tooth enamel formation

Intramembrane proteases are critically involved in signal transduction and membrane protein turnover. Signal-peptide-peptidase-like 2a (SPPL2A), a presenilin-homologue residing in lysosomes/late endosomes, cleaves type II-oriented transmembrane proteins. We recently identified SPPL2A as the enzyme controlling turnover and functions of the invariant chain (CD74) of the major histocompatibility complex II (MHCII) and demonstrated critical importance of this process for B cell development. Surprisingly, we found that SPPL2A is critical for formation of dental enamel. In Sppl2a knockout mice, enamel of the erupted incisors was chalky white and rapidly eroded after eruption. SPPL2A was found to be expressed in enamel epithelium during secretory and maturation stage amelogenesis. Mineral content of enamel in Sppl2a⁻/⁻ incisors was inhomogeneous and reduced by ∼20% compared to wild-type mice with the most pronounced reduction at the mesial side. Frequently, disruption of the enamel layer and localized detachment of the most superficial enamel layer was observed in the knockout incisors leading to an uneven enamel surface. In Sppl2a null mice, morphology and function of secretory stage ameloblasts were not noticeably different from that of wild-type mice. However, maturation stage ameloblasts showed reduced height and a characteristic undulation of the ameloblast layer with localized adherence of the cells to the outer enamel. This was reflected in a delayed and incomplete resorption of the proteinaceous enamel matrix. Thus, we conclude that intramembrane proteolysis by SPPL2A is essential for maintaining cellular homeostasis of ameloblasts. Because modulation of SPPL2A activity appears to be an attractive therapeutic target to deplete B cells and treat autoimmunity, interference with tooth enamel formation should be investigated as a possible adverse effect of pharmacological SPPL2A inhibitors in humans.

The dynamics of supernumerary tooth development are differentially regulated by Sprouty genes

In mice, a toothless diastema separates the single incisor from the three molars in each dental quadrant. In the prospective diastema of the embryo, small rudimentary buds are found that are presumed to be rudiments of suppressed teeth. A supernumerary tooth occurs in the diastema of adult mice carrying mutations in either Spry2 or Spry4. In the case of Spry2 mutants, the origin of the supernumerary tooth involves the revitalization of a rudimentary tooth bud (called R2), whereas its origin in the Spry4 mutants is not known. In addition to R2, another rudimentary primordium (called MS) arises more anteriorly in the prospective diastema. We investigated the participation of both rudiments (MS and R2) in supernumerary tooth development in Spry2 and Spry4 mutants by comparing morphogenesis, proliferation, apoptosis, size and Shh expression in the dental epithelium of MS and R2 rudiments. Increased proliferation and decreased apoptosis were found in MS and R2 at embryonic day (ED) 12.5 and 13.5 in Spry2(-/-) embryos. Apoptosis was also decreased in both rudiments in Spry4(-/-) embryos, but the proliferation was lower (similar to WT mice), and supernumerary tooth development was accelerated, exhibiting a cap stage by ED13.5. Compared to Spry2(-/-) mice, a high number of Spry4(-/-) supernumerary tooth primordia degenerated after ED13.5, resulting in a low percentage of supernumerary teeth in adults. We propose that Sprouty genes were implicated during evolution in reduction of the cheek teeth in Muridae, and their deletion can reveal ancestral stages of murine dental evolution.