Lipids in predentine and dentine

Overburdened ferroptotic stress impairs tooth morphogenesis

The role of regulated cell death in organ development, particularly the impact of non-apoptotic cell death, remains largely uncharted. Ferroptosis, a non-apoptotic cell death pathway known for its iron dependence and lethal lipid peroxidation, is currently being rigorously investigated for its pathological functions. The balance between ferroptotic stress (iron and iron-dependent lipid peroxidation) and ferroptosis supervising pathways (anti-lipid peroxidation systems) serves as the key mechanism regulating the activation of ferroptosis. Compared with other forms of regulated necrotic cell death, ferroptosis is critically related to the metabolism of lipid and iron which are also important in organ development. In our study, we examined the role of ferroptosis in organogenesis using an ex vivo tooth germ culture model, investigating the presence and impact of ferroptotic stress on tooth germ development. Our findings revealed that ferroptotic stress increased during tooth development, while the expression of glutathione peroxidase 4 (Gpx4), a crucial anti-lipid peroxidation enzyme, also escalated in dental epithelium/mesenchyme cells. The inhibition of ferroptosis was found to partially rescue erastin-impaired tooth morphogenesis. Our results suggest that while ferroptotic stress is present during tooth organogenesis, its effects are efficaciously controlled by the subsequent upregulation of Gpx4. Notably, an overabundance of ferroptotic stress, as induced by erastin, suppresses tooth morphogenesis.

Genetische Störungen der Zahnentwicklung und Dentition

Die Zähne sind Organe, die aus ektodermalen epithelialen Aussackungen im Bereich des 1. Kiemenbogens entstehen, gesteuert von epitheliomesenchymalen Interaktionen. Dabei spielen zahlreiche Signalmoleküle speziell der 4 großen Familien TGF-β, FGF, Hedgehog und WNT sowie diverse Transkriptionsfaktoren eine Rolle. Eine Beteiligung der Retinoide an der Odontogenese ist durch umfangreiche Befunde belegt, auch wenn die Inaktivierung relevanter Gene in Mausmodellen meist keine Zahnanomalien verursacht. Die Zahnentwicklung wird klassischerweise in verschiedene Stadien eingeteilt: Entstehung der Zahnleiste, der Zahnknospe, der Schmelzkappe, der Schmelzglocke, die Wurzelbildung und der Zahndurchbruch. Anomalien der Zahnentwicklung können isoliert oder gemeinsam mit anderen Symptomen im Zusammenhang mit Syndromen auftreten. Sie können genetisch bedingt sein oder unter Einwirkung teratogener Stoffe während der Bildung und Mineralisierung der Zahnkeime zustande kommen. Dentibukkale Entwicklungsanomalien treten im Kontext seltener Erkrankungen auf und finden zunehmend Beachtung, da sie bei bestimmten Erkrankungen in der Diagnostik und als prädikative Faktoren wichtige Anhaltspunkte geben können. Allerdings ist hierfür eine interdisziplinäre und internationale Kooperation notwendig, die bislang erst in Ansätzen verwirklicht wurde.

Peritubular dentin, a vertebrate apatitic mineralized tissue without collagen: role of a phospholipid-proteolipid complex

Peritubular dentin (PTD), a highly mineralized annular ring surrounding each odontoblastic process within the dentin, is an enigmatic component in vertebrate teeth. To characterize its structure and composition, we have coupled in situ scanning electron microscopic (SEM) and time-of-flight secondary ion mass spectrometric (TOF-SIMS) analysis of the surface composition of intact bovine coronal dentin with the isolation of intact PTD from hypochlorite-treated dentin and its subsequent TOF-SIMS and direct chemical analysis. The isolated PTD is shown to be a mineralized but porous structure complexed with a high-molecular mass calcium-proteolipid-phospholipid-phosphate complex, which cannot be extracted from the dentin prior to demineralization. The TOF-SIMS and direct amino acid analysis data confirm that the PTD protein is rich in glutamic acid but does not contain collagen. Phosphatidylcholine, phosphatidylserine, and phosphatidylinositol are present, along with a mannose-rich glycan and chondroitin-4- and chondroitin-6-sulfate glycosaminoglycans. PTD apatite, well described in the literature, must therefore form in this noncollagenous proteolipid-phospholipid complex without the intervention of collagen; nevertheless, as shown by SEM, the apatite is formed in small platy crystals, as in the bulk of the intertubular dentin (ITD). We hypothesize that the porous nature of the PTD and its proteolipid-phospholipid complexes may be involved in regulating communication between the ITD and internal PTD tubule fluids and the odontoblasts, similar to the involvement of such lipid complexes in neural, brain, and nuclear transport functions. Thus, the PTD should not be considered solely as a passive structural element in some teeth but as part of the system that allows for the vital function of the dentin.

Leukodystrophy associated with oligodontia in a large inbred family: fortuitous association or new entity?

We describe a large inbred Syrian pedigree with an autosomal recessive neurodegenerative disorder. The clinical picture of the affected patients is oligodontia, and a degenerative neurological condition with onset around age 12, characterized by progressive ataxia and pyramidal syndrome. Abnormalities in the white matter and cortical atrophy were assessed by magnetic resonance imaging. Differential diagnosis and the possibility of a fortuitous association or the report of a hitherto unreported dento-leukoencephalopathy are discussed.

Phospholipids in amelogenesis and dentinogenesis

Phospholipids have been identified in enamel and dentin. Before demineralization, a group of phospholipids extracted by lipid solvents was associated with cell membranes and is therefore closely related to cell growth and intracellular regulations. After demineralization, a second group of phospholipids, associated with the extracellular matrix, was extracted; this group is probably linked to the mineralized phase. Using imidazole-osmium tetroxide fixation of rat incisors, we stained cellular unsaturated fatty acids, so that we could visualize the membrane domains, coated pits, and endocytic inclusions. Filipin, a probe for cholesterol, varied in density along the plasma membrane of secretory ameloblasts, and allowed us to visualize membrane remnants inside the forming enamel. With respect to phospholipids located in the extracellular matrix, the malachite-green-glutaraldehyde (MGA) method or iodoplatinate (IP) reaction retains and visualizes enamel and dentin phospholipids. In predentin, aggregates appearing as granules and filaments, or liposome-like structures, were located in the spaces between collagen fibrils. In dentin, organic envelopes coating the crystals, also named "crystal-ghost" structures, outlined groups of collagen fibrils. Histochemical data provided evidence that phospholipids are co-distributed or interact with proteoglycans. Radioautography after IP reaction established that [3H] choline was detected in dentin as early as 30 min after the intravenous injection of the labeled precursor, before any labeling was seen in odontoblasts and predentin. This suggests that blood-serum-labeled phospholipids pass between odontoblasts, cross the distal permeable junctional complex, and diffuse in dentin prior to any cellular uptake and phospholipid synthesis. Pharmacologically and genetically induced pathology also supports the suggestion that phospholipids play an important role in the formation and mineralization of dental tissues.

Fragmentation of the distal portion of Tomes' processes of secretory ameloblasts in the forming enamel of rat incisors

In order to investigate enamel and dentin phospholipid metabolic pathways, two separate experiments were carried out. Firstly, rats were given chloroquine, a drug which induces a lipidosis-like disease. Extensive accumulation inside lysosomes was seen in all the groups of cells in the forming part of the rat incisor, except secretory ameloblasts which were unaffected by the drug. Secondly, the uptake and fate of 3H-choline were studied by radioautography on rats fed either normally or on an essential fatty acid deficient diet (EFAD). Four hours after the injection of the precursor, incorporation reached a maximum then decreased gradually. At 4 days the forming enamel displayed higher silver grain density than any other compartment. In EFAD rats 3H-choline incorporation was decreased drastically in each compartment except in the forming enamel which was not affected by the deficiency. The longer retention of the labeling in the forming enamel and the lack of lysosomal accumulation in chloroquine-treated secretory ameloblasts support the hypothesis that fragments of the distal Tomes' process are released during enamel formation. Disconnected from the cells, membrane remnants are neither reinternalized nor subjected to further degradation inside lysosomes.

HPLC-OPLC-MS investigation of change of formaldehyde and its generators in human teeth of different physiological stage

Taking into consideration the unquestionable intracellular occurrence of formaldehyde (HCHO) and its generators in cells of plant, animal and human organisms as well as in body fluids it was resolved to determine their levels in hard tissues of physiologically and pathologically changed teeth. The aim of the work was to determine the relationship between the level of HCHO and the levels of its generators in pathologically changed teeth, mainly carietic teeth as tooth caries is still a serious and commonly occurring problem. The occurrence of HCHO (captured as its dimedone adduct) and some of its potential generators was demonstrated in the hard tissues of healthy and pathological human teeth by means of OPLC, HPLC and MS analyses. It was established that the measurable level of HCHO was increased in the carietic teeth in comparison with healthy ones. In the case of paradontic tooth sample, a dramatic increase of HCHO was observed and at the same time the level of betaines was decreased considerably. The obtained results give a new insight into the pathology of hard tissues of teeth in strong correlation with the phases of stress syndrome.

Radioautographic study of the incorporation of (3H)-choline into the phospholipids of secretory ameloblasts and enamel of normal and essential-fatty-acid-deficient rats

(3H)-choline, a precursor for phosphatidylcholine (PC) and sphingomyelin (SM), was injected into rats killed after 4, 24, 48, and 96 hrs. Radioautography carried out on malachite-green/aldehyde-fixed tissues demonstrated that labeled choline was incorporated into cells and further released into the extracellular matrix. In predentin, labeling decreased rapidly, whereas in dentin, silver grains formed a stable band. In contrast, labeling was still high at 48 and 96 hrs in secretory ameloblasts as well as in the forming enamel. This indicates that ameloblasts are actively involved in the synthesis of membranes. Membrane remnants of the ameloblasts could be released into the forming enamel. In rats fed with an essential fatty-acid-deficient (EFAD) diet for 42 days, (3H)-choline uptake was delayed and reduced in pulp cells and odontoblasts, and consequently the migration of labeled phospholipids into dentin. The influence of the EFAD diet on secretory ameloblasts was limited. No difference was detected between normally fed and EFAD-fed rats in the forming enamel.

Influence of specimen preparation on the identification of phospholipids by the phospholipase A2-gold method in mineralizing cartilage and bone

The role of phospholipids in biological mineralization has been hypothesized but not fully elucidated. In order to identify phospholipids at the ultrastructural level in the mineralizing extracellular matrix, rat epiphyseal cartilage and metaphyseal bone have been labeled with the phospholipase A(2) (PLA(2))-gold method. The specificity and the efficiency of phospholipid detection have been evaluated by postembedding labeling of sections from epoxy- or hydrophilic resin-embedded samples, and by preembedding labeling of cryosectioned samples. The efficiency of the labeling was higher in cryosections than in hydrophilic resin-embedded specimens, while lower efficiency was found in epoxy resin-embedded samples. A 2- to 6-fold increase of the labeling density in calcified with respect to uncalcified areas of cartilage and bone has been found, depending on the specimen preparation used. The labeling intensity was significantly higher, at the periphery of the calcifying nodules in the epiphyseal cartilage matrix and in the calcifying osteoid, while the fully calcified bone matrix presented a weak labeling. Matrix vesicles, which are considered a possible source of extracellular phospholipids, appeared labeled in cryosections and in epoxy resin-embedded samples after a preincubation with PLA(2), which also increased the labeling of the intracellular membranes. The localization of phospholipids in the areas of initial mineralization suggests some hypotheses on the possible involvement of these molecules in the mineral phase deposition process.