OE L10 Enamel Proteins Flashcards
Where are the ameloblasts in direct contact with the crystallites?
At the apical end of the rod.
Deposition of mineral occurs perpendicular to the surface creating a rod tail, small amount from ameloblast edges creates interrod enamel.
Do crystallites run in the same direction in the rod tail and rod head?
No, crystallites run in different directions in the rod tail and rod head.
What do crystallites look like at the secretory stage compared to the maturation stage?
Secretory stage: extremely thin
Maturation stage: thick
Are hydroxyapatite crystals in enamel the same as those found in dentine and bone?
No.
- Crystals in enamel are tens to hundreds of um long, have an aspect ratio (length/width) of at least 1000
- Crystals in dentine and bone are usually only 100nm in length and only a few nm in width
What is the overall role of enamel proteins?
Enamel proteins act as nucleators and modulators of hydroxyapatite formation.
Specialised proteins which are largely replaced by HAP crystals during mineraliastion.
What term describes a tissue composed of both organic and inorganic materials?
A bioceramic.
Enamel is a non-collagenous bioceramic.
What are the 3 biological processes in creating enamel?
- Enamel ECM synthesis and assembly
- ECM proccesing and ultimate degradation
- Supramolecular control of HAP crystal formation and growth
Are there specific genes related to the mineralisation of enamel?
No, however, the overall genetic programme controls the timing and relative quanities of genes transcribed, proteins are translated and assemble into a matrix, this matrix guides crystal formation.
When does HAP mineral formation occur?
- During assembly of the matrix
- During modification of the matrix
What is the origin of enamel?
Oral epithelial origin.
VS dentine which is derived from mesenchyme and neural crest and can be regenerated.
What are the 5 enamel specific proteins that have arisen from gene duplication?
- Amelogenin
- Ameloblastin
- Enamelin
- Amelotin
- Apin/ODAM
What family do these 5 proteins belong to?
The secretory calcium-binding phosphoprotein family (SCPP)
Which enamel proteins are synthesised during secretion?
- Amelogenin
- Ameloblastin
- Enamelin
Form the enamel matrix template
Which enamel proteins are synthesised from maturation and onwards?
- Amelotin
- Apin/ODAM
Synthesised at transition between maturation stage and the reduced enamel epithelium
How are enamel proteins synthesised and secreted?
- Proteins synthesised in same vesicles and released (e.g. amelotin and apin in same vesicle).
- Vesicles released from Tomes process
Describe the main features of amelogenin.
- Most abundant enamel matrix protein (80-90%)
- Expressed from genes on X and Y chromosomes, critical copy on X chromosome
- Can isolate DNA from teeth to determine sex, single type of amelogenin = female, 2 types = male
What are the 2 forms of amelogenin?
- TRAP: proteolytically processed form
- LRAP: alternatively spliced products of amelogenin
Describe the domain structure of an amelogenin molecule.
- Rigid central domain made of proline, histidine and glutamine
- N-terminal tri-tyrosine domain
- C-terminal hydrophobic segmenet followed by mineral binding hydrophilic domain
Describe the assembly of amelogenin molecules.
- Amelogenin self-assembles into spherical structures containing many amelogenin molecules
- One sphere usually contains 50-100 amelogenin molecules
- Spheres are called nanospheres
- Size of nanospheres corresponds to spacing of crystallites in the tissue, suggest amelogenin plays a role in organising the framework of the enamel
What does the tri-tyrosine motif of amelogenin bind?
Binds to carbohydrates such as N-acetyl-glucosamine. Allows amelogenin to interact with other matrix components such as enamelins.
What does the C-terminal domain of amelogenin bind to?
Binds to hydroxyapatite. Changes size of crystals and produces crystals of a very unique aspect ration (long and thin).
What faces do crystals have?
3 faces.
- A: top face covered in water
- B: side face with amelogenin bound to it
- C: if both A and B phases are occupied, the crystal must grow in C direction (along C-axis)
Explains unusually long crystallites.
What 2 sets of proteinases are present in enamel development and when?
- Metalloproteinases: active during secretory stage
- Serine proteinases: active during maturation stage
Why are proteinases important in mineralisation?
Proteinases drive maturation by degrading proteins that inhibit mineral deposition.
Give an example of a matrix metalloproteinase and its basic features.
Enamelysin (MMP-20)
- Expressed primarily during secretory and transition stages
- Produced by ameloblasts and odontoblasts
- Secreted from secretory face of Tomes’ process
- Cleaves amelogenin
- Cleaves Kallikrein 4
Give an example of a serine proteinase and its basic features.
Enamel matrix serine proteinase 1 (Kallikrein 4)
- Cleaved by MMP-20 to be activated
- Activity is pH sensitive (5.7)
- Allows crystals to now grow in thickness instead of just length as it was doing before
Describe knockout mice studies for 2 mutations in enamelysin.
- Heterozygous: 50% normal proteloytic activity, sufficient for normal enamel formation
- Homozygous: no proteolytic activity, amelogenin remains in tissue and prevents mineralisation from taking place
In humans, what is the result of a mutaion affecting the hemopexin domain of enamelysin.
Recessive hypomaturation amelogenesis imperfecta.
- Normal sized crowns
- Dull, brownish coloured teeth
- Little difference in radiotranslucency of dentine and enamel (shows that enamel is significantly under mineralised)
Overall function of enamelysin (MMP-20)
Activation of proteases to then cause bulk degradation of protein and allow for mineralisation.
Without MMP-20 mineralisation cannot occur.
What is the result of mutations in EMSP 1 (Kallikrein 4).
Autosomal recessive hypomaturation amelogenesis imperfecta.
- Both alleles must be affected
- Dramatic under mineralisation
- Normal shaped crowns
- Teeth yellow/brown
- Teeth have rough surface
- Teeth wear rapidly
What is the result of mutations in enamelin.
2 types
- Autosomal dominant amelogenesis imperfecta: hypoplastic enamel pitting
- Autosomal recessive amelogenesis imperfecta: majority enamel absent, exposed dentine, open bite malocclusion as secondary consequence of hypersensitivity
Describe the structure and functions of enamelin.
- Synthesised during secretory phase
- Large acidic protein
- High affinity for HAP
- Suggested to interact with crystallites and possibly be involved in nucleation or elongation
What is tuftelin?
- Not a true enamel protein
- Found at ADJ, not enamel specific and widely expressed in other tissues
Describe the main features of ameloblastin.
- Synthesised during secretory phase contributes to enamel matrix
- 2 domains connected by flexible linker
- Functions as an ECM organiser and an adhesion molecule for secretory ameloblasts binding to forming tissue
- Maintains phenotype of ameloblasts
What is the result of mutations in ameloblastin?
Recessive hypoplastic amelogenesis imperfecta:
- Teeth have rough surface
What is the distribution of amelogenin, ameloblastin and enamelin in the enamel?
- N-terminus amelogenin is present throughout the entire tissue
- C-terminus amelogenin is only present in the outer layer of enamel, near the ameloblasts
- Ameloblastin is present throughout the entire tissue (13-17 kDa form) in the sheath space between rods
- Enamelin is only present within the rod core
How do enamel proteins and minor organic components support the overall tissue?
Presence of minor inorganic components e.g. enamel proteins, allows differential movement between adjacent rods therefore reducing stress of the tissue.
Profound plasticing effect. Prevents cracks/fractures.
