Archaeological preservation of amelogenesis pathways

This study analyzes archaeological dental enamel proteomes to characterize the effects of in vivo amelogenesis-related protein cleavage and phosphorylation, ultimately proposing a novel phosphorylation-based marker for genetic sex estimation in human evolutionary research.

The Gist

Imagine your teeth as tiny, indestructible time capsules. While the soft parts of your body (like skin and organs) rot away quickly, the hard outer shell of your teeth—enamel—is the toughest material in your body. It's so tough that it can trap proteins (the building blocks of life) for millions of years, acting like a fossilized hard drive that still holds data long after the computer has turned to dust.

This paper is like a team of digital archaeologists trying to read that hard drive. They didn't just look for what proteins were there; they wanted to understand how those proteins were processed while the tooth was still growing inside the person's mouth, and how that processing helps us solve mysteries today.

Here is the breakdown of their discovery, using some everyday analogies:

1. The "Construction Site" Analogy

Think of making a tooth like building a skyscraper.

• The Workers: Special cells called ameloblasts are the construction crew. They secrete proteins (the bricks and mortar) to build the enamel.
• The Cleanup Crew: Once the bricks are laid, the crew doesn't just leave them there. They use two specific "scissors" (enzymes called MMP20 and KLK4) to trim the excess material and shape the final structure.
• The Quality Control Stamps: Another enzyme (FAM20C) acts like a stamping machine, adding a tiny chemical tag (a phosphate group) to specific spots on the bricks to make them stick together better.

The Big Question: When we find an ancient tooth thousands of years later, do we just see a pile of random bricks? Or can we still see the specific cuts made by the "scissors" and the "stamps" left behind by the original construction crew?

2. The Discovery: The Blueprint is Still Visible

The researchers took ancient teeth from medieval skeletons and modern baby teeth, dissolved the hard enamel to extract the proteins, and used a high-tech microscope (Mass Spectrometry) to read the molecular code.

They found that the construction site's history is still written in the stone.

• The Scissors Marks: Even after centuries, the proteins show clear signs of being cut by the "scissors" (MMP20 and KLK4) exactly where the scientists expected them to be cut during the tooth's growth. It's like finding a piece of paper that has been torn in a specific pattern, proving it was torn by a specific pair of scissors, not just shredded by time.
• The Stamps: They also found the "stamps" (phosphorylation) still attached to the proteins. This confirms that the biological process of tooth formation leaves a permanent signature that survives the march of time.

3. The "Gender Detective" Tool

One of the most exciting parts of the study is a new way to tell if an ancient skeleton is male or female.

Usually, scientists look for a specific protein called Amelogenin. There are two versions:

• AMELX: Found on the X chromosome (females have two, males have one).
• AMELY: Found on the Y chromosome (males only).

The Old Way: Scientists count how much of each protein they find. If they see only AMELX, it's likely a female. If they see both, it's a male. But this is tricky because AMELY is very rare (like finding a needle in a haystack), so sometimes they miss it and accidentally call a male a female.

The New Way (The "Phosphate Stamp"):
The researchers discovered a special "stamp" (phosphorylation) that appears on a specific spot of the male protein (AMELY) but never on the female version.

• Analogy: Imagine every male tooth has a tiny, invisible "M" sticker on a specific brick. Even if you can't find the whole brick, if you see that specific "M" sticker, you know for sure the tooth belongs to a male.
• Why it matters: This "sticker" is a much more reliable clue than just counting the bricks. It helps scientists avoid mistakes when identifying the sex of ancient remains.

4. Why This Matters

This study is like upgrading the software for reading ancient DNA.

• Better Reading: By knowing exactly where the "scissors" cut and where the "stamps" were placed, scientists can now program their computers to look for these specific patterns. This helps them identify more proteins and get a clearer picture of who these ancient people were.
• Evolutionary History: Since these patterns are preserved, we can compare ancient teeth to modern ones to see how our biology has changed (or stayed the same) over millions of years.

In a Nutshell

The authors showed that ancient teeth aren't just silent rocks. They are biological archives that still hold the "blueprints" of how they were built. By learning to read the specific cuts and chemical tags left behind by the body's own construction crew, we can unlock more secrets about our ancestors, including a more accurate way to tell if they were boys or girls.

Technical Summary

1. Problem Statement

Dental enamel is a premier source for ancient proteins (palaeoproteomics) due to its extreme durability, with proteins surviving for over 20 million years. However, the standard view of the enamel proteome is that it is small and static, consisting of roughly 12 unique proteins.

• The Gap: During tooth development (amelogenesis), enamel proteins undergo specific biological processing:
  1. Proteolytic Cleavage: They are digested in vivo by two proteases unique to enamel: Matrix Metalloproteinase 20 (MMP20) and Kallikrein 4 (KLK4).
  2. Post-Translational Modification (PTM): They undergo serine phosphorylation by the kinase FAM20C.
• The Question: It remains unclear whether these specific in vivo cleavage patterns and phosphorylation signatures are preserved in archaeological samples or if they are lost to diagenesis (post-mortem degradation). Furthermore, current methods for estimating genetic sex in ancient remains rely primarily on the presence/absence of AMELY peptides, which can be unreliable due to low abundance. The authors aim to determine if the biological "fingerprint" of amelogenesis survives in ancient enamel and if these patterns can improve sex estimation.

2. Methodology

The study utilized a comparative approach between modern and archaeological samples using high-resolution mass spectrometry.

• Sample Collection:
  • Archaeological: 10 adult permanent maxillary incisors from the Saint Eusebius church graveyard in Arnhem, Netherlands (Medieval/Post-Medieval, 1350–1829 AD).
  • Modern: 10 deciduous (baby) maxillary incisors from the Ratón Pérez Collection (Spain).
  • Controls: Laboratory blanks were included to monitor contamination.
• Protein Extraction & Preparation:
  • Enamel was drilled from the incisal surface, avoiding dentine.
  • Samples were demineralized using 5% HCl at 3°C for 48 hours.
  • Peptides were cleaned and desalted using C18 StageTips.
  • Note: No enzymatic digestion (e.g., trypsin) was performed, relying on the in vivo cleavage products already present in the sample.
• Mass Spectrometry (LC-MS/MS):
  • Analyzed using an EasyLC system coupled to an Exploris 480 mass spectrometer.
  • Data Dependent Acquisition (DDA) mode with High-Energy Collision Dissociation (HCD).
• Bioinformatics:
  • Search Engines: Data was analyzed using MaxQuant (targeting a specific list of enamel proteins) and PEAKS 7 (searching against the full human reference proteome).
  • Modifications: Variable modifications included deamidation (NQ), oxidation (MP), and phosphorylation (STY).
  • Sex Estimation: Genetic sex was determined by comparing Label-Free Quantification (LFQ) intensities of specific peptides unique to AMELX (X-chromosome) and AMELY (Y-chromosome).

3. Key Results

A. Proteome Composition and Preservation

• Identification: 12 core enamel proteins were identified, with Amelogenin (AMELX/AMELY), Ameloblastin (AMBN), and Enamelin (ENAM) being the most abundant.
• Deciduous vs. Permanent: Permanent teeth showed significantly higher MS/MS identification rates (17–30%) compared to modern deciduous teeth (5–17%). The lower rate in deciduous teeth is attributed to the lack of in vivo enzymatic digestion steps in the extraction protocol for modern samples, whereas archaeological samples benefit from natural in vivo cleavage.
• Additional Proteins: The study identified 92 additional proteins (including immune system markers like Immunoglobulins) in the archaeological samples, suggesting the enamel proteome is larger than previously assumed.

B. Preservation of In Vivo Cleavage Patterns

The study confirmed that the specific cleavage patterns induced by MMP20 and KLK4 during tooth formation are preserved in archaeological samples:

• Ameloblastin (AMBN): Showed coverage of the N-terminal 15 kDa, 23 kDa, and C-terminal fragments, matching experimental porcine data.
• Enamelin (ENAM): The stable 32 kDa cleavage product (positions 136–241) was the most abundant region, with sporadic coverage elsewhere, consistent with known resistance to MMP20.
• Amelogenin (AMELX/AMELY): Displayed near-complete sequence coverage, particularly in the Tyrosine-Rich Amelogenin Polypeptide (TRAP) region (6 kDa), confirming that the in vivo digestion products survive millennia.
• Cleavage Sites: The data revealed specific cleavage preferences (e.g., frequent cleavage N-terminal to Serine, Valine, Tyrosine) that align with known MMP20/KLK4 activity.

C. FAM20C Serine Phosphorylation

• The study detected serine phosphorylation in AMBN, ENAM, AMELX, and AMELY.
• Most sites followed the canonical S-X-E and S-X-S(phos) motifs, confirming a biological origin (FAM20C activity) rather than diagenetic modification.
• Some non-canonical sites were observed, the origin of which (biological vs. diagenetic) requires further study.

D. Genetic Sex Estimation

• Standard Method: The study successfully confirmed genetic sex for 19/20 individuals using the standard peptide intensity ratio of AMELX vs. AMELY.
• New Phosphorylation Marker: A novel sex-specific marker was identified at position 66 of AMELY.
  • In males, the serine at position 66 in AMELY is phosphorylated.
  • In females (AMELX), the corresponding serine is not phosphorylated.
  • This phosphorylation was observed in both deciduous and permanent teeth, offering a potential secondary marker for sex estimation when AMELY peptide counts are low.

4. Significance and Contributions

1. Validation of Biological Signatures: The paper provides definitive evidence that the complex biological processing of enamel (proteolytic cleavage and phosphorylation) is preserved in archaeological contexts. This validates the use of these patterns to distinguish endogenous ancient proteins from environmental contaminants.
2. Refinement of Palaeoproteomic Analysis: The authors argue that standard data analysis pipelines often miss these specific PTMs and cleavage products. They recommend explicitly including serine phosphorylation and non-enzymatic cleavage patterns in database searches to increase peptide identification rates and accuracy.
3. Improved Sex Estimation: The discovery of a phosphorylation-based marker for AMELY offers a robust alternative or supplementary method for determining genetic sex in ancient remains, addressing the issue of false negatives when AMELY peptides are scarce.
4. Evolutionary Context: By confirming that these pathways are preserved, the study opens new avenues for using enamel proteomics to study evolutionary changes in tooth development mechanisms across hominins and other mammals.

Conclusion

This study demonstrates that the "fingerprint" of tooth formation—specifically the cleavage by MMP20/KLK4 and phosphorylation by FAM20C—is robustly preserved in archaeological dental enamel. Recognizing these patterns not only improves the reliability of ancient protein identification but also provides new, biologically grounded markers for genetic sex estimation, enhancing the utility of dental enamel in human evolutionary research.