In the quiet world of a laboratory sea snail, scientists have discovered a biological contradiction that challenges our fundamental understanding of how brain cells communicate.
You won't find it on any restaurant menu, but the California sea hare (Aplysia californica) has been a delicacy in neuroscience laboratories for decades. With only about 20,000 oversized neurons—compared to the 86 billion in the human brain—this giant sea snail has helped unravel the mysteries of learning and memory. Now, this humble mollusk has revealed yet another secret, one that involves a molecular mirror image and a peptide that shouldn't work according to neuroscience textbooks.
The California sea hare has only ~20,000 neurons compared to 86 billion in humans, making it an ideal model for neuroscience research.
Discovery of a functional mirror-image peptide that challenges established biological principles.
Proteins, the workhorses of life, are built exclusively from L-amino acids—think of them as the "left-handed" building blocks of biology. For years, scientists believed this molecular left-handedness was an unbreakable rule of life. D-amino acids, the "right-handed" mirror images, were considered rare biological accidents—until now.
In 2018, researchers made a startling discovery in the Aplysia nervous system. The allatotropin-related peptide (ATRP), a neuropeptide that regulates feeding behavior, exists in two forms: the normal all-L version and an unusual version containing a single D-amino acid at position 2 (d2-ATRP) 1 .
"Left-handed" building blocks used in virtually all proteins
"Right-handed" mirror images previously considered biological accidents
This discovery suggests that molecular mirror images might be more common and functionally diverse in biology than previously imagined.
How did researchers discover these mirror-image peptides in the sea snail's brain? The investigation involved sophisticated biochemical sleuthing:
When researchers analyzed peptides from Aplysia cerebral ganglia using liquid chromatography-mass spectrometry, they noticed something peculiar. Instead of one peak corresponding to ATRP, they found two distinct peaks with identical mass but different retention times—a classic signature of diastereomers (molecules with the same chemical formula but different spatial arrangements) 1 .
The team then used aminopeptidase M, an enzyme that rapidly degrades typical all-L peptides. The later-eluting peak disappeared quickly, while the earlier-eluting peak resisted degradation—strong evidence that this species contained a D-amino acid near its N-terminus that protected it from enzymatic breakdown 1 .
Finally, researchers spiked the samples with synthetic versions of potential ATRP variants bearing different D-amino acids. The earlier-eluting endogenous peak co-eluted precisely with synthetic d2-ATRP (containing D-phenylalanine at position 2), confirming its identity 1 .
LC-MS analysis reveals two peaks for ATRP with identical mass but different retention times, indicating the presence of mirror-image molecules.
The most surprising finding emerged when researchers tested how both peptide versions interacted with their newly identified receptor.
| Property | l-ATRP (all-L version) | d2-ATRP (D-amino acid version) |
|---|---|---|
| Receptor Activation | Potent agonist 1 | Potent agonist 1 |
| Neuronal Activity | Increases excitability in feeding circuits 1 | Increases excitability in feeding circuits 1 |
| Stability in Plasma | Rapidly degraded 1 | Highly stable 1 |
| Percentage in CNS | ~70% 1 | ~30% 1 |
In most previously known DAACPs, the D-amino acid version was essential for activity, while the all-L version was ineffective. The Aplysia ATRP system breaks this pattern—both forms are active.
The dramatically different stability between the two forms reveals a potential regulatory mechanism. The D-amino acid version acts like a slow-release version of the peptide, protected from degradation.
This suggests that for some peptides, the primary role of L-to-D residue isomerization may not be receptor activation but rather extending the peptide's lifespan in the extracellular space 1 .
| Tool/Technique | Function in Research |
|---|---|
| Liquid Chromatography-Mass Spectrometry (LC-MS) | Separating and identifying peptides from complex biological mixtures 1 |
| Aminopeptidase M Digestion | Screening for D-amino acid containing peptides based on enzymatic resistance 1 |
| Synthetic Peptide Standards | Confirming identities of endogenous peptides through co-elution experiments 1 |
| pNEX3 Expression Vector | Introducing genes into Aplysia neurons to study receptor function 6 |
| Electrophysiological Recording | Measuring changes in neuronal excitability in response to peptides 6 |
Critical for detecting the two ATRP variants with identical mass but different properties.
Aminopeptidase M digestion revealed the unusual stability of d2-ATRP.
Confirmed that both peptide variants increase neuronal excitability.
The discovery in Aplysia has ripple effects far beyond marine biology. D-amino acids and DAACPs have been linked to various human conditions:
D-aspartate and D-serine have been found in brain tissues of Alzheimer's patients, and DAACPs have been isolated from cataract patients 2 .
Non-enzymatic racemization (spontaneous L-to-D conversion) occurs slowly in long-lived proteins, making D-amino acid content a potential marker of protein age 2 .
Gut bacteria produce D-amino acids that are incorporated into their cell walls, potentially influencing host physiology 2 .
The Aplysia findings open new possibilities for peptide therapeutics. Incorporating D-amino acids could enhance drug stability without necessarily compromising activity, potentially leading to longer-lasting treatments with lower dosing frequencies 1 2 .
The Aplysia ATRP story represents a paradigm shift in neurobiology. Rather than being mere curiosities or pathological accidents, D-amino acid containing peptides appear to be a sophisticated regulatory mechanism fine-tuning neural communication.