VNO Receptor Genetics: V1R and V2R in Humans

Vomeronasal Receptors Overview

Vomeronasal receptors are the molecular machinery that allows the VNO to detect pheromones. There are two main families:

Receptor Type	Classification	Typical Ligands
V1R	Class A GPCR	Small volatile molecules
V2R	Class C GPCR	Peptides, proteins, MHC molecules

Both families are G-protein coupled receptors (GPCRs) that trigger intracellular signaling cascades when activated by pheromones.

V1R Receptors

Function in Other Mammals

In species with functional VNOs (mice, rats):

• Location: Apical layer of VNO sensory epithelium
• Expression: Monogenic (one receptor per neuron)
• Detection: Small volatile molecules, urinary pheromones
• Examples: Male mouse urine compounds, female estrus signals

V1R Gene Numbers Across Species

The V1R repertoire varies dramatically:

Species	Functional V1Rs	Pseudogenes
Platypus	270	Unknown
Mouse	187	168
Rat	102	50+
Dog	8	33
Human	~2	~115
Chimpanzee	0	All

“The differences in size of the V1R repertoire are considered to be the largest variation in gene family size in all mammalian gene families.”

Human V1R Status

In humans:

• ~200 V1R sequences identified in the genome
• Only 4 have intact open reading frames (ORFs)
• Only ~2 are potentially functional
• The rest are pseudogenes (non-functional)

A pseudogene contains mutations that prevent:

• Proper protein expression
• Functional receptor formation
• Signal transduction

V2R Receptors

Function in Other Mammals

V2R receptors are more complex:

• Location: Basal layer of VNO sensory epithelium
• Structure: Large extracellular N-terminal domain
• Detection: Larger molecules (peptides, proteins)
• Special role: Detect MHC class I peptides (important for mate choice)

Human V2R Status

The situation for V2R is even more definitive:

“V2R genes have completely degenerated in humans, chimpanzee, macaque, cow, and dog. In these species only V2R pseudogenes are found.”

Humans have zero functional V2R genes.

This means the entire V2R-mediated detection system is non-functional in humans.

TRPC2: The Critical Ion Channel

Beyond receptors, the VNO requires the TRPC2 ion channel for signal transduction.

Role of TRPC2

When a VNO receptor binds a pheromone:

1. Receptor activates G-protein
2. G-protein activates phospholipase C
3. This opens TRPC2 channels
4. Calcium/sodium influx depolarizes the neuron
5. Signal sent to brain

Human TRPC2

TRPC2 is a pseudogene in humans.

This is perhaps the most critical finding because:

• Even if some V1Rs were functional
• Without TRPC2, no signal transduction is possible
• The entire VNO signaling pathway is broken

When Did This Happen?

Molecular clock analysis suggests:

1. 23-25 million years ago — V1R pseudogenization began
2. Before human/chimp split — Most V1Rs already non-functional
3. Primate evolution — Correlated with increased reliance on vision

Evolutionary Pressure

The loss of VNO function correlates with:

• Development of trichromatic color vision
• Increased brain size and cognition
• Shift toward visual/social communication
• Reduced reliance on chemical signals

Implications for Human Pheromones

What This Means

1. Classical VNO-based pheromone detection is impossible in humans

   • No functional receptors
   • No signal transduction channel
   • No neural pathway

2. If humans detect pheromones, it must be through other pathways:

   • Main olfactory epithelium (functional odorant receptors)
   • Cranial Nerve Zero
   • Unknown mechanisms

3. Research from rodents may not apply:

   • Rodent pheromone studies often focus on VNO
   • Human mechanisms likely differ fundamentally

The Silver Lining

The main olfactory system has:

• ~400 functional odorant receptor genes
• Ability to detect some steroids
• Connection to limbic system (emotion, behavior)

Some odorant receptors can bind:

• Androstenone
• Androstenol
• Other steroid compounds

So pheromone detection may occur through this pathway instead.

OR7D4: A Relevant Example

One well-studied olfactory receptor, OR7D4, detects androstenone:

• Located in main olfactory epithelium (not VNO)
• Genetic variants affect perception (pleasant vs. unpleasant)
• Demonstrates steroid detection via olfactory system

This supports the idea that:

• Pheromone-like compounds are detected
• Detection uses olfactory receptors, not VNO receptors
• Genetic variation explains individual differences

Key Takeaways

1. Human V1R repertoire is 98% pseudogenes
2. Human V2R genes are 100% non-functional
3. TRPC2 (essential ion channel) is a pseudogene
4. VNO-based pheromone detection is genetically impossible in humans
5. Loss occurred ~23-25 million years ago during primate evolution
6. Alternative pathways (MOE, CN0) may compensate
7. OR7D4 proves steroids CAN be detected via olfactory system

References

1. Young JM, et al. “Different evolutionary processes shaped the mouse and human olfactory receptor gene families.” Hum Mol Genet. 2002;11(5):535-46.

2. Liman ER, Innan H. “Relaxed selective pressure on an essential component of pheromone transduction in primate evolution.” PNAS. 2003;100(6):3328-32.

3. Zhang J, Webb DM. “Evolutionary deterioration of the vomeronasal pheromone transduction pathway in catarrhine primates.” PNAS. 2003;100(14):8337-41.

4. Keller A, et al. “Genetic variation in a human odorant receptor alters odour perception.” Nature. 2007;449(7161):468-72.