🌊 Applied Day 7 — Master-Level, Fun, Fully Theoretical Summary

Aquaporins: Structure, Function, Physiology, and Roles Beyond Water Transport

(Only theoretical content — all practical/experimental details removed as instructed.)

⭐ 1. What Are Aquaporins?

Aquaporins (AQPs) are membrane water channels that allow extremely fast and highly selective water movement across cell membranes.

Why do we need them?

Because:

• Water crosses lipid bilayers very slowly by simple diffusion.
• Many cells need rapid, regulated water movement (red blood cells, kidney epithelial cells, salivary glands, etc.).

So evolution created aquaporins, which:

• Increase water permeability by ×100 or more
• Allow bidirectional flow depending on osmotic gradients
• Work passively (no energy required)

⭐ 2. Historical Discovery 🧬

The first aquaporin (AQP1) was discovered by accident by Peter Agre’s group.

The story in short:

1. They were studying the Rhesus blood group protein.
2. While purifying membrane proteins from RBCs, they saw an unexpected ~28 kDa band.
3. Attempts to remove it failed, so they purified it instead.
4. Someone suggested: “Maybe it’s the missing water channel.”
5. They tested this by injecting AQP mRNA into Xenopus oocytes (frog eggs).
   • Normal oocytes are water-impermeable.
   • After expressing AQP, they burst in distilled water → proof of a water channel.
6. Agre received the Nobel Prize (2003) for this.

⭐ 3. Aquaporin Structure 🧱

Aquaporins are tetramers in the membrane.

Each monomer:

• Has six transmembrane helices
• Has its own pore — each monomer is an independent water channel
• Has cytoplasmic N- and C-termini
• Has a narrow pore (~3 Å) lined with amino acids that force water to flip orientation

Why flip water?

Because flipping disrupts continuous hydrogen-bonded chains → prevents protons (H⁺) from “hopping” through. This is why aquaporins conduct water only, not ions.

⭐ 4. Selectivity and Variants

Most AQPs conduct only water, but some conduct:

• Glycerol (aquaglyceroporins)
• Urea
• Ammonia
• Hydrogen peroxide (H₂O₂) → peroxiporins

They are found in:

• Plasma membrane
• Endoplasmic reticulum
• Vesicles
• Mitochondria

Different AQPs have different cellular and tissue distributions.

⭐ 5. Kidney Physiology: Why Aquaporins Matter 💧👩‍⚕️

The kidneys filter ~180 L of fluid/day but produce only 0.5–2 L of urine.

This means ~99% of filtered water is reabsorbed, mostly through:

• Proximal tubule
• Descending limb of Henle

This part is constitutive (always active, not regulated).

Fine-tuning occurs in the collecting duct → controlled by aquaporins and hormones.

⭐ 6. Aquaporins in the Collecting Duct

Three key aquaporins work together:

AQP2

• Located in the apical membrane (facing the tubular lumen)
• Regulated (the “gatekeeper” of water reabsorption)
• Moves between storage vesicles ↔ membrane

AQP3 & AQP4

• Located in basolateral membrane
• Provide the exit pathway for water into the bloodstream

Together they create a transcellular highway: Lumen → AQP2 → cell → AQP3/4 → blood

⭐ 7. Hormonal Regulation: Vasopressin (ADH) 🧪

Water balance is controlled by:

• Blood osmolality
• Blood pressure

When osmolality ↑ or blood pressure ↓:

1. Hypothalamic osmoreceptors detect it
2. Vasopressin (AVP, also called ADH) is released
3. ADH acts on V₂ receptors in collecting duct
4. Increases cAMP → activates PKA
5. AQP2 gets phosphorylated
6. AQP2-containing vesicles insert into the apical membrane
7. Water permeability ↑ dramatically

Without ADH, the collecting duct is nearly water-impermeable.

⭐ 8. Pathophysiology: When Aquaporins Fail

AQP2 mutations → severe, lethal dehydration

Patients can produce ~20 L urine/day (nephrogenic diabetes insipidus).

Common medical causes of AQP2 dysregulation:

• Lithium treatment → downregulates AQP2
• Heart failure → too much ADH → excessive water retention
• Chronic kidney disease → impaired regulation

Regulators and drugs that specifically target AQP2 trafficking currently do not exist, and developing them is an active research field.

⭐ 9. Beyond Water: Aquaporins in Cancer 🧬

Omics studies have shown:

• Some AQPs (especially AQP5) are overexpressed in cancer
• High AQP expression correlates with:
  • lymph node metastasis
  • poor prognosis

Why? Not because of water transport, but because of roles in cell signaling and cytoskeletal organization.

🔬 Key cancer-related mechanisms seen in AQP-overexpressing cells:

1) Migration changes

AQP5 overexpression disrupts coordinated epithelial migration:

• Leader–follower organization breaks down
• Cells detach spontaneously
• Looks like early metastatic behavior

2) RAS pathway activation

AQP5 has a serine residue in an intracellular loop that activates RAS signaling (a major oncogenic pathway). Mutation of this residue abolishes the detachment phenotype.

3) Cell–cell junction disruption

AQP overexpression affects typical junction proteins:

• AQP1, AQP4, AQP5 → weaken junctions
• AQP3 → strengthens junctions (interesting because AQP3 is found in tissues under mechanical stress)

Loss of junction integrity is a hallmark of metastasis.

4) Loss of epithelial polarity

AQP5-overexpressing 3D cysts lose normal apical–basal polarity.

In patient tissue samples:

• AQP5 and key polarity protein Scribble show mutually exclusive patterns, suggesting polarity disruption in vivo.

⭐ 10. Conceptual Summary

Aquaporins are more than water channels. They are dynamic proteins that:

1. Regulate kidney water balance via rapid trafficking
2. Coordinate with hormonal signaling (ADH/vasopressin)
3. Influence cell behavior beyond transport:
   • migration
   • adhesion
   • polarity
   • signaling (e.g., RAS)
4. Are implicated in cancer progression

Theoretical take-home message: Aquaporins participate in physiology, cell biology, and pathology far beyond membrane water transport.