Part 6 · Chapter 6.4

Aquaporins

Aquaporins (AQPs) are tetrameric membrane channels that conduct water — and sometimes small neutral solutes — with remarkable speed and selectivity. Peter Agre’s 1992 discovery (Nobel 2003) resolved decades of physiological puzzlement about how cells achieve water permeabilities far above simple lipid- bilayer values. 13 AQP family members have specialised tissue distributions.

1. Water Conduction Mechanism

AQP1 monomer has a narrow 2.8 Å central pore that admits water (2.6 Å) but excludes hydrated Na⁺/K⁺/Cl^-(3.6–4.0 Å hydrated) and protons. Two asparagine residues in NPA motifs enforce a directional hydrogen-bond dipole that reorients water molecules, breaking the Grotthuss proton-wire mechanism. Per monomer, AQP1 conducts ~3 × 10⁹ H₂O s^-1.

2. AVP-Regulated AQP2

Collecting-duct water reabsorption is regulated by arginine vasopressin (AVP) acting on V2R (a G_s-coupled GPCR). cAMP-PKA phosphorylates AQP2 at Ser256, triggering trafficking of AQP2-containing vesicles to the apical membrane:

\[ \text{AVP} \to \text{V2R} \to \text{G}_s \to \text{cAMP} \to \text{PKA-P-AQP2} \to \text{apical insertion} \to \text{H}_2\text{O}\ \text{reabsorption} \]

Central diabetes insipidus (absent AVP) and nephrogenic diabetes insipidus (V2R or AQP2 mutations) both produce massive polyuria — up to 20 L/day urine output.

Simulation: AQP Family & Permeabilities

Python

script.py23 lines

import numpy as np, matplotlib
matplotlib.use('Agg')
import matplotlib.pyplot as plt

aqps = ['AQP1','AQP2','AQP3','AQP4','AQP5','AQP7','AQP9','AQP11']
Pf = [1.0, 0.8, 0.4, 0.9, 0.7, 0.3, 0.2, 0.5]
tissue = ['RBC/prox tubule','Collecting duct','Skin/kidney','Brain astrocyte',
          'Salivary/lung','Adipocyte','Liver/immune','ER']
permeability = ['H2O only','H2O (AVP-regulated)','H2O+glycerol','H2O',
                'H2O','Glycerol+H2O','Glycerol+urea+H2O','H2O']

fig, ax = plt.subplots(figsize=(11, 6), facecolor='#0a0f1e')
ax.set_facecolor('#111e2f'); ax.tick_params(colors='#bae6fd')
for s in ax.spines.values(): s.set_color('#334155')
ax.barh(aqps, Pf, color='#60a5fa', edgecolor='#bae6fd')
ax.set_xlabel('Relative water permeability Pf (vs AQP1)', color='#bae6fd')
ax.set_title('AQP family: 13 members, water and small solutes',
             color='#67e8f9', fontweight='bold')
ax.grid(True, color='#334155', alpha=0.3, axis='x')
plt.tight_layout()
plt.savefig('output.png', dpi=120, bbox_inches='tight', facecolor='#0a0f1e')
print('AQP1: 3 x 10^9 H2O/s per monomer; no proton permeation')
print('AQP2: AVP/V2R-regulated trafficking to apical membrane (antidiuresis)')

Click Run to execute the Python code

Code will be executed with Python 3 on the server

3. Family Specialisations

AQP1: RBC, proximal tubule, microvascular endothelium.
AQP2: collecting duct apical (AVP-regulated).
AQP3, 7, 9, 10: aquaglyceroporins — permeate glycerol + water. Adipose lipolysis, liver metabolism.
AQP4: astrocyte endfeet, brain water balance, target of neuromyelitis optica (NMO) IgG autoantibodies.
AQP5: salivary, lacrimal, and lung secretion.

Key References

• Preston, G. M., Carroll, T. P., Guggino, W. B. & Agre, P. (1992). “Appearance of water channels in Xenopus oocytes expressing red cell CHIP28 protein.” Science, 256, 385–387.

• Verkman, A. S. (2012). “Aquaporins in clinical medicine.” Annu. Rev. Med., 63, 303–316.

• Knepper, M. A., Kwon, T. H. & Nielsen, S. (2015). “Molecular physiology of water balance.” N. Engl. J. Med., 372, 1349–1358.

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