aquaporins

Aquaporins (AQPs) are a family of integral membrane proteins that form pores allowing water molecules to pass through the plasma membrane down osmotic gradients while excluding ions and solutes, enabling extremely rapid water flux. There are 13 known mammalian isoforms (AQP0-12) with tissue-specific expression: AQP1 is found in proximal tubule, thin descending limb, and red blood cells, mediating constitutive water reabsorption; AQP2 is the principal ADH (vasopressin)-regulated channel in the principal cells of the renal collecting duct; AQP3 and AQP4 are located on the basolateral membrane of collecting duct cells, providing water egress after AQP2-mediated entry; AQP0 is important in lens transparency; AQP4 is also key in astrocyte water balance (relevant in cerebral edema and neuromyelitis optica, where AQP4 is the autoantibody target). Physiologically, ADH binds V2 receptors on collecting duct principal cells, activating a Gs-adenylate cyclase-cAMP-PKA pathway that causes translocation of AQP2-containing vesicles to the apical membrane, increasing water permeability and permitting concentrated urine formation. Clinically, this pathway is central to understanding diabetes insipidus: central DI results from inadequate ADH secretion (treated with desmopressin), while nephrogenic DI results from renal resistance to ADH due to mutations in the AQP2 gene or the V2 receptor (AVPR2), or can be acquired from lithium toxicity, hypercalcemia, or hypokalemia, all of which impair AQP2 trafficking/expression. Aquaporins are also important in SIADH pathophysiology (inappropriately increased AQP2 activity causing hyponatremia) and are therapeutic targets: vaptans (V2 receptor antagonists like tolvaptan) block ADH signaling and are used in SIADH and autosomal dominant polycystic kidney disease. AQP4 antibodies are diagnostic for neuromyelitis optica spectrum disorder (Devic disease), distinguishing it from multiple sclerosis.