Water and Ion Movement Across Membranes Flashcards

This set of flashcards covers the discovery, structure, biological roles, and clinical significance of aquaporins in humans and plants, with a focus on their dual function and impact on cancer and edema.

Aquaporins

Specialized water channels that allow water to cross the plasma membrane; they can also exhibit ion channel activity and are involved in cell migration, invasion, and metastasis.

CHIP28 (hAQP1)

The first human aquaporin discovered by Peter Agre in 1988, originally found in red blood cells.

AtTIP1;1

The first plant aquaporin discovered by Christopher Maurel in 1993, localized to the tonoplast membrane.

Peter Agre and Roderick McKinon

Scientists who were awarded the Nobel prize in 2003 for their work related to aquaporins.

AQP5 (Human Body Role)

Responsible for osmotic water transport across the cells into sweat ducts.

AQP0

An aquaporin expressed in lens fiber cells that plays a role in cell adhesion; its mass is approximately $26$ to $34\,kDa$.

Plant Aquaporin Subfamilies

Classified into five main groups: plasma membrane intrinsic proteins (PIPs), Tonoplast intrinsic proteins (TIPs), nodulin 26 like intrinsic proteins (NIPs), small basic intrinsic proteins (SIPs), and X intrinsic proteins (XIPs).

Aquaporin Structural Features

Typically consists of six TMs (transmembrane domains), N and C termini located intracellularly, 2 NPA motifs, and 2 hemipores, often functioning as tetramers.

NPA motifs

Two specific structural motifs present in aquaporins; one is located near a serine residue that can be phosphorylated to regulate channel activity.

Gating of Plant Aquaporins

Mechanism of regulation occurring through phosphorylation of serine residues, tetramerization, pH (lower pH inhibits activity), cations ($Ca^{2+}$, $Mg^{2+}$), reactive oxygen species (ROS), and phytohormones like ABA.

Xenopus Oocytes

Biological model used to assay osmotic water permeability by injecting cRNA and measuring the rate of volume increase (water entry) via video camera when placed in hypotonic saline.

Osmotic swelling test

An assay used to test for AQP blockers; an effective blocker results in no change to the cell, while water entry via AQP4 in low extracellular salt causes the cell to swell and burst.

Dual Nature of Aquaporins

The ability of specific aquaporins to facilitate both water movement and ion permeation, such as hAQP1 and AtPIP2;1 acting as cation channels ($Na^+$) or hAQP0 and hAQP6 acting as anion channels.

AQP4 Antagonists vs. Agonists in Edema

Antagonists are used early to block water entry into the brain at edema onset; Agonists are used later to increase water exit from the brain during edema resolution.

AQP4 Knockout (KO) Mouse

A research model that showed decreased swelling and improved neurological scores 24 hours after an ischemic stroke compared to wild type (WT) mice.

Astrocytomas

Brain tumors that express high levels of AQP1 and AQP4, which correlates positively with brain tumor edema.

AQP5 (Cancer Role)

An aquaporin associated with Prostrate Cancer that promotes EMT (epithelial-mesenchymal transition) and colocalises with ion channels.

AqB011 and AqB013

Arylsulfonamide derivatives used as inhibitors to block water and ion movement, thereby inhibiting cell migration and invasion in AQP1-expressing cells.

Bacopaside II

A compound identified by the Drug Discovery and Physiology group for its role in inhibiting water and ion movement through aquaporins.

Li+ hot spots

Leading edges of migrating hAQP1-expressing cells where ion entry occurs, detected using a sensor developed by Pei et al.