Challenges and achievements in the therapeutic modulation of aquaporin functionality

Abstract

Aquaporin (AQP) water and solute channels have basic physiological functions throughout the human body. AQP-facilitated water permeability across cell membranes is required for rapid reabsorption of water from pre-urine in the kidneys and for sustained near isosmolar water fluxes e.g. in the brain, eyes, inner ear, and lungs. Cellular water permeability is further connected to cell motility. AQPs of the aquaglyceroporin subfamily are necessary for lipid degradation in adipocytes and glycerol uptake into the liver, as well as for skin moistening. Modulation of AQP function is desirable in several pathophysiological situations, such as nephrogenic diabetes insipidus, Sjögren's syndrome, Menière's disease, heart failure, or tumors to name a few. Attempts to design or to find effective small molecule AQP inhibitors have yielded only a few hits. Challenges reside in the high copy number of AQP proteins in the cell membranes, and spatial restrictions in the protein structure. This review gives an overview on selected physiological and pathophysiological conditions in which modulation of AQP functions appears beneficial and discusses first achievements in the search of drug-like AQP inhibitors.

Introduction

A survey of the currently used pharmacological agents and classification of their respective drug targets puts a figure on the large predominance of compounds that address receptor molecules (44% of all human drug targets; Rask-Andersen et al., 2011). Within the class of receptor drug targets almost half represent G-protein coupled receptors and one fifth are ligand-gated ion channel receptors, followed by tyrosine-kinase receptors. The next large target group contains various enzymes (29% of the drug targets) with a major focus on lipid mediator-producing oxidoreductases, such as the cyclooxygenases, COX, i.e. the targets of nonsteroidal anti-inflammatory drugs. The drug target class of transport and channel proteins ranks third (15% of all drug targets). Here, mainly ion channels of the heart and circular system are modulated in their function to treat arrhythmia and hypertension as well as neurotransmitter transporters of the brain for neurological disorders. The new anti-diabetic class of gliflozins, that inhibit the renal sodium-dependent glucose transporter, SGLT2, represents one of the very few cases in which high-level transport of a nutrient at millimolar concentrations is targeted.

This brief overview shows that the great majority of drugs interfere fairly directly with signal transduction, be it at the receptor, the small-molecule transmitter, or the action potential level. The advantages are obvious: the drugs compete with very low transmitter concentrations, which can be as low as in the femtomolar range, and the target proteins exhibit explicit binding sites, which allow for high-affinity interactions. Both are prerequisites for optimizing drug compounds towards low-dose application and specificity of action.

The situation of transmembrane water transport facilitated by one of the thirteen different human aquaporin channel proteins (AQP0–12) is vastly opposite in both, substrate concentration and affinity aspects. Water represents the most abundant molecule in the human body (about 60% of the total body mass) and its concentration in the body fluids is 55 M, i.e. 10,000 times higher than that of the most important energy carrier molecule glucose. With respect to substrate affinity, AQP proteins appear inconspicuous to passing water molecules by mimicking the hydrogen bond situation and binding energy of the aqueous bulk. Thus, the energy barrier for water permeation through the AQP channel following an osmotic gradient is hardly higher than diffusion in free solution. Structure-wise, AQPs are rigid proteins and exhibit little thermal fluctuations of the amino acid residues in the channel region in order to maintain a 20 Å long and very narrow channel pathway of only 2–4 Å in diameter open for water (orthodox AQPs; Murata et al., 2000) or small, uncharged solutes, mainly glycerol and chemically resembling compounds (aquaglyceroporins; Fu et al., 2000), posing major space limitations for putative inhibitor molecules. On top of that, AQPs tend to populate the cell membranes in large numbers, i.e. the membrane of a single erythrocyte contains about 200,000 AQP copies (Solomon et al., 1983, Denker et al., 1988).

Despite the challenges due to the AQP structure and protein abundance, it appears worthwhile to search for small molecule modulators due to the many and central roles AQPs play in physiology and pathophysiology. A recent review by Verkman et al. (2014) provides a comprehensive and excellent overview on AQP-related disorders and pharmacological intervention attempts. In this paper, we will focus – after going through physiology and AQP-related therapeutic possibilities – on options for compound screening, and the protein structural and chemical aspects of AQP modulator design.