Comparative and Evolutionary Physiology of Water Channels

Abstract

Experiments with a variety of plant and animal tissues over the past 250 years have given rise to concepts of osmosis, osmotic pressure and permeability that are used to this day. The frog skin and mammalian red blood cells proved particularly useful in establishing that water transport across membranes was mediated by pores rather than simple diffusion. Comparative studies identified membrane intrinsic proteins (MIPs) in a variety of tissues that included red blood cells, bovine lens, plant cell membranes, fruit fly brain and microbial membranes. From these studies emerged the concept of water conducting channels called aquaporins (Aqps) that were present in virtually all living organisms. The discovery of Aqps and methodology for identifying them by RT-PCR cloning resulted in an enormous literature that provided mechanistic bases for physiological functions related to ionic and osmotic regulation. Among the vertebrates mammalian Aqps are the best studied due to their importance for biomedical issues such as diabetes insipidus and the availability of knock out (KO) models where specific deficiencies can be studied. From an evolutionary perspective, fish have multiple copies of most of the canonical Aqps that have been characterized in mammals. The development of genomic technologies is beginning to identify episodes of whole genome duplication, beginning in the earliest chordates, that resulted in the variety of Aqps that serve osmoregulatory functions in living vertebrate taxa. Comparative studies are beginning to describe how different Aqps have been coopted to osmoregulatory epithelia but more research is needed to understand the coordination of apical and basolateral membrane function.