Membrane Osmotic Permeability Research Articles

How PIP1 Plant Aquaporins Make a Difference in Terms of Cytosolic pH Sensing and Membrane Osmotic Permeability

Plant cells express in their plasma membrane (PM) a complex set of aquaporins, responsible of increasing the osmotic permeability (Pf) and thus allowing faster water adjustments. This set includes PIP aquaporins, which also sense cytosolic acidification blocking the water pathway through their pores. A subgroup known as PIP1 have proven in many different plant species to additionally request physical interaction with a PIP2 aquaporin reflecting a differential modulatory mechanism. Here we explore how a fruit specific PIP1 aquaporin from Fragaria x ananassa -FaPIP1;1- contributes to modulate the PM in terms of Pf and pH sensing. Being already characterized FaPIP2;1 properties (Alleva et al., 2010 J Exp Bot) we designed a FaPIP2;1 mutant with the initial purpose of occluding its pore by replacing a highly conserved residue (N228D). The experimental approach included i) functional studies (measuring Pf in Xenopus oocytes) and ii) localization studies (fluorescence confocal analysis employing YPF-FaPIPs). Our results show that i) FaPIP1;1 is capable of reaching the PM when coexpressed with N228D as well as FaPIP2;1, ii) the coexpression of FaPIP2;1-FaPIP1;1 resulted in higher Pf than N228D-FaPIP1;1, iii) pH sensitivity measured in terms of EC50 are the same for both coexpressions. Our results allows us to hypothesized that the Pf showed by the coexpression of FaPIP1;1-N228D could represent FaPIP1;1 water transport capacity, being N228D inactive as a water channel. The contribution of FaPIP1;1 is relevant in terms of increasing faster water adjustments in the PM and allowing blocking the pores at a more physiological pH.

Epithelial water transport in a balanced gradient system

The relationship between epithelial fluid transport, standing osmotic gradients, and standing hydrostatic pressure gradients has been investigated using a perturbation expansion of the governing equations. The assumptions used in the expansion are: (a) the volume of lateral intercellular space per unit volume of epithelium is small; (b) the membrane osmotic permeability is much larger than the solute permeability. We find that the rate of fluid reabsorption is set by the rate of active solute transport across lateral membranes. The fluid that crosses the lateral membranes and enters the intercellular cleft is driven longitudinally by small gradients in hydrostatic pressure. The small hydrostatic pressure in the intercellular space is capable of causing significant transmembrane fluid movement, however, the transmembrane effect is countered by the presence of a small standing osmotic gradient. Longitudinal hydrostatic and osmotic gradients balance such that their combined effect on transmembrane fluid flow is zero, whereas longitudinal flow is driven by the hydrostatic gradient. Because of this balance, standing gradients within intercellular clefts are effectively uncoupled from the rate of fluid reabsorption, which is driven by small, localized osmotic gradients within the cells. Water enters the cells across apical membranes and leaves across the lateral intercellular membranes. Fluid that enters the intercellular clefts can, in principle, exit either the basal end or be secreted from the apical end through tight junctions. Fluid flow through tight junctions is shown to depend on a dimensionless parameter, which scales the resistance to solute flow of the entire cleft relative to that of the junction. Estimates of the value of this parameter suggest that an electrically leaky epithelium may be effectively a tight epithelium in regard to fluid flow.