Regulation of Epithelial Cell Functions by the Osmolality and Hydrostatic Pressure Gradients: A Possible Role of the Tight Junction as a Sensor

Abstract

Epithelia act as a barrier to the external environment. The extracellular environment constantly changes, and the epithelia are required to regulate their function in accordance with the changes in the environment. It has been reported that a difference of the environment between the apical and basal sides of epithelia such as osmolality and hydrostatic pressure affects various epithelial functions including transepithelial transport, cytoskeleton, and cell proliferation. In this paper, we review the regulation of epithelial functions by the gradients of osmolality and hydrostatic pressure. We also examine the significance of this regulation in pathological conditions especially focusing on the role of the hydrostatic pressure gradient in the pathogenesis of carcinomas. Furthermore, we discuss the mechanism by which epithelia sense the osmotic and hydrostatic pressure gradients and the possible role of the tight junction as a sensor of the extracellular environment to regulate epithelial functions.

Keywords: cancer; hydrostatic pressure; osmolality; sensor; tight junction.

Figure 1

Effects of the osmotic gradient in various epithelia. (A) Freeze-fracture electron micrographs in the jejunal epithelium. Apical osmolality was increased to 600 mOsm with mannitol and cells were fixed 20 min after the osmotic changes. Apical hyperosmolality increased tight junction (TJ) strand number and depth. Scale bar = 200 nm. From Madara. J. Cell Biol. 1983 [11] with permission. (B) Transmission electron micrographs in the bladder epithelium. Apical osmolality was increased with 240 mM urea and cells were fixed 10 min after the osmotic changes. Apical hyperosmolality induced bleb formation between TJ strands. Scale bar = 200 nm. From Wade et al., Am. J. Physiol. 1973 [15] with permission. (C) Effects of basal hypoosmolality on Xenopus A6 cells. (a) Basal osmolality was decreased by the reduction of NaCl concentration or counterbalanced by the addition of sucrose, and permeability of sodium and chloride (P_Na and P_Cl) were calculated from transepithelial resistance and dilution potentials in the presence of Na⁺, K⁺ and Cl^– channel blockers. Basal hypoosmolality increased P_Na and P_Cl with the selective increase of P_Na. (b) Immunofluorescence of claudin-1 and occludin. Cells were fixed 30 min after the osmotic changes. Basal hypoosmolality altered claudin-1 localization to the apical end and claudin-1 showed colocalization with occludin. Scale bar = 5 µm. Modified from Tokuda et al., Biochem. Biophys. Res. Commun. 2008 [16] and Tokuda et al., Biochem. Biophys. Res. Commun. 2010 [17] with permission. (D) Effects of apical hypoosmolality on Madin–Darby canine kidney (MDCK) II cells. (a) Apical osmolality was decreased by the reduction of NaCl concentration or counterbalanced by the addition of sucrose. Apical hypoosmolality induced the reduction of cation selectivity. (b) Immunofluorescence of claudin-2 or claudin-3 in wild-type and claudin-2 knockout cells. Cells were fixed 30 min after the osmotic changes. Apical hypoosmolality altered the shape of cell–cell contact from zigzag to more straight shape in wild-type cells but not in claudin-2 knockout cells. Scale bar = 10 µm. Modified from Tokuda et al., PLoS ONE. 2016 [18] with permission.

Figure 2

Effects of the hydrostatic pressure (HP) gradient in renal distal tubule cells and podocytes. (A) Effects of HP on Xenopus A6 cells. (a) Time course of transepithelial conductance. 8 cmH₂O HP was applied from apical, basal, or both sides from time 0 to 60 min. The HP from basal side increased transepithelial conductance with reversibility. (b) Immunofluorescence of F-actin and claudin-1. 8 cmH₂O HP from basal side was applied from time 0 to 60 min. HP from basal side increased cell height and altered actin structure and claudin-1 localization with reversibility. Modified from Tokuda et al., Biochem. Biophys. Res. Commun. 2009 [51] with permission. (B) Transmission electron micrographs of podocytes. 1 cmH₂O HP was applied from basal side for three days. Podocyte cells showed more round shape and had wide intercellular space when the HP was applied from basal side. Scale bar = 5 µm. From Coers et al., Pathobiology. 1996 [52] with permission.

Figure 3

Effects of the HP gradient on carcinogenic properties of epithelia. (A) Scanning electron micrographs in MDCK I cells. 0.6 cmH₂O HP was applied from apical or basal side for four days. A bumpy surface with cell masses was observed when the HP was applied from a basal side. Scale bar = 10 µm. (B) Transmission electron micrographs in EpH4 cells. 0.6 cmH₂O HP was applied from apical or basal side for four days. HP from a basal side induced stratification. Cavities were observed within the stratification. Scale bar = 5 µm. (C) Immunofluorescence of F-actin and ZO-1 in Caco2 cells. 0.6 cmH₂O HP was applied from apical or basal side for eight days. HP from a basal side induced stratification. ZO-1 was localized at the cavities within the stratification. Scale bar = 5 µm. Modified from Tokuda et al., PLoS ONE. 2015 [113] with permission.

Figure 4

Theoretical speculation of the mechanism about how epithelia sense osmotic and HP gradients. (A) A model of epithelium in the HP from basal side. Steep HP gradients are formed at TJs and apical and basal cell membranes. (B) The water movement through apical and basal cell membranes in apical hyperosmolality (left), hyperosmolality in both sides (middle), and hypoosmolality in both sides (right). Hyper = hyperosmolality; Hypo = hypoosmolality. (C) Possible mechanism of bleb formation between TJ strands in apical hyperosmolality. (D) Effects of HP from basal side (left), increase of HP in both side (middle) and decrease of HP in both sides (right) on apical and basal cell membranes. (E) Possible effects of HP from basal side on F-actin in the lateral side and cell height.

Figure 5

Possible mechanism of TJs as a sensor.