There are three way possible for the transport of molecules throug enterocytes.
Transcellular transport
Because of the enterocyte physical structure, it is very unlikely that macromolecules can cross the lipid bilayer into the cytosol. Indeed, its plasma membrane being composed of a lipid bilayer in which membrane-bound proteins and glycoproteins are situated, the molecules can only be transported by receptors present at the enterocyte membrane surface. The major pathway for molecules is a transcellular transport called endocytosis which is a receptor-mediated mechanism (Fig 1.10). Besides transport of molecules, endocytosis is also involved in regulation of cell-surface receptor expression, maintenance of cell polarity, and antigen presentation (Conner and Schmid, 2003, Drubin and Nelson, 1996). In enterocytes, three other pinocytosis mechanisms have been proposed: (i) clathrin-mediated endocytosis, which is the major pinocytic pathway; (ii) caveolin-mediated endocytosis, which is a slow process consequently unlikely contributing significantly to bulk fluid-phase uptake; (iii) and clathrin-caveolin-independent endocytosis, which remains poorly understood (Snoeck et al., 2005, Conner and Schmid, 2003) (Fig 1.11).
Another way to absorb nutrients, ions or water is via epithelial transporters which are integral membrane protein pumps or channels (Snoeck et al., 2005).
Nutrients, such as sugars, ions, and amino acids, enter the enterocyte cytoplasm at the apical membrane by these transporters and exit the basolateral membrane.
Some are adenosine triphosphate (ATP)-dependent and others rely on the osmotic gradient between the lumen of the intestine and the mucosa (Fig 1.10).
Paracellular transport
For some molecules, another way to go through the intestinal epithelial barrier is paracellular transport. Instead of crossing through enterocytes, molecules go through the tights junctions, which are dynamic structures, between adjacent epithelial cells (Fig 1.10). However, the passage of large molecules through tight junctions is not possible; they have to be cleaved into peptides or amino acids for proteins, or saccharose, glucose, for example, for polysaccharides. Chantret et al compared 20 different human colonic carcinoma cell lines in order to determine which line spontaneously developed normal human intestinal cell characteristics, such: (i) the organization of the cells into a polarized monolayer; (ii) the presence of an apical brush border; and (iii) the presence of brush border-associated hydrolases such as sucrase-isomaltase, lactase, or alkaline phosphatase (Chantret et al., 1988).
The only human colonic carcinoma cell line able to do all three was the human colonic adenocarcinoma derived epithelial cell line-2 (CaCo-2) concluding that only the CaCo-2 cell line showed the differentiation characteristics of mature enterocytes. Moreover, in another study, Grasset et al showed that the CaCo-2 cells exhibited a similar electrical properties as enterocytes, by displaying a measurable TEER and a positive Isc reflecting ion and molecules transport through cells similarly to human enterocytes (Grasset et al., 1984).
Water uptake in the intestine
Water transport across the cell membranes takes place via several routes: (i) by osmosis, via the lipid bilayer and the water channels called aquaporins (AQP) (Fig 1.12 A.); (ii) by co-transport, via co-transporters or uniporters such as the Potassium-Chloride co-transporter (KCC) (Fig 1.12 B.); (iii) or by co-transport and osmosis at the same time, via specific co-transporter such as the Sodium-glucose co-transporter 1 (SGLT-1) (Fig 1.12 C.) (Zeuthen, 2010)
AQP are integral membrane proteins expressed in various organs and tissues. They serve as channels in the transfer of water and, sometimes, small molecules across the membrane (Takata et al., 2004, Agre et al., 2002) (Fig 1.13).
The water is transferred through the cell membranes via the osmotic pressure
Fig 1. 10: Model of ions transport in polarized enterocytes. On the apical surface, there are four major ions transporters. The H+-ATPase (1) and H+-K+-ATPase (2) are ATP-dependent, they need energy for the ions absorption and/or secretion. Via the hydrolysis of one molecule of ATP into ADP and inorganic phosphate (Pi), ions are absorbed or secreted or both. Currently, two of these channels have been identified through which Cl -can be secreted into the intestinal lumen, thus creating the driving osmotic gradient for fluid secretion, namely: (i) CFRT which is embedded as dimers into the membrane, it is regulated by the phosphorylation of protein kinase A (PKA) and it is a cyclic adenosine monophosphate (cAMP)- dependent mechanism; (ii) CaCC which is an Ca2+-dependent pathway for Cl- secretion, when the cytosolic concentration of Ca2+ increases, the secretion of ions Cl- into the lumen is increasing too.
On the basolateral surface, there are four major transporters too. Three of them have a role in the chloride secretion: (i) the Na+-K+-ATPase channel (5) is ATP-dependent and provides the energetic requirements for active Cl- secretion by transporting three Na+ ions out of the cell and two K+ ions into the cell; (ii) the Na+-K+-2 Cl- co-transporter (NKCC) which mediates Cl- uptake at the basolateral pole and thereby provides the substrate for the apical Cl- secretion; (iii) potassium channels (4) maintain cellular electroneutrality by compensating for Cl- efflux and keeping the cell in a state of hyperpolarization. The last main basolateral receptor is kidney anion exchanger 1 (kAE1) (3) which has a role in the absorption of Cl- ions from the basolateral side; it exchanges HCO3- anion from the cytosol with Cl- extracellular.
Fig 1. 11: The different types of pinocytosis in mammalian cells (reproduced and modified from Conner et al, 2003, with permission). These pathways differ with regard to the size of the endocytic vesicle, the nature of endocytosed molecules, receptors and lipids, and the mechanism of vesicle formation
The macropynocitosis is unlikely to occur by enterocytes since this mechanism involves extensive membrane ruffling and sampling of large volumes of the extracellular milieu. The clathrin-mediated endocytosis involves the concentration of high-affinity transmembrane receptors and their ligands into “clathrin-coated pits” for the formation of a coated vesicle bringing the molecules into the cytosol to be recycle by endosomes. Caveolae, which are flask-shaped invaginations of the plasma membrane, are involved in the caveolin-mediated endocytosis. These invaginations demarcate cholesterol and sphingolipid-rich microdomains in which many different signalling molecules and membrane transporters are concentrated.
regulation and driven by a hydrostatic gradient within the hemipore. 10 AQP channels have been identified so far, AQP0 to AQP10 (Agre et al., 2002). Some are aquaglyceroporins; they are water and glycerol permeable. AQP1 is the main AQP channel, it is found in any type of organs or tissues. In the intestine, AQP5 is mainly found in the small intestine; and AQP3, which is an aquaglyceroporin, is mainly found in the colon (Matsuzaki et al., 2004, Takata et al., 2004, Agre et al., 2002).
Co-transporters and uniporters are membrane transport proteins. They can transport one or several molecules through the cell plasma membrane. The tight coupling between water and molecules in co-transport allows co-transporters to function as molecular water pumps (Fig 1.12 B.). The free energy contained in the transported molecules gradient can be transferred to the water flux. In some co-transporters, like the sodium-glucose co-transporter 1 (SGLT-1), both modes of transport can occur (Fig 1.12 C.). They co-transport water through the plasma membrane but also possess water channel properties (osmosis). The transport of water via these transporters is bimodal; osmosis and co-transport can be done in parallel (Zeuthen, 2010).