Epithelia

Chapters

Lodish 4th edition: Chapter 21 pages 921 - 924
Moyes and Schulte: Chapter 3 and 11

 

Epithelia: stomach, intestine and kidney etc.

We are going to use a few examples of different epithelial to illustrate how ATPase pumps and transporters work together


 

Epithelial cells have a specialized structure with a clear separation between the apical surface and the basal surface
Usually the apical side faces the lumen of the stomach, intestine, kidney etc and the basal side faces the body cavity or blood
The apical side will have a completely different complement of proteins compared to the basal side and this difference is maintained by a series of specialized junctions found near the apical side



 

From Lodish, Molecular Cell Biology


 

There are two types of junctions at the apical side of vertebrate epithelial cells. Adherens junctions that function to keep the cells in contact and tight junctions. We will talk about tight junctions for this part.
Tight junctions are created by two sets of proteins from apposing membranes that form a tight seal when brought into contact. A specialized set of proteins called claudins and occludins are found at these junctions and are necessary for its formation





 

From Lodish, Molecular Cell Biology


 

The junction is impermeable meaning that water, ions etc can not pass between the cells. This is an essential function of tight junctions. Just think what would happen if your stomach lining lost its tight junctions



 

From Lodish, Molecular Cell Biology


 

You can test the barrier function by adding an electron dense ion like lanthanum to one side of the epithelium and then do EM analysis. In this example the ion can not cross the junction.


 

Stomach parietal cells


 

From www.med.monash.edu.au
Figure 1. Schematic diagram showing the organization of the gastric mucosa, gastric gland, gastric parietal cell and the gastric H/K ATPase.


The parietal cells are one of a group of specialized epithelia cells that line the stomach.


 

Diagram of a parietal cell. The lower half is an inactive cell the up half an active one. Note the presence of many mitochondria (M) and the extensive microvilli (MV) in the active cell.


From Lodish, Molecular Cell Biology


 

The parietal cells of the stomach generate the HCl acidic stomach fluid. The pH of the stomach is 1 therefore the [H+] concentration is 106 times greater than inside the parietal cells themselves pH 7.

The H+/K+ ATPase is necessary to drive the H+ up a very strong concentration gradient. Electrical potential is not altered because the export of one H+ is matched by the import of one K+. But as H+ is pumped out of the cell the pH inside becomes alkaline. So to counteract this the HCO3-/Cl- transporter functions to remove HCO3- (thus removing OH-) to maintain pH. But the consequence of this is that Cl- levels rise inside the cell so there are leak Cl- channels in the apical membrane to allow Cl- efflux into the stomach lining and to match the loss of negative Cl- ions the leak K+ channel allows K+ efflux through this membrane as well. Therefore there is excess K+ ions in the stomach fluid which allows for the H+/K+ ATPase to work in the first place. Whew!!!!

These cells are full of mitochondria to create the necessary ATP reserves and thus these cells generate lots of CO2 which can easily be converted to CO2 + H2O -> HCO3- + H+ to generate the abundant H+ necessary in this cell.

Intestinal cells and glucose transport

Movement of sugars and amino acids from the intestinal lumen into the blood is a two-stage process.


 

From Lodish, Molecular Cell Biology




 

From Alberts, Molecular biology of the cell.
Figure 11-12. Transcellular transport. The transcellular transport of glucose across an intestinal epithelial cell depends on the nonuniform distribution of transport proteins in the cell's plasma membrane.


The transport of glucose from the lumen of the intestine lumen to the extracellular fluid (from where it passes into the blood) requires a number of different transporters. Glucose is brought into the cell through the apical domain of the membrane by a Na+ -powered glucose symporter we discussed previously. Glucose then passes out of the cell (down its concentration gradient) by passive transport or "facilitated diffusion" through a GLUT2 transporter in the basal and lateral membrane domains. GLUT2 is very similar to the GLUT1 we discussed before.
NOTE: Important features to allow this include the Na+/K+ ATPase pump to maintain the Na+ gradient (i.e. a low Na+ within the cell) and the leak K+ channel to ensure a electrical gradient.
Another important feature is the tight junctions which maintain the permeability barrier to block leakage across the epithelium AND maintain the polarized location of membrane proteins.

Proximal kidney cells

The job of the kidney is to filter the blood and remove excess nitrates and other metabolic byproducts but in addition ensure that all the important molecules such as glucose, proteins, Na+, H2O etc are reabsorbed and returned to the blood.

We are only going to use the reabsorption processes that occurs in the proximal tubule of the kidney to illustrate the function of transporters in epithelial cells. The cells that line the kidney tubules are epithelial cells with the same polarized structure as the other epithelia discussed, ie. distinct apical/basal domains and tight junctions.

A diagram of the kidney showing Bowman's capsule where the blood is filtered into the kidney. The filter will not allow cells to pass but all fluid and solutes in the blood will pass through. As the blood filtrate moves further down the kidney tubules most solutes are reaborbed. Those that are left behind become concentrated as the filtrate becomes urine.



 

From Moyes and Schulte




 

The filtrate will move along the proximal tubule and during this process reabsorption occurs




 

Reabsorption is by either active or passive transport. Reabsorption consumes a large amount of ATP and oxygen, the kidney receives ~25% of cardiac output. The proximal tubule reabsorbs 60% of all solute, which includes 100% of glucose and amino acids, 90% of bicarbonate and 80-90% of inorganic phosphate, Na+ and water. All solutes are absorbed actively.
Transport of all solute is driven by the Na+/K+ ATPase on the basolateral side of the cell which maintains a low intracellular Na+ concentration and creates a chemical gradient for the movement of Na+ from the blood filtrate into the cell.
 


 

Glucose

100% of glucose is reabsorbed from the blood filtrate and returned to the blood.

This is easy in the first part of the tubule, the epithelial cells transport glucose against a relatively small glucose concentration gradient. In this case the cells use a 1Na+/glucose cotransporter protein. This has a high transport rate but cannot transport glucose up a steep concentration gradient and can only create a internal glucose concentration ¼100 times that of the extracellular filtrate or the forming urine.

Further down the kidney tubule the epithelial cells have to remove the remaining glucose against a more than 100-fold glucose concentration gradient. These cells contain use the 2Na+/glucose transporter discussed above for intestinal epithelial cells and thus can continue the transport of glucose up a very steep concentration gradient.
 

H2O

H2O is absorbed passively by following solutes back into cell. Most of the solute reabsorption is active, with water being freely permeable and therefore moving by osmosis. When the reabsorption of solute from the tubule occurs, there is a fall in solute concentration and hence osmotic activity within the tubule. Water then moves because of osmotic forces to the area outside the tubule where the concentration of solutes is higher.

Na+

The major mechanism to remove Na+ from the filtrate is to pair its movement into the cell with the movement of other solutes. On the basal side of the cell Na+/K+ ATPase plus the leak K+ ensures a strong concentration gradient and negative Em for the favourable transport of Na+ into the cell from the filtrate. The cells have used this to harness Na+ to cotransport a wide range of solutes from the filtrate including:
- Na+/Glucose cotransporter
- Na+/amino acid cotransporter
- Na+/Cl- cotransporter
- Na+/K+ cotransporter

These are all found on the apical membrane that lines the lumen of the proximal kidney tubule.

 

Osmolarity and water balance


 

From Molecular biology of the cell.
Figure 11-16. Response of a human red blood cell to changes in osmolarity of the extracellular fluid. The cell swells or shrinks as water moves into or out of the cell down its concentration gradient.




 

From Molecular Cell Biology


Aquaporin is one of the channels that allows water to cross the membrane.


 

From Molecular Cell Biology



To test the function of aquaporin, mRNA from the cloned gene was injected into Xenopus oocytes. Frog oocytes do not express aquaporin. These photographs show control oocytes (bottom image in each panel) and microinjected oocytes (top image in each panel) at the indicated times after transfer from an isotonic salt solution (0.1 mM) to a hypotonic salt solution (0.035 mM).
The experimental oocytes swell because water now flows down its concentration gradient into the oocytes.

From Molecular Biology of the Cell
 

Cystic fibrosis

Cystic fibrosis is a fatal disease caused by a genetic mutation a transport protein. It is the most common lethal inherited disease of Caucasians, with approximately one in 2500 newborns affected. This transport protein is found in the lungs, pancreas, and sweat glands. The complication in the lungs is the most severe problem in people with this disease.
The characteristic dysfunction in cystic fibrosis is the production of abnormally thick sticky mucus by several types of epithelial cells. The symptoms include respiratory disease due to the airways in the lungs being blocked by thick plugs of mucus, usually followed by the development of chronic bacterial infections.

The protein that is mutated in CF is called the cystic fibrosis transmembrane conductance regulator (CFTR) and is a Cl- transporter. Cells isolated from CF patients were shown to be defective in Cl- transport. Cells from normal lung tissue secrete Cl-. The movement of Cl- from the cell causes Na+ to move into the lung lumen and water passively follows these solutes. It is the movement of water that is so necessary to keep the mucus lining the lungs hydrated and thin. In the absence of Cl- transport, both Na+ and water transport doesn't occur and the musus is abnormally thick.

CFTR was cloned

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