Hard S'Not Just Sticky Mess Quiz / Test | Biology

Quiz Answer Key and Fun Facts

1. Mucins belong to a group called the MUC family and are glycoproteins. What does the term "glycoprotein" mean?

Answer: A protein ligated to carbohydrate chains

Proteins are synthesised on ribosomes and those which are to become glycoproteins are generally fed into the endoplasmic reticulum. In the endoplasmic reticulum, enzymes specifically add preformed glycan chains to certain asparagine residues - these are called N glycans, since "N" is the one letter code for asparagine.

The protein may then be transported to the Golgi apparatus in the cell, where sugars can be added to the oxygens of serines and threonines - these are called O glycans. Mucins are highly O-glycosylated and so proteins which are to become mucins tend to have regions with many serines and threonines.

2. Some mucins are secreted and can form gels - what we call mucus! What special linkages, characterised by oxidised cysteine residues, are necessary for the formation of these gels?

Answer: Disulphide bridges

Disulphide bridges describe a covalent bond between two sulphur atoms found in the amino acid cysteine. Gel-forming mucins contain a central domain rich in serine and threonine (which allow O-linked glycan formation), and peripheral regions rich in cysteine allowing disulphide bonds to form with other mucins.

3. One of the most important functions of mucus is the formation of a protective barrier. For this reason, mucus is found at surfaces that are exposed to the environment. What name is given to these surfaces?

Answer: Epithelia

Mucus is found in the respiratory tract, the digestive system, the uritogenital tract, the eyes and the ears. Its characteristic stickiness means that bacteria become trapped and are unable to reach the epithelia which lies below the mucus and therefore cannot cause infection.

4. Mucins are highly negatively charged. Why is this important to their function as gel-forming agents?

Answer: It allows them to become highly hydrated

Mucins contain many glycans. Some of the sugar residues are negatively charged and sugar modifications such as sulfation add further negative charge. In the extracellular environment (such as the gut lumen) many water molecules are attracted to the mucus and high levels of hydration mean the mucus occupies a large volume. Also, hydration is important in forming a gel that can act as a lubricant.

5. Disorders arising from problems with mucus can tell us a lot about its function. In one disorder, mucus in the respiratory tract is under-hydrated and so becomes thick and sticky. This mucus cannot be moved and so bacteria remains and infections develop. Which of these disorders is being described?

Answer: Cystic fibrosis

Cystic fibrosis is the most common inherited recessive disorder amongst Caucasians. It is caused by a mutation in a protein involved in extruding chloride ions from the cell to the extracellular space. A loss of the ability to move these ions means that water also fails to exit the cell and so mucus is insufficiently hydrated. Beating hair cells in the respiratory tract called cilia cannot move this overly sticky mucus and so trapped bacteria remain and can lead to infections.

6. The role of mucus as a protective barrier was also seen when scientists inactivated the gene coding for MUC2, which is a major constituent of the mucus barrier in the intestines. What name is given to this approach of inactivating genes to deduce function?

Answer: Gene knockout

Gene knockout can lead to symptoms similar to known diseases, meaning that the genetic component of these diseases can be deduced. Knockout of MUC2 in mice led to rapid development of intestinal tumours, suggesting that the MUC2 is important in forming mucus in the intestines and protecting the intestinal epithelia from carcinogenic food particles and bacteria.

7. Mucus is more than just a barrier. In the stomach, it can hold bicarbonate ions that are secreted by epithelial cells of the stomach. Why is this important?

Answer: It protects the stomach lining from the acidic environment

MUC5AC and MUC6 are the two main gel-forming mucins in the stomach. The mucus holds bicarbonate ions which form a pH gradient, from pH2 (acidic) in the lumen to pH7 at the stomach lining, preventing damage to the lining cells.

8. The 2005 Nobel Prize in medicine was awarded for the discovery that stomach ulcers are caused by bacteria. Further investigation into stomach mucus revealed an incredible complexity in the defence against these bacteria. Which of these is *not* a protective function of mucins against stomach ulcer-causing bacteria?

Answer: Mucins attract anti-pathogenic cells that destroy the bacteria

MUC5AC forms the top layer of the mucus and presents decoy receptors that specifically bind to pathogens, which become immobilised. The general stickiness of the mucus also impedes the bacteria non-specifically. MUC6 - also a gel-forming mucin - is the next line of defense.

This mucin has been shown to interfere with bacterial cell wall formation and so is thought to have antibiotic properties. Finally, if the ulcer-causing bacteria manages to pass both MUC5AC and MUC6, they may bind to transmembrane mucins on the stomach epithelia which can be shed, preventing bacteria from entering the stomach cells.

9. Some cells have become specially adapted to release mucus en masse. They hold mucins in dehydrated granules and release them upon external stimuli. What are these cells called?

Answer: Goblet cells

Goblet cells are specialised epithelial cells, an example of which is found in the respiratory tract. Goblet cells in the respiratory tract secret MUC5AC - a major gel-forming mucin in this part of the body. In addition, MUC5B is secreted by sub-mucosal glands in the respiratory tract and, together, these mucins contribute to the majority of mucus that we cough up. Goblet cells hold mucins in dehydrated granules, meaning that the granules are compact. Since mucins are highly negatively charged, positive calcium ions are also held in the granule to minimise the charge repulsion. Upon release, calcium ions are believed to be "mopped up" by negatively charged ions and water molecules associate with the mucins, causing them to expand and form mucus.

10. An interesting finding was that mucins, as well as forming mucus, can also be involved in cell signalling. Specifically, some mucins (such as MUC4) have been shown to act in a similar way to growth factors, resulting in cell proliferation. Based on this finding, which of the following disorders is most likely to arise as a result of over-expression of these mucins?

Answer: Cancer

MUC4 is a transmembrane mucin that has a domain that is similar to the epidermal growth factor. Interaction with the appropriate receptor therefore triggers a signalling cascade leading to cell proliferation. MUC1 over-expression is also a common hallmark of cancer. Tumour cells expressing high levels of MUC1 are highly likely to metastasize, since this mucin can interact with ligands on other cells, allowing them to transmigrate into the bloodstream.

Source: Author doublemm

This quiz was reviewed by FunTrivia editor WesleyCrusher before going online.
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S'Not Just Sticky Mess Trivia Quiz

Mucus is more than just a sticky mess. It is a highly ordered, complex substance that is essential to our survival. This quiz takes a look at the biology of mucins, which are a major constituent of mucus, and which play several other important functions.

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