Lysosomal storage disorders

Question 1

1. What are GAGs? Describe the general structure. What is another name for this group of compounds?

Answer 1

Glycosaminoglycans (GAGs) are long, unbranched chains of negatively charged sugars. They are repeated disaccharide units.
GAGs are also named mucopolysaccharides.

Question 2

2. What are proteoglycans? Do they mainly contain sugars or protein?

Answer 2

Proteoglycans contain many GAGs linked to a protein chain. They contain mainly
sugars (95%) and a small amount of protein.

Question 3

3. Describe the structure and function of a proteoglycan monomer.

Answer 3

A proteoglycan monomer has a central core protein and via a trihexoside, many
disaccharide units are linked.

       As the GAGs are negatively charged, they repel each other and lead to the bottle-
       brush structure. They are surrounded by many water molecules, 
       and produce the “slippery “consistency found in mucus and synovial fluid.

Question 4

4. Describe the synthesis of hyaluronic acid and the structure and function of a proteoglycan aggregate. Where is it assembled?

Answer 4

Hyaluronic acid is synthesized by an integral membrane protein, which elongates the GAG as it is released via ABC-transporters into the extracellular space. Hyaluronic acid is not sulfated and it is not linked to a core protein.

A proteoglycan aggregate contains a long central strand of hyaluronic acid (up to
50 000 units). Many proteoglycan monomers are associated but not covalently
bound.

        Link-proteins stabilize the connection between hyaluronic acid and the 
        proteoglycan monomers. This aggregate can function as shock absorber and 
        lubricant in joints. 

        The proteoglycan aggregate is assembled in the extracellular space. Hyaluronic 
         acid is also synthesized into the extracellular space, the proteoglycans, however, 
         are synthesized inside of cells and then released into the extracellular space.

Question 5

5. What is the general rule for GAGs regarding their disaccharide units?

Answer 5

The general rule for GAGs is that they contain many disaccharide units with
disaccharide - position-1 : glucuronic acid or iduronic acid
disaccharide - position-2: an amino sugar or a N-acetyl- amino sugar

Question 6

6. GAG synthesis uses UDP-glucuronic acid. Is there also the use of a possible UDP-iduronic acid?

Answer 6

No, iduronic acid residues are formed enzymatically from some glucuronic acid residues in GAGs.

Question 7

7. Where are most GAGs found, intracellular or extracellular? What is an important function?

Answer 7

Most GAGs are found extracellular and they can be part of a proteoglycan
monomer or be then assembled to a proteoglycan aggregate.

        GAGs are important components of the fluid in the joints and vitreous humor of 
         the eye and they render the solutions highly compressible. A deficiency of GAGs 
         can lead to osteoarthritis.

Question 8

8. Which GAG does not follow this general rule of being extracellular? Which function does this GAG perform?

Answer 8

Heparin is not found extracellular but it is found inside of mast cells lining the
arteries of lungs, liver and skin.
After the controlled release into the blood, heparin can act as anticoagulant. It is
also used for heparin injections to prevent blood clotting during surgery.

Question 9

9. Describe the synthesis of a proteoglycan monomer. Where does it take place, and what is used?

Answer 9

The core protein enters the RER. Then the protein is O-glycosylated at serine
residues by glycosyl transferases in Golgi. A trihexoside linkage (xylose,
galactose, galactose) is formed and after that UDP-glucuronic acid and mostly
UDP-amino sugars are used for the continued synthesis of many disaccharide
units.

Some glucuronic acid residues of the carbohydrate chain can be changed to iduronic acid residues by an epimerase.

Sulfation occurs immediately after the individual disaccharide units are formed.

Question 10

10. Why is it important to sulfate specific GAGs ? What happens when sulfation is defective?

Answer 10

Sulfation, using PAPS, makes the GAGs even more negatively charged.
A defect in sulfation can lead to chondrodystrophy, which is a disorder of
development and the maintenance of the skeletal system.

Question 11

11. Which one is highly sulfated, hyaluronic acid or is it heparin? Which GAG permits cell migration in the ECM?

Answer 11

Heparin is a highly sulfated GAG, it is one of the most negatively charged
compounds. Hyaluronic acid is an unsulfated GAG.

Cell migration is permitted by hyaluronic acid in the ECM. This is important 
        during embryogenesis but is also related to spread of cancer cells in adults.

Question 12

12. Describe glycoproteins! What is the role of glycoproteins in the plasma membrane? What other functions are performed by glycoproteins?

Answer 12

Glycoproteins contain mainly protein with some, often branched, sugars attached
in O- or N-glycosylation. The amounts of carbohydrate is however very variable.

  Glycoproteins and glycosphingolipids are part of the glycocalyx to the 
  extracellular side of the plasma membrane.

   Glycoproteins are involved with cell surface recognition, cell surface  
   antigenicity, be part of the extracellular matrix and the mucins are also found 
   extracellular in mucus.

Question 13

13. Are the sugars in blood group proteins linked in O-linkage or are they linked in N-linkage?

Answer 13

The sugars of blood groups are linked via O-linkage to proteins at their serine or threonine residues

Question 14

14. Are most plasma proteins glycoproteins? Is albumin a glycoprotein?

Answer 14

Most plasma proteins are glycoproteins, albumin is the exception and it is
not a glycoprotein.

Question 15

15. What are mucins? Where aret hey mainly found in humans?

Answer 15

Mucins are glycoproteins that have a very high amount of sugars (about 80%).
They have a long protein core and contain often negatively charged NANA
(N-acetylneuraminic acid or sialic acid) linked to single amino sugars

 Mucins are negatively charged and attract water. Mucins are secreted in mucus of 
 the respiratory and digestive tracts. The attached sugars also make the mucins 
 resistant to proteolysis by digestive enzymes.

Question 16

16. What is the concept of O-glycosylation? What does the name indicate? Which three amino acids residues are commonly subjected to O-glycosylation?

Answer 16

O-glycosylation is performed in the RER/Golgi and includes step by step linkage
of activated sugars in an individual way.

       The first sugar is linked directly to a hydroxyl group (OH) of the protein. This is 
       why this glycosylation is named O-glycosylation 

       O-glycosylation links an activated sugar to serine or threonine residues (or  
       hydroxylysine residues during procollagen synthesis).

       [it seems that the only exception is the glycosylation of a tyrosine residue 
       performed at the beginning of glycogen synthesis by glycogenin]

Question 17

17. What is the concept of N-glycosylation? What does the name indicate and which amino acid is involved? Compare it to O-glycosylation.

Answer 17

N-glycosylation does not individually link sugars directly to the protein chain.
Instead it uses a lipid bound in the ER membrane to form a precursor with several
sugars that will be given in one step to proteins that are N-glycosylated. The name
N-glycosylation indicates that the sugar precursor is attached to the nitrogen of
the side chain of an asparagine residue. All proteins that are N-glycosylated
receive the same carbohydrate precursor.

        Modification of this general precursor in Golgi allows then the generation of a 
        variety of sugar residues in different N-glycosylated proteins. 

        N-glycosylated proteins can contain high-mannose or complex oligosaccharides.

Question 18

18. Which amino acid residue accepts the sugar precursor in N-glycosylation

Answer 18

In N-glycosylation, the precursor that was first built linked to a lipid, will be
transferred to specific asparagine residues of the protein.

        Note, although glutamine is structurally similar to asparagine, it is not involved 
        with N-glycosylation.

Question 19

19. Which lipid is used as precursor for N-glycosylation?

Answer 19

The lipid used for N-glycosylation is dolichol-PP

[its synthesis branches out of the cholesterol synthesis at the level of farnesyl-PP]

Question 20

20. Which sugar is mainly found in the precursor formed during N-glycosylation?
    What is special about the activated form of this sugar?

Answer 20

Mannose is the sugar mainly found in the precursor formed during N-
glycosylation. There are 9 mannose residues in the finished precursor.

        The activated mannose is GDP-mannose, most other sugars are activated using 
         UTP leading to UDP-sugars.

Question 21

21. What is the main cause of lysosomal storage diseases?

Answer 21

Lysosomal storage diseases are due to reduced lysosomal degradation of GAGs
(mucopolysaccharidoses) or of sphingolipids (sphingolipidoses) and sometimes
of glycoproteins or even glycogen ( Pompe disease with deficiency of acid
maltase).

        The synthesis of these compounds is normal and this imbalance leads to their 
         accumulation in lysosomes.

Question 22

22. Are more diseases known where one particular lysosomal enzyme is deficient, or is it more common that a group of lysosomal enzymes is missing? Name four mucopolysaccharidoses that result from a particular enzyme deficiency!

Answer 22

It is more common that one particular lysosomal enzyme is deficient and there are several different diseases known.

In the special case of I-cell disease, however, a group of lysosomal enzymes is missing in lysosomes.

Mucopolysaccharidoses due to deficiency of a particular enzyme are for example: Hunter syndrome, Hurler syndrome, Sanfilippo syndrome subtypes A-D and the Sly syndrome.

Question 23

23. Which enzymes are deficient in Hurler (MPS I) and Hunter Syndrome (MPS II), respectively? Which type of GAGs is mostly affected? Which of these diseases is X-linked?

Answer 23

Hurler Syndrome is due to deficiency of iduronidase and leads to defective
lysosomal degradation of dermatan and heparan sulfates

         Hunter Syndrome is due to deficiency of iduronate sulfatase, and as in Hurler 
         syndrome there is defective degradation of dermatan and heparan sulfates. 

        The X-linked genetic defect is Hunter Syndrome

Question 24

24. Name some physical indications for mucopolysaccharidoses!

Answer 24

a. Coarse facial features
b. Short in stature, skeletal deformities
c. Hepatosplenomegaly
d. Sometimes corneal clouding
e. Mental retardation in severe cases

Question 25

25. What is defective in I-cell disease: the deficiency of one specific lysosomal enzyme or the transport of several lysosomal enzymes from their synthesis in the ER/Golgi into lysosomes? Explain!

Answer 25

I-cell disease is due to a deficiency of a group of several lysosomal enzymes
which leads to large inclusion bodies in cells (naming I-cell disease).

        I-cell disease is the result of a defective transport of several lysosomal enzymes 
        into the lysosomes, and they are released into the blood instead.

        Normally, a mannose 6-P marker in these enzymes is present for the recognition 
        by the mannose 6-P receptor and are packaged into transport vesicles which are 
        send to lysosomes. 

       An enzyme is necessary for the formation of the mannose 6-P marker in these 
       glycoproteins that are meant to be transported into lysosomes and this enzyme is 
       deficient in I-cell disease.

Question 26

26. The lysosomal enzymes are glycosylated before their transport into lysosomes. Are these enzymes O-glycosylated or N-glycosylated?

Answer 26

The lysosomal enzymes need the mannose 6-P marker for transport into
lysosomes. These enzymes are N-glycosylated [many mannose residues] and a
specific mannose residue will be changed to mannose 6-P as marker for transport.

Question 27

27. What are the clinical characteristics of I-cell disease?

Answer 27

. Clinical characteristics are similar to Hurler syndrome,
I-cell disease is also known as mucolipidosis II.

         It is characterized by skeletal abnormalities, restricted joint movement,   
         coarse facial features, severe psychomotor impairment and enlarged heart valves.
         Death occurs often in childhood.

Question 28

Lysosomal storage diseases also include the group of sphingolipidoses. Which enzyme is deficient in the sphingolipidoses Tay Sachs, Fabry, Gaucher and Niemann-Pick Disease, respectively?

Answer 28

28. Tay Sachs Disease: hexosaminidase A
29. Fabry Disease: -galactosidase
30. Gaucher Disease: glucocerebrosidase
31. Niemann Pick Disease: acid sphingomyelinase

Question 29

Which compounds accumulate in Tay Sachs, Gaucher, Fabry, Niemann-Pick Disease, respectively?

Answer 29

32. Tay Sachs Disease: GM2
33. Gaucher Disease : glucocerebrosides
34. Fabry Disease: globosides (ceramide trihexoside)
35. Niemann-Pick Disease Type A and B: sphingomyelin

Question 30

What are “key words” for Tay Sachs, Gaucher, Fabry and Niemann-Pick Disease, respectively? How are these patients clinically described?

Answer 30

36. Tay Sachs: cherry-red macula and onion shell-like inclusions. Patients have rapid, progressive fatal neurodegradation, blindness, muscle weakness, seizures.
37. Gaucher: “crumpled” tissue paper appearance of cytoplasm. Patients have hepatosplenomegaly and osteoporosis of long bones. Most common LSD.
38. Fabry: reddish skin rash (bathing suit distribution). Patients have kidney and heart failure, and burning pain in lower extremities.
39. Niemann Pick: cherry-red macula and foamy appearing cells. Patients have hepatosplenomegaly, and neurodegeneration in Type A.

Question 31

40. Describe the differences between Niemann-Pick Disease Type A and Type B.

Answer 31

Niemann-Pick Disease Type A is the classic infantile form. Patients have severe rapid neurodegeneration and severe mental retardation. Life expectancy is to 2-3 years.

Niemann-Pick Disease Type B is the visceral chronic form.
Patients have almost no neurodegeneration and life expectancy is to early adulthood.