Cellular basis of Therapeutics Flashcards
What are glycolipid storage diseases?
Glycolipid storage diseases are caused mutations in the genes encoding the glycohydrolases that catabolize GSLs within lysosomes.
In these diseases the substrate for the defective enzyme accumulates in the lysosome and the stored GSL leads to cellular dysfunction and disease.
What are the symptoms glycolipid storage diseases?
- Delay in intellectual and physical development.
- Seizures.
- Facial and other bone deformities.
- Joint stiffness and pain.
- Difficulty breathing.
- Problems with vision and hearing.
- Anemia, nosebleeds, and easy bleeding or bruising.
- Swollen abdomen due to enlarged spleen or liver.
What are glycolipids?
Glycolipids are lipids with a carbohydrate attached by a glycosidic bond. Their role is to maintain the stability of the cell membrane and to facilitate cellular recognition, which is crucial to the immune response and in the connections that allow cells to connect to one another to form tissues.
What is glucosylceramide synthase (GCS)?
Glucosylceramide synthase (GCS) is an enzyme inherent to ceramide metabolism. The enzyme catalyzes the transfer of glucose to ceramide, the first committed step in glycolipid biosynthesis.
Where are glycolipids synthesized?
Glycolipids are synthesized in the endoplasmic reticulum and golgi-apparatus where the majority are transported to membranes to maintain the bilayer.
What are glycosphingolipids (GSLs)?
Glycosphingolipids (GSLs), a subclass of glycolipids found in the cell membranes of organisms from bacteria to humans, are the major glycolipids of animals.
GSLs are internalised, recycled or degraded to sphingosines.
Lysosomes have low pH and are the centres for degradation and recycling.
What are lysosomal storage diseases (LSDs)?
Lysosomal storage diseases (LSDs) are inborn errors of metabolism characterized by the accumulation of substrates in excess in various organs’ cells due to the defective functioning of lysosomes. They cause dysfunction of those organs where they accumulate and contribute to great morbidity and mortality.
What do lysosomal hydolases have?
They have a ‘signal patch’ which identifies them as lysosomal enzymes and M6P is attached to them in the Golgi.
Identified first by looking at I-cell patients.
Even though the hydrolases accumulated in the cell medium they were unable to correct other lysosomal storage diseases.
What is the role of Mannose-6-phosphate (M6P)?
The M6P is attached to the hydrolase so it binds to the M6P receptor in the Golgi.
The M6P Receptor complex is then targeted to the lysosome.
Once the hydrolase is delivered the empty receptor recycles to the Golgi to pick up more hydrolases for transport to the lysosome.
Some M6P receptors also traffic from the cell surface and some M6P containing hydrolases escape due to inefficiencies in the sorting process.
Explain the biochemical and cellular basis of glycolipid storage disease:
Most mutations result in the delivery of a defective enzyme with a reduced catalytic activity to lysosomes.
Another (activator) protein required for optimal hydrolase activity is defective or absent.
A mutation that causes misfolding results in defective transport of a lysosomal hydrolase out of the endoplasmic reticulum.
Alternatively, defective transport of a lysosomal hydrolase out of the ER occurs because a multi-enzyme complex that is required for transport cannot form (Cathepsin A / sialidase / β-Galactosidase).
In the Golgi, defective glycosylation could result in an enzyme with reduced catalytic activity.
Alternatively, defective glycosylation with mannose-6-phosphate in the Golgi could produce an enzyme that cannot reach lysosomes.
Defects in other transport steps from the Golgi could also lead to an Glycolpid Storage Disease.
Defects in integral lysosomal membrane proteins with transporter roles eg Niemann-Pick C.
Defects in proteins that are involved in other vital regulatory events of lysosomal function (LAMP2, lysosomal associated membrane protein 2).
How is the severity of the phenotype related to the residual enzyme activity?
The severity of the phenotype is closely related to the residual enzyme activity.
There is a ‘critical threshold’ of enzyme activity.
Above this level, enzyme activity can deal with substrate influx.
Below this, it cannot and there is accumulation of substrate.
It has been demonstrated that small changes in residual enzyme activity can have a profound effect on rate of accumulation of substrate.
In general, the lower the residual activity, the earlier the age at onset and the more severe the disease, although there is considerable overlap, for example, in Gaucher disease.
Therefore in diseases for which enzyme-based therapies are available, residual enzyme activity is of critical importance in determining response to treatment.
The higher the residual activity, the more satisfactory the response.
What are the roles of the activator proteins?
Two genes are known to encode saposins:
One encodes the GM2 activator protein; its defective function results in the AB variant of GM2 gangliosidosis.
The second gene encodes prosaposin which is processed to four homologous saposins (A, B, C and D).
Deficiency of a saposin results in a clinical phenotype that may resemble a lysosomal storage disease.
Mutations in the coding region of Sap B cause a variant form of metachromatic leukodystrophy with sulphatide storage.
Sap C deficiency causes a variant form of Gaucher disease with glucosylceramide storage.
Deficiency of prosaposin results in a combined saposin deficiency with a very severe phenotype, as might be expected.
However, saposin deficiency often results in features of more than one disorder because each saposin activates more than one enzyme.
Give examples of primary GSL storage diseases:
- Gaucher (types 1,2 &3)
- Fabry
- Tay-Sachs
- Sandhoff
- GM1 gangliosidosis
Give examples of lysosomal diseases with secondary storage of GSLs in neurones:
- Niemann-Pick A
- Niemann-Pick C
- MPSI, III A, III B, VI, VII
- alpha-mannosidosis
What is Niemann-Pick C disease?
- Niemann-Pick type C is a rare inherited disease.
- Neurodegeneration and death due to a defective late endosomal protein, NPC1.
- Defective NPC1 leads to intracellular storage of glycolipids and cholesterol.
- The genetic mutations of this type cause cholesterol and other fats to accumulate in the liver, spleen or lungs.
- The brain is eventually affected too.