Tumour Necrosis Factor Receptors Flashcards
Name some diseases when homeostasis is disturbed?
Neurodegeneration, Immunodeficiency and Infertility - cell death outweighs new cell proliferation
Cancer and autoimmunity - cell proliferation outweighs cell death
What is apoptosis?
Most common form of cell death.
Characteristics: Membrane blebbing, Cellular shrinkage, condensation of chromatin
Apoptosis maintains tissue homeostasis by balancing cellular life and death. Deregulated apoptotic pathways disrupt the balance, resulting in diseases or premature cell loss or unrelenting cell death.
What did Hamburger discover in the 1930s?
Removing a limb bud results in reduced numbers of sensory and motor neurons in the spinal cord when compared to control.
Transplantation of a supernumerary limb resulted in increased numbers of sensory and motor neurons in the spinal cord on that side.
He hypothesised that there must be something secreted by the limb bud that helps with the survival of neurons.
What is the neurotrophic hypothesis?
Targets of innervation secrete limiting amounts of survival factors to generate a balance between the size of the target organ and the number of innervating neurons.
How was NGF tested?
NGF was purified in 1960 and in 1986, its function was tested. Axons were shut off in the extracted ganglion in assay. Extract was added to the ganglion, it survived and neurites grew from cell bodies.
What is the NGF family and what are the receptors?
NGF was found to be a part of a family that also includes BDNF, NT-4/5 and NT-3. They bind to Trk receptors:
- TrkA exclusive for NGF
- TrkB exclusive for BDNF and NT-4/5
- TrkC exclusive for NT-3
What experiment was performed to show that NGF is needed for neurons to survive? And what did this show about endocytosis?
Neurons placed in a petri dish. Soma is separated from the neurons by a tight seal. Allows them to be bathed in different media.
Most neurons die when grown without NGF for 30 hours. Neurons can be kept alive by adding NGF to the compartments with the growing neurites.
Receptors are in the plasma membrane are activated and then endocytosed. Retrograde transport of the receptor in the signalling endosome allows it to travel to the soma.
What is the low-affinity NGF receptor?
Known as p75. Not a receptor tyrosine kinase. 1 ECD and TMD. No cytosolic domain. No catalytic activity. Tumour necrosis factor. Contains a death domain, common to TNF receptors. Transmembrane receptor for NGF, BDNF, NT-3/4. Contributes to neuronal and glial cell damage, axonal degradation and dysfunction during injury and cellular stress. But can contribute to survival during development
What are the characteristics of the TNF receptors?
1 ECD only contains cysteine-rich domains, 1TMD and cytosolic domain. Contains death domain to regulate cell death and degradation. Ligands for TNF receptors can be membrane-bound (most cases) or soluble ligands (spliced versions that are released into the cytosol).
Play critical role in the immune system, good targets for pharma industry.
Receptor mutations involved in disease - asthma, spesis, RA, obesity, Lupus, Diabetes and organ transplantation.
What are the two types of TNF receptor signalling?
Inflammatory - lead to the production of cytokines which lead to diseases such as asthma.
Death-inducing - contains a death domain in ICD
What is a death domain?
A globular protein interaction module composed of a bundle of six alpha-helices. Has similarities to the death effector domain and CARD domain. TNFR1, Fas receptor, TRAIL1 and TRAIL2 all have death domains and similar pathway that leads to apoptosis.
What are two key examples of TNF receptors?
p75 low affinity receptor - regulates apoptosis in the nervous system. Ligands: NGF, BDGF, NT-3/4
CD95 or Fas receptor - regulates peripheral tolerance and lymphoid homeostasis in the immune system. Ligand: Fas-ligand
What happens when there’s a mutation in Fas?
Causes autoimmune lymphoproliferative syndrome. Phenotype causes an increase in proliferation of lymphocytes and increased size of lymphatic organs like the lymph nodes or spleen. Non-malignant.
What is the Fas receptor?
Fas ligand is membrane bound and can only act at the cell surface. FasR is regulated by a tyrosine phosphatase called PTPN13 which contains protein-protein interaction domains. Fas interacts with PDZ3 and PDZ5 domains.
In cancer cells, overexpressed PTPN13 causes Fas to be reduced at the cell surface
What is the TNF receptor structure?
TNF-a is a trimer (3 identical subunits). Alternate b-sheet and unstructured loop (jelly-roll structure).
What are the two models for TNFR activation?
- 3 monomeric receptors are far apart from one another at the plasma membrane. Addition of a trimeric ligand causes trimerisation and the clustering of ECD and ICD, 3 death domains will interact, inducing activation.
- PLAD (pre-ligand association domain) is present on the pre-formed trimer. This association doesn’t mean that the receptor is active because although the ECD are close the death domains are still far apart. Requires the ligand to cause conformational change to bring DD together.
What are the two receptor families for NGF?
- TrkA (receptor tyrosine kinase) survival and axonal growth
- Low-affinity p75 (tumour necrosis factor receptor) initiates apoptosis
What is the downstream pathway of the Fas receptor/extrinsic pathway of apoptosis?
Membrane-bound trimeric ligand induces a trimerisation of the receptor at the plasma membrane. Death domain becomes sticky and can bind to other proteins. Fas-associated death domain (FADD) adaptor protein binds to the receptor (has its own death domain and death effector domain). Death domains can interact with one another. The DED is cleaved and interacts with another DED of caspase-8 (initiator caspase). Caspase-8 then activate other caspases.
