Topic 27: Regulated Proteolysis & Apoptosis

explain contact-mediated signaling in regards to drosophila nerve cells

• drosophila nerve cells signal to their neighbors not to become neurons

  • delta (transmembrane inhibitory signal protein) signals to notch (receptor protein)

What is cell-mediated signaling?

• direct contact between signaling cell and cell receiving signal

What does delta binding trigger? What is released as a result?

• triggers proteolysis of Notch releasing a gene regulatory fragment

What are notch target genes frequently involved in?

• regulating differentiation

The activation of notch by the binding of delta can be viewed as a form of what?

• mechanotransduction

what is mechanotransduction?

• the conversion of applied force to an intracellular signal

  • ex: delta signal applied pulling force on notch

The first 3 steps in regulated proteolysis pathway (notch/delta) are referred to as what?

• proteolytic steps (cleavage)

What can activating notch mutations cause?

• T-cell leukemia

  • leukemic T-cells invading liver

What do the notch mutations in human T-cell leukemias generate?

• generate fragments resembling the notch cytoplasmic domain

What is Wnt signaling an example of?

• example if signaling by regulatory proteolysis

What is Beta-catenin in the Wnt pathway?

• another rapidly synthesized, rapidly degraded signaling molecule

In Wnt pathway, what happens to APC in early stages of colon cancer progression?

• it is often lost

what is the difference between pathways with Wnt signal and one without?

• With Wnt signal: transcription of Wnt target genes

  • activated frizzled

  • stable Beta-catenin

  • active dishevelled

    • CK1 and GSK3 activity on activawted LRP

• Without Wnt signal: Wnt target genes off

  • frizzeled not activated

  • unstable beta-catenin

  • inactive dishevelled

    • CK1 and GSK3 activity on axin

What is apoptosis?

• regulated form of cell death that differs from necrosis and lysis

  • it is controlled form of death, a highly regulated program of dismantling cell structures

What are the Hallmarks of apoptosis? (5)

1. Blebbing: formation of bubble-like bulges in plasma membrane caused by breakdown of cytoskelton forcing the membrane to detach and swell outwards

2. Rounding

3. Cytochrome C release

4. externalization of phosphatidylserine

5. fragmentation of nuclei

what does apoptosis physiologically do to cells no linger needed or that have become dangerous?

• removes the cells

What types of things does apoptosis eliminate?

• temporary structures

• excess cells

• infected cells

• transformed cells

how is apoptosis initiated?

• initiated by activating proteases called caspases

How are initiator caspases activated?

• by clustering

What can activated initiator caspases trigger?

• can trigger an amplifying increase in activated executioner caspases

how can apoptosis be monitored?

• by visualizing DNA fragmentation in gels and in vivo

The ladder effect in gels results from what?

• results from cleavages between nucleosomes

What is the TUNEL method?

• nick end labeling

What happens to nick ends in nick end labeling?

• they can be labeled in tissue to reveal apoptotic cells

Caspase activation leads to a release of _____ in which cuts the genome into many small fragments.

• nuclease

Extrinsic pathways of apoptosis

• clusters initiator caspases via ligand binding to “death receptors”

  • extraceullular death cues

Extracellular death cues can results in what?

• the formation of “death inducing signaling complex” (DISC) that clusters and activated caspase 8

Intrinsic pathway of apoptosis

• clusters initiator caspases by cytochrome C release & Apaf oligomerization

What does the cytochrome C release from mitochondria cause in intrinsic pathway?

• causes apoptosis, clustering & activation of caspase 9

In intrinsic pathway: what can inhibit cytochrome C?

• BcI2