Claasen Cell Polarity and Apoptosis

1. migrating fibroblast

2. Cytotoxic T cell

3. Epithelial cell

4. polarized dividing cell (e.g. stem cells —> only one can differentiate the other stays stem cell)

Polarity proteins provide a molecular machinery that can establish and maintain polar identity of cellular domains

Apical: Crumbs / aPKC / Par-6 / Par-3

Basel: Scribble / Discs-large (Dlg) / Lethal giant larvae (Lgl)

• regulatory network of positive and negative feedback amplifies and maintains the polarity cues

• the polarity proteins are conserved throughout animal kingdom and regulate many processes

• loss of polarity is associated with pathological consequences (tumor / metastasis)

• polarity tightly linked with cytoskeleton

Actin filaments:

• fast elongation [at plus end] (Actin filaments (F-actin) grow by addition of ATPbound actin monomers (G-actin) and depolymerize as a consequence of ATP Hydrolysis) and recycling [at minus end]

• Actin filaments form helical double strands with a fixed polarity

• Actin filaments facilitate maintenance of cell shape and dynamic changes of cell shape —> migrating cells use actin to push forward and then contraction

• The Rho GTPases organize actin dynamics

• Myosins are the motorproteins that walk on actin filaments (Cortex contraction / Filament sliding (contraction) / Vesicle transport)

Microtubule:

• similarities: • Polarity • Dynamic growth/shrinkage • ATP/GTP hydrolysis cycles

• Addition of a GTP-bound α/β tubulin dimer generates a GTP cap on the plus end of the microtubule (stabilizes tubule growth and protects from degradation until GTP is hydrolyzed)

• Motorproteins: in plus direction —> Kinesin and in minus direction <— Dynein

• Push and pull forces in the mitotic spindle mediate chromosome segregation

  —> cell division, centrosome, cilia

Push and pull forces in the mitotic spindle mediate chromosome segregation:

• in the equatorial plane the chromosomes are located before they are pulled (and pushed) apart by the microtubles, in the centre the microtubles overlap and plus end directed motorproteins (Kinesins) push the microtubles to each pole

• the centreosomes (the main microtubule organizing center and regulator of cell cycle progression) are located in the centre of microtubles minus end pointing towards them

• at the cell cortex of two poles minus end directed motorproteins (Dyneins) pull on the microtubles also moving the centrosome the each pole

Intermediate filaments:

• support the structure and mechanical stability of many animal cells

• are flexible and tolerate mechanical stress (rubber like), stable and long lasting

• form helical multimers formed from Keratin (epithelial), Vimentin (muscle) or Laminin (nucleus) proteins

• are key for cell-cell (desmosomes) and cell-matrix (hemidesmosome) adhesion

• important for mechanical stability of our epithelial tissue (skin doesnt rip when pulled)

• tissue remodelling: in develope ment (skin between fingers or tail of frog)

• tissue turn-over: lactation cells only needed when child is born

• renewal of damaged cells: after sunburn the skin needs to be renewed

Apoptosis: cell shrinkage, chromatin condensation, blebbing and encloses all contents in blebs to be taken up by phagocytosis by healthy cell

Autophagy: self eating, the cell self digests (specific) contents in autolysosomes inside itself

Necrosis: the cell swells and bursts releasing all contents uncontrolled causing inflammation (proinflammatory molecules e.g. DNA)

• all have different triggers, morphologies and results

Apoptosis is regulated by caspases

• an apoptotic signal by an adaptor protein causes the initiator caspase (8/9) to dimerize

• that activates the effector caspases (3/6/7) by cleavage —> then again this active caspase cleaves mutiple substrates and degrading cellular proteins which leads to apoptosis

• caspases can self amplify by activating more and more caspases in an hierachy by cleavage

• CAD cleaves the DNA between histones after beeing activated by an effector caspase that cleaves the inhibitory iCAD

Extrinsic pathway:

• killer lymphocyt with Fas ligand binds to cell

• death receptors [Fas or TRAILR] activate death inducing signaling complex (DISC) that bind caspases and activate them by cleavage

Intrinsic pathway:

• DNA-damage, ER stress, hypoxia, starvation are lethal stimuli

• they activate BAX and BAK (inhibition by BCL2 they are all one family)

• they trigger mitochondria as central regulator causing them to release cytochrome c

• cytochrome c binds and activates the adaptor protein forming a wheel (apoptosome) and recruiting and activating caspase 9

Extra: Ras/Raf/MEK/ERK pathway inhibits Caspase 9 —> high cell division and low apoptosis = cancer

G1 phase (with G0 arrest phase and G1 control point):

• G1/S-Cyclin with G1/S cyclin dependent kinase (Cdk)

• gap phase for cell growth and preparation of following phase

  • Damaged DNA? sufficient cell volume? favorable environvent?

S-phase:

• S-Cyclin and S-Cdk

• DNA-replication

G2 phase (with G2 control point)

• S-Cyclin and starting M-Cyclin

• like G1 phase

  • unreplicated or damaged DNA? favorable environvent?

M phase (mitose with prophase, prometaphase, metaphase (chromosomes in equatorial axis), anaphase (mitotic spindle pulls apart) and telophase and then cytokinesis) (with M control point)

• S-Cyclin falling off to Metaphase-Anaphase

• M-Cyclin and M-Cdk

• APC/C at meta-ana transition

  • chromosome misalignment?

Cyclin dependent kinases with Cyclin active and fully active when phosphorylated

Cyclin D —> all phases

E —> G1 / S

A —> S + G2

B —> G2 + M (between meta and anaphase

The Cyclin/Cdk complexes target specific effectors:

• DNA replication machinery

• mitotic spindle

Cyclin can be ubiquinated and degraded by proteasomes

P21 or P27 are Cyclin kinase inhibitors (guardians)

P53 as the guardian of the genome is the TF for P21 (and others) stopping the cell cycle, responsible for cell aging (when telomeres are used up no more dividing) and apoptosis