Block3 4.other Mechansisms Of Signal Transduction

Front 1

Wnt/Beta-Catenin pathway (- Wnt)

Back 1

Frizzled is a 7-transmembrane region receptor (not a GPCR). There is a complex in the cytosol sequestering Beta-catenin and phosphorylating it (GSK3, APC, Axin) preventing its ability as a TF and targeting it for ubiquitin-mediated degradation. TCF protein is sitting on DNA, acting as a transcriptional repressor

Front 2

Wnt/Beta-Catenin pathway (+ Wnt)

Back 2

Wnt binds Frz receptor. Causes association of the receptor with LRP, which binds Axin (originally part of the Beta-catenin sequestering complex). Beta-catenin is now free and stable, it goes into nucleus, binds to TCF, and together they become a transcriptional activator. NOTE: Like Hh, Wnt is fairly localized.

Front 3

Hedgehog Processing

Back 3

Precursor undergoes autocleavage and modifications to cholesterol and palmitate. This causes it to stay anchored into the plasma membrane, making it effective for signaling only in a limited range. Kept in a confined area, so cells that express high levels we get morphogenic effects with timing of exposure and only a few cells can be influenced. Why it is so important in development.

Front 4

Hedgehog Signaling (- Hh)

Back 4

Smoothened, a 7-TM receptor, is internalized and imprisoned in the cytosol by the Patched beast 12-TM receptor. Microtubule-bound complex: Fused (Fu, a ser/thr kinase) Costal-2 (Cos2, MT kinesin) Cubitis Interuptis (Ci, a ZF TF). Cubitus is phosphoed by a couple proteins and as a result it is cleaved by Slime. The cleavage product, Ci75, acts as a trans repressor. Overall, Patched acts to repress signaling and target transcription.

Front 5

Hedgehog Signaling (+ Hh)

Back 5

Hh binds to Patched beast -> Smoothened moves back to plasma membrane (and Patched gets sequestered
 into a vesicle actually). Smoothened has high affinity for Cos2, pulling complex
 off of microtubules. Full length Ci now free in cytoplasm (not bound to microtubule → translocates to nucleus → binds cAMP responsive elements (CRE) in DNA with CBP (CRE Binding Protein).

Front 6

NF-kB Pathway

Back 6

Think stress (an infection, ionizing radiation)
. Signals will be mediated through I-kappa-B kinase, which phosphorylates I-kappa-B-alpha and results in its degradation. When this happens, p65/p50 (NF-KB) is freed and can translocate into nucleus and act as a TF. Overall: Free NF-kB = p65/p50 = TF that allows cells to respond to different kinds of stress. It also has a negative feedback mechanism; it increases synthesis of its own inhibitor.

Front 7

Notch/Delta Signaling

Back 7

Pretty simple. Receiving cell has Notch receptor, the transmitting cell has Delta or some other receptor. When Notch binds ligand, secretases are activated that cleave Notch in two spots, generating three pieces. 1st cleavage is extracellular by MMP (matrix metalloprotease) ADAM10. 2nd cleavage is intracellular by g-secretase (presenilin-1, PS-1).The one on interior of the cell can act as a TF

Front 8

MMPs (matrix metalloproteases) Cleave Many Signal Proteins

Back 8

Many signals and TF's are tethered to the cells in which they are synthesized. Almost like a storage form of the signals. Many growth factors exist sequestered in the ECM, MMP's allow these to be utilized. EGF, HB-EGF and TGF-a → bind HER1. Deregulation of these enzymes can lead to cancer for two reasons:
1. Stimulation of too many growth factors
2. Destruction of the ECM allows cancers to metastasize

Front 9

Phosphatidylinositol Derivatives: PI3 Kinase and Akt.

Back 9

Recall that GPCR's can cause activation of phospholipase C, leading to cleavage of PIP2 into IP3 and DAG. RTK's and cytokine receptors can also cause this because SH2 domain on PLC can dock to pTyr scaffolding on the receptors. However, instead of being cleaved, PIP2 (or just PIP actually) can have a phospho group added to it by PI3 Kinase (to the 3rd carbon of inositol). This phospho group is critical in that it makes the molecule into an Akt (PKB) activator. Akt is normally inactive in cytosol; it contains an inhibitory lip domain like Binding to PI-3 derivatives has two results: PKB localized to membrane, and releases inhibition of catalytic site → partially activated PKB. Complete activation is caused by PDK1 and PDK2. Akt (PKB) is responsible for creating anti-apoptotic signals.

Front 10

Cellular Function of PKB

Back 10

1. Phosphorylates and inactivates many pro-apoptotic proteins. Phosphorylates (sequesters) TFs (Forkhead) in cytosol → prevents transcription of pro-apoptotic proteins → promotes cell survival.
2.PKB promotes glucose uptake and storage: Insulin receptor (RTK) activates PKB → movement of GLUT4
glucose transporter from intracellular vesicles to PM → promotes glucose uptake primarily into muscle and fat cells (lowers blood [glucose]).
- In liver and muscle, insulin activates PKB → phosphorylates and inactivates glycogen synthase kinase 3 → prevents inactivation of glycogen synthase (by GSK3) → gycogen synthesis occurs.

Front 11

Medical Relevance of PKB and PTEN

Back 11

PTEN is a phosphatase that removes phosphate groups from ser/thr and tyrosines. The MAJOR function is to remove phosphate at the 3rd carbon of PIP2 derivatives; therefore helping quiet PKB....and helping promote apoptosis. Not surprisingly, PTEN is deleted in many cancers.

Front 12

CLINICAL: LDL Particle and Atherosclerosis. A mechanism to prevent or activate the body's action of forming its own cholesterol

Back 12

Based upon a proteolytic cleavage by Notch mechanism-like secretases, forming a TF that can translocate to the nucleus.
A. High cholesterol: Insig-1(2) binds SCAP, anchoring SCAP-SREBP in ER membrane. No nuclear SREBP TF.
B. Low cholesterol: Dissociation of Insig-1(2) → SCAP-SREBP moves to Golgi → SREBP cleavage → cytosolic nSREBP TF activated to nucleus.