Cell to Cell Communication

Question 1

Ion Channels

Answer 1

1. Allow ions to flow down concentration gradient
2. Classfied by nature of gating
   
   1. Gating
      
      1. Ligand
      2. Voltage
      3. Mechanical
      4. Phosphorylation
      5. Lipids
   2. Species of ions passing through
      
      1. Na+, K+, Ca+, Cl-, HCO3-
   3. Number of gates

Question 2

Cystic Fibrosis

Answer 2

1. Recessive disease
2. Caused by loss of function mutation in CFTR which encodes a Cl-
3. Ligand for Cl- receptor is ATP
4. Mutation leads to
   
   1. Abnormal salt transport across epithelial cell membrane
   2. Results in thick mucus build-up

Question 3

Mutations in Na+ ion channels

Answer 3

1. Leads to defective inactivation and late Na+ currents in:
   
   1. Paramyotonia
   2. Cardiac arrythmia
   3. Epilepsy
   4. Heart and brain related problesm

Question 4

Estrogen and Estrogen Receptor interaction

Answer 4

Estrogen binds to Estrogen receptor

1. Estrogen receptor is bound to chaperone in cytoplasm
2. When estrogen enters the cell, it binds to estrogen receptor
3. Estrogen binding induces a conformational change that displaces the chaperone and exposes nuclear import signal
4. Estrogen receptor wtih estrogen dimerizes with another estrogen receptor with estrogen
5. Dimerized receptors enter nucleus and bind to estrogen response elements in DNA promoters
6. Recruit co-activators to activate gene transcription

Tamoxifen (used for breast cancer treatment) binds to estrogen receptor

1. Tamoxifen, which is metabolized into hydroxytamoxifen (H-tam).
2. Hydroxytamoxifen binds to estrogen receptor, causing chaperone to displace.
3. The receptor dimerizes and translocates to nucleus.
4. It binds to target genes in the ERE and recruites co-repressors (cell growth pathway genes)
5. It represses gene transcription and prevents the over-proliferation of the cell

Question 5

Protein Kinase Signal Transduction

Answer 5

1. Ligand binds to tyrosine kinase receptor
2. Tyrosine kinase receptor dimerizes with another ligand-bound tyrosine kinase receptor
3. The cytoplasmic domains are brought close together and cross-phosphorylate each other.
4. The phosphorylated tyrosine tails attract SH2 domain on Grb.
5. SOS (GEF bound to Grb) is activated.
6. SOS binds to small G-protein Ras and catalyzes exchange of Ras-GDP for Ras-GTP thereby activating Ras.
7. RasGTP activates transcription factor cascade, the final molecule being MAPK that goes on to target gene transcription in nucleus.

*Ras does not need GAP to inactivate it again because RAS has a weak inherent GAP so Ras can hydrolyze GTP for GDP without any help.However, the process can be helped along by GAP proteins, which push process along with proper kinetics and proper timing.

Question 6

Ras and cancer

Answer 6

1. Oncogenic mutations in Ras may turn all of its downstream pathways on
2. Ras can be made constitutively active when its intrinsice GAP will not function such that GTP cannot be hydrolyzed to GDP.

Question 7

Ras-GTP regualtes

Answer 7

1. Cell growth
2. Cytoskeleton
3. Cell adhesion
4. Membrane traffic
5. Anti-apoptosis

Question 8

Physiological roles of 7-alpha-helix receptors

Answer 8

1. Vision: Opsins such as Rhodopsin
   
   1. Convert EM radiation into cellular signals
2. Smell: Olfactory receptors that bind odorant molecules
3. Mood: receptors for neurotransmitters bind serotonin, dopamine, GABA, and glutamate

Question 9

7-alpha-helix receptor signal transduction

Answer 9

Process

1. The binding of the ligand to the 7-alpha-helix receptor functions as a GEF and exchanges GDP for GTP on alpha thereby activating it.
2. The trimeric subunits dissociate when alpha is bound to GTP
3. The G-alpha subunit then carries out some of the main signaling cascade functions downstream.
4. Downstream functions of G-alpha depend on the type of G-alpha associated with 7 alpha helix receptor
   
   1. Gs-alpha–> activates adenylyl cyclase which then activates cAMP which then activates PKA (kinase A)
   2. Gi-alpha –> inhibits adenylyl cylcase which then inactivates PKA (kinase A)
   3. Gq-alpha –> activates phospholipase C which then activates PKC (kinase C)

Question 10

Negative feedback loop of 7-alpha-helix receptor

I.e B-adrenergic receptor for epinephrine and norepinephrine

Answer 10

1. When ligand binds to receptor, Alpha subunit exchanges GDP for GTP
2. Trimeric G-protein dissociates
3. BARK is a kinase that is activated.
4. BARK phosphorylates 7-alpha-helix receptor
5. B-Arrestin binds to phosphoryalted receptor and prevents interaction with Gs.
6. This brings about desensitization to the ligand.

Question 11

Gs alpha pathway

Answer 11

1. Gs-alpha activates adenylyl cyclase
2. Adenylyl cyclase activates cAMP
3. cAMP activates PKA
4. PKA then regulates intracellular calcium:
   
   1. Cytosolic Ca2+ is maintained at a very low level by calcium pumps in plasma membrane and sarcoplasmic recticulum aka smooth endoplasmic recticulum.
   2. Receptor mediated Ca2+ influx triggers many events in cells
      
      1. Muscle contraction
      2. Regulated secretion
      3. Cell division

Question 12

Gq protein and their pathways

Answer 12

1. Gq-GTP activates phospholipase C which is right near the receptor
2. Phospholipase C cleaves PIP2 into DAG and IP3
3. IP3 triggers release of Ca2+ from lumen of smooth endoplasmic recticulum into the cytoplasm
4. DAG binds C1 domain of PKC and Ca2+ binds to C2 domain and phosphotidyl serine on the DAG
5. The pseudosubstrate is removed from PKC active site thereby activating PKC.
6. The cutting off of psedosubstrate results in PKC being constituively active

Question 13

Calmodulin and CAMK

Answer 13

1. Calmodulin-activated protein kinase (CAMK)’s active site is blocked by inhibitory domain
2. 4 Ca2+ bind to Calmodulin which then binds to inhibitory domain of CAMK thereby activating CAMK
3. CANKII is an important mediator of learning and memory

Question 14

Small G-proteins in Ras family

Answer 14

1. Ras- receptor signaling and cell division
2. Rab- traffic of membrane vesicles
3. Ran- nucleus/cytoplasm traffic
4. Rac- actin cytoskeleton