Cell-cell communication Flashcards
What are the 4 main classes of receptor?
Endocrine, paracrine, synaptic, contact dependent
What is contact dependent signalling?
Two cells in close proximity
Proteins on cell surface interact with eachother
Direct contact between cells
e.g. Gap junctions
Where are gap junctions found?
Both of coupled cells or cellular compartments
What is a homotypic connexon?
12 same connexins
What is a heterolytic connexon?
More than 1 connexin used
What is a homomeric connexon?
All 6 proteins are same connexin
What is a connexon?
6 connexins
Characteristics of Dextran based dye
Can’t pass through gap junctions
Characteristics of Lucifer yellow dye
Pass through gap junction into surrounding cells
What does GJA1 do?
Codes for Cx43, formation of craniometaphyseal dysplasia
What does GJA3 do?
Codes for Cx46, formation of cataracts
What does GJA5 do?
Codes for Cx40, formation of AF
What does GJB1 do?
Codes for Cx32, formation of Charcot-Marie-Tooth syndrome
Which connexions are expressed by atrial tissue?
Cx40 and Cx43
What is AF?
Channels contain mutated connexins/ decreased number of connexins with decreased channel activity
Channels form on lateral membrane
Heterogenous or decreased expression of connexins
What does Charcot-Marie-Tooth syndrome affect?
Peripheral nerves
Symptoms of Charcot-Marie-Tooth syndrome
Gait problems, numbness in lower limbs, reflexes lost at ankles and knees
Most common form of CMT
X-linked form - CMT1X
What is CMT1X caused by?
> 400 mutations in GJB1 gene that encodes gap junction protein Cx32
Fails to form functional channels
Decrease in conduction of AP
Why does CMT lead to muscle atrophy?
In peripheral nerves, it decreases the conduction velocity of APs
How to identify LQTS
Abnormal Qt interval on ECG
Why does LQTS occur?
Decrease in depolarising cardiac membrane currents or increase in depolarising cardiac currents late in cardiac cycle
Why might delayed depolarisation occur?
Reductions in either rapidly or slowly activating delayed repolarizing cardiac potassium current
What causes prolonged depolarisation?
Persistent inwards leak of sodium
What is cardiac dysfunction?
Mutations in heart Na+ and K+ channels lead to cardiac channelopathy called LQTS
What is LQTS1 caused by?
Loss of function in KCNQ1 which codes for slow K channel
What is LQTS2 caused by?
Loss of function in KCNH2 which codes for rapid K channel
What is LQTS3 caused by?
Gain of SCN5A gene which encodes for sodium channel, causing failed inactivation of channel
What is a missense mutation?
Nucleotide change means different codon and different amino acid
How is SQTS formed?
Missense mutations cause increase of function of KCNH2 in SQTS1, KCNQ1 in SQTS2 and KCNJ2 in SQTS3
What is Ras?
Monomeric GTP-binding protein
How is RAs activated?
- Most RTKs activate Ras - a monomeric GTP-binding protein (GTP-ase)
- RTKs linked to enzymes - the enzyme is a kinase which phosphorylate molecules
- Tyrosine kinases add phosphate to tyrosine
- Ligand binds to receptor - often receptor is dimer and sometimes tetramer
- Binding of ligand stabilizes dimer, receptors phosphorylate each other and this allows adaptor molecules to bind which determine downstream signaling
- Adaptor molecules recruit Ras guanine exchange factor (GEF) to catalyze conversion of Ras from inactive GDP bound form to active GTP bound form
- This then signals to other molecules
- Most RTKs activate Ras signaling molecule
What is Herceptin used to treat?
Breast cancer - increased signalling downstream of RTLK
How is EGFR treated?
Tyrosine kinase inhibitors
What cancer is EGFR linked to?
Colorectal cancer
What cancer is ERBB2 linked to?
Breast cancer
How are GPCRs activated?
- Absence of ligand when GPCR is inactive - GPCR = G-protein coupled receptors
- GPCRs bind to G proteins which are a trimeric protein, alpha, beta and gamma subunits
- G protein bind GDP when GPCR is inactive but binds GTP when active, also causes trimeric protein to split (alpha and beta/gamma subunits) and two signals
- Alpha, beta/gamma unit and G-protein all exert signals
- G protein hydrolyses GTP to GDP to inactivate GPCR
How are GPCRs inactivated?
- G protein hydrolyses GTP to GDP to inactivate GPCR
- E.g. target protein is adenyl cyclase
- ATP -(adenyl cyclase)→ cAMP -(cAMP phosphodiesterase)→ 5’AMP
- Alpha subunit hydrolyses GDP to GDP and then reassembles with beta/gamma subunits
Symptoms of cholera
Severe water loss, vomiting and muscle cramps
What microorganism causes cholera?
Vibrio cholera
How does cholera cause water loss?
- Cholera toxin binds to enterocytes (epithelial cells in GIT) and the toxin enters the cell by endocytosis
- Toxin formed form A and beta units (B assists body entry)
- The A subunit stimulates fluid secretion by activating cAMP formation
- A subunit activates Gsa
- A subunit catalyses transfer of ADP ribose from NAD to arginine group on alpha subunit of G-alpha-S sub molecules
- ADP ribosylation inhibits the ability of G-alpha-S to hydrolyse GTP and inactivate the molecule - this means that G protein is ‘locked’ in active position and this maintains a persistent activation of adenylate cyclase and formation of cAMP which in turn activates protein kinase A (PKA)
- G protein is in GTP-bound form
- Stimulates adenylate cyclase
- cAMP produced activates PKA
- CFTR is phosphorylated and activated
- Efflux of Cl- ions (and sodium ions and water)
- Due to charge across membrane, sodium ions pass out of cell and water is lost due to change in osmotic gradient
What does defective GPCR signalling result in?
- Decrease in production of G proteins: pseudohypoparathyroidism (genetic loss of G(s) protein a subunits results in no response to parathyroid hormone)
- Decreased signal initiation: whooping cough (pertussis)(a bacteria toxin adds ADP-ribose to the receptor-binding C-terminal tail of G(i) protein alpha subunits, causing reduced responsiveness of G proteins to receptor activation
- Increased signal initiation: essential hypertension - mutations in G protein beta subunits
- Defective signal termination
