L5: Second messengers Flashcards
What are primary messengers?
carry messages between cells
What are secondary messengers?
carry messages within the cells
Compare the complexity and speed of primary and secondary messengers
- Second messengers = more complex mechanisms and can activate multiple parallel pathways
- primary messengers = generally faster
How do second messengers amplify the signal response?
- Second messengers can be synthesized in large quantities as a response to a single primary messenger - leading to signal amplification
How do second messengers show selectivity of response?
- can be active at different times & locations within the cell, allowing for selective responses to specific signals
How can agonists mediate opposite responses via the same second messenger mechanism?
- Agonists can bind to different receptors, leading to different responses via the same second messenger pathway
How can agonists have different effects depending on the tissue they are located in?
- Agonists can bind to the same receptor but have different effects depending on the tissue they are in - influencing the response to the second messenger mechanism
What are the main types of secondary messengers?
- cyclic nucleotides (cAMP and cGMP)
- molecules derived from lipid bilayers (IP3 and DAG)
- gases (NO and CO)
- ions (Calcium)
what are effector enzymes?
- a type of enzyme activated/regulated by a signalling pathway - often involving second messengers or protein kinases
- play crucial role in transmitting signals within cell & mediating various cellular responses to extracellular stimuli
What are some examples of effector enzymes and their substrates?
Adenyl cyclase: ATP → cAMP
Guanylate cyclase: GTP → cGMP
Phospholipase A2: Membrane lipid → Arachidonic acid
Phospholipase C: PIP2 → IP3 and DAG
Nitric oxide synthase: L-arginine → Nitric oxide
What is the function of adenyl cyclase?
- converts ATP to cAMP, which is an important secondary messenger involved in many cellular processes
What is the function of guanylate cyclase?
converts GTP to cGMP - critical secondary messenger involved in various cellular responses
What is the role of phospholipase A2?
- Phospholipase A2 hydrolyses membrane lipids to produce arachidonic acid, which is a precursor to many signalling molecules
What does phospholipase C do?
- Phospholipase C cleaves PIP2 to generate IP3 and DAG, both of which are important secondary messengers in signal transduction
What does nitric oxide synthase produce?
- Nitric oxide synthase converts L-arginine to nitric oxide, which is a gaseous signalling molecule with various physiological functions
Which G protein subunit is associated with increasing cAMP levels?
Galphas is associated with increasing cAMP levels through activation of adenyl cyclase
Which G protein subunit is associated with decreasing cAMP levels?
Galphai is associated with decreasing cAMP levels through inhibition of adenyl cyclase
Which G protein subunit is associated with increasing IP3 and DAG levels?
Galphaq/11 is associated with increasing IP3 and DAG levels by activating phospholipase C
What are the two major effects of ligand-gated ion channels?
- allow the entry of calcium ions
- modify the membrane potential of cells
How do tyrosine kinase-linked receptors initiate cellular responses?
- initiate cellular responses by activating effector enzymes & protein kinases through phosphorylation cascades
What are the types of target proteins phosphorylated by kinases for long-term changes?
- Kinases phosphorylate target proteins, such as:
- ion channels
- transcription factors
- enzymes, to bring about long-term changes in cellular functions
How are the effects of kinases reversed?
- by dephosphorylation of target proteins carried out by phosphatases
Which residues do protein kinases primarily target for phosphorylation?
- primarily target serine, threonine, and tyrosine residues for phosphorylation
What are the classes of kinases based on their substrate specificity?
The classes of kinases include:
- Serine/threonine kinases - PKC, PKA, PKG, MAPK, CaMK
- Receptor Serine/threonine kinases - TGFß receptor
- Dual specificity protein kinases - target tyrosine/threonine residues (e.g., MEK)
- Receptor tyrosine kinases - neurotrophic receptors
- Non-receptor tyrosine kinases - Src family
What is the function of BDNF (Brain-Derived Neurotrophic Factor) in pain processing?
- BDNF alters pain processing by binding to trkB receptors on spinal & peripheral neurons and glia
- leads to altered gene expression, NMDA receptor activation and trafficking, enhancing the neuronal response to glutamate
Describe the activation process of trkB receptors by BDNF
- BDNF binds to trkB receptors, causing conformational change that leads to receptor dimerization
- intrinsic kinase activity of the receptor is enhanced, resulting in auto-phosphorylation and phosphorylation of other tyrosine residues on the receptor
- these phosphorylated residues serve as docking sites for signalling proteins
What signalling proteins are involved in the trkB receptor pathway?
- trkB receptor pathway involves signalling proteins GRB2 (growth factor receptor-bound protein 2), Sos (guanine nucleotide exchange factor), and Ras (small GTPase)
- activation of Ras through the Ras-Sos complex leads to activation of the MAP kinase cascade
What are the downstream effects of the MAP kinase cascade activation?
- the MAP kinase cascade activation alters gene expression through CREB (cAMP response element-binding protein) & affects other cytosolic targets - leading to diverse cellular responses
How does PI-3K (phosphatidylinositol-3 kinase) increase the responsiveness of trkB receptors?
- PI-3K enhances the responsiveness of trkB receptors by increasing the diversity of cellular responses that can be produced with just one molecule of BDNF
Where is BDNF synthesized and transported, and what causes an increase in its synthesis?
- BDNF synthesized in primary sensory neurons & transported to their central terminals
- Its synthesis is increased during local inflammatory processes
Describe the signalling cascade involving cAMP and CREB in memory formation
- In memory formation, cAMP and CREB phosphorylation play a role in long-term memory
- cAMP activates PKA (protein kinase A), and the catalytic subunit of PKA phosphorylates target molecules on serine and threonine residues
- this cascade modulates memory formation
- additionally, spatial and temporal localization of this signalling pathway is facilitated by scaffold proteins and A kinase anchoring proteins (AKAPs)
What are the major effects of cAMP mediated by PKA?
- involve the dissociation of the regulatory subunit from the catalytic subunit
- the catalytic subunit then phosphorylates target molecules on serine and threonine residues
- these target molecules include ion channels, enzymes, and TFs
How is AC (adenyl cyclase) involved in signal amplification?
- AC can phosphorylate multiple ATP molecules, resulting in signal amplification
- it exists in 8 isoforms, and its activation is stimulated by Gas and inhibited by Gai
- Isoforms I, III, and VIII are activated by calcium/calmodulin, while isoforms V and VI are inhibited by calcium (CaM-independent)
What are AKAPs, and what role do they play in signalling cascades?
- AKAPs (A kinase anchoring proteins) are proteins that bind all components of the signalling cascade, forming a small signalling complex near the receptor
- They play a crucial role in allowing spatial-temporal localization of the signalling pathway
How is calcium excitotoxicity avoided in cells?
- Calcium excitotoxicity avoided through various mechanisms that regulate calcium concentration. mechanisms included :
- Plasma membrane or smooth endoplasmic reticulum ATPase
- Secondary transporters like NCKX (Na in, calcium out, potassium out) or sodium calcium exchanger NCX
- Calcium-activated K+ and Cl- channels
- Uptake of calcium into cellular compartments (e.g., endoplasmic reticulum, mitochondria) via ATP-dependent mechanisms (e.g., SERCA)
- Binding of calcium to Ca2+ buffering proteins (e.g., calbindin, calretinin)
- CaM Kinase dependent phosphorylation of Ca2+ channels to decrease their activity
How does calcium regulate various processes in cells?
- Ca2+ ions play critical role in regulating a diverse range of processes in cells, including: memory formation, vesicular trafficking & neurogenesis
- Ca2+ (doesn’t directly bind to most target proteins; instead) binds to calmodulin (CaM), causing conformational change in protein
- calcium-calmodulin complex then alters activity of other proteins by binding to them & changing their shape
- many effects of calcium signalling rely on calcium-calmodulin dependent protein kinases (CaMKs), with one of their targets being CREB (which is involved in memory formation)
What are the consequences of sustained increases in calcium concentration in cells?
- sustained ↑ in Ca2+ concentration can lead to cell death via necrosis or apoptosis.
- often a result of ↑ glutamate NMDA receptor activity or decreased cytoplasmic calcium buffering
- elevated Ca2+ concentrations activate proteases, phospholipases, and endonucleases, which damage the cytoskeleton, cell membrane, and DNA
- avoiding calcium excitotoxicity is essential to maintain cellular health
How does calcium regulation protect cells from calcium excitotoxicity?
- cells utilize various mechanisms to regulate [Ca2+] & prevent Ca2+ excitotoxicity
- mechanisms include: pumping Ca2+ out of the cell through plasma membrane or smooth endoplasmic reticulum ATPases, secondary transporters, and calcium-activated ion channels
- Ca2+ is also taken up into cellular compartments, such as the ER and mitochondria, via ATP-dependent mechanisms
- additionally ca2+ bound to calcium buffering proteins like calbindin and calretinin
- CaM Kinase can also ↓ the activity of Ca2+ channels through phosphorylation, contributing to calcium regulation & cellular protection
