Cellular Signaling PDF

Cellular Signaling Friday, October 25, 2024 12:08 PM Endogenous signaling molecules: originate from inside the body Secreted: exocytosis ○ e.g. hormones, NTs Released: passive diffusion through channels ○ e.g. ATP, prostaglandins ○ ATP can be co-released with hormones a...

Cellular Signaling Friday, October 25, 2024 12:08 PM Endogenous signaling molecules: originate from inside the body Secreted: exocytosis ○ e.g. hormones, NTs Released: passive diffusion through channels ○ e.g. ATP, prostaglandins ○ ATP can be co-released with hormones and NTs Liberation: membrane anchored ligands get cut off by matrix metalloproteinases ○ e.g. growth factors, cytokines Exposed: at membrane but tightly bound to cell surface ○ Ephrins Extracellular matrix proteins ○ e.g. integrins Modes of signaling Endocrine ○ Release hormones ○ Transported over long distances ○ Relatively slow ○ e.g. insulin, parathyroid hormone (PTH), etc. Paracrine ○ Impact local/immediate environment ○ Platelet derived growth factor (PDGF): for wound healing ○ Parathyroid hormone related peptide (PTHRP): similar to PTH but paracrine Synaptic ○ Special type of paracrine signaling ○ For neurons ○ Rapid signaling ○ Usually in direction of pre --> post-synaptic cell ▪ Cannabinoid signaling goes the other way (post --> pre-synaptic cell) ▪ Uses endocannabinoids as the NT  Contact-dependent (juxtacrine) ○ Physical contact between adjacent cells ○ Ligand protrudes from one cell to the other ▪ e.g. ephrin receptors, adhesion GPCRs ○ Important in developmental biology Autocrine ○ Acts on itself ○ e.g. cytokine IF-1, VEGF, NTs that bind to pre-synaptic autoreceptors ○ Functions: ▪ To control secretion activity – "notice" the amount it's secreting Control through negative feedback loop Present in pre-synaptic cells – presynaptic autoreceptors ▪ Can tell if surrounding cells are similar Protease-dependent ○ Proteolytic activation of ligand ○ Irreversible signaling ○ In GPCR, extracellular N-terminus is cleaved by thrombin to expose active ligand ○ Release of membrane-anchored ligand to bind to something else ○ Conversion of pro-hormone to active hormone ▪ Endorphins: intracellular process ▪ Angiotensin II: extracellular process Some notes Some ligands can be endocrine, NT, autocrine, etc. (e.g. noradrenaline) ○ Doesn't matter for the receiving cell as long as they have the receptor for it Transduce: to convert to another form  Affinity: how tightly substrate binds (rate off/rate on) Signals need to be turned off ○ Resorption of agonists can be found a lot for NTs ○ Drugs can target reuptake channels (e.g. cocaine blocks serotonin reuptake) Determinants of signaling Receptor expression level (i.e. more receptors = more response) Receptor variants Intracellular signaling ○ Manages magnitude and speed of signaling G proteins... GDP – inactive GTP – active To get activation... ○ GDP must release ○ Protein is in nucleotide free state ○ There's 10x more GTP than GDP, so it'll bind quickly ▪ GTP DOES NOT HAVE MORE AFFINITY, THERE'S JUST MORE GTP Rate of hydrolysis back to GDP is dependent on protein ○ Usually slow Protein regulators ○ GEF: promote GDP release ○ GAP: promote GTP hydrolysis ○ GDI: inhibit GDP release Phosphorylation... Ubiquitous Reversible between ATP and ADP Phosphorylation doesn't always mean active form Cell signaling 1st messenger --> 2nd messenger --> signaling cascade Signal amplification is present ○ There is some redundancy ○ If 2nd messengers are all saturated, 1st messengers don't have an effect ○ Don't need all receptors to be active – spare receptors Attenuation: pushing back on the signal ○ e.g. 2nd messenger breakdown Scaffolding proteins: brings components together ○ Could also bring in negative regulators though Tachyphylaxis/desensitization ○ Acute – ion channel conformation ○ Substrate depletion ○ Receptor internalization Signal transduction pathways are not linear, they diverge and cross Pleiotropy: one signal elicits multiple outcomes  e.g. GPCR Convergence: multiple signals elicit common outcome In example, it all ultimately leads to conversion of PIP2 --> PIP3 Signal integration: activation of molecule only when multiple substrates are present For drug inhibition, can just inhibit 1 activator Families of Receptors 4 main types of receptors Ligand-gated ion channels ○ Multiple subunits ○ Ligand triggers channel opening ○ Ionotropic GPCR ○ 7 transmembrane domain ○ Extracellular N-terminus ○ Intracellular C-terminus ○ Metabotropic Kinase/enzyme-linked receptors ○ 1 transmembrane domain  ○ Extracellular N-terminus ○ Intracellular C-terminus Nuclear receptors ○ Found in cytosol or nucleus ○ Monomers when inactive ○ Dimers when activate Time frames of cellular response Ligand-gated ion channel: very fast ○ Milliseconds ○ e.g. nAChR GPCR: fast ○ Seconds ○ e.g. muscarinic AChR Kinase/enzyme-linked: slow ○ Some are hours, some are faster ▪ Insulin receptors are fast ○ Most does gene transcription Nuclear receptors: very slow ○ Hours ○ All does gene transcription All have the potential to initiate slow responses (i.e. gene transcription) ○ Only some can also be fast Nuclear receptors Found in cytosol or nucleus Ligands are hydrophobic ○ Diffuses across membrane ○ Binds to nuclear receptors Around 66-100 kDa in size Types: ○ Type 1 steroid receptors ▪ Found in cytosol ▪ Responsible for steroid hormones (e.g. estrogen, testosterone, etc.) ▪ Ligand binding makes receptor dissociate from chaperone protein ▪ Exposes dimerization and signaling domain ▪ Dimerizes in cytosol ▪ Moves to nucleus ○ Type 2 RXR heterodimers ▪ Found in nucleus ▪ Responsible for TH, retinoic acid, etc. ▪ After ligand binding, heterodimerize with retinoid X receptor (RXR) ○ Type 3 ▪ Similar to type 1 ○ Type 4 ▪ Small category of nuclear receptors ▪ Can be monomers or dimers Structure ○ Ligand binding domain (LBD) ▪ Ligand binding will expose dimerization domain ○ DNA binding domain (DBD) ○ Transactivation domain (AF) ○ Type 1 – face to face ▪ DBD are in opposite directions ▪ Binds to palindromic sequences in DNA ○ Type 2 – tail to tail ▪ DBD are in same direction ▪ Binds to direct repeat sequences in DNA Enzyme-linked receptors Responsible for hormones and growth factors 3 main groups ○ RTK ○ Cytokine receptors lacking enzymatic activity ○ Natriuretic peptide receptors with cAMP activity Insulin receptors: a type of RTK ○ Is actually a dimer at first with α and β subunits ▪ Function as a monomer ○ Transactivation: phosphorylation of Tyr on "partner" receptor ○ This signal translocate GLUT4 to membrane Some RTK can heterodimerize ○ Allows for functional flexibility ○ Can still form homodimers, but heterodimers have increased activity Gene transcription pathways ○ RTK/Ras/Raf/MAP kinase pathway 1. Transphosphorylation of RTK 2. Adaptor protein Grb2 binds to receptor 3. Recruits Sos protein 4. Recruits Ras GEF 5. Ras GEF is activated by GTP 6. Recruits Raf 7. Initiates kinase cascade to alter gene expression ○ Cytokine pathway 1. These receptors have NO kinase activity on their own 2. After dimerization, recruit Janus kinase (JAK) from cytosol 3. JAK phosphorylates receptor 4. Recruits and phosphorylates STAT protein 5. STAT protein dimerizes 6. Enters nucleus as transcription factor  Natriuretic peptide receptor (NPR) ○ 3 types ○ 2 types have guanylyl cyclase domain that converts GTP --> cGMP ○ NPR-C ▪ NO intracellular domain ▪ NO activity ▪ Sequesters A/B/CNP to act as an inhibitory sink Ligand-gated ion channels Ionotropic receptors Combination of subunits vary between ligand and non-ligand binding subunits ○ Give rise to many subtypes Tend to be activated by small molecules (e.g. choline, ACh, etc.) Cys-loop superfamily ○ Very common ○ Pentamers or tetramers ○ Each subunit has 4 transmembrane domains ○ Both C and N-terminus are extracellular ○ N-terminus loop has a disulfide bond ○ e.g. 5HT3, nAChR, GABA, glycine, etc. Glutamate receptors ○ Tetramers ○ Each subunit has 3 transmembrane domains ○ Extracellular N-terminus ○ Intracellular C-terminus ○ Has a pore loop that forms pore lining/selectivity filter  P2X purinergic receptors ○ Trimers ○ Each subunit has 2 transmembrane domains ○ Intracellular N and C terminus ○ Large extracellular loop that binds to ATP (the ligand) Some channels can partially open if some ligands are bound ○ Only fully opens when all ligands are bound G protein coupled receptors (GPCR) Largest family of receptor proteins Binding of ligand leads to alteration in... ○ Concentration of intracellular mediators ○ Ion permeability at membrane Can be activated by lots of things 3 extracellular and 3 intracellular loops