Cell Signaling
Evolution
Saccharomyces cerevisiae: \(a\) and \(\alpha\) mating types secretes a specific factor that binds to the receptors only on the other type of cell.
Quorum Sensing: monitor the local density of bacterial cells.
Biofilm: aggregation of bacterial cells adhered to a surface.
Myxobacteria share information about nutrient availability. When food is scarce, the cells aggregate and form a fruiting body.
Local Signaling
Cell Junctions: Gap Junctions and Plasmodesmata
Cell-Cell Recognition: Direct contact between membrane-bound cell-surface molecules. Important in embryonic development.
Paracrine Signaling: Numerous cells can simultaneously recieve and respond. e.g. growth factor
Synaptic Signaling: Electrical Signal + Neurotransmitter to target cell.
Endocrine Signaling: Travel through circulatory system. e.g. plant growth regulators, insulin, etc.
Three Stages of Cell Signaling
Reception, Transduction, Response
Earl W. Sutherland, Nobel Prize 1971
Reception
Ligand: Signal Molecule, highly specific. Cell-surface receptors represent 30% of all human proteins, 1% of proteins whose structures have been determined by X-ray. \(\beta\) adrenergic receptor
Breast cancer patients have poor prognosis if their cancer cells have excessive levels of RTK HER2.
G Protein-Coupled Receptors (GPCRs)
- G protein bound with GDP (Inactive)
- Ligand binds to receptor, causing receptor's shape to change. Receptor's cytoplasmic side binds an inactive G protein, causing GTP to displace GDP and thus activates the G protein.
- G protein dissociates from the receptor, binds to an enzyme and activate the enzyme.
- G protein also functions as a GTPase enzyme, hydrolyzes its GTP to GDP + Pi and becomes inactive again.
Receptor: Seven transmembrane \(\alpha\) helices.
Malfunctions of associated G proteins involved in many diseases, including bacterial infections (cholera, pertussis and botulism). 60% of all medicines today exert their effects by infuencing G protein pathways.
Receptor Tyrosine Kinases (RTKs)
- Monomers: ligand-binding site, \(\alpha\) helix spanning the membrane and an intracellular tail with multiple tyrosines.
- Dimer: binding of ligand cause monomers to associate (dimerization)
- Dimerization activates tyrosine kinase region, each tyrosine kinase adds a phosphate from an ATP to a tyrosine on the other monomer.
- Receptor activated, recognized by relay proteins. Each proteins binds to a specific phosphorylated tyrosine, triggering an transduction pathway.
Tyrosine Kinase: catalyzes the transfer of a phosphate group from ATP to tyrosine (an amino acid) on a substrate protein.
One RTK may activate ten or more different transduction pathways. Abnormal RTKs that function in the absence of ligands are associated with various cancers. (because it coordinates cell growth and reproduction)
問題：啟動怎樣的pathway是該細胞中的relay protein決定，還是RTK的種類決定呢？
Ligand-Gated Ion Channel Receptors
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Ligand binds to receptor and the gate opens, specific ions (such as \(Na^+\) and \(Ca^{2+}\)) can flow through the channel and change the concentration rapidly.
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Ligand dissociates, the gate closes.
Important in nervous system, also present on the ER.
Intracellular Receptors
e.g. Hydrophobic: steroid hormones, thyroid hormones; Small: nitric oxide
Aldosterone (a steroid hormone) pass through plasma membrane, binds to receptor protein and enters nucleus. Bound protein acts as a transcription factor, turns on genes that control water and sodium flow in kidney and eventually affect blood volume.
Transduction
Protein Kinase: phosphorylate either serine or threonine.
Phosphorylation Cascade: 2% of human genes are thought to code for protein kinases
Protein Phosphatases: remove phosphate groups from proteins, turning off transduction pathway.
Seond Messengers
Cyclic AMP
- adenylyl cyclase converts ATP to cAMP in response to siganl. Concentration of cAMP can boosted 20x in seconds.
- phosphodiesterase converts cAMP to AMP.
- Other components: GPCRs, protein kinases.
- Activates protein kinase A (serine/threonine kinase)
- Inhibitory G protein blocks activation of adenylyl cyclase.
Viagra: treatment for erectile dysfunciton. cGMP acts as a signaling molecule to relax smooth muscel cells in artery walls, leads to dilation of blood vessels.
Vibrio cholerae: modifies G protein causing it to stuck in activated form, continues to make cAMPs and leads to secretion of salt into intestines.
Calcium Ions and Inositol Triphosphate (\(IP_3\))
Increase \(Ca^{2+}\) : muscle cell contraction, secretion, division, greening in response to light. \(Ca^{2+}\) concentration in cytosol is much lower than outside the shell & ER.
- Ligand + Receptor activates Phospholipase C
- Phospholipase C cleaves \(PIP_2\) into DAG and \(IP_3\)
- DAG functions as 2nd messenger, \(IP_3\) binds to \(IP_3\)-gated calcium channel in ER membrane
- \(Ca^{2+}\) flow out of ER, activating more pathways
Response
Amplification
G protein (100), cAMP, protein kinase, phosphorylase kinase, glycogen phosphorylase => Release glucose 1-phosphate from glycogen
Specificity
Response of a signal depends on a the particular collection of signal receptor proteins, relay proteins, and proteins needed to carry out the response.
Efficiency
Scaffolding Proteins: scaffolding proteins in brain cells permanently hold together networks of signaling pathway proteins at synapses.
Wiskott-Aldrich Syndrome (WAS): abnormal bleeding, eczema caused by absent WAS protein and disrupted pathways.
Termination
Concentration of signaling molecules falls, fewer receptors are bound. GTPase hydrolyzes GTP bound to G protein; Phosphodiesterase converts cAMP to AMP; Phosphatase - Kinase, etc.
Apoptosis
Controlled Cell Suicide - protects neighboring cells from damage.
Caenorhabditis elegans: Ced-9 (in the outer mitochondrial membrane) acts as a brake, inhibits Ced-4. Death signal disables Ced-9 and thus activates Ced-4, proteases (caspases, Ced-3) and nucleases
Human: 15 different caspases, including cytochrome c (functions in ATP synthesis). e.g. form pores in the mitochondrial membrane. DNA suffered irreparable damage, excessive protein misfolding, development of nervous system, operation of immune system and morphogenesis.
Parkinson's and Alzheimer's Disease: In Alzheimer's disease, accumulation of aggregated proteins in neuronal cells triggers apoptosis. Programmed Cell Death in Parkinson's Disease
Cell Cycle
- Phases: G1 (1st gap), S (synthesis), G2 (2nd gap), M (mitotic).
- Checkpoints: G1 (restriction point), G2 amd M checkpoint.
Cyclin and Cyclin-dependent Kinases (Cdks): Activity of Cdks rises and falls with concentration of cyclin.
Maturation-Promoting Factor (MPF): Act as kinases and activates kinases, promotes fragmentation of nuclear envelope during prometaphase, chromosome condensation, spindle formation during prophase, etc.
G2 Checkpoint
- Synthesis of Cyclin in late S phase to G2 phase.
- Cyclin combine with Cdks, producing MPF. MPFs accumulate and passes G2 checkpoint.
- MPF promotes mitosis, peaks during metaphase.
- MPF degrades during anaphase - G1
Internal and External Signals
Internal Signals
- Chromosomes properly attached to spindle (kinetochores), activate separase to cleave cohesins (M checkpoint).
External Factors
Growth Factor, like Platelet-Derived Growth Factor (PDGF) required for division of cultured fibroblasts (G1 checkpoint). When injury occurs, platelets release PDGF, resulting in proliferation of fibroblasts to heal the wound.
Density-Dependent Inhibition: crowded cells stop dividing.
Anchorage Dependence: must attached to a substratum (i.e. culture flask or extracellular matrix of a tissue).
Loss of Cell Cycle Control in Cancer
Transformation: HeLa cells divide indefinitely vs normal cells 20-50 divisions
Metastasis: spread of cancer cells
Chemotherapy: Toxic to actively dividing cells. e.g. Taxol freezes mitotic spindle by preventing microtubule depolymerization. Side effects: intestinal cells (nausea), hair follicle cells and immune system.
References
N. Campbell, J. Reece, L. Urry M. Cain, S Wasserman, P. Minorsky and R. Jackson. Biology: A Global Approach. Pearson Education 2014