Lecture: Cell Signaling

Lecture: Cell Signaling
Introduction / Overview of Cell Communication Local vs. Long Distance Signaling Cell to Cell Contact (ex: cell junctions & cell-cell recognition) Secretion of Local Regulators (local) Neurotransmitters and Neurons (local) Hormones traveling in blood (long distance) III Stages of Cell Signaling  #1 - Reception (of signal = ligand) & Receptors * 3 receptors: (1) G-protein couple receptors (2) Receptor Tyrosine Kinases (3) Ion Channel Receptors  #2 – Transduction (a multistep pathway) * Phospholylation by Kinases * 2nd messengers – ex: cyclic AMP (cAMP) and Calcium & IP3  #3 – Cellular Response * Cytoplasmic * Nuclear

Overview: The Cellular Internet
Cell-to-cell communication is essential for life of unicellular and multicellular organisms Cells can communicate to the cell(s) next to them or cells from much further away in another part of the body How do cells receive a signal? Once cells receive the signal, how does this lead to changes inside the cell (response)? Biologists have discovered some universal mechanisms of cellular regulation The combined effects of multiple signals determine cell response For example, the dilation of blood vessels is controlled by multiple molecules Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Evolution of Cell Signaling
A signal transduction pathway is a series of steps by which a signal on a cell’s surface is converted into a specific cellular response Signal transduction pathways convert signals on a cell’s surface into cellular responses Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Local and Long-Distance Signaling
Cells in a multicellular organism communicate by chemical messengers Local = factors (ex: growth factors), neurotransmitters Long Distance = hormones Animal and plant cells have cell junctions that directly connect the cytoplasm of adjacent cells In local signaling, animal cells may communicate by direct contact, or cell-cell recognition Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Gap junctions between animal cells Plasmodesmata between plant cells
Fig. 11-4 Plasma membranes Gap junctions between animal cells Plasmodesmata between plant cells (a) Cell junctions Figure 11.4 Communication by direct contact between cells (b) Cell-cell recognition

(a) Paracrine signaling (b) Synaptic signaling
Fig. 11-5ab Local signaling Target cell Electrical signal along nerve cell triggers release of neurotransmitter Neurotransmitter diffuses across synapse Secreting cell Secretory vesicle Figure 11.5 Local and long-distance cell communication in animals Local regulator diffuses through extracellular fluid Target cell is stimulated (a) Paracrine signaling (b) Synaptic signaling

Long-distance signaling
Fig. 11-5c Long-distance signaling Endocrine cell Blood vessel Hormone travels in bloodstream to target cells Figure 11.5 Local and long-distance cell communication in animals Target cell (c) Hormonal signaling

The Three Stages of Cell Signaling: A Preview
The first research into cell signaling involved how the hormone epinephrine acts on cells Research suggested that cells receiving signals went through three processes: Reception Transduction Response Animation: Overview of Cell Signaling Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Plasma membrane 1 Reception Receptor Signaling molecule 1
Fig EXTRACELLULAR FLUID CYTOPLASM Plasma membrane 1 1 Reception Receptor Figure 11.6 Overview of cell signaling Signaling molecule

Plasma membrane 1 Reception Transduction Receptor Signaling molecule 1
Fig EXTRACELLULAR FLUID CYTOPLASM Plasma membrane 1 1 Reception 2 Transduction Receptor Relay molecules in a signal transduction pathway Figure 11.6 Overview of cell signaling Signaling molecule

Plasma membrane 1 Reception Transduction Response Receptor Activation
Fig EXTRACELLULAR FLUID CYTOPLASM Plasma membrane 1 Reception 2 Transduction 3 Response Receptor Activation of cellular response Relay molecules in a signal transduction pathway Figure 11.6 Overview of cell signaling Signaling molecule

Reception: A signal molecule binds to a receptor protein, causing it to change shape
The binding between a signal molecule (ligand) and receptor is highly specific A shape change in a receptor is often the initial transduction of the signal Most signal receptors are plasma membrane proteins Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Receptors in the Plasma Membrane
Most water-soluble signal molecules bind to specific sites on receptor proteins in the plasma membrane There are three main types of membrane receptors: G protein-coupled receptors Receptor tyrosine kinases Ion channel receptors Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

A G protein-coupled receptor is a plasma membrane receptor that works with the help of a G protein
The G protein acts as an on/off switch: If GDP is bound to the G protein, the G protein is inactive Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Signaling-molecule binding site
Fig. 11-7a Signaling-molecule binding site Figure 11.7 Membrane receptors—G protein-coupled receptors, part 1 Segment that interacts with G proteins G protein-coupled receptor

Fig. 11-7b Plasma membrane G protein-coupled receptor Inactive enzyme
Activated receptor Signaling molecule GDP G protein (inactive) Enzyme GDP GTP CYTOPLASM 1 2 Activated enzyme Figure 11.7 Membrane receptors—G protein-coupled receptors, part 2 GTP GDP P i Cellular response 3 4

Receptor tyrosine kinases are membrane receptors that attach phosphates to tyrosines
A receptor tyrosine kinase can trigger multiple signal transduction pathways at once Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fully activated receptor tyrosine kinase
Fig. 11-7c Signaling molecule (ligand) Ligand-binding site Signaling molecule  Helix Tyr Tyr Tyrosines Tyr Tyr Tyr Tyr Tyr Tyr Tyr Tyr Tyr Tyr Tyr Tyr Tyr Tyr Tyr Tyr Receptor tyrosine kinase proteins Dimer CYTOPLASM 1 2 Activated relay proteins Figure 11.7 Membrane receptors—receptor tyrosine kinases Cellular response 1 Tyr Tyr P Tyr Tyr P Tyr Tyr P P Tyr Tyr P Tyr Tyr P Tyr Tyr P P Cellular response 2 Tyr Tyr P Tyr Tyr P Tyr P Tyr P 6 ATP 6 ADP Activated tyrosine kinase regions Fully activated receptor tyrosine kinase Inactive relay proteins 3 4

A ligand-gated ion channel receptor acts as a gate when the receptor changes shape
When a signal molecule binds as a ligand to the receptor, the gate allows specific ions, such as Na+ or Ca2+, through a channel in the receptor Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

1 Signaling molecule (ligand) Gate closed Ions Plasma membrane
Fig. 11-7d 1 Signaling molecule (ligand) Gate closed Ions Plasma membrane Ligand-gated ion channel receptor 2 Gate open Cellular response Figure 11.7 Membrane receptors—ion channel receptors 3 Gate closed

Concept 11.3: Transduction: Cascades of molecular interactions relay signals from receptors to target molecules in the cell Signal transduction usually involves multiple steps Multistep pathways can amplify a signal: A few molecules can produce a large cellular response Multistep pathways provide more opportunities for coordination and regulation of the cellular response Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Protein Phosphorylation and Dephosphorylation
In many pathways, the signal is transmitted by a cascade of protein phosphorylations Protein kinases transfer phosphates from ATP to protein, a process called phosphorylation Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Phosphorylation cascade
Fig. 11-9 Signaling molecule Receptor Activated relay molecule Inactive protein kinase 1 Active protein kinase 1 Inactive protein kinase 2 ATP Phosphorylation cascade ADP Active protein kinase 2 P PP P i Figure 11.9 A phosphorylation cascade Inactive protein kinase 3 ATP ADP Active protein kinase 3 P PP P i Inactive protein ATP ADP P Active protein Cellular response PP P i

Small Molecules and Ions as Second Messengers
The extracellular signal molecule that binds to the receptor is a pathway’s “first messenger” Second messengers are small, nonprotein, water-soluble molecules or ions that spread throughout a cell by diffusion Cyclic AMP and calcium ions are common second messengers Second messengers participate in pathways initiated by G protein-coupled receptors and receptor tyrosine kinases Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Cyclic AMP Cyclic AMP (cAMP) is one of the most widely used second messengers Adenylyl cyclase, an enzyme in the plasma membrane, converts ATP to cAMP in response to an extracellular signal Many signal molecules trigger formation of cAMP Other components of cAMP pathways are G proteins, G protein-coupled receptors, and protein kinases cAMP usually activates protein kinase A, which phosphorylates various other proteins Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fig. 11-10 Adenylyl cyclase Phosphodiesterase Pyrophosphate P P ATP
cAMP AMP Figure Cyclic AMP

First messenger Adenylyl cyclase G protein GTP G protein-coupled
Fig First messenger Adenylyl cyclase G protein G protein-coupled receptor GTP ATP Second messenger cAMP Figure cAMP as second messenger in a G-protein-signaling pathway Protein kinase A Cellular responses

Calcium Ions and Inositol Triphosphate (IP3)
Calcium ions (Ca2+) act as a second messenger in many pathways Calcium is an important second messenger because cells can regulate its concentration Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

EXTRACELLULAR FLUID Plasma membrane Ca2+ pump ATP Mitochondrion
Fig EXTRACELLULAR FLUID Plasma membrane Ca2+ pump ATP Mitochondrion Nucleus CYTOSOL Ca2+ pump Endoplasmic reticulum (ER) Figure The maintenance of calcium ion concentrations in an animal cell Ca2+ pump ATP Key High [Ca2+] Low [Ca2+]

Animation: Signal Transduction Pathways
A signal relayed by a signal transduction pathway may trigger an increase in calcium in the cytosol Pathways leading to the release of calcium involve inositol triphosphate (IP3) and diacylglycerol (DAG) as additional second messengers Animation: Signal Transduction Pathways Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

EXTRA- CELLULAR FLUID Signaling molecule (first messenger) G protein
Fig EXTRA- CELLULAR FLUID Signaling molecule (first messenger) G protein DAG GTP G protein-coupled receptor PIP2 Phospholipase C IP3 (second messenger) IP3-gated calcium channel Figure Calcium and IP3 in signaling pathways Endoplasmic reticulum (ER) Ca2+ CYTOSOL

Fig EXTRA- CELLULAR FLUID Signaling molecule (first messenger) G protein DAG GTP G protein-coupled receptor PIP2 Phospholipase C IP3 (second messenger) IP3-gated calcium channel Figure Calcium and IP3 in signaling pathways Endoplasmic reticulum (ER) Ca2+ Ca2+ (second messenger) CYTOSOL

Fig EXTRA- CELLULAR FLUID Signaling molecule (first messenger) G protein DAG GTP G protein-coupled receptor PIP2 Phospholipase C IP3 (second messenger) IP3-gated calcium channel Figure Calcium and IP3 in signaling pathways Endoplasmic reticulum (ER) Various proteins activated Cellular responses Ca2+ Ca2+ (second messenger) CYTOSOL

Concept 11.4: Response: Cell signaling leads to regulation of transcription or cytoplasmic activities The cell’s response to an extracellular signal is sometimes called the “output response” Ultimately, a signal transduction pathway leads to regulation of one or more cellular activities The response may occur in the cytoplasm or may involve action in the nucleus Signaling pathways can also affect the physical characteristics of a cell, for example, cell shape Figure Nuclear responses to a signal: the activation of a specific gene by a growth factor Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Nuclear and Cytoplasmic Responses
Many signaling pathways regulate the synthesis of enzymes or other proteins, usually by turning genes on or off in the nucleus The final activated molecule may function as a transcription factor Other pathways regulate the activity of enzymes Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Growth factor Reception Receptor Phosphorylation cascade Transduction
Fig Growth factor Reception Receptor Phosphorylation cascade Transduction CYTOPLASM Inactive transcription factor Active transcription factor Figure Nuclear responses to a signal: the activation of a specific gene by a growth factor Response P DNA Gene NUCLEUS mRNA

Signal Amplification Enzyme cascades amplify the cell’s response
At each step, the number of activated products is much greater than in the preceding step Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Glucose-1-phosphate (108 molecules)
Fig Reception Binding of epinephrine to G protein-coupled receptor (1 molecule) Transduction Inactive G protein Active G protein (102 molecules) Inactive adenylyl cyclase Active adenylyl cyclase (102) ATP Cyclic AMP (104) Inactive protein kinase A Active protein kinase A (104) Figure Cytoplasmic response to a signal: the stimulation of glycogen breakdown by epinephrine Inactive phosphorylase kinase Active phosphorylase kinase (105) Inactive glycogen phosphorylase Active glycogen phosphorylase (106) Response Glycogen Glucose-1-phosphate (108 molecules)

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