Generalized Signaling Pathway

• Release signal molecule from signaling cell

• Bind and activate receptor in target cell

• Activate intracellular signaling pathway

• Alter effector protein(s) to change cell behavior

Forms of Cell Signaling

• Paracrine: signal molecule secreted and acts locally• Contact-dependent: signal molecule remains bound

to signaling cell

• Endocrine: signal molecule secreted into bloodstream

• Synaptic: neurotransmitter acts at synaptic contacts

• Gap junctions: allow passage of small signaling molecules

• Autocrine: signaling cell signals to cells of the same type and to itself

Speed of Signaling Response

• Alterations to proteins already present occurs rapidly• New gene expression and protein synthesis occurs slowly

• Cell responds to set of signals based on its receptors• Signals act in different combinations to effect behavior of

cell

Response to Combinations of Signals

• Different receptors• Same receptor coupled to different intracellular signaling

machinery

Different Responses to Same Signal

• Ion-channel-linked: neurotransmitter controls opening of ion channel

• G-protein-linked: intracellular signaling mediated by trimeric GTP-binding protein

Types of Cell Surface Receptors

• Enzyme-linked: function as enzyme or directly associated with enzyme

• Chain of molecular interactions between receptor and target protein

• Small intracellular mediators (second messengers)

• Intracellular signaling proteins

Relaying Signal Within Cell

Some Ways for Intracellular Signaling Proteins to Relay Signal

• Generate small intracellular mediators • Induced to interact with other signaling proteins• Act as molecular switches

- Phosphorylation and de-phosphorylation- GTP binding and hydrolysis- Other mechanisms

Induced Interactions of Signaling Proteins

• Intracellular signaling propagated by inducing protein interactions• Interaction domains in signaling proteins• Adaptor proteins

Molecular Switches Regulated by Phosphorylation

• Covalent attachment of phosphate by kinases, removal by phosphatases

• Different signaling proteins can be activated by phosphorylation or de-phosphorylation

Molecular Switches Regulated by GTP Binding

• Active when GTP-bound; inactive when GDP-bound• GTP binding by guanine nucleotide exchange

factors, hydrolysis by GTPase activating proteins

Target Cell Desensitization

• Decreased response to signal at particular level following prolonged exposure

• Negative feedback with short delay• Signaling machinery modified, less responsive

Receptor-Mediated Activation of G Proteins

• Trimeric G protein ()• Inactive when GDP bound to • Activation of receptor causes

dissociation of from and exchange of GDP for GTP

• Activated mediates signaling

Turning off G proteins

• Stimulation of GTPase activity of by interaction with target signaling protein or specific RGS

• Receptors linked to Gs activate adenylyl cyclase

• Converts ATP to cAMP, which acts a small intracellular mediator• Activates cAMP-dependent protein kinase (PKA)

Signaling by cAMP

Cholera & Pertussis Toxins

• Gs subunit activates adenylyl cyclase• Cholera toxin: ADP ribosylation of Gs (); cannot

hydrolyze GTP

• Gi subunit inhibits adenylyl cyclase• Pertussis toxin: ADP ribosylation of Gi (); prevents

binding to receptors

• Receptors linked to Gq activate phospholipase C-• PI(4,5)P2 cleaved to diacylglycerol and IP3

• IP3 opens channels in ER causing influx of Ca2+ into cytosol

• Diacylglycerol activates protein kinase C (PKC)

Signaling by Inositol Phospholipids

Cleavage by Phospholipase C-

• G protein-linked receptor kinases phosphorylate activated receptor

• Arrestin binding prevents G protein interaction, promotes endocytosis

Receptor Desensitization

Ca2+ as Small Intracellular Mediator

• Involved in PKC activation• Binds calmodulin to regulate other proteins• Activates CaM-kinases• Activates calcineurin, a phosphatase,

whose substrates include NFAT

Calcineurin-NFAT Signaling in T Cells

• Calcineurin dephosphorylates, activates NFAT

• NFAT turns on genes for T cell activation

• Cyclosporin inhibits calcineurin and thereby NFAT

Signaling Through Enzyme-Linked Receptors

• Ligand binds to extracellular side of receptor• Activates enzyme activity present or

associated with cytoplasmic side

– Receptor tyrosine kinases– Tyrosine kinase associated receptors– Receptor tyrosine phosphatases– Receptor serine/threonine kinases– Receptor guanylyl cyclases

• Classes:

Receptor Tyrosine Kinases

Examples of RTK subfamilies

• Ligand-induced receptor dimerization/oligomerization• Cross-phosphorylation of receptors

Activation of Receptor Tyrosine Kinases

Recruiting Intracellular Signaling Proteins

• Tyrosine phosphorylation generates binding sites for specific signaling proteins

• Most proteins that bind to activated receptor have SH2 domains

Examples of SH2-Containing Proteins

• Enzymes, for example phospholipase C-γ• Proteins that function as adaptors

Receptor-Mediated Activation of Ras

• Ras is monomeric GTP binding protein• Grb-2 adaptor has SH2 domain that binds activated receptor• Grb-2 also binds Ras GEF, which activates Ras

• Chain of three protein kinases that are sequentially activated• Ras activates first kinase in chain

MAP Kinase Pathway

• Diffuses across plasma membrane from signaling cell into target cell• NO activates guanylyl cyclase in smooth muscle to cause relaxation• Viagra inhibits cGMP phosphodiesterase

Signaling by Nitric Oxide

• Ligand-activated gene regulatory proteins• Ligands diffuse through plasma membrane,

include steroid hormones, thyroid hormone, retinoic acid

Signaling by Nuclear Receptors

Selective Estrogen Receptor Modulators

• Antagonists in some cell types and agonists in others• Tamoxifen as breast cancer therapy• Raloxifene to help prevent osteoporosis