Intracellular Signal Transduction Pathways and Cascades

Outline : I. Review cell to cell communication II. Ligands III. Receptors IV. Activation of Intracellular signaling pathways and production of second messengers V.Effector Proteins VI.Mechanisms to terminate signal transduction Main Concepts 1- Receptors and ligands mediate signal transduction 2-Signals can be amplified by the activation of multiple down stream pathways: ex. cAMP, IP3, PIP3 3-Signal transduction results in a change in cellular function- ex. Ion channel function, cytoskeletal organization (cell shape), or gene expression 4- Termination of signaling: desensitization, internalization, downregulation

Signal Transduction- The process of converting a signal from outside the cell to a functional change within the cell

Two Main Types of Cell-Cell Communication Direct: Gap Junctions Indirect: Receptors and Signal Transduction

Defined by: Where the signal originates Where the signal is released (distance travelled) Location of the target cells What types of cells are signaling

What is the Signal? Chemical communication between cells Molecule (ligand) that will bind to a receptor. Intended to be amplified to elicit a response

Three classes of cell signaling molecules

Receptors propagate the SIGNAL across the cell membrane Intracellular Receptors: Ligand must be hydrophobic and able to pass directly through plasma membrane Ex. estrogen Cell Surface Receptors: Ligands can be either hydrophilic or hydrophobic Three major classes defined by mechanism used to transduce ligand-binding into intracellular signaling events.

Receptors are proteins that mediate a biological change following ligand binding. Ligand + receptor [Ligand-receptor complex] Non-covalent Interactions (reversible) Ligand binding site(s) on the outside (extracellular domain) and sites for protein-protein interactions on inside (cytoplasmic domain). High affinity and selectivity for their ligands. Function via signal transduction, though by very different molecular mechanisms. Can regulate ion channels and therefore affect the membrane potential or excitability of neurons. Have mechanisms for desensitization. Are regulated by phosphorylation. Cell surface receptors are membrane spanning proteins that undergo allosteric changes in response to ligand binding.

How well an agonist can activate a receptor once it is bound- refers to response Efficacy How strongly a ligand binds to the receptor- Equilibrium dissociation constant = K D Affinity Specificity = Selectivity How well a receptor can distinguish among different ligands Receptor Properties Reversibility Location = Localization A ligand binds to its the receptor through non-covalent interactions Where is the receptor localized in the cell, on the plasma membrane or in intracellular vesicles. Where in the plasma membrane-synaptic or extrasynaptic

G Protein Coupled Receptors: GPCRs Largest family of cell surface receptors with >1000 genes (a lot of these are odorant receptors) Targets of the majority of therapeutic drugs (over 50% of all prescription pharmaceuticals on the market) Core structure: 7 transmembrane  -helices (extracellular N-term, intracellular C- term) Respond to a massive number and variety of ligands Divergent ligands –Photons-Retinal; small organic molecules; neurotransmitters and neuromodulators; glycoproteins; hormones GPCRs have mechanisms for signal amplification, can generate second messengers, and regulate protein kinases and protein phosphorylation.

GPCRs (metabotropic receptors) compared to ligand gated ion channels (LGICs; ionotropic receptors) Slow speed: (0.1 sec to min to hours) Signal transduction results in a change in cellular function- including ion channel function, cytoskeletal organization (cell shape) or gene expression. Mostly modulatory, rather than direct mediation of rapid synaptic response. i.e. neuromodulation. Receptors can be presynaptic, postsynaptic, or on the cell body Targets and effects Amplification

Why Target Molecules? Receptors propagate the signal across the postsynaptic cell membrane Intracellular signaling cascades propagate and amplify the signal that results in phosphorylation (or dephosphorylation) of target molecules

What Target Molecules? Kinases – – Transfer phosphate groups from ATP to substrates to activate them (phosphorylate) Phosphatases – – Remove inorganic phosphates from activated molecules (dephosphorylate) Kinase cascade

EVERYBODY STAND, PLEASE!! Effector Second messenger Later effector

What Responses? Transcriptional modification Activation (or inhibition) of surface receptors – Ion channels – Ion pumps – Neurotransmitter receptors Triggering of intracellular events – Ca 2+ release from intracellular stores (ER)

Figure 15-1 Molecular Biology of the Cell (© Garland Science 2008) Channel or transporter altered [ion] membrane potential Intracellular cascades amplify the signal, providing extremely high sensitivity. The activation of only a few receptors can produce a response Amplification allows for the induction of responses in cells with low density of receptors or the induction of responses at low concentration of signaling molecules

Second messengers contribute to regulation of cellular proteins by phosphorylation/dephosphorylation

Presynaptic receptors-inhibit (a few enhance) neurotransmitter release Development-Neurite outgrowth—Go family, CB1 receptor Dendritic spine remodeling and postsynaptic organization Neuroprotection—A1 and A2 adenosine receptor, a 2A -AR Plasticity –muscarinic receptors, learning & memory Behavior –dopamine, serotonin, opioid, adrenergic Examples of neuronal functions regulated by GPCRs This huge variety of functions suggests that there might be more than one type of GPCR signaling cascade

Specific GPCR cascades

Receptors and heterotrimeric GPCRS

Mechanisms of protein kinase activation

One Major Target of cAMP: cAMP Dependent Protein Kinase (PKA) Consensus Sequence RRXS/TX

Downstream Targets of PKA

PKA regulates CREB and Transcription HAT

Regulation of tyrosine hydroxylase by protein phosphorylation

GPCR regulation: desensitization and down-regulation LE RE How is GPCR signaling terminated?

Main Concepts 1- Receptors and ligands mediate signal transduction 2-Signals can be amplified by the activation of multiple down stream pathways: ex. cAMP 3-Signal transduction results in a change in cellular function- ex. Ion channel function, cytoskeletal organization (cell shape) or gene expression 4- Termination of signaling: desensitization, internalization, downregulation