BIOCHEM 2.7 - SIGNAL TRANSDUCTION

• Signal Transduction is the focus
• Signal mediation and transduction down a pathway

Objectives

• Identify signaling mediators for receptors or pathways

• Emphasis is on G-coupled protein receptors, inositol triphosphate, integrin signaling, PI3K, and insulin signaling

• Recognize steps leading to signal amplification

• Explain how low levels of ligand lead to biological responses

• Signal transduction is collaborative or concomitant

• Summarize signal transduction mechanisms like coincident, gated, and feedback

• Assess consequences of changes in initiation, propagation, or termination/reset of a pathway

• Considering ligands, receptors, mediators, or transcription factors

• Three major components to all signaling pathways can cause adverse consequences if altered

Basics of Signal Propagation

• Amplification happens during this signaling
• Receptors trigger mediators, which trigger additional mediators
• Mediators include proteins, ions, and phospholipids

GPCR: cAMP Signaling Pathway

• Step 1: Unoccupied receptor does not interact with Gs protein.
• Step 2: Occupied receptor changes shape interacting with Gs protein, which releases GDP and binds GTP.
• Step 3: Alpha subunit of Gs protein dissociates activating adenylyl cyclase.
• Step 4: When hormone is no longer present, the receptor reverts to its resting state
• GTP on the alpha subunit is hydrolyzed to GDP, and adenylyl cyclase is deactivated

Metabolism of cAMP

• cAMP is involved in propagation, can also be defined as signal transduction Propagation is amplification step
• Low levels of ligand activity trigger receptors
• Mediators/signal transducers increase signal at each cascade stage
• Process: intramolecular attack of 3' hydroxyl group of the ribose
• Against the alpha-phosphoryl group of ATP, phosphodiester bond
• Termination via hydrolysis of cAMP to 5'AMP by cAMP specific phosphodiesterases

Protein Kinase A

• Protein Kinase phosphorylates proteins on serine and threonine residues
• Protein Kinase binds cAMP
• Functions as a mediator
• Regulates activity of ion channels and activity of phosphatases that regulate signaling
• Protein Kinase A activates molecules like CREB -Recruits coactivator proteins and induces transcription
• Protein Kinase Also regulates depending on the activation to set the correct scene
• cAMP activates Protein Kinase to either do other jobs or express transcription factors

Balancing Stimulation and Inhibition

• Stimulation and inhibition can be on or off depending on the product

• Inhibitory G-proteins function like stimulatory G-proteins

• Alpha subunit inhibits adenylate cyclase activation

• Critical balance between Stimulation and Inhibition must be found

• Pertussis toxin prevents the inhibitory pathway from functioning

• Cholera toxin prevents GTPase activity, preventing GTP to GDP exchange

GPCR: IP3 Signaling Pathway

• Step 1: Hormone binds to a specific receptor

• Step 2: Occupied receptor interacts with Gq protein.

• Step 3: Alpha Subunit of Gq releases GDP and binds GTP.

• Step 4: Alpha Subunit of Gq dissociates from Beta + alpha subunits activating phospholipase C.

• Step 5: Active phospholipase C cleaves phosphatidylinositol 4,5-bisphosphate (PIP2) to inositol trisphosphate (IP3) and diacylglycerol.

• Step 6: IP3 binds to a specific receptor on the endoplasmic reticulum, causing the release of sequestered Ca2+.

• Step 7: Calcium and diacylglycerol activate protein kinase

• Step 8: Protein Kinase C catalyzes phosphorylation of cellular proteins that mediate cellular responses

• IP3 controls Ca++ Channels

• DAG stays on the membrane and activates protein kinase C

Inositol Signaling and Phospholipase C

• Triggering second messengers
• Inositol 1,4,5-triphosphate (IP3) triggers intracellular calcium stores
• Diacylglycerol (DAG) activates protein kinase C
• You want to trigger channels when there is a conc gradient for Ca+2 to to move down , otherwise the activity Doesn't mat

Activity of Mediators

• Inositol Triphosphate acts as the ligand for the ligand gated calcium Channel
• Diacylglycerol (along with calcium) activates PKC

Signaling and Nitric Oxide Synthase

• Production of reactive nitrogen species is triggered by calcium via calcium dependent nitric oxide synthase (NOS)
• Calcium binds to the calmodulin domain in the calcium-dependent NOS
• Binding to calmodulin results in shifts to the corresponding protein which initiates activation

Insulin Signaling

• Insulin receptor is a tyrosine kinase
• Multiple mediators can be triggered based on specific phosphorylation patterns
• Insulin signaling can be growth stimulatory (mitogenic) in nature via Ras signaling
• Phosphatidylinositol3-kinase (PI3K) signaling leads to downstream activation of serine/threonine kinase via activation of AKT
• AKT mediates metabolic effects of insulin in muscle, liver, and adipose

Integrins

• Signaling through integrins enables the ECM to influence multiple aspects of cellular behavior
• Downstream effects: cellular changes in migration, proliferation, and differentiation
• Signaling can be self-promoting: changes in gene expression for more integrins to increase the number of receptors Signaling mediation overlaps with other/growth stimulatory signaling
• Plays a vital role in wound healing

Collaborative and Concomitant Signaling

• Integrins demonstrate collaboration and concomitant signaling
• Concomitant = signal independently to trigger the same molecule
• Pathways include: Ras-MAPK, PI3K-Akt, AKT (protein kinase B), and Rho
• Collaboration = work together or one activates the other
• Integrins enable growth factor-dependent signaling of the same molecule

General Mechanisms Review

• Coincident mechanisms converge at a single unit that leads to output
• Detector differentiates between the pathways and achieve desired outcome
• Gated mechanisms represent a regulatory control of one pathway on another
• The actions of one pathway contribute to or regulate the other
• Modifies response in a cell-type dependent manner
• Feedback = modified gated mechanism

Explore signal transduction pathways, focusing on G-coupled protein receptors, inositol triphosphate, and integrin signaling. Learn to identify signaling mediators and understand signal amplification. Investigate how changes in pathway components can lead to adverse consequences.