Cell Communication Processes

Cell Communication

• Cell communication is a crucial process for coordinating cellular activities.
• Different types of cell communication exist, each with a unique mechanism.

Types of Cell Communication

• Endocrine: Signaling molecules (hormones) travel through the bloodstream to target cells.
• Paracrine: Signaling molecules act locally on nearby target cells.
• Neuroal: Neurotransmitters are released at synapses to transmit signals between neurons and target cells.
• Contact-dependent: Signaling molecules are bound to the surface of signaling cells, interacting with receptor proteins on the surface of target cells.

Contact-Dependent Signaling Detail

• Membrane-bound inhibitory signal molecule (Delta) interacts with the receptor molecule (Notch) of nearby cells.
• This can inhibit certain developmental processes or prevent cells from growing in certain ways.

One Transmitter System

• Acetylcholine is involved in different cell types with differing actions.
• Heart muscle: Decreases rate and force of contraction.
• Salivary gland cell: Secretion.
• Skeletal muscle cell: Contraction.
• Acetylcholine's function varies depending on the target cell's receptor.

Different Signals Integrated

• Diverse signals combine to influence cellular decisions.
• Signals can promote survival, division, differentiation, or death.

Typical Signaling Cascade

• Extracellular signal molecule binds to a receptor protein.
• Intracellular signaling proteins relay the signal.
• Target proteins produce a cellular response (e.g., altered metabolism, gene expression, cytoskeletal movement).

Structuring the Signaling Process

• Relay: Signal is passed from one molecule to the next.
• Modulation: Strength or direction of signal can be adjusted.
• Amplification: Initial signal is amplified to elicit an enhanced cellular response.
• Divergence: A single signal can trigger multiple responses.

Different Types of Receptors

• Cell-surface receptors bind to hydrophilic signaling molecules.
• Intracellular receptors bind to hydrophobic signaling molecules, allowing them to pass through the cell membrane.

Cell Membrane Permeable Ligands of Cystolic Receptors

• Examples including cortisol, estradiol, testosterone, and thyroxine are identified as ligands that can permeate cell membranes and interact with intracellular receptors.

Steroid Receptor-Mediated Signaling

• Steroid hormones (like cortisol) bind to intracellular receptors.
• The hormone-receptor complex moves into the nucleus.
• The complex alters gene expression, activating specific genes.

Types of Receptors (summary)

• Ion channel-linked receptors: Open or close channels in response to specific signals, influencing ion flow.
• G-protein-linked receptors: Coupled to G proteins, which activate enzymes, producing second messenger molecules.
• Enzyme-linked receptors: Have an enzyme domain that is activated after binding to a ligand, initiating a phosphorylation cascade.

Role of Phosphorylation as an Activity Switch

• Phosphorylation can activate or deactivate target proteins and initiate cascades.
• GTP binding proteins also act as switches by activating cellular responses.

G-protein Coupled Receptor - 7 Transmembrane Receptor

• A common type of cell-surface receptor involved in many signaling pathways.
• The receptor spans the cell membrane seven times.
• It couples to G proteins, which can activate or inactivate various downstream targets.

GPCR - G-protein Function Steps

• The receptor binds a signal molecule, triggering a conformational change.
• The altered receptor activates a G protein.
• The activated G protein can activate various downstream effectors.

GPCR Function (target protein focus)

• The activated alpha subunit of a G protein dissociates from the beta and gamma subunits, activating the target protein.
• The activated alpha subunit hydrolyses GTP to GDP, inactivating the subunit and allowing it to re-associate with the beta-gamma complex, ending the signaling cascade.

GPCR - G-protein Function: G-protein coupled receptor and enzyme function

• Example cascades using acetylcholine and K+ channels are featured and provide specific examples of how signaling pathways can initiate diverse effector functions.

Synthesis and Breakdown of cAMP Second Messenger

• Adenylyl cyclase converts ATP to cAMP (a second messenger) in response to a signal.
• cAMP activates protein kinase A (PKA), initiating a signaling cascade.
• cAMP is broken down to AMP by phosphodiesterase, terminating the signal.

Genomic and Direct Routes of Signaling

• Extracellular signals can alter protein function quickly.
• Signals also can change gene expression for slower, more lasting effects.
• Both genomic and direct pathways are used depending on the signal and the cell response.

GPCR - PKA Transcription Example

• This presents a specific example.
• A signal activates adenylyl cyclase, leading to a cAMP increase.
• cAMP activates PKA, which phosphorylates proteins (transcription factors) in the nucleus.
• PKA affects target gene expression, causing a cellular response.

GPCR - PLC- Ca²⁺ Example

• Signalling pathway that involves inositol phospholipid (IP₃).
• Ligand binding starts the pathway resulting in increased cytosolic calcium levels.
• The increase in calcium triggers numerous downstream effects.

Special Anatomy of a Photoreceptor Cell in the Eye

• Photoreceptor cells in the eye have a specific structure.
• An outer segment contains membranes with rhodopsin (light-sensitive protein).
• Light interacting with rhodopsin triggers a cascade, converting light information into neural signals.

Amplification in Phototransduction

• A single photon of light can trigger a large response in photoreceptor cells.
• The amplification is mediated by several cascading steps.
• These steps increase the sensitivity of the visual system.

Receptor Tyrosine Kinase Signaling

• Ligands binding to receptor tyrosine kinase (RTK) proteins activates the receptor intracellular signaling.
• The activated receptor tyrosine kinases trigger a series of cellular responses.

Ras-MAP Kinase Pathway

• The diagram highlights the multi-step activation leading from the initial receptor activation to changes in protein activity and gene expression..
• Key molecules and their functions are visible, leading to a comprehensive understanding of the pathway.

Mutant Receptors Example

• Specific mutations in receptor structure can alter the effectiveness of signal cascades.

Introducing Specific Mutations and Defining the Order of Proteins in the Pathway

• Mutations in proteins in the pathway can block signaling if mutated.
• This aids in understanding what order proteins need to be in to trigger the cascade.

Mutant Ras Causes Continuous Activation

• Constitutively active Ras can constantly transmit the signal even without an external signal present. This has implications for potential uncontrolled cellular responses.

Cytokine Receptor Signaling

• Different signalling pathway initiated by cytokine receptors, not RTK or G-protein mediated examples
• The pathway has specific steps and components.

Growth Factor Signaling

• Different signalling pathways initiated by TGF-β receptor, not RTK or G-protein mediated examples.
• The pathway has specific steps and components.

Signals from Different Inputs Integrate

• Diverse signalling inputs can converge on common pathways.
• Multiple inputs can converge on the same target gene regulatory molecules.
• This can unify the outcomes of multifaceted cellular stimuli.

Explore the intricate mechanisms of cell communication in this quiz. Learn about the various types of signaling such as endocrine, paracrine, and contact-dependent signaling. Understand how these processes are essential for coordination of cellular activities.