Cell Signaling Vocabulary

Cell Signalling Overview

• What is Cell Signalling?

  • Molecular mechanisms by which cells:

    • Detect external stimuli

    • Respond to stimuli

    • Send messages to other cells

Signal Transduction

• Definition:

  • The conversion of an impulse or stimulus from one physical or chemical form to another.

  • Example: Your phone detects a radio signal and converts it into sound.

• Process:

  • Begins when a receptor on the target cell receives an extracellular signal.

  • Produces intracellular signalling molecules that alter cell behaviour.

Types of Cell Signalling

1. Endocrine Signalling

• Characteristics:

  • Signal molecules (hormones) produced in endocrine cells, secreted into the bloodstream.

  • Examples:

    • Insulin: Regulates glucose uptake, secreted by pancreatic β cells.

    • Steroid Hormones: Estradiol (from ovaries), testosterone (from testes).

• Hormone Definition:

  • An extracellular signal molecule secreted and transported in the bloodstream to induce effects on target tissues.

2. Paracrine Signalling

• Characteristics:

  • Signal molecules diffuse locally through extracellular fluid.

  • Act as local mediators on nearby cells.

• Examples:

  • Histamine: Induces inflammation by making blood vessels leaky.

  • Epidermal Growth Factor (EGF): Stimulates cell proliferation.

3. Neuronal Signalling

• Characteristics:

  • Messages delivered quickly and specifically to target cells via electrical signals along axons.

  • Release neurotransmitters at synapses.

• Examples of Neurotransmitters:

  • Acetylcholine: Excitatory at nerve-muscle synapses.

  • GABA: Inhibitory in the central nervous system.

4. Contact-Dependent Signalling

• Characteristics:

  • Short-range signalling through direct cell contact.

  • No secreted molecules are required.

• Example:

  • Delta molecule interacts with target cell receptors during embryonic development, influencing differentiation.

Communication Analogies

• Endocrine Signalling: Like broadcasting information over radio.

• Paracrine Signalling: Similar to posting flyers locally.

• Neuronal Signalling: Equivalent to a phone call for long-distance, precise communication.

• Contact-Dependent Signalling: Comparable to a face-to-face conversation.

Extracellular Signal Molecules

• Classes:

  • Large, hydrophilic molecules cannot cross the plasma membrane, binding to cell surface receptors.

  • Small, hydrophobic molecules can pass through the membrane and bind to intracellular receptors.

Responses to Signalling

• Variability:

  • The same signal can induce different responses in different cell types, depending on downstream effectors and pathways.

• Combinations of Signals:

  • Cells may require multiple signals to survive or to grow and differentiate.

  • Lack of survival signals can lead to apoptosis (cell suicide).

Speed of Cellular Response

• Fast Responses:

  • Changes in metabolism, secretion, and movement occur quickly.

• Slow Responses:

  • Changes in gene expression and protein synthesis that affect differentiation or growth occur slowly.

Cell-Surface Receptors

• Function:

  • Detect signals outside the cell and relay them across the membrane, initiating the signal transduction pathway.

  • Ligands refer to signal molecules that bind to receptors.

Intracellular Signalling Pathways

• Functions:

  • Relay signals, amplify responses, detect and integrate multiple signals, distribute signals to various effector proteins, engage in feedback regulation.

Feedback Regulation

• Importance:

  • Adjusts responses to signals.

  • Positive Feedback: Enhances response to the signal.

  • Negative Feedback: Inhibits earlier components in the pathway.

Molecular Switches in Signalling

• Types:

  • Activated by phosphorylation or GTP binding.

• Phosphorylation:

  • Kinases add phosphate to activate, while phosphatases remove phosphate to deactivate.

• GTP-binding Proteins:

  • Activated by exchanging GDP for GTP, switch off by hydrolyzing GTP to GDP.

Types of Cell-Surface Receptors

1. Ion-Channel-Coupled Receptors

• Open in response to signal molecules, altering membrane potential and generating electrical currents.

2. G-Protein-Coupled Receptors (GPCRs)

• Activated by signal molecules, interact with G proteins to turn enzymes on or off.

3. Enzyme-Coupled Receptors

• Act as enzymes or associate with other enzymes, activating intracellular enzymatic activity upon ligand binding.

Summary of Cell Signalling

• Four key types: Endocrine, paracrine, neuronal, contact-dependent.

• Cell surface receptors and intracellular receptors play crucial roles.

• Intracellular signalling pathways are essential for many cell functions.

Cell Signalling Overview

What is Cell Signalling?

Cell signalling encompasses the molecular mechanisms through which cells detect external stimuli, respond to these stimuli, and communicate with other cells in their environment. This process is crucial for maintaining homeostasis, facilitating growth, and orchestrating numerous physiological processes in multicellular organisms.

Signal Transduction

Definition:

Signal transduction is the intricate process of converting an impulse or stimulus from one physical or chemical form to another. This mechanism ensures that internal and external signals are effectively communicated within and between cells, enabling appropriate cellular responses.

Example:

An illustrative example is how a mobile phone detects a radio signal and converts it into auditory information, highlighting how an external signal can be transformed into a response.

Process:

This process begins when a receptor, typically a protein on the target cell's surface, binds to an extracellular signal (ligand). This binding initiates a cascade of intracellular signalling events that ultimately alters the target cell's behavior and function.

Types of Cell Signalling

1. Endocrine SignallingCharacteristics:In this type of signalling, signal molecules (hormones) are synthesized in endocrine cells, released into the bloodstream, and transported throughout the body to distant target tissues.Examples:

   • Insulin: A vital hormone that regulates glucose uptake, secreted by pancreatic β cells in response to rising blood sugar levels.

   • Steroid Hormones: These include sex hormones such as estradiol (produced by ovaries) and testosterone (produced by testes), playing critical roles in reproductive and developmental processes.Hormone Definition:Hormones are extracellular signal molecules that, once secreted, travel via the bloodstream to specific target tissues, where they induce a variety of physiological effects.

2. Paracrine SignallingCharacteristics:Paracrine signals are local mediators that diffuse through the extracellular fluid to affect nearby cells rather than traveling through the bloodstream.Examples:

   • Histamine: Released during allergic reactions and inflammation, histamine increases vascular permeability and causes blood vessels to leak, contributing to swelling and redness.

   • Epidermal Growth Factor (EGF): This factor promotes cell proliferation and differentiation, crucial in wound healing and tissue regeneration.

3. Neuronal SignallingCharacteristics:Neuronal signalling transmits messages rapidly and specifically to target cells through electrical signals traveling along axons.Neurotransmitters are released at synapses, facilitating communication between neurons and other cell types.Examples of Neurotransmitters:

   • Acetylcholine: Plays an excitatory role at nerve-muscle junctions, crucial for muscle contraction.

   • GABA (Gamma-Aminobutyric Acid): Functions as the main inhibitory neurotransmitter in the central nervous system, helping to control neuronal excitability.

4. Contact-Dependent SignallingCharacteristics:This type of signalling requires direct contact between neighbouring cells, facilitating communication without the need for secreted molecules.Example:The Delta molecule interacts with target cell receptors during embryonic development, influencing cell differentiation and tissue patterning.

Communication Analogies

• Endocrine Signalling: Comparable to broadcasting information over a radio; signals reach distant targets affecting widespread areas.

• Paracrine Signalling: Similar to posting flyers in a community; messages are intended for nearby areas and affect local cells.

• Neuronal Signalling: Like a phone call offering direct and immediate communication to a specific individual without interference.

• Contact-Dependent Signalling: Analogous to having a face-to-face conversation, where messages are conveyed through direct interaction.

Extracellular Signal Molecules

Classes:

Extracellular signal molecules can be categorized based on their properties:

• Large, Hydrophilic Molecules: These molecules cannot cross the plasma membrane and depend on binding to cell surface receptors to exert their effects.

• Small, Hydrophobic Molecules: These can penetrate the plasma membrane, allowing them to bind to intracellular receptors, influencing gene expression directly.

Responses to Signalling

Variability:

The same extracellular signal can evoke different cellular responses depending on the specific target cell type, available intracellular machinery, downstream effectors, and signalling pathways involved.

Combinations of Signals:

Cells often require multiple signals for survival, growth, and differentiation. The absence of essential survival signals can trigger programmed cell death, known as apoptosis.

Speed of Cellular Response

Fast Responses:

These responses involve immediate changes in cellular functions, such as metabolism, secretion of molecules, and cell movement, which can occur within seconds or minutes.

Slow Responses:

Conversely, slow responses relate to alterations in gene expression and subsequent protein synthesis, impacting processes like differentiation and growth that typically take minutes to hours.

Cell-Surface Receptors

Function:

Cell-surface receptors are critical for detecting external signals and transmitting them across the plasma membrane to initiate the signal transduction cascade that will lead to a definitive cellular response.

Ligands:

A ligand is a signal molecule that specifically binds to a receptor to trigger its activity, activating the receptor and setting off downstream processes.

Intracellular Signalling Pathways

Functions:

Intracellular signalling pathways are essential for various aspects of cellular function, including:

• Relaying signals from receptors to effector proteins

• Amplifying responses to ensure a robust reaction to a stimulus

• Detecting and integrating multiple incoming signals

• Distributing signals to multiple effector proteins

• Engaging in feedback regulation to fine-tune responses efficiently.

Feedback Regulation

Importance:

Feedback regulation is a fundamental component of cell signalling that ensures cells can adjust their responses appropriately.

• Positive Feedback: Strengthens and amplifies the response to an initial signal, often seen in processes like blood clotting.

• Negative Feedback: Inhibits earlier components in the signalling pathway, serving as a mechanism for maintaining homeostasis and preventing overactivity.

Molecular Switches in Signalling

Types:

Molecular switches are critical in controlling signalling pathways and are typically activated through:

• Phosphorylation: Kinases add phosphate groups to target proteins to activate them, while phosphatases remove these phosphate groups, thus deactivating the proteins.

• GTP-binding Proteins: These proteins are activated by exchanging GDP for GTP; they switch off through hydrolyzing GTP back to GDP, thus ensuring precise control over cellular signals.

Types of Cell-Surface Receptors

1. Ion-Channel-Coupled Receptors: These receptors open in response to ligand binding, leading to changes in membrane potential and generating electrical currents, critical for rapid signalling in nervous and muscle tissue.

2. G-Protein-Coupled Receptors (GPCRs): A large family of receptors activated by diverse signal molecules that interact with G proteins to modulate the activity of various downstream enzymes, influencing multiple physiological processes.

3. Enzyme-Coupled Receptors: These receptors either have intrinsic enzymatic activity or associate with other enzymes, triggering intracellular enzymatic activity upon ligand binding, crucial for many growth and differentiation signals.

Summary of Cell Signalling

The process of cell signalling is multifaceted, encompassing four principal types: endocrine, paracrine, neuronal, and contact-dependent signalling. Cell surface receptors and intracellular receptors play vital roles in detecting signals and initiating the necessary cellular responses. The intricate networks of intracellular signalling pathways are paramount for regulating diverse cell functions, ensuring appropriate physiological outcomes in response to ever-changing internal and external environments.