Lec 15/16 Cell Signaling

Summary of Key Concepts

Speed of Responses Across Different Signaling Pathways

Cell signaling pathways exhibit varying speeds of response depending on the nature of the signal and the mechanism involved:

• Electrical signals (e.g., nerve impulses) are extremely rapid, occurring in milliseconds.

• Protein phosphorylation processes take seconds to minutes, allowing for quick but slightly slower responses compared to electrical signals.

• Gene expression changes involve protein synthesis and typically take hours, representing slower responses that lead to long-term changes in cell behavior.

Distinct Cell Responses in Analog Biological Systems

Analog biological systems can produce a variety of distinct cell responses through different signaling mechanisms:

• All-or-None response: This type of response is binary, with no intermediate states. Once a certain threshold is exceeded, the signaling pathway is fully activated.

• Oscillatory behavior: The system reacts in cycles, often seen in processes like circadian rhythms where the response is periodic.

• Adaptation: The system adjusts to ongoing stimulation, allowing cells to remain sensitive to changes in signal intensity rather than absolute concentrations.

Molecular Mechanisms Behind Circadian Rhythms and Feedback Systems

Circadian rhythms are regulated through intricate feedback systems:

• Negative feedback loops are crucial for maintaining oscillatory behaviors. Specific proteins (e.g., Kai-A, Kai-B, Kai-C) interact and phosphorylate to establish rhythms.

• Kai-C phosphorylates itself slowly, aided by Kai-A. Phosphorylated Kai-C activates Kai-B, which then inhibits further phosphorylation events, creating a negative feedback loop that regulates the circadian rhythm.

Adaptation Mechanisms in Cell Signaling

Adaptation mechanisms allow cells to remain sensitive to changes in environmental signals through various negative feedback processes:

• Receptor sequestration: Receptors are temporarily removed from the cell surface.

• Receptor downregulation: The number of receptors on the cell surface is reduced.

• Receptor inactivation: Receptors are rendered inactive.

• Inactivation of signaling proteins: Signaling proteins are deactivated.

• Production of inhibitory proteins: Proteins that inhibit signaling pathways are produced.

These mechanisms ensure that cells do not become desensitized to persistent stimuli and can continue to respond to new signals effectively.

Key Terms Integration

• Extracellular signal (primary messenger): Initiates the signaling process.

• Cell surface receptor: Detects the signal and transmits it inside the cell.

• Second messenger: Relays the signal within the cell.

• Mediators: Amplify the signal and affect cell response.

• Cell response: The final action taken by the cell (growth, differentiation, death, etc.).

Types of Signaling

• Paracrine signaling: Local action affecting nearby cells.

• Autocrine signaling: Cells respond to signals they produce themselves.

• Endocrine signaling: Long-distance signaling through hormones, leading to systemic responses.

• Contact-dependent signaling: Requires direct contact between cells.

• Synaptic signaling: Specialized form of signaling in neurons.

Feedback Mechanisms

• Positive feedback: Can lead to bistable (switch-like) behavior, creating lasting changes after the signal is removed.

• Negative feedback: Used for homeostasis, can create complex systems with simple functions. Short delays lead to transient responses; longer delays produce oscillations.