Cell Signaling

Cell-Cell Communication

Cell signaling is the process by which cells communicate with each other to coordinate their activities. This is essential for the regulation of physiological processes in multicellular organisms.

• Signals can be ions, small molecules, steroids, or nonsteroid hormones.

• Most cellular responses to signals involve either:

  • Regulating gene expression (protein synthesis)

  • Activating or inactivating already existing proteins (often faster)

• Adding a phosphate group (phosphorylation) can activate or inactivate proteins.

• Signals can also be transmitted via direct cell-to-cell contact (e.g., gap junctions).

Examples of Signals

• Ions

• Small molecules

• Steroid hormones

• Nonsteroid hormones

Hormones

Hormones are chemical messengers that facilitate long-distance communication between cells, especially in animals.

• Two main communication systems in animals: nervous system (neurons) and endocrine system (hormones).

• Hormones travel via the bloodstream and can reach nearly every cell in the body.

• Despite widespread distribution, hormones only affect specific target cells that have the appropriate receptors.

Hormone Target Cells

• Each hormone or signaling molecule acts only on certain cells (target cells).

• Target cells have receptors that specifically bind the hormone or signaling molecule.

• Example: Adrenaline/epinephrine has different effects in the liver versus the heart.

Steroid Hormones

Steroid hormones are lipid-soluble molecules that can cross cell membranes and directly affect gene expression.

• Enter cells by diffusing through the plasma membrane.

• Bind to intracellular receptors.

• Activate genes and stimulate the production of specific proteins.

• Resulting proteins alter cellular activity.

• Structurally similar to cholesterol.

• Examples: Testosterone, cortisol.

• Transported in blood via carrier proteins.

Nonsteroid Hormones

Nonsteroid hormones are generally not lipid-soluble and cannot cross the cell membrane. They exert their effects by binding to receptors on the cell surface.

• Structurally similar to proteins or amino acids.

• Bind to receptors on the surface of target cells.

• Cannot cross the cell membrane.

• Trigger a cascade of events inside the cell, often involving activation of enzymes.

• Effects are typically rapid (seconds to minutes).

• Examples: Many peptide hormones.

Receptors

Properties of Receptors

Receptors are proteins that bind signaling molecules and initiate a cellular response.

• Receptors are not static; their numbers and sensitivity can change over time.

• Chronic stimulation can lead to changes in receptor number (e.g., drug tolerance).

• Some drugs (e.g., beta-blockers) can block receptors to alter physiological responses.

Signal Processing

Signal Transduction

Signal transduction is the process by which a cell converts an extracellular signal into a functional response.

• Involves conversion of the signal from one form to another (extracellular to intracellular).

• Often involves activation of a series of proteins (signal cascade).

• Signal is usually amplified during transduction.

• Signals can be diversified, leading to multiple effects from a single signal.

Common Components of Signal Transduction Pathways

• Second messengers (e.g., small molecules or ions) that activate protein kinases.

• Protein kinases: enzymes that phosphorylate (add phosphate groups to) other proteins.

• Most cell types have a large variety of protein kinases.

Phosphorylation Cascades

Phosphorylation cascades are sequences of events where one protein kinase activates another, amplifying the signal.

• Protein kinases activate/inactivate proteins by adding phosphate groups.

• Phosphorylation cascades can amplify the signal many times.

• Cascades are terminated by phosphatases, which remove phosphate groups.

Key Equations:

• Phosphorylation:

• Dephosphorylation:

Crosstalk

Cell signaling pathways often interact and integrate information, rather than acting in isolation.

• Signaling networks are complex, similar to neural networks or computer microprocessors.

• Many drugs target signaling proteins, especially G-protein coupled receptors.

• Over one-third of drugs act on G-protein coupled receptors.

Unicellular Organisms and Cell Signaling

Even unicellular organisms use cell signaling to coordinate activities, especially in populations.

• Microbiome bacteria can make and respond to signals (e.g., serotonin).

• Population density signaling is called quorum sensing.

• Quorum sensing can regulate medically relevant behaviors, such as biofilm formation.

• Example: Bacteria forming biofilms (e.g., dental plaque).

• Protists like slime molds use G-protein coupled signaling.

Summary Table: Steroid vs. Nonsteroid Hormones