Cell Signaling
Types of Cell Signaling
Contact-Dependent Signaling
Explained that signals cannot move; they depend on the signaling cell and contacting other cells.
The nature of contact-dependent signaling involves the signaling cell activating the receiving cell to respond to the signal.
A receptor inside the receiving cell gets cut and part enters the nucleus to activate DNA responses.
Example: Illustration of a signaling cell and a receiving cell side by side, visual representation.
Other Types of Signaling
Three additional key types of signaling discussed:
Autocrine signaling: Signals that act on the same cell.
Paracrine signaling: Signals that act on nearby cells.
Juxtacrine signaling: Indicated as synonymous with contact-dependent signaling, where cells are adjacent.
Cell Response to Signals
General Mechanism
Receptor Activation: Involvement of conformational changes upon signal binding.
Signal Transduction: Process by which extracellular signals are transmitted to the inside of a cell through a series of enzymatic reactions.
Defined as the mechanism where information from signals is conveyed via intracellular proteins undergoing conformational changes.
Cellular Response: Three mechanisms discussed for how cells respond to signals:
Activation or inactivation of enzymes (salivary glands example with activated salivary amylase).
Example: Seeing food that you like activates salivary enzymes; vice versa for disliked food.
Changes in cytoskeletal proteins (e.g., polymerization or depolymerization of microtubules).
Changes in transcription factors affecting gene expression.
Activation of genes in maturation versus deactivation of younger-active genes.
Phosphorylation
Phosphorylation as a crucial post-translational modification that can activate or inactivate proteins depending on the context.
Discusses enzymes involved in phosphorylation:
Kinases: Enzymes that transfer phosphate groups, typically from ATP.
Phosphatases: Enzymes that remove phosphate groups.
Types of Receptors
Classification by Location
Cell Surface Receptors (Extracellular): Bind hydrophilic signals such as ions and hormones on the plasma membrane.
Important for binding signals that cannot permeate the membrane.
Intracellular Receptors: Found in cytosol or nucleus, bind mostly hydrophobic signals that can diffuse through membranes.
G-Protein Coupled Receptors (GPCR)
Defined as multipass proteins through the membrane (seven passes).
G proteins consist of three subunits (alpha, beta, gamma) that act as molecular switches.
Activation of G Proteins: Involves binding of GTP instead of GDP, prompts conformational changes allowing communication inside the cell.
Two pathways based on activated subunits (either alpha-GTP or beta-gamma dimer) activating target proteins on the membrane or cytosol.
Example of Cellular Response: Growth factor signaling leading to cellular proliferation when involved with cancer cell receptors (e.g., HER2 in breast cancer).
Ligand-Gated Ion Channels
Characterized as receptors that open in response to signal binding, allowing ion flow across the membrane.
Discussed examples, such as calcium and sodium channels, which enable swift cellular responses.
Direct Transcription Factors
Explains a mechanism for intracellular receptors where the signal binds, leading to a confirmation change and the release of an inhibitor.
Signal-bound receptor complex enters the nucleus and binds directly to DNA to initiate transcription.