cell phys exam 3 lect 22

Introduction to signaling mechanisms used by cells to influence the activity of cells and tissues at targeted locations within the body.
Signaling is broad and not merely about sending out a signal.
General overview of mechanisms and concepts covered in upcoming lectures, including detailed discussion on gap junctions and contact-dependent signaling.

Ligand: A molecule that can be of virtually any size, represented by a key binding to a specific site on a target molecule, typically a protein.
Receptor: The target of a ligand; it recognizes and responds to a signaling molecule (ligand), facilitating signal transduction. The term "receptor" was first coined in 1948.
Agonist: A type of ligand that selectively interacts with a receptor, inducing a conformational change that affects cell function and induces physiological activity.
Antagonist: A ligand that interacts with a receptor and inhibits its function, preventing agonists from affecting the receptor.
Second Messenger: Small molecules generated inside the cell as a result of receptor activation, which amplify the signaling effect and are capable of diffusing within the cell.

Gap Junctions: Allow cells to share cytoplasm and transmit electrical signals, found in various tissues (e.g., liver, heart).
- Cells communicate through direct cytoplasmic connections.

Requires direct physical contact between cells through membrane-bound signals.
Juxtacrine Signaling: A newer term describing signaling in close proximity, dependent on the presence of signaling molecules and receptors on neighboring cell membranes.

Hormonal Signaling: Molecules released into the bloodstream to affect distant targets (endocrine signaling). Various methods such as autocrine, paracrine, endocrine, and lumacrine signaling categorize the distances and methods of signaling.
- Lumacrine: Signaling within a hollow tube where the signal affects downstream locations.
- Lactocrine: Signaling molecules released into the mammary system affecting offspring.

Existence and History: Known for as long as there have been animals; first described in 1965.
Structure: Composed of connexins that cross the membrane four times. A connexin forms a connexon, and two connexons from adjacent cells connect to form a gap junction.
Functionality: Gap junctions facilitate the passage of signaling molecules and ions between cells (typically up to 1,200 Dalton in size).
- Molecules that can pass: glucose (180 Da), small ions, cyclic AMP (cAMP), and small drugs (300-800 Da).
Electrical Coupling: Important for coordinated contractions in cardiac muscle and transmitting electrical signals; arrhythmias can arise from defects in gap junctions.

Example 1: Leukocyte Diapedesis
- Involves glycoproteins on leukocytes binding to receptors on endothelial cells, leading to cells rolling along blood vessel linings before diapedesis.
Example 2: Synapse Formation
- Presynaptic cells express neurexins that interact with neuroligands on postsynaptic cells. This interaction forms a neighborhood of proteins that are crucial for neurotransmitter release.

Discusses secreted molecules, how they diffuse, their routes, and the duration of their effects. Key categories include:
- Endocrine Signaling: Hormonal signaling via hormones such as vasopressin, affecting wide bodily functions.
- Neurotransmitter Signaling: Fast-acting signaling cognate to nervous system functions, e.g., acetylcholine on muscle receptors.
- Autocoids: e.g., bradykinin affecting vascular smooth muscle relaxation.
- Cytokines: e.g., interleukin-2 in immune function.

These receptors play critical roles in various signaling pathways, including hormonal and neurotransmitter reactions.
- For vasopressin:
- V1 receptor (vascular smooth muscle): increases vascular resistance
- V2 receptor (kidney collecting ducts): increases water retention through aquaporin modulation.
Discussion around similarities between oxytocin and vasopressin, their physiological effects during parturition, and potential side effects (e.g., fluid retention).

The ongoing discussion of signaling mechanisms serves to elucidate how cells communicate and respond within the body, setting a framework for further exploration in the lectures ahead.
Importance of understanding cell signaling in the context of physiology and pathological conditions, as well as its implications for therapeutic interventions and biomedical research questions.