Introduction to Homeostasis and G-Protein Signaling

Etymology and Core Definitions of Homeostasis

• Etymology of the Term:

  • Derived from the Greek words: "Homeo" (meaning same) and "stasis" (meaning to stay the same).

  • Definition: Homeostasis is the ability of an organism to maintain stable internal conditions despite changes occurring outside the body or within the body.

• The Concept of Mechanism:

  • A mechanism is defined as a process, pathway, or function that occurs at a physical or chemical level as a response to environmental changes.

  • Mechanisms are essential for maintaining life-sustaining processes including survival, growth, and development.

• Dynamic Equilibrium and Set Points:

  • Homeostasis is not a static state but a dynamic equilibrium.

  • It involves a limited range of deviations that the body can tolerate from an average value known as a set point.

  • If a deviation from the set point becomes too great, homeostatic mechanisms are activated to bring the body back toward that set point.

• Interconnectivity of Systems:

  • While anatomy and physiology often "chunk" the body into organ systems for easier understanding, all systems and organs interact continuously.

  • A key feature of this continuity is the role of fluids, specifically extracellular fluid (ECF) and interstitial fluid.

  • These fluids bathe all cells and organs, acting as the medium for communication and coordination via processes such as diffusion, osmosis, and various transport mechanisms.

General Model of Homeostatic Control

• Flow of Homeostatic Response:

  1. Organism is in a state of homeostasis.

  2. An internal or external change occurs.

  3. A deviation from the homeostatic set point is produced.

  4. The body initiates compensation mechanisms.

  5. Success: If compensation is successful, the body returns to homeostatic levels.

  6. Failure: If compensation fails, the result is disease, illness, or sickness.

Negative Feedback Mechanisms

• Definition and Function:

  • Negative feedback is the primary process used to monitor the internal environment and detect deviations.

  • The term "negative" refers to the fact that the response decreases or opposes the initial stimulus, effectively turning off the loop once balance is restored.

  • It serves a corrective nature to keep parameters close to the set point.

• Compotents of the Negative Feedback Loop (Reflex Pattern):

  • Stimulus: A deviation from the homeostatic set point.

  • Receptor (Sensor): Detects the change/stimulus.

  • Integration Center (Control Center): Typically the central nervous system, endocrine system, or a local integrator. It processes the information and generates an output.

  • Effector: A system (like muscles or glands) that carries out the response.

  • Response: The adaptive output that brings the body back into balance.

• The "Teeter-Totter" Analogy:

  • Homeostasis acts like a teeter-totter in a balanced position. A stimulus tips the balance; the integration center detects this and triggers an effector to move the teeter-totter back to the center.

Examples of Negative Feedback

• Temperature Regulation:

  • Heat Response: If internal activity (exercise) or external heat increases body temperature, thermal receptors detect the change. The hypothalamus (monitoring center) triggers the autonomic nervous system to produce sweat. Evaporation of sweat lowers skin temperature to restore balance.

  • Cold Response: If there is a decrease in temperature, the hypothalamic thermoregulatory center is activated. This may trigger shivering (to generate heat) and increased metabolism to raise internal body temperature.

• Oxygen Regulation (Erythropoiesis):

  • Stimulus: Hypoxia (low oxygen in tissues) or hypoxemia (low oxygen in the blood).

  • Causes: Intense exercise, high altitudes (e.g., visiting Aspen, Colorado, where atmospheric oxygen concentration is lower), or hypoventilation.

  • Mechanism: Specific cells in the kidney (and to a lesser extent, the liver) detect the low oxygen levels and stimulate bone marrow to increase red blood cell production.

  • Result: Increased red blood cells contain more hemoglobin, a protein that binds to and carries oxygen, thereby restoring homeostatic oxygen delivery to tissues.

Positive Feedback Mechanisms

• Definition and Function:

  • A self-amplifying cycle where an initial stimulus evokes a response that further increases the stimulus.

  • This leads to rapid changes or "switching" the body to a different state.

  • Because they are self-accelerating, they can lead to a "vicious cycle" and destabilize the body if unchecked; they usually require an external event to shut them down.

• Examples of Positive Feedback:

  • Economic Example: Prices rise, increasing the cost of living. This leads to increased wages, which increases production costs, which then further increases prices.

  • Blood Clotting: Triggered by damage to a blood vessel. It involves an exponential increase in activity from zero to quickly form a clot and prevent hemorrhage.

  • Childbirth:

    1. The baby's head pushes against the cervix.

    2. A sensory signal is sent to the hypothalamus.

    3. The hypothalamus stimulates the release of oxytocin.

    4. Oxytocin causes uterine contractions.

    5. Contractions push the baby harder against the cervix.

    6. The cycle continues until the baby is born (the external event that stops the loop).

Energy Molecules and Intracellular Signaling

• Purines:

  • Adenine and Guanine are the two purines (double-ring structures).

  • ATP (Adenosine Triphosphate): The primary energy currency of the cell. High energy is stored in the bonds of the third phosphate group due to the electronegativity of the oxygens.

  • GTP (Guanosine Triphosphate): Important for physiological work and protein conformational changes rather than pure energy storage.

• Phosphorylation and Kinases:

  • Kinases: A group of enzymes that transfer a phosphate group from ATP to a substrate (usually a protein).

  • This phosphorylation causes a conformational change that can activate or deactivate the protein or change how it binds to others.

• Cyclic AMP (cAMP) as a Second Messenger:

  • Adenylyl Cyclase: An enzyme that converts ATP into cyclic AMP.

  • Functions of cAMP:

    1. Activates Protein Kinase A (PKA), which regulates the heart and autonomic functions.

    2. Opens nucleotide-gated ion channels (e.g., the "dark current" in vision).

• Functions of GTP and G-Proteins:

  • Used in the carboxylic acid cycle for substrate-level phosphorylation to make ATP.

  • Critical for protein synthesis (initiation, elongation, termination).

  • Necessary for microtubule assembly.

  • Small G-Proteins: Involved in hormone/growth factor signaling and intracellular transport (ER to Golgi to exocytosis). Small G-proteins associated with growth factors are often oncogenes (genes that can cause cancer if they malfunction).

  • Large G-Proteins: Heterotrimeric proteins (Alpha $\alpha$, Beta $\beta$, and Gamma $\gamma$ subunits).

G-Protein Coupled Receptors (GPCRs)

• Structure:

  • Referred to as 7-transmembrane receptors because the protein spans the plasma membrane seven times.

  • It has an amino terminal outside the cell and short loops inside/outside.

  • It contains binding sites for ligands like acetylcholine, norepinephrine, oxytocin, or endocannabinoids.

• The Activation Cycle:

  • GEF (Guanine Nucleotide Exchange Factor): A protein (often the receptor itself) that promotes the release of GDP so GTP can bind, activating the G-protein.

  • GAP (GTPase Activating Protein): A protein that promotes the hydrolysis of GTP back into GDP/inorganic phosphate, inactivating the G-protein.

• Amplification Signal:

  • G-protein signaling allows for enormous amplification. One receptor can activate many aeGdenzymes, which in turn produce thousands of second messengers.

  • Affinities: Neuromuscular junctions require high concentrations of neurotransmitter ($10^{-3}\,mol/dm^3$), whereas GPCRs can be activated by sub-micromolar or nanomolar ranges ($10^{-9}\,mol/dm^3$).

Families of G-Alpha Subunits

1. $G_s$ (Stimulatory): Stimulates adenylyl cyclase to increase cAMP levels. Found in olfactory receptors.

2. $G_i / G_o$ (Inhibitory): Inhibits adenylyl cyclase. Includes transducin (vision) and certain taste receptors (sweet, umami).

3. $G_q$: Coupled to Phospholipase C (PLC). Uses calcium and diacylglycerol (DAG) as signals.

4. $G_{12/13}$: Couples to a small G-protein called Rogue; involved in hormone responses and blood neurotransmitters.

Pathophysiology and Toxins

• Pertussis Toxin:

  • Produced by Bordetella pertussis (Whooping Cough).

  • Prevents $G_i$ from interacting with adenylyl cyclase, leading to excessive mucus and respiratory issues.

  • Prevented by DTaP or Tdap vaccines.

• Cholera Toxin:

  • Produced by Vibrio cholerae.

  • Inhibits the GTPase activity of $G_s$, causing excessive cAMP production.

  • Leads to severe diarrhea and dehydration. Treated with oral hydration; oral vaccines are available for high-risk conditions.

Questions & Discussion

• Question: What exactly is a kinase?

• Answer: A kinase is an enzyme that phosphorylates a protein by taking a phosphate from ATP and adding it to that protein. This changes the protein's conformation and its activity level.