Signaling Systems

• Cells share three jobs:

  • Survive: Maintaining viability.

  • Sense: Detecting environmental needs and changes.

  • Respond: Executing a physiological action based on sensed data.

  • Reproduce: (neurons do NOT)

• Signaling systems focus primarily on how cells "Sense" what they need to do, primarily through communication between cells.

Proteins

• For any action a cell performs, a protein is responsible for doing the work. Any response by a cell is simply a change in the Total Activity for one or more proteins.

• The formula for Total Activity of a Protein is:     $\text{Total Activity} = (\text{Specific Activity of each molecule}) \times (\text{Amount of protein/Number of molecules})$ only some do both

  • hydrophilic signals affect specific activity while lipophilic affects amount

• Signaling systems must alter Total Activity through one of two methods:

  • Altering Specific Activity: Changing the functionality of existing protein molecules

  • Altering Gene Expression: Changing the number of protein molecules through synthesis

Signaling

• Definition: Signaling implies that information is transferred from one location to another to bring about a response by changing protein total activity.

• Direct Electrical Communication:

  • Uses Gap Junctions between cells.

  • This is the fastest form of communication in the body.

  • Found in:

    • Cardiac muscle.

    • Some smooth muscle.

    • Some neurons (though evidence suggests this in neurons may be related to seizures; more research is needed).

      • all neurons depolarize at same time and repolarize during slow phase of sleep

  • Functional Syncytium: All neurons/cells connected via gap junctions depolarize and repolarize at the same time. While extremely fast (e.g., during the slow phase of sleep), it lacks flexibility because one cell must behave exactly like all the cells it is connected to.

• Chemical Signals:

  • These are slower than electrical signals but allow for much more flexible and diverse responses.

  • There are three main types:

• 1. Neurotransmitters:

  • The fastest type of chemical signal.

  • Characteristics: Fast onset, short duration response.

  • Examples: Glutamate, GABA, Glycine, ACh.

  • Physiological examples: EPSP (Excitatory Postsynaptic Potential) and IPSP (Inhibitory Postsynaptic Potential), lasting approximately $10-20\text{ ms}$.

    • exists and available stored in vesicles

• 2. Hormones:

  • Signals that circulate via the blood. They generally have a slower onset and longer duration compared to neurotransmitters.

    • endocrine

  • Polypeptides:

    • Primarily alter Specific Activity.

    • Result in a relatively faster onset and shorter duration within the hormone group.

    • Examples: Insulin, Somatotropin, IGF-1 (Growth factor), FSH.

  • Steroids:

    • Primarily alter Gene Expression.

    • Result in a slower onset and longer duration of response.

    • Examples: Cortisol, Estradiol, Aldosterone.

• 3. Local Signals:

  • These have short lifespans and do not allow for circulation in the blood. They act faster than most hormones.

  • Paracrines:

    • Released by one cell and affect neighboring cells

    • conversion of histamine in mast cells, stored in secretory granules respond to local injury, infection, dilates blood vessels

      • later WBCs metabolize histamine

    • prostaglandins: maintains normal function

      • constantly released by mammalian tissues

      • arachidonic acid cleaved from membrane phospholipids

      • appropriate stimuli for phospholipase A2 (G prot)

        • e.g. epinephrine, bradykinin, thrombin, angiotensin II

        

    • Onset and duration vary.

    • Examples: Histamine, Prostaglandins, Kinins, NGF (Nerve Growth Factor).

  • 
    

  • Autocrines:

    • Affect the behavior of the same cell that released them.

    • Often co-released with neurotransmitters to prevent more release (negative feedback).

    • Example: Adenosine.

How do signals alter cell behavior?

• Hydrophilic Signals - polypeptides:

  • Cannot pass through the cell membrane.

  • Components:

    • Signal: Circulates in blood or interstitial fluid (ISF). Often rapidly catabolized or metabolized to an inactive form.

    • R (Receptor): Specific for a particular signal; the signal is useless without it.

    • E (Effector): Protein that generates a Second Messenger signal.

  • Action: Primarily acts by changing Specific Activity via amplification through a coupling mechanism.

• Lipophilic Signals - steroids:

  • Can pass through membranes freely.

  • Synthesis: Cannot be synthesized ahead of time and stored in vesicles (as they would diffuse out). They are synthesized "on-demand" only when the appropriate stimulus is present.

  • Receptors: Located within target cells, either in the cytosol or the nucleus.

  • Transport: Use specific Transport Proteins (e.g., Albumin) to aid distribution and decrease catabolism in the blood.

  • Action: Primarily works on DNA/Nucleus to change Gene Expression (Transcription of mRNA → translation by ribosomes → New protein molecules).

  • Note: Estrogen, Progesterone, and Thyroid Hormone receptors are specifically located in the nucleus.

• Local Effects:

  • Thromboxanes (from Platelets): Promote vasoconstriction and platelet aggregation. Aspirin decreases platelet plugs by inhibiting this.

  • Prostacyclin (PGI2): Made by endothelial cells; prevents platelet aggregation and keeps blood flowing.

  • Leukotrienes: Can promote Bronchoconstriction (involved in Asthma) and increase vascular permeability during immune responses.

• Inactivation: Responses are rapid and short-lived due to quick inactivation:

  1. 15-hydroxyprostaglandin DHase: An enzyme found in most tissues that metabolizes eicosanoids.

  2. "Suicide Enzyme": Cyclo-oxygenase destroys itself once it has been activated.

Second Messenger Systems

• Acetylcholine (ACh): Binds to Nicotinic and Muscarinic receptors (Cholinergic).

• Norepinephrine (NE): Binds to $\alpha_1$, $\alpha_2$, $\beta_1$, and $\beta_2$ receptors.

• GABA: Binds to $GABA_A$ and $GABA_B$ receptors.

• effector: ion channels “ionophore” change permeability

• Steps:

  • neurotransmitter binds ligand gated channel

  • Behavioral Changes in the Cell:

    • Depolarization: Excites the cell. Typically occurs when $Na^+$ flows into the cell.

      • if to threshold → AP

    • Hyperpolarization: Inhibits the cell. Typically occurs when $K^+$ flows out of the cell.

• Inactivation of Response:

  • Occurs when the signal is removed and the gate in the channel closes.

Sometimes channels are selective for Ca

• Signaling Sequence:

  • Signal binds receptor $\rightarrow$ $Ca^{2+}$ channel opens $\rightarrow$ $Ca^{2+}$ enters the cell.

  • $Ca^{2+}$ acts as a second messenger by binding to a Calcium-Binding Protein (CBP) such as Calmodulin.

  • This forms an active complex (e.g., $Ca^{2+}$-Calmodulin).

  • The active complex activates target proteins, which may include protein kinases (initiating a cascade of phosphorylations) or metabolic enzymes, leading to changed cell behavior.

• Inactivation Mechanisms:

  • The primary signal detaches from the receptor, stopping $Ca^{2+}$ entry.

  • Calcium ATPases (pumps) are constantly active to maintain low cytosolic levels.

  • $Ca^{2+}$ is pumped:

    1. Out of the cell.

    2. Into the mitochondria.

    3. Into the Smooth Endoplasmic Reticulum (ER) or Sarcoplasmic Reticulum (in skeletal and cardiac muscle).

  • As free $[Ca^{2+}]$ drops, it dissociates from the Calcium-Binding Protein, inactivating it.

Tyrosine Kinase (TK) Receptors

• Catalytic Receptors

• trans-membranous protein.

• Structure and Activation:

  • Extracellular domain: Signal binding site.

  • Intracellular domain: Tyrosine Kinase (TK) domain.

  • Binding of the signal causes the TK to phosphorylate itself (autophosphorylation) using ATP.

• Function:

  • Once activated, the TK uses ATP to phosphorylate other enzyme molecules at their Tyrosine R groups (side chains).

  • This changes the specific activity of those enzymes, potentially activating metabolic processes (often anabolic) or inhibiting catabolic processes.

  • Some phosphorylated proteins are protein kinases themselves, allowing for a broad range of enzyme activity changes from a single receptor.

• Note: phosphates added to tyrosine R groups

• some proteins phosphorylated by TK are protein kinases themselves so broad range of enzyme activities can be changed by one TK receptor

• if phosphate catabolic increase activity

• if anabolic decrease activity

Biological Significance:

• TK receptors are often associated with anabolic signals like Insulin and Growth Factors (e.g., EGF - Epidermal Growth Factor, PDGF - Platelet-Derived Growth Factor).

• They are critical in regulating cell development, differentiation, and proliferation.

G-Proteins

• G-Proteins (GTP-binding proteins) are membranous proteins activated when a signal binds to a receptor → cascade

• glucagon or epinephrine bind to receptors → adenylate cyclase → cAMP → active protein kinase

  • systems generate cAMP

  • G protein diffuse in membrane to activate effector proteins

• G-Protein Subunits and Cycle:

  • Resting State: The G-protein exists as a trimer with $\alpha$, $\beta$, and $\gamma$ subunits. GDP is bound to the $\alpha$ subunit.

  • Activation: Signal binds receptor $\rightarrow$ The receptor prefers binding to the G-protein $\rightarrow$ GTP replaces GDP on the $\alpha$ subunit.

  • Effector Activation: The $\alpha$-GTP complex detaches from the $\beta/\gamma$ dimer, the $\beta/\gamma$ dimer diffuses through the membrane, activating an effector (like Adenylate Cyclase).

• Inactivation:

  • The signal unbinds from the receptor.

  • $\beta/\gamma$ dimer reattach with GDP

  • G-proteins have intrinsic GTPase activity causing them to auto inactivate themselves

  • effector inactivates

• Specific G-Protein Families:

  • $G_s$: Stimulates Adenylate Cyclase.

  • $G_i$: Inhibits Adenylate Cyclase.

  • $G_p$: Activates Phospholipase C.

• Pathological Note:

  • Cholera Toxin inactivates the GTPase activity in the $\alpha$ subunit, keeping the G-protein permanently "on." This results in excessive cAMP production, leading to severe diarrhea and potential death.

Cyclic AMP

• cAMP Mechanism (e.g., Glucagon or Epinephrine in glycogen metabolism):

  • Signal $\rightarrow$ G-protein $\rightarrow$ Adenylate Cyclase $\rightarrow$ ATP is converted to cAMP + $PP_i$.

  • cAMP activates cAMP-Dependent Protein Kinase (Protein Kinase A).

  • Inactivation: Phosphodiesterase (constantly working) converts cAMP to AMP. This enzyme is inhibited by Theophylline and Caffeine,

    • cAMP prolongs signaling effects as it inhibits phosphatase

• Phospholipase C (PLC) Cascade:

  • Signal binds receptor $\rightarrow$ Activates G-protein $\rightarrow$ Activates Effector: Phospholipase C.

  • PLC cleaves a membrane phospholipid, Phosphatidyl-inositol-diphosphate ($PIP_2$), into two second messengers:

    1. Diacylglycerol (DAG): Hydrophobic; remains in the cell membrane.

    2. Inositol Tri-phosphate ($IP_3$): Hydrophilic; enters the cytosol.

• Cellular Response to PLC:

  • $IP_3$ binds to the Smooth ER, causing the release of stored $Ca^{2+}$.

  • $Ca^{2+}$ and DAG together activate Protein Kinase C (PKC).

  • PKC phosphorylates target proteins to change cell function.

• Notable Caveats and Connections:

  • Phorbol Esters are carcinogenic because they irreversibly activate PKC.

  • DAG can be converted into Monoacylglycerol (MAG) and Arachidonic Acid.

  • Arachidonic Acid is a substrate for Prostaglandin Synthase, allowing signaling to spread to neighboring cells.

  • phophatase removes PO4 from pO4 protein and IP3 (inactivates them)

    • Ca pumped back into ER, Mito and out of cell

Nitric Oxide (NO) Signaling

• NO is a recently discovered gas signal that can diffuse freely through cell membranes to reach neighboring cells.

  • stable anesthetic

• It was previously known as Endothelium-Derived Relaxing Factor (EDRF)

• Physical Properties:

  • It is a highly unstable free radical.

  • It has a short half-life of only 6-10 seconds before disintegrating.

• Function:

  • Produces vasodilation by relaxing vascular smooth muscle (Mechanism used by drugs like Viagra).

  • Inhibits platelet aggregation.

  • Functions as a neurotransmitter.

  • Mediates tumoricidal and bactericidal actions in macrophages.

• Target:

  • NO activates Guanylate Cyclase, which produces cyclic GMP (cGMP) as a second messenger.

Regulation

• Target cells can exhibit Decreased Sensitivity when signal molecules remain abundant over time.

• Mechanisms of Down-Regulation:

  • Receptor Phosphorylation: Receptors stay in the membrane but can no longer activate G-proteins.

  • Receptor-Mediated Endocytosis: The binding of a signal stimulates the cell to internalize the receptors into vesicles.

    • Receptor Sequestration: If the vesicles can be exocytosed back to the membrane.

    • Receptor Down-Regulation: If the receptors are subsequently catabolized/broken down.

  • G prot structure can change and cause them to be inactive

• Clinical and Practical Examples:

  • Type II Diabetes: A pathological example of receptor down-regulation.

  • Drug Tolerance: Heroin use leads to receptor down-regulation, requiring higher doses to achieve the same effect.

  • Disease Treatment: In Parkinson's, drugs may lose effectiveness as receptors down-regulate over time.

• Cross-Talk: Multiple signal cascades often interact, leading to highly complex and integrated cellular responses.