Learning Objectives

• Compare and contrast endocrine, paracrine and autocrine signaling.
• Describe selected receptor subtypes (Alpha, beta, and subtypes a, i, q).
• Describe internal responses in general terms such as phosphorylation, dephosphorylation and the second messenger concept.
• Describe second messengers generated by cyclization reactions such as $cAMP$ and $cGMP$.
• Describe second messengers generated by breakdown of membrane components such as generation of IP3 and DAG from phosphatidylinositol.

- Describe how thyroid and other hormones that act in the nucleus cause altered transcription of specific genes. Give an example of amplification of stimulus by signal cascades.

Main types of chemical communication

• Autocrine: Chemical messages that act on the same cell that produces them. This type of signaling is typically local and short-range, often involved in regulating cell growth or immune responses.
• Paracrine: Chemical messages that travel to nearby cells. This signaling relies on local mediators that are quickly degraded or sequestered, preventing widespread effects. An example is neurotransmission at a synapse.
• Endocrine: Chemical messages that travel in the blood (hormones) to distant target cells. Endocrine signals are typically long-range and can evoke slow, sustained responses.

- Signaling molecules include: hormones (endocrine), neurotransmitters (nervous system), and cytokines (immune system).

Steps in G-Protein-Coupled Receptor (GPCR) signaling

1. Receptor binds hormone: The ligand binds to the extracellular domain of the GPCR, inducing a conformational change in the receptor.
2. G protein exchanges GDP for GTP and dissociates: The activated receptor catalyzes the exchange of GDP for GTP on the $G\alpha$ subunit. This leads to the dissociation of the heterotrimeric G protein into an active $G\alpha$-GTP subunit and a G${\beta\gamma}$ dimer, both of which can then signal to downstream effectors.
3. Target protein binds GTP-bound $G\alpha$ ($G\alpha$-GTP): The active $G\alpha$-GTP subunit (and/or G${\beta\gamma}$ dimer) interacts with and modulates the activity of an effector protein (e.g., adenylyl cyclase or phospholipase C).
4. GTP is hydrolyzed to GDP: The $G\alpha$ subunit possesses intrinsic GTPase activity, which hydrolyzes GTP back to GDP. This causes $G\alpha$ to dissociate from the target protein, becoming inactive.
5. G protein reassociates: The $G\alpha$-GDP subunit reassociates with the G${\beta\gamma}$ dimer and the receptor, returning to its inactive heterotrimeric state, ready for another activation cycle. This rapid deactivation ensures precise control and reversibility of the signal.

- General cascade involves receptor activation (\rightarrow) heterotrimeric G-protein activation (\rightarrow) effector enzyme (e.g., adenylyl cyclase or phospholipase C) (\rightarrow) production of second messengers ($cAMP$, $cGMP$, IP3, DAG) (\rightarrow) downstream kinases and targets. This cascade often leads to signal amplification, where a single activated receptor can activate multiple G proteins, each producing many second messenger molecules.

Subunits of heterotrimeric G-proteins ($G\alpha$ and G${\beta\gamma}$)

• ${G\alpha_s}$
  • Action: Stimulates adenylyl cyclase (AC), leading to an increase in intracellular $cAMP$ levels, which typically activates Protein Kinase A (PKA).
  • Physiological uses: Glucagon and epinephrine to regulate metabolic enzymes (e.g., glycogenolysis); regulatory polypeptide hormones to control steroid hormone and thyroid hormone synthesis; by some neurotransmitters (e.g., dopamine) to control ion channels.
• ${G\alpha_{i/o}}$
  • Action: Inhibits adenylyl cyclase, leading to a decrease in intracellular $cAMP$ levels, thereby reducing PKA activity. Signal can also flow through the G${\beta\gamma}$-subunits.
  • Examples: Epinephrine; many neurotransmitters (acetylcholine, dopamine, serotonin) often mediate inhibitory effects through ${G\alpha_{i/o}}$.
• ${G\alpha_t}$
  • Action: Stimulates cGMP phosphodiesterase (PDE), which breaks down $cGMP$. This is a crucial role in the transducin pathway for light detection in the eye, where a decrease in $cGMP$ leads to channel closure.
• ${G\alpha_{q/11}}$
  • Action: Activates phospholipase C$\beta$ (PLC$\beta$), which hydrolyzes ${PIP_2}$ to generate the second messengers IP3 and DAG.
  • Examples: Epinephrine, acetylcholine, histamine, thyroid-stimulating hormone (TSH), interleukin-8, somatostatin, angiotensin. These often mediate effects related to calcium mobilization and Protein Kinase C activation.
• ${G\alpha_{12/13}}$
  • Action: Activates Rho-GEF (guanosine nucleotide exchange factor), which in turn activates Rho GTPases. This influences cytoskeletal elements, cell shape, and motility.
• G${\beta\gamma}$-subunits (G${\beta\gamma}$)
  • Can modulate signaling downstream and participate in signaling alongside $G\alpha$ subunits. Their roles are diverse, including direct regulation of ion channels (e.g., K+ channels in cardiac muscle) and activation of certain kinases or other effector proteins.

- Note: Distinct $G\alpha$ subunits provide specificity for activation or inhibition of AC; $G<em>s$ stimulates AC to raise $cAMP$; $G</em>i$ inhibits AC to lower $cAMP$.

Distinct roles for $G<em>s$ and $G</em>i$

• $G_s$ (stimulatory) activates adenylyl cyclase, increasing $cAMP$ levels, which then promotes the activity of PKA and its downstream effects.
• $G_i$ (inhibitory) inhibits adenylyl cyclase, decreasing $cAMP$ levels, thereby reducing PKA activity. This provides a mechanism for turning off or attenuating $cAMP$-mediated responses.

- G${\beta\gamma}$-subunits can contribute to signaling even when $G\alpha$ is in a particular state, adding additional regulatory layers by directly interacting with various effectors.

Activation of Protein Kinase A (PKA) by $cAMP$

• Increase in $cAMP$ activates protein kinase A (PKA), a $cAMP$-dependent protein kinase.
• PKA is a tetramer consisting of two catalytic (C) subunits and two regulatory (R) subunits. In its inactive state, the R subunits bind to and inhibit the C subunits.
• Activation involves a conformational change: When $cAMP$ levels rise, $cAMP$ binds to the regulatory subunits. This binding induces a conformational change in the R subunits, causing them to dissociate from and release the active catalytic subunits.
• Consequences: The free catalytic subunits of PKA can then phosphorylate a wide range of target proteins on serine or threonine residues. This leads to diverse cellular responses, including modulation of enzyme activities (e.g., in glycogen metabolism), regulation of ion channels, and phosphorylation of transcription factors (e.g., CREB) to alter gene expression.

- Important relation: $\text{PKA} \approx \mathrm{C}<em>2\mathrm{R}</em>2$ (tetrameric holoenzyme). Each regulatory subunit typically binds two molecules of $cAMP$.

Activation of Protein Kinase G (PKG) by $cGMP$

• $cGMP$ is primarily generated by guanylyl cyclase (GC), which can be membrane-bound (activated by peptide hormones like ANP) or soluble (activated by nitric oxide).
• PKG is the main target of $cGMP$. Upon binding $cGMP$, PKG is activated and phosphorylates target proteins, often on serine/threonine residues.
• $cGMP$ can directly activate some ion channels and pumps that modulate cytoplasmic $Ca^{2+}$ in smooth muscle cells, contributing to relaxation.
• $cGMP$ levels are regulated by phosphodiesterase-5 (PDE-5), which hydrolyzes $cGMP$ to inactive GMP. Inhibition of PDE-5 (e.g., by sildenafil) prolongs $cGMP$ signaling.
• There is also a soluble form of guanylyl cyclase (sGC) in the cytoplasm that serves as a receptor for nitric oxide (NO). NO signaling leads to activation of sGC and production of $cGMP$, initiating a cascade that results in smooth muscle relaxation and vasodilation by activating PKG.

- Reaction references:- NO signaling leads to activation of soluble GC and production of $cGMP$: $\text{GTP} \xrightarrow{\text{sGC}} \text{cGMP} + \text{PP}_i$

Phosphatidylinositol signaling: IP3 and DAG ($G_q$ pathway)

• Phosphatidylinositol (PI), a minor lipid component of the plasma membrane, is sequentially phosphorylated to generate phosphatidylinositol 4,5-bisphosphate ($PIP_2$).
• Phospholipase C (PLC), specifically PLC$\beta$ activated by $G\alpha<em>q$, hydrolyzes ${PIP</em>2}$ to generate two crucial second messengers: inositol trisphosphate (IP3) and diacylglycerol (DAG).
• IP3 mobilizes $Ca^{2+}$: IP3 diffuses into the cytosol and binds to IP3-gated $Ca^{2+}$ channels on the endoplasmic reticulum, causing the rapid release of stored $Ca^{2+}$ into the cytoplasm.
• DAG activates protein kinase C (PKC): DAG remains embedded in the plasma membrane and, in conjunction with the increased intracellular $Ca^{2+}$ (mobilized by IP3), activates Protein Kinase C (PKC). Activated PKC then phosphorylates a variety of target proteins, leading to diverse cellular responses.
• The signaling lipids can be recycled to generate phosphatidylinositol again, maintaining the cell's lipid balance.

- Reaction: $\text{PIP}<em>2 \xrightarrow{\text{PLC}} \text{IP}</em>3 + \text{DAG}$

Muscarinic receptor signaling on endothelial cells (example of PLC pathway)

• Receptor: Muscarinic (M3) receptor, found on endothelial cells, couples via $G_q$ to PLC$\beta$.
• GTP binding to $G\alpha<em>q$ activates PLC$\beta$: This leads to the hydrolysis of ${PIP</em>2}$ and the generation of DAG and IP3, raising their intracellular concentrations.
• IP3 increases intracellular $Ca^{2+}$: The released $Ca^{2+}$ binds to calmodulin, activating the complex $Ca^{2+}$/calmodulin.
• $Ca^{2+}$/calmodulin activates endothelial nitric oxide synthase (eNOS): Activated eNOS catalyzes the production of nitric oxide (NO) from L-arginine.
• NO production and diffusion: The NO produced by endothelial cells diffuses rapidly into adjacent vascular smooth muscle cells. In these smooth muscle cells, NO activates soluble guanylyl cyclase, increasing $cGMP$ levels, which leads to activation of PKG and ultimately smooth muscle relaxation (vasodilation).
• DAG activates PKC: Simultaneously, DAG activates PKC, promoting downstream phosphorylation events within the endothelial cell itself, leading to other functional changes.

- Result: Changes in enzyme activity and receptor activity, modulated by acetylcholine signaling, primarily driving vasodilation through NO production.

Phosphorylation and dephosphorylation as regulatory mechanisms

• Protein phosphorylation, typically on serine, threonine, or tyrosine residues, often activates or modulates protein function. It acts as a molecular switch, rapidly altering protein conformation and activity.
• Two major enzyme classes are involved in this critical regulatory balance:
  • Kinases: Enzymes that catalyze the addition of a phosphate group from ATP to a protein (phosphorylation).
  • Phosphatases: Enzymes that catalyze the removal of a phosphate group from a protein (dephosphorylation).
• The dynamic balance between these two processes controls most signal transduction pathways. This allows for precise and rapid control over cellular processes, as phosphorylation cascades can also amplify signals.

- Visual concept: Phosphorylation converts an inactive form to an active form (or vice versa) depending on the substrate and the specific phosphorylation site.

Steroid and thyroid hormone receptor superfamily

• Lipophilic hormones: act through intracellular (nuclear) receptors that regulate gene transcription.
• These compounds are water-insoluble and are transported in the blood bound to carrier proteins, protecting them from degradation and ensuring their delivery to target cells.
• Receptors have modular intracellular/nuclear architectures, allowing them to bind to specific DNA sequences (Hormone Response Elements, HREs) and directly regulate transcription.

- Family name: steroid/thyroid hormone receptor superfamily.

Intracellular receptors and specific receptor types

• Glucocorticoid receptor (GR): Binds cortisol and synthetic glucocorticoids; anti-inflammatory and metabolic effects.
• Mineralocorticoid receptor (MR): Binds aldosterone; regulates salt and water balance.
• Progesterone receptor (PR): Binds progesterone; involved in reproductive functions.
• Estrogen receptors (ER$\alpha$, ER$\beta$): Bind estrogen; involved in reproductive system, bone, and cardiovascular health.
• Androgen receptor (AR): Binds testosterone and dihydrotestosterone; involved in male sexual development and secondary characteristics.
• Vitamin D receptor (VDR): Binds 1,25-dihydroxy vitamin D; regulates calcium and phosphate homeostasis.
• Thyroid hormone receptor (TR): Binds thyroid hormones (T3, T4); regulates metabolism, growth, and development.
• Retinoic acid receptor (RAR): Binds retinoic acid; involved in embryonic development and cell differentiation.
• Peroxisome proliferator-activated receptors (PPAR$\alpha$, PPAR$\gamma$, PPAR$\delta$): Bind fatty acids and their derivatives; involved in lipid metabolism and inflammation.
• FXR (bile acid receptor): Regulates bile acid and lipid metabolism.
• LXR (Liver X receptor): Regulates cholesterol and lipid metabolism.
• SXR/PXR (Steroid and xenobiotic receptor / pregnane X receptor): Responds to a wide range of endogenous steroids and xenobiotics; involved in drug metabolism.
• CAR (Constitutive androstane receptor): Regulates the expression of drug-metabolizing enzymes and transporters.

- These receptors are intracellular/nuclear receptors that function as ligand-activated transcription factors, controlling the expression of specific genes.

Transactivation and modular structure of intracellular receptors

• Transactivation: The process by which a ligand-induced conformational change in the receptor enables it to bind to specific DNA sequences (Hormone Response Elements, HREs), recruit co-activator proteins, and directly initiate or modulate gene transcription.
• Intracellular receptors are often described as modular due to their distinct functional domains:
  • Ligand-binding domain (LBD): Binds the specific hormone and undergoes a conformational change upon binding.
  • DNA-binding domain (DBD): Contains zinc fingers that recognize and bind to specific HREs in the promoter regions of target genes.

- Transactivation domain (TAD): Interacts with co-activator or co-repressor proteins to either activate or repress gene transcription.

Steroid hormones and gene transcription

• Steroid hormones, being lipophilic, easily pass through the plasma membrane and bind to a specific intracellular receptor, which can be located in the cytoplasm or nucleus.
• Upon ligand binding, the hormone-receptor complex undergoes a conformational change, often dissociates from heat shock proteins, and then translocates to the nucleus (if initially cytoplasmic).
• In the nucleus, the activated hormone-receptor complex typically dimerizes and binds to specific Hormone Response Elements (HREs) on the DNA, located in the regulatory regions of target genes.

- The resulting transcriptional changes lead to biological effects by increasing or decreasing the synthesis of specific proteins; the pattern of gene expression depends on the specific hormone, the target cell type, and the presence of other transcription factors that can synergize or antagonize the receptor's action.

Thyroid hormone receptor signaling

• Thyroid hormone receptor (TR) signaling in target cells. TRs are typically bound to DNA even in the absence of hormone, often in complexes with co-repressors, actively repressing gene transcription.
• When thyroid hormone (T3) binds to the receptor, it induces a conformational change that causes the dissociation of co-repressors and the recruitment of co-activators.

- This shift from co-repressor to co-activator binding results in altered transcription of specific genes, leading to diverse physiological effects (e.g., cardiovascular effects via ${ \beta }$-receptors).

Vitamin D receptor (VDR) signaling

• Binding of 1,25-dihydroxy vitamin D (the active form of vitamin D) to VDR occurs in the cytoplasm or nucleus.
• The VDR then heterodimerizes with retinoid X receptor (RXR).

- The VDR/RXR heterodimer significantly increases its affinity for the vitamin D response element (VDRE) in the DNA, leading to transcriptional activation of genes involved in calcium and phosphate homeostasis.

Estrogen receptor signaling

• Estrogen diffuses into the cell and binds to its receptor (ER$\alpha$ or ER$\beta$), which can be cytoplasmic or nuclear.

- The estrogen receptor complex then undergoes dimerization and binds to estrogen response elements (EREs) in the DNA to regulate transcription of genes involved in female reproductive physiology, bone density, and cardiovascular health.

Summary of key concepts

• Mode of communication varies with biological needs/outcomes (autocrine, paracrine, endocrine).
• GPCRs are the most common receptor type and mediate diverse biological roles through heterotrimeric G proteins and second messengers.
• $G<em>i$ and $G</em>s$ are distinct $G\alpha$ subunits; $G<em>i$ inhibits adenylyl cyclase, while $G</em>s$ stimulates it, leading to opposing effects on $cAMP$ levels.
• $cGMP$ is a second messenger generated by activation of guanylyl cyclase (membrane-bound or soluble) and acts via PKG and other targets, notably in smooth muscle relaxation.
• Regulation is primarily through the dynamic balance of phosphorylation by kinases and dephosphorylation by phosphatases, acting as molecular switches.

- Intracellular or nuclear receptors regulate a broad range of physiological actions by altering gene transcription, leading to slower but sustained effects compared to membrane receptors.

Practical implications and connections

• Pharmacology: Many drugs target GPCRs (e.g., adrenergic agonists/antagonists for cardiovascular conditions, antihistamines) and PDE inhibitors (PDE-5 inhibitors like sildenafil affecting the NO-$cGMP$ axis for erectile dysfunction and pulmonary hypertension).
• NO-$cGMP$ pathway: This pathway is critical for smooth muscle relaxation, impacting vascular tone (blood pressure regulation) and erectile function.
• Nuclear receptor signaling: This mechanism underlies the therapeutic effects of steroids (e.g., anti-inflammatory glucocorticoids), thyroid hormone (e.g., for hypothyroidism), vitamin D (e.g., for bone health), and other lipophilic hormones on gene expression.

- Understanding second messengers and amplification: These concepts help explain how even a small hormonal signal can yield large and diverse downstream responses within a cell, making signaling pathways highly efficient and versatile.

Quick reference formulas and key reactions

• Adenylyl cyclase reaction (cAMP formation): $\text{ATP} \xrightarrow{\text{AC}} \text{cAMP} + \text{PP}_i$
• Guanylyl cyclase reaction (cGMP formation): $$\text{GTP} \xrightarrow{\text{GC}} \text{cGMP