Signal Transduction — MCQs

Signal Transduction Indian Medical PG Practice Questions and MCQs

Question 31: Oxytocin causes muscle contraction through which mechanism?

A. cAMP
B. Ion channels
C. IP3-DAG (Correct Answer)
D. Nuclear receptors

Explanation: **Explanation:** Oxytocin exerts its physiological effects (primarily uterine contraction and milk ejection) by binding to the **Oxytocin Receptor (OXTR)**, which is a member of the **G-protein coupled receptor (GPCR)** family, specifically coupled to the **Gq subclass**. 1. **Mechanism of Action (Why C is correct):** When oxytocin binds to its receptor, it activates **Phospholipase C (PLC)**. PLC cleaves the membrane phospholipid Phosphatidylinositol 4,5-bisphosphate (PIP2) into two second messengers: * **Inositol triphosphate (IP3):** Binds to receptors on the sarcoplasmic reticulum, causing a rapid release of **intracellular Calcium (Ca²⁺)**. * **Diacylglycerol (DAG):** Activates Protein Kinase C (PKC). The surge in intracellular calcium binds to calmodulin, activating Myosin Light Chain Kinase (MLCK), which leads to smooth muscle contraction. 2. **Why other options are incorrect:** * **A (cAMP):** This is the second messenger for Gs/Gi-coupled receptors (e.g., Glucagon, ACTH, ADH-V2). cAMP typically causes smooth muscle *relaxation* (e.g., Beta-2 agonists). * **B (Ion channels):** While calcium enters via channels, the *primary* signal transduction for the oxytocin receptor is metabolic (GPCR), not a direct ligand-gated ion channel (like Nicotinic ACh receptors). * **D (Nuclear receptors):** These are used by lipid-soluble hormones (Steroids, Thyroid hormones, Vitamin D) that act as transcription factors. Oxytocin is a peptide hormone and cannot cross the cell membrane. **High-Yield Clinical Pearls for NEET-PG:** * **Gq-coupled receptors mnemonic:** "**HAV 1 M&M**" (H1, Alpha-1, V1-ADH, M1, M3) and **Oxytocin**. * **V1 vs. V2:** ADH acts via **Gq (IP3-DAG)** on V1 receptors (vasoconstriction) but via **Gs (cAMP)** on V2 receptors (renal water reabsorption). * Oxytocin is synthesized in the **paraventricular nucleus** of the hypothalamus and stored in the posterior pituitary.

Question 32: Patients with both Graves' disease and Cushing syndrome are overproducing hormones that have which one of the following in common?

A. Reacting with receptors in the cell membrane
B. Utilizing second messengers
C. Binding to intracellular receptors (Correct Answer)
D. Binding to RNA to produce physiologically active proteins

Explanation: **Explanation:** The core of this question lies in identifying the signaling mechanisms of the specific hormones involved in Graves' disease and Cushing syndrome. **1. Why the Correct Answer is Right:** * **Graves’ Disease** involves the overproduction of **Thyroid Hormones (T3, T4)**. * **Cushing Syndrome** involves the overproduction of **Glucocorticoids (Cortisol)**. Both Thyroid hormones and Steroid hormones are lipophilic. They cross the plasma membrane and **bind to intracellular receptors** (Cortisol binds to cytoplasmic receptors which then translocate; T3/T4 bind directly to nuclear receptors). These hormone-receptor complexes act as transcription factors, binding to Hormone Response Elements (HRE) on DNA to regulate gene expression. **2. Why the Incorrect Options are Wrong:** * **Option A & B:** These describe the mechanism for **water-soluble hormones** (e.g., Catecholamines, Peptides like Insulin or Glucagon). These hormones cannot cross the lipid bilayer and must bind to cell membrane receptors, often triggering second messengers like cAMP, IP3/DAG, or cGMP. * **Option D:** Hormone-receptor complexes bind to **DNA**, not RNA. They stimulate or inhibit the transcription of DNA into mRNA, which is then translated into proteins. **Clinical Pearls for NEET-PG:** * **Nuclear Receptors (Group I):** Steroids (Cortisol, Aldosterone, Estrogen, Progesterone, Testosterone), Thyroid hormones (T3/T4), Retinoic acid, and Vitamin D. * **Mnemonic for Intracellular Receptors:** "PET TV" (Progesterone, Estrogen, Testosterone, Thyroid, Vitamin D/Vitamin A). * **Graves' Disease:** Caused by Thyroid Stimulating Immunoglobulins (TSI) mimicking TSH. * **Cushing Syndrome:** Characterized by hypercortisolism; remember that Cortisol is the "stress hormone" from the adrenal cortex.

Question 33: What is true about the G-protein receptor complex?

A. It interacts with the transmembrane domain.
B. It exchanges GTP for GDP.
C. Adenyl cyclase activation leads to increased cAMP.
D. All of the above. (Correct Answer)

Explanation: **Explanation:** The G-Protein Coupled Receptor (GPCR) system is the most common mechanism for signal transduction across cell membranes. 1. **Interaction with Transmembrane Domain (Option A):** GPCRs are characterized by a **7-transmembrane (serpentine)** α-helical structure. When a ligand binds to the extracellular domain, it induces a conformational change in the transmembrane segments, which then activates the intracellular G-protein complex (comprising α, β, and γ subunits). 2. **GTP/GDP Exchange (Option B):** In its inactive state, the Gα subunit is bound to **GDP**. Upon receptor activation, the G-protein undergoes a conformational change that causes it to **exchange GDP for GTP**. This leads to the dissociation of the Gα-GTP complex from the βγ subunits, allowing it to interact with effector enzymes. 3. **Adenyl Cyclase and cAMP (Option C):** In the $G_s$ (stimulatory) pathway, the activated Gα subunit stimulates the enzyme **Adenyl Cyclase**, which catalyzes the conversion of ATP to **cyclic AMP (cAMP)**. cAMP then acts as a second messenger to activate Protein Kinase A (PKA). Since all three statements accurately describe the functional cycle of the G-protein complex, **Option D is correct.** **High-Yield Clinical Pearls for NEET-PG:** * **Termination of Signal:** The Gα subunit has intrinsic **GTPase activity**, which hydrolyzes GTP back to GDP, acting as a "built-in" molecular switch to turn off the signal. * **Toxins:** * **Cholera toxin:** Inhibits GTPase activity of $G_s$, leading to permanent activation and massive cAMP levels (secretory diarrhea). * **Pertussis toxin:** Inhibits $G_i$ (inhibitory G-protein), preventing the inhibition of Adenyl Cyclase. * **Second Messengers:** While $G_s/G_i$ affect cAMP, the **$G_q$ pathway** activates Phospholipase C, leading to $IP_3$ (increases $Ca^{2+}$) and DAG (activates Protein Kinase C).

Question 34: Bone resorption is enhanced by:

A. PGD2
B. PGF2
C. PGE2 (Correct Answer)
D. PGI2

Explanation: ### Explanation **Correct Option: C (PGE2)** Prostaglandin E2 (PGE2) is a potent stimulator of bone resorption. It acts primarily by increasing the expression of **RANKL** (Receptor Activator of Nuclear Factor kappa-B Ligand) on osteoblasts and marrow stromal cells. RANKL then binds to its receptor, RANK, on osteoclast precursors, leading to their differentiation and activation. Additionally, PGE2 inhibits the production of **Osteoprotegerin (OPG)**, a decoy receptor that normally blocks RANKL, further shifting the balance toward bone breakdown. **Analysis of Incorrect Options:** * **A (PGD2):** Primarily involved in allergic and inflammatory responses, smooth muscle contraction, and sleep regulation. It does not play a significant role in bone remodeling. * **B (PGF2α):** Known for its role in luteolysis and uterine contraction. While it may have minor effects on bone cells, it is not the primary prostaglandin associated with bone resorption. * **D (PGI2 - Prostacyclin):** Produced by vascular endothelium, it is a potent vasodilator and inhibitor of platelet aggregation. It generally has an inhibitory effect on osteoclast activity, the opposite of PGE2. **High-Yield Facts for NEET-PG:** * **Mechanism:** PGE2 acts via G-protein coupled receptors (EP receptors), mainly increasing intracellular cAMP. * **Clinical Correlation:** Chronic inflammation (like Periodontitis or Rheumatoid Arthritis) leads to high local PGE2 levels, resulting in localized bone loss. * **NSAIDs Connection:** Non-steroidal anti-inflammatory drugs (NSAIDs) inhibit COX enzymes, reducing PGE2 levels, which is why they can sometimes delay bone healing or be used to manage heterotopic ossification. * **Osteoblast Paradox:** While PGE2 stimulates resorption via RANKL, in certain pulsatile or low-dose conditions, it can also have anabolic (bone-forming) effects. However, in the context of standard physiology and exams, it is the classic mediator of **resorption**.

Question 35: All of the following are transcription factors activated by signal transduction pathways of hypertrophy, except?

A. GATA4
B. NFAT
C. MEF2
D. MLL1 (Correct Answer)

Explanation: ### Explanation **Correct Answer: D. MLL1** **1. Why MLL1 is the Correct Answer:** Cardiac hypertrophy is a compensatory response to mechanical stress or neurohumoral stimuli (like Angiotensin II or Endothelin-1). This process involves the activation of specific signal transduction pathways (calcineurin, MAPK, and PI3K) that trigger a "fetal gene program." **MLL1 (Mixed Lineage Leukemia 1)** is a histone methyltransferase involved in epigenetic regulation and chromatin remodeling, particularly in hematopoiesis and leukemogenesis. While it regulates gene expression, it is **not** a primary transcription factor directly activated by the classical signaling cascades of cardiac hypertrophy. **2. Analysis of Incorrect Options:** * **GATA4 (Option A):** A zinc-finger transcription factor that is a central regulator of cardiac development and hypertrophy. It is activated via the MAPK pathway and induces the expression of genes like ANP (Atrial Natriuretic Peptide). * **NFAT (Option B):** Nuclear Factor of Activated T-cells is the primary downstream target of the **Calcineurin** pathway. Increased intracellular calcium activates calcineurin, which dephosphorylates NFAT, allowing it to enter the nucleus and trigger hypertrophic gene transcription. * **MEF2 (Option C):** Myocyte Enhancer Factor 2 works in synergy with GATA4 and NFAT. It is a key target of the MAP kinase and Calcium/Calmodulin-dependent kinase (CaMK) pathways during the remodeling process. **3. High-Yield Clinical Pearls for NEET-PG:** * **The "Fetal Gene Program":** Hypertrophy involves a shift from adult isoforms to fetal isoforms (e.g., α-MHC to **β-MHC** and induction of **ANP/BNP**). * **Key Enzyme:** **Calcineurin** is the most high-yield phosphatase associated with pathological cardiac hypertrophy. * **Pathological vs. Physiological:** Pathological hypertrophy (hypertension) involves fibrosis and GATA4/NFAT; physiological hypertrophy (exercise) primarily involves the **PI3K/Akt** pathway.

Question 36: What is true about the G-protein receptor complex?

A. It interacts with the transmembrane domain. (Correct Answer)
B. It converts GTP to GDP.
C. Adenyl cyclase activation leads to increased cAMP.
D. GPCRs do not undergo phosphorylation.

Explanation: ### Explanation **1. Why Option A is Correct:** G-Protein Coupled Receptors (GPCRs) are characterized by a **7-transmembrane (7-TM) α-helical domain** (also called serpentine receptors). The ligand binds to the extracellular side or within the transmembrane cleft, inducing a conformational change. This change allows the **cytoplasmic loops** and the **transmembrane segments** to interact with the heterotrimeric G-protein (α, β, and γ subunits), specifically facilitating the exchange of GDP for GTP on the α-subunit. **2. Analysis of Incorrect Options:** * **Option B:** The G-protein complex does not convert GTP to GDP to initiate signaling; rather, it acts as a **GTPase**. It hydrolyzes GTP to GDP to **terminate** the signal. The activation step involves replacing GDP with a *new* GTP molecule. * **Option C:** While Adenyl cyclase activation *does* increase cAMP, this is a **downstream effect** of the G-protein action, not a property of the receptor complex itself. In the context of "what is true about the receptor complex structure/interaction," Option A is a more fundamental structural fact. (Note: In some exam contexts, C is a true statement, but A defines the receptor's primary structural mechanism). * **Option D:** GPCRs **do** undergo phosphorylation. This is a key mechanism for **desensitization**. G-protein-coupled receptor kinases (GRKs) phosphorylate the cytoplasmic tail, allowing **β-arrestin** to bind, which uncouples the receptor from G-proteins and triggers internalization. **3. High-Yield Clinical Pearls for NEET-PG:** * **Structure:** GPCRs are the largest family of cell-surface receptors. * **Vibrio cholerae:** Cholera toxin causes ADP-ribosylation of the **Gs protein**, inhibiting its GTPase activity, leading to constitutive cAMP production and permanent activation of chloride channels (CFTR). * **Bordetella pertussis:** Pertussis toxin ADP-ribosylates the **Gi protein**, locking it in the "off" state, preventing the inhibition of adenyl cyclase. * **Second Messengers:** Remember the Gq pathway (Phospholipase C → IP3/DAG → Ca²⁺/Protein Kinase C).

Question 37: To which end of the G protein-coupled receptor are the G proteins attached?

A. Aminoterminal end
B. Carboxyterminal end (Correct Answer)
C. Third transmembrane domain
D. Seventh transmembrane domain

Explanation: **Explanation:** G-protein-coupled receptors (GPCRs) are **seven-transmembrane (serpentine)** proteins that cross the lipid bilayer seven times. The structural orientation is critical for their function: 1. **Why Option B is Correct:** The **Carboxy-terminal (C-terminus)** and the **third intracellular loop (IL3)** are located on the **cytosolic side** of the plasma membrane. Heterotrimeric G-proteins ($\alpha, \beta, \gamma$ subunits) are peripheral membrane proteins anchored to the inner leaflet of the plasma membrane. Therefore, the G-protein binds to the intracellular domains of the receptor, specifically the C-terminus and the IL3, to initiate signal transduction upon ligand binding. 2. **Why Other Options are Incorrect:** * **Option A (Amino-terminal end):** The N-terminus is located on the **extracellular side**. Its primary role is often involved in ligand binding (especially for peptide hormones) and receptor trafficking, not G-protein coupling. * **Options C & D (Transmembrane domains):** The 3rd and 7th transmembrane domains are hydrophobic segments embedded within the lipid bilayer. While they form the structural core and the ligand-binding pocket (for small molecules like epinephrine), they do not directly "attach" to the G-protein, which resides in the aqueous environment of the cytosol. **High-Yield Clinical Pearls for NEET-PG:** * **Molecular Switch:** G-proteins act as switches; they are **active when bound to GTP** and inactive when bound to GDP. * **Termination:** The intrinsic **GTPase activity** of the $\alpha$-subunit hydrolyzes GTP to GDP, terminating the signal. * **Toxins:** *Vibrio cholerae* toxin causes permanent activation of $G_s$ (by ADP-ribosylation), leading to persistent cAMP elevation and secretory diarrhea. * **Second Messengers:** GPCRs typically work through cAMP (via Adenylyl Cyclase) or $IP_3/DAG$ (via Phospholipase C).

Question 38: IP3 facilitates which ion entry into the cytoplasm?

A. Na+
B. K+
C. Ca++ (Correct Answer)
D. Mg++

Explanation: **Explanation:** The correct answer is **C. Ca++**. **Mechanism of Action:** In the Phosphoinositide signaling pathway, a ligand binds to a G-protein coupled receptor (GPCR), activating Phospholipase C (PLC). PLC cleaves the membrane phospholipid Phosphatidylinositol 4,5-bisphosphate (PIP2) into two second messengers: **Diacylglycerol (DAG)** and **Inositol 1,4,5-trisphosphate (IP3)**. IP3 is water-soluble and diffuses into the cytoplasm, where it binds to specific **IP3-gated Calcium channels** located on the membrane of the **Endoplasmic Reticulum (ER)** (or Sarcoplasmic Reticulum in muscles). This binding triggers the opening of these channels, allowing stored Ca++ ions to flow down their concentration gradient into the cytoplasm. Increased cytosolic Ca++ then activates various cellular processes, often by binding to Calmodulin. **Analysis of Incorrect Options:** * **A (Na+) & B (K+):** These ions are primarily involved in maintaining membrane potential and nerve impulse conduction via voltage-gated or ligand-gated channels (like the Nicotinic ACh receptor), but they are not directly regulated by the IP3 second messenger system. * **D (Mg++):** Magnesium acts as a cofactor for many enzymes (especially those involving ATP), but it does not serve as a primary signaling ion released by the IP3 pathway. **High-Yield Clinical Pearls for NEET-PG:** * **DAG vs. IP3:** While IP3 releases Ca++, DAG remains in the membrane to activate **Protein Kinase C (PKC)**. * **Lithium Connection:** Lithium, used in Bipolar Disorder, inhibits the recycling of Inositol (Inositol monophosphatase), thereby depleting PIP2 levels and dampening this signaling pathway. * **H1, α1, V1, M1, M3:** These receptors (mnemonic: "HAV 1 M&M") all utilize the Gq-PLC-IP3-Ca++ pathway.

Question 39: All of the following hormones use cyclic AMP as a second messenger except?

A. Glucagon
B. Estrogen (Correct Answer)
C. Epinephrine
D. Luteinizing hormone

Explanation: **Explanation:** The core concept tested here is the classification of hormones based on their chemical nature and receptor location. **1. Why Estrogen is the correct answer:** Hormones are broadly divided into two groups. **Group I hormones** (Steroids, Thyroid hormones, Calcitriol, and Retinoids) are lipophilic. They cross the cell membrane and bind to **intracellular or nuclear receptors**. Their mechanism of action involves changing gene expression directly rather than using second messengers. **Estrogen**, being a steroid hormone, follows this mechanism and does not utilize cAMP. **2. Why the other options are incorrect:** Options A, C, and D are **Group II hormones**. These are water-soluble (peptides or catecholamines) and cannot cross the lipid bilayer. They bind to cell surface receptors (G-Protein Coupled Receptors - GPCRs) and activate the **Adenylyl Cyclase system**, which converts ATP to **cAMP** (the second messenger). * **Glucagon:** Uses cAMP to trigger glycogenolysis in the liver. * **Epinephrine:** Acts via $\beta$-adrenergic receptors to increase cAMP. * **Luteinizing Hormone (LH):** A glycoprotein hormone that uses cAMP to stimulate steroidogenesis in gonads. **Clinical Pearls for NEET-PG:** * **Mnemonic for cAMP-mediated hormones:** "FLAT ChAMP" (FSH, LH, ACTH, TSH, CRH, hCG, ADH (V2), MSH, PTH) + Glucagon and Calcitonin. * **ANP and Nitric Oxide (NO)** use **cGMP** as a second messenger. * **Insulin and Growth Hormone** use the **Tyrosine Kinase** pathway. * **Oxytocin and GnRH** use the **IP3/DAG** (Calcium) pathway.

Question 40: Which of the following hormones does not utilize cAMP as a second messenger?

A. TSH
B. Insulin (Correct Answer)
C. LH
D. FSH

Explanation: **Explanation:** The correct answer is **Insulin**. The mechanism of action for hormones depends on their receptor type and the specific second messenger system they activate. **1. Why Insulin is the correct answer:** Insulin (along with IGF-1 and Growth Hormone) utilizes the **Receptor Tyrosine Kinase (RTK)** pathway rather than the cAMP pathway. Upon binding to its α-subunits, the β-subunits undergo autophosphorylation, activating **Insulin Receptor Substrates (IRS-1/2)**. This triggers the PI3K/Akt pathway and the MAP kinase pathway to regulate glucose uptake and gene expression. It does not involve Adenylate Cyclase or cAMP. **2. Why the other options are incorrect:** * **TSH, LH, and FSH:** These are all glycoprotein hormones secreted by the anterior pituitary. They bind to **G-Protein Coupled Receptors (GPCRs)** linked to the **Gs protein**. This activates Adenylate Cyclase, which converts ATP to **cAMP**, subsequently activating Protein Kinase A (PKA). Therefore, they all utilize cAMP as their primary second messenger. **High-Yield Clinical Pearls for NEET-PG:** * **cAMP Users (FLAT ChAMP):** **F**SH, **L**H, **A**CTH, **T**SH, **C**RH, **h**CG, **A**DH (V2 receptor), **M**SH, **P**TH, and Glucagon. * **cGMP Users:** ANP, BNP, and Nitric Oxide (Vasodilators). * **IP3/DAG Users (GOAT):** **G**nRH, **O**xytocin, **A**DH (V1 receptor), **T**RH. * **Tyrosine Kinase (Intrinsic):** Insulin, IGF-1, PDGF. * **JAK/STAT (Non-intrinsic TK):** Growth Hormone, Prolactin, Erythropoietin.

Practice by Chapter

Cell Surface Receptors: Types and Functions

Practice Questions

G-Protein Coupled Receptors

Practice Questions

Enzyme-Linked Receptors

Practice Questions

Second Messengers in Signal Transduction

Practice Questions

cAMP and cGMP Signaling

Practice Questions

Calcium as Second Messenger

Practice Questions

Inositol Phosphate Pathway

Practice Questions

MAP Kinase Cascades

Practice Questions

JAK-STAT Signaling Pathway

Practice Questions

Insulin Signaling Pathway

Practice Questions

Nuclear Receptors and Gene Regulation

Practice Questions

Defects in Signal Transduction and Disease

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