Signal Transduction Pathways — MCQs
Which of the following is a G protein coupled receptor?
What is the primary function of G-proteins in cellular signaling?
All these hormones primarily use cyclic adenosine monophosphate (cAMP) as their main second messenger pathway, except:
Which of the following act through G protein coupled receptors?
Many signaling pathways involve the generation of inositol trisphosphate (IP3) and diacylglycerol (DAG). These molecules are involved in the regulation of which cellular processes?
Action of alpha-subunit of G protein is:
Which of the following genes is most commonly mutated in Juvenile myelomonocytic leukemia (JMML)?
Which of the following genes is least likely to be involved in the development of carcinoma of the colon?
The parvocellular pathway from lateral geniculate nucleus to visual cortex is most sensitive for the stimulus of
Which substance is most likely to increase in the rods of the retina when the light is turned on?
Signal Transduction Pathways Indian Medical PG Practice Questions and MCQs
Question 1: Which of the following is a G protein coupled receptor?
- A. M2 muscarinic receptor (Correct Answer)
- B. NMDA receptor
- C. Insulin receptors
- D. Steroid receptors
Explanation: ***M2 muscarinic receptor***- The **M2 muscarinic receptor** is a classic example of a **G protein-coupled receptor (GPCR)** [1]. When a ligand binds to a G-protein-coupled receptor, it triggers a mechanism where GDP is exchanged for GTP, causing the G-protein's alpha subunit to separate and initiate signaling pathways [1]. These heterotrimeric G-proteins couple cell surface receptors to catalytic units that form second messengers or directly to ion channels [1]. GPCRs are important regulators of nerve activity in the CNS and are receptors for neurotransmitters of the peripheral autonomic nervous system, with acetylcholine (ACh) being a ligand that regulates functions of glands and smooth muscle [2]. The **M2 muscarinic receptor** specifically activates an **inhibitory G protein (G_i)**, leading to a decrease in **cAMP** and opening of **potassium channels**. The effects of metabotropic receptors, like GPCRs, can last tens of seconds to minutes, contrasting with the brief effects of ionotropic receptors [4].*NMDA receptor*- The **NMDA receptor** is a **ligand-gated ion channel** that allows the influx of calcium and sodium ions [3]. It does not couple to G proteins, but directly mediates ion flow upon activation by **glutamate** and **glycine**. Ligand-gated ion channels open a central transmembrane ion channel when a neurotransmitter binds to sites on its extracellular domain [3].*Steroid*- **Steroid hormones** primarily act on **intracellular receptors** that, once activated, translocate to the nucleus to regulate gene expression. They are not cell surface receptors and do not utilize G protein signaling.*Insulin receptors*- **Insulin receptors** are **receptor tyrosine kinases** that, upon binding insulin, undergo autophosphorylation and activate intracellular signaling pathways. They signal through a cascade of protein phosphorylations, not through G proteins.
Question 2: What is the primary function of G-proteins in cellular signaling?
- A. Signal transducers (Correct Answer)
- B. Mediators of hormone action
- C. Molecules that bind hormones
- D. Intracellular signaling molecules
Explanation: ***Signal transducers*** - G-proteins act as **molecular switches**, converting extracellular signals received by G protein-coupled receptors (GPCRs) into intracellular responses. - They bind **GTP** in their active state and **hydrolyze it to GDP** to become inactive, regulating downstream effectors like enzymes and ion channels. *Mediators of hormone action* - While G-proteins are involved in the action of many hormones, this describes a *result* of their function rather than their fundamental role. - Their primary function is to transduce signals, which then mediates hormone effects. *Molecules that bind hormones* - **Receptors**, not G-proteins, are primarily responsible for binding hormones or other ligands. - G-proteins are activated *after* a receptor binds a ligand and undergoes a conformational change. *Intracellular signaling molecules* - This statement is true, but it's a broad category. **Signal transducers** specifically highlights their role in converting one form of signal to another. - Many molecules operate intracellularly, but G-proteins' unique role is in linking receptor activation to effector modulation.
Question 3: All these hormones primarily use cyclic adenosine monophosphate (cAMP) as their main second messenger pathway, except:
- A. Dopamine (Correct Answer)
- B. Glucagon
- C. vasopressin
- D. Corticotropin
Explanation: ***Dopamine*** - **Dopamine** has dual signaling mechanisms depending on receptor subtype, making it unique among the listed hormones. - **D1-like receptors** (D1, D5) couple to Gs proteins and **increase cAMP** levels. - **D2-like receptors** (D2, D3, D4) couple to Gi proteins and **decrease/inhibit cAMP** production. - Since dopamine's effects are mediated through both cAMP-increasing and cAMP-decreasing pathways with significant physiological roles for both, it does **not primarily use cAMP** as a straightforward second messenger like the other hormones listed. - Therefore, dopamine is the exception as it has mixed cAMP signaling rather than primarily activating the cAMP pathway. *Corticotropin (ACTH)* - **Corticotropin** (ACTH) binds to melanocortin-2 receptors (MC2R) on the adrenal cortex and **primarily utilizes the cAMP pathway**. - Activation of adenylyl cyclase leads to increased intracellular cAMP, which activates protein kinase A (PKA). - This stimulates the synthesis and release of glucocorticoids (primarily cortisol). *Glucagon* - **Glucagon** binds to its G-protein coupled receptors on hepatocytes, leading to activation of adenylyl cyclase and increased intracellular **cAMP**. - The cAMP then activates protein kinase A, mediating glucagon's metabolic effects including **glycogenolysis and gluconeogenesis**. - This is a classic example of cAMP-mediated hormone action. *Vasopressin* - **Vasopressin** (ADH) primarily acts through **V2 receptors** in the renal collecting ducts, which use the **cAMP pathway** to increase water reabsorption (its primary physiological function). - V1 receptors (vasoconstriction) use the IP3/DAG pathway, but this is a secondary effect. - Since vasopressin's main clinical action is via cAMP-mediated V2 receptors, it primarily uses cAMP as its second messenger.
Question 4: Which of the following act through G protein coupled receptors?
- A. Ach Muscarinic receptors (Correct Answer)
- B. Insulin receptors
- C. Ach Nicotinic receptors
- D. GABA-A receptors
Explanation: ***Ach Muscarinic receptors*** - All five **muscarinic acetylcholine receptors (M1-M5)** are **G protein-coupled receptors (GPCRs)** that mediate the parasympathetic nervous system's effects. - Activation of these receptors leads to downstream signaling through various G proteins, influencing cellular functions like heart rate and smooth muscle contraction. *Insulin receptors* - Insulin receptors are **receptor tyrosine kinases (RTKs)**, not GPCRs. - Upon insulin binding, they undergo autophosphorylation and activate intracellular signaling cascades involving **IRS proteins**, leading to glucose uptake. *Ach Nicotinic receptors* - Nicotinic acetylcholine receptors are **ligand-gated ion channels**, not GPCRs. - They open an ion pore in response to acetylcholine binding, allowing ions like sodium to flow through, resulting in rapid depolarization. *GABA-A receptors* - GABA-A receptors are also **ligand-gated ion channels**, specifically anion channels that are permeable to chloride ions. - When GABA binds, they open, allowing chloride influx, which typically hyperpolarizes the neuron and inhibits neural activity.
Question 5: Many signaling pathways involve the generation of inositol trisphosphate (IP3) and diacylglycerol (DAG). These molecules are involved in the regulation of which cellular processes?
- A. DNA replication and cell division
- B. Protein synthesis and degradation
- C. Lipid metabolism and fatty acid synthesis
- D. Calcium signaling and protein kinase C activation (Correct Answer)
Explanation: ***Calcium signaling and protein kinase C activation*** - **Inositol trisphosphate (IP3)** binds to **IP3 receptors** on the **endoplasmic reticulum**, triggering the release of **calcium ions (Ca²⁺)** into the cytoplasm, thereby initiating **intracellular calcium signaling cascades**. - **Diacylglycerol (DAG)** remains in the **plasma membrane** and activates **protein kinase C (PKC)**, which phosphorylates various target proteins to regulate cellular responses. - Together, IP3 and DAG form a classic **second messenger system** downstream of **phospholipase C (PLC)** activation, mediating responses from **G-protein coupled receptors (GPCRs)** and **receptor tyrosine kinases (RTKs)**. - This is the **primary and direct function** of the IP3/DAG signaling pathway in cell biology. *Lipid metabolism and fatty acid synthesis* - While DAG is structurally a **lipid intermediate** and can serve as a precursor in lipid metabolism, this is **not its primary role in signaling pathways**. - The question specifically asks about their involvement in **signaling pathways**, where IP3 and DAG function as **second messengers**, not metabolic intermediates. - Their generation from **PIP2 cleavage** by phospholipase C is a **signaling event**, not a metabolic one. *DNA replication and cell division* - Cell division is regulated by **cyclins**, **cyclin-dependent kinases (CDKs)**, and **checkpoint proteins**. - While IP3/DAG signaling can have **downstream effects** on cell proliferation through PKC activation, this is not their primary or direct role. - DNA replication machinery is not directly regulated by IP3 or DAG. *Protein synthesis and degradation* - **Protein synthesis** is primarily regulated by the **mTOR pathway**, **eIF factors**, and **ribosomal proteins**. - **Protein degradation** is mediated by the **ubiquitin-proteasome system** and **autophagy-lysosome pathway**. - While PKC activation can have indirect effects on these processes, IP3/DAG are not primary regulators of protein synthesis or degradation.
Question 6: Action of alpha-subunit of G protein is:
- A. Conversion of GDP to GTP
- B. Internalization of receptors
- C. Binding of agonist
- D. Breakdown of GTP to GDP (Correct Answer)
Explanation: ***Breakdown of GTP to GDP*** - The **alpha-subunit** of a G protein possesses intrinsic **GTPase activity**, meaning it can hydrolyze bound **GTP into GDP** and inorganic phosphate. - This **GTP hydrolysis** is crucial for **inactivating the G protein**, causing the alpha subunit to reassociate with the beta-gamma dimer and terminate the signaling cascade. *Conversion of GDP to GTP* - The conversion of **GDP to GTP** is a **replacement event** that occurs when an activated receptor promotes the dissociation of GDP from the alpha subunit, allowing GTP to bind. - This binding event **activates the G protein**, but the alpha subunit itself does not catalyze this conversion. *Internalization of receptors* - **Receptor internalization** is a process typically mediated by **clathrin-coated pits** and **endosomes**, where receptors are brought into the cell. - This process is involved in **receptor desensitization** and recycling, not a direct action of the G protein alpha subunit. *Binding of agonist* - The **binding of an agonist** occurs at the **receptor site**, specifically to the extracellular or transmembrane domains of the G protein-coupled receptor (GPCR). - This event **activates the receptor**, which then interacts with the G protein, but the alpha subunit does not directly bind the agonist.
Question 7: Which of the following genes is most commonly mutated in Juvenile myelomonocytic leukemia (JMML)?
- A. PTPN11 (Correct Answer)
- B. KRAS
- C. PTEN
- D. APC
Explanation: ***PTPN11*** - **PTPN11** encodes the SHP2 protein, a non-receptor protein tyrosine phosphatase that plays a crucial role in the **RAS-MAPK signaling pathway**. - **Germline or somatic mutations** in PTPN11 are found in approximately **35% of JMML cases**, making it the **most commonly mutated gene** in this disorder. - These gain-of-function mutations lead to constitutive activation of RAS signaling, driving the myeloproliferative phenotype characteristic of JMML. - PTPN11 mutations are also associated with **Noonan syndrome**, and patients with Noonan syndrome have an increased risk of developing JMML. *KRAS* - **KRAS** is a proto-oncogene encoding a GTPase in the RAS-MAPK pathway. Mutations cause constitutive activation and uncontrolled cell proliferation. - KRAS mutations are found in approximately **15-20% of JMML cases**, making it the **second most common** genetic alteration in this disease. - While definitely associated with JMML, KRAS mutations are **less frequent than PTPN11 mutations**. *PTEN* - **PTEN** is a tumor suppressor gene regulating the PI3K-AKT pathway, involved in various cancers including Cowden syndrome and endometrial cancer. - PTEN mutations are **not associated with JMML** pathogenesis. *APC* - The **APC gene** is a tumor suppressor in the Wnt signaling pathway, critical for colon epithelial regulation. - APC mutations cause **familial adenomatous polyposis (FAP)** and colorectal cancer, but are **not implicated in JMML**.
Question 8: Which of the following genes is least likely to be involved in the development of carcinoma of the colon?
- A. K-ras
- B. Beta-Catenin (Correct Answer)
- C. APC
- D. Mismatch Repair Genes
Explanation: ***Beta-Catenin*** - While **beta-catenin protein accumulation** is critical in colorectal cancer pathogenesis (primarily through APC mutations), direct mutations in the **CTNNB1 gene** (encoding beta-catenin) are **rare in colorectal cancer** (~5% of cases) [1]. - Most colorectal cancers achieve beta-catenin activation indirectly through **APC inactivation**, making beta-catenin gene mutations the least likely mechanism among the listed options [1]. - This contrasts with other cancers (e.g., hepatocellular carcinoma, endometrial cancer) where direct CTNNB1 mutations are more common. *APC* - The **adenomatous polyposis coli (APC) gene** is mutated in approximately **80% of sporadic colorectal cancers**, representing the earliest and most common genetic alteration in the **adenoma-carcinoma sequence** [1]. - APC loss leads to beta-catenin accumulation and constitutive **Wnt pathway activation**, driving uncontrolled cell proliferation [2]. - Germline APC mutations cause **familial adenomatous polyposis (FAP)** [5]. *K-ras* - **K-ras oncogene** mutations occur in **30-50% of colorectal cancers**, typically as an intermediate event in the adenoma-carcinoma progression [1]. - These activating mutations lead to constitutive signaling through the **MAPK pathway**, promoting cell proliferation and survival independent of growth factor signals. *Mismatch Repair Genes* - **Mismatch repair (MMR) genes** (MLH1, MSH2, MSH6, PMS2) are involved in **15-20% of all colorectal cancers** [4]. - Germline mutations cause **Lynch syndrome (HNPCC)** (~3% of CRCs) [5]. - Sporadic **MLH1 promoter hypermethylation** accounts for 12-15% of colorectal cancers, leading to **microsatellite instability (MSI-high)** tumors [3]. - MMR deficiency represents an alternative, well-established pathway of colorectal carcinogenesis. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. The Gastrointestinal Tract, p. 819. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Neoplasia, pp. 304-305. [3] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. The Gastrointestinal Tract, pp. 819-821. [4] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. Common Clinical Problems From Alimentary System Disease, pp. 373-374. [5] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. The Gastrointestinal Tract, pp. 821-822.
Question 9: The parvocellular pathway from lateral geniculate nucleus to visual cortex is most sensitive for the stimulus of
- A. Fine spatial detail (Correct Answer)
- B. Saccadic eye movements
- C. Luminance contrast
- D. Temporal frequency
Explanation: ***Fine spatial detail*** - The **parvocellular pathway** is specialized for processing **high-acuity vision**, including color and fine spatial resolution. - Its neurons have **small receptive fields** and respond best to detailed patterns and stationary objects. *Saccadic eye movements* - **Saccadic eye movements** are rapid eye movements controlled by various brain regions, but their generation and control are not directly the primary function of the parvocellular pathway. - These movements are involved in scanning the visual field to bring objects of interest onto the fovea, which is then analyzed by the parvocellular pathway. *Luminance contrast* - While the parvocellular pathway does respond to contrast, the **magnocellular pathway** is more specialized for detecting rapid changes in **luminance contrast** and motion. - The magnocellular pathway has larger receptive fields and processes information about movement and depth. *Temporal frequency* - **Temporal frequency**, or the rate of flicker or motion, is primarily processed by the **magnocellular pathway**. - This pathway is optimized for detecting movement and rapid changes in the visual scene, rather than fine spatial details.
Question 10: Which substance is most likely to increase in the rods of the retina when the light is turned on?
- A. Cyclic guanosine monophosphate (cGMP)
- B. Metarhodopsin II (Correct Answer)
- C. Cyclic adenosine monophosphate (cAMP)
- D. Rhodopsin
Explanation: ***Metarhodopsin II*** - When **light strikes rhodopsin**, it undergoes a conformational change, forming **metarhodopsin II**, which is the active form that initiates the phototransduction cascade. - **Metarhodopsin II** activates a **G-protein (transducin)**, leading to a decrease in cGMP and subsequent rod hyperpolarization. *Cyclic guanosine monophosphate (cGMP)* - **Light activation** of rhodopsin triggers a cascade that **decreases cGMP concentration** in the rods, leading to closing of cGMP-gated sodium channels. - In the **dark**, cGMP levels are high, keeping the sodium channels open and the rod depolarized. *Cyclic adenosine monophosphate (cAMP)* - **cAMP** is a significant second messenger in many cellular processes but is **not directly involved in the primary phototransduction pathway** in rods. - Its levels do not acutely increase in response to light in the same manner as molecules in the phototransduction cascade. *Rhodopsin* - **Rhodopsin** is the **light-sensitive pigment** located in the rod outer segment membranes. - When light is turned on, rhodopsin is **converted** into its active form, metarhodopsin II, meaning the amount of intact rhodopsin itself will decrease, not increase.
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