Signal Transduction Indian Medical PG Practice Questions and MCQs
Practice Indian Medical PG questions for Signal Transduction. These multiple choice questions (MCQs) cover important concepts and help you prepare for your exams.
Signal Transduction Indian Medical PG 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
Signal Transduction 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.
Signal Transduction Indian Medical PG Question 2: Which of the following statements about G protein-coupled receptors (GPCRs) is true?
- A. The three subunits alpha, beta, and gamma must remain together as a complex for G protein to function.
- B. G proteins can act as either inhibitory or excitatory based on the type of alpha subunit. (Correct Answer)
- C. G proteins bind directly to hormones to become activated.
- D. In the resting state, G proteins are bound to GTP.
Signal Transduction Explanation: ***G proteins can act as either inhibitory or excitatory based on the type of alpha subunit.***
- Different classes of Gα subunits (e.g., **Gαs**, **Gαi**, **Gαq**) couple to diverse downstream effectors, leading to either **stimulation** (excitatory) or **inhibition** of cellular processes.
- For example, **Gαs** activates adenylyl cyclase, while **Gαi** inhibits it, demonstrating their opposing roles.
*The three subunits alpha, beta, and gamma must remain together as a complex for G protein to function.*
- Upon activation, the **Gα subunit dissociates** from the **Gβγ dimer**, and both free units can then independently modulate effector molecules.
- For the G protein to function in signal transduction, the α subunit often separates from the βγ dimer to interact with its target enzyme or ion channel.
*G proteins bind directly to hormones to become activated.*
- **GPCRs** (the receptors themselves) bind to hormones or other ligands on the **extracellular side** of the membrane.
- The binding of the ligand to the GPCR induces a conformational change in the receptor, which then activates the associated G protein on the intracellular side.
*In the resting state, G proteins are bound to GTP.*
- In the **resting (inactive) state**, the Gα subunit of the trimeric G protein is bound to **GDP**.
- Activation occurs when the GPCR facilitates the exchange of **GDP for GTP** on the Gα subunit.
Signal Transduction Indian Medical PG Question 3: Which of the following statements best describes the mechanism of action of insulin on target cells?
- A. Insulin binds to a receptor on the outer surface of the plasma membrane, activating adenylate cyclase through the Gs protein.
- B. Insulin binds to a cytoplasmic receptor and is transferred as a hormone receptor complex to the nucleus to modulate gene expression.
- C. Insulin enters the cell and causes the release of calcium ions from intracellular stores.
- D. Insulin binds to a transmembrane receptor on the outer surface of the plasma membrane, activating the tyrosine kinase in the cytosolic domain of the receptor. (Correct Answer)
Signal Transduction Explanation: ***Insulin binds to a transmembrane receptor on the outer surface of the plasma membrane, activating the tyrosine kinase in the cytosolic domain of the receptor.***
- **Insulin** is a **peptide hormone** and cannot freely pass through the lipid bilayer, thus it binds to a **transmembrane receptor** on the cell surface.
- This binding leads to the activation of the receptor's intrinsic **tyrosine kinase activity** in the intracellular domain, initiating a signaling cascade.
*Insulin binds to a cytoplasmic receptor and is transferred as a hormone receptor complex to the nucleus to modulate gene expression.*
- This mechanism describes the action of **steroid hormones**, which are lipid-soluble and can cross the cell membrane, binding to **intracellular receptors**.
- **Insulin** acts via a **cell surface receptor** and its downstream effects are mediated through signal transduction pathways, not direct nuclear translocation.
*Insulin binds to a receptor on the outer surface of the plasma membrane, activating adenylate cyclase through the Gs protein.*
- This mechanism is characteristic of **G-protein coupled receptors (GPCRs)**, which activate or inhibit enzymes like adenylate cyclase via G-proteins to produce second messengers like cyclic AMP.
- The **insulin receptor** is a **receptor tyrosine kinase**, not a GPCR, and does not directly activate adenylate cyclase via Gs protein.
*Insulin enters the cell and causes the release of calcium ions from intracellular stores.*
- While some hormones and neurotransmitters can trigger the release of intracellular **calcium ions**, this is typically mediated by specific pathways (e.g., GPCRs linked to phospholipase C).
- **Insulin** does not directly enter target cells to cause calcium release; its actions are primarily mediated through receptor tyrosine kinase signaling pathways.
Signal Transduction Indian Medical PG Question 4: Which of the following are causes of GnRH dependent precocious puberty?
1. Constitutional
2. Tubercular Encephalitis
3. McCune-Albright syndrome
4. Primary hypothyroidism
- A. 1 and 2 only
- B. 1, 2 and 3 (Correct Answer)
- C. 2, 3 and 4
- D. 1, 3 and 4
Signal Transduction Explanation: ***1, 2 and 3***
- **Constitutional precocious puberty** is the most common form of central (GnRH-dependent) precocious puberty, where the hypothalamic-pituitary-gonadal axis matures prematurely without an underlying organic cause [1].
- **Tubercular encephalitis** can cause CNS lesions that stimulate the hypothalamus, leading to premature GnRH release and subsequent central precocious puberty [1].
- **McCune-Albright syndrome** is primarily associated with GnRH-independent precocious puberty, but in a small percentage of cases, chronic stimulation of the ovaries (due to activating GNAS mutations) can eventually lead to secondary central (GnRH-dependent) precocious puberty via an exhausted feedback mechanism.
*1 and 2 only*
- This option is incorrect because while constitutional precocious puberty and tubercular encephalitis are causes of GnRH-dependent precocious puberty, McCune-Albright syndrome can also lead to GnRH-dependent precocious puberty secondarily.
- It omits a valid cause, making it an incomplete answer.
*2, 3 and 4*
- This option incorrectly includes **primary hypothyroidism** as a cause of GnRH-dependent precocious puberty. Primary hypothyroidism is associated with GnRH-independent (peripheral) precocious puberty due to elevated TSH cross-reacting with FSH receptors.
- It also omits **constitutional precocious puberty**, which is the most common cause of GnRH-dependent precocious puberty [1].
*1, 3 and 4*
- This option incorrectly includes **primary hypothyroidism** as a cause of GnRH-dependent precocious puberty; it is a cause of GnRH-independent precocity.
- While constitutional precocious puberty is a correct inclusion, the presence of primary hypothyroidism makes this option incorrect for GnRH-dependent causes.
Signal Transduction Indian Medical PG Question 5: Agent that acts through tyrosine kinase receptor is
- A. Insulin (Correct Answer)
- B. MSH
- C. TSH
- D. TRH
Signal Transduction Explanation: ***Insulin***
- **Insulin** binds to its receptor, which is a **tyrosine kinase receptor**, leading to autophosphorylation and the activation of intracellular signaling pathways.
- This activation is crucial for glucose uptake and metabolism by various cells in the body.
*MSH*
- **Melanocyte-stimulating hormone (MSH)** acts primarily through **G protein-coupled receptors**, specifically melanocortin receptors.
- These receptors activate adenylyl cyclase, leading to an increase in intracellular cAMP.
*TSH*
- **Thyroid-stimulating hormone (TSH)** also acts via a **G protein-coupled receptor** on thyroid follicular cells.
- Its binding stimulates adenylyl cyclase, increasing cAMP and thus thyroid hormone synthesis and release.
*TRH*
- **Thyrotropin-releasing hormone (TRH)** binds to **G protein-coupled receptors** on pituitary thyrotrophs.
- This interaction activates the phospholipase C pathway, leading to the release of TSH.
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