4. Secondary messengers 2

What is the typical concentration of free cytosolic $Ca^{2+}$ in a resting cell?

Approximately $0.1\ \mu mol/L$.

What is the normal extracellular concentration range for Ca^{2+} ions?

2.2-2.6\ mmol/L.

Which organelle serves as a major intracellular store for Ca^{2+} with significantly higher concentrations than the cytosol?

The Endoplasmic Reticulum (or Sarcoplasmic Reticulum).

Which type of voltage-gated Ca^{2+} channels open upon membrane depolarisation to allow ion influx?

L-type Ca^{2+} channels.

Name the Ca^{2+}-ATPase responsible for transporting Ca^{2+} from the cytosol into the endoplasmic reticulum.

SERCA (Sarco/Endoplasmic Reticulum Ca^{2+}-ATPase).

What is the function of the Plasma Membrane Ca^{2+}-ATPase (PMCA)?

It extrudes Ca^{2+} out of the cell into the extracellular space.

How many Na^{+} ions are typically imported for every one Ca^{2+} ion extruded by the Na^{+}/Ca^{2+} exchanger (NCX)?

Three Na^{+} ions.

In which two cell types is the Na^{+}/Ca^{2+} exchanger (NCX) particularly important for maintaining Ca^{2+} homeostasis?

Cardiomyocytes and neurons.

Which Ca^{2+}-binding protein acts as a primary cytosolic Ca^{2+} sensor and regulates various enzymes?

Calmodulin (CaM).

Where is the Ca^{2+}-buffer protein calreticulin primarily located within the cell?

Within the lumen of the endoplasmic reticulum.

Name a cytosolic protein that serves specifically as a Ca^{2+} buffer to maintain low free ion concentrations.

Calbindin.

Give an example of a short-term physiological response mediated by Ca^{2+} signalling.

Muscle contraction (excitation-contraction coupling) or neurotransmitter exocytosis.

List one long-term cellular response regulated by Ca^{2+} as a secondary messenger.

Regulation of gene expression, cell proliferation, or apoptosis.

Which Ca^{2+}/phospholipid-dependent kinase is activated by the combination of DAG and increased cytosolic Ca^{2+}?

Protein Kinase C (PKC).

What clinical condition is associated with a defective ryanodine receptor (RyR1) leading to uncontrolled Ca^{2+} release?

Malignant hyperthermia.

How does the drug Verapamil lower blood pressure in smooth muscle?

By blocking L-type Ca^{2+} channels to reduce ion influx and promote relaxation.

To which muscle protein is Calmodulin (CaM) structurally and functionally homologous?

Troponin C.

Which Ca^{2+}/CaM-dependent phosphatase modulates signalling by dephosphorylating target proteins?

Calcineurin (Phosphoprotein Phosphatase 2B).

Name the Ca^{2+}-dependent protease involved in various cellular degradation processes.

Calpain.

Which enzyme converts glycogen phosphorylase into its active form under the control of Ca^{2+}?

Phosphorylase kinase.

Phospholipase C (PLC) hydrolyses PIP_2 to produce which two secondary mediators?

IP_3 (Inositol-1,4,5-trisphosphate) and DAG (Diacylglycerol).

What is the specific role of IP_3 in the phosphoinositide signalling system?

It mobilises Ca^{2+} from intracellular stores like the endoplasmic reticulum.

What is the primary function of Diacylglycerol (DAG) in the plasma membrane?

It activates Protein Kinase C (PKC).

Which enzyme releases Arachidonic Acid (AA) from membrane phospholipids?

Phospholipase A_2 (PLA_2).

Arachidonic acid is converted into prostaglandins and thromboxanes via which metabolic pathway?

The Cyclooxygenase (COX) pathway.

Which COX isoform is inducible and primarily associated with inflammatory responses?

COX-2.

What are the primary products of the Lipoxygenase (LOX) pathway of arachidonic acid metabolism?

Leukotrienes (e.g. LTB_4, LTC_4).

Which leukotrienes are specifically associated with bronchospasm in asthma?

LTC_4, LTD_4, and LTE_4.

Which enzyme generates PIP_3 from PIP_2 to initiate the AKT signalling pathway?

Phosphatidylinositol-3-kinase (PI3K).

What is the role of the p85 subunit in the PI3K heterodimer?

It is the regulatory subunit that binds to phosphorylated tyrosine residues on RTKs via SH2 domains.

Which protein kinase is recruited to the membrane by PIP_3 and subsequently activates AKT?

PDK1 (Phosphoinositide-dependent kinase-1).

Name the phosphatase that acts as a negative regulator of the PI3K pathway by converting PIP_3 back to PIP_2.

PTEN (Phosphatase and Tensin Homolog).

List two metabolic effects of activated AKT (Protein Kinase B).

Increased glucose transport (GLUT4) and increased protein synthesis.

What is the primary function of the mTORC1 complex in the cell?

It acts as a nutrient and growth sensor to regulate protein synthesis and inhibit autophagy.

Which specific amino acid is a known activator of the mTORC1 signalling pathway?

Leucine.

How does mTORC1 activity change during periods of cellular starvation?

It is inhibited, leading to the activation of autophagy.

Which mTOR complex is primarily responsible for regulating the actin cytoskeleton and cell mobility?

mTORC2.

The enzyme sphingomyelinase breaks down sphingomyelin to produce which stress-related mediator?

Ceramide.

What is the typical cellular outcome of increased intracellular ceramide levels?

Induction of apoptosis.

Which enzyme phosphorylates sphingosine to generate the signalling molecule Sphingosine-1-phosphate (S1P)?

Sphingosine kinase (SphK1).

How does S1P influence cell fate compared to ceramide?

S1P promotes cell survival and migration, whereas ceramide promotes apoptosis.

What is the clinical significance of a sphingomyelinase defect in Niemann-Pick disease?

Ceramide deficiency leads to reduced apoptosis.

In the phosphoinositide system, which heterotrimeric G-protein subunit typically activates Phospholipase C (PLC)?

The G_q alpha-subunit.

Which secondary mediator is the first to be discovered in cellular signalling?

cAMP (cyclic adenosine monophosphate).

What is the effect of AKT activation on cellular apoptosis?

It decreases apoptosis (promotes cell survival).

Which enzyme converts ATP into cAMP?

Adenylate cyclase.

Which second messenger is generated by the action of guanylyl cyclase?

cGMP.

The combination of Ca^{2+} and which protein is required to activate CaMKII?

Calmodulin (CaM).

Which Ca^{2+}-binding protein is found specifically in the heart to regulate Ca^{2+} levels?

Sorcin.

Which secondary mediator acts on IP_3 receptors located on the endoplasmic reticulum membrane?

IP_3 (Inositol-1,4,5-trisphosphate).

What does the 'L' in L-type Ca^{2+} channel stand for?

Long-lasting.

Which signalling pathway is predominantly involved in regulating smooth muscle actin-myosin complexes?

The Ca^{2+}/Calmodulin pathway (via Myosin Light Chain Kinase).

What is the biochemical precursor for all eicosanoids, including prostaglandins?

Arachidonic acid (AA).

Which protein domain allows PDK1 and AKT to bind to PIP_3 at the plasma membrane?

The PH (Pleckstrin Homology) domain.

Which mTOR complex is inhibited by the drug rapamycin?

mTORC1.

Name the transcription factor that creates a positive feedback loop by inducing SphK1 after activation by intracellular S1P.

NF-\kappa B.

How do NSAIDs reduce inflammation biochemically?

By inhibiting Cyclooxygenase (COX) enzymes to reduce prostaglandin synthesis.

Which ion gradient is used by the NCX to drive the export of Ca^{2+}?

The Na^{+} gradient.

Which enzyme converts S1P back into sphingosine or degrades it?

The source material specifies formation via SphK1; degradation/reversion is implied as regulation, though the specific degradation enzyme name is not in the text.

What effect does high ATP (energy levels) have on mTORC1 activity?

It activates mTORC1.

Which protein mediates the adhesion and membrane stabilisation effects of Ca^{2+}?

Annexins.

Which secondary messenger pathway is activated by PDGF binding to its RTK to promote cell survival?

The PI3K-AKT pathway.

What is the function of the p110 subunit in PI3K?

It is the catalytic subunit responsible for phosphorylating PIP_2.

Which secondary mediator is lipid-soluble and remains in the plasma membrane to activate PKC?

DAG (Diacylglycerol).

Name a physiological process that involves stimulus-secretion coupling mediated by Ca^{2+}.

Insulin secretion from pancreatic beta cells or neurotransmitter release.

Which enzyme is the target for LTB_4 function in inflammation?

None; LTB_4 acts as a chemotactic agent to activate polymorphonuclear leukocytes.

What effect do corticosteroids have on the activity of Phospholipase A_2 (PLA_2)?

They inhibit PLA_2, thereby reducing arachidonic acid release.

Which enzyme is constitutive (always present) in most tissues for basic prostaglandin production?

COX-1.

Which lipid mediator regulates cell migration by binding to extracellular receptors such as S1PR1?

Sphingosine-1-phosphate (S1P).

Which organelle's Ca^{2+} concentration is roughly 10^{-4}\ M?

The Endoplasmic/Sarcoplasmic Reticulum.

Which pump is ATP-dependent and maintains low cytosolic Ca^{2+} by pumping it out of the cell?

PMCA (Plasma Membrane Ca^{2+}-ATPase).

What is the relationship between mTORC1 and autophagy?

mTORC1 inhibits autophagy.

Which Ca^{2+}-dependent enzyme is responsible for glycogenolysis control?

Phosphorylase kinase.

What effect does AKT have on glucose transport?

It increases glucose transport by promoting GLUT4 translocation to the membrane.

Which signalling molecule creates a 'cell fate decision' balance with S1P?

Ceramide.

What is the effector molecule for the PI3K pathway activated by insulin?

PIP_3.

How does Ca^{2+} contribute to adaptive neuronal processes?

Via regulation of synaptic plasticity.

Which Ca^{2+}-binding protein is located on myosin light chains to facilitate muscle contraction?

Myosin light chains themselves act as Ca^{2+}-binding proteins.

Which enzyme is stimulated by growth factors to produce S1P?

Sphingosine kinase (SphK).

What determines whether mTORC1 is 'ON' or 'OFF' regarding cell growth?

Nutrient availability, energy levels (ATP), and growth factor signals.