Cell Communication and Signal Transduction Pathways in Biology

What is a signal molecule?

A chemical messenger that facilitates communication between cells.

What process involves the binding of extracellular signals to receptors in target cells?

Signal transduction.

What are the three requirements for cell communication?

A signaling cell, a chemical messenger, and a target cell with a receptor.

What are the two main types of signals used in cell communication?

Extracellular signals and intracellular signals.

What is the role of relay molecules in a signal transduction pathway?

They activate in succession to pass a message down the pathway.

What types of targets can be affected by cell signaling?

Cytoplasmic targets (enzymes, cytoskeleton, membrane proteins) and nuclear targets (transcription factors, splicing factors).

Why must signal transduction pathways be turned off?

To prevent unnecessary responses when the extracellular signal is no longer present.

What can result from a failure to properly activate or deactivate a signal transduction pathway?

Diseases such as diabetes or cancer.

What is the function of insulin in the body?

To lower blood glucose levels after meals by facilitating glucose transport into cells.

What characterizes Type 1 diabetes?

Autoimmune destruction of insulin-secreting beta cells in the pancreas.

What is a common treatment for Type 1 diabetes?

Injections of synthetic insulin.

What is Type 2 diabetes characterized by?

Insulin resistance in tissues, leading to high insulin levels in the blood.

What are growth factors?

Paracrine signals that regulate cell division and numbers in nearby tissues.

What happens if a mutation occurs in a growth factor signal transduction pathway?

Cell division may continue uncontrollably, potentially leading to cancer.

What is an oncogene?

A mutated gene that can lead to cancer, often involved in cell division regulation.

What is the difference between conventional cancer treatments and targeted cancer therapies?

Conventional treatments non-specifically inhibit cell division, while targeted therapies block specific overactive proteins in cancer.

Give an example of targeted cancer therapy.

Tarceva and Iressa, which block overactive EGF receptors in lung adenocarcinoma patients.

What is hyperglycemia?

High levels of glucose in the blood, which can be toxic and lead to serious health issues.

What are the different types of receptors involved in cell communication?

Receptor kinases, G protein-coupled receptors, intracellular receptors, and ion channel receptors.

What are second messengers in signal transduction?

Cyclic AMP (cAMP) and calcium ions (Ca2+).

What is the role of a receptor in cell communication?

A receptor binds to a ligand, changing its conformation and activating a site inside the cell to transmit the signal.

What happens if a cell does not express a receptor for a particular ligand?

The cell is blind to that signal and cannot respond to it.

What are protein kinases?

Enzymes that transfer a phosphate group from ATP to a protein, thereby phosphorylating it.

How are protein kinases classified?

By location (cytoplasmic or receptor kinases) and by what they phosphorylate (tyrosine, serine/threonine, or lipids).

What is the function of receptor tyrosine kinases?

They are involved in signaling pathways for growth factors and insulin, activated by ligand binding leading to dimerization and autophosphorylation.

What occurs during the activation of receptor tyrosine kinases?

Ligand binding causes dimerization, activating the tyrosine kinase, which leads to autophosphorylation.

What is the significance of the phosphate groups on activated receptor tyrosine kinases?

They provide a scaffold for binding other signaling proteins, activating multiple downstream signaling pathways.

What are G protein-coupled receptors (GPCRs)?

Receptors that include many classic hormones and neurotransmitters, characterized by seven a-helices across the membrane.

What is the role of GDP and GTP in G protein function?

GDP is bound in the inactive state, while GTP is bound in the active state, enabling the G protein to interact with other components.

What are heterotrimeric G proteins?

Large G proteins activated by GPCRs, consisting of three subunits: alpha, beta, and gamma.

What are monomeric G proteins?

Small G proteins that are components of intracellular signaling pathways, not directly coupled to receptors.

What is the function of intracellular receptors?

They bind to steroid hormones, moving into the nucleus to function as transcription factors that activate gene transcription.

What is a ligand-gated ion channel?

An ion channel receptor that opens in response to a ligand binding, allowing ions to flow into the cell.

How can ion channel receptors be activated?

By extracellular ligands, intracellular ligands, or changes in membrane voltage.

What is the process of receptor dimerization?

The linking of two receptors together upon ligand binding, crucial for the activation of receptor tyrosine kinases.

What is autophosphorylation?

The process where each receptor in a dimer adds phosphate groups to the other, activating the receptor's signaling capabilities.

What is the role of calcium ions (Ca2+) in cell signaling?

They act as a second messenger, generating intracellular responses when they flow into the cell.

What types of ligands can bind to intracellular receptors?

Small hydrophobic molecules, such as steroid hormones like testosterone and estrogen.

What happens when a ligand binds to a GPCR?

It activates the receptor, causing a conformational change that allows interaction with a G protein.

What is the significance of receptor specificity?

Receptors are specific for one ligand, determining the cell's response to various signals.

What are the two main classes of protein kinases?

Serine/threonine kinases and tyrosine kinases.

What is the function of lipid kinases?

They add phosphate groups to lipids rather than proteins.

What is the general pathway of receptor tyrosine kinase signaling?

1. Ligand binds to receptor. 2. Dimerization and activation. 3. Autophosphorylation. 4. Binding of other molecules to initiate signaling.

How many distinct kinase genes are there in the genome?

More than 500 distinct kinase genes.

What is the role of the hormone-receptor complex in the nucleus?

It functions as a transcription factor that activates the transcription of specific genes.

What activates ion channels?

Ion channels can be activated by extracellular ligands, intracellular ligands, or changes in membrane voltage.

What determines the direction of ion flow through channels?

The direction of flow depends on the electrochemical gradient of the ions.

What are the first discovered second messengers?

Ca2+ ions and cyclic AMP (cAMP).

Why are second messengers important?

They are produced in large amounts and diffuse widely to spread the signal throughout the cytoplasm.

Why is Ca2+ considered a good signal?

Because cells maintain low concentrations of Ca2+ in the cytosol, so even a small increase represents a significant signal.

How is the Ca2+ signal generated?

By the binding of extracellular messenger molecules to G protein-coupled receptors, activating phospholipase C, which generates inositol trisphosphate (IP3).

What is the role of inositol trisphosphate (IP3) in Ca2+ signaling?

IP3 diffuses through the cytoplasm and opens ligand-gated Ca2+ ion channels in the endoplasmic reticulum membrane.

What is the function of cyclic AMP (cAMP)?

cAMP activates kinases such as protein kinase A (PKA), which phosphorylates target proteins to elicit a cellular response.

What amplifies signals in signal transduction pathways?

G proteins, second messengers like Ca2+ and cAMP, and kinase cascades.

How does a kinase cascade amplify a signal?

By phosphorylating and activating other kinases, increasing the number of affected molecules geometrically.

What is an example of signal amplification in response to epinephrine?

Activation of one receptor can lead to the production of 100,000,000 glucose molecules from glycogen.

What happens when a signal transduction pathway is overactive?

It can lead to uncontrolled cellular responses, such as excessive cell division in cancer.

What are oncogenes?

Genes that cause cancer due to gain of function mutations in kinases or G proteins, leading to overactive signaling pathways.

What are tumor suppressor genes?

Genes that normally turn off signaling pathways to inhibit cell division; loss of function mutations can lead to cancer.

How does cholera toxin affect signal transduction?

It inactivates a GTPase activating protein (GAP), causing a G protein to remain permanently active, leading to excessive chloride transport and water loss.

What is the effect of caffeine on signal transduction?

Caffeine inhibits cAMP phosphodiesterase, preventing cAMP inactivation, and also blocks adenosine receptors, increasing wakefulness.

What is Brody disease?

A genetic disease caused by a defect in a Ca2+ pump, leading to symptoms like cramps and slow muscle relaxation due to impaired Ca2+ signaling.

What is the role of Ca2+ pumps in muscle contraction?

They help turn off the Ca2+ signal by pumping Ca2+ back into the sarcoplasmic reticulum after muscle contraction.

What is the function of phosphatases in signal transduction?

They turn off signaling pathways by dephosphorylating target proteins.

What is the role of G proteins in signal transduction?

They act as molecular switches that relay signals from receptors to target enzymes or ion channels.

What is the significance of the electrochemical gradient for ions?

It determines the direction and flow of ions through ion channels.

What is the primary messenger in signal transduction?

The extracellular signal that initiates the signaling cascade.

What is the function of calmodulin in Ca2+ signaling?

It binds Ca2+ ions and activates kinases that propagate the signal.

What happens during the fight or flight response regarding glycogen?

Epinephrine triggers the breakdown of glycogen to glucose for rapid energy.

What is the role of adenylyl cyclase in signal transduction?

It converts ATP to cyclic AMP (cAMP) in response to extracellular signals.

How does a loss of function mutation in tumor suppressor genes lead to cancer?

It prevents the genes from turning off signaling pathways that inhibit cell division, leading to uncontrolled growth.