Cell Cycle and Signaling Pathways Quiz

Cell Structure Overview

• Cell signaling is a chapter in the broader field of cell biology, covering how cells communicate and respond to signals.

Extracellular Signal Molecules and Receptors

• Extracellular signal molecules bind to specific receptors on the cell surface or within the cell.
• Cell-surface receptors bind hydrophilic signal molecules.
• Intracellular receptors bind small, hydrophobic signal molecules, often carried by carrier proteins.

Intracellular Signaling Pathways

• Extracellular signals initiate intracellular signaling pathways.
• These pathways involve intracellular signaling proteins that relay and amplify the signal.
• They often involve phosphorylation cascades or GTP-binding proteins.
• Effector proteins ultimately carry out the cellular response, altering metabolism, gene expression, or cytoskeletal movement.

Types of Receptors

• Receptor types include steroid hormone receptors, G-protein-coupled receptors (GPCRs), receptor protein-tyrosine kinases (RTKs), non-receptor protein-tyrosine kinases, and ligand-gated channels.
• Specific receptors exist for B and T cells, showcasing the body’s immune response.

Signaling by Phosphorylation

• Kinases and phosphatases are frequent components of intracellular signaling pathways, where phosphorylation/dephosphorylation are key mechanisms.
• Phosphorylation often activates proteins, while dephosphorylation inactivates them.
• GTP-binding proteins are essential parts of signaling pathways, switching between active (GTP-bound) and inactive (GDP-bound) states.

cAMP Phosphodiesterase (PDE)

• PDE enzymes break down the cyclic AMP (cAMP) second messenger molecule, which can affect thyroid cell proliferation.
• Inhibiting PDE would increase cAMP levels.

Receptor Tyrosine Kinases (RTKs)

• These receptors, activated by growth factors, initiate downstream signaling cascades that can lead to cell proliferation.
• The activated receptor tyrosine kinase can phosphorylate other proteins (downstream signaling molecules).
• SH2 domains bind to specific phosphotyrosine-containing peptide sequences on the activated receptors.

Non-receptor Tyrosine Kinases

• These are not intrinsic to the receptor itself but are associated proteins involved in signaling.
• Ligand binding triggers the dimerization of the receptor, enabling cross-phosphorylation and thus activation of associated nonreceptor tyrosine kinases.
• Kinases phosphorylate tyrosine residues on the receptor, creating sites for downstream signaling molecules.

Integrin Signaling via FAK

• Integrins are receptors that connect cells to the extracellular matrix (ECM), influencing cell behavior.
• The non-receptor tyrosine kinase FAK is crucial in integrin signaling.
• Integrin binding leads to FAK activation by autophosphorylation.
• Src then phosphorylates FAK, creating more downstream signaling sites.

Ras Proteins and Ras-MAP Kinase Cascades

• Ras is an important G protein, crucial for multiple signaling pathways, embedded in cell membranes via a lipid anchor.
• Ras proteins are activated by GTP/GDP binding and play a significant role in cell signaling.
• Oncogenic Ras versions cause unregulated cell proliferation, commonly implicated in cancer growth.

Generalized Ras-MAP Kinase Cascade

• Growth factor binding triggers receptor activation, initiating a complex series of protein phosphorylations.
• This results in the activation of transcription factors, ultimately regulating cell proliferation via downstream signaling pathways.
• The cascade typically includes three kinase levels (MAPKKK, MAPKK, MAPK).

ERK Downstream Signaling

• Activated ERK (extracellular signal-regulated kinase) translocates to the nucleus.
• ERK then phosphorylates transcription factors such as Elk-1. These complexes (with SRF) bind to the serum response element (SRE) in the DNA.
• Consequently, genes related to cell proliferation are upregulated, leading to more cell growth.

MAP Kinase Activation

• Mammalian cells have multiple interconnected MAP kinase cascades to control diverse cellular functions, such as proliferation, differentiation, survival, inflammation, and cell death responses.
• Different cascades respond to distinct stimuli (such as growth factors versus inflammatory cytokines), with shared components and crosstalk between them.
• Several key kinases are important to consider (e.g. ERK, JNK, p38).

Scaffold Proteins

• Scaffold proteins organize signaling cascades to bring components into close proximity, increasing the efficiency of signal transduction.
• Through proximity, signaling cascades can be regulated and occur faster, increasing the efficacy and precision of the proteins involved in the pathways.

Second Messengers

• Second messengers are intracellular substances, triggered by a first messenger (ligand), released into the cell interior after receptor binding on the cell surface.
• They amplify signals and may affect various cellular processes; include phospholipid-derived molecules (such as inositol triphosphate) as well as cyclic AMP (cAMP) and nitric oxide.

Phosphatidylinositol-Derived Second Messengers

• Phosphatidylinositols, PI(4,5)P2 and PI(3,4,5)P3, are critical to cellular messaging.
• Specific enzymes act on these key molecules in cell membranes, and the resultant molecules relay signals or induce cellular pathways.

PI 3-Kinase/Akt Pathway

• The PI3-kinase/Akt pathway is crucial for regulating cell growth, survival, and metabolism.
• PI-3-kinase phosphorylates PIP2, forming PIP3, which recruits other proteins, including Akt, to the cell membrane.
• Akt activates diverse downstream targets by phosphorylation.

FOXO Regulation

• FOXO is a transcription factor affecting gene expression and is regulated by Akt/PI3K pathways.
• Growth factor signaling inhibits FOXO nuclear activity, which plays a key role in cell growth/survival.
• In the absence of growth factors, FOXO is de-phosphorylated and translocates to the nucleus, turning on genes essential to survival.

NF-κB Signaling

• NF-κB (a transcription factor) is part of a critical signaling pathway activated by tumor necrosis factor receptor (TNF) activation.
• TNF receptor activation initially leads to the recruitment of adaptor proteins to initiate the IKB kinase pathway.
• Phosphorylated IKB is targeted for ubiquitin and proteasome degradation, releasing NF-κB to enter the nucleus and promote target gene expression.

Feedback Loops and Signaling Dynamics

• Feedback loops are key parts of signaling pathways, as they maintain a balance and allow for fine-tuning of cellular responses.
• These critical loops, involving gene activation/regulation of IKB for NF-κB, affect signaling dynamics and cellular activity.

Crosstalk between Pathways

• Signaling pathways frequently interact, influencing each other through positive or negative feedback loops.
• The ERK and PI-3-kinase/Akt pathways exhibit significant crosstalk, showing how different signaling systems may influence each other.

The Eukaryotic Cell Cycle

• Eukaryotic cells are governed by a conserved cell cycle with distinct phases regulated by multiple checkpoints controlling the cell cycle (G1, S, G2, M).
• Cell cycles in different organisms have variations in the length of their phases.

Embryonic Cell Cycles

• Embryonic cell cycles lack G1 and G2 phases during early divisions, focusing primarily on cell division (S and M phases).
• Key cell cycle research is based on temperature-sensitive mutant yeast used to elucidate and test these mechanisms.

DNA Content Determination

• Flow cytometry is a useful tool to evaluate the DNA content of cells during the cell cycle.
• Distinctive DNA content peaks allow the identification of cells in particular phases (G1/G2/M/S).

Cell Cycle Regulation in Budding Yeast

• Budding yeast (Saccharomyces cerevisiae) illustrates a key cell cycle regulation point in G1 called Start.
• Regulation here depends on factors like nutrient availability and cell size.

Animal Cell Cycles and Growth Factors

• Animal cell cycles have a restriction point in G1, analogous to Start in yeast.
• Cell entry into quiescence in G0 occurs in the absence of growth factors.

Cell Cycle Checkpoints

• Cell cycle checkpoints ensure accuracy and integrity. Damaged DNA is repaired or the cycle is halted to ensure accurate replication.
• These include checkpoints in G1, G2, and mitosis to address any DNA damage. DNA replication problems can also trigger arrests at these checkpoints. The spindle assembly checkpoint is an example of a mitosis-specific checkpoint.

Eukaryotic Cell Cycle Regulators

• Eukaryotic cell cycle progression is governed by a conserved set of protein kinases called Cyclin-dependent kinases (CDKs) and their regulatory subunits, called cyclins.
• These kinases, and their cyclin partners, are fundamental to driving cell cycle progression.

Maturation Promoting Factor (MPF)

• MPF is a protein kinase complex (cyclin-dependent kinase and cyclin) that triggers entry into M (mitosis) phase, based on levels of cyclin.
• In frog oocytes, MPF activation is triggered by progesterone, initiating cell division.

Cyclin Regulation

• Cyclins are key regulators, fluctuating throughout the cell cycle and in response to extracellular signals.
• Specific cyclin pairs drive the cell through various phases (e.g., cyclin D and cyclin-dependent kinases Cdk4 and Cdk6).

Cdk Inhibitors

• Cyclin-dependent kinases are also regulated by inhibitors, including those belonging to the Ink4 and Cip/Kip family.
• These inhibitors are essential in monitoring the cycle progression and ensuring that there are no irregularities.

D-Type Cyclins

• In animal cells, D-type cyclins are important for activating Cdk4/Cdk6, and they are a crucial link between the activity of growth factors and cell cycle progression.
• These drive cell cycle progression through the restriction point.

Rb Protein Regulation

• In its underphosphorylated form, Rb binds to the E2F transcription factor, preventing cell cycle progression.
• Phosphorylation of Rb by cyclin-dependent kinases (CDK4/6) releases E2F and activates genes required for cell cycle progression.