Cell Cycle and Nuclear Receptors Quiz

Cell Communication and Signal Transduction Pathway

• Cells communicate with each other and the environment.
• Signals are sent through the cell to regulate function.
• Quorum sensing in bacteria is defined.
• General steps in signal transduction are explained.
• Proteins involved in different types of cellular signaling are explained.

Signaling Types

• External signal from environment: Response to stimuli (temperature, light, pressure), foreign cells (bacteria, viruses), and chemical/antigens
• External signal from the organism: Hormones
• Cell-cell contact
• Internal signal from the cell: Autocrine, signal cascades

Bacteria – 2 Component System

• All bacteria have 2-component signaling systems.
• Sensor: Histidine protein kinase
• Effector: Response regulator (aspartic acid protein kinase)
• Kinases: Proteins which transfer a phosphate anion to another protein.
• Phosphorylation: Adding a phosphate anion to a protein.
• Phosphatases: Proteins which remove a phosphate from another protein.

Bacteria – 2 Component System Steps

• Receive input stimulus (osmolarity, pH, temperature, antibiotics).
• Autophosphorylation of Sensor (histidine kinase).
• Transfer of phosphate to Response Regulator (aspartic acid).
• Response - Gene regulation (turn on or off).
• Dephosphorylation of effector (turn off signal).
• Phosphorylated aspartate is unstable

Prokaryotic Communication

• Bacteria communicate with other bacteria to effectively invade host tissue.
• Bacteria may need to produce an enzyme to break down host defense proteins to gain a foothold.
• The enzyme must be effective against the host, requiring the bacteria to know how many bacteria are present.

Bacteria Quorum Sensing

• Cell density-dependent response.
• Bacteria sense the presence of enough bacteria to activate relevant genes. Bacteria are sending out an autoinducer (AI).
• The AI migrates back into the bacteria, and the concentration of AI increases as the bacteria grow.
• When a trigger point occurs, all bacteria activate a gene to produce a response or attack the host. (e.g. LasA in cystic fibrosis patients)

Quorum Sensing

• Bacteria transcribe Gene A to make protein A.
• Protein A creates and releases AI into the host environment.
• As the bacterial colony density increases, the concentration of AI increases and diffuses back into the bacteria.
• Eventually, the high concentration of AI turns on other genes to produce effective responses needed to attack host tissue.

Prokaryotic Cell Communication

• Bacteria use 2-component signaling and Quorum Sensing in an integrated fashion.
• Component systems can have several relay points.
• Bacteria receive multiple signals simultaneously (pH changes, temperature, host immune responses).
• All signals are integrated to create a response to survive.

Host-Pathogen Relationship

• Hosts and pathogens are in a constant evolutionary state.
• As one evolves to survive, it puts selective pressure on the other to evolve and survive.
• A bacteria may evolve to enter the host cell better.
• The host will respond by evolving to prevent bacteria entry.
• The host may evolve to recognize the bacteria faster.
• The bacteria may evolve to disguise itself to avoid recognition.
• The cycle continues; constant communication between the host cells, pathogen and the environment.

Bacterial Quorum Sensing Communication

• Bacteria may communicate with the host system through hormone recognition.
• Conversely, the host may sense bacteria through hormone receptors.

Eukaryotic Signal Transduction

• General Steps: Synthesis of signaling molecule, release of signaling molecule by the signaling cell, transport of signal to target cell, interaction of signal with target cell (membrane, cytosol, nucleus), change in cell behavior (up or down gene regulation), removal of signal from target cell.

Eukaryotic Signal Transduction Properties

• Fast response: Mediated through peptide hormones and catecholamines; fully active stored in vesicles; short half-life (seconds-minutes)
• Slow response: Mediated through steroid and growth hormones; synthesis is slow; stored in an inactive precursor form; released by diffusion; travels a longer distance; slower degradation half-life (hours-days).

Eukaryotic Cell to Cell Signal Transduction

• Three Primary Mechanisms: Contact (signaling cell directly contacts target cell, major mechanism of immune system), Gap Junctions (signaling across specialized channels-connexons), Secretion (release of signals into interstitial/bloodstream fluid).

Eukaryotic Signal Transduction

• Types of Secretion: Endocrine (cell remote from target cell-hormones), Paracrine (adjacent cells-neurotransmitters, immune response), Autocrine (target produces its own signaling molecule-growth factors, community effect).

Exocytosis

• Vesicles fuse with plasma membrane and release proteins/signal molecules.
• Two secretory pathways: Constitutive (continuously producing proteins, etc., for normal functioning), Regulated (specialized process of secretory cells; selecting proteins for release through clathrin-coated vesicles).

Signal Transduction

• Various types of signal transduction pathways are present in higher organisms (G-Protein Coupled Receptors, Receptor Tyrosine-Linked Kinases, Receptors with Intrinsic Enzymatic Activity). This includes receptor tyrosine kinases (receptor tyrosine phosphatases, receptor serine/threonine kinases, receptor guanylate cyclases).
• Includes Nuclear Receptors and Apoptosis

Signal Transduction and Secondary Messengers

• Signaling cascades involve a relay of protein activation (phosphorylation).
• Many signaling cascades use secondary messengers, including cAMP, lipids (DAG), and calcium.

G-Protein Coupled Receptors

• Membrane-bound receptors linking G-proteins to outside signals.
• Ligand binding domains on the ectoplasmic face; cytosolic domains activating G-proteins.
• Examples: Beta-Adrenergic Receptor, Rhodopsin
• Seven membrane-spanning regions, N-terminal segment on exoplasmic face, C-terminal segment on cytosolic face.
• Includes light-activated receptors, odorant receptors, hormone/neurotransmitter receptors (e.g., GABA).

G-Proteins

• Proteins which bind GTP and GDP.
• Act as on/off switches; inactive (GDP), active (GTP).
• Activate downstream proteins and interact with membrane receptors.

Note

• Terminology should be clear-distinguish receptors from ligands/proteins that interact with the receptor.
• Keep G-Protein terms separate from receptor terms.

Phosphatidyinositol Signaling

• PI is phosphorylated twice to produce PI(4,5)P2.
• PI-Phospholipase cleaves the phosphorylated inositol to produce secondary messengers, including IP3 and DAG.
• IP3 is a ligand for Calcium channels on the smooth ER, causing calcium release, and DAG activates Protein Kinase C.

Enzymatic Activity

• Receptor Tyrosine Kinases: Receptors that can auto-phosphorylate and pass signals to cell effectors.
• Receptor Tyrosine-Linked Kinases: Receptors lacking kinase activity, depending on another molecule for phosphorylation and passing signals to effectors. Linked to Tyrosine Kinases.

Intrinsic Enzymatic Activity

• Receptor Tyrosine Kinases (RTKs): Transmembrane protein receptors with ligand binding domains (ectoplasmic) and activating domains (cytoplasmic).
• Auto-phosphorylation transfers phosphates to substrate. Subsequent cascade effects activate downstream proteins (e.g., Growth Factor Receptor).

Receptor Tyrosine Kinases

• RTK signaling pathways regulate gene expression via effects on cell proliferation, differentation, survival, and cellular metabolism.
• RTKs transmit a growth signal, activating Ras, a GTPase protein that transmits the signal to downstream components.

Effector: Activation of Ras

• Ras is a proto-oncogene mediating growth hormone mitogenic stimuli.
• It's a membrane-anchored protein.
• Inactive Ras is bound to GDP, while active Ras is bound to GTP.
• Active Ras activates Raf, activating the MAP kinase pathway, leading to cell proliferation.

Activation of Ras

• GEFs (Guanine Exchange Factors) replace GDP with GTP to activate Ras.
• GAPs (GTPase Activating Proteins) enable Ras to hydrolyze GTP to GDP (inactivating Ras).

MAP Kinase Pathway

• MAP Kinase cascade uses signal transduction through several Protein Tyrosine Kinases.
• Ras activates Raf. Multi-step phosphorylation cascade of downstream protein kinases.
• Gene expression is eventually activated by a transcription factor via MAP kinases.

Nuclear Receptors

• Type 1: Nuclear receptors bound to HSP in the cytoplasm/nucleus. Ligand-dependent, activate transcription (e.g., steroid, estrogen, progesterone, glucocorticoid receptors).
• Type 2: Nuclear receptors located on the gene to repress genes. Ligand-dependent transcription (e.g., thyroid, retinoid, Vitamin D receptors).

Receptor Domain

• DNA Binding Domain: Highly conserved region.
• Ligand Binding Domain: Provides specificity to the ligand.
• N-Terminal Domain: Variable; contains phosphorylation sites, activates co-activators or co-repressors.

Type I – Nuclear Receptors

• Steroid binding to the receptor causes conformational changes and its release from the carrier. Translocation to the nucleus; Receptor/ligand complex dimerization; DNA interaction; Gene Transcription.

Nuclear Receptors – Steroid #1 and #2

• Receptor-ligand complex binding, coactivator recruitment, DNA binding, and transcription are the steps for both steroid #1 and #2.

Selective Estrogen Response Modulators (SERMS)

• SERMs are receptor modulators that can be agonists or antagonists in tissues.
• Examples include Raloxifene (bone tissue agonist), and Tamoxifen (breast cancer antagonist).

Type II Receptors (Thyroid)

• Translocation to nucleus. Binding to receptors. Remove co-repressor signal. Activate gene transcription.

Apoptosis

• Signaling pathway that leads to cell death: Controlled cell-death pathway, normal development (web removal), bacterial/viral infection (mediated by immune system), DNA damage and mutation.

Apoptosis – Initiation

• External through "Death Domain" receptors.
• Internal through stress detection.
• Targeting by an immune cell.
• Activation of caspases (cysteine/aspartic acid cleavage proteases); cell shrinkage; nuclear breakup; surface bleb formation; engulfment by phagocytes.

The Mitochondria and Apoptosis

• Internal cellular damage causes stress, sensed by the endoplasmic reticulum (RER; affects protein synthesis).
• Signal sent to activate proapoptotic proteins (Bax, Bad).
• Activation of Bax/Bad causes mitochondrial membrane permeability, releasing cytochrome c (a messenger).
• Cytochrome c triggers caspase-9 activation, activating the procaspases, which in turn activates downstream effector caspases.

Receptor-mediated Apoptosis Pathway

• Tumor Necrosis Factor (TNF) binds to its receptor, activating death domains to recruit adaptor proteins FADD and TRADD.
• This provides caspase-8 activation.
• Caspase-8 cleaves caspase-3, which is the major executioner caspase targeting the nucleus.

Integration of Signals

• Eukaryotes integrate simultaneous signals.
• Convergence: Multiple signals use the same effector.
• Divergence: Similar ligand triggers different pathways based on tissue/receptor type or other signals
• Crosstalk: Different signaling cascades communicate. Multiple downstream events can arise from one single signaling event.

p53 Normally Inactive

• p53 is a transcription factor normally inactive.
• Good growth: Ubiquitin-mediated degradation by MDM2.
• DNA damage: Activation of p53 and p21 production. MDM2 can no longer target p53 leading to cell-cycle arrest. p21 blocks cyclin/CDK from being active.

Cell Division Cycle Mutants: Yeast

• Researchers create mutants in yeast using temperature sensitivity as a marker (grow at 25°C but not 35°C)
• Shifting growth temperatures from high to low indicates if mutations disrupt the cell cycle.

Test your knowledge on the intricacies of the cell cycle, particularly the roles of various cyclins and nuclear receptors. This quiz covers key concepts such as the G1 to S phase transition, ubiquitin ligase functions, and the specifics of nuclear receptor activity. Perfect for students of molecular biology and biochemistry.