Cell Signaling
Why do cells need signaling? Where can these signals come from/where do they go? How are they transmitted
-cell signaling = how cells communicate with each other; "ability of cells to perceive signals from outside the cell and alter cellular activities in response"
-signals may come from the environment OR from other cells
-transmitted across cell membrane(s), or via interacting with INTEGRAL membrane proteins
How does the MAPK pathway begin?
Hint: What is Ras?
-Ras: "type of G-protein"
--> Ras activity controlled by whether it is bound to GDP (inactive) or GTP (active)
--> for receptor signal transduction, the phosphorylated receptor brings Ras activating protein to Ras at the plasma membrane
What is mitosis?
What is the purpose of mitosis in eukaryotes?
1) Mitosis: "division of genetic material in nucleus that produces daughter cells GENETICALLY IDENTICAL to parent cell"
**Mitosis is only involved in production of SOMATIC (body) CELLS**
-growth/wound repair in MULTIcellular eukaryotes
-ASEXUAL reproduction in UNIcellular organisms
What is cancer, biologically-speaking? What are different types of cancer-causing mutations?
Cancer: "disease of individual cells that is a result of uncontrolled proliferation"
-proliferation occurs as a result of genetic changes within individual cells
-multiple mutations that promote the cancer phenotype accumulate over time
-Types of cancer causing mutations can be found in genes that regulate:
1) cell growth & division
2) apoptosis (programmed cell death)
3) metastasis (ability to travel through bloodstream)
4) angiogenesis (ability to generate blood vessels)
Name & describe the 2 types of signaling molecules & receptors.
What type of receptor is a steroid hormone receptor?
1) Lipid soluble: "molecule can cross plasma membrane to enter cell; uses intracellular receptors where signal PHYSICALLY ENTERS the cell"
2) Lipid-insoluble: "large, hydrophilic molecules that CANNOT cross plasma membrane; use cell surface receptor where signal is transducer across the cell but does NOT physically enter the cell"
-steroid hormone receptor = lipid soluble, binds to intracellular receptor
What are the general steps involved in cell signaling?
1) Signal --> receptor
-factor outside the cell
-protein on/in cell binds to signal, and BINDNG ACTIVITY CHANGES RECEPTOR ACTIVITY
2) receptor --> signaling intermediates
-factors within the cell are altered in response to receptor activation
**allows for SIGNAL AMPLIFICATION**
3) Signal intermediates --> effectors
-ultimate mediators of HOW the cell responds (often transcription factors)
4) effectors --> cell response
-signal deactivation
What does Ras activate?
-Ras activates series of KINASES, called the MAPK pathway
-MAPKKK is activated by DIRECT interaction with RasGTP, which leads to conformational change
-MAPKKK phosphorylates MAPKK
-MAPKK phosphorylates MAPK
Draw the cell cycle + all its stages
-What determines the lengths of each stage?
-Lengths of each stage of cell cycle (aka, alternation of interphase + M phase stages) can vary in different tissues/organisms
What are tumor suppressor genes? What types of mutations do they promote? Ex?
What are proto-oncogenes? What types of mutations do they promote? Ex?
1) Tumor suppressor genes: "genes that protect a cell from cancer"
-loss of function mutation that promotes cancer
-BOTH copies of tumor suppressor alleles must be DEFECTIVE to lead to ABNORMAL development; if only ONE is damaged, second can still produce enough correct protein on its own
-ex: Rb, p53
2) Proto oncogenes: "genes that cause cancer when hyperactive"
-gain of function mutations that promote cancer (oncogenes)
-ex: Ras, telomerase
What are protein kinases? How do they relate to phosphorylation?
1) Protein Kinases:
-enzymes
-use ATP as a substrate
-have a specific set of substrate(s)
2) phosphorylation acts to CHANGE protein CONFORMATION by affecting folding of the protein
Name & describe the 3 types of short-range signaling.
When is short-range signaling important? What types of cells is it used for?
1) Paracrine signaling (binds to receptors and stimulates nearby cells)
2) Juxtacrine signaling (contact-dependent signaling, requires cells to be in close contact)
3) Autocrine signaling (binds to receptors on cell that secreted the signal)
-short-range important for development + tissue maintenance
-found in ALL cell types
What happens once MAPK is activated?
-Active (phosphorylated) MAPK moves into the nucleus & activates transcription factors through phosphorylation
-gene involved in cell division get transcribed, which causes a change in gene expression
What are the components of interphase? What occurs?
Interphase: G1, S, G2 stages
-chromosomes replicated during S phase & centrosomes are duplicated
-Organelles replicated + cell growth occurs during G1 & G2
How do most cancers arise (is there genetic predisposition?)
Give 2 examples of genes that can be inherited that have shown to cause a predisposition to cancer
1) MOST cancers arise "de novo" -- aka with NO genetic predisposition
2) SOME cancer can arise due to genetic predisposition-- which occurs when a mutant version of 1 allele is inherited
ex: BRCA 1 +/-, RB +/-
How is kinase signaling turned ON/OFF?
-Kinases: "add phosphate group to proteins using ATP hydrolysis"
-Phosphatases: "REMOVE phosphate group by hydrolysis"
--> Conformation + activity of protein is greatly changed by phosphorylation/de-phosphorylation
Name & describe the 2 main types of long-range signaling.
What types of cells use long-range signaling?
1) Synaptic: "essential for NERVOUS systems to function"
2) Endocrine: "essential for physiology + HOMEOSTASIS"
-long-range signaling is used in SPECIALIZED cells
How does amplification occur?
-Signal arrives and binds at receptor
-receptor changes conformation, bound G-protein replaces GDP with GTP
-activated alpha subunit of G-protein binds to & activates protein (enzyme)
-Enzyme produces secondary messengers + triggers response!
-signaling molecule then detaches, and receptor is inactivated
-G-protein hydrolyzes GTP to GDP is inactivated, and secondary messengers are degraded
-Signal is DE-activated!
**COMMON THEME: conformation change in protein CHANGES its activity***
What occurs in each stage of mitosis?
1) prophase: chromosomes condense, microtubules form & migrate to opposite poles
2) prometaphase: nuclear envelope breaks down, microtubules contact chromosomes at kinetochores
3) metaphase: chromosomes line up in the middle of the cell (metaphase plate)
4) anaphase: sister chromatids separate & are pulled to OPPOSITE sides of cells; they are now individual chromosomes
5) telophase: chromosomes de-condense, nuclear envelope re-forms
6) Cytokinesis: once mitosis is complete, cytoplasm divides & forms 2 identical daughter cells
Explain what a Ras oncogene is, how could you treat this?
-Ras oncogenic mutations PREVENT Ras from hydrolyzing GTP --> GDP, thus Ras is locked into an "ON" state
-do NOT respond to receptor kinase antibody treatment, why?
-currently NO drugs to treat hyper-activated ras protein, so patients get chemotherapy
What is a checkpoint? Where are each of the 3 checkpoints in the cell cycle/what do you need to pass?
-Checkpoint: "interactions between regulatory molecules that determine whether a cell proceeds with division"
1) G1 checkpoint
-cell size adequate
-nutrients sufficient
-social signals (growth factors) present
-DNA undamaged
*MOST important checkpoint for MOST cells*
2) G2 checkpoint
-chromosomes have replicated successfully
-DNA undamaged
-Activated MPF present
3) Spindle Assembly Checkpoint (M phase)
-all chromosomes properly attached to spindle apparatus
3 domains:
1) extracellular signal binding domain
2) transmembrane domain
3) cytoplasmic enzymatic domain
-ligand binding changes receptor conformation --> DIMERIZATION (conformational change in receptor = change in receptor's activity)
-kinse domains of dimerized receptors phosphorylate both each other + their targets --> ATP hydrolysis is used for phosphorylation
**important in MAPK pathway!
What are the differences between phosphoprotein activation/deactivation and G-protein activation/deactivation?
1) Phosphoprotein:
-change in protein conformation by COVALENT addition of phosphate
-removal of phosphate by a phosphatase to inactivate protein
-addition of non-hydrolyzable form of ATP INHIBITS signaling
2) G-protein:
-change in protein conformation by NON-covalent binding of GTP
-GTP hydrolyzes itself to inactivate G-protein
-addition of non-hydrolyzable form of GTP ACTIVATES signaling
What is a mitosis promoting factor? How does its concentration change during the cell cycle? Why is this important?
MPF: "composed of 2 distinct subunits: a protein kinase which catalyzes phosphorylation of a target protein by ATP hydrolysis + a cyclin"
-concentration of MPF protein kinase does NOT change much during cell cycle; however, MPF cyclin concentration INCREASES during interphase, then peaks in M phase before decreasing again
-MPF protein kinase is a cyclin-dependent kinase that is active ONLY when bound to cyclin subunit
-when cyclin concentrations are HIGH, MORE MPF is active & target proteins are phosphorylated, causing initiation of mitosis
**moving through a checkpoint REQUIRES CDK activity
*moving through a checkpoint requires the activity of a cyclin-dependent kinase*
What is tumor angiogenesis?
How is MPF deactivated? What type of feedback regulation is this and whY?
M-cyclin is ubiquitinated by an enzyme that is activated during anaphase (of mitosis)
-enzyme targets cyclin to a proteasome, which destroys the cyclin
-NEGATIVE feedback regulation b/c a process is being SLOWED DOWN by one of its PRODUCTS