Mechanisms of Disease I Flashcards
How does a zygote turn into a mature multicellular organism?
Cell growth and differentiation are the basic mechanisms responsible for turning a zygote into a mature multicellular organism
- Cell growth = a bigger organism more cells
- Differentiation = cells become complex (usually) an end to growth
- Cell growth precedes differentiation, but with some overlap
What 3 groups do diseases related cell growth and differentiation
fall into?
• Developmental conditions
- Can be related to cell growth or differentiation (or both)
- E.g. Neural tube defects like spina bifida
• Neoplasia (and metaplasia)
- E.g. cancer, tumours
• Others, e.g. cardiac hypertrophy
What are the 2 main forms of cell growth?
- hypertrophy (bigger cells)
* hyperplasia (more cells) - most common
Hypertrophy
- Hypertrophy is simply cells growing bigger
- More proteins, more membrane etc etc.
- Elevated protein synthesis is a big driver of increased cell size
- The heart is a classic example
Hyperplasia
- Hyperplasia – more cells – is caused by cell division, or proliferation
- example is the cell cycle
Differentiation
– Exit from the cell cycle
• Differentiated cells are “post-mitotic”
– A program of cell type-specific gene expression
– Cell morphology and function changes
diagram
Is there anything in common with Hypertrophy and Hyperplasia?
• Yes – the mechanisms governing them
• Cell growth and differentiation are governed by the integration of multiple signals:
– intra- and extracellular signals (checks on cellular physiology, growth and inhibitory factors, cell adhesion
etc.)
• Signals converge on the promoters of key genes
– Promoters act as “co-incidence detectors”
– Express gene YES/NO? How much?
Extracellular Signals Recap
(RECAP)
• Ligand – Receptor – Intracellular cascade
• Three broad classes:
• Paracrine: produced locally to stimulate proliferation of a different cell type that has the appropriate cell surface receptor
• Autocrine: produced by a cell that also expresses the appropriate cell surface receptor
• Endocrine: like conventional hormones, released systemically for distant effects
Extracellular Signals in Cell Growth and differentiation
Proteins that:
• Stimulate proliferation and promote survival
- Mitogens e.g. Growth factors and interleukins (EGF, FGF, NGF, PDGF, IGF1, IL2, IL4)
- Induce differentiation and inhibit proliferation, e.g. TGF-beta
- Can do either, e.g. Wnt ligands
• Induce apoptosis, e.g. TNFα and other members of the TNF family
diagram
Phases of the cell cycle
diagram
FACs analysis of cell DNA Content
If a DNA stain is applied, FACs can measure the DNA content of every cell in a population - diagram 1 +2
Fluorescence Microscopy
Blue= DNA Red = γ-tubulin Green = CHEK2 Yellow = centrioles (γ-tubulin and CHEK2 colocalised)
diagram
Revision mitosis slides
- Prophase (1)
- Nucleus becomes less definite
- Microtubular spindle apparatus assembles
- Centrioles migrate to poles
- Prometaphase
- Nuclear membrane breaks down
- Kinetochores attach to spindle in nuclear region
- Metaphase (2)
- Chromosomes align in equatorial plane
- Anaphase (3)
- Chromatids separate and migrate to opposite poles
- Telophase (4)
- Daughter nuclei form
- Cytokinesis
- Division of cytoplasm
- Chromosomes decondense
Cell cycle checkpoints
Controls (involving specific protein kinases and phosphatases) ensure the strict alternation of mitosis and DNA replication.
diagram
CYCLIN-DEPENDENT
KINASES (CDK)
diagram
Regulation of Cyclin-CDK activity
• Cycles of synthesis (gene expression) and
destruction (by proteasome)
• Post translational modification by phosphorylation
• May result in activation, inhibition or destruction
• Dephosphorylation
• Binding of cyclin-dependent kinase inhibitors (CDKIs)
RETINOBLASTOMA
PROTEIN (RB)
diagram
SEQUENTIAL ACTIVITIES
IN THE CYCLE
diagram
Revision slide -SEQUENCE OF EVENTS
TRIGGERED BY GROWTH
FACTORS
- Growth factor signalling activates early gene expression (transcription factors – FOS, JUN, MYC)
- Early gene products stimulate delayed gene expression (includes Cyclin D, CDK2/4 and E2F transcription factors)
- E2F sequestered by binding to unphosphorylated retinoblastoma protein (RB)
- G1 cyclin-CDK complexes hypophosphorylate RB and then G1/S cyclin-CDK complexes hyperphosphorylate RB releasing E2F
- E2F stimulates expression of more Cyclin E and S-phase proteins (e.g. DNA polymerase, thymidine kinase, Proliferating Cell Nuclear Antigen etc.)
- S-phase cyclin-CDK and G2/M cyclin-CDK complexes build up in inactive forms. These switches are activated by post-translational modification or removal of inhibitors, driving the cell through S-phase and mitosis.
Checkpoint reminder - What if there is DNA damage?
diagram
DNA damage triggers cell cycle arrest or apoptosis
ROLE OF TP53
diagram
TP53 AND CANCER
• TP53 loss-of-function mutations are amongst the most frequent in cancer
– Prevent cell cycle arrest - faster growth
– Prevent apoptosis - don’t die
– Prevent DNA repair - more mutations = more heterogeneity, more adaption, cancer progression
Chemotherapy pt 1
CHEMOTHERAPY
• Traditional chemotherapeutic drugs act on the cell cycle
– Objective: stop proliferation, induce apoptosis
• S-phase drugs cause DNA damage, e.g.
– 5-fluorouracil (prevents synthesis of thymidine)
– Cisplatin (binds to DNA causing damage and blocking repair)
Chemotherapy pt 2
- M-Phase drugs target the mitotic spindle
- Vinca alkaloids
- stabilize free tubulin
- prevent microtubule polymerization
- arrest cells in mitosis
- Paclitaxel (Taxol)
- stabilizes microtubules
- preventing de-polymerization
- arrests cell in mitosis
- Not just cancer: colchicine (similar mode of action to vinca alkaloids) is used for immune-suppression
revision slides- summary of key steps
• Growth factors binding to receptors induce gene expression
• G1 and G1/S Cyclin-CDK complexes phosphorylate RB in the absence of inhibition by CKIs (expression of these is regulated by TP53 or TGF-beta)
• E2F released, stimulating expression of genes required for S-phase
• Cell replicates DNA (expression of S-phase Cyclin-CDK complexes)
• If all DNA replicated, G2/M Cyclin-CDK complexes cause cell to enter
mitosis. If chromosomes aligned on spindle, exit from mitosis is triggered
• If process fails, TP53 initiates apoptosis
