Transcript Slide 1
Cell Fate Quiescence Proliferation Apoptosis Differentiation Senescence Necrosis Cell states are mutually exclusive Quiescence Resting cells Post- or pre-mitotic Growth arrested Metabolically active Naïve (Potential to differentiate) Differentiated cells Not differentiating! Potential to proliferate Not proliferating! In G0 of the cell cycle Stem cells Resident pool of progenitor cells Proliferation Maintain cell number Differentiation Accomplish function Proliferation (Cell Division /Mitosis) Cellular division of non-terminally differentiated cells Mitotic stimulation by hormones, growth factors and cytokines Stimulation of Immediate Early Genes (transcription factors c-fos, c-myc, c-jun) Needed to maintain cell population Protein products of IEGs regulate transcription of late genes Change expression of proteins in the cell cycle Functional tissue Regulation of cell cycle / mitosis The cell cycle (a one-way street) Stages cells pass through during cell division G0 (Quiescence) G0 Senescence Terminal differentiation G2-M Restriction Checkpoint 2 Gap Gap DNA synthesis RNA transcription low G1-S Restriction Checkpoint 1 Cell cycle is regulated by cyclins and CDKs Degradation of structural components and cyclins Production of protein complexes for M-phase Cyclin are the regulatory subunit of the CyclinDependent Kinases (CDKs) Activity of CDKs regulated by the availability of the corresponding cyclins Production of protein complexes for S-phase Phosphorylation of complexes for chromatin synthesis Degradation of cell cycle inhibitors (ubiquitination) G1-S restriction Primary regulation point for the control of the cell cycle Growth factors Immediate early Genes Delayed response genes Cyclin E, D-cyclins, E2F-1, CDK2/4 Needed to pass G1-S restriction E2F Group of transcription factors Required for transcription of genes for G1-S transition (DNA replication) Normally (Growth Factor deprived) E2F inhibited by retinoblastoma (Rb) Rb X G1 to S Binds activation domain of E2F T.F. Disrupts transcription Binds histone deacetylase (HDAC) Folds DNA into nucleosome (DNA/histone complex) Hinders TF access Growth factors [Cyclin] ↑ G1 CDK/cyclin complex formed Active kinase Rb-P Dissociates from E2F Transcription of genes for G1 to S The cell cycle is regulated by the fine balance of cyclin expression CDK2/cyclin E complex is essential for G1 to S transition The cell cycle is regulated by the fine balance of cyclin expression CDK2/cyclin E complex is essential for G1 to S transition GFs [D-cyclins] ↑ Complex with CDK 4/6 More cyclin-D D-cyclin -CDK4 ↑↑↑ P-Rb E2F ↑ Cyclin E ↑ Sequests p27KIP Complexes with CDK2 but inhibited by p27KIP CDK2-cyclin E Active G1 to S DNA damage inhibits G1 to S transition until the DNA is repaired Normally p53 tumour suppressor is degraded when DNA is intact DNA damage S-phase p53 stability ↑ [p53] ↓ [p53] ↑ Cells stuck in G1 until DNA damage repaired [p21CIP] ↑ (CDK inhibitor) Binds G1 CDKcyclin complex Alternatively cells may undergo apoptosis Programmed cell death Knockout mice Indicates the in vivo function of the protein p21CIP-/- Apoptosis No inhibition of G1 CDK/cyclin complex Into S-phase with DNA damage No proliferation Tumouragenesis? p21CIP is anti-apoptotic!! NO! Ratio p21 : p53 important P21 : p53 ↑ P21 : p53 ↓ Growth arrest Apoptosis Fine tuning of expression of cyclins, cell cycle inhibitors and tumour suppressors to allow mitogenesis Knockout mice Rb -/- No inhibition of E2F X G1 to S Uncontrolled proliferation? NO! E2F ↑↑↑ Apoptosis Activates p19ARF P21 : p53 ↓ Binds to and inhibits MDM2 (Degrades p53) p53 ↑ There is a fine balance between proliferation and apoptosis G2 to M restriction G2 to M transition also finely controlled Cyclin B synthesis and phosphorylation Controls breakdown of nuclear lamina Multiple checks that prevent uncontrolled proliferation (tumourigenesis) Apoptosis (programmed cell death) Apoptosis = Falling off Ordered set of events Blebbing Nuclear fragmentation Chromatin condensation and fragmentation Cellular fragmentation into apoptotic bodies Phagocytosis Important during embryogenesis Syndactyly Development of free and independent digits Massive death of interdigital mesenchymal cells Brain development ~ 50% neurones undergo apoptosis before adulthood (selection) Immune system Majority of lymphocytes undergo apoptosis during negative selection of antigen receptor Man Turnover of several billion cells / day! External activation Stimulated by cell stress Nutrient deprivation Radiation Heat Viral infection Cytokines Hypoxia GCs Common pathway Apoptosis vs Necrosis Injury Apoptotic pathways Extrinsic pathway Intrinsic pathway Death-inducing signal complex Loss of membrane integrity ATP synthesis stops Change in redox potential Caspase = Cysteine proteases Components of the apoptotic pathway are always present in the cell Waiting for stimulus Anti-apoptotic pathway keeps cells viable Insufficient apoptosis Cancer Activating mutations in BCL-2 Many oncogenes sensitise the cells to apoptosis Also requires apoptosis to be turned off Autoimmunity (Failure to eliminate autoimmune antibodies) Persistent infections (Failure to eliminate infected cells) Excessive apoptosis Neurodegeneration Alzheimer’s disease, Parkinson’s disease, Huntington’s disease Autoimmunity (Uncontrolled apoptosis in specific organs) AIDS (depletion of T lymphocytes) Ischemia (stroke, myocardial infarction) Differentiation Differential expression or repression of genes to confer phenotype and function All diploid cells contain the full complement of genetic material Whole organism from a single cell Inhibition of proliferation Specialised function not conferred until terminal differentiation Pre-Phenotype A Progenitor cell Pre-Phenotype B Pre-Phenotype C Pre-Phenotype D ? Phenotype A Phenotype B Phenotype C Phenotype D Differentiation Differential expression or repression of genes to confer phenotype and function All diploid cells contain the full complement of genetic material Whole organism from a single cell Inhibition of proliferation Specialised function not conferred until terminal differentiation Pre- adipocyte Mesenchymal stromal cell Dedifferentiation Pre- osteoblast Pre- myoblast Pre- chondrocyte Transdifferentiation ? Adipocyte Osteoblast Myoblast Chondrocyte Terminal differentiation Expression of specific markers indicates state of differentiation Adipose-derived stromal cells Cell-cell contact is essential! Coordinated and Synchronized expression of genes Adipocyte Differentiation medium Lipid droplets Terminal differentiation Senescence (aging) Characterised by inability of cells to proliferate Cells are viable but function is compromised Larger, fatter, express β galactosidase Limits the number of cell divisions and possibly somatic mutations An alternative to apoptosis to prevent spread of cancer Progressive telomere shortening Alters RNA splicing in proteins such as progerin (lamin A) which compromise functionality (protective, non-coding DNA at end of chromosome) Many cancers are immortal Stem cells Up-regulation of telomerase (90% tumours) Also upregulate telomerase Adds non-coding DNA onto the telomeres Proliferation needed to supply tissue No telomere shortening Aging Gompetz-Makeham law of mortality Mortality rate increases with age Genetically determined Man at 80 years Mouse at 3 years Old enough for reproductive success! Longevity Differences in Antioxidant enzymes DNA repair Free radical production Hormone signalling over age Stress Chronic activation of the HPA axis Damage and repair Aging ↑ Cell Fate Quiescence Proliferation Apoptosis Differentiation Senescence