FUNBIO 15 Cell Function - Cell Death and Differentiation PDF

Cell Function: Cell Death and Differentiation Class Foundation Year Course Fundamentals of Human Biology Code FUNBIO.15 Lecturer Tom Hodgkinson Date 29/10/24 Learning Outcomes At the end of this lecture, the learner will be able to ALO1: Describe the process of cell death...

Cell Function: Cell Death and Differentiation Class Foundation Year Course Fundamentals of Human Biology Code FUNBIO.15 Lecturer Tom Hodgkinson Date 29/10/24 Learning Outcomes At the end of this lecture, the learner will be able to ALO1: Describe the process of cell death or apoptosis. ALO2: Define the role of mitochondria in apoptosis. ALO3: Understand the process of cell differentiation, and the concept of unipotent and pluripotent cells. ALO4: Describe the role of the organelles in cell differentiation and in tissue formation. ALO5: Explain the cellular processes triggered in cancer cell formation. ALO6: Define mesenchymal cells and discuss their differentiation into muscle and connective tissue. ALO7: Differentiate between adult and embryonic stem cells. ALO8: Discuss the ethical issues involved in stem cells research. Q: In what circumstances might it be beneficial for a cell trigger its own death? - Genetic mutation/ damage - Irreparable damage – e.g., radiation, chemical injury, pathogenic attack - Cell stress, e.g., heat, pH changes - Cell death induced by cytotoxic T cells or activation of death receptors on cell surface - Mitochondrial stress or injury - Injury to cell membrane - Growth factor/ hormone gradients, e.g., embryonic development - Cell cycle dysregulation/ cancer Apoptosis- Programmed cell death Necrosis - traumatic cell death that results from acute cellular injury. Apoptosis - highly regulated and controlled process of cell death. 2002 Nobel Prize in medicine was awarded to Syndney Brenner, H. Robert Horvitz and John Sulston for their work identifying the genes that control apoptosis. Occurs in multicellular organisms and some eukaryotic, single-celled microorganisms, such as yeast. Biochemical events lead to characteristic changes to cell morphology and eventually death. Cell shrinkage Blebbing Apoptotic body Phagocytosis Chromatin DNA condensation formation condensation mRNA Decay Nuclear fragmentation Organelle collapse Average adult human loses 50-70 billion cells each day due to apoptosis. Separation of fingers and toes in developing human embryo occurs because cells between undergo apoptosis. Unlike necrosis, apoptosis produces cell fragments called apoptotic bodies that phagocytes engulf and remove before the contents of the cell can spill out onto surrounding cells and damage them. Apoptosis cannot stop once it has begun, so it is a highly regulated process. Mechanisms of Apoptosis Apoptosis can initiate through two pathways: Intrinsic pathway – the cell kills itself because it senses cell stress internally. Extrinsic pathway – the cell kills itself because of signals from other cells. Weak external signals can also cause stress, triggering the intrinsic pathway. Both pathways induce cell death by activating proteases called caspases. Initiator caspases Executioner caspases Indiscriminate protein degradation Intrinsic Pathway (Mitochondrial Pathway) Mitochondria are essential to multicellular life- without them the cell can’t respire aerobically– some apoptotic pathways work this way. Apoptotic proteins target mitochondria to: Increase formation of mitochondrial membrane pores - Mitochondrial swelling Cytochrome C Increase mitochondrial membrane permeability - causes apoptotic effectors to leak out BAX Dissipate potential of mitochondrial membrane BAK APAF-1 - Nitric oxide may do this to increase membrane permeability During apoptosis, Bax and Bak form a heterodimer Pro-Caspase-9 that forms pores in mitochondrial membrane. SMACs Apoptosome Cytochrome C is released - forms the apoptosome by forming a complex with apoptotic protease activating factor-1 (Apaf-1) and procaspase-9. Pro-caspase-9 is cleaved into active caspase-9, which Caspase-9 cleaves pro-caspase-3 into active caspase-3. Mitochondria also release SMACs (second IAPs mitochondria-derived activator of caspases) - Apoptosis Caspase-3 SMACs bind to proteins that inhibit apoptosis (IAPs) Extrinsic Pathway FAS L Requires an external signal – typically from an immune cell (e.g. cytotoxic lymphocyte). FAS Two main mechanisms/models: R i) TNF-induced (Tumor necrosis factor) model ii) Fas-Fas ligand-mediated model FADD FADD Both involve receptors of the TNF receptor family coupled to extrinsic signals. Pro- Caspase 8 Fas (First apoptosis signal) receptor (FAS R) is a transmembrane receptor that binds to the Fas ligand (FasL). Binding of Fas-FasL leads to formation of the Bid death inducing signalling complex (DISC), which includes Fas associated Death Domains (FADD). Caspase 8 Leads to Pro-caspase 8 cleavage and activation. tBid Caspase-8 can also truncate Bid (BH3 interacting- domain death agonist). Caspase 3 Link between TNFa and apoptosis - important role in several human diseases, including autoimmune Intrinsic Pathway diseases. Apoptosis Cytochrome C Explain cellular processes triggered in cancer cell formation Hypothetical cell progression to cancer Normal cells have tightly controlled regulatory Healthy cell mechanisms that prevent unwanted growth and cell division. Initial mutation in proto- oncogene that results in In cancer cells these mechanisms are disrupted. increased cell growth or division As a consequence of this unchecked division and growth, manner cancers eventually form a tumour or neoplasm. Next mutation inactivates cell cycle check point/ inhibitor Metastasis – spreading of cancer cells to different parts of the body (Proto)oncogenes – cancer activating genes. Additional mutation inactivates Mutations or abnormal activity of gene products genome stability factor or DNA involved in regulating growth, cell division, survival repair mechanism and development Often in cancer, oncogene mutations are coupled Rapid accumulation of with loss of expression or function of tumour mutations and unchecked suppressor genes that regulate the cell cycle or growth/ division control cell growth Cancer cell Same Genome - Diverse Structure & Function White Blood Cells Neuron Hepatocytes (Liver) Intestinal Epithelia Osteocytes (Bone) Differential Gene Expression Gene Expression – When the information coded into a gene (DNA sequence) is used to synthesise a functional product (usually a protein) Neuron Osteocytes (Bone) GENE A ON OFF GENE B ON ON GENE C OFF ON PROTEIN A & PROTEIN B PROTEIN B & PROTEIN C Structure and function Structure and function of neuron cell of osteocyte Differential Gene Expression Genes Genes Expressed Expressed by by Neurons Osteocytes Genes expressed by both neurons and osteocytes…. - ‘Housekeeping genes’ – essential to function of the cell - Cell metabolism, protein synthesis, DNA repair, DNA replication etc How Does A Cell Decide What Genes To Express? - Micro-environment DNA Packaging - Cell-Cell and Cell-extracellular matrix binding Heterochromatin - Soluble factors and cues - Mechanical environment - Lineage commitment of parent cell - DNA packaging & epigenetics Euchromatin Stem Cell Determination & Differentiation Stem cell – ‘is an undifferentiated cell that can self-renew and can produce either more stem cells or more differentiated cells Potency- The range or degree of differentiation potential a stem cell possesses. Totipotent- Can form any cell type in the body and the extra embryological tissues Pluripotent- Can form any cell type in the body  Stem Cell Determination & Differentiation Unipotent Differentiated cell- Functional cells of the tissue Multipotent- Can form specific cell types only Pluripotent- Can form any cell type in the body Multipotent- Can form specific cell types only Unipotent Differentiated cell- Functional cells of the tissue Stem Cell Determination & Differentiation Differentiation is a pathway whereby cells make genetic commitments These commitments gradually restrict the development of the descendants of that cell to a limited set of final tissue types Cell determination is progressive fixation of the fate of a cell’s descendants When this path is complete the appearance of the cell may change significantly as the structure and function of the cell matures For example, mesenchyme cells are undifferentiated mesodermal cells of the embryo which will differentiate into muscle and connective tissue Stem Cell Determination & Differentiation The final step leading to cell specialization is called – Cell Differentiation Once this point is reached the process is irreversible (mostly) The Human Body contains about 250 recognisably different types of cells These organise themselves into diverse and complex structures forming tissues and then organs such as the eye, the hand or the brain But remember all these cells are derived from the Zygote A Differentiated Cell Nucleus Still Contains All The Genes Needed For Development Sheep mammary cell Enucleated egg cell Breakthrough with mammalian experiments came in 1996 – Dolly the sheep (born 1997) Electrical shock – stimulates first cell Ian Wilmut, Keith Campbell and co-workers divisions Egg cytoplasm at the Roslin Institute in Edinburgh, Scotland, containing donor cell nucleus Cloned a sheep using the nuclei from a mammary gland cell and an enucleated egg. Blastocyst Stage of Embryonic Transfer to development host mother (Solomon Fig 17.4 p363) Lamb Ethical Questions In Human Cloning Human reproductive cloning has the goal of producing a newborn human that is genetically identical to another human adult Human therapeutic cloning involves duplication of human ES cells or iPSC cells for scientific study or medical purposes; Reproductive cloning – unpredictable outcomes, violation of human dignity, questions about identity and individuality, parent-child relationship, informed consent, impact on society. Therapeutic cloning– embryo destruction, slippery slope argument, commodification of human life Human Embryonic Stem (ES) Cells Smooth muscle cells Neuron Blastocyst Red blood 1 2 ES cells are present in 3 cells liquid drops of this stem cell culture. Feeder cells ES cells Human Embryonic Stem (ES) Cells Pros Cons Pluripotent – make any cell Ethical concerns – destruction of human embryo Regenerative Medicine – repair damaged tissues Legal and regulatory challenges Disease modelling – make disease-specific cell Immune rejection lines Tumor formation Drug Discovery - more accurate cells/ tissues Technically challenging and expensive Alternative technologies now exist Unregulated ES Application Can Lead To Teratoma Formation Due to pluripotency and capacity for cell disivion poorly controlled application can lead to the formation of tumors called teratomas. Pluripotency of a cell line can also be confirmed by whether a teratoma contains tissues derived from each of the embryonic germ layers: endoderm, mesoderm, and ectoderm. Adult stem cells Stem cell populations have been identified in almost every tissue of the adult body. Responsible for replacing damaged cells, repairing tissue and tissue maintenance. Can be harvested from multiple tissues, including bone marrow, peripheral blood, adipose tissue Varying potency but appear to be ultimately limited by developmental lineage, i.e., multipotent. Mesenchymal stem cells (MSCs) and haematopoietic stem cells (HSCs) most commonly used clinically at present. Human Adult Stem Cells Pros Cons Ethical acceptance Limited differentiation potential Low risk of immune rejection (can be Abundance/ harvesting donor site autologous and observed to be immune privileged) Ageing/ mutation risk Clinical track record/ safety – e.g., bone marrow transplants Limited disease modelling Tissue specific regeneration Technically challenging and expensive Induced Pluripotent Stem Cells (iPSCs) Researchers are now able to reprogram the developmental state of mature cells to become like pluripotent embryonic cells. Can do this by delivering a few key transcription factors or even just by a chemical cocktail. Most common method is still delivery of OCT4, SOX2, KLF-4, c-MYC. Induced Pluripotent Stem Cells (iPSCs) Human Induced Pluripotent Stem Cells (iPSCs) Pros Cons Ethical acceptance Genomic instability Patient-specific models Tumorigenic potential Disease modelling/ cell Technically challenging biobanks and expensive Drug discovery Regulatory challenges Tissue regeneration Incomplete understanding Reduced immune rejection Ageing/ mutation risk Stem Cells In The Clinical Scenario May be the same person (autologous) or a different person (allogeneic) Stem Cells In The Clinical Scenario Stem Cells In The Clinical Scenario Stem Cells In The Clinical Scenario Summary and Learning Outcomes ALO1: Describe the process of cell death or apoptosis. ALO2: Define the role of mitochondria in apoptosis ALO3: Understand the process of cell differentiation, and the concept of unipotential and pluripotential cells. ALO4: Describe the role of the organelles in cell differentiation and in tissue formation. ALO5: Explain the cellular processes triggered in cancer cell formation. ALO6: Define mesenchymal cells and discuss their differentiation into muscle and connective tissue. ALO7: Differentiate between adult and embryonic stem cells. ALO8: Discuss the ethical issues involved in stem cells research.  Learning Resources Online Resources Textbooks https://www.science.org/doi/10.1126/science.1164270 Solomon Chapter 17 p362 – Developmental Genetics https://www.cell.com/fulltext/S0092-8674(06)00976-7 Thank you & Any Questions name EMAIL