Mechanisms of Cell Death Etiology of cell death Major Factors Accidental Genetic Necrosis Apoptosis Necrosis The sum of the morphologic changes that follow cell death in a living tissue or organ Apoptosis a physiological process that includes specific suicide signals leading to cell death The road to necrosis Homeostatic steady state Cellular adaptations Reversible cell injury Irreversible cell injury Cell death Necrosis Pathogenesis of necrosis Necrosis consequences of cell injury Types of necrosis Coagulation necrosis ischemia Liquefaction necrosis escape of hydrolases Enzymatic fat necrosis escape of lipases Caseous necrosis e g bacterial liquefaction Gangrenous necrosis ischemic bacterial liquefaction Necrosis a pathological response to cellular injury Apoptosis a physiological response to specific suicide signals or lack of survival signals Chromatin clumps Chromatin condenses and migrates to nuclear membrane Internucleosomal cleavage leads to laddering of DNA at the nucleosomal repeat length ca 200 bp Mitochondria swell and rupture Cytoplasm shrinks without membrane rupture Plasma membrane lyses Blebbing of plasma and nuclear membranes Cell contents spill out Cell contents are packaged in membrane bounded bodies internal organelles still functioning to be engulfed by neighbours General inflammatory response is triggered Epitopes appear on plasma membrane marking cell as a phagocytic target No spillage no inflammation www chembio uoguelph ca APOPTOSIS AS A PHYSIOLOGICALLY IMPORTANT PROCESS In embryonic and fetal development Tissue developmental programs which control sculpting of embryonic form Developmental organization of the nervous system Elimination of self reactive components of the immune system In the adult On stimulation by T lymphocytes In response to DNA damage or abnormality e g by radiation viral infection or transformation In certain organs and tissues on withdrawal of supporting hormones In addition there are often apoptotic centers in tumors accounting for the paradox of slow gross enlargement in the face of rapid cell proliferation and the rare spontaneous remission www chembio uoguelph ca APOPTOSIS in C elegans C elegans genome 19099 genes 790 seven pass transmembrane receptors 480 zinc finger proteins and 410 protein kinases The life cycle of C elegans from egg to sexual maturity and new eggs is about 3 days ced 1 3 4 and 9 Cell death determining proteins in C elegans are closely related to mammalian apoptosisregulating genes The adult hermaphrodite consists of exactly 959 somatic cells of precisely determined lineage and function Individual cells are named and their relationships to their neighbors are known Overall the 959 somatic cells of adult C elegans arise from 1090 original cells exactly 131 somatic cells undergo programmed cell death in the wild type worm Of the 1090 cells 302 are neurons and many of the programmed deaths also lie in the neuronal lineage www chembio uoguelph ca Autophagic cell death type II programmed cell death meaning that the cytoplasm is actively destroyed long before nuclear changes become apparent Classical apoptotic cell death meaning that the chromatin marginates and the cell and nucleus fragment before morphological changes are seen in intracellular organelles Nature Immunology 4 416 423 2003 Nature Reviews Molecular Cell Biology 2 589 598 2001 Nuclear alterations in different forms of programmed cell death The use of chromatin condensation as a criterion to distinguish apoptosis from apoptosis like PCD has been inconsistent in the scientific literature and the potential for overlapping definitions and errors is large The following examples of classical apoptosis c e and apoptosis like PCD b d f g i might provide a general guideline Examples of control chromatin a and caspase independent chromatin margination triggered directly by microinjection of AIF b Caspase dependent strong chromatin compaction c versus caspase independent AIF driven lumpy chromatin condensation d in PCD of mouse embryonic stem cells e Caspase dependent chromatin compaction to crescent shaped masses at the nuclear periphery and chromatin fragmentation to two compact spheres f or caspaseindependent lumpy chromatin condensation without nuclear fragmentation in colchicine induced neuronal cell death Incomplete lumpy chromatin condensation compare with b d in caspaseindependent apoptosis like PCD triggered by Hsp70 depletion g or the active form of vitamin D i and in caspase dependent TNF induced apoptosis like PCD i in caspase 3 deficient MCF 7 cells APOPTOSIS SIGNALS Mitochondriadependent apoptosis Caspasedependent apoptosis Caspaseindependent apoptosis Nature Reviews Cancer 2 647 656 2002 Death Receptordependent apoptosis Caspasedependent apoptosis Caspaseindependent necrosis The target sequence for Ced 3 and caspases Cys catalytic Asp targeting proteases consists of a tetrapeptide with C terminal Asp D Procaspase 1 can be a substrate for Caspase 1 and autocatalytic activation is common among caspases Thus activation shows positive feedback characteristics consistent with a binary on off regulation Ectopic expression of caspases in mammalian cells induces apoptosis This is the strongest evidence for proteolytic mediation of apoptosis The key intracellular event appears to be caspase activation by proteolytic cascade Multiple caspases control different apoptotic pathways and provide functional redundancy Caspase 1 ICE is one of a family of related proteases which are coexpressed and cDNAs of all caspases can be recovered from a single cell line This appears to provide redundancy for an important function and circumvents what was once an embarrassment that caspase 1 knockout mice still undergo apoptosis The full range of apoptotic processes in mammalian cells involve several caspases some of which have specialized purposes a whole subgroup is involved in cytokine activation rather than apoptosis Some caspases are initiators i e their targets are downstream effector caspases The target sequences of several caspases resemble the activating sequences of other caspases In many cases caspases can target their own activating sequences suggesting autocatalytic positive feedback www chembio uoguelph ca Mammalian Caspases Earnshaw et al 1999 In vivo substrates of effector caspases Nuclear Lamins nucleoplasmin the SR protein 70K U1 hnRNP C RNA Pol I upstream binding factor the p53 regulator MDM2 pRB p27 Kip and p21Cip DNA related MCM3 Repair enzymes including