Cell death in neural development

Cell death in neural development

1. Ways to die

   • Programmed cell death
     • Apoptosis (Type 1)
       • Most well studied
       • Activated by extrinsic and intrinsic signals
       • Important in development and disease
     • Autophagy (Type 2)
       • Less well studied
       • Triggered by starvation, Protein aggregates
       • Important in disease
         • Tumor supressive
         • Neurodegeneration
     • Cytoplasmic (Type 3)
   • Necrosis

2. Apoptosis

   • Experiments
     • Knockout of ced-3 (Cell death abnormal) or egl-1 (Egg laying defective)
       • Too many cells
         • These are 'pro-apoptotic' proteins
     • Knockout of ced-9
       • Too much cell death
         • ''Anti-apoptotic" protein
   • Homologs of c.elegans cell death proteins conserved in vertebrates - regulate cell death
     • With some new elements
       • Bax (Bcl-2 associated)*
         • *Bcl-2homology(BH)domain proteins, including BH3-only, Bax
       • Caspase 9 (cysteine-aspartic acid protease)
         • Two classes of caspase
           • Executioner (or effector) caspase (e.g.caspase1)
           • Initiator caspase (e.g. caspase 9)
         • Caspase cascade
           • At least 10 mammalian caspases
             • Activated by auto-proteolysis
               • Cascade effect
         • Also have many other substrates
           • DNA repair enzymes (PARP)
           • Nuclear Lamins
           • Maybe as many as 5% of cell proteins are targets!
         • Two paths to activation
           • Intrinsic Pathway
             • Pro-apoptotic BH domain proteins (Bax, Bak) permeablise mitochondrial wall
               • BalancebetweenBcl-2(anti- apoptotic) and Bax/Bak controls initiation
                 • Released cytochrome c binds APAF-1, which binds and activates procaspase 9
           • Extrinsic pathway
             • Due to activation of Death Receptors by external ligands, e.g. TNF
               • Death receptors have intracellular Death Domains which bind adaptor proteins and procaspase 8 into a Death Inducing Signalling Complex (DISC), thus releasing active caspase 8

3. Role in development

   • Cell death is critical
     • Loss of caspase 9 leads to exencephaly
     • Why cell death?
       • PCD occurs in all tissues in development and throughout life
         • Why? (see Raff 1996, Cell v86, p173, for great discussion)
       • Size control
         • Most tissues are inter-dependent, you must keep things in proportion
         • Axons receive ‘trophic support’ from their targets (and vice-versa)
       • Removal of transient structures
         • E.g. subplate neurons in cortex, Ti1 axons in grasshopper limb: both are scaffolds for later axons and die once their job is done
       • Functional tuning - matching
         • Elimination of synapses
         • Viktor Hamburger 1900-2001 Student of Hans Spemann, mentor to Levi-Montalcini and Stanley Cohen, discoverers of NGF.
           • What factors underlie this interdependence?
             • Neurotrophins
               • lack of NGF promotes survival and in some contexts its absence actively promotes cell death.
               • Over expression of p75NTR in Trk-expressing cells drives cell death
                 • Lowered expression of p75NTR promotes cell survival
                   • Binding of a neurotrophin to p74NTR in the absence of its appropriate Trk drives death
                     • p75NTR acts as a complex ‘death switch’ to tune neuronal survival to innervation of appropriate targets
                       • Because of its 'death switch' properties, P75NTR is known as a 'dependence receptor'
                         • There are now a large group of ‘dependence receptors’ defined by their ability to induce PCD in the absence of their respective ligands via caspase- dependent cleavage of their ICDs to release, poorly- defined, proapoptotic peptides
                           • Interestingly, many of the identified dependence receptors are known guidance receptors (also include EphA4 and TrkC)
                             • DCC = Deleted in Colon Cancer
               • Electrical activity can also modify response to death signals through activation of neurotrophin expression
   • Its not all about cells dying
     • The pruning back of initially exuberant growth is a widespread feature of NS development
       • e.g. cortico-spinal and cortico-collicular projections are initially similar, but are differentially pruned
       • Many features of pruning are shared with Wallerian degeneration and with apoptosis, including
         • Cell fragmentation
         • Clearing by phagocytosis
         • Led to the speculation that PCD pathways may be involved in both...
           • Local caspase activity directs pruning
             • DRONC is the caspase9 (ie initiator caspase) equivalent in drosophila
             • Dendrites of local sensory neurons are pruned by local degeneration, involving branch severing, blebbing and fragmentation cf. Wallerian deg
             • Bloackade of DRONC (Lof or DN) blocks pruning
             • A reporter of caspase activation (based on PARP, an Effector caspase substrate), shows local activity at sites of branch severing
               • Evidence suggests caspases do not initiate severing of branches but are involved in the subsequent process of engulfment by phagocytes
             • Caspases also involved in axon degeneration

4. See ppt for last 4 slides

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