Lecture 23: Programmed Cell Death and Neurotrophic Factor

What are the major hallmarks of embryonic development?

• generation of new cells

• growth of tissues and organs

• acquisition of new phenotypes and function

What did M. Ernst, A. Glucksmann, and R. Collin do?

• first description of the wide-spread cell death in the nervous system

• received much skepticisms in the scientific community

What did Rita Levi-Montalcini and Giuseppe Levi do?

upon limb bud (peripheral target) removal, neurons in the spinal ganglia in the chick embryo develop normally up to a certain stage, after which a gradual neuron loss ensues

Who is coined with the first description of normal embryonic neuronal death and regression?

Hamburger and Levi-Montalcini

What did Hamburger and Levi-Montalcini propose?

that both normal cell death (without limb removal) and death following limb removal (regression) result from a lack of target-derived substances necessary for growth and survival

How does the removal of a developing limb bud affect the number of motor neurons?

reduces motor neurons

How does the transplantation of an extra limb bud affect the number of motor neurons?

motor neurons are increased

What is the neurotrophic hypothesis?

• developing neurons survive only when they successfully compete for target-derived trophic molecules that are internalized at the nerve terminal and are transported retrogradely along the axon to the cell body

• neurons that fail to obtain a sufficient blood flow of trophic molecules from the target site undergo programmed cell death

Describe the Levi-Montalcini, Meyer, and hamburger axonal halo assay.

• sympathetic or sensory ganglia removed from developing animals and cultured 24 hour in vitro

• NGF treatment results in a dense halo of axonal outgrowth

• this phenotype and assay was first used to purify and characterize NGF as a neurotrophin

List the experimental immunosympathectomy. What does this prove?

1. antibodies that selectively block NGF activity were administered to newborn mice

2. deprive newborn mice of endogenous NGF in vivo

3. complete loss of sympathetic neurons

• NGF is required for the survival of these neurons

What is programmed cell death (PCD)?

the spatially and temporally reproducible and species-specific loss of large numbers of individual cells during development

True or False: Programmed does not equate to genetic predetermination.

true

What does PCD occur in?

• all vertebrates and invertebrates

• many different cells and tissues

• unicellular and multicellular organisms

Why is PCD important for nervous system development?

• pathway formation

• balance of genesis and removal

True or False: PCD occurs in virtually all types of developing neurons and glia cells during development from the time of proliferation until synapse/circuit establishment.

true

Why does PCD occur?

• establish optimal (adaptive) levels of connectivity between neuronal populations

• eliminate aberrant cells or connections

• regulate the size of progenitor populations

What does the dysregulation of PCD cause?

• developmental and congenital neurological defects

• cancers

Why does PCD occur in the PNS?

eliminate aberrant connections and excess glia

When does PCD occur in the PNS?

during synapse formation

What is the result of PCD in the PNS?

refined neural connections

Where does neurogenesis occur in the CNS?

ventricular zone

When does PCD occur in the CNS?

during neurogenesis and migration

Why does PCD occur in the CNS?

eliminate aberrant cells and connections

What occurs in the ventricular zone?

cell proliferation

What follows proliferation?

migration and process formation

Where do migrating neurons settle?

• nucleus

• ganglion

• cortical layer

During regulation, which cells are overproduced? What compensates this overproduction?

• oligodendrocytes and Schwann cells

• many die by PCD to achieve proper number of these cells for optimal myelination of the available axons

What is the PCD of glia cells regulated by?

axon-derived trophic signals

Characterize necrosis.

• initial swelling of cells

• cytoplasmic vacuolization

• breakdown of organelles

• rupture of cell membrane

• release of cellular contents

• shrinkage and loss of chromatin

Characterize apoptosis.

• shrinking cells

• chromatin condensation

• cell membrane and organelles remain intact

• break up of cytoplasm and nucleus into membrane bound apoptotic bodies

What is autophagy?

the regulated degradation of cytoplasmic constituents by autophagosomes

What is cytoplasmic cell death?

cytoplasmic vacuolation characterized by vacuoles of ER and mitochondria

True or False: The apoptosis pathway is evolutionary conserved.

true

What are the pro apoptotic factors released from the mitochondria?

• Cyt C

• Smac

• AIF

What type of dimers are neurotrophins released as? What does this this type of dimer contain?

released as homodimers with the conserved region containing a cysteine knot in the cord of the molecule

What is TRk receptor tyrosine kinase activated by?

NGF binding and dimerization

What binds to p75NTR?

• Nogo receptor

• Lingo-1

• Sotillin

• all NTFs

What do p75NTR lack? What can they still do?

lack a cytosolic kinase domain, but interact with other signaling proteins involved in neurite outgrowth and myelin-based growth

What does the survival of sympathetic and sensory neurons require?

NGF or NT-3

NGF KO mice shows phenotype similar to TrkA KO mice. Speciifcally, what is the phenotype?

• a few subpopulation of peripheral neurons including sympathetic and some sensory neurons are lost

• survival of CNS neurons is largely unchanged

What are the additional NTFs that support the survival of different classes of neurons?

• PLC

• Ras-MAP kinase

• PI-3 kinase

Where do PNS neurons receive trophic support?

• retrograde support from targets

• anterograde support from afferent inputs

Where do CNS neurons receive trophic support?

• incoming neurons

• targets

True or False: CNS neurons generally have many more synaptic connections than PNS neurons.

true

What are the functions of neurotrophines (NGF, BDNF, NT-3, NT-4/5, NT-6)?

• neuronal survival

• differentiation

What are the functions of ciliary neurotrophic factor (CNTF) and leukemia inhibitory factor (LIF)?

• survival of ciliary neurons

• cholinergic survival and differentiation

What are the functions of glial cell line-derived neurotrophic factor (GDNF)?

• neuronal survival

• dopaminergic cell differentiation

What are the functions of insulin-like growth factor (IGF), fibroblast growth factor (FGF), and hepatocyte growth factor (HGF)?

neuronal survival and differentiation

What are the non-survival functions of NTFs?

• cell fate decisions (stem cell)

• axon guidance

• neurite growth and branching

• dendrite development

• synapse development

• synapse stabilization / plasticity

• LTP, learning and memory

• pain physiology and pathophysiology

• neurotransmitter phenotype

• ion channel regulation / function

• neurological disease and degeneration

• psychiatric disease: depression

How are NTFs produced?

from the cleavage of their large precursors by furins and prohormone convertase

True or False: Both NTFs and proneurotrophins (large precursors) contribute to physiological functions.

true

What do proneurotrophins preferentially bind to?

p75NTR

Where are NTFs secreted?

at presynaptic and postsyanptic sites

What are the two modes of NTF secretion?

• constitutive mode

• activity-dependent mode

What is the constitutive mode?

NTFs are secreted from cell bodies and processes

What is the activity-dependent mode?

NTFs are secreted from dendrites and axons

What is crucial for NTF release? What does this mean?

• high frequency stimulation is crucial for release

• thus regulated release of NTFs is restricted to active synapses

What does secretion of NTFs regulate?

synaptic transmission and plasticity

List the steps in synthesis and secretion of BDNF.

1. synthesis of unprocessed BDNF protein

2. prodomain binds sortilin and stimulates folding of mature domain in golgi

3. motif located in mature domain binds CPE and sorts protein to regulated pathway

4. intracellular cleavage by furin or PC1

5. extracellular cleavage by plasmin or MMP

What is the mechanism underlying axonal transport of trophic signals from the nerve terminal to neuronal soma?

1. neurotrophin binds to its receptor Trk

2. internalized together

3. Trk remains active

4. at the endosome, recruits signaling proteins

5. dynein-dependent retrograde axonal transport

6. at the soma, initiate signaling cascade that results in gene expression

What is PCD regulated by?

• neurotrophic factors (PTFs) provided by targets, afferents, and critical survival promoting signals that prevent PCD

• neuronal activity and synaptic transmaission

• nonneuronal cells

• regulating signaling pathways that modulate the expression of pro- and anti-apoptotic gene products

True or False: PCD is present in neurodegenerative diseases, traumatic CNS injury, and neurological disorders.

true

What can PCD be triggered by?

• free radicals

• excitatory amino acids

• eicosanoids

• lipid degeneration products

• tissue cations

• inflammation

• immune responses